Internet Engineering Task Force R. Sparks
Internet-Draft dynamicsoft
Expires: August 27, 2001 February 26, 2001
SIP Call Control - Transfer
draft-ietf-sip-cc-transfer-04
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Copyright Notice
Copyright (C) The Internet Society (2001). All Rights Reserved.
Abstract
This document defines a SIP extension, REFER, and demonstrates its
use to provide Call Transfer capabilities. This work is part of the
Call Control Framework.
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Table of Contents
1. Overview . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Changes from draft-sparks-sip-cc-transfer-03 . . . . . . . 3
3. The REFER Method . . . . . . . . . . . . . . . . . . . . . 4
3.1 The Refer-To Header . . . . . . . . . . . . . . . . . . . 4
3.1.1 Examples . . . . . . . . . . . . . . . . . . . . . . . . . 4
3.2 The Referred-By Header . . . . . . . . . . . . . . . . . . 5
3.2.1 A PGP based signature-scheme . . . . . . . . . . . . . . . 5
3.2.2 Examples . . . . . . . . . . . . . . . . . . . . . . . . . 6
3.3 Header Field Support for the REFER Method . . . . . . . . 6
3.4 Message Body Inclusion . . . . . . . . . . . . . . . . . . 7
3.5 Responses within the REFER transaction . . . . . . . . . . 7
3.6 Behavior of SIP User Agents . . . . . . . . . . . . . . . 8
3.6.1 Accessing the referred-to resource . . . . . . . . . . . . 8
3.6.2 Reporting on the results of the reference . . . . . . . . 8
3.6.2.1 Using NOTIFY . . . . . . . . . . . . . . . . . . . . . . . 8
3.6.2.2 The body of the NOTIFY . . . . . . . . . . . . . . . . . . 8
3.7 Behavior of SIP Registrars/Redirect Servers . . . . . . . 9
3.8 Behavior of SIP Proxies . . . . . . . . . . . . . . . . . 9
3.9 Prototypical REFER callflow . . . . . . . . . . . . . . . 10
3.10 Security Considerations . . . . . . . . . . . . . . . . . 11
3.10.1 Circumventing privacy . . . . . . . . . . . . . . . . . . 11
3.10.2 Circumventing security . . . . . . . . . . . . . . . . . . 12
3.10.3 Limiting the breach . . . . . . . . . . . . . . . . . . . 12
4. Call Transfer . . . . . . . . . . . . . . . . . . . . . . 13
4.1 Actors and Roles . . . . . . . . . . . . . . . . . . . . . 13
4.2 Requirements . . . . . . . . . . . . . . . . . . . . . . . 13
4.3 Using REFER to achieve Call Transfer . . . . . . . . . . . 14
4.4 Unattended Transfer . . . . . . . . . . . . . . . . . . . 14
4.4.1 Successful Unattended Transfer . . . . . . . . . . . . . . 15
4.4.2 Failed Unattended Transfer . . . . . . . . . . . . . . . . 16
4.5 Unattended Transfer with Consultation Hold . . . . . . . . 16
4.5.1 Variation 1 : Exposes transfer target . . . . . . . . . . 17
4.5.2 Variation 2 : Protects transfer target . . . . . . . . . . 18
4.5.3 Consultation Hold in the presence of forking proxies . . . 18
4.6 Attended Transfer . . . . . . . . . . . . . . . . . . . . 19
4.7 Transfer with multiple parties . . . . . . . . . . . . . . 19
5. Open Issues . . . . . . . . . . . . . . . . . . . . . . . 20
5.1 REFER is now dependent on sip-events . . . . . . . . . . . 20
5.2 Registering the events with IANA . . . . . . . . . . . . . 20
6. Acknowledgments . . . . . . . . . . . . . . . . . . . . . 20
References . . . . . . . . . . . . . . . . . . . . . . . . 20
Author's Address . . . . . . . . . . . . . . . . . . . . . 21
Full Copyright Statement . . . . . . . . . . . . . . . . . 22
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1. Overview
This document defines a SIP[1] extension, REFER, and details its use
to provide Call Transfer capabilities. This is part of a family of
Call Control extensions described in the Call Control Framework[2]
document.
Editor's note: Per consensus at the February interim WG meeting, the
two parts of this document, the definition of REFER and its use to
achieve transfer, will be separated into separate documents. That
separation will take place with the next revision of this document.
The mechanisms discussed here are most closely related to
traditional unattended and consultation hold transfers. Discussion
of attended transfer (where all parties are briefly in a conference)
is deferred until the conferencing features in this framework are
addressed.
This work has roots in draft-ietf-sip-cc-01[4] but some basic
semantics are different. In particular, transfers are achieved
through a new method that does not terminate the original signaling
relationship. By disassociating transfers from the processing of
BYE, these changes facilitate recovery of failed transfers and
clarify state management in the participating entities.
2. Changes from draft-sparks-sip-cc-transfer-03
Editor's note: The changes for this revision focused on the changes
to the NOTIFY response mechanism discussed at February's interim
meeting, and clarification to the REFER method itself. As mentioned
above, the intent is to separate this document into two. The split
was postponed a version to ensure the edits to REFER/NOTIFY would be
reflected in the ID repository in time for discussion at IETF 50.
o Modified contents of NOTIFYs to reflect the consensus at the
interim meeting. One event type, with an application/sip body
representing the information to be returned.
o Added message detail to the prototypical flow
o More explicitly stated that REFER MAY occur outside an existing
call-leg
o Reinforced that REFER can be record-routed
o Made the presence of a Contact header mandatory
o Changed the positive response to a REFER request to 202 Accepted
instead of 200 OK
o Corrected editing errors in examples and message diagrams
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3. The REFER Method
REFER is a SIP method as defined by RFC2543[1]. The REFER method
indicates that the recipient (identified by the Request-URI) should
contact a third party using the contact information provided in the
method. A success response indicates that the recipient was able to
contact the third party.
Unless stated otherwise, the protocol for emitting and responding to
a REFER request are identical to those for a BYE request in [1]. The
behavior of SIP entities not implementing the REFER (or any other
unknown) method is explicitly defined in [1].
A REFER request MAY be placed outside the scope of a call-leg
created with an INVITE. REFER MAY be Record-Routed, hence MUST
contain a single Contact header. REFERs occurring inside an existing
call-leg MUST follow the Route/Record-Route logic of that call-leg.
REFERs occurring outside an existing call-leg effectively create a
new call-leg following the behavior of SUBSCRIBE specified [3].
3.1 The Refer-To Header
Refer-To is a request-header as defined by [1]. It may only appear
in a REFER request.
Refer-To = ("Refer-To" | "r") ":" URL
A REFER method MUST contain exactly one Refer-To header.
The Refer-To header MAY be encrypted as part of end-end encryption.
The Contact header is an important part of the Route/Record-Route
mechanism and is not available for this task.
3.1.1 Examples
Refer-To: sip:alice@atlanta.com
Refer-To: sip:bob@biloxi.com?Accept-Contact=sip:bobsdesk.
biloxi.com&Call-ID=55432@alicepc.atlanta.com
Refer-To: sip:carol@cleveland.com;method=SUBSCRIBE
Refer-To: http://www.ietf.org
Long headers are line-wrapped here for clarity only.
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3.2 The Referred-By Header
Referred-By is a request-header as defined by [1]. It can appear in
any request. It conveys the identity of the original REFERrer to the
referred-to party, optionally proving the identity and that the
REFERrer actually issued this reference.
Referred-By = ("Referred-By" | "b") ":" referrer-url ";"
( referenced-url
| ( referenced-url ";" ref-signature )
| ( ref-signature ";" referenced-url )
)
referrer-url = ( name-addr | addr-spec )
referenced-url = "ref" "=" "<" URL ">"
ref-signature = signature-scheme *( ";" sig-scheme-params )
signature-scheme = "scheme" "=" token
sig-scheme-parms = token "=" ( token | quoted-string )
The referrer-url contains the SIP URL of the party sending the REFER
request. The referenced-url contains a copy of the URL placed in the
Refer-To: header. Any occurrences of < or > in the referenced-url
MUST be escaped. The ref-signature contains a signature over the
concatenation of referrer-url and referenced-url. An example
signature scheme is given in section 3.1.2.
A REFER request MUST contain exactly one Referred-By header.
The Referred-By header SHOULD be signed to help detection of REFERs
from unauthorized third parties. A signed Referred-By header SHOULD
include a Date header in the referrer-url to facilitate detection of
replay attacks.
A UA MAY reject a request containing an unsigned Referred-By header.
A UA SHOULD verify the signature on any Referred-By header it
receives.
The Referred-By header MAY be encrypted as part of end-end
encryption.
3.2.1 A PGP based signature-scheme
One signature-scheme for Referred-By headers uses PGP as follows:
signature-scheme = "scheme" "=" "pgp"
sig-scheme-parms = pgp-version | signed-by | pgp-signature
pgp-version, signed-by and pgp-signature are defined in section 15.1
of RFC2543, with the modification that the signature is computed
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across the concatenation of the referrer-url and the referenced-url.
3.2.2 Examples
Referred-By: sip:alice@atlanta.com;ref=<http:www.ietf.org>
Referred-By: "Bob" <sip:bob@biloxi.com>;
ref=<sip:alice@atlanta.com>;scheme=pgp;
pgp-version="5.0";signature="the signature"
(Note that in the last example, the signature would be over the
string "sip:bob@biloxi.comsip:alice@atlanta.com")
3.3 Header Field Support for the REFER Method
This table adds a column to tables 4 and 5 in [1], describing header
presence in a REFER method. See [1] for a key for the symbols used.
A row for the Refer-To: and Referred-By request-header should be
inferred, each mandatory for REFER. Refer-To is not applicable for
all other methods. Referred-By is a general Request header. The enc
and e-e columns in [1] apply to the REFER method unmodified.
Header Where REFER
Accept R -
Accept-Encoding R -
Accept-Language R o
Allow R -
Allow 405 m
Authorization R o
Call-ID gc m
Contact R m
Contact 1xx -
Contact 2-6xx o
Content-Encoding e -
Content-Length e o
Content-Type e -
CSeq gc m
Date g o
Encryption g o
Expires R o
From gc m
Hide R o
Max-Forwards R o
Organization g o
Priority R -
Proxy-Authenticate 407 o
Proxy-Authorization R o
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Proxy-Require R o
Require R o
Retry-After R -
Retry-After 404,480,486 o
Retry-After 503 o
Retry-After 600,603 o
Response-Key R o
Record-Route R o
Record-Route 2xx o
Route R o
Server r o
Subject R -
Timestamp g o
To gc(1) m
Unsupported 420 o
User-Agent g o
Via gc(2) m
Warning r o
WWW-Authenticate 401 o
3.4 Message Body Inclusion
A REFER method may contain a body which SHOULD be processed
according to its Content-Type.
3.5 Responses within the REFER transaction
An agent responding to a REFER Method MUST return a 400 Bad Request
if the request contained zero or more than one Refer-To headers. An
agent responding to a REFER Method MUST return a 400 Bad Request if
the request contained zero or more than one Referred-By headers. An
agent (including proxies generating local responses) MAY return a
100 Trying or any appropriate 400-600 class response as prescribed
by [1]. If the recipient's agent decides to contact the resource in
the Refer-To header, a 202 Accepted response MUST be returned before
the REFER transaction expires.
Editor's note - earlier versions of this draft required the agent
responding to REFER to wait until the referred action completed
before sending a final response to the REFER. That final response
reflected the success or failure of the referred action. This was
infeasible due to the transaction timeout rules defined for
non-INVITE requests in [1]. A REFER must always receive an immediate
(within the lifetime of a non-INVITE transaction) final response.
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3.6 Behavior of SIP User Agents
3.6.1 Accessing the referred-to resource
A UA receiving a well-formed REFER request SHOULD request approval
from the user to proceed (this request could be interactive or
through configuration). Upon receiving approval from the user, the
UA MUST contact the resource identified by the URL in the Refer-To:
header. Note that if the URL is a SIP URL, it could contain header
fields such as Call-Id that will be used to form the resulting
request. If the URL is a SIP URL, the Referred-By header in the
REFER request should be copied into the request sent to the
referred-to resource.
3.6.2 Reporting on the results of the reference
3.6.2.1 Using NOTIFY
Once it is known whether the reference succeeded or failed, the UA
receiving the REFER SHOULD notify the agent sending the refer using
the NOTIFY mechanism defined in Event Notification in SIP[3] as if
the the REFER had established a subscription. In particular:
o Each NOTIFY should reflect the To:, From:, and Call-ID headers
from the REFER as if they had arrived in a SUBSCRIBE.
o Each NOTIFY MUST contain an event header of Event: refer
o Each NOTIFY MUST contain a body of type "application/sip". The
contents of this body are detailed in Section 3.6.2.2
o Analogous to the case for SUBSCRIBE described in [3], the agent
that issued the REFER MUST be prepared to receive a NOTIFY before
the REFER transaction completes.
Open Issue: This makes REFER dependent on sip-events (Section 5.1)
Open Issue: The refer event will need to be registered with IANA
(Section 5.2)
3.6.2.2 The body of the NOTIFY
Each NOTIFY MUST contain a body of type "application/sip". This body
MUST begin with a SIP Response Status-Line as defined in [1]. The
response class in this status line indicates the success of the
referred action. The body MAY contain other SIP headers to provide
information about the outcome of the referenced action.
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A minimal, but complete, implementation can respond with a single
NOTIFY containing either the body:
SIP/2.0 200 OK
if the reference was successful or the body:
SIP/2.0 503 Service Unavailable
if the reference failed.
An implementation MAY include more of a SIP message in that body to
convey more information. Warning headers received in responses to
the referred action are good candidates. In fact, if the reference
was to a SIP URL, entire response to the referenced action could be
returned. However, doing so could have grave security repercussions
(see Section 3.10). Implementers must carefully consider what they
choose to include.
Note that if the reference was to a non-SIP URL, status in any
NOTIFYs to the referrer must still be in the form of SIP Response
Status-Lines. The minimal implementation discussed above is
sufficient to provide a basic indication of success or failure. For
example, if a client receives a REFER to a HTTP URL, and is
successful in accessing the resource, its NOTIFY to the referrer can
contain the application/sip body of "SIP/2.0 200 OK"
3.7 Behavior of SIP Registrars/Redirect Servers
Registrars and Redirect Servers SHOULD return a 603 to a REFER
request, unless they are also playing some other SIP role.
3.8 Behavior of SIP Proxies
SIP Proxies do not require modification to support the REFER method.
Specifically, as required by [1], a proxy should process a REFER
request the same way it processes an OPTIONS request.
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3.9 Prototypical REFER callflow
Agent A Agent B
| |
| REFER |
|----------------------->|
| 202 Accepted |
|<-----------------------|
| |
| |------->
| | (whatever)
| |<------
| |
| NOTIFY |
|<-----------------------|
| 200 OK |
|----------------------->|
| |
| |
Here are examples of what the four messages between Agent A and
Agent B might look like if the reference to (whatever) that Agent B
makes is successful:
Message One
REFER sip:b@agentland SIP/2.0
Via: SIP/2.0/UDP agenta.agentland
To: sip:b@agentland
From: sip:a@agentland;tag=193402342
Call-ID: 898234234@agenta.agentland
CSeq: 93809823 REFER
Refer-To: (whatever URL)
Referred-By: <sip:a@agentland>;ref=<whatever URL>;
scheme=pgp;pgp-version="5.0";signature="signature goes here"
Contact: sip:a@agentland
Content-Length: 0
Message Two
SIP/2.0 202 Accepted
Via: SIP/2.0/UDP agenta.agentland
To: sip:b@agentland;tag=4992881234
From: sip:a@agentland;tag=193402342
Call-ID: 898234234@agenta.agentland
CSeq: 93809823 REFER
Contact: sip:b@agentland
Content-Length: 0
Message Three
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NOTIFY sip:a@agentland SIP/2.0
Via: SIP/2.0/UDP agentb.agentland
To: sip:a@agentland;tag=193402342
From: sip:b@agentland;tag=4992881234
Call-ID: 898234234@agenta.agentland
CSeq: 1993402 NOTIFY
Event: refer
Contact: sip:b@agentland
Content-Type: application/sip
Content-Length: 16
SIP/2.0 200 OK
Message Four
SIP/2.0 200 OK
Via: SIP/2.0/UDP agentb.agentland
To: sip:a@agentland;tag=193402342
From: sip:b@agentland;tag=4992881234
Call-ID: 898234234@agenta.agentland
CSeq: 1993402 NOTIFY
Contact: sip:a@agentland
Content-Length: 0
3.10 Security Considerations
The security requirements of [1] apply to the REFER method.
This mechanism relies on providing contact information for the
referred-to resource to the party being referred. Care should be
taken to provide a suitably restricted URI if the referred to
resource should be protected.
Care should be taken when implementing the logic that determines
whether or not to accept the REFER request. A UA not capable of
accessing non-SIP URLs SHOULD NOT accept REFER requests to them.
Using application/sip bodies to return the progress and results of a
REFER request is extremely powerful. Careless use of that capability
will compromise security and privacy. Here are a couple of simple,
somewhat contrived, examples to demonstrate the potential for harm.
3.10.1 Circumventing privacy
Suppose Alice has a user-agent that accepts REFER requests to SIP
INVITE URLs, and NOTIFYs the referrer of the progress of the INVITE
by copying each response to the INVITE into the body of a NOTIFY.
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Suppose further that Carol has a reason to avoid Mallory and has
configured her system at her proxy to only accept calls from a
certain set of people she trusts (including Alice), so that Mallory
doesn't learn when she's around, or what user agent she's actually
using.
Mallory can send a REFER to Alice, with a Refer-To: indicating
Carol. If Alice can reach Carol, the 200 OK Carol sends gets
returned to Mallory in a NOTIFY, letting him know not only that
Carol is around, but also the IP address of the agent she's using.
3.10.2 Circumventing security
Suppose Alice, with the same user agent as above, is working at a
company that is working on the greatest SIP device ever invented -
the SIP FOO. The company has been working for months building the
device and the marketing materials, carefully keeping the idea, even
the name of the idea secret (since a FOO is one of those things that
anybody could do if they'd just had the idea first). FOO is up and
running, and anyone at the company can use it, but it's not
available outside the company firewall.
Mallory has heard rumor that Alice's company is onto something big,
and has even managed to get his hands on a URL that he suspects
might have something to do with it. He sends a REFER to ALICE with
the mysterious URL and as Alice connects to the FOO, Mallory gets
NOTIFYs with bodies containing
Server: FOO/v0.9.7
3.10.3 Limiting the breach
For each of these cases, and in general, returning a carefully
selected subset of the information available about the progress of
the reference through the NOTIFYs mitigates risk. The minimal
implementation described in Section 3.6.2.2 exposes the least
information about what the agent operating on the REFER request has
done, and is least likely to be a useful tool for malicious users.
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4. Call Transfer
4.1 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:
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.
4.2 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 unattended transfer failure
for instance) will be lost.
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3. The Transferor MUST know whether or not the transfer was
successful (this is significantly different from the
requirements of draft-ietf-sip-cc-01).
4.3 Using REFER to achieve Call Transfer
A REFER 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.
4.4 Unattended Transfer
Unattended 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 diagrams below show indicate the first line of each
message. All messages in a particular diagram share the same
Call-ID. In these diagrams, media is managed through reINVITE holds,
but other mechanisms (mixing multiple media streams at the UA or
using the conferencing extensions for example) are valid.
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4.4.1 Successful Unattended Transfer
Transferor Transferee Transfer
| | Target
| INVITE | |
|<-------------------| |
| 200 OK | |
|------------------->| |
| ACK | |
|<-------------------| |
| INVITE (hold) | |
|------------------->| |
| 200 OK | |
|<-------------------| |
| ACK | |
|------------------->| |
| REFER | |
|------------------->| |
| 202 Accepted | |
|<-------------------| |
| | INVITE |
| |------------------->|
| | 200 OK |
| |<-------------------|
| | ACK |
| |------------------->|
| NOTIFY (200 OK) | |
|<-------------------| |
| 200 OK | |
|------------------->| |
| BYE | |
|------------------->| |
| 200 OK | |
|<-------------------| |
| | BYE |
| |<-------------------|
| | 200 OK |
| |------------------->|
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4.4.2 Failed Unattended Transfer
Transferor Transferee Transfer
| | Target
| | |
| INVITE | |
|<-------------------| |
| 200 OK | |
|------------------->| |
| ACK | |
|<-------------------| |
| INVITE (hold) | |
|------------------->| |
| 200 OK | |
|<-------------------| |
| ACK | |
|------------------->| |
| REFER | |
|------------------->| |
| 202 Accepted | |
|<-------------------| |
| | INVITE |
| |------------------->|
| | 486 Busy Here |
| |<-------------------|
| | ACK |
| |------------------->|
| NOTIFY (503 Service Unavailable) |
| or NOTIFY (486 Busy Here) |
|<-------------------| |
| 200 OK | |
|------------------->| |
| INVITE (unhold) | |
|------------------->| |
| 200 OK | |
|<-------------------| |
| ACK | |
|------------------->| |
| BYE | |
|------------------->| |
| 200 OK | |
|<-------------------| |
4.5 Unattended 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 Unattended
Transfer as described above.
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4.5.1 Variation 1 : Exposes 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 unattended 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.
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 | |
|------------------->| |
| 202 Accepted | |
|<-------------------| |
Call-ID:1 | | INVITE/200 OK/ACK |
| |------------------->|
Call-ID:1 | NOTIFY (200 OK) | |
|<-------------------| |
| 200 OK | |
|------------------->| |
Call-ID:1 | BYE/200 OK | |
|------------------->| |
Call-ID:1 | | BYE/200 OK |
| |<-------------------|
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4.5.2 Variation 2 : Protects 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.
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 | INVITE (hold)/200 OK/ACK |
|---------------------------------------->|
Call-ID:2 | REFER | |
|---------------------------------------->|
| 202 Accepted | |
|<----------------------------------------|
Call-ID:2 | | INVITE/200 OK/ACK |
| |<-------------------|
Call-ID:2 | NOTIFY (200 OK) | |
|<----------------------------------------|
| | 200 OK |
|---------------------------------------->|
Call-ID:1 | BYE/200 OK | |
|------------------->| |
Call-ID:2 | BYE/200 OK | |
|---------------------------------------->|
Call-ID:2 | | BYE/200 OK |
| |------------------->|
4.5.3 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 4.5.1 for
example, the URL 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. 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
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issue the REFER request with the Refer-To: header containing the
Contact 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 URL it
used to reach the Transfer Target, augmented with an Accept-Contact
header containing the Contact the Transfer Target provided.
4.6 Attended Transfer
In an attended transfer, the three actors participate in an ad-hoc
conference as part of the event. Discussion of the implementation of
attended transfer is thus deferred until the conferencing portion of
the Call Control framework has been addressed.
4.7 Transfer with multiple parties
In this example the Originator places call to the Facilitator who
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 | |
| |<-----------| |
| |202 Accepted| |
| |----------->| |
2 | |INVITE/200 OK/ACK |
| |---------------------->|"This is Fred"
2 | |NOTIFY (200 OK) | "Please hold for
| |----------->| | Mary"
| |200 OK | |
| |<-----------| |
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2 | |BYE/200 OK | |
| |<-----------| |
3 | | |BYE/200 OK|
| | |--------->|
2 | |INVITE (hold)/200 OK/ACK
| |---------------------->|
1 |REFER | | |
|<-----------| | |
|202 Accepted| | |
|----------->| | |
1 |INVITE/200 OK/ACK | |
|----------------------------------->| "Hey Fred"
1 |NOTIFY (200 OK) | | "Hello Mary"
|----------->| | |
|200 OK | | |
|<-----------| | |
1 |BYE/200 OK | | |
|<-----------| | |
2 | |BYE/200 OK | |
| |---------------------->|
1 |BYE/200 OK | | |
|<-----------------------------------| "See you later"
5. Open Issues
5.1 REFER is now dependent on sip-events
By introducing NOTIFY, this work is prevented from moving to RFC
until the sip-events draft moves to that level (that work is
currently an individual submission). What needs to happen to our
deliverable schedule to allow for completing the sip-events work?
5.2 Registering the events with IANA
When we near the end of the process, the refer event will need to be
registered with IANA per [3].
6. Acknowledgments
This draft is a collaborative product of the SIP working group. The
editor thanks the following for their early contributions to this
work: Ben Campbell, Chris Cunningham, Steve Donovan, Alan Johnston,
Kevin Summers and Dean Willis.
References
[1] Handley, M., Schulzrinne, H., Schooler, E. and J. Rosenberg,
"SIP: Session Initiation Protocol", RFC 2543, March 1999.
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[2] Campbell, B., "Framework for SIP Call Control Extensions",
draft-ietf-sip-cc-framework-00 (work in progress), March 2000.
[3] Roach, A., "Event Notification in SIP",
draft-roach-sip-subscibe-notify-03 (work in progress), February
2001.
[4] Schulzrinne, H. and J. Rosenberg, "SIP Call Control Services",
draft-ietf-sip-cc-01 (work in progress - expired), June 1999.
Author's Address
Robert J. Sparks
dynamicsoft
5100 Tennyson Parkway
Suite 1200
Plano, TX 75024
email: rsparks@dynamicsoft.com
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Full Copyright Statement
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