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MMUSIC                                                      J. Rosenberg
Internet-Draft                                             Cisco Systems
Expires: April 22, 2006                                 October 19, 2005


Interactive Connectivity Establishment (ICE): A Methodology for Network
     Address Translator (NAT) Traversal for Offer/Answer Protocols
                        draft-ietf-mmusic-ice-06

Status of this Memo

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   This Internet-Draft will expire on April 22, 2006.

Copyright Notice

   Copyright (C) The Internet Society (2005).

Abstract

   This document describes a protocol for Network Address Translator
   (NAT) traversal for multimedia session signaling protocols based on
   the offer/answer model, such as the Session Initiation Protocol
   (SIP).  This protocol is called Interactive Connectivity
   Establishment (ICE).  ICE makes use of existing protocols, such as
   Simple Traversal of UDP Through NAT (STUN) and Traversal Using Relay
   NAT (TURN).  ICE makes use of STUN in peer-to-peer cooperative
   fashion, allowing participants to discover, create and verify mutual



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   connectivity.

Table of Contents

   1.   Introduction . . . . . . . . . . . . . . . . . . . . . . . .   4
   2.   Terminology  . . . . . . . . . . . . . . . . . . . . . . . .   4
   3.   Overview of ICE  . . . . . . . . . . . . . . . . . . . . . .   8
   4.   Sending the Initial Offer  . . . . . . . . . . . . . . . . .  10
   5.   Receipt of the Offer and Generation of the Answer  . . . . .  11
   6.   Processing the Answer  . . . . . . . . . . . . . . . . . . .  11
   7.   Common Procedures  . . . . . . . . . . . . . . . . . . . . .  11
     7.1  Gathering Candidates . . . . . . . . . . . . . . . . . . .  12
     7.2  Prioritizing the Candidates and Choosing an Active One . .  15
     7.3  Encoding Candidates into SDP . . . . . . . . . . . . . . .  17
     7.4  Forming Candidate Pairs  . . . . . . . . . . . . . . . . .  19
     7.5  Ordering the Candidate Pairs . . . . . . . . . . . . . . .  22
     7.6  Performing the Connectivity Checks . . . . . . . . . . . .  23
     7.7  Sending a Binding Request for Connectivity Checks  . . . .  27
     7.8  Receiving a Binding Request for Connectivity Checks  . . .  29
     7.9  Promoting a Candidate to Active  . . . . . . . . . . . . .  31
     7.10   Learning New Candidates from Connectivity Checks . . . .  31
       7.10.1   On Receipt of a Binding Request  . . . . . . . . . .  32
       7.10.2   On Receipt of a Binding Response . . . . . . . . . .  35
     7.11   Subsequent Offer/Answer Exchanges  . . . . . . . . . . .  37
       7.11.1   Sending of a Subsequent Offer  . . . . . . . . . . .  37
       7.11.2   Receiving the Offer and Sending an Answer  . . . . .  39
       7.11.3   Receiving the Answer . . . . . . . . . . . . . . . .  41
     7.12   Binding Keepalives . . . . . . . . . . . . . . . . . . .  41
     7.13   Sending Media  . . . . . . . . . . . . . . . . . . . . .  42
   8.   Guidelines for Usage with SIP  . . . . . . . . . . . . . . .  43
   9.   Interactions with Forking  . . . . . . . . . . . . . . . . .  44
   10.  Interactions with Preconditions  . . . . . . . . . . . . . .  45
   11.  Example  . . . . . . . . . . . . . . . . . . . . . . . . . .  45
   12.  Grammar  . . . . . . . . . . . . . . . . . . . . . . . . . .  68
   13.  Security Considerations  . . . . . . . . . . . . . . . . . .  69
     13.1   Attacks on Connectivity Checks . . . . . . . . . . . . .  69
     13.2   Attacks on Address Gathering . . . . . . . . . . . . . .  72
     13.3   Attacks on the Offer/Answer Exchanges  . . . . . . . . .  73
     13.4   Insider Attacks  . . . . . . . . . . . . . . . . . . . .  73
       13.4.1   The Voice Hammer Attack  . . . . . . . . . . . . . .  73
       13.4.2   STUN Amplification Attack  . . . . . . . . . . . . .  74
   14.  IANA Considerations  . . . . . . . . . . . . . . . . . . . .  74
     14.1   candidate Attribute  . . . . . . . . . . . . . . . . . .  74
     14.2   remote-candidate Attribute . . . . . . . . . . . . . . .  75
   15.  IAB Considerations . . . . . . . . . . . . . . . . . . . . .  75
     15.1   Problem Definition . . . . . . . . . . . . . . . . . . .  75
     15.2   Exit Strategy  . . . . . . . . . . . . . . . . . . . . .  76
     15.3   Brittleness Introduced by ICE  . . . . . . . . . . . . .  76



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     15.4   Requirements for a Long Term Solution  . . . . . . . . .  77
     15.5   Issues with Existing NAPT Boxes  . . . . . . . . . . . .  78
   16.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . .  78
   17.  References . . . . . . . . . . . . . . . . . . . . . . . . .  78
     17.1   Normative References . . . . . . . . . . . . . . . . . .  78
     17.2   Informative References . . . . . . . . . . . . . . . . .  79
        Author's Address . . . . . . . . . . . . . . . . . . . . . .  81
        Intellectual Property and Copyright Statements . . . . . . .  82











































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1.  Introduction

   A multimedia session signaling protocol is a protocol that exchanges
   control messages between a pair of agents for the purposes of
   establishing the flow of media traffic between them.  This media flow
   is distinct from the flow of control messages, and may take a
   different path through the network.  Examples of such protocols are
   the Session Initiation Protocol (SIP) [3], the Real Time Streaming
   Protocol (RTSP) [17] and the International Telecommunications Union
   (ITU) H.323.

   These protocols, by nature of their design, are difficult to operate
   through Network Address Translators (NAT).  Because their purpose is
   to establish a flow of media packets, they tend to carry IP addresses
   within their messages, which is known to be problematic through NAT
   [18].  The protocols also seek to create a media flow directly
   between participants, so that there is no application layer
   intermediary between them.  This is done to reduce media latency,
   decrease packet loss, and reduce the operational costs of deploying
   the application.  However, this is difficult to accomplish through
   NAT.  A full treatment of the reasons for this is beyond the scope of
   this specification.

   Numerous solutions have been proposed for allowing these protocols to
   operate through NAT.  These include Application Layer Gateways
   (ALGs), the Middlebox Control Protocol [20], Simple Traversal of UDP
   through NAT (STUN) [1], Traversal Using Relay NAT [16], and Realm
   Specific IP [21] [22] along with session description extensions
   needed to make them work, such as the Session Description Protocol
   (SDP) [7] attribute for the Real Time Control Protocol (RTCP) [2].
   Unfortunately, these techniques all have pros and cons which make
   each one optimal in some network topologies, but a poor choice in
   others.  The result is that administrators and implementors are
   making assumptions about the topologies of the networks in which
   their solutions will be deployed.  This introduces complexity and
   brittleness into the system.  What is needed is a single solution
   which is flexible enough to work well in all situations.

   This specification provides that solution for media streams
   established by signaling protocols based on the offer-answer model,
   RFC 3264 [5].  It is called Interactive Connectivity Establishment,
   or ICE.  ICE makes use of STUN and TURN, but uses them in a specific
   methodology which avoids many of the pitfalls of using any one alone.

2.  Terminology

   Several new terms are introduced in this specification:




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   Agent: As defined in RFC 3264, an agent is the protocol
      implementation involved in the offer/answer exchange.  There are
      two agents involved in an offer/answer exchange.

   Peer: From the perspective of one of the agents in a session, its
      peer is the other agent.  Specifically, from the perspective of
      the offerer, the peer is the answerer.  From the perspective of
      the answerer, the peer is the offerer.

   Transport Address: The combination of an IP address and port.

   Local Transport Address: A local transport address is a transport
      address that has been allocated from the operating system on the
      host.  This includes transport addresses obtained through Virtual
      Private Networks (VPNs) and transport addresses obtained through
      Realm Specific IP (RSIP) [21] (which lives at the operating system
      level).  Transport addresses are typically obtained by binding to
      an interface.

   m/c line: The media and connection lines in the SDP, which together
      hold the transport address used for the receipt of media.

   Derived Transport Address: A derived transport address is a transport
      address which is derived from a local transport address.  The
      derived transport address is related to the associated local
      transport address in that packets sent to the derived transport
      address are received on the socket bound to its associated local
      transport address.  Derived addresses are obtained using protocols
      like STUN and TURN, and more generally, any UNSAF protocol [23].

   Associated Local Transport Address: When a peer sends a packet to a
      transport address, the associated local transport address is the
      local transport address at which those packets will actually
      arrive.  For a local transport address, its associated local
      transport address is the same as the local transport address
      itself.  For STUN derived and TURN derived transport addresses,
      however, they are not the same.  The associated local transport
      address is the one from which the STUN or TURN transport was
      derived.

   Peer Derived Transport Address: A peer derived transport address is a
      derived transport address learned from a STUN server running
      within a peer in a media session.

   TURN Derived Transport Address: A derived transport address obtained
      from a TURN server.





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   STUN Derived Transport Address: A derived transport address obtained
      from a STUN server whose address has been provisioned or
      discovered by the UA.  This, by definition, excludes Peer Derived
      Transport Addresses.

   Candidate: A sequence of transport addresses that form an atomic set
      for usage with a particular media session.  Here, atomic means
      that all of transport addresses in the candidate need to work
      before the candidate will be used for actual media transport.  In
      the case of RTP, there can be one or more transport addresses per
      candidate.  In the most common case, there are two - one for RTP,
      and another for RTCP.  If the agent doesn't use RTCP, there would
      be just one.  If Generic Forward Error Correction (FEC) [19] is in
      use, there may be more than two.  The transport addresses that
      compose a candidate are all of the same type - local, STUN
      derived, TURN derived or peer derived.

   Local Candidate: A candidate whose transport addresses are local
      transport addresses.

   STUN Candidate: A candidate whose transport addresses are STUN
      derived transport addresses.

   TURN Candidate: A candidate whose transport addresses are TURN
      derived transport addresses.

   Peer Derived Candidate: A candidate whose transport addresses are
      peer derived transport addresses.

   Generating Candidate: The candidate from which a peer derived
      candidate is derived.

   Active Candidate: The candidate that is in use for exchange of media.
      This is the one that an agent places in the m/c line of an offer
      or answer.

   Candidate ID: An identifier for a candidate.

   Component: When a media stream, and as a consequence, its candidate,
      require several IP addresses and ports to work atomically, each of
      the constituent IP addresses and ports represents a component of
      that media stream.  For example, RTP-based media streams typically
      have two components - one for RTP, and one for RTCP.

   Component ID: An integer, starting with one within each candidate and
      incrementing by one for each component, which identifies the
      component.




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   Transport Address ID (tid): An identifier for a transport address,
      formed by concatenating the candidate ID with the component ID,
      separated by a "colon".

   Candidate Pair: The combination of a candidate from one agent along
      with a candidate from its peer.

   Native Candidate: From the perspective of each agent, the candidate
      in a candidate pair which represents a set of addresses obtained
      by that agent.

   Remote Candidate: From the perspective of each agent, the candidate
      in a candidate pair which represents the set of addresses obtained
      by that agents peer.

   Transport Address Pair: The combination of the transport address for
      one component of a candidate with the transport address of the
      same component for the matching candidate in a candidate pair.

   Transport Address Pair ID: An identifier for a transport address
      pair.  Formed by concatenating the native transport address ID
      with the remote transport address ID, separated by a "colon".

   Matching Transport Address Pair: When a STUN Binding Request is
      received on a local transport address, the matching transport
      address pair is the transport address pair whose connectivity is
      being checked by that Binding Request.

   Candidate Pair Priority Ordering: An ordering of candidate pairs
      based on a combination of the qvalues of each candidate and the
      candidate IDs of each candidate.

   Candidate Pair Check Ordering: An ordering of candidate pairs that is
      similar to the candidate pair priority ordering, except that the
      active candidate appears at the top of the list, regardless of its
      priority.

   Transport Address Pair Check Ordering: An ordering of transport
      address pairs that determines the sequence of connectivity checks
      performed for the pairs.

   Transport Address Pair Count: The number of transport address pairs
      in a candidate pair.  This is equal to the minimum of the number
      of transport addresses in the native candidate and the number of
      transport addresses in the remote candidate.






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3.  Overview of ICE

   ICE makes the fundamental assumption that clients exist in a network
   of segmented connectivity.  This segmentation is the result of a
   number of addressing realms in which a client can simultaneously be
   connected.  We use "realms" here in the broadest sense.  A realm is
   defined purely by connectivity.  Two clients are in the same realm
   if, when they exchange the addresses each has in that realm, they are
   able to send packets to each other.  This includes IPv6 and IPv4
   realms, which actually use different address spaces, in addition to
   private networks connected to the public Internet through NAT.

   The key assumption in ICE is that a client cannot know, apriori,
   which address realms it shares with any peer it may wish to
   communicate with.  Therefore, in order to communicate, it has to try
   connecting to addresses in all of the realms.


          Agent A          TURN,STUN Servers          Agent B
             |(1) Gather Addresses |                     |
             |-------------------->|                     |
             |(2) Offer            |                     |
             |------------------------------------------>|
             |                     |(3) Gather Addresses |
             |                     |<--------------------|
             |(4) Answer           |                     |
             |<------------------------------------------|
             |(5) STUN Check       |                     |
             |<------------------------------------------|
             |(6) STUN Check       |                     |
             |------------------------------------------>|
             |(7) Offer            |                     |
             |------------------------------------------>|
             |(8) Answer           |                     |
             |<------------------------------------------|
             |(9) Media            |                     |
             |<------------------------------------------|
             |(10) Media           |                     |
             |------------------------------------------>|


                                 Figure 1

   The basic flow of operation for ICE is shown in Figure 1.  Before the
   offerer establishes a session, it obtains local transport addresses
   from its operating system on as many interfaces as it has access to.
   These interfaces can include IPv4 and IPv6 interfaces, in addition to
   Virtual Private Network (VPN) interfaces or ones associated with



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   RSIP.  It then obtains transport addresses for the media from each
   interface.  Though the ICE framework can support any type of
   transport protocol, this specification only defines mechanisms for
   UDP.  In addition, the agent obtains derived transport addresses from
   each local transport address using protocols such as STUN and TURN.
   These are paced at a fixed rate in order to limit network load and
   avoid NAT overload.  The local and derived transport addresses are
   formed into candidates, each of which represents a possible set of
   transport addresses that might be viable for a media stream.

   Each candidate is listed in a set of a=candidate attributes in the
   offer.  Each candidate is given a priority.  Priority is a matter of
   local policy, but typically, lowest priority would be given to
   transport addresses learned from a TURN server (i.e., TURN derived
   transport addresses).  Each candidate is also assigned a distinct ID,
   called a candidate ID.

   The agent will choose one of its candidates as its active candidate
   for inclusion in the connection and media lines in the offer.  Media
   can be sent to this candidate immediately following its validation.
   Media is not sent without validation in order to avoid denial-of-
   service attacks.  In particular, without ICE, an offerer can send an
   offer to another agent, and list the IP address and port of a target
   in the offer.  If the agent is an automata that answers a call
   automatically, it will do so and then proceed to send media to the
   target.  This provides substantial packet amplifications.  ICE fixes
   this by using STUN-based validation of addresses.

   The offer is then sent to the answerer.  This specification does not
   address the issue of how the signaling messages themselves traverse
   NAT.  It is assumed that signaling protocol specific mechanisms are
   used for that purpose.  The answerer follows a similar process as the
   offerer followed; it obtains addresses from local interfaces, obtains
   derived transport addresses from those, and then groups them into
   candidates for inclusion in a=candidate attributes in the answer.  It
   picks one candidate as its active candidate and places it into the
   m/c line in the answer.

   Once the offer/answer exchange has completed, both agents pair up the
   candidates, and then determine an ordered set of transport address
   pairs.  This ordering is based primarily on the priority of the
   candidates, with the exception of the active candidate, whose
   addresses are at the top of the list.  Both agents start at the top
   of this list, beginning a connectivity check for that transport
   address pair.  At a fixed interval, checks for the next transport
   address on the list begin.  This results in a pacing of the
   connectivity checks.  These connectivity checks are performed through
   peer-to-peer STUN requests, sent from one agent to the other.  In



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   addition to pacing the checks out at regular intervals, the offerer
   will generate a connectivity check for a transport address pair when
   it receives one from its peer.  As soon as the active candidate has
   been verified by the STUN checks, media can begin to flow.  Once a
   higher priority candidate has been verified by the offerer, it ceases
   additional connectivity checks, and sends an updated offer which
   promotes this higher priority candidate to the m/c-line.  That
   candidate is also listed in a=candidate attributes, resulting in
   periodic STUN keepalives through the duration of the media session.

   If an agent receives a STUN connectivity check with a new source IP
   address and port, or a response to such a check with a new IP address
   and port indicated in the MAPPED-ADDRESS attribute, this new address
   might be a viable candidate for the receipt of media.  This happens
   when there is a symmetric NAT between the agents.  In such a case,
   the agents algorithmically construct a new candidate.  Like other
   candidates, connectivity checks begin for it, and if they succeed,
   its transport addresses can be used for receipt of media by promoting
   it to the m/c-line.

   The gathering of addresses and connectivity checks take time.  As a
   consequence, in order to have no impact on the call setup time or
   post-pickup delay for SIP, these offer/answer exchanges and checks
   happen while the call is ringing.

4.  Sending the Initial Offer

   When an agent wishes to begin a session by sending an initial offer,
   it starts by gathering transport addresses, as described in
   Section 7.1.  This will produce a set of candidates, including local
   ones, STUN-derived ones, and TURN-derived ones.

   This process of gathering candidates can actually happen at any time
   before sending the initial offer.  A agent can pre-gather transport
   addresses, using a user interface cue (such as picking up the phone,
   or entry into an address book) as a hint that communications is
   imminent.  Doing so eliminates any additional perceivable call setup
   delays due to address gathering.

   When it comes time to offer communications, the agent determines a
   priority for each candidate and identifies the active candidate that
   will be used for receipt of media, as described in Section 7.2.

   The next step is to construct the offer message.  For each media
   stream, it places its candidates into a=candidate attributes in the
   offer and puts its active candidate into the m/c line.  The process
   for doing this is described in Section 7.3.  The offer is then sent.




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5.  Receipt of the Offer and Generation of the Answer

   Upon receipt of the offer message, the agent checks if the offer
   contains any a=candidate attributes.  If it does, the offerer
   supports ICE.  In that case, it starts gathering candidates, as
   described in Section 7.1, and prioritizes them as described in
   Section 7.2.  This processing is done immediately on receipt of the
   offer, to prepare for the case where the user should accept the call,
   or early media needs to be generated.  By gathering candidates (and
   performing connectivity checks) while the user is being alerted to
   the request for communications, session establishment delays due to
   that gathering can be eliminated.

   The agent then constructs its answer, encoding its candidates into
   a=candidate attributes and including the active one in the m/c-line,
   as described in Section 7.3.  The agent then forms candidate pairs as
   described in Section 7.4.  These are ordered as described in
   Section 7.5.  The agent then begins connectivity checks, as described
   in Section 7.6.  It follows the logic in Section 7.10 on receipt of
   Binding Requests and responses to learn new candidates from the
   checks themselves.

   Transmission of media is performed according to the procedures in
   Section 7.13.

6.  Processing the Answer

   There are two possible cases for processing of the answer.  If the
   answerer did not support ICE, the answer will not contain any
   a=candidate attributes.  As a result, the offerer knows that it
   cannot perform its connectivity checks.  In this case, it proceeds
   with normal media processing as if ICE was not in use.  The
   procedures for sending media, described in Section 7.13, MUST be
   followed however.

   If the answer contains candidates, it implies that the answerer
   supports ICE.  The agent then forms candidate pairs as described in
   Section 7.4.  These are ordered as described in Section 7.5.  The
   agent then begins connectivity checks, as described in Section 7.6.
   It follows the logic in Section 7.10 on receipt of Binding Requests
   and responses to learn new candidates from the checks themselves.

   Transmission of media is performed according to the procedures in
   Section 7.13.

7.  Common Procedures

   This section discusses procedures that are common between offerer and



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   answerer.

7.1  Gathering Candidates

   An agent gathers candidates when it believes that communications is
   imminent.  For offerers, this occurs before sending an offer
   (Section 4).  For answerers, it occurs before sending an answer
   (Section 5).

   Each candidate has one or more components, each of which is
   associated with a sequence number, starting at 1 for the first
   component of each candidate, and incrementing by 1 for each
   additional component within that candidate.  These components
   represent a set of transport addresses for which connectivity must be
   validated.  For a particular media stream, all of the candidates
   SHOULD have the same number of components.  The number of components
   that are needed are a function of the type of media stream.

   For traditional RTP-based media streams, it is RECOMMENDED that there
   be two components per candidate - one for RTP and one for RTCP.  The
   component with the component ID of 1 MUST be RTP, and the one with
   component ID of 2 MUST be RTCP.  If an agent doesn't implement RTCP,
   it SHOULD have a single component for the RTP stream (which will have
   a component ID of 1 by definition).  Each component of a candidate
   has a single transport address.

   The first step is to gather local candidates.  Local candidates are
   obtained by binding to ephemeral ports on an interface (physical or
   virtual, including VPN interfaces) on the host.  The process for
   gathering local candidates depends on the transport protocol.
   Procedures are specified here for UDP.  Extensions to ICE that define
   procedures for other transport protocols MUST specify how local
   transport addresses are gathered.

   For each UDP media stream the agent wishes to use, the agent SHOULD
   obtain a set of candidates (one for each interface) by binding to N
   ephemeral UDP ports on each interface, where N is the number of
   components needed for the candidate.  For RTP, N is typically two.
   If a host has K local interfaces, this will result in K candidates
   for each UDP stream , requiring K*N local transport addresses.

   Once the agent has obtained local candidates, it obtains candidates
   with derived transport addresses.  The process for gathering derived
   candidates depends on the transport protocol.  Procedures are
   specified here for UDP.  Extensions to ICE that define procedures for
   other transport protocols MUST specify how derived transport
   addresses are gathered.




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   Agents which serve end users directly, such as softphones,
   hardphones, terminal adapters and so on, MUST implement STUN and
   SHOULD use it to obtain STUN candidates.  These devices SHOULD
   implement and SHOULD use TURN to obtain TURN candidates.  They MAY
   implement and MAY use other protocols that provide derived transport
   addresses, such as TEREDO [31].  Usage of STUN and TURN is at SHOULD
   strength to allow for provider variation.  If it is not to be used,
   it is RECOMMENDED that it be implemented and just disabled through
   configuration, so that it can re-enabled through configuration if
   conditions change in the future.

   Agents which represent network servers under the control of a service
   provider, such as gateways to the telephone network, media servers,
   or conferencing servers that are targeted at deployment only in
   networks with public IP addresses MAY use STUN, TURN or other similar
   protocols to obtain candidates.

      Why would these types of endpoints even bother to implement ICE?
      The answer is that such an implementation greatly facilitates NAT
      traversal for clients that connect to it.  The ability to process
      STUN connectivity checks allows for clients to obtain peer-derived
      transport addresses that can be used by the network server to
      reach them without a relay, even through symmetric NAT.
      Furthermore, implementation of the STUN connectivity checks allows
      for NAT bindings along the way to be kept open.  ICE also provides
      numerous security properties that are independent of NAT
      traversal, and would benefit any multimedia endpoint.  See
      Section 13 for a discussion on these benefits.

   Obtaining STUN, TURN and other derived candidates requires
   transmission of packets which have the effect of creating bindings on
   NAT devices between the client and the STUN or TURN servers.
   Experience has shown that many NAT devices have upper limits on the
   rate at which they will create new bindings.  Furthermore,
   transmission of these packets on the network makes use of bandwidth
   and needs to be rate limited by the agent.  As a consequence, a
   client SHOULD pace its STUN and TURN transactions, such that the
   start of each new transaction occurs at least Ta seconds after the
   start of the previous transaction.  The value of Ta SHOULD be
   configurable, and SHOULD have a default of 50ms.  Note that this
   pacing applies only to the start of a new transaction; pacing of
   retransmissions within a STUN or TURN transaction is governed by the
   retransmission rules defined in those protocols.

   To obtain STUN candidates, the client takes a local UDP candidate,
   and for each configured STUN server, produces a STUN candidate.  It
   is anticipated that clients may have a multiplicity of STUN servers
   that it discovers or is configured with in network environments where



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   there are multiple layers of NAT.  To produce the STUN candidate from
   the local candidate, it follows the procedures of Section 9 of RFC
   3489 for each local transport address in the local candidate.  It
   obtains a shared secret from the STUN server and then initiates a
   Binding Request transaction from each local transport address to that
   server.  The Binding Response will provide the client with its STUN
   derived transport address in the MAPPED-ADDRESS attribute.  If the
   client had K local candidates, this will produce S*K STUN candidates,
   where S is the number of STUN servers.

   It is anticipated that clients may have a multiplicity of TURN
   servers configured or discovered in network environments where there
   are multiple layers of NAT, and that layering is known to the
   provider of the client.  To obtain TURN candidates, for each
   configured TURN server, the client initiates an Allocate Request
   transaction using the procedures of Section 8 of [16] from each
   transport address of a particular local candidate.  The Allocate
   Response will provide the client with its TURN derived transport
   address in the MAPPED-ADDRESS attribute.  Once the TURN allocations
   against a particular TURN server succeed from all of the transport
   addresses in a particular local candidate, the client SHOULD NOT
   attempt any further TURN allocations to that particular server from
   the transport addresses in any other local candidates.  This is to
   reduce the number of bindings allocated from the NATs.  Only a single
   TURN candidate is needed from a particular TURN server.  The order in
   which local candidates are tried against the TURN server is a matter
   of local policy.

   Since a client will pace its STUN and TURN allocations at a rate of
   one new transaction every Ta seconds, it will take a certain amount
   of time for these allocations to occur.  It is RECOMMENDED that
   implementations have a configurable upper bound on the total number
   of such allocations they will perform before generation of their
   offer or answer.  Any allocations not completed at that point SHOULD
   be abandoned, but MAY continue and be used in an updated offer once
   they complete.  A default value of 10 is RECOMMENDED.  Since the
   total number of allocations that could be done (based on the number
   of STUN servers, TURN servers and local interfaces) might exceed this
   value, clients SHOULD prioritize their allocations and perform higher
   priority ones first.  It is RECOMMENDED that STUN allocations be
   prioritized over TURN allocations.

   Once the allocations are complete, any redundant candidates are
   discarded.  A candidate is redundant if its transport addresses for
   each component match the transport addresses for each component of
   another candidate.





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7.2  Prioritizing the Candidates and Choosing an Active One

   The prioritization process takes the set of candidates and associates
   each with a priority.  This priority reflects the desire that the
   agent has to receive media on that address, and is assigned as a
   value from 0 to 1 (1 being most preferred).  Priorities are ordinal,
   so that their significance is only meaningful relative to other
   candidates from that agent for a particular media stream.  Candidates
   MAY have the same priority.  However, it is RECOMMENDED that each
   candidate have a distinct priority.  Doing so improves the efficiency
   of ICE.

   This specification makes no normative recommendations on how the
   prioritization is done.  However, some useful guidelines are
   suggested on how such a prioritization can be determined.

   One criteria for choosing one candidate over another is whether or
   not that candidate involves the use of a relay.  That is, if media is
   sent to that candidate, will the media first transit a relay before
   being received.  TURN candidates make use of relays (the TURN
   server), as do any local candidates associated with a VPN server.
   When media is transited through a relay, it can increase the latency
   between transmission and reception.  It can increase the packet
   losses, because of the additional router hops that may be taken.  It
   may increase the cost of providing service, since media will be
   routed in and right back out of a relay run by the provider.  If
   these concerns are important, candidates with this property can be
   listed with lower priority.

   Another criteria for choosing one candidate over another is IP
   address family.  ICE works with both IPv4 and IPv6.  It therefore
   provides a transition mechanism that allows dual-stack hosts to
   prefer connectivity over IPv6, but to fall back to IPv4 in case the
   v6 networks are disconnected (due, for example, to a failure in a
   6to4 relay) [26].  It can also help with hosts that have both a
   native IPv6 address and a 6to4 address.  In such a case, higher
   priority could be afforded to the native v6 address, followed by the
   6to4 address, followed by a native v4 address.  This allows a site to
   obtain and begin using native v6 addresss immediately, yet still
   fallback to 6to4 addresses when communicating with agents in other
   sites that do not yet have native v6 connectivity.

   Another criteria for choosing one candidate over another is security.
   If a user is a telecommuter, and therefore connected to their
   corporate network and a local home network, they may prefer their
   voice traffic to be routed over the VPN in order to keep it on the
   corporate network when communicating within the enterprise, but use
   the local network when communicating with users outside of the



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   enterprise.

   Another criteria for choosing one address over another is topological
   awareness.  This is most useful for candidates which make use of
   relays (including TURN and VPN).  In those cases, if an agent has
   preconfigured or dynamically discovered knowledge of the topological
   proximity of the relays to itself, it can use that to select closer
   relays with higher priority.

   There may be transport-specific reasons for preferring one candidate
   over another.  In such a case, specifications defining usage of ICE
   with other transport protocols SHOULD document such considerations.

   Once the candidates have been prioritized, one may be selected as the
   active one.  This is the candidate that will be used for actual
   exchange of media if and when its validated, until replaced by an
   updated offer or answer.  The active candidate will also be used to
   receive media from ICE-unaware peers.  As such, it is RECOMMENDED
   that one be chosen based on the likelihood of that candidate to work
   with the peer that is being contacted.  Unfortunately, it is
   difficult to ascertain which candidate that might be.  As an example,
   consider a user within an enterprise.  To reach non-ICE capable
   agents within the enterprise, a local candidate has to be used, since
   the enterprise policies may prevent communication between elements
   using a relay on the public network.  However, when communicating to
   peers outside of the enterprise, a TURN-based candidate from a
   publically accessible TURN server is needed.

   Indeed, the difficulty in picking just one address that will work is
   the whole problem that motivated the development of this
   specification in the first place.  As such, it is RECOMMENDED that
   the active candidate be a TURN derived candidate from a TURN server
   providing public IP addresses.  Furthermore, ICE is only truly
   effective when it is supported on both sides of the session.  It is
   therefore most prudent to deploy it to close-knit communities as a
   whole, rather than piecemeal.  In the example above, this would mean
   that ICE would ideally be deployed completely within the enterprise,
   rather than just to parts of it.

   An additional consideration for selection of the active candidate is
   the switching of media stream destinations between the initial offer
   and the subsequent offer.  If the active candidate pair in the
   initial offer is be validated, media will flow once that pair is
   validated.  When the ICE checks complete and yield a higher priority
   candidate pair, there will be an updated offer/answer exchange that
   will change the active candidate.  This will result in a change in
   the destination of the media packets.  This may also cause a
   different path for the media packets.  That path might have different



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   delay and jitter characteristics.  As a consequence, the jitter
   buffers may see a glitch, causing possible media artifacts.  If these
   issues are a concern, the initial offer MAY omit an active candidate.
   In such a case, an updated offer will need to be sent immediately
   when communicating with an ICE-unaware agent, setting an active
   candidate.

   There may be transport-specific reasons for selection of an active
   candidate.  In such a case, specifications defining usage of ICE with
   other transport protocols SHOULD document such considerations.

7.3  Encoding Candidates into SDP

   For each candidate for a media stream, the agent includes a series of
   a=candidate attributes as media-level attributes, one for each
   component in the candidate.  Each candidate has a unique identifier,
   called the candidate-id.  The candidate-id MUST be chosen randomly
   and contain at least 128 bits of randomness (this does not mean that
   the candidate-id is 128 bits long; just that it has at least 128 bits
   of randomness).  It is chosen only when the candidate is placed into
   the SDP for the first time; subsequent offers or answers within the
   same session containing that same candidate MUST use the same
   candidate-id used previously.

   Each component of the candidate has an identifier, called the
   component-id.  The component-id is a sequence number.  For each
   candidate, it starts at one, and increments by one for each
   component.  As discussed below, ICE will perform connectivity checks
   such that, between a pair of candidates, checks only occur between
   transport addresses with the same component-id.  As a consequence, if
   one candidate has three components, and it is paired with a candidate
   that has two, there will only be two transport address pairs and two
   connectivity checks.

   ICE will work without a standardized mapping between the components
   of a media stream and the numerical value of the component-id.  This
   allows ICE to be used with media streams with multiple components
   without development of standards around such a mapping.  However, a
   specific mapping has been defined in this specification for RTP -
   component-id 1 corresponds to RTP, and component-id of 2 corresponds
   to RTCP.  Like the candidate-id, the component-id is assigned at the
   time the candidate is first placed into the SDP; subsequent offers or
   answers within the same session containing that same candidate MUST
   use the same component-id used previously.

   The transport, addr and port of the a=candidate attribute (all
   defined in Section 12) are set to the transport protocol, unicast
   address and port of the tranport address.  A Fully Qualified Domain



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   Name (FQDN) for a host MAY be used in place of a unicast address.  In
   that case, when receiving an offer or answer containing an FQDN in an
   a=candidate attribute, the FQDN is looked up in the DNS using an A or
   AAAA record, and the resulting IP address is used for the remainder
   of ICE processing.  The qvalue is set to the priority of the
   candidate, and MUST be the same for all components of the candidate.
   Each transport address also includes a password that will be used for
   securing the STUN connectivity checks.  This password MUST be chosen
   randomly with 128 bits of randomness (though it can be longer than
   128 bits).  Like the candidate-id, it is chosen when the candidate is
   placed into an SDP for the first time for a particular session;
   subsequent offers and answers within the same session conveying the
   same candidate MUST use the same password.  The converse is true; if
   a new offer is generated as part of a new multimedia session, a new
   password (and candidate-id) would be used even if the transport
   address from a previous session was being recycled.

   The combination of candidate-id and component-id uniquely identify
   each transport address.  As a consequence, each transport address has
   a unique identifier, called the tid.  The tid is formed by
   concatenating the candidate-id with the component-id, separated by
   the colon (":").  The tid is not explicitly encoded in the SDP; it is
   derived from the candidate-id and component-id, which are present in
   the SDP.  The usage of the colon as a separator allows the
   candidate-id and component-id to be extracted from the tid, since the
   colon is not a valid character for the candidate-id.

   The tid gets combined, through further concatenation, with the tid of
   a transport address from the remote candidate (separated again by
   another colon) to form the username that is placed in the STUN checks
   between the peers.  This allows the STUN message to uniquely identify
   the pairing whose connectivity it is checking.  The tid is needed as
   a unique identifier because the IP address within the candidate fails
   to provide that uniqueness as a consequence of NAT.

   Consider agents A, B, and C. A and B are within private enterprise 1,
   which is using 10.0.0.0/8.  C is within private enterprise 2, which
   is also using 10.0.0.0/8.  As it turns out, B and C both have IP
   address 10.0.1.1.  A sends an offer to C. C, in its answer, provides
   A with its transport addresses.  In this case, thats 10.0.1.1:8866
   and 8877.  As it turns out, B is in a session at that same time, and
   is also using 10.0.1.1:8866 and 8877.  This means that B is prepared
   to accept STUN messages on those ports, just as C is.  A will send a
   STUN request to 10.0.1.1:8866 and 8877.  However, these do not go to
   C as expected.  Instead, they go to B. If B just replied to them, A
   would believe it has connectivity to C, when in fact it has
   connectivity to a completely different user, B. To fix this, tid
   takes on the role of a unique identifier.  C provides A with an



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   identifier for its transport address, and A provides one to C. A
   concatenates these two identifiers (with a colon between) and uses
   the result as the username in its STUN query to 10.0.1.1:8866.  This
   STUN query arrives at B. However, the username is unknown to B, and
   so the request is rejected.  A treats the rejected STUN request as if
   there were no connectivity to C (which is actually true).  Therefore,
   the error is avoided.

   An unfortunate consequence of the non-uniqueness of IP addresses is
   that, in the above example, B might not even be an ICE agent.  It
   could be any host, and the port to which the STUN packet is directed
   could be any ephemeral port on that host.  If there is an application
   listening on this socket for packets, and it is not prepared to
   handle malformed packets for whatever protocol is in use, the
   operation of that application could be affected.  Fortunately, since
   the ports exchanged in SDP are ephemeral and ususally drawn from the
   dynamic or registered range, the odds are good that the port is not
   used to run a server on host B, but rather is the agent side of some
   protocol.  This decreases the probability of hitting a port in-use,
   due to the transient nature of port usage in this range.  However,
   the possibility of a problem does exist, and network deployers should
   be prepared for it.  Note that this is not a problem specific to ICE;
   stray packets can arrive at a port at any time for any type of
   protocol, especially ones on the public Internet.  As such, this
   requirement is just restating a general design guideline for Internet
   applications - be prepared for unknown packets on any port.

   The active candidate, if there is one, is placed into the m/c lines
   of the SDP.  For RTP streams, this is done by placing the RTP address
   and port into the c and m lines in the SDP respectively.  If the
   agent is utilizing RTCP, it MUST encode its address and port using
   the a=rtcp attribute as defined in RFC 3605 [2].  If RTCP is not in
   use, the agent MUST signal that using b=RS:0 and b=RR:0 as defined in
   RFC 3556 [8].

   If there is no active candidate, the agent MUST include an a=inactive
   attribute.  The RTP address and port in the m/c-line is
   inconsequential, since it won't be used.

   Encoding of candidates may involve transport protocol specific
   considerations.  There are none for UDP.  However, extensions that
   define usage of ICE with other transport protocols SHOULD specify any
   special encoding considerations.

7.4  Forming Candidate Pairs

   Once the offer/answer exchange has completed, both agents will have a
   set of candidates for each media stream.  Each agent forms a set of



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   candidate pairs for each media stream by combining each of its
   candidates with each of the candidates of its peer.  Candidates can
   be paired up only if their transport protocols are identical.  If an
   offer/answer exchange took place for a session comprised of an audio
   and a video stream, and each agent had two candidates per media
   stream, there would be 8 candidate pairs, 4 for audio and 4 for
   video.  One agent can offer two candidates for a media stream, and
   the answer can contain three candidates for the same media stream.
   In that case, there would be six candidate pairs.

   Each candidate has a number of components, each of which has a
   transport address.  Within a candidate pair, the components
   themselves are paired up such that transport addresses with the same
   component ID are combined to form a transport address pair.
   Returning to the previous example, for each of the 8 candidate pairs,
   there would be two transport address pairs - one for RTP, and one for
   RTCP.  If one candidate has more components than the other, those
   extra components will not be part of a transport address pair, won't
   be validated, and will effectively be treated as if they weren't
   included in the candidate pair in the first place.

   The relationship between a candidate, candidate pair, transport
   address, transport address pair and component are shown in Figure 2.
   This figure shows the relationships as seen by the agent that owns
   the candidate with candidate ID "L".  This candidate has two
   components with transport addresses A and B respectively.  This
   candidate is called the native candidate, since it is the one owned
   by the agent in question.  The candidate owned by its peer is called
   the remote candidate.  As the figure shows, there is a single
   candidate pair, and two components in each candidate.  The native
   candidate has a candidate-id of "L", and the remote candidate has a
   candidate-id of "R".  Since the two component-ids are 1 and 2,
   candidate "L" has two transport addresses with transport address IDs
   of "L:1" and "L:2" respectively.  Similarly, candidate "R" has two
   transport addresses with transport address IDs of "R:1" and "R:2"
   respectively.

   Furthermore, each transport address pair is associated with an ID,
   the transport address pair ID.  This ID is equal to the concatenation
   of the tid of the native transport address with the tid of the remote
   transport address, separated by a colon.  This means that the
   identifiers are seen differenly for each agent.  For the agent that
   owns candidate "L", there are two transport address pairs.  One
   contains transport address "L:1" and "R:1", with a transport address
   pair ID of "L:1:R:1".  The other contains transport address "L:2" and
   "R:2", with a transport address pair ID of "L:2:R:2".  For the agent
   that owns candidate "R", the identifiers for these two transport
   address pairs are reversed; it would be "R:1:L:1" for the first one



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   and "R:2:L:2" for the second.


              ...............................................
              .                                             .
              .                                             .
              .  .............               .............  .
              .  .  tid=L:1  .               .  tid=R:1  .  .
              .  .    --     .               .    --     .  . component
     component.  .   | A|------------------------| C|    .  .   id=1
       id=1   .  .    --     .   Transport   .    --     .  .
              .  .           .    Address    .           .  .
              .  .           .     Pair      .           .  .
              .  .           .  id=L:1:R:1   .           .  .
              .  .           .               .           .  .
              .  .           .               .           .  .
              .  .  tid=L:2  .               .  tid=R:2  .  .
    component .  .    --     .               .    --     .  .
      id=2    .  .   | B|------------------------| D|         component
              .  .    --     .   Transport   .    --     .  .   id=2
              .  .           .    Address    .           .  .
              .  .           .     Pair      .           .  .
              .  .           .   id=L:2:R:2  .           .  .
              .  .           .               .           .  .
              .  .............               .............  .
              .     Native                      Remote      .
              .    Candidate                   Candidate    .
              .      id=L                        id=R       .
              .                                             .
              .                                             .
              ...............................................

                              Candidate Pair


                                 Figure 2

   If a candidate pair was created as a consequence of an offer
   generated by an agent, then that agent is said to be the offerer of
   that candidate pair and all of its transport address pairs.
   Similarly, the other agent is said to be the answerer of that
   candidate pair and all of its transport address pairs.  As a
   consequence, each agent has a particular role, either offerer or
   answerer, for each transport address pair.  This role is important;
   when a candidate pair is to be promoted to active, the offerer is the
   one which performs the updated offer.





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7.5  Ordering the Candidate Pairs

   For the same reason that the STUN and TURN allocations are paced at a
   rate of Ta transactions per second, so too are the connectivity
   checks paced, also at a rate of Ta transactions per second.  However,
   in order to rapidly converge on a valid candidate pair that is
   mutually desirable, the candidate pairs are ordered, and the checks
   start with the candidate pair at the top of the list.  Rapid
   convergence of ICE depends on both the offerer and answerer coming to
   the same conclusion on the ordering of candidate pairs.

   Recall that when each candidate is encoded into SDP, it contains a
   qvalue between 1 and 0, with 1 being the highest priority.  Peer-
   derived candidates, learned through the procedures described in
   Section 7.10 also have a priority between 0 and 1.  For each media
   stream, the native candidates are ordered based on their qvalues,
   with higher q-values coming first.  Amongst candidates with the same
   qvalue, they are ordered based on candidate ID, using lexicographic
   order where C1 is placed before C2, if C2 precedes C1.  In other
   words, if the qvalues are the same, the candidates are sorted in
   reverse order.  This is actually important; as discussed in
   Section 13, it allows peer-derived candidates to be preferred over
   native ones.  The result of these two ordering rules will be an
   ordered list of candidates.  The first candidate in this list is
   given a sequence number of 1, the next is given a sequence number of
   2, and so on.  This same procedure is done for the remote candidates.
   The result is that each candidate pair has two sequence numbers, one
   for the native candidate, and one for the remote candidate.

   First, all of the candidate pairs for whom the smaller of the two
   sequence numbers equals 1 are taken first.  Then, all of those for
   whom the smaller of the two sequence numbers equals 2 are taken next,
   and so on.  Amongst those pairs that share the same value for their
   smaller sequence number, they are ordered by the larger of their two
   sequence numbers (smallest first).  Amongst those pairs that share
   the same value for their smaller sequence number and the same value
   for their larger sequence number, the larger of the two candidate IDs
   in each pair are selected, and the pairs are lexicographically
   ordered in reverse by that candidate ID, largest first.

   As an example, consider two agents, A and B. One offers two
   candidates for a media stream with candidate IDs of "g9" and "88",
   with q-values of 1.0 and 0.8 respectively.  The other answers with
   three candidates with candidate IDs of "h8", "65" and "kl", with
   q-values of 0.3, 0.2 and 0.1 respectively.  The following table shows
   the rank ordering of the six candidate pairs.  The column labeled
   "Max SN" is the larger of the two sequence numbers in the candidate
   pair, and "Min SN" is the minimum.  The column labeled "Max Cand.



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   ID" is the value of the larger of the two candidate IDs in the
   candidate pair.



   Order    A     A       A       B     B      B                   Max
          Cand. Cand.    Cand.  Cand. Cand.   Cand.   Max    Min   Cand.
            ID  q-value   SN      ID  q-value  SN     SN     SN     ID
   ---------------------------------------------------------------------
    1      g9    1.0      1       h8    0.3    1       1      1    h8
    2      88    0.8      2       h8    0.3    1       2      1    h8
    3      g9    1.0      1       65    0.2    2       2      1    g9
    4      g9    1.0      1       k1    0.1    3       3      1    k1
    5      88    0.8      2       65    0.2    2       2      2    88
    6      88    0.8      2       k1    0.1    3       3      2    k1

   This ordering is then modified slightly by taking the candidate pair
   corresponding to the active candidate, if there is one, and promoting
   it to the top of the list.  This allows the current active candidate
   to be tested first.  As discussed below, media is not sent until the
   corresponding candidate is verified, necessitating rapid verification
   of the active candidate.  This modified ordering is called the
   candidate pair check ordering, since it reflects the order in which
   connectivity checks will be done.  If there was no active candidate,
   the candidate pair check ordering and the candidate pair priority
   ordering will be identical.

   Within each candidate pair there will be a set of transport address
   pairs, one for each component ID.  Those pairs are ordered by
   component ID.  The result is an absolute ordering of all transport
   address pairs for a media stream, sorted first by the order of their
   candidate pairs (with the exception of the active candidate),
   followed by the order of their component IDs.  This ordering is
   called the transport address pair check ordering.

   Ordering of candidates may involve transport protocol specific
   considerations.  There are none for UDP.  However, extensions that
   define usage of ICE with other transport protocols SHOULD specify any
   special ordering considerations.

7.6  Performing the Connectivity Checks

   Connectivity checks are performed by sending peer-to-peer STUN
   Binding Requests.  These checks result in a candidate progressing
   through a state machine that captures the progress of connectivity
   checks.  The specific state machine and the procedures for the
   connectivity checks are specific to the transport protocol.  This
   specification defines rules for UDP.  Extensions to ICE that describe



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   other transport protocols SHOULD describe the state machine and the
   procedures for connectivity checks.

   The set of states visited by the offerer and answerer are depicted
   graphically in Figure 4


                                         |
                                         |Start
                                         |
                                         |
                                         V
                                   +------------+
                                   |            |
                                   |            |
                                   |  Waiting   |----------------+
                                   |            |                |
                                   |            |                |
                                   +------------+                |
                                         |                       |
                                         | Timer Ta              | Get Req
                                         | --------.             | -------
                                         | Send Req              | Send Res,
                                         V                       | Send Req
                  Get Res          +------------+      Get Req   |
                  -------          |            |      -------   |
                     -             |            |      Send Res  |
                   +---------------| Testing    |-----------+    |
                   |               |            |           |    |
                   |               |            |           |    |
                   |               +------------+           |    |
                   |                     |                  |    |
                   |                     | Error            |    |
                   |                     | -----            |    |
    Timer Tr       |                     |   -              |    |
    --------       V                     V                  V    V
    Send Req +------------+        +------------+        +------------+
       +-----|            |        |            |        |            |
       |     |   Recv-    |        |            |        |   Send-    |
       |     |   Valid    |------->|  Invalid   |<-------|   Valid    |
       |     |            |        |            |        |            |
       +---->|            | Error  |            |  Error |            |
             +------------+ -----  +------------+  ----- +------------+
                   |          -          ^           -         |
                   |                     | Error               |
                   |                     | -----               |
                   |                     |   -                 |
                   |               +------------+              |



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                   |               |            |              |
                   |               |            |              |
                   +-------------->|   Valid    |<-------------+
                      Get Req      |            |     Get Res
                      -------      |            |     -------
                      Send Res     +------------+        -
                                     |       ^
                                     |       |
                                     |       |
                                     +-------+
                                      Timer Tr
                                      --------
                                      Send Req


                                 Figure 4

   The state machine has six states - waiting, testing, Recv-Valid,
   Send-Valid, Valid and Invalid.  Initially, all transport address
   pairs start in the waiting state.  In this state, the agent waits for
   one of two events - a chance to send a Binding Request, or receipt of
   a Binding Request.

   Since there is an instance of the state machine for each transport
   address pair, Binding Requests and responses need to be matched to
   the specific state machine for which they apply.  This is done by
   computing the matching transport address pair for each Binding
   Request.  This is done by examining the USERNAME of the incoming
   Binding Request.  The USERNAME directly contains the transport
   address pair ID.  Requests that are sent by an agent as part of the
   processing described here encode the transport address pair in the
   USERNAME.  Binding Responses are matched to their requests using the
   STUN transaction ID, and then mapped to the transport address pair
   from that.

   Every Ta seconds, the agent starts a new connectivity check for a
   transport address pair.  The check is started for the first transport
   address pair in the transport address pair check ordered list (which
   will be the active candidate) that is in the Waiting state.  The
   state machine for this transport address pair is moved to the Testing
   state, and the agent sends a connectivity check using a STUN Binding
   Request, as outlined in Section 7.7.  Once a STUN connectivity check
   begins, the processing of the check follows the rules for STUN.
   Specifically, retransmits of STUN requests are done as specified in
   RFC 3489, and furthermore, if a transaction fails and needs to be
   retried, that retry can happen rapidly, as described below.  It
   doesn't "count" against the rate limit of 1/Ta checks per second.  In
   addition, the keepalives that are generated for a valid pair do not



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   count against the rate limit either.  The rate limit applies strictly
   to the start of connectivity checks by the answerer for a transport
   address pair that has been newly signaled through an offer/answer
   exchange.

   In addition, if, while in the Waiting state, an agent receives a
   Binding Request matching that transport address pair, and this
   Binding Request generates a successful response, the agent moves into
   the Send-Valid state, and sends a connectivity check of its own using
   a STUN Binding Request, as outlined in Section 7.7.  If the Binding
   Request didn't generate a success response, there is no change in
   state or generation of a Binding Request.

   If, while in the Testing state, the agent receives a successful
   response to its STUN request, it moves into the Recv-Valid state.  In
   this state, the agent knows that packets can flow in both directions.
   However, its peer agent doesn't yet know that; all it knows is that
   it has been able to receive a packet.  Thus, in this state, the agent
   awaits receipt of the Binding Request sent by its peer, as the
   response to that request is what informs its peer that packets can
   flow in both directions.

   If, while in the Send-Valid state, the agent receives a successful
   response to its STUN request, it moves to the Valid state.  In this
   state, the agent knows that packets can flow in each direction.  It
   also knows that its peer has sent it the STUN Request whose response
   will demonstrate to the peer that packets can flow in each direction.

   If, while in the Recv-Valid state, the agent receives a STUN Binding
   Request from its peer that results in a successful response, the
   agent moves into the Valid state.  Receipt of a request whose
   response was not a successful one does not result in a change in
   state.

   In any state, if the STUN transaction results in an error, the state
   machine moves into the invalid state.

   If a transport address pair is in the Recv-Valid or Valid state, an
   agent MUST generate a new STUN Binding Request transaction every Tr
   seconds.  This transaction ensures that NAT bindings for the
   transport address pair remain open while the candidate is under
   consideration.  The transaction is performed as outlined in
   Section 7.7.  These transactions can also be used to keep the
   bindings alive when the candidate is promoted to active, as described
   in Section 7.12.  Tr SHOULD be configurable, and SHOULD default to 15
   seconds.  If the transaction results in an error, the state machine
   moves to the invalid state.  This happens in cases where the NAT
   bindings expire (e.g., due to binding timeouts or NAT failures).



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   The candidate pair itself has a state, which is derived from the
   states of its transport address pairs.  If at least one of the
   transport address pairs in a candidate pair is in the invalid state,
   the state of the candidate pair is considered to be invalid.  If the
   candidate pair enters this state, an agent SHOULD move the state
   machines for all of the other transport address pairs in this
   candidate pair into the invalid state as well.  This will ensure that
   connectivity checks never start for those transport address pairs.
   Furthermore, if checks are already in progress for one of those
   transport address pairs, the agent SHOULD cease them.

   If all of the transport address pairs making up the candidate pair
   are Valid, the candidate pair is considered valid.  If all of the
   transport address pairs making up the candidate pair are either Valid
   or Recv-Valid, and at least one is Recv-Valid, the candidate pair is
   considered to be Recv-Valid.  If all of the transport address pairs
   making up the candidate pair are either Valid or Send-Valid, and at
   least one is Send-Valid, the candidate pair is considered to be Send-
   Valid.  If all of the transport address pairs in a candidate pair are
   in the Waiting state, the candidate pair is in the waiting state.  If
   all of the transport address pairs in the candidate pair are either
   in the Waiting or Testing states, and at least one is in the Testing
   state, the state of the candidate pair is Testing.  Otherwise, the
   state of the candidate pair is considered Indeterminate.

   A candidate itself also has a state.  If a candidate is present in at
   least one valid candidate pair, that candidate is said to be valid.
   If all of the candidate pairs containing that candidate are invalid,
   the candidate itself is invalid.  Otherwise, the candidate's state is
   Indeterminate.

   If a native candidate becomes valid, and is more preferred than the
   active one, the offerer sends an updated offer with this newly
   validated candidate promoted to the m/c-line.  This process is
   discussed in more detail in Section 7.9.

7.7  Sending a Binding Request for Connectivity Checks

   An agent performs a Binding Request transaction by sending a STUN
   Binding Request from its native transport address, and sending it to
   the remote transport address.  The meaning of "sending from its
   native transport address" depends on the type of transport protocol
   and the type of transport address (local, STUN-derived, TURN-derived,
   or peer-derived).  This specification defines the meaning for UDP.
   Specifications defining other transport protocols must define what
   this means for them.

   For UDP-based local transport addresses, sending from the local



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   transport address has the meaning one would expect - the request is
   sent such that the source IP address and port For STUN derived UDP
   transport addresses, it is sent by sending from the local transport
   address used to derive that STUN address.  For TURN derived UDP
   transport addresses, it is sent by using TURN mechanisms to send the
   request through the TURN server (using the SEND primitive).  Sending
   the request through the TURN server neccesarily requires that the
   request be sent from the client, using the local transport address
   used to derive the TURN transport address.

   The Binding Request sent by the agent MUST contain the USERNAME
   attribute.  This attribute MUST be set to the transport address pair
   ID of the corresponding transport address pair as seen by its peer.
   Thus, for the first transport address pair in Figure 2, if the agent
   on the left sends the STUN Binding Request, the USERNAME will have
   the value R:1:L:1.  If the agent on the right sends the STUN Binding
   Request, the USERNAME will have the value L:1:R:1.  To be clear, the
   USERNAME that is used is NOT the one seen locally, but rather the one
   as seen by its peer.  The request SHOULD contain the MESSAGE-
   INTEGRITY attribute, computed according to RFC 3489 procedures.  The
   key used as input to the HMAC is the password provided by the peer
   for this remote transport address.  The Binding Request MUST NOT
   contain the CHANGE-REQUEST or RESPONSE-ADDRESS attribute.

   The STUN transaction will generate either a timeout, or a response.
   If the response is a 420, 500, or 401, the agent should try again as
   described in RFC 3489 (as mentioned above, it need not wait Ta
   seconds to try again).  Either initially, or after such a retry, the
   STUN transaction might produce a non-recoverable failure response
   (error codes 400, 430, 431, or 600) or a failure result inapplicable
   to this usage of STUN and thus unrecoverable (432, 433).  If this
   happens, an error event is generated into the state machine, and the
   transport address pair enters the invalid state.

   If the STUN transaction times out, the client SHOULD NOT retry.  The
   only reason a retry might succeed is if there was severe packet loss
   during the duration of the check, or the answer was significantly
   delayed, also due to packet loss.  However, STUN Binding Request
   transactions run for 9.5 seconds, which is well beyond the typical
   tolerance for a session establishment.  The retries come with a
   penalty of additional traffic, which can be used to launch DoS
   attacks Section 13.4.2.  The only reason to not follow the SHOULD NOT
   is if the agent has adjusted the STUN transaction timers to be more
   aggressive.

   If the Binding Response is a 200, the agent SHOULD check for the
   MESSAGE-INTEGRITY attribute and verify it, as discussed in RFC 3489.
   Indeed, this check SHOULD be done for all responses.  This will



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   result in the response being discarded (eventually leading to a
   timeout), if the integrity check fails.

7.8  Receiving a Binding Request for Connectivity Checks

   As a result of providing a list of candidates in its offer or answer,
   an agent will receive STUN Binding Request messages.  An agent MUST
   be prepared to receive STUN Binding Requests on each local transport
   address from the moment it sends an offer or answer that contains a
   candidate with that local transport address.  Similarly, it MUST be
   prepared to receive STUN Binding Requests on a local transport
   address the moment it sends an offer or answer that contains a STUN
   or TURN candidate derived from a local candidate containing that
   local transport address.  It can cease listening for STUN messages on
   that local transport address after sending an updated offer or answer
   which does not include any candidates with transport addresses that
   are equal to or derived from that local transport address.

   The agent does not need to provide STUN service on any other IP
   addresses or ports, unlike the STUN usage described in [1].  The need
   to run the service on multiple ports is to support receipt of Binding
   Requests with the CHANGE-REQUEST attribute.  However, that attribute
   is not used when STUN is used for connectivity checks.  A server
   SHOULD reject, with a 400 answer, any STUN requests with a CHANGE-
   REQUEST attribute whose value is non-zero.  The CHANGED-ADDRESS
   attribute in a BindingAnswer is set to the transport address on which
   the server is running.

   Furthermore, there is no need to support TLS or to be prepared to
   receive SharedSecret request messages.  Those messages are used to
   obtain shared secrets to be used with BindingRequests.  However, with
   ICE, these shared secrets are exchanged through the offer/answer
   exchange itself.

   One of the candidates may be in use as the active candidate.  For the
   transport addresses comprising that candidate, the agent will receive
   both STUN requests and media packets on its associated local
   transport addresses.  The agent MUST be able to disambiguate them.
   In the case of RTP/RTCP, this disambiguation is easy.  RTP and RTCP
   packets start with the bits 0b10 (v=2).  The first two bits in STUN
   are always 0b00.  This disambiguation also works for packets sent
   using Secure RTP [25], since the RTP header is in the clear.
   Disambiguating STUN with other media stream protocols may be more
   complicated.  However, it can always be possible with arbitrarily
   high probabilities by selecting an appropriately random username (see
   below).

   Processing of the Binding Request proceeds in two steps.  The first



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   is generation of the response, and the second is side-effect
   processing.  Generation of the response follows the general
   procedures of RFC 3489.  The USERNAME is considered valid if its
   topmost portion (the part up to, but not including the second colon)
   corresponds to a transport address ID known to the agent.  The
   password associated with that transport address ID is used to verify
   the MESSAGE-INTEGRITY attribute, if one was present in the request.
   If the USERNAME was not valid, the agent generates a 430.  Otherwise,
   the success response will include the MAPPED-ADDRESS attribute, which
   is used for learning new candidates, as described in Section 7.10.
   The MAPPED-ADDRESS attribute is populated with the source IP address
   and port of the Binding Request.  For Binding Requests received over
   TURN-derived transport addresses, this MUST be the source IP address
   and port of the Binding Request when it arrived at the TURN relay,
   prior to forwarding towards the agent.  That source transport address
   will be present in the REMOTE-ADDRESS attribute of a TURN Data
   Indication message, if the Binding Request were delivered through a
   Data Indication.  If the Binding Request was not encapsulated in a
   Data Indication, that source address is equal to the current active
   destination for the TURN session.

   The side effect processing involves changes to the state machine for
   a transport address pair.  This processing cannot be done until the
   initial offer/answer exchange has completed.  As a consequence, if
   the answerer received a Binding Request that generated a success
   response, but had not yet received the answer to its offer, it waits
   for the answer, and when it arrives, then performs the side effect
   processing.

   The agent takes the entire contents of the USERNAME, and compares
   them against the transport address pair identifiers as seen by that
   agent for each transport address pair.  If there is no match, nothing
   is done - this should never happen for compliant implementations.  If
   there is a match, the resulting transport address pair is called the
   matching transport address pair.  The state machine for the matching
   transport address pair is then updated based on the receipt of a STUN
   Binding Request, and the resulting actions described in Section 7.6
   are undertaken.

   An agent will continue to receive periodic STUN transactions on a
   local transport address as long as it had listed that transport
   address, or one derived from it, in an a=candidate attribute in its
   most recent offer or answer, and the state machine indicates that
   Binding Requests are periodically sent (as is the case for UDP).  It
   MUST process any such transactions according to this section.  It is
   possible that a transport address pair that was previously valid may
   become invalidated as a result of a subsequent failed STUN
   transaction.



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7.9  Promoting a Candidate to Active

   As a consequence of the connectivity checks, each agent will change
   the states for each transport address pair, and consequently, for the
   candidate pairs.  When a candidate pair becomes valid, and the agent
   is in the role of offerer for that candidate pair, the agent follows
   the logic in this section.  The rules only apply to the offerer of a
   candidate pair in order to eliminate the possibility of both agents
   simultaneously offering an update to promote a candidate to active.

   If this candidate pair is the first one in the candidate pair
   priority ordered list, the agent SHOULD send an updated offer as
   described in Section 7.11.1.  If this candidate pair is not the first
   on that list, but it is the first on the candidate pair check ordered
   list, it means that this candidate pair is the active one, and its
   connectivity has been verified.  This is good news; the currently
   active candidate is working.  Media can now flow as described in
   Section 7.13 (media will never flow prior to validation).  However,
   no updated offer is sent at this time.

   If this candidate pair is not the first on the candidate pair
   priority ordered list or the candidate pair check ordered list, and
   the wait-state timer has not yet been set, the agent sets this timer
   to Tws seconds.  Tws SHOULD be configurable, and SHOULD have a
   default of 100ms.  This timer allows for a higher priority
   connectivity check to complete, in the event its STUN Binding Request
   was lost or delayed in the network.  If, prior to the wait-state
   timer firing, another connectivity check completes and a candidate
   pair is validated, there is no need to reset or cancel the timer.
   Once the timer fires, the agent SHOULD issue an updated offer as
   described in Section 7.11.1.

7.10  Learning New Candidates from Connectivity Checks

   ICE makes use of candidate addresses learned through protocols like
   STUN, as described in Section 7.1.  These addresses are learned when
   STUN requests are sent to configured STUN servers.  However, the
   peer-to-peer STUN connectivity checks can themselves provide
   additional candidates that ICE can make use of.  This happens, for
   example, when two agents are separated by a symmetric NAT.  When the
   agent behind the symmetric NAT sends a Binding Request to the other
   agent (which can have a public address or be behind any type of NAT
   except for symmetric), the symmetric NAT will create a new NAT
   binding for this Binding Request.  Because of the properties of
   symmetric NAT, that binding can be used be the agent on the public
   side of the symmetric NAT to send packets back to the agent behind
   the symmetric NAT.




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   To do this, ICE agents perform additional processing on the receipt
   of STUN Binding Requests and responses, beyond the logic described in
   Section 7.7 and Section 7.8.  This logic is described below.

7.10.1  On Receipt of a Binding Request

   When a STUN Binding Request is received which generates a success
   response, that Binding Request would have been associated with a
   matching transport address pair and corresponding candidate pair.
   The source IP and port of this Binding Request are compared to the IP
   address and port of the remote transport address in the matching
   transport address pair.  Note that, in this case, we are comparing
   actual IP addresses and ports - not tids.  In addition, if the
   Binding Request arrived through a TURN derived transport address, the
   source IP and port of this binding request used for the comparison
   are those in the Binding Request when it arrived at the TURN relay,
   prior to forwarding towards the agent.  That source transport address
   will be present in the REMOTE-ADDRESS attribute of a TURN Data
   Indication message, if the Binding Request were delivered through a
   Data Indication.  If the Binding Request was not encapsulated in a
   Data Indication, that source address is equal to the current active
   destination for the TURN session.

   The comparison of the source IP and port of the Binding Request and
   the IP address and port of the remote transport address in the
   matching transport address pair may not match.  One reason this could
   happen is if there was a NAT between the two agents.  If they do not
   match, the source IP and port of the Binding Request (and again, for
   TURN derived transport address, this refers to the source IP address
   and port of the packet when it arrived at the relay) are compared to
   the IP address and ports across the transport address pairs in *all*
   remote candidates.  If there is still no match, it means that the
   source IP and port might represent another valid remote transport
   address.  Such a transport address is called a peer-derived transport
   address.

   To use it, that address needs to be associated with a candidate
   (called a peer-derived candidate).  In this case, however, the
   candidate isn't signaled through an offer/answer exchange; it is
   constructed dynamically from information in the STUN request.  Like
   all other candidates, the peer-derived candidate has a candidate ID.
   The candidate ID is derived from the candidate IDs of the matching
   candidate pair.  In particular, the candidate ID is constructed by
   concatenating the remote candidate ID with the native candidate ID
   (without the colon).

   On receipt of a STUN Binding Request whose source IP and port don't
   match the transport address in any remote candidate, the agent



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   constructs the candidate ID that represents the peer-derived
   candidate, and checks to see if that candidate exists.  It may
   already exist if it had been constructed as a consequence of a
   previous application of this logic on receipt of a Binding Request
   for a different transport address pair of the same candidate pair.
   If there is not yet a peer derived candidate with that candidate ID,
   the agent creates it, and assigns it the newly computed candidate ID.
   The priority of the peer-derived candidate MUST be set to the
   priority of its generating candidate - the remote candidate in the
   matching transport address pair.  Note that, at this time, the peer
   derived candidate has no transport addresses in it.

   Newly created or not, the agent extracts the component ID from the
   matching transport address pair, and sees if a transport address with
   that same component ID exists in the peer derived candidate.  If not
   (and it shouldn't), the agent adds a transport address to the peer-
   derived candidate.  This transport address is equal to the source IP
   address and port from the incoming STUN Binding Request.  It is
   assigned the component ID equal to the component ID in the matching
   transport address pair.  This transport address will have a tid,
   equal to the concatenation of the candidate ID for this new
   candidate, and the component ID, separated by a colon.

   The peer-derived candidate becomes usable once the number of
   transport addresses in it equals the transport address pair count of
   the candidate pair from which it is derived.  Initially, the peer-
   derived candidate will start with a single transport address.  More
   are added as the connectivity checks for the original candidate pair
   take place.  Once the peer-derived candidate becomes usable, it has
   to be paired up with native candidates.  However, unlike the
   procedures of Section 7.5, which pair up each remote candidate with
   each native candidate, this peer-derived candidate is only paired up
   with the native candidate from the candidate pair from which it was
   derived.  This creates a new candidate pair, and a set of new
   transport address pairs.

   Recall that, for each candidate pair, one agent plays the role of
   offerer, and the other of answerer.  For peer-derived candidates, the
   agent that receives the STUN request and follows the processing in
   this section acts as the answerer.

   Figure 5 provides a pictorial representation of the peer derived
   candidate (the one with id=RL) and its pairing with the native
   candidate with id L. The candidate with ID R is referred to as the
   generating candidate.  The peer-derived candidate is effectively an
   alternate for that generating candidate, but is only paired with a
   specific native candidate.  Note that, for a particular generating
   candidate, there can be many peer derived candidates, up to one for



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   each native candidate.


                 .............               .............
                 .  tid=L:1  .               .  tid=R:1  .
        component.    --     .    id=L:1:R:1 .    --     .component
          id=1   .   | A|------------------------| C|    .  id=1
                 .    -- -------+            .    --     .
                 .           .  |            .           .
                 .           .  |            .           .   Generating
                 .           .  |            .           .   Candidate
                 .           .  |            .           .
                 .           .  |            .           .
                 .  tid=L:2  .  |            .  tid=R:2  .
        component.    --     .  | id=L:2:R:2 .    --     .component
          id=2   .   | B|-------C----------------| D|    .  id=2
                 .    -- -----+ |            .    --     .
                 .           .| |            .           .
                 .           .| |            .           .
                 .           .| |            .           .
                 .           .| |            .           .
                 .............| |            .............
                    Native    | |               Remote
                   Candidate  | |              Candidate
                     id=L     | |                id=R
                              | |
                              | |
                             .| |
                              | |
                              | |
                              | |
                              | |             .............
                              | |             .  tid=RL:1 .
                              | | id=L:1:RL:1 .    --     .component
                              | +-----------------| C|    .  id=1
                              |               .    --     .
                              |               .           .
                              |               .           .   Peer Derived
                              |               .           .   Candidate
                              |               .           .
                              |               .           .
                              |               .  tid=RL:2 .
                              |   id=L:2:RL:2 .    --     .component
                              +-------------------| D|    .  id=2
                                              .    --     .
                                              .           .
                                              .           .
                                              .           .



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                                              .           .
                                              .............
                                                 Remote
                                                Candidate
                                                  id=RL

                                 Figure 5

   The new transport address pairs have a state machine associated with
   them.  The state that is entered, and actions to take as a
   consequence, are specific to the transport protocol.  For UDP, the
   procedures are defined here.  Extensions that define processing for
   other transport protocols SHOULD describe the behavior.

   For UDP, the state machine enters the Send-Valid state.  Effectively,
   the Binding Request just received "counts" as a validation in this
   direction, even though it was formally done for a different candidate
   pair.  In addition, the agent SHOULD generate a Binding Request for
   each transport address in this new candidate pair, as described in
   Section 7.7.  The transport address pairs are inserted into the
   ordered list of pairs based on the ordering described in Section 7.5
   and processing follows the logic described in Section 7.6.

7.10.2  On Receipt of a Binding Response

   The procedures on receipt of a Binding Response are nearly identical
   to those for receipt of a Binding Request as described above.

   When a successful STUN Binding Response is received, it will be
   associated with a matching transport address pair and corresponding
   candidate pair.  This matching is done based on comparison of
   candidate IDs.  The value of the MAPPED-ADDRESS attribute of the
   Binding Response are compared to the IP address and port of the
   native transport address in the matching transport address pair.
   Note that, in this case, we are comparing actual IP addresses and
   ports - not tids.  These may not match if there was a NAT between the
   two agents.  If they do not match, the value of the MAPPED-ADDRESS
   attribute of the Binding Response are compared to the IP address and
   ports across the transport address pairs in *all* native candidates.
   If there is still no match, it means that the MAPPED-ADDRESS might
   represent another valid remote transport address.

   To use it, that address needs to be associated with a candidate.  In
   this case, however, the candidate isn't signaled through an offer/
   answer exchange; it is constructed dynamically from information in
   the STUN response.  Such a candidate is called a peer-derived
   candidate.  Like all other candidates, the peer-derived candidate has
   a candidate ID.  The candidate ID is derived from the candidate IDs



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   of the matching candidate pair.  In particular, the candidate ID is
   constructed by concatenating the native candidate ID with the remote
   candidate ID (without the colon).

   On receipt of a STUN Binding Response whose MAPPED-ADDRESS didn't
   match the transport address in any native candidate, the agent
   constructs the candidate ID that represents the peer-derived
   candidate, and checks to see if that candidate exists.  It may
   already exist if it had been constructed as a consequence of a
   previous application of this logic on receipt of a Binding Response
   for a different transport address pair of the same candidate pair.
   If there is not yet a peer derived candidate with that candidate ID,
   the agent creates it, and assigns it the newly computed candidate ID.
   The priority of the new candidate MUST be set to the priority of the
   generating candidate - the native candidate in the matching transport
   address pair.  Note that, at this time, the peer derived candidate
   has no transport addresses in it.

   Newly created or not, the agent extracts the component ID from the
   matching transport address pair, and sees if a transport address with
   that same component ID exists in the peer derived candidate.  If not
   (and it shouldn't), the agent adds a transport address to the peer-
   derived candidate.  This transport address is equal to the MAPPED-
   ADDRESS from the STUN Binding Response.  It is assigned the component
   ID equal to the component ID in the matching transport address pair.
   This transport address will have a tid, equal to the concatenation of
   the candidate ID for this new candidate, and the component ID,
   separated by a colon.

   The peer-derived candidate becomes usable once the number of
   transport addresses in it equals the transport address pair count of
   candidate pair from which it is derived.  Initially, the peer-derived
   candidate will start with a single transport address.  More are added
   as the connectivity checks for the original candidate pair take
   place.  Once the peer-derived candidate becomes usable, it has to be
   paired up with remote candidates.  However, unlike the procedures of
   Section 7.5, which pair up each remote candidate with each native
   candidate, the peer-derived candidate is only paired up with the
   remote candidate from the matching candidate pair .  This creates a
   new candidate pair, and a set of new transport address pairs.

   Recall that, for each candidate pair, one agent plays the role of
   offerer, and the other of answerer.  For peer-derived candidates, the
   agent that receives the STUN request and follows the processing in
   this section acts as the answerer.

   The new transport address pairs have a state machine associated with
   them.  The state that is entered, and actions to take as a



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   consequence, are specific to the transport protocol.  For UDP, the
   procedures are defined here.  Extensions that define processing for
   other transport protocols SHOULD describe the behavior.

   For UDP, the state machine enters the Recv-Valid state.  Effectively,
   the Binding Response just received "counts" as a validation in this
   direction, even though it was formally done for a different candidate
   pair.  The transport address pairs are inserted into the ordered list
   of pairs based on the ordering described in Section 7.5, and
   processing follows the logic described in Section 7.6.

7.11  Subsequent Offer/Answer Exchanges

   An agent MAY issue an updated offer at any time.  This updated offer
   may be sent for reasons having nothing to do with ICE processing (for
   example, the addition of a video stream in a multimedia session), or
   it may be due to a change in ICE-related parameters.  For example, if
   an agent acquires a new candidate after the initial offer/answer
   exchange, it may seek to add it.

   However, agents SHOULD follow the logic described in Section 7.9 to
   determine when to send an updated offer as a consequence of promoting
   a candidate to active.

   If there are any aspects of this processing that are specific to the
   transport protocol, those SHOULD be called out in ICE extensions that
   define operation with other transport protocols.  There are no
   additional considerations for UDP.

7.11.1  Sending of a Subsequent Offer

   The offer MAY contain a new active candidate in the m/c line.  This
   candidate SHOULD be the native candidate from the highest candidate
   pair in the candidate pair priority ordered list whose state is
   valid.  If there are no candidate pairs in this state, the highest
   one whose state is partially valid SHOULD be used.  If there are no
   candidate pairs in this state, the candidate pair that is most likely
   to work with this peer, as described in Section 7.2, SHOULD be used.
   The candidate is encoded into the m/c line in an updated offer as
   described in Section 7.3.

   If the candidate pair whose native candidate was encoded into the
   m/c-line was valid or partially valid, the agent MUST include an
   a=remote-candidate attribute into the offer.  This attribute MUST
   contain the candidate ID of the remote candidate in the candidate
   pair.  It is used by the recipient of the offer in selecting its
   candidate for the answer.




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   The meaning of a=candidate attributes within a subsequent offer have
   the same meaning as they do in an initial offer.  They are a request
   for the peer to attempt (or continue to attempt if the candidate was
   provided previously) a connectivity check using STUN from each of its
   own candidates.  When an updated offer is sent, there are several
   dispositions regarding the candidates:

   retained: A candidate is retained if the candidate ID for the
      candidate is included in the new offer, and matches the candidate
      ID for a candidate in the previous offer or answer.  In this case,
      all of the information about the candidate - its qvalue and
      components, and the IP addresses, ports, STUN passwords and
      transport protocols of its components, MUST be the same as the
      previous offer or answer from the agent.  If the agent wants to
      change them, this is accomplished by changing the candidate ID as
      well.  That will have the effect of removing the old candidate and
      adding a new one with the updated information.

   removed: A candidate is removed if its candidate ID appeared in a
      previous offer or answer, and that candidate ID is not present in
      the new offer.

   added: A candidate is added if its candidate ID appeared in the new
      offer, but was not present in a previous offer or answer from that
      agent.

   The following rules are used to determine the disposition of the each
   of the current native candidates in the new offer:

   o  If a candidate is invalid, and all peer-derived candidates
      generated from it are invalid as well, it SHOULD be removed.

   o  If the candidate in the m/c-line is valid, all other candidates
      SHOULD be removed.  This has the effect of stopping connectivity
      checks of other candidates.  This SHOULD would not be followed if
      an agent wanted to keep a candidate ready for usage should, for
      some reason, the active candidate later become invalid.

   o  If the candidate in the m/c-line is valid, and it is not peer-
      derived, that candidate MUST be retained.  If the candidate in the
      m/c-line is peer-derived, its generating candidate MUST be
      retained, even if it is itself invalid.

   o  If the candidate in the m/c-line has not been validated, all other
      candidates that are not invalid, or candidates for whom their
      derived candidates are not invalid, SHOULD be retained.





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   o  Peer derived candidates MUST NOT be added; they continue to be
      used as long as their generating candidate was retained.  Peer
      derived candidates are learned exclusively through the STUN
      connectivity checks.

   A new candidate MAY be added.  This can happen when the candidate is
   a new one, learned since the previous offer/answer exchange, and it
   has a higher priority than the currently active candidate.  It can
   also occur when an agent wishes to restart checks for a transport
   address it had tried previously.  Effectively, changing the candidate
   ID value in an updated offer will "restart" connectivity checks for
   that candidate.

   If a candidate is removed, the agent takes the following steps:

   1.  The agent eliminates any candidate pairs whose native candidate
       equalled the candidate that was removed.  Equality is based on
       comparison of candidate IDs.

   2.  The agent eliminates any candidate pairs that had a native
       candidate that is a peer derived candidate generated from the
       candidate that was removed.

   3.  The candidate pairs that are eliminated are removed from the
       candidate pair priority ordered list and candidate pair check
       ordered list.  As a consequence of this, if connectivity checks
       had not yet begun for the candidate pair, they won't.

   4.  If connectivity checks were already in progress for transport
       addresses in that candidate pair, the agent SHOULD immediately
       terminate them.  No further retransmissions take place, and no
       further transactions from that candidate will be made.

   5.  If the removed candidate was a TURN-derived candidate, the agent
       SHOULD de-allocate its transport addresses from the TURN server.
       If a local candidate was removed, and all of its derived
       candidates were also removed (including any peer-derived
       candidates), local operating system resources for each of the
       transport addresses in the local candidate SHOULD be de-
       allocated.


7.11.2  Receiving the Offer and Sending an Answer

   To generate the answer, the answerer has to decide which transport
   addresses to include in the m/c line, and which to include in
   candidate attributes.




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   Rules for choosing transport addresses for the m/c-line are as
   follows.  The agent examines the transport addresses in the m/c-line
   of the offer.  It compares these with the transport addresses in the
   remote candidates of candidate pairs whose states are Valid.  If
   there is matching candidate pair in that state, the agent MUST pick
   the native candidate from one of those pairs, and use that candidate
   as the active one.  If none of the matching pairs are in the Valid
   state, the agent checks if there are any matching pairs in the Send-
   Valid state.  If there are, the agent looks for the a=remote-
   candidate attribute in the offer.  If present, and the candidate ID
   listed there is one of the native candidate IDs amongst the matching
   pairs, that candidate ID MUST be used as the active one.  If the
   a=remote-candidate attribute was not present in the offer, or there
   were no matching candidate pairs in the Send-Valid state, the
   candidate that is most likely to work with this peer, as described in
   Section 7.2, SHOULD be used.

   The a=remote-candidate exists to eliminate a race condition between
   the updated offer and the response to the STUN Binding Request that
   moved a candidate into the valid state.  If the answer arrives at the
   agent prior to the Binding Response, the candidate pair that was
   validated by the offer will still be in the Send-Valid state.  To
   eliminate this condition, the identity of the validated candidate is
   included in the offer itself.

   Like the offerer, the answer can decide, for each of its candidates,
   whether they are retained or removed.  The same rules defined in
   Section 7.11.1 for determining their disposition apply to the
   answerer.  Similarly, if a candidate is removed, the same rules in
   Section 7.11.1 regarding removal of canididate pairs and freeing of
   resources apply.

   Once the answer is sent, the answerer will have the set of native and
   remote candidates before this offer/answer exchange, and the set of
   native and remote candidates afterwards.  The agent then pairs up the
   native and remote candidates which were added or retained.
   Furthermore, for candidate pairs containing a peer derived transport
   address, those pairs continue as long as both candidates are
   retained.  A peer derived candidate continues to be used as long as
   its generating parent continues to be used.  This leads to a set of
   current candidate pairs.

   If a candidate pair existed previously, but as a consequence of the
   offer/answer exchange, either its native or remote candidate has been
   removed, the agent takes the following steps:

   1.  The candidate pair is removed from the candidate pair priority
       ordered list and candidate pair check ordered list.  As a



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       consequence of this, if connectivity checks had not yet begun for
       the candidate pair, they won't.

   2.  If connectivity checks were already in progress for that
       candidate pair, the agent SHOULD immediately terminate any STUN
       transactions in progress from that candidate.  No further
       retransmissions take place, and no further transactions from that
       candidate will be made.

   3.  If the agent receives a STUN Binding Request for that candidate
       pair, the agent SHOULD generate a 430 response.

   If a candidate pair existed previously, and continues to exist, no
   changes are made; any STUN transactions in progress for that
   candidate pair continue, and it remains on the candidate pair
   priority ordered list and candidate pair check ordered list.

   If a candidate pair is new (because either its native candidate is
   new, or its remote candidate is new, or both), the agent takes the
   role of answerer for this candidate pair.  The new candidate pair is
   inserted into the candidate pair priority ordered list and candidate
   pair check ordered list.  STUN connectivity checks will start for
   them based on the logic described in Section 7.6.

7.11.3  Receiving the Answer

   Once the answer is received, the answerer will have the set of native
   and remote candidates before this offer/answer exchange, and the set
   of native and remote candidates afterwards.  It then follows the same
   logic described in Section 7.11.2, pairing up the candidate pairs,
   removing ones that are no longer in use, and beginning of processing
   for ones that are new.

7.12  Binding Keepalives

   Once a candidate is promoted to active, and media begins flowing, it
   is still necessary to keep the bindings alive at intermediate NATs
   for the duration of the session.  Normally, the media stream packets
   themselves (e.g., RTP) meet this objective.  However, several cases
   merit further discussion.  Firstly, in some RTP usages, such as SIP,
   the media streams can be "put on hold".  This is accomplished by
   using the SDP "sendonly" or "inactive" attributes, as defined in RFC
   3264 [5].  RFC 3264 directs implementations to cease transmission of
   media in these cases.  However, doing so may cause NAT bindings to
   timeout, and media won't be able to come off hold.

   Secondly, some RTP payload formats, such as the payload format for
   text conversation [34], may send packets so infrequently that the



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   interval exceeds the NAT binding timeouts.

   Thirdly, if silence suppression is in use, long periods of silence
   may cause media transmission to cease sufficiently long for NAT
   bindings to time out.

   To prevent these problems, ICE implementations MUST continue to list
   their active transport addresses in a=candidate lines for UDP-based
   media streams.  As a consequence of this, STUN packets will be
   transmitted periodically independently of the transmission (or lack
   thereof) of media packets.  This provides a media independent, RTP
   independent, and codec independent solution for keeping the NAT
   bindings alive.  STUN Binding Requests cannot be used for TCP-based
   transports because the media protocol may not provide framing
   services to support this.  As such, application layer keepalives MUST
   be used in this case.

   If an ICE implementation is communciating with one that does not
   support ICE, keepalives MUST still be sent.  Indeed, these keepalives
   are essential even if neither endpoint implements ICE.  As such, this
   specification defines keepalive behavior generally, for endpoints
   that support ICE, and those that do not.

   All endpoints MUST send keepalives for each media session.  These
   keepalives MUST be sent regardless of whether the media stream is
   currently inactive, sendonly, recvonly or sendrecv.  The keepalive
   SHOULD be sent using a format which is supported by its peer.  ICE
   endpoints allow for STUN-based keepalives for UDP streams, and as
   such, STUN keepalives MUST be used when an agent is communicating
   with a peer that supports ICE.  An agent can determine that its peer
   supports ICE by the presence of the a=candidate attributes for each
   media session.  If the peer does not support ICE, the choice of a
   packet format for keepalives is a matter of local implementation.  A
   format which allows packets to easily be sent in the absence of
   actual media content is RECOMMENDED.  Examples of formats which
   readily meet this goal are RTP No-Op [29] and RTP comfort noise [27].

   STUN-based keepalives will be sent periodically every Tr seconds as a
   consequence of the rules in in Section 7.7.  If STUN keepalives are
   not in use (because the peer does not support ICE or because of TCP),
   an agent SHOULD ensure that a media packet is sent every Tr seconds.
   If one is not sent as a consequence of normal media communications, a
   keepalive packet using one of the formats discussed above SHOULD be
   sent.

7.13  Sending Media

   An agent MUST NOT send media packets until the active candidate has



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   entered either the Valid or Recv-Valid state.  This is to prevent a
   particularly destructive denial-of-service attack described in
   Section 13.4.1.

   It is important to note that an agent always sends media to the
   address in the m/c-line, not to a validated candidate.  To use a
   candidate, it must be promoted to the m/c-line through an updated
   offer/answer exchange.

   When an agent sends media packets, it MUST send them from the same IP
   address and port it has advertised in the m/c-line.  This provides a
   property known as symmetry, which is an essential facet of NAT
   traversal.

   In the case of a STUN-derived transport address, this means that the
   RTP packets are sent from the local transport address used to obtain
   the STUN address.  In the case of a TURN-derived transport address,
   this means that media packets are sent through the TURN server (using
   the TURN SEND primitive).  For local transport addresses, media is
   sent from that local transport address.

   This symmetric behavior MUST be followed by an agent even if its peer
   in the session doesn't support ICE.

8.  Guidelines for Usage with SIP

   SIP [3] makes use of the offer/answer model, and is one of the
   primary targets for usage of ICE.  SIP allows for offer/answer
   exchanges to occur in many different combinations of messages,
   including INVITE/200 OK and 200 OK/ACK.  When support for reliable
   provisional responses (RFC 3262 [13]) and UPDATE (RFC 3311 [28]) are
   added, additional combinations of messages that can be used for
   offer/answer exchanges are added.  As such, this section provides
   some guidance on good ways to make use of SIP with ICE.

   ICE requires a series of STUN-based connectivity checks to take place
   between endpoints, along with an updated offer/answer exchange to use
   a validated candidate.  These exchanges require time to complete.  If
   the initial offer/answer exchange were to take place in the INVITE
   and 200 OK response respectively, the connectivity checks and updated
   offer would all occur after the called party answered.  This will
   result in a potential increase in the post-pickup delay.  This delay
   refers to the time between when a user "answers the phone" and when
   any speech they utter can be delivered to the caller.

   To eliminate any increase in post-pickup delay due to ICE, it is
   RECOMMENDED that the initial offer/answer exchange take place in an
   INVITE and a 18x provisional response.  As a consequence, support for



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   RFC 3262 is RECOMMENDED with ICE.  The STUN connectivity checks will
   then take place while the called party is being "rung".  To deliver
   the updated offer prior to the user answering the call, it is
   RECOMMENDED that it be delivered with an UPDATE request.  This will
   allow ICE to have completed prior to the called party even answering
   the session invitation.

   If RFC 3262 and RFC 3311 are not supported by both agents, tuning can
   still take place to reduce post-pickup delays.  In particular, the
   answerer SHOULD include its answer in an unreliable 18x response.
   RFC 3261 requires that the same answer also be placed in a 200 OK,
   which is delivered reliably.  However, placing it in a 18x gives the
   offerer an early preview of the answer, and allows the connectivity
   checks to all occur prior to the user answering the call.  However,
   the updated offer with the highest priority valid candidate promoted
   to the m/c-line cannot occur until after the 200 OK, in which case it
   SHOULD be done with a re-INVITE.  Fortunately, if the active
   candidates in the initial offer/answer exchange end up being valid
   anyway, media can flow as soon as the user answers the call (or even
   before hand, if early media is needed).  The additional offer/answer
   exchange in the re-INVITE would merely improve the situation by using
   a higher priority candidate pair.

   One of the difficulties in including the answer in the 18x, and then
   using it for connectivity checks, is that the 18x might be lost.  In
   such a case, the STUN connectivity check from the answerer to the
   offerer (UAS to UAC) will pend indefinitely.  To prevent this, it is
   RECOMMENDED that a SIP UA retransmit its 18x periodically, using the
   same exponential backoff defined in RFC 3262, until such time as a
   Binding Response is received for any of the Binding Requests it sent.

   As discussed in Section 13, offer/answer exchanges SHOULD be secured
   against eavesdropping and man-in-the-middle attacks.  To do that, the
   usage of SIPS is RECOMMENDED when used in concert with ICE.

9.  Interactions with Forking

   SIP allows INVITE requests carrying offers to fork, which means that
   they are delivered to multiple user agents.  Each of those user
   agents then provides an answer to the offer in the INVITE.  The
   result is that a single offer generated by the UAC produces multiple
   answers.

   ICE interacts very well with forking.  Indeed, ICE fixes some of the
   problems associated with forking.  Once the offer/answer exchange has
   completed, the UAC will have an answer from each UAS that received
   the INVITE.  The ICE connectivity checks that ensue will carry
   transport address pair IDs that correlate each of those checks (and



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   thus their corresponding IP addresses and ports) with a specific
   remote user agent.  As these checks happen before any media is
   transmitted, ICE allows a UAC to disambiguate subsequent media
   traffic by looking at the source IP address and port, and then
   correlate that traffic with a particular remote UA.  When SIP is used
   without ICE, the incoming media traffic cannot be disambiguated
   without an additional offer/answer exchange.

10.  Interactions with Preconditions

   Because ICE involves multiple addresses and pre-session activities,
   its interactions with preconditions merits further discussion.

   Quality of Service (QoS) preconditions, which are defined in RFC 3312
   [9] and RFC 4032 [10], apply only to the IP addresses and ports
   listed in the m/c lines in an offer/answer.  If ICE changes the
   address and port where media is received, this change is reflected in
   the m/c lines of a new offer/answer.  As such, it appears like any
   other re-INVITE would, and is fully treated in RFC 3312 and 4032,
   which applies without regard to the fact that the m/c lines are
   changing due to ICE negotiations ocurring "in the background".

   ICE also has (purposeful) interactions with connectivity
   preconditions [14].  Those interactions are described there.

11.  Example

   This section provides an example ICE call flow.  Two agents, L and R,
   are using ICE.  Both agents have a single IPv4 interface, and are
   configured with a single TURN and single STUN server each (indeed,
   the same one for each).  As a consequence, each agent will end up
   with three candidates - a local candidate, a TURN-derived candidate,
   and a STUN-derived candidate.  The agents are seeking to communicate
   using a single RTP-based voice stream.  As a consequence, each
   candidate has two components - one for RTP and one for RTCP.  Agent L
   is behind a symmetric NAT, and agent R is on the public Internet.

   To facilitate understanding, transport addresses are listed in a
   mnemonic form.  This form is <entity&rt;-<type&rt;-<seq-no&rt;, where
   <entity&rt; refers to the entity whose interface the transport
   address is on, and is one of "L", "R", "STUN", "TURN", or "NAT".  The
   <type&rt; is either "PUB" for transport addresses that are public,
   and "PRIV" for transport addresses that are private.  Finally, <seq-
   no&rt; is a sequence number that is different for each transport
   address of the same type on a particular entity.

   The STUN server has advertised transport address STUN-PUB-1 for STUN
   requests, and the TURN server has advertised transport address TURN-



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   PUB-1 for TURN allocations.

   In addition, candidate IDs are also listed in mnemonic form.  Agent L
   uses candidate ID L1 for its local candidate, L2 for its STUN derived
   candidate, and L3 for its TURN derived candidate.  Agent R uses R1
   for its local candidate and R2 for its TURN derived candidate.  The
   passwords for each transport address are LPASS1 through LPASS6 for
   agent L, and RPASS1 through RPASS4 for agent R.

   In example SDP messages, $<token&rt;.IP is used to refer to the value
   of the IP address of the transport address with mnemonic name
   "token".  Similarly, $<token&rt;.PORT is used to refer to the value
   of the port of the transport address with mnemonic name "token".

   In the call flow example in Figure 6, STUN and TURN messages are
   annotated with several attributes.  The "S=" attribute indicates the
   source transport address of the message.  The "D=" attribute
   indicates the destination transport address of the message.  The
   "MA=" attribute is used in STUN Binding Response messages, or STUN
   Binding Response messages carried in a TURN SEND or TURN DATA
   message, and refers to the value of the MAPPED-ADDRESS attribute in
   the STUN Binding Response.  The "RA=" attribute is used in TURN DATA
   messages, and refers to the value of the REMOTE-ADDRESS attribute.
   The "U=" attribute is used in STUN Binding Requests, and corresponds
   to the STUN USERNAME.  Finally, the "DA=" attribute is used in TURN
   SEND messages, and refers to the value of the DESTINATION-ADDRESS
   attribute.

   The call flow example omits STUN authentication operations.


             L             NAT           STUN             R
             |              |              |              |
             |              |              |              |
             |              |              |              |
             |RTP STUN alloc.              |              |
             |              |              |              |
             |              |              |              |
             |              |              |              |
             |(1) STUN Req  |              |              |
             |S=L-PRIV-1    |              |              |
             |D=STUN-PUB-1  |              |              |
             |------------->|              |              |
             |              |              |              |
             |              |              |              |
             |              |(2) STUN Req  |              |
             |              |S=NAT-PUB-1   |              |
             |              |D=STUN-PUB-1  |              |



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             |              |------------->|              |
             |              |              |              |
             |              |(3) STUN Res  |              |
             |              |S=STUN-PUB-1  |              |
             |              |D=NAT-PUB-1   |              |
             |              |MA=NAT-PUB-1  |              |
             |              |<-------------|              |
             |              |              |              |
             |(4) STUN Res  |              |              |
             |S=STUN-PUB-1  |              |              |
             |D=L-PRIV-1    |              |              |
             |MA=NAT-PUB-1  |              |              |
             |<-------------|              |              |
             |              |              |              |
             |              |              |              |
             |              |              |              |
             |RTCP STUN alloc.             |              |
             |Ta secs. later|              |              |
             |              |              |              |
             |              |              |              |
             |              |              |              |
             |(5) STUN Req  |              |              |
             |S=L-PRIV-2    |              |              |
             |D=STUN-PUB-1  |              |              |
             |------------->|              |              |
             |              |              |              |
             |              |              |              |
             |              |(6) STUN Req  |              |
             |              |S=NAT-PUB-2   |              |
             |              |D=STUN-PUB-1  |              |
             |              |------------->|              |
             |              |              |              |
             |              |(7) STUN Res  |              |
             |              |S=STUN-PUB-1  |              |
             |              |D=NAT-PUB-2   |              |
             |              |MA=NAT-PUB-2  |              |
             |              |<-------------|              |
             |              |              |              |
             |(8) STUN Res  |              |              |
             |S=STUN-PUB-1  |              |              |
             |D=L-PRIV-2    |              |              |
             |MA=NAT-PUB-2  |              |              |
             |<-------------|              |              |
             |              |              |              |
             |              |              |              |
             |              |              |              |
             |RTP TURN alloc.              |              |
             |Ta secs. later|              |              |



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             |              |              |              |
             |              |              |              |
             |              |              |              |
             |(9) TURN Req  |              |              |
             |S=L-PRIV-1    |              |              |
             |D=TURN-PUB-1  |              |              |
             |------------->|              |              |
             |              |              |              |
             |              |              |              |
             |              |(10) TURN Req |              |
             |              |S=NAT-PUB-3   |              |
             |              |D=TURN-PUB-1  |              |
             |              |------------->|              |
             |              |              |              |
             |              |(11) TURN Res |              |
             |              |S=TURN-PUB-1  |              |
             |              |D=NAT-PUB-3   |              |
             |              |MA=TURN-PUB-2 |              |
             |              |<-------------|              |
             |              |              |              |
             |(12) TURN Res |              |              |
             |S=TURN-PUB-1  |              |              |
             |D=L-PRIV-1    |              |              |
             |MA=TURN-PUB-2 |              |              |
             |<-------------|              |              |
             |              |              |              |
             |              |              |              |
             |              |              |              |
             |RTCP TURN alloc.             |              |
             |Ta secs. later|              |              |
             |              |              |              |
             |              |              |              |
             |              |              |              |
             |(13) TURN Req |              |              |
             |S=L-PRIV-2    |              |              |
             |D=TURN-PUB-1  |              |              |
             |------------->|              |              |
             |              |              |              |
             |              |              |              |
             |              |(14) TURN Req |              |
             |              |S=NAT-PUB-4   |              |
             |              |D=TURN-PUB-1  |              |
             |              |------------->|              |
             |              |              |              |
             |              |(15) TURN Res |              |
             |              |S=TURN-PUB-1  |              |
             |              |D=NAT-PUB-4   |              |
             |              |MA=TURN-PUB-3 |              |



Rosenberg                Expires April 22, 2006                [Page 48]

Internet-Draft                     ICE                      October 2005


             |              |<-------------|              |
             |              |              |              |
             |(16) TURN Res |              |              |
             |S=TURN-PUB-1  |              |              |
             |D=L-PRIV-2    |              |              |
             |MA=TURN-PUB-3 |              |              |
             |<-------------|              |              |
             |              |              |              |
             |              |              |              |
             |              |              |              |
             |              |              |              |
             |(17) Offer    |              |              |
             |------------------------------------------->|
             |              |              |              |
             |              |              |              |
             |              |              |              |
             |              |              |              |
             |              |              |              |RTP STUN alloc.
             |              |              |              |
             |              |              |              |
             |              |              |              |
             |              |              |(18) STUN Req |
             |              |              |S=R-PUB-1     |
             |              |              |D=STUN-PUB-1  |
             |              |              |<-------------|
             |              |              |              |
             |              |              |(19) STUN Res |
             |              |              |S=STUN-PUB-1  |
             |              |              |D=R-PUB-1     |
             |              |              |MA=R-PUB-1    |
             |              |              |------------->|
             |              |              |              |
             |              |              |              |
             |              |              |              |
             |              |              |              |RTCP STUN alloc.
             |              |              |              |Ta secs. later
             |              |              |              |
             |              |              |              |
             |              |              |              |
             |              |              |(20) STUN Req |
             |              |              |S=R-PUB-2     |
             |              |              |D=STUN-PUB-1  |
             |              |              |<-------------|
             |              |              |              |
             |              |              |(21) STUN Res |
             |              |              |S=STUN-PUB-1  |
             |              |              |D=R-PUB-2     |
             |              |              |MA=R-PUB-2    |



Rosenberg                Expires April 22, 2006                [Page 49]

Internet-Draft                     ICE                      October 2005


             |              |              |------------->|
             |              |              |              |
             |              |              |              |
             |              |              |              |
             |              |              |              |RTP TURN alloc.
             |              |              |              |Ta secs. later
             |              |              |              |
             |              |              |              |
             |              |              |              |
             |              |              |(22) TURN Req |
             |              |              |S=R-PUB-1     |
             |              |              |D=TURN-PUB-1  |
             |              |              |<-------------|
             |              |              |              |
             |              |              |(23) TURN Res |
             |              |              |S=TURN-PUB-1  |
             |              |              |D=R-PUB-1     |
             |              |              |MA=TURN-PUB-4 |
             |              |              |------------->|
             |              |              |              |
             |              |              |              |
             |              |              |              |
             |              |              |              |RTCP TURN alloc.
             |              |              |              |Ta secs. later
             |              |              |              |
             |              |              |              |
             |              |              |              |
             |              |              |(24) TURN Req |
             |              |              |S=R-PUB-2     |
             |              |              |D=TURN-PUB-1  |
             |              |              |<-------------|
             |              |              |              |
             |              |              |(25) TURN Res |
             |              |              |S=TURN-PUB-1  |
             |              |              |D=R-PUB-2     |
             |              |              |MA=TURN-PUB-5 |
             |              |              |------------->|
             |              |              |              |
             |              |              |              |
             |              |              |              |
             |              |              |              |
             |(26) answer   |              |              |
             |<-------------------------------------------|
             |              |              |              |
             |              |              |              |
             |              |              |              |
             |              |              |              |Validate
             |              |              |              |TURN-PUB-4 to TURN-PUB-2



Rosenberg                Expires April 22, 2006                [Page 50]

Internet-Draft                     ICE                      October 2005


             |              |              |              |
             |              |              |              |
             |              |              |(27) TURN SEND|
             |              |              |S=R-PUB-1     |
             |              |              |D=TURN-PUB-1  |
             |              |              |DA=TURN-PUB-2 |
             |              |              |<-------------|
             |              |              |              |
             |              |              |STUN Req.     |
             |              |              |S=TURN-PUB-4  |
             |              |              |D=TURN-PUB-2  |
             |              |              |U=L3:1:R2:1   |
             |              |              |              |
             |              |              |              |
             |              |              |              |
             |              |              |              |
             |              |              |              |
             |              |              |Discard       |
             |              |              |              |
             |              |              |              |
             |              |              |              |
             |              |              |              |
             |Validate      |              |              |
             |TURN-PUB-2 to TURN-PUB-4     |              |
             |              |              |              |
             |              |              |              |
             |(28) TURN SEND|              |              |
             |S=L-PRIV-1    |              |              |
             |D=TURN-PUB-1  |              |              |
             |DA=TURN-PUB-4 |              |              |
             |------------->|              |              |
             |              |              |              |
             |              |(29) TURN SEND|              |
             |              |S=NAT-PUB-3   |              |
             |              |D=TURN-PUB-1  |              |
             |              |DA=TURN-PUB-4 |              |
             |              |------------->|              |
             |              |              |              |
             |              |              |STUN Req.     |
             |              |              |S=TURN-PUB-2  |
             |              |              |D=TURN-PUB-4  |
             |              |              |U=R2:1:L3:1   |
             |              |              |              |
             |              |              |              |
             |              |              |(30) TURN DATA|
             |              |              |S=TURN-PUB-1  |
             |              |              |D=R-PUB-1     |
             |              |              |RA=TURN-PUB-2 |



Rosenberg                Expires April 22, 2006                [Page 51]

Internet-Draft                     ICE                      October 2005


             |              |              |------------->|
             |              |              |(31) TURN SEND|
             |              |              |S=R-PUB-1     |
             |              |              |D=TURN-PUB-1  |
             |              |              |DA=TURN-PUB-2 |
             |              |              |MA=TURN-PUB-2 |
             |              |              |<-------------|
             |              |              |              |
             |              |              |STUN Res.     |
             |              |              |S=TURN-PUB-4  |
             |              |              |D=TURN-PUB-2  |
             |              |              |MA=TURN-PUB-2 |
             |              |              |              |
             |              |(32) TURN DATA|              |
             |              |S=TURN-PUB-1  |              |
             |              |D=NAT-PUB-3   |              |
             |              |RA=TURN-PUB-4 |              |
             |              |MA=TURN-PUB-2 |              |
             |              |<-------------|              |
             |(33) TURN DATA|              |              |
             |S=TURN-PUB-1  |              |              |
             |D=L-PRIV-1    |              |              |
             |RA=TURN-PUB-4 |              |              |
             |MA=TURN-PUB-2 |              |              |
             |<-------------|              |              |
             |              |              |              |
             |              |              |              |
             |              |              |              |
             |              |              |              |Validate
             |              |              |              |TURN-PUB-4 to TURN-PUB-2
             |              |              |              |
             |              |              |              |
             |              |              |(34) TURN SEND|
             |              |              |S=R-PUB-1     |
             |              |              |D=TURN-PUB-1  |
             |              |              |DA=TURN-PUB-2 |
             |              |              |<-------------|
             |              |              |              |
             |              |              |STUN Req.     |
             |              |              |S=TURN-PUB-4  |
             |              |              |D=TURN-PUB-2  |
             |              |              |U=L3:1:R2:1   |
             |              |              |              |
             |              |              |              |
             |              |(35) TURN DATA|              |
             |              |S=TURN-PUB-1  |              |
             |              |D=NAT-PUB-3   |              |
             |              |RA=TURN-PUB-4 |              |



Rosenberg                Expires April 22, 2006                [Page 52]

Internet-Draft                     ICE                      October 2005


             |              |<-------------|              |
             |              |              |              |
             |(36) TURN DATA|              |              |
             |S=TURN-PUB-1  |              |              |
             |D=L-PRIV-1    |              |              |
             |RA=TURN-PUB-4 |              |              |
             |<-------------|              |              |
             |(37) TURN SEND|              |              |
             |S=L-PRIV-1    |              |              |
             |D=TURN-PUB-1  |              |              |
             |DA=TURN-PUB-4 |              |              |
             |MA=TURN-PUB-4 |              |              |
             |------------->|              |              |
             |              |(38) TURN SEND|              |
             |              |S=NAT-PUB-3   |              |
             |              |D=TURN-PUB-1  |              |
             |              |DA=TURN-PUB-4 |              |
             |              |MA=TURN-PUB-4 |              |
             |              |------------->|              |
             |              |              |              |
             |              |              |STUN Res.     |
             |              |              |S=TURN-PUB-2  |
             |              |              |D=TURN-PUB-4  |
             |              |              |MA=TURN-PUB-4 |
             |              |              |              |
             |              |              |(39) TURN DATA|
             |              |              |S=TURN-PUB-1  |
             |              |              |D=R-PUB-1     |
             |              |              |RA=TURN-PUB-2 |
             |              |              |MA=TURN-PUB-4 |
             |              |              |------------->|
             |              |              |              |
             |              |              |              |
             |              |              |              |
             |              |              |              |Validate
             |              |              |              |TURN-PUB-5 to TURN-PUB-3
             |              |              |              |
             |              |              |              |
             |              |              |(40) TURN SEND|
             |              |              |S=R-PUB-2     |
             |              |              |D=TURN-PUB-1  |
             |              |              |DA=TURN-PUB-3 |
             |              |              |<-------------|
             |              |              |              |
             |              |              |STUN Req.     |
             |              |              |S=TURN-PUB-5  |
             |              |              |D=TURN-PUB-3  |
             |              |              |U=L3:2:R2:2   |



Rosenberg                Expires April 22, 2006                [Page 53]

Internet-Draft                     ICE                      October 2005


             |              |              |              |
             |              |              |              |
             |              |              |              |
             |              |              |              |
             |              |              |              |
             |              |              |Discard       |
             |              |              |              |
             |              |              |              |
             |              |              |              |
             |              |              |              |
             |Validate      |              |              |
             |TURN-PUB-3 to TURN-PUB-5     |              |
             |              |              |              |
             |              |              |              |
             |(41) TURN SEND|              |              |
             |S=L-PRIV-2    |              |              |
             |D=TURN-PUB-1  |              |              |
             |DA=TURN-PUB-5 |              |              |
             |------------->|              |              |
             |              |              |              |
             |              |(42) TURN SEND|              |
             |              |S=NAT-PUB-4   |              |
             |              |D=TURN-PUB-1  |              |
             |              |DA=TURN-PUB-5 |              |
             |              |------------->|              |
             |              |              |              |
             |              |              |STUN Req.     |
             |              |              |S=TURN-PUB-3  |
             |              |              |D=TURN-PUB-5  |
             |              |              |U=R2:2:L3:2   |
             |              |              |              |
             |              |              |              |
             |              |              |(43) TURN DATA|
             |              |              |S=TURN-PUB-1  |
             |              |              |D=R-PUB-2     |
             |              |              |RA=TURN-PUB-3 |
             |              |              |------------->|
             |              |              |(44) TURN SEND|
             |              |              |S=R-PUB-2     |
             |              |              |D=TURN-PUB-1  |
             |              |              |DA=TURN-PUB-3 |
             |              |              |MA=TURN-PUB-3 |
             |              |              |<-------------|
             |              |              |              |
             |              |              |STUN Res.     |
             |              |              |S=TURN-PUB-5  |
             |              |              |D=TURN-PUB-3  |
             |              |              |MA=TURN-PUB-3 |



Rosenberg                Expires April 22, 2006                [Page 54]

Internet-Draft                     ICE                      October 2005


             |              |              |              |
             |              |(45) TURN DATA|              |
             |              |S=TURN-PUB-1  |              |
             |              |D=NAT-PUB-4   |              |
             |              |RA=TURN-PUB-5 |              |
             |              |MA=TURN-PUB-3 |              |
             |              |<-------------|              |
             |(46) TURN DATA|              |              |
             |S=TURN-PUB-1  |              |              |
             |D=L-PRIV-2    |              |              |
             |RA=TURN-PUB-5 |              |              |
             |MA=TURN-PUB-3 |              |              |
             |<-------------|              |              |
             |              |              |              |
             |              |              |              |
             |              |              |              |
             |              |              |              |Validate
             |              |              |              |TURN-PUB-5 to TURN-PUB-3
             |              |              |              |
             |              |              |              |
             |              |              |(47) TURN SEND|
             |              |              |S=R-PUB-2     |
             |              |              |D=TURN-PUB-1  |
             |              |              |DA=TURN-PUB-3 |
             |              |              |<-------------|
             |              |              |              |
             |              |              |STUN Req.     |
             |              |              |S=TURN-PUB-5  |
             |              |              |D=TURN-PUB-3  |
             |              |              |U=L3:2:R2:2   |
             |              |              |              |
             |              |              |              |
             |              |(48) TURN DATA|              |
             |              |S=TURN-PUB-1  |              |
             |              |D=NAT-PUB-4   |              |
             |              |RA=TURN-PUB-5 |              |
             |              |<-------------|              |
             |              |              |              |
             |(49) TURN DATA|              |              |
             |S=TURN-PUB-1  |              |              |
             |D=L-PRIV-2    |              |              |
             |RA=TURN-PUB-5 |              |              |
             |<-------------|              |              |
             |(50) TURN SEND|              |              |
             |S=L-PRIV-2    |              |              |
             |D=TURN-PUB-1  |              |              |
             |DA=TURN-PUB-5 |              |              |
             |MA=TURN-PUB-5 |              |              |



Rosenberg                Expires April 22, 2006                [Page 55]

Internet-Draft                     ICE                      October 2005


             |------------->|              |              |
             |              |(51) TURN SEND|              |
             |              |S=NAT-PUB-4   |              |
             |              |D=TURN-PUB-1  |              |
             |              |DA=TURN-PUB-5 |              |
             |              |MA=TURN-PUB-5 |              |
             |              |------------->|              |
             |              |              |              |
             |              |              |STUN Res.     |
             |              |              |S=TURN-PUB-3  |
             |              |              |D=TURN-PUB-5  |
             |              |              |MA=TURN-PUB-5 |
             |              |              |              |
             |              |              |(52) TURN DATA|
             |              |              |S=TURN-PUB-1  |
             |              |              |D=R-PUB-2     |
             |              |              |RA=TURN-PUB-3 |
             |              |              |MA=TURN-PUB-5 |
             |              |              |------------->|
             |              |              |              |
             |              |              |              |
             |              |              |              |
             |              |              |              |
             |RTP flows     |              |              |
             |              |              |              |
             |              |              |              |
             |(53) TURN SEND|              |              |
             |S=L-PRIV-1    |              |              |
             |D=TURN-PUB-1  |              |              |
             |DA=TURN-PUB-4 |              |              |
             |------------->|              |              |
             |              |              |              |
             |              |(54) TURN SEND|              |
             |              |S=NAT-PUB-3   |              |
             |              |D=TURN-PUB-1  |              |
             |              |DA=TURN-PUB-4 |              |
             |              |------------->|              |
             |              |              |              |
             |              |              |              |
             |              |              |RTP           |
             |              |              |S=TURN-PUB-2  |
             |              |              |D=TURN-PUB-4  |
             |              |              |              |
             |              |              |              |
             |              |              |(55) TURN DATA|
             |              |              |S=TURN-PUB-1  |
             |              |              |D=R-PUB-1     |
             |              |              |RA=TURN-PUB-2 |



Rosenberg                Expires April 22, 2006                [Page 56]

Internet-Draft                     ICE                      October 2005


             |              |              |------------->|
             |              |              |              |
             |              |              |              |
             |              |              |              |
             |              |              |              |
             |              |              |              |RTP flows
             |              |              |              |
             |              |              |              |
             |              |              |(56) TURN SEND|
             |              |              |S=R-PUB-1     |
             |              |              |D=TURN-PUB-1  |
             |              |              |DA=TURN-PUB-2 |
             |              |              |<-------------|
             |              |              |              |
             |              |              |              |
             |              |              |RTP           |
             |              |              |S=TURN-PUB-4  |
             |              |              |D=TURN-PUB-2  |
             |              |              |              |
             |              |              |              |
             |              |(57) TURN DATA|              |
             |              |S=TURN-PUB-1  |              |
             |              |D=NAT-PUB-3   |              |
             |              |RA=TURN-PUB-4 |              |
             |              |<-------------|              |
             |              |              |              |
             |(58) TURN DATA|              |              |
             |S=TURN-PUB-1  |              |              |
             |D=L-PRIV-1    |              |              |
             |RA=TURN-PUB-4 |              |              |
             |<-------------|              |              |
             |              |              |              |
             |              |              |              |
             |              |              |              |
             |Validate      |              |              |
             |L-PRIV-1 to R-PUB-1          |              |
             |              |              |              |
             |              |              |              |
             |(59) STUN Req.|              |              |
             |S=L-PRIV-1    |              |              |
             |D=R-PUB-1     |              |              |
             |U=R1:1:L1:1   |              |              |
             |------------->|              |              |
             |              |              |              |
             |              |(60) STUN Req.|              |
             |              |S=NAT-PUB-5   |              |
             |              |D=R-PUB-1     |              |
             |              |U=R1:1:L1:1   |              |



Rosenberg                Expires April 22, 2006                [Page 57]

Internet-Draft                     ICE                      October 2005


             |              |---------------------------->|
             |              |              |              |
             |              |(61) STUN Res.|              |
             |              |S=R-PUB-1     |              |
             |              |D=NAT-PUB-5   |              |
             |              |MA=NAT-PUB-5  |              |
             |              |<----------------------------|
             |              |              |              |
             |(62) STUN Res.|              |              |
             |S=R-PUB-1     |              |              |
             |D=L-PRIV-1    |              |              |
             |MA-NAT-PUB-5  |              |              |
             |<-------------|              |              |
             |              |              |              |
             |              |              |              |
             |              |              |              |
             |              |              |              |Validate
             |              |              |              |R-PUB-1 to L-PRIV-1
             |              |              |              |
             |              |              |              |
             |              |(63) STUN Req.|              |
             |              |S=R-PUB-1     |              |
             |              |D=L-PRIV-1    |              |
             |              |U=L1:1:R1:1   |              |
             |              |<----------------------------|
             |              |              |              |
             |              |              |              |
             |              |              |              |
             |              |              |              |
             |              |Discard       |              |
             |              |              |              |
             |              |              |              |
             |              |              |              |
             |              |              |              |
             |              |              |              |Validate
             |              |              |              |R-PUB-2 to L-PRIV-2
             |              |              |              |
             |              |              |              |
             |              |(64) STUN Req.|              |
             |              |S=R-PUB-2     |              |
             |              |D=L-PRIV-2    |              |
             |              |U=L1:2:R1:2   |              |
             |              |<----------------------------|
             |              |              |              |
             |              |              |              |
             |              |              |              |
             |              |              |              |
             |              |Discard       |              |



Rosenberg                Expires April 22, 2006                [Page 58]

Internet-Draft                     ICE                      October 2005


             |              |              |              |
             |              |              |              |
             |              |              |              |
             |              |              |              |
             |Validate      |              |              |
             |L-PRIV-2 to R-PUB-2          |              |
             |              |              |              |
             |              |              |              |
             |(65) STUN Req.|              |              |
             |S=L-PRIV-2    |              |              |
             |D=R-PUB-2     |              |              |
             |U=R1:2:L1:2   |              |              |
             |------------->|              |              |
             |              |              |              |
             |              |(66) STUN Req.|              |
             |              |S=NAT-PUB-6   |              |
             |              |D=R-PUB-2     |              |
             |              |U=R1:2:L1:2   |              |
             |              |---------------------------->|
             |              |              |              |
             |              |(67) STUN Res.|              |
             |              |S=R-PUB-2     |              |
             |              |D=NAT-PUB-6   |              |
             |              |MA=NAT-PUB-6  |              |
             |              |<----------------------------|
             |              |              |              |
             |(68) STUN Res.|              |              |
             |S=R-PUB-2     |              |              |
             |D=L-PRIV-2    |              |              |
             |MA=NAT-PUB-6  |              |              |
             |<-------------|              |              |
             |              |              |              |
             |              |              |              |
             |              |              |              |
             |              |              |              |Validate
             |              |              |              |R-PUB-1 to NAT-PUB-5
             |              |              |              |
             |              |              |              |
             |              |(69) STUN Req.|              |
             |              |S=R-PUB-1     |              |
             |              |D=NAT-PUB-5   |              |
             |              |U=L1R1:1:R1:1 |              |
             |              |<----------------------------|
             |              |              |              |
             |(70) STUN Req.|              |              |
             |S=R-PUB-1     |              |              |
             |D=L-PRIV-1    |              |              |
             |U=L1R1:1:R1:1 |              |              |



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             |<-------------|              |              |
             |              |              |              |
             |(71) STUN Res.|              |              |
             |S=L-PRIV-1    |              |              |
             |D=R-PUB-1     |              |              |
             |MA=R-PUB-1    |              |              |
             |------------->|              |              |
             |              |              |              |
             |              |(72) STUN Res.|              |
             |              |S=NAT-PUB-5   |              |
             |              |D=R-PUB-1     |              |
             |              |MA=R-PUB-1    |              |
             |              |---------------------------->|
             |              |              |              |
             |              |              |              |
             |              |              |              |
             |              |              |              |Validate
             |              |              |              |R-PUB-2 to NAT-PUB-6
             |              |              |              |
             |              |              |              |
             |              |(73) STUN Req.|              |
             |              |S=R-PUB-2     |              |
             |              |D=NAT-PUB-6   |              |
             |              |U=L1R1:2:R1:2 |              |
             |              |<----------------------------|
             |              |              |              |
             |(74) STUN Req.|              |              |
             |S=R-PUB-2     |              |              |
             |D=L-PRIV-2    |              |              |
             |U=L1R1:2:R1:2 |              |              |
             |<-------------|              |              |
             |              |              |              |
             |(75) STUN Res.|              |              |
             |S=L-PRIV-2    |              |              |
             |D=R-PUB-2     |              |              |
             |MA=R-PUB-2    |              |              |
             |------------->|              |              |
             |              |              |              |
             |              |(76) STUN Res.|              |
             |              |S=NAT-PUB-6   |              |
             |              |D=R-PUB-2     |              |
             |              |MA=R-PUB-2    |              |
             |              |---------------------------->|
             |              |              |              |
             |              |              |              |
             |              |              |              |
             |              |              |              |
             |(77) Offer    |              |              |



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             |------------------------------------------->|
             |              |              |              |
             |              |              |              |
             |              |              |              |
             |              |              |              |
             |(78) Answer   |              |              |
             |<-------------------------------------------|
             |              |              |              |
             |              |              |              |
             |              |              |              |
             |              |              |              |
             |              |              |              |
             |              |              |              |

                                 Figure 6

   First, agent L obtains a STUN derived transport address for its RTP
   packets (messages 1-4).  Recall that the NAT is symmetric.  Here, it
   creates a binding of NAT-PUB-1 for this UDP request, and this becomes
   the STUN derived transport address for RTP.  Agent L repeats this
   process for RTCP (messages 5-8) Ta seconds later, and obtains NAT-
   PUB-2 as its STUN derived transport address for RTCP.  The two
   transport addresses are the two components of the STUN derived
   candidate that agent L has just obtained.

   Next, agent L will allocate a TURN derived transport address for RTP
   (messages 9-12) and RTCP (messages 13-16).  This produces TURN-PUB-2
   and TURN-PUB-3 for RTP and RTCP, respectively.

   With its three candidates, agent L prioritizes them, choosing the
   local candidate as highest priority, followed by the STUN derived
   candidate, followed by the TURN-derived candidate.  It chooses its
   TURN derived candidate as the active candidate, and encodes it into
   the m/c-line.  The resulting offer (message 17) looks like:


       v=0
       o=jdoe 2890844526 2890842807 IN IP4 $L-PRIV-1.IP
       s=
       c=IN IP4 $TURN-PUB-2.IP
       t=0 0
       m=audio $TURN-PUB-2.PORT RTP/AVP 0
       a=rtpmap:0 PCMU/8000
       a=rtcp:$TURN-PUB-3.PORT
       a=candidate $L1 1 $L-PASS1 UDP 1.0 $L-PRIV-1.IP $L-PRIV-1.PORT
       a=candidate $L1 2 $L-PASS2 UDP 1.0 $L-PRIV-2.IP $L-PRIV-2.PORT
       a=candidate $L2 1 $L-PASS3 UDP 0.7 $NAT-PUB-1.IP $NAT-PUB-1.PORT
       a=candidate $L2 2 $L-PASS4 UDP 0.7 $NAT-PUB-2.IP $NAT-PUB-2.PORT



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       a=candidate $L3 1 $L-PASS5 UDP 0.3 $TURN-PUB-2.IP $TURN-PUB-2.PORT
       a=candidate $L3 2 $L-PASS6 UDP 0.3 $TURN-PUB-3.IP $TURN-PUB-3.PORT

   This offer is received at agent R. Agent R will gather its STUN
   derived RTP transport address (messages 18-19) and RTCP address
   (messages 20-21).  Since the result of the STUN allocations did not
   provide a new set of transport addresses, there will not be a
   separate candidate for them.  Agent R then gathers its TURN derived
   RTP transport address (messages 22-23) and TURN derived RTCP
   transport addresses (messages 24-25).  Agent R now has two
   candidates.  It prioritizes the local candidate with higher priority
   than the TURN candidate, and selects the TURN candidate as the active
   candidate.  Its resulting answer looks like:


       v=0
       o=bob 2808844564 2808844564 IN IP4 $R-PUB-1.IP
       s=
       c=IN IP4 $TURN-PUB-4.IP
       t=0 0
       m=audio $TURN-PUB-4.PORT RTP/AVP 0
       a=rtpmap:0 PCMU/8000
       a=rtcp:$TURN-PUB-5.PORT
       a=candidate $R1 1 $R-PASS1 UDP 1.0 $R-PUB-1.IP $R-PUB-1.PORT
       a=candidate $R1 2 $R-PASS2 UDP 1.0 $R-PUB-2.IP $R-PUB-2.PORT
       a=candidate $R2 1 $R-PASS3 UDP 0.3 $TURN-PUB-4.IP $TURN-PUB-4.PORT
       a=candidate $R2 2 $R-PASS4 UDP 0.3 $TURN-PUB-5.IP $TURN-PUB-5.PORT

   Next, agents L and R form candidate pairs and the transport address
   check ordered list.  This list will start with the two components in
   the currently active candidate pair - TURN.  Agent R begins its
   checks (message 27).  It will check connectivity between the active
   candidate pair, starting with the first component, which is
   TURN-PUB-4 for agent R and TURN-PUB-2 for agent L. The state machine
   for that transport address pair moves to the Testing state.  Since
   this is a TURN-derived transport address for agent R, it utilizes the
   TURN SEND mechanism to deliver the Binding Request.  The DESTINATION-
   ADDRESS is TURN-PUB-2.

   The TURN server will extract the content of the TURN message, which
   is a STUN Binding Request, and deliver it to the destination, TURN-
   PUB-4.  This request will be sent from the TURN address allocated to
   R, which is TURN-PUB-4.  As both interfaces are on the TURN server,
   this message is sent to itself (and thus the lack of a message number
   in the sequence diagram above).  Note that the USERNAME in the
   Binding Request is L3:1:R2:1, which represents the transport address
   pair ID.  This message gets discarded by the TURN server since, as of
   yet, there are no permissions established for the TURN-PUB-2



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   allocation.  However, it did have the side effect of establishing a
   permission on the TURN-PUB-4 binding, allowing incoming packets from
   TURN-PUB-2.

   Once L gets the offer, it will attempt to validate the first
   transport address pair in the transport address pair check ordered
   list, which will be the active candidate.  The state machine for this
   transport address pair moves into the Testing state.  Like agent R
   did, it will use the TURN SEND primitive to send a STUN Binding
   Request from its TURN derived transport address, TURN-PUB-2, to TURN-
   PUB-4 (message 28).  This packet traverses the NAT (message 29) and
   arrives at the TURN server.  The TURN server will unwrap the contents
   of the packet and send them from TURN-PUB-2 to TURN-PUB-4.  It will
   also, as a consequence, add a permission for TURN-PUB-4.  The
   contents of the packet are a STUN Binding Request with USERNAME R2:1:
   L3:1 (note how this is the flip of the USERNAME in the Binding
   Request sent by agent R).  This is also a packet from the TURN server
   to itself.  However, now, the packet is not discarded, as a
   permission had been installed as a consequence of the "suicide
   packet" from agent R (a suicide packet is a packet that has no hope
   of traversing a far end NAT, but serves the purpose of enabling a
   permission in a near end NAT so that a packet from the peer can be
   returned).  Thus, the TURN server will relay the received STUN
   request towards agent R (message 30).  This is delivered as a TURN
   DATA Indication.  Notice how the REMOTE-ADDRESS is TURN-PUB-2; this
   is important as it will be used to construct the STUN Binding
   Response.

   Agent R will receive the DATA Indication, and unwrap its contents to
   find the Binding Request.  The state machine for this transport
   address pair is currently in the Testing state.  It therefore moves
   into the Send-Valid state, and it generates a Binding Response.
   However, the MAPPED-ADDRESS in the Binding Response is constructed
   using the source IP address and port that were seen by the TURN
   server when the Binding Request arrived at TURN-PUB-4, which is the
   looped message between messages 29 and 30.  This source address is
   TURN-PUB-2, which is the value of the REMOTE-ADDRESS attribute in
   message 30.  Thus, the STUN Binding Response will contain TURN-PUB-2
   in the MAPPED-ADDRESS, and is to be sent to TURN-PUB-2.  To send the
   response, agent R takes the STUN Binding Response and encapsulates it
   in a TURN SEND primitive, setting the DESTINATION-ADDRESS to TURN-
   PUB-2.  This is shown in message 31.

   The TURN server will receive this SEND request, and unwrap its
   contents to find the STUN Binding Response.  It sends it to the value
   of the DESTINATION-ADDRESS attribute, and sends it from the TURN
   address allocated to R, which is TURN-PUB-4.  This, once again,
   results in a looped message to itself, and it arrives at TURN-PUB-2.



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   Now, however, there is a permission installed for TURN-PUB-4.  The
   TURN server will therefore forward the packet to agent L. To do so,
   it constructs a TURN DATA Indication containing the contents of the
   packet.  It sets the REMOTE-ADDRESS to the source transport address
   of the request it received (TURN-PUB-4), and forwards it to agent L
   (message 32).  This traverses the NAT (message 33) and arrives at
   agent L. As a consequence of the receipt of a Binding Response, the
   state machine for this transport address pair moves to the Recv-Valid
   state.  The agent also examines the MAPPED-ADDRESS of the STUN
   response.  It is TURN-PUB-2.  This is the same as the native
   transport address of this transport address pair, and thus doesn't
   represent a new transport address that might have been learned.

   A short while later, agent R will attempt a retransmission of its
   STUN Binding Request that was lost (the contents of message 27 that
   were discarded by the TURN server due to lack of permission).  This
   time, however, a permission has been installed and the retransmission
   will work.  So, it sends the Binding Request again (message 34,
   identical to message 27).  This is looped by the TURN server to
   itself again, but this time there is a permission in place when it
   arrives at TURN-PUB-2.  As such, the request is forwarded towards
   agent L this time, in a TURN DATA Indication (message 35).  This
   traverses the NAT (message 36) and arrives at agent L. Agent L
   extracts the contents of the request, which are a STUN Binding
   Request.  This causes the state machine to move from Recv-Valid to
   Valid.  It generates a STUN Binding Response, and sets the MAPPED-
   ADDRESS to the value of the REMOTE-ADDRESS in message 36
   (TURN-PUB-4).  This Binding Response is sent to TURN-PUB-4, which is
   accomplished through a TURN SEND primitive (message 37).  This SEND
   Request traverses the NAT (message 38) and is received by the TURN
   server.  Its contents are decapsulated, and sent to TURN-PUB-4, which
   is again a loop on the same host.  This packet is then sent towards
   agent R in a DATA Indication (message 39).  The contents of the DATA
   Indication are extracted, and the agent sees a successful Binding
   Response.  It therefore moves the state machine from the Send-Valid
   state to the Valid state.  At this point, the transport address pair
   is in the Valid state for both agents.

   Approximately Ta seconds after agent R sent message 27, agent R will
   start checks for the next transport address pair in its transport
   address pair check ordered list.  This is the second component of the
   same candidate pair, used for RTCP.  This sequence, messages 40
   through 52, are identical to the ones for RTP, but differ only in the
   specific transport addresses.

   Once that validation happens, the second transport address pair has
   been validated.  The candidate pair moves into the valid state, and
   both candidates are considered valid.  The active candidate has now



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   been validated, and media can begin to flow.  It will do so through
   the TURN server; indeed, it is relayed "twice" through the TURN
   server.  Even though there is a single TURN server, it is logically
   acting as two separate TURN servers.  Indeed, had L and R used two
   separate TURN servers, media would be relayed through both TURN
   servers.

   The actual media flows are shown as well.  It is important to note
   that, since the ICE checks have not yet concluded on the candidate
   that will ultimately be used, no TURN Set Active Destinations have
   been sent.  As a consequence, media that is sent through the TURN
   servers has to be sent using TURN Send requests.  This introduces
   some overhead, but is a transient condition.  In message 53, agent L
   sends an RTP packet to agent R using a SEND request.  It is sent to
   TURN-PUB-4.  This traverses the NAT (message 54), and arrives at the
   TURN server.  It is decapsulated, looped to itself, and arrives at
   TURN-PUB-4.  From there, it is encapsulated in a DATA Indication and
   sent to agent R (message 55).  In the reverse direction, agent R will
   send an RTP packet using a TURN SEND primitive (message 56), and send
   it to TURN-PUB-2.  This is received by the TURN server, decapsulated,
   and sent to TURN-PUB-2 from TURN-PUB-4.  This is again a loop within
   the same host, arriving at TURN-PUB-4.  The contents of the packet
   are sent to agent L through a TURN DATA Indication (message 57),
   which traverses the NAT (message 58) to arrive at agent R. Since this
   call flow is already long enough, RTCP packet transmission is not
   shown.

   Approximately Ta seconds after it sends message 41, agent L goes to
   the next transport address pair in its transport address pair check
   ordered list that is in the Waiting state.  This will be the RTP
   candidate for the top priority candidate pair, which is L-PRIV-1 on
   agent L and R-PUB-1 on agent R. This is a local candidate for each
   agent.  To perform the check, agent R sends a STUN Binding Request
   from L-PRIV-1 to R-PUB-1 (message 59).  Note the USERNAME of
   R1:1:L1:1, which identifies this transport address pair.  This
   traverses the NAT (message 60).  Since the NAT is symmetric and this
   is a new destination IP address, the NAT allocates a new transport
   address on its public side, NAT-PUB-5, and places this in the source
   IP address and port.  This packet arrives at agent R. Agent R finds a
   matching transport address pair in the Waiting state.  The state
   machine transitions to the Send-Valid state.  It sends the Binding
   response, with a MAPPED-ADDRESS equal to NAT-PUB-5 (message 61),
   which traverses the NAT and arrives at agent L (message 62).  Agent
   R, in addition to sending the response, will also send a Binding
   Request.  It is important to remember that this Binding Request is
   sent to the remote address in the transport address pair (L-PRIV-1),
   and NOT to the source IP address and port of the Binding Request
   (NAT-PUB-5); that will happen later.  This attempt is shown in



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   message 63.  However, since the L-PRIV-1 is private, the packet is
   discarded in the network.

   Now, as a consequence of receiving message 60, agent R will have
   constructed a peer-derived candidate.  The candidate ID for this
   candidate is L1R1, and it initially contains a single transport
   address pair, NAT-PUB-5 and R-PUB-1.  However, the candidate isn't
   yet usable until the other component gets added.  Similarly, agent L
   will have constructed the same peer-derived candidate, with the same
   candidate ID and the same transport address pair.

   Some Ta seconds after sending message 40, agent R will move to the
   next transport address pair in the transport address pair check
   ordered list whose state is Waiting.  This is the RTCP component of
   the highest priority candidate pair.  It will attempt a connectivity
   check, from R-PUB-2 to L-PRIV-2 (message 64).  Since L-PRIV-1 is
   private, this message is discarded.

   Some Ta seconds after sending message 59, agent L will move to the
   next transport address pair in the transport address pair check
   ordered list whose state is Waiting.  This is the RTCP component of
   the highest priority candidate pair.  It will attempt a connectivity
   check, from L-PRIV-2 to R-PUB-2 (message 65), which operates nearly
   identically to messages 59-62, with the exception of the specific
   addresses.  Here, the NAT will create a new binding for the RTCP,
   NAT-PUB-6, and this transport address is new for both participants.
   On receipt of this Binding Request at agent R (message 66), agent R
   constructs the candidate ID for the peer-derived candidate, L1R1, and
   finds it already exists.  As such, this new transport address is
   added, and the peer-derived candidate becomes complete and usable.
   Agent L does the same thing on receipt of message 68.  This candidate
   will have the same priority as its generating candidate L1 (1.0), and
   is paired up with R1 (also at priority 1.0).  Since L1R1 has the same
   priority as L1 itself, the ordering algorithm in Section 7.5 will use
   the reverse lexicographic order of the candidate ID iself to
   determine order.  L1R1 is larger than L1, so that the peer-derived
   candidate will come before its generating candidate.  As a
   consequence, the peer-derived candidate pair will have a higher
   priority than its generating candidate, and appear just before it in
   the candidate pair priority ordered list.

   As a consequence, after agent R sends message 67 and completes the
   peer-derived candidate, it will move the two transport addresses in
   the peer derived candidate into the Send-Valid state, and send a
   Binding Request for each in rapid succession (agent L will have moved
   both into the Recv-Valid state upon receipt of message 68).  The
   first of these connectivity checks are for the RTP component, from
   R-PUB-1 to NAT-PUB-5 (message 69).  Note the USERNAME in the STUN



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   Binding Request, L1R1:1:R1:1, which identifies the peer-derived
   transport address pair.  This will succesfully traverse the NAT and
   be delivered to agent L (message 70).  The receipt of this request
   moves the state machine for this transport address pair from Recv-
   Valid to Valid, and a Binding Response is sent (message 71).  This
   passes through the NAT and arrives at agent R (message 72).  This
   causes its state machine to enter the Valid state as well.  The
   MAPPED-ADDRESS, R-PUB-1, is not new to agent R and thus does not
   result in the creation of a new peer-derived candidate.

   Messages 73 through 76 show the same basic flow for RTCP.  Upon
   receipt of message 76, both transport address pairs are Valid at both
   agents, causing the peer derived candidate to become valid.  Timer
   Tws is set at agent L, and fires without any higher priority
   candidate pairs becoming validated.  As such, it now decides to send
   an updated offer to promote the peer-derived candidate to active.
   This offer (message 77) looks like:


       v=0
       o=jdoe 2890844526 2890842808 IN IP4 $L-PRIV-1.IP
       s=
       c=IN IP4 $NAT-PUB-5.IP
       t=0 0
       m=audio $NAT-PUB-5.PORT RTP/AVP 0
       a=rtpmap:0 PCMU/8000
       a=rtcp:$NAT-PUB-6.PORT
       a=remote-candidate:R1
       a=candidate $L1 1 $L-PASS1 UDP 1.0 $L-PRIV-1.IP $L-PRIV-1.PORT
       a=candidate $L1 2 $L-PASS2 UDP 1.0 $L-PRIV-2.IP $L-PRIV-2.PORT

   There are several important things to note in this offer.  Firstly,
   note how the m/c-line now contains NAT-PUB-5 and NAT-PUB-6, the peer
   derived transport addresses it learned through the ICE processing.
   Secondly, note how there remains a candidate encoded into the
   a=candidate attributes.  This is candidate L1, NOT candidate L1R1.
   Recall that the peer-derived candidates are never encoded into the
   SDP.  Rather, their generating candidate is encoded.  This will cause
   keepalives to take place for the genreating candidate if valid
   (though its not) and any of its derived candidates, which is what we
   want.  Finally, notice the inclusion of the a=remote-candidate
   attribute.  Since agent L doesn't know whether agent R received
   messages 72 or 76, it doesnt know whether the state of the candidate
   is Recv-Valid or Valid at agent R. So, it has to tell agent R that,
   in case its Recv-Valid, to please use it anyway.

   The answer generated by agent R looks like:




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       v=0
       o=bob 2808844564 2808844565 IN IP4 $R-PUB-1.IP
       s=
       c=IN IP4 $R-PUB-1.IP
       t=0 0
       m=audio $R-PUB-1.PORT RTP/AVP 0
       a=rtpmap:0 PCMU/8000
       a=rtcp:$R-PUB-2.PORT
       a=candidate $R1 1 $R-PASS1 UDP 1.0 $R-PUB-1.IP $R-PUB-1.PORT
       a=candidate $R1 2 $R-PASS2 UDP 1.0 $R-PUB-2.IP $R-PUB-2.PORT

   With this, media can now flow directly between endpoints.  The
   removal of the TURN-based candidates from the offer/answer exchange
   will cause the TURN allocations to be removed.

12.  Grammar

   This specification defines two new SDP attributes - the "candidate"
   and "remote-candidate" attributes.

   The candidate attribute MUST be present within a media block of the
   SDP.  It contains a transport address for a candidate that can be
   used for connectivity checks.  There may be multiple candidate
   attributes in a media block.

   The syntax of this attribute is defined using Augmented BNF as
   defined in RFC 2234 [11]:


   candidate-attribute   = "candidate" ":" candidate-id SP component-id SP
                           password SP
                           transport SP
                           qvalue SP   ;qvalue from RFC 3261
                           addr SP     ;addr from RFC 3266
                           port        ;port from RFC 2327
                           *(SP extension-att-name SP
                                extension-att-value)

   transport             = "UDP" / transport-extension
   transport-extension   = token
   candidate-id          = 1*base64-char
   password              = 1*base64-char
   base64-char           = ALPHANUM / DIGIT / "+" / "/"
   component-id          = 1*DIGIT
   extension-att-name    = token
   extension-att-value   = token





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   The candidate-id is used to group together the transport addresses
   for a particular candidate.  It MUST be constructed with at least 128
   bits of randomness.  It MUST have the same value for all transport
   addresses within the same candidate.  It MUST have a different value
   for transport addresses within different candidates for the same
   media stream.  The password MUST also be constructed with at least
   128 bits of randomness, and MUST differ for each transport address.
   Both of these use a syntax that is defined to be equal to the base64
   alphabet [4], which allows the candidate-id and password to be
   generated by performing a base64 encoding of a randomly generated 128
   bit value (note, however, that this does not mean that the
   candidate-id or password is base64 decoded when use in STUN
   messages).  The component-id is a positive integer, which identifies
   the specific component of the candidate.  It MUST start at 1 and MUST
   increment by 1 for each component of a particular candidate.

   The addr production is taken from [12], allowing for IPv4 addresses,
   IPv6 addresses and FQDNs.  The port production is taken from RFC 2327
   [7].  The token production is taken from RFC 3261 [3].  The transport
   production indicates the transport protocol for the candidate.  This
   specification only defines UDP.  However, extensibility is provided
   to allow for future transport protocols to be used with ICE, such as
   TCP or the Datagram Congestion Control Protocol (DCCP) [32].

   The a=candidate attribute can itself be extended.  The grammar allows
   for new name/value pairs to be added at the end of the attribute.  An
   implementation MUST ignore any name/value pairs it doesn't
   understand.

   The syntax of the "remote-candidate" attribute is defined using
   Augmented BNF as defined in RFC 2234 [11]:


   remote-candidate-att = "remote-candidate" ":" candidate-id

   This attribute MUST be present in an offer when the candidate in the
   m/c-line is part of a candidate pair that is in the valid or
   partially valid state.

13.  Security Considerations

   There are several types of attacks possible in an ICE system.  This
   section considers these attacks and their countermeasures.

13.1  Attacks on Connectivity Checks

   An attacker might also attempt to disrupt the STUN-based connectivity
   checks.  Ultimately, all of these attacks fool an agent into thinking



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   something incorrect about the results of the connectivity checks.
   The possible false conclusions an attacker can try and cause are:

   False Invalid An attacker can fool a pair of agents into thinking a
      candidate pair is invalid, when it isn't.  This can be used to
      cause an agent to prefer a different candidate (such as one
      injected by the attacker), or to disrupt a call by forcing all
      candidates to fail.

   False Valid An attacker can fool a pair of agents into thinking a
      candidate pair is valid, when it isn't.  This can cause an agent
      to proceed with a session, but then not be able to receive any
      media.

   False Peer-Derived Candidate An attacker can cause an agent to
      discover a new peer-derived candidate, when it shouldn't have.
      This can be used to redirect media streams to a DoS target or to
      the attacker, for eavesdropping or other purposes.

   False Valid on False Candidate An attacker has already convinced an
      agent that there is a candidate with an address that doesn't
      actually route to that agent (for example, by injecting a false
      peer-derived candidate or false STUN-derived candidate).  It must
      then launch an attack that forces the agents to believe that this
      candidate is valid.

   Of the various techniques for creating faked STUN messages described
   in RFC 3489, many are not applicable for the connectivity checks.
   Compromises of STUN servers are not much of a concern, since the STUN
   servers are embedded in endpoints and distributed throughout the
   network.  Thus, compromising the STUN server is equivalent to
   comprimising the endpoint, and if that happens, far more problematic
   attacks are possible than those against ICE.  Similarly, DNS attacks
   are irrelevant since STUN servers are not discovered via DNS, they
   are signaled via SIP.  Injection of fake responses and relaying
   modified requests all can be handled in ICE with the countermeasures
   discussed below.

   To force the false invalid result, the attacker has to wait for the
   connectivity check for one of the agents to be sent.  When it is, the
   attacker needs to inject a fake response with an unrecoverable error
   response, such as a 600.  This attack only needs to be launched
   against one of the agents in order to invalidate the candidate pair.
   However, since the candidate is, in fact, valid, the original request
   may reach the peer agent, and result in a success response.  The
   attacker needs to force this packet or its response to be dropped,
   through a DoS attack, layer 2 network disruption, or other technique.
   If it doesn't do this, the success response will also reach the



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   originator, alerting it to a possible attack.  This will cause the
   agent to abandon the candidate, which is the desired result in any
   case.  Fortunately, this attack is mitigated completely through the
   STUN message integrity mechanism.  The attacker needs to inject a
   fake response, and in order for this response to be processed, the
   attacker needs the password.  If the offer/answer signaling is
   secured, the attacker will not have the password.

   Forcing the fake valid result works in a similar way.  The agent
   needs to wait for the Binding Request from each agent, and inject a
   fake success response.  The attacker won't need to worry about
   disrupting the actual response since, if the candidate is not valid,
   it presumably wouldn't be received anyway.  However, like the fake
   invalid attack, this attack is mitigated completely through the STUN
   message integrity and offer/answer security techniques.

   Forcing the false peer-derived candidate result can be done either
   with fake requests or responses, or with replays.  We consider the
   fake requests and responses case first.  It requires the attacker to
   send a Binding Request to one agent with a source IP address and port
   for the false transport address.  In addition, the attacker must wait
   for a Binding Request from the other agent, and generate a fake
   response with a MAPPED-ADDRESS attribute.  This attack is best
   launched against a candidate pair that is likely to be invalid, so
   the attacker doesnt need to contend with the actual responses to the
   real connectivity checks.  Like the other attacks described here,
   this attack is mitigated by the STUN message integrity mechanisms and
   secure offer/answer exchanges.

   Forcing the false peer-derived candidate result with packet replays
   is different.  The attacker waits until one of the agents sends a
   Binding Request for one of the transport address pairs.  It then
   intercepts this request, and replays it towards the other agent with
   a faked source IP address.  It must also prevent the original request
   from reaching the remote agent, either by launching a DoS attack to
   cause the packet to be dropped, or forcing it to be dropped using
   layer 2 mechanisms.  The replayed packet is received at the other
   agent, and accepted, since the integrity check passes (the integrity
   check cannot and does not cover the source IP address and port).  It
   is then responded to.  This response will contain a MAPPED-ADDRESS
   with the false transport address.  It is passed to the this false
   address.  The attacker must then intercept it and relay it towards
   the originator.

   The other agent will then initiate a connectivity check towards that
   transport address.  This validation needs to succeed.  This requires
   the attacker to force a false valid on a false candidate.  Injecting
   of fake requests or responses to achieve this goal is prevented using



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   the integrity mechanisms of STUN and the offer/answer exchange.
   Thus, this attack can only be launched through replays.  To do that,
   the attacker must intercept the Binding Request towards this false
   transport address, and replay it towards the other agent.  Then, it
   must intercept the response and replay that back as well.

   This attack is very hard to launch unless the attacker themself is
   identified by the fake transport address.  This is because it
   requires the attacker to intercept and replay packets sent by two
   different hosts.  If both agents are on different networks (for
   example, across the public Internet), this attack can be hard to
   coordinate, since it needs to occur against two different endpoints
   on different parts of the network at the same time.

   If the attacker themself is identified by the fake transport address,
   the attack is easier to coordinate.  However, if SRTP is used [25],
   the attacker will not be able to play the media packets, they will
   only be able to discard them, effectively disabling the media stream
   for the call.  However, this attack requires the agent to disrupt
   packets in order to block the connectivity check from reaching the
   target.  In that case, if the goal is to disrupt the media stream,
   its much easier to just disrupt it with the same mechanism, rather
   than attack ICE.

13.2  Attacks on Address Gathering

   ICE endpoints make use of STUN for gathering addresses from a STUN
   server in the network.  This is corresponds to the binding
   acquisition use case discussed in Section 10.1 of RFC 3489.  As a
   consequence, the attacks against STUN itself that are described in
   Section 12 of RFC 3489 can still be used against the STUN address
   gathering operations that occur in ICE.

   However, the additional mechanisms provided by ICE actually
   counteract such attacks, making binding acquisition with STUN  more
   secure when combined with ICE than without ICE.

   Consider an attacker which is able to provide an agent with a faked
   MAPPED-ADDRESS in a STUN Binding Request that is used for address
   gathering.  This is the primary attack primitive described in Section
   12 of RFC 3489.  This address will be used as a STUN derived
   candidate in the ICE exchange.  For this candidate to actually be
   used for media, the attacker must also attack the connectivity
   checks, and in particular, force a false valid on a false candidate.
   This attack is very hard to launch if the false address identifies a
   third party, and is prevented by SRTP if it identifies the attacker
   themself.




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   If the attacker elects not to attack the connectivity checks, the
   worst it can do is prevent the STUN-derived address from being used.
   However, if the peer agent has at least one address that is reachable
   by the agent under attack, the STUN connectivity checks themselves
   will provide a STUN-derived address that can be used for the exchange
   of media.  Peer derived candidates are preferred over the candidate
   they are generated from for this reason.  As such, an attack solely
   on the STUN address gathering will normally have no impact on a call
   at all.

13.3  Attacks on the Offer/Answer Exchanges

   An attacker that can modify or disrupt the offer/answer exchanges
   themselves can readily launch a variety of attacks with ICE.  They
   could direct media to a target of a DoS attack, they could insert
   themselves into the media stream, and so on.  These are similar to
   the general security considerations for offer/answer exchanges, and
   the security considerations in RFC 3264 [5] apply.  These require
   techniques for message integrity and encryption for offers and
   answers, which are satisfied by the SIPS mechanism when SIP is used.
   As such, the usage of SIPS with ICE is RECOMMENDED.

13.4  Insider Attacks

   In addition to attacks where the attacker is a third party trying to
   insert fake offers, answers or stun messages, there are several
   attacks possible with ICE when the attacker is an authenticated and
   valid participant in the ICE exchange.

13.4.1  The Voice Hammer Attack

   The voice hammer attack is an amplification attack, of the variety
   discussed in Section 3 of [30].  In this attack, the attacker
   initiates sessions to other agents, and includes the IP address and
   port of a DoS target in the m/c-line of their SDP.  This causes
   substantial amplification; a single offer/answer exchange can create
   a continuing flood of media packets, possibly at high rates (consider
   video sources).  This attack is not speific to ICE, but ICE can help
   provide remediation.

   Specifically, if ICE is used, the agent receiving the malicious SDP
   will first peform connectivity checks to the target of media before
   sending it there.  If this target is a third party host, the checks
   will not succeed, and media is never sent.

   Unfortunately, ICE doesn't help if its not used, in which case an
   attacker could simply send the offer without the ICE parameters.
   However, in environments where the set of clients are known, and



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   limited to ones that support ICE, the server can reject any offers or
   answers that don't indicate ICE support.

13.4.2  STUN Amplification Attack

   The STUN amplification attack is similar to the voice hammer.
   However, instead of voice packets being directed to the target, STUN
   connectivity checks are directed to the target.  This attack is
   accomplished by having the offerer send an offer with a large number
   of candidates, say 50.  The answerer receives the offer, and starts
   its checks, which are directed at the target, and consequently, never
   generate a response.  The answerer will start a new connectivity
   check every 50ms, and each check is a STUN transaction consisting of
   9 retransmits of a message 64 bytes in length.  This produces a
   fairly substantial 92 kbps, just in STUN requests.

   It is impossible to eliminate the amplification, but the volume can
   be reduced through a variety of heuristics.  For example, agents can
   limit the number of candidates they'll accept in an offer or answer,
   they can increase the value of Ta, or exponentially increase Ta as
   time goes on.  All of these ultimately trade off the time for the ICE
   exchanges to complete, with the amount of traffic that gets sent.

14.  IANA Considerations

   This specification defines two new SDP attribute per the procedures
   of Appendix B of RFC 2327.  The required information for the
   registrations are included here.

14.1  candidate Attribute

   Contact Name: Jonathan Rosenberg, jdrosen@jdrosen.net.

   Attribute Name: candidate

   Long Form: candidate

   Type of Attribute: media level

   Charset Considerations: The attribute is not subject to the charset
      attribute.

   Purpose: This attribute is used with Interactive Connectivity
      Establishment (ICE), and provides one of many possible candidate
      addresses for communication.  These addresses are validated with
      an end-to-end connectivity check using Simple Traversal of UDP
      with NAT (STUN).




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   Appropriate Values: See Section 12 of RFC XXXX [Note to RFC-ed:
      please replace XXXX with the RFC number of this specification].


14.2  remote-candidate Attribute

   Contact Name: Jonathan Rosenberg, jdrosen@jdrosen.net.

   Attribute Name: remote-candidate

   Long Form: remote-candidate

   Type of Attribute: media level

   Charset Considerations: The attribute is not subject to the charset
      attribute.

   Purpose: This attribute is used with Interactive Connectivity
      Establishment (ICE), and provides the identity of the remote
      candidate that the offerer wishes the answerer to use in its
      answer.

   Appropriate Values: See Section 12 of RFC XXXX [Note to RFC-ed:
      please replace XXXX with the RFC number of this specification].


15.  IAB Considerations

   The IAB has studied the problem of "Unilateral Self Address Fixing",
   which is the general process by which a agent attempts to determine
   its address in another realm on the other side of a NAT through a
   collaborative protocol reflection mechanism [23].  ICE is an example
   of a protocol that performs this type of function.  Interestingly,
   the process for ICE is not unilateral, but bilateral, and the
   difference has a signficant impact on the issues raised by IAB.  The
   IAB has mandated that any protocols developed for this purpose
   document a specific set of considerations.  This section meets those
   requirements.

15.1  Problem Definition

   From RFC 3424 any UNSAF proposal must provide:

      Precise definition of a specific, limited-scope problem that is to
      be solved with the UNSAF proposal.  A short term fix should not be
      generalized to solve other problems; this is why  "short term
      fixes usually aren't".




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   The specific problems being solved by ICE are:

      Provide a means for two peers to determine the set of transport
      addresses which can be used for communication.

      Provide a means for resolving many of the limitations of other
      UNSAF mechanisms by wrapping them in an additional layer of
      processing (the ICE methodology).

      Provide a means for a agent to determine an address that is
      reachable by another peer with which it wishes to communicate.


15.2  Exit Strategy

   From RFC 3424, any UNSAF proposal must provide:

      Description of an exit strategy/transition plan.  The better short
      term fixes are the ones that will naturally see less and less use
      as the appropriate technology is deployed.

   ICE itself doesn't easily get phased out.  However, it is useful even
   in a globally connected Internet, to serve as a means for detecting
   whether a router failure has temporarily disrupted connectivity, for
   example.  However, what ICE does is help phase out other UNSAF
   mechanisms.  ICE effectively selects amongst those mechanisms,
   prioritizing ones that are better, and deprioritizing ones that are
   worse.  Local IPv6 addresses can be preferred.  As NATs begin to
   dissipate as IPv6 is introduced, derived transport addresses from
   other UNSAF mechanisms simply never get used, because higher priority
   connectivity exists.  Therefore, the servers get used less and less,
   and can eventually be remove when their usage goes to zero.

   Indeed, ICE can assist in the transition from IPv4 to IPv6.  It can
   be used to determine whether to use IPv6 or IPv4 when two dual-stack
   hosts communicate with SIP (IPv6 gets used).  It can also allow a
   network with both 6to4 and native v6 connectivity to determine which
   address to use when communicating with a peer.

15.3  Brittleness Introduced by ICE

   From RFC3424, any UNSAF proposal must provide:

      Discussion of specific issues that may render systems more
      "brittle".  For example, approaches that involve using data at
      multiple network layers create more dependencies, increase
      debugging challenges, and make it harder to transition.




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   ICE actually removes brittleness from existing UNSAF mechanisms.  In
   particular, traditional STUN (the usage described in RFC 3489) has
   several points of brittleness.  One of them is the discovery process
   which requires a agent to try and classify the type of NAT it is
   behind.  This process is error-prone.  With ICE, that discovery
   process is simply not used.  Rather than unilaterally assessing the
   validity of the address, its validity is dynamically determined by
   measuring connectivity to a peer.  The process of determining
   connectivity is very robust.  The only potential problem is that
   bilaterally fixed addresses through STUN can expire if traffic does
   not keep them alive.  However, that is substantially less brittleness
   than the STUN discovery mechanisms.

   Another point of brittleness in STUN, TURN, and any other unilateral
   mechanism is its absolute reliance on an additional server.  ICE
   makes use of a server for allocating unilateral addresses, but allows
   agents to directly connect if possible.  Therefore, in some cases,
   the failure of a STUN or TURN server would still allow for a call to
   progress when ICE is used.

   Another point of brittleness in traditional STUN is that it assumes
   that the STUN server is on the public Internet.  Interestingly, with
   ICE, that is not necessary.  There can be a multitude of STUN servers
   in a variety of address realms.  ICE will discover the one that has
   provided a usable address.

   The most troubling point of brittleness in traditional STUN is that
   it doesn't work in all network topologies.  In cases where there is a
   shared NAT between each agent and the STUN server, traditional STUN
   may not work.  With ICE, that restriction can be lifted.

   Traditional STUN also introduces some security considerations.
   Fortunately, those security considerations are also mitigated by ICE.

15.4  Requirements for a Long Term Solution

   From RFC 3424, any UNSAF proposal must provide:

      Identify requirements for longer term, sound technical solutions
      -- contribute to the process of finding the right longer term
      solution.

   Our conclusions from STUN remain unchanged.  However, we feel ICE
   actually helps because we believe it can be part of the long term
   solution.






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15.5  Issues with Existing NAPT Boxes

   From RFC 3424, any UNSAF proposal must provide:

      Discussion of the impact of the noted practical issues with
      existing, deployed NA[P]Ts and experience reports.

   A number of NAT boxes are now being deployed into the market which
   try and provide "generic" ALG functionality.  These generic ALGs hunt
   for IP addresses,  either in text or binary form within a packet, and
   rewrite them if they match a binding.  This will interfere with
   proper operation of any UNSAF mechanism, including ICE.

16.  Acknowledgements

   The authors would like to thank Flemming Andreasen, Dan Wing, Douglas
   Otis, Francois Audet and Magnus Westerlund for their comments and
   input.

17.  References

17.1  Normative References

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

   [2]   Huitema, C., "Real Time Control Protocol (RTCP) attribute in
         Session Description Protocol (SDP)", RFC 3605, October 2003.

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

   [4]   Josefsson, S., "The Base16, Base32, and Base64 Data Encodings",
         RFC 3548, July 2003.

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

   [6]   Mealling, M., "The IETF XML Registry", BCP 81, RFC 3688,
         January 2004.

   [7]   Handley, M. and V. Jacobson, "SDP: Session Description
         Protocol", RFC 2327, April 1998.

   [8]   Casner, S., "Session Description Protocol (SDP) Bandwidth
         Modifiers for RTP Control Protocol (RTCP) Bandwidth", RFC 3556,



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         July 2003.

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

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

   [11]  Crocker, D. and P. Overell, "Augmented BNF for Syntax
         Specifications: ABNF", RFC 2234, November 1997.

   [12]  Olson, S., Camarillo, G., and A. Roach, "Support for IPv6 in
         Session Description Protocol (SDP)", RFC 3266, June 2002.

   [13]  Rosenberg, J. and H. Schulzrinne, "Reliability of Provisional
         Responses in Session Initiation Protocol (SIP)", RFC 3262,
         June 2002.

   [14]  Andreasen, F., "Connectivity Preconditions for Session
         Description Protocol Media Streams",
         draft-ietf-mmusic-connectivity-precon-00 (work in progress),
         May 2005.

   [15]  Yon, D., "Connection-Oriented Media Transport in the Session
         Description Protocol  (SDP)", draft-ietf-mmusic-sdp-comedia-10
         (work in progress), November 2004.

   [16]  Rosenberg, J., "Traversal Using Relay NAT (TURN)",
         draft-rosenberg-midcom-turn-08 (work in progress),
         September 2005.

17.2  Informative References

   [17]  Schulzrinne, H., Rao, A., and R. Lanphier, "Real Time Streaming
         Protocol (RTSP)", RFC 2326, April 1998.

   [18]  Senie, D., "Network Address Translator (NAT)-Friendly
         Application Design Guidelines", RFC 3235, January 2002.

   [19]  Rosenberg, J. and H. Schulzrinne, "An RTP Payload Format for
         Generic Forward Error Correction", RFC 2733, December 1999.

   [20]  Srisuresh, P., Kuthan, J., Rosenberg, J., Molitor, A., and A.
         Rayhan, "Middlebox communication architecture and framework",
         RFC 3303, August 2002.

   [21]  Borella, M., Lo, J., Grabelsky, D., and G. Montenegro, "Realm



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         Specific IP: Framework", RFC 3102, October 2001.

   [22]  Borella, M., Grabelsky, D., Lo, J., and K. Taniguchi, "Realm
         Specific IP: Protocol Specification", RFC 3103, October 2001.

   [23]  Daigle, L. and IAB, "IAB Considerations for UNilateral Self-
         Address Fixing (UNSAF) Across Network Address Translation",
         RFC 3424, November 2002.

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

   [25]  Baugher, M., McGrew, D., Naslund, M., Carrara, E., and K.
         Norrman, "The Secure Real-time Transport Protocol (SRTP)",
         RFC 3711, March 2004.

   [26]  Carpenter, B. and K. Moore, "Connection of IPv6 Domains via
         IPv4 Clouds", RFC 3056, February 2001.

   [27]  Zopf, R., "Real-time Transport Protocol (RTP) Payload for
         Comfort Noise (CN)", RFC 3389, September 2002.

   [28]  Rosenberg, J., "The Session Initiation Protocol (SIP) UPDATE
         Method", RFC 3311, October 2002.

   [29]  Andreasen, F., "A No-Op Payload Format for RTP",
         draft-ietf-avt-rtp-no-op-00 (work in progress), May 2005.

   [30]  Rescorla, E. and M. Handley, "Internet Denial of Service
         Considerations", draft-iab-dos-03 (work in progress),
         September 2005.

   [31]  Huitema, C., "Teredo: Tunneling IPv6 over UDP through NATs",
         draft-huitema-v6ops-teredo-05 (work in progress), April 2005.

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

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

   [34]  Hellstrom, G., "RTP Payload for Text Conversation",
         draft-ietf-avt-rfc2793bis-09 (work in progress), August 2004.






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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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