WebDAV Ordering: Web Distributed Authoring and Versioning (WebDAV) Ordered Collections Protocol

Network Working Group                                       J. Whitehead
Request for Comments: 3648                               U.C. Santa Cruz
Category: Standards Track                                J. Reschke, Ed.
                                                              greenbytes
                                                           December 2003


           Web Distributed Authoring and Versioning (WebDAV)
                      Ordered Collections Protocol

Status of this Memo

   This document specifies an Internet standards track protocol for the
   Internet community, and requests discussion and suggestions for
   improvements.  Please refer to the current edition of the "Internet
   Official Protocol Standards" (STD 1) for the standardization state
   and status of this protocol.  Distribution of this memo is unlimited.

Copyright Notice

   Copyright (C) The Internet Society (2003).  All Rights Reserved.

Abstract

   This specification extends the Web Distributed Authoring and
   Versioning (WebDAV) Protocol to support the server-side ordering of
   collection members.  Of particular interest are orderings that are
   not based on property values, and so cannot be achieved using a
   search protocol's ordering option and cannot be maintained
   automatically by the server.  Protocol elements are defined to let
   clients specify the position in the ordering of each collection
   member, as well as the semantics governing the ordering.

Table of Contents

   1.  Notational Conventions . . . . . . . . . . . . . . . . . . . .  3
   2.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   3.  Terminology  . . . . . . . . . . . . . . . . . . . . . . . . .  4
   4.  Overview of Ordered Collections  . . . . . . . . . . . . . . .  5
       4.1.  Additional Collection properties . . . . . . . . . . . .  6
             4.1.1.  DAV:ordering-type (protected). . . . . . . . . .  6
   5.  Creating an Ordered Collection . . . . . . . . . . . . . . . .  7
       5.1.  Overview . . . . . . . . . . . . . . . . . . . . . . . .  7
       5.2.  Example: Creating an Ordered Collection. . . . . . . . .  8
   6.  Setting the Position of a Collection Member. . . . . . . . . .  8
       6.1.  Overview . . . . . . . . . . . . . . . . . . . . . . . .  8
       6.2.  Examples: Setting the Position of a Collection Member. . 10
       6.3.  Examples: Renaming a member of an ordered collection . . 10
   7.  Changing a Collection Ordering: ORDERPATCH method. . . . . . . 11
       7.1.  Example: Changing a Collection Ordering. . . . . . . . . 13
       7.2.  Example: Failure of an ORDERPATCH Request. . . . . . . . 14
   8.  Listing the Members of an Ordered Collection . . . . . . . . . 16
       8.1.  Example: PROPFIND on an Ordered Collection . . . . . . . 17
   9.  Relationship to versioned collections. . . . . . . . . . . . . 19
       9.1.  Collection Version Properties. . . . . . . . . . . . . . 20
             9.1.1.  Additional semantics for
                     DAV:version-controlled-binding-set (protected) . 20
             9.1.2.  DAV:ordering-type (protected). . . . . . . . . . 20
       9.2.  Additional CHECKIN semantics . . . . . . . . . . . . . . 20
       9.3.  Additional CHECKOUT Semantics. . . . . . . . . . . . . . 20
       9.4.  Additional UNCHECKOUT, UPDATE, and MERGE Semantics . . . 21
   10. Capability Discovery . . . . . . . . . . . . . . . . . . . . . 21
       10.1. Example: Using OPTIONS for the Discovery of Support for
             Ordering . . . . . . . . . . . . . . . . . . . . . . . . 22
       10.2. Example: Using Live Properties for the Discovery of
             Ordering . . . . . . . . . . . . . . . . . . . . . . . . 22
   11. Security Considerations. . . . . . . . . . . . . . . . . . . . 23
       11.1.  Denial of Service and DAV:ordering-type . . . . . . . . 23
   12. Internationalization Considerations. . . . . . . . . . . . . . 24
   13. IANA Considerations. . . . . . . . . . . . . . . . . . . . . . 24
   14. Intellectual Property Statement. . . . . . . . . . . . . . . . 25
   15. Contributors . . . . . . . . . . . . . . . . . . . . . . . . . 25
   16. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 25
   17. Normative References . . . . . . . . . . . . . . . . . . . . . 26
   A.  Extensions to the WebDAV Document Type Definition. . . . . . . 27
   Index. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 29
   Full Copyright Statement . . . . . . . . . . . . . . . . . . . . . 30

1.  Notational Conventions

   Since this document describes a set of extensions to the WebDAV
   Distributed Authoring Protocol [RFC2518], which is itself an
   extension to the HTTP/1.1 protocol, the augmented BNF used here to
   describe protocol elements is exactly the same as described in
   Section 2.1 of HTTP [RFC2616].  Since this augmented BNF uses the
   basic production rules provided in Section 2.2 of HTTP, these rules
   apply to this document as well.

   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 as a purely notational
   convention.  WebDAV request and response bodies can not 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:

   1. element names use the "DAV:" namespace,

   2. element ordering is irrelevant,

   3. extension elements (elements not already defined as valid child
      elements) may be added anywhere, except where explicitly stated
      otherwise,

   4. extension attributes (attributes not already defined as valid for
      this element) may be added anywhere, except where explicitly
      stated otherwise.

2.  Introduction

   This specification builds on the collection infrastructure provided
   by the WebDAV Distributed Authoring Protocol, adding support for the
   server-side ordering of collection members.

   There are many scenarios in which it is useful to impose an ordering
   on a collection at the server, such as expressing a recommended
   access order, or a revision history order.  The members of a
   collection might represent the pages of a book, which need to be
   presented in order if they are to make sense, or an instructor might
   create a collection of course readings that she wants to be displayed
   in the order they are to be read.

   Orderings may be based on property values, but this is not always the
   case.  The resources in the collection may not have properties that
   can be used to support the desired ordering.  Orderings based on
   properties can be obtained using a search protocol's ordering option,
   but orderings not based on properties cannot.  These orderings
   generally need to be maintained by a human user.

   The ordering protocol defined here focuses on support for such
   human-maintained orderings.  Its protocol elements allow clients to
   specify the position of each collection member in the collection's
   ordering, as well as the semantics governing the order.  The protocol
   is designed to allow additional support in the future for orderings
   that are maintained automatically by the server.

   The remainder of this document is structured as follows: Section 3
   defines terminology that will be used throughout the specification.
   Section 4 provides an overview of ordered collections.  Section 5
   describes how to create an ordered collection, and Section 6
   discusses how to set a member's position in the ordering of a
   collection.  Section 7 explains how to change a collection ordering.
   Section 8 discusses listing the members of an ordered collection.
   Section 9 discusses the impact on version-controlled collections (as
   defined in [RFC3253]).  Section 10 describes capability discovery.
   Sections 11 through 13 discuss security, internationalization, and
   IANA considerations.  The remaining sections provide supporting
   information.

3.  Terminology

   The terminology used here follows that in [RFC2518] and [RFC3253].
   Definitions of the terms resource, Uniform Resource Identifier (URI),
   and Uniform Resource Locator (URL) are provided in [RFC2396].

   Ordered Collection

      A collection for which the results from a PROPFIND request are
      guaranteed to be in the order specified for that collection.

   Unordered Collection

      A collection for which the client cannot depend on the
      repeatability of the ordering of results from a PROPFIND request.

   Client-Maintained Ordering

      An ordering of collection members that is maintained on the server
      based on client requests specifying the position of each
      collection member in the ordering.

   Server-Maintained Ordering

      An ordering of collection members that is maintained automatically
      by the server, based on a client's choice of ordering semantics.

   Ordering Semantics

      In general, ordering semantics are the set of structures or
      meanings applied to the ordering of the member of a specific
      collection.  Within this document, "ordering semantics" refers
      specifically to the structure specified in the DAV:ordering-type
      property.  See Section 4.1.1 for more information on
      DAV:ordering-type.

   This document uses the terms "precondition", "postcondition" and
   "protected property" as defined in [RFC3253].  Servers MUST report
   pre-/postcondition failures as described in section 1.6 of this
   document.

4.  Overview of Ordered Collections

   If a collection is not ordered, the client cannot depend on the
   repeatability of the ordering of results from a PROPFIND request.  By
   specifying an ordering for a collection, a client requires the server
   to follow that ordering whenever it responds to a PROPFIND request on
   that collection.

   Server-side orderings may be client-maintained or server-maintained.
   For client-maintained orderings, a client must specify the ordering
   position of each of the collection's members, either when the member
   is added to the collection (using the Position header (Section 6)) or
   later (using the ORDERPATCH (Section 7) method).  For server-
   maintained orderings, the server automatically positions each of the
   collection's members according to the ordering semantics.  This
   specification supports only client-maintained orderings, but is
   designed to allow the future extension with server-maintained
   orderings.

   A collection that supports ordering is not required to be ordered.

   If a collection is ordered, each of its internal member URIs MUST
   appear in the ordering exactly once, and the ordering MUST NOT
   include any URIs that are not internal members of the collection.
   The server is responsible for enforcing these constraints on
   orderings.  The server MUST remove an internal member URI from the
   ordering when it is removed from the collection.  Removing an
   internal member MUST NOT affect the ordering of the remaining
   internal members.  The server MUST add an internal member URI to the
   ordering when it is added to the collection.

   Only one ordering can be attached to any collection.  Multiple
   orderings of the same resources can be achieved by creating multiple
   collections referencing those resources, and attaching a different
   ordering to each collection.

   An ordering is considered to be part of the state of a collection
   resource.  Consequently, the ordering is the same no matter which URI
   is used to access the collection and is protected by locks or access
   control constraints on the collection.

4.1.  Additional Collection properties

   A DAV:allprop PROPFIND request SHOULD NOT return any of the
   properties defined in this document.

4.1.1.  DAV:ordering-type (protected)

   The DAV:ordering-type property indicates whether the collection is
   ordered and, if so, uniquely identifies the semantics of the
   ordering.  It may also point to an explanation of the semantics in
   human and/or machine-readable form.  At a minimum, this allows human
   users who add members to the collection to understand where to
   position them in the ordering.  This property cannot be set using
   PROPPATCH.  Its value can only be set by including the Ordering-Type
   header with a MKCOL request or by submitting an ORDERPATCH request.

   Ordering types are identified by URIs that uniquely identify the
   semantics of the collection's ordering.  The following two URIs are
   predefined:

   DAV:custom: The value DAV:custom indicates that the collection is
      ordered, but the semantics governing the ordering are not being
      advertised.

   DAV:unordered: The value DAV:unordered indicates that the collection
      is not ordered.  That is, the client cannot depend on the
      repeatability of the ordering of results from a PROPFIND request.

   An ordering-aware client interacting with an ordering-unaware server
   (e.g., one that is implemented only according to [RFC2518]) SHOULD
   assume that the collection is unordered if a collection does not have
   the DAV:ordering-type property.

   <!ELEMENT ordering-type (href) >

5.  Creating an Ordered Collection

5.1.  Overview

   When a collection is created, the client MAY request that it be
   ordered and specify the semantics of the ordering by using the new
   Ordering-Type header (defined below) with a MKCOL request.

   For collections that are ordered, the client SHOULD identify the
   semantics of the ordering with a URI in the Ordering-Type header,
   although the client MAY simply set the header value to DAV:custom to
   indicate that the collection is ordered but the semantics of the
   ordering are not being advertised.  Setting the value to a URI that
   identifies the ordering semantics provides the information a human
   user or software package needs to insert new collection members into
   the ordering intelligently.  Although the URI in the Ordering-Type
   header MAY point to a resource that contains a definition of the
   semantics of the ordering, clients SHOULD NOT access that resource to
   avoid overburdening its server.  A value of DAV:unordered in the
   Ordering-Type header indicates that the client wants the collection
   to be unordered.  If the Ordering-Type header is not present, the
   collection will be unordered.

   Additional Marshalling:

      Ordering-Type = "Ordering-Type" ":" absoluteURI
      ; absoluteURI: see RFC2396, section 3

      The URI "DAV:unordered" indicates that the collection is not
      ordered, while "DAV:custom" indicates that the collection is to be
      ordered, but the semantics of the ordering is not being
      advertised.  Any other URI value indicates that the collection is
      ordered, and identifies the semantics of the ordering.

   Additional Preconditions:

      (DAV:ordered-collections-supported): the server MUST support
      ordered collections in the part of the URL namespace identified by
      the request URL.

   Additional Postconditions:

      (DAV:ordering-type-set): if the Ordering-Type header was present,
      the request MUST have created a new collection resource with the
      DAV:ordering-type being set according to the Ordering-Type request
      header.  The collection MUST be ordered unless the ordering type
      is "DAV:unordered".

5.2.  Example: Creating an Ordered Collection

   >> Request:

   MKCOL /theNorth/ HTTP/1.1
   Host: example.org
   Ordering-Type: http://example.org/orderings/compass.html

    >> Response:

   HTTP/1.1 201 Created

   In this example, a new ordered collection was created.  Its
   DAV:ordering-type property has the URI from the Ordering-Type header
   as its value http://example.org/orderings/compass.html.  In this
   case, the URI identifies the semantics governing a client-maintained
   ordering.  As new members are added to the collection, clients or end
   users can use the semantics to determine where to position the new
   members in the ordering.

6.  Setting the Position of a Collection Member

6.1.  Overview

   When a new member is added to a collection with a client-maintained
   ordering (for example, with PUT, COPY, or MKCOL), its position in the
   ordering can be set with the new Position header.  The Position
   header allows the client to specify that an internal member URI
   should be first in the collection's ordering, last in the
   collection's ordering, immediately before some other internal member
   URI in the collection's ordering, or immediately after some other
   internal member URI in the collection's ordering.

   If the Position request header is not used when adding a member to an
   ordered collection, then:

   o  If the request is replacing an existing resource, the server MUST
      preserve the present ordering.

   o  If the request is adding a new internal member URI to the
      collection, the server MUST append the new member to the end of
      the ordering.

   Note to implementers: this specification does not mandate a specific
   implementation of MOVE operations within the same parent collection.
   Therefore, servers may either implement this as a simple rename
   operation (preserving the collection member's position), or as a
   sequence of "remove" and "add" (causing the semantics of "adding a
   new member" to apply).  Future revisions of this specification may
   specify this behaviour more precisely based on future implementation
   experience.

   Additional Marshalling:

      Position = "Position" ":" ("first" | "last" |
                                (("before" | "after") segment))

      segment is defined in Section 3.3 of [RFC2396].

      The segment is interpreted relative to the collection to which the
      new member is being added.

      When the Position header is present, the server MUST insert the
      new member into the ordering at the specified location.

      The "first" keyword indicates that the new member is placed in the
      beginning position in the collection's ordering, while "last"
      indicates that the new member is placed in the final position in
      the collection's ordering.  The "before" keyword indicates that
      the new member is added to the collection's ordering immediately
      prior to the position of the member identified in the segment.
      Likewise, the "after" keyword indicates that the new member is
      added to the collection's ordering immediately following the
      position of the member identified in the segment.

      If the request is replacing an existing resource and the Position
      header is present, the server MUST remove the internal member URI
      from its current position, and insert it at the newly requested
      position.

   Additional Preconditions:

      (DAV:collection-must-be-ordered): the target collection MUST be
      ordered.

      (DAV:segment-must-identify-member): the referenced segment MUST
      identify a resource that exists and is different from the affected
      resource.

   Additional Postconditions:

      (DAV:position-set): if a Position header is present, the request
      MUST create the new collection member at the specified position.

6.2.  Examples: Setting the Position of a Collection Member

   >> Request:

   COPY /~user/dav/spec08.html HTTP/1.1
   Host: example.org
   Destination: http://example.org/~slein/dav/spec08.html
   Position: after requirements.html

    >> Response:

   HTTP/1.1 201 Created

   This request resulted in the creation of a new resource at
   example.org/~slein/dav/spec08.html.  The Position header in this
   example caused the server to set its position in the ordering of the
   /~slein/dav/ collection immediately after requirements.html.

   >> Request:

   MOVE /i-d/draft-webdav-prot-08.txt HTTP/1.1
   Host: example.org
   Destination: http://example.org/~user/dav/draft-webdav-prot-08.txt
   Position: first

    >> Response:

   HTTP/1.1 409 Conflict
   Content-Type: text/xml; charset="utf-8"
   Content-Length: xxxx

   <?xml version="1.0" encoding="utf-8" ?>
   <D:error xmlns:D="DAV:">
     <D:collection-must-be-ordered/>
   </D:error>

   In this case, the server returned a 409 (Conflict) status code
   because the /~user/dav/ collection is an unordered collection.
   Consequently, the server was unable to satisfy the Position header.

6.3.  Examples: Renaming a member of an ordered collection

   The following sequence of requests will rename a collection member
   while preserving its position, independently of how the server
   implements the MOVE operation:

   1. PROPFIND collection with depth 1, retrieving the DAV:ordering-type
      property (an interactive client has already likely done this in
      order to display the collection's content).

   2. If the DAV:ordering-type property is present and does not equal
      "dav:unordered" (thus if the collection is ordered), determine the
      current position (such as "first" or "after x") and setup the
      Position header accordingly.

   3. Perform the MOVE operation, optionally supplying the Position
      header computed in the previous step.

7.  Changing a Collection Ordering: ORDERPATCH method

   The ORDERPATCH method is used to change the ordering semantics of a
   collection, to change the order of the collection's members in the
   ordering, or both.

   The server MUST apply the changes in the order they appear in the
   order XML element.  The server MUST either apply all the changes or
   apply none of them.  If any error occurs during processing, all
   executed changes MUST be undone and a proper error result returned.

   If an ORDERPATCH request changes the ordering semantics, but does not
   completely specify the order of the collection members, the server
   MUST assign a position in the ordering to each collection member for
   which a position was not specified.  These server-assigned positions
   MUST follow the last position specified by the client.  The result is
   that all members for which the client specified a position are at the
   beginning of the ordering, followed by any members for which the
   server assigned positions.  Note that the ordering of the server-
   assigned positions is not defined by this document, therefore servers
   can use whatever rule seems reasonable (for instance, alphabetically
   or by creation date).

   If an ORDERPATCH request does not change the ordering semantics, any
   member positions not specified in the request MUST remain unchanged.

   A request to reposition a collection member to the same place in the
   ordering is not an error.

   If an ORDERPATCH request fails, the server state preceding the
   request MUST be restored.

   Additional Marshalling:

      The request body MUST be DAV:orderpatch element.

      <!ELEMENT orderpatch (ordering-type?, order-member*) >

      <!ELEMENT order-member (segment, position) >
      <!ELEMENT position (first | last | before | after)>
      <!ELEMENT segment (#PCDATA)>
      <!ELEMENT first EMPTY >
      <!ELEMENT last EMPTY >
      <!ELEMENT before segment >
      <!ELEMENT after segment >

      PCDATA value: segment, as defined in section 3.3 of [RFC2396].

      The DAV:ordering-type property is modified according to the
      DAV:ordering-type element.

      The ordering of internal member URIs in the collection identified
      by the Request-URI is changed based on instructions in the order-
      member XML elements.  Specifically, in the order that they appear
      in the request.  The order-member XML elements identify the
      internal member URIs whose positions are to be changed, and
      describe their new positions in the ordering.  Each new position
      can be specified as first in the ordering, last in the ordering,
      immediately before some other internal member URI, or immediately
      after some other internal member URI.

      If a response body for a successful request is included, it MUST
      be a DAV:orderpatch-response XML element.  Note that this document
      does not define any elements for the ORDERPATCH response body, but
      the DAV:orderpatch-response element is defined to ensure
      interoperability between future extensions that do define elements
      for the ORDERPATCH response body.

      <!ELEMENT orderpatch-response ANY>

      Since multiple changes can be requested in a single ORDERPATCH
      request, the server MUST return a 207 (Multi-Status) response
      (defined in [RFC2518]), containing DAV:response elements for
      either the request-URI (when the DAV:ordering-type could not be
      modified) or URIs of collection members to be repositioned (when
      an individual positioning request expressed as DAV:order-member
      could not be fulfilled) if any problems are encountered.

   Preconditions:

      (DAV:collection-must-be-ordered): see Section 6.1.

      (DAV:segment-must-identify-member): see Section 6.1.

   Postconditions:

      (DAV:ordering-type-set): if the request body contained a
      DAV:ordering-type element, the request MUST have set the
      DAV:ordering-type property of the collection to the value
      specified in the request.

      (DAV:ordering-modified): if the request body contained DAV:order-
      member elements, the request MUST have set the ordering of
      internal member URIs in the collection identified by the request-
      URI based upon the instructions in the DAV:order-member elements.

7.1.  Example: Changing a Collection Ordering

   Consider an ordered collection /coll-1, with bindings ordered as
   follows:

   three.html
   four.html
   one.html
   two.html

    >> Request:

   ORDERPATCH /coll-1/ HTTP/1.1
   Host: example.org
   Content-Type: text/xml; charset="utf-8"
   Content-Length: xxx

   <?xml version="1.0" ?>
   <d:orderpatch xmlns:d="DAV:">
      <d:ordering-type>
         <d:href>http://example.org/inorder.ord</d:href>
      </d:ordering-type>
      <d:order-member>
         <d:segment>two.html</d:segment>
         <d:position><d:first/></d:position>
      </d:order-member>
      <d:order-member>
         <d:segment>one.html</d:segment>
         <d:position><d:first/></d:position>
      </d:order-member>
      <d:order-member>
         <d:segment>three.html</d:segment>
         <d:position><d:last/></d:position>
      </d:order-member>
      <d:order-member>
         <d:segment>four.html</d:segment>
         <d:position><d:last/></d:position>
      </d:order-member>
   </d:orderpatch>

    >> Response:

   HTTP/1.1 200 OK

   In this example, after the request has been processed, the
   collection's ordering semantics are identified by the URI http://
   example.org/inorder.ord.  The value of the collection's
   DAV:ordering-type property has been set to this URI.  The request
   also contains instructions for changing the positions of the
   collection's internal member URIs in the ordering to comply with the
   new ordering semantics.  As the DAV:order-member elements are
   required to be processed in the order they appear in the request,
   two.html is moved to the beginning of the ordering, and then one.html
   is moved to the beginning of the ordering.  Then three.html is moved
   to the end of the ordering, and finally four.html is moved to the end
   of the ordering.  After the request has been processed, the
   collection's ordering is as follows:

   one.html
   two.html
   three.html
   four.html

7.2.  Example: Failure of an ORDERPATCH Request

   Consider a collection /coll-1/ with members ordered as follows:

   nunavut.map
   nunavut.img
   baffin.map
   baffin.desc
   baffin.img
   iqaluit.map
   nunavut.desc
   iqaluit.img
   iqaluit.desc
    >> Request:

   ORDERPATCH /coll-1/ HTTP/1.1
   Host: www.nunanet.com
   Content-Type: text/xml; charset="utf-8"
   Content-Length: xxx

   <?xml version="1.0" ?>
   <d:orderpatch xmlns:d="DAV:">
      <d:order-member>
         <d:segment>nunavut.desc</d:segment>
         <d:position>
            <d:after>
               <d:segment>nunavut.map</d:segment>
            </d:after>
         </d:position>
      </d:order-member>
      <d:order-member>
         <d:segment>iqaluit.map</d:segment>
         <d:position>
            <d:after>
               <d:segment>pangnirtung.img</d:segment>
            </d:after>
         </d:position>
      </d:order-member>
   </d:orderpatch>

    >> Response:

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

   <?xml version="1.0" ?>
   <d:multistatus xmlns:d="DAV:">
     <d:response>
       <d:href>http://www.nunanet.com/coll-1/iqaluit.map</d:href>
       <d:status>HTTP/1.1 403 Forbidden</d:status>
       <d:responsedescription>
         <d:error><d:segment-must-identify-member/></d:error>
         pangnirtung.img is not a collection member.
       </d:responsedescription>
     </d:response>
   </d:multistatus>
   In this example, the client attempted to position iqaluit.map after a
   URI that is not an internal member of the collection /coll-1/.  The
   server responded to this client error with a 403 (Forbidden) status
   code, indicating the failed precondition DAV:segment-must-identify-
   member.  Because ORDERPATCH is an atomic method, the request to
   reposition nunavut.desc (which would otherwise have succeeded) failed
   as well, but does not need to be expressed in the multistatus
   response body.

8.  Listing the Members of an Ordered Collection

   A PROPFIND request is used to retrieve a listing of the members of an
   ordered collection, just as it is used to retrieve a listing of the
   members of an unordered collection.

   However, when responding to a PROPFIND on an ordered collection, the
   server MUST order the response elements according to the ordering
   defined on the collection.  If a collection is unordered, the client
   cannot depend on the repeatability of the ordering of results from a
   PROPFIND request.

   In a response to a PROPFIND with Depth: infinity, members of
   different collections may be interleaved.  That is, the server is not
   required to do a breadth-first traversal.  The only requirement is
   that the members of any ordered collection appear in the order
   defined for that collection.  Thus, for the hierarchy illustrated in
   the following figure, where collection A is an ordered collection
   with the ordering B C D,

                          A
                         /|\
                        / | \
                       B  C  D
                      /  /|\
                     E  F G H

   it would be acceptable for the server to return response elements in
   the order A B E C F G H D or "A B E C H G F D" as well (if C is
   unordered).  In this response, B, C, and D appear in the correct
   order, separated by members of other collections.  Clients can use a
   series of Depth: 1 PROPFIND requests to avoid the complexity of
   processing Depth: infinity responses based on depth-first traversals.

8.1.  Example: PROPFIND on an Ordered Collection

   Suppose a PROPFIND request is submitted to /MyColl/, which has its
   members ordered as follows.

   /MyColl/
      lakehazen.html
      siorapaluk.html
      iqaluit.html
      newyork.html

    >> Request:

   PROPFIND /MyColl/ HTTP/1.1

   Host: example.org
   Depth: 1
   Content-Type: text/xml; charset="utf-8"
   Content-Length: xxxx

   <?xml version="1.0" ?>
   <D:propfind xmlns:D="DAV:">
     <D:prop xmlns:J="http://example.org/jsprops/">
       <D:ordering-type/>
       <D:resourcetype/>
       <J:latitude/>
    </D:prop>
   </D:propfind>

    >> Response:

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

   <?xml version="1.0" ?>
   <D:multistatus xmlns:D="DAV:"
                  xmlns:J="http://example.org/jsprops/">
      <D:response>
         <D:href>http://example.org/MyColl/</D:href>
         <D:propstat>
            <D:prop>
               <D:ordering-type>
                  <D:href>DAV:custom</D:href>
               </D:ordering-type>
               <D:resourcetype><D:collection/></D:resourcetype>
            </D:prop>
            <D:status>HTTP/1.1 200 OK</D:status>
         </D:propstat>
         <D:propstat>
            <D:prop>
               <J:latitude/>
            </D:prop>
            <D:status>HTTP/1.1 404 Not Found</D:status>
         </D:propstat>
      </D:response>
      <D:response>
         <D:href>http://example.org/MyColl/lakehazen.html</D:href>
         <D:propstat>
            <D:prop>
               <D:resourcetype/>
               <J:latitude>82N</J:latitude>
            </D:prop>
            <D:status>HTTP/1.1 200 OK</D:status>
         </D:propstat>
         <D:propstat>
            <D:prop>
               <D:ordering-type/>
            </D:prop>
            <D:status>HTTP/1.1 404 Not Found</D:status>
         </D:propstat>
      </D:response>
      <D:response>
         <D:href
         >http://example.org/MyColl/siorapaluk.html</D:href>
         <D:propstat>
            <D:prop>
               <D:resourcetype/>
               <J:latitude>78N</J:latitude>
            </D:prop>
            <D:status>HTTP/1.1 200 OK</D:status>
         </D:propstat>
         <D:propstat>
            <D:prop>
               <D:ordering-type/>
            </D:prop>
            <D:status>HTTP/1.1 404 Not Found</D:status>
         </D:propstat>
      </D:response>
      <D:response>
         <D:href>http://example.org/MyColl/iqaluit.html</D:href>
         <D:propstat>
            <D:prop>
               <D:resourcetype/>
               <J:latitude>62N</J:latitude>
            </D:prop>
            <D:status>HTTP/1.1 200 OK</D:status>
         </D:propstat>
         <D:propstat>
            <D:prop>
               <D:ordering-type/>
            </D:prop>
            <D:status>HTTP/1.1 404 Not Found</D:status>
         </D:propstat>
      </D:response>
      <D:response>
         <D:href>http://example.org/MyColl/newyork.html</D:href>
         <D:propstat>
            <D:prop>
               <D:resourcetype/>
               <J:latitude>45N</J:latitude>
            </D:prop>
            <D:status>HTTP/1.1 200 OK</D:status>
         <D:propstat>
            <D:prop>
               <D:ordering-type/>
            </D:prop>
            <D:status>HTTP/1.1 404 Not Found</D:status>
         </D:propstat>
         </D:propstat>
      </D:response>
   </D:multistatus>

   In this example, the server responded with a list of the collection
   members in the order defined for the collection.

9.  Relationship to versioned collections

   The Versioning Extensions to WebDAV [RFC3253] introduce the concept
   of versioned collections, recording both the dead properties and the
   set of internal version-controlled bindings.  This section defines
   how this feature interacts with ordered collections.

   This specification considers both the ordering type (DAV:ordering-
   type property) and the ordering of collection members to be part of
   the state of a collection.  Therefore, both MUST be recorded upon
   CHECKIN or VERSION-CONTROL, and both MUST be restored upon CHECKOUT,
   UNCHECKOUT or UPDATE (where for compatibility with RFC 3253, only the
   ordering of version-controlled members needs to be maintained).

9.1.  Collection Version Properties

9.1.1.  Additional semantics for DAV:version-controlled-binding-set
        (protected)

   For ordered collections, the DAV:version-controlled-binding elements
   MUST appear in the ordering defined for the checked-in ordered
   collection.

9.1.2.  DAV:ordering-type (protected)

   The DAV:ordering-type property records the DAV:ordering-type property
   of the checked-in ordered collection.

9.2.  Additional CHECKIN semantics

   Additional Postconditions:

      (DAV:initialize-version-controlled-bindings-ordered): If the
      request-URL identified a both ordered and version-controlled
      collection, then the child elements of DAV:version-controlled-
      binding-set of the new collection version MUST appear in the
      ordering defined for that collection.

      (DAV:initialize-collection-version-ordering-type): If the
      request-URL identified a both ordered and version-controlled
      collection, then the DAV:ordering-type property of the new
      collection version MUST be a copy of the collection's
      DAV:ordering-type property.

9.3.  Additional CHECKOUT Semantics

   Additional Postconditions:

      (DAV:initialize-version-history-bindings-ordered): If the request
      has been applied to a collection version with a DAV:ordering-type
      other than "DAV:unordered", the bindings in the new working
      collection MUST be ordered according to the collection version's
      DAV:version-controlled-binding-set property.

      (DAV:initialize-ordering-type): If the request has been applied to
      a collection version, the DAV:ordering-type property of the new
      working collection MUST be initialized from the collection
      version's DAV:ordering-type property.

9.4.  Additional UNCHECKOUT, UPDATE, and MERGE Semantics

   Additional Postconditions:

      (DAV:update-version-controlled-collection-members-ordered): If the
      request modified the DAV:checked-in version of a version-
      controlled collection and the DAV:ordering-type for the checked-in
      version is not unordered ("DAV:unordered"), the version-controlled
      members MUST be ordered according to the checked-in version's
      DAV:version-controlled-binding-set property.  The ordering of
      non-version-controlled members is server-defined.

      (DAV:update-version-ordering-type): If the request modified the
      DAV:checked-in version of a version-controlled collection, the
      DAV:ordering-type property MUST be updated from the checked-in
      version's property.

10.  Capability Discovery

   Sections 9.1 and 15 of [RFC2518] describe the use of compliance
   classes with the DAV header in responses to OPTIONS, indicating which
   parts of the Web Distributed Authoring protocols the resource
   supports.  This specification defines an OPTIONAL extension to
   [RFC2518].  It defines a new compliance class, called ordered-
   collections, for use with the DAV header in responses to OPTIONS
   requests.  If a collection resource does support ordering, its
   response to an OPTIONS request may indicate that it does, by listing
   the new ORDERPATCH method as one it supports, and by listing the new
   ordered-collections compliance class in the DAV header.

   When responding to an OPTIONS request, only a collection or a null
   resource can include ordered-collections in the value of the DAV
   header.  By including ordered-collections, the resource indicates
   that its internal member URIs can be ordered.  It implies nothing
   about whether any collections identified by its internal member URIs
   can be ordered.

   Furthermore, RFC 3253 [RFC3253] introduces the live properties
   DAV:supported-method-set (section 3.1.3) and DAV:supported-live-
   property-set (section 3.1.4).  Servers MUST support these properties
   as defined in RFC 3253.

10.1.  Example: Using OPTIONS for the Discovery of Support for
       Ordering

    >> Request:

   OPTIONS /somecollection/ HTTP/1.1
   Host: example.org

    >> Response:

   HTTP/1.1 200 OK
   Allow: OPTIONS, GET, HEAD, POST, PUT, DELETE, TRACE, COPY, MOVE
   Allow: MKCOL, PROPFIND, PROPPATCH, LOCK, UNLOCK, ORDERPATCH
   DAV: 1, 2, ordered-collections

   The DAV header in the response indicates that the resource
   /somecollection/ is level 1 and level 2 compliant, as defined in
   [RFC2518].  In addition, /somecollection/ supports ordering.  The
   Allow header indicates that ORDERPATCH requests can be submitted to
   /somecollection/.

10.2.  Example: Using Live Properties for the Discovery of Ordering

    >> Request:
   PROPFIND /somecollection HTTP/1.1
   Depth: 0
   Content-Type: text/xml; charset="utf-8"
   Content-Length: xxx

   <?xml version="1.0" encoding="UTF-8" ?>
   <propfind xmlns="DAV:">
     <prop>
       <supported-live-property-set/>
       <supported-method-set/>
     </prop>
   </propfind>

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

   <?xml version="1.0" encoding="utf-8" ?>
   <multistatus xmlns="DAV:">
     <response>
       <href>http://example.org/somecollection</href>
       <propstat>
         <prop>
           <supported-live-property-set>
             <supported-live-property>
               <prop><ordering-type/></prop>
             </supported-live-property>
             <!-- ... other live properties omitted for brevity ... -->
           </supported-live-property-set>
           <supported-method-set>
             <supported-method name="COPY" />
             <supported-method name="DELETE" />
             <supported-method name="GET" />
             <supported-method name="HEAD" />
             <supported-method name="LOCK" />
             <supported-method name="MKCOL" />
             <supported-method name="MOVE" />
             <supported-method name="OPTIONS" />
             <supported-method name="ORDERPATCH" />
             <supported-method name="POST" />
             <supported-method name="PROPFIND" />
             <supported-method name="PROPPATCH" />
             <supported-method name="PUT" />
             <supported-method name="TRACE" />
             <supported-method name="UNLOCK" />
           </supported-method-set>
         </prop>
         <status>HTTP/1.1 200 OK</status>
       </propstat>
     </response>
   </multistatus>

   Note that actual responses MUST contain a complete list of supported
   live properties.

11.  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 and the WebDAV
   Distributed Authoring Protocol specification also apply to this
   protocol specification.  In addition, ordered collections introduce a
   new security concern.  This issue is detailed here.

11.1.  Denial of Service and DAV:ordering-type

   There may be some risk of denial of service at sites that are
   advertised in the DAV:ordering-type property of collections.
   However, it is anticipated that widely-deployed applications will use
   hard-coded values for frequently-used ordering semantics rather than
   looking up the semantics at the location specified by DAV:ordering-
   type.  This risk will be further reduced if clients observe the
   recommendation of Section 5.1 that requests not be sent to the URI in
   DAV:ordering-type.

12.  Internationalization Considerations

   This specification follows the practices of [RFC2518] by encoding all
   human-readable content using [XML] and in the treatment of names.
   Consequently, this specification complies with the IETF Character Set
   Policy [RFC2277].

   WebDAV applications MUST support the character set tagging, character
   set encoding, and the language tagging functionality of the XML
   specification.  This constraint ensures that the human-readable
   content of this specification complies with [RFC2277].

   As in [RFC2518], names in this specification fall into three
   categories: names of protocol elements such as methods and headers,
   names of XML elements, and names of properties.  The naming of
   protocol elements follows the precedent of HTTP using English names
   encoded in USASCII for methods and headers.  The names of XML
   elements used in this specification are English names encoded in
   UTF-8.

   For error reporting, [RFC2518] follows the convention of HTTP/1.1
   status codes, including with each status code a short, English
   description of the code (e.g., 423 Locked).  Internationalized
   applications will ignore this message, and display an appropriate
   message in the user's language and character set.

   This specification introduces no new strings that are displayed to
   users as part of normal, error-free operation of the protocol.

   For the rationale of these decisions and advice for application
   implementers, see [RFC2518].

13.  IANA Considerations

   This document uses the namespaces defined by [RFC2518] for properties
   and XML elements.  All other IANA considerations mentioned in
   [RFC2518] also apply to this document.

14.  Intellectual Property Statement

   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 of the 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
   IETF's procedures 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 implementors 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 which may cover technology that may be required to practice
   this standard.  Please address the information to the IETF Executive
   Director.

15.  Contributors

   This document has benefited from significant contributions from Geoff
   Clemm, Jason Crawford, Jim Davis, Chuck Fay and Judith Slein.

16.  Acknowledgements

   This document has benefited from thoughtful discussion by Jim Amsden,
   Steve Carter, Tyson Chihaya, Ken Coar, Ellis Cohen, Bruce Cragun,
   Spencer Dawkins, Mark Day, Rajiv Dulepet, David Durand, Lisa
   Dusseault, Roy Fielding, Yaron Goland, Fred Hitt, Alex Hopmann,
   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 others.

17.  Normative References

   [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
             Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC2277] Alvestrand, H., "IETF Policy on Character Sets and
             Languages", BCP 18, RFC 2277, January 1998.

   [RFC2396] Berners-Lee, T., Fielding, R. and L. Masinter, "Uniform
             Resource Identifiers (URI): Generic Syntax", RFC 2396,
             August 1998.

   [RFC2518] Goland, Y., Whitehead, E., Faizi, A., Carter, S. and D.
             Jensen, "HTTP Extensions for Distributed Authoring --
             WEBDAV", RFC 2518, February 1999.

   [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.

   [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.

   [XML]     Bray, T., Paoli, J., Sperberg-McQueen, C. and E. Maler,
             "Extensible Markup Language (XML) 1.0 (2nd ed)", W3C REC-
             xml, October 2000, <http://www.w3.org/TR/2000/REC-xml-
             20001006>.

Appendix A. Extensions to the WebDAV Document Type Definition

   <!ELEMENT orderpatch (ordering-type?, order-member*) >
   <!ELEMENT order-member (segment, position) >
   <!ELEMENT ordering-type (href) >
   <!ELEMENT position (first | last | before | after)>
   <!ELEMENT first EMPTY >
   <!ELEMENT last EMPTY >
   <!ELEMENT before segment >
   <!ELEMENT after segment >
   <!ELEMENT segment (#PCDATA)>

Index

   C
      Client-Maintained Ordering  4
      Condition Names
         DAV:collection-must-be-ordered (pre)  9
         DAV:initialize-collection-version-ordering-type (post)  20
         DAV:initialize-ordering-type (post)  21
         DAV:initialize-version-controlled-bindings-ordered (post)  20
         DAV:initialize-version-history-bindings-ordered (post)  20
         DAV:ordered-collections-supported (pre)  7
         DAV:ordering-modified (post)  13
         DAV:ordering-type-set (post)  7, 13
         DAV:position-set (post)  9
         DAV:segment-must-identify-member (pre)  9
         DAV:update-version-controlled-collection-members-ordered
             (post) 21
         DAV:update-version-ordering-type (post)  21

   D
      DAV header
         compliance class 'ordered-collections'  21
      DAV:collection-must-be-ordered precondition  9
      DAV:custom ordering type  6
      DAV:initialize-collection-version-ordering-type postcondition  20
      DAV:initialize-ordering-type postcondition  21
      DAV:initialize-version-controlled-bindings-ordered
          postcondition 20
      DAV:initialize-version-history-bindings-ordered postcondition  20
      DAV:ordered-collections-supported precondition  7
      DAV:ordering-modified postcondition  13
      DAV:ordering-type property  6
      DAV:ordering-type-set postcondition  7, 13
      DAV:position-set postcondition  9
      DAV:segment-must-identify-member precondition  9
      DAV:unordered ordering type  6
      DAV:update-version-controlled-collection-members-ordered
          postcondition  21
      DAV:update-version-ordering-type postcondition  21

   H
      Headers
         Ordering-Type  7
         Position  9

   M
      Methods
         ORDERPATCH  11

   O
      Ordered Collection  4
      Ordering Semantics  5
      Ordering-Type header  7
      ORDERPATCH method  11

   P
      Position header  9
      Properties
         DAV:ordering-type  6

   S
      Server-Maintained Ordering  5

   U
      Unordered Collection  4

Authors' Addresses

   Jim Whitehead
   UC Santa Cruz, Dept. of Computer Science
   1156 High Street
   Santa Cruz, CA  95064
   US

   EMail: ejw@cse.ucsc.edu


   Julian F. Reschke, Ed.
   greenbytes GmbH
   Salzmannstrasse 152
   Muenster, NW  48159
   Germany

   Phone: +49 251 2807760
   Fax:   +49 251 2807761
   EMail: julian.reschke@greenbytes.de
   URI:   http://greenbytes.de/tech/webdav/

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   Funding for the RFC Editor function is currently provided by the
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