Internet DRAFT - draft-nottingham-rfc5988bis
draft-nottingham-rfc5988bis
Network Working Group M. Nottingham
Internet-Draft August 11, 2017
Obsoletes: 5988 (if approved)
Intended status: Standards Track
Expires: February 12, 2018
Web Linking
draft-nottingham-rfc5988bis-08
Abstract
This specification defines a model for the relationships between
resources on the Web ("links") and the type of those relationships
("link relation types").
It also defines the serialisation of such links in HTTP headers with
the Link header field.
Note to Readers
_RFC EDITOR: please remove this section before publication_
This is a work-in-progress to revise RFC5988.
The issues list can be found at https://github.com/mnot/I-D/labels/
rfc5988bis.
The most recent (often, unpublished) draft is at
https://mnot.github.io/I-D/rfc5988bis/.
Recent changes are listed at https://github.com/mnot/I-D/commits/gh-
pages/rfc5988bis.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
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material or to cite them other than as "work in progress."
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This Internet-Draft will expire on February 12, 2018.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Notational Conventions . . . . . . . . . . . . . . . . . 3
1.2. Conformance and Error Handling . . . . . . . . . . . . . 4
2. Links . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
2.1. Link Relation Types . . . . . . . . . . . . . . . . . . . 5
2.1.1. Registered Relation Types . . . . . . . . . . . . . . 5
2.1.2. Extension Relation Types . . . . . . . . . . . . . . 7
2.2. Target Attributes . . . . . . . . . . . . . . . . . . . . 8
3. Link Serialisation in HTTP Headers . . . . . . . . . . . . . 8
3.1. Link Target . . . . . . . . . . . . . . . . . . . . . . . 9
3.2. Link Context . . . . . . . . . . . . . . . . . . . . . . 9
3.3. Relation Type . . . . . . . . . . . . . . . . . . . . . . 10
3.4. Target Attributes . . . . . . . . . . . . . . . . . . . . 11
3.4.1. Serialisation-Defined Attributes . . . . . . . . . . 11
3.4.2. Extension Attributes . . . . . . . . . . . . . . . . 12
3.5. Link Header Field Examples . . . . . . . . . . . . . . . 12
4. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 14
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4.1. Link HTTP Header Field Registration . . . . . . . . . . . 14
4.2. Link Relation Type Registry . . . . . . . . . . . . . . . 14
4.3. Link Relation Application Data Registry . . . . . . . . . 14
5. Security Considerations . . . . . . . . . . . . . . . . . . . 14
6. Internationalisation Considerations . . . . . . . . . . . . . 15
7. References . . . . . . . . . . . . . . . . . . . . . . . . . 15
7.1. Normative References . . . . . . . . . . . . . . . . . . 15
7.2. Informative References . . . . . . . . . . . . . . . . . 17
Appendix A. Notes on Other Link Serialisations . . . . . . . . . 18
A.1. Link Serialisation in HTML . . . . . . . . . . . . . . . 18
A.2. Link Serialisation in Atom . . . . . . . . . . . . . . . 18
Appendix B. Algorithms for Parsing Link Header Fields . . . . . 19
B.1. Parsing a Header Set for Links . . . . . . . . . . . . . 19
B.2. Parsing a Link Field Value . . . . . . . . . . . . . . . 20
B.3. Parsing Parameters . . . . . . . . . . . . . . . . . . . 22
B.4. Parsing a Quoted String . . . . . . . . . . . . . . . . . 23
Appendix C. Changes from RFC5988 . . . . . . . . . . . . . . . . 24
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 25
1. Introduction
This specification defines a model for the relationships between
resources on the Web ("links") and the type of those relationships
("link relation types").
HTML [W3C.REC-html5-20141028] and Atom [RFC4287] both have well-
defined concepts of linking; Section 2 generalises this into a
framework that encompasses linking in these formats and (potentially)
elsewhere.
Furthermore, Section 3 defines an HTTP header field for conveying
such links.
1.1. Notational Conventions
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in BCP
14 [RFC2119],[RFC8174] when, and only when, they appear in all
capitals, as shown here.
This document uses the Augmented Backus-Naur Form (ABNF) notation of
[RFC7230], including the #rule, and explicitly includes the following
rules from it: quoted-string, token, SP (space), BWS (bad
whitespace), OWS (optional whitespace), RWS (required whitespace)
LOALPHA, DIGIT.
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Additionally, the following rules are included from [RFC3986]: URI
and URI-Reference; from [RFC6838]: type-name and subtype-name; from
[W3C.REC-css3-mediaqueries-20120619]: media-query-list; and from
[RFC5646]: Language-Tag.
1.2. Conformance and Error Handling
The requirements regarding conformance and error handling highlighted
in [RFC7230], Section 2.5 apply to this document.
2. Links
In this specification, a link is a typed connection between two
resources, and is comprised of:
o A _link context_,
o a _link relation type_ (Section 2.1),
o a _link target_, and
o optionally, _target attributes_ (Section 2.2).
A link can be viewed as a statement of the form "_link context_ has a
_link relation type_ resource at _link target_, which has _target
attributes_".
For example, https://www.example.com/ has a "canonical" resource at
https://example.com, which has a "type" of "text/html".
Link contexts and link targets are both IRIs [RFC3987]. However, in
the common case, the link context will also be a URI [RFC3986],
because many protocols (such as HTTP) do not support dereferencing
IRIs. Likewise, the link target will be sometimes be converted to a
URI (see [RFC3987], Section 3.1) in serialisations that do not
support IRIs (such as the Link header field defined in Section 3).
This specification does not place restrictions on the cardinality of
links; there can be multiple links to and from a particular target,
and multiple links of the same or different types between a given
context and target. Likewise, the relative ordering of links in any
particular serialisation, or between serialisations (e.g., the Link
header field and in-content links) is not specified or significant in
this specification; applications that wish to consider ordering
significant can do so.
Links are conveyed in _link serialisations_; they are the "bytes on
the wire", and can occur in various forms. For example, Atom
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[RFC4287] and HTML [W3C.REC-html5-20141028] both defined
serialisations of links into their respective formats, and Section 3
defines how to serialise links in HTTP header fields.
This specification does not define a general syntax for links across
different serialisations, nor does it mandate a specific context for
any given link; it is expected that serialisations of links will
specify both aspects.
Finally, links are used by _link applications_. Generally, an
application will define the link relation type(s) it uses, along with
the serialisation(s) that they might occur within. For example, the
application "Web browsing" looks for the "stylesheet" link relation
type in the HTML link serialisation (and optionally in the Link
header field), whereas the application "AtomPub" uses the "edit" and
"edit-media" link relations in the Atom serialisation.
2.1. Link Relation Types
In the simplest case, a link relation type identifies the semantics
of a link. For example, a link with the relation type "copyright"
indicates that the current link context has a copyright resource at
the link target.
Link relation types can also be used to indicate that the target
resource has particular attributes, or exhibits particular
behaviours; for example, a "service" link implies that the link
target can be used as part of a defined protocol (in this case, a
service description).
Relation types are not to be confused with media types [RFC2046];
they do not identify the format of the representation that results
when the link is dereferenced. Rather, they only describe how the
current context is related to another resource.
Relation types SHOULD NOT infer any additional semantics based upon
the presence or absence of another link relation type, or its own
cardinality of occurrence. An exception to this is the combination
of the "alternate" and "stylesheet" registered relation types, which
has special meaning in HTML for historical reasons.
There are two kinds of relation types: registered and extension.
2.1.1. Registered Relation Types
Well-defined relation types can be registered as tokens for
convenience and/or to promote reuse by other applications, using the
procedure in Section 2.1.1.1.
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Registered relation type names MUST conform to the reg-rel-type rule
(see Section 3.3), and MUST be compared character-by-character in a
case-insensitive fashion. They SHOULD be appropriate to the
specificity of the relation type; i.e., if the semantics are highly
specific to a particular application, the name should reflect that,
so that more general names are available for less specific use.
Registered relation types MUST NOT constrain the media type of the
link context, and MUST NOT constrain the available representation
media types of the link target. However, they can specify the
behaviours and properties of the target resource (e.g., allowable
HTTP methods, request and response media types that are required be
supported).
Historically, registered relation types have been identified with a
URI [RFC3986] by prefixing their names with an application-defined
base URI (e.g., see Appendix A.2). This practice is NOT RECOMMENDED,
because the resulting strings will not be considered equivalent to
the registered relation types by other applications. Applications
that do use such URIs internally MUST NOT use them in link
serialisations that do not explicitly accommodate them.
2.1.1.1. Registering Link Relation Types
The link relations registry is located at
https://www.iana.org/assignments/link-relations/. Registration
requests can be made by following the instructions located there, or
by sending an e-mail to the "link-relations@ietf.org" mailing list.
Registration requests consist of at least the following information:
o *Relation Name*: The name of the relation type
o *Description*: A short English description of the type's
semantics. It SHOULD be stated in terms of the relationship
between the link context and link target.
o *Reference*: Reference to the document that specifies the link
relation type, preferably including a URI that can be used to
retrieve a copy of the document. An indication of the relevant
section(s) can also be included, but is not required.
The expert(s) can define additional fields to be collected in the
registry.
General requirements for registered relation types are described in
Section 2.1.1.
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Registrations MUST reference a freely available, stable
specification.
Note that relation types can be registered by third parties
(including the expert(s)), if the expert(s) determine that an
unregistered relation type is widely deployed and not likely to be
registered in a timely manner otherwise. Such registrations still
are subject to the requirements defined, including the need to
reference a specification.
2.1.1.2. Registration Request Processing
Relation types are registered using the Specification Required policy
(see Section 4.6 of [RFC8126]), which implies review and approval by
a designated expert.
The goal of the registry is to reflect common use of links on the
Internet. Therefore, the expert(s) should be strongly biased towards
approving registrations, unless they are abusive, frivolous, not
likely to be used on the Internet, or actively harmful to the
Internet and/or the Web (not merely aesthetically displeasing, or
architecturally dubious). As stated in Section 2.1.1, the expert(s)
can withhold registration of names that are too general for the
proposed application.
The expert(s) will clearly identify any issues which cause a
registration to be refused. Advice about the semantics of a proposed
link relation type can be given, but if it does not block
registration, this should be explicitly stated.
When a request is approved, the expert(s) will inform IANA, and the
registration will be processed. The IESG is the final arbiter of any
objection.
2.1.2. Extension Relation Types
Applications that don't wish to register a relation type can use an
extension relation type, which is a URI [RFC3986] that uniquely
identifies the relation type. Although the URI can point to a
resource that contains a definition of the semantics of the relation
type, clients SHOULD NOT automatically access that resource to avoid
overburdening its server.
The URI used for an extension relation type SHOULD be under the
control of the person or party defining it, or be delegated to them.
When extension relation types are compared, they MUST be compared as
strings (after converting to URIs if serialised in a different
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format) in a case-insensitive fashion, character-by-character.
Because of this, all-lowercase URIs SHOULD be used for extension
relations.
Note that while extension relation types are required to be URIs, a
serialisation of links can specify that they are expressed in another
form, as long as they can be converted to URIs.
2.2. Target Attributes
_Target attributes_ are a list of key/value pairs that describe the
link or its target; for example, a media type hint.
They can be defined both by individual link relation types and by
link serialisations.
This specification does not attempt to coordinate the name of target
attributes, their cardinality or use. Those creating and maintaining
serialisations SHOULD coordinate their target attributes to avoid
conflicts in semantics or syntax, and MAY define their own registries
of target attributes.
The names of target attributes SHOULD conform to the token rule, but
SHOULD NOT include any of the characters "%", "'" or "*", for
portability across serializations, and MUST be compared in a case-
insensitive fashion.
Target attribute definitions SHOULD specify:
o The serialisation of their values into Unicode or a subset
thereof, to maximise their chances of portability across link
serialisations.
o The semantics and error handling of multiple occurrences of the
target attribute on a given link.
This specification does define target attributes for use in the Link
HTTP header field in Section 3.4.
3. Link Serialisation in HTTP Headers
The Link header field provides a means for serialising one or more
links into HTTP headers.
The ABNF for the field value is:
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Link = #link-value
link-value = "<" URI-Reference ">" *( OWS ";" OWS link-param )
link-param = token BWS [ "=" BWS ( token / quoted-string ) ]
Note that any "link-param" can be generated with values using either
the "token" or the "quoted-string" syntax, and therefore recipients
MUST be able to parse both forms. In other words, the following
parameters are equivalent:
x=y
x="y"
Previous definitions of the Link header did not equate the token and
quoted-string forms explicitly; the "title" parameter was always
quoted, and the "hreflang" parameter was always a token. Senders
wishing to maximize interoperability will send them in those forms.
Individual "link-param"s specify their syntax in terms of the value
after any necessary unquoting (as per [RFC7230], Section 3.2.6).
This specification establishes the link-params "rel", "anchor", and
"rev" (which are part of the general link model), as well as
"hreflang", "media", "title", "title*", and "type" (which are target
attributes defined by the serialization.
3.1. Link Target
Each link-value conveys one target IRI as a URI-Reference (after
conversion to one, if necessary; see [RFC3987], Section 3.1) inside
angle brackets ("<>"). If the URI-Reference is relative, parsers
MUST resolve it as per [RFC3986], Section 5. Note that any base IRI
appearing in the message's content is not applied.
3.2. Link Context
By default, the context of a link conveyed in the Link header field
is the URL of the representation it is associated with, as defined in
[RFC7231], Section 3.1.4.1, serialised as a URI.
When present, the anchor parameter overrides this with another URI,
such as a fragment of this resource, or a third resource (i.e., when
the anchor value is an absolute URI). If the anchor parameter's
value is a relative URI, parsers MUST resolve it as per [RFC3986],
Section 5. Note that any base URI from the body's content is not
applied.
The ABNF for the "anchor" parameter's value is:
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URI-Reference ; Section 4.1 of {{RFC3986}}
Link application can choose to ignore links with an anchor parameter.
For example, the application in use might not allow the link context
to be assigned to a different resource. In such cases, the entire
link is to be ignored; link applications MUST NOT process the link
without applying the anchor.
Note that depending on HTTP status code and response headers, the
link context might be "anonymous" (i.e., no link context is
available). For example, this is the case on a 404 response to a GET
request.
3.3. Relation Type
The relation type of a link conveyed in the Link header field is
conveyed in the "rel" parameter's value. The "rel" parameter MUST be
present but MUST NOT appear more than once in a given link-value;
occurrences after the first MUST be ignored by parsers.
The "rel" parameter can, however, contain multiple link relation
types. When this occurs, it establishes multiple links that share
the same context, target, and target attributes.
The "rev" parameter has been used in the past to indicate that the
semantics of the relationship are in the reverse direction. That is,
a link from A to B with REL="X" expresses the same relationship as a
link from B to A with REV="X". "rev" is deprecated by this
specification because it often confuses authors and readers; in most
cases, using a separate relation type is preferable.
The ABNF for the "rel" and "rev" parameters' values is:
relation-type *( 1*SP relation-type )
where:
relation-type = reg-rel-type / ext-rel-type
reg-rel-type = LOALPHA *( LOALPHA / DIGIT / "." / "-" )
ext-rel-type = URI ; Section 3 of {{RFC3986}}
Note that extension relation types are REQUIRED to be absolute URIs
in Link header fields, and MUST be quoted when they contain
characters not allowed in tokens, such as semicolon (";") or comma
(",") (as these characters are used as delimiters in the header field
itself).
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3.4. Target Attributes
The Link header field defines several target attributes specific to
this serialisation, and also allows extension target attributes.
Target attributes are serialised in the Link header field as
parameters (see [RFC7231], Section 3.1.1.1 for the definition of
their syntax).
3.4.1. Serialisation-Defined Attributes
The "hreflang", "media", "title", "title*", and "type" link-params
can be translated to serialisation-defined target attributes for the
link.
The "hreflang" attribute, when present, is a hint indicating what the
language of the result of dereferencing the link should be. Note
that this is only a hint; for example, it does not override the
Content-Language header field of a HTTP response obtained by actually
following the link. Multiple "hreflang" attributes on a single link-
value indicate that multiple languages are available from the
indicated resource.
The ABNF for the "hreflang" parameter's value is:
Language-Tag
The "media" attribute, when present, is used to indicate intended
destination medium or media for style information (see
[W3C.REC-html5-20141028], Section 4.2.4). Its value MUST be quoted
if it contains a semicolon (";") or comma (","). There MUST NOT be
more than one "media" attribute in a link-value; occurrences after
the first MUST be ignored by parsers.
The ABNF for the "media" parameter's value is:
media-query-list
The "title" attribute, when present, is used to label the destination
of a link such that it can be used as a human-readable identifier
(e.g., a menu entry) in the language indicated by the Content-
Language header field (if present). The "title" attribute MUST NOT
appear more than once in a given link; occurrences after the first
MUST be ignored by parsers.
The "title*" link-param can be used to encode this attribute in a
different character set, and/or contain language information as per
[I-D.ietf-httpbis-rfc5987bis]. The "title*" link-param MUST NOT
appear more than once in a given link-value; occurrences after the
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first MUST be ignored by parsers. If the attribute does not contain
language information, its language is indicated by the Content-
Language header field (when present).
If both the "title" and "title*" link-param appear in a link,
applications SHOULD use the "title*" link-param's value for the
"title" attribute.
The "type" attribute, when present, is a hint indicating what the
media type of the result of dereferencing the link should be. Note
that this is only a hint; for example, it does not override the
Content-Type header field of a HTTP response obtained by actually
following the link. The "type" attribute MUST NOT appear more than
once in a given link-value; occurrences after the first MUST be
ignored by parsers.
The ABNF for the "type" parameter's value is:
type-name "/" subtype-name ; see {{RFC6838}}, Section 4.2
3.4.2. Extension Attributes
Other link-params are link-extensions, and are to be considered as
target attributes.
Such target attributes MAY be defined to use the encoding in
[I-D.ietf-httpbis-rfc5987bis] (e.g., "example" and "example*"). When
both forms are present, they SHOULD be considered to be the same
target attribute; applications SHOULD use the value of the name
ending in "*" (after [I-D.ietf-httpbis-rfc5987bis] decoding), but MAY
fall back to the other value if there is an error in decoding it, or
if they do not support decoding.
3.5. Link Header Field Examples
For example:
Link: <http://example.com/TheBook/chapter2>; rel="previous";
title="previous chapter"
indicates that "chapter2" is previous to this resource in a logical
navigation path.
Similarly,
Link: </>; rel="http://example.net/foo"
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indicates that the root resource ("/") is related to this resource
with the extension relation type "http://example.net/foo".
This link:
Link: </terms>; rel="copyright"; anchor="#foo"
indicates that the linked copyright terms only apply to the portion
of the document indicated by the (media type-specific) fragment
identifier "foo".
The example below shows an instance of the Link header field encoding
multiple links, and also the use of RFC 5987 encoding to encode both
non-ASCII characters and language information.
Link: </TheBook/chapter2>;
rel="previous"; title*=UTF-8'de'letztes%20Kapitel,
</TheBook/chapter4>;
rel="next"; title*=UTF-8'de'n%c3%a4chstes%20Kapitel
Here, both links have titles encoded in UTF-8, use the German
language ("de"), and the second link contains the Unicode code point
U+00E4 ("LATIN SMALL LETTER A WITH DIAERESIS").
Note that link-values can convey multiple links between the same link
target and link context; for example:
Link: <http://example.org/>;
rel="start http://example.net/relation/other"
Here, the link to "http://example.org/" has the registered relation
type "start" and the extension relation type
"http://example.net/relation/other".
Finally, this header field:
Link: <https://example.org/>; rel="start",
<https://example.org/index>; rel="index"
is equivalent to these:
Link: <https://example.org/>; rel="start"
Link: <https://example.org/index>; rel="index"
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4. IANA Considerations
4.1. Link HTTP Header Field Registration
This specification updates the Message Header registry entry for
"Link" in HTTP [RFC3864] to refer to this document.
Header field: Link
Applicable protocol: http
Status: standard
Author/change controller:
IETF (iesg@ietf.org)
Internet Engineering Task Force
Specification document(s):
[this document]
4.2. Link Relation Type Registry
This specification updates the registration procedures for the Link
Relation Type registry; see Section 2.1.1.1.
IANA will direct any incoming requests regarding the registry to this
document and, if defined, the processes established by the expert(s);
typically, this will mean referring them to the registry Web page.
Note that the expert(s) are allowed (as per Section 2.1.1.1) to
define additional fields to be collected in the registry.
4.3. Link Relation Application Data Registry
This specification removes the Link Relation Application Data
Registry, as it has not been used, and future use is not anticipated.
IANA is instructed to remove it.
5. Security Considerations
The content of the Link header field is not secure, private or
integrity-guaranteed. Use of Transport Layer Security (TLS) with
HTTP ([RFC2818]) is currently the only end-to-end way to provide
these properties.
Link applications ought to consider the attack vectors opened by
automatically following, trusting, or otherwise using links gathered
from HTTP header fields.
For example, Link header fields that use the "anchor" parameter to
associate a link's context with another resource cannot be trusted
since they are effectively assertions by a third party that could be
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incorrect or malicious. Applications can mitigate this risk by
specifying that such links should be discarded unless some
relationship between the resources is established (e.g., they share
the same authority).
Dereferencing links has a number of risks, depending on the
application in use. For example, the Referer header [RFC7231] can
expose information about the application's state (including private
information) in its value. Likewise, cookies [RFC6265] are another
mechanism that, if used, can become an attack vector. Applications
can mitigate these risks by carefully specifying how such mechanisms
should operate.
The Link header field makes extensive use of IRIs and URIs. See
[RFC3987] Section 8 for security considerations relating to IRIs.
See [RFC3986] Section 7 for security considerations relating to URIs.
See [RFC7230] Section 9 for security considerations relating to HTTP
header fields.
6. Internationalisation Considerations
Link targets may need to be converted to URIs in order to express
them in serialisations that do not support IRIs. This includes the
Link HTTP header field.
Similarly, the anchor parameter of the Link header field does not
support IRIs, and therefore IRIs must be converted to URIs before
inclusion there.
Relation types are defined as URIs, not IRIs, to aid in their
comparison. It is not expected that they will be displayed to end
users.
Note that registered Relation Names are required to be lower-case
ASCII letters.
7. References
7.1. Normative References
[I-D.ietf-httpbis-rfc5987bis]
Reschke, J., "Indicating Character Encoding and Language
for HTTP Header Field Parameters", draft-ietf-httpbis-
rfc5987bis-05 (work in progress), February 2017.
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[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<http://www.rfc-editor.org/info/rfc2119>.
[RFC3864] Klyne, G., Nottingham, M., and J. Mogul, "Registration
Procedures for Message Header Fields", BCP 90, RFC 3864,
DOI 10.17487/RFC3864, September 2004,
<http://www.rfc-editor.org/info/rfc3864>.
[RFC3986] Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
Resource Identifier (URI): Generic Syntax", STD 66,
RFC 3986, DOI 10.17487/RFC3986, January 2005,
<http://www.rfc-editor.org/info/rfc3986>.
[RFC3987] Duerst, M. and M. Suignard, "Internationalized Resource
Identifiers (IRIs)", RFC 3987, DOI 10.17487/RFC3987,
January 2005, <http://www.rfc-editor.org/info/rfc3987>.
[RFC5646] Phillips, A., Ed. and M. Davis, Ed., "Tags for Identifying
Languages", BCP 47, RFC 5646, DOI 10.17487/RFC5646,
September 2009, <http://www.rfc-editor.org/info/rfc5646>.
[RFC6838] Freed, N., Klensin, J., and T. Hansen, "Media Type
Specifications and Registration Procedures", BCP 13,
RFC 6838, DOI 10.17487/RFC6838, January 2013,
<http://www.rfc-editor.org/info/rfc6838>.
[RFC7230] Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
Protocol (HTTP/1.1): Message Syntax and Routing",
RFC 7230, DOI 10.17487/RFC7230, June 2014,
<http://www.rfc-editor.org/info/rfc7230>.
[RFC7231] Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
Protocol (HTTP/1.1): Semantics and Content", RFC 7231,
DOI 10.17487/RFC7231, June 2014,
<http://www.rfc-editor.org/info/rfc7231>.
[RFC8126] Cotton, M., Leiba, B., and T. Narten, "Guidelines for
Writing an IANA Considerations Section in RFCs", BCP 26,
RFC 8126, DOI 10.17487/RFC8126, June 2017,
<http://www.rfc-editor.org/info/rfc8126>.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <http://www.rfc-editor.org/info/rfc8174>.
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[W3C.REC-css3-mediaqueries-20120619]
Rivoal, F., "Media Queries", World Wide Web Consortium
Recommendation REC-css3-mediaqueries-20120619, June 2012,
<http://www.w3.org/TR/2012/
REC-css3-mediaqueries-20120619>.
7.2. Informative References
[RFC2046] Freed, N. and N. Borenstein, "Multipurpose Internet Mail
Extensions (MIME) Part Two: Media Types", RFC 2046,
DOI 10.17487/RFC2046, November 1996,
<http://www.rfc-editor.org/info/rfc2046>.
[RFC2818] Rescorla, E., "HTTP Over TLS", RFC 2818,
DOI 10.17487/RFC2818, May 2000,
<http://www.rfc-editor.org/info/rfc2818>.
[RFC4287] Nottingham, M., Ed. and R. Sayre, Ed., "The Atom
Syndication Format", RFC 4287, DOI 10.17487/RFC4287,
December 2005, <http://www.rfc-editor.org/info/rfc4287>.
[RFC6265] Barth, A., "HTTP State Management Mechanism", RFC 6265,
DOI 10.17487/RFC6265, April 2011,
<http://www.rfc-editor.org/info/rfc6265>.
[W3C.REC-html5-20141028]
Hickson, I., Berjon, R., Faulkner, S., Leithead, T.,
Navara, E., O'Connor, T., and S. Pfeiffer, "HTML5",
World Wide Web Consortium Recommendation REC-
html5-20141028, October 2014,
<http://www.w3.org/TR/2014/REC-html5-20141028>.
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Appendix A. Notes on Other Link Serialisations
Header fields (Section 3) are only one serialisation of links; other
specifications have defined alternative serialisations.
A.1. Link Serialisation in HTML
HTML motivated the original syntax of the Link header field, and many
of the design decisions in this document are driven by a desire to
stay compatible with it.
In HTML, the link element can be mapped to links as specified here by
using the "href" attribute for the target URI, and "rel" to convey
the relation type, as in the Link header field. The context of the
link is the URI associated with the entire HTML document. HTML also
defines several attributes on links that can be seen as target
attributes, including "media", "hreflang", "type" and "sizes".
Section 4.8 of HTML5 ([W3C.REC-html5-20141028]) defines modern HTML
links. That document links to the Microformats Wiki as a registry;
over time, the IANA registry ought to mirror its contents, and
ideally eventually replace it (although that depends on the HTML
community).
Surveys of existing HTML content have shown that unregistered link
relation types that are not URIs are (perhaps inevitably) common.
Consuming HTML implementations ought not consider such unregistered
short links to be errors, but rather relation types with a local
scope (i.e., their meaning is specific and perhaps private to that
document).
Finally, the HTML specification gives a special meaning when the
"alternate" relation types coincides with other relation types in the
same link. Such links ought to be serialised in the Link header
field using a single list of relation-types (e.g., rel="alternate
stylesheet") to preserve this relationship.
A.2. Link Serialisation in Atom
Atom [RFC4287] is a link serialisation that conveys links in the
atom:link element, with the "href" attribute indicating the link
target and the "rel" attribute containing the relation type. The
context of the link is either a feed locator or an entry ID,
depending on where it appears; generally, feed-level links are
obvious candidates for transmission as a Link header field.
When serialising an atom:link into a Link header field, it is
necessary to convert link targets (if used) to URIs.
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Atom defines extension relation types in terms of IRIs. This
specification re-defines them as URIs, to simplify and reduce errors
in their comparison.
Atom allows registered link relation types to be serialised as
absolute URIs using a prefix, "http://www.iana.org/assignments/
relation/". This prefix is specific to the Atom serialisation.
Furthermore, link relation types are always compared in a case-
sensitive fashion; therefore, registered link relation types SHOULD
be converted to their registered form (usually, lowercase) when
serialised in an Atom document.
Note also that while the Link header field allows multiple relations
to be serialised in a single link, atom:link does not. In this case,
a single link-value may map to several atom:link elements.
As with HTML, atom:link defines some attributes that are not
explicitly mirrored in the Link header field syntax, but they can
also be used as link-extensions to maintain fidelity.
Appendix B. Algorithms for Parsing Link Header Fields
This appendix outlines a set of non-normative algorithms: for parsing
the Link header(s) out of a header set, parsing a link header field
value, and algorithms for parsing generic parts of the field value.
These algorithms are more permissive than the ABNF defining the
syntax might suggest; the error handling embodied in them is a
reasonable approach, but not one that is required. As such they are
advisory only, and in cases where there is disagreement, the correct
behaviour is defined by the body of this specification.
B.1. Parsing a Header Set for Links
This algorithm can be used to parse the Link header fields that a
HTTP header set contains. Given a "header_set" of (string
"field_name", string "field_value") pairs, assuming ASCII encoding,
it returns a list of link objects.
1. Let "field_values" be a list containing the members of
"header_set" whose "field_name" is a case-insensitive match for
"link".
2. Let "links" be an empty list.
3. For each "field_value" in "field_values":
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1. Let "value_links" be the result of _Parsing A Link Field
Value_ (Appendix B.2) from "field_value".
2. Append each member of "value_links" to "links".
4. Return "links".
B.2. Parsing a Link Field Value
This algorithm parses zero or more comma-separated link-values from a
Link header field. Given a string "field_value", assuming ASCII
encoding, it returns a list of link objects.
1. Let "links" be an empty list.
2. While "field_value" has content:
1. Consume any leading OWS.
2. If the first character is not "<", return "links".
3. Discard the first character ("<").
4. Consume up to but not including the first ">" character or
end of "field_value" and let the result be "target_string".
5. If the next character is not ">", return "links".
6. Discard the leading ">" character.
7. Let "link_parameters", be the result of _Parsing Parameters_
(Appendix B.3) from "field_value" (consuming zero or more
characters of it).
8. Let "target" be the result of relatively resolving (as per
[RFC3986], Section 5.2) "target_string". Note that any base
URI carried in the payload body is NOT used.
9. Let "relations_string" be the second item of the first tuple
of "link_parameters" whose first item matches the string
"rel", or the empty string ("") if it is not present.
10. Split "relations_string" on RWS (removing it in the process)
into a list of strings "relation_types".
11. Let "context_string" be the second item of the first tuple
of "link_parameters" whose first item matches the string
"anchor". If it is not present, "context_string" is the URL
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of the representation carrying the Link header [RFC7231],
Section 3.1.4.1, serialised as a URI. Where the URL is
anonymous, "context_string" is null.
12. Let "context" be the result of relatively resolving (as per
[RFC3986], Section 5.2) "context_string", unless
"context_string" is null in which case "context" is null.
Note that any base URI carried in the payload body is NOT
used.
13. Let "target_attributes" be an empty list.
14. For each tuple ("param_name", "param_value") of
"link_parameters":
1. If "param_name" matches "rel" or "anchor", skip this
tuple.
2. If "param_name" matches "media", "title", "title*" or
"type" and "target_attributes" already contains a tuple
whose first element matches the value of "param_name",
skip this tuple.
3. Append ("param_name", "param_value") to
"target_attributes".
15. Let "star_param_names" be the set of "param_name"s in the
("param_name", "param_value") tuples of "link_parameters"
where the last character of "param_name" is an asterisk
("*").
16. For each "star_param_name" in "star_param_names":
1. Let "base_param_name" be "star_param_name" with the last
character removed.
2. If the implementation does not choose to support an
internationalised form of a parameter named
"base_param_name" for any reason (including, but not
limited to, it being prohibited by the parameter's
specification), remove all tuples from "link_parameters"
whose first member is "star_param_name" and skip to the
next "star_param_name".
3. Remove all tuples from "link_parameters" whose first
member is "base_param_name".
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4. Change the first member of all tuples in
"link_parameters" whose first member is
"star_param_name" to "base_param_name".
17. For each "relation_type" in "relation_types":
1. Case-normalise "relation_type" to lowercase.
2. Append a link object to "links" with the target
"target", relation type of "relation_type", context of
"context", and target attributes "target_attributes".
3. Return "links".
B.3. Parsing Parameters
This algorithm parses the parameters from a header field value.
Given an ASCII string "input", it returns a list of (string
"parameter_name", string "parameter_value") tuples that it contains.
"input" is modified to remove the parsed parameters.
1. Let "parameters" be an empty list.
2. While "input" has content:
1. Consume any leading OWS.
2. If the first character is not ";", return "parameters".
3. Discard the leading ";" character.
4. Consume any leading OWS.
5. Consume up to but not including the first BWS, "=", ";", ","
character or end of "input" and let the result be
"parameter_name".
6. Consume any leading BWS.
7. If the next character is "=":
1. Discard the leading "=" character.
2. Consume any leading BWS.
3. If the next character is DQUOTE, let "parameter_value"
be the result of _Parsing a Quoted String_
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(Appendix B.4) from "input" (consuming zero or more
characters of it).
4. Else, consume the contents up to but not including the
first ";", "," character or end of "input" and let the
results be "parameter_value".
5. If the last character of "parameter_name" is an asterisk
("*"), decode "parameter_value" according to
[I-D.ietf-httpbis-rfc5987bis]. Continue processing
"input" if an unrecoverable error is encountered.
8. Else:
1. Let "parameter_value" be an empty string.
9. Case-normalise "parameter_name" to lowercase.
10. Append ("parameter_name", "parameter_value") to
"parameters".
11. Consume any leading OWS.
12. If the next character is "," or the end of "input", stop
processing "input" and return "parameters".
B.4. Parsing a Quoted String
This algorithm parses a quoted string, as per [RFC7230],
Section 3.2.6. Given an ASCII string "input", it returns an unquoted
string. "input" is modified to remove the parsed string.
1. Let "output" be an empty string.
2. If the first character of "input" is not DQUOTE, return "output".
3. Discard the first character.
4. While "input" has content:
1. If the first character is a backslash ("\"):
1. Discard the first character.
2. If there is no more "input", return "output".
3. Else, consume the first character and append it to
"output".
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2. Else, if the first character is DQUOTE, discard it and return
"output".
3. Else, consume the first character and append it to "output".
5. Return "output".
Appendix C. Changes from RFC5988
This specification has the following differences from its
predecessor, RFC5988:
o The initial relation type registrations were removed, since
they've already been registered by 5988.
o The introduction has been shortened.
o The Link Relation Application Data Registry has been removed.
o Incorporated errata.
o Updated references.
o Link cardinality was clarified.
o Terminology was changed from "target IRI" and "context IRI" to
"link target" and "link context" respectively.
o Made assigning a URI to registered relation types serialisation-
specific.
o Removed misleading statement that the link header field is
semantically equivalent to HTML and Atom links.
o More carefully defined and used "link serialisations" and "link
applications."
o Clarified the cardinality of target attributes (generically and
for "type").
o Corrected the default link context for the Link header field, to
be dependent upon the identity of the representation (as per
RFC7231).
o Defined a suggested parsing algorithm for the Link header.
o The value space of target attributes and their definition has been
specified.
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o The ABNF has been updated to be compatible with [RFC7230]. In
particular, whitespace is now explicit.
o Some parameters on the HTTP header field can now appear as a
token.
o Parameters on the HTTP header can now be value-less.
o Handling of quoted strings is now defined by [RFC7230].
o The "type" header field parameter now needs to be quoted (as
"token" does not allow "/").
Author's Address
Mark Nottingham
EMail: mnot@mnot.net
URI: https://www.mnot.net/
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