Network Working GroupM. Nottingham
Internet-DraftJune 16, 2006
Intended status: Informational
Expires: December 18, 2006

HTTP Header Linking

draft-nottingham-http-link-header-00

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Copyright Notice

Copyright © The Internet Society (2006). All Rights Reserved.

Abstract

This specification clarifies the status of the Link HTTP header and introduces the complimentary Profile and Link-Template HTTP headers.


1. Introduction

A means of indicating the relationships between documents on the Web has been available for some time in HTML, and was considered as a HTTP header in [RFC2068], but removed from [RFC2616], due to a lack of implementation experience.

There have since surfaced many cases where a means of including this information in HTTP headers has proved useful. However, because it was removed, the status of the Link header is unclear, leading some to consider minting new application-specific HTTP headers instead of reusing it.

Additionally, the complementary "profile" mechanism -- which is often used to disambiguate link relationship types -- is not available as a HTTP header.

This specification seeks to address these shortcomings. It also introduces a new header, Link-Template, that allows the structure of links to be described.

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 BCP 14, [RFC2119], as scoped to those conformance targets.

This specification uses the Augmented Backus-Naur Form (ABNF) notation of [RFC2616], and explicitly includes the following rules from it: quoted-string, token, SP (space), ALPHA (letters), DIGIT (decimal digit). Additionally, the following rules are included from [RFC3986]: URI-Reference, reserved, unreserved.

4. The Profile Header Field

The Profile entity-header field provides a means to indicate the meta data profile of the entity. Commonly, it is used to disambiguate the meaning of relationships in links.

Note that this URI MAY be used as either an identifier (e.g., to uniquely identify links, without dereferencing the URI), or as a link that is intended to be dereferenced.

The Profile field is semantically equivalent to the profile attribute of the HEAD element in HTML [W3C.REC-html401-19991224]. Note, however, that its use is not limited to HTML entities.

    Profile       = "Profile" ":" #("<" URI-Reference ">")

For example:

    Profile: <http://www.acme.com/profiles/core>
    Profile: <http://example.com/p1>, </profiles/other-profile>

6. IANA Considerations

This specification requires registration of two Message Header Fields for HTTP [RFC3864]. Note that "Link" is already present in the registry; this registration only updates its specification document.

Header field: Link
Applicable protocol: http
Status: standard
Author/change controller:
    IETF  (iesg@ietf.org)
    Internet Engineering Task Force
Specification document(s):
   [ this document ]
Header field: Profile
Applicable protocol: http
Status: standard
Author/change controller:
    IETF  (iesg@ietf.org)
    Internet Engineering Task Force
Specification document(s):
   [ this document ]
Header field: Link-Template
Applicable protocol: http
Status: standard
Author/change controller:
    IETF  (iesg@ietf.org)
    Internet Engineering Task Force
Specification document(s):
   [ this document ]

7. Security Considerations

The content of both the Link and Profile headers are not secure, private or integrity-guaranteed, and due caution should be excercised when using them.

Applications that take advantage of these mechanisms should consider the attack vectors opened by automatically following, trusting, or otherwise using links gathered from HTTP headers.

8. References

8.1. 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.
[RFC3986]
Berners-Lee, T., Fielding, R., and L. Masinter, “Uniform Resource Identifier (URI): Generic Syntax”, STD 66, RFC 3986, January 2005.
[W3C.REC-html401-19991224]
Jacobs, I., Raggett, D., and A. Hors, “HTML 4.01 Specification”, World Wide Web Consortium Recommendation REC-html401-19991224, December 1999, <http://www.w3.org/TR/1999/REC-html401-19991224>.

8.2. Informative References

[RFC2068]
Fielding, R., Gettys, J., Mogul, J., Nielsen, H., and T. Berners-Lee, “Hypertext Transfer Protocol -- HTTP/1.1”, RFC 2068, January 1997.

A. Acknowledgements

This specification lifts the definition of the Link header from RFC2068; credit for it belongs entirely to the authors of and contributors to that document.

The semantics and much of the syntax of the Profile header was defined by HTML 4.01; credit for them belongs to the authors of and contributors to that document.

Joe Gregorio, Marc Hadley and David Orchard contributed to the design of the Link-Template mechanism.

Author's Address

Mark Nottingham
EMail: mnot@pobox.com
URI: http://www.mnot.net/

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