WDIFF

draft-nottingham-http-link-header-03.txt --> draft-nottingham-http-link-header-04.txt

Network Working Group M. Nottingham
Internet-Draft December 1, 2008 February 25, 2009
Updates: 4287 (if approved)
Intended status: Standards Track
Expires: June 4, August 29, 2009

Link Relations and HTTP Header Linking
draft-nottingham-http-link-header-03
draft-nottingham-http-link-header-04

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Abstract

This document specifies relation types for Web links, and defines a
registry for them. It also defines how to send such links in HTTP
headers with the Link header-field.

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Table of Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Notational Conventions . . . . . . . . . . . . . . . . . . . . 3
3. Links . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
4. Link Relation Types . . . . . . . . . . . . . . . . . . . . . 4
5. The Link Header Field . . . . . . . . . . . . . . . . . . . . 5
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 6 7
7. Security Considerations . . . . . . . . . . . . . . . . . . . 10 11
8. Internationalisation Considerations . . . . . . . . . . . . . 11
9. References . . . . . . . . . . . . . . . . . . . . . . . . . . 10
8.1. 12
9.1. Normative References . . . . . . . . . . . . . . . . . . . 10
8.2. 12
9.2. Informative References . . . . . . . . . . . . . . . . . . 11 12
Appendix A. Notes on Using the Link Header with HTML HTML4 . . . . . . 11 13
Appendix B. Notes on Using the Link Header with Atom . . . . . . 12 14
Appendix C. Acknowledgements . . . . . . . . . . . . . . . . . . 13 15
Appendix D. Document history . . . . . . . . . . . . . . . . . . 13 15
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 14
Intellectual Property and Copyright Statements . . . . . . . . . . 15 17

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1. Introduction

A means of indicating the relationships between documents resources on the Web,
as well as indicating the type of those relationships, has been
available for some time in HTML [W3C.REC-html401-19991224], and more
recently in Atom [RFC4287]. These mechanisms, although conceptually
similar, are separate. separately specified. However, links between resources
need not be format-specific; it can be useful to have typed links
that are independent of the format, especially when a resource has
representations in multiple formats.

This

To this end, this document defines a framework for typed link relations, independent of the links that
isn't specific to a particular serialisation or context they occur in. of use. It
does so by clarifying the status of re-defining the link relation registry established by Atom, Atom
to have a broader scope, and registering in adding to it the relations that are
defined by HTML.

Furthermore, an HTTP header-field for conveying typed links was
defined in [RFC2068], but removed from [RFC2616], due to a lack of
implementation experience. Since then, it has been implemented in
some User-Agents (e.g., for stylesheets), and several additional use
cases for doing
so have surfaced. However, because Because it was removed, the status of the Link
header is unclear, leading some to consider minting new
application-specific application-
specific HTTP headers instead of reusing it. This document addresses
this by re-specifying the Link header with updated but backwards-compatible backwards-
compatible syntax.

[[ Feedback is welcome on the ietf-http-wg@w3.org mailing list,
although this is NOT a work item of the HTTPBIS WG. ]]

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
[RFC2616], and explicitly includes the following rules from it:
quoted-string, token, SP (space). Additionally, the following rules
are included from [RFC3986]: URI and URI-Reference, and from
[RFC4288]: type-name.

3. Links

In the context of this specification, a link is comprised of: a typed connection between two

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o A target URI ([RFC3986]), February 2009

resources that are identified by IRIs [RFC3987], and is comprised of:
o A context of use, IRI, and
o A link relation type (Section 4), and
o A link direction (outbound or inbound). target IRI.

A link can be viewed as a statement of the form "(subject) "(context IRI) has a
(relation type) resource at (object)", where for an outbound link (target IRI)."

Note that in the subject
is common case, the context of use and the object is the resource identified by
the target URI, and for an inbound link the subject is the resource
identified by IRI will also be a URI
[RFC3986], because common protocols (such as HTTP) do not support
dereferencing IRIs. Likewise, the target IRI will be converted to a
URI and in serialisations that do not support IRIs (e.g., the object is Link
header).

This specification does not place restrictions on the context cardinality of use.

This
links; there can be multiple links from and to a particular IRI, and
multiple links of different types between two given IRIs.

Additionally, this specification does not define a general syntax for
expressing links, nor the mandate a specific context for a any given link;
it is expected that applications of link relations links will specify both aspects.
One such application is communication of links through HTTP headers,
specified in Section 5.

Such applications may further constrain or extend links (e.g.,
associating a media type hint, only allowing links in one direction). hint).

4. Link Relation Types

A link relation type identifies the semantics of a link. For
example, an outbound a link with the relation type "copyright" indicates that the
resource identified by the target IRI is a statement of the copyright
terms applying to the current context of the link. IRI.

Relation types are not to be confused with media types [RFC4288];
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.

As such, relation types are not format-specific, and MUST NOT specify
a particular format or media type that they are to be used with.
Likewise, the context IRI for a given link is usually determined by
the serialisation of the link (e.g., the Link header, a HTML
document, etc.); a relation type SHOULD NOT specify what its context of its
use is.

Relation types are URIs. Although specific applications of links may
specify the use of URI-References, they must also indicate how to
resolve them to absolute URIs.

Although anyone may mint a URI to be used as a relation type, it is
expected that a few standard types will predominate. To facilitate
this, Section 6.2 establishes an IANA registry of context IRI.

Consuming implementations SHOULD ignore relation types that
share a common base URI. they do
not understand or have no need to process.

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5. The Link Header Field

The Link entity-header field provides a means for conveying one or
more links in February 2009

There are two kinds of relation types; registered and extension.

4.1. Registered Relation Types

Commonly-used relation types with a clear meaning that are shared
across applications can be registered as tokens for convenience and
to promote reuse. For example, "self" and "alternate" are registered
relation types, because they are broadly useful.

This draft establishes an IANA registry of such relation types; see
Section 6.2.

Registered relation types MUST conform to the token rule, and SHOULD
conform to the sgml-name rule for compatibility with deployed
implementations;

sgml-name = ALPHA *( ALPHA | DIGIT | "." | "-" )

Names that differ only in case from existing entries (e.g., "Foo" and
"foo") MUST NOT be registered.

Registered relation types MUST be compared in a case-insensitive
fashion.

Although they are specified as tokens, applications wishing to
internally refer to an extension relation type using a URI MAY do so
by considering it relative to the base URI
"http://www.iana.org/assignments/relation/". However, the URI form
of a registered relation type SHOULD NOT be serialised when an
application specifies the use of a relation type, because a consuming
implementation may not recognise it.

4.2. Extension Relation Types

Applications that don't merit a registered relation type may use an
extension relation type. An extension relation type is a URI
[RFC3986] that, when dereferenced, SHOULD yield a document describing
that relation type.

Extension relation types MUST be compared in a case-sensitive
fashion, character-by-character.

5. The Link Header Field

The Link entity-header field provides a means for conveying one or
more links in HTTP headers. It is semantically equivalent to the
<LINK> element in HTML, as well as the atom:link feed-level element

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in Atom [RFC4287].

Link = "Link" ":" #link-value
link-value = "<" URI-Reference ">" *( ";" link-param ) )
link-param = ( ( "rel" "=" relation-type relation-types )
| ( "rev" "=" relation-type relation-types )
| ( "type" "=" type-name )
| ( "title" "=" quoted-string )
| ( "title*" "=" enc2231-string )
| ( "anchor" "=" <"> URI-Reference <"> )
| ( link-extension ) )
link-extension = token [ "=" ( token | quoted-string ) ]
relation-type
enc2231-string = URI-Reference <extended-value, see <xref target="RFC2231"/>,
Section 7>
relation-types = relation-type |
<"> URI-Reference relation-type *( SP URI-Reference) relation-type ) <">
relation-type = reg-relation-type | ext-relation-type
reg-relation-type = token
ext-relation-type = URI

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.

Each link-value conveys one target URI IRI as a URI-Reference (after
conversion, if necessary) inside angle brackets ("<>"). If it the URI-
Reference is relative, it MUST be resolved as per [RFC3986]. Note
that
because it is conveyed in a header, any base URIs IRI from the body's content are is not
applied to it.

The applied.

By default, the context of links a link conveyed in the Link header field
is the IRI associated with the representation that it occurs in. When
present, the header is part of.

Each link-value MUST have at least one "rel" or "rev" anchor parameter whose
value indicates the relation type. If overrides this with another URI, such
as a fragment of this resource, or a third resource (i.e., when the "rel" parameter
anchor value is used,
it indicates that an absolute URI).

Normally, the link's direction for that relation type of a link is
outbound; if conveyed in the "rel"
parameter's value. The "rev" parameter has also been used for this
purpose historically by some formats, and is used, the given relation type's
direction is inbound.

If the relation-type is a relative URI, included here for
compatibility with those uses, but its base URI MUST be
considered use is not encouraged nor
defined by this specification.

Note that extension relation types are REQUIRED to be "http://www.iana.org/assignments/relation/", absolute URIs
in Link headers, and the
corresponding value MUST be present in the link relation registry.

Relation-types that include quoted if they contain a semicolon (";")
or comma (",") MUST be
quoted.

The title parameter MAY be used to label the destination of a link (",").

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The title parameter is used to label the destination of a link such
that it can be used as identification within a human-readable
menu. identifier (e.g. a menu
entry). The title* parameter MAY be used to instead to encode this
label in an alternate character set, and/or contain language
information as per [RFC2231]. When using the enc2231-string syntax,
producers MUST NOT use a charset value other than 'ISO-8859-1' or
'UTF-8'.

Note that link-values may contain convey multiple relations; links between the same
target and context IRIs; for example

Link: <http://example.org/>; rel="index start";
rel="http://example.net/relation/other";
rev=copyright rel=index;
rel="start http://example.net/relation/other"

Here, the link to "http://example.org/" has outbound links of the registered relation
types
"http://www.iana.org/assignments/relation/index",
"http://www.iana.org/assignments/relation/start", "index" and
"http://example.net/relation/other", as well as an inbound link of "start", and the extension relation type "http://www.iana.org/assignments/relation/copyright".
"http://example.net/relation/other".

6. IANA Considerations

6.1. Link Header 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 ]

6.2. Link Relation Type Registry

This specification establishes the Link Relation Type Registry,
located at <http://www.iana.org/assignments/relation/>, and updates
Atom [RFC4287] to refer to it in place of the "Registry of Link
Relations".

The semantics of requirements for registered relation types is are described in
Section 4. This
registry is intended to contain widely-used, standard relation 4.1.

Relation types
so that they may be used in "short form" (i.e., as a relative URI) in
applications that allow this.

Registered relation types have an implicit base URI registered on the advice of
<http://www.iana.org/assignments/relation/>, and their name SHOULD be
limited to a Designated Expert
(appointed by the sgml-name rule for simplicity and backwards-
compatibility. IESG or their delegate), with a Specification
Required (using terminology from [RFC5226]).

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sgml-name = ALPHA *( ALPHA | DIGIT | "." | "-" )

Names that differ only February 2009

Registration requests consist of the completed registration template
below, typically published in case (e.g., "Foo" and "foo") MUST NOT be
registered.

New relation types can be registered an RFC or Open Standard (in the sense
described by IETF Review, as outlined in
[RFC5226]. Specifications [RFC2026], section 7). However, to allow for the
allocation of new values should use prior to publication, the following
template: Designated Expert may
approve registration once they are satisfied that an RFC (or other
Open Standard) will be published.

The registration template is:

o Relation Name:
o Description:
o Reference:

Upon receiving a registration request (usually via IANA), the
Designated Expert should request review and comment from the
apps-discuss@ietf.org mailing list (or a successor designated by the
APPS Area Directors). Before a period of 30 days has passed, the
Designated Expert will either approve or deny the registration
request, communicating this decision both to the review list and to
IANA. Denials should include an explanation and, if applicable,
suggestions as to how to make the request successful.

The Link Relation Type registry's initial contents are:

o Relation Name: alternate
o Description: Designates a substitute for the link's context.
o Reference: [W3C.REC-html401-19991224]

o Relation Name: appendix
o Description: Refers to an appendix.
o Reference: [W3C.REC-html401-19991224]

o Relation Name: bookmark
o Description: Refers to a bookmark or entry point.
o Reference: [W3C.REC-html401-19991224]

o Relation Name: chapter
o Description: Refers to a chapter in a collection of resources.
o Reference: [W3C.REC-html401-19991224]

o Relation Name: contents
o Description: Refers to a table of contents.
o Reference: [W3C.REC-html401-19991224]

o Relation Name: copyright
o Description: Refers to a copyright statement that applies to the
link's context.

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o Reference: [W3C.REC-html401-19991224]

o Relation Name: current
o Description: Refers to a resource containing the most recent
item(s) in a collection of resources.
o Reference: [RFC5005] Reference: [RFC5005]

o Relation Name: describedby
o Description: Refers to a resource providing information about the
link's context.
o Documentation: <http://www.w3.org/TR/powder-dr/>

o Relation Name: edit

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o Description: Refers to a resource that can be used to edit the
link's context.
o Reference: [RFC5023]

o Relation Name: edit-media
o Description: Refers to a resource that can be used to edit media
associated with the link's context.
o Reference: [RFC5023]

o Relation Name: enclosure
o Description: Identifies a related resource that is potentially
large and might require special handling.
o Reference: [RFC4287]

o Relation Name: first
o Description: A URI An IRI that refers to the furthest preceding resource
in a series of resources.
o Reference: <http://www.iana.org/assignments/link-relations/first>

o Relation Name: glossary
o Description: Refers to a glossary of terms.
o Reference: [W3C.REC-html401-19991224]

o Relation Name: help
o Description: Refers to a resource offering help (more information,
links to other sources information, etc.)
o Reference: [W3C.REC-html401-19991224]

o Relation Name: index
o Description: Refers to an index.
o Reference: [W3C.REC-html401-19991224]

o Relation Name: last

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o Description: A URI An IRI that refers to the furthest following resource
in a series of resources.
o Reference: <http://www.iana.org/assignments/link-relations/last>

o Relation Name: license
o Description: Refers to a license associated with the link's
context.
o Reference: [RFC4946]

o Relation Name: next
o Description: Refers to the next resource in a ordered series of
resources.
o Reference: [W3C.REC-html401-19991224]

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o Relation Name: next-archive
o Description: Refers to the immediately following archive resource.
o Reference: [RFC5005]

o Relation Name: payment
o Description: indicates a resource where payment is accepted.
o Reference:
<http://www.iana.org/assignments/link-relations/payment>

o Relation Name: prev
o Description: Refers to the previous resource in an ordered series
of resources. Synonym for "previous".
o Reference: [W3C.REC-html401-19991224]

o Relation Name: previous
o Description: Refers to the previous resource in an ordered series
of resources. Synonym for "prev".
o Reference: [W3C.REC-html401-19991224]

o Relation Name: prev-archive
o Description: Refers to the immediately preceding archive resource.
o Reference: [RFC5005]

o Relation Name: related
o Description: Identifies a related resource.
o Reference: [RFC4287]

o Relation Name: replies
o Description: Identifies a resource that is a reply to the context
of the link.
o Reference: [RFC4685]

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o Relation Name: section
o Description: Refers to a section in a collection of resources.
o Reference: [W3C.REC-html401-19991224]

o Relation Name: self
o Description: Conveys an identifier for the link's context.
o Reference: [RFC4287]

o Relation Name: start
o Description: Refers to the first resource in a collection of
resources.
o Reference: [W3C.REC-html401-19991224]

o Relation Name: stylesheet

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o Description: Refers to an external style sheet.
o Reference: [W3C.REC-html401-19991224]

o Relation Name: subsection
o Description: Refers to a resource serving as a subsection in a
collection of resources.
o Reference: [W3C.REC-html401-19991224]

o Relation Name: via
o Description: Identifies a resource that is the source of the
information in the link's context.
o Reference: [RFC4287]

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.

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,
Link headers that use the "anchor" parameter to associate a link's
context with another resource should be treated with due caution.

8. Internationalisation Considerations

Target IRIs may need to be converted to URIs in order to serialise
them in applications that do not support IRIs. This includes the
Link HTTP header.

Similarly, the anchor parameter of the Link header does not support

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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.

9. References

8.1.

9.1. Normative References

[RFC2026] Bradner, S., "The Internet Standards Process -- Revision
3", BCP 9, RFC 2026, October 1996.

[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.

[RFC2231] Freed, N. and K. Moore, "MIME Parameter Value and Encoded
Word Extensions: Character Sets, Languages, and
Continuations", RFC 2231, November 1997.

[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, June 1999.

[RFC3864] Klyne, G., Nottingham, M., and J. Mogul, "Registration
Procedures for Message Header Fields", BCP 90, RFC 3864,
September 2004.

[RFC3986] Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
Resource Identifier (URI): Generic Syntax", STD 66,
RFC 3986, January 2005.

[RFC3987] Duerst, M. and M. Suignard, "Internationalized Resource
Identifiers (IRIs)", RFC 3987, January 2005.

[RFC4288] Freed, N. and J. Klensin, "Media Type Specifications and
Registration Procedures", BCP 13, RFC 4288, December 2005.

[RFC5226] Narten, T. and H. Alvestrand, "Guidelines for Writing an

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IANA Considerations Section in RFCs", BCP 26, RFC 5226,
May 2008.

8.2.

9.2. Informative References

[RFC2068] Fielding, R., Gettys, J., Mogul, J., Nielsen, H., and T.
Berners-Lee, "Hypertext Transfer Protocol -- HTTP/1.1",

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RFC 2068, January 1997.

[RFC4287] Nottingham, M. and R. Sayre, "The Atom Syndication
Format", RFC 4287, December 2005.

[RFC4685] Snell, J., "Atom Threading Extensions", RFC 4685,
September 2006.

[RFC4946] Snell, J., "Atom License Extension", RFC 4946, July 2007.

[RFC5005] Nottingham, M., "Feed Paging and Archiving", RFC 5005,
September 2007.

[RFC5023] Gregorio, J. and B. de hOra, "The Atom Publishing
Protocol", RFC 5023, October 2007.

[W3C.REC-html401-19991224]
Raggett, D., Hors, A., and I. Jacobs, "HTML 4.01
Specification", W3C REC REC-html401-19991224,
December 1999.

Appendix A. Notes on Using the Link Header with HTML HTML4

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 these uses.

In HTML4, the link element can be mapped to links as specified here
by using the "href" attribute for the target URI, and "rel" and rev" to convey
both the relation type and its direction, type, as in the Link header. The context of the
link is generally the URI associated with the entire HTML document.

HTML4 also has a "rev" parameter for links that allows a link's
relation to be reversed. The Link header has a "rev" parameter to
allow the expression of these links in HTTP headers, but its use is
not encouraged, due to the confusion this mechanism causes as well as
conflicting interpretations among HTML versions.

All of the link relations defined by HTML4 have been included in the
link relation registry, so they can be used without modification.
However, extension link relations work differently in HTML4 and the
Link header; the former uses a document-wide "profile" URI to scope
the relations, while the latter allows the use of full URIs on
individual relations.

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Therefore, when using the profile mechanism in HTML4, it is necessary
to map the profiled link relations to URIs when expressed in Link

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headers. For example, in HTML:

<html>
<head profile="http://example.com/profile1/">
<link rel="foo" href="/foo"> href="/bar">
</head>
[...]

could be represented as a header like this;

Link: </foo>; </bar>; rel="http://example.com/profile1/foo"

Profile authors should note this when creating profile URIs; it may
be desirable to use URIs that end in a delimiter (e.g., "/" or "#"),
to make extracting the specific relation in use easier.

Surveys of existing HTML defines content have shown that unregistered link
relation values as case-insensitive, while the Link
header's syntax does not. Therefore, it is important types that are not URIs are (perhaps inevitably) common.
Consuming HTML implementations should not consider such unregistered
short links to case-
normalise be errors, but rather relation values in HTML before comparing or converting them types with a local
scope (i.e., their meaning is specific and perhaps private to Link headers.

HTML that
document).

HTML4 also defines several attributes on links that are not
explicitly defined by the Link header. Although most of these are believed to
be defunct, they These attributes can be used
serialised as link-extensions. link-extensions to maintain fidelity.

Appendix B. Notes on Using the Link Header with Atom

Atom conveys links in the atom:link element, with the "href"
attribute indicating the target URI IRI and the "rel" attribute
containing the relation type. The context of the link is either a
feed IRI or an entry, entry ID, depending on where it appears; generally, feed-
level
feed-level links are candidates for transmission as a Link header. Since
atom:link only specifies "rel", only outbound links are allowed by
non-extended Atom syntax.

When serialising an atom:link into a Link header, it is necessary to
convert target IRIs (if used) to URIs. Additionally, since the base URI for
link relations in Link headers is fixed,

Atom defines extension relation types
(i.e,. those not in the registry) must terms of IRIs. This
specification defines them as URIs, to aid in their comparison.

Atom allows registered link relation types to be represented serialised as
absolute
URIs. URIs, because a base URI is defined for the registry. Such
relation types SHOULD be converted to the appropriate registered form
(e.g., "http://www.iana.org/assignments/relation/self" to "self") so

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that they are not mistaken for extension relation types.

Note also that while the Link header allows multiple relations to be
associated with a single link, atom:link does not. In this case, a

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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 may also be
used as link-extensions.

Appendix C. Acknowledgements

This specification lifts the idea and definition for the Link header
from RFC2068; credit for it belongs entirely to the authors of and
contributors to that document. The link relation registrations
themselves are sourced from several documents; see the applicable
references.

The author would like to thank thank the many people who commented upon,
encouraged and gave feedback to this draft, especially including
Frank Ellermann, Roy Fielding and Julian Reschke.

Appendix D. Document history

[[ to be removed by the RFC editor before publication as an RFC. ]]

-04

o Defined context as a resource, rather than a representation.
o Removed concept of link directionality; relegated to a deprecated
Link header extension.
o Relation types split into registered (non-URI) and extension
(URI).
o Changed wording around finding URIs for registered relation types.
o Changed target and context URIs to IRIs (but not extension
relation types).
o Add RFC2231 encoding for title parameter, explicit BNF for title*.
o Add i18n considerations.
o Specify how to compare relation types.
o Changed registration procedure to Designated Expert.
o Softened language around presence of relations in the many people who commented upon,
encouraged and gave feedback registry.
o Added describedby relation.
o Re-added 'anchor' parameter, along with security consideration for
third-party anchors.

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o Softened language around HTML4 attributes that aren't directly
accommodated.
o Various tweaks to this draft, especially including
Frank Ellermann abstract, introduction and Julian Reschke.

Appendix D. Document history

[[ to be removed by the RFC editor before publication as an RFC. ]] examples.

-03

o Inverted focus from Link headers to link relations.
o Specified was a link relation type is.
o Based on discussion, re-added 'rev'.
o Changed IESG Approval to IETF Consensus for relation registrations
(i.e., require a document).
o Updated RFC2434 reference to RFC5226.
o Registered relations SHOULD conform to sgml-name.
o Cautioned against confusing relation types with media types.

-02

o Dropped XLink language.
o Removed 'made' example.
o Removed 'rev'. Can still be used as an extension.
o Added HTML reference to introduction.
o Required relationship values that have a ; or , to be quoted.
o Changed base URI for relation values.
o Noted registry location.
o Added advisory text about HTML profile URIs.
o Disallowed registration of relations that only differ in case.

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o Clarified language about IRIs in Atom.
o Added descriptions for 'first', 'last', and 'payment', referring
to current IANA registry entries, as these were sourced from
e-mail. Will this cause self-referential implosion?
o Explicitly updates RFC4287.
o Added 'type' parameter.
o Removed unnecessary advice about non-HTML relations in HTML
section.

-01

o Changed syntax of link-relation to one or more URI; dropped
Profile.
o Dropped anchor parameter; can still be an extension.
o Removed Link-Template header; can be specified by templates spec
or elsewhere.
o Straw-man for link relation registry.

-00

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o Initial draft; normative text lifted from RFC2068.

Author's Address

Mark Nottingham

Email: mnot@mnot.net
URI: http://www.mnot.net/

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