Network Working Group J. Snell
Internet-Draft October 9, 2012
Intended status: Informational
Expires: April 12, 2013
HTTP Link and Unlink Methods
draft-snell-link-method-01
Abstract
This specification defines the semantics of the Link and Unlink HTTP
methods.
Status of this Memo
This Internet-Draft is submitted to IETF in full conformance with the
provisions of BCP 78 and BCP 79.
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This Internet-Draft will expire on April 12, 2013.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. LINK . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. UNLINK . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
4. Relationship to other HTTP Methods and Discoverability of
Links . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
5. Example . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
6. Security Considerations . . . . . . . . . . . . . . . . . . . . 7
7. Normative References . . . . . . . . . . . . . . . . . . . . . 7
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 8
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1. Introduction
This specification updates the HTTP LINK and UNLINK methods
originally defined in [RFC2068]. These were originally defined as
"additional request methods" that were later dropped entirely from
follow-on iterations of the HTTP specification due to previous lack
of interest or use.
TODO: Fill in explanation as to why this is needed.
In this document, the key words "MUST", "MUST NOT", "REQUIRED",
"SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY",
and "OPTIONAL" are to be interpreted as described in [RFC2119].
2. LINK
The LINK method is used to establish one or more relationships
between an existing resource identified by the effective request URI
and other resources. Metadata contained within Link header fields
[RFC5988] provide the information about which other resources are
being connected to the target resource and the type of relationship
being established. A payload within a LINK request message has no
defined semantics.
The semantics of the LINK method change to a "conditional LINK" if
the request message includes an If-Modified-Since, If-Unmodified-
Since, If-Match, If-None-Match, or If-Range header field
([I-D.ietf-httpbis-p4-conditional]). A conditional LINK requests
that the relationship be established only under the circumstances
described by the conditional header field(s).
LINK request messages are idempotent. For any pair of resources,
only a single relationship of any given type can exist. However,
multiple relationships of different types can be established between
the same pair of resources.
LINK request messages are not safe, however, in that establishing a
relationship causes an inherent change to the state of the target
resource.
Responses to LINK requests are not cacheable. If a LINK request
passes through a cache that has one or more stored responses for the
effective request URI, those stored responses will be invalidated
(see Section 6 of [I-D.ietf-httpbis-p6-cache]).
A single LINK request message can contain multiple Link header
fields, each of which establishes a separate relationship with the
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target resource. In such cases, the server MUST accept the entire
set of relationships atomically. If any of the specified
relationships cannot be created, the server MUST NOT create any of
them.
A successful response to a Link request that results in either the
creation or modification of a relationship SHOULD be 200 (OK) if the
response includes a representation describing the status, 201
(Created) if the action results in the creation of a new resource
that represents the newly established relationship, 202 (Accepted) if
the action has not yet been enacted, or 204 (No Content) if the
action has been enacted but the response does not include a
representation.
The LINK method MAY be overridden by human intervention (or other
means) on the origin server. The client cannot be guaranteed that
the operation has been carried out, even if the status code returned
from the origin server indicates that the action has been completed
successfully. However, the server SHOULD NOT indicate success
unless, at the time the response is given, it intends to create or
update the specified relationships.
If the LINK request message attempts to create or update an existing
relationship and the server does not intend to comply with the
request for any reason other than a client or server error, the
server can return a 304 (Not Modified) response to indicate that no
modifications have been made.
3. UNLINK
The UNLINK method is used to remove one or more relationships between
the existing resource identified by the effective request URI and
other resources. Metadata contained within Link header fields
[RFC5988] provide the information about the resources to which
relationships of a specific type are to be removed. A payload within
an UNLINK request message has no defined semantics.
The semantics of the UNLINK method change to a "conditional UNLINK"
if the request message includes an If-Modified-Since, If-Unmodified-
Since, If-Match, If-None-Match, or If-Range header field
([I-D.ietf-httpbis-p4-conditional]). A conditional UNLINK requests
that the relationship be removed only under the circumstances
described by the conditional header field(s).
UNLINK request messages are idempotent.
UNLINK request messages are not safe, however, in that removing a
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relationship causes an inherent change to the state of the target
resource.
Responses to UNLINK requests are not cacheable. If an UNLINK request
passes through a cache that has one or more stored responses for the
effective request URI, those stored responses will be invalidated
(see Section 6 of [I-D.ietf-httpbis-p6-cache]).
A single UNLINK request message can contain multiple Link header
fields, each of which identifies a separate relationship to remove.
In such cases, the server MUST remove the entire set of relationships
atomically. If any of the specified relationships cannot be removed,
the server MUST NOT remove any of them.
A successful response indicating the removing of the relationship
SHOULD be 200 (OK) if the response includes a representation
describing the status, 202 (Accepted) if the action has not yet been
enacted, or 204 (No Content) if the action has been enacted but the
response does not include a representation.
The UNLINK method MAY be overridden by human intervention (or other
means) on the origin server. The client cannot be guaranteed that
the operation has been carried out, even if the status code returned
from the origin server indicates that the action has been completed
successfully. However, the server SHOULD NOT indicate success
unless, at the time the response is given, it intends to remove the
specified relationships.
If the UNLINK request message attempts to remove an existing
relationship and the server does not intend to remove or otherwise
alter the existing relationship for any reason other than a client or
server error, the server can return a 304 (Not Modified) response to
indicate that no modifications have been made.
4. Relationship to other HTTP Methods and Discoverability of Links
The use of the LINK and UNLINK request methods to manage
relationships between resources has no direct bearing on the use or
appearance of Link header fields within any other HTTP request or
response message involving the same effective request URI. Nor do
the methods have any direct normative impact on the use of link-like
structures within the resource representations returned by a server
for any particular resource.
Whether and how to represent relationships managed using LINK and
UNLINK is left solely at the discretion of the server implementation.
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This specification does not define a means of discovering or
enumerating the relationships that have been established using the
LINK request method.
5. Example
There exists a broad range of possible use cases for the LINK and
UNLINK methods. The examples that follow illustrate a subset of
those cases.
Example 1: Creating two separate links between an image and the
profiles of two people associated with the image:
LINK /images/my_dog.jpg HTTP/1.1
Host: example.org
Link: <http://example.com/profiles/joe>; rel="tag"
Link: <http://example.com/profiles/sally>; rel="tag"
Example 2: Removing an existing Link relationship between two
resources:
UNLINK /images/my_dog.jpg HTTP/1.1
Host: example.org
Link: <http://example.com/profiles/sally>; rel="tag"
Example 3: Establish a "pingback" or "trackback" style link to a blog
entry about an article
LINK /articles/an_interesting_article HTTP/1.1
Host: example.org
Link: <http://example.com/my_blog_post>; rel="about"
Example 4: Establish a link between two semantically related
resources:
LINK /some-resource HTTP/1.1
Host: example.org
Link: <http://example.com/schemas/my_schema>; rel="describedBy"
Example 5: Add an existing resource to a collection:
LINK /some-collection-resource HTTP/1.1
Host: example.org
Link: <http://example.com/my-member-resource>; rel="item"
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Example 6: Link one resource to another that monitors its current
state (e.g. pub/sub)
LINK /my-resource HTTP/1.1
Host: example.org
Link: <http://example.com/my-monitor>; rel="monitor"
6. Security Considerations
The LINK and UNLINK methods are subject to the same general security
considerations as all HTTP methods as described in
[I-D.ietf-httpbis-p2-semantics].
Implementers need to be aware of the possible ways the LINK method
can be abused as a means of propagating inappropriate links to
external resources. For instance, the unregulated acceptance of LINK
requests can be used as a vector for spam or malware distribution.
Because the LINK and UNLINK methods cause changes to a resource's
state, the server is responsible for determining the client's
authorization to make such changes.
7. Normative References
[I-D.ietf-httpbis-p2-semantics]
Fielding, R. and J. Reschke, "Hypertext Transfer Protocol
(HTTP/1.1): Semantics and Content",
draft-ietf-httpbis-p2-semantics-21 (work in progress),
October 2012.
[I-D.ietf-httpbis-p4-conditional]
Fielding, R. and J. Reschke, "Hypertext Transfer Protocol
(HTTP/1.1): Conditional Requests",
draft-ietf-httpbis-p4-conditional-21 (work in progress),
October 2012.
[I-D.ietf-httpbis-p6-cache]
Fielding, R., Nottingham, M., and J. Reschke, "Hypertext
Transfer Protocol (HTTP/1.1): Caching",
draft-ietf-httpbis-p6-cache-21 (work in progress),
October 2012.
[RFC2068] Fielding, R., Gettys, J., Mogul, J., Nielsen, H., and T.
Berners-Lee, "Hypertext Transfer Protocol -- HTTP/1.1",
RFC 2068, January 1997.
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[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC5988] Nottingham, M., "Web Linking", RFC 5988, October 2010.
Author's Address
James M Snell
Email: jasnell@gmail.com
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