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Applications HTTP Working Group HTTP content-encoding 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. Discussion of this draft takes place on the HTTPBIS working group mailing list (ietf-http-wg@w3.org), which is archived at . Working Group information can be found at and ; source code and issues list for this draft can be found at . The changes in this draft are summarized in .

In HTTP, content codings allow for payload encodings such as for compression or integrity checks (, Section 3.1.2). 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 (, 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) (, Section 6.5.13), recommending to include the "Accept-Encoding" header field when appropriate.

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

Section 5.3.4 of 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 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 SHOULD respond with a 415 status and SHOULD include an "Accept-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 SHOULD 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.

A client submits a POST request using the "compress" content coding (, Section 3.1.2.1):

The server rejects request because it only allows the "gzip" content coding:

...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:

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 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.

This specification does not introduce any new security considerations beyond those discussed in Section 9 of .

HTTP header fields are registered within the "Message Headers" registry located at , as defined by . 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 Name Protocol Status Reference Accept-Encoding http standard , Section 5.3.4, and of this document

HTTP status codes are registered within the "Status Code" registry located at . 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 Media Type , Section 6.5.13, and of this document

Harvard University

sob@harvard.edu

Adobe Systems Incorporated

fielding@gbiv.com

greenbytes GmbH

julian.reschke@greenbytes.de

Adobe Systems Incorporated

fielding@gbiv.com

greenbytes GmbH

julian.reschke@greenbytes.de

Nine by Nine

GK-IETF@ninebynine.org

BEA Systems

mnot@pobox.com

HP Labs

JeffMogul@acm.org

Clarified that the information returned in Accept-Encoding is per resource, not per server. Added some deployment considerations. Updated HTTP/1.1 references.

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.

First Working Group draft; updated boilerplate accordingly.

Apply editorial improvements suggested by Mark Nottingham.

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

Thanks go to the members of the and HTTPbis Working Group, namely Amos Jeffries, Mark Nottingham, Pete Resnick, and Ted Hardie.