draft-ietf-sip-rfc3312-update-03.txt   rfc4032.txt 
SIP Working Group G. Camarillo Network Working Group G. Camarillo
Internet-Draft Ericsson Request for Comments: 4032 Ericsson
Expires: March 30, 2005 P. Kyzivat Updates: 3312 P. Kyzivat
Cisco Systems Category: Standards Track Cisco Systems
September 29, 2004 March 2005
Update to the Session Initiation Protocol (SIP) Preconditions
Framework
draft-ietf-sip-rfc3312-update-03.txt
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Copyright Notice Copyright Notice
Copyright (C) The Internet Society (2004). Copyright (C) The Internet Society (2005).
Abstract Abstract
This document updates the framework for preconditions in SIP. We This document updates RFC 3312, which defines the framework for
provide guidelines for authors of new precondition types and describe preconditions in SIP. We provide guidelines for authors of new
how to use SIP preconditions in situations that involve session precondition types and describe how to use SIP preconditions in
mobility. situations that involve session mobility.
Table of Contents Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3 2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 2
3. Defining New Precondition Types . . . . . . . . . . . . . . . 3 3. Defining New Precondition Types . . . . . . . . . . . . . . . 3
3.1 Precondition Type Tag . . . . . . . . . . . . . . . . . . 3 3.1. Precondition Type Tag . . . . . . . . . . . . . . . . . 3
3.2 Status Type . . . . . . . . . . . . . . . . . . . . . . . 4 3.2. Status Type . . . . . . . . . . . . . . . . . . . . . . 3
3.3 Precondition Strength . . . . . . . . . . . . . . . . . . 4 3.3. Precondition Strength . . . . . . . . . . . . . . . . . 3
3.4 Suspending and Resuming Session Establishment . . . . . . 4 3.4. Suspending and Resuming Session Establishment . . . . . 3
4. Issues Related to Session Mobility . . . . . . . . . . . . . . 5 4. Issues Related to Session Mobility . . . . . . . . . . . . . . 4
4.1 Update to RFC 3312 . . . . . . . . . . . . . . . . . . . . 6 4.1. Update to RFC 3312 . . . . . . . . . . . . . . . . . . . 5
4.2 Desired Status . . . . . . . . . . . . . . . . . . . . . . 8 4.2. Desired Status . . . . . . . . . . . . . . . . . . . . . 7
5. Security Considerations . . . . . . . . . . . . . . . . . . . 8 5. Security Considerations . . . . . . . . . . . . . . . . . . . 7
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 8 6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 7
7. Acknowledges . . . . . . . . . . . . . . . . . . . . . . . . . 8 7. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 8
8. References . . . . . . . . . . . . . . . . . . . . . . . . . . 8 8. References . . . . . . . . . . . . . . . . . . . . . . . . . . 8
8.1 Normative References . . . . . . . . . . . . . . . . . . . . 8 8.1. Normative References . . . . . . . . . . . . . . . . . . 8
8.2 Informational References . . . . . . . . . . . . . . . . . . 9 8.2. Informational References . . . . . . . . . . . . . . . . 8
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . 9 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 9
Intellectual Property and Copyright Statements . . . . . . . . 10 Full Copyright Statement . . . . . . . . . . . . . . . . . . . . . 10
1. Introduction 1. Introduction
RFC 3312 [3] defines the framework for SIP [2] preconditions, which RFC 3312 [3] defines the framework for SIP [2] preconditions, which
is a generic framework that allows SIP UAs (User Agents) to suspend is a generic framework that allows SIP UAs (User Agents) to suspend
the establishment of a session until a set of preconditions are met. the establishment of a session until a set of preconditions are met.
Although only Quality of Service (QoS) preconditions have been Although only Quality of Service (QoS) preconditions have been
defined so far, this framework supports different preconditions defined so far, this framework supports different types of
types. (QoS preconditions are defined by RFC 3312 [3] as well.) preconditions. (QoS preconditions are defined by RFC 3312 as well).
This document updates RFC 3312 [3]. We provide guidelines for This document updates RFC 3312, provides guidelines for authors of
authors of new precondition types and explain which topics they need new precondition types and explains which topics they need to discuss
to discuss when defining them. In addition, we update some of the when defining them. In addition, it updates some of the procedures
procedures in RFC 3312 to be able to use SIP preconditions in in RFC 3312 for using SIP preconditions in situations that involve
situations that involve session mobility, as described below. session mobility as described below.
RFC 3312 [3] focuses on media sessions that do not move around. That RFC 3312 focuses on media sessions that do not move around. That is,
is, media is sent between the same end-points throughout the duration media is sent between the same end-points throughout the duration of
of the session. Nevertheless, media sessions established by SIP are the session. Nevertheless, media sessions established by SIP are not
not always static. always static.
SIP offers mechanisms to provide session mobility, namely re-INVITEs SIP offers mechanisms to provide session mobility, namely re-INVITEs
and UPDATEs [5]. While existing implementations of RFC 3312 [3] can and UPDATEs [5]. While existing implementations of RFC 3312 can
probably handle session mobility, there is a need to explicitly point probably handle session mobility, there is a need to explicitly point
out the issues involved and make a slight update to some of the out the issues involved and make a slight update on some of the
procedures defined there. With the updated procedures defined in procedures defined there in. With the updated procedures defined in
this document, messages carrying precondition information become more this document, messages carrying precondition information become more
explicit about the current status of the preconditions. 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 2. Terminology
In this document, the key words "MUST", "MUST NOT", "REQUIRED", In this document, the key words "MUST", "MUST NOT", "REQUIRED",
"SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT
RECOMMENDED", "MAY", and "OPTIONAL" are to be interpreted as RECOMMENDED", "MAY", and "OPTIONAL" are to be interpreted as
described in BCP 14, RFC 2119 [1] and indicate requirement levels for described in BCP 14, RFC 2119 [1] and indicate requirement levels for
compliant implementations. compliant implementations.
3. Defining New Precondition Types 3. Defining New Precondition Types
Specifications defining new precondition types need to discuss the Specifications defining new precondition types need to discuss the
topics described in this section. Having clear definitions of new topics described in this section. Having clear definitions of new
precondition types is essential to ensure interoperability among precondition types is essential to ensure interoperability among
different implementations. different implementations.
3.1 Precondition Type Tag 3.1. Precondition Type Tag
New precondition types MUST have an associated 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 (e.g., "qos" is the tag for QoS preconditions). Authors of new
for precondition types can be found at: preconditions MUST register new precondition types and their tags
with the IANA by following the instructions in Section 15 of RFC
http://www.iana.org/assignments/sip-precond-types 3312.
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 3.2. Status Type
RFC 3312 [3] defines two status types: end-to-end and segmented. RFC 3312 defines two status types: end-to-end and segmented.
Specifications defining new precondition types MUST indicate which of Specifications defining new precondition types MUST indicate which
these status applies to the new precondition. New preconditions can status applies to the new precondition. New preconditions can use
use only one status type or both. For example, the QoS preconditions only one status type or both. For example, the QoS preconditions
defined in RFC 3312 can use both [3]. defined in RFC 3312 can use both.
3.3 Precondition Strength 3.3. Precondition Strength
RFC 3312 [3] defines optional and mandatory preconditions. RFC 3312 defines optional and mandatory preconditions.
Specifications defining new precondition types MUST describe whether Specifications defining new precondition types MUST describe whether
or not optional preconditions are applicable, and in case they are, or not optional preconditions are applicable, and in case they are,
what is the expected behavior of a UA on reception of optional what is the expected behavior of a UA on reception of optional
preconditions. preconditions.
3.4 Suspending and Resuming Session Establishment 3.4. Suspending and Resuming Session Establishment
Section 6 of RFC 3312 [3] describes the behavior of UAs from the Section 6 of RFC 3312 describes the behavior of UAs from the moment
moment session establishment is suspended due to a set of session establishment is suspended, due to a set of preconditions,
preconditions until is resumed when these preconditions are met. In until it is resumed when these preconditions are met. In general,
general, the called user is not alterted until the preconditions are the called user is not alerted until the preconditions are met.
met.
Still, in addition to not alerting the user, each precondition type In addition to not alerting the user, each precondition type MUST
MUST define any extra actions UAs should perform or refrain from define any extra actions UAs should perform or refrain from
performing when session establishment is suspended. The behavior of performing when session establishment is suspended. The behavior of
media streams during session suspension is therefore part of the media streams during session suspension is therefore part of the
definition of a particular precondition type. Some precondition definition of a particular precondition type. Some precondition
types may allow media streams to send and receive packets during types may allow media streams to send and receive packets during
session suspension; others may not. Consequently, the following session suspension; others may not. Consequently, the following
paragraph from RFC 3312 only appplies to QoS preconditions: paragraph from RFC 3312 only applies to QoS preconditions:
While session establishment is suspended, user agents SHOULD not While session establishment is suspended, user agents SHOULD not
send any data over any media stream. In the case of RTP, neither send any data over any media stream. In the case of RTP, neither
RTP nor RTCP packets are sent. RTP nor RTCP packets are sent.
As a clarification to the previous paragraph, the control messages To clarify the previous paragraph, the control messages used to
used to establish connections in connection-oriented transport establish connections in connection-oriented transport protocols
protocols (e.g., TCP SYNs) are not affected by the previous rule. (e.g., TCP SYNs) are not affected by the previous rule. So, user
So, user agents follow standard rules (e.g., the SDP a:setup agents follow standard rules (e.g., the SDP 'setup' attribute [7]) to
attribute [7]) to decide when to establish the connection, regardless decide when to establish the connection, regardless of QoS
of the presence of QoS preconditions. preconditions.
New precondition types MUST also describe the behaviour of UAs on New precondition types MUST also describe the behaviour of UAs on
reception of a re-INVITE or an UPDATE with preconditions for an reception of a re-INVITE or an UPDATE with preconditions for an
ongoing session. ongoing session.
4. Issues Related to Session Mobility 4. Issues Related to Session Mobility
Section 5 oft RFC 3312 [3] describes how to use SIP [2] preconditions Section 5 of RFC 3312 describes how to use SIP [2] preconditions with
with the offer/answer model [4]. RFC 3312 gives a set of rules that the offer/answer model [4]. RFC 3312 gives a set of rules that allow
allow a user agent to communicate changes in the current status of a user agent to communicate changes in the current status of the
the preconditions to the remote user agent. preconditions to the remote user agent.
The idea is that a given user agent knows about the current status of 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 some part of the preconditions (e.g., send direction of the QoS
precondition) through local information (e.g., an RSVP RESV is precondition) through local information (e.g., an RSVP RESV is
received indicating that resource reservation was successful). The received indicating that resource reservation was successful). The
UAC (User Agent Client) informs the UAS (User Agent Server) about UAC (User Agent Client) informs the UAS (User Agent Server) about
changes in the current status by sending an offer to the UAS. The 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 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 about the status of the part of the preconditions the UAS has local
information about. information about.
Note, however, that UASs do not usually send updates about the Note, however, that UASs do not usually send updates about the
current status to the UAC because UASs are the ones resuming current status to the UAC because UASs are the ones resuming
session establishment when all the preconditions are met. session establishment when all the preconditions are met.
Therefore, rather than performing an offer/answer exchange to Therefore, rather than performing an offer/answer exchange to
inform the UAC that all the preconditions are met, they simply inform the UAC that all the preconditions are met, they simply
send a 180 (Ringing) response indicating that session send a 180 (Ringing) response indicating that session
establishment has been resumed. establishment has been resumed.
While RFC 3312 [3] allows to update current status information using While RFC 3312 allows updating current status information using the
offers as described above, it does not allow to downgrade current methods described above, it does not allow downgrading current status
status values in answers, as shown in the third row of Table 3 of RFC values in answers, as shown in the third row of Table 3 of RFC 3312.
3312. However, such downgrades are sometimes needed. Figure 1 shows Figure 1 shows how performing such a downgrade in an answer would
an example where performing such a downgrade in an answer would be sometimes be needed.
needed.
3pcc 3pcc
A Controller B C A Controller B C
| | | | | | | |
|<-dialog 1->|<-dialog 2->| | |<-dialog 1->|<-dialog 2->| |
| | | | | | | |
| *********************** | | | *********************** | |
|* MEDIA *| | |* MEDIA *| |
| *********************** | | | *********************** | |
skipping to change at page 6, line 30 skipping to change at page 5, line 36
| ******************************** | | ******************************** |
| | | | | | | |
| | | | | | | |
Figure 1: Session mobility using 3pcc Figure 1: Session mobility using 3pcc
The 3pcc (Third Party Call Control) [6] controller in Figure 1 has 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 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 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 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) 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 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 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 media stream in the send direction is "Yes". After contacting C
contacting C establishing dialog 3, sends back an answer to A. This establishing dialog 3, the controller sends back an answer to A.
answer contains a new destination for the media (C) and should have This answer contains a new destination for the media (C) and should
downgraded the current status of the media stream to "No", since have downgraded the current status of the media stream to "No", since
there is no reservation of resources between A and C. there is no reservation of resources between A and C.
4.1 Update to RFC 3312 4.1. Update to RFC 3312
Below there are a set of new rules that update RFC 3312 [3] to Below is a set of new rules that update RFC 3312 to address the
address the issues above. issues above.
The rule below applies to offerers that are moving a media stream to The rule below applies to offerers moving a media stream to a new
a new address: address:
When a stream is being moved to a new transport address, the offerer 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 MUST set all current status values about which it does not have local
information about to "No". information about to "No".
Note that for streams using segmented status (as opposed to Note that for streams using segmented status (as opposed to end-to-
end-to-end status), the fact that the address for the media stream at end status), the fact that the address for the media stream at the
the local segment changes may or may not affect the status of the local segment changes may or may not affect the status of
preconditions at the remote segment. However, moving an existing preconditions at the remote segment. However, moving an existing
stream to a new location, from the preconditions point of view, is stream to a new location, from the preconditions point of view, is
like establishing a new stream. Therefore, it is appropriate to set like establishing a new stream. Therefore, it is appropriate to set
all the current status values to "No" and start a new precondition all the current status values to "No" and start a new precondition
negotiation from scratch. negotiation from scratch.
The updated table and the rules below applies to an answerer that is The updated table and rules below apply to an answerer that is moving
moving a media stream. That is, the offerer was not aware of the a media stream. The offerer was not aware of the move when it
move when it generated the offer. generated the offer.
Table 3 of RFC 3312 [3] needs to be updated to allow answers to Table 3 of RFC 3312 needs to be updated to allow answerers to
downgrade current status values. The following table shows the downgrade current status values. The following table shows the
result. result.
Transac. status table Local status table New values transac./local Transac status table Local status table New values transac./local
____________________________________________________________________ ____________________________________________________________________
no no no/no no no no/no
yes yes yes/yes yes yes yes/yes
yes no depends on local info yes no depends on local info
no yes depends on local info no yes depends on local info
An answerer MUST downgrade the current status values that received in An answerer MUST downgrade the current status values received in the
the offer if it has local information about them or if the media offer if it has local information about them or if the media stream
stream is being moved to a new transport address. is being moved to a new transport address.
Note that for streams using segmented status the address change at 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 answerer may or may not affect the status of the preconditions at
the offerer's segment. However, as stated above, moving an existing the offerer's segment. However, as stated above, moving an existing
stream to a new location, from the preconditions point of view, is stream to a new location, from the preconditions point of view, is
like establishing a new stream. Therefore, it is appropriate to set like establishing a new stream. Therefore, it is appropriate to set
all the current status values to "No" and start a new precondition all the current status values to "No" and start a new precondition
negotiation from scratch. negotiation from scratch.
The new table below applies to an offerer that receives an answer The new table below applies to an offerer that receives an answer
that updates or downgrades its local status tables. that updates or downgrades its local status tables.
Offerers should update their local status tables when they receive an Offerers should update their local status tables when they receive an
answer as shown in the following table. answer as shown in the following table.
Transac. status table Local status table New value Local Status Transac. status table Local status table New value Local Status
_________________________________________________________________ _________________________________________________________________
no no no no no no
yes yes yes yes yes yes
yes no yes yes no yes
no yes no no yes no
4.2 Desired Status 4.2. Desired Status
The desired status that a UA wants for a media stream after the 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 stream is moved to a new transport address may be different than the
desired status negotiated for the stream originally. A UA, for desired status negotiated for the stream originally. A UA, for
instance, may require mandatory QoS over a low-bandwidth link but be instance, may require mandatory QoS over a low bandwidth link but be
satisfied with optional QoS when the stream is moved to a satisfied with optional QoS when the stream is moved to a high
high-bandwidth link. bandwidth link.
If the new desired status is higher than the previous one (e.g., If the new desired status is higher than the previous one (e.g.,
optional to mandatory), the UA, following RFC 3312 procedures, may optional to mandatory), the UA, following RFC 3312 procedures, may
upgrade its desired status in an offer or in an answer. If the new 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 desired status is lower that the previous one (i.e., mandatory to
optional), the UA, following RFC 3312 procedures as well, may optional), the UA, following RFC 3312 procedures as well, may
downgrade its desired status only in an offer (i.e., not in an downgrade its desired status only in an offer (i.e., not in an
answer.) answer.)
5. Security Considerations 5. Security Considerations
An attacker adding preconditions to a session description or An attacker adding preconditions to a session description or
modifying existing preconditions could keep sessions from being modifying existing preconditions could prevent establishment of
established. An attacker removing preconditions from a session sessions. An attacker removing preconditions from a session
description could force sessions to be established without meeting description could force sessions to be established without meeting
mandatory preconditions. mandatory preconditions.
It is thus strongly RECOMMENDED that integrity protection be applied Thus, it is strongly RECOMMENDED that integrity protection be applied
to the SDP session descriptions. S/MIME is the natural choice to to the SDP session descriptions. S/MIME is the natural choice to
provide such end-to-end integrity protection, as described in RFC provide such end-to-end integrity protection, as described in RFC
3261 [2]. 3261 [2].
6. IANA Considerations 6. IANA Considerations
This document has no 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.
7. Acknowledges 7. Acknowledgement
Dave Oran and Allison Mankin provided useful comments on this Dave Oran and Allison Mankin provided useful comments on this
document. document.
8. References 8. References
8.1 Normative References 8.1. Normative References
[1] Bradner, S., "Key words for use in RFCs to Indicate Requirement [1] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC 2119, March 1997. Levels", BCP 14, RFC 2119, March 1997.
[2] Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston, A., [2] Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston, A.,
Peterson, J., Sparks, R., Handley, M. and E. Schooler, "SIP: Peterson, J., Sparks, R., Handley, M., and E. Schooler, "SIP:
Session Initiation Protocol", RFC 3261, June 2002. Session Initiation Protocol", RFC 3261, June 2002.
[3] Camarillo, G., Marshall, W. and J. Rosenberg, "Integration of [3] Camarillo, G., Marshall, W., and J. Rosenberg, "Integration of
Resource Management and Session Initiation Protocol (SIP)", RFC Resource Management and Session Initiation Protocol (SIP)", RFC
3312, October 2002. 3312, October 2002.
8.2 Informational References 8.2. Informational References
[4] Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model with [4] Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model with
Session Description Protocol (SDP)", RFC 3264, June 2002. Session Description Protocol (SDP)", RFC 3264, June 2002.
[5] Rosenberg, J., "The Session Initiation Protocol (SIP) UPDATE [5] Rosenberg, J., "The Session Initiation Protocol (SIP) UPDATE
Method", RFC 3311, October 2002. Method", RFC 3311, October 2002.
[6] Rosenberg, J., Peterson, J., Schulzrinne, H. and G. Camarillo, [6] Rosenberg, J., Peterson, J., Schulzrinne, H., and G. Camarillo,
"Best Current Practices for Third Party Call Control (3pcc) in "Best Current Practices for Third Party Call Control (3pcc) in
the Session Initiation Protocol (SIP)", BCP 85, RFC 3725, April the Session Initiation Protocol (SIP)", BCP 85, RFC 3725, April
2004. 2004.
[7] Yon, D., "Connection-Oriented Media Transport in the Session [7] Yon, D. and Camarillo, G., "TCP-Based Media Transport in the
Description Protocol (SDP)", draft-ietf-mmusic-sdp-comedia-08 Session Description Protocol (SDP)", Work In Progress, November
(work in progress), July 2004. 2004.
Authors' Addresses Authors' Addresses
Gonzalo Camarillo Gonzalo Camarillo
Ericsson Ericsson
Hirsalantie 11 Hirsalantie 11
Jorvas 02420 Jorvas 02420
Finland Finland
EMail: Gonzalo.Camarillo@ericsson.com EMail: Gonzalo.Camarillo@ericsson.com
Paul Kyzivat Paul Kyzivat
Cisco Systems Cisco Systems
1414 Massachusetts Avenue, BXB500 C2-2 1414 Massachusetts Avenue, BXB500 C2-2
Boxborough, MA 01719 Boxborough, MA 01719
USA USA
EMail: pkyzivat@cisco.com EMail: pkyzivat@cisco.com
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 End of changes. 

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