SIPPING WG R. Sparks
Internet-Draft dynamicsoft
Expires: April 25, 2003 A. Johnston
WorldCom
October 25, 2002
Session Initiation Protocol Call Control - Transfer
draft-ietf-sipping-cc-transfer-00
Status of this Memo
This document is an Internet-Draft and is in full conformance with
all provisions of Section 10 of RFC2026.
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This Internet-Draft will expire on April 25, 2003.
Copyright Notice
Copyright (C) The Internet Society (2002). All Rights Reserved.
Abstract
This document describes providing Call Transfer capabilities in the
Session Initiation Protocol (SIP). This work is part of the SIP
Multiparty Call Control Framework.
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Table of Contents
1. Overview . . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Changes from draft-sparks-sip-cc-transfer-06 . . . . . . . . 3
3. Actors and Roles . . . . . . . . . . . . . . . . . . . . . . 3
4. Requirements . . . . . . . . . . . . . . . . . . . . . . . . 4
5. Using REFER to achieve Call Transfer . . . . . . . . . . . . 5
6. Basic Transfer . . . . . . . . . . . . . . . . . . . . . . . 5
6.1 Successful Transfer . . . . . . . . . . . . . . . . . . . . 6
6.2 Failed Transfer . . . . . . . . . . . . . . . . . . . . . . 7
6.2.1 Target Busy . . . . . . . . . . . . . . . . . . . . . . . . 7
6.2.2 Transfer Target does not answer . . . . . . . . . . . . . . 8
7. Transfer with Consultation Hold . . . . . . . . . . . . . . 9
7.1 Exposing transfer target . . . . . . . . . . . . . . . . . . 9
7.2 Protecting transfer target . . . . . . . . . . . . . . . . . 11
7.3 Attended Transfer . . . . . . . . . . . . . . . . . . . . . 12
7.4 Recovery when one party does not support REFER . . . . . . . 13
7.5 Consultation Hold in the presence of forking proxies . . . . 14
7.6 Aborting a Consultation Hold . . . . . . . . . . . . . . . . 16
8. Transfer with multiple parties . . . . . . . . . . . . . . . 16
9. Open Issues . . . . . . . . . . . . . . . . . . . . . . . . 18
10. To Do . . . . . . . . . . . . . . . . . . . . . . . . . . . 18
11. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . 18
References . . . . . . . . . . . . . . . . . . . . . . . . . 18
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . 19
Full Copyright Statement . . . . . . . . . . . . . . . . . . 20
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1. Overview
This document describes providing Call Transfer capabilities and
requirements in SIP [1]. This work is part of the Multiparty Call
Control Framework [2].
The mechanisms discussed here are most closely related to traditional
basic and consultation hold transfers. This document does not
discuss transfer scenarios involving ad-hoc conferences (where all
parties involved are briefly in a conference until this transferor
drops out).
This document details the use of REFER method [3] and Replaces [4]
header field to achieve call transfer.
2. Changes from draft-sparks-sip-cc-transfer-06
o Fixed Call-ID errors
o Added Replaces requirement
o Added Supported, Allow, etc requirement
o Removed use of Accept-Contact header
o Added language about creation of routable Contact URI
o Updated flows for refer-06 behavior
o Editorial cleanup
3. Actors and Roles
There are three actors in a given transfer event, each playing one of
the following roles:
Transferee - the party being transferred to the Transfer
Target.
Transferor - the party initiating the transfer
Transfer Target - the new party being introduced into a call with
the Transferee.
The following roles are used to describe transfer requirements and
scenarios:
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Originator - wishes to place a call to the Recipient. This actor
is the source of the first INVITE in a session, to
either a Facilitator or a Screener.
Facilitator - receives a call or out-of-band request from the
Originator, establishes a call to the Recipient
through the Screener, and connects the Originator to
the Recipient.
Screener - receives a call ultimately intended for the Recipient
and transfers the calling party to the Recipient if
appropriate.
Recipient - the party the Originator is ultimately connected to.
4. Requirements
1. Any party in a SIP session MUST be able to transfer any other
party in that session at any point in that session.
2. The Transferor and the Transferee MUST NOT be removed from a
session as part of a transfer transaction.
At first glance, requirement 2 may seem to indicate
that the user experience in a transfer must be
significantly different from what a current PBX or
Centrex user expects. As the call-flows in this
document show, this is not the case. A client MAY
preserve the current experience. In fact, without
this requirement, some forms of the current
experience (ringback on transfer failure
for instance) will be lost.
3. The Transferor MUST know whether or not the transfer was
successful (this is significantly different from the requirements
of the earlier BYE-Also approach to transfer).
4. The Transferee MUST be able to replace an existing dialog with a
new dialog.
5. The Transferor and Transferee SHOULD indicate their support for
the primitives required to achieve transfer.
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5. Using REFER to achieve Call Transfer
A REFER [3] can be issued by the Transferor to cause the Transferee
to issue an INVITE to the Transfer-Target. Note that a successful
REFER transaction does not terminate the session between the
Transferor and the Transferee. If those parties wish to terminate
their session, they must do so with a subsequent BYE request. The
media negotiated between the transferee and the transfer target is
not affected by the media that had been negotiated between the
transferor and the transferee. In particular, the INVITE issued by
the Transferee will have the same SDP body it would have if he
Transferee had initiated that INVITE on its own. Further, the
disposition of the media streams between the Transferor and the
Transferee is not altered by the REFER method. Agents may alter a
session's media through additional signaling. For example, they may
make use of the SIP hold re-INVITE [1] or the conferencing extensions
provided by this framework.
6. Basic Transfer
Basic Transfer consists of the Transferor providing the Transfer
Target's contact to the Transferee. The Transferee attempts to
establish a session using that contact and reports the results of
that attempt to the Transferor. The signaling relationship between
the Transferor and Transferee is not terminated, so the call is
recoverable if the Transfer Target cannot be reached. Note that the
Transfer Target's contact information has been exposed to the
Transferee. The provided contact can be used to make new calls in
the future.
The participants in a basic transfer should indicate support for the
REFER and NOTIFY methods in Allow header fields in INVITE, 200 OK to
INVITE, and OPTIONS.
The diagrams below show indicate the first line of each message. The
first column of the figure shows the Call-ID used in that particular
message. In these diagrams, media is managed through re-INVITE
holds, but other mechanisms (mixing multiple media streams at the UA
or using the conferencing extensions for example) are valid.
Each of the flows below shows the dialog between the Transferor and
the Transferee remaining connected (on hold) during the REFER
process. While this provides the greatest flexibility for recovery
from failure, it is not necessary. If the Transferor's agent does
not wish to participate in the remainder of the REFER process and has
no intention of assisting with recovery from transfer failure, it
could emit a BYE to the Transferee as soon as the REFER transaction
completes. This flow is sometimes known as "unattended transfer".
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6.1 Successful Transfer
Transferor Transferee Transfer
| | Target
| INVITE | |
Call-ID:1 |<-------------------| |
| 200 OK | |
Call-ID:1 |------------------->| |
| ACK | |
Call-ID:1 |<-------------------| |
| INVITE (hold) | |
Call-ID:1 |------------------->| |
| 200 OK | |
Call-ID:1 |<-------------------| |
| ACK | |
Call-ID:1 |------------------->| |
| REFER | |
Call-ID:1 |------------------->| |
| 202 Accepted | |
Call-ID:1 |<-------------------| |
| NOTIFY (100 Trying)| |
Call-ID:1 |<-------------------| |
| 200 OK | |
Call-ID:1 |------------------->| |
| | INVITE |
Call-ID:2 | |------------------->|
| | 200 OK |
Call-ID:2 | |<-------------------|
| | ACK |
Call-ID:2 | |------------------->|
| NOTIFY (200 OK) | |
Call-ID:1 |<-------------------| |
| 200 OK | |
Call-ID:1 |------------------->| |
| BYE | |
Call-ID:1 |------------------->| |
| 200 OK | |
Call-ID:1 |<-------------------| |
| | BYE |
Call-ID:2 | |<-------------------|
| | 200 OK |
Call-ID:2 | |------------------->|
Figure 1. Basic Transfer Call Flow.
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6.2 Failed Transfer
This section shows examples of failed transfer attempts. After the
transfer failure occurs, the Transferor takes the Transferee off hold
and resumes the session.
6.2.1 Target Busy
Transferor Transferee Transfer
| | Target
| | |
| INVITE | |
Call-ID:1 |<-------------------| |
| 200 OK | |
Call-ID:1 |------------------->| |
| ACK | |
Call-ID:1 |<-------------------| |
| INVITE (hold) | |
Call-ID:1 |------------------->| |
| 200 OK | |
Call-ID:1 |<-------------------| |
| ACK | |
Call-ID:1 |------------------->| |
| REFER | |
Call-ID:1 |------------------->| |
| 202 Accepted | |
Call-ID:1 |<-------------------| |
| NOTIFY (100 Trying)| |
Call-ID:1 |<-------------------| |
| 200 OK | |
Call-ID:1 |------------------->| |
| | INVITE |
Call-ID:2 | |------------------->|
| | 486 Busy Here |
Call-ID:2 | |<-------------------|
| | ACK |
Call-ID:2 | |------------------->|
| NOTIFY (503 Service Unavailable) |
| or NOTIFY (486 Busy Here) |
Call-ID:1 |<-------------------| |
| 200 OK | |
Call-ID:1 |------------------->| |
| INVITE (unhold) | |
Call-ID:1 |------------------->| |
| 200 OK | |
Call-ID:1 |<-------------------| |
| ACK | |
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Call-ID:1 |------------------->| |
| BYE | |
Call-ID:1 |------------------->| |
| 200 OK | |
Call-ID:1 |<-------------------| |
Figure 2. Failed Transfer - Target Busy
6.2.2 Transfer Target does not answer
Transferor Transferee Transfer
| | Target
| INVITE | |
Call-ID:1 |<-------------------| |
| 200 OK | |
Call-ID:1 |------------------->| |
| ACK | |
Call-ID:1 |<-------------------| |
| INVITE (hold) | |
Call-ID:1 |------------------->| |
| 200 OK | |
Call-ID:1 |<-------------------| |
| ACK | |
Call-ID:1 |------------------->| |
| REFER | |
Call-ID:1 |------------------->| |
| 202 Accepted | |
Call-ID:1 |<-------------------| |
| NOTIFY (100 Trying)| |
Call-ID:1 |<-------------------| |
| 200 OK | |
Call-ID:1 |------------------->| |
| | INVITE |
Call-ID:2 | |------------------->|
| | 180 Ringing |
Call-ID:2 | |<-------------------|
| | (Transferee gets tired of waiting)
| | CANCEL |
Call-ID:2 | |------------------->|
| | 200 OK (CANCEL) |
Call-ID:2 | |<-------------------|
| | 487 Request Cancelled (INVITE)
Call-ID:2 | |<-------------------|
| | ACK |
Call-ID:2 | |------------------->|
| NOTIFY (487 Request Cancelled) |
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Call-ID:1 |<-------------------| |
| 200 OK | |
Call-ID:1 |------------------->| |
| INVITE (unhold) | |
Call-ID:1 |------------------->| |
| 200 OK | |
Call-ID:1 |<-------------------| |
| ACK | |
Call-ID:1 |------------------->| |
| BYE | |
Call-ID:1 |------------------->| |
| 200 OK | |
Call-ID:1 |<-------------------| |
Figure 3. Failed Transfer - Target Does Not Answer.
7. Transfer with Consultation Hold
Transfer with Consultation Hold involves a session between the
transferor and the transfer target before the transfer actually takes
place. This is implemented with SIP Hold and Transfer as described
above.
7.1 Exposing transfer target
The transferor places the transferee on hold, establishes a call with
the transfer target to alert them to the impending transfer,
terminates the connection with the transfer target, then proceeds
with transfer as above. This variation can be used to provide an
experience similar to that expected by current PBX and Centrex users.
To (hopefully) improve clarity, non-REFER transactions have been
collapsed into one indicator with the arrow showing the direction of
the request.
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Transferor Transferee Transfer
| | Target
| | |
Call-ID:1 | INVITE/200 OK/ACK | |
|<-------------------| |
Call-ID:1 | INVITE (hold)/200 OK/ACK |
|------------------->| |
Call-ID:2 | INVITE/200 OK/ACK | |
|---------------------------------------->|
Call-ID:2 | BYE/200 OK | |
|---------------------------------------->|
Call-ID:1 | REFER | |
|------------------->| |
Call-ID:1 | 202 Accepted | |
|<-------------------| |
Call-ID:1 | NOTIFY (100 Trying)| |
|<-------------------| |
Call-ID:1 | 200 OK | |
|------------------->| |
Call-ID:3 | | INVITE/200 OK/ACK |
| |------------------->|
Call-ID:1 | NOTIFY (200 OK) | |
|<-------------------| |
Call-ID:1 | 200 OK | |
|------------------->| |
Call-ID:1 | BYE/200 OK | |
|------------------->| |
Call-ID:3 | | BYE/200 OK |
| |<-------------------|
Figure 4. Transfer with Consultation Hold - Exposing Transfer
Target.
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7.2 Protecting transfer target
The transferor places the transferee on hold, establishes a call with
the transfer target and then reverses their roles, transferring the
original transfer target to the original transferee. This has the
advantage of hiding information about the original transfer target
from the original transferee. On the other hand, the Transferee's
experience is different that in current systems. The Transferee is
effectively "called back" by the Transfer Target.
One of the problems with this simplest implementation of a target
protecting transfer is that the transferee is receiving a new call
from the transfer-target. Unless the transferee's agent has a
reliable way to associate this new call with the call it already has
with the transferor, it will have to alert the new call on another
appearance. If this, or some other call-waiting-like UI were not
available, the transferee might be stuck returning a Busy-Here to the
transfer target, effectively preventing the transfer. There are many
ways that that correlation could be provided. The dialog parameters
could be provided directly as header parameters in the Refer-To: URI
for example. The Replaces mechanism [4] uses this approach and
solves this problem nicely.
For the flow below, dialog1 means dialog identifier 1, and consists
of the parameters of the Replaces header for dialog 1. In [4] this
is the Call-ID, To-tag and From-tag.
Note that the transferee's agent emits a BYE to the transferor's
agent as an immediate consequence of processing the Replaces header.
The Transferor knows that both the Transferee and the Transfer Target
support the Replaces header from the Supported: replaces header
contained in the 200 OK responses from both.
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Transferor Transferee Transfer
| | Target
| | |
dialog1 | INVITE/200 OK/ACK | |
|<-------------------| |
dialog1 | INVITE (hold)/200 OK/ACK |
|------------------->| |
dialog2 | INVITE/200 OK/ACK | |
|---------------------------------------->|
dialog2 | INVITE (hold)/200 OK/ACK |
|---------------------------------------->|
dialog2 | REFER (Refer-To:sip:Transferee?Replaces=dialog1)
|---------------------------------------->|
dialog2 | 202 Accepted | |
|<----------------------------------------|
dialog2 | NOTIFY (100 Trying)| |
|<----------------------------------------|
dialog2 | | 200 OK |
|---------------------------------------->|
dialog3 | | INVITE (Replaces:dialog1)/200 OK/ACK
| |<-------------------|
dialog1 | BYE/200 OK | |
|<-------------------| |
dialog2 | NOTIFY (200 OK) | |
|<----------------------------------------|
dialog2 | | 200 OK |
|---------------------------------------->|
dialog2 | BYE/200 OK | |
|---------------------------------------->|
| | (transferee and target converse)
dialog3 | | BYE/200 OK |
| |------------------->|
Figure 5. Transfer Protecting Transfer Target.
7.3 Attended Transfer
The transferor places the transferee on hold, establishes a call with
the transfer target to alert them to the impending transfer, places
the target on hold, then proceeds with transfer using an escaped
Replaces header field in the Refer-To header. This is another common
service expected by current PBX and Centrex users.
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Transferor Transferee Transfer
| | Target
| | |
dialog1 | INVITE/200 OK/ACK | |
|<-------------------| |
dialog1 | INVITE (hold)/200 OK/ACK |
|------------------->| |
dialog2 | INVITE/200 OK/ACK | |
|---------------------------------------->|
dialog2 | INVITE (hold)/200 OK/ACK |
|---------------------------------------->|
dialog1 | REFER (Refer-To:sip:TransferTarget?Replaces=dialog2)
|------------------->| |
dialog1 | 202 Accepted | |
|<-------------------| |
dialog1 | NOTIFY (100 Trying)| |
|<-------------------| |
dialog1 | 200 OK | |
|------------------->| |
dialog3 | | INVITE (Replaces:dialog2)/200 OK/ACK
| |------------------->|
dialog2 | BYE/200 OK | |
|<----------------------------------------|
dialog1 | NOTIFY (200 OK) | |
|<-------------------| |
dialog1 | 200 OK | |
|------------------->| |
dialog1 | BYE/200 OK | |
|------------------->| |
dialog3 | | BYE/200 OK |
| |<-------------------|
Figure 6. Attended Transfer Call Flow.
7.4 Recovery when one party does not support REFER
If protecting or exposing the transfer target is not a concern, it is
possible to complete a transfer with consultation hold when only the
transferor and one other party support REFER. Note that a 405 Method
Not Allowed might be returned instead of the 501 Not Implemented
response.
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Transferor Transferee Transfer
| | Target
| | |
dialog1 | INVITE/200 OK/ACK | |
|<-------------------| |
dialog1 | INVITE (hold)/200 OK/ACK |
|------------------->| |
dialog2 | INVITE/200 OK/ACK | |
|---------------------------------------->|
dialog2 | INVITE (hold)/200 OK/ACK |
|---------------------------------------->|
dialog1 | REFER (Refer-To:sip:TransferTarget?Replaces=dialog2)
|------------------->| |
dialog1 | 501 Not Implemented |
|<-------------------| |
dialog2 | REFER (Refer-To:sip:Transferee?Replaces=dialog1)
|---------------------------------------->|
dialog2 | 202 Accepted | |
|<----------------------------------------|
dialog2 | NOTIFY (100 Trying)| |
|<----------------------------------------|
dialog2 | | 200 OK |
|---------------------------------------->|
dialog3 | | INVITE (Replaces:dialog1)/200 OK/ACK
| |<-------------------|
dialog2 | NOTIFY (200 OK) | |
|<----------------------------------------|
| | 200 OK |
|---------------------------------------->|
dialog1 | BYE/200 OK | |
|<-------------------| |
dialog2 | BYE/200 OK | |
|---------------------------------------->|
dialog3 | | BYE/200 OK |
| |------------------->|
Figure 7. Recovery when one party does not support REFER.
7.5 Consultation Hold in the presence of forking proxies
It is worth noting that the examples given above abstract away any
proxies that might be between the three parties. In the examples of
Figures 4, 5, and 6, the URI used to reach the Transfer Target may go
through a forking proxy. There is no guarantee that the Transferee's
and Transferor's invitations to the Transfer Target will reach the
same endpoint. If the proxy forked in parallel, both invitations
could cause multiple endpoints to ring.
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To increase the probability of the desired behavior of having the
referred invite reach and ring only the same endpoint as the
consultation invite, the Transferor SHOULD issue the REFER request
with the Refer-To: header containing the Contact URI the Transfer
Target provided in its 200 OK to the Transferor's INVITE. If that
REFER fails, the Transferor SHOULD issue another REFER with the
Refer-To: header containing the URI it used to reach the Transfer
Target, augmented with a Require: replaces header field.
This ensures that only the endpoint which has the active dialog will
accept the INVITE request.
Note that the Require:replaces header is necessary so that a if the
request is forked to a different endpoint which does not support
Replaces, the endpoint will respond with a 420 Bad Extension instead
of accepting the INVITE.
Note that any RFC 3261 compliant will generate a Contact URI which is
routable outside a dialog as per Section 8.1.1.8 of RFC 3261. For a
UA which requires all request to be routed through a proxy (such as
for NAT/firewall traversal or screening/feature reasons), special
care must be taken in constructing the Contact URI. One approach is
to construct a URI which is unique for the device which resolves to
the proxy. A registration would then be required to bind this URI to
a URI which resolves directly to the device.
For example, consider a UA with a username carol and a hostname
server51.chicago.com. A normal Contact would be automatically
generated in the form:
Contact: sip:carol@serv51.chicago.com
However, if this UA requires that all requests come through a proxy
server at p1.chicago.com then this Contact will not work as the proxy
will be bypassed.
Another approach would involve a Contact of the form:
Contact: sip:serv51@chicago.com
in which this sip:serv51@chicago.com URI would be registered by the
UA against a Contact:
Contact: sip:carol@serv51.chicago.com
which resolves directly to the UA.
This means that a UA would first register a URI that corresponds to
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the device. Then, it would register a users URI (AOR) and use the
device URI that it registered as the Contact URI.
An alternative approach (which is not compatible with unextended RFC
2543 or 3261) would be to include an escaped loose Route header field
in the Contact URI:
Contact: sip:carol@serv51.chicago.com?Route=sip:p1.chicago.com;lr
This would result in a request being sent to
sip:carol@serv51.chicago.com with a loose Route header forcing
routing to sip:p1.chicago.com first.
Open Issue: This syntax, while allowed in a redirection, is not
permitted in an INVITE or 200 OK response per Table 1 in RFC 3261.
Other approaches may also be used to generate this globally routable
Contact URI.
7.6 Aborting a Consultation Hold
In any of the consultation hold flows above, the Transferor may
decide to terminate its attempt to contact the Transfer target before
that session is established. Most frequently, that will be the end
of the scenario, but in some circumstances, the transferor may wish
to proceed with the transfer action. For example, he may wish to
complete the transfer knowing that the transferee will end up
eventually talking to the transfer-target's voice-mail service. Some
PBX systems support this feature, sometimes called "semi-attended
transfer", that is effectively a hybrid between a fully attended
transfer and an unattended transfer. A true implementation of this
feature requires a short ad-hoc conference between all parties, which
ensures that no media clipping occurs. This flow is outside the
scope of this document.
For flows that expose the transfer target, this simply becomes a
basic transfer.
This scenario is far more complicated for flows that protect the
transfer target. Since no session is established between the
transferor and the transfer target, the transfer target's agent would
have to honor out-of-session REFERs, and somehow indicate what's
happening via its user interface (this scenario is most likely to
occur when the transfer-target is away from his agent).
8. Transfer with multiple parties
In this example the Originator places call to the Facilitator who
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reaches the Recipient through the Screener. The Recipient's contact
information is exposed to the Facilitator and the Originator. This
example is provided for clarification of the semantics of the REFER
method only and should not be used as the design of an
implementation.
Originator Facilitator Screener Recipient
Call-ID | | | |
1 |INVITE/200 OK/ACK | |"Get Fred for me!"
|----------->| | | "Right away!"
1 |INVITE (hold)/200 OK/ACK | |
|<-----------| | |
2 | |INVITE/200 OK/ACK |"I have a call
| |----------->| |from Mary for Fred"
2 | |INVITE (hold)/200 OK/ACK "Hold please"
| |<-----------| |
3 | | |INVITE/200 OK/ACK
| | |--------->|"You have a call
| | | |from Mary"
| | | | "Put her through"
3 | | |INVITE (hold)/200 OK/ACK
| | |--------->|
2 | |REFER | |
| |<-----------| |
2 | |202 Accepted| |
| |----------->| |
2 | |NOTIFY (100 Trying) |
| |----------->| |
2 | |200 OK | |
| |<-----------| |
2 | |INVITE/200 OK/ACK |
| |---------------------->|"This is Fred"
2 | |NOTIFY (200 OK) | "Please hold for
| |----------->| | Mary"
2 | |200 OK | |
| |<-----------| |
2 | |BYE/200 OK | |
| |<-----------| |
3 | | |BYE/200 OK|
| | |--------->|
2 | |INVITE (hold)/200 OK/ACK
| |---------------------->|
1 |REFER | | |
|<-----------| | |
1 |202 Accepted| | |
|----------->| | |
1 |NOTIFY (100 Trying) | |
|----------->| | |
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1 |200 OK | | |
|<-----------| | |
1 |INVITE/200 OK/ACK | |
|----------------------------------->| "Hey Fred"
1 |NOTIFY (200 OK) | | "Hello Mary"
|----------->| | |
1 |200 OK | | |
|<-----------| | |
1 |BYE/200 OK | | |
|<-----------| | |
2 | |BYE/200 OK | |
| |---------------------->|
1 |BYE/200 OK | | |
|<-----------------------------------| "See you later"
Figure 8. Transfer with Multiple Parties Example.
9. Open Issues
The creation of globally routable Contact URIs and the use of escaped
Route headers in Contact header fields in INVITE and 200 OK
responses.
10. To Do
Add message details to all the call flows.
11. Acknowledgments
This draft is a collaborative product of the SIP working group.
Thanks to Rohan Mahy for his input on the use of Replaces in
transfer.
References
[1] 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.
[2] Mahy, R., Campbell, B., Johnston, A., Petrie, D., Rosenberg, J.
and R. Sparks, "A Multi-party Application Framework for SIP",
draft-ietf-sipping-cc-framework-01 (work in progress), June
2002.
[3] Sparks, R., "The REFER Method", draft-ietf-sip-refer-06 (work in
progress), July 2002.
[4] Mahy, R., Biggs, B. and R. Dean, "The SIP Replaces Header",
Sparks & Johnston Expires April 25, 2003 [Page 18]
Internet-Draft SIP CC Transfer October 2002
draft-ietf-sipping-replaces-02 (work in progress), April 2002.
Authors' Addresses
Robert J. Sparks
dynamicsoft
5100 Tennyson Parkway
Suite 1200
Plano, TX 75024
EMail: rsparks@dynamicsoft.com
Alan Johnston
WorldCom
100 South 4th Street
St. Louis, MO 63104
EMail: alan.johnston@wcom.com
Sparks & Johnston Expires April 25, 2003 [Page 19]
Internet-Draft SIP CC Transfer October 2002
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Sparks & Johnston Expires April 25, 2003 [Page 20]
