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Versions: 00 01 02 03 04 05 06 07 08 09 10 11 12 13 RFC 4437

WEBDAV Working Group                                     J. Slein, Xerox
INTERNET DRAFT                             E.J. Whitehead Jr., UC Irvine
<draft-ietf-webdav-redirectref-protocol-02.txt>      J. Davis, CourseNet
                                                      G. Clemm, Rational
                                                         C. Fay, FileNet
                                                        J. Crawford, IBM
                                                       December 17, 1999
Expires June 17, 2000

                        WebDAV Redirect Reference Resources

Status of this Memo

This document is an Internet-Draft and is in full conformance with all
provisions of Section 10 of RFC2026. 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
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     The list of current Internet-Drafts can be accessed at
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Distribution of this document is unlimited. Please send comments to the
Distributed Authoring and Versioning (WebDAV) working group at <w3c-
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Discussions of the WEBDAV working group are archived at URL:
<http://lists.w3.org/Archives/Public/w3c-dist-auth/>.

Abstract

This is one of a pair of specifications that extend the WebDAV
Distributed Authoring Protocol to enable clients to create new access
paths to existing resources.  The two protocol extensions have very
different characteristics that make them useful for different sorts of
applications.

The present specification defines redirect reference resources.  A
redirect reference resource is a resource whose default response is an
HTTP/1.1 302 (Found) status code, redirecting the client to a different
resource, the target resource.  A redirect reference makes it possible
to access the target resource indirectly, through any URI mapped to the
redirect reference resource.  There are no integrity guarantees
associated with redirect reference resources.

The related specification, RFC xxxx, defines bindings, and the BIND
method for creating them.  Creating a new binding to a resource
indirectly creates one or more new URIs mapped to that resource, which
can then be used to access it.  Servers are required to insure the
integrity of any bindings that they allow to be created.


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Table of Contents

1       Notational Conventions........................................3
2       Introduction..................................................3
3       Terminology...................................................4
4       Overview of Redirect Reference Resources......................5
5       Creating a Redirect Reference Resource........................6
5.1     MKRESOURCE....................................................6
5.2     Example: Creating a Redirect Reference Resource with
        MKRESOURCE....................................................7
6       Operations on Redirect Reference Resources....................8
6.1     Example: GET on a Redirect Reference Resource.................9
6.2     Example: PUT on a Redirect Reference Resource with Apply-To-
        Redirect-Ref..................................................9
6.3     Example: PROPPATCH on a Redirect Reference Resource..........10
7       Operations on Collections That Contain Redirect Reference
        Resources....................................................10
7.1     MOVE and DELETE on Collections That Contain Redirect
        References...................................................11
7.2     LOCK on a Collection That Contains Redirect References.......11
7.3     Example: PROPFIND on a Collection with Redirect Reference
        Resources....................................................12
7.4     Example: PROPFIND with Apply-To-Redirect-Ref on a Collection
        with Redirect Reference Resources............................13
7.5     Example: COPY on a Collection That Contains a Redirect
        Reference Resource...........................................15
7.6     Example: LOCK on a Collection That Contains a Redirect
        Reference Resource...........................................15
8       Operations on Targets of Redirect Reference Resources........17
9       Relative URIs in DAV:reftarget...............................17
9.1     Example: Resolving a Relative URI in a MKRESOURCE Request....17
9.2     Example: Resolving a Relative URI in a Multi-Status Response.18
10      Redirect References to Collections...........................19
11      Headers......................................................20
11.1    Redirect-Ref Response Header.................................20
11.2    Apply-To-Redirect-Ref Request Header.........................20
12      Properties...................................................20
12.1    reftarget Property...........................................20
12.2    location Pseudo-Property.....................................20
13      XML Elements.................................................21
13.1    redirectref XML Element......................................21
14      Extensions to the DAV:response XML Element for Multi-Status
        Responses....................................................21
15      Capability Discovery.........................................21
15.1    Example: Discovery of Support for Redirect Reference
        Resources....................................................22
16      Security Considerations......................................22
16.1    Privacy Concerns.............................................22
16.2    Redirect Loops...............................................22
16.3    Redirect Reference Resources and Denial of Service...........23
16.4    Private Locations May Be Revealed............................23
17      Internationalization Considerations..........................23
18      IANA Considerations..........................................24
19      Copyright....................................................24
20      Intellectual Property........................................24

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21      Acknowledgements.............................................24
22      References...................................................24
23      Authors' Addresses...........................................25
24      Appendices...................................................25
24.1    Appendix 1: Extensions to the WebDAV Document Type
        Definition...................................................25

1 Notational Conventions

Since this document describes a set of extensions to the WebDAV
Distributed Authoring Protocol [WebDAV], 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].
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].

2 Introduction

This is one of a pair of specifications that extend 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 redirect reference resources defined here provide a mechanism for
creating alternative access paths to existing resources.  A redirect

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reference resource is a resource in one collection whose purpose is to
forward requests to another resource (its target), usually in a
different collection.  In this way, it allows clients to submit requests
to the target resource from another collection.  It redirects most
requests to the target resource using the HTTP 302 (Found) status code,
thereby providing a form of mediated access to the target resource.

The companion specification, RFC xxxx, defines the BIND method, a
different mechanism for allowing clients to create alternative access
paths to existing WebDAV-compliant resources. The BIND method lets
clients associate a new URI with an existing WebDAV resource.  This URI
can then be used to submit requests to the resource.  Since URIs of
WebDAV-compliant 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.

Redirect references and bindings have very different characteristics:

A redirect reference is a resource, and so can have properties and a
body of its own.  Properties of a redirect reference resource can
contain such information as who created the reference, when, and why.
Since redirect reference resources are implemented using HTTP 302
responses, it generally takes two round trips to submit a request to the
intended resource.  Servers are not required to enforce the integrity of
redirect references.  Redirect references work equally well for local
resources and for resources that reside on a different server from the
reference.

By contrast, 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.

The remainder of this document is structured as follows: Section 3
defines terms that will be used throughout the specification.  Section 4
provides an overview of redirect reference resources.  Section 5
discusses how to create a redirect reference resource.  Section 6
defines the semantics of existing methods when applied to redirect
reference resources, and Section 7 discusses their semantics when
applied to collections that contain redirect reference resources.
Sections 8 through 10 discuss several other issues raised by the
existence of redirect reference resources.  Sections 11 through 14
define the new headers, properties, and XML elements required to support
redirect reference resources.  Section 15 discusses capability
discovery.  Sections 16 through 18 present the security,
internationalization, and IANA concerns raised by this specification.
The remaining sections provide a variety of supporting information.

3 Terminology


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The terminology used here follows and extends that in the WebDAV
Distributed Authoring Protocol specification [WebDAV]. Definitions of
the terms resource, Uniform Resource Identifier (URI), and Uniform
Resource Locator (URL) are provided in [URI].

Reference Resource
     A resource whose purpose is to forward requests to another
     resource.  Reference resources are an alternative mechanism to
     bindings (defined in [B]) for allowing clients to create multiple
     URIs that can be used to submit requests to the same resource.

Redirect Reference Resource
     A resource that allows clients to forward requests to another
     resource using the HTTP 1.1 302 (Found) response mechanism.  The
     client is aware that this type of reference resource is mediating
     between it and the target resource.

Direct Reference Resource
     Direct Reference Resources are out of scope for this
     specification, but are defined here for contrast with redirect
     reference resources.   A direct reference resource automatically
     forwards requests to another resource, in a way that is
     transparent to the client.

Non-Reference Resource
     A resource that is not a reference to another resource.

Target Resource
     The resource to which requests are forwarded by a reference
     resource.

4 Overview of Redirect Reference Resources

For all operations submitted to a redirect reference resource, the
default response is a 302 (Found), accompanied by the Redirect-Ref
header (defined in Section 11.1 below) and the Location header set to
the URI of the target resource.  With this information, the client can
resubmit the request to the URI of the target resource.

A redirect reference resource never automatically forwards requests to
its target resource.  It is this characteristic that distinguishes
redirect reference resource from direct reference resources and from
bindings.  It is also what insures that redirect reference resources
will be simple to implement and that cross-server references will be
possible.  If the redirect reference resource were required to forward
requests automatically, the server would need proxy capabilities in
order to support cross-server references.

If the client is aware that it is operating on a redirect reference
resource, it can resolve the reference by retrieving the reference
resource's DAV:reftarget property (defined in Section 12.1 below), whose
value contains the URI of the target resource.  It can then submit
requests to the target resource.

A redirect reference resource is a new type of resource. To distinguish

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redirect reference resources from non-reference resources, a new value
of the DAV:resourcetype property (defined in [WebDAV]), DAV:redirectref,
is defined in Section 13.1 below.

Since a redirect reference resource is a resource, it can have its own
properties and body, and methods can be applied to the reference
resource as well as to its target resource.  The Apply-To-Redirect-Ref
request header (defined in Section 11.2 below) is provided so that
referencing-aware clients can control whether an operation is applied to
the redirect reference resource or to its target resource.  The Apply-
To-Redirect-Ref header can be used with most requests to redirect
reference resources.  This header is particularly useful with PROPFIND,
to retrieve the reference resource's own properties.

5 Creating a Redirect Reference Resource

The MKRESOURCE method is used to create new redirect reference
resources.  As defined in Section 5.1, MKRESOURCE can be used to create
a resource of any type other than standard data containers and
collections.  In order to create a redirect reference resource using
MKRESOURCE, the values of two properties must be set in the body of the
MKRESOURCE request.  The value of DAV:resourcetype MUST be set to
DAV:redirectref, a new value of DAV:resourcetype defined in Section
13.1.  The value of DAV:reftarget MUST be set to the URI of the target
resource.

Used in this way, the MKRESOURCE method creates a redirect reference
resource whose target is identified by the DAV:reftarget property.  It
creates a new binding between the new redirect reference resource and
the last path segment of the Request-URI.  The new binding is added to
its parent collection, identified by the Request-URI minus its trailing
slash (if present) and final segment.

5.1 MKRESOURCE

The MKRESOURCE method requests the creation of a resource and
initialization of its properties.  It allows resources other than
standard data containers and collections to be created and their
properties initialized in one atomic operation.

Preconditions:

A resource MUST NOT exist at the Request-URI.

Request Marshalling:

The location of the new resource to be created is specified by the
Request-URI.

The request body of the MKRESOURCE method MUST consist of the
DAV:propertyupdate XML element defined in Section 12.13 of [WebDAV].

Postconditions:

If the response status code is 201, a new resource exists at the

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Request-URI.

The body of the new resource is empty.

The properties of the new resource are as specified by the
DAV:propertyupdate request body, using PROPPATCH semantics. If the
DAV:propertyupdate does not specify a DAV:resourcetype, the resource
will be a standard data container.

If the response status code is not 201, then a new resource is not
created at the Request-URI, and any existing resource at the Request-URI
is unaffected.

Response Marshalling:

Responses from a MKRESOURCE request SHOULD NOT be cached, as MKRESOURCE
has non-idempotent semantics.

The following status codes can be expected in responses to MKRESOURCE:

201 (Created): The new resource was successfully created.

207 (Multi-Status): This response is generated if an error was
encountered while initializing the properties of the resource, in which
case the response is as defined in Section 8.2.1 of [WebDAV].

403 (Forbidden): The server does not allow the creation of the requested
resource type at the requested location, or the parent collection of the
Request-URI exists but cannot accept members.

409 (Conflict): A resource cannot be created at the Request-URI because
the parent collection for the resource does not exist, or because there
is already a resource at that request-URL.

423 (Locked): The Request-URI is locked, and the lock token was not
passed with the request.

507 (Insufficient Storage): The server does not have sufficient space to
record the state of the resource.

5.2 Example: Creating a Redirect Reference Resource with MKRESOURCE

>> Request:

MKRESOURCE /~whitehead/dav/spec08.ref HTTP/1.1
Host: www.ics.uci.edu
Content-Type: text/xml; charset="utf-8"
Content-Length: xxx

<?xml version="1.0" encoding="utf-8" ?>
<D:propertyupdate xmlns:D="DAV:">
   <D:set>
      <D:prop>
         <D:resourcetype><D:redirectref/></D:resourcetype>
         <D:reftarget>

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            <D:href>/i-d/draft-webdav-protocol-08.txt</D:href>
         </D:reftarget>
      </D:prop>
   </D:set>
</D:propertyupdate>

>> Response:

HTTP/1.1 201 Created

This request resulted in the creation of a new redirect reference
resource at www.ics.uci.edu/~whitehead/dav/spec08.ref, which points to
the resource identified by the DAV:reftarget property. In this example,
the target resource is identified by the URI http://www.ics.uci.edu/i-
d/draft-webdav-protocol-08.txt. The redirect reference resource's
DAV:resourcetype property is set to DAV:redirectref.

6 Operations on Redirect Reference Resources

Although non-referencing-aware clients cannot create reference
resources, they should be able to submit requests through the reference
resources created by reference-aware WebDAV clients.  They should be
able to follow any references to their targets.  To make this possible,
a server that receives any request made via a redirect reference
resource MUST return a 302 (Found) status code, unless the request
includes an Apply-To-Redirect-Ref header. The client and server MUST
follow [HTTP] Section 10.3.3 "302 Found," but with these additional
rules:

o The Location response header MUST contain an absolute URI that
  identifies the target of the reference resource.

o The response MUST include the Redirect-Ref header.  This header
  allows reference-aware WebDAV clients to recognize the resource as a
  reference resource and understand the reason for the redirection.

A reference-aware WebDAV client can act on this response in one of two
ways.  It can, like a non-referencing client, resubmit the request to
the URI in the Location header in order to operate on the target
resource.  Alternatively, it can resubmit the request to the URI of the
redirect reference resource with the Apply-To-Redirect-Ref header in
order to operate on the reference resource itself.  If the Apply-To-
Redirect-Ref header is present, the request MUST be applied to the
reference resource itself, and a 302 response MUST NOT be returned.

A reference-aware client may know before submitting its request that the
Request-URI identifies a redirect reference resource. In this case, if
the client wants to apply the method to the reference resource, it can
save the round trip caused by the 302 response by using an Apply-To-
Redirect-Ref header in its initial request to the URI.

A few methods need additional explanation:

The Apply-To-Redirect-Ref header can be used with GET or HEAD to
retrieve the entity headers of a redirect reference resource.  When

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Apply-To-Redirect-Ref is used with GET or HEAD, the Redirect-Ref entity
header MUST be returned, along with all HTTP headers that make sense for
reference resources (for example, Cache-Control, Age, ETag, Expires, and
Last-Modified).

A redirect reference resource MAY have a body, though none is defined
for it in this specification.  The PUT method can be used, with Apply-
To-Redirect-Ref, to create or replace the body of a redirect reference
resource.

Since MKCOL and MKRESOURCE fail when applied to existing resources, if
the client attempts to resubmit the request to the target resource, the
request MUST fail (unless the reference resource is a dangling
reference).  Similarly, if the client attempts to resubmit the request
to the reference resource with an Apply-To-Redirect-Ref header, the
request MUST fail.

Since ORDERPATCH applies only to collections, an ORDERPATCH request with
an Apply-To-Redirect-Ref header on a redirect reference resource MUST
fail.

6.1 Example: GET on a Redirect Reference Resource

>> Request:

GET /bar.html HTTP/1.1
Host: www.foo.com

>> Response:

HTTP/1.1 302 Found
Location: http://www.svr.com/Internet/xxspec08.html
Redirect-Ref:

Since /bar.html is a redirect reference resource and the Apply-To-
Redirect-Ref header is not included in the request, the response is a
302 (Found).  The Redirect-Ref header informs a reference-aware client
that this is not an ordinary HTTP 1.1 redirect, but is a redirect
reference resource.  The URI of the target resource is provided in the
Location header so that the client can resubmit the request to the
target resource.

6.2 Example: PUT on a Redirect Reference Resource with Apply-To-
Redirect-Ref

>> Request:

PUT /bar.html HTTP/1.1
Host: www.foo.com
Apply-To-Redirect-Ref:
Content-Type: text/xml; charset="utf-8"
Content-Length: xxxx

. . . some content . . .


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>> Response:

HTTP/1.1 200 OK

Although /bar.html is a redirect reference resource, the presence of the
Apply-To-Redirect-Ref header prevents a 302 response, and instead causes
the request to be applied to the reference resource.  The result in this
case is that the reference resource is replaced by a non-reference
resource having the content submitted with the request.

6.3 Example: PROPPATCH on a Redirect Reference Resource

Request:

PROPPATCH /bar.html HTTP/1.1
Host: www.foo.com
Content-Type: text/xml; charset="utf-8"
Content-Length: xxxx

   <?xml version="1.0" encoding="utf-8" ?>
   <D:propertyupdate xmlns:D="DAV:"
   xmlns:Z="http://www.w3.com/standards/z39.50/">
     <D:set>
          <D:prop>
               <Z:authors>
                    <Z:Author>Jim Whitehead</Z:Author>
                    <Z:Author>Roy Fielding</Z:Author>
               </Z:authors>
          </D:prop>
     </D:set>
     <D:remove>
          <D:prop><Z:Copyright-Owner/></D:prop>
     </D:remove>
   </D:propertyupdate>

Response:

HTTP/1.1 302 Found
Location: http://www.svr.com/Internet/xxspec08.html
Redirect-Ref:

Since /bar.html is a redirect reference resource and the Apply-To-
Redirect-Ref header is not included in the request, the response is a
302 (Found).  The Redirect-Ref header informs a reference-aware client
that this is not an ordinary HTTP 1.1 redirect, but is a redirect
reference resource.  The URI of the target resource is provided in the
Location header so that the client can resubmit the request to the
target resource.

7 Operations on Collections That Contain Redirect Reference Resources

A URI of a redirect reference resource can be an internal member URI of
a collection just as the URI of a non-reference resource can.  Any
operation on a collection with Depth: 1 or Depth: infinity applies to
redirect reference resources in the collection just as it applies to any

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other resources in the collection.  The methods that can accept a Depth
header are PROPFIND, COPY, MOVE, DELETE, and LOCK.

Consistent with the rules in Section 6, the response for each redirect
reference encountered while processing a collection MUST be a 302
(Found) unless a Apply-To-Redirect-Ref header is included with the
request.  The overall response will therefore be a 207 (Multi-Status).
Since a Location header and Redirect-Ref header cannot be returned for
each redirect reference encountered, the same information is provided
using properties in the response elements for those resources.  The
DAV:location pseudo-property and the DAV:resourcetype property MUST be
included with the 302 status code.  This necessitates an extension to
the syntax of the DAV:response element that was defined in [WebDAV].
The extension is defined in Section 14 below.

A referencing-aware client can tell from the DAV:resourcetype property
that the collection contains a redirect reference resource.  The
DAV:location pseudo-property contains the absolute URI of the target
resource.  A referencing-aware client can either use the URI value of
the DAV:location pseudo-property to resubmit its request to the target
resource, or it can submit the request to the redirect reference
resource with Apply-To-Redirect-Ref.

It is recommended that future editors of [WebDAV] define the
DAV:location pseudo-property in [WebDAV], so that non-referencing
clients will also be able to use the response to operate on the target
resource.  (This will also enable clients to operate on traditional
HTTP/1.1 302 responses in Multi-Status responses.) Until then, non-
referencing clients will not be able to process 302 responses from
redirect reference resources encountered while processing a collection.

The Apply-To-Redirect-Ref header (defined in Section 11.2) MAY be used
with any request on a collection.  If present, it will be applied to all
redirect reference resources encountered while processing the
collection.

7.1 MOVE and DELETE on Collections That Contain Redirect References

DELETE removes the binding that corresponds to the Request-URI.  MOVE
removes that binding and creates a new binding to the same resource.  In
cases where DELETE and MOVE are applied to a collection, these
operations affect all the descendents of the collection, but they do so
indirectly.  There is no need to visit each descendent in order to
process the request.  Consequently, even if there are redirect reference
resources in a tree that is being deleted or moved, there will be no 302
responses from the redirect reference resources.

7.2 LOCK on a Collection That Contains Redirect References

LOCK poses special problems because it is atomic. An attempt to lock
(with Depth: infinity) a collection that contains redirect references
will always fail.  The Multi-Status response will contain a 302 response
for each redirect reference.

Reference-aware clients can lock the collection by using Apply-To-

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Redirect-Ref, and, if desired, lock the targets of the redirect
references individually.

Non-referencing clients must resort to locking each resource
individually.

7.3 Example: PROPFIND on a Collection with Redirect Reference Resources

Suppose a PROPFIND request with Depth = infinity is submitted to the
following collection, with the members shown here:

http://www.svr.com/MyCollection/
     (non-reference resource) diary.html
     (redirect reference resource) nunavut

>> Request:

PROPFIND /MyCollection/ HTTP/1.1
Host: www.svr.com
Depth: infinity
Content-Type: text/xml
Content-Length: xxxx

<?xml version="1.0" ?>
<D:propfind xmlns:D="DAV: ">
   <D:prop xmlns:J="http://www.svr.com/jsprops/">
      <D:resourcetype/>
      <J:keywords/>
   </D:prop>
</D:propfind>

>> Response:

HTTP/1.1 207 Multi-Status
Content-Type: text/xml
Content-Length: xxxx

<?xml version="1.0" ?>
<D:multistatus xmlns:D="DAV:"
               xmlns:J="http://www.svr.com/jsprops/">
   <D:response>
      <D:href>http://www.svr.com/MyCollection/</D:href>
      <D:propstat>
         <D:prop>
            <D:resourcetype><D:collection/></D:resourcetype>
            <J:keywords>diary, interests, hobbies</J:keywords>
         </D:prop>
         <D:status>HTTP/1.1 200 OK</D:status>
      </D:propstat>
   </D:response>
   <D:response>
      <D:href>http://www.svr.com/MyCollection/diary.html</D:href>
      <D:propstat>
         <D:prop>
            <D:resourcetype/>

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            <J:keywords>diary, travel, family, history</J:keywords>
         </D:prop>
         <D:status>HTTP/1.1 200 OK</D:status>
      </D:propstat>
   </D:response>
   <D:response>
      <D:href>http://www.svr.com/MyCollection/nunavut</D:href>
      <D:status>HTTP/1.1 302 Found</D:status>
      <D:prop>
         <D:location>
            <D:href>http://www.inac.gc.ca/art/inuit/</D:href>
         </D:location>
         <D:resourcetype><D:redirectref/></D:resourcetype>
      </D:prop>
   </D:response>
</D:multistatus>

In this example the Depth header is set to infinity, and the Apply-To-
Redirect-Ref header is not used.  The collection contains one URI that
identifies a redirect reference resource.  The response element for the
redirect reference resource has a status of 302 (Found), and includes a
DAV:prop element with the DAV:location pseudo-property and the
DAV:resourcetype property to allow clients to retrieve the properties of
its target resource.  (The response element for the redirect reference
resource does not include the requested properties.  The client can
submit another PROPFIND request to the URI in the DAV:location pseudo-
property to retrieve those properties.)

7.4 Example: PROPFIND with Apply-To-Redirect-Ref on a Collection with
Redirect Reference Resources

Suppose a PROPFIND request with Apply-To-Redirect-Ref and Depth =
infinity is submitted to the following collection, with the members
shown here:

/MyCollection/
     (non-reference resource) diary.html
     (redirect reference resource) nunavut

>> Request:

PROPFIND /MyCollection/ HTTP/1.1
Host: www.svr.com
Depth: infinity
Apply-To-Redirect-Ref:
Content-Type: text/xml
Content-Length: xxxx

<?xml version="1.0" ?>
<D:propfind xmlns:D="DAV:">
   <D:prop>
      <D:resourcetype/>
      <D:reftarget/>
   </D:prop>
</D:propfind>

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>> Response:

HTTP/1.1 207 Multi-Status
Content-Type: text/xml
Content-Length: xxxx

<?xml version="1.0" ?>
<D:multistatus xmlns:D="DAV:">
   <D:response>
      <D:href>http://www.svr.com/MyCollection/</D:href>
      <D:propstat>
         <D:prop>
            <D:resourcetype><D:collection/></D:resourcetype>
         </D:prop>
         <D:status>HTTP/1.1 200 OK</D:status>
      </D:propstat>
      <D:propstat>
         <D:prop> <D:reftarget/> </D:prop>
         <D:status>HTTP/1.1 404 Not Found</D:status>
      </D:propstat>
   </D:response>
   <D:response>
      <D:href>http://www.svr.com/MyCollection/diary.html</D:href>
      <D:propstat>
         <D:prop>
            <D:resourcetype/>
         </D:prop>
         <D:status>HTTP/1.1 200 OK</D:status>
      </D:propstat>
      <D:propstat>
         <D:prop> <D:reftarget/> </D:prop>
         <D:status>HTTP/1.1 404 Not Found</D:status>
      </D:propstat>
   </D:response>
   <D:response>
      <D:href>http://www.svr.com/MyCollection/nunavut</D:href>
      <D:propstat>
         <D:prop>
            <D:resourcetype><D:redirectref/></D:resourcetype>
            <D:reftarget>
               <D:href>http://www.inac.gc.ca/art/inuit/</D:href>
            </D:reftarget>
         </D:prop>
         <D:status>HTTP/1.1 200 OK</D:status>
      </D:propstat>
   </D:response>
</D:multistatus>

Since the Apply-To-Redirect-Ref header is present, the response shows
the properties of the redirect reference resource in the collection
rather than the properties of its target. The Apply-To-Redirect-Ref
header also prevents a 302 response from being returned for the redirect
reference resource.


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7.5 Example: COPY on a Collection That Contains a Redirect Reference
Resource

Suppose a COPY request is submitted to the following collection, with
the members shown:

/MyCollection/
     (non-reference resource) diary.html
     (redirect reference resource) nunavut with target
                                /Someplace/nunavut.map

>> Request:

COPY /MyCollection/ HTTP/1.1
Host: www.svr.com
Depth: infinity
Destination: http://www.svr.com/OtherCollection/

>> 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.svr.com/MyCollection/nunavut</D:href>
      <D:status>HTTP/1.1 302 Found</D:status>
         <D:prop>
            <D:location>
               <D:href>
                  http://www.svr.com//Someplace/nunavut.map
               </D:href>
            </D:location>
            <D:resourcetype><D:redirectref/></D:resourcetype>
         </D:prop>
   </D:response>
</D:multistatus>

In this case, since /MyCollection/nunavut is a redirect reference
resource, the COPY operation was only a partial success.  The redirect
reference resource was not copied, but a 302 response was returned for
it.  So the resulting collection is as follows:

/OtherCollection/
      (non-reference resource) diary.html

7.6 Example: LOCK on a Collection That Contains a Redirect Reference
Resource

Suppose a LOCK request is submitted to the following collection, with
the members shown:

/MyCollection/

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     (non-reference resource) diary.html
     (redirect reference resource) nunavut

>> Request:

LOCK /MyCollection/ HTTP/1.1
Host: www.svr.com
Content-Type: text/xml
Content-Length: nnnn
Authorizaton: Digest username="jas",
   realm=jas@webdav.sb.aol.com, nonce=". . . ",
   uri="/MyCollection/tuva",
   response=". . . ", opaque=". . . "

<?xml version="1.0" ?>
<D:lockinfo xmlns:D="DAV:">
   <D:lockscope><D:exclusive/></D:lockscope>
   <D:locktype><D:write/></D:locktype>
   <D:owner>
      <D:href>http://www.svr.com/~jas/contact.html</D:href>
   </D:owner>
</D:lockinfo>

>> Response:

HTTP/1.1 207 Multi-Status
Content-Type: text/xml
Content-Length: nnnn

<?xml version="1.0" ?>
<D:multistatus xmlns:D="Dav:">
   <D:response>
      <D:href>http://www.svr.com/MyCollection/</D:href>
      <D:propstat>
         <D:prop><D:lockdiscovery/></D:prop>
         <D:status>HTTP/1.1 424 Failed Dependency</D:status>
      </D:propstat>
   </D:response>
   <D:response>
      <D:href>http://www.svr.com/MyCollection/diary.html</D:href>
      <D:status>HTTP/1.1 424 Failed Dependency</D:status>
   </D:response>
   <D:response>
      <D:href>http://www.svr.com/MyCollection/nunavut</D:href>
      <D:status>HTTP/1.1 302 Found</D:status>
      <D:prop>
         <D:location>
            <D:href>http://www.inac.gc.ca/art/inuit/</D:href>
         </D:location>
         <D:resourcetype><D:redirectref/></D:resourcetype>
      </D:prop>
   </D:response>
</D:multistatus>

The server returns a 302 response code for the redirect reference

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resource in the collection.  Consequently, neither the collection nor
any of the resources identified by its internal member URIs were locked.
A referencing-aware client can submit a separate LOCK request to the URI
in the DAV:location pseudo-property returned for the redirect reference
resource, and can resubmit the LOCK request with the Apply-To-Redirect-
Ref header to the collection.  At that point both the reference resource
and its target resource will be locked (as well as the collection and
all the resources identified by its other members).

8 Operations on Targets of Redirect Reference Resources

Operations on targets of redirect reference resources have no effect on
the reference resource.

9 Relative URIs in DAV:reftarget

The URI in the href in a DAV:reftarget property MAY be a relative URI.
In this case, the base URI to be used for resolving the relative URI to
absolute form is the URI used in the HTTP message to identify the
redirect reference resource to which the DAV:reftarget property belongs.

When DAV:reftarget occurs in the body of a MKRESOURCE request, the base
URI is constructed as follows: Its scheme component is "http", its
authority component is the value of the Host header in the request, and
its path component is the Request-URI in the request.  See Section 5 of
[URI] for a discussion of relative URI references and how to resolve
them.

When DAV:reftarget appears in the context of a Multi-Status response, it
is in a DAV:response element that contains a single DAV:href element.
The value of this DAV:href element serves as the base URI for resolving
a relative URI in DAV:reftarget.  The value of DAV:href may itself be
relative, in which case it must be resolved first in order to serve as
the base URI for the relative URI in DAV:reftarget.  If the DAV:href
element is relative, its base URI is constructed from the scheme
component "http", the value of the Host header in the request, and the
request-URI.

9.1 Example: Resolving a Relative URI in a MKRESOURCE Request

>> Request:

MKRESOURCE /north/inuvik HTTP/1.1
Host: www.somehost.edu
Content-Type: text/xml; charset="utf-8"
Content-Length: xxx

<?xml version="1.0" encoding="utf-8" ?>
<D:propertyupdate xmlns:D="DAV:">
   <D:set>
      <D:prop>
         <D:resourcetype><D:redirectref/></D:resourcetype>
         <D:reftarget>
            <D:href>mapcollection/inuvik.gif</D:href>
         </D:reftarget>

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      </D:prop>
   </D:set>
</D:propertyupdate>

>> Response:

HTTP/1.1 201 Created

In this example, the base URI is http://www.somehost.edu/north/inuvik.
Then, following the rules in [URI] Section 5, the relative URI in
DAV:reftarget resolves to the absolute URI
http://www.somehost.edu/north/mapcollection/inuvik.gif.

9.2 Example: Resolving a Relative URI in a Multi-Status Response

>> Request:

PROPFIND /geog/ HTTP/1.1
Host: www.xxsvr.com
Apply-To-Redirect-Ref:
Depth: 1
Content-Type: text/xml
Content-Length: nnn

<?xml version="1.0" ?>
<D:propfind xmlns:D="DAV:">
   <D:prop>
      <D:resourcetype/>
      <D:reftarget/>
   </D:prop>
</D:propfind>

>> Response:

HTTP/1.1 207 Multi-Status
Content-Type: text/xml
Content-Length: nnn

<?xml version="1/0" ?>
<D:multistatus xmlns:D="DAV:">
   <D:response>
      <D:href>/geog/</D:href>
      <D:propstat>
         <D:prop>
            <D:resourcetype><D:collection/></D:resourcetype>
         </D:prop>
         <D:status>HTTP/1.1 200 OK</D:status>
     </D:propstat>
     <D:propstat>
         <D:prop><D:reftarget/></D:prop>
         <D:status>HTTP/1.1 404 Not Found</D:status>
     </D:propstat>
   </D:response>
   <D:response>
      <D:href>/geog/stats.html</D:href>

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      <D:propstat>
         <D:prop>
            <D:resourcetype><D:redirectref/></D:resourcetype>
            <D:reftarget><D:href>statistics/population/1997.html
                 </D:href></D:reftarget>
         </D:prop>
         <D:status>HTTP/1.1 200 OK</D:status>
      </D:propstat>
   </D:response>
</D:multistatus>

In this example, the relative URI statistics/population/1997.html is
returned as the value of reftarget for the reference resource identified
by href /geog/stats.html.  The href is itself a relative URI, which
resolves to http://www.xxsrv.com/geog/stats.html.  This is the base URI
for resolving the relative URI in reftarget.  The absolute URI of
reftarget is http://www.xxsrv.com/geog/statistics/population/1997.html.

10 Redirect References to Collections

In a Request-URI /segment1/segment2/segment3, any of the three segments
may identify a redirect reference resource.  (See [URI], Section 3.3,
for definitions of "path" and "segment".)  If any segment in a Request-
URI identifies a redirect reference resource, the response is a 302.
The value of the Location header in the 302 response is as follows:

The leftmost path segment of the request-URI that identifies a redirect
reference resource, together with all path segments and separators to
the left of it, is replaced by the value of the redirect reference
resource's DAV:reftarget property (resolved to an absolute URI).  The
remainder of the request-URI is concatenated to this path.

Note: If the DAV:reftarget property ends with a "/" and the remainder of
the Request-URI is non-empty (and therefore must begin with a "/"), the
final "/" in the DAV:reftarget property is dropped before the remainder
of the Request-URI is appended.

Consider Request-URI /x/y/z.html.  Suppose that /x/ is a redirect
reference resource whose target resource is collection /a/, which
contains redirect reference resource y whose target resource is
collection /b/, which contains redirect reference resource z.html whose
target resource is /c/d.html.

/x/ -----> /a/
           /a/y/ -----> /b/
                        /b/z.html -----> /c/d.html

In this case the client must follow up three separate 302 responses
before finally reaching the target resource.  The server responds to the
initial request with a 302 with Location: /a/y/z.html, and the client
resubmits the request to /a/y/z.html.  The server responds to this
request with a 302 with Location: /b/z.html, and the client resubmits
the request to /b/z.html.  The server responds to this request with a
302 with Location: /c/d.html, and the client resubmits the request to
/c/d.html.  This final request succeeds.

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11 Headers

11.1 Redirect-Ref Response Header

Redirect-Ref = "Redirect-Ref:"

The Redirect-Ref header is used in all 302 responses from redirect
reference resources.  Its presence informs reference-aware clients that
the response is not a plain HTTP/1.1 redirect, but is a response from a
redirect reference resource.

11.2 Apply-To-Redirect-Ref Request Header

Apply-To-Redirect-Ref = "Apply-To-Redirect-Ref" ":"

The optional Apply-To-Redirect-Ref header can be used on any request to
a redirect reference resource.  When it is used, the request MUST be
applied to the reference resource itself, and a 302 response MUST NOT be
returned.

If the Apply-To-Redirect-Ref header is used on a request to any other
sort of resource besides a redirect reference resource, the server
SHOULD ignore it.

12 Properties

12.1 reftarget Property

Name:       reftarget
Namespace:  DAV:
Purpose:    A property of redirect reference resources that provides an
            efficient way for clients to discover the URI of the target
            resource.  This is a read-only property after its initial
            creation. Its value can only be set in a MKRESOURCE request.
Value:      href containing the URI of the target resource.  This value
            MAY be a relative URI.  The reftarget property can occur in
            the entity bodies of MKRESOURCE requests and of responses to
            PROPFIND requests.

<!ELEMENT reftarget href >

12.2 location Pseudo-Property

Name:       location
Namespace:  DAV:
Purpose:    For use with 302 (Found) response codes in Multi-Status
            responses.  It contains the absolute URI of the temporary
            location of the resource.  In the context of redirect
            reference resources, this value is the absolute URI of the
            target resource.  It is analogous to the Location header in
            HTTP 302 responses defined in [HTTP] Section 10.3.3 "302
            Found."  Including the location pseudo-property in a Multi-
            Status response requires an extension to the syntax of the
            DAV:response element defined in [WebDAV], which is defined

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            in Section 14 below.  This pseudo-property is not expected
            to be stored on the reference resource. It is modeled as a
            property only so that it can be returned inside a DAV:prop
            element in a Multi-Status response.
Value:      href containing the absolute URI of the target resource.

<!ELEMENT location href >

13 XML Elements

13.1 redirectref XML Element

Name:       redirectref
Namespace:  DAV:
Purpose:    Used as the value of the DAV:resourcetype property to
            specify that the resource type is a redirect reference
            resource.

<!ELEMENT redirectref EMPTY >

14 Extensions to the DAV:response XML Element for Multi-Status Responses

As described in Section 7, the DAV:location pseudo-property and the
DAV:resourcetype property may be returned in the DAV:response element of
a 207 Multi-Status response, to allow clients to resubmit their requests
to the target resource of a redirect reference resource.

Whenever these properties are included in a Multi-Status response, they
are placed in a DAV:prop element associated with the href to which they
apply.  This structure provides a framework for future extensions by
other standards that may need to include additional properties in their
responses.

Consequently, the definition of the DAV:response XML element changes to
the following:

<!ELEMENT response (href, ((href*, status, prop?) | (propstat+)),
responsedescription?) >

15 Capability Discovery

Sections 9.1 and 15 of [WebDAV] describe the use of compliance classes
with the DAV header in responses to OPTIONS, to indicate which parts of
the WebDAV Distributed Authoring protocols the resource supports. This
specification defines an OPTIONAL extension to [WebDAV].  It defines a
new compliance class, called redirectrefs, for use with the DAV header
in responses to OPTIONS requests.  If a resource does support redirect
references, its response to an OPTIONS request may indicate that it
does, by listing the new redirectrefs compliance class in the DAV
headerand by listing the MKRESOURCE method as one it supports.

When responding to an OPTIONS request, any type of resource can include
redirectrefs in the value of the DAV header.  Doing so indicates that
the server permits a redirect reference resource at the request URI.


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15.1 Example: Discovery of Support for Redirect Reference Resources

>> Request:

OPTIONS /somecollection/someresource HTTP/1.1
HOST: somehost.org

>> Response:

HTTP/1.1 200 OK
Date: Tue, 20 Jan 1998 20:52:29 GMT
Connection: close
Accept-Ranges: none
Allow: OPTIONS, GET, HEAD, POST, PUT, DELETE, TRACE, COPY, MOVE, MKCOL,
PROPFIND, PROPPATCH, LOCK, UNLOCK, MKRESOURCE
Public: OPTIONS, GET, HEAD, POST, PUT, DELETE, TRACE, COPY, MOVE, MKCOL,
PROPFIND, PROPPATCH, LOCK, UNLOCK, BIND, MKRESOURCE, ORDERPATCH
DAV: 1, 2, redirectrefs

The DAV header in the response indicates that the resource
/somecollection/someresource is level 1 and level 2 compliant, as
defined in [WebDAV].  In addition, /somecollection/someresource supports
redirect reference resources.  The Allow header indicates that
MKRESOURCE requests can be submitted to /somecollection/someresource.
The Public header shows that other Request-URIs on the server support
additional methods.

16 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, redirect reference resources introduce
several new security concerns and increase the risk of some existing
threats.  These issues are detailed below.

16.1 Privacy Concerns

By creating redirect reference resources on a trusted server, it is
possible for a hostile agent to induce users to send private information
to a target on a different server.   This risk is mitigated somewhat,
since clients are required to notify the user of the redirection for any
request other than GET or HEAD. (See [HTTP], Section 10.3.3 302 Found.)

16.2 Redirect Loops

Although redirect loops were already possible in HTTP 1.1, the
introduction of the MKRESOURCE method creates a new avenue for clients
to create loops accidentally or maliciously.  If the reference resource
and its target are on the same server, the server may be able to detect
MKRESOURCE requests that would create loops. See also [HTTP], Section
10.3 "Redirection 3xx."


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16.3 Redirect Reference Resources and Denial of Service

Denial of service attacks were already possible by posting URLs that
were intended for limited use at heavily used Web sites.  The
introduction of MKRESOURCE creates a new avenue for similar denial of
service attacks.  Clients can now create redirect reference resources at
heavily used sites to target locations that were not designed for heavy
usage.

16.4 Private Locations May Be Revealed

There are several ways that redirect reference resources may reveal
information about directory structures.  First, the DAV:reftarget
property of every redirect reference resource contains the URI of the
target resource.  Anyone who has access to the reference resource can
discover the directory path that leads to the target resource.   The
owner of the target resource may have wanted to limit knowledge of this
directory structure.

Sufficiently powerful access control mechanisms can control this risk to
some extent.  Property-level access control could prevent users from
examining the DAV:reftarget property.  (The Location header returned in
responses to requests on redirect reference resources reveals the same
information, however.)  In some environments, the owner of a resource
might be able to use access control to prevent others from creating
references to that resource.

This risk is no greater than the similar risk posed by HTML links.

17 Internationalization Considerations

This specification follows the practices of [WebDAV] in encoding all
human-readable content using XML [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 [WebDAV}, 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.  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, [WebDAV] 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

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as part of normal, error-free operation of the protocol.

For rationales for these decisions and advice for application
implementors, see [WebDAV].

18 IANA Considerations

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

19 Copyright

To be supplied by the RFC Editor.

20 Intellectual Property

To be supplied by the RFC Editor.

21 Acknowledgements

This draft has benefited from thoughtful discussion by Jim Amsden, Peter
Carlson, Steve Carter, Tyson Chihaya, Ken Coar, Ellis Cohen, 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 others.

22 References

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

[URI] T. Berners-Lee, R. Fielding, L. Masinter, "Uniform Resource
Identifiers (URI): Generic Syntax." RFC 2396. MIT/LCS, U.C. Irvine,
Xerox. August, 1998.

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

[XML] T. Bray, J. Paoli, C.M. Sperberg-McQueen, "Extensible Markup
Language (XML)."  World Wide Web Consortium Recommendation REC-xml-
19980210. http://www.w3.org/TR/1998/REC-xml-19980210.

[HTTP] R. Fielding, J. Gettys, J. Mogul, H. Frystyk, L. Masinter, P.
Leach, T. Berners-Lee, "Hypertext Transfer Protocol -- HTTP/1.1." RFC
2616.  UC Irvine, Compaq, W3C, Xerox, Microsoft.  June, 1999.

[WebDAV] Y. Y. Goland, E. J. Whitehead, Jr., A. Faizi, S. R. Carter, D.
Jensen, "HTTP Extensions for Distributed Authoring - WebDAV." RFC 2518.
Microsoft, U.C. Irvine, Netscape, Novell.  February, 1999.

[B] J. Slein, E.J. Whitehead Jr., J. Davis, G. Clemm, C. Fay, J.

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Crawford, "WebDAV Bindings." Internet Draft (work in progress) draft-
ietf-webdav-binding-protocol-02. Xerox, UC Irvine, CourseNet, Rational,
FileNet, IBM. December, 1999.

23 Authors' Addresses

J. Slein
Xerox Corporation
800 Phillips Road, 105-50C
Webster, NY 14580
Email: jslein@crt.xerox.com

E. J. Whitehead, Jr.
Dept. of Information and Computer Science
University of California, Irvine
Irvine, CA 92697-3425
Email: ejw@ics.uci.edu

J. Davis
CourseNet Systems
170 Capp Street
San Francisco, CA 94110
Email: jrd3@alum.mit.edu

G. Clemm
Rational Software Corporation
20 Maguire Road
Lexington, MA 02173-3104
Email: gclemm@rational.com

C. Fay
FileNet Corporation
3565 Harbor Boulevard
Costa Mesa, CA 92626-1420
Email: cfay@filenet.com

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

24 Appendices

24.1 Appendix 1: Extensions to the WebDAV Document Type Definition

<!--============= XML Elements from Section 13 ================-->
<!ELEMENT redirectref EMPTY >
<!--============= Property Elements from Section 12 ===========-->
<!ELEMENT reftarget href>
<!ELEMENT location href>
<!--====== Changes to the DAV:response Element from Section 14 ====-->
<!ELEMENT response (href, ((href*, status, prop?) | (propstat+)),
responsedescription?) >


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Expires June 17, 2000

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Html markup produced by rfcmarkup 1.109, available from https://tools.ietf.org/tools/rfcmarkup/