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Versions: (draft-camarillo-mmusic-connection-precon)
00 01
MMUSIC Working Group G. Camarillo
Internet-Draft Ericsson
Expires: June 2, 2005 December 2, 2004
Connection-Establishment Preconditions in the Session Initiation
Protocol (SIP)
draft-ietf-mmusic-connection-precon-01.txt
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Copyright Notice
Copyright (C) The Internet Society (2004).
Abstract
This document defines the connection-establishment precondition type
for the SIP preconditions framework. Connection-establishment
preconditions are met when a transport connection (e.g., a TCP
connection) is successfully established between two endpoints.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Precondition Tag . . . . . . . . . . . . . . . . . . . . . . . 3
4. Status Type . . . . . . . . . . . . . . . . . . . . . . . . . 3
5. Direction Tag . . . . . . . . . . . . . . . . . . . . . . . . 3
6. Precondition Strength . . . . . . . . . . . . . . . . . . . . 4
7. Suspending and Resuming Session Establishment . . . . . . . . 4
8. Example . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
9. Security Considerations . . . . . . . . . . . . . . . . . . . 6
10. IANA Considerations . . . . . . . . . . . . . . . . . . . . 6
11. Normative References . . . . . . . . . . . . . . . . . . . . 6
Author's Address . . . . . . . . . . . . . . . . . . . . . . . 7
Intellectual Property and Copyright Statements . . . . . . . . 8
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1. Introduction
RFC 3312 [3] defines a framework for preconditions for SIP [2],
which is updated by [5]. This document defines a new precondition
type for that framework: connection-establishment preconditions.
UAs (User Agents) use connection-establishment preconditions when
they need to know whether a transport connection (e.g., a TCP
connection) has been established successfully and is ready to carry
user data.
We define the connection-establishment precondition type following
the guidelines provided in [5] to extend the SIP preconditions
framework.
2. Terminology
In this document, the key words "MUST", "MUST NOT", "REQUIRED",
"SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT
RECOMMENDED", "MAY", and "OPTIONAL" are to be interpreted as
described in BCP 14, RFC 2119 [1] and indicate requirement levels for
compliant implementations.
3. Precondition Tag
The precondition tag associated with the connection-establishment
preconditions is "conn". This precondition tag is registered with
the IANA in Section 10.
4. Status Type
RFC 3312 [3] defines two status types, end-to-end and segmented, but
only the end-to-end status type applies to connection-establishment
preconditions. So, connection-establishment preconditions MUST use
the end-to-end status type and MUST NOT use the segmented status
type.
5. Direction Tag
RFC 3312 [3] defines four direction tags: none, send, recv, and
sendrecv. Once a transport connection is established, they indicate
in which directions the connection can carry user data. For example,
a successfully-established TCP connection (i.e., in ESTABLISHED
statate) would have an associated direction tag of sendrecv because
it can carry data in both directions.
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6. Precondition Strength
RFC 3312 [3] defines optional and mandatory preconditions, but only
mandatory preconditions apply to connection-establishment
preconditions. So, connection-establishment preconditions MUST NOT
use optional preconditions.
7. Suspending and Resuming Session Establishment
According to [5], documents defining new precondition types need to
describe the behavior of UAs from the moment session establishment is
suspended due to a set of preconditions until is resumed when these
preconditions are met.
While session establishment is suspended due to
connection-establishment preconditions, user agents SHOULD not send
any user data over the media streams affected by the preconditions.
Additionally, the UAS (User Agent Server) SHOULD NOT alert the called
user.
Offers with connection-establishment preconditions in re-INVITEs or
UPDATEs follow the rules given in Section 6 of RFC 3312 [3].
Both user agents SHOULD continue using the old session parameters
until all the mandatory preconditions are met. At that moment,
the user agents can begin using the new session parameters.
8. Example
The following example uses connection-establishment preconditions.
Both UAs use a radio access network that does not allow them to send
any data (not even a TCP SYN) until a radio bearer has been setup for
the connection. Figure 1 shows the message flow of this example (the
PRACK transaction has been omitted for clarity):
A B
| INVITE |
| a=curr:conn e2e none |
| a=des:conn mandatory e2e sendrecv |
| a=setup:holdconn |
|----------------------------------->|
| |
| 183 Session Progress |
| a=curr:conn e2e none |
| a=des:conn mandatory e2e sendrecv |
| a=setup:holdconn |
|<-----------------------------------|
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| |
| UPDATE |
| a=curr:conn e2e none |
| a=des:conn mandatory e2e sendrecv |
A's radio | a=setup:actpass |
bearer is +----------------------------------->|
up | |
| 200 OK |
| a=curr:conn e2e none |
| a=des:conn mandatory e2e sendrecv |
| a=setup:active |
|<-----------------------------------|
| |
| |
| |
| | B's radio
|<---TCP Connection Establishment--->+ bearer is up
| | B sends TCP SYN
| |
| |
| 180 Ringing | TCP connection
|<-----------------------------------+ is up
| | B alerts the user
| |
Figure 1: Message flow with two types of preconditions
A sends an INVITE requesting connection-establishment preconditions.
The setup attribute in the offer is set to holdconn because A cannot
send or receive any data before setting up a radio bearer for the
connection.
B agrees to use connection-establishment preconditions by sending a
183 (Session Progress) response. The setup attribute in the answer
is also set to holdconn because B, like A, cannot send or receive any
data before setting up a radio bearer for the connection.
When A's radio bearer is ready, A sends an UPDATE to B with a setup
attribute with a value of actpass. This attribute indicates that A
can perform an active or a passive TCP open. A is letting B choose
which endpoint will initiate the connection.
Since B's radio bearer is not ready yet, B chooses to be the one
initiating the connection and indicates so with a setup attribute
with a value of active. At a later point, when B's radio bearer is
ready, B initiates the TCP connection towards A.
Once the TCP connection is established successfully, B alerts the
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callee and sends a 180 (Ringing) response.
9. Security Considerations
An attacker adding preconditions to a session description or
modifying existing preconditions could keep sessions from being
established. An attacker removing preconditions from a session
description could force sessions to be established without meeting
mandatory preconditions.
It is thus strongly RECOMMENDED that integrity protection be applied
to the SDP session descriptions. S/MIME [4] is the natural choice to
provide such end-to-end integrity protection, as described in RFC
3261 [2].
10. IANA Considerations
This document defines a new precondition type:
connection-establishment. It needs to be registered by the IANA
under the registry for Precondition Types used with SIP.
Pecondition-Type Description Reference
---------------- ----------------------------------- ---------
conn Connection-establishment preconditions [RFCXXXX]
11 Normative 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] Camarillo, G., Marshall, W. and J. Rosenberg, "Integration of
Resource Management and Session Initiation Protocol (SIP)", RFC
3312, October 2002.
[4] Peterson, J., "S/MIME Advanced Encryption Standard (AES)
Requirement for the Session Initiation Protocol (SIP)", RFC
3853, July 2004.
[5] Camarillo, G., "Update to the Session Initiation Protocol (SIP)
Preconditions Framework", draft-ietf-sip-rfc3312-update-03 (work
in progress), September 2004.
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Author's Address
Gonzalo Camarillo
Ericsson
Hirsalantie 11
Jorvas 02420
Finland
EMail: Gonzalo.Camarillo@ericsson.com
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