SIP WG R. Mahy
Internet-Draft Cisco Systems, Inc.
Expires: October 28, 2002 B. Biggs
R. Dean
April 29, 2002
The SIP Replaces Header
draft-ietf-sip-replaces-01.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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Copyright Notice
Copyright (C) The Internet Society (2002). All Rights Reserved.
Abstract
This document proposes a new header for use with the SIP call control
architecture. The Replaces header is used in peer-to-peer call
control 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. Syntax . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
3.1 The Replaces Header . . . . . . . . . . . . . . . . . . . . . 4
3.2 Formal Syntax . . . . . . . . . . . . . . . . . . . . . . . . 4
3.3 Examples . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
3.4 New option tag for Require and Supported headers . . . . . . . 5
3.5 687 Response Code: "Dialog Terminated" . . . . . . . . . . . . 5
4. User Agent Behavior: Receiving a Replaces Header . . . . . . . 5
4.1 Matching Dialogs . . . . . . . . . . . . . . . . . . . . . . . 5
4.2 Matching with Explicit Local Tags . . . . . . . . . . . . . . 6
4.3 Matching with the * to-tag . . . . . . . . . . . . . . . . . . 6
4.4 Replaces Semantics . . . . . . . . . . . . . . . . . . . . . . 7
4.5 Proxy behavior . . . . . . . . . . . . . . . . . . . . . . . . 7
4.6 Consequences of a forking proxy not supporting this extension 8
5. Usage Examples . . . . . . . . . . . . . . . . . . . . . . . . 9
5.1 Replacing an Active Dialog . . . . . . . . . . . . . . . . . . 9
5.2 Replacing an Early Dialog initiated by someone else . . . . . 10
5.3 Replacing an Early Dialog you initiated . . . . . . . . . . . 12
5.4 Handling Replaces for a Terminated Dialog . . . . . . . . . . 14
5.5 An Error Case . . . . . . . . . . . . . . . . . . . . . . . . 15
5.6 Backwards compatibility with RFC2543 User Agents . . . . . . . 16
5.7 Replacing with to-tag set to '*' . . . . . . . . . . . . . . . 18
5.8 A non-compliant forking proxy . . . . . . . . . . . . . . . . 19
6. Security Considerations . . . . . . . . . . . . . . . . . . . 21
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 21
7.1 Registration of "Replaces" SIP header . . . . . . . . . . . . 21
7.2 Registration of "replaces" SIP Option-tag . . . . . . . . . . 22
7.3 Registration of "687" SIP Response code . . . . . . . . . . . 22
8. To Do and Open Issues . . . . . . . . . . . . . . . . . . . . 22
8.1 Open Issues: . . . . . . . . . . . . . . . . . . . . . . . . . 22
8.2 To Do: . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22
8.3 Changes Since -00 . . . . . . . . . . . . . . . . . . . . . . 22
9. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 23
Normative References . . . . . . . . . . . . . . . . . . . . . 23
Informational References . . . . . . . . . . . . . . . . . . . 23
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . 24
Full Copyright Statement . . . . . . . . . . . . . . . . . . . 25
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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 [3].
Throughout this document, an "established dialog" means an active SIP
dialog in which the request that created the dialog has received a
successful (2xx-class) final response (ex: 200 OK). An "early
dialog" means transaction and dialog state that exists after a
request is sent which would create a new dialog, but before a final
response is received for the initial request.
2. Overview
This document describes a SIP [1] extension for distributed call
control as part of the SIP multiparty applications architecture
framework [6]. The Replaces header is used in peer-to-peer call
control to logically replace an existing SIP dialog with a new SIP
dialog.
INVITEs are requests which can be accepted, rejected or declined. A
User Agent that accepts a request with call-control semantics agrees
to take responsibility for setting up the appropriate requested media
relationships.
In the parlance of the SIP multiparty architecture, the "Replaces"
header is used to replace one participant with another in a
conversation space. 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.
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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 [2] method as used in
cc-transfer [7], they may be used independently.
3. Syntax
3.1 The Replaces Header
The Replaces header indicates that the dialog identified by the
header 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 here only for INVITE requests. The Replaces header MAY be
encrypted as part of end-to-end encryption.
This document adds the following entry to Table 3 of [1]:
Header field where proxy ACK BYE CAN INV OPT REG
------------ ----- ----- --- --- --- --- --- ---
Replaces R - - - o - -
SUB NOT REF INF UPD PRA
--- --- --- --- --- ---
Replaces R - - - - - -
Note that the Replaces header has specific call control semantics.
If both a Replaces header and another header with contradictory
semantics are present in a request, the request MUST be rejected with
a 400 "Bad Request" response.
3.2 Formal Syntax
The following syntax specification uses the augmented Backus-Naur
Form (BNF) as described in RFC-2234 [4].
Replaces = "Replaces" HCOLON replaces-values
*(COMMA replaces-values)
replaces-values = callid *( SEMI replaces-param )
callid = token [ "@" token ]
replaces-param = to-tag | from-tag | extension-param
to-tag = "to-tag" EQUAL ( UUID | "*" )
from-tag = "from-tag" EQUAL UUID
extension-param = token [ EQUAL ( token | quoted-string ) ]
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A Replaces header MUST contain exactly one to-tag and exactly one
from-tag, as they are required for unique dialog matching. Since we
rely on the tags for matching purposes, implementations which support
Replaces MUST support the SIP specification, which requires tags.
For compatibility with early dialogs and dialogs initiated by RFC2543
[5] compliant UAs, a tag of zero must match both tags of zero and
null tags.
3.3 Examples
Replaces: 98732@sip.billybiggs.com
;from-tag=r33th4x0r
;to-tag=ff87ff
Replaces: 12345@149.112.118.3;to-tag=12345;from-tag=54321
Replaces: 87134@171.161.34.23;to-tag=24796;from-tag=0
Replaces: 12345@149.112.118.3;to-tag=*;from-tag=24583
3.4 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 in the Supported header. UAs that want explicit
failure notification if Replaces is not supported MAY include the
"replaces" option in the Require header.
Example:
Require: replaces, 100rel
3.5 687 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.
4. User Agent Behavior: Receiving a Replaces Header
4.1 Matching Dialogs
The Replaces header contains information used to match an existing
SIP dialog (call-id, to-tag, and from-tag). Upon receiving an INVITE
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with the Replaces header, the UA MUST attempt to match this
information with an established 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. The to-tag of "*" is a special
token which matches all local tags. When this special "*" token is
present, the matching semantics are slightly different.
4.2 Matching with Explicit Local Tags
If the Replaces header matches more than one dialog, the UA MAY use
other headers if present (ex: the Referred-By header) to attempt to
match a single dialog. If a single matching dialog is not found, 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.
If the Replaces header matches a dialog which was not created with an
INVITE, the UAS MUST reject the request with an appropriate response.
If the Replaces header matches a dialog which has already terminated,
the UA SHOULD decline the request with a 603 Declined response. This
prevents phantom ringing in cases like example 6.4.
Once a matching call-leg is found, the UAS MAY authenticate the
INVITE request. If the request is successfully authenticated or
already preauthorized, the UAS SHOULD proceed with processing. The
UAS MAY prompt the user to accept or reject unauthenticated requests.
The UAS MAY reject the request with any appropriate response (for
example: 603 "Decline", 403 "Forbidden", or 488 "Not Acceptable
Here")
4.3 Matching with the * to-tag
If the Replaces header matches more than one dialog, the UA MAY use
other headers if present (ex: the Referred-By header) to attempt to
match a single dialog. If a single matching dialog is not found, the
UA MUST act as if no match is found.
If the Replaces header matches a dialog which was not created with an
INVITE, the UAS MUST reject the request with an appropriate response.
If the Replaces header matches a dialog which has already been
terminated with a CANCEL or BYE, the UAS MUST reject the INVITE and
return a 481 Call/Transaction Does Not Exist response.
If no match is found, the UAS MUST ignore the Replaces header and
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continue processing the INVITE as if it was not present.
Once a matching call-leg is found, the UAS MAY authenticate the
INVITE request. If the request is successfully authenticated or
already preauthorized, the UAS SHOULD proceed with processing. The
UAS MAY prompt the user to accept or reject unauthenticated requests.
The UAS MAY reject the request with any appropriate response (for
example: 603 "Decline", 403 "Forbidden", or 488 "Not Acceptable
Here")
4.4 Replaces Semantics
If the Replaces header matches an established active dialog, the UA
SHOULD attempt 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 by sending a BYE. 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 response and leave the matched dialog unchanged.
If the Replaces header matches an early dialog that was initiated by
the UA, the UA SHOULD attempt to accept the new INVITE. If the UA
cannot accept the new INVITE, the UA MUST return an appropriate
response and leave the matched dialog unchanged. If the UA
successfully accepts the new INVITE, the UA MUST reassign the
resources of the early dialog to the new INVITE, and CANCEL the
replaced early dialog.
If the Replaces header matches an early dialog that was not initiated
by the UA, the UA SHOULD attempt to provisionally accept the new
INVITE. In other words, the UA should attempt whatever steps are
necessary to return a provisional or final response suitable for the
state of the resources used by the matched dialog.
If this is successful, the UA MUST reassign the resources of the
early dialog to the new INVITE, and respond to the replaced early
dialog with a 687 "Transaction Terminated" response (defined earlier
in this document).
4.5 Proxy behavior
A proxy which does not fork merely needs to pass the Replaces header
transparently as described in SIP.
A forking proxy which supports this specification that would normally
fork a request for the Request-URI in the request MUST examine that
INVITE request for a Replaces header.
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The proxy MUST attempt to match the Call-ID, to-tag, and from-tag
with a transaction that that proxy (or cluster of proxies) previously
forked. If the Replaces header includes a "*" in the to-tag, then
the request will match all branches that match the Call-ID, and from-
tag.
If the proxy cannot locate a matching transaction, it continues
processing the request as if the Replaces header did not exist.
If the proxy locates a matching transaction, if SHOULD treat the new
request exactly as if it were the matching request. In other words,
all the branching logic and state (including timers such as call
forward timers) of the matched request should be copied and used for
the new request. The proxy will skip contacts which have already
failed and forward the new INVITE request to all the Contacts for
which the replaced request has active matching branches. The proxy
MUST NOT forward the original INVITE on to new branches.
Furthermore, if the proxy forwards the new request to more than one
active branch, it MUST wait until it receives a non-"100 Trying"
response from all active branches of the new request, before sending
CANCEL requests to any of the replaced branches. This prevents a
race condition which is described in Section 7.8.
If necessary, the proxy MUST continue to forward the new INVITE
request to new branches as if it were the replaced INVITE.
OPEN ISSE: This is a lot of work. Do we really want to do
this or just punt on early attended transfer?
4.6 Consequences of a forking proxy not supporting this extension
Forking proxies which do not support this extension may inadvertently
forward an INVITE request with a Replaces header to a different set
of Contacts than the original request it was intended to replace.
This may occur as a result of many policy-based Contact selection
algorithms, including time-of-day, load-sharing, presence-based, or
caller-based policies.
A parallel forking proxy may cause a race condition whereby only one
branch of many caused by the initial INVITE is replaced. This race
condition is illustrated in the example in Section 7.8.
Call forwarding timers may be reset for active branches. For
example, take a forking proxy which implements a call forwarding
service after 12 seconds of inactivity. The original INVITE may have
been tried already for 10 seconds. The replacement INVITE is likely
to begin this timer again at 12 seconds.
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5. Usage Examples
The following non-normative examples are not intended to enumerate
all the possibilities for the usage of these extensions, but rather
to provide examples or ideas only. For more examples, please see
service-examples [9].
5.1 Replacing an Active Dialog
In this 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 she retrieves the "parked" call from phone2 by sending an
INVITE with Replaces to Bob with the dialog information Bob shared
with the Parking Place. How did Alice get this information? Maybe
she subscribed to this information from the Parking Place, or went to
a website and clicked on a URL.
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->| |
| *4 |<--200-----------| *5 |
| |---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>
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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>
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
Message *4: Bob -> Alice@phone2
SIP/2.0 200 OK
To: <sip:bob@sip.org>;tag=9343
From: <sip:alice@phone2.sip.org>;tag=8983
Call-ID: 09870@phone2.sip.org
CSeq: 1 INVITE
Contact: <sip:alice@phone2.sip.org>
Message *5: Bob -> Parking Place
BYE sip:parkingplace@sip.org SIP/2.0
To: <sip:parkingplace@sip.org>;tag=6472
From: <sip:bob@sip.org>;tag=7743
Call-ID: 425928@bobster.sip.org
CSeq: 2 BYE
Contact: <sip:bob@bobster.sip.org>
5.2 Replacing an Early Dialog initiated by someone else
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
place in the queue.
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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
Call-ID: 425928@dhcp23311.acme.com
CSeq: 1 INVITE
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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>
5.3 Replacing an Early Dialog you initiated
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 subscribes to the call-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----------->| |
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*2 |<----180---------------| Bob hears desk phone |
| | ringing from lab but |
| | isn't REGISTERed yet |
| | |
| |<--SUB callpackage/200--|
| |---NOTIFY/200---------->|
*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
Call-ID: 09870@labpc.sip.org
CSeq: 1 INVITE
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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>
5.4 Handling Replaces for a Terminated Dialog
In this example, Alice, Bob, and Cathy participate in a 3-way call
mixed locally by Bob's UA. Bob's UA is programmed to revert to a
simple 2-party call when any party hangs up (including Bob).
Ordinarily this would be a very polite feature--Cathy and Alice could
continue to talk after Bob hangsup. If all three hang up at about
the same time, but Bob hangs up first (this will happen about one-
third of the time), an INVITE with Replaces header can arrive at
Cathy's UA shortly after she has hung up. Because Cathy's UA needs
to keep transaction state around for a while anyway (typically 32
seconds), the dialog information in the Replaces header should match
a terminated dialog. Cathy declines the INVITE, and cleanup proceeds
normally.
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Alice Bob Cathy
| |
| Alice, Bob, and Cathy are participants in |
| a 3-way call mixed by Bob |
| |
|<=====================>#<======================>|
| | |
| All three hang up at | |
| about the same time | |
| | |
| Bob's UA tries to | |
| setup a 2-way call | |
| btwn Alice and Cathy | |
| | |
|<---REFER--------------| |
|----INVITE with Replaces--->XX (lost or late) |
| | |
| |<-----BYE/200-----------| the dialog is
| | | already dead
|----INVITE with Replaces----------------------->| so
|<---603 Declined--------------------------------| Cathy Declines
|----ACK---------------------------------------->|
|----NOTIFY/200-------->| |
| | |
|<-----BYE/200--------->| |
| (either side sends) | |
| | |
| | |
5.5 An Error Case
The following example illustrates one reason an INVITE with Replaces
may fail. In this example, both Bob and Cathy have a common audio
codec with Alice, but Bob and Cathy do not share a common codec.
When Cathy receives an INVITE from Bob with the Replaces header,
Cathy determines she cannot communicate, sends a 488 response to Bob,
and maintains her session with Alice.
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Alice Bob Cathy
| | |
|--INVITE/200/ACK------>| |
| | |
|<=audio w/GSM codec===>| |
| | |
|----INVITE/200/ACK-------------------------------->|
| | |
|<===audio with G.729 codec========================>|
| | |
| | |
|--REFER/200----------->| |
| |--INVITE w/Replaces------->|
| | |
| | no codec in common! |
| | |
| |<-488 Not Acceptable Here--|
|<--NOTIFY/200----------|--ACK--------------------->|
| | |
|<=====================>| |
|<=================================================>|
| | |
5.6 Backwards compatibility with RFC2543 User Agents
In this example, both Alice and Bob use tags, but Alice wishes to
replace a dialog at Bob that was initiated by a User Agent that does
not support tags.
RFC2543
User Agent
Alice Bob (no tags)
| | |
| |<---------INVITE-----------| *1
| |----------200--------------| *2
| |<---------ACK--------------|
| | |
| |<=========================>|
| | |
| | |
*3 |--INVITE w/Replaces--->| |
*4 |<----200 OK------------|----------BYE------------->| *5
|-----ACK-------------->|<---------200--------------|
| | |
|<=====================>| |
| | |
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Message *1: Oldtimer (RFC 2543 User Agent)-> Bob
INVITE sip:bob@sip.org SIP/2.0
To: <sip:bob@sip.org>
From: <sip:oldtimer@sip.org>
Call-ID: 425928@test-ua.sip.org
CSeq: 1 INVITE
Contact: <sip:oldtimer@test-ua.sip.org>
Message *2: Bob -> Oldtimer
SIP/2.0 200 OK
To: <sip:bob@sip.org>;tag=3245
From: <sip:oldtimer@sip.org>
Call-ID: 425928@test-ua.sip.org
CSeq: 1 INVITE
Contact: <sip:bob@bobster.sip.org>
Message *3: Alice -> 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>
Replaces: 425928@test-ua.sip.org;to-tag=3245;from-tag=0
Message *4: Bob -> Alice
SIP/2.0 200 OK
To: <sip:bob@sip.org>;tag=9343
From: <sip:alice@phone2.sip.org>;tag=8983
Call-ID: 09870@phone2.sip.org
CSeq: 1 INVITE
Contact: <sip:bob@bobster.sip.org>
Message *5: Bob -> Oldtimer
BYE sip:oldtimer@test-ua.sip.org SIP/2.0
To: <sip:oldtimer@sip.org>
From: <sip:bob@sip.org>;tag=3245
Call-ID: 425928@test-ua.sip.org
CSeq: 2 BYE
Contact: <sip:bob@bobster.sip.org>
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5.7 Replacing with to-tag set to '*'
In this example, Alice and Bob have a stable call. Bob call Cathy,
who is available at a multiplicity of Contacts. Cathy's proxy first
forwards INVITEs to her desk and lab for 6 seconds, and then tries
here home and car for 6 seconds, finally arriving at her voicemail
server if none of these contacts answer. Bob intends to talk with
Cathy, and then transfer Alice to Cathy, but for whatever reason,
becomes impatient and transfers Alice's call before an established
dialog is created. Bob's intent is for Alice to reach Cathy at any
of her contacts as opposed to at a single contact, so he asks her to
send an INVITE replacing his entire call with the wildcard to-tag
parameter. Cathy's proxy replaces Bob's entire chain of proxy
forwarding logic and call forwarding timers with the new INVITE from
Alice.
Cathy's Contacts
desk lab home car VM
Alice Bob Proxy C1 C2 C3 C4 C5
| | | | | | | |
| stable call | | | | | | |
|<============>| | | | | | |
| |--INVITE-(1)-->| | | | | |
| | |-INVITE-(1a)->| | | | |
| | |-INVITE-(1b)------>| | | |
| | |<---18x-(1a)--| | | | |
| |<----18x-(1a)--|<---18x-(1b)-------| | | |
| |<----18x-(1b)--| | | | | |
| | | proxy waits | | | | |
| | | 6 seconds | | | | |
| | | | | | | |
| | |-CANCEL/200-->| | | | |
| | |-CANCEL/200------->| | | |
| | |<-487/ACK (1a)| | | | |
| | |<-487/ACK (1b)-----| | | |
| | |-INVITE-(1c)----------->| | |
| | |-INVITE-(1d)---------------->| |
| | |<---18x-(1c)------------| | |
| |<----18x-(1c)--|<---18x-(1d)-----------------| |
| |<----18x-(1d)--| | | | | |
| | | proxy sets | | | | |
| | Bob gets | timer for | | | | |
| | impatient | 6 more secs | | | | |
|<-REFER/202---| | | | | | |
|--INVITE w/Replace to-tag=* ->| | | | | |
| | |-INVITE w/Replaces(2c)->| | |
| | |-INVITE w/Replaces(2d)------>| |
| | |<---18x-(2c)------------| | |
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|<---18x (2c)------------------|<-687/ACK (1c)----------| | |
| | |<---18x-(2d)-----------------| |
|<---18x (2d)------------------|<-687/ACK (1d)---------------| |
| |<----687 (1)---| | | | | |
| |-----ACK (1)-->| | | | | |
|<---BYE/200---| | call forward | | | | |
| | | timer expires| | | | |
| | | | | | | |
| | |-CANCEL/200------------>| | |
| | |-CANCEL/200----------------->| |
| | |<-487/ACK (2c)----------| | |
| | |<-487/ACK (2d)----------| | |
| | |-INVITE-(2e)--------------------->|
| | |<---200-(2e)----------------------|
|<---200 (2e)------------------| | | | | |
|----ACK--------------------------------------------------------->|
|--NOTIFY/200->| | | | | | |
5.8 A non-compliant forking proxy
The following example illustrates two of the consequences of a
forking proxy not supporting the Replaces extension. An annoying but
less serious consequence is that the proxy is likely to start call
forwarding timers from scratch (see the "#" sign in the call flow),
resulting in longer delays than in the example in Section 5.7. A
more serious consequence is that if an INVITE with a Replaces header
is lost, as in INVITE 2d, a race condition can arise whereby a
parallel branch (2c) of the same INVITE with Replaces (2) request
will trigger a 687 to be sent to terminate the replaced branch (1c).
Since the proxy does not understand the linkage between the matching
INVITE and the replacement INVITE, the proxy will CANCEL remaining
branches (1d) for INVITE 1. When the proxy later retransmits INVITE
2d, the request will fail, because the matching dialog was previously
cancelled.
Note however that despite these two problems (the race condition
being unlikely, and the timing problem being merely annoying), the
proxy will still work well enough that Alice's INVITE will continue
to follow the proxy's chain of contact logic past the set of contacts
originally visited by Bob. This is the intent of an INVITE request
with a Replaces header with the wildcard to-tag parameter.
The most serious consequence of a forking proxy not supporting
Replaces is that it may decide to fork the replacement INVITE request
to a completely orthogonal set of Contacts. In this case, neither a
specific-dialog replacement, nor a wildcard replacement will work.
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Cathy's Contacts
desk lab home car VM
Alice Bob Proxy C1 C2 C3 C4 C5
| | | | | | | |
| stable call | | | | | | |
|<============>| | | | | | |
| |--INVITE-(1)-->| | | | | |
| | |-INVITE-(1a)->| | | | |
| | |-INVITE-(1b)------>| | | |
| | |<---18x-(1a)--| | | | |
| |<----18x-(1a)--|<---18x-(1b)-------| | | |
| |<----18x-(1b)--| | | | | |
| | | proxy waits | | | | |
| | | 6 seconds | | | | |
| | | | | | | |
| | |-CANCEL/200-->| | | | |
| | |-CANCEL/200------->| | | |
| | |<-487/ACK (1a)| | | | |
| | |<-487/ACK (1b)-----| | | |
| | |-INVITE-(1c)----------->| | |
| | |-INVITE-(1d)---------------->| |
| | |<---18x-(1c)------------| | |
| |<----18x-(1c)--|<---18x-(1d)---------------->| |
| |<----18x-(1d)--| | | | | |
| | | | | | | |
| | Bob gets | | | | | |
| | impatient | | | | | |
|<-REFER/202---| | | | | | |
|--INVITE w/Replace to-tag=* ->| | | | | |
| | |-INVITE (2a)->| | | | |
| | | Replaces * | | | | |
| | |-INVITE (2b)->| | | | |
| | | Replaces * | | | | |
| | |<---481-(2a)--| | | | |
| | |<---420-(2b)--| | | | |
| | # proxy sets | | | | | |
| | timer for |-INVITE w/Replaces(2c)->| | |
| | 6 more secs|-INVITE w/Replaces(2d)-LOST..|* |
| | |<---18x-(2c)------------| | |
|<---18x (2c)------------------|<-687/ACK (1c)----------| |* |
| |<----687 (1)---|-CANCEL/200-(1d)------------>|* |
| |-----ACK (1)-->|<-487/ACK (1d)---------------| |
|<---BYE/200---| | | | | | |
| | |-INVITE (2d) retransmitted-->|* |
| | |<-481/ACK (2d)---------------|* |
| | | | | | | |
| | | call forward | | | | |
| | | timer expires| | | | |
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| | | | | | | |
| | |-CANCEL/200------------>| | |
| | |<-487/ACK (2c)----------| | |
| | |-INVITE-(2e)--------------------->|
| | |<---200-(2e)----------------------|
|<---200 (2e)------------------| | | | | |
|----ACK--------------------------------------------------------->|
|--NOTIFY/200->| | | | | | |
6. Security Considerations
This extension can be used to disconnect or replace participants of a
multimedia conversation with an attacker. As such, invitations with
the Replaces header SHOULD only be accepted in a dialog in which the
peer has been properly authenticated using a standard SIP mechanism,
and for which message integrity is checked so that the header cannot
be added or modified in transit.
The extensions proposed in this document do not significantly change
the relative security of SIP devices. Currently in SIP, an
eavesdropper who learns the Call-ID, To, and From headers can easily
modify or destroy a dialog using a reINVITE. In practice, dialog
information (Call-ID, to-tag, and from-tag) for most uses of Replaces
is obtained via subscription to a "call-package" event package or via
transitivity using the REFER method. Encryption of SIP signaling to
insure confidentiality of this information is 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.
7. IANA Considerations
7.1 Registration of "Replaces" SIP header
Name of Header: Replaces
Short form: none
Registrant: Rohan Mahy
rohan@cisco.com
Normative description: section 4.1 of this document
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7.2 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
Registrant: Rohan Mahy
rohan@cisco.com
7.3 Registration of "687" SIP Response code
Number of response code: 687
Default reason phrase: Dialog Terminated
Registrant: Rohan Mahy
rohan@cisco.com
Normative description: section 4.3 of this document
8. To Do and Open Issues
8.1 Open Issues:
- Are the proposals for early dialog and terminated dialog matching
acceptable? - Is the proposed tag matching scheme for pre-bis UAs
acceptable?
8.2 To Do:
- Update references
8.3 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.
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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
9. 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] Sparks, R., "The Refer Method", draft-ietf-sip-refer-02 (work in
progress), October 2001.
[3] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC 2119, March 1997.
[4] Crocker, D. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", RFC 2234, November 1997.
Informational References
[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,
"Third Party Call Control in SIP", draft-rosenberg-sip-3pcc-03
(work in progress), November 2001.
[9] Johnston, A., "SIP Service Examples", draft-ietf-sipping-
service-examples-01 (work in progress), April 2002.
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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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Full Copyright Statement
Copyright (C) The Internet Society (2002). All Rights Reserved.
This document and translations of it may be copied and furnished to
others, and derivative works that comment on or otherwise explain it
or assist in its implementation may be prepared, copied, published
and distributed, in whole or in part, without restriction of any
kind, provided that the above copyright notice and this paragraph are
included on all such copies and derivative works. However, this
document itself may not be modified in any way, such as by removing
the copyright notice or references to the Internet Society or other
Internet organizations, except as needed for the purpose of
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English.
The limited permissions granted above are perpetual and will not be
revoked by the Internet Society or its successors or assigns.
This document and the information contained herein is provided on an
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TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING
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HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
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Acknowledgement
Funding for the RFC Editor function is currently provided by the
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Mahy, et al. Expires October 28, 2002 [Page 25]
