Hypertext Transfer Protocol (HTTP) Client-Initiated Content-Encoding
RFC 7694

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/rfc7694.

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.

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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 ([RFC7231], Section 3.1.2).  In
   particular, the "gzip" content coding ([RFC7230], 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 ([RFC7231],
   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) ([RFC7231],
   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 [RFC2119].

   This document reuses terminology defined in the base HTTP
   specifications, namely Section 2 of [RFC7230] and Section 3.1.2 of
   [RFC7231].

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3.  Using the 'Accept-Encoding' Header Field in Responses

   Section 5.3.4 of [RFC7231] 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 [RFC7231] 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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4.  Example

   A client submits a POST request using the "compress" content coding
   ([RFC7231], 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 [RFC7231] 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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   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
   [RFC7231]) and HTTP/2 (Section 10 of [RFC7540]).

   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 [RFC7230]), which, when used over a secure channel, can enable
   side-channel attacks such as BREACH (see Section 10.6 of [RFC7540]
   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          |
   |                 |          |          | [RFC7231] and Section 3   |
   |                 |          |          | of this document          |
   +-----------------+----------+----------+---------------------------+

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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 [RFC7231] and Section |
   |       | Media Type      | 3 of this document                      |
   +-------+-----------------+-----------------------------------------+

8.  References

8.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <http://www.rfc-editor.org/info/rfc2119>.

   [RFC7230]  Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
              Protocol (HTTP/1.1): Message Syntax and Routing",
              RFC 7230, DOI 10.17487/RFC7230, June 2014,
              <http://www.rfc-editor.org/info/rfc7230>.

   [RFC7231]  Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
              Protocol (HTTP/1.1): Semantics and Content", RFC 7231,
              DOI 10.17487/RFC7231, June 2014,
              <http://www.rfc-editor.org/info/rfc7231>.

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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   [RFC7540]  Belshe, M., Peon, R., and M. Thomson, Ed., "Hypertext
              Transfer Protocol Version 2 (HTTP/2)", RFC 7540,
              DOI 10.17487/RFC7540, May 2015,
              <http://www.rfc-editor.org/info/rfc7540>.

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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