Internet Engineering Task Force (IETF) G. Clemm
Request for Comments: 5842 IBM
Category: Experimental J. Crawford
ISSN: 2070-1721 IBM Research
J. Reschke, Ed.
greenbytes
J. Whitehead
U.C. Santa Cruz
April 2010
Binding Extensions to
Web Distributed Authoring and Versioning (WebDAV)
Abstract
This specification defines bindings, and the BIND method for creating
multiple bindings to the same resource. Creating a new binding to a
resource causes at least one new URI to be mapped to that resource.
Servers are required to ensure the integrity of any bindings that
they allow to be created.
Status of This Memo
This document is not an Internet Standards Track specification; it is
published for examination, experimental implementation, and
evaluation.
This document defines an Experimental Protocol for the Internet
community. This document is a product of the Internet Engineering
Task Force (IETF). It represents the consensus of the IETF
community. It has received public review and has been approved for
publication by the Internet Engineering Steering Group (IESG). Not
all documents approved by the IESG are a candidate for any level of
Internet Standard; see Section 2 of RFC 5741.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
http://www.rfc-editor.org/info/rfc5842.
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Copyright Notice
Copyright (c) 2010 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
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the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
This document may contain material from IETF Documents or IETF
Contributions published or made publicly available before November
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material may not have granted the IETF Trust the right to allow
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Without obtaining an adequate license from the person(s) controlling
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outside the IETF Standards Process, and derivative works of it may
not be created outside the IETF Standards Process, except to format
it for publication as an RFC or to translate it into languages other
than English.
Table of Contents
1. Introduction ....................................................4
1.1. Terminology ................................................5
1.2. Method Preconditions and Postconditions ....................6
2. Overview of Bindings ............................................7
2.1. Bindings to Collections ....................................7
2.1.1. Bind Loops ..........................................8
2.2. URI Mappings Created by a New Binding ......................8
2.3. COPY and Bindings ..........................................9
2.3.1. Example: COPY with "Depth: infinity" in
Presence of Bind Loops .............................11
2.3.2. Example: COPY Updating Multiple Bindings ...........13
2.3.3. Example: COPY with "Depth: infinity" with
Multiple Bindings to a Leaf Resource ...............14
2.4. DELETE and Bindings .......................................15
2.5. MOVE and Bindings .........................................15
2.5.1. Example: Simple MOVE ...............................16
2.5.2. Example: MOVE Request Causing a Bind Loop ..........16
2.6. PROPFIND and Bindings .....................................18
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2.7. Determining Whether Two Bindings Are to the Same
Resource ..................................................18
2.8. Discovering the Bindings to a Resource ....................19
3. Properties .....................................................19
3.1. DAV:resource-id Property ..................................20
3.2. DAV:parent-set Property ...................................20
3.2.1. Example for DAV:parent-set Property ................20
4. BIND Method ....................................................21
4.1. Example: BIND .............................................24
5. UNBIND Method ..................................................24
5.1. Example: UNBIND ...........................................26
6. REBIND Method ..................................................26
6.1. Example: REBIND ...........................................28
6.2. Example: REBIND in Presence of Locks and Bind Loops .......29
7. Additional Status Codes ........................................31
7.1. 208 Already Reported ......................................31
7.1.1. Example: PROPFIND by Bind-Aware Client .............32
7.1.2. Example: PROPFIND by Non-Bind-Aware Client .........34
7.2. 508 Loop Detected .........................................34
8. Capability Discovery ...........................................34
8.1. OPTIONS Method ............................................34
8.2. 'DAV' Request Header ......................................34
9. Relationship to Locking in WebDAV ..............................35
9.1. Example: Locking and Multiple Bindings ....................36
10. Relationship to WebDAV Access Control Protocol ................37
11. Relationship to Versioning Extensions to WebDAV ...............37
12. Security Considerations .......................................40
12.1. Privacy Concerns .........................................40
12.2. Bind Loops ...............................................40
12.3. Bindings and Denial of Service ...........................40
12.4. Private Locations May Be Revealed ........................40
12.5. DAV:parent-set and Denial of Service .....................41
13. Internationalization Considerations ...........................41
14. IANA Considerations ...........................................41
15. Acknowledgements ..............................................41
16. References ....................................................41
16.1. Normative References .....................................41
16.2. Informative References ...................................42
Index .............................................................42
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1. Introduction
This specification extends the WebDAV Distributed Authoring Protocol
([RFC4918]) to enable clients to create new access paths to existing
resources. This capability is useful for several reasons:
URIs of WebDAV-compliant resources are hierarchical and correspond to
a hierarchy of collections in resource space. The WebDAV Distributed
Authoring Protocol makes it possible to organize these resources into
hierarchies, placing them into groupings, known as collections, which
are more easily browsed and manipulated than a single flat
collection. However, hierarchies require categorization decisions
that locate resources at a single location in the hierarchy, a
drawback when a resource has multiple valid categories. For example,
in a hierarchy of vehicle descriptions containing collections for
cars and boats, a description of a combination car/boat vehicle could
belong in either collection. Ideally, the description should be
accessible from both. Allowing clients to create new URIs that
access the existing resource lets them put that resource into
multiple collections.
Hierarchies also make resource sharing more difficult, since
resources that have utility across many collections are still forced
into a single collection. For example, the mathematics department at
one university might create a collection of information on fractals
that contains bindings to some local resources but also provides
access to some resources at other universities. For many reasons, it
may be undesirable to make physical copies of the shared resources on
the local server, for example, to conserve disk space, to respect
copyright constraints, or to make any changes in the shared resources
visible automatically. Being able to create new access paths to
existing resources in other collections or even on other servers is
useful for this sort of case.
The BIND method, defined here, provides a mechanism for allowing
clients to create alternative access paths to existing WebDAV
resources. HTTP [RFC2616] and WebDAV [RFC4918] methods are able to
work because there are mappings between URIs and resources. A method
is addressed to a URI, and the server follows the mapping from that
URI to a resource, applying the method to that resource. Multiple
URIs may be mapped to the same resource, but until now, there has
been no way for clients to create additional URIs mapped to existing
resources.
BIND lets clients associate a new URI with an existing WebDAV
resource, and this URI can then be used to submit requests to the
resource. Since URIs of WebDAV resources are hierarchical, and
correspond to a hierarchy of collections in resource space, the BIND
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method also has the effect of adding the resource to a collection.
As new URIs are associated with the resource, it appears in
additional collections.
A BIND request does not create a new resource, but simply makes a new
URI for submitting requests to an existing resource available. The
new URI is indistinguishable from any other URI when submitting a
request to a resource. Only one round trip is needed to submit a
request to the intended target. Servers are required to enforce the
integrity of the relationships between the new URIs and the resources
associated with them. Consequently, it may be very costly for
servers to support BIND requests that cross server boundaries.
This specification is organized as follows. Section 1.1 defines
terminology used in the rest of the specification, while Section 2
overviews bindings. Section 3 defines the new properties needed to
support multiple bindings to the same resource. Section 4 specifies
the BIND method, used to create multiple bindings to the same
resource. Section 5 specifies the UNBIND method, used to remove a
binding to a resource. Section 6 specifies the REBIND method, used
to move a binding to another collection.
1.1. Terminology
The terminology used here follows and extends that in the WebDAV
Distributed Authoring Protocol specification [RFC4918].
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119].
This document uses XML DTD fragments ([XML]) as a notational
convention, using the rules defined in Section 17 of [RFC4918].
URI Mapping
A relation between an absolute URI and a resource. For an
absolute URI U and the resource it identifies R, the URI mapping
can be thought of as (U => R). Since a resource can represent
items that are not network retrievable as well as those that are,
it is possible for a resource to have zero, one, or many URI
mappings. Mapping a resource to an "http"-scheme URI makes it
possible to submit HTTP requests to the resource using the URI.
Path Segment
Informally, the characters found between slashes ("/") in a URI.
Formally, as defined in Section 3.3 of [RFC3986].
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Binding
A relation between a single path segment (in a collection) and a
resource. A binding is part of the state of a collection. If two
different collections contain a binding between the same path
segment and the same resource, these are two distinct bindings.
So for a collection C, a path segment S, and a resource R, the
binding can be thought of as C:(S -> R). Bindings create URI
mappings, and hence allow requests to be sent to a single resource
from multiple locations in a URI namespace. For example, given a
collection C (accessible through the URI
http://www.example.com/CollX), a path segment S (equal to
"foo.html"), and a resource R, then creating the binding C: (S ->
R) makes it possible to use the URI
http://www.example.com/CollX/foo.html to access R.
Collection
A resource that contains, as part of its state, a set of bindings
that identify internal member resources.
Internal Member URI
The URI that identifies an internal member of a collection and
that consists of the URI for the collection, followed by a slash
character ('/'), followed by the path segment of the binding for
that internal member.
Binding Integrity
The property of a binding that says that:
* the binding continues to exist, and
* the identity of the resource identified by that binding does
not change,
unless an explicit request is executed that is defined to delete
that binding (examples of requests that delete a binding are
DELETE, MOVE, and -- defined later on -- UNBIND and REBIND).
1.2. Method Preconditions and Postconditions
See Section 16 of [RFC4918] for the definitions of "precondition" and
"postcondition".
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2. Overview of Bindings
Bindings are part of the state of a collection. They define the
internal members of the collection and the names of those internal
members.
Bindings are added and removed by a variety of existing HTTP methods.
A method that creates a new resource, such as PUT, COPY, and MKCOL,
adds a binding. A method that deletes a resource, such as DELETE,
removes a binding. A method that moves a resource (e.g., MOVE) both
adds a binding (in the destination collection) and removes a binding
(in the source collection). The BIND method introduced here provides
a mechanism for adding a second binding to an existing resource.
There is no difference between an initial binding added by PUT, COPY,
or MKCOL and additional bindings added with BIND.
It would be very undesirable if one binding could be destroyed as a
side effect of operating on the resource through a different binding.
In particular, the removal of one binding to a resource (e.g., with a
DELETE or a MOVE) MUST NOT disrupt another binding to that resource,
e.g., by turning that binding into a dangling path segment. The
server MUST NOT reclaim system resources after removing one binding,
while other bindings to the resource remain. In other words, the
server MUST maintain the integrity of a binding. It is permissible,
however, for future method definitions (e.g., a DESTROY method) to
have semantics that explicitly remove all bindings and/or immediately
reclaim system resources.
Note: the collection model described herein is not compatible with
systems in which resources inherit properties based solely on the
access path, as the ability to create additional bindings will
cause a single resource to appear as member of several different
collections at the same time.
2.1. Bindings to Collections
Creating a new binding to a collection makes each resource associated
with a binding in that collection accessible via a new URI, and thus
creates new URI mappings to those resources but no new bindings.
For example, suppose a new binding CollY is created for collection C1
in the figure below. It immediately becomes possible to access
resource R1 using the URI /CollY/x.gif and to access resource R2
using the URI /CollY/y.jpg, but no new bindings for these child
resources were created. This is because bindings are part of the
state of a collection, and they associate a URI that is relative to
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that collection with its target resource. No change to the bindings
in Collection C1 is needed to make its children accessible using
/CollY/x.gif and /CollY/y.jpg.
+-------------------------+
| Root Collection |
| bindings: |
| CollX CollY |
+-------------------------+
| /
| /
| /
+------------------+
| Collection C1 |
| bindings: |
| x.gif y.jpg |
+------------------+
| \
| \
| \
+-------------+ +-------------+
| Resource R1 | | Resource R2 |
+-------------+ +-------------+
2.1.1. Bind Loops
Bindings to collections can result in loops ("cycles"), which servers
MUST detect when processing "Depth: infinity" requests. It is
sometimes possible to complete an operation in spite of the presence
of a loop. For instance, a PROPFIND can still succeed if the server
uses the new status code 208 (Already Reported) defined in
Section 7.1.
However, the 508 (Loop Detected) status code is defined in
Section 7.2 for use in contexts where an operation is terminated
because a loop was encountered.
Support for loops is OPTIONAL: servers MAY reject requests that would
lead to the creation of a bind loop (see DAV:cycle-allowed
precondition defined in Section 4).
2.2. URI Mappings Created by a New Binding
Suppose a binding from "Binding-Name" to resource R is to be added to
a collection, C. Then if C-MAP is the set of URIs that were mapped
to C before the BIND request, then for each URI "C-URI" in C-MAP, the
URI "C-URI/Binding-Name" is mapped to resource R following the BIND
request.
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For example, if a binding from "foo.html" to R is added to a
collection C, and if the following URIs are mapped to C:
http://www.example.com/A/1/
http://example.com/A/one/
then the following new mappings to R are introduced:
http://www.example.com/A/1/foo.html
http://example.com/A/one/foo.html
Note that if R is a collection, additional URI mappings are created
to the descendents of R. Also, note that if a binding is made in
collection C to C itself (or to a parent of C), an infinite number of
mappings are introduced.
For example, if a binding from "myself" to C is then added to C, the
following infinite number of additional mappings to C are introduced:
http://www.example.com/A/1/myself
http://www.example.com/A/1/myself/myself
...
and the following infinite number of additional mappings to R are
introduced:
http://www.example.com/A/1/myself/foo.html
http://www.example.com/A/1/myself/myself/foo.html
...
2.3. COPY and Bindings
As defined in Section 9.8 of [RFC4918], COPY causes the resource
identified by the Request-URI to be duplicated and makes the new
resource accessible using the URI specified in the Destination
header. Upon successful completion of a COPY, a new binding is
created between the last path segment of the Destination header and
the destination resource. The new binding is added to its parent
collection, identified by the Destination header minus its final
segment.
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The following figure shows an example: suppose that a COPY is issued
to URI-3 for resource R (which is also mapped to URI-1 and URI-2),
with the Destination header set to URI-X. After successful
completion of the COPY operation, resource R is duplicated to create
resource R', and a new binding has been created that creates at least
the URI mapping between URI-X and the new resource (although other
URI mappings may also have been created).
URI-1 URI-2 URI-3 URI-X
| | | |
| | | <---- URI Mappings ----> |
| | | |
+---------------------+ +------------------------+
| Resource R | | Resource R' |
+---------------------+ +------------------------+
It might be thought that a COPY request with "Depth: 0" on a
collection would duplicate its bindings, since bindings are part of
the collection's state. This is not the case, however. The
definition of Depth in [RFC4918] makes it clear that a "Depth: 0"
request does not apply to a collection's members. Consequently, a
COPY with "Depth: 0" does not duplicate the bindings contained by the
collection.
If a COPY request causes an existing resource to be updated, the
bindings to that resource MUST be unaffected by the COPY request.
Using the preceding example, suppose that a COPY request is issued to
URI-X for resource R', with the Destination header set to URI-2. The
content and dead properties of resource R would be updated to be a
copy of those of resource R', but the mappings from URI-1, URI-2, and
URI-3 to resource R remain unaffected. If, because of multiple
bindings to a resource, more than one source resource updates a
single destination resource, the order of the updates is server
defined (see Section 2.3.2 for an example).
If a COPY request would cause a new resource to be created as a copy
of an existing resource, and that COPY request has already created a
copy of that existing resource, the COPY request instead creates
another binding to the previous copy, instead of creating a new
resource (see Section 2.3.3 for an example).
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2.3.1. Example: COPY with "Depth: infinity" in Presence of Bind Loops
As an example of how COPY with "Depth: infinity" would work in the
presence of bindings, consider the following collection:
+------------------+
| Root Collection |
| bindings: |
| CollX |
+------------------+
|
|
+-------------------------------+
| Collection C1 |<-------+
| bindings: | |
| x.gif CollY | |
+-------------------------------+ |
| \ (creates loop) |
| \ |
+-------------+ +------------------+ |
| Resource R1 | | Collection C2 | |
+-------------+ | bindings: | |
| y.gif CollZ | |
+------------------+ |
| | |
| +--------+
|
+-------------+
| Resource R2 |
+-------------+
If a COPY request with "Depth: infinity" is submitted to /CollX, with
a destination of /CollA, the outcome of the copy operation is that a
copy of the tree is replicated to the target /CollA:
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+------------------+
| Root Collection |
| bindings: |
| CollX CollA |
+------------------+
| |
| +---------------------------+
| |
+-------------------+ |
| Collection C1 |<------------------+ |
| bindings: | | |
| x.gif CollY | | |
+-------------------+ | |
| \ (creates loop) | |
| \ | |
+-------------+ +-----------------+ | |
| Resource R1 | | Collection C2 | | |
+-------------+ | bindings: | | |
| y.gif CollZ | | |
+-----------------+ | |
| | | |
| +-------+ |
| |
+-------------+ |
| Resource R2 | |
+-------------+ |
|
+-------------------------------+
|
+-------------------+
| Collection C3 |<------------------+
| bindings: | |
| x.gif CollY | |
+-------------------+ |
| \ (creates loop) |
| \ |
+-------------+ +-----------------+ |
| Resource R3 | | Collection C4 | |
+-------------+ | bindings: | |
| y.gif CollZ | |
+-----------------+ |
| | |
| +-------+
|
+-------------+
| Resource R4 |
+-------------+
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Note that the same would apply for more complex loops.
2.3.2. Example: COPY Updating Multiple Bindings
Given the following collection hierarchy:
+------------------+
| Root Collection |
| bindings: |
| CollX CollY |
+------------------+
/ \
/ \
/ \
+--------------------------+ +-----------------+
| Collection C1 | | Collection C2 |
| bindings: | | bindings: |
| x.gif y.gif | | x.gif y.gif |
+--------------------------+ +-----------------+
| | | |
| | | |
+-------------+ +-------------+ +-------------+
| Resource R1 | | Resource R2 | | Resource R3 |
+-------------+ +-------------+ +-------------+
A COPY of /CollX with "Depth: infinity" to /CollY will not result in
a changed hierarchy, and Resource R3 will be updated with the content
of either Resource R1 or Resource R2.
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2.3.3. Example: COPY with "Depth: infinity" with Multiple Bindings to a
Leaf Resource
Given the following collection hierarchy:
+------------------+
| Root Collection |
| bindings: |
| CollX |
+------------------+
|
|
|
+----------------+
| Collection C1 |
| bindings: |
| x.gif y.gif |
+----------------+
| |
| |
+-------------+
| Resource R1 |
+-------------+
A COPY of /CollX with "Depth: infinity" to /CollY results in the
following collection hierarchy:
+------------------+
| Root Collection |
| bindings: |
| CollX CollY |
+------------------+
| \
| \
| \
+----------------+ +-----------------+
| Collection C1 | | Collection C2 |
| bindings: | | bindings: |
| x.gif y.gif | | x.gif y.gif |
+----------------+ +-----------------+
| | | |
| | | |
+-------------+ +-------------+
| Resource R1 | | Resource R2 |
+-------------+ +-------------+
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2.4. DELETE and Bindings
When there are multiple bindings to a resource, a DELETE applied to
that resource MUST NOT remove any bindings to that resource other
than the one identified by the Request-URI. For example, suppose the
collection identified by the URI "/a" has a binding named "x" to a
resource R, and another collection identified by "/b" has a binding
named "y" to the same resource R. Then, a DELETE applied to "/a/x"
removes the binding named "x" from "/a" but MUST NOT remove the
binding named "y" from "/b" (i.e., after the DELETE, "/y/b" continues
to identify the resource R).
When DELETE is applied to a collection, it MUST NOT modify the
membership of any other collection that is not itself a member of the
collection being deleted. For example, if both "/a/.../x" and
"/b/.../y" identify the same collection, C, then applying DELETE to
"/a" must not delete an internal member from C or from any other
collection that is a member of C, because that would modify the
membership of "/b".
If a collection supports the UNBIND method (see Section 5), a DELETE
of an internal member of a collection MAY be implemented as an UNBIND
request. In this case, applying DELETE to a Request-URI has the
effect of removing the binding identified by the final segment of the
Request-URI from the collection identified by the Request-URI minus
its final segment. Although [RFC4918] allows a DELETE to be a non-
atomic operation, when the DELETE operation is implemented as an
UNBIND, the operation is atomic. In particular, a DELETE on a
hierarchy of resources is simply the removal of a binding to the
collection identified by the Request-URI.
2.5. MOVE and Bindings
When MOVE is applied to a resource, the other bindings to that
resource MUST be unaffected; and if the resource being moved is a
collection, the bindings to any members of that collection MUST be
unaffected. Also, if MOVE is used with Overwrite:T to delete an
existing resource, the constraints specified for DELETE apply.
If the destination collection of a MOVE request supports the REBIND
method (see Section 6), a MOVE of a resource into that collection MAY
be implemented as a REBIND request. Although [RFC4918] allows a MOVE
to be a non-atomic operation, when the MOVE operation is implemented
as a REBIND, the operation is atomic. In particular, applying a MOVE
to a Request-URI and a Destination URI has the effect of removing a
binding to a resource (at the Request-URI) and creating a new binding
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to that resource (at the Destination URI). Even when the Request-URI
identifies a collection, the MOVE operation involves only removing
one binding to that collection and adding another.
2.5.1. Example: Simple MOVE
As an example, suppose that a MOVE is issued to URI-3 for resource R
below (which is also mapped to URI-1 and URI-2), with the Destination
header set to URI-X. After successful completion of the MOVE
operation, a new binding has been created that creates the URI
mapping between URI-X and resource R. The binding corresponding to
the final segment of URI-3 has been removed, which also causes the
URI mapping between URI-3 and R to be removed. If resource R were a
collection, old URI-3-based mappings to members of R would have been
removed, and new URI-X-based mappings to members of R would have been
created.
>> Before Request:
URI-1 URI-2 URI-3
| | |
| | | <---- URI Mappings
| | |
+---------------------+
| Resource R |
+---------------------+
>> After Request:
URI-1 URI-2 URI-X
| | |
| | | <---- URI Mappings
| | |
+---------------------+
| Resource R |
+---------------------+
2.5.2. Example: MOVE Request Causing a Bind Loop
Note that in the presence of collection bindings, a MOVE request can
cause the creation of a bind loop.
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Consider the top-level collections C1 and C2 with URIs "/CollW/" and
"/CollX/". C1 also contains an additional binding named "CollY" to
C2:
+------------------+
| Root Collection |
| bindings: |
| CollW CollX |
+------------------+
| |
| |
+------------------+ |
| Collection C1 | |
| bindings: | |
| CollY | |
+------------------+ |
| |
| |
+------------------+
| Collection C2 |
| |
| |
+------------------+
In this case, the MOVE request below would cause a bind loop:
>> Request:
MOVE /CollW HTTP/1.1
Host: example.com
Destination: /CollX/CollZ
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If the request succeeded, the resulting state would be:
+------------------+
| Root Collection |
| bindings: |
| CollX |
+------------------+
|
|
+------------------+ |
| Collection C1 | |
+----> | bindings: | |
| | CollY | |
| +------------------+ |
| | |
| | |
| +------------------+
| | Collection C2 |
| | bindings: |
| | CollZ |
| +------------------+
| |
| |
+-------------------+
2.6. PROPFIND and Bindings
Consistent with [RFC4918], the value of a dead property MUST be
independent of the number of bindings to its host resource or of the
path submitted to PROPFIND. On the other hand, the behavior for each
live property depends on its individual definition (for example, see
[RFC3744], Section 5, Paragraph 2 for a case where the value is
independent of its path and bindings, and [RFC4918], Section 8.8 for
a discussion about the live properties DAV:getetag and DAV:
getlastmodified, which may behave differently).
2.7. Determining Whether Two Bindings Are to the Same Resource
It is useful to have some way of determining whether two bindings are
to the same resource. Two resources might have identical contents
and properties, but not be the same resource (e.g., an update to one
resource does not affect the other resource).
The REQUIRED DAV:resource-id property defined in Section 3.1 is a
resource identifier, which MUST be unique across all resources for
all time. If the values of DAV:resource-id returned by PROPFIND
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requests through two bindings are identical character by character,
the client can be assured that the two bindings are to the same
resource.
The DAV:resource-id property is created, and its value assigned, when
the resource is created. The value of DAV:resource-id MUST NOT be
changed. Even after the resource is no longer accessible through any
URI, that value MUST NOT be reassigned to another resource's DAV:
resource-id property.
Any method that creates a new resource MUST assign a new, unique
value to its DAV:resource-id property. For example, a PUT applied to
a null resource, COPY (when not overwriting an existing target) and
CHECKIN (see [RFC3253], Section 4.4) must assign a new, unique value
to the DAV:resource-id property of the new resource they create.
On the other hand, any method that affects an existing resource must
not change the value of its DAV:resource-id property. Specifically,
a PUT or a COPY that updates an existing resource must not change the
value of its DAV:resource-id property. A REBIND, since it does not
create a new resource, but only changes the location of an existing
resource, must not change the value of the DAV:resource-id property.
2.8. Discovering the Bindings to a Resource
An OPTIONAL DAV:parent-set property on a resource provides a list of
the bindings that associate a collection and a URI segment with that
resource. If the DAV:parent-set property exists on a given resource,
it MUST contain a complete list of all bindings to that resource that
the client is authorized to see. When deciding whether to support
the DAV:parent-set property, server implementers / administrators
should balance the benefits it provides against the cost of
maintaining the property and the security risks enumerated in
Sections 12.4 and 12.5.
3. Properties
The bind feature introduces the properties defined below.
A DAV:allprop PROPFIND request SHOULD NOT return any of the
properties defined by this document. This allows a binding server to
perform efficiently when a naive client, which does not understand
the cost of asking a server to compute all possible live properties,
issues a DAV:allprop PROPFIND request.
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3.1. DAV:resource-id Property
The DAV:resource-id property is a REQUIRED property that enables
clients to determine whether two bindings are to the same resource.
The value of DAV:resource-id is a URI, and may use any registered URI
scheme that guarantees the uniqueness of the value across all
resources for all time (e.g., the urn:uuid: URN namespace defined in
[RFC4122] or the opaquelocktoken: URI scheme defined in [RFC4918]).
<!ELEMENT resource-id (href)>
3.2. DAV:parent-set Property
The DAV:parent-set property is an OPTIONAL property that enables
clients to discover what collections contain a binding to this
resource (i.e., what collections have that resource as an internal
member). It contains an href/segment pair for each collection that
has a binding to the resource. The href identifies the collection,
and the segment identifies the binding name of that resource in that
collection.
A given collection MUST appear only once in the DAV:parent-set for
any given binding, even if there are multiple URI mappings to that
collection.
<!ELEMENT parent-set (parent)*>
<!ELEMENT parent (href, segment)>
<!ELEMENT segment (#PCDATA)>
<!-- PCDATA value: segment, as defined in Section 3.3 of
[RFC3986] -->
3.2.1. Example for DAV:parent-set Property
For example, if collection C1 is mapped to both /CollX and /CollY,
and C1 contains a binding named "x.gif" to a resource R1, then either
[/CollX, x.gif] or [/CollY, x.gif] can appear in the DAV:parent-set
of R1, but not both. But if C1 also had a binding named "y.gif" to
R1, then there would be two entries for C1 in the DAV:parent-set of
R1 (i.e., both [/CollX, x.gif] and [/CollX, y.gif] or, alternatively,
both [/CollY, x.gif] and [/CollY, y.gif]).
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+-------------------------+
| Root Collection |
| bindings: |
| CollX CollY |
+-------------------------+
| /
| /
| /
+-----------------+
| Collection C1 |
| bindings: |
| x.gif y.gif |
+-----------------+
| |
| |
| |
+-------------+
| Resource R1 |
+-------------+
In this case, one possible value for the DAV:parent-set property on
"/CollX/x.gif" would be:
<parent-set xmlns="DAV:">
<parent>
<href>/CollX</href>
<segment>x.gif</segment>
</parent>
<parent>
<href>/CollX</href>
<segment>y.gif</segment>
</parent>
</parent-set>
4. BIND Method
The BIND method modifies the collection identified by the Request-
URI, by adding a new binding from the segment specified in the BIND
body to the resource identified in the BIND body.
If a server cannot guarantee the integrity of the binding, the BIND
request MUST fail. Note that it is especially difficult to maintain
the integrity of cross-server bindings. Unless the server where the
resource resides knows about all bindings on all servers to that
resource, it may unwittingly destroy the resource or make it
inaccessible without notifying another server that manages a binding
to the resource. For example, if server A permits the creation of a
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binding to a resource on server B, server A must notify server B
about its binding and must have an agreement with B that B will not
destroy the resource while A's binding exists. Otherwise, server B
may receive a DELETE request that it thinks removes the last binding
to the resource and destroy the resource while A's binding still
exists. The precondition DAV:cross-server-binding is defined below
for cases where servers fail cross-server BIND requests because they
cannot guarantee the integrity of cross-server bindings.
By default, if there already is a binding for the specified segment
in the collection, the new binding replaces the existing binding.
This default binding replacement behavior can be overridden using the
Overwrite header defined in Section 10.6 of [RFC4918].
If a BIND request fails, the server state preceding the request MUST
be restored. This method is unsafe and idempotent (see [RFC2616],
Section 9.1).
Marshalling:
The request MAY include an Overwrite header.
The request body MUST be a DAV:bind XML element.
<!ELEMENT bind (segment, href)>
If the request succeeds, the server MUST return 201 (Created) when
a new binding was created and 200 (OK) or 204 (No Content) when an
existing binding was replaced.
If a response body for a successful request is included, it MUST
be a DAV:bind-response XML element. Note that this document does
not define any elements for the BIND response body, but the DAV:
bind-response element is defined to ensure interoperability
between future extensions that do define elements for the BIND
response body.
<!ELEMENT bind-response ANY>
Preconditions:
(DAV:bind-into-collection): The Request-URI MUST identify a
collection.
(DAV:bind-source-exists): The DAV:href element MUST identify a
resource.
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(DAV:binding-allowed): The resource identified by the DAV:href
supports multiple bindings to it.
(DAV:cross-server-binding): If the resource identified by the DAV:
href element in the request body is on another server from the
collection identified by the Request-URI, the server MUST support
cross-server bindings (servers that do not support cross-server
bindings can use this condition code to signal the client exactly
why the request failed).
(DAV:name-allowed): The name specified by the DAV:segment is
available for use as a new binding name.
(DAV:can-overwrite): If the collection already contains a binding
with the specified path segment, and if an Overwrite header is
included, the value of the Overwrite header MUST be "T".
(DAV:cycle-allowed): If the DAV:href element identifies a
collection, and if the Request-URI identifies a collection that is
a member of that collection, the server MUST support cycles in the
URI namespace (servers that do not support cycles can use this
condition code to signal the client exactly why the request
failed).
(DAV:locked-update-allowed): If the collection identified by the
Request-URI is write-locked, then the appropriate token MUST be
specified in an If request header.
(DAV:locked-overwrite-allowed): If the collection already contains
a binding with the specified path segment, and if that binding is
protected by a write lock, then the appropriate token MUST be
specified in an If request header.
Postconditions:
(DAV:new-binding): The collection MUST have a binding that maps
the segment specified in the DAV:segment element in the request
body to the resource identified by the DAV:href element in the
request body.
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4.1. Example: BIND
>> Request:
BIND /CollY HTTP/1.1
Host: www.example.com
Content-Type: application/xml; charset="utf-8"
Content-Length: 172
<?xml version="1.0" encoding="utf-8" ?>
<D:bind xmlns:D="DAV:">
<D:segment>bar.html</D:segment>
<D:href>http://www.example.com/CollX/foo.html</D:href>
</D:bind>
>> Response:
HTTP/1.1 201 Created
Location: http://www.example.com/CollY/bar.html
The server added a new binding to the collection,
"http://www.example.com/CollY", associating "bar.html" with the
resource identified by the URI
"http://www.example.com/CollX/foo.html". Clients can now use the URI
"http://www.example.com/CollY/bar.html" to submit requests to that
resource.
5. UNBIND Method
The UNBIND method modifies the collection identified by the Request-
URI by removing the binding identified by the segment specified in
the UNBIND body.
Once a resource is unreachable by any URI mapping, the server MAY
reclaim system resources associated with that resource. If UNBIND
removes a binding to a resource, but there remain URI mappings to
that resource, the server MUST NOT reclaim system resources
associated with the resource.
If an UNBIND request fails, the server state preceding the request
MUST be restored. This method is unsafe and idempotent (see
[RFC2616], Section 9.1).
Marshalling:
The request body MUST be a DAV:unbind XML element.
<!ELEMENT unbind (segment)>
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If the request succeeds, the server MUST return 200 (OK) or 204
(No Content) when the binding was successfully deleted.
If a response body for a successful request is included, it MUST
be a DAV:unbind-response XML element. Note that this document
does not define any elements for the UNBIND response body, but the
DAV:unbind-response element is defined to ensure interoperability
between future extensions that do define elements for the UNBIND
response body.
<!ELEMENT unbind-response ANY>
Preconditions:
(DAV:unbind-from-collection): The Request-URI MUST identify a
collection.
(DAV:unbind-source-exists): The DAV:segment element MUST identify
a binding in the collection identified by the Request-URI.
(DAV:locked-update-allowed): If the collection identified by the
Request-URI is write-locked, then the appropriate token MUST be
specified in the request.
(DAV:protected-url-deletion-allowed): If the binding identified by
the segment is protected by a write lock, then the appropriate
token MUST be specified in the request.
Postconditions:
(DAV:binding-deleted): The collection MUST NOT have a binding for
the segment specified in the DAV:segment element in the request
body.
(DAV:lock-deleted): If the internal member URI of the binding
specified by the Request-URI and the DAV:segment element in the
request body was protected by a write lock at the time of the
request, that write lock must have been deleted by the request.
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5.1. Example: UNBIND
>> Request:
UNBIND /CollX HTTP/1.1
Host: www.example.com
Content-Type: application/xml; charset="utf-8"
Content-Length: 117
<?xml version="1.0" encoding="utf-8" ?>
<D:unbind xmlns:D="DAV:">
<D:segment>foo.html</D:segment>
</D:unbind>
>> Response:
HTTP/1.1 200 OK
The server removed the binding named "foo.html" from the collection,
"http://www.example.com/CollX". A request to the resource named
"http://www.example.com/CollX/foo.html" will return a 404 (Not Found)
response.
6. REBIND Method
The REBIND method removes a binding to a resource from a collection,
and adds a binding to that resource into the collection identified by
the Request-URI. The request body specifies the binding to be added
(segment) and the old binding to be removed (href). It is
effectively an atomic form of a MOVE request, and MUST be treated the
same way as MOVE for the purpose of determining access permissions.
If a REBIND request fails, the server state preceding the request
MUST be restored. This method is unsafe and idempotent (see
[RFC2616], Section 9.1).
Marshalling:
The request MAY include an Overwrite header.
The request body MUST be a DAV:rebind XML element.
<!ELEMENT rebind (segment, href)>
If the request succeeds, the server MUST return 201 (Created) when
a new binding was created and 200 (OK) or 204 (No Content) when an
existing binding was replaced.
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If a response body for a successful request is included, it MUST
be a DAV:rebind-response XML element. Note that this document
does not define any elements for the REBIND response body, but the
DAV:rebind-response element is defined to ensure interoperability
between future extensions that do define elements for the REBIND
response body.
<!ELEMENT rebind-response ANY>
Preconditions:
(DAV:rebind-into-collection): The Request-URI MUST identify a
collection.
(DAV:rebind-source-exists): The DAV:href element MUST identify a
resource.
(DAV:cross-server-binding): If the resource identified by the DAV:
href element in the request body is on another server from the
collection identified by the Request-URI, the server MUST support
cross-server bindings (servers that do not support cross-server
bindings can use this condition code to signal the client exactly
why the request failed).
(DAV:name-allowed): The name specified by the DAV:segment is
available for use as a new binding name.
(DAV:can-overwrite): If the collection already contains a binding
with the specified path segment, and if an Overwrite header is
included, the value of the Overwrite header MUST be "T".
(DAV:cycle-allowed): If the DAV:href element identifies a
collection, and if the Request-URI identifies a collection that is
a member of that collection, the server MUST support cycles in the
URI namespace (servers that do not support cycles can use this
condition code to signal the client exactly why the request
failed).
(DAV:locked-update-allowed): If the collection identified by the
Request-URI is write-locked, then the appropriate token MUST be
specified in the request.
(DAV:protected-url-modification-allowed): If the collection
identified by the Request-URI already contains a binding with the
specified path segment, and if that binding is protected by a
write lock, then the appropriate token MUST be specified in the
request.
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(DAV:locked-source-collection-update-allowed): If the collection
identified by the parent collection prefix of the DAV:href URI is
write-locked, then the appropriate token MUST be specified in the
request.
(DAV:protected-source-url-deletion-allowed): If the DAV:href URI
is protected by a write lock, then the appropriate token MUST be
specified in the request.
Postconditions:
(DAV:new-binding): The collection MUST have a binding that maps
the segment specified in the DAV:segment element in the request
body, to the resource that was identified by the DAV:href element
in the request body.
(DAV:binding-deleted): The URL specified in the DAV:href element
in the request body MUST NOT be mapped to a resource.
(DAV:lock-deleted): If the URL specified in the DAV:href element
in the request body was protected by a write lock at the time of
the request, that write lock must have been deleted by the
request.
6.1. Example: REBIND
>> Request:
REBIND /CollX HTTP/1.1
Host: www.example.com
Content-Type: application/xml; charset="utf-8"
Content-Length: 176
<?xml version="1.0" encoding="utf-8" ?>
<D:rebind xmlns:D="DAV:">
<D:segment>foo.html</D:segment>
<D:href>http://www.example.com/CollY/bar.html</D:href>
</D:rebind>
>> Response:
HTTP/1.1 200 OK
The server added a new binding to the collection,
"http://www.example.com/CollX", associating "foo.html" with the
resource identified by the URI
"http://www.example.com/CollY/bar.html" and removes the binding named
"bar.html" from the collection identified by the URI
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"http://www.example.com/CollY". Clients can now use the URI
"http://www.example.com/CollX/foo.html" to submit requests to that
resource, and requests on the URI
"http://www.example.com/CollY/bar.html" will fail with a 404 (Not
Found) response.
6.2. Example: REBIND in Presence of Locks and Bind Loops
To illustrate the effects of locks and bind loops on a REBIND
operation, consider the following collection:
+------------------+
| Root Collection |
| bindings: |
| CollW |
+------------------+
|
|
|
+-------------------------------+
| Collection C1 |<--------+
| LOCKED infinity | |
| (lock token L1) | |
| bindings: | |
| CollX CollY | |
+-------------------------------+ |
| | |
| | (creates loop) |
| | |
+-----------------+ +------------------+ |
| Collection C2 | | Collection C3 | |
| (inherit lock) | | (inherit lock) | |
| (lock token L1) | | (lock token L1) | |
| bindings: | | bindings: | |
| {none} | | y.gif CollZ | |
+-----------------+ +------------------+ |
| | |
| +-----+
|
+---------------------------+
| Resource R2 |
| (lock inherited from C1) |
| (lock token L1) |
+---------------------------+
(where L1 is "urn:uuid:f92d4fae-7012-11ab-a765-00c0ca1f6bf9").
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Note that the binding between CollZ and C1 creates a loop in the
containment hierarchy. Servers are not required to support such
loops, though the server in this example does.
The REBIND request below will remove the segment "CollZ" from C3 and
add a new binding from "CollA" to the collection C2.
REBIND /CollW/CollX HTTP/1.1
Host: www.example.com
If: (<urn:uuid:f92d4fae-7012-11ab-a765-00c0ca1f6bf9>)
Content-Type: application/xml; charset="utf-8"
Content-Length: 152
<?xml version="1.0" encoding="utf-8" ?>
<D:rebind xmlns:D="DAV:">
<D:segment>CollA</D:segment>
<D:href>/CollW/CollY/CollZ</D:href>
</D:rebind>
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The outcome of the REBIND operation is:
+------------------+
| Root Collection |
| bindings: |
| CollW |
+------------------+
|
|
|
+-------------------------------+
| Collection C1 |
| LOCKED infinity |
| (lock token L1) |
| bindings: |
| CollX CollY |
+-------------------------------+
| ^ |
| | |
+-----------------+ | +------------------+
| Collection C2 | | | Collection C3 |
|(inherited lock) | | | (inherited lock) |
|(lock token L1) | | | (lock token L1) |
| bindings: | | | bindings: |
| CollA | | | y.gif |
+-----------------+ | +------------------+
| | |
+---------------+ |
(creates loop) |
+---------------------------+
| Resource R2 |
| (inherited lock from C1) |
| (lock token L1) |
+---------------------------+
7. Additional Status Codes
7.1. 208 Already Reported
The 208 (Already Reported) status code can be used inside a DAV:
propstat response element to avoid enumerating the internal members
of multiple bindings to the same collection repeatedly. For each
binding to a collection inside the request's scope, only one will be
reported with a 200 status, while subsequent DAV:response elements
for all other bindings will use the 208 status, and no DAV:response
elements for their descendants are included.
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Note that the 208 status will only occur for "Depth: infinity"
requests, and that it is of particular importance when the multiple
collection bindings cause a bind loop as discussed in Section 2.2.
A client can request the DAV:resource-id property in a PROPFIND
request to guarantee that they can accurately reconstruct the binding
structure of a collection with multiple bindings to a single
resource.
For backward compatibility with clients not aware of the 208 status
code appearing in multistatus response bodies, it SHOULD NOT be used
unless the client has signaled support for this specification using
the "DAV" request header (see Section 8.2). Instead, a 508 status
should be returned when a binding loop is discovered. This allows
the server to return the 508 as the top-level return status, if it
discovers it before it started the response, or in the middle of a
multistatus, if it discovers it in the middle of streaming out a
multistatus response.
7.1.1. Example: PROPFIND by Bind-Aware Client
For example, consider a PROPFIND request on /Coll (bound to
collection C), where the members of /Coll are /Coll/Foo (bound to
resource R) and /Coll/Bar (bound to collection C).
>> Request:
PROPFIND /Coll/ HTTP/1.1
Host: www.example.com
Depth: infinity
DAV: bind
Content-Type: application/xml; charset="utf-8"
Content-Length: 152
<?xml version="1.0" encoding="utf-8" ?>
<D:propfind xmlns:D="DAV:">
<D:prop>
<D:displayname/>
<D:resource-id/>
</D:prop>
</D:propfind>
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>> Response:
HTTP/1.1 207 Multi-Status
Content-Type: application/xml; charset="utf-8"
Content-Length: 1241
<?xml version="1.0" encoding="utf-8" ?>
<D:multistatus xmlns:D="DAV:">
<D:response>
<D:href>http://www.example.com/Coll/</D:href>
<D:propstat>
<D:prop>
<D:displayname>Loop Demo</D:displayname>
<D:resource-id>
<D:href
>urn:uuid:f81d4fae-7dec-11d0-a765-00a0c91e6bf8</D:href>
</D:resource-id>
</D:prop>
<D:status>HTTP/1.1 200 OK</D:status>
</D:propstat>
</D:response>
<D:response>
<D:href>http://www.example.com/Coll/Foo</D:href>
<D:propstat>
<D:prop>
<D:displayname>Bird Inventory</D:displayname>
<D:resource-id>
<D:href
>urn:uuid:f81d4fae-7dec-11d0-a765-00a0c91e6bf9</D:href>
</D:resource-id>
</D:prop>
<D:status>HTTP/1.1 200 OK</D:status>
</D:propstat>
</D:response>
<D:response>
<D:href>http://www.example.com/Coll/Bar</D:href>
<D:propstat>
<D:prop>
<D:displayname>Loop Demo</D:displayname>
<D:resource-id>
<D:href
>urn:uuid:f81d4fae-7dec-11d0-a765-00a0c91e6bf8</D:href>
</D:resource-id>
</D:prop>
<D:status>HTTP/1.1 208 Already Reported</D:status>
</D:propstat>
</D:response>
</D:multistatus>
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7.1.2. Example: PROPFIND by Non-Bind-Aware Client
In this example, the client isn't aware of the 208 status code
introduced by this specification. As the "Depth: infinity" PROPFIND
request would cause a loop condition, the whole request is rejected
with a 508 status.
>> Request:
PROPFIND /Coll/ HTTP/1.1
Host: www.example.com
Depth: infinity
Content-Type: application/xml; charset="utf-8"
Content-Length: 125
<?xml version="1.0" encoding="utf-8" ?>
<D:propfind xmlns:D="DAV:">
<D:prop> <D:displayname/> </D:prop>
</D:propfind>
>> Response:
HTTP/1.1 508 Loop Detected
7.2. 508 Loop Detected
The 508 (Loop Detected) status code indicates that the server
terminated an operation because it encountered an infinite loop while
processing a request with "Depth: infinity". This status indicates
that the entire operation failed.
8. Capability Discovery
8.1. OPTIONS Method
If the server supports bindings, it MUST return the compliance class
name "bind" as a field in the "DAV" response header (see [RFC4918],
Section 10.1) from an OPTIONS request on any resource implemented by
that server. A value of "bind" in the "DAV" header MUST indicate
that the server supports all MUST-level requirements and REQUIRED
features specified in this document.
8.2. 'DAV' Request Header
Clients SHOULD signal support for all MUST-level requirements and
REQUIRED features by submitting a "DAV" request header containing the
compliance class name "bind". In particular, the client MUST
understand the 208 status code defined in Section 7.1.
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9. Relationship to Locking in WebDAV
Locking is an optional feature of WebDAV ([RFC4918]). The base
WebDAV specification and this protocol extension have been designed
in parallel, making sure that all features of WebDAV can be
implemented on a server that implements this protocol as well.
Unfortunately, WebDAV uses the term "lock-root" inconsistently. It
is introduced in Section 6.1 of [RFC4918], point 2, as:
2. A resource becomes directly locked when a LOCK request to a
URL of that resource creates a new lock. The "lock-root" of the
new lock is that URL. If at the time of the request, the URL is
not mapped to a resource, a new empty resource is created and
directly locked.
On the other hand, [RFC4918], Section 9.10.1 states:
A LOCK request to an existing resource will create a lock on the
resource identified by the Request-URI, provided the resource is
not already locked with a conflicting lock. The resource
identified in the Request-URI becomes the root of the lock.
Servers that implement both WebDAV locking and support for multiple
bindings MUST use the first interpretation: the lock-root is the URI
through which the lock was created, not a resource. This URI, and
potential aliases of this URI ([RFC4918], Section 5), are said to be
"protected" by the lock.
As defined in the introduction to Section 7 of [RFC4918], write
operations that modify the state of a locked resource require that
the lock token is submitted with the request. Consistent with
WebDAV, the state of the resource consists of the content ("any
variant"), dead properties, lockable live properties (item 1), plus,
for a collection, all its bindings (item 2). Note that this, by
definition, does not depend on the Request-URI to which the write
operation is applied (the locked state is a property of the resource,
not its URI).
However, the lock-root is the URI through which the lock was
requested. Thus, the protection defined in item 3 of the list does
not apply to additional URIs that may be mapped to the same resource
due to the existence of multiple bindings.
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9.1. Example: Locking and Multiple Bindings
Consider a root collection "/", containing the two collections C1 and
C2, named "/CollX" and "/CollY", and a child resource R, bound to C1
as "/CollX/test" and bound to C2 as "/CollY/test":
+-------------------------+
| Root Collection |
| bindings: |
| CollX CollY |
+-------------------------+
| |
| |
| |
+---------------+ +---------------+
| Collection C1 | | Collection C2 |
| bindings: | | bindings: |
| test | | test |
+---------------+ +---------------+
| |
| |
| |
+------------------+
| Resource R |
+------------------+
Given a host name of "www.example.com", applying a depth-zero write
lock to "/CollX/test" will lock the resource R, and the lock-root of
this lock will be "http://www.example.com/CollX/test".
Thus, the following operations will require that the associated lock
token is submitted with the "If" request header ([RFC4918], Section
10.4):
o a PUT or PROPPATCH request modifying the content or lockable
properties of resource R (as R is locked) -- no matter which URI
is used as request target, and
o a MOVE, REBIND, UNBIND, or DELETE request causing "/CollX/test"
not to be mapped to resource R anymore (be it addressed to
"/CollX" or "/CollX/test").
The following operations will not require submission of the lock
token:
o a DELETE request addressed to "/CollY" or "/CollY/test", as it
does not affect the resource R, nor the lock-root,
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o for the same reason, an UNBIND request removing the binding "test"
from collection C2, or the binding "CollY" from the root
collection, and
o similarly, a MOVE or REBIND request causing "/CollY/test" not
being mapped to resource R anymore.
Note that despite the lock-root being
"http://www.example.com/CollX/test", an UNLOCK request can be
addressed through any URI mapped to resource R, as UNLOCK operates on
the resource identified by the Request-URI, not that URI (see
[RFC4918], Section 9.11).
10. Relationship to WebDAV Access Control Protocol
Note that the WebDAV Access Control Protocol has been designed for
compatibility with systems that allow multiple URIs to map to the
same resource (see [RFC3744], Section 5):
Access control properties (especially DAV:acl and DAV:inherited-
acl-set) are defined on the resource identified by the Request-URI
of a PROPFIND request. A direct consequence is that if the
resource is accessible via multiple URI, the value of access
control properties is the same across these URI.
Furthermore, note that BIND and REBIND behave the same as MOVE with
respect to the DAV:acl property (see [RFC3744], Section 7.3).
11. Relationship to Versioning Extensions to WebDAV
Servers that implement Workspaces ([RFC3253], Section 6) and Version-
Controlled Collections ([RFC3253], Section 14) already need to
implement BIND-like behavior in order to handle UPDATE and UNCHECKOUT
semantics.
Consider a workspace "/ws1/", containing the version-controlled,
checked-out collections C1 and C2, named "/ws1/CollX" and "/ws1/
CollY", and a version-controlled resource R, bound to C1 as "/ws1/
CollX/test":
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+-------------------------+
| Workspace |
| bindings: |
| CollX CollY |
+-------------------------+
| |
| |
| |
+---------------+ +---------------+
| Collection C1 | | Collection C2 |
| bindings: | | |
| test | | |
+---------------+ +---------------+
|
|
|
+------------------+
| Resource R |
+------------------+
Moving "/ws1/CollX/test" into "/ws1/CollY", checking in C2, but
undoing the checkout on C1 will undo part of the MOVE request, thus
restoring the binding from C1 to R, but keeping the new binding from
C2 to R:
>> Request:
MOVE /ws1/CollX/test HTTP/1.1
Host: www.example.com
Destination: /ws1/CollY/test
>> Response:
HTTP/1.1 204 No Content
>> Request:
CHECKIN /ws1/CollY/ HTTP/1.1
Host: www.example.com
>> Response:
HTTP/1.1 201 Created
Cache-Control: no-cache
Location: http://repo.example.com/his/17/ver/42
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>> Request:
UNCHECKOUT /ws1/CollX/ HTTP/1.1
Host: www.example.com
>> Response:
HTTP/1.1 200 OK
Cache-Control: no-cache
As a result, both C1 and C2 would have a binding to R:
+-------------------------+
| Workspace |
| bindings: |
| CollX CollY |
+-------------------------+
| |
| |
| |
+---------------+ +---------------+
| Collection C1 | | Collection C2 |
| bindings: | | bindings: |
| test | | test |
+---------------+ +---------------+
| |
| |
| |
+------------------+
| Resource R |
+------------------+
The MOVE semantics defined in Section 3.15 of [RFC3253] already
require that "/ws1/CollX/test" and "/ws1/CollY/test" will have the
same version history (as exposed in the DAV:version-history
property). Furthermore, the UNCHECKOUT semantics (which in this case
is similar to UPDATE, see Section 14.11 of [RFC3253]) require:
If a new version-controlled member is in a workspace that already
has a version-controlled resource for that version history, then
the new version-controlled member MUST be just a binding (i.e.,
another name for) that existing version-controlled resource.
Thus, "/ws1/CollX/test" and "/ws1/CollY/test" will be bindings to the
same resource R, and have identical DAV:resource-id properties.
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12. Security Considerations
This section is provided to make WebDAV implementers aware of the
security implications of this protocol.
All of the security considerations of HTTP/1.1 ([RFC2616], Section
15) and the WebDAV Distributed Authoring Protocol specification
([RFC4918], Section 20) also apply to this protocol specification.
In addition, bindings introduce several new security concerns and
increase the risk of some existing threats. These issues are
detailed below.
12.1. Privacy Concerns
In a context where cross-server bindings are supported, creating
bindings on a trusted server may make it possible for a hostile agent
to induce users to send private information to a target on a
different server.
12.2. Bind Loops
Although bind loops were already possible in HTTP 1.1, the
introduction of the BIND method creates a new avenue for clients to
create loops accidentally or maliciously. If the binding and its
target are on the same server, the server may be able to detect BIND
requests that would create loops. Servers are required to detect
loops that are caused by bindings to collections during the
processing of any requests with "Depth: infinity".
12.3. Bindings and Denial of Service
Denial-of-service attacks were already possible by posting URIs that
were intended for limited use at heavily used Web sites. The
introduction of BIND creates a new avenue for similar denial-of-
service attacks. If cross-server bindings are supported, clients can
now create bindings at heavily used sites to target locations that
were not designed for heavy usage.
12.4. Private Locations May Be Revealed
If the DAV:parent-set property is maintained on a resource, the
owners of the bindings risk revealing private locations. The
directory structures where bindings are located are available to
anyone who has access to the DAV:parent-set property on the resource.
Moving a binding may reveal its new location to anyone with access to
DAV:parent-set on its resource.
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12.5. DAV:parent-set and Denial of Service
If the server maintains the DAV:parent-set property in response to
bindings created in other administrative domains, it is exposed to
hostile attempts to make it devote resources to adding bindings to
the list.
13. Internationalization Considerations
All internationalization considerations mentioned in Section 19 of
[RFC4918] also apply to this document.
14. IANA Considerations
Section 7 defines the HTTP status codes 208 (Already Reported) and
508 (Loop Detected), which have been added to the HTTP Status Code
Registry.
15. Acknowledgements
This document is the collaborative product of the authors and Tyson
Chihaya, Jim Davis, Chuck Fay and Judith Slein. It has benefited
from thoughtful discussion by Jim Amsden, Peter Carlson, Steve
Carter, Ken Coar, Ellis Cohen, Dan Connolly, Bruce Cragun, Cyrus
Daboo, Spencer Dawkins, Mark Day, Werner Donne, Rajiv Dulepet, David
Durand, Lisa Dusseault, Stefan Eissing, Roy Fielding, Yaron Goland,
Joe Hildebrand, Fred Hitt, Alex Hopmann, James Hunt, Marcus Jager,
Chris Kaler, Manoj Kasichainula, Rohit Khare, Brian Korver, Daniel
LaLiberte, Steve Martin, Larry Masinter, Jeff McAffer, Alexey
Melnikov, Surendra Koduru Reddy, Max Rible, Sam Ruby, Bradley
Sergeant, Nick Shelness, John Stracke, John Tigue, John Turner, Kevin
Wiggen, and other members of the concluded WebDAV working group.
16. References
16.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2616] Fielding, R., Gettys, J., Mogul, J., Frystyk, H.,
Masinter, L., Leach, P., and T. Berners-Lee, "Hypertext
Transfer Protocol -- HTTP/1.1", RFC 2616, June 1999.
[RFC3986] Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
Resource Identifier (URI): Generic Syntax", STD 66,
RFC 3986, January 2005.
Clemm, et al. Experimental [Page 41]
RFC 5842 Binding Extensions to WebDAV April 2010
[RFC4918] Dusseault, L., Ed., "HTTP Extensions for Web Distributed
Authoring and Versioning (WebDAV)", RFC 4918, June 2007.
[XML] Bray, T., Paoli, J., Sperberg-McQueen, C., Maler, E., and
F. Yergeau, "Extensible Markup Language (XML) 1.0 (Fifth
Edition)", W3C REC-xml-20081126, November 2008,
<http://www.w3.org/TR/2008/REC-xml-20081126/>.
16.2. Informative References
[RFC3253] Clemm, G., Amsden, J., Ellison, T., Kaler, C., and J.
Whitehead, "Versioning Extensions to WebDAV (Web
Distributed Authoring and Versioning)", RFC 3253,
March 2002.
[RFC3744] Clemm, G., Reschke, J., Sedlar, E., and J. Whitehead, "Web
Distributed Authoring and Versioning (WebDAV) Access
Control Protocol", RFC 3744, May 2004.
[RFC4122] Leach, P., Mealling, M., and R. Salz, "A Universally
Unique IDentifier (UUID) URN Namespace", RFC 4122,
July 2005.
Index
2
208 Already Reported (status code) 31, 41
5
508 Loop Detected (status code) 34, 41
B
BIND method 21
Marshalling 22
Postconditions 23
Preconditions 22
Binding 6
Binding Integrity 6-7, 21
C
Collection 6
Condition Names
DAV:bind-into-collection (pre) 22
DAV:bind-source-exists (pre) 22
DAV:binding-allowed (pre) 23
DAV:binding-deleted (post) 25, 28
DAV:can-overwrite (pre) 23, 27
DAV:cross-server-binding (pre) 23, 27
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DAV:cycle-allowed (pre) 23, 27
DAV:lock-deleted (post) 25, 28
DAV:locked-overwrite-allowed (pre) 23
DAV:locked-source-collection-update-allowed (pre) 28
DAV:locked-update-allowed (pre) 23, 25, 27
DAV:name-allowed (pre) 23, 27
DAV:new-binding (post) 23, 28
DAV:protected-source-url-deletion-allowed (pre) 28
DAV:protected-url-deletion-allowed (pre) 25
DAV:protected-url-modification-allowed (pre) 27
DAV:rebind-into-collection (pre) 27
DAV:rebind-source-exists (pre) 27
DAV:unbind-from-collection (pre) 25
DAV:unbind-source-exists (pre) 25
D
DAV header
compliance class 'bind' 34
DAV:bind-into-collection precondition 22
DAV:bind-source-exists precondition 22
DAV:binding-allowed precondition 23
DAV:binding-deleted postcondition 25, 28
DAV:can-overwrite precondition 23, 27
DAV:cross-server-binding precondition 23, 27
DAV:cycle-allowed precondition 23, 27
DAV:lock-deleted postcondition 25, 28
DAV:locked-overwrite-allowed precondition 23
DAV:locked-source-collection-update-allowed precondition 28
DAV:locked-update-allowed precondition 23, 25, 27
DAV:name-allowed precondition 23, 27
DAV:new-binding postcondition 23, 28
DAV:parent-set property 20
DAV:protected-source-url-deletion-allowed precondition 28
DAV:protected-url-deletion-allowed precondition 25
DAV:protected-url-modification-allowed precondition 27
DAV:rebind-into-collection precondition 27
DAV:rebind-source-exists precondition 27
DAV:resource-id property 19
DAV:unbind-from-collection precondition 25
DAV:unbind-source-exists precondition 25
I
Internal Member URI 6
L
Locking 35
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M
Methods
BIND 21
REBIND 26
UNBIND 24
P
Path Segment 5
Properties
DAV:parent-set 20
DAV:resource-id 19
R
REBIND method 26
Marshalling 26
Postconditions 28
Preconditions 27
S
Status Codes
208 Already Reported 31, 41
508 Loop Detected 34, 41
U
UNBIND method 24
Marshalling 24
Postconditions 25
Preconditions 25
URI Mapping 5
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Authors' Addresses
Geoffrey Clemm
IBM
550 King Street
Littleton, MA 01460
EMail: [email protected]
Jason Crawford
IBM Research
P.O. Box 704
Yorktown Heights, NY 10598
EMail: [email protected]
Julian F. Reschke (editor)
greenbytes GmbH
Hafenweg 16
Muenster, NW 48155
Germany
EMail: [email protected]
Jim Whitehead
UC Santa Cruz, Dept. of Computer Science
1156 High Street
Santa Cruz, CA 95064
EMail: [email protected]
Clemm, et al. Experimental [Page 45]
