Network Working GroupG. Clemm
Internet-DraftIBM
Updates: draft-ietf-webdav-rfc2518bis (if approved)J. Crawford
Intended status: Standards TrackIBM Research
Expires: August 12, 2006J. Reschke
greenbytes
J. Whitehead
U.C. Santa Cruz
February 8, 2006

Binding Extensions to Web Distributed Authoring and Versioning (WebDAV)
draft-ietf-webdav-bind-13

Note: a later version of this document has been published as RFC5842.

Status of this Memo

By submitting this Internet-Draft, each author represents that any applicable patent or other IPR claims of which he or she is aware have been or will be disclosed, and any of which he or she becomes aware will be disclosed, in accordance with Section 6 of BCP 79.

Internet-Drafts are working documents of the Internet Engineering Task Force (IETF), its areas, and its working groups. Note that other groups may also distribute working documents as Internet-Drafts.

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”.

The list of current Internet-Drafts can be accessed at http://www.ietf.org/ietf/1id-abstracts.txt.

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This Internet-Draft will expire on August 12, 2006.

Copyright Notice

Copyright © The Internet Society (2006). All Rights Reserved.

Abstract

This specification defines bindings, and the BIND method for creating multiple bindings to the same resource. Creating a new binding to a resource causes at least one new URI to be mapped to that resource. Servers are required to insure the integrity of any bindings that they allow to be created.

Editorial Note (To be removed by RFC Editor before publication)

Please send comments to the Distributed Authoring and Versioning (WebDAV) working group at <mailto:w3c-dist-auth@w3.org>, which may be joined by sending a message with subject "subscribe" to <mailto:w3c-dist-auth-request@w3.org>. Discussions of the WEBDAV working group are archived at <http://lists.w3.org/Archives/Public/w3c-dist-auth/>.

<http://www.webdav.org/bind/draft-ietf-webdav-bind-issues.html> lists all registered issues since draft 02.


Table of Contents

Issues list

IdTypeStatusDateRaised By
ED_updateseditclosed2003-12-30julian.reschke@greenbytes.de
editeditopen2004-05-30julian.reschke@greenbytes.de
webdav-reveditopen2006-01-30julian.reschke@greenbytes.de
 I  edit   (type: edit, status: open)
julian.reschke@greenbytes.de2004-05-30 Umbrella issue for editorial fixes/enhancements.
Associated changes in this document: del-, 1.1, 2.6, del-1, 2.7, 3.2, 4, 5, 6, 8.1, del-2, del-2, 9, A.
 I  webdav-rev   (type: edit, status: open)
julian.reschke@greenbytes.de2006-01-30 Update from RFC2518 to RFC2518bis.
2006-02-07Resolution: Partly resolved: removed own explanation of DTD syntax, removed own definition of precondition/postcondition, removed reference to broken RFC2518 language about DELETE and UNLOCK, removed own definition of DAV: request header, updated "Rationale for Distinguishing Bindings from URI Mappings" to reflect the changes in draft-ietf-webdav-rfc2518bis-12, making proposals for more changes so that the issue can be closed (see also <http://ietf.cse.ucsc.edu:8080/bugzilla/show_bug.cgi?id=227> and <http://greenbytes.de/tech/webdav/draft-ietf-webdav-rfc2518bis-12.html#rfc.section.5.2">).
Associated changes in this document: 1, 1, 1.1, 1.1, 1.2, 1.3, 2.3, 2.3, 2.4, 2.4, 2.5, 2.6, 2, 3.1, 4, 8.1, 8.1, 8.2, 8.2, 11, 12, 14.1.
 I  ED_updates   (type: edit, status: closed)
julian.reschke@greenbytes.de2003-12-30 Shouldn't the BIND spec be labelled as "updating" RFC2518 and/or RFC3253?
julian.reschke@greenbytes.de2004-01-05 It seems that there was consensus to say that BIND does update RFC2518, while there's no consensus about the relationship to RFC3253. As this is a minor editorial question, I propose to just say "updated 2518" and to close the issue.
2006-02-07Resolution: Previously: State "updates 2518". Changed to: "updated draft-ietf-webdav-rfc2518bis".

1. Introduction

This specification extends the WebDAV Distributed Authoring Protocol  I ([draft-ietf-webdav-rfc2518bis]) to enable clients to create new access paths to existing resources. This capability is useful for several reasons:

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 BIND method defined here provides a mechanism for allowing clients to create alternative access paths to existing WebDAV resources. HTTP [RFC2616] and WebDAV  I [RFC2518][draft-ietf-webdav-rfc2518bis] methods are able to work because there are mappings between URIs and resources. A method is addressed to a URI, and the server follows the mapping from that URI to a resource, applying the method to that resource. Multiple URIs may be mapped to the same resource, but until now there has been no way for clients to create additional URIs mapped to existing resources.

BIND lets clients associate a new URI with an existing WebDAV resource, and this URI can then be used to submit requests to the resource. Since URIs of WebDAV resources are hierarchical, and correspond to a hierarchy of collections in resource space, the BIND method also has the effect of adding the resource to a collection. As new URIs are associated with the resource, it appears in additional collections.

A BIND request does not create a new resource, but simply makes available a new URI for submitting requests to an existing resource. The new URI is indistinguishable from any other URI when submitting a request to a resource. Only one round trip is needed to submit a request to the intended target. Servers are required to enforce the integrity of the relationships between the new URIs and the resources associated with them. Consequently, it may be very costly for servers to support BIND requests that cross server boundaries.

This specification is organized as follows. Section 1.1 defines terminology used in the rest of the specification, while Section 2 overviews bindings. Section 3 defines the new properties needed to support multiple bindings to the same resource. Section 4 specifies the BIND method, used to create multiple bindings to the same resource. Section 5 specifies the UNBIND method, used to remove a binding to a resource. Section 6 specifies the REBIND method, used to move a binding to another collection.

1.1. Terminology

The terminology used here follows and extends that in the WebDAV Distributed Authoring Protocol specification  I [RFC2518][draft-ietf-webdav-rfc2518bis].

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  I  purely notational convention. WebDAV request and response bodies cannot be validated due to the specific extensibility rules defined in  I sSection 23 of [RFC2518] and due to the fact that all XML elements defined by this specification use the XML namespace name "DAV:". In particular:

notational convention, using the rules defined in Section 17 of [draft-ietf-webdav-rfc2518bis].

URI Mapping

Path Segment

Binding

Collection

Internal Member URI

1.2. Rationale for Distinguishing Bindings from URI Mappings

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.

In [draft-ietf-webdav-rfc2518bis], the definition of collection state has been partly updated so that it doesn't depend on the access URL anymore. However, there are some more changes needed in the subsequent paragraphs to complete this change. The authors of this specification recommend updating paragraphs 2, 3 and 4 of Section 5.2 of [draft-ietf-webdav-rfc2518bis] to read:

  • A collection MUST contain at most one mapping for a given path segment, i.e., it is illegal to have the same path segment mapped to more than one resource. Properties defined on collections behave exactly as do properties on non-collection resources.
  • For all WebDAV compliant resources A and B, identified by URLs "U" and "V" respectively, such that "V" is equal to "U/SEGMENT", A MUST be a collection that contains a mapping from "SEGMENT" to B. So, if resource B with URL "http://example.com/bar/blah" is WebDAV compliant and if resource A with URL "http://example.com/bar/" is WebDAV compliant, then resource A must be a collection and must contain a mapping from "blah" to B.
  • Collection resources MAY have mappings to non-WebDAV compliant resources in the HTTP URL namespace hierarchy but are not required to do so. For example, if the resource X with URL "http://example.com/bar/blah" is not WebDAV compliant and the resource A with "URL http://example.com/bar/" identifies a collection, then A may or may not have a mapping from "blah" to X.

(See also <http://ietf.cse.ucsc.edu:8080/bugzilla/show_bug.cgi?id=227#c5>).

1.3. Method Preconditions and Postconditions

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.

See Section 16 of [draft-ietf-webdav-rfc2518bis] for the definitions of "precondition" and "postcondition".

2. Overview of Bindings

Bindings are part of the state of a collection. They define the internal members of the collection, and the names of those internal members.

Bindings are added and removed by a variety of existing HTTP methods. A method that creates a new resource, such as PUT, COPY, and MKCOL, adds a binding. A method that deletes a resource, such as DELETE, removes a binding. A method that moves a resource (e.g. MOVE) both adds a binding (in the destination collection) and removes a binding (in the source collection). The BIND method introduced here provides a mechanism for adding a second binding to an existing resource. There is no difference between an initial binding added by PUT, COPY, or MKCOL, and additional bindings added with BIND.

It would be very undesirable if one binding could be destroyed as a side effect of operating on the resource through a different binding. In particular, the removal of one binding to a resource (e.g. with a DELETE or a MOVE) MUST NOT disrupt another binding to that resource, e.g. by turning that binding into a dangling path segment. The server MUST NOT reclaim system resources after removing one binding, while other bindings to the resource remain. In other words, the server MUST maintain the integrity of a binding. It is permissible, however, for future method definitions (e.g., a DESTROY method) to have semantics that explicitly remove all bindings and/or immediately reclaim system resources.

2.1. Bindings to Collections

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.

+-------------------------+
| Root Collection         |
|  bindings:              |
|  CollX          CollY   |
+-------------------------+
    |            /          
    |           /
    |          /
+------------------+  
| Collection C1    |  
| bindings:        |
| x.gif     y.jpg  |  
+------------------+  
    |          \               
    |           \               
    |            \              
+-------------+   +-------------+
| Resource R1 |   | Resource R2 |
+-------------+   +-------------+

2.1.1. Bind loops

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. For instance, a PROPFIND can still succeed if the server uses the new status code 208 (Already Reported) defined in Section 7.1.

However, the 506 (Loop Detected) status code is defined in Section 7.2 for use in contexts where an operation is terminated because a loop was encountered.

2.2. URI Mappings Created by a new Binding

Suppose a binding from "Binding-Name" to resource R is to 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
   ...

2.3. COPY and Bindings

As defined in  I Section 8.8 of [RFC2518]Section 9.8 of [draft-ietf-webdav-rfc2518bis], 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  I [RFC2518][draft-ietf-webdav-rfc2518bis] 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.

2.3.1. Example: COPY with 'Depth: infinity' in presence of bind loops

As an example of how COPY with Depth infinity would work in the presence of bindings, consider the following collection:

+------------------+
| Root Collection  |
|  bindings:       |
|  CollX           |
+------------------+
    |
    |
+-------------------------------+
| Collection C1                 |<-------+
| bindings:                     |        |
| x.gif      CollY              |        |
+-------------------------------+        |
    |            \        (creates loop) |
    |             \                      |
+-------------+   +------------------+   |
| Resource R1 |   | Collection C2    |   |
+-------------+   | bindings:        |   |
                  | y.gif     CollZ  |   |
                  +------------------+   |
                      |         |        |
                      |         +--------+
                      |
                  +-------------+
                  | Resource R2 | 
                  +-------------+

If a COPY with Depth infinity is submitted to /CollX, with destination of /CollA, the outcome of the copy operation is:

+------------------+
| Root Collection  |
|  bindings:       |
|  CollX     CollA |
+------------------+
   |           |
   |           +---------------------------+
   |                                       |
+-------------------+                      |
| Collection C1     |<------------------+  |
| bindings:         |                   |  |
| x.gif      CollY  |                   |  |
+-------------------+                   |  |
   |            \        (creates loop) |  |
   |             \                      |  |
+-------------+   +-----------------+   |  |
| Resource R1 |   | Collection C2   |   |  |
+-------------+   | bindings:       |   |  |
                  | y.gif     CollZ |   |  |
                  +-----------------+   |  |
                      |         |       |  |
                      |         +-------+  |
                      |                    |
                  +-------------+          |
                  | Resource R2 |          |
                  +-------------+          |
                                           |
           +-------------------------------+
           |
+-------------------+
| Collection C3     |<------------------+
| bindings:         |                   |
| x.gif      CollY  |                   |
+-------------------+                   |
   |            \        (creates loop) |
   |             \                      |
+-------------+   +-----------------+   |
| Resource R3 |   | Collection C4   |   |
+-------------+   | bindings:       |   |
                  | y.gif     CollZ |   |
                  +-----------------+   |
                      |         |       |
                      |         +-------+
                      |
                  +-------------+
                  | Resource R4 |
                  +-------------+

2.3.2. Example: COPY with 'Depth: infinity' with multiple bindings to a leaf resource

Given the following collection hierarchy:

+------------------+
| Root Collection  |
|  bindings:       |
|  CollX           |
+------------------+
   |            
   |  
   |               
+----------------+ 
| Collection C1  | 
| bindings:      | 
| x.gif    y.gif | 
+----------------+
   |         |     
   |         |    
 +-------------+   
 | Resource R1 |    
 +-------------+

A COPY of /CollX with Depth infinity to /CollY results in the following collection hierarchy:

+------------------+
| Root Collection  |
|  bindings:       |
|  CollX     CollY |
+------------------+
   |              \
   |               \
   |                \           
+----------------+  +-----------------+
| Collection C1  |  | Collection C2   |
| bindings:      |  | bindings:       |
| x.gif    y.gif |  | x.gif     y.gif |
+----------------+  +-----------------+
   |         |          |         |
   |         |          |         |
 +-------------+      +-------------+
 | Resource R1 |      | Resource R2 |
 +-------------+      +-------------+

2.4. DELETE and Bindings

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). I  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  I [RFC2518][draft-ietf-webdav-rfc2518bis] 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.

2.5. MOVE and Bindings

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  I [RFC2518][draft-ietf-webdav-rfc2518bis] 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). 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      |
+---------------------+                  

2.6. PROPFIND and Bindings

Consistent with  I [RFC2518][draft-ietf-webdav-rfc2518bis], the value of a dead property MUST be independent of the number of bindings to its host resource or of the path submitted to PROPFIND. On the other hand, the behaviour for each live property depends on its individual definition (for example, see [RFC3744],  I sSection 5, paragraph 2).

 I  

del-1. UNLOCK and Bindings

Due to the specific language used in  I sSection 8.11 of [RFC2518], it might be thought that an UNLOCK request to a locked resource would unlock just the particular binding expressed by the Request-URI, rather than the resource identified by that URI. This is not the case, however. Section 6 of [RFC2518] clearly states that locks are on resources, not URIs, so the server MUST allow UNLOCK to be used to unlock a locked resource through any binding to that resource. The authors of this specification anticipate and recommend that future revisions of [RFC2518] maintain this behavior.

2.7. Determining Whether Two Bindings Are to the Same Resource

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 character by character, 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 applied to a null resource, COPY (when not overwriting an existing target) and CHECKIN (see [RFC3253],  I sSection 4.4) must assign a new, unique value to the DAV:resource-id property of the new resource they create.

On the other hand, any method that affects an existing resource must not change the value of its DAV:resource-id property. Specifically, a PUT or a COPY that updates an existing resource must not change the value of its DAV:resource-id property. A REBIND, 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.

2.8. Discovering the Bindings to a Resource

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 10.4 and 10.5.

3. Properties

The bind feature introduces the properties defined below.

A DAV:allprop PROPFIND request SHOULD NOT return any of the properties defined by this document. This allows a binding server to perform efficiently when a naive client, which does not understand the cost of asking a server to compute all possible live properties, issues a DAV:allprop PROPFIND request.

3.1. DAV:resource-id Property

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 urn:uuid: URN namespace defined in [RFC4122] or the opaquelocktoken: URI scheme defined in  I [RFC2518][draft-ietf-webdav-rfc2518bis]).

<!ELEMENT resource-id (href)>

3.2. DAV:parent-set Property

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.

<!ELEMENT parent-set (parent)*>
<!ELEMENT parent (href, segment)>
<!ELEMENT segment (#PCDATA)>
<!-- PCDATA value: segment, as defined in  I sSection 3.3 of
     [RFC3986] -->

3.2.1. Example for DAV:parent-set property

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. both [/CollX, x.gif] and [/CollX, y.gif] or, alternatively, both [/CollY, x.gif] and [/CollY, y.gif]).

+-------------------------+
| Root Collection         |
|  bindings:              |
|  CollX          CollY   |
+-------------------------+
    |            /          
    |           /
    |          /
+-----------------+  
| Collection C1   |  
| bindings:       |
| x.gif    y.gif  |  
+-----------------+  
     |      |               
     |      |               
     |      |              
 +--------------+ 
 | Resource  R1 |
 +--------------+

In this case, one possible value for DAV:parent-set property on "/CollX/x.gif" would be:

  <parent-set xmlns="DAV:">
    <parent>
      <href>/CollX</href>
      <segment>x.gif</segment>
    </parent>
    <parent>
      <href>/CollX</href>
      <segment>y.gif</segment>
    </parent>
  </parent-set>

4. BIND Method

The BIND method modifies the collection identified by the Request-URI, by adding a new binding from the segment specified in the BIND body to the resource identified in the BIND body.

If a server cannot guarantee the integrity of the binding, the BIND request MUST fail. Note that it is especially difficult to maintain the integrity of cross-server bindings. Unless the server where the resource resides knows about all bindings on all servers to that resource, it may unwittingly destroy the resource or make it inaccessible without notifying another server that manages a binding to the resource. For example, if server A permits creation of a binding to a resource on server B, server A must notify server B about its binding and must have an agreement with B that B will not destroy the resource while A's binding exists. Otherwise server B may receive a DELETE request that it thinks removes the last binding to the resource and destroy the resource while A's binding still exists. The precondition DAV:cross-server-binding is defined below for cases where servers fail cross-server BIND requests because they cannot guarantee the integrity of cross-server bindings.

By default, if there already is a binding for the specified segment in the collection, the new binding replaces the existing binding. This default binding replacement behavior can be overridden using the Overwrite header defined in  I Section 9.6 of [RFC2518]Section 9.6 of [draft-ietf-webdav-rfc2518bis].

If a BIND request fails, the server state preceding the request MUST be restored. This method is unsafe and idempotent (see [RFC2616],  I sSection 9.1).

Marshalling:

Preconditions:

Postconditions:

4.1. Example: BIND

>> 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.

5. UNBIND Method

The UNBIND method modifies the collection identified by the Request-URI, by removing the binding identified by the segment specified in the UNBIND body.

Once a resource is unreachable by any URI mapping, the server MAY reclaim system resources associated with that resource. If UNBIND removes a binding to a resource, but there remain URI mappings to that resource, the server MUST NOT reclaim system resources associated with the resource.

If an UNBIND request fails, the server state preceding the request MUST be restored. This method is unsafe and idempotent (see [RFC2616],  I sSection 9.1).

Marshalling:

Preconditions:

Postconditions:

5.1. Example: UNBIND

>> 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.

6. REBIND Method

The REBIND method removes a binding to a resource from a collection, and adds a binding to that resource into the collection identified by the Request-URI. The request body specifies the binding to be added (segment) and the old binding to be removed (href). It is effectively an atomic form of a MOVE request, and MUST be treated the same way as MOVE for the purpose of determining access permissions.

If a REBIND request fails, the server state preceding the request MUST be restored. This method is unsafe and idempotent (see [RFC2616],  I sSection 9.1).

Marshalling:

Preconditions:

Postconditions:

6.1. Example: REBIND

>> 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.

6.2. Example: REBIND in presence of locks and bind loops

To illustrate the effects of locks and bind loops on a REBIND operation, consider the following collection:

+------------------+
| Root Collection  |
|  bindings:       |
|  CollW           |
+------------------+
     |
     |
     |
+-------------------------------+
| Collection C1                 |<--------+
| LOCKED infinity               |         |
| (lock token L1)               |         |
| bindings:                     |         |
| CollX               CollY     |         |
+-------------------------------+         |
     |                  |                 |
     |                  |  (creates loop) |
     |                  |                 |
+-----------------+  +------------------+ |
| Collection C2   |  | Collection C3    | |
| (inherit lock)  |  | (inherit lock)   | | 
| (lock token L1) |  | (lock token L1)  | |
| bindings:       |  | bindings:        | |
|  {none}         |  | y.gif     CollZ  | |
+-----------------+  +------------------+ |
                       |            |     |
                       |            +-----+
                       |
                   +---------------------------+
                   | Resource R2               |
                   | (lock inherited from C1)  |
                   | (lock token L1)           |
                   +---------------------------+

(where L1 is "opaquelocktoken:f92d4fae-7012-11ab-a765-00c0ca1f6bf9").

Note that the binding between CollZ and C1 creates a loop in the containment hierarchy. Servers are not required to support such loops, though the server in this example does.

The REBIND request below will remove the segment "CollZ" from C3 and add a new binding from "CollA" to the collection C2.

REBIND /CollW/CollX HTTP/1.1
Host: www.example.com
If: (<opaquelocktoken:f92d4fae-7012-11ab-a765-00c0ca1f6bf9>)
Content-Type: text/xml; charset="utf-8"
Content-Length: xxx

<?xml version="1.0" encoding="utf-8" ?>
<D:rebind xmlns:D="DAV:">
  <D:segment>CollA</D:segment>
  <D:href>/CollW/CollY/CollZ</D:href>
</D:rebind>

The outcome of the REBIND operation is:

+------------------+
| Root Collection  |
|  bindings:       |
|  CollW           |
+------------------+
     |
     |
     |
+-------------------------------+
| Collection C1                 |
| LOCKED infinity               |
| (lock token L1)               |
| bindings:                     |         
| CollX                  CollY  |         
+-------------------------------+         
     |              ^      |             
     |              |      |              
+-----------------+ | +------------------+ 
| Collection C2   | | | Collection C3    | 
|(inherited lock) | | | (inherited lock) |
|(lock token L1)  | | | (lock token L1)  |
| bindings:       | | | bindings:        |
| CollA           | | | y.gif            |
+-----------------+ | +------------------+
    |               |    |           
    +---------------+    |          
     (creates loop)      |
                   +---------------------------+
                   | Resource R2               |
                   | (inherited lock from C1)  |
                   | (lock token L1)           |
                   +---------------------------+

7. Additional Status Codes

7.1. 208 Already Reported

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:resource-id 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.

7.1.1. Example: PROPFIND by bind-aware client

For example, consider a PROPFIND request on /Coll (bound to collection C), where the members of /Coll are /Coll/Foo (bound to resource R) and /Coll/Bar (bound to collection C).

>> Request:

PROPFIND /Coll/ HTTP/1.1
Host: www.example.com
Depth: infinity
DAV: bind
Content-Type: text/xml; charset="utf-8"
Content-Length: xxx

<?xml version="1.0" encoding="utf-8" ?>
<D:propfind xmlns:D="DAV:">
  <D:prop>
   <D:displayname/>
   <D:resource-id/>
  </D:prop>
</D:propfind>

>> Response:

HTTP/1.1 207 Multi-Status
Content-Type: text/xml; charset="utf-8"
Content-Length: xxx

<?xml version="1.0" encoding="utf-8" ?>
<D:multistatus xmlns:D="DAV:">
  <D:response>
    <D:href>http://www.example.com/Coll/</D:href>
    <D:propstat>
      <D:prop>
        <D:displayname>Loop Demo</D:displayname>
        <D:resource-id>
          <D:href
>urn:uuid:f81d4fae-7dec-11d0-a765-00a0c91e6bf8</D:href>
        </D:resource-id>
      </D:prop>
      <D:status>HTTP/1.1 200 OK</D:status>
    </D:propstat>
  </D:response>
  <D:response>
    <D:href>http://www.example.com/Coll/Foo</D:href>
    <D:propstat>
      <D:prop>
        <D:displayname>Bird Inventory</D:displayname>
        <D:resource-id>
          <D:href
>urn:uuid:f81d4fae-7dec-11d0-a765-00a0c91e6bf9</D:href>
        </D:resource-id>
      </D:prop>
      <D:status>HTTP/1.1 200 OK</D:status>
    </D:propstat>
  </D:response>
  <D:response>
    <D:href>http://www.example.com/Coll/Bar</D:href>
    <D:propstat>
      <D:prop>
        <D:displayname>Loop Demo</D:displayname>
        <D:resource-id>
          <D:href
>urn:uuid:f81d4fae-7dec-11d0-a765-00a0c91e6bf8</D:href>
        </D:resource-id>
      </D:prop>
      <D:status>HTTP/1.1 208 Already Reported</D:status>
    </D:propstat>
  </D:response>
</D:multistatus>

7.1.2. Example: PROPFIND by non-bind-aware client

In this example, the client isn't aware of the 208 status code introduced by this specification. As the "Depth: infinity" PROPFIND request would cause a loop condition, the whole request is rejected with a 506 status.

>> Request:

PROPFIND /Coll/ HTTP/1.1
Host: www.example.com
Depth: infinity
Content-Type: text/xml; charset="utf-8"
Content-Length: xxx

<?xml version="1.0" encoding="utf-8" ?>
<D:propfind xmlns:D="DAV:">
   <D:prop> <D:displayname/> </D:prop>
</D:propfind>

>> Response:

HTTP/1.1 506 Loop Detected

7.2. 506 Loop Detected

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.

8. Capability discovery

8.1. OPTIONS method

If the server supports bindings, it MUST return the compliance class name "bind" as a field in the "DAV" response header (see  I [RFC2518][draft-ietf-webdav-rfc2518bis],  I sSection  I 910.1) 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.

8.2. 'DAV' request header

del-2. Generic syntax

This specification introduces the 'DAV' request header that allows clients to signal compliance to specific WebDAV features. It has the same syntax as the response header defined in [RFC2518],  I sSection 9.1, but MAY be used with any method.

Note that clients MUST NOT submit a specific compliance class name in the request header unless the specification defining this compliance class specifically defines its semantics for clients.

Note that if a server chooses to vary the result of a request based on values in the "DAV" header, the response either MUST NOT be cacheable or the server MUST mark the response accordingly using the "Vary" header (see [RFC2616],  I sSection 14.44).

del-3. Client compliance class 'bind'

Clients SHOULD signal support for all MUST level requirements and REQUIRED features by submitting a "DAV" request header containing the compliance class name "bind". In particular, the client MUST understand the 208 status code defined in Section 7.1.

Clients SHOULD signal support for all MUST level requirements and REQUIRED features by submitting a "DAV" request header containing the compliance class name "bind". In particular, the client MUST understand the 208 status code defined in Section 7.1.

9. Relationship to WebDAV Access Control Protocol

BIND and REBIND behave the same as MOVE with respect to the DAV:acl property (see [RFC3744],  I sSection 7.3).

10. Security Considerations

This section is provided to make WebDAV implementors 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, bindings introduce several new security concerns and increase the risk of some existing threats. These issues are detailed below.

10.1. Privacy Concerns

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.

10.2. Bind Loops

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".

10.3. Bindings, and Denial of Service

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.

10.4. Private Locations May Be Revealed

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.

10.5. DAV:parent-set and Denial of Service

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.

11. Internationalization Considerations

All internationalization considerations mentioned in  I [RFC2518][draft-ietf-webdav-rfc2518bis] also apply to this document.

12. IANA Considerations

All IANA considerations mentioned in  I [RFC2518][draft-ietf-webdav-rfc2518bis] also apply to this document.

13. Acknowledgements

This document is the collaborative product of the authors and Tyson Chihaya, Jim Davis, Chuck Fay and Judith Slein. This draft has benefited from thoughtful discussion by Jim Amsden, Peter Carlson, Steve Carter, Ken Coar, Ellis Cohen, Dan Connolly, Bruce Cragun, Spencer Dawkins, Mark Day, Rajiv Dulepet, David Durand, Lisa Dusseault, Stefan Eissing, Roy Fielding, Yaron Goland, Joe Hildebrand, Fred Hitt, Alex Hopmann, James Hunt, Marcus Jager, Chris Kaler, Manoj Kasichainula, Rohit Khare, Brian Korver, Daniel LaLiberte, Steve Martin, Larry Masinter, Jeff McAffer, Surendra Koduru Reddy, Max Rible, Sam Ruby, Bradley Sergeant, Nick Shelness, John Stracke, John Tigue, John Turner, Kevin Wiggen, and other members of the WebDAV working group.

14. References

14.1. Normative References

[RFC2518]Goland, Y., Whitehead, E., Faizi, A., Carter, S., and D. Jensen, “HTTP Extensions for Distributed Authoring -- WEBDAV”, RFC 2518, February 1999.
[draft-ietf-webdav-rfc2518bis]Dusseault, L., Ed., “HTTP Extensions for Distributed Authoring - WebDAV RFC2518 bis”, Internet-Draft draft-ietf-webdav-rfc2518bis-12 (work in progress), February 2006, <http://greenbytes.de/tech/webdav/draft-ietf-webdav-rfc2518bis-12.html>.
[RFC2119]Bradner, S., “Key words for use in RFCs 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.
[RFC3986]Berners-Lee, T., Fielding, R., and L. Masinter, “Uniform Resource Identifier (URI): Generic Syntax”, STD 66, RFC 3986, January 2005.
[XML]Bray, T., Paoli, J., Sperberg-McQueen, C., Maler, E., and F. Yergeau, “Extensible Markup Language (XML) 1.0 (Third Edition)”, W3C REC-xml-20040204, February 2004, <http://www.w3.org/TR/2004/REC-xml-20040204>.

14.2. Informative References

[RFC3253]Clemm, G., Amsden, J., Ellison, T., Kaler, C., and J. Whitehead, “Versioning Extensions to WebDAV (Web Distributed Authoring and Versioning)”, RFC 3253, March 2002.
[RFC3744]Clemm, G., Reschke, J., Sedlar, E., and J. Whitehead, “Web Distributed Authoring and Versioning (WebDAV) Access Control Protocol”, RFC 3744, May 2004.
[RFC4122]Leach, P., Mealling, M., and R. Salz, “A Universally Unique IDentifier (UUID) URN Namespace”, RFC 4122, July 2005.

A. Change Log (to be removed by RFC Editor before publication)

A.1. Since draft-ietf-webdav-bind-02

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".

A.2. Since draft-ietf-webdav-bind-03

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.

A.3. Since draft-ietf-webdav-bind-04

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

A.4. Since draft-ietf-webdav-bind-05

Editorial fixes. Add and resolve issues "1.3_error_negotiation", "2.5_language" and "7.1.1_add_resource_id". Add historical issue "4_LOCK_BEHAVIOR" and it's resolution for better tracking.

A.5. Since draft-ietf-webdav-bind-06

Rewrite Editorial Note. Open and resolve issues "2.6_identical", "specify_safeness_and_idempotence" and "ED_rfc2026_ref".

A.6. Since draft-ietf-webdav-bind-07

Add more index items (no change tracking). Add and resolve issues "2.3_copy_to_same", "bind_properties", "bind_vs_ACL", "6_rebind_intro" and "rfc2396bis" (actually an action item). Fix XML DTD fragment in section 3.3. Make spelling of "Request-URI" consistent.

A.7. Since draft-ietf-webdav-bind-08

Resolved editorial issues raised by Jim Whitehead in <http://lists.w3.org/Archives/Public/w3c-dist-auth/2004OctDec/0129.html>. Add and resolve issues "atomicity", "2_allow_destroy", "2.1_separate_loop_discussion", "2.1.1_bind_loops_vs_locks", "2.3_copy_depth_infinity", "2.3_copy_example", "2.3_copy_vs_loops", "2.6_resource-id_vs_versions", "3.2_example" and "6_rebind_premissions". Add issue "2.6_when_do_ids_change". Re-open and resolve "6_rebind_intro".

A.8. Since draft-ietf-webdav-bind-09

Add and resolve issue "6.1_rebind_vs_locks", adding proposed example text. Add action item "3.1_uuids". Close issue "2.6_when_do_ids_change". Add and resolve issues "2.6_bindings_vs_properties" and "uri_draft_ref".

A.9. Since draft-ietf-webdav-bind-10

Resolve action item "3.1_uuids". Add and resolve issue "2.7_unlock_vs_bindings". Revisit issue "2.6_bindings_vs_properties", and remove the part of the sentence that speaks about live properties. Update "rfc2396bis" references to "RFC3986". Add issue "9_ns_op_and_acl" and add potential resolution. Align artwork where applicable (new xml2rfc1.29rc2 feature).

A.10. Since draft-ietf-webdav-bind-11

Updated [draft-mealling-uuid-urn] to [RFC4122]. Add statement about live properties in Section 2.6.

 I  

A.11. Since draft-ietf-webdav-bind-12

Updated Author's address. Uppercase "Section" when referring to other documents.

Updating from RFC2518 to RFC2518bis:

Index

2 5 B C D I M P R S U

Authors' Addresses

Geoffrey Clemm
IBM
20 Maguire Road
Lexington, MA 02421
EMail: geoffrey.clemm@us.ibm.com

Jason Crawford
IBM Research
P.O. Box 704
Yorktown Heights, NY 10598
EMail: ccjason@us.ibm.com

Julian F. Reschke
greenbytes GmbH
Hafenweg 16
Muenster, NW 48155
Germany
EMail: julian.reschke@greenbytes.de

Jim Whitehead
UC Santa Cruz, Dept. of Computer Science
1156 High Street
Santa Cruz, CA 95064
EMail: ejw@cse.ucsc.edu

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