draft-ietf-httpbis-p3-payload-19.txt   draft-ietf-httpbis-p3-payload-20.txt 
HTTPbis Working Group R. Fielding, Ed. HTTPbis Working Group R. Fielding, Ed.
Internet-Draft Adobe Internet-Draft Adobe
Obsoletes: 2616 (if approved) Y. Lafon, Ed. Intended status: Standards Track Y. Lafon, Ed.
Intended status: Standards Track W3C Expires: January 17, 2013 W3C
Expires: September 13, 2012 J. Reschke, Ed. J. Reschke, Ed.
greenbytes greenbytes
March 12, 2012 July 16, 2012
HTTP/1.1, part 3: Message Payload and Content Negotiation HTTP/1.1, part 3: Message Payload and Content Negotiation
draft-ietf-httpbis-p3-payload-19 draft-ietf-httpbis-p3-payload-20
Abstract Abstract
The Hypertext Transfer Protocol (HTTP) is an application-level This part is now obsolete. Please see HTTPbis, Part 2.
protocol for distributed, collaborative, hypertext information
systems. HTTP has been in use by the World Wide Web global
information initiative since 1990. This document is Part 3 of the
seven-part specification that defines the protocol referred to as
"HTTP/1.1" and, taken together, obsoletes RFC 2616.
Part 3 defines HTTP message content, metadata, and content
negotiation.
Editorial Note (To be removed by RFC Editor)
Discussion of this draft should take place on the HTTPBIS working
group mailing list (ietf-http-wg@w3.org), which is archived at
<http://lists.w3.org/Archives/Public/ietf-http-wg/>.
The current issues list is at
<http://tools.ietf.org/wg/httpbis/trac/report/3> and related
documents (including fancy diffs) can be found at
<http://tools.ietf.org/wg/httpbis/>.
The changes in this draft are summarized in Appendix E.20.
Status of This Memo Status of This Memo
This Internet-Draft is submitted in full conformance with the This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79. provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet- working documents as Internet-Drafts. The list of current Internet-
Drafts is at http://datatracker.ietf.org/drafts/current/. Drafts is at http://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress." material or to cite them other than as "work in progress."
This Internet-Draft will expire on September 13, 2012. This Internet-Draft will expire on January 17, 2013.
Copyright Notice Copyright Notice
Copyright (c) 2012 IETF Trust and the persons identified as the Copyright (c) 2012 IETF Trust and the persons identified as the
document authors. All rights reserved. document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal This document is subject to BCP 78 and the IETF Trust's Legal
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skipping to change at page 2, line 39 skipping to change at page 2, line 19
modifications of such material outside the IETF Standards Process. modifications of such material outside the IETF Standards Process.
Without obtaining an adequate license from the person(s) controlling Without obtaining an adequate license from the person(s) controlling
the copyright in such materials, this document may not be modified the copyright in such materials, this document may not be modified
outside the IETF Standards Process, and derivative works of it may outside the IETF Standards Process, and derivative works of it may
not be created outside the IETF Standards Process, except to format not be created outside the IETF Standards Process, except to format
it for publication as an RFC or to translate it into languages other it for publication as an RFC or to translate it into languages other
than English. than English.
Table of Contents Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 5 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Terminology . . . . . . . . . . . . . . . . . . . . . . . 5
1.2. Conformance and Error Handling . . . . . . . . . . . . . . 5
1.3. Syntax Notation . . . . . . . . . . . . . . . . . . . . . 6
1.3.1. Core Rules . . . . . . . . . . . . . . . . . . . . . . 6
1.3.2. ABNF Rules defined in other Parts of the
Specification . . . . . . . . . . . . . . . . . . . . 6
2. Protocol Parameters . . . . . . . . . . . . . . . . . . . . . 7
2.1. Character Encodings (charset) . . . . . . . . . . . . . . 7
2.2. Content Codings . . . . . . . . . . . . . . . . . . . . . 7
2.2.1. Content Coding Registry . . . . . . . . . . . . . . . 8
2.3. Media Types . . . . . . . . . . . . . . . . . . . . . . . 8
2.3.1. Canonicalization and Text Defaults . . . . . . . . . . 9
2.3.2. Multipart Types . . . . . . . . . . . . . . . . . . . 10
2.4. Language Tags . . . . . . . . . . . . . . . . . . . . . . 10
3. Payload . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
3.1. Payload Header Fields . . . . . . . . . . . . . . . . . . 11
3.2. Payload Body . . . . . . . . . . . . . . . . . . . . . . . 11
4. Representation . . . . . . . . . . . . . . . . . . . . . . . . 11
4.1. Representation Header Fields . . . . . . . . . . . . . . . 12
4.2. Representation Data . . . . . . . . . . . . . . . . . . . 13
5. Content Negotiation . . . . . . . . . . . . . . . . . . . . . 13
5.1. Server-driven Negotiation . . . . . . . . . . . . . . . . 14
5.2. Agent-driven Negotiation . . . . . . . . . . . . . . . . . 16
6. Header Field Definitions . . . . . . . . . . . . . . . . . . . 16
6.1. Accept . . . . . . . . . . . . . . . . . . . . . . . . . . 16
6.2. Accept-Charset . . . . . . . . . . . . . . . . . . . . . . 19
6.3. Accept-Encoding . . . . . . . . . . . . . . . . . . . . . 19
6.4. Accept-Language . . . . . . . . . . . . . . . . . . . . . 21
6.5. Content-Encoding . . . . . . . . . . . . . . . . . . . . . 22
6.6. Content-Language . . . . . . . . . . . . . . . . . . . . . 23
6.7. Content-Location . . . . . . . . . . . . . . . . . . . . . 23
6.8. Content-Type . . . . . . . . . . . . . . . . . . . . . . . 25
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 25
7.1. Header Field Registration . . . . . . . . . . . . . . . . 25
7.2. Content Coding Registry . . . . . . . . . . . . . . . . . 26
8. Security Considerations . . . . . . . . . . . . . . . . . . . 26
8.1. Privacy Issues Connected to Accept Header Fields . . . . . 27
9. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 27
10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 27
10.1. Normative References . . . . . . . . . . . . . . . . . . . 27
10.2. Informative References . . . . . . . . . . . . . . . . . . 29
Appendix A. Differences between HTTP and MIME . . . . . . . . . . 30
A.1. MIME-Version . . . . . . . . . . . . . . . . . . . . . . . 30
A.2. Conversion to Canonical Form . . . . . . . . . . . . . . . 31
A.3. Conversion of Date Formats . . . . . . . . . . . . . . . . 31
A.4. Introduction of Content-Encoding . . . . . . . . . . . . . 31
A.5. No Content-Transfer-Encoding . . . . . . . . . . . . . . . 32
A.6. Introduction of Transfer-Encoding . . . . . . . . . . . . 32
A.7. MHTML and Line Length Limitations . . . . . . . . . . . . 32
Appendix B. Additional Features . . . . . . . . . . . . . . . . . 32
Appendix C. Changes from RFC 2616 . . . . . . . . . . . . . . . . 33
Appendix D. Collected ABNF . . . . . . . . . . . . . . . . . . . 33
Appendix E. Change Log (to be removed by RFC Editor before
publication) . . . . . . . . . . . . . . . . . . . . 35
E.1. Since RFC 2616 . . . . . . . . . . . . . . . . . . . . . . 35
E.2. Since draft-ietf-httpbis-p3-payload-00 . . . . . . . . . . 35
E.3. Since draft-ietf-httpbis-p3-payload-01 . . . . . . . . . . 36
E.4. Since draft-ietf-httpbis-p3-payload-02 . . . . . . . . . . 36
E.5. Since draft-ietf-httpbis-p3-payload-03 . . . . . . . . . . 36
E.6. Since draft-ietf-httpbis-p3-payload-04 . . . . . . . . . . 37
E.7. Since draft-ietf-httpbis-p3-payload-05 . . . . . . . . . . 37
E.8. Since draft-ietf-httpbis-p3-payload-06 . . . . . . . . . . 37
E.9. Since draft-ietf-httpbis-p3-payload-07 . . . . . . . . . . 38
E.10. Since draft-ietf-httpbis-p3-payload-08 . . . . . . . . . . 38
E.11. Since draft-ietf-httpbis-p3-payload-09 . . . . . . . . . . 39
E.12. Since draft-ietf-httpbis-p3-payload-10 . . . . . . . . . . 39
E.13. Since draft-ietf-httpbis-p3-payload-11 . . . . . . . . . . 40
E.14. Since draft-ietf-httpbis-p3-payload-12 . . . . . . . . . . 40
E.15. Since draft-ietf-httpbis-p3-payload-13 . . . . . . . . . . 40
E.16. Since draft-ietf-httpbis-p3-payload-14 . . . . . . . . . . 40
E.17. Since draft-ietf-httpbis-p3-payload-15 . . . . . . . . . . 41
E.18. Since draft-ietf-httpbis-p3-payload-16 . . . . . . . . . . 41
E.19. Since draft-ietf-httpbis-p3-payload-17 . . . . . . . . . . 41
E.20. Since draft-ietf-httpbis-p3-payload-18 . . . . . . . . . . 41
Index . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41
1. Introduction 1. Introduction
This document defines HTTP/1.1 message payloads (a.k.a., content), This part is now obsolete. Please see HTTPbis, Part 2. See also
the associated metadata header fields that define how the payload is <http://trac.tools.ietf.org/wg/httpbis/trac/ticket/351>.
intended to be interpreted by a recipient, the request header fields
that might influence content selection, and the various selection
algorithms that are collectively referred to as HTTP content
negotiation.
This document is currently disorganized in order to minimize the
changes between drafts and enable reviewers to see the smaller errata
changes. A future draft will reorganize the sections to better
reflect the content. In particular, the sections on entities will be
renamed payload and moved to the first half of the document, while
the sections on content negotiation and associated request header
fields will be moved to the second half. The current mess reflects
how widely dispersed these topics and associated requirements had
become in [RFC2616].
1.1. Terminology
This specification uses a number of terms to refer to the roles
played by participants in, and objects of, the HTTP communication.
content negotiation
The mechanism for selecting the appropriate representation when
servicing a request. The representation in any response can be
negotiated (including error responses).
selected representation
The current representation of the target resource that would have
been selected in a successful response if the same request had
used the method GET and excluded any conditional request header
fields.
1.2. Conformance and Error Handling
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119].
This document defines conformance criteria for several roles in HTTP
communication, including Senders, Recipients, Clients, Servers, User-
Agents, Origin Servers, Intermediaries, Proxies and Gateways. See
Section 2 of [Part1] for definitions of these terms.
An implementation is considered conformant if it complies with all of
the requirements associated with its role(s). Note that SHOULD-level
requirements are relevant here, unless one of the documented
exceptions is applicable.
This document also uses ABNF to define valid protocol elements
(Section 1.3). In addition to the prose requirements placed upon
them, Senders MUST NOT generate protocol elements that are invalid.
Unless noted otherwise, Recipients MAY take steps to recover a usable
protocol element from an invalid construct. However, HTTP does not
define specific error handling mechanisms, except in cases where it
has direct impact on security. This is because different uses of the
protocol require different error handling strategies; for example, a
Web browser may wish to transparently recover from a response where
the Location header field doesn't parse according to the ABNF,
whereby in a systems control protocol using HTTP, this type of error
recovery could lead to dangerous consequences.
1.3. Syntax Notation
This specification uses the Augmented Backus-Naur Form (ABNF)
notation of [RFC5234] with the list rule extension defined in Section
1.2 of [Part1]. Appendix D shows the collected ABNF with the list
rule expanded.
The following core rules are included by reference, as defined in
[RFC5234], Appendix B.1: ALPHA (letters), CR (carriage return), CRLF
(CR LF), CTL (controls), DIGIT (decimal 0-9), DQUOTE (double quote),
HEXDIG (hexadecimal 0-9/A-F/a-f), LF (line feed), OCTET (any 8-bit
sequence of data), SP (space), and VCHAR (any visible US-ASCII
character).
1.3.1. Core Rules
The core rules below are defined in [Part1]:
OWS = <OWS, defined in [Part1], Section 3.2.1>
token = <token, defined in [Part1], Section 3.2.4>
word = <word, defined in [Part1], Section 3.2.4>
1.3.2. ABNF Rules defined in other Parts of the Specification
The ABNF rules below are defined in other parts:
absolute-URI = <absolute-URI, defined in [Part1], Section 2.7>
partial-URI = <partial-URI, defined in [Part1], Section 2.7>
qvalue = <qvalue, defined in [Part1], Section 4.3.1>
2. Protocol Parameters
2.1. Character Encodings (charset)
HTTP uses charset names to indicate the character encoding of a
textual representation.
A character encoding is identified by a case-insensitive token. The
complete set of tokens is defined by the IANA Character Set registry
(<http://www.iana.org/assignments/character-sets>).
charset = token
Although HTTP allows an arbitrary token to be used as a charset
value, any token that has a predefined value within the IANA
Character Set registry MUST represent the character encoding defined
by that registry. Applications SHOULD limit their use of character
encodings to those defined within the IANA registry.
HTTP uses charset in two contexts: within an Accept-Charset request
header field (in which the charset value is an unquoted token) and as
the value of a parameter in a Content-Type header field (within a
request or response), in which case the parameter value of the
charset parameter can be quoted.
Implementors need to be aware of IETF character set requirements
[RFC3629] [RFC2277].
2.2. Content Codings
Content coding values indicate an encoding transformation that has
been or can be applied to a representation. Content codings are
primarily used to allow a representation to be compressed or
otherwise usefully transformed without losing the identity of its
underlying media type and without loss of information. Frequently,
the representation is stored in coded form, transmitted directly, and
only decoded by the recipient.
content-coding = token
All content-coding values are case-insensitive. HTTP/1.1 uses
content-coding values in the Accept-Encoding (Section 6.3) and
Content-Encoding (Section 6.5) header fields. Although the value
describes the content-coding, what is more important is that it
indicates what decoding mechanism will be required to remove the
encoding.
compress
See Section 4.2.1 of [Part1].
deflate
See Section 4.2.2 of [Part1].
gzip
See Section 4.2.3 of [Part1].
2.2.1. Content Coding Registry
The HTTP Content Coding Registry defines the name space for the
content coding names.
Registrations MUST include the following fields:
o Name
o Description
o Pointer to specification text
Names of content codings MUST NOT overlap with names of transfer
codings (Section 4 of [Part1]), unless the encoding transformation is
identical (as is the case for the compression codings defined in
Section 4.2 of [Part1]).
Values to be added to this name space require IETF Review (see
Section 4.1 of [RFC5226]), and MUST conform to the purpose of content
coding defined in this section.
The registry itself is maintained at
<http://www.iana.org/assignments/http-parameters>.
2.3. Media Types
HTTP uses Internet Media Types [RFC2046] in the Content-Type
(Section 6.8) and Accept (Section 6.1) header fields in order to
provide open and extensible data typing and type negotiation.
media-type = type "/" subtype *( OWS ";" OWS parameter )
type = token
subtype = token
The type/subtype MAY be followed by parameters in the form of
attribute/value pairs.
parameter = attribute "=" value
attribute = token
value = word
The type, subtype, and parameter attribute names are case-
insensitive. Parameter values might or might not be case-sensitive,
depending on the semantics of the parameter name. The presence or
absence of a parameter might be significant to the processing of a
media-type, depending on its definition within the media type
registry.
A parameter value that matches the token production can be
transmitted as either a token or within a quoted-string. The quoted
and unquoted values are equivalent.
Note that some older HTTP applications do not recognize media type
parameters. When sending data to older HTTP applications,
implementations SHOULD only use media type parameters when they are
required by that type/subtype definition.
Media-type values are registered with the Internet Assigned Number
Authority (IANA). The media type registration process is outlined in
[RFC4288]. Use of non-registered media types is discouraged.
2.3.1. Canonicalization and Text Defaults
Internet media types are registered with a canonical form. A
representation transferred via HTTP messages MUST be in the
appropriate canonical form prior to its transmission except for
"text" types, as defined in the next paragraph.
When in canonical form, media subtypes of the "text" type use CRLF as
the text line break. HTTP relaxes this requirement and allows the
transport of text media with plain CR or LF alone representing a line
break when it is done consistently for an entire representation.
HTTP applications MUST accept CRLF, bare CR, and bare LF as
indicating a line break in text media received via HTTP. In
addition, if the text is in a character encoding that does not use
octets 13 and 10 for CR and LF respectively, as is the case for some
multi-byte character encodings, HTTP allows the use of whatever octet
sequences are defined by that character encoding to represent the
equivalent of CR and LF for line breaks. This flexibility regarding
line breaks applies only to text media in the payload body; a bare CR
or LF MUST NOT be substituted for CRLF within any of the HTTP control
structures (such as header fields and multipart boundaries).
If a representation is encoded with a content-coding, the underlying
data MUST be in a form defined above prior to being encoded.
2.3.2. Multipart Types
MIME provides for a number of "multipart" types -- encapsulations of
one or more representations within a single message body. All
multipart types share a common syntax, as defined in Section 5.1.1 of
[RFC2046], 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.
In general, HTTP treats a multipart message body no differently than
any other media type: strictly as payload. HTTP does not use the
multipart boundary as an indicator of message body length. In all
other respects, an HTTP user agent SHOULD follow the same or similar
behavior as a MIME user agent would upon receipt of a multipart type.
The MIME header fields within each body-part of a multipart message
body do not have any significance to HTTP beyond that defined by
their MIME semantics.
If an application receives an unrecognized multipart subtype, the
application MUST treat it as being equivalent to "multipart/mixed".
Note: The "multipart/form-data" type has been specifically defined
for carrying form data suitable for processing via the POST
request method, as described in [RFC2388].
2.4. Language Tags
A language tag, as defined in [RFC5646], identifies a natural
language spoken, written, or otherwise conveyed by human beings for
communication of information to other human beings. Computer
languages are explicitly excluded. HTTP uses language tags within
the Accept-Language and Content-Language fields.
In summary, a language tag is composed of one or more parts: A
primary language subtag followed by a possibly empty series of
subtags:
language-tag = <Language-Tag, defined in [RFC5646], Section 2.1>
White space is not allowed within the tag and all tags are case-
insensitive. The name space of language subtags is administered by
the IANA (see
<http://www.iana.org/assignments/language-subtag-registry>).
Example tags include:
en, en-US, es-419, az-Arab, x-pig-latin, man-Nkoo-GN
See [RFC5646] for further information.
3. Payload
HTTP messages MAY transfer a payload if not otherwise restricted by
the request method or response status code. The payload consists of
metadata, in the form of header fields, and data, in the form of the
sequence of octets in the message body after any transfer-coding has
been decoded.
A "payload" in HTTP is always a partial or complete representation of
some resource. We use separate terms for payload and representation
because some messages contain only the associated representation's
header fields (e.g., responses to HEAD) or only some part(s) of the
representation (e.g., the 206 status code).
3.1. Payload Header Fields
HTTP header fields that specifically define the payload, rather than
the associated representation, are referred to as "payload header
fields". The following payload header fields are defined by
HTTP/1.1:
+-------------------+--------------------------+
| Header Field Name | Defined in... |
+-------------------+--------------------------+
| Content-Length | Section 3.3.2 of [Part1] |
| Content-Range | Section 5.2 of [Part5] |
+-------------------+--------------------------+
3.2. Payload Body
A payload body is only present in a message when a message body is
present, as described in Section 3.3 of [Part1]. The payload body is
obtained from the message body by decoding any Transfer-Encoding that
might have been applied to ensure safe and proper transfer of the
message.
4. Representation
A "representation" is information in a format that can be readily
communicated from one party to another. A resource representation is
information that reflects the state of that resource, as observed at
some point in the past (e.g., in a response to GET) or to be desired
at some point in the future (e.g., in a PUT request).
Most, but not all, representations transferred via HTTP are intended
to be a representation of the target resource (the resource
identified by the effective request URI). The precise semantics of a
representation are determined by the type of message (request or
response), the request method, the response status code, and the
representation metadata. For example, the above semantic is true for
the representation in any 200 (OK) response to GET and for the
representation in any PUT request. A 200 response to PUT, in
contrast, contains either a representation that describes the
successful action or a representation of the target resource, with
the latter indicated by a Content-Location header field with the same
value as the effective request URI. Likewise, response messages with
an error status code usually contain a representation that describes
the error and what next steps are suggested for resolving it.
4.1. Representation Header Fields
Representation header fields define metadata about the representation
data enclosed in the message body or, if no message body is present,
about the representation that would have been transferred in a 200
response to a simultaneous GET request with the same effective
request URI.
The following header fields are defined as representation metadata:
+-------------------+------------------------+
| Header Field Name | Defined in... |
+-------------------+------------------------+
| Content-Encoding | Section 6.5 |
| Content-Language | Section 6.6 |
| Content-Location | Section 6.7 |
| Content-Type | Section 6.8 |
| Expires | Section 3.3 of [Part6] |
+-------------------+------------------------+
Additional header fields define metadata about the selected
representation, which might differ from the representation included
in the message for responses to some state-changing methods. The
following header fields are defined as selected representation
metadata:
+-------------------+------------------------+
| Header Field Name | Defined in... |
+-------------------+------------------------+
| ETag | Section 2.3 of [Part4] |
| Last-Modified | Section 2.2 of [Part4] |
+-------------------+------------------------+
4.2. Representation Data
The representation body associated with an HTTP message is either
provided as the payload body of the message or referred to by the
message semantics and the effective request URI. The representation
data is in a format and encoding defined by the representation
metadata header fields.
The data type of the representation data is determined via the header
fields Content-Type and Content-Encoding. These define a two-layer,
ordered encoding model:
representation-data := Content-Encoding( Content-Type( bits ) )
Content-Type specifies the media type of the underlying data, which
defines both the data format and how that data SHOULD be processed by
the recipient (within the scope of the request method semantics).
Any HTTP/1.1 message containing a payload body SHOULD include a
Content-Type header field defining the media type of the associated
representation unless that metadata is unknown to the sender. If the
Content-Type header field is not present, it indicates that the
sender does not know the media type of the representation; recipients
MAY either assume that the media type is "application/octet-stream"
([RFC2046], Section 4.5.1) or examine the content to determine its
type.
In practice, resource owners do not always properly configure their
origin server to provide the correct Content-Type for a given
representation, with the result that some clients will examine a
response body's content and override the specified type. Clients
that do so risk drawing incorrect conclusions, which might expose
additional security risks (e.g., "privilege escalation").
Furthermore, it is impossible to determine the sender's intent by
examining the data format: many data formats match multiple media
types that differ only in processing semantics. Implementers are
encouraged to provide a means of disabling such "content sniffing"
when it is used.
Content-Encoding is used to indicate any additional content codings
applied to the data, usually for the purpose of data compression,
that are a property of the representation. If Content-Encoding is
not present, then there is no additional encoding beyond that defined
by the Content-Type.
5. Content Negotiation
HTTP responses include a representation which contains information
for interpretation, whether by a human user or for further
processing. Often, the server has different ways of representing the
same information; for example, in different formats, languages, or
using different character encodings.
HTTP clients and their users might have different or variable
capabilities, characteristics or preferences which would influence
which representation, among those available from the server, would be
best for the server to deliver. For this reason, HTTP provides
mechanisms for "content negotiation" -- a process of allowing
selection of a representation of a given resource, when more than one
is available.
This specification defines two patterns of content negotiation;
"server-driven", where the server selects the representation based
upon the client's stated preferences, and "agent-driven" negotiation,
where the server provides a list of representations for the client to
choose from, based upon their metadata. In addition, there are other
patterns: some applications use an "active content" pattern, where
the server returns active content which runs on the client and, based
on client available parameters, selects additional resources to
invoke. "Transparent Content Negotiation" ([RFC2295]) has also been
proposed.
These patterns are all widely used, and have trade-offs in
applicability and practicality. In particular, when the number of
preferences or capabilities to be expressed by a client are large
(such as when many different formats are supported by a user-agent),
server-driven negotiation becomes unwieldy, and might not be
appropriate. Conversely, when the number of representations to
choose from is very large, agent-driven negotiation might not be
appropriate.
Note that in all cases, the supplier of representations has the
responsibility for determining which representations might be
considered to be the "same information".
5.1. Server-driven Negotiation
If the selection of the best representation for a response is made by
an algorithm located at the server, it is called server-driven
negotiation. Selection is based on the available representations of
the response (the dimensions over which it can vary; e.g., language,
content-coding, etc.) and the contents of particular header fields in
the request message or on other information pertaining to the request
(such as the network address of the client).
Server-driven negotiation is advantageous when the algorithm for
selecting from among the available representations is difficult to
describe to the user agent, or when the server desires to send its
"best guess" to the client along with the first response (hoping to
avoid the round-trip delay of a subsequent request if the "best
guess" is good enough for the user). In order to improve the
server's guess, the user agent MAY include request header fields
(Accept, Accept-Language, Accept-Encoding, etc.) which describe its
preferences for such a response.
Server-driven negotiation has disadvantages:
1. It is impossible for the server to accurately determine what
might be "best" for any given user, since that would require
complete knowledge of both the capabilities of the user agent and
the intended use for the response (e.g., does the user want to
view it on screen or print it on paper?).
2. Having the user agent describe its capabilities in every request
can be both very inefficient (given that only a small percentage
of responses have multiple representations) and a potential
violation of the user's privacy.
3. It complicates the implementation of an origin server and the
algorithms for generating responses to a request.
4. It might limit a public cache's ability to use the same response
for multiple user's requests.
Server-driven negotiation allows the user agent to specify its
preferences, but it cannot expect responses to always honor them.
For example, the origin server might not implement server-driven
negotiation, or it might decide that sending a response that doesn't
conform to them is better than sending a 406 (Not Acceptable)
response.
Many of the mechanisms for expressing preferences use quality values
to declare relative preference. See Section 4.3.1 of [Part1] for
more information.
HTTP/1.1 includes the following header fields for enabling server-
driven negotiation through description of user agent capabilities and
user preferences: Accept (Section 6.1), Accept-Charset (Section 6.2),
Accept-Encoding (Section 6.3), Accept-Language (Section 6.4), and
User-Agent (Section 10.10 of [Part2]). However, an origin server is
not limited to these dimensions and MAY vary the response based on
any aspect of the request, including aspects of the connection (e.g.,
IP address) or information within extension header fields not defined
by this specification.
Note: In practice, User-Agent based negotiation is fragile,
because new clients might not be recognized.
The Vary header field (Section 3.5 of [Part6]) can be used to express
the parameters the server uses to select a representation that is
subject to server-driven negotiation.
5.2. Agent-driven Negotiation
With agent-driven negotiation, selection of the best representation
for a response is performed by the user agent after receiving an
initial response from the origin server. Selection is based on a
list of the available representations of the response included within
the header fields or body of the initial response, with each
representation identified by its own URI. Selection from among the
representations can be performed automatically (if the user agent is
capable of doing so) or manually by the user selecting from a
generated (possibly hypertext) menu.
Agent-driven negotiation is advantageous when the response would vary
over commonly-used dimensions (such as type, language, or encoding),
when the origin server is unable to determine a user agent's
capabilities from examining the request, and generally when public
caches are used to distribute server load and reduce network usage.
Agent-driven negotiation suffers from the disadvantage of needing a
second request to obtain the best alternate representation. This
second request is only efficient when caching is used. In addition,
this specification does not define any mechanism for supporting
automatic selection, though it also does not prevent any such
mechanism from being developed as an extension and used within
HTTP/1.1.
This specification defines the 300 (Multiple Choices) and 406 (Not
Acceptable) status codes for enabling agent-driven negotiation when
the server is unwilling or unable to provide a varying response using
server-driven negotiation.
6. Header Field Definitions
This section defines the syntax and semantics of HTTP/1.1 header
fields related to the payload of messages.
6.1. Accept
The "Accept" header field can be used by user agents to specify
response media types that are acceptable. Accept header fields can
be used to indicate that the request is specifically limited to a
small set of desired types, as in the case of a request for an in-
line image.
Accept = #( media-range [ accept-params ] )
media-range = ( "*/*"
/ ( type "/" "*" )
/ ( type "/" subtype )
) *( OWS ";" OWS parameter )
accept-params = OWS ";" OWS "q=" qvalue *( accept-ext )
accept-ext = OWS ";" OWS token [ "=" word ]
The asterisk "*" character is used to group media types into ranges,
with "*/*" indicating all media types and "type/*" indicating all
subtypes of that type. The media-range MAY include media type
parameters that are applicable to that range.
Each media-range MAY be followed by one or more accept-params,
beginning with the "q" parameter for indicating a relative quality
factor. The first "q" parameter (if any) separates the media-range
parameter(s) from the accept-params. Quality factors allow the user
or user agent to indicate the relative degree of preference for that
media-range, using the qvalue scale from 0 to 1 (Section 4.3.1 of
[Part1]). The default value is q=1.
Note: Use of the "q" parameter name to separate media type
parameters from Accept extension parameters is due to historical
practice. Although this prevents any media type parameter named
"q" from being used with a media range, such an event is believed
to be unlikely given the lack of any "q" parameters in the IANA
media type registry and the rare usage of any media type
parameters in Accept. Future media types are discouraged from
registering any parameter named "q".
The example
Accept: audio/*; q=0.2, audio/basic
SHOULD be interpreted as "I prefer audio/basic, but send me any audio
type if it is the best available after an 80% mark-down in quality".
A request without any Accept header field implies that the user agent
will accept any media type in response. If an Accept header field is
present in a request and none of the available representations for
the response have a media type that is listed as acceptable, the
origin server MAY either honor the Accept header field by sending a
406 (Not Acceptable) response or disregard the Accept header field by
treating the response as if it is not subject to content negotiation.
A more elaborate example is
Accept: text/plain; q=0.5, text/html,
text/x-dvi; q=0.8, text/x-c
Verbally, this would be interpreted as "text/html and text/x-c are
the preferred media types, but if they do not exist, then send the
text/x-dvi representation, and if that does not exist, send the text/
plain representation".
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/plain, text/plain;format=flowed, */*
have the following precedence:
1. text/plain;format=flowed
2. text/plain
3. text/*
4. */*
The media type quality factor associated with a given type is
determined by finding the media range with the highest precedence
which matches that type. For example,
Accept: text/*;q=0.3, text/html;q=0.7, text/html;level=1,
text/html;level=2;q=0.4, */*;q=0.5
would cause the following values to be associated:
+-------------------+---------------+
| Media Type | Quality Value |
+-------------------+---------------+
| text/html;level=1 | 1 |
| text/html | 0.7 |
| text/plain | 0.3 |
| image/jpeg | 0.5 |
| text/html;level=2 | 0.4 |
| text/html;level=3 | 0.7 |
+-------------------+---------------+
Note: A user agent might be provided with a default set of quality
values for certain media ranges. However, unless the user agent is a
closed system which cannot interact with other rendering agents, this
default set ought to be configurable by the user.
6.2. Accept-Charset
The "Accept-Charset" header field can be used by user agents to
indicate what character encodings are acceptable in a response
payload. This field allows clients capable of understanding more
comprehensive or special-purpose character encodings to signal that
capability to a server which is capable of representing documents in
those character encodings.
Accept-Charset = 1#( ( charset / "*" )
[ OWS ";" OWS "q=" qvalue ] )
Character encoding values (a.k.a., charsets) are described in
Section 2.1. Each charset MAY be given an associated quality value
which represents the user's preference for that charset. The default
value is q=1. An example is
Accept-Charset: iso-8859-5, unicode-1-1;q=0.8
The special value "*", if present in the Accept-Charset field,
matches every character encoding which is not mentioned elsewhere in
the Accept-Charset field. If no "*" is present in an Accept-Charset
field, then all character encodings not explicitly mentioned get a
quality value of 0.
A request without any Accept-Charset header field implies that the
user agent will accept any character encoding in response. If an
Accept-Charset header field is present in a request and none of the
available representations for the response have a character encoding
that is listed as acceptable, the origin server MAY either honor the
Accept-Charset header field by sending a 406 (Not Acceptable)
response or disregard the Accept-Charset header field by treating the
response as if it is not subject to content negotiation.
6.3. Accept-Encoding
The "Accept-Encoding" header field can be used by user agents to
indicate what response content-codings (Section 2.2) are acceptable
in the response. An "identity" token is used as a synonym for "no
encoding" in order to communicate when no encoding is preferred.
Accept-Encoding = #( codings [ OWS ";" OWS "q=" qvalue ] )
codings = content-coding / "identity" / "*"
Each codings value MAY be given an associated quality value which
represents the preference for that encoding. The default value is
q=1.
For example,
Accept-Encoding: compress, gzip
Accept-Encoding:
Accept-Encoding: *
Accept-Encoding: compress;q=0.5, gzip;q=1.0
Accept-Encoding: gzip;q=1.0, identity; q=0.5, *;q=0
A server tests whether a content-coding for a given representation is
acceptable, according to an Accept-Encoding field, using these rules:
1. The special "*" symbol in an Accept-Encoding field matches any
available content-coding not explicitly listed in the header
field.
2. If the representation has no content-coding, then it is
acceptable by default unless specifically excluded by the Accept-
Encoding field stating either "identity;q=0" or "*;q=0" without a
more specific entry for "identity".
3. If the representation's content-coding is one of the content-
codings listed in the Accept-Encoding field, then it is
acceptable unless it is accompanied by a qvalue of 0. (As
defined in Section 4.3.1 of [Part1], a qvalue of 0 means "not
acceptable".)
4. If multiple content-codings are acceptable, then the acceptable
content-coding with the highest non-zero qvalue is preferred.
An Accept-Encoding header field with a combined field-value that is
empty implies that the user agent does not want any content-coding in
response. If an Accept-Encoding header field is present in a request
and none of the available representations for the response have a
content-coding that is listed as acceptable, the origin server SHOULD
send a response without any content-coding.
A request without an Accept-Encoding header field implies that the
user agent will accept any content-coding in response, but a
representation without content-coding is preferred for compatibility
with the widest variety of user agents.
Note: Most HTTP/1.0 applications do not recognize or obey qvalues
associated with content-codings. This means that qvalues will not
work and are not permitted with x-gzip or x-compress.
6.4. Accept-Language
The "Accept-Language" header field can be used by user agents to
indicate the set of natural languages that are preferred in the
response. Language tags are defined in Section 2.4.
Accept-Language =
1#( language-range [ OWS ";" OWS "q=" qvalue ] )
language-range =
<language-range, defined in [RFC4647], Section 2.1>
Each language-range can be given an associated quality value which
represents an estimate of the user's preference for the languages
specified by that range. The quality value defaults to "q=1". For
example,
Accept-Language: da, en-gb;q=0.8, en;q=0.7
would mean: "I prefer Danish, but will accept British English and
other types of English". (see also Section 2.3 of [RFC4647])
For matching, Section 3 of [RFC4647] defines several matching
schemes. Implementations can offer the most appropriate matching
scheme for their requirements.
Note: The "Basic Filtering" scheme ([RFC4647], Section 3.3.1) is
identical to the matching scheme that was previously defined in
Section 14.4 of [RFC2616].
It might be contrary to the privacy expectations of the user to send
an Accept-Language header field with the complete linguistic
preferences of the user in every request. For a discussion of this
issue, see Section 8.1.
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.
Note: When making the choice of linguistic preference available to
the user, we remind implementors of the fact that users are not
familiar with the details of language matching as described above,
and ought to be provided appropriate guidance. As an example,
users might assume that on selecting "en-gb", they will be served
any kind of English document if British English is not available.
A user agent might suggest in such a case to add "en" to get the
best matching behavior.
6.5. Content-Encoding
The "Content-Encoding" header field indicates what content-codings
have been applied to the representation beyond those inherent in the
media type, and thus what decoding mechanisms must be applied in
order to obtain the media-type referenced by the Content-Type header
field. Content-Encoding is primarily used to allow a representation
to be compressed without losing the identity of its underlying media
type.
Content-Encoding = 1#content-coding
Content codings are defined in Section 2.2. An example of its use is
Content-Encoding: gzip
The content-coding is a characteristic of the representation.
Typically, the representation body is stored with this encoding and
is only decoded before rendering or analogous usage. However, a
transforming proxy MAY modify the content-coding if the new coding is
known to be acceptable to the recipient, unless the "no-transform"
cache-control directive is present in the message.
If the media type includes an inherent encoding, such as a data
format that is always compressed, then that encoding would not be
restated as a Content-Encoding even if it happens to be the same
algorithm as one of the content-codings. Such a content-coding would
only be listed if, for some bizarre reason, it is applied a second
time to form the representation. Likewise, an origin server might
choose to publish the same payload data as multiple representations
that differ only in whether the coding is defined as part of Content-
Type or Content-Encoding, since some user agents will behave
differently in their handling of each response (e.g., open a "Save as
..." dialog instead of automatic decompression and rendering of
content).
A representation that has a content-coding applied to it MUST include
a Content-Encoding header field (Section 6.5) that lists the content-
coding(s) applied.
If multiple encodings have been applied to a representation, the
content codings MUST be listed in the order in which they were
applied. Additional information about the encoding parameters MAY be
provided by other header fields not defined by this specification.
If the content-coding of a representation in a request message is not
acceptable to the origin server, the server SHOULD respond with a
status code of 415 (Unsupported Media Type).
6.6. Content-Language
The "Content-Language" header field describes the natural language(s)
of the intended audience for the representation. Note that this
might not be equivalent to all the languages used within the
representation.
Content-Language = 1#language-tag
Language tags are defined in Section 2.4. The primary purpose of
Content-Language is to allow a user to identify and differentiate
representations according to the user's own preferred language.
Thus, if the body content is intended only for a Danish-literate
audience, the appropriate field is
Content-Language: da
If no Content-Language is specified, the default is that the content
is intended for all language audiences. This might 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 a
representation 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 would properly only include "en".
Content-Language MAY be applied to any media type -- it is not
limited to textual documents.
6.7. Content-Location
The "Content-Location" header field supplies a URI that can be used
as a specific identifier for the representation in this message. In
other words, if one were to perform a GET on this URI at the time of
this message's generation, then a 200 response would contain the same
representation that is enclosed as payload in this message.
Content-Location = absolute-URI / partial-URI
The Content-Location value is not a replacement for the effective
Request URI (Section 5.5 of [Part1]). It is representation metadata.
It has the same syntax and semantics as the header field of the same
name defined for MIME body parts in Section 4 of [RFC2557]. However,
its appearance in an HTTP message has some special implications for
HTTP recipients.
If Content-Location is included in a response message and its value
is the same as the effective request URI, then the response payload
SHOULD be considered a current representation of that resource. For
a GET or HEAD request, this is the same as the default semantics when
no Content-Location is provided by the server. For a state-changing
request like PUT or POST, it implies that the server's response
contains the new representation of that resource, thereby
distinguishing it from representations that might only report about
the action (e.g., "It worked!"). This allows authoring applications
to update their local copies without the need for a subsequent GET
request.
If Content-Location is included in a response message and its value
differs from the effective request URI, then the origin server is
informing recipients that this representation has its own, presumably
more specific, identifier. For a GET or HEAD request, this is an
indication that the effective request URI identifies a resource that
is subject to content negotiation and the selected representation for
this response can also be found at the identified URI. For other
methods, such a Content-Location indicates that this representation
contains a report on the action's status and the same report is
available (for future access with GET) at the given URI. For
example, a purchase transaction made via a POST request might include
a receipt document as the payload of the 200 response; the Content-
Location value provides an identifier for retrieving a copy of that
same receipt in the future.
If Content-Location is included in a request message, then it MAY be
interpreted by the origin server as an indication of where the user
agent originally obtained the content of the enclosed representation
(prior to any subsequent modification of the content by that user
agent). In other words, the user agent is providing the same
representation metadata that it received with the original
representation. However, such interpretation MUST NOT be used to
alter the semantics of the method requested by the client. For
example, if a client makes a PUT request on a negotiated resource and
the origin server accepts that PUT (without redirection), then the
new set of values for that resource is expected to be consistent with
the one representation supplied in that PUT; the Content-Location
cannot be used as a form of reverse content selection that identifies
only one of the negotiated representations to be updated. If the
user agent had wanted the latter semantics, it would have applied the
PUT directly to the Content-Location URI.
A Content-Location field received in a request message is transitory
information that SHOULD NOT be saved with other representation
metadata for use in later responses. The Content-Location's value
might be saved for use in other contexts, such as within source links
or other metadata.
A cache cannot assume that a representation with a Content-Location
different from the URI used to retrieve it can be used to respond to
later requests on that Content-Location URI.
If the Content-Location value is a partial URI, the partial URI is
interpreted relative to the effective request URI.
6.8. Content-Type
The "Content-Type" header field indicates the media type of the
representation. In the case of responses to the HEAD method, the
media type is that which would have been sent had the request been a
GET.
Content-Type = media-type
Media types are defined in Section 2.3. An example of the field is
Content-Type: text/html; charset=ISO-8859-4
Further discussion of Content-Type is provided in Section 4.2.
7. IANA Considerations
7.1. Header Field Registration
The Message Header Field Registry located at <http://www.iana.org/
assignments/message-headers/message-header-index.html> shall be
updated with the permanent registrations below (see [RFC3864]):
+-------------------+----------+----------+--------------+
| Header Field Name | Protocol | Status | Reference |
+-------------------+----------+----------+--------------+
| Accept | http | standard | Section 6.1 |
| Accept-Charset | http | standard | Section 6.2 |
| Accept-Encoding | http | standard | Section 6.3 |
| Accept-Language | http | standard | Section 6.4 |
| Content-Encoding | http | standard | Section 6.5 |
| Content-Language | http | standard | Section 6.6 |
| Content-Location | http | standard | Section 6.7 |
| Content-Type | http | standard | Section 6.8 |
| MIME-Version | http | standard | Appendix A.1 |
+-------------------+----------+----------+--------------+
The change controller is: "IETF (iesg@ietf.org) - Internet
Engineering Task Force".
7.2. Content Coding Registry
The registration procedure for HTTP Content Codings is now defined by
Section 2.2.1 of this document.
The HTTP Content Codings Registry located at
<http://www.iana.org/assignments/http-parameters> shall be updated
with the registration below:
+----------+------------------------------------------+-------------+
| Name | Description | Reference |
+----------+------------------------------------------+-------------+
| compress | UNIX "compress" program method | Section |
| | | 4.2.1 of |
| | | [Part1] |
| deflate | "deflate" compression mechanism | Section |
| | ([RFC1951]) used inside the "zlib" data | 4.2.2 of |
| | format ([RFC1950]) | [Part1] |
| gzip | Same as GNU zip [RFC1952] | Section |
| | | 4.2.3 of |
| | | [Part1] |
| identity | reserved (synonym for "no encoding" in | Section 6.3 |
| | Accept-Encoding header field) | |
+----------+------------------------------------------+-------------+
8. Security Considerations
This section is meant to inform application developers, information
providers, and users of the security limitations in HTTP/1.1 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.
8.1. Privacy Issues Connected to Accept Header Fields
Accept header fields can reveal information about the user to all
servers which are accessed. The Accept-Language header field in
particular can reveal information the user would consider to be of a
private nature, because the understanding of particular languages is
often strongly correlated to the membership of a particular ethnic
group. User agents which offer the option to configure the contents
of an Accept-Language header field to be sent in every request are
strongly encouraged to let the configuration process include a
message which makes the user aware of the loss of privacy involved.
An approach that limits the loss of privacy would be for a user agent
to omit the sending of Accept-Language header fields by default, and
to ask the user whether or not to start sending Accept-Language
header fields to a server if it detects, by looking for any Vary
header fields generated by the server, that such sending could
improve the quality of service.
Elaborate user-customized accept header fields sent in every request,
in particular if these include quality values, can be used by servers
as relatively reliable and long-lived user identifiers. Such user
identifiers would allow content providers to do click-trail tracking,
and would allow collaborating content providers to match cross-server
click-trails or form submissions of individual users. Note that for
many users not behind a proxy, the network address of the host
running the user agent will also serve as a long-lived user
identifier. In environments where proxies are used to enhance
privacy, user agents ought to be conservative in offering accept
header configuration options to end users. As an extreme privacy
measure, proxies could filter the accept header fields in relayed
requests. General purpose user agents which provide a high degree of
header configurability SHOULD warn users about the loss of privacy
which can be involved.
9. Acknowledgments
See Section 9 of [Part1].
10. References
10.1. Normative References
[Part1] Fielding, R., Ed., Lafon, Y., Ed., and J. Reschke, Ed.,
"HTTP/1.1, part 1: URIs, Connections, and Message
Parsing", draft-ietf-httpbis-p1-messaging-19 (work in
progress), March 2012.
[Part2] Fielding, R., Ed., Lafon, Y., Ed., and J. Reschke, Ed.,
"HTTP/1.1, part 2: Message Semantics",
draft-ietf-httpbis-p2-semantics-19 (work in progress),
March 2012.
[Part4] Fielding, R., Ed., Lafon, Y., Ed., and J. Reschke, Ed.,
"HTTP/1.1, part 4: Conditional Requests",
draft-ietf-httpbis-p4-conditional-19 (work in progress),
March 2012.
[Part5] Fielding, R., Ed., Lafon, Y., Ed., and J. Reschke, Ed.,
"HTTP/1.1, part 5: Range Requests and Partial Responses",
draft-ietf-httpbis-p5-range-19 (work in progress),
March 2012.
[Part6] Fielding, R., Ed., Lafon, Y., Ed., Nottingham, M., Ed.,
and J. Reschke, Ed., "HTTP/1.1, part 6: Caching",
draft-ietf-httpbis-p6-cache-19 (work in progress),
March 2012.
[RFC1950] Deutsch, L. and J-L. Gailly, "ZLIB Compressed Data Format
Specification version 3.3", RFC 1950, May 1996.
[RFC1951] Deutsch, P., "DEFLATE Compressed Data Format Specification
version 1.3", RFC 1951, May 1996.
[RFC1952] Deutsch, P., Gailly, J-L., Adler, M., Deutsch, L., and G.
Randers-Pehrson, "GZIP file format specification version
4.3", RFC 1952, May 1996.
[RFC2045] Freed, N. and N. Borenstein, "Multipurpose Internet Mail
Extensions (MIME) Part One: Format of Internet Message
Bodies", RFC 2045, November 1996.
[RFC2046] Freed, N. and N. Borenstein, "Multipurpose Internet Mail
Extensions (MIME) Part Two: Media Types", RFC 2046,
November 1996.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC4647] Phillips, A., Ed. and M. Davis, Ed., "Matching of Language
Tags", BCP 47, RFC 4647, September 2006.
[RFC5234] Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", STD 68, RFC 5234, January 2008.
[RFC5646] Phillips, A., Ed. and M. Davis, Ed., "Tags for Identifying
Languages", BCP 47, RFC 5646, September 2009.
10.2. Informative References
[RFC1945] Berners-Lee, T., Fielding, R., and H. Nielsen, "Hypertext
Transfer Protocol -- HTTP/1.0", RFC 1945, May 1996.
[RFC2049] Freed, N. and N. Borenstein, "Multipurpose Internet Mail
Extensions (MIME) Part Five: Conformance Criteria and
Examples", RFC 2049, November 1996.
[RFC2068] Fielding, R., Gettys, J., Mogul, J., Nielsen, H., and T.
Berners-Lee, "Hypertext Transfer Protocol -- HTTP/1.1",
RFC 2068, January 1997.
[RFC2076] Palme, J., "Common Internet Message Headers", RFC 2076,
February 1997.
[RFC2277] Alvestrand, H., "IETF Policy on Character Sets and
Languages", BCP 18, RFC 2277, January 1998.
[RFC2295] Holtman, K. and A. Mutz, "Transparent Content Negotiation
in HTTP", RFC 2295, March 1998.
[RFC2388] Masinter, L., "Returning Values from Forms: multipart/
form-data", RFC 2388, August 1998.
[RFC2557] Palme, F., Hopmann, A., Shelness, N., and E. Stefferud,
"MIME Encapsulation of Aggregate Documents, such as HTML
(MHTML)", RFC 2557, March 1999.
[RFC2616] Fielding, R., Gettys, J., Mogul, J., Frystyk, H.,
Masinter, L., Leach, P., and T. Berners-Lee, "Hypertext
Transfer Protocol -- HTTP/1.1", RFC 2616, June 1999.
[RFC3629] Yergeau, F., "UTF-8, a transformation format of ISO
10646", STD 63, RFC 3629, November 2003.
[RFC3864] Klyne, G., Nottingham, M., and J. Mogul, "Registration
Procedures for Message Header Fields", BCP 90, RFC 3864,
September 2004.
[RFC4288] Freed, N. and J. Klensin, "Media Type Specifications and
Registration Procedures", BCP 13, RFC 4288, December 2005.
[RFC5226] Narten, T. and H. Alvestrand, "Guidelines for Writing an
IANA Considerations Section in RFCs", BCP 26, RFC 5226,
May 2008.
[RFC5322] Resnick, P., "Internet Message Format", RFC 5322,
October 2008.
[RFC6151] Turner, S. and L. Chen, "Updated Security Considerations
for the MD5 Message-Digest and the HMAC-MD5 Algorithms",
RFC 6151, March 2011.
[RFC6266] Reschke, J., "Use of the Content-Disposition Header Field
in the Hypertext Transfer Protocol (HTTP)", RFC 6266,
June 2011.
Appendix A. Differences between HTTP and MIME
HTTP/1.1 uses many of the constructs defined for Internet Mail
([RFC5322]) and the Multipurpose Internet Mail Extensions (MIME
[RFC2045]) to allow a message body to be transmitted in an open
variety of representations and with extensible mechanisms. However,
RFC 2045 discusses mail, and HTTP has a few features that are
different from those described in MIME. These differences were
carefully chosen to optimize performance over binary connections, to
allow greater freedom in the use of new media types, to make date
comparisons easier, and to acknowledge the practice of some early
HTTP servers and clients.
This appendix describes specific areas where HTTP differs from MIME.
Proxies and gateways to strict MIME environments SHOULD be aware of
these differences and provide the appropriate conversions where
necessary. Proxies and gateways from MIME environments to HTTP also
need to be aware of the differences because some conversions might be
required.
A.1. MIME-Version
HTTP is not a MIME-compliant protocol. However, HTTP/1.1 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 indicates that the message is in
full conformance with the MIME protocol (as defined in [RFC2045]).
Proxies/gateways are responsible for ensuring full conformance (where
possible) when exporting HTTP messages to strict MIME environments.
MIME-Version = 1*DIGIT "." 1*DIGIT
MIME version "1.0" is the default for use in HTTP/1.1. However,
HTTP/1.1 message parsing and semantics are defined by this document
and not the MIME specification.
A.2. Conversion to Canonical Form
MIME requires that an Internet mail body-part be converted to
canonical form prior to being transferred, as described in Section 4
of [RFC2049]. Section 2.3.1 of this document describes the forms
allowed for subtypes of the "text" media type when transmitted over
HTTP. [RFC2046] requires that content with a type of "text"
represent line breaks as CRLF and forbids the use of CR or LF outside
of line break sequences. HTTP allows CRLF, bare CR, and bare LF to
indicate a line break within text content when a message is
transmitted over HTTP.
Where it is possible, a proxy or gateway from HTTP to a strict MIME
environment SHOULD translate all line breaks within the text media
types described in Section 2.3.1 of this document to the RFC 2049
canonical form of CRLF. Note, however, that this might be
complicated by the presence of a Content-Encoding and by the fact
that HTTP allows the use of some character encodings which do not use
octets 13 and 10 to represent CR and LF, respectively, as is the case
for some multi-byte character encodings.
Conversion will break any cryptographic checksums applied to the
original content unless the original content is already in canonical
form. Therefore, the canonical form is recommended for any content
that uses such checksums in HTTP.
A.3. Conversion of Date Formats
HTTP/1.1 uses a restricted set of date formats (Section 8 of [Part2])
to simplify the process of date comparison. Proxies and gateways
from other protocols SHOULD ensure that any Date header field present
in a message conforms to one of the HTTP/1.1 formats and rewrite the
date if necessary.
A.4. Introduction of Content-Encoding
MIME does not include any concept equivalent to HTTP/1.1's Content-
Encoding header field. Since this acts as a modifier on the media
type, proxies and gateways from HTTP to MIME-compliant protocols MUST
either change the value of the Content-Type header field or decode
the representation before forwarding the message. (Some experimental
applications of Content-Type for Internet mail have used a media-type
parameter of ";conversions=<content-coding>" to perform a function
equivalent to Content-Encoding. However, this parameter is not part
of the MIME standards).
A.5. No Content-Transfer-Encoding
HTTP does not use the Content-Transfer-Encoding field of MIME.
Proxies and gateways from MIME-compliant protocols to HTTP MUST
remove any Content-Transfer-Encoding prior to delivering the response
message to an HTTP client.
Proxies and gateways from HTTP to MIME-compliant protocols are
responsible for ensuring that the message is in the correct format
and encoding for safe transport on that protocol, where "safe
transport" is defined by the limitations of the protocol being used.
Such a proxy or gateway SHOULD label the data with an appropriate
Content-Transfer-Encoding if doing so will improve the likelihood of
safe transport over the destination protocol.
A.6. Introduction of Transfer-Encoding
HTTP/1.1 introduces the Transfer-Encoding header field (Section 3.3.1
of [Part1]). Proxies/gateways MUST remove any transfer-coding prior
to forwarding a message via a MIME-compliant protocol.
A.7. MHTML and Line Length Limitations
HTTP implementations which share code with MHTML [RFC2557]
implementations need to be aware of MIME line length limitations.
Since HTTP does not have this limitation, HTTP does not fold long
lines. MHTML messages being transported by HTTP follow all
conventions of MHTML, including line length limitations and folding,
canonicalization, etc., since HTTP transports all message-bodies as
payload (see Section 2.3.2) and does not interpret the content or any
MIME header lines that might be contained therein.
Appendix B. Additional Features
[RFC1945] and [RFC2068] document protocol elements used by some
existing HTTP implementations, but not consistently and correctly
across most HTTP/1.1 applications. Implementors are advised to be
aware of these features, but cannot rely upon their presence in, or
interoperability with, other HTTP/1.1 applications. Some of these
describe proposed experimental features, and some describe features
that experimental deployment found lacking that are now addressed in
the base HTTP/1.1 specification.
A number of other header fields, such as Content-Disposition and
Title, from SMTP and MIME are also often implemented (see [RFC6266]
and [RFC2076]).
Appendix C. Changes from RFC 2616
Clarify contexts that charset is used in. (Section 2.1)
Registration of Content Codings now requires IETF Review
(Section 2.2.1)
Remove the default character encoding for text media types; the
default now is whatever the media type definition says.
(Section 2.3.1)
Change ABNF productions for header fields to only define the field
value. (Section 6)
Remove definition of Content-MD5 header field because it was
inconsistently implemented with respect to partial responses, and
also because of known deficiencies in the hash algorithm itself (see
[RFC6151] for details). (Section 6)
Remove ISO-8859-1 special-casing in Accept-Charset. (Section 6.2)
Remove base URI setting semantics for Content-Location due to poor
implementation support, which was caused by too many broken servers
emitting bogus Content-Location header fields, and also the
potentially undesirable effect of potentially breaking relative links
in content-negotiated resources. (Section 6.7)
Remove reference to non-existant identity transfer-coding value
tokens. (Appendix A.5)
Remove discussion of Content-Disposition header field, it is now
defined by [RFC6266]. (Appendix B)
Appendix D. Collected ABNF
Accept = [ ( "," / ( media-range [ accept-params ] ) ) *( OWS "," [
OWS media-range [ accept-params ] ] ) ]
Accept-Charset = *( "," OWS ) ( charset / "*" ) [ OWS ";" OWS "q="
qvalue ] *( OWS "," [ OWS ( charset / "*" ) [ OWS ";" OWS "q="
qvalue ] ] )
Accept-Encoding = [ ( "," / ( codings [ OWS ";" OWS "q=" qvalue ] ) )
*( OWS "," [ OWS codings [ OWS ";" OWS "q=" qvalue ] ] ) ]
Accept-Language = *( "," OWS ) language-range [ OWS ";" OWS "q="
qvalue ] *( OWS "," [ OWS language-range [ OWS ";" OWS "q=" qvalue ]
] )
Content-Encoding = *( "," OWS ) content-coding *( OWS "," [ OWS
content-coding ] )
Content-Language = *( "," OWS ) language-tag *( OWS "," [ OWS
language-tag ] )
Content-Location = absolute-URI / partial-URI
Content-Type = media-type
MIME-Version = 1*DIGIT "." 1*DIGIT
OWS = <OWS, defined in [Part1], Section 3.2.1>
absolute-URI = <absolute-URI, defined in [Part1], Section 2.7>
accept-ext = OWS ";" OWS token [ "=" word ]
accept-params = OWS ";" OWS "q=" qvalue *accept-ext
attribute = token
charset = token
codings = content-coding / "identity" / "*"
content-coding = token
language-range = <language-range, defined in [RFC4647], Section 2.1>
language-tag = <Language-Tag, defined in [RFC5646], Section 2.1>
media-range = ( "*/*" / ( type "/*" ) / ( type "/" subtype ) ) *( OWS
";" OWS parameter )
media-type = type "/" subtype *( OWS ";" OWS parameter )
parameter = attribute "=" value
partial-URI = <partial-URI, defined in [Part1], Section 2.7>
qvalue = <qvalue, defined in [Part1], Section 4.3.1>
subtype = token
token = <token, defined in [Part1], Section 3.2.4>
type = token
value = word
word = <word, defined in [Part1], Section 3.2.4>
ABNF diagnostics:
; Accept defined but not used
; Accept-Charset defined but not used
; Accept-Encoding defined but not used
; Accept-Language defined but not used
; Content-Encoding defined but not used
; Content-Language defined but not used
; Content-Location defined but not used
; Content-Type defined but not used
; MIME-Version defined but not used
Appendix E. Change Log (to be removed by RFC Editor before publication)
E.1. Since RFC 2616
Extracted relevant partitions from [RFC2616].
E.2. Since draft-ietf-httpbis-p3-payload-00
Closed issues:
o <http://tools.ietf.org/wg/httpbis/trac/ticket/8>: "Media Type
Registrations" (<http://purl.org/NET/http-errata#media-reg>)
o <http://tools.ietf.org/wg/httpbis/trac/ticket/14>: "Clarification
regarding quoting of charset values"
(<http://purl.org/NET/http-errata#charactersets>)
o <http://tools.ietf.org/wg/httpbis/trac/ticket/16>: "Remove
'identity' token references"
(<http://purl.org/NET/http-errata#identity>)
o <http://tools.ietf.org/wg/httpbis/trac/ticket/25>: "Accept-
Encoding BNF"
o <http://tools.ietf.org/wg/httpbis/trac/ticket/35>: "Normative and
Informative references"
o <http://tools.ietf.org/wg/httpbis/trac/ticket/46>: "RFC1700
references"
o <http://tools.ietf.org/wg/httpbis/trac/ticket/55>: "Updating to
RFC4288"
o <http://tools.ietf.org/wg/httpbis/trac/ticket/65>: "Informative
references"
o <http://tools.ietf.org/wg/httpbis/trac/ticket/66>: "ISO-8859-1
Reference"
o <http://tools.ietf.org/wg/httpbis/trac/ticket/68>: "Encoding
References Normative"
o <http://tools.ietf.org/wg/httpbis/trac/ticket/86>: "Normative up-
to-date references"
E.3. Since draft-ietf-httpbis-p3-payload-01
Ongoing work on ABNF conversion
(<http://tools.ietf.org/wg/httpbis/trac/ticket/36>):
o Add explicit references to BNF syntax and rules imported from
other parts of the specification.
E.4. Since draft-ietf-httpbis-p3-payload-02
Closed issues:
o <http://tools.ietf.org/wg/httpbis/trac/ticket/67>: "Quoting
Charsets"
o <http://tools.ietf.org/wg/httpbis/trac/ticket/105>:
"Classification for Allow header"
o <http://tools.ietf.org/wg/httpbis/trac/ticket/115>: "missing
default for qvalue in description of Accept-Encoding"
Ongoing work on IANA Message Header Field Registration
(<http://tools.ietf.org/wg/httpbis/trac/ticket/40>):
o Reference RFC 3984, and update header field registrations for
headers defined in this document.
E.5. Since draft-ietf-httpbis-p3-payload-03
Closed issues:
o <http://tools.ietf.org/wg/httpbis/trac/ticket/67>: "Quoting
Charsets"
o <http://tools.ietf.org/wg/httpbis/trac/ticket/113>: "language tag
matching (Accept-Language) vs RFC4647"
o <http://tools.ietf.org/wg/httpbis/trac/ticket/121>: "RFC 1806 has
been replaced by RFC2183"
Other changes:
o <http://tools.ietf.org/wg/httpbis/trac/ticket/68>: "Encoding
References Normative" -- rephrase the annotation and reference
BCP97.
E.6. Since draft-ietf-httpbis-p3-payload-04
Closed issues:
o <http://tools.ietf.org/wg/httpbis/trac/ticket/132>: "RFC 2822 is
updated by RFC 5322"
Ongoing work on ABNF conversion
(<http://tools.ietf.org/wg/httpbis/trac/ticket/36>):
o Use "/" instead of "|" for alternatives.
o Introduce new ABNF rules for "bad" whitespace ("BWS"), optional
whitespace ("OWS") and required whitespace ("RWS").
o Rewrite ABNFs to spell out whitespace rules, factor out header
field value format definitions.
E.7. Since draft-ietf-httpbis-p3-payload-05
Closed issues:
o <http://tools.ietf.org/wg/httpbis/trac/ticket/118>: "Join
"Differences Between HTTP Entities and RFC 2045 Entities"?"
Final work on ABNF conversion
(<http://tools.ietf.org/wg/httpbis/trac/ticket/36>):
o Add appendix containing collected and expanded ABNF, reorganize
ABNF introduction.
Other changes:
o Move definition of quality values into Part 1.
E.8. Since draft-ietf-httpbis-p3-payload-06
Closed issues:
o <http://tools.ietf.org/wg/httpbis/trac/ticket/80>: "Content-
Location isn't special"
o <http://tools.ietf.org/wg/httpbis/trac/ticket/155>: "Content
Sniffing"
E.9. Since draft-ietf-httpbis-p3-payload-07
Closed issues:
o <http://tools.ietf.org/wg/httpbis/trac/ticket/13>: "Updated
reference for language tags"
o <http://tools.ietf.org/wg/httpbis/trac/ticket/110>: "Clarify rules
for determining what entities a response carries"
o <http://tools.ietf.org/wg/httpbis/trac/ticket/154>: "Content-
Location base-setting problems"
o <http://tools.ietf.org/wg/httpbis/trac/ticket/155>: "Content
Sniffing"
o <http://tools.ietf.org/wg/httpbis/trac/ticket/188>: "pick IANA
policy (RFC5226) for Transfer Coding / Content Coding"
o <http://tools.ietf.org/wg/httpbis/trac/ticket/189>: "move
definitions of gzip/deflate/compress to part 1"
Partly resolved issues:
o <http://tools.ietf.org/wg/httpbis/trac/ticket/148>: "update IANA
requirements wrt Transfer-Coding values" (add the IANA
Considerations subsection)
o <http://tools.ietf.org/wg/httpbis/trac/ticket/149>: "update IANA
requirements wrt Content-Coding values" (add the IANA
Considerations subsection)
E.10. Since draft-ietf-httpbis-p3-payload-08
Closed issues:
o <http://tools.ietf.org/wg/httpbis/trac/ticket/81>: "Content
Negotiation for media types"
o <http://tools.ietf.org/wg/httpbis/trac/ticket/181>: "Accept-
Language: which RFC4647 filtering?"
E.11. Since draft-ietf-httpbis-p3-payload-09
Closed issues:
o <http://tools.ietf.org/wg/httpbis/trac/ticket/122>: "MIME-Version
not listed in P1, general header fields"
o <http://tools.ietf.org/wg/httpbis/trac/ticket/143>: "IANA registry
for content/transfer encodings"
o <http://tools.ietf.org/wg/httpbis/trac/ticket/155>: "Content
Sniffing"
o <http://tools.ietf.org/wg/httpbis/trac/ticket/200>: "use of term
"word" when talking about header structure"
Partly resolved issues:
o <http://tools.ietf.org/wg/httpbis/trac/ticket/196>: "Term for the
requested resource's URI"
E.12. Since draft-ietf-httpbis-p3-payload-10
Closed issues:
o <http://tools.ietf.org/wg/httpbis/trac/ticket/69>: "Clarify
'Requested Variant'"
o <http://tools.ietf.org/wg/httpbis/trac/ticket/80>: "Content-
Location isn't special"
o <http://tools.ietf.org/wg/httpbis/trac/ticket/90>: "Delimiting
messages with multipart/byteranges"
o <http://tools.ietf.org/wg/httpbis/trac/ticket/109>: "Clarify
entity / representation / variant terminology"
o <http://tools.ietf.org/wg/httpbis/trac/ticket/136>: "confusing
req. language for Content-Location"
o <http://tools.ietf.org/wg/httpbis/trac/ticket/167>: "Content-
Location on 304 responses"
o <http://tools.ietf.org/wg/httpbis/trac/ticket/183>: "'requested
resource' in content-encoding definition"
o <http://tools.ietf.org/wg/httpbis/trac/ticket/220>: "consider
removing the 'changes from 2068' sections"
Partly resolved issues:
o <http://tools.ietf.org/wg/httpbis/trac/ticket/178>: "Content-MD5
and partial responses"
E.13. Since draft-ietf-httpbis-p3-payload-11
Closed issues:
o <http://tools.ietf.org/wg/httpbis/trac/ticket/123>: "Factor out
Content-Disposition"
E.14. Since draft-ietf-httpbis-p3-payload-12
Closed issues:
o <http://tools.ietf.org/wg/httpbis/trac/ticket/224>: "Header
Classification"
o <http://tools.ietf.org/wg/httpbis/trac/ticket/276>: "untangle
ABNFs for header fields"
o <http://tools.ietf.org/wg/httpbis/trac/ticket/277>: "potentially
misleading MAY in media-type def"
E.15. Since draft-ietf-httpbis-p3-payload-13
Closed issues:
o <http://tools.ietf.org/wg/httpbis/trac/ticket/20>: "Default
charsets for text media types"
o <http://tools.ietf.org/wg/httpbis/trac/ticket/178>: "Content-MD5
and partial responses"
o <http://tools.ietf.org/wg/httpbis/trac/ticket/276>: "untangle
ABNFs for header fields"
o <http://tools.ietf.org/wg/httpbis/trac/ticket/281>: "confusing
undefined parameter in media range example"
E.16. Since draft-ietf-httpbis-p3-payload-14
None.
E.17. Since draft-ietf-httpbis-p3-payload-15
Closed issues:
o <http://tools.ietf.org/wg/httpbis/trac/ticket/285>: "Strength of
requirements on Accept re: 406"
E.18. Since draft-ietf-httpbis-p3-payload-16
Closed issues:
o <http://tools.ietf.org/wg/httpbis/trac/ticket/186>: "Document
HTTP's error-handling philosophy"
E.19. Since draft-ietf-httpbis-p3-payload-17
Closed issues:
o <http://tools.ietf.org/wg/httpbis/trac/ticket/323>: "intended
maturity level vs normative references"
E.20. Since draft-ietf-httpbis-p3-payload-18
Closed issues:
o <http://tools.ietf.org/wg/httpbis/trac/ticket/330>: "is ETag a
representation header field?"
o <http://tools.ietf.org/wg/httpbis/trac/ticket/338>: "Content-
Location doesn't constrain the cardinality of representations"
o <http://tools.ietf.org/wg/httpbis/trac/ticket/346>: "make IANA
policy definitions consistent"
Index
A
Accept header field 16
Accept-Charset header field 19
Accept-Encoding header field 19
Accept-Language header field 21
C
Coding Format
compress 7
deflate 8
gzip 8
compress (Coding Format) 7
content negotiation 5
Content-Encoding header field 22
Content-Language header field 23
Content-Location header field 23
Content-Transfer-Encoding header field 32
Content-Type header field 25
D
deflate (Coding Format) 8
G
Grammar
Accept 17
Accept-Charset 19
Accept-Encoding 19
accept-ext 17
Accept-Language 21
accept-params 17
attribute 9
charset 7
codings 19
content-coding 7
Content-Encoding 22
Content-Language 23
Content-Location 23
Content-Type 25
language-range 21
language-tag 10
media-range 17
media-type 8
MIME-Version 30
parameter 9
subtype 8
type 8
value 9
gzip (Coding Format) 8
H
Header Fields
Accept 16
Accept-Charset 19
Accept-Encoding 19
Accept-Language 21
Content-Encoding 22
Content-Language 23
Content-Location 23
Content-Transfer-Encoding 32
Content-Type 25
MIME-Version 30
M
MIME-Version header field 30
P
payload 11
R
representation 11
S
selected representation 5
Authors' Addresses Authors' Addresses
Roy T. Fielding (editor) Roy T. Fielding (editor)
Adobe Systems Incorporated Adobe Systems Incorporated
345 Park Ave 345 Park Ave
San Jose, CA 95110 San Jose, CA 95110
USA USA
EMail: fielding@gbiv.com EMail: fielding@gbiv.com
skipping to change at page 43, line 45 skipping to change at page 3, line 37
EMail: ylafon@w3.org EMail: ylafon@w3.org
URI: http://www.raubacapeu.net/people/yves/ URI: http://www.raubacapeu.net/people/yves/
Julian F. Reschke (editor) Julian F. Reschke (editor)
greenbytes GmbH greenbytes GmbH
Hafenweg 16 Hafenweg 16
Muenster, NW 48155 Muenster, NW 48155
Germany Germany
Phone: +49 251 2807760
Fax: +49 251 2807761
EMail: julian.reschke@greenbytes.de EMail: julian.reschke@greenbytes.de
URI: http://greenbytes.de/tech/webdav/ URI: http://greenbytes.de/tech/webdav/
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