Internet Engineering Task Force                  Flemming Andreasen
   MMUSIC Working Group                                      Dave Oran
   INTERNET-DRAFT                                             Dan Wing
   EXPIRES: August 2005                                  Cisco Systems
                                                        February, 2005

                     Connectivity Preconditions for
               Session Description Protocol Media Streams
        <draft-andreasen-mmusic-connectivityprecondition-02.txt>


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Copyright Notice

   Copyright (C) The Internet Society (2005).  All Rights Reserved.

Abstract

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









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


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

   The concept of a Session Description Protocol (SDP) [SDP]
   precondition in the Session Initiation Protocol (SIP) [SIP] is
   defined in [RFC3312] and [RFC3312upd].  A precondition 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 one or more Network Address Translators (NATs) or
   firewalls in the media path, the media stream may not be received by
   the far end.  The connectivity precondition defined in this document
   ensures, that session progress is delayed until media stream
   connectivity has been verified, or the session itself is abandoned.

3  Connectivity Precondition Definition

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

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





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   RFC 3312 defines support for two kinds of status types, namely
   segmented and end-to-end.  The connectivity precondition-type
   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 mandatory 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 delay of session establishment defined here implies that
   alerting of the called party MUST NOT occur until the precondition
   has been satisfied.  Packets may be both sent and received on the
   media streams in question, however such packets SHOULD be limited to
   packets that are necessary to verify connectivity between the two
   endpoints involved on the media stream, i.e. the underlying media
   stream SHOULD NOT be cut through.  For example, STUN packets [STUN],
   RTP No-Op packets and corresponding RTCP reports, as well as TCP SYN
   and ACK packets can be exchanged on media streams that support them
   as a way of verifying connectivity.

   The direction attributes defined in RFC 3312 are interpreted as
   follows:

   * send:  This party is sending packets on the media stream to the
     other party, and the other party has received at least one of
     those packets, i.e., there is connectivity in the forward
     (sending) direction.

   * recv:  The other party is sending packets on the media stream to
     this party, and this party has received at least one of those
     packets, i.e., there is connectivity in the backwards (receiving)
     direction.

   When the media stream consists of multiple destination addresses,
   connectivity to all of them MUST be verified in order for the
   precondition to be met.  In the case of RTP-based media streams,
   RTCP connectivity however is not a requirement.

   Note that a "send" connectivity precondition from the offerer's
   point of view corresponds to a "recv" connectivity precondition from
   the answerer's point of view, and vice versa.  If media stream
   connectivity in both directions is required before session
   establishment or modification continues, the desired status MUST be
   set to "sendrecv".

   Connectivity preconditions may have a strength-tag of either
   "mandatory" or "optional".  When a mandatory connectivity



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   precondition is offered, and the answerer cannot satisfy the
   connectivity precondition, e.g., because the offer does not include
   parameters that enable connectivity to be verified without media cut
   through, the offer MUST be rejected as described in RFC 3312.  When
   an optional connectivity precondition is offered, the answerer MUST
   generate its answer SDP as soon as possible; since session progress
   is not delayed in this case, it is not known whether the associated
   media streams will have connectivity.  If the answerer wants to
   delay session progress until connectivity has been verified, the
   answerer MUST increase the strength of the connectivity precondition
   by using a strength-tag of "mandatory" in the answer.

     Note that use of a "mandatory" precondition requires the presence
     of a SIP "Require" header with the option tag "precondition": Any
     SIP UA that does not support a mandatory precondition will reject
     such requests.  To get around this issue, an optional connectivity
     precondition and the SIP "Supported" header with the option tag
     "precondition" can be used instead.

   Offers with connectivity preconditions in re-INVITEs or UPDATEs
   follow the rules given in Section 6 of RFC 3312, i.e.:

     "Both user agents SHOULD continue using the old session parameters
     until all the mandatory preconditions are met.  At that moment,
     the user agents can begin using the new session parameters."

   It should be noted, that connectivity may not exist between two
   entities initially, e.g., when one or both entities are behind a
   symmetric NAT.  Subsequent packet exchanges however may create the
   necessary address bindings in the NAT(s) thereby creating
   connectivity.  The ICE methodology [ICE] for example ensures that
   such bindings are created following an offer/answer exchange.

3.1 Verifying Connectivity

   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.

     Editor's Note: The above obviously only applies to RTP-based media
     streams.

   The above definitions of send and receive connectivity preconditions
   beg two questions: How does the sender of a packet know the other
   party received it, and how does the receiver of a packet know who
   sent it (in particular, the correlation between an incoming media
   packet and a particular SIP dialog may not be obvious).  The



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   determination depends on the exact method being used to verify
   connectivity, however the following provides procedures for three
   specific approaches:

   * RTP No-Op [no-op]:  The sender of an RTP No-Op payload can verify
     send connectivity by examining the RTCP report being returned.  In
     particular, the source SSRC in the RTCP report block is used for
     correlation.  The RTCP report block also contains the SSRC of the
     sender of the report and the SSRC of incoming RTP No-Op packets
     identifies the sender of the RTP packet.  Thus, once send
     connectivity has been ascertained, receipt of an RTP No-Op packet
     from the same SSRC provides the necessary correlation to determine
     receive connectivity.  Alternatively, the duality of send and
     receive preconditions can be exploited, with one side confirming
     when his send precondition is satisfied, which in turn implies the
     other sides recv precondition is satisfied.

   * ICE [ICE]:     The STUN binding request message sent to check
     connectivity contains a transaction ID which is returned in the
     STUN binding response, thus send connectivity is verified easily.
     STUN binding requests also contain a username and a password which
     ICE communicates via SIP.  When an incoming STUN message is
     received, it is therefore easy to determine the source of that
     message and hence receive connectivity can be determined that way.

     ICE presents the peer with a number of alternative candidate
     addresses for a particular media stream.  Once connectivity has
     been verified for one of those candidate addresses, connectivity
     has been verified, regardless of whether this candidate address is
     the one that ends up being used.  If a media stream consists of
     multiple destination addresses, verification of a candidate
     address for each must occur in order for the precondition to be
     satisfied.

   * TCP [TCP]:     TCP connections are bidirectional and hence there
     is no difference between send and recv connectivity preconditions.
     Once the TCP three-way hand shake has completed (SYN, SYN-ACK,
     ACK), the TCP connection is established and data can be sent and
     received by either party, i.e. both a send and a receive
     connectivity precondition has been satisfied.

4  Examples

   The call flow of Figure 1 shows a basic session establishment with
   the Session Initiation Protocol using SDP connectivity preconditions
   and RTP No-Op.  Note that not all SDP details are provided in the
   following.







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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

   SDP1: A includes a mandatory 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".  A's
   local status table (see RFC 3312) for the connectivity precondition
   is as follows:

       Direction |  Current | Desired Strength |  Confirm
      -----------+----------+------------------+----------
         send    |    no    |   mandatory      |    no
         recv    |    no    |   mandatory      |    no

   and the resulting offer SDP is:

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





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   SDP2: When B receives the offer, B sees the mandatory sendrecv
   connectivity precondition.  B can ascertain connectivity to A
   ("send" from B's point of view) by use of the RTP No-Op, however B
   wants A to inform it about connectivity in the other direction
   ("recv" from B's point of view).  B's local status table therefore
   looks as follows:

       Direction |  Current | Desired Strength |  Confirm
      -----------+----------+------------------+----------
         send    |    no    |   mandatory      |    no
         recv    |    no    |   mandatory      |    no

   Since B wants to ask A for confirmation about the "recv" (from B's
   point of view) connectivity precondition, the resulting answer SDP
   becomes:

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

   Meanwhile, B performs a connectivity check to A, which succeeds and
   hence B's local status table is updated as follows:

       Direction |  Current | Desired Strength |  Confirm
      -----------+----------+------------------+----------
         send    |    yes   |   mandatory      |    no
         recv    |    no    |   mandatory      |    no

   Since the "recv" connectivity precondition (from B's point of view)
   is still not satisfied, session establishment remains suspended.

   SDP3: When A receives the answer SDP, A notes that confirmation was
   requested for B's "recv" connectivity precondition, which is the
   "send" precondition from A's point of view.  A performs a
   connectivity check to B, which succeeds, and A's local status table
   becomes:

       Direction |  Current | Desired Strength |  Confirm
      -----------+----------+------------------+----------
         send    |    yes   |   mandatory      |    yes
         recv    |    no    |   mandatory      |    no

   Since B asked for confirmation about the "send" connectivity (from
   A's point of view), A now sends an UPDATE (5) to B to confirm the
   connectivity from A to B:






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

   SDP4:  Upon receiving the updated offer, B now knows that there is
   connectivity from A to B and updates the local status table as
   follows ("send" from A corresponds to "recv" from B's point of
   view):

       Direction |  Current | Desired Strength |  Confirm
      -----------+----------+------------------+----------
         send    |    yes   |   mandatory      |    no
         recv    |    yes   |   mandatory      |    no

   B 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:cntv e2e sendrecv
     a=des:cntv mandatory e2e sendrecv

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

5  Security Considerations

   In addition to the general security considerations for preconditions
   provided in RFC 3312, the following security issues, which are
   specific to connectivity preconditions, should be considered.

   Connectivity preconditions rely on mechanisms beyond SDP, e.g. RTP
   No-Op [no-op] or STUN [stun], to establish and verify connectivity
   between an offerer and an answerer.  An attacker that prevents those
   mechanism from succeeding can prevent media sessions from being
   established and hence it is RECOMMENDED that such mechanisms are
   adequately secured by message authentication and integrity
   protection.  Also, the mechanisms SHOULD consider how to prevent
   denial of service attacks.  Similarly, an attacker that can forge
   packets for these mechanisms can enable sessions to be established
   when there in fact is no media connectivity, which may lead to a
   poor user experience.  Authentication and integrity protection of
   such mechanisms can prevent this type of attacks and hence use of it
   is RECOMMENDED.






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6  IANA Considerations

   IANA is hereby requested to register a RFC 3312 precondition type
   called "cntv" 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.

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.

   [SIP] J. Rosenberg, H. Schulzrinne, G. Camarillo, A. Johnston, J.
   Peterson, R. Sparks, M. Handley, E. Schooler, "SIP: Session
   Initiation Protocol", RFC 3261, June 2002.

10 Informative References

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





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   [no-op] F. Andreasen, D. Oran, and D. Wing, "RTP No-Op Payload
   Format", Work in Progress

   [stun] J. Rosenberg, J. Weinberger, C. Huitema, R. Mahy, "STUN -
   Simple Traversal of User Datagram Protocol (UDP) Through Network
   Address Translators (NATs)", RFC 3489, March 2003.

   [RFC3312upd] G. Camarillo and P. Kyzivat, "Update to the Session
   Initiation Protocol (SIP) Preconditions Framework", IETF, work in
   progress.

   [ICE] J. Rosenberg, "Interactive Connectivity Establishment (ICE): A
   Methodology for Network Address Translator (NAT) Traversal for
   Multimedia Session Establishment Protocols", IETF, work in progress.

   [TCP]  J. Postel, "Transmission Control Protocol", RFC 793,
   September 1981.

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   THE INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED
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Copyright Statement

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Acknowledgment

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








































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