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Versions: (draft-rosenberg-simple-xcap) 00 01 02 03 04 05 06 07 08 09 10 11 12 RFC 4825

SIMPLE                                                      J. Rosenberg
Internet-Draft                                             Cisco Systems
Expires: November 6, 2006                                    May 5, 2006


   The Extensible Markup Language (XML) Configuration Access Protocol
                                 (XCAP)
                       draft-ietf-simple-xcap-11

Status of this Memo

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   This Internet-Draft will expire on November 6, 2006.

Copyright Notice

   Copyright (C) The Internet Society (2006).

Abstract

   This specification defines the Extensible Markup Language (XML)
   Configuration Access Protocol (XCAP).  XCAP allows a client to read,
   write and modify application configuration data, stored in XML format
   on a server.  XCAP maps XML document sub-trees and element attributes
   to HTTP URIs, so that these components can be directly accessed by
   HTTP.





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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  4
   2.  Overview of Operation  . . . . . . . . . . . . . . . . . . . .  4
   3.  Terminology  . . . . . . . . . . . . . . . . . . . . . . . . .  5
   4.  Definitions  . . . . . . . . . . . . . . . . . . . . . . . . .  5
   5.  Application Usages . . . . . . . . . . . . . . . . . . . . . .  7
     5.1.  Application Unique ID (AUID) . . . . . . . . . . . . . . .  7
     5.2.  Default Namespace Binding  . . . . . . . . . . . . . . . .  8
     5.3.  Data Validation  . . . . . . . . . . . . . . . . . . . . .  8
     5.4.  Data Semantics . . . . . . . . . . . . . . . . . . . . . . 10
     5.5.  Naming Conventions . . . . . . . . . . . . . . . . . . . . 10
     5.6.  Resource Interdependencies . . . . . . . . . . . . . . . . 11
     5.7.  Authorization Policies . . . . . . . . . . . . . . . . . . 11
     5.8.  Data Extensibility . . . . . . . . . . . . . . . . . . . . 12
     5.9.  Documenting Application Usages . . . . . . . . . . . . . . 12
     5.10. Guidelines for Creating Application Usages . . . . . . . . 13
   6.  URI Construction . . . . . . . . . . . . . . . . . . . . . . . 14
     6.1.  XCAP Root  . . . . . . . . . . . . . . . . . . . . . . . . 15
     6.2.  Document Selector  . . . . . . . . . . . . . . . . . . . . 16
     6.3.  Node Selector  . . . . . . . . . . . . . . . . . . . . . . 18
     6.4.  Namespace Bindings for the Selector  . . . . . . . . . . . 22
   7.  Client Operations  . . . . . . . . . . . . . . . . . . . . . . 24
     7.1.  Create or Replace a Document . . . . . . . . . . . . . . . 25
     7.2.  Delete a Document  . . . . . . . . . . . . . . . . . . . . 25
     7.3.  Fetch a Document . . . . . . . . . . . . . . . . . . . . . 26
     7.4.  Create or Replace an Element . . . . . . . . . . . . . . . 26
     7.5.  Delete an Element  . . . . . . . . . . . . . . . . . . . . 28
     7.6.  Fetch an Element . . . . . . . . . . . . . . . . . . . . . 29
     7.7.  Create or Replace an Attribute . . . . . . . . . . . . . . 30
     7.8.  Delete an Attribute  . . . . . . . . . . . . . . . . . . . 31
     7.9.  Fetch an Attribute . . . . . . . . . . . . . . . . . . . . 31
     7.10. Fetch Namespace Bindings . . . . . . . . . . . . . . . . . 31
     7.11. Conditional Operations . . . . . . . . . . . . . . . . . . 32
   8.  Server Behavior  . . . . . . . . . . . . . . . . . . . . . . . 34
     8.1.  POST Handling  . . . . . . . . . . . . . . . . . . . . . . 34
     8.2.  PUT Handling . . . . . . . . . . . . . . . . . . . . . . . 35
       8.2.1.  Locating the Parent  . . . . . . . . . . . . . . . . . 35
       8.2.2.  Verifying Document Content . . . . . . . . . . . . . . 36
       8.2.3.  Creation . . . . . . . . . . . . . . . . . . . . . . . 37
       8.2.4.  Replacement  . . . . . . . . . . . . . . . . . . . . . 41
       8.2.5.  Validation . . . . . . . . . . . . . . . . . . . . . . 42
       8.2.6.  Conditional Processing . . . . . . . . . . . . . . . . 42
       8.2.7.  Resource Interdependencies . . . . . . . . . . . . . . 43
     8.3.  GET Handling . . . . . . . . . . . . . . . . . . . . . . . 43
     8.4.  DELETE Handling  . . . . . . . . . . . . . . . . . . . . . 44
     8.5.  Managing Etags . . . . . . . . . . . . . . . . . . . . . . 45
   9.  Cache Control  . . . . . . . . . . . . . . . . . . . . . . . . 45



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   10. Namespace Binding Format . . . . . . . . . . . . . . . . . . . 46
   11. Detailed Conflict Reports  . . . . . . . . . . . . . . . . . . 46
     11.1. Document Structure . . . . . . . . . . . . . . . . . . . . 46
     11.2. XML Schema . . . . . . . . . . . . . . . . . . . . . . . . 48
   12. XCAP Server Capabilities . . . . . . . . . . . . . . . . . . . 52
     12.1. Application Unique ID (AUID) . . . . . . . . . . . . . . . 53
     12.2. XML Schema . . . . . . . . . . . . . . . . . . . . . . . . 53
     12.3. Default Namespace  . . . . . . . . . . . . . . . . . . . . 54
     12.4. MIME Type  . . . . . . . . . . . . . . . . . . . . . . . . 54
     12.5. Validation Constraints . . . . . . . . . . . . . . . . . . 54
     12.6. Data Semantics . . . . . . . . . . . . . . . . . . . . . . 54
     12.7. Naming Conventions . . . . . . . . . . . . . . . . . . . . 55
     12.8. Resource Interdependencies . . . . . . . . . . . . . . . . 55
     12.9. Authorization Policies . . . . . . . . . . . . . . . . . . 55
   13. Examples . . . . . . . . . . . . . . . . . . . . . . . . . . . 55
   14. Security Considerations  . . . . . . . . . . . . . . . . . . . 58
   15. IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 59
     15.1. XCAP Application Unique IDs  . . . . . . . . . . . . . . . 59
     15.2. MIME Types . . . . . . . . . . . . . . . . . . . . . . . . 59
       15.2.1. application/xcap-el+xml MIME Type  . . . . . . . . . . 60
       15.2.2. application/xcap-att+xml MIME Type . . . . . . . . . . 61
       15.2.3. application/xcap-ns+xml MIME Type  . . . . . . . . . . 61
       15.2.4. application/xcap-error+xml MIME Type . . . . . . . . . 62
       15.2.5. application/xcap-caps+xml MIME Type  . . . . . . . . . 63
     15.3. URN Sub-Namespace Registrations  . . . . . . . . . . . . . 64
       15.3.1. urn:ietf:params:xml:ns:xcap-error  . . . . . . . . . . 64
       15.3.2. urn:ietf:params:xml:ns:xcap-caps . . . . . . . . . . . 65
     15.4. XML Schema Registrations . . . . . . . . . . . . . . . . . 66
       15.4.1. XCAP Error Schema Registration . . . . . . . . . . . . 66
       15.4.2. XCAP Capabilities Schema Registration  . . . . . . . . 66
   16. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 66
   17. References . . . . . . . . . . . . . . . . . . . . . . . . . . 67
     17.1. Normative References . . . . . . . . . . . . . . . . . . . 67
     17.2. Informative References . . . . . . . . . . . . . . . . . . 68
   Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 70
   Intellectual Property and Copyright Statements . . . . . . . . . . 71















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1.  Introduction

   In many communications applications, such as Voice over IP, instant
   messaging, and presence, it is necessary for network servers to
   access per-user information in the process of servicing a request.
   This per-user information resides within the network, but is managed
   by the end user themselves.  Its management can be done through a
   multiplicity of access points, including the web, a wireless handset,
   or a PC application.

   There are many examples of per-user information.  One is presence
   [20] authorization policy, which defines rules about which watchers
   are allowed to subscribe to a presentity, and what information they
   are allowed to access.  Another is presence lists, which are lists of
   users whose presence is desired by a watcher [26].  One way to obtain
   presence information for the list is to subscribe to a resource which
   represents that list [21].  In this case, the Resource List Server
   (RLS) requires access to this list in order to process a SIP [16]
   SUBSCRIBE [28] request for it.  Another way to obtain presence for
   the users on the list is for a watcher to subscribe to each user
   individually.  In that case, it is convenient to have a server store
   the list, and when the client boots, it fetches the list from the
   server.  This would allow a user to access their resource lists from
   different clients.

   This specification describes a protocol that can be used to
   manipulate this per-user data.  It is called the Extensible Markup
   Language (XML) Configuration Access Protocol (XCAP).  XCAP is a set
   of conventions for mapping XML documents and document components into
   HTTP URIs, rules for how the modification of one resource affects
   another, data validation constraints, and authorization policies
   associated with access to those resources.  Because of this
   structure, normal HTTP primitives can be used to manipulate the data.
   XCAP is based heavily on ideas borrowed from the Application
   Configuration Access Protocol (ACAP) [25], but it is not an extension
   of it, nor does it have any dependencies on it.  Like ACAP, XCAP is
   meant to support the configuration needs for a multiplicity of
   applications, rather than just a single one.


2.  Overview of Operation

   Each application that makes use of XCAP specifies an application
   usage (Section 5).  This application usage defines the XML schema [2]
   for the data used by the application, along with other key pieces of
   information.  The principal task of XCAP is to allow clients to read,
   write, modify, create and delete pieces of that data.  These
   operations are supported using HTTP/1.1 [6].  An XCAP server acts as



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   a repository for collections of XML documents.  There will be
   documents stored for each application.  Within each application,
   there are documents stored for each user.  Each user can have a
   multiplicity of documents for a particular application.  To access
   some component of one of those documents, XCAP defines an algorithm
   for constructing a URI that can be used to reference that component.
   Components refer to any element or attribute within the document.
   Thus, the HTTP URIs used by XCAP point to a document, or to pieces of
   information that are finer grained than the XML document itself.  An
   HTTP resource which follows the naming conventions and validation
   constraints defined here is called an XCAP resource.

   Since XCAP resources are also HTTP resources, they can be accessed
   using HTTP methods.  Reading an XCAP resource is accomplished with
   HTTP GET, creating or modifying one is done with HTTP PUT, and
   removing one of the resources is done with an HTTP DELETE.  XCAP
   resources do not represent processing scripts; as a result, POST
   operations to HTTP URIs representing XCAP resources are not defined.
   Properties that HTTP associates with resources, such as entity tags,
   also apply to XCAP resources.  Indeed, entity tags are particularly
   useful in XCAP, as they allow a number of conditional operations to
   be performed.

   XML documents which are equivalent for the purposes of many
   applications may differ in their physical representation.  With XCAP
   resources, the canonical form with comments [19] of an XML document
   determines the logical equivalence.  In other words, the canonical
   specification determines, how significant whitespace MUST be
   processed and for example, that new inserted attributes may appear in
   any order within the physical representation.


3.  Terminology

   In this document, the key words "MUST", "MUST NOT", "REQUIRED",
   "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY",
   and "OPTIONAL" are to be interpreted as described in RFC 2119 [7] and
   indicate requirement levels for compliant implementations.


4.  Definitions

   The following terms are used throughout this document:

   XCAP Resource: An HTTP resource representing an XML document, an
      element within an XML document, or an attribute of an element
      within an XML document that follows the naming and validation
      constraints of XCAP.



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   XCAP Server: An HTTP server that understands how to follow the naming
      and validation constraints defined in this specification.

   XCAP Client: An HTTP client that understands how to follow the naming
      and validation constraints defined in this specification.

   Application: A collection of software components within a network
      whose operation depends on data managed and stored on an XCAP
      server.

   Application Usage: Detailed information on the interaction of an
      application with the XCAP server.

   Application Unique ID (AUID): A unique identifier within the
      namespace of application unique IDs created by this specification
      that differentiates XCAP resources accessed by one application
      from XCAP resources accessed by another.

   Naming Conventions: The part of an application usage that specifies
      well-known URIs used by an application, or more generally,
      specifies the URIs that are typically accessed by an application
      during its processing.

   XCAP User Identifier (XUI): The XUI is a string, valid as a path
      element in an HTTP URI, that is associated with each user served
      by the XCAP server.

   XCAP Root: A context that contains all of the documents across all
      application usages and users that are managed by the server.

   Document Selector: A sequence of path segments, with each segment
      being separated by a "/", that identify the XML document within an
      XCAP root that is being selected.

   Node Selector: A sequence of path segments, with each segment being
      separated by a "/", that identify the XML node (element or
      attribute) being selected within a document.

   Node Selector Separator: A single path segment equal to two tilde
      characters "~~" that is used to separate the document selector
      from the node selector within an HTTP URI.

   Document URI: The HTTP URI containing the XCAP root and document
      selector, resulting in the selection of a specific document.  As a
      result, performing a GET against the document URI would retrieve
      the document.





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   Node URI: The HTTP URI containing the XCAP root, document selector,
      node selector separator and node selector, resulting in the
      selection of a specific XML node.

   XCAP Root URI: An HTTP URI that representing the XCAP root.  Although
      a syntactically valid URI, the XCAP Root URI does not correspond
      to an actual resource on an XCAP server.  Actual resources are
      created by appending additional path information to the XCAP Root
      URI.

   Global Tree: A URI that represents the parent for all global
      documents for a particular application usage within a particular
      XCAP root.

   Home Directory: A URI that represents the parent for all documents
      for a particular user for a particular application usage within a
      particular XCAP root.

   Positional Insertion: A PUT operation that results in the insertion
      of a new element into a document such that its position relative
      to other children of the same parent is set by the client.


5.  Application Usages

   Each XCAP resource on a server is associated with an application.  In
   order for an application to use those resources, application specific
   conventions must be specified.  Those conventions include the XML
   schema that defines the structure and constraints of the data, well
   known URIs to bootstrap access to the data, and so on.  All of those
   application specific conventions are defined by the application
   usage.

5.1.  Application Unique ID (AUID)

   Each application usage is associated with a name, called an
   Application Unique ID (AUID).  This name uniquely identifies the
   application usage within the namespace of application usages, and is
   different from AUIDs used by other applications.  AUIDs exist in one
   of two namespaces.  The first namespace is the IETF namespace.  This
   namespace contains a set of tokens, each of which is registered with
   IANA.  These registrations occur with the publication of standards
   track RFCs [27] based on the guidelines in Section 15.  The second
   namespace is the vendor-proprietary namespace.  Each AUID in that
   namespace is prefixed with the reverse domain name of the
   organization creating the AUID, followed by a period, followed by any
   vendor defined token.  As an example, the example.com domain can
   create an AUID with the value "com.example.foo" but cannot create one



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   with the value "org.example.foo".  AUIDs within the vendor namespace
   do not need to be registered with IANA.  The vendor namespace is also
   meant to be used in lab environments where no central registry is
   needed.  The syntax for AUIDs, expressed in ABNF [12] (and using some
   of the BNF defined in RFC 3986 [13]) is:


   AUID             =  global-auid / vendor-auid
   global-auid      =  auid
   auid             =  1*auid-char
   vendor-auid      =  rev-hostname "." auid
   rev-hostname     =  toplabel *( "." domainlabel  )
   domainlabel      =  alphanum
                       / alphanum *( alphanum / "-" ) alphanum
   toplabel         =  ALPHA / ALPHA *( alphanum / "-" ) alphanum
   auid-char        =  auid-unreserved / pct-encoded / sub-delims
                       / ":" / "@"
   auid-unreserved  = ALPHA / DIGIT / "-" / "_" / "~"

   The allowed characters for the auid production is a subset of the
   pchar production defined in RFC3986.  In particular, it omits the
   ".", which allows for the auid to be separated from the reverse
   hostname.

5.2.  Default Namespace Binding

   In order for the XCAP server to match a URI to an element or
   attribute of a document, any XML namespace prefixes used within the
   URI must be expanded [3].  This expansion requires a namespace
   binding context.  That context maps namespace prefixes to namespace
   URIs.  It also defines a default namespace that applies to elements
   without namespace prefixes.  The namespace binding context comes from
   two sources.  Firstly, the mapping of namespace prefixes to namespace
   URIs is obtained from the URI itself (see Section 6.4).  However, the
   default namespace URI is defined by the application usage itself, and
   applies to all URIs referencing resources within that application
   usage.  All application usages MUST define a namespace URI that
   represents the default namespace to be used when evaluating URIs.
   The default namespace does not apply to elements or attributes within
   the documents themselves.  However, if a document contains a URI
   representing an XCAP resource, the default namespace defined by the
   application usage applies to that URI as well.

5.3.  Data Validation

   One of the responsibilities of an XCAP server is to validate the
   content of each XCAP resource when an XCAP client tries to modify
   one.  This is done using two mechanisms.  Firstly, all application



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   usages MUST describe their document contents using XML schema [2].
   The application usage MUST also identify the MIME type for documents
   compliant to that schema.

   Unfortunately, XML schemas cannot represent every form of data
   constraint.  As an example, one XML element may contain an integer
   which defines the maximum number of instances of another element.
   This constraint cannot be represented with XML schema.  However, such
   constraints may be important to the application usage.  The
   application usage defines any additional constraints beyond those in
   the schema.

   Of particular importance are uniqueness constraints.  In many cases,
   an application will require that there only be one instance of some
   element or attribute within a particular scope.  Each uniqueness
   constraint needs to be specified by identifying the field, or
   combinations of fields, that need to be unique, and then identifying
   the scope in which that uniqueness applies.  One typical scope is the
   set of all elements of a certain name within the same parent.
   Another typical scope is the set of all URIs valid within a
   particular domain.  In some cases these constraints can be specified
   using XML schema, which provides the <unique> element for this
   purpose.  Other uniqueness constraints, such as URI uniqueness across
   a domain, cannot be expressed by schema.  Whether or not the schema
   is used to express some of the uniqueness requirements, the
   application usage MUST specify all uniqueness requirements when it
   defines its data validation needs.

   For example, the resource lists application usage [22] requires that
   each <list> element have a unique value for the "name" attribute
   within a single parent.  As another example, the RLS services
   application usage [22] requires that the value of the "uri" attribute
   of the <service> element be a URI that is unique within the domain of
   the URI.

   URI constraints represent another form of constraints.  These are
   constraints on the scheme or structure of the scheme specific part of
   the URI.  These kinds of constraints cannot be expressed in an XML
   schema.  If these constraints are important to an application usage,
   they need to be explicitly called out.

   Another important data constraint is referential integrity.
   Referential integrity is important when the name or value of an
   element or attribute is used as a key to select another element or
   attribute.  An application usage MAY specify referential integrity
   constraints.  However, XCAP servers are not a replacement for
   Relational Database Management Systems (RDBMS), and therefore servers
   are never responsible for maintaining referential integrity.  XCAP



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   clients are responsible for making all of the appropriate changes to
   documents in order to maintain referential integrity.

   Another constraint is character encoding.  XML allows documents to be
   encoded using several different character sets.  However, this
   specification mandates that all documents used with XCAP MUST be
   encoded using UTF-8.  This cannot be changed by an application usage.

   The data validation information is consumed by both clients, which
   use them to make sure they construct requests that will be accepted
   by the server, and by servers, which validate the constraints when
   they receive a request (with the exception of referential integrity
   constraints, which are not validated by the server).

5.4.  Data Semantics

   For each application usage, the data present in the XML document has
   a well defined semantic.  The application usage defines that
   semantic, so that a client can properly construct a document in order
   to achieve the desired result.  They are not used by the server, as
   it is purposefully unaware of the semantics of the data it is
   managing.  The data semantics are expressed in English prose by the
   application usage.

   One particularly important semantic is the base URI to be used for
   the resolution of any relative URI references pointed to XCAP
   resources.  As discussed below, relative URI references pointing to
   XCAP resources cannot be resolved using the retrieval URI as the base
   URI.  Therefore, it is up to the application usage to specify the
   base URI.

5.5.  Naming Conventions

   In addition to defining the meaning of the document in the context of
   a particular application, an application usage has to specify how the
   applications obtain the documents they need.  In particular, it needs
   to define any well-known URIs used for bootstrapping purposes, and
   document any other conventions on the URIs used by an application.
   It should also document how documents reference each other.  These
   conventions are called naming conventions.

   For many application usages, users need only a single document.  In
   such a case, it is RECOMMENDED that the application usage require
   that this document be called "index" and exist within the users home
   directory.

   As an example, the RLS services application usage allows an RLS to
   obtain the contents of a resource list when the RLS receives a



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   SUBSCRIBE request for a SIP URI identifying an RLS service.  The
   application usage specifies that the list of service definitions is
   present within a specific document with a specific name within the
   global tree.  This allows the RLS to perform a single XCAP request to
   fetch the service definition for the service associated with the SIP
   URI in a SUBSCRIBE request.

   Naming conventions are used by XCAP clients to construct their URIs.
   The XCAP server does not make use of them.

5.6.  Resource Interdependencies

   When a user modifies an XCAP resource, the content of many other
   resources is affected.  For example, when a user deletes an XML
   element within a document, it does so by issuing a DELETE request
   against the URI for the element resource.  However, deleting this
   element also deletes all child elements and their attributes, each of
   which is also an XCAP resource.  As such, manipulation of one
   resource affects the state of other resources.

   For the most part, these interdependencies are fully specified by the
   XML schema used by the application usage.  However, in some
   application usages, there is a need for the server to relate
   resources together, and such a relationship cannot be specified
   through a schema.  This occurs when changes in one document will
   affect another document.  Typically, this is the case when an
   application usage is defining a document that acts as a collection of
   information defined in other documents.

   As an example, when a user creates a new RLS service (that is, it
   creates a new <service> element within an RLS services document), the
   server adds that element to a read-only global list of services
   maintained by the server in the global tree.  This read-only global
   list is accessed by the RLS when processing a SIP SUBSCRIBE request.

   Resource interdependencies are used by both XCAP clients and servers.

5.7.  Authorization Policies

   By default, each user is able to access (read, modify, and delete)
   all of the documents below their home directory, and any user is able
   to read documents within the global directory.  However, only trusted
   users, explicitly provisioned into the server, can modify global
   documents.

   The application usage can specify a different authorization policy
   that applies to all documents associated with that application usage.
   An application usage can also specify whether another application



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   usage is used to define the authorization policies.  An application
   usage for setting authorization policies can also be defined
   subsequent to the definition of the the main application usage.  In
   such a case, the main application usage needs only to specify that
   such a usage will be defined in the future.

   If an application usage does not wish to change the default
   authorization policy, it can merely state that the default policy is
   used.

   The authorization policies defined by the application usage are used
   by the XCAP server during its operation.

5.8.  Data Extensibility

   An XCAP server MUST understand an application usage in order to
   process an HTTP request made against a resource for that particular
   application usage.  However, it is not required for the server to
   understand all of the contents of a document used by an application
   usage.  A server is required to understand the baseline schema
   defined by the application usage.  However, those schemas can define
   points of extensibility where new content can be added from other
   namespaces and corresponding schemas.  Sometimes, the server will
   understand those namespaces and therefore have access to their
   schemas.  Sometimes, it will not.

   A server MUST allow for documents that contain elements from
   namespaces not known to the server.  In such a case, the server
   cannot validate that such content is schema compliant; it will only
   verify that the XML is well-formed.

   If a client wants to verify that a server supports a particular
   namespace before operating on a resource, it can query the server for
   its capabilities using the XCAP Capabilities application usage,
   discussed in Section 12.

5.9.  Documenting Application Usages

   Application usages are documented in specifications which convey the
   information described above.  In particular, an application usage
   specification MUST provide the following information:

   o  Application Unique ID (AUID): If the application usage is meant
      for general use on the Internet, the application usage MUST
      register the AUID into the IETF tree using the IANA procedures
      defined in Section 15.





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   o  XML Schema

   o  Default Namespace

   o  MIME Type

   o  Validation Constraints

   o  Data Semantics

   o  Naming Conventions

   o  Resource Interdependencies

   o  Authorization Policies

5.10.  Guidelines for Creating Application Usages

   The primary design task when creating a new application usage is to
   define the schema.  Although XCAP can be used with any XML document,
   intelligent schema design will improve the efficiency and utility of
   the document when it is manipulated with XCAP.

   XCAP provides three fundamental ways to select elements amongst a set
   of siblings - by the expanded name of the element, by its position,
   or by the value of a specific attribute.  Positional selection always
   allows a client to get exactly what it wants.  However, it requires a
   client to cache a copy of the document in order to construct the
   predicate.  Furthermore, if a client performs a PUT, it requires the
   client to reconstruct the PUT processing that a server would follow
   in order to update its local cached copy.  Otherwise, the client will
   be forced to re-GET the document after every PUT, which is
   inefficient.  As such, it is a good idea to design schemas such that
   common operations can be performed without requiring the client to
   cache a copy of the document.

   Without positional selection, a client can pick the element at each
   step by its expanded name or the value of an attribute.  Many schemas
   include elements that can be repeated within a parent (often,
   minOccurs equals zero or one, and maxOccurs is unbounded).  As such,
   all of the elements have the same name.  This leaves the attribute
   value as the only way to select an element.  Because of this, if an
   application usage expects user to manipulate elements or attributes
   that are descendants of an element which can repeat, that element
   SHOULD include, in its schema, an attribute which can be suitably
   used as a unique index.  Furthermore, the naming conventions defined
   by that application usage SHOULD specify this uniqueness constraint
   explicitly.



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   URIs often make a good choice for such unique index.  They have
   fundamental uniqueness properties, and are also usually of semantic
   significance in the application usage.  However, care must be taken
   when using a URI as an attribute value.  URI equality is usually
   complex.  However, attribute equality is performed by the server
   using XML rules, which are based on case sensitive string comparison.
   Thus, XCAP will match URIs based on lexical equality, not functional
   equality.  In such cases, an application usage SHOULD consider these
   implications carefully.

   XCAP provides the ability of a client to operate on a single element,
   attribute or document at a time.  As a result, it may be possible
   that common operations the client might perform will require a
   sequence of multiple requests.  This is inefficient, and introduces
   the possibility of failure conditions when another client modifies
   the document in the middle of a sequence.  In such a case, the client
   will be forced to detect this case using entity tags (discussed below
   in Section 7.11), and undo its previous changes.  This is very
   difficult.

   As a result, the schemas SHOULD be defined so that common operations
   generally require a single request to perform.  Consider an example.
   Lets say an application usage is defining permissions for users to
   perform certain operations.  The schema can be designed in two ways.
   The top level of the tree can identify users, and within each user,
   there can be the permissions associated with the user.  In an
   alternative design, the top level of the tree identifies each
   permission, and within that permission, the set of users who have it.
   If, in this application usage, it is common to change the permission
   for a user from one value to another, the former schema design is
   better for xcap; it will require a single PUT to make such a change.
   In the latter case, either the entire document needs to be replaced
   (which is a single operation), or two PUT operations need to occur -
   one to remove the user from the old permission, and one to add the
   user to the new permission.

   Naming conventions form another key part of the design of an
   application usage.  The application usage should be certain that XCAP
   clients know where to "start" to retrieve and modify documents of
   interest.  Generally, this will involve the specification of a well-
   known document at a well-known URI.  That document can contain
   references to other documents that the client needs to read or
   modify.


6.  URI Construction

   In order to manipulate an XCAP resource, the data must be represented



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   by an HTTP URI.  XCAP defines a specific naming convention for
   constructing these URIs.  The URI is constructed by concatenating the
   XCAP root with the document selector with the node selector separator
   with a percent-encoded form of the node selector.  This is followed
   by an optional query component that defines namespace bindings used
   in evaluating the URI.  The XCAP root is the enclosing context in
   which all XCAP resources live.  The document selector is a path that
   identifies a document within the XCAP root.  The node selector
   separator is a path segment with a value of double tilde ("~~"), and
   SHOULD NOT be percent-encoded, as advised in Section 2.3 of RFC 3986
   [13].  URIs containing %7E%7E should be normalized to ~~ for
   comparison; they are equivalent.  The node selector separator is
   piece of syntactic sugar that separates the document selector from
   the node selector.  The node selector is an expression that
   identifies a component of the document, such as an element or
   attribute.

   The sections below describe these components in more detail.

6.1.  XCAP Root

   The root of the XCAP hierarchy is called the XCAP root.  It defines
   the context in which all other resources exist.  The XCAP root is
   represented with an HTTP URI, called the XCAP Root URI.  This URI is
   a valid HTTP URI; however, it doesn't point to any resource that
   actually exists on the server.  Its purpose is to identify the root
   of the tree within the domain where all XCAP documents are stored.
   It can be any valid HTTP URI, but MUST NOT contain a query component
   (a complete XCAP URI may have a query component, but it is not part
   of the XCAP root URI).  It is RECOMMENDED that it be equal to
   xcap.domain where domain is the domain of the provider.  As an
   example, "http://xcap.example.com" might be used as the XCAP root URI
   within the example.com domain.  Typically, the XCAP root URI is
   provisioned into client devices.  If not explicitly provisioned,
   clients SHOULD assume the form xcap.domain where domain is the domain
   of their service provider (for SIP, this would be the domain part of
   their Address-of-Record (AOR)).  A server or domain MAY support
   multiple XCAP root URIs.  In such a case, it is effectively operating
   as if it were serving separate domains.  There is never information
   carryover or interactions between resources in different XCAP root
   URIs.

   When a client generates an HTTP request to a URI identifying an XCAP
   resource, RFC 2616 procedures for the construction of the Request-URI
   apply.  In particular, the authority component of the URI may not be
   present in the Request-URI if the request is sent directly to the
   origin server.




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   The XCAP root URI can also be a relative HTTP URI.  It is the
   responsibility of the application usage to specify the base URI for
   an HTTP URI representing an XCAP resource whenever such a URI appears
   within a document defined by that application usage.  Generally
   speaking, it is unsafe to use the retrieval URI as the base URI.
   This is because any URI that points to an ancestor for a particular
   element or attribute can contain content including that element or
   attribute.  If that element or attribute contained a relative URI
   reference, it would be resolved relative to whatever happened to be
   used to retrieve the content, and this will often not be the base URI
   defined by the application usage.

6.2.  Document Selector

   Each document within the XCAP root is identified by its document
   selector.  The document selector is a sequence of path segments,
   separated by a slash ("/").  These path segments define a
   hierarchical structure for organizing documents within any XCAP root.
   The first path segment MUST be the XCAP AUID.  So, continuing the
   example above, all of the documents used by the resource lists
   application would be under "http://xcap.example.com/resource-lists".

   It is assumed that each application will have data that is set by
   users, and/or it will have global data that applies to all users.  As
   a result, beneath each AUID there are two sub-trees.  One, called
   "users", holds the documents that are applicable to specific users,
   and the other, called "global", holds documents applicable to all
   users.  The subtree beneath "global" is called the global tree.  The
   path segment after the AUID MUST either be "global" or "users".

   Within the "users" tree are zero or more sub-trees, each of which
   identifies documents that apply to a specific user.  Each user known
   to the server is associated with a username, called the XCAP User
   Identifier (XUI).  Typically, an endpoint is provisioned with the
   value of the XUI.  For systems that support SIP applications, it is
   RECOMMENDED that the XUI be equal to the Address-of-Record (AOR) for
   the user (i.e., sip:joe@example.com).  Since SIP endpoints generally
   know their AOR, they will also know their XUI.  As a consequence, if
   no XUI is explicitly provisioned, a SIP UA SHOULD assume it is equal
   to their AOR.  This XUI MUST be used as the path segment beneath the
   "users" segment.  Since the SIP URI allows for characters which are
   not permitted in HTTP URI path segments (such as the '?' and '/'
   characters, which are permitted in the user part of the SIP URI), any
   such characters MUST be percent encoded.  The subtree beneath an XUI
   for a particular user is called their home directory.  "User" in this
   context should be interpreted loosely; a user might correspond to
   device, for example.




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   XCAP does not itself define what it means for documents to "apply" to
   a user, beyond specification of a baseline authorization policy,
   described below in Section 8.  Each application usage can specify
   additional authorization policies which depend on data used by the
   application itself.

   The remainder of the document selector (the path following "global"
   or the XUI) points to specific documents for that application usage.
   Subdirectories are permitted, but are NOT RECOMMENDED.  XCAP provides
   no way to create sub-directories or to list their contents, thus
   limiting their utility.  If subdirectories are used, there MUST not
   be a document in a directory with the same name as a sub-directory.

   The final path segment in the document selector identifies the actual
   document in the hierarchy.  This is equivalent to a filename, except
   that XCAP does not require that its document resources be stored as
   files in a file system.  However, the term "filename" is used to
   describe the final path segment in the document selector.  In
   traditional filesystems, the filename would have a filename
   extension, such as ".xml".  There is nothing in this specification
   that requires or prevents such extensions from being used in the
   filename.  In some cases, the application usage will specify a naming
   convention for documents, and those naming conventions may or may not
   specify a file extension.  For example, in the RLS services
   application usage [22], documents in the user's home directory with
   the filename "index" will be used by the server to compute the global
   index, which is also a document with the filename "index".  Barring
   specific guidelines in the application usage, if a user has a single
   document for a particular application usage, this SHOULD be called
   "index".

   When the naming conventions in an application usage do not constrain
   the filename conventions (or, more generally, the document selector),
   an application will know the filename (or more generally, the
   document selector) because it is included as a reference in a
   document which is at a well known location.  As another example,
   within the index document defined by RLS services, the <service>
   element has a child element called <resource-list> whose content is a
   URI pointing to a resource list within the users home directory.

   As a result, if the user creates a new document, and then references
   that document from a well-known document (such as the index document
   above), it doesn't matter whether the user includes an extension in
   the filename or not, as long as the user is consistent and maintains
   referential integrity.

   As an example, the path segment
   "/resource-lists/users/sip:joe@example.com/index" is a document



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   selector.  Concatenating the XCAP root URI with the document selector
   produces the HTTP URI "http://xcap.example.com/resource-lists/users/
   sip:joe@example.com/index".  In this URI, the AUID is "resource-
   lists", and the document is in the user tree with the XUI
   "sip:joe@example.com" with filename "index".

6.3.  Node Selector

   The node selector specifies specific nodes of the XML document which
   are to be accessed.  A node refers to an XML element, an attribute of
   an element, or a set of namespace bindings.  The node selector is an
   expression which identifies an element, attribute or set of namespace
   bindings.  Its grammar is:


   node-selector          = element-selector ["/" terminal-selector]
   terminal-selector      = attribute-selector / namespace-selector /
                            extension-selector
   element-selector       = step *( "/" step)
   step                   = by-name / by-pos / by-attr / by-pos-attr /
                            extension-selector
   by-name                = NameorAny
   by-pos                 = NameorAny "[" position "]"
   position               = 1*DIGIT
   attr-test              = ( "@" att-name "=" <"> att-value <"> ) /
                            ( "@" att-name "=" <'> att-value <'> )
   by-attr                = NameorAny "[" attr-test "]"
   by-pos-attr            = NameorAny "[" position "]" "[" attr-test "]"
   NameorAny              = QName / "*"   ; QName from XML Namespaces
   att-name               = QName
   att-value              = AttValue      ; from XML specification
   attribute-selector     = "@" att-name
   namespace-selector     = "namespace::*"
   extension-selector     = 1*( %x00-2e / %x30-ff )  ; anything but "/"

   The QName grammar is defined in the XML namespaces [3] specification,
   and the AttValue grammar is defined in the XML specification XML 1.0
   [1].

   The extension-selector is included for purposes of extensibility.  It
   can be composed of any character except the slash, which is the
   delimeter amongst steps.  Any characters in an extension that cannot
   be represented in a URI MUST be percent-encoded before placement into
   a URI.

   Note that the double quote, left square bracket and right square
   bracket characters, which are meaningful to XCAP, cannot be directly
   represented in the HTTP URI.  As a result, they are percent-encoded



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   when placed within the HTTP URI.  In addition to these characters, an
   apostrophe (') character can be used as a delimiter within XPath
   expressions.  Furthermore, since XML allows for non-ASCII characters,
   the names of elements and attributes may not be directly
   representable in a URI.  Any such characters MUST be represented by
   converting them to an octet sequence corresponding to their
   representation in UTF-8, and then percent-encoding that sequence of
   octets.

   Similarly, the XML specification defines the QName production for the
   grammar for element and attribute names, and the AttValue production
   for the attribute values.  Unfortunately, the characters permitted by
   these productions include some that are not allowed for pchar, which
   is the production for the allowed set of characters in path segments
   in the URI.  The AttValue production allows many such characters
   within the US-ASCII set, including the space.  Those characters MUST
   be percent- encoded when placed in the URI.  Furthermore, QName and
   AttValue allow many Unicode characters, outside of US-ASCII.  When
   these characters need to be represented in the HTTP URI, they are
   percent- encoded.  To do this, the data should be encoded first as
   octets according to the UTF-8 character encoding [18] and then only
   those octets that do not correspond to characters in the pchar set
   should be percent-encoded.  For example, the character A would be
   represented as "A", the character LATIN CAPITAL LETTER A WITH GRAVE
   would be represented as "%C3%80", and the character KATAKANA LETTER A
   would be represented as "%E3%82%A2".

   As a result, the grammar above represents the expressions processed
   by the XCAP server internally after it has decoded the URI.  The on-
   the-wire format is dictated by RFC 3986 [13].  In the discussions and
   examples below, when the node selectors are not part of an HTTP URI,
   they are presented in their internal format prior to encoding.  If an
   example includes a node selector within an HTTP URI, it is presented
   in its percent-encoded form.

   The node selector is based on the concepts in XPath [10].  Indeed,
   the node selector expression, before it is percent-encoded for
   representation in the HTTP URI, happens to be a valid XPath
   expression.  However, XPath provides a set of functionality far
   richer than is needed here, and its breadth would introduce much
   unneeded complexity into XCAP.

   To determine the XML element, attribute or namespace bindings
   selected by the node selector, processing begins at the root node of
   the XML document.  The first step in the element selector is then
   taken.  Each step chooses a single XML element within the current
   document context.  The document context is the point within the XML
   document from which a specific step is evaluated.  The document



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   context begins at the root node of the document.  When a step
   determines an element within that context, that element becomes the
   new context for evaluation of the next step.  Each step can select an
   element by its name (expanded), by a combination of name and
   attribute value, by name and position, or by name, position and
   attribute.  In all cases, the name can be wildcarded, so that all
   elements get selected.

   The selection operation operates as follows.  Within the current
   document context, the children of that context are enumerated in
   document order.  If the context is the root node of the document, its
   child element is the root element of the document.  If the context is
   an element, its children are all of the children of that element
   (naturally).  Next, those elements whose name is not a match for
   NameorAny are discarded.  An element name is a match if NameorAny is
   the wildcard, or, if its not a wildcard, the element name matches
   NameorAny.  Matching is discussed below.  The result is an ordered
   list of elements.

   The elements in the list are further filtered by the predicates,
   which are the expressions in square brackets following NameorAny.
   Each predicate further prunes the elements from the current ordered
   list.  These predicates are evaluated in order.  If the content of
   the predicate is a position, the position-th element is selected
   (that is, treat "position" as a variable, and take the element whose
   position equals that variable), and all others are discarded.  If
   there are fewer elements in the list than the value of position, the
   result is a no-match.

   If the content of the predicate is an attribute name and value, all
   elements possessing that attribute with that value are selected, and
   all others are discarded.  Note that, although a document can have
   namespace declarations within elements, those elements cannot be
   selected using a namespace declaration as a predicate.  That is, a
   step like "el-name[@xmlns='namespace']" will never match an element,
   even if there is an element in the list that specifies a default
   namespace of "namespace".  In other words, a namespace node is NOT an
   attribute.  If the namespaces in scope for an element are needed,
   they can be selected using the namespace-selector described below.
   If there are no elements with attributes having the given name and
   value, the result is a no-match.

   After the predicates have been applied, the result will be a no-
   match, one element, or multiple elements.  If the result is multiple
   elements, the node selector is invalid.  Each step in a node selector
   MUST produce a single element to form the context for the next step.
   This is more restrictive than general XPath expressions, which allow
   a context to contain multiple nodes.  If the result is a no-match,



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   the node selector is invalid.  The node selector is only valid if a
   single element was selected.  This element becomes the context for
   the evaluation of the next step in the node selector expression.

   The last location step is either the previously described element
   selector or a "terminal selector".  If the terminal selector is an
   attribute selector, the server checks to see if there is an attribute
   with the same expanded name in the current element context.  If there
   is not, the result is considered a no-match.  Otherwise, that
   attribute is selected.  If the terminal selector is a namespace
   selector, the result is equal to the set of namespace bindings in
   scope for the element, including the possible default namespace
   declaration.  This specification defines a syntax for representing
   namespace bindings, so they can be returned to the client in an HTTP
   response.

   As a result, once the entire node selector is evaluated against the
   document, the result will either be a no-match, invalid, a single
   element, a single attribute or a set of namespace bindings.

   Matching of element names is performed as follows.  The element being
   compared in the step has its name expanded as described in XML
   namespaces [3].  The element name in the step is also expanded.  This
   expansion requires that any namespace prefix is converted to its
   namespace URI.  Doing that requires a set of bindings from prefixes
   to namespace URIs.  This set of bindings is obtained from the query
   component of the URI (see Section 6.4).  If the prefix of the QName
   of an element is empty, the corresponding URI is then the default
   namespace URI defined by the application usage, or null if not
   defined.  Comparisons are then performed as described in XML
   namespaces [3].  Note that the namespace prefix expansions described
   here are different than those specified in the XPath 1.0
   specification, but are closer to those currently defined by the XPath
   2.0 specification [24].

   Matching of attribute names proceeds in a similar way.  The attribute
   in the document has its name expanded as described in XML namespaces
   [3].  If the attribute name in the attribute selector has a namespace
   prefix, its name is expanded using the namespace bindings obtained
   from the query component of the URI.  An unprefixed attribute QName
   is in no namespace.

   Comments, text content (including whitespace), and processing
   instructions can be present in a document, but cannot be selected by
   the expressions defined here.  Of course, if such information is
   present in a document, and a user selects an XML element enclosing
   that data, that information would be included in a resulting GET, for
   example.  Furthermore, whitespace is respected by XCAP.  If a client



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   PUTs an element or document that contains whitespace, the server
   retains that whitespace, and will return the element or document back
   to the client with exactly the same whitespace.  Similarly, when an
   element is inserted, no additional whitespace is added around the
   inserted element, and the element gets inserted in a very specific
   location relative to any whitespace, comments or processing
   instructions around it.  Section 8.2.3 describes where the insertion
   occurs.

   As an example, consider the following XML document:


   <?xml version="1.0"?>
   <watcherinfo xmlns="urn:ietf:params:xml:ns:watcherinfo"
                version="0" state="full">
     <watcher-list resource="sip:professor@example.net"
                   package="presence">
       <watcher status="active"
                id="8ajksjda7s"
                duration-subscribed="509"
                event="approved">sip:userA@example.net</watcher>
       <watcher status="pending"
                id="hh8juja87s997-ass7"
                display-name="Mr. Subscriber"
                event="subscribe">sip:userB@example.org</watcher>
     </watcher-list>
   </watcherinfo>

   Figure 3: Example XML Document

   Assuming that the default namespace URI for this application usage is
   "urn:ietf:params:xml:ns:watcherinfo", the node selector watcherinfo/
   watcher-list/watcher[@id="8ajksjda7s"] would select the following XML
   element:


   <watcher status="active"
       id="8ajksjda7s"
       duration-subscribed="509"
       event="approved">sip:userA@example.net</watcher>

6.4.  Namespace Bindings for the Selector

   In order to expand the namespace prefixes used in the node selector,
   a set of bindings from those namespace prefixes to namespace URI must
   be used.  Those bindings are contained in the query component of the
   URI.  If no query component is present, it means that only the
   default namespace URI (as identified by the application usage) is



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   defined.  The query component is formatted as a valid xpointer
   expression [5] after suitable URI encoding as defined in Section 4.1
   of the Xpointer framework.  This xpointer expression SHOULD only
   contain expressions from the xmlns() scheme [4].  A server compliant
   to this specification MUST ignore any xpointer expressions not from
   the xmlns() scheme.  The xmlns() xpointer expressions define the set
   of namespace bindings in use for evaluating the URI.

   Note that xpointer expressions were originally designed for usage
   within fragment identifiers of URIs.  However, within XCAP, they are
   used within query components of URIs.

   The following example shows a more complex matching operation, this
   time including the usage of namespace bindings.  Consider the
   following document:


   <?xml version="1.0"?>
   <foo xmlns="urn:test:default-namespace">
     <ns1:bar xmlns:ns1="urn:test:namespace1-uri"
              xmlns="urn:test:namespace1-uri">
       <baz/>
       <ns2:baz xmlns:ns2="urn:test:namespace2-uri"/>
     </ns1:bar>
     <ns3:hi xmlns:ns3="urn:test:namespace3-uri">
       <there/>
     </ns3:hi>
   </foo>

   Assume that this document has a document URI of
   "http://xcap.example.com/test/users/sip:joe@example.com/index", where
   "test" is the application usage.  This application usage defines a
   default namespace URI of "urn:test:default-namespace".  The XCAP URI:


   http://xcap.example.com/test/users/sip:joe@example.com/index/
   ~~/foo/a:bar/b:baz?xmlns(a=urn:test:namespace1-uri)
   xmlns(b=urn:test:namespace1-uri)

   will select the first <baz> child element of the <bar> element in the
   document.  The XCAP URI:


   http://xcap.example.com/test/users/sip:joe@example.com/index/
   ~~/foo/a:bar/b:baz?xmlns(a=urn:test:namespace1-uri)
   xmlns(b=urn:test:namespace2-uri)

   will select the second <baz> child element of the <bar> element in



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   the document.  The following XCAP URI will also select the second
   element in the document:


   http://xcap.example.com/test/users/sip:joe@example.com/index/
   ~~/d:foo/a:bar/b:baz?xmlns(a=urn:test:namespace1-uri)
   xmlns(b=urn:test:namespace2-uri)
   xmlns(d=urn:test:default-namespace)


7.  Client Operations

   An XCAP client is an HTTP/1.1 compliant client.  Specific data
   manipulation tasks are accomplished by invoking the right set of HTTP
   methods with the right set of headers on the server.  This section
   describes those in detail.

   In all cases where the client modifies a document, by deleting or
   inserting a document, element or attribute resource, the client
   SHOULD verify that, if the operation were to succeed, the resulting
   document would meet the data constraints defined by the application
   usage, including schema validation.  For example, if the client
   performs a PUT operation to
   "http://xcap.example.com/rls-services/users/joe/mybuddies", rls-
   services is the application unique ID, and the constraints defined by
   it SHOULD be followed.

   The client will know what URI to use based on the naming conventions
   described by the application usage.

   If the document, after modification, does not meet the data
   constraints, the server will reject it with a 409.  The 409 response
   may contain an XML body, formatted according to the schema in
   Section 11.2, which provides further information on the nature of the
   error.  The client MAY use this information to try and alter the
   request so that this time, it might succeed.  The client SHOULD NOT
   simply retry the request without changing some aspect of it.

   In some cases, the application usage will dictate a uniqueness
   constraint that the client cannot guarantee on its own.  One such
   example is that a URI has to be unique within a domain.  Typically,
   the client is not the owner of the domain, and so it cannot be sure
   that a URI is unique.  In such a case, the client can either generate
   a sufficiently random identifier, or it can pick a "vanity"
   identifier in the hopes that it is not taken.  In either case, if the
   identifier is not unique, the server will reject the request with a
   409 and suggest alternatives that the client can use to try again.
   If the server does not suggest alternatives, the client SHOULD



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   attempt to use random identifiers with increasing amounts of
   randomness.

   HTTP also specifies that PUT and DELETE requests are idempotent.
   This means that, if the client performs a PUT on a document and it
   succeeds, it can perform the same PUT, and the resulting document
   will look the same.  Similarly, when a client performs a DELETE, if
   it succeeds, a subsequent DELETE to the same URI will generate a 404;
   the resource no longer exists on the server since it was deleted by
   the previous DELETE operation.  To maintain this property, the client
   SHOULD construct its URIs such that, after the modification has taken
   place, the URI in the request will point to the resource just
   inserted for PUT (i.e., the body of the request), and will point to
   nothing for DELETE.  If this property is maintained, it is the case
   that GET to the URI in the PUT will return the same content (i.e.,
   GET(PUT(X)) == x).  This property implies idempotency.  Although a
   request can still be idempotent if it does not possess this property,
   XCAP does not permit such requests.  If the client's request does not
   have this property, the server will reject the request with a 409 and
   indicate a cannot-insert error condition.

   If the result of the PUT is a 200 or 201 response, the operation was
   successful.  Other response codes to any request, such as a
   redirection, are processed as per RFC 2616 [6].

7.1.  Create or Replace a Document

   To create or replace a document, the client constructs a URI that
   references the location where the document is to be placed.  This URI
   MUST be a document URI, and therefore contain the XCAP root and
   document selector.  The client then invokes a PUT method on that URI.

   The MIME content type MUST be the type defined by the application
   usage.  For example, it would be "application/rls-services+xml" for
   an RLS services [22] document, and not "application/xml".

   If the Request-URI identifies a document that already exists in the
   server, the PUT operation replaces that document with the content of
   the request.  If the Request-URI does not identify an existing
   document, the document is created on the server at that specific URI.

7.2.  Delete a Document

   To delete a document, the client constructs a URI that references the
   document to be deleted.  This URI MUST be a document URI.  The client
   then invokes a DELETE operation on the URI to delete the document.





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7.3.  Fetch a Document

   As one would expect, fetching a document is trivially accomplished by
   performing an HTTP GET request with the Request URI set to the
   document URI.

7.4.  Create or Replace an Element

   To create or replace an XML element within an existing document, the
   client constructs a URI whose document selector points to the
   document to be modified.  The node selector MUST be present in the
   URI, delimited from the document selector with the node selector
   separator.  The query component MUST be present if the node selector
   makes use of namespace prefixes, in which case the xmlns()
   expressions in the query component MUST define those prefixes.  To
   create this element within the document, the node selector is
   constructed such that it is a no-match against the current document,
   but if the element in the body of the request was added to the
   document as desired by the client, the node selector would select
   that element.  To replace an element in the document, the node
   selector is constructed so that it is a match against the element in
   the current document to be replaced, as well as a match to the new
   element (present in the body of the PUT request) that is to replace
   it.

   Oftentimes, the client will wish to insert an element into a document
   in a certain position relative to other children of the same parent.
   This is called a positional insertion.  They often arise because the
   schema constrains where the element can occur, or because ordering of
   elements is significant within the schema.  To accomplish this, the
   client can use a node selector of the following form:


     parent/*[position][unique-attribute-value]

   Here, "parent" is an expression for the parent of the element to be
   inserted. "position" is the position amongst the existing child
   elements of this parent where the new element is to be inserted.
   "unique-attribute-value" is an attribute name and value for the
   element to be inserted, which is different from the current element
   in "position".  The second predicate is needed so that the overall
   expression is a no-match when evaluated against the current children.
   Otherwise, the PUT would replace the existing element in that
   position.  Note that in addition to wildcard "*" a QName can also be
   used as a node test.  The insert logic is described in more detail in
   Section 8.2.3.

   Consider the example document in Figure 3.  The client would like to



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   insert a new <watcher> element as the second element underneath
   <watcher-list>.  However, it cannot just PUT to a URI with the
   watcherinfo/watcher-list/*[2] node selector; this node selector would
   select the existing 2nd child element of <watcher-list> and replace
   it.  Thus, the PUT has to be made to a URI with watcherinfo/
   watcher-list/*[2][@id="hhggff"] as the node selector, where "hhggff"
   is the value of the "id" attribute of the new element to be inserted.
   This node-selector is a no-match against the current document, and
   would be a match against the new element if it was inserted as the
   2nd child element of <watcher-list>.

   The "*" indicates that all element children of <watcher-info> are to
   be considered when computing the position for insertion.  If, instead
   of a wildcard *, an element name (QName) was present, the expression
   above would insert the new element as the position-th element amongst
   those with the same expanded name (see Section 8.2.3 for a discussion
   on insertion rules).

   Once the client constructs the URI, it invokes the HTTP PUT method.
   The content in the request MUST be an XML element.  Specifically, it
   contains the element, starting with the opening bracket for the begin
   tag for that element, including the attributes and content of that
   element (whether it be text or other child elements), and ending with
   the closing bracket for the end tag for that element.  The MIME type
   in the request MUST be "application/xcap-el+xml", defined in
   Section 15.2.1.  If the node selector, when evaluated against the
   current document, results in a no-match, the server performs a
   creation operation.  If the node selector, when evaluated against the
   current document, is a match for an element in the current document,
   the server replaces it with the content of the PUT request.  This
   replacement is complete; that is, the old element (including its
   attributes, namespace declarations and content: text, element,
   comment and processing instruction nodes) are removed, and the new
   one, including its attributes, namespace declarations and content, is
   put in its place.

   To be certain that element insertions have the GET(PUT(x))==x
   property, the client can check that the attribute predicates in the
   final path segment of the URI match the attributes of the element in
   the body of the request.  As an example of an request that would not
   have this property and therefore not be idempotent, consider the
   following PUT request (URIs are line-folded for readability):









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   PUT
   /rls-services/users/sip:bill@example.com/index/~~/rls-services/
   service%5b@uri=%22sip:good-friends@example.com%22%5d
    HTTP/1.1
   Content-Type:application/xcap-el+xml
   Host: xcap.example.com

   <service uri="sip:mybuddies@example.com">
     <resource-list>http://xcap.example.com/resource-lists/users
   /sip:joe@example.com/index/~~/resource-lists/list%5b@name=%22l1%22%5d
   </resource-list>
     <packages>
      <package>presence</package>
     </packages>
   </service>

   This request will fail with a 409.  The Request URI contains a final
   path segment with a predicate based on attributes -
   @uri="sip:good-friends@example.com".  However, this will not match
   the value of the "uri" attribute in the element in the body
   (sip:mybuddies@example.com).

   The GET(PUT(x))==x property introduces some limitations on the types
   of operations possible.  It will not be possible to replace an
   element with one that has a new value for an attribute that is the
   sole unique element identifier, if the URI contained a node selector
   which was using the previous value of that attribute for purposes of
   selecting the element.  This is exactly the use case in the example
   above.  To get around this limitation, the selection can be done by
   position instead of attribute value, or the parent of the element to
   be replaced can be selected, and then the body of the PUT operation
   would contain the parent, the child to be replaced, and all other
   siblings.

7.5.  Delete an Element

   To delete an element from a document, the client constructs a URI
   whose document selector points to the document containing the element
   to be deleted.  The node selector MUST identify a single element.
   The node selector MUST be present following the node selector
   separator, and identify the specific element to be deleted.
   Furthermore, the node selector MUST match no element after the
   deletion of the target element.  This is required to maintain the
   idempotency property of HTTP deletions.  The query component MUST be
   present if the node selector makes use of namespace prefixes, in
   which case the xmlns() expressions in the query component MUST define
   those prefixes.




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   If the client wishes to delete an element in a specific position,
   this is referred to as a positional deletions.  Like a positional
   insertion, the node selector has the following form:


     parent/*[position][unique-attribute-value]

   Where "parent" is an expression for the parent of the element to be
   deleted, "position" is the position of the element to be deleted
   amongst the existing child elements of this parent, and "unique-
   attribute-value" is an attribute name and value for the element to be
   deleted, where this attribute name and value are different than any
   of the siblings of the element.

   Positional deletions without using a unique attribute name and value
   are possible, but only in limited cases where idempotency is
   guaranteed.  In particular, if a DELETE operation refers to an
   element by name and position alone (parent/elname[n]), this is
   permitted only when the element to be deleted is the last element
   amongst all its siblings with that name.  Similarly, if a DELETE
   operation refers to an element by position alone (parent/*[n]), this
   is permitted only when the elemented to be deleted is the last
   amongst all sibling elements, regardless of name.

   The client then invokes the HTTP DELETE method.  The server will
   remove the element from the document (including its attributes,
   namespace declarations and its descendant nodes, such as any
   children).

7.6.  Fetch an Element

   To fetch an element of a document, the client constructs a URI whose
   document selector points to the document containing the element to be
   fetched.  The node selector MUST be present following the node
   selector separator, and must identify the element to be fetched.  The
   query component MUST be present if the node selector makes use of
   namespace prefixes, in which case the xmlns() expressions in the
   query component MUST define those prefixes.

   The client then invokes the GET method.  The 200 OK response will
   contain that XML element.  Specifically, it contains the content of
   the XML document, starting with the opening bracket for the begin tag
   for that element, and ending with the closing bracket for the end tag
   for that element.  This will, as a result, include all attributes,
   namespace declarations and descendant nodes: elements, comments, text
   and processing instructions of that element.





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7.7.  Create or Replace an Attribute

   To create or replace an attribute in an existing element of a
   document, the client constructs a URI whose document selector points
   to the document to be modified.  The node selector, following the
   node selector separator, MUST be present.  The node selector MUST be
   constructed such that, if the attribute was created or replaced as
   desired, the node selector would select that attribute.  If the node
   selector, when evaluated against the current document, results in a
   no-match, it is a creation operation.  If it matches an existing
   attribute, it is a replacement operation.  The query component MUST
   be present if the node selector makes use of namespace prefixes, in
   which case the xmlns() expressions in the query component MUST define
   those prefixes.

   The client then invokes the HTTP PUT method.  The content defined by
   the request MUST be the value of the attribute, compliant to the
   grammar for AttValue as defined in XML 1.0 [1].  Note that, unlike
   when AttValue is present in the URI, there is no percent-encoding
   which only applies to URIs.  This request MUST be sent with the
   Content-Type of "application/xcap-att+xml" as defined in
   Section 15.2.2.  The server will add that attribute such that, if the
   node selector is evaluated on the resulting document, it returns the
   attribute present in the request.

   To be certain that attribute insertions have the GET(PUT(x))==x
   property, the client can check that any attribute predicate in the
   path segment that selects the element into which the attribute is
   inserted, matches a different attribute than the one being inserted
   by the request.  As an example of a request that would not have this
   property and therefore not be idempotent, consider the following PUT
   request (URIs are line folded for readability):


   PUT
   /rls-services/users/sip:bill@example.com/index/~~/rls-services
   /service%5b@uri=%22sip:good-friends@example.com%22%5d/@uri
    HTTP/1.1
   Content-Type:application/xcap-att+xml
   Host: xcap.example.com

   "sip:bad-friends@example.com"

   As with element insertions and replacements, the GET(PUT(x))==x
   property introduces limitations on attribute insertions and
   replacements.  It will not be possible to replace the attribute value
   of an attribute, when that attribute is the sole unique element
   identifier, and the URI contains a node selector that uses the



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   previous value of the attribute to select the affected element.  This
   is the use case in the example above.  Instead, the element can be
   selected positionally, or its entire parent replaced.

   This request will fail with a 409.

7.8.  Delete an Attribute

   To delete attributes from the document, the client constructs a URI
   whose document selector points to the document containing the
   attributes to be deleted.  The node selector MUST be present
   following the node selector separator, and evaluate to an attribute
   in the document to be deleted.  The query component MUST be present
   if the node selector makes use of namespace prefixes, in which case
   the xmlns() expressions in the query component MUST define those
   prefixes.

   The client then invokes the HTTP DELETE method.  The server will
   remove the attribute from the document.

7.9.  Fetch an Attribute

   To fetch an attribute of a document, the client constructs a URI
   whose document selector points to the document containing the
   attribute to be fetched.  The node selector MUST be present following
   the node selector separator, containing an expression identifying the
   attribute whose value is to be fetched.  The query component MUST be
   present if the node selector makes use of namespace prefixes, in
   which case the xmlns() expressions in the query component MUST define
   those prefixes.

   The client then invokes the GET method.  The 200 OK response will
   contain an "application/xcap-att+xml" document with the specified
   attribute, formatted according to the grammar of AttValue as defined
   in the XML 1.0 specifications.

7.10.  Fetch Namespace Bindings

   If a client wishes to insert an element or attribute into a document,
   and that element or attribute are part of a namespace declared
   elsewhere in the document, the client will need to know the namespace
   bindings in order to construct the XML content in the request.  If
   the client has a cached copy of the document, it will know the
   bindings.  However, if it doesn't have the whole document cached, it
   can be useful to fetch just the bindings that are in scope for an
   element, in order to construct a subsequent PUT request.

   To get those bindings, the client constructs a URI whose document



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   selector points to the document containing the element whose
   namespace bindings are to be fetched.  The node selector MUST be
   present following the node selector separator, containing an
   expression identifying the desired namespace bindings.  The query
   component MUST be present if the node selector makes use of namespace
   prefixes, in which case the xmlns() expressions in the query
   component MUST define those prefixes.

   The client then invokes the GET method.  The 200 OK response will
   contain an "application/xcap-ns+xml" document with the namespace
   definitions.  The format for this document is defined in Section 10.

   A client cannot set the namespace prefixes in scope for an element.
   As such, a node selector that identifies namespace prefixes MUST NOT
   appear in a PUT or DELETE request.

7.11.  Conditional Operations

   The HTTP specification defines several header fields that can be used
   by a client to make the processing of the request conditional.  In
   particular, the If-None-Match and If-Match header fields allow a
   client to make them conditional on the current value of the entity
   tag for the resource.  These conditional operations are particularly
   useful for XCAP resources.

   For example, it is anticipated that clients will frequently wish to
   cache the current version of a document.  So, when the client starts
   up, it will fetch the current document from the server and store it.
   When it does so, the GET response will contain the entity tag for the
   document resource.  Each resource within a document maintained by the
   server will share the same value of the entity tag.  As a result, the
   entity tag returned by the server for the document resource is
   applicable to element and attribute resources within the document.

   If the client wishes to insert or modify an element or attribute
   within the document, but it wants to be certain that the document
   hasn't been modified since the client last operated on it, it can
   include an If-Match header field in the request, containing the value
   of the entity tag known to the client for all resources within the
   document.  If the document has changed, the server will reject this
   request with a 412 response.  In that case, the client will need to
   flush its cached version, fetch the entire document, and store the
   new entity tag returned by the server in the 200 OK to the GET
   request.  It can then retry the request, placing the new entity tag
   in the If-Match header field.  If this succeeds, the Etag header
   field in the response to PUT contains the entity tag for the resource
   that was just inserted or modified.  Because all resources in a
   document share the same value for their entity tag, this entity tag



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   value can be applied to the modification of other resources.

   A client can also conditionally delete elements or attributes by
   including an If-Match header field in DELETE requests.  Note that the
   200 OK responses to a DELETE will contain an Etag header field,
   containing the entity tag for all of the other resources in the
   document, even though the resource identified by the DELETE request
   no longer exists.

   When a client uses conditional PUT and DELETE operations, it can
   apply those changes to its local cached copy, and update the value of
   the entity tag for the locally cached copy based on the Etag header
   field returned in the response.  As long as no other clients try to
   modify the document, the client will be able to perform conditional
   operations on the document without ever having to perform separate
   GET operations to synchronize the document and it's entity tags with
   the server.  If another client tries to modify the document, this
   will be detected by the conditional mechanisms, and the client will
   need to perform a GET to resynchronize its copy unless it has some
   other means to learn about the change.

   If a client does not perform a conditional operation, but did have a
   cached copy of the document, that cached copy will become invalid
   once the operation is performed (indeed, it may have become invalid
   even beforehand).  Unconditional operations should only be performed
   by clients when knowledge of the entire document is not important for
   the operation to succeed.

   In another example, a client may wish to insert a new element into a
   document, but wants to be sure that the insertion will only take
   place if that element does not exist.  In other words, the client
   wants the PUT operation to be a creation, not a replacement.  To
   accomplish that, the client can insert the If-None-Match header field
   into the PUT request, with a value of *.  This tells the server to
   reject the request with a 412 if resource exists.

   As another example, a when a client fetches a document, and there is
   an older version cached, it is useful for clients to use a
   conditional GET in order to reduce network usage if the cached copy
   is still valid.  This is done by including, in the GET request, the
   If-None-Match header field with a value equal to the current etag
   held by the client for the document.  The server will only generate a
   200 OK reponse if the etag held by the server differs than that held
   by the client.  If it doesn't differ, the server will respond with a
   304 response.






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8.  Server Behavior

   An XCAP server is an HTTP/1.1 compliant origin server.  The behaviors
   mandated by this specification relate to the way in which the HTTP
   URI is interpreted and the content is constructed.

   An XCAP server MUST be explicitly aware of the application usage
   against which requests are being made.  That is, the server must be
   explicitly configured to handle URIs for each specific application
   usage, and must be aware of the constraints imposed by that
   application usage.

   When the server receives a request, the treatment depends on the URI.
   If the URI refers to an application usage not understood by the
   server, the server MUST reject the request with a 404 (Not Found)
   response.  If the URI refers to a user that is not recognized by the
   server, it MUST reject the request with a 404 (Not Found).  If the
   URI includes extension-selectors that the server doesn't understand,
   it MUST reject the request with a 404 (Not Found).

   Next, the server authenticates the request.  All XCAP servers MUST
   implement HTTP Digest [11].  Furthermore, servers MUST implement HTTP
   over TLS, RFC 2818 [14].  It is RECOMMENDED that administrators use
   an HTTPS URI as the XCAP root URI, so that the digest client
   authentication occurs over TLS.

   Next, the server determines if the client has authorization to
   perform the requested operation on the resource.  The application
   usage defines the authorization policies.  An application usage may
   specify that the default is used.  This default is described in
   Section 5.7.

   Next, the server makes sure that it can properly evaluate the request
   URI.  The server MUST check the node selector in the request URI, if
   present.  If any qualified names are present that use a namespace
   prefix, and that prefix is not defined in an xmlns() expression in
   the query component of the request URI, the server MUST reject the
   request with a 400 response.

   After checking the namespace prefix definitions, the specific
   behavior depends on the method and what the URI refers to.

8.1.  POST Handling

   XCAP resources do not represent processing scripts.  As a result,
   POST operations to HTTP URIs representing XCAP resources are not
   defined.  A server receiving such a request for an XCAP resource
   SHOULD return a 405.



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8.2.  PUT Handling

   The behavior of a server in receipt of a PUT request is as specified
   in HTTP/1.1 Section 9.6 - the content of the request is placed at the
   specified location.  This section serves to define the notion of
   "placement" and "specified location" within the context of XCAP
   resources.

   If the request URI contained a namespace-selector, the server MUST
   reject the request with a 405 (Method Not Allowed) and MUST include
   an Allow header field including the GET method.

8.2.1.  Locating the Parent

   The first step the server performs is to locate the parent, whether
   it is a directory or element, in which the resource is to be placed.
   To do that, the server removes the last path segment from the URI.
   The rest of the URI refers to the parent.  This parent can be a
   document, element, or prefix of a document selector (called a
   directory, even though this specification does not mandate that
   documents are actually stored in a filesystem).  This URI is called
   the parent URI.  The path segment that was removed is called the
   target selector, and the node (element, document or attribute) it
   describes is called the target node.

   If the parent URI has no node selector separator, it is referring to
   the directory into which the document should be inserted.  In normal
   XCAP operations, this will be either the users home directory or the
   global directory, which will always exist on the server.  However, if
   an application usage is making use of subdirectories (despite the
   fact that this is not recommended), it is possible that the directory
   into which the document should be inserted does not exist.  In this
   case, the server MUST return a 409 response, and SHOULD include a
   detailed conflict report including the <no-parent> element.  Detailed
   conflict reports are discussed in Section 11.  If the directory does
   exist, the server checks to see if there is a document with the same
   filename as the target node.  If there is, the operation is the
   replacement operation discussed in Section 8.2.4.  If it does not
   exist, it is the creation operation, discussed in Section 8.2.3.

   If the parent URI has a node selector separator, the document
   selector is extracted, and that document is retrieved.  If the
   document does not exist, the server MUST return a 409 response, and
   SHOULD include a detailed conflict report including the <no-parent>
   element.  If it does exist, the node selector is extracted, and
   decoded (recall that the node selector is percent-encoded).  The node
   selector is applied to the document based on the matching operations
   discussed in Section 6.3.  If the result is a no-match or invalid,



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   the server MUST return a 409 response, and SHOULD include a detailed
   conflict report including the <no-parent> element.

   If the node-selector is valid, the server examines the target
   selector, and evaluates it within the context of the parent node.  If
   the target node exists within the parent, the operation is a
   replacement, as described in Section 8.2.4.  If it does not exist, it
   is the creation operation, discussed in Section 8.2.3.

   Before performing the replacement or creation, as determined based on
   the logic above, the server validates the content of the request as
   described in Section 8.2.2

8.2.2.  Verifying Document Content

   If the PUT request is for a document (the request URI had no node
   selector separator), the content of the request body has to be a
   well-formed XML document.  If it is not, the server MUST reject the
   request with a 409 response code.  That response SHOULD include a
   detailed conflict report including the <not-well-formed> element.  If
   the document is well-formed but not UTF-8 encoded, the server MUST
   reject the request with a 409 response code.  That response SHOULD
   include a detailed conflict report including the <not-utf-8> element.
   If the MIME type in the Content-Type header field of the request is
   not equal to the MIME type defined for the application usage, the
   server MUST reject the request with a 415.

   If the PUT request is for an element, the content of the request body
   has to be a well-balanced region of an XML document, also known as an
   XML fragment body in The XML Fragment Interchange [23] specification,
   including only a single element.  If it is not, the server MUST
   reject the request with a 409 response code.  That response SHOULD
   include a detailed conflict report including the <not-xml-frag>
   element.  If the fragment body is well-balanced but contains
   characters outside of the UTF-8 character set, the server MUST reject
   the request with a 409 response code.  That response SHOULD include a
   detailed conflict report including the <not-utf-8> element.  If the
   MIME type in the Content-Type header field of the request is not
   equal to "application/xcap-el+xml", the server MUST reject the
   request with a 415.

   If the PUT request is for an attribute, the content of the request
   body has to be a sequence of characters that comply with the grammar
   for AttValue as defined above.  If it is not, the server MUST reject
   the request with a 409 response code.  That response SHOULD include a
   detailed conflict report including the <not-xml-att-value> element.
   If the attribute value is valid but contains characters outside of
   the UTF-8 character set, the server MUST reject the request with a



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   409 response code.  That response SHOULD include a detailed conflict
   report including the <not-utf-8> element.If the MIME type in the
   Content-Type header field of the request is not equal to
   "application/xcap-att+xml", the server MUST reject the request with a
   415.

8.2.3.  Creation

   The steps in this sub-section are followed if the PUT request will
   result in the creation of a new document, element or attribute.

   If the PUT request is for a document, the content of the request body
   is placed into the directory, and its filename is associated with the
   target node, which is a document.

   If the PUT request is for an element, the server inserts the content
   of the request body as a new child element of the parent element
   selected in Section 8.2.1.  The insertion is done such that, the
   request URI, when evaluated, would now point to the element which was
   inserted.  There exist three possible ways how new elements are
   positioned.

   First, if there were no other sibling elements with the same expanded
   name, and the insertion is not positionally constrained, the new
   element is inserted such that it is the last element amongst all
   element siblings.  Furthermore, if there were comment, text, or
   processing instruction nodes after the former last element, they MUST
   occur prior to the insertion of the new element.  This case occurs
   when one of the following are true:

   o  The element name in the target selector is not wildcarded.  There
      could be an attribute selector (in which case, it would have to
      match an attribute of the element being inserted), and the
      position in the target selector will either be absent or have a
      value of 1 (a value greater than one would always result in
      rejection of the request, since this is the first element with the
      given name underneath the parent).

   o  The element name in the target selector is wildcarded, but there
      are no other elements underneath the same parent.  There could be
      an attribute selector (in which case, it would have to match an
      attribute of the element being inserted), and the position in the
      target selector will either be absent or have a value of 1 (a
      value greater than one would always result in rejection of the
      request, since this is the first element underneath the parent).

   o  The element name in the target selector is wildcarded, and there
      are other elements underneath the same parent.  However, there is



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      an attribute selector which matches none of the attributes in the
      other sibling elements underneath the parent, but does match an
      attribute of the element to be inserted.  The position in the
      target selector is absent.

   Secondly, if there were sibling elements with the same name already
   in the document, but the insertion is positionally unconstrained, the
   server MUST insert the element such that it is in the "earliest last"
   position.  "Earliest last" means that the new element MUST be
   inserted so that there are no elements after it with the same
   expanded name, and for all insertion positions where this is true, it
   is inserted such that as many sibling nodes (element, comment, text
   or processing instruction) appear after it as possible.  This case
   occurs when the target selector is defined by a by-name or by-attr
   production, and there is no position indicated.

   Lastly, if the element is positionally constrained, the server MUST
   insert the element so that it is in the "earliest nth" position.
   When n>1 and NameofAny is not a wildcard, the element MUST be
   inserted so that there are n-1 sibling elements before it with the
   same expanded name.  If there are not n-1 sibling elements with the
   same expanded name, the request will fail.  When n>1 and NameorAny is
   a wildcard, the element MUST be inserted so that there are n-1
   sibling elements before it, each of which can have any expanded name.
   If there are not n-1 sibling elements in the document, the request
   will fail.  In both of these cases, the new element is inserted such
   that as many sibling nodes appear after it as possible.  When n=1 and
   NameorAny is not a wildcard, the insertion is positionally
   constrained when an element with the same expanded name already
   appears as a child of the same parent.  In this case, the new element
   MUST appear just before the existing first element with this same
   expanded name.  When n=1 and NameorAny is wildcarded, the insertion
   is positionally constrained when there is also an attribute selector
   that didn't match the first sibling of the parent (if it did match,
   or was absent, this wouldn't have been an insertion).  In this case,
   the new element MUST appear just before all existing elements
   regardless of their expanded name.

   In practice, this insertion logic keeps elements with the same
   expanded names closely together.  This simplifies the application
   logic when the content model is described by XML schema with
   <sequence> rules and maxOccurs="unbounded" cardinalities, like:









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   <xs:element name="foobar">
     <xs:complexType>
       <xs:sequence>
         <xs:element ref="foo" maxOccurs="unbounded" />
         <xs:element ref="bar" maxOccurs="unbounded" />
       </xs:sequence>
     </xs:complexType>
   </xs:element>

   Not any <bar> element may precede one or multiple <foo> elements.
   Either <bar> or <foo> elements may easily be added without wildcards
   and positional constraints.  Note that if "minOccurs" cardinality of
   <foo> element were zero and <foo> elements don't exist yet, a
   positional predicate with the * wildcard MUST be used.

   The whole insert logic is best described by complete examples,
   consider the following document:


   <?xml version="1.0"?>
   <root>
    <el1 att="first"/>
    <el1 att="second"/>
    <!-- comment -->
    <el2 att="first/>
   </root>

   A PUT request whose content is <el1 att="third"/> and whose node
   selector is root/el1[@att="third"] would result in the following
   document:


   <?xml version="1.0"?>
   <root>
    <el1 att="first"/>
    <el1 att="second"/><el1 att="third">
    <!-- comment -->
    <el2 att="first/>
   </root>

   Notice how it has been inserted as the third <el1> element in the
   document, and just before the comment and whitespace nodes.  It would
   have been inserted in exactly the same place if the node selector had
   been root/el1[3][@att="third"] or root/*[3][@att="third"].

   If the content of the request had been <el3 att="first"/> and the
   node selector was root/el3, it would result in the following
   document:



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   <?xml version="1.0"?>
   <root>
    <el1 att="first"/>
    <el1 att="second"/>
    <!-- comment -->
    <el2 att="first/>
   <el3 att="first"/></root>

   A PUT request whose content is <el2 att="2"/> and whose node selector
   is root/el2[@att="2"] would result in the following document:


   <?xml version="1.0"?>
   <root>
    <el1 att="first"/>
    <el1 att="second"/>
    <!-- comment -->
    <el2 att="first"/><el2 att="2"/>
   </root>

   It would have been inserted in exactly the same place if the node
   selector had been root/el2[2][@att="2"].  However, a selector root/
   *[2][@att="2"] would result in the following document:


   <?xml version="1.0"?>
   <root>
    <el1 att="first"/><el2 att="2"/>
    <el1 att="second"/>
    <!-- comment -->
    <el2 att="first"/>
   </root>

   Lastly, if the node selector had been root/el2[1][@att="2"] the
   result would be:


   <?xml version="1.0"?>
   <root>
    <el1 att="first"/>
    <el1 att="second"/>
    <!-- comment -->
    <el2 att="2"/><el2 att="first"/>
   </root>

   It is possible that the element cannot be inserted such that the
   request URI, when evaluated, returns the content provided in the
   request.  Such a request is not allowed for PUT.  This happens when



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   the element in the body is not described by the expression in the
   target selector.  An example of this case is described in
   Section 7.4.  If this happens the server MUST NOT perform the
   insertion, and MUST reject the request with a 409 response.  The body
   of the response SHOULD contain a detailed conflict report containing
   the <cannot-insert> element.  It is important to note that schema
   compliance does not play a role while performing the insertion.  That
   is, the decision of where the element gets inserted is dictated
   entirely by the structure of the request-URI, the current document,
   and the rules in this specification.

   If the PUT request is for an attribute, the server inserts the
   content of the request body as the value of the attribute.  The name
   of the attribute is equal to the att-name from the attribute-selector
   in the target selector.

   Assuming that the insertion can be accomplished, the server verifies
   that the insertion results in a document that meets the constraints
   of the application usage.  This is dicussed in Section 8.2.5.

8.2.4.  Replacement

   The steps in this sub-section are followed if the PUT request will
   result in the replacement of a document, element or attribute with
   the contents of the request.

   If the PUT request is for a document, the content of the request body
   is placed into the directory, replacing the document with the same
   filename.

   If the PUT request is for an element, the server replaces the target
   node with the content of the request body.  As in the creation case,
   it is possible that, after replacement, the request URI does not
   select the element that was just inserted.  If this happens the
   server MUST NOT perform the replacement, and MUST reject the request
   with a 409 response.  The body of the response SHOULD contain a
   detailed conflict report containing the <cannot-insert> element.

   If the PUT request is for an attribute, the server sets the value of
   the selected attribute to the content of the request body.  It is
   possible in the replacement case (but not in the creation case),
   that, after replacement of the attribute, the request URI no longer
   selects the attribute that was just replaced.  The scenario in which
   this can happen is discussed in Section 7.7.  If this is the case,
   the server MUST NOT perform the replacement, and MUST reject the
   request with a 409 response.  The body of the response SHOULD contain
   a detailed conflict report containing the <cannot-insert> element.




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8.2.5.  Validation

   Once the document, element or attribute has been tentatively inserted
   the server needs to verify that the resulting document meets the data
   constraints outlined by the application usage.

   First, the server checks that the final document is compliant to the
   schema.  If it is not, the server MUST NOT perform the insertion.  It
   MUST reject the request with a 409 response.  That response SHOULD
   contain a detailed conflict report containing the <schema-validation-
   error> element.

   Next, the server checks for any uniqueness constraints identified by
   the application usage.  If the application usage required that a
   particular element or attribute had a unique value within a specific
   scope, the server would check that this uniqueness property still
   exists.  If the application usage required that a URI within the
   document was unique within the domain, the server checks whether it
   is the case.  If any of these uniqueness constraints are not met, the
   server MUST NOT perform the insertion.  It MUST reject the request
   with a 409 response.  That response SHOULD contain a detailed
   conflict report containing the <uniqueness-failure> element.  That
   element can contain suggested values that the client can retry with.
   These SHOULD be values that, at the time the server generates the
   409, would meet the uniqueness constraints.

   The server also checks for URI constraints and other non-schema data
   constraints.  If the document fails one of these constraints, the
   server MUST NOT perform the insertion.  It MUST reject the request
   with a 409 response.  That response SHOULD contain a detailed
   conflict report containing the <constraint-failure> element.  That
   element indicates that the document failed non-schema data
   constraints explicitly called out by the application usage.

   Element or attribute removals have similar constraints.  The server
   checks the document for schema validity and compliance to constraints
   defined by the application usage, and rejects the request as
   described above if either check fails.

8.2.6.  Conditional Processing

   A PUT request for an XCAP resource, like any other HTTP resource, can
   be made conditional through usage of the If-Match and If-None-Match
   header fields.  For a replacement, these are processed as defined in
   [6].  For an insertion of an element or attribute, conditional
   operations are permitted.  The entity tag that is used for the
   procedures in [6] is the one for all of the resources within the same
   document as the parent of the element or attribute being inserted.



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   One way to think of this is that, logically speaking, on receipt of
   the PUT request, the XCAP server instantiates the resource referenced
   by the request and assigns it its entity tag, and then applies the
   processing of the request.

8.2.7.  Resource Interdependencies

   Because XCAP resources include elements, attributes and documents,
   each of which has its own HTTP URI, the creation or modification of
   one resource affects the state of many others.  For example,
   insertion of a document creates resources on the server for all of
   the elements and attributes within that document.  After the server
   has performed the insertion associated with the PUT, the server MUST
   create and/or modify those resources affected by that PUT.  The
   structure of the document completely defines the inter-relationship
   between those resources.

   However, the application usage can specify other resource inter-
   dependencies.  The server MUST create or modify the resources
   specified by the application usage.

   If the creation or replacement was successful, and the resource
   interdependencies are resolved, the server returns a 201 Created or
   or 200 OK, respectively.  Note that a 201 Created is generated for
   creation of new documents, elements, or attributes.  A 200 OK
   response to PUT MUST not contain any content.  Per the
   recommendations of RFC 2616, the 201 can contain a Location header
   field and entity that identify the resource that was created.

8.3.  GET Handling

   The semantics of GET are as specified in RFC 2616.  This section
   clarifies the specific content to be returned for a particular URI
   that represents an XCAP resource.

   If the request URI contains only a document selector, the server
   returns the document specified by the URI if it exists, else returns
   a 404 response.  The MIME type of the body of the 200 OK response
   MUST be the MIME type defined by that application usage (i.e.,
   "application/resource-lists+xml").

   If the request URI contains a node selector, the server obtains the
   document specified by the document selector, and if it is found,
   evaluates the node-selector within that document.  If no document is
   found, or if the node-selector is a no-match or invalid, the server
   returns a 404 response.  Otherwise, the server returns a 200 OK
   response.  If the node selector identifies an XML element, that
   element is returned in the 200 OK response as an XML fragment body



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   containing the selected element.  The MIME type of the response MUST
   be "application/xcap-el+xml".  If the node selector identifies an XML
   attribute, the value of that attribute is returned in the body of the
   response.  The MIME type of the response MUST be "application/
   xcap-att+xml".  If the node selector identifies a set of namespace
   bindings, the server computes the set of namespace bindings in scope
   for the element (including the default) and encodes it using the
   "application/xcap-ns+xml" format defined in Section 10.  That
   document is then returned in the body of the response.

   GET operations can be conditional, and follow the procedures defined
   in [6].

8.4.  DELETE Handling

   The semantics of DELETE are as specified in RFC 2616.  This section
   clarifies the specific content to be deleted for a particular URI
   that represents an XCAP resource.

   If the request URI contained a namespace-selector, the server MUST
   reject the request with a 405 (Method Not Allowed) and MUST include
   an Allow header field including the GET method.

   If the request URI contains only a document selector, the server
   deletes the document specified by the URI if it exists and returns a
   200 OK, else returns a 404 response.

   If the request URI contains a node selector, the server obtains the
   document specified by the document selector, and if it is found,
   evaluates the node-selector within that document.  If no document is
   found, or if the node-selector is a no-match or invalid (note that it
   will be invalid if multiple elements or attributes are selected), the
   server returns a 404 response.  Otherwise, the server removes the
   specified element or attribute from the document and performs the
   validation checks defined in Section 8.2.5.  Note that this deletion
   does not include any white space around the element that was deleted;
   the XCAP server MUST preserve surrounding whitespace.  It is possible
   that, after deletion, the request URI selects another element in the
   document.  If this happens the server MUST NOT perform the deletion,
   and MUST reject the request with a 409 response.  The body of the
   response SHOULD contain a detailed conflict report containing the
   <cannot-delete> element.  If the deletion will cause a failure of one
   of the constraints, the deletion MUST NOT take place.  The server
   follows the procedures in Section 8.2.5 for computing the 409
   response.  If the deletion results in a document that is still valid,
   the server MUST perform the deletion, process the resource
   interdependencies defined by the application usage, and return a 200
   OK response.



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   DELETE operations can be conditional, and follow the procedures
   defined in [6].

   Before the server returns the 200 OK response to a DELETE, it MUST
   process the resource interdependencies as defined in Section 8.2.7.

8.5.  Managing Etags

   An XCAP server MUST maintain entity tags for all resources that it
   maintains.  This specification introduces the additional constraint
   that when one resource within a document (including the document
   itself) changes, that resource is assigned a new etag, and all other
   resources within that document MUST be assigned the same etag value.
   An XCAP server MUST include the Etag header field in all 200 or 201
   responses to PUT, GET, and DELETE, assuming the document itself still
   exists after the operation.  In the case of a DELETE, the entity tag
   refers to the value of the entity tag for all other resources after
   the deletion of the specified resource has occurred.  One way to
   think about this is that the server is, logically speaking, re-
   instantiating the resource for a brief instant, so that it can have
   an entity tag that can be reasonably returned in the response to
   DELETE.

   XCAP resources do not introduce new requirements on the strength of
   the entity tags; as in RFC 2616, weak ones MAY be used if performance
   constraints or other conditions make usage of strong ones untenable
   for some reason.

   As a result of this constraint, when a client makes a change to an
   element or attribute within a document, the response to that
   operation will convey the entity tag of the resource that was just
   affected.  Since the client knows that this entity tag value is
   shared by all of the other resources in the document, the client can
   make conditional requests against other resources using that entity
   tag.


9.  Cache Control

   An XCAP resource is a valid HTTP resource, and therefore, it can be
   cached by clients and network caches.  Network caches, however, will
   not be aware of the interdependencies between XCAP resources.  As
   such, a change to an element in a document by a client will
   invalidate other XCAP resources affected by the change.  For
   application usages contain data that is likely to be dynamic or
   written by clients, servers SHOULD indicate a no-cache directive.





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10.  Namespace Binding Format

   A node-selector can identify a set of namespace bindings that are in
   scope for a particular element.  In order to convey these bindings in
   a GET response, a way is needed to encode them.

   Encoding is trivially done by including a single XML element in an
   XML fragment body.  This element has the same local-name as the
   element whose namespace bindings are desired, and also the same
   namespace-prefix.  The element has an xmlns attribute identifying the
   default namespace in scope, and an xmlns:prefix declaration for each
   prefix that is in scope.

   For example, consider the XML document in Section 6.4.  The node-
   selector df:foo/df2:bar/df2:baz/namespace::* will select the
   namespaces in scope for the <baz> element in the document, assuming
   the request is accompanied by a query component that contains
   xmlns(df=urn:test:default-namespace) and
   xmlns(df2=urn:test:namespace1-uri).  A GET containing this node
   selector and namespace bindings will produce the following result:


   <baz xmlns="urn:test:namespace1-uri"
        xmlns:ns1="urn:tes:namespace1-uri"/>

   It is important to note that the client does not need to know the
   actual namespace bindings in order to construct the URI.  It does
   need to know the namespace URI for each element in the node-selector.
   The namespace bindings present in the query component are defined by
   the client, mapping those URI to a set of prefixes.  The bindings
   returned by the server are the actual bindings used in the document.


11.  Detailed Conflict Reports

   In cases where the server returns a 409 error response, that response
   will usually include a document in the body of the response which
   provides further details on the nature of the error.  This document
   is an XML document, formatted according to the schema of
   Section 11.2.  Its MIME type, registered by this specification, is
   "application/xcap-error+xml".

11.1.  Document Structure

   The document structure is simple.  It contains the root element
   <xcap-error>.  The content of this element is a specific error
   condition.  Each error condition is represented by a different
   element.  This allows for different error conditions to provide



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   different data about the nature of the error.  All error elements
   support a "phrase" attribute, which can contain text meant for
   rendering to a human user.

   The following error elements are defined by this specification:

   <not-well-formed>: This indicates that the body of the request was
      not a well-formed XML document.

   <not-xml-frag>: This indicates that the request was supposed to
      contain a valid XML fragment body, but did not.  Most likely this
      is because the XML in the body was malformed or not balanced.

   <no-parent>: This indicates that an attempt to insert a document,
      element, or attribute failed because the directory, document or
      element into which the insertion was supposed to occur does not
      exist.  This error element can contain an optional <ancestor>
      element, which provides an HTTP URI that represents the closest
      parent that would be a valid point of insertion.  This HTTP URI
      MAY be a relative URI, relative to the document itself.  Because
      this is a valid HTTP URI, its node selector component MUST be
      percent-encoded.

   <schema-validation-error>: This element indicates that the document
      was not compliant to the schema after the requested operation was
      performed.

   <not-xml-att-value>: This indicates that the request was supposed to
      contain a valid XML attribute value, but did not.

   <cannot-insert>: This indicates that the requested PUT operation
      could not be performed because a GET of that resource after the
      PUT would not yield the content of the PUT request.

   <cannot-delete>: This indicates that the requested DELETE operation
      could not be performed because it would not be idempotent.

   <uniqueness-failure>: This indicates that the requested operation
      would result in a document that did not meet a uniqueness
      constraint defined by the application usage.  For each URI,
      element or attribute specified by the client which is not unique,
      an <exists> element is present as the content of the error
      element.  Each <exists> element has a "field" attribute that
      contains a relative URI identifying the XML element or attribute
      whose value needs to be unique, but wasn't.  The relative URI is
      relative to the document itself, and will therefore start with the
      root element.  The query component of the URI MUST be present if
      the node selector portion of the URI contains namespace prefixes.



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      Qualified elements attached with default namespace declaration are
      selected as described in Section 6.4.  Since the "field" node
      selector is a valid HTTP URI, it MUST be percent-encoded.  The
      <exists> element can optionally contain a list of <alt-value>
      elements.  Each one is a suggested alternate value which does not
      currently exist on the server.

   <constraint-failure>: This indicates that the requested operation
      would result in a document that failed a data constraint defined
      by the application usage, but not enforced by the schema or a
      uniqueness constraint.

   <extension>: This indicates an error condition that is defined by an
      extension to XCAP.  Clients which do not understand the content of
      the extension element MUST discard the xcap-error document and
      treat the error as an unqualified 409.

   As an example, the following document indicates that the user
   attempted to create an RLS service using the URI
   sip:friends@example.com, but that URI already exists:


   <?xml version="1.0" encoding="UTF-8"?>
   <xcap-error xmlns="urn:ietf:params:xml:ns:xcap-error">
    <uniqueness-failure>
     <exists field="rls-services/service/@uri">
       <alt-value>sip:mybuddies@example.com</alt-value>
     </exists>
    </uniqueness-failure>
   </xcap-error>

11.2.  XML Schema


   <?xml version="1.0" encoding="UTF-8"?>
   <xs:schema targetNamespace="urn:ietf:params:xml:ns:xcap-error"
    xmlns="urn:ietf:params:xml:ns:xcap-error"
    xmlns:xs="http://www.w3.org/2001/XMLSchema"
    elementFormDefault="qualified"
    attributeFormDefault="unqualified">

    <xs:element name="error-element" abstract="true"/>

    <xs:element name="xcap-error">
     <xs:annotation>
      <xs:documentation>Indicates the reason for the error.
     </xs:documentation>
     </xs:annotation>



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     <xs:complexType>
      <xs:sequence>
       <xs:element ref="error-element"/>
      </xs:sequence>
     </xs:complexType>
    </xs:element>

    <xs:element name="extension" substitutionGroup="error-element">
     <xs:complexType>
      <xs:sequence>
       <xs:any namespace="##any" processContents="lax"
               minOccurs="0" maxOccurs="unbounded"/>
      </xs:sequence>
     </xs:complexType>
    </xs:element>

    <xs:element name="schema-validation-error"
     substitutionGroup="error-element">
     <xs:annotation>
      <xs:documentation>This element indicates
   that the document was not compliant to the schema after the requested
   operation was performed.</xs:documentation>
     </xs:annotation>
     <xs:complexType>
      <xs:attribute name="phrase" type="xs:string" use="optional"/>
     </xs:complexType>
    </xs:element>

    <xs:element name="not-xml-frag" substitutionGroup="error-element">
     <xs:annotation>
      <xs:documentation>This indicates that the request was supposed to
   contain a valid XML fragment body, but did not.</xs:documentation>
     </xs:annotation>
     <xs:complexType>
      <xs:attribute name="phrase" type="xs:string" use="optional"/>
     </xs:complexType>
    </xs:element>

    <xs:element name="no-parent" substitutionGroup="error-element">
     <xs:annotation>
      <xs:documentation>This indicates that an attempt to insert
   an element, attribute or document failed because the document or
   element into which the insertion was
   supposed to occur does not exist</xs:documentation>
     </xs:annotation>
     <xs:complexType>
      <xs:sequence>
       <xs:element name="ancestor" type="xs:anyURI" minOccurs="0">



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        <xs:annotation>
         <xs:documentation>Contains an HTTP URI that points to the
   element which is the closest ancestor that does exist.
         </xs:documentation>
        </xs:annotation>
       </xs:element>
      </xs:sequence>
      <xs:attribute name="phrase" type="xs:string" use="optional"/>
     </xs:complexType>
    </xs:element>

    <xs:element name="cannot-insert" substitutionGroup="error-element">
     <xs:annotation>
      <xs:documentation>This indicates that the requested
   PUT operation could not be performed because a GET of that resource
   after the PUT would not yield the content of the PUT request.
      </xs:documentation>
     </xs:annotation>
     <xs:complexType>
      <xs:attribute name="phrase" type="xs:string" use="optional"/>
     </xs:complexType>
    </xs:element>

    <xs:element name="not-xml-att-value"
     substitutionGroup="error-element">
     <xs:annotation>
      <xs:documentation>This indicates that the
   request was supposed to contain a valid XML attribute value, but did
   not.</xs:documentation>
     </xs:annotation>
     <xs:complexType>
      <xs:attribute name="phrase" type="xs:string" use="optional"/>
     </xs:complexType>
    </xs:element>

    <xs:element name="uniqueness-failure"
     substitutionGroup="error-element">
     <xs:annotation>
      <xs:documentation>This indicates that the
   requested operation would result in a document that did not meet a
   uniqueness constraint defined by the application usage.
      </xs:documentation>
     </xs:annotation>
     <xs:complexType>
      <xs:sequence>
       <xs:element name="exists" maxOccurs="unbounded">
        <xs:annotation>
         <xs:documentation>For each URI,



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   element or attribute specified by the client which is not unique,
   one of these is present.</xs:documentation>
        </xs:annotation>
        <xs:complexType>
         <xs:sequence minOccurs="0">
          <xs:element name="alt-value" type="xs:string"
           maxOccurs="unbounded">
           <xs:annotation>
            <xs:documentation>An optional set of alternate values can be
   provided.</xs:documentation>
           </xs:annotation>
          </xs:element>
         </xs:sequence>
         <xs:attribute name="field" type="xs:string" use="required"/>
        </xs:complexType>
       </xs:element>
      </xs:sequence>
      <xs:attribute name="phrase" type="xs:string" use="optional"/>
     </xs:complexType>
    </xs:element>

    <xs:element name="not-well-formed"
     substitutionGroup="error-element">
     <xs:annotation>
      <xs:documentation>This indicates that the body of the request was
   not a well-formed document.</xs:documentation>
     </xs:annotation>
     <xs:complexType>
      <xs:attribute name="phrase" type="xs:string" use="optional"/>
     </xs:complexType>
    </xs:element>

    <xs:element name="constraint-failure"
     substitutionGroup="error-element">
     <xs:annotation>
      <xs:documentation>This indicates that the
   requested operation would result in a document that failed a data
   constraint defined by the application usage, but not enforced by the
   schema or a uniqueness constraint.</xs:documentation>
     </xs:annotation>
     <xs:complexType>
      <xs:attribute name="phrase" type="xs:string" use="optional"/>
     </xs:complexType>
    </xs:element>

    <xs:element name="cannot-delete" substitutionGroup="error-element">
     <xs:annotation>
      <xs:documentation>This indicates that the requested DELETE



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   operation could not be performed because it would not be
   idempotent.</xs:documentation>
     </xs:annotation>
     <xs:complexType>
      <xs:attribute name="phrase" type="xs:string" use="optional"/>
     </xs:complexType>
    </xs:element>

    <xs:element name="not-utf-8" substitutionGroup="error-element">
     <xs:annotation>
      <xs:documentation>This indicates that request could not be
         completed because it would have produced a document not
         encoded in UTF-8.</xs:documentation>
     </xs:annotation>
     <xs:complexType>
      <xs:attribute name="phrase" type="xs:string" use="optional"/>
     </xs:complexType>
    </xs:element>
   </xs:schema>



12.  XCAP Server Capabilities

   XCAP can be extended through the addition of new application usages
   and extensions to the core protocol.  Application usages may define
   MIME types with XML schemas that allow new extension nodes from new
   namespaces.  It will often be necessary for a client to determine
   what extensions, application usages or namespaces a server supports
   before making a request.  To enable that, this specification defines
   an application usage with the AUID "xcap-caps".  All XCAP servers
   MUST support this application usage.  This usage defines a single
   document within the global tree which lists the capabilities of the
   server.  Clients can read this well-known document, and therefore
   learn the capabilities of the server.

   The structure of the document is simple.  The root element is <xcap-
   caps>.  Its children are <auids>, <extensions>, and <namespaces>.
   Each of these contain a list of AUIDs, extensions and namespaces
   supported by the server.  Extensions are named by tokens defined by
   the extension, and typically define new selectors.  Namespaces are
   identified by their namespace URI.  Since all XCAP servers support
   the "xcap-caps" AUID, it MUST be listed in the <auids> element and
   the "urn:ietf:params:xml:ns:xcap-caps" namespace MUST be listed in
   the <namespaces> element.

   The following sections provide the information needed to define this
   application usage.



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12.1.  Application Unique ID (AUID)

   This specification defines the "xcap-caps" AUID within the IETF tree,
   via the IANA registration in Section 15.

12.2.  XML Schema


   <?xml version="1.0" encoding="UTF-8"?>
   <xs:schema targetNamespace="urn:ietf:params:xml:ns:xcap-caps"
    xmlns="urn:ietf:params:xml:ns:xcap-caps"
    xmlns:xs="http://www.w3.org/2001/XMLSchema"
    elementFormDefault="qualified" attributeFormDefault="unqualified">
    <xs:element name="xcap-caps">
     <xs:annotation>
      <xs:documentation>Root element for xcap-caps</xs:documentation>
     </xs:annotation>
     <xs:complexType>
      <xs:sequence>
       <xs:element name="auids">
        <xs:annotation>
         <xs:documentation>List of supported AUID.</xs:documentation>
        </xs:annotation>
        <xs:complexType>
         <xs:sequence minOccurs="0" maxOccurs="unbounded">
          <xs:element name="auid" type="auidType"/>
         </xs:sequence>
        </xs:complexType>
       </xs:element>
       <xs:element name="extensions" minOccurs="0">
        <xs:annotation>
         <xs:documentation>List of supported extensions.
         </xs:documentation>
        </xs:annotation>
        <xs:complexType>
         <xs:sequence minOccurs="0" maxOccurs="unbounded">
          <xs:element name="extension" type="extensionType"/>
         </xs:sequence>
        </xs:complexType>
       </xs:element>
       <xs:element name="namespaces">
        <xs:annotation>
         <xs:documentation>List of supported namespaces.
         </xs:documentation>
        </xs:annotation>
        <xs:complexType>
         <xs:sequence minOccurs="0" maxOccurs="unbounded">
          <xs:element name="namespace" type="namespaceType"/>



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         </xs:sequence>
        </xs:complexType>
       </xs:element>
       <xs:any namespace="##other" processContents="lax"
        minOccurs="0" maxOccurs="unbounded"/>
      </xs:sequence>
     </xs:complexType>
    </xs:element>
    <xs:simpleType name="auidType">
     <xs:annotation>
      <xs:documentation>AUID Type</xs:documentation>
     </xs:annotation>
     <xs:restriction base="xs:string"/>
    </xs:simpleType>
    <xs:simpleType name="extensionType">
     <xs:annotation>
      <xs:documentation>Extension Type</xs:documentation>
     </xs:annotation>
     <xs:restriction base="xs:string"/>
    </xs:simpleType>
    <xs:simpleType name="namespaceType">
     <xs:annotation>
      <xs:documentation>Namespace type</xs:documentation>
     </xs:annotation>
     <xs:restriction base="xs:anyURI"/>
    </xs:simpleType>
   </xs:schema>


12.3.  Default Namespace

   The default namespace used in evaluating a URI is
   urn:ietf:params:xml:ns:xcap-caps.

12.4.  MIME Type

   Documents conformant to this schema are known by the MIME type
   "application/xcap-caps+xml", registered in Section 15.2.5.

12.5.  Validation Constraints

   There are no additional validation constraints associated with this
   application usage.

12.6.  Data Semantics

   Data semantics are defined above.




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12.7.  Naming Conventions

   A server MUST maintain a single instance of the document in the
   global tree, using the filename "index".  There MUST NOT be an
   instance of this document in the users tree.

12.8.  Resource Interdependencies

   There are no resource interdependencies in this application usage
   beyond those defined by the schema.

12.9.  Authorization Policies

   This application usage does not change the default authorization
   policy defined by XCAP.


13.  Examples

   This section goes through several examples, making use of the
   resource-lists and rls-services [22] XCAP application usages.

   First, a user Bill creates a new document (see Section 7.1).  This
   document is a new resource-list, initially with a single list, called
   friends, with no users in it:


   PUT
   /resource-lists/users/sip:bill@example.com/index HTTP/1.1
   Content-Type:application/resource-lists+xml
   Host: xcap.example.com

   <?xml version="1.0" encoding="UTF-8"?>
   <resource-lists xmlns="urn:ietf:params:xml:ns:resource-lists">
     <list name="friends">
     </list>
   </resource-lists>

   Next, Bill creates an RLS services document defining a single RLS
   service referencing this list.  This service has a URI of
   sip:myfriends@example.com (URIs are line-folded for readability):










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   PUT
   /rls-services/users/sip:bill@example.com/index HTTP/1.1
   Content-Type:application/rls-services+xml
   Host: xcap.example.com

   <?xml version="1.0" encoding="UTF-8"?>
   <rls-services xmlns="urn:ietf:params:xml:ns:rls-services">
   <service uri="sip:myfriends@example.com">
     <resource-list>http://xcap.example.com/resource-lists/users/
   sip:bill@example.com/index/~~/resource-lists/
   list%5b@name=%22friends%22%5d
   </resource-list>
     <packages>
      <package>presence</package>
     </packages>
    </service>
   </rls-services>

   Next, Bill creates an element in the resource-lists document
   (Section 7.4).  In particular, he adds an entry to the list:


   PUT
   /resource-lists/users/sip:bill@example.com/index
   /~~/resource-lists/list%5b@name=%22friends%22%5d/entry HTTP/1.1
   Content-Type:application/xcap-el+xml
   Host: xcap.example.com

   <entry uri="sip:bob@example.com">
       <display-name>Bob Jones</display-name>
     </entry>

   Next, Bill fetches the document (Section 7.3):


   GET
   /resource-lists/users/sip:bill@example.com/index HTTP/1.1

   And the result is (note how white space text nodes appear in the
   document):











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   HTTP/1.1 200 OK
   Etag: "wwhha"
   Content-Type: application/resource-lists+xml

   <?xml version="1.0" encoding="UTF-8"?>
   <resource-lists xmlns="urn:ietf:params:xml:ns:resource-lists">
     <list name="friends">
     <entry uri="sip:bob@example.com">
       <display-name>Bob Jones</display-name>
     </entry></list>
   </resource-lists>

   Next, Bill adds another entry to the list, which is another list that
   has three entries.  This is another element creation (Section 7.4):


   PUT
   /resource-lists/users/sip:bill@example.com/index/~~/
   resource-lists/list%5b@name=%22friends%22%5d/
   list%5b@name=%22close-friends%22%5d HTTP/1.1
   Content-Type: application/xcap-el+xml
   Host: xcap.example.com

   <list name="close-friends">
      <entry uri="sip:joe@example.com">
        <display-name>Joe Smith</display-name>
      </entry>
      <entry uri="sip:nancy@example.com">
        <display-name>Nancy Gross</display-name>
      </entry>
      <entry uri="sip:petri@example.com">
        <display-name>Petri Aukia</display-name>
      </entry>
   </list>

   Then, Bill decides he doesn't want Petri on the list, so he deletes
   the entry (Section 7.5):


   DELETE
   /resource-lists/users/sip:bill@example.com/index/
   ~~/resource-lists/list/list/
   entry%5b@uri=%22sip:petri@example.com%22%5d HTTP/1.1
   Host: xcap.example.com

   Bill decides to check on the URI for Nancy, so he fetches a
   particular attribute (Section 7.6):




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   GET
   /resource-lists/users/sip:bill@example.com/index/
   ~~/resource-lists/list/list/entry%5b2%5d/@uri HTTP/1.1
   Host: xcap.example.com

   and the server responds:


   HTTP/1.1 200 OK
   Etag: "ad88"
   Content-Type:application/xcap-att+xml

   "sip:nancy@example.com"


14.  Security Considerations

   Frequently, the data manipulated by XCAP contains sensitive
   information.  To avoid eavesdroppers from seeing this information, it
   is RECOMMENDED that an admistrator hand out an https URI as the XCAP
   root URI.  This will result in TLS-encrypted communications between
   the client and server, preventing any eavesdropping.

   Client and server authentication are also important.  A client needs
   to be sure it is talking to the server it believes it is contacting.
   Otherwise, it may be given false information, which can lead to
   denial of service attacks against a client.  To prevent this, a
   client SHOULD attempt to upgrade [15] any connections to TLS.
   Similarly, authorization of read and write operations against the
   data is important, and this requires client authentication.  As a
   result, a server SHOULD challenge a client using HTTP Digest [11] to
   establish its identity, and this SHOULD be done over a TLS
   connection.

   Because XCAP is a usage of HTTP and not a separate protocol, it runs
   on the same port numbers as HTTP traffic normally does.  This makes
   it difficult to apply port-based filtering rules in firewalls to
   separate the treatment of XCAP traffic from other HTTP traffic.
   However, this problem exists broadly today because HTTP is used to
   access a wide breadth of content, all on the same port, and XCAP
   views application configuration documents as just another type of
   HTTP content.  As such, separate treatment of XCAP traffic from other
   HTTP traffic requires firewalls to examine the URL itself.  There is
   no foolproof way to identify a URL as pointing to an XCAP resource.
   However, the presence of the double tilde (~~) is a strong hint that
   the URL points to an XML element or attribute.  As always, care must
   be taken in looking for the double-tilde due to the breadth of ways
   in which a URI can be encoded on-the-wire [29] [13].



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15.  IANA Considerations

   There are several IANA considerations associated with this
   specification.

15.1.  XCAP Application Unique IDs

   This specification instructs IANA to create a new registry for XCAP
   application unique IDs (AUIDs).  This registry is defined as a table
   that contains three colums:

   AUID: This will be a string provided in the IANA registrations into
      the registry.

   Description: This is text that is supplied by the IANA registration
      into the registry.

   Document: This is a reference to the RFC containing the registration.

   This specification instructs IANA to create this table with an
   initial entry.  The resulting table would look like:


   Application Unique         Description            Document
     ID (AUID)
   -----------------------------------------------------------

   xcap-caps                  Capabilities of an     RFC XXXX
                              XCAP server

   [[NOTE TO IANA/RFC-EDITOR: Please replace XXXX with the RFC number of
   this specification.]]

   XCAP AUIDs are registered by the IANA when they are published in
   standards track RFCs.  The IANA Considerations section of the RFC
   must include the following information, which appears in the IANA
   registry along with the RFC number of the publication.

      Name of the AUID.  The name MAY be of any length, but SHOULD be no
      more than twenty characters long.  The name MUST consist of
      alphanum and mark [16] characters only.

      Descriptive text that describes the application usage.

15.2.  MIME Types

   This specification requests the registration of several new MIME
   types according to the procedures of RFC 2048 [8] and guidelines in



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   RFC 3023 [9].

15.2.1.  application/xcap-el+xml MIME Type

   MIME media type name: application

   MIME subtype name: xcap-el+xml

   Mandatory parameters: none

   Optional parameters: Same as charset parameter application/xml as
      specified in RFC 3023 [9].

   Encoding considerations: Same as encoding considerations of
      application/xml as specified in RFC 3023 [9].

   Security considerations: See Section 10 of RFC 3023 [9].

   Interoperability considerations: none.

   Published specification: RFC XXXX [[NOTE TO RFC EDITOR: Please
      replace XXXX with the published RFC number of this
      specification.]].

   Applications which use this media type: This document type has been
      used to support transport of XML fragment bodies in RFC XXXX
      [[NOTE TO RFC EDITOR: Please replace XXXX with the published RFC
      number of this specification.]], the XML Configuration Access
      Protocol (XCAP).

   Additional Information:

      Magic Number: None

      File Extension: .xel

      Macintosh file type code: "TEXT"

      Personal and email address for further information: Jonathan
         Rosenberg, jdrosen@jdrosen.net

      Intended usage: COMMON

      Author/Change controller: The IETF.







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15.2.2.  application/xcap-att+xml MIME Type

   MIME media type name: application

   MIME subtype name: xcap-att+xml

   Mandatory parameters: none

   Optional parameters: Same as charset parameter application/xml as
      specified in RFC 3023 [9].

   Encoding considerations: Same as encoding considerations of
      application/xml as specified in RFC 3023 [9].

   Security considerations: See Section 10 of RFC 3023 [9].

   Interoperability considerations: none.

   Published specification: RFC XXXX [[NOTE TO RFC EDITOR: Please
      replace XXXX with the published RFC number of this
      specification.]].

   Applications which use this media type: This document type has been
      used to support transport of XML attribute values in RFC XXXX
      [[NOTE TO RFC EDITOR: Please replace XXXX with the published RFC
      number of this specification.]], the XML Configuration Access
      Protocol (XCAP).

   Additional Information:

      Magic Number: None

      File Extension: .xav

      Macintosh file type code: "TEXT"

      Personal and email address for further information: Jonathan
         Rosenberg, jdrosen@jdrosen.net

      Intended usage: COMMON

      Author/Change controller: The IETF.

15.2.3.  application/xcap-ns+xml MIME Type







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   MIME media type name: application

   MIME subtype name: xcap-ns+xml

   Mandatory parameters: none

   Optional parameters: Same as charset parameter application/xml as
      specified in RFC 3023 [9].

   Encoding considerations: Same as encoding considerations of
      application/xml as specified in RFC 3023 [9].

   Security considerations: See Section 10 of RFC 3023 [9].

   Interoperability considerations: none.

   Published specification: RFC XXXX [[NOTE TO RFC EDITOR: Please
      replace XXXX with the published RFC number of this
      specification.]].

   Applications which use this media type: This document type has been
      used to support transport of XML fragment bodies in RFC XXXX
      [[NOTE TO RFC EDITOR: Please replace XXXX with the published RFC
      number of this specification.]], the XML Configuration Access
      Protocol (XCAP).

   Additional Information:

      Magic Number: None

      File Extension: .xns

      Macintosh file type code: "TEXT"

      Personal and email address for further information: Jonathan
         Rosenberg, jdrosen@jdrosen.net

      Intended usage: COMMON

      Author/Change controller: The IETF.

15.2.4.  application/xcap-error+xml MIME Type

   MIME media type name: application







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   MIME subtype name: xcap-error+xml

   Mandatory parameters: none

   Optional parameters: Same as charset parameter application/xml as
      specified in RFC 3023 [9].

   Encoding considerations: Same as encoding considerations of
      application/xml as specified in RFC 3023 [9].

   Security considerations: See Section 10 of RFC 3023 [9].

   Interoperability considerations: none.

   Published specification: RFC XXXX [[NOTE TO RFC EDITOR: Please
      replace XXXX with the published RFC number of this
      specification.]].

   Applications which use this media type: This document type conveys
      error conditions defined in RFC XXXX.  [[NOTE TO RFC EDITOR:
      Please replace XXXX with the published RFC number of this
      specification.]]

   Additional Information:

      Magic Number: None

      File Extension: .xer

      Macintosh file type code: "TEXT"

      Personal and email address for further information: Jonathan
         Rosenberg, jdrosen@jdrosen.net

      Intended usage: COMMON

      Author/Change controller: The IETF.

15.2.5.  application/xcap-caps+xml MIME Type

   MIME media type name: application

   MIME subtype name: xcap-caps+xml

   Mandatory parameters: none






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   Optional parameters: Same as charset parameter application/xml as
      specified in RFC 3023 [9].

   Encoding considerations: Same as encoding considerations of
      application/xml as specified in RFC 3023 [9].

   Security considerations: See Section 10 of RFC 3023 [9].

   Interoperability considerations: none.

   Published specification: RFC XXXX [[NOTE TO RFC EDITOR: Please
      replace XXXX with the published RFC number of this
      specification.]].

   Applications which use this media type: This document type conveys
      capabililites of an XML Configuration Access Protocol (XCAP)
      server, as defined in RFC XXXX.  [[NOTE TO RFC EDITOR: Please
      replace XXXX with the published RFC number of this
      specification.]]

   Additional Information:

      Magic Number: None

      File Extension: .xca

      Macintosh file type code: "TEXT"

      Personal and email address for further information: Jonathan
         Rosenberg, jdrosen@jdrosen.net

      Intended usage: COMMON

      Author/Change controller: The IETF.

15.3.  URN Sub-Namespace Registrations

   This specification registers several new XML namespaces, as per the
   guidelines in RFC 3688 [17].

15.3.1.  urn:ietf:params:xml:ns:xcap-error

   URI: The URI for this namespace is urn:ietf:params:xml:ns:xcap-error

   Registrant Contact: IETF, SIMPLE working group, (simple@ietf.org),
      Jonathan Rosenberg (jdrosen@jdrosen.net).





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   XML:


                BEGIN
                <?xml version="1.0"?>
                <!DOCTYPE html PUBLIC "-//W3C//DTD XHTML Basic 1.0//EN"
                   "http://www.w3.org/TR/xhtml-basic/xhtml-basic10.dtd">
                <html xmlns="http://www.w3.org/1999/xhtml">
                <head>
                  <meta http-equiv="content-type"
                     content="text/html;charset=iso-8859-1"/>
                  <title>XCAP Error Namespace</title>
                </head>
                <body>
                  <h1>Namespace for XCAP Error Documents</h1>
                  <h2>urn:ietf:params:xml:ns:xcap-error</h2>
                  <p>See <a href="[URL of published RFC]">RFCXXXX [[NOTE
   TO RFC-EDITOR/IANA: Please replace XXXX with the RFC Number of
   this specification]]</a>.</p>
                </body>
                </html>
                END

15.3.2.  urn:ietf:params:xml:ns:xcap-caps

   URI: The URI for this namespace is urn:ietf:params:xml:ns:xcap-caps

   Registrant Contact: IETF, SIMPLE working group, (simple@ietf.org),
      Jonathan Rosenberg (jdrosen@jdrosen.net).

   XML:




















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                BEGIN
                <?xml version="1.0"?>
                <!DOCTYPE html PUBLIC "-//W3C//DTD XHTML Basic 1.0//EN"
                   "http://www.w3.org/TR/xhtml-basic/xhtml-basic10.dtd">
                <html xmlns="http://www.w3.org/1999/xhtml">
                <head>
                  <meta http-equiv="content-type"
                     content="text/html;charset=iso-8859-1"/>
                  <title>XCAP Capabilities Namespace</title>
                </head>
                <body>
                  <h1>Namespace for XCAP Capability Documents</h1>
                  <h2>urn:ietf:params:xml:ns:xcap-caps</h2>
                  <p>See <a href="[URL of published RFC]">RFCXXXX [[NOTE
   TO RFC-EDITOR/IANA: Please replace XXXX with the RFC Number of
   this specification]]</a>.</p>
                </body>
                </html>
                END

15.4.  XML Schema Registrations

   This section registers two XML schemas per the procedures in [17].

15.4.1.  XCAP Error Schema Registration

   URI: urn:ietf:params:xml:schema:xcap-error

   Registrant Contact: IETF, SIMPLE working group, (simple@ietf.org),
      Jonathan Rosenberg (jdrosen@jdrosen.net).

   XML Schema: The XML for this schema can be found as the sole content
      of Section 11.2.

15.4.2.  XCAP Capabilities Schema Registration

   URI: urn:ietf:params:xml:schema:xcap-caps

   Registrant Contact: IETF, SIMPLE working group, (simple@ietf.org),
      Jonathan Rosenberg (jdrosen@jdrosen.net).

   XML Schema: The XML for this schema can be found as the sole content
      of Section 12.2.


16.  Acknowledgements

   The author would like to thank Jari Urpalainen, who has contributed



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   many important comments and has assisted with edit passes in the
   document.  The author would also like to thank Ben Campbell, Eva-
   Maria Leppanen, Hisham Khartabil, Chris Newman, Joel Halpern, , Lisa
   Dusseault, Tim Bray, Pete Cordell, Jeroen van Bemmel, Christian
   Schmidt, and Spencer Dawkins for their input and comments.  A special
   thanks to Ted Hardie for his input and support.


17.  References

17.1.  Normative References

   [1]   Yergeau, F., Bray, T., Paoli, J., Sperberg-McQueen, C., and E.
         Maler, "Extensible Markup Language (XML) 1.0 (Third Edition)",
         W3C REC REC-xml-20040204, February 2004.

   [2]   Maloney, M., Beech, D., Thompson, H., and N. Mendelsohn, "XML
         Schema Part 1: Structures Second Edition", W3C REC REC-
         xmlschema-1-20041028, October 2004.

   [3]   Bray, T., Hollander, D., and A. Layman, "Namespaces in XML",
         W3C REC REC-xml-names-19990114, January 1999.

   [4]   DeRose, S., Daniel, R., Maler, E., and J. Marsh, "XPointer
         xmlns() Scheme", W3C REC REC-xptr-xmlns-20030325, March 2003.

   [5]   Grosso, P., Maler, E., Marsh, J., and N. Walsh, "XPointer
         Framework", W3C REC REC-xptr-framework-20030325, March 2003.

   [6]   Fielding, R., Gettys, J., Mogul, J., Frystyk, H., Masinter, L.,
         Leach, P., and T. Berners-Lee, "Hypertext Transfer Protocol --
         HTTP/1.1", RFC 2616, June 1999.

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

   [8]   Freed, N., Klensin, J., and J. Postel, "Multipurpose Internet
         Mail Extensions (MIME) Part Four: Registration Procedures",
         BCP 13, RFC 2048, November 1996.

   [9]   Murata, M., St. Laurent, S., and D. Kohn, "XML Media Types",
         RFC 3023, January 2001.

   [10]  Clark, J. and S. DeRose, "XML Path Language (XPath) Version
         1.0", W3C REC REC-xpath-19991116, November 1999.

   [11]  Franks, J., Hallam-Baker, P., Hostetler, J., Lawrence, S.,
         Leach, P., Luotonen, A., and L. Stewart, "HTTP Authentication:



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         Basic and Digest Access Authentication", RFC 2617, June 1999.

   [12]  Crocker, D. and P. Overell, "Augmented BNF for Syntax
         Specifications: ABNF", RFC 4234, October 2005.

   [13]  Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
         Resource Identifier (URI): Generic Syntax", STD 66, RFC 3986,
         January 2005.

   [14]  Rescorla, E., "HTTP Over TLS", RFC 2818, May 2000.

   [15]  Khare, R. and S. Lawrence, "Upgrading to TLS Within HTTP/1.1",
         RFC 2817, May 2000.

   [16]  Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston, A.,
         Peterson, J., Sparks, R., Handley, M., and E. Schooler, "SIP:
         Session Initiation Protocol", RFC 3261, June 2002.

   [17]  Mealling, M., "The IETF XML Registry", BCP 81, RFC 3688,
         January 2004.

   [18]  Yergeau, F., "UTF-8, a transformation format of ISO 10646",
         STD 63, RFC 3629, November 2003.

   [19]  Boyer, J., "Canonical XML Version 1.0", W3C REC REC-xml-c14n-
         20010315, March 2001.

17.2.  Informative References

   [20]  Rosenberg, J., "A Presence Event Package for the Session
         Initiation Protocol (SIP)", RFC 3856, August 2004.

   [21]  Roach, A., Rosenberg, J., and B. Campbell, "A Session
         Initiation Protocol (SIP) Event Notification Extension for
         Resource Lists", draft-ietf-simple-event-list-07 (work in
         progress), January 2005.

   [22]  Rosenberg, J., "Extensible Markup Language (XML) Formats for
         Representing Resource Lists",
         draft-ietf-simple-xcap-list-usage-05 (work in progress),
         February 2005.

   [23]  Grosso, P. and D. Veillard, "XML Fragment Interchange", W3C
         CR CR-xml-fragment-20010212, February 2001.

   [24]  Berglund, A., Boag, S., Chamberlin, D., Fernandez, M., Kay, M.,
         Robie, J., and J. Simeon, "XML Path Language (XPath) 2.0", W3C
         CR CR-xpath20-20051103, November 2005.



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   [25]  Newman, C. and J. Myers, "ACAP -- Application Configuration
         Access Protocol", RFC 2244, November 1997.

   [26]  Day, M., Rosenberg, J., and H. Sugano, "A Model for Presence
         and Instant Messaging", RFC 2778, February 2000.

   [27]  Narten, T. and H. Alvestrand, "Guidelines for Writing an IANA
         Considerations Section in RFCs", BCP 26, RFC 2434,
         October 1998.

   [28]  Roach, A., "Session Initiation Protocol (SIP)-Specific Event
         Notification", RFC 3265, June 2002.

   [29]  Duerst, M. and M. Suignard, "Internationalized Resource
         Identifiers (IRIs)", RFC 3987, January 2005.




































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Author's Address

   Jonathan Rosenberg
   Cisco Systems
   600 Lanidex Plaza
   Parsippany, NJ  07054
   US

   Phone: +1 973 952-5000
   Email: jdrosen@cisco.com
   URI:   http://www.jdrosen.net








































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