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RFC 4235 - An INVITE-Initiated Dialog Event Package for the Sess


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Network Working Group                                       J. Rosenberg
Request for Comments: 4235                                 Cisco Systems
Category: Standards Track                                 H. Schulzrinne
                                                     Columbia University
                                                            R. Mahy, Ed.
                                                            SIP Edge LLC
                                                           November 2005

            An INVITE-Initiated Dialog Event Package for the
                   Session Initiation Protocol (SIP)

Status of this Memo

   This document specifies an Internet standards track protocol for the
   Internet community, and requests discussion and suggestions for
   improvements.  Please refer to the current edition of the "Internet
   Official Protocol Standards" (STD 01) for the standardization state
   and status of this protocol.  Distribution of this memo is unlimited.

Copyright Notice

   Copyright (C) The Internet Society (2005).

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 and to receive notification of the changes in state of INVITE-
   initiated dialog usages in which the subscribed-to user is involved.

Table of Contents

   1. Introduction ....................................................3
   2. Terminology .....................................................4
   3. Dialog Event Package ............................................4
      3.1. Event Package Name .........................................4
      3.2. Event Package Parameters ...................................4
      3.3. SUBSCRIBE Bodies ...........................................5
      3.4. Subscription Duration ......................................6
      3.5. NOTIFY Bodies ..............................................6
      3.6. Notifier Processing of SUBSCRIBE Requests ..................7
      3.7. Notifier Generation of NOTIFY Requests .....................8
           3.7.1. The Dialog State Machine ............................8
           3.7.2. Applying the State Machine .........................11

      3.8. Subscriber Processing of NOTIFY Requests ..................12
      3.9. Handling of Forked Requests ...............................12
      3.10. Rate of Notifications ....................................13
      3.11. State Agents .............................................13
   4. Dialog Information Format ......................................13
      4.1. Structure of Dialog Information ...........................13
           4.1.1. Dialog Element .....................................14
           4.1.2. State Element ......................................15
           4.1.3. Duration Element ...................................15
           4.1.4. Replaces Element ...................................15
           4.1.5. Referred-By Element ................................16
           4.1.6. Local and Remote Elements ..........................16
      4.2. Sample Notification Body ..................................17
      4.3. Constructing Coherent State ...............................18
      4.4. Schema ....................................................19
   5. Definition of New Media Feature Parameters .....................22
      5.1. The "sip.byeless" Parameter ...............................22
      5.2. The "sip.rendering" parameter .............................23
   6. Examples .......................................................24
      6.1. Basic Example .............................................24
      6.2. Emulating a Shared-Line Phone System ......................26
      6.3. Minimal Dialog Information with Privacy ...................31
   7. Security Considerations ........................................32
   8. IANA Considerations ............................................32
      8.1. application/dialog-info+xml MIME Registration .............33
      8.2. URN Sub-Namespace Registration for
           urn:ietf:params:xml:ns:dialog-info ........................34
      8.3. Schema Registration .......................................34
      8.4. Media Feature Parameter Registration ......................34
           8.4.1. sip.byeless ........................................35
           8.4.2. sip.rendering ......................................35
   9. Acknowledgements ...............................................36
   10. References ....................................................36
      10.1. Normative References .....................................36
      10.2. Informative References ...................................37

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 [16], watcher
   information [17], and message waiting indicators [18], amongst
   others.  This document defines an event package for INVITE-initiated
   dialog usages.  Dialogs refer to the SIP relationship established
   between two SIP peers [2].  Dialogs can be created by many methods,
   although RFC 3261 defines only one: the INVITE method.  RFC 3265 [1]
   defines the SUBSCRIBE and NOTIFY methods, which also create new
   dialog usages.  However, using this package to model state for non-
   session dialog usages is out of the scope of this specification.

   A variety of applications are enabled through knowledge of INVITE
   dialog usage state.  Some examples include:

      Automatic Callback: In this basic Public Switched Telephone
         Network (PSTN) application, user A calls user B but 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 up,
         they hear ringing, while they are being connected to B.  To
         implement this with SIP, a mechanism is required for A to
         receive a notification when the dialogs at B are complete.

      Presence-Enabled Conferencing: In this application, user A wishes
         to set up a conference call with users B and C.  Rather than
         being scheduled, the call is 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.  In the
         second way, the server subscribes to the dialog state of those
         users and receives notifications as this state 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 receive an
         Instant Message (IM) on 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.

   This document also defines two new callee capability [10] feature
   parameters:

      o "sip.byeless", which indicates that a SIP user agent (UA) is not
         capable of terminating a session itself (for example, in some
         announcement or recording services, or in some call centers) in
         which the UA is no longer interested in participating; and

      o "sip.rendering", which positively describes whether the user
         agent is rendering any of the media it is receiving.  These
         feature parameters are useful in many of the same applications
         that motivated the dialog package, such as conferencing,
         presence, and the shared-line example described in Section 6.2.

2.  Terminology

   In this document, the key words "MUST", "MUST NOT", "REQUIRED",
   "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY",
   and "OPTIONAL" are to be interpreted as described in RFC 2119 [9] and
   indicate requirement levels for compliant implementations.

3.  Dialog Event Package

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

3.1.  Event Package Name

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

3.2.  Event Package Parameters

   This package defines four Event Package parameters:  call-id, to-tag,
   from-tag, and include-session-description.  If a subscription to a
   specific dialog is requested, the first three of these parameters
   MUST be present, to identify the dialog that is being subscribed to.
   The to-tag is matched against the local tag, the from-tag is matched
   against the remote tag, and the call-id is matched against the
   Call-ID.  The include-session-description parameter indicates whether
   the subscriber would like to receive the session descriptions
   associated with the subscribed dialog usage or usages.

   It is also possible to subscribe to the set of dialogs created as a
   result of a single INVITE sent by a UAC (user agent client).  In that
   case, the call-id and to-tag MUST be present.  The to-tag is matched
   against the local tag and the call-id is matched against the Call-ID.

   The ABNF for these parameters is shown below.  It refers to many
   constructions from the ABNF of RFC3261, such as EQUAL, DQUOTE, and
   token.

   call-id     =  "call-id" EQUAL ( token / DQUOTE callid DQUOTE )
                    ;; NOTE: any DQUOTEs inside callid MUST be escaped!
   from-tag    =  "from-tag" EQUAL token
   to-tag      =  "to-tag" EQUAL token
   with-sessd  =  "include-session-description"

   If any call-ids contain embedded double-quotes, those double-quotes
   MUST be escaped using the backslash-quoting mechanism.  Note that the
   call-id parameter may need to be expressed as a quoted string.  This
   is because the ABNF for the callid production and the word
   production, which is used by callid (both from RFC 3261 [1]), allow
   some characters (such as "@", "[", and ":") that are not allowed
   within a token.

3.3.  SUBSCRIBE Bodies

   A SUBSCRIBE request for a dialog package MAY contain a body.  This
   body defines a filter to be applied to the subscription.  Filter
   documents are not specified in this document, and at the time of
   writing, they are expected to be the subject of future
   standardization activity.

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

   o  If the Event header field contained dialog identifiers, a
      notification is generated every time there is a change in the
      state of any matching dialogs for the user identified in the
      request URI of the SUBSCRIBE.

   o  If there were no dialog identifiers in the Event header field, a
      notification is generated every time there is any change in the
      state of any dialogs for the user identified in the request URI of
      the SUBSCRIBE with the following exceptions.  If the target
      (Contact) URI of a subscriber is equivalent to the remote target
      URI of a specific dialog, then the dialog element for that dialog
      is suppressed for that subscriber.  (The subscriber is already a
      party in the dialog directly, so these notifications are

      superfluous.)  If no dialogs remain after suppressing dialogs, the
      entire notification to that subscriber is suppressed and the
      version number in the dialog-info element is not incremented for
      that subscriber.  Implicit filtering for one subscriber does not
      affect notifications to other subscribers.

   o  Notifications do not normally contain full state; rather, they
      only indicate the state of the dialog(s) whose state has changed.
      The exceptions are a NOTIFY sent in response to a SUBSCRIBE, and a
      NOTIFY that contains no dialog elements.  These NOTIFYs contain
      the complete view of dialog state.

   o  The notifications contain the identities of the participants in
      the dialog, the target URIs, and the dialog identifiers.  Session
      descriptions are not included unless explicitly requested and
      explicitly authorized.

3.4.  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
   typically long.  Clients SHOULD 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 or
   dialogs (and they are authorized to find out just the state of those
   dialogs).  In that case, when the dialogs terminate, so too does the
   subscription.  In these cases, the value of the subscription duration
   is largely irrelevant; it SHOULD be longer than the typical duration
   of a dialog.  We recommend a default duration of two hours, which is
   likely to cover most dialogs.

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

3.5.  NOTIFY Bodies

   As described in RFC 3265 [1], 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 in a
   package-specific default format if the Accept header field was
   omitted from the SUBSCRIBE.

   In this event package, the body of the notification contains a dialog
   information document.  This document describes the state of one or
   more dialogs associated with the subscribed resource.  All

   subscribers and notifiers MUST support the "application/
   dialog-info+xml" data format described in Section 4.  The subscribe
   request MAY contain an Accept header field.  If no such header field
   is present, it has a default value of "application/dialog-info+xml".
   If the header field is present, it MUST include "application/
   dialog-info+xml", and it MAY include any other types capable of
   representing dialog state.

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

3.6.  Notifier Processing of SUBSCRIBE Requests

   The dialog information for a user contains sensitive information.
   Therefore, all subscriptions SHOULD be authenticated and then
   authorized before approval.  All implementors of this package MUST
   support the digest authentication mechanism as a baseline.  The
   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 user B is that user 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 at least one call.  Specifically, they
   should not be able to find out any more information than if they sent
   an INVITE.  (This concept of a "virtual" dialog is discussed more in
   Section 3.7.2, and an example of such a notification body is shown
   below).

      <?xml version="1.0"?>
      <dialog-info xmlns="urn:ietf:params:xml:ns:dialog-info"
                   version="0" state="full"
                   entity="sip:alice@example.com">
        <dialog id="as7d900as8">
          <state>confirmed</state>
        </dialog>
      </dialog-info>

   A user agent that registers with the address-of-record X SHOULD
   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 become aware of each
   other's state.  This is useful for applications such as
   single-line-extension, also known as shared lines.

      Note that many implementations of "shared-lines" have a feature
      that allows details of calls on a shared address-of-record to be
      made private.  This is a completely reasonable authorization
      policy that could result in notifications that contain only the id
      attribute of the dialog element and the state element when
      shared-line privacy is requested, and notifications with more
      complete information when shared-line privacy is not requested.

3.7.  Notifier Generation of NOTIFY Requests

   Notifications are generated for the dialog package when an INVITE
   request is sent, 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 or participation information 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.

3.7.1.  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
   (user agent client) and UAS (usage agent server).  We have chosen to
   handle the first issue by extending the dialog finite state machine
   (FSM) to include the states between transmission of the INVITE and
   the creation of actual dialogs through receipt of 1xx and 2xx
   responses.  As a result, this specification supports the notion of
   dialog state for dialogs before they are fully instantiated.

   We have also chosen to use a single FSM for both UAC and UAS.

                +----------+            +----------+
                |          | 1xx-notag  |          |
                |          |----------->|          |
                |  Trying  |            |Proceeding|-----+
                |          |---+  +-----|          |     |
                |          |   |  |     |          |     |
                +----------+   |  |     +----------+     |
                     |   |     |  |          |           |
                     |   |     |  |          |           |
                     +<--C-----C--+          |1xx-tag    |
                     |   |     |             |           |
            cancelled|   |     |             V           |
             rejected|   |     |1xx-tag +----------+     |
                     |   |     +------->|          |     |2xx
                     |   |              |          |     |
                     +<--C--------------|  Early   |-----C---+ 1xx-tag
                     |   |   replaced   |          |     |   | w/new tag
                     |   |              |          |<----C---+ (new FSM
                     |   |              +----------+     |      instance
                     |   |   2xx             |           |      created)
                     |   +----------------+  |           |
                     |                    |  |2xx        |
                     |                    |  |           |
                     V                    V  V           |
                +----------+            +----------+     |
                |          |            |          |     |
                |          |            |          |     |
                |Terminated|<-----------| Confirmed|<----+
                |          |  error     |          |
                |          |  timeout   |          |
                +----------+  replaced  +----------+
                              local-bye   |      ^
                              remote-bye  |      |
                                          |      |
                                          +------+
                                           2xx w. new tag
                                            (new FSM instance
                                             created)

                               Figure 3

   The FSM for dialog state is shown in Figure 3.  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 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 (the
   call identifier and local tag) are known.  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 been received, the FSM would
   have transitioned to the Confirmed state.

   If, after transitioning to the Early or Confirmed states, the UAC
   receives another 1xx or 2xx respectively with a different tag,
   another instance of the FSM is created, initialized into the Early or
   Confirmed state, respectively.  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 in the common case of no
   forking.

   If the UAC sends a CANCEL and then subsequently receives a 487 to its
   INVITE transaction, all FSMs spawned from that INVITE transition to
   the Terminated state with the event "cancelled".  If the UAC receives
   a new invitation (with a Replaces [13] header) that replaces the
   current Early or Confirmed dialog, all INVITE transactions spawned
   from the replaced invitation transition to the Terminated state with
   the event "replaced".  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 "local-bye" and "remote-bye" events as
   appropriate), 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 receives a CANCEL
   request and then generates 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 generates
   any other non-2xx final response to the INVITE request, the FSM
   transitions to the Terminated state with the event "rejected".  If
   the UAS receives a new invitation (with a Replaces [13] header field)
   that replaces the current Confirmed dialog, the replaced invitation
   transitions to the Terminated state with the event "replaced".  Once

   in the Confirmed state, the other 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 states to the Terminated state.

3.7.2.  Applying the State Machine

   The notifier MAY generate a NOTIFY request on any event transition of
   the FSM.  Whether it does or not is policy dependent.  However, some
   general guidelines are provided.

   When the subscriber is unauthenticated, or it is authenticated but
   represents a third party with no specific authorization policies, it
   is RECOMMENDED that subscriptions to an individual dialog or to a
   specific set of dialogs be forbidden.  Only subscriptions to all
   dialogs (i.e., there are no dialog identifiers in the Event header
   field) are permitted.  In that case, actual dialog states across all
   dialogs will not be reported.  Rather, a single "virtual" dialog FSM
   will be used, and event transitions on that FSM will 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 or Confirmed 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,
   Early, or Confirmed 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, Proceeding, Early or
   Confirmed state.  The choice of state to use depends on whether the
   UA wishes to let unknown users know that their phone is ringing, as
   opposed to being in an active call.

   It is RECOMMENDED that, in the absence of any preference, Confirmed
   is used in all cases as shown in the example in Section 3.6.
   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 minimal information to be conveyed.  A subscriber cannot
   know how many calls are in progress, or with whom, just that there
   exists a call.  This is the same information they would receive if

   they simply sent an INVITE to the user instead; a 486 (Busy Here)
   response would indicate that they are on 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 dialogs that have been subscribed to be
   reported, along with complete dialog IDs.  This means either all of
   the dialogs, if no dialog identifiers were present in the Event
   header field, or the specific set of dialogs identified by the Event
   header field parameters.

   The notifier SHOULD generate a NOTIFY request on any change in the
   characteristics associated with the dialog.  Since these include
   Contact URIs, Contact parameters, and session descriptions, receipt
   of re-INVITEs and UPDATE requests [3] that modify this information
   MAY trigger notifications.

3.8.  Subscriber Processing of NOTIFY Requests

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

   Typically, the NOTIFY for the dialog package will contain information
   about only 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;
   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 4.3 for information on
   constructing coherent information from an application/dialog-info+xml
   document.

3.9.  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 to reach multiple UAs.

   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.

3.10.  Rate of Notifications

   For reasons of congestion control, it is important that the rate of
   notifications not be excessive.  It is RECOMMENDED that the server
   not generate notifications for a single subscriber faster than once
   every 1 second.

3.11.  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.  However, in some
   cases, a network agent may also know the state of the dialogs held by
   a user.  Such state agents MAY be used with this package.

4.  Dialog Information Format

   Dialog information is an XML document [4] 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 [5], using the namespace identifier
   'ietf' defined by [6] and extended by [7].  This URN is:

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

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

4.1.  Structure of Dialog Information

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

   o  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 non-negative 32 bit integer.

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

   o  entity: This attribute contains a URI that identifies the user
      whose dialog information is reported in the remainder of the
      document.  This user is referred to as the "observed user".

   The dialog-info element has a series of zero or more dialog sub-
   elements.  Each of those represents a specific dialog.  An example:

      <?xml version="1.0"?>
      <dialog-info xmlns="urn:ietf:params:xml:ns:dialog-info"
                   version="0" notify-state="full"
                   entity="sip:alice@example.com">
      </dialog-info>

4.1.1.  Dialog Element

   The dialog element reports information about a specific dialog or
   "half-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 or "half-dialog".  This is a different identifier
   than the dialog ID defined in RFC 3261 [2], but related to it.

   For a caller, the id is created when an INVITE request is sent.  When
   a 1xx response with a tag, or a 2xx response is received, the dialog
   is formally created.  The id remains unchanged.  However, if an
   additional 1xx or 2xx is received, resulting in the creation of
   another dialog (and resulting FSM), that dialog is allocated a new
   id.

   For a callee, the id is created when an INVITE outside of an existing
   dialog is received.  When a 2xx or a 1xx with a tag is sent, creating
   the dialog, the id remains unchanged.

   The id MUST be unique amongst all current dialogs at a UA.

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

      o  call-id: This attribute is a string that represents the call-id
         component of the dialog identifier.  (Note that single and
         double quotes inside a call-id must be escaped using &quote;
         for " and &apos; for ' .)

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

      o  remote-tag: This attribute is a string that 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 the generation of an INVITE for which no final
         responses or provisional responses with tags has been received.

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

      <?xml version="1.0"?>
      <dialog-info xmlns="urn:ietf:params:xml:ns:dialog-info"
                   version="0" state="partial"
                   entity="sip:alice@example.com">
        <dialog id="as7d900as8" call-id="a84b4c76e66710"
                local-tag="1928301774" direction="initiator">
      ...
        </dialog>
      </dialog-info>

   The sub-elements of the dialog element provide additional information
   about the dialog.  Some of these sub-elements provide more detail
   about the dialog itself, while the local and remote sub-elements
   describe characteristics of the participants involved in the dialog.
   The only mandatory sub-element is the state element.

4.1.2.  State Element

   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 that caused any transition into the terminated state, and an
   optional code attribute that indicates the response code associated
   with any transition caused by a response to the original INVITE.

      <state event="rejected" code="486">terminated</state>

4.1.3.  Duration Element

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

      <duration>145</duration>

4.1.4.  Replaces Element

   The "replaces" element is used to correlate a new dialog with one it
   replaced as a result of an invitation with a Replaces header field.
   This element is present in the replacement dialog only (the newer
   dialog) and contains attributes with the call-id, local-tag, and
   remote-tag of the replaced dialog.

      <replaces call-id="hg287s98s89"
             local-tag="6762h7" remote-tag="09278hsb"/>

4.1.5.  Referred-By Element

   The "referred-by" element is used to correlate a new dialog with a
   REFER [12] request that triggered it.  The element is present in a
   dialog that was triggered by a REFER request that contained a
   Referred-By [11] header field and contains the (optional) display
   name attribute and the Referred-By URI as its value.

      <referred-by display="Bob">sip:bob@example.com</referred-by>

4.1.6.  Local and Remote Elements

   The "local" and "remote" elements are sub-elements of the dialog
   element that contain information about the local and remote
   participants, respectively.  They both have a number of optional
   sub-elements that indicate the identity conveyed by the participant,
   the target URI, the feature-tags of the target, and the
   session-description of the participant.

4.1.6.1.  Identity Element

   The "identity" element indicates a local or remote URI, as defined in
   [2] as appropriate.  It has an optional attribute, display, that
   contains the display name from the appropriate URI.

      Note that multiple identities (for example a sip: URI and a tel:
      URI) could be included if they all correspond to the participant.
      To avoid repeating identity information in each request, the
      subscriber can assume that the identity URIs are the same as in
      previous notifications if no identity elements are present in the
      corresponding local or remote element.  If any identity elements
      are present in the local or remote part of a notification, the new
      list of identity tags completely supersedes the old list in the
      corresponding part.

      <identity display="Anonymous">
           sip:anonymous@anonymous.invalid</identity>

4.1.6.2.  Target Element

   The "target" contains the local or remote target URI constructed by
   the user agent for this dialog, as defined in RFC 3261 [2] in a "uri"
   attribute.

   It can contain a list of Contact header parameters in param sub-
   elements (such as those defined in [10]).  The param element contains
   two required attributes, pname and pval.  Boolean parameters are
   represented by the explicit pval values, "true" and "false" (for
   example, when a feature parameter is explicitly negated).  Parameters
   that have no value at all are represented by the explicit pval value
   "true".   The param element itself has no contents.  To avoid
   repeating Contact information in each request, the subscriber can
   assume that the target URI and parameters are the same as in previous
   notifications if no target element is present in the corresponding
   local or remote element.  If a target element is present in the local
   or remote part of a notification, the new target tag and list of
   parameter tags completely supersedes the old target and parameter
   list in the corresponding part.  Note that any quoting (including
   extra angle-bracket quoting used to quote string values in [10]) or
   backslash escaping MUST be removed before being placed in a pval
   attribute.  Any remaining single quotes, double quotes, and
   ampersands MUST be properly XML escaped.

      <target uri="sip:alice@pc33.example.com">
        <param pname="isfocus" pval="true"/>
        <param pname="class" pval="business"/>
        <param pname="description" pval="Alice's desk &amp; office"/>
        <param pname="sip.rendering" pval="no"/>
      </target>

4.1.6.3.  Session Description Element

   The session-description element contains the session description used
   by the observed user for its end of the dialog.  This element should
   generally NOT be included in the notifications, unless it was
   explicitly requested by the subscriber.  It has a single attribute,
   "type", which indicates the MIME media type of the session
   description.  To avoid repeating session description information in
   each request, the subscriber can assume that the session description
   is the same as in previous notifications if no session description
   element is present in the corresponding local or remote element.

4.2.  Sample Notification Body

   <?xml version="1.0" encoding="UTF-8"?>
   <dialog-info xmlns="urn:ietf:params:xml:ns:dialog-info"
    xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
     xsi:schemaLocation="urn:ietf:params:xml:ns:dialog-info"
     version="1" state="full">
     <dialog id="123456">
        <state>confirmed</state>
        <duration>274</duration>

        <local>
          <identity display="Alice">sip:alice@example.com</identity>
          <target uri="sip:alice@pc33.example.com">
            <param pname="isfocus" pval="true"/>
            <param pname="class" pval="personal"/>
          </target>
        </local>
        <remote>
          <identity display="Bob">sip:bob@example.org</identity>
          <target uri="sip:bobster@phone21.example.org"/>
        </remote>
     </dialog>
   </dialog-info>

4.3.  Constructing Coherent State

   The dialog information subscriber maintains a table listing the
   dialogs, with a row for each dialog.  Each row is indexed by an ID
   that is present in the "id" attribute of the "dialog" element.  Each
   row contains 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 is compared to the "version" attribute in the new document.
   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 and the document is processed.  If the
   document did not contain full state, the subscriber SHOULD generate a
   refresh request (SUBSCRIBE) 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
   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 and then 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 compares the ID in the "id"
   attribute of the "dialog" element with the ID associated with the
   row.  If the dialog does not 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.

4.4.  Schema

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

      <?xml version="1.0" encoding="UTF-8"?>
      <xs:schema
        targetNamespace="urn:ietf:params:xml:ns:dialog-info"
        xmlns:xs="http://www.w3.org/2001/XMLSchema"
        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>
            <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="replaces" minOccurs="0" maxOccurs="1">
                <xs:complexType>
                  <xs:attribute name="call-id" type="xs:string"
                    use="required"/>
                  <xs:attribute name="local-tag" type="xs:string"
                    use="required"/>
                  <xs:attribute name="remote-tag" type="xs:string"
                    use="required"/>
                </xs:complexType>
              </xs:element>
              <xs:element name="referred-by" type="tns:nameaddr"
                minOccurs="0" maxOccurs="1"/>
              <xs:element name="route-set" minOccurs="0" maxOccurs="1">
                <xs:complexType>
                  <xs:sequence>
                    <xs:element name="hop" type="xs:string"
                        minOccurs="1" maxOccurs="unbounded"/>
                  </xs:sequence>
                </xs:complexType>
              </xs:element>
              <xs:element name="local" type="tns:participant"
                minOccurs="0" maxOccurs="1"/>
              <xs:element name="remote" type="tns:participant"
                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:complexType name="participant">
          <xs:sequence>
            <xs:element name="identity" type="tns:nameaddr"
              minOccurs="0" maxOccurs="1"/>
            <xs:element name="target" minOccurs="0" maxOccurs="1">
              <xs:complexType>
                <xs:sequence>
                  <xs:element name="param" minOccurs="0"
                    maxOccurs="unbounded">
                    <xs:complexType>
                      <xs:attribute name="pname" type="xs:string"
                        use="required"/>
                      <xs:attribute name="pval" type="xs:string"
                        use="required"/>
                    </xs:complexType>
                  </xs:element>
                </xs:sequence>
                <xs:attribute name="uri" type="xs:string"
                                           use="required"/>
              </xs:complexType>
            </xs:element>
            <xs:element name="session-description" type="tns:sessd"
              minOccurs="0" maxOccurs="1"/>
            <xs:element name="cseq" type="xs:nonNegativeInteger"
              minOccurs="0" maxOccurs="1"/>
            <xs:any namespace="##other" processContents="lax"
              minOccurs="0" maxOccurs="unbounded"/>
          </xs:sequence>
        </xs:complexType>

        <xs:complexType name="nameaddr">
          <xs:simpleContent>
            <xs:extension base="xs:anyURI">
              <xs:attribute name="display-name" type="xs:string"
                use="optional"/>
            </xs:extension>
          </xs:simpleContent>
        </xs:complexType>
        <xs:complexType name="sessd">
          <xs:simpleContent>
            <xs:extension base="xs:string">
              <xs:attribute name="type" type="xs:string"
                                          use="required"/>
            </xs:extension>
          </xs:simpleContent>

        </xs:complexType>

        <xs:element name="state">
          <xs:complexType>
            <xs:simpleContent>
              <xs:extension base="xs:string">
                <xs:attribute name="event" use="optional">
                  <xs:simpleType>
                    <xs:restriction base="xs:string">
                      <xs:enumeration value="cancelled"/>
                      <xs:enumeration value="rejected"/>
                      <xs:enumeration value="replaced"/>
                      <xs:enumeration value="local-bye"/>
                      <xs:enumeration value="remote-bye"/>
                      <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:positiveInteger">
                      <xs:minInclusive value="100"/>
                      <xs:maxInclusive value="699"/>
                    </xs:restriction>
                  </xs:simpleType>
                </xs:attribute>
              </xs:extension>
            </xs:simpleContent>
          </xs:complexType>
        </xs:element>
      </xs:schema>

5.  Definition of New Media Feature Parameters

   This section defines two new media feature parameters that are useful
   as input to user presence, in conferencing applications, and in
   applications like the shared-line example described in Section 6.2.
   These feature parameters are especially useful in combination with
   the dialog package, as they allow an authorized third party to become
   aware of these characteristics.

5.1.  The "sip.byeless" Parameter

   The "sip.byeless" media feature parameter is a new boolean parameter,
   defined in this document, that provides a positive indication that
   the user agent setting the parameter is unable to terminate sessions
   on its own (for example, by sending a BYE request).  For example,

   continuous announcement services and certain recording services are
   unable to determine when it would be desirable to terminate a
   session, and therefore they do not have the ability to terminate
   sessions at all.  Also, many human call centers are configured so
   that they never terminate sessions.  (This is to prevent call center
   agents from accidentally disconnecting the caller).  (Note that per
   [10], this parameter name must be preceded by a "+" character when
   used in a SIP Contact header field.)

      Contact: <sip:recording-service@host.example.net>
          ;automaton;+sip.byeless

5.2.  The "sip.rendering" Parameter

   The "sip.rendering" media feature parameter is a new string
   parameter, defined in this document, that can provide a positive
   indication whether the user agent setting the parameter is currently
   rendering any of the media it is receiving in the context of a
   specific session.  It MUST only be used in a Contact header field in
   a dialog created using the INVITE request.

   This parameter has three legal values: "yes", "no", and "unknown".
   The value "yes" indicates positive knowledge that the user agent is
   rendering at least one of the streams of media that it is receiving.
   The value "no" indicates positive knowledge that the user agent is
   rendering none of the media that it is receiving.  The value
   "unknown" indicates that the user agent does not know whether the
   media associated with the session is being rendered (which may be the
   case if the user agent is acting as a 3pcc (Third Party Call Control)
   [19] controller).

   The "sip.rendering" parameter is useful in applications such as
   shared appearances, conference status monitoring, or as an input to
   user presence.

      Contact: <sip:musak-onhold@host.example.net>
        ;automaton;+sip.rendering="no"

6.  Examples

6.1.  Basic Example

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

      INVITE sip:bob@example.com SIP/2.0
      Via: SIP/2.0/UDP pc33.example.com;branch=z9hG4bKnashds8
      Max-Forwards: 70
      To: Bob <sip:bob@example.com>
      From: Alice <sip:alice@example.com>;tag=1928301774
      Call-ID: a84b4c76e66710
      CSeq: 314159 INVITE
      Contact: <sip:alice@pc33.example.com>
      Content-Type: application/sdp
      Content-Length: 142

      [SDP not shown]

   The XML document in a notification from Alice might look like:

      <?xml version="1.0"?>
      <dialog-info xmlns="urn:ietf:params:xml:ns:dialog-info"
                   version="0"
                   state="full"
                   entity="sip:alice@example.com">
        <dialog id="as7d900as8" call-id="a84b4c76e66710"
                local-tag="1928301774" direction="initiator">
          <state>trying</state>
        </dialog>
      </dialog-info>

   If the following 180 response is received:

      SIP/2.0 180 Ringing
      Via: SIP/2.0/UDP pc33.example.com;branch=z9hG4bKnashds8
      To: Bob <sip:bob@example.com>;tag=456887766
      From: Alice <sip:alice@example.com>;tag=1928301774
      Call-ID: a84b4c76e66710
      CSeq: 314159 INVITE
      Contact: <sip:bob@host.example.com>

   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:alice@example.com">
        <dialog id="as7d900as8" call-id="a84b4c76e66710"
                local-tag="1928301774" remote-tag="456887766"
                direction="initiator">
          <state>early</state>
        </dialog>
      </dialog-info>

   If it receives a second 180 with a different tag:

      SIP/2.0 180 Ringing
      Via: SIP/2.0/UDP pc33.example.com;branch=z9hG4bKnashds8
      To: Bob <sip:bob@example.com>;tag=hh76a
      From: Alice <sip:alice@example.com>;tag=1928301774
      Call-ID: a84b4c76e66710
      CSeq: 314159 INVITE
      Contact: <sip:jack@host.example.com>

   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:alice@example.com">
        <dialog id="as7d900as8" call-id="a84b4c76e66710"
                local-tag="1928301774" remote-tag="456887766"
                direction="initiator">
          <state>early</state>
        </dialog>
        <dialog id="as7d900as8" call-id="a84b4c76e66710"
                local-tag="1928301774" remote-tag="hh76a"
                direction="initiator">
          <state>early</state>
        </dialog>
      </dialog-info>

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

      <?xml version="1.0"?>
      <dialog-info xmlns="urn:ietf:params:xml:ns:dialog-info"
                   version="3"
                   state="partial"
                   entity="sip:alice@example.com">
        <dialog id="as7d900as8" call-id="a84b4c76e66710"
                local-tag="1928301774" remote-tag="hh76a"
                direction="initiator">
          <state>confirmed</state>
        </dialog>
      </dialog-info>

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

      <?xml version="1.0"?>
      <dialog-info xmlns="urn:ietf:params:xml:ns:dialog-info"
                   version="4"
                   state="partial"
                   entity="sip:alice@example.com">
        <dialog id="as7d900as8" call-id="a84b4c76e66710"
                local-tag="1928301774" remote-tag="hh76a"
                direction="initiator">
          <state event="cancelled">terminated</state>
        </dialog>
      </dialog-info>

6.2.  Emulating a Shared-Line Phone System

   The following example shows how a SIP telephone user agent can
   provide detailed state information and also emulate a shared-line
   telephone system (the phone "lies" about having a dialog while it is
   merely offhook).

   Idle:

      <?xml version="1.0"?>
      <dialog-info xmlns="urn:ietf:params:xml:ns:dialog-info"
                   version="0" state="full"
                   entity="sip:alice@example.com">
      </dialog-info>

   Seized:

      <?xml version="1.0"?>
      <dialog-info xmlns="urn:ietf:params:xml:ns:dialog-info"
                   version="1" state="partial"
                   entity="sip:alice@example.com">
        <dialog id="as7d900as8">
          <state>trying</state>
        </dialog>
      </dialog-info>

   Dialing:

      <?xml version="1.0"?>
      <dialog-info xmlns="urn:ietf:params:xml:ns:dialog-info"
                   version="2" state="partial"
                   entity="sip:alice@example.com">
        <dialog id="as7d900as8" call-id="a84b4c76e66710"
                local-tag="1928301774" direction="initiator">
          <state>trying</state>
          <local>
            <identity display="Alice Smith">
               sip:alice@example.com
            </identity>
            <target uri="sip:alice@pc33.example.com"/>
          </local>
          <remote>
            <identity>sip:bob@example.net</identity>
          </remote>
        </dialog>
      </dialog-info>

   Ringing:

      <?xml version="1.0"?>
      <dialog-info xmlns="urn:ietf:params:xml:ns:dialog-info"
                   version="3" state="partial"
                   entity="sip:alice@example.com">
        <dialog id="as7d900as8" call-id="a84b4c76e66710"
                local-tag="1928301774"
                remote-tag="07346y131" direction="initiator">
          <state code="180">early</state>
          <remote>
            <target uri="sip:bobster@host2.example.net"/>
          </remote>
        </dialog>
      </dialog-info>

   Answered (by voicemail):

      <?xml version="1.0"?>
      <dialog-info xmlns="urn:ietf:params:xml:ns:dialog-info"
                   version="4" state="partial"
                   entity="sip:alice@example.com">
        <dialog id="as7d900as8" call-id="a84b4c76e66710"
                local-tag="1928301774"
                remote-tag="07346y131" direction="initiator">
          <state reason="cancelled">terminated</state>
        </dialog>
        <dialog id="zxcvbnm3" call-id="a84b4c76e66710"
                local-tag="1928301774"
                remote-tag="8736347" direction="initiator">
          <state code="200">confirmed</state>
          <remote>
            <target uri="sip:bob-is-not-here@vm.example.net">
              <param pname="actor" pval="msg-taker"/>
              <param pname="automaton" pval="true"/>
              <param pname="+sip.byeless" pval="true"/>
            </target>
          </remote>
        </dialog>
      </dialog-info>

   Alice would rather talk to Bob's assistant (Cathy Jones) than to
   Bob's voicemail.  She indicates this preference by pressing a key
   (perhaps "0" in North America or "9" in Europe).  Bob's voicemail
   system then acts on this keypress by transferring [20] Alice's call
   to Cathy's AOR.

      <?xml version="1.0"?>
      <dialog-info xmlns="urn:ietf:params:xml:ns:dialog-info"
                   version="5" state="partial"
                   entity="sip:alice@example.com">
        <dialog id="zxcvbnm3" call-id="a84b4c76e66710"
                local-tag="1928301774"
                remote-tag="8736347" direction="initiator">
          <state reason="replaced">terminated</state>
        </dialog>
        <dialog id="sfhjsjk12" call-id="o34oii1"
                local-tag="8903j4"
                remote-tag="78cjkus" direction="receiver">
          <state reason="replaced">confirmed</state>
          <replaces call-id="a84b4c76e66710"
                local-tag="1928301774"
                remote-tag="8736347"/>
          <referred-by>
            sip:bob-is-not-here@vm.example.net
          </referred-by>
          <local>
            <target uri="sip:alice@pc33.example.com"/>
              <param pname="+sip.rendering" pval="yes"/>
          </local>
          <remote>
            <identity display="Cathy Jones">
               sip:cjones@example.net
            </identity>
            <target uri="sip:line3@host3.example.net">
              <param pname="actor" pval="attendant"/>
              <param pname="automaton" pval="false"/>
            </target>
          </remote>
        </dialog>
      </dialog-info>

   Alice and Cathy talk, Cathy adds Alice to a local conference:

      <?xml version="1.0"?>
      <dialog-info xmlns="urn:ietf:params:xml:ns:dialog-info"
                   version="6" state="partial"
                   entity="sip:alice@example.com">
        <dialog id="sfhjsjk12" call-id="o34oii1"
                local-tag="8903j4"
                remote-tag="78cjkus" direction="receiver">
          <state>confirmed</state>
          <remote>
            <target uri="sip:confid-34579@host3.example.net">
              <param pname="isfocus" pval="true"/>
            </target>
          </remote>
        </dialog>
      </dialog-info>

   Alice puts Cathy on hold:

      <?xml version="1.0"?>
      <dialog-info xmlns="urn:ietf:params:xml:ns:dialog-info"
                   version="7" state="partial"
                   entity="sip:alice@example.com">
        <dialog id="sfhjsjk12" call-id="o34oii1"
                local-tag="8903j4"
                remote-tag="78cjkus" direction="receiver">
          <state>confirmed</state>
          <local>
            <target uri="sip:alice@pc33.example.com"/>
              <param pname="+sip.rendering" pval="no"/>
            </target>
          </local>
        </dialog>
      </dialog-info>

   Cathy hangs up:

      <?xml version="1.0"?>
      <dialog-info xmlns="urn:ietf:params:xml:ns:dialog-info"
                   version="8" state="partial"
                   entity="sip:alice@example.com">
        <dialog id="sfhjsjk12" call-id="o34oii1"
                local-tag="8903j4"
                remote-tag="78cjkus" direction="receiver">
          <state reason="remote-bye">terminated</state>
        </dialog>
        <dialog id="08hjh1345">
          <state>trying</state>
        </dialog>
      </dialog-info>

   Alice hangs up:

      <?xml version="1.0"?>
      <dialog-info xmlns="urn:ietf:params:xml:ns:dialog-info"
                   version="9" state="full"
                   entity="sip:alice@example.com">
      </dialog-info>

6.3.  Minimal Dialog Information with Privacy

   The following example shows the same user agent providing minimal
   information to maintain privacy for services like automatic callback.

   Onhook:

      <?xml version="1.0"?>
      <dialog-info xmlns="urn:ietf:params:xml:ns:dialog-info"
                   version="0" state="full"
                   entity="sip:alice@example.com">
      </dialog-info>

   Offhook:  (implementation/policy choice for Alice to transition to
   this "state" when "seized", when Trying, when Proceeding, or when
   Confirmed.)

      <?xml version="1.0"?>
      <dialog-info xmlns="urn:ietf:params:xml:ns:dialog-info"
                   version="1" state="full"
                   entity="sip:alice@example.com">
        <dialog id="1">
          <state>confirmed</state>
        </dialog>
      </dialog-info>

   Onhook: (implementation/policy choice for Alice to transition to this
   "state" when terminated, or when no longer "seized")

      <?xml version="1.0"?>
      <dialog-info xmlns="urn:ietf:params:xml:ns:dialog-info"
                   version="2" state="full"
                   entity="sip:alice@example.com">
      </dialog-info>

7.  Security Considerations

   Subscriptions to dialog state can reveal sensitive information.  For
   this reason, Section 3.6 discusses authentication and authorization
   of subscriptions, and provides guidelines on sensible authorization
   policies.  All implementations of this package MUST support the
   digest authentication mechanism.

   Since the data in notifications is sensitive as well, end-to-end SIP
   encryption mechanisms using S/MIME MAY be used to protect it.  User
   agents that implement the dialog package SHOULD also implement SIP
   over TLS [15] and the sips: scheme.

8.  IANA Considerations

   This document registers a new MIME type, application/dialog-info+xml;
   a new XML namespace; and two new media feature parameters in the SIP
   tree.

8.1.  MIME Registration for application/dialog-info+xml Type

   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 [8].

   Encoding considerations: Same as encoding considerations of
      application/xml as specified in RFC 3023 [8].

   Security considerations: See Section 10 of RFC 3023 [8] and Section 7
      of this specification.

   Interoperability considerations: none.

   Published specification: This document.

   Applications that 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: .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.

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

   This section registers a new XML namespace, per the guidelines in
   [7].

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

   Registrant Contact: The IESG, <iesg@ietf.org>
   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>urn:ietf:params:xml:ns:dialog-info</h2>
        <p>See <a href="ftp://ftp.rfc-editor.org/in-notes/rfc4235.txt">
             RFC4235</a>.</p>
      </body>
      </html>
      END

8.3.  Schema Registration

   This specification registers a schema, per the guidelines in [7].

      URI: urn:ietf:params:xml:schema:dialog-info

   Registrant Contact: The IESG, <iesg@ietf.org>

      XML: The XML can be found as the sole content of Section 4.4.

8.4.  Media Feature Parameter Registration

   This section registers two new media feature tags, per the procedures
   defined in RFC 2506 [14].  The tags are placed into the sip tree,
   which is defined in [10].

8.4.1. Media Feature Tag sip.byeless
       Media feature tag name sip.byeless

   ASN.1 Identifier 19

   Summary of the media feature indicated by this tag: This feature tag
   is a boolean flag.  When set it indicates that the device is
   incapable of terminating a session autonomously.

   Values appropriate for use with this feature tag: Boolean.

   The feature tag is intended primarily for use in the following
   applications, protocols, services, or negotiation mechanisms: This
   feature tag is most useful in a communications application for
   describing the capabilities of an application, such as an
   announcement service, recording service, conference, or call center.

   Examples of typical use: Call centers and media services.

   Related standards or documents: RFC 4235
   Security Considerations: This media feature tag can be used in ways
   that affect application behaviors or may reveal private information.
   For example, a conferencing or other application may decide to
   terminate a call prematurely if this media feature tag is set.
   Therefore, if an attacker can modify the values of this tag, they may
   be able to affect the behavior of applications.  As a result of this,
   applications that utilize this media feature tag SHOULD provide a
   means for ensuring its integrity.  Similarly, this feature tag should
   only be trusted as valid when it comes from the user or user agent
   described by the tag.  As a result, protocols for conveying this
   feature tag SHOULD provide a mechanism for guaranteeing authenticity.

8.4.2.  Media Feature Tag sip.rendering

   Media feature tag name: sip.rendering

   ASN.1 Identifier: 20

   Summary of the media feature indicated by this tag: This feature tag
      contains one of three string values indicating if the device is
      rendering any media from the current session ("yes"), none of the
      media from the current session ("no"), or if this status is not
      known to the device ("unknown").

   Values appropriate for use with this feature tag: String.

   The feature tag is intended primarily for use in the following
      applications, protocols, services, or negotiation mechanisms: This
      feature tag is most useful in a communications application, for
      describing the state of a device (such as a phone or PDA) during a
      multimedia session.

   Examples of typical use: Conferencing, telephone shared-line
      emulation, and presence applications.

   Related standards or documents: RFC 4235

   Security Considerations: This media feature tag can be used in ways
      that affect application behaviors or may reveal private
      information.  For example, a conferencing or other application may
      decide to terminate a call prematurely if this media feature tag
      is set to "no".  Therefore, if an attacker can modify the values
      of this tag, they may be able to affect the behavior of
      applications.  As a result of this, applications that utilize this
      media feature tag SHOULD provide a means for ensuring its
      integrity.  Similarly, this feature tag should only be trusted as
      valid when it comes from the user or user agent described by the
      tag.  As a result, protocols for conveying this feature tag SHOULD
      provide a mechanism for guaranteeing authenticity.

9.  Acknowledgements

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

10.  References

10.1.  Normative References

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

   [2]   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.

   [3]   Rosenberg, J., "The Session Initiation Protocol (SIP) UPDATE
         Method", RFC 3311, October 2002.

   [4]   Paoli, J., Sperberg-McQueen, C., Bray, T., 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]   Moats, R., "A URN Namespace for IETF Documents", RFC 2648,
         August 1999.

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

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

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

   [10]  Rosenberg, J., Schulzrinne, H., and P. Kyzivat, "Indicating
         User Agent Capabilities in the Session Initiation Protocol
         (SIP)", RFC 3840, August 2004.

   [11]  Sparks, R., "The Session Initiation Protocol (SIP) Referred-By
         Mechanism", RFC 3892, September 2004.

   [12]  Sparks, R., "The Session Initiation Protocol (SIP) Refer
         Method", RFC 3515, April 2003.

   [13]  Mahy, R., Biggs, B., and R. Dean, "The Session Initiation
         Protocol (SIP) "Replaces" Header", RFC 3891, September 2004.

   [14]  Holtman, K., Mutz, A., and T. Hardie, "Media Feature Tag
         Registration Procedure", BCP 31, RFC 2506, March 1999.

   [15]  Dierks, T. and C. Allen, "The TLS Protocol Version 1.0", RFC
         2246, January 1999.

10.2.  Informative References

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

   [17]  Rosenberg, J., "A Watcher Information Event Template-Package
         for the Session Initiation Protocol (SIP)", RFC 3857, August
         2004.

   [18]  Mahy, R., "A Message Summary and Message Waiting Indication
         Event Package for the Session Initiation Protocol (SIP)", RFC
         3842, August 2004.

   [19]  Rosenberg, J., Peterson, J., Schulzrinne, H., and G. Camarillo,
         "Best Current Practices for Third Party Call Control (3pcc) in
         the Session Initiation Protocol (SIP)", BCP 85, RFC 3725, April
         2004.

   [20]  Sparks, R., "Session Initiation Protocol Call Control -
         Transfer", Work in Progress, July 2005.

Authors' Addresses

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

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

   Henning Schulzrinne
   Columbia University
   M/S 0401
   1214 Amsterdam Ave.
   New York, NY  10027
   US

   EMail: schulzrinne@cs.columbia.edu
   URI:   http://www.cs.columbia.edu/~hgs

   Rohan Mahy (editor)
   SIP Edge LLC

   EMail: rohan@ekabal.com

Full Copyright Statement

   Copyright (C) The Internet Society (2005).

   This document is subject to the rights, licenses and restrictions
   contained in BCP 78, and except as set forth therein, the authors
   retain all their rights.

   This document and the information contained herein are provided on an
   "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS
   OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE INTERNET
   ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED,
   INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE
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Acknowledgement

   Funding for the RFC Editor function is currently provided by the
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