Internet Engineering Task Force SIP WG
Internet Draft G. Camarillo
Ericsson
P. Kyzivat
Cisco
draft-camarillo-sip-rfc3312-update-00.txt
August 28, 2003
Expires: February, 2004
Interactions of Preconditions with Session
Mobility in the Session Initiation Protocol (SIP)
STATUS OF THIS MEMO
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Abstract
This document describes how to use SIP preconditions in situations
that involve session mobility. This document updates RFC3312, which
defines the framework for SIP preconditions.
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Table of Contents
1 Introduction ........................................ 3
2 Terminology ......................................... 3
3 Issues Related to Session Mobility .................. 3
4 Update to RFC 3312 .................................. 4
5 Security Considerations ............................. 6
6 Authors' Addresses .................................. 6
7 Normative References ................................ 6
8 Informative References .............................. 7
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1 Introduction
RFC 3312 [1] defines the framework for SIP [2] preconditions and
focuses on media sessions that do not move around. That is, media is
sent between the same end-points throughout the duration of the
session.
However, media sessions established by SIP are not always static. SIP
offers mechanisms to provide session mobility, namely re-INVITEs and
UPDATEs [5]. While existing implementations of RFC 3312 [1] can
probably handle session mobility, there is a need to explicitly point
out the issues involved and make a slight update to some of the
procedures defined there. With the updated procedures defined in this
document, messages carrying precondition information become more
explicit about the current status of the preconditions.
2 Terminology
In this document, the key words "MUST", "MUST NOT", "REQUIRED",
"SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY",
and "OPTIONAL" are to be interpreted as described in RFC 2119 [3].
3 Issues Related to Session Mobility
Section 5 of RFC 3312 [1] describes how to use SIP [2] preconditions
with the offer/answer model [4]. RFC 3312 gives a set of rules that
allow a user agent to communicate changes in the current status of
the preconditions to the remote user agent.
The idea is that a given user agent knows about the current status of
some part of the preconditions (e.g., send direction of the QoS
precondition) through local information (e.g., an RSVP RESV is
received indicating that resource reservation was successful). The
UAC informs the UAS about changes in the current status by sending an
offer to the UAS. The UAS, in turn, could (if needed) send an offer
to the UAC informing it about the status of the part of the
preconditions the UAS has local information about.
Note, however, that UASs do not usually send updates about
the current status to the UAC because UASs are the ones
resuming session establishment when all the preconditions
are met. Therefore, rather than performing an offer/answer
exchange to inform the UAC that all the preconditions are
met, they simply send a 180 (Ringing) response indicating
that session establishment has been resumed.
While RFC 3312 [1] allows to update current status information using
offers as described above, it does not allow to downgrade current
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status values in answers, as shown in the third row of Table 3 of RFC
3312. However, such downgrades are sometimes needed. Figure 1 shows
an example where performing such a downgrade in an answer would be
needed.
3pcc
A controller B C
| | | |
|<-dialog 1->|<-dialog 2->| |
| | | |
| *********************** | |
|* MEDIA *| |
| *********************** | |
| | | |
| | | |
|<-dialog 1->|<------dialog 3----->|
| | | |
| ******************************** |
|* MEDIA *|
| ******************************** |
| | | |
| | | |
Figure 1: Session Mobility using 3pcc
The 3pcc [6] controller in Figure 1 has established a session
between A and B using dialog 1 towards A and dialog 2 towards B. At
that point, the controller wants A to have a session with C instead
of B. To transfer A to C (configuration shown at the bottom of Figure
1), the controller sends an empty (no offer) re-INVITE to A. Since A
does not know that the session will be moved, its offer in the 200 OK
states that the current status of the media stream in the send
direction is "Yes". The controller, after contacting C establishing
dialog 3, sends back an answer to A. This answer contains a new
destination for the media (C) and should have downgraded the current
status of the media stream to "No", since there is no reservation of
resources between A and C.
4 Update to RFC 3312
Below there are a set of new rules that update RFC 3312 [1] to
address the issues above.
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The rule below applies to offerers that are moving a media stream to
a new address:
When a stream is being moved to a new transport address, the offerer
MUST set all the current status values it does not have local
information about to "No".
Note that for streams using segmented status (as opposed to end-to-
end status), the fact that the address for the media stream at the
local segment changes may or may not affect the status of the
preconditions at the remote segment. However, moving an existing
stream to a new location, from the preconditions point of view, is
like establishing a new stream. Therefore, it is appropriate to set
all the current status values to "No" and start a new precondition
negotiation from scratch.
The updated table and the rules below applies to an answerer that is
moving a media stream. That is, the offerer was not aware of the move
when it generated the offer.
Table 3 of RFC 3312 [1] needs to be updated to allow answers to
downgrade current status values. Table 1 below shows the result.
Transac. status table Local status table New values transac./local
____________________________________________________________________
no no no/no
yes yes yes/yes
yes no depends on local info
no yes depends on local info
Table 1: Possible values for the "Current" fields
An answerer MUST downgrade the current status values that received in
the offer if it has local information about them or if the media
stream is being moved to a new transport address.
Note that for streams using segmented status the address change at
the answerer may or may not affect the status of the preconditions at
the offerer's segment. However, as stated above, moving an existing
stream to a new location, from the preconditions point of view, is
like establishing a new stream. Therefore, it is appropriate to set
all the current status values to "No" and start a new precondition
negotiation from scratch.
The new table below applies to an offerer that receives an answer
that updates or downgrades its local status tables.
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Offerers should update their local status tables when they receive an
answer as shown in Table 2.
Transac. status table Local status table New value Local Status
_________________________________________________________________
no no no
yes yes yes
yes no yes
no yes no
Table 2: Possible values for the "Current" fields after an answer
5 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 is the natural choice to
provide such end-to-end integrity protection, as described in RFC
3261 [2].
6 Authors' Addresses
Gonzalo Camarillo
Ericsson
Advanced Signalling Research Lab.
FIN-02420 Jorvas
Finland
electronic mail: Gonzalo.Camarillo@ericsson.com
Paul Kyzivat
Cisco Systems
1414 Massachusetts Avenue, BXB500 C2-2
Boxborough, MA 01719
USA
electronic mail: pkyzivat@cisco.com
7 Normative References
[1] "Integration of resource management and session initiation
protocol (SIP)," RFC 3312, Internet Engineering Task Force, Oct.
2002.
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[2] J. Rosenberg, H. Schulzrinne, G. Camarillo, A. R. Johnston, J.
Peterson, R. Sparks, M. Handley, and E. Schooler, "SIP: session
initiation protocol," RFC 3261, Internet Engineering Task Force, June
2002.
[3] S. Bradner, "Key words for use in RFCs to indicate requirement
levels," RFC 2119, Internet Engineering Task Force, Mar. 1997.
[4] J. Rosenberg and H. Schulzrinne, "An offer/answer model with
session description protocol (SDP)," RFC 3264, Internet Engineering
Task Force, June 2002.
8 Informative References
[5] J. Rosenberg, "The session initiation protocol (SIP) UPDATE
method," RFC 3311, Internet Engineering Task Force, Oct. 2002.
[6] J. Rosenberg, J. L. Peterson, H. Schulzrinne, and G. Camarillo,
"Best current practices for third party call control in the session
initiation protocol," internet draft, Internet Engineering Task
Force, July 2003. Work in progress.
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