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Differences from draft-ietf-http-v10-spec-00.txt
Hypertext Transfer Protocol -- HTTP/1.0
Status of this Memo
This document is an Internet-Draft. Internet-Drafts are working
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Distribution of this document is unlimited. Please send comments to
the HTTP working group at <http-wg@cuckoo.hpl.hp.com>. Discussions
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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 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 reflects preferred usage
of the protocol referred to as "HTTP/1.0".
Table of Contents
1. Introduction
1.1 Purpose
1.2 Overall Operation
1.3 Terminology
2. Notational Conventions and Generic Grammar
2.1 Augmented BNF
2.2 Basic Rules
3. Protocol Parameters
3.1 HTTP Version
3.2 Uniform Resource Identifiers
3.2.1 General Syntax
3.2.2 http URL
3.3 Date/Time Formats
3.3.1 Full Date
3.3.2 Delta Seconds
3.4 Media Types
3.4.1 Canonicalization and Text Defaults
3.4.2 Multipart Types
3.5 Character Set Encodings
3.6 Encoding Mechanisms
3.7 Transfer Encodings
3.8 Language Tags
3.9 Quality Values
3.10 Product Tokens
4. HTTP Message
4.1 Message Types
4.2 Message Headers
4.3 General Message Header Fields
5. Request
5.1 Request-Line
5.2 Method
5.2.1 GET
5.2.2 HEAD
5.2.3 POST
5.2.4 PUT
5.2.5 DELETE
5.2.6 LINK
5.2.7 UNLINK
5.3 Request-URI
5.4 Request Header Fields
6. Response
6.1 Status-Line
6.2 Status Codes and Reason Phrases
6.2.1 Informational 1xx
6.2.2 Successful 2xx
6.2.3 Redirection 3xx
6.2.4 Client Error 4xx
6.2.5 Server Errors 5xx
6.3 Response Header Fields
7. Entity
7.1 Entity Header Fields
7.2 Entity Body
7.2.1 Type
7.2.2 Length
8. Header Field Definitions
8.1 Accept
8.2 Accept-Charset
8.3 Accept-Encoding
8.4 Accept-Language
8.5 Allow
8.6 Authorization
8.7 Content-Encoding
8.8 Content-Language
8.9 Content-Length
8.10 Content-Transfer-Encoding
8.11 Content-Type
8.12 Date
8.13 Expires
8.14 Forwarded
8.15 From
8.16 If-Modified-Since
8.17 Last-Modified
8.18 Link
8.19 Location
8.20 MIME-Version
8.21 Orig-URI
8.22 Pragma
8.23 Public
8.24 Referer
8.25 Retry-After
8.26 Server
8.27 Title
8.28 URI
8.29 User-Agent
8.30 WWW-Authenticate
9. Content Negotiation
10. Access Authentication
10.1 Basic Authentication Scheme
11. Security Considerations
11.1 Authentication of Clients
11.2 Idempotent Methods
11.3 Abuse of Server Log Information
11.4 Transfer of Sensitive Information
12. Acknowledgments
13. References
14. Authors' Addresses
Appendix A. Internet Media Type message/http
Appendix B. Tolerant Applications
Appendix C. Relationship to MIME
C.1 Conversion to Canonical Form
C.1.1 Representation of Line Breaks
C.1.2 Default Character Set Encoding
C.2 Default Content-Transfer-Encoding
C.3 Introduction of Content-Encoding
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 preferred usage of the protocol
referred to as "HTTP/1.0". This specification does not necessarily
reflect the "current practice" of any single HTTP server or client
implementation. It does, however, seek to remain compatible with
existing implementations wherever possible, and is the reference
for future implementations of HTTP/1.0.
Practical information systems require more functionality than
simple retrieval, including search, front-end update, and
annotation. HTTP/1.0 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) [3],
as a location (URL) [5] or name (URN) [18], 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 [8] and the
Multipurpose Internet Mail Extensions (MIME) [6].
HTTP/1.0 is also used for communication between user agents and
various gateways, allowing hypermedia access to existing Internet
protocols like SMTP [14], NNTP [12], FTP [16], Gopher [2], and
WAIS [9]. HTTP/1.0 is designed to allow such gateways, via proxy
servers, without any loss of the data conveyed by those earlier
protocols.
1.2 Overall Operation
The HTTP protocol is based on a request/response paradigm. A
requesting program (termed a client) establishes a connection with
a receiving program (termed 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 status line, including its protocol version and a success or
error code, followed by a MIME-like message containing server
information, entity metainformation, and possible body content. It
should be noted that a given program may be capable of being both a
client and a server; our use of those terms refers only to the role
being performed by the program during a particular connection,
rather than to the program's purpose in general.
On the Internet, the communication generally takes place over a
TCP/IP connection. The default port is TCP 80 [17], but other ports
can be used. This does not preclude the HTTP/1.0 protocol from
being implemented on top of any other protocol on the Internet, or
on other networks. 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.
For most implementations, the connection is established by the
client prior to each request and closed by the server after sending
the response. However, this is not a feature of the protocol and is
not required by this specification. Both clients and servers must
be capable of handling cases where either party closes the
connection prematurely, due to user action, automated time-out, or
program failure. In any case, the closing of the connection by
either or both parties always terminates the current request,
regardless of its status.
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 virtual circuit established between two parties for the
purpose of communication.
message
A structured sequence of octets transmitted via the connection
as the basic component of communication.
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.
entity
A particular representation or rendition of a 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
A program that establishes connections for the purpose of
sending requests.
user agent
The client program which is closest to the user and which
initiates requests at their behest.
server
A 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 forwarding requests. Proxies are often used
to act as a portal through a network firewall. A proxy server
accepts requests from other clients and services them either
internally or by passing them, with possible translation, on to
other servers. A caching proxy is a proxy server with a local
cache of server responses -- some requested resources can be
serviced from the cache rather than from the origin server. Some
proxy servers also act as origin servers.
gateway
A proxy which services HTTP requests by translation into
protocols other than HTTP. The reply sent from the remote server
to the gateway is likewise translated into HTTP before being
forwarded to the user agent.
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 [8]. Implementors will need to be familiar with the
notation in order to understand this specification. The augmented
BNF includes the following constructs:
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.
#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, zero or more 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.
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 character set
encoding is defined by [19].
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)>
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 3.4.
CRLF = CR LF
HTTP/1.0 headers can be folded onto multiple lines if the
continuation lines begin with linear whitespace characters. All
linear whitespace, including folding, has the same semantics as SP.
LWS = [CRLF] 1*( SP | HT )
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
token = 1*<any CHAR except CTLs or tspecials>
tspecials = "(" | ")" | "<" | ">" | "@"
| "," | ";" | ":" | "\" | <">
| "/" | "[" | "]" | "?" | "="
| SP | HT
Comments can be included in HTTP header fields by surrounding the
comment text with parentheses.
comment = "(" *( ctext | comment ) ")"
ctext = <any text excluding "(" and ")">
Note: Use of comments within HTTP headers is generally
discouraged, since they are rarely seen by human eyes and
hence only increase network traffic. However, they may be
useful for messages posted or retrieved via NNTP and SMTP
gateways.
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>
The backslash character ("\") may be used as a single-character
quoting mechanism only within quoted-string and comment constructs.
quoted-pair = "\" CHAR
When left unquoted and not within a comment, HTTP uses angle
brackets to delimit machine-processable addresses; any LWS inside
the angle brackets should be ignored.
addr-string = ( "<" *(qatext) ">" )
qatext = <any CHAR except "<", ">", and CTLs,
but including LWS>
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 set
encodings other than US-ASCII only when encoded according to the
rules of RFC 1522 [13].
text = <any OCTET except CTLs,
but including LWS>
Recipients of header field text containing octets outside the
US-ASCII character set encoding may assume that they are ISO-8859-1
characters if there is no other encoding indicated by an RFC 1522
mechanism.
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
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 servers must be able to recognize the format of the
Request-Line for all lower-version requests, to understand any
valid request in the format of the immediately-prior major version
(<major-1>), to understand any valid request in the format of their
own native major version (<major>) with the same or lower minor
version, and to respond appropriately with a message within the
same <major> protocol version used by the client, even when the
response is simply an error message.
HTTP clients must be able to recognize the format of the
Status-Line for all lower-version responses, to understand any
valid response in the format of the immediately-prior major version
(<major-1>), and to understand any valid response in the format of
their own native major version (<major>) with the same or lower
minor version. The following hypothetical example illustrates the
required behavior.
o A valid HTTP/3.5 request is received and the server's native
protocol version is
o Less than 3.0: it should attempt to understand the request
and respond (possibly with an error) in its native format;
o Major number of 3: It should understand the request and
respond in its native format;
o Major number of 4: It should understand the request and
respond with a version 3 message;
o Major number higher than 4: It should attempt to understand
the request and respond (possibly with an error) with a
version 3 message;
o User agent receives a response to an HTTP/3.5 request, and the
response version is
o Less than 2.0: It should attempt to understand the response
and unobtrusively warn the user of the version mismatch;
o 2.0--3.4: It should understand the response and be aware
that its request may not have been fully understood by the
server;
o 3.5 or higher 3: It should understand the response and can
assume that the server understood all aspects of the request
if the response does not indicate an error;
o 4.0 or higher: It should attempt to understand the response
and unobtrusively warn the user of the version mismatch.
Proxies must be careful in forwarding requests that are received in
a format different than that of the proxy's native version. Since
the protocol version indicates the protocol capability of the
sender, a proxy 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 must either downgrade the request
version or respond with an error. Requests with a version lower
than that of the proxy's native format may be upgraded by the proxy
before being forwarded; the proxy's response to that request must
follow the normal server requirements.
3.2 Uniform Resource Identifiers
URIs have been known by many names: WWW addresses, Universal
Document Identifiers, Universal Resource Identifiers [3], and
finally the combination of Uniform Resource Locators (URL) [5] and
Names (URN) [18]. 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/1.0 can be represented in absolute form or relative to
some known base URI [10], 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
hex = "A" | "B" | "C" | "D" | "E" | "F"
| "a" | "b" | "c" | "d" | "e" | "f" | DIGIT
reserved = ";" | "/" | "?" | ":" | "@" | "&" | "="
safe = "$" | "-" | "_" | "." | "+"
extra = "!" | "*" | "'" | "(" | ")" | ","
national = <any OCTET excluding CTLs, SP,
ALPHA, DIGIT, reserved, safe, and extra>
For more information on URL syntax and semantics, see RFC 1738 [5]
and RFC 1808 [10]. The BNF above includes characters--all those
marked as national--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. In fact, the only real requirement for HTTP is that the
URI not contain any LWS; any other invalid URI can be identified
and rejected by the server.
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 = <FQDN or IP address, as defined in RFC 1738>
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.
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
3.3.1 Full Date
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
The first format is preferred as an Internet standard and
represents a fixed-length subset of that defined by RFC 1123 [7]
(an update to RFC 822 [8]). The second format is in common use, but
is based on the obsolete RFC 850 [11] date format and lacks a four-
digit year. HTTP/1.0 clients and servers must accept all three
formats, though they must never generate the third (asctime)
format. Future clients and servers must only generate the RFC 1123
format for representing date/time stamps in HTTP/1.0 requests and
responses.
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 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"
Comments and/or extra LWS are not permitted inside an HTTP-date
value generated by a conformant application.
Note: HTTP/1.0 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. This format should only be used to
represent short time periods or periods that cannot start until
receipt of the message.
delta-seconds = 1*DIGIT
3.4 Media Types
HTTP uses Internet Media Types [15] (formerly referred to as MIME
Content-Types [6]) in order to provide open and extensible data
typing and type negotiation. For mail applications, where there is
no type negotiation between sender and receiver, it is reasonable
to put strict limits on the set of allowed media types. With HTTP,
however, user agents can identify acceptable media types as part of
the connection, and thus are allowed more freedom in the use of non-
registered types. The following grammar for media types is a
superset of that for MIME because it does not restrict itself to
the official IANA and x-token types.
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 not case-
sensitive. Parameter values may or may not be case-sensitive,
depending on the semantics of the parameter name. LWS should not be
generated between the type and subtype, nor between an attribute
and its value.
If a given media-type value has been registered by the IANA, any
use of that value must be indicative of the registered data format.
Although HTTP allows the use of non-registered media types, such
usage must not conflict with the IANA registry. Data providers are
strongly encouraged to register their media types with IANA via the
procedures outlined in RFC 1590 [15].
All media-type's registered by IANA must be preferred over
extension tokens. However, HTTP does not limit conforming
applications to the use of officially registered media types, nor
does it encourage the use of an "x-" prefix for unofficial types
outside of explicitly short experimental use between consenting
applications.
3.4.1 Canonicalization and Text Defaults
Media types are registered in a canonical form. In general, entity
bodies transferred via HTTP must be represented in the appropriate
canonical form prior to transmission. If the body has been encoded
via a Content-Encoding and/or Content-Transfer-Encoding, the data
must be in canonical form prior to that encoding. However, HTTP
modifies the canonical form requirements for media of primary type
"text" and for "application" types consisting of text-like records.
HTTP redefines the canonical form of text media to allow multiple
octet sequences to indicate a text line break. In addition to the
preferred form of CRLF, HTTP applications must accept a bare CR or
LF alone as representing a single line break in text media.
Furthermore, if the text media is represented in a character set
encoding which does not use octets 13 and 10 for CR and LF
respectively, as is the case for some multi-byte character set
encodings, HTTP allows the use of whatever octet sequence(s) is
defined by that character set encoding to represent the equivalent
of CRLF, bare CR, and bare LF. It is assumed that any recipient
capable of using such a character set encoding will know the
appropriate octet sequence for representing line breaks within that
character set encoding.
Note: This interpretation of line breaks applies only to the
contents of an Entity-Body and only after any Content-
Transfer-Encoding and/or Content-Encoding has been removed.
All other HTTP constructs use CRLF exclusively to indicate a
line break. Encoding mechanisms define their own line break
requirements.
A recipient of an HTTP text entity should translate the received
entity line breaks to the local line break conventions before
saving the entity external to the application and its cache;
whether this translation takes place immediately upon receipt of
the entity, or only when prompted by the user, is entirely up to
the individual application.
HTTP also redefines the default character set encoding for text
media in an entity body. If a textual media type defines a charset
parameter with a registered default value of "US-ASCII", HTTP
changes the default to be "ISO-8859-1". Since the ISO-8859-1 [20]
character set encoding is a superset of US-ASCII [19], this has no
effect upon the interpretation of entity bodies which only contain
octets within the US-ASCII set (0 - 127). The presence of a charset
parameter value in a Content-Type header field overrides the
default.
It is recommended but not required that the character set encoding
of an entity body be labelled as the lowest common denominator of
the character codes used within a document, with the exception that
no label is preferred over the labels US-ASCII or ISO-8859-1.
3.4.2 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 [17] 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. Ideally, an HTTP user agent should follow the same or similar
behavior as a MIME user agent does upon receipt of a multipart type.
As in MIME [6], 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. Unlike in
MIME, multipart body-parts may contain HTTP header fields which are
significant to the meaning of that part.
A URI-header field (Section 8.28) should be included in the body-
part for each enclosed entity that can be identified by a URI.
3.5 Character Set Encodings
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.
However, since this is more commonly referred to as a character
encoding, this document will refer to them as character set
encodings. Character set encodings are identified by case-
insensitive tokens. The complete set of tokens are defined by the
IANA Character Set registry [17]. However, because that registry
does not define a single, consistent token for each character set
encoding, we define here the preferred names for those character
set encodings most likely to be used with HTTP entities. This set
of charset values includes those registered by RFC 1521 [6] -- the
US-ASCII [19] and ISO-8859 [20] character set encodings -- 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 [17] must represent the character set
encoding defined by that registry. Applications are encouraged, but
not required, to limit their use of character set encodings to
those defined by the IANA registry.
3.6 Encoding Mechanisms
Encoding mechanism values are used to indicate an encoding
transformation that has been or can be applied to a resource.
Encoding mechanisms 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.
encoding-mechanism = "gzip" | "compress" | token
Note: For historical reasons, HTTP/1.0 applications should
consider "x-gzip" and "x-compress" to be equivalent to
"gzip" and "compress", respectively.
All encoding-mechanism values are case-insensitive. HTTP/1.0 uses
encoding-mechanism values in the Accept-Encoding (Section 8.3) and
Content-Encoding (Section 8.7) header fields. Although the value
describes the encoding-mechanism, 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 encoding-mechanism formats. Two values are defined by this
specification:
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. Gzip is available from the GNU project at
<URL:ftp://prep.ai.mit.edu/pub/gnu/>.
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.
3.7 Transfer Encodings
Transfer encoding 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. Current transfer encodings are only used with entities
destined for or retrieved from MIME-conformant systems, and thus
will rarely occur in an HTTP/1.0 message. This differs from an
encoding-mechanism in that the transfer encoding is a property of
the message, not of the original resource.
transfer-encoding = "binary" | "8bit" | "7bit"
| "quoted-printable" | "base64"
| token
All transfer-encoding values are case-insensitive. HTTP/1.0 may use
transfer-encoding values in the Content-Transfer-Encoding
(Section 8.10) header field.
Note: Transfer encodings were designed for MIME with the
assumption of their being used only within the context of
Internet mail and SMTP. "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).
The values "7bit", "8bit", and "binary" are used to indicate that
no transfer encoding has been performed. Instead, they describe the
sort of encoding that might be needed for transmission through an
unsafe transport system. Binary indicates that the body may contain
any set of octets. 8bit adds the restrictions that CR and LF
characters only occur as part of CRLF line separators, all lines
are short (less than 1000 octets), and no NULs (octet 0) are
present. 7bit adds a further restriction that all octets are 7-bit
US-ASCII characters.
The "quoted-printable" and "base64" values indicate that the
associated encoding (as defined in MIME [6]) has been applied to
the body. These encodings consist entirely of 7-bit US-ASCII
characters.
3.8 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.
The HTTP/1.0 protocol uses language tags within the
Accept-Language, Content-Language, and URI-header fields.
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 not case-
sensitive. The namespace 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.
In the context of the Accept-Language header (Section 8.4), a
language tag is not to be interpreted as a single token, as per
RFC 1766, but as a hierarchy. A server should consider that it has a
match when a language tag received in an Accept-Language header
matches the initial portion of the language tag of a document. An
exact match should be preferred. This interpretation allows a
browser to send, for example:
Accept-Language: en-US, en; ql=0.95
when the intent is to access, in order of preference, documents in
US-English ("en-US"), 'plain' or 'international' English ("en"),
and any other variant of English (initial "en-").
Note: Using the language tag as a hierarchy does not imply
that all languages with a common prefix will be understood
by those fluent in one or more of those languages; it simply
allows the user to request this commonality when it is true
for that user.
3.9 Quality Values
HTTP content negotiation (Section 9) uses short "floating point"
numbers to indicate the relative importance ("weight") of various
negotiable parameters. The calculated weights are normalized to a
real number in the range 0 through 1, where 0 is the minimum and 1
the maximum value. In order to discourage misuse of this feature,
HTTP/1.0 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 ] )
| ( "." 0*3DIGIT )
| ( "1" [ "." 0*3("0") ] )
"Quality values" is a slight misnomer, since these values actually
measure relative degradation in perceived quality. Thus, a value of
"0.8" represents a 20% degradation from the optimum rather than a
statement of 80% quality.
3.10 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 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 [8] for transferring entities. Both messages may include
optional header fields (a.k.a. "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 5.4
| 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 6.3
| 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 client from using content negotiation and 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 5.4), Response-Header (Section 6.3), and
Entity-Header (Section 7.1) fields, follow the same generic format
as that given in Section 3.1 of RFC 822 [8]. Each header field
consists of a name followed by a colon (":") and the field value.
Field names are never case-sensitive. 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 LWS.
HTTP-header = field-name ":" [ field-value ] CRLF
field-name = 1*<any CHAR, excluding CTLs, SP, and ":">
field-value = *( field-content | comment | 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.
4.3 General Message 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
communicating parties or the content being transferred. Although
none of the General-Header fields are required, they are all
strongly recommended where their use is appropriate, and should be
understood by all future HTTP/1.0 clients and servers. These
headers apply only to the message being transmitted.
General-Header = Date ; Section 8.12
| Forwarded ; Section 8.14
| MIME-Version ; Section 8.20
| Pragma ; Section 8.22
General header field names can be extended only via a change in the
protocol version. Unknown 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 requested, 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 5.4
| 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
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".
5.2 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" | "HEAD" | "PUT" | "POST"
| "DELETE" | "LINK" | "UNLINK"
| extension-method
extension-method = token
The list of methods acceptable by a specific resource can be
specified in an "Allow" Entity-Header (Section 8.5). However, the
client is always notified through the return code of the response
whether a method is currently allowed on a specific resource, as
this 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 unknown or not implemented by the server.
The methods GET and HEAD must be supported by all general-purpose
servers. Servers which provide Last-Modified dates for resources
must also support the conditional GET method.
The set of common methods for HTTP/1.0 is described below. Although
this set can be easily expanded, additional methods cannot be
assumed to share the same semantics for separately extended clients
and servers. In order to maintain compatibility, the semantic
definition for extension methods should be registered with the
IANA [17].
5.2.1 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 8.16. 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.
5.2.2 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.
5.2.3 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 [4], 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 client can suggest a URI for identifying the new resource by
including a URI-header field in the request. However, the server
should treat that URI as advisory and may store the entity under a
different URI or without any URI.
The client may apply relationships between the new resource and
other existing resources by including Link header fields, as
described in Section 8.18. The server may use the Link information
to perform other operations as a result of the new resource being
added. For example, lists and indexes might be updated. However, no
mandatory operation is imposed on the origin server. The origin
server may also generate its own or additional links to other
resources.
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 the allocated URI, all
applicable Link header fields, and 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.
5.2.4 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 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 an appendage. 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. The user
agent should be informed of these URIs via one or more URI header
fields in the response. The Location header field should be used to
identify the exact location URI if it is different than the
Request-URI.
A valid Content-Length is required on all HTTP/1.0 PUT 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.
The client can create or modify relationships between the enclosed
entity and other existing resources by including Link header
fields, as described in Section 8.18. As with POST, the server may
use the Link information to perform other operations as a result of
the request. However, no mandatory operation is imposed on the
origin server. The origin server may generate its own or additional
links to other resources.
The actual method for determining how the resource is placed, and
what happens to its predecessor, is defined entirely by the origin
server. If version control is implemented by the origin server,
then Link relationships should be defined by the server to help
identify and control revisions to a resource; suggested
relationship names include "Derived-From", "Obsoletes", and
"Updates".
Note: The model of sending an entire PUT request within a
single message, without first checking if the server is
willing to accept that data, will break if the server is
unwilling to accept the request or desires some form of
authentication beforehand. Worse, the client won't be
notified of the reason for error if a TCP reset is received
prior to reading the response buffer (see note in
Section 6.2.4). It should therefore be recognized that
HTTP/1.0 PUT and large POST requests will only work reliably
if the client's intentions and server's desires are
negotiated prior to the request.
5.2.5 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.
5.2.6 LINK
The LINK method establishes one or more Link relationships between
the existing resource identified by the Request-URI and other
existing resources. The difference between LINK and other methods
allowing links to be established between resources is that the LINK
method does not allow any Entity-Body to be sent in the request and
does not result in the creation of new resources.
5.2.7 UNLINK
The UNLINK method removes one or more Link relationships from the
existing resource identified by the Request-URI. These
relationships may have been established using the LINK method or by
any other method supporting the Link header. The removal of a link
to a resource does not imply that the resource ceases to exist or
becomes inaccessible for future references.
5.3 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. Note that this is not the case for any of the methods
defined by this document; however, it may be true of extension
methods. One example would be
OPTIONS * HTTP/1.0
The absoluteURI form is only allowed when the request is being made
to a proxy server. The proxy is requested to forward the request
and return the response. If the request is idempotent and a
response is cached, the proxy may return the cached message if it
passes any restrictions in the Pragma and Expires header fields.
Note that the proxy may forward the request on to another proxy or
directly to the origin 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/hypertext/WWW/TheProject.html HTTP/1.0
The most common form of Request-URI is that used to identify a
resource on an origin server. In this case, only the absolute path
of the URI (abs_path) is transmitted. 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 /hypertext/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).
5.4 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. All header fields are optional and conform to the generic
HTTP-header syntax.
Request-Header = Accept ; Section 8.1
| Accept-Charset ; Section 8.2
| Accept-Encoding ; Section 8.3
| Accept-Language ; Section 8.4
| Authorization ; Section 8.6
| From ; Section 8.15
| If-Modified-Since ; Section 8.16
| Orig-URI ; Section 8.21
| Referer ; Section 8.24
| User-Agent ; Section 8.29
Request-Header field names can be extended only via a change in the
protocol version. Unknown 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 ]
Full-Response = Status-Line ; Section 6.1
*( General-Header ; Section 4.3
| Response-Header ; Section 6.3
| 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 never generate such a response.
6.2 Status Codes and Reason Phrases
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.
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.
Status-Code = "200" ; OK
| "201" ; Created
| "202" ; Accepted
| "203" ; Non-Authoritative Information
| "204" ; No Content
| "300" ; Multiple Choices
| "301" ; Moved Permanently
| "302" ; Moved Temporarily
| "303" ; See Other
| "304" ; Not Modified
| "400" ; Bad Request
| "401" ; Unauthorized
| "402" ; Payment Required
| "403" ; Forbidden
| "404" ; Not Found
| "405" ; Method Not Allowed
| "406" ; None Acceptable
| "407" ; Proxy Authentication Required
| "408" ; Request Timeout
| "409" ; Conflict
| "410" ; Gone
| "411" ; Authorization Refused
| "500" ; Internal Server Error
| "501" ; Not Implemented
| "502" ; Bad Gateway
| "503" ; Service Unavailable
| "504" ; Gateway Timeout
| extension-code
extension-code = 3DIGIT
Reason-Phrase = *<text, excluding CR, LF>
HTTP status codes are extensible and should be registered with the
IANA. 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 unknown response as being equivalent to the x00 status
code of that class. For example, if an unknown status code of 421
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 are
encouraged to present the entity returned with the response to the
user, since that entity is likely to include human-readable
information which will explain the unusual status.
Each Status-Code is described below, including a description of
which method(s) it can follow and any metainformation required in
the response.
6.2.1 Informational 1xx
This class of status codes 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 standard
HTTP/1.0 request. However, they may be useful for experimental
applications which are outside the scope of this specification.
6.2.2 Successful 2xx
This class of status codes indicates that the client's request was
successfully received, understood, and accepted.
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 being
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;
PUT, DELETE, LINK, UNLINK
an entity describing the result of the action;
If the entity corresponds to a resource, the response may include a
Location header field giving the actual location of that specific
resource for later reference.
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 URI-header field of the response, with the most
specific URL for the resource given by a Location header field. The
origin server is encouraged, but not obliged, to actually create
the resource before using this Status-Code. If the action cannot be
carried out immediately, or within a clearly defined timeframe, the
server should respond with 202 (accepted) instead.
Of the methods defined by this specification, only PUT and 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.
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).
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 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.
6.2.3 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 can sometimes be carried out by the user agent without
interaction with the user, but it is strongly recommended that this
only take place if the method used in the request is idempotent
(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
The requested resource is available at one or more locations and a
preferred location could not be determined via content negotiation.
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.
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. If the server has a
preferred choice, it should include its URL in a Location field;
user agents not capable of complex selection may use the Location
value for automatic redirection.
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 are
encouraged to automatically relink references to the Request-URI to
one or more of the new references returned by the server, where
possible.
If the new URI is a single location, its URL must be given by the
Location field in the response. If more than one URI exists for the
resource, the primary URL should be given in the Location field and
the other URIs given in one or more URI-header fields. The Entity-
Body 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 using
the PUT, POST, or DELETE methods, 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.
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.
If the new URI is a single location, its URL must be given by the
Location field in the response. If more than one URI exists for the
resource, the primary URL should be given in the Location field and
the other URIs given in one or more URI-header fields. The Entity-
Body 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 using
the PUT, POST, or DELETE methods, 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.
303 See Other
The requested resource resides under a different URI and should be
accessed 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 resource is
not a replacement reference for the original Request-URI.
If the new URI is a single location, its URL must be given by the
Location field in the response. If more than one URI exists for the
resource, the primary URL should be given in the Location field and
the other URIs given in one or more URI-header fields. The Entity-
Body of the response should contain a short hypertext note with a
hyperlink to the new URI(s).
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 shall
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 and which may have
changed independently of the entity's Last-Modified date. Examples
of relevant header fields include: Date, Server, and Expires.
6.2.4 Client Error 4xx
The 4xx class of status codes 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 is encouraged to 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.
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 it having
a malformed syntax. The client is discouraged from repeating the
request without modifications.
401 Unauthorized
The request requires user authentication. The response must include
a WWW-Authenticate header field (Section 8.30) containing a
challenge applicable to the requested resource. The client may
repeat the request with a suitable Authorization header field. HTTP
access authentication is explained in Section 10.
402 Payment Required
This code is not currently supported, but is reserved for future
use.
403 Forbidden
The server understood the request, but is refusing to perform the
request because of an unspecified reason. Authorization will not
help and the request should not be repeated. This status code can
be used if the server does not want to make public why the request
has not been fulfilled.
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. 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.
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 method's for the
requested resource.
406 None Acceptable
The server has found a resource matching the Request-URI, but not
one that satisfies the conditions identified by the Accept and
Accept-Encoding request headers. 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. 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.
407 Proxy Authentication Required
This code is reserved for future use. It is similar to 401
(unauthorized), but indicates that the client must first
authenticate itself with the proxy. HTTP/1.0 does not provide a
means for proxy authentication.
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.
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 may 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 PUT. In this case, the response entity
may contain a list of the differences between the two versions.
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 are encouraged to
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.
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.
411 Authorization Refused
The request credentials provided by the client were rejected by the
server or insufficient to grant authorization to access the
resource. This is similar to the 403 (forbidden) response, but
allows more information to be provided to the user. The content of
the response should contain a description of the problem and may
suggest corrective action. HTTP access authentication is explained
in Section 10.
The response must include a WWW-Authenticate header field
(Section 8.30) containing a challenge applicable to the requested
resource. If the challenge is different from that assumed by the
last request, the client may repeat the request with a suitable
Authorization header field after obtaining the user's approval.
6.2.5 Server Errors 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 is encouraged to 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 received an invalid response from the gateway or
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. 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.
504 Gateway Timeout
The server did not receive a timely response from the gateway or
upstream server it accessed in attempting to complete the request.
6.3 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 are not intended to give
information about an Entity-Body returned in the response, but
about the server itself.
Response-Header= Location ; Section 8.19
| Public ; Section 8.23
| Retry-After ; Section 8.25
| Server ; Section 8.26
| WWW-Authenticate ; Section 8.30
Response-Header field names can be extended only via a change in
the protocol version. Unknown 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.
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 8.5
| Content-Encoding ; Section 8.7
| Content-Language ; Section 8.8
| Content-Length ; Section 8.9
| Content-Transfer-Encoding ; Section 8.10
| Content-Type ; Section 8.11
| Expires ; Section 8.13
| Last-Modified ; Section 8.17
| Link ; Section 8.18
| Title ; Section 8.27
| URI-header ; Section 8.28
| extension-header
extension-header=HTTP-header
The extension-header mechanism allows additional Entity-Header to
be defined without changing the protocol, but these fields cannot
be assumed to be recognizable by the recipient. Unknown 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/1.0 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. This specification defines two
request methods, "POST" and "PUT", that allow an entity-body. In
general, the presence of an entity-body in a request is signaled by
the inclusion of a Content-Length and/or Content-Transfer-Encoding
header field in the request message headers. HTTP/1.0 requests
containing content 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 content header fields
may lead one to believe they do. The responses 204 (no content) and
304 (not modified) must not include a message body.
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,
Content-Encoding, and Content-Transfer-Encoding. These define a
three-layer, ordered encoding model:
entity-body <-
Content-Transfer-Encoding( Content-Encoding( Content-Type ) )
The default for both encodings is none (i.e., the identity
function). A Content-Type specifies the media type of the
underlying data. A Content-Encoding may be used to indicate any
additional encoding mechanisms applied to the type, usually for the
purpose of data compression, that is a property of the resource
requested. A Content-Transfer-Encoding may be applied by a
transport agent to ensure safe and proper transfer of the message.
Note that the Content-Transfer-Encoding is a property of the
message, not of the resource.
The Content-Type header field has no default value. If and only if
the media type is not given by a Content-Type header, as is always
the case for Simple-Response messages, the receiver may attempt to
guess the media type via inspection of its content and/or the name
extension(s) of the URL used to specify the resource. If the media
type remains unknown, the receiver 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 several 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
Content-Type (for types with an explicit end-of-body delimiter),
the Content-Transfer-Encoding (for packetized encodings), or 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. Furthermore, there is no guarantee that an
HTTP/1.0 server will recognize types with an explicit end-of-body
delimiter, and there is no packetized Content-Transfer-Encoding
defined for HTTP/1.0. Therefore, HTTP/1.0 requests containing
content 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. Header Field Definitions
This section defines the syntax and semantics of all standard
HTTP/1.0 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.
8.1 Accept
The Accept 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 of that type. The set of ranges given by the client should
represent what types are acceptable given the context of the
request. The Accept field should only be used when the request is
specifically limited to a set of desired types, as in the case of a
request for an in-line image, or to indicate qualitative
preferences for specific media types.
The field may be folded onto several lines and more than one
occurrence of the field is allowed, with the semantics being the
same as if all the entries had been in one field value.
Accept = "Accept" ":" #(
media-range
[ ";" "q" "=" qvalue ]
[ ";" "mxb" "=" 1*DIGIT ] )
media-range = ( "*/*"
| ( type "/" "*" )
| ( type "/" subtype )
) *( ";" parameter )
The parameter q is used to indicate the quality factor, which
represents the user's preference for that range of media types. The
parameter mxb gives the maximum acceptable size of the Entity-Body,
in decimal number of octets, for that range of media types.
Section 9 describes the content negotiation algorithm which makes
use of these values. The default values are: q=1 and mxb=undefined
(i.e., infinity).
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."
If no Accept header is present, then it is assumed that the client
accepts all media types with quality factor 1. This is equivalent
to the client sending the following accept header field:
Accept: */*; q=1
or
Accept: */*
A more elaborate example is
Accept: text/plain; q=0.5, text/html,
text/x-dvi; q=0.8; mxb=100000, 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
Entity-Body in text/x-dvi if the entity is less than 100000 bytes,
otherwise send text/plain."
Note: In earlier versions of this document, the mxs
parameter defined the maximum acceptable delay in seconds
before the response would arrive. This has been removed as
the server has no means of obtaining a useful reference
value. However, this does not prevent the client from
internally measuring the response time and optimizing the
Accept header field accordingly.
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;version=2.0, */*
have the following precedence:
1) text/html;version=2.0
2) text/html
3) text/*
4) */*
The quality value 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;version=2.0,
*/*;q=0.5
would cause the following values to be associated:
text/html;version=2.0 = 1
text/html = 0.7
text/plain = 0.3
image/jpeg = 0.5
text/html;level=3 = 0.7
It must be emphasized that the Accept field should only be used
when it is necessary to restrict the response media types to a
subset of those possible or when the user has been permitted to
specify qualitative values for ranges of media types. If no quality
factors have been set by the user, and the context of the request
is such that the user agent is capable of saving the entity to a
file if the received media type is unknown, then the only
appropriate value for Accept is "*/*".
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 completely closed system which cannot
interact with other rendering agents, this default set
should be configurable by the user.
8.2 Accept-Charset
The Accept-Charset request header field can be used to indicate a
list of preferred character set encodings other than the default
US-ASCII and ISO-8859-1. This field allows clients capable of
understanding more comprehensive or special-purpose character set
encodings to signal that capability to a server which is capable of
representing documents in those character set encodings.
Accept-Charset = "Accept-Charset" ":" #charset
Character set encoding values are described in Section 3.5. An
example is
Accept-Charset: iso-8859-5, unicode-1-1
The value of this field should not include "US-ASCII" or
"ISO-8859-1", since those values are always assumed by default. If
a resource is only available in a character set encoding other than
the defaults, and that character set encoding is not listed in the
Accept-Charset field, it is only acceptable for the server to send
the entity if the character set encoding can be identified by an
appropriate charset parameter on the media type or within the
format of the media type itself.
Note: User agents are not required to be able to render the
characters associated with the ISO-8859-1 character set
encoding. However, they must be able to interpret their
meaning to whatever extent is required to properly handle
messages in that character set encoding.
8.3 Accept-Encoding
The Accept-Encoding request header field is similar to Accept, but
restricts the encoding-mechanism values which are acceptable in the
response.
Accept-Encoding = "Accept-Encoding" ":"
#( encoding-mechanism )
An example of its use is
Accept-Encoding: compress, gzip
If no Accept-Encoding field is present in a request, the server
should assume that the client will accept any encoding-mechanism.
8.4 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" ":"
#( language-tag [ ";" "ql" "=" qvalue ] )
The language-tag is described in Section 3.8. Each language may be
given an associated quality value which represents an estimate of
the user's comprehension of that language. The quality value
defaults to "ql=1" (100% comprehension) for listed languages. This
value may be used in the server's content negotiation algorithm
(Section 9). For example,
Accept-Language: da, en-gb;ql=0.8, de;ql=0.55
would mean: "I prefer Danish, but will accept British English (with
80% comprehension) or German (with a 55% comprehension)."
If the server cannot fulfill the request with one or more of the
languages given, or if the languages only represent a subset of a
multi-linguistic Entity-Body, it is acceptable to serve the request
in an unspecified language. This is equivalent to asssigning a
quality value of "ql=0.001" to any unlisted language.
If no Accept-Language header is present in the request, the server
should assume that all languages are equally acceptable.
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.
8.5 Allow
The Allow 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. 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" ":" #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 field value should be
followed. This field has no default value; if left undefined, the
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 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 must use the Public
response header field (Section 8.23) if they wish to describe what
methods are implemented on the server as a whole.
8.6 Authorization
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
containing the authentication information of the user agent for the
realm of the resource being requested.
Authorization = "Authorization" ":" 1#credentials
HTTP access authentication is described in Section 10. If a request
is authenticated and a realm specified, the same credentials should
be valid for all other requests within this realm, until the server
indicates otherwise with a 411 (authorization refused) response.
8.7 Content-Encoding
The Content-Encoding header field is used as a modifier to the
media-type. When present, its value indicates what additional
encoding mechanisms have been applied to the resource, and thus
what decoding mechanisms 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" ":" 1#encoding-mechanism
Encoding mechanisms are defined in Section 3.6. An example of its
use is
Content-Encoding: 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.
If multiple encodings have been applied to a resource, the
encoding-mechanisms must be listed in the order in which they were
applied. Additional information about the encoding parameters may
be provided by other Entity-Header fields not defined by this
specification.
8.8 Content-Language
The Content-Language 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.
Content-Language = "Content-Language" ":" #language-tag
Language tags are defined in Section 3.8. The primary purpose of
Content-Language is to allow a selective consumer to identify and
differentiate resources according to the consumer's own preferred
language. Thus, if the body content is intended only for a Danish-
literate audience, the appropriate field is
Content-Language: dk
If no Content-Language is specified, the default is that the
content is intended for all language audiences. This may mean that
the sender does not consider it to be specific to any natural
language, or that the sender does not know for which language it is
intended.
Multiple languages may be listed for content that is intended for
multiple audiences. For example, a rendition of the "Treaty of
Waitangi," presented simultaneously in the original Maori and
English versions, would call for
Content-Language: mi, en
However, just because multiple languages are present within an
entity does not mean that it is intended for multiple linguistic
audiences. An example would be a beginner's language primer, such
as "A First Lesson in Latin," which is clearly intended to be used
by an English-literate audience. In this case, the Content-Language
should only include "en".
Content-Language may be applied to any media type -- it should not
be limited to textual documents.
8.9 Content-Length
The Content-Length 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
Although it is not required, applications are strongly encouraged
to use this field to indicate the size of the Entity-Body to be
transferred, regardless of the media type of the entity.
Any Content-Length greater than or equal to zero is a valid value.
Section 7.2.2 describes how to determine the length of an
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.
8.10 Content-Transfer-Encoding
The Content-Transfer-Encoding (CTE) header indicates what (if any)
type of transformation has been applied to the entity in order to
safely transfer it between the sender and the recipient. This
differs from the Content-Encoding in that the CTE is a property of
the message, not of the original resource.
Content-Transfer-Encoding = "Content-Transfer-Encoding" ":"
transfer-encoding
Transfer encodings are defined in Section 3.7. Because all HTTP
transactions take place on an 8-bit clean connection, the default
Content-Transfer-Encoding for all messages is binary. However, HTTP
may be used to transfer MIME messages which already have a defined
CTE. An example is:
Content-Transfer-Encoding: quoted-printable
Many older HTTP/1.0 applications do not understand the
Content-Transfer-Encoding header. However, since it may appear in
any MIME message (i.e., entities retrieved via a gateway to a MIME-
conformant protocol), future HTTP/1.0 applications must understand
it upon receipt. Gateways are the only HTTP applications that would
generate a CTE.
8.11 Content-Type
The Content-Type 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 3.4. An example of the field is
Content-Type: text/html; charset=ISO-8859-4
The Content-Type header field has no default value. Further
discussion of methods for identifying the media type of an entity
is provided in Section 7.2.1.
8.12 Date
The Date header 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 default 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 PUT and POST
requests, and even then it is optional. A received message which
does not have a Date header field should be assigned one by the
receiver if and only if the message will be cached by that receiver
or gatewayed via a protocol which requires a Date.
Only one Date header field is allowed per message. 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) usage.
8.13 Expires
The Expires field gives the date/time after which the entity should
be considered stale. This allows information providers to suggest
the volatility of the resource. Caching clients, including proxies,
must not cache this copy of the resource beyond the date given,
unless its status has been updated by a later check of the origin
server. 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 are strongly
encouraged to 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
The Expires field has no default value. 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, copies of 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. 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.
Note: Applications are encouraged to be tolerant of bad or
misinformed implementations of the Expires header. In
particular, recipients may wish to recognize a delta-seconds
value (any decimal integer) as representing the number of
seconds after receipt of the message that its contents
should be considered expired. Likewise, 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.
8.14 Forwarded
The Forwarded header is to be used by proxies to indicate the
intermediate steps between the user agent and the server on
requests, and between the origin server and the client on
responses. It is analogous to the "Received" field of RFC 822 [8]
and is intended to be used for tracing transport problems and
avoiding request loops.
Forwarded = "Forwarded" ":" #( "by" URI [ "(" product ")" ]
[ "for" FQDN ] )
FQDN = <Fully-Qualified Domain Name>
For example, a message could be sent from a client on
ptsun00.cern.ch to a server at www.ics.uci.edu port 80, via an
intermediate HTTP proxy at info.cern.ch port 8000. The request
received by the server at www.ics.uci.edu would then have the
following Forwarded header field:
Forwarded: by http://info.cern.ch:8000/ for ptsun00.cern.ch
Multiple Forwarded header fields are allowed and should represent
each proxy that has forwarded the message. It is strongly
recommended that proxies used as a portal through a network
firewall do not, by default, send out information about the
internal hosts within the firewall region. This information should
only be propagated if explicitly enabled. If not enabled, the for
token and FQDN should not be included in the field value, and any
Forwarded headers already present in the message (those added
behind the firewall) should be removed.
8.15 From
The From 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 [8] (as updated by RFC 1123 [7]):
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 does not have to
correspond to 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.
8.16 If-Modified-Since
The If-Modified-Since 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
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.
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 the 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.
Note: The same functionality can be obtained, though with
much greater overhead, by issuing a HEAD request and
following it with a GET request if the server indicates that
the entity has been modified.
8.17 Last-Modified
The Last-Modified 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 receiver
should interpret it: if the receiver 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-mod date.
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.
8.18 Link
The Link header 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. The Link field is semantically equivalent to the <LINK>
element in HTML [4].
Link = "Link" ":" #("<" URI ">"
[ ";" "rel" "=" relationship ]
[ ";" "rev" "=" relationship ]
[ ";" "title" "=" quoted-string ] )
relationship = sgml-name
| ( <"> sgml-name *( SP sgml-name) <"> )
sgml-name = ALPHA *( ALPHA | DIGIT | "." | "-" )
Relation values are not case-sensitive and may be extended within
the constraints of the sgml-name syntax. There are no predefined
link relationship values for HTTP/1.0. The title parameter may be
used to label the destination of a link such that it can be used as
identification within a human-readable menu. Examples of usage
include:
Link: <http://www.cern.ch/TheBook/chapter2>; rel="Previous"
Link: <mailto:timbl@w3.org>; rev="Made"; title="Tim Berners-Lee"
The first example indicates that the entity is previous to chapter2
in a logical navigation path. The second indicates that the person
responsible for making the resource available is identified by the
given e-mail address.
8.19 Location
The Location response header field defines the exact location of
the resource that was identified by the Request-URI. For 2xx
responses, the location should be the URL needed to retrieve that
same resource again (i.e., if variants of that resource are
available, the value of the Location field should locate the
variant chosen by the server if it has its own specific URL). For
3xx responses, the location should 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
If no base URL is provided by or within the entity, the value of
the Location field should be used as the base for resolving
relative URLs [10].
8.20 MIME-Version
HTTP is not a MIME-conformant protocol (see Appendix C). However,
HTTP/1.0 messages may include a single MIME-Version header field to
indicate what version of the MIME protocol was used to construct
the message. Use of the MIME-Version header field should indicate
that the message is in full compliance with the MIME protocol (as
defined in [6]). Unfortunately, current versions of HTTP/1.0
clients and servers use this field indiscriminately, and thus
receivers must not take it for granted that the message is indeed
in full compliance with MIME. Gateways are responsible for ensuring
this compliance (where possible) when exporting HTTP messages to
strict MIME environments. Future HTTP/1.0 applications must only
use MIME-Version when the message is intended to be MIME-conformant.
MIME-Version = "MIME-Version" ":" 1*DIGIT "." 1*DIGIT
MIME version "1.0" is the default for use in HTTP/1.0. However,
HTTP/1.0 message parsing and semantics are defined by this document
and not the MIME specification.
8.21 Orig-URI
The Orig-URI request header field allows the client to specify, for
the server's benefit, the original Uniform Resource Identifier
(Section 3.2) of the resource being requested, as it was obtained
from the user or the referring resource. This allows a server to
differentiate between internally-ambiguous URLs (such as the root
"/" URL of a server harboring multiple virtual hostnames), to learn
about new URNs used to reference resources on the server, and to
provide some additional assistance in identifying and redirecting
moved resources and resource fragments.
Orig-URI = "Orig-URI" ":" absoluteURI [ "#" fragment ]
Example:
Orig-URI: http://www.w3.org/
The URI must be in absolute form and should include the fragment if
one is given to the client. It should include exactly what was
referenced by the Referer resource, with the exception that a
relative reference must first be resolved to its absolute form.
8.22 Pragma
The Pragma message header field is used to specify directives that
should be applied to all intermediaries along the request/response
chain. The directives typically specify behavior intended to
prevent intermediate proxies or caches from adversely interfering
with the request or response. All pragma directives specify
optional behavior from the viewpoint of the protocol; however, some
systems may require that behavior be consistent with the
directives. HTTP/1.0 defines semantics for the "no-cache" and
"max-age" directives.
Pragma = "Pragma" ":" #pragma-directive
pragma-directive = "no-cache"
| "max-age" "=" delta-seconds
| extension-pragma
extension-pragma = token [ "=" word ]
When the "no-cache" directive is present in a request message, a
caching intermediary 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.
When the "no-cache" directive is present in a response message,
caching intermediaries are requested to not cache this response.
This allows an origin server to state that the message is intended
for only one recipient and may not be a valid response for other
requests.
When the "max-age" directive is present in a request message, a
caching intermediary should forward the request toward the origin
server if it has no cached copy, or refresh its cached copy if it
is older than the age value given (in seconds) prior to returning a
response. A cached copy's "age" is determined by the cached
message's Date header field, or the equivalent as stored by the
cache manager. In most cases, a cached copy can be refreshed by
forwarding a conditional GET request toward the origin server with
the stored message's Date value in the If-Modified-Since field. If
a 304 (not modified) response is received, the cache should replace
the cached message's Date with that of the 304 response and send
this refreshed message as the response. Any other response should
be forwarded directly to the requestor and, depending on the
response code and the discretion of the cache manager, may replace
the message in the cache.
When the "max-age" directive is present in a cached response
message, a caching intermediary should refresh the message if it is
older than the age value given (in seconds) at the time of a new
request for that resource. The behavior should be equivalent to
what would occur if the request had included that pragma directive.
If both the new request and the cached message have max-age
specified, then the lesser of the two values should be used.
Pragma directives must be passed through by a proxy, regardless of
their significance to that proxy, since the directives may be
applicable to all intermediaries along the request/response chain.
It is not possible to specify a pragma for a specific proxy;
however, any pragma directive not relevant to a proxy should be
ignored.
Pragma directives do not apply to the end-points of a
request/response chain. For example, a user agent's internal (non-
shared) cache and/or history mechanism should ignore all pragma
directives in received messages. Similarly, pragma directives are
not applicable to the origin of a resource, though they may be
applicable to a server's internal response cache.
8.23 Public
The Public response header field lists the set of non-standard
methods supported by the server. The purpose of this field is
strictly to inform the recipient of the capabilities of the server
regarding unusual methods. The methods listed may or may not be
applicable to the Request-URI; the Allow header field (Section 8.5)
should be used to indicate methods allowed for a particular URI.
This does not prevent a client from trying other methods. The field
value should not include the methods predefined for HTTP/1.0 in
Section 5.2.
Public = "Public" ":" #method
Example of use:
Public: OPTIONS, MGET, MHEAD
This header field applies only to the server directly connected to
the client (i.e., the nearest neighbor in a chain of connections).
If the response passes through a proxy, the proxy must either
remove the Public header field or replace it with one applicable to
its own capabilities.
8.24 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 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://info.cern.ch/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.
8.25 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.
Retry-After = "Retry-After" ":" ( HTTP-date | delta-seconds )
Two examples of its use are
Retry-After: Wed, 14 Dec 1994 18:22:54 GMT
Retry-After: 120
In the latter example, the delay is 2 minutes.
8.26 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 3.10) 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 )
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. Instead, it
should include a Forwarded field (as described in Section 8.14).
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.
8.27 Title
The Title header field indicates the title of the entity
Title = "Title" ":" *text
An example of the field is
Title: Hypertext Transfer Protocol -- HTTP/1.0
This field is isomorphic with the <TITLE> element in HTML [4].
8.28 URI
The URI-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.
URI-header = "URI" ":" #( "<" ( absoluteURI | relativeURI ) ">"
[ ";" vary ] *( ";" characteristic) )
vary = "vary" "="
( vary-dimension | ( <"> 1#vary-dimension <"> ) )
vary-dimension = "type" | "charset" | "language" | "encoding"
| "user-agent" | "version" | token
characteristic = ( "type={" media-type "}" )
| ( "language={" 1#language-tag "}" )
| ( "encoding={" 1#encoding-mechanism "}" )
| ( "length=" 1*DIGIT )
| ( "qs=" qvalue )
Any URI specified in this field can be either absolute or relative
to the Request-URI.
If the Location header field is present in a 2xx response, its
value defines an implicit URI header with the characteristic
parameters defined by the associated Content-* header fields.
The URI-header may be used by a client performing a POST request to
suggest a URI for the new entity. Whether or not the suggested URI
is used is entirely up to the server to decide. In any case, the
server's response must include the actual URI(s) of the new
resource if one is successfully created (status 201).
If a URI refers to a set of variants, then the dimensions of that
variance must be given with a vary parameter. One example is:
URI: <http://info.cern.ch/hypertext/WWW/TheProject.multi>;
vary="type,language"
which indicates that the URI covers a group of entities that vary
in media type and natural language. A request for that URI will
result in a response that depends upon the client's request headers
for Accept and Accept-Language. Similar dimensions exist for the
Accept-Encoding, Accept-Charset, and User-Agent header fields, as
demonstrated in the following example.
URI: <TheProject.ps>; vary="encoding,version";
type={application/postscript},
<TheProject.html>; vary="user-agent,charset,version";
type={text/html},
<TheProject.html3;v=25>; type={text/html; level=3}; qs=0.9
User agents may use this information to notify the user of
additional formats.
The vary parameter has an important effect on cache management,
particularly for caching intermediaries which service a diverse set
of user agents. Since the response to one user agent may differ
from the response to a second user agent if the two agents have
differing request profiles, a caching intermediary must keep track
of the content metainformation for resources with varying
dimensions. Thus, the vary parameter tells the intermediary what
entity headers must be part of the key for caching that URI. When
the caching proxy gets a request for that URI, it must forward the
request toward the origin server if the request profile includes a
variant dimension that has not already been cached.
If the origin server provides the characteristics of each
identified resource as part of the URI header, then the recipient
may improve its cached response behavior by attempting to duplicate
the content negotiation that would be provided by the server. This
is not required by the protocol, but may improve the accuracy or
timeliness of responses to the end-user.
8.29 User-Agent
The User-Agent 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
always include this field with requests. The field can contain
multiple product tokens (Section 3.10) 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 )
Example:
User-Agent: CERN-LineMode/2.15 libwww/2.17b3
The User-Agent field may include additional information within
comments.
Note: Some current proxy applications append their product
information to the list in the User-Agent field. This is no
longer recommended, since it makes machine interpretation of
these fields ambiguous. Instead, proxies should use the
Forwarded header described in Section 8.14.
8.30 WWW-Authenticate
The WWW-Authenticate header field must be included in 401
(unauthorized) and 411 (authorization refused) response messages.
The field value consists of a challenge that indicates the
authentication scheme and parameters applicable to the Request-URI.
WWW-Authenticate = "WWW-Authenticate" ":" challenge
The HTTP access authentication process is described in Section 10.
9. Content Negotiation
Content negotiation is an optional feature of the HTTP protocol. It
is designed to allow for selection of a preferred content
representation, within a single request-response round-trip, and
without intervention from the user. However, this may not always be
desirable for the user and is sometimes unnecessary for the content
provider. Implementors are encouraged to provide mechanisms whereby
the amount of preemptive content negotiation, and the parameters of
that negotiation, are configurable by the user and server
maintainer.
The first step in the negotiation algorithm is for the server to
determine whether or not there are any content variants for the
requested resource. Content variants may be in the form of multiple
preexisting entities or a set of dynamic conversion filters. These
variants make up the set of entities which may be sent in response
to a request for the given Request-URI. In most cases, there will
only be one available form of the resource, and thus a single
"variant".
For each variant form of the resource, the server identifies a set
of quality values (Section 3.9) which act as weights for measuring
the desirability of that resource as a response to the current
request. The calculated weights are all real numbers in the range
0 through 1, where 0 is the minimum and 1 the maximum value. The
maximum acceptable bytes for each media range and the size of the
resource variant are also factors in the equation.
The following parameters are included in the calculation:
qs Source quality is measured by the content provider as
representing the amount of degradation from the original
source. For example, a picture originally in JPEG form
would have a lower qs when translated to the XBM format,
and much lower qs when translated to an ASCII-art
representation. Note, however, that this is a function of
the source -- an original piece of ASCII-art may degrade in
quality if it is captured in JPEG form. The qs value should
be assigned to each variant by the content provider; if no
qs value has been assigned, the default is generally
"qs=1". A server may define its own default qs value based
on the resource characteristics, but only if individual
resources can override those defaults.
qe Encoding quality is measured by comparing the variant's
applied encoding-mechanisms (Section 3.6) to those listed
in the request message's Accept-Encoding field. If the
variant has no assigned Content-Encoding, or if no Accept-
Encoding field is present, the value assigned is "qe=1". If
all of the variant's content encodings are listed in the
Accept-Encoding field, then the value assigned is "qe=1".
If any of the variant's content encodings are not listed in
the provided Accept-Encoding field, then the value assigned
is "qe=0.001".
qc Charset quality is measured by comparing the variant media-
type's charset parameter value (if any) to those character
set encodings (Section 3.5) listed in the request message's
Accept-Charset field. If the variant's media-type has no
charset parameter, or the variant's charset is US-ASCII or
ISO-8859-1, or if no Accept-Charset field is present, then
the value assigned is "qc=1". If the variant's charset is
listed in the Accept-Charset field, then the value assigned
is "qc=1". Otherwise, if the variant's charset is not
listed in the provided Accept-Encoding field, then the
value assigned is "qc=0.001".
ql Language quality is measured by comparing the variant's
assigned language tag(s) (Section 3.8) to those listed in
the request message's Accept-Language field. If no variant
has an assigned Content-Language, or if no Accept-Language
field is present, the value assigned is "ql=1". If at least
one variant has an assigned content language, but the one
currently under consideration does not, then it should be
assigned the value "ql=0.5". If any of the variant's
content languages are listed in the Accept-Language field,
then the value assigned is the maximum of the "ql"
parameter values for those language tags (Section 8.4); if
there was no exact match and at least one of the Accept-
Language field values is a complete subtag prefix of the
content language tag(s), then the "ql" parameter value of
the largest matching prefix is used. If none of the
variant's content language tags or tag prefixes are listed
in the provided Accept-Language field, then the value
assigned is "ql=0.001".
q Media type quality is measured by comparing the variant's
assigned media type (Section 3.4) to those listed in the
request message's Accept field. If no Accept field is
given, then the value assigned is "q=1". If at least one
listed media range (Section 8.1) matches the variant's
media type, then the "q" parameter value assigned to the
most specific of those matched is used (e.g.,
"text/html;version=3.0" is more specific than "text/html",
which is more specific than "text/*", which in turn is more
specific than "*/*"). If no media range in the provided
Accept field matches the variant's media type, then the
value assigned is "q=0".
mxb The maximum number of bytes in an Entity-Body that the
client will accept is also obtained from the matching of
the variant's assigned media type to those listed in the
request message's Accept field. If no Accept field is
given, or if no media range in the provided Accept field
matches the variant's media type, then the value assigned
is "mxb=undefined" (i.e., infinity). Otherwise, the value
used is that given to the "mxb" parameter in the media
range chosen above for the q value.
bs The actual number of bytes in the Entity-Body for the
variant when it is included in a response message. This
should equal the value of Content-Length.
The mapping function is defined as:
Q(qs,qe,qc,ql, { if mxb=undefined, then (qs*qe*qc*ql*q) }
q,mxb,bs) = { if mxb >= bs, then (qs*qe*qc*ql*q) }
{ if mxb < bs, then 0 }
The variants with a maximal value for the Q function represent the
preferred representation(s) of the entity; those with a Q values
less than the maximal value are therefore excluded from further
consideration. If multiple representations exist that only vary by
Content-Encoding, then the smallest representation (lowest bs) is
preferred.
If no variants remain with a value of Q greater than zero (0), the
server should respond with a 406 (none acceptable) response
message. If multiple variants remain with an equally high Q value,
the server may either choose one from those available and respond
with 200 (ok) or respond with 300 (multiple choices) and include an
entity describing the choices. In the latter case, the entity
should either be of type "text/html', such that the user can choose
from among the choices by following an exact link, or of some type
that would allow the user agent to perform the selection
automatically.
The 300 (multiple choices) response can be given even if the server
does not perform any winnowing of the representation choices via
the content negotiation algorithm described above. Furthermore, it
may include choices that were not considered as part of the
negotiation algorithm and resources that may be located at other
servers.
Servers that make use of content negotiated resources are strongly
encouraged to include URI response headers which accurately
describe the available variants and include the relevant parameters
necessary for the client (user agent or proxy) to evaluate those
variants.
The algorithm presented above assumes that the user agent has
correctly implemented the protocol and is accurately communicating
its intentions in the form of Accept-related header fields. The
server may alter its response if it knows that the particular
version of user agent software making the request has incorrectly
or inadequately implemented these fields.
10. Access Authentication
HTTP provides a simple challenge-response authorization style 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 semicolon-separated list of attribute-value
pairs which carry the parameters necessary for achieving
authentication via that scheme.
auth-scheme = "basic" | 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 a challenge
applicable to the requested resource.
challenge = auth-scheme 1*SP realm *( ";" auth-param )
realm = "realm" "=" 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 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 or 411 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 = auth-scheme [ 1*LWS encoded-cookie ]
*(";" auth-param )
encoded-cookie = <any valid base64 [6] encoded string,
except not limited to 76 char/line>
The domain over which credentials can be automatically applied by a
user agent is determined by the authorization space. If a request
is authenticated, the credentials can be reused for all other
requests within that authorization space for a period of time
determined by the authentication scheme, parameters, and/or user
preference.
If the server does not wish to accept the credentials sent with a
request, it should return either a 403 (forbidden) or 411
(authorization refused) response. In the latter case, 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 at the transport level, via message
encapsulation, and/or 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. HTTP/1.0 does not provide a means
for a client to be authenticated with a proxy.
Note: The names Proxy-Authenticate and Proxy-Authorization
have been suggested as headers, analogous to
WWW-Authenticate and Authorization, but applying only to the
immediate connection with a proxy.
10.1 Basic Authentication Scheme
The basic authentication scheme is based on the model that the
client 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. The
server will service the request only if it can validate the user-ID
and password for the domain of the Request-URI.
basic-challenge= "Basic" SP realm
The client sends the user-ID and password, separated by a single
colon ":" character, within a base64 [6] encoded-cookie in the
credentials.
basic-credentials="Basic" SP basic-cookie
basic-cookie = <base64 encoding of userid-password>
userid-password= [ token ] ":" *text
There are no optional authentication parameters for the basic
scheme. For example, 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 are
encouraged to implement the scheme in order to communicate with
servers that use it.
11. 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.
11.1 Authentication of Clients
As mentioned in Section 10.1, 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 to be
employed to increase security.
11.2 Idempotent Methods
The writers of client software should be aware that the software
represents the user in their interactions over the net, 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" and
should not have side effects. This allows the client software to
represent other methods, such as POST, PUT and DELETE, in a special
way, so that the user is aware of the fact that an non-idempotent
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
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.
11.3 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.
11.4 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 apriori
method of determining the sensitivity of any particular piece of
information within the context of any given request. Therefore,
applications are encouraged to supply as much control over this
information as possible to the provider of that information. Four
header fields are worth special mention in this context: Server,
Forwarded, 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 are
encouraged to make the Server header field a configurable option.
Proxies which serve as a gateway through a network firewall should
take special precautions regarding the transfer of header
information that identifies the hosts behind the firewall. In
particular, they should remove, or replace with sanitized versions,
any Forwarded fields generated behind the firewall.
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.
12. Acknowledgments
This specification makes heavy use of the augmented BNF and generic
constructs defined by David H. Crocker for RFC 822 [8]. Similarly,
it reuses many of the definitions provided by Nathaniel Borenstein
and Ned Freed for MIME [6]. 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 three
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, Haringkon W. Lie,
Ari Luotonen, Rob McCool, 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.
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
Marc Hedlund Koen Holtman
Alex Hopmann Bob Jernigan
Shel Kaphan Martijn Koster
Dave Kristol Daniel LaLiberte
Albert Lunde John C. Mallery
Larry Masinter Mitra
Gavin Nicol Bill Perry
Jeffrey Perry Owen Rees
David Robinson Marc Salomon
Rich Salz Jim Seidman
Chuck Shotton Eric W. Sink
Simon E. Spero Robert S. Thau
Francois Yergeau Mary Ellen Zurko
13. References
[1] H. Alvestrand. "Tags for the identification of languages."
RFC 1766, UNINETT, March 1995.
[2] F. Anklesaria, M. McCahill, P. Lindner, D. Johnson, D. Torrey,
and B. Alberti. "The Internet Gopher Protocol: A distributed
document search and retrieval protocol." RFC 1436, University
of Minnesota, March 1993.
[3] T. Berners-Lee. "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.
[4] T. Berners-Lee and D. Connolly. "HyperText Markup Language
Specification - 2.0." Work in Progress
(draft-ietf-html-spec-04.txt), MIT/W3C, June 1995.
[5] T. Berners-Lee, L. Masinter, and M. McCahill. "Uniform Resource
Locators (URL)." RFC 1738, CERN, Xerox PARC, University of
Minnesota, October 1994.
[6] N. Borenstein 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.
[7] R. Braden. "Requirements for Internet hosts - application and
support." STD 3, RFC 1123, IETF, October 1989.
[8] D. H. Crocker. "Standard for the Format of ARPA Internet Text
Messages." STD 11, RFC 822, UDEL, August 1982.
[9] 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.
[10] R. Fielding. "Relative Uniform Resource Locators." RFC 1808,
UC Irvine, June 1995.
[11] M. Horton and R. Adams. "Standard for interchange of USENET
messages." RFC 1036 (Obsoletes RFC 850), AT&T Bell
Laboratories, Center for Seismic Studies, December 1987.
[12] B. Kantor 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.
[13] K. Moore. "MIME (Multipurpose Internet Mail Extensions) Part
Two: Message Header Extensions for Non-ASCII Text." RFC 1522,
University of Tennessee, September 1993.
[14] J. Postel. "Simple Mail Transfer Protocol." STD 10, RFC 821,
USC/ISI, August 1982.
[15] J. Postel. "Media Type Registration Procedure." RFC 1590,
USC/ISI, March 1994.
[16] J. Postel and J. K. Reynolds. "File Transfer Protocol (FTP)."
STD 9, RFC 959, USC/ISI, October 1985.
[17] J. Reynolds and J. Postel. "Assigned Numbers." STD 2, RFC 1700,
USC/ISI, October 1994.
[18] K. Sollins and L. Masinter. "Functional Requirements for
Uniform Resource Names." RFC 1737, MIT/LCS, Xerox Corporation,
December 1994.
[19] US-ASCII. Coded Character Set - 7-Bit American Standard Code
for Information Interchange. Standard ANSI X3.4-1986, ANSI,
1986.
[20] 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.
14. Authors' Addresses
Tim Berners-Lee
Director, W3 Consortium
MIT Laboratory for Computer Science
545 Technology Square
Cambridge, MA 02139, U.S.A.
Tel: +1 (617) 253 5702
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.
Tel: +1 (714) 824-4049
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.
Tel: +1 (617) 258 8143
Fax: +1 (617) 258 8682
Email: frystyk@w3.org
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 [15].
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 StatusLine and servers
tolerant when parsing the RequestLine. 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.
C. Relationship to MIME
HTTP/1.0 reuses many of the constructs defined for Internet Mail
(RFC 822 [8]) and the Multipurpose Internet Mail Extensions
(MIME [6]) to allow entities to be transmitted in an open variety
of representations and with extensible mechanisms. However, HTTP is
not a MIME-conforming application. HTTP's performance requirements
differ substantially from those of Internet mail. Since it is not
limited by the restrictions of existing mail protocols and
gateways, HTTP does not obey some of the constraints imposed by
RFC 822 and MIME for mail transport.
This appendix describes specific areas where HTTP differs from
MIME. Gateways to MIME-compliant protocols must be aware of these
differences and provide the appropriate conversions where
necessary. No conversion should be necessary for a MIME-conforming
entity to be transferred using HTTP.
C.1 Conversion to Canonical Form
MIME requires that an entity be converted to canonical form prior
to being transferred, as described in Appendix G of RFC 1521 [6].
Although HTTP does require media types to be transferred in
canonical form, it changes the definition of "canonical form" for
text-based media types as described in Section 3.4.1.
C.1.1 Representation of Line Breaks
MIME requires that the canonical form of any text type represent
line breaks as CRLF and forbids the use of CR or LF outside of line
break sequences. Since HTTP allows CRLF, bare CR, and bare LF
(or the octet sequence(s) to which they would be translated for the
given character set encoding) to indicate a line break within text
content, recipients of an HTTP message cannot rely upon receiving
MIME-canonical line breaks in text.
Where it is possible, a gateway from HTTP to a MIME-conformant
protocol should translate all line breaks within text/* media types
to the MIME canonical form of CRLF. However, this may be
complicated by the presence of a Content-Encoding and by the fact
that HTTP allows the use of some character set encodings which do
not use octets 13 and 10 to represent CR and LF, as is the case for
some multi-byte character set encodings.
C.1.2 Default Character Set Encoding
MIME requires that all subtypes of the top-level Content-Type
"text" have a default character set encoding of US-ASCII [19].
In contrast, HTTP defines the default character set encoding for
"text" to be ISO-8859-1 [20] (a superset of US-ASCII). Therefore,
if a text/* media type given in the Content-Type header field does
not already include an explicit charset parameter, the parameter
;charset="iso-8859-1"
should be added by the gateway if the entity contains any octets
greater than 127.
C.2 Default Content-Transfer-Encoding
The default Content-Transfer-Encoding (CTE) for all MIME messages
is "7bit". In contrast, HTTP defines the default CTE to be
"binary". Therefore, if an entity does not include an explicit CTE
header field, the gateway should apply either the
"quoted-printable" or "base64" transfer encodings and add the
appropriate Content-Transfer-Encoding field. At a minimum, the
explicit CTE field of
Content-Transfer-Encoding: binary
should be added by the gateway if it is unwilling to apply a
mail-safe transfer encoding.
C.3 Introduction of Content-Encoding
MIME does not include any concept equivalent to HTTP's
Content-Encoding header field. Since this acts as a modifier on the
media type, gateways to MIME-conformant protocols should either
change the value of the Content-Type header field or decode the
Entity-Body before forwarding the message.
Note: Some experimental applications of Content-Type for
Internet mail have used a media-type parameter of
";conversions=<encoding-mechanisms>" to perform an
equivalent function as Content-Encoding. However, this
parameter is not part of the MIME specification at the time
of this writing.
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