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Versions: 00 01 02 03 04 05 06 rfc4235                                  
Internet Engineering Task Force                               SIPPING WG
Internet Draft                                              J. Rosenberg
                                                             dynamicsoft
                                                          H. Schulzrinne
                                                             Columbia U.
draft-ietf-sipping-dialog-package-00.txt
June 24, 2002
Expires: December 2002


   A Session Initiation Protocol (SIP) Event Package for Dialog State

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://www.ietf.org/ietf/1id-abstracts.txt

   To view the list Internet-Draft Shadow Directories, see
   http://www.ietf.org/shadow.html.


Abstract

   This document defines a dialog event package for the SIP Events
   architecture, along with a data format used in notifications for this
   package. The dialog package allows users to subscribe to another
   user, an receive notifications about the changes in state of INVITE
   initiated dialogs that the user is involved in.










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                           Table of Contents



   1          Introduction ........................................    4
   2          Dialog Event Package ................................    5
   2.1        Event Package Name ..................................    5
   2.2        Event Package Parameters ............................    5
   2.3        SUBSCRIBE Bodies ....................................    5
   2.4        Subscription Duration ...............................    5
   2.5        NOTIFY Bodies .......................................    6
   2.6        Notifier Processing of SUBSCRIBE Requests ...........    6
   2.7        Notifier Generation of NOTIFY Requests ..............    7
   2.7.1      The Dialog State Machine ............................    7
   2.7.2      Applying the state machine ..........................    9
   2.8        Subscriber Processing of NOTIFY Requests ............   10
   2.9        Handling of Forked Requests .........................   11
   2.10       Rate of Notifications ...............................   11
   2.11       State Agents ........................................   11
   3          Dialog Information Format ...........................   11
   3.1        Structure of Dialog Information .....................   12
   3.1.1      Dialog Element ......................................   12
   3.1.2      State ...............................................   13
   3.1.3      Local URI ...........................................   13
   3.1.4      Remote URI ..........................................   13
   3.1.5      Local Session Description ...........................   13
   3.1.6      Remote Session Description ..........................   13
   3.1.7      Remote Target .......................................   13
   3.1.8      Local CSeq ..........................................   14
   3.1.9      Remote CSeq .........................................   14
   3.1.10     Duration ............................................   14
   3.2        Constructing Coherent State .........................   14
   3.3        Schema ..............................................   15
   3.4        Example .............................................   18
   4          Security Considerations .............................   20
   5          IANA Considerations .................................   20
   5.1        application/dialog-info+xml MIME Registration .......   20
   5.2        URN Sub-Namespace Registration for
   urn:ietf:params:xml:ns:dialog-info .............................   21
   6          Acknowledgements ....................................   22
   7          Authors Addresses ...................................   22
   8          Normative References ................................   22
   9          Informative References ..............................   23





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1 Introduction

   The SIP Events framework [1] defines general mechanisms for
   subscription to, and notification of, events within SIP networks. It
   introduces the notion of a package, which is a specific
   "instantiation" of the events mechanism for a well-defined set of
   events. Packages have been defined for user presence [8], watcher
   information [9], and message waiting indicators [10], amongst others.
   Here, we define an event package for INVITE initiated dialogs.
   Dialogs refer to the SIP relationship established between two SIP
   peers [2].

   There are a variety of applications enabled through the knowledge of
   dialog state. Some examples include:

        Automatic Callback: In this basic PSTN application, user A calls
             user B. User B is busy. User A would like to get a callback
             when user B hangs up. When B hangs up, user A's phone
             rings. When A picks it up, they here ringing, and are being
             connected to B. In VoIP, this requires A to receive a
             notification when the dialogs at A are complete.

        Presence-Enabled Conferencing: In this application, a user A
             wishes to set up a conference call with users B and C.
             Rather than scheduling it, it is to be created
             automatically when A, B and C are all available. To do
             this, the server providing the application would like to
             know whether A, B and C are "online", not idle, and not in
             a phone call. Determining whether or not A, B and C are in
             calls can be done in two ways. In the first, the server
             acts as a call stateful proxy for users A, B and C, and
             therefore knows their call state. This won't always be
             possible, however, and it introduces scalability,
             reliability, and operational complexities. Rather, the
             server would subscriber to the dialog state of those users,
             and receive notifications as it changes. This enables the
             application to be provided in a distributed way; the server
             need not reside in the same domain as the users.

        IM Conference Alerts: In this application, a user can get an IM
             sent to their phone whenever someone joins a conference
             that the phone is involved in. The IM alerts are generated
             by an application separate from the conference server.

   In general, the dialog package allows for construction of distributed
   applications, where the application requires information on dialog
   state, but is not co-resident with the end user on which that state
   resides.



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2 Dialog Event Package

   This section provides the details for defining a SIP Events package,
   as specified by [1].

2.1 Event Package Name

   The name of this event package is "dialog". This package name is
   carried in the Event and Allow-Events header, as defined in [1].

2.2 Event Package Parameters

   This package does not define any event package parameters.

2.3 SUBSCRIBE Bodies

   A SUBSCRIBE for a dialog package MAY contain a body. This body
   defines a filter to apply to the subscription.

   A SUBSCRIBE for a dialog package MAY be sent without a body. This
   implies the default subscription filtering policy. The default policy
   is:

        o Notifications are generated every time there is any change in
          the state of any dialogs for the user identified in the
          request URI of the SUBSCRIBE.

        o Notifications do not normally contain full state; rather, they
          only indicate the state of the dialog whose state has changed.
          The exception is a NOTIFY sent in response to a SUBSCRIBE.
          These NOTIFYs contain the complete view of dialog state.

        o The notifications contain the identities of the participants
          in the dialog, and the dialog identifiers. Additional
          information, such as the route set, remote target URI, CSeq
          numbers, SDP information, and so on, are not included normally
          unless explicitly requested and/or explicitly authorized.


        OPEN ISSUE: It might be useful to define a filter within
        this document that would allow a subscriber to ask about a
        specific dialog.

2.4 Subscription Duration

   Dialog state changes fairly quickly; once established, a typical
   phone call lasts a few minutes (this is different for other session
   types, of course). However, the interval between new calls is



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   typically infrequent. As such, we arbitrarily choose a default
   duration of one hour, and RECOMMEND that clients specify an explicit
   duration.

   There are two distinct use cases for dialog state. The first is when
   a subscriber is interested in the state of a specific dialog (and
   they are authorized to find out about just the state of that dialog).
   In that case, when the dialog terminates, so too does the
   subscription. In these cases, the value of the subscription duration
   is largely irrelevant, and SHOULD be longer than the typical duration
   of a dialog, about two hours would cover most dialogs.

   In another case, a subscriber is interested in the state of all call
   legs for a specific user. In these cases, a shorter interval makes
   more sense. The default is one hour for these subscriptions.

2.5 NOTIFY Bodies

   The body of the notification contains a dialog information document.
   The format of this document is described in Section 3. Its MIME type
   is "application/dialog-info+xml". All subscribers MUST support this
   format, and MUST list its type in any Accept header in the SUBSCRIBE.
   When no Accept header is present in the SUBSCRIBE, its default value
   is "application/dialog-info+xml".

   Other dialog information formats might be defined in the future. In
   that case, the subscriptions MAY indicate support for other formats.
   However, they MUST always support and list "application/dialog-
   info+xml" as an allowed format.

   Of course, the notifications generated by the server MUST be in one
   of the formats specified in the Accept header in the SUBSCRIBE
   request.

2.6 Notifier Processing of SUBSCRIBE Requests

   The dialog information for a user contains very sensitive
   information. Therefore, all subscriptions SHOULD be authenticated and
   then authorized before approval. Authorization policy is at the
   discretion of the administrator, as always. However, a few
   recommendations can be made.

   It is RECOMMENDED that if the policy of a user is that A is allowed
   to call them, dialog subscriptions from user A be allowed. However,
   the information provided in the notifications does not contain any
   dialog identification information; merely an indication of whether
   the user is in one or more calls, or not. Specifically, they should
   not be able to find out any more information than if they sent an



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   INVITE.

   It is RECOMMENDED that if a user agent registers with the address-
   of-record X, that this user agent authorize subscriptions that come
   from any entity that can authenticate itself as X. Complete
   information on the dialog state SHOULD be sent in this case. This
   authorization behavior allows a group of devices representing a
   single user to all become aware of each other's state. This is useful
   for applications such as single-line-extension.

2.7 Notifier Generation of NOTIFY Requests

   Notifications are generated for the dialog package when a new dialog
   comes into existence at a UA, or when the state or characteristics of
   an existing dialog changes. Therefore, a model of dialog state is
   needed in order to determine precisely when to send notifications,
   and what their content should be. The SIP specification has a
   reasonably well defined lifecycle for dialogs. However, it is not
   explicitly modelled. This specification provides an explicit model of
   dialog state through a finite state machine.

   It is RECOMMENDED that NOTIFY requests only contain information on
   the dialogs whose state has changed. However, if a notifier receives
   a SUBSCRIBE request, the triggered NOTIFY SHOULD contain the state of
   all dialogs that the subscriber is authorized to see.

2.7.1 The Dialog State Machine

   Modelling of dialog state is complicated by two factors. The first is
   forking, which can cause a single INVITE to generate many dialogs at
   a UAC. The second is the differing views of state at the UAC and UAS.
   We have chosen to handle the first issue by extending the dialog FSM
   to include the states between transmission of the INVITE and the
   creation of actual dialogs through receipt of 1xx and 2xx responses.
   We have also chosen to use a single FSM for both UAC and UAS.


   The FSM for dialog state is shown in Figure 1. The FSM is best
   understood by considering the UAC and UAS cases separately.

   The FSM is created in the "trying" state when the UAC sends an INVITE
   request. Upon receipt of a 1xx without a tag (the "1xx-notag" event),
   the FSM transitions to the "proceeding" state. Note that there is no
   actual dialog yet, as defined by the SIP specification. However,
   there is a "half-dialog", in the sense that two of the three
   components of the dialog ID are known (the call identifier and local
   tag). If a 1xx with a tag is received, the FSM transitions to the
   early state. The full dialog identifier is now defined. Had a 2xx



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               +----------+            +----------+
               |          | 1xx-notag  |          |
               |          |----------->|          |
               |  Trying  |            |Proceeding|-----+
               |          |---+  +-----|          |     |
               |          |   |  |     |          |     |
               +----------+   |  |     +----------+     |
                    |   |     |  |          |           |
                    |   |     |  |          |           |
                    +<--C-----C--+          |1xx-tag    |
                    |   |     |             |           |
           cancelled|   |     |             V           |
            rejected|   |     |1xx-tag +----------+     |
                    |   |     +------->|          |     |2xx
                    |   |              |          |     |
                    +<--C--------------|  Early   |     |
                    |   |              |          |     |
                    |   |              |          |     |
                    |   |              +----------+     |
                    |   |   2xx             |           |
                    |   +----------------+  |           |
                    |                    |  |2xx        |
                    |                    |  |           |
                    V                    V  V           |
               +----------+            +----------+     |
               |          |            |          |     |
               |          |            |          |     |
               |Terminated|<-----------| Confirmed|<----+
               |          |    hungup  |          |
               |          |    error   |          |
               +----------+    timeout +----------+





   Figure 1: Dialog finite state machine


   been received, the FSM would have transitioned to the "confirmed"
   state.




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   If, after transitioning to the "early" or "confirmed" states, the UAC
   receives another 1xx or 2xx with a different tag, another instance of
   the FSM is created, initialized into the "early" or "confirmed" state
   depending on the response code. The benefit of this approach is that
   there will be a single FSM representing the entire state of the
   invitation and resulting dialog when dealing with the common case of
   no forking.

   If the UAC should send a CANCEL, and then subsequently receive a 487
   to its INVITE transaction, all FSMs spawned from that INVITE
   transition to the "terminated" state with the event "canceled". If
   the INVITE transaction terminates with a non-2xx response for any
   other reason, all FSMs spawned from that INVITE transition to the
   terminated state with the event "rejected".

   Once in the confirmed state, the call is active. It can transition to
   the terminated state if the UAC sends a BYE or receives a BYE
   (corresponding to the "hungup" event), if a mid-dialog request
   generates a 481 or 408 response (corresponding to the "error" event),
   or a mid-dialog request generates no response (corresponding to the
   "timeout" event).

   From the perspective of the UAS, when an INVITE is received, the FSM
   is created in the "trying" state. If it sends a 1xx without a tag,
   the FSM transitions to the "proceeding" state. If a 1xx is sent with
   a tag, the FSM transitions to the "early" state, and if a 2xx is
   sent, it transitions to the "confirmed" state. If the UAS should
   receive a CANCEL request and then generate a 487 response to the
   INVITE (which can occur in the proceeding and early states), the FSM
   transitions to the terminated state with the event "cancelled". If
   the UAS should generate any other non-2xx final response to the
   INVITE request, the FSM transitions to the terminated state with the
   event "rejected". Once in the "confirmed" state, the transitions to
   the "terminated" state occur for the same reasons they do in the case
   of UAC.


        There should never be a transition from the "trying" state
        to the "terminated" state with the event "cancelled", since
        the SIP specification prohibits transmission of CANCEL
        until a provisional response is received. However, this
        transition is defined in the FSM just to unify the
        transitions from trying, proceeding, and early to the
        terminated state.

2.7.2 Applying the state machine

   The notifier MAY generate a NOTIFY request on any event transition of



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   the FSM. Whether it does or not is policy dependent. However, some
   general guidelines are provided.

   When the subscriber is unauthenticated, or is authenticated, but
   represents a third party with no specific authorization policies, it
   is RECOMMENDED that actual dialog states across all dialogs not be
   reported. Rather, a single "virtual" dialog FSM be used, and event
   transitions on that FSM be reported. If there is any dialog at the UA
   whose state is "confirmed", the virtual FSM is in the "confirmed"
   state. If there are no dialogs at the UA in the confirmed state, but
   there is at least one in the "early" state, the virtual FSM is in the
   "early" state. If there are no dialogs in the confirmed or early
   states, but there is at least one in the "proceeding" state, the
   virtual FSM is in the "proceeding" state. If there are no dialogs in
   the confirmed, early, or proceeding states, but there is at least one
   in the "trying" state, the virtual FSM is in the "trying" state.
   Furthermore, it is RECOMMENDED that the notifications of changes in
   the virtual FSM machine not convey any information except the state
   of the FSM and its event transitions - no dialog identifiers (which
   are ill-defined in this model in any case). The use of this virtual
   FSM allows for minimal information to be conveyed. A subscriber
   cannot know how many calls are in progress, or with whom, just that
   there exists a call.

   When the subscriber is authenticated, and has authenticated itself
   with the same address-of-record that the UA itself uses, if no
   explicit authorization policy is defined, it is RECOMMENDED that all
   state transitions on all dialogs be reported, along with complete
   dialog IDs.

   The notifier MAY generate a NOTIFY request on any change in the
   characteristics associated with the dialog. Since these include CSeq
   numbers and SDP, receipt of re-INVITEs and UPDATE requests [11] which
   modify this information MAY trigger notifications.

2.8 Subscriber Processing of NOTIFY Requests

   The SIP Events framework expects packages to specify how a subscriber
   processes NOTIFY requests in any package specific ways, and in
   particular, how it uses the NOTIFY requests to contruct a coherent
   view of the state of the subscribed resource.

   Typically, the NOTIFY for the dialog package will only contain
   information about those dialogs whose state has changed. To construct
   a coherent view of the total state of all dialogs, a subscriber to
   the dialog package will need to combine NOTIFYs received over time.

   Notifications within this package can convey partial information;



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   that is, they can indicate information about a subset of the state
   associated with the subscription. This means that an explicit
   algorithm needs to be defined in order to construct coherent and
   consistent state. The details of this mechanism are specific to the
   particular document type. See Section 3.2 for information on
   constructing coherent information from an application/dialog-info+xml
   document.

2.9 Handling of Forked Requests

   Since dialog state is distributed across the UA for a particular
   user, it is reasonable and useful for a SUBSCRIBE request for dialog
   state to fork, and reach multiple UA.

   As a result, a forked SUBSCRIBE request for dialog state can install
   multiple subscriptions. Subscribers to this package MUST be prepared
   to install subscription state for each NOTIFY generated as a result
   of a single SUBSCRIBE.

2.10 Rate of Notifications

   For reasons of congestion control, it is important that the rate of
   notifications not become excessive. As a result, it is RECOMMENDED
   that the server not generate notifications for a single subscriber at
   a rate faster than once every 5 seconds.

2.11 State Agents

   Dialog state is ideally maintained in the user agents in which the
   dialog resides. Therefore, the elements that maintain the dialog are
   the ones best suited to handle subscriptions to it. Therefore, the
   usage of state agents is NOT RECOMMENDED for this package.

3 Dialog Information Format

   Dialog information is an XML document [3] that MUST be well-formed
   and SHOULD be valid. Dialog information documents MUST be based on
   XML 1.0 and MUST be encoded using UTF-8. This specification makes use
   of XML namespaces for identifying dialog information documents and
   document fragments. The namespace URI for elements defined by this
   specification is a URN [4], using the namespace identifier 'ietf'
   defined by [5] and extended by [6]. This URN is:



   urn:ietf:params:xml:ns:dialog-info





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   A dialog information document begins with the root element tag
   "dialog-info".

3.1 Structure of Dialog Information

   A dialog information document starts with a dialog-info element. This
   element has three mandatory attributes:

        version: This attribute allows the recipient of dialog
             information documents to properly order them. Versions
             start at 0, and increment by one for each new document sent
             to a subscriber. Versions are scoped within a subscription.
             Versions MUST be representable using a 32 bit integer.

        state: This attribute indicates whether the document contains
             the full dialog information, or whether it contains only
             information on those dialogs which have changed since the
             previous document (partial).

        entity: This attribute contains a URI that identifies the user
             whose dialog information is reported in the remainder of
             the document.

   The dialog-info element has a series of dialog sub-elements. Each of
   those represents a specific dialog.

3.1.1 Dialog Element

   The dialog element reports information on a specific dialog. It has a
   single mandatory attribute, id. The id attribute provides a single
   string that can be used as an identifier for this dialog. This is a
   different identifier than the formal dialog ID defined in SIP [2].
   This identifier exists before the dialog exists, and remains
   unchanged when the actual dialog is established.

   There are a number of optional attributes which provide
   identification information about the dialog:

        call-id: This attribute is a string which represents the call-id
             component of the dialog identifier.

        local-tag: This attribute is a string which represents the
             local-tag component of the dialog identifier.

        remote-tag: This attribute is a string which represents the
             remote-tag component of the dialog identifier. The remote
             tag attribute won't be present if there is only a "half-
             dialog", resulting from generation of a request that can



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             create a dialog.

        direction: This attribute is either initiator or recipient, and
             indicates whether the notifier was the initiator of the
             dialog, or the recipient of the INVITE that created it.

   The sub-elements of the dialog element provide additional information
   about the dialog. The only mandatory one is status.

3.1.2 State

   The state element indicates the state of the dialog. Its value is an
   enumerated type describing one of the states in the FSM above. It has
   an optional event attribute that can be used to indicate the event
   which caused the transition into the current state, and an optional
   code attribute that indicates the response code associated with the
   transition, assuming the event was 1xx-tag, 1xx-notag, or 2xx.

3.1.3 Local URI

   The local-uri element indicates the local URI, as defined in [2]. It
   has an optional attribute, display-name, that contains the display
   name from the local URI.

3.1.4 Remote URI

   The remote-uri element indicates the remote URI, as defined in [2].
   It has an optional attribute, display-name, that contains the display
   name from the remote URI.

3.1.5 Local Session Description

   The local-session-description element contains the session
   description used by the notifier for its end of the dialog. This
   element should generally NOT be included in the notifications, unless
   explicitly requested by the subscriber. It has a single attribute,
   type, which indicates the MIME media type of the session description.

3.1.6 Remote Session Description

   The remote-session-description element contains the session
   description used by the peer of the notifier for its end of the
   dialog. This element should generally NOT be included in the
   notifications, unless explicitly requested by the subscriber. It has
   a single attribute, type, which indicates the MIME media type of the
   session description.

3.1.7 Remote Target



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   The remote-target contains the remote-target URI as constructed by
   the user agent for this dialog, as defined in RFC BBBB [2]. This
   element should generally not be included in notifications, unless
   explicitly requested by the subscriber.

3.1.8 Local CSeq

   The local-cseq element contains the most recent value of the CSeq
   header used by the UA in an outgoing request on the dialog. This
   element should generally NOT be included in the notifications, unless
   explicitly requested by the subscriber. If no CSeq has yet been
   defined, the value of the element is -1.

3.1.9 Remote CSeq

   The remote-cseq element contains the most recent value of the CSeq
   header seen by the UA in an incoming request on the dialog. This
   element should generally NOT be included in the notifications, unless
   explicitly requested by the subscriber. If no CSeq has yet been
   defined, the value of the element is -1.

3.1.10 Duration

   The duration element contains the amount of time, in seconds, since
   the FSM was created.

3.2 Constructing Coherent State

   The dialog information subscriber maintains a table for the list of
   dialogs. The table contains a row for each dialog. Each row is
   indexed by an ID, present in the "id" attribute of the "dialog"
   element. The contents of each row contain the state of that dialog as
   conveyed in the document. The table is also associated with a version
   number. The version number MUST be initialized with the value of the
   "version" attribute from the "dialog-info" element in the first
   document received. Each time a new document is received, the value of
   the local version number, and the "version" attribute in the new
   document, are compared. If the value in the new document is one
   higher than the local version number, the local version number is
   increased by one, and the document is processed. If the value in the
   document is more than one higher than the local version number, the
   local version number is set to the value in the new document, the
   document is processed, and the subscriber SHOULD generate a refresh
   request to trigger a full state notification. If the value in the
   document is less than the local version, the document is discarded
   without processing.

   The processing of the dialog information document depends on whether



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   it contains full or partial state. If it contains full state,
   indicated by the value of the "state" attribute in the "dialog-info"
   element, the contents of the table are flushed. They are repopulated
   from the document. A new row in the table is created for each
   "dialog" element. If the document contains partial state, as
   indicated by the value of the "state" attribute in the "dialog-info"
   element, the document is used to update the table. For each "dialog"
   element in the document, the subscriber checks to see whether a row
   exists for that dialog. This check is done by comparing the ID in the
   "id" attribute of the "dialog" element with the ID associated with
   the row. If the dialog doesn't exist in the table, a row is added,
   and its state is set to the information from that "dialog" element.
   If the dialog does exist, its state is updated to be the information
   from that "dialog" element. If a row is updated or created, such that
   its state is now terminated, that entry MAY be removed from the table
   at any time.

3.3 Schema

   The following is the schema for the application/dialog-info+xml type:



   <?xml version="1.0" encoding="UTF-8"?>
   <xs:schema xmlns:xs="http://www.w3.org/2001/XMLSchema"
          targetNamespace="urn:ietf:params:xml:ns:dialog-info"
          xmlns:tns="urn:ietf:params:xml:ns:dialog-info"
          elementFormDefault="qualified" attributeFormDefault="unqualified" >
   <!-- This import brings in the XML language attribute xml:lang-->
     <xs:import namespace="http://www.w3.org/XML/1998/namespace"
                schemaLocation="http://www.w3.org/2001/03/xml.xsd" />
     <xs:element name="dialog-info">
       <xs:complexType>
         <xs:sequence>
         <xs:element ref="tns:dialog" minOccurs="0" maxOccurs="unbounded"/>
         <xs:any namespace="##other" processContents="lax" minOccurs="0"
                 maxOccurs="unbounded"/>
         </xs:sequence>
         <xs:attribute name="version" type="xs:nonNegativeInteger"
                       use="required"/>
         <xs:attribute name="state" use="required">
           <xs:simpleType>
             <xs:restriction base="xs:string">
               <xs:enumeration value="full"/>
               <xs:enumeration value="partial"/>
             </xs:restriction>
           </xs:simpleType>
         </xs:attribute>



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         <xs:attribute name="entity" type="xs:anyURI" use="required"/>
       </xs:complexType>
     </xs:element>
     <xs:element name="dialog">
     <xs:complexType>
       <xs:sequence>
       <xs:element ref="tns:state" minOccurs="1" maxOccurs="1"/>
       <xs:element name="duration" type="xs:nonNegativeInteger"
                   minOccurs="0" maxOccurs="1"/>
       <xs:element name="local-uri" type="tns:nameaddr"
                   minOccurs="0" maxOccurs="1"/>
       <xs:element name="remote-uri" type="tns:nameaddr"
                   minOccurs="0" maxOccurs="1"/>
       <xs:element name="local-session-description" type="tns:sessd"
                   minOccurs="0" maxOccurs="1"/>
       <xs:element name="remote-session-description" type="tns:sessd"
                   minOccurs="0" maxOccurs="1"/>
       <xs:element name="remote-target" type="tns:nameaddr"
                   minOccurs="0" maxOccurs="1"/>
       <xs:element name="local-cseq" type="tns:cseq" minOccurs="0"
                   maxOccurs="1"/>
       <xs:element name="remote-cseq" type="tns:cseq" minOccurs="0"
                   maxOccurs="1"/>
       <xs:any namespace="##other" processContents="lax" minOccurs="0"
               maxOccurs="unbounded"/>
       </xs:sequence>
       <xs:attribute name="id" type="xs:string" use="required"/>
       <xs:attribute name="call-id" type="xs:string" use="optional"/>
       <xs:attribute name="local-tag" type="xs:string" use="optional"/>
       <xs:attribute name="remote-tag" type="xs:string" use="optional"/>
       <xs:attribute name="direction" use="optional">
       <xs:simpleType>
         <xs:restriction base="xs:string">
         <xs:enumeration value="initiator"/>
         <xs:enumeration value="recipient"/>
         </xs:restriction>
       </xs:simpleType>
       </xs:attribute>
     </xs:complexType>
     </xs:element>
     <xs:element name="state">
     <xs:complexType>
       <xs:simpleContent>
       <xs:restriction base="xs:string">
       <xs:enumeration value="trying"/>
       <xs:enumeration value="proceeding"/>
       <xs:enumeration value="early"/>
       <xs:enumeration value="confirmed"/>



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       <xs:enumeration value="terminated"/>
       <xs:attribute name="event" use="optional">
       <xs:simpleType>
         <xs:restriction base="xs:string">
         <xs:enumeration value="1xx-notag"/>
         <xs:enumeration value="1xx-tag"/>
         <xs:enumeration value="2xx"/>
         <xs:enumeration value="cancelled"/>
         <xs:enumeration value="rejected"/>
         <xs:enumeration value="hungup"/>
         <xs:enumeration value="error"/>
         <xs:enumeration value="timeout"/>
         </xs:restriction>
       </xs:simpleType>
       </xs:attribute>
       <xs:attribute name="code" use="optional">
       <xs:simpleType>
         <xs:restriction base="xs:nonNegativeInteger">
         <xs:maxInclusive value="699"/>
         <xs:minInclusive value="100"/>
         </xs:restriction>
       </xs:simpleType>
       </xs:attribute>
       </xs:restriction>
       </xs:simpleContent>
     </xs:complexType>
     </xs:element>
     <xs:complexType name="nameaddr">
       <xs:simpleContent>
       <xs:extension base="xs:anyURI">
       <xs:attribute name="display-name" type="xs:string"/>
       </xs:extension>
       </xs:simpleContent>
     </xs:complexType>
     <xs:complexType name="sessd">
       <xs:simpleContent>
       <xs:restriction base="xs:string">
       <xs:attribute name="type" type="xs:string"/>
       </xs:restriction>
       </xs:simpleContent>
     </xs:complexType>
     <xs:complexType name="cseq">
       <xs:simpleContent>
       <xs:restriction base="xs:integer">
       <xs:minInclusive value="-1"/>
       </xs:restriction>
       </xs:simpleContent>
     </xs:complexType>



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   </xs:schema>




3.4 Example

   For example, if a UAC sends an INVITE that looks like, in part:



   INVITE sip:callee@foo.com SIP/2.0
   From: sip:caller@bar.com;tag=123
   To: sip:callee@foo.com
   Call-ID: 987@1.2.3.4



   The XML document in a notification might look like:



   <?xml version="1.0"?>
   <dialog-info xmlns="urn:ietf:params:xml:ns:dialog-info"
                version="0"
                state="full"
                entity="sip:caller@bar.com">
     <dialog id="as7d900as8" call-id="987@1.2.3.4"
             local-tag="123" direction="initiator">
       <state>trying</state>
     </dialog>
   </dialog-info>



   If a 1xx with a tag is received, the XML document in a notification
   might look like:



   <?xml version="1.0"?>
   <dialog-info xmlns="urn:ietf:params:xml:ns:dialog-info"
                version="1"
                state="full"
                entity="sip:caller@bar.com">
     <dialog id="as7d900as8" call-id="987@1.2.3.4"
             local-tag="123" remote-tag="456" direction="initiator">
       <state>early</state>



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     </dialog>
   </dialog-info>



   If it receives a second 1xx, with a different tag, this results in
   the creation of a second dialog:



   <?xml version="1.0"?>
   <dialog-info xmlns="urn:ietf:params:xml:ns:dialog-info"
                version="2"
                state="full"
                entity="sip:caller@bar.com">
     <dialog id="as7d900as8" call-id="987@1.2.3.4"
             local-tag="123" remote-tag="456" direction="initiator">
       <state>early</state>
     </dialog>
     <dialog id="as7d900as8" call-id="987@1.2.3.4"
             local-tag="123" remote-tag="789" direction="initiator">
       <state>early</state>
     </dialog>
   </dialog-info>



   If a 200 OK is received on the second dialog, it moves to confirmed:



   <?xml version="1.0"?>
   <dialog-info xmlns="urn:ietf:params:xml:ns:dialog-info"
                version="3"
                state="partial"
                entity="sip:caller@bar.com">
     <dialog id="as7d900as8" call-id="987@1.2.3.4"
             local-tag="123" remote-tag="789" direction="initiator">
       <state event="2xx">confirmed</state>
     </dialog>
   </dialog-info>



   32 seconds later, the other early dialog terminates because no 2xx is
   received for it. This implies that it was successfully cancelled, and
   therefore the following notification is sent:




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   <?xml version="1.0"?>
   <dialog-info xmlns="urn:ietf:params:xml:ns:dialog-info"
                version="4"
                state="partial"
                entity="sip:caller@bar.com">
     <dialog id="as7d900as8" call-id="987@1.2.3.4"
             local-tag="123" remote-tag="456" direction="initiator">
       <state event="cancelled">terminated</state>
     </dialog>
   </dialog-info>



4 Security Considerations

   Subscriptions to dialog state can reveal very sensitive information.
   For this reason, this specification recommends authentication and
   authorization of subscriptions, and provides guidelines on sensible
   authorization policies.

   Since the data in notifications is sensitive as well, end-to-end SIP
   encryption mechanisms using S/MIME MAY be used to protect it.

5 IANA Considerations

   This document registers a new MIME type, application/dialog-info+xml
   and registers a new XML namespace.

5.1 application/dialog-info+xml MIME Registration

        MIME media type name: application

        MIME subtype name: dialog-info+xml

        Mandatory parameters: none

        Optional parameters: Same as charset parameter application/xml
             as specified in RFC 3023 [7].

        Encoding considerations: Same as encoding considerations of
             application/xml as specified in RFC 3023 [7].

        Security considerations: See Section 10 of RFC 3023 [7] and
             Section 4 of this specification.

        Interoperability considerations: none.

        Published specification: This document.



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        Applications which use this media type: This document type has
             been used to support SIP applications such as call return
             and auto-conference.

        Additional Information:

             Magic Number: None

             File Extension: .dif or .xml

             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.

5.2 URN Sub-Namespace Registration for urn:ietf:params:xml:ns:dialog-
   info

   This section registers a new XML namespace, as per the guidelines in
   [6].

        URI: The URI for this namespace is
             urn:ietf:params:xml:ns:dialog-info.

        Registrant Contact: IETF, SIMPLE working group,
             <simple@mailman.dynamicsoft.com>, 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>Dialog Information Namespace</title>
             </head
             <body>
               <h1>Namespace for Dialog Information</h1>
               <h2>application/dialog-info+xml</h2>



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               <p>See <a href="[[[URL of published RFC]]]">RFCXXXX</a>.</p>
             </body>
             </html>
             END



6 Acknowledgements

   The author would like to thank Sean Olson for his comments.

7 Authors Addresses




   Jonathan Rosenberg
   dynamicsoft
   72 Eagle Rock Avenue
   First Floor
   East Hanover, NJ 07936
   email: jdrosen@dynamicsoft.com

   Henning Schulzrinne
   Columbia University
   M/S 0401
   1214 Amsterdam Ave.
   New York, NY 10027-7003
   email: schulzrinne@cs.columbia.edu



8 Normative References

   [1] A. Roach, "SIP-specific event notification," Internet Draft,
   Internet Engineering Task Force, Mar. 2002.  Work in progress.

   [2] J. Rosenberg, H. Schulzrinne, et al.  , "SIP: Session initiation
   protocol," Internet Draft, Internet Engineering Task Force, Feb.
   2002.  Work in progress.

   [3] W. W. W. C. (W3C), "Extensible markup language (xml) 1.0." The
   XML 1.0 spec can be found at http://www.w3.org/TR/1998/REC-xml-
   19980210.

   [4] R. Moats, "URN syntax," RFC 2141, Internet Engineering Task
   Force, May 1997.




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   [5] R. Moats, "A URN namespace for IETF documents," RFC 2648,
   Internet Engineering Task Force, Aug. 1999.

   [6] M. Mealling, "The IANA XML registry," Internet Draft, Internet
   Engineering Task Force, Nov. 2001.  Work in progress.

   [7] M. Murata, S. S. Laurent, and D. Kohn, "XML media types," RFC
   3023, Internet Engineering Task Force, Jan. 2001.

9 Informative References

   [8] J. Rosenberg, "Session initiation protocol (SIP) extensions for
   presence," Internet Draft, Internet Engineering Task Force, May 2002.
   Work in progress.

   [9] J. Rosenberg, "A session initiation protocol (SIP)event
   template-package for watcher information," Internet Draft, Internet
   Engineering Task Force, May 2002.  Work in progress.

   [10] R. Mahy, "A message summary and message waiting indication event
   package for the session initiation protocol (SIP)," Internet Draft,
   Internet Engineering Task Force, June 2002.  Work in progress.

   [11] J. Rosenberg, "The session initiation protocol UPDATE method,"
   Internet Draft, Internet Engineering Task Force, May 2002.  Work in
   progress.


   Full Copyright Statement

   Copyright (c) The Internet Society (2002). All Rights Reserved.

   This document and translations of it may be copied and furnished to
   others, and derivative works that comment on or otherwise explain it
   or assist in its implementation may be prepared, copied, published
   and distributed, in whole or in part, without restriction of any
   kind, provided that the above copyright notice and this paragraph are
   included on all such copies and derivative works. However, this
   document itself may not be modified in any way, such as by removing
   the copyright notice or references to the Internet Society or other
   Internet organizations, except as needed for the purpose of
   developing Internet standards in which case the procedures for
   copyrights defined in the Internet Standards process must be
   followed, or as required to translate it into languages other than
   English.

   The limited permissions granted above are perpetual and will not be
   revoked by the Internet Society or its successors or assigns.



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   This document and the information contained herein is provided on an
   "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
   TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING
   BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION
   HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
   MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.













































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