WDIFF

draft-nottingham-http-link-header-02.txt --> draft-nottingham-http-link-header-03.txt

Network Working Group M. Nottingham
Internet-Draft July 3, December 1, 2008
Updates: 4287 (if approved)
Intended status: Standards Track
Expires: January June 4, 2009

Link Relations and HTTP Header Linking
draft-nottingham-http-link-header-02
draft-nottingham-http-link-header-03

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This Internet-Draft will expire on January June 4, 2009.

Abstract

This document clarifies the status of the Link HTTP header specifies relation types for Web links, and
attempts defines a
registry for them. It also defines how to consolidate link relations send such links in a single registry. HTTP
headers with the Link header-field.

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Table of Contents

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

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1. Introduction

A means of indicating the relationships between documents on the Web Web,
as well as indicating the type of those relationships, has been
available for some time in HTML [W3C.REC-html401-19991224], and was considered as more
recently in Atom [RFC4287]. These mechanisms, although conceptually
similar, are separate. 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 document defines typed link relations, independent of the
context they occur in. It does so by clarifying the status of the
link relation registry established by Atom, and registering in it the
relations that are defined by HTML.

Furthermore, an HTTP header header-field for conveying typed links was
defined in [RFC2068], but removed from [RFC2616], due to a lack of
implementation experience.

There have since surfaced many Since then, several use cases where a means of including this
information in HTTP headers has proved useful. for doing
so have surfaced. However, because it was removed, the status of the
Link header is unclear, leading some to consider minting new
application-specific HTTP headers instead of reusing it. This
document seeks to address these shortcomings.

Additionally, formats other than HTML -- namely, Atom [RFC4287] --
have also defined generic linking mechanisms that are similar to
those in HTML, but not identical. This document aims to reconcile
these differences when such links are expressed as headers.

[[ NOTE: This is a straw-man draft that is intended to give a ROUGH
idea of what it would take to align and consolidate the HTML and Atom
link relations into a single registry with reasonable extensibility
rules. In particular; a) it changes the registry for Atom link
relations, and the process for registration; b) it assigns more
generic semantics to several existing link relations, both Atom and
HTML; c) it changes the syntax of addresses this by re-specifying the Link header (in the case where
extensions are present). with updated
but 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. type-name.

3. Links

In the context of this specification, a link is comprised of:

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3. The Link Header Field

The Link entity-header field provides Relations December 2008

o A target URI ([RFC3986]), and
o A context of use, and
o A link relation type (Section 4), and
o A link direction (outbound or inbound).

A link can be viewed as a means statement of the form "(subject) has a
(relation type) at (object)", where for describing an outbound link the subject
is 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 the target URI and the object is the context of use.

This specification does not define a
relationship between two resources, generally between general syntax for expressing
links, nor the specific context for a given link; it is expected that
applications of link relations 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).

4. Link Relation Types

A link relation type identifies the
entity associated semantics of a link. For
example, an outbound link with the header and some other relation type "copyright"
indicates that the resource identified is a statement of the
copyright terms applying to the current context of the link.

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. An entity
MAY include multiple

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, 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 relation types that
share a common base URI.

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5. The Link header 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
in Atom [RFC4287].

Link = "Link" ":" #("<" #link-value
link-value = "<" URI-Reference ">" *( ";" link-param ) )
link-param = ( ( "rel" "=" relationship relation-type )
| ( "rev" "=" relation-type )
| ( "type" "=" type-name )
| ( "title" "=" quoted-string )
| ( link-extension ) )
link-extension = token [ "=" ( token | quoted-string ) ]

relationship
relation-type = URI-Reference |
<"> URI-Reference *( SP URI-Reference) <"> )

Relationship values that include a semicolon (";") or comma (",")
MUST be quoted.

The title parameter MAY be used to label the destination of a link
such that it can be used as identification within a human-readable
menu.

For example:

Link: <http://www.cern.ch/TheBook/chapter2>; <http://example.com/TheBook/chapter2>; rel="previous";
title="previous chapter"

This

indicates that chapter2 is previous to this resource in a logical
navigation path.

3.1. Link Relationships

Relationship values are

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

The context of links conveyed in the Link header field is the
representation that identify the header is part of.

Each link-value MUST have at least one "rel" or "rev" parameter whose
value indicates the relation type. If the "rel" parameter is used,
it indicates that the link's direction for that relation type of link. is
outbound; if the "rev" parameter is used, the given relation type's
direction is inbound.

If the
relationship relation-type is a relative URI, its base URI MUST be
considered to be "http://www.iana.org/assignments/relation/", and the
corresponding value MUST be present in the link relation registry.

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

The title parameter MAY be used to label the destination of a link

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such that it can be used as identification within a human-readable
menu.

Note that link-values may contain multiple relations; for example

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

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

6. IANA Considerations

4.1.

6.1. Link Header Registration

This specification requires that updates the Message Header Registry entry for
"Link" in HTTP [RFC3864] be updated to refer to this document,
and located at "http://www.iana.org/assignments/relation/". document.

Header field: Link
Applicable protocol: http
Status: standard
Author/change controller:
IETF (iesg@ietf.org)
Internet Engineering Task Force
Specification document(s):
[ this document ]

4.2.

6.2. Link Relation Type Registry

This specification is intended to update Atom to become the reference
for establishes the Link Relation registry, and clarifies its nature Type Registry,
located at <http://www.iana.org/assignments/relation/>, and use.

A Link relation is a way updates
Atom [RFC4287] to refer to it in place of indicating the semantics "Registry of a link. Link
relations are not format-specific, and MUST NOT specify a particular
format or media type
Relations".

The semantics of relation types is described in Section 4. This
registry is intended to contain widely-used, standard relation types
so that they are to may be used with.

The security considerations of following in "short form" (i.e., as a particular link are not
determined by the link's relative URI) in
applications that allow this.

Registered relation type; they are determined by the
specific context types have an implicit base URI of the use
<http://www.iana.org/assignments/relation/>, and the media type of the response.

Likewise, a link relation their name SHOULD NOT specify what the context of its
use is, although the media type of be
limited to the dereferenced link may
constrain how it is applied. sgml-name rule for simplicity and backwards-
compatibility.

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

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

New relations MAY relation types can be registered, subject to IESG Approval, registered by IETF Review, as outlined in [RFC2434]. Requests should be made by email to IANA,
which will then forward the request to the IESG, requesting approval.
The request
[RFC5226]. Specifications of new values should use the following
template:

o Relation Name:
o Description:
o Reference:

The Link Relation registry replaces the Atom Link Relation registry,
using the same address with the following contents:

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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. statement that applies to the
link's context.
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]

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 that refers to the furthest preceding resource
in a series of resources.

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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
o Description: A URI 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".

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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]

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 headers header-field is not secure, private or integrity-
guaranteed,
integrity-guaranteed, and due caution should be excercised exercised when using
it.

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Applications that take advantage of these mechanisms typed links should consider the
attack vectors opened by automatically following, trusting, or
otherwise using links gathered from HTTP headers.

8. References

6.1.

8.1. Normative References

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

[RFC2434] Narten, T. and H. Alvestrand, "Guidelines for Writing an
IANA Considerations Section in RFCs", BCP 26, RFC 2434,
October 1998.

[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.

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

6.2. 5226,
May 2008.

8.2. Informative References

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

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[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 Links

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, as in the Link
header. The context of the link is generally the entire HTML
document.

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
headers. For example, in HTML:

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

could be represented as a header like this;

Link: </foo>; 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.

HTML defines link relation values as case-insensitive, while the Link
header's syntax does not. Therefore, it is important to case-
normalise relation values in HTML before comparing or converting them
to Link headers.

HTML also defines several attributes on links that are not explicitly

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defined by the Link header. Although most of these are believed to
be defunct, they can be used as link-extensions.

Appendix B. Notes on Using the Link Header with Atom Links

Atom conveys links in the atom:link element. element, with the "href"
attribute indicating the target URI and the "rel" attribute
containing the relation type. The context of the link is either a
feed or an entry, depending on where it appears; generally, 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 IRIs (if used) to URIs. Additionally, since the base URI for
link relations in Link headers is fixed, extension links relation types
(i.e,. those not in the registry) MUST must be represented as absolute
URIs.

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
single

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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 of 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 the many people who commented upon,
encouraged and gave feedback to this draft, especially including
Frank Ellermann and Julian Reschke.

Appendix D. Document history

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

-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.

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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

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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Full Copyright Statement

This document is subject to the rights, licenses and restrictions
contained in BCP 78, and except as set forth therein, the authors
retain all their rights.

This document and the information contained herein are provided on an
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