Request for Comments: 3891 Cisco Systems, Inc.
Category: Standards Track B. Biggs
R. Dean
September 2004
The Session Initiation Protocol (SIP) "Replaces" Header
Status of this Memo
This document specifies an Internet standards track protocol for the
Internet community, and requests discussion and suggestions for
improvements. Please refer to the current edition of the "Internet
Official Protocol Standards" (STD 1) for the standardization state
and status of this protocol. Distribution of this memo is unlimited.
Copyright Notice
Copyright (C) The Internet Society (2004).
Abstract
This document defines a new header for use with Session Initiation
Protocol (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 "Call
Pickup". Note that the definition of these example features is non-
normative.
Table of Contents
1. Overview. . . . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Conventions . . . . . . . . . . . . . . . . . . . . . . . . . 4
3. User Agent Server Behavior: Receiving a Replaces Header . . . 4
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. . . . . . . . . . . . . . . . . . . . . . . . 9
7.1. Replacing an Early Dialog at the Originator . . . . . . 9
8. Security Considerations . . . . . . . . . . . . . . . . . . . 11
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 13
9.1. Registration of "Replaces" SIP Header . . . . . . . . . 13
9.2. Registration of "replaces" SIP Option-tag . . . . . . . 13
10. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 13
11. References. . . . . . . . . . . . . . . . . . . . . . . . . . 13
11.1. Normative References. . . . . . . . . . . . . . . . . . 13
11.2. Informative References. . . . . . . . . . . . . . . . . 14
12. Authors’ Addresses. . . . . . . . . . . . . . . . . . . . . . 15
13. Full Copyright Statement. . . . . . . . . . . . . . . . . . . 16
1. Overview
This document describes a SIP [1] extension header field as part of
the SIP multiparty applications architecture framework [10]. 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 [11] 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 dialog 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 [8] method as used in a
Transfer [12], 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
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 mechanism. Perhaps she subscribed to this information from the
Parking Place (using the session dialog package [13]), 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 |
|<--NOTIFY/200 (trying)-----------|--INVITE/200/ACK-->|
|<--NOTIFY/200 (success)----------|<=================>|
|------------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
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
2. 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 BCP 14, RFC 2119 [2].
This document refers frequently to the terms "confirmed dialog" and
"early dialog". These are defined in Section 12 of SIP [1].
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 User Agent (UA) attempts to match this
information with a confirmed or early dialog. The User Agent Server
(UAS) matches the to-tag and from-tag parameters 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 matched invitation was just terminated, the
replacement request should fail as well. Declining the request with
a 600-class response prevents an irritating race-condition where the
UA rings or alerts for a replacement call which is not wanted.)
If the Replaces header field matches an active dialog, the UA MUST
verify that the initiator of the new INVITE is authorized to replace
the matched dialog. If the initiator of the new INVITE has been
successfully authenticated as equivalent to the user who is being
replaced, then the replacement is authorized. For example, if the
user being replaced and the initiator of the replacement dialog share
the same credentials for Digest authentication [6], or they sign the
replacement request with S/MIME [7] with the same private key and
present the (same) corresponding certificate used in the original
dialog, then the replacement is authorized.
Alternatively, the Referred-By mechanism [4] 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 the replacement request contains a
Referred-By header that corresponds to the user being replaced, the
UA SHOULD treat the replacement as if the replacement was authorized
by the replaced party. The Referred-By header SHOULD reference a
corresponding, valid Refererred-By Authenticated Identity Body [5].
The UA MAY apply other local policy to authorize the remainder of the
request. In other words, the UAS may apply a different policy to the
replacement dialog than was applied to the replaced dialog.
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.
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 checks
for the presence of the "early-only" flag in the Replaces header
field. (This flag allows the UAC to prevent a potentially
undesirable race condition described in Section 7.1.) If the flag is
present, the UA rejects the request with a 486 Busy response.
Otherwise, 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
User Agent Client (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. If the user agent only
wishes to replace an early dialog (as in the Call Pickup example in
Section 7.1), the UAC MAY also include the "early-only" parameter in
the Replaces header field. A UAC MUST NOT send an INVITE with a
Replaces header field that attempts to replace an early dialog which
was not originated by the target of the INVITE with a 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.
For example, if Alice replaces Cathy in an early dialog with Bob, but
Bob 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. Although this specification takes reasonable
precautions to prevent unexpected behavior in the face of forking,
implementations SHOULD only address replacement requests (i.e., set
the Request-URI of the replacement request) to the SIP Contact URI of
the target.
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 other 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, PRACK, and PUBLISH are defined
respectively in [15], [16], [8], [17], [18], [19], and [20].
Header field where proxy ACK BYE CAN INV OPT REG MSG
------------ ----- ----- --- --- --- --- --- --- ---
Replaces R - - - o - - -
SUB NOT REF INF UPD PRA PUB
--- --- --- --- --- --- ---
Replaces R - - - - - - -
The following syntax specification uses the augmented Backus-Naur
Form (BNF) as described in RFC 2234 [3]. The syntax below relies on
a number of productions from SIP [1].
Replaces = "Replaces" HCOLON callid *(SEMI replaces-param)
replaces-param = to-tag / from-tag / early-flag / generic-param
to-tag = "to-tag" EQUAL token
from-tag = "from-tag" EQUAL token
early-flag = "early-only"
A Replaces header field 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 RFC 2543 [9] compliant UAs, a
tag of zero matches both tags of zero and null. A Replaces header
field MAY contain the early-flag.
Examples:
Replaces: 98732@sip.example.com
;from-tag=r33th4x0r
;to-tag=ff87ff
Replaces: 12adf2f34456gs5;to-tag=12345;from-tag=54321;early-only
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 SIP
Service Examples [14]. 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
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 of taking 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
CSeq: 1 INVITE
Contact: <sip:bob@labpc.example.org>
Replaces: 425928@phone.example.org
;to-tag=7743;from-tag=6472;early-only
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