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IETF RFC 7694



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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 top


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]. Reschke Standards Track PAGE 2 top

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. Reschke Standards Track PAGE 3 top

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. Reschke Standards Track PAGE 4 top

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 | +-----------------+----------+----------+---------------------------+ Reschke Standards Track PAGE 5 top

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>. Reschke Standards Track PAGE 6 top

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/ Reschke Standards Track PAGE 7 top

RFC TOTAL SIZE: 13676 bytes PUBLICATION DATE: Wednesday, November 4th, 2015 LEGAL RIGHTS: The IETF Trust (see BCP 78)


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