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IETF RFC 7694
Last modified on Wednesday, November 4th, 2015
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Internet Engineering Task Force (IETF) J. Reschke
Request for Comments: 7694 greenbytes
Category: Standards Track November 2015
ISSN: 2070-1721
Hypertext Transfer Protocol (HTTP) Client-Initiated Content-Encoding
 Abstract
In HTTP, content codings allow for payload encodings such as for
compression or integrity checks. In particular, the "gzip" content
coding is widely used for payload data sent in response messages.
Content codings can be used in request messages as well; however,
discoverability is not on par with response messages. This document
extends the HTTP "Accept-Encoding" header field for use in responses,
to indicate the content codings that are supported in requests.
Status of This Memo
This is an Internet Standards Track document.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 5741.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
http://www.rfc-editor.org/info/RFC 7694.
Copyright Notice
Copyright (c) 2015 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Reschke Standards Track PAGE 1 
RFC 7694 HTTP CICE November 2015
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Notational Conventions . . . . . . . . . . . . . . . . . . . 2
3. Using the 'Accept-Encoding' Header Field in Responses . . . . 3
4. Example . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
5. Deployment Considerations . . . . . . . . . . . . . . . . . . 4
6. Security Considerations . . . . . . . . . . . . . . . . . . . 5
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 5
7.1. Header Field Registry . . . . . . . . . . . . . . . . . . 5
7.2. Status Code Registry . . . . . . . . . . . . . . . . . . 6
8. References . . . . . . . . . . . . . . . . . . . . . . . . . 6
8.1. Normative References . . . . . . . . . . . . . . . . . . 6
8.2. Informative References . . . . . . . . . . . . . . . . . 6
Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . 7
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 7
1. Introduction
In HTTP, content codings allow for payload encodings such as for
compression or integrity checks ([RFC 7231], Section 3.1.2). In
particular, the "gzip" content coding ([RFC 7230], Section 4.2) is
widely used for payload data sent in response messages.
Content codings can be used in request messages as well; however,
discoverability is not on par with response messages. This document
extends the HTTP "Accept-Encoding" header field ([RFC 7231],
Section 5.3.4) for use in responses, to indicate the content codings
that are supported in requests. It furthermore updates the
definition of status code 415 (Unsupported Media Type) ([RFC 7231],
Section 6.5.13), recommending that the "Accept-Encoding" header field
be included when appropriate.
2. Notational Conventions
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 [RFC 2119].
This document reuses terminology defined in the base HTTP
specifications, namely Section 2 of [RFC 7230] and Section 3.1.2 of
[RFC 7231].
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RFC 7694 HTTP CICE November 2015
3. Using the 'Accept-Encoding' Header Field in Responses
Section 5.3.4 of [RFC 7231] defines "Accept-Encoding" as a request
header field only.
This specification expands that definition to allow "Accept-Encoding"
as a response header field as well. When present in a response, it
indicates what content codings the resource was willing to accept in
the associated request. A field value that only contains "identity"
implies that no content codings were supported.
Note that this information is specific to the associated request; the
set of supported encodings might be different for other resources on
the same server and could change over time or depend on other aspects
of the request (such as the request method).
Section 6.5.13 of [RFC 7231] defines status code 415 (Unsupported
Media Type) to apply to problems related to both media types and
content codings.
Servers that fail a request due to an unsupported content coding
ought to respond with a 415 status and ought to include an "Accept-
Encoding" header field in that response, allowing clients to
distinguish between issues related to content codings and media
types. In order to avoid confusion with issues related to media
types, servers that fail a request with a 415 status for reasons
unrelated to content codings MUST NOT include the "Accept-Encoding"
header field.
It is expected that the most common use of "Accept-Encoding" in
responses will have the 415 (Unsupported Media Type) status code, in
response to optimistic use of a content coding by clients. However,
the header field can also be used to indicate to clients that content
codings are supported, to optimize future interactions. For example,
a resource might include it in a 2xx response when the request
payload was big enough to justify use of a compression coding but the
client failed do so.
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RFC 7694 HTTP CICE November 2015
4. Example
A client submits a POST request using the "compress" content coding
([RFC 7231], Section 3.1.2.1):
POST /edit/ HTTP/1.1
Host: example.org
Content-Type: application/atom+xml;type=entry
Content-Encoding: compress
...compressed payload...
The server rejects the request because it only allows the "gzip"
content coding:
HTTP/1.1 415 Unsupported Media Type
Date: Fri, 09 May 2014 11:43:53 GMT
Accept-Encoding: gzip
Content-Length: 68
Content-Type: text/plain
This resource only supports the "gzip" content coding in requests.
At this point, the client can retry the request with the supported
"gzip" content coding.
Alternatively, a server that does not support any content codings in
requests could answer with:
HTTP/1.1 415 Unsupported Media Type
Date: Fri, 09 May 2014 11:43:53 GMT
Accept-Encoding: identity
Content-Length: 61
Content-Type: text/plain
This resource does not support content codings in requests.
5. Deployment Considerations
Servers that do not support content codings in requests already are
required to fail a request that uses a content coding.
Section 6.5.13 of [RFC 7231] defines the status code 415 (Unsupported
Media Type) for this purpose, so the only change needed is to include
the "Accept-Encoding" header field with the value "identity" in that
response.
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RFC 7694 HTTP CICE November 2015
Servers that do support some content codings are required to fail
requests with unsupported content codings as well. To be compliant
with this specification, servers will need to use the status code 415
(Unsupported Media Type) to signal the problem and will have to
include an "Accept-Encoding" header field that enumerates the content
codings that are supported. As the set of supported content codings
is usually static and small, adding the header field ought to be
trivial.
6. Security Considerations
This specification only adds discovery of supported content codings
and diagnostics for requests failing due to unsupported content
codings. As such, it doesn't introduce any new security
considerations over those already present in HTTP/1.1 (Section 9 of
[RFC 7231]) and HTTP/2 (Section 10 of [RFC 7540]).
However, the point of better discoverability and diagnostics is to
make it easier to use content codings in requests. This might lead
to increased usage of compression codings such as gzip (Section 4.2
of [RFC 7230]), which, when used over a secure channel, can enable
side-channel attacks such as BREACH (see Section 10.6 of [RFC 7540]
and [BREACH]). At the time of publication, it was unclear how
BREACH-like attacks can be applied to compression in HTTP requests.
7. IANA Considerations
7.1. Header Field Registry
HTTP header fields are registered within the "Message Headers"
registry located at <http://www.iana.org/assignments/
message-headers>, as defined by [BCP90].
This document updates the definition of the "Accept-Encoding" header
field. The "Permanent Message Header Field Names" registry has been
updated as follows:
+-----------------+----------+----------+---------------------------+
| Header Field | Protocol | Status | Reference |
| Name | | | |
+-----------------+----------+----------+---------------------------+
| Accept-Encoding | http | standard | Section 5.3.4 of |
| | | | [RFC 7231] and Section 3 |
| | | | of this document |
+-----------------+----------+----------+---------------------------+
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RFC 7694 HTTP CICE November 2015
7.2. Status Code Registry
HTTP status codes are registered within the "HTTP Status Codes"
registry located at <http://www.iana.org/assignments/
http-status-codes>.
This document updates the definition of the status code 415
(Unsupported Media Type). The "HTTP Status Codes" registry has been
updated as follows:
+-------+-----------------+-----------------------------------------+
| Value | Description | Reference |
+-------+-----------------+-----------------------------------------+
| 415 | Unsupported | Section 6.5.13 of [RFC 7231] and Section |
| | Media Type | 3 of this document |
+-------+-----------------+-----------------------------------------+
8. References
8.1. Normative References
[RFC 2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC 2119, March 1997,
<http://www.rfc-editor.org/info/RFC 2119>.
[RFC 7230] Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
Protocol (HTTP/1.1): Message Syntax and Routing",
RFC 7230, DOI 10.17487/RFC 7230, June 2014,
<http://www.rfc-editor.org/info/RFC 7230>.
[RFC 7231] Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
Protocol (HTTP/1.1): Semantics and Content", RFC 7231,
DOI 10.17487/RFC 7231, June 2014,
<http://www.rfc-editor.org/info/RFC 7231>.
8.2. Informative References
[BCP90] Klyne, G., Nottingham, M., and J. Mogul, "Registration
Procedures for Message Header Fields", BCP 90, RFC 3864,
September 2004, <http://www.rfc-editor.org/info/bcp90>.
[BREACH] Gluck, Y., Harris, N., and A. Prado, "BREACH: Reviving the
CRIME Attack", July 2013,
<http://breachattack.com/resources/
BREACH%20-%20SSL,%20gone%20in%2030%20seconds.pdf>.
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RFC 7694 HTTP CICE November 2015
[RFC 7540] Belshe, M., Peon, R., and M. Thomson, Ed., "Hypertext
Transfer Protocol Version 2 (HTTP/2)", RFC 7540,
DOI 10.17487/RFC 7540, May 2015,
<http://www.rfc-editor.org/info/RFC 7540>.
Acknowledgements
Thanks go to the Hypertext Transfer Protocol Working Group
participants, namely Amos Jeffries, Ben Campbell, Mark Nottingham,
Pete Resnick, Stephen Farrell, and Ted Hardie.
Author's Address
Julian F. Reschke
greenbytes GmbH
Hafenweg 16
Muenster, NW 48155
Germany
Email: julian.reschke@greenbytes.de
URI: http://greenbytes.de/tech/webdav/
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RFC TOTAL SIZE: 13676 bytes
PUBLICATION DATE: Wednesday, November 4th, 2015
LEGAL RIGHTS: The IETF Trust (see BCP 78)
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