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Versions: 00 01 02 03 04 05 06 07 RFC 5898

MMUSIC Working Group                                        F. Andreasen
Internet-Draft                                       Cisco Systems, Inc.
Intended status: Standards Track                            G. Camarillo
Expires: April 27, 2009                                         Ericsson
                                                                 D. Oran
                                                                 D. Wing
                                                     Cisco Systems, Inc.
                                                        October 24, 2008


   Connectivity Preconditions for Session Description Protocol Media
                                Streams
              draft-ietf-mmusic-connectivity-precon-05.txt

Status of this Memo

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   applicable patent or other IPR claims of which he or she is aware
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   This Internet-Draft will expire on April 27, 2009.

Abstract

   This document defines a new connectivity precondition for the Session
   Description Protocol (SDP) precondition framework.  A connectivity
   precondition can be used to delay session establishment or
   modification until media stream connectivity has been successfully
   verified.  The method of verification may vary depending on the type
   of transport used for the media.  For unreliable datagram transports
   such as UDP, verification involves probing the stream with data or



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   control packets.  For reliable connection-oriented transports such as
   TCP, verification can be achieved simply by successful connection
   establishment or by probing the connection with data or control
   packets, depending on the situation.


Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Terminology  . . . . . . . . . . . . . . . . . . . . . . . . .  3
   3.  Connectivity Precondition Definition . . . . . . . . . . . . .  3
     3.1.  Syntax . . . . . . . . . . . . . . . . . . . . . . . . . .  3
     3.2.  Operational Semantics  . . . . . . . . . . . . . . . . . .  4
     3.3.  Status Type  . . . . . . . . . . . . . . . . . . . . . . .  4
     3.4.  Direction Tag  . . . . . . . . . . . . . . . . . . . . . .  5
     3.5.  Precondition Strength  . . . . . . . . . . . . . . . . . .  5
   4.  Verifying Connectivity . . . . . . . . . . . . . . . . . . . .  6
     4.1.  Media Stream to Dialog Correlation . . . . . . . . . . . .  7
     4.2.  Explicit Connectivity Verification Mechanisms  . . . . . .  7
     4.3.  Verifying Connectivity for Connection-Oriented
           Transports . . . . . . . . . . . . . . . . . . . . . . . .  9
   5.  Connectivity and Other Precondition Types  . . . . . . . . . .  9
   6.  Examples . . . . . . . . . . . . . . . . . . . . . . . . . . .  9
   7.  Security Considerations  . . . . . . . . . . . . . . . . . . . 14
   8.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 15
   9.  Change Log . . . . . . . . . . . . . . . . . . . . . . . . . . 15
     9.1.  Changes since -03  . . . . . . . . . . . . . . . . . . . . 15
     9.2.  Changes since -02  . . . . . . . . . . . . . . . . . . . . 15
     9.3.  Changes since -01  . . . . . . . . . . . . . . . . . . . . 15
   10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 15
     10.1. Normative References . . . . . . . . . . . . . . . . . . . 15
     10.2. Informative References . . . . . . . . . . . . . . . . . . 16
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 17
   Intellectual Property and Copyright Statements . . . . . . . . . . 18

















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1.  Introduction

   The concept of a Session Description Protocol (SDP) [RFC4566]
   precondition in the Session Initiation Protocol (SIP) [RFC3261] is
   defined in [RFC3312] (updated by [RFC4032]).  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 a session (i.e., prior to starting alerting the callee).

   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
   [RFC0793], 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.

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


2.  Terminology

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


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 [RFC3312] as follows:








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

   This precondition tag is registered with the IANA in Section 8.

3.2.  Operational Semantics

   According to [RFC4032], documents defining new precondition types
   need to describe the behavior of UAs (User Agents) from the moment
   session establishment is suspended due to a set of preconditions,
   until it 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., until media stream
   connectivity has been established in the desired direction or
   directions).  The delay of session establishment defined here implies
   that alerting of the called party does 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.  That is, the underlying media stream
   SHOULD NOT be cut through.  For example, STUN packets
   [I-D.ietf-behave-rfc3489bis], RTP [RFC3550] No-Op
   [I-D.ietf-avt-rtp-no-op] packets and their 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.

   Some media streams are described by a single 'm' line but,
   nevertheless, involve multiple addresses.  For example, [RFC5109]
   specifies how to send FEC (Forward Error Correction) information as a
   separate stream (the address for the FEC stream is provided in an
   'a=fmtp' line).  When a 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 MAY be verified, but it is not a requirement.

3.3.  Status Type

   [RFC3312] 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.






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3.4.  Direction Tag

   The direction attributes defined in [RFC3312] are interpreted as
   follows:

   o  send: the party that generated the session description is sending
      packets on the media stream to the other party, and the other
      party has received at least one of those packets.  That is, there
      is connectivity in the forward (sending) direction.

   o  recv: the other party is sending packets on the media stream to
      the party that generated the session description, and this party
      has received at least one of those packets.  That is, there is
      connectivity in the backwards (receiving) direction.

   o  sendrecv: both the send and recv conditions hold.

   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 needs to
   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 [RFC3312].

   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



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   "precondition" can be used instead.

   Offers with connectivity preconditions in re-INVITEs or UPDATEs
   follow the rules given in Section 6 of [RFC3312].  That is:

      "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."


4.  Verifying Connectivity

   Media stream connectivity is ascertained by use of a connectivity
   verification mechanism between the media endpoints.  A connectivity
   verification mechanism may be an explicit mechanism, such as ICE
   [I-D.ietf-mmusic-ice] or ICE TCP [I-D.ietf-mmusic-ice-tcp], or it may
   be an implicit mechanism, such as TCP.  Explicit mechanisms provide
   specifications for when connectivity between two endpoints using an
   offer/answer exchange is ascertained, whereas implicit mechanisms do
   not.  The verification mechanism is negotiated as part of the normal
   offer/answer exchange, however it is not identified explicitly.  More
   than one mechanism may be negotiated, but the offerer and answerer
   need not use the same.  The following rules guide which connectivity
   verification mechanism to use:


   1.  if an explicit connectivity verification mechanism (e.g., ICE) is
       negotiated, the precondition is met when the mechanism verifies
       connectivity successfully, otherwise

   2.  if a connection-oriented transport (e.g., TCP) is negotiated, the
       precondition is met when the connection is established.

   3.  in other cases, an implicit verification mechanism may be
       provided by the transport itself or the media stream data using
       the transport (e.g., RTP No-Op)

   4.  if none of the above apply, connectivity cannot be verified
       reliably and the connectivity precondition will never be
       satisfied if requested.

   This document does not mandate any particular connectivity
   verification mechanism; however, in the following, we provide
   additional considerations for verification mechanisms.







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4.1.  Media Stream to Dialog Correlation

   SIP and SDP do not provide any inherent capabilities for associating
   an incoming media stream packet with a particular dialog.  Thus, when
   an offerer is trying to ascertain connectivity, and an incoming media
   stream packet is received, the offerer may not know which dialog had
   its "recv" connectivity verified.  Explicit connectivity verification
   mechanisms therefore typically provide a means to correlate the media
   stream, whose connectivity is being verified, with a particular SIP
   dialog.  However, some connectivity verification mechanisms may not
   provide such a correlation.  In the absence of a dialog-to-media-
   stream correlation mechanism (e.g., ICE), a UAS (User Agent Server)
   MUST NOT require the offerer to confirm a connectivity precondition.

4.2.  Explicit Connectivity Verification Mechanisms

   Explicit connectivity verification mechanisms typically use probe
   traffic with some sort of feedback to inform the sender whether
   reception was successful.  Below we provide two examples of such
   mechanisms, and how they are used with connectivity preconditions:

   Interactive Connectivity Establishment (ICE) [I-D.ietf-mmusic-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.  It also
   provides correlation with a particular SIP dialog.

   ICE implementations may be either Full or Lite (see
   [I-D.ietf-mmusic-ice]).  Full implementations generate and respond to
   STUN Binding Requests, whereas Lite implementations only respond to
   them.  With ICE, one side is a controlling agent, and the other side
   is a controlled agent.  A Full implementation can take on either
   role, whereas a Lite implementation can only be a controlled agent.
   The controlling agent decides which valid candidate to use and
   informs the controlled agent of it by identifying the pair as the
   nominated pair.  This leads to the following connectivity
   precondition rules:


   o  A Full implementation ascertains both "send" and "recv"
      connectivity when it operates as a STUN client and has sent a STUN
      Binding Request that resulted in a successful check for all the
      components of the media stream (as defined further in ICE).





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   o  A Full or a Lite implementation ascertains "recv" connectivity
      when it operates as a STUN server and has received a STUN Binding
      Request that resulted in a successful response for all the
      components of the media stream (as defined further in ICE).

   o  A Lite implementation ascertains "send" and "recv" connectivity
      when the controlling agent has informed it of the nominated pair
      for all the components of the media stream.

   A simpler and slightly more delay-prone alternative to the above
   rules is for all ICE implementations to ascertain "send" and "recv"
   connectivity for a media stream when the ICE state for that media
   stream has moved to Completed.

   Note that 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.  Also note, that when ICE
   is used to verify connectivity preconditions, the precondition is not
   satisfied until 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, which is
   imposed by ICE.  Finally, it should be noted, that although
   connectivity has been ascertained, a new offer/answer exchange may be
   required before media can flow (per ICE).

   RTP No-Op [I-D.ietf-avt-rtp-no-op] enables the sender of an RTP No-Op
   payload to 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.

   The above are merely examples of explicit connectivity verification
   mechanisms.  Other techniques can be used as well.  It is however
   RECOMMENDED that ICE be supported by entities that support
   connectivity preconditions.  Use of ICE has the benefit of working
   for all 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



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   connectivity preconditions have a common way of ascertaining
   connectivity.

4.3.  Verifying Connectivity for Connection-Oriented Transports

   Connection-oriented transport protocols generally provide an implicit
   connectivity verification mechanism.  Connection establishment
   involves sending traffic in both directions thereby verifying
   connectivity at the transport protocol level.  When a three-way (or
   more) handshake for connection establishment succeeds, bi-directional
   communication is confirmed and both the "send" and "recv"
   preconditions are satisfied whether requested or not.  In the case of
   TCP for example, once the TCP three-way handshake 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).  SCTP [RFC4960]
   connections have similar semantics as TCP and SHOULD be treated the
   same.

   When a connection-oriented transport is part of an offer, it may be
   passive, active, or active/passive [RFC4145].  When it is passive,
   the offerer expects the answerer to initiate the connection
   establishment, and when it is active, the offerer wants to initiate
   the connection establishment.  When it is active/passive, the
   answerer decides.  As noted earlier, lack of a media-stream-to-dialog
   correlation mechanism can make it difficult to guarantee with whom
   connectivity has been ascertained.  When the offerer takes on the
   passive role, the offerer will not necessarily know which SIP dialog
   originated an incoming connection request.  If the offerer instead is
   active, this problem is avoided.


5.  Connectivity and Other Precondition Types

   The role of a connectivity precondition is to ascertain media stream
   connectivity before establishing or modifying a session.  The
   underlying intent is for the two parties to be able to exchange media
   packets successfully.  Connectivity by itself however may not fully
   satisfy this.  Quality of Service for example may be required for the
   media stream; this can be addressed by use of the "qos" precondition
   defined in [RFC3312].  Similarly, succesful security parameter
   negotiation may be another prequisite; this can be addressed by use
   of the "sec" precondition defined in [RFC5027].


6.  Examples

   The first example uses the connectivity precondition with TCP in the



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   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
   required 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 [RFC4145] because
   A cannot send or receive any data before setting up a radio bearer



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   for the connection.

   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 knows the
   "sendrecv" precondition is satisfied, and B proceeds with the session
   (i.e., 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 (the required PRACK
   transaction and some SDP details have been omitted for clarity).  The
   message flow for this scenario is shown in Figure 2 below.


























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                  A                                            B

                  |                                            |
                  |-------------(1) INVITE SDP1--------------->|
                  |                                            |
                  |<------(2) 183 Session Progress SDP2--------|
                  |                                            |
                  |<~~~~~ Connectivity check to A ~~~~~~~~~~~~~|
                  |~~~~~ Connectivity to A OK ~~~~~~~~~~~~~~~~>|
                  |                                            |
                  |~~~~~ Connectivity check to B ~~~~~~~~~~~~~>|
                  |<~~~~ Connectivity to B OK ~~~~~~~~~~~~~~~~~|
                  |                                            |
                  |-------------(3) UPDATE SDP3--------------->|
                  |                                            |
                  |<--------(4) 200 OK (UPDATE) SDP4-----------|
                  |                                            |
                  |<-------------(5) 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 [RFC3312]) 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: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"



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   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:conn e2e none
     a=des:conn mandatory e2e sendrecv
     a=conf:conn e2e recv

   Meanwhile, B performs a successful send connectivity check to A by
   sending an RTP No-Op packet to A and receiving a corresponding RTCP
   report.  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 successful
   send connectivity check to B by sending an RTP No-Op packet to B and
   receiving a corresponding RTCP report, and A's local status table
   becomes:










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       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 192.0.2.1
     a=curr:conn e2e send
     a=des:conn mandatory e2e sendrecv

   B has both send and recv connectivity confirmed at this point and the
   session can continue.


7.  Security Considerations

   In addition to the general security considerations for preconditions
   provided in [RFC3312], the following security issues, which are
   specific to connectivity preconditions, should be considered.

   Connectivity preconditions rely on mechanisms beyond SDP such as
   TCP[RFC0793] connection establishment, RTP No-Op
   [I-D.ietf-avt-rtp-no-op], or STUN [I-D.ietf-behave-rfc3489bis] 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 [RFC3853] provides such end-
   to-end integrity protection, as described in [RFC3261].







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8.  IANA Considerations

   IANA is hereby requested to register a new precondition type under
   the Precondition Types used with SIP subregistry, which is located
   under the Session Initiation Protocol (SIP) Parameters registry.


   Precondition-Type  Description                          Reference
   -----------------  -----------------------------------  ---------
   conn               Connectivity precondition            [RFCxxxx]

   [Note to the RFC Editor: replace RFCxxxx with the number assigned to
   this RFC.]


9.  Change Log

9.1.  Changes since -03

   Minor fixes here and there.

9.2.  Changes since -02

   Connectivity preconditions are now mechanism agnostic.  Clarified
   when and how to use ICE, RTP No-Op, and connection establishment
   procedures to check connectivity.  Clarified relation with other
   precondition types.

9.3.  Changes since -01

   There are no changes since the previous version of the document.


10.  References

10.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC5109]  Li, A., "RTP Payload Format for Generic Forward Error
              Correction", RFC 5109, December 2007.

   [RFC3261]  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.




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

   [RFC3550]  Schulzrinne, H., Casner, S., Frederick, R., and V.
              Jacobson, "RTP: A Transport Protocol for Real-Time
              Applications", STD 64, RFC 3550, July 2003.

   [RFC3853]  Peterson, J., "S/MIME Advanced Encryption Standard (AES)
              Requirement for the Session Initiation Protocol (SIP)",
              RFC 3853, July 2004.

   [RFC4032]  Camarillo, G. and P. Kyzivat, "Update to the Session
              Initiation Protocol (SIP) Preconditions Framework",
              RFC 4032, March 2005.

   [RFC4566]  Handley, M., Jacobson, V., and C. Perkins, "SDP: Session
              Description Protocol", RFC 4566, July 2006.

10.2.  Informative References

   [RFC0793]  Postel, J., "Transmission Control Protocol", STD 7,
              RFC 793, September 1981.

   [RFC4145]  Yon, D. and G. Camarillo, "TCP-Based Media Transport in
              the Session Description Protocol (SDP)", RFC 4145,
              September 2005.

   [RFC4960]  Stewart, R., "Stream Control Transmission Protocol",
              RFC 4960, September 2007.

   [RFC5027]  Andreasen, F. and D. Wing, "Security Preconditions for
              Session Description Protocol (SDP) Media Streams",
              RFC 5027, October 2007.

   [I-D.ietf-avt-rtp-no-op]
              Andreasen, F., "A No-Op Payload Format for RTP",
              draft-ietf-avt-rtp-no-op-04 (work in progress), May 2007.

   [I-D.ietf-mmusic-ice]
              Rosenberg, J., "Interactive Connectivity Establishment
              (ICE): A Protocol for Network Address  Translator (NAT)
              Traversal for Offer/Answer Protocols",
              draft-ietf-mmusic-ice-19 (work in progress), October 2007.

   [I-D.ietf-mmusic-ice-tcp]
              Rosenberg, J., "TCP Candidates with Interactive
              Connectivity Establishment (ICE)",



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              draft-ietf-mmusic-ice-tcp-07 (work in progress),
              July 2008.

   [I-D.ietf-behave-rfc3489bis]
              Rosenberg, J., Mahy, R., Matthews, P., and D. Wing,
              "Session Traversal Utilities for (NAT) (STUN)",
              draft-ietf-behave-rfc3489bis-18 (work in progress),
              July 2008.


Authors' Addresses

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

   Email: fandreas@cisco.com


   Gonzalo  Camarillo
   Ericsson
   Hirsalantie 11
   Jorvas  02420
   Finland

   Email: Gonzalo.Camarillo@ericsson.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  94301
   USA

   Email: dwing@cisco.com





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