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MMUSIC                                                      J. Rosenberg
Internet-Draft                                             Cisco Systems
Expires: December 28, 2006                                 June 26, 2006


Interactive Connectivity Establishment (ICE): A Methodology for Network
     Address Translator (NAT) Traversal for Offer/Answer Protocols
                        draft-ietf-mmusic-ice-09

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   This Internet-Draft will expire on December 28, 2006.

Copyright Notice

   Copyright (C) The Internet Society (2006).

Abstract

   This document describes a protocol for Network Address Translator
   (NAT) traversal for multimedia session signaling protocols based on
   the offer/answer model, such as the Session Initiation Protocol
   (SIP).  This protocol is called Interactive Connectivity
   Establishment (ICE).  ICE makes use of the Simple Traversal of UDP
   through NAT (STUN), applying its binding discovery, connectivity
   check and relay usages.




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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   4
   2.  Overview of ICE . . . . . . . . . . . . . . . . . . . . . . .   4
   3.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .  15
   4.  Sending the Initial Offer . . . . . . . . . . . . . . . . . .  18
   5.  Receipt of the Offer and Generation of the Answer . . . . . .  19
   6.  Processing the Answer . . . . . . . . . . . . . . . . . . . .  19
   7.  Common Procedures . . . . . . . . . . . . . . . . . . . . . .  20
     7.1.  Gathering Candidates  . . . . . . . . . . . . . . . . . .  20
     7.2.  Prioritizing the Candidates and Choosing an Operating
           One . . . . . . . . . . . . . . . . . . . . . . . . . . .  25
     7.3.  Encoding Candidates into SDP  . . . . . . . . . . . . . .  27
     7.4.  Forming Candidate Pairs . . . . . . . . . . . . . . . . .  31
     7.5.  Ordering the Candidate Pairs  . . . . . . . . . . . . . .  33
     7.6.  Performing the Connectivity Checks  . . . . . . . . . . .  36
     7.7.  Sending a Binding Request for Connectivity Checks . . . .  42
     7.8.  Receiving a Binding Request for Connectivity Checks . . .  44
     7.9.  Promoting a Candidate to Operating  . . . . . . . . . . .  46
     7.10. Learning New Candidates from Connectivity Checks  . . . .  47
       7.10.1.  On Receipt of a Binding Request  . . . . . . . . . .  47
       7.10.2.  On Receipt of a Binding Response . . . . . . . . . .  51
     7.11. Subsequent Offer/Answer Exchanges . . . . . . . . . . . .  53
       7.11.1.  Sending of a Subsequent Offer  . . . . . . . . . . .  53
       7.11.2.  Receiving the Offer and Sending an Answer  . . . . .  56
       7.11.3.  Receiving the Answer . . . . . . . . . . . . . . . .  59
     7.12. Binding Keepalives  . . . . . . . . . . . . . . . . . . .  59
     7.13. Sending Media . . . . . . . . . . . . . . . . . . . . . .  61
     7.14. Receiving Media . . . . . . . . . . . . . . . . . . . . .  63
   8.  Guidelines for Usage with SIP . . . . . . . . . . . . . . . .  64
   9.  Interactions with Forking . . . . . . . . . . . . . . . . . .  66
   10. Interactions with Preconditions . . . . . . . . . . . . . . .  67
   11. Examples  . . . . . . . . . . . . . . . . . . . . . . . . . .  67
     11.1. Basic Example . . . . . . . . . . . . . . . . . . . . . .  68
     11.2. Advanced Example  . . . . . . . . . . . . . . . . . . . .  72
   12. Grammar . . . . . . . . . . . . . . . . . . . . . . . . . . .  93
   13. Security Considerations . . . . . . . . . . . . . . . . . . .  95
     13.1. Attacks on Connectivity Checks  . . . . . . . . . . . . .  95
     13.2. Attacks on Address Gathering  . . . . . . . . . . . . . .  98
     13.3. Attacks on the Offer/Answer Exchanges . . . . . . . . . .  99
     13.4. Insider Attacks . . . . . . . . . . . . . . . . . . . . .  99
       13.4.1.  The Voice Hammer Attack  . . . . . . . . . . . . . .  99
       13.4.2.  STUN Amplification Attack  . . . . . . . . . . . . .  99
   14. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 100
     14.1. candidate Attribute . . . . . . . . . . . . . . . . . . . 100
     14.2. remote-candidate Attribute  . . . . . . . . . . . . . . . 100
     14.3. ice-pwd Attribute . . . . . . . . . . . . . . . . . . . . 101
   15. IAB Considerations  . . . . . . . . . . . . . . . . . . . . . 101



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     15.1. Problem Definition  . . . . . . . . . . . . . . . . . . . 102
     15.2. Exit Strategy . . . . . . . . . . . . . . . . . . . . . . 102
     15.3. Brittleness Introduced by ICE . . . . . . . . . . . . . . 103
     15.4. Requirements for a Long Term Solution . . . . . . . . . . 104
     15.5. Issues with Existing NAPT Boxes . . . . . . . . . . . . . 104
   16. Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . 104
   17. References  . . . . . . . . . . . . . . . . . . . . . . . . . 105
     17.1. Normative References  . . . . . . . . . . . . . . . . . . 105
     17.2. Informative References  . . . . . . . . . . . . . . . . . 106
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . . 108
   Intellectual Property and Copyright Statements  . . . . . . . . . 109








































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1.  Introduction

   RFC 3264 [4] defines a two-phase exchange of Session Description
   Protocol (SDP) messages [5] for the purposes of establishment of
   multimedia sessions.  This offer/answer mechanism is used by
   protocols such as the Session Initiation Protocol (SIP) [2].

   Protocols using offer/answer are difficult to operate through Network
   Address Translators (NAT).  Because their purpose is to establish a
   flow of media packets, they tend to carry IP addresses within their
   messages, which is known to be problematic through NAT [15].  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 [17], Simple Traversal of UDP
   through NAT (STUN) [14] and its revision [12], the STUN Relay Usage
   [13], and Realm Specific IP [18] [19] along with session description
   extensions needed to make them work, such as the Session Description
   Protocol (SDP) [5] attribute for the Real Time Control Protocol
   (RTCP) [1].  Unfortunately, these techniques all have pros and cons
   which make each one optimal in some network topologies, but a poor
   choice in others.  The result is that administrators and implementors
   are making assumptions about the topologies of the networks in which
   their solutions will be deployed.  This introduces complexity and
   brittleness into the system.  What is needed is a single solution
   which is flexible enough to work well in all situations.

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


2.  Overview of ICE

   A typical architecture for an ICE deployment is shown in Figure 1.
   The figure shows two endpoints (known as agents in RFC 3264
   terminology) which we call L and R (for left and right, which helps
   visualize call flows).  Both L and R are behind a NAT.  The type of



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   NAT and its properties are unknown.  Indeed, it is not known whether
   the agent is behind a NAT at all, or whether there are multiple NATs
   between it and the network.  Agents A and B are capable of engaging
   in an offer/answer exchange [4] by which they can exchange SDP
   messages, whose purpose is to set up a media session between A and B.
   Of course, the offer/answer exchange itself must be capable of
   traversing the NAT.  Such traversal is facilitated through signaling
   elements such as SIP servers, and is outside the scope of this
   specification.  Different solutions are applied for traversal of the
   signaling that carries the offer/answer exchange, and for the media
   set up by that offer/answer exchange.  This is because of the vastly
   different requirements on latency, packet loss, and overall bandwidth
   between the signaling and media.  For example, usage of a signaling
   intermediary, such as a SIP proxy, as a relay for all signaling at
   all times, is acceptable, whereas usage of relays at all times for
   media is highly undesirable.

   In addition to the agents, a SIP server and NATs, ICE is typically
   used in concert with STUN servers in the network.  Each agent can
   have its own STUN server, or they can be the same.



                              +-------+
                              | SIP   |
           +-------+          | Srvr  |          +-------+
           | STUN  |          |       |          | STUN  |
           | Srvr  |          +-------+          | Srvr  |
           |       |                             |       |
           +-------+                             +-------+







               +--------+                   +--------+
               |  NAT   |                   |  NAT   |
               +--------+                   +--------+



           +-------+                             +-------+
           | Agent |                             | Agent |
           |   L   |                             |   R   |
           |       |                             |       |
           +-------+                             +-------+



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   Figure 1

   Prior to initiating an offer, the offering agent (L in this example)
   starts by performing a process known as address gathering.  This
   process allows the client to obtain one or more transport addresses,
   one more of which might be viable addresses at which the agent can
   receive incoming media packets from the other agent, which we call
   its peer.  A transport address is just the combination of an IP
   address and port.  With ICE, an agent will actually provide its peer
   with all of its possible transport addresses, and ICE will figure out
   which one to actually use.

   Naturally, one viable transport address is one obtained directly from
   a local interface the client has towards the network.  Such a
   transport address is called a local transport address.  The local
   interface could be one on a local layer 2 network technology, such as
   ethernet or WiFi, or it could be one that is obtained through a
   tunnel mechanism, such as a Virtual Private Network (VPN) or Mobile
   IP (MIP).  In all cases, these appear to the agent as a local
   interface from which ports (and thus transport addresses) can be
   allocated.

   If an agent is multihomed, it can obtain a transport address from
   each interface.  Depending on the location of the peer on the IP
   network, the agent may be reachable through one of those interfaces,
   or through another.  Consider, for example, an agent which has a
   local interface to a private net 10 network, and also to the public
   Internet.  A transport address from the net10 interface will be
   directly reachable when communicating with a peer on the same private
   net 10 network, while a transport address from the public interface
   will be directly reachable when communicating with a peer on the
   public Internet.  Rather than trying to guess which interface will
   work prior to sending an offer, the offering agent includes both
   transport addresses in its offer.

   Indeed, when using a media technology like the Real Time Transport
   Protocol (RTP), an agent needs two transport addresses on each
   interface - one for the RTP, and one for the Real Time Control
   Protocol (RTCP).  Other media technologies may require a multiplicity
   of transport addresses to be used and treated as a bundle.  Each of
   these transport addresses is called a component.  There are two
   components in an RTP stream - the RTP itself, and the RTCP.  In ICE,
   the set of transport addresses that represent an atomic grouping on
   which communications is possible is called a candidate.  In the
   example so far, the agent would obtain two candidates - one from the
   net 10 interface, and one from the interface on the public Internet.
   Each candidate would contain two transport addresses, corresponding
   to each of the two components.



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   Once the agent has obtained local transport addresses, it uses STUN
   to obtain additional transport addresses.  To do this, it would send
   a STUN Binding Request, using the Binding Discovery Usage [12] or the
   Relay Usage [13] from a local transport address, to its STUN server.
   It is assumed that the address of the STUN server is configured, or
   learned in some way.  Indeed, an agent might even have multiple STUN
   servers.  As a consequence of communicating with the STUN server, the
   agent can learn potentially two new types of transport addresses -
   server reflexive transport addresses and relayed transport addresses.
   The relationship of these addresses to the local transport address is
   shown in Figure 2.


                 To Internet

                     |
                     |
                     |  /------------  Relayed
                     | /               Address
                 +--------+
                 |        |
                 |  STUN  |
                 | Server |
                 |        |
                 +--------+
                     |
                     |
                     | /------------  Server
                     |/               Reflexive
               +------------+         Address
               |    NAT     |
               +------------+
                     |
                     | /------------  Local
                     |/               Address
                 +--------+
                 |        |
                 | Agent  |
                 |        |
                 +--------+

   Figure 2

   The local transport address is resident on the agent itself.  Through
   either the Binding Discovery Usage or the Relay Usage, the agent can
   discover its server reflexive transport address.  This is the address
   on the public side of the NAT, facing the STUN server.  It is the
   transport address allocated to the agent on the public side of the



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   NAT as a consequence of the transmission of the STUN request through
   the NAT, to the STUN server.  The NAT will allocate a binding,
   mapping this server reflexive transport address to the local
   transport address.  Packets received at the NAT, targeted towards the
   server reflexive transport address, will have their destination
   address rewritten to the local transport address by the NAT, and then
   be forwarded to the agent.  When there are multiple NATs between the
   agent and the STUN server, the STUN request will create a binding on
   each NAT, but only the outermost server reflexive transport address
   will be discovered by the agent.

   In addition, through the Relay Usage, the agent can request that the
   STUN server itself allocate a transport address from one of its local
   interfaces, and establish a binding that maps that transport address
   (called a relayed transport address, naturally) towards the source
   transport address of the STUN request, which will actually be equal
   to the server reflexive transport address allocated by the outermost
   NAT.  Consequently, packets sent to the relayed transport address
   will be routed by the IP network towards the STUN server.  The STUN
   server will receive them, rewrite the destination address to be equal
   to the server reflexive transport address, and forward them.  They
   will then arrive at the NAT, where the destination address is
   rewritten once again, and the packet forward finally to the agent at
   its local address.

   Since the server reflexive transport addresses and relayed transport
   addresses and obtained from a local transport address, they are said
   to be derived transport addresses, since they are derived from (and
   ultimately map to) their associated local transport address.  During
   the process of address gathering, the agent will obtain as many
   transport addresses of a given type as are needed for the media
   session.  For example, with RTP, two transport addresses are needed
   for a candidate.  The agent will obtain two server reflexive
   transport addresses (each derived from a local transport address),
   and they would be used to constitute a server reflexive candidate.
   The local transport addresses make up a local candidate, and the
   relayed transport addresses make up a relayed candidate.














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              Server                         Server
             Reflexive                      Reflexive
             Candidate                      Candidate
           ..............                 ..............
           .            .                 .            .
           .  +-+  +-+  .                 .  +-+  +-+  .
           .  | |  | |  .                 .  | |  | |  .
           .  +-+  +-+  .                 .  +-+  +-+  .
           .   ^    ^   .                 .   ^    ^   .
           ....|....|....                 ....|....|....
               |    |                         |    |
               |    |                         |    |
           ....|....|....                 ....|....|....
           .   |    |   .                 .   |    |   .
           .  +-+  +-+  .   Local         .  +-+  +-+  .   Local
           .  | |  | |  . Candidate       .  | |  | |  . Candidate
           .  +-+  +-+  .                 .  +-+  +-+  .
           .   |    |   .                 .   |    |   .
           ....|....|....                 ....|....|....
               |    |                         |    |
               |    |                         |    |
           ....|....|....                 ....|....|....
           .   V    V   .                 .   V    V   .
           .  +-+  +-+  .                 .  +-+  +-+  .
           .  | |  | |  .                 .  | |  | |  .
           .  +-+  +-+  .                 .  +-+  +-+  .
           .            .                 .            .
           ..............                 ..............
              Relayed                        Relayed
             Candidate                      Candidate


                                  Legend
                                  ------

                           +-+
                           | | Transport Address
                           +-+

                          ---> Derived From

                           ...
                           . . Candidate
                           ...


   Figure 3




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   The relationship between these various transport addresses and
   candidates is shown pictorially in Figure 3.  The figure shows our
   example agent with two local interfaces, each of which provides two
   transport address pairs to make up two candidates.  From those two
   local candidates, a server reflexive and relayed candidate are
   derived.

   Once the agent has completed gathering its candidates, it assigns
   each a candidate identifier, called the candidate ID.  The candidate
   ID is a random number used to uniquely identify each candidate, and
   is used in the connectivity checks discussed below.  The components
   of each candidate are ordered numerically, starting at one, such that
   each transport address has a component ID.  For example, in an RTP
   candidate there are two components, component ID 1 and component ID
   2.  Each transport address pair is therefore uniquely identified by a
   combination of its candidate ID and its component ID.  The
   combination of the two is called, unsurprisingly, a transport address
   ID, or tid for short.

   The agent will place all of its candidates in an offer, using a new
   SDP attribute called the candidate attribute.  This attribute
   contains the actual transport address, the candidate ID and component
   ID, and a q-value.  The q-value is used for the agent to prioritize
   its candidates.  An agent will typically prefer to receive media at
   particular candidates over other candidates, based on local policy.
   For example, an agent would normally prefer to receive interactive
   voice RTP packets at its local candidate as opposed to its relayed
   candidate, due to the extra latency incurred by traveling through the
   relay.

   The candidate attribute will also include an indicator of the type of
   candidate (server reflexive, local, relayed), and its related
   transport address.  For server reflexive transport addresses, the
   related transport address is the local transport address from which
   it was derived.  For relayed transport addresses, the related
   transport address is the server reflexive address towards the relay.
   The related transport address for reflexive candidates is used by the
   ICE algorithm itself, as explained below.  For relayed candidates,
   the related transport address is not used by ICE directly; it is
   useful for diagnostic purposes and for Quality of Service mechanisms
   that require knowledge of addresses closer to the agent.

   Finally, the agent chooses one of its candidates for inclusion in the
   m and c lines (called the m/c-line collectively).  Assuming that
   candidate is verified as functional by the ICE connectivity checks
   described below, this is the actual IP address and port to which
   media will be sent.  The candidate selected for inclusion in the m/c-
   line of an offer (or an answer) is called the operating candidate,



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   since it is the one that is the in-use destination for receipt of
   media traffic.

   Once the operating candidate is chosen, the agent sends the offer.
   Through the wonders or SIP or other signaling protocols, this offer
   is delivered to the peer, which must now select its answer.  To
   create the answer, the agent starts by gathering addresses, in
   exactly the same way the offered did.  It includes those as
   candidates in its answer, and selects one as the operating candidate,
   just like the offered did.  It then sends the answer.

   Each agent then pairs up each of its candidates with the candidates
   of its peer.  From the perspective of the offerer, the set of
   candidates it sent in its offer are called its native candidates, and
   the ones received in the answer are the remote candidates.
   Similarly, from the perspective of the answerer, the set of
   candidates it sent in its answer are the native candidates, and the
   ones received in the offer are the remote candidates.  Both agents
   pair up each of their native candidates with each of the remote
   candidates, producing a set of candidate pairs.  If there were N
   native candidates and M remote candidates, there will be N*M
   candidate pairs.  Within each candidate pair, the transport addresses
   themselves are paired up one for one, resulting in transport address
   pairs as well.  The transport addresses are paired up such that they
   have identical component IDs.  Each transport address pair has an ID,
   called the transport address pair ID, formed by concatenating the
   transport address IDs of its two transport addresses.

   Once the pairing is done, the transport address pairs are ordered in
   such a way that both the offerer and answerer will end up with the
   same order.  This ordering is done by using the q-values each side
   provided, along with the candidate IDs to help break ties.  Then,
   each side begins a process known as connectivity checks.
   Connectivity checks are STUN transactions, using the connectivity
   check usage of STUN, sent from the native transport address to the
   remote transport address of a particular transport address pair.  If
   an agent sends a STUN request and gets a successful response, the
   transport address pair is said to be Receive Valid, or Recv Valid for
   short, since the agent knows that its peer was able to receive a
   packet.  If an agent receives a request and sends a response, the
   transport address pair is said to be Send Valid, since the agent
   knows that its peer was able to send it a packet.  When transactions
   in both directions complete, the transport address pair is said to be
   Valid.  The idea behind ICE is that if a transport address pair is
   valid, it means that agents were able to succesfully exchange IP
   packets in both directions.  Consequently, any media packets, which
   are sent to and from exactly the same IP addresses and ports, should
   also work, since they don't differ in their IP addresses or ports.



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   It's important to point out that, when used with ICE, an agent will
   always send and receive media on the same transport address.  That
   is, if an agent includes a transport address of 192.0.2.1:2444
   (meaning an IP address of 192.0.2.1 and port of 2444) in its SDP for
   receiving RTP packets (and also STUN connectivity check), it will not
   only receive STUN requests and RTP packets on this transport address,
   it will also send STUN requests and RTP packets from this transport
   address.  This property, known as symmetric RTP, is essential for
   proper operation of ICE.  Peer reflexive transport addresses,
   discussed further below, will generally only work when symmetric RTP
   is used.  Symmetric RTP is also key for keeping NAT bindings alive.

   Since there can be quite a few transport address pairs to check,
   performing all of the connectivity checks in parallel can cause
   substantial load on the network.  Instead, each agent will start at
   the top of the ordered list they each created, and every 50ms, begin
   a new connectivity check.

   In order to succesfully process a STUN connectivity check, an agent
   must be able to correlate the STUN request or response with the
   transport address pair whose connectivity the STUN message is meant
   to validate.  To perform this correlation, the STUN connectivity
   checks contain a USERNAME attribute formed in a special way.  In
   particular, the USERNAME contains the actual transport address pair
   ID, which, as described above, is formed by concatenating the
   transport address IDs of each of the candidates.  The USERNAME is
   used in conjunction with an authentication and message integrity
   operation on the STUN message that requires a password.  This
   password is conveyed in the offer/answer exchange, and is a random
   number valid only for the duration of the media session.  This
   ensures that, if the signaling channel carrying the offer/answer
   exchange is secure, the agent can be certain that its STUN
   connectivity checks are taking place with the agent which responded
   to the signaling.

   Because each agent is receiving STUN requests on the same IP address
   and port that media will later be sent to, each agent is effectively
   acting as its own mini STUN server, implementing the connectivity
   check usage described in [12].  Like all STUN servers, when the agent
   sends a STUN response to a request, the response includes the XOR-
   MAPPED-ADDRESS attribute that contains the source IP address and port
   that the request came from.  In certain deployment scenarios, and in
   particular where one of the agents is behind a NAT whose address and
   port mapping properties are address and port dependent [32], this
   source IP address and port may differ from the server reflexive ones
   allocated by the peer during the address gathering phase.  This
   source IP address and port, conveyed in the XOR-MAPPED-ADDRESS
   attribute of the STUN response, therefore constitutes a new transport



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   address, called a peer reflexive transport address, which can be used
   for communications.


                                      +-------+
                                      | STUN  |
                                      | Srvr  |
                                      |       |
                                      +-------+
                                           ^
                                           |
                                           |
                                           |
                                           |
             +--------------------------+  |
             |                     NAT-2|  |NAT-1
             |                      +-----------+
             |                      |  APD NAT  |
             |                      +-----------+
             |                          |  |
             |                           \ |
             VL1                          \|R1
         +-------+                    +-------+
         | Agent |                    | Agent |
         |   L   |                    |   R   |
         |       |                    |       |
         +-------+                    +-------+

   Figure 4

   Consider the example of Figure 4.  The agent on the left, agent L,
   has a single interface and is not behind a NAT.  Consequently, it
   ends up with a single candidate with a single transport address
   (normally two for RTP, but we'll consider just one for ease of
   explanation), transport address L1.  It sends an offer to agent R,
   which is behind one of these Address and Port Dependent (APD) mapping
   NATs.  Agent R has a local transport address R1, and obtains a server
   reflexive transport address from its STUN server, transport address
   NAT-1.  Now, when agent R sends a connectivity check from its local
   transport address (R1) to L's local transport address (L1), this
   check will traverse the NAT.  The connectivity check itself will
   create a new mapping in the NAT and be allocated a new binding on the
   NAT - NAT-2.  This STUN request arrives at L, which generates a STUN
   response containing transport address NAT-2.  Agent R, noticing that
   this is not the same as its other two transport addresses, treats
   this as a new peer reflexive transport address.

   This new peer reflexive transport address is paired up with the



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   remote transport address containing the STUN server from which that
   transport address was learned (transport address L1 in the example
   above).  This becomes a new transport address pair, and connectivity
   checks are run on it as well.

   Once all of the transport address pairs in a candidate pair have been
   validated, that candidate pair is ready to be used.  Media starts
   being sent on it immediately, and the offerer will send an updated
   offer, now containing the agents half of the validated candidate pair
   in the m/c-line.  This is called "promoting a candidate to
   operating".  The updated offer only contains a single candidate
   attribute - the one for the operating candidate.  It also contains an
   attribute, called the remote-candidate attribute, which tells the
   answerer the remote candidate in the validated candidate pair.  The
   answerer uses this attribute, along with its own view on the states
   of the candidate pairs, to place a candidate in the m/c-line and
   populate the candidate attributes in its answer.

   It is important to understand that, when ICE is in use, media is not
   sent to a candidate without validation, even if that candidate
   appears in the m/c-line.  This is in order to avoid denial-of-service
   attacks.  In particular, without ICE, an offerer can send an offer to
   another agent, and list the IP address and port of a target in the
   offer.  If the agent is an automata that answers a call
   automatically, it will do so and then proceed to send media to the
   target.  This provides substantial packet amplifications.  ICE fixes
   this by requiring that an agent never send media packets unless it
   has sent a STUN message towards the target of the RTP packets, and
   received a reply from that target.  See Section 7.13 for details.

   A summary of this overall behavior is shown in the basic call flow in
   Figure 5.



















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


   Figure 5


3.  Terminology

   Several new terms are introduced in this specification:

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

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

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

   Local Transport Address: A local transport address is a transport
      address that has been allocated from the operating system on the
      host.  This includes transport addresses obtained through Virtual
      Private Networks (VPNs) and transport addresses obtained through
      Realm Specific IP (RSIP) [18] (which lives at the operating system
      level).  Transport addresses are typically obtained by binding to
      an interface.



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   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 obtained 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 more generally, any UNSAF protocol [20].

   Reflexive Transport Address: As defined in [12], a derived transport
      address learned by a client which identifies that client as seen
      by another host on an IP network, typically a STUN server.  When
      there is an intervening NAT between the client and the other host,
      the reflexive transport address represents the binding allocated
      to the client on the public side of the NAT.  Reflexive transport
      addresses are learned from the XOR-MAPPED-ADDRESS attribute in
      STUN Binding Responses and Allocate Responses [13].

   Server Reflexive Transport Address: A server reflexive transport
      address is a reflexive address that is reflected off of a server,
      distinct from the peer, whose address is configured or learned by
      the client prior to an offer/answer exchange.

   Peer Reflexive Transport Address: A peer reflexive transport address
      is a reflexive address that is reflected off of the peer.  Peer
      reflexive transport addresses are learned by connectivity checks.

   Relayed Transport Address: A derived transport address that
      terminates on a server, and is forwarded towards the client.  The
      STUN Allocate Request, defined as part of the STUN relay usage
      [13] can be used to obtain a relayed transport address, for
      example.

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

   Candidate: A sequence of transport addresses that form an atomic set
      for usage with a particular media session.  Here, atomic means
      that all of transport addresses in the candidate need to work
      before the candidate will be used for actual media transport.  In



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      the case of RTP, there can be one or more transport addresses per
      candidate.  In the most common case, there are two - one for RTP,
      and another for RTCP.  If the agent doesn't use RTCP, there would
      be just one.  If Generic Forward Error Correction (FEC) [16] is in
      use, there may be more than two.  The transport addresses that
      compose a candidate are all of the same type - local, server
      reflexive, peer reflexive or relayed.

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

   Server Reflexive Candidate: A candidate whose transport addresses are
      server reflexive transport addresses.

   Peer Reflexive Candidate: A candidate whose transport addresses are
      peer reflexive transport addresses.

   Relayed Candidate: A candidate whose transport addresses are relayed
      transport addresses.

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

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

   Candidate ID: An identifier for a candidate.

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

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

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

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






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

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

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

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

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

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

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

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

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


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, server reflexive ones, and relayed 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, the agent determines a
   priority for each candidate and identifies the operating candidate
   that will be used for receipt of media, as described in Section 7.2.

   The next step is to construct the offer message.  For each media
   stream, it places its candidates into a=candidate attributes in the
   offer and puts its operating candidate into the m/c line.  The
   process for doing this is described in Section 7.3.  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 the offer does, the offerer
   supports ICE.  In that case, it starts gathering candidates, as
   described in Section 7.1, and prioritizes them as described in
   Section 7.2.  This processing is done immediately on receipt of the
   offer, to prepare for the case where the user should accept the call,
   or early media needs to be generated.  By gathering candidates (and
   performing connectivity checks) while the user is being alerted to
   the request for communications, session establishment delays are
   reduced.

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

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


6.  Processing the Answer

   There are two possible cases for processing of the answer.  If the
   answerer did not support ICE, the answer will not contain any
   a=candidate attributes.  As a result, the offerer knows that it



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   cannot perform its connectivity checks.  In this case, it proceeds
   with normal media processing as if ICE was not in use.  However, it
   SHOULD send media with the symmetric property described in
   Section 7.13, and follow the keepalive procedures in Section 7.12.

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

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


7.  Common Procedures

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

7.1.  Gathering Candidates

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

   Each candidate has one or more components, each of which is
   associated with a sequence number, starting at 1 for the first
   component of each candidate, and incrementing by 1 for each
   additional component within that candidate.  These components
   represent a set of transport addresses for which connectivity must be
   validated.  For a particular media stream, all of the candidates
   SHOULD have the same number of components.  The number of components
   that are needed are a function of the type of media stream.  All of
   the components in a candidate MUST be of the same type - server
   reflexive, relayed, or local, and obtained from the same server in
   the case of server reflexive or relayed candidates.  For local
   candidates, each component MUST be obtained from the same interface.
   For server reflexive and relayed candidates, each component MUST be
   derived from a component with the same component ID, all of which
   come from a single local candidate.

   For traditional RTP-based media streams, it is RECOMMENDED that there
   be two components per candidate - one for RTP and one for RTCP.  The
   component with the component ID of 1 MUST be RTP, and the one with
   component ID of 2 MUST be RTCP.  If an agent doesn't implement RTCP,



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   it SHOULD have a single component for the RTP stream (which will have
   a component ID of 1 by definition).  Each component of a candidate
   has a single transport address.

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

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

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

   Agents which serve end users directly, such as softphones,
   hardphones, terminal adapters and so on, MUST implement the STUN
   Binding Discovery usage and SHOULD use it to obtain server reflexive
   candidates.  These devices SHOULD implement the STUN Relay usage, and
   SHOULD use its Allocate request to obtain both server reflexive and
   relayed candidates.  They MAY implement and MAY use other protocols
   that provide derived transport addresses, such as TEREDO [29].

   The requirement to use the relay Usage is at SHOULD strength to allow
   for provider variation.  If it is not to be used, it is RECOMMENDED
   that it be implemented and just disabled through configuration, so
   that it can re-enabled through configuration if conditions change in
   the future.

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





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      Why would these types of endpoints even bother to implement ICE?
      The answer is that such an implementation greatly facilitates NAT
      traversal for clients that connect to it.  Consider a PC softphone
      behind a NAT whose mapping policy is address and port dependent.
      The softphone initiates a call through a gateway that implements
      ICE.  The gateway doesn't obtain any server reflexive or relayed
      transport addresses, but it implements ICE, and consequently, is
      prepared to receive STUN connectivity checks on its local
      transport addresses.  The softphone will send a STUN connectivity
      to check to that local transport address, causing the NAT to
      allocate a new binding for the softphone.  The connectivity check
      will inform the softphone of this address, allowing it to be used
      by the gateway as a peer reflexive remote candidate.  This allows
      direct media transmission between the gateway and softphone,
      without the need for relays.  Furthermore, implementation of the
      STUN connectivity checks allows for NAT bindings along the way to
      be kept open.  ICE also provides numerous security properties that
      are independent of NAT traversal, and would benefit any multimedia
      endpoint.  See Section 13 for a discussion on these benefits.

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

   Derived candidates can be obtained from the STUN Binding Discovery
   usage or the STUN Relay usage.  The latter is preferred since it will
   provide the client with both a server reflexive and a relayed
   transport address with a single transaction.  It is possible that
   some STUN servers will only support the Relay usage or only the
   Binding Discovery usage, in which case a client might be configured
   with different servers depending on the usage.

   To obtain both server reflexive and relayed candidates using the STUN
   Relay Usage, the client takes a local UDP candidate, and for each
   configured STUN server, produces both candidates.  It is anticipated
   that clients may have a multiplicity of STUN servers configured or
   discovered in network environments where there are multiple layers of
   NAT, and where that layering is known to the provider of the client.



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   To obtain these candidates, for each configured STUN server, the
   client initiates an Allocate Request transaction using the procedures
   of Section 8.1.2 of [13] from each transport address of a particular
   local candidate.  The Allocate Response will provide the client with
   its server reflexive transport address (obtained from the XOR-MAPPED-
   ADDRESS attribute) and its relayed transport address in the RELAY-
   ADDRESS attribute.  Indeed, these two transport addresses are related
   to each other.  The relay will forward packets received on the
   relayed transport address towards that server reflexive transport
   address.  As such, the server reflexive transport address is said to
   be the associated server reflexive transport address for that relayed
   address.  Once the Allocate requests have given a client a relayed
   transport address for all transport addresses in a relayed candidate,
   there is no reason for a client to obtain further relayed candidates
   through the same STUN server.  Thus, if there are other local
   candidates from which the client has not yet obtained relayed
   transport address, the client SHOULD NOT bother to obtain them.
   Instead, it SHOULD use the STUN Binding Discovery usage and obtain
   just server reflexive addresses from that STUN server.  The order in
   which local candidates are tried against the STUN server to obtain
   relayed candidates is a matter of local policy.

   To obtain server reflexive candidates using the STUN Binding
   Discovery usage, the client takes a local UDP candidate, and for each
   configured STUN server, produces a server reflexive candidate.  To
   produce the server reflexive candidate from the local candidate, it
   follows the procedures of Section 12.2 of [12] for each local
   transport address in the local candidate.  The Binding Response will
   provide the client with its server reflexive transport address.  If
   the client had K local candidates, this will produce S*K server
   reflexive candidates, where S is the number of STUN servers.

   Since a client will pace its STUN transactions (both Binding and
   Allocate requests) at a total rate of one new transaction every Ta
   seconds, it will take a certain amount of time to complete the
   address gathering phase.  It is RECOMMENDED that implementations have
   a configurable upper bound on the total amount of time allotted to
   address gathering.  Any transactions not completed at that point
   SHOULD be abandoned, but MAY continue and be used in an updated offer
   once they complete.  A default value of 5s is RECOMMENDED.  Since the
   total number of allocations that could be done (based on the number
   of STUN servers and local interfaces) might exceed this value,
   clients SHOULD prioritize their local candidates and STUN servers,
   performing transactions from the highest priority local candidates to
   the highest priority STUN servers first.  A STUN server would
   typically be higher priority if it supports the STUN Relay Usage,
   since such a server provides two transport addresses with one
   transaction.



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   Once the allocations are complete, any redundant candidates are
   discarded.  Candidate A is redundant with candidate B if the
   transport addresses of each component match, and each component of
   their associated local candidates match.  For example, consider a set
   of candidates with a single component.  One candidate is a local
   candidate, and its one component has a transport address of 10.0.1.1:
   4458.  A reflexive transport address is derived from this local
   transport address, producing a 10.0.1.1:4458.  These two candidates
   are identical, and also have identical associated local transport
   addresses, so they are redundant.


          +----------+
          | STUN Srvr|
          +----------+
               |
               |
             -----
           //     \\
          |         |
         |  B:net10  |
          |         |
           \\     //
             -----
               |
               |
          +----------+
          |   NAT    |
          +----------+
               |
               |
             -----
           //     \\
          |    A    |
         |192.168/16 |
          |         |
           \\     //
             -----
               |
               |
               |192.168.1.1        -----
          +----------+           //     \\           +----------+
          |          |          |         |          |          |
          | Offerer  |---------|  C:net10  |---------| Answerer |
          |          |10.0.1.1  |         | 10.0.1.2 |          |
          +----------+           \\     //           +----------+
                                   -----




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   Figure 6

   Consider the more complicated case of Figure 6.  In this case, the
   offerer is multi-homed.  It has one interface, 10.0.1.1, on network
   C, which is a net 10 private network.  The Answerer is on this same
   network.  The offerer is also connected to network A, which is
   192.168/16.  The offerer has an interface of 192.168.1.1 on this
   network.  There is a NAT on this network, natting into network B,
   which is another net10 private network, but not connected to network
   C. There is a STUN server on network B.

   The offerer obtains local transport address on its interface on
   network C (10.0.1.1:2498) and a local transport address on its
   interface on network A (192.168.1.1:3344).  It performs a STUN query
   to its configured STUN server from 192.168.1.1:3344.  This query
   passes through the NAT, which happens to assign the binding 10.0.1.1:
   2498.  The STUN server reflects this in the STUN Binding Response.
   Now, the offerer has obtained a candidate with a transport address it
   already has (10.0.1.1:2498), but from a new interface.  It therefore
   keeps it.  When it performs its connectivity checks, the offerer will
   end up sending packets from both interfaces, and those sent from its
   interface on network C will succeed.

7.2.  Prioritizing the Candidates and Choosing an Operating One

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

   This specification makes no normative statements 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 an intermediary.  That is, if
   media is sent to that candidate, will the media first transit an
   intermediate server before being received.  Relayed candidates are
   clearly one type of candidates that involve an intermediary.  Another
   are local candidates associated with a VPN server.  When media is
   transited through an intermediary, it can increase the latency



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   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 an intermediary 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) [23].  It can also help with hosts that have both a
   native IPv6 address and a 6to4 address.  In such a case, higher
   priority could be afforded to the native v6 address, followed by the
   6to4 address, followed by a native v4 address.  This allows a site to
   obtain and begin using native v6 addresses 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 that make use of
   relays.  In those cases, if an agent has preconfigured or dynamically
   discovered knowledge of the topological proximity of the relays to
   itself, it can use that to select closer relays with higher priority.

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

   Once the candidates have been prioritized, one may be selected as the
   operating one.  This is the candidate that will be used for actual
   exchange of media if and when its validated, until a higher priority
   candidate is validated.  The operating 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



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   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 relayed candidate from a
   publically accessible STUN 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 operating candidate be a relayed candidate from a STUN server
   providing public IP addresses in response to an Allocate request.
   Furthermore, ICE is only truly effective when it is supported on both
   sides of the session.  It is therefore most prudent to deploy it to
   close-knit communities as a whole, rather than piecemeal.  In the
   example above, this would mean that ICE would ideally be deployed
   completely within the enterprise, rather than just to parts of it.

   An additional consideration for selection of the operating candidate
   is the switching of media stream destinations between the initial
   offer and the subsequent offer.  The operating candidate pair in the
   initial offer is validated first, and if that validation succeeds,
   media will immediately begin to flow between the pair.  When the ICE
   checks complete and yield a higher priority candidate pair, media
   will begin to flow to it (there will also be an updated offer/answer
   exchange that changes the operating candidate).  This will result in
   a change in the destination of the media packets.  This may also
   cause a different path for the media packets.  That path might have
   different delay and jitter characteristics.  As a consequence, the
   jitter buffers may see a glitch, causing possible media artifacts.
   If these issues are a concern, the initial offer MAY omit an
   operating candidate.  This is done by including an m/c-line with an
   a=inactive attribute.  In such a case, an updated offer will need to
   be sent immediately when communicating with an ICE-unaware agent,
   setting an operating candidate.

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

7.3.  Encoding Candidates into SDP

   For each candidate for a media stream, the agent includes a series of
   a=candidate attributes as media-level attributes, one for each
   component in the candidate.  Each candidate has a unique identifier,
   called the candidate ID.  The candidate ID MUST be chosen randomly
   and contain at least 24 bits of randomness.  This means that a
   candidate ID must be at least 4 characters long, since each character
   in the base64 alphabet used for candidate IDs contains at most 6 bits
   of randomness.  A candidate ID MAY be longer than 4 characters, and



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   different candidate IDs MAY have different lengths.  It is chosen
   only when the candidate is placed into the SDP for the first time;
   subsequent offers or answers within the same session containing that
   same candidate MUST use the same candidate ID used previously. 24
   bits is sufficient because the candidate ID is not providing security
   (the much more random password is).  Its sole purpose is to make it
   highly unlikely that both the offerer and answerer select the same
   value for a candidate for the same media stream.  Different values
   for the candidate ID are required to break ties in the procedure that
   is used to order the candidate pairs.

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

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

   The transport, addr and port of the a=candidate attribute (all
   defined in Section 12) are set to the transport protocol, unicast
   address and port of the tranport address.  A Fully Qualified Domain
   Name (FQDN) for a host MAY be used in place of a unicast address.  In
   that case, when receiving an offer or answer containing an FQDN in an
   a=candidate attribute, the FQDN is looked up in the DNS using an A or
   AAAA record, and the resulting IP address is used for the remainder
   of ICE processing.  The qvalue is set to the priority of the
   candidate, and MUST be the same for all components of the candidate.

   The agent MUST include a type for the transport address by populating
   the candidate-types production with the appropriate value - "local"
   for local transport addresses, "srflx" for server reflexive
   candidates, and "relay" for relayed candidates.  If the transport
   address is server reflexive, the agent MUST include the rel-addr and
   rel-port productions containing the associated local transport



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   address for that server reflexive transport address.  There are
   environments in which the policy of an agent is such that it never
   provides local transport addresses in its offers or answers, for fear
   of revealing internal topology to external hosts.  In such cases, an
   agent MAY include a random transport address instead, as long as it
   is the same transport address for all server reflexive candidates
   derived from the same actual local transport address.  This is
   because the transport address in the rel-addr and rel-port production
   are used by the ICE algorithm itself for correlation purposes.

   If the tranport address is relayed, the agent SHOULD include the rel-
   addr and rel-port productions, containing the associated server
   reflexive transport address.  When a relayed address is obtained from
   a STUN relay, the associated server reflexive transport address is
   the value from the XOR-MAPPED-ADDRESS that was returned in the same
   STUN response which provided the relayed address to the agent.
   Though not used directly with ICE, the rel-addr and rel-port
   attributes are essential for proper functioning of QoS mechanisms,
   such as those defined by 3gpp and Packetcable.

   The rel-addr and rel-port production MUST NOT be present for a local
   transport address.

   All of the candidates for a media stream share a password that is
   used for securing the STUN connectivity checks.  The password will be
   used to process the MESSAGE-INTEGRITY attribute for STUN requests
   received by the agent.  The password for candidates for different
   media streams MAY be the same, or MAY be different.  This password
   MUST be chosen randomly with 128 bits of randomness (though it can be
   longer than 128 bits).  This password is contained in the a=ice-pwd
   attribute, present as a session or media level attribute.  Since each
   character of the ice-pwd attribute can represent six bits of
   randomness, the ice-pwd attribute will always be at least 22
   characters long.  New passwords MUST be selected for each new
   session, even if the transport address from a previous session is
   being recycled.

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




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

   Consider agents A, B, and C. A and B are within private enterprise 1,
   which is using 10.0.0.0/8.  C is within private enterprise 2, which
   is also using 10.0.0.0/8.  As it turns out, B and C both have IP
   address 10.0.1.1.  A sends an offer to C. C, in its answer, provides
   A with its transport addresses.  In this case, that is 10.0.1.1:8866
   and 10.0.1.1:8877.  As it turns out, B is in a session at that same
   time, and is also using 10.0.1.1:8866 and 10.0.1.1:8877.  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 and another to
   10.0.1.1:8877.  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, the transport address ID 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 (with a colon between) and uses the result as the
   username in its STUN query to 10.0.1.1:8866.  This STUN query arrives
   at B. However, the username is unknown to B, and so the request is
   rejected.  A treats the rejected STUN request as if there were no
   connectivity to C (which is actually true).  Therefore, the error is
   avoided.

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



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   applications - be prepared for unknown packets on any port.

   The operating candidate, if there is one, is placed into the m/c
   lines of the SDP.  For RTP streams, this is done by placing the RTP
   address and port into the c and m lines in the SDP respectively.  If
   the agent is utilizing RTCP, it MUST encode its address and port
   using the a=rtcp attribute as defined in RFC 3605 [1].  If RTCP is
   not in use, the agent MUST signal that using b=RS:0 and b=RR:0 as
   defined in RFC 3556 [6].

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

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

   Once an offer or answer are sent, an agent MUST be prepared to
   receive both STUN and media packets on each candidate.  As discussed
   in Section 7.13, media packets can be sent to a candidate prior to
   its promotion to operating.

7.4.  Forming Candidate Pairs

   Once the offer/answer exchange has completed, both agents will have a
   set of candidates for each media stream.  Each agent forms a set of
   candidate pairs for each media stream by combining each of its
   candidates with each of the candidates of its peer.  Candidates can
   be paired up only if their transport protocols are identical.  Each
   candidate has a number of components, each of which has a transport
   address.  Within a candidate pair, the components themselves are
   paired up such that transport addresses with the same component ID
   are combined to form a transport address pair.  If one candidate has
   more components than the other, those extra components will not be
   part of a transport address pair, won't be validated, and will
   effectively be treated as if they weren't included in the candidate
   pair in the first place.

   For example, if an offer/answer exchange took place for a session
   comprised of an audio and a video stream, and each agent had two
   candidates per media stream, there would be 8 candidate pairs, 4 for
   audio and 4 for video.  For each of the 8 candidate pairs, there
   would be two transport address pairs - one for RTP, and one for RTCP.

   The relationship between a candidate, candidate pair, transport
   address, transport address pair and component are shown in Figure 7.



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   This figure shows the relationships as seen by the agent that owns
   the candidate with candidate ID "L".  This candidate has two
   components with transport addresses A and B respectively.  This
   candidate is called the native candidate, since it is the one owned
   by the agent in question.  The candidate owned by its peer is called
   the remote candidate.  As the figure shows, there is a single
   candidate pair, and two components in each candidate.  The native
   candidate has a candidate ID of "L", and the remote candidate has a
   candidate ID of "R".  Since the two component IDs are 1 and 2,
   candidate "L" has two transport addresses with transport address IDs
   of "L:1" and "L:2" respectively.  Similarly, candidate "R" has two
   transport addresses with transport address IDs of "R:1" and "R:2"
   respectively.  Note that these candidate IDs are not actually legal
   since they are not sufficiently random.  However, we use "L" and "R"
   to keep the figures readable.

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























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

                              Candidate Pair


   Figure 7

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

7.5.  Ordering the Candidate Pairs

   Recall that when each candidate is encoded into SDP, it contains a
   qvalue between 1 and 0, with 1 being the highest priority.  Peer



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   reflexive candidates, learned through the procedures described in
   Section 7.10 also have a priority between 0 and 1.  For each media
   stream, the native candidates are ordered based on their qvalues,
   with higher q-values coming first.  Amongst candidates with the same
   qvalue, they are ordered based on candidate ID, using reverse ASCII
   sort order.  For example, the candidate with candidate ID "lagDx"
   sorts before the candidate with ID "bad79", and both of those follow
   the candidate with ID "m8zz".

   The usage of a reverse ASCII sort order is important; as discussed in
   Section 13, it allows peer-derived candidates to be preferred over
   native ones.

   The result of these ordering rules will be an ordered list of
   candidates.  The first candidate in this list is given a sequence
   number of 1, the next is given a sequence number of 2, and so on.
   This same procedure is done for the remote candidates.  The result is
   that each candidate pair has two sequence numbers, one for the native
   candidate, and one for the remote candidate.

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

   The resulting ordering of candidate pairs is called the candidate
   pair priority ordered list.

   As an example, consider two agents, A and B. One offers two
   candidates for a media stream with candidate IDs of "g9g9" and
   "8888", with q-values of 1.0 and 0.8 respectively.  The other answers
   with three candidates with candidate IDs of "h8h8", "6565" and
   "klkl", with q-values of 0.3, 0.2 and 0.1 respectively.  The
   following table shows the rank ordering of the six candidate pairs.
   The column labeled "Max SN" is the larger of the two sequence numbers
   in the candidate pair, and "Min SN" is the minimum.  The column
   labeled "Max Cand.  ID" is the value of the larger of the two
   candidate IDs in the candidate pair.







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   Order    A     A       A       B     B      B                   Max
          Cand. Cand.    Cand.  Cand. Cand.   Cand.   Max    Min   Cand.
            ID  q-value   SN      ID  q-value  SN     SN     SN     ID
   ---------------------------------------------------------------------
    1      g9g9  1.0      1     h8h8   0.3     1       1      1    h8h8
    2      8888  0.8      2     h8h8   0.3     1       2      1    h8h8
    3      g9g9  1.0      1     6565   0.2     2       2      1    g9g9
    4      g9g9  1.0      1     klkl   0.1     3       3      1    klkl
    5      8888  0.8      2     6565   0.2     2       2      2    8888
    6      8888  0.8      2     klkl   0.1     3       3      2    klkl

   The candidate pair priority ordered list is then used to obtain an
   ordered list of transport address pairs, on which the agent will, in
   order, attempt to send STUN connectivity checks.  This list, called
   the transport address pair check ordered list, is very similar to the
   candidate pair priority ordered list, but differs in two important
   respects.  Firstly, the candidate pairs matching the operating
   candidate pair (there can actually be more than one) get promoted to
   the top of the list.  This allows the operating candidate pair to be
   validated first.  Secondly, many of the checks would be redundant,
   and a filtering algorithm is used to eliminate these redundant
   checks.

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

   To form the transport address pair check ordered list, the candidate
   list is first modified by taking the candidate pairs corresponding to
   the operating candidate pair, and promoting them to the top of the
   list.  A candidate pair matches the operating candidate pair when its
   native and remote transport address match the native and remote
   transport addresses in the m/c-line, respectively.  In unusual
   circumstances, there may be more than one such candidate pair.  In
   such a case, they should be promoted such that the higher priority
   candidate pairs appear first.  In addition, it is possible that none
   of the candidate pairs match the operating candidate pair.  In that
   case, no candidate pairs are promoted.

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




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   The next step is to remove redundant transport addresses.  Starting
   at the top of the list, the agent moves down from one transport
   address pair to the next.  If a transport address pair under
   consideration has the same remote transport address as a previous
   pair, based on transport address pair ID comparisons, and the native
   transport address from that previous pair has the same origination
   transport address as the one under consideration (based on IP address
   and port comparison), the one under consideration is removed from the
   list.

   The origination transport address is the address that the agent would
   send from in order to emit a packet with that native transport
   address as a source transport address.  For a local transport
   address, the origination transport address is equal to that local
   transport address.  For a server reflexive transport address, the
   origination transport address is equal to the local transport address
   from which it was derived.  For relayed addresses, packets are
   emitted by explicitly sending them through the relay.  Consequently,
   the origination transport address is equal to the relayed address.

   After the agent has gone through the entire list, the result is the
   transport address pair check ordered list.

   The pairs that get removed are redundant since the agent would send a
   STUN connectivity check using the same source and destination
   addresses as a previous check.  Consequently, the connectivity check
   will provide no information to the remote agent except for the
   transport address pair ID its associated with.  These turn out to be
   unnecesary due to the STUN processing rules outlined below.

7.6.  Performing the Connectivity Checks

   Connectivity checks are a STUN usage defined in [12].  They are
   performed by sending peer-to-peer STUN Binding Requests.  These
   checks result in a transport address pair progressing through a state
   machine that captures the progress of the connectivity checks.  The
   specific state machine and the procedures for the connectivity checks
   are specific to the transport protocol.  This specification defines
   rules for UDP.  The state machine processing described in this
   section MUST be followed by agents.  Extensions to ICE that describe
   other transport protocols SHOULD describe the state machine and the
   procedures for connectivity checks.

   The set of states for a transport address pair visited by the offerer
   and answerer are depicted graphically in Figure 9.  Note that this
   state machine exists for all transport address pairs, including ones
   pruned from the transport address pair check ordered list.




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                                         |
                                         |Start
                                         |
                                         |
                                         V
                                   +------------+
                 +-----------------|            |
                 |                 |            |
                 |            +----|  Waiting   |----------------+
                 |            |    |            |                |
                 |            |    |            |                |
                 |      Miss  |    +------------+                |
                 |      ----  |          |                       |
        Match Res|        -   |          | Selected              | Match Req
        ---------|            |          | --------.             | -------
            -    |            |          | Send Req    Match Req | Send Req
                 |            |          V             --------- |
                 |  Match Res |    +------------+      Re-Xmit   |
                 |  --------- |    |            |      Req       |
                 |      -     |    |            |                |
                 |     +------c----| Testing    |-----------+    |
                 |     |      |    |            |           |    |
                 |     |      |    |            |           |    |
                 |     |      |    +------------+           |    |
                 |     |      |          |                  |    |
                 |     |      |          | Error or         |    |
                 |     |      |          | Miss             |    |
    Timer Tr     |     |      |          | -----            |    |
    --------     V     V      |          V   -              V    V
    Send Req +------------+   |    +------------+        +------------+
       +-----|            |   +--->|            |        |            |
       |     |   Recv-    |        |            |        |   Send-    |
       |     |   Valid    |------->|  Invalid   |<-------|   Valid    |
       |     |            |        |            |        |            |
       +---->|            | Error, |            | Error, |            |
             +------------+ Miss   +------------+ Miss   +------------+
                   |        -----        ^        -----        |
                   |          -          | Error,   -          |
                   |                     | Miss                |
                   |                     | -----               |
                   |                     |   -                 |
                   |               +------------+              |
                   |               |            |              |
                   |               |            |              |
                   +-------------->|   Valid    |<-------------+
                    Match Req      |            |   Match Res
                    ---------      |            |   ---------
                        -          +------------+       -



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                                     |       ^
                                     |       |
                                     |       |
                                     +-------+
                                      Timer Tr
                                      --------
                                      Send Req

   Figure 9

   The state machine has six states - Waiting, Testing, Recv-Valid,
   Send-Valid, Valid and Invalid.  In the Waiting state, the agent is
   waiting to send or receive a connectivity check for the pair.  In the
   Testing state, the agent has sent a connectivity check and is
   awaiting a response.  In the Recv-Valid state, the agent knows that
   its peer can receive packets from it on this transport address pair.
   In the Send-Valid state, the agent knows that its peer can send
   packets to it.  In the Valid state, the agent knows that its peer can
   both send and receive packets from it.

   Initially, all transport address pairs start in the Waiting state.
   In this state, the agent waits for one of three events - a chance to
   send a Binding Request, receipt of a Binding Request, or receipt of a
   Binding Response.

   Since there is an instance of the state machine for each transport
   address pair, Binding Requests and responses need to be matched to
   the specific state machine for which they were meant to apply.  As
   described below, the Binding Request may not be a match for the
   transport address pair it was meant to validate.  To find the
   transport address pair it was meant to validate, called the target
   transport address pair, the agent examinines the USERNAME of the
   incoming Binding Request.  The USERNAME directly contains the
   transport address pair ID for the pair it was meant to validate.
   Binding Responses are matched to their requests using the STUN
   transaction ID, and then mapped to the transport address pair from
   that.

   For each media stream, the agent starts a new connectivity check for
   a transport address pair every Tb*RND seconds.  Tb SHOULD scale
   linearly with the number of media streams, so that the pace of
   connectivity checks overall is invariant to the number of media
   streams.  Consequently, it is RECOMMENDED that Tb have a default
   value of N*50ms, where N is the number of media streams.  RND is a
   random number chosen uniformly between 0.7 and 1.3, and it helps to
   avoid synchronization between the transmission of connectivity checks
   for different media streams.  On average, if there are N media
   streams, the checks across all media streams will be paced out at a



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   total of N/Tb checks per second.  The check is started for the first
   transport address pair in the transport address pair check ordered
   list that is in the Waiting state.  The "Selected" event is passed to
   the state machine for this transport address pair, causing it to be
   moved to the Testing state.  The agent then sends a connectivity
   check using a STUN Binding Request, as outlined in Section 7.7.

   Once a STUN connectivity check begins, the processing of the check
   follows the rules for STUN.  Specifically, retransmits of STUN
   requests are done as specified in [12], and furthermore, if a
   transaction fails and needs to be retried, that retry can happen
   rapidly, as described below.  It doesn't "count" against the average
   rate limit of 1/Tb checks per second per media stream.  In addition,
   the keepalives that are generated for a valid pair do not count
   against the rate limit either.  The rate limit applies strictly to
   the start of connectivity checks for a transport address pair that
   has been newly signaled through an offer/answer exchange.

   When an agent receives a Binding Request, which per the processing
   rules of Section 7.8 produces a succesful response, the agent
   examines the source transport address of the request.  If the native
   transport address was relayed, this would be the source as seen by
   the relay.  For the STUN relay usage, that source transport address
   will be present in the REMOTE-ADDRESS attribute of a STUN Data
   Indication message, if the Binding Request was delivered through a
   Data Indication.  If the Binding Request was not encapsulated in a
   Data Indication, that source transport address is equal to the
   current active destination for the STUN relay session.

   If the source transport address matches the remote transport address
   of the target transport address pair, the Binding Request is
   considered to be a match for the target transport address pair.
   Consequently, a Match Req event is passed to the state machine for
   the target transport address pair.  If the state machine was in the
   Waiting or Testing state, the state machine moves into the Send-Valid
   state.  If it was previously in the Waiting state, the agent sends a
   connectivity check of its own for the target transport address pair,
   as outlined in Section 7.7.  If it was in the Testing state, it
   retransmits a Binding Request for the transaction in progress.  This
   retransmission is one that would not normally occur based on the
   procedures in [12].  ICE "prods" the STUN transaction state machine
   to send an extra retransmit, in addition to the one which is
   scheduled to be sent next.  This helps speed up bidirectional
   connectivity verification when one agent is behind a NAT with an
   address and port dependent filtering behavior [32].

   If the source transport addresses in the Binding Request was not a
   match for the remote transport address, the Binding Request is



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   considered to be a miss for the target transport address pair.
   Consequently, a Miss event is passed to the state machine of the
   target transport address pair, and it immediately moves into the
   Invalid state.  Typically, the source transport address won't match
   when there was a NAT between the sender and receiver with an address
   and port dependent mapping property, though there are other cases in
   which this can happen.

   Though it was a miss for the target transport address pair, the
   connectivity check may have been a match for a different transport
   address pair.  To determine this, the agent checks the source
   transport address of the Binding Request against all of the other
   remote transport addresses of transport address pairs for the same
   media stream that use the same transport protocol and share the same
   native transport address (based on transport address ID comparison)
   of the target.  Of those that match (assuming at least one matches),
   it refines the set further by selecting only those for whom the
   origination transport address of the remote transport address matches
   the origination transport address of the remote transport address in
   the target transport address pair.  The origination transport address
   for a remote transport address is obtained from information signaled
   in the SDP, and depends on the type.  For a local transport address,
   the origination address equals that local transport address.  For a
   server reflexive transport address, the origination address is
   obtained from the related address information provided in the SDP.
   For a relayed transport address, the origination transport address
   quals that relayed transport address.  For these three types, the
   type is signaled in the SDP.  For a peer derived transport address,
   the origination address is the same as the origination address of the
   generating transport address.

   If there was a match (there can only be either one or zero matches),
   this match is called the alternate.  In many cases, the alternate
   transport address pair will not be in the transport address pair
   check ordered list; it will have been one of the ones pruned.
   Indeed, this is why it was pruned - a check on the remaining
   transport address pairs can serve to validate it.  The state machine
   for the alternate is passed the Match Req event.  If it was in the
   Waiting state, this causes it to move into the Send-Valid state, and
   a connectivity check is generated for the alternate transport address
   pair.  It may have been in the Testing state, in which case it moves
   move into the Send-Valid state, and the agent restransmits the
   Binding Request for the transaction in progress.  If it was the in
   the Recv-Valid state, this causes it to move into the Valid state.

   If no alternate could be found, it means that a new remote transport
   address and corresponding origination transport address have been
   discovered.  In this case, the agent follows the procedures of



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   Section 7.10.1 to create a new transport address pair and state
   machine for it.

   If the Binding Request didn't generate a success response, an Error
   event is passed to the state machine of the target, causing it to
   move into the Invalid state.

   If the agent receives a successful response to its STUN request, it
   agent examines the transport address in the XOR-MAPPED-ADDRESS
   attribute of the response.  This will be a peer reflexive transport
   address.  If the peer reflexive transport address matches (based on
   IP address and port comparison) the native transport address of the
   target transport address pair, a Match Res event is passed to the
   state machine of the target.  If the state machine was in the Testing
   state, the state machine moves into the Recv-Valid state.  If it was
   in the Send-Valid state, it moves into the Valid state.

   If, however, the transport addresses didn't match, a Miss event is
   passed to the state machine of the target, and it immediately moves
   into the Invalid state.  The agent checks the peer reflexive
   transport address against all of the other native transport addresses
   for transport address pairs for the same media stream with the same
   transport protocol and the same remote transport address (based on
   comparison of transport address ID) as the target.  Of those that
   match (assuming at least one matches), it refines the set further by
   selecting only those for whom the origination transport address of
   the native transport address matches the origination address of the
   native transport address in the target transport address pair.  The
   resulting transport address pair (there can be only zero or one) is
   called the alternate.  In many cases, the alternate transport address
   pair will not be in the transport address pair check ordered list; it
   will have been one of the ones pruned.  The state machine for the
   alternate is passed the Match Res event.  If it was in the Waiting
   state, this causes it to move into the Recv-Valid state.  It may have
   been in the Testing state, in which case it moves move into the Recv-
   Valid state.  If it was the in the Send-Valid state, this causes it
   to move into the Valid state.

   If no alternate could be found, the Binding Response will create a
   new peer reflexive transport address, and the procedures of
   Section 7.10.2 are followed to create a new transport address pair
   and state machine for it.

   In any state, if the STUN transaction results in an error, the state
   machine moves into the Invalid state.  A STUN transaction produces an
   "error" based on the processing in Section 7.7, which indicates which
   STUN response codes constitute an error as far as ICE processing is
   concerned.



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   If a transport address pair is in the Recv-Valid or Valid state, an
   agent MUST generate a new STUN Binding Request transaction every Tr
   seconds.  This transaction ensures that NAT bindings for the
   transport address pair remain open while the candidate is under
   consideration.  The transaction is performed as outlined in
   Section 7.7.  These transactions can also be used to keep the NAT
   bindings alive when the candidate is promoted to operating, as
   described in Section 7.12.  Tr SHOULD be configurable, and SHOULD
   default to 15 seconds.  These STUN transactions are processed in the
   same way as any other, and can result in new peer derived transport
   addresses, or can fail and cause the transport address pair to be
   invalidated.

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

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

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

7.7.  Sending a Binding Request for Connectivity Checks

   An agent performs a connectivity check on a transport address pair by
   sending a STUN Binding Request from its native transport address, and



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   sending it to the remote transport address.  Sending from its native
   transport address is done by sending it from its origination
   transport address.  As mentioned above, the origination transport
   address depends on the type of transport protocol and the type of
   transport address (local, reflexive, or relayed).  This specification
   defines the meaning for UDP.  Specifications defining other transport
   protocols must define what this means for them.

   For UDP-based local transport addresses, sending from the local
   transport address has the meaning one would expect - the request is
   sent such that the source IP address and port equal that of the local
   transport address.  For reflexive transport addresses, it is sent by
   sending from the associated local transport address used to derive
   that reflexive address.  For relayed transport addresses, it is sent
   by using STUN mechanisms to send the request through the STUN relay
   (using the Send request).  Sending the request through the STUN relay
   server necesarily requires that the request be sent from the client,
   using the local transport address used to derive the relayed
   transport address.

   The Binding Request sent by the agent MUST contain the USERNAME
   attribute.  This attribute MUST be set to the transport address pair
   ID of the corresponding transport address pair as seen by its peer.
   Thus, for the first transport address pair in Figure 7, if the agent
   on the left sends the STUN Binding Request, the USERNAME will have
   the value R:1:L:1.  If the agent on the right sends the STUN Binding
   Request, the USERNAME will have the value L:1:R:1.  To be clear, the
   USERNAME that is used is NOT the one seen locally, but rather the one
   as seen by its peer.  The request SHOULD contain the MESSAGE-
   INTEGRITY attribute, computed according to [12].  The key used as
   input to the HMAC is the password provided by the peer for this
   remote transport address.  This password will be identical for all
   remote transport addresses for the same media stream.

   Note that all ICE implementations are required to be compliant to
   [12], as opposed to the older [14].  Consequently, all connectivity
   checks will contain the magic cookie in the STUN header, and cause
   the STUN server embedded in each ICE implementation to include XOR-
   MAPPED-ADDRESS attributes in the response, rather than MAPPED-
   ADDRESS.

   Once created, the STUN transaction is linked to the transport address
   pair so that, when the response is received, the state machine on the
   linked transport address pair can be updated.

   The STUN transaction will generate either a timeout, or a response.
   If the response is a 420, 500, or 401, the agent should try again as
   described in [12] (as mentioned above, it need not wait the roughly



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   Tb seconds to try again).  Either initially, or after such a retry,
   the STUN transaction might produce a non-recoverable failure response
   or a failure result inapplicable to this usage of STUN and thus
   unrecoverable.  If this happens, an error event is generated into the
   state machine, and the transport address pair enters the invalid
   state.

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

   If the Binding Response is a 200, the agent SHOULD check for the
   MESSAGE-INTEGRITY attribute and verify it, as discussed in [12].
   Indeed, this check SHOULD be done for all responses.  This will
   result in the response being discarded (eventually leading to a
   timeout), if the integrity check fails.

7.8.  Receiving a Binding Request for Connectivity Checks

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

   As discussed in [12], since the username and password for STUN
   requests are exchanged through another mechanism - here, ICE - the
   Shared Secret Request mechanism is not needed and need not be
   implemented by agents that provide the connectivity check usage.

   One of the candidates may be in use as the operating candidate, or
   may become promoted to the operating candidate in the next offer/
   answer exchange as a consequence of a successful validation.  In
   either case, both media and STUN packets will be sent to the



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   transport addresses comprising that candidate, causing both to
   receive on their associated local transport addresses.  The agent
   MUST be able to disambiguate them.  This is done trivially by looking
   for the STUN magic cookie as the value of the second 32-bit word in
   the packet.  If present, it identifies a STUN packet.

   Processing of the Binding Request proceeds in two steps.  The first
   is generation of the response, and the second is ICE-specific
   processing.  Generation of the response follows the general
   procedures of [12], and is independent of the state machinery
   described in Section 7.6.  The USERNAME is considered valid if one of
   the candidate IDs sent in an offer or answer is a prefix of the
   USERNAME (this will always be the case, even for peer reflexive
   candidates), and for the component indicated in the USERNAME, the
   associated local transport address matches the local transport
   address on which the request was received.  The password associated
   with that candidate ID, which was provided by the agent to its peer,
   is used to verify the MESSAGE-INTEGRITY attribute, if one was present
   in the request.  If the USERNAME is not valid, the agent generates a
   430.  Otherwise, the success response will include the XOR-MAPPED-
   ADDRESS attribute, which is used for learning new candidates, as
   described in Section 7.10.  The XOR-MAPPED-ADDRESS attribute is
   constructed using the source IP address and port of the Binding
   Request.  For Binding Requests received over relayed transport
   addresses, this MUST be the source IP address and port of the Binding
   Request when it arrived at the relay, prior to forwarding towards the
   agent.  That source transport address will be present in the REMOTE-
   ADDRESS attribute of a STUN Data Indication message, if the Binding
   Request was delivered through a Data Indication.  If the Binding
   Request was not encapsulated in a Data Indication, that source
   address is equal to the current active destination for the STUN relay
   session.

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

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



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   Binding Request, and the resulting actions described in Section 7.6
   are undertaken.

   An agent will continue to receive periodic STUN connectivity checks
   on a local transport address as long as it had listed that transport
   address, or one derived from it, in an a=candidate attribute in its
   most recent offer or answer and the transport address is for UDP.
   Whether STUN keepalives are used for other transport protocols is
   defined by the specifications for that transport protocol.  The agent
   processes any such transactions according to this section.  It is
   possible that a transport address pair that was previously valid may
   become invalidated as a result of a subsequent failed STUN
   transaction.

7.9.  Promoting a Candidate to Operating

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

   The agent locates the candidate pair in the candidate pair priority
   ordered list.  If it is the highest priority candidate pair, the
   agent SHOULD send an updated offer immediately as described in
   Section 7.11.1.  If it is not the highest priority candidate pair,
   and the states of all lower priority candidate pairs are Invalid, the
   agent SHOULD send an updated offer immediately.  If it is not the
   highest priority candidate pair, and the state of at least one of the
   lower priority candidate pairs is Indeterminate, the agent does
   nothing.  Tests have yet to begin for higher priority candidate
   pairs.  If it is not the highest priority candidate pair, and none of
   the lower priority candidate pairs have a state of Indeterminate, the
   agents starts a timer, called the wait-state timer, but only if this
   timer is not already running.  The timer is set to fire in Tws
   seconds.  Tws SHOULD be configurable, and SHOULD have a default of
   Tws = max(0, 200ms - N*Tb), where N is the number of components for
   the candidates for this media stream.  The 200ms allows for a single
   STUN retransmission (which takes 100ms) and an RTT of 100ms.  This
   timer allows for a higher priority connectivity check to complete, in
   the event its STUN Binding Request was lost or delayed in the
   network.  Note that the timer goes to zero as the number of
   components increases.  If, prior to the wait-state timer firing,
   another connectivity check completes and a candidate pair is
   validated, there is no need to reset or cancel the timer.  Once the



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   timer fires, the agent SHOULD issue an updated offer as described in
   Section 7.11.1.  This updated offer will use the highest priority
   candidate pair in Valid state when the timer fires.

7.10.  Learning New Candidates from Connectivity Checks

   ICE makes use of reflexive addresses, which are addresses that inform
   an agent of its transport address as seen by another host.  An
   initial offer or answer generated by an agent includes server
   reflexive addresses, which are learned from a configured or
   discovered STUN server in the network.  However, the connectivity
   checks themselves can inform an agent of reflexive addresses, and in
   particular, ones that are reflexive towards its peer.  These are
   called peer reflexive candidates.  A new peer reflexive candidate is
   typically observed when two agents are separated by a NAT with the
   address-dependent or address and port dependent mapping properties
   [32].  However, in unusual topologies, peer reflexive candidates can
   be observed even when there are only NATs with the endpoint
   independent mapping property.  Because STUN and the media packets are
   sent on the same port, regardless of the filtering properties of the
   NAT (whether endpoint independent, address dependent, or address and
   port dependent), this reflexive address can be used by the peer for
   sending STUN and media packets back towards the agent.

   To obtain and use these peer reflexive transport addresses, ICE
   agents MUST perform the additional processing on the receipt of STUN
   Binding Requests and responses described in the following two
   subsections.  These procedures are not just applied in the (hopefully
   increasingly rare) case of address and port dependent mapping NATs.
   They are also needed for behave-compliant NATs [32].

7.10.1.  On Receipt of a Binding Request

   The procedures in this section are followed when an agent receives a
   STUN Binding Request matched to a target transport address pair whose
   source transport address (where the source is the one seen by the
   relay for requests received on a relayed transport address) doesn't
   match any of the existing remote transport addresses, or where the
   source matches, but the origination transport address does not.  This
   source address and its associated origination transport address
   become a new remote transport address.

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



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   the target candidate pair.  In particular, the candidate ID is
   constructed by concatenating the remote candidate ID with the native
   candidate ID (without the colon).  The password for the new candidate
   equals that of the remote candidate ID in the target candidate pair
   (note that, this password would be the same for all remote candidates
   for the same media line).

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

   The remote candidate is then paired up with a native candidate.
   However, unlike the procedures of Section 7.5, which pair up each
   remote candidate with each native candidate, this peer reflexive
   candidate is only paired up with a the native candidate from the
   candidate pair from which it was derived.  This creates a new
   candidate pair.  This new candidate pair is inserted into the
   candidate pair priority ordered list based on the ordering rules
   defined in Section 7.5.  Note that no entries are added to the
   transport address pair check ordered list.

   Recall that, for each candidate pair, one agent plays the role of
   offerer, and the other of answerer.  For a peer-reflexive candidate,
   the role is identical to that of its generating candidate.

   Newly created or not, the agent extracts the component ID from the
   matching transport address pair, and sees if a transport address with
   that same component ID exists in the peer reflexive candidate.  If it
   does, the agent does nothing further.  This can happen in unusual
   cases when there is a NAT reboot in the middle of a STUN transaction,
   causing two requests in the same transaction two produce two
   different transport addresses.  If there is no transport address with
   the same component ID in the peer reflexive candidate, the agent adds
   a transport address to the peer reflexive candidate.  This transport
   address is equal to the source IP address and port from the incoming
   STUN Binding Request (and in the case of Binding Request received on
   a relayed transport address, the one seen by the relay), and has a
   transport protocol equal to that of the incoming STUN request.  It is



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   assigned the component ID equal to the component ID in the target
   transport address pair.  This new transport address will have a
   transport address ID, equal to the concatenation of the candidate ID
   for this new candidate, and the component ID, separated by a colon.
   The type of the transport address is considered to be peer reflexive,
   though this is never signaled through SDP and so there is no
   candidate-types value defined for it.  Recall that each transport
   address is associated with an origination transport address.  For
   server reflexive candidates, the origination transport address is
   signaled through SDP.  For peer reflexive transport addresses, it is
   inherited from the origination transport address of the generating
   transport address.  If the generating transport address was a local
   transport address, then the origination transport address is that
   transport address.  If the generating transport address was server
   reflexive, the origination transport address is the related transport
   address that was signaled for that server reflexive candidate.  If
   the generating transport address was relayed, the origination
   transport address is the relayed transport address itself.  Whether
   and how other candidate attributes defined by extensions are
   inherited depends on the extension.

   The newly added transport address is paired up with the native
   transport address with the same component ID.  Initially, the peer
   reflexive candidate will start with a single transport address a
   transport address pair.  More are added as the connectivity checks
   for the original candidate pair take place.

   Figure 10 provides a pictorial representation of the peer reflexive
   candidate (the one with id=RL) and its pairing with the native
   candidate with ID L. The candidate with ID R is the generating
   candidate.  The peer reflexive candidate is effectively an alternate
   for that generating candidate, but is only paired with a specific
   native candidate.  Note that, for a particular generating candidate,
   there can be many peer derived candidates, up to one for each native
   candidate.  Also note that candidate IDs with values "L" and "R" and
   "RL" are not actually permitted, since all candidate IDs must be at
   least four characters long.  These shortened candidate IDs are used
   to keep the figure readable.













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

   Figure 10

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

   For UDP, the state machine enters the Send-Valid state.  Effectively,
   the Binding Request just received "counts" as a validation in this
   direction, even though it was formally done for a different transport
   address pair.  In addition, the agent generates a Binding Request for
   the new transport address pair, as described in Section 7.7.
   Processing of the response follows the logic described in
   Section 7.6.




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   As with all candidate pairs, the state of this new candidate pair is
   derived from the states of its transport address pairs.  Until the
   number of transport address pairs in the candidate pair equals the
   transport address pair count of the candidate pair from which it is
   derived, the state of the candidate pair is Indeterminate.  Once they
   are equal, the state is derived just like any other candidate pair.

7.10.2.  On Receipt of a Binding Response

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

   The procedures in this section are followed when an agent receives a
   STUN Binding Response matched to a transport address pair whose XOR-
   MAPPED-ADDRESS doesn't match any of the existing native transport
   addresses.  The XOR-MAPPED-ADDRESS becomes a new native transport
   address.

   To use it, the XOR-MAPPED-ADDRESS needs to be associated with a
   candidate (called a peer-derived candidate).  In this case, however,
   the candidate isn't signaled through an offer/answer exchange; it is
   constructed dynamically from information in the STUN response.  Like
   all other candidates, the peer-derived candidate has a candidate ID.
   The candidate ID is derived from the candidate IDs of the target
   candidate pair.  In particular, the candidate ID is constructed by
   concatenating the native candidate ID with the remote candidate ID
   (without the colon).  The password for the new candidate equals that
   of the native candidate ID in the matching candidate pair (note that,
   this password would be the same for all native candidates for the
   same media line).

   When the Binding Response is received, the agent constructs the
   candidate ID that represents the peer reflexive candidate, and checks
   to see if that candidate exists.  It may already exist if it had been
   constructed as a consequence of a previous application of this logic
   on receipt of a Binding Response for a different transport address
   pair of the same candidate pair.  If there is not yet a peer
   reflexive candidate with that candidate ID, the agent creates it, and
   assigns it the newly computed candidate ID.  The priority of the
   peer-derived candidate is set to the priority of its generating
   candidate - the native candidate in the target transport address
   pair.  Note that, at this time, the peer derived candidate has no
   transport addresses in it.  The native candidate is then paired up
   with a remote candidate.  However, unlike the procedures of
   Section 7.5, which pair up each native candidate with each remote
   candidate, this peer reflexive candidate is only paired up with the
   remote candidate from the target candidate pair.  This creates a new
   candidate pair.  This new candidate pair is inserted into the



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   candidate pair priority ordered list based on the ordering rules
   defined in Section 7.5.  Note that no entries are added to the
   transport address pair check ordered list.

   Recall that, for each candidate pair, one agent plays the role of
   offerer, and the other of answerer.  For a peer-reflexive candidate,
   the role is identical to that of its generating candidate.

   Newly created or not, the agent extracts the component ID from the
   target transport address pair, and sees if a transport address with
   that same component ID exists in the peer reflexive candidate.  If it
   does, the agent does nothing further.  This can happen in unusual
   cases when there is a NAT reboot in the middle of a STUN transaction,
   causing two requests in the same transaction two produce two
   different transport addresses.  If there is no transport address with
   the same component ID in the peer reflexive candidate, the agent adds
   a transport address to the peer reflexive candidate.  This transport
   address is equal to the XOR-MAPPED-ADDRESS from the incoming STUN
   Binding Response, and has a transport protocol equal to the one used
   for the Binding Response.  It is assigned the component ID equal to
   the component ID in the matching transport address pair.  This
   transport address will have a transport address ID, equal to the
   concatenation of the candidate ID for this new candidate, and the
   component ID, separated by a colon.  The type of the transport
   address is considered to be peer reflexive, though this is never
   signaled through SDP and so there is no candidate-types value defined
   for it.  Recall that each transport address is associated with an
   origination transport address.  For server reflexive candidates, the
   origination transport address is signaled through SDP.  For peer
   reflexive transport addresses, it is inherited from the origination
   transport address of the generating transport address.  If the
   generating transport address was a local transport address, then the
   origination transport address is that transport address.  If the
   generating transport address was server reflexive, the origination
   transport address is the related transport address that was signaled
   for that server reflexive candidate.  If the generating transport
   address was relayed, the origination transport address is the relayed
   transport address itself.  Whether and how other candidate attributes
   defined by extensions are inherited depends on the extension.

   The newly added transport address is paired up with the remote
   transport address with the same component ID.  Initially, the peer
   reflexive candidate will start with a single transport address a
   transport address pair.  More are added as the connectivity checks
   for the original candidate pair take place.

   The new transport address pair has a state machine associated with
   it.  The state that is entered, and actions to take as a consequence,



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

   For UDP, the state machine enters the Recv-Valid state.  Effectively,
   the Binding Response just received "counts" as a validation in this
   direction, even though it was formally done for a different candidate
   pair.  The peer will likely generate a Binding Request for this
   candidate pair; processing of the request follows the logic described
   in Section 7.6.

   As with all candidate pairs, the state of this new candidate pair is
   derived from the states of its transport address pairs.  Until the
   number of transport address pairs in the candidate pair equals the
   transport address pair count of the candidate pair from which it is
   derived, the state of the candidate pair is Indeterminate.  Once they
   are equal, the state is derived just like any other candidate pair.

7.11.  Subsequent Offer/Answer Exchanges

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

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

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

7.11.1.  Sending of a Subsequent Offer

   The offer MAY contain a new operating candidate in the m/c line.
   This candidate SHOULD be the native candidate from the highest
   priority candidate pair in the candidate pair priority ordered list
   whose state is Valid.  If there are no candidate pairs in this state,
   the highest one whose state is Send-Valid or Recv-Valid SHOULD be
   used.  If there are no candidate pairs in these states, the candidate
   pair that is most likely to work with this peer, as described in
   Section 7.2, SHOULD be used.  The candidate is encoded into the m/c
   line in an updated offer as described in Section 7.3.  Note that,
   while peer-derived candidates never appear in a=candidate attributes



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   (only their generating candidates appear there), a peer-derived
   candidate can appear in the m/c line if it has been selected for
   usage for media.

   If the candidate pair whose native candidate was encoded into the
   m/c-line was Valid, Send-Valid or Recv-Valid, the agent MUST include
   an a=remote-candidate attribute into the offer.  This attribute MUST
   contain the candidate ID of the remote candidate in the candidate
   pair.  It is used by the recipient of the offer in selecting its
   candidate for the answer.  Because the native candidate in the m/c-
   line will typically be Valid, Send-Valid or Recv-Valid in every offer
   after the initial one, the a=remote-candidate attribute will
   typically be used in all subsequent offers.

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

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

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

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

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

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

   o  If the candidate in the m/c-line is valid, all other lower
      priority candidates SHOULD be removed.  This has the effect of



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      stopping connectivity checks of other candidates.  This SHOULD
      would not be followed if an agent wanted to keep a candidate ready
      for usage if, for some reason, the operating candidate later
      become invalid.

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

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

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

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

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

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

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

   3.  The candidate pairs that are eliminated are removed from the
       candidate pair priority ordered list.  Their corresponding
       transport address pairs are removed from the transport address
       pair check ordered list.  As a consequence of this, if
       connectivity checks had not yet begun for the candidate pair,
       they won't.  If a transport address pair had been pruned from the
       transport address pair check ordered list because it was
       redundant with one of the transport address pairs which was just
       removed, that transport address pair is added back to the list.




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

   5.  If the removed candidate was a relayed candidate, the agent
       SHOULD de-allocate its transport addresses from the STUN relay if
       it is not using those resources elswhere.  If a local candidate
       was removed, and all of its derived candidates were also removed
       (including any peer reflexive candidates), local operating system
       resources for each of the transport addresses in the local
       candidate SHOULD be de-allocated, as long as it is not using
       those resources elsewhere.  The resources may be in use elsewhere
       if they were included in an initial offer which generated
       multiple answers (as can happen with SIP forking).  In such a
       case, a subsequent offer which removes the candidate will not
       imply its removal with the other branches; each becomes a
       separate offer/answer relationship.

   Subsequent offers MUST contain a=ice-pwd attributes that specify the
   password for the candidates for each media stream.  If any of the
   candidates for a particular m-line are the same as the previous
   offer, the ICE password for that m-line MUST be the same.  If all of
   the candidates for a particular m-line are different from the
   previous offer, the ICE password for that m-line MAY be different.
   Note that it is permissible to use a session-level attribute in one
   offer, but to provide the same password as a media-level attribute in
   a subsequent offer.  This is not a change in password, just a change
   in its representation.

7.11.2.  Receiving the Offer and Sending an Answer

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

   The first step in the process is to look for the a=remote-candidate
   attribute in the offer.  The a=remote-candidate exists to eliminate a
   race condition between the updated offer and the response to the STUN
   Binding Request that moved a candidate into the Valid state.  This
   race condition is shown in Figure 11.  On receipt of message 5, agent
   A can move its transport address pair state machine into the Valid
   state.  It sends a STUN response to the request (message 6), but this
   is lost.  Agent A proceeds with an updated offer (message 7), which
   is received at agent B. As far as agent B is concerned, the transport
   address pair is still in the Send-Valid state.  It will move into the
   Valid state only on receipt of the STUN response in message 10.



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   Thus, upon receipt of the offer, agent B cannot determine which
   candidate to include in its answer.  To eliminate this condition, the
   identity of the validated candidate is included in the offer itself.
   Note, however, that the answerer will not send media until it has
   received this STUN response.


          Agent A               Network               Agent B
             |(1) Offer            |                     |
             |------------------------------------------>|
             |(2) Answer           |                     |
             |<------------------------------------------|
             |(3) STUN Req.        |                     |
             |------------------------------------------>|
             |(4) STUN Res.        |                     |
             |<------------------------------------------|
             |(5) STUN Req.        |                     |
             |<------------------------------------------|
             |(6) STUN Res.        |                     |
             |-------------------->|                     |
             |                     |Lost                 |
             |(7) Offer            |                     |
             |------------------------------------------>|
             |(8) Answer           |                     |
             |<------------------------------------------|
             |(9) STUN Req.        |                     |
             |<------------------------------------------|
             |(10) STUN Res.       |                     |
             |------------------------------------------>|


   Figure 11

   If the a=remote-candidate attribute is present, the agent examines
   the transport addresses in the m/c-line of the offer.  It compares
   these with the transport addresses in the remote candidates of all
   candidate pairs.  If there is no match, no further processing of the
   a=remote-candidate attribute is done.  If there is at least one
   match, the agent compares the native candidate ID of each matching
   pair with the value of the a=remote-candidate attribute.  If there is
   a match, that candidate pair is selected.  For each transport address
   pair in that candidate pair, if the state of the transport address
   pair is Send-Valid, the agent considers the state to be Valid just
   for the purpose of constructing the answer.  In particular, it will
   impact selection of the candidate for the m/c-line and the set of
   additional candidates to include or exclude from the answer.
   However, the actual state MUST remain Send-Valid.  This state will be
   used to determine when it is safe to send media.  Keeping it at Send-



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   Valid is necessary to prevent against DoS attacks.

   Note that the a=remote-candidate attribute SHOULD NOT be included in
   the answer, and if included, will just be ignored by the offerer,
   since it is not used in any processing of the answer.

   Rules for choosing transport addresses for the m/c-line are as
   follows.  The agent examines the transport addresses in the m/c-line
   of the offer.  It compares these with the transport addresses in the
   remote candidates of candidate pairs whose states are Valid.  If
   there is a matching candidate pair in that state, the pair with the
   highest priority MUST be chosen, and the native candidate from that
   pair used as the operating candidate.  If there were no matching
   candidate pairs in the Valid state (possibly because the transport
   addresses in the m/c-line in the offer didn't match any of the remote
   candiadtes), the candidate that is most likely to work with this
   peer, as described in Section 7.2, SHOULD be used.  Note that this
   candidate may be Valid as a consequence of being temporarily changed
   to such by the a=remote-candidate attribute.

   Like the offerer, the answerer can decide, for each of its
   candidates, whether they are retained or removed.  The same rules
   defined in Section 7.11.1 for determining their disposition apply to
   the answerer.  Similarly, if a candidate is removed, the same rules
   in Section 7.11.1 regarding removal of canididate pairs and freeing
   of resources apply.  As with selection of the candidate for the m/c-
   line, the state of one of the candidates may be Valid as a
   consequence of being temporarily changed to such by the a=remote-
   candidate attribute.

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

   If a candidate pair existed previously, but as a consequence of the
   offer/answer exchange, it no longer exists, the agent takes the
   following steps:

   1.  The candidate pair is removed from the candidate pair priority
       ordered list.  Their corresponding transport address pairs are
       removed from the transport address pair check ordered list.  As a
       consequence of this, if connectivity checks had not yet begun for
       the candidate pair, they won't.  If a transport address pair had
       been pruned from the transport address pair check ordered list



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       because it was redundant with one of the transport address pairs
       which was just removed, that transport address pair is added back
       to the list.

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

   3.  If the agent receives a STUN Binding Request for that candidate
       pair, however, processing occurs as defined in Section 7.8.

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

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

7.11.3.  Receiving the Answer

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

7.12.  Binding Keepalives

   Once a candidate is promoted to operating, and media begins flowing,
   it is still necessary to keep the bindings alive at intermediate NATs
   for the duration of the session.  Normally, the media stream packets
   themselves (e.g., RTP) meet this objective.  However, several cases
   merit further discussion.  Firstly, in some RTP usages, such as SIP,
   the media streams can be "put on hold".  This is accomplished by
   using the SDP "sendonly" or "inactive" attributes, as defined in RFC
   3264 [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.



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   Secondly, some RTP payload formats, such as the payload format for
   text conversation [31], 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 operating candidate in a=candidate lines for UDP-based media
   streams.  As a consequence of this, STUN packets will be transmitted
   periodically independently of the transmission (or lack thereof) of
   media packets.  These will be received on the same IP address and
   port as the media streams.  The agent determines whether the packet
   is media or STUN by looking for the magic cookie in bits 32-63 of the
   data.  If present, it indicates that the packet is STUN, and if not,
   indicates that it is media.  This provides a media independent, RTP
   independent, and codec independent solution for keeping the NAT
   bindings alive.  However, an ICE implementation MUST be prepared for
   the transport address received in an m/c-line to not correspond to
   any a=candidate attributes.

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

   All endpoints MUST send keepalives for each media session.  These
   keepalives MUST be sent regardless of whether the media stream is
   currently inactive, sendonly, recvonly or sendrecv.  The keepalive
   SHOULD be sent using a format which is supported by its peer.  ICE
   endpoints allow for STUN-based keepalives for UDP streams, and as
   such, STUN keepalives MUST be used when an agent is communicating
   with a peer that supports ICE.  An agent can determine that its peer
   supports ICE by the presence of the a=candidate attributes for each
   media session.  If the peer does not support ICE, the choice of a
   packet format for keepalives is a matter of local implementation.  A
   format which allows packets to easily be sent in the absence of
   actual media content is RECOMMENDED.  Examples of formats which
   readily meet this goal are RTP No-Op [28] and RTP comfort noise [24].
   If the peer doesn't support any formats that are particularly well
   suited for keepalives, an agent SHOULD send RTP packets with an
   incorrect version number, or some other form of error which would
   cause them to be discarded by the peer.

   STUN-based keepalives will be sent periodically every Tr seconds as a
   consequence of the rules in in Section 7.7.  If STUN keepalives are



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   not in use (because the peer does not support ICE), an agent SHOULD
   ensure that a media packet is sent every Tr seconds.  If one is not
   sent as a consequence of normal media communications, a keepalive
   packet using one of the formats discussed above SHOULD be sent.

7.13.  Sending Media

   When an agent receives an offer and sends an answer, or when it
   receives an answer to an offer it sent, it begins connectivity
   checks.  If there is a candidate that corresponds to the m/c-line,
   these checks will include validation of the operating candidate pair.
   In that case, an agent SHOULD NOT send media on the operating
   candidate pair until that candidate pair has reached the Valid or
   Recv-Valid state.  This is to help prevent a denial-of-service
   attack, described in Section 13.  Once the operating candidate pair
   reaches the Valid or Recv-Valid state, an agent MAY start sending
   media to that candidate pair.  If there is no candidate that
   corresponds to the m/c-line, the m/c-line cannot be validated, and
   media is sent to it as described in RFC 3264 [4].  Under normal
   conditions, there will be a candidate for the m/c-line.  Indeed - ICE
   itself requires that an agent include one.  However, actual SIP
   deployments have seen usage of network intermediaries which
   manipulate the m/c-line of offers and answers.  Should such elements
   ignore the candidate attributes, it would manifest itself like an
   agent which did not include a candidate for the m/c-line.  For this
   reason, this use case is explicitly supported by ICE.

   Offer/answer exchanges are used with protocols, like SIP, which
   require media to be sent "early", from the answerer to the offer,
   prior to completion of the initial offer/answer exchange.  It is
   highly desirable (and sometimes necessary) for this early media to
   use the candidate pair ultimately selected by ICE connectivity
   checks.  For this reason, ICE provides an early media mechanism that
   allows for a candidate pair to be used in one direction prior to its
   promotion to operating in a subsequent offer/answer exchange.  Note
   that, with ICE, early media pertains to media sent to a candidate
   pair until its promotion to operating in a subsequent offer/answer
   exchange.  This is a broader definition than is used in [26], which
   defines early media as media sent prior to acceptance of a call.

   As a consequence of the connectivity checks, an agent will change the
   states for each transport address pair, and consequently, for the
   candidate pairs.  When a candidate pair becomes Valid or Recv-Valid,
   and there is a candidate pair for the m/c-line, and the candidate
   pair is not equal to the operating candidate pair, and the agent is
   in the role of answerer for that candidate pair, the agent checks the
   position of that pair in the candidate pair priority ordered list.
   If it is the first, the agent selects this candidate pair for early



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   media.  If this candidate pair is not the first on the candidate pair
   priority ordered list, but is higher priority than the operating
   candidate pair, and the early media wait-state timer has not yet been
   set, the agent sets this timer to Tws seconds.  Though the early
   media wait state timer has the same value as the wait state timer
   described in Section 7.9, these are different timers and indeed are
   set by different entites.  The early media wait state timer allows
   for a higher priority connectivity check to complete, in the event
   its STUN Binding Request or Response was lost or delayed in the
   network.  If, prior to the early media wait-state timer firing,
   another connectivity check completes and a candidate pair enters the
   Valid or Recv-Valid states, there is no need to reset or cancel the
   timer.  Once the timer fires, the agent SHOULD select the highest
   priority candidate pair in the Valid or Recv-Valid state for which
   the agent has the role of answerer, and use that candidate pair for
   early media.

   ICE processing will ensure that, under almost all circumstances, the
   candidate pair selected by the answerer for early media will also be
   the one selected by the offerer for eventual promotion to operating.
   The early media state implies that the answerer knows that this
   candidate pair is to be used, but the offerer doesn't know yet that
   it will eventually be validated.  It is for this reason that the
   candidate pair can be used for early media.

   If a candidate pair is selected for early media, an agent MAY send
   media on that candidate pair, even if it is not the same as the
   operating candidate pair.  However, to deal with cases in which the
   offerer and answerer do not agree on the eventual selection of this
   candidate for promotion to operating (a rare but possible case), the
   agent MUST discontinue using the candidate pair for sending media Tlo
   seconds after the next opportunity its peer would have to send an
   updated offer.  In the case of an answer delivered in a 200 OK to an
   offer in a SIP INVITE (regardless of whether that same answer
   appeared in an earlier unreliable provisional response), this would
   be Tlo seconds after receipt of the ACK.  Tlo SHOULD be configurable
   and SHOULD have a default of 5 seconds.  This time represents the
   amount of time it should take the offerer to perform its connectivity
   checks, arrive at the same conclusion about the viability of the
   early candidate, and then generate an updated offer promoting it to
   operating.  If, after Tlo seconds, no updated offer arrives, the
   answerer MUST cease using the early candidate.  Media MAY be sent to
   the operating candidate pair if it is in the Valid or Recv-Valid
   state.

   If an updated offer does arrive prior to the expiration of the timer,
   the agent MUST execute the procedures in Section 7.11.2, which will
   result in the selection of a candidate for the m/c-line in the



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   answer.  At that point, the procedures of this section SHOULD be
   restarted by the answerer.  This implies that the operating candidate
   pair, if Valid or Recv-Valid, will be used.  If a higher priority
   candidate pair subsequently enters the Valid or Recv-Valid state, it
   may end up being used as an early candidate.

   To use a candidate pair, whether it is early or operating, media is
   sent to the IP addresses and ports of the components in the remote
   candidate, and sends that media from the IP addresses and ports of
   the components in the native candidate.  Transport addresses are
   paired up based on component ID.  For example, if a remote candidate
   has two components R1 and R2, and the native candidate has two
   components L1 and L2, media packets are sent from L1 to R1 and from
   L2 to R2.  This provides a property known as symmetry.  This
   symmetric behavior MUST be followed by an agent even if its peer in
   the session doesn't support ICE.

   The definition of sending media "from" a particular transport address
   depends on the type of transport address.  In the case of a server
   reflexive 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 relayed transport address, this means that media
   packets are sent through the relay server (for STUN relays, this
   would be using the Send request).  For local transport addresses,
   media is sent from that local transport address.  For peer reflexive
   transport addresses, media is sent from the local transport address
   used to obtain the reflexive address.

   ICE has interactions with jitter buffer adaptation mechanisms.  An
   RTP stream can begin using one candidate, and switch to another one.
   The newer candidate may result in RTP packets taking a different path
   through the network - one with different delay characteristics.  As
   discussed below, agents are encouraged to re-adjust jitter buffers
   when there are changes in source or destination address.
   Furthermore, many audio codecs use the marker bit to signal the
   beginning of a talkspurt, for the purposes of jitter buffer
   adaptation.  For such codecs, it is RECOMMENDED that the sender
   change the marker bit when an agent switches transmission of media
   from one candidate pair to another.

7.14.  Receiving Media

   ICE implementations MUST be prepared to receive media on a candidate
   pair if it is in the role of offerer for that candidate pair, even if
   that candidate pair is not currently operating.  This is a
   consequence of the early media mechanism described in the previous
   section.




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   If an agent determines that its peer supports ICE (an offerer knows
   this when the answer contains a=candidate attributes), it SHOULD
   discard any media packets received on a candidate pair prior to the
   candidate pair entering the Send Valid state.  This helps eliminate
   certain attacks, as discussed in Section 13.  Note that, in cases of
   forking, an agent may get multiple answers to its offer, each for a
   different peer.  Consequently, if would only discard media packets
   received on a candidate pair once it has determined that all forked
   targets support ICE.

   It is RECOMMENDED that, when an agent receives an RTP packet with a
   new source or destination IP address for a particular media stream,
   that the agent re-adjust its jitter buffers.

   RFC 3550 [21] describes an algorithm in Section 8.2 for detecting
   SSRC collisions and loops.  These algorithms are based, in part, on
   seeing different source IP addresses and ports with the same SSRC.
   However, when ICE is used, such changes will naturally occur as the
   media streams switch between candidates.  An agent will be able to
   determine that a media stream is from the same peer as a consequence
   of the STUN exchange that proceeds media transmission.  Thus, if
   there is a change in source IP address and port, but the media
   packets come from the same peer agent, this SHOULD NOT be treated as
   an SSRC collision.


8.  Guidelines for Usage with SIP

   SIP [2] makes use of the offer/answer model, and is one of the
   primary targets for usage of ICE.  SIP allows for offer/answer
   exchanges to occur in many different combinations of messages,
   including INVITE/200 OK and 200 OK/ACK.  When support for reliable
   provisional responses (RFC 3262 [11]) and UPDATE (RFC 3311 [25]) are
   added, additional combinations of messages that can be used for
   offer/answer exchanges are added.  As such, this section provides
   some guidance on good ways to make use of SIP with ICE.

   ICE requires a series of STUN-based connectivity checks to take place
   between endpoints.  These checks start from the answerer on
   generation of its answer, and start from the offerer when it receives
   the answer.  These checks can take time to complete, and as such, the
   selection of messages to use with offers and answers can effect
   perceived user latency.  Two latency figures are of particular
   interest.  These are the post-pickup delay and the post-dial delay.
   The post-pickup delay refers to the time between when a user "answers
   the phone" and when any speech they utter can be delivered to the
   caller.  The post-dial delay refers to the time between when a user
   enters the destination address for the user, and ringback begins as a



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   consequence of having succesfully started ringing the phone of the
   called party.

   To reduce post-dial delays, it is RECOMMENDED that the caller begin
   gathering candidates prior to actually sending its initial INVITE.
   This can be started upon user interface cues that a call is pending,
   such as activity on a keypad or the phone going offhook.

   To reduce post-pickup delays, ICE allows for media to be sent from
   the answerer to the offerer on a candidate pair, prior to its
   promotion to operating.  However, this requires the answerer to have
   generated its answer and sent it.  In most cases, it will require
   this answer to be received by the offerer.  The reason is that
   connectivity checks or RTP packets from the answerer to the offerer
   will not be forwarded by NATs towards the offerer until the offerer
   has established a permission in the NAT by generating a packet
   towards the answerer.

   For this reason, if an offer is received in an INVITE request, the
   UAS SHOULD immediately gather its candidates and then generate an
   answer in a provisional response.  When reliable provisional
   responses are not used, the SDP in the provisional response is the
   answer, and that exact same answer reappears in the 200 OK.  To deal
   with possible losses of the provisional response, it SHOULD be
   retransmitted until some indication of receipt.  This indication can
   either be through PRACK [11], or through the receipt of a STUN
   Binding Request with a correct username and password.  Even if PRACK
   is not used, the provisional response SHOULD be retransmitted using
   the exponential backoff described in [11].  Furthermore, once the
   answer has been sent, the agent SHOULD begin its connectivity checks.
   Once a candidate reaches the Valid or Recv-Valid state, the UAS has a
   known-valid path for media packets towards the UAC.  This point is
   called the connected point in ICE.

   Once the UAS reaches the connected point, media can be sent from the
   UAS towards the UAC without any additional delays.  However, between
   the receipt of the INVITE and the connected point, any media that
   needs to be sent towards the caller (such as SIP early media [26]
   cannot be transmitted.  For this reason, implementations MAY choose
   to delay alerting the called party until the connected point is
   reached.  In the case of a PSTN gateway, this would mean that the
   setup message into the PSTN is delayed until the connected point.
   Doing this increases the post-dial delay, but has the effect of
   eliminating 'ghost rings'.  Ghost rings are cases where the called
   party hears the phone ring, picks up, but hears nothing and cannot be
   heard.  This technique works without requiring support for, or usage
   of, preconditions [7], since its a localized decision.  It also has
   the benefit of guaranteeing that not a single packet of early media



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   will get clipped.  If an agent chooses to delay local alerting in
   this way, it SHOULD generate a 180 response once alerting begins.

   A slight variation of this approach is to wait for a connectivity
   check to succeed to a higher priority candidate pair than the
   operating one.  This allows for the agent to only ever send media,
   early or otherwise, to a single candidate, which will work better
   with jitter buffers, at the expense of even greater post-dial delays.

   Note that, prior to the promotion of a candidate pair to operating,
   the offerer will not be able to send using the candidate pair.  When
   used with SIP, if the initial offer is sent in the INVITE, and the
   answer is sent in both the provisional and final 200 OK response, the
   offerer will not be able to send media until it sends a re-INVITE and
   receives the 200 OK response to that re-INVITE.  This can take
   several hundred milliseconds.  If this latency is an issue (it is
   generally not considered an issue for voice systems), reliable
   provisional responses [11] MAY be used, in which case an UPDATE [25]
   can be used to send an updated offer prior to the call being
   answered.

   As discussed in Section 13, offer/answer exchanges SHOULD be secured
   against eavesdropping and man-in-the-middle attacks.  To do that, the
   usage of SIPS [2] is RECOMMENDED when used in concert with ICE.


9.  Interactions with Forking

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

   ICE interacts very well with forking.  Indeed, ICE fixes some of the
   problems associated with forking.  Once the offer/answer exchange has
   completed, the UAC will have an answer from each UAS that received
   the INVITE.  The ICE connectivity checks that ensue will carry
   transport address pair IDs that correlate each of those checks (and
   thus their corresponding IP addresses and ports) with a specific
   remote user agent.  As these checks happen before any media is
   transmitted, ICE allows a UAC to disambiguate subsequent media
   traffic by looking at the source IP address and port, and then
   correlate that traffic with a particular remote UA.  When SIP is used
   without ICE, the incoming media traffic cannot be disambiguated
   without an additional offer/answer exchange.





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10.  Interactions with Preconditions

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

   Quality of Service (QoS) preconditions, which are defined in RFC 3312
   [7] and RFC 4032 [8], apply only to the IP addresses and ports listed
   in the m/c lines in an offer/answer.  If ICE changes the address and
   port where media is received, this change is reflected in the m/c
   lines of a new offer/answer.  As such, it appears like any other re-
   INVITE would, and is fully treated in RFC 3312 and 4032, which
   applies without regard to the fact that the m/c lines are changing
   due to ICE negotiations ocurring "in the background".

   However, usage of early candidates with QoS preconditions is NOT
   RECOMMENDED, since QoS will only be reserved for the candidate pair
   in the m/c-line.  An agent SHOULD only send to the operating
   candidate (once it enters the Valid or Recv-Valid states) if QoS
   preconditions are used for a media session.

   ICE also has (purposeful) interactions with connectivity
   preconditions [27].  Those interactions are described there.


11.  Examples

   This section provides two examples.  One is a very basic example, and
   the other is more elaborate.  A common configuration and setup is
   used in both cases.

   Two agents, L and R, are using ICE.  Both agents have a single IPv4
   interface.  For agent L, it is 10.0.1.1, and for agent R, 192.0.2.1.
   Both are configured with a single STUN server each (indeed, the same
   one for each), which is listening for STUN requests at an IP address
   of 192.0.2.2 and port 3478.  This STUN server supports both the
   Binding Discovery usage and the Relay usage.  Agent L is behind a
   NAT, and agent R is on the public Internet.  The public side of the
   NAT has an IP address of 192.0.2.3.

   To facilitate understanding, transport addresses are listed using
   variables that have mnemonic names.  This format of the anem is
   entity-type-seqno, where entity refers to the entity whose interface
   the transport address is on, and is one of "L", "R", "STUN", or
   "NAT".  The type is either "PUB" for transport addresses that are
   public, and "PRIV" for transport addresses that are private.
   Finally, seq-no is a sequence number that is different for each
   transport address of the same type on a particular entity.  Each
   variable has an IP address and port, denoted by varname.IP and



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   varname.PORT, respectively, where varname is the name of the
   variable.

   In addition, candidate IDs are also listed using variables that have
   mnemonic names.  Agent L uses candidate ID L1 for its local
   candidate, L2 for its server reflexive candidate, and L3 for its
   relayed candidate.  Agent R uses R1 for its local candidate and R2
   for its relayed candidate.  The password is LPASS for each candidate
   from agent L, and RPASS for each candidate from agent R.

   The STUN server has advertised transport address STUN-PUB-1 (which is
   192.0.2.2:3478) for both the binding discovery usage and the relay
   usage.

   In the call flow itself, STUN messages are annotated with several
   attributes.  The "S=" attribute indicates the source transport
   address of the message.  The "D=" attribute indicates the destination
   transport address of the message.  The "MA=" attribute is used in
   STUN Binding Response messages, STUN Binding Response messages
   carried in a STUN Send Request or Data Indication, and in a Allocate
   Response, and refers to the reflexive transport address derived from
   the XOR-MAPPED-ADDRESS attribute.  The "RA=" attribute is used in
   STUN Data Indications, and refers to the value of the REMOTE-ADDRESS
   attribute.  The "U=" attribute is used in STUN Requests, and
   corresponds to the STUN USERNAME.  The "DA=" attribute is used in
   STUN Send requests, and refers to the value of the DESTINATION-
   ADDRESS attribute.  The "R=" attribute is used in Allocate responses,
   and it indicates the value of the RELAY-ADDRESS attribute.

   The call flow examples omit STUN authentication operations.

11.1.  Basic Example

   In this example, the NAT has an endpoint independent mapping property
   and an address dependent filtering property.  Neither agent is using
   the STUN relay usage, only the binding discovery usage.  As a
   consequence, agent L will end up with two candidates - a local
   candidate and a server reflexive candidate.  Agent R will have one -
   a local candidate (the reflexive candidate will be identical to the
   local one, and thus discarded).  The agents are seeking to
   communicate using a single RTP-based voice stream.  RTCP is not used.
   As a consequence, each candidate has one component.


             L             NAT           STUN             R
             |RTP STUN alloc.              |              |
             |(1) STUN Req  |              |              |
             |S=$L-PRIV-1   |              |              |



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             |D=$STUN-PUB-1 |              |              |
             |------------->|              |              |
             |              |(2) STUN Req  |              |
             |              |S=$NAT-PUB-1  |              |
             |              |D=$STUN-PUB-1 |              |
             |              |------------->|              |
             |              |(3) STUN Res  |              |
             |              |S=$STUN-PUB-1 |              |
             |              |D=$NAT-PUB-1  |              |
             |              |MA=$NAT-PUB-1 |              |
             |              |<-------------|              |
             |(4) STUN Res  |              |              |
             |S=$STUN-PUB-1 |              |              |
             |D=$L-PRIV-1   |              |              |
             |MA=$NAT-PUB-1 |              |              |
             |<-------------|              |              |
             |(5) Offer     |              |              |
             |------------------------------------------->|
             |              |              |              |RTP STUN alloc.
             |              |              |(6) STUN Req  |
             |              |              |S=$R-PUB-1    |
             |              |              |D=$STUN-PUB-1 |
             |              |              |<-------------|
             |              |              |(7) STUN Res  |
             |              |              |S=$STUN-PUB-1 |
             |              |              |D=$R-PUB-1    |
             |              |              |MA=$R-PUB-1   |
             |              |              |------------->|
             |(8) answer    |              |              |
             |<-------------------------------------------|
             |              |(9) Bind Req  |              |
             |              |S=$R-PUB-1    |              |
             |              |D=$NAT-PUB-1  |              |
             |              |<----------------------------|
             |              |Dropped       |              |
             |(10) Bind Req |              |              |
             |S=$L-PRIV-1   |              |              |
             |D=$R-PUB-1    |              |              |
             |------------->|              |              |
             |              |(11) Bind Req |              |
             |              |S=$NAT-PUB-1  |              |
             |              |D=$R-PUB-1    |              |
             |              |---------------------------->|
             |              |(12) Bind Res |              |
             |              |S=$R-PUB-1    |              |
             |              |D=$NAT-PUB-1  |              |
             |              |MA=$NAT-PUB-1 |              |
             |              |<----------------------------|



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             |(13) Bind Res |              |              |
             |S=$R-PUB-1    |              |              |
             |D=$L-PRIV-1   |              |              |
             |MA=$NAT-PUB-1 |              |              |
             |<-------------|              |              |
             |RTP flows     |              |              |
             |              |(14) Bind Req |              |
             |              |S=$R-PUB-1    |              |
             |              |D=$NAT-PUB-1  |              |
             |              |<----------------------------|
             |(15) Bind Req |              |              |
             |S=$R-PUB-1    |              |              |
             |D=$L-PRIV-1   |              |              |
             |<-------------|              |              |
             |(16) Bind Res |              |              |
             |S=$L-PRIV-1   |              |              |
             |D=$R-PUB-1    |              |              |
             |MA=$R-PUB-1   |              |              |
             |------------->|              |              |
             |              |(17) Bind Res |              |
             |              |S=$NAT-PUB-1  |              |
             |              |D=$R-PUB-1    |              |
             |              |MA=$R-PUB-1   |              |
             |              |---------------------------->|
             |              |              |              |RTP flows



   Figure 12

   First, agent L obtains a server reflexive transport address for its
   RTP packets (messages 1-4).  Recall that the NAT has the address and
   port independent mapping property.  Here, it creates a binding of
   NAT-PUB-1 for this UDP request, and this becomes the server reflexive
   transport address for RTP, the sole component of its server reflexive
   candidate.

   With its two candidates, agent L prioritizes them, choosing the local
   candidate as highest priority, followed by the server reflexive
   candidate.  It chooses its server reflexive candidate as the
   operating candidate, and encodes it into the m/c-line.  The resulting
   offer (message 5) looks like (lines folded for clarity):









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       v=0
       o=jdoe 2890844526 2890842807 IN IP4 $L-PRIV-1.IP
       s=
       c=IN IP4 $NAT-PUB-1.IP
       t=0 0
       a=ice-pwd:$LPASS
       m=audio $NAT-PUB-1.PORT RTP/AVP 0
       a=rtpmap:0 PCMU/8000
       a=candidate:$L1 1 UDP 1.0 $L-PRIV-1.IP $L-PRIV-1.PORT typ local
       a=candidate:$L2 1 UDP 0.7 $NAT-PUB-1.IP $NAT-PUB-1.PORT typ srflx raddr
   $L-PRIV-1.IP rport $L-PRIV-1.PORT

   The offer, with the variables replaced with their values, will look
   like (lines folded for clarity):


       v=0
       o=jdoe 2890844526 2890842807 IN IP4 10.0.1.1
       s=
       c=IN IP4 192.0.2.3
       t=0 0
       a=ice-pwd:asd88fgpdd777uzjYhagZg
       m=audio 45664 RTP/AVP 0
       a=rtpmap:0 PCMU/8000
       a=candidate:8hhY 1 UDP 1.0 10.0.1.1 8998 typ local
       a=candidate:Bzo8 1 UDP 0.7 192.0.2.3 45664 typ srflx raddr
   10.0.1.1 rport 8998

   This offer is received at agent R. Agent R will gather its server
   reflexive transport address (messages 6-7).  Since R is not behind a
   NAT, this address is identical to its local transport address, and
   was obtained from its local transport address, and thus does not
   represent a separate candidate.  It therefore ends up with a single
   local candidate with a single component for RTP.  Its resulting
   answer looks like:


       v=0
       o=bob 2808844564 2808844564 IN IP4 $R-PUB-1.IP
       s=
       c=IN IP4 $R-PUB-1.IP
       t=0 0
       a=ice-pwd:$RPASS
       m=audio $R-PUB-1.PORT RTP/AVP 0
       a=rtpmap:0 PCMU/8000
       a=candidate:$R1 1 UDP 1.0 $R-PUB-1.IP $R-PUB-1.PORT typ local

   With the variables filled in:



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       v=0
       o=bob 2808844564 2808844564 IN IP4 192.0.2.1
       s=
       c=IN IP4 192.0.2.1
       t=0 0
       a=ice-pwd:YH75Fviy6338Vbrhrlp8Yh
       m=audio 3478 RTP/AVP 0
       a=rtpmap:0 PCMU/8000
       a=candidate:9uB6 1 UDP 1.0 192.0.2.1 3478 typ local

   Next, agents L and R form candidate pairs, the candidate pair
   priority ordered list and transport address pair check ordered list.
   The candidate pair priority ordered list will have two entries, and
   be identical for L and R. The highest priority one will be the one
   containing L2 and R1 (since its the operating candidate pair), and
   the second one will be L1 and R1.  The transport address pair check
   ordered list initially starts with two entries.  For agent L, this
   will be L2:1:R1:1 and L1:1:R1:1.  However, after the trimming
   operation, agent L will remove the second transport address pair,
   since it shares the same origination transport address as the first
   (L-PRIV-1 for both).  However, R will keep both transport address
   pairs.

   Agent R begins its connectivity check (message 9) for transport
   address pair L2:1:R1:1 (note that, from its perspective, the
   transport address pair has the ID R1:1:L2:1, and this ID would appear
   in the USERNAME of STUN requests it receives).  Since the NAT has a
   filtering policy of address dependent, the connectivity check is
   discarded.

   When agent L gets the answer, it begins its connectivity check for
   L2:1:R1:1 (messages 10-13), which succeed, placing the transport
   address pair and resulting candidate pair into the Recv-Valid state.
   L can now send media to R. When agent R receives the connectivity
   check (message 11), it is a match for the transport address pair, and
   the state of the transport address pair moves to Send-Valid.  Agent R
   begins its connectivity checks (messages 14-17).  When the check
   arrives at the NAT (message 14), it is permitted to pass since a
   permission was created towards R-PUB-1 as a consequence of message
   10.  This check arrives at agent L, which generates a success
   response (message 16), and updates the state of the transport address
   pair to Valid.  This response arrives at agent R, which also updates
   the state of the transport address pair to Valid.  Now, media can
   flow from agent R to agent L as well.

11.2.  Advanced Example

   In this more advanced example, The NAT has address and port dependent



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   mapping and filtering properties.  Both agents use the STUN relay
   usage in addition to the binding discovery usage.  As a consequence,
   agent L will end up with three candidates - a local candidate, a
   relayed candidate, and a server reflexive candidate.  Agent R will
   have two - a local candidate and a relayed candidate (the server
   reflexive candidate will equal the local candidate and thus not be
   used).  The agents are seeking to communicate using a single RTP-
   based voice stream, but are using RTCP.  As a consequence, each
   candidate has two components - one for RTP and one for RTCP.


             L             NAT           STUN             R
             |              |              |              |
             |              |              |              |
             |              |              |              |
             |RTP Alloc.    |              |              |
             |              |              |              |
             |              |              |              |
             |              |              |              |
             |(1) Alloc Req |              |              |
             |S=L-PRIV-1    |              |              |
             |D=STUN-PUB-1  |              |              |
             |------------->|              |              |
             |              |              |              |
             |              |              |              |
             |              |(2) Alloc Req |              |
             |              |S=NAT-PUB-1   |              |
             |              |D=STUN-PUB-1  |              |
             |              |------------->|              |
             |              |(3) Alloc Res |              |
             |              |S=STUN-PUB-1  |              |
             |              |D=NAT-PUB-1   |              |
             |              |R=STUN-PUB-2  |              |
             |              |MA=NAT-PUB-1  |              |
             |              |<-------------|              |
             |(4) Alloc Res |              |              |
             |S=STUN-PUB-1  |              |              |
             |D=L-PRIV-1    |              |              |
             |R=STUN-PUB-2  |              |              |
             |MA=NAT-PUB-1  |              |              |
             |<-------------|              |              |
             |              |              |              |
             |              |              |              |
             |              |              |              |
             |RTCP Alloc.   |              |              |
             |Ta secs. later|              |              |
             |              |              |              |
             |              |              |              |



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             |              |              |              |
             |(5) Alloc Req |              |              |
             |S=L-PRIV-2    |              |              |
             |D=STUN-PUB-1  |              |              |
             |------------->|              |              |
             |              |              |              |
             |              |              |              |
             |              |(6) Alloc Req |              |
             |              |S=NAT-PUB-2   |              |
             |              |D=STUN-PUB-1  |              |
             |              |------------->|              |
             |              |(7) Alloc Res |              |
             |              |S=STUN-PUB-1  |              |
             |              |D=NAT-PUB-2   |              |
             |              |R=STUN-PUB-3  |              |
             |              |MA=NAT-PUB-2  |              |
             |              |<-------------|              |
             |(8) Alloc Res |              |              |
             |S=STUN-PUB-1  |              |              |
             |D=L-PRIV-2    |              |              |
             |R=STUN-PUB-3  |              |              |
             |MA=NAT-PUB-2  |              |              |
             |<-------------|              |              |
             |              |              |              |
             |              |              |              |
             |              |              |              |
             |              |              |              |
             |(9) Offer     |              |              |
             |------------------------------------------->|
             |              |              |              |
             |              |              |              |
             |              |              |              |
             |              |              |              |
             |              |              |              |RTP Alloc.
             |              |              |              |
             |              |              |              |
             |              |              |              |
             |              |              |(10) Alloc Req|
             |              |              |S=R-PUB-1     |
             |              |              |D=STUN-PUB-1  |
             |              |              |<-------------|
             |              |              |(11) Alloc Res|
             |              |              |S=STUN-PUB-1  |
             |              |              |D=R-PUB-1     |
             |              |              |R=STUN-PUB-4  |
             |              |              |MA=R-PUB-1    |
             |              |              |------------->|
             |              |              |              |



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             |              |              |              |
             |              |              |              |
             |              |              |              |RTCP Alloc.
             |              |              |              |Ta secs. later
             |              |              |              |
             |              |              |              |
             |              |              |              |
             |              |              |(12) Alloc Req|
             |              |              |S=R-PUB-2     |
             |              |              |D=STUN-PUB-1  |
             |              |              |<-------------|
             |              |              |(13) Alloc Res|
             |              |              |S=STUN-PUB-1  |
             |              |              |D=R-PUB-2     |
             |              |              |R=STUN-PUB-5  |
             |              |              |MA=R-PUB-2    |
             |              |              |------------->|
             |              |              |              |
             |              |              |              |
             |              |              |              |
             |              |              |              |
             |(14) answer   |              |              |
             |<-------------------------------------------|
             |              |              |              |
             |              |              |              |
             |              |              |              |
             |              |              |              |Validate
             |              |              |              |STUN-PUB-4 to STUN-PUB-2
             |              |              |              |
             |              |              |              |
             |              |              |(15) Send Ind |
             |              |              |S=R-PUB-1     |
             |              |              |D=STUN-PUB-1  |
             |              |              |DA=STUN-PUB-2 |
             |              |              |<-------------|
             |              |              |              |
             |              |              |Bind Req.     |
             |              |              |S=STUN-PUB-4  |
             |              |              |D=STUN-PUB-2  |
             |              |              |U=L3:1:R2:1   |
             |              |              |              |
             |              |              |              |
             |              |              |              |
             |              |              |              |
             |              |              |              |
             |              |              |Discard       |
             |              |              |              |
             |              |              |              |



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             |              |              |              |
             |              |              |              |
             |Validate      |              |              |
             |STUN-PUB-2 to STUN-PUB-4     |              |
             |              |              |              |
             |              |              |              |
             |(16) Send Ind |              |              |
             |S=L-PRIV-1    |              |              |
             |D=STUN-PUB-1  |              |              |
             |DA=STUN-PUB-4 |              |              |
             |------------->|              |              |
             |              |              |              |
             |              |(17) Send Ind |              |
             |              |S=NAT-PUB-1   |              |
             |              |D=STUN-PUB-1  |              |
             |              |DA=STUN-PUB-4 |              |
             |              |------------->|              |
             |              |              |              |
             |              |              |Bind Req.     |
             |              |              |S=STUN-PUB-2  |
             |              |              |D=STUN-PUB-4  |
             |              |              |U=R2:1:L3:1   |
             |              |              |              |
             |              |              |              |
             |              |              |(18) Data Ind |
             |              |              |S=STUN-PUB-1  |
             |              |              |D=R-PUB-1     |
             |              |              |RA=STUN-PUB-2 |
             |              |              |------------->|
             |              |              |(19) Send Ind |
             |              |              |S=R-PUB-1     |
             |              |              |D=STUN-PUB-1  |
             |              |              |DA=STUN-PUB-2 |
             |              |              |MA=STUN-PUB-2 |
             |              |              |<-------------|
             |              |              |              |
             |              |              |Bind Res.     |
             |              |              |S=STUN-PUB-4  |
             |              |              |D=STUN-PUB-2  |
             |              |              |MA=STUN-PUB-2 |
             |              |              |              |
             |              |(20) Data Ind |              |
             |              |S=STUN-PUB-1  |              |
             |              |D=NAT-PUB-1   |              |
             |              |RA=STUN-PUB-4 |              |
             |              |MA=STUN-PUB-2 |              |
             |              |<-------------|              |
             |(21) Data Ind |              |              |



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             |S=STUN-PUB-1  |              |              |
             |D=L-PRIV-1    |              |              |
             |RA=STUN-PUB-4 |              |              |
             |MA=STUN-PUB-2 |              |              |
             |<-------------|              |              |
             |              |              |              |
             |              |              |              |
             |              |              |              |
             |              |              |              |Validate
             |              |              |              |STUN-PUB-4 to STUN-PUB-2
             |              |              |              |
             |              |              |              |
             |              |              |(22) Send Ind |
             |              |              |S=R-PUB-1     |
             |              |              |D=STUN-PUB-1  |
             |              |              |DA=STUN-PUB-2 |
             |              |              |<-------------|
             |              |              |              |
             |              |              |Bind Req.     |
             |              |              |S=STUN-PUB-4  |
             |              |              |D=STUN-PUB-2  |
             |              |              |U=L3:1:R2:1   |
             |              |              |              |
             |              |              |              |
             |              |(23) Data Ind |              |
             |              |S=STUN-PUB-1  |              |
             |              |D=NAT-PUB-1   |              |
             |              |RA=STUN-PUB-4 |              |
             |              |<-------------|              |
             |              |              |              |
             |(24) Data Ind |              |              |
             |S=STUN-PUB-1  |              |              |
             |D=L-PRIV-1    |              |              |
             |RA=STUN-PUB-4 |              |              |
             |<-------------|              |              |
             |(25) Send Ind |              |              |
             |S=L-PRIV-1    |              |              |
             |D=STUN-PUB-1  |              |              |
             |DA=STUN-PUB-4 |              |              |
             |MA=STUN-PUB-4 |              |              |
             |------------->|              |              |
             |              |(26) Send Ind |              |
             |              |S=NAT-PUB-1   |              |
             |              |D=STUN-PUB-1  |              |
             |              |DA=STUN-PUB-4 |              |
             |              |MA=STUN-PUB-4 |              |
             |              |------------->|              |
             |              |              |              |



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             |              |              |Bind Res.     |
             |              |              |S=STUN-PUB-2  |
             |              |              |D=STUN-PUB-4  |
             |              |              |MA=STUN-PUB-4 |
             |              |              |              |
             |              |              |(27) Data Ind |
             |              |              |S=STUN-PUB-1  |
             |              |              |D=R-PUB-1     |
             |              |              |RA=STUN-PUB-2 |
             |              |              |MA=STUN-PUB-4 |
             |              |              |------------->|
             |              |              |              |
             |              |              |              |
             |              |              |              |
             |              |              |              |Validate
             |              |              |              |STUN-PUB-5 to STUN-PUB-3
             |              |              |              |
             |              |              |              |
             |              |              |(28) Send Ind |
             |              |              |S=R-PUB-2     |
             |              |              |D=STUN-PUB-1  |
             |              |              |DA=STUN-PUB-3 |
             |              |              |<-------------|
             |              |              |              |
             |              |              |Bind Req.     |
             |              |              |S=STUN-PUB-5  |
             |              |              |D=STUN-PUB-3  |
             |              |              |U=L3:2:R2:2   |
             |              |              |              |
             |              |              |              |
             |              |              |              |
             |              |              |              |
             |              |              |              |
             |              |              |Discard       |
             |              |              |              |
             |              |              |              |
             |              |              |              |
             |              |              |              |
             |Validate      |              |              |
             |STUN-PUB-3 to STUN-PUB-5     |              |
             |              |              |              |
             |              |              |              |
             |(29) Send Ind |              |              |
             |S=L-PRIV-2    |              |              |
             |D=STUN-PUB-1  |              |              |
             |DA=STUN-PUB-5 |              |              |
             |------------->|              |              |
             |              |              |              |



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             |              |(30) Send Ind |              |
             |              |S=NAT-PUB-2   |              |
             |              |D=STUN-PUB-1  |              |
             |              |DA=STUN-PUB-5 |              |
             |              |------------->|              |
             |              |              |              |
             |              |              |Bind Req.     |
             |              |              |S=STUN-PUB-3  |
             |              |              |D=STUN-PUB-5  |
             |              |              |U=R2:2:L3:2   |
             |              |              |              |
             |              |              |              |
             |              |              |(31) Data Ind |
             |              |              |S=STUN-PUB-1  |
             |              |              |D=R-PUB-2     |
             |              |              |RA=STUN-PUB-3 |
             |              |              |------------->|
             |              |              |(32) Send Ind |
             |              |              |S=R-PUB-2     |
             |              |              |D=STUN-PUB-1  |
             |              |              |DA=STUN-PUB-3 |
             |              |              |MA=STUN-PUB-3 |
             |              |              |<-------------|
             |              |              |              |
             |              |              |Bind Res.     |
             |              |              |S=STUN-PUB-5  |
             |              |              |D=STUN-PUB-3  |
             |              |              |MA=STUN-PUB-3 |
             |              |              |              |
             |              |(33) Data Ind |              |
             |              |S=STUN-PUB-1  |              |
             |              |D=NAT-PUB-2   |              |
             |              |RA=STUN-PUB-5 |              |
             |              |MA=STUN-PUB-3 |              |
             |              |<-------------|              |
             |(34) Data Ind |              |              |
             |S=STUN-PUB-1  |              |              |
             |D=L-PRIV-2    |              |              |
             |RA=STUN-PUB-5 |              |              |
             |MA=STUN-PUB-3 |              |              |
             |<-------------|              |              |
             |              |              |              |
             |              |              |              |
             |              |              |              |
             |              |              |              |Validate
             |              |              |              |STUN-PUB-5 to STUN-PUB-3
             |              |              |              |
             |              |              |              |



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             |              |              |(35) Send Ind |
             |              |              |S=R-PUB-2     |
             |              |              |D=STUN-PUB-1  |
             |              |              |DA=STUN-PUB-3 |
             |              |              |<-------------|
             |              |              |              |
             |              |              |Bind Req.     |
             |              |              |S=STUN-PUB-5  |
             |              |              |D=STUN-PUB-3  |
             |              |              |U=L3:2:R2:2   |
             |              |              |              |
             |              |              |              |
             |              |(36) Data Ind |              |
             |              |S=STUN-PUB-1  |              |
             |              |D=NAT-PUB-2   |              |
             |              |RA=STUN-PUB-5 |              |
             |              |<-------------|              |
             |              |              |              |
             |(37) Data Ind |              |              |
             |S=STUN-PUB-1  |              |              |
             |D=L-PRIV-2    |              |              |
             |RA=STUN-PUB-5 |              |              |
             |<-------------|              |              |
             |(38) Send Ind |              |              |
             |S=L-PRIV-2    |              |              |
             |D=STUN-PUB-1  |              |              |
             |DA=STUN-PUB-5 |              |              |
             |MA=STUN-PUB-5 |              |              |
             |------------->|              |              |
             |              |(39) Send Ind |              |
             |              |S=NAT-PUB-2   |              |
             |              |D=STUN-PUB-1  |              |
             |              |DA=STUN-PUB-5 |              |
             |              |MA=STUN-PUB-5 |              |
             |              |------------->|              |
             |              |              |              |
             |              |              |Bind Res.     |
             |              |              |S=STUN-PUB-3  |
             |              |              |D=STUN-PUB-5  |
             |              |              |MA=STUN-PUB-5 |
             |              |              |              |
             |              |              |(40) Data Ind |
             |              |              |S=STUN-PUB-1  |
             |              |              |D=R-PUB-2     |
             |              |              |RA=STUN-PUB-3 |
             |              |              |MA=STUN-PUB-5 |
             |              |              |------------->|
             |              |              |              |



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             |              |              |              |
             |              |              |              |
             |              |              |              |
             |RTP flows     |              |              |
             |              |              |              |
             |              |              |              |
             |(41) Send Ind |              |              |
             |S=L-PRIV-1    |              |              |
             |D=STUN-PUB-1  |              |              |
             |DA=STUN-PUB-4 |              |              |
             |------------->|              |              |
             |              |              |              |
             |              |(42) Send Ind |              |
             |              |S=NAT-PUB-1   |              |
             |              |D=STUN-PUB-1  |              |
             |              |DA=STUN-PUB-4 |              |
             |              |------------->|              |
             |              |              |              |
             |              |              |              |
             |              |              |RTP           |
             |              |              |S=STUN-PUB-2  |
             |              |              |D=STUN-PUB-4  |
             |              |              |              |
             |              |              |              |
             |              |              |(43) Data Ind |
             |              |              |S=STUN-PUB-1  |
             |              |              |D=R-PUB-1     |
             |              |              |RA=STUN-PUB-2 |
             |              |              |------------->|
             |              |              |              |
             |              |              |              |
             |              |              |              |
             |              |              |              |
             |              |              |              |RTP flows
             |              |              |              |
             |              |              |              |
             |              |              |(44) Send Ind |
             |              |              |S=R-PUB-1     |
             |              |              |D=STUN-PUB-1  |
             |              |              |DA=STUN-PUB-2 |
             |              |              |<-------------|
             |              |              |              |
             |              |              |              |
             |              |              |RTP           |
             |              |              |S=STUN-PUB-4  |
             |              |              |D=STUN-PUB-2  |
             |              |              |              |
             |              |              |              |



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             |              |(45) Data Ind |              |
             |              |S=STUN-PUB-1  |              |
             |              |D=NAT-PUB-1   |              |
             |              |RA=STUN-PUB-4 |              |
             |              |<-------------|              |
             |              |              |              |
             |(46) Data Ind |              |              |
             |S=STUN-PUB-1  |              |              |
             |D=L-PRIV-1    |              |              |
             |RA=STUN-PUB-4 |              |              |
             |<-------------|              |              |
             |              |              |              |
             |              |              |              |
             |              |              |              |
             |Validate      |              |              |
             |L-PRIV-1 to R-PUB-1          |              |
             |              |              |              |
             |              |              |              |
             |(47) Bind Req.|              |              |
             |S=L-PRIV-1    |              |              |
             |D=R-PUB-1     |              |              |
             |U=R1:1:L1:1   |              |              |
             |------------->|              |              |
             |              |              |              |
             |              |(48) Bind Req.|              |
             |              |S=NAT-PUB-3   |              |
             |              |D=R-PUB-1     |              |
             |              |U=R1:1:L1:1   |              |
             |              |---------------------------->|
             |              |              |              |
             |              |(49) Bind Res.|              |
             |              |S=R-PUB-1     |              |
             |              |D=NAT-PUB-3   |              |
             |              |MA=NAT-PUB-3  |              |
             |              |<----------------------------|
             |              |              |              |
             |(50) Bind Res.|              |              |
             |S=R-PUB-1     |              |              |
             |D=L-PRIV-1    |              |              |
             |MA-NAT-PUB-3  |              |              |
             |<-------------|              |              |
             |              |              |              |
             |              |              |              |
             |              |              |              |
             |              |              |              |Validate
             |              |              |              |R-PUB-1 to L-PRIV-1
             |              |              |              |
             |              |              |              |



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             |              |(51) Bind Req.|              |
             |              |S=R-PUB-1     |              |
             |              |D=L-PRIV-1    |              |
             |              |U=L1:1:R1:1   |              |
             |              |<----------------------------|
             |              |              |              |
             |              |              |              |
             |              |              |              |
             |              |              |              |
             |              |Discard       |              |
             |              |              |              |
             |              |              |              |
             |              |              |              |
             |              |              |              |
             |              |              |              |Validate
             |              |              |              |R-PUB-2 to L-PRIV-2
             |              |              |              |
             |              |              |              |
             |              |(52) Bind Req.|              |
             |              |S=R-PUB-2     |              |
             |              |D=L-PRIV-2    |              |
             |              |U=L1:2:R1:2   |              |
             |              |<----------------------------|
             |              |              |              |
             |              |              |              |
             |              |              |              |
             |              |              |              |
             |              |Discard       |              |
             |              |              |              |
             |              |              |              |
             |              |              |              |
             |              |              |              |
             |Validate      |              |              |
             |L-PRIV-2 to R-PUB-2          |              |
             |              |              |              |
             |              |              |              |
             |(53) Bind Req.|              |              |
             |S=L-PRIV-2    |              |              |
             |D=R-PUB-2     |              |              |
             |U=R1:2:L1:2   |              |              |
             |------------->|              |              |
             |              |              |              |
             |              |(54) Bind Req.|              |
             |              |S=NAT-PUB-4   |              |
             |              |D=R-PUB-2     |              |
             |              |U=R1:2:L1:2   |              |
             |              |---------------------------->|
             |              |              |              |



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             |              |(55) Bind Res.|              |
             |              |S=R-PUB-2     |              |
             |              |D=NAT-PUB-4   |              |
             |              |MA=NAT-PUB-4  |              |
             |              |<----------------------------|
             |              |              |              |
             |(56) Bind Res.|              |              |
             |S=R-PUB-2     |              |              |
             |D=L-PRIV-2    |              |              |
             |MA=NAT-PUB-4  |              |              |
             |<-------------|              |              |
             |              |              |              |
             |              |              |              |
             |              |              |              |
             |              |              |              |Validate
             |              |              |              |R-PUB-1 to NAT-PUB-3
             |              |              |              |
             |              |              |              |
             |              |(57) Bind Req.|              |
             |              |S=R-PUB-1     |              |
             |              |D=NAT-PUB-3   |              |
             |              |U=L1R1:1:R1:1 |              |
             |              |<----------------------------|
             |              |              |              |
             |(58) Bind Req.|              |              |
             |S=R-PUB-1     |              |              |
             |D=L-PRIV-1    |              |              |
             |U=L1R1:1:R1:1 |              |              |
             |<-------------|              |              |
             |              |              |              |
             |(59) Bind Res.|              |              |
             |S=L-PRIV-1    |              |              |
             |D=R-PUB-1     |              |              |
             |MA=R-PUB-1    |              |              |
             |------------->|              |              |
             |              |              |              |
             |              |(60) Bind Res.|              |
             |              |S=NAT-PUB-3   |              |
             |              |D=R-PUB-1     |              |
             |              |MA=R-PUB-1    |              |
             |              |---------------------------->|
             |              |              |              |
             |              |              |              |
             |              |              |              |
             |              |              |              |Validate
             |              |              |              |R-PUB-2 to NAT-PUB-4
             |              |              |              |
             |              |              |              |



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             |              |(61) Bind Req.|              |
             |              |S=R-PUB-2     |              |
             |              |D=NAT-PUB-4   |              |
             |              |U=L1R1:2:R1:2 |              |
             |              |<----------------------------|
             |              |              |              |
             |(62) Bind Req.|              |              |
             |S=R-PUB-2     |              |              |
             |D=L-PRIV-2    |              |              |
             |U=L1R1:2:R1:2 |              |              |
             |<-------------|              |              |
             |              |              |              |
             |(63) Bind Res.|              |              |
             |S=L-PRIV-2    |              |              |
             |D=R-PUB-2     |              |              |
             |MA=R-PUB-2    |              |              |
             |------------->|              |              |
             |              |              |              |
             |              |(64) Bind Res.|              |
             |              |S=NAT-PUB-4   |              |
             |              |D=R-PUB-2     |              |
             |              |MA=R-PUB-2    |              |
             |              |---------------------------->|
             |              |              |              |
             |              |              |              |
             |              |              |              |
             |              |              |              |
             |(65) Offer    |              |              |
             |------------------------------------------->|
             |              |              |              |
             |              |              |              |
             |              |              |              |
             |              |              |              |
             |(66) Answer   |              |              |
             |<-------------------------------------------|
             |              |              |              |
             |              |              |              |
             |              |              |              |
             |              |              |              |
             |              |              |              |
             |              |              |              |


   Figure 17

   First, agent L obtains both server reflexive and relayed transport
   addresses for its RTP packets, using a STUN Allocate request, which
   will provide it with both types of addresses (messages 1-4).  Recall



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   that the NAT has the address and port dependent mapping property.
   Here, it creates a binding of NAT-PUB-1 for this UDP request, and
   this becomes the server reflexive transport address for RTP.  The
   relayed transport address is STUN-PUB-2, allocated by the STUN
   server.  Agent L repeats this process for RTCP (messages 5-8) Ta
   seconds later, and obtains NAT-PUB-2 as its server reflexive
   transport address for RTCP and STUN-PUB-3 for its relayed transport
   address.

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


       v=0
       o=jdoe 2890844526 2890842807 IN IP4 $L-PRIV-1.IP
       s=
       c=IN IP4 $STUN-PUB-2.IP
       t=0 0
       a=ice-pwd:$LPASS
       m=audio $STUN-PUB-2.PORT RTP/AVP 0
       a=rtpmap:0 PCMU/8000
       a=rtcp:$STUN-PUB-3.PORT
       a=candidate:$L1 1 UDP 1.0 $L-PRIV-1.IP $L-PRIV-1.PORT
       a=candidate:$L1 2 UDP 1.0 $L-PRIV-2.IP $L-PRIV-2.PORT
       a=candidate:$L2 1 UDP 0.7 $NAT-PUB-1.IP $NAT-PUB-1.PORT
       a=candidate:$L2 2 UDP 0.7 $NAT-PUB-2.IP $NAT-PUB-2.PORT
       a=candidate:$L3 1 UDP 0.3 $STUN-PUB-2.IP $STUN-PUB-2.PORT
       a=candidate:$L3 2 UDP 0.3 $STUN-PUB-3.IP $STUN-PUB-3.PORT

   This offer is received at agent R. Agent R will gather its server
   reflexive and relayed transport addresses for RTP from an Allocate
   request (messages 10-11).  Since the server reflexive transport
   address matches its local transport address, no separate candidate is
   used for it.  The agent then gathers its server reflexive and relayed
   transport addresses for RTCP (messages 12-13).  It prioritizes the
   local candidate with higher priority than the relayed candidate, and
   selects the relayed candidate as the operating candidate.  Its
   resulting answer looks like:










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

   Next, agents L and R form candidate pairs and the transport address
   pair check ordered list.  This list will start with the two
   components in the currently operating candidate pair - relayed
   candidates.  Agent R begins its checks (message 15).  It will check
   connectivity between the operating candidate pair, starting with the
   first component, which is STUN-PUB-4 for agent R and STUN-PUB-2 for
   agent L. The state machine for that transport address pair moves to
   the Testing state.  Since this is a relayed transport address for
   agent R, it utilizes the STUN Send Indication to deliver the Binding
   Request.  The DESTINATION-ADDRESS is STUN-PUB-2.

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

   Once L gets the offer, it will attempt to validate the first
   transport address pair in the transport address pair check ordered
   list, which will be the operating candidate.  The state machine for
   this transport address pair moves into the Testing state.  Like agent
   R did, it will use the STUN Send Indication to send a STUN Binding
   Request from its relayed transport address, STUN-PUB-2, to STUN-PUB-4
   (message 16).  This packet traverses the NAT (message 17) and arrives
   at the STUN server.  The STUN server will unwrap the contents of the
   packet and send them from STUN-PUB-2 to STUN-PUB-4.  It will also, as
   a consequence, add a permission for STUN-PUB-4.  The contents of the



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   packet are a STUN Binding Request with USERNAME R2:1:L3:1 (note how
   this is the flip of the USERNAME in the Binding Request sent by agent
   R).  This is also a packet from the STUN server to itself.  However,
   now, the packet is not discarded, as a permission had been installed
   as a consequence of the "suicide packet" from agent R (a suicide
   packet is a packet that has no hope of traversing a far end NAT, but
   serves the purpose of enabling a permission in a near end NAT so that
   a packet from the peer can be returned).  Thus, the STUN server will
   relay the received STUN request towards agent R (message 18).  This
   is delivered as a STUN Data Indication.  Notice how the REMOTE-
   ADDRESS is STUN-PUB-2; this is important as it will be used to
   construct the STUN Binding Response.

   Agent R will receive the Data Indication, and unwrap its contents to
   find the Binding Request.  The state machine for this transport
   address pair is currently in the Testing state.  It therefore moves
   into the Send-Valid state, and it generates a Binding Response.
   However, the XOR-MAPPED-ADDRESS in the Binding Response is
   constructed using the source IP address and port that were seen by
   the STUN server when the Binding Request arrived at STUN-PUB-4, which
   is the looped message between messages 17 and 18.  This source
   address is STUN-PUB-2, which is the value of the REMOTE-ADDRESS
   attribute in message 18.  Thus, the STUN Binding Response will
   contain STUN-PUB-2 in the XOR-MAPPED-ADDRESS, and is to be sent to
   STUN-PUB-2.  To send the response, agent R takes the STUN Binding
   Response and encapsulates it in a STUN Send indication, setting the
   DESTINATION-ADDRESS to STUN-PUB-2.  This is shown in message 19.

   The STUN server will receive this Send Indication, and unwrap its
   contents to find the STUN Binding Response.  It sends it to the value
   of the DESTINATION-ADDRESS attribute, and sends it from the relayed
   address allocated to R, which is STUN-PUB-4.  This, once again,
   results in a looped message to itself, and it arrives at STUN-PUB-2.
   Now, however, there is a permission installed for STUN-PUB-4.  The
   STUN server will therefore forward the packet to agent L. To do so,
   it constructs a STUN Data Indication containing the contents of the
   packet.  It sets the REMOTE-ADDRESS to the source transport address
   of the request it received (STUN-PUB-4), and forwards it to agent L
   (message 20).  This traverses the NAT (message 21) and arrives at
   agent L. As a consequence of the receipt of a Binding Response, the
   state machine for this transport address pair moves to the Recv-Valid
   state.  The agent also examines the XOR-MAPPED-ADDRESS of the STUN
   response.  It indicates STUN-PUB-2.  This is the same as the native
   transport address of this transport address pair, and thus doesn't
   represent a new transport address that might have been learned.

   Because of the receipt of message 18, the transport address pair
   moved from Testing to Send-Valid, causing R to attempt a



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   retransmission of its STUN Binding Request that was lost (the
   contents of message 15 that were discarded by the STUN server due to
   lack of permission).  This time, however, a permission has been
   installed and the retransmission will work.  So, it sends the Binding
   Request again (message 22, identical to message 15).  This is looped
   by the STUN server to itself again, but this time there is a
   permission in place when it arrives at STUN-PUB-2.  As such, the
   request is forwarded towards agent L this time, in a STUN Data
   Indication (message 23).  This traverses the NAT (message 24) and
   arrives at agent L. Agent L extracts the contents of the request,
   which are a STUN Binding Request.  This causes the state machine to
   move from Recv-Valid to Valid.  It generates a STUN Binding Response,
   and sets the XOR-MAPPED-ADDRESS based on the value of the REMOTE-
   ADDRESS in message 24 (STUN-PUB-4).  This Binding Response is sent to
   STUN-PUB-4, which is accomplished through a STUN Send Indication
   (message 25).  This Send Indication traverses the NAT (message 26)
   and is received by the STUN server.  Its contents are decapsulated,
   and sent to STUN-PUB-4, which is again a loop on the same host.  This
   packet is then sent towards agent R in a Data Indication (message
   27).  The contents of the DATA Indication are extracted, and the
   agent sees a successful Binding Response.  It therefore moves the
   state machine from the Send-Valid state to the Valid state.  At this
   point, the transport address pair is in the Valid state for both
   agents.

   Approximately Tb seconds after agent R sent message 15, agent R will
   start checks for the next transport address pair in its transport
   address pair check ordered list.  This is the second component of the
   same candidate pair, used for RTCP.  This sequence, messages 28
   through 40, are identical to the ones for RTP, but differ only in the
   specific transport addresses.

   Once that validation happens, the second transport address pair has
   been validated.  The candidate pair moves into the valid state, and
   both candidates are considered valid.  The operating candidate has
   now been validated, and media can begin to flow.  It will do so
   through the STUN server; indeed, it is relayed "twice" through the
   STUN server.  Even though there is a single STUN server, it is
   logically acting as two separate STUN servers.  Indeed, had L and R
   used two separate STUN servers, media would be relayed through both
   STUN servers in a trapezoid configuration.

   The actual media flows are shown as well.  It is important to note
   that, since the ICE checks have not yet concluded on the candidate
   that will ultimately be used, no STUN Set Active Destinations have
   been sent.  As a consequence, media that is sent through the STUN
   servers has to be sent using STUN Send indications.  This introduces
   some overhead, but is a transient condition.  In message 41, agent L



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   sends an RTP packet to agent R using a Send indication.  It is sent
   to STUN-PUB-4.  This traverses the NAT (message 42), and arrives at
   the STUN server.  It is decapsulated, looped to itself, and arrives
   at STUN-PUB-4.  From there, it is encapsulated in a Data Indication
   and sent to agent R (message 43).  In the reverse direction, agent R
   will send an RTP packet using a STUN Send indication (message 42),
   and send it to STUN-PUB-2.  This is received by the STUN server,
   decapsulated, and sent to STUN-PUB-2 from STUN-PUB-4.  This is again
   a loop within the same host, arriving at STUN-PUB-4.  The contents of
   the packet are sent to agent L through a STUN Data Indication
   (message 45), which traverses the NAT (message 46) to arrive at agent
   L. Since this call flow is already long enough, RTCP packet
   transmission is not shown.

   Approximately Tb seconds after it sends message 29, agent L goes to
   the next transport address pair in its transport address pair check
   ordered list that is in the Waiting state.  This will be the RTP
   candidate for the top priority candidate pair, which is L-PRIV-1 on
   agent L and R-PUB-1 on agent R. This is a local candidate for each
   agent.  To perform the check, agent L sends a STUN Binding Request
   from L-PRIV-1 to R-PUB-1 (message 47).  Note the USERNAME of
   R1:1:L1:1, which identifies this transport address pair.  This
   traverses the NAT (message 48).  Since the NAT has the address and
   port dependent mapping property, and this is a new destination IP
   address, the NAT allocates a new transport address on its public
   side, NAT-PUB-3, and places this in the source IP address and port.
   This packet arrives at agent R. Agent R finds a matching transport
   address pair in the Waiting state.  The state machine transitions to
   the Send-Valid state.  It sends the Binding response, with a XOR-
   MAPPED-ADDRESS indicating NAT-PUB-3 (message 49), which traverses the
   NAT and arrives at agent L (message 50).  Agent R, in addition to
   sending the response, will also send a Binding Request.  It is
   important to remember that this Binding Request is sent to the remote
   address in the transport address pair (L-PRIV-1), and NOT to the
   source IP address and port of the Binding Request (NAT-PUB-3); that
   will happen later.  This attempt is shown in message 51.  However,
   since the L-PRIV-1 is private, the packet is discarded in the
   network.

   Now, as a consequence of receiving message 48, agent R will have
   constructed a peer-derived candidate.  The candidate ID for this
   candidate is L1R1, and it initially contains a single transport
   address pair, NAT-PUB-3 and R-PUB-1.  However, the candidate isn't
   yet usable until the other component gets added.  Similarly, agent L
   will have constructed the same peer-derived candidate, with the same
   candidate ID and the same transport address pair.

   Some Tb seconds after sending message 28, agent R will move to the



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   next transport address pair in the transport address pair check
   ordered list whose state is Waiting.  This is the RTCP component of
   the highest priority candidate pair.  It will attempt a connectivity
   check, from R-PUB-2 to L-PRIV-2 (message 52).  Since L-PRIV-1 is
   private, this message is discarded.

   Some Tb seconds after sending message 47, agent L will move to the
   next transport address pair in the transport address pair check
   ordered list whose state is Waiting.  This is the RTCP component of
   the highest priority candidate pair.  It will attempt a connectivity
   check, from L-PRIV-2 to R-PUB-2 (message 53), which operates nearly
   identically to messages 47-50, with the exception of the specific
   addresses.  Here, the NAT will create a new binding for the RTCP,
   NAT-PUB-4, and this transport address is new for both participants.
   On receipt of this Binding Request at agent R (message 54), agent R
   constructs the candidate ID for the peer-derived candidate, L1R1, and
   finds it already exists.  As such, this new transport address is
   added, and the peer-derived candidate becomes complete and usable.
   Agent L does the same thing on receipt of message 56.  This candidate
   will have the same priority as its generating candidate L1 (1.0), and
   is paired up with R1 (also at priority 1.0).  Since L1R1 has the same
   priority as L1 itself, the ordering algorithm in Section 7.5 will use
   the reverse ASCII sort order of the candidate ID iself to determine
   order.  L1R1 is larger than L1, so that the peer-derived candidate
   will come before its generating candidate.  As a consequence, the
   peer-derived candidate pair will have a higher priority than its
   generating candidate, and appear just before it in the candidate pair
   priority ordered list.

   As a consequence, after agent R sends message 55 and completes the
   peer-derived candidate, it will move the two transport addresses in
   the peer derived candidate into the Send-Valid state, and send a
   Binding Request for each in rapid succession (agent L will have moved
   both into the Recv-Valid state upon receipt of message 56).  The
   first of these connectivity checks are for the RTP component, from
   R-PUB-1 to NAT-PUB-3 (message 57).  Note the USERNAME in the STUN
   Binding Request, L1R1:1:R1:1, which identifies the peer-derived
   transport address pair.  This will succesfully traverse the NAT and
   be delivered to agent L (message 58).  The receipt of this request
   moves the state machine for this transport address pair from Recv-
   Valid to Valid, and a Binding Response is sent (message 59).  This
   passes through the NAT and arrives at agent R (message 60).  This
   causes its state machine to enter the Valid state as well.  The
   reflexive transport address, R-PUB-1, is not new to agent R and thus
   does not result in the creation of a new peer-derived candidate.

   Messages 61 through 64 show the same basic flow for RTCP.  Upon
   receipt of message 64, both transport address pairs are Valid at both



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   agents, causing the peer derived candidate to become valid.  Timer
   Tws is set at agent L, and fires without any higher priority
   candidate pairs becoming validated.  At agent R, media can now be
   sent on this candidate pair from answerer (agent R) to offerer (agent
   L).  Agent L sends an updated offer to promote the peer-derived
   candidate to operating.  This offer (message 65) looks like:


       v=0
       o=jdoe 2890844526 2890842808 IN IP4 $L-PRIV-1.IP
       s=
       c=IN IP4 $NAT-PUB-3.IP
       t=0 0
       a=ice-pwd:$LPASS
       m=audio $NAT-PUB-3.PORT RTP/AVP 0
       a=rtpmap:0 PCMU/8000
       a=rtcp:$NAT-PUB-4.PORT
       a=remote-candidate:R1
       a=candidate:$L1 1 UDP 1.0 $L-PRIV-1.IP $L-PRIV-1.PORT
       a=candidate:$L1 2 UDP 1.0 $L-PRIV-2.IP $L-PRIV-2.PORT

   There are several important things to note in this offer.  Firstly,
   note how the m/c-line now contains NAT-PUB-3 and NAT-PUB-4, the peer
   derived transport addresses it learned through the ICE processing.
   Secondly, note how there remains a candidate encoded into the
   a=candidate attributes.  This is candidate L1, NOT candidate L1R1.
   Recall that the peer-derived candidates are never encoded into the
   SDP.  Rather, their generating candidate is encoded.  This will cause
   keepalives to take place for the generating candidate if valid
   (though its not) and any of its derived candidates, which is what we
   want.  Finally, notice the inclusion of the a=remote-candidate
   attribute.  Since agent L doesn't know whether agent R received
   messages 60 or 64, it doesnt know whether the state of the candidate
   is Send-Valid or Valid at agent R. So, it has to tell agent R that,
   in case its Send-Valid, to please use it anyway.

   The answer generated by agent R looks like:


       v=0
       o=bob 2808844564 2808844565 IN IP4 $R-PUB-1.IP
       s=
       c=IN IP4 $R-PUB-1.IP
       t=0 0
       a=ice-pwd:$RPASS
       m=audio $R-PUB-1.PORT RTP/AVP 0
       a=rtpmap:0 PCMU/8000
       a=rtcp:$R-PUB-2.PORT



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       a=candidate:$R1 1 UDP 1.0 $R-PUB-1.IP $R-PUB-1.PORT
       a=candidate:$R1 2 UDP 1.0 $R-PUB-2.IP $R-PUB-2.PORT

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


12.  Grammar

   This specification defines three new SDP attributes - the
   "candidate", "remote-candidate" and "ice-pwd" attributes.

   The candidate attribute is a media-level attribute only.  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.
   There is no requirement that a=candidate attribute which indicate
   components for the same candidate appear one right after the other or
   in component ID order.

   The syntax of this attribute is defined using Augmented BNF as
   defined in RFC 4234 [9]:


   candidate-attribute   = "candidate" ":" candidate-id SP component-id SP
                           transport SP
                           qvalue SP   ;qvalue from RFC 3261
                           connection-address SP     ;from RFC 4566
                           port         ;port from RFC 4566
                           [SP cand-type]
                           [SP rel-addr]
                           [SP rel-port]
                           *(SP extension-att-name SP
                                extension-att-value)

   transport             = "UDP" / transport-extension
   transport-extension   = token              ; from RFC 3261
   candidate-id          = 1*base64-char

   base64-char           = ALPHA / DIGIT / "+" / "/"
   component-id          = 1*DIGIT
   cand-type             = "typ" SP candidate-types
   candidate-types       = "local" / "srflx" / "relay" / token
   rel-addr              = "raddr" SP connection-address
   rel-port              = "rport" SP port
   extension-att-name    = byte-string    ;from RFC 4566
   extension-att-value   = byte-string




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   The candidate-id is used to group together the transport addresses
   for a particular candidate.  It MUST be constructed with at least 24
   bits of randomness.  It MUST have the same value for all transport
   addresses within the same candidate.  It MUST have a different value
   for transport addresses within different candidates for the same
   media stream.  The candidate-id uses a syntax that is defined to be
   equal to the base64 alphabet [3], which allows the candidate-id to be
   generated by performing a base64 encoding of a randomly generated
   value (note, however, that this does not mean that the candidate-id
   or password is base64 decoded when use in STUN messages).  In
   addition, if content is base64 encoded to generate the candidate-id,
   it MUST NOT be padded with '='.  Section 2.2 of RFC 3548 indicates
   that some base64 usages do not require padding, and it requests that
   such usages call out that fact.  ICE is one such usage.  This is
   because the data is never decoded.  The component-id is a positive
   integer, which identifies the specific component of the candidate.
   It MUST start at 1 and MUST increment by 1 for each component of a
   particular candidate.

   The addr production is taken from [10], allowing for IPv4 addresses,
   IPv6 addresses and FQDNs.  The port production is taken from RFC 4566
   [5].  The token production is taken from RFC 3261 [2].  The transport
   production indicates the transport protocol for the candidate.  This
   specification only defines UDP.  However, extensibility is provided
   to allow for future transport protocols to be used with ICE, such as
   TCP or the Datagram Congestion Control Protocol (DCCP) [30].

   The cand-type production encodes the type of transport address.  This
   specification defines the values "local" for a local transport
   address, "srflx" for a server reflexive transport address, and
   "relay" for a relayed transport address.  The set of candidate types
   is extensible for the future.  Note that there is no value defined
   for peer reflexive transport addresses.  This is because these
   transport addresses are never carried in the SDP itself; they are
   learned implicitly through connectivity checks.  Inclusion of the
   candidate type is optional.

   The rel-addr and rel-port productions convey information on related
   transport addresses.  For a server reflexive transport address, the
   rel-addr and rel-port contain the associated local transport address.
   For a relayed transport address, the rel-addr and rel-port contain
   the server reflexive transport address towards the relay.  If rel-
   addr is present, rel-port MUST be present, and if rel-port is
   present, rel-addr MUST be present.  If the candidate type is "local",
   rel-addr and rel-port MUST NOT be present.  If the candidate type is
   "srflx" or "relayed", both rel-addr and rel-port MUST be present.

   The a=candidate attribute can itself be extended.  The grammar allows



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   for new name/value pairs to be added at the end of the attribute.  An
   implementation MUST ignore any name/value pairs it doesn't
   understand.

   The syntax of the "remote-candidate" attribute is defined using
   Augmented BNF as defined in RFC 4234 [9]:


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

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

   The syntax of the "ice-pwd" attribute is defined as:


   ice-pwd-att           = "ice-pwd" ":" password
   password              = 1*base64-char

   The "ice-pwd" attribute can appear at either the session-level or
   media-level.  When present in both, the value in the media-level
   takes precedence.  Thus, the value at the session level is
   effectively a default that applies to all media streams, unless
   overriden by a media-level value.  It MUST have at least 128 bits of
   randomness.  Like the candidate ID, its syntax is taken from the
   base64 alphabet, allowing the password to be generted from a base64
   encoding of a 128 bit value.  In addition, if content is base64
   encoded to generate the candidate ID, it MUST NOT be padded with '='.


13.  Security Considerations

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

13.1.  Attacks on Connectivity Checks

   An attacker might attempt to disrupt the STUN-based connectivity
   checks.  Ultimately, all of these attacks fool an agent into thinking
   something incorrect about the results of the connectivity checks.
   The possible false conclusions an attacker can try and cause are:

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



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

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

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

   Of the various techniques for creating faked STUN messages described
   in [12], many are not applicable for the connectivity checks.
   Compromises of STUN servers are not much of a concern, since the STUN
   servers are embedded in endpoints and distributed throughout the
   network.  Thus, compromising the STUN server is equivalent to
   comprimising the endpoint, and if that happens, far more problematic
   attacks are possible than those against ICE.  Similarly, DNS attacks
   are irrelevant since STUN servers are not discovered via DNS, they
   are signaled via SIP.  Injection of fake responses and relaying
   modified requests all can be handled in ICE with the countermeasures
   discussed below.

   To force the false invalid result, the attacker has to wait for the
   connectivity check for one of the agents to be sent.  When it is, the
   attacker needs to inject a fake response with an unrecoverable error
   response, such as a 600.  This attack only needs to be launched
   against one of the agents in order to invalidate the candidate pair.
   However, since the candidate is, in fact, valid, the original request
   may reach the peer agent, and result in a success response.  The
   attacker needs to force this packet or its response to be dropped,
   through a DoS attack, layer 2 network disruption, or other technique.
   If it doesn't do this, the success response will also reach the
   originator, alerting it to a possible attack.  This will cause the
   agent to abandon the candidate, which is the desired result in any
   case.  Fortunately, this attack is mitigated completely through the
   STUN message integrity mechanism.  The attacker needs to inject a
   fake response, and in order for this response to be processed, the
   attacker needs the password.  If the offer/answer signaling is
   secured, the attacker will not have the password.

   Forcing the fake valid result works in a similar way.  The agent



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

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

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

   The other agent will then initiate a connectivity check towards that
   transport address.  This validation needs to succeed.  This requires
   the attacker to force a false valid on a false candidate.  Injecting
   of fake requests or responses to achieve this goal is prevented using
   the integrity mechanisms of STUN and the offer/answer exchange.
   Thus, this attack can only be launched through replays.  To do that,
   the attacker must intercept the Binding Request towards this false
   transport address, and replay it towards the other agent.  Then, it
   must intercept the response and replay that back as well.

   This attack is very hard to launch unless the attacker themself is
   identified by the fake transport address.  This is because it
   requires the attacker to intercept and replay packets sent by two



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   different hosts.  If both agents are on different networks (for
   example, across the public Internet), this attack can be hard to
   coordinate, since it needs to occur against two different endpoints
   on different parts of the network at the same time.

   If the attacker themself is identified by the fake transport address,
   the attack is easier to coordinate.  However, if SRTP is used [22],
   the attacker will not be able to play the media packets, they will
   only be able to discard them, effectively disabling the media stream
   for the call.  However, this attack requires the agent to disrupt
   packets in order to block the connectivity check from reaching the
   target.  In that case, if the goal is to disrupt the media stream,
   its much easier to just disrupt it with the same mechanism, rather
   than attack ICE.

13.2.  Attacks on Address Gathering

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

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

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

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




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13.3.  Attacks on the Offer/Answer Exchanges

   An attacker that can modify or disrupt the offer/answer exchanges
   themselves can readily launch a variety of attacks with ICE.  They
   could direct media to a target of a DoS attack, they could insert
   themselves into the media stream, and so on.  These are similar to
   the general security considerations for offer/answer exchanges, and
   the security considerations in RFC 3264 [4] apply.  These require
   techniques for message integrity and encryption for offers and
   answers, which are satisfied by the SIPS mechanism [2] when SIP is
   used.  As such, the usage of SIPS with ICE is RECOMMENDED.

13.4.  Insider Attacks

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

13.4.1.  The Voice Hammer Attack

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

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

   Unfortunately, ICE doesn't help if its not used, in which case an
   attacker could simply send the offer without the ICE parameters.
   However, in environments where the set of clients are known, and
   limited to ones that support ICE, the server can reject any offers or
   answers that don't indicate ICE support.

13.4.2.  STUN Amplification Attack

   The STUN amplification attack is similar to the voice hammer.
   However, instead of voice packets being directed to the target, STUN
   connectivity checks are directed to the target.  This attack is
   accomplished by having the offerer send an offer with a large number
   of candidates, say 50.  The answerer receives the offer, and starts
   its checks, which are directed at the target, and consequently, never



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   generate a response.  The answerer will start a new connectivity
   check every 50ms, and each check is a STUN transaction consisting of
   9 retransmits of a message 64 bytes in length.  This produces a
   fairly substantial 92 kbps, just in STUN requests.

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


14.  IANA Considerations

   This specification defines three new SDP attribute per the procedures
   of Section 8.2.4 of [5].  The required information for the
   registrations are included here.

14.1.  candidate Attribute

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

   Attribute Name: candidate

   Long Form: candidate

   Type of Attribute: media level

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

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

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

14.2.  remote-candidate Attribute

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







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   Attribute Name: remote-candidate

   Long Form: remote-candidate

   Type of Attribute: media level

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

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

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

14.3.  ice-pwd Attribute

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

   Attribute Name: ice-pwd

   Long Form: ice-pwd

   Type of Attribute: session level

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

   Purpose: This attribute is used with Interactive Connectivity
      Establishment (ICE), and provides the password used to protect
      STUN connectivity checks.

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


15.  IAB Considerations

   The IAB has studied the problem of "Unilateral Self Address Fixing",
   which is the general process by which a agent attempts to determine
   its address in another realm on the other side of a NAT through a
   collaborative protocol reflection mechanism [20].  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



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   document a specific set of considerations.  This section meets those
   requirements.

15.1.  Problem Definition

   From RFC 3424 any UNSAF proposal must provide:

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

   The specific problems being solved by ICE are:

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

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

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

15.2.  Exit Strategy

   From RFC 3424, any UNSAF proposal must provide:

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

   ICE itself doesn't easily get phased out.  However, it is useful even
   in a globally connected Internet, to serve as a means for detecting
   whether a router failure has temporarily disrupted connectivity, for
   example.  ICE also helps prevent certain security attacks which have
   nothing to do with NAT.  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



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   network with both 6to4 and native v6 connectivity to determine which
   address to use when communicating with a peer.

15.3.  Brittleness Introduced by ICE

   From RFC3424, any UNSAF proposal must provide:

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

   ICE actually removes brittleness from existing UNSAF mechanisms.  In
   particular, traditional STUN (as described in [14]) 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.

   Another point of brittleness in STUN 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 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.

   Consequently, ICE serves to repair the brittleness introduced in
   other UNSAF mechanisms, and does not introduce any additional
   brittleness into the system.





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15.4.  Requirements for a Long Term Solution

   From RFC 3424, any UNSAF proposal must provide:

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

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

15.5.  Issues with Existing NAPT Boxes

   From RFC 3424, any UNSAF proposal must provide:

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

   A number of NAT boxes are now being deployed into the market which
   try and provide "generic" ALG functionality.  These generic ALGs hunt
   for IP addresses, either in text or binary form within a packet, and
   rewrite them if they match a binding.  This interferes with
   traditional STUN.  However, the update to STUN [12] uses an encoding
   which hides these binary addresses from generic ALGs.  Since [12] is
   required for all ICE implementations, this NAPT problem does not
   impact ICE.

   Existing NAPT boxes have non-deterministic and typically short
   expiration times for UDP-based bindings.  This requires
   implementations to send periodic keepalives to maintain those
   bindings.  ICE uses a default of 15s, which is a very conservative
   estimate.  Eventually, over time, as NAT boxes become compliant to
   behave [32], this minimum keepalive will become deterministic and
   well-known, and the ICE timers can be adjusted.  Having a way to
   discover the minimum keepalive interval would be far better still.


16.  Acknowledgements

   The authors would like to thank Flemming Andreasen, Rohan Mahy, Dean
   Willis, Dan Wing, Douglas Otis, Tim Moore, Francois Audet, Bill May
   and Philip Matthews for their comments and input.  A special thanks
   goes to Magnus Westerlund for doing several detailed reviews on the
   various revisions of this specification.  His input led to many
   substantive improvements in this document.





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17.  References

17.1.  Normative References

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

   [2]   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.

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

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

   [5]   Handley, M., "SDP: Session Description Protocol",
         draft-ietf-mmusic-sdp-new-26 (work in progress), January 2006.

   [6]   Casner, S., "Session Description Protocol (SDP) Bandwidth
         Modifiers for RTP Control Protocol (RTCP) Bandwidth", RFC 3556,
         July 2003.

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

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

   [9]   Crocker, D. and P. Overell, "Augmented BNF for Syntax
         Specifications: ABNF", RFC 4234, October 2005.

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

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

   [12]  Rosenberg, J., "Simple Traversal of UDP Through Network Address
         Translators (NAT) (STUN)", draft-ietf-behave-rfc3489bis-03
         (work in progress), March 2006.

   [13]  Rosenberg, J., "Obtaining Relay Addresses from Simple Traversal
         of UDP Through NAT (STUN)", draft-ietf-behave-turn-00 (work in
         progress), March 2006.



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17.2.  Informative References

   [14]  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.

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

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

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

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

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

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

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

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

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

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

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

   [26]  Camarillo, G. and H. Schulzrinne, "Early Media and Ringing Tone
         Generation in the Session Initiation Protocol (SIP)", RFC 3960,
         December 2004.

   [27]  Andreasen, F., "Connectivity Preconditions for Session



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         Description Protocol Media Streams",
         draft-ietf-mmusic-connectivity-precon-02 (work in progress),
         June 2006.

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

   [29]  Huitema, C., "Teredo: Tunneling IPv6 over UDP through Network
         Address Translations (NATs)", RFC 4380, February 2006.

   [30]  Kohler, E., Handley, M., and S. Floyd, "Datagram Congestion
         Control Protocol (DCCP)", RFC 4340, March 2006.

   [31]  Hellstrom, G. and P. Jones, "RTP Payload for Text
         Conversation", RFC 4103, June 2005.

   [32]  Audet, F. and C. Jennings, "NAT Behavioral Requirements for
         Unicast UDP", draft-ietf-behave-nat-udp-07 (work in progress),
         June 2006.
































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Internet-Draft                     ICE                         June 2006


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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Internet-Draft                     ICE                         June 2006


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