SIP WG R. Mahy
Internet-Draft Cisco Systems, Inc.
Expires: August 31, 2003 B. Biggs
R. Dean
March 2, 2003
The Session Inititation Protocol (SIP) "Replaces" Header
draft-ietf-sip-replaces-03.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 August 31, 2003.
Copyright Notice
Copyright (C) The Internet Society (2003). 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 . . . . . . . . . . . . . . . . . . . . . . . 7
6. Syntax . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
6.1 The Replaces Header . . . . . . . . . . . . . . . . . . . . 7
6.2 New option tag for Require and Supported headers . . . . . . 8
7. Usage Examples . . . . . . . . . . . . . . . . . . . . . . . 8
7.1 Replacing an Early Dialog at the originator . . . . . . . . 8
8. Security Considerations . . . . . . . . . . . . . . . . . . 10
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . 12
9.1 Registration of "Replaces" SIP header . . . . . . . . . . . 12
9.2 Registration of "replaces" SIP Option-tag . . . . . . . . . 12
10. Changes . . . . . . . . . . . . . . . . . . . . . . . . . . 13
10.1 Changes Since -02 . . . . . . . . . . . . . . . . . . . . . 13
10.2 Changes Since -01 . . . . . . . . . . . . . . . . . . . . . 13
10.3 Changes Since -00 . . . . . . . . . . . . . . . . . . . . . 13
11. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . 14
Normative References . . . . . . . . . . . . . . . . . . . . 14
Informational References . . . . . . . . . . . . . . . . . . 14
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . 15
Intellectual Property and Copyright Statements . . . . . . . 16
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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 [7] 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 related User
Agents.
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 [8], they may be used independently.
For example, Alice is talking to Bob from phone1. She transfers Bob
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to a Parking Place while she goes to the lab. When she gets there
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 (using the session dialog package [9]), 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@example.org SIP/2.0
To: <sip:parkingplace@example.org>
From: <sip:bob@example.org>;tag=7743
Call-ID: 425928@bobster.example.org
CSeq: 1 INVITE
Contact: <sip:bob@bobster.example.org>
Referred-By: <sip:alice@phone1.example.org>
Message *2: Parking Place -> Bob
SIP/2.0 200 OK
To: <sip:parkingplace@example.org>;tag=6472
From: <sip:bob@example.org>;tag=7743
Call-ID: 425928@bobster.example.org
CSeq: 1 INVITE
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Contact: <sip:parkplace@monopoly.example.org>
Message *3: Alice@phone2 -> Bob
INVITE sip:bob@bobster.example.org
To: <sip:bob@example.org>
From: <sip:alice@phone2.example.org>;tag=8983
Call-ID: 09870@phone2.example.org
CSeq: 1 INVITE
Contact: <sip:alice@phone2.example.org>
Require: replaces
Replaces: 425928@bobster.example.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 parameters
are matched as if they were tags present in an incoming request. In
other words the to-tag parameter is compared to the local tag, and
the from-tag parameter 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 a 481 response.
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 an 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
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authenticated successfully as equivalent to the user who is being
replaced, then the replacement is authorized. In addition, the UA
MAY use other authorization mechanisms defined for this purpose in
standards track extensions. Extensions could define other mechanisms
for transitively asserting authorization of a replacement. For
example, the Referred-By mechanism [11] defines a mechanism that the
UAS can use to verify that a replacement request was sent on behalf
of the other participant in the matched dialog (in this case,
triggered by a REFER request).
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 this UA, it returns a 481 (Call/Transaction Does Not
Exist) response to the new INVITE, and leaves the matched dialog
unchanged. Note that since Replaces matches only a single dialog, the
replacement dialog will not be retargeted according to the same
forking logic as the original request which created the early dialog.
(Currently no use cases have been identified for replacing just a
single dialog in this circumstance.)
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. A UAC MUST NOT send an INVITE with Replaces
header field which attempts to replace an early dialog which was not
originated by the target of the INVITE with Replaces header field.
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.
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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.
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 2 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. MESSAGE, SUBSCRIBE and
NOTIFY, REFER, INFO, UPDATE, and PRACK are defined respectively in
[12], [13], [4], [14], [15], and [16].
Header field where proxy ACK BYE CAN INV OPT REG MSG
------------ ----- ----- --- --- --- --- --- --- ---
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].
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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.
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
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 [10]. Via and Max-Forwards headers are omitted for
clarity and brevity.
7.1 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
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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
| | |
*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@example.org SIP/2.0
To: <sip:bob@example.org>
From: <sip:alice@example.org>;tag=7743
Call-ID: 425928@phone.example.org
CSeq: 1 INVITE
Contact: <sip:alice@phone.example.org>
Message *2: Bob's desk phone -> Alice
SIP/2.0 180 Ringing
To: <sip:bob@example.org>;tag=6472
From: <sip:alice@example.org>;tag=7743
Call-ID: 425928@phone.example.org
CSeq: 1 INVITE
Contact: <sip:bob@bobster.example.org>
Message *3: Bob in lab -> Alice
INVITE sip:alice@phone.example.org
To: <sip:alice@example.org>
From: <sip:bob@example.org>;tag=8983
Call-ID: 09870@labpc.example.org
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CSeq: 1 INVITE
Contact: <sip:bob@labpc.example.org>
Replaces: 425928@phone.example.org;to-tag=7743;from-tag=6472
Message *4: Alice -> Bob in lab
SIP/2.0 200 OK
To: <sip:alice@example.org>;tag=9232
From: <sip:bob@example.org>;tag=8983
Call-ID: 09870@labpc.example.org
CSeq: 1 INVITE
Contact: <sip:alice@phone.example.org>
Message *5: Alice -> Bob's desk
CANCEL sip:bob@example.org SIP/2.0
To: <sip:bob@example.org>
From: <sip:alice@example.org>;tag=7743
Call-ID: 425928@phone.example.org
CSeq: 1 CANCEL
Contact: <sip:alice@phone.example.org>
Message *6: Bob's desk -> Alice
SIP/2.0 200 OK
To: <sip:bob@example.org>
From: <sip:alice@example.org>;tag=7743
Call-ID: 425928@phone.example.org
CSeq: 1 CANCEL
Contact: <sip:bob@bobster.example.org>
Message *7: Bob's desk -> Alice
SIP/2.0 487 Request Terminated
To: <sip:bob@example.org>;tag=6472
From: <sip:alice@example.org>;tag=7743
Call-ID: 425928@phone.example.org
CSeq: 1 INVITE
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.
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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 (Digest or S/MIME), and authorized to request a replacement
of the target dialog.
How a User Agent determines which requests are legitimately
authorized to make dialog replacements is non-trivial and depends on
a considerable amount of local policy configuration. In general,
there are four cases when a authorization for a replacement is
reasonable or warranted. (Note that clearly expressing most of these
relationships is facilitated by using S/MIME.)
1. Replacement made by a party considered equivalent to the replaced
party
2. Replacement made on behalf of the replaced party (perhaps
transitively)
3. Replacement made by a former participant
4. Replacement made by a specifically authorized party
Starting with #1 for example, if an executive and an assistant both
receive requests for a shared address-of-record, if so configured,
either should be able to replace dialogs of the other for the shared
identity. Both could even potentially share the same keying material
(Digest or S/MIME), or one could hold an authorization document
signed by the other expressing this relationship. Likewise in a call
center environment, each call center agent could possess credentials
which supervisors also have access to.
The most common case of a replacement is on the request of the
replaced participant (who no longer wants to be involved). This is
the case in many features such as completing an Attended Transfer and
converting a 3-way call to a point-to-point call. Such replacements
are typically triggered by a REFER request from the replaced
participant. The Referred-By mechanism defines one way to identify
the apparent original requester and an S/MIME-based signature
mechanism to secure this information.
In the example in section 2, Alice sends an INVITE with Replaces to
Bob. Alice was a former participant in the conversation and had a
previous dialog relationship with Bob. Alice can use the same Digest
or SMIME credentials she used to authenticate with Bob during the
original call to prove that she was a former participant. Note that
this justification for replacing calls is more dangerous than the
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others, and in most cases another way to authorize the replacing
participant is available. Implementation SHOULD NOT rely on this
method as an authorization mechanism.
The last scenario is the easiest to secure but the least likely to be
useful in practice. It is unlikely that an arbitrary host in the
Internet is aware of any special authorization relationship between
the replaced and the replacing parties. However, this case may be
useful in some environments. Since this usage does not effectively
degrade the security of the solution it is still allowed.
Some mechanisms for obtaining the dialog information needed by the
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. Since manipulating this dialog information
could cause User Agents to replace the wrong dialog, use of message
integrity protection for this information is STRONGLY RECOMMENDED,
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 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
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10. Changes
10.1 Changes Since -02
o Removed the ability to match an early dialog at the receiver of
the matching dialog, since all the use cases apparently needing
this feature actually need to match an entire set of targets in a
chain of proxy forking logic. Also removed all references to the
687 response code which was only used for this purpose.
o Added more detail in section 3 and section 8 about how to
authorize replacements.
10.2 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.
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
10.3 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.
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o Split normative and non-normative references
11. Acknowledgments
Thanks to Robert Sparks, Alan Johnston, Dan Petrie, 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., Schulzrinne, H., Camarillo, G., Johnston, A.,
Peterson, J., Sparks, R., Handley, M. and E. Schooler, "SIP:
Session Initiation Protocol", RFC 3261, June 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.
Informational References
[4] Sparks, R., "The SIP Refer Method", draft-ietf-sip-refer-07
(work in progress), December 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-01 (work in progress), July
2002.
[7] 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-02 (work
in progress), June 2002.
[8] Sparks, R. and A. Johnston, "Session Initiation Protocol Call
Control - Transfer", draft-ietf-sipping-cc-transfer-01 (work in
progress), February 2003.
[9] Rosenberg, J. and H. Schulzrinne, "A Session Initiation
Protocol (SIP) Event Package for Dialog State",
draft-ietf-sipping-dialog-package-00 (work in progress), June
2002.
[10] Johnston, A. and S. Donovan, "Session Initiation Protocol
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Service Examples", draft-ietf-sipping-service-examples-03 (work
in progress), November 2002.
[11] Sparks, R., "The SIP Referred-By Mechanism",
draft-ietf-sip-referredby-01 (work in progress), February 2003.
[12] Campbell, B., Rosenberg, J., Schulzrinne, H., Huitema, C. and
D. Gurle, "Session Initiation Protocol (SIP) Extension for
Instant Messaging", RFC 3428, December 2002.
[13] Roach, A., "Session Initiation Protocol (SIP)-Specific Event
Notification", RFC 3265, June 2002.
[14] Donovan, S., "The SIP INFO Method", RFC 2976, October 2000.
[15] Rosenberg, J., "The Session Initiation Protocol (SIP) UPDATE
Method", RFC 3311, October 2002.
[16] jdrosen@dynamicsoft.com and schulzrinne@cs.columbia.edu,
"Reliability of Provisional Responses in Session Initiation
Protocol (SIP)", RFC 3262, June 2002.
Authors' Addresses
Rohan Mahy
Cisco Systems, Inc.
170 West Tasman Drive
San Jose, CA 95134
USA
EMail: rohan@cisco.com
Billy Biggs
EMail: bbiggs@dumbterm.net
Rick Dean
EMail: rfc@fdd.com
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Mahy, et al. Expires August 31, 2003 [Page 16]
Internet-Draft Replaces March 2003
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