Web Linking
draft-nottingham-rfc5988bis-01

Network Working Group                                      M. Nottingham
Internet-Draft                                               May 3, 2016
Obsoletes: 5988 (if approved)
Intended status: Standards Track
Expires: November 4, 2016

                              Web Linking
                     draft-nottingham-rfc5988bis-01

Abstract

   This specification defines a way to indicate the relationships
   between resources on the Web ("links") and the type of those
   relationships ("link relation types").

   It also defines the use of such links in HTTP headers with the Link
   header field.

Note to Readers

   This is a work-in-progress to revise RFC5988.

   The issues list can be found at https://github.com/mnot/I-D/labels/
   rfc5988bis .

   The most recent (often, unpublished) draft is at
   https://mnot.github.io/I-D/rfc5988bis/ .

   Recent changes are listed at https://github.com/mnot/I-D/commits/gh-
   pages/rfc5988bis .

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 4, 2016.

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Copyright Notice

   Copyright (c) 2016 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
   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 Simplified 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.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Notational Conventions  . . . . . . . . . . . . . . . . . . .   3
   3.  Links . . . . . . . . . . . . . . . . . . . . . . . . . . . .   3
   4.  Link Relation Types . . . . . . . . . . . . . . . . . . . . .   5
     4.1.  Registered Relation Types . . . . . . . . . . . . . . . .   5
       4.1.1.  Registering Link Relation Types . . . . . . . . . . .   6
     4.2.  Extension Relation Types  . . . . . . . . . . . . . . . .   7
   5.  Link Serialisation in the Link HTTP Header Field  . . . . . .   7
     5.1.  Link Target . . . . . . . . . . . . . . . . . . . . . . .   8
     5.2.  Link Context  . . . . . . . . . . . . . . . . . . . . . .   8
     5.3.  Relation Type . . . . . . . . . . . . . . . . . . . . . .   9
     5.4.  Target Attributes . . . . . . . . . . . . . . . . . . . .   9
     5.5.  Examples  . . . . . . . . . . . . . . . . . . . . . . . .  10
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  11
     6.1.  Link HTTP Header Field Registration . . . . . . . . . . .  11
     6.2.  Link Relation Type Registry . . . . . . . . . . . . . . .  11
   7.  Security Considerations . . . . . . . . . . . . . . . . . . .  12
   8.  Internationalisation Considerations . . . . . . . . . . . . .  12
   9.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  13

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     9.1.  Normative References  . . . . . . . . . . . . . . . . . .  13
     9.2.  Informative References  . . . . . . . . . . . . . . . . .  14
   Appendix A.  Link Serialisation in HTML . . . . . . . . . . . . .  16
   Appendix B.  Link Serialisation in Atom . . . . . . . . . . . . .  16
   Appendix C.  Changes from RFC5988 . . . . . . . . . . . . . . . .  17
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .  18

1.  Introduction

   This specification defines a way to indicate the relationships
   between resources on the Web ("links") and the type of those
   relationships ("link relation types").

   HTML [W3C.REC-html5-20141028] and Atom [RFC4287] both have well-
   defined concepts of linking; this specification generalises this into
   a framework that encompasses linking in these formats and
   (potentially) elsewhere.

   Furthermore, this specification formalises an HTTP header field for
   conveying such links, having been originally defined in
   Section 19.6.2.4 of [RFC2068], but removed from [RFC2616].

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
   [RFC7230], including the #rule, 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 [RFC6838]: type-name and subtype-name; from
   [W3C.CR-css3-mediaqueries-20090915]: media_query_list; from
   [RFC5646]: Language-Tag; and from [I-D.ietf-httpbis-rfc5987bis], ext-
   value and parmname.

3.  Links

   In this specification, a link is a typed connection between two
   resources, and is comprised of:

   o  A _link context_,

   o  a _link relation type_ (Section 4),

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   o  a _link target_, and

   o  optionally, _target attributes_.

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

   Link contexts and link targets are both IRIs [RFC3987].  However, in
   the common case, the link context will also be a URI [RFC3986],
   because many protocols (such as HTTP) do not support dereferencing
   IRIs.  Likewise, the link target will be sometimes be converted to a
   URI (see [RFC3987], Section 3.1) in places that do not support IRIs
   (such as the Link header field defined in Section 5).

   This specification does not place restrictions on the cardinality of
   links; there can be multiple links to and from a particular target,
   and multiple links of the same or different types between a given
   context and target.  Likewise, the relative ordering of links in any
   particular serialisation, or between serialisations (e.g., the Link
   header field 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, cardinality
   or use, but individual link relations, link serialisations and link
   applications can do so.  This specification does provide common
   target attributes for use in the Link HTTP header field.

   Links are conveyed in _link serialisations_; they are the "bytes on
   the wire", and can occur in various forms.  This specification does
   not define a general syntax for links, nor does it 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.

   Finally, links are consumed by _link applications_. Generally, an
   application will define the link relation types it uses, along with
   the serialisations that they might occur within.  For example, the
   application "Web browsing" looks for the "stylesheet" link relation
   type in the HTML link serialisation.

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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 link target is a
   statement of the copyright terms applying to the current link
   context.

   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 [RFC6838];
   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 HTML 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, using the
   procedure in Section 4.1.1.

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

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   Historically, applications have sometimes referred to registered
   relation types with a URI [RFC3986] (e.g., Appendix B) by prefixing
   their names with an application-defined base URI.  This practice is
   NOT RECOMMENDED, because the resulting strings will not be considered
   equivalent to the registered relation types by other processors.
   Applications that do use such URIs internally MUST NOT use them in
   link serialisations that do not explicitly accommodate them.

4.1.1.  Registering 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 Expert(s) will establish procedures for requesting registrations,
   and make them available from the registry page.

   Registration requests consist of at least the following information:

   o  Relation Name:

   o  Description:

   o  Reference:

   The Expert(s) MAY define additional fields to be collected in the
   registry.

   General requirements for registered relation types are described in
   Section 4.1.

   See the registry for examples of the description field; generally, it
   SHOULD identify the semantics in terms of the link's context and
   target.

   Registrations MUST reference a freely available, stable
   specification.

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

   Decisions (or lack thereof) made by the Expert(s) 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).

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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] 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.

   The URI used for an extension relation type SHOULD be under the
   control of the person or party defining it, or be delegated to them.
   These URIs also SHOULD NOT use the base URI defined by an application
   for registered relation types (as per Section 4.1).

   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 XML QNames [W3C.REC-xml-names-20091208]) 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.

5.  Link Serialisation in the Link HTTP Header Field

   The Link header field provides a means for serialising one or more
   links into HTTP headers.

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   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" "="
                  ( media_query_list | ( <"> media_query_list <"> ) )
                )
                | ( "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 RFC5987-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

5.1.  Link Target

   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.  Link Context

   By default, the context of a link conveyed in the Link header field
   is identity of the representation it is associated with, as defined
   in [RFC7231], Section 3.1.4.1, serialised as a URI.

   When present, the anchor parameter overrides this with another URI,
   such as a fragment of this resource, or a third resource (i.e., when

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   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.

   Consuming implementations can choose to ignore links with an anchor
   parameter.  For example, the application in use might not allow the
   link context 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
   link context might be "anonymous" (i.e., no link context is
   available).  For instance, this is the case on a 404 response to a
   GET request.

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.  That is,
   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 field itself).

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 field 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.

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   The "media" parameter, when present, is used to indicate intended
   destination medium or media for style information (see
   [W3C.REC-html5-20141028], Section 4.2.4).  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 field (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.ietf-httpbis-rfc5987bis].  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 field (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 field of a HTTP response obtained by actually
   following the link.  The "type" parameter MUST NOT appear more than
   once in a given link-value; occurrences after the first MUST be
   ignored by parsers.

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".

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   The example below shows an instance of the Link header field encoding
   multiple links, and also the use of RFC 5987 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 link
   target and link context; 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

   In addition to the actions below, IANA should terminate the Link
   Relation Application Data Registry, as it has not been used, and
   future use is not anticipated.

6.1.  Link HTTP Header Field 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):
       [RFC&rfc.number;]

6.2.  Link Relation Type Registry

   This specification updates the registration procedures for the Link
   Relation Type registry; see Section 4.1.1.  The Expert(s) and IANA
   will interact as outlined below.

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   IANA will direct any incoming requests regarding the registry to the
   processes established by the Expert(s); typically, this will mean
   referring them to the registry HTML page.

   The Expert(s) will provide registry data to IANA in an agreed form
   (e.g. a specific XML format).  IANA will publish: * The raw registry
   data * The registry data, transformed into HTML * The registry data
   in any alternative formats provided by the Expert(s)

   Each published document will be at a URL agreed to by IANA and the
   Expert(s), and IANA will set HTTP response headers on them as
   (reasonably) requested by the Expert(s).

   Additionally, the HTML generated by IANA will: * Take directions from
   the Expert(s) as to the content of the HTML page's introductory text
   and markup * Include a stable HTML fragment identifier for each
   registered link relation

   All registry data documents MUST include Simplified BSD License text
   as described in Section 4.e of the Trust Legal Provisions
   (<http://trustee.ietf.org/license-info>).

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 Transport Layer Security (TLS) with HTTP ([RFC2818] and
   [RFC2817]) is currently the only end-to-end way to provide such
   protection.

   Link applications ought to consider the attack vectors opened by
   automatically following, trusting, or otherwise using links gathered
   from HTTP headers.  In particular, Link header fields that use the
   "anchor" parameter to associate a link's context with another
   resource should be treated with due caution.

   The Link 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
   [RFC7230] for security considerations relating to HTTP headers.

8.  Internationalisation Considerations

   Link targets 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 field.

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   Similarly, the anchor parameter of the Link header field 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.

   Note that registered Relation Names are required to be lower-case
   ASCII letters.

9.  References

9.1.  Normative References

   [I-D.ietf-httpbis-rfc5987bis]
              Reschke, J., "Indicating Character Encoding and Language
              for HTTP Header Field Parameters", draft-ietf-httpbis-
              rfc5987bis-01 (work in progress), March 2016.

   [RFC2026]  Bradner, S., "The Internet Standards Process -- Revision
              3", BCP 9, RFC 2026, DOI 10.17487/RFC2026, October 1996,
              <http://www.rfc-editor.org/info/rfc2026>.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <http://www.rfc-editor.org/info/rfc2119>.

   [RFC3864]  Klyne, G., Nottingham, M., and J. Mogul, "Registration
              Procedures for Message Header Fields", BCP 90, RFC 3864,
              DOI 10.17487/RFC3864, September 2004,
              <http://www.rfc-editor.org/info/rfc3864>.

   [RFC3986]  Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
              Resource Identifier (URI): Generic Syntax", STD 66,
              RFC 3986, DOI 10.17487/RFC3986, January 2005,
              <http://www.rfc-editor.org/info/rfc3986>.

   [RFC3987]  Duerst, M. and M. Suignard, "Internationalized Resource
              Identifiers (IRIs)", RFC 3987, DOI 10.17487/RFC3987,
              January 2005, <http://www.rfc-editor.org/info/rfc3987>.

   [RFC5226]  Narten, T. and H. Alvestrand, "Guidelines for Writing an
              IANA Considerations Section in RFCs", BCP 26, RFC 5226,
              DOI 10.17487/RFC5226, May 2008,
              <http://www.rfc-editor.org/info/rfc5226>.

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   [RFC5646]  Phillips, A., Ed. and M. Davis, Ed., "Tags for Identifying
              Languages", BCP 47, RFC 5646, DOI 10.17487/RFC5646,
              September 2009, <http://www.rfc-editor.org/info/rfc5646>.

   [RFC6838]  Freed, N., Klensin, J., and T. Hansen, "Media Type
              Specifications and Registration Procedures", BCP 13,
              RFC 6838, DOI 10.17487/RFC6838, January 2013,
              <http://www.rfc-editor.org/info/rfc6838>.

   [RFC7230]  Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
              Protocol (HTTP/1.1): Message Syntax and Routing",
              RFC 7230, DOI 10.17487/RFC7230, June 2014,
              <http://www.rfc-editor.org/info/rfc7230>.

   [RFC7231]  Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
              Protocol (HTTP/1.1): Semantics and Content", RFC 7231,
              DOI 10.17487/RFC7231, June 2014,
              <http://www.rfc-editor.org/info/rfc7231>.

   [W3C.CR-css3-mediaqueries-20090915]
              Lie, H., Celik, T., Glazman, D., 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>.

9.2.  Informative References

   [RFC2068]  Fielding, R., Gettys, J., Mogul, J., Frystyk, H., and T.
              Berners-Lee, "Hypertext Transfer Protocol -- HTTP/1.1",
              RFC 2068, DOI 10.17487/RFC2068, January 1997,
              <http://www.rfc-editor.org/info/rfc2068>.

   [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,
              DOI 10.17487/RFC2616, June 1999,
              <http://www.rfc-editor.org/info/rfc2616>.

   [RFC2817]  Khare, R. and S. Lawrence, "Upgrading to TLS Within
              HTTP/1.1", RFC 2817, DOI 10.17487/RFC2817, May 2000,
              <http://www.rfc-editor.org/info/rfc2817>.

   [RFC2818]  Rescorla, E., "HTTP Over TLS", RFC 2818,
              DOI 10.17487/RFC2818, May 2000,
              <http://www.rfc-editor.org/info/rfc2818>.

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   [RFC4287]  Nottingham, M., Ed. and R. Sayre, Ed., "The Atom
              Syndication Format", RFC 4287, DOI 10.17487/RFC4287,
              December 2005, <http://www.rfc-editor.org/info/rfc4287>.

   [W3C.REC-html-rdfa-20150317]
              Sporny, M., "HTML+RDFa 1.1 - Second Edition", World Wide
              Web Consortium Recommendation REC-html-rdfa-20150317,
              March 2015,
              <http://www.w3.org/TR/2015/REC-html-rdfa-20150317>.

   [W3C.REC-html5-20141028]
              Hickson, I., Berjon, R., Faulkner, S., Leithead, T.,
              Navara, E., O&#039;Connor, E., and S. Pfeiffer, "HTML5",
              World Wide Web Consortium Recommendation REC-
              html5-20141028, October 2014,
              <http://www.w3.org/TR/2014/REC-html5-20141028>.

   [W3C.REC-xml-names-20091208]
              Bray, T., Hollander, D., Layman, A., Tobin, R., and H.
              Thompson, "Namespaces in XML 1.0 (Third Edition)", World
              Wide Web Consortium Recommendation REC-xml-names-20091208,
              December 2009,
              <http://www.w3.org/TR/2009/REC-xml-names-20091208>.

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Appendix A.  Link Serialisation in HTML

   HTML [W3C.REC-html5-20141028] motivated the original syntax of the
   Link header field, and many of the design decisions in this document
   are driven by a desire to stay compatible with it.

   In HTML, 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 field.  The context of the
   link is the URI associated with the entire HTML document.

   All of the link relation types defined by HTML 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 HTML, including:

   o  As absolute URIs,

   o  using the RDFa [W3C.REC-html-rdfa-20150317] convention of mapping
      token prefixes to URIs (in a manner similar to XML name spaces).

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

   Surveys of existing HTML content have shown that unregistered link
   relation types that are not URIs are (perhaps inevitably) common.
   Consuming HTML implementations ought 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).

   HTML also defines several attributes on links that can be see as
   target attributes, including "media", "hreflang", "type" and "sizes".

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

Appendix B.  Link Serialisation in Atom

   Atom [RFC4287] is a link serialisation that conveys links in the
   atom:link element, with the "href" attribute indicating the link
   target and the "rel" attribute containing the relation type.  The
   context of the link is either a feed locator or an entry ID,
   depending on where it appears; generally, feed-level links are
   obvious candidates for transmission as a Link header field.

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   When serialising an atom:link into a Link header field, it is
   necessary to convert link targets (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 using a prefix, "http://www.iana.org/assignments/
   relation/".  This prefix is specific to the Atom serialisation.

   Furthermore, 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 field 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 field syntax, but they can
   also be used as link-extensions to maintain fidelity.

Appendix C.  Changes from RFC5988

   This specification has the following differences from its
   predecessor, RFC5988:

   o  The initial relation type registrations were removed, since
      they've already been registered by 5988.

   o  The introduction has been shortened.

   o  The Link Relation Application Data Registry has been removed.

   o  Incorporated errata.

   o  Updated references.

   o  Link cardinality was clarified.

   o  Terminology was changed from "target IRI" and "context IRI" to
      "link target" and "link context" respectively.

   o  Made assigning a URI to registered relation types application-
      specific.

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   o  Removed misleading statement that the link header field is
      semantically equivalent to HTML and Atom links.

   o  More carefully defined how the Experts and IANA should interact.

   o  More carefully defined and used "link serialisations" and "link
      applications."

   o  Clarified the cardinality of target attributes (generically and
      for "type").

   o  Corrected the default link context for the Link header field, to
      be dependent upon the identity of the representation (as per
      RFC7231).

Author's Address

   Mark Nottingham

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

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