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

Internet Engineering Task Force (IETF)                      J. Rosenberg
Request for Comments: 5874                                   jdrosen.net
Category: Standards Track                                  J. Urpalainen
ISSN: 2070-1721                                                    Nokia
                                                                May 2010


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

Abstract

   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 reports which document has changed and its
   former and new entity tags.  It can report the differences between
   versions of the document, using an XML patch format.  It can report
   existing element and attribute content when versions of an XCAP
   server document change.  XCAP diff documents can be delivered to diff
   clients using a number of means, including a Session Initiation
   Protocol (SIP) event package.

Status of This Memo

   This is an Internet Standards Track document.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Further information on
   Internet Standards is available in Section 2 of RFC 5741.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   http://www.rfc-editor.org/info/rfc5874.

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   document authors.  All rights reserved.

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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Terminology  . . . . . . . . . . . . . . . . . . . . . . . . .  4
   3.  Structure of an XCAP Diff Document . . . . . . . . . . . . . .  5
   4.  XML Schema . . . . . . . . . . . . . . . . . . . . . . . . . .  8
   5.  Example Document . . . . . . . . . . . . . . . . . . . . . . . 11
   6.  Basic Requirements for a System Exchanging XCAP Diff
       Documents  . . . . . . . . . . . . . . . . . . . . . . . . . . 11
   7.  Security Considerations  . . . . . . . . . . . . . . . . . . . 13
   8.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 14
     8.1.  application/xcap-diff+xml MIME Type  . . . . . . . . . . . 14
     8.2.  URN Sub-Namespace Registration for
           urn:ietf:params:xml:ns:xcap-diff . . . . . . . . . . . . . 15
     8.3.  Schema Registration  . . . . . . . . . . . . . . . . . . . 15
   9.  Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . . 16
   10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 16
     10.1. Normative References . . . . . . . . . . . . . . . . . . . 16
     10.2. Informative References . . . . . . . . . . . . . . . . . . 17
   Appendix A.  Informative Examples  . . . . . . . . . . . . . . . . 18
     A.1.  Indicating Existing, Changed, or Removed Documents . . . . 18
     A.2.  Indicating Actual Changes of Documents . . . . . . . . . . 21
     A.3.  Indicating XCAP Component Contents . . . . . . . . . . . . 23

1.  Introduction

   The Extensible Markup Language (XML) Configuration Access Protocol
   (XCAP) [RFC4825] is a protocol that allows XCAP clients to manipulate
   XML documents stored on a server.  These XML documents serve as
   configuration information for application protocols.  As an example,
   resource list [RFC4662] subscriptions (also known as presence lists)
   allow a SIP 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 SIP client.
   This application requires the server, called a Resource List Server
   (RLS), to have access to the list of presentities [RFC2778].  This
   list needs to be manipulated by XCAP clients so they can add and
   remove their friends as they desire.

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

   To deal with this problem, clients can use a Session Initiation
   Protocol (SIP) [RFC3261] event package [RFC3265] to subscribe to
   change events [RFC5875] in XCAP documents.  This notification needs
   to indicate the specific resource that changed and how it changed.
   One solution for the format of such a change notification would be a
   content indirection object [RFC4483].  Though content indirection can
   tell a client that a document has changed, it provides it with a 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 XCAP client made at around the
   same time.  Furthermore, content indirections don't indicate how a
   document changed; they are only able to indicate that it did change.

   To resolve these problems, this document defines a data format that
   can convey the fact that an XML document managed by XCAP has changed.
   This data format is an XML document format, called an XCAP diff
   document.  This format reports which document has changed and its
   former and new entity tags.  It can report the differences between
   versions of the document, using an XML patch format [RFC5261], which
   indicate how to transform the locally cached XCAP document from the
   version prior to the change to the version after it.  Its intent is
   to reduce the required overall bandwidth and the number of separate
   transmissions.  It can also report existing element and attribute
   content when versions of an XML document change at an XCAP server.

   XML documents that are equivalent for the purposes of many
   applications may differ in their physical representation.  Similar to
   XCAP, the canonical form with comments [W3C.REC-xml-c14n-20010315] of
   an XML document determines the logical equivalence when this format
   is used to patch locally cached XCAP documents.

2.  Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in RFC 2119 [RFC2119] 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
      which the XCAP diff document is reporting a change.

   Diff document:  The XML document defined by this specification that
      reports on a set of changes in an XCAP document resource.  It is
      delivered from a server to a diff client by a transport that is
      not defined by this specification.

   XCAP server:  A protocol entity that manages XCAP documents and their
      entity tags.  It usually contains an integrated diff notifier.

   Diff notifier:  This is the entity of a server that generates XCAP
      diff documents based on its knowledge of a set of XCAP documents
      and their changes, and it transmits the generated diff documents
      to a diff client within a session.

   Diff client:  A client that consumes XCAP diff documents in order to
      construct a locally cached document that is equivalent to a
      specific version of a document resource stored at an XCAP server.
      It is typically a SIP User Agent (UA) and an XCAP client.

   XCAP Client:  A client that updates and retrieves documents stored at
      an XCAP server.  It can also patch element and attribute content
      of XCAP documents located at an XCAP server.

   Locally cached resource:  A resource that has typically been
      downloaded by HTTP from an XCAP server to a diff client.  It may
      have been patched locally by a diff client based on the XCAP diff
      document information.  It is equivalent to a single version in its
      change history at an XCAP server.  Version history of XCAP
      documents is indicated by HTTP entity tags (ETags).

   ETag:  A strong HTTP entity tag whose value is set by an XCAP server.
      Documents at an XCAP server are updated by XCAP clients.  The XCAP
      server assigns a new ETag value to each document version according
      to the HTTP specification.

3.  Structure of an XCAP Diff Document

   An XCAP diff document is an XML [W3C.REC-xml-20060816] 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 [RFC2141], using the namespace identifier
   'ietf' defined by [RFC2648] and extended by [RFC3688].  This URN is:

      urn:ietf:params:xml:ns:xcap-diff

   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
   only represent changes in documents within the same XCAP root.  The
   content of the <xcap-diff> element is a sequence of <document>,
   <element>, and <attribute> elements followed by any number of
   elements from other namespaces for the purposes of extensibility.
   Wherever the XML schema (see Section 4) allows extension elements or
   attributes, any such unknown content MUST be ignored by the diff
   client.

   Each <document> element specifies changes in a specific document
   within the XCAP root.  If several <document> elements pinpoint the
   same specific document, i.e., for example, the full entity tag (ETag)
   change history is indicated, the corresponding patches MUST be able
   to be applied in the given XCAP diff document order.

      Note: This requirement simplifies applications that process XCAP
      diff documents since there's no need to sort patch instructions
      when applying them.

   The <document> element has one mandatory attribute, "sel", and two
   optional attributes, "new-etag" and "previous-etag".  The "sel"
   attribute of the <document> element 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 entity tag
   (ETag) for the document after the application of the changes,
   assuming the document exists after those changes.  The "previous-
   etag" attribute provides an identifier for the document instance
   prior to the change.  If the change being reported is the removal of
   a document, only the "previous-etag" MUST be included and the "new-
   etag" attribute MUST NOT be present.  The "new-etag" attribute MUST
   only exist alone when the document either exists or it was just
   created (no patch included).  Both attributes are present when a
   patch (or series of XCAP operations) has been applied to the
   resource.  Also, both attributes MAY be used to indicate an ETag
   change without any document modifications (patches).

   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
   only requirement then is that the sequence of events, when executed
   serially, will result in the transformation of the document with the
   ETag "previous-etag" to the one whose ETag is "new-etag".  Also, the
   series of operations do not have to be the same exact series of
   operations that occurred at the server.

   Each <document> element contains either a sequence of patching
   instructions or an indication that the body hasn't semantically
   changed.  The latter means that the document has been assigned a new
   ETag but its content is unchanged and it is indicated by the <body-
   not-changed> element.  Patching instructions are described by the
   <add>, <replace>, and <remove> elements.  These elements use the
   corresponding add, replace, and remove types defined in [RFC5261],
   and define a set of patch operations that can be applied to transform
   the locally cached document.  See [RFC5261] for instructions on how
   this transformation is effected.  The <document> element can also
   contain elements from other namespaces for the purposes of
   extensibility.  The <add>, <replace>, and <remove> elements allow
   extension attributes from any namespace.

   Figure 1 shows <document> element content and how the corresponding
   resource or metadata changes.  In practice, an external document
   retrieval means HTTP GET requests for target resources.  The asterisk
   character '*' means that a <document> element has child element(s):
   <add>, <replace>, or <remove>, or alternatively only a <body-not-
   changed> element.  The hyphen character '-' means that the
   corresponding content (attribute or element) doesn't exist in a
   <document> element.  The 'xxx' and 'yyy' are values of entity tags
   (ETag) of an XCAP document.

   +-----------+----------+-----------+----------+-------------------+
   | previous- | new-     | <add>     | <body-   | locally cached    |
   | etag      | etag     | <replace> | not-     | XCAP resource/    |
   |           |          | <remove>  | changed> | metadata change   |
   +-----------+----------+-----------+----------+-------------------+
   | xxx       | yyy      | *         | -        | resource patched, |
   |           |          |           |          | patch included    |
   +-----------+----------+-----------+----------+-------------------+
   | xxx       | yyy      | -         | -        | resource patched, |
   |           |          |           |          | external document |
   |           |          |           |          | retrieval         |
   +-----------+----------+-----------+----------+-------------------+
   | xxx       | yyy      | -         | *        | only ETag changed |
   +-----------+----------+-----------+----------+-------------------+
   | -         | yyy      | -         | -        | resource created  |
   |           |          |           |          | or exists,        |
   |           |          |           |          | external document |
   |           |          |           |          | retrieval         |
   +-----------+----------+-----------+----------+-------------------+
   | xxx       | -        | -         | -        | resource removed  |
   +-----------+----------+-----------+----------+-------------------+

   Figure 1: <document> element content / corresponding resource changes

   Each <element> element indicates the existing element content of an
   XCAP document.  It has one mandatory attribute, "sel", and
   optionally, an "exists" attribute and extension attributes from any
   namespace.  The "sel" attribute of the <element> element identifies
   an XML element of an XCAP document.  It is a percent-encoded relative
   URI following XCAP conventions when selecting elements.  The XCAP
   Node Selector MUST always locate a unique node, the "exists"
   attribute thus shows whether an element exists or not in the XCAP
   document.  When the "exists" attribute is absent from the <element>
   element, the indicated element still exists in the XCAP document.
   The located element exists as a child element of the <element>
   element.  In a corner case where the content of this element cannot
   be presented for some reason (e.g., the payload is too large)
   although it exists in the XCAP document, the <element> element MUST
   NOT have any child nodes.

   As the located XML element is typically namespace qualified, all
   needed namespace declarations MUST exist within the <xml-diff>
   document.  The possible local namespace declarations within the
   located element exist unmodified as in the source document, similar
   to XCAP conventions.  Other namespace references MUST be resolved
   from the context of the <element> or its parent elements.  The
   prefixes of qualified names (QNames) [W3C.REC-xml-names-20060816] of
   XML nodes also remain as they originally exist in the source XCAP
   document.

   Each <attribute> element indicates the existing attribute content of
   an XCAP document.  It has one mandatory attribute, "sel", and
   optionally, an "exists" attribute and extension attributes from any
   namespace.  The "sel" attribute of the <attribute> element identifies
   an XML attribute of an XCAP document.  It is a percent-encoded
   relative URI following XCAP conventions when selecting attributes.
   The "exists" attribute indicates whether or not an attribute exists
   in the XCAP document.  When the "exists" attribute is absent from the
   <attribute> element, the indicated attribute still exists in the XCAP
   document.  The child text node of the <attribute> element indicates
   the value of the located attribute.  Note that if the attribute is
   namespace qualified, the query parameter of the XCAP URI indicates
   the attached namespace URI and the prefix in the XCAP source
   document.

   Namespaces of the "sel" attribute of the <attribute> and <element>
   elements MUST also be resolved properly.  Section 6.4. of [RFC4825]
   describes the rules when using namespace prefixes in XCAP Node
   Selectors.  Without a namespace prefix in an element selector, an
   XCAP Default Document Namespace MUST be applied.  The namespace
   resolving rules of Patch operation elements: <add>, <replace>, and
   <remove> are described in Section 4.2.1 of [RFC5261].

4.  XML Schema

   The XML Schema for the XCAP diff format.

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

      <!-- include patch-ops -->
      <xs:include
       schemaLocation="urn:ietf:params:xml:schema:patch-ops"/>

      <!-- document root -->
      <xs:element name="xcap-diff">
       <xs:complexType>
        <xs:sequence minOccurs="0">
         <xs:sequence minOccurs="0" maxOccurs="unbounded">
          <xs:choice>
           <xs:element name="document" type="documentType"/>
           <xs:element name="element" type="elementType"/>
           <xs:element name="attribute" type="attributeType"/>
          </xs:choice>
         </xs:sequence>
         <xs:any namespace="##other" processContents="lax"
                 minOccurs="0" maxOccurs="unbounded"/>
        </xs:sequence>
        <xs:attribute name="xcap-root" type="xs:anyURI" use="required"/>
        <xs:anyAttribute processContents="lax"/>
       </xs:complexType>
      </xs:element>

      <!-- xcap document type -->
      <xs:complexType name="documentType">
       <xs:choice minOccurs="0">
        <xs:element name="body-not-changed" type="emptyType"/>
        <xs:sequence minOccurs="0" maxOccurs="unbounded">
         <xs:choice>
          <xs:element name="add">
           <xs:complexType mixed="true">
            <xs:complexContent>
             <xs:extension base="add">
              <xs:anyAttribute processContents="lax"/>
             </xs:extension>
            </xs:complexContent>
           </xs:complexType>
          </xs:element>
          <xs:element name="remove">
           <xs:complexType>
            <xs:complexContent>
             <xs:extension base="remove">
              <xs:anyAttribute processContents="lax"/>
             </xs:extension>
            </xs:complexContent>
           </xs:complexType>
          </xs:element>
          <xs:element name="replace">
           <xs:complexType mixed="true">
            <xs:complexContent>
             <xs:extension base="replace">
              <xs:anyAttribute processContents="lax"/>
             </xs:extension>
            </xs:complexContent>
           </xs:complexType>
          </xs:element>
          <xs:any namespace="##other" processContents="lax"/>
         </xs:choice>
        </xs:sequence>
       </xs:choice>
       <xs:attribute name="sel" type="xs:anyURI" use="required"/>
       <xs:attribute name="new-etag" type="xs:string"/>
       <xs:attribute name="previous-etag" type="xs:string"/>
       <xs:anyAttribute processContents="lax"/>
      </xs:complexType>

      <!-- xcap element type -->
      <xs:complexType name="elementType">
       <xs:complexContent mixed="true">
        <xs:restriction base="xs:anyType">
         <xs:sequence>
          <xs:any processContents="lax" namespace="##any"
                  minOccurs="0" maxOccurs="1"/>
         </xs:sequence>
         <xs:attribute name="sel" type="xs:string"
                       use="required"/>
         <xs:attribute name="exists" type="xs:boolean"/>
         <xs:anyAttribute processContents="lax"/>
        </xs:restriction>
       </xs:complexContent>
      </xs:complexType>

      <!-- xcap attribute type -->
      <xs:complexType name="attributeType">
       <xs:simpleContent>
        <xs:extension base="xs:string">
         <xs:attribute name="sel" type="xs:string"
                       use="required"/>
         <xs:attribute name="exists" type="xs:boolean"/>
         <xs:anyAttribute processContents="lax"/>
        </xs:extension>
       </xs:simpleContent>
      </xs:complexType>

      <!-- empty type -->
      <xs:complexType name="emptyType"/>
     </xs:schema>

5.  Example Document

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

   <?xml version="1.0" encoding="UTF-8"?>
   <d:xcap-diff xmlns:d="urn:ietf:params:xml:ns:xcap-diff"
                xmlns="urn:ietf:params:xml:ns:rls-services"
                xcap-root="http://xcap.example.com/root/">

    <d:document new-etag="7ahggs"
              sel="resource-lists/users/sip:joe@example.com/coworkers"
              previous-etag="8a77f8d"/>

    <d:element sel="rls-services/users/sip:joe@example.com/index/~~
   /*/service%5b@uri='sip:marketing@example.com'%5d"
             xmlns:rl="urn:ietf:params:xml:ns:resource-lists"
       ><service uri="sip:marketing@example.com">
         <list name="marketing">
           <rl:entry uri="sip:joe@example.com"/>
           <rl:entry uri="sip:sudhir@example.com"/>
         </list>
         <packages>
           <package>presence</package>
         </packages>
       </service></d:element>

    <d:attribute
    sel="rls-services/users/sip:joe@example.com/index/~~/*/service/@uri"
         >sip:marketing@example.com</d:attribute>

   </d:xcap-diff>

   This indicates that the document with the URI "http://
   xcap.example.com/root/resource-lists/users/sip:joe@example.com/
   coworkers" has changed.  Its previous entity tag is "8a77f8d" and its
   new one is "7ahggs", but actual changes are not shown.  The <service>
   element exists in the rls-services "index" document and its full
   content is shown.  Note that the <service> element is attached with a
   default namespace declaration within the original document.
   Similarly, "uri" attribute content is shown from the same "index"
   document as an illustrative example.

6.  Basic Requirements for a System Exchanging XCAP Diff Documents

   Documents at an XCAP server are identified by URIs, and updated by
   XCAP clients with HTTP (PUT and DELETE) methods.  The XCAP server
   assigns a new entity tag value for each document version.  An entity
   tag value is defined by Section 3.11 of RFC 2616 [RFC2616]: "An
   entity tag MUST be unique across all versions of all entities
   associated with a particular resource".  These entity tags are used
   to protect requests from making overriding changes when multiple XCAP
   clients update the same XCAP document.  An entity tag value can be
   interpreted as a unique identifier to a specific version of an XCAP
   document in its change history.

   The entity tag values of XCAP resources also enable a reliable way to
   update the locally cached XCAP resource copies in an XCAP diff
   implementation.  When a diff client applies XCAP diff document
   changes, it MUST apply a resource state change only if entity tag
   values match with octet-by-octet equivalence according to the table
   defined in Figure 1.  If a diff client notices inconsistencies and/or
   errors when it applies reported resource changes, it SHOULD tear down
   the session.

   State changes of an XCAP document MUST be delivered reliably from a
   diff notifier to a diff client, and a diff client MUST be able to
   apply all changes of an XCAP document in the same chronological order
   that occurred at an XCAP server.  When using an unreliable transport
   with retransmissions, the application protocol used with the XCAP
   diff MUST ensure that duplicates are dropped.  If an XCAP diff
   delivery is lost, the diff session MUST be torn down.  Note that a
   diff notifier can easily notice a lost notification when a diff
   client must respond to each XCAP diff delivery.

   A diff notifier doesn't necessarily report all of these XCAP document
   updates with ETags; it MAY skip over some intermediate version of a
   document, for example, with rapidly changing resources.  However, it
   MUST always report changes consistently to a diff client so that it
   can properly update the latest state (content and ETag) of its
   locally cached resources.

      As an example, an XCAP document is updated by different 'a', 'b',
      and 'c' versions identified with the same corresponding ETag
      values in a relatively short period.  The first reported
      notification contains the 'a' "new-tag" information (no "previous-
      etag" attribute), and the diff notifier decides to skip the update
      notification identified by the 'b' ETag value.  The second
      notification to a diff client MUST then contain the 'a' "previous-
      etag" and 'c' "new-etag" values with optional corresponding
      content changes (from version 'a' to 'c').

   Since XCAP documents are typically confidential, diff notifiers MUST
   obey the XCAP authorization rules.  In practice, this means following
   the read privilege rules of XCAP resources when notifying the
   authenticated diff clients of changes.  Transport SHOULD be secured
   by encryption.

      Note: This format specification doesn't define how to select the
      resources whose differences a diff notifier should report.  It
      also doesn't define whether actual content changes should be
      reported.  Typically, however, a diff client starts a session by
      sending a resource listing request.  Then it compares the remote
      resource listings with locally cached ones, and probably downloads
      those resources that aren't locally cached or whose entity tags
      differ.  When a diff client receives an XCAP diff with a
      "previous-etag" value that matches its current cached copy of a
      document, it can apply the diffs to the cached copy.  As it takes
      some time to download reference documents, and diff notifications
      appear after actual resource state changes, several round trips
      may be needed before a full synchronization is achieved,
      especially with rapidly changing resources.

7.  Security Considerations

   XCAP diff documents can include changes from one version of a
   document to another version.  As a consequence, if the document
   itself is sensitive and requires confidentiality, integrity, or
   authentication, then the same applies to the XCAP diff format.
   Therefore, protocols that transport XCAP diff documents must provide
   sufficient security capabilities for transporting the document
   itself.  Confidential XCAP documents are typically transported using
   TLS-encrypted (Transport Layer Security) [RFC5246] communication; see
   RFC 4825 [RFC4825] for further security details.

   When this format is used to report content changes of XCAP documents,
   all security considerations of RFC 5261 [RFC5261] apply.  Very
   frequent updates of XCAP documents and/or many diff clients per
   subscribed resource impose a Denial-of-Service attack possibility to
   the servers processing XCAP diff documents.  An efficient patch
   processing and throttling can, however, decrease the required overall
   processings and transactions.

   The SIP event package framework specified in RFC 3265 [RFC3265] is
   the most typical use-case for this format.  Then, an end-to-end SIP
   encryption mechanism, such as Secure/Multipurpose Internet Mail
   Extensions (S/MIME) described in Section 26.2.4 of RFC 3261
   [RFC3261], SHOULD be used.  If that is not available, it is
   RECOMMENDED that TLS [RFC5246] be used between elements to provide
   hop-by-hop authentication and encryption mechanisms as described in
   Section 26.2.2 ("SIPS URI Scheme") and Section 26.3.2.2 ("Interdomain
   Requests") of RFC 3261 [RFC3261].  Event packages MAY also have other
   specific threats that MUST be considered on an application-by-
   application basis.

8.  IANA Considerations

   There are several IANA considerations associated with this
   specification.

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 the charset parameter application/xml as
   specified in RFC 3023 [RFC3023].

   Encoding considerations: Same as the encoding considerations of
   application/xml as specified in RFC 3023 [RFC3023].

   Security considerations: See Section 10 of RFC 3023 [RFC3023] and
   Section 7 of RFC 5874.

   Interoperability considerations: none.

   Published specification: This document.

   Applications that use this media type: This document type has been
   used to support manipulation of resource lists [RFC4826] using XCAP.

   Additional Information:

      Magic Number: None

      File Extension: .xdf

      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.

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
   [RFC3688].

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

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

      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 Diff Namespace</title>
  </head>
  <body>
    <h1>Namespace for XCAP Diff</h1>
    <h2>urn:ietf:params:xml:ns:xcap-diff</h2>
    <p>See <a
       href="http://www.rfc-editor.org/rfc/rfc5874.txt">RFC5874</a>.</p>
  </body>
  </html>
  END

8.3.  Schema Registration

   This section registers a new XML schema per the procedures in
   [RFC3688].

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

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

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

9.  Acknowledgments

   The authors would like to thank Pavel Dostal, Jeroen van Bemmel,
   Martin Hynar, Anders Lindgren, Mary Barnes, Ben Campbell, Francis
   Dupont, David Harrington, Alexey Melnikov, Dan Romascanu, and Robert
   Sparks for their valuable comments.

10.  References

10.1.  Normative References

   [W3C.REC-xml-20060816]
             Paoli, J., Bray, T., Yergeau, F., Maler, E., and C.
             Sperberg-McQueen, "Extensible Markup Language (XML) 1.0
             (Fourth Edition)", World Wide Web Consortium
             FirstEdition REC-xml- 20060816, August 2006, <http://
             www.w3.org/TR/2006/REC-xml-20060816>.

   [W3C.REC-xml-c14n-20010315]
             Boyer, J., "Canonical XML Version 1.0", World Wide Web
             Consortium Recommendation REC-xml-c14n-20010315,
             March 2001, <http://www.w3.org/TR/2001/
             REC-xml-c14n-20010315>.

   [W3C.REC-xml-names-20060816]
              Hollander, D., Layman, A., and T. Bray, "Namespaces in XML
             1.0 (Second Edition)", World Wide Web Consortium
             FirstEdition REC-xml-names-20060816, August 2006,
             <http://www.w3.org/TR/ 2006/REC-xml-names-20060816>.

   [RFC2141] Moats, R., "URN Syntax", RFC 2141, May 1997.

   [RFC2616] Fielding, R., Gettys, J., Mogul, J., Frystyk, H., Masinter,
             L., Leach, P., and T. Berners-Lee, "Hypertext Transfer
             Protocol -- HTTP/1.1", RFC 2616, June 1999.

   [RFC3023] Murata, M., St. Laurent, S., and D.  Kohn, "XML Media
             Types", RFC 3023, January 2001.

   [RFC2648] Moats, R., "A URN Namespace for IETF Documents", RFC 2648,
             August 1999.

   [RFC3688] Mealling, M., "The IETF XML Registry", BCP 81, RFC 3688,
             January 2004.

   [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
             Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC4825] Rosenberg, J., "The Extensible Markup Language (XML)
             Configuration Access Protocol (XCAP)", RFC 4825, May 2007.

   [RFC5261] Urpalainen, J., "An Extensible Markup Language (XML) Patch
             Operations Framework Utilizing XML Path Language (XPath)
             Selectors", RFC 5261, September 2008.

   [RFC5246] Dierks, T. and E. Rescorla, "The Transport Layer Security
             (TLS) Protocol Version 1.2", RFC 5246, August 2008.

10.2.  Informative References

   [RFC5875] Urpalainen, J. and D. Willis, "An Extensible Markup
             Language (XML) Configuration Access Protocol (XCAP) Diff
             Event Package", RFC 5875, May 2010.

   [RFC2778] Day, M., Rosenberg, J., and H. Sugano, "A Model for
             Presence and Instant Messaging", RFC 2778, February 2000.

   [RFC3261] 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.

   [RFC3265] Roach, A., "Session Initiation Protocol (SIP)-Specific
             Event Notification", RFC 3265, June 2002.

   [RFC4662] Roach, A., Campbell, B., and J.  Rosenberg, "A Session
             Initiation Protocol (SIP) Event Notification Extension for
             Resource Lists", RFC 4662, August 2006.

   [RFC4826] Rosenberg, J., "Extensible Markup Language (XML) Formats
             for Representing Resource Lists", RFC 4826, May 2007.

   [RFC4483] Burger, E., "A Mechanism for Content Indirection in Session
             Initiation Protocol (SIP) Messages", RFC 4483, May 2006.

Appendix A.  Informative Examples

   These informative examples illustrate basic features of XCAP diff
   format.

   The following documents exist at an XCAP server (xcap.example.com)
   with an imaginary "tests" application usage (there's no default
   document namespace defined in this imaginary application usage).

   http://xcap.example.com/tests/users/sip:joe@example.com/index:

   <?xml version="1.0" encoding="UTF-8"?>
   <doc id="bar">
     <note>This is a sample document</note>
   </doc>

   and then

   http://xcap.example.com/tests/users/sip:john@example.com/index:

   <?xml version="1.0" encoding="UTF-8"?>
   <doc>
     <note>This is another sample document</note>
   </doc>

A.1.  Indicating Existing, Changed, or Removed Documents

   Firstly, an XCAP diff document can indicate what documents exist in a
   collection.  An XCAP diff document may then be:

   <?xml version="1.0" encoding="UTF-8"?>
   <xcap-diff xmlns="urn:ietf:params:xml:ns:xcap-diff"
              xcap-root="http://xcap.example.com/">

    <document new-etag="7ahggs"
              sel="tests/users/sip:joe@example.com/index"/>

    <document new-etag="terteer"
              sel="tests/users/sip:john@example.com/index"/>

   </xcap-diff>

   This listing indicates current ETags of existing documents and their
   relative URIs.

   Let's say that Joe adds a new document to his collection:

   PUT /tests/users/sip:joe@example.com/another_document HTTP/1.1
   Host: xcap.example.com
   ....
   Content-Type: application/xml
   Content-Length: [XXX]

   <?xml version="1.0" encoding="UTF-8"?>
   <doc>
     <note>This is another sample document</note>
   </doc>

   The requests result header has an HTTP ETag "terteer" for this new
   document.

   Then an XCAP diff document may then indicate only the creation of
   this single new document:

   <?xml version="1.0" encoding="UTF-8"?>
   <xcap-diff xmlns="urn:ietf:params:xml:ns:xcap-diff"
              xcap-root="http://xcap.example.com/">

    <document new-etag="terteer"
              sel="tests/users/sip:joe@example.com/another_document"/>

   </xcap-diff>

   A "new-etag" without a "previous-etag" attribute indicates a creation
   of a new document.

   Then Joe decides to modify an existing resource:

   PUT /tests/users/sip:joe@example.com/another_document HTTP/1.1
   Host: xcap.example.com
   ....
   Content-Type: application/xml
   Content-Length: [XXX]

   <?xml version="1.0" encoding="UTF-8"?>
   <doc>
     <note>This is a modified document</note>
   </doc>

   The reported new HTTP ETag is "huwiias".

   Then an XCAP diff document may be:

   <?xml version="1.0" encoding="UTF-8"?>
   <xcap-diff xmlns="urn:ietf:params:xml:ns:xcap-diff"
              xcap-root="http://xcap.example.com/">

    <document previous-etag="terteer" new-etag="huwiias"
              sel="tests/users/sip:joe@example.com/another_document"/>

   </xcap-diff>

   Both "previous-etag" and "new-etag" attributes signal that a
   modification has happened to a resource, but actual changes are not
   shown.

   Let's say that Joe then removes a document from his collection:

   DELETE /tests/users/sip:joe@example.com/another_document HTTP/1.1
   Host: xcap.example.com

   This HTTP DELETE request results in the unlinking of the resource,
   and the XCAP diff may be:

   <?xml version="1.0" encoding="UTF-8"?>
   <xcap-diff xmlns="urn:ietf:params:xml:ns:xcap-diff"
              xcap-root="http://xcap.example.com/">

    <document previous-etag="huwiias"
              sel="tests/users/sip:joe@example.com/another_document"/>

   </xcap-diff>

   Thus, a "previous-etag" without a "new-etag" attribute indicates the
   removal of a resource.

A.2.  Indicating Actual Changes of Documents

   Secondly, XCAP diff documents are capable of showing actual changes
   to documents with [RFC5261] patching semantics.

   Now Joe's XCAP client utilizes the XCAP patching capability to add a
   new element to a document:

   PUT /tests/users/sip:joe@example.com/index/~~/doc/foo HTTP/1.1
   Host: xcap.example.com
   ....
   Content-Type: application/xcap-el+xml
   Content-Length: [XXX]

   <foo>this is a new element</foo>

   Since the insertion of the element is successful, Joe's XCAP client
   receives the new HTTP ETag "fgherhryt3" of the updated "index"
   document.

   Immediately thereafter, Joe's XCAP client issues another HTTP request
   (this request could even be pipelined):

   PUT /tests/users/sip:joe@example.com/index/~~/doc/bar HTTP/1.1
   Host: xcap.example.com
   ....
   Content-Type: application/xcap-el+xml
   Content-Length: [XXX]

   <bar>this is a bar element
   </bar>

   The reported new HTTP ETag of "index" is now "dgdgdfgrrr".

   And then Joe's XCAP client issues yet another HTTP request:

   PUT /tests/users/sip:joe@example.com/index/~~/doc/foobar HTTP/1.1
   Host: xcap.example.com
   ....
   Content-Type: application/xcap-el+xml
   Content-Length: [XXX]

   <foobar>this is a foobar element</foobar>

   The reported new ETag of "index" is now "63hjjsll".

   XCAP diff format document may then indicate these XCAP component
   changes by:

   <?xml version="1.0" encoding="UTF-8"?>
   <d:xcap-diff xmlns:d="urn:ietf:params:xml:ns:xcap-diff"
                xcap-root="http://xcap.example.com/">

    <d:document previous-etag="7ahggs3"
                sel="tests/users/sip:joe@example.com/index"
                new-etag="63hjjsll">
     <d:add sel="*"
       ><foo>this is a new element</foo><bar>this is a bar element
   </bar><foobar>this is a foobar element</foobar></d:add>
    </d:document>

   </d:xcap-diff>

   Note how several XCAP component modifications were aggregated
   together, and full history information got lost.

   Alternatively, the content could have been:

   <?xml version="1.0" encoding="UTF-8"?>
   <d:xcap-diff xmlns:d="urn:ietf:params:xml:ns:xcap-diff"
              xcap-root="http://xcap.example.com/">

    <d:document previous-etag="7ahggs"
                sel="tests/users/sip:joe@example.com/index"
                new-etag="fgherhryt3">
      <d:add sel="*"
       ><foo>this is a new element</foo></d:add></d:document>

    <d:document previous-etag="fgherhryt3"
                sel="tests/users/sip:joe@example.com/index"
                new-etag="dgdgdfgrrr">
      <d:add sel="*"
       ><bar>this is a bar element
   </bar></d:add></d:document>

    <d:document previous-etag="dgdgdfgrrr"
                sel="tests/users/sip:joe@example.com/index"
                new-etag="63hjjsll">
      <d:add sel="*"
       ><foobar>this is a foobar element</foobar></d:add></d:document>

   </d:xcap-diff>
   This shows the full ETag change history of a document, and ETags
   change chronologically in the reported XML document order.

A.3.  Indicating XCAP Component Contents

   Lastly, the XCAP diff format can also indicate the existing full
   contents of XCAP components, i.e., elements or attributes:

   <?xml version="1.0" encoding="UTF-8"?>
   <d:xcap-diff xmlns:d="urn:ietf:params:xml:ns:xcap-diff"
              xcap-root="http://xcap.example.com/">

    <d:attribute sel="tests/users/sip:joe@example.com/index/~~/doc/@id"
     >bar</d:attribute>

    <d:element sel="tests/users/sip:joe@example.com/index/~~/*/foo"
       ><foo>this is a new element</foo></d:element>

   </d:xcap-diff>

   Note that the HTTP ETag value of the new document is not shown as it
   is irrelevant for this use-case.

   Then Joe's XCAP client removes the "id" attribute:

   DELETE /tests/users/sip:joe@example.com/index/~~/doc/@id HTTP/1.1
   Host: xcap.example.com
   ....
   Content-Length: 0

   And the XCAP diff document may then be:

   <?xml version="1.0" encoding="UTF-8"?>
   <xcap-diff xmlns="urn:ietf:params:xml:ns:xcap-diff"
              xcap-root="http://xcap.example.com/">

    <attribute sel="tests/users/sip:joe@example.com/index/~~/doc/@id"
     exists="0"/>

   </xcap-diff>

   This indicates that the subscribed attribute was removed from the
   document.  The element content in this use-case may be discarded from
   the XCAP diff document, for example, when the size of XCAP diff
   document would be impractically large to the transport layer.

Authors' Addresses

   Jonathan Rosenberg
   jdrosen.net
   Monmouth, NJ
   US

   EMail: jdrosen@jdrosen.net
   URI:   http://www.jdrosen.net


   Jari Urpalainen
   Nokia
   Itamerenkatu 11-13
   Helsinki  00180
   Finland

   Phone: +358 7180 37686
   EMail: jari.urpalainen@nokia.com