Web Linking
draft-nottingham-rfc5988bis-00
The information below is for an old version of the document.
| Document | Type |
This is an older version of an Internet-Draft that was ultimately published as RFC 8288.
|
|
|---|---|---|---|
| Author | Mark Nottingham | ||
| Last updated | 2015-11-03 | ||
| RFC stream | (None) | ||
| Formats | |||
| Reviews |
GENART Telechat review
(of
-07)
by Stewart Bryant
Ready w/nits
GENART Last Call review
(of
-05)
by Stewart Bryant
Ready w/nits
|
||
| Additional resources | |||
| Stream | Stream state | (No stream defined) | |
| Consensus boilerplate | Unknown | ||
| RFC Editor Note | (None) | ||
| IESG | IESG state | Became RFC 8288 (Proposed Standard) | |
| Telechat date | (None) | ||
| Responsible AD | (None) | ||
| Send notices to | (None) |
draft-nottingham-rfc5988bis-00
Network Working Group M. Nottingham
Internet-Draft November 4, 2015
Obsoletes: 5988 (if approved)
Intended status: Standards Track
Expires: May 7, 2016
Web Linking
draft-nottingham-rfc5988bis-00
Abstract
This specification defines a way to indicate the relationships
between resources on the Web ("links") and the type of those
relationships ("link relation types").
It also defines the use of such links in HTTP headers with the Link
header field.
Note to Readers
This is a work-in-progress to revise RFC5988.
The issues list can be found at https://github.com/mnot/I-D/labels/
rfc5988bis .
The most recent (often, unpublished) draft is at
https://mnot.github.io/I-D/rfc5988bis/ .
Recent changes are listed at https://github.com/mnot/I-D/commits/gh-
pages/rfc5988bis .
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at http://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on May 7, 2016.
Nottingham Expires May 7, 2016 [Page 1]
Internet-Draft Web Linking November 2015
Copyright Notice
Copyright (c) 2015 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
This document may contain material from IETF Documents or IETF
Contributions published or made publicly available before November
10, 2008. The person(s) controlling the copyright in some of this
material may not have granted the IETF Trust the right to allow
modifications of such material outside the IETF Standards Process.
Without obtaining an adequate license from the person(s) controlling
the copyright in such materials, this document may not be modified
outside the IETF Standards Process, and derivative works of it may
not be created outside the IETF Standards Process, except to format
it for publication as an RFC or to translate it into languages other
than English.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Notational Conventions . . . . . . . . . . . . . . . . . . . 3
3. Links . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
4. Link Relation Types . . . . . . . . . . . . . . . . . . . . . 4
4.1. Registered Relation Types . . . . . . . . . . . . . . . . 5
4.1.1. Registering Link Relation Types . . . . . . . . . . . 5
4.2. Extension Relation Types . . . . . . . . . . . . . . . . 6
5. The Link Header Field . . . . . . . . . . . . . . . . . . . . 7
5.1. Link Target . . . . . . . . . . . . . . . . . . . . . . . 7
5.2. Link Context . . . . . . . . . . . . . . . . . . . . . . 8
5.3. Relation Type . . . . . . . . . . . . . . . . . . . . . . 8
5.4. Target Attributes . . . . . . . . . . . . . . . . . . . . 8
5.5. Examples . . . . . . . . . . . . . . . . . . . . . . . . 9
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 10
6.1. Link HTTP Header Registration . . . . . . . . . . . . . . 10
6.2. Link Relation Type Registry . . . . . . . . . . . . . . . 11
7. Security Considerations . . . . . . . . . . . . . . . . . . . 11
8. Internationalisation Considerations . . . . . . . . . . . . . 12
9. References . . . . . . . . . . . . . . . . . . . . . . . . . 12
Nottingham Expires May 7, 2016 [Page 2]
Internet-Draft Web Linking November 2015
9.1. Normative References . . . . . . . . . . . . . . . . . . 12
9.2. Informative References . . . . . . . . . . . . . . . . . 13
Appendix A. Using the Link Header with the HTML Format . . . . . 15
Appendix B. Using the Link Header with the Atom Format . . . . . 15
Appendix C. Changes from RFC5988 . . . . . . . . . . . . . . . . 16
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 17
1. Introduction
This specification defines a way to indicate 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; this specification generalises this into
a framework that encompasses linking in these formats and
(potentially) elsewhere.
Furthermore, this specification formalises an HTTP header field for
conveying such links, having been originally defined in
Section 19.6.2.4 of [RFC2068], but removed from [RFC2616].
2. Notational Conventions
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in BCP 14, [RFC2119], as
scoped to those conformance targets.
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), LOALPHA, DIGIT.
Additionally, the following rules are included from [RFC3986]: URI
and URI-Reference; from [RFC6838]: type-name and subtype-name; from
[W3C.CR-css3-mediaqueries-20090915]: media_query_list; from
[RFC5646]: Language-Tag; and from [I-D.ietf-httpbis-rfc5987bis], ext-
value and parmname.
3. 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 4),
Nottingham Expires May 7, 2016 [Page 3]
Internet-Draft Web Linking November 2015
o a link target, and
o optionally, 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 [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 places that do not support IRIs
(such as the Link header field defined in Section 5).
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 and in-content links) is not specified or significant in this
specification; applications that wish to consider ordering
significant can do so.
Target attributes are a set of key/value pairs that describe the link
or its target; for example, a media type hint. This specification
does not attempt to coordinate their names or use, but does provide
common target attributes for use in the Link HTTP header.
Finally, this specification does not define a general syntax for
expressing links, nor does it mandate a specific context for any
given link; it is expected that serialisations of links will specify
both aspects. One such serialisation is communication of links
through HTTP headers, specified in Section 5.
4. 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 resource identified by the link target is a
statement of the copyright terms applying to the current link
context.
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 identified
resource is part of a defined protocol (in this case, a service
description).
Nottingham Expires May 7, 2016 [Page 4]
Internet-Draft Web Linking November 2015
Relation types are not to be confused with media types [RFC6838];
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.
4.1. 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 4.1.1.
Registered relation type names MUST conform to the reg-rel-type rule,
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 must be
supported).
Applications that wish to refer to a registered relation type with a
URI [RFC3986] MAY do so by prepending
"http://www.iana.org/assignments/relation/" to its name. Note that
the resulting strings are not considered equivalent to the registered
relation types by many processors, and SHOULD NOT be serialised
unless the application using link relations specifically allows them.
4.1.1. Registering Link Relation Types
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 [RFC5226]).
The Expert(s) will establish procedures for requesting registrations,
and make them available from the registry page.
Nottingham Expires May 7, 2016 [Page 5]
Internet-Draft Web Linking November 2015
Registration requests consist of at least the following information:
o Relation Name:
o Description:
o Reference:
The Expert(s) MAY define additional fields to be collected in the
registry.
General requirements for registered relation types are described in
Section 4.1.
See the registry for examples of the description field; generally, it
SHOULD identify the semantics in terms of the link's context and
target.
Registrations MUST reference a freely available, stable
specification.
Note that relation types can be registered by third parties, if the
Expert(s) determine that an unregistered relation type is widely
deployed and not likely to be registered in a timely manner.
Decisions (or lack thereof) made by the Expert(s) can be first
appealed to Application Area Directors (contactable using app-
ads@tools.ietf.org email address or directly by looking up their
email addresses on http://www.iesg.org/ website) and, if the
appellant is not satisfied with the response, to the full IESG (using
the iesg@iesg.org mailing list).
4.2. 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.
When extension relation types are compared, they MUST be compared as
strings (after converting to URIs if serialised in a different
format, such as a XML QNames [W3C.REC-xml-names-20091208]) in a case-
insensitive fashion, character-by-character. Because of this, all-
lowercase URIs SHOULD be used for extension relations.
Nottingham Expires May 7, 2016 [Page 6]
Internet-Draft Web Linking November 2015
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.
5. The Link Header Field
The Link entity-header field provides a means for serialising one or
more links in HTTP headers.
Link = "Link" ":" #link-value
link-value = "<" URI-Reference ">" *( ";" link-param )
link-param = ( ( "rel" "=" relation-types )
| ( "anchor" "=" <"> URI-Reference <"> )
| ( "rev" "=" relation-types )
| ( "hreflang" "=" Language-Tag )
| ( "media" "="
( media_query_list | ( <"> media_query_list <"> ) )
)
| ( "title" "=" quoted-string )
| ( "title*" "=" ext-value )
| ( "type" "=" ( media-type | quoted-mt ) )
| ( link-extension ) )
link-extension = ( parmname [ "=" ( ptoken | quoted-string ) ] )
| ( ext-name-star "=" ext-value )
ext-name-star = parmname "*" ; reserved for RFC5987-profiled
; extensions. Whitespace NOT
; allowed in between.
ptoken = 1*ptokenchar
ptokenchar = "!" | "#" | "$" | "%" | "&" | "'" | "("
| ")" | "*" | "+" | "-" | "." | "/" | DIGIT
| ":" | "<" | "=" | ">" | "?" | "@" | ALPHA
| "[" | "]" | "^" | "_" | "`" | "{" | "|"
| "}" | "~"
media-type = type-name "/" subtype-name
quoted-mt = <"> media-type <">
relation-types = relation-type
| <"> relation-type *( 1*SP relation-type ) <">
relation-type = reg-rel-type | ext-rel-type
reg-rel-type = LOALPHA *( LOALPHA | DIGIT | "." | "-" )
ext-rel-type = URI
5.1. Link Target
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
from the message's content is not applied.
Nottingham Expires May 7, 2016 [Page 7]
Internet-Draft Web Linking November 2015
5.2. Link Context
By default, the context of a link conveyed in the Link header field
is the IRI of the requested resource.
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.
Consuming implementations 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; consuming implementations 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 instance, this is the case on a 404 response to a
GET request.
5.3. Relation Type
The relation type of a link 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.
Note that extension relation types are REQUIRED to be absolute URIs
in Link headers, and MUST be quoted if they contain a semicolon (";")
or comma (",") (as these characters are used as delimiters in the
header itself).
5.4. Target Attributes
The "hreflang", "media", "title", "title*", "type", and any link-
extension link-params are considered to be target attributes for the
link.
Nottingham Expires May 7, 2016 [Page 8]
Internet-Draft Web Linking November 2015
The "hreflang" parameter, 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 of a HTTP response obtained by actually
following the link. Multiple "hreflang" parameters on a single link-
value indicate that multiple languages are available from the
indicated resource.
The "media" parameter, 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 (","), and there MUST NOT
be more than one "media" parameter in a link-value.
The "title" parameter, 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 (if present). The "title" parameter MUST NOT appear
more than once in a given link-value; occurrences after the first
MUST be ignored by parsers.
The "title*" parameter can be used to encode this label in a
different character set, and/or contain language information as per
[I-D.ietf-httpbis-rfc5987bis]. The "title*" parameter MUST NOT
appear more than once in a given link-value; occurrences after the
first MUST be ignored by parsers. If the parameter does not contain
language information, its language is indicated by the Content-
Language header (when present).
If both the "title" and "title*" parameters appear in a link-value,
processors SHOULD use the "title*" parameter's value.
The "type" parameter, 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 of a HTTP response obtained by actually following
the link. There MUST NOT be more than one type parameter in a link-
value.
5.5. 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.
Nottingham Expires May 7, 2016 [Page 9]
Internet-Draft Web Linking November 2015
Similarly,
Link: </>; rel="http://example.net/foo"
indicates that the root resource ("/") is related to this resource
with the extension relation type "http://example.net/foo".
The example below shows an instance of the Link header 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".
6. IANA Considerations
In addition to the actions below, IANA should terminate the Link
Relation Application Data Registry, as it has not been used, and
future use is not anticipated.
6.1. Link HTTP Header Registration
This specification updates the Message Header registry entry for
"Link" in HTTP [RFC3864] to refer to this document.
Nottingham Expires May 7, 2016 [Page 10]
Internet-Draft Web Linking November 2015
Header field: Link
Applicable protocol: http
Status: standard
Author/change controller:
IETF (iesg@ietf.org)
Internet Engineering Task Force
Specification document(s):
[RFC&rfc.number;]
6.2. Link Relation Type Registry
This specification updates the registration procedures for the Link
Relation Type registry; see Section 4.1.1. The Expert(s) and IANA
will interact as outlined below.
IANA will direct any incoming requests regarding the registry to the
processes established by the Expert(s); typically, this will mean
referring them to the registry HTML page.
The Expert(s) will provide registry data to IANA in an agreed form
(e.g. a specific XML format). IANA will publish: * The raw registry
data * The registry data, transformed into HTML * The registry data
in any alternative formats provided by the Expert(s)
Each published document will be at a URL agreed to by IANA and the
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 text
and markup * Include a stable HTML fragment identifier for each
registered link relation
All registry data documents MUST include Simplified BSD License text
as described in Section 4.e of the Trust Legal Provisions
(<http://trustee.ietf.org/license-info>).
7. Security Considerations
The content of the Link header field is not secure, private or
integrity-guaranteed, and due caution should be exercised when using
it. Use of Transport Layer Security (TLS) with HTTP ([RFC2818] and
[RFC2817]) is currently the only end-to-end way to provide such
protection.
Applications that take advantage of typed links should consider the
attack vectors opened by automatically following, trusting, or
otherwise using links gathered from HTTP headers. In particular,
Nottingham Expires May 7, 2016 [Page 11]
Internet-Draft Web Linking November 2015
Link headers that use the "anchor" parameter to associate a link's
context with another resource should be treated with due caution.
The Link entity-header field makes extensive use of IRIs and URIs.
See [RFC3987] for security considerations relating to IRIs. See
[RFC3986] for security considerations relating to URIs. See
[RFC7230] for security considerations relating to HTTP headers.
8. 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.
Similarly, the anchor parameter of the Link header 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.
9. References
9.1. Normative References
[I-D.ietf-httpbis-rfc5987bis]
Reschke, J., "Indicating Character Encoding and Language
for HTTP Header Field Parameters", draft-ietf-httpbis-
rfc5987bis-00 (work in progress), October 2015.
[RFC2026] Bradner, S., "The Internet Standards Process -- Revision
3", BCP 9, RFC 2026, DOI 10.17487/RFC2026, October 1996,
<http://www.rfc-editor.org/info/rfc2026>.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
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>.
Nottingham Expires May 7, 2016 [Page 12]
Internet-Draft Web Linking November 2015
[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>.
[RFC5226] Narten, T. and H. Alvestrand, "Guidelines for Writing an
IANA Considerations Section in RFCs", BCP 26, RFC 5226,
DOI 10.17487/RFC5226, May 2008,
<http://www.rfc-editor.org/info/rfc5226>.
[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>.
[W3C.CR-css3-mediaqueries-20090915]
Lie, H., Celik, T., Glazman, D., and A. Kesteren, "Media
Queries", World Wide Web Consortium CR CR-css3-
mediaqueries-20090915, September 2009,
<http://www.w3.org/TR/2009/CR-css3-mediaqueries-20090915>.
9.2. 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>.
Nottingham Expires May 7, 2016 [Page 13]
Internet-Draft Web Linking November 2015
[RFC2817] Khare, R. and S. Lawrence, "Upgrading to TLS Within
HTTP/1.1", RFC 2817, DOI 10.17487/RFC2817, May 2000,
<http://www.rfc-editor.org/info/rfc2817>.
[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>.
[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]
Hickson, I., Berjon, R., Faulkner, S., Leithead, T.,
Navara, E., O'Connor, E., and S. Pfeiffer, "HTML5",
World Wide Web Consortium Recommendation REC-
html5-20141028, October 2014,
<http://www.w3.org/TR/2014/REC-html5-20141028>.
[W3C.REC-xml-names-20091208]
Bray, T., Hollander, D., Layman, A., Tobin, R., and H.
Thompson, "Namespaces in XML 1.0 (Third Edition)", World
Wide Web Consortium Recommendation REC-xml-names-20091208,
December 2009,
<http://www.w3.org/TR/2009/REC-xml-names-20091208>.
Nottingham Expires May 7, 2016 [Page 14]
Internet-Draft Web Linking November 2015
Appendix A. Using the Link Header with the HTML Format
HTML motivated the original syntax of the Link header, 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. The context of the link is
the URI associated with the entire HTML document.
All of the link relation types defined by HTML have been included in
the Link Relation Type registry, so they can be used without
modification. However, there are several potential ways to serialise
extension relation types into HTML, including:
o As absolute URIs,
o using the RDFa [W3C.REC-html-rdfa-20150317] convention of mapping
token prefixes to URIs (in a manner similar to XML name spaces).
Individual applications of linking will therefore need to define how
their extension links should be serialised into HTML.
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).
HTML also defines several attributes on links that can be see as
target attributes, including "media", "hreflang", "type" and "sizes".
Finally, the HTML specification gives a special meaning when the
"alternate" and "stylesheet" relation types coincide in the same
link. Such links ought to be serialised in the Link header using a
single list of relation-types (e.g., rel="alternate stylesheet") to
preserve this relationship.
Appendix B. Using the Link Header with the Atom Format
Atom 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.
Nottingham Expires May 7, 2016 [Page 15]
Internet-Draft Web Linking November 2015
When serialising an atom:link into a Link header, 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. Such relation types SHOULD be converted to the
appropriate registered form (e.g.,
"http://www.iana.org/assignments/relation/self" to "self") so that
they are not mistaken for extension relation types.
Furthermore, Atom 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 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 syntax, but they can also be
used as link-extensions to maintain fidelity.
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.
Nottingham Expires May 7, 2016 [Page 16]
Internet-Draft Web Linking November 2015
o A convention for assigning a URI to registered relation types was
defined.
o Removed misleading statement that the link header field is
semantically equivalent to HTML and Atom links.
o More carefully defined how the Experts and IANA should interact.
Author's Address
Mark Nottingham
EMail: mnot@mnot.net
URI: http://www.mnot.net/
Nottingham Expires May 7, 2016 [Page 17]