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rfc:rfc5842

Internet Engineering Task Force (IETF) G. Clemm Request for Comments: 5842 IBM Category: Experimental J. Crawford ISSN: 2070-1721 IBM Research

                                                       J. Reschke, Ed.
                                                            greenbytes
                                                          J. Whitehead
                                                       U.C. Santa Cruz
                                                            April 2010
                       Binding Extensions to
         Web Distributed Authoring and Versioning (WebDAV)

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 ensure the integrity of any bindings that
 they allow to be created.

Status of This Memo

 This document is not an Internet Standards Track specification; it is
 published for examination, experimental implementation, and
 evaluation.
 This document defines an Experimental Protocol for the Internet
 community.  This document is a product of the Internet Engineering
 Task Force (IETF).  It represents the consensus of the IETF
 community.  It has received public review and has been approved for
 publication by the Internet Engineering Steering Group (IESG).  Not
 all documents approved by the IESG are a candidate for any level of
 Internet Standard; see Section 2 of RFC 5741.
 Information about the current status of this document, any errata,
 and how to provide feedback on it may be obtained at
 http://www.rfc-editor.org/info/rfc5842.

Clemm, et al. Experimental [Page 1] RFC 5842 Binding Extensions to WebDAV April 2010

Copyright Notice

 Copyright (c) 2010 IETF Trust and the persons identified as the
 document authors.  All rights reserved.
 This document is subject to BCP 78 and the IETF Trust's Legal
 Provisions Relating to IETF Documents
 (http://trustee.ietf.org/license-info) in effect on the date of
 publication of this document.  Please review these documents
 carefully, as they describe your rights and restrictions with respect
 to this document.  Code Components extracted from this document must
 include Simplified BSD License text as described in Section 4.e of
 the Trust Legal Provisions and are provided without warranty as
 described in the Simplified BSD License.
 This document may contain material from IETF Documents or IETF
 Contributions published or made publicly available before November
 10, 2008.  The person(s) controlling the copyright in some of this
 material may not have granted the IETF Trust the right to allow
 modifications of such material outside the IETF Standards Process.
 Without obtaining an adequate license from the person(s) controlling
 the copyright in such materials, this document may not be modified
 outside the IETF Standards Process, and derivative works of it may
 not be created outside the IETF Standards Process, except to format
 it for publication as an RFC or to translate it into languages other
 than English.

Table of Contents

 1. Introduction ....................................................4
    1.1. Terminology ................................................5
    1.2. Method Preconditions and Postconditions ....................6
 2. Overview of Bindings ............................................7
    2.1. Bindings to Collections ....................................7
         2.1.1. Bind Loops ..........................................8
    2.2. URI Mappings Created by a New Binding ......................8
    2.3. COPY and Bindings ..........................................9
         2.3.1. Example: COPY with "Depth: infinity" in
                Presence of Bind Loops .............................11
         2.3.2. Example: COPY Updating Multiple Bindings ...........13
         2.3.3. Example: COPY with "Depth: infinity" with
                Multiple Bindings to a Leaf Resource ...............14
    2.4. DELETE and Bindings .......................................15
    2.5. MOVE and Bindings .........................................15
         2.5.1. Example: Simple MOVE ...............................16
         2.5.2. Example: MOVE Request Causing a Bind Loop ..........16
    2.6. PROPFIND and Bindings .....................................18

Clemm, et al. Experimental [Page 2] RFC 5842 Binding Extensions to WebDAV April 2010

    2.7. Determining Whether Two Bindings Are to the Same
         Resource ..................................................18
    2.8. Discovering the Bindings to a Resource ....................19
 3. Properties .....................................................19
    3.1. DAV:resource-id Property ..................................20
    3.2. DAV:parent-set Property ...................................20
         3.2.1. Example for DAV:parent-set Property ................20
 4. BIND Method ....................................................21
    4.1. Example: BIND .............................................24
 5. UNBIND Method ..................................................24
    5.1. Example: UNBIND ...........................................26
 6. REBIND Method ..................................................26
    6.1. Example: REBIND ...........................................28
    6.2. Example: REBIND in Presence of Locks and Bind Loops .......29
 7. Additional Status Codes ........................................31
    7.1. 208 Already Reported ......................................31
         7.1.1. Example: PROPFIND by Bind-Aware Client .............32
         7.1.2. Example: PROPFIND by Non-Bind-Aware Client .........34
    7.2. 508 Loop Detected .........................................34
 8. Capability Discovery ...........................................34
    8.1. OPTIONS Method ............................................34
    8.2. 'DAV' Request Header ......................................34
 9. Relationship to Locking in WebDAV ..............................35
    9.1. Example: Locking and Multiple Bindings ....................36
 10. Relationship to WebDAV Access Control Protocol ................37
 11. Relationship to Versioning Extensions to WebDAV ...............37
 12. Security Considerations .......................................40
    12.1. Privacy Concerns .........................................40
    12.2. Bind Loops ...............................................40
    12.3. Bindings and Denial of Service ...........................40
    12.4. Private Locations May Be Revealed ........................40
    12.5. DAV:parent-set and Denial of Service .....................41
 13. Internationalization Considerations ...........................41
 14. IANA Considerations ...........................................41
 15. Acknowledgements ..............................................41
 16. References ....................................................41
    16.1. Normative References .....................................41
    16.2. Informative References ...................................42
 Index .............................................................42

Clemm, et al. Experimental [Page 3] RFC 5842 Binding Extensions to WebDAV April 2010

1. Introduction

 This specification extends the WebDAV Distributed Authoring Protocol
 ([RFC4918]) 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, for example, 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 [RFC4918] 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

Clemm, et al. Experimental [Page 4] RFC 5842 Binding Extensions to WebDAV April 2010

 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 a new
 URI for submitting requests to an existing resource available.  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 [RFC4918].
 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 notational
 convention, using the rules defined in Section 17 of [RFC4918].
 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 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 [RFC3986].

Clemm, et al. Experimental [Page 5] RFC 5842 Binding Extensions to WebDAV April 2010

 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.
 Binding Integrity
    The property of a binding that says that:
  • the binding continues to exist, and
  • the identity of the resource identified by that binding does

not change,

    unless an explicit request is executed that is defined to delete
    that binding (examples of requests that delete a binding are
    DELETE, MOVE, and -- defined later on -- UNBIND and REBIND).

1.2. Method Preconditions and Postconditions

 See Section 16 of [RFC4918] for the definitions of "precondition" and
 "postcondition".

Clemm, et al. Experimental [Page 6] RFC 5842 Binding Extensions to WebDAV April 2010

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.
    Note: the collection model described herein is not compatible with
    systems in which resources inherit properties based solely on the
    access path, as the ability to create additional bindings will
    cause a single resource to appear as member of several different
    collections at the same time.

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 they associate a URI that is relative to

Clemm, et al. Experimental [Page 7] RFC 5842 Binding Extensions to WebDAV April 2010

 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 ("cycles"), 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 508 (Loop Detected) status code is defined in
 Section 7.2 for use in contexts where an operation is terminated
 because a loop was encountered.
 Support for loops is OPTIONAL: servers MAY reject requests that would
 lead to the creation of a bind loop (see DAV:cycle-allowed
 precondition defined in Section 4).

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.

Clemm, et al. Experimental [Page 8] RFC 5842 Binding Extensions to WebDAV April 2010

 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 Section 9.8 of [RFC4918], 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.

Clemm, et al. Experimental [Page 9] RFC 5842 Binding Extensions to WebDAV April 2010

 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 that 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 [RFC4918] 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 (see Section 2.3.2 for an example).
 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 (see Section 2.3.3 for an example).

Clemm, et al. Experimental [Page 10] RFC 5842 Binding Extensions to WebDAV April 2010

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 request with "Depth: infinity" is submitted to /CollX, with
 a destination of /CollA, the outcome of the copy operation is that a
 copy of the tree is replicated to the target /CollA:

Clemm, et al. Experimental [Page 11] RFC 5842 Binding Extensions to WebDAV April 2010

              +------------------+
              | 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 |
                                +-------------+

Clemm, et al. Experimental [Page 12] RFC 5842 Binding Extensions to WebDAV April 2010

 Note that the same would apply for more complex loops.

2.3.2. Example: COPY Updating Multiple Bindings

 Given 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 |   | Resource R3 |
          +-------------+  +-------------+   +-------------+
 A COPY of /CollX with "Depth: infinity" to /CollY will not result in
 a changed hierarchy, and Resource R3 will be updated with the content
 of either Resource R1 or Resource R2.

Clemm, et al. Experimental [Page 13] RFC 5842 Binding Extensions to WebDAV April 2010

2.3.3. 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 |
                 +-------------+      +-------------+

Clemm, et al. Experimental [Page 14] RFC 5842 Binding Extensions to WebDAV April 2010

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).
 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 [RFC4918] 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 [RFC4918] 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

Clemm, et al. Experimental [Page 15] RFC 5842 Binding Extensions to WebDAV April 2010

 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.

2.5.1. Example: Simple MOVE

 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 that 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      |
             +---------------------+

2.5.2. Example: MOVE Request Causing a Bind Loop

 Note that in the presence of collection bindings, a MOVE request can
 cause the creation of a bind loop.

Clemm, et al. Experimental [Page 16] RFC 5842 Binding Extensions to WebDAV April 2010

 Consider the top-level collections C1 and C2 with URIs "/CollW/" and
 "/CollX/".  C1 also contains an additional binding named "CollY" to
 C2:
                                   +------------------+
                                   | Root Collection  |
                                   |  bindings:       |
                                   |  CollW    CollX  |
                                   +------------------+
                                       |          |
                                       |          |
                          +------------------+    |
                          | Collection C1    |    |
                          |  bindings:       |    |
                          |           CollY  |    |
                          +------------------+    |
                                       |          |
                                       |          |
                                   +------------------+
                                   | Collection C2    |
                                   |                  |
                                   |                  |
                                   +------------------+
 In this case, the MOVE request below would cause a bind loop:
 >> Request:
 MOVE /CollW HTTP/1.1
 Host: example.com
 Destination: /CollX/CollZ

Clemm, et al. Experimental [Page 17] RFC 5842 Binding Extensions to WebDAV April 2010

 If the request succeeded, the resulting state would be:
                                   +------------------+
                                   | Root Collection  |
                                   |  bindings:       |
                                   |           CollX  |
                                   +------------------+
                                                  |
                                                  |
                          +------------------+    |
                          | Collection C1    |    |
                   +----> |  bindings:       |    |
                   |      |           CollY  |    |
                   |      +------------------+    |
                   |                   |          |
                   |                   |          |
                   |               +------------------+
                   |               | Collection C2    |
                   |               |  bindings:       |
                   |               | CollZ            |
                   |               +------------------+
                   |                   |
                   |                   |
                   +-------------------+

2.6. PROPFIND and Bindings

 Consistent with [RFC4918], 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 behavior for each
 live property depends on its individual definition (for example, see
 [RFC3744], Section 5, Paragraph 2 for a case where the value is
 independent of its path and bindings, and [RFC4918], Section 8.8 for
 a discussion about the live properties DAV:getetag and DAV:
 getlastmodified, which may behave differently).

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

Clemm, et al. Experimental [Page 18] RFC 5842 Binding Extensions to WebDAV April 2010

 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], Section 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 12.4 and 12.5.

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.

Clemm, et al. Experimental [Page 19] RFC 5842 Binding Extensions to WebDAV April 2010

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 [RFC4918]).
 <!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 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 Section 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:parent-set of
 R1 (i.e., both [/CollX, x.gif] and [/CollX, y.gif] or, alternatively,
 both [/CollY, x.gif] and [/CollY, y.gif]).

Clemm, et al. Experimental [Page 20] RFC 5842 Binding Extensions to WebDAV April 2010

                      +-------------------------+
                      | Root Collection         |
                      |  bindings:              |
                      |  CollX          CollY   |
                      +-------------------------+
                          |            /
                          |           /
                          |          /
                      +-----------------+
                      | Collection C1   |
                      | bindings:       |
                      | x.gif    y.gif  |
                      +-----------------+
                           |      |
                           |      |
                           |      |
                       +-------------+
                       | Resource R1 |
                       +-------------+
 In this case, one possible value for the 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 the creation of a

Clemm, et al. Experimental [Page 21] RFC 5842 Binding Extensions to WebDAV April 2010

 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 Section 10.6 of [RFC4918].
 If a BIND request fails, the server state preceding the request MUST
 be restored.  This method is unsafe and idempotent (see [RFC2616],
 Section 9.1).
 Marshalling:
    The request MAY include an Overwrite header.
    The request body MUST be a DAV:bind XML element.
    <!ELEMENT bind (segment, href)>
    If the request succeeds, the server MUST return 201 (Created) when
    a new binding was created and 200 (OK) or 204 (No Content) when an
    existing binding was replaced.
    If a response body for a successful request is included, it MUST
    be a DAV:bind-response XML element.  Note that this document does
    not define any elements for the BIND response body, but the DAV:
    bind-response element is defined to ensure interoperability
    between future extensions that do define elements for the BIND
    response body.
    <!ELEMENT bind-response ANY>
 Preconditions:
    (DAV:bind-into-collection): The Request-URI MUST identify a
    collection.
    (DAV:bind-source-exists): The DAV:href element MUST identify a
    resource.

Clemm, et al. Experimental [Page 22] RFC 5842 Binding Extensions to WebDAV April 2010

    (DAV:binding-allowed): The resource identified by the DAV:href
    supports multiple bindings to it.
    (DAV:cross-server-binding): If the resource identified by the DAV:
    href element in the request body is on another server from the
    collection identified by the Request-URI, the server MUST support
    cross-server bindings (servers that do not support cross-server
    bindings can use this condition code to signal the client exactly
    why the request failed).
    (DAV:name-allowed): The name specified by the DAV:segment is
    available for use as a new binding name.
    (DAV:can-overwrite): If the collection already contains a binding
    with the specified path segment, and if an Overwrite header is
    included, the value of the Overwrite header MUST be "T".
    (DAV:cycle-allowed): If the DAV:href element identifies a
    collection, and if the Request-URI identifies a collection that is
    a member of that collection, the server MUST support cycles in the
    URI namespace (servers that do not support cycles can use this
    condition code to signal the client exactly why the request
    failed).
    (DAV:locked-update-allowed): If the collection identified by the
    Request-URI is write-locked, then the appropriate token MUST be
    specified in an If request header.
    (DAV:locked-overwrite-allowed): If the collection already contains
    a binding with the specified path segment, and if that binding is
    protected by a write lock, then the appropriate token MUST be
    specified in an If request header.
 Postconditions:
    (DAV:new-binding): The collection MUST have a binding that maps
    the segment specified in the DAV:segment element in the request
    body to the resource identified by the DAV:href element in the
    request body.

Clemm, et al. Experimental [Page 23] RFC 5842 Binding Extensions to WebDAV April 2010

4.1. Example: BIND

 >> Request:
 BIND /CollY HTTP/1.1
 Host: www.example.com
 Content-Type: application/xml; charset="utf-8"
 Content-Length: 172
 <?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
 Location: http://www.example.com/CollY/bar.html
 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], Section 9.1).
 Marshalling:
    The request body MUST be a DAV:unbind XML element.
    <!ELEMENT unbind (segment)>

Clemm, et al. Experimental [Page 24] RFC 5842 Binding Extensions to WebDAV April 2010

    If the request succeeds, the server MUST return 200 (OK) or 204
    (No Content) when the binding was successfully deleted.
    If a response body for a successful request is included, it MUST
    be a DAV:unbind-response XML element.  Note that this document
    does not define any elements for the UNBIND response body, but the
    DAV:unbind-response element is defined to ensure interoperability
    between future extensions that do define elements for the UNBIND
    response body.
    <!ELEMENT unbind-response ANY>
 Preconditions:
    (DAV:unbind-from-collection): The Request-URI MUST identify a
    collection.
    (DAV:unbind-source-exists): The DAV:segment element MUST identify
    a binding in the collection identified by the Request-URI.
    (DAV:locked-update-allowed): If the collection identified by the
    Request-URI is write-locked, then the appropriate token MUST be
    specified in the request.
    (DAV:protected-url-deletion-allowed): If the binding identified by
    the segment is protected by a write lock, then the appropriate
    token MUST be specified in the request.
 Postconditions:
    (DAV:binding-deleted): The collection MUST NOT have a binding for
    the segment specified in the DAV:segment element in the request
    body.
    (DAV:lock-deleted): If the internal member URI of the binding
    specified by the Request-URI and the DAV:segment element in the
    request body was protected by a write lock at the time of the
    request, that write lock must have been deleted by the request.

Clemm, et al. Experimental [Page 25] RFC 5842 Binding Extensions to WebDAV April 2010

5.1. Example: UNBIND

 >> Request:
 UNBIND /CollX HTTP/1.1
 Host: www.example.com
 Content-Type: application/xml; charset="utf-8"
 Content-Length: 117
 <?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], Section 9.1).
 Marshalling:
    The request MAY include an Overwrite header.
    The request body MUST be a DAV:rebind XML element.
    <!ELEMENT rebind (segment, href)>
    If the request succeeds, the server MUST return 201 (Created) when
    a new binding was created and 200 (OK) or 204 (No Content) when an
    existing binding was replaced.

Clemm, et al. Experimental [Page 26] RFC 5842 Binding Extensions to WebDAV April 2010

    If a response body for a successful request is included, it MUST
    be a DAV:rebind-response XML element.  Note that this document
    does not define any elements for the REBIND response body, but the
    DAV:rebind-response element is defined to ensure interoperability
    between future extensions that do define elements for the REBIND
    response body.
    <!ELEMENT rebind-response ANY>
 Preconditions:
    (DAV:rebind-into-collection): The Request-URI MUST identify a
    collection.
    (DAV:rebind-source-exists): The DAV:href element MUST identify a
    resource.
    (DAV:cross-server-binding): If the resource identified by the DAV:
    href element in the request body is on another server from the
    collection identified by the Request-URI, the server MUST support
    cross-server bindings (servers that do not support cross-server
    bindings can use this condition code to signal the client exactly
    why the request failed).
    (DAV:name-allowed): The name specified by the DAV:segment is
    available for use as a new binding name.
    (DAV:can-overwrite): If the collection already contains a binding
    with the specified path segment, and if an Overwrite header is
    included, the value of the Overwrite header MUST be "T".
    (DAV:cycle-allowed): If the DAV:href element identifies a
    collection, and if the Request-URI identifies a collection that is
    a member of that collection, the server MUST support cycles in the
    URI namespace (servers that do not support cycles can use this
    condition code to signal the client exactly why the request
    failed).
    (DAV:locked-update-allowed): If the collection identified by the
    Request-URI is write-locked, then the appropriate token MUST be
    specified in the request.
    (DAV:protected-url-modification-allowed): If the collection
    identified by the Request-URI already contains a binding with the
    specified path segment, and if that binding is protected by a
    write lock, then the appropriate token MUST be specified in the
    request.

Clemm, et al. Experimental [Page 27] RFC 5842 Binding Extensions to WebDAV April 2010

    (DAV:locked-source-collection-update-allowed): If the collection
    identified by the parent collection prefix of the DAV:href URI is
    write-locked, then the appropriate token MUST be specified in the
    request.
    (DAV:protected-source-url-deletion-allowed): If the DAV:href URI
    is protected by a write lock, then the appropriate token MUST be
    specified in the request.
 Postconditions:
    (DAV:new-binding): The collection MUST have a binding that maps
    the segment specified in the DAV:segment element in the request
    body, to the resource that was identified by the DAV:href element
    in the request body.
    (DAV:binding-deleted): The URL specified in the DAV:href element
    in the request body MUST NOT be mapped to a resource.
    (DAV:lock-deleted): If the URL specified in the DAV:href element
    in the request body was protected by a write lock at the time of
    the request, that write lock must have been deleted by the
    request.

6.1. Example: REBIND

 >> Request:
 REBIND /CollX HTTP/1.1
 Host: www.example.com
 Content-Type: application/xml; charset="utf-8"
 Content-Length: 176
 <?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

Clemm, et al. Experimental [Page 28] RFC 5842 Binding Extensions to WebDAV April 2010

 "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 "urn:uuid:f92d4fae-7012-11ab-a765-00c0ca1f6bf9").

Clemm, et al. Experimental [Page 29] RFC 5842 Binding Extensions to WebDAV April 2010

 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: (<urn:uuid:f92d4fae-7012-11ab-a765-00c0ca1f6bf9>)
 Content-Type: application/xml; charset="utf-8"
 Content-Length: 152
 <?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>

Clemm, et al. Experimental [Page 30] RFC 5842 Binding Extensions to WebDAV April 2010

 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.

Clemm, et al. Experimental [Page 31] RFC 5842 Binding Extensions to WebDAV April 2010

 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 signaled support for this specification using
 the "DAV" request header (see Section 8.2).  Instead, a 508 status
 should be returned when a binding loop is discovered.  This allows
 the server to return the 508 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: application/xml; charset="utf-8"
 Content-Length: 152
 <?xml version="1.0" encoding="utf-8" ?>
 <D:propfind xmlns:D="DAV:">
   <D:prop>
    <D:displayname/>
    <D:resource-id/>
   </D:prop>
 </D:propfind>

Clemm, et al. Experimental [Page 32] RFC 5842 Binding Extensions to WebDAV April 2010

 >> Response:
 HTTP/1.1 207 Multi-Status
 Content-Type: application/xml; charset="utf-8"
 Content-Length: 1241
 <?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>

Clemm, et al. Experimental [Page 33] RFC 5842 Binding Extensions to WebDAV April 2010

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 508 status.
 >> Request:
 PROPFIND /Coll/ HTTP/1.1
 Host: www.example.com
 Depth: infinity
 Content-Type: application/xml; charset="utf-8"
 Content-Length: 125
 <?xml version="1.0" encoding="utf-8" ?>
 <D:propfind xmlns:D="DAV:">
    <D:prop> <D:displayname/> </D:prop>
 </D:propfind>
 >> Response:
 HTTP/1.1 508 Loop Detected

7.2. 508 Loop Detected

 The 508 (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 [RFC4918],
 Section 10.1) 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

 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.

Clemm, et al. Experimental [Page 34] RFC 5842 Binding Extensions to WebDAV April 2010

9. Relationship to Locking in WebDAV

 Locking is an optional feature of WebDAV ([RFC4918]).  The base
 WebDAV specification and this protocol extension have been designed
 in parallel, making sure that all features of WebDAV can be
 implemented on a server that implements this protocol as well.
 Unfortunately, WebDAV uses the term "lock-root" inconsistently.  It
 is introduced in Section 6.1 of [RFC4918], point 2, as:
    2.  A resource becomes directly locked when a LOCK request to a
    URL of that resource creates a new lock.  The "lock-root" of the
    new lock is that URL.  If at the time of the request, the URL is
    not mapped to a resource, a new empty resource is created and
    directly locked.
 On the other hand, [RFC4918], Section 9.10.1 states:
    A LOCK request to an existing resource will create a lock on the
    resource identified by the Request-URI, provided the resource is
    not already locked with a conflicting lock.  The resource
    identified in the Request-URI becomes the root of the lock.
 Servers that implement both WebDAV locking and support for multiple
 bindings MUST use the first interpretation: the lock-root is the URI
 through which the lock was created, not a resource.  This URI, and
 potential aliases of this URI ([RFC4918], Section 5), are said to be
 "protected" by the lock.
 As defined in the introduction to Section 7 of [RFC4918], write
 operations that modify the state of a locked resource require that
 the lock token is submitted with the request.  Consistent with
 WebDAV, the state of the resource consists of the content ("any
 variant"), dead properties, lockable live properties (item 1), plus,
 for a collection, all its bindings (item 2).  Note that this, by
 definition, does not depend on the Request-URI to which the write
 operation is applied (the locked state is a property of the resource,
 not its URI).
 However, the lock-root is the URI through which the lock was
 requested.  Thus, the protection defined in item 3 of the list does
 not apply to additional URIs that may be mapped to the same resource
 due to the existence of multiple bindings.

Clemm, et al. Experimental [Page 35] RFC 5842 Binding Extensions to WebDAV April 2010

9.1. Example: Locking and Multiple Bindings

 Consider a root collection "/", containing the two collections C1 and
 C2, named "/CollX" and "/CollY", and a child resource R, bound to C1
 as "/CollX/test" and bound to C2 as "/CollY/test":
                       +-------------------------+
                       | Root Collection         |
                       |  bindings:              |
                       |  CollX          CollY   |
                       +-------------------------+
                           |                |
                           |                |
                           |                |
                  +---------------+  +---------------+
                  | Collection C1 |  | Collection C2 |
                  | bindings:     |  | bindings:     |
                  |     test      |  |     test      |
                  +---------------+  +---------------+
                           |               |
                           |               |
                           |               |
                          +------------------+
                          |    Resource R    |
                          +------------------+
 Given a host name of "www.example.com", applying a depth-zero write
 lock to "/CollX/test" will lock the resource R, and the lock-root of
 this lock will be "http://www.example.com/CollX/test".
 Thus, the following operations will require that the associated lock
 token is submitted with the "If" request header ([RFC4918], Section
 10.4):
 o  a PUT or PROPPATCH request modifying the content or lockable
    properties of resource R (as R is locked) -- no matter which URI
    is used as request target, and
 o  a MOVE, REBIND, UNBIND, or DELETE request causing "/CollX/test"
    not to be mapped to resource R anymore (be it addressed to
    "/CollX" or "/CollX/test").
 The following operations will not require submission of the lock
 token:
 o  a DELETE request addressed to "/CollY" or "/CollY/test", as it
    does not affect the resource R, nor the lock-root,

Clemm, et al. Experimental [Page 36] RFC 5842 Binding Extensions to WebDAV April 2010

 o  for the same reason, an UNBIND request removing the binding "test"
    from collection C2, or the binding "CollY" from the root
    collection, and
 o  similarly, a MOVE or REBIND request causing "/CollY/test" not
    being mapped to resource R anymore.
 Note that despite the lock-root being
 "http://www.example.com/CollX/test", an UNLOCK request can be
 addressed through any URI mapped to resource R, as UNLOCK operates on
 the resource identified by the Request-URI, not that URI (see
 [RFC4918], Section 9.11).

10. Relationship to WebDAV Access Control Protocol

 Note that the WebDAV Access Control Protocol has been designed for
 compatibility with systems that allow multiple URIs to map to the
 same resource (see [RFC3744], Section 5):
    Access control properties (especially DAV:acl and DAV:inherited-
    acl-set) are defined on the resource identified by the Request-URI
    of a PROPFIND request.  A direct consequence is that if the
    resource is accessible via multiple URI, the value of access
    control properties is the same across these URI.
 Furthermore, note that BIND and REBIND behave the same as MOVE with
 respect to the DAV:acl property (see [RFC3744], Section 7.3).

11. Relationship to Versioning Extensions to WebDAV

 Servers that implement Workspaces ([RFC3253], Section 6) and Version-
 Controlled Collections ([RFC3253], Section 14) already need to
 implement BIND-like behavior in order to handle UPDATE and UNCHECKOUT
 semantics.
 Consider a workspace "/ws1/", containing the version-controlled,
 checked-out collections C1 and C2, named "/ws1/CollX" and "/ws1/
 CollY", and a version-controlled resource R, bound to C1 as "/ws1/
 CollX/test":

Clemm, et al. Experimental [Page 37] RFC 5842 Binding Extensions to WebDAV April 2010

                       +-------------------------+
                       | Workspace               |
                       |  bindings:              |
                       |  CollX          CollY   |
                       +-------------------------+
                           |                |
                           |                |
                           |                |
                  +---------------+  +---------------+
                  | Collection C1 |  | Collection C2 |
                  | bindings:     |  |               |
                  |     test      |  |               |
                  +---------------+  +---------------+
                           |
                           |
                           |
                          +------------------+
                          |    Resource R    |
                          +------------------+
 Moving "/ws1/CollX/test" into "/ws1/CollY", checking in C2, but
 undoing the checkout on C1 will undo part of the MOVE request, thus
 restoring the binding from C1 to R, but keeping the new binding from
 C2 to R:
 >> Request:
 MOVE /ws1/CollX/test HTTP/1.1
 Host: www.example.com
 Destination: /ws1/CollY/test
 >> Response:
 HTTP/1.1 204 No Content
 >> Request:
 CHECKIN /ws1/CollY/ HTTP/1.1
 Host: www.example.com
 >> Response:
 HTTP/1.1 201 Created
 Cache-Control: no-cache
 Location: http://repo.example.com/his/17/ver/42

Clemm, et al. Experimental [Page 38] RFC 5842 Binding Extensions to WebDAV April 2010

 >> Request:
 UNCHECKOUT /ws1/CollX/ HTTP/1.1
 Host: www.example.com
 >> Response:
 HTTP/1.1 200 OK
 Cache-Control: no-cache
 As a result, both C1 and C2 would have a binding to R:
                       +-------------------------+
                       | Workspace               |
                       |  bindings:              |
                       |  CollX          CollY   |
                       +-------------------------+
                           |                |
                           |                |
                           |                |
                  +---------------+  +---------------+
                  | Collection C1 |  | Collection C2 |
                  | bindings:     |  | bindings:     |
                  |     test      |  |     test      |
                  +---------------+  +---------------+
                           |                |
                           |                |
                           |                |
                          +------------------+
                          |    Resource R    |
                          +------------------+
 The MOVE semantics defined in Section 3.15 of [RFC3253] already
 require that "/ws1/CollX/test" and "/ws1/CollY/test" will have the
 same version history (as exposed in the DAV:version-history
 property).  Furthermore, the UNCHECKOUT semantics (which in this case
 is similar to UPDATE, see Section 14.11 of [RFC3253]) require:
    If a new version-controlled member is in a workspace that already
    has a version-controlled resource for that version history, then
    the new version-controlled member MUST be just a binding (i.e.,
    another name for) that existing version-controlled resource.
 Thus, "/ws1/CollX/test" and "/ws1/CollY/test" will be bindings to the
 same resource R, and have identical DAV:resource-id properties.

Clemm, et al. Experimental [Page 39] RFC 5842 Binding Extensions to WebDAV April 2010

12. Security Considerations

 This section is provided to make WebDAV implementers aware of the
 security implications of this protocol.
 All of the security considerations of HTTP/1.1 ([RFC2616], Section
 15) and the WebDAV Distributed Authoring Protocol specification
 ([RFC4918], Section 20) 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.

12.1. 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.

12.2. 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".

12.3. 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.

12.4. 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.

Clemm, et al. Experimental [Page 40] RFC 5842 Binding Extensions to WebDAV April 2010

12.5. 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.

13. Internationalization Considerations

 All internationalization considerations mentioned in Section 19 of
 [RFC4918] also apply to this document.

14. IANA Considerations

 Section 7 defines the HTTP status codes 208 (Already Reported) and
 508 (Loop Detected), which have been added to the HTTP Status Code
 Registry.

15. Acknowledgements

 This document is the collaborative product of the authors and Tyson
 Chihaya, Jim Davis, Chuck Fay and Judith Slein.  It has benefited
 from thoughtful discussion by Jim Amsden, Peter Carlson, Steve
 Carter, Ken Coar, Ellis Cohen, Dan Connolly, Bruce Cragun, Cyrus
 Daboo, Spencer Dawkins, Mark Day, Werner Donne, 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, Alexey
 Melnikov, Surendra Koduru Reddy, Max Rible, Sam Ruby, Bradley
 Sergeant, Nick Shelness, John Stracke, John Tigue, John Turner, Kevin
 Wiggen, and other members of the concluded WebDAV working group.

16. References

16.1. Normative References

 [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.

Clemm, et al. Experimental [Page 41] RFC 5842 Binding Extensions to WebDAV April 2010

 [RFC4918]  Dusseault, L., Ed., "HTTP Extensions for Web Distributed
            Authoring and Versioning (WebDAV)", RFC 4918, June 2007.
 [XML]      Bray, T., Paoli, J., Sperberg-McQueen, C., Maler, E., and
            F. Yergeau, "Extensible Markup Language (XML) 1.0 (Fifth
            Edition)", W3C REC-xml-20081126, November 2008,
            <http://www.w3.org/TR/2008/REC-xml-20081126/>.

16.2. 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.

Index

 2
    208 Already Reported (status code)  31, 41
 5
    508 Loop Detected (status code)  34, 41
 B
    BIND method  21
       Marshalling  22
       Postconditions  23
       Preconditions  22
    Binding  6
    Binding Integrity  6-7, 21
 C
    Collection  6
    Condition Names
       DAV:bind-into-collection (pre)  22
       DAV:bind-source-exists (pre)  22
       DAV:binding-allowed (pre)  23
       DAV:binding-deleted (post)  25, 28
       DAV:can-overwrite (pre)  23, 27
       DAV:cross-server-binding (pre)  23, 27

Clemm, et al. Experimental [Page 42] RFC 5842 Binding Extensions to WebDAV April 2010

       DAV:cycle-allowed (pre)  23, 27
       DAV:lock-deleted (post)  25, 28
       DAV:locked-overwrite-allowed (pre)  23
       DAV:locked-source-collection-update-allowed (pre)  28
       DAV:locked-update-allowed (pre)  23, 25, 27
       DAV:name-allowed (pre)  23, 27
       DAV:new-binding (post)  23, 28
       DAV:protected-source-url-deletion-allowed (pre)  28
       DAV:protected-url-deletion-allowed (pre)  25
       DAV:protected-url-modification-allowed (pre)  27
       DAV:rebind-into-collection (pre)  27
       DAV:rebind-source-exists (pre)  27
       DAV:unbind-from-collection (pre)  25
       DAV:unbind-source-exists (pre)  25
 D
    DAV header
       compliance class 'bind'  34
    DAV:bind-into-collection precondition  22
    DAV:bind-source-exists precondition  22
    DAV:binding-allowed precondition  23
    DAV:binding-deleted postcondition  25, 28
    DAV:can-overwrite precondition  23, 27
    DAV:cross-server-binding precondition  23, 27
    DAV:cycle-allowed precondition  23, 27
    DAV:lock-deleted postcondition  25, 28
    DAV:locked-overwrite-allowed precondition  23
    DAV:locked-source-collection-update-allowed precondition  28
    DAV:locked-update-allowed precondition  23, 25, 27
    DAV:name-allowed precondition  23, 27
    DAV:new-binding postcondition  23, 28
    DAV:parent-set property  20
    DAV:protected-source-url-deletion-allowed precondition  28
    DAV:protected-url-deletion-allowed precondition  25
    DAV:protected-url-modification-allowed precondition  27
    DAV:rebind-into-collection precondition  27
    DAV:rebind-source-exists precondition  27
    DAV:resource-id property  19
    DAV:unbind-from-collection precondition  25
    DAV:unbind-source-exists precondition  25
 I
    Internal Member URI  6
 L
    Locking  35

Clemm, et al. Experimental [Page 43] RFC 5842 Binding Extensions to WebDAV April 2010

 M
    Methods
       BIND  21
       REBIND  26
       UNBIND  24
 P
    Path Segment  5
    Properties
       DAV:parent-set  20
       DAV:resource-id  19
 R
    REBIND method  26
       Marshalling  26
       Postconditions  28
       Preconditions  27
 S
    Status Codes
       208 Already Reported  31, 41
       508 Loop Detected  34, 41
 U
    UNBIND method  24
       Marshalling  24
       Postconditions  25
       Preconditions  25
    URI Mapping  5

Clemm, et al. Experimental [Page 44] RFC 5842 Binding Extensions to WebDAV April 2010

Authors' Addresses

 Geoffrey Clemm
 IBM
 550 King Street
 Littleton, MA  01460
 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 (editor)
 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

Clemm, et al. Experimental [Page 45]

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