Binding Extensions to Web Distributed Authoring and Versioning (WebDAV)

The terminology used here follows and extends that in the WebDAV Distributed Authoring Protocol specification [RFC2518].¶

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].¶

This document uses XML DTD fragments ([XML]) as a purely notational convention. WebDAV request and response bodies cannot be validated due to the specific extensibility rules defined in section 23 of [RFC2518] and due to the fact that all XML elements defined by this specification use the XML namespace name "DAV:". In particular: ¶

• Element names use the "DAV:" namespace.
• Element ordering is irrelevant.
• Extension elements/attributes (elements/attributes not already defined as valid child elements) may be added anywhere, except when explicitly stated otherwise.

URI Mapping ¶

• A relation between an absolute URI and a resource. For an absolute URI U and the resource it identifies R, the URI mapping can be thought of as (U => R). Since a resource can represent items that are not network retrievable, as well as those that are, it is possible for a resource to have zero, one, or many URI mappings. Mapping a resource to an "http" scheme URI makes it possible to submit HTTP protocol requests to the resource using the URI.

Path Segment ¶

• Informally, the characters found between slashes ("/") in a URI. Formally, as defined in section 3.3 of [RFC2396].

Binding ¶

• A relation between a single path segment (in a collection) and a resource. A binding is part of the state of a collection. If two different collections contain a binding between the same path segment and the same resource, these are two distinct bindings. So for a collection C, a path segment S, and a resource R, the binding can be thought of as C:(S -> R). Bindings create URI mappings, and hence allow requests to be sent to a single resource from multiple locations in a URI namespace. For example, given a collection C (accessible through the URI http://www.example.com/CollX), a path segment S (equal to "foo.html"), and a resource R, then creating the binding C: (S -> R) makes it possible to use the URI http://www.example.com/CollX/foo.html to access R.

Collection ¶

• A resource that contains, as part of its state, a set of bindings that identify internal member resources.

Internal Member URI ¶

• The URI that identifies an internal member of a collection, and that consists of the URI for the collection, followed by a slash character ('/'), followed by the path segment of the binding for that internal member.

In [RFC2518], the state of a collection is defined as containing a list of internal member URIs. If there are multiple mappings to a collection, then the state of the collection is different when you refer to it via a different URI. This is undesirable, since ideally a collection's membership should remain the same, independent of which URI was used to reference it.¶

The notion of binding is introduced to separate the final segment of a URI from its parent collection's contribution. This done, a collection can be defined as containing a set of bindings, thus permitting new mappings to a collection without modifying its membership. The authors of this specification anticipate and recommend that future revisions of [RFC2518] will update the definition of the state of a collection to correspond to the definition in this document.¶

A "precondition" of a method describes the state on the server that must be true for that method to be performed. A "postcondition" of a method describes the state on the server that must be true after that method has completed. If a method precondition or postcondition for a request is not satisfied, the response status of the request MUST be either 403 (Forbidden) if the request should not be repeated because it will always fail, or 409 (Conflict) if it is expected that the user might be able to resolve the conflict and resubmit the request.¶

In order to allow better client handling of 403 and 409 responses, a distinct XML element type is associated with each method precondition and postcondition of a request. When a particular precondition is not satisfied or a particular postcondition cannot be achieved, the appropriate XML element MUST be returned as the child of a top-level DAV:error element in the response body, unless otherwise negotiated by the request. In a 207 Multi-Status response, the DAV:error element would appear in the appropriate DAV:responsedescription element.¶

Bindings to collections can result in loops, which servers MUST detect when processing "Depth: infinity" requests. It is sometimes possible to complete an operation in spite of the presence of a loop. However, the 506 (Loop Detected) status code is defined in Section 7 for use in contexts where an operation is terminated because a loop was encountered.¶

Creating a new binding to a collection makes each resource associated with a binding in that collection accessible via a new URI, and thus creates new URI mappings to those resources but no new bindings.¶

For example, suppose a new binding CollY is created for collection C1 in the figure below. It immediately becomes possible to access resource R1 using the URI /CollY/x.gif and to access resource R2 using the URI /CollY/y.jpg, but no new bindings for these child resources were created. This is because bindings are part of the state of a collection, and associate a URI that is relative to that collection with its target resource. No change to the bindings in Collection C1 is needed to make its children accessible using /CollY/x.gif and /CollY/y.jpg.¶

Suppose a binding from "Binding-Name" to resource R ↑ I ↓isto be added to a collection, C. Then if C-MAP is the set of URIs that were mapped to C before the BIND request, then for each URI "C-URI" in C-MAP, the URI "C-URI/Binding-Name" is mapped to resource R following the BIND request.¶

For example, if a binding from "foo.html" to R is added to a collection C, and if the following URIs are mapped to C:

http://www.example.com/A/1/
http://example.com/A/one/

then the following new mappings to R are introduced:

http://www.example.com/A/1/foo.html
http://example.com/A/one/foo.html

Note that if R is a collection, additional URI mappings are created to the descendents of R. Also, note that if a binding is made in collection C to C itself (or to a parent of C), an infinite number of mappings are introduced.¶

For example, if a binding from "myself" to C is then added to C, the following infinite number of additional mappings to C are introduced:

http://www.example.com/A/1/myself
http://www.example.com/A/1/myself/myself
   ...

and the following infinite number of additional mappings to R are introduced:

http://www.example.com/A/1/myself/foo.html
http://www.example.com/A/1/myself/myself/foo.html
   ...

As defined in Section 8.8 of [RFC2518], COPY causes the resource identified by the Request-URI to be duplicated, and makes the new resource accessible using the URI specified in the Destination header. Upon successful completion of a COPY, a new binding is created between the last path segment of the Destination header, and the destination resource. The new binding is added to its parent collection, identified by the Destination header minus its final segment.¶

The following figure shows an example: Suppose that a COPY is issued to URI-3 for resource R (which is also mapped to URI-1 and URI-2), with the Destination header set to URI-X. After successful completion of the COPY operation, resource R is duplicated to create resource R', and a new binding has been created which creates at least the URI mapping between URI-X and the new resource (although other URI mappings may also have been created).

URI-1   URI-2    URI-3                           URI-X
   |       |        |                              |
   |       |        |   <---- URI Mappings ---->   |
   |       |        |                              |
+---------------------+                 +------------------------+
|     Resource R      |                 |     Resource R'        |
+---------------------+                 +------------------------+

It might be thought that a COPY request with "Depth: 0" on a collection would duplicate its bindings, since bindings are part of the collection's state. This is not the case, however. The definition of Depth in [RFC2518] makes it clear that a "Depth: 0" request does not apply to a collection's members. Consequently, a COPY with "Depth: 0" does not duplicate the bindings contained by the collection.¶

If a COPY request causes an existing resource to be updated, the bindings to that resource MUST be unaffected by the COPY request. Using the preceding example, suppose that a COPY request is issued to URI-X for resource R', with the Destination header set to URI-2. The content and dead properties of resource R would be updated to be a copy of those of resource R', but the mappings from URI-1, URI-2, and URI-3 to resource R remain unaffected. If because of multiple bindings to a resource, more than one source resource updates a single destination resource, the order of the updates is server defined.¶

If a COPY request would cause a new resource to be created as a copy of an existing resource, and that COPY request has already created a copy of that existing resource, the COPY request instead creates another binding to the previous copy, instead of creating a new resource.¶

When there are multiple bindings to a resource, a DELETE applied to that resource MUST NOT remove any bindings to that resource other than the one identified by the request URI. For example, suppose the collection identified by the URI "/a" has a binding named "x" to a resource R, and another collection identified by "/b" has a binding named "y" to the same resource R. Then a DELETE applied to "/a/x" removes the binding named "x" from "/a" but MUST NOT remove the binding named "y" from "/b" (i.e. after the DELETE, "/y/b" continues to identify the resource R). In particular, although Section 8.6.1 of [RFC2518] states that during DELETE processing, a server "MUST remove any URI for the resource identified by the Request-URI from collections which contain it as a member", a server that supports the binding protocol MUST NOT follow this requirement.¶

When DELETE is applied to a collection, it MUST NOT modify the membership of any other collection that is not itself a member of the collection being deleted. For example, if both "/a/.../x" and "/b/.../y" identify the same collection, C, then applying DELETE to "/a" MUST NOT delete an internal member from C or from any other collection that is a member of C, because that would modify the membership of "/b".¶

If a collection supports the UNBIND method (see Section 5), a DELETE of an internal member of a collection MAY be implemented as an UNBIND request. In this case, applying DELETE to a Request-URI has the effect of removing the binding identified by the final segment of the Request-URI from the collection identified by the Request-URI minus its final segment. Although [RFC2518] allows a DELETE to be a non-atomic operation, when the DELETE operation is implemented as an UNBIND, the operation is atomic. In particular, a DELETE on a hierarchy of resources is simply the removal of a binding to the collection identified by the Request-URI.¶

When MOVE is applied to a resource, the other bindings to that resource MUST be unaffected, and if the resource being moved is a collection, the bindings to any members of that collection MUST be unaffected. Also, if MOVE is used with Overwrite:T to delete an existing resource, the constraints specified for DELETE apply.¶

If the destination collection of a MOVE request supports the REBIND method (see Section 6), a MOVE of a resource into that collection MAY be implemented as a REBIND request. Although [RFC2518] allows a MOVE to be a non-atomic operation, when the MOVE operation is implemented as a REBIND, the operation is atomic. In particular, applying a MOVE to a Request-URI and a Destination URI has the effect of removing a binding to a resource (at the Request-URI), and creating a new binding to that resource (at the Destination URI).  I ↓Even when the Request-URI identifies a collection, the MOVE operation involves only removing one binding to that collection and adding another. ¶

As an example, suppose that a MOVE is issued to URI-3 for resource R below (which is also mapped to URI-1 and URI-2), with the Destination header set to URI-X. After successful completion of the MOVE operation, a new binding has been created which creates the URI mapping between URI-X and resource R. The binding corresponding to the final segment of URI-3 has been removed, which also causes the URI mapping between URI-3 and R to be removed. If resource R were a collection, old URI-3 based mappings to members of R would have been removed, and new URI-X based mappings to members of R would have been created.¶

>> Before Request:

 URI-1   URI-2    URI-3
   |       |        |                               
   |       |        |      <---- URI Mappings
   |       |        |
+---------------------+                  
|     Resource R      |
+---------------------+                  

>> After Request:

 URI-1   URI-2    URI-X
   |       |        |                               
   |       |        |      <---- URI Mappings
   |       |        |
+---------------------+                  
|     Resource R      |
+---------------------+                  

It is useful to have some way of determining whether two bindings are to the same resource. Two resources might have identical contents and properties, but not be the same resource (e.g. an update to one resource does not affect the other resource).¶

The REQUIRED DAV:resource-id property defined in Section 3.1 is a resource identifier, which MUST be unique across all resources for all time. If the values of DAV:resource-id returned by PROPFIND requests through two bindings are identical, the client can be assured that the two bindings are to the same resource.¶

The DAV:resource-id property is created, and its value assigned, when the resource is created. The value of DAV:resource-id MUST NOT be changed. Even after the resource is no longer accessible through any URI, that value MUST NOT be reassigned to another resource's DAV:resource-id property.¶

Any method that creates a new resource MUST assign a new, unique value to its DAV:resource-id property. For example, a PUT or a COPY that creates a new resource must assign a new, unique value to the DAV:resource-id property of that new resource.¶

On the other hand, any method that affects an existing resource MUST NOT change the value of its DAV:resource-id property. For example, a PUT or a COPY that updates an existing resource must not change the value of its DAV:resource-id property. A MOVE, since it does not create a new resource, but only changes the location of an existing resource, must not change the value of the DAV:resource-id property.¶

An OPTIONAL DAV:parent-set property on a resource provides a list of the bindings that associate a collection and a URI segment with that resource. If the DAV:parent-set property exists on a given resource, it MUST contain a complete list of all bindings to that resource that the client is authorized to see. When deciding whether to support the DAV:parent-set property, server implementers / administrators should balance the benefits it provides against the cost of maintaining the property and the security risks enumerated in Sections 8.4 and 8.5. ¶

The DAV:resource-id property is a REQUIRED property that enables clients to determine whether two bindings are to the same resource. The value of DAV:resource-id is a URI, and may use any registered URI scheme that guarantees the uniqueness of the value across all resources for all time (e.g. the opaquelocktoken: scheme defined in [RFC2518]).¶

The DAV:parent-set property is an OPTIONAL property that enables clients to discover what collections contain a binding to this resource (i.e. what collections have that resource as an internal member). It contains an of href/segment pair for each collection that has a binding to the resource. The href identifies the collection, and the segment identifies the binding name of that resource in that collection.¶

A given collection MUST appear only once in the DAV:parent-set for any given binding, even if there are multiple URI mappings to that collection. For example, if collection C1 is mapped to both /CollX and /CollY, and C1 contains a binding named "x.gif" to a resource R1, then either [/CollX, x.gif] or [/CollY, x.gif] can appear in the DAV:parent-set of R1, but not both. But if C1 also had a binding named "y.gif" to R1, then there would be two entries for C1 in the DAV:binding-set of R1 (i.e. either both [/CollX, x.gif] and [/CollX, y.gif] or alternatively, both [/CollY, x.gif] and [/CollY, y.gif]).¶

>> Request:

BIND /CollY HTTP/1.1
Host: www.example.com
Content-Type: text/xml; charset="utf-8"
Content-Length: xxx

<?xml version="1.0" encoding="utf-8" ?>
<D:bind xmlns:D="DAV:">
   <D:segment>bar.html</D:segment>
   <D:href>http://www.example.com/CollX/foo.html</D:href>
</D:bind>

>> Response:

HTTP/1.1 201 Created

The server added a new binding to the collection, "http://www.example.com/CollY", associating "bar.html" with the resource identified by the URI "http://www.example.com/CollX/foo.html". Clients can now use the URI "http://www.example.com/CollY/bar.html", to submit requests to that resource.¶

>> Request:

UNBIND /CollX HTTP/1.1
Host: www.example.com
Content-Type: text/xml; charset="utf-8"
Content-Length: xxx

<?xml version="1.0" encoding="utf-8" ?>
<D:unbind xmlns:D="DAV:">
   <D:segment>foo.html</D:segment>
</D:unbind>

>> Response:

HTTP/1.1 200 OK

The server removed the binding named "foo.html" from the collection, "http://www.example.com/CollX". A request to the resource named "http://www.example.com/CollX/foo.html" will return a 404 (Not Found) response.¶

>> Request:

REBIND /CollX HTTP/1.1
Host: www.example.com
Content-Type: text/xml; charset="utf-8"
Content-Length: xxx

<?xml version="1.0" encoding="utf-8" ?>
<D:rebind xmlns:D="DAV:">
   <D:segment>foo.html</D:segment>
   <D:href>http://www.example.com/CollY/bar.html</D:href>
</D:rebind>

>> Response:

HTTP/1.1 200 OK

The server added a new binding to the collection, "http://www.example.com/CollX", associating "foo.html" with the resource identified by the URI "http://www.example.com/CollY/bar.html", and removes the binding named "bar.html" from the collection identified by the URI "http://www.example.com/CollY". Clients can now use the URI "http://www.example.com/CollX/foo.html" to submit requests to that resource, and requests on the URI "http://www.example.com/CollY/bar.html" will fail with a 404 (Not Found) response.¶

The 208 (Already Reported) status code can be used inside a DAV:propstat response element to avoid enumerating the internal members of multiple bindings to the same collection repeatedly. For each binding to a collection inside the request's scope, only one will be reported with a 200 status, while subsequent DAV:response elements for all other bindings will use the 208 status, and no DAV:response elements for their descendants are included.¶

Note that the 208 status will only occur for "Depth: infinity" requests, and that it is of particular importance when the multiple collection bindings cause a bind loop as discussed in Section 2.2.¶

A client can request the DAV:resourceid property in a PROPFIND request to guarantee that they can accurately reconstruct the binding structure of a collection with multiple bindings to a single resource.¶

For backward compatibility with clients not aware of the 208 status code appearing in multistatus response bodies, it SHOULD NOT be used unless the client has signalled support for this specification using the "DAV" request header (see Section 8.2). Instead, a 506 status should be returned when a binding loop is discovered. This allows the server to return the 506 as the top level return status, if it discovers it before it started the response, or in the middle of a multistatus, if it discovers it in the middle of streaming out a multistatus response.¶

The 506 (Loop Detected) status code indicates that the server terminated an operation because it encountered an infinite loop while processing a request with "Depth: infinity". This status indicates that the entire operation failed.¶

If the server supports bindings, it MUST return the compliance class name "bind" as a field in the "DAV" response header (see [RFC2518], section 9.1) from an OPTIONS request on any resource implemented by that server. A value of "bind" in the "DAV" header MUST indicate that the server supports all MUST level requirements and REQUIRED features specified in this document.¶

In a context where cross-server bindings are supported, creating bindings on a trusted server may make it possible for a hostile agent to induce users to send private information to a target on a different server.¶

Although bind loops were already possible in HTTP 1.1, the introduction of the BIND method creates a new avenue for clients to create loops accidentally or maliciously. If the binding and its target are on the same server, the server may be able to detect BIND requests that would create loops. Servers are required to detect loops that are caused by bindings to collections during the processing of any requests with "Depth: infinity".¶

Denial of service attacks were already possible by posting URIs that were intended for limited use at heavily used Web sites. The introduction of BIND creates a new avenue for similar denial of service attacks. If cross-server bindings are supported, clients can now create bindings at heavily used sites to target locations that were not designed for heavy usage.¶

If the DAV:parent-set property is maintained on a resource, the owners of the bindings risk revealing private locations. The directory structures where bindings are located are available to anyone who has access to the DAV:parent-set property on the resource. Moving a binding may reveal its new location to anyone with access to DAV:parent-set on its resource.¶

If the server maintains the DAV:parent-set property in response to bindings created in other administrative domains, it is exposed to hostile attempts to make it devote resources to adding bindings to the list.¶

Add and resolve issues "2.3_COPY_SHARED_BINDINGS" and "2.3_MULTIPLE_COPY". Add issue "5.1_LOOP_STATUS" and proposed resolution, but keep it open. Add issues "ED_references" and "4_507_status". Started work on index. Rename document to "Binding Extensions to Web Distributed Authoring and Versioning (WebDAV)". Rename "References" to "Normative References". Close issue "ED_references". Close issue "4_507_status".¶

Add and close issues "9.2_redirect_loops", "ED_authors" and "ED_updates". Add section about capability discovery (DAV header). Close issues "5.1_LOOP_STATUS". Add and resolve new issue "5.1_506_STATUS_STREAMING". Update XML spec reference. Add issue "locking" and resolve as invalid.¶

Add and close issues "6_precondition_binding_allowed" and "6_lock_behaviour". Add mailing list and issues list pointers to front.¶