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WEBDAV Working Group                                     J. Slein, Xerox
INTERNET DRAFT                                           J. Davis, Xerox
<draft-ietf-webdav-collection-protocol-02>            A. Babich, FileNet
                                           E.J. Whitehead Jr., UC Irvine
                                                       November 10, 1998
Expires May 10, 1999

                        WebDAV Advanced Collections Protocol

Status of this Memo

This document is an Internet-Draft. 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
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Abstract

The base WebDAV protocol [WebDAV] provides basic support for
collections.  It defines a MKCOL method for creating collections and
specifies how other HTTP and WebDAV methods interact with collections.
It supports internal members of collections, which it defines as members
whose URIs are immediately relative to the URI of the collection.

Many applications, however, need more powerful collections.  There are
two areas in particular where more powerful functionality is often
needed: referential resources and ordering.

This draft specifies extensions to the base WebDAV protocol to support
these more powerful collections.

Table of Contents

1       Terminology ............................................... 3
2       Introduction .............................................. 4
3       Referential Resources ..................................... 4
3.1     Scope ..................................................... 4
3.2     Overview .................................................. 5

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3.3     Creating Referential Resources ............................ 7
3.3.1   The MKREF Method .......................................... 7
3.3.2   Status Codes .............................................. 8
3.3.3   Example ................................................... 8
3.4     Deleting, Copying, and Moving Referential Resources ....... 9
3.5     Listing Referential Members of a Collection ............... 9
3.6     Other WebDAV Operations on Redirect Referential Resources . 9
3.7     Other WebDAV Operations on Direct Referential Resources .. 10
3.8     HTTP Operations on Redirect Referential Resources ........ 10
3.9     HTTP Operations on Direct Referential Resources .......... 11
3.10    Operations on Targets of Referential Resources ........... 12
3.11    Discovering a Target’s References ........................ 12
3.12    Behavior of Dangling Direct References ................... 13
3.13    Summary of Referencing Headers Required in Responses ..... 16
4       Ordered Collections ...................................... 16
4.1     Overview ................................................. 16
4.2     Creating an Ordered Collection ........................... 16
4.2.1   Overview ................................................. 16
4.2.2   Status Codes ............................................. 17
4.2.3   Example .................................................. 17
4.3     Setting the Position of a Collection Member .............. 17
4.3.1   Overview ................................................. 18
4.3.2   Status Codes ............................................. 18
4.3.3   Examples ................................................. 18
4.4     Changing the Semantics of a Collection Ordering .......... 19
4.5     Changing the Position of a Collection Member ............. 19
4.5.1   The ORDERPATCH Method .................................... 19
4.5.2   Status Codes ............................................. 19
4.5.3   Example .................................................. 19
4.5.4   Design Rationale ......................................... 21
5       New Headers .............................................. 21
5.1     Ref-Target Entity Header ................................. 21
5.2     Ref-Type Entity Header ................................... 22
5.3     Ref-Integrity Entity Header .............................. 22
5.4     Re-Direct Request Header ................................. 23
5.5     Hide-Target Entity Header ................................ 23
5.6     Resource-Type Entity Header .............................. 23
5.7     Ordered Entity Header .................................... 23
5.8     Position Request Header .................................. 24
5.9     DAV-Collections Entity Header ............................ 24
6       New Properties ........................................... 24
6.1     reftarget Property ....................................... 25
6.2     refintegrity Property .................................... 25
6.3     reftype Property ......................................... 25
6.4     references Property ...................................... 25
6.5     orderingtype Property .................................... 26
7       New XML Elements ......................................... 26
7.1     reference XML Element .................................... 26
7.2     direct XML Element ....................................... 26
7.3     redirect XML Element ..................................... 26
7.4     weak XML Element ......................................... 26
7.5     unordered XML Element .................................... 27
7.6     custom XML Element ....................................... 27
7.7     order XML Element ........................................ 27
7.8     ordermember XML Element .................................. 27

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7.9     position XML Element ..................................... 28
7.10    first XML Element ........................................ 28
7.11    last XML Element ......................................... 28
7.12    before XML Element ....................................... 28
7.13    after XML Element ........................................ 28
8       Extensions to the DAV:multistatus XML Element ............ 29
9       Capability Discovery ..................................... 29
9.1     Using OPTIONS ............................................ 29
9.2     Example .................................................. 29
10      Dependencies on Other Specifications ..................... 30
11      Security Considerations .................................. 30
11.1    Redirect Loops ........................................... 30
11.2    References and Denial of Service ......................... 30
11.3    Maintaining Referential Integrity May Reveal Private Locations30
11.4    DAV:references and Denial of Service ..................... 30
11.5    DAV:references and Malicious Deletion of Resources ....... 31
11.6    Malicious Modifications of Ordering ...................... 31
11.7    Denial of Service and DAV:orderingtype ................... 31
12      Internationalization Considerations ...................... 31
13      IANA Considerations ...................................... 31
14      Copyright ................................................ 32
15      Intellectual Property .................................... 32
16      Acknowledgements ......................................... 32
17      References ............................................... 32
18      Authors' Addresses ....................................... 32
19      Appendices ............................................... 33
19.1    Appendix 1 - Extensions to the WebDAV Document Type Definition33


1 Terminology

The terminology used here follows and extends that in the base WebDAV
protocol specification [WebDAV].

Collection
     A resource that contains member resources

Member Resource
     A resource contained by a collection

Referential Resource (or Reference)
     A resource that has no content of its own, but rather is a
     reference to another resource

Ordinary Resource
     A resource that is not a reference to another resource

Target Resource
     The resource referenced by a referential resource

Direct Reference
     A reference that is resolved by the server, giving the client the
     illusion that it is operating directly on the target resource

Redirect Reference

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     A reference that must be resolved by the client

Strong Reference
     A reference whose referential integrity is guaranteed by the
     server

Weak Reference
     A reference whose referential integrity is not guaranteed by the
     server

Referential Integrity
     A server guarantees the integrity of a reference if it ensures
     that the reference will not be broken, or enables the reference's
     owner to ensure that the reference will not be broken.

2 Introduction

The simple collections that the base WebDAV specification supports are
powerful enough to be widely useful.  They provide for the hierarchical
organization of resources, with mechanisms for creating and deleting
collections, copying and moving them, locking them, adding resources to
them and deleting resources from them, and getting listings of their
members.  Delete, copy, move, list, and lock operations can be applied
recursively, so that a client can operate on whole hierarchies with a
single request.

Many applications, however, need more powerful collections.  There are
two areas in particular where more powerful functionality is often
needed: references and ordering.

References make it possible for many collections, on the same or
different servers, to share the same resource.  Because the collections
share the resource by referencing it, only one physical copy of the
resource need exist, and any changes made in the resource are visible
from all the collections that reference it.

It is useful for many applications to be able to impose an ordering on a
collection. Orderings may be based on property values, but they may be
completely independent of any properties on the collection’s member
resources.  Orderings based on properties can be obtained using a search
protocol [DASL], but orderings not based on properties need some other
mechanism.

Since these two areas are independent of each other, servers may elect
to comply with the Referential Resources section of this specification
or with the Ordered Collections section or both.  A server that supports
referencing MUST support redirect references, and MAY support direct
references.  A server MUST advertise its compliance with particular
parts of this specification through its response to an OPTIONS request,
as specified in Section 9 below.

3 Referential Resources

3.1 Scope


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[WebDAVReq] distinguishes between "weak" references and "strong"
references, and also between "redirect" references and "direct"
references.  This specification supports weak references, direct
references, and redirect references, and is designed so that it can be
extended to support strong references in the future.

Strong references are references whose integrity is guaranteed by the
server; weak references are those whose integrity is not guaranteed.
Strong references and weak references are both useful in different
contexts.  Some applications cannot tolerate broken links.  A software
development application, for example, must be able to rely on the
integrity of references to component modules. Such applications must be
able to request strong references.  Other applications may want to
reference target resources on multiple servers, where referential
integrity cannot be guaranteed, and may be less concerned about possible
broken references.

Several considerations led to the decision not to support strong
references in the current specification.  First, there are many possible
policies that applications and services might use to enforce referential
integrity.

o Delete strong references when their targets are deleted.
o Decline to delete targets of strong references.
o Notify strong references when their targets have been deleted.
o Let owners of resources decide whether strong references to them are
  allowed.

There appears to be no common practice in this area.  Moreover, some of
the policies have significant security risks.

o Moving a target of strong references could be a security risk to the
  owner of the target by revealing secret locations on the target's
  server.
o A strong reference could be a security risk to the owner of the
  reference by revealing secret locations on his server.
o The presence of strong references to resources on a server could make
  it impossible to reclaim space on that server by moving or deleting
  those target resources.

These considerations together led to the decision not to support strong
references in the short term.

3.2 Overview

A referential resource is a resource that has no content of its own, but
instead references another resource.  The resource it references may be
in the same collection as the reference, or in any other collection.
This target resource may be a collection or a simple resource or another
reference, or any other sort of resource that may be defined in the
future.  A resource may be the target of any number of referential
resources.  To make it possible to distinguish references from ordinary
resources, a new value of the DAV:resourcetype property is defined here.
The DAV:resourcetype property of all references MUST have the value
DAV:reference.

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Redirect references are references that must be resolved by the client.
They are simple for servers to implement, straightforward for clients to
use, and have only limited security implications.  Any server that
complies with WebDAV referencing MUST provide redirect references.

To resolve a redirect reference, the client retrieves the DAV:reftarget
property, whose value is the URI of the target resource.  Every
reference, direct or redirect, MUST have the DAV:reftarget property.  If
a client wishes to operate on the target resource of a redirect
reference, it resolves the reference, and then invokes the operation on
the target resource.

The redirect reference appears to the client as an independent resource
with its own properties.  Operations with the URI of the redirect
reference as their request-URI affect the reference, not its target
resource.

Direct references, in contrast, are resolved by the server.  They give
the client the illusion that it is operating directly on the target
resource.  These references are more complex for the server to
implement, and raise additional security issues.  Consequently, servers
are not required to provide direct references in order to be compliant
with WebDAV referencing.

The default behavior of a direct reference is to apply most operations
directly to its target, so that the client is not aware that a reference
is mediating between it and the target resource.  The exceptions are
operations that affect membership in a collection rather than the state
of the target resource.  At present, the operations that fall into this
category are DELETE, MOVE, and COPY.  These operations are applied to
the reference itself rather than to its target, so that only the
collection containing the reference is affected.

The Re-Direct request header is also provided, to force an operation to
be applied to the direct reference itself rather than its target.  In
effect, it makes the reference behave like a redirect reference for that
request.  This header is particularly useful with PROPFIND, to allow
clients to view the reference’s own properties.

To distinguish between direct and redirect references, a new DAV:reftype
property is defined, with the values DAV:direct and DAV:redirect.  Every
reference MUST have the DAV:reftype property.  The DAV:reftype property
of a direct reference MUST have the value DAV:direct.  The DAV:reftype
property of a redirect reference MUST have the value DAV:redirect.

Although strong references are not currently supported, a new
DAV:refintegrity property is defined in anticipation of future support
for strong references.  DAV:refintegrity will allow clients to
distinguish between weak and strong references.  All references MUST
have this property.  Although the only value currently defined for
DAV:refintegrity is DAV:weak, other values may be defined in the future.

***** If a server wants to enforce referential integrity outside this
protocol, a value of DAV:weak is misleading.

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3.3 Creating Referential Resources

3.3.1 The MKREF Method

Referential resources are created using the MKREF method.  The request-
URI of the MKREF request identifies the resource to be created.  The
required Ref-Target header contains the URI of the target resource.

An optional Ref-Integrity general header is defined below, primarily for
future support for strong references.  The only value currently defined
for this header is "DAV:weak", although other values may be used by
private agreement.  "DAV:weak" is the default value if the header is not
present.

***** Does Ref-Integrity = DAV:weak mean that the server need not
enforce referential integrity, or that it must not enforce referential
integrity for the new resource?  We have a requirement that there be
some way to request the server not to enforce referential integrity for
a particular reference.  You might also want to change from don’t-
enforce to enforce at different times in the life of the reference.

An optional Ref-Type general header is defined below, with values
"DAV:direct" and "DAV:redirect". The default value is "DAV:redirect" if
the header is not present.

The optional Hide-Target request header is defined below, to enable the
creator of a direct reference to specify that the DAV:reftarget property
be hidden from clients.  If the Hide-Target header is not present, the
server MUST assume that the DAV:reftarget property is not to be hidden.
If the Hide-Target header is used with Ref-Type header set to
DAV:redirect, the server MUST ignore the Hide-Target header.  The Ref-
Target header MUST never be returned in response to any request for a
direct reference created with the Hide-Target header.

***** How useful is this really? Isn’t it the owner of the target (not
of the reference) who wants to control whether the location of the
target is hidden?  True, we have a scenario where the person creating
the reference wants to hide the location of the target, but that’s
because the owner of the reference and the owner of the target are the
same person in that example.

An optional Position request header supports ordered collections by
allowing the client creating a reference to specify where the new member
is to be placed in the collection's ordering.  (This header can also be
used with PUT to create an ordinary resource at a specific position in
the collection ordering.)

When a server processes a MKREF request, it MUST set the
DAV:resourcetype property (defined in [WebDAV]) of the new resource to
be DAV:reference.

When a server processes a MKREF request, it MUST set the DAV:reftarget
property to the URI of the target resource.


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When a server processes a MKREF request, it MUST set the
DAV:refintegrity property and the DAV:reftype property based on the
values of the Ref-Integrity and Ref-Type headers.

The client MUST NOT send any content with the MKREF request, and so MUST
NOT use the Content-Length or Transfer-Encoding headers.  (See [HTTP].)

If a MKREF request is submitted for an existing resource, the existing
resource's content and headers will be overwritten.  This behavior is
analogous to the behavior of the HTTP PUT method.  Live properties may
get new values at the server's discretion; dead properties will retain
their existing values.  If the Position header is absent in this case
and the collection is ordered, the server MUST leave the member at its
previous position in the collection ordering.  If the Position header is
present and the collection is ordered, the server MUST remove the member
from its previous position, and then insert it at the requested
position.

3.3.2 Status Codes

201 (Created): The reference was successfully created.

200 (OK): The reference was successfully created, replacing an existing
resource at the same location.

400 (Bad Request): The client attempted to send content with the
request, or set an invalid value for the Ref-Target, Ref-Integrity, Ref-
Type, or Position header.

409 (Conflict): Several conditions may produce this response.  There may
be no resource at the location specified in Ref-Target, on a server that
prohibits dangling references.  The request may be attempting to create
the reference in a collection that does not exist.  The request may be
attempting to position the reference before or after a resource that is
not in the collection, or before or after itself.  The request may be
attempting to position the reference in an unordered collection.

***** These are not conflicts with the state of the resource at the
request-URI, but conflicts with the state of the parent collection or
the target resource.  Is it still appropriate to use 409?

507 (Insufficient Storage): There is not enough space on the server to
complete the request.

3.3.3 Example

Request:

MKREF /~whitehead/dav/spec08.ref HTTP/1.1
HOST: www.ics.uci.edu
Ref-Target: <http://www.ics.uci.edu/i-d/draft-webdav-protocol-08.txt>

Response:

HTTP/1.1 201 Created

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This request resulted in the creation of a new referential resource at
www.ics.uci.edu/~whitehead/dav/spec08.ref, which points to the resource
identified by the Ref-Target header.  Its DAV:resourcetype property is
set to DAV:reference.  Its DAV:reftarget property is set to the URI of
its target resource.  Its DAV:refintegrity property is set to the
default value of DAV:weak.  Its DAV:reftype property is set to the
default value of DAV:redirect.

3.4 Deleting, Copying, and Moving Referential Resources

The DELETE method should be used to delete referential resources.  For
both direct and redirect references, DELETE affects the reference
itself, and not its target.

A MOVE operation on a referential resource moves the referential
resource to a different location, but has no effect on the location of
its target. The DAV:reftarget property is unchanged after a MOVE unless
the Ref-Target header is used to change it.

A COPY operation on a referential resource copies the referential
resource, not its target resource, to another location. The
DAV:reftarget property of the destination resource is the same as the
DAV:reftarget of the source resource, unless the Ref-Target header is
used to change it.

3.5 Listing Referential Members of a Collection

Since a referential member of a collection is just a resource in the
collection, a listing of members of the collection shows referential
members along with ordinary members.  That is, a WebDAV PROPFIND request
on a collection resource with Depth = 1 or infinity MUST return a
response XML element for each ordinary member and for each referential
member.

For a redirect reference, the properties returned by the PROPFIND are
the properties of the reference itself.  If Depth = infinity in the
PROPFIND request, the server MUST NOT follow redirect references into
any collections to which they may refer.

For a direct reference, the properties returned by the PROPFIND are the
properties of its target resource.  If Depth = infinity in the PROPFIND
request, the server MUST follow direct references into any collections
to which they may refer.  That is, if the target resource is a
collection, the server MUST list the members of that collection.  If the
Re-Direct header is used with PROPFIND, the server MUST treat any direct
references like redirect references, as described in the previous
paragraph.

3.6 Other WebDAV Operations on Redirect Referential Resources

Operations on a redirect reference affect only the reference, and not
its target resource.

A LOCK operation on a redirect reference locks the reference, not its

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target.  A LOCK on the collection with Depth = 1 or infinity locks any
redirect references that are members of the collection.  It does not
lock the targets of those redirect references.

A PROPPATCH on a redirect reference modifies the properties of the
reference, not the properties of its target resource.

A PROPFIND on a redirect reference returns the properties of the
reference, not the properties of its target resource.

3.7 Other WebDAV Operations on Direct Referential Resources

With the exception of DELETE, MOVE, and COPY, which were discussed
above, operations on direct references affect the target resource, not
the reference, unless the Re-Direct header is used.

Unless the Re-Direct header is used, a LOCK operation on a direct
reference locks the target.  A lock on a direct reference to a
collection locks the target collection.  A lock on a collection with
Depth = 1 or infinity locks the targets of the direct references in the
collection.

Unless the Re-Direct header is used, a PROPPATCH on a direct reference
modifies the properties of its target resource, not the properties of
the reference itself.

Unless the Re-Direct header is used, a PROPFIND on a direct reference
returns the properties of its target resource, not the properties of the
reference itself.

If the Re-Direct header is used with a LOCK, PROPPATCH, or PROPFIND
request on a direct reference, it behaves as if it were being applied to
a redirect reference, as specified in Section 3.6.

3.8 HTTP Operations on Redirect Referential Resources

Although existing HTTP clients cannot create referential resources, they
should be able to read collections created by reference-aware WebDAV
clients.  They should be able to follow any references in those
collections to their targets.  To make this possible, a server that
receives a GET or HEAD on a redirect reference MUST return a 302(Moved
Temporarily) status code.  The server MUST follow [HTTP] Section 10.3.3
"302 Moved Temporarily," but with these additional rules:

o The Location header MUST contain the target URI of the reference.

o The response MUST include all referencing entity headers that make
  sense for redirect references: Ref-Type, Ref-Target, and Ref-
  Integrity.

o The response MUST also include those HTTP headers that make sense for
  referential resources, at a minimum: Cache-Control, Age, ETag,
  Expires, and Last-Modified.

The second and third of these rules preserve normal GET and HEAD

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behavior for reference-aware WebDAV clients.

POST cannot be applied to a redirect reference.  A reference cannot
accept another entity as its subordinate.  Depending upon the nature of
the target resource, however, it might make sense to apply POST to the
target.  A server that receives a POST request on a redirect reference
MUST return a 302 (Moved Temporarily).  The rules for constructing and
using the response are the same as for GET and HEAD, except that there
is no requirement to return any headers other than Ref-Type. This header
allows reference-aware WebDAV clients to recognize the resource as a
reference and understand the reason for the redirection.

PUT cannot be applied to a redirect reference.  To replace one redirect
reference with another, MKREF MUST be used.  To replace a redirect
reference with an ordinary resource, the reference MUST first be deleted
with DELREF, after which a PUT MUST be used to create the ordinary
resource.

Existing HTTP clients that do not understand referential resources need
to be accommodated, however.  To enable these clients to operate
reasonably on redirect references, a server that receives a PUT request
on a redirect reference MUST return a 302 (Moved Temporarily).  The
client and server MUST follow [HTTP] Section 10.3.3 "302 Moved
Temporarily," but with these additional rules:

o The Location response header MUST contain the target URI of the
  reference.

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

o The response MUST include an entity body for display to users.  The
  entity body explains that the requested resource is a reference to
  another resource, and allows the user to choose whether to replace
  the target resource or to replace the reference.

This last rule is needed for PUT, but not for GET, HEAD, or POST.  Only
for PUT does it make sense for the user to confirm that the operation is
to be performed at the request-URI.  GET or HEAD will already have
returned all useful information about the request-URI.  POST makes no
sense for the redirect reference at the request-URI.  But the user might
really want to replace the redirect reference with the entity in the PUT
request.

To enable existing HTTP clients to retrieve the capabilities of the
target resource, a server that receives an OPTIONS request on a redirect
reference MUST return a 302 (Moved Temporarily).  At the same time,
reference-aware WebDAV clients may need to retrieve the capabilities of
the reference itself.  Consequently, the server MUST provide a choice as
to whether the method should be applied to the reference or its target.
Consequently, the same rules apply for OPTIONS as for PUT above.

3.9 HTTP Operations on Direct Referential Resources


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GET, HEAD, PUT, POST, and OPTIONS on direct references are passed
through to their target resources.  GET returns the content and headers
of the target resource, HEAD returns the headers of the target resource,
PUT replaces the content of the target resource, POST performs the
expected function at the target resource, and OPTIONS reports the
communication options available at the target resource.

The Re-Direct request header may be used with GET, HEAD, or OPTIONS to
view the headers or capabilities of the reference, rather than its
target.  If the Re-Direct header is used, the method’s behavior is as
described in Section 3.8.

The Re-Direct request header must not be used with PUT or POST, which
cannot be applied to references.  If a server receives a PUT or POST
request on a direct reference with the Re-Direct header, it MUST respond
with a 400 (Bad Request).

***** Confirm behavior for PUT / POST with Re-Direct.

3.10 Operations on Targets of Referential Resources

In general, operations on targets of weak referential resources have no
effect on the referential resource.  However, servers that choose to
maintain the integrity of references are free to make changes to the
state of references when moving or deleting their targets.

When moving a target resource, a server MAY insert an optional step into
the semantics of MOVE as defined in [WebDAV] for the purpose of
maintaining referential integrity.  Between the copy step and the delete
step of a MOVE, the server MAY perform an update step, which changes the
DAV:reftarget property of any references to the target to reflect its
new location.

When deleting a target resource, a server MAY perform any internal
operations necessary to implement its policy on preserving referential
integrity.  For example, it might delete any references to the deleted
target, or it might flag them as having a deleted target, or it might
replace references with copies of the target.

3.11 Discovering a Target’s References

An optional DAV:references property on the target resource provides a
list of referential resources whose DAV:reftarget property points to
that target resource.  If present, DAV:references is a read-only
property, maintained by the server.  By retrieving this property, a
client can discover the URIs of the references that point to the target,
and so can also discover the collections of which those URIs are
members.  As for all DAV: properties, this specification is silent as to
how the DAV:references property is implemented on the server.

The DAV:references property is expected to be supported mainly by
Document Management Systems (DMSs) and other servers that will maintain
the property only for references within their own domain.  It is not in
general possible for a server to maintain the property for references on
other servers.  If a reference on a different server points to the

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target, the server where the target is located is unlikely to know about
that reference.  This protocol provides no mechanism for one server to
notify another of the creation of a reference to one of its resources.
Consequently, the list of references in DAV:reftarget may be incomplete.

Rationale: A number of scenarios require clients to navigate from a
target resource to the references that point to it, and to the
collections that contain those references.  This capability is
particularly important for DMSs, which may populate their collections
entirely by reference.  Their clients may need to determine, for any
object in the DMS, what collections it belongs to.  It is also important
in other contexts.  For example, some servers enforce referential
integrity by refusing to delete any resource that is referenced by other
resources.  In such an environment, the client must be able to discover
the references in order to delete them before attempting to delete the
target.

Once a search capability [DASL] is available, it may be possible for a
client to discover the references that point to a given target by
searching for resources with DAV:reftarget equal to the URI of the
target resource.  However, it will be much more efficient for the client
to retrieve a list of references from the target resource.

***** Only if DASL provides search of structured properties or we define
DAV:reftarget as a string value, not an href.

Risks: When deciding whether to support the DAV:references property,
server implementors / administrators should balance the efficiencies it
provides in discovering references against the cost of maintaining the
property and the security risks enumerated in Sections 11.4 and 11.5.

3.12 Behavior of Dangling Direct References

***** Think about chains of direct references.

GET and HEAD respond with 404 (Not Found), but the Ref-Type and Ref-
Target headers are included in the response, so that the client can tell
that it is the target, and not the reference, that was not found. (If
the Hide-Target header was used when creating the reference, then Ref-
Target is not included.)

PUT, MKCOL, and MKREF succeed, creating a new resource at the location
specified by the reference’s DAV:reftarget property.

POST fails with 404 (Not Found), since a nonexistent resource cannot
accept another resource as its subordinate.  The Ref-Type and Ref-Target
headers are included in the response, so that the client can tell that
it is the target, and not the reference, that was not found. (If the
Hide-Target header was used when creating the reference, then Ref-Target
is not included.)

OPTIONS fails with 404 (Not Found). The Ref-Type and Ref-Target headers
are included in the response, so that the client can tell that it is the
target, and not the reference, that was not found. (If the Hide-Target
header was used when creating the reference, then Ref-Target is not

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included.)

PROPFIND responds with a 207 Multistatus, including a 404 (Not Found) as
the status for the target resource.  The DAV:reftype and DAV:reftarget
properties of the references are included in the response.  (If the
Hide-Target header was used when creating the reference, then
DAV:reftarget is not included.)  Their presence informs the client that
it is the target, not the reference, that was not found. These two
elements are extensions to the DAV:multistatus element as defined in
{WEBDAV].

For example,

Request:

PROPFIND /collection1/directref7 HTTP/1.1
Host: www.somehost.com
Content-Type: text/xml
Content-Length: xxxx

<?xml version="1.0" ?>
<D:propfind xmlns:D="DAV:">
   <D:prop xmlns:X="http://www.somehost.com/schemas/x">
      <X:author/>
      <X:title/>
   </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:">
   <D:response>
      <D:href>http://www.somehost.com/collection1/directref7</D:href>
      <D:status>HTTP/1.1 404 Not Found</D:status>
      <D:reftype><D:direct/></D:reftype>
      <D:reftarget>
         <D:href>http://www.somehost.com/collection2/file19</D:href>
      </D:reftarget>
   </D:response>
   <D:responsedescription>Target resource not
found.</D:responsedescription>
</D:multistatus>

PROPPATCH responds with a 207 Multistatus, including a 404 (Not Found)
as the status for the target resource.  The DAV:reftype and
DAV:reftarget properties of the references are included in the response.
(If the Hide-Target header was used when creating the reference, then
DAV:reftarget is not included.)  Their presence informs the client that
it is the target, not the reference, that was not found. These two
elements are extensions to the DAV:multistatus element as defined in

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{WEBDAV].

***** Is a response to a PROPPATCH required to be a 207 (Multi-Status),
or could it just be a 404 (Not Found)?

For example,

Request:

PROPPATCH /collection1/directref7 HTTP/1.1
Host: www.somehost.com
Content-Type: text/xml
Content-Length: xxxx

<?xml version="1.0" ?>
<D:propertyupdate xmlns:D="DAV:">
   <D:set>
      <D:prop xmlns:X="http://www.somehost.com/schemas/x">
         <X:author>Pauloosie Padluq</X:author>
         <X:title>South Baffin Carvers</X:title>
      </D:prop>
   </D:set>
</D:propertyupdate>

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.somehost.com/collection1/directref7</D:href>
      <D:status>HTTP/1.1 404 Not Found</D:status>
      <D:reftype><D:direct/></D:reftype>
      <D:reftarget>
         <D:href>http://www.somehost.com/collection2/file19</D:href>
      </D:reftarget>
   </D:response>
   <D:responsedescription>Target resource not
found.</D:responsedescription>
</D:multistatus>

MOVE, COPY, and DELETE succeed.  For MOVE and COPY, the reference at the
destination will be broken, just as the reference at the source was.

If a single resource is being LOCKed or UNLOCKed, the response is a 404
(Not Found).  The Ref-Type and Ref-Target headers are included in the
response. (If the Hide-Target header was used when creating the
reference, then Ref-Target is not included.) Their presence informs the
client that it is the target, not the reference, that was not found.

If the dangling reference is a member of a collection that is being
LOCKed or UNLOCKed, the response is a 207 (Multi-Status).  The

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DAV:status element for the dangling reference is a 404 (Not Found).  The
DAV:reftype and DAV:reftarget properties of the references are included
in the response.  (If the Hide-Target header was used when creating the
reference, then DAV:reftarget is not included.)  Their presence informs
the client that it is the target, not the reference, that was not found.
These two elements are extensions to the DAV:multistatus element as
defined in {WEBDAV].

3.13 Summary of Referencing Headers Required in Responses

This section summarizes the rules that determine which referencing
headers must be included in responses to requests on references.

o Every response to a request on a reference MUST include the Ref-Type
  header, so that the client knows that it was operating on a
  reference, and whether the operation was applied to the reference
  itself or to its target.

o Every response to a request on a direct reference MUST include the
  Ref-Target header, unless the reference was created with the Hide-
  Target header.  This allows the client to tell what resource was
  affected by the operation.  A request that includes the Re-Direct
  header is treated as if it were a request on a redirect reference,
  and so the response NEED NOT include the Ref-Target header.

o Every response to a GET or HEAD request on a redirect reference (or
  on a direct reference with the Re-Direct header) MUST include all the
  referencing entity headers that make sense for redirect references:
  Ref-Type, Ref-Target, and Ref-Integrity.

4 Ordered Collections

4.1 Overview

Collections on a compliant server may be ordered, but need not be.  It
is up to the client to decide whether a given collection is ordered and,
if so, to specify the semantics to be used for ordering its members.  If
a collection is ordered, each of its members must be in the ordering
exactly once, and the ordering must not include any resource that is not
a member of 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.

The server is responsible for enforcing these constraints on orderings.
The server MUST remove a resource from the ordering when it is removed
from the collection. The server MUST add a resource to the ordering when
it is added to the collection.

When responding to a PROPFIND on a collection, the server MUST order the
response elements according to the ordering defined on the collection.

4.2 Creating an Ordered Collection

4.2.1 Overview

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When a collection is created, the client can request that it be ordered
and specify the semantics of the ordering by using the new Ordered
header in the MKCOL request, setting its value to the URI of the
semantics to be used.  If the client does not want the collection to be
ordered, it may omit the Ordered header, or use it with the value
"DAV:unordered".

Every collection MUST have the new DAV:orderingtype property, which
indicates whether the collection is ordered and, if so, identifies the
semantics of the ordering.  A value of DAV:unordered indicates that that
collection is not ordered.  That is, the client cannot depend on the
repeatability of the ordering of results from a PROPFIND request.  For
collections that are ordered, DAV:orderingtype SHOULD be set to an href
that identifies the semantics of the ordering, allowing a human user or
software package to insert new collection members into the ordering
intelligently.  Although the href MAY point to a resource that contains
a definition of the semantics of the ordering, clients are discouraged
from accessing that resource, in order to avoid overburdening its
server.  The DAV:orderingtype property MAY be set to DAV:custom to
indicate that the collection is to be ordered, but the semantics of the
ordering is not being advertised.

If the Ordered header is present on a MKCOL request, the server MUST set
the collection's DAV:orderingtype property to the value of the Ordered
header.  If the Ordered header is not present, the server MUST treat the
request as if it had an Ordered header with the value "DAV:unordered",
meaning that the collection is not ordered.  If the collection is
ordered, the server MUST respond to PROPFIND requests on the collection
using the specified ordering.

4.2.2 Status Codes

No new error conditions are introduced.

4.2.3 Example

Request:

MKCOL /theNorth/ HTTP/1.1
Host: www.server.org
Ordered: <http://www.server.org/orderings/compass.html>

Response:

HTTP/1.1 201 Created

In this example, a new, ordered collection was created.  Its
DAV:orderingtype property has as its value the URI from the Ordered
header.  In this case, the URI points to a description of the semantics
governing the 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.

4.3 Setting the Position of a Collection Member

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4.3.1 Overview

When a new member is added to a collection with MKREF, PUT, COPY, MOVE,
or LOCK, its position in the ordering can be set with the new Position
header.  The Position header allows the client to specify that the
member should be first in the collection's ordering, last in the
collection's ordering, before some other collection member in the
collection's ordering, or after some other collection member in the
collection's ordering.

The server MUST insert the new member into the ordering at the location
specified in the Position header, if one is present (and if the
collection is ordered); otherwise, it MUST append the new member to the
end of the ordering (if the collection is ordered).  If a PUT or MKREF
causes an existing resource to be replaced, and if the Position header
is absent, the server MUST leave the member at its previous position in
thee collection ordering.  If the Position header is present, the server
MUST remove the member from its previous position, and then insert it at
the requested position.

4.3.2 Status Codes

201 (Created): The resource was successfully created.

409 (Conflict): The request may be attempting to position the resource
before or after a resource that is not in the collection, or before or
after itself, or it may be attempting to position the resource in an
unordered collection.

4.3.3 Examples

Request:

MKREF /~whitehead/dav/spec08.ref HTTP/1.1
HOST: www.ics.uci.edu
Ref-Target: <http://www.ics.uci.edu/i-d/draft-webdav-protocol-08.txt>
Position: After <requirements.html>

Response:

HTTP/1.1 201 Created

This request resulted in the creation of a new referential resource at
www.ics.uci.edu/~whitehead/dav/spec08.ref, which points to the resource
identified by the Ref-Target header.  The Position header in this
example caused the server to set its position in the ordering of the
/~whitehead/dav/ collection immediately after the requirements.html
resource.

Request:

MOVE /i-d/draft-webdav-protocol-08.txt HTTP/1.1
Host: www.ics.uci.edu
Destination: </~whitehead/dav/draft-webdav-protocol-08.txt>

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Position: First

Response:

HTTP/1.1 409 Conflict

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

4.4 Changing the Semantics of a Collection Ordering

After a collection has been created, a client can change its ordering
semantics, or change an ordered collection to an unordered collection or
vice versa, by using PROPPATCH to change the value of its
DAV:orderingtype property.  The client is then responsible for updating
the ordering of the collection members according to the new semantics.
PROPPATCH is defined in [WebDAV], Section 7.2.

4.5 Changing the Position of a Collection Member

4.5.1 The ORDERPATCH Method

To change the positions of collection members in the collection's
ordering, the client MUST use an ORDERPATCH request with a request body
containing an order XML element.  The request-URI of an ORDERPATCH
request is the URI of the collection whose ordering is to be updated.
The order XML element identifies the member resources whose positions
are to be changed, and describes their new positions in the ordering.
Each new position can be specified as first in the ordering, last in the
ordering, before some other collection member in the ordering, or after
some other collection member in the ordering.  The server MUST apply the
changes in the order they appear in the order element.

4.5.2 Status Codes

Since multiple changes can be requested in a single ORDERPATCH request,
the server MUST return a 207 Multi-Status response, as defined in
[WebDAV].

Within the 207 Multi-Status response, the following status codes are
possible:

200 (OK): The change in ordering was successfully made.

409 (Conflict): An attempt was made to position the resource before or
after a resource that is not in the collection, or before or after
itself, or an attempt was made to position the resource in an unordered
collection.

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

4.5.3 Example


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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
Content-Length: xxx

<?xml version="1.0" ?>
<d:order xmlns:d="DAV:">
   <d:ordermember>
      <d:href>nunavut.desc</d:href>
      <d:position>
         <d:after>
            <d:href>nunavut.map</d:href>
         </d:after>
      </d:position>
   </d:ordermember>
   <d:ordermember>
      <d:href>iqaluit.img</d:href>
      <d:position>
         <d:last/>
      </d:position>
   </d:ordermember>
</d:order>

Response:

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

<?xml version="1.0" ?>
<d:multistatus xmlns:d="DAV:">
   <d:response>
      <d:href>http://www.nunanet.com/coll-1/nunavut.desc</d:href>
      <d:status>HTTP/1.1 200 OK</d:status>
   </d:response>
   <d:response>
      <d:href>http://www.nunanet.com/coll-1/iqaluit.img</d:href>
      <d:status>HTTP/1.1 200 OK</d:status>
   </d:response>
</d:multistatus>


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In this example, after the request has been processed, the collection's
ordering is as follows:

nunavut.map
nunavut.desc
nunavut.img
baffin.map
baffin.desc
baffin.img
iqaluit.map
iqaluit.desc
iqaluit.img

4.5.4 Design Rationale

The decision to introduce the new ORDERPATCH method was made after
investigating the possibility of using the existing MOVE method with a
Position header.  The use of MOVE initially looked appealingly simple:

MOVE /root/coll-1/foo HTTP/1.1
Host: www.somehost.com
Destination: </root/coll-1/foo>
Position: First

Unfortunately, several features of the semantics of MOVE make it
unsuitable for changing the position of a collection member in the
collection's ordering.  First, [WebDAV] defines MOVE as logically
equivalent to a copy followed by a delete of the source resource.  This
definition makes it impossible to MOVE a resource to a destination URL
that is the same as the source URL.  The resource would be deleted
rather than moved.  Second, [WebDAV] states that when moving a resource
to a destination where a resource already exists, the Overwrite header
must be "T", and in this case the server must DELETE the resource at the
destination before performing the MOVE.  Again, this makes it impossible
to MOVE a resource to the same location.  Finally, [WebDAV] states that
locks are lost on a MOVE, an outcome that seems undesirable in this
case.

5 New Headers

***** Decide which headers intended for MKREF also can be used with
MOVE, COPY, LOCK.  Is it possible to create a referential resource by
invoking LOCK?  If so, Ref-Type, Ref-Target, and Ref-Integrity would all
have to be usable with LOCK.  We might need a Resource-Type header to
say that it is a reference we are creating.  What about MOVE and COPY?
At the moment, we’re saying you can change Ref-Type, Ref-Target, and
Ref-Integrity on a MOVE or COPY.  It would be pretty weird to change
Resource-Type.  What about Hide-Target?

5.1 Ref-Target Entity Header

Ref-Target = "Ref-Target" ":" Coded-url

Coded-url is defined in [WebDAV], Section 8.4.


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The Ref-Target header is used with the MKREF method to identify the
target resource of the new referential resource being created.  It is a
required header in MKREF requests.  This header may also be used with
COPY and MOVE requests to change the target of the destination
reference.

Servers MUST include the Ref-Target header in the following responses
unless the Hide-Target header was used when the reference was created,
in which case the Ref-Target header MUST NOT be included in responses:

o Responses to GET and HEAD requests on redirect references

o Responses to all requests on direct references, unless the request
  included the Re-Direct header

5.2 Ref-Type Entity Header

Ref-Type = "Ref-Type" ":" ("DAV:direct" | "DAV:redirect")

The Ref-Type header is defined to distinguish between direct and
redirect references.  The possible values of this header are DAV:direct
and DAV:redirect.  If the header is not present on a MKREF request, the
server MUST treat the request as if it has a Ref-Type header with the
value DAV:redirect.  This header may also be used with a COPY or MOVE
request on a reference.  If this header is not present on a COPY or MOVE
request, the DAV:reftype property MUST be treated like any other live
property, as specified in [WebDAV] sections 7.8.2 and 7.9.1.

Servers MUST include the Ref-Target header in the following responses:

o Responses to all requests on both direct and redirect references

5.3 Ref-Integrity Entity Header

Ref-Integrity = "Ref-Integrity" ":" ("DAV:weak")

The Ref-Integrity header is defined to allow future support for strong
references.  It specifies whether the server should enforce referential
integrity for a referential resource being created with MKREF.  The only
value currently defined for the Ref-Integrity header is "DAV:weak",
which means that the server need not enforce referential integrity for
the newly created reference.  Other values may be used by private
agreement between the client and server.  If the header is not present
on a MKREF request, the server MUST treat the request as if it has a
Ref-Integrity header set to "DAV:weak".  This header may also be used
with COPY and MOVE requests.  If this header is not present on a COPY or
MOVE request, the DAV:refintegrity property MUST be treated like any
other live property, as specified in [WebDAV] sections 7.8.2 and 7.9.1.

***** Again, does DAV:weak mean that the server need not maintain
referential integrity, or that it must not?

Servers MUST include the Ref-Integrity header in the following
responses:


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o Responses to GET and HEAD requests on redirect references (or on
  direct references if the request included the Re-Direct header)

5.4 Re-Direct Request Header

Re-Direct = "Re-Direct" ":"

The optional Re-Direct header can be used on any request to a direct
reference except PUT or POST.  It forces the reference to behave like a
redirect reference for that request.  The operation will be applied to
the reference itself, not to its target.  If the Re-Direct header is
used on a request to any other sort of resource besides a direct
reference, the server SHOULD ignore it.  If the Re-Direct header is used
with a PUT or POST request on a direct reference, the server MUST
respond with a 400 (Bad Request).

***** Confirm behavior for exception cases.

***** What if there is a chain of direct references?  Suppose I want to
see the properties of the second reference in the chain - how would I do
that?

5.5 Hide-Target Entity Header

Hide-Target = "Hide-Target" ":"

The optional Hide-Target header is for use when creating a new direct
reference.  It specifies that the URI of the target resource is to be
hidden.  If this header is used when creating a direct reference, the
server MUST NOT include the Ref-Target header in any response to a
request on the reference. If the Hide-Target header is not present, the
server MUST assume that the DAV:reftarget property is not to be hidden.
If the Hide-Target header is used in a request with Ref-Type =
DAV:redirect, the server MUST ignore the Hide-Target header.

***** If we want to let clients change their minds about Hide-Target
after a reference has been created, we need a DAV:hidetarget property to
allow that.

5.6 Resource-Type Entity Header

Resource-Type = "Resource-Type" ":" ["DAV:collection" | "DAV:reference"
                                     |""]

The Resource-Type header contains the value of the DAV:resourcetype
property.  It is used with LOCK, MOVE, or COPY to set or change the
resource type.

***** Not needed unless you can create a reference with LOCK, or change
resource type with MOVE or COPY.

5.7 Ordered Entity Header

Ordered = "Ordered" ":" ("DAV:unordered" | "DAV:custom" | Coded-url)


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The Ordered header may be used with MKCOL to request that the new
collection be ordered and to specify its ordering semantics.  A value of
"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. A Coded-url value indicates that the collection
is ordered, and identifies the semantics of the ordering, allowing a
human user or software package to insert new collection members into the
ordering intelligently.  The Coded-url MAY point to a resource that
contains a definition of the semantics of the ordering.  A value of
"DAV:custom" indicates that the collection is to be ordered, but the
semantics of the ordering is not being advertised.

If the Ordered header is not present on a MKCOL request, the server MUST
treat the request as if it had an Ordered header with the value
"DAV:unordered".

5.8 Position Request Header

Position = "Position" ":" ("First" | "Last" |
                           (("Before" | "After") Coded-url))

The Position header may be used with MKREF, PUT, COPY, MOVE, or LOCK to
tell the server where in the collection ordering to position the
resource being added to the collection.  It may be used for both
ordinary and referential members.

If the Coded-url is a relative URL, it is interpreted relative to the
collection in which the resource is being created.

If the Position request header is not used, then:

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

If the request is adding a new member to the collection, the server MUST
append the new member to the end of the ordering (if the collection is
ordered).

5.9 DAV-Collections Entity Header

DAV-Collections = "DAV-Collections" ":" 1#collection-capability
collection-capability = "DAV:basicref" | "DAV:directref" |
"DAV:orderedcoll"

The DAV-Collections header is used in responses to OPTIONS requests, to
enumerate the collection-related capabilities of the resource.  A value
of "DAV:basicref" indicates that the resource provides basic referencing
(redirect references).  A value of "DAV:directref" indicates that the
resource provides direct references.  If a resource provides direct
references, it MUST also provide redirect references.  A value of
"DAV:orderedcoll" indicates that the resource provides ordered
collections.

6 New Properties


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6.1 reftarget Property

Name:       reftarget
Namespace:  DAV:
Purpose:    A required property of referential resources that provides
            an efficient way for clients to discover the URI of the
            target resource.  This is a read-only property, whose value
            can only be set by using the Ref-Target header with a MKREF,
            COPY, or MOVE request.
Value:      URI of the target resource.

<!ELEMENT reftarget href>

6.2 refintegrity Property

Name:       refintegrity
Namespace:  DAV:
Purpose:    A required property of a referential resource that indicates
            whether the server guarantees referential integrity for that
            reference.  The refintegrity property is defined to allow
            future support for strong references.  The only value
            currently defined for refintegrity is weak, which means that
            the server need not [does not?] enforce referential
            integrity for the reference.  Other values may be used by
            private agreement between the client and server.  This is a
            readonly property, whose value can only be set by using the
            Ref-Integrity header with a MKREF, COPY, or MOVE request.
Value:      weak

<!ELEMENT refintegrity (weak)>

6.3 reftype Property

Name:       reftype
Namespace:  DAV
Purpose:    A required property of a referential resource that
            identifies the reference as direct or redirect.  This is a
            read-only property, whose value can only be set by using
            the Ref-Type header with a MKREF, COPY, or MOVE request.
Value:      direct | redirect

<!ELEMENT reftype (direct | redirect)>

6.4 references Property

Name:       references
Namespace:  DAV:
Purpose:    Enables clients to discover, for any target resource, what
            references point to it and what collections contain it by
            reference.  This is an optional property.  If present, it is
            a read-only property, maintained by the server.
Value:      List of the URIs of the references that point to the target
            resource.

<!ELEMENT references (href*)>

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6.5 orderingtype Property

Name:       orderingtype
Namespace:  DAV:
Purpose:    Indicates whether the collection is ordered and, if so,
            uniquely identifies the semantics of the ordering being
            used.  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.
Value:      unordered for an unordered collection, or a URI that
            uniquely identifies the semantics of the collection's
            ordering.  The URI MAY point to a definition of the ordering
            semantics.  The value custom may be used for a collection
            that is to be ordered, but for which the semantics are not
            being advertised.

<!ELEMENT orderingtype (arbitrary | custom | href) >

7 New XML Elements

7.1 reference XML Element

Name:       reference
Namespace:  DAV:
Purpose:    A new value of the DAV:resourcetype property that identifies
            its resource as a referential resource.  The
            DAV:resourcetype property of a referential resource MUST
            have this value.
Value:      EMPTY

<!ELEMENT reference EMPTY >

7.2 direct XML Element

Name:       direct
Namespace:  DAV:
Purpose:    A value for the DAV:reftype property that identifies its
            resource as a direct reference.
Value:      EMPTY

<!ELEMENT direct EMPTY >

7.3 redirect XML Element

Name:       redirect
Namespace:  DAV:
Purpose:    A value for the DAV:reftype property that identifies its
            resource as a redirect reference.
Value:      EMPTY

<!ELEMENT redirect EMPTY >

7.4 weak XML Element

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Name:       weak
Namespace:  DAV:
Purpose:    The only value currently defined for the DAV:refintegrity
            property.  It means that the server need not [does not?]
            enforce referential integrity for the reference to which the
            property belongs.
Value:      EMPTY

<!ELEMENT weak EMPTY >

7.5 unordered XML Element

Name:       unordered
Namespace:  DAV:
Purpose:    A value of the DAV:orderingtype property that 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.
Value:      EMPTY

<!ELEMENT unordered EMPTY >

7.6 custom XML Element

Name:       custom
Namespace:  DAV:
Purpose:    A value of the DAV:orderingtype property that indicates that
            the collection is ordered, but the semantics of the ordering
            are not being advertised.
Value:      EMPTY

<!ELEMENT custom EMPTY >

7.7 order XML Element

Name:       order
Namespace:  DAV:
Purpose:    For use with the new ORDERPATCH method.  Describes a change
            to be made in a collection ordering.
Value:      A description of the new positions of collection members in
            the collection's ordering.

<!ELEMENT order (member+) >

7.8 ordermember XML Element

Name:       ordermember
Namespace:  DAV:
Purpose:    Occurs in the order XML Element, and describes the new
            position of a single collection member in the collection's
            ordering.
Value:      An href containing the relative URI of the collection
            member, and a description of its new position in the
            ordering.  The href XML element is defined in [WebDAV],

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            Section 11.3.

<!ELEMENT ordermember (href, position) >

7.9 position XML Element

Name:       position
Namespace:  DAV:
Purpose:    Occurs in the member XML element.  Describes the new
            position in a collection's ordering of one of the
            collection's members.
Value:      The new position can be described as first in the
            collection's ordering, last in the collection's ordering,
            before some other member of the collection, or after some
            other member of the collection.

<!ELEMENT position (first | last | before | after)>

7.10 first XML Element

Name:       first
Namespace:  DAV:
Purpose:    Occurs in the position XML element.  Describes the
            collection member's position as first in the collection's
            ordering.
Value:      EMPTY

<!ELEMENT first EMPTY >

7.11 last XML Element

Name:       last
Namespace:  DAV:
Purpose:    Occurs in the position XML element.  Describes the
            collection member's position as last in the collection's
            ordering.
Value:      EMPTY

<!ELEMENT last EMPTY >

7.12 before XML Element

Name:       before
Namespace:  DAV:
Purpose:    Occurs in the position XML element.  Describes the
            collection member's position as coming before some other
            collection member in the collection's ordering.
Value:      href of the member it precedes in the ordering

<!ELEMENT before href >

7.13 after XML Element

Name:       after
Namespace:  DAV:

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Purpose:    Occurs in the position XML element.  Describes the
            collection member's position as coming after some other
            collection member in the collection's ordering.
Value:      href of the member it follows in the ordering

<!ELEMENT after href >

8 Extensions to the DAV:multistatus XML Element

As described in Section 3.12, the DAV:reftype property and the
DAV:reftarget property may be returned in the DAV:response element of a
207 Multi-Status response, to indicate that a problem is not with a
direct reference, but with its target resource.

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

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

9 Capability Discovery

9.1 Using OPTIONS

Since referencing and ordering are independent capabilities, a resource
MAY support either or both.  A resource that provides referencing MUST
support redirect references, and MAY in addition support direct
references.  A response to an OPTIONS request MUST indicate which of
these capabilities the resource supports.

This specification defines two new methods: MKREF in support of
referencing, and ORDERPATCH in support of ordering.  The response MUST
indicate which of these methods the resource allows.  In addition, the
response MUST include the DAV-Collections header, listing those of the
three collection-related capabilities the resource provides.  Possible
values for the DAV-Collections header are DAV:basicref, DAV:directref,
and DAV:orderedcoll.

9.2 Example

Request:

OPTIONS /somecollection/ 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, MKREF, ORDERPATCH
Public: OPTIONS, GET, HEAD, POST, PUT, DELETE, TRACE, COPY, MOVE, MKCOL,
PROPFIND, PROPPATCH, LOCK, UNLOCK, MKREF, ORDERPATCH

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DAV-Collections: DAV:basicref, DAV:directref, DAV:orderedcoll

This response indicates that the resource /somecollection/ provides
basic referencing, direct references, and ordered collections.

10 Dependencies on Other Specifications

TBD

11 Security Considerations

This section is provided to detail issues concerning security
implications of which WebDAV applications need to be aware.

All of the security considerations of HTTP/1.1 and the base WebDAV
protocol also apply to WebDAV collections.  In addition, referential
resources and ordered collections introduce several new security
concerns and increase the risk of some existing threats.  These issues
are detailed below.

11.1 Redirect Loops

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

11.2 References and Denial of Service

The introduction of referential resources creates a new avenue for
denial of service attacks. Clients can create heavily used references to
target locations that were not designed for heavy usage.

11.3 Maintaining Referential Integrity May Reveal Private Locations

Although this specification does not require servers to maintain
referential integrity, it does not prevent them from doing so.  If a
server updates a reference’s DAV:reftarget property when its target
resource is moved, there is the risk that a private location will be
revealed in the new value of DAV:reftarget.  Clients can avoid this risk
by doing a COPY followed by a DELETE rather than a MOVE.

If the owner of the target also owns the direct reference to it, the
Hide-Target header can be used when creating the direct reference to
prevent others from discovering the location of the target through that
reference.

11.4 DAV:references and Denial of Service

If the server maintains the DAV:references property in response to
references created in other administrative domains, it is exposed to
hostile attempts to make it devote resources to adding references to the
list.

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11.5 DAV:references and Malicious Deletion of Resources

Servers that support the DAV:references property should insure that
clients cannot create editable properties with the name DAV:references.
An editable DAV:references property would constitute a security risk on
servers that enforce referential integrity by deleting references when
their target is deleted.  These servers could be tricked into deleting a
resource by listing it in the DAV:references property of some target
resource.

11.6 Malicious Modifications of Ordering

Particularly in large collections, moving a collection member to a
different position in the ordering can make it very difficult for users
to find.

11.7 Denial of Service and DAV:orderingtype

There may be some risk of denial of service at sites that are advertised
in the DAV:orderingtype 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:orderingtype.

12 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 [Alvestrand].

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 [Alvestrand].

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.

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

13 IANA Considerations


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TBD

14 Copyright

15 Intellectual Property

16 Acknowledgements

This draft has benefited from thoughtful discussion by Steve Carter,
Geoffrey Clemm, Ellis Cohen, Bruce Cragun, Spencer Dawkins, Rajiv
Dulepet, David Durand, Chuck Fay, Roy Fielding, Yaron Goland, Fred Hitt,
Alex Hopmann, Marcus Jager, Chris Kaler, Rohit Khare, Daniel LaLiberte,
Steve Martin, Surendra Koduru Reddy, Sam Ruby, Bradley Sergeant, Nick
Shelness, John Stracke, John Tigue, John Turner, and others.

17 References

[WebDAV] Y. Y. Goland, E. J. Whitehead, Jr., A. Faizi, S. R. Carter, D.
Jensen, "HTTP Extensions for Distributed Authoring - WebDAV." Draft-
ietf-webdav-protocol-09. Internet Draft, work in progress.  Microsoft,
U.C. Irvine, Netscape, Novell. November, 1998.

[DASL] Saveen Reddy, D. Jensen, Surendra Reddy, R. Henderson, J. Davis,
A. Babich, "DAV Searching & Locating." Draft-reddy-dasl-protocol-03.
Internet Draft, work in progress. Microsoft, Novell, Oracle, Netscape,
Xerox, Filenet.  November, 1998.

[WebDAVReq] J. Slein, J. Davis, "Requirements for Advanced Collection
Functionality in WebDAV." Draft-ietf-webdav-collection-reqts-02.
Internet Draft, work in progress.  Xerox, 1998.

[HTTP] R. Fielding, J. Gettys, J. Mogul, H. Frystyk, T. Berners-Lee,
"Hypertext Transfer Protocol -- HTTP/1.1." RFC 2068.  UC Irvine, DEC,
MIT/LCS.  January, 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.

18 Authors' Addresses

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

J. Davis
Xerox Corporation
3333 Coyote Hill Road
Palo Alto, CA 94304
Email: jdavis@parc.xerox.com

A. Babich
FileNet Corporation

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3565 Harbor Boulevard
Costa Mesa, CA 92626-1420
Email: ababich@filenet.com

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

19 Appendices

19.1 Appendix 1 - Extensions to the WebDAV Document Type Definition

<!--============= XML Elements from Section 7 =======================-->
<!ELEMENT reference EMPTY >
<!ELEMENT direct EMPTY >
<!ELEMENT redirect EMPTY >
<!ELEMENT weak EMPTY >
<!ELEMENT unordered EMPTY >
<!ELEMENT custom EMPTY >
<!ELEMENT order (member+) >
<!ELEMENT ordermember (href, position) >
<!ELEMENT position (first | last | before | after)>
<!ELEMENT first EMPTY >
<!ELEMENT last EMPTY >
<!ELEMENT before href >
<!ELEMENT after href >
<!--============= Property Elements from Section 6 ==================-->
<!ELEMENT reftarget href>
<!ELEMENT refintegrity (weak)>
<!ELEMENT reftype (direct | redirect)>
<!ELEMENT references (href*)>
<!ELEMENT orderingtype (arbitrary | custom | href) >
<!--======== Changes to the DAV:multistatus Element from Section 8 ==-->
<!ELEMENT response (href, ((href*, status, reftype?, reftarget?) |
(propstat+)), responsedescription?) >

Expires May 10, 1999

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