MMUSIC Working Group                                        F. Andreason Andreasen
Internet-Draft                                        Cisco System, Inc.
Expires: October 26, 2005 April 22, 2006                                     G. Camarillo
                                                                 D. Oran
                                                      Cisco Systems, Inc
                                                                 D. Wing
                                                     Cisco Systems, Inc.
                                                          April 24,
                                                        October 19, 2005

   Connectivity Preconditions for Session Description Protocol Media

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Copyright Notice

   Copyright (C) The Internet Society (2005).


   This document defines a new connectivity precondition for the Session
   Description Protocol precondition framework described in RFC 3312
   (and its update, RFC4032).  A connectivity precondition can be used
   to delay session establishment or modification until media stream
   connectivity has been verified successfully.  The method of
   verification may vary depending on the type of transport used for the
   media.  For reliable connection-oriented transports such as TCP
   verification is achieved by successful connection establishment.  For
   unreliable datagram transports such as UDP, verification involves
   probing the stream with data or control packets.

   NOTE: This document is the result of a merge of two prior documents
   with overlapping scope: draft-ietf-mmusic-connectivityprecondition-02
   and draft-ietf-mmusic-connection-precon-02.  The former covered the
   case of datagram unreliable transports; the latter the case of
   connection-oriented reliable transports.  The merged version covers
   these two but also describes operations in hybrid cases of unreliable
   connection-oriented transports and reliable datagram transports.

Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  4  3
   2.  Terminology  . . . . . . . . . . . . . . . . . . . . . . . . .  4  3
   3.  Connectivity Precondition Definition . . . . . . . . . . . . .  4
     3.1  3
     3.1.  Syntax . . . . . . . . . . . . . . . . . . . . . . . . . .  4
     3.2  3
     3.2.  Operational semantics  . . . . . . . . . . . . . . . . . .  5
     3.3  4
     3.3.  Status type  . . . . . . . . . . . . . . . . . . . . . . .  5
     3.4  4
     3.4.  Direction tag  . . . . . . . . . . . . . . . . . . . . . .  5
     3.5  4
     3.5.  Precondition strength  . . . . . . . . . . . . . . . . . .  6  5
   4.  Verifying connectivity . . . . . . . . . . . . . . . . . . . .  7
     4.1  6
     4.1.  Procedures for connection-oriented transports  . . . . . .  8
     4.2  7
     4.2.  Procedures for datagram transports . . . . . . . . . . . .  8
   5.  Examples . . . . . . . . . . . . . . . . . . . . . . . . . . .  9
   6.  Security Considerations  . . . . . . . . . . . . . . . . . . . 14
   7.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 14
   8.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 15
     8.1.  Normative References . . . . . . . . . . . . . . . . . . . 15
     8.2.  Informative References . . . . . . . . . . . . . . . . . . 15
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . 16 . . 17
   Intellectual Property and Copyright Statements . . . . . . . . 17 . . 18

1.  Introduction

   The concept of a Session Description Protocol (SDP) [2] precondition
   in the Session Initiation Protocol (SIP) [SIP] is defined in RFC3312
   [4] (updated by RFC4032 [6]).  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, RFC3312
   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, through an SDP offer-answer exchange, 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.  In cases where
   the media is carried over a connection-oriented transport such as TCP
   [8], the connection-establishment procedures may fail.  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.

   The connectivity precondition type defined in this document follows
   the guidelines provided in RFC4032 [6] to extend the SIP
   preconditions framework.

2.  Terminology

   In this document, the key words "MUST", "MUST NOT", "REQUIRED",
   RECOMMENDED", "MAY", and "OPTIONAL" are to be interpreted as
   described in BCP 14, RFC 2119 [1] and indicate requirement levels for
   compliant implementations.

3.  Connectivity Precondition Definition


3.1.  Syntax

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

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

   This precondition tag is registered with the IANA in Section 7.


3.2.  Operational semantics

   According to RFC4032 [6], documents defining new precondition types
   need to describe 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.  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 is 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 does not occur until the precondition has been

   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

   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.


3.3.  Status type

   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.


3.4.  Direction tag

   The direction attributes defined in RFC 3312 are interpreted as

   o  send: The party who generated the session description (the offerer
      in an offer-answer exchange) 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.
   o  recv: The other party (the answerer in an offer-answer exchange)
      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.
   o  sendrecv: Both the send and recv conditions hold.  In the case of
      a connection-oriented transport such as TCP, once established the
      connection would usually have an associated direction tag of
      sendrecv because it can carry data in both directions.

   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".


3.5.  Precondition strength

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

   When a mandatory connectivity 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 [12] [7] methodology for example ensures that such
   bindings are created following an offer/answer exchange.

4.  Verifying connectivity

   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). obvious) ?

   Media stream connectivity can be ascertained in a variety of ways.
   This document does not mandate any particular mechanism for doing so,
   however the appropriate machinery is likely to vary depending on the
   type of transport used for media carriage.  In order to comply with
   the intent of an endpoint requiring connectivity preconditions, the
   following general principles apply:

   o  The 3-way handshake connection establishment procedures of a
      reliable transport protocol such as TCP are usually adequate to
      demonstrate bi-directional connectivity (and hence "sendrecv"
      media capability).  Probe packets sent over the connection are
      generally not required to satisfy the precondition.
   o  A pure datagram transport such as UDP (whether carrying RTP or
      some other protocol) by itself provides no useful feedback about
      connectivity.  Hence, some sort of probe traffic is necessary to
      ascertain whether packets are being received successfully.
   o  Connectivity preconditions are used to verify connectivity based
      on the address information exchanged in offers and answers.  When
      overlapping IP address spaces are used (e.g. because one or both
      endpoints are behind a Network Address Translator), it is possible
      to inadvertently verify connectivity with an unrelated entity.  In
      order to address this issue, a correlation mechanism is needed
      between media stream packets on one side and offers and answers on
      the other side.  ICE [7] defines one such correlation mechanism,
      however use of it is above and beyond the connection-oriented
      connectivity preconditions defined here.

   o  Some connection-oriented transport protocols may allow the data
      transfer phase to operate in an unreliable mode (today there is no
      standards-track IETF protocol which exhibits this characteristic).
      In such cases the success of connection establishment may not
      definitively demonstrate connectivity in the data phase, and hence
      probe traffic MAY be necessary to ascertain if the precondition is
   o  Hybrid protocols such as DCCP [13] [14] provide their own feedback
      channel and initialization procedures, which can serve to verify
      connectivity without the use of explicit probe traffic.

   The determination depends on the exact method being used to verify


4.1.  Procedures for connection-oriented transports

   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.  Implementations SHOULD NOT require the receipt of
   probe traffic in order to consider the precondition satisfied.

   SCTP [8] [9] connections have similar semantics as TCP and SHOULD be
   treated the same as TCP.

4.2  Procedures for datagram transports


   When a connection-oriented transport is part of connectivity on datagram transports usually entail an offer, it may be
   passive, active, or active/passive [12].  When it is passive, the sending of probe traffic with some form of feedback
   offerer expects the answerer to inform initiate the
   sender whether reception was successful.  Any of connection
   establishment, and when it is active, the following
   techniques MAY be used.  Other techniques which meet offerer wants to initiate
   the requirement
   of Section 4 above MAY also be used.

   o  RTP no-op [11]: The sender of an RTP No-Op payload can verify send
      connectivity by examining connection establishment.  When it is active/passive, the RTCP report(s) being returned.  In
   answerer decides.

   SIP and SDP do not provide any inherent capabilities for associating
   an incoming media stream packet with a particular dialog.  Thus, when
   the offerer is passive and an incoming connection is being
   established, the offerer cannot guarantee that the packet is
   associated with a particular dialog.  When SIP forking is being used,
   this implies that the offerer cannot determine which of the early
   dialogs now has its recv connectivity precondition satisfied - a
   correlation mechanism is missing.  This turns out not to be a problem
   however, since the successful completion of the connection-
   establishment procedure itself (e.g. receipt of SYN-ACK in the case
   of TCP) informs the answerer that the precondition has been
   satisfied, and hence there is no need for the offerer to explicitly
   inform the answerer of this (by sending a SIP UPDATE message).  In
   the absence of a correlation mechanism (e.g.  ICE), an answerer
   therefore MUST NOT require the offerer to confirm a connectivity
   precondition on a connection-oriented transport.

4.2.  Procedures for datagram transports

   Verification of connectivity on datagram transports usually entails
   the sending of probe traffic with some form of feedback to inform the
   sender whether reception was successful.  Techniques that can be used
   to verify connectivity on datagram transports include:

   o  ICE [7]: ICE provides one or more candidate addresses in signaling
      between the offerer and the answerer and then uses STUN Binding
      Requests to determine which pairs of candidate addresses have
      connectivity.  Each STUN Binding Request contains a password which
      is communicated in the SDP as well; this enables correlation
      between STUN Binding Requests and candidate addresses for a
      particular media stream.  In ICE, connectivity is always checked
      in both directions by following a state machine with a set of
      states for the offerer and a set of states for the answerer: The
      offerer ascertains "recv" connectivity for a particular transport
      address pair by transitioning into the "validating" state, whereas
      "send" connectivity is ascertained by transitioning into the
      "valid" state.  The answerer ascertains both "send" and "recv"
      connectivity for a particular transport address pair by
      transitioning into the "send-valid" state.  As a consequence of
      this, there is never a need for the answerer to request
      confirmation of the connectivity precondition when using ICE: the
      answerer can determine the status locally.  When ICE is used to
      verify connectivity preconditions, the precondition is satisfied
      as soon as one of the candidates becomes valid, i.e. connectivity
      has been verified for all the component transport addresses used
      by the media stream.  For example, with an RTP-based media stream
      where RTCP is not suppressed, connectivity must be ascertained for
      both RTP and RTCP; this is a tightening of the general operational
      semantics provided in Section 3.2 imposed by ICE.  Finally, it
      should be noted, that though connectivity has been ascertained, a
      new offer/answer exchange may be required before media can
      actually flow (per ICE).
   o  RTP no-op [13]: The sender of an RTP No-Op payload can verify send
      connectivity by examining the RTCP report(s) 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.
   o  STUN [2]:

   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 [12] communicates via SIP.  When an incoming STUN message is
      received, it is therefore easy to determine the source above are merely examples of techniques 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  Other
   techniques which meet the precondition to requirements of Section 4 above can be
      satisfied. used
   as well.  It is however RECOMMENDED that the No-Op RTP payload format ICE be supported by entities
   that support connectivity preconditions.  This
   will preconditions for datagram transports.  Use
   of ICE has the benefit of working for all datagram based media
   streams (not just RTP) as well as facilitate NAT and firewall
   traversal, which may otherwise interfere with connectivity.
   Furthermore, the ICE recommendation provides a baseline to ensure
   that all entities that require probe traffic to support the
   connectivity preconditions have at least one common way of
   ascertaining connectivity.

5.  Examples

   The first example uses the connectivity precondition with TCP in the
   context of a session involving a wireless access medium.  Both UAs
   use a radio access network that does not allow them to send any data
   (not even a TCP SYN) until a radio bearer has been setup for the
   connection.  Figure 1 shows the message flow of this example (the
   PRACK transaction has been omitted for clarity):

               A                                    B
               |  INVITE                            |
               |  a=curr:conn e2e none              |
               |  a=des:conn mandatory e2e sendrecv |
               |  a=setup:holdconn                  |
               |                                    |
               |  183 Session Progress              |
               |  a=curr:conn e2e none              |
               |  a=des:conn mandatory e2e sendrecv |
               |  a=setup:holdconn                  |
               |                                    |
               |  UPDATE                            |
               |  a=curr:conn e2e none              |
               |  a=des:conn mandatory e2e sendrecv |
     A's radio |  a=setup:actpass                   |
     bearer is +----------------------------------->|
     up        |                                    |
               |  200 OK                            |
               |  a=curr:conn e2e none              |
               |  a=des:conn mandatory e2e sendrecv |
               |  a=setup:active                    |
               |                                    |
               |                                    |
               |                                    |
               |                                    | B's radio
               |<---TCP Connection Establishment--->+ bearer is up
               |                                    | B sends TCP SYN
               |                                    |
               |                                    |
               |  180 Ringing                       | TCP connection
               |<-----------------------------------+ is up
               |                                    | B alerts the user
               |                                    |

   Figure 1: Message flow with two types of  preconditions

   A sends an INVITE requesting connection-establishment preconditions.
   The setup attribute in the offer is set to holdconn because A cannot
   send or receive any data before setting up a radio bearer for the

   B agrees to use the connectivity precondition by sending a 183
   (Session Progress) response.  The setup attribute in the answer is
   also set to holdconn because B, like A, cannot send or receive any
   data before setting up a radio bearer for the connection.

   When A's radio bearer is ready, A sends an UPDATE to B with a setup
   attribute with a value of actpass.  This attribute indicates that A
   can perform an active or a passive TCP open.  A is letting B choose
   which endpoint will initiate the connection.

   Since B's radio bearer is not ready yet, B chooses to be the one
   initiating the connection and indicates so with a setup attribute
   with a value of active.  At a later point, when B's radio bearer is
   ready, B initiates the TCP connection towards A.

   Once the TCP connection is established successfully, B alerts the
   callee and sends a 180 (Ringing) response.

   The second example shows a basic SIP session establishment using SDP
   connectivity preconditions and RTP No-Op.  Note that not all SDP
   details are provided in the following. below shows the message flow
   for this scenario shown in Figure 2 below.

                  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 2: Connectivity precondition with RTP no-op

   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
   a=rtpmap:96 no-op/8000
   a=curr:conn e2e none
   a=des:conn mandatory e2e sendrecv

   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

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

       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:

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

6.  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.
   TCP[8] connection establishment, RTP No-Op [11] [13] or STUN [9], [10], 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.

   It is also strongly RECOMMENDED that integrity protection be applied
   to the SDP session descriptions.  S/MIME [5] is the natural choice to
   provide such end-to-end integrity protection, as described in RFC
   3261 [3].

7.  IANA Considerations

   IANA is hereby requested to register a RFC 3312 precondition type
   called "conn" with the name "Connectivity precondition".  The
   reference for this precondition type is the current 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]  Handley, M. and V. Jacobson, "SDP: Session Description
        Protocol", RFC 2327, April 1998.

   [3]  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.

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

   [5]  Peterson, J., "S/MIME Advanced Encryption Standard (AES)
        Requirement for the Session Initiation Protocol (SIP)",
        RFC 3853, July 2004.

   [6]  Camarillo, G. and P. Kyzivat, "Update to the Session Initiation
        Protocol (SIP) Preconditions Framework", RFC 4032, March 2005.


   [7]  Rosenberg, J., "Interactive Connectivity Establishment (ICE): A
        Methodology for Network  Address Translator (NAT) Traversal for
        Offer/Answer Protocols", draft-ietf-mmusic-ice-05 (work in
        progress), July 2005.

8.2.  Informative References


   [8]   Postel, J., "Transmission Control Protocol", STD 7, RFC 793,
         September 1981.


   [9]   Stone, J., Stewart, R., and D. Otis, "Stream Control
         Transmission Protocol (SCTP) Checksum Change", RFC 3309,
         September 2002.


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


   [11]  Schulzrinne, H. and S. Casner, "RTP Profile for Audio and Video
         Conferences with Minimal Control", STD 65, RFC 3551, July 2003.


   [12]  Yon, D. and G. Camarillo, "TCP-Based Media Transport in the
         Session Description Protocol (SDP)", RFC 4145, September 2005.

   [13]  Andreasen, F., "RTP "A No-Op Payload Format",
         draft-wing-avt-rtp-noop-01 (work in progress), October 2004.

   [12]  Rosenberg, J., "Interactive Connectivity Establishment (ICE): A
         Methodology for Network  Address Translator (NAT) Traversal Format for
         Multimedia Session Establishment Protocols",
         draft-ietf-mmusic-ice-04 RTP",
         draft-wing-avt-rtp-noop-03 (work in progress), February May 2005.


   [14]  Kohler, E., "Datagram Congestion Control Protocol (DCCP)",
         draft-ietf-dccp-spec-11 (work in progress), March 2005.

Authors' Addresses

   Flemming Andreasen
   Cisco System, Inc.
   499 Thornall Street, 8th Floor
   Edison, NJ  08837


   Gonzalo  Camarillo
   Hirsalantie 11
   Jorvas  02420


   David Oran
   Cisco Systems, Inc
   7 Ladyslipper Lane
   Acton, MA  01720


   Dan Wing
   Cisco Systems, Inc.
   170 West Tasman Drive
   San Jose, CA  94301


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