Web Linkingmnot@mnot.nethttps://www.mnot.net/
General
link relationThis 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.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to revise RFC5988.The issues list can be found at https://github.com/mnot/I-D/labels/rfc5988bis.The most recent (often, unpublished) draft is at https://mnot.github.io/I-D/rfc5988bis/.Recent changes are listed at https://github.com/mnot/I-D/commits/gh-pages/rfc5988bis.This specification defines a model for the relationships between resources on the Web
(“links”) and the type of those relationships (“link relation types”).HTML and Atom both have well-defined concepts of linking;
generalises this into a framework that encompasses linking in these formats and
(potentially) elsewhere.Furthermore, defines an HTTP header field for conveying such links.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 , when, and only when, they appear in
all capitals, as shown here.This document uses the Augmented Backus-Naur Form (ABNF) notation of , including the
#rule, and explicitly includes the following rules from it: quoted-string, token, SP (space),
BWS (bad whitespace), OWS (optional whitespace), RWS (required whitespace) LOALPHA, DIGIT.Additionally, the following rules are included from : URI and URI-Reference; from
: type-name and subtype-name; from :
media-query-list; and from : Language-Tag.The requirements regarding conformance and error handling highlighted in , Section 2.5
apply to this document.In this specification, a link is a typed connection between two resources, and is comprised of:A link context,a link relation type (),a link target, andoptionally, target attributes ().A link can be viewed as a statement of the form “link context has a link relation type resource
at link target, which has target attributes”.Link contexts and link targets are both IRIs . However, in the common case, the link
context will also be a URI , because many protocols (such as HTTP) do not support
dereferencing IRIs. Likewise, the link target will be sometimes be converted to a URI (see
, Section 3.1) in serialisations that do not support IRIs (such as the Link header field
defined in ).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 and HTML both defined
serialisations of links into their respective formats, and 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.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 ; 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.Well-defined relation types can be registered as tokens for convenience and/or to promote reuse by
other applications, using the procedure in .Registered relation type names MUST conform to the reg-rel-type rule (see ), 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 by prefixing
their names with an application-defined base URI (e.g., see ). 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.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:Relation Name: The name of the relation typeDescription: 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.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) MAY define additional fields to be collected in the registry.General requirements for registered relation types are described in .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.Relation types are registered on the advice of a Designated Expert (appointed by the IESG or their
delegate), with a Specification Required (using terminology from Section 4.1 of ).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
, the Experts MAY withhold registration of names that are too general for the
proposed application.The Expert(s) MUST clearly identify any issues which cause a registration to be refused. Advice
about the semantics of a proposed link relation type can be given, but if it does not block
registration, this SHOULD be explicitly stated.When a request is approved, the Expert(s) will inform IANA, and the registration will be processed.
The IESG is the final arbiter of any objection.Applications that don’t wish to register a relation type can use an extension relation type, which
is a URI that uniquely identifies the relation type. Although the URI can point to a
resource that contains a definition of the semantics of the relation type, clients SHOULD NOT
automatically access that resource to avoid overburdening its server.The URI used for an extension relation type SHOULD be under the control of the person or party
defining it, or be delegated to them.When extension relation types are compared, they MUST be compared as strings (after converting to
URIs if serialised in a different format) in a case-insensitive fashion, character-by-character.
Because of this, all-lowercase URIs SHOULD be used for extension relations.Note that while extension relation types are required to be URIs, a serialisation of links can
specify that they are expressed in another form, as long as they can be converted to URIs.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:The serialisation of their values into Unicode or a subset thereof, to maximise their chances of
portability across link serialisations.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
.The Link header field provides a means for serialising one or more links into HTTP headers.The ABNF for the field value is:Note that any link-param can be generated with values using either the token or the
quoted-string syntax, and therefore recipients MUST be able to parse both forms. Individual
link-params specify their syntax in terms of the value after any necessary unquoting (as per
, Section 3.2.6).This specification defines the link-params “rel”, “anchor”, “rev”, “hreflang”, “media”, “title”,
“title*”, and “type”; see , and .Each link-value conveys one target IRI as a URI-Reference (after conversion to one, if necessary;
see , Section 3.1) inside angle brackets (“<>”). If the URI-Reference is relative,
parsers MUST resolve it as per , Section 5. Note that any base IRI appearing in the
message’s content is not applied.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 , 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 , Section 5. Note
that any base URI from the body’s content is not applied.The ABNF for the anchor parameter’s value is: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.The relation type of a link conveyed in the Link header field is conveyed in the “rel” parameter’s
value. The “rel” parameter MUST NOT appear more than once in a given link-value; occurrences after
the first MUST be ignored by parsers.The “rev” parameter has been used in the past to indicate that the semantics of the relationship
are in the reverse direction. That is, a link from A to B with REL=”X” expresses the same
relationship as a link from B to A with REV=”X”. “rev” is deprecated by this specification because
it often confuses authors and readers; in most cases, using a separate relation type is preferable.The ABNF for the rel and rev parameters’ values is:where: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).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 , Section 3.1.1.1 for the definition of their syntax).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:The “media” attribute, when present, is used to indicate intended destination medium or media for
style information (see , 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: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 . The “title*” link-param MUST
NOT appear more than once in a given link-value; occurrences after the first MUST be ignored by
parsers. If the attribute does not contain language information, its language is indicated by the
Content-Language header field (when present).If both the “title” and “title*” link-param appear in a link, 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: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 (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
decoding), but MAY fall back to the other value if there is an
error in decoding it, or if they do not support decoding.For example:indicates that “chapter2” is previous to this resource in a logical navigation path.Similarly,indicates that the root resource (“/”) is related to this resource with the extension relation type
“http://example.net/foo”.This link: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.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: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:is equivalent to these:This specification updates the Message Header registry entry for “Link” in HTTP to
refer to this document.This specification updates the registration procedures for the Link Relation Type registry; see
. The Expert(s) and IANA are expected interact as outlined below.The Expert(s) will provide registry data to IANA in a mutually-agreed form (e.g. a specific XML
format). IANA will publish:The raw registry dataThe registry data, transformed into HTMLThe registry data alternative formats provided by the Expert(s) (if any)If IANA’s internal processes require making changes to registry data and/or adding registry entries,
IANA will inform the Expert(s) of this in a mutually agreed way.Each published document will be at a URL mutually agreed to by IANA and the Expert(s), and IANA
will set HTTP response headers on them as (reasonably) requested by the Expert(s).Additionally, the HTML generated by IANA will:Take directions from the Expert(s) as to the content of the HTML page’s introductory textInclude a stable HTML fragment identifier for each registered link relationAll registry data documents MUST include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions ().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 ) to define additional fields to be
collected in the registry.This specification terminates the Link Relation Application Data Registry, as it has not been used,
and future use is not anticipated. IANA is instructed to remove it.The content of the Link header field is not secure, private or integrity-guaranteed. Use of
Transport Layer Security (TLS) with HTTP () 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. In particular, Link header fields that
use the “anchor” parameter to associate a link’s context with another resource are to be treated
with due caution.The Link header field makes extensive use of IRIs and URIs. See Section 8 for security
considerations relating to IRIs. See Section 7 for security considerations relating to
URIs. See Section 9 for security considerations relating to HTTP header fields.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.Key words for use in RFCs to Indicate Requirement LevelsIn many standards track documents several words are used to signify the requirements in the specification. These words are often capitalized. This document defines these words as they should be interpreted in IETF documents. This document specifies an Internet Best Current Practices for the Internet Community, and requests discussion and suggestions for improvements.Registration Procedures for Message Header FieldsThis specification defines registration procedures for the message header fields used by Internet mail, HTTP, Netnews and other applications. This document specifies an Internet Best Current Practices for the Internet Community, and requests discussion and suggestions for improvements.Uniform Resource Identifier (URI): Generic SyntaxA Uniform Resource Identifier (URI) is a compact sequence of characters that identifies an abstract or physical resource. This specification defines the generic URI syntax and a process for resolving URI references that might be in relative form, along with guidelines and security considerations for the use of URIs on the Internet. The URI syntax defines a grammar that is a superset of all valid URIs, allowing an implementation to parse the common components of a URI reference without knowing the scheme-specific requirements of every possible identifier. This specification does not define a generative grammar for URIs; that task is performed by the individual specifications of each URI scheme. [STANDARDS-TRACK]Internationalized Resource Identifiers (IRIs)This document defines a new protocol element, the Internationalized Resource Identifier (IRI), as a complement of the Uniform Resource Identifier (URI). An IRI is a sequence of characters from the Universal Character Set (Unicode/ISO 10646). A mapping from IRIs to URIs is defined, which means that IRIs can be used instead of URIs, where appropriate, to identify resources. The approach of defining a new protocol element was chosen instead of extending or changing the definition of URIs. This was done in order to allow a clear distinction and to avoid incompatibilities with existing software. Guidelines are provided for the use and deployment of IRIs in various protocols, formats, and software components that currently deal with URIs.Media Type Specifications and Registration ProceduresThis document defines procedures for the specification and registration of media types for use in HTTP, MIME, and other Internet protocols. This memo documents an Internet Best Current Practice.Guidelines for Writing an IANA Considerations Section in RFCsMany protocols make use of identifiers consisting of constants and other well-known values. Even after a protocol has been defined and deployment has begun, new values may need to be assigned (e.g., for a new option type in DHCP, or a new encryption or authentication transform for IPsec). To ensure that such quantities have consistent values and interpretations across all implementations, their assignment must be administered by a central authority. For IETF protocols, that role is provided by the Internet Assigned Numbers Authority (IANA).In order for IANA to manage a given namespace prudently, it needs guidelines describing the conditions under which new values can be assigned or when modifications to existing values can be made. If IANA is expected to play a role in the management of a namespace, IANA must be given clear and concise instructions describing that role. This document discusses issues that should be considered in formulating a policy for assigning values to a namespace and provides guidelines for authors on the specific text that must be included in documents that place demands on IANA.This document obsoletes RFC 2434. This document specifies an Internet Best Current Practices for the Internet Community, and requests discussion and suggestions for improvements.Tags for Identifying LanguagesThis document describes the structure, content, construction, and semantics of language tags for use in cases where it is desirable to indicate the language used in an information object. It also describes how to register values for use in language tags and the creation of user-defined extensions for private interchange. This document specifies an Internet Best Current Practices for the Internet Community, and requests discussion and suggestions for improvements.Hypertext Transfer Protocol (HTTP/1.1): Message Syntax and RoutingThe Hypertext Transfer Protocol (HTTP) is a stateless application-level protocol for distributed, collaborative, hypertext information systems. This document provides an overview of HTTP architecture and its associated terminology, defines the "http" and "https" Uniform Resource Identifier (URI) schemes, defines the HTTP/1.1 message syntax and parsing requirements, and describes related security concerns for implementations.Hypertext Transfer Protocol (HTTP/1.1): Semantics and ContentThe Hypertext Transfer Protocol (HTTP) is a stateless \%application- level protocol for distributed, collaborative, hypertext information systems. This document defines the semantics of HTTP/1.1 messages, as expressed by request methods, request header fields, response status codes, and response header fields, along with the payload of messages (metadata and body content) and mechanisms for content negotiation.Media QueriesIndicating Character Encoding and Language for HTTP Header Field ParametersBy default, header field values in Hypertext Transfer Protocol (HTTP) messages cannot easily carry characters outside the US-ASCII coded character set. RFC 2231 defines an encoding mechanism for use in parameters inside Multipurpose Internet Mail Extensions (MIME) header field values. This document specifies an encoding suitable for use in HTTP header fields that is compatible with a simplified profile of the encoding defined in RFC 2231. This document obsoletes RFC 5987.Ambiguity of Uppercase vs Lowercase in RFC 2119 Key WordsRFC 2119 specifies common key words that may be used in protocol specifications. This document aims to reduce the ambiguity by clarifying that only UPPERCASE usage of the key words have the defined special meanings.Multipurpose Internet Mail Extensions (MIME) Part Two: Media TypesThis second document defines the general structure of the MIME media typing system and defines an initial set of media types. [STANDARDS-TRACK]HTTP Over TLSThis memo describes how to use Transport Layer Security (TLS) to secure Hypertext Transfer Protocol (HTTP) connections over the Internet. This memo provides information for the Internet community.The Atom Syndication FormatThis document specifies Atom, an XML-based Web content and metadata syndication format. [STANDARDS-TRACK]HTML5Header fields () are only one serialisation of links; other specifications have defined
alternative serialisations.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 () 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.Atom 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.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.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.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.Let field_values be a list containing the members of header_set whose field_name is a
case-insensitive match for “link”.Let links be an empty list.For each field_value in field_values: Let value_links be the result of Parsing A Link Field Value () from
field_value.Append each member of value_links to links.Return links.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.Let links be an empty list.While field_value has content: Consume any leading OWS.If the first character is not “<”, return links.Discard the first character (“<”).Consume up to but not including the first “>” character or end of field_value and let the
result be target_string.If the next character is not “>”, return links.Discard the leading “>” character.Let link_parameters, be the result of Parsing Parameters () from
field_value (consuming zero or more characters of it).Let target be the result of relatively resolving (as per , Section 5.2)
target_string. Note that any base URI carried in the payload body is NOT used.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.Split relations_string on RWS (removing it in the process) into a list of strings
relation_types.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 of
the representation carrying the Link header , Section 3.1.4.1, serialised as a
URI. Where the URL is anonymous, context_string is null.Let context be the result of relatively resolving (as per , 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.Let target_attributes be an empty list.For each tuple (param_name, param_value) of link_parameters: If param_name matches “rel” or “anchor”, skip this tuple.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.Append (param_name, param_value) to target_attributes.Let star_param_names be the set of param_names in the (param_name, param_value) tuples
of link_parameters where the last character of param_name is an asterisk (“*”).For each star_param_name in star_param_names: Let base_param_name be star_param_name with the last character removed.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.Remove all tuples from link_parameters whose first member is base_param_name.Change the first member of all tuples in link_parameters whose first member is
star_param_name to base_param_name.For each relation_type in relation_types: Case-normalise relation_type to lowercase.Append a link object to links with the target target, relation type of
relation_type, context of context, and target attributes target_attributes.Return links.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.Let parameters be an empty list.While input has content: Consume any leading OWS.If the first character is not “;”, return parameters.Discard the leading “;” character.Consume any leading OWS.Consume up to but not including the first BWS, “=”, “;”, “,” character or end of input and
let the result be parameter_name.Consume any leading BWS.If the next character is “=”: Discard the leading “=” character.Consume any leading BWS.If the next character is DQUOTE, let parameter_value be the result of Parsing a Quoted
String () from input (consuming zero or more characters of it).Else, consume the contents up to but not including the first “;”, “,” character or end of
input and let the results be parameter_value.If the last character of parameter_name is an asterisk (“*”), decode parameter_value
according to . Continue processing input if an unrecoverable
error is encountered.Else: Let parameter_value be an empty string.Case-normalise parameter_name to lowercase.Append (parameter_name, parameter_value) to parameters.Consume any leading OWS.If the next character is “,” or the end of input, stop processing input and return
parameters.This algorithm parses a quoted string, as per , Section 3.2.6. Given an ASCII string
input, it returns an unquoted string. input is modified to remove the parsed string.Let output be an empty string.If the first character of input is not DQUOTE, return output.Discard the first character.While input has content: If the first character is a backslash (“\”): Discard the first character.If there is no more input, return output.Else, consume the first character and append it to output.Else, if the first character is DQUOTE, discard it and return output.Else, consume the first character and append it to output.Return output.This specification has the following differences from its predecessor, RFC5988:The initial relation type registrations were removed, since they’ve already been registered by
5988.The introduction has been shortened.The Link Relation Application Data Registry has been removed.Incorporated errata.Updated references.Link cardinality was clarified.Terminology was changed from “target IRI” and “context IRI” to “link target” and “link context”
respectively.Made assigning a URI to registered relation types serialisation-specific.Removed misleading statement that the link header field is semantically equivalent to HTML and
Atom links.More carefully defined how the Experts and IANA should interact.More carefully defined and used “link serialisations” and “link applications.”Clarified the cardinality of target attributes (generically and for “type”).Corrected the default link context for the Link header field, to be dependent upon the identity
of the representation (as per RFC7231).Defined a suggested parsing algorithm for the Link header.The value space of target attributes and their definition has been specified.The ABNF has been updated to be compatible with . In particular, whitespace is now
explicit.Some parameters on the HTTP header field can now appear as a token.Handling of quoted strings is now defined by .The type header field parameter now needs to be quoted (as token does not allow “/”).