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

Web Linking
draft-nottingham-http-link-header-07

Abstract

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

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Table of Contents

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 ietf-http-wg@w3.org 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

6.1 Link HTTP Header Registration

This specification updates the Message Header Registry entry for "Link" in HTTP [RFC3864] to refer to this 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 ]

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]@ietf.org 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 iesg@iesg.org 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

[RFC2026]Bradner, S., “The Internet Standards Process -- Revision 3”, BCP 9, RFC 2026, October 1996.
[RFC2119]Bradner, S., “Key words for use in RFCs to Indicate Requirement Levels”, BCP 14, RFC 2119, March 1997.
[RFC2231]Freed, N. and K. Moore, “MIME Parameter Value and Encoded Word Extensions: Character Sets, Languages, and Continuations”, RFC 2231, November 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.
[RFC3987]Duerst, M. and M. Suignard, “Internationalized Resource Identifiers (IRIs)”, RFC 3987, January 2005.
[RFC4288]Freed, N. and J. Klensin, “Media Type Specifications and Registration Procedures”, BCP 13, RFC 4288, December 2005.
[RFC4646]Phillips, A. and M. Davis, “Tags for Identifying Languages”, RFC 4646, September 2006.
[RFC5226]Narten, T. and H. Alvestrand, “Guidelines for Writing an IANA Considerations Section in RFCs”, BCP 26, RFC 5226, May 2008.

9.2 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.
[RFC4287]Nottingham, M. and R. Sayre, “The Atom Syndication Format”, RFC 4287, December 2005.
[RFC4685]Snell, J., “Atom Threading Extensions”, RFC 4685, September 2006.
[RFC4946]Snell, J., “Atom License Extension”, RFC 4946, July 2007.
[RFC5005]Nottingham, M., “Feed Paging and Archiving”, RFC 5005, September 2007.
[RFC5023]Gregorio, J. and B. de hOra, “The Atom Publishing Protocol”, RFC 5023, October 2007.
[W3C.CR-css3-mediaqueries-20090915]Glazman, D., Çelik, T., Lie, H., and A. Kesteren, “Media Queries”, World Wide Web Consortium CR CR-css3-mediaqueries-20090915, September 2009, <http://www.w3.org/TR/2009/CR-css3-mediaqueries-20090915>.
[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>.
[W3C.REC-rdfa-syntax-20081014]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, <http://www.w3.org/TR/2008/REC-rdfa-syntax-20081014>.
[W3C.REC-xhtml-basic-20080729]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, <http://www.w3.org/TR/2008/REC-xhtml-basic-20080729>.

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]@ietf.org 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 },
      text
    }*
  }+
}

A.2 Example

<RelationTypes>
  <RelationType name="example"
                reference="http://www.example.org/example_spec">
    <description>This is an example relation type.</description>
    <field name="foo">This is the value of the Foo field.</field>
  </RelationType>
  <!-- ... -->
</RelationTypes>
				

B. Notes on Using the Link Header with the HTML4 Format

HTML motivated the original syntax of the Link header, and many of the design decisions in this document are driven by a desire to stay compatible with these uses.

In HTML4, the link element can be mapped to links as specified here by using the "href" attribute for the target URI, and "rel" to convey the relation type, as in the Link header. The context of the link is the URI associated with the entire HTML document.

HTML4 also has a "rev" parameter for links that allows a link's relation to be reversed. The Link header does not define a corresponding "rev" parameter to allow the expression of these links in HTTP headers, due to the confusion this mechanism causes as well as conflicting interpretations (briefly, some hold that rev reverses the direction of the link, while others that it reverses the semantics of the relation itself).

All of the link relation types defined by HTML4 have been included in the link relation type registry, so they can be used without modification. However, there are several potential ways to serialise extension relation types into HTML4, including

Individual applications of linking will therefore need to define how their extension links should be serialised into HTML4.

Surveys of existing HTML content have shown that unregistered link relation types that are not URIs are (perhaps inevitably) common. Consuming HTML implementations should not consider such unregistered short links to be errors, but rather relation types with a local scope (i.e., their meaning is specific and perhaps private to that document).

HTML4 also defines several attributes on links that are not explicitly defined by the Link header. These attributes can be serialised as link-extensions to maintain fidelity.

Finally, the HTML4 specification gives a special meaning when the "alternate" and "stylesheet" relation types coincide in the same link. Such links should be serialised in the Link header using a single list of relation-types (e.g., rel="alternate stylesheet") to preserve this relationship.

C. Notes on Using the Link Header with the Atom Format

Atom conveys links in the atom:link element, with the "href" attribute indicating the target IRI and the "rel" attribute containing the relation type. The context of the link is either a feed IRI or an entry ID, depending on where it appears; generally, feed-level links are obvious candidates for transmission as a Link header.

When serialising an atom:link into a Link header, it is necessary to convert target IRIs (if used) to URIs.

Atom defines extension relation types in terms of IRIs. This specification re-defines them as URIs, to simplify and reduce errors in their comparison.

Atom allows registered link relation types to be serialised as absolute URIs. Such relation types SHOULD be converted to the appropriate registered form (e.g., "http://www.iana.org/assignments/relation/self" to "self") so that they are not mistaken for extension relation types.

Furthermore, Atom link relation types are always compared in a case-sensitive fashion; therefore, registered link relation types SHOULD be converted to their registered form (usually, lower case) when serialised in an Atom document.

Note also that while the Link header allows multiple relations to be serialised in a single link, atom:link does not. In this case, a single link-value may map to several atom:link elements.

As with HTML, atom:link defines some attributes that are not explicitly mirrored in the Link header syntax, but they may also be used as link-extensions to maintain fidelity.

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.

E. Document history

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

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Author's Address

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