SIMPLE                                                      J. Rosenberg
Internet-Draft                                               dynamicsoft
Expires: August 16, 2004                               February 16, 2004

   A Session Initiation Protocol (SIP) Event Package for Modification
  Events for the Extensible Markup Language (XML) Configuration Access
                   Protocol (XCAP) Managed Documents

Status of this Memo

   This document is an Internet-Draft and is in full conformance with
   all provisions of Section 10 of RFC2026.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF), its areas, and its working groups. Note that other
   groups may also distribute working documents as Internet-Drafts.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time. It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   The list of current Internet-Drafts can be accessed at http://

   The list of Internet-Draft Shadow Directories can be accessed at

   This Internet-Draft will expire on August 16, 2004.

Copyright Notice

   Copyright (C) The Internet Society (2004). All Rights Reserved.


   This specification defines a Session Initiation Protocol (SIP) event
   package for finding out about changes to documents managed by the
   Extensible Markup Language (XML) Configuration Access Protocol
   (XCAP). XCAP allows a client to manipulate XML documents on a server
   which contain configuration information for application protocols.
   Multiple clients can potentially access the same document, in which
   case the other clients would like to be notified of a change in the
   document made by another. This event package allows a client to do

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Table of Contents

   1.   Introduction . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.   Document Change Event Package  . . . . . . . . . . . . . . .   4
   2.1  Package Name . . . . . . . . . . . . . . . . . . . . . . . .   4
   2.2  Event Package Parameters . . . . . . . . . . . . . . . . . .   4
   2.3  SUBSCRIBE Bodies . . . . . . . . . . . . . . . . . . . . . .   5
   2.4  Subscription Duration  . . . . . . . . . . . . . . . . . . .   5
   2.5  NOTIFY Bodies  . . . . . . . . . . . . . . . . . . . . . . .   5
   2.6  Notifier Processing of SUBSCRIBE Requests  . . . . . . . . .   6
   2.7  Notifier Generation of NOTIFY Requests . . . . . . . . . . .   7
   2.8  Subscriber Processing of NOTIFY Requests . . . . . . . . . .   7
   2.9  Handling of Forked Requests  . . . . . . . . . . . . . . . .   8
   2.10 Rate of Notifications  . . . . . . . . . . . . . . . . . . .   8
   2.11 State Agents . . . . . . . . . . . . . . . . . . . . . . . .   8
   3.   application/xml-change+xml Media Type  . . . . . . . . . . .   9
   3.1  XML Schema . . . . . . . . . . . . . . . . . . . . . . . . .  11
   3.2  Example Document . . . . . . . . . . . . . . . . . . . . . .  11
   4.   Security Considerations  . . . . . . . . . . . . . . . . . .  13
   5.   IANA Considerations  . . . . . . . . . . . . . . . . . . . .  14
   5.1  SIP Event Package  . . . . . . . . . . . . . . . . . . . . .  14
   5.2  application/xcap-change+xml MIME Type  . . . . . . . . . . .  14
   5.3  URN Sub-Namespace Registration for
        urn:ietf:params:xml:ns:xcap-change . . . . . . . . . . . . .  15
        Normative References . . . . . . . . . . . . . . . . . . . .  17
        Informative References . . . . . . . . . . . . . . . . . . .  18
        Author's Address . . . . . . . . . . . . . . . . . . . . . .  18
        Intellectual Property and Copyright Statements . . . . . . .  19

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1. Introduction

   The Extensible Markup Language (XML) Configuration Access Protocol
   (XCAP) [10] 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 [11] 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 information must be made known to
   all clients which have cached copies of the document, so that they
   can fetch the most recent one.

   This problem is addressed by this specification, which provides a
   Session Initiation Protocol (SIP) [1]event package [2] for
   subscribing to changes in documents managed by an XCAP server. This
   package would be used by any client which is retaining a cached copy
   of a document obtained by XCAP, so that it can find out when a change
   has been made, invalidating its cached copy. In fact, the
   notifications generated by this package indicate the specific change
   which occurred in the document, so the client can decide whether the
   change is significant enough to warrant a refetch from the XCAP

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2. Document Change Event Package

   The SIP event framework [2] defines a SIP extension for subscribing
   to, and receiving notifications of, events. It leaves the definition
   of many aspects of these events to concrete extensions, known as
   event packages. This document qualifies as an event package. This
   section fills in the information required for all event packages by
   RFC 3265.

2.1 Package Name

   The name of this package is "xcap-change". As specified in RFC 3265
   [2], this value appears in the Event header field present in
   SUBSCRIBE and NOTIFY requests.

2.2 Event Package Parameters

   The SIP event framework allows event packages to define additional
   parameters carried in the Event header field. This package defines a
   single event header parameter, called "doc-component", which
   specifies a particular document and document component which is being
   to subscribed to. The request-URI specifies the user whose data is
   being subscribed to. To subscribe to global data, a user agent would
   subscribed to a special user name that is configured into the UA. The
   name "global-xcap-user" is RECOMMENDED, and SHOULD be used if no
   explicit user name is provisioned. By default, a subscription is for
   all XCAP data associated with that user. The header field parameter
   allows the subscription to specify a specific document and document

   The format of this header field parameter is a quoted string. The
   value of this string is the portion of an XCAP URI to the right of
   the directory for the user, in the case of user data, or to the right
   of the global directory for global data. The XCAP URI can only refer
   to a document or to an element within that document. When the URI
   represents a document, the subscription is to changes anywhere in the
   document. When it is to an element, the subscription is to changes
   that occur in the attributes or content of that element, including
   all children. For example, if a user wishes to subscribe to http://,
   the event header parameter would be "mydir/friends.xml".

      OPEN ISSUE: There is no way to specify that a subscription is to
      multiple documents. Multiple subscriptions would be needed for
      that. Is that limitation OK for now? A filter can fix that down
      the road.

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2.3 SUBSCRIBE Bodies

   A SUBSCRIBE request MAY contain a body. The purpose of the body
   depends on its type. Subscriptions will normally not contain bodies.
   The Request-URI, which identifies the user whose data is being
   subscribed to, combined with the event package name and parameter, is
   sufficient for this package.

   One type of body that can be included in a SUBSCRIBE request is a
   filter document. These filters request that only document change
   events generate notifications, or would ask for a restriction on the
   set of data returned in NOTIFY requests. Filter documents are not
   specified in this document, and at the time of writing, are expected
   to be the subject of future standardization activity.

   Honoring of these filters is at the policy discretion of the

   If the SUBSCRIBE request does not contain a filter, this tells the
   notifier that no filter is to be applied. The notifier SHOULD send
   NOTIFY requests at the discretion of its own policy.

2.4 Subscription Duration

   Generally speaking, changes to application configuration data are
   relatively infrequent. Of course, this depends on the type of
   application, but generally configuration data is static. As a result,
   notifications are expected infrequently, and subscriptions will
   typically be held for long periods of time. This would argue for long
   subscription refresh intervals. For this reason, the default
   subscription duration is two hours. Of course, a different duration
   can be requested by a client, or set by a server, using the Expires
   header field, as per RFC 3265 [2].

2.5 NOTIFY Bodies

   As described in RFC 3265 [2], the NOTIFY message will contain bodies
   that describe the state of the subscribed resource. This body is in a
   format listed in the Accept header field of the SUBSCRIBE, or a
   package-specific default if the Accept header field was omitted from

   In this event package, the body of the notification contains a
   document change document. This document describes the current version
   of an XML document managed by XCAP, in addition to the changes in
   this document from the previous version. Note that a listing of
   changes from the previous version is only sent in a NOTIFY triggered
   by a change to the document. NOTIFY requests sent in response to an

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   initial SUBSCRIBE, or a SUBSCRIBE refresh, only indicate the current
   version of the XML document. They do not contain the actual full
   contents of the XML document. In other words, the resource being
   subscribed to is NOT the XML document itself, but rather, the version
   history for the document.

      OPEN ISSUE: This is the main issue in this specification. There
      are three potential scopes. The first is that subscription is to
      the document itself, in which case a full state update in a NOTIFY
      contains the current document. The second is a subscription to the
      revision history, which gives you changes, but not the full state
      of the document. The third option is to just subscribe to the
      etag, so that you know that something changed, but the
      notifications don't tell you anything about what changed. Which do
      we need? The latter is the simplest, good for the case where third
      party modification of documents is rare.

   All subscribers and notifiers MUST support the "application/
   xcap-change+xml" data format described in Section 3. The subscribe
   request MAY contain an Accept header field. If no such header field
   is present, it has a default value of "application/xcap-change+xml".
   If the header field is present, it MUST include "application/
   xcap-change+xml", and MAY include any other types capable of
   representing changes in XCAP documents.

2.6 Notifier Processing of SUBSCRIBE Requests

   This subsection defines package-specific processing at the notifier
   when it receives a SUBSCRIBE request. General processing rules for
   requests are covered in Section 8.2 of RFC 3261 [1], in addition to
   general SUBSCRIBE processing in RFC 3265 [2].

   A notifier for this package SHOULD authenticate all subscribers.
   Generally, subscribers will have a pre-existing relationship with the
   notifier. This is because the principle application of this package
   is for a client of XCAP (which will have a relationship with the XCAP
   server) to find out about changes in cached documents. Therefore, the
   HTTP Digest mechanism in SIP is a good match for authentication, and
   MUST be supported by all clients and servers. Note that this
   authentication mechanism is already mandatory for all SIP-compliant

   Once authenticated, the server SHOULD authorize the subscriber.
   Generally, this authorization policy SHOULD mirror the authorization
   policy defined in an XCAP application usage for read access. Thats
   because this package provides a form of read-access, and the
   permissions should not differ based on whether the read is performed
   with XCAP or with a SIP SUBSCRIBE request.

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2.7 Notifier Generation of NOTIFY Requests

   RFC 3265 details the formatting and structure of NOTIFY messages.
   However, packages are mandated to provide detailed information on
   when to send a NOTIFY, how to compute the state of the resource, how
   to generate neutral or fake state information, and whether state
   information is complete or partial. This section describes those
   details for the presence event package.

   A notifier MAY send a notification at any time. Typically, it will
   send one after a document managed by an XCAP server has changed as
   the result of an XCAP operation. This notification contains an
   application/xcap-change+xml document that specifies the current
   version (as a server modification time) for the XML document being
   subscribed to. It also contains information about what changed -
   whether a new element or attribute was added, whether an existing one
   changed, or whether an existing one was deleted, and indicates
   against which version those changes were made. The xcap-change
   document also contains a hash of the new XML document.

   Notifications sent in response to SUBSCRIBE requests (either initial
   or refresh), or sent when there is a change in subscription state,
   will normally only contain the current version of the XML document
   being subscribed to.

   The body of the NOTIFY MUST be sent using one of the types listed in
   the Accept header field in the most recent SUBSCRIBE request, or
   using the type "application/xcap-change+xml" if no Accept header
   field was present.

2.8 Subscriber Processing of NOTIFY Requests

   RFC 3265 [2] leaves it to event packages to describe the process
   followed by the subscriber upon receipt of a NOTIFY request,
   including any logic required to form a coherent resource state.

   In this package, the notifications can be optionally used by the
   client to determine the state of the XML document being subscribed
   to. When a client receives a notification, it checks the version
   against which the changes are relative. If this is not the same as
   the version currently cached by the client, the client SHOULD use
   XCAP to fetch the latest version of the document. If it is the same,
   the client applies the change to its local cache of the document. To
   apply the changes, the client follows the procedures defined by XCAP
   [10] as if it were the HTTP server. After applying the changes, the
   client applies the mandatory XML canonicalization defined in the
   Canonical XML 1.0 [3] specification, and computes an HMAC [12] using
   SHA1 over this canonical document, with a key whose value is 0x2238a.

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   The resulting string is compared with the hash attribute of the
   xml-change document. If they match, the client can be sure that it
   has the most up to date version. If they don't match, the client
   SHOULD fetch the most current version of the document.

   Of course, this mechanism for computing the most current document
   from the hash is optional. A client can elect to ignore the
   information on what changed and simply fetch the most recent document
   every time it gets a change indication where the new version is not
   the same as the one cached by the client.

2.9 Handling of Forked Requests

   RFC 3265 [2] requires each package to describe handling of forked
   SUBSCRIBE requests.

   This specification only allows a single dialog to be constructed as a
   result of emitting an initial SUBSCRIBE request. Section 4.4.9 of RFC
   3265 [2] describes the processing that is required to guarantee the
   creation of a single dialog in response to a SUBSCRIBE request.

2.10 Rate of Notifications

   RFC 3265 [2] requires each package to specify the maximum rate at
   which notifications can be sent. A notifier SHOULD NOT generate
   notifications at a rate of more than once every five seconds.

2.11 State Agents

   RFC 3265 [2] requires each package to consider the role of state
   agents in the package, and if they are used, to specify how
   authentication and authorization are done.

   State agents play no role in this package.

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3. application/xml-change+xml Media Type

   An xml-change document is an XML [4] document that MUST be
   well-formed and SHOULD be valid. XML-change documents MUST be based
   on XML 1.0 and MUST be encoded using UTF-8. This specification makes
   use of XML namespaces for identifying xml-change documents and
   document fragments. The namespace URI for elements defined by this
   specification is a URN [5], using the namespace identifier 'ietf'
   defined by [7] and extended by [8]. This URN is:


   An xml-change document begins with the root element tag "documents".
   It consists of any number of "document" sub-elements, each of which
   conveys information on a particular document. Other elements from
   different namespaces MAY be present for the purposes of
   extensibility; elements or attributes from unknown namespaces MUST be

   Each "document" element consists of zero or more "change" elements,
   each of which conveys information about a specific change to the
   document. Other elements from different namespaces MAY be present for
   the purposes of extensibility; elements or attributes from unknown
   namespaces MUST be ignored. There are four attributes associated with
   the "document" element:

   uri: specifies the HTTP URI that identifies the document.

   new-etag: specifies the etag of the document after the application of
      the change. This attribute is mandatory.

   previous-etag: specifies the etag of the version of the document
      against which the change was made. This attribute MUST be present
      if any change sub-elements are present.

   hash: specifies an HMAC of the new document, represented in canonical
      form. See Section 2.8 for details on how this value is computed.
      This attribute MUST be present if any change sub-elements are

   Each "change" element contains text. This text contains the exact
   value present in the HTTP request that caused the change in the
   document. If that content was XML, it is represented in the
   xml-change document as CDATA. There are two attributes associated
   with this element:

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   uri: contains the URI that the HTTP request contained in the
      Request-URI. This attribute is mandatory.

   method: contains the method of the HTTP request. This attribute is

      OPEN ISSUE: Probably it would be better to describe the changes
      more generically, rather than binding them to the specifics of
      XCAP. Indeed, if there is any non-determinism in how the server
      computes the document resulting from an XCAP operation, this
      approach will not work. We can either use patch or define a
      specific XML patch format.

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3.1 XML Schema

   <?xml version="1.0" encoding="UTF-8"?>
   <xs:schema targetNamespace="urn:ietf:params:xml:ns:xml-change"
   elementFormDefault="qualified" attributeFormDefault="unqualified">
    <xs:element name="documents">
       <xs:element name="tns:document" maxOccurs="unbounded">
          <xs:element name="tns:change" type="xs:string" maxOccurs="unbounded"/>
          <xs:any namespace="##other" processContents="lax" minOccurs="0"
         <xs:attribute name="tns:uri" type="xs:anyURI" use="required"/>
         <xs:attribute name="tns:new-etag" type="xs:string" use="required"/>
         <xs:attribute name="tns:previous-etag" type="xs:string" use="optional"/>
         <xs:attribute name="tns:hash" type="xs:string" use="optional"/>
       <xs:any namespace="##other" processContents="lax" minOccurs="0"
    <xs:simpleType name="httpMethod">
     <xs:restriction base="xs:string">
      <xs:enumeration value="PUT"/>
      <xs:enumeration value="DELETE"/>

3.2 Example Document

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

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   <?xml version="1.0" encoding="UTF-8"?>
   <documents xmlns:xsi="">
        hash="<hash value>">
       <change method="DELETE"

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4. Security Considerations

   The security considerations for this package are similar to those of
   XCAP. The configuration data can contain sensitive information, and
   both the client and the server need to authenticate each other. As
   such, a notifier for this package MUST support HTTP Digest to
   authenticate subscribers. Notifiers and subscribers MAY use SIPs S/
   MIME feature to provide authentication and message integrity.

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5. IANA Considerations

   There are several IANA considerations associated with this

5.1 SIP Event Package

   This specification registers an event package, based on the
   registration procedures defined in RFC 3265 [2]. The following is the
   information required for such a registration:

   Package Name: xml-change

   Package or Template-Package: This is a package.

   Published Document: RFC XXXX (Note to RFC Editor: Please fill in XXXX
      with the RFC number of this specification).

   Person to Contact: Jonathan Rosenberg,

5.2 application/xcap-change+xml MIME Type

      MIME media type name: application

      MIME subtype name: xcap-change+xml

      Mandatory parameters: none

      Optional parameters: Same as charset parameter application/xml as
      specified in RFC 3023 [6].

      Encoding considerations: Same as encoding considerations of
      application/xml as specified in RFC 3023 [6].

      Security considerations: See Section 10 of RFC 3023 [6] and
      Section 4 of this specification.

      Interoperability considerations: none.

      Published specification: This document.

      Applications which use this media type: This document type has
      been used to support manipulation of presence lists [13] using

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      Additional Information:

         Magic Number: None

         File Extension: .xcd or .xml

         Macintosh file type code: "TEXT"

         Personal and email address for further information: Jonathan

         Intended usage: COMMON

         Author/Change controller: The IETF.

5.3 URN Sub-Namespace Registration for

   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


                <?xml version="1.0"?>
                <!DOCTYPE html PUBLIC "-//W3C//DTD XHTML Basic 1.0//EN"
                <html xmlns="">
                  <meta http-equiv="content-type"
                  <title>XCAP Change Namespace</title>
                  <h1>Namespace for XCAP Change</h1>
                  <p>See <a href="[[[URL of published RFC]]]">RFCXXXX</a>.</p>

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Normative References

   [1]   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.

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

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

   [4]   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.

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

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

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

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

   [9]   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.

   [10]  Rosenberg, J., "The Extensible Markup Language (XML)
         Configuration Access Protocol (XCAP)",
         draft-ietf-simple-xcap-01 (work in progress), October 2003.

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Informative References

   [11]  Roach, A., Rosenberg, J. and B. Campbell, "A Session Initiation
         Protocol (SIP) Event Notification Extension for  Resource
         Lists", draft-ietf-simple-event-list-04 (work in progress),
         June 2003.

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

   [13]  Rosenberg, J., "An Extensible Markup Language (XML)
         Configuration Access Protocol (XCAP)  Usage for Presence
         Lists", draft-ietf-simple-xcap-list-usage-01 (work in
         progress), October 2003.

Author's Address

   Jonathan Rosenberg
   600 Lanidex Plaza
   Parsippany, NJ  07054

   Phone: +1 973 952-5000

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Intellectual Property Statement

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Full Copyright Statement

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   This document and the information contained herein is provided on an

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   Funding for the RFC Editor function is currently provided by the
   Internet Society.

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