|Internet Engineering Task Force (IETF)||M. Nottingham|
|Request for Comments: 5988||October 2010|
|Category: Standards Track|
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.
This is an Internet Standards Track document.
This document is a product of the Internet Engineering Task Force (IETF). It represents the consensus of the IETF community. It has received public review and has been approved for publication by the Internet Engineering Steering Group (IESG). Further information on Internet Standards is available in Section 2 of RFC 5741.
Information about the current status of this document, any errata, and how to provide feedback on it may be obtained at http://www.rfc-editor.org/info/rfc5988.
Copyright © 2010 IETF Trust and the persons identified as the document authors. All rights reserved.
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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 redefining 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 Section 22.214.171.124 of [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.
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; from [RFC5646]: Language-Tag; and from [RFC5987], ext-value and parmname.
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 (firstname.lastname@example.org) Internet Engineering Task Force Specification document(s): [RFC5988]
This specification also establishes the Link Relation Application Field registry, to allow entries in the Link Relation Type registry to be extended with application-specific data (hereafter, "app data") specific to all instances of a given link relation type.
Application data is 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:
The Description SHOULD identify the value space of the app data. The Default Value MUST be appropriate to entries to which the app data does not apply.
Entries that pre-date the addition of app data will automatically be considered to have the default value for that app data; 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 app data as well as the registrant of the existing entry (if possible).
Registration requests should be sent to the email@example.com mailing list, marked clearly in the subject line (e.g., "NEW APP DATA - example" to register "example" app data).
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 can be brought to the IESG's attention (using the firstname.lastname@example.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.
The content of the Link header field is not secure, private or integrity-guaranteed, and due caution should be exercised when using it. Use of Transport Layer Security (TLS) with HTTP ([RFC2818] and [RFC2817]) is currently the only end-to-end way to provide such protection.
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.
The Link entity-header field makes extensive use of IRIs and URIs. See [RFC3987] for security considerations relating to IRIs. See [RFC3986] for security considerations relating to URIs. See [RFC2616] for security considerations relating to HTTP headers.
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.
|[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.|
|[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.|
|[RFC5226]||Narten, T. and H. Alvestrand, “Guidelines for Writing an IANA Considerations Section in RFCs”, BCP 26, RFC 5226, May 2008.|
|[RFC5646]||Phillips, A. and M. Davis, “Tags for Identifying Languages”, BCP 47, RFC 5646, September 2009.|
|[RFC5987]||Reschke, J., “Character Set and Language Encoding for Hypertext Transfer Protocol (HTTP) Header Field Parameters”, RFC 5987, August 2010.|
|[RFC2068]||Fielding, R., Gettys, J., Mogul, J., Nielsen, H., and T. Berners-Lee, “Hypertext Transfer Protocol -- HTTP/1.1”, RFC 2068, January 1997.|
|[RFC2817]||Khare, R. and S. Lawrence, “Upgrading to TLS Within HTTP/1.1”, RFC 2817, May 2000.|
|[RFC2818]||Rescorla, E., “HTTP Over TLS”, RFC 2818, May 2000.|
|[RFC4287]||Nottingham, M., Ed. and R. Sayre, Ed., “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.|
|[RFC5829]||Brown, A., Clemm, G., and J. Reschke, “Link Relation Types for Simple Version Navigation between Web Resources”, RFC 5829, April 2010.|
|[W3C.CR-css3-mediaqueries-20090915]||van Kesteren, A., Glazman, D., Lie, H., and T. Çelik, “Media Queries”, W3C Candidate Recommendation CR-css3-mediaqueries-20090915, September 2009, <http://www.w3.org/TR/2009/CR-css3-mediaqueries-20090915/>.|
Latest version available at <http://www.w3.org/TR/css3-mediaqueries/>.
|[W3C.CR-curie-20090116]||Birbeck, M. and S. McCarron, “CURIE Syntax 1.0”, W3C Candidate Recommendation CR-curie-20090116, January 2009, <http://www.w3.org/TR/2009/CR-curie-20090116>.|
Latest version available at <http://www.w3.org/TR/curie>.
|[W3C.REC-html401-19991224]||Le Hors, A., Raggett, D., and I. Jacobs, “HTML 4.01 Specification”, W3C Recommendation REC-html401-19991224, December 1999, <http://www.w3.org/TR/1999/REC-html401-19991224>.|
Latest version available at <http://www.w3.org/TR/html401>.
|[W3C.REC-rdfa-syntax-20081014]||Adida, B., Birbeck, M., McCarron, S., and S. Pemberton, “RDFa in XHTML: Syntax and Processing”, W3C Recommendation REC-rdfa-syntax-20081014, October 2008, <http://www.w3.org/TR/2008/REC-rdfa-syntax-20081014>.|
Latest version available at <http://www.w3.org/TR/rdfa-syntax>.
|[W3C.REC-xhtml-basic-20080729]||Baker, M., Ishikawa, M., Stark, P., Matsui, S., Wugofski, T., and T. Yamakami, “XHTML™ Basic 1.1”, W3C Recommendation REC-xhtml-basic-20080729, July 2008, <http://www.w3.org/TR/2008/REC-xhtml-basic-20080729>.|
Latest version available at <http://www.w3.org/TR/xhtml-basic>.
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.
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.
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, lowercase) 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 can also be used as link-extensions to maintain fidelity.
This specification lifts the idea and definition for the Link header from RFC 2068; 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.