SIMPLE                                                      J. Rosenberg
Internet-Draft                                             Cisco Systems
Expires: April 25, 2006                                 October 22, 2005

  An Extensible Markup Language (XML) Document Format for Indicating A
     Change in XML Configuration Access  Protocol (XCAP) Resources

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Copyright Notice

   Copyright (C) The Internet Society (2005).


   This specification defines a document format that can be used to
   indicate that a change has occurred in a document managed by the
   Extensible Markup Language (XML) Configuration Access Protocol
   (XCAP).  This format indicates the document that has changed and its
   former and new entity tags.  XCAP diff documents can be delivered to
   clients using a number of means, including the Session Initiation
   Protocol (SIP) event package for configuration data.  By subscribing
   to this event package, clients can learn about document changes made

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   by other clients.  The XCAP diff format is extensible, so that
   additional information, such as a description of the actual change,
   can be included.

Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Terminology  . . . . . . . . . . . . . . . . . . . . . . . . .  4
   3.  Structure of an XCAP Diff Document . . . . . . . . . . . . . .  4
   4.  XML Schema . . . . . . . . . . . . . . . . . . . . . . . . . .  6
   5.  Example Document . . . . . . . . . . . . . . . . . . . . . . .  6
   6.  Usage with the Config Framework  . . . . . . . . . . . . . . .  7
   7.  Security Considerations  . . . . . . . . . . . . . . . . . . .  8
   8.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . .  8
     8.1   application/xcap-diff+xml MIME Type  . . . . . . . . . . .  8
     8.2   URN Sub-Namespace Registration for
           urn:ietf:params:xml:ns:xcap-diff . . . . . . . . . . . . .  9
     8.3   Schema Registration  . . . . . . . . . . . . . . . . . . . 10
   9.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 10
     9.1   Normative References . . . . . . . . . . . . . . . . . . . 10
     9.2   Informative References . . . . . . . . . . . . . . . . . . 11
       Author's Address . . . . . . . . . . . . . . . . . . . . . . . 12
       Intellectual Property and Copyright Statements . . . . . . . . 13

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1.  Introduction

   The Extensible Markup Language (XML) Configuration Access Protocol
   (XCAP) [8] is a protocol that allows clients to manipulate XML
   documents stored on a server.  These XML documents serve as
   configuration information for application protocols.  As an example,
   resource list [12] subscriptions (also known as presence lists) allow
   a client to have a single SIP subscription to a list of users, where
   the list is maintained on a server.  The server will obtain presence
   for those users and report it back to the client.  This application
   requires the server, called a Resource List Server (RLS), to have
   access to the list of presentities.  This list needs to be
   manipulated by clients so they can add and remove their friends as
   they desire.

   Complexities arise when multiple clients attempt to simultaneously
   manipulate a document, such as a presence list.  Frequently, a client
   will keep a copy of the current list in memory, so it can render it
   to users.  However, if another client modifies the document, the
   cached version becomes stale.  This modification event must be made
   known to all clients which have cached copies of the document, so
   that they can fetch the most recent one.

   To deal with this problem, clients can use the Session Initiation
   Protocol (SIP) [10] event package [11] for subscribing to changes in
   configuration and profile information [9], including application data
   that resides on an XCAP server.  With that package, a user gets
   notified that a particular document has changed.  This notification
   can include the full content of the new document, or it can be a
   content indirection [15].  Though content indirection can tell a
   client that a document has changed, it provides it with MIME
   Content-ID indicating the new version of the document.  The MIME
   Content-ID is not the same as the entity tag, which is used by XCAP
   for document versioning.  As such, a client cannot easily ascertain
   whether an indication of a change in a document is due to a change it
   just made, or due to a change another client made at around the same

   In addition, when an XCAP client inserts a new element or attribute
   into an existing document, the client has no way to know whether the
   insertion was done against its cached version of the document.  The
   reasons for this are described in Section 7.10 of XCAP.  To help a
   client ascertain whether this has occurred after performing the
   insertion, the XCAP response needs to contain a document which
   indicates the entity tags before and after the document was modified.

   To resolve these problems, this document defines a data format which
   can convey the fact that an XML document has changed.  This data

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   format is an XML document format, called an XCAP diff document.  This
   format can indicate that a document has changed, and provide its
   previous and new entity tags.  This specification also explains how
   this format is used in conjunction with the configuration profile

2.  Terminology

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

   This specification also defines the following additional terms:

   Document: When the term document is used without the "XCAP diff" in
      front of it, it refers to the XCAP document resource about whom
      the XCAP diff document is reporting a change.

   XCAP diff document: The XML document defined by this specification
      that reports on a set of changes in an XCAP document resource.

   Server: Typically an XCAP server, this is a protocol entity that
      generates XCAP diff documents based on its knowledge of a set of
      XCAP documents.

   Client: Typically an XCAP client and SIP User Agent (UA) that acts as
      a subscriber to the configuration event package, this is a
      protocol entity that consumes XCAP diff documents in order to
      reconstruct the document stored on the server.

3.  Structure of an XCAP Diff Document

   An XCAP diff document is an XML [2] document that MUST be well-formed
   and SHOULD be valid.  XCAP diff documents MUST be based on XML 1.0
   and MUST be encoded using UTF-8.  This specification makes use of XML
   namespaces for identifying XCAP diff documents and document
   fragments.  The namespace URI for elements defined by this
   specification is a URN [3], using the namespace identifier 'ietf'
   defined by [5] and extended by [6].  This URN is:


   An XCAP diff document begins with the root element tag <xcap-diff>.
   This element has a single mandatory attribute, "xcap-root".  The
   value of this attribute is the XCAP root URI for the documents in
   which the changes have taken place.  A single XCAP diff document can

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   only represent changes in documents within the same XCAP root.  The
   content of the <xcap-diff> element is a sequence of <document>
   elements.  Each <document> element specifies changes in a specific
   document within the XCAP root.  It has one mandatory attribute, "doc-
   selector", and a three optional attributes, "new-etag", "previous-
   etag" and "hash".  The "doc-selector" identifies the specific
   document within the XCAP root for which changes are indicated.  Its
   content MUST be a relative path reference, with the base URI being
   equal to the XCAP root URI.  The "new-etag" attribute provides the
   etag for the document after the application of the changes, assuming
   the document exists after those changes.  If the change being
   reported is the deletion of the document, the "new-etag" attribute
   will not be present.  A server MUST include the "new-etag" unless the
   document does not exist subsequent to the changes reported in the
   XCAP diff document.  The "previous-etag" attribute provides an
   identifier for the document instance prior to the change.  If the
   document did not exist prior to the change (that is, the change was
   the creation of the document), the "previous-etag" is not present.

   The "previous-etag" and "new-etag" need not have been sequentially
   assigned etags at the server.  An XCAP diff document can indicate
   changes that have occurred over a series of XCAP operations.

   The optional "hash" attribute provides an HMAC of the document
   instance whose etag is "new-etag", once that document is represented
   in canonical form.  To compute this value, the server MUST apply the
   mandatory XML canonicalization defined in the Canonical XML 1.0 [1]
   specification, and then computes an HMAC [13] using SHA1 over this
   canonical document, with a key whose value is 0x2238a.  The result is
   the value of the "hash" attribute.  This attribute is optional, and a
   server MAY elect not to include it.  Even if present, a client MAY
   elect to ignore it.

   This contents of the <document> element are extensible, and can
   include elements from other namespaces.  It is anticipated that
   extensions would be defined that allow the actual change in the
   document to be reported.

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4.  XML Schema

   <?xml version="1.0" encoding="UTF-8"?>
   <xs:schema targetNamespace="urn:ietf:params:xml:ns:xcap-diff"
    elementFormDefault="qualified" attributeFormDefault="unqualified">
    <xs:element name="document">
       <xs:any namespace="##other" minOccurs="0" maxOccurs="unbounded"/>
      <xs:attribute name="doc-selector" type="xs:anyURI" use="required"/>
      <xs:attribute name="new-etag" type="xs:string" use="optional"/>
      <xs:attribute name="previous-etag" type="xs:string" use="optional"/>
      <xs:attribute name="hash" type="xs:string" use="optional"/>
      <xs:anyAttribute namespace="##other" processContents="lax"/>
    <xs:element name="xcap-diff">
       <xs:element ref="document"/>
      <xs:attribute name="xcap-root" type="xs:anyURI" use="required"/>

5.  Example Document

   The following is an example of a document compliant to the schema.

   <?xml version="1.0" encoding="UTF-8"?>
   <xcap-diff xmlns="urn:ietf:params:xml:ns:xcap-diff"
    <document new-etag="7ahggs"

   This indicates that the document with URI has
   changed.  Its previous entity tag is 8a77f8d and its new one is

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6.  Usage with the Config Framework

   The framework for user agent profile delivery [9] defines an event
   package which can be used to subscribe to user, device, application
   or local-network data that defines the configuration of a client.
   This data can be present in an XCAP server.  Normally, content
   indirection [15] will be used as the NOTIFY body format, to indicate
   the specific document that has changed, and should be re-fetched.
   However, if the client includes an Accept header field including the
   MIME type "application/xcap-diff+xml", the server has the option of
   returning documents in this format instead.

   When the client performs an initial subscription, the rules in [9]
   are used to select the set of documents which the subscription
   applies to.  Upon initial subscription, the server does not know
   which instances of each document (where each instance is identified
   by an etag) the client currently posessses, if any.  Indeed, upon
   startup, the client will not have any documents.  The initial NOTIFY
   in this case MUST include a <document> element for each document
   associated with the subscription.  The "previous-etag" attribute MUST
   be absent, and the "new-etag" attribute MUST be present and contain
   the entity tag for the current version of that document resource.  An
   XCAP diff document structured this way is called a "reference" XCAP
   diff document.  It establishes the baseline etags and document URIs
   for the documents covered by the subscription.

   Upon receipt of this document, the client can determine whether its
   local instance documents, if any, match the etags in the XCAP diff
   document.  If they do not match, the client SHOULD perform a
   conditional GET for each document.  The document URI is constructed
   by appending the XCAP root in the "xcap-root" attribute of the <xcap-
   diff> element to the escape coded "doc-selector" from each <document>
   element.  The request is made conditional by including an If-Match
   header field, with the value of the etag from each <document>
   element.  So long as the documents haven't changed between the NOTIFY
   and the GET, the client will obtain the reference versions that the
   server will use for subsequent notifications.

   If the conditional GET should fail, the client SHOULD generate a
   SUBSCRIBE refresh request to trigger a new NOTIFY.  The server will
   always generate a "reference" XML diff document on receipt of a
   SUBSCRIBE refresh.  This establishes a new set of baseline etags, and
   the client can then attempt to do another fetch.  It is anticipated
   that future extensions to the profile delivery framework will allow a
   client to include, in its SUBSCRIBE request, an indicator of the
   current version of the documents it holds.  That would obviate the

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   need for a potentially never-ending stream of SUBSCRIBE/GET sequences
   should the documents be rapidly changing, for some reason.

   Once the client has obtained the versions of the documents identified
   in the reference XML diff, it can process NOTIFY requests on that
   subscription.  To process the NOTIFY requests, it makes sure that its
   current version matches the version in the "previous-etag" attribute
   of the <document> element.  If not, the client can then fetch the
   updated document from the server.  If they do match, the client has
   the most current version.

7.  Security Considerations

   XCAP diff documents are not very sensitive; they only contain entity
   tags and the URI for documents.  An attacker that is able to examine
   such a document cannot access or modify the referenced document
   unless it has also managed to attack XCAP itself.  Thus, there is no
   requirement for message confidentiality.  However, an attacker that
   can modify XCAP diff documents in transit could fool a client into
   thinking that a document hasn't changed, when it has, or vice-a-
   versa.  Therefore, protocols which transport XCAP Diff documents
   SHOULD provide message integrity.

8.  IANA Considerations

   There are several IANA considerations associated with this

8.1  application/xcap-diff+xml MIME Type

      MIME media type name: application

      MIME subtype name: xcap-diff+xml

      Mandatory parameters: none

      Optional parameters: Same as charset parameter application/xml as
      specified in RFC 3023 [4].

      Encoding considerations: Same as encoding considerations of
      application/xml as specified in RFC 3023 [4].

      Security considerations: See Section 10 of RFC 3023 [4] and
      Section 7 of RFCXXXX [[NOTE TO RFC-EDITOR/IANA: Please replace
      XXXX with the RFC number of this specification.]].

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      Interoperability considerations: none.

      Published specification: This document.

      Applications which use this media type: This document type has
      been used to support manipulation of resource lists [14] using

      Additional Information:

         Magic Number: None

         File Extension: .xdf

         Macintosh file type code: "TEXT"

         Personal and email address for further information: Jonathan

         Intended usage: COMMON

         Author/Change controller: The IETF.

8.2  URN Sub-Namespace Registration for urn:ietf:params:xml:ns:xcap-diff

   This section registers a new XML namespace, as per the guidelines in

      URI: The URI for this namespace is

      Registrant Contact: IETF, SIMPLE working group, (,
      Jonathan Rosenberg (


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                <?xml version="1.0"?>
                <!DOCTYPE html PUBLIC "-//W3C//DTD XHTML Basic 1.0//EN"
                <html xmlns="">
                  <meta http-equiv="content-type"
                  <title>XCAP Diff Namespace</title>
                  <h1>Namespace for XCAP Diff</h1>
                  <p>See <a href="[URL of published RFC]">RFCXXXX[[NOTE
   TO IANA/RFC-EDITOR: Please replace XXXX with the RFC number of this

8.3  Schema Registration

   This section registers a new XML schema per the procedures in [6].

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

      Registrant Contact: IETF, SIMPLE working group, (,
      Jonathan Rosenberg (

      The XML for this schema can be found as the sole content of
      Section 4.

9.  References

9.1  Normative References

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

   [2]  Bray, T., Paoli, J., Sperberg-McQueen, C., and E. Maler,
        "Extensible Markup Language (XML) 1.0 (Second Edition)", W3C
        FirstEdition REC-xml-20001006, October 2000.

   [3]  Moats, R., "URN Syntax", RFC 2141, May 1997.

   [4]  Murata, M., St. Laurent, S., and D. Kohn, "XML Media Types",

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        RFC 3023, January 2001.

   [5]  Moats, R., "A URN Namespace for IETF Documents", RFC 2648,
        August 1999.

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

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

   [8]  Rosenberg, J., "The Extensible Markup Language (XML)
        Configuration Access Protocol (XCAP)", draft-ietf-simple-xcap-07
        (work in progress), June 2005.

   [9]  Petrie, D., "A Framework for Session Initiation Protocol User
        Agent Profile Delivery", draft-ietf-sipping-config-framework-07
        (work in progress), July 2005.

9.2  Informative References

   [10]  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.

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

   [12]  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.

   [13]  Krawczyk, H., Bellare, M., and R. Canetti, "HMAC: Keyed-Hashing
         for Message Authentication", RFC 2104, February 1997.

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

   [15]  Burger, E., "A Mechanism for Content Indirection in Session
         Initiation Protocol (SIP)  Messages",
         draft-ietf-sip-content-indirect-mech-05 (work in progress),
         October 2004.

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Author's Address

   Jonathan Rosenberg
   Cisco Systems
   600 Lanidex Plaza
   Parsippany, NJ  07054

   Phone: +1 973 952-5000

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