< draft-nottingham-rfc5988bis-01.txt   draft-nottingham-rfc5988bis-02.txt >
Network Working Group M. Nottingham Network Working Group M. Nottingham
Internet-Draft May 3, 2016 Internet-Draft November 22, 2016
Obsoletes: 5988 (if approved) Obsoletes: 5988 (if approved)
Intended status: Standards Track Intended status: Standards Track
Expires: November 4, 2016 Expires: May 26, 2017
Web Linking Web Linking
draft-nottingham-rfc5988bis-01 draft-nottingham-rfc5988bis-02
Abstract Abstract
This specification defines a way to indicate the relationships This specification defines a way to indicate the relationships
between resources on the Web ("links") and the type of those between resources on the Web ("links") and the type of those
relationships ("link relation types"). relationships ("link relation types").
It also defines the use of such links in HTTP headers with the Link It also defines the serialisation of such links in HTTP headers with
header field. the Link header field.
Note to Readers Note to Readers
This is a work-in-progress to revise RFC5988. This is a work-in-progress to revise RFC5988.
The issues list can be found at https://github.com/mnot/I-D/labels/ The issues list can be found at https://github.com/mnot/I-D/labels/
rfc5988bis . rfc5988bis .
The most recent (often, unpublished) draft is at The most recent (often, unpublished) draft is at
https://mnot.github.io/I-D/rfc5988bis/ . https://mnot.github.io/I-D/rfc5988bis/ .
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Internet-Drafts are working documents of the Internet Engineering Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet- working documents as Internet-Drafts. The list of current Internet-
Drafts is at http://datatracker.ietf.org/drafts/current/. Drafts is at http://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress." material or to cite them other than as "work in progress."
This Internet-Draft will expire on November 4, 2016. This Internet-Draft will expire on May 26, 2017.
Copyright Notice Copyright Notice
Copyright (c) 2016 IETF Trust and the persons identified as the Copyright (c) 2016 IETF Trust and the persons identified as the
document authors. All rights reserved. document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of (http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents publication of this document. Please review these documents
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the copyright in such materials, this document may not be modified the copyright in such materials, this document may not be modified
outside the IETF Standards Process, and derivative works of it may outside the IETF Standards Process, and derivative works of it may
not be created outside the IETF Standards Process, except to format not be created outside the IETF Standards Process, except to format
it for publication as an RFC or to translate it into languages other it for publication as an RFC or to translate it into languages other
than English. than English.
Table of Contents Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Notational Conventions . . . . . . . . . . . . . . . . . . . 3 2. Notational Conventions . . . . . . . . . . . . . . . . . . . 3
3. Links . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 3. Links . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
4. Link Relation Types . . . . . . . . . . . . . . . . . . . . . 5 4. Link Relation Types . . . . . . . . . . . . . . . . . . . . . 5
4.1. Registered Relation Types . . . . . . . . . . . . . . . . 5 4.1. Registered Relation Types . . . . . . . . . . . . . . . . 5
4.1.1. Registering Link Relation Types . . . . . . . . . . . 6 4.1.1. Registering Link Relation Types . . . . . . . . . . . 6
4.2. Extension Relation Types . . . . . . . . . . . . . . . . 7 4.2. Extension Relation Types . . . . . . . . . . . . . . . . 7
5. Link Serialisation in the Link HTTP Header Field . . . . . . 7 5. Target Attributes . . . . . . . . . . . . . . . . . . . . . . 7
5.1. Link Target . . . . . . . . . . . . . . . . . . . . . . . 8 6. Link Serialisation in HTTP Headers . . . . . . . . . . . . . 8
5.2. Link Context . . . . . . . . . . . . . . . . . . . . . . 8 6.1. Link Target . . . . . . . . . . . . . . . . . . . . . . . 8
5.3. Relation Type . . . . . . . . . . . . . . . . . . . . . . 9 6.2. Link Context . . . . . . . . . . . . . . . . . . . . . . 8
5.4. Target Attributes . . . . . . . . . . . . . . . . . . . . 9 6.3. Relation Type . . . . . . . . . . . . . . . . . . . . . . 9
5.5. Examples . . . . . . . . . . . . . . . . . . . . . . . . 10 6.4. Target Attributes . . . . . . . . . . . . . . . . . . . . 9
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 11 6.4.1. Serialisation-Defined Attributes . . . . . . . . . . 10
6.1. Link HTTP Header Field Registration . . . . . . . . . . . 11 6.4.2. Extension Attributes . . . . . . . . . . . . . . . . 11
6.2. Link Relation Type Registry . . . . . . . . . . . . . . . 11 6.5. Examples . . . . . . . . . . . . . . . . . . . . . . . . 11
7. Security Considerations . . . . . . . . . . . . . . . . . . . 12 7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 12
8. Internationalisation Considerations . . . . . . . . . . . . . 12 7.1. Link HTTP Header Field Registration . . . . . . . . . . . 12
9. References . . . . . . . . . . . . . . . . . . . . . . . . . 13 7.2. Link Relation Type Registry . . . . . . . . . . . . . . . 12
9.1. Normative References . . . . . . . . . . . . . . . . . . 13
9.2. Informative References . . . . . . . . . . . . . . . . . 14 8. Security Considerations . . . . . . . . . . . . . . . . . . . 13
Appendix A. Link Serialisation in HTML . . . . . . . . . . . . . 16 9. Internationalisation Considerations . . . . . . . . . . . . . 13
Appendix B. Link Serialisation in Atom . . . . . . . . . . . . . 16 10. References . . . . . . . . . . . . . . . . . . . . . . . . . 14
Appendix C. Changes from RFC5988 . . . . . . . . . . . . . . . . 17 10.1. Normative References . . . . . . . . . . . . . . . . . . 14
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 18 10.2. Informative References . . . . . . . . . . . . . . . . . 15
Appendix A. Notes on Other Link Serialisations . . . . . . . . . 17
A.1. Link Serialisation in HTML . . . . . . . . . . . . . . . 17
A.2. Link Serialisation in Atom . . . . . . . . . . . . . . . 18
Appendix B. Algorithm for Parsing Link Headers . . . . . . . . . 18
Appendix C. Changes from RFC5988 . . . . . . . . . . . . . . . . 21
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 22
1. Introduction 1. Introduction
This specification defines a way to indicate the relationships This specification defines a way to indicate the relationships
between resources on the Web ("links") and the type of those between resources on the Web ("links") and the type of those
relationships ("link relation types"). relationships ("link relation types").
HTML [W3C.REC-html5-20141028] and Atom [RFC4287] both have well- HTML [W3C.REC-html5-20141028] and Atom [RFC4287] both have well-
defined concepts of linking; this specification generalises this into defined concepts of linking; this specification generalises this into
a framework that encompasses linking in these formats and a framework that encompasses linking in these formats and
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2. Notational Conventions 2. Notational Conventions
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in BCP 14, [RFC2119], as document are to be interpreted as described in BCP 14, [RFC2119], as
scoped to those conformance targets. scoped to those conformance targets.
This document uses the Augmented Backus-Naur Form (ABNF) notation of This document uses the Augmented Backus-Naur Form (ABNF) notation of
[RFC7230], including the #rule, and explicitly includes the following [RFC7230], including the #rule, and explicitly includes the following
rules from it: quoted-string, token, SP (space), LOALPHA, DIGIT. 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 Additionally, the following rules are included from [RFC3986]: URI
and URI-Reference; from [RFC6838]: type-name and subtype-name; from and URI-Reference; from [RFC6838]: type-name and subtype-name; from
[W3C.CR-css3-mediaqueries-20090915]: media_query_list; from [W3C.CR-css3-mediaqueries-20090915]: media_query_list; from
[RFC5646]: Language-Tag; and from [I-D.ietf-httpbis-rfc5987bis], ext- [RFC5646]: Language-Tag; and from [I-D.ietf-httpbis-rfc5987bis], ext-
value and parmname. value and parmname.
3. Links 3. Links
In this specification, a link is a typed connection between two In this specification, a link is a typed connection between two
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value and parmname. value and parmname.
3. Links 3. Links
In this specification, a link is a typed connection between two In this specification, a link is a typed connection between two
resources, and is comprised of: resources, and is comprised of:
o A _link context_, o A _link context_,
o a _link relation type_ (Section 4), o a _link relation type_ (Section 4),
o a _link target_, and o a _link target_, and
o optionally, _target attributes_. o optionally, _target attributes_ (Section 5).
A link can be viewed as a statement of the form "{link context} has a 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 {link relation type} resource at {link target}, which has {target
attributes}". attributes}".
Link contexts and link targets are both IRIs [RFC3987]. However, in Link contexts and link targets are both IRIs [RFC3987]. However, in
the common case, the link context will also be a URI [RFC3986], the common case, the link context will also be a URI [RFC3986],
because many protocols (such as HTTP) do not support dereferencing because many protocols (such as HTTP) do not support dereferencing
IRIs. Likewise, the link target will be sometimes be converted to a IRIs. Likewise, the link target will be sometimes be converted to a
URI (see [RFC3987], Section 3.1) in places that do not support IRIs URI (see [RFC3987], Section 3.1) in places that do not support IRIs
(such as the Link header field defined in Section 5). (such as the Link header field defined in Section 6).
This specification does not place restrictions on the cardinality of This specification does not place restrictions on the cardinality of
links; there can be multiple links to and from a particular target, links; there can be multiple links to and from a particular target,
and multiple links of the same or different types between a given and multiple links of the same or different types between a given
context and target. Likewise, the relative ordering of links in any context and target. Likewise, the relative ordering of links in any
particular serialisation, or between serialisations (e.g., the Link particular serialisation, or between serialisations (e.g., the Link
header field and in-content links) is not specified or significant in header field and in-content links) is not specified or significant in
this specification; applications that wish to consider ordering this specification; applications that wish to consider ordering
significant can do so. significant can do so.
_Target attributes_ are a set of key/value pairs that describe the
link or its target; for example, a media type hint. This
specification does not attempt to coordinate their names, cardinality
or use, but individual link relations, link serialisations and link
applications can do so. This specification does provide common
target attributes for use in the Link HTTP header field.
Links are conveyed in _link serialisations_; they are the "bytes on Links are conveyed in _link serialisations_; they are the "bytes on
the wire", and can occur in various forms. This specification does the wire", and can occur in various forms. This specification does
not define a general syntax for links, nor does it mandate a specific not define a general syntax for links, nor does it mandate a specific
context for any given link; it is expected that serialisations of context for any given link; it is expected that serialisations of
links will specify both aspects. One such serialisation is links will specify both aspects. One such serialisation is
communication of links through HTTP headers, specified in Section 5. communication of links through HTTP headers, specified in Section 6.
Finally, links are consumed by _link applications_. Generally, an Finally, links are consumed by _link applications_. Generally, an
application will define the link relation types it uses, along with application will define the link relation types it uses, along with
the serialisations that they might occur within. For example, the the serialisations that they might occur within. For example, the
application "Web browsing" looks for the "stylesheet" link relation application "Web browsing" looks for the "stylesheet" link relation
type in the HTML link serialisation. type in the HTML link serialisation.
4. Link Relation Types 4. Link Relation Types
In the simplest case, a link relation type identifies the semantics In the simplest case, a link relation type identifies the semantics
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particular application, the name should reflect that, so that more particular application, the name should reflect that, so that more
general names are available for less specific use. general names are available for less specific use.
Registered relation types MUST NOT constrain the media type of the Registered relation types MUST NOT constrain the media type of the
link context, and MUST NOT constrain the available representation link context, and MUST NOT constrain the available representation
media types of the link target. However, they can specify the media types of the link target. However, they can specify the
behaviours and properties of the target resource (e.g., allowable behaviours and properties of the target resource (e.g., allowable
HTTP methods, request and response media types that must be HTTP methods, request and response media types that must be
supported). supported).
Historically, applications have sometimes referred to registered Historically, registered relation types have been identified with a
relation types with a URI [RFC3986] (e.g., Appendix B) by prefixing URI [RFC3986] by prefixing their names with an application-defined
their names with an application-defined base URI. This practice is base URI (e.g., see Appendix A.2). This practice is NOT RECOMMENDED,
NOT RECOMMENDED, because the resulting strings will not be considered because the resulting strings will not be considered equivalent to
equivalent to the registered relation types by other processors. the registered relation types by other processors. Applications that
Applications that do use such URIs internally MUST NOT use them in do use such URIs internally MUST NOT use them in link serialisations
link serialisations that do not explicitly accommodate them. that do not explicitly accommodate them.
4.1.1. Registering Link Relation Types 4.1.1. Registering Link Relation Types
Relation types are registered on the advice of a Designated Expert Relation types are registered on the advice of a Designated Expert
(appointed by the IESG or their delegate), with a Specification (appointed by the IESG or their delegate), with a Specification
Required (using terminology from [RFC5226]). Required (using terminology from [RFC5226]).
The Expert(s) will establish procedures for requesting registrations, The Expert(s) will establish procedures for requesting registrations,
and make them available from the registry page. and make them available from the registry page.
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Applications that don't wish to register a relation type can use an Applications that don't wish to register a relation type can use an
extension relation type, which is a URI [RFC3986] that uniquely extension relation type, which is a URI [RFC3986] that uniquely
identifies the relation type. Although the URI can point to a identifies the relation type. Although the URI can point to a
resource that contains a definition of the semantics of the relation resource that contains a definition of the semantics of the relation
type, clients SHOULD NOT automatically access that resource to avoid type, clients SHOULD NOT automatically access that resource to avoid
overburdening its server. overburdening its server.
The URI used for an extension relation type SHOULD be under the 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. control of the person or party defining it, or be delegated to them.
These URIs also SHOULD NOT use the base URI defined by an application
for registered relation types (as per Section 4.1).
When extension relation types are compared, they MUST be compared as When extension relation types are compared, they MUST be compared as
strings (after converting to URIs if serialised in a different strings (after converting to URIs if serialised in a different
format, such as a XML QNames [W3C.REC-xml-names-20091208]) in a case- format, such as a XML QNames [W3C.REC-xml-names-20091208]) in a case-
insensitive fashion, character-by-character. Because of this, all- insensitive fashion, character-by-character. Because of this, all-
lowercase URIs SHOULD be used for extension relations. lowercase URIs SHOULD be used for extension relations.
Note that while extension relation types are required to be URIs, a Note that while extension relation types are required to be URIs, a
serialisation of links can specify that they are expressed in another serialisation of links can specify that they are expressed in another
form, as long as they can be converted to URIs. form, as long as they can be converted to URIs.
5. Link Serialisation in the Link HTTP Header Field 5. Target Attributes
_Target attributes_ are a set of key/value pairs that describe the
link or its target; for example, a media type hint.
This specification does not attempt to coordinate the name of target
attributes, their cardinality or use; they are defined both by
individual link relations and by link serialisations.
Serialisations SHOULD coordinate their target attributes to avoid
conflicts in semantics or syntax. Relation types MAY define
additional target attributes specific to them.
The names of target attributes SHOULD conform to the parmname rule
for portability across serializations, and MUST be compared in a
case-insensitive fashion.
Target attribute definitions SHOULD specify:
o Their serialisation 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
attribute on a given link.
This specification does define target attributes for use in the Link
HTTP header field in Section 6.4.
6. Link Serialisation in HTTP Headers
The Link header field provides a means for serialising one or more The Link header field provides a means for serialising one or more
links into HTTP headers. links into HTTP headers.
Link = "Link" ":" #link-value The ABNF for the field value is given below:
link-value = "<" URI-Reference ">" *( ";" link-param )
link-param = ( ( "rel" "=" relation-types )
| ( "anchor" "=" <"> URI-Reference <"> )
| ( "rev" "=" relation-types )
| ( "hreflang" "=" Language-Tag )
| ( "media" "="
( media_query_list | ( <"> media_query_list <"> ) )
)
| ( "title" "=" quoted-string )
| ( "title*" "=" ext-value )
| ( "type" "=" ( media-type | quoted-mt ) )
| ( link-extension ) )
link-extension = ( parmname [ "=" ( ptoken | quoted-string ) ] )
| ( ext-name-star "=" ext-value )
ext-name-star = parmname "*" ; reserved for RFC5987-profiled
; extensions. Whitespace NOT
; allowed in between.
ptoken = 1*ptokenchar
ptokenchar = "!" | "#" | "$" | "%" | "&" | "'" | "("
| ")" | "*" | "+" | "-" | "." | "/" | DIGIT
| ":" | "<" | "=" | ">" | "?" | "@" | ALPHA
| "[" | "]" | "^" | "_" | "`" | "{" | "|"
| "}" | "~"
media-type = type-name "/" subtype-name
quoted-mt = <"> media-type <">
relation-types = relation-type
| <"> relation-type *( 1*SP relation-type ) <">
relation-type = reg-rel-type | ext-rel-type
reg-rel-type = LOALPHA *( LOALPHA | DIGIT | "." | "-" )
ext-rel-type = URI
5.1. Link Target 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 6.2,
Section 6.3 and Section 6.4.
6.1. Link Target
Each link-value conveys one target IRI as a URI-Reference (after Each link-value conveys one target IRI as a URI-Reference (after
conversion to one, if necessary; see [RFC3987], Section 3.1) inside conversion to one, if necessary; see [RFC3987], Section 3.1) inside
angle brackets ("<>"). If the URI-Reference is relative, parsers angle brackets ("<>"). If the URI-Reference is relative, parsers
MUST resolve it as per [RFC3986], Section 5. Note that any base IRI MUST resolve it as per [RFC3986], Section 5. Note that any base IRI
from the message's content is not applied. from the message's content is not applied.
5.2. Link Context 6.2. Link Context
By default, the context of a link conveyed in the Link header field By default, the context of a link conveyed in the Link header field
is identity of the representation it is associated with, as defined is identity of the representation it is associated with, as defined
in [RFC7231], Section 3.1.4.1, serialised as a URI. in [RFC7231], Section 3.1.4.1, serialised as a URI.
When present, the anchor parameter overrides this with another 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 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 the anchor value is an absolute URI). If the anchor parameter's
value is a relative URI, parsers MUST resolve it as per [RFC3986], 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 Section 5. Note that any base URI from the body's content is not
applied. applied.
The ABNF for the "anchor" parameter's value is: ~~~ abnf2616 URI-
Reference ~~~
Consuming implementations can choose to ignore links with an anchor Consuming implementations can choose to ignore links with an anchor
parameter. For example, the application in use might not allow the parameter. For example, the application in use might not allow the
link context to be assigned to a different resource. In such cases, link context to be assigned to a different resource. In such cases,
the entire link is to be ignored; consuming implementations MUST NOT the entire link is to be ignored; consuming implementations MUST NOT
process the link without applying the anchor. process the link without applying the anchor.
Note that depending on HTTP status code and response headers, the Note that depending on HTTP status code and response headers, the
link context might be "anonymous" (i.e., no link context is link context might be "anonymous" (i.e., no link context is
available). For instance, this is the case on a 404 response to a available). For instance, this is the case on a 404 response to a
GET request. GET request.
5.3. Relation Type 6.3. Relation Type
The relation type of a link is conveyed in the "rel" parameter's The relation type of a link conveyed in the Link header field is
value. The "rel" parameter MUST NOT appear more than once in a given conveyed in the "rel" parameter's value. The "rel" parameter MUST
link-value; occurrences after the first MUST be ignored by parsers. 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 The "rev" parameter has been used in the past to indicate that the
semantics of the relationship are in the reverse direction. That is, 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 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 link from B to A with REV="X". "rev" is deprecated by this
specification because it often confuses authors and readers; in most specification because it often confuses authors and readers; in most
cases, using a separate relation type is preferable. cases, using a separate relation type is preferable.
The ABNF for the "rel" and "rev" parameters' values is: ~~~ abnf2616
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
Note that extension relation types are REQUIRED to be absolute URIs Note that extension relation types are REQUIRED to be absolute URIs
in Link headers, and MUST be quoted if they contain a semicolon (";") in Link headers, and MUST be quoted if they contain a semicolon (";")
or comma (",") (as these characters are used as delimiters in the or comma (",") (as these characters are used as delimiters in the
header field itself). header field itself).
5.4. Target Attributes 6.4. Target Attributes
The "hreflang", "media", "title", "title*", "type", and any link- The Link header field defines several target attributes specific to
extension link-params are considered to be target attributes for the 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).
6.4.1. Serialisation-Defined Attributes
The "hreflang", "media", "title", "title*", and "type" link-params
can be translated to serialisation-defined target attributes for the
link. link.
The "hreflang" parameter, when present, is a hint indicating what the The "hreflang" attribute, when present, is a hint indicating what the
language of the result of dereferencing the link should be. Note language of the result of dereferencing the link should be. Note
that this is only a hint; for example, it does not override the that this is only a hint; for example, it does not override the
Content-Language header field of a HTTP response obtained by actually Content-Language header field of a HTTP response obtained by actually
following the link. Multiple "hreflang" parameters on a single link- following the link. Multiple "hreflang" attributes on a single link-
value indicate that multiple languages are available from the value indicate that multiple languages are available from the
indicated resource. indicated resource.
The "media" parameter, when present, is used to indicate intended The ABNF for the "hreflang" parameter's value is: ~~~ abnf2616
Language-Tag ~~~
The "media" attribute, when present, is used to indicate intended
destination medium or media for style information (see destination medium or media for style information (see
[W3C.REC-html5-20141028], Section 4.2.4). Its value MUST be quoted [W3C.REC-html5-20141028], Section 4.2.4). Its value MUST be quoted
if it contains a semicolon (";") or comma (","), and there MUST NOT if it contains a semicolon (";") or comma (","). There MUST NOT be
be more than one "media" parameter in a link-value. more than one "media" attribute in a link-value; occurrences after
the first MUST be ignored by parsers.
The "title" parameter, when present, is used to label the destination The ABNF for the "media" parameter's value is: ~~~ abnf2616
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 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- (e.g., a menu entry) in the language indicated by the Content-
Language header field (if present). The "title" parameter MUST NOT Language header field (if present). The "title" attribute MUST NOT
appear more than once in a given link-value; occurrences after the appear more than once in a given link; occurrences after the first
first MUST be ignored by parsers. MUST be ignored by parsers.
The "title*" parameter can be used to encode this label in a The "title*" link-param can be used to encode this attribute in a
different character set, and/or contain language information as per different character set, and/or contain language information as per
[I-D.ietf-httpbis-rfc5987bis]. The "title*" parameter MUST NOT [I-D.ietf-httpbis-rfc5987bis]. The "title*" link-param MUST NOT
appear more than once in a given link-value; occurrences after the appear more than once in a given link-value; occurrences after the
first MUST be ignored by parsers. If the parameter does not contain first MUST be ignored by parsers. If the attribute does not contain
language information, its language is indicated by the Content- language information, its language is indicated by the Content-
Language header field (when present). Language header field (when present).
If both the "title" and "title*" parameters appear in a link-value, If both the "title" and "title*" link-param appear in a link,
processors SHOULD use the "title*" parameter's value. processors SHOULD use the "title*" link-param's value for the "title"
attribute.
The "type" parameter, when present, is a hint indicating what the The "type" attribute, when present, is a hint indicating what the
media type of the result of dereferencing the link should be. Note 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 that this is only a hint; for example, it does not override the
Content-Type header field of a HTTP response obtained by actually Content-Type header field of a HTTP response obtained by actually
following the link. The "type" parameter MUST NOT appear more than following the link. The "type" attribute MUST NOT appear more than
once in a given link-value; occurrences after the first MUST be once in a given link-value; occurrences after the first MUST be
ignored by parsers. ignored by parsers.
5.5. Examples The ABNF for the "type" parameter's value is: ~~~ abnf2616 type-name
"/" subtype-name ~~~
6.4.2. 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.
6.5. Examples
For example: For example:
Link: <http://example.com/TheBook/chapter2>; rel="previous"; Link: <http://example.com/TheBook/chapter2>; rel="previous";
title="previous chapter" title="previous chapter"
indicates that "chapter2" is previous to this resource in a logical indicates that "chapter2" is previous to this resource in a logical
navigation path. navigation path.
Similarly, Similarly,
Link: </>; rel="http://example.net/foo" Link: </>; rel="http://example.net/foo"
indicates that the root resource ("/") is related to this resource indicates that the root resource ("/") is related to this resource
with the extension relation type "http://example.net/foo". with the extension relation type "http://example.net/foo".
The example below shows an instance of the Link header field encoding 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 multiple links, and also the use of RFC 5987 encoding to encode both
non-ASCII characters and language information. non-ASCII characters and language information.
Link: </TheBook/chapter2>; Link: </TheBook/chapter2>;
rel="previous"; title*=UTF-8'de'letztes%20Kapitel, rel="previous"; title*=UTF-8'de'letztes%20Kapitel,
</TheBook/chapter4>; </TheBook/chapter4>;
rel="next"; title*=UTF-8'de'n%c3%a4chstes%20Kapitel rel="next"; title*=UTF-8'de'n%c3%a4chstes%20Kapitel
Here, both links have titles encoded in UTF-8, use the German Here, both links have titles encoded in UTF-8, use the German
language ("de"), and the second link contains the Unicode code point language ("de"), and the second link contains the Unicode code point
U+00E4 ("LATIN SMALL LETTER A WITH DIAERESIS"). U+00E4 ("LATIN SMALL LETTER A WITH DIAERESIS").
Note that link-values can convey multiple links between the same link Note that link-values can convey multiple links between the same link
target and link context; for example: target and link context; for example:
Link: <http://example.org/>; Link: <http://example.org/>;
rel="start http://example.net/relation/other" rel="start http://example.net/relation/other"
Here, the link to "http://example.org/" has the registered relation Here, the link to "http://example.org/" has the registered relation
type "start" and the extension relation type type "start" and the extension relation type
"http://example.net/relation/other". "http://example.net/relation/other".
6. IANA Considerations 7. IANA Considerations
In addition to the actions below, IANA should terminate the Link In addition to the actions below, IANA should terminate the Link
Relation Application Data Registry, as it has not been used, and Relation Application Data Registry, as it has not been used, and
future use is not anticipated. future use is not anticipated.
6.1. Link HTTP Header Field Registration 7.1. Link HTTP Header Field Registration
This specification updates the Message Header registry entry for This specification updates the Message Header registry entry for
"Link" in HTTP [RFC3864] to refer to this document. "Link" in HTTP [RFC3864] to refer to this document.
Header field: Link Header field: Link
Applicable protocol: http Applicable protocol: http
Status: standard Status: standard
Author/change controller: Author/change controller:
IETF (iesg@ietf.org) IETF (iesg@ietf.org)
Internet Engineering Task Force Internet Engineering Task Force
Specification document(s): Specification document(s):
[RFC&rfc.number;] [RFC&rfc.number;]
6.2. Link Relation Type Registry 7.2. Link Relation Type Registry
This specification updates the registration procedures for the Link This specification updates the registration procedures for the Link
Relation Type registry; see Section 4.1.1. The Expert(s) and IANA Relation Type registry; see Section 4.1.1. The Expert(s) and IANA
will interact as outlined below. will interact as outlined below.
IANA will direct any incoming requests regarding the registry to the IANA will direct any incoming requests regarding the registry to the
processes established by the Expert(s); typically, this will mean processes established by the Expert(s); typically, this will mean
referring them to the registry HTML page. referring them to the registry Web page.
The Expert(s) will provide registry data to IANA in an agreed form The Expert(s) will provide registry data to IANA in an agreed form
(e.g. a specific XML format). IANA will publish: * The raw registry (e.g. a specific XML format). IANA will publish:
data * The registry data, transformed into HTML * The registry data
in any alternative formats provided by the Expert(s) o The raw registry data
o The registry data, transformed into HTML
o The registry data in any alternative formats provided by the
Expert(s)
Each published document will be at a URL agreed to by IANA and the Each published document will be at a URL agreed to by IANA and the
Expert(s), and IANA will set HTTP response headers on them as Expert(s), and IANA will set HTTP response headers on them as
(reasonably) requested by the Expert(s). (reasonably) requested by the Expert(s).
Additionally, the HTML generated by IANA will: * Take directions from Additionally, the HTML generated by IANA will:
the Expert(s) as to the content of the HTML page's introductory text
and markup * Include a stable HTML fragment identifier for each o Take directions from the Expert(s) as to the content of the HTML
registered link relation page's introductory text and markup
o Include a stable HTML fragment identifier for each registered link
relation
All registry data documents MUST include Simplified BSD License text All registry data documents MUST include Simplified BSD License text
as described in Section 4.e of the Trust Legal Provisions as described in Section 4.e of the Trust Legal Provisions
(<http://trustee.ietf.org/license-info>). (<http://trustee.ietf.org/license-info>).
7. Security Considerations 8. Security Considerations
The content of the Link header field is not secure, private or The content of the Link header field is not secure, private or
integrity-guaranteed, and due caution should be exercised when using integrity-guaranteed, and due caution should be exercised when using
it. Use of Transport Layer Security (TLS) with HTTP ([RFC2818] and it. Use of Transport Layer Security (TLS) with HTTP ([RFC2818] and
[RFC2817]) is currently the only end-to-end way to provide such [RFC2817]) is currently the only end-to-end way to provide such
protection. protection.
Link applications ought to consider the attack vectors opened by Link applications ought to consider the attack vectors opened by
automatically following, trusting, or otherwise using links gathered automatically following, trusting, or otherwise using links gathered
from HTTP headers. In particular, Link header fields that use the from HTTP headers. In particular, Link header fields that use the
"anchor" parameter to associate a link's context with another "anchor" parameter to associate a link's context with another
resource should be treated with due caution. resource should be treated with due caution.
The Link header field makes extensive use of IRIs and URIs. See The Link header field makes extensive use of IRIs and URIs. See
[RFC3987] for security considerations relating to IRIs. See [RFC3987] for security considerations relating to IRIs. See
[RFC3986] for security considerations relating to URIs. See [RFC3986] for security considerations relating to URIs. See
[RFC7230] for security considerations relating to HTTP headers. [RFC7230] for security considerations relating to HTTP headers.
8. Internationalisation Considerations 9. Internationalisation Considerations
Link targets may need to be converted to URIs in order to express Link targets may need to be converted to URIs in order to express
them in serialisations that do not support IRIs. This includes the them in serialisations that do not support IRIs. This includes the
Link HTTP header field. Link HTTP header field.
Similarly, the anchor parameter of the Link header field does not Similarly, the anchor parameter of the Link header field does not
support IRIs, and therefore IRIs must be converted to URIs before support IRIs, and therefore IRIs must be converted to URIs before
inclusion there. inclusion there.
Relation types are defined as URIs, not IRIs, to aid in their Relation types are defined as URIs, not IRIs, to aid in their
comparison. It is not expected that they will be displayed to end comparison. It is not expected that they will be displayed to end
users. users.
Note that registered Relation Names are required to be lower-case Note that registered Relation Names are required to be lower-case
ASCII letters. ASCII letters.
9. References 10. References
9.1. Normative References 10.1. Normative References
[I-D.ietf-httpbis-rfc5987bis] [I-D.ietf-httpbis-rfc5987bis]
Reschke, J., "Indicating Character Encoding and Language Reschke, J., "Indicating Character Encoding and Language
for HTTP Header Field Parameters", draft-ietf-httpbis- for HTTP Header Field Parameters", draft-ietf-httpbis-
rfc5987bis-01 (work in progress), March 2016. rfc5987bis-03 (work in progress), July 2016.
[RFC2026] Bradner, S., "The Internet Standards Process -- Revision
3", BCP 9, RFC 2026, DOI 10.17487/RFC2026, October 1996,
<http://www.rfc-editor.org/info/rfc2026>.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997, DOI 10.17487/RFC2119, March 1997,
<http://www.rfc-editor.org/info/rfc2119>. <http://www.rfc-editor.org/info/rfc2119>.
[RFC3864] Klyne, G., Nottingham, M., and J. Mogul, "Registration [RFC3864] Klyne, G., Nottingham, M., and J. Mogul, "Registration
Procedures for Message Header Fields", BCP 90, RFC 3864, Procedures for Message Header Fields", BCP 90, RFC 3864,
DOI 10.17487/RFC3864, September 2004, DOI 10.17487/RFC3864, September 2004,
<http://www.rfc-editor.org/info/rfc3864>. <http://www.rfc-editor.org/info/rfc3864>.
skipping to change at page 14, line 30 skipping to change at page 15, line 26
Protocol (HTTP/1.1): Semantics and Content", RFC 7231, Protocol (HTTP/1.1): Semantics and Content", RFC 7231,
DOI 10.17487/RFC7231, June 2014, DOI 10.17487/RFC7231, June 2014,
<http://www.rfc-editor.org/info/rfc7231>. <http://www.rfc-editor.org/info/rfc7231>.
[W3C.CR-css3-mediaqueries-20090915] [W3C.CR-css3-mediaqueries-20090915]
Lie, H., Celik, T., Glazman, D., and A. Kesteren, "Media Lie, H., Celik, T., Glazman, D., and A. Kesteren, "Media
Queries", World Wide Web Consortium CR CR-css3- Queries", World Wide Web Consortium CR CR-css3-
mediaqueries-20090915, September 2009, mediaqueries-20090915, September 2009,
<http://www.w3.org/TR/2009/CR-css3-mediaqueries-20090915>. <http://www.w3.org/TR/2009/CR-css3-mediaqueries-20090915>.
9.2. Informative References 10.2. Informative References
[RFC2068] Fielding, R., Gettys, J., Mogul, J., Frystyk, H., and T. [RFC2068] Fielding, R., Gettys, J., Mogul, J., Frystyk, H., and T.
Berners-Lee, "Hypertext Transfer Protocol -- HTTP/1.1", Berners-Lee, "Hypertext Transfer Protocol -- HTTP/1.1",
RFC 2068, DOI 10.17487/RFC2068, January 1997, RFC 2068, DOI 10.17487/RFC2068, January 1997,
<http://www.rfc-editor.org/info/rfc2068>. <http://www.rfc-editor.org/info/rfc2068>.
[RFC2616] Fielding, R., Gettys, J., Mogul, J., Frystyk, H., [RFC2616] Fielding, R., Gettys, J., Mogul, J., Frystyk, H.,
Masinter, L., Leach, P., and T. Berners-Lee, "Hypertext Masinter, L., Leach, P., and T. Berners-Lee, "Hypertext
Transfer Protocol -- HTTP/1.1", RFC 2616, Transfer Protocol -- HTTP/1.1", RFC 2616,
DOI 10.17487/RFC2616, June 1999, DOI 10.17487/RFC2616, June 1999,
skipping to change at page 15, line 17 skipping to change at page 16, line 13
December 2005, <http://www.rfc-editor.org/info/rfc4287>. December 2005, <http://www.rfc-editor.org/info/rfc4287>.
[W3C.REC-html-rdfa-20150317] [W3C.REC-html-rdfa-20150317]
Sporny, M., "HTML+RDFa 1.1 - Second Edition", World Wide Sporny, M., "HTML+RDFa 1.1 - Second Edition", World Wide
Web Consortium Recommendation REC-html-rdfa-20150317, Web Consortium Recommendation REC-html-rdfa-20150317,
March 2015, March 2015,
<http://www.w3.org/TR/2015/REC-html-rdfa-20150317>. <http://www.w3.org/TR/2015/REC-html-rdfa-20150317>.
[W3C.REC-html5-20141028] [W3C.REC-html5-20141028]
Hickson, I., Berjon, R., Faulkner, S., Leithead, T., Hickson, I., Berjon, R., Faulkner, S., Leithead, T.,
Navara, E., O&#039;Connor, E., and S. Pfeiffer, "HTML5", Navara, E., O&#039;Connor, T., and S. Pfeiffer, "HTML5",
World Wide Web Consortium Recommendation REC- World Wide Web Consortium Recommendation REC-
html5-20141028, October 2014, html5-20141028, October 2014,
<http://www.w3.org/TR/2014/REC-html5-20141028>. <http://www.w3.org/TR/2014/REC-html5-20141028>.
[W3C.REC-xml-names-20091208] [W3C.REC-xml-names-20091208]
Bray, T., Hollander, D., Layman, A., Tobin, R., and H. Bray, T., Hollander, D., Layman, A., Tobin, R., and H.
Thompson, "Namespaces in XML 1.0 (Third Edition)", World Thompson, "Namespaces in XML 1.0 (Third Edition)", World
Wide Web Consortium Recommendation REC-xml-names-20091208, Wide Web Consortium Recommendation REC-xml-names-20091208,
December 2009, December 2009,
<http://www.w3.org/TR/2009/REC-xml-names-20091208>. <http://www.w3.org/TR/2009/REC-xml-names-20091208>.
Appendix A. Link Serialisation in HTML Appendix A. Notes on Other Link Serialisations
Header fields (Section 6) are only one serialisation of links; other
specifications have defined alternative serialisations.
A.1. Link Serialisation in HTML
HTML [W3C.REC-html5-20141028] motivated the original syntax of the HTML [W3C.REC-html5-20141028] motivated the original syntax of the
Link header field, and many of the design decisions in this document Link header field, and many of the design decisions in this document
are driven by a desire to stay compatible with it. are driven by a desire to stay compatible with it.
In HTML, the link element can be mapped to links as specified here by 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 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 the relation type, as in the Link header field. The context of the
link is the URI associated with the entire HTML document. link is the URI associated with the entire HTML document.
skipping to change at page 16, line 36 skipping to change at page 17, line 41
Individual applications of linking will therefore need to define how Individual applications of linking will therefore need to define how
their extension links should be serialised into HTML. their extension links should be serialised into HTML.
Surveys of existing HTML content have shown that unregistered link Surveys of existing HTML content have shown that unregistered link
relation types that are not URIs are (perhaps inevitably) common. relation types that are not URIs are (perhaps inevitably) common.
Consuming HTML implementations ought not consider such unregistered Consuming HTML implementations ought not consider such unregistered
short links to be errors, but rather relation types with a local short links to be errors, but rather relation types with a local
scope (i.e., their meaning is specific and perhaps private to that scope (i.e., their meaning is specific and perhaps private to that
document). document).
HTML also defines several attributes on links that can be see as HTML also defines several attributes on links that can be seen as
target attributes, including "media", "hreflang", "type" and "sizes". target attributes, including "media", "hreflang", "type" and "sizes".
Finally, the HTML specification gives a special meaning when the Finally, the HTML specification gives a special meaning when the
"alternate" and "stylesheet" relation types coincide in the same "alternate" and "stylesheet" relation types coincide in the same
link. Such links ought to be serialised in the Link header field link. Such links ought to be serialised in the Link header field
using a single list of relation-types (e.g., rel="alternate using a single list of relation-types (e.g., rel="alternate
stylesheet") to preserve this relationship. stylesheet") to preserve this relationship.
Appendix B. Link Serialisation in Atom A.2. Link Serialisation in Atom
Atom [RFC4287] is a link serialisation that conveys links in the Atom [RFC4287] is a link serialisation that conveys links in the
atom:link element, with the "href" attribute indicating the link atom:link element, with the "href" attribute indicating the link
target and the "rel" attribute containing the relation type. The target and the "rel" attribute containing the relation type. The
context of the link is either a feed locator or an entry ID, context of the link is either a feed locator or an entry ID,
depending on where it appears; generally, feed-level links are depending on where it appears; generally, feed-level links are
obvious candidates for transmission as a Link header field. obvious candidates for transmission as a Link header field.
When serialising an atom:link into a Link header field, it is When serialising an atom:link into a Link header field, it is
necessary to convert link targets (if used) to URIs. necessary to convert link targets (if used) to URIs.
skipping to change at page 17, line 29 skipping to change at page 18, line 38
serialised in an Atom document. serialised in an Atom document.
Note also that while the Link header field allows multiple relations 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, 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. a single link-value may map to several atom:link elements.
As with HTML, atom:link defines some attributes that are not As with HTML, atom:link defines some attributes that are not
explicitly mirrored in the Link header field syntax, but they can explicitly mirrored in the Link header field syntax, but they can
also be used as link-extensions to maintain fidelity. 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.
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).
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".
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 Appendix C. Changes from RFC5988
This specification has the following differences from its This specification has the following differences from its
predecessor, RFC5988: predecessor, RFC5988:
o The initial relation type registrations were removed, since o The initial relation type registrations were removed, since
they've already been registered by 5988. they've already been registered by 5988.
o The introduction has been shortened. o The introduction has been shortened.
skipping to change at page 17, line 50 skipping to change at page 22, line 8
o Incorporated errata. o Incorporated errata.
o Updated references. o Updated references.
o Link cardinality was clarified. o Link cardinality was clarified.
o Terminology was changed from "target IRI" and "context IRI" to o Terminology was changed from "target IRI" and "context IRI" to
"link target" and "link context" respectively. "link target" and "link context" respectively.
o Made assigning a URI to registered relation types application- o Made assigning a URI to registered relation types serialisation-
specific. specific.
o Removed misleading statement that the link header field is o Removed misleading statement that the link header field is
semantically equivalent to HTML and Atom links. semantically equivalent to HTML and Atom links.
o More carefully defined how the Experts and IANA should interact. o More carefully defined how the Experts and IANA should interact.
o More carefully defined and used "link serialisations" and "link o More carefully defined and used "link serialisations" and "link
applications." applications."
o Clarified the cardinality of target attributes (generically and o Clarified the cardinality of target attributes (generically and
for "type"). for "type").
o Corrected the default link context for the Link header field, to o Corrected the default link context for the Link header field, to
be dependent upon the identity of the representation (as per be dependent upon the identity of the representation (as per
RFC7231). 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 Author's Address
Mark Nottingham Mark Nottingham
EMail: mnot@mnot.net EMail: mnot@mnot.net
URI: http://www.mnot.net/ URI: https://www.mnot.net/
 End of changes. 63 change blocks. 
146 lines changed or deleted 367 lines changed or added

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