Network Working Group J.M. Snell Internet-Draft April 22, 2013 Intended status: Informational Expires: October 24, 2013 HTTP Link and Unlink Methods draft-snell-link-method-02 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. 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 October 24, 2013. Copyright Notice Copyright (c) 2013 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. Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2 2. LINK . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2 3. UNLINK . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 Snell Expires October 24, 2013 [Page 1] Internet-Draft HTTP Link and Unlink Methods April 2013 4. Relationship to other HTTP Methods and Discoverability of Links . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 5. Example . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 6. Security Considerations . . . . . . . . . . . . . . . . . . . 6 7. Normative References . . . . . . . . . . . . . . . . . . . . 6 Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 7 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. Establishing a relationship causes an inherent change to the state of the target resource. Snell Expires October 24, 2013 [Page 2] Internet-Draft HTTP Link and Unlink Methods April 2013 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 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 Snell Expires October 24, 2013 [Page 3] Internet-Draft HTTP Link and Unlink Methods April 2013 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. Removing a 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 Snell Expires October 24, 2013 [Page 4] Internet-Draft HTTP Link and Unlink Methods April 2013 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. 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: ; rel="tag" Link: ; rel="tag" Example 2: Removing an existing Link relationship between two resources: UNLINK /images/my_dog.jpg HTTP/1.1 Host: example.org Link: ; 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: ; rel="about" Example 4: Establish a link between two semantically related resources: Snell Expires October 24, 2013 [Page 5] Internet-Draft HTTP Link and Unlink Methods April 2013 LINK /some-resource HTTP/1.1 Host: example.org Link: ; rel="describedBy" Example 5: Add an existing resource to a collection: LINK /some-collection-resource HTTP/1.1 Host: example.org Link: ; rel="item" 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: ; 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-22 (work in progress), February 2013. [I-D.ietf-httpbis-p4-conditional] Fielding, R. and J. Reschke, "Hypertext Transfer Protocol (HTTP/1.1): Conditional Requests", draft-ietf- httpbis-p4-conditional-22 (work in progress), February 2013. Snell Expires October 24, 2013 [Page 6] Internet-Draft HTTP Link and Unlink Methods April 2013 [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-22 (work in progress), February 2013. [RFC2068] Fielding, R., Gettys, J., Mogul, J., Nielsen, H., and T. Berners-Lee, "Hypertext Transfer Protocol -- HTTP/1.1", RFC 2068, January 1997. [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 Snell Expires October 24, 2013 [Page 7]