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  INTERNET-DRAFT                   G. Clemm
  draft-ietf-webdav-bind-02        IBM
                                   J. Crawford
                                   IBM Research
                                   J. Reschke
                                   Greenbytes
                                   J. Slein
                                   Xerox
                                   E.J. Whitehead
                                   U.C. Santa Cruz
  
  Expires December 27, 2003        June 27, 2003
  
                        Binding Extensions to WebDAV
  
  Status of this Memo
  This document is an Internet-Draft and is in full conformance with all
  provisions of RFC 2026, Section 10.
  
  Internet-Drafts are working documents of the Internet Engineering Task
  Force (IETF), its areas, and its working groups.  Note that other groups
  may also distribute working documents as Internet-Drafts.
  
  Internet-Drafts are draft documents valid for a maximum of six months
  and may be updated, replaced, or obsoleted by other documents at any
  time.  It is inappropriate to use Internet-Drafts as reference material
  or to cite them other than as "work in progress."
  
  The list of current Internet-Drafts can be accessed at
  http://www.ietf.org/ietf/1id-abstracts.txt
  
  The list of Internet-Draft Shadow Directories can be accessed at
  http://www.ietf.org/shadow.html.
  
  
  Abstract
  This specification defines bindings, and the BIND method for creating
  multiple bindings to the same resource.  Creating a new binding to a
  resource causes at least one new URI to be mapped to that resource.
  Servers are required to insure the integrity of any bindings that they
  allow to be created.
  
  
  
  
  
  
  
  
  
  
  
  
  
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  Table of Contents
  
  1 INTRODUCTION............................................3
  1.1 Terminology...........................................4
  1.2 Rationale for Distinguishing Bindings from URI
      Mappings..............................................6
  1.3 Method Preconditions and Postconditions...............6
  
  2 OVERVIEW OF BINDINGS....................................7
  2.1 Bindings to Collections...............................7
  2.2 URI Mappings Created by a new Binding.................8
  2.3 COPY and Bindings.....................................9
  2.4 DELETE and Bindings..................................10
  2.5 MOVE and Bindings....................................10
  2.6 Determining Whether Two Bindings Are to the Same
      Resource.............................................11
  2.7 Discovering the Bindings to a Resource...............12
  
  3 PROPERTIES.............................................12
  3.1 DAV:resource-id Property.............................12
  3.2 DAV:parent-set Property..............................13
  
  4 BIND METHOD............................................13
  4.1 Example: BIND........................................15
  
  5 UNBIND METHOD..........................................15
  5.1 Example: UNBIND......................................16
  
  6 REBIND METHOD..........................................17
  6.1 Example: REBIND......................................18
  
  7 ADDITIONAL STATUS CODES................................19
  7.1 506 Loop Detected....................................19
  
  8 SECURITY CONSIDERATIONS................................20
  8.1 Privacy Concerns.....................................20
  8.2 Redirect Loops.......................................21
  8.3 Bindings, and Denial of Service......................21
  8.4 Private Locations May Be Revealed....................21
  8.5 DAV:parent-set and Denial of Service.................21
  
  9 INTERNATIONALIZATION CONSIDERATIONS....................21
  
  10  IANA CONSIDERATIONS..................................21
  
  11  INTELLECTUAL PROPERTY................................22
  
  12  ACKNOWLEDGEMENTS.....................................22
  
  13  REFERENCES...........................................22
  
  14  AUTHORS' ADDRESSES...................................23
  
  
  
  
  
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  1  INTRODUCTION
  
       This specification extends the WebDAV Distributed Authoring
       Protocol to enable clients to create new access paths to existing
       resources.  This capability is useful for several reasons:
  
       URIs of WebDAV-compliant resources are hierarchical and correspond
       to a hierarchy of collections in resource space.  The WebDAV
       Distributed Authoring Protocol makes it possible to organize these
       resources into hierarchies, placing them into groupings, known as
       collections, which are more easily browsed and manipulated than a
       single flat collection.  However, hierarchies require
       categorization decisions that locate resources at a single location
       in the hierarchy, a drawback when a resource has multiple valid
       categories. For example, in a hierarchy of vehicle descriptions
       containing collections for cars and boats, a description of a
       combination car/boat vehicle could belong in either collection.
       Ideally, the description should be accessible from both. Allowing
       clients to create new URIs that access the existing resource lets
       them put that resource into multiple collections.
  
       Hierarchies also make resource sharing more difficult, since
       resources that have utility across many collections are still
       forced into a single collection. For example, the mathematics
       department at one university might create a collection of
       information on fractals that contains bindings to some local
       resources, but also provides access to some resources at other
       universities.  For many reasons, it may be undesirable to make
       physical copies of the shared resources on the local server: to
       conserve disk space, to respect copyright constraints, or to make
       any changes in the shared resources visible automatically. Being
       able to create new access paths to existing resources in other
       collections or even on other servers is useful for this sort of
       case.
  
       The BIND method defined here provides a mechanism for allowing
       clients to create alternative access paths to existing WebDAV
       resources. HTTP and WebDAV methods are able to work because there
       are mappings between URIs and resources.  A method is addressed to
       a URI, and the server follows the mapping from that URI to a
       resource, applying the method to that resource.  Multiple URIs may
       be mapped to the same resource, but until now there has been no way
       for clients to create additional URIs mapped to existing resources.
  
       BIND lets clients associate a new URI with an existing WebDAV
       resource, and this URI can then be used to submit requests to the
       resource.  Since URIs of WebDAV resources are hierarchical, and
       correspond to a hierarchy of collections in resource space, the
       BIND method also has the effect of adding the resource to a
       collection.  As new URIs are associated with the resource, it
       appears in additional collections.
  
  
  
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       A BIND request does not create a new resource, but simply makes
       available a new URI for submitting requests to an existing
       resource.  The new URI is indistinguishable from any other URI when
       submitting a request to a resource.  Only one round trip is needed
       to submit a request to the intended target.  Servers are required
       to enforce the integrity of the relationships between the new URIs
       and the resources associated with them.  Consequently, it may be
       very costly for servers to support BIND requests that cross server
       boundaries.
  
       This specification is organized as follows.  Section 1.1 defines
       terminology used in the rest of the specification, while Section
       1.3 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 [RFC2518].
  
       This document uses XML DTD fragments as a purely notational
       convention. WebDAV request and response bodies cannot be validated
       due to the specific extensibility rules defined in section 23 of
       [RFC2518] and due to the fact that all XML elements defined by this
       specification use the XML namespace name "DAV:". In particular:
       - Element names use the "DAV:" namespace.
       - Element ordering is irrelevant.
       - Extension elements/attributes (elements/attributes not already
       defined as valid child elements) may be added anywhere, except when
       explicitly stated otherwise.
  
     URI Mapping
  
       A relation between an absolute URI and a resource.  For an absolute
       URI U and the resource it identifies R, the URI mapping can be
       thought of as (U => R).  Since a resource can represent items that
       are not network retrievable, as well as those that are, it is
       possible for a resource to have zero, one, or many URI mappings.
       Mapping a resource to an "http" scheme URI makes it possible to
       submit HTTP protocol requests to the resource using the URI.
  
     Path Segment
  
       Informally, the characters found between slashes ("/") in a URI.
       Formally, as defined in section 3.3 of [RFC2396].
  
  
  
  
  
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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.
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
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     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.
  
  
  1.2 Rationale for Distinguishing Bindings from URI Mappings
  
       In [RFC2518], the state of a collection is defined as containing a
       list of internal member URIs.  If there are multiple mappings to a
       collection, then the state of the collection is different when you
       refer to it via a different URI. This is undesirable, since ideally
       a collection's membership should remain the same, independent of
       which URI was used to reference it.
  
       The notion of binding is introduced to separate the final segment
       of a URI from its parent collectionÂ’s contribution. This done, a
       collection can be defined as containing a set of bindings, thus
       permitting new mappings to a collection without modifying its
       membership.  The authors of this specification anticipate and
       recommend that future revisions of [RFC2518] will update the
       definition of the state of a collection to correspond to the
       definition in this document.
  
  
  1.3 Method Preconditions and Postconditions
  
       A "precondition" of a method describes the state on the server that
       must be true for that method to be performed.  A "postcondition" of
       a method describes the state on the server that must be true after
       that method has completed.  If a method precondition or
       postcondition for a request is not satisfied, the response status
       of the request MUST be either 403 (Forbidden) if the request should
       not be repeated because it will always fail, or 409 (Conflict) if
       it is expected that the user might be able to resolve the conflict
       and resubmit the request.
  
       In order to allow better client handling of 403 and 409 responses,
       a distinct XML element type is associated with each method
       precondition and postcondition of a request.  When a particular
       precondition is not satisfied or a particular postcondition cannot
       be achieved, the appropriate XML element MUST be returned as the
       child of a top-level DAV:error element in the response body, unless
       otherwise negotiated by the request.  In a 207 Multi-Status
       response, the DAV:error element would appear in the appropriate
       DAV:responsedescription element.
  
  
  
  
  
  
  
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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.
  
  
  2.1 Bindings to Collections
  
       Bindings to collections can result in loops, which servers MUST
       detect when processing "Depth: infinity" requests.  It is sometimes
       possible to complete an operation in spite of the presence of a
       loop.  However, the 506 (Loop Detected) status code is defined in
       Section 5 for use in contexts where an operation is terminated
       because a loop was encountered.
  
       Creating a new binding to a collection makes each resource
       associated with a binding in that collection accessible via a new
       URI, and thus creates new URI mappings to those resources but no
       new bindings.
  
       For example, suppose a new binding CollY is created for collection
       C1 in the figure below.  It immediately becomes possible to access
       resource R1 using the URI /CollY/x.gif and to access resource R2
       using the URI /CollY/y.jpg, but no new bindings for these child
       resources were created.  This is because bindings are part of the
       state of a collection, and associate a URI that is relative to that
       collection with its target resource.  No change to the bindings in
       Collection C1 is needed to make its children accessible using
       /CollY/x.gif and /CollY/y.jpg.
  
  
  
  
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       +-------------------------+
       | Root Collection         |
       |  bindings:              |
       |  CollX          CollY   |
       +-------------------------+
           |            /
           |           /
           |          /
       +------------------+
       | Collection C1    |
       | bindings:        |
       | x.gif     y.jpg  |
       +------------------+
           |          \
           |           \
           |            \
       +-------------+   +-------------+
       | Resource R1 |   | Resource R2 |
       +-------------+   +-------------+
  
  
  2.2 URI Mappings Created by a new Binding
  
       Suppose a binding from "Binding-Name" to resource R to be added to
       a collection, C.  Then if C-MAP is the set of URIs that were mapped
       to C before the BIND request, then for each URI "C-URI" in C-MAP,
       the URI "C-URI/Binding-Name" is mapped to resource R following the
       BIND request.
  
       For example, if a binding from "foo.html" to R is added to a
       collection C, and if the following URIs are mapped to C:
  
       http://www.example.com/A/1/
       http://example.com/A/one/
  
       then the following new mappings to R are introduced:
  
       http://www.example.com/A/1/foo.html
       http://example.com/A/one/foo.html
  
       Note that if R is a collection, additional URI mappings are created
       to the descendents of R.  Also, note that if a binding is made in
       collection C to C itself (or to a parent of C), an infinite number
       of mappings are introduced.
  
       For example, if a binding from "myself" to C is then added to C,
       the following infinite number of additional mappings to C are
       introduced:
  
       http://www.example.com/A/1/myself
       http://www.example.com/A/1/myself/myself
          ...
  
  
  
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       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 8.8 of [RFC2518], COPY causes the resource
       identified by the Request-URI to be duplicated, and makes the new
       resource accessible using the URI specified in the Destination
       header.  Upon successful completion of a COPY, a new binding is
       created between the last path segment of the Destination header,
       and the destination resource. The new binding is added to its
       parent collection, identified by the Destination header minus its
       final segment.
  
       The following figure shows an example: Suppose that a COPY is
       issued to URI-3 for resource R (which is also mapped to URI-1 and
       URI-2), with the Destination header set to URI-X.  After successful
       completion of the COPY operation, resource R is duplicated to
       create resource R', and a new binding has been created which
       creates at least the URI mapping between URI-X and the new resource
       (although other URI mappings may also have been created).
  
       URI-1   URI-2    URI-3                           URI-X
          |       |        |                              |
          |       |        |   <---- URI Mappings ---->   |
          |       |        |                              |
       +---------------------+                 +------------------------+
       |     Resource R      |                 |     Resource R'        |
       +---------------------+                 +------------------------+
  
       It might be thought that a COPY request with "Depth: 0" on a
       collection would duplicate its bindings, since bindings are part of
       the collection's state.  This is not the case, however.  The
       definition of Depth in [RFC2518] makes it clear that a "Depth: 0"
       request does not apply to a collection's members.  Consequently, a
       COPY with "Depth: 0" does not duplicate the bindings contained by
       the collection.
  
       If a COPY causes one or more existing resources to be updated, the
       bindings to those resources MUST be unaffected by the COPY.  Using
       the preceding example, suppose that a COPY 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.
  
  
  
  
  
  
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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).  In particular, although
       Section 8.6.1 of [RFC2518] states that during DELETE processing, a
       server "MUST remove any URI for the resource identified by the
       Request-URI from collections which contain it as a member", a
       server that supports the binding protocol MUST NOT follow this
       requirement.
  
       When DELETE is applied to a collection, it MUST NOT modify the
       membership of any other collection that is not itself a member of
       the collection being deleted.  For example, if both "/a/.../x" and
       "/b/.../y" identify the same collection, C, then applying DELETE to
       "/a" MUST NOT delete an internal member from C or from any other
       collection that is a member of C, because that would modify the
       membership of "/b".
  
       If a collection supports the UNBIND method (see Section 5), a
       DELETE of an internal member of a collection MAY be implemented as
       an UNBIND request.  In this case, applying DELETE to a Request-URI
       has the effect of removing the binding identified by the final
       segment of the Request-URI from the collection identified by the
       Request-URI minus its final segment. Although [RFC2518] allows a
       DELETE to be a non-atomic operation, when the DELETE operation is
       implemented as an UNBIND, the operation is atomic.  In particular,
       a DELETE on a hierarchy of resources is simply the removal of a
       binding to the collection identified by the Request-URI.
  
  
  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 [RFC2518] allows
       a MOVE to be a non-atomic operation, when the MOVE operation is
       implemented as a REBIND, the operation is atomic.  In particular,
       applying a MOVE to a Request-URI and a Destination URI has the
       effect of removing a binding to a resource (at the Request-URI),
       and creating a new binding to that resource (at the Destination
       URI).
  
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       As an example, suppose that a MOVE is issued to URI-3 for resource
       R below (which is also mapped to URI-1 and URI-2), with the
       Destination header set to URI-X.  After successful completion of
       the MOVE operation, a new binding has been created which creates
       the URI mapping between URI-X and resource R.  The binding
       corresponding to the final segment of URI-3 has been removed, which
       also causes the URI mapping between URI-3 and R to be removed.  If
       resource R were a collection, old URI-3 based mappings to members
       of R would have been removed, and new URI-X based mappings to
       members of R would have been created.
  
       >> Before Request:
  
        URI-1   URI-2    URI-3
          |       |        |
          |       |        |      <---- URI Mappings
          |       |        |
       +---------------------+
       |     Resource R      |
       +---------------------+
  
       >> After Request:
  
        URI-1   URI-2    URI-X
          |       |        |
          |       |        |      <---- URI Mappings
          |       |        |
       +---------------------+
       |     Resource R      |
       +---------------------+
  
       Although [RFC2518] allows a MOVE on a collection to be a non-atomic
       operation, a MOVE implemented as a REBIND MUST be atomic.  Even
       when the Request-URI identifies a collection, the MOVE operation
       involves only removing one binding to that collection and adding
       another.  There are no operations on bindings to any of its
       children, so the case of MOVE on a collection is the same as the
       case of MOVE on a non-collection resource.  Both are atomic.
  
  
  2.6 Determining Whether Two Bindings Are to the Same Resource
  
       It is useful to have some way of determining whether two bindings
       are to the same resource.  Two resources might have identical
       contents and properties, but not be the same resource (e.g. an
       update to one resource does not affect the other resource).
  
       The REQUIRED DAV:resource-id property defined in Section 3.1 is a
       resource identifier, which MUST be unique across all resources for
       all time.  If the values of DAV:resource-id returned by PROPFIND
       requests through two bindings are identical, the client can be
       assured that the two bindings are to the same resource.
  
  
  
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       The DAV:resource-id property is created, and its value assigned,
       when the resource is created.  The value of DAV:resource-id MUST
       NOT be changed.  Even after the resource is no longer accessible
       through any URI, that value MUST NOT be reassigned to another
       resource's DAV:resource-id property.
  
       Any method that creates a new resource MUST assign a new, unique
       value to its DAV:resource-id property.  For example, a PUT or a
       COPY that creates a new resource must assign a new, unique value to
       the DAV:resource-id property of that new resource.
  
       On the other hand, any method that affects an existing resource
       MUST NOT change the value of its DAV:resource-id property.  For
       example, a PUT or a COPY that updates an existing resource must not
       change the value of its DAV:resource-id property.  A MOVE, since it
       does not create a new resource, but only changes the location of an
       existing resource, must not change the value of the DAV:resource-id
       property.
  
  
  2.7 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 8.4 and 8.5.
  
  
  3  PROPERTIES
  
       The bind feature introduces the following properties for a
       resource.
  
       A DAV:allprop PROPFIND request SHOULD NOT return any of the
       properties defined by this document.  This allows a binding server
       to perform efficiently when a naive client, which does not
       understand the cost of asking a server to compute all possible live
       properties, issues a DAV:allprop PROPFIND request.
  
  
  3.1 DAV:resource-id Property
  
       The DAV:resource-id property is a REQUIRED property that enables
       clients to determine whether two bindings are to the same resource.
       The value of DAV:resource-id is a URI, and may use any registered
       URI scheme that guarantees the uniqueness of the value across all
       resources for all time (e.g. the opaquelocktoken: scheme defined in
       [RFC2518]).
  
  
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       <!ELEMENT resource-id (href)>
  
  3.2 DAV:parent-set Property
  
       The DAV:parent-set property is an OPTIONAL property that enables
       clients to discover what collections contain a binding to this
       resource (i.e. what collections have that resource as an internal
       member).  It contains an of href/segment pair for each collection
       that has a binding to the resource.  The href identifies the
       collection, and the segment identifies the binding name of that
       resource in that collection.
  
       A given collection MUST appear only once in the DAV:parent-set for
       any given binding, even if there are multiple URI mappings to that
       collection.  For example, if collection C1 is mapped to both /CollX
       and /CollY, and C1 contains a binding named "x.gif" to a resource
       R1, then either [/CollX, x.gif] or [/CollY, x.gif] can appear in
       the DAV:parent-set of R1, but not both.  But if C1 also had a
       binding named "y.gif" to R1, then there would be two entries for C1
       in the DAV:binding-set of R1 (i.e. either both [/CollX, x.gif] and
       [/CollX, y.gif] or alternatively, both [/CollY, x.gif] and [/CollY,
       y.gif]).
  
       <!ELEMENT parent-set (parent)*>
       <!ELEMENT parent (href, segment)>
       <!ELEMENT segment (#PCDATA)>
       PCDATA value: segment, as defined in section 3.3 of [RFC2396]
  
  4  BIND METHOD
  
       The BIND method modifies the collection identified by the Request-
       URI, by adding a new binding from the segment specified in the BIND
       body to the resource identified in the BIND body.
  
       If a server cannot guarantee the integrity of the binding, the BIND
       request MUST fail.  Note that it is especially difficult to
       maintain the integrity of cross-server bindings.  Unless the server
       where the resource resides knows about all bindings on all servers
       to that resource, it may unwittingly destroy the resource or make
       it inaccessible without notifying another server that manages a
       binding to the resource.  For example, if server A permits creation
       of a binding to a resource on server B, server A must notify server
       B about its binding and must have an agreement with B that B will
       not destroy the resource while A's binding exists.  Otherwise
       server B may receive a DELETE request that it thinks removes the
       last binding to the resource and destroy the resource while A's
       binding still exists. Status code 507 (Cross-server Binding
       Forbidden) is defined in Section 7.1 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.
  
  
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       This default binding replacement behavior can be overridden using
       the Overwrite header defined in Section 9.6 of [RFC2518].
  
     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) 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.
  
       (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.
  
       (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.
  
  
  
  
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       (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.
  
  
  4.1 Example: BIND
  
       >> Request:
  
       BIND /CollY HTTP/1.1
       Host: www.example.com
       Content-Type: text/xml; charset="utf-8"
       Content-Length: xxx
  
       <?xml version="1.0" encoding="utf-8" ?>
       <D:bind xmlns:D="DAV:">
          <D:segment>bar.html</D:segment>
          <D:href>http://www.example.com/CollX/foo.html</D:href>
       </D:bind>
  
       >> Response:
  
       HTTP/1.1 201 Created
  
       The server added a new binding to the collection,
       "http://www.example.com/CollY", associating "bar.html" with the
       resource identified by the URI
       "http://www.example.com/CollX/foo.html".  Clients can now use the
       URI "http://www.example.com/CollY/bar.html", to submit requests to
       that resource.
  
  
  5  UNBIND METHOD
  
       The UNBIND method modifies the collection identified by the
       Request-URI, by removing the binding identified by the segment
       specified in the UNBIND body.
  
       Once a resource is unreachable by any URI mapping, the server MAY
       reclaim system resources associated with that resource. If UNBIND
       removes a binding to a resource, but there remain URI mappings to
  
  
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       that resource, the server MUST NOT reclaim system resources
       associated with the resource.
  
     Marshalling:
  
       The request body MUST be a DAV:unbind XML element.
  
       <!ELEMENT unbind (segment)>
  
       If the request succeeds, the server MUST return 200 (OK) 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.
  
  
  5.1 Example: UNBIND
  
       >> Request:
  
       UNBIND /CollX HTTP/1.1
       Host: www.example.com
       Content-Type: text/xml; charset="utf-8"
       Content-Length: xxx
  
       <?xml version="1.0" encoding="utf-8" ?>
  
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       <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 one
       collection, and adds a binding to that resource into another
       collection.  It is effectively an atomic form of a MOVE request.
  
     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) when an existing binding was
       replaced.
  
       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: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
  
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       collection identified by the request-URI, the server MUST support
       cross-server bindings.
  
       (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.
  
       (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.
  
       (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.
  
  
  6.1 Example: REBIND
  
       >> Request:
  
       REBIND /CollX HTTP/1.1
       Host: www.example.com
       Content-Type: text/xml; charset="utf-8"
       Content-Length: xxx
  
  
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       <?xml version="1.0" encoding="utf-8" ?>
       <D:rebind xmlns:D="DAV:">
          <D:segment>foo.html</D:segment>
          <D:href>http://www.example.com/CollY/bar.html</D:href>
       </D:rebind>
  
       >> Response:
  
       HTTP/1.1 200 OK
  
       The server added a new binding to the collection,
       "http://www.example.com/CollX", associating "foo.html" with the
       resource identified by the URI
       "http://www.example.com/CollY/bar.html", and removes the binding
       named "bar.html" from the collection identified by the URI
       "http://www.example.com/CollY".  Clients can now use the URI
       "http://www.example.com/CollX/foo.html" to submit requests to that
       resource, and requests on the URI
       "http://www.example.com/CollY/bar.html" will fail with a 404 (Not
       Found) response.
  
  
  7  ADDITIONAL STATUS CODES
  
  
  7.1 506 Loop Detected
  
       The 506 (Loop Detected) status code indicates that the server
       terminated an operation because it encountered an infinite loop
       while processing a request with "Depth: infinity".
  
       When this status code is the top-level status code for the
       operation, it indicates that the entire operation failed.
  
       When this status code occurs inside a multi-status response, it
       indicates only that a loop is being terminated, but does not
       indicate failure of the operation as a whole.
  
       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
       Content-Type: text/xml; charset="utf-8"
       Content-Length: xxx
  
       <?xml version="1.0" encoding="utf-8" ?>
       <D:propfind xmlns:D="DAV:">
          <D:prop> <D:displayname/> </D:prop>
       </D:propfind>
  
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       >> Response:
  
       HTTP/1.1 207 Multi-Status
       Content-Type: text/xml; charset="utf-8"
       Content-Length: xxx
  
       <?xml version="1.0" encoding="utf-8" ?>
       <D:multistatus xmlns:D="DAV:">
          <D:response>
             <D:href>http://www.example.com/Coll/</D:href>
             <D:propstat>
                <D:prop>
                   <D:displayname>Loop Demo</D:displayname>
                </D: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: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:status>HTTP/1.1 506 Loop Detected</D:status>
          </D:response>
       </D:multistatus>
  
  8  SECURITY CONSIDERATIONS
  
       This section is provided to make WebDAV applications aware of the
       security implications of this protocol.
  
       All of the security considerations of HTTP/1.1 and the WebDAV
       Distributed Authoring Protocol specification also apply to this
       protocol specification.  In addition, bindings introduce several
       new security concerns and increase the risk of some existing
       threats.  These issues are detailed below.
  
  
  8.1 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.
  
  
  
  
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  8.2 Redirect Loops
  
       Although redirect 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".
  
  
  8.3 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.
  
  
  8.4 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.
  
  
  8.5 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.
  
  
  9  INTERNATIONALIZATION CONSIDERATIONS
  
       All internationalization considerations mentioned in [RFC2518] also
       apply to this document.
  
  
  10 IANA CONSIDERATIONS
  
       All IANA considerations mentioned in [RFC2518] also apply to this
       document.
  
  
  
  
  
  
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  11 INTELLECTUAL PROPERTY
  
       The following notice is copied from RFC 2026, Section 10.4, and
       describes the position of the IETF concerning intellectual property
       claims made against this document.
  
       The IETF takes no position regarding the validity or scope of any
       intellectual property or other rights that might be claimed to
       pertain to the implementation or use other technology described in
       this document or the extent to which any license under such rights
       might or might not be available; neither does it represent that it
       has made any effort to identify any such rights.  Information on
       the procedures of the IETF with respect to rights in standards-
       track and standards-related documentation can be found in BCP-11.
       Copies of claims of rights made available for publication and any
       assurances of licenses to be made available, or the result of an
       attempt made to obtain a general license or permission for the use
       of such proprietary rights by implementers or users of this
       specification can be obtained from the IETF Secretariat.
  
       The IETF invites any interested party to bring to its attention any
       copyrights, patents or patent applications, or other proprietary
       rights that may cover technology that may be required to practice
       this standard.  Please address the information to the IETF
       Executive Director.
  
  
  12 ACKNOWLEDGEMENTS
  
       This draft is the collaborative product of the authors and Tyson
       Chihaya, Jim Davis, and Chuck Fay.  This draft has benefited from
       thoughtful discussion by Jim Amsden, Peter Carlson, Steve Carter,
       Ken Coar, Ellis Cohen, Dan Connolly, Bruce Cragun, Spencer Dawkins,
       Mark Day, Rajiv Dulepet, David Durand, Roy Fielding, Yaron Goland,
       Fred Hitt, Alex Hopmann, James Hunt, Marcus Jager, Chris Kaler,
       Manoj Kasichainula, Rohit Khare, Daniel LaLiberte, Steve Martin,
       Larry Masinter, Jeff McAffer, Surendra Koduru Reddy, Max Rible, Sam
       Ruby, Bradley Sergeant, Nick Shelness, John Stracke, John Tigue,
       John Turner, Kevin Wiggen, and other members of the WebDAV working
       group.
  
  
  13 REFERENCES
  
       [RFC2026] S.Bradner, "The Internet Standards Process", RFC 2026,
       October 1996.
  
       [RFC2119] S.Bradner, "Key words for use in RFCs to Indicate
       Requirement Levels", RFC 2119, March 1997.
  
       [RFC2277] H.Alvestrand, "IETF Policy on Character Sets and
       Languages." RFC 2277,  January 1998.
  
  
  
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       [RFC2396] T. Berners-Lee, R. Fielding, L. Masinter, "Uniform
       Resource Identifiers (URI): Generic Syntax." RFC 2396, August 1998.
  
       [RFC2518] Y.Goland, E.Whitehead, A.Faizi, S.R.Carter, D.Jensen,
       "HTTP Extensions for Distributed Authoring - WEBDAV", RFC 2518,
       February 1999.
  
       [RFC2616] R.Fielding, J.Gettys, J.C.Mogul, H.Frystyk, L.Masinter,
       P.Leach, and T.Berners-Lee, "Hypertext Transfer Protocol --
       HTTP/1.1", RFC 2616, June 1999.
  
       [XML] T. Bray, J. Paoli, C.M. Sperberg-McQueen, "Extensible Markup
       Language (XML) 1.0 (Second Edition)"  W3C Recommendation 6 October
       2000. http://www.w3.org/TR/2000/REC-xml-20001006.
  
  
  14 AUTHORS' ADDRESSES
  
       Geoffrey Clemm
       Rational Software Corporation
       20 Maguire Road
       Lexington, MA 02173-3104
       Email: geoffrey.clemm@us.ibm.com
  
       Jason Crawford
       IBM Research
       P.O. Box 704
       Yorktown Heights, NY 10598
       Email: ccjason@us.ibm.com
  
       Julian F. Reschke
       greenbytes GmbH
       Salzmannstrasse 152
       Muenster, NW 48159, Germany
       Email: julian.reschke@greenbytes.de
  
       Judy Slein
       Xerox Corporation
       800 Phillips Road, 105-50C
       Webster, NY 14580
       Email: jslein@crt.xerox.com
  
       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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