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MMUSIC                                                      J. Rosenberg
Internet-Draft                                             Cisco Systems
Expires: April 26, 2007                                 October 23, 2006


Interactive Connectivity Establishment (ICE): A Methodology for Network
     Address Translator (NAT) Traversal for Offer/Answer Protocols
                        draft-ietf-mmusic-ice-12

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   This Internet-Draft will expire on April 26, 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 . . . . . . . . . . . . . . . . . . . . . . . . .  5
   2.  Overview of ICE  . . . . . . . . . . . . . . . . . . . . . . .  5
     2.1.  Gathering Candidate Addresses  . . . . . . . . . . . . . .  7
     2.2.  Connectivity Checks  . . . . . . . . . . . . . . . . . . .  9
     2.3.  Sorting Candidates . . . . . . . . . . . . . . . . . . . . 10
     2.4.  Frozen Candidates  . . . . . . . . . . . . . . . . . . . . 11
     2.5.  Security for Checks  . . . . . . . . . . . . . . . . . . . 11
     2.6.  Concluding ICE . . . . . . . . . . . . . . . . . . . . . . 11
     2.7.  Passive-Only Agents  . . . . . . . . . . . . . . . . . . . 12
   3.  Terminology  . . . . . . . . . . . . . . . . . . . . . . . . . 13
   4.  Choosing a Mode  . . . . . . . . . . . . . . . . . . . . . . . 15
   5.  Sending the Initial Offer  . . . . . . . . . . . . . . . . . . 15
     5.1.  Gathering Candidates . . . . . . . . . . . . . . . . . . . 16
     5.2.  Prioritizing Candidates  . . . . . . . . . . . . . . . . . 18
     5.3.  Choosing In-Use Candidates . . . . . . . . . . . . . . . . 20
     5.4.  Encoding the SDP . . . . . . . . . . . . . . . . . . . . . 20
   6.  Receiving the Initial Offer  . . . . . . . . . . . . . . . . . 22
     6.1.  Verifying ICE Support  . . . . . . . . . . . . . . . . . . 22
     6.2.  Determining Role . . . . . . . . . . . . . . . . . . . . . 23
     6.3.  Gathering Candidates . . . . . . . . . . . . . . . . . . . 23
     6.4.  Prioritizing Candidates  . . . . . . . . . . . . . . . . . 23
     6.5.  Choosing In Use Candidates . . . . . . . . . . . . . . . . 23
     6.6.  Encoding the SDP . . . . . . . . . . . . . . . . . . . . . 23
     6.7.  Forming the Check Lists  . . . . . . . . . . . . . . . . . 23
     6.8.  Performing Periodic Checks . . . . . . . . . . . . . . . . 26
   7.  Receipt of the Initial Answer  . . . . . . . . . . . . . . . . 27
     7.1.  Verifying ICE Support  . . . . . . . . . . . . . . . . . . 27
     7.2.  Determining Role . . . . . . . . . . . . . . . . . . . . . 27
     7.3.  Forming the Check List . . . . . . . . . . . . . . . . . . 27
     7.4.  Performing Periodic Checks . . . . . . . . . . . . . . . . 27
   8.  Connectivity Checks  . . . . . . . . . . . . . . . . . . . . . 27
     8.1.  Client Procedures  . . . . . . . . . . . . . . . . . . . . 28
       8.1.1.  Sending the Request  . . . . . . . . . . . . . . . . . 28
       8.1.2.  Processing the Response  . . . . . . . . . . . . . . . 29
     8.2.  Server Procedures  . . . . . . . . . . . . . . . . . . . . 30
   9.  Concluding ICE . . . . . . . . . . . . . . . . . . . . . . . . 32
   10. Subsequent Offer/Answer Exchanges  . . . . . . . . . . . . . . 33
     10.1. Generating the Offer . . . . . . . . . . . . . . . . . . . 33
     10.2. Receiving the Offer and Generating an Answer . . . . . . . 34
     10.3. Updating the Check and Valid Lists . . . . . . . . . . . . 35
   11. Keepalives . . . . . . . . . . . . . . . . . . . . . . . . . . 37
   12. Media Handling . . . . . . . . . . . . . . . . . . . . . . . . 38
     12.1. Sending Media  . . . . . . . . . . . . . . . . . . . . . . 38
     12.2. Receiving Media  . . . . . . . . . . . . . . . . . . . . . 39
   13. Usage with SIP . . . . . . . . . . . . . . . . . . . . . . . . 39
     13.1. Latency Guidelines . . . . . . . . . . . . . . . . . . . . 39



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     13.2. Interactions with Forking  . . . . . . . . . . . . . . . . 40
     13.3. Interactions with Preconditions  . . . . . . . . . . . . . 41
     13.4. Interactions with Third Party Call Control . . . . . . . . 41
   14. Grammar  . . . . . . . . . . . . . . . . . . . . . . . . . . . 42
   15. Example  . . . . . . . . . . . . . . . . . . . . . . . . . . . 44
   16. Security Considerations  . . . . . . . . . . . . . . . . . . . 49
     16.1. Attacks on Connectivity Checks . . . . . . . . . . . . . . 49
     16.2. Attacks on Address Gathering . . . . . . . . . . . . . . . 52
     16.3. Attacks on the Offer/Answer Exchanges  . . . . . . . . . . 52
     16.4. Insider Attacks  . . . . . . . . . . . . . . . . . . . . . 52
       16.4.1. The Voice Hammer Attack  . . . . . . . . . . . . . . . 53
       16.4.2. STUN Amplification Attack  . . . . . . . . . . . . . . 53
   17. Definition of Connectivity Check Usage . . . . . . . . . . . . 54
     17.1. Applicability  . . . . . . . . . . . . . . . . . . . . . . 54
     17.2. Client Discovery of Server . . . . . . . . . . . . . . . . 54
     17.3. Server Determination of Usage  . . . . . . . . . . . . . . 54
     17.4. New Requests or Indications  . . . . . . . . . . . . . . . 54
     17.5. New Attributes . . . . . . . . . . . . . . . . . . . . . . 54
     17.6. New Error Response Codes . . . . . . . . . . . . . . . . . 55
     17.7. Client Procedures  . . . . . . . . . . . . . . . . . . . . 55
     17.8. Server Procedures  . . . . . . . . . . . . . . . . . . . . 55
     17.9. Security Considerations for Connectivity Check . . . . . . 55
   18. IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 55
     18.1. SDP Attributes . . . . . . . . . . . . . . . . . . . . . . 55
       18.1.1. candidate Attribute  . . . . . . . . . . . . . . . . . 55
       18.1.2. remote-candidates Attribute  . . . . . . . . . . . . . 56
       18.1.3. ice-passive Attribute  . . . . . . . . . . . . . . . . 56
       18.1.4. ice-pwd Attribute  . . . . . . . . . . . . . . . . . . 57
       18.1.5. ice-ufrag Attribute  . . . . . . . . . . . . . . . . . 57
     18.2. STUN Attributes  . . . . . . . . . . . . . . . . . . . . . 58
   19. IAB Considerations . . . . . . . . . . . . . . . . . . . . . . 58
     19.1. Problem Definition . . . . . . . . . . . . . . . . . . . . 58
     19.2. Exit Strategy  . . . . . . . . . . . . . . . . . . . . . . 59
     19.3. Brittleness Introduced by ICE  . . . . . . . . . . . . . . 59
     19.4. Requirements for a Long Term Solution  . . . . . . . . . . 60
     19.5. Issues with Existing NAPT Boxes  . . . . . . . . . . . . . 60
   20. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 61
   21. References . . . . . . . . . . . . . . . . . . . . . . . . . . 61
     21.1. Normative References . . . . . . . . . . . . . . . . . . . 61
     21.2. Informative References . . . . . . . . . . . . . . . . . . 62
   Appendix A.  Passive-Only ICE  . . . . . . . . . . . . . . . . . . 64
   Appendix B.  Design Motivations  . . . . . . . . . . . . . . . . . 66
     B.1.  Pacing of STUN Transactions  . . . . . . . . . . . . . . . 66
     B.2.  Candidates with Multiple Bases . . . . . . . . . . . . . . 67
     B.3.  Purpose of the Translation . . . . . . . . . . . . . . . . 69
     B.4.  Importance of the STUN Username  . . . . . . . . . . . . . 69
     B.5.  The Candidate Pair Sequence Number Formula . . . . . . . . 70
     B.6.  The Frozen State . . . . . . . . . . . . . . . . . . . . . 71



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     B.7.  The remote-candidates attribute  . . . . . . . . . . . . . 71
     B.8.  Why are Keepalives Needed? . . . . . . . . . . . . . . . . 72
     B.9.  Why Prefer Peer Reflexive Candidates?  . . . . . . . . . . 73
     B.10. Why Send an Updated Offer? . . . . . . . . . . . . . . . . 73
   Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 74
   Intellectual Property and Copyright Statements . . . . . . . . . . 75













































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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 L and R 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 L and R.
   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
   agent 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 will



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   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:


   L                        R
   -                        -
   STUN request ->             \  L's
             <- STUN response  /  check

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

   Figure 3

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

   At the end of this handshake, both L and R know that they can send
   (and receive) messages end-to-end in both directions.

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 5.2 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,



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   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
   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.

2.6.  Concluding ICE

   ICE checks are performed in a specific sequence, so that high



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   priority pairs are checked first, followed by lower priority ones.
   One way to conclude ICE is to declare victory as soon as a check for
   each component of each media stream completes successfully.  Indeed,
   this is a reasonable algorithm, and details for it are provided
   below.  However, it is possible that packet losses will cause a
   higher priority check to take longer to complete, and allowing ICE to
   run a little longer might produce better results.  More
   fundamentally, however, the prioritization defined by this
   specification may not yield "optimal" results.  As an example, if the
   aim is to select low latency media paths, usage of a relay is a hint
   that latencies may be higher, but it is nothing more than a hint.  An
   actual RTT measurement could be made, and it might demonstrate that a
   pair with lower priority is actually better than one with higher
   priority.

   Consequently, ICE assigns one of the agents in the role of the
   controlling agent, and the other as passive.  The controlling agent
   runs a selection algorithm, through which it can decide when to
   conclude ICE checks, and which pairs get selected.  When a
   controlling agent selects a pair for a particular component of a
   media stream, it generates a check for that pair and includes a flag
   in the check indicating that the pair has been selected.  This will
   cause the passive agent to cease any other checks it has lined up for
   that component, and mark the pair validated by that check as
   "selected".  Once there is a selected pair for each component of a
   media stream, the ICE checks for that media stream are considered to
   be completed, and media can flow in each direction for that stream,
   as shown in Figure 4.  Once all of the media streams are completed,
   the controlling endpoint sends an updated offer if the currently in-
   use candidates don't match the ones it selected.


   L                        R
   -                        -
   STUN request + flag ->      \  L's
             <- STUN response  /  check

   -> RTP Data
                  <- RTP Data

   Figure 4

2.7.  Passive-Only Agents

   ICE requires both sides of a call to support it.  However, certain
   agents, such as those in gateways to the PSTN, media servers,
   conferencing servers, and voicemail servers, are known to not be
   behind a NAT or firewall.  To make it easier for these devices to



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   support ICE, they can operate in a "passive-only" mode (in contrast
   to a "full" mode).  In passive-only mode, they don't need to gather
   candidates and don't act as the controlling agent.  They only need to
   respond to checks, generate triggered checks, and follow the rules
   for sending media and keepalives.


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.

   Component: A component is a single transport address that is used to
      support a media stream.  For media streams based on RTP, there are
      two components per media stream - one for RTP, and one for RTCP.

   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.







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   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.

   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.





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   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 for a media stream that
      have been validated by a successful STUN transaction.

   Controlling Agent: The STUN agent which is responsible for selecting
      the final choice of candidate pairs and signaling them through
      STUN and an updated offer, if needed.

   Passive Agent: The STUN agent which waits for the controlling agent
      to select the final choice of candidate pairs.


4.  Choosing a Mode

   The first step in ICE processing is selection of a mode.  An ICE
   agent can operate in either full mode or passive-only mode.  An agent
   MUST NOT act in passive-only mode unless the following are all true:

   1.  The device definitively knows that it has a public IP address.
       Usage of tests and heuristics like those defined in RFC 3489 [13]
       are not sufficient to make this determination.  Rather, knowledge
       comes from explicit configuration due to known location in the
       network.  Typically, this limits passive-only mode to devices
       like PSTN gateways, conferencing servers, voicemail servers and
       so on.

   2.  The device will only provide one candidate for each component of
       each media stream, matching the values in the m/c-line for each
       media stream.

   Full mode is meant for general purpose endpoints, such as softphones,
   hard-phones, and other devices that may or may not be placed in
   networks with public addresses.


5.  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



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   inclusion in the m/c-line, and then formulate and send the SDP.  Each
   of these steps is described in the subsections below.

5.1.  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 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 the 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
   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.

   Agents implementing passive-only mode MUST NOT gather server
   reflexive or relayed candidates.  Agents implementing full mode
   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.

   The full-mode 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 full-mode agent chooses another such pair (the
   order is inconsequential), and sends a STUN request to the server



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   from that host candidate.  If the full-mode agent is using both
   relayed and server reflexive candidates, this request MUST be a STUN
   Allocate request from the relay usage [12].  If the full-mode 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
   20ms.  Note that this pacing applies only to starting STUN
   transactions with source and destination transport addresses (i.e.,
   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, a full-mode agent eliminates redundant candidates.  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, all agents assign each candidate 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



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   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.

5.2.  Prioritizing Candidates

   The prioritization process results in the assignment of a priority to
   each candidate.  An agent does this by determining a preference for
   each type of candidate (server reflexive, peer 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 = (2^24)*(type preference) +
              (2^8)*(local preference) +
              (2^0)*(256 - component ID)


   The type preference MUST be an integer from 0 to 126 inclusive, and
   represents the preference for the type of the candidate (where the
   types are local, server reflexive, peer reflexive and relayed).  A
   126 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 higher than that of server reflexive candidates.  Note that
   candidates gathered based on the procedures of Section 5.1 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 256
   inclusive.  The local preference MUST be an integer from 0 to 65535
   inclusive.  It represents a preference for the particular interface
   from which the candidate was obtained, in cases where an agent is
   multihomed. 65535 represents the highest preference, and a zero, the
   lowest.  When there is only a single interface, this value SHOULD be
   set to 65535.  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



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   selection of these values.

   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 126, server reflexive candidates have a type
   preference of 100, peer reflexive have a type prefence of 110, 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



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   knowledge of the topological proximity of the relays to itself, it
   can use that to assign higher local preferences to candidates
   obtained from closer relays.

5.3.  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
   full mode agents select relayed candidates to be in-use.  Passive-
   only agents will, naturally, select their only candidates - the host
   candidates - to be in use.

5.4.  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



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   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, and these component IDs MUST
   be between 1 and 256.

   The candidate attribute also includes the priority, which is the
   value determined for the candidate as described in Section 5.2, and
   the foundation, which is the value determined for the candidate as
   described in Section 5.1.  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
   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.




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   If an agent is operating in passive-only mode, it MUST include the
   "a=ice-passive" session level attribute in its offer.  If an agent is
   in full mode, it MUST NOT include this attribute.

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


6.  Receiving the Initial Offer

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

6.1.  Verifying ICE Support

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

   o  There is at least one a=candidate attribute for each media stream
      in the offer 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 11, which describes keepalive procedures.

   In addition, if the offer contains the "a=ice-passive" attribute, and
   the answerer is also passive-only, the agent MUST process the SDP
   based on normal RFC 3264 procedures, as if it didn't support ICE,
   with the exception of Section 11, which describes keepalive



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   procedures.

6.2.  Determining Role

   If the agent is in passive-only mode, it assumes the passive role for
   this session.  If the agent is in full-mode, but its peer is in
   passive-only mode (as indicated by the a=ice-passive attribute in the
   SDP), the agent assumes the controlling role for this session.  If
   the agent and its peer are both in full-mode, the agent which
   generated the offer which started the ICE processing takes on the
   controlling role, and the other takes the passive role.

   Based on this definition, once roles are determined for a session,
   they persist unless ICE is restarted, as discussed below.  A restart
   causes a new selection of roles.

6.3.  Gathering Candidates

   The process for gathering candidates at the answerer is identical to
   the process for the offerer as described in Section 5.1.  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.

6.4.  Prioritizing Candidates

   The process for prioritizing candidates at the answerer is identical
   to the process followed by the offerer, as described in Section 5.2.

6.5.  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 5.3.

6.6.  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 5.4.

6.7.  Forming the Check Lists

   A full-mode agent MUST form the check lists as described in this
   section.  A passive-only agent MAY do so, but there is no need.

   There is one check list per in-use media stream resulting from the
   offer/answer exchange.  A media stream is in-use as long as its port
   is non-zero (which is used in RFC 3264 to reject a media stream).



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   Consequently, a media stream is in-use even if it is marked as
   a=inactive or has a bandwidth value of zero.  Each check list is a
   sequence of STUN connectivity checks that are performed by the agent.
   To form the check list for a media stream, 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 for a media stream
   (called local candidates) and pairs them with the candidates it
   received from its peer (called remote candidates) for that media
   stream.  A local candidate is paired with a remote candidate if and
   only if the two candidates 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.

   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.
   The priority for a pair is computed as:

      pair priority = 2^32*MIN(O-P,A-P) + 2*MAX(O-P,A-P) + (O-P>A-P:1?0)

   Where O-P>A-P:1?0 is an expression whose value is 1 if O-P is greater
   than A-P, and 0 otherwise.  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



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   higher up on the priority list.  The result is called the check list
   for that media stream, 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.

   Each check in the check list is associated with a state.  This state
   is assigned once the check list for each media stream has been
   computed.  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.

   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 in each check list to the
   Frozen state.  Then, it takes the first check in the check list for
   the first media stream (a media stream is the first media stream when
   it is described by the first m-line in the SDP offer and answer), and
   sets its state to Waiting.  It then finds all of the other checks in
   that check list with the same component ID, but different
   foundations, and sets all of their states to Waiting as well.  Once
   this is done, one of the check lists will have some number of checks
   in the Waiting state, and the other check lists will have all of
   their checks in the Frozen state.  A check list with at least one
   check that is not Frozen is called an active check list.

   The check list itself is associated with a state, which captures the
   state of ICE checks for that media stream.  There are two states:







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   Running: In this state, ICE checks are still in progress for this
      media stream.

   Completed: In this state, the controlling agent has signaled that a
      candidate pair has been selected for each component.
      Consequently, no further ICE checks are performed.

   When a check list is first constructed as the consequence of an
   offer/answer exchange, it is placed in the Running state.

   ICE processing across all media streams also has a state associated
   with it.  This state is equal to Running while checks are in
   progress.  The state is Completed when all checks have been
   completed, Rules for transitioning between states are described
   below.

6.8.  Performing Periodic Checks

   An agent performs two types of checks.  The first type are periodic
   checks.  These checks occur periodically for each media stream, and
   involve choosing the highest priority check in the Waiting state from
   each 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.  Full mode agents MUST
   generate periodic checks, and all agents MUST generate triggered
   checks.  This section describes how periodic checks are performed,
   and thus applies only to full mode agents.

   Once the full-mode agent has computed the check lists as described in
   Section 6.7, it sets a timer for each active check list.  The timer
   fires every Ta/N seconds, where N is the number of active check lists
   (initially, there is only one active check list).  Implementations
   MAY set the timer to fire less frequently than this.  Ta is the same
   value used to pace the gathering of candidates, as described in
   Section 5.1.  The first timer for each active check list 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 full-mode agent MUST find the highest
   priority check in that 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 8.1.1.  If
   none of the checks in that 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-



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   Progress.  If there are no checks in either the Waiting or Frozen
   state, then the timer for that check list is stopped.

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


7.  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, determines its role, forms the check list and begins
   performing periodic checks.

7.1.  Verifying ICE Support

   The answerer will proceed with the ICE procedures defined in this
   specification if there is at least one a=candidate attribute for each
   media stream in the answer it just received.  If this condition is
   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 11,
   which describes keepalive procedures.

7.2.  Determining Role

   The offerer follows the same procedures described for the answerer in
   Section 6.2.

7.3.  Forming the Check List

   The offerer follows the same procedures described for the answerer in
   Section 6.7.

7.4.  Performing Periodic Checks

   The offerer follows the same procedures described for the answerer in
   Section 6.8.


8.  Connectivity Checks

   This section describes how connectivity checks are performed.  All
   ICE implementations are required to be compliant to [11], as opposed



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   to the older [13].

8.1.  Client Procedures

8.1.1.  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.

   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.

   A full-mode agent MUST include the PRIORITY attribute in its Binding
   Request.  This attribute MAY be omitted for passive-only agents.  The
   attribute MUST be set equal to the priority that would be assigned,
   based on the algorithm in Section 5.2, 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.

   The controlling agent MAY include the USE-CANDIDATE attribute in the
   Binding Request.  The passive agent MUST NOT include it in its
   Binding Request.  This attribute signals that the controlling agent
   wishes to cease checks for this component, and use the candidate pair
   resulting from the check for this component.  Section 9 provides



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   guidance on determining when to include it.

   If the agent is using Diffserv Codepoint markings [25] in its media
   packets, it SHOULD apply those same markings to its connectivity
   checks.

8.1.2.  Processing the Response

   If the STUN transaction generates an unrecoverable failure response
   or times out, a full-mode agent sets the state of the check to Failed
   (passive-only agents do not maintain the state machinery).  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 destination IP address and port of
   the response match the source IP address and port that the Binding
   Request was sent from.  If these do not match, the processing
   described in the remainder of this section MUST NOT be performed.  In
   addition, a full-mode agent sets the state of the check to Failed.

   If the check succeeds, processing continues.  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.  It is very
   important to note that this validated pair will often not be
   identical to the check itself; in many cases, the local candidate
   (learned through the mapped address in the response) will be
   different than the local candidate the request was sent from.

   Next, the agent computes the priority for the pair based on the
   priority of each candidate, using the algorithm in Section 6.7.  For
   a passive-only agent, the priority of the local candidate is the one
   it signaled for the candidate in its SDP, and the priority of the
   remote candidate is known either from the SDP, or if not there, from
   the value of the PRIORITY attribute in the Binding Request which
   triggered the check that just completed.  For a full-mode agent, if
   the local candidate was not one it signaled in its SDP, the priority
   of the local candidate might additionally be equal to the PRIORITY
   attribute the agent placed in the Binding Request which just
   completed.

   Once the priority of the candidate pair has been computed, the pair
   is added to the valid list for that media stream.  If the response is
   a consequence of a triggered check, and the request which caused the
   triggered check included the USE-CANDIDATE attribute, the candidate



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   pair is additionally marked as selected.  If a full-mode agent had
   included the USE-CANDIDATE attribute in the request that produced the
   success response, the agent marks the candidate pair as selected.

   Next, a full-mode agent updates its ICE states.  The full-mode 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 5.1.  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).

   Next, the full-mode agent changes the state for this check to
   Succeeded.  The full-mode agent sees if the success of this check can
   cause other checks to be unfrozen.  If the check had a component ID
   of one, the full-mode 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
   full-mode agent MUST change the state for all other Frozen checks for
   the first component of different media streams (and thus in different
   check lists) but the same foundation, to Waiting.

8.2.  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 a transport address 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
   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



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   transport address used for STUN processing (namely, generation of the
   XOR-MAPPED-ADDRESS attribute) is the transport address 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.

   If the agent is using Diffserv Codepoint markings [25] in its media
   packets, it SHOULD apply those same markings to its responses to
   Binding Requests.

   If the STUN request resulted in an error response, no further
   processing is performed.

   Otherwise, 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 not be
   amongst the candidates signaled previously from the peer in its SDP).
   The username fragment and password of the peer are readily determined
   from the SDP and from the check that was just received.  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.  Using that username fragment, the agent can check the SDP
   messages received from its peer (there may be more than one in cases
   of forking), and find this username fragment.  The corresponding
   password is then selected.  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 the
   triggered check and the rest of the processing described in the
   remainder of this section.

   The remainder of the processing in this section applies to state
   updates performed by full-mode agents.

   If the source transport address of the request does not match any
   existing remote candidates, it represents a new peer reflexive remote
   candidate.  The full-mode agent gives the candidate 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.  This candidate is then added to the list of remote
   candidates.  However, the full-mode agent does not pair this
   candidate with any local candidates.




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   A full-mode agent knows the priorities for the local and remote
   candidates of the triggered check described above, 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.  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, and the Binding Request just received contained the
   USE-CANDIDATE attribute, it means that the pair resulting from that
   previous check has been selected.  The agent MUST take the candidate
   pair in the valid list that was learned from that previous successful
   check, and mark it as selected.  If there was a check on the check
   list with this same local and remote candidates, and its state was
   Failed, 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, and its state is set to In-Progress.


9.  Concluding ICE

   Concluding ICE involves selection of pairs by the controlling agent,
   updating of state machinery by full-mode agents, and possibly the
   generation of an updated offer by the controlling agent.  Since a
   passive-only agent can never be in the controlling role, the
   processing in this section only applies to full-mode agents.

   The controlling agent can use any algorithm it likes for deciding
   when to select a candidate pair.  However, it MUST eventually include
   a USE-CANDIDATE attribute in a check for each component of each media
   stream.  The following trivial algorithm chooses the first candidate
   pair that validates for each media stream: the controlling agent
   includes the USE-CANDIDATE attribute in every check it sends.

   Once a candidate pair in the Valid list is marked as selected, a
   full-mode agent MUST NOT generate any further periodic checks for
   that component of that media stream, and SHOULD cease any
   retransmissions in progress for checks for that component of that
   media stream.  Once there is at least one candidate pair for each
   component of a media stream that is marked as selected, a full-mode
   agent MUST change the state of processing for its check list to
   Completed.  Once all of the media streams enter the Completed state,
   the controlling agent takes the highest priority candidate pair for
   each component of each media stream which has been marked as
   selected.  If any of those candidate pairs differ from the in-use
   candidates in m/c-lines of the most recent offer/answer exchange, the



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   controlling agent MUST generate an updated offer as described in
   Section 10.


10.  Subsequent Offer/Answer Exchanges

   An agent MAY generate a subsequent offer at any time.  However, the
   rules in Section 9 will cause the controlling agent to send an
   updated offer at the conclusion of ICE processing when ICE has
   selected different candidate pairs from the in-use pairs.

10.1.  Generating the Offer

   An agent MAY change the ice-pwd and/or ice-ufrag for a media stream
   in an offer.  Doing so is a signal to restart ICE processing for that
   media stream.  When an agent restarts ICE for a media stream, it MUST
   NOT include the a=remote-candidates attribute, since the state of the
   media stream would not be Completed at this point.  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.

   An agent MUST restart ICE processing if the offer is being generated
   for the purposes of changing the target of the media stream.  In
   other words, if an agent wants to generated an updated offer which,
   had ICE not been in use, would result in a new value for the
   transport address in the m/c-line, the agent MUST restart ICE for
   that media stream.  This implies that setting the IP address in the c
   line to 0.0.0.0 will cause an ICE restart.  Consequently, ICE
   implementations SHOULD NOT utilize this mechanism for call hold, and
   instead use a=inactive as described in [4]

   If an agent removes a media stream by setting its port to zero, it
   MUST NOT include any candidate attributes for that media stream.

   When a full-mode agent generates an updated offer, the set of
   candidate attributes to include for each media stream depend on the
   state of ICE processing for that media stream.  If the processing for
   that media stream is in the Completed state, a full-mode agent MUST
   include a candidate attribute for the local candidate of each pair
   that has been chosen for use by ICE for that media stream.  A pair is
   chosen if it is the highest priority selected pair in the valid list
   for a component of that media stream.  A full-mode agent SHOULD NOT
   include any other candidate attributes for that media stream.  If ICE
   processing for a media stream is in the Running state, the agent MUST
   include all current candidates (including peer reflexive candidates
   learned through ICE processing) for that media stream.  It MAY



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   include candidates it did not offer previously, but which it has
   gathered since the last offer/answer exchange.  If a media stream is
   new or ICE checks are restarting for that stream, a full-mode agent
   includes the set of candidates it wishes to utilize.  This MAY
   include some, none, or all of the previous candidates for that stream
   in the case of a restart, and MAY include a totally new set of
   candidates gathered as described in Section 5.1.

   A passive-only agent includes its one and only candidate for each
   component of each media stream in an a=candidate attribute in any
   subsequent offer.

   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 8.1.2.  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 for full-mode agents also depends on the
   state of ICE processing.  If ICE processing for a media stream is in
   the Completed state, the m/c-line MUST use the local candidate from
   the highest priority selected pair in the valid list for each
   component of that media stream.  If ICE processing is in the Running
   state, a full-mode agent SHOULD populate the m/c-line for that media
   stream based on the considerations in Section 5.3.

   A passive agent populates the m/c-lines with its one and only one
   candidate for each component of each media stream.

   In addition, the controlling agent MUST include the a=remote-
   candidates attribute for each media stream that is in the Completed
   state.  The attribute contains the remote candidates from the highest
   priority selected pair in the valid list for each component of that
   media stream.

   An agent MUST NOT change its mode (passive-only or full) by adding or
   removing the a=ice-passive attribute from an updated offer, unless
   ICE processing is being restarted for all media streams in the offer.

   Note that an agent can add a new media stream at any time, even if
   ICE has long finished for the existing media streams.  Based on the
   rules described here, checks will begin for this new stream as if it
   was in an initial offer.

10.2.  Receiving the Offer and Generating an Answer

   When receiving a subsequent offer within an existing session, an



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   agent MUST re-apply the verification procedures in Section 6.1
   without regard to the results of verification from any previous
   offer/answer exchanges.  Indeed, it is possible that a previous
   offer/answer exchange resulted in ICE not being used, but it is used
   as a consequence of a subsequent exchange.

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

   The rules for inclusion of candidate attributes in an answer are
   identical to the rules followed by the offerer as described in
   Section 10.1.

   However, the rules for setting the contents of the m/c-line are
   different.  For a full-mode agent, processing of the offer depends on
   the presence or absence of the a=remote-candidates attribute in a
   media stream.  If present, it means that the offerer (acting as the
   controlling agent) 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 full-mode agent MUST populate the m/c-line with the
   candidates from the a=remote-candidates attribute.

   If the offer did not contain the a=remote-candidates attribute, or
   the agent is a passive-only agent, the agent follows the same
   procedures for populating the m/c-line as described for the offerer
   in Section 10.1.

   An agent MUST NOT include the a=remote-candidates attribute in an
   answer.  An agent MUST NOT change the a=ice-ufrag or a=ice-pwd
   attributes in an answer relative to the last SDP it provided.  Such a
   change can only take place in an offer.  However, if the offer
   contained a change in the a=ice-ufrag or a=ice-pwd attributes
   compared to the previous SDP from the peer, it is a signal that ICE
   is restarting for this media stream.

   An agent MUST NOT change its mode from a previous answer unless,
   based on the offer, ICE procedures are being restarted for all media
   streams in the offer.  In that case, it MAY change its mode.

10.3.  Updating the Check and Valid Lists

   Once the subsequent offer/answer exchange has completed, each agent
   needs to determine the impact, if any, on the Check and Valid lists.
   Unless there is an ICE restart, an offer/answer exchange has no



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   impact on the state of ICE processing for each media stream; that is
   determined entirely by the checks themselves.  An updated offer/
   answer exchange can impact the transmission rules for media, as
   described in Section 12.1.

   If the offer had a change in the ice-ufrag and/or ice-pwd for a media
   stream, the agent MUST start a new Valid list for that media stream.
   However, it retains the old Valid list for the purposes of sending
   media until ICE processing completes, at which point the old Valid
   list is discarded and the new one is utilized to determine media and
   keepalive targets.  A full-mode agent MUST also flush the check list
   for the affected media streams, and then recompute the check list and
   its states as described in Section 6.7.

   If the subsequent offer added a new media stream, a full-mode agent
   MUST create a new check list for it (and an empty Valid list to start
   of course), as described in Section 6.7.

   If the subsequent offer removed a media stream, or an answer rejected
   an offered media stream, an agent MUST flush the Valid list for that
   media stream.  It MUST terminate any STUN transactions in progress
   for that media stream.  A full-mode agent MUST remove the check list
   for that media stream and cancel any pending periodic checks for it.

   If a media stream existed previously, and remains after the offer/
   answer exchange, the agent MUST NOT modify the Valid list for that
   media stream.  However, if a full-mode agent is in the Running state
   for that media stream, the check list is updated.  To do that, the
   full-mode agent recomputes the check lists using the procedures
   described in Section 6.7.  If a check on the new check lists was also
   on the previous check lists, and its state was Waiting, In-Progress,
   Succeeded or Failed, its state is copied over.  If a check on the new
   check lists does not have a state (because its a new check on an
   existing check list, or a check on a new check list, or the check was
   on an old check list but its state was not copied over) its state is
   set to Frozen.

   If none of the check lists are active (meaning that the checks in
   each check list are Frozen), the full-mode agent sets the first check
   in the check list for the first media stream to Waiting, and then
   sets the state of all other checks in that check list for the same
   component ID and with the same foundation to Waiting as well.

   Next, the full-mode agent goes through each 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 in the same check
   list with the same foundation whose component IDs are not one, have
   their state set to Waiting.  If, for a particular check list, there



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   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 (and thus in different check
   lists) with the same foundation to Waiting.


11.  Keepalives

   STUN connectivity checks are also used to keep NAT bindings open once
   ICE processing has completed.  This is accomplished by periodically
   generating a check on the candidate pair currently being used for
   media.

   Specifically, once ICE processing allows media to begin flowing, as
   described in Section 12.1, 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 creates a connectivity check for
   each component of that media stream.  This check is sent on the
   candidate pair currently being used to send media, as described in
   Section 12.1.

   This specification makes no recommendations on the behaviors should
   the keepalive itself fail.  However, an agent SHOULD NOT blindly
   restart ICE processing for that stream; if the keepalive was lost due
   to congestion, the ICE restart will only aggravate the problem.

   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, and regardless of
   the presence or value of the bandwidth attribute.  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 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



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   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
   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.


12.  Media Handling

12.1.  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 the state of a media stream is Running, there is no old Valid list
   for that media stream (which would be due to an ICE restart), a full-
   mode agent MUST NOT send media.  For passive-only agents, which do
   not retain states about ICE processing, it MUST NOT send media until
   there is a selected candidate pair in either the old or new Valid
   list for each component of the media stream.

   When an agent sends media, it MUST send it using the highest priority
   selected pair for each component in either the old Valid list for a
   media stream (if it exists), else the new Valid list for that media
   stream.  In several cases, this will not be the same candidate pairs
   present in the m/c-line.  When ICE first completes, if the selected
   pairs aren't a match for the m/c-line, an updated offer/answer
   exchange will take place to remedy this disparity.  However, until
   that update offer arrives, there will not be a match.  Furthermore,
   in very unusual cases, the m/c-lines in the updated offer/answer will
   not be a match.

   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



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   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.

12.2.  Receiving Media

   ICE implementations MUST be prepared to receive media on any
   candidates provided in the most recent offer/answer exchange.

   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 transport addresses 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 transport address, but the media packets
   come from the same peer agent, this SHOULD NOT be treated as an SSRC
   collision.


13.  Usage with SIP

13.1.  Latency Guidelines

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



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   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 and timers described
   in [9].  Note, however, that if PRACK is not used, the rules for when
   an agent can send an updated offer or answer do not change from those
   specified in RFC 3262, even though the provisional response has been
   delivered "reliably".  Specifically, if the offer contained an
   INVITE, the same answer appears in all of the 1xx and in the 2xx
   response to the INVITE.  Only after that 2xx has been sent can an
   updated offer/answer exchange occur.

   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 [24] 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.

   As discussed in Section 16, 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.

13.2.  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



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   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.

13.3.  Interactions with Preconditions

   Quality of Service (QoS) preconditions, which are defined in RFC 3312
   [6] and RFC 4032 [7], apply only to the transport addresses listed in
   the m/c lines in an offer/answer.  If ICE changes the transport
   address where media is received, this change is reflected in the m/c
   lines of a new offer/answer.  As such, it appears like any other re-
   INVITE would, and is fully treated in RFC 3312 and 4032, which 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.  Note
   that the procedures described in Section 13.1 describe their own type
   of "preconditions", albeit with less functionality than those
   provided by the explicit preconditions in [26].

13.4.  Interactions with Third Party Call Control

   ICE works with Flows I, III 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 II is fundamentally incompatible
   with ICE; each agent will believe itself to be the answerer and thus
   never generate a re-INVITE.

   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 restart ICE for each media stream and go
   through the process of gathering new candidates.  Furthermore, that
   list of candidates SHOULD include the ones currently in-use.






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14.  Grammar

   This specification defines five new SDP attributes - the "candidate",
   "remote-candidates", "ice-passive", "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
   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.



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   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
   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 5.4.

   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-passive" candidate is:


   ice-passive           = "ice-passive"

   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



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   is effectively a default that applies to all media streams, unless
   overriden by a media-level value.


15.  Example

   Two agents, L and R, are using ICE.  Both are full-mode ICE
   implementations.  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 only the Binding
   Discovery usage; relays are not used in this example.  Agent L is
   behind a NAT, and agent R is on the public Internet.  The NAT has an
   endpoint 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 the binding discovery 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 |              |              |



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



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


   Figure 10

   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 126 for the host candidate and 100
   for the server reflexive.  The local preference is 65535.  Based on
   this, the priority of the host candidate is 2130706178 and for the
   server reflexive candidate is 1694498562.  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):








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       v=0
       o=jdoe 2890844526 2890842807 IN IP4 $L-PRIV-1.IP
       s=
       c=IN IP4 $NAT-PUB-1.IP
       t=0 0
       a=ice-pwd:asd88fgpdd777uzjYhagZg
       a=ice-ufrag:8hhY
       m=audio $NAT-PUB-1.PORT RTP/AVP 0
       a=rtpmap:0 PCMU/8000
       a=candidate:1 1 UDP 2130706178 $L-PRIV-1.IP $L-PRIV-1.PORT typ local
       a=candidate:2 1 UDP 1694498562 $NAT-PUB-1.IP $NAT-PUB-1.PORT typ srflx raddr
   $L-PRIV-1.IP rport $L-PRIV-1.PORT

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


       v=0
       o=jdoe 2890844526 2890842807 IN IP4 10.0.1.1
       s=
       c=IN IP4 192.0.2.3
       t=0 0
       a=ice-pwd:asd88fgpdd777uzjYhagZg
       a=ice-ufrag:8hhY
       m=audio 45664 RTP/AVP 0
       a=rtpmap:0 PCMU/8000
       a=candidate:1 1 UDP 2130706178 10.0.1.1 8998 typ local
       a=candidate:2 1 UDP 1694498562 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 2130706178.  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



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       a=rtpmap:0 PCMU/8000
       a=candidate:1 1 UDP 2130706178 $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
       a=rtpmap:0 PCMU/8000
       a=candidate:1 1 UDP 2130706178 192.0.2.1 3478 typ local

   Since neither side indicated that they are passive-only, the agent
   which sent the offer that began ICE processing (agent L) becomes the
   controlling agent.

   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 4.57566E+18 (note
   that an implementation would represent this as a 64 bit integer so as
   not to lose precision).  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 4.57566E+18, and the
   second has a local candidate of $R_PUB_1 and remote candidate of
   $NAT_PUB_1 and priority 3.63891E+18.

   Agent R begins its connectivity check (message 9) for the first pair
   (between the two host candidates).  Since R is the passive agent for
   this session, the check omits the USE-CANDIDATE attribute.  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).  It implements the default
   algorithm for candidate selection, and thus includes a USE-CANDIDATE
   attribute in this check.  Since the check succeeds, agent L creates a
   new pair, whose 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.  In addition, it is marked as
   selected since the Binding Request contained the USE-CANDIDATE



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   attribute.  Since there is a selected candidate in the Valid list for
   the one component of this media stream, ICE processing for this
   stream moves into the Completed state.  Agent L can now send media if
   it so chooses.

   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 14-17) 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 for that media stream.  Since
   the check was generated in the reverse direction of a check that
   contained the USE-CANDIDATE attribute, the candidate pair is marked
   as selected.  Consequently, processing for this stream moves into the
   Completed state, and agent R can also send media.


16.  Security Considerations

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

16.1.  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.

   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.





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   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
   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



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   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.
   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.





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16.2.  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.

16.3.  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.

16.4.  Insider Attacks

   In addition to attacks where the attacker is a third party trying to



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   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.

16.4.1.  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.

16.4.2.  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
   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.




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      OPEN ISSUE: Need better remediation for this.


17.  Definition of Connectivity Check Usage

   STUN [11] requires that new usages provide a specific set of
   information as part of their formal definition.  This section meets
   the requirements spelled out there.

17.1.  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.

   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].

17.2.  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 transport address of the STUN server.  Note that the STUN server
   is a logical entity, and is not a physically distinct server in this
   usage.

17.3.  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.

17.4.  New Requests or Indications

   This usage does not define any new message types.

17.5.  New Attributes

   This usage defines two new attributes, PRIORITY and USE-CANDIDATE.

   The PRIORITY attribute indicates the priority that is to be
   associated with a peer reflexive candidate, should one be discovered



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   by this check.  It is a 32 bit unsigned integer, and has an attribute
   type of 0x0024.

   The USE-CANDIDATE attribute indicates that the candidate pair
   resulting from this check should be used for transmission of media.
   The attribute has no content (the Length field of the attribute is
   zero); it serves as a flag.  It has an attribute type of 0x0025.

17.6.  New Error Response Codes

   This usage does not define any new error response codes.

17.7.  Client Procedures

   Client procedures are defined in Section 8.1.

17.8.  Server Procedures

   Server procedures are defined in Section 8.2.

17.9.  Security Considerations for Connectivity Check

   Security considerations for the connectivity check are discussed in
   Section 16.


18.  IANA Considerations

   This specification registers new SDP attributes and new STUN
   attributes.

18.1.  SDP Attributes

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

18.1.1.  candidate Attribute

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

   Attribute Name: candidate

   Long Form: candidate







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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 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 14 of RFC XXXX [Note to RFC-ed:
      please replace XXXX with the RFC number of this specification].

18.1.2.  remote-candidates Attribute

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

   Attribute Name: remote-candidates

   Long Form: remote-candidates

   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 14 of RFC XXXX [Note to RFC-ed:
      please replace XXXX with the RFC number of this specification].

18.1.3.  ice-passive Attribute

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

   Attribute Name: ice-passive

   Long Form: ice-passive

   Type of Attribute: session level







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

   Purpose: This attribute is used with Interactive Connectivity
      Establishment (ICE), and indicates that an agent can only operate
      in ICE's passive mode.

   Appropriate Values: See Section 14 of RFC XXXX [Note to RFC-ed:
      please replace XXXX with the RFC number of this specification].

18.1.4.  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 14 of RFC XXXX [Note to RFC-ed:
      please replace XXXX with the RFC number of this specification].

18.1.5.  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.

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





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   Appropriate Values: See Section 14 of RFC XXXX [Note to RFC-ed:
      please replace XXXX with the RFC number of this specification].

18.2.  STUN Attributes

   This section registers two new STUN attributes per the procedures in
   [11].


      0x0024 PRIORITY
      0x0025 USE-CANDIDATE


19.  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.

19.1.  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).






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

19.2.  Exit Strategy

   From RFC 3424, any UNSAF proposal must provide:

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

   ICE itself doesn't easily get phased out.  However, it is useful even
   in a globally connected Internet, to serve as a means for detecting
   whether a router failure has temporarily disrupted connectivity, for
   example.  ICE also helps prevent certain security attacks which have
   nothing to do with NAT.  However, what ICE does is help phase out
   other UNSAF mechanisms.  ICE effectively selects amongst those
   mechanisms, prioritizing ones that are better, and deprioritizing
   ones that are worse.  Local IPv6 addresses can be preferred.  As NATs
   begin to dissipate as IPv6 is introduced, 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.

19.3.  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



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   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
   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.

19.4.  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.

19.5.  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



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   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.

   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.


20.  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.


21.  References

21.1.  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



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         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.

   [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
         Underneath NAT (STUN)", draft-ietf-behave-turn-02 (work in
         progress), October 2006.

21.2.  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.



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   [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.

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

   [25]  Blake, S., Black, D., Carlson, M., Davies, E., Wang, Z., and W.
         Weiss, "An Architecture for Differentiated Services", RFC 2475,
         December 1998.

   [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-08 (work in progress),
         October 2006.

   [31]  Jennings, C. and R. Mahy, "Managing Client Initiated



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         Connections in the Session Initiation Protocol  (SIP)",
         draft-ietf-sip-outbound-04 (work in progress), June 2006.


Appendix A.  Passive-Only ICE

   ICE allows for two modes of operation in an agent - passive-only and
   full.  Passive-only mode is applicable to entities like PSTN
   gateways, media servers and conferencing servers that are always
   publicly connected and are not behind a firewall or NAT.

   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 16 for a more
   complete discussion on this attack.

   Because of the benefits of implementing ICE in endpoints that don't
   themselves require NAT traversal, ICE reduces the cost of
   implementation by allowing them to run in passive-only mode.  The
   rules for passive-only endpoints are described throughout the



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   specification.  What follows is an informative summary to give
   implementors a good sense of what is required:

   o  A passive-only agent obtains candidates just from its host
      interfaces, just like it would do without ICE.  It doesn't need to
      implement the STUN Binding Discovery usage [11] or the relay usage
      [12] to gather server reflexive or relayed candidates.  It needs
      to assign its candidates a foundation ID; however it can use the
      IP address itself as the foundation ID.

   o  The prioritization in Section 5.2 is trivially accomplished for
      passive-only agents utilizing RTP.  The type preference is set to
      126 and the local preference to 65535, resulting in a priority of
      2130706431 for RTP and 2130706430 for RTCP.

   o  In use candidates Section 5.3 are trivially selected - they are
      equal to the host candidates.

   o  A passive-only agent will need to select a username and password
      for each session.  An SDP offer (and answer) constructed by an
      RTP-based audio-only agent will contain two a=candidate lines,
      which mirror the RTP and RTCP transport addresses in the m/c-line.
      Each a=candidate line contains the priority and foundation
      computed above, and indicates that it is a host candidate
      Section 5.4.

   o  A passive-only agent doesn't need to construct check lists or
      maintain the states of ICE processing Section 6.7.  It only needs
      to maintain the valid list, which are the list of checks it has
      completed.  Once it places its candidate lines into an offer or
      answer, it waits for the receipt of checks.

   o  A passive-only agent doesn't generate periodic checks.  It only
      generates triggered checks, which are checks that are created as a
      consequence of receiving a check.  A passive-only agent does need
      to be able to respond to a STUN check it receives.

   o  A passive-only agent does not add the PRIORITY or USE-CANDIDATE
      attributes to its STUN requests.  Its STUN requests only contain
      the USERNAME and MESSAGE-INTEGRITY attributes, set based on the
      username fragments and passwords exchanged in the offer and
      answer.

   o  Handling of subsequent offer/answer exchanges is done trivially -
      the passive-only agent includes its one and only candidate for
      each component of each media stream in an a=candidate attribute
      and in the m/c-line, just like an initial offer or answer.




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   o  A passive-only agent never needs to compute or include the
      a=remote-candidates attribute in any offer it sends.  It never
      needs to generate an updated offer as a consequence of ICE
      processing.

   o  A passive-only agent sends media once a selected candidate pair
      appears in its Valid list for that media stream.


Appendix B.  Design Motivations

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

B.1.  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



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   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
   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.

B.2.  Candidates with Multiple Bases

   Section 5.1 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.

B.3.  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.

B.4.  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.

B.5.  The Candidate Pair Sequence Number Formula

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

      pair priority = 2^32*MIN(O-P,A-P) + 2*MAX(O-P,A-P) + (O-P>A-P:1?0)

   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 decreasing 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 offerers priority is used as the tie breaker in the
   last part of the expression.  The factor of 2*32 is used since the
   priority of a single candidate is always less than 2*32, resulting in
   the pair priority being a "concatenation" of the two component
   priorities.  This creates the desired sorting property.

B.6.  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.

B.7.  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.
   Note, however, that agent B will not send media until it has received
   this STUN response.









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          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

B.8.  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
   may cause media transmission to cease sufficiently long for NAT
   bindings to time out.

   For these reasons, the media packets themselves cannot be relied



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   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.

B.9.  Why Prefer Peer Reflexive Candidates?

   Section 5.2 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 16.  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.

B.10.  Why Send an Updated Offer?

   Section 12.1 describes rules for sending media.  Both agents can send
   media once ICE checks complete, without waiting for an updated offer.
   Indeed, the only purpose of the updated offer is to "correct" the
   m/c-line so that it matches where media is being sent, based on ICE
   procedures.

   This begs the 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.












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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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