Individual SubmissionJ. Snell
Internet-DraftFebruary 2008
Intended status: Informational
Expires: August 2008

Prefer Header for HTTP

draft-snell-http-prefer-02

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Abstract

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

1. Introduction

This specification defines a new HTTP header that can be used by a client to request that certain behaviors be implemented by a server while 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].

2. The Prefer Request Header

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.

  Prefer       =  "Prefer" ":" 1#preference

  preference   =  "return-no-content" |
                  "return-content" | 
                  "return-status" | 
                  preference-extension
  preference-extension =  token [ "=" ( token | quoted-string )
                          *prefer-params ]
  prefer-params =  ";" token [ "=" ( token | quoted-string ) ]

This header is defined with an extensible syntax to allow for future values included in the Registry of Preferences (see Section 7.1). A server that does not recognize or is unable to comply with particular preference values in the Prefer header 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.

An HTTP proxy MAY choose to honor a preference even if the origin server does not. The Prefer request-header MUST be forwarded by the proxy if the request is forwarded.

3. The Preference-Applied Response Header

The Preference-Applied response header MAY be included in the response message to indicate which Prefer request header values were honored by the server and applied to the request.

  Prefer       =  "Preference-Applied" ":" 1#preference

4. The "return-no-content" Preference

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.

5. The "return-content" Preference

The "return-content" token indicates that the client prefers that the server include an entity representing the current state of the resource in the response to a successful request.

6. The "return-status" Preference

The "return-status" token indicates that the client prefers that the server include an entity describing the status of the request in the response to a successful request.

7. IANA Considerations

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

    Header field name: Prefer
    
    Applicable Protocol: HTTP
    
    Status: standard
    
    Author/Change controller: IETF
    
    Specification document: this specification

7.1. The Registry of Preferences

This registry is maintained by IANA and initially contains three values: "return-no-content", "return-content" and "return-status". New assignments are subjects to IESG approval, as outlined in [RFC2434]. Requests should be made by email to IANA, which will then forward the request to the IESG, requesting approval. The request should use the following template:

  • Preference: (A value for the Prefer request header that conforms to the syntax rule given in Section 2)
  • Description:
  • Expected server behavior:
  • Security considerations:

8. Security Considerations

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.

9. Normative References

[RFC2119]
Bradner, S., “Key words for use in RFCs to Indicate Requirement Levels”, BCP 14, RFC 2119, March 1997.
[RFC2434]
Narten, T. and H. Alvestrand, “Guidelines for Writing an IANA Considerations Section in RFCs”, BCP 26, RFC 2434, October 1998.
[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.

Appendix A. Acknowledgements

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

Appendix B. Changes

TODO

Appendix C. Notes to RFC Editor

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

Appendix D. Editorial Notes

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

Author's Address

James M Snell
EMail: jasnell@gmail.com
URI: http://www.snellspace.com

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