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rfc:rfc7463

Internet Engineering Task Force (IETF) A. Johnston, Ed. Request for Comments: 7463 Avaya Updates: 3261, 4235 M. Soroushnejad, Ed. Category: Standards Track V. Venkataramanan ISSN: 2070-1721 Sylantro Systems Corp.

                                                            March 2015
     Shared Appearances of a Session Initiation Protocol (SIP)
                      Address of Record (AOR)

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 of an Address
 of Record (AOR) since SIP does not have the concept of lines.  This
 feature is commonly offered in IP Centrex services and IP Private
 Branch Exchange (IPBX) offerings and is likely to be implemented on
 SIP IP telephones and SIP feature servers used in a business
 environment.  This feature allows several user agents (UAs) to share
 a common AOR, learn about calls placed and received by other UAs in
 the group, and pick up or join calls within the group.  This document
 discusses use cases, lists requirements, and defines extensions to
 implement this feature.  This specification updates RFCs 3261 and
 4235.

Status of This Memo

 This is an Internet Standards Track document.
 This document is a product of the Internet Engineering Task Force
 (IETF).  It represents the consensus of the IETF community.  It has
 received public review and has been approved for publication by the
 Internet Engineering Steering Group (IESG).  Further information on
 Internet Standards is available in Section 2 of RFC 5741.
 Information about the current status of this document, any errata,
 and how to provide feedback on it may be obtained at
 http://www.rfc-editor.org/info/rfc7463.

Copyright Notice

 Copyright (c) 2015 IETF Trust and the persons identified as the
 document authors.  All rights reserved.

Johnston, et al. Standards Track [Page 1] RFC 7463 SIP Shared Appearances March 2015

 This document is subject to BCP 78 and the IETF Trust's Legal
 Provisions Relating to IETF Documents
 (http://trustee.ietf.org/license-info) in effect on the date of
 publication of this document.  Please review these documents
 carefully, as they describe your rights and restrictions with respect
 to this document.  Code Components extracted from this document must
 include Simplified BSD License text as described in Section 4.e of
 the Trust Legal Provisions and are provided without warranty as
 described in the Simplified BSD License.
 This document may contain material from IETF Documents or IETF
 Contributions published or made publicly available before November
 10, 2008.  The person(s) controlling the copyright in some of this
 material may not have granted the IETF Trust the right to allow
 modifications of such material outside the IETF Standards Process.
 Without obtaining an adequate license from the person(s) controlling
 the copyright in such materials, this document may not be modified
 outside the IETF Standards Process, and derivative works of it may
 not be created outside the IETF Standards Process, except to format
 it for publication as an RFC or to translate it into languages other
 than English.

Table of Contents

 1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
 2.  Conventions Used in This Document . . . . . . . . . . . . . .   5
 3.  Usage Scenarios . . . . . . . . . . . . . . . . . . . . . . .   5
   3.1.  Executive/Assistant Arrangement . . . . . . . . . . . . .   5
   3.2.  Call Group  . . . . . . . . . . . . . . . . . . . . . . .   5
   3.3.  Single Line Extension . . . . . . . . . . . . . . . . . .   6
   3.4.  Changing UAs  . . . . . . . . . . . . . . . . . . . . . .   6
 4.  Requirements and Implementation . . . . . . . . . . . . . . .   6
   4.1.  Requirements  . . . . . . . . . . . . . . . . . . . . . .   6
   4.2.  Implementation  . . . . . . . . . . . . . . . . . . . . .   8
 5.  Normative Description . . . . . . . . . . . . . . . . . . . .  10
   5.1.  Elements  . . . . . . . . . . . . . . . . . . . . . . . .  11
   5.2.  Shared Appearance Dialog Package Extensions . . . . . . .  11
     5.2.1.  The <appearance> Element  . . . . . . . . . . . . . .  11
     5.2.2.  The <exclusive> Element . . . . . . . . . . . . . . .  12
     5.2.3.  The <joined-dialog> Element . . . . . . . . . . . . .  12
     5.2.4.  The <replaced-dialog> Element . . . . . . . . . . . .  13
   5.3.  Shared Appearance User Agents . . . . . . . . . . . . . .  13
     5.3.1.  Appearance Numbers and Call Context . . . . . . . . .  16
     5.3.2.  Appearance Numbers and Call Control . . . . . . . . .  17
     5.3.3.  Appearance Numbers and Transfer . . . . . . . . . . .  18
   5.4.  Appearance Agent  . . . . . . . . . . . . . . . . . . . .  18
 6.  XML Schema Definition . . . . . . . . . . . . . . . . . . . .  21
 7.  Alert-Info Appearance Parameter Definition  . . . . . . . . .  23

Johnston, et al. Standards Track [Page 2] RFC 7463 SIP Shared Appearances March 2015

 8.  User Interface Considerations . . . . . . . . . . . . . . . .  24
   8.1.  Appearance Number Rendering . . . . . . . . . . . . . . .  24
     8.1.1.  Single Appearance UAs . . . . . . . . . . . . . . . .  24
     8.1.2.  Dual Appearance UAs . . . . . . . . . . . . . . . . .  24
     8.1.3.  Shared Appearance UAs with Fixed Appearance Number  .  25
     8.1.4.  Shared Appearance UAs with Variable Appearance
             Numbers . . . . . . . . . . . . . . . . . . . . . . .  25
     8.1.5.  Example User Interface Issues . . . . . . . . . . . .  25
   8.2.  Call State Rendering  . . . . . . . . . . . . . . . . . .  26
 9.  Interoperability with Non-shared Appearance UAs . . . . . . .  26
   9.1.  Appearance Assignment . . . . . . . . . . . . . . . . . .  26
   9.2.  Appearance Release  . . . . . . . . . . . . . . . . . . .  27
   9.3.  UAs Supporting Dialog Events but Not Shared Appearance  .  27
 10. Provisioning Considerations . . . . . . . . . . . . . . . . .  27
 11. Example Message Flows . . . . . . . . . . . . . . . . . . . .  28
   11.1.  Registration and Subscription  . . . . . . . . . . . . .  28
   11.2.  Appearance Selection for Incoming Call . . . . . . . . .  32
   11.3.  Outgoing Call without Appearance Seizure . . . . . . . .  35
   11.4.  Outgoing Call with Appearance Seizure  . . . . . . . . .  38
   11.5.  Outgoing Call without Using an Appearance Number . . . .  42
   11.6.  Appearance Release . . . . . . . . . . . . . . . . . . .  44
   11.7.  Appearance Pickup  . . . . . . . . . . . . . . . . . . .  45
   11.8.  Call between UAs within the Group  . . . . . . . . . . .  50
   11.9.  Consultation Hold with Appearances . . . . . . . . . . .  52
   11.10. Joining or Bridging an Appearance  . . . . . . . . . . .  55
   11.11. Loss of Appearance during Allocation . . . . . . . . . .  58
   11.12. Appearance Seizure Contention Race Condition . . . . . .  59
   11.13. Appearance Agent Subscription to UAs . . . . . . . . . .  60
   11.14. Appearance Pickup Race Condition Failure . . . . . . . .  62
   11.15. Appearance Seizure Incoming/Outgoing Contention Race
          Condition  . . . . . . . . . . . . . . . . . . . . . . .  65
 12. Security Considerations . . . . . . . . . . . . . . . . . . .  66
 13. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  67
   13.1.  SIP Event Header Field Parameter: shared . . . . . . . .  67
   13.2.  SIP Alert-Info Header Field Parameter: appearance  . . .  68
   13.3.  URN Sub-Namespace Registration: sa-dialog-info . . . . .  68
   13.4.  XML Schema Registration  . . . . . . . . . . . . . . . .  68
 14. References  . . . . . . . . . . . . . . . . . . . . . . . . .  69
   14.1.  Normative References . . . . . . . . . . . . . . . . . .  69
   14.2.  Informative References . . . . . . . . . . . . . . . . .  70
 Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . . .  71
 Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  71

1. Introduction

 The feature and functionality requirements for SIP user agents (UAs)
 supporting business telephony applications differ greatly from basic
 SIP UAs, both in terms of services and end-user experience.  In

Johnston, et al. Standards Track [Page 3] RFC 7463 SIP Shared Appearances March 2015

 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 [RFC6665], 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 [RFC5359].
 This specification details a method for implementing a group
 telephony feature known variously 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 SIP events [RFC6665] and
 publication [RFC3903] (carrying the SIP dialog state event package
 [RFC4235]) for exchanging status among UAs.
 In traditional telephony, the line is physical.  A common scenario in
 telephony is for a number of business telephones to share a single or
 a small number of lines.  The sharing or appearance of these lines
 between a number of phones is what gives this feature its name.  A
 common scenario in SIP is for a number of business telephones to
 share a single or a small number of Address of Record (AOR) URIs.
 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 of an
 AOR has a visual display (lamp that can change color or blink or a
 screen icon) and a button (used to select the appearance) where each
 appearance number is associated with a different dialog to/from the
 AOR.  The telephony concept of line appearance is still relevant to
 SIP due to the user interface considerations.  It is important to
 keep the appearance number construct because:
 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.
 The purpose of the appearance number is to identify active calls to
 facilitate sharing between users (e.g., passing a call from one user
 to another).  If a telephone has enough buttons/lamps, the appearance
 number could be the positional sequence number of the button.  If

Johnston, et al. Standards Track [Page 4] RFC 7463 SIP Shared Appearances March 2015

 not, it may still be desirable to present the call state, but the
 appearance number should be displayed so that users know which call,
 for example, is on hold on which key.
 In this document, except for the usage scenarios in the next section,
 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 main differences relate to
 the user interface considerations of the device.

3.1. Executive/Assistant Arrangement

 The appearances on the executive's UA 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. Call Group

 Users with similar business needs or tasks can be assigned to
 specific groups and share an AOR.  For example, an IT department
 staff of five might answer a help line that 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 appearance.  Another phone can request to be added/joined/
 bridged to an existing appearance resulting in a conference call.

Johnston, et al. Standards Track [Page 5] RFC 7463 SIP Shared Appearances March 2015

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
 seizes 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.

3.4. Changing UAs

 A user is on a call on one UA and wishes to change devices and
 continue the call on another UA.  They place the call on hold, note
 the appearance number of the call, then walk to another UA.  They are
 able to identify the same appearance number on the other UA, pick up
 the call, and continue the conversation.

4. Requirements and Implementation

 The next section details the requirements and discusses the
 implementation of the shared appearances feature.

4.1. Requirements

 The basic requirements of the shared appearances 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:   A UA must be able to join (also called bridge or conference
          together) or pick up (take) an active call of 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 participate in the shared appearance group
          without manual configuration of group members.
 REQ-5:   The mechanism must scale for large numbers of appearances
          and large numbers of UAs without introducing excessive
          messaging traffic.
 REQ-6:   Each call or session (incoming or outgoing) should be
          assigned a common "appearance" number from a managed pool

Johnston, et al. Standards Track [Page 6] RFC 7463 SIP Shared Appearances March 2015

          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 status of all
          appearances of the group.
 REQ-8:   There must be mechanisms to resolve appearance contention
          among the UAs in the group.  Contention in this context
          means an appearance number being associated with multiple
          dialogs that are not mixed or otherwise associated.
 REQ-9:   The mechanism must allow all UAs receiving an incoming
          session request to utilize 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 that 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, seize, 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 a private conversation with the
          external party and other UAs must not be allowed to join or
          take the call.  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 shared appearance 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 seize a
          particular appearance number for outgoing requests prior to
          sending the actual request.  This is often called seizure.

Johnston, et al. Standards Track [Page 7] RFC 7463 SIP Shared Appearances March 2015

 REQ-16:  The mechanism should support a way for a UA to seize a
          particular appearance number and also send the request at
          the same time.  This is needed when an automatic ringdown
          feature (a telephone configured to immediately dial a phone
          number when it goes off-hook) is combined with shared
          appearances.  In this case, seizing the line is integrated
          with dialing.

4.2. Implementation

 This section non-normatively discusses the implementation of the
 shared appearances feature.  The normative description is in
 Section 5.  Many of the requirements for this service can be met
 using standard SIP mechanisms such as:
 o  A SIP Forking Proxy and Registrar/Location Service meets REQ-1.
 o  The SIP Dialog Package meets REQ-2.
 o  The combination of the SIP Replaces and Join header fields meets
    REQ-3.
 o  The use of a State Agent for the Dialog Package meets REQ-4 and
    REQ-5.
 REQ-6 suggests the need for an entity that 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 UA that has taken on this functionality for a group.  The
 Appearance Agent knows or is able to determine the dialog state of
 all members of the group.
 While the appearance resource could be managed cooperatively by a
 group of UAs without any central control, this is outside the scope
 of this document.  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.
 To best meet REQ-9, the appearance number for an incoming INVITE
 needs to be contained in the INVITE, in addition to being delivered
 in the dialog package NOTIFY.  Otherwise, if the NOTIFY is delayed or

Johnston, et al. Standards Track [Page 8] RFC 7463 SIP Shared Appearances March 2015

 lost, a UA in the group might receive an incoming INVITE but might
 not know which appearance number to render during alerting.
 This specification defines an extension parameter, which is
 normatively defined in Section 7, for the Alert-Info header field in
 RFC 3261 to carry the appearance number:
 Alert-Info: <urn:alert:service:normal>;appearance=1
 The following list describes the operation of the shared appearances
 feature.
 1.  A UA is configured with the AOR of a shared appearance group.  It
     registers against the AOR, then attempts a dialog state
     subscription to the AOR.  If the subscription fails, loops back
     to itself, or returns an error, it knows there is no State Agent
     and, hence, no Appearance Agent and this feature is not
     implemented.
 2.  If the subscription receives a 200 OK, the UA knows there is a
     State Agent and that the feature is implemented.  The UA then
     follows the steps in this list.
 3.  Information learned about the dialog state of other UAs in the
     group is rendered to the user.
 4.  Incoming calls are forked to all UAs in the group, and any may
     answer.  UAs receive the appearance number to use in rendering
     the incoming call in a NOTIFY from the Appearance Agent and in
     the INVITE itself.  The UA will also receive a notification if
     the call is answered by another UA in the group so this
     information can be rendered to the user.
 5.  For outgoing calls, the operation depends on the implementation.
     If the user seizes a particular appearance number for the call,
     the UA publishes the trying state dialog information with the
     desired appearance number and waits for a 2xx response before
     sending the INVITE.
 6.  For outgoing calls, if the user does not seize a particular
     appearance or does not care, the INVITE can be sent immediately,
     and the appearance number learned as the call progresses from a
     notification from the Appearance Agent.
 7.  For outgoing calls, if the user does not want an appearance
     number assigned (such as during a consultation call or if a UA is
     fetching 'service media' such as music on hold [RFC7088]), the UA

Johnston, et al. Standards Track [Page 9] RFC 7463 SIP Shared Appearances March 2015

     also publishes prior to sending the INVITE but does not include
     an appearance number in the publication.
 8.  Established calls within the group may be joined (bridged) or
     taken (picked) by another UA.  Information in the dialog package
     notifications can be used to construct Join or Replaces header
     fields.  Since the same appearance number is used for these types
     of operations, this information is published prior to sending the
     INVITE Join or INVITE Replaces.
 9.  The Appearance Agent may not have direct access to the complete
     dialog state of some or all of the UAs in the group.  If this is
     the case, the Appearance Agent will subscribe to the dialog state
     of individual UAs in the group to obtain this information.  In
     any case, the Appearance Agent will send normal notifications
     (via the subscriptions established by the UAs in step 1) every
     time the aggregate dialog state of the AOR changes, including
     when calls are placed, answered, placed on and off hold, and hung
     up.

5. Normative Description

 This section normatively describes the shared appearances feature
 extensions.  The following definitions are used throughout this
 document:
 Appearance number:  An appearance number is a positive integer
    associated with one or more dialogs of an AOR.  Appearance numbers
    are managed by an Appearance Agent and displayed and rendered to
    the user by UAs that support this specification.  When an
    appearance number is assigned or requested, generally the assigned
    number is the smallest positive integer that is not currently
    assigned as an appearance number to a dialog for this AOR.  This
    specification does not define an upper limit on appearance
    numbers; however, using appearance numbers that are not easily
    represented using common integer representations is likely to
    cause failures.
 Seizing:  An appearance can be reserved prior to a call being placed
    by seizing the appearance.  An appearance can be seized by
    communicating an artificial state of "trying" prior to actually
    initiating a dialog (i.e., sending the INVITE), in order to appear
    as if it were already initiating a dialog.
 Selecting (or Not-Seizing):  An appearance is merely selected (i.e.,
    not seized) if there is no such communication of artificial state
    of "trying" prior to initiating a dialog: i.e., the state is

Johnston, et al. Standards Track [Page 10] RFC 7463 SIP Shared Appearances March 2015

    communicated when the dialog is actually initiated.  The
    appearance number is learned after the INVITE is sent.

5.1. Elements

 A complete system to implement this feature consists of:
 1.  UAs that support publications, subscriptions, and notifications
     for the SIP dialog event package and the shared appearance dialog
     package extensions and behavior.
 2.  An Appearance Agent consisting of a State Agent for the dialog
     event package that implements an Event State Compositor (ESC) and
     the shared appearance dialog package extensions and behavior.
     The Appearance Agent also has logic for assigning and releasing
     appearance numbers and resolving appearance number contention.
 3.  A forking proxy server that can communicate with the State Agent.
 4.  A registrar that supports the registration event package.
 The behavior of these elements is described normatively in the
 following sections after the definitions of the dialog package
 extensions.

5.2. Shared Appearance Dialog Package Extensions

 This specification defines four new elements as extensions to the SIP
 Dialog Event package [RFC4235].  The schema is defined in Section 6.
 The elements are <appearance>, <exclusive>, <joined-dialog>, and
 <replaced-dialog>, which are sub-elements of the <dialog> element.

5.2.1. The <appearance> Element

 The <appearance> element, a child of the <dialog> element, is used to
 convey the appearance number of the dialog described by the parent
 <dialog> element.  When sent by a UA in a PUBLISH with parent
 <dialog> with state attribute "trying" to the Appearance Agent, the
 UA is requesting assignment of the given appearance number to the
 current or future dialog with the given dialog identifiers.  When an
 <appearance> element is sent by the Appearance Agent in a NOTIFY, it
 indicates that the appearance number has been assigned to the
 specified dialog.
 Note that a <dialog-info> element describes the contained dialogs
 from the point of view of the UA (named by the "entity" attribute),
 regardless of whether the containing request is sent by the UA or the
 Appearance Agent.  In particular, if the UA sent a request within the

Johnston, et al. Standards Track [Page 11] RFC 7463 SIP Shared Appearances March 2015

 described dialog, the To header field URI would match the <remote>
 <identity> value and the to-tag parameter would match the remote-tag
 attribute.  Similarly, the From header field URI would match the
 <local> <identity> value and the from-tag parameter would match the
 local-tag attribute.

5.2.2. The <exclusive> Element

 The <exclusive> element, a child of the <dialog> element, is a
 boolean, which, when true, indicates that the UA is not willing to
 accept an INVITE with a Join or Replaces header field targeted to the
 dialog described by the <dialog> element that is the parent of the
 <exclusive> element.  For example, some shared appearance systems
 only allow call pickup when the call is on hold.  In this case, the
 <exclusive> element should be set to "false" when the call is held
 and "true" when the call is not held, rather than having the
 "exclusive" value implied by the hold state.
 It is important to note that this element is a hint.  In order to
 prevent another UA from taking or joining a call, a UA can, in
 addition to setting the <exclusive> tag, not report full dialog
 information to the Appearance Agent.  Not having the full dialog
 information (Call-ID, remote-tag, and local-tag) prevents another UA
 from constructing a Join or Replaces header field.  Although a UA may
 set <exclusive> to "true", the UA must still be ready to reject an
 INVITE Join relating to this dialog.  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 Replaces
 requests containing those dialog identifiers with a 403 (Forbidden)
 response.
    Note that exclusivity has nothing to do with appearance number
    selection or seizing -- instead, it is about call control
    operations that can be performed on a dialog.
 If the <exclusive> element is not present, it is assumed to be false.

5.2.3. The <joined-dialog> Element

 The <joined-dialog> element, a child of the <dialog> element, is used
 to convey dialog identifiers of any other dialogs that are joined
 (mixed or bridged) with the dialog.  Only the UA that is the common
 endpoint of the mixed dialogs (and thus controlling the mixing
 operation) should include this element in publications to the

Johnston, et al. Standards Track [Page 12] RFC 7463 SIP Shared Appearances March 2015

 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.  Joined dialogs will share the same
 appearance number.
 If the <joined-dialog> element is not present, it is assumed that the
 dialog is not joined or to be joined to any other dialog.

5.2.4. The <replaced-dialog> Element

 The <replaced-dialog> element, a child of the <dialog> element, is
 used to convey dialog identifiers of any other dialogs that will be
 or have been replaced with this dialog.  For example, a UA in the
 group picking up a call on another UA by sending an INVITE with
 Replaces would include this element for the replacing dialog.
 Replaced dialogs will share the same appearance number.
 If the <replaced-dialog> element is not present, it is assumed that
 the dialog has not replaced or is not to replace to any other dialog.

5.3. Shared Appearance User Agents

 UAs that support the shared appearances feature use the dialog state
 package [RFC4235] with the shared appearance extensions and the
 'shared' Event header field parameter defined in Section 13.
 UAs use the dialog package extensions in Section 5.2 along with
 SUBSCRIBE [RFC6665], NOTIFY [RFC6665], and PUBLISH [RFC3903].
 SUBSCRIBE, NOTIFY, and PUBLISH requests for the dialog event package
 include the 'shared' Event header field parameter as required by this
 specification.
    The presence of the 'shared' Event header field parameter tells
    the Appearance Agent that the UA supports this specification.
 Upon initialization, the UA MUST subscribe to the dialog event
 package of the AOR and refresh the subscription per the SIP Events
 Framework [RFC6665].  If the SUBSCRIBE request fails, then no
 Appearance Agent may be present and this feature is not active for
 this AOR.  The UA MAY periodically retry the subscription to see if
 conditions have changed at intervals no shorter than four hours.
    Four hours was chosen to limit the subscription test to six per
    day per UA.  Increasing this interval would reduce this failure
    traffic but take longer for a newly activated Appearance Agent to
    be discovered.

Johnston, et al. Standards Track [Page 13] RFC 7463 SIP Shared Appearances March 2015

 UAs can also use the presence of the 'shared' Event header field
 parameter in NOTIFYs to discover the presence of an Appearance Agent
 for the AOR.
 UAs that implement the shared appearances feature, call pickup,
 joining, and bridging MUST support sending an INVITE with Replaces
 [RFC3891]  or Join [RFC3911].  The User Agent Client (UAC) needs to
 include the to-tag and from-tag information in the Replaces or Join
 header so that the correct dialog will be matched by the User Agent
 Server (UAS) per the rules in RFCs 3891 and 3911.
 All UAs that implement the shared appearances feature and support
 INVITE MUST support receiving an INVITE with a Replaces [RFC3891] or
 a Join [RFC3911] header field.
 When publishing or notifying dialog package information, a UA
 includes the largest set of 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.  While this dialog
 identification information is optional in [RFC4235], it is essential
 in the shared appearances feature, allowing call control operations.
 When placing calls on hold, use the "+sip.rendering=no" feature tag
 to indicate this in dialog package notifications.  Using the full SDP
 session description instead forces the endpoint to do a lot of extra
 parsing, unnecessarily complicating the code and inviting errors.
    The accurate rendering of the idle/active/alerting/hold state of
    other UAs in the group is an important part of the shared
    appearances feature.
 A UA that does not need to seize a particular appearance number (or
 doesn't care) would just send an INVITE as normal to place an
 outbound call.
 If the call is an emergency call, a UA MUST never wait for a
 confirmed seizure before sending an INVITE.  Instead, the emergency
 call MUST proceed without waiting for the PUBLISH transaction.
 If a UA requires a particular appearance number, the a UA MUST send a
 dialog package PUBLISH request and wait for a 2xx response before
 sending the INVITE.  This is required in the following situations:
 1.  When the user seizes a particular appearance number for an
     outgoing call (e.g., seizing the appearance and going "off-hook",
     if the UA's user interface uses this metaphor).

Johnston, et al. Standards Track [Page 14] RFC 7463 SIP Shared Appearances March 2015

 2.  When the user has requested that an appearance number not be used
     for an outgoing call (i.e., during a consultation call, a
     'service media' call such as for music on hold [RFC7088], or for
     a call not considered part of the shared appearance group).
 3.  When the user has selected to join (or bridge) an existing call.
 4.  When the user has selected to replace (or take) an existing call.
 Note that when a UA seizes an appearance prior to establishment of a
 dialog (numbers 1 and 2 in the above list), not all dialog
 information will be available.  In particular, when a UA publishes an
 attempt to seize an appearance prior to knowing the destination URI,
 minimal or no dialog information may be available.  For example, in
 some cases, only the local target URI for the call will be known: not
 any dialog information.  If the From tag and Call-ID were not present
 in the initial PUBLISH, a new PUBLISH MUST be sent as soon as this
 information is available.
    The first publication will cause the Appearance Agent to reserve
    the appearance number for this UA.  If the publication does not
    have any dialog identifiers (e.g., Call-ID or local-tag), the
    Appearance Agent cannot assign the appearance number to a
    particular dialog of the UA until the second publication, which
    will contain some dialog identifiers.
 This publication state is refreshed as described in [RFC3903] during
 the early dialog state or the Appearance Agent may reassign the
 appearance number.  Once the dialog has transitioned to the confirmed
 state, no publication refreshes are necessary.
    This specification assumes that the Appearance Agent has other
    means besides UA publication to learn about the state of UA
    dialogs.  In this specification, PUBLISH is used to indicate
    desired and intended appearance number operations.  Once a dialog
    transitions from early to confirmed, this role is over; hence, no
    publication refreshes are needed.
 Appearance numbers are a shorthand label for active and pending
 dialogs related to an AOR.  Many of the features and services built
 using this extension rely on the correct rendering of this
 information to the human user.  In addition, the group nature of the
 feature means that the rendering must be similar between different
 vendors and different models.  Failure to do so will greatly reduce
 the value and usefulness of these protocol extensions.  In a
 correctly designed user interface for this feature, the appearances
 number for each active and pending dialog is explicitly (i.e., by
 appearance number) or implicitly (using a user interface metaphor

Johnston, et al. Standards Track [Page 15] RFC 7463 SIP Shared Appearances March 2015

 that makes the numbering and ordering clear to the user) rendered to
 the user.  The far-end identity of each dialog (e.g., the remote
 party identity) is not a useful replacement for the appearance
 number.  The state of each appearance is also to be rendered (idle,
 active, busy, joined, etc.).  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.
 Appearance numbers of dialogs can be learned by dialog package
 notifications containing the <appearance> element from the Appearance
 Agent or from the 'appearance' Alert-Info parameter in an incoming
 INVITE.  Should they conflict, the dialog package notification takes
 precedence.
 A user may select an appearance number but then abandon placing a
 call (go back on-hook).  In this case, the UA frees up the appearance
 number by removing the event state with a PUBLISH as described in
 [RFC3903].  A failure to do this will require unnecessary operations
 by the Appearance Agent and tie up appearance numbers that could
 otherwise be used by other UAs in the shared appearance group.
 A UA SHOULD register against the AOR only if it is likely the UA will
 be answering incoming calls.  If the UA is mainly going to be
 monitoring the status of the shared appearance group calls and
 picking or joining calls, the UA SHOULD only subscribe to the AOR and
 not register against the AOR.  If a monitoring UA registers rather
 than just subscribing, it generates large amounts of unnecessary
 network traffic.
    All subscribed UAs will receive dialog package NOTIFYs of trying
    state for incoming INVITEs.
 A UA MUST NOT insert an 'appearance' parameter into an Alert-Info
 header field in an INVITE or other request.
    The Appearance Agent is solely responsible for doing this.

5.3.1. Appearance Numbers and Call Context

 There are cases where two separate dialogs at a UA are not mixed but
 share the same 'context'.  That is, they relate to each other and
 should not be treated the same as any other two dialogs within the
 group.  One example of this is a 'consultation call' where a user
 puts an existing dialog on hold, then calls another user, before
 switching back to the original dialog.  Another case, described
 below, occurs during transfer operations, where for a transient
 period, a UA is involved in dialogs with two other UAs, but the
 dialogs are related, and should not be treated as independent
 dialogs.  These cases are best handled by not assigning an appearance

Johnston, et al. Standards Track [Page 16] RFC 7463 SIP Shared Appearances March 2015

 number to a newly created dialog when it shares a context with an
 existing dialog.  But if the preexisting dialog is terminated, its
 appearance number should be reassigned to the newly created dialog.
 A UA that wants to place a call but does not have an appearance
 number assigned sends a PUBLISH before sending the INVITE.  The
 PUBLISH does not have an 'appearance' element present, but it does
 have the 'shared' Event header field parameter present.  If the
 Appearance Agent policy does not allow calls without an assigned
 appearance number, a 400 (Bad Request) response is sent by the
 Appearance Agent and the UA will republish either selecting/seizing
 an appearance number or send the INVITE without publishing, in which
 case the Appearance Agent will assign one.
    Note that if an Appearance Agent rejects calls without an
    appearance number, certain operations such as consultation calls,
    transfer, and music on hold may be negatively impacted.

5.3.2. Appearance Numbers and Call Control

 When an INVITE is generated to attempt to bridge or take a call
 (i.e., contains Join or Replaces with a dialog identifier of another
 dialog in the shared appearance group), the UA MUST first send a
 PUBLISH to the Appearance Agent.  This PUBLISH will contain:
 1.  The appearance number of the joined or replaced call in the
     <appearance> element
 2.  The dialog information from the Join header field in the <joined-
     dialog> element, if the dialog is being joined
 3.  The dialog information from the Replaces header field in the
     <replaced-dialog> element, if the dialog is being replaced
    Note that this information is provided to the Appearance Agent so
    that it can provide proper appearance assignment behavior.  If the
    INVITE Join or Replaces was sent without publishing first, the
    Appearance Agent might assign a new appearance number to this
    INVITE, which would be a mistake.  With Join, the publication has
    the <joined-dialog> element to prevent the Appearance Agent from
    generating a 400 (Bad Request) response due to the reuse of an
    appearance number.  For Replaces, the purpose of the <replaced-
    dialog> is to prevent a race condition where the BYE could cause
    the appearance number to be released when it should stay with the
    replacing dialog.

Johnston, et al. Standards Track [Page 17] RFC 7463 SIP Shared Appearances March 2015

5.3.3. Appearance Numbers and Transfer

 During a transfer operation, it is important that the appearance
 number not change during the operation.  Consider the example of
 Alice, a member of a shared appearance group, who is talking to
 Carol, who is outside the shared appearance group.  Carol transfers
 Alice to David, who is also outside the shared appearance group.  For
 example, if Alice is using appearance 3 for the session with Carol,
 the resulting session with David should also use appearance number 3.
 Otherwise, an appearance number change can cause a "jump" on the UI
 and confusion to the user.  There are two possible scenarios using
 the terminology of RFC 5589: Alice is the transferee in any type of
 transfer (receives the REFER) or the transfer target in an attended
 transfer (receives the INVITE with Replaces).
 If Alice is the transferee, the triggered INVITE from the REFER is
 treated as a consultation call.  Alice SHOULD publish requesting that
 the Appearance Agent not assign an appearance number for this INVITE.
 When the transfer completes, Alice SHOULD publish again to move the
 appearance number from the dialog with Carol to the dialog with
 David.  If a PUBLISH is sent to move the appearance number, the
 publication MUST be sent prior to sending the BYE to Carol to avoid a
 race condition where the Appearance Agent reassigns the appearance
 number after seeing the BYE.
 If Alice is the target, the incoming INVITE will contain a Replaces
 header field.  As a result, the Appearance Agent will have reused the
 appearance number of the dialog with Carol, and this appearance
 number will continue to be used after the dialog with Carol has been
 terminated.

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 Appearance Agent MUST support the appearance dialog package
 extensions defined in Section 5.2 and use the 'shared' Event header
 field parameter.  The Appearance Agent MUST support publications and
 subscriptions for this event package.
 The Appearance Agent MUST have a way of discovering the state of all
 dialogs associated with the AOR.  If this information is not
 available from a call stateful proxy or Back-to-Back User Agent
 (B2BUA), the Appearance Agent can use the registration event package
 [RFC3680] to learn of UAs associated with the AOR and subscribe to
 their dialog event state.  An Appearance Agent can also subscribe to
 a UA's dialog event state in order to reconstruct state.  As a
 result, the registrar MUST support the registration event package.

Johnston, et al. Standards Track [Page 18] RFC 7463 SIP Shared Appearances March 2015

 Dialog package notifications are recommended by RFC 4235 to "only
 contain information on the dialogs whose state or participation
 information has changed."  This specification extends RFC 4235 as
 follows.  The Appearance Agent SHOULD send dialog event state
 notifications whenever the following events happen to UAs in the AOR
 group:
 1.  A call is received, placed, answered, or terminated.
 2.  A call is placed on or off hold.
 3.  A call is joined or replaced.
 4.  An appearance number is reserved or released.
 The Appearance Agent MUST allocate an appearance number for all
 incoming calls and send immediate notifications to the UAs subscribed
 to the shared group AOR.  A new appearance number is allocated except
 for an incoming INVITE with a Join or Replaces header field.  For
 this case, the appearance number should match the appearance number
 of the dialog being joined or replaced.  If the INVITE Replaces or
 Join comes from outside the shared appearance group, the Appearance
 Agent will include a <joined-dialog> or <replaced-dialog> element in
 the NOTIFY containing the dialog information from the Replaces or
 Joined header field.
 The Appearance Agent MUST be able to communicate with the forking
 proxy to learn about incoming calls and also to pass the appearance
 number to the proxy or ensure the Alert-Info header field is included
 in the INVITE with the appropriate appearance number.
    Note that UAs need to be able to handle incoming INVITEs without
    an appearance number assigned.  This could be caused by a failure
    of the Appearance Agent or other error condition.  Although the
    proper rendering of the INVITE may not be possible, this is better
    than ignoring or failing the INVITE.
 An Appearance Agent SHOULD assign an appearance number to an outgoing
 dialog if a PUBLISH has not been received selecting/seizing a
 particular appearance number.
    Note that if the shared appearance group has appearance-unaware
    UAs making calls, the Appearance Agent will still allocate
    appearance numbers for INVITEs sent by those UAs.
 An Appearance Agent receiving a PUBLISH with an appearance number
 checks to make sure the publication is valid.  An appearance number
 can be assigned to only one dialog unless there is a <joined-dialog>

Johnston, et al. Standards Track [Page 19] RFC 7463 SIP Shared Appearances March 2015

 or <replaced-dialog> element indicating that the dialog will be/has
 been replaced or joined.  A 400 (Bad Request) response is returned if
 the chosen appearance number is invalid, and an immediate NOTIFY
 SHOULD be sent to the UA containing full dialog event state.
 An Appearance Agent receiving a PUBLISH without an appearance number
 but with the 'shared' Event header field parameter present interprets
 this as a request by the UA to not assign an appearance number.  If
 the Appearance Agent policy does not allow this, a 400 (Bad Request)
 response is returned.  If policy does allow this, a 200 (OK) response
 is returned and no appearance number is allocated.  An Appearance
 Agent does not have to share this dialog information (i.e., send a
 NOTIFY) with other UAs in the group as the information will not be
 rendered by the other UAs.
 The Appearance Agent allocates an appearance number to a dialog from
 the time the appearance is requested via a PUBLISH or from the
 receipt of an INVITE to the time when the last dialog associated with
 the appearance is terminated, including all dialogs that are joined
 or replaced.  During the early dialog state, the Appearance Agent
 controls the rate of dialog state publication using the Expires
 header field in 200 (OK) responses to PUBLISH requests.  An interval
 of 3 minutes is RECOMMENDED.  After the dialog associated with the
 publication has been confirmed, the expiration of the publication
 state has no effect on the appearance allocation.  If the publication
 contains no dialog state information, the Appearance Agent MUST
 reserve the appearance number for the UA but cannot assign the
 appearance to any particular dialog of the UA.  When the publication
 state is updated with any dialog information, the appearance number
 can then be assigned to the particular dialog.  A UA that has been
 allocated an appearance number using a PUBLISH MAY free up the
 appearance number by removing the event state with a PUBLISH as
 described in [RFC3903].
 If an INVITE is sent by a member of the group to the shared AOR
 (i.e., they call their own AOR), the Appearance Agent MUST assign two
 appearance numbers.  The first appearance number will be the one
 selected or assigned to the outgoing INVITE.  The second appearance
 number will be another one assigned by the Appearance Agent for the
 INVITE as it is forked back to the members of the group.
    The is to preserve a common behavior in legacy systems.
 If an INVITE is sent by a member of the group using the shared AOR or
 sent to the shared AOR and no appearance number is available, the
 proxy MAY reject the INVITE with a 403 (Forbidden) response code.

Johnston, et al. Standards Track [Page 20] RFC 7463 SIP Shared Appearances March 2015

 Appearance numbers are only used for dialogs in which one or more UAs
 associated with the group AOR are participants.  If an incoming
 INVITE to the group AOR is forwarded to another AOR, the appearance
 number is immediately freed up and can be assigned to another dialog.

6. XML Schema Definition

 The 'appearance', 'joined-dialog', 'replaced-dialog', 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:

Johnston, et al. Standards Track [Page 21] RFC 7463 SIP Shared Appearances March 2015

 <?xml version="1.0" encoding="UTF-8"?>
   <xs:schema
     targetNamespace="urn:ietf:params:xml:ns:sa-dialog-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="replaced-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>

Johnston, et al. Standards Track [Page 22] RFC 7463 SIP Shared Appearances March 2015

7. Alert-Info Appearance Parameter Definition

 This specification extends [RFC3261] to add an 'appearance' parameter
 to the Alert-Info header field and also to allow proxies to modify or
 delete the Alert-Info header field.
 The changes to the ABNF [RFC5234] in RFC 3261 are:
      alert-param      =  LAQUOT absoluteURI RAQUOT *( SEMI
                            (generic-param / appearance-param) )
      appearance-param = "appearance" EQUAL 1*DIGIT
 A proxy inserting an 'appearance' Alert-Info parameter follows normal
 Alert-Info policies.  To indicate the appearance number for this
 dialog, the proxy adds the Alert-Info header field with the
 'appearance' parameter to the INVITE.  If an Alert-Info is already
 present, the proxy adds the 'appearance' parameter to the Alert-Info
 header field.  If an appearance number parameter is already present
 (associated with another AOR or by mistake), the value is rewritten
 adding the new appearance number.  There MUST NOT be more than one
 appearance parameter in an Alert-Info header field.
 If no special ringtone is desired, a normal ringtone SHOULD be
 indicated using the urn:alert:service:normal in the Alert-Info, as
 per [RFC7462].  The appearance number present in an Alert-Info header
 field SHOULD be rendered by the UA to the user, following the
 guidelines in Section 5.3.  If the INVITE is forwarded to another
 AOR, the appearance parameter in the Alert-Info SHOULD be removed
 before forwarding outside the group.
 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 is a variety of ways the proxy can 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 shared 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 for the Proxy to first send the incoming INVITE to the
    Appearance Agent, which would redirect to the shared appearance

Johnston, et al. Standards Track [Page 23] RFC 7463 SIP Shared Appearances March 2015

    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 seize 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.

8. 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.

8.1. 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.
 The following guidelines suggest how the appearance number should be
 handled in three typical user interface implementations.

8.1.1. Single Appearance UAs

 These devices are constrained by only having the capability of
 displaying status indications for a single appearance.  The UA SHOULD
 still send messages annotated with appearance number "1".  Any call
 indications for appearances other than for number "1" SHOULD be
 rejected with a 480 (Temporarily Unavailable) or 486 (Busy Here)
 response.  Note that this means that a single appearance UA cannot
 answer its own call to the shared AOR, since this call would use a
 second appearance number.

8.1.2. 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.
 Only appearance numbers "1" and "2" will be used by these UAs.

Johnston, et al. Standards Track [Page 24] RFC 7463 SIP Shared Appearances March 2015

8.1.3. 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 rejected, and not mapped to a
 free button.  Users of these devices often seize specific appearance
 numbers for outgoing calls, and the UA will need to seize the
 appearance number and wait for confirmation from the Appearance Agent
 before proceeding with calls.

8.1.4. Shared Appearance UAs with Variable Appearance Numbers

 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.  These devices can typically
 make calls without waiting for confirmation from the Appearance Agent
 on the appearance number.

8.1.5. Example User Interface Issues

 The problems faced by each style of user interface are readily seen
 in this example:
 1.  A call arrives at the shared appearance group and is assigned an
     appearance number of "1".  All UAs should be able to render to
     the user the arrival of this call.
 2.  Another call arrives at the shared appearance group and is
     assigned an appearance number of "2".  The single appearance UA
     should not present this call to the user.  Other UAs should have
     no problems presenting this call distinctly from the first call.
 3.  The first call clears, releasing appearance number "1".  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 "1" is now free, but that there is still a call on
     appearance number "2".
 4.  A third call arrives and is assigned the appearance number of
     "1".  All UAs should be able to render the arrival of this new
     call to the user.  Multiple appearance UAs should continue to
     indicate the presence of the second call, and they should also

Johnston, et al. Standards Track [Page 25] RFC 7463 SIP Shared Appearances March 2015

     ensure that the presentation order is related to the appearance
     number and not the order of call arrival.

8.2. Call State Rendering

 UAs that implement the shared appearances 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, means that an appearance number is
 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 shared appearance
 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.

9. Interoperability with Non-shared Appearance UAs

 It is desirable to allow a basic UA that does not directly support
 shared appearance to be part of a shared appearance group.  To
 support this, the Proxy must collaborate with the Appearance Agent.
 This is not required in the basic shared appearance architecture;
 consequently, shared appearance interoperability with non-shared
 appearance UAs will not be available in all shared appearance
 deployments.
 First, a UA that does not support dialog events or the shared
 appearances feature will be discussed.  Then, a UA that does support
 dialog events but not the shared appearances feature will be
 discussed.

9.1. Appearance Assignment

 A UA that has no knowledge of appearances will only have appearance
 numbers for outgoing calls if assigned by the Appearance Agent.  If
 the non-shared appearance UA does not support Join or Replaces, all
 dialogs SHOULD be marked "exclusive" to indicate that these options
 are not available.  Marking these dialogs "exclusive" provides a
 better user experience and avoids extra SIP messaging failures.

Johnston, et al. Standards Track [Page 26] RFC 7463 SIP Shared Appearances March 2015

9.2. Appearance Release

 In all cases, the Appearance Agent must be aware of the 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 that Record-
 Routes for dialogs from the non-shared-appearance-aware UA, the
 Appearance Agent will know about the state of dialogs including hold,
 etc.  This information could be determined from inspection of non-
 end-to-end-encrypted INVITE and re-INVITE messages and added to the
 dialog information conveyed to other UAs.

9.3. UAs Supporting Dialog Events but Not Shared Appearance

 Interoperability with UAs that support dialog events but not the
 shared appearances feature is more straightforward.  As before, all
 appearance number assignments must be done by the Appearance Agent.
 The Appearance Agent SHOULD still include appearance information in
 NOTIFYs -- this UA will simply ignore this extra information.  This
 type of UA will also ignore appearance number limitations and may
 attempt to join or replace dialogs marked exclusive.  As a result,
 the Proxy or UAs need to reject such requests or the dialogs will be
 joined or taken.

10. Provisioning Considerations

 UAs can automatically discover if this feature is active for an AOR
 by looking for the 'shared' Event header field parameter in a
 response to a dialog package SUBSCRIBE to the AOR, so no provisioning
 for this is needed.
 The registrar will need to be provisioned to accept either first or
 third party registrations for the shared AOR.  First party
 registration means the To and From URIs in the REGISTER request are
 the shared AOR URI.  Third-party registration means the To URI is the
 shared AOR URI and the From URI is a different AOR, perhaps that of
 the individual user.  Either the credentials of the shared AOR or the
 user MUST be accepted by the registrar and the Appearance Agent,
 depending on the authorization policy in place for the domain.
 If the Appearance Agent needs to subscribe to the dialog state of the
 UAs, then the Appearance Agent and the UAs need to be provisioned
 with credentials so the UAs can authenticate the Appearance Agent.
 In some cases, UAs in the shared appearance group might have a UI
 limitation on the number of appearances that can be rendered.

Johnston, et al. Standards Track [Page 27] RFC 7463 SIP Shared Appearances March 2015

 Typically, this will be hard-phones with buttons/lamps instead of
 more flexible UIs.  In this case, it can be useful for the Appearance
 Agent to know this maximum number.  This can allow the Appearance
 Agent to apply policy when this limit is reached, e.g., deny a call.
 However, this mechanism does not provide any way to discover this by
 protocol means.

11. Example Message Flows

 The next section shows call flow and message examples.  The flows and
 descriptions are non-normative.  Note that, in these examples, all
 INVITEs sent by a UA in the group will be From the shared AOR
 (sip:HelpDesk@example.com in this case), and all INVITES sent to the
 group will have a Request-URI of the shared AOR.  Any other requests
 would not apply to this feature and would be handled using normal SIP
 mechanisms.
 Note that the first 12 examples assume the Appearance Agent is aware
 of dialog state events.  The example in Section 11.13 shows the case
 where this is not the case, and, as a result, the Appearance Agent
 initiates a subscription to users of the shared AOR.  Any of the
 other call flow examples could have shown this mode of operation as
 it is equally valid.

11.1. Registration and Subscription

 Bob and Alice are in a shared appearance group identified by the
 shared appearance AOR sip:HelpDesk@example.com.  Bob REGISTERs using
 contact sip:bob@ua2.example.com.  Alice REGISTERs with contact
 sip:alice@ua1.example.com.
 UAs for Alice and Bob subscribe to the dialog package for the
 appearance AOR and publish dialog state to the Appearance Agent.
 Message exchanges between the Registrar, Appearance Agent, Alice, and
 Bob are shown below.  The call flow examples below do not show the
 authentication of subscriptions, publications, and notifications.  It
 should be noted that for security purposes, all publications and
 subscriptions must be authorized before they are accepted.
 Also note that registrations and subscriptions must all be refreshed
 by Alice at intervals determined by the expiration intervals returned
 by the Registrar or Appearance Agent.
 Registrar     Appearance Agent          Alice             Bob
 |                    |                    |                |
 |                    |                    |                |
 |<--------------------------- REGISTER F1<|                |
 |                    |                    |                |

Johnston, et al. Standards Track [Page 28] RFC 7463 SIP Shared Appearances March 2015

 |>F2 200 OK ----------------------------->|                |
 |                    |                    |                |
 |                    |<----- SUBSCRIBE F3<|                |
 |                    |                    |                |
 |                    |>F4 200 OK -------->|                |
 |                    |                    |                |
 |                    |>F5 NOTIFY -------->|                |
 |                    |                    |                |
 |                    |<-------- 200 OK F6<|                |
 |                    |                    |                |
 |<-------------------------------------------- REGISTER F7<|
 |                    |                    |                |
 |>F8 200 OK ---------------------------------------------->|
 |                    |                    |                |
 |                    |<---------------------- SUBSCRIBE F9<|
 |                    |                    |                |
 |                    |>F10 200 OK ------------------------>|
 |                    |                    |                |
 |                    |>F11 NOTIFY ------------------------>|
 |                    |                    |                |
 |                    |<------------------------ 200 OK F12<|
 |                    |                    |                |
 Figure 1. Registration and Subscription Example
 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:HelpDesk@example.com>
 CSeq: 2 REGISTER
 Call-ID: d3281184-518783de-cc23d6bb
 Contact: <sip:alice@ua1.example.com>
 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=z9hG4bK527b54da8ACC7B09
 CSeq: 2 REGISTER
 Call-ID: d3281184-518783de-cc23d6bb

Johnston, et al. Standards Track [Page 29] RFC 7463 SIP Shared Appearances March 2015

 From: <sip:alice@example.com>;tag=CDF9A668-909E2BDD
 To: <sip:HelpDesk@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 notifies Alice of the status.
 F3 Alice ----> Appearance Agent
 SUBSCRIBE sip:HelpDesk@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:HelpDesk@example.com>
 CSeq: 91 SUBSCRIBE
 Call-ID: ef4704d9-bb68aa0b-474c9d94
 Contact: <sip:alice@ua1.example.com>
 Event: dialog;shared
 Accept: application/dialog-info+xml
 Max-Forwards: 70
 Expires: 3700
 Content-Length: 0
 F4 Appearance Agent ----> Alice
 SIP/2.0 200 OK
 Via: SIP/2.0/UDP ua1.example.com;branch=z9hG4bKf10fac97E7A76D6A
 CSeq: 91 SUBSCRIBE
 Call-ID: ef4704d9-bb68aa0b-474c9d94
 From: <sip:alice@example.com>;tag=925A3CAD-CEBB276E
 To: <sip:HelpDesk@example.com>;tag=1636248422222257
 Allow-Events: dialog
 Expires: 3700
 Contact: <sip:appearanceagent.example.com>
 Content-Length: 0
 F5 Appearance Agent ----> Alice
 NOTIFY sip:alice@ua1.example.com SIP/2.0
 From: <sip:HelpDesk@example.com>;tag=1636248422222257
 To: <sip:alice@example.com>;tag=925A3CAD-CEBB276E
 Call-ID: ef4704d9-bb68aa0b-474c9d94
 CSeq: 232 NOTIFY
 Via: SIP/2.0/UDP appearanceagent.example.com;branch=z9hG4bK1846
 Max-Forwards: 70

Johnston, et al. Standards Track [Page 30] RFC 7463 SIP Shared Appearances March 2015

 Content-Type: application/dialog-info+xml
 Event: dialog;shared
 Subscription-State: active;expires=3000
 Contact: <sip:appearanceagent.example.com>
 Content-Length: ...
 <?xml version="1.0"?>
 <dialog-info xmlns="urn:ietf:params:xml:ns:dialog-info"
              version="40"
              state="full"
              entity="sip:HelpDesk@example.com">
 </dialog-info>
 F6 Alice ----> Appearance Agent
 SIP/2.0 200 OK
 Via: SIP/2.0/UDP appearanceagent.example.com;branch=z9hG4bK1846
 From: <sip:HelpDesk@example.com>;tag=1636248422222257
 To: <sip:alice@example.com>;tag=925A3CAD-CEBB276E
 CSeq: 232 NOTIFY
 Call-ID: ef4704d9-bb68aa0b-474c9d94
 Contact: <sip:alice@ua1.example.com>
 Content-Length: 0
 F7 Bob ----> Registrar
 REGISTER sip:registrar.example.com SIP/2.0
 Via: SIP/2.0/UDP ua2.example.com;branch=z9hG4b53b54d87B
 From: <sip:bob@example.com>;tag=34831131
 To: <sip:HelpDesk@example.com>
 CSeq: 72 REGISTER
 Call-ID: 139490230230249348
 Contact: <sip:bob@ua2.example.com>
 Max-Forwards: 70
 Expires: 3600
 Content-Length: 0
 F8 Registrar ----> Bob
 SIP/2.0 200 OK
 Via: SIP/2.0/UDP ua2.example.com;branch=z9hG4b53b54d87B
 From: <sip:bob@example.com>;tag=34831131
 To: <sip:HelpDesk@example.com>;tag=fkwlwqi1
 CSeq: 72 REGISTER
 Call-ID: 139490230230249348

Johnston, et al. Standards Track [Page 31] RFC 7463 SIP Shared Appearances March 2015

 Contact: <sip:alice@ua1.example.com>;expires=3200
 Contact: <sip:bob@ua2.example.com>;expires=3600
 Content-Length: 0

11.2. Appearance Selection for Incoming Call

 In the call flow below, Bob and Alice are in a shared appearance
 group.  Carol places a call to the shared appearance group AOR.  The
 Appearance Agent sends NOTIFYs to Alice and Bob telling them what
 appearance the call is using.  Both Alice and Bob's devices are
 alerted of the incoming call.  Bob answers the call.
 Note that it is possible that both Alice and Bob answer the call and
 send 200 (OK) responses to Carol.  It is up to Carol to resolve this
 situation.  Typically, Carol will send ACKs to both 200 OKs but send
 a BYE to terminate one of the dialogs.  As a result, either Alice or
 Bob will receive the BYE and publish that their dialog is over.
 However, if Carol answers both Alice and Bob and keeps both dialogs
 active, then the Appearance Agent will need to resolve the situation
 by moving either Alice or Bob's dialog to a different appearance.
 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.  Note that the order of
 F2 - F5 and F7 - F8 could be reversed.
            Forking     Appearance
 Carol      Proxy         Agent         Alice      Bob
 |            |             |             |         |
 |>F1 INVITE >|             |             |         |
 |            |< - - - - - >|             |         |
 |            |             |>F2 NOTIFY ----------->|
 |            |             |             |         |
 |            |             |<F3 200 OK -----------<|
 |            |             |             |         |
 |            |             |>F4 NOTIFY ->|         |
 |            |             |             |         |
 |            |             |<-200 OK F5-<|         |
 |<- 100 F6 -<|             |             |         |
 |            |>F7 INVITE (appearance=1) ---------->|
 |            |             |             |         |
 |            |>F8 INVITE (appearance=1) >|         |
 |            |             |             |         |
 |            |<-------------------- Ringing 180 F9<|
 |< 180 F10 -<|             |             |         |

Johnston, et al. Standards Track [Page 32] RFC 7463 SIP Shared Appearances March 2015

 |            |<--------- 180 Ringing F11<|         |
 |< 180 F12 -<|             |             |         |
 |            |             |             |         |
 |            |<------------------------ 200 OK F13<|
 |< 200 F14 -<|             |             |         |
 |            |             |             |         |
 |            |>F15 CANCEL -------------->|         |
 |            |             |             |         |
 |            |<-------------- 200 OK F16<|         |
 |            |             |             |         |
 |            |<Request Cancelled 487 F17<|         |
 |            |             |             |         |
 |            |>F18 ACK ----------------->|         |
 |>F19 ACK -->|             |             |         |
 |            |>F20 ACK --------------------------->|
 |            |             |             |         |
 |<=============Both way RTP established===========>|
 |            |             |             |         |
 |            |< - - - - - >|             |         |
 |            |             |             |         |
 |            |             |>F21 NOTIFY >|         |
 |            |             |             |         |
 |            |             |<- 200 F22 -<|         |
 |            |             |             |         |
 |            |             |>F23 NOTIFY ---------->|
 |            |             |             |         |
 |            |             |<F24 200 OK ----------<|
 |            |             |                       |
 Figure 2. Appearance Selection for Incoming Call Example
 F4 Appearance Agent ----> Alice
 NOTIFY sip:alice@ua1.example.com SIP/2.0
 From: <sip:HelpDesk@example.com>;tag=151702541050937
 To: <sip:alice@example.com>;tag=18433323-C3D237CE
 Call-ID: 1e361d2f-a9f51109-bafe31d4
 CSeq: 12 NOTIFY
 Via: SIP/2.0/UDP appearanceagent.example.com;branch=z9hG4bK1403
 Max-Forwards: 70
 Content-Type: application/dialog-info+xml
 Event: dialog;shared
 Subscription-State: active;expires=2800
 Contact: <sip:appearanceagent.example.com>
 Content-Length: ...
 <?xml version="1.0"?>

Johnston, et al. Standards Track [Page 33] RFC 7463 SIP Shared Appearances March 2015

 <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:HelpDesk@example.com">
    <dialog id="2a7294823093f5274e3fd2ec54a2d76c"
         call-id="14-1541707345"
         remote-tag="44BAD75D-E3128D42"
         direction="recipient">
       <sa:appearance>1</sa:appearance>
       <state>trying</state>
       <remote>
         <identity>sip:carol@ua.example.com</identity>
       </remote>
    </dialog>
 </dialog-info>
 F7 Proxy ----> Bob
 INVITE sip:bob@ua2.example.com SIP/2.0
 Via: SIP/2.0/UDP ua3.example.com;branch=z9hG4bK4324ea
 Via: SIP/2.0/UDP proxy.example.com;branch=z9hG4bK38432ji
 From: <sip:carol@example.com>;tag=44BAD75D-E3128D42
 To: <sip:HelpDesk@example.com>
 CSeq: 106 INVITE
 Call-ID: 14-1541707345
 Contact: <sip:carol@ua3.example.com>
 Max-Forwards: 69
 Alert-Info: <urn:alert:service:normal>;appearance=1
 Content-Type: application/sdp
 Content-Length: ...
 v=0
 o=- 1102980499 1102980499 IN IP4 ua3.example.com
 s=
 c=IN IP4 ua3.example.com
 t=0 0
 m=audio 2238 RTP/AVP 0 8 101
 a=rtpmap:0 PCMU/8000
 a=rtpmap:8 PCMA/8000
 a=rtpmap:101 telephone-event/8000
 F21 Appearance Agent ----> Alice
 NOTIFY sip:alice@ua1.example.com SIP/2.0
 From: <sip:HelpDesk@example.com>;tag=151702541050937

Johnston, et al. Standards Track [Page 34] RFC 7463 SIP Shared Appearances March 2015

 To: <sip:alice@example.com>;tag=18433323-C3D237CE
 Call-ID: 1e361d2f-a9f51109-bafe31d4
 CSeq: 13 NOTIFY
 Via: SIP/2.0/UDP appearanceagent.example.com;branch=z9hG4bK4164F03j
 Max-Forwards: 70
 Content-Type: application/dialog-info+xml
 Event: dialog;shared
 Subscription-State: active;expires=2500
 Contact: <sip:appearanceagent.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="17"
              state="partial"
              entity="sip:HelpDesk@example.com">
    <dialog id="2a7294823093f5274e3fd2ec54a2d76c"
         call-id="14-1541707345"
         remote-tag="44BAD75D-E3128D42"
         local-tag="7349dsfjkFD03s"
         direction="recipient">
       <sa:appearance>1</sa:appearance>
       <state>confirmed</state>
       <local>
         <target>sip:bob@ua2.example.com</target>
       </local>
       <remote>
         <identity>sip:carol@ua.example.com</identity>
       </remote>
    </dialog>
 </dialog-info>

11.3. Outgoing Call without Appearance Seizure

 In this scenario, Bob's UA places a call without first selecting/
 seizing an appearance number.  After Bob sends the INVITE, the
 appearance assigns an appearance number for it and notifies both
 Alice and Bob.
 Carol        Proxy           Alice     Appearance Agent         Bob
 |              |               |              |                  |
 |              |               |              |                  |
 |              |<------------------------------------- INVITE F1<|
 |              |               |              |                  |
 |              |>F2 100 Trying --------------------------------->|

Johnston, et al. Standards Track [Page 35] RFC 7463 SIP Shared Appearances March 2015

 |<-- INVITE F3<|               |              |                  |
 |              |< - - - - - - - - - - - - - ->|                  |
 |              |               |              |                  |
 |              |               |<-- NOTIFY F4<|                  |
 |              |               |              |                  |
 |              |               |>F5 200 OK -->|                  |
 |              |               |              |------- NOTIFY F6>|
 |              |               |              |                  |
 |              |               |              |<F7 200 OK ------<|
 |>F8 180  ---->|               |              |                  |
 |              |>F9 180 Ringing -------------------------------->|
 |              |               |              |                  |
 |              |< - - - - - - - - - - - - - ->|                  |
 |              |               |              |                  |
 |              |               |<- NOTIFY F10<|                  |
 |              |               |              |                  |
 |              |               |>F11 200 OK ->|                  |
 |              |               |              |------ NOTIFY F12>|
 |              |               |              |                  |
 |              |               |              |<F13 200 OK -----<|
 |>F14 200 OK ->|               |              |                  |
 |              |>F15 200 OK ------------------------------------>|
 |              |               |              |                  |
 |              |<--------------------------------------- ACK F16<|
 |<---- ACK F17<|               |              |                  |
 |              |               |              |                  |
 |<================= Both way RTP established ===================>|
 |              |               |              |                  |
 |              |< - - - - - - - - - - - - - ->|                  |
 |              |               |              |                  |
 |              |               |<- NOTIFY F18<|                  |
 |              |               |              |                  |
 |              |               |>F19 200 OK ->|                  |
 |              |               |              |------ NOTIFY F20>|
 |              |               |              |                  |
 |              |               |              |<F21 200 OK -----<|
 |              |               |              |                  |
 Figure 3. Outgoing Call without Appearance Seizure Example
 F1 Bob ----> Proxy
 INVITE sip:carol@example.com SIP/2.0
 Via: SIP/2.0/UDP ua2.example.com;branch=z9hG4bK98c87c52123A08BF
 From: <sip:HelpDesk@example.com>;tag=15A3DE7C-9283203B
 To: <sip:carol@example.com>
 CSeq: 1 INVITE

Johnston, et al. Standards Track [Page 36] RFC 7463 SIP Shared Appearances March 2015

 Call-ID: f3b3cbd0-a2c5775e-5df9f8d5
 Contact: <sip:bob@ua2.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
 F4 Appearance Agent ----> Alice
 NOTIFY sip:alice@ua1.example.com SIP/2.0
 Via: SIP/2.0/UDP appearanceagent.example.com;branch=z9hG4bK81d84f62
 From: <sip:HelpDesk@example.com>;tag=1636248422222257
 To: <sip:alice@example.com>;tag=925A3CAD-CEBB276E
 Call-ID: ef4704d9-bb68aa0b-474c9d94
 CSeq: 233 NOTIFY
 Max-Forwards: 70
 Content-Type: application/dialog-info+xml
 Event: dialog;shared
 Subscription-State: active;expires=2200
 Contact: <sip:appearanceagent.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:HelpDesk@example.com">
     <dialog id="fa02538339df3ce597f9e3e3699e28fc"
          call-id="f3b3cbd0-a2c5775e-5df9f8d5"
          local-tag="15A3DE7C-9283203B"    direction="initiator">
             <sa:appearance>1</sa:appearance>
             <sa:exclusive>false</sa:exclusive>
                <state>trying</state>
                 <local>
                     <target uri="sip:bob@ua2.example.com">
                     </target>

Johnston, et al. Standards Track [Page 37] RFC 7463 SIP Shared Appearances March 2015

                    </local>
         </dialog>
 </dialog-info>
 F6 Appearance Agent ----> Bob
 NOTIFY sip:bob@ua1.example.com SIP/2.0
 From: <sip:HelpDesk@example.com>;tag=497585728578386
 To: <sip:bob@example.com>;tag=633618CF-B9C2EDA4
 Call-ID: a7d559db-d6d7dcad-311c9e3a
 CSeq: 7 NOTIFY
 Via: SIP/2.0/UDP appearanceagent.example.com
  ;branch=z9hG4bK1711759878512309
 Max-Forwards: 70
 Content-Type: application/dialog-info+xml
 Event: dialog;shared
 Subscription-State: active;expires=2000
 Contact: <sip:appearanceagent.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="78"
              state="partial"
              entity="sip:HelpDesk@example.com">
     <dialog id="02538339hfgdf3ce597f9e3egkl3699e28fc"
          call-id="f3b3cbd0-a2c5775e-5df9f8d5"
          local-tag="15A3DE7C-9283203B"    direction="initiator">
             <sa:appearance>1</sa:appearance>
             <sa:exclusive>false</sa:exclusive>
                <state>trying</state>
                 <local>
                     <target uri="sip:bob@ua2.example.com">
                     </target>
                    </local>
         </dialog>
 </dialog-info>

11.4. Outgoing Call with Appearance Seizure

 In this scenario, Bob's UA sends out a dialog event PUBLISH with
 state (trying) selecting/seizing an appearance number before sending
 the INVITE.  After receiving the 200 (OK) 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.  Bob's UA puts as

Johnston, et al. Standards Track [Page 38] RFC 7463 SIP Shared Appearances March 2015

 much of the dialog information from F7 as can be determined in
 advance.  In this case, the minimum of the Contact URI is included,
 which allows the Appearance Agent to correlate the INVITE with the
 PUBLISH.
 Carol        Proxy           Alice     Appearance Agent         Bob
 |              |               |              |                  |
 |              |               |              |<----- PUBLISH F1<|
 |              |               |              |                  |
 |              |               |              |>F2 200 OK ------>|
 |              |               |              |                  |
 |              |               |<-- NOTIFY F3<|                  |
 |              |               |              |                  |
 |              |               |>F4 200 OK -->|                  |
 |              |               |              |------- NOTIFY F5>|
 |              |               |              |                  |
 |              |               |              |<F6 200 OK ------<|
 |              |               |              |                  |
 |              |<------------------------------------- INVITE F7<|
 |              |               |              |                  |
 |              |>F8 100 Trying --------------------------------->|
 |<-- INVITE F9<|               |              |                  |
 |              |               |              |<---- PUBLISH F10<|
 |              |               |              |                  |
 |              |               |              |>F11 200 OK ----->|
 |              |               |              |                  |
 |>F12 180  --->|               |              |                  |
 |              |>F13 180 Ringing ------------------------------->|
 |              |               |              |                  |
 |              |< - - - - - - - - - - - - - ->|                  |
 |              |               |              |                  |
 |              |               |<- NOTIFY F14<|                  |
 |              |               |              |                  |
 |              |               |>F15 200 OK ->|                  |
 |              |               |              |------ NOTIFY F16>|
 |              |               |              |                  |
 |              |               |              |<F17 200 OK -----<|
 |>F18 200 OK ->|               |              |                  |
 |              |>F19 200 OK ------------------------------------>|
 |              |               |              |                  |
 |              |<--------------------------------------- ACK F20<|
 |<---- ACK F21<|               |              |                  |
 |              |               |              |                  |
 |<================= Both way RTP established ===================>|
 |              |               |              |                  |
 |              |< - - - - - - - - - - - - - ->|                  |
 |              |               |              |                  |
 |              |               |<- NOTIFY F22<|                  |

Johnston, et al. Standards Track [Page 39] RFC 7463 SIP Shared Appearances March 2015

 |              |               |              |                  |
 |              |               |>F23 200 OK ->|                  |
 |              |               |              |------ NOTIFY F24>|
 |              |               |              |                  |
 |              |               |              |<F25 200 OK -----<|
 |              |               |              |                  |
 Figure 4. Outgoing Call with Appearance Seizure Example
 F1 to F4: Bob uses the shared appearance of the Help Desk on his UA
 to place an outgoing call (e.g., he goes off-hook).  Before sending
 the outgoing INVITE request, Bob publishes to the Appearance Agent
 reserving appearance number 1.  The Appearance Agent notifies Alice
 (and all other UAs, including Bob) of the event by sending NOTIFYs.
 F1 Bob ----> Appearance Agent
 PUBLISH sip:HelpDesk@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:HelpDesk@example.com>
 CSeq: 7 PUBLISH
 Call-ID: 44fwF144-F12893K38424
 Contact: <sip:bob@ua2.example.com>
 Event: dialog;shared
 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="full"
              entity="sip:HelpDesk@example.com">
     <dialog id="id3d4f9c83" direction="initiator">
         <sa:appearance>1</sa:appearance>
         <sa:exclusive>false</sa:exclusive>
         <state>trying</state>
         <local>
             <target uri="sip:bob@ua2.example.com">
             </target>
         </local>
     </dialog>
 </dialog-info>

Johnston, et al. Standards Track [Page 40] RFC 7463 SIP Shared Appearances March 2015

 F2 Appearance Agent  ----> Bob
 SIP/2.0 200 OK
 Via: SIP/2.0/UDP ua2.example.com;branch=z9hG4bK61314d6446383E79
 From: <sip:bob@example.com>;tag=44150CC6-A7B7919D
 To: <sip:HelpDesk@example.com>
 CSeq: 7 PUBLISH
 Call-ID: 44fwF144-F12893K38424
 Contact: <sip:bob@ua2.example.com>
 Event: dialog;shared
 SIP-Etag: 482943245
 Allow-Events: dialog
 Expires: 60
 Content-Length: 0
 F7 Bob ---> Proxy
 INVITE sip:carol@example.com SIP/2.0
 Via: SIP/2.0/UDP ua2.example.com;branch=z9hG4bK342122
 Max-Forwards: 70
 From: <sip:HelpDesk@example.com>;tag=15A3DE7C-9283203B
 To: <sip:carol@example.com>
 Call-ID: f3b3cbd0-a2c5775e-5df9f8d5
 CSeq: 31 INVITE
 Contact: <sip:bob@ua2.example.com>
 Content-Type: application/sdp
 Content-Length: ...
 (SDP Not Shown)
 F10 Bob ----> Appearance Agent
 PUBLISH sip:HelpDesk@example.com SIP/2.0
 Via: SIP/2.0/UDP ua2.example.com;branch=z9hG4bK6d644638E7
 From: <sip:bob@example.com>;tag=0CCf6-A7FdsB79D
 To: <sip:HelpDesk@example.com>
 CSeq: 437 PUBLISH
 Call-ID: fwF14d4-F1FFF2F2893K38424
 Contact: <sip:bob@ua2.example.com>
 Event: dialog;shared
 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"

Johnston, et al. Standards Track [Page 41] RFC 7463 SIP Shared Appearances March 2015

              xmlns:sa="urn:ietf:params:xml:ns:sa-dialog-info"
              version="6"
              state="full"
              entity="sip:HelpDesk@example.com">
     <dialog id="id3d4f9c83"
          call-id="f3b3cbd0-a2c5775e-5df9f8d5"
          local-tag="15A3DE7C-9283203B"
                                          direction="initiator">
         <sa:appearance>1</sa:appearance>
         <sa:exclusive>false</sa:exclusive>
         <state>trying</state>
         <local>
             <target uri="sip:bob@ua2.example.com">
             </target>
         </local>
         <remote>
             <identity uri="sip:carol@example.com">
             </identity>
         </remote>
     </dialog>
 </dialog-info>

11.5. Outgoing Call without Using an Appearance Number

 In this scenario, Bob's UA sends out a dialog event PUBLISH with
 state (trying) indicating that he does not want to utilize an
 appearance number for this dialog.  The PUBLISH does not have an
 appearance element but does have the 'shared' Event header field
 parameter.  As a result, the Appearance Agent knows the UA does not
 wish to use an appearance number for this call.  If the Appearance
 Agent does not wish to allow this, it would reject the PUBLISH with a
 400 (Bad Request) response and the UA would know to re-PUBLISH
 selecting/seizing an appearance number.
 Carol        Proxy           Alice     Appearance Agent         Bob
 |              |               |              |                  |
 |              |               |              |<----- PUBLISH F1<|
 |              |               |              |                  |
 |              |               |              |>F2 200 OK ------>|
 |              |               |              |                  |
 |              |               |<-- NOTIFY F3<|                  |
 |              |               |              |                  |
 |              |               |>F4 200 OK -->|                  |
 |              |               |              |------- NOTIFY F5>|
 |              |               |              |                  |
 |              |               |              |<F6 200 OK ------<|
 |              |               |              |                  |
 |              |<------------------------------------- INVITE F7<|

Johnston, et al. Standards Track [Page 42] RFC 7463 SIP Shared Appearances March 2015

 |              |               |              |                  |
 |              |>F8 100 Trying --------------------------------->|
 |<-- INVITE F9<|               |              |                  |
 |              |               |              |<---- PUBLISH F10<|
 |              |               |              |                  |
 |              |               |              |>F11 200 OK ----->|
 |              |               |              |                  |
 |>F12 180  --->|               |              |                  |
 |              |>F13 180 Ringing ------------------------------->|
 |              |               |              |                  |
 |              |< - - - - - - - - - - - - - ->|                  |
 |              |               |              |                  |
 |              |               |<- NOTIFY F14<|                  |
 |              |               |              |                  |
 |              |               |>F15 200 OK ->|                  |
 |              |               |              |------ NOTIFY F16>|
 |              |               |              |                  |
 |              |               |              |<F17 200 OK -----<|
 |>F18 200 OK ->|               |              |                  |
 |              |>F19 200 OK ------------------------------------>|
 |              |               |              |                  |
 |              |<--------------------------------------- ACK F20<|
 |<---- ACK F21<|               |              |                  |
 |              |               |              |                  |
 |<================= Both way RTP established ===================>|
 |              |               |              |                  |
 |              |< - - - - - - - - - - - - - ->|                  |
 |              |               |              |                  |
 |              |               |<- NOTIFY F22<|                  |
 |              |               |              |                  |
 |              |               |>F23 200 OK ->|                  |
 |              |               |              |------ NOTIFY F24>|
 |              |               |              |                  |
 |              |               |              |<F25 200 OK -----<|
 |              |               |              |                  |
 Figure 5. Outgoing Call without using an Appearance Number Example
 F1 Bob ----> Appearance Agent
 PUBLISH sip:appearanceagent.example.com SIP/2.0
 Via: SIP/2.0/UDP ua2.example.com;branch=z9hG4bK61314d6446383E79
 From: <sip:bob@example.com>;tag=4415df82k39sf
 To: <sip:HelpDesk@example.com>
 CSeq: 7 PUBLISH
 Call-ID: 44fwF144-F12893K38424
 Contact: <sip:bob@ua2.example.com>

Johnston, et al. Standards Track [Page 43] RFC 7463 SIP Shared Appearances March 2015

 Event: dialog;shared
 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="full"
              entity="sip:HelpDesk@example.com">
     <dialog id="id3d4f9c83" direction="initiator">
         <sa:exclusive>false</sa:exclusive>
         <state>trying</state>
         <local>
             <target uri="sip:bob@ua2.example.com">
             </target>
         </local>
     </dialog>
 </dialog-info>
 Note that F7 would be the same as the previous example.

11.6. Appearance Release

 Bob and Carol are in a dialog, created, for example as in
 Section 11.3.  Carol sends a BYE to Bob to terminate the dialog and
 the Appearance Agent de-allocates the appearance number used, sending
 notifications out to the UAs in the shared group.
 Carol        Proxy           Alice     Appearance Agent         Bob
 |              |               |              |                  |
 |              |               |              |                  |
 |<================= Both way RTP established ===================>|
 |              |               |              |                  |
 |>F22 BYE ---->|               |              |                  |
 |              |>F23 BYE --------------------------------------->|
 |              |               |              |                  |
 |              |<------------------------------------ 200 OK F24<|
 |<--200 OK F25<|               |              |                  |
 |              |< - - - - - - - - - - - - - ->|                  |
 |              |               |              |                  |
 |              |               |<- NOTIFY F26<|                  |
 |              |               |              |                  |
 |              |               |>F27 200 OK ->|                  |
 |              |               |              |------ NOTIFY F28>|
 |              |               |              |                  |
 |              |               |              |<F29 200 OK -----<|

Johnston, et al. Standards Track [Page 44] RFC 7463 SIP Shared Appearances March 2015

 Figure 6. Appearance Release Example
 F28 Appearance Agent ----> Bob
 NOTIFY sip:bob@ua1.example.com SIP/2.0
 From: <sip:HelpDesk@example.com>;tag=497585728578386
 To: <sip:bob@example.com>
 Call-ID: a7d559db-d6d7dcad-311c9e3a
 CSeq: 7 NOTIFY
 Via: SIP/2.0/UDP appearanceagent.example.com
  ;branch=z9hG4bK759878512309
 Max-Forwards: 70
 Content-Type: application/dialog-info+xml
 Event: dialog;shared
 Subscription-State: active;expires=1800
 Contact: <sip:appearanceagent.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:HelpDesk@example.com">
     <dialog id="fa02538339df3ce597f9e3e3699e28fc"
          call-id="f3b3cbd0-a2c5775e-5df9f8d5"
          local-tag="15A3DE7C-9283203B"
          remote-tag="65a98f7c-1dd2-11b2-88c6-b0316298f7c"
                                          direction="initiator">
             <sa:appearance>1</sa:appearance>
             <sa:exclusive>false</sa:exclusive>
                <state>terminated</state>
                 <local>
                     <target uri="sip:bob@ua2.example.com">
                     </target>
                    </local>
         </dialog>
 </dialog-info>

11.7. Appearance Pickup

 In this scenario, Bob has an established dialog with Carol created
 using the call flows of Figures 1 or 2.  Bob then places Carol on
 hold.  Alice receives a notification of this and renders this on
 Alice's UI.  Alice subsequently picks up the held call and has a
 established session with Carol.  Finally, Carol hangs up.  Alice must
 PUBLISH F32 to indicate that the INVITE F38 will be an attempt to

Johnston, et al. Standards Track [Page 45] RFC 7463 SIP Shared Appearances March 2015

 pickup the dialog between Carol and Bob and, hence, may use the same
 appearance number.  This example also shows Secure SIP (sips) being
 used.
 Carol        Proxy           Alice     Appearance Agent         Bob
 |              |               |              |                  |
 |<================= Both way RTP established ===================>|
 |              |               |              |                  |
 |              |<------------------------------(hold) INVITE F22<|
 |<- INVITE F23<|               |              |                  |
 |              |               |              |                  |
 |>F24 200 OK ->|               |              |                  |
 |              |>F25 200 OK ------------------------------------>|
 |              |               |              |                  |
 |              |<--------------------------------------- ACK F26<|
 |<---- ACK F27<|               |              |                  |
 |              |< - - - - - - - - - - - - - ->|                  |
 |              |               |              |                  |
 |              |               |<- NOTIFY F28<|                  |
 |              |               |              |                  |
 |              |               |>F29 200 OK ->|                  |
 |              |               |              |>F30 NOTIFY ----->|
 |              |               |              |                  |
 |              |               |              |<----- 200 OK F31<|
 |              |               |              |                  |
 |              |  Alice decides to pick up the call              |
 |              |               |              |                  |
 |              |               |>F32 PUBLISH->|                  |
 |              |               |              |                  |
 |              |               |<- 200 OK F33<|                  |
 |              |               |              |                  |
 |              |               |<- NOTIFY F34<|                  |
 |              |               |              |                  |
 |              |               |>F35 200 OK ->|                  |
 |              |               |              |>F36 NOTIFY ----->|
 |              |               |              |                  |
 |              |               |              |<----- 200 OK F37<|
 |              |<-- INVITE F38<|              |                  |
 |<- INVITE F39<|(w/ Replaces)  |              |                  |
 |( w/ Replaces)|               |              |                  |
 |>F40 200 OK ->|               |              |                  |
 |              |>F41 200 OK -->|              |                  |
 |              |               |              |                  |
 |              |< - - - - - - - - - - - - - ->|                  |
 |              |               |              |                  |
 |              |               |              |>F42 NOTIFY ----->|
 |              |               |              |                  |
 |              |               |              |<----- 200 OK F43<|

Johnston, et al. Standards Track [Page 46] RFC 7463 SIP Shared Appearances March 2015

 |              |               |<- NOTIFY F44<|                  |
 |              |               |              |                  |
 |              |               |>F45 200 OK ->|                  |
 |              |               |              |                  |
 |              |<----- ACK F46<|              |                  |
 |<---- ACK F47<|               |              |                  |
 |              |               |              |                  |
 |<= Both way RTP established =>|              |                  |
 |              |               |              |                  |
 |>F48 BYE ---->|               |              |                  |
 |              |>F49 BYE --------------------------------------->|
 |              |               |              |                  |
 |              |<------------------------------------ OK 200 F50<|
 |<- 200 OK F51<|               |              |                  |
 |              |               |              |                  |
 |              |< - - - - - - - - - - - - - ->|                  |
 |              |               |              |                  |
 |              |               |<- NOTIFY F52<|                  |
 |              |               |              |                  |
 |              |               |>F53 200 OK ->|                  |
 |              |               |              |                  |
 |              |               |              |>F54 NOTIFY ----->|
 |              |               |              |                  |
 |              |               |              |<----- 200 OK F55<|
 Figure 7. Appearance Pickup Example
 F28 Appearance ----> Alice
 NOTIFY sips:alice@ua1.example.com SIP/2.0
 From: <sips:HelpDesk@example.com>;tag=151702541050937
 To: <sips:alice@example.com>;tag=18433323-C3D237CE
 Call-ID: 1e361d2f-a9f51109-bafe31d4
 CSeq: 12 NOTIFY
 Via: SIP/2.0/TLS appearanceagent.example.com
  ;branch=z9hG4bK1403
 Max-Forwards: 70
 Content-Type: application/dialog-info+xml
 Event: dialog;shared
 Subscription-State: active;expires=1800
 Contact: <sips:appearanceagent.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="10"

Johnston, et al. Standards Track [Page 47] RFC 7463 SIP Shared Appearances March 2015

              state="partial"
              entity="sips:HelpDesk@example.com">
    <dialog id="id3d4f9c83"
          call-id="f3b3cbd0-a2c5775e-5df9f8d5"
          local-tag="15A3DE7C-9283203B"
          remote-tag="65a98f7c-1dd2-11b2-88c6-b0316298f7c"
          direction="initiator">
          <sa:appearance>1</sa:appearance>
          <sa:exclusive>false</sa:exclusive>
          <state>active</state>
          <local>
            <target uri="sips:bob@ua2.example.com">
              <param pname="+sip.rendering" pval="no"/>
            </target>
          </local>
          <remote>
            <identity>sips:carol@example.com</identity>
            <target uri="sips:carol@ua3.example.com" />
          </remote>
     </dialog>
 </dialog-info>
 F32 Alice ----> Appearance Agent
 PUBLISH sips:HelpDesk@example.com SIP/2.0
 Via: SIP/2.0/TLS ua2.example.com;branch=z9hG4bKa5d6cf61F5FBC05A
 From: <sips:HelpDesk@example.com>;tag=44150CC6-A7B7919D
 To: <sips:alice@example.com>;tag=428765950880801
 CSeq: 11 PUBLISH
 Call-ID: 87837Fkw87asfds
 Contact: <sips:alice@ua2.example.com>
 Event: dialog;shared
 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="full"
              entity="sips:HelpDesk@example.com">
    <dialog id="id3d4f9c83"
        call-id="3d57cd17-47deb849-dca8b6c6"
                       local-tag="8C4183CB-BCEAB710" >
          <sa:appearance>1</sa:appearance>
          <sa:exclusive>false</sa:exclusive>

Johnston, et al. Standards Track [Page 48] RFC 7463 SIP Shared Appearances March 2015

          <sa:replaced-dialog
            call-id="f3b3cbd0-a2c5775e-5df9f8d5"
            from-tag="15A3DE7C-9283203B"
            to-tag="65a98f7c-1dd2-11b2-88c6-b03162323164+65a98f7c" />
          <state>trying</state>
          <local>
            <target uri="sips:alice@ua1.example.com">
              <param pname="+sip.rendering" pval="yes"/>
            </target>
          </local>
          <remote>
              <target uri="sips:carol@ua3.example.com" />
          </remote>
     </dialog>
 </dialog-info>
 F38 Alice ----> Proxy
 INVITE sips:carol@example.com SIP/2.0
 Via: SIP/2.0/TLS ua1.example.com;branch=z9hG4bK4ea695b5B376A60C
 From: <sips:HelpDesk@example.com>;tag=8C4183CB-BCEAB710
 To: <sips:carol@example.com:5075>
 CSeq: 1 INVITE
 Call-ID: 3d57cd17-47deb849-dca8b6c6
 Contact: <sips:alice@ua1.example.com>
 <all-one-line>
 Replaces: f3b3cbd0-a2c5775e-5df9f8d5;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
 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

Johnston, et al. Standards Track [Page 49] RFC 7463 SIP Shared Appearances March 2015

11.8. Call between UAs within the Group

 In this scenario, Bob calls Alice who is also in the shared
 appearance group.  Only one appearance number is used for this
 dialog.  This example also shows the use of the 'exclusive' tag to
 indicate that other UAs in the group can not join or take this
 dialog.
 Carol        Proxy           Alice     Appearance Agent         Bob
 |              |               |              |                  |
 |              |<--------------------  INVITE (to Alice's UA) F1<|
 |              |               |              |                  |
 |              |< - - - - - - - - - - - - - ->|                  |
 |              |               |              |                  |
 |              |               |              |                  |
 |              |               |<-- NOTIFY F2<|                  |
 |              |               |              |                  |
 |              |               |>F3 200 OK -->|                  |
 |              |               |              |>F4 NOTIFY ------>|
 |              |               |              |                  |
 |              |               |              |<------ 200 OK F5<|
 |              |>F6 INVITE --->|              |                  |
 |              | (appearance=1)|              |                  |
 |              |               |              |                  |
 |              |<------ 180 F7<|              |                  |
 |              |               |              |                  |
 |              |>F8 180  --------------------------------------->|
 |              |               |              |                  |
 |              |< - - - - - - - - - - - - - ->|                  |
 |              |               |              |                  |
 |              |               |<-- NOTIFY F9<|                  |
 |              |               |              |                  |
 |              |               |>F10 200 OK ->|                  |
 |              |               |              |>F11 NOTIFY ----->|
 |              |               |              |                  |
 |              |               |              |<----- 200 OK F12<|
 |              |<-- 200 OK F13<|              |                  |
 |              |               |              |                  |
 |              |>F14 200 OK ------------------------------------>|
 |              |               |              |                  |
 |              |<--------------------------------------- ACK F15<|
 |              |               |              |                  |
 |              |>F16 ACK ----->|              |                  |
 |              |               |              |                  |
 |              |               |<======= RTP established =======>|
 |              |               |              |                  |
 |              |< - - - - - - - - - - - - - ->|                  |
 |              |               |              |                  |

Johnston, et al. Standards Track [Page 50] RFC 7463 SIP Shared Appearances March 2015

 |              |               |<- NOTIFY F17<|                  |
 |              |               |              |                  |
 |              |               |>F18 200 OK ->|                  |
 |              |               |              |>F19 NOTIFY ----->|
 |              |               |              |                  |
 |              |               |              |<----- 200 OK F24<|
 |              |               |              |                  |
 Figure 8. Call between UAs within the Group Example
 F19 Appearance Agent ----> Bob
 NOTIFY sip:bob@ua1.example.com SIP/2.0
 From: <sip:HelpDesk@example.com>;tag=497585728578386
 To: <sip:bob@example.com>;tag=633618CF-B9C2EDA4
 Call-ID: a7d559db-d6d7dcad-311c9e3a
 CSeq: 7 NOTIFY
 Via: SIP/2.0/UDP appearanceagent.example.com
  ;branch=z9hG4bK1711759878512309
 Max-Forwards: 70
 Content-Type: application/dialog-info+xml
 Event: dialog;shared
 Subscription-State: active;expires=1500
 Contact: <sip:appearanceagent.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="10"
              state="partial"
              entity="sip:HelpDesk@example.com">
    <dialog id="3xdsd4f9c83"
            call-id="b3cbd0-ad2c5775e-5df9f8d5"
               local-tag="34322kdfr234f"
            remote-tag="3153DE7C-928203B"
            direction="initiator">
          <sa:exclusive>true</sa:exclusive>
          <sa:appearance>1</sa:appearance>
          <state>confirmed</state>
          <local>
            <target uri="sip:bob@ua2.example.com">
            </target>
          </local>
          <remote>
            <identity>sip:HelpDesk@example.com</identity>
            <target uri="sip:alice@ua1.example.com" />

Johnston, et al. Standards Track [Page 51] RFC 7463 SIP Shared Appearances March 2015

          </remote>
     </dialog>
     <dialog id="4839589"
            call-id="b3cbd0-ad2c5775e-5df9f8d5"
            local-tag="3153DE7C-928203B"
               remote-tag="34322kdfr234f"
            direction="responder">
          <sa:exclusive>true</sa:exclusive>
          <sa:appearance>1</sa:appearance>
          <state>confirmed</state>
          <local>
             <target uri="sip:alice@ua1.example.com" />
          </local>
          <remote>
            <identity>sip:HelpDesk@example.com</identity>
            <target uri="sip:bob@ua2.example.com" />
          </remote>
     </dialog>
 </dialog-info>

11.9. Consultation Hold with Appearances

 In this scenario, Bob has a call with Carol.  Bob makes a
 consultation call to Alice by putting Carol on hold and calling
 Alice.  Bob's UA chooses not to have an appearance number for the
 call to Alice since it is treating it as part of the call to Carol.
 He indicates this in the PUBLISH F32, which contains the 'shared'
 Event header field parameter but no <appearance> element.  The
 PUBLISH is sent before the INVITE to Alice to ensure no appearance
 number is assigned by the Appearance Agent.  Finally, Bob hangs up
 with Alice and resumes the call with Carol.  Dialog notifications of
 the consultation call are not shown, as they are not used.
 Note that if Carol hangs up while Bob is consulting with Alice, Bob
 can decide if he wants to reuse the appearance number used with Carol
 for the call with Alice.  If not, Bob publishes the termination of
 the dialog with Carol and the Appearance Agent will re-allocate the
 appearance.  If he wants to keep the appearance, Bob will publish the
 termination of the dialog with Carol and also publish the appearance
 with the dialog with Alice.  This will result in Bob keeping the
 appearance number until he reports the dialog with Alice terminated.
 Note that the call flow would be similar if Bob called a music on
 hold server instead of Alice to implement a music on hold service as
 described in [RFC7088].

Johnston, et al. Standards Track [Page 52] RFC 7463 SIP Shared Appearances March 2015

 Carol        Proxy           Alice     Appearance Agent         Bob
 |              |               |              |                  |
 |<================= Both way RTP established ===================>|
 |              |               |              |                  |
 |              |<------------------------------(hold) INVITE F22<|
 |<- INVITE F23<|               |              |                  |
 |              |               |              |                  |
 |>F24 200 OK ->|               |              |                  |
 |              |>F25 200 OK ------------------------------------>|
 |              |               |              |                  |
 |              |<--------------------------------------- ACK F26<|
 |<---- ACK F27<|               |              |                  |
 |              |< - - - - - - - - - - - - - ->|                  |
 |              |               |              |                  |
 |              |               |<- NOTIFY F28<|                  |
 |              |               |              |                  |
 |              |               |>F29 200 OK ->|                  |
 |              |               |              |>F30 NOTIFY ----->|
 |              |               |              |                  |
 |              |               |              |<----- 200 OK F31<|
 |              |               |              |                  |
 |              |  Bob makes a consultation call to Alice         |
 |              |               |              |                  |
 |              |               |              |<---- PUBLISH F32<|
 |              |               |              |                  |
 |              |               |              |>F33 200 OK ----->|
 |              |               |              |                  |
 |              |<------------------------------------ INVITE F34<|
 |              |               |              |                  |
 |              |>F35 INVITE -->|              |                  |
 |              |               |              |                  |
 |              |<-- 200 OK F36<|              |                  |
 |              |               |              |                  |
 |              |>F37 200 OK ------------------------------------>|
 |              |               |              |                  |
 |              |<--------------------------------------- ACK F38<|
 |              |               |              |                  |
 |              |>F39 ACK ----->|              |                  |
 |              |               |              |                  |
 |              |               |<======= RTP established =======>|
 |              |               |              |                  |
 |              |            Bob hangs up with Alice              |
 |              |               |              |                  |
 |              |<--------------------------------------- BYE F40<|
 |              |               |              |                  |
 |              |>F41 BYE ----->|              |                  |
 |              |               |              |                  |
 |              |<-- 200 OK F42<|              |                  |

Johnston, et al. Standards Track [Page 53] RFC 7463 SIP Shared Appearances March 2015

 |              |               |              |                  |
 |              |>F43 200 OK ------------------------------------>|
 |              |               |              |                  |
 |              |<----------------------------(unhold) INVITE F44<|
 |<- INVITE F45<|               |              |                  |
 |              |               |              |                  |
 |>F46 200 OK ->|               |              |                  |
 |              |>F47 200 OK ------------------------------------>|
 |              |               |              |                  |
 |              |<--------------------------------------- ACK F48<|
 |<---- ACK F49<|               |              |                  |
 |              |< - - - - - - - - - - - - - ->|                  |
 |              |               |              |                  |
 |              |               |<- NOTIFY F50<|                  |
 |              |               |              |                  |
 |              |               |>F51 200 OK ->|                  |
 |              |               |              |>F52 NOTIFY ----->|
 |              |               |              |                  |
 |              |               |              |<----- 200 OK F53<|
 |              |               |              |                  |
 |<================= Both way RTP established ===================>|
 |              |               |              |                  |
 Figure 9. Consultation Hold with Appearances Example
 F32 Bob ----> Appearance Agent
 PUBLISH sip:HelpDesk@example.com SIP/2.0
 Via: SIP/2.0/UDP ua2.example.com;branch=z9hG4bKa5d6cf61F5FBC05A
 From: <sip:bob@example.com>;tag=44150CC6-A7B7919D
 To: <sip:HelpDesk@example.com>;tag=428765950880801
 CSeq: 11 PUBLISH
 Call-ID: 44fwF144-F12893K38424
 Contact: <sip:bob@ua2.example.com>
 Event: dialog;shared
 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="full"
              entity="sip:HelpDesk@example.com">
    <dialog id="id3d4f9c83"
            call-id="b3cbd0-ad2c5775e-5df9f8d5"
            local-tag="3153DE7C-928203B"

Johnston, et al. Standards Track [Page 54] RFC 7463 SIP Shared Appearances March 2015

            direction="initiator">
          <sa:exclusive>true</sa:exclusive>
          <state>trying</state>
          <local>
            <target uri="sip:bob@ua2.example.com">
            </target>
          </local>
          <remote>
            <identity>sip:HelpDesk@example.com</identity>
            <target uri="sip:alice@ua1.example.com" />
          </remote>
     </dialog>
 </dialog-info>

11.10. Joining or Bridging 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
   |            |             |             |         |
   |<=============Both way RTP established===========>|
   |            |             |             |         |
   |            |             |< PUBLISH F22|         |
   |            |             |             |         |
   |            |             |>F23 200 OK >|         |
   |            |             |             |         |
   |            |<---- INVITE (w/ Join) F24<|         |
   |            |             |             |         |
   |            |>F25 INVITE (w/Join)---------------->|
   |            |             |             |         |
   |            |<---- OK 200 Contact:Bob;isfocus F26<|
   |            |             |             |         |
   |            |< - - - - - >|             |         |
   |            |             |             |         |
   |            |             |>F27 NOTIFY >|         |
   |            |             |             |         |
   |            |             |< 200 OK F28<|         |
   |            |             |             |         |
   |            |             |>F29 NOTIFY ---------->|
   |            |             |             |         |
   |            |             |<F30 200 OK ----------<|
   |            |             |             |         |
   |            |>F31 200 OK Contact:B----->|         |
   |            |             |             |         |
   |            |<----------------- ACK F32<|         |

Johnston, et al. Standards Track [Page 55] RFC 7463 SIP Shared Appearances March 2015

   |            |             |             |         |
   |            |>ACK F33---------------------------->|
   |            |             |             |         |
   |            |<-----INVITE Contact:Bob;isfocus F34<|
   |<-INVITE F35|             |             |         |
   |            |             |             |         |
   |>F26 200 -->|             |             |         |
   |            |>F37 200 OK ------------------------>|
   |            |             |             |         |
   |            |<--------------------------- ACK F38<|
   |<--- ACK F39|             |             |         |
   |            |             |             |<==RTP==>|
   |<=============Both way RTP established===========>|
   |            |             |             |         |
   |            |< - - - - - >|             |         |
   |            |             |             |         |
   |            |             |>F40 NOTIFY >|         |
   |            |             |             |         |
   |            |             |< 200 OK F41<|         |
   |            |             |             |         |
   |            |             |>F42 NOTIFY ---------->|
   |            |             |             |         |
   |            |             |<F43 200 OK ----------<|
   |            |             |             |         |
 Figure 10. Joining or Bridging an Appearance Example
 F22 Alice ----> Appearance Agent
 PUBLISH sip:HelpDesk@example.com SIP/2.0
 Via: SIP/2.0/UDP ua2.example.com;branch=z9hG4bKa5d6cf61F5FBC05A
 From: <sip:alice@example.com>;tag=44150CC6-A7B7919D
 To: <sip:HelpDesk@example.com>;tag=428765950880801
 CSeq: 11 PUBLISH
 Call-ID: 87837Fkw87asfds
 Contact: <sip:alice@ua2.example.com>
 Event: dialog;shared
 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="full"
              entity="sip:HelpDesk@example.com:5060">
    <dialog id="id3d4f9c83"

Johnston, et al. Standards Track [Page 56] RFC 7463 SIP Shared Appearances March 2015

        call-id="dc95da63-60db1abd-d5a74b48"
        local-tag="605AD957-1F6305C2" >
          <sa:appearance>1</sa:appearance>
          <sa:exclusive>false</sa:exclusive>
          <sa:joined-dialog
            call-id="14-1541707345"
            from-tag="44BAD75D-E3128D42"
            to-tag="d3b06488-1dd1-11b2-88c5-b03162323164+d3e48f4c" />
          <state>trying</state>
          <local>
            <target uri="sip:alice@ua1.example.com">
            </target>
          </local>
          <remote>
              <target uri="sip:bob@example.com" />
          </remote>
     </dialog>
 </dialog-info>
 F24 Alice ----> Proxy
 INVITE sip:bob@ua.example.com SIP/2.0
 Via: SIP/2.0/UDP ua1.example.com;branch=z9hG4bKcc9d727c2C29BE31
 From: <sip:HelpDesk@example.com>;tag=605AD957-1F6305C2
 To: <sip:bob@ua.example.com>
 CSeq: 2 INVITE
 Call-ID: dc95da63-60db1abd-d5a74b48
 Contact: <sip:alice@ua1.example.com>
 <all-one-line>
 Join: 14-1541707345;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

Johnston, et al. Standards Track [Page 57] RFC 7463 SIP Shared Appearances March 2015

11.11. Loss of Appearance during Allocation

 Bob reserves an appearance with a PUBLISH, sends an INVITE to Carol,
 then becomes unreachable.  When he fails to refresh his publication
 to the appearance agent, the Appearance Agent declares the dialog
 terminated and frees up the appearance using NOTIFYs F14 and F16.
 After retransmitting the NOTIFY to Bob (in not shown messages F17,
 F18, etc.), the subscription is terminated.
 Carol        Proxy           Alice     Appearance Agent         Bob
 |              |               |              |                  |
 |              |               |              |<----- PUBLISH F1<|
 |              |               |              |                  |
 |              |               |              |>F2 200 OK ------>|
 |              |               |              |                  |
 |              |               |<-- NOTIFY F3<|                  |
 |              |               |              |                  |
 |              |               |>F4 200 OK -->|                  |
 |              |               |              |------- NOTIFY F5>|
 |              |               |              |                  |
 |              |               |              |<F6 200 OK ------<|
 |              |               |              |                  |
 |              |<------------------------------------- INVITE F7<|
 |              |               |              |                  |
 |              |>F8 100 Trying --------------------------------->|
 |<-- INVITE F9<|               |              |                  |
 |              |               |              |<---- PUBLISH F10<|
 |              |               |              |                  |
 |              |               |              |>F11 200 OK ----->|
 |              |               |              |                  |
 |>F12 180  --->|               |              |                  |
 |              |>F13 180 Ringing ------------------------------->|
 |              |               |              |                  |
 |              |               |              | Bob goes offline |
 |              |               |              |                  |
 |              |               | Appearance selection times out  |
 |              |               |              |                  |
 |              |               |<- NOTIFY F14<|                  |
 |              |               |              |                  |
 |              |               |>F15 200 OK ->|                  |
 |              |               |              |------ NOTIFY F16>|
 |              |               |              |                  |
 |              |               |   NOTIFY is retransmitted       |
 Figure 11. Loss of Appearance during Allocation Example

Johnston, et al. Standards Track [Page 58] RFC 7463 SIP Shared Appearances March 2015

11.12. Appearance Seizure Contention Race Condition

 Bob and Alice both try to reserve appearance 2 by publishing at the
 same time.  The Appearance Agent allocates the appearance to Bob by
 sending a 200 OK and denies it to Alice by sending a 400 (Bad
 Request) response.  After the NOTIFY F5, Alice learns that Bob is
 using appearance 2.  Alice then attempts to reserve appearance 3 by
 publishing, which is then accepted.

Johnston, et al. Standards Track [Page 59] RFC 7463 SIP Shared Appearances March 2015

 Carol        Proxy           Alice     Appearance Agent         Bob
 |              |               |              |                  |
 |              |               |              |<----- PUBLISH F1<|
 |              |               |              |        (appearance=2)
 |              |               |>F2 PUBLISH ->|                  |
 |              |               |     (appearance=2)              |
 |              |               |              |                  |
 |              |               |              |>F3 200 OK ------>|
 |              |               |<---- F4 400 <|                  |
 |              |               |              |                  |
 |              |               |<-- NOTIFY F5<|                  |
 |              |               |              |                  |
 |              |               |>F6 200 OK -->|                  |
 |              |               |              |------- NOTIFY F7>|
 |              |               |              |                  |
 |              |               |              |<F8 200 OK ------<|
 |              |               |              |                  |
 |              |<------------------------------------- INVITE F9<|
 |              |               |              |                  |
 |              |>F10 100 Trying -------------------------------->|
 |<- INVITE F11<|               |              |                  |
 |              |               |              |<---- PUBLISH F12<|
 |              |               |              |        (appearance=2)
 |              |               |              |>F13 200 OK ----->|
 |              |               |>F14 PUBLISH->|                  |
 |              |               |     (appearance=3)              |
 |              |               |              |                  |
 |              |               |<--- F15 200 <|                  |
 |              |               |              |                  |
 |              |               |<- NOTIFY F16<|                  |
 |              |               |              |                  |
 |              |               |>F17 200 OK ->|                  |
 Dave           |               |              |------ NOTIFY F18>|
 |              |               |              |                  |
 |              |               |              |<F19 200 OK -----<|
 |              |<-- INVITE F20<|              |                  |
 |              |               |              |                  |
 |              |>F21 100 ----->|              |                  |
 |<- INVITE F22<|               |              |                  |
 Figure 12. Appearance Seizure Contention Race Condition Example

11.13. Appearance Agent Subscription to UAs

 In this scenario, the Appearance Agent does not have any way of
 knowing Bob's dialog state information, except through Bob.  This
 could be because the Appearance Agent is not part of a B2BUA, or

Johnston, et al. Standards Track [Page 60] RFC 7463 SIP Shared Appearances March 2015

 perhaps Bob is remotely registering.  When Bob registers, the
 Appearance Agent receives a registration event package notification
 from the registrar.  The Appearance Agent then SUBSCRIBEs to Bob's
 dialog event state using Event:dialog in the SUBSCRIBE.  Whenever
 Bob's dialog state changes, Bob's UA sends a NOTIFY to the Appearance
 Agent which then notifies the other UAs in the group.
 Carol        Proxy           Alice     Appearance Agent         Bob
 |              |               |              |                  |
 |              |<----------------------------------- REGISTER F1<|
 |              |               |              |                  |
 |              |>F2 200 OK ------------------------------------->|
 |              |               |              |                  |
 |              |>F3 NOTIFY ------------------>|                  |
 |              |               |              |                  |
 |              |<------------------ 200 OK F4<|                  |
 |              |               |              |---- SUBSCRIBE F5>|
 |              |               |              |                  |
 |              |               |              |<F6 200 OK ------<|
 |              |               |              |                  |
 |              |               |              |<------ NOTIFY F7<|
 |              |               |              |                  |
 |              |               |              |>F8 200 OK ------>|
 |              |               |              |                  |
 |              |               |              |<--- SUBSCRIBE F9<|
 |              |               |              |                  |
 |              |               |              |>F10 200 OK ----->|
 |              |               |              |                  |
 |              |               |              |------ NOTIFY F11>|
 |              |               |              |                  |
 |              |               |              |<F12 200 OK -----<|
 |              |               |              |                  |
 |              |<------------------------------------ INVITE F13<|
 |              |               |              |                  |
 |              |>F14 100 Trying -------------------------------->|
 |<- INVITE F15<|               |              |                  |
 |              |               |              |<----- NOTIFY F16<|
 |              |               |              |                  |
 |              |               |              |>F17 200 OK ----->|
 |              |               |<- NOTIFY F18<|                  |
 |              |               |              |                  |
 |              |               |>F19 200 OK ->|                  |
 |              |               |              |------ NOTIFY F20>|
 |              |               |              |                  |
 |              |               |              |<F21 200 OK -----<|
 |>F22 180  --->|               |              |                  |
 |              |>F23 180 Ringing ------------------------------->|
 |              |               |              |                  |

Johnston, et al. Standards Track [Page 61] RFC 7463 SIP Shared Appearances March 2015

 |              |               |              |<----- NOTIFY F24<|
 |              |               |              |                  |
 |              |               |              |>F25 200 OK ----->|
 |              |               |<- NOTIFY F26<|                  |
 |              |               |              |                  |
 |              |               |>F27 200 OK ->|                  |
 |              |               |              |------ NOTIFY F28>|
 |              |               |              |                  |
 |              |               |              |<F29 200 OK -----<|
 |>F30 200 OK ->|               |              |                  |
 |              |>F31 200 OK ------------------------------------>|
 |              |               |              |                  |
 |              |               |              |<----- NOTIFY F32<|
 |              |               |              |                  |
 |              |               |              |>F33 200 OK ----->|
 |              |               |              |                  |
 |              |<--------------------------------------- ACK F34<|
 |<---- ACK F35<|               |              |                  |
 |              |               |              |                  |
 |<================= Both way RTP established ===================>|
 |              |               |              |                  |
 |              |               |<- NOTIFY F36<|                  |
 |              |               |              |                  |
 |              |               |>F37 200 OK ->|                  |
 |              |               |              |------ NOTIFY F38>|
 |              |               |              |                  |
 |              |               |              |<F39 200 OK -----<|
 |              |               |              |                  |
 Figure 13. Appearance Agent Subscription to UAs Example

11.14. Appearance Pickup Race Condition Failure

 In this scenario, Bob has an established dialog with Carol created
 using the call flows of Figure 1 or Figure 2.  Bob then places Carol
 on hold.  Alice receives a notification of this and renders this on
 Alice's UI.  Alice attempts to pick up the call but Carol hangs up
 before the pickup can complete.  Alice cancels the pickup attempt
 with the PUBLISH F48.  Note that the call flow for a failed Join
 would be almost identical.
 Carol        Proxy           Alice     Appearance Agent         Bob
 |              |               |              |                  |
 |<================= Both way RTP established ===================>|
 |              |               |              |                  |
 |              |<------------------------------(hold) INVITE F22<|
 |<- INVITE F23<|               |              |                  |

Johnston, et al. Standards Track [Page 62] RFC 7463 SIP Shared Appearances March 2015

 |              |               |              |                  |
 |>F24 200 OK ->|               |              |                  |
 |              |>F25 200 OK ------------------------------------>|
 |              |               |              |                  |
 |              |<--------------------------------------- ACK F26<|
 |<---- ACK F27<|               |              |                  |
 |              |               |              |                  |
 |              |< - - - - - - - - - - - - - ->|                  |
 |              |               |              |                  |
 |              |               |<- NOTIFY F28<|                  |
 |              |               |              |                  |
 |              |               |>F29 200 OK ->|                  |
 |              |               |              |>F30 NOTIFY ----->|
 |              |               |              |                  |
 |              |               |              |<----- 200 OK F31<|
 |              |               |              |                  |
 |              |  Alice decides to pick up the call              |
 |              |               |              |                  |
 |              |               |>F32 PUBLISH->|                  |
 |              |               |              |                  |
 |              |               |<- 200 OK F33<|                  |
 |              |               |              |                  |
 |              |               |<- NOTIFY F34<|                  |
 |              |               |              |                  |
 |              |               |>F35 200 OK ->|                  |
 |              |               |              |>F36 NOTIFY ----->|
 |              |               |              |                  |
 |              |               |              |<----- 200 OK F37<|
 |>F38 BYE ---->|               |              |                  |
 |              |>F39 BYE --------------------------------------->|
 |              |               |              |                  |
 |              |<------------------------------------ OK 200 F40<|
 |<- 200 OK F41<|               |              |                  |
 |              |<-- INVITE F42<|              |                  |
 |<- INVITE F43<|(w/ Replaces)  |              |                  |
 |( w/ Replaces)|               |              |                  |
 |              |               |              |                  |
 |>F44 481 ---->|               |              |                  |
 |              |>F45 481 ----->|              |                  |
 |<---- ACK F46<|               |              |                  |
 |              |<----- ACK F47<|              |                  |
 |              |               |>F48 PUBLISH->|                  |
 |              |               |              |                  |
 |              |               |<- 200 OK F49<|                  |
 |              |               |              |                  |
 |              |               |<- NOTIFY F50<|                  |
 |              |               |              |                  |
 |              |               |>F51 200 OK ->|                  |

Johnston, et al. Standards Track [Page 63] RFC 7463 SIP Shared Appearances March 2015

 |              |               |              |>F52 NOTIFY ----->|
 |              |               |              |                  |
 |              |               |              |<----- 200 OK F53<|
 Figure 14. Appearance Pickup Race Condition Failure Example
 F48 Alice ----> Appearance Agent
 PUBLISH sip:HelpDesk@example.com SIP/2.0
 Via: SIP/2.0/UDP ua2.example.com;branch=z9hG4bKa5d6cf61F5FBC05A
 From: <sip:alice@example.com>;tag=44150CC6-A7B7919D
 To: <sip:HelpDesk@example.com>;tag=428765950880801
 CSeq: 11 PUBLISH
 Call-ID: 87837Fkw87asfds
 Contact: <sip:alice@ua2.example.com>
 Event: dialog;shared
 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="full"
              entity="sip:HelpDesk@example.com">
    <dialog id="id3d4f9c83"
        call-id="dc95da63-60db1abd-d5a74b48"
        local-tag="605AD957-1F6305C2" >
          <sa:appearance>1</sa:appearance>
          <sa:exclusive>false</sa:exclusive>
          <sa:replaced-dialog
            call-id="14-1541707345"
            from-tag="44BAD75D-E3128D42"
            to-tag="d3b06488-1dd1-11b2-88c5-b03162323164+d3e48f4c" />
          <state>terminated</state>
          <local>
            <target uri="sip:alice@ua1.example.com">
            </target>
          </local>
          <remote>
              <target uri="sip:carol@ua3.example.com" />
          </remote>
     </dialog>
 </dialog-info>

Johnston, et al. Standards Track [Page 64] RFC 7463 SIP Shared Appearances March 2015

11.15. Appearance Seizure Incoming/Outgoing Contention Race Condition

 Alice tries to seize appearance 2 at the same time appearance 2 is
 allocated to an incoming call.  The Appearance Agent resolves the
 conflict by sending a 400 (Bad Request) to Alice.  After the NOTIFY
 F6, Alice learns that the incoming call is using appearance 2.  Alice
 republishes for appearance 3, which is accepted.  Note that this
 example shows the INVITE being received before the NOTIFY from the
 Appearance Agent.

Johnston, et al. Standards Track [Page 65] RFC 7463 SIP Shared Appearances March 2015

 Carol        Proxy           Alice     Appearance Agent         Bob
 |              |               |              |                  |
 |>-- INVITE F1>|               |              |                  |
 |              |< - - - - - - - - - - - - - ->|                  |
 |              |               |              |                  |
 |              |               |>F2 PUBLISH ->|                  |
 |              |               |     (appearance=2)              |
 |              |               |              |                  |
 |              |>F3 INVITE (appearance=2) ---------------------->|
 |              |               |              |                  |
 |              |>F4 INVITE     |              |                  |
 |              |(appearance=2)>|              |                  |
 |              |               |<---- F5 400 <|                  |
 |              |               |              |                  |
 |              |               |<-- NOTIFY F6<|                  |
 |              |               |              |                  |
 |              |               |>F7 200 OK -->|                  |
 |              |               |              |------- NOTIFY F8>|
 |              |               |              |                  |
 |              |               |              |<F9 200 OK ------<|
 |              |               |              |                  |
 |              |               |>F10 PUBLISH->|                  |
 |              |               |     (appearance=3)              |
 |              |               |              |                  |
 |              |               |< F11 200 OK <|                  |
 |              |               |              |                  |
 |              |               |<- NOTIFY F12<|                  |
 |              |               |              |                  |
                |               |>F13 200 OK ->|                  |
 Dave           |               |              |------ NOTIFY F14>|
 |              |               |              |                  |
 |              |               |              |<F15 200 OK -----<|
 |              |<-- INVITE F16<|              |                  |
 |              |               |              |                  |
 |              |>F17 100 ----->|              |                  |
 |<- INVITE F18<|               |              |                  |
 Figure 15. Appearance Seizure Incoming/Outgoing Contention
            Race Condition Example

12. Security Considerations

 Since multiple line appearance features are implemented using
 semantics provided by SIP [RFC3261], the SIP Event Package for Dialog
 State [RFC4235], and the SIP Event Framework [RFC6665] and [RFC3903],
 security considerations in these documents apply to this document as
 well.

Johnston, et al. Standards Track [Page 66] RFC 7463 SIP Shared Appearances March 2015

 To provide confidentiality, NOTIFY or PUBLISH message bodies that
 provide the dialog state information and the dialog identifiers MAY
 be encrypted end-to-end using the standard mechanisms such as S/MIME
 described in [RFC3261].  Alternatively, sending the NOTIFY and
 PUBLISH requests over TLS also provides confidentiality, although on
 a hop-by-hop basis.  All SUBSCRIBEs and PUBLISHes between the UAs and
 the Appearance Agent MUST be authenticated.  Without proper
 authentication and confidentiality, a third party could learn
 information about dialogs associated with a AOR and could try to use
 this information to hijack or manipulate those dialogs using SIP call
 control primitives.
 This feature relies on standard SIP call control primitives such as
 Replaces and Join.  Proper access controls on their use MUST be used
 so that only members of the shared appearance group can use these
 mechanisms.  All INVITEs with Replaces or Join header fields MUST
 only be accepted if the peer requesting dialog replacement or joining
 has been properly authenticated using a standard SIP mechanism (such
 as Digest or S/MIME), and authorized to request a replacement.
 Otherwise, a third party could disrupt or hijack existing dialogs in
 the shared appearance group.
 For an emergency call, a UA MUST NOT wait for a confirmed seizure of
 an appearance before sending an INVITE.  Waiting for confirmation
 could inadvertently delay or block the emergency call, which by its
 nature needs to be placed as expeditiously as possible.  Instead, a
 emergency call MUST proceed regardless of the status of the PUBLISH
 transaction.

13. IANA Considerations

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

13.1. SIP Event Header Field Parameter: shared

 This document defines the 'shared' header field parameter in the
 Event header field in the "Header Field Parameters and Parameter
 Values" registry defined by [RFC3968].
                                                  Predefined
Header Field                  Parameter Name      Values     Reference
----------------------------  ------------------  ---------- ---------
Event                         shared               No        RFC 7463

Johnston, et al. Standards Track [Page 67] RFC 7463 SIP Shared Appearances March 2015

13.2. SIP Alert-Info Header Field Parameter: appearance

 This document defines the 'appearance' parameter in the Alert-Info
 header in the "Header Field Parameters and Parameter Values" registry
 defined by [RFC3968].
                                                  Predefined
 Header Field                  Parameter Name     Values     Reference
 ----------------------        ---------------    ---------  ---------
 Alert-Info                    appearance         No         RFC 7463

13.3. 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.b.johnston@gmail.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:sa-dialog-info</h2>
      <p>See <a href="http://www.rfc-editor.org/rfc/rfc7463.txt">
         RFC 7463</a>.</p>
     </body>
     </html>
    END

13.4. XML Schema Registration

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

Johnston, et al. Standards Track [Page 68] RFC 7463 SIP Shared Appearances March 2015

    URI: urn:ietf:params:xml:schesa:sa-dialog-info.
    Registrant Contact: IETF BLISS working group, <bliss@ietf.org>,
     Alan Johnston <alan.b.johnston@gmail.com>
    The XML for this schema can be found in Section 6.

14. References

14.1. Normative References

 [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
            Requirement Levels", BCP 14, RFC 2119, March 1997,
            <http://www.rfc-editor.org/info/rfc2119>.
 [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, <http://www.rfc-editor.org/info/rfc3261>.
 [RFC3515]  Sparks, R., "The Session Initiation Protocol (SIP) Refer
            Method", RFC 3515, April 2003,
            <http://www.rfc-editor.org/info/rfc3515>.
 [RFC3688]  Mealling, M., "The IETF XML Registry", BCP 81, RFC 3688,
            January 2004, <http://www.rfc-editor.org/info/rfc3688>.
 [RFC3840]  Rosenberg, J., Schulzrinne, H., and P. Kyzivat,
            "Indicating User Agent Capabilities in the Session
            Initiation Protocol (SIP)", RFC 3840, August 2004,
            <http://www.rfc-editor.org/info/rfc3840>.
 [RFC3891]  Mahy, R., Biggs, B., and R. Dean, "The Session Initiation
            Protocol (SIP) "Replaces" Header", RFC 3891, September
            2004, <http://www.rfc-editor.org/info/rfc3891>.
 [RFC3903]  Niemi, A., "Session Initiation Protocol (SIP) Extension
            for Event State Publication", RFC 3903, October 2004,
            <http://www.rfc-editor.org/info/rfc3903>.
 [RFC3911]  Mahy, R. and D. Petrie, "The Session Initiation Protocol
            (SIP) "Join" Header", RFC 3911, October 2004,
            <http://www.rfc-editor.org/info/rfc3911>.

Johnston, et al. Standards Track [Page 69] RFC 7463 SIP Shared Appearances March 2015

 [RFC3968]  Camarillo, G., "The Internet Assigned Number Authority
            (IANA) Header Field Parameter Registry for the Session
            Initiation Protocol (SIP)", BCP 98, RFC 3968, December
            2004, <http://www.rfc-editor.org/info/rfc3968>.
 [RFC4235]  Rosenberg, J., Schulzrinne, H., and R. Mahy, "An INVITE-
            Initiated Dialog Event Package for the Session Initiation
            Protocol (SIP)", RFC 4235, November 2005,
            <http://www.rfc-editor.org/info/rfc4235>.
 [RFC5234]  Crocker, D. and P. Overell, "Augmented BNF for Syntax
            Specifications: ABNF", STD 68, RFC 5234, January 2008,
            <http://www.rfc-editor.org/info/rfc5234>.
 [RFC6665]  Roach, A., "SIP-Specific Event Notification", RFC 6665,
            July 2012, <http://www.rfc-editor.org/info/rfc6665>.
 [RFC7462]  Liess, L., Ed., Jesske, R., Johnston, A., Worley, D., and
            P. Kyzivat, "URNs for the Alert-Info Header Field of the
            Session Initiation Protocol (SIP)", RFC 7462, March 2015,
            <http://www.rfc-editor.org/info/rfc7462>.

14.2. Informative References

 [RFC3680]  Rosenberg, J., "A Session Initiation Protocol (SIP) Event
            Package for Registrations", RFC 3680, March 2004,
            <http://www.rfc-editor.org/info/rfc3680>.
 [RFC4579]  Johnston, A. and O. Levin, "Session Initiation Protocol
            (SIP) Call Control - Conferencing for User Agents", BCP
            119, RFC 4579, August 2006,
            <http://www.rfc-editor.org/info/rfc4579>.
 [RFC5359]  Johnston, A., Sparks, R., Cunningham, C., Donovan, S., and
            K. Summers, "Session Initiation Protocol Service
            Examples", BCP 144, RFC 5359, October 2008,
            <http://www.rfc-editor.org/info/rfc5359>.
 [RFC7088]  Worley, D., "Session Initiation Protocol Service Example
            -- Music on Hold", RFC 7088, February 2014,
            <http://www.rfc-editor.org/info/rfc7088>.

Johnston, et al. Standards Track [Page 70] RFC 7463 SIP Shared Appearances March 2015

Acknowledgements

 The following individuals were part of the shared appearance 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, and Theo Zourzouvillys.
 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 and JD Smith of
    Siemens Communications; Dale R. Worley of Pingtel; and Graeme
    Dollar of Yahoo, Inc.
 Also thanks to Geoff Devine, Paul Kyzivat, Jerry Yin, John Elwell,
 Dan York, Spenser Dawkins, Martin Dolly, and Brett Tate for their
 comments.
 Thanks to Carolyn Beeton, Francois Audet, Andy Hutton, Tim Ross, Raji
 Chinnappa, and Harsh Mendiratta for their detailed review of the
 document.

Authors' Addresses

 Alan Johnston (editor)
 Avaya
 St. Louis, MO
 United States
 EMail: alan.b.johnston@gmail.com
 Mohsen Soroushnejad (editor)
 Sylantro Systems Corp.
 EMail: msoroush@gmail.com

Johnston, et al. Standards Track [Page 71] RFC 7463 SIP Shared Appearances March 2015

 Venkatesh Venkataramanan
 Sylantro Systems Corp.
 EMail: vvenkatar@gmail.com

Johnston, et al. Standards Track [Page 72]

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