Network Working Group M. Nottingham Internet-Draft January 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 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 http://www.ietf.org/ietf/1id-abstracts.txt. The list of Internet-Draft Shadow Directories can be accessed at http://www.ietf.org/shadow.html. 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 (http://trustee.ietf.org/license-info) in effect on the date of publication of this document. Please review these documents Nottingham Expires July 23, 2010 [Page 1]
Internet-Draft Web Linking January 2010 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. Nottingham Expires July 23, 2010 [Page 2]
Internet-Draft Web Linking January 2010 Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3 2. Notational Conventions . . . . . . . . . . . . . . . . . . . . 3 3. Links . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 4. Link Relation Types . . . . . . . . . . . . . . . . . . . . . 4 4.1. Registered Relation Types . . . . . . . . . . . . . . . . 5 4.2. Extension Relation Types . . . . . . . . . . . . . . . . . 6 5. The Link Header Field . . . . . . . . . . . . . . . . . . . . 6 5.1. Target IRI . . . . . . . . . . . . . . . . . . . . . . . . 7 5.2. Context IRI . . . . . . . . . . . . . . . . . . . . . . . 7 5.3. Relation Type . . . . . . . . . . . . . . . . . . . . . . 7 5.4. Target Attributes . . . . . . . . . . . . . . . . . . . . 7 5.5. Examples . . . . . . . . . . . . . . . . . . . . . . . . . 8 6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 9 6.1. Link HTTP Header Registration . . . . . . . . . . . . . . 9 6.2. Link Relation Type Registry . . . . . . . . . . . . . . . 10 6.3. Link Relation Field Registry . . . . . . . . . . . . . . . 14 7. Security Considerations . . . . . . . . . . . . . . . . . . . 15 8. Internationalisation Considerations . . . . . . . . . . . . . 16 9. References . . . . . . . . . . . . . . . . . . . . . . . . . . 16 9.1. Normative References . . . . . . . . . . . . . . . . . . . 16 9.2. Informative References . . . . . . . . . . . . . . . . . . 17 Appendix A. Link Relation Registry Format . . . . . . . . . . . . 18 A.1. Relax NG Grammar . . . . . . . . . . . . . . . . . . . . . 18 A.2. Example . . . . . . . . . . . . . . . . . . . . . . . . . 19 Appendix B. Notes on Using the Link Header with the HTML4 Format . . . . . . . . . . . . . . . . . . . . . . . 19 Appendix C. Notes on Using the Link Header with the Atom Format . . . . . . . . . . . . . . . . . . . . . . . 20 Appendix D. Acknowledgements . . . . . . . . . . . . . . . . . . 21 Appendix E. Document history . . . . . . . . . . . . . . . . . . 21 Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 24 Nottingham Expires July 23, 2010 [Page 3]
Internet-Draft Web Linking January 2010 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. Nottingham Expires July 23, 2010 [Page 4]
Internet-Draft Web Linking January 2010 3. Links In this specification, a link is a typed connection between two resources that are identified by IRIs [RFC3987], and is comprised of: o A context IRI, and o a link relation type (Section 4), and o a target IRI, and o optionally, target attributes. 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], because many protocols (such as HTTP) do not support dereferencing IRIs. Likewise, the target IRI will be converted to a URI (see [RFC3987], 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. 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 Nottingham Expires July 23, 2010 [Page 5]
Internet-Draft Web Linking January 2010 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]; 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. 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. Registered relation type names MUST conform to the reg-relation-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 MAY specify the behaviours and properties of the target resource (e.g., allowable methods, request and response media types which must be supported). Additionally, specific applications of linking may have additional per-relation type attributes which are advantageous to register. For example, some link relations might not be appropriate to use in particular contexts, or might have common behaviour such as whether their content should be archived with the page. To accommodate this, new per-entry fields MAY be added to the registry, by registering them in the Link Relation Field Registry Section 6.3. Nottingham Expires July 23, 2010 [Page 6]
Internet-Draft Web Linking January 2010 4.2. Extension Relation Types Applications that don't wish to register a relation type may use an extension relation type, which is a URI [RFC3986] 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 URIs 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 MAY specify that they are expressed in another form, as long as they can be converted to URIs. 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]. 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*" "=" enc2231-string ) | ( "type" "=" type-name "/" subtype-name ) | ( link-extension ) ) link-extension = token [ "=" ( token | quoted-string ) ] enc2231-string = <extended-initial-value [RFC2231] Section 7> relation-types = relation-type | <"> relation-type *( 1*SP relation-type ) <"> relation-type = reg-relation-type | ext-relation-type reg-relation-type = LOALPHA *( LOALPHA | DIGIT | "." | "-" ) ext-relation-type = URI Nottingham Expires July 23, 2010 [Page 7]
Internet-Draft Web Linking January 2010 5.1. Target IRI Each link-value conveys one target IRI as a URI-Reference (after conversion to one, if necessary; see [RFC3987], Section 3.1) inside angle brackets ("<>"). If the URI-Reference is relative, parsers MUST resolve it as per [RFC3986], Section 5. Note that any base IRI from the message's content is not applied. 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 and explicitly specified by use by an application, 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], Section 5. Note that any base URI from the body's content is not applied. The anchor parameter MUST be ignored by consuming implementations, unless its use is specified by the application in use. 5.3. Relation Type The relation type of a link is conveyed in the "rel" parameter's value. Note that the "rev" parameter has also been used by some formats, and MAY be accommodated as a link-extension, but its use is neither encouraged nor defined by this specification. The "rel" parameter MUST NOT appear more than once in a given link- value; occurrences after the first MUST be ignored by parsers. 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 (","). 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- Nottingham Expires July 23, 2010 [Page 8]
Internet-Draft Web Linking January 2010 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], Section 6.13. Note that this may be updated by [W3C.CR-css3-mediaqueries-20090915]). 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). 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 MAY be used encode this label in a different character set, and/or contain language information as per [RFC2231]. When using the enc2231-string syntax, producers MUST NOT use a charset value other than 'ISO-8859-1' or 'UTF-8'. The "title*" parameter MAY appear more than once in a given link-value, but each occurrence MUST indicate a different language; occurrences after the first for a given language MUST be ignored by parsers. When presenting links to users, agents SHOULD use the most appropriate "title*" value, according to user preferences. If an appropriate "title*" value cannot be found, the "title" parameter's value, if available, can be used. 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. 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, Nottingham Expires July 23, 2010 [Page 9]
Internet-Draft Web Linking January 2010 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 may 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". 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 ] Nottingham Expires July 23, 2010 [Page 10]
Internet-Draft Web Linking January 2010 6.2. Link Relation Type Registry This specification establishes the Link Relation Type Registry, and updates Atom [RFC4287] 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. ]] 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]). The requirements for registered relation types are described in Section 4.1. Registration requests consist of the completed registration template below, typically published in an RFC or Open Standard (in the sense described by [RFC2026], 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. The registration template is: o Relation Name: o Description: o Reference: o Notes: [optional] o Fields: [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. 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(s) Nottingham Expires July 23, 2010 [Page 11]
Internet-Draft Web Linking January 2010 will update the registry XML file (using the format described in Appendix A and send it to the [TBD-2]@ietf.org mailing list (which SHOULD NOT be centrally archived, 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. 6.2.2. Initial Registry Contents The Link Relation Type registry's initial contents are: o Relation Name: alternate o Description: Designates a substitute for the link's context. o Reference: [W3C.REC-html401-19991224] o Relation Name: appendix o Description: Refers to an appendix. o Reference: [W3C.REC-html401-19991224] o Relation Name: bookmark o Description: Refers to a bookmark or entry point. o Reference: [W3C.REC-html401-19991224] o Relation Name: chapter o Description: Refers to a chapter in a collection of resources. o Reference: [W3C.REC-html401-19991224] o Relation Name: contents o Description: Refers to a table of contents. o Reference: [W3C.REC-html401-19991224] o Relation Name: copyright o Description: Refers to a copyright statement that applies to the link's context. o Reference: [W3C.REC-html401-19991224] o Relation Name: current o Description: Refers to a resource containing the most recent item(s) in a collection of resources. o Reference: [RFC5005] o Relation Name: describedby Nottingham Expires July 23, 2010 [Page 12]
Internet-Draft Web Linking January 2010 o Description: Refers to a resource providing information about the link's context. o Documentation: <http://www.w3.org/TR/powder-dr/#assoc-linking> o Relation Name: edit o Description: Refers to a resource that can be used to edit the link's context. o Reference: [RFC5023] o Relation Name: edit-media o Description: Refers to a resource that can be used to edit media associated with the link's context. o Reference: [RFC5023] o Relation Name: enclosure o Description: Identifies a related resource that is potentially large and might require special handling. o Reference: [RFC4287] o Relation Name: first o Description: An IRI that refers to the furthest preceding resource in a series of resources. o Reference: [this document] o Notes: this relation type pre-exists this specification, and did not indicate a reference. Originally requested by Mark Nottingham in December 2004. o Relation Name: glossary o Description: Refers to a glossary of terms. o Reference: [W3C.REC-html401-19991224] o Relation Name: help o Description: Refers to a resource offering help (more information, links to other sources information, etc.) o Reference: [W3C.REC-html401-19991224] o Relation Name: index o Description: Refers to an index. o Reference: [W3C.REC-html401-19991224] o Relation Name: last o Description: An IRI that refers to the furthest following resource in a series of resources. o Reference: [this document] o Notes: this relation type pre-exists this specification, and did not indicate a reference. Originally requested by Mark Nottingham in December 2004. Nottingham Expires July 23, 2010 [Page 13]
Internet-Draft Web Linking January 2010 o Relation Name: license o Description: Refers to a license associated with the link's context. o Reference: [RFC4946] o Relation Name: next o Description: Refers to the next resource in a ordered series of resources. o Reference: [W3C.REC-html401-19991224] o Relation Name: next-archive o Description: Refers to the immediately following archive resource. o Reference: [RFC5005] o Relation Name: payment o Description: indicates a resource where payment is accepted. o Reference: [this document] o Notes: this relation type pre-exists this specification, and did not indicate a reference. Requested by Joshua Kinberg and Robert Sayre. o Relation Name: prev o Description: Refers to the previous resource in an ordered series of resources. Synonym for "previous". o Reference: [W3C.REC-html401-19991224] o Relation Name: previous o Description: Refers to the previous resource in an ordered series of resources. Synonym for "prev". o Reference: [W3C.REC-html401-19991224] o Relation Name: prev-archive o Description: Refers to the immediately preceding archive resource. o Reference: [RFC5005] o Relation Name: related o Description: Identifies a related resource. o Reference: [RFC4287] o Relation Name: replies o Description: Identifies a resource that is a reply to the context of the link. o Reference: [RFC4685] o Relation Name: section o Description: Refers to a section in a collection of resources. Nottingham Expires July 23, 2010 [Page 14]
Internet-Draft Web Linking January 2010 o Reference: [W3C.REC-html401-19991224] o Relation Name: self o Description: Conveys an identifier for the link's context. o Reference: [RFC4287] o Relation Name: service o Description: Indicates a URI that can be used to retrieve a service document. o Reference: [RFC5023] o Notes: When used in an Atom document, this relation type specifies Atom Publishing Protocol service documents by default. Requested by James Snell. o Relation Name: start o Description: Refers to the first resource in a collection of resources. o Reference: [W3C.REC-html401-19991224] o Relation Name: stylesheet o Description: Refers to an external style sheet. o Reference: [W3C.REC-html401-19991224] o Relation Name: subsection o Description: Refers to a resource serving as a subsection in a collection of resources. o Reference: [W3C.REC-html401-19991224] o Relation Name: up o Description: Refers to a parent document in a hierarchy of documents. o Reference: [this document] o Notes: this relation type pre-exists this specification, and did not indicate a reference. Requested by Noah Slater. o Relation Name: via o Description: Identifies a resource that is the source of the information in the link's context. o Reference: [RFC4287] 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 Nottingham Expires July 23, 2010 [Page 15]
Internet-Draft Web Linking January 2010 terminology from [RFC5226]). Registration requests consist of the completed registration template below; o Field Name: o Description: o Default Value: o 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, Nottingham Expires July 23, 2010 [Page 16]
Internet-Draft Web Linking January 2010 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. Nottingham Expires July 23, 2010 [Page 17]
Internet-Draft Web Linking January 2010 [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", BCP 47, 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., Celik, 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] Raggett, D., Hors, A., and I. Jacobs, "HTML 4.01 Specification", W3C REC REC-html401-19991224, December 1999. [W3C.REC-rdfa-syntax-20081014] Pemberton, S., Birbeck, M., Adida, B., and S. McCarron, "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>. Nottingham Expires July 23, 2010 [Page 18]
Internet-Draft Web Linking January 2010 [W3C.REC-xhtml-basic-20080729] Baker, M., Wugofski, T., Ishikawa, M., Stark, P., Matsui, S., and T. Yamakami, "XHTML[TM] Basic 1.1", World Wide Web Consortium Recommendation REC-xhtml-basic-20080729, July 2008, <http://www.w3.org/TR/2008/REC-xhtml-basic-20080729>. 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]@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 }* }+ } Nottingham Expires July 23, 2010 [Page 19]
Internet-Draft Web Linking January 2010 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> 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. 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 o As absolute URIs, or o using the document-wide "profile" attribute's URI as a prefix for relation types, or o using the RDFa [W3C.REC-rdfa-syntax-20081014] convention of mapping token prefixes to URIs (in a manner similar to XML name spaces) (note that RDFa is only defined to work in XHTML [W3C.REC-xhtml-basic-20080729], but is sometimes used in HTML4). 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 Nottingham Expires July 23, 2010 [Page 20]
Internet-Draft Web Linking January 2010 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. 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. Nottingham Expires July 23, 2010 [Page 21]
Internet-Draft Web Linking January 2010 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. 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. ]] -07 o Allowed multiple spaces between relation types. o Relaxed requirements for registered relations. o Removed Defining New Link Serialisations appendix. o Added Field registry. o Added registry XML format. o Changed registration procedure to use mailing list(s), giving the Designated Experts more responsibility for the smooth running of the registry. o Loosened prohibition against media-specific relation types to SHOULD NOT. o Disallowed registration of media-specific relation types (can still be used as extension types). o Clarified that parsers are responsible for resolving relative URIs. o Fixed ABNF for extended-initial-value. o Fixed title* parameter quoting in example. o Added notes for registered relations that lack a reference. o Added hreflang parameter. o Clarified status of 'rev'. o Removed advice to use @profile in HTML4. o Clarified what multiple *title and hreflang attributes mean. Nottingham Expires July 23, 2010 [Page 22]
Internet-Draft Web Linking January 2010 o Disallowed multiple type, rel and title attributes. o Removed text about absolute URI form of registered relations. o Required registered relations to conform to sgml-name (now just rel-relation-type). o Required registered relations to be lowercase. o Made comparison of extension relations case insensitive. o Clarified requirements on registered relation types regarding media types, etc. o Allowed applications to ignore links with anchor parameters if they're concerned. o Made 'rev' text a bit less confusing. o Extension relation URIs SHOULD be all-lowercase. o Added media parameter. o Required applications to specifically call out use of anchor parameter. -06 o Added "up" and "service" relation types. o Fixed "type" attribute syntax and added prose. o Added note about RDFa and XHTML to HTML4 notes. o Removed specific location for the registry, since IANA seems to have its own ideas about that. -05 o Clarified how to resolve relative URIs in the 'anchor' parameter. o Tweaked language about dereferencing relation type URIs. o Separated out examples. o Made target-parameters more explicit in the model. o Discourage special semantics between different relations, or based upon cardinality. o Grandfathered in special semantics of 'alternate stylesheet' for HTML4. o Note that extension types can be serialised in ways other than as URIs, as long as they can be converted to URIs. o Change default context of a link header to that of the requested resource. o Use this document as reference for relations that don't have a formal definition other than the registry entries; avoids circular references. o Noted that ordering of links is not significant or defined in this spec, but may be in specific applications. o Adjusted uses of 'application' to 'serialisation' where appropriate. o Added 'Defining New Link Serialisations' section. Nottingham Expires July 23, 2010 [Page 23]
Internet-Draft Web Linking January 2010 o Added note about case sensitivity when comparing registered relation types in Atom. -04 o Defined context as a resource, rather than a representation. o Removed concept of link directionality; relegated to a deprecated Link header extension. o Relation types split into registered (non-URI) and extension (URI). o Changed wording around finding URIs for registered relation types. o Changed target and context URIs to IRIs (but not extension relation types). o Add RFC2231 encoding for title parameter, explicit BNF for title*. o Add i18n considerations. o Specify how to compare relation types. o Changed registration procedure to Designated Expert. o Softened language around presence of relations in the registry. o Added describedby relation. o Re-added 'anchor' parameter, along with security consideration for third-party anchors. o Softened language around HTML4 attributes that aren't directly accommodated. o Various tweaks to abstract, introduction and examples. -03 o Inverted focus from Link headers to link relations. o Specified was a link relation type is. o Based on discussion, re-added 'rev'. o Changed IESG Approval to IETF Consensus for relation registrations (i.e., require a document). o Updated RFC2434 reference to RFC5226. o Registered relations SHOULD conform to sgml-name. o Cautioned against confusing relation types with media types. -02 o Dropped XLink language. o Removed 'made' example. o Removed 'rev'. Can still be used as an extension. o Added HTML reference to introduction. o Required relationship values that have a ; or , to be quoted. o Changed base URI for relation values. o Noted registry location. o Added advisory text about HTML profile URIs. Nottingham Expires July 23, 2010 [Page 24]
Internet-Draft Web Linking January 2010 o Disallowed registration of relations that only differ in case. o Clarified language about IRIs in Atom. o 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? o Explicitly updates RFC4287. o Added 'type' parameter. o Removed unnecessary advice about non-HTML relations in HTML section. -01 o Changed syntax of link-relation to one or more URI; dropped Profile. o Dropped anchor parameter; can still be an extension. o Removed Link-Template header; can be specified by templates spec or elsewhere. o Straw-man for link relation registry. -00 o Initial draft; normative text lifted from RFC2068. Author's Address Mark Nottingham Email: mnot@mnot.net URI: http://www.mnot.net/ Nottingham Expires July 23, 2010 [Page 25]
