Network Working Group                                      M. Nottingham
Internet-Draft                                            April 19, 2017
Obsoletes: 5988 (if approved)
Intended status: Standards Track
Expires: October 21, 2017


                              Web Linking
                     draft-nottingham-rfc5988bis-05

Abstract

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

   It also defines the serialisation 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   The most recent (often, unpublished) draft is at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 October 21, 2017.



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Copyright Notice

   Copyright (c) 2017 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
     1.1.  Notational Conventions  . . . . . . . . . . . . . . . . .   3
   2.  Links . . . . . . . . . . . . . . . . . . . . . . . . . . . .   3
     2.1.  Link Relation Types . . . . . . . . . . . . . . . . . . .   5
       2.1.1.  Registered Relation Types . . . . . . . . . . . . . .   5
       2.1.2.  Extension Relation Types  . . . . . . . . . . . . . .   7
     2.2.  Target Attributes . . . . . . . . . . . . . . . . . . . .   7
   3.  Link Serialisation in HTTP Headers  . . . . . . . . . . . . .   8
     3.1.  Link Target . . . . . . . . . . . . . . . . . . . . . . .   8
     3.2.  Link Context  . . . . . . . . . . . . . . . . . . . . . .   9
     3.3.  Relation Type . . . . . . . . . . . . . . . . . . . . . .   9
     3.4.  Target Attributes . . . . . . . . . . . . . . . . . . . .  10
       3.4.1.  Serialisation-Defined Attributes  . . . . . . . . . .  10
       3.4.2.  Extension Attributes  . . . . . . . . . . . . . . . .  11
     3.5.  Link Header Field Examples  . . . . . . . . . . . . . . .  11
   4.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  12
     4.1.  Link HTTP Header Field Registration . . . . . . . . . . .  13
     4.2.  Link Relation Type Registry . . . . . . . . . . . . . . .  13
   5.  Security Considerations . . . . . . . . . . . . . . . . . . .  14



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   6.  Internationalisation Considerations . . . . . . . . . . . . .  14
   7.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  15
     7.1.  Normative References  . . . . . . . . . . . . . . . . . .  15
     7.2.  Informative References  . . . . . . . . . . . . . . . . .  16
   Appendix A.  Notes on Other Link Serialisations . . . . . . . . .  17
     A.1.  Link Serialisation in HTML  . . . . . . . . . . . . . . .  17
     A.2.  Link Serialisation in Atom  . . . . . . . . . . . . . . .  17
   Appendix B.  Algorithm for Parsing Link Headers . . . . . . . . .  18
   Appendix C.  Changes from RFC5988 . . . . . . . . . . . . . . . .  21
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .  22

1.  Introduction

   This specification defines a model for 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; Section 2 generalises this into a
   framework that encompasses linking in these formats and (potentially)
   elsewhere.

   Furthermore, Section 3 defines an HTTP header field for conveying
   such links.

1.1.  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), BWS (bad
   whitespace), OWS (optional whitespace), RWS (required whitespace)
   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; and from
   [RFC5646]: Language-Tag.

2.  Links

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




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   o  A _link context_,

   o  a _link relation type_ (Section 2.1),

   o  a _link target_, and

   o  optionally, _target attributes_ (Section 2.2).

   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 serialisations that do not
   support IRIs (such as the Link header field defined in Section 3).

   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.

   Links are conveyed in _link serialisations_; they are the "bytes on
   the wire", and can occur in various forms.  For example, Atom
   [RFC4287] and HTML [W3C.REC-html5-20141028] both defined
   serialisations of links into their respective formats, and Section 3
   defines how to serialise links in HTTP header fields.

   This specification does not define a general syntax for links across
   different serialisations, nor does it mandate a specific context for
   any given link; it is expected that serialisations of links will
   specify both aspects.

   Finally, links are consumed by _link applications_. Generally, an
   application will define the link relation type(s) it uses, along with
   the serialisation(s) that they might occur within.  For example, the
   application "Web browsing" looks for the "stylesheet" link relation
   type in the HTML link serialisation, whereas the application
   "AtomPub" uses the "edit" and "edit-media" link relations.






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2.1.  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 current link context has a copyright resource at
   the link target.

   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 link
   target can be used as 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.

2.1.1.  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 2.1.1.1.

   Registered relation type names MUST conform to the reg-rel-type rule
   (see Section 3.3), 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 are required be
   supported).

   Historically, registered relation types have been identified with a
   URI [RFC3986] by prefixing their names with an application-defined



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   base URI (e.g., see Appendix A.2).  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.

2.1.1.1.  Registering Link Relation Types

   The link relations registry is located at
   https://www.iana.org/assignments/link-relations/ .  Registration
   requests can be made by following the instructions located there, or
   by sending an e-mail to the "link-relations@ietf.org" mailing list.

   Registration requests consist of at least the following information:

   o  *Relation Name*: The name of the relation type

   o  *Description*: A short English description of the type's
      semantics.  It SHOULD be stated in terms of the relationship
      between the link context and link target.

   o  *Reference*: Reference to the document that specifies the link
      relation type, preferably including a URI that can be used to
      retrieve a copy of the document.  An indication of the relevant
      section(s) MAY also be included, but is not required.

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

   General requirements for registered relation types are described in
   Section 2.1.1.

   Registrations MUST reference a freely available, stable
   specification.

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

2.1.1.2.  Registration Request Processing

   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 goal of the registry is to reflect common use of links on the
   Internet.  Therefore, the Expert(s) SHOULD be strongly biased towards



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   approving registrations, unless they are abusive, frivolous, not
   likely to be used on the Internet, or actively harmful to the
   Internet and/or the Web (not merely aesthetically displeasing, or
   architecturally dubious).  As stated in Section 2.1.1, the Experts
   MAY withhold registration of names that are too general for the
   proposed application.

   The Expert(s) MUST clearly identify any issues which cause a
   registration to be refused.  Advice about the semantics of a proposed
   link relation type can be given, but if it does not block
   registration, this SHOULD be explicitly stated.

   When a request is approved, the Expert(s) will inform IANA, and the
   registration will be processed.  The IESG is the final arbiter of any
   objection.

2.1.2.  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.

   When extension relation types are compared, they MUST be compared as
   strings (after converting to URIs if serialised in a different
   format) 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.

2.2.  Target Attributes

   _Target attributes_ are a list of key/value pairs that describe the
   link or its target; for example, a media type hint.

   They can be defined both by individual link relation types and by
   link serialisations.

   This specification does not attempt to coordinate the name of target
   attributes, their cardinality or use.  Serialisations SHOULD



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   coordinate their target attributes to avoid conflicts in semantics or
   syntax.

   The names of target attributes SHOULD conform to the token rule, but
   SHOULD NOT include any of the characters "%", "'" or "*", for
   portability across serializations, and MUST be compared in a case-
   insensitive fashion.

   Target attribute definitions SHOULD specify:

   o  The serialisation of their values into Unicode or a subset
      thereof, to maximise their chances of portability across link
      serialisations.

   o  The semantics and error handling of multiple occurrences of the
      target attribute on a given link.

   This specification does define target attributes for use in the Link
   HTTP header field in Section 3.4.

3.  Link Serialisation in HTTP Headers

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

   The ABNF for the field value is:

     Link       = #link-value
     link-value = "<" URI-Reference ">" *( OWS ";" OWS link-param )
     link-param = token BWS "=" BWS ( token / quoted-string )

   Note that any "link-param" can be generated with values using either
   the "token" or the "quoted-string" syntax, and therefore recipients
   MUST be able to parse both forms.  Individual "link-param"s specify
   their syntax in terms of the value after any necessary unquoting (as
   per [RFC7230], Section 3.2.6).

   This specification defines the link-params "rel", "anchor", "rev",
   "hreflang", "media", "title", "title*", and "type"; see Section 3.2,
   Section 3.3 and Section 3.4.

3.1.  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.



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3.2.  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
   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 ABNF for the "anchor" parameter's value is:

     URI-Reference

   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 example, this is the case on a 404 response to a GET
   request.

3.3.  Relation Type

   The relation type of a link conveyed in the Link header field 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.

   The ABNF for the "rel" and "rev" parameters' values is:

     relation-type *( 1*SP relation-type )

   where:




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     relation-type  = reg-rel-type | ext-rel-type
     reg-rel-type   = LOALPHA *( LOALPHA | DIGIT | "." | "-" )
     ext-rel-type   = URI

   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).

3.4.  Target Attributes

   The Link header field defines several target attributes specific to
   this serialisation, and also allows extension target attributes.
   Target attributes are serialised in the Link header field as
   parameters (see [RFC7231], Section 3.1.1.1 for the definition of
   their syntax).

3.4.1.  Serialisation-Defined Attributes

   The "hreflang", "media", "title", "title*", and "type" link-params
   can be translated to serialisation-defined target attributes for the
   link.

   The "hreflang" attribute, 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" attributes on a single link-
   value indicate that multiple languages are available from the
   indicated resource.

   The ABNF for the "hreflang" parameter's value is:

     Language-Tag

   The "media" attribute, 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 (",").  There MUST NOT be
   more than one "media" attribute in a link-value; occurrences after
   the first MUST be ignored by parsers.

   The ABNF for the "media" parameter's value is:

     media_query_list

   The "title" attribute, when present, is used to label the destination
   of a link such that it can be used as a human-readable identifier



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   (e.g., a menu entry) in the language indicated by the Content-
   Language header field (if present).  The "title" attribute MUST NOT
   appear more than once in a given link; occurrences after the first
   MUST be ignored by parsers.

   The "title*" link-param can be used to encode this attribute in a
   different character set, and/or contain language information as per
   [I-D.ietf-httpbis-rfc5987bis].  The "title*" link-param MUST NOT
   appear more than once in a given link-value; occurrences after the
   first MUST be ignored by parsers.  If the attribute does not contain
   language information, its language is indicated by the Content-
   Language header field (when present).

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

   The "type" attribute, 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" attribute MUST NOT appear more than
   once in a given link-value; occurrences after the first MUST be
   ignored by parsers.

   The ABNF for the "type" parameter's value is:

     type-name "/" subtype-name

3.4.2.  Extension Attributes

   Other link-params are link-extensions, and are to be considered as
   target attributes.

   Such target attributes MAY be defined to use the encoding in
   [I-D.ietf-httpbis-rfc5987bis] (e.g., "example" and "example*").  When
   both forms are present, they SHOULD be considered to be the same
   target attribute; processors SHOULD use the value of the name ending
   in "*" (after [I-D.ietf-httpbis-rfc5987bis] decoding), but MAY fall
   back to the other value if there is an error in decoding it, or if
   they do not support decoding.

3.5.  Link Header Field Examples

   For example:

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



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   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".

   This link:

   Link: </terms>; rel="copyright"; anchor="#foo"

   indicates that the linked copyright terms only apply to the portion
   of the document indicated by the (media type-specific) fragment
   identifier "foo".

   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".

4.  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.





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4.1.  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;]

4.2.  Link Relation Type Registry

   This specification updates the registration procedures for the Link
   Relation Type registry; see Section 2.1.1.1.  The Expert(s) and IANA
   are expected interact as outlined below.

   The Expert(s) will provide registry data to IANA in a mutually-agreed
   form (e.g. a specific XML format).  IANA will publish:

   o  The raw registry data

   o  The registry data, transformed into HTML

   o  The registry data alternative formats provided by the Expert(s)
      (if any)

   If IANA's internal processes require making changes to registry data
   and/or adding registry entries, IANA will inform the Expert(s) of
   this in a mutually agreed way.

   Each published document will be at a URL mutually 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:

   o  Take directions from the Expert(s) as to the content of the HTML
      page's introductory text

   o  Include a stable HTML fragment identifier for each registered link
      relation






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   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>).

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

5.  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 are to 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.

6.  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.

   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.








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7.  References

7.1.  Normative References

   [I-D.ietf-httpbis-rfc5987bis]
              Reschke, J., "Indicating Character Encoding and Language
              for HTTP Header Field Parameters", draft-ietf-httpbis-
              rfc5987bis-05 (work in progress), February 2017.

   [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>.

   [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>.





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   [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>.

7.2.  Informative References

   [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>.

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




















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Appendix A.  Notes on Other Link Serialisations

   Header fields (Section 3) are only one serialisation of links; other
   specifications have defined alternative serialisations.

A.1.  Link Serialisation in HTML

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

   HTML5 ([W3C.REC-html5-20141028]) Section 4.8 defines modern HTML
   links.  That document links to the Microformats Wiki as a registry;
   over time, the IANA registry ought to mirror its contents, and
   ideally eventually replace it (although that depends on the HTML
   community).

   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).

   Finally, the HTML specification gives a special meaning when the
   "alternate" relation types coincides with other relation types 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.

A.2.  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.

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



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   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 B.  Algorithm for Parsing Link Headers

   Given a HTTP header field-value "field_value" as a string assuming
   ASCII encoding, the following algorithm can be used to parse it into
   the model described by this specification:

   1.  Let "links" be an empty list.

   2.  Create "link_strings" by splitting "field_value" on ","
       characters, excepting "," characters within quoted strings as per
       [RFC7230], Section 3.2.6, or which form part of link's URI-
       Reference (i.e. between "<" and ">" characters where the "<" is
       immediately preceded by OWS and either a "," character or the
       beginning of the "field_value" string).

   3.  For each "link_string" in "link_strings":

       1.   Let "target_string" be the string between the first "<" and
            first ">" characters in "link_string".  If they do not
            appear, or do not appear in that order, fail parsing.

       2.   Let "rest" be the remaining characters (if any) after the
            first ">" character in "link_string".

       3.   Split "rest" into an array of strings "parameter_strings",
            on the ";" character, excepting ";" characters within quoted
            strings as per [RFC7230], Section 3.2.6.



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       4.   Let "link_parameters" be an empty array.

       5.   For each item "parameter" in "parameter_strings":

            1.  Remove OWS from the beginning and end of "parameter".

            2.  Skip this item if "parameter" matches the empty string
                ("").

            3.  Split "parameter" into "param_name" and "param_value" on
                the first "=" character.  If "parameter" does not
                contain "=", let "param_name" be "parameter" and
                "param_value" be null.

            4.  Remove OWS from the end of "param_name" and the
                beginning of "param_value".

            5.  Case-normalise "param_name" to lowercase.

            6.  If the first and last characters of "param_value" are
                both DQUOTE:

                1.  Remove the first and last characters of
                    "param_value".

                2.  Replace quoted-pairs within "param_value" with the
                    octet following the backslash, as per [RFC7230],
                    Section 3.2.6.

            7.  If the last character of "param_name" is an asterisk
                ("*"), decode "param_value" according to
                [I-D.ietf-httpbis-rfc5987bis].  Skip this item if an
                unrecoverable error is encountered.

            8.  Append the tuple ("param_name", "param_value") to
                "link_parameters".

       6.   Let "target" be the result of relatively resolving (as per
            [RFC3986], Section 5.2) "target_string".  Note that any base
            URI carried in the payload body is NOT used.

       7.   Let "relations_string" be the second item of the first tuple
            of "link_parameters" whose first item matches the string
            "rel", or the empty string ("") if it is not present.

       8.   Split "relations_string" into an array of strings
            "relation_types", on RWS (removing all whitespace in the
            process).



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       9.   Let "context_string" be the second item of the first tuple
            of "link_parameters" whose first item matches the string
            "anchor".  If it is not present, "context_string" is the
            identity of the representation carrying the Link header
            [RFC7231], Section 3.1.4.1, serialised as a URI.  Where the
            identity is "anonymous" "context_string" is null.

       10.  Let "context" be the result of relatively resolving (as per
            [RFC3986], Section 5.2) "context_string", unless
            "context_string" is null in which case "context" is null.
            Note that any base URI carried in the payload body is NOT
            used.

       11.  Let "target_attributes" be an empty array.

       12.  For each tuple ("param_name", "param_value") of
            "link_parameters":

            1.  If "param_name" matches "rel" or "anchor", skip this
                tuple.

            2.  If "param_name" matches "media", "title", "title*" or
                "type" and "target_attributes" already contains a tuple
                whose first element matches the value of "param_name",
                skip this tuple.

            3.  Append ("param_name", "param_value") to
                "target_attributes".

       13.  Let "star_param_names" be the set of "param_name"s in the
            ("param_name", "param_value") tuples of "link_parameters"
            where the last character of "param_name" is an asterisk
            ("*").

       14.  For each "star_param_name" in "star_param_names":

            1.  Let "base_param_name" be "star_param_name" with the last
                character removed.

            2.  If the implementation does not choose to support an
                internationalised form of a parameter named
                "base_param_name" for any reason (including, but not
                limited to, it being prohibited by the parameter's
                specification), remove all tuples from "link_parameters"
                whose first member is "star_param_name" and skip to the
                next "star_param_name".





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            3.  Remove all tuples from "link_parameters" whose first
                member is "base_param_name".

            4.  Change the first member of all tuples in
                "link_parameters" whose first member is
                "star_param_name" to "base_param_name".

       15.  For each "relation_type" in "relation_types":

            1.  Case-normalise "relation_type" to lowercase.

            2.  Append a link object to "links" with the target
                "target", relation type of "relation_type", context of
                "context", and target attributes "target_attributes".

   4.  Return "links".

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 serialisation-
      specific.

   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."



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   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).

   o  Defined a suggested parsing algorithm for the Link header.

   o  The value space of target attributes and their definition has been
      specified.

   o  The ABNF has been updated to be compatible with [RFC7230].  In
      particular, whitespace is now explicit.

   o  Some parameters on the HTTP header field can now appear as a
      token.

   o  Handling of quoted strings is now defined by [RFC7230].

   o  The "type" header field parameter now needs to be quoted (as
      "token" does not allow "/").

Author's Address

   Mark Nottingham

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






















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