Web Linking
draft-nottingham-rfc5988bis-08

Network Working Group                                      M. Nottingham
Internet-Draft                                           August 11, 2017
Obsoletes: 5988 (if approved)
Intended status: Standards Track
Expires: February 12, 2018

                              Web Linking
                     draft-nottingham-rfc5988bis-08

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

   _RFC EDITOR: please remove this section before publication_

   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   The most recent (often, unpublished) draft is at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."

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   This Internet-Draft will expire on February 12, 2018.

Copyright Notice

   Copyright (c) 2017 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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   the Trust Legal Provisions and are provided without warranty as
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   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
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   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
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   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
     1.2.  Conformance and Error Handling  . . . . . . . . . . . . .   4
   2.  Links . . . . . . . . . . . . . . . . . . . . . . . . . . . .   4
     2.1.  Link Relation Types . . . . . . . . . . . . . . . . . . .   5
       2.1.1.  Registered Relation Types . . . . . . . . . . . . . .   5
       2.1.2.  Extension Relation Types  . . . . . . . . . . . . . .   7
     2.2.  Target Attributes . . . . . . . . . . . . . . . . . . . .   8
   3.  Link Serialisation in HTTP Headers  . . . . . . . . . . . . .   8
     3.1.  Link Target . . . . . . . . . . . . . . . . . . . . . . .   9
     3.2.  Link Context  . . . . . . . . . . . . . . . . . . . . . .   9
     3.3.  Relation Type . . . . . . . . . . . . . . . . . . . . . .  10
     3.4.  Target Attributes . . . . . . . . . . . . . . . . . . . .  11
       3.4.1.  Serialisation-Defined Attributes  . . . . . . . . . .  11
       3.4.2.  Extension Attributes  . . . . . . . . . . . . . . . .  12
     3.5.  Link Header Field Examples  . . . . . . . . . . . . . . .  12
   4.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  14

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     4.1.  Link HTTP Header Field Registration . . . . . . . . . . .  14
     4.2.  Link Relation Type Registry . . . . . . . . . . . . . . .  14
     4.3.  Link Relation Application Data Registry . . . . . . . . .  14
   5.  Security Considerations . . . . . . . . . . . . . . . . . . .  14
   6.  Internationalisation Considerations . . . . . . . . . . . . .  15
   7.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  15
     7.1.  Normative References  . . . . . . . . . . . . . . . . . .  15
     7.2.  Informative References  . . . . . . . . . . . . . . . . .  17
   Appendix A.  Notes on Other Link Serialisations . . . . . . . . .  18
     A.1.  Link Serialisation in HTML  . . . . . . . . . . . . . . .  18
     A.2.  Link Serialisation in Atom  . . . . . . . . . . . . . . .  18
   Appendix B.  Algorithms for Parsing Link Header Fields  . . . . .  19
     B.1.  Parsing a Header Set for Links  . . . . . . . . . . . . .  19
     B.2.  Parsing a Link Field Value  . . . . . . . . . . . . . . .  20
     B.3.  Parsing Parameters  . . . . . . . . . . . . . . . . . . .  22
     B.4.  Parsing a Quoted String . . . . . . . . . . . . . . . . .  23
   Appendix C.  Changes from RFC5988 . . . . . . . . . . . . . . . .  24
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .  25

1.  Introduction

   This specification defines a model for 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", "NOT RECOMMENDED", "MAY", and
   "OPTIONAL" in this document are to be interpreted as described in BCP
   14 [RFC2119],[RFC8174] when, and only when, they appear in all
   capitals, as shown here.

   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.

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   Additionally, the following rules are included from [RFC3986]: URI
   and URI-Reference; from [RFC6838]: type-name and subtype-name; from
   [W3C.REC-css3-mediaqueries-20120619]: media-query-list; and from
   [RFC5646]: Language-Tag.

1.2.  Conformance and Error Handling

   The requirements regarding conformance and error handling highlighted
   in [RFC7230], Section 2.5 apply to this document.

2.  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 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_".

   For example, https://www.example.com/ has a "canonical" resource at
   https://example.com, which has a "type" of "text/html".

   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

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   [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 used 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 (and optionally in the Link
   header field), whereas the application "AtomPub" uses the "edit" and
   "edit-media" link relations in the Atom serialisation.

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 [RFC2046];
   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.

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   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
   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 applications.  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) can also be included, but is not required.

   The expert(s) can define additional fields to be collected in the
   registry.

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

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   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 otherwise.  Such registrations still
   are subject to the requirements defined, including the need to
   reference a specification.

2.1.1.2.  Registration Request Processing

   Relation types are registered using the Specification Required policy
   (see Section 4.6 of [RFC8126]), which implies review and approval by
   a designated expert.

   The goal of the registry is to reflect common use of links on the
   Internet.  Therefore, the expert(s) should be strongly biased towards
   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 expert(s)
   can withhold registration of names that are too general for the
   proposed application.

   The expert(s) will 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

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   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.  Those creating and maintaining
   serialisations SHOULD coordinate their target attributes to avoid
   conflicts in semantics or syntax, and MAY define their own registries
   of target attributes.

   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:

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     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.  In other words, the following
   parameters are equivalent:

     x=y
     x="y"

   Previous definitions of the Link header did not equate the token and
   quoted-string forms explicitly; the "title" parameter was always
   quoted, and the "hreflang" parameter was always a token.  Senders
   wishing to maximize interoperability will send them in those 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 establishes the link-params "rel", "anchor", and
   "rev" (which are part of the general link model), as well as
   "hreflang", "media", "title", "title*", and "type" (which are target
   attributes defined by the serialization.

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
   appearing in the message's content is not applied.

3.2.  Link Context

   By default, the context of a link conveyed in the Link header field
   is the URL 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:

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     URI-Reference ; Section 4.1 of {{RFC3986}}

   Link application 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; link applications 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 be
   present but MUST NOT appear more than once in a given link-value;
   occurrences after the first MUST be ignored by parsers.

   The "rel" parameter can, however, contain multiple link relation
   types.  When this occurs, it establishes multiple links that share
   the same context, target, and target attributes.

   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:

     relation-type  = reg-rel-type / ext-rel-type
     reg-rel-type   = LOALPHA *( LOALPHA / DIGIT / "." / "-" )
     ext-rel-type   = URI ; Section 3 of {{RFC3986}}

   Note that extension relation types are REQUIRED to be absolute URIs
   in Link header fields, and MUST be quoted when they contain
   characters not allowed in tokens, such as semicolon (";") or comma
   (",") (as these characters are used as delimiters in the header field
   itself).

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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
   (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

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   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,
   applications 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 ; see {{RFC6838}}, Section 4.2

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; applications 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"

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

   Similarly,

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

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   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".

   Finally, this header field:

   Link: <https://example.org/>; rel="start",
         <https://example.org/index>; rel="index"

   is equivalent to these:

   Link: <https://example.org/>; rel="start"
   Link: <https://example.org/index>; rel="index"

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4.  IANA Considerations

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):
       [this document]

4.2.  Link Relation Type Registry

   This specification updates the registration procedures for the Link
   Relation Type registry; see Section 2.1.1.1.

   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.

   Note that the expert(s) are allowed (as per Section 2.1.1.1) to
   define additional fields to be collected in the registry.

4.3.  Link Relation Application Data Registry

   This specification removes the Link Relation Application Data
   Registry, as it has not been used, and future use is not anticipated.
   IANA is instructed to remove it.

5.  Security Considerations

   The content of the Link header field is not secure, private or
   integrity-guaranteed.  Use of Transport Layer Security (TLS) with
   HTTP ([RFC2818]) is currently the only end-to-end way to provide
   these properties.

   Link applications ought to consider the attack vectors opened by
   automatically following, trusting, or otherwise using links gathered
   from HTTP header fields.

   For example, Link header fields that use the "anchor" parameter to
   associate a link's context with another resource cannot be trusted
   since they are effectively assertions by a third party that could be

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   incorrect or malicious.  Applications can mitigate this risk by
   specifying that such links should be discarded unless some
   relationship between the resources is established (e.g., they share
   the same authority).

   Dereferencing links has a number of risks, depending on the
   application in use.  For example, the Referer header [RFC7231] can
   expose information about the application's state (including private
   information) in its value.  Likewise, cookies [RFC6265] are another
   mechanism that, if used, can become an attack vector.  Applications
   can mitigate these risks by carefully specifying how such mechanisms
   should operate.

   The Link header field makes extensive use of IRIs and URIs.  See
   [RFC3987] Section 8 for security considerations relating to IRIs.
   See [RFC3986] Section 7 for security considerations relating to URIs.
   See [RFC7230] Section 9 for security considerations relating to HTTP
   header fields.

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.

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.

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   [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>.

   [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>.

   [RFC8126]  Cotton, M., Leiba, B., and T. Narten, "Guidelines for
              Writing an IANA Considerations Section in RFCs", BCP 26,
              RFC 8126, DOI 10.17487/RFC8126, June 2017,
              <http://www.rfc-editor.org/info/rfc8126>.

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <http://www.rfc-editor.org/info/rfc8174>.

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   [W3C.REC-css3-mediaqueries-20120619]
              Rivoal, F., "Media Queries", World Wide Web Consortium
              Recommendation REC-css3-mediaqueries-20120619, June 2012,
              <http://www.w3.org/TR/2012/
              REC-css3-mediaqueries-20120619>.

7.2.  Informative References

   [RFC2046]  Freed, N. and N. Borenstein, "Multipurpose Internet Mail
              Extensions (MIME) Part Two: Media Types", RFC 2046,
              DOI 10.17487/RFC2046, November 1996,
              <http://www.rfc-editor.org/info/rfc2046>.

   [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>.

   [RFC6265]  Barth, A., "HTTP State Management Mechanism", RFC 6265,
              DOI 10.17487/RFC6265, April 2011,
              <http://www.rfc-editor.org/info/rfc6265>.

   [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".

   Section 4.8 of HTML5 ([W3C.REC-html5-20141028]) 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.  Algorithms for Parsing Link Header Fields

   This appendix outlines a set of non-normative algorithms: for parsing
   the Link header(s) out of a header set, parsing a link header field
   value, and algorithms for parsing generic parts of the field value.

   These algorithms are more permissive than the ABNF defining the
   syntax might suggest; the error handling embodied in them is a
   reasonable approach, but not one that is required.  As such they are
   advisory only, and in cases where there is disagreement, the correct
   behaviour is defined by the body of this specification.

B.1.  Parsing a Header Set for Links

   This algorithm can be used to parse the Link header fields that a
   HTTP header set contains.  Given a "header_set" of (string
   "field_name", string "field_value") pairs, assuming ASCII encoding,
   it returns a list of link objects.

   1.  Let "field_values" be a list containing the members of
       "header_set" whose "field_name" is a case-insensitive match for
       "link".

   2.  Let "links" be an empty list.

   3.  For each "field_value" in "field_values":

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       1.  Let "value_links" be the result of _Parsing A Link Field
           Value_ (Appendix B.2) from "field_value".

       2.  Append each member of "value_links" to "links".

   4.  Return "links".

B.2.  Parsing a Link Field Value

   This algorithm parses zero or more comma-separated link-values from a
   Link header field.  Given a string "field_value", assuming ASCII
   encoding, it returns a list of link objects.

   1.  Let "links" be an empty list.

   2.  While "field_value" has content:

       1.   Consume any leading OWS.

       2.   If the first character is not "<", return "links".

       3.   Discard the first character ("<").

       4.   Consume up to but not including the first ">" character or
            end of "field_value" and let the result be "target_string".

       5.   If the next character is not ">", return "links".

       6.   Discard the leading ">" character.

       7.   Let "link_parameters", be the result of _Parsing Parameters_
            (Appendix B.3) from "field_value" (consuming zero or more
            characters of it).

       8.   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.

       9.   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.

       10.  Split "relations_string" on RWS (removing it in the process)
            into a list of strings "relation_types".

       11.  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 URL

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            of the representation carrying the Link header [RFC7231],
            Section 3.1.4.1, serialised as a URI.  Where the URL is
            anonymous, "context_string" is null.

       12.  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.

       13.  Let "target_attributes" be an empty list.

       14.  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".

       15.  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
            ("*").

       16.  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".

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

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

       17.  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".

   3.  Return "links".

B.3.  Parsing Parameters

   This algorithm parses the parameters from a header field value.
   Given an ASCII string "input", it returns a list of (string
   "parameter_name", string "parameter_value") tuples that it contains.
   "input" is modified to remove the parsed parameters.

   1.  Let "parameters" be an empty list.

   2.  While "input" has content:

       1.   Consume any leading OWS.

       2.   If the first character is not ";", return "parameters".

       3.   Discard the leading ";" character.

       4.   Consume any leading OWS.

       5.   Consume up to but not including the first BWS, "=", ";", ","
            character or end of "input" and let the result be
            "parameter_name".

       6.   Consume any leading BWS.

       7.   If the next character is "=":

            1.  Discard the leading "=" character.

            2.  Consume any leading BWS.

            3.  If the next character is DQUOTE, let "parameter_value"
                be the result of _Parsing a Quoted String_

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                (Appendix B.4) from "input" (consuming zero or more
                characters of it).

            4.  Else, consume the contents up to but not including the
                first ";", "," character or end of "input" and let the
                results be "parameter_value".

            5.  If the last character of "parameter_name" is an asterisk
                ("*"), decode "parameter_value" according to
                [I-D.ietf-httpbis-rfc5987bis].  Continue processing
                "input" if an unrecoverable error is encountered.

       8.   Else:

            1.  Let "parameter_value" be an empty string.

       9.   Case-normalise "parameter_name" to lowercase.

       10.  Append ("parameter_name", "parameter_value") to
            "parameters".

       11.  Consume any leading OWS.

       12.  If the next character is "," or the end of "input", stop
            processing "input" and return "parameters".

B.4.  Parsing a Quoted String

   This algorithm parses a quoted string, as per [RFC7230],
   Section 3.2.6.  Given an ASCII string "input", it returns an unquoted
   string. "input" is modified to remove the parsed string.

   1.  Let "output" be an empty string.

   2.  If the first character of "input" is not DQUOTE, return "output".

   3.  Discard the first character.

   4.  While "input" has content:

       1.  If the first character is a backslash ("\"):

           1.  Discard the first character.

           2.  If there is no more "input", return "output".

           3.  Else, consume the first character and append it to
               "output".

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       2.  Else, if the first character is DQUOTE, discard it and return
           "output".

       3.  Else, consume the first character and append it to "output".

   5.  Return "output".

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 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).

   o  Defined a suggested parsing algorithm for the Link header.

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

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   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  Parameters on the HTTP header can now be value-less.

   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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