< draft-nottingham-rfc5988bis-04.txt   draft-nottingham-rfc5988bis-05.txt >
Network Working Group M. Nottingham Network Working Group M. Nottingham
Internet-Draft February 3, 2017 Internet-Draft April 19, 2017
Obsoletes: 5988 (if approved) Obsoletes: 5988 (if approved)
Intended status: Standards Track Intended status: Standards Track
Expires: August 7, 2017 Expires: October 21, 2017
Web Linking Web Linking
draft-nottingham-rfc5988bis-04 draft-nottingham-rfc5988bis-05
Abstract Abstract
This specification defines a way to indicate the relationships This specification defines a model for the relationships between
between resources on the Web ("links") and the type of those resources on the Web ("links") and the type of those relationships
relationships ("link relation types"). ("link relation types").
It also defines the serialisation of such links in HTTP headers with It also defines the serialisation of such links in HTTP headers with
the Link 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 .
skipping to change at page 1, line 49 skipping to change at page 1, line 49
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 August 7, 2017. This Internet-Draft will expire on October 21, 2017.
Copyright Notice Copyright Notice
Copyright (c) 2017 IETF Trust and the persons identified as the Copyright (c) 2017 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
skipping to change at page 2, line 35 skipping to change at page 2, line 35
Without obtaining an adequate license from the person(s) controlling Without obtaining an adequate license from the person(s) controlling
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 1.1. Notational Conventions . . . . . . . . . . . . . . . . . 3
3. Links . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 2. Links . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
4. Link Relation Types . . . . . . . . . . . . . . . . . . . . . 5 2.1. Link Relation Types . . . . . . . . . . . . . . . . . . . 5
4.1. Registered Relation Types . . . . . . . . . . . . . . . . 5 2.1.1. Registered Relation Types . . . . . . . . . . . . . . 5
4.1.1. Registering Link Relation Types . . . . . . . . . . . 6 2.1.2. Extension Relation Types . . . . . . . . . . . . . . 7
4.1.2. Registration Request Processing . . . . . . . . . . . 7 2.2. Target Attributes . . . . . . . . . . . . . . . . . . . . 7
4.2. Extension Relation Types . . . . . . . . . . . . . . . . 7 3. Link Serialisation in HTTP Headers . . . . . . . . . . . . . 8
5. Target Attributes . . . . . . . . . . . . . . . . . . . . . . 8 3.1. Link Target . . . . . . . . . . . . . . . . . . . . . . . 8
6. Link Serialisation in HTTP Headers . . . . . . . . . . . . . 8 3.2. Link Context . . . . . . . . . . . . . . . . . . . . . . 9
6.1. Link Target . . . . . . . . . . . . . . . . . . . . . . . 9 3.3. Relation Type . . . . . . . . . . . . . . . . . . . . . . 9
6.2. Link Context . . . . . . . . . . . . . . . . . . . . . . 9 3.4. Target Attributes . . . . . . . . . . . . . . . . . . . . 10
6.3. Relation Type . . . . . . . . . . . . . . . . . . . . . . 9 3.4.1. Serialisation-Defined Attributes . . . . . . . . . . 10
6.4. Target Attributes . . . . . . . . . . . . . . . . . . . . 10 3.4.2. Extension Attributes . . . . . . . . . . . . . . . . 11
6.4.1. Serialisation-Defined Attributes . . . . . . . . . . 10 3.5. Link Header Field Examples . . . . . . . . . . . . . . . 11
6.4.2. Extension Attributes . . . . . . . . . . . . . . . . 11 4. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 12
6.5. Examples . . . . . . . . . . . . . . . . . . . . . . . . 12 4.1. Link HTTP Header Field Registration . . . . . . . . . . . 13
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 13 4.2. Link Relation Type Registry . . . . . . . . . . . . . . . 13
7.1. Link HTTP Header Field Registration . . . . . . . . . . . 13 5. Security Considerations . . . . . . . . . . . . . . . . . . . 14
7.2. Link Relation Type Registry . . . . . . . . . . . . . . . 13 6. Internationalisation Considerations . . . . . . . . . . . . . 14
8. Security Considerations . . . . . . . . . . . . . . . . . . . 14 7. References . . . . . . . . . . . . . . . . . . . . . . . . . 15
9. Internationalisation Considerations . . . . . . . . . . . . . 14 7.1. Normative References . . . . . . . . . . . . . . . . . . 15
10. References . . . . . . . . . . . . . . . . . . . . . . . . . 14 7.2. Informative References . . . . . . . . . . . . . . . . . 16
10.1. Normative References . . . . . . . . . . . . . . . . . . 15
10.2. Informative References . . . . . . . . . . . . . . . . . 16
Appendix A. Notes on Other Link Serialisations . . . . . . . . . 17 Appendix A. Notes on Other Link Serialisations . . . . . . . . . 17
A.1. Link Serialisation in HTML . . . . . . . . . . . . . . . 17 A.1. Link Serialisation in HTML . . . . . . . . . . . . . . . 17
A.2. Link Serialisation in Atom . . . . . . . . . . . . . . . 17 A.2. Link Serialisation in Atom . . . . . . . . . . . . . . . 17
Appendix B. Algorithm for Parsing Link Headers . . . . . . . . . 18 Appendix B. Algorithm for Parsing Link Headers . . . . . . . . . 18
Appendix C. Changes from RFC5988 . . . . . . . . . . . . . . . . 21 Appendix C. Changes from RFC5988 . . . . . . . . . . . . . . . . 21
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 22 Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 22
1. Introduction 1. Introduction
This specification defines a way to indicate the relationships This specification defines a model for 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; Section 2 generalises this into a
a framework that encompasses linking in these formats and framework that encompasses linking in these formats and (potentially)
(potentially) elsewhere. elsewhere.
Furthermore, this specification formalises an HTTP header field for Furthermore, Section 3 defines an HTTP header field for conveying
conveying such links, having been originally defined in such links.
Section 19.6.2.4 of [RFC2068], but removed from [RFC2616].
2. Notational Conventions 1.1. 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), BWS (bad rules from it: quoted-string, token, SP (space), BWS (bad
whitespace), OWS (optional whitespace), RWS (required whitespace) whitespace), OWS (optional whitespace), RWS (required whitespace)
LOALPHA, DIGIT. 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; and from [W3C.CR-css3-mediaqueries-20090915]: media_query_list; and from
[RFC5646]: Language-Tag.. [RFC5646]: Language-Tag.
3. Links 2. 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 2.1),
o a _link target_, and o a _link target_, and
o optionally, _target attributes_ (Section 5). o optionally, _target attributes_ (Section 2.2).
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 serialisations that do not
(such as the Link header field defined in Section 6). support IRIs (such as the Link header field defined in Section 3).
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.
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. For example, Atom the wire", and can occur in various forms. For example, Atom
[RFC4287] and HTML [W3C.REC-html5-20141028] both defined [RFC4287] and HTML [W3C.REC-html5-20141028] both defined
serialisations of links into their respective formats, and Section 6 serialisations of links into their respective formats, and Section 3
defines how to serialise links in HTTP header fields. defines how to serialise links in HTTP header fields.
This specification does not define a general syntax for links across This specification does not define a general syntax for links across
different serialisations, nor does it mandate a specific context for different serialisations, nor does it mandate a specific context for
any given link; it is expected that serialisations of links will any given link; it is expected that serialisations of links will
specify both aspects. specify both aspects.
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 type(s) it uses, along with
the serialisations that they might occur within. For example, the the serialisation(s) 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, whereas the application type in the HTML link serialisation, whereas the application
"AtomPub" uses the "edit" and "edit-media" link relations. "AtomPub" uses the "edit" and "edit-media" link relations.
4. Link Relation Types 2.1. 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
of a link. For example, a link with the relation type "copyright" of a link. For example, a link with the relation type "copyright"
indicates that the resource identified by the link target is a indicates that the current link context has a copyright resource at
statement of the copyright terms applying to the current link the link target.
context.
Link relation types can also be used to indicate that the target Link relation types can also be used to indicate that the target
resource has particular attributes, or exhibits particular resource has particular attributes, or exhibits particular
behaviours; for example, a "service" link implies that the identified behaviours; for example, a "service" link implies that the link
resource is part of a defined protocol (in this case, a service target can be used as part of a defined protocol (in this case, a
description). service description).
Relation types are not to be confused with media types [RFC6838]; Relation types are not to be confused with media types [RFC6838];
they do not identify the format of the representation that results they do not identify the format of the representation that results
when the link is dereferenced. Rather, they only describe how the when the link is dereferenced. Rather, they only describe how the
current context is related to another resource. current context is related to another resource.
Relation types SHOULD NOT infer any additional semantics based upon Relation types SHOULD NOT infer any additional semantics based upon
the presence or absence of another link relation type, or its own the presence or absence of another link relation type, or its own
cardinality of occurrence. An exception to this is the combination cardinality of occurrence. An exception to this is the combination
of the "alternate" and "stylesheet" registered relation types, which of the "alternate" and "stylesheet" registered relation types, which
has special meaning in HTML for historical reasons. has special meaning in HTML for historical reasons.
There are two kinds of relation types: registered and extension. There are two kinds of relation types: registered and extension.
4.1. Registered Relation Types 2.1.1. Registered Relation Types
Well-defined relation types can be registered as tokens for Well-defined relation types can be registered as tokens for
convenience and/or to promote reuse by other applications, using the convenience and/or to promote reuse by other applications, using the
procedure in Section 4.1.1. procedure in Section 2.1.1.1.
Registered relation type names MUST conform to the reg-rel-type rule Registered relation type names MUST conform to the reg-rel-type rule
(see Section 6.3), and MUST be compared character-by-character in a (see Section 3.3), and MUST be compared character-by-character in a
case-insensitive fashion. They SHOULD be appropriate to the case-insensitive fashion. They SHOULD be appropriate to the
specificity of the relation type; i.e., if the semantics are highly specificity of the relation type; i.e., if the semantics are highly
specific to a particular application, the name should reflect that, specific to a particular application, the name should reflect that,
so that more general names are available for less specific use. so that more 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 are required be
supported). supported).
Historically, registered relation types have been identified with a Historically, registered relation types have been identified with a
URI [RFC3986] by prefixing their names with an application-defined URI [RFC3986] by prefixing their names with an application-defined
base URI (e.g., see Appendix A.2). This practice is NOT RECOMMENDED, base URI (e.g., see Appendix A.2). This practice is NOT RECOMMENDED,
because the resulting strings will not be considered equivalent to because the resulting strings will not be considered equivalent to
the registered relation types by other processors. Applications that the registered relation types by other processors. Applications that
do use such URIs internally MUST NOT use them in link serialisations do use such URIs internally MUST NOT use them in link serialisations
that do not explicitly accommodate them. that do not explicitly accommodate them.
4.1.1. Registering Link Relation Types 2.1.1.1. Registering Link Relation Types
Any party can request registration of a link relation type.
Registration requests can be sent to the "link-relations@ietf.org" The link relations registry is located at
mailing list. The Expert(s) MAY establish alternate means of https://www.iana.org/assignments/link-relations/ . Registration
requesting registrations, which SHOULD be linked to from the registry requests can be made by following the instructions located there, or
page. by sending an e-mail to the "link-relations@ietf.org" mailing list.
Registration requests consist of at least the following information: Registration requests consist of at least the following information:
o *Relation Name*: The name of the relation type o *Relation Name*: The name of the relation type
o *Description*: A short English description of the type's o *Description*: A short English description of the type's
semantics. It SHOULD be stated in terms of the relationship semantics. It SHOULD be stated in terms of the relationship
between the link context and link target. between the link context and link target.
o *Reference*: Reference to the document that specifies the link o *Reference*: Reference to the document that specifies the link
relation type, preferably including a URI that can be used to relation type, preferably including a URI that can be used to
retrieve a copy of the document. An indication of the relevant retrieve a copy of the document. An indication of the relevant
section(s) MAY also be included, but is not required. section(s) MAY also be included, but is not required.
The Expert(s) MAY define additional fields to be collected in the The Expert(s) MAY define additional fields to be collected in the
registry. registry.
General requirements for registered relation types are described in General requirements for registered relation types are described in
Section 4.1. Section 2.1.1.
Registrations MUST reference a freely available, stable Registrations MUST reference a freely available, stable
specification. specification.
Note that relation types can be registered by third parties Note that relation types can be registered by third parties
(including the Expert(s)), if the Expert(s) determine that an (including the Expert(s)), if the Expert(s) determine that an
unregistered relation type is widely deployed and not likely to be unregistered relation type is widely deployed and not likely to be
registered in a timely manner. registered in a timely manner.
4.1.2. Registration Request Processing 2.1.1.2. Registration Request Processing
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 goal of the registry is to reflect common use of links on the The goal of the registry is to reflect common use of links on the
Internet. Therefore, the Expert(s) SHOULD be strongly biased towards Internet. Therefore, the Expert(s) SHOULD be strongly biased towards
approving registrations, unless they are abusive, frivolous, not approving registrations, unless they are abusive, frivolous, not
likely to be used on the Internet, or actively harmful to the likely to be used on the Internet, or actively harmful to the
Internet and/or the Web (not merely aesthetically displeasing, or Internet and/or the Web (not merely aesthetically displeasing, or
architecturally dubious). architecturally dubious). As stated in Section 2.1.1, the Experts
MAY withhold registration of names that are too general for the
proposed application.
The Expert(s) MUST clearly identify any issues which cause a The Expert(s) MUST clearly identify any issues which cause a
registration to be refused. Advice about the syntax or semantics of registration to be refused. Advice about the semantics of a proposed
a proposed link relation type can be given, but if it does not block link relation type can be given, but if it does not block
registration, this SHOULD be explicitly stated. registration, this SHOULD be explicitly stated.
When a request is approved, the Expert(s) will inform IANA, and the 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 registration will be processed. The IESG is the final arbiter of any
objection. objection.
4.2. Extension Relation Types 2.1.2. Extension Relation Types
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.
skipping to change at page 8, line 5 skipping to change at page 7, line 42
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) in a case-insensitive fashion, character-by-character. format) in a case-insensitive fashion, character-by-character.
Because of this, all-lowercase URIs SHOULD be used for extension Because of this, all-lowercase URIs SHOULD be used for extension
relations. 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. Target Attributes 2.2. Target Attributes
_Target attributes_ are a list of key/value pairs that describe the _Target attributes_ are a list of key/value pairs that describe the
link or its target; for example, a media type hint. link or its target; for example, a media type hint.
They can be defined both by individual link relation types and by They can be defined both by individual link relation types and by
link serialisations. link serialisations.
This specification does not attempt to coordinate the name of target This specification does not attempt to coordinate the name of target
attributes, their cardinality or use. Serialisations SHOULD attributes, their cardinality or use. Serialisations SHOULD
coordinate their target attributes to avoid conflicts in semantics or coordinate their target attributes to avoid conflicts in semantics or
skipping to change at page 8, line 33 skipping to change at page 8, line 22
Target attribute definitions SHOULD specify: Target attribute definitions SHOULD specify:
o The serialisation of their values into Unicode or a subset o The serialisation of their values into Unicode or a subset
thereof, to maximise their chances of portability across link thereof, to maximise their chances of portability across link
serialisations. serialisations.
o The semantics and error handling of multiple occurrences of the o The semantics and error handling of multiple occurrences of the
target attribute on a given link. target attribute on a given link.
This specification does define target attributes for use in the Link This specification does define target attributes for use in the Link
HTTP header field in Section 6.4. HTTP header field in Section 3.4.
6. Link Serialisation in HTTP Headers 3. 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.
The ABNF for the field value is given below: The ABNF for the field value is:
Link = #link-value Link = #link-value
link-value = "<" URI-Reference ">" *( OWS ";" OWS link-param ) link-value = "<" URI-Reference ">" *( OWS ";" OWS link-param )
link-param = token BWS "=" BWS ( token / quoted-string ) link-param = token BWS "=" BWS ( token / quoted-string )
Note that any "link-param" can be generated with values using either Note that any "link-param" can be generated with values using either
the "token" or the "quoted-string" syntax, and therefore recipients the "token" or the "quoted-string" syntax, and therefore recipients
MUST be able to parse both forms. Individual "link-param"s specify MUST be able to parse both forms. Individual "link-param"s specify
their syntax in terms of the value after any necessary unquoting (as their syntax in terms of the value after any necessary unquoting (as
per [RFC7230], Section 3.2.6). per [RFC7230], Section 3.2.6).
This specification defines the link-params "rel", "anchor", "rev", This specification defines the link-params "rel", "anchor", "rev",
"hreflang", "media", "title", "title*", and "type"; see Section 6.2, "hreflang", "media", "title", "title*", and "type"; see Section 3.2,
Section 6.3 and Section 6.4. Section 3.3 and Section 3.4.
6.1. Link Target 3.1. 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.
6.2. Link Context 3.2. 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
skipping to change at page 9, line 42 skipping to change at page 9, line 30
URI-Reference 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 example, this is the case on a 404 response to a GET
GET request. request.
6.3. Relation Type 3.3. Relation Type
The relation type of a link conveyed in the Link header field is The relation type of a link conveyed in the Link header field is
conveyed in the "rel" parameter's value. The "rel" parameter MUST conveyed in the "rel" parameter's value. The "rel" parameter MUST
NOT appear more than once in a given link-value; occurrences after NOT appear more than once in a given link-value; occurrences after
the first MUST be ignored by parsers. 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
skipping to change at page 10, line 27 skipping to change at page 10, line 14
relation-type = reg-rel-type | ext-rel-type relation-type = reg-rel-type | ext-rel-type
reg-rel-type = LOALPHA *( LOALPHA | DIGIT | "." | "-" ) reg-rel-type = LOALPHA *( LOALPHA | DIGIT | "." | "-" )
ext-rel-type = URI 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).
6.4. Target Attributes 3.4. Target Attributes
The Link header field defines several target attributes specific to The Link header field defines several target attributes specific to
this serialisation, and also allows extension target attributes. this serialisation, and also allows extension target attributes.
Target attributes are serialised in the Link header field as Target attributes are serialised in the Link header field as
parameters (see [RFC7231], Section 3.1.1.1 for the definition of parameters (see [RFC7231], Section 3.1.1.1 for the definition of
their syntax). their syntax).
6.4.1. Serialisation-Defined Attributes 3.4.1. Serialisation-Defined Attributes
The "hreflang", "media", "title", "title*", and "type" link-params The "hreflang", "media", "title", "title*", and "type" link-params
can be translated to serialisation-defined target attributes for the can be translated to serialisation-defined target attributes for the
link. link.
The "hreflang" attribute, 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" attributes on a single link- following the link. Multiple "hreflang" attributes on a single link-
skipping to change at page 11, line 47 skipping to change at page 11, line 33
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" attribute 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.
The ABNF for the "type" parameter's value is: The ABNF for the "type" parameter's value is:
type-name "/" subtype-name type-name "/" subtype-name
6.4.2. Extension Attributes 3.4.2. Extension Attributes
Other link-params are link-extensions, and are to be considered as Other link-params are link-extensions, and are to be considered as
target attributes. target attributes.
Such target attributes MAY be defined to use the encoding in Such target attributes MAY be defined to use the encoding in
[I-D.ietf-httpbis-rfc5987bis] (e.g., "example" and "example_"). When [I-D.ietf-httpbis-rfc5987bis] (e.g., "example" and "example*"). When
both forms are present, they SHOULD be considered to be the same both forms are present, they SHOULD be considered to be the same
target attribute; processors SHOULD use the value of the name ending target attribute; processors SHOULD use the value of the name ending
in "_" (after [I-D.ietf-httpbis-rfc5987bis] decoding), but MAY fall 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 back to the other value if there is an error in decoding it, or if
they do not support decoding. they do not support decoding.
6.5. Examples 3.5. Link Header Field 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".
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 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
skipping to change at page 13, line 5 skipping to change at page 12, line 46
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".
7. IANA Considerations 4. 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.
7.1. Link HTTP Header Field Registration 4.1. 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;]
7.2. Link Relation Type Registry 4.2. 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 2.1.1.1. The Expert(s) and IANA
will interact as outlined below. are expected interact as outlined below.
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.
The Expert(s) will provide registry data to IANA in an agreed form The Expert(s) will provide registry data to IANA in a mutually-agreed
(e.g. a specific XML format). IANA will publish: form (e.g. a specific XML format). IANA will publish:
o The raw registry data o The raw registry data
o The registry data, transformed into HTML o The registry data, transformed into HTML
o The registry data in any alternative formats provided by the o The registry data alternative formats provided by the Expert(s)
Expert(s) (if any)
Each published document will be at a URL agreed to by IANA and the If IANA's internal processes require making changes to registry data
Expert(s), and IANA will set HTTP response headers on them as and/or adding registry entries, IANA will inform the Expert(s) of
this in a mutually agreed way.
Each published document will be at a URL mutually agreed to by IANA
and the Expert(s), and IANA will set HTTP response headers on them as
(reasonably) requested by the Expert(s). (reasonably) requested by the Expert(s).
Additionally, the HTML generated by IANA will: Additionally, the HTML generated by IANA will:
o Take directions from the Expert(s) as to the content of the HTML o Take directions from the Expert(s) as to the content of the HTML
page's introductory text page's introductory text
o Include a stable HTML fragment identifier for each registered link o Include a stable HTML fragment identifier for each registered link
relation 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>).
8. Security Considerations IANA will direct any incoming requests regarding the registry to this
document and, if defined, the processes established by the Expert(s);
typically, this will mean referring them to the registry Web page.
5. Security Considerations
The content of the Link header field is not secure, private or 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 are to 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.
9. Internationalisation Considerations 6. 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.
10. References 7. References
10.1. Normative References
7.1. 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-04 (work in progress), November 2016. rfc5987bis-05 (work in progress), February 2017.
[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 16, line 16 skipping to change at page 16, line 16
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>.
10.2. Informative References 7.2. Informative References
[RFC2068] Fielding, R., Gettys, J., Mogul, J., Frystyk, H., and T.
Berners-Lee, "Hypertext Transfer Protocol -- HTTP/1.1",
RFC 2068, DOI 10.17487/RFC2068, January 1997,
<http://www.rfc-editor.org/info/rfc2068>.
[RFC2616] Fielding, R., Gettys, J., Mogul, J., Frystyk, H.,
Masinter, L., Leach, P., and T. Berners-Lee, "Hypertext
Transfer Protocol -- HTTP/1.1", RFC 2616,
DOI 10.17487/RFC2616, June 1999,
<http://www.rfc-editor.org/info/rfc2616>.
[RFC2817] Khare, R. and S. Lawrence, "Upgrading to TLS Within [RFC2817] Khare, R. and S. Lawrence, "Upgrading to TLS Within
HTTP/1.1", RFC 2817, DOI 10.17487/RFC2817, May 2000, HTTP/1.1", RFC 2817, DOI 10.17487/RFC2817, May 2000,
<http://www.rfc-editor.org/info/rfc2817>. <http://www.rfc-editor.org/info/rfc2817>.
[RFC2818] Rescorla, E., "HTTP Over TLS", RFC 2818, [RFC2818] Rescorla, E., "HTTP Over TLS", RFC 2818,
DOI 10.17487/RFC2818, May 2000, DOI 10.17487/RFC2818, May 2000,
<http://www.rfc-editor.org/info/rfc2818>. <http://www.rfc-editor.org/info/rfc2818>.
[RFC4287] Nottingham, M., Ed. and R. Sayre, Ed., "The Atom [RFC4287] Nottingham, M., Ed. and R. Sayre, Ed., "The Atom
Syndication Format", RFC 4287, DOI 10.17487/RFC4287, Syndication Format", RFC 4287, DOI 10.17487/RFC4287,
December 2005, <http://www.rfc-editor.org/info/rfc4287>. December 2005, <http://www.rfc-editor.org/info/rfc4287>.
[W3C.REC-html-rdfa-20150317]
Sporny, M., "HTML+RDFa 1.1 - Second Edition", World Wide
Web Consortium Recommendation REC-html-rdfa-20150317,
March 2015,
<http://www.w3.org/TR/2015/REC-html-rdfa-20150317>.
[W3C.REC-html5-20141028] [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, T., 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>.
Appendix A. Notes on Other Link Serialisations Appendix A. Notes on Other Link Serialisations
Header fields (Section 6) are only one serialisation of links; other Header fields (Section 3) are only one serialisation of links; other
specifications have defined alternative serialisations. specifications have defined alternative serialisations.
A.1. Link Serialisation in HTML A.1. Link Serialisation in HTML
HTML motivated the original syntax of the Link header field, and many 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 of the design decisions in this document are driven by a desire to
stay compatible with it. 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
 End of changes. 63 change blocks. 
132 lines changed or deleted 124 lines changed or added

This html diff was produced by rfcdiff 1.48. The latest version is available from http://tools.ietf.org/tools/rfcdiff/