    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.

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    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

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          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

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    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

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       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].

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       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".

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    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

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       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.

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       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.

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       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).

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    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:

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

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       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.

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

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    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

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       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.

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       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

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       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

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       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.

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    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]).

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

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       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.

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          (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.

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    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)>

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          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.

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    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.

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          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.

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          (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

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       "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").

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       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>

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       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.

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       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>

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       >> 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>

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    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.

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    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.

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    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,

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       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":

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

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       >> 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.

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    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.

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    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.

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       [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

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             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

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       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

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    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

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