draft-ietf-sip-rfc3312-update-00.txt   draft-ietf-sip-rfc3312-update-01.txt 
Internet Engineering Task Force SIP WG SIP Working Group G. Camarillo
Internet Draft G. Camarillo Internet-Draft Ericsson
Ericsson Expires: November 30, 2004 P. Kyzivat
P. Kyzivat Cisco Systems
Cisco June 2004
draft-ietf-sip-rfc3312-update-00.txt
November 19, 2003
Expires: May, 2004
Interactions of Preconditions with Session Update to the Session Initiation Protocol (SIP) Preconditions
Mobility in the Session Initiation Protocol (SIP) Framework
draft-ietf-sip-rfc3312-update-01.txt
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Copyright (C) The Internet Society (2004). All Rights Reserved.
Abstract Abstract
This document describes how to use SIP preconditions in situations This document updates the framework for preconditions in SIP. We
that involve session mobility. This document updates RFC3312, which provide guidelines for authors of new precondition types and describe
defines the framework for SIP preconditions. how to use SIP preconditions in situations that involve session
mobility.
Table of Contents Table of Contents
1 Introduction ........................................ 3 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2 Terminology ......................................... 3 2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3
3 Issues Related to Session Mobility .................. 3 3. Defining New Precondition Types . . . . . . . . . . . . . . . 3
4 Update to RFC 3312 .................................. 4 3.1 Precondition Type Tag . . . . . . . . . . . . . . . . . . 3
5 Desired Status ...................................... 6 3.2 Status Type . . . . . . . . . . . . . . . . . . . . . . . 4
6 Security Considerations ............................. 6 3.3 Precondition Strength . . . . . . . . . . . . . . . . . . 4
7 Authors' Addresses .................................. 6 3.4 Suspending and Resuming Session Establishment . . . . . . 4
8 Normative References ................................ 7 4. Issues Related to Session Mobility . . . . . . . . . . . . . . 5
9 Informative References .............................. 7 4.1 Update to RFC 3312 . . . . . . . . . . . . . . . . . . . . 6
4.2 Desired Status . . . . . . . . . . . . . . . . . . . . . . 8
5. Security Considerations . . . . . . . . . . . . . . . . . . . 8
6. Acknowledges . . . . . . . . . . . . . . . . . . . . . . . . . 8
7. References . . . . . . . . . . . . . . . . . . . . . . . . . . 8
7.1 Normative References . . . . . . . . . . . . . . . . . . . . 8
7.2 Informational References . . . . . . . . . . . . . . . . . . 9
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . 9
Intellectual Property and Copyright Statements . . . . . . . . 10
1 Introduction 1. Introduction
RFC 3312 [1] defines the framework for SIP [2] preconditions and RFC 3312 [3] defines the framework for SIP [2] preconditions, which
focuses on media sessions that do not move around. That is, media is is a generic framework that allows SIP UAs (User Agents) to suspend
sent between the same end-points throughout the duration of the the establishment of a session until a set of preconditions are met.
session. 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.)
However, media sessions established by SIP are not always static. SIP This document updates RFC 3312 [3]. We provide guidelines for authors
offers mechanisms to provide session mobility, namely re-INVITEs and of new precondition types and explain which topics they need to
UPDATEs [5]. While existing implementations of RFC 3312 [1] can 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 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 to some of the
procedures defined there. With the updated procedures defined in this procedures defined there. With the updated procedures defined in this
document, messages carrying precondition information become more document, messages carrying precondition information become more
explicit about the current status of the preconditions. explicit about the current status of the preconditions.
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", "MAY", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT
and "OPTIONAL" are to be interpreted as described in RFC 2119 [3]. RECOMMENDED", "MAY", and "OPTIONAL" are to be interpreted as
described in BCP 14, RFC 2119 [1] and indicate requirement levels for
compliant implementations.
3 Issues Related to Session Mobility 3. Defining New Precondition Types
Section 5 of RFC 3312 [1] describes how to use SIP [2] preconditions 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:
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 users 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 keep from
performing when session establishment is suspended. So, the behavior
of media streams during session suspension is 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 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 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 allow a user agent to communicate changes in the current status of
the preconditions to the remote user agent. the 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 informs the UAS about changes in the current status by sending an UAC (User Agent Client) informs the UAS (User Agent Server) about
offer to the UAS. The UAS, in turn, could (if needed) send an offer changes in the current status by sending an offer to the UAS. The
to the UAC informing it about the status of the part of the UAS, in turn, could (if needed) send an offer to the UAC informing it
preconditions the UAS has local information about. 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 Note, however, that UASs do not usually send updates about the
the current status to the UAC because UASs are the ones current status to the UAC because UASs are the ones resuming
resuming session establishment when all the preconditions session establishment when all the preconditions are met.
are met. Therefore, rather than performing an offer/answer Therefore, rather than performing an offer/answer exchange to
exchange to inform the UAC that all the preconditions are inform the UAC that all the preconditions are met, they simply
met, they simply send a 180 (Ringing) response indicating send a 180 (Ringing) response indicating that session
that session establishment has been resumed. establishment has been resumed.
While RFC 3312 [1] allows to update current status information using While RFC 3312 [3] allows to update current status information using
offers as described above, it does not allow to downgrade current 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 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 3312. However, such downgrades are sometimes needed. Figure 1 shows
an example where performing such a downgrade in an answer would be an example where performing such a downgrade in an answer would be
needed. needed.
3pcc 3pcc
A controller B C A Controller B C
| | | | | | | |
|<-dialog 1->|<-dialog 2->| | |<-dialog 1->|<-dialog 2->| |
| | | | | | | |
| *********************** | | | *********************** | |
|* MEDIA *| | |* MEDIA *| |
| *********************** | | | *********************** | |
| | | | | | | |
| | | | | | | |
|<-dialog 1->|<------dialog 3----->| |<-dialog 1->|<------dialog 3----->|
| | | | | | | |
| ******************************** | | ******************************** |
|* MEDIA *| |* MEDIA *|
| ******************************** | | ******************************** |
| | | | | | | |
| | | | | | | |
Figure 1: Session Mobility using 3pcc Figure 1: Session mobility using 3pcc
The 3pcc [6] controller in Figure 1 has established a session The 3pcc (Third Party Call Control) [6] controller in Figure 1 has
between A and B using dialog 1 towards A and dialog 2 towards B. At established a session between A and B using dialog 1 towards A and
that point, the controller wants A to have a session with C instead dialog 2 towards B. At that point, the controller wants A to have a
of B. To transfer A to C (configuration shown at the bottom of Figure session with C instead of B. To transfer A to C (configuration shown
1), the controller sends an empty (no offer) re-INVITE to A. Since A at the bottom of Figure 1, the controller sends an empty (no offer)
does not know that the session will be moved, its offer in the 200 OK re-INVITE to A. Since A does not know that the session will be moved,
states that the current status of the media stream in the send its offer in the 200 OK states that the current status of the media
direction is "Yes". The controller, after contacting C establishing stream in the send direction is "Yes". The controller, after
dialog 3, sends back an answer to A. This answer contains a new contacting C establishing dialog 3, sends back an answer to A. This
destination for the media (C) and should have downgraded the current answer contains a new destination for the media (C) and should have
status of the media stream to "No", since there is no reservation of downgraded the current status of the media stream to "No", since
resources between A and C. there is no reservation of resources between A and C.
4 Update to RFC 3312 4.1 Update to RFC 3312
Below there are a set of new rules that update RFC 3312 [1] to Below there are a set of new rules that update RFC 3312 [3] to
address the issues above. address the issues above.
The rule below applies to offerers that are moving a media stream to The rule below applies to offerers that are moving a media stream to
a new address: a new 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 the current status values it does not have local
information about to "No". information about to "No".
Note that for streams using segmented status (as opposed to end-to- Note that for streams using segmented status (as opposed to
end status), the fact that the address for the media stream at the end-to-end status), the fact that the address for the media stream at
local segment changes may or may not affect the status of the the local segment changes may or may not affect the status of the
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 the rules below applies to an answerer that is
moving a media stream. That is, the offerer was not aware of the move moving a media stream. That is, the offerer was not aware of the move
when it generated the offer. when it generated the offer.
Table 3 of RFC 3312 [1] needs to be updated to allow answers to Table 3 of RFC 3312 [3] needs to be updated to allow answers to
downgrade current status values. Table 1 below shows the result. downgrade current status values. The following table shows the
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
Table 1: Possible values for the "Current" fields
An answerer MUST downgrade the current status values that received in An answerer MUST downgrade the current status values that received in
the offer if it has local information about them or if the media the offer if it has local information about them or if the media
stream is being moved to a new transport address. stream 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 Table 2. 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
Table 2: Possible values for the "Current" fields after an answer 4.2 Desired Status
5 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 high- satisfied with optional QoS when the stream is moved to a
bandwidth link. high-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 (e.g., 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.)
6 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 keep sessions from being
established. An attacker removing preconditions from a session established. 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 It is thus 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].
7 Authors' Addresses 6. Acknowledges
Dave Oran and Allison Mankin provided useful comments on this
document.
7. References
7.1 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.
7.2 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 SDP",
draft-ietf-mmusic-sdp-comedia-05 (work in progress), March 2003.
Authors' Addresses
Gonzalo Camarillo Gonzalo Camarillo
Ericsson Ericsson
Advanced Signalling Research Lab. Hirsalantie 11
FIN-02420 Jorvas Jorvas 02420
Finland Finland
electronic mail: 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
electronic mail: pkyzivat@cisco.com
8 Normative References
[1] "Integration of resource management and session initiation
protocol (SIP)," RFC 3312, Internet Engineering Task Force, Oct.
2002.
[2] J. Rosenberg, H. Schulzrinne, G. Camarillo, A. R. Johnston, J. EMail: pkyzivat@cisco.com
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.
9 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, Intellectual Property Statement
"Best current practices for third party call control in the session
initiation protocol," Internet Draft draft-ietf-sipping-3pcc-05,
Internet Engineering Task Force, Oct. 2003. Work in progress.
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