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Versions: 00 01 02                                                      
   Internet Engineering Task Force                  Flemming Andreasen
   MMUSIC Working Group                                      Dave Oran
   INTERNET-DRAFT                                             Dan Wing
   EXPIRES: August 2004                                  Cisco Systems
                                                        February, 2004

                     Connectivity Preconditions for
               Session Description Protocol Media Streams
        <draft-andreasen-mmusic-connectivityprecondition-00.txt>


Status of this memo

   This document is an Internet-Draft and is in full conformance with
   all provisions of Section 10 of RFC2026.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF), its areas, and its working groups. Note that
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   Drafts.

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Copyright Notice

   Copyright (C) The Internet Society (2004).  All Rights Reserved.

Abstract

   This document defines a new connectivity precondition for the
   Session Description Protocol precondition framework described in RFC
   3312.  A connecitivity precondition can be used to delay session
   establishment or modification until media stream connectivity has
   been verified successfully.














INTERNET-DRAFT         Connectivity Preconditions       February, 2004



1.   Notational Conventions..........................................2
2.   Introduction....................................................2
3.   Connectivity Precondition Definition............................2
4.   Examples........................................................3
5.   Security Considerations.........................................5
6.   IANA Considerations.............................................5
7.   Acknowledgements................................................5
8.   Authors' Addresses..............................................6
9.   Normative References............................................6
10.  Informative References..........................................6
Intellectual Property Statement......................................7
Acknowledgement......................................................8


1. Notational Conventions

   The key words "MUST", "MUST NOT", "REQUIRED", "MUST", "MUST NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in [RFC2119].

2. Introduction

   RFC 3312 defines the concept of a Session Description Protocol (SDP)
   [SDP] precondition, which is a condition that has to be satisfied
   for a given media stream in order for session establishment or
   modification to proceed.  When the precondition is not met, session
   progress is delayed until the precondition is satisfied, or the
   session establishment fails.  For example, RFC 3312 defines the
   Quality of Service precondition, which is used to ensure
   availability of network resources prior to establishing (i.e.
   alerting) a call.

   SIP sessions are typically established in order to setup one or more
   media streams.  Even though a media stream may be negotiated
   successfully, the actual media stream itself may fail.  For example,
   when there is a NAT or firewall in the media path, the media stream
   may not be received.  The connectivity precondition defined in this
   document ensures, that session progress is delayed until media
   stream connectivity is established, or the session itself is
   abandoned.

3. Connectivity Precondition Definition

   The connectivity precondition type is defined by the string "con"
   and hence we modify the grammar found in RFC 3312 as follows:

     precondition-type  =  "con" | "qos" | token

   RFC 3312 defines support for two kinds of status types, namely
   segmented and end-to-end.  The connectivity precondition-type



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   defined here MUST be used with the end-to-end status type; use of
   the segmented status type is undefined.

   An entity that wishes to delay session establishment or modification
   until media stream connectivity has been established uses this
   precondition-type in an offer.  When a connectivity precondition is
   received in an offer, session establishment or modification MUST be
   delayed until the connectivity precondition has been met, i.e. media
   stream connectivity has been established in the desired
   direction(s).

   The direction attributes are interpreted as follows:

   * send:  The offerer/answerer is sending media stream packets to the
     other party, and the offerer/answer knows the other party is
     receiving those media stream packets.

   * recv:  The offerer/answerer knows that the other party has
     ascertained media stream connectivity to it.

   If media stream connectivity in both directions is required, the
   desired status should be set to "sendrecv".

   Media stream connectivity can be ascertained in different ways and
   this document does not mandate any particular mechanism for doing
   so.  It is however RECOMMENDED that the No-Op RTP payload format
   defined in [no-op] is supported by entities that support
   connectivity preconditions.  This will ensure that all entities that
   support the connectivity preconditions have at least one common way
   of ascertaining connectivity.

4. Examples

   The call flow of Figure 1 shows a basic session establishment using
   connectivity preconditions and using RTP no-op.  Note that not all
   SDP details are provided in the following.

   SDP1: A includes the end-to-end connectivity precondition with a
   desired status of "sendrecv"; this will ensure media stream
   connectivity in both directions before continuing with the session
   setup.  Since media stream connectivity in either direction is
   unknown at this point, the current status is set to "none":

     m=audio 20000 RTP/AVP 0 96
     c=IN IP4 192.0.2.1
     a=rtpmap:96 no-op/8000
     a=curr:con e2e none
     a=des:con mandatory e2e sendrecv

   SDP2: When B receives the offer, B sees the bidirectional
   connectivity preconditions.  B can ascertain connectivity to A by



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   use of the RTP no-op, however B needs A to inform it about
   connectivity in the other direction.  Consequently, B includes a
   request for confirmation in the "receive" direction:

     m=audio 30000 RTP/AVP 0 96
     a=rtpmap:96 no-op/8000
     c=IN IP4 192.0.2.4
     a=curr:con e2e none
     a=des:con mandatory e2e sendrecv
     a=conf:con e2e recv

   Meanwhile, B performs a connectivity check to A, which succeeds.
   Also, A performs a connectivity check to B, which also succeeds.

   SDP3: A now sends an UPDATE (5) to B to confirm the connectivity
   from A to B, as requested by B:

     m=audio 20000 RTP/AVP 0 96
     a=rtpmap:96 no-op/8000
     c=IN IP4 192.0.2.1
     a=curr:con e2e send
     a=des:qos mandatory e2e sendrecv

   SDP4:  Upon receiving the updated offer, B now knows that there is
   connectivity from A to B; connectivity from B to A was verified
   earlier.  B therefore responds with an answer (6) which contains the
   current status of the connectivity precondition (i.e., sendrecv)
   from B's point of view:

     m=audio 30000 RTP/AVP 0 96
     a=rtpmap:96 no-op/8000
     c=IN IP4 192.0.2.4
     a=curr:con e2e sendrecv
     a=des:con mandatory e2e sendrecv

   At this point in time, session establishment resumes and B returns a
   180 (Ringing) response (7).

















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                  A                                            B

                  |                                            |
                  |-------------(1) INVITE SDP1--------------->|
                  |                                            |
                  |<------(2) 183 Session Progress SDP2--------|
                  |                                            |
                  |<~~~~~ Connectivity check to A ~~~~~~~~~~~~~|
                  |                                            |
                  |----------------(3) PRACK------------------>|
                  |                                            |
                  |~~~~~ Connectivity to A OK ~~~~~~~~~~~~~~~~>|
                  |                                            |
                  |<-----------(4) 200 OK (PRACK)--------------|
                  |                                            |
                  |~~~~~ Connectivity check to B ~~~~~~~~~~~~~>|
                  |<~~~~ Connectivity to B OK ~~~~~~~~~~~~~~~~~|
                  |                                            |
                  |-------------(5) UPDATE SDP3--------------->|
                  |                                            |
                  |<--------(6) 200 OK (UPDATE) SDP4-----------|
                  |                                            |
                  |<-------------(7) 180 Ringing---------------|
                  |                                            |
                  |                                            |
                  |                                            |

                Figure 1: Example using the connectivity precondition



5. Security Considerations

   TBD

6. IANA Considerations

   IANA is hereby requested to register a RFC 3312 precondition type
   called "con" with the name "Connectivity precondition".  The
   reference for this precondition type is the current document.

7. Acknowledgements

   The concept of a "connectivity precondition" is the result of
   discussions with numerous people over a long period of time; the
   authors greatly appreciate these contributions.








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8. Authors' Addresses

   Flemming Andreasen
   Cisco Systems, Inc.
   499 Thornall Street, 8th Floor
   Edison, New Jersey  08837 USA
   EMail: fandreas@cisco.com

   David Oran
   Cisco Systems, Inc.
   7 Ladyslipper Lane
   Acton, MA 01720  USA
   EMail: oran@cisco.com

   Dan Wing
   Cisco Systems, Inc.
   170 West Tasman Drive
   San Jose, CA  95134  USA
   EMail: dwing@cisco.com

9. Normative References

   [RFC3312] G. Camarillo, W. Marshall, J. Rosenberg, "Integration of
   Resource Management and Session Initiation Protocol (SIP)", RFC
   3312, October 2002.

   [RFC2327] M. Handley and V. Jacobson, "SDP: Session Description
   Protocol", RFC 2327, April 1998.

10.  Informative References

   [RFC3551] H. Schulzrinne, and S. Casner "RTP Profile for Audio and
   Video Conferences with Minimal Control", RFC 3550, July 2003.

   [NO-OP] F. Andreasen, D. Oran, and D. Wing, "RTP No-Op Payload
   Format", February 2004, http://search.ietf.org/internet-drafts/
   draft-wing-avt-rtp-noop-00.txt, Work in Progress

















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Acknowledgement

   Funding for the RFC Editor function is currently provided by the
   Internet Society.


















































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