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MMUSIC                                                      J. Rosenberg
Internet-Draft                                             Cisco Systems
Expires: March 4, 2007                                   August 31, 2006


Interactive Connectivity Establishment (ICE): A Methodology for Network
     Address Translator (NAT) Traversal for Offer/Answer Protocols
                        draft-ietf-mmusic-ice-10

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   This Internet-Draft will expire on March 4, 2007.

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
   Underneath NAT (STUN) protocol, applying its binding discovery and
   relay usages, in addition to defining a new usage for checking
   connectivity between peers.



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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  4
   2.  Overview of ICE  . . . . . . . . . . . . . . . . . . . . . . .  4
     2.1.  Gathering Candidate Addresses  . . . . . . . . . . . . . .  6
     2.2.  Connectivity Checks  . . . . . . . . . . . . . . . . . . .  8
     2.3.  Sorting Candidates . . . . . . . . . . . . . . . . . . . . 10
     2.4.  Frozen Candidates  . . . . . . . . . . . . . . . . . . . . 10
     2.5.  Security for Checks  . . . . . . . . . . . . . . . . . . . 11
   3.  Terminology  . . . . . . . . . . . . . . . . . . . . . . . . . 11
   4.  Sending the Initial Offer  . . . . . . . . . . . . . . . . . . 13
     4.1.  Gathering Candidates . . . . . . . . . . . . . . . . . . . 13
     4.2.  Prioritizing Candidates  . . . . . . . . . . . . . . . . . 15
     4.3.  Choosing In-Use Candidates . . . . . . . . . . . . . . . . 18
     4.4.  Encoding the SDP . . . . . . . . . . . . . . . . . . . . . 19
   5.  Receiving the Initial Offer  . . . . . . . . . . . . . . . . . 20
     5.1.  Verifying ICE Support  . . . . . . . . . . . . . . . . . . 20
     5.2.  Gathering Candidates . . . . . . . . . . . . . . . . . . . 21
     5.3.  Prioritizing Candidates  . . . . . . . . . . . . . . . . . 21
     5.4.  Choosing In Use Candidates . . . . . . . . . . . . . . . . 21
     5.5.  Encoding the SDP . . . . . . . . . . . . . . . . . . . . . 21
     5.6.  Forming the Check List . . . . . . . . . . . . . . . . . . 21
     5.7.  Performing Periodic Checks . . . . . . . . . . . . . . . . 23
   6.  Receipt of the Initial Answer  . . . . . . . . . . . . . . . . 24
     6.1.  Verifying ICE Support  . . . . . . . . . . . . . . . . . . 24
     6.2.  Forming the Check List . . . . . . . . . . . . . . . . . . 24
     6.3.  Performing Periodic Checks . . . . . . . . . . . . . . . . 24
   7.  Connectivity Checks  . . . . . . . . . . . . . . . . . . . . . 24
     7.1.  Applicability  . . . . . . . . . . . . . . . . . . . . . . 24
     7.2.  Client Discovery of Server . . . . . . . . . . . . . . . . 25
     7.3.  Server Determination of Usage  . . . . . . . . . . . . . . 25
     7.4.  New Requests or Indications  . . . . . . . . . . . . . . . 25
     7.5.  New Attributes . . . . . . . . . . . . . . . . . . . . . . 25
     7.6.  New Error Response Codes . . . . . . . . . . . . . . . . . 25
     7.7.  Client Procedures  . . . . . . . . . . . . . . . . . . . . 25
       7.7.1.  Sending the Request  . . . . . . . . . . . . . . . . . 25
       7.7.2.  Processing the Response  . . . . . . . . . . . . . . . 26
     7.8.  Server Procedures  . . . . . . . . . . . . . . . . . . . . 27
     7.9.  Security Considerations for Connectivity Check . . . . . . 29
   8.  Completing the ICE Checks  . . . . . . . . . . . . . . . . . . 29
   9.  Subsequent Offer/Answer Exchanges  . . . . . . . . . . . . . . 30
     9.1.  Generating the Offer . . . . . . . . . . . . . . . . . . . 30
     9.2.  Receiving the Offer and Generating an Answer . . . . . . . 31
     9.3.  Updating the Check and Valid Lists . . . . . . . . . . . . 32
   10. Keepalives . . . . . . . . . . . . . . . . . . . . . . . . . . 33
   11. Media Handling . . . . . . . . . . . . . . . . . . . . . . . . 34
     11.1. Sending Media  . . . . . . . . . . . . . . . . . . . . . . 34
     11.2. Receiving Media  . . . . . . . . . . . . . . . . . . . . . 35



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   12. Usage with SIP . . . . . . . . . . . . . . . . . . . . . . . . 35
     12.1. Latency Guidelines . . . . . . . . . . . . . . . . . . . . 35
     12.2. Interactions with Forking  . . . . . . . . . . . . . . . . 37
     12.3. Interactions with Preconditions  . . . . . . . . . . . . . 37
     12.4. Interactions with Third Party Call Control . . . . . . . . 38
   13. Grammar  . . . . . . . . . . . . . . . . . . . . . . . . . . . 38
   14. Example  . . . . . . . . . . . . . . . . . . . . . . . . . . . 40
   15. Security Considerations  . . . . . . . . . . . . . . . . . . . 46
     15.1. Attacks on Connectivity Checks . . . . . . . . . . . . . . 46
     15.2. Attacks on Address Gathering . . . . . . . . . . . . . . . 49
     15.3. Attacks on the Offer/Answer Exchanges  . . . . . . . . . . 49
     15.4. Insider Attacks  . . . . . . . . . . . . . . . . . . . . . 50
       15.4.1. The Voice Hammer Attack  . . . . . . . . . . . . . . . 50
       15.4.2. STUN Amplification Attack  . . . . . . . . . . . . . . 50
   16. IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 51
     16.1. candidate Attribute  . . . . . . . . . . . . . . . . . . . 51
     16.2. remote-candidates Attribute  . . . . . . . . . . . . . . . 51
     16.3. ice-pwd Attribute  . . . . . . . . . . . . . . . . . . . . 52
     16.4. ice-ufrag Attribute  . . . . . . . . . . . . . . . . . . . 52
   17. IAB Considerations . . . . . . . . . . . . . . . . . . . . . . 53
     17.1. Problem Definition . . . . . . . . . . . . . . . . . . . . 53
     17.2. Exit Strategy  . . . . . . . . . . . . . . . . . . . . . . 53
     17.3. Brittleness Introduced by ICE  . . . . . . . . . . . . . . 54
     17.4. Requirements for a Long Term Solution  . . . . . . . . . . 55
     17.5. Issues with Existing NAPT Boxes  . . . . . . . . . . . . . 55
   18. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 56
   19. References . . . . . . . . . . . . . . . . . . . . . . . . . . 56
     19.1. Normative References . . . . . . . . . . . . . . . . . . . 56
     19.2. Informative References . . . . . . . . . . . . . . . . . . 57
   Appendix A.  Design Motivations  . . . . . . . . . . . . . . . . . 58
     A.1.  Applicability to Gateways and Servers  . . . . . . . . . . 59
     A.2.  Pacing of STUN Transactions  . . . . . . . . . . . . . . . 60
     A.3.  Candidates with Multiple Bases . . . . . . . . . . . . . . 61
     A.4.  Purpose of the Translation . . . . . . . . . . . . . . . . 63
     A.5.  Importance of the STUN Username  . . . . . . . . . . . . . 63
     A.6.  The Candidate Pair Sequence Number Formula . . . . . . . . 64
     A.7.  The Frozen State . . . . . . . . . . . . . . . . . . . . . 65
     A.8.  The remote-candidates attribute  . . . . . . . . . . . . . 65
     A.9.  Why are Keepalives Needed? . . . . . . . . . . . . . . . . 66
     A.10. Why Prefer Peer Reflexive Candidates?  . . . . . . . . . . 67
     A.11. Why Can't Offerers Send Media When a Pair Validates  . . . 67
   Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 69
   Intellectual Property and Copyright Statements . . . . . . . . . . 70








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1.  Introduction

   RFC 3264 [4] defines a two-phase exchange of Session Description
   Protocol (SDP) messages [10] for the purposes of establishment of
   multimedia sessions.  This offer/answer mechanism is used by
   protocols such as the Session Initiation Protocol (SIP) [3].

   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 [14].  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 [15], Simple Traversal
   Underneath NAT (STUN) [13] and its revision [11], the STUN Relay
   Usage [12], and Realm Specific IP [17] [18] along with session
   description extensions needed to make them work, such as the Session
   Description Protocol (SDP) [10] attribute for the Real Time Control
   Protocol (RTCP) [2].  Unfortunately, these techniques all have pros
   and cons which make each one optimal in some network topologies, but
   a poor choice in others.  The result is that administrators and
   implementors are making assumptions about the topologies of the
   networks in which their solutions will be deployed.  This introduces
   complexity and brittleness into the system.  What is needed is a
   single solution which is flexible enough to work well in all
   situations.

   This specification provides that solution for media streams
   established by signaling protocols based on the offer-answer model.
   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

   In a typical ICE deployment, we have two endpoints (known as agents
   in RFC 3264 terminology) which want to communicate.  They are able to
   communicate indirectly via some signaling system such as SIP, by



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   which they can perform an offer/answer exchange of SDP [4] messages.
   Note that ICE is not intended for NAT traversal for SIP, which is
   assumed to be provided via some other mechanism [31].  At the
   beginning of the ICE process, the agents are ignorant of their own
   topologies.  In particular, they might or might not be behind a NAT
   (or multiple tiers of NATs).  ICE allows the agents to discover
   enough information about their topologies to find a path or paths by
   which they can communicate.

   Figure Figure 1 shows a typical environment for ICE deployment.  The
   two endpoints are labelled L and R (for left and right, which helps
   visualize call flows).  Both L and R are behind NATs -- though as
   mentioned before, they don't know that.  The type of NAT and its
   properties are also unknown.  Agents A and B are capable of engaging
   in an offer/answer exchange by which they can exchange SDP messages,
   whose purpose is to set up a media session between A and B.
   Typically, this exchange will occur through a SIP server.

   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  |
           |       |         /         \         |       |
           +-------+        /           \        +-------+
                           /             \
                          /               \
                         /                 \
                        /                   \
                       /  <-  Signalling ->  \
                      /                       \
                     /                         \
               +--------+                   +--------+
               |  NAT   |                   |  NAT   |
               +--------+                   +--------+
                 /                                \
                /                                  \
               /                                    \
           +-------+                             +-------+
           | Agent |                             | Agent |
           |   L   |                             |   R   |
           |       |                             |       |



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           +-------+                             +-------+

   Figure 1

   The basic idea behind ICE is as follows: each agent has a variety of
   candidate transport addresses it could use to communicate with the
   other agent.  These might include:

   o  It's directly attached network interface (or interfaces in the
      case of a multihomed machine

   o  A translated address on the public side of a NAT (a "server
      reflexive" address)

   o  The address of a media relay the agent is using.

   Potentially, any of L's candidate transport addresses can be used to
   communicate with any of R's transport addresses.  In practice,
   however, many combinations will not work.  For instance, if L and R
   are both behind NATs then their directly interface addresses are
   unlikely to be able to communicate directly (this is why ICE is
   needed, after all!).  The purpose of ICE is to discover which pairs
   of addresses will work.  The way that ICE does this is to
   systematically try all possible pairs (in a carefully sorted order)
   until it finds one or more that works.

2.1.  Gathering Candidate Addresses

   In order to execute ICE, an agent has to identify all of its address
   candidates.  Naturally, one viable candidate is one obtained directly
   from a local interface the client has towards the network.  Such a
   candidate is called a HOST CANDIDATE.  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 a candidate) can be allocated.

   If an agent is multihomed, it can obtain a candidate from each
   interface.  Depending on the location of the peer on the IP network
   relative to the agent, the agent may be reachable by the peer 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 candidate from the net10 interface
   will be directly reachable when communicating with a peer on the same
   private net 10 network, while a candidate 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



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   work prior to sending an offer, the offering agent includes both
   candidates in its offer.

   Once the agent has obtained host candidates, it uses STUN to obtain
   additional candidates.  These come in two flavors: translated
   addresses on the public side of a NAT (SERVER REFLEXIVE CANDIDATES)
   and addresses of media relays (RELAYED CANDIDATES).  The relationship
   of these candidates to the host candidate is shown in Figure 2.  Both
   types of candidates are discovered using STUN.


                 To Internet

                     |
                     |
                     |  /------------  Relayed
                     | /               Candidate
                 +--------+
                 |        |
                 |  STUN  |
                 | Server |
                 |        |
                 +--------+
                     |
                     |
                     | /------------  Server
                     |/               Reflexive
               +------------+         Candidate
               |    NAT     |
               +------------+
                     |
                     | /------------  Host
                     |/               Candidate
                 +--------+
                 |        |
                 | Agent  |
                 |        |
                 +--------+

   Figure 2

   To find a server reflexive candidate, the agent sends a STUN Binding
   Request, using the Binding Discovery Usage [11] from each host
   candidate, to its STUN server.  (It is assumed that the address of
   the STUN server is configured, or learned in some way.)  When the
   agents sends the Binding Request, the NAT (assuming there is one)
   will allocate a binding, mapping this server reflexive candidate to
   the host candidate.  Outgoing packets sent from the host candidate



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   will be translated by the NAT to the server reflexive candidate.
   Incoming packets sent to the server relexive candidate will be
   translated by the NAT to the host candidate and forwarded to the
   agent.  We call the host candidate associated with a given server
   reflexive candidate the BASE.

Note

   "Base" refers to the address you'd send from for a particular
   candidate.  Thus, as a degenerate case host candidates also have a
   base, but it's the same as the host candidate.

   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 candidate will be discovered by the agent.
   If the agent is not behind a NAT, then the base candidate will be the
   same as the server reflexive candidate and the server reflexive
   candidate can be ignored.

   The final type of candidate is a RELAYED candidate.  The STUN Relay
   Usage [12] allows a STUN server to act as a media relay, forwarding
   traffic between L and R. In order to send traffic to L, R sends
   traffic to the media relay which forwards it to L and vice versa.
   The same thing happens in the other direction.

   Traffic from L to R has its addresses rewritten twice: first by the
   NAT and second by the STUN relay server.  Thus, the address that R
   knows about and the one that it wants to send to is the one on the
   STUN relay server.  This address is the final kind of candidate,
   which we call a RELAYED CANDIDATE.

2.2.  Connectivity Checks

   Once L has gathered all of its candidates, it orders them highest to
   lowest priority and sends them to R over the signalling channel.  The
   candidates are carried in attributes in the SDP offer.  When R
   receives the offer, it performs the same gathering process and
   responds with its own list of candidates.  At the end of this
   process, each agent has a complete list of both its candidates and
   its peer's candidates and is ready to perform connectivity checks by
   pairing up the candidates to see which pair works.

   The basic principle of the connectivity checks is simple:

   1.  Sort the candidate pairs in priority order.

   2.  Send checks on each candidate pair in priority order.




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   3.  Acknowledge checks received from the other agent.

   A complete connectivity check for a single candidate pair is a simple
   4-message handshake:


   A                        B
   -                        -
   STUN request ->             \  A's
             <- STUN response  /  check

              <- STUN request  \  B's
   STUN response ->            /  check

   Figure 3

   As an optimization, as soon as B gets A's check message he
   immediately sends his own check message to A on the same candidate
   pair.  This accelerates the process of finding a valid candidate.

   At the end of this handshake, both A and B know that they can send
   (and receive) messages end-to-end in both directions.  Note that as
   soon as B receives A's STUN response it knows that the B->A path
   works and it can start sending media on that path right away, as
   shown below.  This allows for 'early media' to flow as fast as
   possible:


   A                        B
   -                        -
   STUN request ->             \  A's
             <- STUN response  /  check

              <- STUN request  \  B's
   STUN response ->            /  check
                  <- RTP Data

   Figure 4

   Once any connectivity check for a candidate for a given media
   component succeeds, ICE uses that candidate and immediately abandons
   all other connectivity checks for that component.  Note that due to
   race conditions and packet loss, this may mean that the "best"
   candidate isn't selected, but it does guarantee the selection of a
   candidate that works, and because of the sorting process it will
   generally be one of the most preferred ones.





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2.3.  Sorting Candidates

   Because the algorithm above searches all candidate pairs, if a
   working pair exists it will eventually find it no matter what order
   the candidates are tried in.  In order to produce faster (and better)
   results, the candidates are sorted in a specified order.  The
   algorithm is described in Section 4.2 but follows two general
   principles:

   o  Each agent gives its candidates a numeric priority which is sent
      along with the candidate to the peer

   o  The local and remote priorities are combined so that each agent
      has the same ordering for the candidate pairs.

   The second property is important for getting ICE to work when there
   are NATs in front of A and B. Frequently, NATs will not allow packets
   in from a host until the agent behind the NAT has sent a packet
   towards that host.  Consequently, ICE checks in each direction will
   not succeed until both sides have sent a check through their
   respective NATs.

   In general the priority algorithm is designed so that candidates of
   similar type get similar priorities and so that more direct routes
   are favored over indirect ones.  Within those guidelines, however,
   agents have a fair amount of discretion about how to tune their
   algorithms.

2.4.  Frozen Candidates

   The previous description only addresses the case where the agents
   wish to establish a single media component--i.e., a single flow with
   a single host-port quartet.  However, in many cases (in particular
   RTP and RTCP) the agents actually need to establish connectivity for
   more than one flow.

   The naive way to attack this problem would be to simply do
   independent ICE exchanges for each media component.  This is
   obviously inefficient because the network properties are likely to be
   very similar for each component (especially because RTP and RTCP are
   typically run on adjacent ports).  Thus, it should be possible to
   leverage information from one media component in order to determine
   the best candidates for another.  ICE does this with a mechanism
   called "frozen candidates."

   The basic principle behind frozen candidates is that initially only
   the candidates for a single media component are tested.  The other
   media components are marked "frozen".  When the connectivity checks



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   for the first component succeed, the corresponding candidates for the
   other components are unfrozen and checked immediately.  This avoids
   repeated checking of components which are superficially more
   attractive but in fact are likely to fail.

   While we've described "frozen" here as a separate mechanism for
   expository purposes, in fact it is an integral part of ICE and the
   the ICE prioritization algorithm automatically ensures that the right
   candidates are unfrozen and checked in the right order.

2.5.  Security for Checks

   Because ICE is used to discover which addresses can be used to send
   media between two agents, it is important to ensure that the process
   cannot be hijacked to send media to the wrong location.  Each STUN
   connectivity check is covered by a message authentication code (MAC)
   computed using a key exchanged in the signalling channel.  This MAC
   provides message integrity and data origin authentication, thus
   stopping an attacker from forging or modifying connectivity check
   messages.  The MAC also aids in disambiguating ICE exchanges from
   forked calls.


3.  Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in RFC 2119 [1].

   This specification makes use of the following terminology:

   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.

   Candidate: A transport address that is to be tested by ICE procedures
      in order to determine its suitability for usage for receipt of
      media.






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   Host Candidate: A candidate obtained by binding to a specific port
      from an interface on the host.  This includes both physical
      interfaces and logical ones, such as ones obtained through Virtual
      Private Networks (VPNs) and Realm Specific IP (RSIP) [17] (which
      lives at the operating system level).

   Server Reflexive Candidate: A candidate obtained by sending a STUN
      request from a host candidate to a STUN server, distinct from the
      peer, whose address is configured or learned by the client prior
      to an offer/answer exchange.

   Peer Reflexive Candidate: A candidate obtained by sending a STUN
      request from a host candidate to the STUN server running on a
      peer's candidate.

   Relayed Candidate: A candidate obtained by sending a STUN Allocate
      request from a host candidate to a STUN server.  The relayed
      candidate is resident on the STUN server, and the STUN server
      relays packets back towards the agent.

   Translation: The translation of a relayed candidate is the transport
      address that the relay will forward a packet to, when one is
      received at the relayed candidate.  For relayed candidates learned
      through the STUN Allocate request, the translation of the relayed
      candidate is the server reflexive candidate returned by the
      Allocate response.

   Base: The base of a server reflexive candidate is the host candidate
      from which it was derived.  A host candidate is also said to have
      a base, equal to that candidate itself.  Similarly, the base of a
      relayed candidate is that candidate itself.

   Foundation: Each candidate has a foundation, which is an identifier
      that is distinct for two candidates that have different types,
      different interface IP addresses for their base, and different IP
      addresses for their STUN servers.  Two candidates have the same
      foundation when they are of the same type, their bases have the
      same IP address, and, for server reflexive or relayed candidates,
      they come from the same STUN server.  Foundations are used to
      correlate candidates, so that when one candidate is found to be
      valid, candidates sharing the same foundation can be tested next,
      as they are likely to also be valid.

   Local Candidate: A candidate that an agent has obtained and included
      in an offer or answer it sent.






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   Remote Candidate: A candidate that an agent received in an offer or
      answer from its peer.

   In-Use Candidate: A candidate is in-use when it appears in the m/c-
      line of an active media stream.

   Candidate Pair: A pairing containing a local candidate and a remote
      candidate.

   Check: A candidate pair where the local candidate is a transport
      address from which an agent can send a STUN connectivity check.

   Check List: An ordered set of STUN checks that an agent is to
      generate towards a peer.

   Periodic Check: A connectivity check generated by an agent as a
      consequence of a timer that fires periodically, instructing it to
      send a check.

   Triggered Check: A connectivity check generated as a consequence of
      the receipt of a connectivity check from the peer.

   Valid List: An ordered set of candidate pairs that have been
      validated by a successful STUN transaction.


4.  Sending the Initial Offer

   In order to send the initial offer in an offer/answer exchange, an
   agent must gather candidates, priorize them, choose ones for
   inclusion in the m/c-line, and then formulate and send the SDP.  Each
   of these steps is described in the subsections below.

4.1.  Gathering Candidates

   An agent gathers candidates when it believes that communications is
   imminent.  An offerer can do this based on a user interface cue, or
   based on an explicit request to initiate a session.  Every candidate
   is an IP address and port (also known as a transport address).  It
   also has a type and a base.  Three types are defined and gathered by
   this specification - host candidates, server reflexive candidates,
   and relayed candidates.  The base of a candidate is candidate that an
   agent must send from when using that candidate.

   The first step is to gather host candidates.  Host 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 host candidates depends on the transport



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   protocol.  Procedures are specified here for UDP.

   For each UDP media stream the agent wishes to use, the agent SHOULD
   obtain a candidate for each component of the media stream on each
   interface that the host has.  It obtains each candidate by binding to
   a UDP port on the specific interface.  A host candidate (and indeed
   every candidate) is always associated with a specific component for
   which it is a candidate.  Each component has an ID assigned to it,
   called the component ID.  For RTP-based media streams, the RTP itself
   has a component ID of 1, and RTCP a component ID of 2.  If an agent
   is using RTCP it MUST obtain a candidate for it.  If an agent is
   using both RTP and RTCP, it would end up with 2*K host candidates if
   an agent has K interfaces.

   The base for each host candidate is set to the candidate itself.

   Once the agent has obtained host candidates, it obtains server
   reflexive and relayed candidates.  The process for gathering server
   reflexive and relayed candidates depends on the transport protocol.
   Procedures are specified here for UDP.

   Agents which serve end users directly, such softphones, hardphones,
   terminal adapters and so on, SHOULD obtain relayed candidates and
   MUST obtain server reflexive candidates.  The requirement to obtain
   relayed candidates is at SHOULD strength to allow for provider
   variation.  If they are not 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 skip obtaining server reflexive
   and relayed candidates.

   The agent next pairs each host candidate with the STUN server with
   which it is configured or has discovered by some means.  This
   specification only considers usage of a single STUN server.  Every Ta
   seconds, the agent chooses another such pair (the order is
   inconsequential), and sends a STUN request to the server from that
   host candidate.  If the agent is using both relayed and server
   reflexive candidates, this request MUST be a STUN Allocate request
   from the relay usage [12].  If the agent is using only server
   reflexive candidates, the request MUST be a STUN Binding request
   using the binding discovery usage [11].

   The value of Ta SHOULD be configurable, and SHOULD have a default of
   50ms.  Note that this pacing applies only to starting STUN
   transactions with source and destination transport addresses (i.e.,



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   the host candidate and STUN server respectively) for which a STUN
   transaction has not previously been sent.  Consequently,
   retransmissions of a STUN request are governed entirely by the
   retransmission rules defined in [11].  Similarly, retries of a
   request due to recoverable errors (such as an authentication
   challenge) happen immediately and are not paced by timer Ta.  Because
   of this pacing, it will take a certain amount of time to obtain all
   of the server reflexive and relayed candidates.  Implementations
   should be aware of the time required to do this, and if the
   application requires a time budget, limit the amount of candidates
   which are gathered.

   An Allocate Response will provide the client with a server reflexive
   candidate (obtained from the mapped address) and a relayed candidate
   in the RELAY-ADDRESS attribute.  A Binding Response will provide the
   client with a only server reflexive candidate (also obtained from the
   mapped address).  The base of the server reflexive candidate is the
   host candidate from which the Allocate or Binding request was sent.
   The base of a relayed candidate is that candidate itself.  A server
   reflexive candidate obtained from an Allocate response is the called
   the "translation" of the relayed candidate obtained from the same
   response.  The agent will need to remember the translation for the
   relayed candidate, since it is placed into the SDP.  If a relayed
   candidate is identical to a host candidate (which can happen in rare
   cases), the relayed candidate MUST be discarded.  Proper operation of
   ICE depends on each base being unique.

   Next, redundant candidates are eliminated.  A candidate is redundant
   if its transport address equals another candidate, and its base
   equals the base of that other candidate.  Note that two candidates
   can have the same transport address yet have different bases, and
   these would not be considered redundant.

   Finally, each candidate is assigned a foundation.  The foundation is
   an identifier, scoped within a session.  Two candidates MUST have the
   same foundation ID when they are of the same type (host, relayed,
   server reflexive, peer reflexive or relayed), their bases have the
   same IP address (the ports can be different), and, for reflexive and
   relayed candidates, the STUN servers used to obtain them have the
   same IP address.  Similarly, two candidates MUST have different
   foundations if their types are different, their bases have different
   IP addresses, or the STUN servers used to obtain them have different
   IP addresses.

4.2.  Prioritizing Candidates

   The prioritization process results in the assignment of a priority to
   each candidate.  An agent does this by determining a preference for



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   each type of candidate (server reflexive, per reflexive, relayed and
   host), and, when the agent is multihomed, choosing a preference for
   its interfaces.  These two preferences are then combined to compute
   the priority for a candidate.  That priority MUST be computed using
   the following formula:


   priority = 1000*(type preference) +
               100*(local preference) +
                10*(stream ID) +
                 1*(10 - component ID)

   The type preference MUST be an integer from 0 to 9 inclusive, and
   represents the preference for the type of the candidate (where the
   types are local, server reflexive, peer reflexive and relayed).  A 9
   is the highest preference, and a 0 is the lowest.  Setting the value
   to a 0 means that candidates of this type will only be used as a last
   resort.  The type preference MUST be identical for all candidates of
   the same type and MUST be different for candidates of different
   types.  The type preference for peer reflexive candidates MUST be
   lower than that of server reflexive candidates.  Note that candidates
   gathered based on the procedures of Section 4.1 will never be peer
   reflexive candidates; candidates of these type are learned from the
   STUN connectivity checks performed by ICE.  The component ID is the
   component ID for the candidate, and MUST be between 1 and 10
   inclusive.  The stream ID is an integer, starting at 9, that
   decrements by one for each media stream in the session.  When
   signaled in the SDP, the first m-line is the one with stream ID 9,
   the next with stream ID 8, the next with stream ID 7, and so on.  In
   essence, the stream ID indicates the position of that media stream in
   the SDP itself.  The stream ID MUST be less than or equal to 9, and
   therefore ICE only works with multimedia sessions with 10 or fewer
   media streams.  The local preference MUST be an integer from 0 to 9
   inclusive.  It represents a preference for the particular interface
   from which the candidate was obtained, in cases where an agent is
   multihomed.  A nine represents the highest preference, and a zero,
   the lowest.  When there is only a single interface, this value SHOULD
   be set to nine.  Generally speaking, if there are multiple candidates
   for a particular component for a particular media stream which have
   the same type, the local preference MUST be unique for each one.  In
   this specification, this only happens for multi-homed hosts.

   These rules guarantee that there is a unique priority for each
   candidate.  This priority will be used by ICE to determine the order
   of the connectivity checks and the relative preference for
   candidates.  Consequently, what follows are some guidelines for
   selection of these values.




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   One criteria for selection of the type and local preference values is
   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 host candidates
   obtained from a VPN interface.  When media is transited through an
   intermediary, it can increase the latency between transmission and
   reception.  It can increase the packet losses, because of the
   additional router hops that may be taken.  It may increase the cost
   of providing service, since media will be routed in and right back
   out of an intermediary run by the provider.  If these concerns are
   important, the type preference for relayed candidates can be set
   lower than the type preference for reflexive and host candidates.
   Indeed, it is RECOMMENDED that in this case, host candidates have a
   type preference of nine, server reflexive candidates have a type
   preference of 5, peer reflexive have a type prefence of 6, and
   relayed candidates have a type preference of zero.  Furthermore, if
   an agent is multi-homed and has multiple interfaces, the local
   preference for host candidates from a VPN interface SHOULD have a
   priority of 0.

   Another criteria for selection of preferences 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) [22].  It can also help with hosts that have both a native
   IPv6 address and a 6to4 address.  In such a case, lower local
   preferences could be assigned to the v6 interface, followed by the
   6to4 interfaces, followed by the v4 interfaces.  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 selecting preferences 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.  In such a case,
   a VPN interface would have a higher local preference than any other
   interfaces.

   Another criteria for selecting preferences 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 assign higher local preferences to candidates



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   obtained from closer relays.

   There may be transport-specific reasons for assigning preferences to
   candidates.  In such a case, specifications defining usage of ICE
   with other transport protocols SHOULD document such considerations.

4.3.  Choosing In-Use Candidates

   A candidate is said to be "in-use" if it appears in the m/c-line of
   an offer or answer.  When communicating with an ICE peer, being in-
   use implies that, should these candidates be selected by the ICE
   algorithm, bidirectional media can flow and the candidates can be
   used.  If a candidate is selected by ICE but is not in-use, only
   unidirectional media can flow and only for a brief time; the
   candidate must be made in-use through an updated offer/answer
   exchange.  When communicating with a peer that is not ICE-aware, the
   in-use candidates will be used exclusively for the exchange of media,
   as defined in normal offer/answer procedures.

   An agent MUST choose a set of candidates, one for each component of
   each active media stream, to be in-use.  A media stream is active if
   it does not contain the a=inactive SDP attribute.

   It is RECOMMENDED that in-use candidates be chosen based on the
   likelihood of those candidates to work with the peer that is being
   contacted.  Unfortunately, it is difficult to ascertain which
   candidates that might be.  As an example, consider a user within an
   enterprise.  To reach non-ICE capable agents within the enterprise,
   host candidates have to be used, since the enterprise policies may
   prevent communication between elements using a relay on the public
   network.  However, when communicating to peers outside of the
   enterprise, relayed candidates from a publically accessible STUN
   server are needed.

   Indeed, the difficulty in picking just one transport 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
   relayed candidates be selected to be in-use.  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.

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




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4.4.  Encoding the SDP

   The agent includes a single a=candidate media level attribute in the
   SDP for each candidate for that media stream.  The a=candidate
   attribute contains the IP address, port and transport protocol for
   that candidate.  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 candidate attribute also includes the component ID for that
   candidate.  For media streams based on RTP, candidates for the actual
   RTP media MUST have a component ID of 1, and candidates for RTCP MUST
   have a component ID of 2.  Other types of media streams which require
   multiple components MUST develop specifications which define the
   mapping of components to component IDs.

   The candidate attribute also includes the priority, which is the
   value determined for the candidate as described in Section 4.2, and
   the foundation, which is the value determined for the candidate as
   described in Section 4.1.  The agent SHOULD include a type for each
   candidate by populating the candidate-types production with the
   appropriate value - "host" for host candidates, "srflx" for server
   reflexive candidates, "prflx" for peer reflexive candidates (though
   these never appear in an initial offer/answer exchange), and "relay"
   for relayed candidates.  The related address MUST NOT be included if
   a type was not included.  If a type was included, the related address
   SHOULD be present for server reflexive, peer reflexive and relayed
   candidates.  If a candidate is server or peer reflexive, the related
   address is equal to the base for that server or peer reflexive
   candidate.  If the candidate is relayed, the related address is equal
   to the translation of the relayed address.  If the candidiate is a
   host candidate, there is no related address and the rel-addr
   production MUST be omitted.

   STUN connectivity checks between agents make use of a short term
   credential that is exchanged in the offer/answer process.  The
   username part of this credential is formed by concatenating a
   username fragment from each agent, separated by a colon.  Each agent
   also provides a password, used to compute the message integrity for
   requests it receives.  As such, an SDP MUST contain the ice-ufrag and
   ice-pwd attributes, containing the username fragment and password
   respectively.  These can be either session or media level attributes,
   and thus common across all candidates for all media streams, or all
   candidates for a particular media stream, respectively.  However, if
   two media streams have identical ice-ufrag's, they MUST have
   identical ice-pwd's.  The ice-ufrag and ice-pwd attributes MUST be
   chosen randomly at the beginning of a session.  The ice-ufrag



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   attribute MUST contain at least 24 bits of randomness, and the ice-
   pwd attribute MUST contain at least 128 bits of randomness.  This
   means that the ice-ufrag attribute will be at least 4 characters
   long, and the ice-pwd at least 22 characters long, since the grammar
   for these attributes allows for 6 bits of randomness per character.
   The attributes MAY be longer than 4 and 21 characters respectively,
   of course.

   The m/c-line is populated with the candidates that are in-use.  For
   streams based on RTP, this is done by placing the RTP candidate into
   the m and c lines respectively.  If the agent is utilizing RTCP, it
   MUST encode the RTCP candidate into the m/c-line using the a=rtcp
   attribute as defined in RFC 3605 [2].  If RTCP is not in use, the
   agent MUST signal that using b=RS:0 and b=RR:0 as defined in RFC 3556
   [5].

   There MUST be a candidate attribute for each component of the media
   stream in the m/c-line.

   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 11.1, media packets can be sent to a candidate prior to
   its appearence in the m/c-line.


5.  Receiving the Initial Offer

   When an agent receives an initial offer, it will check if the offeror
   supports ICE, gather candidates, prioritize them, choose one for in-
   use, encode and send an answer, and then form a check list and begin
   connectivity checks.

5.1.  Verifying ICE Support

   The agent will proceed with the ICE procedures defined in this
   specification if the following are both true:

   o  There is at least one a=candidate attribute for each media stream
      in the SDP it just received.

   o  For each media stream, at least one of the candidates is a match
      for its respective in-use component in the m/c-line.

   If both of these conditions are not met, the agent MUST process the
   SDP based on normal RFC 3264 procedures, without using any of the ICE
   mechanisms described in the remainder of this specification, with the
   exception of Section 10, which describes keepalive procedures.




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5.2.  Gathering Candidates

   The process for gathering candidates at the answerer is identical to
   the process for the offerer as described in Section 4.1.  It is
   RECOMMENDED that this process begin immediately on receipt of the
   offer, prior to user acceptance of a session.  Such gathering MAY
   even be done pre-emptively when an agent starts.

5.3.  Prioritizing Candidates

   The process for prioritizing candidates at the answerer is identical
   to the process followed by the offerer, as described in Section 4.2.

5.4.  Choosing In Use Candidates

   The process for selecting in-use candidates at the answerer is
   identical to the process followed by the offerer, as described in
   Section 4.3.

5.5.  Encoding the SDP

   The process for encoding the SDP at the answerer is identical to the
   process followed by the offerer, as described in Section 4.4.

5.6.  Forming the Check List

   Next, the agent forms the check list.  The check list is a sequence
   of STUN connectivity checks that are performed by the agent.  To form
   the check list, the agent forms candidate pairs, computes a candidate
   pair priority, orders the pairs by priority, prunes them, and sets
   their states.  These steps are described in this section.

   First, the agent takes each of its candidates (called local
   candidates) and pairs them with the candidates it received from its
   peer (called remote candidates).  A local candidate is paired with a
   remote candidate if and only if the two candidates are for the same
   media stream, have the same component ID, and have the same IP
   address version.  It is possible that some of the local candidates
   don't get paired with a remote candidate, and some of the remote
   candidates don't get paired with local candidates.  This can happen
   if one agent didn't include candidates for the all of the components
   for a media stream.  In the case of RTP, for example, this would
   happen when one agent provided candidates for RTCP, and the other did
   not.  If this happens, the number of components for that media stream
   is effectively reduced, and considered to be equal to the minimum
   across both agents of the maximum component ID provided by each agent
   across all components for the media stream.




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   Once the pairs are formed, a candidate pair priority is computed.
   Let O-P be the priority for the candidate provided by the offerer.
   Let A-P be the priority for the candidate provided by the answerer.
   Let O-IP be the IP address (without the port) of the candidate
   provided by the offerer.  Let SZ be two to the power of 32 for IPv4
   candidates, and two to the power of 128 for IPv6 candidates.  The
   priority for a pair is computed as:

      pair priority = 10000*MIN(O-P,A-P) + MAX(O-P,A-P) + O-IP/SZ

      OPEN ISSUE: This can be larger than 32 bits.  Should consider ways
      of reducing that.

   This formula ensures a unique priority for each pair in most cases.
   One the priority is assigned, the agent sorts the candidate pairs in
   decreasing order of priority.  If two pairs have identical priority,
   the ordering amongst them is arbitrary.

   This sorted list of candidate pairs is used to determine a sequence
   of connectivity checks that will be performed.  Each check involves
   sending a request from a local candidate to a remote candidate.
   Since an agent cannot send requests directly from a reflexive
   candidate, but only from its base, the agent next goes through the
   sorted list of candidate pairs.  For each pair where the local
   candidate is server reflexive, the server reflexive candidate MUST be
   replaced by its base.  Once this has been done, the agent MUST remove
   redundant pairs.  A pair is redundant if its local and remote
   candidates are identical to the local and remote candidates of a pair
   higher up on the priority list.  The result is called the check list,
   and each candidate pair on it is called a check.

   Each check is also said to have a foundation, which is merely the
   combination of the foundations of the local and remote candidates in
   the check.

   Finally, each check in the check list is associated with a state.
   There are five potential values that the state can have:

   Waiting: This check has not been performed, and can be performed as
      soon as it is the highest priority Waiting check on the check
      list.

   In-Progress: A request has been sent for this check, but the
      transaction is in progress.







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   Succeeded: This check was already done and produced a successful
      result.

   Failed: This check was already done and failed, either never
      producing any response or producing an unrecoverable failure
      response.

   Frozen: This check hasn't been performed, and it can't yet be
      performed until some other check succeeds, allowing it to move
      into the Waiting state.

   First, the agent sets all of the checks to the Frozen state.  Then,
   it sets the first check in the check list to Waiting.  It then finds
   all of the other checks for the same media stream and with the same
   component ID, but different foundations, and sets all of their states
   to Waiting.

5.7.  Performing Periodic Checks

   An agent performs two types of checks.  The first type are periodic
   checks.  These checks occur periodically, and involve choosing the
   highest priority check in the Waiting state from the check list, and
   performing it.  The other type of check is called a triggered check.
   This is a check that is performed on receipt of a connectivity check
   from the peer.  This section describes how periodic checks are
   performed.

   Once the agent has computed the check list as described in
   Section 5.6, it sets a timer that fires every Ta seconds.  This is
   the same value used to pace the gathering of candidates, as described
   in Section 4.1.  The first timer fires immediately, so that the agent
   performs a connectivity check the moment the offer/answer exchange
   has been done, followed by the next periodic check Ta seconds later.

   When the timer fires, the agent MUST find the highest priority check
   in the check list that is in the Waiting state.  The agent then sends
   a STUN check from the local candidate of that check to the remote
   candidate of that check.  The procedures for forming the STUN request
   for this purpose are described in Section 7.7.1.  If none of the
   checks in the check list are in the Waiting state, but there are
   checks in the Frozen state, the highest priority check in the Frozen
   state is moved into the Waiting state, and that check is performed.
   When a check is performed, its state is set to In-Progress.  If there
   are no checks in either the Waiting or Frozen state, then timer Ta is
   stopped.

   Performing the connectivity check requires the agent to know the
   username fragment for the local and remote candidates, and the



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   password for the remote candidate.  For periodic checks, the remote
   username fragment and password are learned directly from the SDP
   received from the peer, and the local username fragment is known by
   the agent.


6.  Receipt of the Initial Answer

   This section describes the procedures that an agent follows when it
   receives the answer from the peer.  It verifies that its peer
   supports ICE, forms the check list and begins performing periodic
   checks.

6.1.  Verifying ICE Support

   The offerer follows the same procedures described for the answerer in
   Section 5.1.

6.2.  Forming the Check List

   The offerer follows the same procedures described for the answerer in
   Section 5.6.

6.3.  Performing Periodic Checks

   The offerer follows the same procedures described for the answerer in
   Section 5.7.


7.  Connectivity Checks

   This section describes how connectivity checks are performed.
   Connectivity checks are a STUN usage, and the behaviors described
   here meet the guidelines for definitions of new usages as outlined in
   [11]

   Note that all ICE implementations are required to be compliant to
   [11], as opposed to the older [13].

7.1.  Applicability

   This STUN usage provides a connectivity check between two peers
   participating in an offer/answer exchange.  This check serves to
   validate a pair of candidates for usage of exchange of media.
   Connectivity checks also allow agents to discover reflexive
   candidates towards their peers, called peer reflexive candidates.
   Finally, connectivity checks serve to keep NAT bindings alive.




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   It is fundamental to this STUN usage that the addresses and ports
   used for media are the same ones used for the Binding Requests and
   responses.  Consequently, it will be necessary to demultiplex STUN
   traffic from whatever the media traffic is.  This demultiplexing is
   done using the techniques described in [11].

7.2.  Client Discovery of Server

   The client does not follow the DNS-based procedures defined in [11].
   Rather, the remote candidate of the check to be performed is used as
   the IP address and port of the STUN server.  Note that the STUN
   server is a logical entity, and is not a physically distinct server
   in this usage.

7.3.  Server Determination of Usage

   The server is aware of this usage because it signaled this port
   through the offer/answer exchange.  Any STUN packets received on this
   port will be for the connectivity check usage.

7.4.  New Requests or Indications

   This usage does not define any new message types.

7.5.  New Attributes

   This usage defines a new attribute, PRIORITY.  This attribute
   indicates the priority that is to be associated with a peer reflexive
   candidate, should one be discovered by this check.  It is a 32 bit
   unsigned integer, and has an attribute type of 0x0024.

7.6.  New Error Response Codes

   This usage does not define any new error response codes.

7.7.  Client Procedures

   This section defines additional procedures for the Binding Request
   transaction, beyond those described in [11].

7.7.1.  Sending the Request

   The agent acting as the client generates a connectivity check either
   periodically, or triggered.  In either case, the check is generated
   by sending a Binding Request from a local candidate, to a remote
   candidate.  The agent must know the username fragment for both
   candidates and the password for the remote candidate.




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   A Binding Request serving as a connectivity check MUST utilize a STUN
   short term credential.  Rather than being learned from a Shared
   Secret request, the short term credential is exchanged in the offer/
   answer procedures.  In particular, the username is formed by
   concatenating the username fragment provided by the peer with the
   username fragment of the agent sending the request, separated by a
   colon (":").  The password is equal to the password provided by the
   peer.  For example, consider the case where agent A is the offerer,
   and agent B is the answerer.  Agent A included a username fragment of
   AFRAG for its candidates, and a password of APASS.  Agent B provided
   a username fragment of BFRAG and a password of BPASS.  A connectivity
   check from A to B (and its response of course) utilize the username
   BFRAG:AFRAG and a password of BPASS.  A connectivity check from B to
   A (and its response) utilize the username AFRAG:BFRAG and a password
   of APASS.

   All Binding Requests for the connectivity check usage MUST contain
   the PRIORITY attribute.  This MUST be set equal to the priority that
   would be assigned, based on the algorithm in Section 4.2, to a peer
   reflexive candidate learned from this check.  Such a peer reflexive
   candidate has a stream ID, component ID and local preference that are
   equal to the host candidate from which the check is being sent, but a
   type preference equal to the value associated with peer reflexive
   candidates.

   The Binding Request by an agent MUST include the USERNAME and
   MESSAGE-INTEGRITY attributes.  That is, an agent MUST NOT wait to be
   challenged for short term credentials.  Rather, it MUST provide them
   in the Binding Request right away.

7.7.2.  Processing the Response

   If the STUN transaction generates an unrecoverable failure response
   or times out, the agent sets the state of the check to Failed.  The
   remainder of this section applies to processing of successful
   responses (any response from 200 to 299).

   The agent MUST check that the source IP address and port of the
   response equals the destination IP address and port that the Binding
   Request was sent to, and that the source IP address and port of the
   request match the destination IP address and port that the Binding
   Response was received on.  If these do not match, the agent sets the
   state of the check to Failed.  The processing described in the
   remainder of this section MUST NOT be performed.

   Otherwise, the source transport address of the response matched the
   destination transport address of the request.  The agent changes the
   state for this check to Succeeded.  Next, the agent sees if the



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   success of this check can cause other checks to be unfrozen.  If the
   check had a component ID of one, the agent MUST change the states for
   all other Frozen checks for the same media stream and same
   foundation, but different component IDs, to Waiting.  If the
   component ID for the check was equal to the number of components for
   the media stream, the agent MUST change the state for all other
   Frozen checks for the first component of different media streams but
   the same foundation, to Waiting.

   Next, the agent checks the mapped address from the STUN response.  If
   the transport address does not match any of the local candidates that
   the agent knows about, the mapped address representes a new peer
   reflexive candidate.  Its type is equal to peer reflexive.  Its base
   is set equal to the candidate from which the STUN check was sent.
   Its username fragment and password are identical to the candidate
   from which the check was sent.  It is assigned the priority value
   that was placed in the PRIORITY attribute of the request.  Its
   foundation is selected as described in Section 4.1.  The peer
   reflexive candidate is then added to the list of local candidates
   known by the agent (though it is not paired with other remote
   candidates at this time).

   In addition, the agent creates a candidate pair whose local candidate
   equals the mapped address of the response, and whose remote candidate
   equals the destination address to which the request was sent.  This
   is called a validated pair, since it has been validated by a STUN
   connectivity check.  The agent will know, either from the SDP or
   through the PRIORITY attribute that was present in a STUN request,
   the priorities of the local and remote candidates of the validated
   pair.  Based on these priorities, a priority for the validated pair
   itself is computed if it was not already known, using the algorithm
   in Section 5.6, and the pair is added to the valid list.

7.8.  Server Procedures

   An agent MUST be prepared to receive a Binding Request on the base of
   each candidate it included in its most recent offer or answer.
   Receipt of a Binding Request on an IP address and port that the agent
   had included in a candidate attribute is an indication that the
   connectivity check usage applies to the request.

   The agent MUST use a short term credential to authenticate the
   request and perform a message integrity check.  The agent MUST accept
   a credential if the username consists of two values separated by a
   colon, where the first value is equal to the username fragment
   generated by the agent in an offer or answer for a session in-
   progress, and the password is equal to the password for that username
   fragment.  It is possible (and in fact very likely) that an offeror



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   will receive a Binding Request prior to receiving the answer from its
   peer.  However, the request can be processed without receiving this
   answer, and a response generated.

   For requests being received on a relayed candidate, the source IP
   address and port used for STUN processing (namely, generation of the
   XOR-MAPPED-ADDRESS attribute) is the IP address and port as seen by
   the relay.  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.

   When the agent receives a STUN Binding Request for which it generates
   a successful response, the agent checks the source transport address
   of the request.  If this transport address does not match any
   existing remote candidates, it represents a new peer reflexive remote
   candidate.  This candidate is given a priority equal to the PRIORITY
   attribute from the request.  The type of the candidate is equal to
   peer reflexive.  Its foundation is set to an arbitrary value,
   different from the foundation for all other remote candidates.  The
   username fragment for this candidate is equal to the bottom half (the
   part after the colon) of the username in the Binding Request that was
   just received.  The password for this username fragment is taken from
   the SDP from the peer.  If agent has not yet received this SDP (a
   likely case for the offerer in the initial offer/answer exchange), it
   MUST wait for the SDP to be received, and then proceed with rest of
   the processing described in the remainder of this section.  This
   candidate is then added to the list of remote candidates.  However,
   it is not paired with any local candidates.

   Next, the agent MUST generate a triggered check in the reverse
   directon if it has not already sent such a check.  The triggered
   check has a local candidate equal to the candidate on which the STUN
   request was received, and a remote candidate equal to the source
   transport address where the request came from (which may be a newly
   formed peer reflexive candidate).  The agent knows the priorities for
   the local and remote candidates of this check, and so can compute the
   priority for the check itself.  If there is already a check on the
   check list with this same local and remote candidates, and the state
   of that check is Waiting or Frozen, its state is changed to In-
   Progress and the check is performed.  If there was already a check on
   the check list with this same local and remote candidates, and its
   state was In-Progress, the agent SHOULD generate an immediate
   retransmit of the Binding Request.  This is to facilitate rapid
   completion of ICE when both agents are behind NAT.  If there was a
   check in the list already and its state was Succeeded or Failed,



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   nothing further is done.  If there was no matching check on the check
   list, it is inserted into the check list based on its priority, its
   state is set to In-Progress, and the check is performed.

7.9.  Security Considerations for Connectivity Check

   Security considerations for the connectivity check are discussed in
   Section 15.


8.  Completing the ICE Checks

   When a pair is added to the valid list, and the agent was the offeror
   in the most recent offer/answer exchange, the agent MUST check to see
   if there is a pair on the validated list for each component of each
   media stream.  If there is, the offeror MUST stop timer Ta, and MUST
   cease retransmitting any Binding Requests for transactions in
   progress.  It MUST ignore any responses which may subsequently arrive
   to transactions previously in progress.  The offeror MUST generate an
   updated offer as described in Section 9.  It does this regardless of
   whether the highest priority pairs in the check list match the
   current in-use candidate pairs.

   When a pair is aded to the valid list, and the agent was the answerer
   in the most recent offer/answer exchange, the agent MAY begin sending
   media using that candidate pair, as described in Section 11.1.  In
   addition, if there is a candidate pair on the valid list for each
   component of each media stream, the answerer MUST stop timer Ta, and
   MUST cease retransmitting any Binding Requests for transactions in
   progress.  It MUST ignore any responses which may subsequently arrive
   to transactions previously in progress.

   Note that only agent that was the answerer in the most recent offer/
   answer exchange gets to send media right away.  The offeror must wait
   for a subsequent offer/answer exchange if the valid candidates don't
   match those in the m/c-line.

      OPEN ISSUE: It is possible that higher priority checks may still
      succeed, if we allowed things to continue.  This can happen for
      several reasons.  First, an in-progress check of higher priority
      had some packet loss and thus hasn't completed.  Timer Tws was
      meant to handle this (I removed this timer from -10 to simplify).
      More interestingly, higher priority checks may have not been done
      because a triggered check of lower priority succeeded.  This
      happens in cases where the number of checks at each agent are
      assymetric.  It is possible to fix both of these problems by
      delaying the completion of the ICE procedures for a bit more time.
      This adds complexity and latency.  The basic algorithm would be



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      this.  You take the lowest priority pair in the valid list.  You
      keep doing checks as long as there are higher priority checks on
      the list in the Waiting state.  If there are none, you wait a
      brief time (say 50ms) and then consider ICE finished.


9.  Subsequent Offer/Answer Exchanges

   An agent MAY generate a subsequent offer at any time.  However, the
   rules in Section 7.7.2 will cause the offerer to generate an updated
   offer when the candidates in the valid list are not all in-use.

9.1.  Generating the Offer

   When an agent generates an updated offer, the set of candidate
   attributes to include depend on the state of ICE processing.  If ICE
   is "done", which occurs when the valid list includes a candidate pair
   for each component of each media stream, the agent MUST include a
   candidate attribute for each local candidate amongst the pairs in the
   valid list (including peer reflexive candidates), and SHOULD NOT
   include any others.  This will cause STUN keepalives to be sent for
   the in-use candidates, and thats it.

   If, however, the valid list does not yet include a candidate pair for
   each component of each media stream, the agent SHOULD include all
   current candidates, including any peer reflexive candidates it has
   learned since the last offer or answer it sent.  This MAY include
   candidates it did not offer previously, but which it has gathered
   since the last offer/answer exchange.

   If a candidate was sent in a previous offer/answer exchange, it
   SHOULD have the same priority.  For a peer reflexive candidate, the
   priority SHOULD be the same as determined by the processing in
   Section 7.7.2.  The foundation SHOULD be the same.  The username
   fragments and passwords for a media stream SHOULD remain the same as
   the previous offer or answer.

   Population of the m/c-lines also depends on the state of ICE
   processing.  If, for a particular media stream, the valid list has
   candidate pairs for all of the components of that media stream, those
   pairs are used.  In particular, the m/c-line would be constructed by
   from the local candidate from each of those candidate pairs.  In
   addition, the agent MUST include the a=remote-candidates attribute
   for that media stream, and include in it the remote candidates for
   each of the pairs that were used.

   If, for a particular media stream, the valid list does not have pairs
   for all of the components of the stream, the agent SHOULD populate



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   the m/c-line for that media stream based on the considerations in
   Section 4.3.

   The agent MUST use the same ice-pwd and ice-ufrag for a media stream
   as its previous offer or answer.  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.

9.2.  Receiving the Offer and Generating an Answer

   When the answerer generates its answer, it must decide what
   candidates to include in the answer, and how to populate the m/c-
   line.

   For each media stream in the offer, the agent checks to see if the
   stream contained the remote-candidates attribute.  If it did, it
   means that the offerer believed that ICE processing has completed for
   that media stream.  In this case, the remote-candidates attribute
   contains the candidates that the answerer is supposed to use.  It is
   possible that the agent doesn't even know of these candidates yet;
   they will be discovered shortly through a response to an in-progress
   check.  The agent MUST populate the m/c-line with the candidates from
   the a=remote-candidates attribute.  In addition, it MUST include an
   a=candidate attribute in its answer for each candidate in the
   a=remote-candidates attribute.  If the agent is not aware of the
   candidate yet, it will need to generate a priority value for it.  The
   type preference in the computation is peer-reflexive, and the stream
   ID and component ID are known from the offer.  The agent chooses an
   arbitrary local preference value if it is multi-homed, since it won't
   yet know the interface associated with this candidate.

   If a media stream does not yet contain the a=remote-candidates
   attribute, it means that the offerer believes that ICE checks are
   still in progress for that media stream.  In this case, the answerer
   SHOULD include an a=candidate attribute for all of the candidates for
   that media stream it knows about (including peer-reflexive
   candidates).  The m/c-line is populated based on the considerations
   in Section 4.3.

   Construction of the ice-pwd and ice-ufrag are identical to the
   procedures followed by the offerer, as described in Section 9.1.

   Note that the a=remote-candidates 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.





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9.3.  Updating the Check and Valid Lists

   Once the subsequent offer/answer exchange has completed, each agent
   needs to compute the new check list resulting from this exchange, and
   then remove any pairs from the valid list which are no longer usable.
   Once these adjustments are made, ICE processing continues using these
   new lists.

   Each agent recomputes the check list using the procedures described
   in Section 5.6.  If a check on this new check list was also on the
   previous check list, and its state was Waiting, In-Progress,
   Succeeded or Failed, its state is copied over.  If a check on the new
   check list does not have a state (because its a new check or its
   state was not copied over), and it is for the component with
   component ID 1 and for the media stream with stream ID 9, its state
   is set to Waiting.  All other pairs without a state have their state
   set to Frozen.

   Next, the agent goes through the check list, starting with the
   highest priority check.  If a check has a state of Succeeded, and it
   has a component ID of 1, then all Frozen checks for the same media
   stream and same foundation whose component IDs are not one, have
   their state set to Waiting.  If, for a particular media stream, there
   are checks for each component of that media stream in the Succeeded
   state, the agent moves the state of all Frozen checks for the first
   component of all other media streams with the same foundation to
   Waiting.

   If a check was on the old check list, but was not on the new check
   list, and had a state of In-Progress, the corresponding STUN
   transaction is abandoned.  No further retransmits will be sent for
   the STUN request, and any response that might be received is ignored.

   Next, the agent prunes the valid list.  For each pair on the valid
   list, the agent examines each candidate in the pair.  If the
   candidate was not peer reflexive, and was not present in the most
   recent offer/answer exchange, the candidate pair is removed from the
   valid list.

      OPEN ISSUE: This means that you cannot forcefully remove a peer
      reflexive candidate.  This feature was possible, at much
      complexity, in previous versions of the spec.  An alternative is
      to remove a peer reflexive candidate if it was not present in the
      offer/answer, and was discovered more than 500ms ago.







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10.  Keepalives

   STUN connectivity checks are also used to keep NAT bindings open once
   a session is underway.  This is accomplished by periodically re-
   starting the check process, as described in this section.

   Once the initial offer/answer exchange has taken place, the agent
   sets a timer to fire in Tr seconds.  Tr SHOULD be configurable and
   SHOULD have a default of 15 seconds.  When Tr fires, the agent MUST
   reset the states for all of the checks in the check list using the
   procedures defined in Section 5.6 and then begin performing periodic
   checks as described in Section 5.7.  By the time the timer fires for
   the first time, the check list will include only the in-use
   candidates.  Reperforming these checks will therefore performing a
   period keepalive.

      OPEN ISSUE: ICE isn't saying anything about what happens if these
      periodic keepalives should fail.  It they do, something really bad
      has happened, like a NAT reboot or failure.  I think we should
      keep that out of scope.

   When an ICE agent is communicating with an agent that is not ICE-
   aware, keepalives still need to be utilized.  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 [27] and RTP comfort noise [23].
   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
   described above.  If STUN keepalives are not in use (because the peer
   does not support ICE), an agent SHOULD ensure that a media packet is



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   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.


11.  Media Handling

11.1.  Sending Media

   Agents always send media using a candidate pair.  An agent will send
   media to the remote candidate in the pair (setting the destination
   address and port of the packet equal to that remote candidate), and
   will send it from the local candidate.  When the local candidate is
   server or peer reflexive, media is originated from the base.  Media
   sent from a relayed candidate is sent through that relay, using
   procedures defined in [12].

   If an agent was the offerer in the most recent offer/answer exchange,
   when it sends media, it MUST use the candidates in the m/c-line for
   each media stream.  However, it MUST only send media once those
   candidates also appear in the valid list.  If the candidates in the
   m/c-line are not the ones that are ultimately selected by ICE, this
   implies that the offerer will need to wait for the subsequent offer/
   answer exchange to complete before it can send media.

   If an agent was the answerer in the most recent offer/answer
   exchange, the rules are different.  When the agent wishes to send
   media, and the candidate pairs in the m/c-lines are also the highest
   priority ones in the valid list for each media stream, it uses those
   candidate pairs.  If, however, the highest priority pairs in the
   valid list for a media stream are not the same as the ones in the
   m/c-lines, the agent MUST use the highest priority pairs in the valid
   list.  However, the agent MUST discontinue using those candidate
   pairs 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
   candidate pair, and then generate an updated offer.  If, after Tlo
   seconds, no updated offer arrives, the answerer MUST cease sending
   media, and will need to wait for the updated offer.

      OPEN ISSUE: In previous versions of ICE, once this timer fired,
      you just sent media to the one in the m/c-line.  This causes the
      media streams to flip back and forth between addresses, which I am



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      trying to avoid.  Since this timer should never go off anyway, I
      removed this feature.

   ICE has interactions with jitter buffer adaptation mechanisms.  An
   RTP stream can begin using one candidate, and switch to another one,
   though this happens rarely with ICE.  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.

11.2.  Receiving Media

   ICE implementations MUST be prepared to receive media on any
   candidates provided in the most recent offer/answer exchange.  In
   order to avoid attacks described in Section 15, when an agent
   receives a media packet, and it knows its peer supports ICE, it MUST
   verify that it has received a check (for which a successful response
   was generated) on the same 5-tuple as the received media packet (that
   is, the source and destination transport addresses of the media
   packet match those of the check).  If no such check has succeeded,
   the agent MUST silently discard the media packet.

   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 [20] 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 sometimes 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.


12.  Usage with SIP

12.1.  Latency Guidelines

   ICE requires a series of STUN-based connectivity checks to take place



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   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
   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.

   If an offer is received in an INVITE request, the callee 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 [9], or through the receipt of a successful STUN Binding
   Request.  Even if PRACK is not used, the provisional response SHOULD
   be retransmitted using the exponential backoff described in [9].
   Furthermore, once the answer has been sent, the agent SHOULD begin
   its connectivity checks.  Once candidate pairs for each component of
   a media stream enter the valid list, the callee can begin sending
   media on that media stream.

   However, prior to this point, any media that needs to be sent towards
   the caller (such as SIP early media [25] cannot be transmitted.  For
   this reason, implementations SHOULD delay alerting the called party
   until candidates for each component of each media stream have entered
   the valid list.  In the case of a PSTN gateway, this would mean that
   the setup message into the PSTN is delayed until this 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 [6], since its a localized decision.  It also has the
   benefit of guaranteeing that not a single packet of media will get
   clipped, so that post-pickup delay is zero.  If an agent chooses to
   delay local alerting in this way, it SHOULD generate a 180 response
   once alerting begins.



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   Based on the rules in Section 11.1, the offerer will not be able to
   send media until the highest priority valid candidates match the m/c-
   line.  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 generally 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 [9] MAY be used, in
   which case an UPDATE [24] can be used to send an updated offer prior
   to the call being answered.

   As discussed in Section 15, offer/answer exchanges SHOULD be secured
   against eavesdropping and man-in-the-middle attacks.  To do that, the
   usage of SIPS [3] is RECOMMENDED when used in concert with ICE.

12.2.  Interactions with Forking

   ICE interacts very well with forking.  Indeed, ICE fixes some of the
   problems associated with forking.  Without ICE, when a call forks and
   the caller receives multiple incoming media streams, it cannot
   determine which media stream corresponds to which callee.

   With ICE, this problem is resolved.  The connectivity checks which
   occur prior to transmission of media carry username fragments, which
   in turn are correlated to a specific callee.  Subsequent media
   packets which arrive on the same 5-tuple as the connectivity check
   will be associated with that same callee.  Thus, the caller can
   perform this correlation as long as it has received an answer.

   Section 11.2 introduces a requirement for agents receiving media;
   namely, that media should be discarded until a check has been
   received from that peer.  Unfortunately, this mechanism doesn't work
   well in forking situations where a subset of the recipients are not
   ICE-aware.  Those recipients will not send checks, and media from
   them will be discarded.

      OPEN ISSUE: Obviously this is an issue.  Need to either remove
      this feature of ICE or find a way to make it work better in
      forking situations.

12.3.  Interactions with Preconditions

   Quality of Service (QoS) preconditions, which are defined in RFC 3312
   [6] and RFC 4032 [7], 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-



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   INVITE would, and is fully treated in RFC 3312 and 4032, which apply
   without regard to the fact that the m/c lines are changing due to ICE
   negotiations ocurring "in the background".

   Indeed, an agent SHOULD NOT indicate that Qos preconditions have been
   met until the ICE checks have completed and selected the candidate
   pairs to be used for media.

   ICE also has (purposeful) interactions with connectivity
   preconditions [26].  Those interactions are described there.

      OPEN ISSUE: Are these preconditions really needed with ICE?  ICE
      provides a connectivity precondition on its own using the
      mechanisms described above.

12.4.  Interactions with Third Party Call Control

   ICE works with Flows I and IV as described in [16].  Flow I works
   without the controller supporting or being aware of ICE.  Flow IV
   will work as long as the controller passes along the ICE attributes
   without alteration.  Flow III may disrupt ICE processing, since it
   will distort the stream ID values used in the computation of
   priorities.  When there is but a single media stream, Flow III will
   work as long as the controller passes through the ICE attributes
   unmodified.  Flow II is fundamentally incompatible with ICE; each
   agent will believe itself to be the answerer and thus never generate
   a re-INVITE.

      OPEN ISSUE: Its really too bad flow III doesn't work with
      multimedia; should consider ways to make it work.  There are
      several ways.

   The flows for continued operation, as described in Section 7 of RFC
   3725, require additional behavior of ICE implementations to support.
   In particular, if an agent receives a mid-dialog re-INVITE that
   contains no offer, it MUST go through the process of gathering
   candidates, prioritizing them and generating an offer, as if this was
   an initial offer for a session.  Furthermore, that list of candidates
   SHOULD include the ones currently in-use.


13.  Grammar

   This specification defines four new SDP attributes - the "candidate",
   "remote-candidates", "ice-ufrag" 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



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   checks.

   The syntax of this attribute is defined using Augmented BNF as
   defined in RFC 4234 [8]:


   candidate-attribute   = "candidate" ":" foundation SP component-id SP
                           transport SP
                           priority SP
                           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)

   foundation            = 1*ice-char
   component-id          = 1*DIGIT
   transport             = "UDP" / transport-extension
   transport-extension   = token              ; from RFC 3261
   priority              = 1*DIGIT
   cand-type             = "typ" SP candidate-types
   candidate-types       = "host" / "srflx" / "prflx" / "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
   ice-char              = ALPHA / DIGIT / "+" / "/"


   The foundation is composed of one or more ice-char.  The component-id
   is a positive integer, which identifies the specific component for
   which the transport address is a candidate.  It MUST start at 1 and
   MUST increment by 1 for each component of a particular candidate.
   The connect-address production is taken from RFC 4566 [10], allowing
   for IPv4 addresses, IPv6 addresses and FQDNs.  The port production is
   also taken from RFC 4566 [10].  The token production is taken from
   RFC 3261 [3].  The transport production indicates the transport
   protocol for the candidate.  This specification only defines UDP.
   However, extensibility is provided to allow for future transport
   protocols to be used with ICE, such as TCP or the Datagram Congestion
   Control Protocol (DCCP) [28].

   The cand-type production encodes the type of candidate.  This
   specification defines the values "host", "srflx", "prflx" and "relay"
   for host, server reflexive, peer reflexive and relayed candidates,
   respectively.  The set of candidate types is extensible for the



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   future.  Inclusion of the candidate type is optional.  The rel-addr
   and rel-port productions convey information the related transport
   addresses.  Rules for inclusion of these values is described in
   Section 4.4.

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

   The syntax of the "remote-candidates" attribute is defined using
   Augmented BNF as defined in RFC 4234 [8].  The remote-candidates
   attribute is a media level attribute only.


   remote-candidate-att = "remote-candidates" ":" remote-candidate
                           0*(SP remote-candidate)
   remote-candidate = component-ID SP connection-address SP port

   The attribute contains a connection-address and port for each
   component.  The ordering of components is irrelevant.  However, a
   value MUST be present for each component of a media stream.

   The syntax of the "ice-pwd" and "ice-ufrag" attributes are defined
   as:


   ice-pwd-att           = "ice-pwd" ":" password
   ice-ufrag-att         = "ice-ufrag" ":" ufrag
   password              = 22*ice-char
   ufrag                 = 4*ice-char

   The "ice-pwd" and "ice-ufrag" attributes 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.


14.  Example

   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 NAT has an endpoint



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   independent mapping property and an address dependent filtering
   property.  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.  The format of the name 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
   varname.PORT, respectively, where varname is the name of the
   variable.

   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.  However, neither agent is using 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 and refers to the mapped address.

   The call flow examples omit STUN authentication operations and RTCP,
   and focus on RTP for a single media stream.


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



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



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             |D=$L-PRIV-1   |              |              |
             |<-------------|              |              |
             |(18) Bind Res |              |              |
             |S=$L-PRIV-1   |              |              |
             |D=$R-PUB-1    |              |              |
             |MA=$R-PUB-1   |              |              |
             |------------->|              |              |
             |              |(19) Bind Res |              |
             |              |S=$NAT-PUB-1  |              |
             |              |D=$R-PUB-1    |              |
             |              |MA=$R-PUB-1   |              |
             |              |---------------------------->|
             |RTP flows     |              |              |


   Figure 9

   First, agent L obtains a host candidate from its local interface (not
   shown), and from that, sends a STUN Binding Request to the STUN
   server to get a server reflexive candidate (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 candidate for RTP.

   Agent L sets a type preference of 9 for the host candidate and 5 for
   the server reflexive.  The local preference is 9.  Based on this, the
   priority of the host candidate is 9909 and for the server reflexive
   candidate is 5909.  The host candidate is assigned a foundation of 1,
   and the server reflexive, a foundation of 2.  It chooses its server
   reflexive candidate as the in-use candidate, and encodes it into the
   m/c-line.  The resulting offer (message 5) looks like (lines folded
   for clarity):


       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:asd88fgpdd777uzjYhagZg
       a=ice-ufrag:8hhY
       m=audio $NAT-PUB-1.PORT RTP/AVP 0
       a=rtpmap:0 PCMU/8000
       a=candidate:1 1 UDP 9909 $L-PRIV-1.IP $L-PRIV-1.PORT typ local
       a=candidate:2 1 UDP 5909 $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



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   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
       a=ice-ufrag:8hhY
       m=audio 45664 RTP/AVP 0
       a=rtpmap:0 PCMU/8000
       a=candidate:1 1 UDP 9909 10.0.1.1 8998 typ local
       a=candidate:2 1 UDP 5909 192.0.2.3 45664 typ srflx raddr
   10.0.1.1 rport 8998

   This offer is received at agent R. Agent R will obtain a host
   candidate, and from it, obtain a server reflexive candidate (messages
   6-7).  Since R is not behind a NAT, this candidate is identical to
   its host candidate, and they share the same base.  It therefore
   discards this candidate and ends up with a single host candidate.
   With identical type and local preferences as L, the priority for this
   candidate is 9909.  It chooses a foundation of 1 for its single
   candidate.  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:YH75Fviy6338Vbrhrlp8Yh
       a=ice-ufrag:9uB6
       m=audio $R-PUB-1.PORT RTP/AVP 0
       a=rtpmap:0 PCMU/8000
       a=candidate:1 1 UDP 9909 $R-PUB-1.IP $R-PUB-1.PORT typ local

   With the variables filled in:


       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
       a=ice-ufrag:9uB6
       m=audio 3478 RTP/AVP 0



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       a=rtpmap:0 PCMU/8000
       a=candidate:1 1 UDP 9909 192.0.2.1 3478 typ local

   Agents L and R both pair up the candidates.  They both initially have
   two.  However, agent L will prune the pair containing its server
   reflexive candidate, resulting in just one.  At agent L, this pair
   (the check) has a local candidate of $L_PRIV_1 and remote candidate
   of $R_PUB_1, and has a candidate pair priority of 99099909.039.  At
   agent R, there are two checks.  The highest priority has a local
   candidate of $R_PUB_1 and remote candidate of $L_PRIV_1 and has a
   priority of 99099909.039, and the second has a local candidate of
   $R_PUB_1 and remote candidate of $NAT_PUB_1 and priority 59099909.75.

   Agent R begins its connectivity check (message 9) for the first pair
   (between the two host candidates).  The host candidate from agent L
   is private and behind a different NAT, and thus this check is
   discarded.

   When agent L gets the answer, it performs its one and only
   connectivity check (messages 10-13).  This will succeed.  This causes
   agent L to create a new pair, whos local candidate is from the mapped
   address in the binding response (NAT-PUB-1 from message 13) and whose
   remote candidate is the destination of the request (R-PUB-1 from
   message 10).  This is added to the valid list.  At this point, agent
   L examines the valid list and sees that there is a candidate there
   for each component of each media stream (which is just RTP for the
   single audio stream).  It therefore considers ICE checks complete and
   sends an updated offer (message 14).  This offer serves only to
   remove the candidate that was not selected and indicate the remote
   candidates; the m/c-line remains unchanged.  This offer looks like:


       v=0
       o=jdoe 2890844528 2890842809 IN IP4 10.0.1.1
       s=
       c=IN IP4 192.0.2.3
       t=0 0
       a=ice-pwd:asd88fgpdd777uzjYhagZg
       a=ice-ufrag:8hhY
       m=audio 45664 RTP/AVP 0
       a=remote-candidates 1 192.0.2.1 3478
       a=rtpmap:0 PCMU/8000
       a=candidate:2 1 UDP 5909 192.0.2.3 45664 typ srflx raddr
   10.0.1.1 rport 8998

   Agent R can construct the answer.  Since the remote-candidates listed
   in the offer match the ones that agent R had already selected for the
   m/c-line in the previous answer, there is no change there.  Its



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   answer therefore looks like:


       v=0
       o=bob 2808844565 2808844566 IN IP4 192.0.2.1
       s=
       c=IN IP4 192.0.2.1
       t=0 0
       a=ice-pwd:YH75Fviy6338Vbrhrlp8Yh
       a=ice-ufrag:9uB6
       m=audio 3478 RTP/AVP 0
       a=rtpmap:0 PCMU/8000
       a=candidate:1 1 UDP 9909 192.0.2.1 3478 typ local

   Upon receipt of the check from agent L (message 11), agent R will
   generate its triggered check.  This check happens to match the next
   one on its check list - from its host candidate to agent L's server
   reflexive candidate.  This check (messages 16-19) will succeed.
   Consequently, agent R constructs a new candidate pair using the
   mapped address from the response as the local candidate (R-PUB-1) and
   the destination of the request (NAT-PUB-1) as the remote candidate.
   This pair is added to the valid list.  Since this pair matches the
   pair in the m/c-lines, agent R can send media as well.


15.  Security Considerations

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

15.1.  Attacks on Connectivity Checks

   An attacker might attempt to disrupt the STUN 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.

   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.




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   False Peer-Reflexive Candidate: An attacker can cause an agent to
      discover a new peer reflexive 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 reflexive candidate or false server reflexive 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 [11], 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 usually irrelevant since STUN servers are not typically
   discovered via DNS, they are signaled via IP addresses embedded in
   SDP.  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 from 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.  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.  Fortunately, this attack is mitigated completely
   through the STUN message integrity mechanism.  The attacker needs to
   inject a fake response, and in order for this response to be
   processed, the attacker needs the password.  If the offer/answer
   signaling is secured, the attacker will not have the password.

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

   Forcing the false peer reflexive candidate result can be done either



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   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 candidate.  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 containing the false candidate.
   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 reflexive candidate result with packet replays
   is different.  The attacker waits until one of the agents sends a
   check.  It 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 candidate, and will be sent to that
   false candidate.  The attacker must then intercept it and relay it
   towards the originator.

   The other agent will then initiate a connectivity check towards that
   false candidate.  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 check towards this false candidate,
   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 candidate.  This is because it requires the
   attacker to intercept and replay packets sent by two different hosts.
   If both agents are on different networks (for example, across the
   public Internet), this attack can be hard to coordinate, since it
   needs to occur against two different endpoints on different parts of
   the network at the same time.

   If the attacker themself is identified by the fake candidate the
   attack is easier to coordinate.  However, if SRTP is used [21], 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.



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   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.

15.2.  Attacks on Address Gathering

   ICE endpoints make use of STUN for gathering candidates rom a STUN
   server in the network.  This is corresponds to the Binding Discovery
   usage of STUN described in [11].  As a consequence, the attacks
   against STUN itself that are described in that specification can
   still be used against the binding discovery usage when utilized with
   ICE.

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

   Consider an attacker which is able to provide an agent with a faked
   mapped address in a STUN Binding Request that is used for address
   gathering.  This is the primary attack primitive described in [11].
   This address will be used as a server reflexive 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 server reflexive candidate from being
   used.  However, if the peer agent has at least one candidate that is
   reachable by the agent under attack, the STUN connectivity checks
   themselves will provide a peer reflexive candidate that can be used
   for the exchange of media.  Peer reflexive candidates are generally
   preferred over server reflexive candidates.  As such, an attack
   solely on the STUN address gathering will normally have no impact on
   a session at all.

15.3.  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 [3] when SIP is
   used.  As such, the usage of SIPS with ICE is RECOMMENDED.



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15.4.  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.

15.4.1.  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 specific 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.

15.4.2.  STUN Amplification Attack

   The STUN amplification attack is similar to the voice hammer.
   However, instead of voice packets being directed to the target, STUN
   connectivity checks are directed to the target.  This attack is
   accomplished by having the offerer send an offer with a large number
   of candidates, say 50.  The answerer receives the offer, and starts
   its checks, which are directed at the target, and consequently, never
   generate a response.  The answerer will start a new connectivity
   check every 50ms, and each check is a STUN transaction consisting of
   9 retransmits of a message 65 bytes in length (plus 28 bytes for the
   IP/UDP header) that runs for 7.9 seconds, for a total of 105 bytes/
   second per transaction on average.  In the worst case, there can be
   158 transactions in progress at once (7.9 seconds divided by 50ms),
   for a total of 132 kbps, just for STUN requests.

   It is impossible to eliminate the amplification, but the volume can
   be reduced through a variety of heuristics.  For example, agents can
   limit the number of candidates they'll accept in an offer or answer,
   they can increase the value of Ta, or exponentially increase Ta as



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   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.

      OPEN ISSUE: Need better remediation for this.  Especially an issue
      if we reduce Ta to be as fast as media packets themselves, in
      which case this attack is as equally devastating as the voice
      hammer.


16.  IANA Considerations

   This specification defines four new SDP attributes per the procedures
   of Section 8.2.4 of [10].  The required information for the
   registrations are included here.

16.1.  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 Underneath
      NAT (STUN).

   Appropriate Values: See Section 13 of RFC XXXX [Note to RFC-ed:
      please replace XXXX with the RFC number of this specification].

16.2.  remote-candidates Attribute

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

   Attribute Name: remote-candidates

   Long Form: remote-candidates







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   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
      candidates that the offerer wishes the answerer to use in its
      answer.

   Appropriate Values: See Section 13 of RFC XXXX [Note to RFC-ed:
      please replace XXXX with the RFC number of this specification].

16.3.  ice-pwd Attribute

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

   Attribute Name: ice-pwd

   Long Form: ice-pwd

   Type of Attribute: session or 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 password used to protect
      STUN connectivity checks.

   Appropriate Values: See Section 13 of RFC XXXX [Note to RFC-ed:
      please replace XXXX with the RFC number of this specification].

16.4.  ice-ufrag Attribute

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

   Attribute Name: ice-ufrag

   Long Form: ice-ufrag

   Type of Attribute: session or media level

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






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   Purpose: This attribute is used with Interactive Connectivity
      Establishment (ICE), and provides the fragments used to construct
      the username in STUN connectivity checks.

   Appropriate Values: See Section 13 of RFC XXXX [Note to RFC-ed:
      please replace XXXX with the RFC number of this specification].


17.  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 [19].  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.
   Indeed, ICE can be considered a B-SAF (Bilateral Self-Address Fixing)
   protocol, rather than an UNSAF protocol.  Regardless, the IAB has
   mandated that any protocols developed for this purpose document a
   specific set of considerations.  This section meets those
   requirements.

17.1.  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.

17.2.  Exit Strategy

   From RFC 3424, any UNSAF proposal must provide:




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      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, server reflexive and
   relayed candidates (both forms of UNSAF mechanisms) simply never get
   used, because higher priority connectivity exists to the native host
   candidates.  Therefore, the servers get used less and less, and can
   eventually be remove when their usage goes to zero.

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

17.3.  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 RFC 3489 [13]) 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 traditional 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



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   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 is removed.

   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.

17.4.  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.

17.5.  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 [11] uses an encoding
   which hides these binary addresses from generic ALGs.  Since [11] is
   required for all ICE implementations, this NAPT problem does not
   impact ICE.




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   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 [30], this minimum keepalive will become deterministic and
   well-known, and the ICE timers can be adjusted.  Having a way to
   discover and control the minimum keepalive interval would be far
   better still.


18.  Acknowledgements

   The authors would like to thank Flemming Andreasen, Rohan Mahy, Dean
   Willis, Eric Cooper, Dan Wing, Douglas Otis, Tim Moore, and Francois
   Audet for their comments and input.  A special thanks goes to Bill
   May, who suggested several of the concepts in this specification,
   Philip Matthews, who suggested many of the key performance
   optimizations in this specification, Eric Rescorla, who drafted the
   text in the introduction, and Magnus Westerlund, for doing several
   detailed reviews on the various revisions of this specification.


19.  References

19.1.  Normative References

   [1]   Bradner, S., "Key words for use in RFCs to Indicate Requirement
         Levels", BCP 14, RFC 2119, March 1997.

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

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

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

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

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




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

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

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

   [10]  Handley, M., Jacobson, V., and C. Perkins, "SDP: Session
         Description Protocol", RFC 4566, July 2006.

   [11]  Rosenberg, J., "Simple Traversal Underneath Network Address
         Translators (NAT) (STUN)", draft-ietf-behave-rfc3489bis-04
         (work in progress), July 2006.

   [12]  Rosenberg, J., "Obtaining Relay Addresses from Simple Traversal
         of UDP Through NAT (STUN)", draft-ietf-behave-turn-01 (work in
         progress), June 2006.

19.2.  Informative References

   [13]  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.

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

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

   [16]  Rosenberg, J., Peterson, J., Schulzrinne, H., and G. Camarillo,
         "Best Current Practices for Third Party Call Control (3pcc) in
         the Session Initiation Protocol (SIP)", BCP 85, RFC 3725,
         April 2004.

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

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

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



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

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

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

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

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

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

   [26]  Andreasen, F., "Connectivity Preconditions for Session
         Description Protocol Media Streams",
         draft-ietf-mmusic-connectivity-precon-02 (work in progress),
         June 2006.

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

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

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

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

   [31]  Jennings, C. and R. Mahy, "Managing Client Initiated
         Connections in the Session Initiation Protocol  (SIP)",
         draft-ietf-sip-outbound-04 (work in progress), June 2006.


Appendix A.  Design Motivations

   ICE contains a number of normative behaviors which may themselves be
   simple, but derive from complicated or non-obvious thinking or use



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   cases which merit further discussion.  Since these design motivations
   are not neccesary to understand for purposes of implementation, they
   are discussed here in an appendix to the specification.  This section
   is non-normative.

A.1.  Applicability to Gateways and Servers

   Section 4.1 discusses procedures for gathering candidates, including
   host, server reflexive and relayed.  In that section, recommendations
   are given for when an agent should obtain each of these three types.
   In particular, for agents embedded in PSTN gateways, media servers,
   conferencing servers, and so on, ICE specifies that an agent can
   stick with just host candidates, since it has a public IP address.

   This leads to an important question - why would such an endpoint even
   bother with ICE?  If it has a public IP address, what additional
   value do the ICE procedures bring?  There are many, actually.

   First, doing so 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 candidates, but it implements ICE,
   and consequently, is prepared to receive STUN connectivity checks on
   its host candidates.  The softphone will send a STUN connectivity
   check to the gateway, which passes through the intervending NAT.
   This causes the NAT to allocate a new binding for the softphone.  The
   connectivity is received by the gateway, and will cause it gateway to
   send a check back to the softphone, at this newly created candidate.
   A successful response confirms that this candidate is usable, and the
   gateway can send media immediately to the softphone.  This allows
   direct media transmission between the gateway and softphone, without
   the need for relays, even though the softphone was behind a 'bad'
   NAT.

   Second, implementation of the STUN connectivity checks allows for NAT
   bindings along the way to be kept open.  Keeping these bindings open
   is essential for continued communications between the gateway and
   softphone.

   Third, ICE prevents a fairly destructive attack in multimedia
   systems, called the voice hammer.  The STUN connectivity check used
   by an ICE endpoint allows it to be certain that the target of media
   packets is, in fact, the same entity that requested the packets
   through the offer/answer exchange.  See Section 15 for a more
   complete discussion on this attack.





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A.2.  Pacing of STUN Transactions

   STUN transactions used to gather candidates and to verify
   connectivity are paced out at an approximate rate of one new
   transaction every Ta seconds, where Ta has a default of 50ms.  Why
   are these transactions paced, and why was 50ms chosen as default?

   Sending of these STUN requests will often 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, the
   pacing ensures that the NAT devices does not get overloaded and that
   traffic is kept at a reasonable rate.

   Another aspect of the STUN requests is their bandwidth usage.  In
   ICE, each STUN request contains the STUN 20 byte header, in addition
   to the USERNAME, MESSAGE-INTEGRITY and PRIORITY attributes.  The
   USERNAME attribute contains a 4-byte attribute overhead, plus the
   username value itself.  This username is the concatenation of the two
   fragments, plus a colon.  Each fragment is supposed to be at least 4
   bytes long, making the total length of the USERNAME attribute (4*2 +
   1 + 4) = 13 bytes.  The MESSAGE-INTEGRITY attribute is 4 bytes of
   overhead plus 20 bytes value, for 24 bytes.  The PRIORITY attribute
   is 4 bytes of overhead plus 4 bytes of value, for 8 bytes.  Thus, the
   total length of the STUN Binding Request is (20 + 13 + 24 + 8) = 65
   bytes, with 28 bytes of overhead for IP and UDP for a total of 93
   bytes.  The response contains the STUN 20 byte header, the XOR-
   MAPPED-ADDRESS, and MESSAGE-INTEGRITY attributes.  XOR-MAPPED-ADDRESS
   has 4 bytes overhead plus an 8 byte value, for a total of 12 bytes.
   Thus, each STUN response is (20 + 12 + 24) = 56 bytes plus 28 bytes
   of UDP/IP overhead for a total of 84 bytes.  Checks typically fall
   into one of two cases.  If a check works, each transaction has a
   single request and a single response, for a total of 2 packets and
   177 bytes over one RTT interval.  Assuming a fairly agressive RTT of
   70ms, this produces 20.23 kbps, but only briefly.  If a check fails
   because the pair is invalid, there will be nine requests and no
   responses.  This produces 837 bytes over 7.9s, for a total of 105.9
   bps, but over a long period of time.

      OPEN ISSUE: The bandwidth computations are pretty complex because
      ICE is not a CBR stream, and its bandwidth utilization depends on
      how many transactions it ends up generating before it finishes.
      Need to work this model more.

   Given that these numbers are close to, if not greater than, the
   bandwidths utilized by many voice codecs, this seems a reasonable



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   value to use.

      OPEN ISSUE: There is some debate about whether to reduce this
      pacing interval smaller, say 20ms, to speed up ICE, or perhaps
      make it equal to the bandwidth that would be utilized by the media
      streams themselves.

A.3.  Candidates with Multiple Bases

   Section 4.1 talks about merging together candidates that are
   identical but have different bases.  When can an agent have two
   candidates that have the same IP address and port, but different
   bases?  Consider the topology of Figure 16:






































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          +----------+
          | 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 |          |
          +----------+           \\     //           +----------+
                                   -----


   Figure 16

   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 a host candidate on its interface on network C



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   (10.0.1.1:2498) and a host candidate 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 server reflexive candidate with a transport address that
   is identical to a host candidate (10.0.1.1:2498).  However, the
   server reflexive candidate has a base of 192.168.1.1:3344, and the
   host candidate has a base of 10.0.1.1:2498.

A.4.  Purpose of the Translation

   When a candidate is relayed, the SDP offer or answer contain both the
   relayed candidate and its translation.  However, the translation is
   never used by ICE itself.  Why is it present in the message?

   There are two motivations for its inclusion.  The first is
   diagnostic.  It is very useful to know the relationship between the
   different types of candidates.  By including the translation, an
   agent can know which relayed candidate is associated with which
   reflexive candidate, which in turn is associated with a specific host
   candidate.  When checks for one candidate succeed and not the others,
   this provides useful diagnostics on what is going on in the network.

   The second reason has to do with off-path Quality of Service (QoS)
   mechanisms.  When ICE is used in environments such as PacketCable 2.0
   [[TODO: need PC2.0 reference]], proxies will, in addition to
   performing normal SIP operations, inspect the SDP in SIP messages,
   and extract the IP address and port for media traffic.  They can then
   interact, through policy servers, with access routers in the network,
   to establish guaranteed QoS for the media flows.  This QoS is
   provided by classifying the RTP traffic based on 5-tuple, and then
   providing it a guaranteed rate, or marking its Diffserv codepoints
   appropriately.  When a residential NAT is present, and a relayed
   candidate gets selected for media, this relayed candidate will be a
   transport address on an actual STUN relay.  That address says nothing
   about the actual transport address in the access router that would be
   used to classify packets for QoS treatment.  Rather, the translation
   of that relayed address is needed.  By carrying the translation in
   the SDP, the proxy can use that transport address to request QoS from
   the access router.

A.5.  Importance of the STUN Username

   ICE requires the usage of message integrity with STUN using its short
   term credential functionality.  The actual short term credential is
   formed by exchanging username fragments in the SDP offer/answer
   exchange.  The need for this mechanism goes beyond just security; it



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   is actual required for correct operation of ICE in the first place.

   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 host candidates.  In this case, those candidates are
   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
   as host candidates.  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
   STUN short term credential mechanisms are used.  The username
   fragments are sufficiently random that it is highly unlikely that B
   would be using the same values as A. Consequently, B would reject the
   STUN request since the credentials were invalid.  In essence, the
   STUN username fragments provide a form of transient host identifiers,
   bound to a particular offer/answer session.

   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
   applications - be prepared for unknown packets on any port.

A.6.  The Candidate Pair Sequence Number Formula

   The sequence number for a candidate pair has an odd form.  It is:

      PAIR-SN = 10000*MAX(O-SN,A-SN) + MIN(O-SN,A-SN) + O-IP/SZ

   Why is this?  When the candidate pairs are sorted based on this



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   value, the resulting sorting has the MAX/MIN property.  This means
   that the pairs are first sorted based on increasing value of the
   maximum of the two sequence numbers.  For pairs that have the same
   value of the maximum sequence number, the minimum sequence number is
   used to sort amongst them.  If the max and the min sequence numbers
   are the same, the IP address of the offerers candidate serves as a
   tie breaker.  The factor of 1000 is used since there will always be
   fewer than a 1000 candidates, and thus the largest value a sequence
   number (and thus the minimum sequence number) can have is always less
   than 1000.  This creates the desired sorting property.

   Recall that candidate sequence numbers are assigned such that, for a
   particular set of candidates of the same type, the RTP components
   have lower sequence numbers than the corresponding RTCP component.
   Also recall that, if an agent prefers host candidates to server
   reflexive to relayed, sequence numbers for host candidates are always
   lower than server reflexive which are always lower than relayed.
   Because of this,

A.7.  The Frozen State

   The Frozen state is used for two purposes.  Firstly, it allows ICE to
   first perform checks for the first component of a media stream.  Once
   a successful check has completed for the first component, the other
   components of the same type and local preference will get performed.
   Secondly, when there are multiple media streams, it allows ICE to
   first check candidates for a single media stream, and once a set of
   candidates has been found, candidates of that same type for other
   media streams can be checked first.  This effectively 'caches' the
   results of a check for one media stream, and applies them to another.
   For example, if only the relayed candidates for audio (which were the
   last resort candidates) succeed, ICE will check the relayed
   candidates for video first.

A.8.  The remote-candidates attribute

   The a=remote-candidates attribute 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 list.  This
   race condition is shown in Figure 17.  On receipt of message 4, agent
   A adds a candidate pair to the valid list.  If there was only a
   single media stream with a single component, agent A could now send
   an updated offer.  However, the check from agent B has not yet
   generated a response, and agent B receives the updated offer (message
   7) before getting the response (message 10).  Thus, it does not yet
   know that this particular pair is valid.  To eliminate this
   condition, the actual candidates at B that were selected by the
   offerer (the remote candidates) are included in the offer itself.



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   Note, however, that agent B 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 17

A.9.  Why are Keepalives Needed?

   Once media begins flowing on a candidate pair, 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.

   Secondly, some RTP payload formats, such as the payload format for
   text conversation [29], may send packets so infrequently that the
   interval exceeds the NAT binding timeouts.

   Thirdly, if silence suppression is in use, long periods of silence



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   may cause media transmission to cease sufficiently long for NAT
   bindings to time out.

   For these reasons, the media packets themselves cannot be relied
   upon.  ICE defines a simple periodic keepalive that operates
   indpendently of media transmission.  This makes its bandwidth
   requirements highly predictable, and thus amenable to QoS
   reservations.

A.10.  Why Prefer Peer Reflexive Candidates?

   Section 4.2 describes procedures for computing the priority of
   candidate based on its type and local preferences.  That section
   requires that the type preference for peer reflexive candidates
   always be lower than server reflexive.  Why is that?  The reason has
   to do with the security considerations in Section 15.  It is much
   easier for an attacker to cause an agent to use a false server
   reflexive candidate than it is for an attacker to cause an agent to
   use a false peer reflexive candidate.  Consequently, attacks against
   the STUN binding discovery usage are thwarted by ICE by preferring
   the peer reflexive candidates.

A.11.  Why Can't Offerers Send Media When a Pair Validates

   Section 11.1 describes rules for sending media.  The rules are
   asymmetric, and not the same for offerers and answerers.  In
   particular, an answerer can send media right away to a candidate pair
   once it validates, even if it doesnt match the pairs in the m/c-line.
   THe offerer cannot - it must wait for an updated offer/answer
   exchange.  Why is that?

   This, in fact, relates to a bigger question - why is the updated
   offer/answer exchange needed at all?  Indeed, in a pure offer/answer
   environment, it would not be.  The offerer and answerer will agree on
   the candidates to use through ICE, and then can begin using them.  As
   far as the agents themselves are concerned, the updated offer/answer
   provides no new information.  However, in practice, numerous
   components along the signaling path look at the SDP information.
   These include entities performing off-path QoS reservations, NAT
   traversal components such as ALGs and Session Border Controllers
   (SBCs) and diagnostic tools that passively monitor the network.  For
   these tools to continue to function without change, the core property
   of SDP - that the m/c-lines represent the addresses used for media -
   must be retained.  For this reason, an updated offer must be sent.

   To ensure that an updated offerer is sent, ICE purposefully prevents
   the offerer from sending media until that offer is sent.  It
   furthermore restricts the answerer in how long it can send media



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   until an updated offer is received.  This provides protocol
   incentives for sending the updated offer.

   The updated offer also helps ensure that ICE did the right thing.  In
   very unusual cases, the offerer and answerer might not agree on the
   candidates selected by ICE.  This would be detected in the updated
   offer/answer exchange, allowing them to restart ICE procedures to fix
   the problem.











































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Author's Address

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

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








































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