Network Working Group G. Camarillo
Request for Comments: 4032 Ericsson
Updates: 3312 P. Kyzivat
Category: Standards Track Cisco Systems
March 2005
Update to the Session Initiation Protocol (SIP)
Preconditions Framework
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 (2005).
Abstract
This document updates RFC 3312, which defines 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.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 2
3. Defining New Precondition Types . . . . . . . . . . . . . . . 3
3.1. Precondition Type Tag . . . . . . . . . . . . . . . . . 3
3.2. Status Type . . . . . . . . . . . . . . . . . . . . . . 3
3.3. Precondition Strength . . . . . . . . . . . . . . . . . 3
3.4. Suspending and Resuming Session Establishment . . . . . 3
4. Issues Related to Session Mobility . . . . . . . . . . . . . . 4
4.1. Update to RFC 3312 . . . . . . . . . . . . . . . . . . . 5
4.2. Desired Status . . . . . . . . . . . . . . . . . . . . . 7
5. Security Considerations . . . . . . . . . . . . . . . . . . . 7
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 7
7. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 8
8. References . . . . . . . . . . . . . . . . . . . . . . . . . . 8
8.1. Normative References . . . . . . . . . . . . . . . . . . 8
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8.2. Informational References . . . . . . . . . . . . . . . . 8
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 9
Full Copyright Statement . . . . . . . . . . . . . . . . . . . . . 10
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 types of
preconditions. (QoS preconditions are defined by RFC 3312 as well).
This document updates RFC 3312, provides guidelines for authors of
new precondition types and explains which topics they need to discuss
when defining them. In addition, it updates some of the procedures
in RFC 3312 for using SIP preconditions in situations that involve
session mobility as described below.
RFC 3312 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 can
probably handle session mobility, there is a need to explicitly point
out the issues involved and make a slight update on some of the
procedures defined there in. With the updated procedures defined in
this document, messages carrying precondition information become more
explicit about the current status of the preconditions.
Specifically, we now allow answers to downgrade current status values
(this was disallowed by RFC 3312). We consider moving an existing
stream to a new location as equivalent to establishing a new stream.
Therefore, answers moving streams to new locations set all the
current status values in their answers to "No" and start a new
precondition negotiation from scratch.
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.
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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). Authors of new
preconditions MUST register new precondition types and their tags
with the IANA by following the instructions in Section 15 of RFC
3312.
3.2. Status Type
RFC 3312 defines two status types: end-to-end and segmented.
Specifications defining new precondition types MUST indicate which
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. Precondition Strength
RFC 3312 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 describes the behavior of UAs from the moment
session establishment is suspended, due to a set of preconditions,
until it is resumed when these preconditions are met. In general,
the called user is not alerted until the preconditions are met.
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
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types may allow media streams to send and receive packets during
session suspension; others may not. Consequently, the following
paragraph from RFC 3312 only applies 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.
To clarify 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 'setup' attribute [7]) to
decide when to establish the connection, regardless 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 of RFC 3312 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.
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While RFC 3312 allows updating current status information using the
methods described above, it does not allow downgrading current status
values in answers, as shown in the third row of Table 3 of RFC 3312.
Figure 1 shows how performing such a downgrade in an answer would
sometimes 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 (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". After contacting C
establishing dialog 3, the controller 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 is a set of new rules that update RFC 3312 to address the
issues above.
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The rule below applies to offerers moving a media stream to a new
address:
When a stream is being moved to a new transport address, the offerer
MUST set all current status values about which 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
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 rules below apply to an answerer that is moving
a media stream. The offerer was not aware of the move when it
generated the offer.
Table 3 of RFC 3312 needs to be updated to allow answerers 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 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 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
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 (i.e., 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 prevent establishment of
sessions. An attacker removing preconditions from a session
description could force sessions to be established without meeting
mandatory preconditions.
Thus, it is 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
The IANA registration requirements for the preconditions framework
are defined in RFC 3312. Any new preconditions are governed by the
IANA Considerations there.
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7. Acknowledgement
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:
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. and Camarillo, G., "TCP-Based Media Transport in the
Session Description Protocol (SDP)", Work In Progress, November
2004.
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Authors' Addresses
Gonzalo Camarillo
Ericsson
Hirsalantie 11
Jorvas 02420
Finland
EMail:
[email protected]
Paul Kyzivat
Cisco Systems
1414 Massachusetts Avenue, BXB500 C2-2
Boxborough, MA 01719
USA
EMail:
[email protected]
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RFC 4032 SIP Preconditions Framework March 2005
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