Network Working GroupM. Nottingham
Internet-DraftJanuary 19, 2010
Updates: 4287 (if approved)
Intended status: Standards Track
Expires: July 23, 2010

Web Linking


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

Status of this Memo

This Internet-Draft is submitted to IETF in full conformance with the provisions of BCP 78 and BCP 79.

Internet-Drafts are working documents of the Internet Engineering Task Force (IETF), its areas, and its working groups. Note that other groups may also distribute working documents as Internet-Drafts.

Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as “work in progress”.

The list of current Internet-Drafts can be accessed at

The list of Internet-Draft Shadow Directories can be accessed at

This Internet-Draft will expire on July 23, 2010.

Copyright Notice

Copyright (c) 2010 IETF Trust and the persons identified as the document authors. All rights reserved.

This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents ( in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights and restrictions with respect to this document. Code Components extracted from this document must include Simplified BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the BSD License.

This document may contain material from IETF Documents or IETF Contributions published or made publicly available before November 10, 2008. The person(s) controlling the copyright in some of this material may not have granted the IETF Trust the right to allow modifications of such material outside the IETF Standards Process. Without obtaining an adequate license from the person(s) controlling the copyright in such materials, this document may not be modified outside the IETF Standards Process, and derivative works of it may not be created outside the IETF Standards Process, except to format it for publication as an RFC or to translate it into languages other than English.

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 or application. It does so by re-defining the link relation registry established by Atom to have a broader domain, 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), LOALPHA, DIGIT.

Additionally, the following rules are included from [RFC3986]: URI and URI-Reference; from [RFC4288]: type-name and subtype-name; from [W3C.REC-html401-19991224]: MediaDesc, and from [RFC4646]: Language-Tag.

6. IANA Considerations

6.3. Link Relation Field Registry

This specification also establishes the Link Relation Field Registry, to allow entries in the Link Relation Type Registry to be extended with application-specific data (hereafter, "fields").

Fields are registered on the advice of a Designated Expert (appointed by the IESG or their delegate), with a Specification Required (using terminology from [RFC5226]).

Registration requests consist of the completed registration template below;

  • Field Name:
  • Description:
  • Default Value:
  • Notes: [optional]

The Description SHOULD identify the value space of the field. The Default Value MUST be appropriate to entries which the field does not apply to.

Entries that pre-date the addition of a field will automatically be considered to have the default value for that field; if there are exceptions, the modification of such entries should be coordinated by the Designated Expert(s), in consultation with the author of the proposed field as well as the registrant of the existing entry (if possible).

Registration requests should be sent to the [TBD] mailing list, marked clearly in the subject line (e.g,. "NEW EXTENSION FIELD").

Within at most 14 days of the request, the Designated Expert will either approve or deny the registration request, communicating this decision to the review list. Denials should include an explanation and, if applicable, suggestions as to how to make the request successful. Registration requests that are undetermined for a period longer than 21 days MAY be brought to the IESG's attention (using the mailing list) for resolution.

When a registration request is successful, the Designated Expert will forward it to IANA for publication. IANA should only accept registry updates from the Designated Expert(s), and should direct all requests for registration to the review mailing list.

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.
Phillips, A. and M. Davis, “Tags for Identifying Languages”, RFC 4646, September 2006.
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.
Glazman, D., Çelik, T., Lie, H., and A. Kesteren, “Media Queries”, World Wide Web Consortium CR CR-css3-mediaqueries-20090915, September 2009, <>.
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, <>.

Appendix A. Link Relation Registry Format

To facilitate applications that wish to use registry data, this specification defines an XML-based format for the registry entries.

Each registered relation type is represented by a RelationType element, and if any of the Field values are other than the default value identified in the Field Registry, they will be represented by field elements.

Note that this format is NOT that which IANA publishes the registry in, because doing so would subject IANA's servers to, potentially, very high load (e.g., if Web browsers were to automatically update their copies of the registry). Instead, this format is published to the [TBD-2] mailing list, so that interested implementors can subscribe and distribute the machine-readable document using their own infrastructure.

A.1. Relax NG Grammar

element RelationTypes {
  element RelationType {
    attribute name { text },
    attribute reference { text },
    element description { text }, 
    element notes { text }?,
    element field {
      attribute name { text },

A.2. Example

  <RelationType name="example"
    <description>This is an example relation type.</description>
    <field name="foo">This is the value of the Foo field.</field>
  <!-- ... -->

Appendix 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 type 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, Eran Hammer-Lahav, and Julian Reschke.

Appendix E. Document history

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









Author's Address

Mark Nottingham