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MMUSIC                                                      J. Rosenberg
Internet-Draft                                             Cisco Systems
Expires: January 18, 2006                                  July 17, 2005


Interactive Connectivity Establishment (ICE): A Methodology for Network
     Address Translator (NAT) Traversal for Offer/Answer Protocols
                        draft-ietf-mmusic-ice-05

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Copyright Notice

   Copyright (C) The Internet Society (2005).

Abstract

   This document describes a methodology for Network Address Translator
   (NAT) traversal for multimedia session signaling protocols, such as
   the Session Initiation Protocol (SIP).  This methodology 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 connectivity.



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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  4
   2.  Terminology  . . . . . . . . . . . . . . . . . . . . . . . . .  4
   3.  Overview of ICE  . . . . . . . . . . . . . . . . . . . . . . .  6
   4.  Sending the Initial Offer  . . . . . . . . . . . . . . . . . .  8
   5.  Receipt of the Offer and Generation of the Answer  . . . . . .  9
   6.  Processing the Answer  . . . . . . . . . . . . . . . . . . . .  9
   7.  Common Procedures  . . . . . . . . . . . . . . . . . . . . . . 10
     7.1   Gathering Candidates . . . . . . . . . . . . . . . . . . . 10
     7.2   Encoding Candidates into SDP . . . . . . . . . . . . . . . 13
     7.3   Prioritizing the Transport Addresses and Choosing an
           Active One . . . . . . . . . . . . . . . . . . . . . . . . 15
     7.4   Connectivity Checks  . . . . . . . . . . . . . . . . . . . 17
       7.4.1   UDP Connectivity Checks  . . . . . . . . . . . . . . . 19
         7.4.1.1   Send Validation  . . . . . . . . . . . . . . . . . 19
         7.4.1.2   Receive Validation . . . . . . . . . . . . . . . . 20
         7.4.1.3   Learning New Candidates from Connectivity
                   Checks . . . . . . . . . . . . . . . . . . . . . . 22
           7.4.1.3.1   On Receipt of a Binding Request  . . . . . . . 23
           7.4.1.3.2   On Receipt of a Binding Response . . . . . . . 26
       7.4.2   TCP Connectivity Checks  . . . . . . . . . . . . . . . 26
         7.4.2.1   Connection Establishment . . . . . . . . . . . . . 26
         7.4.2.2   Sending STUN Binding Requests  . . . . . . . . . . 27
         7.4.2.3   Receiving STUN Requests  . . . . . . . . . . . . . 29
     7.5   Promoting a Valid Candidate to Active  . . . . . . . . . . 30
       7.5.1   Minimum Requirements . . . . . . . . . . . . . . . . . 30
       7.5.2   Suggested Algorithm  . . . . . . . . . . . . . . . . . 31
     7.6   Subsequent Offer/Answer Exchanges  . . . . . . . . . . . . 33
       7.6.1   Sending of an Offer  . . . . . . . . . . . . . . . . . 33
       7.6.2   Receiving the Offer and Sending an Answer  . . . . . . 34
       7.6.3   Receiving the Answer . . . . . . . . . . . . . . . . . 36
     7.7   Binding Keepalives . . . . . . . . . . . . . . . . . . . . 37
     7.8   Sending Media  . . . . . . . . . . . . . . . . . . . . . . 38
   8.  Interactions with Forking  . . . . . . . . . . . . . . . . . . 38
   9.  Interactions with Preconditions  . . . . . . . . . . . . . . . 38
   10.   Example  . . . . . . . . . . . . . . . . . . . . . . . . . . 39
   11.   Grammar  . . . . . . . . . . . . . . . . . . . . . . . . . . 39
   12.   Security Considerations  . . . . . . . . . . . . . . . . . . 40
   13.   IANA Considerations  . . . . . . . . . . . . . . . . . . . . 42
   14.   IAB Considerations . . . . . . . . . . . . . . . . . . . . . 42
     14.1  Problem Definition . . . . . . . . . . . . . . . . . . . . 42
     14.2  Exit Strategy  . . . . . . . . . . . . . . . . . . . . . . 43
     14.3  Brittleness Introduced by ICE  . . . . . . . . . . . . . . 43
     14.4  Requirements for a Long Term Solution  . . . . . . . . . . 44
     14.5  Issues with Existing NAPT Boxes  . . . . . . . . . . . . . 45
   15.   Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 45
   16.   References . . . . . . . . . . . . . . . . . . . . . . . . . 45



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     16.1  Normative References . . . . . . . . . . . . . . . . . . . 45
     16.2  Informative References . . . . . . . . . . . . . . . . . . 46
       Author's Address . . . . . . . . . . . . . . . . . . . . . . . 47
       Intellectual Property and Copyright Statements . . . . . . . . 48















































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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) [16] 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 in
   life is to establish a flow of packets, they tend to carry IP
   addresses within their messages, which is known to be problematic
   through NAT [17].  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 [18], Simple Traversal of UDP
   through NAT (STUN) [1], Traversal Using Relay NAT [14], and Realm
   Specific IP [19] [20] 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 protocols based on the
   offer-answer model, RFC 3264 [4].  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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   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 offeror.

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

   Local Transport Address: A local transport address 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) [19] (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 [21].

   Candidate Transport Address: A transport address advertised by a
      agent in an offer or answer.  A candidate transport address can
      either by a local transport address or a derived transport
      address.

   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.

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

   Candidate: A sequence of candidate transport addresses that form an
      atomic set for usage with a particular media stream.  In the case
      of RTP, there are two candidate transport addresses per candidate:
      one for RTP, and another for RTCP.  Connectivity is verified to
      all of the candidate transport addresses within a candidate before
      that candidate is used.  The transport addresses that compose a



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

   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.


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.















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          Agent A          TURN,STUN Servers          Agent B
             |(1) Gather Addresses |                     |
             |-------------------->|                     |
             |(2) Offer            |                     |
             |------------------------------------------>|
             |                     |(3) Gather Addresses |
             |                     |<--------------------|
             |(4) Answer           |                     |
             |<------------------------------------------|
             |(5) Media            |                     |
             |<------------------------------------------|
             |(6) Media            |                     |
             |------------------------------------------>|
             |(7) STUN Checks      |                     |
             |<------------------------------------------|
             |(8) STUN Checks      |                     |
             |------------------------------------------>|
             |(9) Offer            |                     |
             |------------------------------------------>|
             |(10) Answer          |                     |
             |<------------------------------------------|
             |(11) Media           |                     |
             |<------------------------------------------|
             |(12) Media           |                     |
             |------------------------------------------>|


                                 Figure 1

   The basic flow of operation for ICE is shown in Figure 1.  Before the
   offeror 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
   RSIP.  For media protocols that support both UDP and TCP (such as the
   Real Time Transport Protocol (RTP) [22], which can run over either),
   it obtains both TCP and UDP transport addresses.  In addition, the
   agent obtains derived transport addresses from each local transport
   address using protocols such as STUN and TURN.  Each local and
   derived transport address becomes a candidate for receipt of media
   traffic.

   The agent will choose one of its candidate transport addresses as its
   initial media transport address for inclusion in the connection and
   media lines in the offer.  This transport address will be utilized
   for media traffic while connectivity is verified to all of the
   candidates.  Since these checks may take time to execute, media
   clipping will occur if the media transport address is not reachable



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   by the peer.  To minimize the probability of clipping, the transport
   address that is most likely to work is chosen.  This is normally a
   TURN-derived tranport address, but others can be utilized based on
   local policy.

   Each candidate transport address (including the one being used as the
   media transport address) is listed in an a=candidate attribute in the
   offer.  Each candidate is given a preference.  Preference is a matter
   of local policy, but typically, lowest preference 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 transport ID (tid).

   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
   offeror followed; it obtains addresses from local interfaces, obtains
   derived transport addresses from those, and the combination becomes
   its set of candidate transport addresses.  It picks one as its
   initial media transport address and places it into the m/c line in
   the answer, and then lists all of them in the a=candidate attributes
   in the answer, along with a preference and tid.

   Once the offer/answer exchange has completed, each agent sends media
   from its media transport address to the media transport address of
   its peer.  This media stream may or may not work, depending on
   whether or not the media transport address is reachable.  In parallel
   with the transmission of media, a connectivity check begins.  This
   check makes use of STUN messages sent from each candidate to each
   other candidate.  These checks will allow each agent to determine
   whether it can send packets from a particular candidate to a
   candidate from its peer, and whether packets can be sent back.  If,
   after a certain period of time, an agent determines that a pair of
   candidates works, and has a higher priority than the transport
   addresses currently in use for media (perhaps because the ones in use
   don't work), it sends a new offer that "promotes" its candidate into
   the m/c line.  This causes the media traffic to switch to this new
   transport address.

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



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   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, it determines a priority
   for each candidate and identifies the active candidate that will be
   used for receipt of media, as described in Section 7.3.

   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 7.2.  The offer is then sent.

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 offeror
   supports ICE.  In that case, it starts gathering candidates, as
   described in Section 7.1, and prioritizes them Section 7.3.  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 while the user is
   being alerted to the request for communications, session
   establishment delays due to that gathering can be eliminated.

   At some point, the answerer will decide to accept or reject the
   communications.  A rejection terminates ICE processing.  In the case
   of acceptance, the answer is constructed, and if the offeror
   supported ICE, the candidates are encoded into the SDP as described
   in Section 7.2.  The answer is then sent.  If the offeror supported
   ICE, the answerer begins its connectivity checks as described in
   Section 7.4.

   In addition, and regardless if the offeror supported ICE, the
   answerer can begin sending media packets as it normally would.  It
   sends media according to the procedures in Section 7.8.

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 offeror 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 7.8, MUST be
   followed however.



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   If the answer contains candidates, it implies that the answerer
   supported ICE.  In that case, the offeror begins connectivity checks
   as described in Section 7.4.  It also starts sending media, using the
   candidate in the m/c line, based on the procedures described in
   Section 7.8.

7.  Common Procedures

   This section discusses procedures that are common between offeror and
   answerer.

7.1  Gathering Candidates

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

   Each candidate is composed of a series of transport addresses of the
   same type.  In the case of RTP, the candidate is composed of either
   one or two transport addresses.  Normally there are two - one for
   RTP, and one for RTCP.  However, if RTCP is not in use, a candidate
   will only contain 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.  Specifically, for
   each UDP-only 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
   transport addresses needed for the candidate.  For RTP, N is
   typically two.  For each TCP-only media stream the agent wishes to
   use, the agent SHOULD obtain a set of candidates by binding to N
   ephemeral TCP ports on each interface, where N is the number of
   transport addresses needed for the candidate.  For media streams that
   can support either UDP or TCP, the agent SHOULD obtain a set of
   candidates by binding to N ephemeral UDP and N ephemeral TCP ports on
   each interface, where N is the number of transport addresses needed
   for the candidate.

   If a host has K local interfaces, this will result in K candidates
   for each UDP stream (requiring K*N transport addresses), K candidates
   for each TCP stream (requiring K*N transport addresses), and 2K
   candidates for streams that support UDP and TCP (requiring 2*K*N
   transport addresses).

   Media streams carried using the Real Time Transport Protocol (RTP)
   [22] can run over TCP [27].  As such, it is RECOMMENDED that both UDP



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   and TCP candidates be obtained.  Transmission of real time media over
   UDP is generally preferred to TCP.  However, many network
   environments, for better or for worse, permit only TCP traffic.
   Obtaining a TCP candidate, and then using it in conjunction with a
   TURN relay as described below, allows for ICE to make use of the TCP
   media only when UDP connectivity is non-existent, as it may be in
   these restricted environments.  However, providers of real-time
   communications services may decide that it is preferable to have no
   media at all than it is to have media over TCP.  To allow for choice,
   it is RECOMMENDED that agents be configurable with whether they
   obtain TCP candidates for real time media.

      Having it be configurable, and then configuring it to be off, is
      far better than not having the capability at all.  An important
      goal of this specification is to provide a single mechanism that
      can be used across all types of endpoints.  As such, it is
      preferable to account for provider and network variation through
      configuration, instead of hard-coded limitations in an
      implementation.  Furthermore, network characteristics and
      connectivity assumptions can, and will change over time.  Just
      because a agent is communicating with a server on the public
      network today, doesn't mean that it won't need to communicate with
      one behind a NAT tomorrow.  Just because a agent is behind a full
      cone NAT today, doesn't mean that tomorrow they won't pick up
      their agent and take it to a public network access point where
      there is a symmetric NAT or one that only allows outbound TCP.
      The way to handle these cases and build a reliable system is for
      agents to implement a diverse set of techniques for allocating
      addresses, so that at least one of them is almost certainly going
      to work in any situation.  Implementors should consider very
      carefully any assumptions that they make about deployments before
      electing not to implement one of the mechanisms for address
      allocation.  In particular, implementors should consider whether
      the elements in the system may be mobile, and connect through
      different networks with different connectivity.  They should also
      consider whether endpoints which are under their control, in terms
      of location and network connectivity, would always be under their
      control.  Only in cases where there isn't now, and never will be,
      endpoint mobility or nomadicity of any sort, should a technique be
      omitted.

   Once the agent has obtained local candidates, it obtains candidates
   with derived transport addresses.  Agents which serve end users
   directly, such as softphones, hardphones, terminal adaptors 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 [25].  As with



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   TCP, usage of STUN and TURN is at SHOULD strength to allow for
   provider variation.  If it is not to be used, it is also 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 endpoints that connect to it.  The ability to
      process STUN connectivity checks allows for the network server to
      obtain peer-derived transport addresses that can be used to
      provide relay-free traversal of symmetric NAT for endpoints that
      connect to it.  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 12 for a discussion on these benefits.

   To obtain STUN candidates (which are always UDP), 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 configured in network environments where
   there are multiple layers of NAT, and that layering is known to the
   provider of the client.  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 the 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 configured STUN servers.

   To obtain UDP TURN candidates, the client takes a local UDP
   candidate, and for each configured TURN server, produces a TURN
   candidate.  It is anticipated that clients may have a multiplicity of
   TURN servers configured in network environments where there are
   multiple layers of NAT, and that layering is known to the provider of
   the client.  To produce the TURN candidate from the local candidate,
   it follows the procedures of Section 8 of [14] for each local
   transport address in the local candidate.  It initiates an Allocate
   Request transaction from the local transport address to that server.



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   The Allocate Response will provide the client with its TURN derived
   transport address in the MAPPED-ADDRESS attribute.  If the client had
   K local candidates, this will produce S*K UDP TURN candidates, where
   S is the number of configured TURN servers.

   To obtain a TURN-derived TCP candidates, the client takes a local TCP
   candidate, and for each configured TURN server, produces a TCP TURN
   candidate.  It is anticipated that clients may have a multiplicity of
   TURN servers configured in network environments where there are
   multiple layers of NAT, and that layering is known to the provider of
   the client.  To produce the TURN candidate from the local candidate,
   it iterates through the local transport addresses in the local
   candidate, and for for each one, initiates a TCP connection from the
   same interface the local transport address to the TURN server.  It is
   not neccesary to initiate the connection from the actual port in the
   local transport address.  Following the procedures of Section 8 of
   [14], it initiates an Allocate Request transaction over the
   connection.  The Allocate Response will provide the client with its
   TCP TURN derived transport address in the MAPPED-ADDRESS attribute.
   If the client had K local TCP candidates, this will produce S*K TCP
   TURN candidates, where S is the number of configured TURN servers.

7.2  Encoding Candidates into SDP

   For each candidate to be placed into the SDP, the agent includes a
   series of a=candidate attributes as media-level attributes, one for
   each transport address in the candidate.  Each of the transport
   addresses for the same candidate MUST have the same value of the
   candidate-id attribute.  The a=candidate attributes for different
   candidates MUST be unique within that media stream.  Using a simple
   sequence number, incrementing by one for each candidate for a media
   stream, meets these requirements.  The transport, unicast-address and
   port of the attribute are set to those for the candidate.  The qvalue
   is set to the priority of this candidate (note that, for RTP, the RTP
   and RTCP transport addresses MUST have equal priority values).  The
   tid MUST be chosen randomly with 128 bits of randomness.  The tid is
   chosen only when the transport address is placed into the SDP for the
   first time; subsequent offers or answers within the same session
   containing that same transport address would use the same tid used
   previously.

   The tid serves as a unique identifier for each transport address.  It
   also gets combined, through concatenation, with the tid of a peer
   candidate to form the username and password 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



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   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
   identifier for its transport address, and A provides one to C. A
   concatenates these two identifiers and uses the result as the
   username and password 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 effected.  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, because there are separate transport addresses for RTP and
   RTCP, each will have a distinct tid.

   The active candidate 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 it 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].




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   For media streams that are inherently TCP-based (as opposed to ones
   where TCP is a fallback and would be listed as a candidate but not
   the initial active address), the connections MUST be signaled using
   comedia [13], and those connections MUST be in "holdconn" mode.  This
   has the effect of suspending connection attempts via the comedia
   mechanisms, allowing ICE to open the connections instead.  These
   connections then get removed from holdconn mode when the ICE
   procedures complete and an updated offer/answer exchange takes place
   that promotes one of the existing ICE-established connections to
   active.  Note that this has the result of increasing the post-dial-
   delay for TCP-oriented media, but brings with it substantial security
   and NAT traversal properties.

7.3  Prioritizing the Transport Addresses 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 for a particular media stream.

   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) [24].  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



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   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
   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 a 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.

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

   Once the candidates have been prioritized, one is selected as the
   active one.  This is the candidate that will be used for actual
   exchange of media, until replaced by an updated offer or answer.
   Since the ICE connectivity checks can take a few seconds to execute,
   media clipping can occur is this candidate doesn't work.  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 default address be a TURN 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



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   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.

7.4  Connectivity Checks

   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
   pairings for each media stream by combining each of its UDP
   candidates with each of the UDP candidates of its peer, and by
   combining each of its TCP candidates with each of the TCP candidates
   of its peer.  If candidates for other transport protocols were
   signaled through the offer/answer exchange, a pairing is performed
   between each of those as well.  If an offer/answer exchange took
   place for a session comprised of an audio and a video stream, and
   each stream had two UDP and two TCP candidates from each agent, there
   would be 16 pairings, 8 for audio and 8 for video.  Each of those
   eight would be comprised of four UDP and four TCP.  Note that there
   is no requirement that the number of candidates from each peer be the
   same.  One agent can offer two UDP candidates for a media stream, and
   the answer can contain three UDP candidates for the same media
   stream.  In that case, there would be six UDP pairings.

   Each candidate has a number of transport addresses.  In the case of
   RTP, there are either one or two.  Within the pairing, the transport
   addresses of each candidate are linked together one-to-one to form a
   transport address pair.  In the case of RTP, the result will either
   be one or two transport address pairs - one for RTP, and possibly
   another for RTCP.  The relationship between a candidate, transport
   address, pairing and transport address pair are shown in Figure 2.
   This figure shows the pairing as seen by the agent that owns the
   candidate {A,B}.  The candidate owned by that agent is called the
   native candidate, and the one owned by its peer is the remote
   candidate.  As the figure shows, there is one pairing between two
   candidates, and two transport address pairs ({A,C} and {B,D}).  If
   one of the candidates only had one transport address (in the case
   where RTCP was not being used by one agent), there would only be one
   transport address pair, {A,C}.  Each transport address is associated
   with a tid.  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.  This means that the identifiers are
   different for each agent.  For the agent that owns {A,B}, the
   transport address pair ID is WY for the first transport address pair,
   and XZ for the second.  For the agent that owns {C,D}, it would be
   reversed - YW for the first transport address pair, and ZX for the
   second.



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                ...........................................
                .                                         .
     .......... .                                         . ..........
     .        . .  .............           .............  . .        .
     .        . .  .           .           .           .  . .        .
     .    --  . .  .    --     .           .    --     .  . .   --   .
     .   | A|<<<<<<<<<<| A|--------------------| C|>>>>>>>>>>>>| K|  .
     .    --  . .  .    --     . Transport .    --     .  . .   --   .
     .        . .  . Transport .  Address  . Transport .  . .        .
     .        . .  .  Address  .   Pair    .  Address  .  . .        .
     .        . .  .  tid=W    .   ID=WY   .   tid=Y   .  . .        .
     .        . .  .           .           .           .  . .        .
     .        . .  .           .           .           .  . .        .
     .        . .  .           .           .           .  . .        .
     .    --  . .  .    --     .           .    --     .  . .   --   .
     .   | J|<<<<<<<<<<| B|--------------------| D|>>>>>>>>>>>>| D|  .
     .    --  . .  .    --     . Transport .    --     .  . .   --   .
     .......... .  . Transport .  Address  . Transport .  . ..........
     Associated .  .  Address  .   Pair    .  Address  .  . Associated
     Local      .  .   tid=X   .   ID=XZ   .   tid=Z   .  . Local
     Transport  .  .           .           .           .  . Transport
     Addresses  .  .............           .............  . Addresses
                .       Native              Remote        .
                .     Candidate            Candidate      .
                .        and                  and         .
                . Transport Addresses Transport Addresses .
                .                                         .
                ...........................................

                                   Pairing


                                 Figure 2

   The figure also shows that each transport address has an associated
   local transport address.  The associated local transport address is
   the local transport address at which the agent will receive packets
   sent to the transport address.  For a local transport address, its
   associated local transport address is the same.  That is the case of
   transport address A and D in the diagram.  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.

   Next, each agent begins sending connectivity checks for each
   transport address pair.  The procedure differs for UDP and TCP.





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7.4.1  UDP Connectivity Checks

   An agent considers a UDP pairing validated when all of its transport
   address pairs have been validated.  Each transport address pair is
   validated if an agent successfully completed a STUN Binding Request
   transaction from its native transport address to the corresponding
   remote transport address, and when it has received a STUN Binding
   Request transaction on its native transport address, sent from the
   remote transport address.  This ensures that packets can flow in each
   direction.

   Because validation of a transport address pair involves a STUN
   transaction in each direction, a pair can be in one of five states -
   unknown, invalid, send-valid, receive-valid and valid.  Each
   transport address pair starts in the unknown state.

7.4.1.1  Send Validation

   To validate a transport address pair in the send direction, an agent
   needs to complete a successful STUN Binding Request transaction.
   This means it needs to send a Binding Request from its native
   transport address to the remote transport address, and receive a
   successful Binding Response back.

   For UDP-based transport addresses, an agent initiates a STUN Binding
   Request transaction by sending from its native transport address, and
   sends it to the remote transport address.  The meaning of "sending
   from its native transport address" is clear in the case of a local
   transport address - the request is sent such that the source IP
   address and port of the packet is equal to that local transport
   address.  However, the meaning is different for STUN and TURN derived
   transport addresses.  For STUN derived transport address, it is sent
   by sending from the local transport address used to derive that STUN
   address.  For TURN derived 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 the example above, if
   the agent on the left sends the STUN Binding Request, the USERNAME
   will have the value YW.  The request MAY contain the MESSAGE-
   INTEGRITY attribute, computed according to RFC 3489 procedures.  The
   MESSAGE-INTEGRITY The Binding Request MUST NOT contain the CHANGE-



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   REQUEST or ANSWER-ADDRESS attribute.

   Each of these STUN transactions 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.  Either initially, or after such
   a retry, the STUN transaction might produce a non-recoverable failure
   response (error codes 400, 431, or 600) or a failure result
   inapplicable to this usage of STUN and thus unrecoverable (432, 433).
   If this happens the transport address pair and its corresponding
   candidate is considered invalid.  If the STUN transaction produces a
   430 error or times out, the client SHOULD retry with a new STUN
   Binding Request transaction.  The 430 response code, as described
   below, is generated when the server doesn't recognize the STUN
   username because the BindingRequest was sent received prior to the
   receipt of the answer.  Its ocurrence is a result of a failed race
   between the BindingRequest and the answer.  This is remedied by
   retrying, which allows the "slower" answer to be received.  These
   retry transactions carry the same USERNAME value as the original
   Binding Request, and differ only in their STUN transaction ID.  If
   these retries have not produced a success response after Tg seconds,
   the transport address pair is considered invalid.  Tg SHOULD be
   configurable.  It is RECOMMENDED that it default to 50 seconds.  This
   is a reasonable approximation of the maximum SIP transaction
   duration.

   If the STUN transaction succeeds for a UDP transport address pair
   (producing a success response), and the pair was previously in the
   receive-valid state, it is considered valid.  If the pair was
   previously in the unknown state, it is considered send-valid.

   If a transport address pair is send-valid or valid, 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.  They
   can also be used to keep the bindings alive when the candidate is
   promoted to active, as described in Section 7.7.  Tr SHOULD be
   configurable, and SHOULD default to 15 seconds.  Each new Binding
   Request transaction is processed according to the procedures in this
   Section.  It is possible for a previously valid candidate to later be
   invalidated by a subsequent STUN transaction.  This happens in cases
   where the NAT bindings expire.

7.4.1.2  Receive Validation

   As a result of providing a list of candidates in its offer or answer,
   an ICE implementation 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



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   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 reliably sending
   an updated offer or answer which does not include any candidates
   equal to or derived from that local transport address.  Here,
   "reliably" means that the agent knows that the offer or answer was
   received by its peer.  This knowledge is based on the protocol
   carrying the offer/answer exchanges.  In the case of SIP, if the
   offer is in an INVITE, the agent knows this was received by its peer
   when a 200 OK or reliable provisional response [9] is received with
   the answer.  If the offer is in a reliable provisional response, the
   agent knows it was reliably received when the PRACK arrives.  If an
   answer is in a 200 OK response, the agent knows this was received
   when the ACK is received.

   The agent does not need to provide STUN service on any other IP
   address or port, unlike the STUN usage described in [1].  The need to
   run the service on multiple ports is to support the change flags.
   However, those flags are not needed with ICE, and the server SHOULD
   reject, with a 400 answer, any STUN requests with these flags set.
   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, a shared secret is not needed.  The tid's that are exchanged and
   used to form the STUN USERNAME attribute do not actually require the
   security properties associated with a shared secret in order for ICE
   to operate securely; this is because ICE security is bootstrapped off
   of the protocol carrying the offer/answer exchanges.

   One of the candidates will 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 [23], 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).




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   The STUN Binding Request can only be usefully processed once an
   offer/answer exchange has completed.  As a result, if an offeror
   receives a STUN Binding Request message prior to the receipt of an
   answer to its offer, it MUST reject the request with a 430 response.
   This will cause the answerer to retry, and give time for the answer
   (which is in transit) to arrive at the offerer.

   If the offer/answer exchange has completed, the agent MUST follow the
   procedures defined in RFC 3489 and verify that the USERNAME attribute
   is known to the server.  Here, this is done by taking the USERNAME
   attribute, and comparing it against the transport address pair
   identifiers for each transport address pair as seen by that agent.
   If there is no match, the STUN Binding Request generates a 400.  If
   there is a match, the resulting transport address pair is called the
   matching transport address pair.  The user agent proceeds with the
   processing of the request and generation of a response as per RFC
   3489.  In addition, the if the state of that transport address pair
   was previously unknown, it changes to receive-valid.  If the state
   was previously send-valid, it moves to valid.

   An agent will continue to receive periodic STUN transactions as long
   as it had listed its transport address in an a=candidate attribute.
   It MUST process those 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.

7.4.1.3  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 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.

   To do this, ICE agents dynamically learn new candidates by examining
   the source IP addresses and MAPPED-ADDRESS attributes in STUN Binding
   Requests and Responses respectively.  If they don't match any
   existing candidates, a new candidate is added.  This candidate
   corresponds to the new IP address and port created by the symmetric
   NAT, and is a new point of contact for the agent behind the symmetric



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   NAT.  Since that candidate is only reachable from the very specific
   IP address and port where the STUN request was sent to, the new
   candidate is paired up with that transport address on the other
   agent.  Since all candidates need to have properties, such as tids,
   priorities and candidate IDs, these are all computed algorithmically,
   so that they can be determined by both agents just from the STUN
   message.

   The specific procedures on receipt of a Binding Request and Response
   for accomplishing this are described here.

7.4.1.3.1  On Receipt of a Binding Request

   When a STUN Binding Request is received which generates a success
   response, the source IP address and port of that request is compared
   all existing remote transport addresses.  If there is no match, the
   agent creates a new remote candidate, and adds a transport address to
   it.  It sets the IP address and port of this new remote transport
   address to the IP address and port that was present in the incoming
   Binding Request.  Since this is a new candidate transport address, it
   requires a new tid.  The agent creates one algorithmically, by
   concatenating the tid of the remote transport address in the matching
   transport address pair (recall that the matching transport address
   pair is the one whose transport address pair ID matched the username
   of the incoming Binding Request) with the string representation of
   the source IP address and port from the incoming Binding Request.
   This string representation is defined using the grammar for
   "hostport" from RFC 3261 [3], which defines the familiar notation of
   the IP address and port separated by a colon.

   The priority of the new candidate MUST be set to the priority of the
   remote candidate in the matching transport address pair.  There is no
   need to compute the candidate ID for this new candidate.

   Though this is a valid transport address, the agent does not pair it
   up with each of its own transport addresses.  Rather, it pairs it up
   only with the native transport address from the matching transport
   address pair.  This creates a new transport address pair.  Since
   connectivity has been verified in the receive direction, the agent
   sets its state to receive-valid.  As with all other transport address
   pairs, the agent will attempt to validate send capabilities by
   sending a STUN Binding Request according to the procedures in
   Section 7.4.1.1.

   It is important to note that this process creates a new remote
   transport address, not a whole new remote candidate.  For a whole
   remote candidate to come into existence, all of its component
   transport addresses must come into existence, and all must have been



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   obtained as a result of a STUN Binding Requests between transport
   address pairs in the same pairing.  As an example, consider the
   pairing in Figure 2.  If the peer is behind a symmetric NAT, the
   Binding Request sent from C to A might produce a new remote transport
   address for RTP.  To create a full candidate, a STUN Binding Request
   from D to B has to also create a new remote transport address, to be
   used for RTCP.  If this were to happen, the resulting set of
   relationships is shown in Figure 3.  To simplify the diagram,
   associated local transport address relationships have been omitted.
   Notice how the tids of the new remote candidate have been constructed
   by concatenating the tids of the original remote candidate with the
   newly discovered transport addresses, here, {R,S}.







































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           .............                              .............
           .           .                              .           .
           .    --     .                              .    --     .
           .   | A|---------------------------------------| C|    .
           .    -- -----------+  Transport            .    --     .
           . Transport .      |   Address             . Transport .
           .  Address  .      |    Pair               .  Address  .
           .  tid=W    .      |    ID=WY              .   tid=Y   .
           .           .      |                       .           .
           .           .      |                       .           .
           .           .      |                       .           .
           .    --     .      |                       .    --     .
           .   | B|-----------C---------------------------| D|    .
           .    -- ---------+ |  Transport            .    --     .
           . Transport .    | |   Address             . Transport .
           .  Address  .    | |    Pair               .  Address  .
           .   tid=X   .    | |    ID=XZ              .   tid=Z   .
           .           .    | |                       .           .
           .............    | |                       .............
                            | |                         remote
               native       | |                         candidate
               candidate    | |
                            | |                       .............
                            | |                       .           .
                            | |                       .    --     .
                            | +---------------------------| R|    .
                            |     Transport           .    --     .
                            |      Address            . Transport .
                            |       Pair              .  Address  .
                            |       ID=WYR            .   tid=YR  .
                            |                         .           .
                            |                         .           .
                            |                         .           .
                            |                         .    --     .
                            +-----------------------------| S|    .
                                  Transport           .    --     .
                                   Address            . Transport .
                                    Pair              .  Address  .
                                    ID=XZS            .   tid=ZS  .
                                                      .           .
                                                      .............
                                                       peer-derived
                                                       remote candidate

                                 Figure 3






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7.4.1.3.2  On Receipt of a Binding Response

   When an agent receives a successful Binding Response, it examines the
   MAPPED-ADDRESS attribute in that response.  If the MAPPED-ADDRESS
   does match any of the existing candidate transport addresses, this
   represents a new peer-derived transport address.

   The agent creates a new local candidate, and adds a transport address
   to it.  It sets the IP address and port of this new native transport
   address to the IP address and port that was present in the MAPPED-
   ADDRESS attribute of the Binding Response.  Since this is a new
   candidate transport address, it requires a new tid.  The agent
   creates one algorithmically, by concatenating the tid of the native
   transport address in the transport address pair that was being
   validated by the Binding Request with the string representation of
   the source IP address and port from the MAPPED-ADDRESS attribute.
   This string representation is defined using the grammar for
   "hostport" from RFC 3261 [3], which defines the familiar notation of
   the IP address and port separated by a colon.

   The priority of the new candidate MUST be set to the priority of the
   native candidate that was being validated by the Binding Request.
   The agent SHOULD assign a new candidate ID to this candidate.

   Though this is a valid transport address, the agent does not pair it
   up with each of the remote transport addresses.  Rather, it pairs it
   up only with the remote transport address from the transport address
   pair that was being validated.  This creates a new transport address
   pair.  Since connectivity has been verified in the send direction,
   the agent sets its state to send-valid.  As with all other transport
   address pairs, the agent will attempt to validate receive
   capabilities by waiting for a a STUN Binding Request according to the
   procedures in Section 7.4.1.2.

   It is important to note that this process creates a new native
   transport address, not a whole new candidate.  For a whole native
   candidate to come into existence, all of its component transport
   addresses must come into existence, and all must have been obtained
   as a result of a STUN Binding Requests between transport address
   pairs in the same pairing.

7.4.2  TCP Connectivity Checks

7.4.2.1  Connection Establishment

   Because of the connection-oriented nature of TCP, the connectivity
   checks work differently.  After the offer/answer exchange completes,
   each agent will have a set of TCP candidates at which it is waiting



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   to receive a connection on, and it will have a similar set from its
   peer.  Thus, a pairing of TCP candidates allows for the possibility
   of TCP connections in each direction.  Unlike the UDP checks, where
   the STUN packets are sent from the native transport addresses to the
   remote ones, the TCP connections are not opened from the native TCP
   transport addresses to the remote ones.  This would represent a
   simultaneous open, and represent an unusual condition that would
   either fail, or at best result in a single TCP connection.  Rather,
   ICE desires to attempt two connections, one in each direction, and
   use one of them if both happen to succeed.

   To accomplish this, each agent will attempt to open a connection to
   each remote transport address in the transport address pair, and do
   so "from" its native transport address.  Here, however, "from" means
   something different than the UDP case.  If the native transport
   address is a local transport address, the agent opens the TCP
   connection from the same IP interface used to obtain the local
   transport address, but from a different and ephemeral port.  Indeed,
   that port MUST NOT be the same as the port in the local transport
   address.  If the native transport address is a TURN-derived TCP
   transport address, no attempt is made to open a connection at all.
   TURN-derived TCP transport addresses can only be used in passive
   mode.

   As such, for each TCP transport address pair, there will be either
   zero, one, or two connection attempts.  If the transport address
   pairs are both TURN-derived, there will be zero (both sides passive).
   If one of the transport addresses is local, and the other TURN
   derived, there will be one connection attempt.  The agent owning the
   local transport address will be in active mode, and the agent owning
   the TURN-derived one will be in passive mode.  If both are local
   transport address, there will be two attempts, and each agent will
   act in active mode.

   Because a transport address pair can produce multiple connections,
   validity becomes a property of the TCP connection itself.  A
   transport address pair is considered valid if at least one valid
   connection has been established within it.  An entire pairing is
   valid if all transport address pairs are valid.

7.4.2.2  Sending STUN Binding Requests

   Once the connection is established, the agent which opened the
   connection (that is, acted in active mode) sends a STUN Binding
   Request over that connection.  STUN Binding Requests as described in
   RFC 3489 are not normally sent over UDP, but when used in conjunction
   with ICE for connectivity checks, they are sent over TCP.




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   This unusual operation requires some explanation.  At first glance, a
   successful TCP connection ought to be sufficient.  Clearly,
   connectivity is established, as TCP packets were exchanged in both
   directions via the TCP handshake.  While that is true, the STUN
   Binding Requests serve many purposes, only one of which is to
   literally test connectivity.  The STUN requests also serve as a
   correlation vehicle, allowing the agent to match the source of a
   connection attempt with the offer/answer signaling driving the entire
   mechanism.  For example, in the case of a forked SIP INVITE carrying
   an offer, the UAC may receive two connection attempts to each of its
   passive TCP addresses, one from each branch of the fork.  These are
   readily disambiguated by the STUN Binding Request which will follow,
   as the tid in the USERNAME tells the UAC which branch has initiated
   the connection.

   More importantly, however, the STUN Binding Request is an essential
   part of the security properties of ICE.  Without it, an entity
   eavesdropping the signaling messages would be able to deny service or
   hijack media connections, and such attacks would require encryption
   of the offer/answer exchanges (using a mechanism like SIPS [3]) to
   prevent.  However, when a STUN Binding Request exchange is added,
   these attacks are completely foiled without the need for SIPS,
   raising the overall security of ICE substantially with minimal cost.
   These properties of ICE are discussed thoroughly in Section 12.

   As such, once an agent has actively opened a TCP connection to the
   remote agent, it sends a STUN Binding Request over that connection.
   Recall that STUN messages include length indicators, allowing them to
   be framed over a connection-oriented transport protocol.  The Binding
   Request 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 YW.  The request
   MAY contain the MESSAGE-INTEGRITY attribute, computed according to
   RFC 3489 procedures.  The MESSAGE-INTEGRITY The Binding Request MUST
   NOT contain the CHANGE-REQUEST or ANSWER-ADDRESS attribute.  The STUN
   BindingRequest message SHOULD NOT be retransmitted over the
   connection.

   The STUN 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.  Either initially, or after such a retry, the
   STUN transaction might produce a non-recoverable failure response
   (error codes 400, 431, or 600) or a failure result inapplicable to
   this usage of STUN and thus unrecoverable (432, 433).  If this
   happens the connection is considered invalid.  If the STUN
   transaction produces a 430 error or times out, the client SHOULD



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   retry with a new STUN Binding Request transaction.  The 430 response
   code is a result of a failed race between the BindingRequest and the
   answer.  This is remedied by retrying, which allows the "slower"
   answer to be received.  These retry transactions carry the same
   USERNAME value as the original Binding Request, and differ only in
   their STUN transaction ID.  If these retries have not produced a
   success response after Tg seconds, the connection is considered
   invalid.  Tg SHOULD be configurable.  It is RECOMMENDED that it
   default to 50 seconds.  This is a reasonable approximation of the
   maximum SIP transaction duration.

   If the STUN Binding Request generates a successful response, the
   connection over which it was sent is considered valid.  Furthermore,
   the agent stores the IP address and port from the MAPPED-ADDRESS
   response in the STUN Binding Response.  This is called the "apparent"
   native transport address for the active side of the connection.  It
   will be used later if this connection is used for media transport.

   Once a connection is valid, the agent which initiated the connection
   MUST generate a new STUN Binding Request transaction every Tr
   seconds.  This transaction ensures that NAT bindings for the
   connection remain open while the connection is under consideration as
   a candidate.  Tr SHOULD be configurable, and SHOULD default to 15
   seconds.  Each new Binding Request transaction is processed according
   to the procedures in this section.  It is possible for a previously
   valid candidate to later be invalidated by a subsequent STUN
   transaction.  This happens in cases where the NAT bindings expire.
   Note that, unlike the UDP case, STUN is sent only while a connection
   is is not active for media.  If the connection is used as the active
   connection for media, STUN MUST NOT be sent.

7.4.2.3  Receiving STUN Requests

   When an agent acted as the passive side of a TCP connection, it will
   receive a STUN Binding Request over that connection.

   One of the candidates will 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 [23], 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).



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   The STUN Binding Request can only be usefully processed once an
   offer/answer exchange has completed.  As a result, if an offeror
   receives a STUN Binding Request message prior to the receipt of an
   answer to its offer, it MUST reject the request with a 430 response.
   This will cause the answerer to retry, and give time for the answer
   (which is in transit) to arrive at the offerer.

   If the offer/answer exchange has completed, the agent MUST follow the
   procedures defined in RFC 3489 and verify that the USERNAME attribute
   is known to the server.  Here, this is done by taking the USERNAME
   attribute, and comparing it against the transport address pair
   identifiers for each transport address pair as seen by that agent.
   If there is no match, the STUN Binding Request generates a 400.  If
   there is a match, the resulting transport address pair is called the
   matching transport address pair.  The user agent proceeds with the
   processing of the request and generation of a response as per RFC
   3489.  In addition, the agent stores the source IP address and port
   of the Binding Request, and associates it with the connection.  This
   address is called the "apparent" remote transport address for this
   connection.

   An agent will continue to receive periodic STUN transactions as long
   as it had listed its transport address in an a=candidate attribute.
   It MUST process those 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.

   Note that, unlike the UDP case, there will never be simultaneous
   transmission of media and STUN packets over TCP connections.  This is
   because the connection is listed as on hold according to comedia
   procedures, and no media will be transmitted.  ICE will establish the
   connections as described here.  Once established, an updated offer/
   answer exchange can promote those connections to active usage through
   the comedia "exist" mechanism, as described below.  The additional
   offer/answer exchange provides a barrier synchronization point at
   which a TCP connection switches from ICE control to control by the
   media source and sinks.  Once it is active, STUN packets will no
   longer be sent on the connection.

7.5  Promoting a Valid Candidate to Active

7.5.1  Minimum Requirements

   As the STUN connectivity checks run, they will result in the
   validation of pairings.  Once validated, a pairing can be used by
   promoting it to active.  This promotion occurs by placing the
   transport addresses for the native candidate of the pairing into the



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   m/c line and sending an updated offer.  It MAY promote a candidate
   associated with any validated pairing at any time, as long as the
   candidate had been provided in series of a=candidate attributes in
   the most recent offer (in other words, an agent can't validate a
   candidate, omit that candidate from the a=candidate attribute of an
   offer, and then later on, generate a new offer that promotes the
   candidate to active).  The procedures for doing so are described
   here.

   Any candidates which the agent would like to retain as valid
   candidates are also included in a=candidate lines in the offer.  It
   SHOULD include any candidates learned from the peer-to-peer discovery
   processing of Section 7.4.1.3, and SHOULD include any candidates of
   higher priority than the one just promoted to active.  It SHOULD omit
   candidates of lower priority than the one being promoted to active.
   It SHOULD omit any for whom all pairings that include that candidate
   have become invalid.

   If a candidate is omitted, and that candidate was a TURN-derived
   transport address, the agent SHOULD de-allocate the address from the
   TURN server.  If a local candidate was omitted, along with all of its
   derived transport addresses, local operating system resources for
   that candidate SHOULD be de-allocated.

   Once it has decided on the set of candidates to provide in the
   updated offer, the agent constructs the offer and follows the
   procedures in Section 7.6 which defines general subsequent offer/
   answer processing.

7.5.2  Suggested Algorithm

   ICE leaves substantial variability to implementors around when an
   agent decides to generate a new offer.  However, there are good ways
   to do this, and bad ways.  Perhaps the worst algorithm possible would
   be to generate a new offer every time a candidate with higher
   priority than the active one becomes valid.  This algorithm will
   likely result in a large number of offer/answer exchanges in rapid
   succession, many of which will produce "glare" as each agent will
   independently initiate an exchange.  This will consume CPU and
   network resources for little benefit.  Rather, the ideal algorithm
   strikes a balance between usage of network resources and the desire
   to use the ideal pair of candidates.

   The following algorithm provides a good tradeoff, and usage of this
   algorithm is RECOMMENDED.  The algorithm results in a bounded number
   of additional offer/answer exchanges after the initial one - never
   more than two, and frequently one or zero.  The algorithm almost
   never produces a glare condition.



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   Once the initial offer/answer exchange completes, media flow will
   happen, though not optimally (where optimal is defined by the
   policies used to set the priorities of the candidates), as long as
   the candidate that is active has been validated.  Thus, the objective
   of the algorithm is to quickly make sure that there is a valid path
   for media (to avoid clipping), and then do a single offer/answer
   exchange to use the highest priority pairing that was validated.

   After the initial offer/answer exchange, each agent sets a timer Tu.
   This timer SHOULD have a configurable baseline value, which SHOULD
   default to 3 seconds.  The actual timer is set to this baseline, plus
   a time value chosen uniformly beween -1 and 1 seconds.  This causes
   the actual timer to be randomized so that the timer doesnt fire
   simultaneously at each agent.  In addition, each agent monitors the
   status of the active pairing.  If the active media stream is UDP-
   based, the status of the active candidates is equal to the status of
   the pairing with matching transport addresses.  In the case of TCP-
   based media, the active media stream is never active initially, since
   it always begins with the "holdconn" state.

   If, when Tu fires, the active pairing has not been validated, and
   there exists at least one pairing that has been validated, the agent
   generates a new offer.  This offer promotes its highest priority
   candidate with a validated pairing to the active candidate.  If there
   are no pairings that have been validated when the timer fires, the
   agent waits until one is validated, and once that happens, sets a
   timer to fire randomly between 0 and 2 seconds.  When the timer
   fires, a new offer is generated that promotes the candidate from this
   validating pairing to active.  If the active pairing is validated
   when the timer fires, the agent does nothing at this time.

   If new offer is to be sent, the agent includes the new active
   candidate in the a=candidate attribute list.  It also includes all
   candidates with higher priority than the one that is active,
   including ones it learned from the connectivity checks themselves.

   At this point, media is flowing successfully, since a valid candidate
   is active.  However, it may not be optimal.  So, the next stage of
   the algorithm is to let the connectivity checks continue.  If those
   checks indicate that a pairing between the two highest priority
   candidates from both agents has been validated, each agent sets a
   timer whose value is randomly set between 0 and 2 seconds.  When the
   timer fires, a new offer is generated that promotes the candidate
   from this validating pairing to active.  Otherwise, when the
   connectivity checks have all concluded, such that no pairing exists
   in the invalid state, each agent sets a timer whose value is randomly
   set between 0 and 2 seconds.  When the timer fires, a new offer is
   generated that promotes the candidate from the valid pairing with the



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   highest priority to active.

7.6  Subsequent Offer/Answer Exchanges

   An offer/answer exchange within a session can occur at any time,
   whether it is the result of the algorithm described in Section 7.5.2,
   or because one of the agents wishes to add or remove a media stream,
   or add a codec, and so on.

7.6.1  Sending of an Offer

   The meaning of a=candidate attributes within a subsequent offer have
   the same meaning 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.  As such, an a=candidate attribute is included in
   subsequent offers when (1) connectivity checks haven't concluded yet
   to that candidate, or (2) the checks have concluded, and the
   candidate is currently active.  In that case, STUN is used to keep
   the bindings active.

   If an agent sends an offer which omits candidates it had sent to its
   peer previously, it MUST cease connectivity checks from that
   candidate.  Any pairings that include the absent native candidate are
   discarded.  Any STUN transactions in progress from that candidate are
   immediately terminated - no further retransmissions take place, and
   no further transactions from that candidate will be made.  If a TCP
   connection was opened to or from that candidate, and that connection
   is not listed as the active one in the offer, the connection is torn
   down.

   The offer MAY contain a new active candidate in the m/c line.  If the
   new active transprot address is UDP, candidate is encoded into an
   update offer as described in Section 7.2.  The transport addresses
   constituting the candidate SHOULD also be listed in a=candidate
   attributes, so that STUN can be used as an ongoing keepalive.

   If the new active transport address is TCP, it is more complicated.
   Recall that each TCP connection is opened from one of the agents to
   the other, such that, for each connection, one agent has the active
   role, and the other, the passive.  The ICE mechanisms allow the
   active agent to actually choose a specific connection for use in an
   offer, so long as the agent has used a different ephemeral port for
   each connection it initiated (which is almost always the case).  If,
   however, an agent was in the passive role, it cannot choose a
   specific connection.  Rather, it can choose a specific native
   transport address which may have been used to receive multiple
   connections.  This assymetric behavior brings with it some important



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   security properties, which are discussed in Section 12.

   If the agent was the active one and established the connection, it
   includes its apparent native transport address in the m/c line of the
   SDP (recall that this address was discovered via the STUN exchange
   over the connection).  Note that this is instead of the SHOULD-
   strength recommendation in comedia, which recommends that the port
   number sent by the entity which initiated the connection should be
   '9'.  The actual port number is present to facilitate identification
   of the connection.  The a=setup attribute MUST be present and MUST
   contain the value "active".  The a=connection attribute MUST be
   present and MUST have the value of "existing".

   If the agent was the passive one and was the recipient of the
   connection, it includes its transport address in the m/c line of the
   SDP.  In this case, that address will be the same as the one it had
   placed into the a=candidate line of the SDP.  The a=setup attribute
   MUST be present and MUST contain the value of "passive".  The
   a=connection attribute MUST be present and MUST have the value of
   "existing".

7.6.2  Receiving the Offer and Sending an Answer

   If an agent receives an updated offer with a=candidate attributes, it
   checks to see if it already knows about the listed candidates.  This
   is done by comparing the tid with the candidates it had received in
   the previous offer or answer from the peer.  If the tid is already
   known, processing for that candidate continues as if no offer had
   been made.  Any connectivity checks in progress continue, and any
   ongoing STUN keepalives continue.

   If a candidate which had been listed previously is no longer present
   in the offer, this tells the answerer to cease connectivity checks.
   Any pairings that include the absent remote candidate are discarded.
   Any STUN transactions in progress to that candidate are immediately
   terminated - no further retransmissions take place, and no further
   transactions to that candidate will be made.  If a TCP connection was
   opened to or from that candidate, and that connection is not listed
   as the active one in the offer, the connection is torn down.

   The agent then sends its answer.  Like the offerer, it can add or
   remove candidates from its answer.  If it removed candidates from its
   answer, it ceases STUN connectivity checks from those candidates, and
   any pairings that include those candidates are discarded.  Any STUN
   transactions in progress to that candidate are immediately terminated
   - no further retransmissions take place, and no further transactions
   to that candidate will be made.  If a TCP connection was opened to or
   from that candidate, and that connection is not listed as the active



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   one in the answer, the connection is torn down.

   After transmission of the answer, there may be a set of candidates
   which were new in the offer, and a set that were new in the answer.
   The agent begins connectivity checks as described in Section 7.4,
   pairing each new candidate in its answer with all candidates in the
   offer, and each new candidate in the offer with all of its candidates
   in the answer.

   The m/c line may have also changed, indicating a new active
   candidate.  If the m/c line contains a UDP stream, the agent begins
   sending media to the transport addresses listed there.  In addition,
   it checks to see if those transport addresses correspond to a remote
   candidate in a valid pairing.  So long as the remote agent has
   offered up a candidate that has been validated by ICE, it should be
   the case.  Indeed, there may be a multitude of valid pairings
   containing the transport addresses in the m/c line as the remote
   candidate.  In that case, the agent MUST choose the pairing whose
   native candidate has the highest priority.  It MUST place this
   candidate in the m/c line.  Transmission of media occurs as defined
   in Section 7.8.

   If the m/c line has changed, and now indicates a new TCP candidate,
   the agent examines it.  The comedia "a=connection" attribute will
   normally be present and normally contain the value of "existing".  If
   not present, or if present but with a value of "new", comedia process
   is followed, as apparently the peer has abandoned ICE operation for
   this media stream.  Assuming it contains a value of "existing", the
   agent looks at whether the a=setup attribute is present.  If its
   value is "active", it means that a connection that was initiated by
   the remote agent is to be used.  The agent examines the transport
   address in the m/c line.  It looks for a matching value in the
   apparent remote transport addresses of existing connections.  If it
   matches multiple connections (though it should normally match just
   one), one of those connections is chosen.  The native transport
   address of that connection is then placed into the m/c line of the
   answer.  If no existing connections where matched, an error has
   occured.  The agent SHOULD respond with "holdconn", and then generate
   its own offer with a connection to the peer which it believes is
   valid.

   If the a=setup attribute had a value of "passive", it means that a
   connection that was initiated by the agent itself is to be used.  The
   agent examines the transport address in the m/c line.  It looks for a
   matching value amongst the remote transport addresses in valid
   pairings.  If multiple pairings match, it MUST choose the one whose
   native transport address has the highest priority.  The apparent
   native transport address associated with an active connection



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   initiated by the agent is then placed into the m/c line, and that TCP
   connection is used to send and receive media.  If no pairings match,
   an error has occured.  The agent SHOULD respond with "holdconn", and
   then generate its own offer with a connection to the peer which it
   believes is valid.

7.6.3  Receiving the Answer

   If an agent receives an answer with a=candidate attributes, it checks
   to see if it already knows about the listed candidates.  This is done
   by comparing the tid with the candidates it had received in the
   previous offer or answer from the peer.  If the tid is already known,
   processing for that candidate continues as if no offer had been made.
   Any connectivity checks in progress continue, and any ongoing STUN
   keepalives continue.

   If a candidate which had been listed previously is no longer present
   in the answer, this tells the offerer to cease connectivity checks.
   Any pairings that include the absent remote candidate are discarded.
   Any STUN transactions in progress to that candidate are immediately
   terminated - no further retransmissions take place, and no further
   transactions to that candidate will be made.  If a TCP connection was
   opened to or from that candidate, and that connection is not listed
   as the active one in the answer, the connection is torn down.

   Furthermore, there may be a set of candidates which were new in the
   offer, and a set that were new in the answer.  The agent begins
   connectivity checks as described in Section 7.4, pairing each new
   candidate in its offer with all candidates in the answer, and each
   new candidate in the answer with all of its candidates in the offer.

   The m/c line may have also changed, indicating a new active
   candidate.  If the m/c line contains a UDP stream, the agent begins
   sending media to the transport addresses listed there as defined in
   Section 7.8.  It will send from the m/c line it had signaled in the
   offer.

   If the m/c line has changed, and now indicates a new TCP candidate,
   the agent examines it.  If the agent had, in its offer, indicated the
   desire to use a specific connection that it had initiated, it would
   have used the a=connection attribute with the value of "existing",
   and the a=setup attribute with the value of "active", and have placed
   its apparent native transport address in the m/c line.  In that case,
   the m/c line in the answer will normally have the a=connection
   attribute with the value "existing", which means that the remote
   agent agrees with the usage of that connection.  The transport
   addresses in the m/c line should correspond to the remote transport
   addresses that the agent had initiated its connection to.  If so,



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   that connection is used.

   If the agent had, in its offer, indicated the desire to use any
   connection that had been established to a specific native transport
   address, it would have, in its offer, used the a=connection attribute
   with the value of "existing" and the a=setup attribute with the value
   of "passive", and placed that address in the m/c line.  In that case,
   the m/c line in the answer will normally have the a=connection
   attribute with the value of "existing" and the a=setup attribute with
   the value of "active".  The transport address in the m/c line will
   correspond to the apparent remote transport address.  The agent MUST
   scan its existing connections to the native transport address it had
   advertised in the offer, and find the one whose apparent remote
   transport address matches the m/c line in the answer.  If there is a
   match, that connection is used for sending media.  If there is no
   match, an error has occurred.

7.7  Binding Keepalives

   Once the candidates are 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 RTP packets
   themselves 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 [4].  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 [28], may send packets so infrequently that the
   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 as candidates in a=candidate lines.
   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.

   If an ICE implementation is communciating with one that does not
   support ICE, keepalives MUST still be sent.  In that case, it is
   RECOMMENDED that an agent support the RTP No-Op payload format [15],



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   and send it at least once every 20 seconds if media is not otherwise
   being sent.  This No-Op MUST be sent even if the media stream is
   inactive or recvonly.

7.8  Sending Media

   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
   travresal.

   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.  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 tids
   that correlate each of those checks (and thus their corresponding
   source IP address and port or TCP connection) with a specific remote
   user agent.  As these checks happen before any media is transmitted,
   ICE allows a UAC to disambiguate subsequent media traffic, and
   corelate 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.

9.  Interactions with Preconditions

   Because ICE involves multiple addresses and pre-session activities,
   its interactions with preconditions [10] merits further discussion.

   Quality of Service (QoS) preconditions, which are defined in RFC
   3312, apply only to the IP addresses and ports listed in the m/c



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   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, 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 [12].  As described there, the precondition is
   satisfied once ICE has verified that there exists a valid path of
   connectivity for each media stream to which the precondition applies.
   More specifically, it is satisfied when there is at least one valid
   UDP transport address pairing or TCP connection for such a media
   stream.  Furthermore, when a subsequent offer is made to promote one
   of those valid transport address pairings or connections into the
   m/c-line, the preconditions is marked as met in that same offer/
   answer exchange.

10.  Example

   In the example that follows, messages are labeled with "message name
   A,B" to mean a message from transport address A to B. For STUN
   Requests, this is followed by curly brackets enclosing the username
   (which is also the password).  For STUN answers, this is followed by
   square brackets containing the value of MAPPED ADDRESS.  The example
   shows a flow of two agents where one is behind a full cone NAT, and
   the other is behind a symmetric NAT.

   TODO: Fill in.  This is a big complicated flow!

11.  Grammar

   This specification defines a new SDP attribute.  It is called
   "candidate".  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:












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   candidate-attribute   = "candidate" ":" candidate-id SP tid SP
                           transport SP
                           qvalue SP   ;qvalue from RFC 3261
                           addr SP
                           port SP
                             ;addr, port from RFC 2327
   transport             = "UDP" / "TCP" / transport-extension
   transport-extension   = token
   candidate-id          = 1*DIGIT
   id                    = non-ws-string


   The candidate-id is used to group together the transport addresses
   for a particular candidate.  It MUST be a positive integer whose
   value is less than (2^31 -1).  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 tid production contains an
   identifier, chosen with 128 bits of randomness, that identifies the
   transport address.  The tid of a pair of transport addresses is
   combined to for the username and password of a STUN request from one
   transport address to another.  The transport production indicates the
   transport protocol for the candidate.  This can be either UDP or TCP.
   Extensibility is provided to allow for future transport protocols to
   be used with ICE, such as the Datagram Congestion Control Protocol
   (DCCP) [26].  The unicast-address production is from RFC 2327, and
   contains the IPv4 or IPv6 address of the candidate.  The port
   production contains its port.

12.  Security Considerations

   There are numerous threats in a system using ICE.  This section
   overviews these threats and discusses how they are mitigated.

   STUN itself introduces many security considerations, which receive an
   extensive treatment in RFC 3489.  STUN is used within ICE in two ways
   - one, as a technique for address gathering, and two, as a peer-to-
   peer connectivity check.  All of the security considerations of RFC
   3489 apply directly to the former usage.  However, the latter usage,
   as a peer-to-peer connectivity check, is sufficiently different that
   a discussion of its security considerations is appropriate.

   It remains the case that many attacks are rooted in a single
   primitive - an attacker attempts to inject a STUN response with an
   invalid MAPPED-ADDRESS attribute.  In the usages of STUN described in
   RFC 3489, this injection can occur as a result of compromises of STUN
   servers, attacks on the DNS, rogue NATs, injection of faked responses
   coupled with a dos attack, and replaying modified requests.  With



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   peer-to-peer STUN, 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.  Rogue NATs,
   injection of fake responses and relaying modified requests all can be
   handled in ICE with the countermeasures discussed below.

   Consider an attacker that intercepts a STUN packet used for
   connectivity checks, and replays it using its own source address.  If
   successful, this would fool an endpoint into thinking that this faked
   source address was a valid destination for media (recall that the
   source transport address of received STUN packets is used as a
   potential candidate address).  However, the recipient of the replayed
   packet will not just send media to that candidate.  It will verify it
   with a STUN connectivity check.  This check will be sent to that
   faked source address, and if there is no answer, the address will not
   be used.  The attacker cannot answer the STUN request without access
   to the username and password, which are exchanged as part of the
   signaling.  Thus, if the signaling is protected as recommended above,
   the attacker cannot obtain the username or password.

   If an attacker instead intercepts and replays STUN packets used for
   the purposes of unilateral allocation, a similar result occurs.  The
   target of the attack will be fooled into thinking it has a STUN
   derived transport address that it does not.  Its peer will perform a
   connectivity check to this address, which will fail.  The attacker
   cannot force this check to succeed without access to the username and
   password, which are protected.  Thus, this address will not be used.

   In the worst case, an attacker can generate enough traffic so that
   none of the valid STUN checks or unilateral allocations succeed.
   This would result in a service disruption.  However, this attack is
   no worse than any pure packet flood disruption attack launched
   against any other protocol.  These attacks cannot be prevented by any
   protocol means.

   If an attacker could intercept and modify the contents of the Offer
   or Accept messages, they could disrupt the session, divert the media,
   and otherwise take control over the session.  This attack is
   prevented by encryption, authentication and message integrity of the
   signaling channel used for ICE.

   SIP-based implementations of ICE SHOULD use the sips URI scheme when
   transporting SDP with ICE information, and MAY use S/MIME [3].




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13.  IANA Considerations

   This specification defines one new SDP attribute per the procedures
   of Appendix B of RFC 2327.  The required information for the
   registration is:

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

   Attribute Name: candidate

   Long Form: candidiate

   Type of Attribute: media level

   Charset Considerations: The attribute is not subject the 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).

   Appropriate Values: See Section 11 of RFC XXXX [Note to RFC-ed:
      please replace XXXX with the RFC number of this specification].


14.  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 [21].  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.

14.1  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.


14.2  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.

14.3  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.

14.4  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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14.5  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.

15.  Acknowledgements

   The authors would like to thank Douglas Otis, Francois Audet and
   Magnus Westerland for their comments and input.

16.  References

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

   [5]   Zopf, R., "Real-time Transport Protocol (RTP) Payload for
         Comfort Noise (CN)", RFC 3389, September 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,
         July 2003.



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

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

   [11]  Camarillo, G., "The Alternative Network Address Types Semantics
         (ANAT) for theSession  Description Protocol (SDP) Grouping
         Framework", draft-ietf-mmusic-anat-02 (work in progress),
         October 2004.

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

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

   [14]  Rosenberg, J., "Traversal Using Relay NAT (TURN)",
         draft-rosenberg-midcom-turn-07 (work in progress),
         February 2005.

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

16.2  Informative References

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

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

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

   [19]  Borella, M., Lo, J., Grabelsky, D., and G. Montenegro, "Realm
         Specific IP: Framework", RFC 3102, October 2001.

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

   [21]  Daigle, L. and IAB, "IAB Considerations for UNilateral Self-



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         Address Fixing (UNSAF) Across Network Address Translation",
         RFC 3424, November 2002.

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

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

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

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

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

   [27]  Lazzaro, J., "Framing RTP and RTCP Packets over Connection-
         Oriented Transport", draft-ietf-avt-rtp-framing-contrans-05
         (work in progress), January 2005.

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


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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