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Internet Engineering Task Force (IETF) R. Mahy Request for Comments: 5850 Unaffiliated Category: Informational R. Sparks ISSN: 2070-1721 Tekelec

                                                          J. Rosenberg
                                                             D. Petrie
                                                      A. Johnston, Ed.
                                                              May 2010
         A Call Control and Multi-Party Usage Framework for
               the Session Initiation Protocol (SIP)


 This document defines a framework and the requirements for call
 control and multi-party usage of the Session Initiation Protocol
 (SIP).  To enable discussion of multi-party features and
 applications, we define an abstract call model for describing the
 media relationships required by many of these.  The model and actions
 described here are specifically chosen to be independent of the SIP
 signaling and/or mixing approach chosen to actually set up the media
 relationships.  In addition to its dialog manipulation aspect, this
 framework includes requirements for communicating related information
 and events such as conference and session state and session history.
 This framework also describes other goals that embody the spirit of
 SIP applications as used on the Internet such as the definition of
 primitives (not services), invoker and participant oriented
 primitives, signaling and mixing model independence, and others.

Status of This Memo

 This document is not an Internet Standards Track specification; it is
 published for informational purposes.
 This document is a product of the Internet Engineering Task Force
 (IETF).  It represents the consensus of the IETF community.  It has
 received public review and has been approved for publication by the
 Internet Engineering Steering Group (IESG).  Not all documents
 approved by the IESG are a candidate for any level of Internet
 Standard; see Section 2 of RFC 5741.
 Information about the current status of this document, any errata,
 and how to provide feedback on it may be obtained at

Mahy, et al. Informational [Page 1] RFC 5850 SIP Call Control Framework May 2010

Copyright Notice

 Copyright (c) 2010 IETF Trust and the persons identified as the
 document authors.  All rights reserved.
 This document is subject to BCP 78 and the IETF Trust's Legal
 Provisions Relating to IETF Documents
 ( in effect on the date of
 publication of this document.  Please review these documents
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 the Trust Legal Provisions and are provided without warranty as
 described in the Simplified BSD License.
 This document may contain material from IETF Documents or IETF
 Contributions published or made publicly available before November
 10, 2008.  The person(s) controlling the copyright in some of this
 material may not have granted the IETF Trust the right to allow
 modifications of such material outside the IETF Standards Process.
 Without obtaining an adequate license from the person(s) controlling
 the copyright in such materials, this document may not be modified
 outside the IETF Standards Process, and derivative works of it may
 not be created outside the IETF Standards Process, except to format
 it for publication as an RFC or to translate it into languages other
 than English.

Table of Contents

 1.  Motivation and Background  . . . . . . . . . . . . . . . . . .  4
 2.  Key Concepts . . . . . . . . . . . . . . . . . . . . . . . . .  6
   2.1.  Conversation Space Model . . . . . . . . . . . . . . . . .  7
   2.2.  Relationship between Conversation Space, SIP Dialogs,
         and SIP Sessions . . . . . . . . . . . . . . . . . . . . .  8
   2.3.  Signaling Models . . . . . . . . . . . . . . . . . . . . .  9
   2.4.  Mixing Models  . . . . . . . . . . . . . . . . . . . . . . 10
     2.4.1.  Tightly Coupled  . . . . . . . . . . . . . . . . . . . 11
     2.4.2.  Loosely Coupled  . . . . . . . . . . . . . . . . . . . 12
   2.5.  Conveying Information and Events . . . . . . . . . . . . . 13
   2.6.  Componentization and Decomposition . . . . . . . . . . . . 15
     2.6.1.  Media Intermediaries . . . . . . . . . . . . . . . . . 15
     2.6.2.  Text-to-Speech and Automatic Speech Recognition  . . . 17
     2.6.3.  VoiceXML . . . . . . . . . . . . . . . . . . . . . . . 17
   2.7.  Use of URIs  . . . . . . . . . . . . . . . . . . . . . . . 18
     2.7.1.  Naming Users in SIP  . . . . . . . . . . . . . . . . . 19
     2.7.2.  Naming Services with SIP URIs  . . . . . . . . . . . . 20
   2.8.  Invoker Independence . . . . . . . . . . . . . . . . . . . 22
   2.9.  Billing Issues . . . . . . . . . . . . . . . . . . . . . . 23

Mahy, et al. Informational [Page 2] RFC 5850 SIP Call Control Framework May 2010

 3.  Catalog of Call Control Actions and Sample Features  . . . . . 23
   3.1.  Remote Call Control Actions on Early Dialogs . . . . . . . 24
     3.1.1.  Remote Answer  . . . . . . . . . . . . . . . . . . . . 24
     3.1.2.  Remote Forward or Put  . . . . . . . . . . . . . . . . 24
     3.1.3.  Remote Busy or Error Out . . . . . . . . . . . . . . . 24
   3.2.  Remote Call Control Actions on Single Dialogs  . . . . . . 24
     3.2.1.  Remote Dial  . . . . . . . . . . . . . . . . . . . . . 24
     3.2.2.  Remote On and Off Hold . . . . . . . . . . . . . . . . 25
     3.2.3.  Remote Hangup  . . . . . . . . . . . . . . . . . . . . 25
   3.3.  Call Control Actions on Multiple Dialogs . . . . . . . . . 25
     3.3.1.  Transfer . . . . . . . . . . . . . . . . . . . . . . . 25
     3.3.2.  Take . . . . . . . . . . . . . . . . . . . . . . . . . 26
     3.3.3.  Add  . . . . . . . . . . . . . . . . . . . . . . . . . 27
     3.3.4.  Local Join . . . . . . . . . . . . . . . . . . . . . . 28
     3.3.5.  Insert . . . . . . . . . . . . . . . . . . . . . . . . 28
     3.3.6.  Split  . . . . . . . . . . . . . . . . . . . . . . . . 29
     3.3.7.  Near-Fork  . . . . . . . . . . . . . . . . . . . . . . 29
     3.3.8.  Far-Fork . . . . . . . . . . . . . . . . . . . . . . . 29
 4.  Security Considerations  . . . . . . . . . . . . . . . . . . . 30
 Appendix A.    Example Features  . . . . . . . . . . . . . . . . . 32
 Appendix A.1.  Attended Transfer . . . . . . . . . . . . . . . . . 32
 Appendix A.2.  Auto Answer . . . . . . . . . . . . . . . . . . . . 32
 Appendix A.3.  Automatic Callback  . . . . . . . . . . . . . . . . 32
 Appendix A.4.  Barge-In  . . . . . . . . . . . . . . . . . . . . . 32
 Appendix A.5.  Blind Transfer  . . . . . . . . . . . . . . . . . . 32
 Appendix A.6.  Call Forwarding . . . . . . . . . . . . . . . . . . 33
 Appendix A.7.  Call Monitoring . . . . . . . . . . . . . . . . . . 33
 Appendix A.8.  Call Park . . . . . . . . . . . . . . . . . . . . . 33
 Appendix A.9.  Call Pickup . . . . . . . . . . . . . . . . . . . . 33
 Appendix A.10. Call Return . . . . . . . . . . . . . . . . . . . . 34
 Appendix A.11. Call Waiting  . . . . . . . . . . . . . . . . . . . 34
 Appendix A.12. Click-to-Dial . . . . . . . . . . . . . . . . . . . 34
 Appendix A.13. Conference Call . . . . . . . . . . . . . . . . . . 34
 Appendix A.14. Consultative Transfer . . . . . . . . . . . . . . . 34
 Appendix A.15. Distinctive Ring  . . . . . . . . . . . . . . . . . 35
 Appendix A.16. Do Not Disturb  . . . . . . . . . . . . . . . . . . 35
 Appendix A.17. Find-Me . . . . . . . . . . . . . . . . . . . . . . 35
 Appendix A.18. Hotline . . . . . . . . . . . . . . . . . . . . . . 35
 Appendix A.19. IM Conference Alerts  . . . . . . . . . . . . . . . 35
 Appendix A.20. Inbound Call Screening  . . . . . . . . . . . . . . 35
 Appendix A.21. Intercom  . . . . . . . . . . . . . . . . . . . . . 36
 Appendix A.22. Message Waiting . . . . . . . . . . . . . . . . . . 36
 Appendix A.23. Music on Hold . . . . . . . . . . . . . . . . . . . 36
 Appendix A.24. Outbound Call Screening . . . . . . . . . . . . . . 36
 Appendix A.25. Pre-Paid Calling  . . . . . . . . . . . . . . . . . 37
 Appendix A.26. Presence-Enabled Conferencing . . . . . . . . . . . 37
 Appendix A.27. Single Line Extension/Multiple Line Appearance  . . 37
 Appendix A.28. Speakerphone Paging . . . . . . . . . . . . . . . . 38

Mahy, et al. Informational [Page 3] RFC 5850 SIP Call Control Framework May 2010

 Appendix A.29. Speed Dial  . . . . . . . . . . . . . . . . . . . . 38
 Appendix A.30. Voice Message Screening . . . . . . . . . . . . . . 38
 Appendix A.31. Voice Portal  . . . . . . . . . . . . . . . . . . . 39
 Appendix A.32. Voicemail . . . . . . . . . . . . . . . . . . . . . 40
 Appendix A.33. Whispered Call Waiting  . . . . . . . . . . . . . . 40
 Appendix B.    Acknowledgments . . . . . . . . . . . . . . . . . . 40
 5.  Informative References . . . . . . . . . . . . . . . . . . . . 40

1. Motivation and Background

 The Session Initiation Protocol (SIP) [RFC3261] was defined for the
 initiation, maintenance, and termination of sessions or calls between
 one or more users.  However, despite its origins as a large-scale
 multi-party conferencing protocol, SIP is used today primarily for
 point-to-point calls.  This two-party configuration is the focus of
 the SIP specification and most of its extensions.
 This document defines a framework and the requirements for call
 control and multi-party usage of SIP.  Most multi-party operations
 manipulate SIP dialogs (also known as call legs) or SIP conference
 media policy to cause participants in a conversation to perceive
 specific media relationships.  In other protocols that deal with the
 concept of calls, this manipulation is known as call control.  In
 addition to its dialog or policy manipulation aspect, call control
 also includes communicating information and events related to
 manipulating calls, including information and events dealing with
 session state and history, conference state, user state, and even
 message state.
 Based on input from the SIP community, the authors compiled the
 following set of goals for SIP call control and multi-party
 o  Define primitives, not services.  Allow for a handful of robust
    yet simple mechanisms that can be combined to deliver features and
    services.  Throughout this document, we refer to these simple
    mechanisms as "primitives".  Primitives should be sufficiently
    robust so that when they are combined with each other, they can be
    used to build lots of services.  However, the goal is not to
    define a provably complete set of primitives.  Note that while the
    IETF will NOT standardize behavior or services, it may define
    example services for informational purposes, as in service
    examples [RFC5359].
 o  Be participant oriented.  The primitives should be designed to
    provide services that are oriented around the experience of the
    participants.  The authors observe that end users of features and
    services usually don't care how a media relationship is set up.

Mahy, et al. Informational [Page 4] RFC 5850 SIP Call Control Framework May 2010

    Their ultimate experience is only based on the resulting media and
    other externally visible characteristics.
 o  Be signaling model independent.  Support both a central-control
    and a peer-to-peer feature invocation model (and combinations of
    the two).  Baseline SIP already supports a centralized control
    model described in 3pcc (third party call control) [RFC3725], and
    the SIP community has expressed a great deal of interest in peer-
    to-peer or distributed call control using primitives such as those
    defined in REFER [RFC3515], Replaces [RFC3891], and Join
 o  Be mixing model independent.  The bulk of interesting multi-party
    applications involve mixing or combining media from multiple
    participants.  This mixing can be performed by one or more of the
    participants or by a centralized mixing resource.  The experience
    of the participants should not depend on the mixing model used.
    While most examples in this document refer to audio mixing, the
    framework applies to any media type.  In this context, a "mixer"
    refers to combining media of the same type in an appropriate,
    media-specific way.  This is consistent with the model described
    in the SIP conferencing framework.
 o  Be invoker oriented.  Only the user who invokes a feature or a
    service needs to know exactly which service is invoked or why.
    This is good because it allows new services to be created without
    requiring new primitives from all of the participants; and it
    allows for much simpler feature authorization policies, for
    example, when participation spans organizational boundaries.  As
    discussed in Section 2.7, this also avoids exponential state
    explosion when combining features.  The invoker only has to manage
    a user interface or application programming interface (API) to
    prevent local feature interactions.  All the other participants
    simply need to manage the feature interactions of a much smaller
    number of primitives.
 o  Primitives make full use of URIs (uniform resource identifiers).
    URIs are a very powerful mechanism for describing users and
    services.  They represent a plentiful resource that can be
    extremely expressive and easily routed, translated, and
    manipulated -- even across organizational boundaries.  URIs can
    contain special parameters and informational header fields that
    need only be relevant to the owner of the namespace (domain) of
    the URI.  Just as a user who selects an http: URL need not
    understand the significance and organization of the web site it
    references, a user may encounter a SIP URI that translates into an
    email-style group alias, which plays a pre-recorded message or
    runs some complex call-handling logic.  Note that while this may

Mahy, et al. Informational [Page 5] RFC 5850 SIP Call Control Framework May 2010

    seem paradoxical to the previous goal, both goals can be satisfied
    by the same model.
 o  Make use of SIP header fields and SIP event packages to provide
    SIP entities with information about their environment.  These
    should include information about the status/handling of dialogs on
    other user agents (UAs), information about the history of other
    contacts attempted prior to the current contact, the status of
    participants, the status of conferences, user presence
    information, and the status of messages.
 o  Encourage service decomposition, and design to make use of
    standard components using well-defined, simple interfaces.  Sample
    components include a SIP mixer, recording service, announcement
    server, and voice-dialog server.  (This is not an exhaustive
 o  Include authentication, authorization, policy, logging, and
    accounting mechanisms to allow these primitives to be used safely
    among mutually untrusted participants.  Some of these mechanisms
    may be used to assist in billing, but no specific billing system
    will be endorsed.
 o  Permit graceful fallback to baseline SIP.  Definitions for new SIP
    call control extensions/primitives must describe a graceful way to
    fallback to baseline SIP behavior.  Support for one primitive must
    not imply support for another primitive.
 o  Don't reinvent traditional models, such as the model used for the
    H.450 family of protocols, JTAPI (Java Telephony Application
    Programming Interface), or the CSTA (Computer-supported
    telecommunications applications) call model, as these other models
    do not share the design goals presented in this document.
 Note that the flexibility in this model does have some disadvantages
 in terms of interoperability.  It is possible to build a call control
 feature in SIP using different combinations of primitives.  For a
 discussion of the issues associated with this, see [BLISS-PROBLEM].

2. Key Concepts

 This section introduces a number of key concepts that will be used to
 describe and explain various call control operations and services in
 the remainder of this document.  This includes the conversation space
 model, signaling and mixing models, common components, and the use of

Mahy, et al. Informational [Page 6] RFC 5850 SIP Call Control Framework May 2010

2.1. Conversation Space Model

 This document introduces the concept of an abstract "conversation
 space" as a set of participants who believe they are all
 communicating among one another.  Each conversation space contains
 one or more participants.
 Participants are SIP UAs that send original media to or terminate and
 receive media from other members of the conversation space.
 Logically, every participant in the conversation space has access to
 all the media generated in that space (this is strictly true if all
 participants share a common media type).  A SIP UA that does not
 contribute or consume any media is NOT a participant, nor is a UA
 that merely forwards, transcodes, mixes, or selects media originating
 elsewhere in the conversation space.
    Note that a conversation space consists of zero or more SIP calls
    or SIP conferences.  A conversation space is similar to the
    definition of a "call" in some other call models.
 Participants may represent human users or non-human users (referred
 to as robots or automatons in this document).  Some participants may
 be hidden within a conversation space.  Some examples of hidden
 participants include: robots that generate tones, images, or
 announcements during a conference to announce users arriving and
 departing, a human call center supervisor monitoring a conversation
 between a trainee and a customer, and robots that record media for
 training or archival purposes.
 Participants may also be active or passive.  Active participants are
 expected to be intelligent enough to leave a conversation space when
 they no longer desire to participate.  (An attentive human
 participant is obviously active.)  Some robotic participants (such as
 a voice-messaging system, an instant-messaging agent, or a voice-
 dialog system) may be active participants if they can leave the
 conversation space when there is no human interaction.  Other robots
 (for example, our tone-generating robot from the previous example)
 are passive participants.  A human participant "on hold" is passive.
 An example diagram of a conversation space can be shown as a "bubble"
 or ovals, or as a "set" in curly or square bracket notation.  Each
 set, oval, or bubble represents a conversation space.  Hidden
 participants are shown in lowercase letters.  Examples are given in
 Figure 1.
 Note that while the term "conversation" usually applies to oral
 exchange of information, we apply the conversation space model to any
 media exchange between participants.

Mahy, et al. Informational [Page 7] RFC 5850 SIP Call Control Framework May 2010

 { A , B }                   [ A , b, C, D ]
    .-.                 .---.
   /   \               /     \
  /  A  \             / A   b \
 (       )           (         )
  \  B  /             \ C   D /
   \   /               \     /
    '-'                 '---'
 Figure 1.  Conversation Spaces

2.2. Relationship between Conversation Space, SIP Dialogs, and SIP

 In [RFC3261], a call is "an informal term that refers to some
 communication between peers, generally set up for the purposes of a
 multimedia conversation".  The concept of a conversation space is
 needed because the SIP definition of call is not sufficiently precise
 for the purpose of describing the user experience of multi-party
 Do any other definitions convey the correct meaning?  SIP and SDP
 (Session Description Protocol) [RFC4566] both define a conference as
 "a multimedia session identified by a common session description".  A
 session is defined as "a set of multimedia senders and receivers and
 the data streams flowing from senders to receivers".  The definition
 of "call" in some call models is more similar to our definition of a
 conversation space.
 Some examples of the relationship between conversation spaces, SIP
 dialogs, and SIP sessions are listed below.  In each example, a human
 user will perceive that there is a single call.
 o  A simple two-party call is a single conversation space, a single
    session, and a single dialog.
 o  A locally mixed three-way call is two sessions and two dialogs.
    It is also a single conversation space.
 o  A simple dial-in audio conference is a single conversation space,
    but is represented by as many dialogs and sessions as there are
    human participants.
 o  A multicast conference is a single conversation space, a single
    session, and as many dialogs as participants.

Mahy, et al. Informational [Page 8] RFC 5850 SIP Call Control Framework May 2010

2.3. Signaling Models

 Obviously, to make changes to a conversation space, you must be able
 to use SIP signaling to cause these changes.  Specifically, there
 must be a way to manipulate SIP dialogs (call legs) to move
 participants into and out of conversation spaces.  Although this is
 not as obvious, there also must be a way to manipulate SIP dialogs to
 include non-participant UAs that are otherwise involved in a
 conversation space (e.g., back-to-back user agents or B2BUAs, third
 party call control (3pcc) controllers, mixers, transcoders,
 translators, or relays).
 Implementations may setup the media relationships described in the
 conversation space model using a centralized control model.  One
 common way to implement this using SIP is known as third party call
 control (3pcc) and is described in 3pcc [RFC3725].  The 3pcc approach
 relies on only the following three primitive operations:
 o  Create a new dialog (INVITE)
 o  Modify a dialog (reINVITE)
 o  Destroy a dialog (BYE)
 The main advantage of the 3pcc approach is that it only requires very
 basic SIP support from end systems to support call control features.
 As such, third party call control is a natural way to handle protocol
 conversion and mid-call features.  It also has the advantage and
 disadvantage that new features can/must be implemented in one place
 only (the controller), and it neither requires enhanced client
 functionality nor takes advantage of it.
 In addition, a peer-to-peer approach is discussed at length in this
 document.  The primary drawback of the peer-to-peer model is
 additional complexity in the end system and authentication and
 management models.  The benefits of the peer-to-peer model include:
 o  state remains at the edges,
 o  call signaling need only go through participants involved (there
    are no additional points of failure), and
 o  peers may take advantage of end-to-end message integrity or

Mahy, et al. Informational [Page 9] RFC 5850 SIP Call Control Framework May 2010

 The peer-to-peer approach relies on additional "primitive"
 operations, some of which are identified here.
 o  Replace an existing dialog
 o  Join a new dialog with an existing dialog
 o  Locally perform media forking (multi-unicast)
 o  Ask another user agent (UA) to send a request on your behalf
 The peer-to-peer approach also only results in a single SIP dialog,
 directly between the two UAs.  The 3pcc approach results in two SIP
 dialogs, between each UA and the controller.  As a result, the SIP
 features and extensions that will be used during the dialog are
 limited to the those understood by the controller.  As a result, in a
 situation where both the UAs support an advanced SIP feature but the
 controller does not, the feature will not be able to be used.
 Many of the features, primitives, and actions described in this
 document also require some type of media mixing, combining, or
 selection as described in the next section.

2.4. Mixing Models

 SIP permits a variety of mixing models, which are discussed here
 briefly.  This topic is discussed more thoroughly in the SIP
 conferencing framework [RFC4353] and [RFC4579].  SIP supports both
 tightly coupled and loosely coupled conferencing, although more
 sophisticated behavior is available in tightly coupled conferences.
 In a tightly coupled conference, a single SIP user agent (called the
 focus) has a direct dialog relationship with each participant (and
 may control non-participant user agents as well).  The focus can
 authoritatively publish information about the character and
 participants in a conference.  In a loosely coupled conference, there
 are no coordinated signaling relationships among the participants.
 For brevity, only the two most popular conferencing models are
 significantly discussed in this document (local and centralized
 mixing).  Applications of the conversation spaces model to loosely
 coupled multicast and distributed full unicast mesh conferences are
 left as an exercise for the reader.  Note that a distributed full
 mesh conference can be used for basic conferences, but does not
 easily allow for more complex conferencing actions like splitting,
 merging, and sidebars.

Mahy, et al. Informational [Page 10] RFC 5850 SIP Call Control Framework May 2010

 Call control features should be designed to allow a mixer (local or
 centralized) to decide when to reduce a conference back to a two-
 party call, or drop all the participants (for example, if only two
 automatons are communicating).  The actual heuristics used to release
 calls are beyond the scope of this document, but may depend on
 properties in the conversation space, such as the number of active,
 passive, or hidden participants and the send-only, receive-only, or
 send-and-receive orientation of various participants.

2.4.1. Tightly Coupled

 Tightly coupled conferences utilize a central point for signaling and
 authentication known as a focus [RFC4353].  The actual media can be
 centrally mixed or distributed. (Single) End System Mixing

 The first model we call "end system mixing".  In this model, user A
 calls user B, and they have a conversation.  At some point later, A
 decides to conference in user C.  To do this, A calls C, using a
 completely separate SIP call.  This call uses a different Call-ID,
 different tags, etc.  There is no call set up directly between B and
 C.  No SIP extension or external signaling is needed.  A merely
 decides to locally join two dialogs.
    B     C
     \   /
      \ /
 Figure 2.  End System Mixing Example
 In Figure 2, A receives media streams from both B and C, and mixes
 them.  A sends a stream containing A's and C's streams to B, and a
 stream containing A's and B's streams to C.  Basically, user A
 handles both signaling and media mixing. Centralized Mixing

 In a centralized mixing model, all participants have a pairwise SIP
 and media relationship with the mixer.  Common applications of
 centralized mixing include ad hoc conferences and scheduled dial-in
 or dial-out conferences.  In Figure 3 below, the mixer M receives and
 sends media to participants A, B, C, D, and E.

Mahy, et al. Informational [Page 11] RFC 5850 SIP Call Control Framework May 2010

    B     C
     \   /
      \ /
       M --- A
      / \
     /   \
    D     E
 Figure 3.  Centralized Mixing Example Centralized Signaling, Distributed Media

 In this conferencing model, there is a centralized controller, as in
 the dial-in and dial-out cases.  However, the centralized server
 handles signaling only.  The media is still sent directly between
 participants, using either multicast or multi-unicast.  Participants
 perform their own mixing.  Multi-unicast is when a user sends
 multiple packets (one for each recipient, addressed to that
 recipient).  This is referred to as a "Decentralized Multipoint
 Conference" in [H.323].  Full mesh media with centralized mixing is
 another approach.

2.4.2. Loosely Coupled

 In these models, there is no point of central control of SIP
 signaling.  As in the "Centralized Signaling, Distributed Media" case
 above, all endpoints send media to all other endpoints.
 Consequently, every endpoint mixes their own media from all the other
 sources and sends their own media to every other participant. Large-Scale Multicast Conferences

 Large-scale multicast conferences were the original motivation for
 both the Session Description Protocol (SDP) [RFC4566] and SIP.  In a
 large-scale multicast conference, one or more multicast addresses are
 allocated to the conference.  Each participant joins those multicast
 groups and sends their media to those groups.  Signaling is not sent
 to the multicast groups.  The sole purpose of the signaling is to
 inform participants of which multicast groups to join.  Large-scale
 multicast conferences are usually pre-arranged, with specific start
 and stop times.  However, multicast conferences do not need to be
 pre-arranged, so long as a mechanism exists to dynamically obtain a
 multicast address.

Mahy, et al. Informational [Page 12] RFC 5850 SIP Call Control Framework May 2010 Full Distributed Unicast Conferencing

 In this conferencing model, each participant has both a pairwise
 media relationship and a pairwise signaling relationship with every
 other participant (a full mesh).  This model requires a mechanism to
 maintain a consistent view of distributed state across the group.
 This is a classic, hard problem in computer science.  Also, this
 model does not scale well for large numbers of participants.  For <n>
 participants, the number of media and signaling relationships is
 approximately n-squared.  As a result, this model is not generally
 available in commercial implementations; to the contrary, it is
 primarily the topic of research or experimental implementations.
 Note that this model assumes peer-to-peer signaling.

2.5. Conveying Information and Events

 Participants should have access to information about the other
 participants in a conversation space so that this information can be
 rendered to a human user or processed by an automaton.  Although some
 of this information may be available from the Request-URI or To,
 From, Contact, or other SIP header fields, another mechanism of
 reporting this information is necessary.
 Many applications are driven by knowledge about the progress of calls
 and conferences.  In general, these types of events allow for the
 construction of distributed applications, where the application
 requires information on dialog and conference state, but is not
 necessarily a co-resident with an endpoint user agent or conference
 server.  For example, a focus involved in a conversation space may
 wish to provide URIs for conference status and/or conference/floor
 The SIP Events architecture [RFC3265] defines general mechanisms for
 subscription to and notification of events within SIP networks.  It
 introduces the notion of a package that is a specific "instantiation"
 of the events mechanism for a well-defined set of events.
 Event packages are needed to provide the status of a user's dialogs,
 the status of conferences and their participants, user-presence
 information, the status of registrations, and the status of a user's
 messages.  While this is not an exhaustive list, these are sufficient
 to enable the sample features described in this document.
 The conference event package [RFC4575] allows users to subscribe to
 information about an entire tightly coupled SIP conference.
 Notifications convey information about the participants such as the
 SIP URI identifying each user, their status in the space (active,
 declined, departed), URIs to invoke other features (such as sidebar

Mahy, et al. Informational [Page 13] RFC 5850 SIP Call Control Framework May 2010

 conversations), links to other relevant information (such as floor-
 control policies), and if floor-control policies are in place, the
 user's floor-control status.  For conversation spaces created from
 cascaded conferences, conversation state can be gathered from
 relevant foci and merged into a cohesive set of state.
 The dialog package [RFC4235] provides information about all the
 dialogs the target user is maintaining, in which conversations the
 user is participating, and how these are correlated.  Likewise, the
 registration package [RFC3680] provides notifications when contacts
 have changed for a specific address-of-record (AOR).  The combination
 of these allows a user agent to learn about all conversations
 occurring for the entire registered contact set for an address-of-
 Note that user presence in SIP [RFC3856] has a close relationship
 with these latter two event packages.  It is fundamental to the
 presence model that the information used to obtain user presence is
 constructed from any number of different input sources.  Examples of
 other such sources include calendaring information and uploads of
 presence documents.  These two packages can be considered another
 mechanism that allows a presence agent to determine the presence
 state of the user.  Specifically, a user presence server can act as a
 subscriber for the dialog and registration packages to obtain
 additional information that can be used to construct a presence
 The multi-party architecture may also need to provide a mechanism to
 get information about the status/handling of a dialog (for example,
 information about the history of other contacts attempted prior to
 the current contact).  Finally, the architecture should provide ample
 opportunities to present informational URIs that relate to calls,
 conversations, or dialogs in some way.  For example, consider the SIP
 Call-Info header or Contact header fields returned in a 300-class
 response.  Frequently, additional information about a call or dialog
 can be fetched via non-SIP URIs.  For example, consider a web page
 for package tracking when calling a delivery company or a web page
 with related documentation when joining a dial-in conference.  The
 use of URIs in the multi-party framework is discussed in more detail
 in Section 3.7.
 Finally, the interaction of SIP with stimulus-signaling-based
 applications, which allow a user agent to interact with an
 application without knowledge of the semantics of that application,
 is discussed in the SIP application interaction framework [RFC5629].
 Stimulus signaling can occur with a user interface running locally
 with the client, or with a remote user interface, through media
 streams.  Stimulus signaling encompasses a wide range of mechanisms,

Mahy, et al. Informational [Page 14] RFC 5850 SIP Call Control Framework May 2010

 from clicking on hyperlinks, to pressing buttons, to traditional
 Dual-Tone Multi Frequency (DTMF) input.  In all cases, stimulus
 signaling is supported through the use of markup languages, which
 play a key role in that framework.

2.6. Componentization and Decomposition

 This framework proposes a decomposed component architecture with a
 very loose coupling of services and components.  This means that a
 service (such as a conferencing server or an auto-attendant) need not
 be implemented as an actual server.  Rather, these services can be
 built by combining a few basic components in straightforward or
 arbitrarily complex ways.
 Since the components are easily deployed on separate boxes, by
 separate vendors, or even with separate providers, we achieve a
 separation of function that allows each piece to be developed in
 complete isolation.  We can also reuse existing components for new
 applications.  This allows rapid service creation, and the ability
 for services to be distributed across organizational domains anywhere
 in the Internet.
 For many of these components, it is also desirable to discover their
 capabilities, for example, querying the ability of a mixer to host a
 10-dialog conference or to reserve resources for a specific time.
 These actions could be provided in the form of URIs, provided there
 is an a priori means of understanding their semantics.  For example,
 if there is a published dictionary of operations, a way to query the
 service for the available operations and the associated URIs, the URI
 can be the interface for providing these service operations.  This
 concept is described in more detail in the context of dialog
 operations in Section 3.

2.6.1. Media Intermediaries

 Media intermediaries are not participants in any conversation space,
 although an entity that is also a media translator may also have a
 co-located participant component (for example, a mixer that also
 announces the arrival of a new participant; the announcement portion
 is a participant, but the mixer itself is not).  Media intermediaries
 should be as transparent as possible to the end users -- offering a
 useful, fundamental service without getting in the way of new
 features implemented by participants.  Some common media
 intermediaries are described below.

Mahy, et al. Informational [Page 15] RFC 5850 SIP Call Control Framework May 2010 Mixer

 A SIP mixer is a component that combines media from all dialogs in
 the same conversation in a media-specific way.  For example, the
 default combining for an audio conference might be an N-1
 configuration, while a text mixer might interleave text messages on a
 per-line basis.  More details about how to manipulate the media
 policy used by mixers is discussed in [XCON-CCMP]. Transcoder

 A transcoder translates media from one encoding or format to another
 (for example, GSM (Global System for Mobile communications) voice to
 G.711, MPEG2 to H.261, or text/html to text/plain), or from one media
 type to another (for example, text to speech).  A more thorough
 discussion of transcoding is described in the SIP transcoding
 services invocation [RFC5369]. Media Relay

 A media relay terminates media and simply forwards it to a new
 destination without changing the content in any way.  Sometimes,
 media relays are used to provide source IP address anonymity, to
 facilitate middlebox traversal, or to provide a trusted entity where
 media can be forcefully disconnected. Queue Server

 A queue server is a location where calls can be entered into one of
 several FIFO (first-in, first-out) queues.  A queue server would
 subscribe to the presence of groups or individuals who are interested
 in its queues.  When detecting that a user is available to service a
 queue, the server redirects or transfers the last call in the
 relevant queue to the available user.  On a queue-by-queue basis,
 authorized users could also subscribe to the call state (dialog
 information) of calls within a queue.  Authorized users could use
 this information to effectively pluck (take) a call out of the queue
 (for example, by sending an INVITE with a Replaces header to one of
 the user agents in the queue). Parking Place

 A parking place is a location where calls can be terminated
 temporarily and then retrieved later.  While a call is "parked", it
 can receive media "on hold" such as music, announcements, or
 advertisements.  Such a service could be further decomposed such that
 announcements or music are handled by a separate component.

Mahy, et al. Informational [Page 16] RFC 5850 SIP Call Control Framework May 2010 Announcements and Voice Dialogs

 An announcement server is a server that can play digitized media
 (frequently audio), such as music or recorded speech.  These servers
 are typically accessible via SIP, HTTP (Hyper Text Transport
 Protocol), or RTSP (Real-Time Streaming Protocol).  An analogous
 service is a recording service that stores digitized media.  A
 convention for specifying announcements in SIP URIs is described in
 [RFC4240].  Likewise, the same server could easily provide a service
 that records digitized media.
 A "voice dialog" is a model of spoken interactive behavior between a
 human and an automaton that can include synthesized speech, digitized
 audio, recognition of spoken and DTMF key input, a recording of
 spoken input, and interaction with call control.  Voice dialogs
 frequently consist of forms or menus.  Forms present information and
 gather input; menus offer choices of what to do next.
 Spoken dialogs are a basic building block of applications that use
 voice.  Consider, for example, that a voicemail system, the
 conference-id and passcode collection system for a conferencing
 system, and complicated voice-portal applications all require a
 voice-dialog component.

2.6.2. Text-to-Speech and Automatic Speech Recognition

 Text-to-speech (TTS) is a service that converts text into digitized
 audio.  TTS is frequently integrated into other applications, but
 when separated as a component, it provides greater opportunity for
 broad reuse.  Automatic Speech Recognition (ASR) is a service that
 attempts to decipher digitized speech based on a proposed grammar.
 Like TTS, ASR services can be embedded, or exposed so that many
 applications can take advantage of such services.  A standardized
 (decomposed) interface to access standalone TTS and ASR services is
 currently being developed as described in [RFC4313].

2.6.3. VoiceXML

 VoiceXML is a W3C (World Wide Web Consortium) recommendation that was
 designed to give authors control over the spoken dialog between users
 and applications.  The application and user take turns speaking: the
 application prompts the user, and the user in turn responds.  Its
 major goal is to bring the advantages of web-based development and
 content delivery to interactive voice-response applications.  We
 believe that VoiceXML represents the ideal partner for SIP in the
 development of distributed IVR (interactive voice response) servers.
 VoiceXML is an XML-based scripting language for describing IVR
 services at an abstract level.  VoiceXML supports DTMF recognition,

Mahy, et al. Informational [Page 17] RFC 5850 SIP Call Control Framework May 2010

 speech recognition, text-to-speech, and the playing out of recorded
 media files.  The results of the data collected from the user are
 passed to a controlling entity through an HTTP POST operation.  The
 controller can then return another script, or terminate the
 interaction with the IVR server.
 A VoiceXML server also need not be implemented as a monolithic
 server.  Figure 4 shows a diagram of a VoiceXML browser that is split
 into media and non-media handling parts.  The VoiceXML interpreter
 handles SIP dialog state and state within a VoiceXML document, and
 sends requests to the media component over another protocol.
                     |             |
                     | VoiceXML    |
                     | Interpreter |
                     | (signaling) |
                       ^          ^
                       |          |
                   SIP |          | RTSP
                       |          |
                       |          |
                       v          v
          +-------------+        +-------------+
          |             |        |             |
          |  SIP UA     |   RTP  | RTSP Server |
          |             |<------>|   (media)   |
          |             |        |             |
          +-------------+        +-------------+
 Figure 4.  Decomposed VoiceXML Server

2.7. Use of URIs

 All naming in SIP uses URIs.  URIs in SIP are used in a plethora of
 contexts: the Request-URI; Contact, To, From, and *-Info header
 fields; application/uri bodies; and embedded in email, web pages,
 instant messages, and ENUM records.  The Request-URI identifies the
 user or service for which the call is destined.
 SIP URIs embedded in informational SIP header fields, SIP bodies, and
 non-SIP content can also specify methods, special parameters, header
 fields, and even bodies.  For example:;method=REFER?Refer-To=

Mahy, et al. Informational [Page 18] RFC 5850 SIP Call Control Framework May 2010

 Throughout this document, we discuss call control primitive
 operations.  One of the biggest problems is defining how these
 operations may be invoked.  There are a number of ways to do this.
 One way is to define the primitives in the protocol itself such that
 SIP methods (for example, REFER) or SIP header fields (for example,
 Replaces) indicate a specific call control action.  Another way to
 invoke call control primitives is to define a specific Request-URI
 naming convention.  Either these conventions must be shared between
 the client (the invoker) and the server, or published by or on behalf
 of the server.  The former involves defining URI construction
 techniques (e.g., URI parameters and/or token conventions) as
 proposed in [RFC4240].  The latter technique usually involves
 discovering the URI via a SIP event package, a web page, a business
 card, or an instant message.  Yet, another means to acquire the URIs
 is to define a dictionary of primitives with well-defined semantics
 and provide a means to query the named primitives and corresponding
 URIs that may be invoked on the service or dialogs.

2.7.1. Naming Users in SIP

 An address-of-record, or public SIP address, is a SIP (or Secure SIP
 (SIPS)) URI that points to a domain with a location service that can
 map the URI to set of Contact URIs where the user might be available.
 Typically, the Contact URIs are populated via registration.
 Address-of-Record               Contacts ->
 Callee Capabilities [RFC3840] define a set of additional parameters
 to the Contact header field that define the characteristics of the
 user agent at the specified URI.  For example, there is a mobility
 parameter that indicates whether the UA is fixed or mobile.  When a
 user agent registers, it places these parameters in the Contact
 header fields to characterize the URIs it is registering.  This
 allows a proxy for that domain to have information about the contact
 addresses for that user.
 When a caller sends a request, it can optionally request Caller
 Preferences [RFC3841] by including the Accept-Contact, Request-
 Disposition, and Reject-Contact header fields that request certain
 handling by the proxy in the target domain.  These header fields
 contain preferences that describe the set of desired URIs to which
 the caller would like their request routed.  The proxy in the target
 domain matches these preferences with the Contact characteristics
 originally registered by the target user.  The target user can also

Mahy, et al. Informational [Page 19] RFC 5850 SIP Call Control Framework May 2010

 choose to run arbitrarily complex "Find-me" feature logic on a proxy
 in the target domain.
 There is a strong asymmetry in how preferences for callers and
 callees can be presented to the network.  While a caller takes an
 active role by initiating the request, the callee takes a passive
 role in waiting for requests.  This motivates the use of callee-
 supplied scripts and caller preferences included in the call request.
 This asymmetry is also reflected in the appropriate relationship
 between caller and callee preferences.  A server for a callee should
 respect the wishes of the caller to avoid certain locations, while
 the preferences among locations has to be the callee's choice, as it
 determines where, for example, the phone rings and whether the callee
 incurs mobile telephone charges for incoming calls.
 SIP User Agent implementations are encouraged to make intelligent
 decisions based on the type of participants (active/passive, hidden,
 human/robot) in a conversation space.  This information is conveyed
 via the dialog package or in a SIP header field parameter
 communicated using an appropriate SIP header field.  For example, a
 music on hold service may take the sensible approach that if there
 are two or more unhidden participants, it should not provide hold
 music; or that it will not send hold music to robots.
 Multiple participants in the same conversation space may represent
 the same human user.  For example, the user may use one participant
 device for video, chat, and whiteboard media on a PC and another for
 audio media on a SIP phone.  In this case, the address-of-record is
 the same for both user agents, but the Contacts are different.  In
 this case, there is really only one human participant.  In addition,
 human users may add robot participants that act on their behalf (for
 example, a call recording service or a calendar announcement
 reminder).  Call control features in SIP should continue to function
 as expected in such an environment.

2.7.2. Naming Services with SIP URIs

 A critical piece of defining a session-level service that can be
 accessed by SIP is defining the naming of the resources within that
 service.  This point cannot be overstated.
 In the context of SIP control of application components, we take
 advantage of the fact that the left-hand side of a standard SIP URI
 is a user part.  Most services may be thought of as user automatons
 that participate in SIP sessions.  It naturally follows that the user
 part should be utilized as a service indicator.

Mahy, et al. Informational [Page 20] RFC 5850 SIP Call Control Framework May 2010

 For example, media servers commonly offer multiple services at a
 single host address.  Use of the user part as a service indicator
 enables service consumers to direct their requests without ambiguity.
 It has the added benefit of enabling media services to register their
 availability with SIP Registrars just as any "real" SIP user would.
 This maintains consistency and provides enhanced flexibility in the
 deployment of media services in the network.
 There has been much discussion about the potential for confusion if
 media-service URIs are not readily distinguishable from other types
 of SIP UAs.  The use of a service namespace provides a mechanism to
 unambiguously identify standard interfaces while not constraining the
 development of private or experimental services.
 In SIP, the Request-URI identifies the user or service for which the
 call is destined.  The great advantage of using URIs (specifically,
 the SIP Request-URI) as a service identifier comes because of the
 combination of two facts.  First, unlike in the PSTN (Public Switched
 Telephone Network), where the namespace (dialable telephone numbers)
 is limited, URIs come from an infinite space.  They are plentiful,
 and they are free.  Secondly, the primary function of SIP is call
 routing through manipulations of the Request-URI.  In the traditional
 SIP application, this URI represents a person.  However, the URI can
 also represent a service, as we propose here.  This means we can
 apply the routing services SIP provides to the routing of calls to
 services.  The result -- the problem of service invocation and
 service location becomes a routing problem, for which SIP provides a
 scalable and flexible solution.  Since there is such a vast namespace
 of services, we can explicitly name each service in a finely granular
 way.  This allows the distribution of services across the network.
 For further discussion about services and SIP URIs, see RFC 3087
 Consider a conferencing service, where we have separated the names of
 ad hoc conferences from scheduled conferences, we can program proxies
 to route calls for ad hoc conferences to one set of servers and calls
 for scheduled ones to another, possibly even in a different provider.
 In fact, since each conference itself is given a URI, we can
 distribute conferences across servers, and easily guarantee that
 calls for the same conference always get routed to the same server.
 This is in stark contrast to conferences in the telephone network,
 where the equivalent of the URI -- the phone number -- is scarce.  An
 entire conferencing provider generally has one or two numbers.
 Conference IDs must be obtained through IVR interactions with the
 caller or through a human attendant.  This makes it difficult to
 distribute conferences across servers all over the network, since the
 PSTN routing only knows about the dialed number.

Mahy, et al. Informational [Page 21] RFC 5850 SIP Call Control Framework May 2010

 For more examples, consider the URI conventions of RFC 4240 [RFC4240]
 for media servers and RFC 4458 [RFC4458] for voicemail and IVR
 In practical applications, it is important that an invoker does not
 necessarily apply semantic rules to various URIs it did not create.
 Instead, it should allow any arbitrary string to be provisioned, and
 map the string to the desired behavior.  The administrator of a
 service may choose to provision specific conventions or mnemonic
 strings, but the application should not require it.  In any large
 installation, the system owner is likely to have preexisting rules
 for mnemonic URIs, and any attempt by an application to define its
 own rules may create a conflict.  Implementations should allow an
 arbitrary mix of URIs from these schemes, or any other scheme that
 renders valid SIP URIs, rather than enforce only one particular
 As we have shown, SIP URIs represent an ideal, flexible mechanism for
 describing and naming service resources, regardless of whether the
 resources are queues, conferences, voice dialogs, announcements,
 voicemail treatments, or phone features.

2.8. Invoker Independence

 With functional signaling, only the invoker of features in SIP needs
 to know exactly which feature they are invoking.  One of the primary
 benefits of this approach is that combinations of functional features
 work in SIP call control without requiring complex feature-
 interaction matrices.  For example, let us examine the combination of
 a "transfer" of a call that is "conferenced".
 Alice calls Bob.  Alice silently "conferences in" her robotic
 assistant Albert as a hidden party.  Bob transfers Alice to Carol.
 If Bob asks Alice to Replace her leg with a new one to Carol, then
 both Alice and Albert should be communicating with Carol
 Using the peer-to-peer model, this combination of features works fine
 if A is doing local mixing (Alice replaces Bob's dialog with
 Carol's), or if A is using a central mixer (the mixer replaces Bob's
 dialog with Carol's).  A clever implementation using the 3pcc model
 can generate similar results.
 New extensions to the SIP Call Control Framework should attempt to
 preserve this property.

Mahy, et al. Informational [Page 22] RFC 5850 SIP Call Control Framework May 2010

2.9. Billing Issues

 Billing in the PSTN is typically based on who initiated a call.  At
 the moment, billing in a SIP network is neither consistent with
 itself nor with the PSTN.  (A billing model for SIP should allow for
 both PSTN-style billing and non-PSTN billing.)  The example below
 demonstrates one such inconsistency.
 Alice places a call to Bob.  Alice then blind transfers Bob to Carol
 through a PSTN gateway.  In current usage of REFER, Bob may be billed
 for a call he did not initiate (his UA originated the outgoing
 dialog, however).  This is not necessarily a terrible thing, but it
 demonstrates a security concern (Bob must have appropriate local
 policy to prevent fraud).  Also, Alice may wish to pay for Bob's
 session with Carol.  There should be a way to signal this in SIP.
 Likewise, a Replacement call may maintain the same billing
 relationship as a Replaced call, so if Alice first calls Carol, then
 asks Bob to Replace this call, Alice may continue to receive a bill.
 Further work in SIP billing should define a way to set or discover
 the direction of billing.

3. Catalog of Call Control Actions and Sample Features

 Call control actions can be categorized by the dialogs upon which
 they operate.  The actions may involve a single or multiple dialogs.
 These dialogs can be early or established.  Multiple dialogs may be
 related in a conversation space to form a conference or other
 interesting media topologies.
 It should be noted that it is desirable to provide a means by which a
 party can discover the actions that may be performed on a dialog.
 The interested party may be independent or related to the dialogs.
 One means of accomplishing this is through the ability to define and
 obtain URIs for these actions, as described in Section 2.7.2.
 Below are listed several call control "actions" that establish or
 modify dialogs and relate the participants in a conversation space.
 The names of the actions listed are for descriptive purposes only
 (they are not normative).  This list of actions is not meant to be
 In the examples, all actions are initiated by the user "Alice"
 represented by UA "A".

Mahy, et al. Informational [Page 23] RFC 5850 SIP Call Control Framework May 2010

3.1. Remote Call Control Actions on Early Dialogs

 The following are a set of actions that may be performed on a single
 early dialog.  These actions can be thought of as a set of remote
 control operations.  For example, an automaton might perform the
 operation on behalf of a user.  Alternatively, a user might use the
 remote control in the form of an application to perform the action on
 the early dialog of a UA that may be out of reach.  All of these
 actions correspond to telling the UA how to respond to a request to
 establish an early dialog.  These actions provide useful
 functionality for PDA-, PC-, and server-based applications that
 desire the ability to control a UA.  A proposed mechanism for this
 type of functionality is described in remote call control

3.1.1. Remote Answer

 A dialog is in some early dialog state such as 180 Ringing.  It may
 be desirable to tell the UA to answer the dialog.  That is, tell it
 to send a 200 OK response to establish the dialog.

3.1.2. Remote Forward or Put

 It may be desirable to tell the UA to respond with a 3xx class
 response to forward an early dialog to another UA.

3.1.3. Remote Busy or Error Out

 It may be desirable to instruct the UA to send an error response such
 as 486 Busy Here.

3.2. Remote Call Control Actions on Single Dialogs

 There is another useful set of actions that operate on a single
 established dialog.  These operations are useful in building
 productivity applications for aiding users in controlling their
 phones.  For example, a Customer Relationship Management (CRM)
 application that sets up calls for a user eliminating the need for
 the user to actually enter an address.  These operations can also be
 thought of as remote control actions.  A proposed mechanism for this
 type of functionality is described in remote call control

3.2.1. Remote Dial

 This action instructs the UA to initiate a dialog.  This action can
 be performed using the REFER method.

Mahy, et al. Informational [Page 24] RFC 5850 SIP Call Control Framework May 2010

3.2.2. Remote On and Off Hold

 This action instructs the UA to put an established dialog on hold.
 Though this operation can conceptually be performed with the REFER
 method, there are no semantics defined as to what the referred party
 should do with the SDP.  There is no way to distinguish between the
 desire to go on or off hold on a per-media stream basis.

3.2.3. Remote Hangup

 This action instructs the UA to terminate an early or established
 dialog.  A REFER request with the following Refer-To URI and Target-
 Dialog header field [RFC4538] performs this action.  Note: this
 example does not show the full set of header fields.
 Target-Dialog: 13413098;local-tag=879738;remote-tag=023214

3.3. Call Control Actions on Multiple Dialogs

 These actions apply to a set of related dialogs.

3.3.1. Transfer

 This section describes how call transfer can be achieved using
 centralized (3pcc) and peer-to-peer (REFER) approaches.
 The conversation space changes as follows:
  before            after
 { A , B }  -->   { C , B }
 A replaces itself with C.
 To make this happen using the peer-to-peer approach, "A" would send
 two SIP requests.  A shorthand for those requests is shown below:
 REFER B  Refer-To:C
 To make this happen using the 3pcc approach instead, the controller
 sends requests represented by the shorthand below:
 INVITE C (w/SDP of B)
 reINVITE B (w/SDP of C)

Mahy, et al. Informational [Page 25] RFC 5850 SIP Call Control Framework May 2010

 Features enabled by this action:
  1. blind transfer
  2. transfer to a central mixer (some type of conference or forking)
  3. transfer to park server (park)
  4. transfer to music on hold or announcement server
  5. transfer to a "queue"
  6. transfer to a service (such as voice-dialog service)
  7. transition from local mixer to central mixer
 This action is frequently referred to as "completing an attended
 transfer".  It is described in more detail in [RFC5589].
 Note that if a transfer requires URI hiding or privacy, then the 3pcc
 approach can more easily implement this.  For example, if the URI of
 C needs to be hidden from B, then the use of 3pcc helps accomplish

3.3.2. Take

 The conversation space changes as follows:
 { B , C } --> { B , A }
 A forcibly replaces C with itself.  In most uses of this primitive, A
 is just "un-replacing" itself.
 Using the peer-to-peer approach, "A" sends:
  INVITE B  Replaces: <dialog between B and C>
 Using the 3pcc approach (all requests sent from controller):
  INVITE A (w/SDP of B)
  reINVITE B (w/SDP of A)
 Features enabled by this action:
  1. transferee completes an attended transfer
  2. retrieve from central mixer (not recommended)
  3. retrieve from music on hold or park
  4. retrieve from queue
  5. call center take
  6. voice portal resuming ownership of a call it originated
  7. answering-machine style screening (pickup)
  8. pickup of a ringing call (i.e., early dialog)

Mahy, et al. Informational [Page 26] RFC 5850 SIP Call Control Framework May 2010

 Note that pick up of a ringing call has perhaps some interesting
 additional requirements.  First of all, it is an early dialog as
 opposed to an established dialog.  Secondly, the party that is to
 pick up the call may only wish to do so only while it is an early
 dialog.  That is in the race condition where the ringing UA accepts
 just before it receives signaling from the party wishing to take the
 call, the taking party wishes to yield or cancel the take.  The goal
 is to avoid yanking an answered call from the called party.
 This action is described in Replaces [RFC3891] and in [RFC5589].

3.3.3. Add

 Note that the following four actions are described in [RFC4579].
 This is merely adding a participant to a SIP conference.  The
 conversation space changes as follows:
 { A , B } --> { A , B , C }
 A adds C to the conversation.
 Using the peer-to-peer approach, adding a party using local mixing
 requires no signaling.  To transition from a two-party call or a
 locally mixed conference to central mixing, A could send the
 following requests:
  REFER B  Refer-To: conference-URI
  INVITE conference-URI
 To add a party to a conference:
  REFER C  Refer-To: conference-URI
  REFER conference-URI  Refer-To: C
 Using the 3pcc approach to transition to centrally mixed, the
 controller would send:
  INVITE mixer leg 1 (w/SDP of A)
  INVITE mixer leg 2 (w/SDP of B)
  INVITE C (late SDP)
  reINVITE A (w/SDP of mixer leg 1)
  reINVITE B (w/SDP of mixer leg 2)
  INVITE mixer leg3 (w/SDP of C)

Mahy, et al. Informational [Page 27] RFC 5850 SIP Call Control Framework May 2010

 To add a party to a SIP conference:
  INVITE C (late SDP)
  INVITE conference-URI (w/SDP of C)
 Features enabled:
  1. standard conference feature
  2. call recording
  3. answering-machine style screening (screening)

3.3.4. Local Join

 The conversation space changes like this:
 { A , B } , { A , C }  -->  { A , B , C }
         or like this
 { A , B } , { C , D }  -->  { A , B , C , D }
 A takes two conversation spaces and joins them together into a single
 Using the peer-to-peer approach, A can mix locally, or REFER the
 participants of both conversation spaces to the same central mixer
 (as in Section 3.3.5).
 For the 3pcc approach, the call flows for inserting participants, and
 joining and splitting conversation spaces are tedious yet
 straightforward, so these are left as an exercise for the reader.
 Features enabled:
  1. standard conference feature
  2. leaving a sidebar to rejoin a larger conference

3.3.5. Insert

 The conversation space changes like this:
 { B , C } --> { A , B , C }
 A inserts itself into a conversation space.
 A proposed mechanism for signaling this using the peer-to-peer
 approach is to send a new header field in an INVITE with "joining"
 [RFC3911] semantics.  For example:

Mahy, et al. Informational [Page 28] RFC 5850 SIP Call Control Framework May 2010

 INVITE B Join: <dialog id of B and C>
 If B accepted the INVITE, B would accept responsibility to set up the
 dialogs and mixing necessary (for example, to mix locally or to
 transfer the participants to a central mixer).
 Features enabled:
  1. barge-in
  2. call center monitoring
  3. call recording

3.3.6. Split

 { A , B , C , D } --> { A , B } , { C , D }
 If using a central conference with peer-to-peer
  REFER C  Refer-To: conference-URI (new URI)
  REFER D  Refer-To: conference-URI (new URI)
 Features enabled:
  1. sidebar conversations during a larger conference

3.3.7. Near-Fork

 A participates in two conversation spaces simultaneously:
 { A, B } --> { B , A } & { A , C }
 A is a participant in two conversation spaces such that A sends the
 same media to both spaces, and renders media from both spaces,
 presumably by mixing or rendering the media from both.  We can define
 that A is the "anchor" point for both forks, each of which is a
 separate conversation space.
 This action is purely local implementation (it requires no special
 signaling).  Local features such as switching calls between the
 background and foreground are possible using this media relationship.

3.3.8. Far-Fork

 The conversation space diagram.
 { A, B } --> { A , B } & { B , C }

Mahy, et al. Informational [Page 29] RFC 5850 SIP Call Control Framework May 2010

 A requests B to be the "anchor" of two conversation spaces.
 This is easily set up by creating a conference with two sub-
 conferences and setting the media policy appropriately such that B is
 a participant in both.  Media forking can also be set up using 3pcc,
 as described in Section 5.1 of RFC 3264 [RFC3264] (an offer/answer
 model for SDP).  The session descriptions for forking are quite
 complex.  Controllers should verify that endpoints can handle forked
 media, for example, using prior configuration.
 Features enabled:
  1. barge-in
  2. voice-portal services
  3. whisper
  4. key word detection
  5. sending DTMF somewhere else

4. Security Considerations

 Call control primitives provide a powerful set of features that can
 be dangerous in the hands of an attacker.  To complicate matters,
 call control primitives are likely to be automatically authorized
 without direct human oversight.
 The class of attacks that are possible using these tools includes the
 ability to eavesdrop on calls, disconnect calls, redirect calls,
 render irritating content (including ringing) at a user agent, cause
 an action that has billing consequences, subvert billing (theft-of-
 service), and obtain private information.  Call control extensions
 must take extra care to describe how these attacks will be prevented.
 We can also make some general observations about authorization and
 trust with respect to call control.  The security model is
 dramatically dependent on the signaling model chosen (see Section
 Let us first examine the security model used in the 3pcc approach.
 All signaling goes through the controller, which is a trusted entity.
 Traditional SIP authentication and hop-by-hop encryption and message
 integrity work fine in this environment, but end-to-end encryption
 and message integrity may not be possible.
 When using the peer-to-peer approach, call control actions and
 primitives can be legitimately initiated by a) an existing
 participant in the conversation space, b) a former participant in the
 conversation space, or c) an entity trusted by one of the
 participants.  For example, a participant always initiates a

Mahy, et al. Informational [Page 30] RFC 5850 SIP Call Control Framework May 2010

 transfer; a retrieve from park (a take) is initiated on behalf of a
 former participant, and a barge-in (insert or far-fork) is initiated
 by a trusted entity (an operator, for example).
 Authenticating requests by an existing participant or a trusted
 entity can be done with baseline SIP mechanisms.  In the case of
 features initiated by a former participant, these should be protected
 against replay attacks, e.g., by using a unique name or identifier
 per invocation.  The Replaces header field exhibits this behavior as
 a by-product of its operation (once a Replaces operation is
 successful, the dialog being Replaced no longer exists).  These
 credentials may, for example, need to be passed transitively or
 fetched in an event body.
 To authorize call control primitives that trigger special behavior
 (such as an INVITE with Replaces or Join semantics), the receiving
 user agent may have trouble finding appropriate credentials with
 which to challenge or authorize the request, as the sender may be
 completely unknown to the receiver, except through the introduction
 of a third party.  These credentials need to be passed transitively
 in some way or fetched in an event body, for example.
 Standard SIP privacy and anonymity mechanisms such as [RFC3323] and
 [RFC3325] used during SIP session establishment apply equally well to
 SIP call control operations.  SIP call control mechanisms should
 address privacy and anonymity issues associated with that operation.
 For example, privacy during a transfer operation using REFER is
 discussed in Section 7.2 of [RFC5589]

Mahy, et al. Informational [Page 31] RFC 5850 SIP Call Control Framework May 2010

Appendix A. Example Features

 Primitives are defined in terms of their ability to provide features.
 These example features should require an amply robust set of services
 to demonstrate a useful set of primitives.  They are described here
 briefly.  Note that the descriptions of these features are non-
 normative.  Note also that this document describes a mixture of both
 features originating in the world of telephones and features that are
 clearly Internet oriented.

Appendix A.1. Attended Transfer

 In Attended Transfer [RFC5589], the transferring party establishes a
 session with the transfer target before completing the transfer.

Appendix A.2. Auto Answer

 In Auto Answer, calls to a certain address or URI answer immediately
 via a speakerphone.  The Answer-Mode header field [RFC5373] can be
 used for this feature.

Appendix A.3. Automatic Callback

 In Automatic Callback [RFC5359], Alice calls Bob, but Bob is busy.
 Alice would like Bob to call her automatically when he is available.
 When Bob hangs up, Alice's phone rings.  When Alice answers, Bob's
 phone rings.  Bob answers and they talk.

Appendix A.4. Barge-In

 In Barge-in, Carol interrupts Alice who has an in-progress call with
 Bob.  In some variations, Alice forcibly joins a new conversation
 with Carol, in other variations, all three parties are placed in the
 same conversation (basically a three-way conference).  Barge-in works
 the same as call monitoring except that it must indicate that the
 send media stream be mixed so that all of the other parties can hear
 the stream from the UA that is barging in.

Appendix A.5. Blind Transfer

 In Blind Transfer [RFC5589], Alice is in a conversation with Bob.
 Alice asks Bob to contact Carol, but makes no attempt to contact
 Carol independently.  In many implementations, Alice does not verify
 Bob's success or failure in contacting Carol.

Mahy, et al. Informational [Page 32] RFC 5850 SIP Call Control Framework May 2010

Appendix A.6. Call Forwarding

 In call forwarding [RFC5359], before a dialog is accepted, it is
 redirected to another location, for example, because the originally
 intended recipient is busy, does not answer, is disconnected from the
 network, or has configured all requests to go elsewhere.

Appendix A.7. Call Monitoring

 Call monitoring is a Join operation [RFC3911].  For example, a call
 center supervisor joins an in-progress call for monitoring purposes.
 The monitoring UA sends a Join to the dialog to which it wants to
 listen.  It is able to discover the dialog via the dialog state on
 the monitored UA.  The monitoring UA sends SDP in the INVITE that
 indicates receive-only media.  As the UA is only monitoring, it does
 not matter whether the UA indicates it wishes the send stream to be
 mixed or point to point.

Appendix A.8. Call Park

 In Call Park [RFC5359], a participant parks a call (essentially puts
 the call on hold), and then retrieves it at a later time (typically
 from another location).  Call park requires the ability to put a
 dialog some place, advertise it to users in a pickup group, and to
 uniquely identify it in a means that can be communicated (including
 human voice).  The dialog can be held locally on the UA parking the
 dialog or alternatively transferred to the park service for the
 pickup group.  The parked dialog then needs to be labeled (e.g.,
 orbit 12) in a way that can be communicated to the party that is to
 pick up the call.  The UAs in the pickup group discover the parked
 dialog(s) via the dialog package from the park service.  If the
 dialog is parked locally, the park service merely aggregates the
 parked call states from the set of UAs in the pickup group.

Appendix A.9. Call Pickup

 There are two different features that are called Call Pickup
 [RFC5359].  The first is the pickup of a parked dialog.  The UA from
 which the dialog is to be picked up subscribes to the dialog state of
 the park service or the UA that has locally parked the dialog.
 Dialogs that are parked should be labeled with an identifier.  The
 labels are used by the UA to allow the user to indicate which dialog
 is to be picked up.  The UA picking up the call invoked the URI in
 the call state that is labeled as replace-remote.
 The other call pickup feature involves picking up an early dialog
 (typically ringing).  A party picks up a call that was ringing at
 another location.  One variation allows the caller to choose which

Mahy, et al. Informational [Page 33] RFC 5850 SIP Call Control Framework May 2010

 location, another variation just picks up any call in that user's
 "pickup group".  This feature uses some of the same primitives as the
 pickup of a parked call.  The call state of the UA ringing phone is
 advertised using the dialog package.  The UA that is to pick up the
 early dialog subscribes either directly to the ringing UA or to a
 service aggregating the states for UAs in the pickup group.  The call
 state identifies early dialogs.  The UA uses the call state(s) to
 help the user choose which early dialog is to be picked up.  The UA
 then invokes the URI in the call state labeled as replace-remote.

Appendix A.10. Call Return

 In Call Return, Alice calls Bob.  Bob misses the call or is
 disconnected before he is finished talking to Alice.  Bob invokes
 Call return, which calls Alice, even if Alice did not provide her
 real identity or location to Bob.

Appendix A.11. Call Waiting

 In Call Waiting, Alice is in a call, then receives another call.
 Alice can place the first call on hold, and talk with the other
 caller.  She can typically switch back and forth between the callers.

Appendix A.12. Click-to-Dial

 In Click-to-Dial [RFC5359], Alice looks in her company directory for
 Bob.  When she finds Bob, she clicks on a URI to call him.  Her phone
 rings (or possibly answers automatically), and when she answers,
 Bob's phone rings.  The application or server that hosts the Click-
 to-Dial application captures the URI to be dialed and can set up the
 call using 3pcc or can send a REFER request to the UA that is to dial
 the address.  As users sometimes change their mind or wish to give up
 listing to a ringing or voicemail answered phone, this application
 illustrates the need to also have the ability to remotely hangup a

Appendix A.13. Conference Call

 In a Conference Call [RFC4579], there are three or more active,
 visible participants in the same conversation space.

Appendix A.14. Consultative Transfer

 In Consultative Transfer [RFC5589], the transferring party
 establishes a session with the target and mixes both sessions
 together so that all three parties can participate, then disconnects
 leaving the transferee and transfer target with an active session.

Mahy, et al. Informational [Page 34] RFC 5850 SIP Call Control Framework May 2010

Appendix A.15. Distinctive Ring

 In Distinctive Ring, incoming calls have different ring cadences or
 sample sounds depending on the From party, the To party, or other
 factors.  The target UA either makes a local decision based on
 information in an incoming INVITE (To, From, Contact, Request-URI) or
 trusts an Alert-Info header field [RFC3261] provided by the caller or
 inserted by a trusted proxy.  In the latter case, the UA fetches the
 content described in the URI (typically via HTTP) and renders it to
 the user.

Appendix A.16. Do Not Disturb

 In Do Not Disturb, Alice selects the Do Not Disturb option.  Calls to
 her either ring briefly or not at all and are forwarded elsewhere.
 Some variations allow specially authorized callers to override this
 feature and ring Alice anyway.  Do Not Disturb is best implemented in
 SIP using presence [RFC3856].

Appendix A.17. Find-Me

 In Find-Me, Alice sets up complicated rules for how she can be
 reached (possibly using CPL (Call Processing Language) [RFC3880],
 presence [RFC3856], or other factors).  When Bob calls Alice, his
 call is eventually routed to a temporary Contact where Alice happens
 to be available.

Appendix A.18. Hotline

 In Hotline, Alice picks up a phone and is immediately connected to
 the technical support hotline, for example.  Hotline is also
 sometimes known as a Ringdown line.

Appendix A.19. IM Conference Alerts

 In IM Conference Alerts, a user receives a notification as an instant
 message whenever someone joins a conference in which they are already
 a participant.

Appendix A.20. Inbound Call Screening

 In Inbound Call Screening, Alice doesn't want to receive calls from
 Matt.  Inbound Screening prevents Matt from disturbing Alice.  In
 some variations, this works even if Matt hides his identity.

Mahy, et al. Informational [Page 35] RFC 5850 SIP Call Control Framework May 2010

Appendix A.21. Intercom

 In Intercom, Alice typically presses a button on a phone that
 immediately connects to another user or phone and causes that phone
 to play her voice over its speaker.  Some variations immediately set
 up two-way communications, other variations require another button to
 be pressed to enable a two-way conversation.  The UA initiates a
 dialog using INVITE and the Answer-Mode: Auto header field as
 described in [RFC5373].  The called UA accepts the INVITE with a 200
 OK and automatically enables the speakerphone.
 Alternatively, this can be a local decision for the UA to auto answer
 based upon called-party identification.

Appendix A.22. Message Waiting

 In Message Waiting [RFC3842], Bob calls Alice when she has stepped
 away from her phone.  When she returns, a visible or audible
 indicator conveys that someone has left her a voicemail message.  The
 message waiting indication may also convey how many messages are
 waiting, from whom, at what time, and other useful pieces of

Appendix A.23. Music on Hold

 In Music on Hold [RFC5359], when Alice places a call with Bob on
 hold, it replaces its audio with streaming content such as music,
 announcements, or advertisements.  Music on hold can be implemented a
 number of ways.  One way is to transfer the held call to a holding
 service.  When the UA wishes to take the call off hold, it basically
 performs a take on the call from the holding service.  This involves
 subscribing to call state on the holding service and then invoking
 the URI in the call state labeled as replace-remote.
 Alternatively, music on hold can be performed as a local mixing
 operation.  The UA holding the call can mix in the music from the
 music service via RTP (i.e., an additional dialog) or RTSP or other
 streaming media source.  This approach is simpler (i.e., the held
 dialog does not move so there is less chance of loosing them) from a
 protocol perspective, however it does use more LAN bandwidth and
 resources on the UA.

Appendix A.24. Outbound Call Screening

 In Outbound Call Screening, Alice is paged and unknowingly calls a
 PSTN pay-service telephone number in the Caribbean, but local policy
 blocks her call, and possibly informs her why.

Mahy, et al. Informational [Page 36] RFC 5850 SIP Call Control Framework May 2010

Appendix A.25. Pre-Paid Calling

 In Pre-paid Calling, Alice pays for a certain currency or unit amount
 of calling value.  When she places a call, she provides her account
 number somehow.  If her account runs out of calling value during a
 call, her call is disconnected or redirected to a service where she
 can purchase more calling value.
 For prepaid calling, the user's media always passes through a device
 that is trusted by the pre-paid provider.  This may be the other
 endpoint (for example, a PSTN gateway).  In either case, an
 intermediary proxy or B2BUA can periodically verify the amount of
 time available on the pre-paid account, and use the session-timer
 extension to cause the trusted endpoint (gateway) or intermediary
 (media relay) to send a reINVITE before that time runs out.  During
 the reINVITE, the SIP intermediary can re-verify the account and
 insert another session-timer header field.
 Note that while most pre-paid systems on the PSTN use an IVR to
 collect the account number and destination, this isn't strictly
 necessary for a SIP-originated prepaid call.  SIP requests and SIP
 URIs are sufficiently expressive to convey the final destination, the
 provider of the prepaid service, the location from which the user is
 calling, and the prepaid account they want to use.  If a pre-paid IVR
 is used, the mechanism described below (Voice Portals) can be
 combined as well.

Appendix A.26. Presence-Enabled Conferencing

 In Presence-Enabled Conferencing, Alice wants to set up a conference
 call with Bob and Cathy when they all happen to be available (rather
 than scheduling a predefined time).  The server providing the
 application monitors their status, and calls all three when they are
 all "online", not idle, and not in another call.  This could be
 implemented using conferencing [RFC4579] and presence [RFC3264]

Appendix A.27. Single Line Extension/Multiple Line Appearance

 In Single Line Extension/Multiple Line Appearances, groups of phones
 are all treated as "extensions" of a single line or AOR.  A call for
 one rings them all.  As soon as one answers, the others stop ringing.
 If any extension is actively in a conversation, another extension can
 "pick up" and immediately join the conversation.  This emulates the
 behavior of a home telephone line with multiple phones.  Incoming
 calls ring all the extensions through basic parallel forking.  Each
 extension subscribes to dialog events from each other extension.
 While one user has an active call, any other UA extension can insert

Mahy, et al. Informational [Page 37] RFC 5850 SIP Call Control Framework May 2010

 itself into that conversation (it already knows the dialog
 information) in the same way as barge-in.
 When implemented using SIP, this feature is known as Shared
 Appearances of an AOR [BLISS-SHARED].  Extensions to the dialog
 package are used to convey appearance numbers (line numbers).

Appendix A.28. Speakerphone Paging

 In Speakerphone Paging, Alice calls the paging address and speaks.
 Her voice is played on the speaker of every idle phone in a
 preconfigured group of phones.  Speakerphone paging can be
 implemented using either multicast or through a simple multipoint
 mixer.  In the multicast solution, the paging UA sends a multicast
 INVITE with send-only media in the SDP (see also [RFC3264]).  The
 automatic answer and enabling of the speakerphone is a locally
 configured decision on the paged UAs.  The paging UA sends RTP via
 the multicast address indicated in the SDP.
 The multipoint solution is accomplished by sending an INVITE to the
 multipoint mixer.  The mixer is configured to automatically answer
 the dialog.  The paging UA then sends REFER requests for each of the
 UAs that are to become paging speakers (the UA is likely to send out
 a single REFER that is parallel forked by the proxy server).  The UAs
 performing as paging speakers are configured to automatically answer
 based upon caller identification (e.g., the To field, URI, or
 Referred-To header fields).
 Finally, as a third option, the user agent can send a mass-invitation
 request to a conference server, which would create a conference and
 send INVITEs containing the Answer-Mode: Auto header field to all
 user agents in the paging group.

Appendix A.29. Speed Dial

 In Speed Dial, Alice dials an abbreviated number, enters an alias, or
 presses a special speed-dial button representing Bob.  Her action is
 interpreted as if she specified the full address of Bob.

Appendix A.30. Voice Message Screening

 In Voice Message Screening, Bob calls Alice.  Alice is screening her
 calls, so Bob hears Alice's voicemail greeting.  Alice can hear Bob
 leave his message.  If she decides to talk to Bob, she can take the
 call back from the voicemail system; otherwise, she can let Bob leave
 a message.  This emulates the behavior of a home telephone answering

Mahy, et al. Informational [Page 38] RFC 5850 SIP Call Control Framework May 2010

 At first, this is the same as Call Monitoring (Appendix A.7).  In
 this case, the voicemail service is one of the UAs.  The UA screening
 the message monitors the call on the voicemail service, and also
 subscribes to dialog information.  If the user screening their
 messages decides to answer, they perform a take from the voicemail
 system (for example, send an INVITE with Replaces to the UA leaving
 the message).

Appendix A.31. Voice Portal

 Voice Portal is service that allows users to access a portal site
 using spoken dialog interaction.  For example, Alice needs to
 schedule a working dinner with her co-worker Carol.  Alice uses a
 voice portal to check Carol's flight schedule, find a restaurant near
 her hotel, make a reservation, get directions there, and page Carol
 with this information.  A voice portal is essentially a complex
 collection of voice dialogs used to access interesting content.  One
 of the most desirable call control features of a Voice Portal is the
 ability to start a new outgoing call from within the context of the
 Portal (to make a restaurant reservation, or return a voicemail
 message, for example).  Once the new call is over, the user should be
 able to return to the Portal by pressing a special key, using some
 DTMF sequence (e.g., a very long pound or hash tone), or by speaking
 a key word (e.g., "Main Menu").
 In order to accomplish this, the Voice Portal starts with the
 following media relationship:
 { User , Voice Portal }
 The user then asks to make an outgoing call.  The Voice Portal asks
 the user to perform a far-fork.  In other words, the Voice Portal
 wants the following media relationship:
         { Target , User }  &  { User , Voice Portal }
 The Voice Portal is now just listening for a key word or the
 appropriate DTMF.  As soon as the user indicates they are done, the
 Voice Portal takes the call from the old target, and we are back to
 the original media relationship.
 This feature can also be used by the account number and phone number
 collection menu in a pre-paid calling service.  A user can press a
 DTMF sequence that presents them with the appropriate menu again.

Mahy, et al. Informational [Page 39] RFC 5850 SIP Call Control Framework May 2010

Appendix A.32. Voicemail

 In Voicemail, Alice calls Bob who does not answer or is not
 available.  The call forwards to a voicemail server that plays Bob's
 greeting and records Alice's message for Bob.  An indication is sent
 to Bob that a new message is waiting, and he retrieves the message at
 a later date.  This feature is implemented using features such as
 Call Forwarding (Appendix A.6) and the History-Info header field
 [RFC4244] or voicemail URI convention [RFC4458] and Message Waiting
 [RFC3842] features.

Appendix A.33. Whispered Call Waiting

 In Whispered Call Waiting, Alice is in a conversation with Bob.
 Carol calls Alice.  Either Carol can "whisper" to Alice directly
 ("Can you get lunch in 15 minutes?"), or an automaton whispers to
 Alice informing her that Carol is trying to reach her.

Appendix B. Acknowledgments

 The authors would like to acknowledge Ben Campbell for his
 contributions to the document and thank AC Mahendran, John Elwell,
 and Xavier Marjou for their detailed Working-Group review of the
 document.  The authors would like to thank Magnus Nystrom for his
 review of the document.

5. Informative References

 [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.
 [RFC3264]        Rosenberg, J. and H. Schulzrinne, "An Offer/Answer
                  Model with Session Description Protocol (SDP)",
                  RFC 3264, June 2002.
 [RFC3265]        Roach, A., "Session Initiation Protocol (SIP)-
                  Specific Event Notification", RFC 3265, June 2002.
 [RFC4566]        Handley, M., Jacobson, V., and C. Perkins, "SDP:
                  Session Description Protocol", RFC 4566, July 2006.
 [RFC5359]        Johnston, A., Sparks, R., Cunningham, C., Donovan,
                  S., and K. Summers, "Session Initiation Protocol
                  Service Examples", BCP 144, RFC 5359, October 2008.

Mahy, et al. Informational [Page 40] RFC 5850 SIP Call Control Framework May 2010

 [RFC3725]        Rosenberg, J., Peterson, J., Schulzrinne, H., and G.
                  Camarillo, "Best Current Practices for Third Party
                  Call Control (3pcc) in the Session Initiation
                  Protocol (SIP)", BCP 85, RFC 3725, April 2004.
 [RFC3515]        Sparks, R., "The Session Initiation Protocol (SIP)
                  Refer Method", RFC 3515, April 2003.
 [RFC3891]        Mahy, R., Biggs, B., and R. Dean, "The Session
                  Initiation Protocol (SIP) "Replaces" Header",
                  RFC 3891, September 2004.
 [RFC3911]        Mahy, R. and D. Petrie, "The Session Initiation
                  Protocol (SIP) "Join" Header", RFC 3911,
                  October 2004.
 [BLISS-PROBLEM]  Rosenberg, J., "Basic Level of Interoperability for
                  Session Initiation Protocol (SIP)  Services (BLISS)
                  Problem Statement", Work in Progress, March 2009.
 [RFC4235]        Rosenberg, J., Schulzrinne, H., and R. Mahy, "An
                  INVITE-Initiated Dialog Event Package for the
                  Session Initiation Protocol (SIP)", RFC 4235,
                  November 2005.
 [RFC4575]        Rosenberg, J., Schulzrinne, H., and O. Levin, "A
                  Session Initiation Protocol (SIP) Event Package for
                  Conference State", RFC 4575, August 2006.
 [RFC3680]        Rosenberg, J., "A Session Initiation Protocol (SIP)
                  Event Package for Registrations", RFC 3680,
                  March 2004.
 [RFC3856]        Rosenberg, J., "A Presence Event Package for the
                  Session Initiation Protocol (SIP)", RFC 3856,
                  August 2004.
 [RFC4353]        Rosenberg, J., "A Framework for Conferencing with
                  the Session Initiation Protocol (SIP)", RFC 4353,
                  February 2006.
 [RFC5629]        Rosenberg, J., "A Framework for Application
                  Interaction in the Session Initiation Protocol
                  (SIP)", RFC 5629, October 2009.
 [RFC5369]        Camarillo, G., "Framework for Transcoding with the
                  Session Initiation Protocol (SIP)", RFC 5369,
                  October 2008.

Mahy, et al. Informational [Page 41] RFC 5850 SIP Call Control Framework May 2010

 [XCON-CCMP]      Barnes, M., Boulton, C., Romano, S., and H.
                  Schulzrinne, "Centralized Conferencing Manipulation
                  Protocol", Work in Progress, February 2010.
 [RFC5589]        Sparks, R., Johnston, A., and D. Petrie, "Session
                  Initiation Protocol (SIP) Call Control - Transfer",
                  BCP 149, RFC 5589, June 2009.
 [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.
 [RFC3841]        Rosenberg, J., Schulzrinne, H., and P. Kyzivat,
                  "Caller Preferences for the Session Initiation
                  Protocol (SIP)", RFC 3841, August 2004.
 [RFC3087]        Campbell, B. and R. Sparks, "Control of Service
                  Context using SIP Request-URI", RFC 3087,
                  April 2001.
 [FEATURE-REF]    Audet, F., Johnston, A., Mahy, R., and C. Jennings,
                  "Feature Referral in the Session Initiation Protocol
                  (SIP)", Work in Progress, February 2008.
 [RFC4240]        Burger, E., Van Dyke, J., and A. Spitzer, "Basic
                  Network Media Services with SIP", RFC 4240,
                  December 2005.
 [RFC4458]        Jennings, C., Audet, F., and J. Elwell, "Session
                  Initiation Protocol (SIP) URIs for Applications such
                  as Voicemail and Interactive Voice Response (IVR)",
                  RFC 4458, April 2006.
 [RFC4538]        Rosenberg, J., "Request Authorization through Dialog
                  Identification in the Session Initiation Protocol
                  (SIP)", RFC 4538, June 2006.
 [RFC3880]        Lennox, J., Wu, X., and H. Schulzrinne, "Call
                  Processing Language (CPL): A Language for User
                  Control of Internet Telephony Services", RFC 3880,
                  October 2004.

Mahy, et al. Informational [Page 42] RFC 5850 SIP Call Control Framework May 2010

 [RFC5373]        Willis, D. and A. Allen, "Requesting Answering Modes
                  for the Session Initiation Protocol (SIP)",
                  RFC 5373, November 2008.
 [RFC3842]        Mahy, R., "A Message Summary and Message Waiting
                  Indication Event Package for the Session Initiation
                  Protocol (SIP)", RFC 3842, August 2004.
 [BLISS-SHARED]   Johnston, A., Soroushnejad, M., and V.
                  Venkataramanan, "Shared Appearances of a Session
                  Initiation Protocol (SIP) Address of Record (AOR)",
                  Work in Progress, October 2009.
 [RFC4244]        Barnes, M., "An Extension to the Session Initiation
                  Protocol (SIP) for Request History Information",
                  RFC 4244, November 2005.
 [RFC4313]        Oran, D., "Requirements for Distributed Control of
                  Automatic Speech Recognition (ASR), Speaker
                  Identification/Speaker Verification (SI/SV), and
                  Text-to-Speech (TTS) Resources", RFC 4313,
                  December 2005.
 [RFC3323]        Peterson, J., "A Privacy Mechanism for the Session
                  Initiation Protocol (SIP)", RFC 3323, November 2002.
 [RFC3325]        Jennings, C., Peterson, J., and M. Watson, "Private
                  Extensions to the Session Initiation Protocol (SIP)
                  for Asserted Identity within Trusted Networks",
                  RFC 3325, November 2002.

Mahy, et al. Informational [Page 43] RFC 5850 SIP Call Control Framework May 2010

Authors' Addresses

 Rohan Mahy
 Robert Sparks
 Jonathan Rosenberg
 Dan Petrie
 Alan Johnston (editor)

Mahy, et al. Informational [Page 44]

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