SIP Working Group G. Camarillo
Internet-Draft Ericsson
Expires: March 30, 2005 P. Kyzivat
Cisco Systems
September 29, 2004
Update to the Session Initiation Protocol (SIP) Preconditions
Framework
draft-ietf-sip-rfc3312-update-03.txt
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Copyright (C) The Internet Society (2004).
Abstract
This document updates the framework for preconditions in SIP. We
provide guidelines for authors of new precondition types and describe
how to use SIP preconditions in situations that involve session
mobility.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Defining New Precondition Types . . . . . . . . . . . . . . . 3
3.1 Precondition Type Tag . . . . . . . . . . . . . . . . . . 3
3.2 Status Type . . . . . . . . . . . . . . . . . . . . . . . 4
3.3 Precondition Strength . . . . . . . . . . . . . . . . . . 4
3.4 Suspending and Resuming Session Establishment . . . . . . 4
4. Issues Related to Session Mobility . . . . . . . . . . . . . . 5
4.1 Update to RFC 3312 . . . . . . . . . . . . . . . . . . . . 6
4.2 Desired Status . . . . . . . . . . . . . . . . . . . . . . 8
5. Security Considerations . . . . . . . . . . . . . . . . . . . 8
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 8
7. Acknowledges . . . . . . . . . . . . . . . . . . . . . . . . . 8
8. References . . . . . . . . . . . . . . . . . . . . . . . . . . 8
8.1 Normative References . . . . . . . . . . . . . . . . . . . . 8
8.2 Informational References . . . . . . . . . . . . . . . . . . 9
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . 9
Intellectual Property and Copyright Statements . . . . . . . . 10
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1. Introduction
RFC 3312 [3] defines the framework for SIP [2] preconditions, which
is a generic framework that allows SIP UAs (User Agents) to suspend
the establishment of a session until a set of preconditions are met.
Although only Quality of Service (QoS) preconditions have been
defined so far, this framework supports different preconditions
types. (QoS preconditions are defined by RFC 3312 [3] as well.)
This document updates RFC 3312 [3]. We provide guidelines for
authors of new precondition types and explain which topics they need
to discuss when defining them. In addition, we update some of the
procedures in RFC 3312 to be able to use SIP preconditions in
situations that involve session mobility, as described below.
RFC 3312 [3] 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. Nevertheless, 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 [3] 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", "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. Defining New Precondition Types
Specifications defining new precondition types need to discuss the
topics described in this section. Having clear definitions of new
precondition types is essential to ensure interoperability among
different implementations.
3.1 Precondition Type Tag
New precondition types MUST have an associated precondition type tag
(e.g., "qos" is the tag for QoS preconditions). The IANA registry
for precondition types can be found at:
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http://www.iana.org/assignments/sip-precond-types
Authors of new preconditions MUST register new precondition types,
and their tags, with the IANA following the instructions in Section
15 of RFC 3312 [3].
3.2 Status Type
RFC 3312 [3] defines two status types: end-to-end and segmented.
Specifications defining new precondition types MUST indicate which of
these status applies to the new precondition. New preconditions can
use only one status type or both. For example, the QoS preconditions
defined in RFC 3312 can use both [3].
3.3 Precondition Strength
RFC 3312 [3] defines optional and mandatory preconditions.
Specifications defining new precondition types MUST describe whether
or not optional preconditions are applicable, and in case they are,
what is the expected behavior of a UA on reception of optional
preconditions.
3.4 Suspending and Resuming Session Establishment
Section 6 of RFC 3312 [3] describes 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. In
general, the called user is not alterted until the preconditions are
met.
Still, in addition to not alerting the user, each precondition type
MUST define any extra actions UAs should perform or refrain from
performing when session establishment is suspended. The behavior of
media streams during session suspension is therefore part of the
definition of a particular precondition type. Some precondition
types may allow media streams to send and receive packets during
session suspension; others may not. Consequently, the following
paragraph from RFC 3312 only appplies to QoS preconditions:
While session establishment is suspended, user agents SHOULD not
send any data over any media stream. In the case of RTP, neither
RTP nor RTCP packets are sent.
As a clarification to the previous paragraph, the control messages
used to establish connections in connection-oriented transport
protocols (e.g., TCP SYNs) are not affected by the previous rule.
So, user agents follow standard rules (e.g., the SDP a:setup
attribute [7]) to decide when to establish the connection, regardless
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of the presence of QoS preconditions.
New precondition types MUST also describe the behaviour of UAs on
reception of a re-INVITE or an UPDATE with preconditions for an
ongoing session.
4. Issues Related to Session Mobility
Section 5 oft RFC 3312 [3] 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 (User Agent Client) informs the UAS (User Agent Server) 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 [3] allows to update current status information using
offers as described above, it does not allow to downgrade current
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.
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3pcc
A Controller B C
| | | |
|<-dialog 1->|<-dialog 2->| |
| | | |
| *********************** | |
|* MEDIA *| |
| *********************** | |
| | | |
| | | |
|<-dialog 1->|<------dialog 3----->|
| | | |
| ******************************** |
|* MEDIA *|
| ******************************** |
| | | |
| | | |
Figure 1: Session mobility using 3pcc
The 3pcc (Third Party Call Control) [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.
4.1 Update to RFC 3312
Below there are a set of new rules that update RFC 3312 [3] to
address the issues above.
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
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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 [3] needs to be updated to allow answers to
downgrade current status values. The following table 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
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.
Offerers should update their local status tables when they receive an
answer as shown in the following table.
Transac. status table Local status table New value Local Status
_________________________________________________________________
no no no
yes yes yes
yes no yes
no yes no
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4.2 Desired Status
The desired status that a UA wants for a media stream after the
stream is moved to a new transport address may be different than the
desired status negotiated for the stream originally. A UA, for
instance, may require mandatory QoS over a low-bandwidth link but be
satisfied with optional QoS when the stream is moved to a
high-bandwidth link.
If the new desired status is higher than the previous one (e.g.,
optional to mandatory), the UA, following RFC 3312 procedures, may
upgrade its desired status in an offer or in an answer. If the new
desired status is lower that the previous one (e.g., mandatory to
optional), the UA, following RFC 3312 procedures as well, may
downgrade its desired status only in an offer (i.e., not in 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. IANA Considerations
This document has no IANA considerations.
7. Acknowledges
Dave Oran and Allison Mankin provided useful comments on this
document.
8. References
8.1 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:
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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.
8.2 Informational References
[4] Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model with
Session Description Protocol (SDP)", RFC 3264, June 2002.
[5] Rosenberg, J., "The Session Initiation Protocol (SIP) UPDATE
Method", RFC 3311, October 2002.
[6] Rosenberg, J., Peterson, J., Schulzrinne, H. and G. Camarillo,
"Best Current Practices for Third Party Call Control (3pcc) in
the Session Initiation Protocol (SIP)", BCP 85, RFC 3725, April
2004.
[7] Yon, D., "Connection-Oriented Media Transport in the Session
Description Protocol (SDP)", draft-ietf-mmusic-sdp-comedia-08
(work in progress), July 2004.
Authors' Addresses
Gonzalo Camarillo
Ericsson
Hirsalantie 11
Jorvas 02420
Finland
EMail: Gonzalo.Camarillo@ericsson.com
Paul Kyzivat
Cisco Systems
1414 Massachusetts Avenue, BXB500 C2-2
Boxborough, MA 01719
USA
EMail: pkyzivat@cisco.com
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