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Versions: 00 01 02 03 04 05 06 07 08 09 10 11 12 13 RFC 3744

 INTERNET-DRAFT                   Geoffrey Clemm, Rational Software
 draft-ietf-webdav-acl-09         Anne Hopkins, Microsoft Corporation
                                  Eric Sedlar, Oracle Corporation
                                  Jim Whitehead, U.C. Santa Cruz
 
 Expires January 26, 2003         July 26, 2002
 
 
 
                     WebDAV Access Control Protocol
 
 
 Status of this Memo
 
 This document is an Internet-Draft and is subject to all provisions
 of Section 10 of RFC2026.
 Internet-Drafts are working documents of the Internet Engineering
 Task Force (IETF), its areas, and its working groups. Note that other
 groups may also distribute working documents as Internet-Drafts.
 Internet-Drafts are draft documents valid for a maximum of six months
 and may be updated, replaced, or obsoleted by other documents at any
 time. It is inappropriate to use Internet- Drafts as reference
 material or to cite them other than as "work in progress."
 The list of current Internet-Drafts can be accessed at
 http://www.ietf.org/ietf/1id-abstracts.txt
 The list of Internet-Draft Shadow Directories can be accessed at
 http://www.ietf.org/shadow.html.
 
 Abstract
 
 This document specifies a set of methods, headers, message bodies,
 properties, and reports that define Access Control extensions to the
 WebDAV Distributed Authoring Protocol. This protocol permits a client
 to read and modify access control lists that instruct a server
 whether to allow or deny operations upon a resource (such as
 HyperText Transfer Protocol (HTTP) method invocations) by a given
 principal. A lightweight representation of principals as Web
 resources supports integration of a wide range of user management
 repositories. Search operations allow discovery and manipulation of
 principals using human names.
 
 This document is a product of the Web Distributed Authoring and
 Versioning (WebDAV) working group of the Internet Engineering Task
 Force. Comments on this draft are welcomed, and should be addressed
 to the acl@webdav.org mailing list. Other related documents can be
 found at http://www.webdav.org/acl/, and
 http://www.ics.uci.edu/pub/ietf/webdav/.
 
 
 
 
 
 
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 Table of Contents
 
 1 INTRODUCTION....................................................4
 1.1 Terms.........................................................7
 1.2 Notational Conventions........................................8
 
 2 PRINCIPALS......................................................8
 
 3 PRIVILEGES......................................................9
 3.1 DAV:read Privilege...........................................10
 3.2 DAV:write Privilege..........................................10
 3.3 DAV:write-properties.........................................10
 3.4 DAV:write-content............................................11
 3.5 DAV:unlock...................................................11
 3.6 DAV:read-acl Privilege.......................................12
 3.7 DAV:read-current-user-privilege-set Privilege................12
 3.8 DAV:write-acl Privilege......................................12
 3.9 DAV:all Privilege............................................12
 3.10 Aggregation of Predefined Privileges........................12
 
 4 PRINCIPAL PROPERTIES...........................................13
 4.1 DAV:alternate-URI-set........................................14
 4.2 DAV:principal-URL............................................14
 4.3 DAV:group-member-set.........................................14
 4.4 DAV:group-membership.........................................14
 
 5 ACCESS CONTROL PROPERTIES......................................15
 5.1 DAV:owner....................................................15
   5.1.1 Example: Retrieving DAV:owner............................15
   5.1.2 Example: An Attempt to Set DAV:owner.....................16
 5.2 DAV:supported-privilege-set..................................17
   5.2.1 Example: Retrieving a List of Privileges Supported on a
   Resource.......................................................18
 5.3 DAV:current-user-privilege-set...............................20
   5.3.1 Example: Retrieving the User's Current Set of Assigned
   Privileges.....................................................21
 5.4 DAV:acl......................................................22
   5.4.1 ACE Principal............................................22
   5.4.2 ACE Grant and Deny.......................................24
   5.4.3 ACE Protection...........................................24
   5.4.4 ACE Inheritance..........................................24
   5.4.5 Example: Retrieving a Resource's Access Control List.....25
 5.5 DAV:acl-semantics............................................26
   5.5.1 Example: Retrieving DAV:acl-semantics....................27
 5.6 DAV:inherited-acl-set........................................28
 5.7 DAV:principal-collection-set.................................28
   5.7.1 Example: Retrieving DAV:principal-collection-set.........29
 5.8 Example: PROPFIND to retrieve access control properties......30
 
 
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 6 ACL SEMANTICS..................................................34
 6.1 ACE Combination..............................................34
   6.1.1 DAV:first-match ACE Combination..........................34
   6.1.2 DAV:all-grant-before-any-deny ACE Combination............34
   6.1.3 DAV:specific-deny-overrides-grant ACE Combination........35
 6.2 ACE Ordering.................................................35
   6.2.1 DAV:deny-before-grant ACE Ordering.......................35
 6.3 Allowed ACE..................................................35
   6.3.1 DAV:principal-only-one-ace ACE Constraint................35
   6.3.2 DAV:grant-only ACE Constraint............................35
   6.3.3 DAV:no-invert ACE Constraint.............................36
 6.4 Required Principals..........................................36
 
 7 ACCESS CONTROL AND EXISTING METHODS............................36
 7.1 OPTIONS......................................................36
   7.1.1 Example - OPTIONS........................................36
 7.2 MOVE.........................................................37
 7.3 COPY.........................................................37
 7.4 DELETE.......................................................37
 7.5 LOCK.........................................................37
 
 8 ACCESS CONTROL METHODS.........................................37
 8.1 ACL..........................................................37
   8.1.1 ACL Preconditions........................................38
   8.1.2 Example: the ACL method..................................40
   8.1.3 Example: ACL method failure due to protected ACE
         conflict ................................................41
   8.1.4 Example: ACL method failure due to an inherited ACE
         conflict ................................................42
   8.1.5 Example: ACL method failure due to an attempt to set
         grant and deny in a single ACE ..........................43
 
 9 ACCESS CONTROL REPORTS.........................................44
 9.1 REPORT Method................................................44
 9.2 DAV:acl-principal-prop-set Report............................44
   9.2.1 Example: DAV:acl-principal-prop-set Report...............45
 9.3 DAV:principal-match REPORT...................................46
   9.3.1 Example: DAV:principal-match REPORT......................48
 9.4 DAV:principal-property-search REPORT.........................48
   9.4.1 Matching.................................................51
   9.4.2 Example: successful DAV:principal-property-search REPORT 51
   9.4.3 Example: Unsuccessful DAV:principal-property-search
         REPORT ..................................................53
 9.5 DAV:principal-search-property-set REPORT.....................54
   9.5.1 Example: DAV:principal-search-property-set REPORT........56
 
 10  XML PROCESSING...............................................57
 
 11  INTERNATIONALIZATION CONSIDERATIONS..........................57
 
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 12  SECURITY CONSIDERATIONS......................................58
 12.1 Increased Risk of Compromised Users.........................58
 12.2 Risks of the DAV:read-acl and DAV:current-user-privilege-set
 Privileges.......................................................58
 12.3 No Foreknowledge of Initial ACL.............................59
 
 13  AUTHENTICATION...............................................59
 
 14  IANA CONSIDERATIONS..........................................60
 
 15  INTELLECTUAL PROPERTY........................................60
 
 16  ACKNOWLEDGEMENTS.............................................60
 
 17  REFERENCES...................................................61
 17.1 Normative References........................................61
 17.2 Informational References....................................61
 
 18  AUTHORS' ADDRESSES...........................................62
 
 19  APPENDICES...................................................63
 19.1 WebDAV XML Document Type Definition Addendum................63
 
 
 1  INTRODUCTION
 
   The goal of the WebDAV access control extensions is to provide an
   interoperable mechanism for handling discretionary access control
   for content and metadata managed by WebDAV servers.  WebDAV access
   control can be implemented on content repositories with security as
   simple as that of a UNIX file system, as well as more sophisticated
   models.  The underlying principle of access control is that who you
   are determines what operations you can perform on a resource. The
   "who you are" is defined by a "principal" identifier; users, client
   software, servers, and groups of the previous have principal
   identifiers. The "operations you can perform" are determined by a
   single "access control list" (ACL) associated with a resource.  An
   ACL contains a set of "access control entries" (ACEs), where each
   ACE specifies a principal and a set of privileges that are either
   granted or denied to that principal. When a principal submits an
   operation (such as an HTTP or WebDAV method) to a resource for
   execution, the server evaluates the ACEs in the ACL to determine if
   the principal has permission for that operation.
 
   Since every ACE contains the identifier of a principal, client
   software operated by a human must provide a mechanism for selecting
   this principal. This specification uses http(s) scheme URLs to
   identify principals, which are represented as WebDAV-capable
 
 
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   resources. There is no guarantee that the URLs identifying
   principals will be meaningful to a human. For example,
   http://www.dav.org/u/256432 and
   http://www.dav.org/people/Greg.Stein are both valid URLs that could
   be used to identify the same principal. To remedy this, every
   principal resource has the DAV:displayname property containing a
   human-readable name for the principal.
 
   Since a principal can be identified by multiple URLs, it raises the
   problem of determining exactly which principal is being referenced
   in a given ACE. It is impossible for a client to determine that an
   ACE granting the read privilege to
   http://www.dav.org/people/Greg.Stein also affects the principal at
   http://www.dav.org/u/256432. That is, a client has no mechanism for
   determining that two URLs identify the same principal resource.  As
   a result, this specification requires clients to use just one of
   the many possible URLs for a principal when creating ACEs. A client
   can discover which URL to use by retrieving the DAV:principal-URL
   property (Section 4.2) from a principal resource. No matter which
   of the principal's URLs is used with PROPFIND, the property always
   returns the same URL.
 
   With a system having hundreds to thousands of principals, the
   problem arises of how to allow a human operator of client software
   to select just one of these principals. One approach is to use
   broad collection hierarchies to spread the principals over a large
   number of collections, yielding few principals per collection. An
   example of this is a two level hierarchy with the first level
   containing 36 collections (a-z, 0-9), and the second level being
   another 36, creating collections /a/a/, /a/b/, ..., /a/z/, such
   that a principal with last name "Stein" would appear at /s/t/Stein.
   In effect, this pre-computes a common query, search on last name,
   and encodes it into a hierarchy. The drawback with this scheme is
   that it handles only a small set of predefined queries, and
   drilling down through the collection hierarchy adds unnecessary
   steps (navigate down/up) when the user already knows the
   principal's name. While organizing principal URLs into a hierarchy
   is a valid namespace organization, users should not be forced to
   navigate this hierarchy to select a principal.
 
   This specification provides the capability to perform substring
   searches over a small set of properties on the resources
   representing principals. This permits searches based on last name,
   first name, user name, job title, etc. Two separate searches are
   supported, both via the REPORT method, one to search principal
   resources (DAV:principal-property-search, Section 9.4), the other
   to determine which properties may be searched at all
   (DAV:principal-search-property-set, Section 9.5).
 
 
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   Once a principal has been identified in an ACE, a server evaluating
   that ACE must know the identity of the principal making a protocol
   request, and must validate that that principal is who they claim to
   be, a process known as authentication. This specification
   intentionally omits discussion of authentication, as the HTTP
   protocol already has a number of authentication mechanisms
   [RFC2617].  Some authentication mechanism (such as HTTP Digest
   Authentication, which all WebDAV compliant implementations are
   required to support) must be available to validate the identity of
   a principal.
 
   The following issues are out of scope for this document:
 
      * Access control that applies only to a particular property on a
        resource (excepting the access control properties DAV:acl and
        DAV:current-user-privilege-set), rather than the entire
        resource,
 
      * Role-based security (where a role can be seen as a dynamically
        defined group of principals),
 
      * Specification of the ways an ACL on a resource is initialized,
 
      * Specification of an ACL that applies globally to all
        resources, rather than to a particular resource.
 
      * Creation and maintenance of resources representing people or
        computational agents (principals), and groups of these.
 
   This specification is organized as follows. Section 1.1 defines key
   concepts used throughout the specification, and is followed by a
   more in-depth discussion of principals (Section 2), and privileges
   (Section 3). Properties defined on principals are specified in
   Section 4, and access control properties for content resources are
   specified in Section 5. The semantics of access control lists are
   described in Section 6, including sections on ACE combination
   (Section 6.1), ACE ordering (Section 6.2), and principals required
   to be present in an ACE (Section 6.3.2). Client discovery of access
   control capability using OPTIONS is described in Section 7.1.
   Interactions between access control functionality and existing HTTP
   and WebDAV methods are described in the remainder of Section 7. The
   access control setting method, ACL, is specified in Section 8. Four
   reports that provide limited server-side searching capabilities are
   described in Section 9. Sections on XML processing (Section 10),
   Internationalization considerations (Section 11), security
   considerations (Section 12), and authentication (Section 13) round
   out the specification. An appendix (Section 19.1) provides an XML
   Document Type Definition (DTD) for the XML elements defined in the
   specification.
 
 
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 1.1 Terms
 
   This draft uses the terms defined in HTTP [RFC2616] and WebDAV
   [RFC2518].  In addition, the following terms are defined:
 
 principal
   A "principal" is a distinct human or computational actor that
   initiates access to network resources.  In this protocol, a
   principal is an HTTP resource that represents such an actor.
 
 group
   A "group" is a principal that represents a set of other principals.
 
 privilege
   A "privilege" controls access to a particular set of HTTP
   operations on a resource.
 
 aggregate privilege
   An "aggregate privilege" is a privilege that contains a set of
   other privileges.
 
 abstract privilege
   The modifier "abstract", when applied to a privilege on a resource,
   means the privilege cannot be set in an access control element
   (ACE) on that resource .
 
 access control list (ACL)
   An "ACL" is a list of access control elements that define access
   control to a particular resource.
 
 access control element (ACE)
   An "ACE" either grants or denies a particular set of (non-abstract)
   privileges for a particular principal.
 
 inherited ACE
   An "inherited ACE" is an ACE that is dynamically shared from the
   ACL of another resource. When a shared ACE changes on the primary
   resource, it is also changed on inheriting resources.
 
 protected property
   A "protected property" is one whose value cannot be updated except
   by a method explicitly defined as updating that specific property.
   In particular, a protected property cannot be updated with a
   PROPPATCH request.
 
 
 
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 1.2 Notational Conventions
 
   The augmented BNF used by this document to describe protocol
   elements is described in Section 2.1 of [RFC2616]. Because this
   augmented BNF uses the basic production rules provided in Section
   2.2 of [RFC2616], those rules apply to this document as well.
 
   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in
   this document are to be interpreted as described in [RFC2119].
 
   Definitions of XML elements in this document use XML element type
   declarations (as found in XML Document Type Declarations),
   described in Section 3.2 of [REC-XML]. When an XML element type in
   the "DAV:" namespace is referenced in this document outside of the
   context of an XML fragment, the string "DAV:" will be prefixed to
   the element name.
 
 2  PRINCIPALS
 
   A principal is a network resource that represents a distinct human
   or computational actor that initiates access to network resources.
   Users and groups are represented as principals in many
   implementations; other types of principals are also possible. A URI
   of any scheme MAY be used to identify a principal resource.
   However, servers implementing this specification MUST expose
   principal resources at an http(s) URL, which is a privileged scheme
   that points to resources that have additional properties, as
   described in Section 4. So, a principal resource can have multiple
   URIs, one of which has to be an http(s) scheme URL. Although an
   implementation SHOULD support PROPFIND and MAY support PROPPATCH to
   access and modify information about a principal, it is not required
   to do so.
 
   A principal resource may be a group, where a group is a principal
   that represents a set of other principals, called the members of
   the group.  If a person or computational agent matches a principal
   resource that is a member of a group, they also match the group.
   Membership in a group is recursive, so if a principal is a member
   of group GRPA, and GRPA is a member of group GRPB, then the
   principal is also a member of GRPB.
 
 
 
 
 
 
 
 
 
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 3  PRIVILEGES
 
   Ability to perform a given method on a resource SHOULD be
   controlled by one or more privileges.  Authors of protocol
   extensions that define new HTTP methods SHOULD specify which
   privileges (by defining new privileges, or mapping to ones below)
   are required to perform the method.  A principal with no privileges
   to a resource SHOULD be denied any HTTP access to that resource,
   unless the principal matches an ACE constructed using the DAV:all,
   DAV:authenticated, or DAV:unauthenticated pseudo-principals (see
   Section 5.4.1).
 
   Privileges may be containers of other privileges, in which case
   they are termed "aggregate privileges".  If a principal is granted
   or denied an aggregate privilege, it is semantically equivalent to
   granting or denying each of the aggregated privileges individually.
   For example, an implementation may define add-member and remove-
   member privileges that control the ability to add and remove a
   member of a group.  Since these privileges control the ability to
   update the state of a group, these privileges would be aggregated
   by the DAV:write privilege on a group, and granting the DAV:write
   privilege on a group would also grant the add-member and remove-
   member privileges.
 
   Privileges may be declared to be "abstract" for a given resource,
   in which case they cannot be set in an ACE on that resource.
   Aggregate and non-aggregate privileges are both capable of being
   abstract. Abstract privileges are useful for modeling privileges
   that otherwise would not be exposed via the protocol. Abstract
   privileges also provide server implementations with flexibility in
   implementing the privileges defined in this specification.  For
   example, if a server is incapable of separating the read resource
   capability from the read ACL capability, it can still model the
   DAV:read and DAV:read-acl privileges defined in this specification
   by declaring them abstract, and containing them within a non-
   abstract aggregate privilege (say, read-all) that holds DAV:read,
   and DAV:read-acl. In this way, it is possible to set the aggregate
   privilege, read-all, thus coupling the setting of DAV:read and
   DAV:read-acl, but it is not possible to set DAV:read, or DAV:read-
   acl individually. Since aggregate privileges can be abstract, it is
   also possible to use abstract privileges to group or organize non-
   abstract privileges. Privilege containment loops are not allowed;
   therefore, a privilege MUST NOT contain itself. For example,
   DAV:read cannot contain DAV:read.
 
   The set of privileges that apply to a particular resource may vary
   with the DAV:resourcetype of the resource, as well as between
   different server implementations.  To promote interoperability,
   however, this specification defines a set of well-known privileges
 
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   (e.g. DAV:read, DAV:write, DAV:read-acl, DAV:write-acl, DAV:read-
   current-user-privilege-set, and DAV:all), which can at least be
   used to classify the other privileges defined on a particular
   resource. The access permissions on null resources (defined in
   [RFC2518], Section 3) are solely those they inherit (if any), and
   they are not discoverable (i.e., the access control properties
   specified in Section 5 are not defined on null resources). On the
   transition from null to stateful resource, the initial access
   control list is set by the server's default ACL value policy (if
   any).
 
   Server implementations MAY define new privileges beyond those
   defined in this specification. Privileges defined by individual
   implementations MUST NOT use the DAV: namespace, and instead should
   use a namespace that they control, such as an http scheme URL.
 
 3.1 DAV:read Privilege
 
   The read privilege controls methods that return information about
   the state of the resource, including the resource's properties.
   Affected methods include GET and PROPFIND.  Any implementation-
   defined privilege that also controls access to GET and PROPFIND
   must be aggregated under DAV:readùif an ACL grants access to
   DAV:read, the client may expect that no other privilege needs to be
   granted to have access to GET and PROPFIND.  Additionally, the read
   privilege MAY control the OPTIONS method.
 
   <!ELEMENT read EMPTY>
 
 3.2 DAV:write Privilege
 
   The write privilege controls methods that lock a resource or modify
   the content, dead properties, or (in the case of a collection)
   membership of the resource, such as PUT and PROPPATCH.  Note that
   state modification is also controlled via locking (see section 5.3
   of [WEBDAV]), so effective write access requires that both write
   privileges and write locking requirements are satisfied.  Any
   implementation-defined privilege that also controls access to
   methods modifying content, dead properties or collection membership
   must be aggregated under DAV:write, e.g. if an ACL grants access to
   DAV:write, the client may expect that no other privilege needs to
   be granted to have access to PUT and PROPPATCH.
 
   <!ELEMENT write EMPTY>
 
 3.3 DAV:write-properties
 
   The DAV:write-properties privilege controls methods that modify the
   dead properties of the resource, such as PROPPATCH.  Whether this
 
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   privilege may be used to control access to any live properties is
   determined by the implementation.  Any implementation-defined
   privilege that also controls access to methods modifying dead
   properties must be aggregated under DAV:write-propertiesùe.g. if an
   ACL grants access to DAV:write-properties, the client can safely
   expect that no other privilege needs to be granted to have access
   to PROPPATCH.
 
   <!ELEMENT write-properties EMPTY>
 
 3.4 DAV:write-content
 
   The DAV:write-content privilege controls methods that modify the
   content or (in the case of a collection) membership of the
   resource, such as PUT and DELETE.  Any implementation-defined
   privilege that also controls access to content or alteration of
   collection membership must be aggregated under DAV:write-contentù
   e.g. if an ACL grants access to DAV:write-content, the client can
   safely expect that no other privilege needs to be granted to have
   access to PUT or DELETE.
 
   <!ELEMENT write-content EMPTY>
 
 3.5 DAV:unlock
 
   The DAV:unlock privilege controls the use of the UNLOCK method by a
   principal other than the lock owner (the principal that created a
   lock can always perform an UNLOCK).  While the set of users who may
   lock a resource is most commonly the same set of users who may
   modify a resource, servers may allow various kinds of
   administrators to unlock resources locked by others. Any privilege
   controlling access by non-lock owners to UNLOCK MUST be aggregated
   under DAV:unlock.
 
   A lock owner can always remove a lock by issuing an UNLOCK with the
   correct lock token and authentication credentials. That is, even if
   a principal does not have DAV:unlock privilege, they can still
   remove locks they own. Principals other than the lock owner can
   remove a lock only if they have DAV:unlock privilege and they issue
   an UNLOCK with the correct lock token. Lock timeout is not affected
   by the DAV:unlock privilege.
 
   <!ELEMENT unlock EMPTY>
 
 
 
 
 
 
 
 
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 3.6 DAV:read-acl Privilege
 
   The DAV:read-acl privilege controls the use of PROPFIND to retrieve
   the DAV:acl property of the resource.
 
   <!ELEMENT read-acl EMPTY>
 
 3.7 DAV:read-current-user-privilege-set Privilege
 
   The DAV:read-current-user-privilege-set privilege controls the use
   of PROPFIND to retrieve the DAV:current-user-privilege-set property
   of the resource.
 
   Clients are intended to use this property to visually indicate in
   their UI items that are dependent on the permissions of a resource,
   for example, by graying out resources that are not writeable.
 
   This privilege is separate from DAV:read-acl because there is a
   need to allow most users access to the privileges permitted the
   current user (due to its use in creating the UI), while the full
   ACL contains information that may not be appropriate for the
   current authenticated user. As a result, the set of users who can
   view the full ACL is expected to be much smaller than those who can
   read the current user privilege set, and hence distinct privileges
   are needed for each.
 
   <!ELEMENT read-current-user-privilege-set EMPTY>
 
 3.8 DAV:write-acl Privilege
 
   The DAV:write-acl privilege controls use of the ACL method to
   modify the DAV:acl property of the resource.
 
   <!ELEMENT write-acl EMPTY>
 
 3.9 DAV:all Privilege
 
   DAV:all is an aggregate privilege that contains the entire set of
   privileges that can be applied to the resource.
 
   <!ELEMENT all EMPTY>
 
 3.10 Aggregation of Predefined Privileges
 
   Server implementations are free to aggregate the predefined
   privileges (defined above in Sections 3.1-3.9) subject to the
   following limitations:
 
 
 
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   DAV:read-acl MUST NOT contain DAV:read, DAV:write, DAV:write-acl,
   DAV:write-properties, DAV:write-content, or DAV:read-current-user-
   privilege-set.
 
   DAV:write-acl MUST NOT contain DAV:write, DAV:read, DAV:read-acl,
   or DAV:read-current-user-privilege-set.
 
   DAV:read-current-user-privilege-set MUST NOT contain DAV:write,
   DAV:read, DAV:read-acl, or DAV:write-acl.
 
   DAV:write MUST NOT contain DAV:read, DAV:read-acl, or DAV:read-
   current-user-privilege-set.
 
   DAV:read MUST NOT contain DAV:write, DAV:write-acl, DAV:write-
   properties, or DAV:write-content.
 
   DAV:write MUST contain DAV:write-properties and DAV:write-content.
 
 4  PRINCIPAL PROPERTIES
 
   Principals are manifested to clients as a WebDAV resource,
   identified by a URL.  A principal MUST have a non-empty
   DAV:displayname property (defined in Section 13.2 of [RFC2518]),
   and a DAV:resourcetype property (defined in Section 13.9 of
   [RFC2518]).  Additionally, a principal MUST report the
   DAV:principal XML element in the value of the DAV:resourcetype
   property.  The element type declaration for DAV:principal is:
 
   <!ELEMENT principal EMPTY>
 
   This protocol defines the following additional properties for a
   principal. Since it can be expensive for a server to retrieve
   access control information, the name and value of these properties
   SHOULD NOT be returned by a PROPFIND allprop request (as defined in
   Section 12.14.1 of [RFC2518]).
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
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 4.1 DAV:alternate-URI-set
 
   This protected property, if non-empty, contains the URIs of network
   resources with additional descriptive information about the
   principal. This property identifies additional network resources
   (i.e., it contains one or more URIs) that may be consulted by a
   client to gain additional knowledge concerning a principal. One
   expected use for this property is the storage of an LDAP [RFC2255]
   scheme URL. A user-agent encountering an LDAP URL could use LDAP
   [RFC2589] to retrieve additional machine-readable directory
   information about the principal, and display that information in
   its user interface. Support for this property is REQUIRED, and the
   value is empty if no alternate URI exists for the principal.
 
   <!ELEMENT alternate-URI-set (href*)>
 
 4.2 DAV:principal-URL
 
    A principal may have many URLs, but there must be one primary URL
   that clients can use to uniquely identify a principalùthe
   principal-URL.  This protected property contains the URL that MUST
   be used to identify this principal in an ACL request. Support for
   this property is REQUIRED.
 
   <!ELEMENT principal-URL (href)>
 
 4.3 DAV:group-member-set
 
   This property of a group principal identifies the principals that
   are direct members of this group. Since a group may be a member of
   another group, a group may also have indirect members (i.e. the
   members of its direct members).  A URL in the DAV:group-member-set
   for a principal MUST be the DAV:principal-URL of that principal.
 
   <!ELEMENT group-member-set (href*)>
 
 4.4 DAV:group-membership
 
    This protected property identifies the groups in which the
   principal is directly a member.  Note that a server may allow a
   group to be a member of another group, in which case the DAV:group-
   membership of those other groups would need to be queried in order
   to determine the groups in which the principal is indirectly a
   member. Support for this property is REQUIRED.
 
   <!ELEMENT group-membership (href*)>
 
 
 
 
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 5  ACCESS CONTROL PROPERTIES
 
   This specification defines a number of new properties for WebDAV
   resources.  Access control properties may be retrieved just like
   other WebDAV properties, using the PROPFIND method.  Since it is
   expensive, for many servers, to retrieve access control
   information, a PROPFIND allprop request (as defined in Section
   12.14.1 of [RFC2518]) SHOULD NOT return the names and values of the
   properties defined in this section.
 
   HTTP resources that support the WebDAV Access Control Protocol MUST
   contain the following properties. Null resources (described in
   Section 3 of [RFC2518]) MUST NOT contain the following properties:
 
 5.1 DAV:owner
 
   This protected property identifies a particular principal as being
   the "owner" of the resource. Since the owner of a resource often
   has special access control capabilities (e.g., the owner frequently
   has permanent DAV:write-acl privilege), clients might display the
   resource owner in their user interface.
 
   <!ELEMENT owner (href)>
 
 5.1.1 Example: Retrieving DAV:owner
 
   This example shows a client request for the value of the DAV:owner
   property from a collection resource with URL
   http://www.webdav.org/papers/. The principal making the request is
   authenticated using Digest authentication. The value of DAV:owner
   is the URL http://www.webdav.org/acl/users/gstein, wrapped in the
   DAV:href XML element.
 
   >> Request <<
 
   PROPFIND /papers/ HTTP/1.1
   Host: www.webdav.org
   Content-type: text/xml; charset="utf-8"
   Content-Length: xxx
   Depth: 0
   Authorization: Digest username="jim",
      realm="jim@webdav.org", nonce="...",
      uri="/papers/", response="...", opaque="..."
 
   <?xml version="1.0" encoding="utf-8" ?>
   <D:propfind xmlns:D="DAV:">
     <D:prop>
       <D:owner/>
     </D:prop>
   </D:propfind>
 
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   >> Response <<
 
   HTTP/1.1 207 Multi-Status
   Content-Type: text/xml; charset="utf-8"
   Content-Length: xxx
 
   <?xml version="1.0" encoding="utf-8" ?>
   <D:multistatus xmlns:D="DAV:">
      <D:response>
         <D:href>http://www.webdav.org/papers/</D:href>
         <D:propstat>
            <D:prop>
               <D:owner>
   <D:href>http://www.webdav.org/acl/users/gstein</D:href>
               </D:owner>
            </D:prop>
            <D:status>HTTP/1.1 200 OK</D:status>
        </D:propstat>
      </D:response>
   </D:multistatus>
 
 5.1.2 Example: An Attempt to Set DAV:owner
 
   The following example shows a client request to modify the value of
   the DAV:owner property on the resource with URL
   <http://www.webdav.org/papers>. Since DAV:owner is a protected
   property, the server responds with a 207 (Multi-Status) response
   that contains a 403 (Forbidden) status code for the act of setting
   DAV:owner. Section 8.2.1 of [RFC2518] describes PROPPATCH status
   code information, and Section 11 of [RFC2518] describes the Multi-
   Status response.
 
   >> Request <<
 
   PROPPATCH /papers/ HTTP/1.1
   Host: www.webdav.org
   Content-type: text/xml; charset="utf-8"
   Content-Length: xxx
   Depth: 0
   Authorization: Digest username="jim",
      realm="jim@webdav.org", nonce="...",
      uri="/papers/", response="...", opaque="..."
 
   <?xml version="1.0" encoding="utf-8" ?>
   <D:propertyupdate xmlns:D="DAV:">
      <D:set>
 
 
 
 
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         <D:prop>
            <D:owner>
               <D:href>http://www.webdav.org/acl/users/jim</D:href>
            </D:owner>
         </D:prop>
      </D:set>
   </D:propertyupdate>
 
 
   >> Response <<
 
   HTTP/1.1 207 Multi-Status
   Content-Type: text/xml; charset="utf-8"
   Content-Length: xxx
 
   <?xml version="1.0" encoding="utf-8" ?>
   <D:multistatus xmlns:D="DAV:">
      <D:response>
         <D:href>http://www.webdav.org/papers/</D:href>
         <D:propstat>
            <D:prop><D:owner/></D:prop>
            <D:status>HTTP/1.1 403 Forbidden</D:status>
            <D:responsedescription>
              Failure to set protected property (DAV:owner)
            </D:responsedescription>
         </D:propstat>
      </D:response>
   </D:multistatus>
 
 
 5.2 DAV:supported-privilege-set
 
   This is a protected property that identifies the privileges defined
   for the resource.
 
   <!ELEMENT supported-privilege-set (supported-privilege*)>
 
   Each privilege appears as an XML element, where aggregate
   privileges list as sub-elements all of the privileges that they
   aggregate.
 
   <!ELEMENT supported-privilege
    (privilege, abstract?, description, supported-privilege*)>
   <!ELEMENT privilege ANY>
 
   An abstract privilege MUST NOT be used in an ACE for that resource.
   Servers MUST fail an attempt to set an abstract privilege.
 
   <!ELEMENT abstract EMPTY>
 
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   A description is a human-readable description of what this
   privilege controls access to. Servers MUST indicate the human
   language of the description using the xml:lang attribute and SHOULD
   consider the HTTP Accept-Language request header when selecting one
   of multiple available languages.
 
   <!ELEMENT description #PCDATA>
 
   It is envisioned that a WebDAV ACL-aware administrative client
   would list the supported privileges in a dialog box, and allow the
   user to choose non-abstract privileges to apply in an ACE.  The
   privileges tree is useful programmatically to map well-known
   privileges (defined by WebDAV or other standards groups) into
   privileges that are supported by any particular server
   implementation.  The privilege tree also serves to hide complexity
   in implementations allowing large number of privileges to be
   defined by displaying aggregates to the user.
 
 5.2.1 Example: Retrieving a List of Privileges Supported on a
       Resource
 
   This example shows a client request for the DAV:supported-
   privilege-set property on the resource
   http://www.webdav.org/papers/. The value of the DAV:supported-
   privilege-set property is a tree of supported privileges (using
   "[XML Namespace , localname]" to identify each privilege):
 
      [DAV:, all] (aggregate, abstract)
         |
         +-- [DAV:, read] (aggregate)
                |
                +-- [DAV:, read-acl] (abstract)
                +-- [DAV:, read-current-user-privilege-set] (abstract)
         |
         +-- [DAV:, write] (aggregate)
                |
                +-- [DAV:, write-acl] (abstract)
                +-- [DAV:, write-properties]
                +-- [DAV:, write-content]
         |
         +-- [DAV:, unlock]
 
 
   This privilege tree is not normative (except that it reflects the
   normative aggregation rules given in Section 3.10), and many
   possible privilege trees are possible.
 
 
 
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   >> Request <<
 
   PROPFIND /papers/ HTTP/1.1
   Host: www.webdav.org
   Content-type: text/xml; charset="utf-8"
   Content-Length: xxx
   Depth: 0
   Authorization: Digest username="gclemm",
      realm="gclemm@webdav.org", nonce="...",
      uri="/papers/", response="...", opaque="..."
 
   <?xml version="1.0" encoding="utf-8" ?>
   <D:propfind xmlns:D="DAV:">
     <D:prop>
       <D:supported-privilege-set/>
     </D:prop>
   </D:propfind>
 
   >> Response <<
 
   HTTP/1.1 207 Multi-Status
   Content-Type: text/xml; charset="utf-8"
   Content-Length: xxx
 
   <?xml version="1.0" encoding="utf-8" ?>
   <D:multistatus xmlns:D="DAV:">
     <D:response>
       <D:href>http://www.webdav.org/papers/</D:href>
       <D:propstat>
         <D:prop>
           <D:supported-privilege-set>
             <D:supported-privilege>
               <D:privilege> <D:all/> </D:privilege>
               <D:abstract/>
               <D:description xml:lang="en">Any
   operation</D:description>
               <D:supported-privilege>
                 <D:privilege> <D:read/> </D:privilege>
                 <D:description xml:lang="en">Read any
   object</D:description>
                 <D:supported-privilege>
                   <D:privilege> <D:read-acl/> </D:privilege>
                   <D:abstract/>
                   <D:description xml:lang="en">Read
   ACL</D:description>
                 </D:supported-privilege>
                 <D:supported-privilege>
                   <D:privilege>
 
 
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                     <D:read-current-user-privilege-set/>
                   </D:privilege>
                   <D:abstract/>
                   <D:description xml:lang="en">Read current user
   privilege set property</D:description>
                 </D:supported-privilege>
                 </D:supported-privilege>
                 <D:supported-privilege>
                 <D:privilege> <D:write/> </D:privilege>
                 <D:description xml:lang="en">Write any
   object</D:description>
                 <D:supported-privilege>
                   <D:privilege> <D:write-acl/> </D:privilege>
                   <D:description xml:lang="en">Write
   ACL</D:description>
                   <D:abstract/>
                 </D:supported-privilege>
                 <D:supported-privilege>
                   <D:privilege> <D:write-properties/> </D:privilege>
                   <D:description xml:lang="en">Write
   properties</D:description>
                 </D:supported-privilege>
                 <D:supported-privilege>
                   <D:privilege> <D:write-content/> </D:privilege>
                   <D:description xml:lang="en">Write resource
   content</D:description>
                 </D:supported-privilege>
                 </D:supported-privilege>
                 <D:supported-privilege>
                   <D:privilege> <D:unlock/> </D:privilege>
                   <D:description xml:lang="en">Unlock
   resource</D:description>
                 </D:supported-privilege>
             </D:supported-privilege>
           </D:supported-privilege-set>
         </D:prop>
         <D:status>HTTP/1.1 200 OK</D:status>
       </D:propstat>
     </D:response>
   </D:multistatus>
 
 5.3 DAV:current-user-privilege-set
 
   DAV:current-user-privilege-set is a protected property containing
   the exact set of privileges (as computed by the server) granted to
   the currently authenticated HTTP user. Aggregate privileges and
   their contained privileges are listed. A user-agent can use the
   value of this property to adjust its user interface to make actions
 
 
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   inaccessible (e.g., by graying out a menu item or button) for which
   the current principal does not have permission. This is
   particularly useful for an access control user interface, which can
   be constructed without knowing the ACE combining semantics of the
   server. This property is also useful for determining what
   operations the current principal can perform, without having to
   actually execute an operation.
 
   <!ELEMENT current-user-privilege-set (privilege*)>
   <!ELEMENT privilege ANY>
 
   If the current user is granted a specific privilege, that privilege
   must belong to the set of privileges that may be set on this
   resource. Therefore, each element in the DAV:current-user-
   privilege-set property MUST identify a non-abstract privilege from
   the DAV:supported-privilege-set property.
 
 5.3.1 Example: Retrieving the User's Current Set of Assigned
       Privileges
 
   Continuing the example from Section 5.2.1, this example shows a
   client requesting the DAV:current-user-privilege-set property from
   the resource with URL http://www.webdav.org/papers/. The username
   of the principal making the request is "khare", and Digest
   authentication is used in the request. The principal with username
   "khare" has been granted the DAV:read privilege. Since the DAV:read
   privilege contains the DAV:read-acl and DAV:read-current-user-
   privilege-set privileges (see Section 5.2.1), the principal with
   username "khare" can read the ACL property, and the DAV:current-
   user-privilege-set property. However, the DAV:all, DAV:read-acl,
   DAV:write-acl and DAV:read-current-user-privilege-set privileges
   are not listed in the value of DAV:current-user-privilege-set,
   since (for this example) they are abstract privileges. DAV:write is
   not listed since the principal with username "khare" is not listed
   in an ACE granting that principal write permission.
 
   >> Request <<
 
   PROPFIND /papers/ HTTP/1.1
   Host: www.webdav.org
   Content-type: text/xml; charset="utf-8"
   Content-Length: xxx
   Depth: 0
   Authorization: Digest username="khare",
      realm="khare@webdav.org", nonce="...",
      uri="/papers/", response="...", opaque="..."
 
 
 
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   <?xml version="1.0" encoding="utf-8" ?>
   <D:propfind xmlns:D="DAV:">
     <D:prop>
       <D:current-user-privilege-set/>
     </D:prop>
   </D:propfind>
 
   >> Response <<
 
   HTTP/1.1 207 Multi-Status
   Content-Type: text/xml; charset="utf-8"
   Content-Length: xxx
 
   <?xml version="1.0" encoding="utf-8" ?>
   <D:multistatus xmlns:D="DAV:">
     <D:response>
       <D:href>http://www.webdav.org/papers/</D:href>
       <D:propstat>
         <D:prop>
           <D:current-user-privilege-set>
             <D:privilege> <D:read/> </D:privilege>
           </D:current-user-privilege-set>
         </D:prop>
         <D:status>HTTP/1.1 200 OK</D:status>
       </D:propstat>
     </D:response>
   </D:multistatus>
 
 5.4 DAV:acl
 
   This is a protected property that specifies the list of access
   control entries (ACEs), which define what principals are to get
   what privileges for this resource.
 
   <!ELEMENT acl (ace*) >
 
   Each DAV:ace element specifies the set of privileges to be either
   granted or denied to a single principal.  If the DAV:acl property
   is empty, no principal is granted any privilege.
 
   <!ELEMENT ace (invert | principal, (grant|deny), protected?,
   inherited?)>
 
 5.4.1 ACE Principal
 
   The DAV:principal element identifies the principal to which this
   ACE applies.
 
 
 
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   <!ELEMENT principal ((href)
    | all | authenticated | unauthenticated
    | property | self)>
 
   The current user matches DAV:href only if that user is
   authenticated as being (or being a member of) the principal
   identified by the URL contained by that DAV:href.
 
   The current user always matches DAV:all.
 
   <!ELEMENT all EMPTY>
 
   The current user matches DAV:authenticated only if authenticated.
 
   <!ELEMENT authenticated EMPTY>
 
   The current user matches DAV:unauthenticated only if not
   authenticated.
 
   <!ELEMENT unauthenticated EMPTY>
 
   DAV:all is the union of DAV:authenticated, and DAV:unauthenticated.
   For a given request, the user matches either DAV:authenticated, or
   DAV:unauthenticated, but not both (that is, DAV:authenticated and
   DAV:unauthenticated are disjoint sets).
 
   The current user matches a DAV:property principal in a DAV:acl
   property of a resource only if the value of the identified property
   of that resource contains at most one DAV:href XML element, the URI
   value of DAV:href identifies a principal, and the current user is
   authenticated as being (or being a member of) that principal.  For
   example, if the DAV:property element contained <DAV:owner/>, the
   current user would match the DAV:property principal only if the
   current user is authenticated as matching the principal identified
   by the DAV:owner property of the resource.
 
   <!ELEMENT property ANY>
 
   Alternately, some servers may support ACEs applying to those users
   NOT matching the current principal, e.g. all users not in a
   particular group.  This can be done by wrapping the DAV:principal
   element with DAV:invert.
 
   <!ELEMENT invert principal>
 
 
 
 
 
 
 
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   The current user matches DAV:self in a DAV:acl property of the
   resource only if that resource is a principal and that principal
   matches the current user or, if the principal is a group, a member
   of that group matches the current user.
 
   <!ELEMENT self EMPTY>
 
 5.4.2 ACE Grant and Deny
 
   Each DAV:grant or DAV:deny element specifies the set of privileges
   to be either granted or denied to the specified principal.  A
   DAV:grant or DAV:deny element of the DAV:acl of a resource MUST
   only contain non-abstract elements specified in the DAV:supported-
   privilege-set of that resource.
 
   <!ELEMENT grant (privilege+)>
   <!ELEMENT deny (privilege+)>
   <!ELEMENT privilege ANY>
 
 5.4.3 ACE Protection
 
   A server indicates an ACE is protected by including the
   DAV:protected element in the ACE. If the ACL of a resource contains
   an ACE with a DAV:protected element, an attempt to remove that ACE
   from the ACL MUST fail.
 
   <!ELEMENT protected EMPTY>
 
 5.4.4 ACE Inheritance
 
   The presence of a DAV:inherited element indicates that this ACE is
   inherited from another resource that is identified by the URL
   contained in a DAV:href element.  An inherited ACE cannot be
   modified directly, but instead the ACL on the resource from which
   it is inherited must be modified.
 
   Note that ACE inheritance is not the same as ACL initialization.
   ACL initialization defines the ACL that a newly created resource
   will use (if not specified).  ACE inheritance refers to an ACE that
   is logically shared - where an update to the resource containing an
   ACE will affect the ACE of each resource that inherits that ACE.
   The method by which ACLs are initialized or by which ACEs are
   inherited is not defined by this document.
 
   <!ELEMENT inherited (href)>
 
 
 
 
 
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 5.4.5 Example: Retrieving a Resource's Access Control List
 
   Continuing the example from Sections 5.2.1 and 5.3.1, this example
   shows a client requesting the DAV:acl property from the resource
   with URL http://www.webdav.org/papers/. There are two ACEs defined
   in this ACL:
 
   ACE #1: The group identified by URL
   http://www.webdav.org/acl/groups/maintainers (the group of site
   maintainers) is granted DAV:write privilege. Since (for this
   example) DAV:write contains the DAV:write-acl privilege (see
   Section 5.2.1), this means the "maintainers" group can also modify
   the access control list.
 
   ACE #2: All principals (DAV:all) are granted the DAV:read
   privilege. Since (for this example) DAV:read contains DAV:read-acl
   and DAV:read-current-user-privilege-set, this means all users
   (including all members of the "maintainers" group) can read the
   DAV:acl property and the DAV:current-user-privilege-set property.
 
 
   >> Request <<
 
   PROPFIND /papers/ HTTP/1.1
   Host: www.webdav.org
   Content-type: text/xml; charset="utf-8"
   Content-Length: xxx
   Depth: 0
   Authorization: Digest username="masinter",
      realm="webdav.org", nonce="...",
      uri="/papers/", response="...", opaque="..."
 
   <?xml version="1.0" encoding="utf-8" ?>
   <D:propfind xmlns:D="DAV:">
     <D:prop>
       <D:acl/>
     </D:prop>
   </D:propfind>
 
 
   >> Response <<
 
   HTTP/1.1 207 Multi-Status
   Content-Type: text/xml; charset="utf-8"
   Content-Length: xxx
 
   <?xml version="1.0" encoding="utf-8" ?>
   <D:multistatus xmlns:D="DAV:">
     <D:response>
 
 
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       <D:href>http://www.webdav.org/papers/</D:href>
       <D:propstat>
          <D:prop>
           <D:acl>
             <D:ace>
               <D:principal>
   <D:href>http://www.webdav.org/acl/groups/maintainers</D:href>
               </D:principal>
               <D:grant>
                 <D:privilege> <D:write/> </D:privilege>
               </D:grant>
             </D:ace>
             <D:ace>
               <D:principal>
                 <D:all/>
               </D:principal>
               <D:grant>
                 <D:privilege> <D:read/> </D:privilege>
               </D:grant>
             </D:ace>
           </D:acl>
         </D:prop>
         <D:status>HTTP/1.1 200 OK</D:status>
       </D:propstat>
     </D:response>
   </D:multistatus>
 
 
 5.5    DAV:acl-semantics
 
   This is a protected property that defines the ACL semantics of the
   ACEs specified in the DAV:acl property of this resource.  These
   semantics define how multiple ACEs that match the current user are
   combined, what are the constraints on how ACEs can be ordered, and
   which principals must have an ACE. A client user interface could
   use the value of this property to provide feedback to a human
   operator concerning the impact of proposed changes to an ACL.
   Alternately, a client can use this property to help it determine,
   before submitting an ACL method invocation, what ACL changes it
   needs to make to accomplish a specific goal (or whether that goal
   is even achievable on this server).
 
   Since it is not practical to require all implementations to use the
   same ACL semantics, the DAV:acl-semantics property is used to
   identify the ACL semantics for a particular resource.  The DAV:acl-
   semantics element is defined in Section 6.
 
 
 
 
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 5.5.1     Example: Retrieving DAV:acl-semantics
 
   In this example, the client requests the value of the DAV:acl-
   semantics property. Digest authentication provides credentials for
   the principal operating the client. In this example, the ACE
   combination semantics are DAV:first-match, described in Section
   6.1.1, the ACE ordering semantics are not specified (some value
   other than DAV:deny-before-grant, described in Section 6.2.1), the
   DAV:allowed-ace element states that only one ACE is permitted for
   each principal, and an ACE describing the privileges granted the
   DAV:all principal must exist in every ACL.
 
 
 
   >> Request <<
 
   PROPFIND /papers/ HTTP/1.1
   Host: www.webdav.org
   Content-type: text/xml; charset="utf-8"
   Content-Length: xxx
   Depth: 0
   Authorization: Digest username="srcarter",
      realm="srcarter@webdav.org", nonce="...",
      uri="/papers/", response="...", opaque="..."
 
   <?xml version="1.0" encoding="utf-8" ?>
   <D:propfind xmlns:D="DAV:">
     <D:prop>
       <D:acl-semantics/>
     </D:prop>
   </D:propfind>
 
 
   >> Response <<
 
   HTTP/1.1 207 Multi-Status
   Content-Type: text/xml; charset="utf-8"
   Content-Length: xxx
 
   <?xml version="1.0" encoding="utf-8" ?>
   <D:multistatus xmlns:D="DAV:">
     <D:response>
       <D:href>http://www.webdav.org/papers/</D:href>
       <D:propstat>
           <D:prop>
           <D:acl-semantics>
             <D:ace-combination>
               <D:first-match/>
             </D:ace-combination>
 
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             <D:ace-ordering/>
               <D:allowed-ace>
               <D:principal-only-one-ace/>
             </D:allowed-ace>
             <D:required-principal>
               <D:all/>
             </D:required-principal>
           </D:acl-semantics>
         </D:prop>
         <D:status>HTTP/1.1 200 OK</D:status>
       </D:propstat>
     <D:response>
   </D:multistatus>
 
 
 5.6    DAV:inherited-acl-set
 
   This protected property contains a set of URLs that identify other
   resources that also control the access to this resource.  To have a
   privilege on a resource, not only must the ACL on that resource
   (specified in the DAV:acl property of that resource) grant the
   privilege, but so must the ACL of each resource identified in the
   DAV:inherited-acl-set property of that resource.  Effectively, the
   privileges granted by the current ACL are ANDed with the privileges
   granted by each inherited ACL.
 
   <!ELEMENT inherited-acl-set (href*)>
 
   Note that the ACL semantics of a resource are specified in the
   DAV:acl-semantics property of that resource, and therefore each
   inherited ACL can have different ACL semantics.
 
 5.7 DAV:principal-collection-set
 
   This protected property of a resource contains a set of URLs that
   identify the root collections that contain the principals that are
   available on the server that implements this resource.  A WebDAV
   Access Control Protocol user agent could use the contents of
   DAV:principal-collection-set to retrieve the DAV:displayname
   property (specified in Section 13.2 of [RFC2518]) of all principals
   on that server, thereby yielding human-readable names for each
   principal that could be displayed in a user interface.
 
   <!ELEMENT principal-collection-set (href*)>
   Since different servers can control different parts of the URL
   namespace, different resources on the same host MAY have different
   DAV:principal-collection-set values. The collections specified in
   the DAV:principal-collection-set MAY be located on different hosts
   from the resource. The URLs in DAV:principal-collection-set SHOULD
 
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   be http or https scheme URLs. For security and scalability reasons,
   a server MAY report only a subset of the entire set of known
   principal collections, and therefore clients should not assume they
   have retrieved an exhaustive listing. Additionally, a server MAY
   elect to report none of the principal collections it knows about,
   in which case the property value would be empty.
 
   The value of DAV:principal-collection-set gives the scope of the
   DAV:principal-property-search REPORT (defined in Section 9.4).
   Clients use the DAV:principal-property-search REPORT to populate
   their user interface with a list of principals. Therefore, servers
   that limit a client's ability to obtain principal information will
   interfere with the client's ability to manipulate access control
   lists, due to the difficulty of getting the URL of a principal for
   use in an ACE.
 
 5.7.1 Example: Retrieving DAV:principal-collection-set
 
   In this example, the client requests the value of the
   DAV:principal-collection-set property on the collection resource
   identified by URL http://www.webdav.org/papers/. The property
   contains the two URLs, http://www.webdav.org/acl/users/ and
   http://www.webdav.org/acl/groups/, both wrapped in DAV:href XML
   elements. Digest authentication provides credentials for the
   principal operating the client.
 
   The client might reasonably follow this request with two separate
   PROPFIND requests to retrieve the DAV:displayname property of the
   members of the two collections (/acl/users and /acl/groups). This
   information could be used when displaying a user interface for
   creating access control entries.
 
 
   >> Request <<
 
   PROPFIND /papers/ HTTP/1.1
   Host: www.webdav.org
   Content-type: text/xml; charset="utf-8"
   Content-Length: xxx
   Depth: 0
   Authorization: Digest username="yarong",
      realm="yarong@webdav.org", nonce="...",
      uri="/papers/", response="...", opaque="..."
 
   <?xml version="1.0" encoding="utf-8" ?>
   <D:propfind xmlns:D="DAV:">
     <D:prop>
       <D:principal-collection-set/>
     </D:prop>
   </D:propfind>
 
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   >> Response <<
 
   HTTP/1.1 207 Multi-Status
   Content-Type: text/xml; charset="utf-8"
   Content-Length: xxx
 
   <?xml version="1.0" encoding="utf-8" ?>
   <D:multistatus xmlns:D="DAV:">
    <D:response>
     <D:href>http://www.webdav.org/papers/</D:href>
     <D:propstat>
      <D:prop>
        <D:principal-collection-set>
         <D:href>http://www.webdav.org/acl/users/</D:href>
         <D:href>http://www.webdav.org/acl/groups/</D:href>
        </D:principal-collection-set>
      </D:prop>
      <D:status>HTTP/1.1 200 OK</D:status>
     </D:propstat>
    </D:response>
   </D:multistatus>
 
 5.8 Example: PROPFIND to retrieve access control properties
 
   The following example shows how access control information can be
   retrieved by using the PROPFIND method to fetch the values of the
   DAV:owner, DAV:supported-privilege-set, DAV:current-user-privilege-
   set, and DAV:acl properties.
 
   >> Request <<
 
   PROPFIND /top/container/ HTTP/1.1
   Host: www.foo.org
   Content-type: text/xml; charset="utf-8"
   Content-Length: xxx
   Depth: 0
   Authorization: Digest username="ejw",
      realm="users@foo.org", nonce="...",
      uri="/top/container/", response="...", opaque="..."
 
   <?xml version="1.0" encoding="utf-8" ?>
   <D:propfind xmlns:D="DAV:">
     <D:prop>
       <D:owner/>
       <D:supported-privilege-set/>
       <D:current-user-privilege-set/>
 
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       <D:acl/>
     </D:prop>
   </D:propfind>
 
   >> Response <<
 
   HTTP/1.1 207 Multi-Status
   Content-Type: text/xml; charset="utf-8"
   Content-Length: xxx
 
   <?xml version="1.0" encoding="utf-8" ?>
   <D:multistatus
      xmlns:D="DAV:"
      xmlns:A="http://www.webdav.org/acl/"> <D:response>
     <D:href>http://www.foo.org/top/container/</D:href>
     <D:propstat>
     <D:prop>
       <D:owner>
         <D:href>http://www.foo.org/users/gclemm</D:href> </D:owner>
       <D:supported-privilege-set>
         <D:supported-privilege>
           <D:privilege> <D:all/> </D:privilege>
           <D:abstract/>
           <D:description xml:lang="en">Any operation</D:description>
           <D:supported-privilege>
             <D:privilege> <D:read/> </D:privilege>
             <D:description xml:lang="en">Read any
   object</D:description>
           </D:supported-privilege>
           <D:supported-privilege>
             <D:privilege> <D:write/> </D:privilege>
             <D:abstract/>
             <D:description xml:lang="en">Write any
   object</D:description>
             <D:supported-privilege>
               <D:privilege> <A:create/> </D:privilege>
               <D:description xml:lang="en">Create an
   object</D:description>
             </D:supported-privilege>
             <D:supported-privilege>
               <D:privilege> <A:update/> </D:privilege>
               <D:description xml:lang="en">Update an
   object</D:description>
             </D:supported-privilege>
             <D:supported-privilege>
               <D:privilege> <A:delete/> </D:privilege>
               <D:description xml:lang="en">Delete an
   object</D:description>
 
 
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             </D:supported-privilege>
           </D:supported-privilege>
           <D:supported-privilege>
             <D:privilege> <D:read-acl/> </D:privilege>
             <D:description xml:lang="en">Read the ACL</D:description>
           </D:supported-privilege>
           <D:supported-privilege>
             <D:privilege> <D:write-acl/> </D:privilege>
             <D:description xml:lang="en">Write the
   ACL</D:description>
           </D:supported-privilege>
         </D:supported-privilege>
       </D:supported-privilege-set>
       <D:current-user-privilege-set>
         <D:privilege> <D:read/> </D:privilege>
         <D:privilege> <D:read-acl/> </D:privilege>
       </D:current-user-privilege-set>
       <D:acl>
         <D:ace>
           <D:principal>
             <D:href>http://www.foo.org/users/esedlar</D:href>
             </D:principal>
           <D:grant>
             <D:privilege> <D:read/> </D:privilege>
             <D:privilege> <D:write/> </D:privilege>
             <D:privilege> <D:read-acl/> </D:privilege> </D:grant>
         </D:ace>
         <D:ace>
           <D:principal>
             <D:href>http://www.foo.org/groups/marketing</D:href>
           </D:principal>
           <D:deny>
             <D:privilege> <D:read/> </D:privilege> </D:deny>
         </D:ace>
         <D:ace>
           <D:principal>
             <D:property> <D:owner/> </D:property> </D:principal>
           <D:grant>
             <D:privilege> <D:read-acl/> </D:privilege>
             <D:privilege> <D:write-acl/> </D:privilege> </D:grant>
         </D:ace>
         <D:ace>
           <D:principal> <D:all/> </D:principal>
           <D:grant>
             <D:privilege> <D:read/> </D:privilege></D:grant>
           <D:inherited>
             <D:href>http://www.foo.org/top</D:href> </D:inherited>
         </D:ace> </D:acl>
 
 
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       </D:prop>
       <D:status>HTTP/1.1 200 OK</D:status>
     </D:propstat> </D:response> </D:multistatus>
 
 
   The value of the DAV:owner property is a single DAV:href XML
   element containing the URL of the principal that owns this
   resource.
 
   The value of the DAV:supported-privilege-set property is a tree of
   supported privileges (using "[XML Namespace , localname]" to
   identify each privilege):
 
      [DAV:, all] (aggregate, abstract)
         |
         +-- [DAV:, read]
         +-- [DAV:, write] (aggregate, abstract)
                |
                +-- [http://www.webdav.org/acl, create]
                +-- [http://www.webdav.org/acl, update]
                +-- [http://www.webdav.org/acl, delete]
         +-- [DAV:, read-acl]
         +-- [DAV:, write-acl]
 
 
   The DAV:current-user-privilege-set property contains two
   privileges, DAV:read, and DAV:read-acl. This indicates that the
   current authenticated user only has the ability to read the
   resource, and read the DAV:acl property on the resource.
 
   The DAV:acl property contains a set of four ACEs:
 
   ACE #1: The principal identified by the URL
   http://www.foo.org/users/esedlar is granted the DAV:read,
   DAV:write, and DAV:read-acl privileges.
 
   ACE #2: The principals identified by the URL
   http://www.foo.org/groups/marketing are denied the DAV:read
   privilege.  In this example, the principal URL identifies a group.
 
   ACE #3: In this ACE, the principal is a property principal,
   specifically the DAV:owner property. When evaluating this ACE, the
   value of the DAV:owner property is retrieved, and is examined to
   see if it contains a DAV:href XML element. If so, the URL within
   the DAV:href element is read, and identifies a principal. In this
   ACE, the owner is granted DAV:read-acl, and DAV:write-acl
   privileges.
 
 
 
 
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   ACE #4: This ACE grants the DAV:all principal (all users) the
   DAV:read privilege. This ACE is inherited from the resource
   http://www.foo.org/top, the parent collection of this resource.
 
 6  ACL SEMANTICS
 
   The ACL semantics define how multiple ACEs that match the current
   user are combined, what are the constraints on how ACEs can be
   ordered, and which principals must have an ACE.
 
   <!ELEMENT acl-semantics (ace-combination?, ace-ordering?, allowed-
   ace?, required-principal?)>
 
 
 6.1 ACE Combination
 
   The DAV:ace-combination element defines how privileges from
   multiple ACEs that match the current user will be combined to
   determine the access privileges for that user.  Multiple ACEs may
   match the same user because the same principal can appear in
   multiple ACEs, because multiple principals can identify the same
   user, and because one principal can be a member of another
   principal.
 
   <!ELEMENT ace-combination
    (first-match | all-grant-before-any-deny | specific-deny-
   overrides-grant)>
 
 6.1.1 DAV:first-match ACE Combination
 
   The ACEs are evaluated in the order in which they appear in the
   ACL.  If the first ACE that matches the current user does not grant
   all the privileges needed for the request, the request MUST fail.
 
   <!ELEMENT first-match EMPTY>
 
 6.1.2 DAV:all-grant-before-any-deny ACE Combination
 
   The ACEs are evaluated in the order in which they appear in the
   ACL.  If an evaluated ACE denies a privilege needed for the
   request, the request MUST fail.  If all ACEs have been evaluated
   without the user being granted all privileges needed for the
   request, the request MUST fail.
 
   <!ELEMENT all-grant-before-any-deny EMPTY>
 
 
 
 
 
 
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 6.1.3 DAV:specific-deny-overrides-grant ACE Combination
 
   All ACEs in the ACL are evaluated.  An "individual ACE" is one
   whose principal matches the current user.  A "group ACE" is one
   whose principal is a group that has a member that matches the
   current user.  A privilege is granted if it is granted by an
   individual ACE and not denied by an individual ACE, or if it is
   granted by a group ACE and not denied by an individual or group
   ACE.  A request MUST fail if any of its needed privileges are not
   granted.
 
   <!ELEMENT specific-deny-overrides-grant EMPTY>
 
 6.2 ACE Ordering
 
   The DAV:ace-ordering element defines a constraint on how the ACEs
   can be ordered in the ACL.
 
   <!ELEMENT ace-ordering (deny-before-grant)? >
 
 6.2.1 DAV:deny-before-grant ACE Ordering
 
   This element indicates that all deny ACEs must precede all grant
   ACEs.
 
   <!ELEMENT deny-before-grant EMPTY>
 
 6.3 Allowed ACE
 
   The DAV:allowed-ace XML element specifies constraints on what kinds
   of ACEs are allowed in an ACL.
 
   <!ELEMENT allowed-ace (principal-only-one-ace | grant-only |
                          no-invert | no-acl-inherit)*>
 
 6.3.1 DAV:principal-only-one-ace ACE Constraint
 
   This element indicates that a principal can appear in only one ACE
   per resource.
 
   <!ELEMENT principal-only-one-ace EMPTY>
 
 6.3.2 DAV:grant-only ACE Constraint
 
   This element indicates that ACEs with deny clauses are not allowed.
 
   <!ELEMENT grant-only EMPTY>
 
 
 
 
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 6.3.3 DAV:no-invert ACE Constraint
 
   This element indicates that ACEs with the <invert> element are not
   allowed.
 
   <!ELEMENT no-invert EMPTY>
 
 6.4 Required Principals
 
   The required principal elements identify which principals must have
   an ACE defined in the ACL.
 
   <!ELEMENT required-principal
     (all? | authenticated? | unauthenticated? | self? | href* |
   property*)>
 
   For example, the following element requires that the ACL contain a
   DAV:owner property ACE:
 
   <D:required-principal xmlns:D="DAV:">
     <D:property> <D:owner/> </D:property>
   </D:required-principal>
 
 7  ACCESS CONTROL AND EXISTING METHODS
 
   This section defines the impact of access control functionality on
   existing methods.
 
 7.1 OPTIONS
 
   If the server supports access control, it MUST return "access-
   control" as a field in the DAV response header from an OPTIONS
   request on any resource implemented by that server. A value of
   "access-control" in the DAV header MUST indicate that the server
   supports all MUST level requirements and REQUIRED features
   specified in this document.
 
 7.1.1 Example - OPTIONS
 
   >> Request <<
 
     OPTIONS /foo.html HTTP/1.1
     Host: www.webdav.org
     Content-Length: 0
 
   >> Response <<
 
     HTTP/1.1 200 OK
     DAV: 1, 2, access-control
     Allow: OPTIONS, GET, PUT, PROPFIND, PROPPATCH, ACL
 
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   In this example, the OPTIONS response indicates that the server
   supports access control and that /foo.html can have its access
   control list modified by the ACL method.
 
 7.2 MOVE
 
   When a resource is moved from one location to another due to a MOVE
   request, the non-inherited and non-protected ACEs in the DAV:acl
   property of the resource MUST NOT be modified, or the MOVE request
   fails. Handling of inherited and protected ACEs is intentionally
   undefined to give server implementations flexibility in how they
   implement ACE inheritance and protection.
 
 7.3 COPY
 
   The DAV:acl property on the resource at the destination of a COPY
   MUST be the same as if the resource was created by an individual
   resource creation request (e.g. MKCOL, PUT). Clients wishing to
   preserve the DAV:acl property across a copy need to read the
   DAV:acl property prior to the COPY, then perform an ACL operation
   on the new resource at the destination to restore, insofar as this
   is possible, the original access control list.
 
 7.4 DELETE
 
   The precise combination of privileges and resources necessary to
   permit the DELETE method is intentionally left to the discretion of
   each server implementation. It is envisioned that on some servers,
   DELETE will require write permission on the collection containing
   the resource to be deleted.  On other servers, it might also
   require write permission on the resource being deleted.
 
 7.5 LOCK
 
   A lock on a resource ensures that only the lock owner can modify
   ACEs that are not inherited and not protected  (these are the only
   ACEs that a client can modify with an ACL request). A lock does not
   protect inherited or protected ACEs, since a client cannot modify
   them with an ACL request on that resource.
 
 8  ACCESS CONTROL METHODS
 
 8.1 ACL
 
   The ACL method modifies the access control list (which can be read
   via the DAV:acl property) of a resource.  Specifically, the ACL
 
 
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   method only permits modification to ACEs that are not inherited,
   and are not protected. An ACL method invocation modifies all non-
   inherited and non-protected ACEs in a resource's access control
   list to exactly match the ACEs contained within in the DAV:acl XML
   element (specified in Section 5.4) of the request body. An ACL
   request body MUST contain only one DAV:acl XML element. Unless the
   non-inherited and non-protected ACEs of the DAV:acl property of the
   resource can be updated to be exactly the value specified in the
   ACL request, the ACL request MUST fail.
 
   It is possible that the ACEs visible to the current user in the
   DAV:acl property may only be a portion of the complete set of ACEs
   on that resource. If this is the case, an ACL request only modifies
   the set of ACEs visible to the current user, and does not affect
   any non-visible ACE.
 
   In order to avoid overwriting DAV:acl changes by another client, a
   client SHOULD acquire a WebDAV lock on the resource before
   retrieving the DAV:acl property of a resource that it intends on
   updating.
 
      Implementation Note: Two common operations are to add or remove
      an ACE from an existing access control list. To accomplish this,
      a client uses the PROPFIND method to retrieve the value of the
      DAV:acl property, then parses the returned access control list
      to remove all inherited and protected ACEs (these ACEs are
      tagged with the DAV:inherited and DAV:protected XML elements).
      In the remaining set of non-inherited, non-protected ACEs, the
      client can add or remove one or more ACEs before submitting the
      final ACE set in the request body of the ACL method.
 
 8.1.1 ACL Preconditions
 
   An implementation MAY enforce one or more of the following
   constraints on an ACL request.  If the constraint is violated, a
   403 (Forbidden) or 409 (Conflict) response MUST be returned and the
   indicated XML element MUST be returned as a child of a top level
   DAV:error element in an XML response body.
 
   (DAV:no-ace-conflict): The ACEs submitted in the ACL request MUST
   NOT conflict with each other.  What is considered a conflict
   depends on the ACL semantics of that resource.
 
   (DAV:no-protected-ace-conflict): The ACEs submitted in the ACL
   request MUST NOT conflict with the protected ACEs on the resource.
   For example, if the resource has a protected ACE granting DAV:write
   to a given principal, then it would not be consistent if the ACL
   request submitted an ACE denying DAV:write to the same principal.
 
 
 
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   (DAV:no-inherited-ace-conflict): The ACEs submitted in the ACL
   request MUST NOT conflict with the inherited ACEs on the resource.
   For example, if the resource inherits an ACE from its parent
   collection granting DAV:write to a given principal, then it would
   not be consistent if the ACL request submitted an ACE denying
   DAV:write to the same principal. Note that reporting of this error
   will be implementation-dependent. Implementations have the choice
   to either report this error, or to allow the ACE to be set, and
   then let normal ACE evaluation rules determine whether the new ACE
   has any impact on the privileges available to a specific principal.
 
   (DAV:limited-number-of-aces): The number of ACEs submitted in the
   ACL request MUST NOT exceed the number of ACEs allowed on that
   resource.  However, ACL-compliant servers MUST support at least one
   ACE granting privileges to a single principal, and one ACE granting
   privileges to a group.
 
   (DAV:deny-before-grant): All non-inherited deny ACEs MUST precede
   all non-inherited grant ACEs.
 
   (DAV:principal-only-one-ace): The ACL request MUST NOT result in
   more than one ACE for a given principal.  This precondition applies
   only when the ACL semantics of the resource includes the
   DAV:principal-only-one-ace constraint (defined in Section 6.3.1).
 
   (DAV:grant-only): The ACEs submitted in the ACL request MUST NOT
   include a deny ACE.  This precondition applies only when the ACL
   semantics of the resource includes the DAV:grant-only constraint
   (defined in Section 6.3.2).
 
   (DAV:no-invert):  The ACL request MUST NOT include a DAV:invert
   element.   This precondition applies only when the ACL semantics of
   the resource includes the DAV:no-invert constraint (defined in
   Section 6.3.4).
 
   (DAV:no-abstract): The ACL request MUST NOT attempt to grant or
   deny an abstract privilege (see Section 5.2).
 
   (DAV:not-supported-privilege): The ACEs submitted in the ACL
   request MUST be supported by the resource.
 
   (DAV:missing-required-principal): The result of the ACL request
   MUST have at least one ACE for each principal identified in a
   DAV:required-principal XML element in the ACL semantics of that
   resource (see Section 6.3.2).
 
   (DAV:recognized-principal): Every principal URL in the ACL request
   MUST identify a principal resource.
 
 
 
 
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   (DAV:allowed-principal): The principals specified in the ACEs
   submitted in the ACL request MUST be allowed as principals for the
   resource. For example, a server where only authenticated principals
   can access resources would not allow the DAV:all or
   DAV:unauthenticated principals to be used in an ACE, since these
   would allow unauthenticated access to resources.
 
 8.1.2 Example: the ACL method
 
   In the following example, user "fielding", authenticated by
   information in the Authorization header, grants the principal
   identified by the URL http://www.foo.org/users/esedlar  (i.e., the
   user "esedlar") read and write privileges, grants the owner of the
   resource read-acl and write-acl privileges, and grants everyone
   read privileges.
 
   >> Request <<
 
   ACL /top/container/ HTTP/1.1
   Host: www.foo.org
   Content-Type: text/xml; charset="utf-8"
   Content-Length: xxxx
   Authorization: Digest username="fielding",
      realm="users@foo.org", nonce="...",
      uri="/top/container/", response="...", opaque="..."
 
   <?xml version="1.0" encoding="utf-8" ?>
   <D:acl xmlns:D="DAV:">
     <D:ace>
       <D:principal>
         <D:href>http://www.foo.org/users/esedlar</D:href>
       </D:principal>
       <D:grant>
         <D:privilege> <D:read/> </D:privilege>
         <D:privilege> <D:write/> </D:privilege>
       </D:grant>
     </D:ace>
     <D:ace>
       <D:principal>
         <D:property> <D:owner/> </D:property>
       </D:principal>
       <D:grant>
         <D:privilege> <D:read-acl/> </D:privilege>
         <D:privilege> <D:write-acl/> </D:privilege>
       </D:grant>
     </D:ace>
     <D:ace>
       <D:principal> <D:all/> </D:principal>
 
 
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       <D:grant>
         <D:privilege> <D:read/> </D:privilege>
       </D:grant>
     </D:ace> </D:acl>
 
   >> Response <<
 
   HTTP/1.1 200 OK
 
 8.1.3 Example: ACL method failure due to protected ACE conflict
 
   In the following request, user "fielding", authenticated by
   information in the Authorization header, attempts to deny the
   principal identified by the URL http://www.foo.org/users/esedlar
   (i.e., the user "esedlar") write privileges. Prior to the request,
   the DAV:acl property on the resource contained a protected ACE (see
   Section 5.4.3) granting DAV:owner the DAV:read and DAV:write
   privileges. The principal identified by URL
   http://www.foo.org/users/esedlar is the owner of the resource. The
   ACL method invocation fails because the submitted ACE conflicts
   with the protected ACE, thus violating the semantics of ACE
   protection.
 
   >> Request <<
 
   ACL /top/container/ HTTP/1.1
   Host: www.foo.org
   Content-Type: text/xml; charset="utf-8"
   Content-Length: xxxx
   Authorization: Digest username="fielding",
      realm="users@foo.org", nonce="...",
      uri="/top/container/", response="...", opaque="..."
 
   <?xml version="1.0" encoding="utf-8" ?>
   <D:acl xmlns:D="DAV:">
     <D:ace>
       <D:principal>
         <D:href>http://www.foo.org/users/esedlar</D:href>
       </D:principal>
       <D:deny>
         <D:privilege> <D:write/> </D:privilege>
       </D:deny>
     </D:ace>
   </D:acl>
 
   >> Response <<
 
   HTTP/1.1 403 Forbidden
 
 
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   Content-Type: text/xml; charset="utf-8"
   Content-Length: xxx
 
   <?xml version="1.0" encoding="utf-8" ?>
   <D:error xmlns:D="DAV:">
     <D:no-protected-ace-conflict/>
   </D:error>
 
 8.1.4 Example: ACL method failure due to an inherited ACE conflict
 
   In the following request, user "ejw", authenticated by information
   in the Authorization header, tries to change the access control
   list on the resource http://www.foo.org/top/index.html. This
   resource has two inherited ACEs.
 
   Inherited ACE #1 grants the principal identified by URL
   http://www.foo.org/users/ejw (i.e., the user "ejw")
   http://www.foo.org/privs/write-all and DAV:read-acl privileges. On
   this server, http://www.foo.org/privs/write-all is an aggregate
   privilege containing DAV:write, and DAV:write-acl.
 
   Inherited ACE #2 grants principal DAV:all the DAV:read privilege.
 
   The request attempts to set a (non-inherited) ACE, denying the
   principal identified by the URL http://www.foo.org/users/ejw (i.e.,
   the user "ejw") DAV:write permission. This conflicts with inherited
   ACE #1. Note that the decision to report an inherited ACE conflict
   is specific to this server implementation. Another server
   implementation could have allowed the new ACE to be set, and then
   used normal ACE evaluation rules to determine whether the new ACE
   has any impact on the privileges available to a principal.
 
   >> Request <<
 
   ACL /top/index.html HTTP/1.1
   Host: www.foo.org
   Content-Type: text/xml; charset="utf-8"
   Content-Length: xxxx
   Authorization: Digest username="ejw",
      realm="users@foo.org", nonce="...",
      uri="/top/index.html", response="...", opaque="..."
 
   <?xml version="1.0" encoding="utf-8" ?>
   <D:acl xmlns:D="DAV:" xmlns:F="http://www.foo.org/privs/">
     <D:ace>
         <D:principal>
           <D:href>http://www.foo.org/users/ejw</D:href>
         </D:principal>
         <D:grant><D:write/></D:grant>
 
 
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     </D:ace>
   </D:acl>
 
   >> Response <<
 
   HTTP/1.1 403 Forbidden
   Content-Type: text/xml; charset="utf-8"
   Content-Length: xxx
 
   <?xml version="1.0" encoding="utf-8" ?>
   <D:error xmlns:D="DAV:">
     <D:no-inherited-ace-conflict xmlns:D="DAV:"/>
   </D:error>
 
 8.1.5 Example: ACL method failure due to an attempt to set grant and
       deny in a single ACE.
 
   In this example, user "ygoland", authenticated by information in
   the Authorization header, tries to change the access control list
   on the resource http://www.foo.org/diamond/engagement-ring.gif. The
   ACL request includes a single, syntactically and semantically
   incorrect ACE, which attempts to grant the group identified by the
   URL http://www.foo.org/users/friends DAV:read privilege and deny
   the principal identified by URL http://www.foo.org/users/ygoland-so
   (i.e., the user "ygoland-so") DAV:read privilege. However, it is
   illegal to have multiple principal elements, as well as both a
   grant and deny element in the same ACE, so the request fails due to
   poor syntax.
 
   >> Request <<
 
   ACL /diamond/engagement-ring.gif HTTP/1.1
   Host: www.foo.org
   Content-Type: text/xml; charset="utf-8"
   Content-Length: xxxx
   Authorization: Digest username="ygoland",
      realm="users@foo.org", nonce="...",
      uri="/diamond/engagement-ring.gif", response="...", opaque="..."
 
   <?xml version="1.0" encoding="utf-8" ?>
   <D:acl xmlns:D="DAV:">
       <D:ace>
         <D:principal>
           <D:href>http://www.foo.org/users/friends</D:href>
         </D:principal>
         <D:grant><D:read/></D:grant>
         <D:principal>
           <D:href>http://www.foo.org/users/ygoland-so</D:href>
 
 
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         </D:principal>
         <D:deny><D:read/></D:deny>
       </D:ace>
   </D:acl>
 
   >> Response <<
 
   HTTP/1.1 400 Bad Request
   Content-Length: 0
 
   Note that if the request had been divided into two ACEs, one to
   grant, and one to deny, the request would have been syntactically
   well formed.
 
 9  ACCESS CONTROL REPORTS
 
 9.1 REPORT Method
 
   The REPORT method (defined in Section 3.6 of [RFC3253]) provides an
   extensible mechanism for obtaining information about a resource.
   Unlike the PROPFIND method, which returns the value of one or more
   named properties, the REPORT method can involve more complex
   processing. REPORT is valuable in cases where the server has access
   to all of the information needed to perform the complex request
   (such as a query), and where it would require multiple requests for
   the client to retrieve the information needed to perform the same
   request.
 
   A server that supports the WebDAV Access Control Protocol MUST
   support the DAV:expand-property report (defined in Section 3.8 of
   [RFC3253]).
 
 9.2 DAV:acl-principal-prop-set Report
 
   The DAV:acl-principal-prop-set report returns, for all principals
   in the DAV:acl property (of the Request-URI) that are identified by
   http(s) URLs or by a DAV:property principal, the value of the
   properties specified in the REPORT request body. In the case where
   a principal URL appears multiple times, the DAV:acl-principal-prop-
   set report MUST return the properties for that principal only once.
   Support for this report is REQUIRED.
 
   One expected use of this report is to retrieve the human readable
   name (found in the DAV:displayname property) of each principal
   found in an ACL. This is useful for constructing user interfaces
   that show each ACE in a human readable form.
 
 Marshalling
 
 
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   The request body MUST be a DAV:acl-principal-prop-set XML element.
 
   <!ELEMENT acl-principal-prop-set ANY>
   ANY value: a sequence of one or more elements, with at most one
   DAV:prop element.
   prop: see RFC 2518, Section 12.11
 
 
   This report is only defined when the Depth header has value "0";
   other values result in a 400 (Bad Request) error response. Note
   that [RFC3253], Section 3.6, states that if the Depth header is not
   present, it defaults to a value of "0".
 
   The response body for a successful request MUST be a
   DAV:multistatus XML element (i.e., the response uses the same
   format as the response for PROPFIND).
 
   multistatus: see RFC 2518, Section 12.9
 
   The response body for a successful DAV:acl-principal-prop-set
   REPORT request MUST contain a DAV:response element for each
   principal identified by an http(s) URL listed in a DAV:principal
   XML element of an ACE within the DAV:acl property of the resource
   identified by the Request-URI.
 
 9.2.1 Example: DAV:acl-principal-prop-set Report
 
   Resource http://www.webdav.org/index.html has an ACL with three
   ACEs:
 
   ACE #1: All principals (DAV:all) have DAV:read and DAV:read-
   current-user-privilege-set access.
 
   ACE #2: The principal identified by
   http://www.webdav.org/people/gstein (the user "gstein") is granted
   DAV:write,  DAV:write-acl, DAV:read-acl privileges.
 
   ACE #3: The group identified by
   http://www.webdav.org/groups/authors (the "authors" group) is
   granted DAV:write and DAV:read-acl privileges.
 
   The following example shows a DAV:acl-principal-prop-set report
   requesting the DAV:displayname property. It returns the value of
   DAV:displayname for resources http://www.webdav.org/people/gstein
   and http://www.webdav.org/groups/authors , but not for DAV:all,
   since this is not an http(s) URL.
 
 
 
 
 
 
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   >> Request <<
 
   REPORT /index.html HTTP/1.1
   Host: www.webdav.org
   Content-Type: text/xml; charset="utf-8"
   Content-Length: xxxx
   Depth: 0
 
   <?xml version="1.0" encoding="utf-8" ?>
   <D:acl-principal-prop-set xmlns:D="DAV:">
     <D:prop>
       <D:displayname/>
     </D:prop>
   </D:acl-principal-prop-set>
 
   >> Response <<
 
   HTTP/1.1 207 Multi-Status
   Content-Type: text/xml; charset="utf-8"
   Content-Length: xxxx
 
   <?xml version="1.0" encoding="utf-8" ?>
   <D:multistatus xmlns:D="DAV:">
     <D:response>
       <D:href>http://www.webdav.org/people/gstein</D:href>
       <D:propstat>
         <D:prop>
           <D:displayname>Greg Stein</D:displayname>
         </D:prop>
         <D:status>HTTP/1.1 200 OK</D:status>
       </D:propstat>
     </D:response>
     <D:response>
       <D:href>http://www.webdav.org/groups/authors</D:href>
       <D:propstat>
         <D:prop>
           <D:displayname>Site authors</D:displayname>
         </D:prop>
         <D:status>HTTP/1.1 200 OK</D:status>
       </D:propstat>
     </D:response>
   </D:multistatus>
 
 9.3 DAV:principal-match REPORT
 
   The DAV:principal-match REPORT is used to identify all members (at
   any depth) of the collection identified by the Request-URI that
   match the current user. In particular, if the collection contains
 
 
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   principals, the report can be used to identify all members of the
   collection that match the current user. Alternatively, if the
   collection contains resources that have a property that identifies
   a principal (e.g. DAV:owner), the report can be used to identify
   all members of the collection whose property identifies a principal
   that matches the current user. For example, this report can return
   all of the resources in a collection hierarchy that are owned by
   the current user. Support for this report is REQUIRED.
 
 Marshalling:
   The request body MUST be a DAV:principal-match XML element.
 
   <!ELEMENT principal-match ((principal-property | self), prop?)>
   <!ELEMENT principal-property ANY>
   ANY value: an element whose value identifies a property. The
   expectation is the value of the named property typically contains
   an href element that contains the URI of a principal
   <!ELEMENT self EMPTY>
   prop: see RFC 2518, Section 12.11
 
 
   This report is only defined when the Depth header has value "0";
   other values result in a 400 (Bad Request) error response. Note
   that [RFC3253], Section 3.6, states that if the Depth header is not
   present, it defaults to a value of "0".
 
   The response body for a successful request MUST be a
   DAV:multistatus XML element.
 
   multistatus: see RFC 2518, Section 12.9
 
 
   The response body for a successful DAV:principal-match REPORT
   request MUST contain a DAV:response element for each member of the
   collection that matches the current user. When the DAV:principal-
   property element is used, a match occurs if the current user is
   matched by the principal identified by the URI found in the
   DAV:href element of the property identified by the DAV:principal-
   property element. When the DAV:self element is used in a
   DAV:principal-match report issued against a group, it matches a
   member of the group if that child (a principal resource) identifies
   the same principal as the current user.
 
   If DAV:prop is specified in the request body, the properties
   specified in the DAV:prop element MUST be reported in the
   DAV:response elements.
 
 
 
 
 
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 9.3.1 Example: DAV:principal-match REPORT
 
   The following example identifies the members of the collection
   identified by the URL http://www.webdav.org/doc that are owned by
   the current user. The current user ("gclemm") is authenticated
   using Digest authentication.
 
   >> Request <<
 
   REPORT /doc/ HTTP/1.1
   Host: www.webdav.org
   Authorization: Digest username="gclemm",
      realm="gclemm@webdav.org", nonce="...",
      uri="/papers/", response="...", opaque="..."
   Content-Type: text/xml; charset="utf-8"
   Content-Length: xxxx
   Depth: 0
 
   <?xml version="1.0" encoding="utf-8" ?>
   <D:principal-match xmlns:D="DAV:">
     <D:principal-property>
       <D:owner/>
     </D:principal-property>
   </D:principal-match>
 
   >> Response <<
 
   HTTP/1.1 207 Multi-Status
   Content-Type: text/xml; charset="utf-8"
   Content-Length: xxxx
 
   <?xml version="1.0" encoding="utf-8" ?>
   <D:multistatus xmlns:D="DAV:">
     <D:response>
       <D:href>http://www.webdav.org/doc/foo.html</D:href>
       <D:status>HTTP/1.1 200 OK</D:status>
     </D:response>
     <D:response>
       <D:href>http://www.webdav.org/doc/img/bar.gif</D:href>
       <D:status>HTTP/1.1 200 OK</D:status>
     </D:response>
   </D:multistatus>
 
 9.4 DAV:principal-property-search REPORT
 
   The DAV:principal-property-search REPORT performs a search for all
   principals whose properties contain character data that matches the
   search criteria specified in the request. One expected use of this
   report is to discover the URL of a principal associated with a
 
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   given person or group by searching for them by name. This is done
   by searching over DAV:displayname, which is defined on all
   principals.
 
   The actual search method (exact matching vs. substring matching vs,
   prefix-matching, case-sensitivity) deliberately is left to the
   server implementation to allow implementation on a wide set of
   possible user management systems. In cases where the implementation
   of DAV:principal-property-search is not constrained by the
   semantics of an underlying user management repository, preferred
   default semantics are caseless substring matches.
 
   For implementation efficiency, servers do not typically support
   searching on all properties. A client can discover the set of
   searchable properties by using the DAV:principal-search-property-
   set REPORT, defined in Section 9.5.
 
   Support for the DAV:principal-property-search report is REQUIRED.
 
      Implementation Note: The value of a WebDAV property is a
      sequence of well-formed XML, and hence can include any character
      in the Unicode/ISO-10646 standard, that is, most known
      characters in human languages. Due to the idiosyncrasies of case
      mapping across human languages, implementation of case-
      insensitive matching is non-trivial. Implementors of servers
      that do perform substring matching are strongly encouraged to
      consult [CaseMap], especially Section 2.3 ("Caseless Matching"),
      for guidance when implementing their case-insensitive matching
      algorithms.
 
      Implementation Note: Some implementations of this protocol will
      use an LDAP repository for storage of principal metadata. The
      schema describing each attribute (akin to a WebDAV property) in
      an LDAP repository specifies whether it supports case-sensitive
      or caseless searching. One of the benefits of leaving the search
      method to the discretion of the server implementation is the
      default LDAP attribute search behavior can be used when
      implementing the DAV:principal-property-search report.
 
 Marshalling:
   The request body MUST be a DAV:principal-property-search XML
   element containing a search specification and an optional list of
   properties. For every principal that matches the search
   specification, the response will contain the value of the requested
   properties on that principal.
 
   <!ELEMENT principal-property-search
    ((property-search+), prop?, apply-to-principal-collection-set?) >
 
 
 
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   By default, the report searches all members (at any depth) of the
   collection identified by the Request-URI.  If DAV:apply-to-
   principal-collection-set is specified in the request body, the
   request is applied instead to each collection identified by the
   DAV:prinicipal-collection-set property of the resource identified
   by the Request-URI.
 
   The DAV:property-search element contains a prop element enumerating
   the properties to be searched and a match element, containing the
   search string.
 
   <!ELEMENT property-search (prop, match) >
   prop: see RFC 2518, Section 12.11
 
   <!ELEMENT match #PCDATA >
 
   Multiple property-search elements or multiple elements within a
   DAV:prop element will be interpreted with a logical AND.
 
   This report is only defined when the Depth header has value "0";
   other values result in a 400 (Bad Request) error response. Note
   that [RFC3253], Section 3.6, states that if the Depth header is not
   present, it defaults to a value of "0".
 
   The response body for a successful request MUST be a
   DAV:multistatus XML element.
 
   multistatus: see RFC 2518, Section 12.9
 
   The response body for a successful DAV:principal-property-search
   REPORT request MUST contain  a DAV:response element for each
   principal whose property values satisfy the search specification
   given in DAV:principal-property-search.
 
   The response body for an unsuccessful DAV:principal-property-search
   REPORT request MUST contain, after the XML element indicating the
   failed precondition or postcondition, a DAV:prop element containing
   the property that caused the pre/postcondition to fail.
 
   If DAV:prop is specified in the request body, the properties
   specified in the DAV:prop element MUST be reported in the
   DAV:response elements.
 
 Preconditions:
   (DAV:property-must-be-searchable): All properties specified in the
   DAV:principal-property-search REPORT must be searchable.
 
 
 
 
 
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 9.4.1 Matching
 
   There are several cases to consider when matching strings. The
   easiest case is when a property value is "simple" and has only
   character information item content (see [REC-XML-INFOSET]). For
   example, the search string "julian" would match the DAV:displayname
   property with value "Julian Reschke". Note that the on-the-wire
   marshalling of DAV:displayname in this case is:
 
   <D:displayname xmlns:D="DAV:">Julian Reschke</D:displayname>
 
 
   The name of the property is encoded into the XML element
   information item, and the character information item content of the
   property is "Julian Reschke".
 
   A more complicated case occurs when properties have mixed content
   (that is, compound values consisting of multiple child element
   items, other types of information items, and character information
   item content). Consider the property "aprop" in the namespace
   "http://www.webdav.org/props/", marshalled as:
 
   <W:aprop xmlns:W="http://www.webdav.org/props/">
   {cdata 0}<W:elem1>{cdata 1}</W:elem1>
     <W:elem2>{cdata 2}</W:elem2>{cdata 3}
   </W:aprop>
 
 
   In this case, matching is performed on each individual contiguous
   sequence of character information items. In the example above, a
   search string would be compared to the four following strings:
 
   {cdata 0}
   {cdata 1}
   {cdata 2}
   {cdata 3}
 
 
   That is, four individual matches would be performed, one each for
   {cdata 0}, {cdata 1}, {cdata 2}, and {cdata 3}.
 
 9.4.2 Example: successful DAV:principal-property-search REPORT
 
   In this example, the client requests the principal URLs of all
   users whose DAV:displayname property contains the substring "doE"
   and whose "title" property in the namespace
   "http://BigCorp.com/ns/" (that is, their professional title)
   contains "Sales".  In addition, the client requests five properties
   to be returned with the matching principals:
 
 
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   In the DAV: namespace: displayname
   In the http://www.BigCorp.com/ns/ namespace: department, phone,
   office, salary
 
   The response shows that two principal resources meet the search
   specification, "John Doe" and "Zygdoebert Smith". The property
   "salary" in namespace "http://www.BigCorp.com/ns/" is not returned,
   since the principal making the request does not have sufficient
   access permissions to read this property.
 
 
 
   >> Request <<
 
   REPORT /users/ HTTP/1.1
   Host: www.BigCorp.com
   Content-Type: text/xml; charset=utf-8
   Content-Length: xxxx
   Depth: 0
 
   <?xml version="1.0" encoding="utf-8" ?>
   <D:principal-property-search xmlns:D="DAV:">
     <D:property-search>
       <D:prop>
         <D:displayname/>
       </D:prop>
       <D:match>doE</D:match>
     </D:property-search>
     <D:property-search>
       <D:prop xmlns:B="http://www.BigCorp.com/ns/">
         <B:title/>
       </D:prop>
       <D:match>Sales</D:match>
     </D:property-search>
     <D:prop xmlns:B="http://www.BigCorp.com/ns/">
       <D:displayname/>
       <B:department/>
       <B:phone/>
       <B:office/>
       <B:salary/>
     </D:prop>
   </D:principal-property-search>
 
 
   >> Response <<
 
   HTTP/1.1 207 Multi-Status
   Content-Type: text/xml; charset=utf-8
   Content-Length: xxxx
 
 
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   <?xml version="1.0" encoding="utf-8" ?>
   <D:multistatus xmlns:D="DAV:" xmlns:B="http://BigCorp.com/ns/">
     <D:response>
       <D:href>http://www.BigCorp.com/users/jdoe</D:href>
       <D:propstat>
         <D:prop>
           <D:displayname>John Doe</D:displayname>
           <B:department>Widget Sales</B:department>
           <B:phone>234-4567</B:phone>
           <B:office>209</B:office>
         </D:prop>
         <D:status>HTTP/1.1 200 OK</D:status>
       </D:propstat>
       <D:propstat>
         <D:prop>
           <B:salary/>
         </D:prop>
         <D:status>HTTP/1.1 403 Forbidden</D:status>
       </D:propstat>
     </D:response>
     <D:response>
       <D:href>http://www.BigCorp.com/users/zsmith</D:href>
       <D:propstat>
         <D:prop>
           <D:displayname>Zygdoebert Smith</D:displayname>
           <B:department>Gadget Sales</B:department>
           <B:phone>234-7654</B:phone>
           <B:office>114</B:office>
         </D:prop>
         <D:status>HTTP/1.1 200 OK</D:status>
       </D:propstat>
       <D:propstat>
         <D:prop>
           <B:salary/>
         </D:prop>
         <D:status>HTTP/1.1 403 Forbidden</D:status>
       </D:propstat>
     </D:response>
   </D:multistatus>
 
 
 9.4.3 Example: Unsuccessful DAV:principal-property-search REPORT
 
   In this example, the client requests a search on the non-searchable
   property "phone" in the namespace "http://www.BigCorp.com/ns/".
   The response is a 403 (Forbidden), with a response body containing
 
 
 
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   a DAV:property-must-be-searchable XML element as the value of a
   DAV:error XML element.
 
   >> Request <<
 
   REPORT /users/ HTTP/1.1
   Host: www.BigCorp.com
   Content-Type: text/xml; charset=utf-8
   Content-Length: xxxx
   Depth: 0
 
   <?xml version="1.0" encoding="utf-8" ?>
   <D:principal-property-search xmlns:D="DAV:">
     <D:property-search>
       <D:prop xmlns:B="http://www.BigCorp.com/ns/">
         <B:phone/>
       </D:prop>
       <D:match>232</D:match>
     </D:property-search>
   </D:principal-property-search>
 
 
   >> Response <<
 
   HTTP/1.1 403 Forbidden
   Content-Type: text/xml; charset=utf-8
   Content-Length: xxxx
 
   <?xml version="1.0" encoding="utf-8" ?>
   <D:error xmlns:D="DAV:">
     <D:property-must-be-searchable>
       <D:prop xmlns:B="http://www.BigCorp.com/ns/">
         <B:phone/>
       </D:prop>
     </D:property-must-be-searchable>
   </D:error>
 
 
 9.5 DAV:principal-search-property-set REPORT
 
   The DAV:principal-search-property-set REPORT identifies those
   properties that may be searched using the DAV:principal-property-
   search REPORT (defined in Section 9.4).
 
   Servers MUST support the DAV:principal-search-property-set REPORT
   on all collections identified in the value of a DAV:principal-
   collection-set property.
 
 
 
 
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   An access control protocol user agent could use the results of the
   DAV:principal-search-property-set REPORT to present a query
   interface to the user for retrieving principals.
 
   Support for this report is REQUIRED.
 
      Implementation Note: Some clients will have only limited screen
      real estate for the display of lists of searchable properties.
      In this case, a user might appreciate having the most frequently
      searched properties be displayed on-screen, rather than having
      to scroll through a long list of searchable properties. One
      mechanism for signaling the most frequently searched properties
      is to return them towards the start of a list of properties. A
      client can then preferentially display the list of properties in
      order, increasing the likelihood that the most frequently
      searched properties will appear on-screen, and will not require
      scrolling for their selection.
 
 Marshalling:
   The request body MUST be an empty DAV:principal-search-property-set
   XML element.
 
   This report is only defined when the Depth header has value "0";
   other values result in a 400 (Bad Request) error response. Note
   that [RFC3253], Section 3.6, states that if the Depth header is not
   present, it defaults to a value of "0".
 
   The response body MUST be a DAV:principal-search-property-set XML
   element, containing a DAV:principal-search-property XML element for
   each property that may be searched with the DAV:principal-property-
   search REPORT. A server MAY limit its response to just a subset of
   the searchable properties, such as those likely to be useful to an
   interactive access control client.
 
   <!ELEMENT principal-search-property-set (principal-search-
   property*) >
 
 
   Each DAV:principal-search-property XML element contains exactly one
   searchable property, and a description of the property.
 
   <!ELEMENT principal-search-property (prop, description) >
 
   The DAV:prop element contains one principal property on which the
   server is able to perform a DAV:principal-property-search REPORT.
 
   prop: see RFC 2518, Section 12.11
 
 
 
 
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   The description element is a human-readable description of what
   information this property represents. Servers MUST indicate the
   human language of the description using the xml:lang attribute and
   SHOULD consider the HTTP Accept-Language request header when
   selecting one of multiple available languages.
 
   <!ELEMENT description #PCDATA >
 
 9.5.1 Example: DAV:principal-search-property-set REPORT
 
   In this example, the client determines the set of searchable
   principal properties by requesting the DAV:principal-search-
   property-set REPORT on the root of the server's principal URL
   collection set, identified by http://www.BigCorp.com/users/.
 
 
 
   >> Request <<
 
   REPORT /users/ HTTP/1.1
   Host: www.BigCorp.com
   Content-Type: text/xml; charset="utf-8"
   Content-Length: xxx
   Accept-Language: en, de
   Authorization: BASIC d2FubmFtYWs6cGFzc3dvcmQ=
   Depth: 0
 
   <?xml version="1.0" encoding="utf-8" ?>
   <D:principal-search-property-set xmlns:D="DAV:"/>
 
 
   >> Response <<
 
   HTTP/1.1 200 OK
   Content-Type: text/xml; charset="utf-8"
   Content-Length: xxx
 
   <?xml version="1.0" encoding="utf-8" ?>
   <D:principal-search-property-set xmlns:D="DAV:">
     <D:principal-search-property>
       <D:prop>
         <D:displayname/>
       </D:prop>
       <D:description xml:lang="en">Full name</D:description>
     </D:principal-search-property>
     <D:principal-search-property>
       <D:prop xmlns:B="http://BigCorp.com/ns/">
         <B:title/>
       </D:prop>
 
 
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       <D:description xml:lang="en">Job title</D:description>
     </D:principal-search-property>
   </D:principal-search-property-set>
 
 10 XML PROCESSING
 
   Implementations of this specification MUST support the XML element
   ignore rule, as specified in Section 23.3.2 of [RFC2518], and the
   XML Namespacerecommendation [REC-XML-NAMES].
 
   Note that use of the DAV namespace is reserved for XML elements and
   property names defined in a standards-track or Experimental IETF
   RFC.
 
 
 
 11 INTERNATIONALIZATION CONSIDERATIONS
 
   In this specification, the only human-readable content can be found
   in the description XML element, found within the DAV:supported-
   privilege-set property.  This element contains a human-readable
   description of the capabilities controlled by a privilege.  As a
   result, the description element must be capable of representing
   descriptions in multiple character sets.  Since the description
   element is found within a WebDAV property, it is represented on-
   the-wire as XML [REC-XML], and hence can leverage XML's language
   tagging and character set encoding capabilities. Specifically, XML
   processors must, at minimum, be able to read XML elements encoded
   using the UTF-8 [UTF-8] encoding of the ISO 10646 multilingual
   plane. XML examples in this specification demonstrate use of the
   charset parameter of the Content-Type header, as defined in
   [RFC3023], as well as the XML "encoding" attribute, which together
   provide charset identification information for MIME and XML
   processors. Futhermore, this specification requires server
   implementations to tag description fields with the xml:lang
   attribute (see Section 2.12 of [REC-XML]), which specifies the
   human language of the description. Additionally, server
   implementations should take into account the value of the Accept-
   Language HTTP header to determine which description string to
   return.
 
   For XML elements other than the description element, it is expected
   that implementations will treat the property names, privilege
   names, and values as tokens, and convert these tokens into human-
   readable text in the user's language and character set when
   displayed to a person.  Only a generic WebDAV property display
   utility would display these values in their raw form to a human
   user.
 
 
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   For error reporting, we follow the convention of HTTP/1.1 status
   codes, including with each status code a short, English description
   of the code (e.g., 200 (OK)).  While the possibility exists that a
   poorly crafted user agent would display this message to a user,
   internationalized applications will ignore this message, and
   display an appropriate message in the user's language and character
   set.
 
   Further internationalization considerations for this protocol are
   described in the WebDAV Distributed Authoring protocol
   specification [RFC2518].
 
 12 SECURITY CONSIDERATIONS
 
   Applications and users of this access control protocol should be
   aware of several security considerations, detailed below. In
   addition to the discussion in this document, the security
   considerations detailed in the HTTP/1.1 specification [RFC2616],
   the WebDAV Distributed Authoring Protocol specification [RFC2518],
   and the XML Media Types specification [RFC3023] should be
   considered in a security analysis of this protocol.
 
 12.1 Increased Risk of Compromised Users
 
   In the absence of a mechanism for remotely manipulating access
   control lists, if a single user's authentication credentials are
   compromised, only those resources for which the user has access
   permission can be read, modified, moved, or deleted. With the
   introduction of this access control protocol, if a single
   compromised user has the ability to change ACLs for a broad range
   of other users (e.g., a super-user), the number of resources that
   could be altered by a single compromised user increases. This risk
   can be mitigated by limiting the number of people who have write-
   acl privileges across a broad range of resources.
 
 12.2 Risks of the DAV:read-acl and DAV:current-user-privilege-set
      Privileges
 
   The ability to read the access privileges (stored in the DAV:acl
   property), or the privileges permitted the currently authenticated
   user (stored in the DAV:current-user-privilege-set property) on a
   resource may seem innocuous, since reading an ACL cannot possibly
   affect the resource's state. However, if all resources have world-
   readable ACLs, it is possible to perform an exhaustive search for
   those resources that have inadvertently left themselves in a
   vulnerable state, such as being world-writeable. In particular, the
   property retrieval method PROPFIND, executed with Depth infinity on
   an entire hierarchy, is a very efficient way to retrieve the
 
 
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   DAV:acl or DAV:current-user-privilege-set properties. Once found,
   this vulnerability can be exploited by a denial of service attack
   in which the open resource is repeatedly overwritten. Alternately,
   writeable resources can be modified in undesirable ways.
 
   To reduce this risk, read-acl privileges should not be granted to
   unauthenticated principals, and restrictions on read-acl and read-
   current-user-privilege-set privileges for authenticated principals
   should be carefully analyzed when deploying this protocol. Access
   to the current-user-privilege-set property will involve a tradeoff
   of usability versus security. When the current-user-privilege-set
   is visible, user interfaces are expected to provide enhanced
   information concerning permitted and restricted operations, yet
   this information may also indicate a vulnerability that could be
   exploited. Deployment of this protocol will need to evaluate this
   tradeoff in light of the requirements of the deployment
   environment.
 
 12.3 No Foreknowledge of Initial ACL
 
   In an effort to reduce protocol complexity, this protocol
   specification intentionally does not address the issue of how to
   manage or discover the initial ACL that is placed upon a resource
   when it is created. The only way to discover the initial ACL is to
   create a new resource, then retrieve the value of the DAV:acl
   property. This assumes the principal creating the resource also has
   been granted the DAV:read-acl privilege.
 
   As a result, it is possible that a principal could create a
   resource, and then discover that its ACL grants privileges that are
   undesirable. Furthermore, this protocol makes it possible (though
   unlikely) that the creating principal could be unable to modify the
   ACL, or even delete the resource. Even when the ACL can be
   modified, there will be a short period of time when the resource
   exists with the initial ACL before its new ACL can be set.
 
   Several factors mitigate this risk. Human principals are often
   aware of the default access permissions in their editing
   environments and take this into account when writing information.
   Furthermore, default privilege policies are usually very
   conservative, limiting the privileges granted by the initial ACL.
 
 13 AUTHENTICATION
 
   Authentication mechanisms defined for use with HTTP and WebDAV also
   apply to this WebDAV Access Control Protocol, in particular the
   Basic and Digest authentication mechanisms defined in [RFC2617].
 
 
 
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 14 IANA CONSIDERATIONS
 
   This document uses the namespace defined by [RFC2518] for XML
   elements. All other IANA considerations mentioned in [RFC2518] also
   applicable to this specification.
 
 15 INTELLECTUAL PROPERTY
 
   The following notice is copied from RFC 2026, section 10.4, and
   describes the position of the IETF concerning intellectual property
   claims made against this document.
 
   The IETF takes no position regarding the validity or scope of any
   intellectual property or other rights that might be claimed to
   pertain to the implementation or use other technology described in
   this document or the extent to which any license under such rights
   might or might not be available; neither does it represent that it
   has made any effort to identify any such rights.  Information on
   the IETF's procedures with respect to rights in standards-track and
   standards-related documentation can be found in BCP-11. Copies of
   claims of rights made available for publication and any assurances
   of licenses to be made available, or the result of an attempt made
   to obtain a general license or permission for the use of such
   proprietary rights by implementers or users of this specification
   can be obtained from the IETF Secretariat.
 
   The IETF invites any interested party to bring to its attention any
   copyrights, patents or patent applications, or other proprietary
   rights that may cover technology that may be required to practice
   this standard.  Please address the information to the IETF
   Executive Director.
 
 16 ACKNOWLEDGEMENTS
 
   This protocol is the collaborative product of the WebDAV ACL design
   team: Bernard Chester, Geoff Clemm, Anne Hopkins, Barry Lind, Sean
   Lyndersay, Eric Sedlar, Greg Stein, and Jim Whitehead. The authors
   are grateful for the detailed review and comments provided by Jim
   Amsden, Dylan Barrell, Gino Basso, Murthy Chintalapati, Lisa
   Dusseault, Stefan Eissing, Tim Ellison, Yaron Goland, Dennis
   Hamilton, Laurie Harper, Eckehard Hermann, Ron Jacobs, Chris
   Knight, Remy Maucherat, Larry Masinter, Joe Orton, Peter Raymond,
   Julian Reschke, and Keith Wannamaker. We thank Keith Wannamaker for
   the initial text of the principal property search sections. Prior
   work on WebDAV access control protocols has been performed by Yaron
   Goland, Paul Leach, Lisa Dusseault, Howard Palmer, and Jon Radoff.
   We would like to acknowledge the foundation laid for us by the
   authors of the DeltaV, WebDAV and HTTP protocols upon which this
 
 
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   protocol is layered, and the invaluable feedback from the WebDAV
   working group.
 
 17 REFERENCES
 
 17.1 Normative References
 
   [RFC2119] S.Bradner, "Key words for use in RFCs to Indicate
   Requirement Levels." RFC 2119, BCP 14, March, 1997.
 
   [REC-XML] T. Bray, J. Paoli, C.M. Sperberg-McQueen, "Extensible
   Markup Language (XML)." World Wide Web Consortium Recommendation
   REC-xml.http://www.w3.org/TR/REC-xml
 
   [REC-XML-NAMES] T. Bray, D. Hollander, A. Layman, "Name Spaces in
   XML" World Wide Web Consortium Recommendation REC-xml-names.
   http://www.w3.org/TR/REC-xml-names/
 
   [RFC3253] G. Clemm, J. Amsden, T. Ellison, C. Kaler, J. Whitehead,
   "Versioning Extensions to WebDAV." RFC 3253, March 2002.
 
   [REC-XML-INFOSET] J. Cowan, R. Tobin, "XML Information Set." World
   Wide Web Consortium Recommendation REC-xml-infoset.
   http://www.w3.org/TR/xml-infoset/
 
   [RFC2616] R. Fielding, J. Gettys, J. C. Mogul, H. Frystyk, L.
   Masinter, P. Leach, and T. Berners-Lee, "Hypertext Transfer
   Protocol -- HTTP/1.1." RFC 2616, June, 1999.
 
   [RFC2617] J. Franks, P. Hallam-Baker, J. Hostetler, S. Lawrence, P.
   Leach, A. Luotonen, L. Stewart, "HTTP Authentication: Basic and
   Digest Access Authentication." RFC 2617, June, 1999.
 
   [RFC2518] Y. Goland, E. Whitehead, A. Faizi, S. R. Carter, D.
   Jensen, "HTTP Extensions for Distributed Authoring -- WEBDAV." RFC
   2518, February, 1999.
 
   [RFC2368] P. Hoffman, L. Masinter, J. Zawinski, "The mailto URL
   scheme." RFC 2368, July, 1998.
 
   [RFC3023] M. Murata, S. St.Laurent, D. Kohn, "XML Media Types." RFC
   3023, January, 2001.
 
   [UTF-8] F. Yergeau, "UTF-8, a transformation format of Unicode and
   ISO 10646." RFC 2279, January, 1998.
 
 17.2 Informational References
 
   [RFC2026] S.Bradner, "The Internet Standards Process - Revision 3."
   RFC 2026, BCP 9. Harvard, October, 1996.
 
 
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   [RFC2255] T. Howes, M. Smith, "The LDAP URL Format." RFC 2255.
   Netscape, December, 1997.
 
   [RFC2251] M. Wahl, T. Howes, S. Kille, "Lightweight Directory
   Access Protocol (v3)." RFC 2251. Critical Angle, Netscape, Isode,
   December, 1997.
 
   [CaseMap] M. Davis, "Case Mappings", Unicode Standard Annex #21,
   March 26, 2001.  http://www.unicode.org/unicode/reports/tr21
 
 18 AUTHORS' ADDRESSES
 
   Geoffrey Clemm
   Rational Software
   20 Maguire Road
   Lexington, MA 02421
   Email: geoffrey.clemm@rational.com
 
 
   Anne Hopkins
   Microsoft Corporation
   One Microsoft Way
   Redmond, WA 98052
   Email: annehop@microsoft.com
 
 
   Eric Sedlar
   Oracle Corporation
   500 Oracle Parkway
   Redwood Shores, CA 94065
   Email: esedlar@us.oracle.com
 
 
   Jim Whitehead
   U.C. Santa Cruz
   Dept. of Computer Science
   Baskin Engineering
   1156 High Street
   Santa Cruz, CA 95064
   Email: ejw@cse.ucsc.edu
 
 
 
 
 
 
 
 
 
 
 
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 19 APPENDICES
 
 19.1 WebDAV XML Document Type Definition Addendum
 
   All XML elements defined in this Document Type Definition (DTD)
   belong to the DAV namespace. This DTD should be viewed as an
   addendum to the DTD provided in [RFC2518], section 23.1.
 
   <!-- Privileges -->
 
   <!ELEMENT read EMPTY>
   <!ELEMENT write EMPTY>
   <!ELEMENT write-properties EMPTY>
   <!ELEMENT write-content EMPTY>
   <!ELEMENT unlock EMPTY>
   <!ELEMENT read-acl EMPTY>
   <!ELEMENT read-current-user-privilege-set EMPTY>
   <!ELEMENT write-acl EMPTY>
   <!ELEMENT all EMPTY>
 
 
   <!-- Principal Properties (Section 4) -->
 
   <!ELEMENT principal EMPTY>
 
   <!ELEMENT alternate-URI-set (href*)>
   <!ELEMENT principal-URL (href)>
   <!ELEMENT group-member-set (href*)>
   <!ELEMENT group-membership (href*)>
 
   <!-- Access Control Properties (Section 5) -->
 
   <!-- DAV:owner Property (Section 5.1) -->
 
   <!ELEMENT owner (href prop?)>
   <!ELEMENT prop (see [RFC2518], section 12.11)>
 
 
   <!-- DAV:supported-privilege-set Property (Section 5.2) -->
 
   <!ELEMENT supported-privilege-set (supported-privilege*)>
   <!ELEMENT supported-privilege
    (privilege, abstract?, description, supported-privilege*)>
 
   <!ELEMENT privilege ANY>
   <!ELEMENT abstract EMPTY>
   <!ELEMENT description #PCDATA>
   <!ELEMENT privilege ANY>
 
 
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   <!-- DAV:current-user-privilege-set Property (Section 5.3) -->
 
   <!ELEMENT current-user-privilege-set (privilege*)>
 
 
   <!-- DAV:acl Property (Section 5.4) -->
 
   <!ELEMENT acl (ace)* >
   <!ELEMENT ace (invert | principal, (grant|deny), protected?,
   inherited?)>
   <!ELEMENT invert principal>
 
   <!ELEMENT principal ((href, prop?)
    | all | authenticated | unauthenticated
    | property | self)>
 
   <!ELEMENT prop (see [RFC2518], section 12.11)>
   <!ELEMENT all EMPTY>
   <!ELEMENT authenticated EMPTY>
   <!ELEMENT unauthenticated EMPTY>
   <!ELEMENT property ANY>
   <!ELEMENT self EMPTY>
 
   <!ELEMENT grant (privilege+)>
   <!ELEMENT deny (privilege+)>
   <!ELEMENT privilege ANY>
 
   <!ELEMENT protected EMPTY>
 
   <!ELEMENT inherited (href)>
 
 
   <!-- DAV:inherited-acl-set Property (Section 5.6) -->
 
   <!ELEMENT inherited-acl-set (href*)>
 
 
   <!-- DAV:principal-collection-set Property (Section 5.6) -->
 
   <!ELEMENT principal-collection-set (href*)>
 
 
   <!-- DAV:acl-semantics Property (Section 6) -->
 
   <!ELEMENT acl-semantics (ace-combination?, ace-ordering?, allowed-
   ace?, required-principal?)>
 
 
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   <!ELEMENT ace-combination
    (first-match | all-grant-before-any-deny | specific-deny-
   overrides-grant)>
   <!ELEMENT first-match EMPTY>
   <!ELEMENT all-grant-before-any-deny EMPTY>
   <!ELEMENT specific-deny-overrides-grant EMPTY>
 
   <!ELEMENT ace-ordering (deny-before-grant)? >
   <!ELEMENT deny-before-grant EMPTY>
 
   <!ELEMENT allowed-ace (principal-only-one-ace | grant-only |
                      no-invert | no-acl-inherit)*>
   <!ELEMENT principal-only-one-ace EMPTY>
   <!ELEMENT grant-only EMPTY>
   <!ELEMENT no-invert EMPTY>
   <!ELEMENT no-acl-inherit EMPTY>
 
 
   <!ELEMENT required-principal
     (all? | authenticated? | unauthenticated? | self? | href*
   |property*)>
 
 
   <!-- ACL method preconditions (Section 8.1.1) -->
 
   <!ELEMENT no-ace-conflict EMPTY>
   <!ELEMENT no-protected-ace-conflict EMPTY>
   <!ELEMENT no-inherited-ace-conflict EMPTY>
   <!ELEMENT limited-number-of-aces EMPTY>
   <!ELEMENT no-abstract EMPTY>
   <!ELEMENT not-supported-privilege EMPTY>
   <!ELEMENT missing-required-principal EMPTY>
   <!ELEMENT recognized-principal EMPTY>
   <!ELEMENT allowed-principal EMPTY>
 
 
   <!-- REPORTs (Section 9) -->
 
   <!ELEMENT acl-principal-prop-set ANY>
   ANY value: a sequence of one or more elements, with at most one
   DAV:prop element.
 
   <!ELEMENT principal-match ((principal-property | self), prop?)>
   <!ELEMENT principal-property ANY>
   ANY value: an element whose value identifies a property. The
   expectation is the value of the named property typically contains
   an href element that contains the URI of a principal
 
 
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   <!ELEMENT self EMPTY>
 
   <!ELEMENT principal-property-search ((property-search+), prop?) >
   <!ELEMENT property-search (prop, match) >
   <!ELEMENT match #PCDATA >
 
   <!ELEMENT principal-search-property-set (principal-search-
   property*) >
   <!ELEMENT principal-search-property (prop, description) >
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
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