Network Working GroupM. Nottingham
Internet-DraftJuly 12, 2009
Updates: 4287 (if approved)
Intended status: Standards Track
Expires: January 13, 2010

Web Linking


Status of this Memo

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This document specifies relation types for Web links, and defines a registry for them. It also defines how to send such links in HTTP headers with the Link header-field.

1. Introduction

A means of indicating the relationships between resources on the Web, as well as indicating the type of those relationships, has been available for some time in HTML [W3C.REC-html401-19991224], and more recently in Atom [RFC4287]. These mechanisms, although conceptually similar, are separately specified. However, links between resources need not be format-specific; it can be useful to have typed links that are independent of their serialisation, especially when a resource has representations in multiple formats.

To this end, this document defines a framework for typed links that isn't specific to a particular serialisation. It does so by re-defining the link relation registry established by Atom to have a broader scope, and adding to it the relations that are defined by HTML.

Furthermore, an HTTP header-field for conveying typed links was defined in [RFC2068], but removed from [RFC2616], due to a lack of implementation experience. Since then, it has been implemented in some User-Agents (e.g., for stylesheets), and several additional use cases have surfaced.

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. This document addresses this by re-specifying the Link header as one such serialisation, with updated but backwards-compatible syntax.

[[ Feedback is welcome on the mailing list, although this is NOT a work item of the HTTPBIS WG. ]]

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 document uses the Augmented Backus-Naur Form (ABNF) notation of [RFC2616], and explicitly includes the following rules from it: quoted-string, token, SP (space). Additionally, the following rules are included from [RFC3986]: URI and URI-Reference, and from [RFC4288]: type-name and subtype-name.

6. IANA Considerations

7. Security Considerations

The content of the Link header-field is not secure, private or integrity-guaranteed, and due caution should be exercised when using it.

Applications that take advantage of typed links should consider the attack vectors opened by automatically following, trusting, or otherwise using links gathered from HTTP headers. In particular, Link headers that use the "anchor" parameter to associate a link's context with another resource should be treated with due caution.

8. Internationalisation Considerations

Target IRIs may need to be converted to URIs in order to express them in serialisations that do not support IRIs. This includes the Link HTTP header.

Similarly, the anchor parameter of the Link header does not support IRIs, and therefore IRIs must be converted to URIs before inclusion there.

Relation types are defined as URIs, not IRIs, to aid in their comparison. It is not expected that they will be displayed to end users.

9. References

9.1. Normative References

Bradner, S., “The Internet Standards Process -- Revision 3”, BCP 9, RFC 2026, October 1996.
Bradner, S., “Key words for use in RFCs to Indicate Requirement Levels”, BCP 14, RFC 2119, March 1997.
Freed, N. and K. Moore, “MIME Parameter Value and Encoded Word Extensions: Character Sets, Languages, and Continuations”, RFC 2231, November 1997.
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.
Klyne, G., Nottingham, M., and J. Mogul, “Registration Procedures for Message Header Fields”, BCP 90, RFC 3864, September 2004.
Berners-Lee, T., Fielding, R., and L. Masinter, “Uniform Resource Identifier (URI): Generic Syntax”, STD 66, RFC 3986, January 2005.
Duerst, M. and M. Suignard, “Internationalized Resource Identifiers (IRIs)”, RFC 3987, January 2005.
Freed, N. and J. Klensin, “Media Type Specifications and Registration Procedures”, BCP 13, RFC 4288, December 2005.
Narten, T. and H. Alvestrand, “Guidelines for Writing an IANA Considerations Section in RFCs”, BCP 26, RFC 5226, May 2008.

9.2. Informative References

Fielding, R., Gettys, J., Mogul, J., Nielsen, H., and T. Berners-Lee, “Hypertext Transfer Protocol -- HTTP/1.1”, RFC 2068, January 1997.
Nottingham, M. and R. Sayre, “The Atom Syndication Format”, RFC 4287, December 2005.
Snell, J., “Atom Threading Extensions”, RFC 4685, September 2006.
Snell, J., “Atom License Extension”, RFC 4946, July 2007.
Nottingham, M., “Feed Paging and Archiving”, RFC 5005, September 2007.
Gregorio, J. and B. de hOra, “The Atom Publishing Protocol”, RFC 5023, October 2007.
Klensin, J., “ASCII Escaping of Unicode Characters”, BCP 137, RFC 5137, February 2008.
Jacobs, I., Raggett, D., and A. Hors, “HTML 4.01 Specification”, World Wide Web Consortium Recommendation REC-html401-19991224, December 1999, <>.
Adida, B., Pemberton, S., McCarron, S., and M. Birbeck, “RDFa in XHTML: Syntax and Processing”, World Wide Web Consortium Recommendation REC-rdfa-syntax-20081014, October 2008, <>.
Wugofski, T., Matsui, S., Baker, M., Yamakami, T., Ishikawa, M., and P. Stark, “XHTML™ Basic 1.1”, World Wide Web Consortium Recommendation REC-xhtml-basic-20080729, July 2008, <>.

D. Acknowledgements

This specification lifts the idea and definition for the Link header from RFC2068; credit for it belongs entirely to the authors of and contributors to that document. The link relation registrations themselves are sourced from several documents; see the applicable references.

The author would like to thank the many people who commented upon, encouraged and gave feedback to this specification, especially including Frank Ellermann, Roy Fielding and Julian Reschke.

E. Document history

[[ to be removed by the RFC editor before publication as an RFC. ]]








Author's Address

Mark Nottingham