SIPPING S. Olson
Internet-Draft Microsoft
Expires: December 19, 2003 June 20, 2003
Extensions to the REFER mechanism for Third Party Call Control
draft-olson-sipping-refer-extensions-00
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 December 19, 2003.
Copyright Notice
Copyright (C) The Internet Society (2003). All Rights Reserved.
Abstract
This document proposes a number of extensions to the REFER method
used by the Session Initiation Protocol (SIP) for the purpose of
third party call control.
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Table of Contents
1. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4
3. Requirements . . . . . . . . . . . . . . . . . . . . . . . . . 5
4. Use of the application/dialog-info+xml MIME type with REFER . 6
5. Extending REFER to SIP response codes . . . . . . . . . . . . 11
6. Supressing forking of REFER requests . . . . . . . . . . . . . 18
7. Supressing the NOTIFY associated with a REFER . . . . . . . . 19
8. Replacing complex Refer-To URIs with a MIME body . . . . . . . 21
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 24
9.1 norefersub option tag registration . . . . . . . . . . . . . . 24
9.2 refer-response option tag registration . . . . . . . . . . . . 24
10. Security Considerations . . . . . . . . . . . . . . . . . . . 25
11. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 26
References . . . . . . . . . . . . . . . . . . . . . . . . . . 27
Author's Address . . . . . . . . . . . . . . . . . . . . . . . 27
Intellectual Property and Copyright Statements . . . . . . . . 28
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1. Terminology
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 [1].
To simplify discussions of the REFER method and its extensions, three
new terms will be defined:
REFER-Issuer: the UA issuing the REFER request
REFER-Recipient: the UA receiving the REFER request
REFER-Target: the UA designated in the Refer-To URI
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2. Introduction
The REFER [3] extension to SIP [2] defines a basis for third party
call control that is useful for implementing features such as call
transfer. REFER has a few limitations with respect to implementing
more advanced features in conjunction with the dialog info event
package [7]. These limitations derive in part from the limited
information available in the message/sipfrag [4] content of the
associated NOTIFY. Another limiting factor is the requirement to
compress the semantics of the referred request in the Refer-To URI by
encoding the desired headers as parameters of that URI. Finally,
there are situations where the party issuing the REFER request does
not need the NOTIFY associated with the REFER, perhaps because that
agent is already subscribed to the appropriate dialog package outside
of the REFER request. This represents additional unnecessary traffic
and state in the REFER-Issuer and REFER-recipient.
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3. Requirements
Requirement 1: Allow the REFER-Issuer to watch the progress of a
REFER beyond the end of the INVITE transaction and for the
duration of the dialog
Requirement 2: Expose the dialog information of the dialog
established at the REFER-Recipient as a result of the REFER
Requirement 3: Allow specification of much richer detail of the
request that is generated at the REFER-Recipient as a result of
the REFER beyond just the method and Request-URI. Examples include
caller preferences, portions of the SDP, and user defined headers.
Requirement 4: Allow one user agent to specify that another user
agent answer an incoming call
Requirement 5: Prevent forking of a REFER request and allow targeting
of that REFER request to a specific device or class of device.
Requirement 6: Reduce the number of messages exchanged to perform a
REFER when the REFER-Issuer and REFER-Recipient support richer
call control primitives such as the dialog and conference event
packages.
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4. Use of the application/dialog-info+xml MIME type with REFER
The REFER specification [3] mandates the use of the message/sipfrag
[4] MIME type for NOTIFYs of the refer event package. These NOTIFYs
are sent as part of the implicit subscription created by the REFER.
The purpose of the NOTIFY is to communicate the state of the
transaction between the REFER-Recipient and the REFER-Target that is
created as a result of the REFER
Where the purpose of sending the REFER is actually to establish a
dialog, for example through an INVITE in the Refer-To, the derivative
state of interest is actually the dialog state. While this may be
inferred from the contents of the message/sipfrag body it is a bit
clumsy and in many cases the REFER-Recipient will omit important
details of the dialog in the message/sipfrag body. Two things that
are generally lacking from the message/sipfrag content are the dialog
identifier (Call-ID plus local and remote tags) and the state of the
dialog. Not coincidentally, this is the same information available in
the application/dialog-info+xml MIME type [7] used for the dialog
event package.
To address this shortcoming, it is proposed to extend REFER to allow
the (optional) use of the application/dialog-info+xml MIME type in
place of message/sipfrag. The REFER-Issuer specifies support for this
by including this MIME type in an Accept header. The REFER-Issuer
MUST also support message/sipfrag for compatibility with strictly
RFC3515 compliant implementations. If the REFER-Recipient supports
this extension, it may choose to honor the request or default to
using message/sipfrag dependent on local policy. It is OPTIONAL for
the REFER-Recipient to maintain the subscription for the duration of
the resultant (INVITE) dialog and RECOMMENDED that the subscription
be maintained at least until the dialog is in "confirmed" state.
Figure 1: Example of using application/dialog-info+xml
REFER-Issuer REFER-Recipient REFER-Target
| | |
| M1 REFER (INVITE) | |
|--------------------------->| |
| M2 202 Accepted | |
|<---------------------------| |
| M3 NOTIFY | |
|<---------------------------| |
| M4 200 OK | |
|--------------------------->| |
| | M5 INVITE |
| |----------------------->|
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| M6 NOTIFY | |
|<---------------------------| |
| M7 200 OK | |
|--------------------------->| |
| | M8 180 Ringing |
| |<-----------------------|
| M9 NOTIFY | |
|<---------------------------| |
| M10 200 OK | |
|--------------------------->| |
| | M11 200 OK |
| |<-----------------------|
| M12 NOTIFY | |
|<---------------------------| |
| M13 200 OK | |
|--------------------------->| M14 ACK |
| |----------------------->|
Message flow:
M1: The REFER-Issuer creates a REFER, specifying a preference for the
application/dialog-info+xml MIME type.
REFER sip:b@tradewind.com SIP/2.0
Via: SIP/2.0/TCP issuer.tradewind.com;branch=z9hG4bK-a-1
From: <sip:a@tradewind.com>;tag=1ab
To: <sip:b@tradewind.com>
Call-ID: 1@issuer.tradewind.com
CSeq: 234234 REFER
Max-Forwards: 70
Refer-To: sip:c@tradewind.com;method=INVITE
Accept: application/dialog-info+xml;q=0.5, message/sipfrag;q=0.1
Contact: sip:a@issuer.tradewind.com
Content-Length: 0
M5: The REFER-Recipient creates an appropriate INVITE based on the
REFER and sends it to the REFER-Target.
INVITE sip:c@tradewind.com SIP/2.0
Via: SIP/2.0/TCP recipient.tradewind.com;branch=z9hG4bK-b-1
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From: <sip:b@tradewind.com>;tag=1bc
To: <sip:c@tradewind.com>
Call-ID: 1@recipient.tradewind.com
CSeq: 1234567 INVITE
Max-Forwards: 70
Contact: sip:b@recipient.tradewind.com
Content-Length: ...
<SDP for audio call>
M6: The REFER-Recipient sends a NOTIFY triggered by the INVITE being
sent to the REFER-Target. The body of the NOTIFY contains the
dialog identifier and current state ("trying").
NOTIFY sip:a@issuer.tradewind.com SIP/2.0
Via: SIP/2.0/TCP recipient.tradewind.com;branch=z9hG4bK-b-2
From: <sip:a@tradewind.com>;tag=1ab
To: <sip:b@tradewind.com>;tag=1ba
Call-ID: 1@issuer.tradewind.com
CSeq: 1278784 NOTIFY
Max-Forwards: 70
Event: refer;id=234234
Subscription-State: active;expires=3600
Contact: sip:b@recipient.tradewind.com
Content-Type: application/dialog-info+xml
Content-Length: ...
<?xml version="1.0" ?>
<dialog-info xmlns="urn:ietf:params:xml:ns:dialog-info"
version="1"
state="partial"
entity="sip:b@tradewind.com">
<dialog id="2" call-id="1@recipient.tradewind.com"
local-tag="1bc"
direction="initiator">
<state>trying</state>
</dialog>
</dialog-info>
M9: The REFER-Recipient sends a NOTIFY triggered by the 180 Ringing
received from the REFER-Target. The body of the NOTIFY contains
the dialog identifier and current state ("early").
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NOTIFY sip:a@issuer.tradewind.com SIP/2.0
Via: SIP/2.0/TCP recipient.tradewind.com;branch=z9hG4bK-b-3
From: <sip:a@tradewind.com>;tag=1ab
To: <sip:b@tradewind.com>;tag=1ba
Call-ID: 1@issuer.tradewind.com
CSeq: 1278785 NOTIFY
Max-Forwards: 70
Event: refer;id=234234
Subscription-State: active;expires=3600
Contact: sip:b@recipient.tradewind.com
Content-Type: application/dialog-info+xml
Content-Length: ...
<?xml version="1.0" ?>
<dialog-info xmlns="urn:ietf:params:xml:ns:dialog-info"
version="2"
state="partial"
entity="sip:b@tradewind.com">
<dialog id="2" call-id="1@recipient.tradewind.com"
local-tag="1bc" remote-tag="1cb"
direction="initiator">
<state>early</state>
</dialog>
</dialog-info>
M12: The REFER-Recipient sends a NOTIFY triggered by the 200 OK
received from the REFER-Target. The body of the NOTIFY contains
the dialog identifier and current state ("confirmed").
NOTIFY sip:a@issuer.tradewind.com SIP/2.0
Via: SIP/2.0/TCP recipient.tradewind.com;branch=z9hG4bK-b-4
From: <sip:a@tradewind.com>;tag=1ab
To: <sip:b@tradewind.com>;tag=1ba
Call-ID: 1@issuer.tradewind.com
CSeq: 1278786 NOTIFY
Max-Forwards: 70
Event: refer;id=234234
Subscription-State: active;expires=3600
Contact: sip:b@recipient.tradewind.com
Content-Type: application/dialog-info+xml
Content-Length: ...
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<?xml version="1.0" ?>
<dialog-info xmlns="urn:ietf:params:xml:ns:dialog-info"
version="3"
state="partial"
entity="sip:b@tradewind.com">
<dialog id="2" call-id="1@recipient.tradewind.com"
local-tag="1bc" remote-tag="1cb"
direction="initiator">
<state>confirmed</state>
</dialog>
</dialog-info>
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5. Extending REFER to SIP response codes
REFER is currently defined to trigger the sending of a request from
the REFER-Recipient to the REFER-Target. The intention is most often
to initiate a dialog from the REFER-Recipient to the REFER-Target.
This is an excellent way to generate an action at the REFER-Recipient
based on an event or action that takes places at the REFER-Issuer.
The classic example is call transfer as a result of a user at the
REFER-Issuer taking some action.
With the use of the dialog event package, it is possible for one UA
to monitor events at another UA related to a dialog, such as the
receipt of an INVITE to establish a new dialog. What is lacking is a
way for the watcher to indicate what should be the response to such
an INVITE request. For example, the dialog watcher would like the
recipient of the session initiation request to accept the initiation
(send a 200 OK response to the INVITE request). One motivating
scenario for this is a set of co-operating User Agents (devices) that
belong to the same user. The user, while using one SIP device, wishes
to answer a call that is being received on another of that user's SIP
devices. This gives the user a single UI focus for control while
allowing multiple devices with differing capabilities.
To enable such a scenario, this document proposes an extension to the
SIP(S) URI syntax as defined in SIP [2] The extension is analogous to
the "method" uri-parameter that currently exists to communicate a
method for use in the Refer-To header. A new uri-parameter,
"response", is proposed that is used in conjunction with the "method"
uri-parameter and associated call-id, local tag, and remote tag to
request that the REFER-Recipient send a response within the
identified SIP transaction to the REFER-Target.
The REFER-Issuer MUST specify a "method" parameter in addition to the
"response" parameter. The REFER-Issuer MUST also include the
appropriate local-uri, local-tag, remote-uri, and remote-tag encoded
as From and To headers in the Refer-To URI (or using a message/
sipfrag body as defined later in this document). Note that in order
to satisfy this requirement, the REFER-Issuer must have access to
this information. In particular, it is assumed that the REFER-Issuer
receives the local-uri and remote-uri in the NOTIFY for the dialog
event package from the REFER-Recipient. These elements are optional
in the XML schema. It is anticipated that User Agents that support
these REFER extensions will also include these optional elements in
the application/dialog-info+xml payload (as privacy concerns allow).
To ensure the REFER-Recipient conformant with RFC3515 does not
misintepret this as a REFER to send a request of the specified
method, the REFER-Issuer MUST also include a Require: refer-response
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header in the REFER request. REFER-Recipients which do not understand
this extension will return a 420 response. The REFER-Target does not
need to understand this extension for this to work. Support for this
extension can be queried in advance using a standard OPTIONS request.
The REFER-Issuer MUST request the use of the application/
dialog-info+xml MIME type in NOTIFYs associated with a REFER request
which uses this "response" extension.
The proposed syntax modification follows
uri-parameters = *( ";" uri-parameter)
uri-parameter = transport-param / user-param / method-param
/ ttl-param / maddr-param / lr-param / response-param / other-param
response-param = "response=" 1*3DIGIT
An example call flow follows:
Figure 2: Example of using the "response" uri-parameter in the
Refer-To header
REFER-Issuer REFER-Recipient REFER-Target
| | |
| N1 SUBSCRIBE (dialog) | |
|--------------------------->| |
| N2 202 Accepted | |
|<---------------------------| |
| N3 NOTIFY | |
|<---------------------------| |
| N4 200 OK | |
|--------------------------->| |
| | N5 INVITE |
| |<-----------------------|
| N6 NOTIFY | |
|<---------------------------| |
| N7 200 OK | |
|--------------------------->| |
| | N8 180 Trying |
| |----------------------->|
| N9 NOTIFY | |
|<---------------------------| |
| N10 200 OK | |
|--------------------------->| |
| | |
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| N11 REFER (200) | |
|--------------------------->| |
| N12 200 OK | |
|<---------------------------| N13 200 |
| |----------------------->|
| N14 NOTIFY | |
|<---------------------------| |
| N15 200 OK | |
|--------------------------->| |
Message flow:
N1: The REFER-Issuer subscribes to the dialog event package at the
REFER-Recipient.
SUBSCRIBE sip:b@tradewind.com SIP/2.0
Via: SIP/2.0/TCP issuer.tradewind.com;branch=z9hG4bK-a-1
From: <sip:a@tradewind.com>;tag=1ab
To: <sip:b@tradewind.com>
Call-ID: 1@issuer.tradewind.com
CSeq: 234234 SUBSCRIBE
Max-Forwards: 70
Event: dialog
Accept: application/dialog-info+xml
Contact: sip:a@issuer.tradewind.com
Content-Length: 0
N5: The REFER-Recipient receives an INVITE from the REFER-Target to
start a new call.
INVITE sip:b@tradewind.com SIP/2.0
Via: SIP/2.0/TCP target.tradewind.com;branch=z9hG4bK-c-1
From: <sip:c@tradewind.com>;tag=1cb
To: <sip:b@tradewind.com>
Call-ID: 1@target.tradewind.com
CSeq: 1234567 INVITE
Max-Forwards: 70
Contact: sip:c@target.tradewind.com
Content-Length: ...
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<SDP for an audio call>
N6: The REFER-Recipient sends a NOTIFY triggered by the INVITE
received from the REFER-Target. The body of the NOTIFY contains
the dialog identifier and current state ("trying").
NOTIFY sip:a@issuer.tradewind.com SIP/2.0
Via: SIP/2.0/TCP recipient.tradewind.com;branch=z9hG4bK-b-2
From: <sip:a@tradewind.com>;tag=1ab
To: <sip:b@tradewind.com>;tag=1ba
Call-ID: 1@issuer.tradewind.com
CSeq: 454545 NOTIFY
Max-Forwards: 70
Event: dialog;id=234234
Subscription-State: active;expires=3600
Contact: sip:b@recipient.tradewind.com
Content-Type: application/dialog-info+xml
Content-Length: ...
<?xml version="1.0" ?>
<dialog-info xmlns="urn:ietf:params:xml:ns:dialog-info"
version="1"
state="partial"
entity="sip:b@tradewind.com">
<dialog id="2" call-id="1@target.tradewind.com"
remote-tag="1cb"
direction="recipient">
<local-uri>b@tradewind.com</local-uri>
<remote-uri>c@tradewind.com</remote-uri>
<state>trying</state>
</dialog>
</dialog-info>
N8: The REFER-Recipient sends a 180 Ringing response to the
REFER-Target.
SIP/2.0 180 Ringing
Via: SIP/2.0/TCP target.tradewind.com;branch=z9hG4bK-b-3
From: <sip:c@tradewind.com>;tag=1cb
To: <sip:b@tradewind.com>;tag=1bc
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Call-ID: 1@target.tradewind.com
CSeq: 1234567 INVITE
Contact: sip:b@recipient.tradewind.com
Content-Length: 0
N9: The REFER-Recipient sends a NOTIFY triggered by the 180 Ringing
sent to the REFER-Target. The body of the NOTIFY contains the
dialog identifier and current state ("early").
NOTIFY sip:a@issuer.tradewind.com SIP/2.0
Via: SIP/2.0/TCP recipient.tradewind.com;branch=z9hG4bK-b-4
From: <sip:a@tradewind.com>;tag=1ab
To: <sip:b@tradewind.com>;tag=1ba
Call-ID: 1@issuer.tradewind.com
CSeq: 454546 NOTIFY
Max-Forwards: 70
Event: dialog;id=234234
Subscription-State: active;expires=3600
Contact: sip:b@recipient.tradewind.com
Content-Type: application/dialog-info+xml
Content-Length: ...
<?xml version="1.0" ?>
<dialog-info xmlns="urn:ietf:params:xml:ns:dialog-info"
version="2"
state="partial"
entity="sip:b@tradewind.com">
<dialog id="2" call-id="1@target.tradewind.com"
local-tag="1bc" remote-tag="1cb"
direction="recipient">
<local-uri>b@tradewind.com</local-uri>
<remote-uri>c@tradewind.com</remote-uri>
<state event="1xx-tag">early</state>
</dialog>
</dialog-info>
N11: The REFER-Issuer creates a REFER, specifying that the
REFER-Recipient should send a 200 OK to accept the session
invitation. The From and To headers of the 200 OK are encoded in
the Refer-To URI. The local and remote tags for this are
determined from the information provided in the NOTIFY for the
dialog package. This allows the REFER-Issuer to specify a
particular dialog. Combined with the "method" parameter, this
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identifies a specific transaction within the dialog.
REFER sip:b@tradewind.com SIP/2.0
Via: SIP/2.0/TCP issuer.tradewind.com;branch=z9hG4bK-a-2
From: <sip:a@tradewind.com>;tag=1ab
To: <sip:b@tradewind.com>
Call-ID: 2@issuer.tradewind.com
CSeq: 818181 REFER
Max-Forwards: 70
Accept: application/dialog-info+xml;q=0.5, message/sipfrag;q=0.1
Require: refer-response
Refer-To: sip:c@tradewind.com;method=INVITE;response=200?Call-ID=1%40target.tradewind.com&
To=b%40tradewind.com;tag=1bc&From=c%40tradewind.com;tag=1cb
Contact: sip:a@issuer.tradewind.com
Content-Length: 0
N13: The REFER-Recipient sends a 200 OK to the REFER-Target
constructed using the information in the Refer-To header.
SIP/2.0 200 OK
Via: SIP/2.0/TCP target.tradewind.com;branch=z9hG4bK-c-1
From: <sip:c@tradewind.com>;tag=1cb
To: <sip:b@tradewind.com>;tag=1bc
Call-ID: 1@target.tradewind.com
Supported: refer-response
CSeq: 1234567 INVITE
Contact: sip:b@recipient.tradewind.com
Content-Length: 0
N14: The REFER-Recipient sends a NOTIFY triggered by the 200 OK sent
to the REFER-Target. The body of the NOTIFY contains the dialog
identifier and current state ("confirmed").
NOTIFY sip:a@issuer.tradewind.com SIP/2.0
Via: SIP/2.0/TCP recipient.tradewind.com;branch=z9hG4bK-b-4
From: <sip:a@tradewind.com>;tag=1ab
To: <sip:b@tradewind.com>;tag=1ba
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Call-ID: 1@issuer.tradewind.com
CSeq: 454547 NOTIFY
Max-Forwards: 70
Event: dialog;id=234234
Subscription-State: active;expires=3600
Contact: sip:b@recipient.tradewind.com
Content-Type: application/dialog-info+xml
Content-Length: ...
<?xml version="1.0" ?>
<dialog-info xmlns="urn:ietf:params:xml:ns:dialog-info"
version="3"
state="partial"
entity="sip:b@tradewind.com">
<dialog id="2" call-id="1@target.tradewind.com"
local-tag="1bc" remote-tag="1cb"
direction="recipient">
<local-uri>b@tradewind.com</local-uri>
<remote-uri>c@tradewind.com</remote-uri>
<state event="2xx">confirmed</state>
</dialog>
</dialog-info>
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6. Supressing forking of REFER requests
The REFER specification allows for the possibility of forking a REFER
request which is sent outside of an existing dialog. In some
situations forking a REFER may result in absolutely incorrect
behavior. This is especially true when the REFER is intended to
target a specific device, perhaps with specific capabilities.
Fortunately, there is already a mechanism which can address the need
to indicate that a REFER should NOT be forked. The caller preferences
extension [8] defines both a way to indicate a request not be forked
and a means to target a specific device with the desired
capabilities. No further extension is required. An example of the
usage of caller preferences in conjunction with REFER is shown below.
Example of REFER using Caller Preferences extension
REFER sip:b@tradewind.com SIP/2.0
Via: SIP/2.0/TCP issuer.tradewind.com;branch=z9hG4bK-a-1
From: <sip:a@tradewind.com>;tag=1a
To: <sip:b@tradewind.com>
Call-ID: 1@issuer.tradewind.com
CSeq: 234234 REFER
Max-Forwards: 70
Refer-To: sip:c@tradewind.com;method=INVITE
Require: pref
Request-Dispostion: no-fork
Accept-Contact: *;audio;require
Reject-Contact: *;video
Contact: sip:a@issuer.tradewind.com
Content-Length: 0
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7. Supressing the NOTIFY associated with a REFER
The REFER specification mandates that every REFER create an implicit
subscription between the REFER-Issuer and the REFER-Recipient. This
subscription results in at least one NOTIFY being sent from the
REFER-Recipient to the REFER-Issuer. The REFER-Recipient may choose
to cancel the implicit subscription with this NOTIFY. The
REFER-Issuer may choose to cancel this implicit subscription with an
explicit SUBSCRIBE (Expires: 0) after receipt of the initial NOTIFY
or by sending a 481 response to this initial NOTIFY request.
The purpose of requiring the implicit subcription and initial NOTIFY
is to allow for the situation where the REFER request gets forked and
the REFER-Issuer needs a way to see the multiple dialogs that may be
established as a result of the forked REFER. This is the same
approach used to handle forking of SUBSCRIBE [5] requests. Where the
REFER-Issuer explicitly specifies that forking not occur via caller
preferences, or implicitly does not allow for forking by sending the
REFER within an existing dialog, the requirement that an implicit
subscription be established is unnecessary.
Another purpose of the NOTIFY is to inform the REFER-Issuer of the
progress of the SIP transaction that results from the REFER at the
REFER-Recipient. In the case where the REFER-Issuer is already aware
of the progress of the SIP transaction, such as when the REFER-Issuer
has an explicit subscription to the dialog event package at the
REFER-Recipient, the implicit subscription and resultant NOTIFY
traffic related to the REFER is superfluous and unnecessary network
overhead.
To avoid this unnecessary overhead, it is proposed that the
REFER-Issuer insert a Supported: norefersub header in the REFER
request to indicate to the REFER-Recipient that no implicit
subscription or NOTIFY is needed with respect to this REFER request.
This MUST be used by the REFER-Issuer only when the REFER-Issuer can
be certain that the REFER request will not be forked. The
REFER-Recipient MUST signal support for this extension by inserting a
Supported: norefersub header in the 2xx response to the REFER
request.
Example of REFER which supresses the implicit subscription
REFER sip:b@tradewind.com SIP/2.0
Via: SIP/2.0/TCP issuer.tradewind.com;branch=z9hG4bK-a-1
From: <sip:a@tradewind.com>;tag=1a
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To: <sip:b@tradewind.com>
Call-ID: 1@issuer.tradewind.com
CSeq: 234234 REFER
Max-Forwards: 70
Refer-To: sip:c@tradewind.com;method=INVITE
Request-Dispostion: no-fork
Accept-Contact: *;audio;require
Supported: norefersub
Contact: sip:a@issuer.tradewind.com
Content-Length: 0
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8. Replacing complex Refer-To URIs with a MIME body
Much of the semantics of the REFER request is encapsulated in the
Refer-To URI. This URI will commonly encode the method, From, To,
Call-ID, Accept-Disposition, Accept-Contact [8], and other headers
all in a properly escaped URI. Such a URI can become long, difficult
to debug, and prone to URI escaping errors in SIP implementations.
The situation becomes more complex if the method is itself a REFER
complete with a Refer-To which must contain URI-escaped characters.
This double escaping obfuscates things even more and increases the
chances of improperly escaping/unescaping of the Refer-To URI.
The alternative that is proposed is to make use of the currently
unused REFER body to encapsulate all the information that is
potentially escaped in the Refer-To URI. The possible information
includes the method, headers, and body of the request which is
desired to be created by the REFER-Recipient. The ideal candidate for
this is the message/sipfrag MIME type. This MIME type can express all
of these: the method, Request-URI, headers, and body. This applies to
requests as well as responses (as an extension).
There are two possibilities: use a cid URI [6] in the Refer-To header
to reference the message/sipfrag content in the body of the REFER or
continue to place the URI of the REFER-Target in the Refer-To header
and place all other facets of the request in the message/sipfrag body
with the understanding that the presence of the body implies its
usage for this purpose. A few examples should illuminate things.
Before
REFER sip:b@tradewind.com SIP/2.0
Via: SIP/2.0/TCP issuer.tradewind.com;branch=z9hG4bK-a-1
From: <sip:a@tradewind.com>;tag=1a
To: <sip:b@tradewind.com>
Call-ID: 1@issuer.tradewind.com
CSeq: 234234 REFER
Max-Forwards: 70
Refer-To: sip:c@tradewind.com;method=INVITE;response=200?Call-ID=1%40target.tradewind.com&
From=b%40tradewind.com;tag=2b&To=c%40tradewind.com;tag=1c
Accept: application/dialog-info+xml;q=0.5, message/sipfrag;q=0.1
Require: refer-response
Contact: sip:a@issuer.tradewind.com
Content-Length: 0
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After (placing the REFER-Target in the Refer-To)
REFER sip:b@tradewind.com SIP/2.0
Via: SIP/2.0/TCP issuer.tradewind.com;branch=z9hG4bK-a-1
From: <sip:a@tradewind.com>;tag=1a
To: <sip:b@tradewind.com>
Call-ID: 1@issuer.tradewind.com
CSeq: 234234 REFER
Max-Forwards: 70
Refer-To: sip:c@tradewind.com;method=INVITE
Accept: application/dialog-info+xml;q=0.5, message/sipfrag;q=0.1
Require: refer-response
Contact: sip:a@issuer.tradewind.com
Content-Type: message/sipfrag
Content-Length: ...
SIP/2.0 200 OK
Call-ID: 1@target.tradewind.com
From: b@tradewind.com;tag=2b
To: c@tradewind.com;tag=1c
After (using a cid URI)
REFER sip:b@tradewind.com SIP/2.0
Via: SIP/2.0/TCP issuer.tradewind.com;branch=z9hG4bK-a-1
From: <sip:a@tradewind.com>;tag=1a
To: <sip:b@tradewind.com>
Call-ID: 1@issuer.tradewind.com
CSeq: 234234 REFER
Max-Forwards: 70
Refer-To: cid:1239103912039@issuer.tradewind.com
Accept: application/dialog-info+xml;q=0.5, message/sipfrag;q=0.1
Require: refer-response
Contact: sip:a@issuer.tradewind.com
Content-Type: message/sipfrag
Content-Id: <1239103912039@issuer.tradewind.com>
Content-Length: ...
SIP/2.0 200 OK
Call-ID: 1@target.tradewind.com
From: b@tradewind.com;tag=2b
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To: c@tradewind.com;tag=1c
Note that the target URI and method are still included in the
Refer-To header.
If the REFER-Recipient does not understand this extension, it MUST
return a 4xx response to the REFER request. The REFER-Issuer SHOULD
re-issue the REFER request using URI escaping and only the Refer-To:
URI to convey the same information if possible.
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9. IANA Considerations
9.1 norefersub option tag registration
This document defines a new option tag, norefersub, which specifies
that an implicit subscription for event package refer should not be
created as a result of accepting this REFER request. This option tag
is only meaningful for the REFER request defined in RFC3515.
9.2 refer-response option tag registration
This document defines a new option tag, refer-response, which
specifies that the recipient of the REFER request is expected to
issue a response for the SIP transaction identified within the
Refer-To URI. This option tag is only meaningful for the REFER
request defined in RFC3515.
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10. Security Considerations
No additional concerns above what is defined in RFC3515.
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11. Acknowledgements
The author would like to thank Rohan Mahy for his insightful
comments. The author would also like to thank Sriram Parameswar for
his draft-parameswar-sipping-norefersub-00 proposal which is the
basis for the norefersub option tag proposed in this draft.
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References
[1] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC 2119, March 1997.
[2] 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.
[3] Sparks, R., "The Session Initiation Protocol (SIP) Refer
Method", RFC 3515, April 2003.
[4] Sparks, R., "Internet Media Type message/sipfrag", RFC 3420,
November 2002.
[5] Roach, A., "Session Initiation Protocol (SIP)-Specific Event
Notification", RFC 3265, June 2002.
[6] Levinson, E., "Content-ID and Message-ID Uniform Resource
Locators", RFC 2392, August 1998.
[7] Rosenberg, J. and H. Schulzrinne, "An INVITE Inititiated Dialog
Event Package for the Session Initiation Protocol (SIP",
draft-ietf-sipping-dialog-package-01 (work in progress), March
2003.
[8] Rosenberg, J., Schulzrinne, H. and P. Kyzivat, "Caller
Preferences and Callee Capabilities for the Session Initiation
Protocol (SIP)", draft-ietf-sip-callerprefs-08 (work in
progress), March 2003.
Author's Address
Sean Olson
Microsoft
One Microsoft Way
Redmond, WA 98052
US
Phone: +1-425-707-2846
EMail: seanol@microsoft.com
URI: http://www.microsoft.com/rtc
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