WDIFF

draft-nottingham-http-link-header-00.txt --> draft-nottingham-http-link-header-01.txt

Network Working Group M. Nottingham
Internet-Draft June 16, 2006 March 14, 2008
Expires: December 18, 2006 September 15, 2008

HTTP Header Linking
draft-nottingham-http-link-header-00
draft-nottingham-http-link-header-01

Status of this Memo

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This Internet-Draft will expire on December 18, 2006. September 15, 2008.

Abstract

This specification document clarifies the status of the Link HTTP header and
introduces the complimentary Profile and Link-Template HTTP headers.
attempts to consolidate link relations in a single registry.

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Table of Contents

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

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1. Introduction

A means of indicating the relationships between documents on the Web
has been available for some time in HTML, and was considered as a
HTTP header in [RFC2068], but removed from [RFC2616], due to a lack
of implementation experience.

There have since surfaced many cases where a means of including this
information in HTTP headers has proved useful. 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.

Additionally, the complementary "profile" mechanism -- which is often
used to disambiguate link relationship types -- is not available as a
HTTP header.

This specification document seeks to address these shortcomings. It

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

This document does not attempt to specify how an XLink
[W3C.REC-xlink-20010627] might be described. mapped into the Link header, but
does not prohibit this from being done.

[[ 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 the Link header (in the case where
extensions are present). 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 specification document uses the Augmented Backus-Naur Form (ABNF) notation of
[RFC2616], and explicitly includes the following rules from it:
quoted-string, token, SP (space), ALPHA (letters), DIGIT
(decimal digit). (space). Additionally, the following rules
are included from [RFC3986]: URI-Reference, reserved, unreserved. URI-Reference.

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3. The Link Header Field

The Link entity-header field provides a means for describing a
relationship between two resources, generally between that of the requested
resource
entity associated with the header and some other resource. An entity
MAY include multiple Link values. Links at the metainformation level typically indicate
relationships like hierarchical structure and navigation paths.

The Link header field is semantically equivalent to the <LINK>
element in HTML.

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Link = "Link" ":" #("<" URI-Reference ">"
*( ";" link-param ) )

link-param = ( ( "rel" "=" relationship )
| ( "rev" "=" relationship )
| ( "title" "=" quoted-string )
| ( "anchor" "=" <"> URI-Reference <"> )
| ( link-extension ) )

link-extension = token [ "=" ( token | quoted-string ) ]

relationship = sgml-name URI-Reference | (
<"> sgml-name URI-Reference *( SP sgml-name) URI-Reference) <"> )

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

Relationship values are case-insensitive and MAY be extended within
the constraints of the sgml-name syntax.

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. The anchor parameter
MAY be used to indicate a source anchor other than the entire current
resource, such as a fragment of this resource or a third resource.

Examples of usage include:

Link: <http://www.cern.ch/TheBook/chapter2>; rel="Previous"
Link: <mailto:timbl@w3.org>; rev="Made"; title="Tim Berners-Lee"

The first example indicates that chapter2 is previous to this
resource in a logical navigation path. The second indicates that the
person responsible for making the resource available is identified by
the given e-mail address.

4. The Profile Header Field

The Profile entity-header field provides a means to indicate

3.1. Link Relationships

Relationship values are URIs that identify the meta
data profile type of link. If the entity. Commonly, it
relationship is used to disambiguate the
meaning of relationships in links.

Note that this a relative URI, its base URI MAY MUST be used as either an identifier (e.g., to
uniquely identify links, without dereferencing the URI), or as a link
that is intended considered to be dereferenced.

The Profile field is semantically equivalent to the profile attribute
of
"http://www.iana.org/assignments/link-relations.html#", and the HEAD element value
MUST be present in HTML [W3C.REC-html401-19991224]. Note,
however, that its use is not limited to HTML entities. the link relation registry.

4. IANA Considerations

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Profile = "Profile" ":" #("<" URI-Reference ">")

For example:

Profile: <http://www.acme.com/profiles/core>
Profile: <http://example.com/p1>, </profiles/other-profile>

5. The Link-Template March 2008

4.1. Link Header Field

The Link-Template entity-header field provides a means for describing
the structure of a link between two resources, so Registration

This specification requires that new links can the Message Header Registry entry
for "Link" in HTTP [RFC3864] be generated.

It does so through by allowing brace ("{}") -delimited strings updated to be
interposed throughout a URI reference. These correspond to variables
which, after being replaced with content in a relation-specified
manner, are semantically equivalent refer to the corresponding this document.

Header field: Link header.

For example,

Link-Template: <http://example.com/home/{userid}>; rel="home"
Applicable protocol: http
Status: standard
Author/change controller:
IETF (iesg@ietf.org)
Internet Engineering Task Force
Specification document(s):
[ this document ]

4.2. Link Relation Registry

This link indicates that the "home" link relation can be constructed
if the userid variable specification is known; if it were known intended to be "bob", this
header would be considered equivalent update Atom to

Link: <http://example.com/home/bob>; rel="home"

This specification does not define when or how template variables are
interposed into link templates. become the reference
for the Link relations that wish to allow
templating SHOULD Relation registry, and clarifies its nature and use.

A Link relation is a way of indicating the semantics of a link. Link
relations are not format-specific, and MUST NOT specify such details.

This specification does a particular
format or media type that they are to be used with.

The security considerations of following a particular link are not define
determined by the correct behaviour in link's relation type; they are determined by the face
specific context of a conflict between a Link-Template header the use and a Link header with the same relation. Link relations allowing templating SHOULD specify
their relative precedence.

Applications media type of the response.

Likewise, a link relation SHOULD NOT specify what the context of its
use is, although the media type of the dereferenced link may
constrain how it is applied.

New relations that do not explicitly
allow such templating MAY be registered, subject to IESG Approval, as
outlined in [RFC2434]. Requests should be made by email to IANA,
which will then forward the Link-Template header.

Link-Template = "Link-Template" ":" #("<" template ">"
*( ";" link-param ) )

template = *( uri-char | template-var )

template-var = "{" 1*( uri-char ) "}" request to the IESG, requesting approval.
The request 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:

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

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uri-char = ( reserved | unreserved )

6. IANA Considerations

This specification requires registration March 2008

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 two Message Header Fields
for HTTP [RFC3864]. Note that "Link" is already present 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.
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
o Description: Refers to a resource that can be used to edit the
registry; this registration only updates its specification 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 ]
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: [[ ??? IANA Registry does not reference ]]
o Reference: [[ ??? IANA Registry does not reference ]]

o Relation Name: glossary
o Description: Refers to a glossary of terms.

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Internet-Draft HTTP Header field: Profile
Applicable protocol: http
Status: standard
Author/change controller:
IETF (iesg@ietf.org)
Internet Engineering Task Force
Specification document(s):
[ this document ] Linking March 2008

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: [[ ??? IANA Registry does not reference ]]
o Reference: [[ ??? IANA Registry does not reference ]]

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]

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

o Relation Name: payment
o Description: [[ ??? IANA Registry does not reference ]]
o Reference: [[ ??? IANA Registry does not reference ]]

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]

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

7. Linking March 2008

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
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]

5. Security Considerations

The content of both the Link and Profile headers are is not secure, private or integrity-guaranteed, integrity-
guaranteed, and due caution should be excercised when using them. it.

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

6. References

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6.1. 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.

6.2. 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.

[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.

[W3C.REC-xlink-20010627]
DeRose, S., Maler, E., and D. Orchard, "XML Linking

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Language (XLink) Version 1.0", W3C REC REC-xlink-20010627,
June 2001.

Appendix A. Notes on 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.

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.

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

Furthermore, the registry defines link relations that are not part of
HTML4. When such relations are used, the absolute URI form should be
used; e.g.,

Link: </head>;
rel="http://www.iana.org/assignments/link-relations.html#current"

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

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8. References

8.1. Normative References

[RFC2119] Bradner, S., "Key words for use March 2008

Appendix B. Notes on Atom Links

Atom conveys links in RFCs the atom:link element. When serialising an
atom:link into a Link header, it is necessary to Indicate
Requirement Levels", BCP 14, RFC 2119, March 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 convert any IRIs to
URIs, since HTTP headers cannot directly contain UTF-8.

Additionally, since the base URI 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.

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

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. link relations in Link headers
is fixed, extension links (i.e,. those not in the registry) MUST be
serialised 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 Link header 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 A. C. Acknowledgements

This specification lifts the definition of the Link header from
RFC2068; credit for it belongs entirely to the authors of and
contributors to that document.

The semantics and much of the syntax of the Profile header was
defined by HTML 4.01; credit for them it belongs entirely to the authors of and
contributors to that document.

Joe Gregorio, Marc Hadley and David Orchard contributed to The link relation registrations
themselves are sourced from several documents; see the design applicable
references.

Appendix D. Document history

-01

o Changed syntax of the link-relation to one or more URI; dropped
Profile.
o Dropped anchor parameter; can still be an extension.
o Removed Link-Template mechanism. 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.

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Author's Address

Mark Nottingham

Email: mnot@pobox.com 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
"AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS
OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY, THE IETF TRUST AND
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ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED,
INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE
INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED
WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.

Copyright (C) The Internet Society (2006). This document is subject
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except as set forth therein, the authors retain all their rights.

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Funding for the RFC Editor function is currently provided by the
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Administrative Support Activity (IASA).

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