rtoss

Subversion Repositories:
Compare Path: Rev
With Path: Rev
/ @ 346  →  / @ 347
New file
/httptunnel/README
@@ -0,0 +1,59 @@
httptunnel creates a bidirectional virtual data path tunnelled in HTTP
requests. The requests can be sent via an HTTP proxy if so desired.
 
This can be useful for users behind restrictive firewalls. If WWW
access is allowed through an HTTP proxy, it's possible to use
httptunnel and, say, telnet or PPP to connect to a computer outside
the firewall.
 
If you still don't understand what this is all about, maybe you
can find some useful information in the FAQ file.
 
This program is mostly intended for technically oriented users.
They should know what to do.
 
httptunnel is free software. See COPYING for terms and conditions.
If you like it, I would appreciate if you sent a post card to:
Lars Brinkhoff
Kopmansgatan 2
411 13 Goteborg
Sweden
 
Information and/or latest release should be available from these places:
http://www.nocrew.org/software/httptunnel.html
http://www.gnu.org/software/httptunnel/httptunnel.html
ftp://ftp.nocrew.org/pub/nocrew/unix
ftp://ftp.gnu.org/pub/httptunnel
 
I take no responsibility for what you do with this software. It has
the potential to do dangerous things, like disabling the protection
you system administrator has set up for the local network. Read the
DISCLAIMER file.
 
There are two programs: hts and htc. hts is the httptunnel server
and htc is the client. hts should be installed on a computer outside
the HTTP proxy, and htc should be installed on your local computer.
 
Documentation about how to use the programs should be searched in this
order:
source code
--help output
FAQ
README
 
Having said that, here's a (probably outdated) example:
At host REMOTE, start hts like this:
hts -F localhost:23 8888
At host LOCAL, start htc like this:
htc -F 2323 -P PROXY:8000 REMOTE:8888
or, if using a buffering HTTP proxy:
htc -F 2323 -P PROXY:8000 -B 48K REMOTE:8888
 
Now you can do this at host LOCAL:
telnet localhost 2323
and you will hopefully get a login prompt from host REMOTE.
 
See also
http://metalab.unc.edu/LDP/HOWTO/mini/Firewall-Piercing.html
which is a good introduction to firewall piercing. It also has
describes one way to use httptunnel.
New file
/httptunnel/htc.c
@@ -0,0 +1,674 @@
/*
htc.c
 
Copyright (C) 1999, 2000 Lars Brinkhoff. See COPYING for terms and conditions.
 
htc is the client half of httptunnel. httptunnel creates a virtual
two-way data path tunneled in HTTP requests.
*/
 
#include <stdio.h>
#include <stdlib.h>
#include <unistd_.h>
#include <signal.h>
#include <sys/poll_.h>
#include <sys/time.h>
#include <sys/stat.h>
 
#include "common.h"
#include "base64.h"
 
#define DEFAULT_PROXY_PORT 8080
#define DEFAULT_PROXY_BUFFER_TIMEOUT 500 /* milliseconds */
 
typedef struct
{
char *me;
char *device;
char *host_name;
int host_port;
char *proxy_name;
int proxy_port;
size_t proxy_buffer_size;
int proxy_buffer_timeout;
size_t content_length;
int forward_port;
int use_std;
int use_daemon;
int strict_content_length;
int keep_alive;
int max_connection_age;
char *proxy_authorization;
char *user_agent;
} Arguments;
 
#define NO_PROXY_BUFFER 0
#define NO_PROXY (NULL)
 
int debug_level = 0;
FILE *debug_file = NULL;
 
static void
usage (FILE *f, const char *me)
{
fprintf (f,
"Usage: %s [OPTION]... HOST[:PORT]\n"
"Set up a httptunnel connection to PORT at HOST (default port is %d).\n"
"When a connection is made, I/O is redirected from the source specified\n"
"by the --device, --forward-port or --stdin-stdout switch to the tunnel.\n"
"\n"
" -A, --proxy-authorization USER:PASSWORD proxy authorization\n"
" -z, --proxy-authorization-file FILE proxy authorization file\n"
" -B, --proxy-buffer-size BYTES assume a proxy buffer size of BYTES bytes\n"
" (k, M, and G postfixes recognized)\n"
" -c, --content-length BYTES use HTTP PUT requests of BYTES size\n"
" (k, M, and G postfixes recognized)\n"
" -d, --device DEVICE use DEVICE for input and output\n"
#ifdef DEBUG_MODE
" -D, --debug [LEVEL] enable debugging mode\n"
#endif
" -F, --forward-port PORT use TCP port PORT for input and output\n"
" -h, --help display this help and exit\n"
" -k, --keep-alive SECONDS send keepalive bytes every SECONDS seconds\n"
" (default is %d)\n"
#ifdef DEBUG_MODE
" -l, --logfile FILE specify file for debugging output\n"
#endif
" -M, --max-connection-age SEC maximum time a connection will stay\n"
" open is SEC seconds (default is %d)\n"
" -P, --proxy HOSTNAME[:PORT] use a HTTP proxy (default port is %d)\n"
" -s, --stdin-stdout use stdin/stdout for communication\n"
" (implies --no-daemon)\n"
" -S, --strict-content-length always write Content-Length bytes in requests\n"
" -T, --timeout TIME timeout, in milliseconds, before sending\n"
" padding to a buffering proxy\n"
" -U, --user-agent STRING specify User-Agent value in HTTP requests\n"
" -V, --version output version information and exit\n"
" -w, --no-daemon don't fork into the background\n"
"\n"
"Report bugs to %s.\n",
me, DEFAULT_HOST_PORT, DEFAULT_KEEP_ALIVE,
DEFAULT_MAX_CONNECTION_AGE, DEFAULT_PROXY_PORT,
BUG_REPORT_EMAIL);
}
 
static int
wait_for_connection_on_socket (int s)
{
struct sockaddr addr;
socklen_t len;
int t;
 
len = sizeof addr;
t = accept (s, &addr, &len);
if (t == -1)
return -1;
 
return t;
}
 
static void
parse_arguments (int argc, char **argv, Arguments *arg)
{
int c;
 
/* defaults */
 
arg->me = argv[0];
arg->device = NULL;
arg->forward_port = -1;
arg->host_name = NULL;
arg->host_port = DEFAULT_HOST_PORT;
arg->proxy_name = NO_PROXY;
arg->proxy_port = DEFAULT_PROXY_PORT;
arg->proxy_buffer_size = NO_PROXY_BUFFER;
arg->proxy_buffer_timeout = -1;
arg->content_length = DEFAULT_CONTENT_LENGTH;
arg->use_std = FALSE;
arg->use_daemon = TRUE;
arg->strict_content_length = FALSE;
arg->keep_alive = DEFAULT_KEEP_ALIVE;
arg->max_connection_age = DEFAULT_CONNECTION_MAX_TIME;
arg->proxy_authorization = NULL;
arg->user_agent = NULL;
 
for (;;)
{
int option_index = 0;
static struct option long_options[] =
{
{ "help", no_argument, 0, 'h' },
{ "version", no_argument, 0, 'V' },
{ "no-daemon", no_argument, 0, 'w' },
{ "stdin-stdout", no_argument, 0, 's' },
#ifdef DEBUG_MODE
{ "debug", required_argument, 0, 'D' },
{ "logfile", required_argument, 0, 'l' },
#endif
{ "proxy", required_argument, 0, 'P' },
{ "device", required_argument, 0, 'd' },
{ "timeout", required_argument, 0, 'T' },
{ "keep-alive", required_argument, 0, 'k' },
{ "user-agent", required_argument, 0, 'U' },
{ "forward-port", required_argument, 0, 'F' },
{ "content-length", required_argument, 0, 'c' },
{ "strict-content-length", no_argument, 0, 'S' },
{ "proxy-buffer-size", required_argument, 0, 'B' },
{ "proxy-authorization", required_argument, 0, 'A' },
{ "max-connection-age", required_argument, 0, 'M' },
{ "proxy-authorization-file", required_argument, 0, 'z' },
{ 0, 0, 0, 0 }
};
 
static const char *short_options = "A:B:c:d:F:hk:M:P:sST:U:Vwz:"
#ifdef DEBUG_MODE
"D:l:"
#endif
;
 
c = getopt_long (argc, argv, short_options,
long_options, &option_index);
if (c == -1)
break;
 
switch (c)
{
case 0:
fprintf (stderr, "option %s", long_options[option_index].name);
if (optarg)
fprintf (stderr, " with arg %s", optarg);
fprintf (stderr, "\n");
break;
 
case 'A':
arg->proxy_authorization = optarg;
break;
 
case 'B':
arg->proxy_buffer_size = atoi_with_postfix (optarg);
break;
 
case 'c':
arg->content_length = atoi_with_postfix (optarg);
break;
 
case 'd':
arg->device = optarg;
break;
 
#ifdef DEBUG_MODE
case 'D':
if (optarg)
debug_level = atoi (optarg);
else
debug_level = 1;
break;
 
case 'l':
debug_file = fopen (optarg, "w");
if (debug_file == NULL)
{
fprintf (stderr, "%s: couldn't open file %s for writing\n",
arg->me, optarg);
exit (1);
}
break;
#endif
 
case 'F':
arg->forward_port = atoi (optarg);
break;
 
case 'k':
arg->keep_alive = atoi (optarg);
break;
 
case 'M':
arg->max_connection_age = atoi (optarg);
break;
 
case 'h':
usage (stdout, arg->me);
exit (0);
 
case 'P':
name_and_port (optarg, &arg->proxy_name, &arg->proxy_port);
if (arg->proxy_port == -1)
arg->proxy_port = DEFAULT_PROXY_PORT;
if (arg->proxy_buffer_timeout == -1)
arg->proxy_buffer_timeout = DEFAULT_PROXY_BUFFER_TIMEOUT;
break;
 
case 's':
arg->use_std=TRUE;
arg->use_daemon=FALSE;
break;
 
case 'S':
arg->strict_content_length = TRUE;
break;
 
case 'T':
arg->proxy_buffer_timeout = atoi (optarg);
break;
 
case 'U':
arg->user_agent = optarg;
break;
 
case 'V':
printf ("htc (%s) %s\n", PACKAGE, VERSION);
exit (0);
 
case 'w':
arg->use_daemon=FALSE;
break;
 
case 'z':
{
struct stat s;
char *auth;
int f;
 
f = open (optarg, O_RDONLY);
if (f == -1)
{
fprintf (stderr, "couldn't open %s: %s\n", optarg, strerror (errno));
exit (1);
}
 
if (fstat (f, &s) == -1)
{
fprintf (stderr, "error fstating %s: %s\n", optarg, strerror (errno));
exit (1);
}
 
auth = malloc (s.st_size + 1);
if (auth == NULL)
{
fprintf (stderr, "out of memory whilst allocating "
"authentication string\n");
exit (1);
}
 
if (read_all (f, auth, s.st_size) == -1)
{
fprintf (stderr, "error reading %s: %s\n", optarg, strerror (errno));
exit (1);
}
 
/*
* If file ends with a "\r\n" or "\n", chop them off.
*/
if (s.st_size >= 1 && auth[s.st_size - 1] == '\n')
{
s.st_size -=
(s.st_size >= 2 && auth[s.st_size - 2] == '\r') ? 2 : 1;
}
 
auth[s.st_size] = 0;
arg->proxy_authorization = auth;
}
break;
 
case '?':
break;
 
default:
fprintf (stderr, "?? getopt returned character code 0%o ??\n", c);
}
}
 
if (optind == argc - 1)
{
name_and_port (argv[optind], &arg->host_name, &arg->host_port);
if (arg->host_port == -1)
arg->host_port = DEFAULT_HOST_PORT;
}
else
{
fprintf (stderr, "%s: the destination of the tunnel must be specified.\n"
"%s: try '%s --help' for help.\n",
arg->me, arg->me, arg->me);
exit (1);
}
 
if (arg->device == NULL && arg->forward_port == -1 && !arg->use_std)
{
fprintf (stderr, "%s: one of --device, --forward-port or --stdin-stdout must be used.\n"
"%s: try '%s -help' for help.\n",
arg->me, arg->me, arg->me);
exit (1);
}
 
if ((arg->device != NULL && arg->forward_port != -1) ||
(arg->device != NULL && arg->use_std) ||
(arg->forward_port != -1 && arg->use_std))
{
fprintf (stderr, "%s: only one of --device, --forward-port or --stdin-stdout can be used.\n"
"%s: try '%s --help' for help.\n",
arg->me, arg->me, arg->me);
exit (1);
}
 
/* Removed test ((arg->device == NULL) == (arg->forward_port == -1))
* by Sampo Niskanen - those have been tested already! */
if (arg->host_name == NULL ||
arg->host_port == -1 ||
(arg->proxy_name != NO_PROXY && arg->proxy_port == -1))
{
usage (stderr, arg->me);
exit (1);
}
 
if (debug_level == 0 && debug_file != NULL)
{
fprintf (stderr, "%s: --logfile can't be used without debugging\n",
arg->me);
exit (1);
}
 
if (arg->proxy_name == NO_PROXY)
{
if (arg->proxy_buffer_size != NO_PROXY_BUFFER)
{
fprintf (stderr, "%s: warning: --proxy-buffer-size can't be "
"used without --proxy\n", arg->me);
arg->proxy_buffer_size = NO_PROXY_BUFFER;
}
 
if (arg->proxy_buffer_timeout != -1)
{
fprintf (stderr, "%s: warning: --proxy-buffer-timeout can't be "
"used without --proxy\n", arg->me);
arg->proxy_buffer_timeout = -1;
}
 
if (arg->proxy_authorization != NULL)
{
fprintf (stderr, "%s: warning: --proxy-authorization can't be "
"used without --proxy\n", arg->me);
arg->proxy_authorization = NULL;
}
}
else if (arg->proxy_buffer_size == NO_PROXY_BUFFER)
arg->proxy_buffer_timeout = -1;
}
 
int
main (int argc, char **argv)
{
int s = -1;
int fd = -1;
Arguments arg;
Tunnel *tunnel;
int closed;
 
parse_arguments (argc, argv, &arg);
 
if ((debug_level == 0 || debug_file != NULL) && arg.use_daemon)
daemon (0, 1);
 
#ifdef DEBUG_MODE
if (debug_level != 0 && debug_file == NULL)
debug_file = stderr;
#else
openlog ("htc", LOG_PID, LOG_DAEMON);
#endif
 
log_notice ("htc (%s) %s started with arguments:", PACKAGE, VERSION);
log_notice (" me = %s", arg.me);
log_notice (" device = %s", arg.device ? arg.device : "(null)");
log_notice (" host_name = %s", arg.host_name ? arg.host_name : "(null)");
log_notice (" host_port = %d", arg.host_port);
log_notice (" proxy_name = %s", arg.proxy_name ? arg.proxy_name : "(null)");
log_notice (" proxy_port = %d", arg.proxy_port);
log_notice (" proxy_buffer_size = %d", arg.proxy_buffer_size);
log_notice (" proxy_buffer_timeout = %d", arg.proxy_buffer_timeout);
log_notice (" content_length = %d", arg.content_length);
log_notice (" forward_port = %d", arg.forward_port);
log_notice (" max_connection_age = %d", arg.max_connection_age);
log_notice (" use_std = %d", arg.use_std);
log_notice (" strict_content_length = %d", arg.strict_content_length);
log_notice (" keep_alive = %d", arg.keep_alive);
log_notice (" proxy_authorization = %s",
arg.proxy_authorization ? arg.proxy_authorization : "(null)");
log_notice (" user_agent = %s", arg.user_agent ? arg.user_agent : "(null)");
log_notice (" debug_level = %d", debug_level);
 
 
if (arg.forward_port != -1)
{
struct in_addr addr;
 
addr.s_addr = INADDR_ANY;
s = server_socket (addr, arg.forward_port, 0);
log_debug ("server_socket (%d) = %d", arg.forward_port, s);
if (s == -1)
{
log_error ("couldn't create server socket: %s", strerror (errno));
log_exit (1);
}
}
 
#ifdef DEBUG_MODE
signal (SIGPIPE, log_sigpipe);
#else
signal (SIGPIPE, SIG_IGN);
#endif
 
for (;;)
{
time_t last_tunnel_write;
 
if (arg.device)
{
fd = open_device (arg.device);
log_debug ("open_device (\"%s\") = %d", arg.device, fd);
if (fd == -1)
{
log_error ("couldn't open %s: %s",
arg.device, strerror (errno));
log_exit (1);
}
/* Check that fd is not 0 (clash with --stdin-stdout) */
if (fd == 0)
{
log_notice("changing fd from %d to 3",fd);
if (dup2(fd,3) != 3)
{
log_error ("couldn't dup2(%d,3): %s",fd,strerror(errno));
log_exit (1);
}
}
}
else if (arg.forward_port != -1)
{
log_debug ("waiting for connection on port %d", arg.forward_port);
fd = wait_for_connection_on_socket (s);
log_debug ("wait_for_connection_on_socket (%d) = %d", s, fd);
if (fd == -1)
{
log_error ("couldn't forward port %d: %s",
arg.forward_port, strerror (errno));
log_exit (1);
}
/* Check that fd is not 0 (clash with --stdin-stdout) */
if (fd == 0)
{
log_notice("changing fd from %d to 3",fd);
if (dup2(fd,3) != 3)
{
log_error ("couldn't dup2(%d,3): %s",fd,strerror(errno));
log_exit (1);
}
}
} else if (arg.use_std) {
log_debug ("using stdin as fd");
fd = 0;
if (fcntl(fd,F_SETFL,O_NONBLOCK)==-1)
{
log_error ("couldn't set stdin to non-blocking mode: %s",
strerror(errno));
log_exit (1);
}
/* Usage of stdout (fd = 1) is checked later. */
}
 
log_debug ("creating a new tunnel");
tunnel = tunnel_new_client (arg.host_name, arg.host_port,
arg.proxy_name, arg.proxy_port,
arg.content_length);
if (tunnel == NULL)
{
log_error ("couldn't create tunnel");
log_exit (1);
}
 
if (tunnel_setopt (tunnel, "strict_content_length",
&arg.strict_content_length) == -1)
log_debug ("tunnel_setopt strict_content_length error: %s",
strerror (errno));
 
if (tunnel_setopt (tunnel, "keep_alive",
&arg.keep_alive) == -1)
log_debug ("tunnel_setopt keep_alive error: %s", strerror (errno));
 
if (tunnel_setopt (tunnel, "max_connection_age",
&arg.max_connection_age) == -1)
log_debug ("tunnel_setopt max_connection_age error: %s",
strerror (errno));
 
if (arg.proxy_authorization != NULL)
{
ssize_t len;
char *auth;
 
len = encode_base64 (arg.proxy_authorization,
strlen (arg.proxy_authorization),
&auth);
if (len == -1)
{
log_error ("encode_base64 error: %s", strerror (errno));
}
else
{
char *str = malloc (len + 7);
 
if (str == NULL)
{
log_error ("out of memory when encoding "
"authorization string");
log_exit (1);
}
 
strcpy (str, "Basic ");
strcat (str, auth);
free (auth);
if (tunnel_setopt (tunnel, "proxy_authorization", str) == -1)
log_error ("tunnel_setopt proxy_authorization error: %s",
strerror (errno));
 
free (str);
}
}
 
if (arg.user_agent != NULL)
{
if (tunnel_setopt (tunnel, "user_agent", arg.user_agent) == -1)
log_error ("tunnel_setopt user_agent error: %s",
strerror (errno));
}
 
if (tunnel_connect (tunnel) == -1)
{
log_error ("couldn't open tunnel: %s", strerror (errno));
log_exit (1);
}
if (arg.proxy_name)
log_notice ("connected to %s:%d via %s:%d",
arg.host_name, arg.host_port,
arg.proxy_name, arg.proxy_port);
else
log_notice ("connected to %s:%d", arg.host_name, arg.host_port);
 
closed = FALSE;
time (&last_tunnel_write);
while (!closed)
{
struct pollfd pollfd[2];
int keep_alive_timeout;
int timeout;
time_t t;
int n;
 
pollfd[0].fd = fd;
pollfd[0].events = POLLIN;
pollfd[1].fd = tunnel_pollin_fd (tunnel);
pollfd[1].events = POLLIN;
time (&t);
timeout = 1000 * (arg.keep_alive - (t - last_tunnel_write));
keep_alive_timeout = TRUE;
if (timeout < 0)
timeout = 0;
if (arg.proxy_buffer_timeout != -1 &&
arg.proxy_buffer_timeout < timeout)
{
timeout = arg.proxy_buffer_timeout;
keep_alive_timeout = FALSE;
}
 
log_annoying ("poll () ...");
n = poll (pollfd, 2, timeout);
log_annoying ("... = %d", n);
if (n == -1)
{
log_error ("poll error: %s", strerror (errno));
log_exit (1);
}
else if (n == 0)
{
log_verbose ("poll() timed out");
if (keep_alive_timeout)
{
tunnel_padding (tunnel, 1);
time (&last_tunnel_write);
}
else
{
if (tunnel_maybe_pad (tunnel, arg.proxy_buffer_size) > 0)
time (&last_tunnel_write);
}
continue;
}
handle_input ("device or port", tunnel, fd, pollfd[0].revents,
handle_device_input, &closed);
handle_input ("tunnel", tunnel, fd, pollfd[1].revents,
handle_tunnel_input, &closed);
 
if (pollfd[0].revents & POLLIN)
time (&last_tunnel_write);
}
 
log_debug ("destroying tunnel");
if (fd != 0)
{
close (fd);
}
tunnel_destroy (tunnel);
if (arg.proxy_name)
log_notice ("disconnected from %s:%d via %s:%d",
arg.host_name, arg.host_port,
arg.proxy_name, arg.proxy_port);
else
log_notice ("disconnected from %s%d", arg.host_name, arg.host_port);
}
 
log_debug ("closing server socket");
close (s);
 
log_exit (0);
}
New file
/httptunnel/acconfig.h
@@ -0,0 +1,10 @@
/* Enable debugging mode. */
#undef DEBUG_MODE
 
@BOTTOM@
 
/* Define to 'int' if <sys/socket.h> doesn't define. */
#undef socklen_t
 
/* Define to 0xffffffff if <netinet/in.h> doesn't define. */
#undef INADDR_NONE
New file
/httptunnel/doc/rfc1945.txt
@@ -0,0 +1,3363 @@
 
 
 
 
 
 
Network Working Group T. Berners-Lee
Request for Comments: 1945 MIT/LCS
Category: Informational R. Fielding
UC Irvine
H. Frystyk
MIT/LCS
May 1996
 
 
Hypertext Transfer Protocol -- HTTP/1.0
 
Status of This Memo
 
This memo provides information for the Internet community. This memo
does not specify an Internet standard of any kind. Distribution of
this memo is unlimited.
 
IESG Note:
 
The IESG has concerns about this protocol, and expects this document
to be replaced relatively soon by a standards track document.
 
Abstract
 
The Hypertext Transfer Protocol (HTTP) is an application-level
protocol with the lightness and speed necessary for distributed,
collaborative, hypermedia information systems. It is a generic,
stateless, object-oriented protocol which can be used for many tasks,
such as name servers and distributed object management systems,
through extension of its request methods (commands). A feature of
HTTP is the typing of data representation, allowing systems to be
built independently of the data being transferred.
 
HTTP has been in use by the World-Wide Web global information
initiative since 1990. This specification reflects common usage of
the protocol referred to as "HTTP/1.0".
 
Table of Contents
 
1. Introduction .............................................. 4
1.1 Purpose .............................................. 4
1.2 Terminology .......................................... 4
1.3 Overall Operation .................................... 6
1.4 HTTP and MIME ........................................ 8
2. Notational Conventions and Generic Grammar ................ 8
2.1 Augmented BNF ........................................ 8
2.2 Basic Rules .......................................... 10
3. Protocol Parameters ....................................... 12
 
 
 
Berners-Lee, et al Informational [Page 1]
RFC 1945 HTTP/1.0 May 1996
 
 
3.1 HTTP Version ......................................... 12
3.2 Uniform Resource Identifiers ......................... 14
3.2.1 General Syntax ................................ 14
3.2.2 http URL ...................................... 15
3.3 Date/Time Formats .................................... 15
3.4 Character Sets ....................................... 17
3.5 Content Codings ...................................... 18
3.6 Media Types .......................................... 19
3.6.1 Canonicalization and Text Defaults ............ 19
3.6.2 Multipart Types ............................... 20
3.7 Product Tokens ....................................... 20
4. HTTP Message .............................................. 21
4.1 Message Types ........................................ 21
4.2 Message Headers ...................................... 22
4.3 General Header Fields ................................ 23
5. Request ................................................... 23
5.1 Request-Line ......................................... 23
5.1.1 Method ........................................ 24
5.1.2 Request-URI ................................... 24
5.2 Request Header Fields ................................ 25
6. Response .................................................. 25
6.1 Status-Line .......................................... 26
6.1.1 Status Code and Reason Phrase ................. 26
6.2 Response Header Fields ............................... 28
7. Entity .................................................... 28
7.1 Entity Header Fields ................................. 29
7.2 Entity Body .......................................... 29
7.2.1 Type .......................................... 29
7.2.2 Length ........................................ 30
8. Method Definitions ........................................ 30
8.1 GET .................................................. 31
8.2 HEAD ................................................. 31
8.3 POST ................................................. 31
9. Status Code Definitions ................................... 32
9.1 Informational 1xx .................................... 32
9.2 Successful 2xx ....................................... 32
9.3 Redirection 3xx ...................................... 34
9.4 Client Error 4xx ..................................... 35
9.5 Server Error 5xx ..................................... 37
10. Header Field Definitions .................................. 37
10.1 Allow ............................................... 38
10.2 Authorization ....................................... 38
10.3 Content-Encoding .................................... 39
10.4 Content-Length ...................................... 39
10.5 Content-Type ........................................ 40
10.6 Date ................................................ 40
10.7 Expires ............................................. 41
10.8 From ................................................ 42
 
 
 
Berners-Lee, et al Informational [Page 2]
RFC 1945 HTTP/1.0 May 1996
 
 
10.9 If-Modified-Since ................................... 42
10.10 Last-Modified ....................................... 43
10.11 Location ............................................ 44
10.12 Pragma .............................................. 44
10.13 Referer ............................................. 44
10.14 Server .............................................. 45
10.15 User-Agent .......................................... 46
10.16 WWW-Authenticate .................................... 46
11. Access Authentication ..................................... 47
11.1 Basic Authentication Scheme ......................... 48
12. Security Considerations ................................... 49
12.1 Authentication of Clients ........................... 49
12.2 Safe Methods ........................................ 49
12.3 Abuse of Server Log Information ..................... 50
12.4 Transfer of Sensitive Information ................... 50
12.5 Attacks Based On File and Path Names ................ 51
13. Acknowledgments ........................................... 51
14. References ................................................ 52
15. Authors' Addresses ........................................ 54
Appendix A. Internet Media Type message/http ................ 55
Appendix B. Tolerant Applications ........................... 55
Appendix C. Relationship to MIME ............................ 56
C.1 Conversion to Canonical Form ......................... 56
C.2 Conversion of Date Formats ........................... 57
C.3 Introduction of Content-Encoding ..................... 57
C.4 No Content-Transfer-Encoding ......................... 57
C.5 HTTP Header Fields in Multipart Body-Parts ........... 57
Appendix D. Additional Features ............................. 57
D.1 Additional Request Methods ........................... 58
D.1.1 PUT ........................................... 58
D.1.2 DELETE ........................................ 58
D.1.3 LINK .......................................... 58
D.1.4 UNLINK ........................................ 58
D.2 Additional Header Field Definitions .................. 58
D.2.1 Accept ........................................ 58
D.2.2 Accept-Charset ................................ 59
D.2.3 Accept-Encoding ............................... 59
D.2.4 Accept-Language ............................... 59
D.2.5 Content-Language .............................. 59
D.2.6 Link .......................................... 59
D.2.7 MIME-Version .................................. 59
D.2.8 Retry-After ................................... 60
D.2.9 Title ......................................... 60
D.2.10 URI ........................................... 60
 
 
 
 
 
 
 
Berners-Lee, et al Informational [Page 3]
RFC 1945 HTTP/1.0 May 1996
 
 
1. Introduction
 
1.1 Purpose
 
The Hypertext Transfer Protocol (HTTP) is an application-level
protocol with the lightness and speed necessary for distributed,
collaborative, hypermedia information systems. HTTP has been in use
by the World-Wide Web global information initiative since 1990. This
specification reflects common usage of the protocol referred too as
"HTTP/1.0". This specification describes the features that seem to be
consistently implemented in most HTTP/1.0 clients and servers. The
specification is split into two sections. Those features of HTTP for
which implementations are usually consistent are described in the
main body of this document. Those features which have few or
inconsistent implementations are listed in Appendix D.
 
Practical information systems require more functionality than simple
retrieval, including search, front-end update, and annotation. HTTP
allows an open-ended set of methods to be used to indicate the
purpose of a request. It builds on the discipline of reference
provided by the Uniform Resource Identifier (URI) [2], as a location
(URL) [4] or name (URN) [16], for indicating the resource on which a
method is to be applied. Messages are passed in a format similar to
that used by Internet Mail [7] and the Multipurpose Internet Mail
Extensions (MIME) [5].
 
HTTP is also used as a generic protocol for communication between
user agents and proxies/gateways to other Internet protocols, such as
SMTP [12], NNTP [11], FTP [14], Gopher [1], and WAIS [8], allowing
basic hypermedia access to resources available from diverse
applications and simplifying the implementation of user agents.
 
1.2 Terminology
 
This specification uses a number of terms to refer to the roles
played by participants in, and objects of, the HTTP communication.
 
connection
 
A transport layer virtual circuit established between two
application programs for the purpose of communication.
 
message
 
The basic unit of HTTP communication, consisting of a structured
sequence of octets matching the syntax defined in Section 4 and
transmitted via the connection.
 
 
 
 
Berners-Lee, et al Informational [Page 4]
RFC 1945 HTTP/1.0 May 1996
 
 
request
 
An HTTP request message (as defined in Section 5).
 
response
 
An HTTP response message (as defined in Section 6).
 
resource
 
A network data object or service which can be identified by a
URI (Section 3.2).
 
entity
 
A particular representation or rendition of a data resource, or
reply from a service resource, that may be enclosed within a
request or response message. An entity consists of
metainformation in the form of entity headers and content in the
form of an entity body.
 
client
 
An application program that establishes connections for the
purpose of sending requests.
 
user agent
 
The client which initiates a request. These are often browsers,
editors, spiders (web-traversing robots), or other end user
tools.
 
server
 
An application program that accepts connections in order to
service requests by sending back responses.
 
origin server
 
The server on which a given resource resides or is to be created.
 
proxy
 
An intermediary program which acts as both a server and a client
for the purpose of making requests on behalf of other clients.
Requests are serviced internally or by passing them, with
possible translation, on to other servers. A proxy must
interpret and, if necessary, rewrite a request message before
 
 
 
Berners-Lee, et al Informational [Page 5]
RFC 1945 HTTP/1.0 May 1996
 
 
forwarding it. Proxies are often used as client-side portals
through network firewalls and as helper applications for
handling requests via protocols not implemented by the user
agent.
 
gateway
 
A server which acts as an intermediary for some other server.
Unlike a proxy, a gateway receives requests as if it were the
origin server for the requested resource; the requesting client
may not be aware that it is communicating with a gateway.
Gateways are often used as server-side portals through network
firewalls and as protocol translators for access to resources
stored on non-HTTP systems.
 
tunnel
 
A tunnel is an intermediary program which is acting as a blind
relay between two connections. Once active, a tunnel is not
considered a party to the HTTP communication, though the tunnel
may have been initiated by an HTTP request. The tunnel ceases to
exist when both ends of the relayed connections are closed.
Tunnels are used when a portal is necessary and the intermediary
cannot, or should not, interpret the relayed communication.
 
cache
 
A program's local store of response messages and the subsystem
that controls its message storage, retrieval, and deletion. A
cache stores cachable responses in order to reduce the response
time and network bandwidth consumption on future, equivalent
requests. Any client or server may include a cache, though a
cache cannot be used by a server while it is acting as a tunnel.
 
Any given program may be capable of being both a client and a server;
our use of these terms refers only to the role being performed by the
program for a particular connection, rather than to the program's
capabilities in general. Likewise, any server may act as an origin
server, proxy, gateway, or tunnel, switching behavior based on the
nature of each request.
 
1.3 Overall Operation
 
The HTTP protocol is based on a request/response paradigm. A client
establishes a connection with a server and sends a request to the
server in the form of a request method, URI, and protocol version,
followed by a MIME-like message containing request modifiers, client
information, and possible body content. The server responds with a
 
 
 
Berners-Lee, et al Informational [Page 6]
RFC 1945 HTTP/1.0 May 1996
 
 
status line, including the message's protocol version and a success
or error code, followed by a MIME-like message containing server
information, entity metainformation, and possible body content.
 
Most HTTP communication is initiated by a user agent and consists of
a request to be applied to a resource on some origin server. In the
simplest case, this may be accomplished via a single connection (v)
between the user agent (UA) and the origin server (O).
 
request chain ------------------------>
UA -------------------v------------------- O
<----------------------- response chain
 
A more complicated situation occurs when one or more intermediaries
are present in the request/response chain. There are three common
forms of intermediary: proxy, gateway, and tunnel. A proxy is a
forwarding agent, receiving requests for a URI in its absolute form,
rewriting all or parts of the message, and forwarding the reformatted
request toward the server identified by the URI. A gateway is a
receiving agent, acting as a layer above some other server(s) and, if
necessary, translating the requests to the underlying server's
protocol. A tunnel acts as a relay point between two connections
without changing the messages; tunnels are used when the
communication needs to pass through an intermediary (such as a
firewall) even when the intermediary cannot understand the contents
of the messages.
 
request chain -------------------------------------->
UA -----v----- A -----v----- B -----v----- C -----v----- O
<------------------------------------- response chain
 
The figure above shows three intermediaries (A, B, and C) between the
user agent and origin server. A request or response message that
travels the whole chain must pass through four separate connections.
This distinction is important because some HTTP communication options
may apply only to the connection with the nearest, non-tunnel
neighbor, only to the end-points of the chain, or to all connections
along the chain. Although the diagram is linear, each participant may
be engaged in multiple, simultaneous communications. For example, B
may be receiving requests from many clients other than A, and/or
forwarding requests to servers other than C, at the same time that it
is handling A's request.
 
Any party to the communication which is not acting as a tunnel may
employ an internal cache for handling requests. The effect of a cache
is that the request/response chain is shortened if one of the
participants along the chain has a cached response applicable to that
request. The following illustrates the resulting chain if B has a
 
 
 
Berners-Lee, et al Informational [Page 7]
RFC 1945 HTTP/1.0 May 1996
 
 
cached copy of an earlier response from O (via C) for a request which
has not been cached by UA or A.
 
request chain ---------->
UA -----v----- A -----v----- B - - - - - - C - - - - - - O
<--------- response chain
 
Not all responses are cachable, and some requests may contain
modifiers which place special requirements on cache behavior. Some
HTTP/1.0 applications use heuristics to describe what is or is not a
"cachable" response, but these rules are not standardized.
 
On the Internet, HTTP communication generally takes place over TCP/IP
connections. The default port is TCP 80 [15], but other ports can be
used. This does not preclude HTTP from being implemented on top of
any other protocol on the Internet, or on other networks. HTTP only
presumes a reliable transport; any protocol that provides such
guarantees can be used, and the mapping of the HTTP/1.0 request and
response structures onto the transport data units of the protocol in
question is outside the scope of this specification.
 
Except for experimental applications, current practice requires that
the connection be established by the client prior to each request and
closed by the server after sending the response. Both clients and
servers should be aware that either party may close the connection
prematurely, due to user action, automated time-out, or program
failure, and should handle such closing in a predictable fashion. In
any case, the closing of the connection by either or both parties
always terminates the current request, regardless of its status.
 
1.4 HTTP and MIME
 
HTTP/1.0 uses many of the constructs defined for MIME, as defined in
RFC 1521 [5]. Appendix C describes the ways in which the context of
HTTP allows for different use of Internet Media Types than is
typically found in Internet mail, and gives the rationale for those
differences.
 
2. Notational Conventions and Generic Grammar
 
2.1 Augmented BNF
 
All of the mechanisms specified in this document are described in
both prose and an augmented Backus-Naur Form (BNF) similar to that
used by RFC 822 [7]. Implementors will need to be familiar with the
notation in order to understand this specification. The augmented BNF
includes the following constructs:
 
 
 
 
Berners-Lee, et al Informational [Page 8]
RFC 1945 HTTP/1.0 May 1996
 
 
name = definition
 
The name of a rule is simply the name itself (without any
enclosing "<" and ">") and is separated from its definition by
the equal character "=". Whitespace is only significant in that
indentation of continuation lines is used to indicate a rule
definition that spans more than one line. Certain basic rules
are in uppercase, such as SP, LWS, HT, CRLF, DIGIT, ALPHA, etc.
Angle brackets are used within definitions whenever their
presence will facilitate discerning the use of rule names.
 
"literal"
 
Quotation marks surround literal text. Unless stated otherwise,
the text is case-insensitive.
 
rule1 | rule2
 
Elements separated by a bar ("I") are alternatives,
e.g., "yes | no" will accept yes or no.
 
(rule1 rule2)
 
Elements enclosed in parentheses are treated as a single
element. Thus, "(elem (foo | bar) elem)" allows the token
sequences "elem foo elem" and "elem bar elem".
 
*rule
 
The character "*" preceding an element indicates repetition. The
full form is "<n>*<m>element" indicating at least <n> and at
most <m> occurrences of element. Default values are 0 and
infinity so that "*(element)" allows any number, including zero;
"1*element" requires at least one; and "1*2element" allows one
or two.
 
[rule]
 
Square brackets enclose optional elements; "[foo bar]" is
equivalent to "*1(foo bar)".
 
N rule
 
Specific repetition: "<n>(element)" is equivalent to
"<n>*<n>(element)"; that is, exactly <n> occurrences of
(element). Thus 2DIGIT is a 2-digit number, and 3ALPHA is a
string of three alphabetic characters.
 
 
 
 
Berners-Lee, et al Informational [Page 9]
RFC 1945 HTTP/1.0 May 1996
 
 
#rule
 
A construct "#" is defined, similar to "*", for defining lists
of elements. The full form is "<n>#<m>element" indicating at
least <n> and at most <m> elements, each separated by one or
more commas (",") and optional linear whitespace (LWS). This
makes the usual form of lists very easy; a rule such as
"( *LWS element *( *LWS "," *LWS element ))" can be shown as
"1#element". Wherever this construct is used, null elements are
allowed, but do not contribute to the count of elements present.
That is, "(element), , (element)" is permitted, but counts as
only two elements. Therefore, where at least one element is
required, at least one non-null element must be present. Default
values are 0 and infinity so that "#(element)" allows any
number, including zero; "1#element" requires at least one; and
"1#2element" allows one or two.
 
; comment
 
A semi-colon, set off some distance to the right of rule text,
starts a comment that continues to the end of line. This is a
simple way of including useful notes in parallel with the
specifications.
 
implied *LWS
 
The grammar described by this specification is word-based.
Except where noted otherwise, linear whitespace (LWS) can be
included between any two adjacent words (token or
quoted-string), and between adjacent tokens and delimiters
(tspecials), without changing the interpretation of a field. At
least one delimiter (tspecials) must exist between any two
tokens, since they would otherwise be interpreted as a single
token. However, applications should attempt to follow "common
form" when generating HTTP constructs, since there exist some
implementations that fail to accept anything beyond the common
forms.
 
2.2 Basic Rules
 
The following rules are used throughout this specification to
describe basic parsing constructs. The US-ASCII coded character set
is defined by [17].
 
OCTET = <any 8-bit sequence of data>
CHAR = <any US-ASCII character (octets 0 - 127)>
UPALPHA = <any US-ASCII uppercase letter "A".."Z">
LOALPHA = <any US-ASCII lowercase letter "a".."z">
 
 
 
Berners-Lee, et al Informational [Page 10]
RFC 1945 HTTP/1.0 May 1996
 
 
ALPHA = UPALPHA | LOALPHA
DIGIT = <any US-ASCII digit "0".."9">
CTL = <any US-ASCII control character
(octets 0 - 31) and DEL (127)>
CR = <US-ASCII CR, carriage return (13)>
LF = <US-ASCII LF, linefeed (10)>
SP = <US-ASCII SP, space (32)>
HT = <US-ASCII HT, horizontal-tab (9)>
<"> = <US-ASCII double-quote mark (34)>
 
HTTP/1.0 defines the octet sequence CR LF as the end-of-line marker
for all protocol elements except the Entity-Body (see Appendix B for
tolerant applications). The end-of-line marker within an Entity-Body
is defined by its associated media type, as described in Section 3.6.
 
CRLF = CR LF
 
HTTP/1.0 headers may be folded onto multiple lines if each
continuation line begins with a space or horizontal tab. All linear
whitespace, including folding, has the same semantics as SP.
 
LWS = [CRLF] 1*( SP | HT )
 
However, folding of header lines is not expected by some
applications, and should not be generated by HTTP/1.0 applications.
 
The TEXT rule is only used for descriptive field contents and values
that are not intended to be interpreted by the message parser. Words
of *TEXT may contain octets from character sets other than US-ASCII.
 
TEXT = <any OCTET except CTLs,
but including LWS>
 
Recipients of header field TEXT containing octets outside the US-
ASCII character set may assume that they represent ISO-8859-1
characters.
 
Hexadecimal numeric characters are used in several protocol elements.
 
HEX = "A" | "B" | "C" | "D" | "E" | "F"
| "a" | "b" | "c" | "d" | "e" | "f" | DIGIT
 
Many HTTP/1.0 header field values consist of words separated by LWS
or special characters. These special characters must be in a quoted
string to be used within a parameter value.
 
word = token | quoted-string
 
 
 
 
Berners-Lee, et al Informational [Page 11]
RFC 1945 HTTP/1.0 May 1996
 
 
token = 1*<any CHAR except CTLs or tspecials>
 
tspecials = "(" | ")" | "<" | ">" | "@"
| "," | ";" | ":" | "\" | <">
| "/" | "[" | "]" | "?" | "="
| "{" | "}" | SP | HT
 
Comments may be included in some HTTP header fields by surrounding
the comment text with parentheses. Comments are only allowed in
fields containing "comment" as part of their field value definition.
In all other fields, parentheses are considered part of the field
value.
 
comment = "(" *( ctext | comment ) ")"
ctext = <any TEXT excluding "(" and ")">
 
A string of text is parsed as a single word if it is quoted using
double-quote marks.
 
quoted-string = ( <"> *(qdtext) <"> )
 
qdtext = <any CHAR except <"> and CTLs,
but including LWS>
 
Single-character quoting using the backslash ("\") character is not
permitted in HTTP/1.0.
 
3. Protocol Parameters
 
3.1 HTTP Version
 
HTTP uses a "<major>.<minor>" numbering scheme to indicate versions
of the protocol. The protocol versioning policy is intended to allow
the sender to indicate the format of a message and its capacity for
understanding further HTTP communication, rather than the features
obtained via that communication. No change is made to the version
number for the addition of message components which do not affect
communication behavior or which only add to extensible field values.
The <minor> number is incremented when the changes made to the
protocol add features which do not change the general message parsing
algorithm, but which may add to the message semantics and imply
additional capabilities of the sender. The <major> number is
incremented when the format of a message within the protocol is
changed.
 
The version of an HTTP message is indicated by an HTTP-Version field
in the first line of the message. If the protocol version is not
specified, the recipient must assume that the message is in the
 
 
 
Berners-Lee, et al Informational [Page 12]
RFC 1945 HTTP/1.0 May 1996
 
 
simple HTTP/0.9 format.
 
HTTP-Version = "HTTP" "/" 1*DIGIT "." 1*DIGIT
 
Note that the major and minor numbers should be treated as separate
integers and that each may be incremented higher than a single digit.
Thus, HTTP/2.4 is a lower version than HTTP/2.13, which in turn is
lower than HTTP/12.3. Leading zeros should be ignored by recipients
and never generated by senders.
 
This document defines both the 0.9 and 1.0 versions of the HTTP
protocol. Applications sending Full-Request or Full-Response
messages, as defined by this specification, must include an HTTP-
Version of "HTTP/1.0".
 
HTTP/1.0 servers must:
 
o recognize the format of the Request-Line for HTTP/0.9 and
HTTP/1.0 requests;
 
o understand any valid request in the format of HTTP/0.9 or
HTTP/1.0;
 
o respond appropriately with a message in the same protocol
version used by the client.
 
HTTP/1.0 clients must:
 
o recognize the format of the Status-Line for HTTP/1.0 responses;
 
o understand any valid response in the format of HTTP/0.9 or
HTTP/1.0.
 
Proxy and gateway applications must be careful in forwarding requests
that are received in a format different than that of the
application's native HTTP version. Since the protocol version
indicates the protocol capability of the sender, a proxy/gateway must
never send a message with a version indicator which is greater than
its native version; if a higher version request is received, the
proxy/gateway must either downgrade the request version or respond
with an error. Requests with a version lower than that of the
application's native format may be upgraded before being forwarded;
the proxy/gateway's response to that request must follow the server
requirements listed above.
 
 
 
 
 
 
 
Berners-Lee, et al Informational [Page 13]
RFC 1945 HTTP/1.0 May 1996
 
 
3.2 Uniform Resource Identifiers
 
URIs have been known by many names: WWW addresses, Universal Document
Identifiers, Universal Resource Identifiers [2], and finally the
combination of Uniform Resource Locators (URL) [4] and Names (URN)
[16]. As far as HTTP is concerned, Uniform Resource Identifiers are
simply formatted strings which identify--via name, location, or any
other characteristic--a network resource.
 
3.2.1 General Syntax
 
URIs in HTTP can be represented in absolute form or relative to some
known base URI [9], depending upon the context of their use. The two
forms are differentiated by the fact that absolute URIs always begin
with a scheme name followed by a colon.
 
URI = ( absoluteURI | relativeURI ) [ "#" fragment ]
 
absoluteURI = scheme ":" *( uchar | reserved )
 
relativeURI = net_path | abs_path | rel_path
 
net_path = "//" net_loc [ abs_path ]
abs_path = "/" rel_path
rel_path = [ path ] [ ";" params ] [ "?" query ]
 
path = fsegment *( "/" segment )
fsegment = 1*pchar
segment = *pchar
 
params = param *( ";" param )
param = *( pchar | "/" )
 
scheme = 1*( ALPHA | DIGIT | "+" | "-" | "." )
net_loc = *( pchar | ";" | "?" )
query = *( uchar | reserved )
fragment = *( uchar | reserved )
 
pchar = uchar | ":" | "@" | "&" | "=" | "+"
uchar = unreserved | escape
unreserved = ALPHA | DIGIT | safe | extra | national
 
escape = "%" HEX HEX
reserved = ";" | "/" | "?" | ":" | "@" | "&" | "=" | "+"
extra = "!" | "*" | "'" | "(" | ")" | ","
safe = "$" | "-" | "_" | "."
unsafe = CTL | SP | <"> | "#" | "%" | "<" | ">"
national = <any OCTET excluding ALPHA, DIGIT,
 
 
 
Berners-Lee, et al Informational [Page 14]
RFC 1945 HTTP/1.0 May 1996
 
 
reserved, extra, safe, and unsafe>
 
For definitive information on URL syntax and semantics, see RFC 1738
[4] and RFC 1808 [9]. The BNF above includes national characters not
allowed in valid URLs as specified by RFC 1738, since HTTP servers
are not restricted in the set of unreserved characters allowed to
represent the rel_path part of addresses, and HTTP proxies may
receive requests for URIs not defined by RFC 1738.
 
3.2.2 http URL
 
The "http" scheme is used to locate network resources via the HTTP
protocol. This section defines the scheme-specific syntax and
semantics for http URLs.
 
http_URL = "http:" "//" host [ ":" port ] [ abs_path ]
 
host = <A legal Internet host domain name
or IP address (in dotted-decimal form),
as defined by Section 2.1 of RFC 1123>
 
port = *DIGIT
 
If the port is empty or not given, port 80 is assumed. The semantics
are that the identified resource is located at the server listening
for TCP connections on that port of that host, and the Request-URI
for the resource is abs_path. If the abs_path is not present in the
URL, it must be given as "/" when used as a Request-URI (Section
5.1.2).
 
Note: Although the HTTP protocol is independent of the transport
layer protocol, the http URL only identifies resources by their
TCP location, and thus non-TCP resources must be identified by
some other URI scheme.
 
The canonical form for "http" URLs is obtained by converting any
UPALPHA characters in host to their LOALPHA equivalent (hostnames are
case-insensitive), eliding the [ ":" port ] if the port is 80, and
replacing an empty abs_path with "/".
 
3.3 Date/Time Formats
 
HTTP/1.0 applications have historically allowed three different
formats for the representation of date/time stamps:
 
Sun, 06 Nov 1994 08:49:37 GMT ; RFC 822, updated by RFC 1123
Sunday, 06-Nov-94 08:49:37 GMT ; RFC 850, obsoleted by RFC 1036
Sun Nov 6 08:49:37 1994 ; ANSI C's asctime() format
 
 
 
Berners-Lee, et al Informational [Page 15]
RFC 1945 HTTP/1.0 May 1996
 
 
The first format is preferred as an Internet standard and represents
a fixed-length subset of that defined by RFC 1123 [6] (an update to
RFC 822 [7]). The second format is in common use, but is based on the
obsolete RFC 850 [10] date format and lacks a four-digit year.
HTTP/1.0 clients and servers that parse the date value should accept
all three formats, though they must never generate the third
(asctime) format.
 
Note: Recipients of date values are encouraged to be robust in
accepting date values that may have been generated by non-HTTP
applications, as is sometimes the case when retrieving or posting
messages via proxies/gateways to SMTP or NNTP.
 
All HTTP/1.0 date/time stamps must be represented in Universal Time
(UT), also known as Greenwich Mean Time (GMT), without exception.
This is indicated in the first two formats by the inclusion of "GMT"
as the three-letter abbreviation for time zone, and should be assumed
when reading the asctime format.
 
HTTP-date = rfc1123-date | rfc850-date | asctime-date
 
rfc1123-date = wkday "," SP date1 SP time SP "GMT"
rfc850-date = weekday "," SP date2 SP time SP "GMT"
asctime-date = wkday SP date3 SP time SP 4DIGIT
 
date1 = 2DIGIT SP month SP 4DIGIT
; day month year (e.g., 02 Jun 1982)
date2 = 2DIGIT "-" month "-" 2DIGIT
; day-month-year (e.g., 02-Jun-82)
date3 = month SP ( 2DIGIT | ( SP 1DIGIT ))
; month day (e.g., Jun 2)
 
time = 2DIGIT ":" 2DIGIT ":" 2DIGIT
; 00:00:00 - 23:59:59
 
wkday = "Mon" | "Tue" | "Wed"
| "Thu" | "Fri" | "Sat" | "Sun"
 
weekday = "Monday" | "Tuesday" | "Wednesday"
| "Thursday" | "Friday" | "Saturday" | "Sunday"
 
month = "Jan" | "Feb" | "Mar" | "Apr"
| "May" | "Jun" | "Jul" | "Aug"
| "Sep" | "Oct" | "Nov" | "Dec"
 
Note: HTTP requirements for the date/time stamp format apply
only to their usage within the protocol stream. Clients and
servers are not required to use these formats for user
 
 
 
Berners-Lee, et al Informational [Page 16]
RFC 1945 HTTP/1.0 May 1996
 
 
presentation, request logging, etc.
 
3.4 Character Sets
 
HTTP uses the same definition of the term "character set" as that
described for MIME:
 
The term "character set" is used in this document to refer to a
method used with one or more tables to convert a sequence of
octets into a sequence of characters. Note that unconditional
conversion in the other direction is not required, in that not all
characters may be available in a given character set and a
character set may provide more than one sequence of octets to
represent a particular character. This definition is intended to
allow various kinds of character encodings, from simple single-
table mappings such as US-ASCII to complex table switching methods
such as those that use ISO 2022's techniques. However, the
definition associated with a MIME character set name must fully
specify the mapping to be performed from octets to characters. In
particular, use of external profiling information to determine the
exact mapping is not permitted.
 
Note: This use of the term "character set" is more commonly
referred to as a "character encoding." However, since HTTP and
MIME share the same registry, it is important that the terminology
also be shared.
 
HTTP character sets are identified by case-insensitive tokens. The
complete set of tokens are defined by the IANA Character Set registry
[15]. However, because that registry does not define a single,
consistent token for each character set, we define here the preferred
names for those character sets most likely to be used with HTTP
entities. These character sets include those registered by RFC 1521
[5] -- the US-ASCII [17] and ISO-8859 [18] character sets -- and
other names specifically recommended for use within MIME charset
parameters.
 
charset = "US-ASCII"
| "ISO-8859-1" | "ISO-8859-2" | "ISO-8859-3"
| "ISO-8859-4" | "ISO-8859-5" | "ISO-8859-6"
| "ISO-8859-7" | "ISO-8859-8" | "ISO-8859-9"
| "ISO-2022-JP" | "ISO-2022-JP-2" | "ISO-2022-KR"
| "UNICODE-1-1" | "UNICODE-1-1-UTF-7" | "UNICODE-1-1-UTF-8"
| token
 
Although HTTP allows an arbitrary token to be used as a charset
value, any token that has a predefined value within the IANA
Character Set registry [15] must represent the character set defined
 
 
 
Berners-Lee, et al Informational [Page 17]
RFC 1945 HTTP/1.0 May 1996
 
 
by that registry. Applications should limit their use of character
sets to those defined by the IANA registry.
 
The character set of an entity body should be labelled as the lowest
common denominator of the character codes used within that body, with
the exception that no label is preferred over the labels US-ASCII or
ISO-8859-1.
 
3.5 Content Codings
 
Content coding values are used to indicate an encoding transformation
that has been applied to a resource. Content codings are primarily
used to allow a document to be compressed or encrypted without losing
the identity of its underlying media type. Typically, the resource is
stored in this encoding and only decoded before rendering or
analogous usage.
 
content-coding = "x-gzip" | "x-compress" | token
 
Note: For future compatibility, HTTP/1.0 applications should
consider "gzip" and "compress" to be equivalent to "x-gzip"
and "x-compress", respectively.
 
All content-coding values are case-insensitive. HTTP/1.0 uses
content-coding values in the Content-Encoding (Section 10.3) header
field. Although the value describes the content-coding, what is more
important is that it indicates what decoding mechanism will be
required to remove the encoding. Note that a single program may be
capable of decoding multiple content-coding formats. Two values are
defined by this specification:
 
x-gzip
An encoding format produced by the file compression program
"gzip" (GNU zip) developed by Jean-loup Gailly. This format is
typically a Lempel-Ziv coding (LZ77) with a 32 bit CRC.
 
x-compress
The encoding format produced by the file compression program
"compress". This format is an adaptive Lempel-Ziv-Welch coding
(LZW).
 
Note: Use of program names for the identification of
encoding formats is not desirable and should be discouraged
for future encodings. Their use here is representative of
historical practice, not good design.
 
 
 
 
 
 
Berners-Lee, et al Informational [Page 18]
RFC 1945 HTTP/1.0 May 1996
 
 
3.6 Media Types
 
HTTP uses Internet Media Types [13] in the Content-Type header field
(Section 10.5) in order to provide open and extensible data typing.
 
media-type = type "/" subtype *( ";" parameter )
type = token
subtype = token
 
Parameters may follow the type/subtype in the form of attribute/value
pairs.
 
parameter = attribute "=" value
attribute = token
value = token | quoted-string
 
The type, subtype, and parameter attribute names are case-
insensitive. Parameter values may or may not be case-sensitive,
depending on the semantics of the parameter name. LWS must not be
generated between the type and subtype, nor between an attribute and
its value. Upon receipt of a media type with an unrecognized
parameter, a user agent should treat the media type as if the
unrecognized parameter and its value were not present.
 
Some older HTTP applications do not recognize media type parameters.
HTTP/1.0 applications should only use media type parameters when they
are necessary to define the content of a message.
 
Media-type values are registered with the Internet Assigned Number
Authority (IANA [15]). The media type registration process is
outlined in RFC 1590 [13]. Use of non-registered media types is
discouraged.
 
3.6.1 Canonicalization and Text Defaults
 
Internet media types are registered with a canonical form. In
general, an Entity-Body transferred via HTTP must be represented in
the appropriate canonical form prior to its transmission. If the body
has been encoded with a Content-Encoding, the underlying data should
be in canonical form prior to being encoded.
 
Media subtypes of the "text" type use CRLF as the text line break
when in canonical form. However, HTTP allows the transport of text
media with plain CR or LF alone representing a line break when used
consistently within the Entity-Body. HTTP applications must accept
CRLF, bare CR, and bare LF as being representative of a line break in
text media received via HTTP.
 
 
 
 
Berners-Lee, et al Informational [Page 19]
RFC 1945 HTTP/1.0 May 1996
 
 
In addition, if the text media is represented in a character set that
does not use octets 13 and 10 for CR and LF respectively, as is the
case for some multi-byte character sets, HTTP allows the use of
whatever octet sequences are defined by that character set to
represent the equivalent of CR and LF for line breaks. This
flexibility regarding line breaks applies only to text media in the
Entity-Body; a bare CR or LF should not be substituted for CRLF
within any of the HTTP control structures (such as header fields and
multipart boundaries).
 
The "charset" parameter is used with some media types to define the
character set (Section 3.4) of the data. When no explicit charset
parameter is provided by the sender, media subtypes of the "text"
type are defined to have a default charset value of "ISO-8859-1" when
received via HTTP. Data in character sets other than "ISO-8859-1" or
its subsets must be labelled with an appropriate charset value in
order to be consistently interpreted by the recipient.
 
Note: Many current HTTP servers provide data using charsets other
than "ISO-8859-1" without proper labelling. This situation reduces
interoperability and is not recommended. To compensate for this,
some HTTP user agents provide a configuration option to allow the
user to change the default interpretation of the media type
character set when no charset parameter is given.
 
3.6.2 Multipart Types
 
MIME provides for a number of "multipart" types -- encapsulations of
several entities within a single message's Entity-Body. The multipart
types registered by IANA [15] do not have any special meaning for
HTTP/1.0, though user agents may need to understand each type in
order to correctly interpret the purpose of each body-part. An HTTP
user agent should follow the same or similar behavior as a MIME user
agent does upon receipt of a multipart type. HTTP servers should not
assume that all HTTP clients are prepared to handle multipart types.
 
All multipart types share a common syntax and must include a boundary
parameter as part of the media type value. The message body is itself
a protocol element and must therefore use only CRLF to represent line
breaks between body-parts. Multipart body-parts may contain HTTP
header fields which are significant to the meaning of that part.
 
3.7 Product Tokens
 
Product tokens are used to allow communicating applications to
identify themselves via a simple product token, with an optional
slash and version designator. Most fields using product tokens also
allow subproducts which form a significant part of the application to
 
 
 
Berners-Lee, et al Informational [Page 20]
RFC 1945 HTTP/1.0 May 1996
 
 
be listed, separated by whitespace. By convention, the products are
listed in order of their significance for identifying the
application.
 
product = token ["/" product-version]
product-version = token
 
Examples:
 
User-Agent: CERN-LineMode/2.15 libwww/2.17b3
 
Server: Apache/0.8.4
 
Product tokens should be short and to the point -- use of them for
advertizing or other non-essential information is explicitly
forbidden. Although any token character may appear in a product-
version, this token should only be used for a version identifier
(i.e., successive versions of the same product should only differ in
the product-version portion of the product value).
 
4. HTTP Message
 
4.1 Message Types
 
HTTP messages consist of requests from client to server and responses
from server to client.
 
HTTP-message = Simple-Request ; HTTP/0.9 messages
| Simple-Response
| Full-Request ; HTTP/1.0 messages
| Full-Response
 
Full-Request and Full-Response use the generic message format of RFC
822 [7] for transferring entities. Both messages may include optional
header fields (also known as "headers") and an entity body. The
entity body is separated from the headers by a null line (i.e., a
line with nothing preceding the CRLF).
 
Full-Request = Request-Line ; Section 5.1
*( General-Header ; Section 4.3
| Request-Header ; Section 5.2
| Entity-Header ) ; Section 7.1
CRLF
[ Entity-Body ] ; Section 7.2
 
Full-Response = Status-Line ; Section 6.1
*( General-Header ; Section 4.3
| Response-Header ; Section 6.2
 
 
 
Berners-Lee, et al Informational [Page 21]
RFC 1945 HTTP/1.0 May 1996
 
 
| Entity-Header ) ; Section 7.1
CRLF
[ Entity-Body ] ; Section 7.2
 
Simple-Request and Simple-Response do not allow the use of any header
information and are limited to a single request method (GET).
 
Simple-Request = "GET" SP Request-URI CRLF
 
Simple-Response = [ Entity-Body ]
 
Use of the Simple-Request format is discouraged because it prevents
the server from identifying the media type of the returned entity.
 
4.2 Message Headers
 
HTTP header fields, which include General-Header (Section 4.3),
Request-Header (Section 5.2), Response-Header (Section 6.2), and
Entity-Header (Section 7.1) fields, follow the same generic format as
that given in Section 3.1 of RFC 822 [7]. Each header field consists
of a name followed immediately by a colon (":"), a single space (SP)
character, and the field value. Field names are case-insensitive.
Header fields can be extended over multiple lines by preceding each
extra line with at least one SP or HT, though this is not
recommended.
 
HTTP-header = field-name ":" [ field-value ] CRLF
 
field-name = token
field-value = *( field-content | LWS )
 
field-content = <the OCTETs making up the field-value
and consisting of either *TEXT or combinations
of token, tspecials, and quoted-string>
 
The order in which header fields are received is not significant.
However, it is "good practice" to send General-Header fields first,
followed by Request-Header or Response-Header fields prior to the
Entity-Header fields.
 
Multiple HTTP-header fields with the same field-name may be present
in a message if and only if the entire field-value for that header
field is defined as a comma-separated list [i.e., #(values)]. It must
be possible to combine the multiple header fields into one "field-
name: field-value" pair, without changing the semantics of the
message, by appending each subsequent field-value to the first, each
separated by a comma.
 
 
 
 
Berners-Lee, et al Informational [Page 22]
RFC 1945 HTTP/1.0 May 1996
 
 
4.3 General Header Fields
 
There are a few header fields which have general applicability for
both request and response messages, but which do not apply to the
entity being transferred. These headers apply only to the message
being transmitted.
 
General-Header = Date ; Section 10.6
| Pragma ; Section 10.12
 
General header field names can be extended reliably only in
combination with a change in the protocol version. However, new or
experimental header fields may be given the semantics of general
header fields if all parties in the communication recognize them to
be general header fields. Unrecognized header fields are treated as
Entity-Header fields.
 
5. Request
 
A request message from a client to a server includes, within the
first line of that message, the method to be applied to the resource,
the identifier of the resource, and the protocol version in use. For
backwards compatibility with the more limited HTTP/0.9 protocol,
there are two valid formats for an HTTP request:
 
Request = Simple-Request | Full-Request
 
Simple-Request = "GET" SP Request-URI CRLF
 
Full-Request = Request-Line ; Section 5.1
*( General-Header ; Section 4.3
| Request-Header ; Section 5.2
| Entity-Header ) ; Section 7.1
CRLF
[ Entity-Body ] ; Section 7.2
 
If an HTTP/1.0 server receives a Simple-Request, it must respond with
an HTTP/0.9 Simple-Response. An HTTP/1.0 client capable of receiving
a Full-Response should never generate a Simple-Request.
 
5.1 Request-Line
 
The Request-Line begins with a method token, followed by the
Request-URI and the protocol version, and ending with CRLF. The
elements are separated by SP characters. No CR or LF are allowed
except in the final CRLF sequence.
 
Request-Line = Method SP Request-URI SP HTTP-Version CRLF
 
 
 
Berners-Lee, et al Informational [Page 23]
RFC 1945 HTTP/1.0 May 1996
 
 
Note that the difference between a Simple-Request and the Request-
Line of a Full-Request is the presence of the HTTP-Version field and
the availability of methods other than GET.
 
5.1.1 Method
 
The Method token indicates the method to be performed on the resource
identified by the Request-URI. The method is case-sensitive.
 
Method = "GET" ; Section 8.1
| "HEAD" ; Section 8.2
| "POST" ; Section 8.3
| extension-method
 
extension-method = token
 
The list of methods acceptable by a specific resource can change
dynamically; the client is notified through the return code of the
response if a method is not allowed on a resource. Servers should
return the status code 501 (not implemented) if the method is
unrecognized or not implemented.
 
The methods commonly used by HTTP/1.0 applications are fully defined
in Section 8.
 
5.1.2 Request-URI
 
The Request-URI is a Uniform Resource Identifier (Section 3.2) and
identifies the resource upon which to apply the request.
 
Request-URI = absoluteURI | abs_path
 
The two options for Request-URI are dependent on the nature of the
request.
 
The absoluteURI form is only allowed when the request is being made
to a proxy. The proxy is requested to forward the request and return
the response. If the request is GET or HEAD and a prior response is
cached, the proxy may use the cached message if it passes any
restrictions in the Expires header field. Note that the proxy may
forward the request on to another proxy or directly to the server
specified by the absoluteURI. In order to avoid request loops, a
proxy must be able to recognize all of its server names, including
any aliases, local variations, and the numeric IP address. An example
Request-Line would be:
 
GET http://www.w3.org/pub/WWW/TheProject.html HTTP/1.0
 
 
 
 
Berners-Lee, et al Informational [Page 24]
RFC 1945 HTTP/1.0 May 1996
 
 
The most common form of Request-URI is that used to identify a
resource on an origin server or gateway. In this case, only the
absolute path of the URI is transmitted (see Section 3.2.1,
abs_path). For example, a client wishing to retrieve the resource
above directly from the origin server would create a TCP connection
to port 80 of the host "www.w3.org" and send the line:
 
GET /pub/WWW/TheProject.html HTTP/1.0
 
followed by the remainder of the Full-Request. Note that the absolute
path cannot be empty; if none is present in the original URI, it must
be given as "/" (the server root).
 
The Request-URI is transmitted as an encoded string, where some
characters may be escaped using the "% HEX HEX" encoding defined by
RFC 1738 [4]. The origin server must decode the Request-URI in order
to properly interpret the request.
 
5.2 Request Header Fields
 
The request header fields allow the client to pass additional
information about the request, and about the client itself, to the
server. These fields act as request modifiers, with semantics
equivalent to the parameters on a programming language method
(procedure) invocation.
 
Request-Header = Authorization ; Section 10.2
| From ; Section 10.8
| If-Modified-Since ; Section 10.9
| Referer ; Section 10.13
| User-Agent ; Section 10.15
 
Request-Header field names can be extended reliably only in
combination with a change in the protocol version. However, new or
experimental header fields may be given the semantics of request
header fields if all parties in the communication recognize them to
be request header fields. Unrecognized header fields are treated as
Entity-Header fields.
 
6. Response
 
After receiving and interpreting a request message, a server responds
in the form of an HTTP response message.
 
Response = Simple-Response | Full-Response
 
Simple-Response = [ Entity-Body ]
 
 
 
 
Berners-Lee, et al Informational [Page 25]
RFC 1945 HTTP/1.0 May 1996
 
 
Full-Response = Status-Line ; Section 6.1
*( General-Header ; Section 4.3
| Response-Header ; Section 6.2
| Entity-Header ) ; Section 7.1
CRLF
[ Entity-Body ] ; Section 7.2
 
A Simple-Response should only be sent in response to an HTTP/0.9
Simple-Request or if the server only supports the more limited
HTTP/0.9 protocol. If a client sends an HTTP/1.0 Full-Request and
receives a response that does not begin with a Status-Line, it should
assume that the response is a Simple-Response and parse it
accordingly. Note that the Simple-Response consists only of the
entity body and is terminated by the server closing the connection.
 
6.1 Status-Line
 
The first line of a Full-Response message is the Status-Line,
consisting of the protocol version followed by a numeric status code
and its associated textual phrase, with each element separated by SP
characters. No CR or LF is allowed except in the final CRLF sequence.
 
Status-Line = HTTP-Version SP Status-Code SP Reason-Phrase CRLF
 
Since a status line always begins with the protocol version and
status code
 
"HTTP/" 1*DIGIT "." 1*DIGIT SP 3DIGIT SP
 
(e.g., "HTTP/1.0 200 "), the presence of that expression is
sufficient to differentiate a Full-Response from a Simple-Response.
Although the Simple-Response format may allow such an expression to
occur at the beginning of an entity body, and thus cause a
misinterpretation of the message if it was given in response to a
Full-Request, most HTTP/0.9 servers are limited to responses of type
"text/html" and therefore would never generate such a response.
 
6.1.1 Status Code and Reason Phrase
 
The Status-Code element is a 3-digit integer result code of the
attempt to understand and satisfy the request. The Reason-Phrase is
intended to give a short textual description of the Status-Code. The
Status-Code is intended for use by automata and the Reason-Phrase is
intended for the human user. The client is not required to examine or
display the Reason-Phrase.
 
 
 
 
 
 
Berners-Lee, et al Informational [Page 26]
RFC 1945 HTTP/1.0 May 1996
 
 
The first digit of the Status-Code defines the class of response. The
last two digits do not have any categorization role. There are 5
values for the first digit:
 
o 1xx: Informational - Not used, but reserved for future use
 
o 2xx: Success - The action was successfully received,
understood, and accepted.
 
o 3xx: Redirection - Further action must be taken in order to
complete the request
 
o 4xx: Client Error - The request contains bad syntax or cannot
be fulfilled
 
o 5xx: Server Error - The server failed to fulfill an apparently
valid request
 
The individual values of the numeric status codes defined for
HTTP/1.0, and an example set of corresponding Reason-Phrase's, are
presented below. The reason phrases listed here are only recommended
-- they may be replaced by local equivalents without affecting the
protocol. These codes are fully defined in Section 9.
 
Status-Code = "200" ; OK
| "201" ; Created
| "202" ; Accepted
| "204" ; No Content
| "301" ; Moved Permanently
| "302" ; Moved Temporarily
| "304" ; Not Modified
| "400" ; Bad Request
| "401" ; Unauthorized
| "403" ; Forbidden
| "404" ; Not Found
| "500" ; Internal Server Error
| "501" ; Not Implemented
| "502" ; Bad Gateway
| "503" ; Service Unavailable
| extension-code
 
extension-code = 3DIGIT
 
Reason-Phrase = *<TEXT, excluding CR, LF>
 
HTTP status codes are extensible, but the above codes are the only
ones generally recognized in current practice. HTTP applications are
not required to understand the meaning of all registered status
 
 
 
Berners-Lee, et al Informational [Page 27]
RFC 1945 HTTP/1.0 May 1996
 
 
codes, though such understanding is obviously desirable. However,
applications must understand the class of any status code, as
indicated by the first digit, and treat any unrecognized response as
being equivalent to the x00 status code of that class, with the
exception that an unrecognized response must not be cached. For
example, if an unrecognized status code of 431 is received by the
client, it can safely assume that there was something wrong with its
request and treat the response as if it had received a 400 status
code. In such cases, user agents should present to the user the
entity returned with the response, since that entity is likely to
include human-readable information which will explain the unusual
status.
 
6.2 Response Header Fields
 
The response header fields allow the server to pass additional
information about the response which cannot be placed in the Status-
Line. These header fields give information about the server and about
further access to the resource identified by the Request-URI.
 
Response-Header = Location ; Section 10.11
| Server ; Section 10.14
| WWW-Authenticate ; Section 10.16
 
Response-Header field names can be extended reliably only in
combination with a change in the protocol version. However, new or
experimental header fields may be given the semantics of response
header fields if all parties in the communication recognize them to
be response header fields. Unrecognized header fields are treated as
Entity-Header fields.
 
7. Entity
 
Full-Request and Full-Response messages may transfer an entity within
some requests and responses. An entity consists of Entity-Header
fields and (usually) an Entity-Body. In this section, both sender and
recipient refer to either the client or the server, depending on who
sends and who receives the entity.
 
 
 
 
 
 
 
 
 
 
 
 
 
Berners-Lee, et al Informational [Page 28]
RFC 1945 HTTP/1.0 May 1996
 
 
7.1 Entity Header Fields
 
Entity-Header fields define optional metainformation about the
Entity-Body or, if no body is present, about the resource identified
by the request.
 
Entity-Header = Allow ; Section 10.1
| Content-Encoding ; Section 10.3
| Content-Length ; Section 10.4
| Content-Type ; Section 10.5
| Expires ; Section 10.7
| Last-Modified ; Section 10.10
| extension-header
 
extension-header = HTTP-header
 
The extension-header mechanism allows additional Entity-Header fields
to be defined without changing the protocol, but these fields cannot
be assumed to be recognizable by the recipient. Unrecognized header
fields should be ignored by the recipient and forwarded by proxies.
 
7.2 Entity Body
 
The entity body (if any) sent with an HTTP request or response is in
a format and encoding defined by the Entity-Header fields.
 
Entity-Body = *OCTET
 
An entity body is included with a request message only when the
request method calls for one. The presence of an entity body in a
request is signaled by the inclusion of a Content-Length header field
in the request message headers. HTTP/1.0 requests containing an
entity body must include a valid Content-Length header field.
 
For response messages, whether or not an entity body is included with
a message is dependent on both the request method and the response
code. All responses to the HEAD request method must not include a
body, even though the presence of entity header fields may lead one
to believe they do. All 1xx (informational), 204 (no content), and
304 (not modified) responses must not include a body. All other
responses must include an entity body or a Content-Length header
field defined with a value of zero (0).
 
7.2.1 Type
 
When an Entity-Body is included with a message, the data type of that
body is determined via the header fields Content-Type and Content-
Encoding. These define a two-layer, ordered encoding model:
 
 
 
Berners-Lee, et al Informational [Page 29]
RFC 1945 HTTP/1.0 May 1996
 
 
entity-body := Content-Encoding( Content-Type( data ) )
 
A Content-Type specifies the media type of the underlying data. A
Content-Encoding may be used to indicate any additional content
coding applied to the type, usually for the purpose of data
compression, that is a property of the resource requested. The
default for the content encoding is none (i.e., the identity
function).
 
Any HTTP/1.0 message containing an entity body should include a
Content-Type header field defining the media type of that body. If
and only if the media type is not given by a Content-Type header, as
is the case for Simple-Response messages, the recipient may attempt
to guess the media type via inspection of its content and/or the name
extension(s) of the URL used to identify the resource. If the media
type remains unknown, the recipient should treat it as type
"application/octet-stream".
 
7.2.2 Length
 
When an Entity-Body is included with a message, the length of that
body may be determined in one of two ways. If a Content-Length header
field is present, its value in bytes represents the length of the
Entity-Body. Otherwise, the body length is determined by the closing
of the connection by the server.
 
Closing the connection cannot be used to indicate the end of a
request body, since it leaves no possibility for the server to send
back a response. Therefore, HTTP/1.0 requests containing an entity
body must include a valid Content-Length header field. If a request
contains an entity body and Content-Length is not specified, and the
server does not recognize or cannot calculate the length from other
fields, then the server should send a 400 (bad request) response.
 
Note: Some older servers supply an invalid Content-Length when
sending a document that contains server-side includes dynamically
inserted into the data stream. It must be emphasized that this
will not be tolerated by future versions of HTTP. Unless the
client knows that it is receiving a response from a compliant
server, it should not depend on the Content-Length value being
correct.
 
8. Method Definitions
 
The set of common methods for HTTP/1.0 is defined below. Although
this set can be expanded, additional methods cannot be assumed to
share the same semantics for separately extended clients and servers.
 
 
 
 
Berners-Lee, et al Informational [Page 30]
RFC 1945 HTTP/1.0 May 1996
 
 
8.1 GET
 
The GET method means retrieve whatever information (in the form of an
entity) is identified by the Request-URI. If the Request-URI refers
to a data-producing process, it is the produced data which shall be
returned as the entity in the response and not the source text of the
process, unless that text happens to be the output of the process.
 
The semantics of the GET method changes to a "conditional GET" if the
request message includes an If-Modified-Since header field. A
conditional GET method requests that the identified resource be
transferred only if it has been modified since the date given by the
If-Modified-Since header, as described in Section 10.9. The
conditional GET method is intended to reduce network usage by
allowing cached entities to be refreshed without requiring multiple
requests or transferring unnecessary data.
 
8.2 HEAD
 
The HEAD method is identical to GET except that the server must not
return any Entity-Body in the response. The metainformation contained
in the HTTP headers in response to a HEAD request should be identical
to the information sent in response to a GET request. This method can
be used for obtaining metainformation about the resource identified
by the Request-URI without transferring the Entity-Body itself. This
method is often used for testing hypertext links for validity,
accessibility, and recent modification.
 
There is no "conditional HEAD" request analogous to the conditional
GET. If an If-Modified-Since header field is included with a HEAD
request, it should be ignored.
 
8.3 POST
 
The POST method is used to request that the destination server accept
the entity enclosed in the request as a new subordinate of the
resource identified by the Request-URI in the Request-Line. POST is
designed to allow a uniform method to cover the following functions:
 
o Annotation of existing resources;
 
o Posting a message to a bulletin board, newsgroup, mailing list,
or similar group of articles;
 
o Providing a block of data, such as the result of submitting a
form [3], to a data-handling process;
 
o Extending a database through an append operation.
 
 
 
Berners-Lee, et al Informational [Page 31]
RFC 1945 HTTP/1.0 May 1996
 
 
The actual function performed by the POST method is determined by the
server and is usually dependent on the Request-URI. The posted entity
is subordinate to that URI in the same way that a file is subordinate
to a directory containing it, a news article is subordinate to a
newsgroup to which it is posted, or a record is subordinate to a
database.
 
A successful POST does not require that the entity be created as a
resource on the origin server or made accessible for future
reference. That is, the action performed by the POST method might not
result in a resource that can be identified by a URI. In this case,
either 200 (ok) or 204 (no content) is the appropriate response
status, depending on whether or not the response includes an entity
that describes the result.
 
If a resource has been created on the origin server, the response
should be 201 (created) and contain an entity (preferably of type
"text/html") which describes the status of the request and refers to
the new resource.
 
A valid Content-Length is required on all HTTP/1.0 POST requests. An
HTTP/1.0 server should respond with a 400 (bad request) message if it
cannot determine the length of the request message's content.
 
Applications must not cache responses to a POST request because the
application has no way of knowing that the server would return an
equivalent response on some future request.
 
9. Status Code Definitions
 
Each Status-Code is described below, including a description of which
method(s) it can follow and any metainformation required in the
response.
 
9.1 Informational 1xx
 
This class of status code indicates a provisional response,
consisting only of the Status-Line and optional headers, and is
terminated by an empty line. HTTP/1.0 does not define any 1xx status
codes and they are not a valid response to a HTTP/1.0 request.
However, they may be useful for experimental applications which are
outside the scope of this specification.
 
9.2 Successful 2xx
 
This class of status code indicates that the client's request was
successfully received, understood, and accepted.
 
 
 
 
Berners-Lee, et al Informational [Page 32]
RFC 1945 HTTP/1.0 May 1996
 
 
200 OK
 
The request has succeeded. The information returned with the
response is dependent on the method used in the request, as follows:
 
GET an entity corresponding to the requested resource is sent
in the response;
 
HEAD the response must only contain the header information and
no Entity-Body;
 
POST an entity describing or containing the result of the action.
 
201 Created
 
The request has been fulfilled and resulted in a new resource being
created. The newly created resource can be referenced by the URI(s)
returned in the entity of the response. The origin server should
create the resource before using this Status-Code. If the action
cannot be carried out immediately, the server must include in the
response body a description of when the resource will be available;
otherwise, the server should respond with 202 (accepted).
 
Of the methods defined by this specification, only POST can create a
resource.
 
202 Accepted
 
The request has been accepted for processing, but the processing
has not been completed. The request may or may not eventually be
acted upon, as it may be disallowed when processing actually takes
place. There is no facility for re-sending a status code from an
asynchronous operation such as this.
 
The 202 response is intentionally non-committal. Its purpose is to
allow a server to accept a request for some other process (perhaps
a batch-oriented process that is only run once per day) without
requiring that the user agent's connection to the server persist
until the process is completed. The entity returned with this
response should include an indication of the request's current
status and either a pointer to a status monitor or some estimate of
when the user can expect the request to be fulfilled.
 
204 No Content
 
The server has fulfilled the request but there is no new
information to send back. If the client is a user agent, it should
not change its document view from that which caused the request to
 
 
 
Berners-Lee, et al Informational [Page 33]
RFC 1945 HTTP/1.0 May 1996
 
 
be generated. This response is primarily intended to allow input
for scripts or other actions to take place without causing a change
to the user agent's active document view. The response may include
new metainformation in the form of entity headers, which should
apply to the document currently in the user agent's active view.
 
9.3 Redirection 3xx
 
This class of status code indicates that further action needs to be
taken by the user agent in order to fulfill the request. The action
required may be carried out by the user agent without interaction
with the user if and only if the method used in the subsequent
request is GET or HEAD. A user agent should never automatically
redirect a request more than 5 times, since such redirections usually
indicate an infinite loop.
 
300 Multiple Choices
 
This response code is not directly used by HTTP/1.0 applications,
but serves as the default for interpreting the 3xx class of
responses.
 
The requested resource is available at one or more locations.
Unless it was a HEAD request, the response should include an entity
containing a list of resource characteristics and locations from
which the user or user agent can choose the one most appropriate.
If the server has a preferred choice, it should include the URL in
a Location field; user agents may use this field value for
automatic redirection.
 
301 Moved Permanently
 
The requested resource has been assigned a new permanent URL and
any future references to this resource should be done using that
URL. Clients with link editing capabilities should automatically
relink references to the Request-URI to the new reference returned
by the server, where possible.
 
The new URL must be given by the Location field in the response.
Unless it was a HEAD request, the Entity-Body of the response
should contain a short note with a hyperlink to the new URL.
 
If the 301 status code is received in response to a request using
the POST method, the user agent must not automatically redirect the
request unless it can be confirmed by the user, since this might
change the conditions under which the request was issued.
 
 
 
 
 
Berners-Lee, et al Informational [Page 34]
RFC 1945 HTTP/1.0 May 1996
 
 
Note: When automatically redirecting a POST request after
receiving a 301 status code, some existing user agents will
erroneously change it into a GET request.
 
302 Moved Temporarily
 
The requested resource resides temporarily under a different URL.
Since the redirection may be altered on occasion, the client should
continue to use the Request-URI for future requests.
 
The URL must be given by the Location field in the response. Unless
it was a HEAD request, the Entity-Body of the response should
contain a short note with a hyperlink to the new URI(s).
 
If the 302 status code is received in response to a request using
the POST method, the user agent must not automatically redirect the
request unless it can be confirmed by the user, since this might
change the conditions under which the request was issued.
 
Note: When automatically redirecting a POST request after
receiving a 302 status code, some existing user agents will
erroneously change it into a GET request.
 
304 Not Modified
 
If the client has performed a conditional GET request and access is
allowed, but the document has not been modified since the date and
time specified in the If-Modified-Since field, the server must
respond with this status code and not send an Entity-Body to the
client. Header fields contained in the response should only include
information which is relevant to cache managers or which may have
changed independently of the entity's Last-Modified date. Examples
of relevant header fields include: Date, Server, and Expires. A
cache should update its cached entity to reflect any new field
values given in the 304 response.
 
9.4 Client Error 4xx
 
The 4xx class of status code is intended for cases in which the
client seems to have erred. If the client has not completed the
request when a 4xx code is received, it should immediately cease
sending data to the server. Except when responding to a HEAD request,
the server should include an entity containing an explanation of the
error situation, and whether it is a temporary or permanent
condition. These status codes are applicable to any request method.
 
 
 
 
 
 
Berners-Lee, et al Informational [Page 35]
RFC 1945 HTTP/1.0 May 1996
 
 
Note: If the client is sending data, server implementations on TCP
should be careful to ensure that the client acknowledges receipt
of the packet(s) containing the response prior to closing the
input connection. If the client continues sending data to the
server after the close, the server's controller will send a reset
packet to the client, which may erase the client's unacknowledged
input buffers before they can be read and interpreted by the HTTP
application.
 
400 Bad Request
 
The request could not be understood by the server due to malformed
syntax. The client should not repeat the request without
modifications.
 
401 Unauthorized
 
The request requires user authentication. The response must include
a WWW-Authenticate header field (Section 10.16) containing a
challenge applicable to the requested resource. The client may
repeat the request with a suitable Authorization header field
(Section 10.2). If the request already included Authorization
credentials, then the 401 response indicates that authorization has
been refused for those credentials. If the 401 response contains
the same challenge as the prior response, and the user agent has
already attempted authentication at least once, then the user
should be presented the entity that was given in the response,
since that entity may include relevant diagnostic information. HTTP
access authentication is explained in Section 11.
 
403 Forbidden
 
The server understood the request, but is refusing to fulfill it.
Authorization will not help and the request should not be repeated.
If the request method was not HEAD and the server wishes to make
public why the request has not been fulfilled, it should describe
the reason for the refusal in the entity body. This status code is
commonly used when the server does not wish to reveal exactly why
the request has been refused, or when no other response is
applicable.
 
404 Not Found
 
The server has not found anything matching the Request-URI. No
indication is given of whether the condition is temporary or
permanent. If the server does not wish to make this information
available to the client, the status code 403 (forbidden) can be
used instead.
 
 
 
Berners-Lee, et al Informational [Page 36]
RFC 1945 HTTP/1.0 May 1996
 
 
9.5 Server Error 5xx
 
Response status codes beginning with the digit "5" indicate cases in
which the server is aware that it has erred or is incapable of
performing the request. If the client has not completed the request
when a 5xx code is received, it should immediately cease sending data
to the server. Except when responding to a HEAD request, the server
should include an entity containing an explanation of the error
situation, and whether it is a temporary or permanent condition.
These response codes are applicable to any request method and there
are no required header fields.
 
500 Internal Server Error
 
The server encountered an unexpected condition which prevented it
from fulfilling the request.
 
501 Not Implemented
 
The server does not support the functionality required to fulfill
the request. This is the appropriate response when the server does
not recognize the request method and is not capable of supporting
it for any resource.
 
502 Bad Gateway
 
The server, while acting as a gateway or proxy, received an invalid
response from the upstream server it accessed in attempting to
fulfill the request.
 
503 Service Unavailable
 
The server is currently unable to handle the request due to a
temporary overloading or maintenance of the server. The implication
is that this is a temporary condition which will be alleviated
after some delay.
 
Note: The existence of the 503 status code does not imply
that a server must use it when becoming overloaded. Some
servers may wish to simply refuse the connection.
 
10. Header Field Definitions
 
This section defines the syntax and semantics of all commonly used
HTTP/1.0 header fields. For general and entity header fields, both
sender and recipient refer to either the client or the server,
depending on who sends and who receives the message.
 
 
 
 
Berners-Lee, et al Informational [Page 37]
RFC 1945 HTTP/1.0 May 1996
 
 
10.1 Allow
 
The Allow entity-header field lists the set of methods supported by
the resource identified by the Request-URI. The purpose of this field
is strictly to inform the recipient of valid methods associated with
the resource. The Allow header field is not permitted in a request
using the POST method, and thus should be ignored if it is received
as part of a POST entity.
 
Allow = "Allow" ":" 1#method
 
Example of use:
 
Allow: GET, HEAD
 
This field cannot prevent a client from trying other methods.
However, the indications given by the Allow header field value should
be followed. The actual set of allowed methods is defined by the
origin server at the time of each request.
 
A proxy must not modify the Allow header field even if it does not
understand all the methods specified, since the user agent may have
other means of communicating with the origin server.
 
The Allow header field does not indicate what methods are implemented
by the server.
 
10.2 Authorization
 
A user agent that wishes to authenticate itself with a server--
usually, but not necessarily, after receiving a 401 response--may do
so by including an Authorization request-header field with the
request. The Authorization field value consists of credentials
containing the authentication information of the user agent for the
realm of the resource being requested.
 
Authorization = "Authorization" ":" credentials
 
HTTP access authentication is described in Section 11. If a request
is authenticated and a realm specified, the same credentials should
be valid for all other requests within this realm.
 
Responses to requests containing an Authorization field are not
cachable.
 
 
 
 
 
 
 
Berners-Lee, et al Informational [Page 38]
RFC 1945 HTTP/1.0 May 1996
 
 
10.3 Content-Encoding
 
The Content-Encoding entity-header field is used as a modifier to the
media-type. When present, its value indicates what additional content
coding has been applied to the resource, and thus what decoding
mechanism must be applied in order to obtain the media-type
referenced by the Content-Type header field. The Content-Encoding is
primarily used to allow a document to be compressed without losing
the identity of its underlying media type.
 
Content-Encoding = "Content-Encoding" ":" content-coding
 
Content codings are defined in Section 3.5. An example of its use is
 
Content-Encoding: x-gzip
 
The Content-Encoding is a characteristic of the resource identified
by the Request-URI. Typically, the resource is stored with this
encoding and is only decoded before rendering or analogous usage.
 
10.4 Content-Length
 
The Content-Length entity-header field indicates the size of the
Entity-Body, in decimal number of octets, sent to the recipient or,
in the case of the HEAD method, the size of the Entity-Body that
would have been sent had the request been a GET.
 
Content-Length = "Content-Length" ":" 1*DIGIT
 
An example is
 
Content-Length: 3495
 
Applications should use this field to indicate the size of the
Entity-Body to be transferred, regardless of the media type of the
entity. A valid Content-Length field value is required on all
HTTP/1.0 request messages containing an entity body.
 
Any Content-Length greater than or equal to zero is a valid value.
Section 7.2.2 describes how to determine the length of a response
entity body if a Content-Length is not given.
 
Note: The meaning of this field is significantly different from
the corresponding definition in MIME, where it is an optional
field used within the "message/external-body" content-type. In
HTTP, it should be used whenever the entity's length can be
determined prior to being transferred.
 
 
 
 
Berners-Lee, et al Informational [Page 39]
RFC 1945 HTTP/1.0 May 1996
 
 
10.5 Content-Type
 
The Content-Type entity-header field indicates the media type of the
Entity-Body sent to the recipient or, in the case of the HEAD method,
the media type that would have been sent had the request been a GET.
 
Content-Type = "Content-Type" ":" media-type
 
Media types are defined in Section 3.6. An example of the field is
 
Content-Type: text/html
 
Further discussion of methods for identifying the media type of an
entity is provided in Section 7.2.1.
 
10.6 Date
 
The Date general-header field represents the date and time at which
the message was originated, having the same semantics as orig-date in
RFC 822. The field value is an HTTP-date, as described in Section
3.3.
 
Date = "Date" ":" HTTP-date
 
An example is
 
Date: Tue, 15 Nov 1994 08:12:31 GMT
 
If a message is received via direct connection with the user agent
(in the case of requests) or the origin server (in the case of
responses), then the date can be assumed to be the current date at
the receiving end. However, since the date--as it is believed by the
origin--is important for evaluating cached responses, origin servers
should always include a Date header. Clients should only send a Date
header field in messages that include an entity body, as in the case
of the POST request, and even then it is optional. A received message
which does not have a Date header field should be assigned one by the
recipient if the message will be cached by that recipient or
gatewayed via a protocol which requires a Date.
 
In theory, the date should represent the moment just before the
entity is generated. In practice, the date can be generated at any
time during the message origination without affecting its semantic
value.
 
Note: An earlier version of this document incorrectly specified
that this field should contain the creation date of the enclosed
Entity-Body. This has been changed to reflect actual (and proper)
 
 
 
Berners-Lee, et al Informational [Page 40]
RFC 1945 HTTP/1.0 May 1996
 
 
usage.
 
10.7 Expires
 
The Expires entity-header field gives the date/time after which the
entity should be considered stale. This allows information providers
to suggest the volatility of the resource, or a date after which the
information may no longer be valid. Applications must not cache this
entity beyond the date given. The presence of an Expires field does
not imply that the original resource will change or cease to exist
at, before, or after that time. However, information providers that
know or even suspect that a resource will change by a certain date
should include an Expires header with that date. The format is an
absolute date and time as defined by HTTP-date in Section 3.3.
 
Expires = "Expires" ":" HTTP-date
 
An example of its use is
 
Expires: Thu, 01 Dec 1994 16:00:00 GMT
 
If the date given is equal to or earlier than the value of the Date
header, the recipient must not cache the enclosed entity. If a
resource is dynamic by nature, as is the case with many data-
producing processes, entities from that resource should be given an
appropriate Expires value which reflects that dynamism.
 
The Expires field cannot be used to force a user agent to refresh its
display or reload a resource; its semantics apply only to caching
mechanisms, and such mechanisms need only check a resource's
expiration status when a new request for that resource is initiated.
 
User agents often have history mechanisms, such as "Back" buttons and
history lists, which can be used to redisplay an entity retrieved
earlier in a session. By default, the Expires field does not apply to
history mechanisms. If the entity is still in storage, a history
mechanism should display it even if the entity has expired, unless
the user has specifically configured the agent to refresh expired
history documents.
 
Note: Applications are encouraged to be tolerant of bad or
misinformed implementations of the Expires header. A value of zero
(0) or an invalid date format should be considered equivalent to
an "expires immediately." Although these values are not legitimate
for HTTP/1.0, a robust implementation is always desirable.
 
 
 
 
 
 
Berners-Lee, et al Informational [Page 41]
RFC 1945 HTTP/1.0 May 1996
 
 
10.8 From
 
The From request-header field, if given, should contain an Internet
e-mail address for the human user who controls the requesting user
agent. The address should be machine-usable, as defined by mailbox in
RFC 822 [7] (as updated by RFC 1123 [6]):
 
From = "From" ":" mailbox
 
An example is:
 
From: webmaster@w3.org
 
This header field may be used for logging purposes and as a means for
identifying the source of invalid or unwanted requests. It should not
be used as an insecure form of access protection. The interpretation
of this field is that the request is being performed on behalf of the
person given, who accepts responsibility for the method performed. In
particular, robot agents should include this header so that the
person responsible for running the robot can be contacted if problems
occur on the receiving end.
 
The Internet e-mail address in this field may be separate from the
Internet host which issued the request. For example, when a request
is passed through a proxy, the original issuer's address should be
used.
 
Note: The client should not send the From header field without the
user's approval, as it may conflict with the user's privacy
interests or their site's security policy. It is strongly
recommended that the user be able to disable, enable, and modify
the value of this field at any time prior to a request.
 
10.9 If-Modified-Since
 
The If-Modified-Since request-header field is used with the GET
method to make it conditional: if the requested resource has not been
modified since the time specified in this field, a copy of the
resource will not be returned from the server; instead, a 304 (not
modified) response will be returned without any Entity-Body.
 
If-Modified-Since = "If-Modified-Since" ":" HTTP-date
 
An example of the field is:
 
If-Modified-Since: Sat, 29 Oct 1994 19:43:31 GMT
 
 
 
 
 
Berners-Lee, et al Informational [Page 42]
RFC 1945 HTTP/1.0 May 1996
 
 
A conditional GET method requests that the identified resource be
transferred only if it has been modified since the date given by the
If-Modified-Since header. The algorithm for determining this includes
the following cases:
 
a) If the request would normally result in anything other than
a 200 (ok) status, or if the passed If-Modified-Since date
is invalid, the response is exactly the same as for a
normal GET. A date which is later than the server's current
time is invalid.
 
b) If the resource has been modified since the
If-Modified-Since date, the response is exactly the same as
for a normal GET.
 
c) If the resource has not been modified since a valid
If-Modified-Since date, the server shall return a 304 (not
modified) response.
 
The purpose of this feature is to allow efficient updates of cached
information with a minimum amount of transaction overhead.
 
10.10 Last-Modified
 
The Last-Modified entity-header field indicates the date and time at
which the sender believes the resource was last modified. The exact
semantics of this field are defined in terms of how the recipient
should interpret it: if the recipient has a copy of this resource
which is older than the date given by the Last-Modified field, that
copy should be considered stale.
 
Last-Modified = "Last-Modified" ":" HTTP-date
 
An example of its use is
 
Last-Modified: Tue, 15 Nov 1994 12:45:26 GMT
 
The exact meaning of this header field depends on the implementation
of the sender and the nature of the original resource. For files, it
may be just the file system last-modified time. For entities with
dynamically included parts, it may be the most recent of the set of
last-modify times for its component parts. For database gateways, it
may be the last-update timestamp of the record. For virtual objects,
it may be the last time the internal state changed.
 
An origin server must not send a Last-Modified date which is later
than the server's time of message origination. In such cases, where
the resource's last modification would indicate some time in the
 
 
 
Berners-Lee, et al Informational [Page 43]
RFC 1945 HTTP/1.0 May 1996
 
 
future, the server must replace that date with the message
origination date.
 
10.11 Location
 
The Location response-header field defines the exact location of the
resource that was identified by the Request-URI. For 3xx responses,
the location must indicate the server's preferred URL for automatic
redirection to the resource. Only one absolute URL is allowed.
 
Location = "Location" ":" absoluteURI
 
An example is
 
Location: http://www.w3.org/hypertext/WWW/NewLocation.html
 
10.12 Pragma
 
The Pragma general-header field is used to include implementation-
specific directives that may apply to any recipient along the
request/response chain. All pragma directives specify optional
behavior from the viewpoint of the protocol; however, some systems
may require that behavior be consistent with the directives.
 
Pragma = "Pragma" ":" 1#pragma-directive
 
pragma-directive = "no-cache" | extension-pragma
extension-pragma = token [ "=" word ]
 
When the "no-cache" directive is present in a request message, an
application should forward the request toward the origin server even
if it has a cached copy of what is being requested. This allows a
client to insist upon receiving an authoritative response to its
request. It also allows a client to refresh a cached copy which is
known to be corrupted or stale.
 
Pragma directives must be passed through by a proxy or gateway
application, regardless of their significance to that application,
since the directives may be applicable to all recipients along the
request/response chain. It is not possible to specify a pragma for a
specific recipient; however, any pragma directive not relevant to a
recipient should be ignored by that recipient.
 
10.13 Referer
 
The Referer request-header field allows the client to specify, for
the server's benefit, the address (URI) of the resource from which
the Request-URI was obtained. This allows a server to generate lists
 
 
 
Berners-Lee, et al Informational [Page 44]
RFC 1945 HTTP/1.0 May 1996
 
 
of back-links to resources for interest, logging, optimized caching,
etc. It also allows obsolete or mistyped links to be traced for
maintenance. The Referer field must not be sent if the Request-URI
was obtained from a source that does not have its own URI, such as
input from the user keyboard.
 
Referer = "Referer" ":" ( absoluteURI | relativeURI )
 
Example:
 
Referer: http://www.w3.org/hypertext/DataSources/Overview.html
 
If a partial URI is given, it should be interpreted relative to the
Request-URI. The URI must not include a fragment.
 
Note: Because the source of a link may be private information or
may reveal an otherwise private information source, it is strongly
recommended that the user be able to select whether or not the
Referer field is sent. For example, a browser client could have a
toggle switch for browsing openly/anonymously, which would
respectively enable/disable the sending of Referer and From
information.
 
10.14 Server
 
The Server response-header field contains information about the
software used by the origin server to handle the request. The field
can contain multiple product tokens (Section 3.7) and comments
identifying the server and any significant subproducts. By
convention, the product tokens are listed in order of their
significance for identifying the application.
 
Server = "Server" ":" 1*( product | comment )
 
Example:
 
Server: CERN/3.0 libwww/2.17
 
If the response is being forwarded through a proxy, the proxy
application must not add its data to the product list.
 
Note: Revealing the specific software version of the server may
allow the server machine to become more vulnerable to attacks
against software that is known to contain security holes. Server
implementors are encouraged to make this field a configurable
option.
 
 
 
 
 
Berners-Lee, et al Informational [Page 45]
RFC 1945 HTTP/1.0 May 1996
 
 
Note: Some existing servers fail to restrict themselves to the
product token syntax within the Server field.
 
10.15 User-Agent
 
The User-Agent request-header field contains information about the
user agent originating the request. This is for statistical purposes,
the tracing of protocol violations, and automated recognition of user
agents for the sake of tailoring responses to avoid particular user
agent limitations. Although it is not required, user agents should
include this field with requests. The field can contain multiple
product tokens (Section 3.7) and comments identifying the agent and
any subproducts which form a significant part of the user agent. By
convention, the product tokens are listed in order of their
significance for identifying the application.
 
User-Agent = "User-Agent" ":" 1*( product | comment )
 
Example:
 
User-Agent: CERN-LineMode/2.15 libwww/2.17b3
 
Note: Some current proxy applications append their product
information to the list in the User-Agent field. This is not
recommended, since it makes machine interpretation of these
fields ambiguous.
 
Note: Some existing clients fail to restrict themselves to
the product token syntax within the User-Agent field.
 
10.16 WWW-Authenticate
 
The WWW-Authenticate response-header field must be included in 401
(unauthorized) response messages. The field value consists of at
least one challenge that indicates the authentication scheme(s) and
parameters applicable to the Request-URI.
 
WWW-Authenticate = "WWW-Authenticate" ":" 1#challenge
 
The HTTP access authentication process is described in Section 11.
User agents must take special care in parsing the WWW-Authenticate
field value if it contains more than one challenge, or if more than
one WWW-Authenticate header field is provided, since the contents of
a challenge may itself contain a comma-separated list of
authentication parameters.
 
 
 
 
 
 
Berners-Lee, et al Informational [Page 46]
RFC 1945 HTTP/1.0 May 1996
 
 
11. Access Authentication
 
HTTP provides a simple challenge-response authentication mechanism
which may be used by a server to challenge a client request and by a
client to provide authentication information. It uses an extensible,
case-insensitive token to identify the authentication scheme,
followed by a comma-separated list of attribute-value pairs which
carry the parameters necessary for achieving authentication via that
scheme.
 
auth-scheme = token
 
auth-param = token "=" quoted-string
 
The 401 (unauthorized) response message is used by an origin server
to challenge the authorization of a user agent. This response must
include a WWW-Authenticate header field containing at least one
challenge applicable to the requested resource.
 
challenge = auth-scheme 1*SP realm *( "," auth-param )
 
realm = "realm" "=" realm-value
realm-value = quoted-string
 
The realm attribute (case-insensitive) is required for all
authentication schemes which issue a challenge. The realm value
(case-sensitive), in combination with the canonical root URL of the
server being accessed, defines the protection space. These realms
allow the protected resources on a server to be partitioned into a
set of protection spaces, each with its own authentication scheme
and/or authorization database. The realm value is a string, generally
assigned by the origin server, which may have additional semantics
specific to the authentication scheme.
 
A user agent that wishes to authenticate itself with a server--
usually, but not necessarily, after receiving a 401 response--may do
so by including an Authorization header field with the request. The
Authorization field value consists of credentials containing the
authentication information of the user agent for the realm of the
resource being requested.
 
credentials = basic-credentials
| ( auth-scheme #auth-param )
 
The domain over which credentials can be automatically applied by a
user agent is determined by the protection space. If a prior request
has been authorized, the same credentials may be reused for all other
requests within that protection space for a period of time determined
 
 
 
Berners-Lee, et al Informational [Page 47]
RFC 1945 HTTP/1.0 May 1996
 
 
by the authentication scheme, parameters, and/or user preference.
Unless otherwise defined by the authentication scheme, a single
protection space cannot extend outside the scope of its server.
 
If the server does not wish to accept the credentials sent with a
request, it should return a 403 (forbidden) response.
 
The HTTP protocol does not restrict applications to this simple
challenge-response mechanism for access authentication. Additional
mechanisms may be used, such as encryption at the transport level or
via message encapsulation, and with additional header fields
specifying authentication information. However, these additional
mechanisms are not defined by this specification.
 
Proxies must be completely transparent regarding user agent
authentication. That is, they must forward the WWW-Authenticate and
Authorization headers untouched, and must not cache the response to a
request containing Authorization. HTTP/1.0 does not provide a means
for a client to be authenticated with a proxy.
 
11.1 Basic Authentication Scheme
 
The "basic" authentication scheme is based on the model that the user
agent must authenticate itself with a user-ID and a password for each
realm. The realm value should be considered an opaque string which
can only be compared for equality with other realms on that server.
The server will authorize the request only if it can validate the
user-ID and password for the protection space of the Request-URI.
There are no optional authentication parameters.
 
Upon receipt of an unauthorized request for a URI within the
protection space, the server should respond with a challenge like the
following:
 
WWW-Authenticate: Basic realm="WallyWorld"
 
where "WallyWorld" is the string assigned by the server to identify
the protection space of the Request-URI.
 
To receive authorization, the client sends the user-ID and password,
separated by a single colon (":") character, within a base64 [5]
encoded string in the credentials.
 
basic-credentials = "Basic" SP basic-cookie
 
basic-cookie = <base64 [5] encoding of userid-password,
except not limited to 76 char/line>
 
 
 
 
Berners-Lee, et al Informational [Page 48]
RFC 1945 HTTP/1.0 May 1996
 
 
userid-password = [ token ] ":" *TEXT
 
If the user agent wishes to send the user-ID "Aladdin" and password
"open sesame", it would use the following header field:
 
Authorization: Basic QWxhZGRpbjpvcGVuIHNlc2FtZQ==
 
The basic authentication scheme is a non-secure method of filtering
unauthorized access to resources on an HTTP server. It is based on
the assumption that the connection between the client and the server
can be regarded as a trusted carrier. As this is not generally true
on an open network, the basic authentication scheme should be used
accordingly. In spite of this, clients should implement the scheme in
order to communicate with servers that use it.
 
12. Security Considerations
 
This section is meant to inform application developers, information
providers, and users of the security limitations in HTTP/1.0 as
described by this document. The discussion does not include
definitive solutions to the problems revealed, though it does make
some suggestions for reducing security risks.
 
12.1 Authentication of Clients
 
As mentioned in Section 11.1, the Basic authentication scheme is not
a secure method of user authentication, nor does it prevent the
Entity-Body from being transmitted in clear text across the physical
network used as the carrier. HTTP/1.0 does not prevent additional
authentication schemes and encryption mechanisms from being employed
to increase security.
 
12.2 Safe Methods
 
The writers of client software should be aware that the software
represents the user in their interactions over the Internet, and
should be careful to allow the user to be aware of any actions they
may take which may have an unexpected significance to themselves or
others.
 
In particular, the convention has been established that the GET and
HEAD methods should never have the significance of taking an action
other than retrieval. These methods should be considered "safe." This
allows user agents to represent other methods, such as POST, in a
special way, so that the user is made aware of the fact that a
possibly unsafe action is being requested.
 
 
 
 
 
Berners-Lee, et al Informational [Page 49]
RFC 1945 HTTP/1.0 May 1996
 
 
Naturally, it is not possible to ensure that the server does not
generate side-effects as a result of performing a GET request; in
fact, some dynamic resources consider that a feature. The important
distinction here is that the user did not request the side-effects,
so therefore cannot be held accountable for them.
 
12.3 Abuse of Server Log Information
 
A server is in the position to save personal data about a user's
requests which may identify their reading patterns or subjects of
interest. This information is clearly confidential in nature and its
handling may be constrained by law in certain countries. People using
the HTTP protocol to provide data are responsible for ensuring that
such material is not distributed without the permission of any
individuals that are identifiable by the published results.
 
12.4 Transfer of Sensitive Information
 
Like any generic data transfer protocol, HTTP cannot regulate the
content of the data that is transferred, nor is there any a priori
method of determining the sensitivity of any particular piece of
information within the context of any given request. Therefore,
applications should supply as much control over this information as
possible to the provider of that information. Three header fields are
worth special mention in this context: Server, Referer and From.
 
Revealing the specific software version of the server may allow the
server machine to become more vulnerable to attacks against software
that is known to contain security holes. Implementors should make the
Server header field a configurable option.
 
The Referer field allows reading patterns to be studied and reverse
links drawn. Although it can be very useful, its power can be abused
if user details are not separated from the information contained in
the Referer. Even when the personal information has been removed, the
Referer field may indicate a private document's URI whose publication
would be inappropriate.
 
The information sent in the From field might conflict with the user's
privacy interests or their site's security policy, and hence it
should not be transmitted without the user being able to disable,
enable, and modify the contents of the field. The user must be able
to set the contents of this field within a user preference or
application defaults configuration.
 
We suggest, though do not require, that a convenient toggle interface
be provided for the user to enable or disable the sending of From and
Referer information.
 
 
 
Berners-Lee, et al Informational [Page 50]
RFC 1945 HTTP/1.0 May 1996
 
 
12.5 Attacks Based On File and Path Names
 
Implementations of HTTP origin servers should be careful to restrict
the documents returned by HTTP requests to be only those that were
intended by the server administrators. If an HTTP server translates
HTTP URIs directly into file system calls, the server must take
special care not to serve files that were not intended to be
delivered to HTTP clients. For example, Unix, Microsoft Windows, and
other operating systems use ".." as a path component to indicate a
directory level above the current one. On such a system, an HTTP
server must disallow any such construct in the Request-URI if it
would otherwise allow access to a resource outside those intended to
be accessible via the HTTP server. Similarly, files intended for
reference only internally to the server (such as access control
files, configuration files, and script code) must be protected from
inappropriate retrieval, since they might contain sensitive
information. Experience has shown that minor bugs in such HTTP server
implementations have turned into security risks.
 
13. Acknowledgments
 
This specification makes heavy use of the augmented BNF and generic
constructs defined by David H. Crocker for RFC 822 [7]. Similarly, it
reuses many of the definitions provided by Nathaniel Borenstein and
Ned Freed for MIME [5]. We hope that their inclusion in this
specification will help reduce past confusion over the relationship
between HTTP/1.0 and Internet mail message formats.
 
The HTTP protocol has evolved considerably over the past four years.
It has benefited from a large and active developer community--the
many people who have participated on the www-talk mailing list--and
it is that community which has been most responsible for the success
of HTTP and of the World-Wide Web in general. Marc Andreessen, Robert
Cailliau, Daniel W. Connolly, Bob Denny, Jean-Francois Groff, Phillip
M. Hallam-Baker, Hakon W. Lie, Ari Luotonen, Rob McCool, Lou
Montulli, Dave Raggett, Tony Sanders, and Marc VanHeyningen deserve
special recognition for their efforts in defining aspects of the
protocol for early versions of this specification.
 
Paul Hoffman contributed sections regarding the informational status
of this document and Appendices C and D.
 
 
 
 
 
 
 
 
 
 
Berners-Lee, et al Informational [Page 51]
RFC 1945 HTTP/1.0 May 1996
 
 
This document has benefited greatly from the comments of all those
participating in the HTTP-WG. In addition to those already mentioned,
the following individuals have contributed to this specification:
 
Gary Adams Harald Tveit Alvestrand
Keith Ball Brian Behlendorf
Paul Burchard Maurizio Codogno
Mike Cowlishaw Roman Czyborra
Michael A. Dolan John Franks
Jim Gettys Marc Hedlund
Koen Holtman Alex Hopmann
Bob Jernigan Shel Kaphan
Martijn Koster Dave Kristol
Daniel LaLiberte Paul Leach
Albert Lunde John C. Mallery
Larry Masinter Mitra
Jeffrey Mogul Gavin Nicol
Bill Perry Jeffrey Perry
Owen Rees Luigi Rizzo
David Robinson Marc Salomon
Rich Salz Jim Seidman
Chuck Shotton Eric W. Sink
Simon E. Spero Robert S. Thau
Francois Yergeau Mary Ellen Zurko
Jean-Philippe Martin-Flatin
 
14. References
 
[1] Anklesaria, F., McCahill, M., Lindner, P., Johnson, D.,
Torrey, D., and B. Alberti, "The Internet Gopher Protocol: A
Distributed Document Search and Retrieval Protocol", RFC 1436,
University of Minnesota, March 1993.
 
[2] Berners-Lee, T., "Universal Resource Identifiers in WWW: A
Unifying Syntax for the Expression of Names and Addresses of
Objects on the Network as used in the World-Wide Web",
RFC 1630, CERN, June 1994.
 
[3] Berners-Lee, T., and D. Connolly, "Hypertext Markup Language -
2.0", RFC 1866, MIT/W3C, November 1995.
 
[4] Berners-Lee, T., Masinter, L., and M. McCahill, "Uniform
Resource Locators (URL)", RFC 1738, CERN, Xerox PARC,
University of Minnesota, December 1994.
 
 
 
 
 
 
 
Berners-Lee, et al Informational [Page 52]
RFC 1945 HTTP/1.0 May 1996
 
 
[5] Borenstein, N., and N. Freed, "MIME (Multipurpose Internet Mail
Extensions) Part One: Mechanisms for Specifying and Describing
the Format of Internet Message Bodies", RFC 1521, Bellcore,
Innosoft, September 1993.
 
[6] Braden, R., "Requirements for Internet hosts - Application and
Support", STD 3, RFC 1123, IETF, October 1989.
 
[7] Crocker, D., "Standard for the Format of ARPA Internet Text
Messages", STD 11, RFC 822, UDEL, August 1982.
 
[8] F. Davis, B. Kahle, H. Morris, J. Salem, T. Shen, R. Wang,
J. Sui, and M. Grinbaum. "WAIS Interface Protocol Prototype
Functional Specification." (v1.5), Thinking Machines
Corporation, April 1990.
 
[9] Fielding, R., "Relative Uniform Resource Locators", RFC 1808,
UC Irvine, June 1995.
 
[10] Horton, M., and R. Adams, "Standard for interchange of USENET
Messages", RFC 1036 (Obsoletes RFC 850), AT&T Bell
Laboratories, Center for Seismic Studies, December 1987.
 
[11] Kantor, B., and P. Lapsley, "Network News Transfer Protocol:
A Proposed Standard for the Stream-Based Transmission of News",
RFC 977, UC San Diego, UC Berkeley, February 1986.
 
[12] Postel, J., "Simple Mail Transfer Protocol." STD 10, RFC 821,
USC/ISI, August 1982.
 
[13] Postel, J., "Media Type Registration Procedure." RFC 1590,
USC/ISI, March 1994.
 
[14] Postel, J., and J. Reynolds, "File Transfer Protocol (FTP)",
STD 9, RFC 959, USC/ISI, October 1985.
 
[15] Reynolds, J., and J. Postel, "Assigned Numbers", STD 2, RFC
1700, USC/ISI, October 1994.
 
[16] Sollins, K., and L. Masinter, "Functional Requirements for
Uniform Resource Names", RFC 1737, MIT/LCS, Xerox Corporation,
December 1994.
 
[17] US-ASCII. Coded Character Set - 7-Bit American Standard Code
for Information Interchange. Standard ANSI X3.4-1986, ANSI,
1986.
 
 
 
 
 
Berners-Lee, et al Informational [Page 53]
RFC 1945 HTTP/1.0 May 1996
 
 
[18] ISO-8859. International Standard -- Information Processing --
8-bit Single-Byte Coded Graphic Character Sets --
Part 1: Latin alphabet No. 1, ISO 8859-1:1987.
Part 2: Latin alphabet No. 2, ISO 8859-2, 1987.
Part 3: Latin alphabet No. 3, ISO 8859-3, 1988.
Part 4: Latin alphabet No. 4, ISO 8859-4, 1988.
Part 5: Latin/Cyrillic alphabet, ISO 8859-5, 1988.
Part 6: Latin/Arabic alphabet, ISO 8859-6, 1987.
Part 7: Latin/Greek alphabet, ISO 8859-7, 1987.
Part 8: Latin/Hebrew alphabet, ISO 8859-8, 1988.
Part 9: Latin alphabet No. 5, ISO 8859-9, 1990.
 
15. Authors' Addresses
 
Tim Berners-Lee
Director, W3 Consortium
MIT Laboratory for Computer Science
545 Technology Square
Cambridge, MA 02139, U.S.A.
 
Fax: +1 (617) 258 8682
EMail: timbl@w3.org
 
 
Roy T. Fielding
Department of Information and Computer Science
University of California
Irvine, CA 92717-3425, U.S.A.
 
Fax: +1 (714) 824-4056
EMail: fielding@ics.uci.edu
 
 
Henrik Frystyk Nielsen
W3 Consortium
MIT Laboratory for Computer Science
545 Technology Square
Cambridge, MA 02139, U.S.A.
 
Fax: +1 (617) 258 8682
EMail: frystyk@w3.org
 
 
 
 
 
 
 
 
 
 
Berners-Lee, et al Informational [Page 54]
RFC 1945 HTTP/1.0 May 1996
 
 
Appendices
 
These appendices are provided for informational reasons only -- they
do not form a part of the HTTP/1.0 specification.
 
A. Internet Media Type message/http
 
In addition to defining the HTTP/1.0 protocol, this document serves
as the specification for the Internet media type "message/http". The
following is to be registered with IANA [13].
 
Media Type name: message
 
Media subtype name: http
 
Required parameters: none
 
Optional parameters: version, msgtype
 
version: The HTTP-Version number of the enclosed message
(e.g., "1.0"). If not present, the version can be
determined from the first line of the body.
 
msgtype: The message type -- "request" or "response". If
not present, the type can be determined from the
first line of the body.
 
Encoding considerations: only "7bit", "8bit", or "binary" are
permitted
 
Security considerations: none
 
B. Tolerant Applications
 
Although this document specifies the requirements for the generation
of HTTP/1.0 messages, not all applications will be correct in their
implementation. We therefore recommend that operational applications
be tolerant of deviations whenever those deviations can be
interpreted unambiguously.
 
Clients should be tolerant in parsing the Status-Line and servers
tolerant when parsing the Request-Line. In particular, they should
accept any amount of SP or HT characters between fields, even though
only a single SP is required.
 
The line terminator for HTTP-header fields is the sequence CRLF.
However, we recommend that applications, when parsing such headers,
recognize a single LF as a line terminator and ignore the leading CR.
 
 
 
Berners-Lee, et al Informational [Page 55]
RFC 1945 HTTP/1.0 May 1996
 
 
C. Relationship to MIME
 
HTTP/1.0 uses many of the constructs defined for Internet Mail (RFC
822 [7]) and the Multipurpose Internet Mail Extensions (MIME [5]) to
allow entities to be transmitted in an open variety of
representations and with extensible mechanisms. However, RFC 1521
discusses mail, and HTTP has a few features that are different than
those described in RFC 1521. These differences were carefully chosen
to optimize performance over binary connections, to allow greater
freedom in the use of new media types, to make date comparisons
easier, and to acknowledge the practice of some early HTTP servers
and clients.
 
At the time of this writing, it is expected that RFC 1521 will be
revised. The revisions may include some of the practices found in
HTTP/1.0 but not in RFC 1521.
 
This appendix describes specific areas where HTTP differs from RFC
1521. Proxies and gateways to strict MIME environments should be
aware of these differences and provide the appropriate conversions
where necessary. Proxies and gateways from MIME environments to HTTP
also need to be aware of the differences because some conversions may
be required.
 
C.1 Conversion to Canonical Form
 
RFC 1521 requires that an Internet mail entity be converted to
canonical form prior to being transferred, as described in Appendix G
of RFC 1521 [5]. Section 3.6.1 of this document describes the forms
allowed for subtypes of the "text" media type when transmitted over
HTTP.
 
RFC 1521 requires that content with a Content-Type of "text"
represent line breaks as CRLF and forbids the use of CR or LF outside
of line break sequences. HTTP allows CRLF, bare CR, and bare LF to
indicate a line break within text content when a message is
transmitted over HTTP.
 
Where it is possible, a proxy or gateway from HTTP to a strict RFC
1521 environment should translate all line breaks within the text
media types described in Section 3.6.1 of this document to the RFC
1521 canonical form of CRLF. Note, however, that this may be
complicated by the presence of a Content-Encoding and by the fact
that HTTP allows the use of some character sets which do not use
octets 13 and 10 to represent CR and LF, as is the case for some
multi-byte character sets.
 
 
 
 
 
Berners-Lee, et al Informational [Page 56]
RFC 1945 HTTP/1.0 May 1996
 
 
C.2 Conversion of Date Formats
 
HTTP/1.0 uses a restricted set of date formats (Section 3.3) to
simplify the process of date comparison. Proxies and gateways from
other protocols should ensure that any Date header field present in a
message conforms to one of the HTTP/1.0 formats and rewrite the date
if necessary.
 
C.3 Introduction of Content-Encoding
 
RFC 1521 does not include any concept equivalent to HTTP/1.0's
Content-Encoding header field. Since this acts as a modifier on the
media type, proxies and gateways from HTTP to MIME-compliant
protocols must either change the value of the Content-Type header
field or decode the Entity-Body before forwarding the message. (Some
experimental applications of Content-Type for Internet mail have used
a media-type parameter of ";conversions=<content-coding>" to perform
an equivalent function as Content-Encoding. However, this parameter
is not part of RFC 1521.)
 
C.4 No Content-Transfer-Encoding
 
HTTP does not use the Content-Transfer-Encoding (CTE) field of RFC
1521. Proxies and gateways from MIME-compliant protocols to HTTP must
remove any non-identity CTE ("quoted-printable" or "base64") encoding
prior to delivering the response message to an HTTP client.
 
Proxies and gateways from HTTP to MIME-compliant protocols are
responsible for ensuring that the message is in the correct format
and encoding for safe transport on that protocol, where "safe
transport" is defined by the limitations of the protocol being used.
Such a proxy or gateway should label the data with an appropriate
Content-Transfer-Encoding if doing so will improve the likelihood of
safe transport over the destination protocol.
 
C.5 HTTP Header Fields in Multipart Body-Parts
 
In RFC 1521, most header fields in multipart body-parts are generally
ignored unless the field name begins with "Content-". In HTTP/1.0,
multipart body-parts may contain any HTTP header fields which are
significant to the meaning of that part.
 
D. Additional Features
 
This appendix documents protocol elements used by some existing HTTP
implementations, but not consistently and correctly across most
HTTP/1.0 applications. Implementors should be aware of these
features, but cannot rely upon their presence in, or interoperability
 
 
 
Berners-Lee, et al Informational [Page 57]
RFC 1945 HTTP/1.0 May 1996
 
 
with, other HTTP/1.0 applications.
 
D.1 Additional Request Methods
 
D.1.1 PUT
 
The PUT method requests that the enclosed entity be stored under the
supplied Request-URI. If the Request-URI refers to an already
existing resource, the enclosed entity should be considered as a
modified version of the one residing on the origin server. If the
Request-URI does not point to an existing resource, and that URI is
capable of being defined as a new resource by the requesting user
agent, the origin server can create the resource with that URI.
 
The fundamental difference between the POST and PUT requests is
reflected in the different meaning of the Request-URI. The URI in a
POST request identifies the resource that will handle the enclosed
entity as data to be processed. That resource may be a data-accepting
process, a gateway to some other protocol, or a separate entity that
accepts annotations. In contrast, the URI in a PUT request identifies
the entity enclosed with the request -- the user agent knows what URI
is intended and the server should not apply the request to some other
resource.
 
D.1.2 DELETE
 
The DELETE method requests that the origin server delete the resource
identified by the Request-URI.
 
D.1.3 LINK
 
The LINK method establishes one or more Link relationships between
the existing resource identified by the Request-URI and other
existing resources.
 
D.1.4 UNLINK
 
The UNLINK method removes one or more Link relationships from the
existing resource identified by the Request-URI.
 
D.2 Additional Header Field Definitions
 
D.2.1 Accept
 
The Accept request-header field can be used to indicate a list of
media ranges which are acceptable as a response to the request. The
asterisk "*" character is used to group media types into ranges, with
"*/*" indicating all media types and "type/*" indicating all subtypes
 
 
 
Berners-Lee, et al Informational [Page 58]
RFC 1945 HTTP/1.0 May 1996
 
 
of that type. The set of ranges given by the client should represent
what types are acceptable given the context of the request.
 
D.2.2 Accept-Charset
 
The Accept-Charset request-header field can be used to indicate a
list of preferred character sets other than the default US-ASCII and
ISO-8859-1. This field allows clients capable of understanding more
comprehensive or special-purpose character sets to signal that
capability to a server which is capable of representing documents in
those character sets.
 
D.2.3 Accept-Encoding
 
The Accept-Encoding request-header field is similar to Accept, but
restricts the content-coding values which are acceptable in the
response.
 
D.2.4 Accept-Language
 
The Accept-Language request-header field is similar to Accept, but
restricts the set of natural languages that are preferred as a
response to the request.
 
D.2.5 Content-Language
 
The Content-Language entity-header field describes the natural
language(s) of the intended audience for the enclosed entity. Note
that this may not be equivalent to all the languages used within the
entity.
 
D.2.6 Link
 
The Link entity-header field provides a means for describing a
relationship between the entity and some other resource. An entity
may include multiple Link values. Links at the metainformation level
typically indicate relationships like hierarchical structure and
navigation paths.
 
D.2.7 MIME-Version
 
HTTP messages may include a single MIME-Version general-header field
to indicate what version of the MIME protocol was used to construct
the message. Use of the MIME-Version header field, as defined by RFC
1521 [5], should indicate that the message is MIME-conformant.
Unfortunately, some older HTTP/1.0 servers send it indiscriminately,
and thus this field should be ignored.
 
 
 
 
Berners-Lee, et al Informational [Page 59]
RFC 1945 HTTP/1.0 May 1996
 
 
D.2.8 Retry-After
 
The Retry-After response-header field can be used with a 503 (service
unavailable) response to indicate how long the service is expected to
be unavailable to the requesting client. The value of this field can
be either an HTTP-date or an integer number of seconds (in decimal)
after the time of the response.
 
D.2.9 Title
 
The Title entity-header field indicates the title of the entity.
 
D.2.10 URI
 
The URI entity-header field may contain some or all of the Uniform
Resource Identifiers (Section 3.2) by which the Request-URI resource
can be identified. There is no guarantee that the resource can be
accessed using the URI(s) specified.
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
Berners-Lee, et al Informational [Page 60]
New file
/httptunnel/doc/rfc2045.txt
@@ -0,0 +1,1739 @@
 
 
 
 
 
 
Network Working Group N. Freed
Request for Comments: 2045 Innosoft
Obsoletes: 1521, 1522, 1590 N. Borenstein
Category: Standards Track First Virtual
November 1996
 
 
Multipurpose Internet Mail Extensions
(MIME) Part One:
Format of Internet Message Bodies
 
Status of this Memo
 
This document specifies an Internet standards track protocol for the
Internet community, and requests discussion and suggestions for
improvements. Please refer to the current edition of the "Internet
Official Protocol Standards" (STD 1) for the standardization state
and status of this protocol. Distribution of this memo is unlimited.
 
Abstract
 
STD 11, RFC 822, defines a message representation protocol specifying
considerable detail about US-ASCII message headers, and leaves the
message content, or message body, as flat US-ASCII text. This set of
documents, collectively called the Multipurpose Internet Mail
Extensions, or MIME, redefines the format of messages to allow for
 
(1) textual message bodies in character sets other than
US-ASCII,
 
(2) an extensible set of different formats for non-textual
message bodies,
 
(3) multi-part message bodies, and
 
(4) textual header information in character sets other than
US-ASCII.
 
These documents are based on earlier work documented in RFC 934, STD
11, and RFC 1049, but extends and revises them. Because RFC 822 said
so little about message bodies, these documents are largely
orthogonal to (rather than a revision of) RFC 822.
 
This initial document specifies the various headers used to describe
the structure of MIME messages. The second document, RFC 2046,
defines the general structure of the MIME media typing system and
defines an initial set of media types. The third document, RFC 2047,
describes extensions to RFC 822 to allow non-US-ASCII text data in
 
 
 
Freed & Borenstein Standards Track [Page 1]
RFC 2045 Internet Message Bodies November 1996
 
 
Internet mail header fields. The fourth document, RFC 2048, specifies
various IANA registration procedures for MIME-related facilities. The
fifth and final document, RFC 2049, describes MIME conformance
criteria as well as providing some illustrative examples of MIME
message formats, acknowledgements, and the bibliography.
 
These documents are revisions of RFCs 1521, 1522, and 1590, which
themselves were revisions of RFCs 1341 and 1342. An appendix in RFC
2049 describes differences and changes from previous versions.
 
Table of Contents
 
1. Introduction ......................................... 3
2. Definitions, Conventions, and Generic BNF Grammar .... 5
2.1 CRLF ................................................ 5
2.2 Character Set ....................................... 6
2.3 Message ............................................. 6
2.4 Entity .............................................. 6
2.5 Body Part ........................................... 7
2.6 Body ................................................ 7
2.7 7bit Data ........................................... 7
2.8 8bit Data ........................................... 7
2.9 Binary Data ......................................... 7
2.10 Lines .............................................. 7
3. MIME Header Fields ................................... 8
4. MIME-Version Header Field ............................ 8
5. Content-Type Header Field ............................ 10
5.1 Syntax of the Content-Type Header Field ............. 12
5.2 Content-Type Defaults ............................... 14
6. Content-Transfer-Encoding Header Field ............... 14
6.1 Content-Transfer-Encoding Syntax .................... 14
6.2 Content-Transfer-Encodings Semantics ................ 15
6.3 New Content-Transfer-Encodings ...................... 16
6.4 Interpretation and Use .............................. 16
6.5 Translating Encodings ............................... 18
6.6 Canonical Encoding Model ............................ 19
6.7 Quoted-Printable Content-Transfer-Encoding .......... 19
6.8 Base64 Content-Transfer-Encoding .................... 24
7. Content-ID Header Field .............................. 26
8. Content-Description Header Field ..................... 27
9. Additional MIME Header Fields ........................ 27
10. Summary ............................................. 27
11. Security Considerations ............................. 27
12. Authors' Addresses .................................. 28
A. Collected Grammar .................................... 29
 
 
 
 
 
 
Freed & Borenstein Standards Track [Page 2]
RFC 2045 Internet Message Bodies November 1996
 
 
1. Introduction
 
Since its publication in 1982, RFC 822 has defined the standard
format of textual mail messages on the Internet. Its success has
been such that the RFC 822 format has been adopted, wholly or
partially, well beyond the confines of the Internet and the Internet
SMTP transport defined by RFC 821. As the format has seen wider use,
a number of limitations have proven increasingly restrictive for the
user community.
 
RFC 822 was intended to specify a format for text messages. As such,
non-text messages, such as multimedia messages that might include
audio or images, are simply not mentioned. Even in the case of text,
however, RFC 822 is inadequate for the needs of mail users whose
languages require the use of character sets richer than US-ASCII.
Since RFC 822 does not specify mechanisms for mail containing audio,
video, Asian language text, or even text in most European languages,
additional specifications are needed.
 
One of the notable limitations of RFC 821/822 based mail systems is
the fact that they limit the contents of electronic mail messages to
relatively short lines (e.g. 1000 characters or less [RFC-821]) of
7bit US-ASCII. This forces users to convert any non-textual data
that they may wish to send into seven-bit bytes representable as
printable US-ASCII characters before invoking a local mail UA (User
Agent, a program with which human users send and receive mail).
Examples of such encodings currently used in the Internet include
pure hexadecimal, uuencode, the 3-in-4 base 64 scheme specified in
RFC 1421, the Andrew Toolkit Representation [ATK], and many others.
 
The limitations of RFC 822 mail become even more apparent as gateways
are designed to allow for the exchange of mail messages between RFC
822 hosts and X.400 hosts. X.400 [X400] specifies mechanisms for the
inclusion of non-textual material within electronic mail messages.
The current standards for the mapping of X.400 messages to RFC 822
messages specify either that X.400 non-textual material must be
converted to (not encoded in) IA5Text format, or that they must be
discarded, notifying the RFC 822 user that discarding has occurred.
This is clearly undesirable, as information that a user may wish to
receive is lost. Even though a user agent may not have the
capability of dealing with the non-textual material, the user might
have some mechanism external to the UA that can extract useful
information from the material. Moreover, it does not allow for the
fact that the message may eventually be gatewayed back into an X.400
message handling system (i.e., the X.400 message is "tunneled"
through Internet mail), where the non-textual information would
definitely become useful again.
 
 
 
 
Freed & Borenstein Standards Track [Page 3]
RFC 2045 Internet Message Bodies November 1996
 
 
This document describes several mechanisms that combine to solve most
of these problems without introducing any serious incompatibilities
with the existing world of RFC 822 mail. In particular, it
describes:
 
(1) A MIME-Version header field, which uses a version
number to declare a message to be conformant with MIME
and allows mail processing agents to distinguish
between such messages and those generated by older or
non-conformant software, which are presumed to lack
such a field.
 
(2) A Content-Type header field, generalized from RFC 1049,
which can be used to specify the media type and subtype
of data in the body of a message and to fully specify
the native representation (canonical form) of such
data.
 
(3) A Content-Transfer-Encoding header field, which can be
used to specify both the encoding transformation that
was applied to the body and the domain of the result.
Encoding transformations other than the identity
transformation are usually applied to data in order to
allow it to pass through mail transport mechanisms
which may have data or character set limitations.
 
(4) Two additional header fields that can be used to
further describe the data in a body, the Content-ID and
Content-Description header fields.
 
All of the header fields defined in this document are subject to the
general syntactic rules for header fields specified in RFC 822. In
particular, all of these header fields except for Content-Disposition
can include RFC 822 comments, which have no semantic content and
should be ignored during MIME processing.
 
Finally, to specify and promote interoperability, RFC 2049 provides a
basic applicability statement for a subset of the above mechanisms
that defines a minimal level of "conformance" with this document.
 
HISTORICAL NOTE: Several of the mechanisms described in this set of
documents may seem somewhat strange or even baroque at first reading.
It is important to note that compatibility with existing standards
AND robustness across existing practice were two of the highest
priorities of the working group that developed this set of documents.
In particular, compatibility was always favored over elegance.
 
 
 
 
 
Freed & Borenstein Standards Track [Page 4]
RFC 2045 Internet Message Bodies November 1996
 
 
Please refer to the current edition of the "Internet Official
Protocol Standards" for the standardization state and status of this
protocol. RFC 822 and STD 3, RFC 1123 also provide essential
background for MIME since no conforming implementation of MIME can
violate them. In addition, several other informational RFC documents
will be of interest to the MIME implementor, in particular RFC 1344,
RFC 1345, and RFC 1524.
 
2. Definitions, Conventions, and Generic BNF Grammar
 
Although the mechanisms specified in this set of documents are all
described in prose, most are also described formally in the augmented
BNF notation of RFC 822. Implementors will need to be familiar with
this notation in order to understand this set of documents, and are
referred to RFC 822 for a complete explanation of the augmented BNF
notation.
 
Some of the augmented BNF in this set of documents makes named
references to syntax rules defined in RFC 822. A complete formal
grammar, then, is obtained by combining the collected grammar
appendices in each document in this set with the BNF of RFC 822 plus
the modifications to RFC 822 defined in RFC 1123 (which specifically
changes the syntax for `return', `date' and `mailbox').
 
All numeric and octet values are given in decimal notation in this
set of documents. All media type values, subtype values, and
parameter names as defined are case-insensitive. However, parameter
values are case-sensitive unless otherwise specified for the specific
parameter.
 
FORMATTING NOTE: Notes, such at this one, provide additional
nonessential information which may be skipped by the reader without
missing anything essential. The primary purpose of these non-
essential notes is to convey information about the rationale of this
set of documents, or to place these documents in the proper
historical or evolutionary context. Such information may in
particular be skipped by those who are focused entirely on building a
conformant implementation, but may be of use to those who wish to
understand why certain design choices were made.
 
2.1. CRLF
 
The term CRLF, in this set of documents, refers to the sequence of
octets corresponding to the two US-ASCII characters CR (decimal value
13) and LF (decimal value 10) which, taken together, in this order,
denote a line break in RFC 822 mail.
 
 
 
 
 
Freed & Borenstein Standards Track [Page 5]
RFC 2045 Internet Message Bodies November 1996
 
 
2.2. Character Set
 
The term "character set" is used in MIME to refer to a method of
converting a sequence of octets into a sequence of characters. Note
that unconditional and unambiguous conversion in the other direction
is not required, in that not all characters may be representable by a
given character set and a character set may provide more than one
sequence of octets to represent a particular sequence of characters.
 
This definition is intended to allow various kinds of character
encodings, from simple single-table mappings such as US-ASCII to
complex table switching methods such as those that use ISO 2022's
techniques, to be used as character sets. However, the definition
associated with a MIME character set name must fully specify the
mapping to be performed. In particular, use of external profiling
information to determine the exact mapping is not permitted.
 
NOTE: The term "character set" was originally to describe such
straightforward schemes as US-ASCII and ISO-8859-1 which have a
simple one-to-one mapping from single octets to single characters.
Multi-octet coded character sets and switching techniques make the
situation more complex. For example, some communities use the term
"character encoding" for what MIME calls a "character set", while
using the phrase "coded character set" to denote an abstract mapping
from integers (not octets) to characters.
 
2.3. Message
 
The term "message", when not further qualified, means either a
(complete or "top-level") RFC 822 message being transferred on a
network, or a message encapsulated in a body of type "message/rfc822"
or "message/partial".
 
2.4. Entity
 
The term "entity", refers specifically to the MIME-defined header
fields and contents of either a message or one of the parts in the
body of a multipart entity. The specification of such entities is
the essence of MIME. Since the contents of an entity are often
called the "body", it makes sense to speak about the body of an
entity. Any sort of field may be present in the header of an entity,
but only those fields whose names begin with "content-" actually have
any MIME-related meaning. Note that this does NOT imply thay they
have no meaning at all -- an entity that is also a message has non-
MIME header fields whose meanings are defined by RFC 822.
 
 
 
 
 
 
Freed & Borenstein Standards Track [Page 6]
RFC 2045 Internet Message Bodies November 1996
 
 
2.5. Body Part
 
The term "body part" refers to an entity inside of a multipart
entity.
 
2.6. Body
 
The term "body", when not further qualified, means the body of an
entity, that is, the body of either a message or of a body part.
 
NOTE: The previous four definitions are clearly circular. This is
unavoidable, since the overall structure of a MIME message is indeed
recursive.
 
2.7. 7bit Data
 
"7bit data" refers to data that is all represented as relatively
short lines with 998 octets or less between CRLF line separation
sequences [RFC-821]. No octets with decimal values greater than 127
are allowed and neither are NULs (octets with decimal value 0). CR
(decimal value 13) and LF (decimal value 10) octets only occur as
part of CRLF line separation sequences.
 
2.8. 8bit Data
 
"8bit data" refers to data that is all represented as relatively
short lines with 998 octets or less between CRLF line separation
sequences [RFC-821]), but octets with decimal values greater than 127
may be used. As with "7bit data" CR and LF octets only occur as part
of CRLF line separation sequences and no NULs are allowed.
 
2.9. Binary Data
 
"Binary data" refers to data where any sequence of octets whatsoever
is allowed.
 
2.10. Lines
 
"Lines" are defined as sequences of octets separated by a CRLF
sequences. This is consistent with both RFC 821 and RFC 822.
"Lines" only refers to a unit of data in a message, which may or may
not correspond to something that is actually displayed by a user
agent.
 
 
 
 
 
 
 
 
Freed & Borenstein Standards Track [Page 7]
RFC 2045 Internet Message Bodies November 1996
 
 
3. MIME Header Fields
 
MIME defines a number of new RFC 822 header fields that are used to
describe the content of a MIME entity. These header fields occur in
at least two contexts:
 
(1) As part of a regular RFC 822 message header.
 
(2) In a MIME body part header within a multipart
construct.
 
The formal definition of these header fields is as follows:
 
entity-headers := [ content CRLF ]
[ encoding CRLF ]
[ id CRLF ]
[ description CRLF ]
*( MIME-extension-field CRLF )
 
MIME-message-headers := entity-headers
fields
version CRLF
; The ordering of the header
; fields implied by this BNF
; definition should be ignored.
 
MIME-part-headers := entity-headers
[ fields ]
; Any field not beginning with
; "content-" can have no defined
; meaning and may be ignored.
; The ordering of the header
; fields implied by this BNF
; definition should be ignored.
 
The syntax of the various specific MIME header fields will be
described in the following sections.
 
4. MIME-Version Header Field
 
Since RFC 822 was published in 1982, there has really been only one
format standard for Internet messages, and there has been little
perceived need to declare the format standard in use. This document
is an independent specification that complements RFC 822. Although
the extensions in this document have been defined in such a way as to
be compatible with RFC 822, there are still circumstances in which it
might be desirable for a mail-processing agent to know whether a
message was composed with the new standard in mind.
 
 
 
Freed & Borenstein Standards Track [Page 8]
RFC 2045 Internet Message Bodies November 1996
 
 
Therefore, this document defines a new header field, "MIME-Version",
which is to be used to declare the version of the Internet message
body format standard in use.
 
Messages composed in accordance with this document MUST include such
a header field, with the following verbatim text:
 
MIME-Version: 1.0
 
The presence of this header field is an assertion that the message
has been composed in compliance with this document.
 
Since it is possible that a future document might extend the message
format standard again, a formal BNF is given for the content of the
MIME-Version field:
 
version := "MIME-Version" ":" 1*DIGIT "." 1*DIGIT
 
Thus, future format specifiers, which might replace or extend "1.0",
are constrained to be two integer fields, separated by a period. If
a message is received with a MIME-version value other than "1.0", it
cannot be assumed to conform with this document.
 
Note that the MIME-Version header field is required at the top level
of a message. It is not required for each body part of a multipart
entity. It is required for the embedded headers of a body of type
"message/rfc822" or "message/partial" if and only if the embedded
message is itself claimed to be MIME-conformant.
 
It is not possible to fully specify how a mail reader that conforms
with MIME as defined in this document should treat a message that
might arrive in the future with some value of MIME-Version other than
"1.0".
 
It is also worth noting that version control for specific media types
is not accomplished using the MIME-Version mechanism. In particular,
some formats (such as application/postscript) have version numbering
conventions that are internal to the media format. Where such
conventions exist, MIME does nothing to supersede them. Where no
such conventions exist, a MIME media type might use a "version"
parameter in the content-type field if necessary.
 
 
 
 
 
 
 
 
 
 
Freed & Borenstein Standards Track [Page 9]
RFC 2045 Internet Message Bodies November 1996
 
 
NOTE TO IMPLEMENTORS: When checking MIME-Version values any RFC 822
comment strings that are present must be ignored. In particular, the
following four MIME-Version fields are equivalent:
 
MIME-Version: 1.0
 
MIME-Version: 1.0 (produced by MetaSend Vx.x)
 
MIME-Version: (produced by MetaSend Vx.x) 1.0
 
MIME-Version: 1.(produced by MetaSend Vx.x)0
 
In the absence of a MIME-Version field, a receiving mail user agent
(whether conforming to MIME requirements or not) may optionally
choose to interpret the body of the message according to local
conventions. Many such conventions are currently in use and it
should be noted that in practice non-MIME messages can contain just
about anything.
 
It is impossible to be certain that a non-MIME mail message is
actually plain text in the US-ASCII character set since it might well
be a message that, using some set of nonstandard local conventions
that predate MIME, includes text in another character set or non-
textual data presented in a manner that cannot be automatically
recognized (e.g., a uuencoded compressed UNIX tar file).
 
5. Content-Type Header Field
 
The purpose of the Content-Type field is to describe the data
contained in the body fully enough that the receiving user agent can
pick an appropriate agent or mechanism to present the data to the
user, or otherwise deal with the data in an appropriate manner. The
value in this field is called a media type.
 
HISTORICAL NOTE: The Content-Type header field was first defined in
RFC 1049. RFC 1049 used a simpler and less powerful syntax, but one
that is largely compatible with the mechanism given here.
 
The Content-Type header field specifies the nature of the data in the
body of an entity by giving media type and subtype identifiers, and
by providing auxiliary information that may be required for certain
media types. After the media type and subtype names, the remainder
of the header field is simply a set of parameters, specified in an
attribute=value notation. The ordering of parameters is not
significant.
 
 
 
 
 
 
Freed & Borenstein Standards Track [Page 10]
RFC 2045 Internet Message Bodies November 1996
 
 
In general, the top-level media type is used to declare the general
type of data, while the subtype specifies a specific format for that
type of data. Thus, a media type of "image/xyz" is enough to tell a
user agent that the data is an image, even if the user agent has no
knowledge of the specific image format "xyz". Such information can
be used, for example, to decide whether or not to show a user the raw
data from an unrecognized subtype -- such an action might be
reasonable for unrecognized subtypes of text, but not for
unrecognized subtypes of image or audio. For this reason, registered
subtypes of text, image, audio, and video should not contain embedded
information that is really of a different type. Such compound
formats should be represented using the "multipart" or "application"
types.
 
Parameters are modifiers of the media subtype, and as such do not
fundamentally affect the nature of the content. The set of
meaningful parameters depends on the media type and subtype. Most
parameters are associated with a single specific subtype. However, a
given top-level media type may define parameters which are applicable
to any subtype of that type. Parameters may be required by their
defining content type or subtype or they may be optional. MIME
implementations must ignore any parameters whose names they do not
recognize.
 
For example, the "charset" parameter is applicable to any subtype of
"text", while the "boundary" parameter is required for any subtype of
the "multipart" media type.
 
There are NO globally-meaningful parameters that apply to all media
types. Truly global mechanisms are best addressed, in the MIME
model, by the definition of additional Content-* header fields.
 
An initial set of seven top-level media types is defined in RFC 2046.
Five of these are discrete types whose content is essentially opaque
as far as MIME processing is concerned. The remaining two are
composite types whose contents require additional handling by MIME
processors.
 
This set of top-level media types is intended to be substantially
complete. It is expected that additions to the larger set of
supported types can generally be accomplished by the creation of new
subtypes of these initial types. In the future, more top-level types
may be defined only by a standards-track extension to this standard.
If another top-level type is to be used for any reason, it must be
given a name starting with "X-" to indicate its non-standard status
and to avoid a potential conflict with a future official name.
 
 
 
 
 
Freed & Borenstein Standards Track [Page 11]
RFC 2045 Internet Message Bodies November 1996
 
 
5.1. Syntax of the Content-Type Header Field
 
In the Augmented BNF notation of RFC 822, a Content-Type header field
value is defined as follows:
 
content := "Content-Type" ":" type "/" subtype
*(";" parameter)
; Matching of media type and subtype
; is ALWAYS case-insensitive.
 
type := discrete-type / composite-type
 
discrete-type := "text" / "image" / "audio" / "video" /
"application" / extension-token
 
composite-type := "message" / "multipart" / extension-token
 
extension-token := ietf-token / x-token
 
ietf-token := <An extension token defined by a
standards-track RFC and registered
with IANA.>
 
x-token := <The two characters "X-" or "x-" followed, with
no intervening white space, by any token>
 
subtype := extension-token / iana-token
 
iana-token := <A publicly-defined extension token. Tokens
of this form must be registered with IANA
as specified in RFC 2048.>
 
parameter := attribute "=" value
 
attribute := token
; Matching of attributes
; is ALWAYS case-insensitive.
 
value := token / quoted-string
 
token := 1*<any (US-ASCII) CHAR except SPACE, CTLs,
or tspecials>
 
tspecials := "(" / ")" / "<" / ">" / "@" /
"," / ";" / ":" / "\" / <">
"/" / "[" / "]" / "?" / "="
; Must be in quoted-string,
; to use within parameter values
 
 
 
Freed & Borenstein Standards Track [Page 12]
RFC 2045 Internet Message Bodies November 1996
 
 
Note that the definition of "tspecials" is the same as the RFC 822
definition of "specials" with the addition of the three characters
"/", "?", and "=", and the removal of ".".
 
Note also that a subtype specification is MANDATORY -- it may not be
omitted from a Content-Type header field. As such, there are no
default subtypes.
 
The type, subtype, and parameter names are not case sensitive. For
example, TEXT, Text, and TeXt are all equivalent top-level media
types. Parameter values are normally case sensitive, but sometimes
are interpreted in a case-insensitive fashion, depending on the
intended use. (For example, multipart boundaries are case-sensitive,
but the "access-type" parameter for message/External-body is not
case-sensitive.)
 
Note that the value of a quoted string parameter does not include the
quotes. That is, the quotation marks in a quoted-string are not a
part of the value of the parameter, but are merely used to delimit
that parameter value. In addition, comments are allowed in
accordance with RFC 822 rules for structured header fields. Thus the
following two forms
 
Content-type: text/plain; charset=us-ascii (Plain text)
 
Content-type: text/plain; charset="us-ascii"
 
are completely equivalent.
 
Beyond this syntax, the only syntactic constraint on the definition
of subtype names is the desire that their uses must not conflict.
That is, it would be undesirable to have two different communities
using "Content-Type: application/foobar" to mean two different
things. The process of defining new media subtypes, then, is not
intended to be a mechanism for imposing restrictions, but simply a
mechanism for publicizing their definition and usage. There are,
therefore, two acceptable mechanisms for defining new media subtypes:
 
(1) Private values (starting with "X-") may be defined
bilaterally between two cooperating agents without
outside registration or standardization. Such values
cannot be registered or standardized.
 
(2) New standard values should be registered with IANA as
described in RFC 2048.
 
The second document in this set, RFC 2046, defines the initial set of
media types for MIME.
 
 
 
Freed & Borenstein Standards Track [Page 13]
RFC 2045 Internet Message Bodies November 1996
 
 
5.2. Content-Type Defaults
 
Default RFC 822 messages without a MIME Content-Type header are taken
by this protocol to be plain text in the US-ASCII character set,
which can be explicitly specified as:
 
Content-type: text/plain; charset=us-ascii
 
This default is assumed if no Content-Type header field is specified.
It is also recommend that this default be assumed when a
syntactically invalid Content-Type header field is encountered. In
the presence of a MIME-Version header field and the absence of any
Content-Type header field, a receiving User Agent can also assume
that plain US-ASCII text was the sender's intent. Plain US-ASCII
text may still be assumed in the absence of a MIME-Version or the
presence of an syntactically invalid Content-Type header field, but
the sender's intent might have been otherwise.
 
6. Content-Transfer-Encoding Header Field
 
Many media types which could be usefully transported via email are
represented, in their "natural" format, as 8bit character or binary
data. Such data cannot be transmitted over some transfer protocols.
For example, RFC 821 (SMTP) restricts mail messages to 7bit US-ASCII
data with lines no longer than 1000 characters including any trailing
CRLF line separator.
 
It is necessary, therefore, to define a standard mechanism for
encoding such data into a 7bit short line format. Proper labelling
of unencoded material in less restrictive formats for direct use over
less restrictive transports is also desireable. This document
specifies that such encodings will be indicated by a new "Content-
Transfer-Encoding" header field. This field has not been defined by
any previous standard.
 
6.1. Content-Transfer-Encoding Syntax
 
The Content-Transfer-Encoding field's value is a single token
specifying the type of encoding, as enumerated below. Formally:
 
encoding := "Content-Transfer-Encoding" ":" mechanism
 
mechanism := "7bit" / "8bit" / "binary" /
"quoted-printable" / "base64" /
ietf-token / x-token
 
These values are not case sensitive -- Base64 and BASE64 and bAsE64
are all equivalent. An encoding type of 7BIT requires that the body
 
 
 
Freed & Borenstein Standards Track [Page 14]
RFC 2045 Internet Message Bodies November 1996
 
 
is already in a 7bit mail-ready representation. This is the default
value -- that is, "Content-Transfer-Encoding: 7BIT" is assumed if the
Content-Transfer-Encoding header field is not present.
 
6.2. Content-Transfer-Encodings Semantics
 
This single Content-Transfer-Encoding token actually provides two
pieces of information. It specifies what sort of encoding
transformation the body was subjected to and hence what decoding
operation must be used to restore it to its original form, and it
specifies what the domain of the result is.
 
The transformation part of any Content-Transfer-Encodings specifies,
either explicitly or implicitly, a single, well-defined decoding
algorithm, which for any sequence of encoded octets either transforms
it to the original sequence of octets which was encoded, or shows
that it is illegal as an encoded sequence. Content-Transfer-
Encodings transformations never depend on any additional external
profile information for proper operation. Note that while decoders
must produce a single, well-defined output for a valid encoding no
such restrictions exist for encoders: Encoding a given sequence of
octets to different, equivalent encoded sequences is perfectly legal.
 
Three transformations are currently defined: identity, the "quoted-
printable" encoding, and the "base64" encoding. The domains are
"binary", "8bit" and "7bit".
 
The Content-Transfer-Encoding values "7bit", "8bit", and "binary" all
mean that the identity (i.e. NO) encoding transformation has been
performed. As such, they serve simply as indicators of the domain of
the body data, and provide useful information about the sort of
encoding that might be needed for transmission in a given transport
system. The terms "7bit data", "8bit data", and "binary data" are
all defined in Section 2.
 
The quoted-printable and base64 encodings transform their input from
an arbitrary domain into material in the "7bit" range, thus making it
safe to carry over restricted transports. The specific definition of
the transformations are given below.
 
The proper Content-Transfer-Encoding label must always be used.
Labelling unencoded data containing 8bit characters as "7bit" is not
allowed, nor is labelling unencoded non-line-oriented data as
anything other than "binary" allowed.
 
Unlike media subtypes, a proliferation of Content-Transfer-Encoding
values is both undesirable and unnecessary. However, establishing
only a single transformation into the "7bit" domain does not seem
 
 
 
Freed & Borenstein Standards Track [Page 15]
RFC 2045 Internet Message Bodies November 1996
 
 
possible. There is a tradeoff between the desire for a compact and
efficient encoding of largely- binary data and the desire for a
somewhat readable encoding of data that is mostly, but not entirely,
7bit. For this reason, at least two encoding mechanisms are
necessary: a more or less readable encoding (quoted-printable) and a
"dense" or "uniform" encoding (base64).
 
Mail transport for unencoded 8bit data is defined in RFC 1652. As of
the initial publication of this document, there are no standardized
Internet mail transports for which it is legitimate to include
unencoded binary data in mail bodies. Thus there are no
circumstances in which the "binary" Content-Transfer-Encoding is
actually valid in Internet mail. However, in the event that binary
mail transport becomes a reality in Internet mail, or when MIME is
used in conjunction with any other binary-capable mail transport
mechanism, binary bodies must be labelled as such using this
mechanism.
 
NOTE: The five values defined for the Content-Transfer-Encoding field
imply nothing about the media type other than the algorithm by which
it was encoded or the transport system requirements if unencoded.
 
6.3. New Content-Transfer-Encodings
 
Implementors may, if necessary, define private Content-Transfer-
Encoding values, but must use an x-token, which is a name prefixed by
"X-", to indicate its non-standard status, e.g., "Content-Transfer-
Encoding: x-my-new-encoding". Additional standardized Content-
Transfer-Encoding values must be specified by a standards-track RFC.
The requirements such specifications must meet are given in RFC 2048.
As such, all content-transfer-encoding namespace except that
beginning with "X-" is explicitly reserved to the IETF for future
use.
 
Unlike media types and subtypes, the creation of new Content-
Transfer-Encoding values is STRONGLY discouraged, as it seems likely
to hinder interoperability with little potential benefit
 
6.4. Interpretation and Use
 
If a Content-Transfer-Encoding header field appears as part of a
message header, it applies to the entire body of that message. If a
Content-Transfer-Encoding header field appears as part of an entity's
headers, it applies only to the body of that entity. If an entity is
of type "multipart" the Content-Transfer-Encoding is not permitted to
have any value other than "7bit", "8bit" or "binary". Even more
severe restrictions apply to some subtypes of the "message" type.
 
 
 
 
Freed & Borenstein Standards Track [Page 16]
RFC 2045 Internet Message Bodies November 1996
 
 
It should be noted that most media types are defined in terms of
octets rather than bits, so that the mechanisms described here are
mechanisms for encoding arbitrary octet streams, not bit streams. If
a bit stream is to be encoded via one of these mechanisms, it must
first be converted to an 8bit byte stream using the network standard
bit order ("big-endian"), in which the earlier bits in a stream
become the higher-order bits in a 8bit byte. A bit stream not ending
at an 8bit boundary must be padded with zeroes. RFC 2046 provides a
mechanism for noting the addition of such padding in the case of the
application/octet-stream media type, which has a "padding" parameter.
 
The encoding mechanisms defined here explicitly encode all data in
US-ASCII. Thus, for example, suppose an entity has header fields
such as:
 
Content-Type: text/plain; charset=ISO-8859-1
Content-transfer-encoding: base64
 
This must be interpreted to mean that the body is a base64 US-ASCII
encoding of data that was originally in ISO-8859-1, and will be in
that character set again after decoding.
 
Certain Content-Transfer-Encoding values may only be used on certain
media types. In particular, it is EXPRESSLY FORBIDDEN to use any
encodings other than "7bit", "8bit", or "binary" with any composite
media type, i.e. one that recursively includes other Content-Type
fields. Currently the only composite media types are "multipart" and
"message". All encodings that are desired for bodies of type
multipart or message must be done at the innermost level, by encoding
the actual body that needs to be encoded.
 
It should also be noted that, by definition, if a composite entity
has a transfer-encoding value such as "7bit", but one of the enclosed
entities has a less restrictive value such as "8bit", then either the
outer "7bit" labelling is in error, because 8bit data are included,
or the inner "8bit" labelling placed an unnecessarily high demand on
the transport system because the actual included data were actually
7bit-safe.
 
NOTE ON ENCODING RESTRICTIONS: Though the prohibition against using
content-transfer-encodings on composite body data may seem overly
restrictive, it is necessary to prevent nested encodings, in which
data are passed through an encoding algorithm multiple times, and
must be decoded multiple times in order to be properly viewed.
Nested encodings add considerable complexity to user agents: Aside
from the obvious efficiency problems with such multiple encodings,
they can obscure the basic structure of a message. In particular,
they can imply that several decoding operations are necessary simply
 
 
 
Freed & Borenstein Standards Track [Page 17]
RFC 2045 Internet Message Bodies November 1996
 
 
to find out what types of bodies a message contains. Banning nested
encodings may complicate the job of certain mail gateways, but this
seems less of a problem than the effect of nested encodings on user
agents.
 
Any entity with an unrecognized Content-Transfer-Encoding must be
treated as if it has a Content-Type of "application/octet-stream",
regardless of what the Content-Type header field actually says.
 
NOTE ON THE RELATIONSHIP BETWEEN CONTENT-TYPE AND CONTENT-TRANSFER-
ENCODING: It may seem that the Content-Transfer-Encoding could be
inferred from the characteristics of the media that is to be encoded,
or, at the very least, that certain Content-Transfer-Encodings could
be mandated for use with specific media types. There are several
reasons why this is not the case. First, given the varying types of
transports used for mail, some encodings may be appropriate for some
combinations of media types and transports but not for others. (For
example, in an 8bit transport, no encoding would be required for text
in certain character sets, while such encodings are clearly required
for 7bit SMTP.)
 
Second, certain media types may require different types of transfer
encoding under different circumstances. For example, many PostScript
bodies might consist entirely of short lines of 7bit data and hence
require no encoding at all. Other PostScript bodies (especially
those using Level 2 PostScript's binary encoding mechanism) may only
be reasonably represented using a binary transport encoding.
Finally, since the Content-Type field is intended to be an open-ended
specification mechanism, strict specification of an association
between media types and encodings effectively couples the
specification of an application protocol with a specific lower-level
transport. This is not desirable since the developers of a media
type should not have to be aware of all the transports in use and
what their limitations are.
 
6.5. Translating Encodings
 
The quoted-printable and base64 encodings are designed so that
conversion between them is possible. The only issue that arises in
such a conversion is the handling of hard line breaks in quoted-
printable encoding output. When converting from quoted-printable to
base64 a hard line break in the quoted-printable form represents a
CRLF sequence in the canonical form of the data. It must therefore be
converted to a corresponding encoded CRLF in the base64 form of the
data. Similarly, a CRLF sequence in the canonical form of the data
obtained after base64 decoding must be converted to a quoted-
printable hard line break, but ONLY when converting text data.
 
 
 
 
Freed & Borenstein Standards Track [Page 18]
RFC 2045 Internet Message Bodies November 1996
 
 
6.6. Canonical Encoding Model
 
There was some confusion, in the previous versions of this RFC,
regarding the model for when email data was to be converted to
canonical form and encoded, and in particular how this process would
affect the treatment of CRLFs, given that the representation of
newlines varies greatly from system to system, and the relationship
between content-transfer-encodings and character sets. A canonical
model for encoding is presented in RFC 2049 for this reason.
 
6.7. Quoted-Printable Content-Transfer-Encoding
 
The Quoted-Printable encoding is intended to represent data that
largely consists of octets that correspond to printable characters in
the US-ASCII character set. It encodes the data in such a way that
the resulting octets are unlikely to be modified by mail transport.
If the data being encoded are mostly US-ASCII text, the encoded form
of the data remains largely recognizable by humans. A body which is
entirely US-ASCII may also be encoded in Quoted-Printable to ensure
the integrity of the data should the message pass through a
character-translating, and/or line-wrapping gateway.
 
In this encoding, octets are to be represented as determined by the
following rules:
 
(1) (General 8bit representation) Any octet, except a CR or
LF that is part of a CRLF line break of the canonical
(standard) form of the data being encoded, may be
represented by an "=" followed by a two digit
hexadecimal representation of the octet's value. The
digits of the hexadecimal alphabet, for this purpose,
are "0123456789ABCDEF". Uppercase letters must be
used; lowercase letters are not allowed. Thus, for
example, the decimal value 12 (US-ASCII form feed) can
be represented by "=0C", and the decimal value 61 (US-
ASCII EQUAL SIGN) can be represented by "=3D". This
rule must be followed except when the following rules
allow an alternative encoding.
 
(2) (Literal representation) Octets with decimal values of
33 through 60 inclusive, and 62 through 126, inclusive,
MAY be represented as the US-ASCII characters which
correspond to those octets (EXCLAMATION POINT through
LESS THAN, and GREATER THAN through TILDE,
respectively).
 
(3) (White Space) Octets with values of 9 and 32 MAY be
represented as US-ASCII TAB (HT) and SPACE characters,
 
 
 
Freed & Borenstein Standards Track [Page 19]
RFC 2045 Internet Message Bodies November 1996
 
 
respectively, but MUST NOT be so represented at the end
of an encoded line. Any TAB (HT) or SPACE characters
on an encoded line MUST thus be followed on that line
by a printable character. In particular, an "=" at the
end of an encoded line, indicating a soft line break
(see rule #5) may follow one or more TAB (HT) or SPACE
characters. It follows that an octet with decimal
value 9 or 32 appearing at the end of an encoded line
must be represented according to Rule #1. This rule is
necessary because some MTAs (Message Transport Agents,
programs which transport messages from one user to
another, or perform a portion of such transfers) are
known to pad lines of text with SPACEs, and others are
known to remove "white space" characters from the end
of a line. Therefore, when decoding a Quoted-Printable
body, any trailing white space on a line must be
deleted, as it will necessarily have been added by
intermediate transport agents.
 
(4) (Line Breaks) A line break in a text body, represented
as a CRLF sequence in the text canonical form, must be
represented by a (RFC 822) line break, which is also a
CRLF sequence, in the Quoted-Printable encoding. Since
the canonical representation of media types other than
text do not generally include the representation of
line breaks as CRLF sequences, no hard line breaks
(i.e. line breaks that are intended to be meaningful
and to be displayed to the user) can occur in the
quoted-printable encoding of such types. Sequences
like "=0D", "=0A", "=0A=0D" and "=0D=0A" will routinely
appear in non-text data represented in quoted-
printable, of course.
 
Note that many implementations may elect to encode the
local representation of various content types directly
rather than converting to canonical form first,
encoding, and then converting back to local
representation. In particular, this may apply to plain
text material on systems that use newline conventions
other than a CRLF terminator sequence. Such an
implementation optimization is permissible, but only
when the combined canonicalization-encoding step is
equivalent to performing the three steps separately.
 
(5) (Soft Line Breaks) The Quoted-Printable encoding
REQUIRES that encoded lines be no more than 76
characters long. If longer lines are to be encoded
with the Quoted-Printable encoding, "soft" line breaks
 
 
 
Freed & Borenstein Standards Track [Page 20]
RFC 2045 Internet Message Bodies November 1996
 
 
must be used. An equal sign as the last character on a
encoded line indicates such a non-significant ("soft")
line break in the encoded text.
 
Thus if the "raw" form of the line is a single unencoded line that
says:
 
Now's the time for all folk to come to the aid of their country.
 
This can be represented, in the Quoted-Printable encoding, as:
 
Now's the time =
for all folk to come=
to the aid of their country.
 
This provides a mechanism with which long lines are encoded in such a
way as to be restored by the user agent. The 76 character limit does
not count the trailing CRLF, but counts all other characters,
including any equal signs.
 
Since the hyphen character ("-") may be represented as itself in the
Quoted-Printable encoding, care must be taken, when encapsulating a
quoted-printable encoded body inside one or more multipart entities,
to ensure that the boundary delimiter does not appear anywhere in the
encoded body. (A good strategy is to choose a boundary that includes
a character sequence such as "=_" which can never appear in a
quoted-printable body. See the definition of multipart messages in
RFC 2046.)
 
NOTE: The quoted-printable encoding represents something of a
compromise between readability and reliability in transport. Bodies
encoded with the quoted-printable encoding will work reliably over
most mail gateways, but may not work perfectly over a few gateways,
notably those involving translation into EBCDIC. A higher level of
confidence is offered by the base64 Content-Transfer-Encoding. A way
to get reasonably reliable transport through EBCDIC gateways is to
also quote the US-ASCII characters
 
!"#$@[\]^`{|}~
 
according to rule #1.
 
Because quoted-printable data is generally assumed to be line-
oriented, it is to be expected that the representation of the breaks
between the lines of quoted-printable data may be altered in
transport, in the same manner that plain text mail has always been
altered in Internet mail when passing between systems with differing
newline conventions. If such alterations are likely to constitute a
 
 
 
Freed & Borenstein Standards Track [Page 21]
RFC 2045 Internet Message Bodies November 1996
 
 
corruption of the data, it is probably more sensible to use the
base64 encoding rather than the quoted-printable encoding.
 
NOTE: Several kinds of substrings cannot be generated according to
the encoding rules for the quoted-printable content-transfer-
encoding, and hence are formally illegal if they appear in the output
of a quoted-printable encoder. This note enumerates these cases and
suggests ways to handle such illegal substrings if any are
encountered in quoted-printable data that is to be decoded.
 
(1) An "=" followed by two hexadecimal digits, one or both
of which are lowercase letters in "abcdef", is formally
illegal. A robust implementation might choose to
recognize them as the corresponding uppercase letters.
 
(2) An "=" followed by a character that is neither a
hexadecimal digit (including "abcdef") nor the CR
character of a CRLF pair is illegal. This case can be
the result of US-ASCII text having been included in a
quoted-printable part of a message without itself
having been subjected to quoted-printable encoding. A
reasonable approach by a robust implementation might be
to include the "=" character and the following
character in the decoded data without any
transformation and, if possible, indicate to the user
that proper decoding was not possible at this point in
the data.
 
(3) An "=" cannot be the ultimate or penultimate character
in an encoded object. This could be handled as in case
(2) above.
 
(4) Control characters other than TAB, or CR and LF as
parts of CRLF pairs, must not appear. The same is true
for octets with decimal values greater than 126. If
found in incoming quoted-printable data by a decoder, a
robust implementation might exclude them from the
decoded data and warn the user that illegal characters
were discovered.
 
(5) Encoded lines must not be longer than 76 characters,
not counting the trailing CRLF. If longer lines are
found in incoming, encoded data, a robust
implementation might nevertheless decode the lines, and
might report the erroneous encoding to the user.
 
 
 
 
 
 
Freed & Borenstein Standards Track [Page 22]
RFC 2045 Internet Message Bodies November 1996
 
 
WARNING TO IMPLEMENTORS: If binary data is encoded in quoted-
printable, care must be taken to encode CR and LF characters as "=0D"
and "=0A", respectively. In particular, a CRLF sequence in binary
data should be encoded as "=0D=0A". Otherwise, if CRLF were
represented as a hard line break, it might be incorrectly decoded on
platforms with different line break conventions.
 
For formalists, the syntax of quoted-printable data is described by
the following grammar:
 
quoted-printable := qp-line *(CRLF qp-line)
 
qp-line := *(qp-segment transport-padding CRLF)
qp-part transport-padding
 
qp-part := qp-section
; Maximum length of 76 characters
 
qp-segment := qp-section *(SPACE / TAB) "="
; Maximum length of 76 characters
 
qp-section := [*(ptext / SPACE / TAB) ptext]
 
ptext := hex-octet / safe-char
 
safe-char := <any octet with decimal value of 33 through
60 inclusive, and 62 through 126>
; Characters not listed as "mail-safe" in
; RFC 2049 are also not recommended.
 
hex-octet := "=" 2(DIGIT / "A" / "B" / "C" / "D" / "E" / "F")
; Octet must be used for characters > 127, =,
; SPACEs or TABs at the ends of lines, and is
; recommended for any character not listed in
; RFC 2049 as "mail-safe".
 
transport-padding := *LWSP-char
; Composers MUST NOT generate
; non-zero length transport
; padding, but receivers MUST
; be able to handle padding
; added by message transports.
 
IMPORTANT: The addition of LWSP between the elements shown in this
BNF is NOT allowed since this BNF does not specify a structured
header field.
 
 
 
 
 
Freed & Borenstein Standards Track [Page 23]
RFC 2045 Internet Message Bodies November 1996
 
 
6.8. Base64 Content-Transfer-Encoding
 
The Base64 Content-Transfer-Encoding is designed to represent
arbitrary sequences of octets in a form that need not be humanly
readable. The encoding and decoding algorithms are simple, but the
encoded data are consistently only about 33 percent larger than the
unencoded data. This encoding is virtually identical to the one used
in Privacy Enhanced Mail (PEM) applications, as defined in RFC 1421.
 
A 65-character subset of US-ASCII is used, enabling 6 bits to be
represented per printable character. (The extra 65th character, "=",
is used to signify a special processing function.)
 
NOTE: This subset has the important property that it is represented
identically in all versions of ISO 646, including US-ASCII, and all
characters in the subset are also represented identically in all
versions of EBCDIC. Other popular encodings, such as the encoding
used by the uuencode utility, Macintosh binhex 4.0 [RFC-1741], and
the base85 encoding specified as part of Level 2 PostScript, do not
share these properties, and thus do not fulfill the portability
requirements a binary transport encoding for mail must meet.
 
The encoding process represents 24-bit groups of input bits as output
strings of 4 encoded characters. Proceeding from left to right, a
24-bit input group is formed by concatenating 3 8bit input groups.
These 24 bits are then treated as 4 concatenated 6-bit groups, each
of which is translated into a single digit in the base64 alphabet.
When encoding a bit stream via the base64 encoding, the bit stream
must be presumed to be ordered with the most-significant-bit first.
That is, the first bit in the stream will be the high-order bit in
the first 8bit byte, and the eighth bit will be the low-order bit in
the first 8bit byte, and so on.
 
Each 6-bit group is used as an index into an array of 64 printable
characters. The character referenced by the index is placed in the
output string. These characters, identified in Table 1, below, are
selected so as to be universally representable, and the set excludes
characters with particular significance to SMTP (e.g., ".", CR, LF)
and to the multipart boundary delimiters defined in RFC 2046 (e.g.,
"-").
 
 
 
 
 
 
 
 
 
 
 
Freed & Borenstein Standards Track [Page 24]
RFC 2045 Internet Message Bodies November 1996
 
 
Table 1: The Base64 Alphabet
 
Value Encoding Value Encoding Value Encoding Value Encoding
0 A 17 R 34 i 51 z
1 B 18 S 35 j 52 0
2 C 19 T 36 k 53 1
3 D 20 U 37 l 54 2
4 E 21 V 38 m 55 3
5 F 22 W 39 n 56 4
6 G 23 X 40 o 57 5
7 H 24 Y 41 p 58 6
8 I 25 Z 42 q 59 7
9 J 26 a 43 r 60 8
10 K 27 b 44 s 61 9
11 L 28 c 45 t 62 +
12 M 29 d 46 u 63 /
13 N 30 e 47 v
14 O 31 f 48 w (pad) =
15 P 32 g 49 x
16 Q 33 h 50 y
 
The encoded output stream must be represented in lines of no more
than 76 characters each. All line breaks or other characters not
found in Table 1 must be ignored by decoding software. In base64
data, characters other than those in Table 1, line breaks, and other
white space probably indicate a transmission error, about which a
warning message or even a message rejection might be appropriate
under some circumstances.
 
Special processing is performed if fewer than 24 bits are available
at the end of the data being encoded. A full encoding quantum is
always completed at the end of a body. When fewer than 24 input bits
are available in an input group, zero bits are added (on the right)
to form an integral number of 6-bit groups. Padding at the end of
the data is performed using the "=" character. Since all base64
input is an integral number of octets, only the following cases can
arise: (1) the final quantum of encoding input is an integral
multiple of 24 bits; here, the final unit of encoded output will be
an integral multiple of 4 characters with no "=" padding, (2) the
final quantum of encoding input is exactly 8 bits; here, the final
unit of encoded output will be two characters followed by two "="
padding characters, or (3) the final quantum of encoding input is
exactly 16 bits; here, the final unit of encoded output will be three
characters followed by one "=" padding character.
 
Because it is used only for padding at the end of the data, the
occurrence of any "=" characters may be taken as evidence that the
end of the data has been reached (without truncation in transit). No
 
 
 
Freed & Borenstein Standards Track [Page 25]
RFC 2045 Internet Message Bodies November 1996
 
 
such assurance is possible, however, when the number of octets
transmitted was a multiple of three and no "=" characters are
present.
 
Any characters outside of the base64 alphabet are to be ignored in
base64-encoded data.
 
Care must be taken to use the proper octets for line breaks if base64
encoding is applied directly to text material that has not been
converted to canonical form. In particular, text line breaks must be
converted into CRLF sequences prior to base64 encoding. The
important thing to note is that this may be done directly by the
encoder rather than in a prior canonicalization step in some
implementations.
 
NOTE: There is no need to worry about quoting potential boundary
delimiters within base64-encoded bodies within multipart entities
because no hyphen characters are used in the base64 encoding.
 
7. Content-ID Header Field
 
In constructing a high-level user agent, it may be desirable to allow
one body to make reference to another. Accordingly, bodies may be
labelled using the "Content-ID" header field, which is syntactically
identical to the "Message-ID" header field:
 
id := "Content-ID" ":" msg-id
 
Like the Message-ID values, Content-ID values must be generated to be
world-unique.
 
The Content-ID value may be used for uniquely identifying MIME
entities in several contexts, particularly for caching data
referenced by the message/external-body mechanism. Although the
Content-ID header is generally optional, its use is MANDATORY in
implementations which generate data of the optional MIME media type
"message/external-body". That is, each message/external-body entity
must have a Content-ID field to permit caching of such data.
 
It is also worth noting that the Content-ID value has special
semantics in the case of the multipart/alternative media type. This
is explained in the section of RFC 2046 dealing with
multipart/alternative.
 
 
 
 
 
 
 
 
Freed & Borenstein Standards Track [Page 26]
RFC 2045 Internet Message Bodies November 1996
 
 
8. Content-Description Header Field
 
The ability to associate some descriptive information with a given
body is often desirable. For example, it may be useful to mark an
"image" body as "a picture of the Space Shuttle Endeavor." Such text
may be placed in the Content-Description header field. This header
field is always optional.
 
description := "Content-Description" ":" *text
 
The description is presumed to be given in the US-ASCII character
set, although the mechanism specified in RFC 2047 may be used for
non-US-ASCII Content-Description values.
 
9. Additional MIME Header Fields
 
Future documents may elect to define additional MIME header fields
for various purposes. Any new header field that further describes
the content of a message should begin with the string "Content-" to
allow such fields which appear in a message header to be
distinguished from ordinary RFC 822 message header fields.
 
MIME-extension-field := <Any RFC 822 header field which
begins with the string
"Content-">
 
10. Summary
 
Using the MIME-Version, Content-Type, and Content-Transfer-Encoding
header fields, it is possible to include, in a standardized way,
arbitrary types of data with RFC 822 conformant mail messages. No
restrictions imposed by either RFC 821 or RFC 822 are violated, and
care has been taken to avoid problems caused by additional
restrictions imposed by the characteristics of some Internet mail
transport mechanisms (see RFC 2049).
 
The next document in this set, RFC 2046, specifies the initial set of
media types that can be labelled and transported using these headers.
 
11. Security Considerations
 
Security issues are discussed in the second document in this set, RFC
2046.
 
 
 
 
 
 
 
 
Freed & Borenstein Standards Track [Page 27]
RFC 2045 Internet Message Bodies November 1996
 
 
12. Authors' Addresses
 
For more information, the authors of this document are best contacted
via Internet mail:
 
Ned Freed
Innosoft International, Inc.
1050 East Garvey Avenue South
West Covina, CA 91790
USA
 
Phone: +1 818 919 3600
Fax: +1 818 919 3614
EMail: ned@innosoft.com
 
 
Nathaniel S. Borenstein
First Virtual Holdings
25 Washington Avenue
Morristown, NJ 07960
USA
 
Phone: +1 201 540 8967
Fax: +1 201 993 3032
EMail: nsb@nsb.fv.com
 
 
MIME is a result of the work of the Internet Engineering Task Force
Working Group on RFC 822 Extensions. The chairman of that group,
Greg Vaudreuil, may be reached at:
 
Gregory M. Vaudreuil
Octel Network Services
17080 Dallas Parkway
Dallas, TX 75248-1905
USA
 
EMail: Greg.Vaudreuil@Octel.Com
 
 
 
 
 
 
 
 
 
 
 
 
 
Freed & Borenstein Standards Track [Page 28]
RFC 2045 Internet Message Bodies November 1996
 
 
Appendix A -- Collected Grammar
 
This appendix contains the complete BNF grammar for all the syntax
specified by this document.
 
By itself, however, this grammar is incomplete. It refers by name to
several syntax rules that are defined by RFC 822. Rather than
reproduce those definitions here, and risk unintentional differences
between the two, this document simply refers the reader to RFC 822
for the remaining definitions. Wherever a term is undefined, it
refers to the RFC 822 definition.
 
attribute := token
; Matching of attributes
; is ALWAYS case-insensitive.
 
composite-type := "message" / "multipart" / extension-token
 
content := "Content-Type" ":" type "/" subtype
*(";" parameter)
; Matching of media type and subtype
; is ALWAYS case-insensitive.
 
description := "Content-Description" ":" *text
 
discrete-type := "text" / "image" / "audio" / "video" /
"application" / extension-token
 
encoding := "Content-Transfer-Encoding" ":" mechanism
 
entity-headers := [ content CRLF ]
[ encoding CRLF ]
[ id CRLF ]
[ description CRLF ]
*( MIME-extension-field CRLF )
 
extension-token := ietf-token / x-token
 
hex-octet := "=" 2(DIGIT / "A" / "B" / "C" / "D" / "E" / "F")
; Octet must be used for characters > 127, =,
; SPACEs or TABs at the ends of lines, and is
; recommended for any character not listed in
; RFC 2049 as "mail-safe".
 
iana-token := <A publicly-defined extension token. Tokens
of this form must be registered with IANA
as specified in RFC 2048.>
 
 
 
 
Freed & Borenstein Standards Track [Page 29]
RFC 2045 Internet Message Bodies November 1996
 
 
ietf-token := <An extension token defined by a
standards-track RFC and registered
with IANA.>
 
id := "Content-ID" ":" msg-id
 
mechanism := "7bit" / "8bit" / "binary" /
"quoted-printable" / "base64" /
ietf-token / x-token
 
MIME-extension-field := <Any RFC 822 header field which
begins with the string
"Content-">
 
MIME-message-headers := entity-headers
fields
version CRLF
; The ordering of the header
; fields implied by this BNF
; definition should be ignored.
 
MIME-part-headers := entity-headers
[fields]
; Any field not beginning with
; "content-" can have no defined
; meaning and may be ignored.
; The ordering of the header
; fields implied by this BNF
; definition should be ignored.
 
parameter := attribute "=" value
 
ptext := hex-octet / safe-char
 
qp-line := *(qp-segment transport-padding CRLF)
qp-part transport-padding
 
qp-part := qp-section
; Maximum length of 76 characters
 
qp-section := [*(ptext / SPACE / TAB) ptext]
 
qp-segment := qp-section *(SPACE / TAB) "="
; Maximum length of 76 characters
 
quoted-printable := qp-line *(CRLF qp-line)
 
 
 
 
 
Freed & Borenstein Standards Track [Page 30]
RFC 2045 Internet Message Bodies November 1996
 
 
safe-char := <any octet with decimal value of 33 through
60 inclusive, and 62 through 126>
; Characters not listed as "mail-safe" in
; RFC 2049 are also not recommended.
 
subtype := extension-token / iana-token
 
token := 1*<any (US-ASCII) CHAR except SPACE, CTLs,
or tspecials>
 
transport-padding := *LWSP-char
; Composers MUST NOT generate
; non-zero length transport
; padding, but receivers MUST
; be able to handle padding
; added by message transports.
 
tspecials := "(" / ")" / "<" / ">" / "@" /
"," / ";" / ":" / "\" / <">
"/" / "[" / "]" / "?" / "="
; Must be in quoted-string,
; to use within parameter values
 
type := discrete-type / composite-type
 
value := token / quoted-string
 
version := "MIME-Version" ":" 1*DIGIT "." 1*DIGIT
 
x-token := <The two characters "X-" or "x-" followed, with
no intervening white space, by any token>
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
Freed & Borenstein Standards Track [Page 31]
New file
/httptunnel/doc/rfc2068.txt
@@ -0,0 +1,9075 @@
 
 
 
 
 
 
Network Working Group R. Fielding
Request for Comments: 2068 UC Irvine
Category: Standards Track J. Gettys
J. Mogul
DEC
H. Frystyk
T. Berners-Lee
MIT/LCS
January 1997
 
 
Hypertext Transfer Protocol -- HTTP/1.1
 
Status of this Memo
 
This document specifies an Internet standards track protocol for the
Internet community, and requests discussion and suggestions for
improvements. Please refer to the current edition of the "Internet
Official Protocol Standards" (STD 1) for the standardization state
and status of this protocol. Distribution of this memo is unlimited.
 
Abstract
 
The Hypertext Transfer Protocol (HTTP) is an application-level
protocol for distributed, collaborative, hypermedia information
systems. It is a generic, stateless, object-oriented protocol which
can be used for many tasks, such as name servers and distributed
object management systems, through extension of its request methods.
A feature of HTTP is the typing and negotiation of data
representation, allowing systems to be built independently of the
data being transferred.
 
HTTP has been in use by the World-Wide Web global information
initiative since 1990. This specification defines the protocol
referred to as "HTTP/1.1".
 
Table of Contents
 
1 Introduction.............................................7
1.1 Purpose ..............................................7
1.2 Requirements .........................................7
1.3 Terminology ..........................................8
1.4 Overall Operation ...................................11
2 Notational Conventions and Generic Grammar..............13
2.1 Augmented BNF .......................................13
2.2 Basic Rules .........................................15
3 Protocol Parameters.....................................17
3.1 HTTP Version ........................................17
 
 
 
Fielding, et. al. Standards Track [Page 1]
RFC 2068 HTTP/1.1 January 1997
 
 
3.2 Uniform Resource Identifiers ........................18
3.2.1 General Syntax ...................................18
3.2.2 http URL .........................................19
3.2.3 URI Comparison ...................................20
3.3 Date/Time Formats ...................................21
3.3.1 Full Date ........................................21
3.3.2 Delta Seconds ....................................22
3.4 Character Sets ......................................22
3.5 Content Codings .....................................23
3.6 Transfer Codings ....................................24
3.7 Media Types .........................................25
3.7.1 Canonicalization and Text Defaults ...............26
3.7.2 Multipart Types ..................................27
3.8 Product Tokens ......................................28
3.9 Quality Values ......................................28
3.10 Language Tags ......................................28
3.11 Entity Tags ........................................29
3.12 Range Units ........................................30
4 HTTP Message............................................30
4.1 Message Types .......................................30
4.2 Message Headers .....................................31
4.3 Message Body ........................................32
4.4 Message Length ......................................32
4.5 General Header Fields ...............................34
5 Request.................................................34
5.1 Request-Line ........................................34
5.1.1 Method ...........................................35
5.1.2 Request-URI ......................................35
5.2 The Resource Identified by a Request ................37
5.3 Request Header Fields ...............................37
6 Response................................................38
6.1 Status-Line .........................................38
6.1.1 Status Code and Reason Phrase ....................39
6.2 Response Header Fields ..............................41
7 Entity..................................................41
7.1 Entity Header Fields ................................41
7.2 Entity Body .........................................42
7.2.1 Type .............................................42
7.2.2 Length ...........................................43
8 Connections.............................................43
8.1 Persistent Connections ..............................43
8.1.1 Purpose ..........................................43
8.1.2 Overall Operation ................................44
8.1.3 Proxy Servers ....................................45
8.1.4 Practical Considerations .........................45
8.2 Message Transmission Requirements ...................46
9 Method Definitions......................................48
9.1 Safe and Idempotent Methods .........................48
 
 
 
Fielding, et. al. Standards Track [Page 2]
RFC 2068 HTTP/1.1 January 1997
 
 
9.1.1 Safe Methods .....................................48
9.1.2 Idempotent Methods ...............................49
9.2 OPTIONS .............................................49
9.3 GET .................................................50
9.4 HEAD ................................................50
9.5 POST ................................................51
9.6 PUT .................................................52
9.7 DELETE ..............................................53
9.8 TRACE ...............................................53
10 Status Code Definitions................................53
10.1 Informational 1xx ..................................54
10.1.1 100 Continue ....................................54
10.1.2 101 Switching Protocols .........................54
10.2 Successful 2xx .....................................54
10.2.1 200 OK ..........................................54
10.2.2 201 Created .....................................55
10.2.3 202 Accepted ....................................55
10.2.4 203 Non-Authoritative Information ...............55
10.2.5 204 No Content ..................................55
10.2.6 205 Reset Content ...............................56
10.2.7 206 Partial Content .............................56
10.3 Redirection 3xx ....................................56
10.3.1 300 Multiple Choices ............................57
10.3.2 301 Moved Permanently ...........................57
10.3.3 302 Moved Temporarily ...........................58
10.3.4 303 See Other ...................................58
10.3.5 304 Not Modified ................................58
10.3.6 305 Use Proxy ...................................59
10.4 Client Error 4xx ...................................59
10.4.1 400 Bad Request .................................60
10.4.2 401 Unauthorized ................................60
10.4.3 402 Payment Required ............................60
10.4.4 403 Forbidden ...................................60
10.4.5 404 Not Found ...................................60
10.4.6 405 Method Not Allowed ..........................61
10.4.7 406 Not Acceptable ..............................61
10.4.8 407 Proxy Authentication Required ...............61
10.4.9 408 Request Timeout .............................62
10.4.10 409 Conflict ...................................62
10.4.11 410 Gone .......................................62
10.4.12 411 Length Required ............................63
10.4.13 412 Precondition Failed ........................63
10.4.14 413 Request Entity Too Large ...................63
10.4.15 414 Request-URI Too Long .......................63
10.4.16 415 Unsupported Media Type .....................63
10.5 Server Error 5xx ...................................64
10.5.1 500 Internal Server Error .......................64
10.5.2 501 Not Implemented .............................64
 
 
 
Fielding, et. al. Standards Track [Page 3]
RFC 2068 HTTP/1.1 January 1997
 
 
10.5.3 502 Bad Gateway .................................64
10.5.4 503 Service Unavailable .........................64
10.5.5 504 Gateway Timeout .............................64
10.5.6 505 HTTP Version Not Supported ..................65
11 Access Authentication..................................65
11.1 Basic Authentication Scheme ........................66
11.2 Digest Authentication Scheme .......................67
12 Content Negotiation....................................67
12.1 Server-driven Negotiation ..........................68
12.2 Agent-driven Negotiation ...........................69
12.3 Transparent Negotiation ............................70
13 Caching in HTTP........................................70
13.1.1 Cache Correctness ...............................72
13.1.2 Warnings ........................................73
13.1.3 Cache-control Mechanisms ........................74
13.1.4 Explicit User Agent Warnings ....................74
13.1.5 Exceptions to the Rules and Warnings ............75
13.1.6 Client-controlled Behavior ......................75
13.2 Expiration Model ...................................75
13.2.1 Server-Specified Expiration .....................75
13.2.2 Heuristic Expiration ............................76
13.2.3 Age Calculations ................................77
13.2.4 Expiration Calculations .........................79
13.2.5 Disambiguating Expiration Values ................80
13.2.6 Disambiguating Multiple Responses ...............80
13.3 Validation Model ...................................81
13.3.1 Last-modified Dates .............................82
13.3.2 Entity Tag Cache Validators .....................82
13.3.3 Weak and Strong Validators ......................82
13.3.4 Rules for When to Use Entity Tags and Last-
modified Dates..........................................85
13.3.5 Non-validating Conditionals .....................86
13.4 Response Cachability ...............................86
13.5 Constructing Responses From Caches .................87
13.5.1 End-to-end and Hop-by-hop Headers ...............88
13.5.2 Non-modifiable Headers ..........................88
13.5.3 Combining Headers ...............................89
13.5.4 Combining Byte Ranges ...........................90
13.6 Caching Negotiated Responses .......................90
13.7 Shared and Non-Shared Caches .......................91
13.8 Errors or Incomplete Response Cache Behavior .......91
13.9 Side Effects of GET and HEAD .......................92
13.10 Invalidation After Updates or Deletions ...........92
13.11 Write-Through Mandatory ...........................93
13.12 Cache Replacement .................................93
13.13 History Lists .....................................93
14 Header Field Definitions...............................94
14.1 Accept .............................................95
 
 
 
Fielding, et. al. Standards Track [Page 4]
RFC 2068 HTTP/1.1 January 1997
 
 
14.2 Accept-Charset .....................................97
14.3 Accept-Encoding ....................................97
14.4 Accept-Language ....................................98
14.5 Accept-Ranges ......................................99
14.6 Age ................................................99
14.7 Allow .............................................100
14.8 Authorization .....................................100
14.9 Cache-Control .....................................101
14.9.1 What is Cachable ...............................103
14.9.2 What May be Stored by Caches ...................103
14.9.3 Modifications of the Basic Expiration Mechanism 104
14.9.4 Cache Revalidation and Reload Controls .........105
14.9.5 No-Transform Directive .........................107
14.9.6 Cache Control Extensions .......................108
14.10 Connection .......................................109
14.11 Content-Base .....................................109
14.12 Content-Encoding .................................110
14.13 Content-Language .................................110
14.14 Content-Length ...................................111
14.15 Content-Location .................................112
14.16 Content-MD5 ......................................113
14.17 Content-Range ....................................114
14.18 Content-Type .....................................116
14.19 Date .............................................116
14.20 ETag .............................................117
14.21 Expires ..........................................117
14.22 From .............................................118
14.23 Host .............................................119
14.24 If-Modified-Since ................................119
14.25 If-Match .........................................121
14.26 If-None-Match ....................................122
14.27 If-Range .........................................123
14.28 If-Unmodified-Since ..............................124
14.29 Last-Modified ....................................124
14.30 Location .........................................125
14.31 Max-Forwards .....................................125
14.32 Pragma ...........................................126
14.33 Proxy-Authenticate ...............................127
14.34 Proxy-Authorization ..............................127
14.35 Public ...........................................127
14.36 Range ............................................128
14.36.1 Byte Ranges ...................................128
14.36.2 Range Retrieval Requests ......................130
14.37 Referer ..........................................131
14.38 Retry-After ......................................131
14.39 Server ...........................................132
14.40 Transfer-Encoding ................................132
14.41 Upgrade ..........................................132
 
 
 
Fielding, et. al. Standards Track [Page 5]
RFC 2068 HTTP/1.1 January 1997
 
 
14.42 User-Agent .......................................134
14.43 Vary .............................................134
14.44 Via ..............................................135
14.45 Warning ..........................................137
14.46 WWW-Authenticate .................................139
15 Security Considerations...............................139
15.1 Authentication of Clients .........................139
15.2 Offering a Choice of Authentication Schemes .......140
15.3 Abuse of Server Log Information ...................141
15.4 Transfer of Sensitive Information .................141
15.5 Attacks Based On File and Path Names ..............142
15.6 Personal Information ..............................143
15.7 Privacy Issues Connected to Accept Headers ........143
15.8 DNS Spoofing ......................................144
15.9 Location Headers and Spoofing .....................144
16 Acknowledgments.......................................144
17 References............................................146
18 Authors' Addresses....................................149
19 Appendices............................................150
19.1 Internet Media Type message/http ..................150
19.2 Internet Media Type multipart/byteranges ..........150
19.3 Tolerant Applications .............................151
19.4 Differences Between HTTP Entities and
MIME Entities...........................................152
19.4.1 Conversion to Canonical Form ...................152
19.4.2 Conversion of Date Formats .....................153
19.4.3 Introduction of Content-Encoding ...............153
19.4.4 No Content-Transfer-Encoding ...................153
19.4.5 HTTP Header Fields in Multipart Body-Parts .....153
19.4.6 Introduction of Transfer-Encoding ..............154
19.4.7 MIME-Version ...................................154
19.5 Changes from HTTP/1.0 .............................154
19.5.1 Changes to Simplify Multi-homed Web Servers and
Conserve IP Addresses .................................155
19.6 Additional Features ...............................156
19.6.1 Additional Request Methods .....................156
19.6.2 Additional Header Field Definitions ............156
19.7 Compatibility with Previous Versions ..............160
19.7.1 Compatibility with HTTP/1.0 Persistent
Connections............................................161
 
 
 
 
 
 
 
 
 
 
 
Fielding, et. al. Standards Track [Page 6]
RFC 2068 HTTP/1.1 January 1997
 
 
1 Introduction
 
1.1 Purpose
 
The Hypertext Transfer Protocol (HTTP) is an application-level
protocol for distributed, collaborative, hypermedia information
systems. HTTP has been in use by the World-Wide Web global
information initiative since 1990. The first version of HTTP,
referred to as HTTP/0.9, was a simple protocol for raw data transfer
across the Internet. HTTP/1.0, as defined by RFC 1945 [6], improved
the protocol by allowing messages to be in the format of MIME-like
messages, containing metainformation about the data transferred and
modifiers on the request/response semantics. However, HTTP/1.0 does
not sufficiently take into consideration the effects of hierarchical
proxies, caching, the need for persistent connections, and virtual
hosts. In addition, the proliferation of incompletely-implemented
applications calling themselves "HTTP/1.0" has necessitated a
protocol version change in order for two communicating applications
to determine each other's true capabilities.
 
This specification defines the protocol referred to as "HTTP/1.1".
This protocol includes more stringent requirements than HTTP/1.0 in
order to ensure reliable implementation of its features.
 
Practical information systems require more functionality than simple
retrieval, including search, front-end update, and annotation. HTTP
allows an open-ended set of methods that indicate the purpose of a
request. It builds on the discipline of reference provided by the
Uniform Resource Identifier (URI) [3][20], as a location (URL) [4] or
name (URN) , for indicating the resource to which a method is to be
applied. Messages are passed in a format similar to that used by
Internet mail as defined by the Multipurpose Internet Mail Extensions
(MIME).
 
HTTP is also used as a generic protocol for communication between
user agents and proxies/gateways to other Internet systems, including
those supported by the SMTP [16], NNTP [13], FTP [18], Gopher [2],
and WAIS [10] protocols. In this way, HTTP allows basic hypermedia
access to resources available from diverse applications.
 
1.2 Requirements
 
This specification uses the same words as RFC 1123 [8] for defining
the significance of each particular requirement. These words are:
 
MUST
This word or the adjective "required" means that the item is an
absolute requirement of the specification.
 
 
 
Fielding, et. al. Standards Track [Page 7]
RFC 2068 HTTP/1.1 January 1997
 
 
SHOULD
This word or the adjective "recommended" means that there may
exist valid reasons in particular circumstances to ignore this
item, but the full implications should be understood and the case
carefully weighed before choosing a different course.
 
MAY
This word or the adjective "optional" means that this item is
truly optional. One vendor may choose to include the item because
a particular marketplace requires it or because it enhances the
product, for example; another vendor may omit the same item.
 
An implementation is not compliant if it fails to satisfy one or more
of the MUST requirements for the protocols it implements. An
implementation that satisfies all the MUST and all the SHOULD
requirements for its protocols is said to be "unconditionally
compliant"; one that satisfies all the MUST requirements but not all
the SHOULD requirements for its protocols is said to be
"conditionally compliant."
 
1.3 Terminology
 
This specification uses a number of terms to refer to the roles
played by participants in, and objects of, the HTTP communication.
 
connection
A transport layer virtual circuit established between two programs
for the purpose of communication.
 
message
The basic unit of HTTP communication, consisting of a structured
sequence of octets matching the syntax defined in section 4 and
transmitted via the connection.
 
request
An HTTP request message, as defined in section 5.
 
response
An HTTP response message, as defined in section 6.
 
resource
A network data object or service that can be identified by a URI,
as defined in section 3.2. Resources may be available in multiple
representations (e.g. multiple languages, data formats, size,
resolutions) or vary in other ways.
 
 
 
 
 
 
Fielding, et. al. Standards Track [Page 8]
RFC 2068 HTTP/1.1 January 1997
 
 
entity
The information transferred as the payload of a request or
response. An entity consists of metainformation in the form of
entity-header fields and content in the form of an entity-body, as
described in section 7.
 
representation
An entity included with a response that is subject to content
negotiation, as described in section 12. There may exist multiple
representations associated with a particular response status.
 
content negotiation
The mechanism for selecting the appropriate representation when
servicing a request, as described in section 12. The
representation of entities in any response can be negotiated
(including error responses).
 
variant
A resource may have one, or more than one, representation(s)
associated with it at any given instant. Each of these
representations is termed a `variant.' Use of the term `variant'
does not necessarily imply that the resource is subject to content
negotiation.
 
client
A program that establishes connections for the purpose of sending
requests.
 
user agent
The client which initiates a request. These are often browsers,
editors, spiders (web-traversing robots), or other end user tools.
 
server
An application program that accepts connections in order to
service requests by sending back responses. Any given program may
be capable of being both a client and a server; our use of these
terms refers only to the role being performed by the program for a
particular connection, rather than to the program's capabilities
in general. Likewise, any server may act as an origin server,
proxy, gateway, or tunnel, switching behavior based on the nature
of each request.
 
origin server
The server on which a given resource resides or is to be created.
 
 
 
 
 
 
 
Fielding, et. al. Standards Track [Page 9]
RFC 2068 HTTP/1.1 January 1997
 
 
proxy
An intermediary program which acts as both a server and a client
for the purpose of making requests on behalf of other clients.
Requests are serviced internally or by passing them on, with
possible translation, to other servers. A proxy must implement
both the client and server requirements of this specification.
 
gateway
A server which acts as an intermediary for some other server.
Unlike a proxy, a gateway receives requests as if it were the
origin server for the requested resource; the requesting client
may not be aware that it is communicating with a gateway.
 
tunnel
An intermediary program which is acting as a blind relay between
two connections. Once active, a tunnel is not considered a party
to the HTTP communication, though the tunnel may have been
initiated by an HTTP request. The tunnel ceases to exist when both
ends of the relayed connections are closed.
 
cache
A program's local store of response messages and the subsystem
that controls its message storage, retrieval, and deletion. A
cache stores cachable responses in order to reduce the response
time and network bandwidth consumption on future, equivalent
requests. Any client or server may include a cache, though a cache
cannot be used by a server that is acting as a tunnel.
 
cachable
A response is cachable if a cache is allowed to store a copy of
the response message for use in answering subsequent requests. The
rules for determining the cachability of HTTP responses are
defined in section 13. Even if a resource is cachable, there may
be additional constraints on whether a cache can use the cached
copy for a particular request.
 
first-hand
A response is first-hand if it comes directly and without
unnecessary delay from the origin server, perhaps via one or more
proxies. A response is also first-hand if its validity has just
been checked directly with the origin server.
 
explicit expiration time
The time at which the origin server intends that an entity should
no longer be returned by a cache without further validation.
 
 
 
 
 
 
Fielding, et. al. Standards Track [Page 10]
RFC 2068 HTTP/1.1 January 1997
 
 
heuristic expiration time
An expiration time assigned by a cache when no explicit expiration
time is available.
 
age
The age of a response is the time since it was sent by, or
successfully validated with, the origin server.
 
freshness lifetime
The length of time between the generation of a response and its
expiration time.
 
fresh
A response is fresh if its age has not yet exceeded its freshness
lifetime.
 
stale
A response is stale if its age has passed its freshness lifetime.
 
semantically transparent
A cache behaves in a "semantically transparent" manner, with
respect to a particular response, when its use affects neither the
requesting client nor the origin server, except to improve
performance. When a cache is semantically transparent, the client
receives exactly the same response (except for hop-by-hop headers)
that it would have received had its request been handled directly
by the origin server.
 
validator
A protocol element (e.g., an entity tag or a Last-Modified time)
that is used to find out whether a cache entry is an equivalent
copy of an entity.
 
1.4 Overall Operation
 
The HTTP protocol is a request/response protocol. A client sends a
request to the server in the form of a request method, URI, and
protocol version, followed by a MIME-like message containing request
modifiers, client information, and possible body content over a
connection with a server. The server responds with a status line,
including the message's protocol version and a success or error code,
followed by a MIME-like message containing server information, entity
metainformation, and possible entity-body content. The relationship
between HTTP and MIME is described in appendix 19.4.
 
 
 
 
 
 
 
Fielding, et. al. Standards Track [Page 11]
RFC 2068 HTTP/1.1 January 1997
 
 
Most HTTP communication is initiated by a user agent and consists of
a request to be applied to a resource on some origin server. In the
simplest case, this may be accomplished via a single connection (v)
between the user agent (UA) and the origin server (O).
 
request chain ------------------------>
UA -------------------v------------------- O
<----------------------- response chain
 
A more complicated situation occurs when one or more intermediaries
are present in the request/response chain. There are three common
forms of intermediary: proxy, gateway, and tunnel. A proxy is a
forwarding agent, receiving requests for a URI in its absolute form,
rewriting all or part of the message, and forwarding the reformatted
request toward the server identified by the URI. A gateway is a
receiving agent, acting as a layer above some other server(s) and, if
necessary, translating the requests to the underlying server's
protocol. A tunnel acts as a relay point between two connections
without changing the messages; tunnels are used when the
communication needs to pass through an intermediary (such as a
firewall) even when the intermediary cannot understand the contents
of the messages.
 
request chain -------------------------------------->
UA -----v----- A -----v----- B -----v----- C -----v----- O
<------------------------------------- response chain
 
The figure above shows three intermediaries (A, B, and C) between the
user agent and origin server. A request or response message that
travels the whole chain will pass through four separate connections.
This distinction is important because some HTTP communication options
may apply only to the connection with the nearest, non-tunnel
neighbor, only to the end-points of the chain, or to all connections
along the chain. Although the diagram is linear, each participant
may be engaged in multiple, simultaneous communications. For example,
B may be receiving requests from many clients other than A, and/or
forwarding requests to servers other than C, at the same time that it
is handling A's request.
 
Any party to the communication which is not acting as a tunnel may
employ an internal cache for handling requests. The effect of a cache
is that the request/response chain is shortened if one of the
participants along the chain has a cached response applicable to that
request. The following illustrates the resulting chain if B has a
cached copy of an earlier response from O (via C) for a request which
has not been cached by UA or A.
 
 
 
 
 
Fielding, et. al. Standards Track [Page 12]
RFC 2068 HTTP/1.1 January 1997
 
 
request chain ---------->
UA -----v----- A -----v----- B - - - - - - C - - - - - - O
<--------- response chain
 
Not all responses are usefully cachable, and some requests may
contain modifiers which place special requirements on cache behavior.
HTTP requirements for cache behavior and cachable responses are
defined in section 13.
 
In fact, there are a wide variety of architectures and configurations
of caches and proxies currently being experimented with or deployed
across the World Wide Web; these systems include national hierarchies
of proxy caches to save transoceanic bandwidth, systems that
broadcast or multicast cache entries, organizations that distribute
subsets of cached data via CD-ROM, and so on. HTTP systems are used
in corporate intranets over high-bandwidth links, and for access via
PDAs with low-power radio links and intermittent connectivity. The
goal of HTTP/1.1 is to support the wide diversity of configurations
already deployed while introducing protocol constructs that meet the
needs of those who build web applications that require high
reliability and, failing that, at least reliable indications of
failure.
 
HTTP communication usually takes place over TCP/IP connections. The
default port is TCP 80, but other ports can be used. This does not
preclude HTTP from being implemented on top of any other protocol on
the Internet, or on other networks. HTTP only presumes a reliable
transport; any protocol that provides such guarantees can be used;
the mapping of the HTTP/1.1 request and response structures onto the
transport data units of the protocol in question is outside the scope
of this specification.
 
In HTTP/1.0, most implementations used a new connection for each
request/response exchange. In HTTP/1.1, a connection may be used for
one or more request/response exchanges, although connections may be
closed for a variety of reasons (see section 8.1).
 
2 Notational Conventions and Generic Grammar
 
2.1 Augmented BNF
 
All of the mechanisms specified in this document are described in
both prose and an augmented Backus-Naur Form (BNF) similar to that
used by RFC 822 [9]. Implementers will need to be familiar with the
notation in order to understand this specification. The augmented BNF
includes the following constructs:
 
 
 
 
 
Fielding, et. al. Standards Track [Page 13]
RFC 2068 HTTP/1.1 January 1997
 
 
name = definition
The name of a rule is simply the name itself (without any enclosing
"<" and ">") and is separated from its definition by the equal "="
character. Whitespace is only significant in that indentation of
continuation lines is used to indicate a rule definition that spans
more than one line. Certain basic rules are in uppercase, such as
SP, LWS, HT, CRLF, DIGIT, ALPHA, etc. Angle brackets are used
within definitions whenever their presence will facilitate
discerning the use of rule names.
 
"literal"
Quotation marks surround literal text. Unless stated otherwise, the
text is case-insensitive.
 
rule1 | rule2
Elements separated by a bar ("|") are alternatives, e.g., "yes |
no" will accept yes or no.
 
(rule1 rule2)
Elements enclosed in parentheses are treated as a single element.
Thus, "(elem (foo | bar) elem)" allows the token sequences "elem
foo elem" and "elem bar elem".
 
*rule
The character "*" preceding an element indicates repetition. The
full form is "<n>*<m>element" indicating at least <n> and at most
<m> occurrences of element. Default values are 0 and infinity so
that "*(element)" allows any number, including zero; "1*element"
requires at least one; and "1*2element" allows one or two.
 
[rule]
Square brackets enclose optional elements; "[foo bar]" is
equivalent to "*1(foo bar)".
 
N rule
Specific repetition: "<n>(element)" is equivalent to
"<n>*<n>(element)"; that is, exactly <n> occurrences of (element).
Thus 2DIGIT is a 2-digit number, and 3ALPHA is a string of three
alphabetic characters.
 
#rule
A construct "#" is defined, similar to "*", for defining lists of
elements. The full form is "<n>#<m>element " indicating at least
<n> and at most <m> elements, each separated by one or more commas
(",") and optional linear whitespace (LWS). This makes the usual
form of lists very easy; a rule such as "( *LWS element *( *LWS ","
*LWS element )) " can be shown as "1#element". Wherever this
construct is used, null elements are allowed, but do not contribute
 
 
 
Fielding, et. al. Standards Track [Page 14]
RFC 2068 HTTP/1.1 January 1997
 
 
to the count of elements present. That is, "(element), , (element)
" is permitted, but counts as only two elements. Therefore, where
at least one element is required, at least one non-null element
must be present. Default values are 0 and infinity so that
"#element" allows any number, including zero; "1#element" requires
at least one; and "1#2element" allows one or two.
 
; comment
A semi-colon, set off some distance to the right of rule text,
starts a comment that continues to the end of line. This is a
simple way of including useful notes in parallel with the
specifications.
 
implied *LWS
The grammar described by this specification is word-based. Except
where noted otherwise, linear whitespace (LWS) can be included
between any two adjacent words (token or quoted-string), and
between adjacent tokens and delimiters (tspecials), without
changing the interpretation of a field. At least one delimiter
(tspecials) must exist between any two tokens, since they would
otherwise be interpreted as a single token.
 
2.2 Basic Rules
 
The following rules are used throughout this specification to
describe basic parsing constructs. The US-ASCII coded character set
is defined by ANSI X3.4-1986 [21].
 
OCTET = <any 8-bit sequence of data>
CHAR = <any US-ASCII character (octets 0 - 127)>
UPALPHA = <any US-ASCII uppercase letter "A".."Z">
LOALPHA = <any US-ASCII lowercase letter "a".."z">
ALPHA = UPALPHA | LOALPHA
DIGIT = <any US-ASCII digit "0".."9">
CTL = <any US-ASCII control character
(octets 0 - 31) and DEL (127)>
CR = <US-ASCII CR, carriage return (13)>
LF = <US-ASCII LF, linefeed (10)>
SP = <US-ASCII SP, space (32)>
HT = <US-ASCII HT, horizontal-tab (9)>
<"> = <US-ASCII double-quote mark (34)>
 
 
 
 
 
 
 
 
 
 
Fielding, et. al. Standards Track [Page 15]
RFC 2068 HTTP/1.1 January 1997
 
 
HTTP/1.1 defines the sequence CR LF as the end-of-line marker for all
protocol elements except the entity-body (see appendix 19.3 for
tolerant applications). The end-of-line marker within an entity-body
is defined by its associated media type, as described in section 3.7.
 
CRLF = CR LF
 
HTTP/1.1 headers can be folded onto multiple lines if the
continuation line begins with a space or horizontal tab. All linear
white space, including folding, has the same semantics as SP.
 
LWS = [CRLF] 1*( SP | HT )
 
The TEXT rule is only used for descriptive field contents and values
that are not intended to be interpreted by the message parser. Words
of *TEXT may contain characters from character sets other than ISO
8859-1 [22] only when encoded according to the rules of RFC 1522
[14].
 
TEXT = <any OCTET except CTLs,
but including LWS>
 
Hexadecimal numeric characters are used in several protocol elements.
 
HEX = "A" | "B" | "C" | "D" | "E" | "F"
| "a" | "b" | "c" | "d" | "e" | "f" | DIGIT
 
Many HTTP/1.1 header field values consist of words separated by LWS
or special characters. These special characters MUST be in a quoted
string to be used within a parameter value.
 
token = 1*<any CHAR except CTLs or tspecials>
 
tspecials = "(" | ")" | "<" | ">" | "@"
| "," | ";" | ":" | "\" | <">
| "/" | "[" | "]" | "?" | "="
| "{" | "}" | SP | HT
 
Comments can be included in some HTTP header fields by surrounding
the comment text with parentheses. Comments are only allowed in
fields containing "comment" as part of their field value definition.
In all other fields, parentheses are considered part of the field
value.
 
comment = "(" *( ctext | comment ) ")"
ctext = <any TEXT excluding "(" and ")">
 
 
 
 
 
Fielding, et. al. Standards Track [Page 16]
RFC 2068 HTTP/1.1 January 1997
 
 
A string of text is parsed as a single word if it is quoted using
double-quote marks.
 
quoted-string = ( <"> *(qdtext) <"> )
 
qdtext = <any TEXT except <">>
 
The backslash character ("\") may be used as a single-character quoting
mechanism only within quoted-string and comment constructs.
 
quoted-pair = "\" CHAR
 
3 Protocol Parameters
 
3.1 HTTP Version
 
HTTP uses a "<major>.<minor>" numbering scheme to indicate versions
of the protocol. The protocol versioning policy is intended to allow
the sender to indicate the format of a message and its capacity for
understanding further HTTP communication, rather than the features
obtained via that communication. No change is made to the version
number for the addition of message components which do not affect
communication behavior or which only add to extensible field values.
The <minor> number is incremented when the changes made to the
protocol add features which do not change the general message parsing
algorithm, but which may add to the message semantics and imply
additional capabilities of the sender. The <major> number is
incremented when the format of a message within the protocol is
changed.
 
The version of an HTTP message is indicated by an HTTP-Version field
in the first line of the message.
 
HTTP-Version = "HTTP" "/" 1*DIGIT "." 1*DIGIT
 
Note that the major and minor numbers MUST be treated as separate
integers and that each may be incremented higher than a single digit.
Thus, HTTP/2.4 is a lower version than HTTP/2.13, which in turn is
lower than HTTP/12.3. Leading zeros MUST be ignored by recipients and
MUST NOT be sent.
 
Applications sending Request or Response messages, as defined by this
specification, MUST include an HTTP-Version of "HTTP/1.1". Use of
this version number indicates that the sending application is at
least conditionally compliant with this specification.
 
The HTTP version of an application is the highest HTTP version for
which the application is at least conditionally compliant.
 
 
 
Fielding, et. al. Standards Track [Page 17]
RFC 2068 HTTP/1.1 January 1997
 
 
Proxy and gateway applications must be careful when forwarding
messages in protocol versions different from that of the application.
Since the protocol version indicates the protocol capability of the
sender, a proxy/gateway MUST never send a message with a version
indicator which is greater than its actual version; if a higher
version request is received, the proxy/gateway MUST either downgrade
the request version, respond with an error, or switch to tunnel
behavior. Requests with a version lower than that of the
proxy/gateway's version MAY be upgraded before being forwarded; the
proxy/gateway's response to that request MUST be in the same major
version as the request.
 
Note: Converting between versions of HTTP may involve modification
of header fields required or forbidden by the versions involved.
 
3.2 Uniform Resource Identifiers
 
URIs have been known by many names: WWW addresses, Universal Document
Identifiers, Universal Resource Identifiers , and finally the
combination of Uniform Resource Locators (URL) and Names (URN). As
far as HTTP is concerned, Uniform Resource Identifiers are simply
formatted strings which identify--via name, location, or any other
characteristic--a resource.
 
3.2.1 General Syntax
 
URIs in HTTP can be represented in absolute form or relative to some
known base URI, depending upon the context of their use. The two
forms are differentiated by the fact that absolute URIs always begin
with a scheme name followed by a colon.
 
URI = ( absoluteURI | relativeURI ) [ "#" fragment ]
 
absoluteURI = scheme ":" *( uchar | reserved )
 
relativeURI = net_path | abs_path | rel_path
 
net_path = "//" net_loc [ abs_path ]
abs_path = "/" rel_path
rel_path = [ path ] [ ";" params ] [ "?" query ]
 
path = fsegment *( "/" segment )
fsegment = 1*pchar
segment = *pchar
 
params = param *( ";" param )
param = *( pchar | "/" )
 
 
 
 
Fielding, et. al. Standards Track [Page 18]
RFC 2068 HTTP/1.1 January 1997
 
 
scheme = 1*( ALPHA | DIGIT | "+" | "-" | "." )
net_loc = *( pchar | ";" | "?" )
 
query = *( uchar | reserved )
fragment = *( uchar | reserved )
 
pchar = uchar | ":" | "@" | "&" | "=" | "+"
uchar = unreserved | escape
unreserved = ALPHA | DIGIT | safe | extra | national
 
escape = "%" HEX HEX
reserved = ";" | "/" | "?" | ":" | "@" | "&" | "=" | "+"
extra = "!" | "*" | "'" | "(" | ")" | ","
safe = "$" | "-" | "_" | "."
unsafe = CTL | SP | <"> | "#" | "%" | "<" | ">"
national = <any OCTET excluding ALPHA, DIGIT,
reserved, extra, safe, and unsafe>
 
For definitive information on URL syntax and semantics, see RFC 1738
[4] and RFC 1808 [11]. The BNF above includes national characters not
allowed in valid URLs as specified by RFC 1738, since HTTP servers
are not restricted in the set of unreserved characters allowed to
represent the rel_path part of addresses, and HTTP proxies may
receive requests for URIs not defined by RFC 1738.
 
The HTTP protocol does not place any a priori limit on the length of
a URI. Servers MUST be able to handle the URI of any resource they
serve, and SHOULD be able to handle URIs of unbounded length if they
provide GET-based forms that could generate such URIs. A server
SHOULD return 414 (Request-URI Too Long) status if a URI is longer
than the server can handle (see section 10.4.15).
 
Note: Servers should be cautious about depending on URI lengths
above 255 bytes, because some older client or proxy implementations
may not properly support these lengths.
 
3.2.2 http URL
 
The "http" scheme is used to locate network resources via the HTTP
protocol. This section defines the scheme-specific syntax and
semantics for http URLs.
 
 
 
 
 
 
 
 
 
 
Fielding, et. al. Standards Track [Page 19]
RFC 2068 HTTP/1.1 January 1997
 
 
http_URL = "http:" "//" host [ ":" port ] [ abs_path ]
 
host = <A legal Internet host domain name
or IP address (in dotted-decimal form),
as defined by Section 2.1 of RFC 1123>
 
port = *DIGIT
 
If the port is empty or not given, port 80 is assumed. The semantics
are that the identified resource is located at the server listening
for TCP connections on that port of that host, and the Request-URI
for the resource is abs_path. The use of IP addresses in URL's SHOULD
be avoided whenever possible (see RFC 1900 [24]). If the abs_path is
not present in the URL, it MUST be given as "/" when used as a
Request-URI for a resource (section 5.1.2).
 
3.2.3 URI Comparison
 
When comparing two URIs to decide if they match or not, a client
SHOULD use a case-sensitive octet-by-octet comparison of the entire
URIs, with these exceptions:
 
o A port that is empty or not given is equivalent to the default
port for that URI;
 
o Comparisons of host names MUST be case-insensitive;
 
o Comparisons of scheme names MUST be case-insensitive;
 
o An empty abs_path is equivalent to an abs_path of "/".
 
Characters other than those in the "reserved" and "unsafe" sets (see
section 3.2) are equivalent to their ""%" HEX HEX" encodings.
 
For example, the following three URIs are equivalent:
 
http://abc.com:80/~smith/home.html
http://ABC.com/%7Esmith/home.html
http://ABC.com:/%7esmith/home.html
 
 
 
 
 
 
 
 
 
 
 
 
Fielding, et. al. Standards Track [Page 20]
RFC 2068 HTTP/1.1 January 1997
 
 
3.3 Date/Time Formats
 
3.3.1 Full Date
 
HTTP applications have historically allowed three different formats
for the representation of date/time stamps:
 
Sun, 06 Nov 1994 08:49:37 GMT ; RFC 822, updated by RFC 1123
Sunday, 06-Nov-94 08:49:37 GMT ; RFC 850, obsoleted by RFC 1036
Sun Nov 6 08:49:37 1994 ; ANSI C's asctime() format
 
The first format is preferred as an Internet standard and represents
a fixed-length subset of that defined by RFC 1123 (an update to RFC
822). The second format is in common use, but is based on the
obsolete RFC 850 [12] date format and lacks a four-digit year.
HTTP/1.1 clients and servers that parse the date value MUST accept
all three formats (for compatibility with HTTP/1.0), though they MUST
only generate the RFC 1123 format for representing HTTP-date values
in header fields.
 
Note: Recipients of date values are encouraged to be robust in
accepting date values that may have been sent by non-HTTP
applications, as is sometimes the case when retrieving or posting
messages via proxies/gateways to SMTP or NNTP.
 
All HTTP date/time stamps MUST be represented in Greenwich Mean Time
(GMT), without exception. This is indicated in the first two formats
by the inclusion of "GMT" as the three-letter abbreviation for time
zone, and MUST be assumed when reading the asctime format.
 
HTTP-date = rfc1123-date | rfc850-date | asctime-date
 
rfc1123-date = wkday "," SP date1 SP time SP "GMT"
rfc850-date = weekday "," SP date2 SP time SP "GMT"
asctime-date = wkday SP date3 SP time SP 4DIGIT
 
date1 = 2DIGIT SP month SP 4DIGIT
; day month year (e.g., 02 Jun 1982)
date2 = 2DIGIT "-" month "-" 2DIGIT
; day-month-year (e.g., 02-Jun-82)
date3 = month SP ( 2DIGIT | ( SP 1DIGIT ))
; month day (e.g., Jun 2)
 
time = 2DIGIT ":" 2DIGIT ":" 2DIGIT
; 00:00:00 - 23:59:59
 
wkday = "Mon" | "Tue" | "Wed"
| "Thu" | "Fri" | "Sat" | "Sun"
 
 
 
Fielding, et. al. Standards Track [Page 21]
RFC 2068 HTTP/1.1 January 1997
 
 
weekday = "Monday" | "Tuesday" | "Wednesday"
| "Thursday" | "Friday" | "Saturday" | "Sunday"
 
month = "Jan" | "Feb" | "Mar" | "Apr"
| "May" | "Jun" | "Jul" | "Aug"
| "Sep" | "Oct" | "Nov" | "Dec"
 
Note: HTTP requirements for the date/time stamp format apply only
to their usage within the protocol stream. Clients and servers are
not required to use these formats for user presentation, request
logging, etc.
 
3.3.2 Delta Seconds
 
Some HTTP header fields allow a time value to be specified as an
integer number of seconds, represented in decimal, after the time
that the message was received.
 
delta-seconds = 1*DIGIT
 
3.4 Character Sets
 
HTTP uses the same definition of the term "character set" as that
described for MIME:
 
The term "character set" is used in this document to refer to a
method used with one or more tables to convert a sequence of octets
into a sequence of characters. Note that unconditional conversion
in the other direction is not required, in that not all characters
may be available in a given character set and a character set may
provide more than one sequence of octets to represent a particular
character. This definition is intended to allow various kinds of
character encodings, from simple single-table mappings such as US-
ASCII to complex table switching methods such as those that use ISO
2022's techniques. However, the definition associated with a MIME
character set name MUST fully specify the mapping to be performed
from octets to characters. In particular, use of external profiling
information to determine the exact mapping is not permitted.
 
Note: This use of the term "character set" is more commonly
referred to as a "character encoding." However, since HTTP and MIME
share the same registry, it is important that the terminology also
be shared.
 
 
 
 
 
 
 
 
Fielding, et. al. Standards Track [Page 22]
RFC 2068 HTTP/1.1 January 1997
 
 
HTTP character sets are identified by case-insensitive tokens. The
complete set of tokens is defined by the IANA Character Set registry
[19].
 
charset = token
 
Although HTTP allows an arbitrary token to be used as a charset
value, any token that has a predefined value within the IANA
Character Set registry MUST represent the character set defined by
that registry. Applications SHOULD limit their use of character sets
to those defined by the IANA registry.
 
3.5 Content Codings
 
Content coding values indicate an encoding transformation that has
been or can be applied to an entity. Content codings are primarily
used to allow a document to be compressed or otherwise usefully
transformed without losing the identity of its underlying media type
and without loss of information. Frequently, the entity is stored in
coded form, transmitted directly, and only decoded by the recipient.
 
content-coding = token
 
All content-coding values are case-insensitive. HTTP/1.1 uses
content-coding values in the Accept-Encoding (section 14.3) and
Content-Encoding (section 14.12) header fields. Although the value
describes the content-coding, what is more important is that it
indicates what decoding mechanism will be required to remove the
encoding.
 
The Internet Assigned Numbers Authority (IANA) acts as a registry for
content-coding value tokens. Initially, the registry contains the
following tokens:
 
gzip An encoding format produced by the file compression program "gzip"
(GNU zip) as described in RFC 1952 [25]. This format is a Lempel-
Ziv coding (LZ77) with a 32 bit CRC.
 
compress
The encoding format produced by the common UNIX file compression
program "compress". This format is an adaptive Lempel-Ziv-Welch
coding (LZW).
 
 
 
 
 
 
 
 
 
Fielding, et. al. Standards Track [Page 23]
RFC 2068 HTTP/1.1 January 1997
 
 
Note: Use of program names for the identification of encoding
formats is not desirable and should be discouraged for future
encodings. Their use here is representative of historical practice,
not good design. For compatibility with previous implementations of
HTTP, applications should consider "x-gzip" and "x-compress" to be
equivalent to "gzip" and "compress" respectively.
 
deflate The "zlib" format defined in RFC 1950[31] in combination with
the "deflate" compression mechanism described in RFC 1951[29].
 
New content-coding value tokens should be registered; to allow
interoperability between clients and servers, specifications of the
content coding algorithms needed to implement a new value should be
publicly available and adequate for independent implementation, and
conform to the purpose of content coding defined in this section.
 
3.6 Transfer Codings
 
Transfer coding values are used to indicate an encoding
transformation that has been, can be, or may need to be applied to an
entity-body in order to ensure "safe transport" through the network.
This differs from a content coding in that the transfer coding is a
property of the message, not of the original entity.
 
transfer-coding = "chunked" | transfer-extension
 
transfer-extension = token
 
All transfer-coding values are case-insensitive. HTTP/1.1 uses
transfer coding values in the Transfer-Encoding header field (section
14.40).
 
Transfer codings are analogous to the Content-Transfer-Encoding
values of MIME , which were designed to enable safe transport of
binary data over a 7-bit transport service. However, safe transport
has a different focus for an 8bit-clean transfer protocol. In HTTP,
the only unsafe characteristic of message-bodies is the difficulty in
determining the exact body length (section 7.2.2), or the desire to
encrypt data over a shared transport.
 
The chunked encoding modifies the body of a message in order to
transfer it as a series of chunks, each with its own size indicator,
followed by an optional footer containing entity-header fields. This
allows dynamically-produced content to be transferred along with the
information necessary for the recipient to verify that it has
received the full message.
 
 
 
 
 
Fielding, et. al. Standards Track [Page 24]
RFC 2068 HTTP/1.1 January 1997
 
 
Chunked-Body = *chunk
"0" CRLF
footer
CRLF
 
chunk = chunk-size [ chunk-ext ] CRLF
chunk-data CRLF
 
hex-no-zero = <HEX excluding "0">
 
chunk-size = hex-no-zero *HEX
chunk-ext = *( ";" chunk-ext-name [ "=" chunk-ext-value ] )
chunk-ext-name = token
chunk-ext-val = token | quoted-string
chunk-data = chunk-size(OCTET)
 
footer = *entity-header
 
The chunked encoding is ended by a zero-sized chunk followed by the
footer, which is terminated by an empty line. The purpose of the
footer is to provide an efficient way to supply information about an
entity that is generated dynamically; applications MUST NOT send
header fields in the footer which are not explicitly defined as being
appropriate for the footer, such as Content-MD5 or future extensions
to HTTP for digital signatures or other facilities.
 
An example process for decoding a Chunked-Body is presented in
appendix 19.4.6.
 
All HTTP/1.1 applications MUST be able to receive and decode the
"chunked" transfer coding, and MUST ignore transfer coding extensions
they do not understand. A server which receives an entity-body with a
transfer-coding it does not understand SHOULD return 501
(Unimplemented), and close the connection. A server MUST NOT send
transfer-codings to an HTTP/1.0 client.
 
3.7 Media Types
 
HTTP uses Internet Media Types in the Content-Type (section 14.18)
and Accept (section 14.1) header fields in order to provide open and
extensible data typing and type negotiation.
 
media-type = type "/" subtype *( ";" parameter )
type = token
subtype = token
 
Parameters may follow the type/subtype in the form of attribute/value
pairs.
 
 
 
Fielding, et. al. Standards Track [Page 25]
RFC 2068 HTTP/1.1 January 1997
 
 
parameter = attribute "=" value
attribute = token
value = token | quoted-string
 
The type, subtype, and parameter attribute names are case-
insensitive. Parameter values may or may not be case-sensitive,
depending on the semantics of the parameter name. Linear white space
(LWS) MUST NOT be used between the type and subtype, nor between an
attribute and its value. User agents that recognize the media-type
MUST process (or arrange to be processed by any external applications
used to process that type/subtype by the user agent) the parameters
for that MIME type as described by that type/subtype definition to
the and inform the user of any problems discovered.
 
Note: some older HTTP applications do not recognize media type
parameters. When sending data to older HTTP applications,
implementations should only use media type parameters when they are
required by that type/subtype definition.
 
Media-type values are registered with the Internet Assigned Number
Authority (IANA). The media type registration process is outlined in
RFC 2048 [17]. Use of non-registered media types is discouraged.
 
3.7.1 Canonicalization and Text Defaults
 
Internet media types are registered with a canonical form. In
general, an entity-body transferred via HTTP messages MUST be
represented in the appropriate canonical form prior to its
transmission; the exception is "text" types, as defined in the next
paragraph.
 
When in canonical form, media subtypes of the "text" type use CRLF as
the text line break. HTTP relaxes this requirement and allows the
transport of text media with plain CR or LF alone representing a line
break when it is done consistently for an entire entity-body. HTTP
applications MUST accept CRLF, bare CR, and bare LF as being
representative of a line break in text media received via HTTP. In
addition, if the text is represented in a character set that does not
use octets 13 and 10 for CR and LF respectively, as is the case for
some multi-byte character sets, HTTP allows the use of whatever octet
sequences are defined by that character set to represent the
equivalent of CR and LF for line breaks. This flexibility regarding
line breaks applies only to text media in the entity-body; a bare CR
or LF MUST NOT be substituted for CRLF within any of the HTTP control
structures (such as header fields and multipart boundaries).
 
If an entity-body is encoded with a Content-Encoding, the underlying
data MUST be in a form defined above prior to being encoded.
 
 
 
Fielding, et. al. Standards Track [Page 26]
RFC 2068 HTTP/1.1 January 1997
 
 
The "charset" parameter is used with some media types to define the
character set (section 3.4) of the data. When no explicit charset
parameter is provided by the sender, media subtypes of the "text"
type are defined to have a default charset value of "ISO-8859-1" when
received via HTTP. Data in character sets other than "ISO-8859-1" or
its subsets MUST be labeled with an appropriate charset value.
 
Some HTTP/1.0 software has interpreted a Content-Type header without
charset parameter incorrectly to mean "recipient should guess."
Senders wishing to defeat this behavior MAY include a charset
parameter even when the charset is ISO-8859-1 and SHOULD do so when
it is known that it will not confuse the recipient.
 
Unfortunately, some older HTTP/1.0 clients did not deal properly with
an explicit charset parameter. HTTP/1.1 recipients MUST respect the
charset label provided by the sender; and those user agents that have
a provision to "guess" a charset MUST use the charset from the
content-type field if they support that charset, rather than the
recipient's preference, when initially displaying a document.
 
3.7.2 Multipart Types
 
MIME provides for a number of "multipart" types -- encapsulations of
one or more entities within a single message-body. All multipart
types share a common syntax, as defined in MIME [7], and MUST
include a boundary parameter as part of the media type value. The
message body is itself a protocol element and MUST therefore use only
CRLF to represent line breaks between body-parts. Unlike in MIME, the
epilogue of any multipart message MUST be empty; HTTP applications
MUST NOT transmit the epilogue (even if the original multipart
contains an epilogue).
 
In HTTP, multipart body-parts MAY contain header fields which are
significant to the meaning of that part. A Content-Location header
field (section 14.15) SHOULD be included in the body-part of each
enclosed entity that can be identified by a URL.
 
In general, an HTTP user agent SHOULD follow the same or similar
behavior as a MIME user agent would upon receipt of a multipart type.
If an application receives an unrecognized multipart subtype, the
application MUST treat it as being equivalent to "multipart/mixed".
 
Note: The "multipart/form-data" type has been specifically defined
for carrying form data suitable for processing via the POST request
method, as described in RFC 1867 [15].
 
 
 
 
 
 
Fielding, et. al. Standards Track [Page 27]
RFC 2068 HTTP/1.1 January 1997
 
 
3.8 Product Tokens
 
Product tokens are used to allow communicating applications to
identify themselves by software name and version. Most fields using
product tokens also allow sub-products which form a significant part
of the application to be listed, separated by whitespace. By
convention, the products are listed in order of their significance
for identifying the application.
 
product = token ["/" product-version]
product-version = token
 
Examples:
 
User-Agent: CERN-LineMode/2.15 libwww/2.17b3
Server: Apache/0.8.4
 
Product tokens should be short and to the point -- use of them for
advertising or other non-essential information is explicitly
forbidden. Although any token character may appear in a product-
version, this token SHOULD only be used for a version identifier
(i.e., successive versions of the same product SHOULD only differ in
the product-version portion of the product value).
 
3.9 Quality Values
 
HTTP content negotiation (section 12) uses short "floating point"
numbers to indicate the relative importance ("weight") of various
negotiable parameters. A weight is normalized to a real number in the
range 0 through 1, where 0 is the minimum and 1 the maximum value.
HTTP/1.1 applications MUST NOT generate more than three digits after
the decimal point. User configuration of these values SHOULD also be
limited in this fashion.
 
qvalue = ( "0" [ "." 0*3DIGIT ] )
| ( "1" [ "." 0*3("0") ] )
 
"Quality values" is a misnomer, since these values merely represent
relative degradation in desired quality.
 
3.10 Language Tags
 
A language tag identifies a natural language spoken, written, or
otherwise conveyed by human beings for communication of information
to other human beings. Computer languages are explicitly excluded.
HTTP uses language tags within the Accept-Language and Content-
Language fields.
 
 
 
 
Fielding, et. al. Standards Track [Page 28]
RFC 2068 HTTP/1.1 January 1997
 
 
The syntax and registry of HTTP language tags is the same as that
defined by RFC 1766 [1]. In summary, a language tag is composed of 1
or more parts: A primary language tag and a possibly empty series of
subtags:
 
language-tag = primary-tag *( "-" subtag )
 
primary-tag = 1*8ALPHA
subtag = 1*8ALPHA
 
Whitespace is not allowed within the tag and all tags are case-
insensitive. The name space of language tags is administered by the
IANA. Example tags include:
 
en, en-US, en-cockney, i-cherokee, x-pig-latin
 
where any two-letter primary-tag is an ISO 639 language abbreviation
and any two-letter initial subtag is an ISO 3166 country code. (The
last three tags above are not registered tags; all but the last are
examples of tags which could be registered in future.)
 
3.11 Entity Tags
 
Entity tags are used for comparing two or more entities from the same
requested resource. HTTP/1.1 uses entity tags in the ETag (section
14.20), If-Match (section 14.25), If-None-Match (section 14.26), and
If-Range (section 14.27) header fields. The definition of how they
are used and compared as cache validators is in section 13.3.3. An
entity tag consists of an opaque quoted string, possibly prefixed by
a weakness indicator.
 
entity-tag = [ weak ] opaque-tag
 
weak = "W/"
opaque-tag = quoted-string
 
A "strong entity tag" may be shared by two entities of a resource
only if they are equivalent by octet equality.
 
A "weak entity tag," indicated by the "W/" prefix, may be shared by
two entities of a resource only if the entities are equivalent and
could be substituted for each other with no significant change in
semantics. A weak entity tag can only be used for weak comparison.
 
An entity tag MUST be unique across all versions of all entities
associated with a particular resource. A given entity tag value may
be used for entities obtained by requests on different URIs without
implying anything about the equivalence of those entities.
 
 
 
Fielding, et. al. Standards Track [Page 29]
RFC 2068 HTTP/1.1 January 1997
 
 
3.12 Range Units
 
HTTP/1.1 allows a client to request that only part (a range of) the
response entity be included within the response. HTTP/1.1 uses range
units in the Range (section 14.36) and Content-Range (section 14.17)
header fields. An entity may be broken down into subranges according
to various structural units.
 
range-unit = bytes-unit | other-range-unit
 
bytes-unit = "bytes"
other-range-unit = token
 
The only range unit defined by HTTP/1.1 is "bytes". HTTP/1.1
implementations may ignore ranges specified using other units.
HTTP/1.1 has been designed to allow implementations of applications
that do not depend on knowledge of ranges.
 
4 HTTP Message
 
4.1 Message Types
 
HTTP messages consist of requests from client to server and responses
from server to client.
 
HTTP-message = Request | Response ; HTTP/1.1 messages
 
Request (section 5) and Response (section 6) messages use the generic
message format of RFC 822 [9] for transferring entities (the payload
of the message). Both types of message consist of a start-line, one
or more header fields (also known as "headers"), an empty line (i.e.,
a line with nothing preceding the CRLF) indicating the end of the
header fields, and an optional message-body.
 
generic-message = start-line
*message-header
CRLF
[ message-body ]
 
start-line = Request-Line | Status-Line
 
In the interest of robustness, servers SHOULD ignore any empty
line(s) received where a Request-Line is expected. In other words, if
the server is reading the protocol stream at the beginning of a
message and receives a CRLF first, it should ignore the CRLF.
 
 
 
 
 
 
Fielding, et. al. Standards Track [Page 30]
RFC 2068 HTTP/1.1 January 1997
 
 
Note: certain buggy HTTP/1.0 client implementations generate an
extra CRLF's after a POST request. To restate what is explicitly
forbidden by the BNF, an HTTP/1.1 client must not preface or follow
a request with an extra CRLF.
 
4.2 Message Headers
 
HTTP header fields, which include general-header (section 4.5),
request-header (section 5.3), response-header (section 6.2), and
entity-header (section 7.1) fields, follow the same generic format as
that given in Section 3.1 of RFC 822 [9]. Each header field consists
of a name followed by a colon (":") and the field value. Field names
are case-insensitive. The field value may be preceded by any amount
of LWS, though a single SP is preferred. Header fields can be
extended over multiple lines by preceding each extra line with at
least one SP or HT. Applications SHOULD follow "common form" when
generating HTTP constructs, since there might exist some
implementations that fail to accept anything beyond the common forms.
 
message-header = field-name ":" [ field-value ] CRLF
 
field-name = token
field-value = *( field-content | LWS )
 
field-content = <the OCTETs making up the field-value
and consisting of either *TEXT or combinations
of token, tspecials, and quoted-string>
 
The order in which header fields with differing field names are
received is not significant. However, it is "good practice" to send
general-header fields first, followed by request-header or response-
header fields, and ending with the entity-header fields.
 
Multiple message-header fields with the same field-name may be
present in a message if and only if the entire field-value for that
header field is defined as a comma-separated list [i.e., #(values)].
It MUST be possible to combine the multiple header fields into one
"field-name: field-value" pair, without changing the semantics of the
message, by appending each subsequent field-value to the first, each
separated by a comma. The order in which header fields with the same
field-name are received is therefore significant to the
interpretation of the combined field value, and thus a proxy MUST NOT
change the order of these field values when a message is forwarded.
 
 
 
 
 
 
 
 
Fielding, et. al. Standards Track [Page 31]
RFC 2068 HTTP/1.1 January 1997
 
 
4.3 Message Body
 
The message-body (if any) of an HTTP message is used to carry the
entity-body associated with the request or response. The message-body
differs from the entity-body only when a transfer coding has been
applied, as indicated by the Transfer-Encoding header field (section
14.40).
 
message-body = entity-body
| <entity-body encoded as per Transfer-Encoding>
 
Transfer-Encoding MUST be used to indicate any transfer codings
applied by an application to ensure safe and proper transfer of the
message. Transfer-Encoding is a property of the message, not of the
entity, and thus can be added or removed by any application along the
request/response chain.
 
The rules for when a message-body is allowed in a message differ for
requests and responses.
 
The presence of a message-body in a request is signaled by the
inclusion of a Content-Length or Transfer-Encoding header field in
the request's message-headers. A message-body MAY be included in a
request only when the request method (section 5.1.1) allows an
entity-body.
 
For response messages, whether or not a message-body is included with
a message is dependent on both the request method and the response
status code (section 6.1.1). All responses to the HEAD request method
MUST NOT include a message-body, even though the presence of entity-
header fields might lead one to believe they do. All 1xx
(informational), 204 (no content), and 304 (not modified) responses
MUST NOT include a message-body. All other responses do include a
message-body, although it may be of zero length.
 
4.4 Message Length
 
When a message-body is included with a message, the length of that
body is determined by one of the following (in order of precedence):
 
1. Any response message which MUST NOT include a message-body
(such as the 1xx, 204, and 304 responses and any response to a HEAD
request) is always terminated by the first empty line after the
header fields, regardless of the entity-header fields present in the
message.
 
2. If a Transfer-Encoding header field (section 14.40) is present and
indicates that the "chunked" transfer coding has been applied, then
 
 
 
Fielding, et. al. Standards Track [Page 32]
RFC 2068 HTTP/1.1 January 1997
 
 
the length is defined by the chunked encoding (section 3.6).
 
3. If a Content-Length header field (section 14.14) is present, its
value in bytes represents the length of the message-body.
 
4. If the message uses the media type "multipart/byteranges", which is
self-delimiting, then that defines the length. This media type MUST
NOT be used unless the sender knows that the recipient can parse it;
the presence in a request of a Range header with multiple byte-range
specifiers implies that the client can parse multipart/byteranges
responses.
 
5. By the server closing the connection. (Closing the connection
cannot be used to indicate the end of a request body, since that
would leave no possibility for the server to send back a response.)
 
For compatibility with HTTP/1.0 applications, HTTP/1.1 requests
containing a message-body MUST include a valid Content-Length header
field unless the server is known to be HTTP/1.1 compliant. If a
request contains a message-body and a Content-Length is not given,
the server SHOULD respond with 400 (bad request) if it cannot
determine the length of the message, or with 411 (length required) if
it wishes to insist on receiving a valid Content-Length.
 
All HTTP/1.1 applications that receive entities MUST accept the
"chunked" transfer coding (section 3.6), thus allowing this mechanism
to be used for messages when the message length cannot be determined
in advance.
 
Messages MUST NOT include both a Content-Length header field and the
"chunked" transfer coding. If both are received, the Content-Length
MUST be ignored.
 
When a Content-Length is given in a message where a message-body is
allowed, its field value MUST exactly match the number of OCTETs in
the message-body. HTTP/1.1 user agents MUST notify the user when an
invalid length is received and detected.
 
 
 
 
 
 
 
 
 
 
 
 
 
 
Fielding, et. al. Standards Track [Page 33]
RFC 2068 HTTP/1.1 January 1997
 
 
4.5 General Header Fields
 
There are a few header fields which have general applicability for
both request and response messages, but which do not apply to the
entity being transferred. These header fields apply only to the
message being transmitted.
 
general-header = Cache-Control ; Section 14.9
| Connection ; Section 14.10
| Date ; Section 14.19
| Pragma ; Section 14.32
| Transfer-Encoding ; Section 14.40
| Upgrade ; Section 14.41
| Via ; Section 14.44
 
General-header field names can be extended reliably only in
combination with a change in the protocol version. However, new or
experimental header fields may be given the semantics of general
header fields if all parties in the communication recognize them to
be general-header fields. Unrecognized header fields are treated as
entity-header fields.
 
5 Request
 
A request message from a client to a server includes, within the
first line of that message, the method to be applied to the resource,
the identifier of the resource, and the protocol version in use.
 
Request = Request-Line ; Section 5.1
*( general-header ; Section 4.5
| request-header ; Section 5.3
| entity-header ) ; Section 7.1
CRLF
[ message-body ] ; Section 7.2
 
5.1 Request-Line
 
The Request-Line begins with a method token, followed by the
Request-URI and the protocol version, and ending with CRLF. The
elements are separated by SP characters. No CR or LF are allowed
except in the final CRLF sequence.
 
Request-Line = Method SP Request-URI SP HTTP-Version CRLF
 
 
 
 
 
 
 
 
Fielding, et. al. Standards Track [Page 34]
RFC 2068 HTTP/1.1 January 1997
 
 
5.1.1 Method
 
The Method token indicates the method to be performed on the resource
identified by the Request-URI. The method is case-sensitive.
 
Method = "OPTIONS" ; Section 9.2
| "GET" ; Section 9.3
| "HEAD" ; Section 9.4
| "POST" ; Section 9.5
| "PUT" ; Section 9.6
| "DELETE" ; Section 9.7
| "TRACE" ; Section 9.8
| extension-method
 
extension-method = token
 
The list of methods allowed by a resource can be specified in an
Allow header field (section 14.7). The return code of the response
always notifies the client whether a method is currently allowed on a
resource, since the set of allowed methods can change dynamically.
Servers SHOULD return the status code 405 (Method Not Allowed) if the
method is known by the server but not allowed for the requested
resource, and 501 (Not Implemented) if the method is unrecognized or
not implemented by the server. The list of methods known by a server
can be listed in a Public response-header field (section 14.35).
 
The methods GET and HEAD MUST be supported by all general-purpose
servers. All other methods are optional; however, if the above
methods are implemented, they MUST be implemented with the same
semantics as those specified in section 9.
 
5.1.2 Request-URI
 
The Request-URI is a Uniform Resource Identifier (section 3.2) and
identifies the resource upon which to apply the request.
 
Request-URI = "*" | absoluteURI | abs_path
 
The three options for Request-URI are dependent on the nature of the
request. The asterisk "*" means that the request does not apply to a
particular resource, but to the server itself, and is only allowed
when the method used does not necessarily apply to a resource. One
example would be
 
OPTIONS * HTTP/1.1
 
The absoluteURI form is required when the request is being made to a
proxy. The proxy is requested to forward the request or service it
 
 
 
Fielding, et. al. Standards Track [Page 35]
RFC 2068 HTTP/1.1 January 1997
 
 
from a valid cache, and return the response. Note that the proxy MAY
forward the request on to another proxy or directly to the server
specified by the absoluteURI. In order to avoid request loops, a
proxy MUST be able to recognize all of its server names, including
any aliases, local variations, and the numeric IP address. An example
Request-Line would be:
 
GET http://www.w3.org/pub/WWW/TheProject.html HTTP/1.1
 
To allow for transition to absoluteURIs in all requests in future
versions of HTTP, all HTTP/1.1 servers MUST accept the absoluteURI
form in requests, even though HTTP/1.1 clients will only generate
them in requests to proxies.
 
The most common form of Request-URI is that used to identify a
resource on an origin server or gateway. In this case the absolute
path of the URI MUST be transmitted (see section 3.2.1, abs_path) as
the Request-URI, and the network location of the URI (net_loc) MUST
be transmitted in a Host header field. For example, a client wishing
to retrieve the resource above directly from the origin server would
create a TCP connection to port 80 of the host "www.w3.org" and send
the lines:
 
GET /pub/WWW/TheProject.html HTTP/1.1
Host: www.w3.org
 
followed by the remainder of the Request. Note that the absolute path
cannot be empty; if none is present in the original URI, it MUST be
given as "/" (the server root).
 
If a proxy receives a request without any path in the Request-URI and
the method specified is capable of supporting the asterisk form of
request, then the last proxy on the request chain MUST forward the
request with "*" as the final Request-URI. For example, the request
 
OPTIONS http://www.ics.uci.edu:8001 HTTP/1.1
 
would be forwarded by the proxy as
 
OPTIONS * HTTP/1.1
Host: www.ics.uci.edu:8001
 
after connecting to port 8001 of host "www.ics.uci.edu".
 
The Request-URI is transmitted in the format specified in section
3.2.1. The origin server MUST decode the Request-URI in order to
properly interpret the request. Servers SHOULD respond to invalid
Request-URIs with an appropriate status code.
 
 
 
Fielding, et. al. Standards Track [Page 36]
RFC 2068 HTTP/1.1 January 1997
 
 
In requests that they forward, proxies MUST NOT rewrite the
"abs_path" part of a Request-URI in any way except as noted above to
replace a null abs_path with "*", no matter what the proxy does in
its internal implementation.
 
Note: The "no rewrite" rule prevents the proxy from changing the
meaning of the request when the origin server is improperly using a
non-reserved URL character for a reserved purpose. Implementers
should be aware that some pre-HTTP/1.1 proxies have been known to
rewrite the Request-URI.
 
5.2 The Resource Identified by a Request
 
HTTP/1.1 origin servers SHOULD be aware that the exact resource
identified by an Internet request is determined by examining both the
Request-URI and the Host header field.
 
An origin server that does not allow resources to differ by the
requested host MAY ignore the Host header field value. (But see
section 19.5.1 for other requirements on Host support in HTTP/1.1.)
 
An origin server that does differentiate resources based on the host
requested (sometimes referred to as virtual hosts or vanity
hostnames) MUST use the following rules for determining the requested
resource on an HTTP/1.1 request:
 
1. If Request-URI is an absoluteURI, the host is part of the
Request-URI. Any Host header field value in the request MUST be
ignored.
 
2. If the Request-URI is not an absoluteURI, and the request
includes a Host header field, the host is determined by the Host
header field value.
 
3. If the host as determined by rule 1 or 2 is not a valid host on
the server, the response MUST be a 400 (Bad Request) error
message.
 
Recipients of an HTTP/1.0 request that lacks a Host header field MAY
attempt to use heuristics (e.g., examination of the URI path for
something unique to a particular host) in order to determine what
exact resource is being requested.
 
5.3 Request Header Fields
 
The request-header fields allow the client to pass additional
information about the request, and about the client itself, to the
server. These fields act as request modifiers, with semantics
 
 
 
Fielding, et. al. Standards Track [Page 37]
RFC 2068 HTTP/1.1 January 1997
 
 
equivalent to the parameters on a programming language method
invocation.
 
request-header = Accept ; Section 14.1
| Accept-Charset ; Section 14.2
| Accept-Encoding ; Section 14.3
| Accept-Language ; Section 14.4
| Authorization ; Section 14.8
| From ; Section 14.22
| Host ; Section 14.23
| If-Modified-Since ; Section 14.24
| If-Match ; Section 14.25
| If-None-Match ; Section 14.26
| If-Range ; Section 14.27
| If-Unmodified-Since ; Section 14.28
| Max-Forwards ; Section 14.31
| Proxy-Authorization ; Section 14.34
| Range ; Section 14.36
| Referer ; Section 14.37
| User-Agent ; Section 14.42
 
Request-header field names can be extended reliably only in
combination with a change in the protocol version. However, new or
experimental header fields MAY be given the semantics of request-
header fields if all parties in the communication recognize them to
be request-header fields. Unrecognized header fields are treated as
entity-header fields.
 
6 Response
 
After receiving and interpreting a request message, a server responds
with an HTTP response message.
 
Response = Status-Line ; Section 6.1
*( general-header ; Section 4.5
| response-header ; Section 6.2
| entity-header ) ; Section 7.1
CRLF
[ message-body ] ; Section 7.2
 
6.1 Status-Line
 
The first line of a Response message is the Status-Line, consisting
of the protocol version followed by a numeric status code and its
associated textual phrase, with each element separated by SP
characters. No CR or LF is allowed except in the final CRLF
sequence.
 
 
 
 
Fielding, et. al. Standards Track [Page 38]
RFC 2068 HTTP/1.1 January 1997
 
 
Status-Line = HTTP-Version SP Status-Code SP Reason-Phrase CRLF
 
6.1.1 Status Code and Reason Phrase
 
The Status-Code element is a 3-digit integer result code of the
attempt to understand and satisfy the request. These codes are fully
defined in section 10. The Reason-Phrase is intended to give a short
textual description of the Status-Code. The Status-Code is intended
for use by automata and the Reason-Phrase is intended for the human
user. The client is not required to examine or display the Reason-
Phrase.
 
The first digit of the Status-Code defines the class of response. The
last two digits do not have any categorization role. There are 5
values for the first digit:
 
o 1xx: Informational - Request received, continuing process
 
o 2xx: Success - The action was successfully received, understood,
and accepted
 
o 3xx: Redirection - Further action must be taken in order to
complete the request
 
o 4xx: Client Error - The request contains bad syntax or cannot be
fulfilled
 
o 5xx: Server Error - The server failed to fulfill an apparently
valid request
 
The individual values of the numeric status codes defined for
HTTP/1.1, and an example set of corresponding Reason-Phrase's, are
presented below. The reason phrases listed here are only recommended
-- they may be replaced by local equivalents without affecting the
protocol.
 
Status-Code = "100" ; Continue
| "101" ; Switching Protocols
| "200" ; OK
| "201" ; Created
| "202" ; Accepted
| "203" ; Non-Authoritative Information
| "204" ; No Content
| "205" ; Reset Content
| "206" ; Partial Content
| "300" ; Multiple Choices
| "301" ; Moved Permanently
| "302" ; Moved Temporarily
 
 
 
Fielding, et. al. Standards Track [Page 39]
RFC 2068 HTTP/1.1 January 1997
 
 
| "303" ; See Other
| "304" ; Not Modified
| "305" ; Use Proxy
| "400" ; Bad Request
| "401" ; Unauthorized
| "402" ; Payment Required
| "403" ; Forbidden
| "404" ; Not Found
| "405" ; Method Not Allowed
| "406" ; Not Acceptable
| "407" ; Proxy Authentication Required
| "408" ; Request Time-out
| "409" ; Conflict
| "410" ; Gone
| "411" ; Length Required
| "412" ; Precondition Failed
| "413" ; Request Entity Too Large
| "414" ; Request-URI Too Large
| "415" ; Unsupported Media Type
| "500" ; Internal Server Error
| "501" ; Not Implemented
| "502" ; Bad Gateway
| "503" ; Service Unavailable
| "504" ; Gateway Time-out
| "505" ; HTTP Version not supported
| extension-code
 
extension-code = 3DIGIT
 
Reason-Phrase = *<TEXT, excluding CR, LF>
 
HTTP status codes are extensible. HTTP applications are not required
to understand the meaning of all registered status codes, though such
understanding is obviously desirable. However, applications MUST
understand the class of any status code, as indicated by the first
digit, and treat any unrecognized response as being equivalent to the
x00 status code of that class, with the exception that an
unrecognized response MUST NOT be cached. For example, if an
unrecognized status code of 431 is received by the client, it can
safely assume that there was something wrong with its request and
treat the response as if it had received a 400 status code. In such
cases, user agents SHOULD present to the user the entity returned
with the response, since that entity is likely to include human-
readable information which will explain the unusual status.
 
 
 
 
 
 
 
Fielding, et. al. Standards Track [Page 40]
RFC 2068 HTTP/1.1 January 1997
 
 
6.2 Response Header Fields
 
The response-header fields allow the server to pass additional
information about the response which cannot be placed in the Status-
Line. These header fields give information about the server and about
further access to the resource identified by the Request-URI.
 
response-header = Age ; Section 14.6
| Location ; Section 14.30
| Proxy-Authenticate ; Section 14.33
| Public ; Section 14.35
| Retry-After ; Section 14.38
| Server ; Section 14.39
| Vary ; Section 14.43
| Warning ; Section 14.45
| WWW-Authenticate ; Section 14.46
 
Response-header field names can be extended reliably only in
combination with a change in the protocol version. However, new or
experimental header fields MAY be given the semantics of response-
header fields if all parties in the communication recognize them to
be response-header fields. Unrecognized header fields are treated as
entity-header fields.
 
7 Entity
 
Request and Response messages MAY transfer an entity if not otherwise
restricted by the request method or response status code. An entity
consists of entity-header fields and an entity-body, although some
responses will only include the entity-headers.
 
In this section, both sender and recipient refer to either the client
or the server, depending on who sends and who receives the entity.
 
7.1 Entity Header Fields
 
Entity-header fields define optional metainformation about the
entity-body or, if no body is present, about the resource identified
by the request.
 
 
 
 
 
 
 
 
 
 
 
 
Fielding, et. al. Standards Track [Page 41]
RFC 2068 HTTP/1.1 January 1997
 
 
entity-header = Allow ; Section 14.7
| Content-Base ; Section 14.11
| Content-Encoding ; Section 14.12
| Content-Language ; Section 14.13
| Content-Length ; Section 14.14
| Content-Location ; Section 14.15
| Content-MD5 ; Section 14.16
| Content-Range ; Section 14.17
| Content-Type ; Section 14.18
| ETag ; Section 14.20
| Expires ; Section 14.21
| Last-Modified ; Section 14.29
| extension-header
 
extension-header = message-header
 
The extension-header mechanism allows additional entity-header fields
to be defined without changing the protocol, but these fields cannot
be assumed to be recognizable by the recipient. Unrecognized header
fields SHOULD be ignored by the recipient and forwarded by proxies.
 
7.2 Entity Body
 
The entity-body (if any) sent with an HTTP request or response is in
a format and encoding defined by the entity-header fields.
 
entity-body = *OCTET
 
An entity-body is only present in a message when a message-body is
present, as described in section 4.3. The entity-body is obtained
from the message-body by decoding any Transfer-Encoding that may have
been applied to ensure safe and proper transfer of the message.
 
7.2.1 Type
 
When an entity-body is included with a message, the data type of that
body is determined via the header fields Content-Type and Content-
Encoding. These define a two-layer, ordered encoding model:
 
entity-body := Content-Encoding( Content-Type( data ) )
 
Content-Type specifies the media type of the underlying data.
Content-Encoding may be used to indicate any additional content
codings applied to the data, usually for the purpose of data
compression, that are a property of the requested resource. There is
no default encoding.
 
 
 
 
 
Fielding, et. al. Standards Track [Page 42]
RFC 2068 HTTP/1.1 January 1997
 
 
Any HTTP/1.1 message containing an entity-body SHOULD include a
Content-Type header field defining the media type of that body. If
and only if the media type is not given by a Content-Type field, the
recipient MAY attempt to guess the media type via inspection of its
content and/or the name extension(s) of the URL used to identify the
resource. If the media type remains unknown, the recipient SHOULD
treat it as type "application/octet-stream".
 
7.2.2 Length
 
The length of an entity-body is the length of the message-body after
any transfer codings have been removed. Section 4.4 defines how the
length of a message-body is determined.
 
8 Connections
 
8.1 Persistent Connections
 
8.1.1 Purpose
 
Prior to persistent connections, a separate TCP connection was
established to fetch each URL, increasing the load on HTTP servers
and causing congestion on the Internet. The use of inline images and
other associated data often requires a client to make multiple
requests of the same server in a short amount of time. Analyses of
these performance problems are available [30][27]; analysis and
results from a prototype implementation are in [26].
 
Persistent HTTP connections have a number of advantages:
 
o By opening and closing fewer TCP connections, CPU time is saved,
and memory used for TCP protocol control blocks is also saved.
o HTTP requests and responses can be pipelined on a connection.
Pipelining allows a client to make multiple requests without
waiting for each response, allowing a single TCP connection to be
used much more efficiently, with much lower elapsed time.
o Network congestion is reduced by reducing the number of packets
caused by TCP opens, and by allowing TCP sufficient time to
determine the congestion state of the network.
o HTTP can evolve more gracefully; since errors can be reported
without the penalty of closing the TCP connection. Clients using
future versions of HTTP might optimistically try a new feature, but
if communicating with an older server, retry with old semantics
after an error is reported.
 
HTTP implementations SHOULD implement persistent connections.
 
 
 
 
 
Fielding, et. al. Standards Track [Page 43]
RFC 2068 HTTP/1.1 January 1997
 
 
8.1.2 Overall Operation
 
A significant difference between HTTP/1.1 and earlier versions of
HTTP is that persistent connections are the default behavior of any
HTTP connection. That is, unless otherwise indicated, the client may
assume that the server will maintain a persistent connection.
 
Persistent connections provide a mechanism by which a client and a
server can signal the close of a TCP connection. This signaling takes
place using the Connection header field. Once a close has been
signaled, the client MUST not send any more requests on that
connection.
 
8.1.2.1 Negotiation
 
An HTTP/1.1 server MAY assume that a HTTP/1.1 client intends to
maintain a persistent connection unless a Connection header including
the connection-token "close" was sent in the request. If the server
chooses to close the connection immediately after sending the
response, it SHOULD send a Connection header including the
connection-token close.
 
An HTTP/1.1 client MAY expect a connection to remain open, but would
decide to keep it open based on whether the response from a server
contains a Connection header with the connection-token close. In case
the client does not want to maintain a connection for more than that
request, it SHOULD send a Connection header including the
connection-token close.
 
If either the client or the server sends the close token in the
Connection header, that request becomes the last one for the
connection.
 
Clients and servers SHOULD NOT assume that a persistent connection is
maintained for HTTP versions less than 1.1 unless it is explicitly
signaled. See section 19.7.1 for more information on backwards
compatibility with HTTP/1.0 clients.
 
In order to remain persistent, all messages on the connection must
have a self-defined message length (i.e., one not defined by closure
of the connection), as described in section 4.4.
 
8.1.2.2 Pipelining
 
A client that supports persistent connections MAY "pipeline" its
requests (i.e., send multiple requests without waiting for each
response). A server MUST send its responses to those requests in the
same order that the requests were received.
 
 
 
Fielding, et. al. Standards Track [Page 44]
RFC 2068 HTTP/1.1 January 1997
 
 
Clients which assume persistent connections and pipeline immediately
after connection establishment SHOULD be prepared to retry their
connection if the first pipelined attempt fails. If a client does
such a retry, it MUST NOT pipeline before it knows the connection is
persistent. Clients MUST also be prepared to resend their requests if
the server closes the connection before sending all of the
corresponding responses.
 
8.1.3 Proxy Servers
 
It is especially important that proxies correctly implement the
properties of the Connection header field as specified in 14.2.1.
 
The proxy server MUST signal persistent connections separately with
its clients and the origin servers (or other proxy servers) that it
connects to. Each persistent connection applies to only one transport
link.
 
A proxy server MUST NOT establish a persistent connection with an
HTTP/1.0 client.
 
8.1.4 Practical Considerations
 
Servers will usually have some time-out value beyond which they will
no longer maintain an inactive connection. Proxy servers might make
this a higher value since it is likely that the client will be making
more connections through the same server. The use of persistent
connections places no requirements on the length of this time-out for
either the client or the server.
 
When a client or server wishes to time-out it SHOULD issue a graceful
close on the transport connection. Clients and servers SHOULD both
constantly watch for the other side of the transport close, and
respond to it as appropriate. If a client or server does not detect
the other side's close promptly it could cause unnecessary resource
drain on the network.
 
A client, server, or proxy MAY close the transport connection at any
time. For example, a client MAY have started to send a new request at
the same time that the server has decided to close the "idle"
connection. From the server's point of view, the connection is being
closed while it was idle, but from the client's point of view, a
request is in progress.
 
This means that clients, servers, and proxies MUST be able to recover
from asynchronous close events. Client software SHOULD reopen the
transport connection and retransmit the aborted request without user
interaction so long as the request method is idempotent (see section
 
 
 
Fielding, et. al. Standards Track [Page 45]
RFC 2068 HTTP/1.1 January 1997
 
 
9.1.2); other methods MUST NOT be automatically retried, although
user agents MAY offer a human operator the choice of retrying the
request.
 
However, this automatic retry SHOULD NOT be repeated if the second
request fails.
 
Servers SHOULD always respond to at least one request per connection,
if at all possible. Servers SHOULD NOT close a connection in the
middle of transmitting a response, unless a network or client failure
is suspected.
 
Clients that use persistent connections SHOULD limit the number of
simultaneous connections that they maintain to a given server. A
single-user client SHOULD maintain AT MOST 2 connections with any
server or proxy. A proxy SHOULD use up to 2*N connections to another
server or proxy, where N is the number of simultaneously active
users. These guidelines are intended to improve HTTP response times
and avoid congestion of the Internet or other networks.
 
8.2 Message Transmission Requirements
 
General requirements:
 
o HTTP/1.1 servers SHOULD maintain persistent connections and use
TCP's flow control mechanisms to resolve temporary overloads,
rather than terminating connections with the expectation that
clients will retry. The latter technique can exacerbate network
congestion.
 
o An HTTP/1.1 (or later) client sending a message-body SHOULD monitor
the network connection for an error status while it is transmitting
the request. If the client sees an error status, it SHOULD
immediately cease transmitting the body. If the body is being sent
using a "chunked" encoding (section 3.6), a zero length chunk and
empty footer MAY be used to prematurely mark the end of the
message. If the body was preceded by a Content-Length header, the
client MUST close the connection.
 
o An HTTP/1.1 (or later) client MUST be prepared to accept a 100
(Continue) status followed by a regular response.
 
o An HTTP/1.1 (or later) server that receives a request from a
HTTP/1.0 (or earlier) client MUST NOT transmit the 100 (continue)
response; it SHOULD either wait for the request to be completed
normally (thus avoiding an interrupted request) or close the
connection prematurely.
 
 
 
 
Fielding, et. al. Standards Track [Page 46]
RFC 2068 HTTP/1.1 January 1997
 
 
Upon receiving a method subject to these requirements from an
HTTP/1.1 (or later) client, an HTTP/1.1 (or later) server MUST either
respond with 100 (Continue) status and continue to read from the
input stream, or respond with an error status. If it responds with an
error status, it MAY close the transport (TCP) connection or it MAY
continue to read and discard the rest of the request. It MUST NOT
perform the requested method if it returns an error status.
 
Clients SHOULD remember the version number of at least the most
recently used server; if an HTTP/1.1 client has seen an HTTP/1.1 or
later response from the server, and it sees the connection close
before receiving any status from the server, the client SHOULD retry
the request without user interaction so long as the request method is
idempotent (see section 9.1.2); other methods MUST NOT be
automatically retried, although user agents MAY offer a human
operator the choice of retrying the request.. If the client does
retry the request, the client
 
o MUST first send the request header fields, and then
 
o MUST wait for the server to respond with either a 100 (Continue)
response, in which case the client should continue, or with an
error status.
 
If an HTTP/1.1 client has not seen an HTTP/1.1 or later response from
the server, it should assume that the server implements HTTP/1.0 or
older and will not use the 100 (Continue) response. If in this case
the client sees the connection close before receiving any status from
the server, the client SHOULD retry the request. If the client does
retry the request to this HTTP/1.0 server, it should use the
following "binary exponential backoff" algorithm to be assured of
obtaining a reliable response:
 
1. Initiate a new connection to the server
 
2. Transmit the request-headers
 
3. Initialize a variable R to the estimated round-trip time to the
server (e.g., based on the time it took to establish the
connection), or to a constant value of 5 seconds if the round-trip
time is not available.
 
4. Compute T = R * (2**N), where N is the number of previous retries
of this request.
 
5. Wait either for an error response from the server, or for T seconds
(whichever comes first)
 
 
 
 
Fielding, et. al. Standards Track [Page 47]
RFC 2068 HTTP/1.1 January 1997
 
 
6. If no error response is received, after T seconds transmit the body
of the request.
 
7. If client sees that the connection is closed prematurely, repeat
from step 1 until the request is accepted, an error response is
received, or the user becomes impatient and terminates the retry
process.
 
No matter what the server version, if an error status is received,
the client
 
o MUST NOT continue and
 
o MUST close the connection if it has not completed sending the
message.
 
An HTTP/1.1 (or later) client that sees the connection close after
receiving a 100 (Continue) but before receiving any other status
SHOULD retry the request, and need not wait for 100 (Continue)
response (but MAY do so if this simplifies the implementation).
 
9 Method Definitions
 
The set of common methods for HTTP/1.1 is defined below. Although
this set can be expanded, additional methods cannot be assumed to
share the same semantics for separately extended clients and servers.
 
The Host request-header field (section 14.23) MUST accompany all
HTTP/1.1 requests.
 
9.1 Safe and Idempotent Methods
 
9.1.1 Safe Methods
 
Implementers should be aware that the software represents the user in
their interactions over the Internet, and should be careful to allow
the user to be aware of any actions they may take which may have an
unexpected significance to themselves or others.
 
In particular, the convention has been established that the GET and
HEAD methods should never have the significance of taking an action
other than retrieval. These methods should be considered "safe." This
allows user agents to represent other methods, such as POST, PUT and
DELETE, in a special way, so that the user is made aware of the fact
that a possibly unsafe action is being requested.
 
Naturally, it is not possible to ensure that the server does not
generate side-effects as a result of performing a GET request; in
 
 
 
Fielding, et. al. Standards Track [Page 48]
RFC 2068 HTTP/1.1 January 1997
 
 
fact, some dynamic resources consider that a feature. The important
distinction here is that the user did not request the side-effects,
so therefore cannot be held accountable for them.
 
9.1.2 Idempotent Methods
 
Methods may also have the property of "idempotence" in that (aside
from error or expiration issues) the side-effects of N > 0 identical
requests is the same as for a single request. The methods GET, HEAD,
PUT and DELETE share this property.
 
9.2 OPTIONS
 
The OPTIONS method represents a request for information about the
communication options available on the request/response chain
identified by the Request-URI. This method allows the client to
determine the options and/or requirements associated with a resource,
or the capabilities of a server, without implying a resource action
or initiating a resource retrieval.
 
Unless the server's response is an error, the response MUST NOT
include entity information other than what can be considered as
communication options (e.g., Allow is appropriate, but Content-Type
is not). Responses to this method are not cachable.
 
If the Request-URI is an asterisk ("*"), the OPTIONS request is
intended to apply to the server as a whole. A 200 response SHOULD
include any header fields which indicate optional features
implemented by the server (e.g., Public), including any extensions
not defined by this specification, in addition to any applicable
general or response-header fields. As described in section 5.1.2, an
"OPTIONS *" request can be applied through a proxy by specifying the
destination server in the Request-URI without any path information.
 
If the Request-URI is not an asterisk, the OPTIONS request applies
only to the options that are available when communicating with that
resource. A 200 response SHOULD include any header fields which
indicate optional features implemented by the server and applicable
to that resource (e.g., Allow), including any extensions not defined
by this specification, in addition to any applicable general or
response-header fields. If the OPTIONS request passes through a
proxy, the proxy MUST edit the response to exclude those options
which apply to a proxy's capabilities and which are known to be
unavailable through that proxy.
 
 
 
 
 
 
 
Fielding, et. al. Standards Track [Page 49]
RFC 2068 HTTP/1.1 January 1997
 
 
9.3 GET
 
The GET method means retrieve whatever information (in the form of an
entity) is identified by the Request-URI. If the Request-URI refers
to a data-producing process, it is the produced data which shall be
returned as the entity in the response and not the source text of the
process, unless that text happens to be the output of the process.
 
The semantics of the GET method change to a "conditional GET" if the
request message includes an If-Modified-Since, If-Unmodified-Since,
If-Match, If-None-Match, or If-Range header field. A conditional GET
method requests that the entity be transferred only under the
circumstances described by the conditional header field(s). The
conditional GET method is intended to reduce unnecessary network
usage by allowing cached entities to be refreshed without requiring
multiple requests or transferring data already held by the client.
 
The semantics of the GET method change to a "partial GET" if the
request message includes a Range header field. A partial GET requests
that only part of the entity be transferred, as described in section
14.36. The partial GET method is intended to reduce unnecessary
network usage by allowing partially-retrieved entities to be
completed without transferring data already held by the client.
 
The response to a GET request is cachable if and only if it meets the
requirements for HTTP caching described in section 13.
 
9.4 HEAD
 
The HEAD method is identical to GET except that the server MUST NOT
return a message-body in the response. The metainformation contained
in the HTTP headers in response to a HEAD request SHOULD be identical
to the information sent in response to a GET request. This method can
be used for obtaining metainformation about the entity implied by the
request without transferring the entity-body itself. This method is
often used for testing hypertext links for validity, accessibility,
and recent modification.
 
The response to a HEAD request may be cachable in the sense that the
information contained in the response may be used to update a
previously cached entity from that resource. If the new field values
indicate that the cached entity differs from the current entity (as
would be indicated by a change in Content-Length, Content-MD5, ETag
or Last-Modified), then the cache MUST treat the cache entry as
stale.
 
 
 
 
 
 
Fielding, et. al. Standards Track [Page 50]
RFC 2068 HTTP/1.1 January 1997
 
 
9.5 POST
 
The POST method is used to request that the destination server accept
the entity enclosed in the request as a new subordinate of the
resource identified by the Request-URI in the Request-Line. POST is
designed to allow a uniform method to cover the following functions:
 
o Annotation of existing resources;
 
o Posting a message to a bulletin board, newsgroup, mailing list,
or similar group of articles;
 
o Providing a block of data, such as the result of submitting a
form, to a data-handling process;
 
o Extending a database through an append operation.
 
The actual function performed by the POST method is determined by the
server and is usually dependent on the Request-URI. The posted entity
is subordinate to that URI in the same way that a file is subordinate
to a directory containing it, a news article is subordinate to a
newsgroup to which it is posted, or a record is subordinate to a
database.
 
The action performed by the POST method might not result in a
resource that can be identified by a URI. In this case, either 200
(OK) or 204 (No Content) is the appropriate response status,
depending on whether or not the response includes an entity that
describes the result.
 
If a resource has been created on the origin server, the response
SHOULD be 201 (Created) and contain an entity which describes the
status of the request and refers to the new resource, and a Location
header (see section 14.30).
 
Responses to this method are not cachable, unless the response
includes appropriate Cache-Control or Expires header fields. However,
the 303 (See Other) response can be used to direct the user agent to
retrieve a cachable resource.
 
POST requests must obey the message transmission requirements set out
in section 8.2.
 
 
 
 
 
 
 
 
 
Fielding, et. al. Standards Track [Page 51]
RFC 2068 HTTP/1.1 January 1997
 
 
9.6 PUT
 
The PUT method requests that the enclosed entity be stored under the
supplied Request-URI. If the Request-URI refers to an already
existing resource, the enclosed entity SHOULD be considered as a
modified version of the one residing on the origin server. If the
Request-URI does not point to an existing resource, and that URI is
capable of being defined as a new resource by the requesting user
agent, the origin server can create the resource with that URI. If a
new resource is created, the origin server MUST inform the user agent
via the 201 (Created) response. If an existing resource is modified,
either the 200 (OK) or 204 (No Content) response codes SHOULD be sent
to indicate successful completion of the request. If the resource
could not be created or modified with the Request-URI, an appropriate
error response SHOULD be given that reflects the nature of the
problem. The recipient of the entity MUST NOT ignore any Content-*
(e.g. Content-Range) headers that it does not understand or implement
and MUST return a 501 (Not Implemented) response in such cases.
 
If the request passes through a cache and the Request-URI identifies
one or more currently cached entities, those entries should be
treated as stale. Responses to this method are not cachable.
 
The fundamental difference between the POST and PUT requests is
reflected in the different meaning of the Request-URI. The URI in a
POST request identifies the resource that will handle the enclosed
entity. That resource may be a data-accepting process, a gateway to
some other protocol, or a separate entity that accepts annotations.
In contrast, the URI in a PUT request identifies the entity enclosed
with the request -- the user agent knows what URI is intended and the
server MUST NOT attempt to apply the request to some other resource.
If the server desires that the request be applied to a different URI,
it MUST send a 301 (Moved Permanently) response; the user agent MAY
then make its own decision regarding whether or not to redirect the
request.
 
A single resource MAY be identified by many different URIs. For
example, an article may have a URI for identifying "the current
version" which is separate from the URI identifying each particular
version. In this case, a PUT request on a general URI may result in
several other URIs being defined by the origin server.
 
HTTP/1.1 does not define how a PUT method affects the state of an
origin server.
 
PUT requests must obey the message transmission requirements set out
in section 8.2.
 
 
 
 
Fielding, et. al. Standards Track [Page 52]
RFC 2068 HTTP/1.1 January 1997
 
 
9.7 DELETE
 
The DELETE method requests that the origin server delete the resource
identified by the Request-URI. This method MAY be overridden by human
intervention (or other means) on the origin server. The client cannot
be guaranteed that the operation has been carried out, even if the
status code returned from the origin server indicates that the action
has been completed successfully. However, the server SHOULD not
indicate success unless, at the time the response is given, it
intends to delete the resource or move it to an inaccessible
location.
 
A successful response SHOULD be 200 (OK) if the response includes an
entity describing the status, 202 (Accepted) if the action has not
yet been enacted, or 204 (No Content) if the response is OK but does
not include an entity.
 
If the request passes through a cache and the Request-URI identifies
one or more currently cached entities, those entries should be
treated as stale. Responses to this method are not cachable.
 
9.8 TRACE
 
The TRACE method is used to invoke a remote, application-layer loop-
back of the request message. The final recipient of the request
SHOULD reflect the message received back to the client as the
entity-body of a 200 (OK) response. The final recipient is either the
origin server or the first proxy or gateway to receive a Max-Forwards
value of zero (0) in the request (see section 14.31). A TRACE request
MUST NOT include an entity.
 
TRACE allows the client to see what is being received at the other
end of the request chain and use that data for testing or diagnostic
information. The value of the Via header field (section 14.44) is of
particular interest, since it acts as a trace of the request chain.
Use of the Max-Forwards header field allows the client to limit the
length of the request chain, which is useful for testing a chain of
proxies forwarding messages in an infinite loop.
 
If successful, the response SHOULD contain the entire request message
in the entity-body, with a Content-Type of "message/http". Responses
to this method MUST NOT be cached.
 
10 Status Code Definitions
 
Each Status-Code is described below, including a description of which
method(s) it can follow and any metainformation required in the
response.
 
 
 
Fielding, et. al. Standards Track [Page 53]
RFC 2068 HTTP/1.1 January 1997
 
 
10.1 Informational 1xx
 
This class of status code indicates a provisional response,
consisting only of the Status-Line and optional headers, and is
terminated by an empty line. Since HTTP/1.0 did not define any 1xx
status codes, servers MUST NOT send a 1xx response to an HTTP/1.0
client except under experimental conditions.
 
10.1.1 100 Continue
 
The client may continue with its request. This interim response is
used to inform the client that the initial part of the request has
been received and has not yet been rejected by the server. The client
SHOULD continue by sending the remainder of the request or, if the
request has already been completed, ignore this response. The server
MUST send a final response after the request has been completed.
 
10.1.2 101 Switching Protocols
 
The server understands and is willing to comply with the client's
request, via the Upgrade message header field (section 14.41), for a
change in the application protocol being used on this connection. The
server will switch protocols to those defined by the response's
Upgrade header field immediately after the empty line which
terminates the 101 response.
 
The protocol should only be switched when it is advantageous to do
so. For example, switching to a newer version of HTTP is
advantageous over older versions, and switching to a real-time,
synchronous protocol may be advantageous when delivering resources
that use such features.
 
10.2 Successful 2xx
 
This class of status code indicates that the client's request was
successfully received, understood, and accepted.
 
10.2.1 200 OK
 
The request has succeeded. The information returned with the response
is dependent on the method used in the request, for example:
 
GET an entity corresponding to the requested resource is sent in the
response;
 
HEAD the entity-header fields corresponding to the requested resource
are sent in the response without any message-body;
 
 
 
 
Fielding, et. al. Standards Track [Page 54]
RFC 2068 HTTP/1.1 January 1997
 
 
POST an entity describing or containing the result of the action;
 
TRACE an entity containing the request message as received by the end
server.
 
10.2.2 201 Created
 
The request has been fulfilled and resulted in a new resource being
created. The newly created resource can be referenced by the URI(s)
returned in the entity of the response, with the most specific URL
for the resource given by a Location header field. The origin server
MUST create the resource before returning the 201 status code. If the
action cannot be carried out immediately, the server should respond
with 202 (Accepted) response instead.
 
10.2.3 202 Accepted
 
The request has been accepted for processing, but the processing has
not been completed. The request MAY or MAY NOT eventually be acted
upon, as it MAY be disallowed when processing actually takes place.
There is no facility for re-sending a status code from an
asynchronous operation such as this.
 
The 202 response is intentionally non-committal. Its purpose is to
allow a server to accept a request for some other process (perhaps a
batch-oriented process that is only run once per day) without
requiring that the user agent's connection to the server persist
until the process is completed. The entity returned with this
response SHOULD include an indication of the request's current status
and either a pointer to a status monitor or some estimate of when the
user can expect the request to be fulfilled.
 
10.2.4 203 Non-Authoritative Information
 
The returned metainformation in the entity-header is not the
definitive set as available from the origin server, but is gathered
from a local or a third-party copy. The set presented MAY be a subset
or superset of the original version. For example, including local
annotation information about the resource MAY result in a superset of
the metainformation known by the origin server. Use of this response
code is not required and is only appropriate when the response would
otherwise be 200 (OK).
 
10.2.5 204 No Content
 
The server has fulfilled the request but there is no new information
to send back. If the client is a user agent, it SHOULD NOT change its
document view from that which caused the request to be sent. This
 
 
 
Fielding, et. al. Standards Track [Page 55]
RFC 2068 HTTP/1.1 January 1997
 
 
response is primarily intended to allow input for actions to take
place without causing a change to the user agent's active document
view. The response MAY include new metainformation in the form of
entity-headers, which SHOULD apply to the document currently in the
user agent's active view.
 
The 204 response MUST NOT include a message-body, and thus is always
terminated by the first empty line after the header fields.
 
10.2.6 205 Reset Content
 
The server has fulfilled the request and the user agent SHOULD reset
the document view which caused the request to be sent. This response
is primarily intended to allow input for actions to take place via
user input, followed by a clearing of the form in which the input is
given so that the user can easily initiate another input action. The
response MUST NOT include an entity.
 
10.2.7 206 Partial Content
 
The server has fulfilled the partial GET request for the resource.
The request must have included a Range header field (section 14.36)
indicating the desired range. The response MUST include either a
Content-Range header field (section 14.17) indicating the range
included with this response, or a multipart/byteranges Content-Type
including Content-Range fields for each part. If multipart/byteranges
is not used, the Content-Length header field in the response MUST
match the actual number of OCTETs transmitted in the message-body.
 
A cache that does not support the Range and Content-Range headers
MUST NOT cache 206 (Partial) responses.
 
10.3 Redirection 3xx
 
This class of status code indicates that further action needs to be
taken by the user agent in order to fulfill the request. The action
required MAY be carried out by the user agent without interaction
with the user if and only if the method used in the second request is
GET or HEAD. A user agent SHOULD NOT automatically redirect a request
more than 5 times, since such redirections usually indicate an
infinite loop.
 
 
 
 
 
 
 
 
 
 
Fielding, et. al. Standards Track [Page 56]
RFC 2068 HTTP/1.1 January 1997
 
 
10.3.1 300 Multiple Choices
 
The requested resource corresponds to any one of a set of
representations, each with its own specific location, and agent-
driven negotiation information (section 12) is being provided so that
the user (or user agent) can select a preferred representation and
redirect its request to that location.
 
Unless it was a HEAD request, the response SHOULD include an entity
containing a list of resource characteristics and location(s) from
which the user or user agent can choose the one most appropriate. The
entity format is specified by the media type given in the Content-
Type header field. Depending upon the format and the capabilities of
the user agent, selection of the most appropriate choice may be
performed automatically. However, this specification does not define
any standard for such automatic selection.
 
If the server has a preferred choice of representation, it SHOULD
include the specific URL for that representation in the Location
field; user agents MAY use the Location field value for automatic
redirection. This response is cachable unless indicated otherwise.
 
10.3.2 301 Moved Permanently
 
The requested resource has been assigned a new permanent URI and any
future references to this resource SHOULD be done using one of the
returned URIs. Clients with link editing capabilities SHOULD
automatically re-link references to the Request-URI to one or more of
the new references returned by the server, where possible. This
response is cachable unless indicated otherwise.
 
If the new URI is a location, its URL SHOULD be given by the Location
field in the response. Unless the request method was HEAD, the entity
of the response SHOULD contain a short hypertext note with a
hyperlink to the new URI(s).
 
If the 301 status code is received in response to a request other
than GET or HEAD, the user agent MUST NOT automatically redirect the
request unless it can be confirmed by the user, since this might
change the conditions under which the request was issued.
 
Note: When automatically redirecting a POST request after receiving
a 301 status code, some existing HTTP/1.0 user agents will
erroneously change it into a GET request.
 
 
 
 
 
 
 
Fielding, et. al. Standards Track [Page 57]
RFC 2068 HTTP/1.1 January 1997
 
 
10.3.3 302 Moved Temporarily
 
The requested resource resides temporarily under a different URI.
Since the redirection may be altered on occasion, the client SHOULD
continue to use the Request-URI for future requests. This response is
only cachable if indicated by a Cache-Control or Expires header
field.
 
If the new URI is a location, its URL SHOULD be given by the Location
field in the response. Unless the request method was HEAD, the entity
of the response SHOULD contain a short hypertext note with a
hyperlink to the new URI(s).
 
If the 302 status code is received in response to a request other
than GET or HEAD, the user agent MUST NOT automatically redirect the
request unless it can be confirmed by the user, since this might
change the conditions under which the request was issued.
 
Note: When automatically redirecting a POST request after receiving
a 302 status code, some existing HTTP/1.0 user agents will
erroneously change it into a GET request.
 
10.3.4 303 See Other
 
The response to the request can be found under a different URI and
SHOULD be retrieved using a GET method on that resource. This method
exists primarily to allow the output of a POST-activated script to
redirect the user agent to a selected resource. The new URI is not a
substitute reference for the originally requested resource. The 303
response is not cachable, but the response to the second (redirected)
request MAY be cachable.
 
If the new URI is a location, its URL SHOULD be given by the Location
field in the response. Unless the request method was HEAD, the entity
of the response SHOULD contain a short hypertext note with a
hyperlink to the new URI(s).
 
10.3.5 304 Not Modified
 
If the client has performed a conditional GET request and access is
allowed, but the document has not been modified, the server SHOULD
respond with this status code. The response MUST NOT contain a
message-body.
 
 
 
 
 
 
 
 
Fielding, et. al. Standards Track [Page 58]
RFC 2068 HTTP/1.1 January 1997
 
 
The response MUST include the following header fields:
 
o Date
 
o ETag and/or Content-Location, if the header would have been sent in
a 200 response to the same request
 
o Expires, Cache-Control, and/or Vary, if the field-value might
differ from that sent in any previous response for the same variant
 
If the conditional GET used a strong cache validator (see section
13.3.3), the response SHOULD NOT include other entity-headers.
Otherwise (i.e., the conditional GET used a weak validator), the
response MUST NOT include other entity-headers; this prevents
inconsistencies between cached entity-bodies and updated headers.
 
If a 304 response indicates an entity not currently cached, then the
cache MUST disregard the response and repeat the request without the
conditional.
 
If a cache uses a received 304 response to update a cache entry, the
cache MUST update the entry to reflect any new field values given in
the response.
 
The 304 response MUST NOT include a message-body, and thus is always
terminated by the first empty line after the header fields.
 
10.3.6 305 Use Proxy
 
The requested resource MUST be accessed through the proxy given by
the Location field. The Location field gives the URL of the proxy.
The recipient is expected to repeat the request via the proxy.
 
10.4 Client Error 4xx
 
The 4xx class of status code is intended for cases in which the
client seems to have erred. Except when responding to a HEAD request,
the server SHOULD include an entity containing an explanation of the
error situation, and whether it is a temporary or permanent
condition. These status codes are applicable to any request method.
User agents SHOULD display any included entity to the user.
 
Note: If the client is sending data, a server implementation using
TCP should be careful to ensure that the client acknowledges
receipt of the packet(s) containing the response, before the server
closes the input connection. If the client continues sending data
to the server after the close, the server's TCP stack will send a
reset packet to the client, which may erase the client's
 
 
 
Fielding, et. al. Standards Track [Page 59]
RFC 2068 HTTP/1.1 January 1997
 
 
unacknowledged input buffers before they can be read and
interpreted by the HTTP application.
 
10.4.1 400 Bad Request
 
The request could not be understood by the server due to malformed
syntax. The client SHOULD NOT repeat the request without
modifications.
 
10.4.2 401 Unauthorized
 
The request requires user authentication. The response MUST include a
WWW-Authenticate header field (section 14.46) containing a challenge
applicable to the requested resource. The client MAY repeat the
request with a suitable Authorization header field (section 14.8). If
the request already included Authorization credentials, then the 401
response indicates that authorization has been refused for those
credentials. If the 401 response contains the same challenge as the
prior response, and the user agent has already attempted
authentication at least once, then the user SHOULD be presented the
entity that was given in the response, since that entity MAY include
relevant diagnostic information. HTTP access authentication is
explained in section 11.
 
10.4.3 402 Payment Required
 
This code is reserved for future use.
 
10.4.4 403 Forbidden
 
The server understood the request, but is refusing to fulfill it.
Authorization will not help and the request SHOULD NOT be repeated.
If the request method was not HEAD and the server wishes to make
public why the request has not been fulfilled, it SHOULD describe the
reason for the refusal in the entity. This status code is commonly
used when the server does not wish to reveal exactly why the request
has been refused, or when no other response is applicable.
 
10.4.5 404 Not Found
 
The server has not found anything matching the Request-URI. No
indication is given of whether the condition is temporary or
permanent.
 
 
 
 
 
 
 
 
Fielding, et. al. Standards Track [Page 60]
RFC 2068 HTTP/1.1 January 1997
 
 
If the server does not wish to make this information available to the
client, the status code 403 (Forbidden) can be used instead. The 410
(Gone) status code SHOULD be used if the server knows, through some
internally configurable mechanism, that an old resource is
permanently unavailable and has no forwarding address.
 
10.4.6 405 Method Not Allowed
 
The method specified in the Request-Line is not allowed for the
resource identified by the Request-URI. The response MUST include an
Allow header containing a list of valid methods for the requested
resource.
 
10.4.7 406 Not Acceptable
 
The resource identified by the request is only capable of generating
response entities which have content characteristics not acceptable
according to the accept headers sent in the request.
 
Unless it was a HEAD request, the response SHOULD include an entity
containing a list of available entity characteristics and location(s)
from which the user or user agent can choose the one most
appropriate. The entity format is specified by the media type given
in the Content-Type header field. Depending upon the format and the
capabilities of the user agent, selection of the most appropriate
choice may be performed automatically. However, this specification
does not define any standard for such automatic selection.
 
Note: HTTP/1.1 servers are allowed to return responses which are
not acceptable according to the accept headers sent in the request.
In some cases, this may even be preferable to sending a 406
response. User agents are encouraged to inspect the headers of an
incoming response to determine if it is acceptable. If the response
could be unacceptable, a user agent SHOULD temporarily stop receipt
of more data and query the user for a decision on further actions.
 
10.4.8 407 Proxy Authentication Required
 
This code is similar to 401 (Unauthorized), but indicates that the
client MUST first authenticate itself with the proxy. The proxy MUST
return a Proxy-Authenticate header field (section 14.33) containing a
challenge applicable to the proxy for the requested resource. The
client MAY repeat the request with a suitable Proxy-Authorization
header field (section 14.34). HTTP access authentication is explained
in section 11.
 
 
 
 
 
 
Fielding, et. al. Standards Track [Page 61]
RFC 2068 HTTP/1.1 January 1997
 
 
10.4.9 408 Request Timeout
 
The client did not produce a request within the time that the server
was prepared to wait. The client MAY repeat the request without
modifications at any later time.
 
10.4.10 409 Conflict
 
The request could not be completed due to a conflict with the current
state of the resource. This code is only allowed in situations where
it is expected that the user might be able to resolve the conflict
and resubmit the request. The response body SHOULD include enough
information for the user to recognize the source of the conflict.
Ideally, the response entity would include enough information for the
user or user agent to fix the problem; however, that may not be
possible and is not required.
 
Conflicts are most likely to occur in response to a PUT request. If
versioning is being used and the entity being PUT includes changes to
a resource which conflict with those made by an earlier (third-party)
request, the server MAY use the 409 response to indicate that it
can't complete the request. In this case, the response entity SHOULD
contain a list of the differences between the two versions in a
format defined by the response Content-Type.
 
10.4.11 410 Gone
 
The requested resource is no longer available at the server and no
forwarding address is known. This condition SHOULD be considered
permanent. Clients with link editing capabilities SHOULD delete
references to the Request-URI after user approval. If the server does
not know, or has no facility to determine, whether or not the
condition is permanent, the status code 404 (Not Found) SHOULD be
used instead. This response is cachable unless indicated otherwise.
 
The 410 response is primarily intended to assist the task of web
maintenance by notifying the recipient that the resource is
intentionally unavailable and that the server owners desire that
remote links to that resource be removed. Such an event is common for
limited-time, promotional services and for resources belonging to
individuals no longer working at the server's site. It is not
necessary to mark all permanently unavailable resources as "gone" or
to keep the mark for any length of time -- that is left to the
discretion of the server owner.
 
 
 
 
 
 
 
Fielding, et. al. Standards Track [Page 62]
RFC 2068 HTTP/1.1 January 1997
 
 
10.4.12 411 Length Required
 
The server refuses to accept the request without a defined Content-
Length. The client MAY repeat the request if it adds a valid
Content-Length header field containing the length of the message-body
in the request message.
 
10.4.13 412 Precondition Failed
 
The precondition given in one or more of the request-header fields
evaluated to false when it was tested on the server. This response
code allows the client to place preconditions on the current resource
metainformation (header field data) and thus prevent the requested
method from being applied to a resource other than the one intended.
 
10.4.14 413 Request Entity Too Large
 
The server is refusing to process a request because the request
entity is larger than the server is willing or able to process. The
server may close the connection to prevent the client from continuing
the request.
 
If the condition is temporary, the server SHOULD include a Retry-
After header field to indicate that it is temporary and after what
time the client may try again.
 
10.4.15 414 Request-URI Too Long
 
The server is refusing to service the request because the Request-URI
is longer than the server is willing to interpret. This rare
condition is only likely to occur when a client has improperly
converted a POST request to a GET request with long query
information, when the client has descended into a URL "black hole" of
redirection (e.g., a redirected URL prefix that points to a suffix of
itself), or when the server is under attack by a client attempting to
exploit security holes present in some servers using fixed-length
buffers for reading or manipulating the Request-URI.
 
10.4.16 415 Unsupported Media Type
 
The server is refusing to service the request because the entity of
the request is in a format not supported by the requested resource
for the requested method.
 
 
 
 
 
 
 
 
Fielding, et. al. Standards Track [Page 63]
RFC 2068 HTTP/1.1 January 1997
 
 
10.5 Server Error 5xx
 
Response status codes beginning with the digit "5" indicate cases in
which the server is aware that it has erred or is incapable of
performing the request. Except when responding to a HEAD request, the
server SHOULD include an entity containing an explanation of the
error situation, and whether it is a temporary or permanent
condition. User agents SHOULD display any included entity to the
user. These response codes are applicable to any request method.
 
10.5.1 500 Internal Server Error
 
The server encountered an unexpected condition which prevented it
from fulfilling the request.
 
10.5.2 501 Not Implemented
 
The server does not support the functionality required to fulfill the
request. This is the appropriate response when the server does not
recognize the request method and is not capable of supporting it for
any resource.
 
10.5.3 502 Bad Gateway
 
The server, while acting as a gateway or proxy, received an invalid
response from the upstream server it accessed in attempting to
fulfill the request.
 
10.5.4 503 Service Unavailable
 
The server is currently unable to handle the request due to a
temporary overloading or maintenance of the server. The implication
is that this is a temporary condition which will be alleviated after
some delay. If known, the length of the delay may be indicated in a
Retry-After header. If no Retry-After is given, the client SHOULD
handle the response as it would for a 500 response.
 
Note: The existence of the 503 status code does not imply that a
server must use it when becoming overloaded. Some servers may wish
to simply refuse the connection.
 
10.5.5 504 Gateway Timeout
 
The server, while acting as a gateway or proxy, did not receive a
timely response from the upstream server it accessed in attempting to
complete the request.
 
 
 
 
 
Fielding, et. al. Standards Track [Page 64]
RFC 2068 HTTP/1.1 January 1997
 
 
10.5.6 505 HTTP Version Not Supported
 
The server does not support, or refuses to support, the HTTP protocol
version that was used in the request message. The server is
indicating that it is unable or unwilling to complete the request
using the same major version as the client, as described in section
3.1, other than with this error message. The response SHOULD contain
an entity describing why that version is not supported and what other
protocols are supported by that server.
 
11 Access Authentication
 
HTTP provides a simple challenge-response authentication mechanism
which MAY be used by a server to challenge a client request and by a
client to provide authentication information. It uses an extensible,
case-insensitive token to identify the authentication scheme,
followed by a comma-separated list of attribute-value pairs which
carry the parameters necessary for achieving authentication via that
scheme.
 
auth-scheme = token
 
auth-param = token "=" quoted-string
 
The 401 (Unauthorized) response message is used by an origin server
to challenge the authorization of a user agent. This response MUST
include a WWW-Authenticate header field containing at least one
challenge applicable to the requested resource.
 
challenge = auth-scheme 1*SP realm *( "," auth-param )
 
realm = "realm" "=" realm-value
realm-value = quoted-string
 
The realm attribute (case-insensitive) is required for all
authentication schemes which issue a challenge. The realm value
(case-sensitive), in combination with the canonical root URL (see
section 5.1.2) of the server being accessed, defines the protection
space. These realms allow the protected resources on a server to be
partitioned into a set of protection spaces, each with its own
authentication scheme and/or authorization database. The realm value
is a string, generally assigned by the origin server, which may have
additional semantics specific to the authentication scheme.
 
A user agent that wishes to authenticate itself with a server--
usually, but not necessarily, after receiving a 401 or 411 response-
-MAY do so by including an Authorization header field with the
request. The Authorization field value consists of credentials
 
 
 
Fielding, et. al. Standards Track [Page 65]
RFC 2068 HTTP/1.1 January 1997
 
 
containing the authentication information of the user agent for the
realm of the resource being requested.
 
credentials = basic-credentials
| auth-scheme #auth-param
 
The domain over which credentials can be automatically applied by a
user agent is determined by the protection space. If a prior request
has been authorized, the same credentials MAY be reused for all other
requests within that protection space for a period of time determined
by the authentication scheme, parameters, and/or user preference.
Unless otherwise defined by the authentication scheme, a single
protection space cannot extend outside the scope of its server.
 
If the server does not wish to accept the credentials sent with a
request, it SHOULD return a 401 (Unauthorized) response. The response
MUST include a WWW-Authenticate header field containing the (possibly
new) challenge applicable to the requested resource and an entity
explaining the refusal.
 
The HTTP protocol does not restrict applications to this simple
challenge-response mechanism for access authentication. Additional
mechanisms MAY be used, such as encryption at the transport level or
via message encapsulation, and with additional header fields
specifying authentication information. However, these additional
mechanisms are not defined by this specification.
 
Proxies MUST be completely transparent regarding user agent
authentication. That is, they MUST forward the WWW-Authenticate and
Authorization headers untouched, and follow the rules found in
section 14.8.
 
HTTP/1.1 allows a client to pass authentication information to and
from a proxy via the Proxy-Authenticate and Proxy-Authorization
headers.
 
11.1 Basic Authentication Scheme
 
The "basic" authentication scheme is based on the model that the user
agent must authenticate itself with a user-ID and a password for each
realm. The realm value should be considered an opaque string which
can only be compared for equality with other realms on that server.
The server will service the request only if it can validate the
user-ID and password for the protection space of the Request-URI.
There are no optional authentication parameters.
 
 
 
 
 
 
Fielding, et. al. Standards Track [Page 66]
RFC 2068 HTTP/1.1 January 1997
 
 
Upon receipt of an unauthorized request for a URI within the
protection space, the server MAY respond with a challenge like the
following:
 
WWW-Authenticate: Basic realm="WallyWorld"
 
where "WallyWorld" is the string assigned by the server to identify
the protection space of the Request-URI.
 
To receive authorization, the client sends the userid and password,
separated by a single colon (":") character, within a base64 encoded
string in the credentials.
 
basic-credentials = "Basic" SP basic-cookie
 
basic-cookie = <base64 [7] encoding of user-pass,
except not limited to 76 char/line>
 
user-pass = userid ":" password
 
userid = *<TEXT excluding ":">
 
password = *TEXT
 
Userids might be case sensitive.
 
If the user agent wishes to send the userid "Aladdin" and password
"open sesame", it would use the following header field:
 
Authorization: Basic QWxhZGRpbjpvcGVuIHNlc2FtZQ==
 
See section 15 for security considerations associated with Basic
authentication.
 
11.2 Digest Authentication Scheme
 
A digest authentication for HTTP is specified in RFC 2069 [32].
 
12 Content Negotiation
 
Most HTTP responses include an entity which contains information for
interpretation by a human user. Naturally, it is desirable to supply
the user with the "best available" entity corresponding to the
request. Unfortunately for servers and caches, not all users have
the same preferences for what is "best," and not all user agents are
equally capable of rendering all entity types. For that reason, HTTP
has provisions for several mechanisms for "content negotiation" --
the process of selecting the best representation for a given response
 
 
 
Fielding, et. al. Standards Track [Page 67]
RFC 2068 HTTP/1.1 January 1997
 
 
when there are multiple representations available.
 
Note: This is not called "format negotiation" because the alternate
representations may be of the same media type, but use different
capabilities of that type, be in different languages, etc.
 
Any response containing an entity-body MAY be subject to negotiation,
including error responses.
 
There are two kinds of content negotiation which are possible in
HTTP: server-driven and agent-driven negotiation. These two kinds of
negotiation are orthogonal and thus may be used separately or in
combination. One method of combination, referred to as transparent
negotiation, occurs when a cache uses the agent-driven negotiation
information provided by the origin server in order to provide
server-driven negotiation for subsequent requests.
 
12.1 Server-driven Negotiation
 
If the selection of the best representation for a response is made by
an algorithm located at the server, it is called server-driven
negotiation. Selection is based on the available representations of
the response (the dimensions over which it can vary; e.g. language,
content-coding, etc.) and the contents of particular header fields in
the request message or on other information pertaining to the request
(such as the network address of the client).
 
Server-driven negotiation is advantageous when the algorithm for
selecting from among the available representations is difficult to
describe to the user agent, or when the server desires to send its
"best guess" to the client along with the first response (hoping to
avoid the round-trip delay of a subsequent request if the "best
guess" is good enough for the user). In order to improve the server's
guess, the user agent MAY include request header fields (Accept,
Accept-Language, Accept-Encoding, etc.) which describe its
preferences for such a response.
 
Server-driven negotiation has disadvantages:
 
1. It is impossible for the server to accurately determine what might be
"best" for any given user, since that would require complete
knowledge of both the capabilities of the user agent and the intended
use for the response (e.g., does the user want to view it on screen
or print it on paper?).
 
2. Having the user agent describe its capabilities in every request can
be both very inefficient (given that only a small percentage of
responses have multiple representations) and a potential violation of
 
 
 
Fielding, et. al. Standards Track [Page 68]
RFC 2068 HTTP/1.1 January 1997
 
 
the user's privacy.
 
3. It complicates the implementation of an origin server and the
algorithms for generating responses to a request.
 
4. It may limit a public cache's ability to use the same response for
multiple user's requests.
 
HTTP/1.1 includes the following request-header fields for enabling
server-driven negotiation through description of user agent
capabilities and user preferences: Accept (section 14.1), Accept-
Charset (section 14.2), Accept-Encoding (section 14.3), Accept-
Language (section 14.4), and User-Agent (section 14.42). However, an
origin server is not limited to these dimensions and MAY vary the
response based on any aspect of the request, including information
outside the request-header fields or within extension header fields
not defined by this specification.
 
HTTP/1.1 origin servers MUST include an appropriate Vary header field
(section 14.43) in any cachable response based on server-driven
negotiation. The Vary header field describes the dimensions over
which the response might vary (i.e. the dimensions over which the
origin server picks its "best guess" response from multiple
representations).
 
HTTP/1.1 public caches MUST recognize the Vary header field when it
is included in a response and obey the requirements described in
section 13.6 that describes the interactions between caching and
content negotiation.
 
12.2 Agent-driven Negotiation
 
With agent-driven negotiation, selection of the best representation
for a response is performed by the user agent after receiving an
initial response from the origin server. Selection is based on a list
of the available representations of the response included within the
header fields (this specification reserves the field-name Alternates,
as described in appendix 19.6.2.1) or entity-body of the initial
response, with each representation identified by its own URI.
Selection from among the representations may be performed
automatically (if the user agent is capable of doing so) or manually
by the user selecting from a generated (possibly hypertext) menu.
 
Agent-driven negotiation is advantageous when the response would vary
over commonly-used dimensions (such as type, language, or encoding),
when the origin server is unable to determine a user agent's
capabilities from examining the request, and generally when public
caches are used to distribute server load and reduce network usage.
 
 
 
Fielding, et. al. Standards Track [Page 69]
RFC 2068 HTTP/1.1 January 1997
 
 
Agent-driven negotiation suffers from the disadvantage of needing a
second request to obtain the best alternate representation. This
second request is only efficient when caching is used. In addition,
this specification does not define any mechanism for supporting
automatic selection, though it also does not prevent any such
mechanism from being developed as an extension and used within
HTTP/1.1.
 
HTTP/1.1 defines the 300 (Multiple Choices) and 406 (Not Acceptable)
status codes for enabling agent-driven negotiation when the server is
unwilling or unable to provide a varying response using server-driven
negotiation.
 
12.3 Transparent Negotiation
 
Transparent negotiation is a combination of both server-driven and
agent-driven negotiation. When a cache is supplied with a form of the
list of available representations of the response (as in agent-driven
negotiation) and the dimensions of variance are completely understood
by the cache, then the cache becomes capable of performing server-
driven negotiation on behalf of the origin server for subsequent
requests on that resource.
 
Transparent negotiation has the advantage of distributing the
negotiation work that would otherwise be required of the origin
server and also removing the second request delay of agent-driven
negotiation when the cache is able to correctly guess the right
response.
 
This specification does not define any mechanism for transparent
negotiation, though it also does not prevent any such mechanism from
being developed as an extension and used within HTTP/1.1. An HTTP/1.1
cache performing transparent negotiation MUST include a Vary header
field in the response (defining the dimensions of its variance) if it
is cachable to ensure correct interoperation with all HTTP/1.1
clients. The agent-driven negotiation information supplied by the
origin server SHOULD be included with the transparently negotiated
response.
 
13 Caching in HTTP
 
HTTP is typically used for distributed information systems, where
performance can be improved by the use of response caches. The
HTTP/1.1 protocol includes a number of elements intended to make
caching work as well as possible. Because these elements are
inextricable from other aspects of the protocol, and because they
interact with each other, it is useful to describe the basic caching
design of HTTP separately from the detailed descriptions of methods,
 
 
 
Fielding, et. al. Standards Track [Page 70]
RFC 2068 HTTP/1.1 January 1997
 
 
headers, response codes, etc.
 
Caching would be useless if it did not significantly improve
performance. The goal of caching in HTTP/1.1 is to eliminate the need
to send requests in many cases, and to eliminate the need to send
full responses in many other cases. The former reduces the number of
network round-trips required for many operations; we use an
"expiration" mechanism for this purpose (see section 13.2). The
latter reduces network bandwidth requirements; we use a "validation"
mechanism for this purpose (see section 13.3).
 
Requirements for performance, availability, and disconnected
operation require us to be able to relax the goal of semantic
transparency. The HTTP/1.1 protocol allows origin servers, caches,
and clients to explicitly reduce transparency when necessary.
However, because non-transparent operation may confuse non-expert
users, and may be incompatible with certain server applications (such
as those for ordering merchandise), the protocol requires that
transparency be relaxed
 
o only by an explicit protocol-level request when relaxed by client
or origin server
 
o only with an explicit warning to the end user when relaxed by cache
or client
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
Fielding, et. al. Standards Track [Page 71]
RFC 2068 HTTP/1.1 January 1997
 
 
Therefore, the HTTP/1.1 protocol provides these important elements:
 
1. Protocol features that provide full semantic transparency when this
is required by all parties.
 
2. Protocol features that allow an origin server or user agent to
explicitly request and control non-transparent operation.
 
3. Protocol features that allow a cache to attach warnings to
responses that do not preserve the requested approximation of
semantic transparency.
 
A basic principle is that it must be possible for the clients to
detect any potential relaxation of semantic transparency.
 
Note: The server, cache, or client implementer may be faced with
design decisions not explicitly discussed in this specification. If
a decision may affect semantic transparency, the implementer ought
to err on the side of maintaining transparency unless a careful and
complete analysis shows significant benefits in breaking
transparency.
 
13.1.1 Cache Correctness
 
A correct cache MUST respond to a request with the most up-to-date
response held by the cache that is appropriate to the request (see
sections 13.2.5, 13.2.6, and 13.12) which meets one of the following
conditions:
 
1. It has been checked for equivalence with what the origin server
would have returned by revalidating the response with the origin
server (section 13.3);
 
2. It is "fresh enough" (see section 13.2). In the default case, this
means it meets the least restrictive freshness requirement of the
client, server, and cache (see section 14.9); if the origin server
so specifies, it is the freshness requirement of the origin server
alone.
 
3. It includes a warning if the freshness demand of the client or the
origin server is violated (see section 13.1.5 and 14.45).
 
4. It is an appropriate 304 (Not Modified), 305 (Proxy Redirect), or
error (4xx or 5xx) response message.
 
If the cache can not communicate with the origin server, then a
correct cache SHOULD respond as above if the response can be
correctly served from the cache; if not it MUST return an error or
 
 
 
Fielding, et. al. Standards Track [Page 72]
RFC 2068 HTTP/1.1 January 1997
 
 
warning indicating that there was a communication failure.
 
If a cache receives a response (either an entire response, or a 304
(Not Modified) response) that it would normally forward to the
requesting client, and the received response is no longer fresh, the
cache SHOULD forward it to the requesting client without adding a new
Warning (but without removing any existing Warning headers). A cache
SHOULD NOT attempt to revalidate a response simply because that
response became stale in transit; this might lead to an infinite
loop. An user agent that receives a stale response without a Warning
MAY display a warning indication to the user.
 
13.1.2 Warnings
 
Whenever a cache returns a response that is neither first-hand nor
"fresh enough" (in the sense of condition 2 in section 13.1.1), it
must attach a warning to that effect, using a Warning response-
header. This warning allows clients to take appropriate action.
 
Warnings may be used for other purposes, both cache-related and
otherwise. The use of a warning, rather than an error status code,
distinguish these responses from true failures.
 
Warnings are always cachable, because they never weaken the
transparency of a response. This means that warnings can be passed to
HTTP/1.0 caches without danger; such caches will simply pass the
warning along as an entity-header in the response.
 
Warnings are assigned numbers between 0 and 99. This specification
defines the code numbers and meanings of each currently assigned
warnings, allowing a client or cache to take automated action in some
(but not all) cases.
 
Warnings also carry a warning text. The text may be in any
appropriate natural language (perhaps based on the client's Accept
headers), and include an optional indication of what character set is
used.
 
Multiple warnings may be attached to a response (either by the origin
server or by a cache), including multiple warnings with the same code
number. For example, a server may provide the same warning with texts
in both English and Basque.
 
When multiple warnings are attached to a response, it may not be
practical or reasonable to display all of them to the user. This
version of HTTP does not specify strict priority rules for deciding
which warnings to display and in what order, but does suggest some
heuristics.
 
 
 
Fielding, et. al. Standards Track [Page 73]
RFC 2068 HTTP/1.1 January 1997
 
 
The Warning header and the currently defined warnings are described
in section 14.45.
 
13.1.3 Cache-control Mechanisms
 
The basic cache mechanisms in HTTP/1.1 (server-specified expiration
times and validators) are implicit directives to caches. In some
cases, a server or client may need to provide explicit directives to
the HTTP caches. We use the Cache-Control header for this purpose.
 
The Cache-Control header allows a client or server to transmit a
variety of directives in either requests or responses. These
directives typically override the default caching algorithms. As a
general rule, if there is any apparent conflict between header
values, the most restrictive interpretation should be applied (that
is, the one that is most likely to preserve semantic transparency).
However, in some cases, Cache-Control directives are explicitly
specified as weakening the approximation of semantic transparency
(for example, "max-stale" or "public").
 
The Cache-Control directives are described in detail in section 14.9.
 
13.1.4 Explicit User Agent Warnings
 
Many user agents make it possible for users to override the basic
caching mechanisms. For example, the user agent may allow the user to
specify that cached entities (even explicitly stale ones) are never
validated. Or the user agent might habitually add "Cache-Control:
max-stale=3600" to every request. The user should have to explicitly
request either non-transparent behavior, or behavior that results in
abnormally ineffective caching.
 
If the user has overridden the basic caching mechanisms, the user
agent should explicitly indicate to the user whenever this results in
the display of information that might not meet the server's
transparency requirements (in particular, if the displayed entity is
known to be stale). Since the protocol normally allows the user agent
to determine if responses are stale or not, this indication need only
be displayed when this actually happens. The indication need not be a
dialog box; it could be an icon (for example, a picture of a rotting
fish) or some other visual indicator.
 
If the user has overridden the caching mechanisms in a way that would
abnormally reduce the effectiveness of caches, the user agent should
continually display an indication (for example, a picture of currency
in flames) so that the user does not inadvertently consume excess
resources or suffer from excessive latency.
 
 
 
 
Fielding, et. al. Standards Track [Page 74]
RFC 2068 HTTP/1.1 January 1997
 
 
13.1.5 Exceptions to the Rules and Warnings
 
In some cases, the operator of a cache may choose to configure it to
return stale responses even when not requested by clients. This
decision should not be made lightly, but may be necessary for reasons
of availability or performance, especially when the cache is poorly
connected to the origin server. Whenever a cache returns a stale
response, it MUST mark it as such (using a Warning header). This
allows the client software to alert the user that there may be a
potential problem.
 
It also allows the user agent to take steps to obtain a first-hand or
fresh response. For this reason, a cache SHOULD NOT return a stale
response if the client explicitly requests a first-hand or fresh one,
unless it is impossible to comply for technical or policy reasons.
 
13.1.6 Client-controlled Behavior
 
While the origin server (and to a lesser extent, intermediate caches,
by their contribution to the age of a response) are the primary
source of expiration information, in some cases the client may need
to control a cache's decision about whether to return a cached
response without validating it. Clients do this using several
directives of the Cache-Control header.
 
A client's request may specify the maximum age it is willing to
accept of an unvalidated response; specifying a value of zero forces
the cache(s) to revalidate all responses. A client may also specify
the minimum time remaining before a response expires. Both of these
options increase constraints on the behavior of caches, and so cannot
further relax the cache's approximation of semantic transparency.
 
A client may also specify that it will accept stale responses, up to
some maximum amount of staleness. This loosens the constraints on the
caches, and so may violate the origin server's specified constraints
on semantic transparency, but may be necessary to support
disconnected operation, or high availability in the face of poor
connectivity.
 
13.2 Expiration Model
 
13.2.1 Server-Specified Expiration
 
HTTP caching works best when caches can entirely avoid making
requests to the origin server. The primary mechanism for avoiding
requests is for an origin server to provide an explicit expiration
time in the future, indicating that a response may be used to satisfy
subsequent requests. In other words, a cache can return a fresh
 
 
 
Fielding, et. al. Standards Track [Page 75]
RFC 2068 HTTP/1.1 January 1997
 
 
response without first contacting the server.
 
Our expectation is that servers will assign future explicit
expiration times to responses in the belief that the entity is not
likely to change, in a semantically significant way, before the
expiration time is reached. This normally preserves semantic
transparency, as long as the server's expiration times are carefully
chosen.
 
The expiration mechanism applies only to responses taken from a cache
and not to first-hand responses forwarded immediately to the
requesting client.
 
If an origin server wishes to force a semantically transparent cache
to validate every request, it may assign an explicit expiration time
in the past. This means that the response is always stale, and so the
cache SHOULD validate it before using it for subsequent requests. See
section 14.9.4 for a more restrictive way to force revalidation.
 
If an origin server wishes to force any HTTP/1.1 cache, no matter how
it is configured, to validate every request, it should use the
"must-revalidate" Cache-Control directive (see section 14.9).
 
Servers specify explicit expiration times using either the Expires
header, or the max-age directive of the Cache-Control header.
 
An expiration time cannot be used to force a user agent to refresh
its display or reload a resource; its semantics apply only to caching
mechanisms, and such mechanisms need only check a resource's
expiration status when a new request for that resource is initiated.
See section 13.13 for explanation of the difference between caches
and history mechanisms.
 
13.2.2 Heuristic Expiration
 
Since origin servers do not always provide explicit expiration times,
HTTP caches typically assign heuristic expiration times, employing
algorithms that use other header values (such as the Last-Modified
time) to estimate a plausible expiration time. The HTTP/1.1
specification does not provide specific algorithms, but does impose
worst-case constraints on their results. Since heuristic expiration
times may compromise semantic transparency, they should be used
cautiously, and we encourage origin servers to provide explicit
expiration times as much as possible.
 
 
 
 
 
 
 
Fielding, et. al. Standards Track [Page 76]
RFC 2068 HTTP/1.1 January 1997
 
 
13.2.3 Age Calculations
 
In order to know if a cached entry is fresh, a cache needs to know if
its age exceeds its freshness lifetime. We discuss how to calculate
the latter in section 13.2.4; this section describes how to calculate
the age of a response or cache entry.
 
In this discussion, we use the term "now" to mean "the current value
of the clock at the host performing the calculation." Hosts that use
HTTP, but especially hosts running origin servers and caches, should
use NTP [28] or some similar protocol to synchronize their clocks to
a globally accurate time standard.
 
Also note that HTTP/1.1 requires origin servers to send a Date header
with every response, giving the time at which the response was
generated. We use the term "date_value" to denote the value of the
Date header, in a form appropriate for arithmetic operations.
 
HTTP/1.1 uses the Age response-header to help convey age information
between caches. The Age header value is the sender's estimate of the
amount of time since the response was generated at the origin server.
In the case of a cached response that has been revalidated with the
origin server, the Age value is based on the time of revalidation,
not of the original response.
 
In essence, the Age value is the sum of the time that the response
has been resident in each of the caches along the path from the
origin server, plus the amount of time it has been in transit along
network paths.
 
We use the term "age_value" to denote the value of the Age header, in
a form appropriate for arithmetic operations.
 
A response's age can be calculated in two entirely independent ways:
 
1. now minus date_value, if the local clock is reasonably well
synchronized to the origin server's clock. If the result is
negative, the result is replaced by zero.
 
2. age_value, if all of the caches along the response path
implement HTTP/1.1.
 
Given that we have two independent ways to compute the age of a
response when it is received, we can combine these as
 
corrected_received_age = max(now - date_value, age_value)
 
and as long as we have either nearly synchronized clocks or all-
 
 
 
Fielding, et. al. Standards Track [Page 77]
RFC 2068 HTTP/1.1 January 1997
 
 
HTTP/1.1 paths, one gets a reliable (conservative) result.
 
Note that this correction is applied at each HTTP/1.1 cache along the
path, so that if there is an HTTP/1.0 cache in the path, the correct
received age is computed as long as the receiving cache's clock is
nearly in sync. We don't need end-to-end clock synchronization
(although it is good to have), and there is no explicit clock
synchronization step.
 
Because of network-imposed delays, some significant interval may pass
from the time that a server generates a response and the time it is
received at the next outbound cache or client. If uncorrected, this
delay could result in improperly low ages.
 
Because the request that resulted in the returned Age value must have
been initiated prior to that Age value's generation, we can correct
for delays imposed by the network by recording the time at which the
request was initiated. Then, when an Age value is received, it MUST
be interpreted relative to the time the request was initiated, not
the time that the response was received. This algorithm results in
conservative behavior no matter how much delay is experienced. So, we
compute:
 
corrected_initial_age = corrected_received_age
+ (now - request_time)
 
where "request_time" is the time (according to the local clock) when
the request that elicited this response was sent.
 
Summary of age calculation algorithm, when a cache receives a
response:
 
/*
* age_value
* is the value of Age: header received by the cache with
* this response.
* date_value
* is the value of the origin server's Date: header
* request_time
* is the (local) time when the cache made the request
* that resulted in this cached response
* response_time
* is the (local) time when the cache received the
* response
* now
* is the current (local) time
*/
apparent_age = max(0, response_time - date_value);
 
 
 
Fielding, et. al. Standards Track [Page 78]
RFC 2068 HTTP/1.1 January 1997
 
 
corrected_received_age = max(apparent_age, age_value);
response_delay = response_time - request_time;
corrected_initial_age = corrected_received_age + response_delay;
resident_time = now - response_time;
current_age = corrected_initial_age + resident_time;
 
When a cache sends a response, it must add to the
corrected_initial_age the amount of time that the response was
resident locally. It must then transmit this total age, using the Age
header, to the next recipient cache.
 
Note that a client cannot reliably tell that a response is first-
hand, but the presence of an Age header indicates that a response
is definitely not first-hand. Also, if the Date in a response is
earlier than the client's local request time, the response is
probably not first-hand (in the absence of serious clock skew).
 
13.2.4 Expiration Calculations
 
In order to decide whether a response is fresh or stale, we need to
compare its freshness lifetime to its age. The age is calculated as
described in section 13.2.3; this section describes how to calculate
the freshness lifetime, and to determine if a response has expired.
In the discussion below, the values can be represented in any form
appropriate for arithmetic operations.
 
We use the term "expires_value" to denote the value of the Expires
header. We use the term "max_age_value" to denote an appropriate
value of the number of seconds carried by the max-age directive of
the Cache-Control header in a response (see section 14.10.
 
The max-age directive takes priority over Expires, so if max-age is
present in a response, the calculation is simply:
 
freshness_lifetime = max_age_value
 
Otherwise, if Expires is present in the response, the calculation is:
 
freshness_lifetime = expires_value - date_value
 
Note that neither of these calculations is vulnerable to clock skew,
since all of the information comes from the origin server.
 
If neither Expires nor Cache-Control: max-age appears in the
response, and the response does not include other restrictions on
caching, the cache MAY compute a freshness lifetime using a
heuristic. If the value is greater than 24 hours, the cache must
attach Warning 13 to any response whose age is more than 24 hours if
 
 
 
Fielding, et. al. Standards Track [Page 79]
RFC 2068 HTTP/1.1 January 1997
 
 
such warning has not already been added.
 
Also, if the response does have a Last-Modified time, the heuristic
expiration value SHOULD be no more than some fraction of the interval
since that time. A typical setting of this fraction might be 10%.
 
The calculation to determine if a response has expired is quite
simple:
 
response_is_fresh = (freshness_lifetime > current_age)
 
13.2.5 Disambiguating Expiration Values
 
Because expiration values are assigned optimistically, it is possible
for two caches to contain fresh values for the same resource that are
different.
 
If a client performing a retrieval receives a non-first-hand response
for a request that was already fresh in its own cache, and the Date
header in its existing cache entry is newer than the Date on the new
response, then the client MAY ignore the response. If so, it MAY
retry the request with a "Cache-Control: max-age=0" directive (see
section 14.9), to force a check with the origin server.
 
If a cache has two fresh responses for the same representation with
different validators, it MUST use the one with the more recent Date
header. This situation may arise because the cache is pooling
responses from other caches, or because a client has asked for a
reload or a revalidation of an apparently fresh cache entry.
 
13.2.6 Disambiguating Multiple Responses
 
Because a client may be receiving responses via multiple paths, so
that some responses flow through one set of caches and other
responses flow through a different set of caches, a client may
receive responses in an order different from that in which the origin
server sent them. We would like the client to use the most recently
generated response, even if older responses are still apparently
fresh.
 
Neither the entity tag nor the expiration value can impose an
ordering on responses, since it is possible that a later response
intentionally carries an earlier expiration time. However, the
HTTP/1.1 specification requires the transmission of Date headers on
every response, and the Date values are ordered to a granularity of
one second.
 
 
 
 
 
Fielding, et. al. Standards Track [Page 80]
RFC 2068 HTTP/1.1 January 1997
 
 
When a client tries to revalidate a cache entry, and the response it
receives contains a Date header that appears to be older than the one
for the existing entry, then the client SHOULD repeat the request
unconditionally, and include
 
Cache-Control: max-age=0
 
to force any intermediate caches to validate their copies directly
with the origin server, or
 
Cache-Control: no-cache
 
to force any intermediate caches to obtain a new copy from the origin
server.
 
If the Date values are equal, then the client may use either response
(or may, if it is being extremely prudent, request a new response).
Servers MUST NOT depend on clients being able to choose
deterministically between responses generated during the same second,
if their expiration times overlap.
 
13.3 Validation Model
 
When a cache has a stale entry that it would like to use as a
response to a client's request, it first has to check with the origin
server (or possibly an intermediate cache with a fresh response) to
see if its cached entry is still usable. We call this "validating"
the cache entry. Since we do not want to have to pay the overhead of
retransmitting the full response if the cached entry is good, and we
do not want to pay the overhead of an extra round trip if the cached
entry is invalid, the HTTP/1.1 protocol supports the use of
conditional methods.
 
The key protocol features for supporting conditional methods are
those concerned with "cache validators." When an origin server
generates a full response, it attaches some sort of validator to it,
which is kept with the cache entry. When a client (user agent or
proxy cache) makes a conditional request for a resource for which it
has a cache entry, it includes the associated validator in the
request.
 
The server then checks that validator against the current validator
for the entity, and, if they match, it responds with a special status
code (usually, 304 (Not Modified)) and no entity-body. Otherwise, it
returns a full response (including entity-body). Thus, we avoid
transmitting the full response if the validator matches, and we avoid
an extra round trip if it does not match.
 
 
 
 
Fielding, et. al. Standards Track [Page 81]
RFC 2068 HTTP/1.1 January 1997
 
 
Note: the comparison functions used to decide if validators match
are defined in section 13.3.3.
 
In HTTP/1.1, a conditional request looks exactly the same as a normal
request for the same resource, except that it carries a special
header (which includes the validator) that implicitly turns the
method (usually, GET) into a conditional.
 
The protocol includes both positive and negative senses of cache-
validating conditions. That is, it is possible to request either that
a method be performed if and only if a validator matches or if and
only if no validators match.
 
Note: a response that lacks a validator may still be cached, and
served from cache until it expires, unless this is explicitly
prohibited by a Cache-Control directive. However, a cache cannot do
a conditional retrieval if it does not have a validator for the
entity, which means it will not be refreshable after it expires.
 
13.3.1 Last-modified Dates
 
The Last-Modified entity-header field value is often used as a cache
validator. In simple terms, a cache entry is considered to be valid
if the entity has not been modified since the Last-Modified value.
 
13.3.2 Entity Tag Cache Validators
 
The ETag entity-header field value, an entity tag, provides for an
"opaque" cache validator. This may allow more reliable validation in
situations where it is inconvenient to store modification dates,
where the one-second resolution of HTTP date values is not
sufficient, or where the origin server wishes to avoid certain
paradoxes that may arise from the use of modification dates.
 
Entity Tags are described in section 3.11. The headers used with
entity tags are described in sections 14.20, 14.25, 14.26 and 14.43.
 
13.3.3 Weak and Strong Validators
 
Since both origin servers and caches will compare two validators to
decide if they represent the same or different entities, one normally
would expect that if the entity (the entity-body or any entity-
headers) changes in any way, then the associated validator would
change as well. If this is true, then we call this validator a
"strong validator."
 
However, there may be cases when a server prefers to change the
validator only on semantically significant changes, and not when
 
 
 
Fielding, et. al. Standards Track [Page 82]
RFC 2068 HTTP/1.1 January 1997
 
 
insignificant aspects of the entity change. A validator that does not
always change when the resource changes is a "weak validator."
 
Entity tags are normally "strong validators," but the protocol
provides a mechanism to tag an entity tag as "weak." One can think of
a strong validator as one that changes whenever the bits of an entity
changes, while a weak value changes whenever the meaning of an entity
changes. Alternatively, one can think of a strong validator as part
of an identifier for a specific entity, while a weak validator is
part of an identifier for a set of semantically equivalent entities.
 
Note: One example of a strong validator is an integer that is
incremented in stable storage every time an entity is changed.
 
An entity's modification time, if represented with one-second
resolution, could be a weak validator, since it is possible that
the resource may be modified twice during a single second.
 
Support for weak validators is optional; however, weak validators
allow for more efficient caching of equivalent objects; for
example, a hit counter on a site is probably good enough if it is
updated every few days or weeks, and any value during that period
is likely "good enough" to be equivalent.
 
A "use" of a validator is either when a client generates a request
and includes the validator in a validating header field, or when a
server compares two validators.
 
Strong validators are usable in any context. Weak validators are only
usable in contexts that do not depend on exact equality of an entity.
For example, either kind is usable for a conditional GET of a full
entity. However, only a strong validator is usable for a sub-range
retrieval, since otherwise the client may end up with an internally
inconsistent entity.
 
The only function that the HTTP/1.1 protocol defines on validators is
comparison. There are two validator comparison functions, depending
on whether the comparison context allows the use of weak validators
or not:
 
o The strong comparison function: in order to be considered equal,
both validators must be identical in every way, and neither may be
weak.
o The weak comparison function: in order to be considered equal, both
validators must be identical in every way, but either or both of
them may be tagged as "weak" without affecting the result.
 
The weak comparison function MAY be used for simple (non-subrange)
 
 
 
Fielding, et. al. Standards Track [Page 83]
RFC 2068 HTTP/1.1 January 1997
 
 
GET requests. The strong comparison function MUST be used in all
other cases.
 
An entity tag is strong unless it is explicitly tagged as weak.
Section 3.11 gives the syntax for entity tags.
 
A Last-Modified time, when used as a validator in a request, is
implicitly weak unless it is possible to deduce that it is strong,
using the following rules:
 
o The validator is being compared by an origin server to the actual
current validator for the entity and,
o That origin server reliably knows that the associated entity did
not change twice during the second covered by the presented
validator.
or
 
o The validator is about to be used by a client in an If-Modified-
Since or If-Unmodified-Since header, because the client has a cache
entry for the associated entity, and
o That cache entry includes a Date value, which gives the time when
the origin server sent the original response, and
o The presented Last-Modified time is at least 60 seconds before the
Date value.
or
 
o The validator is being compared by an intermediate cache to the
validator stored in its cache entry for the entity, and
o That cache entry includes a Date value, which gives the time when
the origin server sent the original response, and
o The presented Last-Modified time is at least 60 seconds before the
Date value.
 
This method relies on the fact that if two different responses were
sent by the origin server during the same second, but both had the
same Last-Modified time, then at least one of those responses would
have a Date value equal to its Last-Modified time. The arbitrary 60-
second limit guards against the possibility that the Date and Last-
Modified values are generated from different clocks, or at somewhat
different times during the preparation of the response. An
implementation may use a value larger than 60 seconds, if it is
believed that 60 seconds is too short.
 
If a client wishes to perform a sub-range retrieval on a value for
which it has only a Last-Modified time and no opaque validator, it
may do this only if the Last-Modified time is strong in the sense
described here.
 
 
 
 
Fielding, et. al. Standards Track [Page 84]
RFC 2068 HTTP/1.1 January 1997
 
 
A cache or origin server receiving a cache-conditional request, other
than a full-body GET request, MUST use the strong comparison function
to evaluate the condition.
 
These rules allow HTTP/1.1 caches and clients to safely perform sub-
range retrievals on values that have been obtained from HTTP/1.0
servers.
 
13.3.4 Rules for When to Use Entity Tags and Last-modified Dates
 
We adopt a set of rules and recommendations for origin servers,
clients, and caches regarding when various validator types should be
used, and for what purposes.
 
HTTP/1.1 origin servers:
 
o SHOULD send an entity tag validator unless it is not feasible to
generate one.
o MAY send a weak entity tag instead of a strong entity tag, if
performance considerations support the use of weak entity tags, or
if it is unfeasible to send a strong entity tag.
o SHOULD send a Last-Modified value if it is feasible to send one,
unless the risk of a breakdown in semantic transparency that could
result from using this date in an If-Modified-Since header would
lead to serious problems.
 
In other words, the preferred behavior for an HTTP/1.1 origin server
is to send both a strong entity tag and a Last-Modified value.
 
In order to be legal, a strong entity tag MUST change whenever the
associated entity value changes in any way. A weak entity tag SHOULD
change whenever the associated entity changes in a semantically
significant way.
 
Note: in order to provide semantically transparent caching, an
origin server must avoid reusing a specific strong entity tag value
for two different entities, or reusing a specific weak entity tag
value for two semantically different entities. Cache entries may
persist for arbitrarily long periods, regardless of expiration
times, so it may be inappropriate to expect that a cache will never
again attempt to validate an entry using a validator that it
obtained at some point in the past.
 
HTTP/1.1 clients:
 
o If an entity tag has been provided by the origin server, MUST
use that entity tag in any cache-conditional request (using
If-Match or If-None-Match).
 
 
 
Fielding, et. al. Standards Track [Page 85]
RFC 2068 HTTP/1.1 January 1997
 
 
o If only a Last-Modified value has been provided by the origin
server, SHOULD use that value in non-subrange cache-conditional
requests (using If-Modified-Since).
o If only a Last-Modified value has been provided by an HTTP/1.0
origin server, MAY use that value in subrange cache-conditional
requests (using If-Unmodified-Since:). The user agent should
provide a way to disable this, in case of difficulty.
o If both an entity tag and a Last-Modified value have been
provided by the origin server, SHOULD use both validators in
cache-conditional requests. This allows both HTTP/1.0 and
HTTP/1.1 caches to respond appropriately.
 
An HTTP/1.1 cache, upon receiving a request, MUST use the most
restrictive validator when deciding whether the client's cache entry
matches the cache's own cache entry. This is only an issue when the
request contains both an entity tag and a last-modified-date
validator (If-Modified-Since or If-Unmodified-Since).
 
A note on rationale: The general principle behind these rules is
that HTTP/1.1 servers and clients should transmit as much non-
redundant information as is available in their responses and
requests. HTTP/1.1 systems receiving this information will make the
most conservative assumptions about the validators they receive.
 
HTTP/1.0 clients and caches will ignore entity tags. Generally,
last-modified values received or used by these systems will support
transparent and efficient caching, and so HTTP/1.1 origin servers
should provide Last-Modified values. In those rare cases where the
use of a Last-Modified value as a validator by an HTTP/1.0 system
could result in a serious problem, then HTTP/1.1 origin servers
should not provide one.
 
13.3.5 Non-validating Conditionals
 
The principle behind entity tags is that only the service author
knows the semantics of a resource well enough to select an
appropriate cache validation mechanism, and the specification of any
validator comparison function more complex than byte-equality would
open up a can of worms. Thus, comparisons of any other headers
(except Last-Modified, for compatibility with HTTP/1.0) are never
used for purposes of validating a cache entry.
 
13.4 Response Cachability
 
Unless specifically constrained by a Cache-Control (section 14.9)
directive, a caching system may always store a successful response
(see section 13.8) as a cache entry, may return it without validation
if it is fresh, and may return it after successful validation. If
 
 
 
Fielding, et. al. Standards Track [Page 86]
RFC 2068 HTTP/1.1 January 1997
 
 
there is neither a cache validator nor an explicit expiration time
associated with a response, we do not expect it to be cached, but
certain caches may violate this expectation (for example, when little
or no network connectivity is available). A client can usually detect
that such a response was taken from a cache by comparing the Date
header to the current time.
 
Note that some HTTP/1.0 caches are known to violate this
expectation without providing any Warning.
 
However, in some cases it may be inappropriate for a cache to retain
an entity, or to return it in response to a subsequent request. This
may be because absolute semantic transparency is deemed necessary by
the service author, or because of security or privacy considerations.
Certain Cache-Control directives are therefore provided so that the
server can indicate that certain resource entities, or portions
thereof, may not be cached regardless of other considerations.
 
Note that section 14.8 normally prevents a shared cache from saving
and returning a response to a previous request if that request
included an Authorization header.
 
A response received with a status code of 200, 203, 206, 300, 301 or
410 may be stored by a cache and used in reply to a subsequent
request, subject to the expiration mechanism, unless a Cache-Control
directive prohibits caching. However, a cache that does not support
the Range and Content-Range headers MUST NOT cache 206 (Partial
Content) responses.
 
A response received with any other status code MUST NOT be returned
in a reply to a subsequent request unless there are Cache-Control
directives or another header(s) that explicitly allow it. For
example, these include the following: an Expires header (section
14.21); a "max-age", "must-revalidate", "proxy-revalidate", "public"
or "private" Cache-Control directive (section 14.9).
 
13.5 Constructing Responses From Caches
 
The purpose of an HTTP cache is to store information received in
response to requests, for use in responding to future requests. In
many cases, a cache simply returns the appropriate parts of a
response to the requester. However, if the cache holds a cache entry
based on a previous response, it may have to combine parts of a new
response with what is held in the cache entry.
 
 
 
 
 
 
 
Fielding, et. al. Standards Track [Page 87]
RFC 2068 HTTP/1.1 January 1997
 
 
13.5.1 End-to-end and Hop-by-hop Headers
 
For the purpose of defining the behavior of caches and non-caching
proxies, we divide HTTP headers into two categories:
 
o End-to-end headers, which must be transmitted to the
ultimate recipient of a request or response. End-to-end
headers in responses must be stored as part of a cache entry
and transmitted in any response formed from a cache entry.
o Hop-by-hop headers, which are meaningful only for a single
transport-level connection, and are not stored by caches or
forwarded by proxies.
 
The following HTTP/1.1 headers are hop-by-hop headers:
 
o Connection
o Keep-Alive
o Public
o Proxy-Authenticate
o Transfer-Encoding
o Upgrade
 
All other headers defined by HTTP/1.1 are end-to-end headers.
 
Hop-by-hop headers introduced in future versions of HTTP MUST be
listed in a Connection header, as described in section 14.10.
 
13.5.2 Non-modifiable Headers
 
Some features of the HTTP/1.1 protocol, such as Digest
Authentication, depend on the value of certain end-to-end headers. A
cache or non-caching proxy SHOULD NOT modify an end-to-end header
unless the definition of that header requires or specifically allows
that.
 
A cache or non-caching proxy MUST NOT modify any of the following
fields in a request or response, nor may it add any of these fields
if not already present:
 
o Content-Location
o ETag
o Expires
o Last-Modified
 
 
 
 
 
 
 
 
Fielding, et. al. Standards Track [Page 88]
RFC 2068 HTTP/1.1 January 1997
 
 
A cache or non-caching proxy MUST NOT modify or add any of the
following fields in a response that contains the no-transform Cache-
Control directive, or in any request:
 
o Content-Encoding
o Content-Length
o Content-Range
o Content-Type
 
A cache or non-caching proxy MAY modify or add these fields in a
response that does not include no-transform, but if it does so, it
MUST add a Warning 14 (Transformation applied) if one does not
already appear in the response.
 
Warning: unnecessary modification of end-to-end headers may cause
authentication failures if stronger authentication mechanisms are
introduced in later versions of HTTP. Such authentication
mechanisms may rely on the values of header fields not listed here.
 
13.5.3 Combining Headers
 
When a cache makes a validating request to a server, and the server
provides a 304 (Not Modified) response, the cache must construct a
response to send to the requesting client. The cache uses the
entity-body stored in the cache entry as the entity-body of this
outgoing response. The end-to-end headers stored in the cache entry
are used for the constructed response, except that any end-to-end
headers provided in the 304 response MUST replace the corresponding
headers from the cache entry. Unless the cache decides to remove the
cache entry, it MUST also replace the end-to-end headers stored with
the cache entry with corresponding headers received in the incoming
response.
 
In other words, the set of end-to-end headers received in the
incoming response overrides all corresponding end-to-end headers
stored with the cache entry. The cache may add Warning headers (see
section 14.45) to this set.
 
If a header field-name in the incoming response matches more than one
header in the cache entry, all such old headers are replaced.
 
Note: this rule allows an origin server to use a 304 (Not Modified)
response to update any header associated with a previous response
for the same entity, although it might not always be meaningful or
correct to do so. This rule does not allow an origin server to use
a 304 (not Modified) response to entirely delete a header that it
had provided with a previous response.
 
 
 
 
Fielding, et. al. Standards Track [Page 89]
RFC 2068 HTTP/1.1 January 1997
 
 
13.5.4 Combining Byte Ranges
 
A response may transfer only a subrange of the bytes of an entity-
body, either because the request included one or more Range
specifications, or because a connection was broken prematurely. After
several such transfers, a cache may have received several ranges of
the same entity-body.
 
If a cache has a stored non-empty set of subranges for an entity, and
an incoming response transfers another subrange, the cache MAY
combine the new subrange with the existing set if both the following
conditions are met:
 
o Both the incoming response and the cache entry must have a cache
validator.
o The two cache validators must match using the strong comparison
function (see section 13.3.3).
 
If either requirement is not meant, the cache must use only the most
recent partial response (based on the Date values transmitted with
every response, and using the incoming response if these values are
equal or missing), and must discard the other partial information.
 
13.6 Caching Negotiated Responses
 
Use of server-driven content negotiation (section 12), as indicated
by the presence of a Vary header field in a response, alters the
conditions and procedure by which a cache can use the response for
subsequent requests.
 
A server MUST use the Vary header field (section 14.43) to inform a
cache of what header field dimensions are used to select among
multiple representations of a cachable response. A cache may use the
selected representation (the entity included with that particular
response) for replying to subsequent requests on that resource only
when the subsequent requests have the same or equivalent values for
all header fields specified in the Vary response-header. Requests
with a different value for one or more of those header fields would
be forwarded toward the origin server.
 
If an entity tag was assigned to the representation, the forwarded
request SHOULD be conditional and include the entity tags in an If-
None-Match header field from all its cache entries for the Request-
URI. This conveys to the server the set of entities currently held by
the cache, so that if any one of these entities matches the requested
entity, the server can use the ETag header in its 304 (Not Modified)
response to tell the cache which entry is appropriate. If the
entity-tag of the new response matches that of an existing entry, the
 
 
 
Fielding, et. al. Standards Track [Page 90]
RFC 2068 HTTP/1.1 January 1997
 
 
new response SHOULD be used to update the header fields of the
existing entry, and the result MUST be returned to the client.
 
The Vary header field may also inform the cache that the
representation was selected using criteria not limited to the
request-headers; in this case, a cache MUST NOT use the response in a
reply to a subsequent request unless the cache relays the new request
to the origin server in a conditional request and the server responds
with 304 (Not Modified), including an entity tag or Content-Location
that indicates which entity should be used.
 
If any of the existing cache entries contains only partial content
for the associated entity, its entity-tag SHOULD NOT be included in
the If-None-Match header unless the request is for a range that would
be fully satisfied by that entry.
 
If a cache receives a successful response whose Content-Location
field matches that of an existing cache entry for the same Request-
URI, whose entity-tag differs from that of the existing entry, and
whose Date is more recent than that of the existing entry, the
existing entry SHOULD NOT be returned in response to future requests,
and should be deleted from the cache.
 
13.7 Shared and Non-Shared Caches
 
For reasons of security and privacy, it is necessary to make a
distinction between "shared" and "non-shared" caches. A non-shared
cache is one that is accessible only to a single user. Accessibility
in this case SHOULD be enforced by appropriate security mechanisms.
All other caches are considered to be "shared." Other sections of
this specification place certain constraints on the operation of
shared caches in order to prevent loss of privacy or failure of
access controls.
 
13.8 Errors or Incomplete Response Cache Behavior
 
A cache that receives an incomplete response (for example, with fewer
bytes of data than specified in a Content-Length header) may store
the response. However, the cache MUST treat this as a partial
response. Partial responses may be combined as described in section
13.5.4; the result might be a full response or might still be
partial. A cache MUST NOT return a partial response to a client
without explicitly marking it as such, using the 206 (Partial
Content) status code. A cache MUST NOT return a partial response
using a status code of 200 (OK).
 
If a cache receives a 5xx response while attempting to revalidate an
entry, it may either forward this response to the requesting client,
 
 
 
Fielding, et. al. Standards Track [Page 91]
RFC 2068 HTTP/1.1 January 1997
 
 
or act as if the server failed to respond. In the latter case, it MAY
return a previously received response unless the cached entry
includes the "must-revalidate" Cache-Control directive (see section
14.9).
 
13.9 Side Effects of GET and HEAD
 
Unless the origin server explicitly prohibits the caching of their
responses, the application of GET and HEAD methods to any resources
SHOULD NOT have side effects that would lead to erroneous behavior if
these responses are taken from a cache. They may still have side
effects, but a cache is not required to consider such side effects in
its caching decisions. Caches are always expected to observe an
origin server's explicit restrictions on caching.
 
We note one exception to this rule: since some applications have
traditionally used GETs and HEADs with query URLs (those containing a
"?" in the rel_path part) to perform operations with significant side
effects, caches MUST NOT treat responses to such URLs as fresh unless
the server provides an explicit expiration time. This specifically
means that responses from HTTP/1.0 servers for such URIs should not
be taken from a cache. See section 9.1.1 for related information.
 
13.10 Invalidation After Updates or Deletions
 
The effect of certain methods at the origin server may cause one or
more existing cache entries to become non-transparently invalid. That
is, although they may continue to be "fresh," they do not accurately
reflect what the origin server would return for a new request.
 
There is no way for the HTTP protocol to guarantee that all such
cache entries are marked invalid. For example, the request that
caused the change at the origin server may not have gone through the
proxy where a cache entry is stored. However, several rules help
reduce the likelihood of erroneous behavior.
 
In this section, the phrase "invalidate an entity" means that the
cache should either remove all instances of that entity from its
storage, or should mark these as "invalid" and in need of a mandatory
revalidation before they can be returned in response to a subsequent
request.
 
 
 
 
 
 
 
 
 
 
Fielding, et. al. Standards Track [Page 92]
RFC 2068 HTTP/1.1 January 1997
 
 
Some HTTP methods may invalidate an entity. This is either the entity
referred to by the Request-URI, or by the Location or Content-
Location response-headers (if present). These methods are:
 
o PUT
o DELETE
o POST
 
In order to prevent denial of service attacks, an invalidation based
on the URI in a Location or Content-Location header MUST only be
performed if the host part is the same as in the Request-URI.
 
13.11 Write-Through Mandatory
 
All methods that may be expected to cause modifications to the origin
server's resources MUST be written through to the origin server. This
currently includes all methods except for GET and HEAD. A cache MUST
NOT reply to such a request from a client before having transmitted
the request to the inbound server, and having received a
corresponding response from the inbound server. This does not prevent
a cache from sending a 100 (Continue) response before the inbound
server has replied.
 
The alternative (known as "write-back" or "copy-back" caching) is not
allowed in HTTP/1.1, due to the difficulty of providing consistent
updates and the problems arising from server, cache, or network
failure prior to write-back.
 
13.12 Cache Replacement
 
If a new cachable (see sections 14.9.2, 13.2.5, 13.2.6 and 13.8)
response is received from a resource while any existing responses for
the same resource are cached, the cache SHOULD use the new response
to reply to the current request. It may insert it into cache storage
and may, if it meets all other requirements, use it to respond to any
future requests that would previously have caused the old response to
be returned. If it inserts the new response into cache storage it
should follow the rules in section 13.5.3.
 
Note: a new response that has an older Date header value than
existing cached responses is not cachable.
 
13.13 History Lists
 
User agents often have history mechanisms, such as "Back" buttons and
history lists, which can be used to redisplay an entity retrieved
earlier in a session.
 
 
 
 
Fielding, et. al. Standards Track [Page 93]
RFC 2068 HTTP/1.1 January 1997
 
 
History mechanisms and caches are different. In particular history
mechanisms SHOULD NOT try to show a semantically transparent view of
the current state of a resource. Rather, a history mechanism is meant
to show exactly what the user saw at the time when the resource was
retrieved.
 
By default, an expiration time does not apply to history mechanisms.
If the entity is still in storage, a history mechanism should display
it even if the entity has expired, unless the user has specifically
configured the agent to refresh expired history documents.
 
This should not be construed to prohibit the history mechanism from
telling the user that a view may be stale.
 
Note: if history list mechanisms unnecessarily prevent users from
viewing stale resources, this will tend to force service authors to
avoid using HTTP expiration controls and cache controls when they
would otherwise like to. Service authors may consider it important
that users not be presented with error messages or warning messages
when they use navigation controls (such as BACK) to view previously
fetched resources. Even though sometimes such resources ought not
to cached, or ought to expire quickly, user interface
considerations may force service authors to resort to other means
of preventing caching (e.g. "once-only" URLs) in order not to
suffer the effects of improperly functioning history mechanisms.
 
14 Header Field Definitions
 
This section defines the syntax and semantics of all standard
HTTP/1.1 header fields. For entity-header fields, both sender and
recipient refer to either the client or the server, depending on who
sends and who receives the entity.
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
Fielding, et. al. Standards Track [Page 94]
RFC 2068 HTTP/1.1 January 1997
 
 
14.1 Accept
 
The Accept request-header field can be used to specify certain media
types which are acceptable for the response. Accept headers can be
used to indicate that the request is specifically limited to a small
set of desired types, as in the case of a request for an in-line
image.
 
Accept = "Accept" ":"
#( media-range [ accept-params ] )
 
media-range = ( "*/*"
| ( type "/" "*" )
| ( type "/" subtype )
) *( ";" parameter )
 
accept-params = ";" "q" "=" qvalue *( accept-extension )
 
accept-extension = ";" token [ "=" ( token | quoted-string ) ]
 
The asterisk "*" character is used to group media types into ranges,
with "*/*" indicating all media types and "type/*" indicating all
subtypes of that type. The media-range MAY include media type
parameters that are applicable to that range.
 
Each media-range MAY be followed by one or more accept-params,
beginning with the "q" parameter for indicating a relative quality
factor. The first "q" parameter (if any) separates the media-range
parameter(s) from the accept-params. Quality factors allow the user
or user agent to indicate the relative degree of preference for that
media-range, using the qvalue scale from 0 to 1 (section 3.9). The
default value is q=1.
 
Note: Use of the "q" parameter name to separate media type
parameters from Accept extension parameters is due to historical
practice. Although this prevents any media type parameter named
"q" from being used with a media range, such an event is believed
to be unlikely given the lack of any "q" parameters in the IANA
media type registry and the rare usage of any media type parameters
in Accept. Future media types should be discouraged from
registering any parameter named "q".
 
The example
 
Accept: audio/*; q=0.2, audio/basic
 
SHOULD be interpreted as "I prefer audio/basic, but send me any audio
type if it is the best available after an 80% mark-down in quality."
 
 
 
Fielding, et. al. Standards Track [Page 95]
RFC 2068 HTTP/1.1 January 1997
 
 
If no Accept header field is present, then it is assumed that the
client accepts all media types. If an Accept header field is present,
and if the server cannot send a response which is acceptable
according to the combined Accept field value, then the server SHOULD
send a 406 (not acceptable) response.
 
A more elaborate example is
 
Accept: text/plain; q=0.5, text/html,
text/x-dvi; q=0.8, text/x-c
 
Verbally, this would be interpreted as "text/html and text/x-c are
the preferred media types, but if they do not exist, then send the
text/x-dvi entity, and if that does not exist, send the text/plain
entity."
 
Media ranges can be overridden by more specific media ranges or
specific media types. If more than one media range applies to a given
type, the most specific reference has precedence. For example,
 
Accept: text/*, text/html, text/html;level=1, */*
 
have the following precedence:
 
1) text/html;level=1
2) text/html
3) text/*
4) */*
 
The media type quality factor associated with a given type is
determined by finding the media range with the highest precedence
which matches that type. For example,
 
Accept: text/*;q=0.3, text/html;q=0.7, text/html;level=1,
text/html;level=2;q=0.4, */*;q=0.5
 
would cause the following values to be associated:
 
text/html;level=1 = 1
text/html = 0.7
text/plain = 0.3
image/jpeg = 0.5
text/html;level=2 = 0.4
text/html;level=3 = 0.7
 
Note: A user agent may be provided with a default set of quality
values for certain media ranges. However, unless the user agent is
a closed system which cannot interact with other rendering agents,
 
 
 
Fielding, et. al. Standards Track [Page 96]
RFC 2068 HTTP/1.1 January 1997
 
 
this default set should be configurable by the user.
 
14.2 Accept-Charset
 
The Accept-Charset request-header field can be used to indicate what
character sets are acceptable for the response. This field allows
clients capable of understanding more comprehensive or special-
purpose character sets to signal that capability to a server which is
capable of representing documents in those character sets. The ISO-
8859-1 character set can be assumed to be acceptable to all user
agents.
 
Accept-Charset = "Accept-Charset" ":"
1#( charset [ ";" "q" "=" qvalue ] )
 
Character set values are described in section 3.4. Each charset may
be given an associated quality value which represents the user's
preference for that charset. The default value is q=1. An example is
 
Accept-Charset: iso-8859-5, unicode-1-1;q=0.8
 
If no Accept-Charset header is present, the default is that any
character set is acceptable. If an Accept-Charset header is present,
and if the server cannot send a response which is acceptable
according to the Accept-Charset header, then the server SHOULD send
an error response with the 406 (not acceptable) status code, though
the sending of an unacceptable response is also allowed.
 
14.3 Accept-Encoding
 
The Accept-Encoding request-header field is similar to Accept, but
restricts the content-coding values (section 14.12) which are
acceptable in the response.
 
Accept-Encoding = "Accept-Encoding" ":"
#( content-coding )
 
An example of its use is
 
Accept-Encoding: compress, gzip
 
If no Accept-Encoding header is present in a request, the server MAY
assume that the client will accept any content coding. If an Accept-
Encoding header is present, and if the server cannot send a response
which is acceptable according to the Accept-Encoding header, then the
server SHOULD send an error response with the 406 (Not Acceptable)
status code.
 
 
 
 
Fielding, et. al. Standards Track [Page 97]
RFC 2068 HTTP/1.1 January 1997
 
 
An empty Accept-Encoding value indicates none are acceptable.
 
14.4 Accept-Language
 
The Accept-Language request-header field is similar to Accept, but
restricts the set of natural languages that are preferred as a
response to the request.
 
Accept-Language = "Accept-Language" ":"
1#( language-range [ ";" "q" "=" qvalue ] )
 
language-range = ( ( 1*8ALPHA *( "-" 1*8ALPHA ) ) | "*" )
 
Each language-range MAY be given an associated quality value which
represents an estimate of the user's preference for the languages
specified by that range. The quality value defaults to "q=1". For
example,
 
Accept-Language: da, en-gb;q=0.8, en;q=0.7
 
would mean: "I prefer Danish, but will accept British English and
other types of English." A language-range matches a language-tag if
it exactly equals the tag, or if it exactly equals a prefix of the
tag such that the first tag character following the prefix is "-".
The special range "*", if present in the Accept-Language field,
matches every tag not matched by any other range present in the
Accept-Language field.
 
Note: This use of a prefix matching rule does not imply that
language tags are assigned to languages in such a way that it is
always true that if a user understands a language with a certain
tag, then this user will also understand all languages with tags
for which this tag is a prefix. The prefix rule simply allows the
use of prefix tags if this is the case.
 
The language quality factor assigned to a language-tag by the
Accept-Language field is the quality value of the longest language-
range in the field that matches the language-tag. If no language-
range in the field matches the tag, the language quality factor
assigned is 0. If no Accept-Language header is present in the
request, the server SHOULD assume that all languages are equally
acceptable. If an Accept-Language header is present, then all
languages which are assigned a quality factor greater than 0 are
acceptable.
 
It may be contrary to the privacy expectations of the user to send an
Accept-Language header with the complete linguistic preferences of
the user in every request. For a discussion of this issue, see
 
 
 
Fielding, et. al. Standards Track [Page 98]
RFC 2068 HTTP/1.1 January 1997
 
 
section 15.7.
 
Note: As intelligibility is highly dependent on the individual
user, it is recommended that client applications make the choice of
linguistic preference available to the user. If the choice is not
made available, then the Accept-Language header field must not be
given in the request.
 
14.5 Accept-Ranges
 
The Accept-Ranges response-header field allows the server to indicate
its acceptance of range requests for a resource:
 
Accept-Ranges = "Accept-Ranges" ":" acceptable-ranges
 
acceptable-ranges = 1#range-unit | "none"
 
Origin servers that accept byte-range requests MAY send
 
Accept-Ranges: bytes
 
but are not required to do so. Clients MAY generate byte-range
requests without having received this header for the resource
involved.
 
Servers that do not accept any kind of range request for a resource
MAY send
 
Accept-Ranges: none
 
to advise the client not to attempt a range request.
 
14.6 Age
 
The Age response-header field conveys the sender's estimate of the
amount of time since the response (or its revalidation) was generated
at the origin server. A cached response is "fresh" if its age does
not exceed its freshness lifetime. Age values are calculated as
specified in section 13.2.3.
 
Age = "Age" ":" age-value
 
age-value = delta-seconds
 
Age values are non-negative decimal integers, representing time in
seconds.
 
 
 
 
 
Fielding, et. al. Standards Track [Page 99]
RFC 2068 HTTP/1.1 January 1997
 
 
If a cache receives a value larger than the largest positive integer
it can represent, or if any of its age calculations overflows, it
MUST transmit an Age header with a value of 2147483648 (2^31).
HTTP/1.1 caches MUST send an Age header in every response. Caches
SHOULD use an arithmetic type of at least 31 bits of range.
 
14.7 Allow
 
The Allow entity-header field lists the set of methods supported by
the resource identified by the Request-URI. The purpose of this field
is strictly to inform the recipient of valid methods associated with
the resource. An Allow header field MUST be present in a 405 (Method
Not Allowed) response.
 
Allow = "Allow" ":" 1#method
 
Example of use:
 
Allow: GET, HEAD, PUT
 
This field cannot prevent a client from trying other methods.
However, the indications given by the Allow header field value SHOULD
be followed. The actual set of allowed methods is defined by the
origin server at the time of each request.
 
The Allow header field MAY be provided with a PUT request to
recommend the methods to be supported by the new or modified
resource. The server is not required to support these methods and
SHOULD include an Allow header in the response giving the actual
supported methods.
 
A proxy MUST NOT modify the Allow header field even if it does not
understand all the methods specified, since the user agent MAY have
other means of communicating with the origin server.
 
The Allow header field does not indicate what methods are implemented
at the server level. Servers MAY use the Public response-header field
(section 14.35) to describe what methods are implemented on the
server as a whole.
 
14.8 Authorization
 
A user agent that wishes to authenticate itself with a server--
usually, but not necessarily, after receiving a 401 response--MAY do
so by including an Authorization request-header field with the
request. The Authorization field value consists of credentials
containing the authentication information of the user agent for the
realm of the resource being requested.
 
 
 
Fielding, et. al. Standards Track [Page 100]
RFC 2068 HTTP/1.1 January 1997
 
 
Authorization = "Authorization" ":" credentials
 
HTTP access authentication is described in section 11. If a request
is authenticated and a realm specified, the same credentials SHOULD
be valid for all other requests within this realm.
 
When a shared cache (see section 13.7) receives a request containing
an Authorization field, it MUST NOT return the corresponding response
as a reply to any other request, unless one of the following specific
exceptions holds:
 
1. If the response includes the "proxy-revalidate" Cache-Control
directive, the cache MAY use that response in replying to a
subsequent request, but a proxy cache MUST first revalidate it with
the origin server, using the request-headers from the new request
to allow the origin server to authenticate the new request.
2. If the response includes the "must-revalidate" Cache-Control
directive, the cache MAY use that response in replying to a
subsequent request, but all caches MUST first revalidate it with
the origin server, using the request-headers from the new request
to allow the origin server to authenticate the new request.
3. If the response includes the "public" Cache-Control directive, it
may be returned in reply to any subsequent request.
 
14.9 Cache-Control
 
The Cache-Control general-header field is used to specify directives
that MUST be obeyed by all caching mechanisms along the
request/response chain. The directives specify behavior intended to
prevent caches from adversely interfering with the request or
response. These directives typically override the default caching
algorithms. Cache directives are unidirectional in that the presence
of a directive in a request does not imply that the same directive
should be given in the response.
 
Note that HTTP/1.0 caches may not implement Cache-Control and may
only implement Pragma: no-cache (see section 14.32).
 
Cache directives must be passed through by a proxy or gateway
application, regardless of their significance to that application,
since the directives may be applicable to all recipients along the
request/response chain. It is not possible to specify a cache-
directive for a specific cache.
 
Cache-Control = "Cache-Control" ":" 1#cache-directive
 
cache-directive = cache-request-directive
| cache-response-directive
 
 
 
Fielding, et. al. Standards Track [Page 101]
RFC 2068 HTTP/1.1 January 1997
 
 
cache-request-directive =
"no-cache" [ "=" <"> 1#field-name <"> ]
| "no-store"
| "max-age" "=" delta-seconds
| "max-stale" [ "=" delta-seconds ]
| "min-fresh" "=" delta-seconds
| "only-if-cached"
| cache-extension
 
cache-response-directive =
"public"
| "private" [ "=" <"> 1#field-name <"> ]
| "no-cache" [ "=" <"> 1#field-name <"> ]
| "no-store"
| "no-transform"
| "must-revalidate"
| "proxy-revalidate"
| "max-age" "=" delta-seconds
| cache-extension
 
cache-extension = token [ "=" ( token | quoted-string ) ]
 
When a directive appears without any 1#field-name parameter, the
directive applies to the entire request or response. When such a
directive appears with a 1#field-name parameter, it applies only to
the named field or fields, and not to the rest of the request or
response. This mechanism supports extensibility; implementations of
future versions of the HTTP protocol may apply these directives to
header fields not defined in HTTP/1.1.
 
The cache-control directives can be broken down into these general
categories:
 
o Restrictions on what is cachable; these may only be imposed by the
origin server.
o Restrictions on what may be stored by a cache; these may be imposed
by either the origin server or the user agent.
o Modifications of the basic expiration mechanism; these may be
imposed by either the origin server or the user agent.
o Controls over cache revalidation and reload; these may only be
imposed by a user agent.
o Control over transformation of entities.
o Extensions to the caching system.
 
 
 
 
 
 
 
 
Fielding, et. al. Standards Track [Page 102]
RFC 2068 HTTP/1.1 January 1997
 
 
14.9.1 What is Cachable
 
By default, a response is cachable if the requirements of the request
method, request header fields, and the response status indicate that
it is cachable. Section 13.4 summarizes these defaults for
cachability. The following Cache-Control response directives allow an
origin server to override the default cachability of a response:
 
public
Indicates that the response is cachable by any cache, even if it
would normally be non-cachable or cachable only within a non-shared
cache. (See also Authorization, section 14.8, for additional
details.)
 
private
Indicates that all or part of the response message is intended for a
single user and MUST NOT be cached by a shared cache. This allows an
origin server to state that the specified parts of the response are
intended for only one user and are not a valid response for requests
by other users. A private (non-shared) cache may cache the response.
 
Note: This usage of the word private only controls where the
response may be cached, and cannot ensure the privacy of the
message content.
 
no-cache
Indicates that all or part of the response message MUST NOT be cached
anywhere. This allows an origin server to prevent caching even by
caches that have been configured to return stale responses to client
requests.
 
Note: Most HTTP/1.0 caches will not recognize or obey this
directive.
 
14.9.2 What May be Stored by Caches
 
The purpose of the no-store directive is to prevent the inadvertent
release or retention of sensitive information (for example, on backup
tapes). The no-store directive applies to the entire message, and may
be sent either in a response or in a request. If sent in a request, a
cache MUST NOT store any part of either this request or any response
to it. If sent in a response, a cache MUST NOT store any part of
either this response or the request that elicited it. This directive
applies to both non-shared and shared caches. "MUST NOT store" in
this context means that the cache MUST NOT intentionally store the
information in non-volatile storage, and MUST make a best-effort
attempt to remove the information from volatile storage as promptly
as possible after forwarding it.
 
 
 
Fielding, et. al. Standards Track [Page 103]
RFC 2068 HTTP/1.1 January 1997
 
 
Even when this directive is associated with a response, users may
explicitly store such a response outside of the caching system (e.g.,
with a "Save As" dialog). History buffers may store such responses as
part of their normal operation.
 
The purpose of this directive is to meet the stated requirements of
certain users and service authors who are concerned about accidental
releases of information via unanticipated accesses to cache data
structures. While the use of this directive may improve privacy in
some cases, we caution that it is NOT in any way a reliable or
sufficient mechanism for ensuring privacy. In particular, malicious
or compromised caches may not recognize or obey this directive; and
communications networks may be vulnerable to eavesdropping.
 
14.9.3 Modifications of the Basic Expiration Mechanism
 
The expiration time of an entity may be specified by the origin
server using the Expires header (see section 14.21). Alternatively,
it may be specified using the max-age directive in a response.
 
If a response includes both an Expires header and a max-age
directive, the max-age directive overrides the Expires header, even
if the Expires header is more restrictive. This rule allows an origin
server to provide, for a given response, a longer expiration time to
an HTTP/1.1 (or later) cache than to an HTTP/1.0 cache. This may be
useful if certain HTTP/1.0 caches improperly calculate ages or
expiration times, perhaps due to desynchronized clocks.
 
Note: most older caches, not compliant with this specification, do
not implement any Cache-Control directives. An origin server
wishing to use a Cache-Control directive that restricts, but does
not prevent, caching by an HTTP/1.1-compliant cache may exploit the
requirement that the max-age directive overrides the Expires
header, and the fact that non-HTTP/1.1-compliant caches do not
observe the max-age directive.
 
Other directives allow an user agent to modify the basic expiration
mechanism. These directives may be specified on a request:
 
max-age
Indicates that the client is willing to accept a response whose age
is no greater than the specified time in seconds. Unless max-stale
directive is also included, the client is not willing to accept a
stale response.
 
min-fresh
Indicates that the client is willing to accept a response whose
freshness lifetime is no less than its current age plus the
 
 
 
Fielding, et. al. Standards Track [Page 104]
RFC 2068 HTTP/1.1 January 1997
 
 
specified time in seconds. That is, the client wants a response
that will still be fresh for at least the specified number of
seconds.
 
max-stale
Indicates that the client is willing to accept a response that has
exceeded its expiration time. If max-stale is assigned a value,
then the client is willing to accept a response that has exceeded
its expiration time by no more than the specified number of