TOC 
Network Working GroupM. Nottingham
Internet-DraftMay 5, 2010
Updates: 4287 (if approved) 
Intended status: Standards Track 
Expires: November 6, 2010 


Web Linking
draft-nottingham-http-link-header-10

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

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

Internet-Drafts are working documents of the Internet Engineering Task Force (IETF). Note that other groups may also distribute working documents as Internet-Drafts. The list of current Internet-Drafts is at http://datatracker.ietf.org/drafts/current/.

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.”

This Internet-Draft will expire on November 6, 2010.

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

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Table of Contents

1.  Introduction
2.  Notational Conventions
3.  Links
4.  Link Relation Types
    4.1.  Registered Relation Types
    4.2.  Extension Relation Types
5.  The Link Header Field
    5.1.  Target IRI
    5.2.  Context IRI
    5.3.  Relation Type
    5.4.  Target Attributes
    5.5.  Examples
6.  IANA Considerations
    6.1.  Link HTTP Header Registration
    6.2.  Link Relation Type Registry
    6.3.  Link Relation Application Data Registry
7.  Security Considerations
8.  Internationalisation Considerations
9.  References
    9.1.  Normative References
    9.2.  Informative References
Appendix A.  Link Relation Registry Format
    A.1.  Relax NG Grammar
    A.2.  Example
Appendix B.  Notes on Using the Link Header with the HTML4 Format
Appendix C.  Notes on Using the Link Header with the Atom Format
Appendix D.  Acknowledgements
Appendix E.  Document history
§  Author's Address



 TOC 

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] (Le Hors, A., Raggett, D., and I. Jacobs, “HTML 4.01 Specification,” December 1999.), and more recently in Atom [RFC4287] (Nottingham, M., Ed. and R. Sayre, Ed., “The Atom Syndication Format,” December 2005.). 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 Section 19.6.2.4 of [RFC2068] (Fielding, R., Gettys, J., Mogul, J., Nielsen, H., and T. Berners-Lee, “Hypertext Transfer Protocol -- HTTP/1.1,” January 1997.), but removed from [RFC2616] (Fielding, R., Gettys, J., Mogul, J., Frystyk, H., Masinter, L., Leach, P., and T. Berners-Lee, “Hypertext Transfer Protocol -- HTTP/1.1,” June 1999.), 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.


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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] (Bradner, S., “Key words for use in RFCs to Indicate Requirement Levels,” March 1997.), as scoped to those conformance targets.

This document uses the Augmented Backus-Naur Form (ABNF) notation of [RFC2616] (Fielding, R., Gettys, J., Mogul, J., Frystyk, H., Masinter, L., Leach, P., and T. Berners-Lee, “Hypertext Transfer Protocol -- HTTP/1.1,” June 1999.), and explicitly includes the following rules from it: quoted-string, token, SP (space), LOALPHA, DIGIT.

Additionally, the following rules are included from [RFC3986] (Berners-Lee, T., Fielding, R., and L. Masinter, “Uniform Resource Identifier (URI): Generic Syntax,” January 2005.): URI and URI-Reference; from [RFC4288] (Freed, N. and J. Klensin, “Media Type Specifications and Registration Procedures,” December 2005.): type-name and subtype-name; from [W3C.REC‑html401‑19991224] (Le Hors, A., Raggett, D., and I. Jacobs, “HTML 4.01 Specification,” December 1999.): MediaDesc; from [RFC5646] (Phillips, A. and M. Davis, “Tags for Identifying Languages,” September 2009.): Language-Tag; and from [I‑D.reschke‑rfc2231‑in‑http] (Reschke, J., “Character Set and Language Encoding for Hypertext Transfer Protocol (HTTP) Header Field Parameters,” April 2010.), ext-value and parmname.


 TOC 

3.  Links

In this specification, a link is a typed connection between two resources that are identified by IRIs [RFC3987] (Duerst, M. and M. Suignard, “Internationalized Resource Identifiers (IRIs),” January 2005.), and is comprised of:

A link can be viewed as a statement of the form "{context IRI} has a {relation type} resource at {target IRI}, which has {target attributes}."

Note that in the common case, the context IRI will also be a URI [RFC3986] (Berners-Lee, T., Fielding, R., and L. Masinter, “Uniform Resource Identifier (URI): Generic Syntax,” January 2005.), because many protocols (such as HTTP) do not support dereferencing IRIs. Likewise, the target IRI will be converted to a URI (see [RFC3987] (Duerst, M. and M. Suignard, “Internationalized Resource Identifiers (IRIs),” January 2005.), Section 3.1) in serialisations that do not support IRIs (e.g., the Link header).

This specification does not place restrictions on the cardinality of links; there can be multiple links from and to a particular IRI, and multiple links of different types between two given IRIs. Likewise, the relative ordering of links in any particular serialisation, or between serialisations (e.g., the Link header and in-content links) is not specified or significant in this specification; applications that wish to consider ordering significant can do so.

Target attributes are a set of key/value pairs that describe the link or its target; for example, a media type hint. This specification does not attempt to coordinate their names or use, but does provide common target attributes for use in the Link HTTP header.

Finally, this specification does not define a general syntax for expressing links, nor mandate a specific context for any given link; it is expected that serialisations of links will specify both aspects. One such serialisation is communication of links through HTTP headers, specified in Section 5 (The Link Header Field).


 TOC 

4.  Link Relation Types

In the simplest case, a link relation type identifies the semantics of a link. For example, a link with the relation type "copyright" indicates that the resource identified by the target IRI is a statement of the copyright terms applying to the current context IRI.

Link relation types can also be used to indicate that the target resource has particular attributes, or exhibits particular behaviours; for example, a "service" link implies that the identified resource is part of a defined protocol (in this case, a service description).

Relation types are not to be confused with media types [RFC4288] (Freed, N. and J. Klensin, “Media Type Specifications and Registration Procedures,” December 2005.); they do not identify the format of the representation that results when the link is dereferenced. Rather, they only describe how the current context is related to another resource.

Relation types SHOULD NOT infer any additional semantics based upon the presence or absence of another link relation type, or its own cardinality of occurrence. An exception to this is the combination of the "alternate" and "stylesheet" registered relation types, which has special meaning in HTML4 for historical reasons.

There are two kinds of relation types: registered and extension.


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4.1.  Registered Relation Types

Well-defined relation types can be registered as tokens for convenience and/or to promote reuse by other applications. This specification establishes an IANA registry of such relation types; see Section 6.2 (Link Relation Type Registry).

Registered relation type names MUST conform to the reg-rel-type rule, and MUST be compared character-by-character in a case-insensitive fashion. They SHOULD be appropriate to the specificity of the relation type; i.e., if the semantics are highly specific to a particular application, the name should reflect that, so that more general names are available for less specific use.

Registered relation types MUST NOT constrain the media type of the context IRI, and MUST NOT constrain the available representation media types of the target IRI. However, they can specify the behaviours and properties of the target resource (e.g., allowable HTTP methods, request and response media types which must be supported).

Additionally, specific applications of linking may require additional data to be included in the registry. For example, Web browsers might want to know what kinds of links should be downloaded when they archive a Web page; if this application-specific information is in the registry, new link relation types can control this behaviour without unnecessary coordination.

To accommodate this, per-entry application data can be added to the Link Relation Type Registry, by registering it in the Link Relation Application Data Registry (Section 6.3 (Link Relation Application Data Registry)).


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4.2.  Extension Relation Types

Applications that don't wish to register a relation type can use an extension relation type, which is a URI [RFC3986] (Berners-Lee, T., Fielding, R., and L. Masinter, “Uniform Resource Identifier (URI): Generic Syntax,” January 2005.) that uniquely identifies the relation type. Although the URI can point to a resource that contains a definition of the semantics of the relation type, clients SHOULD NOT automatically access that resource to avoid overburdening its server.

When extension relation types are compared, they MUST be compared as strings (after converting to URIs if serialised in a different format, such as a Curie [W3C.CR‑curie‑20090116] (Birbeck, M. and S. McCarron, “CURIE Syntax 1.0,” January 2009.)) in a case-insensitive fashion, character-by-character. Because of this, all-lowercase URIs SHOULD be used for extension relations.

Note that while extension relation types are required to be URIs, a serialisation of links can specify that they are expressed in another form, as long as they can be converted to URIs.


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5.  The Link Header Field

The Link entity-header field provides a means for serialising one or more links in HTTP headers. It is semantically equivalent to the <LINK> element in HTML, as well as the atom:link feed-level element in Atom [RFC4287] (Nottingham, M., Ed. and R. Sayre, Ed., “The Atom Syndication Format,” December 2005.). 

  Link           = "Link" ":" #link-value
  link-value     = "<" URI-Reference ">" *( ";" link-param )
  link-param     = ( ( "rel" "=" relation-types )
                 | ( "anchor" "=" <"> URI-Reference <"> )
                 | ( "rev" "=" relation-types )
                 | ( "hreflang" "=" Language-Tag )
                 | ( "media" "=" ( MediaDesc | ( <"> MediaDesc <"> ) ) )
                 | ( "title" "=" quoted-string )
                 | ( "title*" "=" ext-value )
                 | ( "type" "=" ( media-type | quoted-mt ) )
                 | ( link-extension ) )
  link-extension = ( parmname [ "=" ( ptoken | quoted-string ) ] )
                 | ( ext-name-star "=" ext-value )
  ext-name-star  = parmname "*" ; reserved for RFC2231-profiled
                                ; extensions. Whitespace NOT
                                ; allowed in between.
  ptoken         = 1*ptokenchar
  ptokenchar     = "!" | "#" | "$" | "%" | "&" | "'" | "("
                 | ")" | "*" | "+" | "-" | "." | "/" | DIGIT
                 | ":" | "<" | "=" | ">" | "?" | "@" | ALPHA
                 | "[" | "]" | "^" | "_" | "`" | "{" | "|"
                 | "}" | "~"
  media-type     = type-name "/" subtype-name
  quoted-mt      = <"> media-type <">
  relation-types = relation-type
                 | <"> relation-type *( 1*SP relation-type ) <">
  relation-type  = reg-rel-type | ext-rel-type
  reg-rel-type   = LOALPHA *( LOALPHA | DIGIT | "." | "-" )
  ext-rel-type   = URI

 TOC 

5.1.  Target IRI

Each link-value conveys one target IRI as a URI-Reference (after conversion to one, if necessary; see [RFC3987] (Duerst, M. and M. Suignard, “Internationalized Resource Identifiers (IRIs),” January 2005.), Section 3.1) inside angle brackets ("<>"). If the URI-Reference is relative, parsers MUST resolve it as per [RFC3986] (Berners-Lee, T., Fielding, R., and L. Masinter, “Uniform Resource Identifier (URI): Generic Syntax,” January 2005.), Section 5. Note that any base IRI from the message's content is not applied.


 TOC 

5.2.  Context IRI

By default, the context of a link conveyed in the Link header field is the IRI of the requested resource.

When present, the anchor parameter overrides this with another URI, such as a fragment of this resource, or a third resource (i.e., when the anchor value is an absolute URI). If the anchor parameter's value is a relative URI, parsers MUST resolve it as per [RFC3986] (Berners-Lee, T., Fielding, R., and L. Masinter, “Uniform Resource Identifier (URI): Generic Syntax,” January 2005.), Section 5. Note that any base URI from the body's content is not applied.

Consuming implementations can choose to ignore links with an anchor parameter. For example, the application in use may not allow the context IRI to be assigned to a different resource. In such cases, the entire link is to be ignored; consuming implementations MUST NOT process the link without applying the anchor.

Note that depending on HTTP status code and response headers, the context IRI might be "anonymous" (i.e., no context IRI is available). For instance, this is the case on a 404 response to a GET request.


 TOC 

5.3.  Relation Type

The relation type of a link is conveyed in the "rel" parameter's value. The "rel" parameter MUST NOT appear more than once in a given link-value; occurrences after the first MUST be ignored by parsers.

The "rev" parameter has been used in the past to indicate that the semantics of the relationship are in the reverse direction. I.e., a link from A to B with REL="X" expresses the same relationship as a link from B to A with REV="X". "rev" is deprecated by this specification because it often confuses authors and readers; in most cases using a separate relation type is preferable.

Note that extension relation types are REQUIRED to be absolute URIs in Link headers, and MUST be quoted if they contain a semicolon (";") or comma (",") (as these characters are used as delimiters in the header itself).


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5.4.  Target Attributes

The "hreflang", "media", "title", "title*", "type" and any link-extension link-params are considered to be target attributes for the link.

The "hreflang" parameter, when present, is a hint indicating what the language of the result of dereferencing the link should be. Note that this is only a hint; for example, it does not override the Content-Language header of a HTTP response obtained by actually following the link. Multiple hreflang parameters on a single link-value indicate that multiple languages are available from the indicated resource.

The "media" parameter, when present, is used to indicate intended destination medium or media for style information (see [W3C.REC‑html401‑19991224] (Le Hors, A., Raggett, D., and I. Jacobs, “HTML 4.01 Specification,” December 1999.), Section 6.13). Note that this may be updated by [W3C.CR‑css3‑mediaqueries‑20090915] (van Kesteren, A., Glazman, D., Lie, H., and T. Çelik, “Media Queries,” September 2009.)). Its value MUST be quoted if it contains a semicolon (";") or comma (","), and there MUST NOT be more than one media parameter in a link-value.

The "title" parameter, when present, is used to label the destination of a link such that it can be used as a human-readable identifier (e.g. a menu entry) in the language indicated by the Content-Language header (if present). The "title" parameter MUST NOT appear more than once in a given link-value; occurrences after the first MUST be ignored by parsers.

The "title*" parameter can be used to encode this label in a different character set, and/or contain language information as per [I‑D.reschke‑rfc2231‑in‑http] (Reschke, J., “Character Set and Language Encoding for Hypertext Transfer Protocol (HTTP) Header Field Parameters,” April 2010.). The "title*" parameter MUST NOT appear more than once in a given link-value; occurrences after the first MUST be ignored by parsers. If the parameter does not contain language information, its language is indicated by the Content-Language header (when present).

If both the "title" and "title*" parameters appear in a link-value, processors SHOULD use the "title*" parameter's value.

The "type" parameter, when present, is a hint indicating what the media type of the result of dereferencing the link should be. Note that this is only a hint; for example, it does not override the Content-Type header of a HTTP response obtained by actually following the link. There MUST NOT be more than one type parameter in a link-value.


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5.5.  Examples

For example: 

Link: <http://example.com/TheBook/chapter2>; rel="previous";
      title="previous chapter"

indicates that "chapter2" is previous to this resource in a logical navigation path.

Similarly, 

Link: </>; rel="http://example.net/foo"

indicates that the root resource ("/") is related to this resource with the extension relation type "http://example.net/foo".

The example below shows an instance of the Link header encoding multiple links, and also the use of RFC 2231 encoding to encode both non-ASCII characters and language information. 

Link: </TheBook/chapter2>;
      rel="previous"; title*=UTF-8'de'letztes%20Kapitel,
      </TheBook/chapter4>;
      rel="next"; title*=UTF-8'de'n%c3%a4chstes%20Kapitel

Here, both links have titles encoded in UTF-8, use the German language ("de"), and the second link contains the Unicode code point U+00E4 ("LATIN SMALL LETTER A WITH DIAERESIS").

Note that link-values can convey multiple links between the same target and context IRIs; for example: 

    Link: <http://example.org/>;
          rel="start http://example.net/relation/other"

Here, the link to "http://example.org/" has the registered relation type "start" and the extension relation type "http://example.net/relation/other".


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6.  IANA Considerations


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6.1.  Link HTTP Header Registration

This specification updates the Message Header Registry entry for "Link" in HTTP [RFC3864] (Klyne, G., Nottingham, M., and J. Mogul, “Registration Procedures for Message Header Fields,” September 2004.) 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 ]

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6.2.  Link Relation Type Registry

This specification establishes the Link Relation Type Registry, and updates Atom [RFC4287] (Nottingham, M., Ed. and R. Sayre, Ed., “The Atom Syndication Format,” December 2005.) to refer to it in place of the "Registry of Link Relations".

[[ Note to IESG: Entries in the Atom registry that are not listed below at the time that IANA implements this change (i.e., those that are registered before this document comes into effect) should be referred to the Designated Expert. ]]


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6.2.1.  Registering new Link Relation Types

Relation types are registered on the advice of a Designated Expert (appointed by the IESG or their delegate), with a Specification Required (using terminology from [RFC5226] (Narten, T. and H. Alvestrand, “Guidelines for Writing an IANA Considerations Section in RFCs,” May 2008.)).

The requirements for registered relation types are described in Section 4.1 (Registered Relation Types).

Registration requests consist of the completed registration template below, typically published in an RFC or Open Standard (in the sense described by [RFC2026] (Bradner, S., “The Internet Standards Process -- Revision 3,” October 1996.), Section 7). However, to allow for the allocation of values prior to publication, the Designated Expert may approve registration once they are satisfied that a specification will be published.

Note that relation types can be registered by third parties, if the Designated Expert determines that an unregistered relation type is widely deployed and not likely to be registered in a timely manner.

The registration template is:

  • Relation Name:
  • Description:
  • Reference:
  • Notes: [optional]
  • Application Data: [optional]

Registration requests should be sent to the [TBD]@ietf.org mailing list, marked clearly in the subject line (e.g,. "NEW RELATION REQUEST").

Within at most 14 days of the request, the Designated Expert(s) 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.

Decisions (or lack thereof) made by the Designated Expert can be first appealed to Application Area Directors (contactable using app-ads@tools.ietf.org email address or directly by looking up their email addresses on http://www.iesg.org/ website) and, if the appellant is not satisfied with the response, to the full IESG (using the iesg@iesg.org mailing list).

When a registration request is successful, the Designated Expert(s) will update the registry XML file (using the format described in Appendix A (Link Relation Registry Format) including the MIT license) and send it to the [TBD-2]@ietf.org mailing list (which SHOULD NOT be centrally archived, so as to avoid load issues from automated agents, and only accept posts from the Designated Expert(s)), so that implementers interested in receiving a machine-readable registry can do so. Simultaneously, they will send a text (not XML) version of the registry 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.


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6.2.2.  Initial Registry Contents

The Link Relation Type registry's initial contents are:

  • Relation Name: first
  • Description: An IRI that refers to the furthest preceding resource in a series of resources.
  • Reference: [this document]
  • Notes: this relation type registration did not indicate a reference. Originally requested by Mark Nottingham in December 2004.

  • Relation Name: last
  • Description: An IRI that refers to the furthest following resource in a series of resources.
  • Reference: [this document]
  • Notes: this relation type registration did not indicate a reference. Originally requested by Mark Nottingham in December 2004.

  • Relation Name: payment
  • Description: indicates a resource where payment is accepted.
  • Reference: [this document]
  • Notes: this relation type registration did not indicate a reference. Requested by Joshua Kinberg and Robert Sayre. It is meant as a general way to facilitate acts of payment, and thus this specification makes no assumptions on the type of payment or transaction protocol. Examples may include a web page where donations are accepted or where goods and services are available for purchase. rel="payment" is not intended to initiate an automated transaction. In Atom documents, a link element with a rel="payment" attribute may exist at the feed/channel level and/or the entry/item level. For example, a rel="payment" link at the feed/channel level may point to a "tip jar" URI, whereas an entry/item containing a book review may include a rel="payment" link that points to the location where the book may be purchased through an online retailer.

  • Relation Name: up
  • Description: Refers to a parent document in a hierarchy of documents.
  • Reference: [this document]
  • Notes: this relation type registration did not indicate a reference. Requested by Noah Slater.


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6.3.  Link Relation Application Data Registry

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] (Narten, T. and H. Alvestrand, “Guidelines for Writing an IANA Considerations Section in RFCs,” May 2008.)).

Registration requests consist of the completed registration template below;

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

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

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 [TBD]@ietf.org mailing list, marked clearly in the subject line (e.g,. "NEW 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 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.


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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. Use of TLS with HTTP ([RFC2818] (Rescorla, E., “HTTP Over TLS,” May 2000.) and [RFC2817] (Khare, R. and S. Lawrence, “Upgrading to TLS Within HTTP/1.1,” May 2000.)) 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] (Duerst, M. and M. Suignard, “Internationalized Resource Identifiers (IRIs),” January 2005.) for security considerations relating to IRIs. See [RFC3986] (Berners-Lee, T., Fielding, R., and L. Masinter, “Uniform Resource Identifier (URI): Generic Syntax,” January 2005.) for security considerations relating to URIs. See [RFC2616] (Fielding, R., Gettys, J., Mogul, J., Frystyk, H., Masinter, L., Leach, P., and T. Berners-Lee, “Hypertext Transfer Protocol -- HTTP/1.1,” June 1999.) for security considerations relating to HTTP headers.


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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.


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9.  References


 TOC 

9.1. Normative References

[I-D.reschke-rfc2231-in-http] Reschke, J., “Character Set and Language Encoding for Hypertext Transfer Protocol (HTTP) Header Field Parameters,” draft-reschke-rfc2231-in-http-12 (work in progress), April 2010 (TXT).
[RFC2026] Bradner, S., “The Internet Standards Process -- Revision 3,” BCP 9, RFC 2026, October 1996 (TXT).
[RFC2119] Bradner, S., “Key words for use in RFCs to Indicate Requirement Levels,” BCP 14, RFC 2119, March 1997 (TXT, HTML, XML).
[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 (TXT, PS, PDF, HTML, XML).
[RFC3864] Klyne, G., Nottingham, M., and J. Mogul, “Registration Procedures for Message Header Fields,” BCP 90, RFC 3864, September 2004 (TXT).
[RFC3986] Berners-Lee, T., Fielding, R., and L. Masinter, “Uniform Resource Identifier (URI): Generic Syntax,” STD 66, RFC 3986, January 2005 (TXT, HTML, XML).
[RFC3987] Duerst, M. and M. Suignard, “Internationalized Resource Identifiers (IRIs),” RFC 3987, January 2005 (TXT).
[RFC4288] Freed, N. and J. Klensin, “Media Type Specifications and Registration Procedures,” BCP 13, RFC 4288, December 2005 (TXT).
[RFC5226] Narten, T. and H. Alvestrand, “Guidelines for Writing an IANA Considerations Section in RFCs,” BCP 26, RFC 5226, May 2008 (TXT).
[RFC5646] Phillips, A. and M. Davis, “Tags for Identifying Languages,” BCP 47, RFC 5646, September 2009 (TXT).

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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 (TXT).
[RFC2817] Khare, R. and S. Lawrence, “Upgrading to TLS Within HTTP/1.1,” RFC 2817, May 2000 (TXT).
[RFC2818] Rescorla, E., “HTTP Over TLS,” RFC 2818, May 2000 (TXT).
[RFC4287] Nottingham, M., Ed. and R. Sayre, Ed., “The Atom Syndication Format,” RFC 4287, December 2005 (TXT, HTML, XML).
[RFC4685] Snell, J., “Atom Threading Extensions,” RFC 4685, September 2006 (TXT).
[RFC4946] Snell, J., “Atom License Extension,” RFC 4946, July 2007 (TXT).
[RFC5005] Nottingham, M., “Feed Paging and Archiving,” RFC 5005, September 2007 (TXT).
[RFC5023] Gregorio, J. and B. de hOra, “The Atom Publishing Protocol,” RFC 5023, October 2007 (TXT).
[RFC5829] Brown, A., Clemm, G., and J. Reschke, “Link Relation Types for Simple Version Navigation between Web Resources,” RFC 5829, April 2010 (TXT).
[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.

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.

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.

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.

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.

Latest version available at http://www.w3.org/TR/xhtml-basic.


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Appendix A.  Link Relation Registry Format

To facilitate applications that wish to use registry data in an automated fashion, 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 app data values are other than the default value identified in the Application Data Registry, they will be represented by appdata 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.


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A.1.  Relax NG Grammar

element RelationTypes {
  element RelationType {
    attribute name { text },
    attribute reference { text },
    element description { text },
    element notes { text }?,
    element appdata {
      attribute name { text },
      text
    }*
  }+
}

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A.2.  Example

<RelationTypes>
  <!--
  Copyright (c) <year> The IETF Trust

  Permission is hereby granted, free of charge, to any person obtaining
  a copy of this software and associated documentation files (the
  "Software"), to deal in the Software without restriction, including
  without limitation the rights to use, copy, modify, merge, publish,
  distribute, sublicense, and/or sell copies of the Software, and to
  permit persons to whom the Software is furnished to do so, subject to
  the following conditions:

  The above copyright notice and this permission notice shall be
  included in all copies or substantial portions of the Software.

  THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND,
  EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF
  MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND
  NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS
  BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN
  ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN
  CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
  SOFTWARE.
  -->
  <RelationType name="example"
                reference="http://www.example.org/example_spec">
    <description>This is an example relation type.</description>
    <appdata name="foo">This is the value of Foo.</appdata>
  </RelationType>
  <!-- ... -->
</RelationTypes>

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Appendix 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.

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.


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Appendix 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 can also be used as link-extensions to maintain fidelity.


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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.


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Appendix E.  Document history

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

-10 (result of IESG review)

  • Clarified media BNF.
  • Added various security considerations.
  • Updated registration procedures.
  • Added more detail to 'payment' relation.
  • Corrected 'hub' relation.

-09

  • Corrected ptoken / ptokenchar BNF.
  • Disallow multiple title* parameters.
  • Prefer title* over title when available.
  • Remove "\" from ptokenchar.
  • Explain why mailing list isn't archived.
  • Define default language for title and title*, based on Content-Language (when present).
  • Adjust MAY requirements.

-08

  • Licensed machine-readable data under MIT.
  • Clarified URI comparison for extension relation types.
  • Various editorial tweaks (thanks, Julian!).
  • Changed "fields" to "appdata" to avoid confusion, and add example to clarify.
  • Defined REV according to HTML2, deprecated.
  • Clarified allowable characters in link-extensions.
  • Changed RFC2231 reference to draft-reschke-rfc2231-in-http.
  • Added hub, latest-version, predecessor-version, successor-version, version-history, working-copy and working-copy-of relation types to initial registry.
  • Adjusted text regarding when anchor parameter is appropriate.

-07

  • Allowed multiple spaces between relation types.
  • Relaxed requirements for registered relations.
  • Removed Defining New Link Serialisations appendix.
  • Added Field registry.
  • Added registry XML format.
  • Changed registration procedure to use mailing list(s), giving the Designated Experts more responsibility for the smooth running of the registry.
  • Loosened prohibition against media-specific relation types to SHOULD NOT.
  • Disallowed registration of media-specific relation types (can still be used as extension types).
  • Clarified that parsers are responsible for resolving relative URIs.
  • Fixed ABNF for extended-initial-value.
  • Fixed title* parameter quoting in example.
  • Added notes for registered relations that lack a reference.
  • Added hreflang parameter.
  • Clarified status of 'rev'.
  • Removed advice to use @profile in HTML4.
  • Clarified what multiple *title and hreflang attributes mean.
  • Disallowed multiple type, rel and title attributes.
  • Removed text about absolute URI form of registered relations.
  • Required registered relations to conform to sgml-name (now just rel-relation-type).
  • Required registered relations to be lowercase.
  • Made comparison of extension relations case insensitive.
  • Clarified requirements on registered relation types regarding media types, etc.
  • Allowed applications to ignore links with anchor parameters if they're concerned.
  • Made 'rev' text a bit less confusing.
  • Extension relation URIs SHOULD be all-lowercase.
  • Added media parameter.
  • Required applications to specifically call out use of anchor parameter.

-06

  • Added "up" and "service" relation types.
  • Fixed "type" attribute syntax and added prose.
  • Added note about RDFa and XHTML to HTML4 notes.
  • Removed specific location for the registry, since IANA seems to have its own ideas about that.

-05

  • Clarified how to resolve relative URIs in the 'anchor' parameter.
  • Tweaked language about dereferencing relation type URIs.
  • Separated out examples.
  • Made target-parameters more explicit in the model.
  • Discourage special semantics between different relations, or based upon cardinality.
  • Grandfathered in special semantics of 'alternate stylesheet' for HTML4.
  • Note that extension types can be serialised in ways other than as URIs, as long as they can be converted to URIs.
  • Change default context of a link header to that of the requested resource.
  • Use this document as reference for relations that don't have a formal definition other than the registry entries; avoids circular references.
  • Noted that ordering of links is not significant or defined in this spec, but may be in specific applications.
  • Adjusted uses of 'application' to 'serialisation' where appropriate.
  • Added 'Defining New Link Serialisations' section.
  • Added note about case sensitivity when comparing registered relation types in Atom.

-04

  • Defined context as a resource, rather than a representation.
  • Removed concept of link directionality; relegated to a deprecated Link header extension.
  • Relation types split into registered (non-URI) and extension (URI).
  • Changed wording around finding URIs for registered relation types.
  • Changed target and context URIs to IRIs (but not extension relation types).
  • Add RFC2231 encoding for title parameter, explicit BNF for title*.
  • Add i18n considerations.
  • Specify how to compare relation types.
  • Changed registration procedure to Designated Expert.
  • Softened language around presence of relations in the registry.
  • Added describedby relation.
  • Re-added 'anchor' parameter, along with security consideration for third-party anchors.
  • Softened language around HTML4 attributes that aren't directly accommodated.
  • Various tweaks to abstract, introduction and examples.

-03

  • Inverted focus from Link headers to link relations.
  • Specified was a link relation type is.
  • Based on discussion, re-added 'rev'.
  • Changed IESG Approval to IETF Consensus for relation registrations (i.e., require a document).
  • Updated RFC2434 reference to RFC5226.
  • Registered relations SHOULD conform to sgml-name.
  • Cautioned against confusing relation types with media types.

-02

  • Dropped XLink language.
  • Removed 'made' example.
  • Removed 'rev'. Can still be used as an extension.
  • Added HTML reference to introduction.
  • Required relationship values that have a ; or , to be quoted.
  • Changed base URI for relation values.
  • Noted registry location.
  • Added advisory text about HTML profile URIs.
  • Disallowed registration of relations that only differ in case.
  • Clarified language about IRIs in Atom.
  • Added descriptions for 'first', 'last', and 'payment', referring to current IANA registry entries, as these were sourced from e-mail. Will this cause self-referential implosion?
  • Explicitly updates RFC4287.
  • Added 'type' parameter.
  • Removed unnecessary advice about non-HTML relations in HTML section.

-01

  • Changed syntax of link-relation to one or more URI; dropped Profile.
  • Dropped anchor parameter; can still be an extension.
  • Removed Link-Template header; can be specified by templates spec or elsewhere.
  • Straw-man for link relation registry.

-00

  • Initial draft; normative text lifted from RFC2068.


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Author's Address

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