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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 Systems
Expires: August 31, 2006                               February 27, 2006


    TCP Candidates with Interactive Connectivity Establishment (ICE
                      draft-ietf-mmusic-ice-tcp-00

Status of this Memo

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   This Internet-Draft will expire on August 31, 2006.

Copyright Notice

   Copyright (C) The Internet Society (2006).

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,
   including the ability to offer a mix of TCP and UDP-based candidates



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   for a single stream.

Table of Contents

   1.   Introduction . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.   Overview of Operation  . . . . . . . . . . . . . . . . . . .   4
   3.   Gathering Addresses  . . . . . . . . . . . . . . . . . . . .   5
   4.   Prioritization . . . . . . . . . . . . . . . . . . . . . . .   8
   5.   Encoding . . . . . . . . . . . . . . . . . . . . . . . . . .   8
   6.   Ordering the Candidate Pairs . . . . . . . . . . . . . . . .   9
   7.   Performing the Connectivity Checks . . . . . . . . . . . . .  10
   8.   Promoting a Candidate to Active  . . . . . . . . . . . . . .  14
   9.   Learning New Candidates from Connectivity Checks . . . . . .  14
   10.  Subsequent Offers  . . . . . . . . . . . . . . . . . . . . .  14
   11.  Binding Keepalives . . . . . . . . . . . . . . . . . . . . .  15
   12.  Sending Media  . . . . . . . . . . . . . . . . . . . . . . .  16
   13.  Security Considerations  . . . . . . . . . . . . . . . . . .  16
   14.  IANA Considerations  . . . . . . . . . . . . . . . . . . . .  17
   15.  References . . . . . . . . . . . . . . . . . . . . . . . . .  17
     15.1   Normative References . . . . . . . . . . . . . . . . . .  17
     15.2   Informative References . . . . . . . . . . . . . . . . .  17
        Author's Address . . . . . . . . . . . . . . . . . . . . . .  18
        Intellectual Property and Copyright Statements . . . . . . .  19




























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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 Simple Traversal of UDP over NAT (STUN) [1].  ICE
   provides a general framework for describing alternates, but 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 run over TCP.  Examples of
   such protocols are web and application sharing and instant messaging
   [8].  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 [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
   both 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 communicatoins 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
   allocated address.  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.

   This specification extends ICE by defining its usage with TCP-based
   candidates.  ICE indicates in each of its sections where there is
   transport-specific logic.  It requests that specifications which
   define usage of ICE with other transport protocols - as this one does
   - define a version of that logic.  This specification does so by



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   following the outline of ICE itself, and calling out the transport
   protocol specific logic needed in each section.

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 transport address pairs and
   candidates.

   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 actpass 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.
   An actpass candidate is one for which the agent will do both.

   Not all types of candidates can be obtained from all types of
   transport addresses.  With local interfaces, agents obtain both
   actpass and active candidates.  Agents don't bother with passive
   ones, since that functionality is subsumed by the acpass candidate.
   Server reflexive candidates, by their nature, are always passive.
   Relayed transport addresses, like local candidates, can produce both
   actpass and active candidates.

   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 agent.  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 not
   paired with active, and passive are not paired with passive.  When a
   connectivity check is to be made for a transport address pair within
   a candidate pair, each agent determines whether it is to make a
   connection attempt for this pair.  If the local candidate is either
   active or actpass, and the remote is either passive or actpass, it
   will make the attempt.  This means that, for candidate pairs where
   both candidates are actpass, both agents will attempt to open a TCP
   connection (this is the so-called simultaneous open in TCP).  In the
   other cases, only one side will try.

   Why have both active and actpass candidates for local and relayed
   transport addresses?  Why not just actpass?  The reason is that NAT



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   treatment of simultaneous opens is currently not well defined, though
   specifications are being developed to address this.  Some NATs
   generate a reset upon receipt of the second TCP SYN packet, which
   will cause the connection attempt to fail.  Therefore, if only
   simultaneous opens are used, connections may often fail.  However,
   only doing 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.  Actpass to actpass are preferred since,
   if it does work, it will not require a relay even when both sides are
   behind the same NAT.

   Once a connection attempt succeeds, the agent which initiated the
   connection sends a STUN Binding Request over the connection, and the
   other agent generates a response.  For simultaneous opens, it is
   possible that both sides will send a Binding Request.  The binding
   request will serve the purpose of correlating the connection to a
   candidate pair.  For candidate pairs where one side was active, the
   STUN Binding Request will always generate a peer derived candidate
   and corresponding candidate pair, which is placed immediately in the
   Valid state, avoiding the need for additional connectivity checks and
   computations of new usernames.  This derived candidate that is then
   associated with the TCP connection.  For all other candidate pairs,
   peer derived candidates are not computed (even when the transport
   address is a new one), and the candidate pair identified by the STUN
   Binding Request is directly linked to the connection.  It is actually
   possible that a single connection can be associated with multiple
   candidate pairs; this happens in several situations, and in
   particular, with connection attempts made to passive candidates.
   However, a single candidate pair is only ever associated with a
   single TCP connection.

   When a TCP-based candidate is promoted to the m/c-line, 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.
   The candidate (or the one which generated it, in the case of a peer-
   derived candidate) remains listed in a candidate attribute so that
   STUN-based keepalives can be used throughout the session.  This
   requires demultiplexing STUN and application traffic on the same TCP
   connection.

3.  Gathering Addresses

   Section 7.1 of ICE defines the procedures for gathering of transport
   addresses for usage in candidates.  These procedures are defined for
   local candidates, server reflexive candidates and relayed candidates.
   ICE indicates that these procedures are transport protocol specific,
   and requires extensions to ICE to define procedures for other



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   transport protocols.  This section defines those procedures for TCP.

   For each TCP-only media stream the agent wishes to use, the agent
   obtains a set of actpass candidates by binding to N ephemeral TCP
   ports on each local interface, where N is the number of transport
   addresses needed for the candidate.  For media streams that can
   support either UDP or TCP, the agent SHOULD obtain a set of actpass
   candidates by binding to N ephemeral UDP and N ephemeral TCP ports on
   each interface, where N is the number of transport addresses needed
   for the candidate.

   It is not necessary to actually allocate active TCP candidates.
   These candidates will be signaled in the offer or answer, but they do
   not include any address and port information - just the STUN
   usernames and priorities.

   Media streams carried using the Real Time Transport Protocol (RTP)
   [4] can run over TCP [5].  As such, it is RECOMMENDED that both UDP
   and TCP candidates be obtained.  However, 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 full
      cone NAT 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 symmetric NAT 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



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      of location and network connectivity, would always be under their
      control.  Only in cases where there isn't now, and never will be,
      endpoint mobility or nomadicity of any sort, should a technique be
      omitted.

   Server reflexive candidates are always passive only.  They are
   derived from the STUN Binding Discovery usage or the STUN Relay
   usage.  The latter is preferred since it will provide the client with
   both a server reflexive and a relayed transport address with a single
   transaction.  It is possible that some STUN servers will only support
   the Relay usage or only the Binding Discovery usage, in which case a
   client might be configured with different servers depending on the
   usage.  It is RECOMMENDED that agents obtain server reflexive TCP
   candidates.  In many cases, the agent will not be able to receive
   incoming TCP connections on a reflexive server address.  However,
   advertising such a transport address through ICE will allow the peer
   agent to perform a connection attempt through a STUN relay server to
   that transport address, thereby creating a permission for that IP
   address on the relay server.  This is essential for allowing two
   clients behind restrictive NATs to rendezvous through the relay.

   Relayed candidates can be both actpass and active.  As with local
   candidates, these candidates do not actually need to be allocated at
   the time of address gathering.  Instead, when the agent needs to open
   a connection from the active relayed candidate, it uses a STUN
   Allocate request to obtain another allocation on the same interface
   as its actpass relayed candidate, and then uses the STUN Connect
   method to open the connection.  This is discussed further below.

   Obtaining server reflexive passive candidates and relayed actpass
   candidates for TCP is nearly identical to the UDP case.  Like UDP, it
   can be accomplished with just the relay usage, or with the binding
   discovery usage and the relay usage separately.  The only difference
   between TCP and UDP is that the client sends its requests to the STUN
   server by first establishing a TCP connection to the server, and then
   sending the STUN request over that connection.  In addition, the
   client will request a TCP-based allocation for the relayed address,
   not a UDP allocation.

   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 server and the subsequent STUN request.  That
   is, every Ta seconds, the agent will open a new TCP connection and
   send a STUN request, ideally an Allocate request, since it will
   provide multiple candidates with one request.






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4.  Prioritization

   Section 7.2 of ICE defines guidelines for prioritizing the set of
   candidates learned through the gathering process.  It specifies that
   if there are considerations that are specific to the transport
   protocol, these considerations should be called out in the ICE
   extension which defines usage with that transport protocol.  This
   section describes considerations specific to TCP.

   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.

   In addition, it is RECOMMENDED that actpass candidates have higher
   priority than active or passive candidates.  As discussed above, this
   allows for simultaneous opens to be preferred when they work, falling
   back to unidirectional opens when they do not.

   Section 7.2 of ICE also defines guidelines for selecting an active
   candidate in the initial offer.  It specifies that if there are
   considerations that are specific to the transport protocol, these
   considerations should be called out in the ICE extension which
   defines usage with that transport protocol.  This section describes
   considerations specific to TCP.

   When TCP-based media streams are used with ICE, the ICE mechanisms
   described here are responsible for opening the connections and
   testing them.  Once validated, they are promoted to active.
   Furthermore, like UDP candidate pairs, once validated, a TCP
   candidate pair can be used immediately in anticipation of an updated
   offer that promotes the candidate to valid.  Due to the time required
   and overhead of TCP connection establishment, it is RECOMMENDED that
   there be no active candidate in the initial offer/answer exchange.
   This avoids opening a connection for temporary usage, followed by
   opening of a subsequent higher priority connection that is then used
   for the remainder of the session.

   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 active candidate be UDP-based.

5.  Encoding

   Section 7.3 of ICE defines procedurs for encoding the candidates into
   an SDP offer or answer.  It specifies that if there are
   considerations that are specific to the transport protocol, these



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   considerations should be called out in the ICE extension which
   defines usage with that transport protocol.  This section describes
   considerations specific to TCP.

   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" for actpass candidates, "tcp-act" for active
   candidates and "tcp-pass" for 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.

   Encoding of the m/c-line, however, requires special considerations
   for TCP.  If there is no active candidate, the media session MUST
   include an a=holdconn attribute as defined in RFC 4145 [3].  This has
   the effect of suspending opening of the TCP connections - exactly the
   desired effect since they are opened by the procedures defined in
   this specification.  The IP address and port encoded into the m/c-
   line are inconsequential, since they are never used.

   Because this specification recommends that the initial offer and
   answer make use of an inactive candidate, a candidate generally
   appears there in subsequent offer/answer exchanges, after that
   candidate has been validated.  Indeed, the ICE procedures will
   actually result in the selection of a candidate pair, which directly
   maps to a TCP connection.  Thus, the purpose of the values in the
   m/c-line are to identify the TCP connection that will be used, using
   the candidate pair as the key.  The candidate pair is signaled by
   having the agent include the native IP address and port of that
   candidate pair in the m/c-line.  In the case of a peer-derived
   candidate pair, the native candidate on the active side will be an
   ephemeral IP address and port.  This is in contrast to RFC 4145,
   which recommends that the active side of a connection place a port
   with value '9'.  In addition, the media session MUST NOT contain the
   a=holdconn attribute.  The media session MUST contain the a=existing
   attribute, indicating that an existing connection is to be used,
   rather than opening a new one.  The a=active, a=passive and a=actpass
   attributes are not relevant when a=existing attribute is present, and
   SHOULD NOT be included.

6.  Ordering the Candidate Pairs

   Section 7.5 of ICE defines procedurs for ordering the candidates into
   an SDP offer or answer.  It specifies that if there are
   considerations that are specific to the transport protocol, these
   considerations should be called out in the ICE extension which
   defines usage with that transport protocol.  This section describes
   considerations specific to TCP.



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   ICE defines two orderings for candidate pairs - a priority order and
   a check order.  These differ only by the position of the active
   candidate in the list.  However, with TCP, prior to validation, there
   is usually no active TCP candidate.  As a consequence, the two lists
   are usually equivalent.

7.  Performing the Connectivity Checks

   Section 7.6 of ICE defines procedures for performing the connectivity
   checks.  These are based on a state machine that captures
   progressions of the checks.  This state machine is specific to the
   transport protocol, and the version for TCP is described here.

   The set of states visited by the offerer and answerer are depicted
   graphically in Figure 1




































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                 +----------+
                 |          |
                 |          |------------------------------------+
                 |  Waiting |                                    |
                 |          |                                    |
                 |          |----------------+                   |
                 |          |                |Get Req.,!active   |
                 +----------+                |----------------   |
                      |Cnxn Succd            |Send Res.          |
                      |----------            |                   |
                      |Send Req              |                   |
                      V                      V                   |
                 +----------+          +----------+              |
                 |          |          |          |              |
                 |          |          |          |              |
                 |  Testing |--------->|  Valid   |              |
                 |          |Send Res, |          |              |
                 |          |!active   |          |              |
                 |          |          |          |              |
                 +----------+          +----------+              |
                      |                                          |
                      |                                          |
                      |                                          |
                      |                                          |
                      |                                          |
                      |                                          |
                      |                +----------+              |
                      |                |          |              |
                      |   Send Res.,   |          |              |
                      |   active       |  Invalid |<-------------+
                      +--------------->|          |    Get Req.,active or
                                       |          |    Bad Request
                                       |          |    ----------------
                                       +----------+    Send Res.



                                 Figure 1

   The state machine has four states - waiting, testing, Valid and
   Invalid.  Initially, all transport address pairs start in the waiting
   state.  It is important to understand that the progression of this
   state machine is driven by the STUN transactions, since it is the
   STUN requests identify the candidate pairs.  This is distinct from
   the process of opening and closing connections, which does not
   directly impact this state machine.  First, however, connections need
   to be opened.




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   Even Ta seconds, the agent performs a new connection attempt.  This
   attempt is started for first transport address pair in the transport
   address pair check ordered list that is in the Waiting state and for
   which the agent is expected to open a connection.  An agent is
   expected to open a connection if its native transport address is
   either active or actpass, and the remote transport address is either
   passive or actpass.  If the candidate pair meets this criteria, the
   agent makes a connection attempt.

   If the native transport address is active, the agent will use an
   ephemeral port for the attempt.  For a local candidate, the agent
   initiates an oubound connection from the local interface, towards the
   remote transport address.  The ephemeral port MUST NOT be the same as
   the port used in an actpass local candidate on the same interface.
   For an active relayed candidate, the procedure is different.  The
   agent will initiate a new TCP connection to its STUN relay server,
   from an ephemeral port, but from the same interface as its current
   connection to that STUN relay server.  As with local candidates, this
   connection to the STUN relay server MUST NOT be from the same port as
   the current local candidate on the same interface.  Once connected,
   it allocates a TCP transport address.  However, it does not need to
   know its IP address and port.  Instead, the agent uses the STUN
   Connect request, and asks the relay to open a TCP connection towards
   the remote transport address in the candidate pair.

   If the native transport address is actpass, the agent initiates the
   connection from that transport address.  For local candidates, this
   is done by initiating an outbound connection directly from the same
   IP address and port it is already listening for incoming connection
   attempts on.  For relayed candidates, the agent asks the relay server
   to initiate a connection from the relayed transport address to the
   remote transport address.  For STUN servers, this is done by issuing
   a STUN Connect request over the existing connection to the server.

   If the connection attempt fails, the agent does nothing.  It does not
   set the state of the candidate pair to invalid.  Indeed, it may still
   yet be valid if its peer is able to open a connection to the agent.
   If the connection attempt succeeds, the agent immediately sends a
   STUN Binding Request according to the procedures of Section 7.7 of
   ICE.  That section indicates that STUN extensions should define any
   transport specific considerations for transmission of the STUN
   request.  In the case of TCP, the STUN request is sent on the
   connection that was just opened.  The STUN request is not
   retransmitted.  STUN messages include length indicators, allowing
   them to be framed over a connection-oriented transport protocol.  At
   this point, the state for the corresponding transport address pair
   moves from waiting to testing.




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   Furthermore, an agent will be listening for incoming TCP connection
   establishment requests on each local acpass transport address.  For
   passive reflexive transport addresses, the agent is already listening
   for incoming requests as a consequence of listening on the local
   actpass transport address.  When an incoming connection request is
   received, it is accepted, and a TCP connection is set up.  However,
   no attempt is made at this time to change the states of the state
   machines.  Those changes are effected only through STUN requests and
   responses.  For relayed actpass transport addresses, the relay is
   listening, and will inform the client of process.  In the case of
   STUN relays, the agent won't actually find out that a connection
   attempt to the server suceeded.  That is not an issue, since the
   acceptance of connections has no impact on ICE processing.  Instead,
   the agent is informed of data that is ultimately sent over that
   connection.  In the case of ICE, that first data will be a STUN
   Binding request.  It is that request which the client needs to
   perform ICE processing.

   STUN Binding Requests and Responses are mapped to transport address
   pairs and their state machines as described in Section 7.6 of ICE.

   If an agent receives a STUN Binding Request, it generates a response
   according to the procedures in Section 7.8 of ICE, including
   generation of the MAPPED-ADDRESS attribute in the response.  If the
   remote transport address is active, the agent moves this transport
   address pair into the Invalid state.  Furthermore, the agent MUST
   compute a peer-derived candidate as described in Section 9.  In
   addition, the TCP connection on which the Binding Request was
   received is then linked with the peer-derived candidate pair.

   If the remote transport address is not active, the agent moves this
   transport address pair into the Valid state.  The TCP connection on
   which the Binding Request was received is then linked with the
   candidate pair.

   If the STUN transaction produces an error, the state machine moves
   into the Invalid state.

   If an agent receives a successful STUN Binding Response, and the
   native transport address is active, the agent moves this transport
   address pair into the Invalid state.  Furthermore, the agent MUST
   compute a peer-derived candidate as described in Section 9.  In
   addition, the TCP connection on which the Binding Request was
   received is then linked with the peer-derived candidate pair.

   If the native transport address is not active, the agent moves this
   transport address pair into the Valid state.  The TCP connection on
   which the Binding Request was received is then linked with the



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   candidate pair.

8.  Promoting a Candidate to Active

   Promotion of a candidate to active occurs as described in Section 7.9
   of ICE.  The only difference to note is that, with TCP, the candidate
   pair priority ordered list and candidate pair check ordered list are
   usually identical, since there is generally no active TCP candidate.
   As a consequence, as soon as a candidate is validated, if it is the
   first in the priority list, an offer is sent immediately.  Otherwise,
   timer Tws is set, and the offer will be sent when it fires.

9.  Learning New Candidates from Connectivity Checks

   Section 7.10 of ICE describes procedures for learning new candidates
   from connectivity checks.  ICE indicates that the behavior of the
   state machines are transport protocol specific, and extensions to ICE
   for new transport protocols are asked to describe the behavior of the
   state machines.  This section does so for TCP.

   Firstly, it is important to realize that a successul TCP connection
   attempt and STUN connectivity check will always result in a peer-
   derived candidate being constructed when one agent was active.  ICE
   talks about learning new peer-derived candidates as a consequence of
   symmetric NAT.  Here, they are learned as a consequence of opening
   TCP connections from an ephemeral port.

   When a new peer-derived candidate is formed as a result of receipt of
   a STUN Binding Request that generates a successful response, the
   state machine for that candidate enters the Valid state.  Unlike UDP,
   a Binding Request is not sent back to the source of the request.
   Similarly, when a new peer-derived candidate is formed as a result of
   receipt of a successful STUN Binding Response, the state machine for
   that candidate enters the Valid state.  In both cases, the new
   candidate pair is inserted into the ordered list of pairs and
   processing follows the logic described in Section 7.

10.  Subsequent Offers

   Section 7.11 of ICE describes procedures for subsequent offer/answer
   exchanges.  ICE indicates that if there are any considerations that
   are transport protocol specific, new transport protocols are asked to
   describe them.  This section does so for TCP.

   The procedures defined in Section 7.11 of ICE apply to TCP as
   defined.  However, if a candidate is not valid, it MUST NOT be placed
   into the m/c-line of a subsequent offer or answer.  Only valid
   candidates are placed into the m/c-line for TCP.  This is in contrast



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   to UDP, where a partially valid one can be used.

   Once the offer/answer exchange has completed, the m/c-lines from each
   agent, when put together, form a transport address pair.  This
   transport address pair is matched to the transport address pairs
   across all of the Valid candidates.  The highest priority candidate
   pair amongst the matching ones is selected, and the TCP connection to
   which it is linked is used.  It is that TCP connection which will be
   used for the transport of media.  Since there is only ever one TCP
   connection associated with a candidate pair, and since a single
   candidate pair is always selected, ICE can guarantee that media is
   transported between peers over a single TCP connection.

   In addition, if a candidate pair is removed as a consequence of the
   processing defined in Section 7.11, and that candidate pair was TCP-
   based, its corresponding TCP connection (if any) is torn down.

   Additional considerations do apply, however, to the usage of RFC 4145
   attributes in the m/c-line.  The offerer will include the a=existing
   attribute in the m-line.  When the answerer receives this, it follows
   the procedures of RFC 4145 to generate the attributes in the
   response.  It MUST indicate that the existing connection is being
   reused, by including an a=existing attribute in the answer.

   Furthermore, RFC 4145 defines the a=existing attribute to mean the
   reuse of the existing connection established as a consequence of RFC
   4145 processing for this media stream.  This specification broadens
   that definition.  The existing connection can also be one established
   as a consequence of the mechanisms defined in this specification, and
   the specific TCP connection to use is identified by the 5-tuple
   constructed from the m/c-line in the offer and the m/c-line in the
   answer, as described above.

   RFC 4145 also describes TCP connection lifecycle management
   procedures.  If the TCP connection used in the m/c-line was opened by
   ICE processing, it is closed by ICE processing as well.  This occurs
   when the session terminates, or when the generating candidate for the
   active one ceases to be retained in a subsequent offer/answer
   exchange.

11.  Binding Keepalives

   STUN-based keepalives are used for TCP-based media streams, just as
   they are for UDP-based media streams, and are performed as described
   in Section 7.12 of ICE.  This requires demultiplexing of STUN and
   application data traffic on the same TCP connection.  For media
   streams based on RTP, this is easily done as follows.  The framing
   mechanism in [5] MUST be used on the TCP connection.  In addition,



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   instead of just an RTP or RTCP packet appearing after the LENGTH
   field, a STUN packet can appear.  The agent determines whether the
   packet is RTP or STUN by looking for the magic cookie in bits 32-63
   of the data.  If present, it indicates that the packet is STUN, and
   if not, indicates that it is RTP.

   In the case of non-RTP traffic, ICE-TCP can be used with any
   application protocol which provides some kind of framing into
   application messages with a well-defined start.  When the application
   framing mechanism points to the start of an application message, the
   agent looks ahead to bits 32-63.  If they equal the magic cookie, the
   message is a STUN message.  Its length is determined by the message
   length in bits 16 to 31 of the STUN packet.  That STUN message is
   extracted and processed, and then the pointer in the data stream
   moves to the end of the STUN packet, and the process begins afresh.
   If bits 32-63 were not equal to the magic cookie, the agent uses
   application protocol specific framing to find the end of the
   application packet, and the process begins afresh.

   The need to perform this demultiplexing, even over TCP, is the
   ugliest part of this specification.  However, it is necessary to
   provide substantial reductions in call setup time possible by sending
   media on a validated candidate prior to its promotion to the m/c-
   line.

12.  Sending Media

   The procedures for sending media in the case of TCP are identical to
   those defined in Section 7.13 of ICE, including the ability to use a
   validated candidate immediately, in anticipation of its promotion
   into the m/c-line of a subsequent offer.  This means that a
   connection can be opened and validated by ICE, and then immediately
   used for application traffic.  This will require the demultiplexing
   described in the previous section to disambiguate STUN and
   application data.

13.  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



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   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.

14.  IANA Considerations

   There are no IANA considerations associated with this specification.

15.  References

15.1  Normative References

   [1]  Rosenberg, J., "Simple Traversal of UDP Through Network Address
        Translators (NAT) (STUN)", draft-ietf-behave-rfc3489bis-02 (work
        in progress), July 2005.

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

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

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

   [5]  Lazzaro, J., "Framing RTP and RTCP Packets over Connection-
        Oriented Transport", draft-ietf-avt-rtp-framing-contrans-06
        (work in progress), September 2005.

   [6]  Rosenberg, J., "Interactive Connectivity Establishment (ICE): A
        Methodology for Network  Address Translator (NAT) Traversal for
        Offer/Answer Protocols", draft-ietf-mmusic-ice-06 (work in
        progress), October 2005.

   [7]  Rosenberg, J., Mahy, R., and C. Huitema, "Obtaining Relay
        Addresses from Simple Traversal of UDP Through NAT (STUN)",
        Internet Draft draft-ietf-behave-turn-00.txt, February 2006.

15.2  Informative References

   [8]  Campbell, B., "The Message Session Relay Protocol",
        draft-ietf-simple-message-sessions-13 (work in progress),
        December 2005.




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Author's Address

   Jonathan Rosenberg
   Cisco Systems
   600 Lanidex Plaza
   Parsippany, NJ  07054
   US

   Phone: +1 973 952-5000
   Email: jdrosen@cisco.com
   URI:   http://www.jdrosen.net








































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