Hypertext Transfer Protocol (HTTP) Client-Initiated 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.¶

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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 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.¶

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].¶

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 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.¶

...at which point the client can retry the request with the supported "gzip" content coding.¶

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.¶

This specification does not introduce any new security considerations beyond those discussed in Section 9 of [RFC7231].¶

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