WebDAV Redirect Reference Resources

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Copyright © The Internet Society (2003). All Rights Reserved.¶

This specification defines redirect reference resources. A redirect reference resource is a resource whose default response is an HTTP/1.1 302 (Found) status code, redirecting the client to a different resource, the target resource. A redirect reference makes it possible to access the target resource indirectly, through any URI mapped to the redirect reference resource. There are no integrity guarantees associated with redirect reference resources.¶

Distribution of this document is unlimited. Please send comments to the Distributed Authoring and Versioning (WebDAV) working group at w3c-dist-auth@w3.org, which may be joined by sending a message with subject "subscribe" to w3c-dist-auth-request@w3.org.¶

Discussions of the WEBDAV working group are archived at URL: http://lists.w3.org/Archives/Public/w3c-dist-auth/.¶

URIs of WebDAV-compliant resources are hierarchical and correspond to a hierarchy of collections in resource space. The WebDAV Distributed Authoring Protocol makes it possible to organize these resources into hierarchies, placing them into groupings, known as collections, which are more easily browsed and manipulated than a single flat collection. However, hierarchies require categorization decisions that locate resources at a single location in the hierarchy, a drawback when a resource has multiple valid categories. For example, in a hierarchy of vehicle descriptions containing collections for cars and boats, a description of a combination car/boat vehicle could belong in either collection. Ideally, the description should be accessible from both. Allowing clients to create new URIs that access the existing resource lets them put that resource into multiple collections.¶

Hierarchies also make resource sharing more difficult, since resources that have utility across many collections are still forced into a single collection. For example, the mathematics department at one university might create a collection of information on fractals that contains bindings to some local resources, but also provides access to some resources at other universities. For many reasons, it may be undesirable to make physical copies of the shared resources on the local server: to conserve disk space, to respect copyright constraints, or to make any changes in the shared resources visible automatically. Being able to create new access paths to existing resources in other collections or even on other servers is useful for this sort of case.¶

The redirect reference resources defined here provide a mechanism for creating alternative access paths to existing resources. A redirect reference resource is a resource in one collection whose purpose is to forward requests to another resource (its target), possibly in a different collection. In this way, it allows clients to submit requests to the target resource from another collection. It redirects most requests to the target resource using the HTTP 302 (Found) status code, thereby providing a form of mediated access to the target resource.¶

A redirect reference is a resource with properties but no body of its own. Properties of a redirect reference resource can contain such information as who created the reference, when, and why. Since redirect reference resources are implemented using HTTP 302 responses, it generally takes two round trips to submit a request to the intended resource. Servers are not required to enforce the integrity of redirect references. Redirect references work equally well for local resources and for resources that reside on a different server from the reference.¶

The remainder of this document is structured as follows: Section 3 defines terms that will be used throughout the specification. Section 4 provides an overview of redirect reference resources. Section 5 discusses how to create a redirect reference resource. Section 6 defines the semantics of existing methods when applied to redirect reference resources, and Section 7 discusses their semantics when applied to collections that contain redirect reference resources. Sections 8 through 10 discuss several other issues raised by the existence of redirect reference resources. Sections 11 through 14 define the new headers, properties, and XML elements required to support redirect reference resources. Section 15 discusses capability discovery. Sections 16 through 18 present the security, internationalization, and IANA concerns raised by this specification. The remaining sections provide a variety of supporting information.¶

Since this document describes a set of extensions to the WebDAV Distributed Authoring Protocol [RFC2518], itself an extension to the HTTP/1.1 protocol, the augmented BNF used here to describe protocol elements is exactly the same as described in Section 2.1 of [RFC2616]. Since this augmented BNF uses the basic production rules provided in Section 2.2 of [RFC2616], these rules apply to this document as well.¶

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 [RFC2119].¶

The terminology used here follows and extends that in the WebDAV Distributed Authoring Protocol specification [RFC2518]. Definitions of the terms resource, Uniform Resource Identifier (URI), and Uniform Resource Locator (URL) are provided in [RFC2396].¶

Redirect Reference Resource ¶

• A resource created to redirect all requests made to it, using 302 (Found), to a defined target resource.

Non-Reference Resource ¶

• A resource that is not a reference to another resource.

Target Resource ¶

• The resource to which requests are forwarded by a reference resource. A target resource can be anything that can be identified by an absolute URI (see [RFC2396], "absoluteURI").

For all operations submitted to a redirect reference resource, the default response is a 302 (Found), accompanied by the Redirect-Ref header (defined in Section 11.1 below) and the Location header set to the URI of the target resource. With this information, the client can resubmit the request to the URI of the target resource.¶

A redirect reference resource never automatically forwards requests to its target resource. Redirect resources bring the same benefits as links in HTML documents. They can be created and maintained without the involvement or even knowledge of their target resource. This reduces the cost of linking between resources."¶

If the client is aware that it is operating on a redirect reference resource, it can resolve the reference by retrieving the reference resource's DAV:reftarget property (defined in Section 12.1 below), whose value contains the URI of the target resource. It can then submit requests to the target resource.¶

A redirect reference resource is a new type of resource. To distinguish redirect reference resources from non-reference resources, a new value of the DAV:resourcetype property (defined in [RFC2518]), DAV:redirectref, is defined in Section 13.1 below.¶

Since a redirect reference resource is a resource, methods can be applied to the reference resource as well as to its target resource. The Apply-To-Redirect-Ref request header (defined in Section 11.2 below) is provided so that referencing-aware clients can control whether an operation is applied to the redirect reference resource or to its target resource. The Apply-To-Redirect-Ref header can be used with most requests to redirect reference resources. This header is particularly useful with PROPFIND, to retrieve the reference resource's own properties.¶

The new MKRESOURCE method is used to create new redirect reference resources. In order to create a redirect reference resource using MKRESOURCE, the values of two properties must be set in the body of the MKRESOURCE request. The value of DAV:resourcetype MUST be set to DAV:redirectref, a new value of DAV:resourcetype defined in Section 13.1. The value of DAV:reftarget MUST be set to the URI of the target resource.¶

Used in this way, the MKRESOURCE method creates a redirect reference resource whose target is identified by the DAV:reftarget property.¶

• The Location response header MUST contain an absolute URI that identifies the target of the reference resource.
• The response MUST include the Redirect-Ref header. This header allows reference-aware WebDAV clients to recognize the resource as a reference resource and understand the reason for the redirection.

A reference-aware WebDAV client can act on this response in one of two ways. It can, like a non-referencing client, resubmit the request to the URI in the Location header in order to operate on the target resource. Alternatively, it can resubmit the request to the URI of the redirect reference resource with the ↑ I ↓Apply-To-Redirect-Ref"Apply-To-Redirect-Ref: T" header in order to operate on the reference resource itself. ↑ I ↓If the Apply-To-Redirect-Ref header is present,In this case, the request MUST be applied to the reference resource itself, and a 302 response MUST NOT be returned.¶

A reference-aware client may know before submitting its request that the Request-URI identifies a redirect reference resource. In this case, if the client wants to apply the method to the reference resource, it can save the round trip caused by the 302 response by using an Apply-To-Redirect-Ref header in its initial request to the URI.¶

As redirect references do not have bodies, GET and PUT requests with ↑ I ↓Apply-To-Redirect-Ref"Apply-To-Redirect-Ref: T" MUST fail with status 403 (forbidden).¶

Consistent with the rules in Section 6, the response for each redirect reference encountered while processing a collection MUST be a 302 (Found) unless a ↑ I ↓Apply-To-Redirect-Ref"Apply-To-Redirect-Ref: T" header is included with the request. The overall response will therefore be a 207 (Multi-Status). Since a Location header and Redirect-Ref header cannot be returned for each redirect reference encountered, the same information is provided using properties in the response elements for those resources. The DAV:location pseudo-property and the DAV:resourcetype property MUST be included with the 302 status code. This necessitates an extension to the syntax of the DAV:response element that was defined in [RFC2518]. The extension is defined in Section 14 below.¶

It is recommended that future editors of [RFC2518] define the DAV:location pseudo-property in [RFC2518], so that non-referencing clients will also be able to use the response to operate on the target resource. (This will also enable clients to operate on traditional HTTP/1.1 302 responses in Multi-Status responses.) Until then, non-referencing clients will not be able to process 302 responses from redirect reference resources encountered while processing a collection.¶

The Apply-To-Redirect-Ref header (defined in Section 11.2) MAY be used with any request on a collection. If present, it will be applied to all redirect reference resources encountered while processing the collection.¶

Operations on targets of redirect reference resources have no effect on the reference resource.¶

The URI in the href in a DAV:reftarget property MAY be a relative URI. In this case, the base URI to be used for resolving the relative URI to absolute form is the URI used in the HTTP message to identify the redirect reference resource to which the DAV:reftarget property belongs.¶

When DAV:reftarget appears in the context of a Multi-Status response, it is in a DAV:response element that contains a single DAV:href element. The value of this DAV:href element serves as the base URI for resolving a relative URI in DAV:reftarget. The value of DAV:href may itself be relative, in which case it must be resolved first in order to serve as the base URI for the relative URI in DAV:reftarget. If the DAV:href element is relative, its base URI is constructed from the scheme component "http", the value of the Host header in the request, and the request-URI.¶

The leftmost path segment of the request-URI that identifies a redirect reference resource, together with all path segments and separators to the left of it, is replaced by the value of the redirect reference resource's DAV:reftarget property (resolved to an absolute URI). The remainder of the request-URI is concatenated to this path.¶

Note: If the DAV:reftarget property ends with a "/" and the remainder of the Request-URI is non-empty (and therefore must begin with a "/"), the final "/" in the DAV:reftarget property is dropped before the remainder of the Request-URI is appended.¶

Consider Request-URI /x/y/z.html. Suppose that /x/ is a redirect reference resource whose target resource is collection /a/, which contains redirect reference resource y whose target resource is collection /b/, which contains redirect reference resource z.html whose target resource is /c/d.html.¶

In this case the client must follow up three separate 302 responses before finally reaching the target resource. The server responds to the initial request with a 302 with Location: /a/y/z.html, and the client resubmits the request to /a/y/z.html. The server responds to this request with a 302 with Location: /b/z.html, and the client resubmits the request to /b/z.html. The server responds to this request with a 302 with Location: /c/d.html, and the client resubmits the request to /c/d.html. This final request succeeds.¶

As described in Section 7, the DAV:location pseudo-property and the DAV:resourcetype property may be returned in the DAV:response element of a 207 Multi-Status response, to allow clients to resubmit their requests to the target resource of a redirect reference resource.¶

Whenever these properties are included in a Multi-Status response, they are placed in a DAV:prop element associated with the href to which they apply. This structure provides a framework for future extensions by other standards that may need to include additional properties in their responses.¶

Sections 9.1 and 15 of [RFC2518] describe the use of compliance classes with the DAV header in responses to OPTIONS, to indicate which parts of the WebDAV Distributed Authoring protocols the resource supports. This specification defines an OPTIONAL extension to [RFC2518]. It defines a new compliance class, called redirectrefs, for use with the DAV header in responses to OPTIONS requests. If a resource does support redirect references, its response to an OPTIONS request may indicate that it does, by listing the new redirectrefs compliance class in the DAV headerand by listing the MKRESOURCE method as one it supports.¶

When responding to an OPTIONS request, any type of resource can include redirectrefs in the value of the DAV header. Doing so indicates that the server permits a redirect reference resource at the request URI.¶

This section is provided to make applications that implement this protocol aware of the security implications of this protocol.¶

All of the security considerations of HTTP/1.1 and the WebDAV Distributed Authoring Protocol specification also apply to this protocol specification. In addition, redirect reference resources introduce several new security concerns and increase the risk of some existing threats. These issues are detailed below.¶

This specification follows the practices of [RFC2518] in encoding all human-readable content using XML [XML] and in the treatment of names. Consequently, this specification complies with the IETF Character Set Policy [RFC2277].¶

WebDAV applications MUST support the character set tagging, character set encoding, and the language tagging functionality of the XML specification. This constraint ensures that the human-readable content of this specification complies with [RFC2277].¶

As in [RFC2518], names in this specification fall into three categories: names of protocol elements such as methods and headers, names of XML elements, and names of properties. Naming of protocol elements follows the precedent of HTTP, using English names encoded in USASCII for methods and headers. The names of XML elements used in this specification are English names encoded in UTF-8.¶

For error reporting, [RFC2518] follows the convention of HTTP/1.1 status codes, including with each status code a short, English description of the code (e.g., 423 Locked). Internationalized applications will ignore this message, and display an appropriate message in the user's language and character set.¶

This specification introduces no new strings that are displayed to users as part of normal, error-free operation of the protocol.¶

For rationales for these decisions and advice for application implementors, see [RFC2518].¶

All IANA considerations mentioned in [RFC2518] also apply to this document.¶

This draftdocument has benefited from thoughtful discussion by Jim Amsden, Peter Carlson, Steve Carter, Tyson Chihaya, Ken Coar, Ellis Cohen, Bruce Cragun, Spencer Dawkins, Mark Day, Rajiv Dulepet, David Durand, Roy Fielding, Yaron Goland, Fred Hitt, Alex Hopmann, James Hunt, Marcus Jager, Chris Kaler, Manoj Kasichainula, Rohit Khare, Daniel LaLiberte, Steve Martin, Larry Masinter, Jeff McAffer, Joe Orton, Surendra Koduru Reddy, Juergen Reuter, Max Rible, Sam Ruby, Bradley Sergeant, Nick Shelness, John Stracke, John Tigue, John Turner, Kevin Wiggen, and others.¶

Copyright © The Internet Society (2003). All Rights Reserved.¶

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