HTTP Working Group                                            J. Reschke
Internet-Draft                                                greenbytes
Intended status: Standards Track                       September 8, 2015
Expires: March 11, 2016


  Hypertext Transfer Protocol (HTTP) Client-Initiated Content-Encoding
                       draft-ietf-httpbis-cice-03

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.

Editorial Note (To be removed by RFC Editor before publication)

   Discussion of this draft takes place on the HTTPBIS working group
   mailing list (ietf-http-wg@w3.org), which is archived at
   <https://lists.w3.org/Archives/Public/ietf-http-wg/>.

   Working Group information can be found at
   <https://tools.ietf.org/wg/httpbis/> and <http://httpwg.github.io/>;
   source code and issues list for this draft can be found at
   <https://github.com/httpwg/http-extensions>.

   The changes in this draft are summarized in Appendix A.6.

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at http://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."




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   This Internet-Draft will expire on March 11, 2016.

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.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . . . 3
   2.  Notational Conventions  . . . . . . . . . . . . . . . . . . . . 3
   3.  Using the 'Accept-Encoding' Header Field in Responses . . . . . 3
   4.  Example . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
   5.  Deployment Considerations . . . . . . . . . . . . . . . . . . . 5
   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  . . . . . . . . . . . . . . . . . . 7
   Appendix A.  Change Log (to be removed by RFC Editor before
                publication) . . . . . . . . . . . . . . . . . . . . . 7
     A.1.  Since draft-reschke-http-cice-00  . . . . . . . . . . . . . 7
     A.2.  Since draft-reschke-http-cice-01  . . . . . . . . . . . . . 7
     A.3.  Since draft-reschke-http-cice-02  . . . . . . . . . . . . . 7
     A.4.  Since draft-ietf-httpbis-cice-00  . . . . . . . . . . . . . 7
     A.5.  Since draft-ietf-httpbis-cice-01  . . . . . . . . . . . . . 8
     A.6.  Since draft-ietf-httpbis-cice-02  . . . . . . . . . . . . . 8
   Appendix B.  Acknowledgements . . . . . . . . . . . . . . . . . . . 8











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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 to include the "Accept-Encoding" header field 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].

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 both media type and content coding related
   problems.

   Servers that fail a request due to an unsupported content coding
   ought to respond with a 415 status and ought to include an "Accept-



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   Encoding" header field in that response, allowing clients to
   distinguish between content coding related issues and media type
   related issues.  In order to avoid confusion with media type related
   problems, 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.

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 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 which 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:







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     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 value "identity" in that
   response.

   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



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   <http://www.iana.org/assignments/message-headers>, as defined by
   [BCP90].

   This document updates the definition of the "Accept-Encoding" header
   field, so the "Permanent Message Header Field Names" registry ought
   to be updated accordingly:

   +-----------------+----------+----------+---------------------------+
   | Header Field    | Protocol | Status   | Reference                 |
   | Name            |          |          |                           |
   +-----------------+----------+----------+---------------------------+
   | Accept-Encoding | http     | standard | [RFC7231], Section 5.3.4, |
   |                 |          |          | and Section 3 of this     |
   |                 |          |          | document                  |
   +-----------------+----------+----------+---------------------------+

7.2.  Status Code Registry

   HTTP status codes are registered within the "Status Code" registry
   located at <http://www.iana.org/assignments/http-status-codes>.

   This document updates the definition of the status code 415
   (Unsupported Media Type), so the "Status Code" registry ought to be
   updated accordingly:

   +-------+------------------+----------------------------------------+
   | Value | Description      | Reference                              |
   +-------+------------------+----------------------------------------+
   | 415   | Unsupported      | [RFC7231], Section 6.5.13, and         |
   |       | Media Type       | Section 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,



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              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>.

   [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>.

Appendix A.  Change Log (to be removed by RFC Editor before publication)

A.1.  Since draft-reschke-http-cice-00

   Clarified that the information returned in Accept-Encoding is per
   resource, not per server.

   Added some deployment considerations.

   Updated HTTP/1.1 references.

A.2.  Since draft-reschke-http-cice-01

   Restrict the scope of A-E from "future requests" to "at the time of
   this request".

   Mention use of A-E in responses other than 415.

   Recommend not to include A-E in a 415 response unless there was
   actually a problem related to content coding.

A.3.  Since draft-reschke-http-cice-02

   First Working Group draft; updated boilerplate accordingly.

A.4.  Since draft-ietf-httpbis-cice-00

   Apply editorial improvements suggested by Mark Nottingham.




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A.5.  Since draft-ietf-httpbis-cice-01

   Clarify that we're also extending the definition of status code 415
   (so update that IANA registry entry as well).

A.6.  Since draft-ietf-httpbis-cice-02

   Removed normative language that required used of Accept-Encoding in
   responses (which would have made existing servers non-compliant).

   Add BREACH like attacks to security considerations
   (<https://github.com/httpwg/http-extensions/issues/94>).

Appendix B.  Acknowledgements

   Thanks go to the members of the and HTTPbis Working Group, 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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