SIP WG R. Mahy
Internet-Draft Cisco Systems, Inc.
Expires: September 30, 2002 B. Biggs
R. Dean
April 2002
The Session Inititation Protocol (SIP) "Replaces" Header
draft-ietf-sip-replaces-02.txt
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 September 30, 2002.
Copyright Notice
Copyright (C) The Internet Society (2002). All Rights Reserved.
Abstract
This document defines a new header for use with SIP multi-party
applications and call control. The Replaces header is used to
logically replace an existing SIP dialog with a new SIP dialog. This
primitive can be used to enable a variety of features, for example:
"Attended Transfer" and "Retrieve from Call Park". Note that
definition of these example features is non-normative.
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Table of Contents
1. Conventions . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Overview . . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. User Agent Server Behavior: Receiving a Replaces Header . . 5
4. User Agent Client Behavior: Sending a Replaces header . . . 6
5. Proxy behavior . . . . . . . . . . . . . . . . . . . . . . . 6
6. Syntax . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
6.1 The Replaces Header . . . . . . . . . . . . . . . . . . . . 7
6.2 New option tag for Require and Supported headers . . . . . . 8
6.3 687 Response Code: "Dialog Terminated" . . . . . . . . . . . 8
7. Usage Examples . . . . . . . . . . . . . . . . . . . . . . . 8
7.1 Replacing an Early Dialog at the receiver . . . . . . . . . 8
7.2 Replacing an Early Dialog at the originator . . . . . . . . 10
8. Security Considerations . . . . . . . . . . . . . . . . . . 12
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . 13
9.1 Registration of "Replaces" SIP header . . . . . . . . . . . 13
9.2 Registration of "replaces" SIP Option-tag . . . . . . . . . 13
9.3 Registration of "687" SIP Response code . . . . . . . . . . 13
10. Changes . . . . . . . . . . . . . . . . . . . . . . . . . . 13
10.1 Changes Since -01 . . . . . . . . . . . . . . . . . . . . . 13
10.2 Changes Since -00 . . . . . . . . . . . . . . . . . . . . . 14
11. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . 14
Normative References . . . . . . . . . . . . . . . . . . . . 14
Informational References . . . . . . . . . . . . . . . . . . 15
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . 15
Full Copyright Statement . . . . . . . . . . . . . . . . . . 17
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1. Conventions
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC-2119 [2].
This document refers frequently to the terms "confirmed dialog" and
"early dialog". These are defined in Section 12 of SIP [1].
2. Overview
This document describes a SIP [1] extension header field as part of
the SIP multiparty applications architecture framework [6]. The
Replaces header is used to logically replace an existing SIP dialog
with a new SIP dialog. This is especially useful in peer-to-peer
call control environments.
One use of the "Replaces" header is to replace one participant with
another in a multimedia conversation. While this functionality is
already available using 3rd party call control [8] style call
control, the 3pcc model requires a central point of control which may
not be desirable in many environments. As such, a method of
performing these same call control primitives in a distributed, peer-
to-peer fashion is very desirable.
Use of a new INVITE with a new header for dialog matching was chosen
over making implicit associations in an incoming INVITE based on
call-id or other fields for the following reasons:
o An INVITE already has the correct semantics for a new call
o Using an explicit Replaces header in a new request makes the
intent of the request obvious.
o A unique call-id may be given to the replacement call. This
avoids call-leg matching problems in any of the clients.
o There are no adverse effects if the header is unsupported.
The Replaces header enables services such as attended call transfer,
retrieve from park, and transition from locally mixed conferences to
two party calls in a distributed peer-to-peer way. This list of
services is not exhaustive. Although the Replaces header is
frequently used in combination with the REFER [4] method as used in
cc-transfer [7], they may be used independently.
For example, Alice is talking to Bob from phone1. She transfers Bob
to a Parking Place while she goes to the lab. When she gets there
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she retrieves the "parked" call from phone2 by sending an INVITE with
a Replaces header field to Bob with the dialog information Bob shared
with the Parking Place. Alice got this information using some out of
band mechansim. Perhaps she subscribed to this information from the
Parking Place, or went to a website and clicked on a URI. A short
call flow for this example follows. (Via and Max-Forwards headers
are omitted for clarity.)
Alice Alice Parking
phone1 phone2 Bob Place
| | | |
|<===============================>| |
| | | |
| Alice transfers Bob to Parking Place |
| | | |
|------------REFER/200----------->| *1 *2 |
| | |--INVITE/200/ACK-->|
|<-----------NOTIFY/200-----------|<=================>|
|------------BYE/200------------->| |
| | | |
| | | |
| Alice later retrieves call from another phone |
| | | |
| *3 |-INV w/Replaces->| |
| |<--200-----------| |
| |---ACK---------->|----BYE/200------->|
| |<===============>| |
| | | |
Message *1: Bob-> Parking Place
INVITE sip:parkingplace@sip.org SIP/2.0
To: <sip:parkingplace@sip.org>
From: <sip:bob@sip.org>;tag=7743
Call-ID: 425928@bobster.sip.org
CSeq: 1 INVITE
Contact: <sip:bob@bobster.sip.org>
Referred-By: <sip:alice@phone1.sip.org>
Message *2: Parking Place -> Bob
SIP/2.0 200 OK
To: <sip:parkingplace@sip.org>;tag=6472
From: <sip:bob@sip.org>;tag=7743
Call-ID: 425928@bobster.sip.org
CSeq: 1 INVITE
Contact: <sip:parkplace@monopoly.sip.org>
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Message *3: Alice@phone2 -> Bob
INVITE sip:bob@bobster.sip.org
To: <sip:bob@sip.org>
From: <sip:alice@phone2.sip.org>;tag=8983
Call-ID: 09870@phone2.sip.org
CSeq: 1 INVITE
Contact: <sip:alice@phone2.sip.org>
Require: replaces
Replaces: 425928@bobster.sip.org;to-tag=7743;from-tag=6472
3. User Agent Server Behavior: Receiving a Replaces Header
The Replaces header contains information used to match an existing
SIP dialog (call-id, to-tag, and from-tag). Upon receiving an INVITE
with a Replaces header, the UA attempts to match this information
with a confirmed or early dialog. The to-tag and from-tag are
matched as if they were present in an incoming request. In other
words the to-tag is compared to the local tag, and the from-tag is
compared to the remote tag.
If more than one Replaces header field is present in an INVITE, or if
a Replaces header field is present in a request other than INVITE,
the UAS MUST reject the request with a 400 Bad Request response.
The Replaces header has specific call control semantics. If both a
Replaces header field and another header field with contradictory
semantics are present in a request, the request MUST be rejected with
a 400 "Bad Request" response.
If the Replaces header field matches more than one dialog, the UA
MUST act as if no match is found.
If no match is found, the UAS rejects the INVITE and returns a 481
Call/Transaction Does Not Exist response. Likewise, if the Replaces
header field matches a dialog which was not created with an INVITE,
the UAS MUST reject the request with an appropriate response (ex:
400, 481, or 501).
If the Replaces header field matches a dialog which has already
terminated, the UA SHOULD decline the request with a 603 Declined
response.
If the Replaces header field matches a active dialog, the UA SHOULD
verify that the initiator of the new INVITE is authorized to replace
the matched dialog. If the initiator of the new INVITE has
authenticated successfully as equivalent to the user who is being
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replaced, then the replacement is authorized. In addition, the UA
MAY use other authorization mechanisms defined for this purpose in
standards track extensions. For example, an extension could define a
mechanism for transitively asserting authorization of a replacement.
If authorization is successful, the UA attempts to accept the new
INVITE, reassign the user interface and other resources of the
matched dialog to the new INVITE, and shut down the replaced dialog.
If the UA cannot accept the new INVITE (for example: it cannot
establish required QoS or keying, or it has incompatible media), the
UA MUST return an appropriate error response and MUST leave the
matched dialog unchanged.
If the Replaces header field matches a confirmed dialog, it accepts
the new INVITE by sending a 200-class response, and shuts down the
replaced dialog by sending a BYE. If the Replaces header field
matches an early dialog that was initiated by the UA, it accepts the
new INVITE by sending a 200-class response, and shuts down the
replaced dialog by sending a CANCEL. If the Replaces header field
matches an early dialog that was not initiated by the UA, the UA
returns a provisional or final response to the new INVITE which is
suitable for the state of the resources used by the matched dialog,
and responds to the replaced early dialog with a 687 "Transaction
Terminated" response (defined earlier in this document).
4. User Agent Client Behavior: Sending a Replaces header
A User Agent that wishes to replace a single existing early or
confirmed dialog with a new dialog of its own, MAY send the target
User Agent an INVITE request containing a Replaces header field. The
UAC places the Call-ID, to-tag, and from-tag information for the
target dialog in a single Replaces header field and sends the new
INVITE to the target.
Note that use of this mechanism does not provide a way to match
multiple dialogs, nor does it provide a way to match an entire call,
an entire transaction, or to follow a chain of proxy forking logic.
For example, if Alice replaces Cathy in an early dialog with Bob, but
he does not answer, Alice's replacement request will not match other
dialogs to which Bob's UA redirects, nor other branches to which his
proxy forwards.
5. Proxy behavior
Proxy Servers do not require any new behavior to support this
extension. They simply pass the Replaces header field transparently
as described in the SIP specification.
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Note that it is possible for a proxy (especially when forking based
on some application layer logic, such as caller screening or time-of-
day routing) to forward an INVITE request containing a Replaces
header field to a completely orthogonal set of Contacts than the
original request it was intended to replace. In this case, the
INVITE request with the Replaces header field will fail.
6. Syntax
6.1 The Replaces Header
The Replaces header field indicates that a single dialog identified
by the header field is to be shut down and logically replaced by the
incoming INVITE in which it is contained. It is a request header
only, and defined only for INVITE requests. The Replaces header
field MAY be encrypted as part of end-to-end encryption. Only a
single Replaces header field value may be present in a SIP request
This document adds the following entry to Table 3 of [1]. Additions
to this table are also provided for extension methods defined at the
time of publication of this document. This is provided as a courtesy
to the reader and is not normative in any way. SUBSCRIBE and NOTIFY,
REFER, INFO, UPDATE, and PRACK are defined respectively in [10], [4],
[11], [12], and [13].
Header field where proxy ACK BYE CAN INV OPT REG
------------ ----- ----- --- --- --- --- --- ---
Replaces R - - - o - -
SUB NOT REF INF UPD PRA
--- --- --- --- --- ---
Replaces R - - - - - -
The following syntax specification uses the augmented Backus-Naur
Form (BNF) as described in RFC-2234 [3].
Replaces = "Replaces" HCOLON callid *(SEMI replaces-param)
replaces-param = to-tag / from-tag / generic-param
to-tag = "to-tag" EQUAL token
from-tag = "from-tag" EQUAL token
A Replaces header MUST contain exactly one to-tag and exactly one
from-tag, as they are required for unique dialog matching. For
compatibility with dialogs initiated by RFC2543 [5] compliant UAs, a
tag of zero matches both tags of zero and null tags.
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Examples:
Replaces: 98732@sip.billybiggs.com
;from-tag=r33th4x0r
;to-tag=ff87ff
Replaces: 12adf2f34456gs5;to-tag=12345;from-tag=54321
Replaces: 87134@171.161.34.23;to-tag=24796;from-tag=0
6.2 New option tag for Require and Supported headers
This specification defines a new Require/Supported header option tag
"replaces". UAs which support the Replaces header MUST include the
"replaces" option tag in a Supported header field. UAs that want
explicit failure notification if Replaces is not supported MAY
include the "replaces" option in a Require header field.
Example:
Require: replaces, 100rel
6.3 687 Response Code: "Dialog Terminated"
This specification defines a new SIP response code. The 687 "Dialog
Terminated" response code indicates that an early dialog has been
completely replaced by a new dialog. A new response code was chosen
from the 6xx class to prevent intervening proxies from attempting to
fork additional branches of the replaced dialog.
7. Usage Examples
The following non-normative examples are not intended to enumerate
all the possibilities for the usage of this extension, but rather to
provide examples or ideas only. For more examples, please see
service-examples [9]. Via and Max-Forwards headers are omitted for
clarity and brevity.
7.1 Replacing an Early Dialog at the receiver
In this example, a Customer tries calling a call center and for some
reason cannot get through properly. The customer calls an Operator
and asks for help. The operator calls the contact center, and upon
receiving a provisional response, assumes that everything is OK and
transfers the Customer to the Call Center, replacing the operator's
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place in the queue.
Call
Operator Customer Center
| | |
|<--INVITE/180/200/ACK--| |
|<=====================>| "Hello, I'm having |
| | trouble calling ..." |
|"OK, I'll try it and | |
| transfer you if it | |
| works for me" | |
| | |
*1 |-----INVITE ----------------------------------->|
*2 |<----182: You are caller number 7---------------|
| | |
| completes transfer | |
| | |
|---REFER/200---------->| |
| |--INVITE with Replaces->| *3
| |<----182: caller #7-----| *4
|<----687 Dialog Terminated----------------------| *5
|-----ACK--------------------------------------->|
|<--NOTIFY/200----------| |
|---BYE/200------------>| |
| | ...time passes.. |
| | |
| | |
| | |
| |<---200 OK--------------|
|<--NOTIFY/200----------|----ACK---------------->|
| | |
| | |
Message *1: Operator -> Call Center
INVITE sip:helpdesk@clueless.org SIP/2.0
To: <sip:helpdesk@clueless.org>
From: <sip:operator@acme.com>;tag=7743
Call-ID: 425928@dhcp23311.acme.com
CSeq: 1 INVITE
Contact: <sip:jdoe@dhcp2311.acme.com>
Accept-Language: en
Message *2: Call Center -> Operator
SIP/2.0 182 You are 7th in Queue
To: <sip:helpdesk@clueless.org>;tag=6472
From: <sip:operator@acme.com>;tag=7743
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Call-ID: 425928@dhcp23311.acme.com
CSeq: 1 INVITE
Contact: <sip:helpdesk@frontline.clueless.org>
Message *3: Customer -> Call Center
INVITE sip:helpdesk@frontline.clueless.org
To: <sip:helpdesk@clueless.org>
From: <sip:customer@acme.com>;tag=8983
Call-ID: 09870@lobby12.acme.com
CSeq: 1 INVITE
Contact: <sip:customer@lobby12.acme.com>
Replaces: 425928@dhcp23311.acme.com;to-tag=7743;from-tag=6472
Accept-Language: en
Referred-By: <sip:jdoe@dhcp2311.acme.com>
Message *4: Call Center -> Customer
SIP/2.0 182 You are 7th in Queue
To: <sip:helpdesk@clueless.org>
From: <sip:customer@acme.com>;tag=8983
Call-ID: 09870@lobby12.acme.com
CSeq: 1 INVITE
Contact: <sip:helpdesk@frontline.clueless.org>
Message *5: Call Center -> Operator
SIP/2.0 687 Dialog Terminated
To: <sip:helpdesk@clueless.org>;tag=6472
From: <sip:operator@acme.com>;tag=7743
Call-ID: 425928@dhcp23311.acme.com
CSeq: 1 INVITE
Contact: <sip:helpdesk@frontline.clueless.org>
7.2 Replacing an Early Dialog at the originator
In this example, Bob just arrived in the lab and hasn't registered
there yet. He hears his desk phone ring. He quickly logs into a
software UA on a nearby computer. Among other things, the software
UA has access to the dialog state of his desk phone. When it notices
that his phone is ringing it offers him the choice to take the call
there. The software UA sends an INVITE with Replaces to Alice. When
Alice's UA receives this new INVITE, it CANCELs her original INVITE
and connects Alice to Bob.
Bob Bob
Alice desk lab
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| | |
*1 |-----INVITE----------->| |
*2 |<----180---------------| Bob hears desk phone |
| | ringing from lab but |
| | isn't REGISTERed yet |
| | |
| |<--fetch dialog state --|
| |---response ----------->|
*3/4 |<-----INVITE with Replaces/200/ACK--------------|
*5/6 |------CANCEL/200------>| |
*7 |<-----487--------------| |
|------ACK------------->| |
| | |
| | |
Message *1: Alice -> Bob's desk phone
INVITE sip:bob@sip.org SIP/2.0
To: <sip:bob@sip.org>
From: <sip:alice@sip.org>;tag=7743
Call-ID: 425928@phone.sip.org
CSeq: 1 INVITE
Contact: <sip:alice@phone.sip.org>
Message *2: Bob's desk phone -> Alice
SIP/2.0 180 Ringing
To: <sip:bob@sip.org>;tag=6472
From: <sip:alice@sip.org>;tag=7743
Call-ID: 425928@phone.sip.org
CSeq: 1 INVITE
Contact: <sip:bob@bobster.sip.org>
Message *3: Bob in lab -> Alice
INVITE sip:alice@phone.sip.org
To: <sip:alice@sip.org>
From: <sip:bob@sip.org>;tag=8983
Call-ID: 09870@labpc.sip.org
CSeq: 1 INVITE
Contact: <sip:bob@labpc.sip.org>
Replaces: 425928@phone.sip.org;to-tag=7743;from-tag=6472
Message *4: Alice -> Bob in lab
SIP/2.0 200 OK
To: <sip:alice@sip.org>;tag=9232
From: <sip:bob@sip.org>;tag=8983
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Call-ID: 09870@labpc.sip.org
CSeq: 1 INVITE
Contact: <sip:alice@phone.sip.org>
Message *5: Alice -> Bob's desk
CANCEL sip:bob@sip.org SIP/2.0
To: <sip:bob@sip.org>
From: <sip:alice@sip.org>;tag=7743
Call-ID: 425928@phone.sip.org
CSeq: 1 CANCEL
Contact: <sip:alice@phone.sip.org>
Message *6: Bob's desk -> Alice
SIP/2.0 200 OK
To: <sip:bob@sip.org>
From: <sip:alice@sip.org>;tag=7743
Call-ID: 425928@phone.sip.org
CSeq: 1 CANCEL
Contact: <sip:bob@bobster.sip.org>
Message *7: Bob's desk -> Alice
SIP/2.0 487 Request Terminated
To: <sip:bob@sip.org>;tag=6472
From: <sip:alice@sip.org>;tag=7743
Call-ID: 425928@phone.sip.org
CSeq: 1 INVITE
Contact: <sip:bob@bobster.sip.org>
8. Security Considerations
The extension specified in this document significantly changes the
relative security of SIP devices. Currently in SIP, even if an
eavesdropper learns the Call-ID, To, and From headers of a dialog,
they cannot easily modify or destroy that dialog if Digest
authentication or end-to-end message integrity are used.
This extension can be used to disconnect participants or replace
participants in a multimedia conversation. As such, invitations with
the Replaces header SHOULD only be accepted if the peer requesting
replacement has been properly authenticated using a standard SIP
mechanism, and authorized to request a replacement of the target
dialog.
Some mechanisms for obtaining the dialog information needed by the
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Replaces header (Call-ID, to-tag, and from-tag) include URIs on a web
page, subscriptions to an appropriate event package, and notifcations
after a REFER request. Use of end-to-end security mechanisms to
encrypt this information is also RECOMMENDED.
This extension was designed to take advantage of future signature or
authorization schemes defined by the SIP Working Group. In general,
call control features would benefit considerably from such work.
9. IANA Considerations
9.1 Registration of "Replaces" SIP header
Name of Header: Replaces
Short form: none
Normative description: section 6.1 of this document
9.2 Registration of "replaces" SIP Option-tag
Name of option: replaces
Description: Support for the SIP Replaces header
SIP headers defined: Replaces
Normative description: This document
9.3 Registration of "687" SIP Response code
Number of response code: 687
Default reason phrase: Dialog Terminated
Normative description: section 6.3 of this document
10. Changes
10.1 Changes Since -01
o Removed the to-tag=* matching mechanism, and related proxy
requirements and examples based on WG consensus at interim meeting
and on the mailing list.
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o Reorganized motivational overview material
o Moved extra examples to service-flows
o Added authorization language in UAS behavior section
o Removed allowance to match on one of multiple matching dialogs
with no tags
o Updated references
10.2 Changes Since -00
o When no dialog matches the Call-ID and tags in a Replaces header,
the UAS now returns a 481 instead of silently accepting the
INVITE.
o Changed the BNF to match the explicit whitespace BNF now used by
SIP.
o Added the to-tag=* matching mechanism.
o Added requirements for forking proxies and a discussion of the
consequences if forking proxies do not support Replaces.
o Added last two examples.
o Split normative and non-normative references
11. Acknowledgments
Thanks to Robert Sparks, Alan Johnston, and Ben Campbell and many
other members of the SIP WG for their continued support of the cause
of distributed call control in SIP.
Normative References
[1] Rosenberg, J. and H. Schulzrinne, "SIP: Session Initiation
Protocol", draft-ietf-sip-rfc2543bis-09 (work in progress),
February 2002.
[2] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC 2119, March 1997.
[3] Crocker, D. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", RFC 2234, November 1997.
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Informational References
[4] Sparks, R., "The Refer Method", draft-ietf-sip-refer-04 (work
in progress), May 2002.
[5] Handley, M., Schulzrinne, H., Schooler, E. and J. Rosenberg,
"SIP: Session Initiation Protocol", RFC 2543, March 1999.
[6] Mahy, R., "A Multi-party Application Framework for SIP", draft-
ietf-sipping-cc-framework-00 (work in progress), March 2002.
[7] Sparks, R., "SIP Call Control - Transfer", draft-ietf-sip-cc-
transfer-05.txt (work in progress), July 2001.
[8] Rosenberg, J., Schulzrinne, H., Camarillo, G. and J. Peterson,
"Best Current Practices for Third Party Call Control in the
Session Initiation Protocol", draft-ietf-sipping-3pcc-00 (work
in progress), May 2002.
[9] Johnston, A., "SIP Service Examples", draft-ietf-sipping-
service-examples-01 (work in progress), April 2002.
[10] Roach, A., "SIP-Specific Event Notification", draft-ietf-sip-
events-05 (work in progress), March 2002.
[11] Donovan, S., "The SIP INFO Method", RFC 2976, October 2000.
[12] Rosenberg, J., "The Session Initiation Protocol UPDATE Method",
draft-ietf-sip-update-02 (work in progress), May 2002.
[13] Rosenberg, J. and H. Schulzrinne, "Reliability of Provisional
Responses in SIP", draft-ietf-sip-100rel-06 (work in progress),
February 2002.
Authors' Addresses
Rohan Mahy
Cisco Systems, Inc.
170 West Tasman Drive
San Jose, CA 95134
USA
EMail: rohan@cisco.com
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Internet-Draft Replaces April 2002
Billy Biggs
EMail: bbiggs@dumbterm.net
Rick Dean
EMail: rfc@fdd.com
Mahy, et al. Expires September 30, 2002 [Page 16]
Internet-Draft Replaces April 2002
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Mahy, et al. Expires September 30, 2002 [Page 17]
