BLISS A. Johnston, Ed.
Internet-Draft Avaya
Expires: September 10, 2009 M. Soroushnejad
V. Venkataramanan
Sylantro Systems Corp
March 9, 2009
Shared Appearances of a Session Initiation Protocol (SIP) Address of
Record (AOR)
draft-ietf-bliss-shared-appearances-02
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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-PBX
offerings and is likely to be implemented on SIP IP telephones and
SIP feature servers used in a business environment. This document
lists requirements and compares implementation options for this
feature. Extensions to the SIP dialog event package are proposed.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 5
2. Conventions used in this document . . . . . . . . . . . . . . 6
3. Usage Scenarios . . . . . . . . . . . . . . . . . . . . . . . 6
3.1. Executive/Assistant Arrangement . . . . . . . . . . . . . 6
3.2. Call Group . . . . . . . . . . . . . . . . . . . . . . . 6
3.3. Single Line Extension . . . . . . . . . . . . . . . . . . 7
4. Requirements and Implementation . . . . . . . . . . . . . . . 7
4.1. Requirements . . . . . . . . . . . . . . . . . . . . . . 7
4.2. Implementation . . . . . . . . . . . . . . . . . . . . . 8
5. Normative Description . . . . . . . . . . . . . . . . . . . . 10
5.1. Elements . . . . . . . . . . . . . . . . . . . . . . . . 10
5.2. Shared Appearance Dialog Package Extensions . . . . . . . 11
5.2.1. The <appearance> element . . . . . . . . . . . . . . . 11
5.2.2. The <exclusive> element . . . . . . . . . . . . . . . 11
5.2.3. The <joined-dialog> element . . . . . . . . . . . . . 11
5.2.4. The <replaced-dialog> element . . . . . . . . . . . . 12
5.3. Shared Appearance User Agents . . . . . . . . . . . . . . 12
5.4. Appearance Agent . . . . . . . . . . . . . . . . . . . . 15
6. XML Schema Definition . . . . . . . . . . . . . . . . . . . . 16
7. User Interface Considerations . . . . . . . . . . . . . . . . 18
7.1. Appearance Number Rendering . . . . . . . . . . . . . . . 18
7.1.1. Single Appearance UAs . . . . . . . . . . . . . . . . 18
7.1.2. Dual Appearance UAs . . . . . . . . . . . . . . . . . 18
7.1.3. Shared Appearance UAs with Fixed Appearance Number . . 18
7.1.4. Shared Appearance UAs with Variable Appearance
Number . . . . . . . . . . . . . . . . . . . . . . . . 19
7.2. Call State Rendering . . . . . . . . . . . . . . . . . . 19
8. Interop with non-Shared Appearance UAs . . . . . . . . . . . . 20
8.1. Appearance Assignment . . . . . . . . . . . . . . . . . . 20
8.2. Appearance Release . . . . . . . . . . . . . . . . . . . 20
8.3. UAs Supporting Dialog Events but Not Shared Appearance . 21
9. Provisioning Considerations . . . . . . . . . . . . . . . . . 21
10. Example Message Flows . . . . . . . . . . . . . . . . . . . . 21
10.1. Registration and Subscription . . . . . . . . . . . . . . 21
10.2. Appearance Selection for Incoming Call . . . . . . . . . 24
10.3. Outgoing Call without Appearance Pre-Selection . . . . . 28
10.4. Outgoing Call with Appearance Pre-Selection . . . . . . . 30
10.5. Outgoing Call without using an Appearance Number . . . . 33
10.6. Appearance Release . . . . . . . . . . . . . . . . . . . 35
10.7. Appearance Pickup . . . . . . . . . . . . . . . . . . . . 36
10.8. Calls between UAs within the Group . . . . . . . . . . . 40
10.9. Consultation Hold with Appearances . . . . . . . . . . . 43
10.10. Joining or Bridging an Appearance . . . . . . . . . . . . 45
10.11. Appearance Allocation - Loss of Appearance . . . . . . . 48
10.12. Appearance Selection Contention Race Condition . . . . . 49
10.13. Appearance Agent Subscription to UAs . . . . . . . . . . 50
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11. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 52
11.1. SIP Event Package Parameter: shared . . . . . . . . . . . 52
11.2. URN Sub-Namespace Registration: sa-dialog-info . . . . . 53
11.3. XML Schema Registration . . . . . . . . . . . . . . . . . 53
12. Appendix A - Incoming Appearance Assignment . . . . . . . . . 54
13. Appendix B - Implementation Options Discussion . . . . . . . . 55
13.1. Appearance Implementation Options . . . . . . . . . . . . 55
13.1.1. URI parameter Approach . . . . . . . . . . . . . . . . 55
13.1.2. Dialog Package Parameter . . . . . . . . . . . . . . . 56
13.1.3. Appearance Selections Mechanisms . . . . . . . . . . . 58
13.2. Comparison . . . . . . . . . . . . . . . . . . . . . . . 61
13.2.1. Comparison of Appearance Selection Methods . . . . . . 62
14. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 63
15. Security Considerations . . . . . . . . . . . . . . . . . . . 63
16. Informative References . . . . . . . . . . . . . . . . . . . . 64
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 65
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1. Introduction
The feature and functionality requirements for SIP user agents (UAs)
supporting business telephony applications differ greatly from basic
SIP user agents, both in terms of services and end user experience.
In addition to basic SIP support [RFC3261], many of the services in a
business environment require the support for SIP extensions such as
REFER [RFC3515], SUBSCRIBE/NOTIFY primitives [RFC3265], PUBLISH
[RFC3903], the SIP Replaces [RFC3891], and Join [RFC3911] header
fields, etc. Many of the popular business services have been
documented in the SIP Service Examples [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 [RFC3265] and
publication [RFC3903] for exchanging status among user agents, and
the SIP dialog state event package [RFC4235] to exchange dialog state
information to achieve the same. Different approaches will be
discussed including the use of URI parameters, feature tags, and
dialog package extensions along with the strengths and weaknesses of
the various approaches.
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. 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 or an
AOR has a visual display (lamp that can change color or blink) and a
button (used to select the appearance). The telephony concept of
line 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").
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2. It is a useful structure for user interface representation.
In this document, we will use the term "appearance" rather than "line
appearance" since SIP does not have the concept of lines. Note that
this does not mean that a conventional telephony user interface
(lamps and buttons) must be used - implementations may use another
metaphor as long as the appearance number is readily apparent to the
user. Each AOR has a separate appearance numbering space. As a
result, a given UA user interface may have multiple occurrences of
the same appearance number, but they will be for different AORs.
2. Conventions used in this document
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC-2119 [RFC2119] and
indicate requirement levels for compliant mechanisms.
3. Usage Scenarios
The following examples are common applications of the Shared
Appearances feature and are mentioned here as informative use cases.
All these example usages can be supported by the Shared Appearances
feature described in this document. The differences relate to the
user interface considerations of the device.
3.1. Executive/Assistant Arrangement
The appearances on the executive's UA 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. An assistant can make outgoing calls using the identity of
either the executive or their own.
3.2. Call Group
Users with similar business needs or tasks can be assigned to
specific groups and share the line appearances of each other on each
others SIP telephony devices. For example, an IT department staff of
five might answer a help line which has three appearances on each
phone in the IT work area. A call answered on one phone can be put
on hold and picked up on another phone. A shout or an IM to another
staff member can result in them taking over a call on a particular
appearance. Another phone can request to be added to an appearance
resulting in a conference call.
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3.3. Single Line Extension
In this scenario, incoming calls are offered to a group of UAs. When
one answers, the other UAs are informed. If another UA in the group
selects the line (i.e. goes off hook), it is immediately bridged or
joined in with the call. This mimics the way residential telephone
extensions usually operate.
4. Requirements and Implementation
The next section details the requirements and discusses the
implementation of the shared appearances of an AOR feature.
4.1. Requirements
The basic requirements of the shared appearance feature can be
summarized as follows:
REQ-1 Incoming calls to the AOR must be offered to a group of UAs and
can be answered by any of them.
REQ-2 Each UA in the group must be able to learn the call status of
the others in the group for the purpose of rendering this information
to the user.
REQ-3 Calls can be joined (also called bridged or conferenced
together) or can be picked up (taken) by another UA in the group in a
secure way.
REQ-4 The mechanism should require the minimal amount of
configuration. UAs registering against the group AOR should be able
to learn about each other and join the appearance group.
REQ-5 The mechanism must scale for large numbers of appearances, n,
and large numbers of UAs, N, without introducing excessive messaging
traffic.
REQ-6 Each call or session (incoming or outgoing) must be assigned a
common "appearance" number from a managed pool administered for the
AOR group. Once the session has terminated, the appearance number is
released back into the pool and can be reused by another incoming or
outgoing session.
REQ-7 Each UA in the group must be able to learn the appearance
status of the the group.
REQ-8 There must be mechanisms to resolve appearance contention among
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the UAs in the group.
REQ-9 The mechanism must allow all UAs receiving an incoming session
request to select the same appearance number at the time of alerting.
REQ-10 The mechanism must have a way of reconstructing appearance
state after an outage that does not result in excessive traffic and
processing.
REQ-11 The mechanism must have backwards compatibility such that a UA
which is unaware of the feature can still register against the group
AOR and make and receive calls.
REQ-12 The mechanism must not allow UAs outside the group to select
or manipulate appearance numbers.
REQ-13 For privacy reasons, there must be a mechanism so that
appearance information is not leaked outside the group of UAs. (e.g.
"So who do you have on line 1?")
REQ-14 The mechanism must support a way for UAs to request
exclusivity on a line appearance. Exclusivity means that the UA
requesting it desires to have a private conversation with the
external party and other UAs must not be allowed to be joined or
taken. 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 select a
particular appearance number for outgoing requests prior to sending
the actual request. This is often called seizure.
REQ-16 The mechanism should support a way for a UA to select a
particular appearance number and also send the request at the same
time. This is needed when a ringdown feature is combined with shared
appearances - in this case, seizing the line is the same thing as
dialing.
4.2. Implementation
Many of the requirements for this service can be met using standard
SIP mechanisms such as:
- A SIP Forking Proxy and Registrar/Location Service meets REQ-1.
- The SIP Dialog Package meets REQ-2.
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- The SIP Replaces and Join header fields meets REQ-3.
- The use of a State Agent for the Dialog Package meets REQ-4 and
REQ-5.
REQ-6 suggests the need for an entity which manages the appearance
resource. Just as conferencing systems commonly have a single point
of control, known as a focus, a Shared Appearance group has a single
point of control of the appearance shared resource. This is defined
as an Appearance Agent for a group. While an Appearance Agent can be
part of a centralized server, it could also be co-resident in a
member User Agent who has taken on this functionality for a group.
The Appearance Agent learns the group state either dialog state
publications from members.
While the appearance resource could be managed co-operatively by a
group of UAs without any central control, this is not discussed in
this draft, but instead is left as a research project for future
standardization. It is also possible that the Appearance Agent logic
could be distributed in all UAs in the group. For example, rules
that govern assigning appearance numbers for incoming requests (e.g.
lowest available appearance number) and rules for contention handling
(e.g. when two UAs request the use of the same appearance number,
hash dialog identifiers and compare with the lowest hash winning)
would need to be defined and implemented.
The next section discusses normal SIP operations used to implement
parts of the shared appearance 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 a 482 Loop Detected, 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 a notification from the Appearance Agent
indicating the appearance number to use in rendering the incoming
call. 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.
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5. For outgoing calls, the operation depends on the user input. If
the user selects a particular appearance number for the call, the
UA publishes this information and waits for a 200 OK before
sending the INVITE.
6. For outgoing calls, if the user does not select 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 wish to select an
appearance (such as during a consultation call), the UA also
publishes this prior to sending the INVITE.
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. In some cases, the Appearance Agent may not have full 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. Normal notifications will be sent every time the
dialog state changes, including calls placed, answered, placed on
and off hold, and hangups.
5. Normative Description
This section normatively describes the shared appearance feature
extensions. For a discussion of various approaches to implement this
feature, see Appendix B.
5.1. Elements
A complete system to implement this feature consists of:
1. User Agents 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
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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 [RFC3265]. The schema is defined in Section 6.
The elements are <appearance>, <exclusive>, <joined-dialog>, and
<replaced-dialog> which are sub-elements of the <dialog> element.
5.2.1. The <appearance> element
The <appearance> element is used convey the appearance number. The
appearance number is a non-negative integer. When sent in a
notification in state Trying to the Appearance Agent, it is used to
request an appearance number. When sent by the Appearance Agent, it
indicates that the appearance number is associated with a dialog.
5.2.2. The <exclusive> element
The <exclusive> element is a boolean used to indicate whether the UA
will accept Join or Replaces INVITEs for this dialog. For example,
some shared appearance systems only allow call pickup when the call
is on hold. In this case, the <exclusive> element should be used to
explicitly indicate this, rather than implicitly by the hold state.
It is important to note that this element is a hint. Although a UA
may set exclusive to true, the UA must still be ready to reject an
INVITE Join relating to this dialog. Also, an INVITE Replaces might
be sent to the non-shared appearance UA to take the call. For this
reason, a UA MAY also not report full dialog identifier information
to the Appearance Agent for calls set to exclusive. If these dialog
identifiers have already been shared with the Appearance Agent, the
UA could send an INVITE Replaces to change them and then not report
the new ones to the Appearance Agent.
If the proxy knows which dialogs are marked exclusive, the proxy MAY
enforce this exclusivity by rejecting INVITE Join and INVITE Replaces
requests containing those dialog identifiers with a 403 Forbidden
response.
5.2.3. The <joined-dialog> element
The <joined-dialog> element is used convey dialog identifiers of any
other dialogs which are joined (mixed or bridged) with the dialog.
Only the UA which is performing the actual media mixing should
include this element in notifications to the Appearance Agent. Note
that this element should still be used even when the Join header
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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.
5.2.4. The <replaced-dialog> element
The <replaced-dialog> element is used convey dialog identifiers of
any other dialogs which 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.
5.3. Shared Appearance User Agents
User Agents that support the Shared Appearance feature MUST support
the dialog state package [RFC4235] with the shared appearance
extensions and the 'shared' dialog event package parameter defined in
this draft.
User Agents MUST support the dialog package extensions in Section 5.2
along with SUBSCRIBE and NOTIFY [RFC3265] and PUBLISH [RFC3903].
SUBSCRIBE, NOTIFY, and PUBLISH requests for the dialog event package
SHOULD include the 'shared' Event header field parameter.
The presence of the 'shared' Event package parameter tells the
Appearance Agent that this UA supports this specification.
Upon initialization and at regular intervals, the UA SHOULD subscribe
to the dialog event package of the AOR. If the SUBSCRIBE request
fails, loops back to itself, or returns a 482 Loop Detected, then no
Appearance Agent is present and this feature is not active for this
AOR. The UA MAY periodically retry the subscription to see if
conditions have changed.
User Agents SHOULD support sending and receiving an INVITE with a
Replaces [RFC3891] header to allow the Call Pickup operation. User
Agents MUST support sending an INVITE with a Join [RFC3911] header
field to initiate bridging.
Note that the Join operation can be implemented outside the UA,
for example, in a B2BUA. This is why UAs must support sending
Join header fields even if they do not necessarily support
receiving them.
When publishing or notifying dialog package information, a UA MUST
include all dialog identification available at the time of
publication, with the exception that a UA may omit information if it
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wishes to prevent other UAs from joining or picking up a call.
Dialog identification includes local and remote target URIs, call-id,
to-tag, and from-tag. When calls are placed on hold, the
"+sip.rendering=no" feature tag MUST be included in dialog package
notifications.
The accurate rendering of the idle/active/alerting/hold state of
other UAs in the group is an important part of the shared
appearance feature.
A UA MUST send dialog package PUBLISH requests in the following
situations:
1. When the user selects a particular appearance number for an
outgoing call (i.e. seizing an appearance or going "off-hook"
with an appearance, if the UA's user interface uses this
metaphor).
2. When the user has requested that an appearance number not be used
for an outgoing call (i.e. during a consultation call 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.
In all these cases, the INVITE SHOULD NOT be sent until the 200 OK
response to the PUBLISH has been received, except for an emergency
call, when a UA MUST never wait for a confirmed seizure before
sending an INVITE. Instead, the emergency call MUST proceed
regardless of the status of PUBLISH transaction.
Note that when a UA selects an appearance prior to establishment of a
dialog (#1 and #2 in above list), not all dialog information will be
available. In particular, when a UA publishes an attempt to select
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 and no dialog
information. If no dialog identification information is present in
the initial PUBLISH, the UA MUST PUBLISH again after receiving the
100 Trying response.
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 SHOULD be refreshed during the early dialog
state or the Appearance Agent may reassign the appearance number.
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Once the dialog has transitioned to the confirmed state, no
publication refreshes are necessary.
UAs SHOULD render information about other appearances to the user.
This includes the state (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. A UA SHOULD render the appearance number to
the user or display appearance status information to the user in a
way that preserves the appearance order.
A UA that does not need to select a particular appearance number (or
doesn't care) would just send an INVITE as normal to place an
outbound call.
A UA wanting to place a call but not have an appearance number
assigned publishes before sending the INVITE without an 'appearance'
element but with the 'shared' event package parameter present. If
the Appearance Agent policy does not allow calls without an assigned
appearance number, a 409 Conflict response will be received, and the
UA will republish either selecting an appearance number or without
one, in which case the Appearance Agent will assign one.
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 appearance number of the joined
or replaced call SHOULD be used. The publication MUST contain the
appearance number of the dialog to be joined or replaced and the
dialog identifier in the 'joined-dialog' or 'replaced-dialog'
elements.
Note that this information is provided to the Appearance Agent so
that it can provide proper appearance assignment behavior. With
Join, the goal is to prevent the Appearance Agent from generating
a 409 Conflict response due to the reuse of an appearance number.
For Replaces, the goal 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.
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.
All subscribed UAs will received NOTIFYs of Trying state for
incoming INVITEs.
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5.4. Appearance Agent
An Appearance Agent defined in this specification MUST implement a
dialog package state agent for the UAs registered against the AOR.
The Appearance Agent MUST support the appearance dialog package
extensions defined in Section 5.2. 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 B2BUA, the Appearance Agent
MAY use the registration event package [RFC3680] to learn of UAs
associated with the AOR and MAY subscribe to their dialog event
state. As a result, the registrar MUST support the registration
event package. 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. 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 to insert in the
Alert-Info header field.
An Appearance Agent SHOULD assign an appearance number to an outgoing
INVITE if a PUBLISH has not been received selecting a particular
appearance number.
Note that if the appearance group has non-shared appearance UAs,
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'
or 'replaced-dialog' element indicating that the dialog will be/has
been replaced or joined. A 409 Conflict 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 package parameter present interprets this
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as a request by the UA to not assign an appearance number. If the
Appearance Agent policy does not allow this, a 409 Conflict response
is returned. If policy does allow this, a 200 OK response is
returned and no appearance number is allocated. In general, the
dialog state will not be shared with the other UAs in the group.
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 terminted, including all dialogs which 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 can not 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.
During dynamic situations, such as during a call pickup or join
action, the Appearance Agent MAY choose to implement rate limiting to
reduce the amount of notification traffic. For example, an
Appearance Agent may choose not to generate immediate NOTIFYs upon
receipt of PUBLISHes. Instead, a single NOTIFY can convey the
effects of a number of PUBLISHes, thus reducing the NOTIFY traffic
within the group.
If an INVITE is sent and no appearance number is available, the proxy
MAY reject the INVITE with a 403 Forbidden response code.
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:
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<?xml version="1.0" encoding="UTF-8"?>
<xs:schema
targetNamespace="urn:ietf:params:xml:ns:sa-dialog-info-info"
xmlns="urn:ietf:params:xml:ns:sa-dialog-info"
xmlns:xs="http://www.w3.org/2001/XMLSchema"
elementFormDefault="qualified">
<xs:element name="joined-dialog" minOccurs="0"
maxOccurs="unbounded">
<xs:complexType>
<xs:attribute name="call-id" type="xs:string"
use="mandatory"/>
<xs:attribute name="local-tag" type="xs:string"
use="mandatory"/>
<xs:attribute name="remote-tag" type="xs:string"
use="mandatory"/>
</xs:complexType>
</xs:element>
<xs:element name="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>
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7. User Interface Considerations
The "appearance number" allocated to a call is an important concept
that enables calls to be handled by multiple devices with
heterogeneous user interfaces in a manner that still allows users to
see a consistent model. Careful treatment of the appearance number
is essential to meet the expectations of the users. Also, rendering
the correct call/appearance state to users is also important.
7.1. Appearance Number Rendering
Since different UAs have different user interface capabilities, it is
usual to find that some UAs have restrictions that others do not.
Perfect interoperability across all UAs is clearly not possible, but
by careful design, interoperability up to the limits of each UA can
be achieved.
The following guidelines suggest how the appearance number should be
handled in three typical user interface implementations.
7.1.1. Single Appearance UAs
These devices are constrained by only having the capability of
displaying status indications for a single appearance. Despite this,
it is important that devices of this type do not ignore the
appearance number. The UA should still send messages annotated with
an appropriate appearance number (i.e. "0"). Any call indications
for appearances other than for number "0" should be rejected with a
486 or 480 response.
7.1.2. Dual Appearance UAs
These devices are essentially single appearance phones that implement
call waiting. They have a very simple user interface that allows
them to switch between two appearances (toggle or flash hook) and
perhaps audible tones to indicate the status of the other appearance.
7.1.3. Shared Appearance UAs with Fixed Appearance Number
This UA is the typical 'business-class' hard-phone. A number of
appearances are typically configured statically and labeled on
buttons, and calls may be managed using these configured appearances.
Any calls outside this range should be ignored, and not mapped to a
free button. Users of these devices often select specific appearance
numbers for outgoing calls, and the UA will need to select the
appearance number and wait for confirmation from the Appearance Agent
before proceeding with calls.
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7.1.4. Shared Appearance UAs with Variable Appearance Number
This UA is typically a soft-phone or graphically rich user interface
hard-phone. In these cases, even the idea of an appearance index may
seem unnecessary. However, for these phones to be able to interwork
successfully with other phone types, it is important that they still
use the appearance index to govern the order of appearance of calls
in progress. No specific guidance on presentation is given except
that the order should be consistent. Thought should also be given to
how an appearance number that has no call associated with it should
be rendered to the user. These devices can typically make calls
without waiting for confirmation from the Appearance Agent on the
appearance number.
The problems faced by each style of user interface are readily seen
in this example:
1. A call arrives at the shared appearance group, and is assigned an
appearance number of 0. All UAs should be able to render to the
user the arrival of this call.
2. Another call arrives at the shared appearance group, and is
assigned an appearance number of 1. The single appearance UA
should not present this call to the user. Other user agents
should have no problems presenting this call distinctly from the
first call.
3. The first call clears, releasing appearance number "0". The
single appearance UA should now be indicating no calls since it
is unable to manage calls other than on the first appearance.
Both shared appearance UAs should clearly show that appearance
number 0 is now free, but that there is still a call on
appearance number 1.
4. A third call arrives, and is assigned the appearance number of 0.
All UAs should be able to render the arrival of this new call to
the user. Multiple appearnce UAs should continue to indicate the
presence of the second call, and should also ensure that the
presentation order is related to the appearance number and not
the order of call arrival.
7.2. Call State Rendering
UAs that implement the shared appearance 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,
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usually indicated by a lamp on, is indicated by an appearance number
being associated with at least one dialog, as reported by the
Appearance Agent. The hold state, often indicated by a blinking
lamp, means the call state from the perspective of the UA in the
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.
8. Interop with non-Shared Appearance UAs
EDITOR'S NOTE: This section needs to be reviewed in light of recent
changes in the specification.
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 interop with non-shared appearance UAs
will not be available in all shared appearance deployments.
First, a UA which does not support dialog events or the shared
appearance feature will be discussed. Then, a UA which does support
dialog events but not the shared appearance feature will be
discussed.
8.1. Appearance Assignment
A UA that has no knowledge of appearances must have appearance
numbers assigned by the Appearance Agent for both incoming and
outgoing calls. If the non-shared appearance UA does not support
Join or Replaces, all dialogs could be marked "exclusive" to indicate
that these options are not available.
8.2. Appearance Release
In all cases the Appearance Agent must be aware of dialog lifetime to
release appearances back into the group.
It is also desirable that any dialog state changes (such as hold,
etc) be made available to other UAs in the group through the Dialog
Event Package. If the Appearance Agent includes a proxy which
Record-Routes for dialogs from the non-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
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INVITE and re-INVITE messages and added to the dialog information
conveyed to other UAs.
8.3. UAs Supporting Dialog Events but Not Shared Appearance
Interoperability with UAs which support dialog events but not the
shared appearance feature is more straightforward. As before, all
appearance number assignment must be done by the Appearance Agent.
This type of UA will be detected by the Appearance Agent by the
absence of the ma event parameter in SUBSCRIBE or PUBLISH messages.
The Appearance Agent can include appearance information in NOTIFYs -
this UA will simply ignore this extra information. This type of UA
will ignore appearance number limitations and may attempt to Join or
Replace dialogs marked exclusive. As a result, the Proxy or UAs may
need to reject such requests.
The need for close cooperation between the Proxy and the Appearance
Agent is not needed as the Appearance Agent will learn about all
dialogs from the UA itself.
9. Provisioning Considerations
Previous versions of this draft required the URI of the Appearance
Agent be provisioned in each UA in the group. Since publication is
now done to the group URI, this provisioning is no longer necessary.
UAs can automatically discover if this feature is active for an AOR
by sending a SUBSCRIBE to the AOR, so no provisioning for this is
needed.
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.
10. Example Message Flows
The next section shows call flow and message examples. The flows and
descriptions are non-normative.
10.1. Registration and Subscription
Bob and Alice are in an appearance group identified by Alice's AOR.
Bob REGISTERs using contact sip:bob@ua2.example.com. Alice REGISTERs
with contact sip:alice@ua1.example.com.
User Agents for Alice and Bob subscribe to the dialog package for the
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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 subscriptions must be
authorized before the same is 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
| | |
| | |
|<--------------------------- REGISTER F1<|
| | |
|>F2 200 OK ----------------------------->|
| | |
| |<----- SUBSCRIBE F3<|
| | |
| |>F4 202 Accepted -->|
| | |
| |>F5 NOTIFY -------->|
| | |
| |<-------- 200 OK F6<|
| | |
Figure 1.
F1-F2: Alice registers AOR with
contact: <sip:alice@ua1.example.com>
F1 Alice ----> Registrar
REGISTER sip:registrar.example.com SIP/2.0
Via: SIP/2.0/UDP ua1.example.com;branch=z9hG4bK527b54da8ACC7B09
From: <sip:alice@example.com>;tag=CDF9A668-909E2BDD
To: <sip:alice@example.com>
CSeq: 2 REGISTER
Call-ID: d3281184-518783de-cc23d6bb
Contact: <sip:alice@ua1.example.com>
Max-Forwards: 70
Expires: 3600
Content-Length: 0
F2 Registrar ----> Alice
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SIP/2.0 200 OK
Via: SIP/2.0/UDP ua1.example.com;branch=z9hG4bKfbf176ef7F1D5FA2
CSeq: 2 REGISTER
Call-ID: d3281184-518783de-cc23d6bb
From: <sip:alice@example.com>;tag=CDF9A668-909E2BDD
To: <sip:alice@example.com>;tag=1664573879820199
Contact: <sip:alice@ua1.example.com>
Expires: 3600
Content-Length: 0
F3 to F6: Alice also subscribes to the events associated with the
Appearance AOR. Appearance Agent also notifies Alice of the status.
F3 Alice ----> Appearance Agent
SUBSCRIBE sip:alice@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:alice@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/2.0 202 Accepted
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:alice@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:alice@example.com>;tag=1636248422222257
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To: <sip:alice@example.com>;tag=925A3CAD-CEBB276E
Call-ID: ef4704d9-bb68aa0b-474c9d94
CSeq: 232 NOTIFY
Via: SIP/2.0/UDP appearanceagent.example.com;branch=z9hG4bK1846
Max-Forwards: 70
Content-Type: application/dialog-info+xml
Event: dialog;shared
Subscription-State: active
Contact: <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:alice@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:alice@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>
Event: dialog;shared
Content-Length: 0
10.2. Appearance Selection for Incoming Call
In the call flow below Bob and Alice are in an appearance group.
Carol places a call to the 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.
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All NOTIFY messages in the call flow below carry dialog events and
only dialog states are mentioned for simplicity. For brevity, the
details of some messages are not shown below.
Forking Appearance
Carol Proxy Agent Alice Bob
| | | | |
|>F1 INVITE >| | | |
| |< - - - - - >| | |
| | |>F2 NOTIFY ----------->|
| | | | |
| | |<F3 200 OK -----------<|
| | | | |
| | |>F4 NOTIFY ->| |
| | | | |
| | |<-200 OK F5-<| |
|<- 100 F6 -<| | | |
| |>F7 INVITE (appearance=1) ---------->|
| | | | |
| |>F8 INVITE (appearance=1) >| |
| | | | |
| |<-------------------- Ringing 180 F9<|
|< 180 F10 -<| | | |
| |<--------- 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 ----------<|
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| | | |
Figure 2.
F4 Appearance Agent ----> Alice
NOTIFY sip:alice@ua1.example.com SIP/2.0
From: <sip:alice@example.com>;tag=151702541050937
To: <sip:alice@example.com>;tag=18433323-C3D237CE
Call-ID: 1e361d2f-a9f51109-bafe31d4
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
Contact: <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="13"
state="partial"
entity="sip:alice@example.com">
<dialog id="2a7294823093f5274e3fd2ec54a2d76c"
call-id="14-1541707345"
remote-tag="44BAD75D-E3128D42"
direction="recipient">
<sa:appearance>1</appearance>
<state>trying</state>
<remote>
<identity>sip:carol@ua.example.com</identity>
</remote>
</dialog>
</dialog-info>
F7 Proxy ----> Bob
INVITE sip:alice@ua3.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:alice@example.com>
CSeq: 106 INVITE
Call-ID: 14-1541707345
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Contact: <sip:carol@ua3.example.com>
Max-Forwards: 69
Alert-Info: <file://ring.pcm>;alert=normal;appearance=1
Content-Type: application/sdp
Content-Length: 223
v=0
o=- 1102980499 1102980499 IN IP4 ua3.example.com
s=
c=IN IP4 ua3.example.com
t=0 0
a=sendrecv
m=audio 2238 RTP/AVP 0 8 101
a=rtpmap:0 PCMU/8000
a=rtpmap:8 PCMA/8000
a=rtpmap:101 telephone-event/8000
F21 Appearance Agent ----> Alice
NOTIFY sip:alice@ua1.example.com SIP/2.0
From: <sip:alice@example.com>;tag=151702541050937
To: <sip:alice@example.com>;tag=18433323-C3D237CE
Call-ID: 1e361d2f-a9f51109-bafe31d4
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
Contact: <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="13"
state="partial"
entity="sip:alice@example.com">
<dialog id="2a7294823093f5274e3fd2ec54a2d76c"
call-id="14-1541707345"
remote-tag="44BAD75D-E3128D42"
local-tag="7349dsfjkFD03s"
direction="recipient">
<sa:appearance>1</appearance>
<state>confirmed</state>
<remote>
<identity>sip:carol@ua.example.com</identity>
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</remote>
</dialog>
</dialog-info>
10.3. Outgoing Call without Appearance Pre-Selection
In this scenario, Bob's UA places a call without first selecting 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 --------------------------------->|
|<-- 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 -----<|
| | | | |
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Figure 3.
F1 Bob ----> Proxy
INVITE sip:carol@example.com SIP/2.0
Via: SIP/2.0/UDP ua2.example.com;branch=z9hG4bK98c87c52123A08BF
From: <sip:bob@example.com>;tag=15A3DE7C-9283203B
To: <sip:carol@example.com>
CSeq: 1 INVITE
Call-ID: f3b3cbd0-a2c5775e-5df9f8d5
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
F6 Appearance Agent ----> Bob
NOTIFY sip:bob@ua1.example.com SIP/2.0
From: <sip:bob@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
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"
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state="partial"
entity="sip:alice@example.com">
<dialog id="fa02538339df3ce597f9e3e3699e28fc"
call-id="f3b3cbd0-a2c5775e-5df9f8d5"
local-tag="15A3DE7C-9283203B" direction="initiator">
<sa:appearance>0</appearance>
<sa:exclusive>false</exclusive>
<state>trying</state>
<local>
<target uri="sip:bob@ua2.example.com">
</target>
</local>
</dialog>
</dialog-info>
10.4. Outgoing Call with Appearance Pre-Selection
In this scenario, Bob's UA sends out a dialog event PUBLISH with
state (trying) selecting 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.
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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<| |
| | | | |
| | |>F23 200 OK ->| |
| | | |------ NOTIFY F24>|
| | | | |
| | | |<F25 200 OK -----<|
| | | | |
Figure 4.
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F1 to F4: Bob uses the shared appearance appearance of Alice on his
UA to place an outgoing call (e.g., he goes off-hook). Before
sending the outgoing INVITE request, Bob publishes to the state agent
that Alice line appearance is in (trying) state. The Appearance
Agent notifies Alice of the same event by forwarding the NOTIFY
payload provided by Bob after appropriately changing the dialog id
field in the XML payload to a unique value towards each of the
entities it is forwarding to (Alice in this example). Note the
shortened expiration interval in F2 of 60 seconds. As long as Bob is
using the appearance number, he must refresh the publication every 60
seconds or loose the appearance.
F1 Bob ----> Appearance Agent
PUBLISH sip:alice@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:alice@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="partial"
entity="sip:alice@example.com">
<dialog id="id3d4f9c83" direction="initiator">
<sa:appearance>0</appearance>
<sa:exclusive>false</exclusive>
<state>trying</state>
<local>
<target uri="sip:bob@ua2.example.com">
</target>
</local>
</dialog>
</dialog-info>
F10 Bob ----> Appearance Agent
PUBLISH sip:alice@example.com SIP/2.0
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Via: SIP/2.0/UDP ua2.example.com;branch=z9hG4bK6d644638E7
From: <sip:bob@example.com>;tag=0CCf6-A7FdsB79D
To: <sip:alice@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"
xmlns:sa="urn:ietf:params:xml:ns:sa-dialog-info"
version="6"
state="partial"
entity="sip:alice@example.com">
<dialog id="id3d4f9c83"
call-id="f3b3cbd0-a2c5775e-5df9f8d5"
local-tag="15A3DE7C-9283203B"
direction="initiator">
<sa:appearance>0</appearance>
<sa:exclusive>false</exclusive>
<state>trying</state>
<local>
<target uri="sip:bob@ua2.example.com">
</target>
</local>
</dialog>
</dialog-info>
10.5. 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' dialog event 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 409 Conflict
response and the UA would know to re-PUBLISH selecting an appearance
number.
Carol Proxy Alice Appearance Agent Bob
| | | | |
| | | |<----- PUBLISH F1<|
| | | | |
| | | |>F2 200 OK ------>|
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| | | | |
| | |<-- 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<| |
| | | | |
| | |>F23 200 OK ->| |
| | | |------ NOTIFY F24>|
| | | | |
| | | |<F25 200 OK -----<|
| | | | |
Figure 5.
F1 Bob ----> Appearance Agent
PUBLISH sip:appearanceagent.example.com SIP/2.0
Via: SIP/2.0/UDP ua2.example.com;branch=z9hG4bK61314d6446383E79
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From: <sip:bob@example.com>;tag=4415df82k39sf
To: <sip:alice@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="partial"
entity="sip:alice@example.com">
<dialog id="id3d4f9c83" direction="initiator">
<sa:exclusive>false</exclusive>
<state>trying</state>
<local>
<target uri="sip:bob@ua2.example.com">
</target>
</local>
</dialog>
</dialog-info>
10.6. Appearance Release
Bob and Carol are in a dialog, created in one of the previous two
call flows. Carol sends a BYE to Bob to terminate the dialog. Bob
publishes the termination of the dialog and the Appearance Agent de-
allocates the appearance number used.
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>|
| | | | |
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| | | |<F29 200 OK -----<|
Figure 6.
F28 Appearance Agent ----> Bob
NOTIFY sip:bob@ua1.example.com SIP/2.0
From: <sip:alice@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
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:alice@example.com">
<dialog id="fa02538339df3ce597f9e3e3699e28fc"
call-id="f3b3cbd0-a2c5775e-5df9f8d5"
local-tag="15A3DE7C-9283203B"
remote-tag="65a98f7c-1dd2-11b2-88c6-b0316298f7c"
direction="initiator">
<sa:appearance>0</appearance>
<sa:exclusive>false</exclusive>
<state>terminated</state>
<local>
<target uri="sip:bob@ua2.example.com">
</target>
</local>
</dialog>
</dialog-info>
10.7. Appearance Pickup
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 subsequently picks up the held call and has a
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established session with Carol. Finally, Carol hangs up.
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<|
| | |<- NOTIFY F44<| |
| | | | |
| | |>F45 200 OK ->| |
| | | | |
| |<----- ACK F46<| | |
|<---- ACK F47<| | | |
| | | | |
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|<= 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.
F28 Appearance ----> Alice
NOTIFY sip:alice@ua1.example.com SIP/2.0
From: <sip:alice@example.com>;tag=151702541050937
To: <sip:alice@example.com>;tag=18433323-C3D237CE
Call-ID: 1e361d2f-a9f51109-bafe31d4
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
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:alice@example.com:5060">
<dialog id="id3d4f9c83"
call-id="f3b3cbd0-a2c5775e-5df9f8d5"
local-tag="15A3DE7C-9283203B"
remote-tag="65a98f7c-1dd2-11b2-88c6-b0316298f7c"
direction="initiator">
<sa:appearance>0</appearance>
<sa:exclusive>false</exclusive>
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<state>active</state>
<local>
<target uri="sip:bob@ua2.example.com">
<param pname="+sip.rendering" pval="no"/>
</target>
</local>
<remote>
<identity>sip:carol@example.com</identity>
<target uri="sip:carol@example.com" />
</remote>
</dialog>
</dialog-info>
F32 Alice ----> Appearance Agent
PUBLISH sip:alice@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:alice@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="partial"
entity="sip:alice@example.com:5060">
<dialog id="id3d4f9c83"
call-id="3d57cd17-47deb849-dca8b6c6"
local-tag="8C4183CB-BCEAB710" >
<sa:appearance>0</appearance>
<sa:exclusive>false</exclusive>
<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="sip:alice@ua1.example.com">
<param pname="+sip.rendering" pval="yes"/>
</target>
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</local>
<remote>
<target uri="sip:carol@example.com" />
</remote>
</dialog>
</dialog-info>
F38 Alice ----> Proxy
INVITE sip:carol@example.com SIP/2.0
Via: SIP/2.0/UDP ua1.example.com;branch=z9hG4bK4ea695b5B376A60C
From: <sip:alice@example.com>;tag=8C4183CB-BCEAB710
To: <sip:carol@example.com:5075>
CSeq: 1 INVITE
Call-ID: 3d57cd17-47deb849-dca8b6c6
Contact: <sip:alice@ua1.example.com>
<all-one-line>
Replaces: f3b3cbd0-a2c5775e-5df9f8d5;to-tag=65a98f7c
-1dd2-11b2-88c6-b03162323164+65a98f7c;from-tag=15A3DE7C-9283203B
</all-one-line>
Max-Forwards: 70
Content-Type: application/sdp
Content-Length: 223
v=0
o=- 1102980497 1102980497 IN IP4 ua1.example.com
s=IP SIP UA
c=IN IP4 ua1.example.com
t=0 0
a=sendrecv
m=audio 2238 RTP/AVP 0 8 101
a=rtpmap:0 PCMU/8000
a=rtpmap:8 PCMA/8000
a=rtpmap:101 telephone-event/8000
10.8. Calls between UAs within the Group
In this scenario, Bob calls Alice who is also in the Appearance
group.
Carol Proxy Alice Appearance Agent Bob
| | | | |
| |<-------------------- INVITE (to Alice's UA) F1<|
| | | | |
| |<- - - - - - ->| | |
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| | | | |
| | |<-- 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<| | |
| | |<- NOTIFY F14<| |
| | | | |
| | |>F15 200 OK ->| |
| | | |>F16 NOTIFY ----->|
| | | | |
| | | |<----- 200 OK F17<|
| | | | |
| |>F18 200 OK ------------------------------------>|
| | | | |
| |<--------------------------------------- ACK F19<|
| | | | |
| |>F20 ACK ----->| | |
| | | | |
| | |<======= RTP established =======>|
| | | | |
| | |<- NOTIFY F21<| |
| | | | |
| | |>F22 200 OK ->| |
| | | |>F23 NOTIFY ----->|
| | | | |
| | | |<----- 200 OK F24<|
| | | | |
Figure 8.
F16 Appearance Agent ----> Bob
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NOTIFY sip:bob@ua1.example.com SIP/2.0
From: <sip:bob@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
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:alice@example.com:5060">
<dialog id="3xdsd4f9c83"
call-id="b3cbd0-ad2c5775e-5df9f8d5"
local-tag="34322kdfr234f"
remote-tag="3153DE7C-928203B"
direction="initiator">
<sa:exclusive>true</exclusive>
<sa:appearance>1</appearance>
<state>connected</state>
<local>
<target uri="sip:bob@ua2.example.com">
</target>
</local>
<remote>
<identity>sip:alice@example.com</identity>
<target uri="sip:alice@ua1.example.com" />
</remote>
</dialog>
<dialog id="4839589"
call-id="b3cbd0-ad2c5775e-5df9f8d5"
local-tag="3153DE7C-928203B"
remote-tag="34322kdfr234f"
direction="responder">
<sa:exclusive>true</exclusive>
<sa:appearance>1</appearance>
<state>connected</state>
<local>
<target uri="sip:alice@ua1.example.com" />
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</local>
<remote>
<identity>sip:alice@example.com</identity>
<target uri="sip:bob@ua2.example.com" />
</remote>
</dialog>
</dialog-info>
10.9. 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 chooses not to have an appearance number for the call to
Alice since he is treating it as part of the call to Carol. He
indicates this in his PUBLISH F32 which 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. Note that the Appearance Agent does not generate
notifications on the dialog state of the consultation call.
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.
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 ----->|
| | | | |
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| | | |<----- 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<| | |
| | | | |
| |>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 ===================>|
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| | | | |
Figure 9.
F32 Bob ----> Appearance Agent
PUBLISH sip:alice@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:alice@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="partial"
entity="sip:alice@example.com:5060">
<dialog id="id3d4f9c83"
call-id="b3cbd0-ad2c5775e-5df9f8d5"
local-tag="3153DE7C-928203B"
direction="initiator">
<sa:exclusive>true</exclusive>
<state>trying</state>
<local>
<target uri="sip:bob@ua2.example.com">
</target>
</local>
<remote>
<identity>sip:alice@example.com</identity>
<target uri="sip:alice@example.com" />
</remote>
</dialog>
</dialog-info>
10.10. 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].
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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<| |
| | | | |
| |>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 ----------<|
| | | | |
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Figure 10.
F22 Alice ----> Appearance Agent
PUBLISH sip:alice@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:alice@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="partial"
entity="sip:alice@example.com:5060">
<dialog id="id3d4f9c83"
call-id="dc95da63-60db1abd-d5a74b48"
local-tag="605AD957-1F6305C2" >
<sa:appearance>0</appearance>
<sa:exclusive>false</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:alice@example.com>;tag=605AD957-1F6305C2
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To: <sip:bob@ua.example.com>
CSeq: 2 INVITE
Call-ID: dc95da63-60db1abd-d5a74b48
Contact: <sip:alice@ua1.example.com>
<all-one-line>
Join: 14-1541707345;to-tag=d3b06488-1dd1-11b2-88c5
-b03162323164+d3e48f4c;from-tag=44BAD75D-E3128D42
</all-one-line>
Max-Forwards: 70
Content-Type: application/sdp
Content-Length: 223
v=0
o=- 1103061265 1103061265 IN IP4 ua1.example.com
s=IP SIP UA
c=IN IP4 ua1.example.com
t=0 0
a=sendrecv
m=audio 2236 RTP/AVP 0 8 101
a=rtpmap:0 PCMU/8000
a=rtpmap:8 PCMA/8000
a=rtpmap:101 telephone-event/8000
10.11. Appearance Allocation - Loss of Appearance
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 R24 and F26.
After retransmitting the NOTIFY F26 to Bob, the subscription is
terminated.
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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.
10.12. Appearance Selection 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 409 Conflict.
After the NOTIFY F24, Alice learns that Bob is using appearance 2.
Alice republishes for appearance 3 which is accepted.
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Carol Proxy Alice Appearance Agent Bob
| | | | |
| | | |<----- PUBLISH F1<|
| | | | (appearance=2)
| | |>F2 PUBLISH ->| |
| | | (appearance=2) |
| | | | |
| | | |>F3 200 OK ------>|
| | |<---- F4 409 <| |
| | | | |
| | |<-- 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.
10.13. Appearance Agent Subscription to UAs
In this scenario, the Appearance Agent does not have any way of
knowing Bob's dialog state information, except through Bob. This
could be because the Appearance Agent is not part of a B2BUA, or
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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. Whenever Bob's dialog state changes, a NOTIFY is
sent to the Appearance Agent who then notifies the other 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 ------------------------------->|
| | | | |
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| | | |<----- 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.
11. IANA Considerations
This section registers the SIP Alert-Info header field parameter
"appearance" and the XML namespace extensions to the SIP Dialog
Package.
11.1. SIP Event Package Parameter: shared
This specification also defines a new event parameter 'shared' for
the Dialog Package. When used in a NOTIFY, it indicates that the
notifier supports the shared appearance feature. When used in a
PUBLISH, it indicates that the publisher has explicit appearance
information contained in the message body. If not present in a
PUBLISH, the Appearance Agent MAY assign an appearance number to any
new dialogs in the message body.
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11.2. URN Sub-Namespace Registration: sa-dialog-info
This section registers a new XML namespace per the procedures
in [RFC3688].
URI: urn:ietf:params:xml:ns:sa-dialog-info.
Registrant Contact: IETF BLISS working group, <bliss@ietf.org>,
Alan Johnston <alan@sipstation.com>
XML:
BEGIN
<?xml version="1.0"?>
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML Basic 1.0//EN"
"http://www.w3.org/TR/xhtml-basic/xhtml-basic10.dtd">
<html xmlns="http://www.w3.org/1999/xhtml">
<head>
<meta http-equiv="content-type"
content="text/html;charset=iso-8859-1"/>
<title>Shared Appearance Dialog Information Namespace</title>
</head>
<body>
<h1>Namespace for Shared Appearance Dialog Information</h1>
<h2>urn:ietf:params:xml:ns:dialog-info</h2>
<p>See <a href="ftp://ftp.rfc-editor.org/in-notes/rfcXXXX.txt">
RFCXXXX</a>.</p>
</body>
</html>
END
11.3. XML Schema Registration
This section registers an XML schema per the procedures in
[RFC3688].
URI: urn:ietf:params:xml:schesa:sa-dialog-info.
Registrant Contact: IETF BLISS working group, <bliss@ietf.org>,
Alan Johnston <alan@sipstation.com>
The XML for this schema can be found in Section 6.
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12. Appendix A - Incoming Appearance Assignment
To best meet REQ-9, the appearance number for an incoming INVITE
should be contained in the INVITE itself.
For the dialog package parameter approach, REQ-9 could be met in two
ways. When an incoming request is received, the Appearance Agent
could send out a NOTIFY with state trying and include the appearance
number to be used for this request. Upon receipt of this NOTIFY, the
UAs could begin alerting using the appearance number selected. This
approach is sub-optimal since the UAs could receive the INVITE but be
unable to begin alerting if the NOTIFY from the Appearance Agent is
delayed or lost
An alternative approach is to define an extension parameter for the
Alert-Info header field in RFC 3261 such as:
Alert-Info: <file://ring.pcm>;alert=normal;appearance=0
This Alert-Info header would indicate to place the call on the first
line appearance instance.
OPEN ISSUE: What URI do we use if no special ring is requested?
The determination as to what value to use in the appearance parameter
can be done at the proxy that forks the incoming request to all the
registered UAs. There are a variety of ways the proxy can use to
determine what value it should use to populate this parameter. For
example, the proxy could fetch this information by initiating a
SUBSCRIBE request with Expires: 0 to the Appearance Agent for the AOR
to fetch the list of lines that are in use. Alternatively, it could
act like a UA that is a part of the appearance group and SUBSCRIBE to
the State-Agent like any other UA. This would ensure that the active
dialog information is available without having to poll on a need
basis. It could keep track of the list of active calls for the
appearance AOR based on how many unique INVITE requests it has forked
to or received from the appearance AOR. Another approach would be
for the Proxy to first send the incoming INVITE to the Appearance
Agent which would redirect to the appearance group URI and escape the
proper Alert-Info header field for the Proxy to recurse and
distribute to the other UAs in the group.
The Appearance Agent needs to know about all incoming requests to the
AOR in order to select the appearance number. One way in which this
could be done is for the Appearance Agent to register against the AOR
with a higher q value. This will result in the INVITE being sent to
the Appearance Agent first, then being offered to the UAs in the
group.
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The changes to RFC 3261 ABNF would be:
alert-param = LAQUOT absoluteURI RAQUOT *( SEMI (generic-param /
appearance-param) )
appearance-param = "appearance" EQUAL *DIGIT
13. Appendix B - Implementation Options Discussion
This section discusses some options on how to implement the Shared
Appearances feature in SIP. This section is non-normative.
13.1. Appearance Implementation Options
This section discusses and compares two methods of implementing,
conveying, and selecting appearances in SIP while meeting the
requirements of Section 4. One approach involves a URI parameter and
is discussed in section 5.1.1. The other approach uses a SIP dialog
package extension parameter and is discussed in section 5.1.2. Both
approaches assume an Appearance Agent. In addition, this section
discusses approaches for incoming appearance indication, REQ-9, and
appearance contention, REQ-8. These approaches will be discussed for
an example appearance group of N phones each with n line appearances.
The usage of the word phone does not imply that this feature is
limited to telephony devices.
13.1.1. URI parameter Approach
Some implementations of this feature utilize a URI parameter such as
"line=3" on the Contact URI. Each appearance is effectively a
logical UA, so each line appearance requires a separate registration.
The number of line appearances needs to be provisioned on each phone.
Each appearance also requires a separate dialog package subscription.
Even using a State Agent for the dialog package, each phone must
maintain n subscriptions to the dialog package.
This results in 2nN total subscriptions and nN registrations for this
implementation.
Since Contact URI parameters will be conveyed by the dialog package,
REQ-7 is met.
REQ-10 can be met by having the Appearance Agent send a SUBSCRIBE to
each UA and line number to obtain the current dialog state - this
will result in nN SUBSCRIBEs and NOTIFYs.
It is not obvious how to meet REQ-11 with this approach. A UA
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registering against the AOR but does not implement the appearance URI
parameter will not include a line appearance number in Contact URIs
and dialog package NOTIFYs. The Appearance Agent will have no way of
indicating to the other UAs the appearance number being used by this
UA, as adding a parameter to the Contact URI would cause call control
operations such as Replaces and Join to fail.
REQs 12 and 13 are difficult to meet with this approach as the line
appearance number will be present in the Request-URI of incoming
requests and the Contact URI in INVITE and 200 OK messages. This
approach will require integrity protection of all dialog creating
requests and responses, and privacy mechanisms to hide the Contact
URI from other UAs.
Also, this approach will require mechanisms to protect against
another UA sending an INVITE directly to a group member with the line
appearance number already set.
13.1.2. Dialog Package Parameter
Instead of the URI parameter approach, consider an extension
parameter "appearance" to the SIP dialog package. The e.g.:
<?xml version="1.0"?>
<dialog-info xmlns="urn:ietf:params:xml:ns:dialog-info"
xmlns:sa="urn:ietf:params:xml:ns:sa-dialog-info"
version="6"
state="partial"
entity="sip:alice@example.com">
<dialog id="id3d4f9c83" from-tag="3423"
to-tag="a3f423j88uju1" direction="initiator">
<sa:appearance>2</appearance>
<sa:exclusive>false</exclusive>
<sa:joined-dialog call-id="sdfg" from-tag="832d1"
to-tag="4542454" />
<sa:joined-dialog call-id="873287876" from-tag="433"
to-tag="jwjwuf5" />
<state>connected</state>
<local>
<target uri="sip:bob@pc.example.com" />
</local>
</dialog>
</dialog-info>
...
In this approach, the appearance number is never carried in a
Request-URI or Contact URI. Instead, it is only present in dialog
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package NOTIFY and PUBLISH messages. As a result, only a single
registration per AOR is required. Also, only a single dialog package
subscription in each direction per AOR.
This results in 2N total subscriptions and N registrations for this
approach.
If the dialog package is extended to carry the appearance number,
then REQ-7 is met.
REQ-10 can be met by having the Appearance Agent send a SUBSCRIBE to
each UA and line number to obtain the current dialog state - this
will result in N SUBSCRIBEs and NOTIFYs.
REQ-11 can be met by this approach. Even though a UA does not
provide an appearance number in dialog package NOTIFYs, the
Appearance Agent can assign one and include it in NOTIFYs to the
other UAs. This parameter would simply be ignored by the UAs that
did not understand the parameter, and have no impact on call control
operations.
REQs 12 and 13 are met because the appearance number is only conveyed
in dialog package NOTIFYs. Integrity and privacy of NOTIFY bodies
can be achieved using normal SIP mechanisms independent of the
security mechanisms used for other requests.
The dialog-package [RFC3265] describes a mechanism whereby shared-
line privacy REQ-14 can be accomplished by suppressing certain dialog
information from being presented to the UAs. The reasoning behind
that is if the UAs were unaware of a dialog's call-id, local-tag and
remote-tag then they will be unable to create requests such as INVITE
with Replaces [RFC3891] and Join [RFC3911] header fields to barge-in
or pickup the line appearance. Below is a quote from section 3.6 of
dialog-package[RFC3265] that describes this approach:
Note that many implementations of "shared-lines" have a feature that
allows details of calls on a shared address-of-record to be made
private. This is a completely reasonable authorization policy that
could result in notifications that contain only the id attribute of
the dialog element and the state element when shared-line privacy is
requested, and notifications with more complete information when
shared-line privacy is not requested.
There are certain fundamental drawbacks in the privacy-by-obscurity
approach described in [RFC3265] . It models exclusivity as a static
property of the appearance AOR. There are situations where
exclusivity needs to be a dynamic property (e.g. boss does not want
secretary to listen-in on a particular part of the conversation). In
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addition, [RFC3265] does not address how a UA can request exclusivity
at the start of a session or mid-session and how that request will be
granted or rejected.
Exclusivity being a dynamic property means that a UA can request it
to be turned on or off in the middle of a session. When exclusivity
is turned off all the UAs that share the line AOR will need to see
the complete dialog information. Once they have that information it
can not be taken back from them. This will not allow exclusivity to
be turned on later on in the dialog lifetime. Therefore, there needs
to be a centralized entity that will actually enforce exclusivity.
The approach proposed for meeting REQ-14 is to include an exclusivity
parameter to the dialog package. This allows a UA to request
exclusivity, by setting the exclusive parameter in notifications.
This could be done prior to a call being made or answered, or during
a call at any time. A UA can remove exclusivity by sending a
notification at any time during a call and setting "exclusive=no".
It also allows a UA to learn that a particular dialog is exclusive by
the presence of this parameter in a NOTIFY. In addition, a UA can
still apply policy to any INVITE Join or Replaces requests it
receives, as per normal SIP call control mechanisms.
With this approach, the number of appearances is centrally managed
and controlled by the Appearance Agent. For UAs with soft keys or
buttons, this gives a great deal of flexibility in system management.
The User Agents in the group could SUBSCRIBE to each other and NOTIFY
dialog state events, but in a large group the User Agents have to
manage a larger number of SUBSCRIPTIONS and NOTIFICATIONS. The State
Agent in the Appearance Agent helps in managing large groups better.
Further, the State Agent can filter dialog state events and NOTIFY
User Agents of the dialog state events which are required for the
application or feature. The State Agent can also SUBSCRIBE to dialog
state events with filters to reduce the number of NOTIFY messages
exchanged between the State Agent and the user agents in the group.
This allows a group of N UAs to each only establish a pair of dialog
state subscriptions (one in each direction) to learn the dialog state
of all other group members. This results in 2N total subscriptions
for the entire group. A full mesh of subscriptions without a state
agent would result in N(N-1) total subscriptions.
13.1.3. Appearance Selections Mechanisms
Regardless of how the appearance number is conveyed by UAs, there is
still the issue of how appearance numbers are selected. For example,
some UAs might have actual buttons and lamps, and pressing a
particular button requires the UA to reserve a particular appearance
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number. For devices with this type of user interface, the selection
must be done before the user continues with the call and dials digits
or a URI. Other UAs with different user interfaces can be flexible
at the time of dialing, updating the display with the appearance
number at a later date. For devices which require advance appearance
selection, there are three options discussed in the following
sections for meeting REQ-15.
13.1.3.1. Floor Control Appearance Selection Mechanism
This approach models each appearance number as a floor (shared
resource) and uses a floor control server to arbitrate exclusive
access (seizure of a particular appearance number). This approach
uses a standard SIP Event State Compositor (ESC), a standard Floor
Control Server that uses the Appearance Agent as Moderator. The
Binary Floor Control Protocol (BFCP) is used between the UAs and the
Floor Control Server. A Registrar/Forking Proxy Server talks to
Appearance Agent about incoming calls. The Appearance Agent acts as
a Moderator for the floor control server and tells forking proxy to
insert the appearance number in incoming and outgoing requests.
Appearance numbers are allocated/selected/reserved in two ways:
For incoming calls, the Forking Proxy interacts with the Appearance
Agent. The Appearance Agent selects an appearance by taking a
particular floor and marking it "moderator controlled". This
appearance number is then included by the Forking Proxy in INVITEs
using the Alert-Info parameter. When a UA answers the call, it takes
the appearance number from the Alert-Info and includes it in the
dialog state publication. It then requests the floor associated with
the appearance number from the floor control server, which forwards
the request to the Appearance Agent (moderator). The Appearance
Agent correlates the floor control request with the dialog state
notification with the dialog ID from the INVITE with the Alert-Info.
If they match, the floor is granted. If they do not match, it means
the floor request is not an answer of the call but is a random
appearance selection by the UA and will be rejected.
For outgoing calls, the UA sends an INVITE and requests a particular
floor from the floor control server. Depending on the User Interface
requirements, the floor request can be done before or after sending
the INVITE. The floor grant policy for most appearances is set to
"first come first serve". Once the floor has been granted and the
call answered, the dialog state publication by the UA will include
the appearance number.
When a call has ended, the UA releases the floor to the floor control
server and this appearance is now available for incoming and outgoing
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calls.
When a UA in the group which does not support BFCP is in a call, the
Appearance Agent will grant the floor associated with that appearance
to that UA. When that call is over, the Appearance Agent will
release the floor. Since the UA will not publish the appearance
number to the ESC, the Appearance Agent will need to do that on their
behalf. If the UA does publish dialog state but without the
appearance number, the Appearance Agent will still need to re-publish
the dialog state including the appearance number. UAs in the group
will be able to recognize these two dialogs as one since they will
have the same SIP dialog ID.
13.1.3.2. INVITE Appearance Selection Mechanism
This is an alternative approach that utilizes sending an INVITE to
select/reserve/seize an appearance number.
A UA that does not need to select a particular appearance number (or
doesn't care) would just send an INVITE as normal. The Appearance
Agent would tell the proxy which appearance number was being used by
inserting this information in a header field in the first non-100
provisional response sent back to the calling UA. The UA would then
PUBLISH this appearance number to the Dialog Event State Compositor
for the AOR which would distribute details of the dialog and the
appearance number to the other UAs in the group.
If an INVITE is sent and no appearance number is available, the proxy
would reject the INVITE with a suitable response code and perhaps a
header field indication.
A UA that does need to select a particular appearance number would
use an approach similar to overlap dialing (multi-stage dialing). An
INVITE would be sent when the appearance number is requested (i.e.
when the button is pressed, before dialing begins). The appearance
number selected would be carried in the INVITE, in a header field or
in the Request-URI, for example. The proxy would reject the INVITE
with a 484 Address Incomplete response (see RFC 3578) if the
appearance number is Available and start a timer. The UA could then
resend the INVITE after the URI has been dialed and then PUBLISH this
appearance number to the ESC. If the appearance number is not
available, another response code such as 403 would be sent. The user
could then select a different appearance number and resend the
INVITE. If no INVITE with a matching Call-ID is received before the
timer expires, the appearance seizure is cancelled and is made
available for other calls.
Note that this approach does not actually require a B2BUA, but it
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does require a proxy that can act as a UAS and communicate with an
Appearance Agent which keeps track of appearance number allocations.
13.1.3.3. PUBLISH Appearance Selection Mechanism
The approach used in previous versions of this draft is to use the
PUBLISH to the event state compositor to select an appearance number.
This approach requires a special event state compositor and special
behavior on the part of the UA.
In the selection of an appearance for requests initiated by UAs in
the group, there is the possibility of contention where more than one
UA select the same appearance number.
One way to solve this and meet REQ-8 is to require UAs to send a
notification (trying) to the Appearance Agent indicating the
appearance number to be used for the session. The Appearance Agent
would confirm the allocation of the appearance number in a NOTIFY
sent to the group UAs. Should the appearance number be unavailable
or otherwise not allowed, there are two options:
- The notification could be rejected with a 500 response and a Retry-
After header field. The Appearance Agent would send an immediate
NOTIFY indicating that the appearance is unavailable. If the NOTIFY
is received before the expiration of the Retry-After time, the
notification state information would become out of date and would be
discarded without resending. The UA would select another appearance
number and send another notification.
- The notification could be accepted but an immediate NOTIFY
generated by the Appearance Agent indicating that the appearance is
unavailable. The UA would then select another appearance number and
PUBLISH again.
UAs would wait for a notification from the Appearance Agent before
sending the INVITE.
13.2. Comparison
In comparing the URI parameter and the dialog package parameter,
there are clear differences in the number of registrations and
subscriptions, with the dialog package approach requiring n times
fewer in both cases.
The security model for the dialog package parameter approach is much
cleaner, since only NOTIFY and PUBLISH requests need integrity and
privacy. The security model for the URI parameter approach would
likely require a B2BUA which introduces many undesirable properties.
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The dialog package parameter approach has better backwards
compatibility than the URI parameter approach.
In summary, the dialog package parameter approach better meets REQs
5, 10, 11, 12, and 13 while the URI parameter approach better meets
REQ-9. However, the combined dialog package parameter approach and
the Alert-Info parameter approach meets REQ-9.
13.2.1. Comparison of Appearance Selection Methods
All three approaches meet REQ-15 and REQ-16.
Previous versions of this draft proposed the publish/notify method of
appearance selection. The advantage of this approach is that the
appearance number is only carried in one place (dialog package XML
documents) and the same protocol/mechanism is used to select and
learn appearance numbers. The disadvantage of this approach is that
a specialized event state compositor must be used, since it is aware
of appearance numbers. Also, concerns have been raised about whether
this approach defines new semantics for publish/notify beyond that in
RFC 3265.
The floor control approach makes good reuse of existing protocols
such as Binary Floor Control Protocol (BFCP) and cleanly models the
state. However, while BFCP can be used in conferencing applications,
it is unlikely most UAs implementing shared appearances would utilize
the protocol. Also, having appearance state in two places (dialog
package XML documents and floor control messages) complicates the
application. Also, BFCP only runs over TCP and requires a separate
offer/answer exchange to establish the connection, making operation
through NATs and firewalls more difficult. The BFCP approach is also
radically different from all current implementations of this feature.
As a result, standardizing this approach would likely result in an
increase in feature interoperability rather than a decrease.
The INVITE selection mechanism is based on overlap dialing. Overlap
dialing is supported in very few SIP UAs and is regarded as a
somewhat archaic leftover from the PSTN. As such, it is not regarded
as a good starting point for a common feature such as shared
appearances.
The PUBLISH selection mechanism reuses the SIP events extensions
which already must be implemented by UAs supporting this feature. In
fact, it results in no additional messages or round trips. It is
also very similar to many current feature implementations today.
Standardizing this approach is likely to increase overall
interoperability of this feature.
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The rest of this document will only discuss the PUBLISH appearance
selection mechanism.
14. Acknowledgements
The following individuals were part of the shared appearance Design
team and have provided input and text to the document (in
alphabetical order):
Martin Dolly, Andrew Hutton, Raj Jain, Fernando Lombardo, Derek
MacDonald, Bill Mitchell, Michael Procter, Theo Zowzouvillys.
Thanks to Chris Boulton for helping with the XML schema.
Much of the material has been drawn from previous work by Mohsen
Soroushnejad, Venkatesh Venkataramanan, Paul Pepper and Anil Kumar,
who in turn received assistance from:
Kent Fritz, John Weald, and Sunil Veluvali of Sylantro Systems, Steve
Towlson, and Michael Procter of Citel Technologies, Rob Harder and
Hong Chen of Polycom Inc, John Elwell, J D Smith of Siemens
Communications, Dale R. Worley of Pingtel, Graeme Dollar of Yahoo
Inc.
Also thanks to Geoff Devine, Paul Kyzivat, Jerry Yin, John Elwell,
Dan York, Spenser Dawkins, Martin Dolly, and Brett Tate for their
comments.
15. Security Considerations
Since multiple line appearance features are implemented using
semantics provided by [RFC3261], Event Package for Dialog State as
define in , and Event Notification [RFC3265], [RFC3903], security
considerations in these documents apply to this draft as well.
Specifically, since dialog state information and the dialog
identifiers are supplied by UA's in an appearance group to other
members, the same is prone to "call hijacks". For example, a rogue
UA could snoop for these identifiers and send an INVITE with Replaces
header containing these call details to take over the call. As such
INVITES with Replaces header MUST be authenticated using the standard
mechanism (like Digest or S/MIME) described in [RFC3261]. before it
is accepted. NOTIFY or PUBLISH message bodies that provide the
dialog state information and the dialog identifiers MAY be encrypted
end-to-end using the standard mechanics. All SUBSCRIBES between the
UA's and the Appearance Agent MUST be authenticated.
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16. Informative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC3261] Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston,
A., Peterson, J., Sparks, R., Handley, M., and E.
Schooler, "SIP: Session Initiation Protocol", RFC 3261,
June 2002.
[RFC3515] Sparks, R., "The Session Initiation Protocol (SIP) Refer
Method", RFC 3515, April 2003.
[RFC3265] Roach, A., "Session Initiation Protocol (SIP)-Specific
Event Notification", RFC 3265, June 2002.
[RFC3903] Niemi, A., "Session Initiation Protocol (SIP) Extension
for Event State Publication", RFC 3903, October 2004.
[RFC3891] Mahy, R., Biggs, B., and R. Dean, "The Session Initiation
Protocol (SIP) "Replaces" Header", RFC 3891,
September 2004.
[RFC5359] Johnston, A., Sparks, R., Cunningham, C., Donovan, S., and
K. Summers, "Session Initiation Protocol Service
Examples", BCP 144, RFC 5359, October 2008.
[RFC4235] Rosenberg, J., Schulzrinne, H., and R. Mahy, "An INVITE-
Initiated Dialog Event Package for the Session Initiation
Protocol (SIP)", RFC 4235, November 2005.
[RFC3911] Mahy, R. and D. Petrie, "The Session Initiation Protocol
(SIP) "Join" Header", RFC 3911, October 2004.
[RFC4579] Johnston, A. and O. Levin, "Session Initiation Protocol
(SIP) Call Control - Conferencing for User Agents",
BCP 119, RFC 4579, August 2006.
[RFC3840] Rosenberg, J., Schulzrinne, H., and P. Kyzivat,
"Indicating User Agent Capabilities in the Session
Initiation Protocol (SIP)", RFC 3840, August 2004.
[RFC3688] Mealling, M., "The IETF XML Registry", BCP 81, RFC 3688,
January 2004.
[RFC3680] Rosenberg, J., "A Session Initiation Protocol (SIP) Event
Package for Registrations", RFC 3680, March 2004.
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Authors' Addresses
Alan Johnston (editor)
Avaya
St. Louis, MO 63124
Email: alan@sipstation.com
Mohsen Soroushnejad
Sylantro Systems Corp
Email: mohsen.soroush@sylantro.com
Venkatesh Venkataramanan
Sylantro Systems Corp
Email: vvenkatar@gmail.com
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