Prefer Header for HTTP

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This specification defines a new HTTP header that can be used by a client to request that certain behaviors be implemented by a server during the processing a request.¶

This specification defines a new HTTP header that can be used by a client to request that certain behaviors be implemented by a server during the processing a request.¶

In this document, the key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" are to be interpreted as described in [RFC2119].¶

The Prefer request-header is used to indicate that particular server behaviors are preferred, but not required, by the client. Prefer is similar in nature to the Expect header defined by [RFC2616] with the exception that servers are allowed to ignore a clients stated preferences.¶

This header is defined with an extensible syntax to allow for future extensions. A server that does not understand or is unable to comply with any of the preference values in the Prefer field of a request MUST ignore those values and MUST NOT stop processing or signal an error.¶

Comparison of preference values is case-insensitive for unquoted tokens and is case-sensitive for quoted-string preference-extensions.¶

The Prefer mechanism is hop-by-hop: that is, an HTTP proxy MAY choose to honor a preference even if the origin server does not. However, the Prefer request-header itself is end-to-end; it MUST be forwarded if the request is forwarded.¶

The "return-no-content" token indicates that the client prefers that the server not include an entity in the response to a successful request. Typically, such responses would use the 204 No Content status code as defined in Section 10.2.5 of [RFC2616], but other status codes can be used as appropriate.¶

The 'Prefer' request header should be added to the permanent registry (see [RFC3864]).¶

Specific preferences requested by a client can introduce security considerations and concerns beyond those discussed in [RFC2616]. Implementors must refer to the specifications and descriptions of those preferences to determine the security considerations relevant to each.¶

6. Normative References

[RFC2119]

Bradner, S., “Key words for use in RFCs to Indicate Requirement Levels”, BCP 14, RFC 2119, March 1997.

[RFC2616]

Fielding, R., Gettys, J., Mogul, J., Frystyk, H., Masinter, L., Leach, P., and T. Berners-Lee, “Hypertext Transfer Protocol -- HTTP/1.1”, RFC 2616, June 1999.

[RFC3864]

Klyne, G., Nottingham, M., and J. Mogul, “Registration Procedures for Message Header Fields”, BCP 90, RFC 3864, September 2004.

The author greatfully acknowledges the input from the IETF HTTP mailing list on the development of this document.¶

TODO¶

The RFC Editor should remove this section and the Changes section.¶

We need to determine how new preference codes are created/registered¶

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