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Internet Engineering Task Force (IETF)                  A. Johnston, Ed.Request for Comments: 7463                                         AvayaUpdates:3261,4235                                 M. Soroushnejad, Ed.Category: Standards Track                              V. VenkataramananISSN: 2070-1721                                   Sylantro Systems Corp.                                                              March 2015Shared 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 inSection 2 of RFC 5741.   Information about the current status of this document, any errata,   and how to provide feedback on it may be obtained athttp://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 toBCP 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 Contents1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .32.  Conventions Used in This Document . . . . . . . . . . . . . .53.  Usage Scenarios . . . . . . . . . . . . . . . . . . . . . . .53.1.  Executive/Assistant Arrangement . . . . . . . . . . . . .53.2.  Call Group  . . . . . . . . . . . . . . . . . . . . . . .53.3.  Single Line Extension . . . . . . . . . . . . . . . . . .63.4.  Changing UAs  . . . . . . . . . . . . . . . . . . . . . .64.  Requirements and Implementation . . . . . . . . . . . . . . .64.1.  Requirements  . . . . . . . . . . . . . . . . . . . . . .64.2.  Implementation  . . . . . . . . . . . . . . . . . . . . .85.  Normative Description . . . . . . . . . . . . . . . . . . . .105.1.  Elements  . . . . . . . . . . . . . . . . . . . . . . . .115.2.  Shared Appearance Dialog Package Extensions . . . . . . .115.2.1.  The <appearance> Element  . . . . . . . . . . . . . .115.2.2.  The <exclusive> Element . . . . . . . . . . . . . . .125.2.3.  The <joined-dialog> Element . . . . . . . . . . . . .125.2.4.  The <replaced-dialog> Element . . . . . . . . . . . .135.3.  Shared Appearance User Agents . . . . . . . . . . . . . .135.3.1.  Appearance Numbers and Call Context . . . . . . . . .165.3.2.  Appearance Numbers and Call Control . . . . . . . . .175.3.3.  Appearance Numbers and Transfer . . . . . . . . . . .185.4.  Appearance Agent  . . . . . . . . . . . . . . . . . . . .186.  XML Schema Definition . . . . . . . . . . . . . . . . . . . .217.  Alert-Info Appearance Parameter Definition  . . . . . . . . .23Johnston, et al.             Standards Track                    [Page 2]

RFC 7463                 SIP Shared Appearances               March 20158.  User Interface Considerations . . . . . . . . . . . . . . . .248.1.  Appearance Number Rendering . . . . . . . . . . . . . . .248.1.1.  Single Appearance UAs . . . . . . . . . . . . . . . .248.1.2.  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 . . . . . . . . . . . . . . . . . . . . . . .258.1.5.  Example User Interface Issues . . . . . . . . . . . .258.2.  Call State Rendering  . . . . . . . . . . . . . . . . . .269.  Interoperability with Non-shared Appearance UAs . . . . . . .269.1.  Appearance Assignment . . . . . . . . . . . . . . . . . .269.2.  Appearance Release  . . . . . . . . . . . . . . . . . . .27     9.3.  UAs Supporting Dialog Events but Not Shared Appearance  .  2710. Provisioning Considerations . . . . . . . . . . . . . . . . .2711. Example Message Flows . . . . . . . . . . . . . . . . . . . .2811.1.  Registration and Subscription  . . . . . . . . . . . . .2811.2.  Appearance Selection for Incoming Call . . . . . . . . .3211.3.  Outgoing Call without Appearance Seizure . . . . . . . .3511.4.  Outgoing Call with Appearance Seizure  . . . . . . . . .3811.5.  Outgoing Call without Using an Appearance Number . . . .4211.6.  Appearance Release . . . . . . . . . . . . . . . . . . .4411.7.  Appearance Pickup  . . . . . . . . . . . . . . . . . . .4511.8.  Call between UAs within the Group  . . . . . . . . . . .5011.9.  Consultation Hold with Appearances . . . . . . . . . . .5211.10. Joining or Bridging an Appearance  . . . . . . . . . . .5511.11. Loss of Appearance during Allocation . . . . . . . . . .5811.12. Appearance Seizure Contention Race Condition . . . . . .5911.13. Appearance Agent Subscription to UAs . . . . . . . . . .6011.14. Appearance Pickup Race Condition Failure . . . . . . . .62     11.15. Appearance Seizure Incoming/Outgoing Contention Race            Condition  . . . . . . . . . . . . . . . . . . . . . . .6512. Security Considerations . . . . . . . . . . . . . . . . . . .6613. IANA Considerations . . . . . . . . . . . . . . . . . . . . .6713.1.  SIP Event Header Field Parameter: shared . . . . . . . .6713.2.  SIP Alert-Info Header Field Parameter: appearance  . . .6813.3.  URN Sub-Namespace Registration: sa-dialog-info . . . . .6813.4.  XML Schema Registration  . . . . . . . . . . . . . . . .6814. References  . . . . . . . . . . . . . . . . . . . . . . . . .6914.1.  Normative References . . . . . . . . . . . . . . . . . .6914.2.  Informative References . . . . . . . . . . . . . . . . .70   Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . . .71   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .711.  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.  InJohnston, 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.  IfJohnston, 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 inRFC 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 20153.3.  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 poolJohnston, 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 inSection 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 orJohnston, 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 inSection 7, for the Alert-Info header field inRFC 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 UAJohnston, 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 isJohnston, 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 inSection 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 theJohnston, 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 theJohnston, 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 inSection 13.   UAs use the dialog package extensions inSection 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 metaphorJohnston, 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 appearanceJohnston, 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 20155.3.3.  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 ofRFC 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 inSection 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 byRFC 4235 to "only   contain information on the dialogs whose state or participation   information has changed."  This specification extendsRFC 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 20157.  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] inRFC 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 inSection 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 appearanceJohnston, 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 20158.1.3.  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 alsoJohnston, 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 20159.2.  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 inSection 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-cc23d6bbJohnston, 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: 70Johnston, 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: 139490230230249348Johnston, 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: 011.2.  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           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=151702541050937Johnston, 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           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 INVITEJohnston, 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            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            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 asJohnston, 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 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            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 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 inSection 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            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 toJohnston, 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            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          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/8000Johnston, et al.             Standards Track                   [Page 49]

RFC 7463                 SIP Shared Appearances               March 201511.8.  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              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              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              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">      <dialogJohnston, 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/8000Johnston, et al.             Standards Track                   [Page 57]

RFC 7463                 SIP Shared Appearances               March 201511.11.  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 ExampleJohnston, et al.             Standards Track                   [Page 58]

RFC 7463                 SIP Shared Appearances               March 201511.12.  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 Example11.13.  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, orJohnston, 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 Example11.14.  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          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 201511.15.  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 Example12.  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               NoRFC 7463Johnston, et al.             Standards Track                   [Page 67]

RFC 7463                 SIP Shared Appearances               March 201513.2.  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         NoRFC 746313.3.  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>      END13.4.  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 inSection 6.14.  References14.1.  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",BCP119,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 2015Acknowledgements   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.comJohnston, et al.             Standards Track                   [Page 71]

RFC 7463                 SIP Shared Appearances               March 2015   Venkatesh Venkataramanan   Sylantro Systems Corp.   EMail: vvenkatar@gmail.comJohnston, et al.             Standards Track                   [Page 72]