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Versions: (draft-rosenberg-mmusic-ice-tcp) 00 01 02 03 04 05 06 07 08 09 10 11 12 13 14 15 16 RFC 6544

MMUSIC                                                      J. Rosenberg
Internet-Draft                                                     Cisco
Intended status: Standards Track                           March 5, 2007
Expires: September 6, 2007


    TCP Candidates with Interactive Connectivity Establishment (ICE
                      draft-ietf-mmusic-ice-tcp-03

Status of this Memo

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   This Internet-Draft will expire on September 6, 2007.

Copyright Notice

   Copyright (C) The IETF Trust (2007).

Abstract

   Interactive Connectivity Establishment (ICE) defines a mechanism for
   NAT traversal for multimedia communication protocols based on the
   offer/answer model of session negotiation.  ICE works by providing a
   set of candidate transport addresses for each media stream, which are
   then validated with peer-to-peer connectivity checks based on Simple
   Traversal of UDP over NAT (STUN).  ICE provides a general framework
   for describing alternates, but only defines UDP-based transport
   protocols.  This specification extends ICE to TCP-based media,



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   including the ability to offer a mix of TCP and UDP-based candidates
   for a single stream.


Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Overview of Operation  . . . . . . . . . . . . . . . . . . . .  4
   3.  Sending the Initial Offer  . . . . . . . . . . . . . . . . . .  5
     3.1.  Gathering Candidates . . . . . . . . . . . . . . . . . . .  5
     3.2.  Prioritization . . . . . . . . . . . . . . . . . . . . . .  7
     3.3.  Choosing Default Candidates  . . . . . . . . . . . . . . .  8
     3.4.  Encoding the SDP . . . . . . . . . . . . . . . . . . . . .  8
   4.  Receiving the Initial Offer  . . . . . . . . . . . . . . . . .  9
     4.1.  Forming the Check Lists  . . . . . . . . . . . . . . . . .  9
   5.  Connectivity Checks  . . . . . . . . . . . . . . . . . . . . .  9
     5.1.  Client Procedures  . . . . . . . . . . . . . . . . . . . .  9
       5.1.1.  Sending the Request  . . . . . . . . . . . . . . . . .  9
     5.2.  Server Procedures  . . . . . . . . . . . . . . . . . . . . 10
   6.  Concluding ICE Processing  . . . . . . . . . . . . . . . . . . 10
   7.  Subsequent Offer/Answer Exchanges  . . . . . . . . . . . . . . 11
     7.1.  ICE Restarts . . . . . . . . . . . . . . . . . . . . . . . 11
   8.  Media Handling . . . . . . . . . . . . . . . . . . . . . . . . 11
     8.1.  Sending Media  . . . . . . . . . . . . . . . . . . . . . . 11
     8.2.  Receiving Media  . . . . . . . . . . . . . . . . . . . . . 11
   9.  Connection Management  . . . . . . . . . . . . . . . . . . . . 12
   10. Security Considerations  . . . . . . . . . . . . . . . . . . . 13
   11. IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 13
   12. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 13
   13. References . . . . . . . . . . . . . . . . . . . . . . . . . . 13
     13.1. Normative References . . . . . . . . . . . . . . . . . . . 13
     13.2. Informative References . . . . . . . . . . . . . . . . . . 14
   Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 14
   Intellectual Property and Copyright Statements . . . . . . . . . . 15

















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1.  Introduction

   Interactive Connectivity Establishment (ICE) [6] defines a mechanism
   for NAT traversal for multimedia communication protocols based on the
   offer/answer model [2] of session negotiation.  ICE works by
   providing a set of candidate transport addresses for each media
   stream, which are then validated with peer-to-peer connectivity
   checks based on Session Traversal Utilities for NAT (STUN) [1].
   However, ICE only defines procedures for UDP-based transport
   protocols.

   There are many reasons why ICE support for TCP is important.
   Firstly, there are media protocols that only run over TCP.  Examples
   of such protocols are web and application sharing and instant
   messaging [9].  For these protocols to work in the presence of NAT,
   unless they define their own NAT traversal mechanisms, ICE support
   for TCP is needed.  In addition, RTP itself can run over TCP (without
   [4] and with TLS [5]).  Typically, it is preferable to run RTP over
   UDP, and not TCP.  However, in a variety of network environments,
   overly restrictive NAT and firewall devices prevent UDP-based
   communications altogether, but general TCP-based communications are
   permitted.  In such environments, sending RTP over TCP, and thus
   establishing the media session, may be preferable to having it fail
   altogether.  With ICE, agents can gather UDP and TCP candidates for
   an RTP-based stream, list the UDP ones with higher priority, and then
   only use the TCP-based ones if the UDP ones fail altogether.  This
   provides a fallback mechanism that allows multimedia communications
   to be highly reliable.

   The usage of RTP over TCP is particularly useful when combined with
   the STUN relay usage [7].  In that usage, one of the agents would
   connect to its STUN relay server using TCP, and obtain a TCP-based
   relayed candidate.  It would offer this to its peer agent as a
   candidate.  The answerer would initiate a TCP connection towards the
   STUN relay server.  When that connection is established, media can
   flow over the connections, through the relay.  The benefit of this
   usage is that it only requires the agents to make outbound TCP
   connections to a server on the public network.  This kind of
   operation is broadly interoperable through NAT and firewall devices.
   Since it is a goal of ICE and this extension to provide highly
   reliable communications that "just works" in as a broad a set of
   network deployments as possible, this usage is particularly
   important.

   The usage of RTP over TCP/TLS is also useful when communicating
   between single-user agents (such as a softphone or hardphone) and an
   agent run by a provider that is meant to service many users, such as
   a PSTN gateway.  In such a deployment, the multi-user agent would act



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   as the TLS server, and have a certificate.  The single-user agent can
   then connect, validate the certificate, but offer none of its own
   (since its not likely to have one).  STUN itself would then provide
   authentication of the softphone to the gateway, leveraging the
   exchange of a short term credential in the SIP signaling.

   This specification extends ICE by defining its usage with TCP
   candidates.  This specification does so by following the outline of
   ICE itself, and calling out the additions and changes necessary in
   each section of ICE to support TCP candidates.


2.  Overview of Operation

   The usage of ICE with TCP is relatively straightforward.  The main
   area of specification is around how and when connections are opened,
   and how those connections relate to candidate pairs.

   When the agents perform address allocations to gather TCP-based
   candidates, three types of candidates can be obtained.  These are
   active candidates, passive candidates, and simultaneous-open
   candidates.  An active candidate is one for which the agent will
   attempt to open an outbound connection, but will not receive incoming
   connection requests.  A passive candidate is one for which the agent
   will receive incoming connection attempts, but not attempt a
   connection.  A simultaneous-open candidate is one for which the agent
   will attempt to open a connection simultaneously with its peer.

   Because this specification requires multiple candidates for a media
   stream, it is not compatible with ICE's lite implementation, and can
   only be used by full implementations.

   When gathering candidates from a host interface, the agent typically
   obtains an active, passive and simultaneous-open candidates.
   Similarly, communications with a STUN server will provide server
   reflexive and relayed versions of all three types.

   When encoding these candidates into offers and answers, the type of
   the candidate is signaled.  In the case of active candidates, an IP
   address and port is present, but it is meaningless, as it is ignored
   by the peer.  As a consequence, active candidates do not need to be
   physically allocated at the time of address gathering.  Rather, the
   physical allocations, which occur as a consequence of a connection
   attempt, occur at the time of the connectivity checks.

   When the candidates are paired together, active candidates are always
   paired with passive, and simultaneous-open candidates with each
   other.  When a connectivity check is to be made on a candidate pair,



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   each agent determines whether it is to make a connection attempt for
   this pair.

      Why have both active and simultaneous-open candidates?  Why not
      just simultaneous-open?  The reason is that NAT treatment of
      simultaneous opens is currently not well defined, though
      specifications are being developed to address this [8].  Some NATs
      block the second TCP SYN packet or improperly process the
      subsequent SYNACK, which will cause the connection attempt to
      fail.  Therefore, if only simultaneous opens are used, connections
      may often fail.  Alternatively, using unidirectional opens (where
      one side is active and the other is passive) is more reliable, but
      will always require a relay if both sides are behind NAT.
      Therefore, in the spirit of the ICE philosophy, both are tried.
      Simultaneous-opens are preferred since, if it does work, it will
      not require a relay even when both sides are behind a different
      NAT.

   The actual processing of generating connectivity checks, managing the
   state of the check list, and updating the Valid list, work
   identically for TCP as they do for UDP.

   ICE requires an agent to demultiplex STUN and application layer
   traffic, since they appear on the same port.  This demultiplexing is
   described by ICE, and is done using the magic cookie and other fields
   of the message.  Stream-oriented transports introduce another
   wrinkle, since they require a way to frame the connection so that the
   application and STUN packets can be extracted in order to determine
   which is which.  For this reason, TCP media streams utilizing ICE use
   the basic framing provided in RFC 4571 [4], even if the application
   layer protocol is not RTP.

   When an updated offer is generated by the controlling endpoint, the
   SDP extensions for connection oriented media [3] are used to signal
   that an existing connection should be used, rather than opening a new
   one.


3.  Sending the Initial Offer

   The offerer MUST be a full ICE implementation.

3.1.  Gathering Candidates

   For each TCP capable media stream the agent wishes to use (including
   ones, like RTP, which can either be UDP or TCP), the agent SHOULD
   obtain two host candidates for each component of the media stream on
   each interface that the host has - one for the simultaneous open, and



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   one for the passive candidate.  If an agent is not capable of acting
   in one of these modes (for example, the TCP connection is being used
   with TLS and the agent can only act as the client), it would omit
   those candidates.

      OPEN ISSUE: What happens with TLS and simultaneous opens?  Who
      sends the ClientHello?

   >

   Providers of real-time communications services may decide that it is
   preferable to have no media at all than it is to have media over TCP.
   To allow for choice, it is RECOMMENDED that agents be configurable
   with whether they obtain TCP candidates for real time media.

      Having it be configurable, and then configuring it to be off, is
      far better than not having the capability at all.  An important
      goal of this specification is to provide a single mechanism that
      can be used across all types of endpoints.  As such, it is
      preferable to account for provider and network variation through
      configuration, instead of hard-coded limitations in an
      implementation.  Furthermore, network characteristics and
      connectivity assumptions can, and will change over time.  Just
      because a agent is communicating with a server on the public
      network today, doesn't mean that it won't need to communicate with
      one behind a NAT tomorrow.  Just because a agent is behind a NAT
      with endpoint indpendent mapping today, doesn't mean that tomorrow
      they won't pick up their agent and take it to a public network
      access point where there is a NAT with address and port dependent
      mapping properties, or one that only allows outbound TCP.  The way
      to handle these cases and build a reliable system is for agents to
      implement a diverse set of techniques for allocating addresses, so
      that at least one of them is almost certainly going to work in any
      situation.  Implementors should consider very carefully any
      assumptions that they make about deployments before electing not
      to implement one of the mechanisms for address allocation.  In
      particular, implementors should consider whether the elements in
      the system may be mobile, and connect through different networks
      with different connectivity.  They should also consider whether
      endpoints which are under their control, in terms of location and
      network connectivity, would always be under their control.  In
      environments where mobility and user control are possible, a
      multiplicity of techniques is essential for reliability.

   Each agent SHOULD "obtain" an active host candidate for each
   component of each TCP capable media stream on each interface that the
   host has.  The agent does not have to actually allocate a port for
   these candidates.  These candidates serve as a placeholder for the



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   creation of the check lists.

   Using each simultaneous-open and passive host TCP candidate as a
   base, the agent SHOULD obtain server reflexive candidate.  In
   addition, the agent SHOULD choose, amongst all host TCP candidates
   for a component that have the same foundation (there will typically
   be two - a passive and simultaneous-open), one of those candidates,
   and from it, obtain two server reflexive candidates - one that will
   be simultaneous-open, and one that will be passive.  If an agent
   requires both a server reflexive and relayed candidate using a
   particular host candidate as a based, it SHOULD obtain both at the
   same time using a STUN Allocate request.  Otherwise, if just a server
   reflexive candidate is required, the agent SHOULD use a STUN Binding
   Request.

   An agent MAY use an additional host TCP candidate to request a UDP-
   based candidate from the server.  Usage of the UDP candidate from the
   relay follows the procedures defined in ICE for UDP candidates.

   Each agent SHOULD "obtain" an active relayed candidate for each
   component of each TCP capable media stream on each interface that the
   host has.  The agent does not have to actually allocate a port for
   these candidates from the relay at this time.  These candidates serve
   as a placeholder for the creation of the check lists.

   Like its UDP counterparts, TCP-based STUN transactions are paced out
   at one every Ta seconds.  This pacing refers to the establishment of
   a TCP connection to the STUN server and the subsequent STUN request.
   That is, every Ta seconds, the agent will open a new TCP connection
   and send a STUN request, either an Allocate or Binding request.

3.2.  Prioritization

   The transport protocol itself is a criteria for choosing one
   candidate over another.  If a particular media stream can run over
   UDP or TCP, the UDP candidates might be preferred over the TCP
   candidates.  This allows ICE to use the lower latency UDP
   connectivity if it exists, but fallback to TCP if UDP doesn't work.

   To accomplish this, the local preference SHOULD be defined as:


   local-preference = (2^12)*(transport-pref) +
                      (2^9)*(direction-pref) +
                      (2^0)*(other-pref)

   When this formulation is used, the transport-pref MUST be between 0
   and 15, with 15 being the most preferred.  The direction-pref MUST be



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   between 0 and 7, with 7 being the most preferred.  Other-pref MUST be
   between 0 and 511, with 511 being the most preferred.  For RTP-based
   media streams, it is RECOMMENDED that UDP have a transport-pref of 15
   and TCP of 6.  It is RECOMMENDED that, for all connection-oriented
   media, simultaneous-open candidates have a direction-pref of 7,
   active of 5 and passive of 2.  If any two candidates have the same
   type-preference, transport-pref, and direction-pref, they MUST have a
   unique other-pref.  With this specification, the only way that can
   happen is with multi-homed hosts, in which case other-pref is a
   preference amongst interfaces.

3.3.  Choosing Default Candidates

   The default candidate is chosen primarily based on the likelihood of
   it working with a non-ICE peer.  When media streams supporting mixed
   modes (both TCP and UDP) are used with ICE, it is RECOMMENDED that,
   for real-time streams (such as RTP), the default candidates be UDP-
   based.  However, the default SHOULD NOT be the simultaneous-open
   candidate.

   If a media stream is inherently TCP-based, the agent SHOULD NOT
   select the simultaneous-open candidate as default.

3.4.  Encoding the SDP

   TCP-based candidates are encoded into a=candidate lines identically
   to the UDP encoding described in [6].  However, the transport
   protocol is set to "tcp-so" for TCP simultaneous-open candidates,
   "tcp-act" for TCP active candidates, and "tcp-pass" for TCP passive
   candidates.  The addr and port encoded into the candidate attribute
   for active candidates MUST be set to IP address that will be used for
   the attempt, but the port MUST be set to 9 (i.e., Discard).  For
   relayed candidates, the IP address that will be used for the attempt
   is the one from a passive or simultaneous-open candidate from the
   same STUN server.

   If the default candidate is TCP, the agent MUST include any SDP
   parameters required for establishing that TCP connection for that
   media stream, in case the peer is not ICE aware.  For example, if a
   TCP-based media stream utilizes RFC 4145 [3], the agent MUST follow
   the procedures defined there for constructing an offer, as if ICE was
   not in use.  For example, if an agent selects its passive candidate
   as default and the media stream utilizes RFC 4145, the agent MUST
   include an a=passive attribute.  Note that these parameters are not
   used by ICE, they are only relevant for non-ICE entities.

   In addition, if a TCP-based candidate is offered, and the default
   candidate is UDP based, the SDP MUST include any parameters that



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   would be required for the TCP stream to be utilized once set up,
   should it be selected by ICE.  This excludes the connection
   parameters from RFC 4145, which are not utilized between ICE peers.
   However, if a TCP candidate was meant to be used for TLS, and the
   default candidate was UDP-based (and of course if it was TCP-based),
   the parameters of RFC 4572 [5] would need to be included in the SDP.
   This signals that the TCP candidate is to be used with TLS.


4.  Receiving the Initial Offer

4.1.  Forming the Check Lists

   When forming candidate pairs, the following types of candidates can
   be paired with each other:



   Local             Remote
   Candidate         Candidate
   ----------------------------
   tcp-so           tcp-so
   tcp-act          tcp-pass
   tcp-pass         tcp-act

   When the agent prunes the check list, it MUST also remove any pair
   for which the local candidate is tcp-pass.

   The remainder of check list processing works like the UDP case.


5.  Connectivity Checks

5.1.  Client Procedures

5.1.1.  Sending the Request

   When an agent wants to send a TCP-based connectivity check, it first
   opens a TCP connection if none yet exists for the 5-tuple on which
   the check is to be sent.  This connection is opened from the local
   candidate of the check to the remote candidate of the check.  If the
   local candidate is tcp-act, the agent MUST open a connection from the
   interface associated with that local candidate.  This connection MUST
   be opened from an unallocated port.  For host candidates, this is
   readily done by connecting from the candidates interface.  For
   relayed candidates, the agent uses the procedures in [7] to initiate
   a new connection from the specified interface on the relay.  [[TODO:
   need to make sure this reconciles with latest TURN]].



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   If the offer/answer exchange, one completed, indicates that the TCP
   candidates for a media stream will utilize TLS (for example, as a
   consequence of the presence of the fingerprint attribute from RFC
   4572), the agent that opened the connection MUST proceed with TLS
   handshakes to secure the link prior to proceeding with STUN checks.

   Once the TCP or TCP/TLS connection is established, connectivity
   checks are sent over the connection.  The agent MUST use the framing
   defined in RFC 4571 [4], even though the data will include both media
   (possibly RTP) and STUN packets.  This framing MUST be used for the
   lifetime of this connection.

   If the TCP connection cannot be established, or the TLS handshakes
   fail, the check is considered to have failed, and a full-mode agent
   MUST update the pair state to Failed in the check list.

5.2.  Server Procedures

   An agent MUST be prepared to receive incoming TCP connection requests
   on any host or relayed TCP candidate that is simultaneous-open or
   passive.  When the connection request is received, the agent MUST
   accept it.  If the offer/answer exchange indicates that TLS is in
   use, the agent MUST be prepared for TLS negotiation, and complete
   that exchange prior to receiving STUN requests.

   The agent MUST use the framing defined in RFC 4571 [4], even though
   the data will include both media (possibly RTP) and STUN packets.
   This framing MUST be used for the lifetime of this connection.

   Once the connection is established, server procedures are identical
   to those for UDP candidates.  Note that STUN requests received on a
   passive TCP or TCP/TLS candidate will typically produce a remote peer
   reflexive candidate.


6.  Concluding ICE Processing

   If there are TCP candidates for a media stream, a controlling agent
   MUST use a regular selection algorithm.

   When ICE processing for a media stream completes, each agent SHOULD
   close all TCP connections except the one between the candidate pairs
   selected by ICE.

      These two rules are related; the closure of connection on
      completion of ICE implies that a regular selection algorithm has
      to be used.  This is because aggressive selection might cause
      transient pairs to be selected.  Once such a pair was selected,



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      the agents would close the other connections, one of which may be
      about to be selected as a better choice.  This race condition may
      result in TCP connections being accidentally closed for the pair
      that ICE selects.


7.  Subsequent Offer/Answer Exchanges

7.1.  ICE Restarts

   If an ICE restart occurs for a media stream with TCP candidate pairs
   that have been selected by ICE, the agents MUST NOT close the
   connections after the restart.  In the offer or answer that causes
   the restart, an agent MAY include a simultaneous-open candidate whose
   transport address matches the previously selected candidate.  If both
   agents do this, the result will be a simultaneous-open candidate pair
   matching an existing TCP connection.  In this case, the agents MUST
   NOT attempt to open a new connection (or start new TLS procedures).
   Instead, that existing connection is reused and STUN checks are
   performed.

   Once the restart completes, if the selected pair does not match the
   previously selected pair, the TCP connection for the previously
   selected pair SHOULD be closed by the agent.


8.  Media Handling

8.1.  Sending Media

   When sending media, if the selected candidate pair matches an
   existing TCP connection, that connection MUST be used for sending
   media.

   The framing defined in RFC 4571 MUST be used when sending media.  For
   media streams that are not RTP-based and do not normally use RFC
   4571, the agent treats the media stream as a byte stream, and assumes
   that it has its own framing of some sort.  It then takes an arbitrary
   number of bytes from the bytestream, and places that as a payload in
   the RFC 4571 frames, including the length.  The recipient can extract
   the bytestream and apply the application-specific framing on it.

8.2.  Receiving Media

   The framing defined in RFC 4571 MUST be used when receiving media.
   For media streams that are not RTP-based and do not normally use RFC
   4571, the agent extracts the payload of each RFC 4571 frame, and
   determines if it is a STUN or an application layer data based on the



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   procedures in [6].  If it is application layer data, the agent
   appends this to the ongoing bytestream collected from the frames.  It
   then parses the bytestream as if it had been directly received over
   the TCP or TCP/TLS connection.  This allows for ICE-tcp to work
   without regard to the framing mechanism used by the application layer
   protocol.


9.  Connection Management

   Once a TCP or TCP/TLS connection is opened by ICE, its lifecycle
   depends on how it is used.  If that candidate pair is selected by ICE
   for usage for media, an agent SHOULD keep the connection open until:

   o  The session terminates

   o  The media stream is removed

   o  An ICE restart takes place, resulting in the selection of a
      different candidate pair.

   In these cases, the agent SHOULD close the connection when that event
   occurs.

   If a connection has been selected by ICE, an agent MAY close it
   anyway.  As described in the next paragraph, this will cause it to be
   reopened almost immediately, and in the interim media cannot be sent.
   Consequently, such closures have a negative effect and are NOT
   RECOMMENDED.  However, there may be cases where an agent needs to
   close a connection for some reason.

   If an agent needs to send media on the selected candidate pair, and
   its TCP connection has closed, either on purpose or due to some
   error, then:

   o  If the agent's local candidate is tcp-act or tcp-so, it MUST
      reopen a connection to the remote candidate of the selected pair.

   o  If the agent's local candidate is tcp-pass, the agent MUST await
      an incoming connection request, and consequently, will not be able
      to send media until it has been opened.

   If the TCP connection is established, and the SDP indicates that TLS
   is in use, the agents MUST redo the TLS handshakes.  Once complete,
   the connection MAY be used for media; re-validation using STUN is not
   required.





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      OPEN ISSUE: Can we use session resumption to avoid redoing this?

   If the TCP connection cannot be established, the controlling agent
   SHOULD restart ICE for this media stream.


10.  Security Considerations

   The main threat in ICE is hijacking of connections for the purposes
   of directing media streams to DoS targets or to malicious users.
   ICE-tcp prevents that by only using TCP connections that have been
   validated.  Validation requires a STUN transaction to take place over
   the connection.  This transaction cannot complete without both
   participants knowing a shared secret exchanged in the rendezvous
   protocol used with ICE, such as SIP.  This shared secret, in turn, is
   protected by that protocol exchange.  In the case of SIP, the usage
   of the sips mechanism is RECOMMENDED.  When this is done, an
   attacker, even if it knows or can guess the port on which an agent is
   listening for incoming TCP connections, will not be able to open a
   connection and send media to the agent.

   A more detailed analysis of this attack and the various ways ICE
   prevents it are described in [6].  Those considerations apply to this
   specification.


11.  IANA Considerations

   There are no IANA considerations associated with this specification.


12.  Acknowledgements

   The authors would like to thank Tim Moore, Francois Audet and Roni
   Even for the reviews and input on this document.


13.  References

13.1.  Normative References

   [1]  Rosenberg, J., "Simple Traversal Underneath Network Address
        Translators (NAT) (STUN)", draft-ietf-behave-rfc3489bis-05 (work
        in progress), October 2006.

   [2]  Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model with
        Session Description Protocol (SDP)", RFC 3264, June 2002.




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

   [4]  Lazzaro, J., "Framing Real-time Transport Protocol (RTP) and RTP
        Control Protocol (RTCP) Packets over Connection-Oriented
        Transport", RFC 4571, July 2006.

   [5]  Lennox, J., "Connection-Oriented Media Transport over the
        Transport Layer Security (TLS) Protocol in the Session
        Description Protocol (SDP)", RFC 4572, July 2006.

   [6]  Rosenberg, J., "Interactive Connectivity Establishment (ICE): A
        Methodology for Network  Address Translator (NAT) Traversal for
        Offer/Answer Protocols", draft-ietf-mmusic-ice-13 (work in
        progress), January 2007.

   [7]  Rosenberg, J., "Obtaining Relay Addresses from Simple Traversal
        Underneath NAT (STUN)", draft-ietf-behave-turn-02 (work in
        progress), October 2006.

13.2.  Informative References

   [8]  Guha, S., "NAT Behavioral Requirements for TCP",
        draft-ietf-behave-tcp-05 (work in progress), February 2007.

   [9]  Campbell, B., "The Message Session Relay Protocol",
        draft-ietf-simple-message-sessions-19 (work in progress),
        February 2007.


Author's Address

   Jonathan Rosenberg
   Cisco
   Edison, NJ
   US

   Email: jdrosen@cisco.com
   URI:   http://www.jdrosen.net












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