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Versions: (draft-rosenberg-sip-3pcc) 00 01 02 03 04 05 06 RFC 3725

Internet Engineering Task Force                               SIPPING WG
Internet Draft                                              J. Rosenberg
                                                             dynamicsoft
                                                             J. Peterson
                                                                 Neustar
                                                          H. Schulzrinne
                                                             Columbia U.
                                                            G. Camarillo
                                                                Ericsson
draft-ietf-sipping-3pcc-00.txt
May 10, 2002
Expires: November 2002


         Best Current Practices for Third Party Call Control
                in the Session Initiation Protocol

STATUS OF THIS MEMO

   This document is an Internet-Draft and is in full conformance with
   all provisions of Section 10 of RFC2026.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF), its areas, and its working groups.  Note that
   other groups may also distribute working documents as Internet-
   Drafts.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress".

   The list of current Internet-Drafts can be accessed at
   http://www.ietf.org/ietf/1id-abstracts.txt

   To view the list Internet-Draft Shadow Directories, see
   http://www.ietf.org/shadow.html.


Abstract

   Third party call control refers to the ability of one entity to
   create a call in which communications is actually between other
   parties. Third party call control is possible using the mechanisms
   specified within the Session Initiation Protocol (SIP). However,
   there are several possible approaches, each with different benefits
   and drawbacks. This document discusses best current practices for the
   usage of the SIP for third party call control.




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                           Table of Contents



   1          Introduction ........................................    3
   2          Terminology .........................................    3
   3          Definitions .........................................    4
   4          3pcc Call Establishment .............................    4
   4.1        Flow I ..............................................    4
   4.2        Flow II .............................................    5
   4.3        Flow III ............................................    7
   4.4        Flow IV .............................................    8
   4.5        Recommendations .....................................    9
   5          Error Handling ......................................   10
   6          Continued Processing ................................   10
   7          3pcc and Early Media ................................   11
   8          Third arty call control and SDP preconditions .......   14
   9          Example Call Flows ..................................   15
   9.1        Click to Dial .......................................   15
   9.2        Mid-Call Announcement Capability ....................   18
   10         Implementation Recommendations ......................   20
   11         Security Considerations .............................   21
   12         IANA Considerations .................................   21
   13         Authors Addresses ...................................   21
   14         Normative References ................................   22
   15         Informative References ..............................   22






















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1 Introduction

   (Note to RFC Editor - please replace all instances of RFC BBBB with
   RFC 3261 when draft-ietf-sip-rfc2543bis is published as an RFC.
   Please replace all instances of RFC MMMM with the RFC number of
   draft-ietf-sip-manyfolks-resource when it issues as an RFC.)

   In the traditional telephony context, third party call control allows
   one entity (which we call the controller) to set up and manage a
   communications relationship between two or more other parties. Third
   party call control (referred to as 3pcc) is often used for operator
   services (where an operator creates a call that connects two
   participants together), and conferencing.

   Similarly, many SIP services are possible through third party call
   control. These include the traditional ones on the PSTN, but also new
   ones such as click-to-dial. Click-to-dial allows a user to click on a
   web page when they wish to speak to a customer service
   representative. The web server then creates a call between the user
   and a customer service representative. The call can be between two
   phones, a phone and an IP host, or two IP hosts.

   Third party call control is possible using only the mechanisms
   specified within RFC BBBB [1]. Indeed, many different call flows are
   possible, each of which will work with SIP compliant user agents.
   However, there are benefits and drawbacks to each of these flows. The
   usage of third party call control also becomes more complex when
   aspects of the call utilize SIP extensions or optional features of
   SIP. In particular, the usage of RFC MMMM [2] (used for coupling of
   signaling to resource reservation) with third party call control is
   non-trivial. Similarly, the usage of early media (where session data
   is exchanged before the call is accepted) with third party call
   control is not trivial.

   This document serves as a best current practice for implementing
   third party call control. Section 4 presents the known call flows
   that can be used to achieve third party call control, and provides
   guidelines on their usage. Section 8 discusses the interactions of
   RFC MMMM [2] with third party call control. Section 7 discusses the
   interactions of early media with third party call control. Section 9
   provides example applications that make usage of the flows
   recommended here.

2 Terminology

   In this document, the key words "MUST", "MUST NOT", "REQUIRED",
   "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY",
   and "OPTIONAL" are to be interpreted as described in RFC 2119 [3] and



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   indicate requirement levels for compliant implementations.

3 Definitions

   The following terms are used throughout this document:

        3pcc: Third Party Call Control, which refers to the general
             ability to manipulate calls between other parties.

        Controller: A controller is a SIP User Agent that wishes to
             create a session between two other user agents.

4 3pcc Call Establishment

   The primary primitive operation of third party call control is the
   establishment of a session between participants A and B.
   Establishment of this session is orchestrated by a third party,
   referred to as the controller.

   This section documents three call flows that the controller can
   utilize in order to provide this primitive operation.

4.1 Flow I



          A              Controller               B
          |(1) INVITE no SDP  |                   |
          |<------------------|                   |
          |(2) 200 offer1     |                   |
          |------------------>|                   |
          |                   |(3) INVITE offer1  |
          |                   |------------------>|
          |                   |(4) 200 OK answer1 |
          |                   |<------------------|
          |                   |(5) ACK            |
          |                   |------------------>|
          |(6) ACK answer1    |                   |
          |<------------------|                   |
          |(7) RTP            |                   |
          |-------------------------------------->|



   Figure 1: 3pcc Flow I



   The call flow for Flow I is shown in Figure 1. The controller first



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   sends an INVITE A (1). This INVITE has no session description. A's
   phone rings, and A answers. This results in a 200 OK (2) that
   contains an offer [4]. The controller needs to send its answer in the
   ACK, as mandated by [1]. To obtain the answer, it sends the offer it
   got from A (offer1) in an INVITE to B (3). B's phone rings. When B
   answers, the 200 OK (4) contains the answer to this offer, answer1.
   The controller sends an ACK to B (5), and then passes answer1 to A in
   an ACK sent to it (6). Because the offer was generated by A, and the
   answer generated by B, the actual media session is between A and B.
   Therefore, media flows between them (7).

   This flow is simple, requires no manipulation of the SDP by the
   controller, and works for any media types supported by both
   endpoints. However, it has a serious timeout problem. User B may not
   answer the call immediately. The result is that the controller cannot
   send the ACK to A right away. This causes A to retransmit the 200 OK
   response periodically. As specified in RFC BBBB Section 13.3.1.4, the
   200 OK will be retransmitted for 64*T1 seconds. If an ACK does not
   arrive by then, the call is considered to have failed. This limits
   the applicability of this flow to scenarios where the controller
   knows that B will answer the INVITE immediately.

4.2 Flow II


   An alternative flow, Flow II, is shown in Figure 2. The controller
   first sends an INVITE user A (1). This is a standard INVITE,
   containing an offer (sdp1) with a single audio media line, one codec,
   a random port number (but not zero), and a connection address of
   0.0.0.0. This creates an initial media stream that is "black holed",
   since no media (or RTCP packets [8]) will flow from A. The INVITE
   causes A's phone to ring.

   When A answers (2), the 200 OK contains an answer, sdp2. the
   controller sends an ACK (4). It then generates a second INVITE (3).
   This INVITE is addressed to user B, and it contains sdp2 as the offer
   to B. Note that the role of sdp2 has changed. In the 200 OK (message
   2), it was an answer, but in the INVITE, it is an offer. Fortunatly,
   all valid answers are valid initial offers. This INVITE causes B's
   phone to ring. When it answers, it generates a 200 OK (5) with an
   answer, sdp3. The controller then generates an ACK (6). Next, it
   sends a re-INVITE to A (7) containing sdp3 as the offer. Once again,
   there has been a reversal of roles. sdp3 was an answer, and now it is
   an offer. Fortunately, an answer to an answer recast as an offer is,
   in turn, a valid offer. This re-INVITE generatea a 200 OK (8) with
   sdp2, assuming that A doesn't decide to change any aspects of the
   session as a result of this re-INVITE. This 200 OK is ACKed (9), and
   then media can flow from A to B. Media from B to A could already



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          A              Controller               B
          |(1) INVITE bh sdp1 |                   |
          |<------------------|                   |
          |(2) 200 sdp2       |                   |
          |------------------>|                   |
          |                   |(3) INVITE sdp2    |
          |                   |------------------>|
          |(4) ACK            |                   |
          |<------------------|                   |
          |                   |(5) 200 OK sdp3    |
          |                   |<------------------|
          |                   |(6) ACK            |
          |                   |------------------>|
          |(7) INVITE sdp3    |                   |
          |<------------------|                   |
          |(8) 200 OK sdp2    |                   |
          |------------------>|                   |
          |(9) ACK            |                   |
          |<------------------|                   |
          |(10) RTP           |                   |
          |-------------------------------------->|



   Figure 2: 3pcc Flow II


   start flowing once message 5 was sent.

   This flow has the advtange that all final responses are immediately
   ACKed. If therefore does not suffer from the timeout and message
   inefficiency problems of flow 1. However, it too has troubles. First
   off, it requires that the controller know the media types to be used
   for the call (since it must generate a "blackhole" SDP, which
   requires media lines). Secondly, the first INVITE to A (1) contains
   media with a 0.0.0.0 connection address. The controller expects that
   the response contains a valid, non-zero connection address for A.
   However, experience has shown that many UAs respond to an offer of a
   0.0.0.0 connection address with an answer containing a 0.0.0.0
   connection address. The offer-answer specification [4] now explicitly
   tells implementors not to do this, but at the time of publication of
   this document, many implementations still did. If A should respond
   with a 0.0.0.0 connection address in sdp2, the flow will not work.

   However, the most serious flaw in this flow is the assumption that
   the 200 OK to the re-INVITE (message 8) contains the same SDP as in
   message 2. This may not be the case. If it is not, the controller



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   needs to re-INVITE B with that SDP (say, sdp4), which may result in
   getting a different SDP, sdp5 , in the 200 OK from B. Then, the
   controller needs to re-INVITE A again, and so on. The result is an
   infinite loop of re-INVITEs. It is possible to break this cycle by
   having very smart UAs which can return the same SDP whenever
   possible, or really smart controllers that can analyze the SDP to
   determine if a re-INVITE is really needed. However, we wish to keep
   this mechanism simple, and avoid SDP awareness in the controller. As
   a result, this flow is not really workable. It is therefore NOT
   RECOMMENDED.

4.3 Flow III



          A                 Controller                  B
          |(1) INVITE no SDP     |                      |
          |<---------------------|                      |
          |(2) 200 offer1        |                      |
          |--------------------->|                      |
          |(3) ACK answer1 (bh)  |                      |
          |<---------------------|                      |
          |                      |(4) INVITE no SDP     |
          |                      |--------------------->|
          |                      |(5) 200 OK offer2     |
          |                      |<---------------------|
          |(6) INVITE offer2'    |                      |
          |<---------------------|                      |
          |(7) 200 answer2'      |                      |
          |--------------------->|                      |
          |                      |(8) ACK answer2       |
          |                      |--------------------->|
          |(9) ACK               |                      |
          |<---------------------|                      |
          |(10) RTP              |                      |
          |-------------------------------------------->|



   Figure 3: 3pcc Flow III



   A thid flow, Flow III, is shown in Figure 3.

   First, the controller sends an INVITE (1) to user A without any SDP
   (which is good, since it means that the controller doesn't need to
   assume anything about the media composition of the session). A's
   phone rings. When A answers, a 200 OK is generated (2) containing its



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   offer, offer1. The controller generates an immediate ACK containing
   an answer (3). This answer is a "black hole" SDP, with its connection
   address set to 0.0.0.0.

   The controller then sends an INVITE to B without SDP (4). This causes
   B's phone to ring. When they answer, a 200 OK is sent, containing
   their offer, offer2 (5). This SDP is used to create a re-INVITE back
   to A (6). That re-INVITE is based on offer2, but may need to be
   reorganized to match up media lines, or to trim media lines. For
   example, if offer1 contained an audio and a video line, in that
   order, but offer2 contained just an audio line, the controller would
   need to add a video line to the offer (setting its port to zero) to
   create offer2'. Since this is a re-INVITE, it should complete quickly
   in the general case. Thats good, since user B is retransmitting their
   200 OK, waiting for an ACK. The SDP in the 200 OK (7) from A,
   answer2', may also need to be reorganized or trimmed before sending
   it an the ACK to B (8) as answer2. Finally, an ACK is sent to A (9),
   and then media can flow.

   This flow has many benefits. First, it will usually operate without
   any spurious retransmissions or timeouts (although this may still
   happen if a re-INVITE is not responded to quickly). Secondly, it does
   not require the controller to guess the media that will be used by
   the participants. Thirdly, it does not assume that a device responds
   properly to an INVITE with SDP containing a connection address of
   0.0.0.0.

   There are some drawbacks. The controller does need to perform SDP
   manipulations. Specifically, it must take some SDP, and generate
   another SDP which has the same media composition, but has connection
   addresses of 0.0.0.0. This is needed for message 3. Secondly, it may
   need to reorder and trim on SDP X, so that its media lines match up
   with those in some other SDP, Y. Thirdly, the offer from B (offer2)
   may have no codecs or media streams in common with the offer from A
   (offer 1). The controller will need to detect this condition, and
   terminate the call. Finally, the flow is far more complicated than
   the simple and elegant Flow I (Figure 1).

4.4 Flow IV


   Flow IV shows a variation on Flow III that reduces its complexity.
   The actual message flow is identical, but the SDP placement and
   construction differs. The initial INVITE (1) contains SDP with no
   media at all, meaning that there are no m lines. This is valid, and
   implies that the media makeup of the session will be established
   later through a re-INVITE [4]. The 200 OK (2) has an answer with no
   media either. This is acknowledged by the controller (3). The flow



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          A                 Controller                  B
          |(1) INVITE offer1     |                      |
          |no media              |                      |
          |<---------------------|                      |
          |(2) 200 answer1       |                      |
          |no media              |                      |
          |--------------------->|                      |
          |(3) ACK               |                      |
          |<---------------------|                      |
          |                      |(4) INVITE no SDP     |
          |                      |--------------------->|
          |                      |(5) 200 OK offer2     |
          |                      |<---------------------|
          |(6) INVITE offer2'    |                      |
          |<---------------------|                      |
          |(7) 200 answer2'      |                      |
          |--------------------->|                      |
          |                      |(8) ACK answer2       |
          |                      |--------------------->|
          |(9) ACK               |                      |
          |<---------------------|                      |
          |(10) RTP              |                      |
          |-------------------------------------------->|



   Figure 4: 3pcc Flow IV


   from this point onwards is identical to Flow III. However, the
   manipuldations required to convert offer2 to offer2', and answer2' to
   answer2, are much simpler. Indeed, no media manipulations are needed
   at all. The only change that is needed is to modify the origin lines,
   so that the origin line in offer2' is valid based on the value in
   offer1 (validify requires that the version increments by one, and
   that the other parameters remain unchanged).

4.5 Recommendations

   Flow I (Figure 1) represents the simplest and the most efficient
   flow. This flow SHOULD be used by a controller if it knows with
   certainty that user B is actually an automata that will answer the
   call immediately. This is the case for devices such as media servers,
   conferencing servers, and messaging servers, for example. Since we
   expect a great deal of third party call control to be to automata,
   special caseing this scenario is reasonable.




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   For calls to unknown entities, or to entities known to represent
   people, it is RECOMMENDED that Flow IV (Figure 4) be used for third
   party call control. Flow III MAY be used instead, but it provides no
   additional benefits over Flow IV. However, Flow II SHOULD NOT be
   used, because of the potential for infinite ping-ponging of re-
   INVITEs.

   Several of these flows use a "black hole" connection address of
   0.0.0.0. This is an IPV4 address with the property that packets sent
   to it will never leave the host which sent them; they are just
   discarded. Those flows are therefore specific to IPv4. For other
   network or address types, an address with an equivalent property
   SHOULD be used.

5 Error Handling

   With all of the call flows in Section 4, one call is established to
   A, and then the controller attempts to establish a call to B.
   However, this call attempt may fail, for any number of reasons. User
   B might be busy (resulting in a 486 response to the INVITE), there
   may not be any media in common, the request may time out, and so on.
   If the call attempt to B should fail, it is RECOMMENDED that the
   controller send a BYE to A. This BYE SHOULD include a Reason header
   [5] which carries the status code from the error response. This will
   inform A of the precise reason for the failure. The information is
   important from a user interface perspective. For example, if A was
   calling from a black phone, and B generated a 486, the BYE will
   contain a Reason code of 486, and this could be used to generate a
   local busy signal so that A knows that B is busy.

6 Continued Processing

   Once the calls are established, both participants believe they are in
   a single point-to-point call. However, they are exchanging media
   directly with each other, rather than with the controller. The
   controller is involved in two dialogs, yet sees no media.

   Since the controller is still a central point for signaling, it now
   has complete control over the call. If it receives a BYE from one of
   the participants, it can create a new BYE and hang up with the other
   participant. This is shown in Figure 5.


   Similarly, if it receives a re-INVITE from one of the participants,
   it can forward it to the other participant. Depending on which flow
   was used, this may require some manipulation on the SDP before
   passing it on.




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          A              Controller               B
          |(1) BYE            |                   |
          |------------------>|                   |
          |(2) 200 OK         |                   |
          |<------------------|                   |
          |                   |(3) BYE            |
          |                   |------------------>|
          |                   |(4) 200 OK         |
          |                   |<------------------|



   Figure 5: Hanging Up with 3PCC


   However, the controller need not "proxy" the SIP messages received
   from one of the parties. Since it is a B2BUA, it can invoke any
   signaling mechanism on each dialog, as it sees fit. For example, if
   the controller receives a BYE from A, it can generate a new INVITE to
   a third party, C, and connect B to that participant instead A call
   flow for this is shown in Figure 6, assuming the case where C
   represents an end user, not an automata. Note that it is just Flow
   IV.


   From here, new parties can be added, removed, transferred, and so on,
   as the controller sees fit.

   It is important to point out that the call need not have been
   established by the controller in order for the processing of this
   section to be used. Rather, the controller could have acted as a
   B2BUA during a call established by A towards B (or vice a versa).

7 3pcc and Early Media

   Early media represents the condition where the session is established
   (as a result of the completion of an offer/answer exchange), yet the
   call itself has not been accepted. This is usually used to convey
   tones or announcements regarding progress of the call. Handling of
   early media in a third party call is straightforward.


   Figure 7 shows the case where user B generates early media before
   answering the call. The flow is almost identical to Flow IV from
   Figure 4. The only difference is that user B generates a reliable
   provisional response (5) [6] instead of a final response, and answer2
   is carried in a PRACK (8) instead of an ACK. When party B finally



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          A           Controller            B                C
          |(1) BYE         |                |                |
          |--------------->|                |                |
          |(2) 200 OK      |                |                |
          |<---------------|                |                |
          |                |(3) INV no media|                |
          |                |-------------------------------->|
          |                |(4) 200 no media|                |
          |                |<--------------------------------|
          |                |(5) ACK         |                |
          |                |-------------------------------->|
          |                |(6) INV no SDP  |                |
          |                |--------------->|                |
          |                |(7) 200 offer3  |                |
          |                |<---------------|                |
          |                |(8) INV offer3' |                |
          |                |-------------------------------->|
          |                |(9) 200 answer3'|                |
          |                |<--------------------------------|
          |                |(10) ACK        |                |
          |                |-------------------------------->|
          |                |(11) ACK answer3|                |
          |                |--------------->|                |
          |                |                |(12) RTP        |
          |                |                |--------------->|



   Figure 6: Alternative to Hangup


   does accept the call (11), there is no change in the session state,
   and therefore, no signaling needs to be done with user A. The
   controller simply ACKs the 200 OK (12) to confirm the dialog.


   The case where user A generates early media is more complicated, and
   is shown in Figure 8. The flow is based on Flow IV. The controller
   sends an INVITE to user A (1), with an offer containing no media
   streams. User A generates a reliable provisional response (2)
   containing an answer with no media streams. The controller PRACKs
   this provisional response (3). Now, the controller sends an INVITE
   without SDP to user B (5). User B's phone rings, and they answer,
   resulting in a 200 OK (6) with an offer, offer2. The controller now
   needs to update the session parameters with user A. However, since
   the call has not been answered, it cannot use a re-INVITE. Rather, it
   uses a SIP UPDATE request (7) [7], passing the offer (after modifying



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          A                 Controller                  B
          |(1) INVITE offer1     |                      |
          |no media              |                      |
          |<---------------------|                      |
          |(2) 200 answer1       |                      |
          |no media              |                      |
          |--------------------->|                      |
          |(3) ACK               |                      |
          |<---------------------|                      |
          |                      |(4) INVITE no SDP     |
          |                      |--------------------->|
          |                      |(5) 183 offer2        |
          |                      |<---------------------|
          |(6) INVITE offer2'    |                      |
          |<---------------------|                      |
          |(7) 200 answer2'      |                      |
          |--------------------->|                      |
          |                      |(8) PRACK answer2     |
          |                      |--------------------->|
          |                      |(9) 200 PRACK         |
          |                      |<---------------------|
          |(10) RTP              |                      |
          |<--------------------------------------------|
          |                      |(11) 200 OK           |
          |                      |<---------------------|
          |                      |(12) ACK              |
          |                      |--------------------->|



   Figure 7: Early Media from User B


   it to get the origin field correct). User A generates its answer in
   the 200 OK to the UPDATE (8). This answer is passed to user B in the
   ACK (9). When user A finally answers (11), there is no change in
   session state, so the controller simply ACKs the 200 OK (12).

   Note that it is likely that there will be clipping of media in this
   call flow. User A is likely a PSTN gateway, and has generated a
   provisional response because of early media from the PSTN side. The
   PSTN will deliver this media even though the gateway does not have
   anywhere to send it, since the initial offer from the controller had
   no media streams. When user B answers, media can begin to flow.
   However, any media sent to the gateway from the PSTN up to that point
   will be lost.




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          A                 Controller                  B
          |(1) INVITE offer1     |                      |
          |no media              |                      |
          |<---------------------|                      |
          |(2) 183 answer1       |                      |
          |no media              |                      |
          |--------------------->|                      |
          |(3) PRACK             |                      |
          |<---------------------|                      |
          |(4) 200 PRACK         |                      |
          |--------------------->|                      |
          |                      |(5) INVITE no SDP     |
          |                      |--------------------->|
          |                      |(6) 200 OK offer2     |
          |                      |<---------------------|
          |(7) UPDATE offer2'    |                      |
          |<---------------------|                      |
          |(8) 200 answer2'      |                      |
          |--------------------->|                      |
          |                      |(9) ACK answer2       |
          |                      |--------------------->|
          |(10) RTP              |                      |
          |-------------------------------------------->|
          |(11) 200 OK           |                      |
          |--------------------->|                      |
          |(12) ACK              |                      |
          |<---------------------|                      |



   Figure 8: Early Media from User A


8 Third arty call control and SDP preconditions

   A SIP extension has been specified that allows for the coupling of
   signaling and resource reservation [2]. This draft relies on
   exchanges of session descriptions before completion of the call
   setup. These flows are initiated when certain SDP parameters are
   passed in the initial INVITE. As a result, the interaction of this
   mechanism with third party call control is not obvious, and worth
   detailing.


   Consider the call flow in Figure 9. The controller follows Flow IV;
   it has no specific requirements for support of the preconditions
   specification [2]. Indeed, there is no mechanism that can be used



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   with Flow IV which allows the controller to request preconditions.
   Therefore, it sends an INVITE (1) with SDP that contains no media
   lines. User A is interested in supporting preconditions, and does not
   want to ring its phone until resources are reserved. Since there are
   no media streams in the INVITE, it can't ring the phone until they
   are conveyed in a subsequent offer. Therefore, it generates a 183
   with the answer, and doesn't alert the user (2). The controller
   PRACKs this (3) and A responds to the PRACK (4).

   At this point, the controller attempts to bring B into the call. It
   sends B an INVITE without SDP (5). B is interested in having
   preconditions for this call. Therefore, it generates its offer in a
   183 that contains the appropriate SDP attributes (6). The controller
   passes this offer to A in an UPDATE request (7). The controller uses
   UPDATE because the call has not been answered yet, and therefore, it
   cannot use a re-INVITE. User A sees that its peer is capable of
   supporting preconditions. Since it desires preconditions for the
   call, it generates an answer in the 200 OK (8) to the UPDATE. This
   answer, in turn, is passed to B in the PRACK for the provisional
   response (9). Now, both sides perform resource reservation. User A
   succeeds first, and passes an updated session description in an
   UPDATE request (13). The controller simply passes this to A (after
   the manipulation of the origin field, as required in Flow IV) in an
   UPDATE (14), and the answer (15) is passed back to A (16). The same
   flow happens, but from B to A, when B's reservation succeeds (17-20).
   Since the preconditions have been met, both sides ring (21 and 22),
   and then both answer (23 and 25), completing the call.

   What is important about this flow is that the controller doesn't know
   anything about preconditions. It merely passes the SDP back and forth
   as needed. The trick is the usage of UPDATE and PRACK to pass the SDP
   when needed. That determination is made entirely based on the
   offer/answer rules described in [6] and [7], and is independent of
   preconditions.

9 Example Call Flows

9.1 Click to Dial

   The first application of this capability we discuss is click to dial.
   In this service, a user is browsing the web page of an e-commerce
   site, and would like to speak to a customer service representative.
   They click on a link, and a call is placed to a customer service
   representative. When the representative picks up, the phone on the
   user's desk rings. When they pick up, the customer service
   representative is there, ready to talk to the user.





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          A                 Controller                  B
          |(1) INVITE offer1     |                      |
          |no media              |                      |
          |<---------------------|                      |
          |(2) 183 answer1       |                      |
          |no media              |                      |
          |--------------------->|                      |
          |(3) PRACK             |                      |
          |<---------------------|                      |
          |(4) 200 OK            |                      |
          |--------------------->|                      |
          |                      |(5) INVITE no SDP     |
          |                      |--------------------->|
          |                      |(6) 183 OK offer2     |
          |                      |des=sendrecv          |
          |                      |conf=recv             |
          |                      |cur=none              |
          |                      |<---------------------|
          |(7) UPDATE offer2'    |                      |
          |des=sendrecv          |                      |
          |conf=recv             |                      |
          |cur=none              |                      |
          |<---------------------|                      |
          |(8) 200 UPDATE        |                      |
          |answer2'              |                      |
          |des=sendrecv          |                      |
          |conf=recv             |                      |
          |cur=none              |                      |
          |--------------------->|                      |
          |                      |(9) PRACK answer2     |
          |                      |des=sendrecv          |
          |                      |conf=recv             |
          |                      |cur=none              |
          |                      |--------------------->|
          |                      |(10) 200 PRACK        |
          |                      |<---------------------|
          |(11) reservation      |                      |
          |-------------------------------------------->|
          |(12) reservation      |                      |
          |<--------------------------------------------|
          |(13) UPDATE offer3    |                      |
          |des=sendrecv          |                      |
          |conf=recv             |                      |
          |cur=recv              |                      |
          |--------------------->|                      |
          |                      |(14) UPDATE offer3'   |
          |                      |des=sendrecv          |
          |                      |conf=recv             |
          |                      |cur=recv              |
          |                      |--------------------->|
          |                      |(15) 200 UPDATE       |
          |                      |answer3'              |
          |                      |des=sendrecv          |
          |                      |conf=recv             |
          |                      |cur=send              |
          |                      |<---------------------|
          |(16) 200 UPDATE       |                      |
          |answer3               |                      |
          |des=sendrecv          |                      |
          |conf=recv             |                      |
          |cur=send              |                      |
          |<---------------------|                      |
          |                      |(17) UPDATE offer4    |
          |                      |des=sendrecv          |
          |                      |conf=recv             |
          |                      |cur=sendrecv          |
          |                      |<---------------------|
          |(18) UPDATE offer4'   |                      |
          |des=sendrecv          |                      |
          |conf=recv             |                      |
          |cur=sendrecv          |                      |
          |<---------------------|                      |
          |(19) 200 UPDATE       |                      |
          |answer4'              |                      |
          |des=sendrecv          |                      |
          |conf=recv             |                      |
          |cur=sendrecv          |                      |
          |--------------------->|                      |
          |                      |(20) 200 UPDATE       |
          |                      |answer4               |
          |                      |des=sendrecv          |
          |                      |conf=recv             |
          |                      |cur=sendrecv          |
          |                      |--------------------->|
          |(21) 180 INVITE       |                      |
          |--------------------->|                      |
          |                      |(22) 180 INVITE       |
          |                      |<---------------------|
          |(23) 200 INVITE       |                      |
          |--------------------->|                      |
          |(24) ACK              |                      |
          |<---------------------|                      |
          |                      |(25) 200 INVITE       |
          |                      |<---------------------|
          |                      |(26) ACK              |
          |                      |--------------------->|



   Figure 9: Call Flow for Preconditions

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  Customer Service       Controller          Users Phone        Users Browser
          |                   |(1) HTTP POST      |                   |
          |                   |<--------------------------------------|
          |                   |(2) HTTP 200 OK    |                   |
          |                   |-------------------------------------->|
          |(3) INVITE offer1  |                   |                   |
          |no media           |                   |                   |
          |<------------------|                   |                   |
          |(4) 200 answer1    |                   |                   |
          |no media           |                   |                   |
          |------------------>|                   |                   |
          |(5) ACK            |                   |                   |
          |<------------------|                   |                   |
          |                   |(6) INVITE no SDP  |                   |
          |                   |------------------>|                   |
          |                   |(7) 200 OK offer2  |                   |
          |                   |<------------------|                   |
          |(8) INVITE offer2' |                   |                   |
          |<------------------|                   |                   |
          |(9) 200 answer2'   |                   |                   |
          |------------------>|                   |                   |
          |                   |(10) ACK answer2   |                   |
          |                   |------------------>|                   |
          |(11) ACK           |                   |                   |
          |<------------------|                   |                   |
          |(12) RTP           |                   |                   |
          |-------------------------------------->|                   |



   Figure 10: Click to Dial Call Flow


   The call flow for this service is given in Figure 10. It is identical
   to that of Figure 4, with the exception that the service is triggered
   through an http GET request when the user clicks on the link.

   We note that this service can be provided through other mechanisms,
   namely PINT [9]. However, there are numerous differences between the
   way in which the service is provided by pint, and the way in which it
   is provided here:

        o The pint solution enables calls only between two PSTN
          endpoints. The solution described here allows calls between
          PSTN phones (through SIP enabled gateways) and native IP
          phones.




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        o When used for calls between two PSTN phones, the solution here
          may result in a portion of the call being routed over the
          Internet. In pint, the call is always routed only over the
          PSTN. This may result in better quality calls with the pint
          solution, depending on the codec in use and QoS capabilities
          of the network routing the Internet portion of the call.

        o The PINT solution requires extensions to SIP (PINT is an
          extension to SIP), whereas the solution described here is done
          with baseline SIP.

        o The PINT solution allows the controller (acting as a PINT
          client) to "step out" once the call is established. The
          solution described here requires the controller to maintain
          call state for the entire duration of the call.

9.2 Mid-Call Announcement Capability

   The third party call control mechanism described here can also be
   used to enable mid-call announcements. Consider a service for pre-
   paid calling cards. Once the pre-paid call is established, the system
   needs to set a timer to fire when they run out of minutes. When this
   timer fires, we would like the user to hear an announcement which
   tells them to enter a credit card to continue. Once they enter the
   credit card info, more money is added to the pre-paid card, and the
   user is reconnected to the destination party.

   We consider here the usage of third party call control just for
   playing the mid-call dialog to collect the credit card information.


   We assume the call is set up so that the controller is in the call as
   a B2BUA. When the timer fires, we wish to connect the caller to a
   media server. The flow for this is shown in Figure 11.  When the
   timer expires, the controller places the called party with a
   connection address of zero (1). This effectively "disconnects" the
   called party. The controller then sends an INVITE without SDP to the
   the pre-paid caller (4). The offer returned from the caller (5) is
   used in an INVITE to the media server which will be collecting digits
   (6). This is an instantiation of Flow II. This flow can only be used
   here because the media server is an automata, and will answer the
   INVITE immediately. If the controller was connecting the pre-paid
   user with another end user, Flow III would need to be used. The media
   server returns an immediate 200 OK (7) with an answer, which is
   passed to the caller in an ACK (8). The result is that the media
   server and the pre-paid caller have their media streams connected.

   The media server plays an announcement, and prompts the user to enter



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    Pre-Paid User        Controller         Called Party        Media Server
          |                   |(1) INV SDP c=0    |                   |
          |                   |------------------>|                   |
          |                   |(2) 200 answer1    |                   |
          |                   |<------------------|                   |
          |                   |(3) ACK            |                   |
          |                   |------------------>|                   |
          |(4) INV no SDP     |                   |                   |
          |<------------------|                   |                   |
          |(5) 200 offer2     |                   |                   |
          |------------------>|                   |                   |
          |                   |(6) INV offer2     |                   |
          |                   |-------------------------------------->|
          |                   |(7) 200 answer2    |                   |
          |                   |<--------------------------------------|
          |(8) ACK answer2    |                   |                   |
          |<------------------|                   |                   |
          |                   |(9) ACK            |                   |
          |                   |-------------------------------------->|
          |(10) RTP           |                   |                   |
          |---------------------------------------------------------->|
          |                   |(11) BYE           |                   |
          |                   |-------------------------------------->|
          |                   |(12) 200 OK        |                   |
          |                   |<--------------------------------------|
          |                   |(13) INV no SDP    |                   |
          |                   |------------------>|                   |
          |                   |(14) 200 offer3    |                   |
          |                   |<------------------|                   |
          |(15) INV offer3'   |                   |                   |
          |<------------------|                   |                   |
          |(16) 200 answer3'  |                   |                   |
          |------------------>|                   |                   |
          |                   |(17) ACK answer3'  |                   |
          |                   |------------------>|                   |
          |(18) ACK           |                   |                   |
          |<------------------|                   |                   |
          |(19) RTP           |                   |                   |
          |-------------------------------------->|                   |



   Figure 11: Mid-Call Announcement







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   a credit card number. After collecting the number, the card number is
   validated. The controller can then hang up the call to the media
   server (11). How the controller can know when to hang up the call is
   outside the scope of this document, and might have been done through
   an HTTP message from the media server to the controller, for example.

   After hanging up with the media server, the controller reconnects the
   user to the original called party. To do this, the controller sends
   an INVITE without SDP to the called party (13). The 200 OK (14)
   contains an offer, offer3. The controller modifies the SDP (as is
   done in Flow III), and passes the offer in an INVITE to the pre-paid
   user (15). The pre-paid user generates an answer in a 200 OK (16)
   which the controller passes to user B in the ACK (17). At this point,
   the caller and called party are reconnected.

10 Implementation Recommendations

   Most of the work involved in supporting third party call control is
   within the controller. A standard SIP UA should be controllable using
   the mechanisms described here. However, third party call control
   relies on a few features that might not be implemented. As such, we
   RECOMMEND that implementors of user agent servers to support the
   following:

        o Re-invites that change the port to which media should be sent

        o Re-invites that change the connection address

        o Re-invites that add a media stream

        o Re-invites that remove a media stream (setting its port to
          zero)

        o Re-invites that add a codec amongst the set in a media stream

        o SDP Connection address of zero

        o Initial invites with a connection address of zero

        o Initial invites with no SDP

        o Initial invites with SDP but no media lines

        o Re-invites with no SDP

        o The UPDATE method [7]

        o Reliability of provisional responses [6]



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11 Security Considerations

   The mechanism described here introduces several security
   considerations. The first issue is that of identity. When the
   controller initiates the call, what identity does it place in the
   From field of the INVITE? The controller could indicate that the call
   is from itself (From: sip:controller@company.com), but in many cases,
   the service is more usable if it "spoofs" the identity of the
   participant that is actually calling. However, to differentiate
   legitimate use of 3pcc from real attacks where a caller is faking an
   identity, user agents SHOULDauthenticate the requests. The controller
   will, of course, authenticate itself as the controller, rather than
   either participant. It is RECOMMENDED that user agents be
   configurable with credentials for entities that are legitimate
   controllers. Note that this will result in SIP messages whose From
   field does not match the identity of originator as determined from
   the authentication mechanism.

   Some of the flows require the controller to manipulate the SDP. If
   S/MIME is used to encrypt or sign the bodies of the request end-to-
   end, third party call control will fail.

12 IANA Considerations

   There are no IANA considerations associated with this specification.

13 Authors Addresses



   Jonathan Rosenberg
   dynamicsoft
   72 Eagle Rock Avenue
   First Floor
   East Hanover, NJ 07936
   email: jdrosen@dynamicsoft.com

   Jon Peterson
   NeuStar, Inc
   1800 Sutter Street, Suite 570
   Concord, CA 94520
   USA
   email: jon.peterson@neustar.com

   Henning Schulzrinne
   Columbia University
   M/S 0401
   1214 Amsterdam Ave.



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   New York, NY 10027-7003
   email: schulzrinne@cs.columbia.edu

   Gonzalo Camarillo
   Ericsson
   Advanced Signalling Research Lab.
   FIN-02420 Jorvas
   Finland
   Phone: +358 9 299 3371
   Fax: +358 9 299 3052
   Email: Gonzalo.Camarillo@ericsson.com



14 Normative References

   [1] J. Rosenberg, H. Schulzrinne, et al.  , "SIP: Session initiation
   protocol," Internet Draft, Internet Engineering Task Force, Feb.
   2002.  Work in progress.

   [2] W. Marshall, G. Camarillo, and J. Rosenberg, "Integration of
   resource management and SIP," Internet Draft, Internet Engineering
   Task Force, Apr. 2002.  Work in progress.

   [3] S. Bradner, "Key words for use in RFCs to indicate requirement
   levels," RFC 2119, Internet Engineering Task Force, Mar. 1997.

   [4] J. Rosenberg and H. Schulzrinne, "An offer/answer model with
   SDP," Internet Draft, Internet Engineering Task Force, Feb. 2002.
   Work in progress.

   [5] H. Schulzrinne, D. Oran, and G. Camarillo, "The reason header
   field for the session initiation protocol," Internet Draft, Internet
   Engineering Task Force, Apr. 2002.  Work in progress.

   [6] J. Rosenberg and H. Schulzrinne, "Reliability of provisional
   responses in SIP," Internet Draft, Internet Engineering Task Force,
   Feb. 2002.  Work in progress.

   [7] J. Rosenberg, "The SIP UPDATE method," Internet Draft, Internet
   Engineering Task Force, Mar. 2002.  Work in progress.

15 Informative References

   [8] H. Schulzrinne, S. Casner, R. Frederick, and V. Jacobson, "RTP: a
   transport protocol for real-time applications," RFC 1889, Internet
   Engineering Task Force, Jan. 1996.




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   [9] S. Petrack and L. Conroy, "The PINT service protocol: Extensions
   to SIP and SDP for IP access to telephone call services," RFC 2848,
   Internet Engineering Task Force, June 2000.


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