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In: MissingRef
BLISS                                                   A. Johnston, Ed.
Internet-Draft                                                     Avaya
Expires: March 28, 2009                                  M. Soroushnejad
                                                       V. Venkataramanan
                                                   Sylantro Systems Corp
                                                               P. Pepper
                                                      Citel Technologies
                                                                A. Kumar
                                                              Yahoo Inc.
                                                      September 24, 2008


 Shared Appearances of a Session Initiation Protocol (SIP)  Address of
                              Record (AOR)
                 draft-ietf-bliss-shared-appearances-00

Status of this Memo

   By submitting this Internet-Draft, each author represents that any
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   This Internet-Draft will expire on March 28, 2009.

Abstract

   This document describes the requirements and implementation of a
   group telephony feature commonly known as Bridged Line Appearance
   (BLA) or Multiple Line Appearance (MLA), or Shared Call/Line
   Appearance (SCA).  When implemented using the Session Initiation
   Protocol (SIP), it is referred to as Shared Appearances (SA) of an



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   Address of Record (AOR) since SIP does not have the concept of lines.
   This feature is commonly offered in the IP Centrex services and IP-
   PBX offerings and is likely to be implemented on SIP IP telephones
   and SIP feature servers used in a business environment.  This
   document lists requirements and compares implementation options for
   this feature.  Extensions to the SIP dialog event package are
   proposed.


Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  4
   2.  Conventions used in this document  . . . . . . . . . . . . . .  5
   3.  Usage Scenarios  . . . . . . . . . . . . . . . . . . . . . . .  5
     3.1.  Executive/Assistant Arrangement  . . . . . . . . . . . . .  5
     3.2.  BLA Call Group . . . . . . . . . . . . . . . . . . . . . .  5
     3.3.  Single Line Extension  . . . . . . . . . . . . . . . . . .  6
   4.  Requirements . . . . . . . . . . . . . . . . . . . . . . . . .  6
   5.  Normative Description  . . . . . . . . . . . . . . . . . . . .  7
     5.1.  Implementation . . . . . . . . . . . . . . . . . . . . . .  7
     5.2.  SA Dialog Package Extensions . . . . . . . . . . . . . . . 10
       5.2.1.  The <appearance> element . . . . . . . . . . . . . . . 11
       5.2.2.  The <exclusive> element  . . . . . . . . . . . . . . . 11
       5.2.3.  The <joined-dialog> element  . . . . . . . . . . . . . 11
     5.3.  Shared Appearance User Agents  . . . . . . . . . . . . . . 11
     5.4.  Appearance Agent . . . . . . . . . . . . . . . . . . . . . 13
   6.  XML Schema Definition  . . . . . . . . . . . . . . . . . . . . 14
   7.  User Interface Considerations  . . . . . . . . . . . . . . . . 15
     7.1.  Appearance Number Rendering  . . . . . . . . . . . . . . . 15
       7.1.1.  Single Appearance UAs  . . . . . . . . . . . . . . . . 16
       7.1.2.  Dual Appearance UAs  . . . . . . . . . . . . . . . . . 16
       7.1.3.  Shared Appearance UAs with Fixed Appearance Number . . 16
       7.1.4.  Shared Appearance UAs with Variable Appearance
               Number . . . . . . . . . . . . . . . . . . . . . . . . 16
     7.2.  Call State Rendering . . . . . . . . . . . . . . . . . . . 17
   8.  Interop with non-SA UAs  . . . . . . . . . . . . . . . . . . . 17
     8.1.  Appearance Assignment  . . . . . . . . . . . . . . . . . . 18
     8.2.  Appearance Release . . . . . . . . . . . . . . . . . . . . 18
     8.3.  UAs Supporting Dialog Events but Not SA  . . . . . . . . . 18
   9.  Provisioning Considerations  . . . . . . . . . . . . . . . . . 19
   10. Example Message Flows  . . . . . . . . . . . . . . . . . . . . 19
     10.1. Registration and Subscription  . . . . . . . . . . . . . . 19
     10.2. Appearance Selection for Outgoing Call . . . . . . . . . . 22
     10.3. Taking an Appearance . . . . . . . . . . . . . . . . . . . 26
     10.4. Appearance Selection for Incoming Call . . . . . . . . . . 33
     10.5. Appearance Publication . . . . . . . . . . . . . . . . . . 37
     10.6. Joining an Appearance  . . . . . . . . . . . . . . . . . . 39
     10.7. Appearance Allocation - Loss of Subscription with UA . . . 42



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     10.8. Appearance Selection Contention Race Condition . . . . . . 43
   11. IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 44
     11.1. SIP Event Package Parameter: sa  . . . . . . . . . . . . . 44
     11.2. URN Sub-Namespace Registration: sa-dialog-info . . . . . . 44
     11.3. XML Schema Registration  . . . . . . . . . . . . . . . . . 45
   12. Appendix A - Incoming Appearance Assignment  . . . . . . . . . 45
   13. Appendix B - Implementation Options Discussion . . . . . . . . 46
     13.1. Appearance Implementation Options  . . . . . . . . . . . . 46
       13.1.1. URI parameter Approach . . . . . . . . . . . . . . . . 46
       13.1.2. Dialog Package Parameter . . . . . . . . . . . . . . . 47
       13.1.3. Appearance Selections Mechanisms . . . . . . . . . . . 50
     13.2. Comparison . . . . . . . . . . . . . . . . . . . . . . . . 52
       13.2.1. Comparison of Appearance Selection Methods . . . . . . 53
   14. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 54
   15. Security Considerations  . . . . . . . . . . . . . . . . . . . 54
   16. Informative References . . . . . . . . . . . . . . . . . . . . 55
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 56
   Intellectual Property and Copyright Statements . . . . . . . . . . 57

































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1.  Introduction

   The feature and functionality requirements for SIP user agents (UAs)
   supporting business telephony applications differ greatly from basic
   SIP user agents, both in terms of services and end user experience.
   In addition to basic SIP support [RFC3261], many of the services in a
   business environment require the support for SIP extensions such as
   REFER [RFC3515], SUBSCRIBE/NOTIFY primitives [RFC3265], PUBLISH
   [RFC3903], the SIP Replaces [RFC3891], and Join [RFC3911], header
   fields, etc.  Many of the popular business services have been
   documented in the SIP Service Examples
   [I-D.ietf-sipping-service-examples].

   This specification details a method for implementing a group
   telephony feature known in telephony as Bridged Line Appearance (BLA)
   or Multiple Line Appearances (MLA), one of the more popular advanced
   features expected of SIP IP telephony devices in a business
   environment.  Other names for this feature include Shared Call/Line
   Appearance (SCA), Shared Call Status and Multiple Call Appearance
   (MCA).  A variant of this feature is known as Single Line Extension.

   This document looks at how this feature can be implemented using
   standard SIP [RFC3261] in conjunction with [RFC3265] and [RFC3903]
   for exchanging status among user agents, and the SIP dialog state
   event package [RFC4235] to exchange dialog state information to
   achieve the same.  Different approaches will be discussed including
   the use of URI parameters, feature tags, and dialog package
   extensions along with the strengths and weaknesses of the various
   approaches.

   A call flow for Single Line Extension was formerly included in the
   SIP Service Examples [I-D.ietf-sipping-service-examples].  However,
   the attempt to implement using standard SIP primitives ultimately
   failed, leading to its removal from that document.  This document
   defines SIP extensions to implement this service.

   In traditional telephony, the line is physical.  A common scenario is
   for a number of business telephones to share a single or a small
   number of Address of Record (AOR) URIs.  The sharing of this AOR
   between multiple UAs is what gives this feature its name.  In
   addition, an AOR can have multiple appearances on a single UA in
   terms of the user interface.  The appearance number relates to the
   user interface for the telephone - typically each appearance or an
   AOR has a visual display (lamp that can change color or blink) and a
   button (used to select the appearance).  The telephony concept of
   line aappearance is still relevant to SIP due to the user interface
   considerations.  It is important to keep the appearance number
   construct because:



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   1.  Human users are used to the concept and will expect it in
       replacement systems (e.g. an overhead page announcement says "Joe
       pickup line 3").
   2.  It is a useful structure for user interface representation.

   In this document, we will use the term "appearance" rather than "line
   appearance" since SIP does not have the concept of lines.  Note that
   this does not mean that a conventional telephony user interface
   (lamps and buttons) must be used - implementations may use another
   metaphor as long as the appearance number is readily apparent to the
   user.  Each AOR has a separate appearance numbering space.  As a
   result, a given UA user interface may have multiple occurrences of
   the same appearance number, but they will be for different AORs.


2.  Conventions used in this document

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in RFC-2119 [RFC2119] and
   indicate requirement levels for compliant mechanisms.


3.  Usage Scenarios

   The following examples are common applications of the Shared
   Appearances feature and are mentioned here as informative use cases.
   All these example usages can be supported by the Shared Appearances
   feature described in this document.  The differences relate to the
   user interface considerations of the device.

3.1.  Executive/Assistant Arrangement

   The appearances on the executive's UA may also appear on the
   assistant's UA.  The assistant may answer incoming calls to the
   executive and then place the call on hold for the executive to pick
   up.  The assistant can always see the state of all calls on the
   executive's UA.

3.2.  BLA Call Group

   Users with similar business needs or tasks can be assigned to
   specific groups and share the line appearances of each other on each
   others SIP telephony devices.  For example, an IT department staff of
   five might answer a help line which has three appearances on each
   phone in the IT work area.  A call answered on one phone can be put
   on hold and picked up on another phone.  A shout or an IM to another
   staff member can result in them taking over a call on a particular



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   appearance.  Another phone can request to be added to an appearance
   resulting in a conference call.

3.3.  Single Line Extension

   In this scenario, incoming calls are offered to a group of UAs.  When
   one answers, the other UAs are informed.  If another UA in the group
   selects the line (i.e. goes off hook), it is immediately bridged or
   joined in with the call.  This mimics the way residential telephone
   extensions usually operate.


4.  Requirements

   The basic requirements of the shared appearance feature can be
   summarized as follows:

   REQ-1 Incoming calls to the AOR must be offered to a group of UAs and
   can be answered by any of them.

   REQ-2 Each UA in the group must be able to learn the call status of
   the others in the group for the purpose of rendering this information
   to the user.

   REQ-3 Calls can be joined (also called bridged or conferenced
   together) or can be picked up (taken) by another UA in the group in a
   secure way.

   REQ-4 The mechanism should require the minimal amount of
   configuration.  UAs registering against the group AOR should be able
   to learn about each other and join the appearance group.

   REQ-5 The mechanism must scale for large numbers of appearances, n,
   and large numbers of UAs, N, without introducing excessive messaging
   traffic.

   REQ-6 Each call or session (incoming or outgoing) must be assigned a
   common "appearance" number from a managed pool administered for the
   AOR group.  Once the session has terminated, the appearance number is
   released back into the pool and can be reused by another incoming or
   outgoing session.

   REQ-7 Each UA in the group must be able to learn the appearance
   status of the the group.

   REQ-8 There must be mechanisms to resolve appearance contention among
   the UAs in the group.




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   REQ-9 The mechanism must allow all UAs receiving an incoming session
   request to select the same appearance number at the time of alerting.

   REQ-10 The mechanism must have a way of reconstructing appearance
   state after an outage that does not result in excessive traffic and
   processing.

   REQ-11 The mechanism must have backwards compatibility such that a UA
   which is unaware of the feature can still register against the group
   AOR and make and receive calls.

   REQ-12 The mechanism must not allow UAs outside the group to select
   or manipulate appearance numbers.

   REQ-13 For privacy reasons, there must be a mechanism so that
   appearance information is not leaked outside the group of UAs. (e.g.
   "So who do you have on line 1?")

   REQ-14 The mechanism must support a way for UAs to request
   exclusivity on a line appearance.  Exclusivity means that the UA
   requesting it desires to have a private conversation with the
   external party and other UAs must not be allowed to barge-in.
   Exclusivity may be requested at the start of an incoming or outgoing
   session or during the session.  An exclusivity request may be
   accepted or rejected by the entity providing the SA service.
   Therefore, the mechanism must provide a way of communicating the
   result back to the requester UA.

   REQ-15 The mechanism should support a way for a UA to select a
   particular appearance number for outgoing requests prior to sending
   the actual request.  This is often called seizure.

   REQ-16 The mechanism should support a way for a UA to select a
   particular appearance number and also send the request at the same
   time.  This is needed when a ringdown feature is combined with shared
   appearances - in this case, seizing the line is the same thing as
   dialing.


5.  Normative Description

   This section normatively describes the SA feature extensions.

5.1.  Implementation

   Many of the requirements for this service can be met using standard
   SIP mechanisms such as:




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   - A SIP Forking Proxy and Registrar/Location Service meets REQ-1.

   - The SIP Dialog Package meets REQ-2.

   - The SIP Replaces and Join header fields meets REQ-3.

   - The SIP Registration Package meets REQ-4.

   - The use of a State Agent for the Dialog Package meets REQ-5.

   REQ-6 suggests the need for an entity which manages the appearance
   resource.  Just as conferencing systems commonly have a single point
   of control, known as a focus, a Shared Appearance group has a single
   point of control of the appearance shared resource.  This is defined
   as an Appearance Agent for a group.  While an Appearance Agent can be
   part of a centralized server, it could also be co-resident in a
   member User Agent who has taken on this functionality for a group.
   The Appearance Agent learns the group state either by subscribing to
   the dialog state of each member UA individually or by dialog state
   publications from members.

   While the appearance resource could be managed co-operatively by a
   group of UAs without any central control, this is not discussed in
   this draft, but instead is left as a research project for future
   standardization.  It is also possible that the Appearance Agent logic
   could be distributed in all UAs in the group.  For example, rules
   that govern assigning appearance numbers for incoming requests (e.g.
   lowest available appearance number) and rules for contention handling
   (e.g. when two UAs request the use of the same appearance number,
   hash dialog identifiers and compare with the lowest hash winning)
   would need to be defined and implemented.

   REQs 6-13 can be implemented using a number of approaches, as
   discussed in the following sections.

   Figure 1 illustrates the SIP components involved in supporting these
   common requirements of the Shared Appearance using standard SIP
   messages including REGISTER, INVITE, SUBSCRIBE, NOTIFY, and PUBLISH.













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   +----------------------------+                            +----+
   |                            |                            |    |
   |     Appearance Agent       |                            | UA |
   |                            |                            |    |
   +----------------------------+                            +----+
       ^ ^ |1)SUBSCRIBE  ^  ^    4)NOTIFY            INVITE     |
       | | |(Event:reg)  |  | registration sip:alice@example.com|
       | | V             |  |     events                        V
       | | +--------------------+        +----------+7)Query+--------+
       | | |    (example.com)   |        |          |<===== |        |
       | | |                    |3) Store| Location |       | Proxy  |
       | | |     Registrar      |=======>|  Service |       |        |
       | | |                    |        |          |=====> |        |
       | | +--------------------+        +----------+8)Resp +--------+
       | |     ^       ^                                       |  |
       | |     |       |  2) REGISTER (alice)                  |  |
       | |     |       |                                       |  |
       | |   +----+ +----+                                     |  |
       | |   |    | |    |                                     |  |
       | |   |UA1 | |UA2 |                                     |  |
       | |   |    | |    |                                     |  |
       | |   +----+ +----+                                     |  |
       | |    ^  ^    ^ ^                                      |  |
       | |    |  |    | |                                      |  |
       | +----+  |    | |                                      |  |
       |         |    | +--------------------------------------+  |
       |         +----+-------------------------------------------+
       |              |              8) INVITE
       +--------------+            sip:alice@example.com
       5-7) SUBSCRIBE and/or PUBLISH
            (Event:dialog)

   Figure 1.

   The next section discusses normal SIP operations used to implement
   parts of the shared appearance feature.

   1.  The Appearance Agent SUBSCRIBES to the registration event package
       as outlined in [RFC3680] for contacts registered to the group
       AOR.  Thus, it has knowledge of all User Agents registered
       against the AOR at any point of time.
   2.  UAs (UA1 and UA2 in Figure 1) belong to the appearance group and,
       after authentication, register against the same AOR (e.g.,
       sip:alice@example.com).
   3.  Each registration is stored in the Location Service.
   4.  The registrar notifies the Appearance Agent of successful
       registration at each UA.




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   5.  UAs PUBLISH their dialog state to the State Agent in the
       Appearance Agent.
   6.  The UAs SUBSCRIBE to the Appearance Agent for the state of all
       dialogs as defined in [RFC3265] .  The Request-URI of the
       SUBSCRIBE could be either the AOR of the group, the Contact URI
       information it received in the incoming subscription from the
       Appearance Agent, or a provisioned URI.
   7.  The UAs PUBLISH their dialog information to the Appearance Agent
       every time their dialog state changes (i.e. receive an INVITE,
       enter alerting state, answer a call, terminate a call, generate
       an INVITE, etc.)
   8.  Forking Proxy forks an incoming INVITE for the AOR address to the
       registered user agents.

   The User Agents in the group could SUBSCRIBE to each other and NOTIFY
   dialog state events, but in a large group the User Agents have to
   manage a larger number of SUBSCRIPTIONS and NOTIFICATIONS.  The State
   Agent in the Appearance Agent helps in managing large groups better.
   Further, the State Agent can filter dialog state events and NOTIFY
   User Agents of the dialog state events which are required for the
   application or feature.  The State Agent can also SUBSCRIBE to dialog
   state events with filters to reduce the number of NOTIFY messages
   exchanged between the State Agent and the user agents in the group.
   This allows a group of N UAs to each only establish a pair of dialog
   state subscriptions (one in each direction) to learn the dialog state
   of all other group members.  This results in 2N total subscriptions
   for the entire group.  A full mesh of subscriptions without a state
   agent would result in N(N-1) total subscriptions.

   The Appearance Agent can select the appearance number for an incoming
   call

   OPEN ISSUE: Do we want to define another mode of operation in which
   UAs only PUBLISH to seize a line appearance?  This assumes the
   Appearance Agent already knows about all dialogs related to the AOR
   and could publish that information to the UAs in the SA group.  This
   approach would simply UA operation and cleanly resolve some race
   conditions.  Should we define this mode in a separate draft?

5.2.  SA Dialog Package Extensions

   This specification defines three new elements as extensions to the
   SIP Dialog Event package [RFC3265] .  The schema is defined in
   Section 7.  The elements are <appearance>, <exclusive>;, and <joined-
   dialog>.  All three elements are sub-elements of the <dialog>
   element.





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5.2.1.  The <appearance> element

   The <appearance> element is used convey the appearance number.  The
   appearance number is a non-negative integer.  When sent in a
   notification in state Trying to the Appearance Agent, it is used to
   request an appearance number.  When sent by the Appearance Agent, it
   indicates that the appearance number is associated with a dialog.

5.2.2.  The <exclusive> element

   The <exclusive> element is a boolean used indicate whether the UA
   will accept Join or Replaces INVITEs for this dialog.  For example,
   some SA systems only allow call pickup when the call is on hold.  In
   this case, the <exclusive> element should be used to explicitly
   indicate this, rather than implicitly by the hold state.

   It is important to note that this element is a hint.  Although a UA
   may set exclusive to true, the UA must still be ready to reject an
   INVITE Join relating to this dialog.  Also, an INVITE Replaces might
   be sent to the non-SA UA to take the call.  For this reason, a UA MAY
   also not report full dialog identifier information to the Appearance
   Agent for calls set to exclusive.  If these dialog identifiers have
   already been shared with the Appearance Agent, the UA could send an
   INVITE Replaces to change them and then not report the new ones to
   the Appearance Agent.

   If the proxy knows which dialogs are marked exclusive, the proxy MAY
   enforce this exclusivity by rejecting INVITE Join and INVITE Replace
   requests containing those dialog identifiers with a 403 Forbidden
   response.

5.2.3.  The <joined-dialog> element

   The <joined-dialog> element is used convey dialog identifiers of any
   other dialogs which are joined (mixed or bridged) with the dialog.
   Only the UA which is performing the actual media mixing should
   include this element in notifications to the Appearance Agent.  Note
   that this element should still be used even when the Join header
   field was not used to join the dialogs.  For example, two separate
   dialogs on a UA could be joined without any SIP call control
   operations.  The UA would report this using this header field.

5.3.  Shared Appearance User Agents

   User Agents that support the Shared Appearance feature MUST support
   the dialog state package [RFC4235] with the SA extensions and the
   "sa" dialog event package parameter defined in this draft.




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   User Agents MUST use the extended package in conjunction with PUBLISH
   [RFC3903] to send local status to the Appearance Agent, and in
   conjunction with SUBSCRIBE and NOTIFY [RFC3265] to learn the status
   or other User Agents.  The publication URI is either a provisioned
   value or the username 'appearance-agent'.  For example, a UA in the
   domain of example.com would publish dialog state to
   sip:appearance-agent@example.com

   OPEN ISSUE: Do we need to specify a publish URI or should it be only
   by provisioning?  Should a UA be able to publish to its own URI for
   this?

   User Agents SHOULD support sending and receiving an INVITE with a
   Replaces [RFC3891] header to allow the Call Pickup operation.  User
   Agents MUST support sending an INVITE a Join [RFC3911] header to
   initiate bridging.  User Agents MUST support receiving an INVITE with
   a Join header, though they are not obligated to support bridging of
   calls, either through policy, preference, or implementation
   limitations such as bandwidth or hardware constraints.

   When publishing dialog package information, a UA MUST include all
   dialog identification available at the time of publication, with the
   exception that a UA may omit information if it wishes to prevent
   other UAs from joining or picking up a call.  Dialog identification
   includes local and remote target URIs, call-id, to-tag, and from-tag.
   When calls are placed on hold, the "+sip.rendering=no" feature tag
   MUST be included in dialog package notifications.

   There are two exceptions to the requirement for seizing an appearance
   before sending an INVITE.  One is an emergency call, which should
   proceed regardless of the status of PUBLISH transaction.  This
   requirement applies to both clients and servers implementing this
   feature.  The other is when the INVITE is an attempt to bridge or
   take a call (i.e. contains Join or Replaces with a dialog identifier
   of another member of the SA group).  In this case, the INVITE Join or
   Replaces should be sent without selecting an appearance.  After the
   INVITE succeeds, the PUBLISH MUST be sent, reusing the appearance
   number of the dialog that has been bridged or taken.

   UAs can tell that a set of dialogs are joined (bridged or mixed)
   together by the presence of one or more <joined-dialog> elements
   containing other SIP dialog identifiers.

   Prior to placing an outbound call, UAs may select or "seize" an
   appearance number by sending a PUBLISH to the Appearance Agent with
   an <appearance> identifying the appearance number selected.  In
   traditional telephony terms, this corresponds to "going off hook"
   with an analog telephone.  Note that when a UA selects an appearance



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   prior to establishment of a dialog, not all dialog information will
   be available.  In particular, when a UA publishes an attempt to
   select an appearance prior to knowing the destination very minimal
   dialog information may be available.

   A UA that does not need to select a particular appearance number (or
   doesn't care) would just send an INVITE as normal to place an
   outbound call.  The Appearance Agent would select an available
   appearance and notify all subscribed UAs of the selection.  The UA
   placing the call then publishes the use of this appearance number.

   If an INVITE is sent and no appearance number is available, the proxy
   would reject the INVITE with a suitable response code and perhaps a
   header field indication.

   For inbound calls which contain a reference to an appearance number
   in the INVITE, a UA MUST PUBLISH the use of an appearance number
   after it responds with a 2xx to establish a dialog.  A UA SHOULD NOT
   PUBLISH the use of an appearance number for incoming requests which
   are in the early state or which are rejected with 4xx, 5xx or 6xx
   responses, as this would create excessive traffic when a large number
   of UAs are sharing an appearance.

   The dialog state package defined in [RFC4235] defines the set of
   messages that MAY be provided by a UA to publish state information of
   dialogs.

   A UA should only register against the AOR if it is likely the UA will
   be answering incoming calls.  If the UA is mainly going to be
   monitoring the status of the SA group calls and picking or joining
   calls, the UA SHOULD only subscribe to the Appearance Agent and not
   register against the AOR.

5.4.  Appearance Agent

   An Appearance Agent defined in this specification MUST implement a
   dialog package state agent for the UAs registered against the AOR.

   The dialog package XML element dialog id (NOT the SIP dialog
   identifier consisting of the Call-ID, To tag, and From tag) is used
   for partial update processing.  RFC 4235 only requires the dialog id
   be unique for the UA, not unique across all UAs associated with the
   same AOR.  As a result, is possible that two UAs in a SA group might
   choose the same XML element dialog id for different dialogs.  If this
   is the case, the Appearance Agent, acting as the dialog package event
   state compositor may need to produce different notification documents
   for the UAs that have conflicting dialog package XML element dialog
   ids.



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   The Appearance Agent MUST support the appearance dialog package
   extensions defined in this specification.

   Even in trivial deployments of two shared appearance enabled user
   agents, race conditions can exist when both user agents attempt to
   utilize an appearance simultaneously (i.e. when the NOTIFY messages,
   that each user agent sends to the other, are active within the
   network during an overlapping time period).  The Appearance Agent can
   use any policy deemed necessary to resolve races and ensure no two
   user agents are not allocated the same appearance number at the same
   time.

   Appearance Agents are responsible for resolving conflicts in the
   appearance resource.  If a UA requests the use of an appearance
   number that is in use by another UA, the Appearance Agent will
   respond based on the presence of the selection attribute in the
   <appearance> element as described in the previous section.

   In the case where a UA is bridging two calls, the Appearance Agent
   may receive dialog package PUBLISHes that contain multiple dialogs
   with the same appearance number.  This is valid and does not
   represent appearance number contention.

   A critical aspect for reliable operation of this feature is the
   ability for all user agents in the BLA group to recover from network
   failures caused at a single UA.  For example, one of the user agents
   in the BLA group may have answered an incoming call, notified the
   dialog state to other members and then experienced a network outage.
   The calling UA could have detected the same (using RTP or some other
   means) and could have hung up.  However, none of the other user
   agents in the BLA group would get notification of this change in
   dialog state and their BLA appearances could stay out of sync for a
   long time; depending on when the network is restored, or when the
   Appearance Agent attempts to refresh its dialog-state subscription
   with the failed UA.  To recover from such a failure, UAs MUST PUBLISH
   with a shorter expiration (expiration interval not smaller than 300
   seconds is RECOMMENDED) following the notification of a "confirmed"
   dialog or when a Appearance Agent honors a "trying" for call
   origination, with the user agents that notified it of this
   information.


6.  XML Schema Definition

   The 'appearance' and 'exclusive' elements are defined within a new
   XML namespace URI.  This namespace is
   "urn:ietf:params:xml:ns:sa-dialog-info".  The schema for these
   elements is:



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


   <xs:element name="joined-dialog" minOccurs="0" maxOccurs="unbounded">
          <xs:complexType>
            <xs:attribute name="call-id" type="xs:string"
              use="mandatory"/>
            <xs:attribute name="local-tag" type="xs:string"
              use="mandatory"/>
            <xs:attribute name="remote-tag" type="xs:string"
              use="mandatory"/>
           </xs:complexType>
        </xs:element>

        <xs:element name="appearance" minOccurs="0" maxOccurs="1">
          <xs:simpleType type="xs:integer">
          </xs:simpleType>
        </xs:element>

        <xs:element name="exclusive" minOccurs="0" maxOccurs="1">
         <xs:simpleType type="xs:boolean">
         </xs:simpleType>
        </xs:element>
  </xs:schema>



7.  User Interface Considerations

   The "appearance number" allocated to a call is an important concept
   that enables calls to be handled by multiple devices with
   heterogeneous user interfaces in a manner that still allows users to
   see a consistent model.  Careful treatment of the appearance number
   is essential to meet the expectations of the users.  Also, rendering
   the correct call/appearance state to users is also important.

7.1.  Appearance Number Rendering

   Since different UAs have different user interface capabilities, it is
   usual to find that some UAs have restrictions that others do not.
   Perfect interoperability across all UAs is clearly not possible, but
   by careful design, interoperability up to the limits of each UA can
   be achieved.



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   The following guidelines suggest how the appearance number should be
   handled in three typical user interface implementations.

7.1.1.  Single Appearance UAs

   These devices are constrained by only having the capability of
   displaying status indications for a single appearance.  Despite this,
   it is important that devices of this type do not ignore the
   appearance number.  The UA should still send messages annotated with
   an appropriate appearance number (i.e. "0").  Any call indications
   for appearances other than for number "0" should be rejected with a
   486 or 480 response.

7.1.2.  Dual Appearance UAs

   These devices are essentially single appearance phones that implement
   call waiting.  They have a very simple user interface that allows
   them to switch between two appearances (toggle or flash hook) and
   perhaps audible tones to indicate the status of the other appearance.

7.1.3.  Shared Appearance UAs with Fixed Appearance Number

   This UA is the typical 'business-class' hard-phone.  A number of
   appearances are typically configured statically and labeled on
   buttons, and calls may be managed using these configured appearances.
   Any calls outside this range should be ignored, and not mapped to a
   free button.  Users of these devices often select specific appearance
   numbers for outgoing calls, and the UA will need to select the
   appearance number and wait for confirmation from the Appearance Agent
   before proceeding with calls.

7.1.4.  Shared Appearance UAs with Variable Appearance Number

   This UA is typically a soft-phone or graphically rich user interface
   hard-phone.  In these cases, even the idea of an appearance index may
   seem unnecessary.  However, for these phones to be able to interwork
   successfully with other phone types, it is important that they still
   use the appearance index to govern the order of appearance of calls
   in progress.  No specific guidance on presentation is given except
   that the order should be consistent.  Thought should also be given to
   how an appearance number that has no call associated with it should
   be rendered to the user.  These devices can typically make calls
   without waiting for confirmation from the Appearance Agent on the
   appearance number.

   The problems faced by each style of user interface are readily seen
   in this example:




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   1.  A call arrives at the SA group, and is assigned an appearance
       number of 0.  All UAs should be able to render to the user the
       arrival of this call.
   2.  Another call arrives at the SA group, and is assigned an
       appearance number of 1.  The single appearance UA should not
       present this call to the user.  Other user agents should have no
       problems presenting this call distinctly from the first call.
   3.  The first call clears, releasing appearance number "0".  The
       single appearance UA should now be indicating no calls since it
       is unable to manage calls other than on the first appearance.
       Both shared appearance UAs should clearly show that appearance
       number 0 is now free, but that there is still a call on
       appearance number 1.
   4.  A third call arrives, and is assigned the appearance number of 0.
       All UAs should be able to render the arrival of this new call to
       the user.  Multiple appearnce UAs should continue to indicate the
       presence of the second call, and should also ensure that the
       presentation order is related to the appearance number and not
       the order of call arrival.

7.2.  Call State Rendering

   UAs that implement the SA feature typically have a user interface
   that provides the state of other appearances in the group.  As dialog
   state NOTIFYs from the Appearance Agent are processed, this
   information can be rendered.  Even the simplest user interface
   typically has three states: idle, active, and hold.  The idle state,
   usually indicated by lamp off, is indicated for an appearance when
   the appearance number is not associated with any dialogs, as reported
   by the Appearance Agent.  The active state, usually indicated by a
   lamp on, is indicated by an appearance number being associated with
   at least one dialog, as reported by the Appearance Agent.  The hold
   state, often indicated by a blinking lamp, means the call state from
   the perspective of the UA in the SA group is hold.  This can be
   determined by the presence of the "sip+rendering=no" feature tag
   [RFC3840] with the local target URI.  Note that the hold state of the
   remote target URI is not relevant to this display.  For joined
   dialogs, the state is rendered as hold only if all local target URIs
   are indicated with the "sip+rendering=no" feature tag.


8.  Interop with non-SA UAs

   It is desirable to allow a basic UA that does not directly support SA
   to be part of a SA group.  To support this the Proxy must collaborate
   with the Appearance Agent.  This is not required in the basic SA
   architecture, consequently SA interop with non-SA UAs will not be
   available in all SA deployments.



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   First, a UA which does not support dialog events or the SA feature
   will be discussed.  Then, a UA which does support dialog events but
   not the SA feature will be discussed.

8.1.  Appearance Assignment

   A UA that has no knowledge of appearances must have appearance
   numbers assigned by the Appearance Agent for both incoming and
   outgoing calls.  If the non-SA UA does not support Join or Replaces,
   all dialogs could be marked "exclusive" to indicate that these
   options are not available.

   OPEN ISSUE: How can an Appearance Agent know that a given UA does not
   support the feature?  Do we need a SIP option tag for this purpose?
   Do we need to be able to correlate a registration with a
   subscription?

8.2.  Appearance Release

   In all cases the Appearance Agent must be aware of dialog lifetime to
   release appearances back into the group.

   It is also desirable that any dialog state changes (such as hold,
   etc) be made available to other UAs in the group through the Dialog
   Event Package.  If the Appearance Agent includes a proxy which
   Record-Routes for dialogs from the non-SA aware UA, the Appearance
   Agent will know about the state of dialogs including hold, etc.  This
   information could be determined from inspection of INVITE and re-
   INVITE messages and added to the dialog information conveyed to other
   UAs.

8.3.  UAs Supporting Dialog Events but Not SA

   Interoperability with UAs which support dialog events but not the SA
   feature is more straightforward.  As before, all appearance number
   assignment must be done by the Appearance Agent.  This type of UA
   will be detected by the Appearance Agent by the absence of the ma
   event parameter in SUBSCRIBE or PUBLISH messages.  The Appearance
   Agent can include appearance information in NOTIFYs - this UA will
   simply ignore this extra information.  This type of UA will ignore
   appearance number limitations and may attempt to Join or Replace
   dialogs marked exclusive.  As a result, the Proxy or UAs may need to
   reject such requests.

   The need for close cooperation between the Proxy and the Appearance
   Agent is not needed as the Appearance Agent will learn about all
   dialogs from the UA itself.




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9.  Provisioning Considerations

   TBD.


10.  Example Message Flows

   The next section shows call flow and message examples.  The flows and
   descriptions are non-normative.

   EDITOR'S NOTE: These flows need to be gone over closely to make sure
   they reflect the latest protocol design.

10.1.  Registration and Subscription

   Bob and Alice are in an appearance group identified by Alice's AOR.
   Bob REGISTERs using contact sip:bob@ua2.example.com.  Furthermore,
   Bob REGISTERs his primary address with contact sip:
   bob@ua2.example.com.  Alice REGISTERs with contact
   sip:alice@ua1.example.com.

   The Appearance Agent subscribes to dialog states of the SA AOR (i.e.,
   sip:alice@example.com) at the User Agents for Alice and Bob. Message
   exchange between the Registrar, Appearance Agent, Alice, and Bob are
   shown below.  The call flow examples below do not show challenging of
   subscriptions or notifications.  It should be noted that for security
   purposes, all subscriptions must be authorized before the same is
   accepted.


  Registrar     Appearance Agent          Alice
  |                    |                    |
  |                    |                    |
  |<--------------------------- REGISTER F1<|
  |                    |                    |
  |>F2 200 OK ----------------------------->|
  |                    |                    |
  |                    |<----- SUBSCRIBE F3<|
  |                    |                    |
  |                    |>F4 202 Accepted -->|
  |                    |                    |
  |                    |>F5 NOTIFY -------->|
  |                    |                    |
  |                    |<-------- OK 200 F6<|
  |                    |                    |

  Figure 2.




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  F1-F2: Alice registers AOR with contact: <sip:alice@ua1.example.com>

  F1 Alice ----> Registrar

  REGISTER sip:registrar.example.com SIP/2.0
  Via: SIP/2.0/UDP ua1.example.com;branch=z9hG4bK527b54da8ACC7B09
  From: <sip:alice@example.com>;tag=CDF9A668-909E2BDD
  To: <sip:alice@example.com>
  CSeq: 2 REGISTER
  Call-ID: d3281184-518783de-cc23d6bb@ua1.example.com
  Contact: <sip:alice@ua1.example.com>
  User-Agent: ABC-UA/1.2.3
  Max-Forwards: 70
  Expires: 3600
  Content-Length: 0



  F2 Registrar ----> Alice

  SIP/2.0 200 OK
  Via: SIP/2.0/UDP ua1.example.com;branch=z9hG4bKfbf176ef7F1D5FA2
  CSeq: 2 REGISTER
  Call-ID: d3281184-518783de-cc23d6bb@ua1.example.com
  From: <sip:alice@example.com>;tag=CDF9A668-909E2BDD
  To: <sip:alice@example.com>;tag=1664573879820199
  Contact:  <sip:alice@ua1.example.com>
  Expires:  3600
  Content-Length: 0



  F3 to F6: Alice also subscribes to the events associated with the
  Appearance AOR. Appearance Agent also notifies Alice of the status.

  F3 Alice ----> Appearance Agent

  SUBSCRIBE sip:sa@stateagent.example.com SIP/2.0
  Via: SIP/2.0/UDP ua1.example.com;branch=z9hG4bKf10fac97E7A76D6A
  From: <sip:alice@example.com>;tag=925A3CAD-CEBB276E
  To: <sip:sa@stateagent.example.com>
  CSeq: 1 SUBSCRIBE
  Call-ID: ef4704d9-bb68aa0b-474c9d94@ua1.example.com
  Contact: <sip:alice@ua1.example.com>
  Event: dialog
  User-Agent: ABC-UA/1.2.3
  Accept: application/dialog-info+xml
  Max-Forwards: 70



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  Expires: 3700
  Content-Length: 0


  F4 Appearance Agent ----> Alice

  SIP/2.0 202 Accepted
  Via: SIP/2.0/UDP ua1.example.com;branch=z9hG4bKf10fac97E7A76D6A
  CSeq: 1 SUBSCRIBE
  Call-ID: ef4704d9-bb68aa0b-474c9d94@ua1.example.com
  From: <sip:alice@example.com>;tag=925A3CAD-CEBB276E
  To: <sip:sa@stateagent.example.com>;tag=1636248422222257
  Allow-Events: dialog
  Expires: 3700
  Contact: <sip:sa@stateagent.example.com>
  Content-Length: 0


  F5 Appearance Agent ----> Alice

  NOTIFY sip:alice@ua1.example.com SIP/2.0
  From: <sip:sa@stateagent.example.com>;tag=1636248422222257
  To: <sip:alice@example.com>;tag=925A3CAD-CEBB276E
  Call-ID: ef4704d9-bb68aa0b-474c9d94@ua1.example.com
  CSeq: 2 NOTIFY
  Via: SIP/2.0/UDP stateagent.example.com;branch=z9hG4bK1846984327225734
  Max-Forwards: 70
  Content-Type: application/dialog-info+xml
  Event: dialog
  Subscription-State: active
  Contact: <sip:sa@stateagent.example.com>
  Content-Length: ...

  <?xml version="1.0"?>
  <dialog-info xmlns="urn:ietf:params:xml:ns:dialog-info"
               version="40"
               state="full"
               entity="sip:alice@example.com">
  </dialog-info>


  F6 Alice ----> Appearance Agent

  SIP/2.0 200 OK
  Via: SIP/2.0/UDP stateagent.example.com;branch=z9hG4bK1846984327225734
  From: <sip:sa@stateagent.example.com>;tag=1636248422222257
  To: <sip:alice@example.com>;tag=925A3CAD-CEBB276E
  CSeq: 2 NOTIFY



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  Call-ID: ef4704d9-bb68aa0b-474c9d94@ua1.example.com
  Contact: <sip:alice@ua1.example.com>
  Event: dialog
  User-Agent: ABC-UA/1.2.3
  Content-Length: 0


10.2.  Appearance Selection for Outgoing Call

   In this scenario, Bob's UA sends out a dialog event NOTIFY with state
   (trying) selecting an appearance number.  After receiving the NOTIFY
   from the Appearance Agent confirming the appearance number, Bob's UA
   sends the INVITE to Carol and establishes a session.  For brevity,
   details of some of the messages are not included in the message
   flows.


Carol        Proxy           Alice     Appearance Agent         Bob
|              |               |              |                  |
|              |               |              |<----- PUBLISH F1<|
|              |               |              |                  |
|              |               |              |>F2 200 OK ------>|
|              |               |              |                  |
|              |               |<-- NOTIFY F3<|                  |
|              |               |              |                  |
|              |               |>F4 200 OK -->|                  |
|              |               |              |------- NOTIFY F5>|
|              |               |              |                  |
|              |               |              |<F6 200 OK ------<|
|              |               |              |                  |
|              |               |              |                  |
|              |<------------------------------------ INVITE F11<|
|              |               |              |                  |
|<- INVITE F12<|               |              |                  |
|              |               |              |                  |
|>F13 180  --->|               |              |                  |
|              |>F14 180 Ringing ------------------------------->|
|>F15 200 OK ->|               |              |                  |
|              |>F16 200 OK ------------------------------------>|
|              |               |              |                  |
|<------------------------------------------------------ ACK F17<|
|              |               |              |                  |
|<================= Both way RTP established ===================>|
|              |               |              |                  |
|              |               |              |<---- PUBLISH F18<|
|              |               |              |                  |
|              |               |              |>F19 200 OK ----->|
|              |               |<- NOTIFY F20<|                  |



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|              |               |              |                  |
|              |               |>F21 200 OK ->|                  |
|              |               |              |------ NOTIFY F22>|
|              |               |              |                  |
|              |               |              |<F23 200 OK -----<|
|              |               |              |                  |

Figure 3.

F1 to F4: Bob uses the BLA appearance of Alice on his UA to place an outgoing call (e.g., he goes off-hook). Before sending the outgoing INVITE request, Bob notifies the sate agent that Alice line appearance is in (trying) state. Appearance Agent notifies Alice of the same event by forwarding the NOTIFY payload provided by Bob after appropriately changing the dialog id field in the XML payload to a unique value towards each of the entities it is forwarding to (Alice in this example).
</t>
<figure><artwork><![CDATA[

F1 Bob ----> Appearance Agent

PUBLISH sip:appearance-agent@example.com SIP/2.0
Via: SIP/2.0/UDP ua2.example.com;branch=z9hG4bK61314d6446383E79
From: <sip:bob@example.com>;tag=44150CC6-A7B7919D
To: <sip:appearance-agent@example.com>;tag=428765950880801
CSeq: 7 PUBLISH
Call-ID: 144-1289338424@example.com
Contact: <sip:bob@ua2.example.com>
Event: dialog
User-Agent: XYZ-UA/4.5.6
Max-Forwards: 70
Content-Type: application/dialog-info+xml
Content-Length: ...

<?xml version="1.0"?>
<dialog-info xmlns="urn:ietf:params:xml:ns:dialog-info"
             xmlns:sa="urn:ietf:params:xml:ns:sa-dialog-info"
             version="6"
             state="partial"
             entity="sip:alice@example.com">
    <dialog id="id3d4f9c83" direction="initiator">
        <sa:appearance>0</appearance>
        <sa:exclusive>false</exclusive>
        <state>trying</state>
        <local>
            <target uri="sip:bob@example.com">
            </target>
        </local>
    </dialog>
</dialog-info>


F2 Appearance Agent ----> Bob




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SIP/2.0 200 OK
Via: SIP/2.0/UDP ua2.example.com;branch=z9hG4bK61314d6446383E79
CSeq: 7 PUBLISH
Call-ID: 144-1289338424@example.com
From: <sip:bob@example.com>;tag=44150CC6-A7B7919D
To: <sip:appearance-agent@example.com>;tag=428765950880801
Allow-Events: dialog
Contact: <sip:appearance-agent@stateagent.example.com>
Content-Length: 0


F3 Appearance Agent ----> Alice

NOTIFY sip:alice@ua1.example.com SIP/2.0
From: <sip:alice@example.com>;tag=497585728578386
To: <sip:alice@example.com>;tag=633618CF-B9C2EDA4
Call-ID: a7d559db-d6d7dcad-311c9e3a@ua1.example.com
CSeq: 7 NOTIFY
Via: SIP/2.0/UDP stateagent.example.com;branch=z9hG4bK1711759878512309
Max-Forwards: 70
Content-Type: application/dialog-info+xml
Event: dialog
Subscription-State: active
Contact: <sip:sa@stateagent.example.com>
Content-Length: ...

<?xml version="1.0"?>
<dialog-info xmlns="urn:ietf:params:xml:ns:dialog-info"
             xmlns:sa="urn:ietf:params:xml:ns:sa-dialog-info"
             version="27"
             state="partial"
             entity="sip:alice@example.com">
    <dialog id="fa02538339df3ce597f9e3e3699e28fc" direction="initiator">
            <sa:appearance>0</appearance>
            <sa:exclusive>false</exclusive>
               <state>trying</state>
                <local>
                    <target uri="sip:bob@example.com">
                    </target>
                   </local>
        </dialog>
</dialog-info>


F4 Alice ----> Appearance Agent

SIP/2.0 200 OK
Via: SIP/2.0/UDP stateagent.example.com;branch=z9hG4bK1711759878512309



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From: <sip:alice@example.com>;tag=497585728578386
To: <sip:alice@example.com>;tag=633618CF-B9C2EDA4
CSeq: 7 NOTIFY
Call-ID: a7d559db-d6d7dcad-311c9e3a@ua1.example.com
Contact: <sip:alice@ua1.example.com>
Event: dialog
User-Agent: ABC-UA/1.2.3
Content-Length: 0

F5 and F6:  The Appearance Agent sends a NOTIFY to Bob confirming appearance number.

F11 to F17: Bob places a call to Carol by sending the INVITE request
towards the Proxy. The INVITE (see F5 message below) includes a
P-Preferred-Identity header <xref target="RFC3325" /> to designate the identity to be
used as the calling party for this call (i.e., Alice instead of Bob).

F11 Bob ----> Proxy

INVITE sip:carol@example.com SIP/2.0
Via: SIP/2.0/UDP ua2.example.com;branch=z9hG4bK98c87c52123A08BF
From: <sip:bob@example.com>;tag=15A3DE7C-9283203B
To: <sip:carol@example.com>
CSeq: 1 INVITE
Call-ID: f3b3cbd0-a2c5775e-5df9f8d5@ua2.example.com
Contact: <sip:bob@ua2.example.com>
User-Agent: XYZ-UA/4.5.6
P-Preferred-Identity: <sip:alice@example.com>
Max-Forwards: 70
Content-Type: application/sdp
Content-Length: 223

v=0
o=- 1102980499 1102980499 IN IP4 ua2.example.com
s=IP SIP UA
c=IN IP4 ua2.example.com
t=0 0
a=sendrecv
m=audio 2236 RTP/AVP 0 8 101
a=rtpmap:0 PCMU/8000
a=rtpmap:8 PCMA/8000
a=rtpmap:101 telephone-event/8000



F18 to F21: Bob notifies the Appearance Agent of the status of the
dialog (i.e., confirmed). Appearance Agent notifies Alice of the
same.




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F18 Bob ----> Appearance Agent

NOTIFY sip:sa@stateagent.example.com SIP/2.0
Via: SIP/2.0/UDP ua2.example.com;branch=z9hG4bKa39d3f69D4E20602
From: <sip:bob@example.com>;tag=44150CC6-A7B7919D
To: <sip:alice@example.com>;tag=428765950880801
CSeq: 9 NOTIFY
Call-ID: 144-1289338424@example.com
Contact: <sip:bob@ua2.example.com>
Event: dialog
User-Agent: XYZ-UA/4.5.6
Subscription-State: active;expires=3342
Max-Forwards: 70
Content-Type: application/dialog-info+xml
Content-Length: ...

<?xml version="1.0"?>
<dialog-info xmlns="urn:ietf:params:xml:ns:dialog-info"
             xmlns:sa="urn:ietf:params:xml:ns:sa-dialog-info"
             version="8"
             state="partial"
             entity="sip:alice@example.com">
    <dialog id="id3d4f9c83"
            call-id="f3b3cbd0-a2c5775e-5df9f8d5@ua2.example.com"
            local-tag="15A3DE7C-9283203B"
            remote-tag="65a98f7c-1dd2-11b2-88c6-b03162323164+65a98f7c"
            direction="initiator">
          <state>confirmed</state>
          <sa:appearance>0</appearance>
          <sa:exclusive>false</exclusive>
          <local>
            <target uri="sip:bob@example.com">
               <param pname="+sip.rendering" pval="yes"/>
            </target>
          </local>
          <remote>
            <identity>sip:carol@example.com</identity>
              <target uri="sip:carol@example.com;user=phone" />
          </remote>
    </dialog>
</dialog-info>

10.3.  Taking an Appearance

   In this scenario, Bob has an established dialog with Carol.  Bob then
   places Carol on hold.  Alice subsequently picks up the held call and
   has a established session with Carol.  Finally, Carol hangs up.  The
   details of the notifications amongst the user agents and the



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   Appearance Agent in updating the status of the BLA group members are
   shown below.  For brevity, details of some of the messages are not
   included in the message flows.


Carol        Proxy           Alice     Appearance Agent         Bob
|              |               |              |                  |
|              |               |              |                  |
|<================= Both way RTP established ===================>|
|              |               |              |                  |
|              |               |              |                  |
|              |<------------------------------(hold) INVITE F16<|
|<- INVITE F17<|               |              |                  |
|              |               |              |                  |
|>F18 200 OK ->|               |              |                  |
|              |>F19 200 OK ------------------------------------>|
|              |               |              |                  |
|<------------------------------------------------------ ACK F20<|
|              |               |              |                  |
|              |               |              |<----- NOTIFY F21<|
|              |               |              |                  |
|              |               |              |>F22 200 OK ----->|
|              |               |<- NOTIFY F23<|                  |
|              |               |              |                  |
|              |               |>F24 200 OK ->|                  |
|              |<-- INVITE F25<|              |                  |
|<- INVITE F26<|(w/ Replaces)  |              |                  |
|( w/ Replaces)|               |              |                  |
|>F27 200 OK ->|               |              |                  |
|              |>F28 200 OK -->|              |                  |
|              |               |              |                  |
|<-------------------- ACK F29<|              |                  |
|              |               |              |                  |
|<= Both way RTP established =>|              |                  |
|              |               |              |                  |
|>F30 BYE ---->|               |              |                  |
|              |>F31 BYE --------------------------------------->|
|              |               |              |                  |
|              |<------------------------------------ OK 200 F32<|
|<- 200 OK F33<|               |              |                  |
|              |               |              |                  |
|              |               |              |<----- NOTIFY F34<|
|              |               |              |                  |
|              |               |              |>F35 200 OK ----->|
|              |               |<- NOTIFY F36<|                  |
|              |               |              |                  |
|              |               |>F37 200 OK ->|                  |
|              |               |              |                  |



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|              |               |>F38 NOTIFY ->|                  |
|              |               |              |                  |
|              |               |<- 200 OK F39<|                  |
|              |               |              |>F40 NOTIFY ----->|
|              |               |              |                  |
|              |               |              |<----- 200 OK F41<|
|>F42 BYE ---->|               |              |                  |
|              |>F43 BYE ----->|              |                  |
|              |               |              |                  |
|              |<-- 200 OK F44<|              |                  |
|<--200 OK F45<|               |              |                  |
|              |               |>F46 NOTIFY ->|                  |
|              |               |              |                  |
|              |               |<- 200 OK F47<|                  |
|              |               |              |>F48 NOTIFY ----->|
|              |               |              |                  |
|              |               |              |<----- OK 200 F49<|

Figure 4.

F16 to F20: Bob places Carol on hold.

F22 to F24: Bob notifies Appearance Agent of the status of the dialog to
indicate the held state. It indicates this by setting the sip.rendering
parameter in the NOTIFY payload to (no). Appearance Agent notifies
Alice of the same.

F22 Bob ----> Appearance Agent

NOTIFY sip:sa@stateagent.example.com SIP/2.0
Via: SIP/2.0/UDP ua2.example.com;branch=z9hG4bK6c78a6c5CA00520E
From: <sip:bob@example.com>;tag=44150CC6-A7B7919D
To: <sip:alice@example.com>;tag=428765950880801
CSeq: 10 NOTIFY
Call-ID: 144-1289338424@example.com
Contact: <sip:bob@ua2.example.com>
Event: dialog
User-Agent: XYZ-UA/4.5.6
Subscription-State: active;expires=3338
Max-Forwards: 70
Content-Type: application/dialog-info+xml
Content-Length: ...

<?xml version="1.0"?>
<dialog-info xmlns="urn:ietf:params:xml:ns:dialog-info"
             xmlns:sa="urn:ietf:params:xml:ns:sa-dialog-info"
             version="9"
             state="partial"



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             entity="sip:alice@example.com">
    <dialog id="id3d4f9c83"
       call-id="f3b3cbd0-a2c5775e-5df9f8d5@ua2.example.com"
       local-tag="15A3DE7C-9283203B"
       remote-tag="65a98f7c-1dd2-11b2-88c6-b03162323164+65a98f7c"
       direction="initiator">
       <state>confirmed</state>
       <sa:appearance>0</appearance>
       <sa:exclusive>false</exclusive>
       <local>
         <target uri="sip:bob@example.com">
             <param pname="+sip.rendering" pval="no"/>
         </target>
       </local>
       <remote>
         <identity>sip:carol@example.com</identity>
           <target uri="sip:carol@example.com" />
        </remote>
    </dialog>
</dialog-info>




F26 to F34 : Alice picks up the held call by sending an INVITE with
Replaces: header (F26). Session is established between Alice and
Carol. The dialog between Carol and Bob is terminated.

F26 Alice ----> Proxy

INVITE sip:carol@example.com SIP/2.0
Via: SIP/2.0/UDP ua1.example.com;branch=z9hG4bK4ea695b5B376A60C
From: <sip:alice@example.com>;tag=8C4183CB-BCEAB710
To: <sip:carol@example.com:5075>
CSeq: 1 INVITE
Call-ID: 3d57cd17-47deb849-dca8b6c6@ua1.example.com
Contact: <sip:alice@ua1.example.com>
User-Agent: ABC-UA/1.2.3
P-Preferred-Identity: <sip:alice@example.com>
<all-one-line>
Replaces: f3b3cbd0-a2c5775e-5df9f8d5@ua2.example.com;to-tag=65a98f7c
-1dd2-11b2-88c6-b03162323164+65a98f7c;from-tag=15A3DE7C-9283203B
</all-one-line>
Max-Forwards: 70
Content-Type: application/sdp
Content-Length: 223

v=0



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o=- 1102980497 1102980497 IN IP4 ua1.example.com
s=IP SIP UA
c=IN IP4 ua1.example.com
t=0 0
a=sendrecv
m=audio 2238 RTP/AVP 0 8 101
a=rtpmap:0 PCMU/8000
a=rtpmap:8 PCMA/8000
a=rtpmap:101 telephone-event/8000


F34 to F41: Bob notifies the Appearance Agent of the termination of
dialog at his UA. Alice notifies the Appearance Agent of the
confirmed state of the dialog at her UA.

F34 Bob ----> Appearance Agent

NOTIFY sip:sa@stateagent.example.com SIP/2.0
Via: SIP/2.0/UDP ua2.example.com;branch=z9hG4bKa5d6cf61F5FBC05A
From: <sip:bob@example.com>;tag=44150CC6-A7B7919D
To: "State_Agent" <sip:alice@example.com>;tag=428765950880801
CSeq: 11 NOTIFY
Call-ID: 144-1289338424@example.com
Contact: <sip:bob@ua2.example.com>
Event: dialog
User-Agent: XYZ-UA/4.5.6
Subscription-State: active;expires=3334
Max-Forwards: 70
Content-Type: application/dialog-info+xml
Content-Length: ...

<?xml version="1.0"?>
<dialog-info xmlns="urn:ietf:params:xml:ns:dialog-info"
             xmlns:sa="urn:ietf:params:xml:ns:sa-dialog-info"
             version="10"
             state="partial"
             entity="sip:alice@example.com:5060">
   <dialog id="id3d4f9c83"
         call-id="f3b3cbd0-a2c5775e-5df9f8d5@ua2.example.com"
         local-tag="15A3DE7C-9283203B"
         remote-tag="65a98f7c-1dd2-11b2-88c6-b03162323164+65a98f7c"
         direction="initiator">
         <sa:appearance>0</appearance>
         <sa:exclusive>false</exclusive>
         <state>terminated</state>
         <local>
           <target uri="sip:bob@example.comsip:bob@example.com">
           </target>



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         </local>
         <remote>
           <identity>sip:carol@example.com</identity>
           <target uri="sip:carol@example.com" />
         </remote>
    </dialog>
</dialog-info>



F38 Alice ----> Appearance Agent

NOTIFY sip:sa@stateagent.example.com SIP/2.0
Via: SIP/2.0/UDP ua1.example.com;branch=z9hG4bK93f44af3518A1572
From: <sip:alice@example.com>;tag=5861255C-14C04045
To: "State_Agent" <sip:alice@example.com>;tag=920163082722420
CSeq: 10 NOTIFY
Call-ID: 143-1840952798@example.com
Contact: <sip:alice@ua1.example.com>
Event: dialog
User-Agent: ABC-UA/1.2.3
Subscription-State: active;expires=3315
Max-Forwards: 70
Content-Type: application/dialog-info+xml
Content-Length: ...

<?xml version="1.0"?>
<dialog-info xmlns="urn:ietf:params:xml:ns:dialog-info"
            xmlns:sa="urn:ietf:params:xml:ns:sa-dialog-info"
            version="9"
             state="partial"
             entity="sip:alice@example.com:5060">
    <dialog id="id402f024e"
        call-id="3d57cd17-47deb849-dca8b6c6@ua1.example.com"
        local-tag="8C4183CB-BCEAB710"
        remote-tag="65a98f7c-1dd2-11b2-88c6-b03162323164+65a98f7c"
        direction="initiator">
        <state>confirmed</state>
        <sa:appearance>0</appearance>
        <sa:exclusive>false</exclusive>
        <local>
          <target uri="sip:alice@example.com">
             <param pname="+sip.rendering" pval="yes"/>
          </target>
        </local>
        <remote>
           <identity>sip:carol@example.com</identity>
           <target uri="sip:carol@example.com" />



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        </remote>
    </dialog>
</dialog-info>


F42 to F59: Carol terminates the dialog with Alice. Alice notifies the
Appearance Agent of the dialog state (terminated). The Appearance Agent
notifies Bob of the same.

F46 Alice ----> Appearance Agent

NOTIFY sip:sa@stateagent.example.com SIP/2.0
Via: SIP/2.0/UDP ua1.example.com;branch=z9hG4bKa46c2f85F29F839C
From: <sip:alice@example.com>;tag=5861255C-14C04045
To: "State_Agent" <sip:alice@example.com>;tag=920163082722420
CSeq: 11 NOTIFY
Call-ID: 143-1840952798@example.com
Contact: <sip:alice@ua1.example.com>
Event: dialog
User-Agent: ABC-UA/1.2.3
Subscription-State: active;expires=3311
Max-Forwards: 70
Content-Type: application/dialog-info+xml
Content-Length: ...

<?xml version="1.0"?>
<dialog-info xmlns="urn:ietf:params:xml:ns:dialog-info"
             xmlns:sa="urn:ietf:params:xml:ns:sa-dialog-info"
             version="10"
             state="partial"
             entity="sip:alice@example.com">
   <dialog id="id402f024e"
      call-id="3d57cd17-47deb849-dca8b6c6@ua1.example.com"
      local-tag="8C4183CB-BCEAB710"
      remote-tag="65a98f7c-1dd2-11b2-88c6-b03162323164+65a98f7c"
      direction="initiator">
      <state>terminated</state>
      <sa:appearance>0</appearance>
      <sa:exclusive>false</exclusive>
      <local>
        <target uri="sip:alice@example.com">
        </target>
      </local>
      <remote>
        <identity>sip:carol@example.com</identity>
        <target uri="sip:carol@example.com" />
      </remote>
   </dialog>



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</dialog-info>


10.4.  Appearance Selection for Incoming Call

   In the call flow below Bob and Alice are in an appearance group
   identified by Alice's AOR.  Carol places a call to Alice.  Both Alice
   and Bob's devices are alerted of the incoming call.  Bob answers the
   call.  He then places Carol on hold.  Alice picks up the held call
   and has a established session with Carol.  Finally, Carol terminates
   the session.  All NOTIFY messages in the call flow below carry dialog
   events and only dialog states are mentioned for simplicity.  For
   brevity, the details of some messages are not shown below.

              Forking     Appearance
   Carol      Proxy         Agent         Alice      Bob
   |            |             |             |         |
   |>F1 INVITE >|             |             |         |
   |            |< - - - - - >|             |         |
   |            |             |>F2 NOTIFY ----------->|
   |            |             |             |         |
   |            |             |<F3 200 OK -----------<|
   |            |             |             |         |
   |            |             |>F4 NOTIFY ->|         |
   |            |             |             |         |
   |            |             |<-200 OK F5-<|         |
   |            |             |             |         |
   |            |>F6 INVITE ------------------------->|
   |            |             |             |         |
   |            |>F7 INVITE --------------->|         |
   |            |             |             |         |
   |<- 100 F8 -<|             |             |         |
   |            |             |             |         |
   |            |<-------------------- Ringing 180 F9<|
   |< 180 F10 -<|             |             |         |
   |            |<--------- 180 Ringing F11<|         |
   |< 180 F12 -<|             |             |         |
   |            |<------------------------ 200 OK F13<|
   |< 200 F14 -<|             |             |         |
   |            |             |             |         |
   |            |>F14 CANCEL -------------->|         |
   |            |             |             |         |
   |            |<-------------- 200 OK F15<|         |
   |            |             |             |         |
   |            |<Request Cancelled 487 F16<|         |
   |            |             |             |         |
   |            |>F17 ACK ----------------->|         |
   |>F18 ACK -->|             |             |         |



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   |            |>F19 ACK --------------------------->|
   |            |             |             |         |
   |<=============Both way RTP established===========>|
   |            |             |             |         |
   |            |             |<---------- NOTIFY F20<|
   |            |             |             |         |
   |            |             |>F21 200 OK ---------->|
   |            |             |             |         |
   |            |             |>F22 NOTIFY >|         |
   |            |             |             |         |
   |            |             |<- 200 F22 -<|         |
   |            |             |             |         |
   |            |             |-------- SUBSCRIBE F23>|
   |            |             |                       |
   |            |             |<F24 200 OK ----------<|
   |            |             |                       |
   |            |             |<---------- NOTIFY F25<|
   |            |             |                       |
   |            |             |>F26 200 OK ---------->|
   |            |             |                       |

   Figure 5.

   F1 to F16: An incoming call from Carol to Alice is forked to
   Bob and Alice. Both Alice and Bob indicate an incoming call
   (e.g., ringing) from Carol. Bob answers the call and two-way
   media is established between Carol and Bob.

   F2 Proxy ----> Bob

   INVITE sip:alice@ua3.example.com SIP/2.0
   Via: SIP/2.0/UDP ua3.example.com;branch=z9hG4bK4324ea695b5B376A
   Via: SIP/2.0/UDP proxy.example.com;branch=z9hG4bK38432ji
   From: <sip:carol@example.com>;tag=94183CB-BCEAB7
   To: <sip:alice@example.com>
   CSeq: 106 INVITE
   Call-ID: 47deb849-dca8b6c6-3d342
   Contact: <sip:carol@ua3.example.com>
   Max-Forwards: 69
   Alert-Info: <file://ring.pcm>;alert=normal;appearance=0
   Content-Type: application/sdp
   Content-Length: 223

   v=0
   o=- 1102980499 1102980499 IN IP4 ua3.example.com
   s=
   c=IN IP4 ua3.example.com
   t=0 0



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   a=sendrecv
   m=audio 2238 RTP/AVP 0 8 101
   a=rtpmap:0 PCMU/8000
   a=rtpmap:8 PCMA/8000
   a=rtpmap:101 telephone-event/8000


   F3 Proxy ----> Alice

   INVITE sip:alice@ua1.example.com SIP/2.0
   Via: SIP/2.0/UDP ua3.example.com;branch=z9hG4bK4324ea695b5B376A
   Via: SIP/2.0/UDP proxy.example.com;branch=z9hG4bK348281
   From: <sip:carol@example.com>;tag=94183CB-BCEAB7
   To: <sip:alice@example.com>
   CSeq: 106 INVITE
   Call-ID: 47deb849-dca8b6c6-3d342
   Contact: <sip:carol@ua3.example.com>
   Max-Forwards: 69
   Alert-Info: <file://ring.pcm>;alert=normal;appearance=0
   Content-Type: application/sdp
   Content-Length: 223

   v=0
   o=- 1102980499 1102980499 IN IP4 ua3.example.com
   s=
   c=IN IP4 ua3.example.com
   t=0 0
   a=sendrecv
   m=audio 2238 RTP/AVP 0 8 101
   a=rtpmap:0 PCMU/8000
   a=rtpmap:8 PCMA/8000
   a=rtpmap:101 telephone-event/8000



   F17 - F20: Bob notifies the Appearance Agent with dialog state
   payload indicating the dialog in confirmed state. Appearance
   Agent notifies Alice of the status of the dialog at Bob.

   F17 Bob ----> Appearance Agent

   NOTIFY sip:sa@stateagent.example.com SIP/2.0
   Via: SIP/2.0/UDP ua2.example.com;branch=z9hG4bK58a0dd68C2D63263
   From: <sip:bob@example.com>;tag=558C18F7-DB9DF7BC
   To: <sip:alice@example.com>;tag=1894685100249086
   CSeq: 14 NOTIFY
   Call-ID: 77-505889516@example.com
   Contact: <sip:bob@ua2.example.com>



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   Event: dialog
   User-Agent: XYZ-UA/4.5.6
   Subscription-State: active;expires=3427
   Max-Forwards: 70
   Content-Type: application/dialog-info+xml
   Content-Length: ...

   <?xml version="1.0"?>
   <dialog-info xmlns="urn:ietf:params:xml:ns:dialog-info"
          xmlns:sa="urn:ietf:params:xml:ns:sa-dialog-info"
          version="13"
          state="partial"
          entity="sip:alice@example.com">
      <dialog id="ida0f8dc17"
           call-id="14-1541707345@example.com"
           local-tag="44BAD75D-E3128D42"
           remote-tag="d3b06488-1dd1-11b2-88c5-b03162323164+d3e48f4c"
           direction="recipient">

         <sa:appearance>0</appearance>
         <sa:exclusive>false</exclusive>
         <state>confirmed</state>
         <local>
           <target uri="sip:alice@example.com">
               <param pname="+sip.rendering" pval="yes"/>
           </target>
         </local>
         <remote>
           <identity>sip:carol@ua.example.com</identity>
         </remote>
      </dialog>
   </dialog-info>


   F19 Appearance Agent ----> Alice

   NOTIFY sip:alice@ua1.example.com SIP/2.0
   From: <sip:alice@example.com>;tag=151702541050937
   To: <sip:alice@example.com>;tag=18433323-C3D237CE
   Call-ID: 1e361d2f-a9f51109-bafe31d4@ua1.example.com
   CSeq: 12 NOTIFY
   Via: SIP/2.0/UDP stateagent.example.com;branch=z9hG4bK14031499568413
   Max-Forwards: 70
   Content-Type: application/dialog-info+xml
   Event: dialog
   Subscription-State: active
   Contact: <sip:sa@stateagent.example.com>
   Content-Length: ...



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   <?xml version="1.0"?>
   <dialog-info xmlns="urn:ietf:params:xml:ns:dialog-info"
                xmlns:sa="urn:ietf:params:xml:ns:sa-dialog-info"
                version="13"
                state="partial"
                entity="sip:alice@example.com">
      <dialog id="2a7294823093f5274e3fd2ec54a2d76c"
           call-id="14-1541707345@example.com"
           local-tag="44BAD75D-E3128D42"
           remote-tag="d3b06488-1dd1-11b2-88c5-b03162323164+d3e48f4c"
           direction="recipient">
         <sa:appearance>0</appearance>
         <sa:exclusive>false</exclusive>
         <state>confirmed</state>
            <local>
           <target uri="sip:alice@example.com">
              <param pname="+sip.rendering" pval="yes"/>
           </target>
         </local>
         <remote>
           <identity>sip:carol@ua.example.com</identity>
         </remote>
      </dialog>
   </dialog-info>



10.5.  Appearance Publication

   This call flow shows the use of PUBLISH between the members of the
   appearance group and the Appearance Agent.


   Carol        Proxy           Alice      Appearance Agent         Bob
     |              |               |              |                  |
     |              |               |              |<----- PUBLISH F1<|
     |              |               |              |                  |
     |              |               |              |>F2 200 OK ------>|
     |              |               |              |                  |
     |              |               |<-- NOTIFY F3<|                  |
     |              |               |              |                  |
     |              |               |>F4 200 OK -->|                  |
     |              |               |              |                  |
     |              |<------------------------------------- INVITE F5<|
     |              |               |              |                  |
     |<-- INVITE F6<|               |              |                  |
     |              |               |              |                  |
     |>F7 180 Ring >|               |              |                  |



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     |              |>F8 180 Ringing -------------------------------->|
     |>F9 200 OK -->|               |              |                  |
     |              |>F10 200 OK ------------------------------------>|
     |              |               |              |                  |
     |<------------------------------------------------------ ACK F11<|
     |              |               |              |                  |
     |<================= Both way RTP established ===================>|
     |              |               |              |                  |
     |              |               |              |<---- PUBLISH F12<|
     |              |               |              |                  |
     |              |               |              |>F13 200 OK ----->|
     |              |               |<- NOTIFY F14<|                  |
     |              |               |              |                  |
     |              |               |>F15 200 OK ->|                  |
     |              |               |              |                  |
     |              |<------------------------------(hold) INVITE F16<|
     |<- INVITE F17<|               |              |                  |
     |              |               |              |                  |
     |>F18 200 OK ->|               |              |                  |
     |              |>F19 200 OK ------------------------------------>|
     |              |               |              |                  |
     |<------------------------------------------------------ ACK F20<|
     |              |               |              |                  |
     |              |               |              |<---- PUBLISH F21<|
     |              |               |              |                  |
     |              |               |              |>F22 200 OK ----->|
     |              |               |<- NOTIFY F23<|                  |
     |              |               |              |                  |
     |              |               |>F24 200 OK ->|                  |
     |              |<-- INVITE F25<|              |                  |
     |<- INVITE F26<|(w/ Replaces)  |              |                  |
     |( w/ Replaces)|               |              |                  |
     |>F27 200 OK ->|               |              |                  |
     |              |>F28 200 OK -->|              |                  |
     |              |               |              |                  |
     |<-------------------- ACK F29<|              |                  |
     |              |               |              |                  |
     |<= Both way RTP established =>|              |                  |
     |              |               |              |                  |
     |>F30 BYE ---->|               |              |                  |
     |              |>F31 BYE --------------------------------------->|
     |              |               |              |                  |
     |              |<------------------------------------ OK 200 F32<|
     |<- 200 OK F33<|               |              |                  |
     |              |               |              |                  |
     |              |               |              |<---- PUBLISH F34<|
     |              |               |              |                  |
     |              |               |              |>F35 200 OK ----->|



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     |              |               |<- NOTIFY F36<|                  |
     |              |               |              |                  |
     |              |               |>F37 200 OK ->|                  |
     |              |               |              |                  |
     |              |               |>F38 PUBLISH >|                  |
     |              |               |              |                  |
     |              |               |<- 200 OK F39<|                  |
     |              |               |              |>F40 NOTIFY ----->|
     |              |               |              |                  |
     |              |               |              |<----- 200 OK F41<|
     |>F42 BYE ---->|               |              |                  |
     |              |>F43 BYE ----->|              |                  |
     |              |               |              |                  |
     |              |<-- 200 OK F44<|              |                  |
     |<--200 OK F45<|               |              |                  |
     |              |               |>F46 PUBLISH >|                  |
     |              |               |              |                  |
     |              |               |<- 200 OK F47<|                  |
     |              |               |              |>F48 NOTIFY ----->|
     |              |               |              |                  |
     |              |               |              |<----- OK 200 F49<|

   Figure 6.


10.6.  Joining an Appearance

   In this call flow, a call answered by Bob is joined by Alice or
   "bridged".  The Join header field is used by Alice to request this
   bridging.  If Bob did not support media mixing, Bob could obtain
   conferencing resources as described in [RFC4579].


   Carol    Forking Proxy Appearance Agent  Alice      Bob
     |            |             |             |         |
     |>F1 INVITE >|             |             |         |
     |            |>F2 INVITE ------------------------->|
     |            |             |             |         |
     |            |>F3 INVITE --------------->|         |
     |            |             |             |         |
     |<-100Try F4<|             |             |         |
     |            |             |             |         |
     |            |<-------------------- Ringing 180 F5<|
     |<180Ring F6<|             |             |         |
     |            |<---------- Ringing 180 F7<|         |
     |<180Ring F8<|             |             |         |
     |            |<------------------------- 200 OK F9<|
     |<-200OK F10<|             |             |         |



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     |            |             |             |         |
     |            |>F11 CANCEL -------------->|         |
     |            |             |             |         |
     |            |<-------------- 200 OK F12<|         |
     |            |             |             |         |
     |            |<Request Cancelled 487 F13<|         |
     |            |             |             |         |
     |            |>F14 ACK ----------------->|         |
     |>F15 ACK -->|             |             |         |
     |            |>F16 ACK --------------------------->|
     |            |             |             |         |
     |<=============Both way RTP established===========>|
     |            |             |             |         |
     |            |             |<---------- NOTIFY F17<|
     |            |             |             |         |
     |            |             |>F18 200 OK ---------->|
     |            |             |             |         |
     |            |             |>F19 NOTIFY >|         |
     |            |             |             |         |
     |            |             |<- 200OK F20<|         |
     |            |             |             |         |
     |            |<---- INVITE (w/ Join) F21<|         |
     |            |             |             |         |
     |            |>F22 INVITE (w/Join)---------------->|
     |            |             |             |         |
     |            |<---- OK 200 Contact:Bob;isfocus F23<|
     |            |             |             |         |
     |            |>F24 200 OK Contact:Bob;isfocus----->|
     |            |             |             |         |
     |            |<----------------- ACK F25<|         |
     |            |             |             |         |
     |            |>ACK F26---------------------------->|
     |            |             |             |         |
     |            |<-----INVITE Contact:Bob;isfocus F27<|
     |<-INVITE F28|             |             |         |
     |>F29 200 -->|             |             |         |
     |            |>F30 200 OK ------------------------>|
     |            |             |             |         |
     |            |<--------------------------- ACK F31<|
     |<--- ACK F32|             |             |         |
     |            |             |             |<==RTP==>|
     |<=============Both way RTP established===========>|

   Figure 7.

   F21 Alice ----> Proxy

   INVITE sip:bob@ua.example.com SIP/2.0



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   Via: SIP/2.0/UDP ua1.example.com;branch=z9hG4bKcc9d727c2C29BE31
   From: <sip:alice@example.com>;tag=605AD957-1F6305C2
   To: <sip:bob@ua.example.com>
   CSeq: 2 INVITE
   Call-ID: dc95da63-60db1abd-d5a74b48@ua1.example.com
   Contact: <sip:alice@ua1.example.com>
   User-Agent: ABC-UA/1.2.3
   P-Preferred-Identity: <sip:alice@example.com>
   <all-one-line>
   Join: 14-1541707345@example.com;to-tag=d3b06488-1dd1-11b2-88c5
   -b03162323164+d3e48f4c;from-tag=44BAD75D-E3128D42
   </all-one-line>
   Max-Forwards: 70
   Content-Type: application/sdp
   Content-Length: 223

   v=0
   o=- 1103061265 1103061265 IN IP4 ua1.example.com
   s=IP SIP UA
   c=IN IP4 ua1.example.com
   t=0 0
   a=sendrecv
   m=audio 2236 RTP/AVP 0 8 101
   a=rtpmap:0 PCMU/8000
   a=rtpmap:8 PCMA/8000
   a=rtpmap:101 telephone-event/8000

























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10.7.  Appearance Allocation - Loss of Subscription with UA


      UA           Appearance Agent         UA1              UA2
      |                |                |                |
      |                | F1: NOTIFY (trying)             |
      |                |<---------------|                |
      |                | F2: 200 OK     |                |
      |                |--------------->|                |
      |                | F3: NOTIFY (trying)             |
      |                |----------------+--------------->|
      |                | F4: 200 OK     |                |
      |                |<---------------+----------------|
      | F5: INVITE     |                |                |
      |<--------------------------------|                |
      | F6: 180 Ringing|                |                |
      |-------------------------------->|                |
      |                |                |                |
      |                |               End               |
      |                |                                 |
      |                |                                 |
      |                | F7: SUBSCRIBE x 6 retries       |
      |                |--------------->                 |
      |                |                                 |
      |                | F8: NOTIFY (terminated)         |
      |                |-------------------------------->|
      |                | F9: 200 OK                      |
      |                |<--------------------------------|
      |                |                                 |

   Figure 8.


   The flow shown in this figure illustrates the failure of a user agent
   after it has obtained an appearance number (F1-F2).  Messages used to
   refresh the subscription from Appearance Agent to UA1 are shown at
   F7.  When the Appearance Agent attempts to refresh its subscription
   but receives no response.  In this case, the Appearance Agent may
   apply policy and free up the appearance number as it wishes.  In this
   case, after a delay, the Appearance Agent frees up the appearance
   number and sends NOTIFY messages (F8) indicating the termination of
   the dialog associated with the shared line.









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10.8.  Appearance Selection Contention Race Condition


      UA       Appearance Agent        UA1              UA2
      |                |                |                |
      |                | F1 NOTIFY (trying appearance=1) |
      |                |<---------------|                |
      |                | F2 NOTIFY (trying appearance=1) |
      |                |<---------------+----------------|
      |                | F3 200 OK      |                |
      |                |--------------->|                |
      |                | F4 200 OK      |                |
      |                |----------------+--------------->|
      |                | F5 NOTIFY  (trying appearance=1)|
      |                |--------------->|                |
      |                | F6 200 OK      |                |
      |                |<---------------|                |
      |                | F7 NOTIFY (trying)              |
      |                |----------------+--------------->|
      |                | F8 200 OK      |                |
      |                |<---------------+----------------|
      | F9 INVITE      |                |                |
      |<--------------------------------|                |
      |                | F10 NOTIFY (trying appearance=2)|
      |                |<---------------+----------------|
      |                | F11 200 OK     |                |
      |                |----------------+--------------->|
      |                | F12 NOTIFY (trying appearance=2)|
      |                |----------------+--------------->|
      |                | F13 200 OK      |               |
      |                |<---------------+----------------|
      | F14 INVITE     |                |                |
      |<-------------------------------------------------|
      |                |                |                |

   Figure 9.


   This figure illustrates two user agents, UA1 and UA2, attempting to
   select the same appearance number (i.e. seize the same line number)
   simultaneously.  This type of race condition is often referred to in
   telephony as a glare condition.  Appearance Agent may use any desired
   policy to decide which UA receives the appearance and which does not.
   In this case UA1 obtains the appearance number, as indicated by the
   NOTIFY from the Appearance Agent with the appearance number.  UA2
   learns that it did not obtain the appearance number since its NOTIFY
   does not contain the appearance number from its NOTIFY.




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11.  IANA Considerations

   This section registers the SIP Alert-Info header field parameter
   "appearance" and the XML namespace extensions to the SIP Dialog
   Package.

11.1.  SIP Event Package Parameter: sa

   This specification also defines a new event parameter "sa" for the
   Dialog Package.

11.2.  URN Sub-Namespace Registration: sa-dialog-info


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

    URI: urn:ietf:params:xml:ns:sa-dialog-info.

    Registrant Contact: IETF BLISS working group, <bliss@ietf.org>,
    Alan Johnston <alan@sipstation.com>

    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>Shared Appearance Dialog Information Namespace</title>
       </head>
       <body>
         <h1>Namespace for Shared Appearance Dialog Information</h1>
         <h2>urn:ietf:params:xml:ns:dialog-info</h2>
         <p>See <a href="ftp://ftp.rfc-editor.org/in-notes/rfcXXXX.txt">
              RFCXXXX</a>.</p>
       </body>
       </html>
    END









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11.3.  XML Schema Registration

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

      URI: urn:ietf:params:xml:schesa:sa-dialog-info.

      Registrant Contact: IETF BLISS working group, <bliss@ietf.org>,
       Alan Johnston <alan@sipstation.com>

      The XML for this schema can be found in Section 7.



12.  Appendix A - Incoming Appearance Assignment

   To best meet REQ-9, the appearance number for an incoming INVITE
   should be contained in the INVITE itself.

   For the dialog package parameter approach, REQ-9 could be met in two
   ways.  When an incoming request is received, the Appearance Agent
   could send out a NOTIFY with state trying and include the appearance
   number to be used for this request.  Upon receipt of this NOTIFY, the
   UAs could begin alerting using the appearance number selected.  This
   approach is sub-optimal since the UAs could receive the INVITE but be
   unable to begin alerting if the NOTIFY from the Appearance Agent is
   delayed or lost

   An alternative approach is to define an extension parameter for the
   Alert-Info header field in RFC 3261 such as:

   Alert-Info: <file://ring.pcm>;alert=normal;appearance=0

   This Alert-Info header would indicate to place the call on the first
   line appearance instance.

   The determination as to what value to use in the appearance parameter
   can be done at the proxy that forks the incoming request to all the
   registered UAs.  There are a variety of ways the proxy can use to
   determine what value it should use to populate this parameter.  For
   example, the proxy could fetch this information by initiating a
   SUBSCRIBE request with Expires: 0 to the Appearance Agent for the AOR
   to fetch the list of lines that are in use.  Alternatively, it could
   act like a UA that is a part of the appearance group and SUBSCRIBE to
   the State-Agent like any other UA.  This would ensure that the active
   dialog information is available without having to poll on a need
   basis.  It could keep track of the list of active calls for the
   appearance AOR based on how many unique INVITE requests it has forked
   to or received from the appearance AOR.  Another approach would be



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   for the Proxy to first send the incoming INVITE to the Appearance
   Agent which would redirect to the appearance group URI and escape the
   proper Alert-Info header field for the Proxy to recurse and
   distribute to the other UAs in the group.

   The Appearance Agent needs to know about all incoming requests to the
   AOR in order to select the appearance number.  One way in which this
   could be done is for the Appearance Agent to register against the AOR
   with a higher q value.  This will result in the INVITE being sent to
   the Appearance Agent first, then being offered to the UAs in the
   group.

   The changes to RFC 3261 ABNF would be:

   alert-param = LAQUOT absoluteURI RAQUOT *( SEMI (generic-param /
   appearance-param) )

   appearance-param = "appearance" EQUAL *DIGIT


13.  Appendix B - Implementation Options Discussion

   This section discusses some options on how to implement the Shared
   Appearances feature in SIP.  This section is non-normative.

13.1.  Appearance Implementation Options

   This section discusses and compares two methods of implementing,
   conveying, and selecting appearances in SIP while meeting the
   requirements of Section 4.  One approach involves a URI parameter and
   is discussed in section 5.1.1.  The other approach uses a SIP dialog
   package extension parameter and is discussed in section 5.1.2.  Both
   approaches assume the common elements and operations of Figure 1.  In
   addition, this section discusses approaches for incoming appearance
   indication, REQ-9, and appearance contention, REQ-8.  These
   approaches will be discussed for an example appearance group of N
   phones each with n line appearances.  The usage of the word phone
   does not imply that this feature is limited to telephony devices.

13.1.1.  URI parameter Approach

   Some implementations of this feature utilize a URI parameter such as
   "line=3" on the Contact URI.  Each appearance is effectively a
   logical UA, so each line appearance requires a separate registration.
   The number of line appearances needs to be provisioned on each phone.
   Each appearance also requires a separate dialog package subscription.
   Even using a State Agent for the dialog package, each phone must
   maintain n subscriptions to the dialog package.



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   This results in 2nN total subscriptions and nN registrations for this
   implementation.

   Since Contact URI parameters will be conveyed by the dialog package,
   REQ-7 is met.

   REQ-10 can be met by having the Appearance Agent send a SUBSCRIBE to
   each UA and line number to obtain the current dialog state - this
   will result in nN SUBSCRIBEs and NOTIFYs.

   It is not obvious how to meet REQ-11 with this approach.  A UA
   registering against the AOR but does not implement the appearance URI
   parameter will not include a line appearance number in Contact URIs
   and dialog package NOTIFYs.  The Appearance Agent will have no way of
   indicating to the other UAs the appearance number being used by this
   UA, as adding a parameter to the Contact URI would cause call control
   operations such as Replaces and Join to fail.

   REQs 12 and 13 are difficult to meet with this approach as the line
   appearance number will be present in the Request-URI of incoming
   requests and the Contact URI in INVITE and 200 OK messages.  This
   approach will require integrity protection of all dialog creating
   requests and responses, and privacy mechanisms to hide the Contact
   URI from other UAs.

   Also, this approach will require mechanisms to protect against
   another UA sending an INVITE directly to a group member with the line
   appearance number already set.

13.1.2.  Dialog Package Parameter

   Instead of the URI parameter approach, consider an extension
   parameter "appearance" to the SIP dialog package.  The e.g.:


















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<?xml version="1.0"?>
<dialog-info xmlns="urn:ietf:params:xml:ns:dialog-info"
             xmlns:sa="urn:ietf:params:xml:ns:sa-dialog-info"
             version="6"
             state="partial"
             entity="sip:alice@example.com">
   <dialog id="id3d4f9c83" from-tag="3423" to-tag="a3f423j88uju1"
                                                     direction="initiator">
       <sa:appearance>2</appearance>
       <sa:exclusive>false</exclusive>
       <sa:joined-dialog call-id="sdfg" from-tag="832d1" to-tag="4542454" />
       <sa:joined-dialog call-id="873287876" from-tag="433" to-tag="jwjwuf5" />
       <state>connected</state>
       <local>
           <target uri="sip:bob@pc.example.com" />
       </local>
   </dialog>
</dialog-info>
...

   In this approach, the appearance number is never carried in a
   Request-URI or Contact URI.  Instead, it is only present in dialog
   package NOTIFY and PUBLISH messages.  As a result, only a single
   registration per AOR is required.  Also, only a single dialog package
   subscription in each direction per AOR.

   This results in 2N total subscriptions and N registrations for this
   approach.

   If the dialog package is extended to carry the appearance number,
   then REQ-7 is met.

   REQ-10 can be met by having the Appearance Agent send a SUBSCRIBE to
   each UA and line number to obtain the current dialog state - this
   will result in N SUBSCRIBEs and NOTIFYs.

   REQ-11 can be met by this approach.  Even though a UA does not
   provide an appearance number in dialog package NOTIFYs, the
   Appearance Agent can assign one and include it in NOTIFYs to the
   other UAs.  This parameter would simply be ignored by the UAs that
   did not understand the parameter, and have no impact on call control
   operations.

   REQs 12 and 13 are met because the appearance number is only conveyed
   in dialog package NOTIFYs.  Integrity and privacy of NOTIFY bodies
   can be achieved using normal SIP mechanisms independent of the
   security mechanisms used for other requests.




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   The dialog-package [RFC3265] describes a mechanism whereby shared-
   line privacy REQ-14 can be accomplished by suppressing certain dialog
   information from being presented to the UAs.  The reasoning behind
   that is if the UAs were unaware of a dialog's call-id, local-tag and
   remote-tag then they will be unable to create requests such as INVITE
   with Replaces [RFC3891] and Join [RFC3911] header fields to barge-in
   or pickup the line appearance.  Below is a quote from section 3.6 of
   dialog-package[RFC3265] that describes this approach:

   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.

   There are certain fundamental drawbacks in the privacy-by-obscurity
   approach described in [RFC3265] .  It models exclusivity as a static
   property of the appearance AOR.  There are situations where
   exclusivity needs to be a dynamic property (e.g. boss does not want
   secretary to listen-in on a particular part of the conversation).  In
   addition, [RFC3265] does not address how a UA can request exclusivity
   at the start of a session or mid-session and how that request will be
   granted or rejected.

   Exclusivity being a dynamic property means that a UA can request it
   to be turned on or off in the middle of a session.  When exclusivity
   is turned off all the UAs that share the line AOR will need to see
   the complete dialog information.  Once they have that information it
   can not be taken back from them.  This will not allow exclusivity to
   be turned on later on in the dialog lifetime.  Therefore, there needs
   to be a centralized entity that will actually enforce exclusivity.

   The approach proposed for meeting REQ-14 is to include an exclusivity
   parameter to the dialog package.  This allows a UA to request
   exclusivity, by setting the exclusive parameter in notifications.
   This could be done prior to a call being made or answered, or during
   a call at any time.  A UA can remove exclusivity by sending a
   notification at any time during a call and setting "exclusive=no".
   It also allows a UA to learn that a particular dialog is exclusive by
   the presence of this parameter in a NOTIFY.  In addition, a UA can
   still apply policy to any INVITE Join or Replaces requests it
   receives, as per normal SIP call control mechanisms.

   With this approach, the number of appearances is centrally managed
   and controlled by the Appearance Agent.  For UAs with soft keys or
   buttons, this gives a great deal of flexibility in system management.



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13.1.3.  Appearance Selections Mechanisms

   Regardless of how the appearance number is conveyed by UAs, there is
   still the issue of how appearance numbers are selected.  For example,
   some UAs might have actual buttons and lamps, and pressing a
   particular button requires the UA to reserve a particular appearance
   number.  For devices with this type of user interface, the selection
   must be done before the user continues with the call and dials digits
   or a URI.  Other UAs with different user interfaces can be flexible
   at the time of dialing, updating the display with the appearance
   number at a later date.  For devices which require advance appearance
   selection, there are three options discussed in the following
   sections for meeting REQ-15.

13.1.3.1.  Floor Control Appearance Selection Mechanism

   This approach models each appearance number as a floor (shared
   resource) and uses a floor control server to arbitrate exclusive
   access (seizure of a particular appearance number).  This approach
   uses a standard SIP Event State Compositor (ESC), a standard Floor
   Control Server that uses the Appearance Agent as Moderator.  The
   Binary Floor Control Protocol (BFCP) is used between the UAs and the
   Floor Control Server.  A Registrar/Forking Proxy Server talks to
   Appearance Agent about incoming calls.  The Appearance Agent acts as
   a Moderator for the floor control server and tells forking proxy to
   insert the appearance number in incoming and outgoing requests.

   Appearance numbers are allocated/selected/reserved in two ways:

   For incoming calls, the Forking Proxy interacts with the Appearance
   Agent.  The Appearance Agent selects an appearance by taking a
   particular floor and marking it "moderator controlled".  This
   appearance number is then included by the Forking Proxy in INVITEs
   using the Alert-Info parameter.  When a UA answers the call, it takes
   the appearance number from the Alert-Info and includes it in the
   dialog state publication.  It then requests the floor associated with
   the appearance number from the floor control server, which forwards
   the request to the Appearance Agent (moderator).  The Appearance
   Agent correlates the floor control request with the dialog state
   notification with the dialog ID from the INVITE with the Alert-Info.
   If they match, the floor is granted.  If they do not match, it means
   the floor request is not an answer of the call but is a random
   appearance selection by the UA and will be rejected.

   For outgoing calls, the UA sends an INVITE and requests a particular
   floor from the floor control server.  Depending on the User Interface
   requirements, the floor request can be done before or after sending
   the INVITE.  The floor grant policy for most appearances is set to



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   "first come first serve".  Once the floor has been granted and the
   call answered, the dialog state publication by the UA will include
   the appearance number.

   When a call has ended, the UA releases the floor to the floor control
   server and this appearance is now available for incoming and outgoing
   calls.

   When a UA in the group which does not support BFCP is in a call, the
   Appearance Agent will grant the floor associated with that appearance
   to that UA.  When that call is over, the Appearance Agent will
   release the floor.  Since the UA will not publish the appearance
   number to the ESC, the Appearance Agent will need to do that on their
   behalf.  If the UA does publish dialog state but without the
   appearance number, the Appearance Agent will still need to re-publish
   the dialog state including the appearance number.  UAs in the group
   will be able to recognize these two dialogs as one since they will
   have the same SIP dialog ID.

13.1.3.2.  INVITE Appearance Selection Mechanism

   This is an alternative approach that utilizes sending an INVITE to
   select/reserve/seize an appearance number.

   A UA that does not need to select a particular appearance number (or
   doesn't care) would just send an INVITE as normal.  The Appearance
   Agent would tell the proxy which appearance number was being used by
   inserting this information in a header field in the first non-100
   provisional response sent back to the calling UA.  The UA would then
   PUBLISH this appearance number to the Dialog Event State Compositor
   for the AOR which would distribute details of the dialog and the
   appearance number to the other UAs in the group.

   If an INVITE is sent and no appearance number is available, the proxy
   would reject the INVITE with a suitable response code and perhaps a
   header field indication.

   A UA that does need to select a particular appearance number would
   use an approach similar to overlap dialing (multi-stage dialing).  An
   INVITE would be sent when the appearance number is requested (i.e.
   when the button is pressed, before dialing begins).  The appearance
   number selected would be carried in the INVITE, in a header field or
   in the Request-URI, for example.  The proxy would reject the INVITE
   with a 484 Address Incomplete response (see RFC 3578) if the
   appearance number is Available and start a timer.  The UA could then
   resend the INVITE after the URI has been dialed and then PUBLISH this
   appearance number to the ESC.  If the appearance number is not
   available, another response code such as 403 would be sent.  The user



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   could then select a different appearance number and resend the
   INVITE.  If no INVITE with a matching Call-ID is received before the
   timer expires, the appearance seizure is cancelled and is made
   available for other calls.

   Note that this approach does not actually require a B2BUA, but it
   does require a proxy that can act as a UAS and communicate with an
   Appearance Agent which keeps track of appearance number allocations.

13.1.3.3.  PUBLISH Appearance Selection Mechanism

   The approach used in previous versions of this draft is to use the
   PUBLISH to the event state compositor to select an appearance number.
   This approach requires a special event state compositor and special
   behavior on the part of the UA.

   In the selection of an appearance for requests initiated by UAs in
   the group, there is the possibility of contention where more than one
   UA select the same appearance number.

   One way to solve this and meet REQ-8 is to require UAs to send a
   notification (trying) to the Appearance Agent indicating the
   appearance number to be used for the session.  The Appearance Agent
   would confirm the allocation of the appearance number in a NOTIFY
   sent to the group UAs.  Should the appearance number be unavailable
   or otherwise not allowed, there are two options:

   - The notification could be rejected with a 500 response and a Retry-
   After header field.  The Appearance Agent would send an immediate
   NOTIFY indicating that the appearance is unavailable.  If the NOTIFY
   is received before the expiration of the Retry-After time, the
   notification state information would become out of date and would be
   discarded without resending.  The UA would select another appearance
   number and send another notification.

   - The notification could be accepted but an immediate NOTIFY
   generated by the Appearance Agent indicating that the appearance is
   unavailable.  The UA would then select another appearance number and
   PUBLISH again.

   UAs would wait for a notification from the Appearance Agent before
   sending the INVITE.

13.2.  Comparison

   In comparing the URI parameter and the dialog package parameter,
   there are clear differences in the number of registrations and
   subscriptions, with the dialog package approach requiring n times



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   fewer in both cases.

   The security model for the dialog package parameter approach is much
   cleaner, since only NOTIFY and PUBLISH requests need integrity and
   privacy.  The security model for the URI parameter approach would
   likely require a B2BUA which introduces many undesirable properties.

   The dialog package parameter approach has better backwards
   compatibility than the URI parameter approach.

   In summary, the dialog package parameter approach better meets REQs
   5, 10, 11, 12, and 13 while the URI parameter approach better meets
   REQ-9.  However, the combined dialog package parameter approach and
   the Alert-Info parameter approach meets REQ-9.

13.2.1.  Comparison of Appearance Selection Methods

   All three approaches meet REQ-15 and REQ-16.

   Previous versions of this draft proposed the publish/notify method of
   appearance selection.  The advantage of this approach is that the
   appearance number is only carried in one place (dialog package XML
   documents) and the same protocol/mechanism is used to select and
   learn appearance numbers.  The disadvantage of this approach is that
   a specialized event state compositor must be used, since it is aware
   of appearance numbers.  Also, concerns have been raised about whether
   this approach defines new semantics for publish/notify beyond that in
   RFC 3265.

   The floor control approach makes good reuse of existing protocols
   such as Binary Floor Control Protocol (BFCP) and cleanly models the
   state.  However, while BFCP can be used in conferencing applications,
   it is unlikely most UAs implementing shared appearances would utilize
   the protocol.  Also, having appearance state in two places (dialog
   package XML documents and floor control messages) complicates the
   application.  Also, BFCP only runs over TCP and requires a separate
   offer/answer exchange to establish the connection, making operation
   through NATs and firewalls more difficult.  The BFCP approach is also
   radically different from all current implementations of this feature.
   As a result, standardizing this approach would likely result in an
   increase in feature interoperability rather than a decrease.

   The INVITE selection mechanism is based on overlap dialing.  Overlap
   dialing is supported in very few SIP UAs and is regarded as a
   somewhat archaic leftover from the PSTN.  As such, it is not regarded
   as a good starting point for a common feature such as shared
   appearances.




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   The PUBLISH selection mechanism reuses the SIP events extensions
   which already must be implemented by UAs supporting this feature.  In
   fact, it results in no additional messages or round trips.  It is
   also very similar to many current feature implementations today.
   Standardizing this approach is likely to increase overall
   interoperability of this feature.

   The rest of this document will only discuss the PUBLISH appearance
   selection mechanism.


14.  Acknowledgements

   The following individuals were part of the SA Design team and have
   provided input and text to the document (in alphabetical order):

   Martin Dolly, Andrew Hutton, Raj Jain, Fernando Lombardo, Derek
   MacDonald, Bill Mitchell, Michael Procter, Theo Zowzouvillys.

   Thanks to Chris Boulton for helping with the XML schema.

   Much of the material has been drawn from previous work by Mohsen
   Soroushnejad, Venkatesh Venkataramanan, Paul Pepper and Anil Kumar,
   who in turn received assistance from:

   Kent Fritz, John Weald, and Sunil Veluvali of Sylantro Systems, Steve
   Towlson, and Michael Procter of Citel Technologies, Rob Harder and
   Hong Chen of Polycom Inc, John Elwell, J D Smith of Siemens
   Communications, Dale R. Worley of Pingtel, Graeme Dollar of Yahoo
   Inc.

   Also thanks to Geoff Devine, Paul Kyzivat, Jerry Yin, John Elwell,
   Dan York, Spenser Dawkins, and Martin Dolly for their comments.


15.  Security Considerations

   Since multiple line appearance features are implemented using
   semantics provided by [RFC3261], Event Package for Dialog State as
   define in , and Event Notification [RFC3265], [RFC3903], security
   considerations in these documents apply to this draft as well.

   Specifically, since dialog state information and the dialog
   identifiers are supplied by UA's in an appearance group to other
   members, the same is prone to "call hijacks".  For example, a rogue
   UA could snoop for these identifiers and send an INVITE with Replaces
   header containing these call details to take over the call.  As such
   INVITES with Replaces header MUST be authenticated using the standard



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   mechanism (like Digest or S/MIME) described in [RFC3261]. before it
   is accepted.  NOTIFY message bodies that provide the dialog state
   information and the dialog identifiers MAY be encrypted end-to-end
   using the standard mechanics.  All SUBSCRIBES between the UA's and
   the Appearance Agent MUST be authenticated.


16.  Informative References

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

   [RFC3261]  Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston,
              A., Peterson, J., Sparks, R., Handley, M., and E.
              Schooler, "SIP: Session Initiation Protocol", RFC 3261,
              June 2002.

   [RFC3515]  Sparks, R., "The Session Initiation Protocol (SIP) Refer
              Method", RFC 3515, April 2003.

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

   [RFC3903]  Niemi, A., "Session Initiation Protocol (SIP) Extension
              for Event State Publication", RFC 3903, October 2004.

   [RFC3891]  Mahy, R., Biggs, B., and R. Dean, "The Session Initiation
              Protocol (SIP) "Replaces" Header", RFC 3891,
              September 2004.

   [I-D.ietf-sipping-service-examples]
              Johnston, A., Sparks, R., Cunningham, C., Donovan, S., and
              K. Summers, "Session Initiation Protocol Service
              Examples", draft-ietf-sipping-service-examples-15 (work in
              progress), July 2008.

   [RFC4235]  Rosenberg, J., Schulzrinne, H., and R. Mahy, "An INVITE-
              Initiated Dialog Event Package for the Session Initiation
              Protocol (SIP)", RFC 4235, November 2005.

   [RFC3680]  Rosenberg, J., "A Session Initiation Protocol (SIP) Event
              Package for Registrations", RFC 3680, March 2004.

   [RFC3911]  Mahy, R. and D. Petrie, "The Session Initiation Protocol
              (SIP) "Join" Header", RFC 3911, October 2004.

   [RFC3325]  Jennings, C., Peterson, J., and M. Watson, "Private
              Extensions to the Session Initiation Protocol (SIP) for



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              Asserted Identity within Trusted Networks", RFC 3325,
              November 2002.

   [RFC4579]  Johnston, A. and O. Levin, "Session Initiation Protocol
              (SIP) Call Control - Conferencing for User Agents",
              BCP 119, RFC 4579, August 2006.

   [RFC3840]  Rosenberg, J., Schulzrinne, H., and P. Kyzivat,
              "Indicating User Agent Capabilities in the Session
              Initiation Protocol (SIP)", RFC 3840, August 2004.


Authors' Addresses

   Alan Johnston (editor)
   Avaya
   St. Louis, MO  63124

   Email: alan@sipstation.com


   Mohsen Soroushnejad
   Sylantro Systems Corp

   Email: mohsen.soroush@sylantro.com


   Venkatesh Venkataramanan
   Sylantro Systems Corp

   Email: vvenkatar@gmail.com


   Paul Pepper
   Citel Technologies

   Email: paul.pepper@citel.com


   Anil Kumar
   Yahoo Inc.

   Email: anil@yahoo-inc.com








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