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MMUSIC                                                      J. Rosenberg
Internet-Draft                                                     Cisco
Obsoletes: 4091 (if approved)                             March 26, 2007
Intended status: Standards Track
Expires: September 27, 2007


Interactive Connectivity Establishment (ICE): A Methodology for Network
     Address Translator (NAT) Traversal for Offer/Answer Protocols
                        draft-ietf-mmusic-ice-15

Status of this Memo

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   This Internet-Draft will expire on September 27, 2007.

Copyright Notice

   Copyright (C) The IETF Trust (2007).

Abstract

   This document describes a protocol for Network Address Translator
   (NAT) traversal for multimedia sessions established with the offer/
   answer model.  This protocol is called Interactive Connectivity
   Establishment (ICE).  ICE makes use of the Session Traversal
   Utilities for NAT (STUN) protocol, applying its binding discovery and
   relay usages, in addition to defining a new usage for checking



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   connectivity between peers.  ICE can be used by any protocol
   utilizing the offer/answer model, such as the Session Initiation
   Protocol (SIP).


Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   6
   2.  Overview of ICE . . . . . . . . . . . . . . . . . . . . . . .   7
     2.1.  Gathering Candidate Addresses . . . . . . . . . . . . . .   9
     2.2.  Connectivity Checks . . . . . . . . . . . . . . . . . . .  11
     2.3.  Sorting Candidates  . . . . . . . . . . . . . . . . . . .  12
     2.4.  Frozen Candidates . . . . . . . . . . . . . . . . . . . .  13
     2.5.  Security for Checks . . . . . . . . . . . . . . . . . . .  14
     2.6.  Concluding ICE  . . . . . . . . . . . . . . . . . . . . .  14
     2.7.  Lite Implementations  . . . . . . . . . . . . . . . . . .  16
   3.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .  16
   4.  Sending the Initial Offer . . . . . . . . . . . . . . . . . .  19
     4.1.  Full Implementation Requirements  . . . . . . . . . . . .  19
       4.1.1.  Gathering Candidates  . . . . . . . . . . . . . . . .  19
         4.1.1.1.  Host Candidates . . . . . . . . . . . . . . . . .  19
         4.1.1.2.  Server Reflexive and Relayed Candidates . . . . .  20
         4.1.1.3.  Eliminating Redundant Candidates  . . . . . . . .  21
         4.1.1.4.  Computing Foundations . . . . . . . . . . . . . .  21
         4.1.1.5.  Keeping Candidates Alive  . . . . . . . . . . . .  22
       4.1.2.  Prioritizing Candidates . . . . . . . . . . . . . . .  22
         4.1.2.1.  Recommended Formula . . . . . . . . . . . . . . .  22
         4.1.2.2.  Guidelines for Choosing Type and Local
                   Preferences . . . . . . . . . . . . . . . . . . .  23
       4.1.3.  Choosing Default Candidates . . . . . . . . . . . . .  24
     4.2.  Lite Implementation . . . . . . . . . . . . . . . . . . .  25
     4.3.  Encoding the SDP  . . . . . . . . . . . . . . . . . . . .  25
   5.  Receiving the Initial Offer . . . . . . . . . . . . . . . . .  27
     5.1.  Verifying ICE Support . . . . . . . . . . . . . . . . . .  27
     5.2.  Determining Role  . . . . . . . . . . . . . . . . . . . .  28
     5.3.  Gathering Candidates  . . . . . . . . . . . . . . . . . .  28
     5.4.  Prioritizing Candidates . . . . . . . . . . . . . . . . .  29
     5.5.  Choosing Default Candidates . . . . . . . . . . . . . . .  29
     5.6.  Encoding the SDP  . . . . . . . . . . . . . . . . . . . .  29
     5.7.  Forming the Check Lists . . . . . . . . . . . . . . . . .  29
       5.7.1.  Forming Candidate Pairs . . . . . . . . . . . . . . .  29
       5.7.2.  Computing Pair Priority and Ordering Pairs  . . . . .  32
       5.7.3.  Pruning the Pairs . . . . . . . . . . . . . . . . . .  32
       5.7.4.  Computing States  . . . . . . . . . . . . . . . . . .  32
     5.8.  Performing Periodic Checks  . . . . . . . . . . . . . . .  35
   6.  Receipt of the Initial Answer . . . . . . . . . . . . . . . .  37
     6.1.  Verifying ICE Support . . . . . . . . . . . . . . . . . .  37
     6.2.  Determining Role  . . . . . . . . . . . . . . . . . . . .  37



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     6.3.  Forming the Check List  . . . . . . . . . . . . . . . . .  37
     6.4.  Performing Periodic Checks  . . . . . . . . . . . . . . .  37
   7.  Performing Connectivity Checks  . . . . . . . . . . . . . . .  37
     7.1.  Client Procedures . . . . . . . . . . . . . . . . . . . .  38
       7.1.1.  Sending the Request . . . . . . . . . . . . . . . . .  38
         7.1.1.1.  PRIORITY and USE-CANDIDATE  . . . . . . . . . . .  38
         7.1.1.2.  ICE-CONTROLLED and ICE-CONTROLLING  . . . . . . .  38
         7.1.1.3.  Forming Credentials . . . . . . . . . . . . . . .  39
         7.1.1.4.  DiffServ Treatment  . . . . . . . . . . . . . . .  39
       7.1.2.  Processing the Response . . . . . . . . . . . . . . .  39
         7.1.2.1.  Failure Cases . . . . . . . . . . . . . . . . . .  39
         7.1.2.2.  Success Cases . . . . . . . . . . . . . . . . . .  40
           7.1.2.2.1.  Discovering Peer Reflexive Candidates . . . .  40
           7.1.2.2.2.  Updating Pair States  . . . . . . . . . . . .  41
           7.1.2.2.3.  Constructing a Valid Pair . . . . . . . . . .  42
           7.1.2.2.4.  Updating the Nominated Flag . . . . . . . . .  42
         7.1.2.3.  Check List and Timer State Updates  . . . . . . .  43
     7.2.  Server Procedures . . . . . . . . . . . . . . . . . . . .  43
       7.2.1.  Additional Procedures for Full Implementations  . . .  44
         7.2.1.1.  Detecting and Repairing Role Conflicts  . . . . .  44
         7.2.1.2.  Computing Mapped Address  . . . . . . . . . . . .  45
         7.2.1.3.  Learning Peer Reflexive Candidates  . . . . . . .  45
         7.2.1.4.  Triggered Checks  . . . . . . . . . . . . . . . .  46
         7.2.1.5.  Updating the Nominated Flag . . . . . . . . . . .  47
       7.2.2.  Additional Procedures for Lite Implementations  . . .  47
   8.  Concluding ICE Processing . . . . . . . . . . . . . . . . . .  47
     8.1.  Nominating Pairs  . . . . . . . . . . . . . . . . . . . .  48
       8.1.1.  Regular Nomination  . . . . . . . . . . . . . . . . .  48
       8.1.2.  Aggressive Nomination . . . . . . . . . . . . . . . .  49
     8.2.  Updating States . . . . . . . . . . . . . . . . . . . . .  49
   9.  Subsequent Offer/Answer Exchanges . . . . . . . . . . . . . .  50
     9.1.  Generating the Offer  . . . . . . . . . . . . . . . . . .  51
       9.1.1.  Procedures for All Implementations  . . . . . . . . .  51
         9.1.1.1.  ICE Restarts  . . . . . . . . . . . . . . . . . .  51
         9.1.1.2.  Removing a Media Stream . . . . . . . . . . . . .  52
         9.1.1.3.  Adding a Media Stream . . . . . . . . . . . . . .  52
       9.1.2.  Procedures for Full Implementations . . . . . . . . .  52
         9.1.2.1.  Existing Media Streams with ICE Running . . . . .  52
         9.1.2.2.  Existing Media Streams with ICE Completed . . . .  53
       9.1.3.  Procedures for Lite Implementations . . . . . . . . .  53
     9.2.  Receiving the Offer and Generating an Answer  . . . . . .  53
       9.2.1.  Procedures for All Implementations  . . . . . . . . .  53
         9.2.1.1.  Detecting ICE Restart . . . . . . . . . . . . . .  54
         9.2.1.2.  New Media Stream  . . . . . . . . . . . . . . . .  54
         9.2.1.3.  Removed Media Stream  . . . . . . . . . . . . . .  54
       9.2.2.  Procedures for Full Implementations . . . . . . . . .  54
         9.2.2.1.  Existing Media Streams with ICE Running and no
                   remote-candidates . . . . . . . . . . . . . . . .  55



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         9.2.2.2.  Existing Media Streams with ICE Completed and
                   no remote-candidates  . . . . . . . . . . . . . .  55
         9.2.2.3.  Existing Media Streams and remote-candidates  . .  55
       9.2.3.  Procedures for Lite Implementations . . . . . . . . .  56
     9.3.  Updating the Check and Valid Lists  . . . . . . . . . . .  56
       9.3.1.  Procedures for Full Implementations . . . . . . . . .  56
         9.3.1.1.  ICE Restarts  . . . . . . . . . . . . . . . . . .  56
         9.3.1.2.  New Media Stream  . . . . . . . . . . . . . . . .  56
         9.3.1.3.  Removed Media Stream  . . . . . . . . . . . . . .  56
         9.3.1.4.  ICE Continuing for Existing Media Stream  . . . .  57
       9.3.2.  Procedures for Lite Implementations . . . . . . . . .  57
   10. Keepalives  . . . . . . . . . . . . . . . . . . . . . . . . .  57
   11. Media Handling  . . . . . . . . . . . . . . . . . . . . . . .  58
     11.1. Sending Media . . . . . . . . . . . . . . . . . . . . . .  58
       11.1.1. Procedures for Full Implementations . . . . . . . . .  59
       11.1.2. Procedures for Lite Implementations . . . . . . . . .  59
       11.1.3. Procedures for All Implementations  . . . . . . . . .  60
     11.2. Receiving Media . . . . . . . . . . . . . . . . . . . . .  60
   12. Usage with SIP  . . . . . . . . . . . . . . . . . . . . . . .  60
     12.1. Latency Guidelines  . . . . . . . . . . . . . . . . . . .  60
       12.1.1. Offer in INVITE . . . . . . . . . . . . . . . . . . .  61
       12.1.2. Offer in Response . . . . . . . . . . . . . . . . . .  62
     12.2. SIP Option Tags and Media Feature Tags  . . . . . . . . .  63
     12.3. Interactions with Forking . . . . . . . . . . . . . . . .  63
     12.4. Interactions with Preconditions . . . . . . . . . . . . .  63
     12.5. Interactions with Third Party Call Control  . . . . . . .  63
   13. Relationship with ANAT  . . . . . . . . . . . . . . . . . . .  64
   14. Extensibility Considerations  . . . . . . . . . . . . . . . .  64
   15. Grammar . . . . . . . . . . . . . . . . . . . . . . . . . . .  65
     15.1. "candidate" Attribute . . . . . . . . . . . . . . . . . .  65
     15.2. "remote-candidates" Attribute . . . . . . . . . . . . . .  67
     15.3. "ice-lite" and "ice-mismatch" Attributes  . . . . . . . .  68
     15.4. "ice-ufrag" and "ice-pwd" Attributes  . . . . . . . . . .  68
     15.5. "ice-options> Attribute . . . . . . . . . . . . . . . . .  69
   16. Example . . . . . . . . . . . . . . . . . . . . . . . . . . .  69
   17. Security Considerations . . . . . . . . . . . . . . . . . . .  76
     17.1. Attacks on Connectivity Checks  . . . . . . . . . . . . .  76
     17.2. Attacks on Address Gathering  . . . . . . . . . . . . . .  78
     17.3. Attacks on the Offer/Answer Exchanges . . . . . . . . . .  79
     17.4. Insider Attacks . . . . . . . . . . . . . . . . . . . . .  79
       17.4.1. The Voice Hammer Attack . . . . . . . . . . . . . . .  80
       17.4.2. STUN Amplification Attack . . . . . . . . . . . . . .  80
     17.5. Interactions with Application Layer Gateways and SIP  . .  81
   18. Definition of Connectivity Check Usage  . . . . . . . . . . .  81
     18.1. Applicability . . . . . . . . . . . . . . . . . . . . . .  82
     18.2. Client Discovery of Server  . . . . . . . . . . . . . . .  82
     18.3. Server Determination of Usage . . . . . . . . . . . . . .  82
     18.4. New Requests or Indications . . . . . . . . . . . . . . .  82



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     18.5. New Attributes  . . . . . . . . . . . . . . . . . . . . .  82
     18.6. New Error Response Codes  . . . . . . . . . . . . . . . .  83
     18.7. Client Procedures . . . . . . . . . . . . . . . . . . . .  83
     18.8. Server Procedures . . . . . . . . . . . . . . . . . . . .  83
     18.9. Security Considerations for Connectivity Check  . . . . .  83
   19. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  83
     19.1. SDP Attributes  . . . . . . . . . . . . . . . . . . . . .  84
       19.1.1. candidate Attribute . . . . . . . . . . . . . . . . .  84
       19.1.2. remote-candidates Attribute . . . . . . . . . . . . .  84
       19.1.3. ice-lite Attribute  . . . . . . . . . . . . . . . . .  85
       19.1.4. ice-mismatch Attribute  . . . . . . . . . . . . . . .  85
       19.1.5. ice-pwd Attribute . . . . . . . . . . . . . . . . . .  86
       19.1.6. ice-ufrag Attribute . . . . . . . . . . . . . . . . .  86
       19.1.7. ice-options Attribute . . . . . . . . . . . . . . . .  86
     19.2. STUN Attributes . . . . . . . . . . . . . . . . . . . . .  87
     19.3. STUN Error Responses  . . . . . . . . . . . . . . . . . .  87
   20. IAB Considerations  . . . . . . . . . . . . . . . . . . . . .  87
     20.1. Problem Definition  . . . . . . . . . . . . . . . . . . .  88
     20.2. Exit Strategy . . . . . . . . . . . . . . . . . . . . . .  88
     20.3. Brittleness Introduced by ICE . . . . . . . . . . . . . .  89
     20.4. Requirements for a Long Term Solution . . . . . . . . . .  89
     20.5. Issues with Existing NAPT Boxes . . . . . . . . . . . . .  90
   21. Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  90
   22. References  . . . . . . . . . . . . . . . . . . . . . . . . .  91
     22.1. Normative References  . . . . . . . . . . . . . . . . . .  91
     22.2. Informative References  . . . . . . . . . . . . . . . . .  92
   Appendix A.  Lite and Full Implementations  . . . . . . . . . . .  93
   Appendix B.  Design Motivations . . . . . . . . . . . . . . . . .  94
     B.1.  Pacing of STUN Transactions . . . . . . . . . . . . . . .  94
     B.2.  Candidates with Multiple Bases  . . . . . . . . . . . . .  95
     B.3.  Purpose of the <rel-addr> and <rel-port> Attributes . . .  97
     B.4.  Importance of the STUN Username . . . . . . . . . . . . .  97
     B.5.  The Candidate Pair Sequence Number Formula  . . . . . . .  98
     B.6.  The remote-candidates attribute . . . . . . . . . . . . .  99
     B.7.  Why are Keepalives Needed?  . . . . . . . . . . . . . . . 100
     B.8.  Why Prefer Peer Reflexive Candidates? . . . . . . . . . . 101
     B.9.  Why Send an Updated Offer?  . . . . . . . . . . . . . . . 101
     B.10. Why are Binding Indications Used for Keepalives?  . . . . 101
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . . 102
   Intellectual Property and Copyright Statements  . . . . . . . . . 103











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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 the IP of media sources and
   sinks within their messages, which is known to be problematic through
   NAT [16].  The protocols also seek to create a media flow directly
   between participants, so that there is no application layer
   intermediary between them.  This is done to reduce media latency,
   decrease packet loss, and reduce the operational costs of deploying
   the application.  However, this is difficult to accomplish through
   NAT.  A full treatment of the reasons for this is beyond the scope of
   this specification.

   Numerous solutions have been proposed for allowing these protocols to
   operate through NAT.  These include Application Layer Gateways
   (ALGs), the Middlebox Control Protocol [17], Simple Traversal
   Underneath NAT (STUN) [15] and its revision, retitled Session
   Traversal Utilities for NAT [12], the STUN Relay Usage [13], and
   Realm Specific IP [19] [20] 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 defines Interactive Connectivity Establishment
   (ICE) as a technique for NAT traversal for media streams established
   by the offer/answer model.  ICE is an extension to the offer/answer
   model, and works by including a multiplicity of IP addresses and
   ports in SDP offers and answers, which are then tested for
   connectivity by peer-to-peer STUN exchanges.  The IP addresses and
   ports included in the SDP are gathered using the STUN binding
   acquisition techniques in [12] and relay allocation procedures in
   [13].







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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 protocol (such as SIP), by
   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 another mechanism [34].  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 potentially find one or more paths by which
   they can communicate.

   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 their own respective
   NATs though they may not be aware of it.  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.

























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

                     Figure 1: ICE Deployment Scenario

   The basic idea behind ICE is as follows: each agent has a variety of
   candidate TRANSPORT ADDRESSES (combination of IP address and port) it
   could use to communicate with the other agent.  These might include:

   o  A transport address on a directly attached network interface or
      interfaces

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

   o  The transport 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 candidate transport addresses.  In
   practice, however, many combinations will not work.  For instance, if
   L and R are both behind NATs, their directly attached 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)



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   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.  A CANDIDATE is a transport address - a combination of IP
   address and port for a particular transport protocol.  This document
   defines three types of candidates, some derived from physical or
   logical network interfaces, others discoverable via STUN.  Naturally,
   one viable candidate is a transport address obtained directly from a
   local interface.  Such a candidate is called a HOST CANDIDATE.  The
   local interface could be 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, such a network
   interface appears to the agent as a local interface from which ports
   (and thus a candidate) can be allocated.

   If an agent is multihomed, it obtains a candidate from each
   interface.  Depending on the location of the PEER (the other agent in
   the session) on the IP network relative to the agent, the agent may
   be reachable by the peer through one or more of those interfaces.
   Consider, for example, an agent which has a local interface to a
   private net 10 network (I1), and a second connected to the public
   Internet (I2).  A candidate from I1 will be directly reachable when
   communicating with a peer on the same private net 10 network, while a
   candidate from I2 will be directly reachable when communicating with
   a peer on the public Internet.  Rather than trying to guess which
   interface will work prior to sending an offer, the offering agent
   includes both candidates in its offer.

   Next, the agent 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.















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                 To Internet

                     |
                     |
                     |  /------------  Relayed
                 Y:y | /               Address
                 +--------+
                 |        |
                 |  STUN  |
                 | Server |
                 |        |
                 +--------+
                     |
                     |
                     | /------------  Server
              X1':x1'|/               Reflexive
               +------------+         Address
               |    NAT     |
               +------------+
                     |
                     | /------------  Local
                 X:x |/               Address
                 +--------+
                 |        |
                 | Agent  |
                 |        |
                 +--------+

                     Figure 2: Candidate Relationships

   To find a server reflexive candidate, the agent sends a STUN Binding
   Request, using the Binding Discovery Usage [12] from each host
   candidate, to its STUN server.  It is assumed that the address of the
   STUN server is manually configured or learned in some unspecified
   way.  It is RECOMMENDED that when an agent has a choice of STUN
   servers (when, for example, they are learned through DNS records and
   multiple results are returned), an agent uses a single STUN server
   (based on its IP address) for all candidates for a particular
   session.  This improves the performance of ICE.

   When the agent sends the Binding Request from IP address and port
   X:x, the NAT (assuming there is one) will allocate a binding X1':x1',
   mapping this server reflexive candidate to the host candidate X:x.
   Outgoing packets sent from the host candidate will be translated by
   the NAT to the server reflexive candidate.  Incoming packets sent to
   the server relexive candidate will be translated by the NAT to the
   host candidate and forwarded to the agent.  We call the host
   candidate associated with a given server reflexive candidate the



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   BASE.

      NOTE: "Base" refers to the address an agent sends 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 is redundant and will be eliminated.

   The final type of candidate is a RELAYED CANDIDATE.  The STUN Relay
   Usage [13] 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 at Y:y, and the relay forwards that to
   X1':x1', which passes through the NAT where it is mapped to X:x and
   delivered to L.

2.2.  Connectivity Checks

   Once L has gathered all of its candidates, it orders them in 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.  It pairs them up, resulting in CANDIDATE
   PAIRS.  To see which pairs work, the agent schedules a series of
   CHECKS.  Each check is a STUN transaction that the client will
   perform on a particular candidate pair by sending a STUN request from
   the local candidate to the remote candidate.

   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.

   3.  Acknowledge checks received from the other agent.

   With both agents performing a check on a candidate pair, the result
   is a 4-way handshake:







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

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

                    Figure 3: Basic Connectivity Check

   It is important to note that the STUN requests are sent to and from
   the exact same IP addresses and ports that will be used for media
   (e.g., RTP and RTCP).  Consequently, agents demultiplex STUN and RTP/
   RTCP using contents of the packets, rather than the port on which
   they are received.  Fortunately, this demultiplexing is easy to do,
   especially for RTP and RTCP.

   Because STUN is used for the connectivity check, the STUN response
   will contain the agent's translated transport address on the public
   side any NATs between the agent and its peer.  If this transport
   address is different from other candidates the agent already learned,
   it represents a new candidate, called a PEER REFLEXIVE CANDIDATE,
   which then gets tested by ICE just the same as any other candidate.

   As an optimization, as soon as R gets L's check message R immediately
   sends a check message to L on the same candidate pair.  This
   accelerates the process of finding a valid candidate, and is called a
   TRIGGERED CHECK.

   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
   resulting list of sorted candidate pairs is called the CHECK LIST.
   The algorithm is described in Section 4.1.2 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.




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   The second property is important for getting ICE to work when there
   are NATs in front of L and R. 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.

   The agent works through this check list by sending a STUN request for
   the next candidate pair on the list every 20ms.  These are called
   PERIODIC CHECKS.

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

2.4.  Frozen Candidates

   The previous description only addresses the case where the agents
   wish to establish a media session with one COMPONENT (a piece of a
   media stream requiring a single transport address; a media stream may
   require multiple components, each of which has to work for the media
   stream as a whole to be work).  Typically, (e.g., with RTP and RTCP)
   the agents actually need to establish connectivity for more than one
   flow.

   The network properties are likely to be very similar for each
   component (especially because RTP and RTCP are sent and received from
   the same IP address).  It is usually 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."

   Each candidate is associated with a property called its FOUNDATION.
   Two candidates have the same foundation when they are "similar" - of
   the same type and obtained from the same interfaces and STUN servers.
   Otherwise, their foundation is different.  A candidate pair has a
   foundation too, which is just the concatenation of the foundations of
   its two candidates.  Initially, only the candidate pairs with unique
   foundations are tested.  The other candidate pairs are marked
   "frozen".  When the connectivity checks for a candidate pair succeed,
   the candidate pairs with the same foundation are unfrozen.  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



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   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 when ICE is used with SIP [3].

2.6.  Concluding ICE

   ICE checks are performed in a specific sequence, so that high
   priority candidate 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.
   In that case, 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 of the CONTROLLED AGENT.  The
   controlling agent gets to nominate which candidate pairs will get
   used for media amongst the ones that are valid.  It can do this in
   one of two ways - using REGULAR NOMINATION or AGGRESSIVE NOMINATION.

   With regular nomination, the controlling agent lets the checks
   continue until at least one valid candidate pair for each media
   stream is found.  Then, it picks amongst those that are valid, and
   sends a second STUN request on its NOMINATED candidate pair, but this
   time with a flag set to tell the peer that this pair has been
   nominated for use.  A This is shown in Figure 4.





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

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

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


                       Figure 4: Regular Nomination

   Once the STUN transaction with the flag completes, both sides cancel
   any future checks for that media stream.  ICE will now send media
   using this pair.  The pair an ICE agent is using for media is called
   the SELECTED PAIR.

   In aggressive nomination, the controlling agent puts the flag in
   every STUN request it sends.  This way, once the first check
   succeeds, ICE processing is complete for that media stream and the
   controlling agent doesn't have to send a second STUN request.  The
   selected pair will be the highest priority valid pair.  Aggressive
   nomination is faster than regular nomination, but gives less
   flexibility.  Aggressive nomination is shown in Figure 5.


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

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


                      Figure 5: Aggressive Nomination

   Once all of the media streams are completed, the controlling endpoint
   sends an updated offer if the candidates in the m and c lines for the
   media stream (called the DEFAULT CANDIDATES) don't match ICE's
   SELECTED CANDIDATES.

   Once ICE is concluded, it can be restarted at any time for one or all
   of the media streams by either agent.  This is done by sending an
   updated offer indicating a restart.




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2.7.  Lite Implementations

   In order for ICE to be used in a call, both agents need to support
   it.  However, certain agents will always be connected to the public
   Internet and have a public IP address at which it can receive packets
   from any correspondent.  To make it easier for these devices to
   support ICE, ICE defines a special type of implementation called LITE
   (in contrast to the normal FULL implementation).  A lite
   implementation doesn't gather candidates; it includes only host
   candidates for any media stream.  When a lite implementation connects
   with a full implementation, the full agent takes the role of the
   controlling agent, and the lite agent takes on the controlled role.
   In addition, lite agents do not need to generate connectivity checks,
   run the state machines, or compute candidate pairs.  Additional
   guidance on when a lite implementation is appropriate, see the
   discussion in Appendix A.

   It is important to note that the lite implementation was added to
   this specification to provide a stepping stone to full
   implementation.  Even for devices that are always connected to the
   public Internet, a full implementation is preferable if achievable.


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].

   Readers should be familiar with the terminology defined in the offer/
   answer model [4], STUN [12] and NAT Behavioral requirements for UDP
   [29]

   This specification makes use of the following additional 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 transport
      protocol (such as UDP or TCP) port.





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   Candidate:  A transport address that is to be tested by ICE
      procedures in order to determine its suitability for usage for
      receipt of media.  Candidates also have properties - their type
      (server reflexive, relayed or host), priority, foundation, and
      base.

   Component:  A component is a piece of a media stream requiring a
      single transport address; a media stream may require multiple
      components, each of which has to work for the media stream as a
      whole to work.  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) [19] (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.  The STUN server's address is configured or learned by the
      client prior to an offer/answer exchange.

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

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

   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:  An arbitrary string that is the same for two candidates
      that have the same type, base IP address, and STUN server.  If any
      of these are different then the foundation will be different.  Two
      candidate pairs with the same foundation pairs are likely to have
      similar network characteristics.  Foundations are used in the
      frozen algorithm.

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





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

   Default Destination/Candidate:  The default destination for a
      component of a media stream is the transport address that would be
      used by an agent that is not ICE aware.  For the RTP component,
      the default IP address is in the c line of the SDP, and the port
      in the m line.  For the RTCP component it is in the rtcp attribute
      when present, and when not present, the IP address in the c line
      and 1 plus the port in the m line.  A default candidate for a
      component is one whose transport address matches the default
      destination for that component.

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

   Check, Connectivity Check, STUN Check:  A STUN Binding Request
      transaction for the purposes of verifying connectivity.  A check
      is sent from the local candidate to the remote candidate of a
      candidate pair.

   Check List:  An ordered set of candidate pairs that an agent will use
      to generate checks.

   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.

   Full:  An ICE implementation that performs the complete set of
      functionality defined by this specification.

   Lite:  An ICE implementation that omits certain functions,
      implementing only as much as is necessary for a peer
      implementation that is full to gain the benefits of ICE.  Lite
      implementations can only act as the controlled agent in a session,
      and do not gather candidates.

   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.  In any session, one agent
      is always controlling.  The other is the controlled agent.




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

   Regular Nomination:  The process of picking a valid candidate pair
      for media traffic by validating the pair with one STUN request,
      and then picking it by sending a second STUN request with a flag
      indicating its nomination.

   Aggressive Nomination:  The process of picking a valid candidate pair
      for media traffic by including a flag in every STUN request, such
      that the first one to produce a valid candidate pair is used for
      media.

   Nominated:  If a valid candidate pair has its nominated flag set, it
      means that it may be selected by ICE for sending and receiving
      media.

   Selected Pair, Selected Candidate:  The candidate pair selected by
      ICE for sending and receiving media is called the selected pair,
      and each of its candidates is called the selected candidate.


4.  Sending the Initial Offer

   In order to send the initial offer in an offer/answer exchange, an
   agent must (1) gather candidates, (2) prioritize them, (3) choose
   default candidates, and then (4) formulate and send the SDP.  The
   first of these four steps differ for full and lite implementations.

4.1.  Full Implementation Requirements

4.1.1.  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 server
   reflexive and relayed candidates are gathered using STUN's Binding
   Discovery and Relay Usages.  The base of a candidate is the candidate
   that an agent must send from when using that candidate.

4.1.1.1.  Host Candidates

   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



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   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.

4.1.1.2.  Server Reflexive and Relayed Candidates

   Agents SHOULD obtain relayed candidates and SHOULD obtain server
   reflexive candidates.  These requirements are at SHOULD strength to
   allow for provider variation.  Use of STUN servers may be unnecessary
   in closed networks where agents are never connected to the public
   Internet or to endpoints outside of the closed network.  In such
   cases, a full implementation would be used for agents that are dual-
   stack or multi-homed, to select a host candidate.  Use of relays is
   expensive, and when ICE is being used, relays will only be utilized
   when both endpoints are behind NATs that perform address and port
   dependent mapping.  Consequently, some deployments might consider
   this use case to be marginal, and elect not to use relays.  If an
   agent does not gather server reflexive or relayed candidates, it is
   RECOMMENDED that the functionality be implemented and just disabled
   through configuration, so that it can re-enabled through
   configuration if conditions change in the future.

   The agent next pairs each host candidate with the STUN server with
   which it is configured or has discovered by some means.  This
   specification only considers usage of a single STUN server.  At that
   very instance, and then every Ta milliseconds thereafter, the agent
   chooses another such pair (the order is inconsequential), and sends a
   STUN request to the server from that host candidate.  If the agent is
   using both relayed and server reflexive candidates, this request MUST
   be a STUN Allocate request using the relay usage [13].  If the agent
   is using only server reflexive candidates, the request MUST be a STUN
   Binding request using the binding discovery usage [12].

   The value of Ta SHOULD be configurable, and SHOULD have a default of
   20ms (see Appendix B.1 for a discussion on the selection of this



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   value).  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 [12].  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 number of candidates
   which are gathered.

   The agent will receive a STUN Binding or Allocate response.  A
   successful Allocate Response will provide the agent with a server
   reflexive candidate (obtained from the mapped address) and a relayed
   candidate in the RELAY-ADDRESS attribute.  If the Allocate request is
   rejected because the server lacks resources to fulfill it, the agent
   SHOULD instead send a Binding Request to obtain a server reflexive
   candidate.  A Binding Response will provide the agent with only a
   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.  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 candidate having a unique base when their transport
   addresses are identical.

4.1.1.3.  Eliminating Redundant Candidates

   Next, the 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.

4.1.1.4.  Computing Foundations

   Finally, the agent assigns each candidate a foundation.  The
   foundation is an identifier, scoped within a session.  Two candidates
   MUST have the same foundation ID when all of the following are true:

   o  they are of the same type (host, relayed, server reflexive, peer
      reflexive or relayed)





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   o  their bases have the same IP address (the ports can be different)

   o  for reflexive and relayed candidates, the STUN servers used to
      obtain them have the same IP address.

   Similarly, two candidates MUST have different foundations if their
   types are different, their bases have different IP addresses, or the
   STUN servers used to obtain them have different IP addresses.

4.1.1.5.  Keeping Candidates Alive

   Once server reflexive and relayed candidates are allocated, they MUST
   be kept alive until ICE processing has completed.  For server
   reflexive candidates learned through the Binding Discovery usage,
   this MUST be another Binding Request from the Binding Discovery
   usage.  For relayed candidates learned through the Relay Usage, this
   MUST be a new Allocate request.  The Allocate request will also
   refresh the server reflexive candidate.

4.1.2.  Prioritizing Candidates

   The prioritization process results in the assignment of a priority to
   each candidate.  Each candidate for a media stream MUST have a unique
   priority that MUST be a positive integer between 1 and (2**32 - 1).
   This priority will be used by ICE to determine the order of the
   connectivity checks and the relative preference for candidates.

   An agent SHOULD compute this priority using the formula in
   Section 4.1.2.1 and choose its parameters using the guidelines in
   Section 4.1.2.2.  If an agent elects to use a different formula, ICE
   will take longer to converge since both agents will not be
   coordinated in their checks.

4.1.2.1.  Recommended Formula

   When using the formula, an agent computes the priority 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
   is computed using the following formula:


   priority = (2^24)*(type preference) +
              (2^8)*(local preference) +
              (2^0)*(256 - component ID)





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   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 4.1.1 will
   never be peer reflexive candidates; candidates of these type are
   learned from the STUN connectivity checks performed by ICE.

   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.  More generally, 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.

   The component ID is the component ID for the candidate, and MUST be
   between 1 and 256 inclusive.

4.1.2.2.  Guidelines for Choosing Type and Local Preferences

   One criteria for selection of the type and local preference values is
   the use of an intermediary, such as a media relay.  With an
   intermediary, if media is sent to that candidate, it will first
   transit the intermediary before being received.  Relayed candidates
   are one type of candidate that involves 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 a media relay run by the provider.
   If these concerns are important, the type preference for relayed
   candidates SHOULD be lower than host candidates.  The RECOMMENDED
   values are 126 for host candidates, 100 for server reflexive
   candidates, 110 for peer reflexive candidates, and 0 for relayed
   candidates.  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.



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   ICE works with both IPv4 and IPv6.  It therefore provides a
   transition mechanism that allows dual-stack hosts to prefer
   connectivity over IPv6, but to fall back to IPv4 in case the v6
   networks are disconnected (due, for example, to a failure in a 6to4
   relay) [24].  It can also help with hosts that have both a native
   IPv6 address and a 6to4 address.  In such a case, higher 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
   interface.

   Another criteria for selecting preferences is topological awareness.
   This is most useful for candidates that make use of relays.  In those
   cases, if an agent has preconfigured or dynamically discovered
   knowledge of the topological proximity of the relays to itself, it
   can use that to assign higher local preferences to candidates
   obtained from closer relays.

4.1.3.  Choosing Default Candidates

   A candidate is said to be default if it would be the target of media
   from a non-ICE peer; that target being called the DEFAULT
   DESTINATION.  If the default candidates are not selected by the ICE
   algorithm when communicating with an ICE-aware peer, an updated
   offer/answer will be required after ICE processing completes in order
   to "correct" the SDP so that the default destination for media
   matches the candidates selected by ICE.  If ICE happens to select the
   default candidates, no updated offer/answer is required.

   An agent MUST choose a set of candidates, one for each component of
   each in-use media stream, to be default.  A media stream is in-use if
   it does not have a port of zero (which is used in RFC 3264 to reject
   a media stream).  Consequently, a media stream is in-use even if it
   is marked as a=inactive [10] or has a bandwidth value of zero.

   It is RECOMMENDED that default candidates be chosen based on the
   likelihood of those candidates to work with the peer that is being
   contacted.  It is RECOMMENDED that the default candidates are the



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   relayed candidates (if relayed candidates are available), server
   reflexive candidates (if server reflexive candidates are available),
   and finally host candidates.

4.2.  Lite Implementation

   For each media stream, the agent allocates a single candidate for
   each component of the media stream from one of its interfaces.  If an
   agent has multiple interfaces, it MUST choose one for each component
   of a particular media stream.  With the lite implementation, ICE
   cannot be used to dynamically choose amongst candidates.  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.

   Each candidate is assigned a foundation.  The foundation MUST be
   different for two candidates from different interfaces, and MUST be
   the same otherwise.  A simple integer that increments for each
   interface will suffice.  In addition, each candidate MUST be assigned
   a unique priority amongst all candidates for the same media stream.
   This priority SHOULD be equal to 2^24*(126) + 2^8*(65535) + 256 minus
   the component ID, which is 2130706432 minus the component ID.

   If an agent has included two candidates for a component, the v4
   candidate SHOULD be selected as the default.  Since a lite
   implementation has a single candidate for a component, each of these
   candidates is considered to be default.

4.3.  Encoding the SDP

   The process of encoding the SDP is identical between full and lite
   implementations.

   The agent will include an m-line for each media stream it wishes to
   use.  The ordering of media streams in the SDP is relevant for ICE.
   ICE will perform its connectivity checks for the first m-line first,
   and consequently media will be able to flow for that stream first.
   Agents SHOULD place their most important media stream, if there is
   one, first in the SDP.

   There will be a candidate attribute for each candidate for a
   particular media stream.  Section 15 provides detailed rules for
   constructing this attribute.  The attribute carries the IP address,
   port and transport protocol for the candidate, in addition to its
   properties that need to be signaled to the peer for ICE to work: the
   priority, foundation, and component ID.  The candidate attribute also
   carries information about the candidate that is useful for



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   diagnostics and other functions: its type and related transport
   addresses.

   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.  The username fragment and password are
   exchanged in the ice-ufrag and ice-pwd attributes, respectively.  In
   addition to providing security, the username provides disambiguation
   and correlation of checks to media streams.  See Appendix B.4 for
   motivation.

   If an agent is a lite implementation, it MUST include an "a=ice-lite"
   session level attribute in its SDP.  If an agent is a full
   implementation, it MUST NOT include this attribute.

   The default candidates are added to the SDP as the default
   destination for media.  For streams based on RTP, this is done by
   placing the IP address and port of the RTP candidate into the c and m
   lines, respectively.  If the agent is utilizing RTCP, it MUST encode
   the RTCP candidate 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].

   The transport addresses that will be the default destination for
   media when communicating with non-ICE peers MUST also be present as
   candidates in one or more a=candidate lines.

   ICE provides for extensibility by allowing an offer or answer to
   contain a series of tokens which identify the ICE extensions used by
   that agent.  If an agent supports an ICE extension, it MUST include
   the token defined for that extension in the ice-options attribute.

   The following is an example SDP message that includes ICE attributes
   (lines folded for readability):














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       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
       b=RS:0
       b=RR:0
       a=rtpmap:0 PCMU/8000
       a=candidate:1 1 UDP 2130706178 10.0.1.1 8998 typ host
       a=candidate:2 1 UDP 1694498562 192.0.2.3 45664 typ srflx raddr
   10.0.1.1 rport 8998

   Once an agent has sent its offer or sent its answer, that agent MUST
   be prepared to receive both STUN and media packets on each candidate.
   As discussed in Section 11.1, media packets can be sent to a
   candidate prior to its appearance as the default destination for
   media in an offer or answer.


5.  Receiving the Initial Offer

   When an agent receives an initial offer, it will check if the offeror
   supports sufficient ICE functionality to proceed (i.e., if both
   offeror and answerer are lite implementations, ICE cannot proceed),
   determine its own role, gather candidates, prioritize them, choose
   default candidates, encode and send an answer, and for full
   implementations, form the check lists and begin connectivity checks.

5.1.  Verifying ICE Support

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

   o  For each media stream, the default destination for at least one
      component of the media stream appears in a candidate attribute.
      For example, in the case of RTP, the IP address and port in the c
      and m line, respectively, appears in a candidate attribute, or the
      value in the rtcp attribute appears in a candidate attribute.

   o  The offer omitted an a=ice-lite attribute or the answerer is a
      full implementation.  In other words, if both agents are lite
      implementations, the agent does not proceed with ICE.

   If any of these conditions are not met, the agent MUST process the
   SDP based on normal RFC 3264 procedures, without using any of the ICE



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   mechanisms described in the remainder of this specification with the
   following exceptions:

   1.  The agent MUST follow the rules of Section 10, which describe
       keepalive procedures for all agents.

   2.  If the agent is not proceeding with ICE because there were
       a=candidate attributes, but none that matched the default
       destination of the media stream, the agent MUST include an a=ice-
       mismatch attribute in its answer.

5.2.  Determining Role

   For each session, each agent takes on a role.  There are two roles -
   controlling, and controlled.  The controlling agent is responsible
   for nominating the candidate pairs that can be used by ICE for each
   media stream, and for generating the updated offer based on ICE's
   selection, when needed.  The controlled agent is told which candidate
   pairs to use for each media stream, and does not generate an updated
   offer to signal this information.

   If one of the agents is a lite implementation, it MUST assume the
   controlled role, and its peer (which will be full; if it was lite,
   ICE would have aborted) MUST assume the controlling role.  If the
   agent and its peer are both full implementations, the agent which
   generated the offer which started the ICE processing takes on the
   controlling role, and the other takes the controlled role.

   In unusual cases it is possible for both agents to mistakenly believe
   they are controlled or controlling.  To deal with such cases, at the
   time an agent determines its role, it MUST select a random number,
   called the tie-breaker, uniformly distributed between 0 and (2**64) -
   1 (that is, a 64 bit positive integer).  This number is used in STUN
   checks to detect and repair this case, as described in
   Section 7.1.1.2.

   Once roles are determined for a session, they persist unless ICE is
   restarted.  A ICE restart (Section 9.1 causes a new selection of
   roles and tie-breakers.

5.3.  Gathering Candidates

   The process for gathering candidates at the answerer is identical to
   the process for the offerer as described in Section 4.1.1 for full
   implementations and Section 4.2 for lite implementations.  It is
   RECOMMENDED that this process begin immediately on receipt of the
   offer, prior to alerting the user.  Such gathering MAY begin when an
   agent starts.



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5.4.  Prioritizing Candidates

   The process for prioritizing candidates at the answerer is identical
   to the process followed by the offerer, as described in Section 4.1.2
   for full implementations and Section 4.2 for lite implementations.

5.5.  Choosing Default Candidates

   The process for selecting default candidates at the answerer is
   identical to the process followed by the offerer, as described in
   Section 4.1.3 for full implementations and Section 4.2 for lite
   implementations.

5.6.  Encoding the SDP

   The process for encoding the SDP at the answerer is identical to the
   process followed by the offerer for both full and lite
   implementations, as described in Section 4.3.

5.7.  Forming the Check Lists

   Forming check lists is done only by full implementations.  Lite
   implementations MUST skip the steps defined in this section.

   There is one check list per in-use media stream resulting from the
   offer/answer exchange.  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.

5.7.1.  Forming Candidate Pairs

   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.  In order to prevent the attacks described in Section 17.4.2,
   agents MAY limit the number of candidates they'll accept in an offer
   or answer.  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.  If this happens, the number of
   components for that media stream is effectively reduced, and
   considered to be equal to the minimum across both agents of the
   maximum component ID provided by each agent across all components for
   the media stream.



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   In the case of RTP, this would happen when one agent provided
   candidates for RTCP, and the other did not.  As another example, the
   offerer can multiplex RTP and RTCP on the same port and signals it
   can do that in the SDP through some new attribute.  However, since
   the offerer doesn't know if the answerer can perform such
   multiplexing, the offerer includes candidates for RTP and RTCP on
   separate ports, so that the offer has two components per media
   stream.  If the answerer can perform such multiplexing, it would
   include just a single component for each candidate - for the combined
   RTP/RTCP mux.  ICE would end up acting as if there was just a single
   component for this candidate.

   The candidate pairs whose local and remote candidates were both the
   default candidates for a particular component is called,
   unsurprisingly, the default candidate pair for that component.  This
   is the pair that would be used to transmit media if both agents had
   not been ICE aware.

   In order to aid understanding, Figure 8 shows the relationships
   between several key concepts - transport addresses, candidates,
   candidate pairs, and check lists, in addition to indicating the main
   properties of candidates and candidate pairs.





























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       +------------------------------------------+
       |                                          |
       | +---------------------+                  |
       | |+----+ +----+ +----+ |   +Type          |
       | || IP | |Port| |Tran| |   +Priority      |
       | ||Addr| |    | |    | |   +Foundation    |
       | |+----+ +----+ +----+ |   +ComponentiD   |
       | |      Transport      |   +RelatedAddr   |
       | |        Addr         |                  |
       | +---------------------+   +Base          |
       |             Candidate                    |
       +------------------------------------------+
       *                                         *
       *    *************************************
       *    *
     +-------------------------------+
    .|                               |
     | Local     Remote              |
     | +----+    +----+   +default?  |
     | |Cand|    |Cand|   +valid?    |
     | +----+    +----+   +nominated?|
     |                    +State     |
     |                               |
     |                               |
     |          Candidate Pair       |
     +-------------------------------+
     *                              *
     *                  ************
     *                  *
     +------------------+
     |  Candidate Pair  |
     +------------------+
     +------------------+
     |  Candidate Pair  |
     +------------------+
     +------------------+
     |  Candidate Pair  |
     +------------------+


            Check
            List


               Figure 8: Conceptual Diagram of a Check List






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5.7.2.  Computing Pair Priority and Ordering Pairs

   Once the pairs are formed, a candidate pair priority is computed.
   Let O be the priority for the candidate provided by the offerer.  Let
   A be the priority for the candidate provided by the answerer.  The
   priority for a pair is computed as:

      pair priority = 2^32*MIN(O,A) + 2*MAX(O,A) + (O>A?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.  Once 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.

5.7.3.  Pruning the Pairs

   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 prune
   the list.  This is done by removing a pair if its local and remote
   candidates are identical to the local and remote candidates of a pair
   higher up on the priority list.  The result is a sequence of ordered
   candidate pairs, called the check list for that media stream.

   In addition, in order to limit the attacks described in
   Section 17.4.2, an agent SHOULD limit the total number of
   connectivity checks they perform across all check lists to 100, by
   discarding the lower priority candidate pairs until there are less
   than 100.

5.7.4.  Computing States

   Each candidate pair in the check list has a foundation and a state.
   The foundation is the combination of the foundations of the local and
   remote candidates in the pair.  The state is assigned once the check
   list for each media stream has been computed.  There are five
   potential values that the state can have:







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

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

   Succeeded:  A check for this pair was already done and produced a
      successful result.

   Failed:  A check for this pair was already done and failed, either
      never producing any response or producing an unrecoverable failure
      response.

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

   As ICE runs, the pairs will move between states as shown in Figure 9.
































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      +-----------+
      |           |
      |           |
      |  Frozen   |
      |           |
      |           |
      +-----------+
            |
            |unfreeze
            |
            V
      +-----------+         +-----------+
      |           |         |           |
      |           | perform |           |
      |  Waiting  |-------->|In-Progress|
      |           |         |           |
      |           |         |           |
      +-----------+         +-----------+
                                  / |
                                //  |
                              //    |
                            //      |
                           /        |
                         //         |
               failure //           |success
                     //             |
                    /               |
                  //                |
                //                  |
              //                    |
             V                      V
      +-----------+         +-----------+
      |           |         |           |
      |           |         |           |
      |   Failed  |         | Succeeded |
      |           |         |           |
      |           |         |           |
      +-----------+         +-----------+

                         Figure 9: Pair State FSM

   The initial states for each pair in the check list are computed by
   performing the following sequence of steps:

   1.  The agent sets all of the pairs in each check list to the Frozen
       state.





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   2.  The agent examines 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).  For that media
       stream, it:

       *  Groups together all of the pairs with the same foundation,

       *  For each group, sets the state of the pair with the lowest
          component ID to Waiting.  If there is more than one such pair,
          the one with the highest priority is used.

   One of the check lists will have some number of pairs in the Waiting
   state, and the other check lists will have all of their pairs in the
   Frozen state.  A check list with at least one pair that is Waiting is
   called an active check list, and a check list with all pairs frozen
   is called a frozen 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:

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

   Completed:  In this state, ICE checks have completed successfully for
      this media stream.

   Failed:  In this state, the ICE checks have not completed
      successfully for this media stream.

   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.

5.8.  Performing Periodic Checks

   Checks are generated only by full implementations.  Lite
   implementations MUST skip the steps described in this section.

   An agent performs periodic checks and triggered checks.  Periodic
   checks occur periodically for each media stream, and involve choosing
   the highest priority pair in the Waiting state from each check list,
   and sending a check on it.  Triggered checks are performed on receipt
   of a connectivity check from the peer (see Section 7.2.1.4).  This



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   section describes how periodic checks are performed.

   Once the agent has computed the check lists as described in
   Section 5.7, 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 4.1.1.  Multiplying by N allows this aggregate check
   throughput to be split between all active check lists.  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 agent MUST:

   o  Find the highest priority pair in that check list that is in the
      Waiting state.

   o  If there is such a pair:

      *  Send a STUN check from the local candidate of that pair to the
         remote candidate of that pair.  The procedures for forming the
         STUN request for this purpose are described in Section 7.1.1.

   o  If there is no such pair:

      *  Find the highest priority pair in that check list that is in
         the Frozen state.

      *  If there is such a pair:

         +  Unfreeze the pair.

         +  Perform a check for that pair, causing its state to
            transition to In-Progress.

      *  If there is no such pair:

         +  Terminate the timer for that check list.

   To compute the message integrity for the check, the agent uses the
   remote username fragment and password learned from the SDP from its
   peer.  The local username fragment is known directly by the agent for
   its own candidate.






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6.  Receipt of the Initial Answer

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

6.1.  Verifying ICE Support

   The offerer 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 10,
   which describes keepalive procedures.

   In some cases, the answer may omit a=candidate attributes for the
   media streams, and instead include an a=ice-mismatch attribute for
   one or more of the media streams in the SDP.  This signals to the
   offerer that the answerer supports ICE, but that ICE processing was
   not used for the session because an intermediary modified the default
   destination for media components without modifying the corresponding
   candidate attributes.  See Section 17 for a discussion of cases where
   this can happen.  This specification provides no guidance on how an
   agent should proceed in such a failure case.

6.2.  Determining Role

   The offerer follows the same procedures described for the answerer in
   Section 5.2.

6.3.  Forming the Check List

   Formation of check lists is performed only by full implementations.
   The offerer follows the same procedures described for the answerer in
   Section 5.7.

6.4.  Performing Periodic Checks

   Periodic checks are performed only by full implementations.  The
   offerer follows the same procedures described for the answerer in
   Section 5.8.


7.  Performing Connectivity Checks

   This section describes how connectivity checks are performed.  All



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   ICE implementations are required to be compliant to [12], as opposed
   to the older [15].  However, whereas a full implementation will both
   generate checks (acting as a STUN client) and receive them (acting as
   a STUN server), a lite implementation will only ever receive checks,
   and thus will only act as a STUN server.

7.1.  Client Procedures

   These procedures define how an agent sends a connectivity check,
   whether it is a periodic or a triggered check.  These procedures are
   only applicable to full implementations.

7.1.1.  Sending the Request

   The check is generated by sending a Binding Request from a local
   candidate, to a remote candidate. [12] describes how Binding Requests
   are constructed and generated.  This section defines additional
   procedures involving the PRIORITY and USE-CANDIDATE attributes,
   defined for the connectivity check usage, and details how credentials
   for message integrity and diffserv markings are computed.

7.1.1.1.  PRIORITY and USE-CANDIDATE

   An agent MUST include the PRIORITY attribute in its Binding Request.
   The attribute MUST be set equal to the priority that would be
   assigned, based on the algorithm in Section 4.1.2, to a peer
   reflexive candidate, should one be learned as a consequence of this
   check (see Section 7.1.2.2.1 for how peer reflexive candidates are
   learned).  This priority value will be computed identically to how
   the priority for the local candidate of the pair was computed, except
   that the type preference is set to the value for peer derived
   candidate types.

   The controlling agent MAY include the USE-CANDIDATE attribute in the
   Binding Request.  The controlled 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 8.1 provides
   guidance on determining when to include it.

7.1.1.2.  ICE-CONTROLLED and ICE-CONTROLLING

   The agent MUST include the ICE-CONTROLLED attribute in the request if
   it is in the controlled role, and MUST include the ICE-CONTROLLING
   attribute in the request if it is in the controlling role.  The
   content of either attribute MUST be the tie breaker that was
   determined in Section 5.2.




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7.1.1.3.  Forming Credentials

   A Binding Request serving as a connectivity check MUST utilize a STUN
   short term credential.  The agent MUST include the USERNAME and
   MESSAGE-INTEGRITY attributes.  An agent MUST NOT wait to be
   challenged for short term credentials.  Rather, it MUST provide them
   in each Binding Request.

   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 L is the offerer, and agent R is the answerer.
   Agent L included a username fragment of LFRAG for its candidates, and
   a password of LPASS.  Agent R provided a username fragment of RFRAG
   and a password of RPASS.  A connectivity check from L to R (and its
   response of course) utilize the username RFRAG:LFRAG and a password
   of RPASS.  A connectivity check from R to L (and its response)
   utilize the username LFRAG:RFRAG and a password of LPASS.

7.1.1.4.  DiffServ Treatment

   If the agent is using Diffserv Codepoint markings [27] in its media
   packets, it SHOULD apply those same markings to its connectivity
   checks.

7.1.2.  Processing the Response

   When a Binding Response is received, it is correlated to its Binding
   Request using the transaction ID, as defined in [12], which then ties
   it to the candidate pair for which the Binding Request was sent.

7.1.2.1.  Failure Cases

   If the STUN transaction generates a 487 (Role Conflict) error
   response, the agent checks whether it had included the ICE-CONTROLLED
   or ICE-CONTROLLING attribute in the Binding Request.  If the request
   had contained the ICE-CONTROLLED attribute, the agent MUST switch to
   the controlling role if it has not already done so.  If the request
   had contained the ICE-CONTROLLING attribute, the agent MUST switch to
   the controlled role if it has not already done so.  Once it has
   switched, the agent MUST immediately retry the request with the ICE-
   CONTROLLING or ICE-CONTROLLED attribute reflecting its new role.
   Note, however, that the tie-breaker value MUST NOT be reselected.

   If the STUN transaction generates an ICMP error, or generates a STUN



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   error response that is unrecoverable (as defined in [12], or times
   out, the agent sets the state of the pair to Failed.

   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.  In other words, the source and destination
   transport addresses in the request and responses are the symmetric.
   If they are not symmetric, the agent sets the state of the pair to
   Failed.

7.1.2.2.  Success Cases

   A check is considered to be a success if all of the following are
   true:

   o  the STUN transaction generated a success response

   o  the source IP address and port of the response equals the
      destination IP address and port that the Binding Request was sent
      to

   o  the destination IP address and port of the response match the
      source IP address and port that the Binding Request was sent from

7.1.2.2.1.  Discovering Peer Reflexive Candidates

   The agent checks the mapped address from the STUN response.  If the
   transport address does not match any of the local candidates that the
   agent knows about, the mapped address represents a new candidate - a
   peer reflexive candidate.  Like other candidates, it has a type,
   base, priority and foundation.  They are computed as follows:

   o  Its type is equal to peer reflexive.

   o  Its base is set equal to the local candidate of the candidate pair
      from which the STUN check was sent.

   o  Its priority is set equal to the value of the PRIORITY attribute
      in the Binding Request.

   o  Its foundation is selected as described in Section 4.1.1.

   This peer reflexive candidate is then added to the list of local
   candidates for the media stream.  Its username fragment and password
   are the same as all other local candidates for that media stream.
   However, the peer reflexive candidate is not paired with other remote



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   candidates.  This is not necessary; a valid pair will be generated
   from it momentarily based on the procedures in Section 7.1.2.2.3.  If
   an agent wishes to pair the peer reflexive candidate with other
   remote candidates besides the one in the valid pair that will be
   generated, the agent MAY generate an updated offer which includes the
   peer reflexive candidate.  This will cause it to be paired with all
   other remote candidates.

7.1.2.2.2.  Updating Pair States

   The agent sets the state of the pair that generated the check to
   Succeeded.  The success of this check might also cause the state of
   other checks to change as well.  The agent MUST perform the following
   two steps:

   1.  The agent changes the states for all other Frozen pairs for the
       same media stream and same foundation to Waiting.  Typically
       these other pairs will have different component IDs but not
       always.

   2.  If there is a pair in the valid list for every component of this
       media stream (where this is the actual number of components being
       used, in cases where the number of components signaled in the SDP
       differs from offerer to answerer), the success of this check may
       unfreeze checks for other media streams.  Note that this step is
       followed not just the first time the valid list under
       consideration has a pair for every component, but every
       subsequent time a check succeeds and adds yet another pair to
       that valid list.  The agent examines the check list for each
       other media stream in turn:

       *  If the check list is active, the agent changes the state of
          all Frozen pairs in that check list whose foundation matches a
          pair in the valid list under consideration, to Waiting.

       *  If the check list is frozen, and there is at least one pair in
          the check list whose foundation matches a pair in the valid
          list under consideration, the state of all pairs in the check
          list whose foundation matches a pair in the valid list under
          consideration are set to Waiting.

       *  If the check list is frozen, and there are no pairs in the
          check list whose foundation matches a pair in the valid list
          under consideration, the agent

          +  Groups together all of the pairs with the same foundation,





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          +  For each group, sets the state of the pair with the lowest
             component ID to Waiting.  If there is more than one such
             pair, the one with the highest priority is used.

7.1.2.2.3.  Constructing a Valid Pair

   Next, the agent constructs 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 valid pair, since it has been validated by a STUN
   connectivity check.  The valid pair may equal the pair that generated
   the check, may equal a different pair in the check list, or may be a
   pair not currently on any check list.  If the pair equals the pair
   that generated the check or is on a check list currently, it is also
   added to the VALID LIST, which is maintained by the agent for each
   media stream.  This list is empty at the start of ICE processing, and
   fills as checks are performed, resulting in valid candidate pairs.

   It will be very common that the pair will not be on any check list.
   Recall that the check list has pairs whose local candidates are never
   server reflexive; those pairs had their local candidates converted to
   the base of the server reflexive candidates, and then pruned if they
   were redundant.  When the response to the STUN check arrives, the
   mapped address will be reflexive if there is a NAT between the two.
   In that case, the valid pair will have a local candidate that doesn't
   match any of the pairs in the check list.

   If the pair is not on any check list, the agent computes the priority
   for the pair based on the priority of each candidate, using the
   algorithm in Section 5.7.  The priority of the local candidate
   depends on its type.  If it is not peer reflexive, it is equal to the
   priority signaled for that candidate in the SDP.  If it is peer
   reflexive, it is equal to the PRIORITY attribute the agent placed in
   the Binding Request which just completed.  The priority of the remote
   candidate is taken from the SDP of the peer.  If the candidate does
   not appear there, then the check must have been a triggered check to
   a new remote candidate.  In that case, the priority is taken as the
   value of the PRIORITY attribute in the Binding Request which
   triggered the check that just completed.  The pair is then added to
   the VALID LIST.

7.1.2.2.4.  Updating the Nominated Flag

   If the agent was a controlling agent, and it had included a USE-
   CANDIDATE attribute in the Binding Request, the valid pair generated
   from that check has its nominated flag set to true.  This flag
   indicates that this valid pair should be used for media if it is the
   highest priority one amongst those whose nominated flag is set.  This



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   may conclude ICE processing for this media stream or all media
   streams; see Section 8.

   If the agent is the controlled agent, the response may result in the
   valid pair having its nominated flag set.  See Section 7.2.1.5 for
   the procedure.

7.1.2.3.  Check List and Timer State Updates

   Regardless of whether the check was successful or failed, the
   completion of the transaction may require updating of check list and
   timer states.

   If all of the pairs in the check list are now either in the Failed or
   Succeeded state, and there is not a pair in the valid list for each
   component of the media stream, the state of the check list is set to
   Failed.  For each frozen check list, the agent:

   o  Groups together all of the pairs with the same foundation,

   o  For each group, sets the state of the pair with the lowest
      component ID to Waiting.  If there is more than one such pair, the
      one with the highest priority is used.

   If none of the pairs in the check list are in the Waiting or Frozen
   state, the check list is no longer considered active, and will not
   count towards the value of N in the computation of timers for
   periodic checks as described in Section 5.8.

7.2.  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 base 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 MESSAGE-INTEGRITY is the output of a hash of the
   password and the STUN packet's contents.  It is possible (and in fact
   very likely) that an offeror will receive a Binding Request prior to
   receiving the answer from its peer.  If this happens, the agent MUST
   generate a response (including computation of the mapped address as
   described in Section 7.2.1.2.  Once the answer is received, it MUST
   proceed with the remaining steps required, namely Section 7.2.1.3,



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   Section 7.2.1.4, and Section 7.2.1.5 for full implementations.  In
   cases where multiple STUN requests are received before the answer,
   this may cause several triggered notifications to all be sent at the
   same time,

   If the agent is using Diffserv Codepoint markings [27] in its media
   packets, it SHOULD apply those same markings to its responses to
   Binding Requests.  The same would apply to any layer 2 markings the
   endpoint might be applying to media packets.

7.2.1.  Additional Procedures for Full Implementations

   This subsection defines the additional server procedures applicable
   to full implementations.

7.2.1.1.  Detecting and Repairing Role Conflicts

   Normally, the rules for selection of a role in Section 5.2 will
   result in each agent selecting a different role - one controlling,
   and one controlled.  However, in unusual call flows, typically
   utilizing third party call control, it is possible for both agents to
   select the same role.  This section describes procedures for checking
   for this case and repairing it.

   An agent MUST examine the Binding Request for either the ICE-
   CONTROLLING or ICE-CONTROLLED attribute.  It MUST follow these
   procedures:

   o  If neither ICE-CONTROLLING or ICE-CONTROLLED are present in the
      request, there is no conflict.

   o  If the agent is in the controlling role, and the ICE-CONTROLLING
      attribute is present in the request:

      *  If the agent's tie-breaker is larger than or equal to the
         contents of the ICE-CONTROLLING attribute, the agent generates
         a Binding Error Response and includes an ERROR-CODE attribute
         with a value of 487 (Role Conflict) but retains its role.

      *  If the agent's tie-breaker is less than the contents of the
         ICE-CONTROLLING attribute, the agent switches to the controlled
         role.

   o  If the agent is in the controlled role, and the ICE-CONTROLLED
      attribute is present in the request:

      *  If the agent's tie-breaker is larger than or equal to the
         contents of the ICE-CONTROLLED attribute, the agent switches to



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         the controlling role.

      *  If the agent's tie-breaker is less than the contents of the
         ICE-CONTROLLED attribute, the agent generates a Binding Error
         Response and includes an ERROR-CODE attribute with a value of
         487 (Role Conflict) but retains its role.

   o  If the agent is in the controlled role and the ICE-CONTROLLING
      attribute was present in the request, or the agent was in the
      controlling role and the ICE-CONTROLLED attribute was present in
      the request, there is no conflict.

   The remaining sections in Section 7.2.1 are followed if the server
   generated a successful response to the Binding Request, even if the
   agent changed roles.

7.2.1.2.  Computing Mapped Address

   For requests being received on a relayed candidate, the source
   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 (a STUN relay
   delivers packets encapsulated in a Data Indication when no active
   destination is set).  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.

7.2.1.3.  Learning Peer Reflexive Candidates

   If the source transport address of the request does not match any
   existing remote candidates, it represents a new peer reflexive remote
   candidate.  This candidate is constructed as follows:

   o  The priority of the candidate is set to the PRIORITY attribute
      from the request.

   o  The type of the candidate is set to peer reflexive.

   o  The foundation of the candidate is set to an arbitrary value,
      different from the foundation for all other remote candidates.  If
      any subsequent offer/answer exchanges contain this peer reflexive
      candidate in the SDP, it will signal the actual foundation for the
      candidate.

   o  The component ID of this candidate is set to the component ID for
      the local candidate to which the request was sent.



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   This candidate is added to the list of remote candidates.  However,
   the agent does not pair this candidate with any local candidates.

7.2.1.4.  Triggered Checks

   Next, the agent constructs a pair whose local candidate is equal to
   the transport address on which the STUN request was received, and a
   remote candidate equal to the source transport address where the
   request came from (which may be peer-reflexive remote candidate that
   was just learned).  Since both candidates are known to the agent, it
   can obtain their priorities and compute the candidate pair priority.
   This pair is then looked up in the check list.  There can be one of
   several outcomes:

   o  If the pair is already on the check list:

      *  If the state of that pair is Waiting or Frozen, its state is
         changed to In-Progress and a check for that pair is performed
         immediately.  This is called a triggered check.

      *  If the state of that pair is In-Progress, the agent SHOULD
         generate an immediate retransmit of the Binding Request for the
         check in progress.  This is to facilitate rapid completion of
         ICE when both agents are behind NAT.  It is RECOMMENDED that,
         after the immediate retransmit, the next retransmission occur T
         milliseconds later, where T is the current STUN retransmit
         interval.  If the immediate retransmit causes the total number
         of requests transmitted to equal the maximum value defined in
         [12], the agent SHOULD NOT send any further retransmits.

      *  If the state of that pair is Failed or Succeeded, no triggered
         check is sent.

   o  If the pair is not already on the check list:

      *  The pair is inserted into the check list based on its priority

      *  Its state is set to In-Progress

      *  A triggered check for that pair is performed immediately.

   If a triggered check is to be generated, it is constructed and
   processed as described in Section 7.1.1.  These procedures require
   the agent to know the transport address, username fragment and
   password for the peer.  The username fragment for the remote
   candidate is equal to 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



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   (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 the username in an SDP (a likely case for
   the offerer in the initial offer/answer exchange), it MUST wait for
   the SDP to be received (since it won't have its peer's ICE password
   without it), and then proceed with the triggered check.

7.2.1.5.  Updating the Nominated Flag

   If the Binding Request received by the agent had the USE-CANDIDATE
   attribute set, and the agent is in the controlled role, the agent
   looks at the state of the pair computed in Section 7.2.1.4:

   o  If the state of this pair is succeeded, it means that the check
      generated by this pair produced a successful response.  This would
      have caused the agent to construct a valid pair when that success
      response was received (see Section 7.1.2.2.3).  The agent now sets
      the nominated flag in the valid pair to true.  This may end ICE
      processing for this media stream; see Section 8.

   o  If the state of this pair is In-Progress, if its check produces a
      successful result, the resulting valid pair has its nominated flag
      set when the response arrives.  This may end ICE processing for
      this media stream when it arrives; see Section 8.

7.2.2.  Additional Procedures for Lite Implementations

   If the check that was just received contained a USE-CANDIDATE
   attribute, the agent constructs a candidate pair whose local
   candidate is equal to the transport address on which the request was
   received, and whose remote candidate is equal to the source transport
   address of the request that was received.  This candidate pair is
   assigned an arbitrary priority, and placed into a list of valid
   candidates pair for that component of that media stream, called the
   valid list.  The agent sets the nominated flag for that pair to true.
   ICE processing is considered complete for a media stream if the valid
   list contains a candidate pair for each component.


8.  Concluding ICE Processing

   The processing rules in this section apply only to full
   implementations.  Concluding ICE involves nominating pairs by the
   controlling agent and updating of state machinery







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8.1.  Nominating Pairs

   The controlling agent nominates pairs to be selected by ICE by using
   one of two techniques: regular nomination or aggressive nomination.
   If its peer has a lite implementation, an agent MUST use a regular
   nomination algorithm.  If its peer is using ICE options (present in
   an ice-options attribute from the peer) that the agent does not
   understand, the agent MUST use a regular nomination algorithm.  If
   its peer is a full implementation and isn't using any ICE options or
   is using ICE options understood by the agent, the agent MAY use
   either the aggressive or the regular nomination algorithm.  However,
   the regular algorithm is RECOMMENDED since it provides greater
   stability.

8.1.1.  Regular Nomination

   With regular nomination, the agent lets some number of checks
   complete, each of which omit the USE-CANDIDATE attribute.  Once one
   or more checks complete successfully for a component of a media
   stream, valid pairs are generated and added to the valid list.  The
   agent lets the checks continue until some stopping criteria is met,
   and then picks amongst the valid pairs based on an evaluation
   criteria.  The criteria for stopping the checks and for evaluating
   the valid pairs is entirely a matter of local optimization.

   When the controlling agent selects the valid pair, it repeats the
   check that produced this valid pair, this time with the USE-CANDIDATE
   attribute.  This check will succeed (since the previous did), causing
   the nominated flag of that and only that pair to be set.
   Consequently, there will be only a single nominated pair in the valid
   list, and when the state of the check list moves to completed, that
   exact pair is selected by ICE for sending and receiving media.

   Regular nomination provides the most flexibility, since the agent has
   control over the stopping and selection criteria for checks.  The
   only requirement is that the agent MUST eventually pick one and only
   one candidate pair and generate a check for that pair with the USE-
   CANDIDATE attribute present.  Regular nomination also improves ICE's
   resilience to variations in implementation (see Section 14).  Regular
   nomination is also more stable, allowing both agents to converge on a
   single pair for media without any transient selections, which can
   happen with the aggressive algorithm.  The drawback of regular
   nomination is that it is guaranteed to increase latencies because it
   requires an additional check to be done.







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8.1.2.  Aggressive Nomination

   With aggressive nomination, the controlling agent includes the USE-
   CANDIDATE attribute in every check it sends.  Once the first check
   for a component succeeds, it will be added to the valid list, have
   its nominated flag set, and then cause ICE processing to cease for
   this check list.  However, because the agent included the USE-
   CANDIDATE attribute in all of its checks, another check may yet
   complete, causing another valid pair to have its nominated flag set.
   ICE always selects the highest priority nominated candidate pair from
   the valid list as the one used for media.  Consequently, the selected
   pair may actually change briefly as ICE checks complete, resulting in
   a set of transient selections until it stabilizes.

8.2.  Updating States

   For both controlling and controlled agents, the state of ICE
   processing depends on the presence of nominated candidate pairs in
   the valid list and on the state of the check list:

   o  If there are no nominated pairs in the valid list for a media
      stream and the state of the check list is Running, ICE processing
      continues.

   o  If there is at least one nominated pair in the valid list for a
      media stream and the state of the check list is Running:

      *  The agent MUST remove all Waiting and Frozen pairs in the check
         list for the same component as the nominated pairs for that
         media stream

      *  If an In-Progress pair in the check list is for the same
         component as a nominated pair, the agent SHOULD cease
         retransmissions for its check if its pair priority is lower
         than the lowest priority nominated pair for that component

   o  Once there is at least one nominated pair in the valid list for
      every component of at least one media stream and the state of the
      check list is Running:

      *  The agent MUST change the state of processing for its check
         list for that media stream to Completed.

      *  The agent MUST continue to respond to any checks it may still
         receive for that media stream, and MUST perform triggered
         checks if required by the processing of Section 7.2.





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      *  The agent MAY begin transmitting media for this media stream as
         described in Section 11.1

   o  Once the state of each check list is Completed:

      *  The agent sets the state of ICE processing overall to
         Completed.

      *  If an agent is controlling, it examines the highest priority
         nominated candidate pair for each component of each media
         stream.  If any of those candidate pairs differ from the
         default candidate pairs in the most recent offer/answer
         exchange, the controlling agent MUST generate an updated offer
         as described in Section 9.  If the controlling agent is using
         an aggressive nomination algorithm, this may result in several
         updated offers as the pairs selected for media change.  An
         agent MAY delay sending the offer for a brief interval (one
         second is RECOMMENDED) in order to allow the selected pairs to
         stabilize.

   o  If the state of the check list is Failed, ICE has not been able to
      complete for this media stream.  The correct behavior depends on
      the state of the check lists for other media streams:

      *  If all check lists are Failed, the agent SHOULD consider the
         session a failure, SHOULD NOT restart ICE, and the controlling
         agent SHOULD terminate the entire session.

      *  If at least one of the check lists for other media streams is
         Completed, the controlling agent SHOULD remove the failed media
         stream from the session in its updated offer.

      *  If none of the check lists for other media streams are
         Completed, but at least one is Running, the agent SHOULD let
         ICE continue.


9.  Subsequent Offer/Answer Exchanges

   Either agent MAY generate a subsequent offer at any time allowed by
   RFC 3264 [4].  The rules in Section 8 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 default
   pairs.  This section defines rules for construction of subsequent
   offers and answers.






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9.1.  Generating the Offer

9.1.1.  Procedures for All Implementations

9.1.1.1.  ICE Restarts

   An agent MAY restart ICE processing for an existing media stream.  An
   ICE restart, as the name implies, will cause all previous state of
   ICE processing to be flushed and checks to start anew.  The only
   difference between an ICE restart and a brand new media session is
   that, during the restart, media can continue to be sent to the
   previously validated pair.

   An agent MUST restart ICE for a media stream if:

   o  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 destination of a media component.

   o  An agent is changing its implementation level.  This typically
      only happens in third party call control use cases, where the
      entity performing the signaling is not the entity receiving the
      media, and it has changed the target of media mid-session to
      another entity that has a different ICE implementation.

   These rules imply that setting the IP address in the c line to
   0.0.0.0 will cause an ICE restart.  Consequently, ICE implementations
   MUST NOT utilize this mechanism for call hold, and instead MUST use
   a=inactive and a=sendonly as described in [4]

   To restart ICE, an agent MUST change both the ice-pwd and the ice-
   ufrag for the media stream in an offer.  Note that it is permissible
   to use a session-level attribute in one offer, but to provide the
   same ice-pwd or ice-ufrag as a media-level attribute in a subsequent
   offer.  This is not a change in password, just a change in its
   representation, and does not cause an ICE restart.

   An agent sets the rest of the fields in the SDP for this media stream
   as it would in an initial offer of this media stream (see
   Section 4.3).  Consequently, the set of candidates MAY include some,
   none, or all of the previous candidates for that stream and MAY
   include a totally new set of candidates gathered as described in
   Section 4.1.1.







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9.1.1.2.  Removing a Media Stream

   If an agent removes a media stream by setting its port to zero, it
   MUST NOT include any candidate attributes for that media stream and
   SHOULD NOT include any other ICE-related attributes defined in
   Section 15 for that media stream.

9.1.1.3.  Adding a Media Stream

   If an agent wishes to add a new media stream, it sets the fields in
   the SDP for this media stream as if this was an initial offer for
   that media stream (see Section 4.3).  This will cause ICE processing
   to begin for this media stream.

9.1.2.  Procedures for Full Implementations

   This section describes additional procedures for full
   implementations, covering existing media streams.

   The username fragments, password, and implementation level MUST
   remain the same as used previously.  If an agent needs to change one
   of these it MUST restart ICE for that media stream.

   Additional behavior depends on the state ICE processing for that
   media stream.

9.1.2.1.  Existing Media Streams with ICE Running

   If an agent generates an updated offer including media stream that
   was previously established, and for which ICE checks are in the
   Running state, the agent follows the procedures defined here.

   An agent MUST include candidate attributes for all local candidates
   it had signaled previously for that media stream.  The properties of
   that candidate as signaled in SDP - the priority, foundation, type
   and related transport address SHOULD remain the same.  The IP
   address, port and transport protocol, which fundamentally identify
   that candidate, MUST remain the same (if they change, it would be a
   new candidate).  The component ID MUST remain the same.  The agent
   MAY include additional candidates it did not offer previously, but
   which it has gathered since the last offer/answer exchange, including
   peer reflexive candidates.

   The agent MAY change the default destination for media.  As with
   initial offers, there MUST be a set of candidate attributes in the
   offer matching this default destination.





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9.1.2.2.  Existing Media Streams with ICE Completed

   If an agent generates an updated offer including media stream that
   was previously established, and for which ICE checks are in the
   Completed state, the agent follows the procedures defined here.

   The default destination for media (i.e., the values of the IP
   addresses and ports in the m and c line used for that media stream)
   MUST be the local candidate from the highest priority nominated pair
   in the valid list for each component.  This "fixes" the default
   destination for media to equal the destination ICE has selected for
   media.

   The agent MUST include a candidate attributes for candidates matching
   the default destination for each component of the media stream, and
   MUST NOT include any other candidates.

   In addition, if the agent is controlling, it MUST include the
   a=remote-candidates attribute for each media stream whose check list
   is in the Completed state.  The attribute contains the remote
   candidates from the highest priority nominated pair in the valid list
   for each component of that media stream.  It is needed to avoid a
   race condition whereby the controlling agent chooses its pairs, but
   the updated offer beats the connectivity checks to the controlled
   agent, which doesn't even know these pairs are valid, let alone
   selected.  See Appendix B.6 for elaboration on this race condition.

9.1.3.  Procedures for Lite Implementations

   This section describes procedures for lite implementations for
   existing streams for which ICE is running.

   A lite implementation MUST include its one and only candidate for
   each component of each media stream in an a=candidate attribute in
   any subsequent offer.  This candidate is formed identically to the
   procedures for initial offers, as described in Section 4.2.

   The username fragments, password, and implementation level MUST
   remain the same as used previously.  If an agent needs to change one
   of these it MUST restart ICE for that media stream.

9.2.  Receiving the Offer and Generating an Answer

9.2.1.  Procedures for All Implementations

   When receiving a subsequent offer within an existing session, an
   agent MUST re-apply the verification procedures in Section 5.1
   without regard to the results of verification from any previous



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   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.

9.2.1.1.  Detecting ICE Restart

   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 indicates
   that ICE is restarting for this media stream.  If all media streams
   are restarting, than ICE is restarting overall.

   If ICE is restarting for a media stream:

   o  The agent MUST change the a=ice-ufrag and a=ice-pwd attributes in
      the answer.

   o  The agent MAY change its implementation level in the answer.

   An agent sets the rest of the fields in the SDP for this media stream
   as it would in an initial answer to this media stream (see
   Section 4.3).  Consequently, the set of candidates MAY include some,
   none, or all of the previous candidates for that stream and MAY
   include a totally new set of candidates gathered as described in
   Section 4.1.1.

9.2.1.2.  New Media Stream

   If the offer contains a new media stream, the agent sets the fields
   in the answer as if it had received an initial offer containing that
   media stream (see Section 4.3).  This will cause ICE processing to
   begin for this media stream.

9.2.1.3.  Removed Media Stream

   If an offer contains a media stream whose port is zero, the agent
   MUST NOT include any candidate attributes for that media stream in
   its answer and SHOULD NOT include any other ICE-related attributes
   defined in Section 15 for that media stream.

9.2.2.  Procedures for Full Implementations

   The username fragments, password, and implementation level MUST
   remain the same as used previously.  If an agent needs to change one
   of these it MUST restart ICE for that media stream by generating an
   offer; ICE cannot be restarted in an answer.

   Additional behaviors depend on the state of ICE processing for that
   media stream.



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9.2.2.1.  Existing Media Streams with ICE Running and no remote-
          candidates

   If ICE is running for a media stream, and the offer for that media
   stream lacked the remote-candidates attribute, the rules for
   construction of the answer are identical to those for the offerer as
   described in Section 9.1.2.1.

9.2.2.2.  Existing Media Streams with ICE Completed and no remote-
          candidates

   If ICE is Completed for a media stream, and the offer for that media
   stream lacked the remote-candidates attribute, the rules for
   construction of the answer are identical to those for the offerer as
   described in Section 9.1.2.2, except that the answerer MUST NOT
   include the a=remote-candidates attribute in the answer.

9.2.2.3.  Existing Media Streams and remote-candidates

   A controlled agent will receive an offer with the a=remote-candidates
   attribute for a media stream when its peer has concluded ICE
   processing for that media stream.  This attribute is present in the
   offer to deal with a race condition between the receipt of the offer,
   and the receipt of the Binding Response which tells the answerer the
   candidate which will be selected by ICE.  See Appendix B.6 for an
   explanation of this race condition.  Consequently, processing of an
   offer with this attribute depends on the winner of the race.

   The agent forms a candidate pair for each component of the media
   stream by:

   o  Setting the remote candidate equal to the offerers default
      destination for that component (e.g., the contents of the m and
      c-lines for RTP, and the a=rtcp attribute for RTCP)

   o  Setting the local candidate equal to the transport address for
      that same component in the a=remote-candidates attribute in the
      offer.

   The agent then sees if each of these candidate pairs are present in
   the valid list.  If a particular pair is not in the valid list, the
   check has "lost" the race.  Call such a pair a "losing pair".

   The agent finds all the pairs in the check list whose remote
   candidates equal the remote candidate in the losing pair:

   o  If none of the pairs are In-Progress, and at least one is Failed,
      it is most likely that a network failure, such as a network



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      partition or serious packet loss, has occurred.  The agent SHOULD
      generate an answer for this media stream as if the remote-
      candidates attribute had not been present, and then restart ICE
      for this stream.

   o  If at least one of the pairs are In-Progress, the agent SHOULD
      wait for those checks to complete, and as each completes, redo the
      processing in this section until there are no losing pairs.

   Once there are no losing pairs, the agent can generate the answer.
   It MUST set the default destination for media to the candidates in
   the remote-candidates attribute from the offer (each of which will
   now be the local candidate of a candidate pair in the valid list).
   It MUST include a candidate attribute in the answer for each
   candidate in the remote-candidates attribute in the offer.

9.2.3.  Procedures for Lite Implementations

   A lite implementation constructs its answer in the same way it does a
   subsequent offer as described in Section 9.1.3

9.3.  Updating the Check and Valid Lists

9.3.1.  Procedures for Full Implementations

9.3.1.1.  ICE Restarts

   The agent MUST remember the highest priority nominated pairs in the
   Valid list for each component of the media stream, called the
   previous selected pairs, prior to the restart.  The agent will
   continue to send media using these pairs, as described in
   Section 11.1.  Once these destinations are noted, the agent MUST
   flush the valid and check lists, and then recompute the check list
   and its states as described in Section 5.7.

9.3.1.2.  New Media Stream

   If the offer/answer exchange added a new media stream, the agent MUST
   create a new check list for it (and an empty Valid list to start of
   course), as described in Section 5.7.

9.3.1.3.  Removed Media Stream

   If the offer/answer exchange 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.  An agent MUST remove the check list
   for that media stream and cancel any pending periodic checks for it.



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9.3.1.4.  ICE Continuing for Existing Media Stream

   The valid list is not affected by an updated offer/answer exchange
   unless ICE is restarting.

   If an agent is in the Running state for that media stream, the check
   list is updated (the check list is irrelevant if the state is
   completed).  To do that, the agent recomputes the check list using
   the procedures described in Section 5.7.  If a pair on the new check
   list was also on the previous check list, and its state was Waiting,
   In-Progress, Succeeded or Failed, its state is copied over.
   Otherwise, its state is set to Frozen.

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

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

9.3.2.  Procedures for Lite Implementations

   If ICE is restarting for a media stream, the agent MUST start a new
   Valid list for that media stream.  It MUST remember the pairs in the
   previous Valid list for each component of the media stream, called
   the previous selected pairs, and continue to send media there as
   described in Section 11.1.


10.  Keepalives

   All endpoints MUST send keepalives for each media session.  These
   keepalives serve the purpose of keeping NAT bindings alive for the
   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.  These keepalives MUST be sent even if ICE is not being
   utilized for the session at all.  The keepalive SHOULD be sent using
   a format which is supported by its peer.  ICE endpoints allow for



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   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 [31] and RTP comfort noise [25].  If the peer
   doesn't support any formats that are particularly well suited for
   keepalives, an agent SHOULD send RTP packets with an incorrect
   version number, or some other form of error which would cause them to
   be discarded by the peer.

   If there has been no packet sent on the candidate pair ICE is using
   for a media component for Tr seconds (where packets include those
   defined for the component (RTP or RTCP) and previous keepalives), an
   agent MUST generate a keepalive on that pair.  Tr SHOULD be
   configurable and SHOULD have a default of 15 seconds.  Alternatively,
   if an agent has a dynamic way to discover the binding lifetimes of
   the intervening NATs, it can use that value to determine Tr.

   If STUN is being used for keepalives, a STUN Binding Indication is
   used [12].  The Binding Indication SHOULD NOT contain integrity
   checks as the messages are simply discarded on receipt regardless of
   contents.  The Indication SHOULD NOT contain the PRIORITY or USE-
   CANDIDATE attributes defined in this document.  The Binding
   Indication is sent using the same local and remote candidates that
   are being used for media.  An agent receiving a Binding Indication
   MUST discard it silently.  Though Binding Indications are used for
   keepalives, an agent MUST be prepared to receive Binding Requests as
   well.  If a Binding Request is received, a response is generated as
   discussed in [12], but there is no impact on ICE processing
   otherwise.

   An agent MUST begin the keepalive processing once ICE has selected
   candidates for usage with media, or media begins to flow, whichever
   happens first.  Keepalives end once the session terminates or the
   media stream is removed.


11.  Media Handling

11.1.  Sending Media

   Procedures for sending media differ for full and lite
   implementations.




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11.1.1.  Procedures for Full Implementations

   Agents always send media using a candidate pair, called the selected
   candidate pair.  An agent will send media to the remote candidate in
   the selected pair (setting the destination address and port of the
   packet equal to that remote candidate), and will send it from the
   local candidate of the selected pair.  When the local candidate is
   server or peer reflexive, media is originated from the base.  Media
   sent from a relayed candidate is sent from the base through that
   relay, using procedures defined in [13].

   The selected pair for a component of a media stream is:

   o  empty if the state of the check list for that media stream is
      Running, and there is no previous selected pair for that component
      due to an ICE restart

   o  equal to the previous selected pair for a component of a media
      stream if the state of the check list for that media stream is
      Running, and there was a previous selected pair for that component
      due to an ICE restart

   o  equal to the highest priority nominated pair for that component in
      the valid list if the state of the check list is Completed

   If the selected pair for at least one component of a media stream is
   empty, an agent MUST NOT send media for any component of that media
   stream.  If the selected pair for each component of a media stream
   has a value, an agent MAY send media for all components of that media
   stream.

   Note that the selected pair for a component of a media stream may not
   equal the default pair for that same component from the most recent
   offer/answer exchange.  When this happens, the selected pair is used
   for media, not the default pair.  When ICE first completes, if the
   selected pairs aren't a match for the default pairs, the controlling
   agent sends an updated offer/answer exchange to remedy this
   disparity.  However, until that updated offer arrives, there will not
   be a match.  Furthermore, in very unusual cases, the default
   candidates in the updated offer/answer will not be a match.

11.1.2.  Procedures for Lite Implementations

   A lite implementation MUST NOT send media until it has a Valid list
   that contains a candidate pair for each component of that media
   stream.  Once that happens, the agent MAY begin sending media
   packets.  To do that, it sends media to the remote candidate in the
   pair (setting the destination address and port of the packet equal to



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   that remote candidate), and will send it from the local candidate.

11.1.3.  Procedures for All Implementations

   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 of media packets.  Furthermore, many
   audio codecs use the marker bit to signal the beginning of a
   talkspurt, for the purposes of jitter buffer adaptation.  For such
   codecs, it is RECOMMENDED that the sender set the marker bit [22]
   when an agent switches transmission of media from one candidate pair
   to another.

11.2.  Receiving Media

   ICE implementations MUST be prepared to receive media on each
   component on any candidates provided for that component in the most
   recent offer/answer exchange (in the case of RTP, this would include
   both RTP and RTCP if candidates were provided for both).

   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 [22] 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.


12.  Usage with SIP

12.1.  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



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   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 successfully started ringing the phone of the
   called party.

   Two cases can be considered - one where the offer is present in the
   initial INVITE, and one where it is in a response.

12.1.1.  Offer in INVITE

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

   If an offer is received in an INVITE request, the answerer SHOULD
   begin to gather its candidates on receipt of the offer and then
   generate an answer in a provisional response once it has completed
   that process.  ICE requires that a provisional response with an SDP
   be transmitted reliably.  This can be done through the existing PRACK
   mechanism [9], or through an optimization that is specific to ICE.
   With this optimization, provisional responses containing an SDP
   answer that begins ICE processing for one or more media streams can
   be sent reliably without RFC 3264.  To do this, the agent retransmits
   the provisional response with th exponential backoff timers described
   in RFC 3262.  Retransmits MUST cease on receipt of a STUN Binding
   Request for one of the media streams signaled in that SDP (because
   receipt of a binding request indicates the offerer has received the
   answer) or on transmission of a 2xx response.  If no Binding Request
   is received prior to the last retransmit, the agent does not consider
   the session terminated.  Despite the fact that the provisional
   response will be delivered reliably, the rules for when an agent can
   send an updated offer or answer do not change from those specified in
   RFC 3262.  Specifically, if the INVITE contained an offer, 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.  This optimization SHOULD NOT be used if both
   agents support PRACK.  Note that the optimization is very specific to
   provisional response carrying answers that start ICE processing; it
   is not a general technique for 1xx reliability.

   Alternatively, an agent MAY delay sending an answer until the 200 OK,
   however this results in a poor user experience and is NOT



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   RECOMMENDED.

   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 answerer 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 [26] MUST NOT 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.

12.1.2.  Offer in Response

   In addition to uses where the offer is in an INVITE, and the answer
   is in the provisional and/or 200 OK response, ICE works with cases
   where the offer appears in the response.  In such cases, which are
   common in third party call control [18], ICE agents SHOULD generate
   their offers in a reliable provisional response (which MUST utilize
   RFC 3262), and not alert the user on receipt of the INVITE.  The
   answer will arrive in a PRACK.  This allows for ICE processing to
   take place prior to alerting, so that there is no post-pickup delay,
   at the expense of increased call setup delays.  Once ICE completes,
   the callee can alert the user and then generate a 200 OK when they
   answer.  The 200 OK would contain no SDP, since the offer/answer
   exchange has completed.

   Alternatively, agents MAY place the offer in a 2xx instead (in which
   case the answer comes in the ACK).  When this happens, the callee
   will alert the user on receipt of the INVITE, and the ICE exchanges
   will take place only after the user answers.  This has the effect of
   reducing call setup delay, but can cause substantial post-pickup
   delays and media clipping.







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12.2.  SIP Option Tags and Media Feature Tags

   [14] specifies a SIP option tag and media feature tag for usage with
   ICE.  ICE implementations using SIP SHOULD support this
   specification, which uses a feature tag in registrations to
   facilitate interoperability through intermediaries.

12.3.  Interactions with Forking

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

   With ICE, this problem is resolved.  The connectivity checks which
   occur prior to transmission of media carry username fragments, which
   in turn are correlated to a specific callee.  Subsequent media
   packets which arrive on the same candidate pair 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.

12.4.  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 as
   the default targets for media in an offer/answer.  If ICE changes the
   transport address where media is received, this change is reflected
   in an updated offer which changes the default destination for media
   to match ICE's selection.  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 destination for media is changing
   due to ICE negotiations occurring "in the background".

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

   ICE also has (purposeful) interactions with connectivity
   preconditions [30].  Those interactions are described there.  Note
   that the procedures described in Section 12.1 describe their own type
   of "preconditions", albeit with less functionality than those
   provided by the explicit preconditions in [30].

12.5.  Interactions with Third Party Call Control

   ICE works with Flows I, III and IV as described in [18].  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



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   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 being used for
   media.


13.  Relationship with ANAT

   RFC 4091 [11] defines a mechanism for indicating that an agent can
   support both IPv4 and IPv6 for a media stream, and it does so by
   including two m-lines, one for v4, and one for v6.  This is similar
   to ICE, which allows for an agent to indicate multiple transport
   addresses using the candidate attribute.  However, ANAT relies on
   static selection to pick between choices, rather than a dynamic
   connectivity check used by ICE.

   This specification deprecates RFC 4091.  Instead, agents wishing to
   support dual-stack will utilize ICE.  Because a dual-stack agent will
   require at least two candidates, one for IPv4 and one for IPv6, dual-
   stack agents MUST be full implementations.  However, agents that are
   implementing dual-stack and are running on closed networks where it
   is known that there are no NAT, MAY include only host candidates in
   their offers, skipping server reflexive and relayed candidates.


14.  Extensibility Considerations

   This specification makes very specific choices about how both agents
   in a session coordinate to arrive at the set of candidate pairs that
   are selected for media.  It is anticipated that future specifications
   will want to alter these algorithms, whether they are simple changes
   like timer tweaks, or larger changes like a revamp of the priority
   algorithm.  When such a change is made, providing interoperability
   between the two agents in a session is critical.

   First, ICE provides the a=ice-options SDP attribute.  Each extension
   or change to ICE is associated with a token.  When an agent
   supporting such an extension or change generates an offer or an
   answer, it MUST include the token for that extension in this
   attribute.  This allows each side to know what the other side is
   doing.  This attribute MUST NOT be present if the agent doesn't



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   support any ICE extensions or changes.

   At this time, no IANA registry or registration procedures are defined
   for these option tags.  At time of writing, it is unclear whether ICE
   changes and extensions will be sufficiently common to warrrant a
   registry.

   One of the complications in achieving interoperability is that ICE
   relies on a distributed algorithm running on both agents to converge
   on an agreed set of candidate pairs.  If the two agents run different
   algorithms, it can be difficult to guarantee convergence on the same
   candidate pairs.  The regular nomination procedure described in
   Section 8 eliminates some of the tight coordination by delegating the
   selection algorithm completely to the controlling agent.
   Consequently, when a controlling agent is communicating with a peer
   that supports options it doesn't know about, the agent MUST run a
   regular nomination algorithm.  When regular nomination is used, ICE
   will converge perfectly even when both agents use different pair
   prioritization algorithms.  One of the keys to such convergence are
   triggered checks, which ensure that the nominated pair is validated
   by both agents.  Consequently, any future ICE enhancements MUST
   preserve triggered checks.

   ICE is also extensible to other media streams beyond RTP, and for
   transport protocols beyond UDP.  Extensions to ICE for non-RTP media
   streams need to specify how many components they utilize, and assign
   component IDs to them, starting at 1 for the most important component
   ID.  Specifications for new transport protocols must define how, if
   at all, various steps in the ICE processing differ from UDP.


15.  Grammar

   This specification defines seven new SDP attributes - the
   "candidate", "remote-candidates", "ice-lite", "ice-mismatch", "ice-
   ufrag", "ice-pwd" and "ice-options" attributes.

15.1.  "candidate" Attribute

   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]:






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   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 / "+" / "/"


   This grammar encodes the primary information about a candidate: its
   IP address, port and transport protocol, and its properties: the
   foundation, component ID, priority, type, and related transport
   address:

   <connection-address>:  is taken from RFC 4566 [10].  It is the IP
      address of the candidate, allowing for IPv4 addresses, IPv6
      addresses and FQDNs.  An IP address SHOULD be used, but an FQDN
      MAY be used in place of an IP address.  In that case, when
      receiving an offer or answer containing an FQDN in an a=candidate
      attribute, the FQDN is looked up in the DNS using an A or AAAA
      record, and the resulting IP address is used for the remainder of
      ICE processing.

   <port>:  is also taken from RFC 4566 [10].  It is the port of the
      candidate.

   <transport>:  indicates the transport protocol for the candidate.
      This specification only defines UDP.  However, extensibility is
      provided to allow for future transport protocols to be used with
      ICE, such as TCP or the Datagram Congestion Control Protocol
      (DCCP) [32].




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   <foundation>:  is composed of one or more <ice-char>.  It is an
      identifier that is equivalent for two candidates that are of the
      same type, share the same base, and come from the same STUN
      server.  The foundation is used to optimize ICE performance in the
      Frozen algorithm.

   <component-id>:  is a positive integer between 1 and 256 which
      identifies the specific component of the media stream for which
      this is a candidate.  It MUST start at 1 and MUST increment by 1
      for each component of a particular 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.  See Section 14 for additional
      discussion on extending ICE to new media streams.

   <priority>:  is a positive integer between 1 and (2**32 - 1).

   <cand-type>:  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.

   <rel-addr> and <rel-port>:  convey transport addresses related to the
      candidate, useful for diagnostics and other purposes. <rel-addr>
      and <rel-port> MUST be present for server reflexive, peer
      reflexive and relayed candidates.  If a candidate is server or
      peer reflexive, <rel-addr> and <rel-port> is equal to the base for
      that server or peer reflexive candidate.  If the candidate is
      relayed, <rel-addr> and <rel-port> is equal to the mapped address
      in the Allocate Response that provided the client with that
      relayed candidate (see Appendix B.3 for a discussion of its
      purpose).  If the candidate is a host candidate <rel-addr> and
      <rel-port> MUST be omitted.

   The 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.

15.2.  "remote-candidates" Attribute

   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.




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   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.  This
   attribute MUST be included in an offer by a controlling agent for a
   media stream that is Completed, and MUST NOT be included in any other
   case.

15.3.  "ice-lite" and "ice-mismatch" Attributes

   The syntax of the "ice-lite" and "ice-mismatch" attributes, both of
   which are flags, is:


   ice-lite               = "ice-lite"
   ice-mismatch           = "ice-mismatch"

   "ice-lite" is a session level attribute only, and indicates that an
   agent is a lite implementation. "ice-mismatch" is a media level
   attribute only, and when present in an answer, indicates that the
   offer arrived with a default destination for a media component that
   didn't have a corresponding candidate attribute.

15.4.  "ice-ufrag" and "ice-pwd" Attributes

   The "ice-ufrag" and "ice-pwd" attributes convey the username fragment
   and password used by ICE for message integrity.  Their syntax is:


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

   The "ice-pwd" and "ice-ufrag" attributes can appear at either the
   session-level or media-level.  When present in both, the value in the
   media-level takes precedence.  Thus, the value at the session level
   is effectively a default that applies to all media streams, unless
   overriden by a media-level value.  Whether present at the session or
   media level, there MUST be an ice-pwd and ice-ufrag attribute for
   each media stream.  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



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   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 22 characters respectively, of course.

15.5.  "ice-options> Attribute

   The "ice-options" attribute is a session level attribute.  It
   contains a series of tokens which identify the options supported by
   the agent.  Its grammar is:


   ice-options           = "ice-options" ":" ice-option-tag
                             0*(SP ice-option-tag)
   ice-option-tag        = 1*ice-char


16.  Example

   The example is based on the simplified topology of Figure 15.





























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                             +-----+
                             |     |
                             |STUN |
                             | Srvr|
                             +-----+
                                |
                     +---------------------+
                     |                     |
                     |      Internet       |
                     |                     |
                     |                     |
                     +---------------------+
                       |                |
                       |                |
                +---------+             |
                |  NAT    |             |
                +---------+             |
                     |                  |
                     |                  |
                     |                  |
                  +-----+            +-----+
                  |     |            |     |
                  |  L  |            |  R  |
                  |     |            |     |
                  +-----+            +-----+

                        Figure 15: Example Topology

   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 in private address space [28], and for agent
   R, 192.0.2.1 on the public Internet.  Both are configured with the
   same STUN server (shown in this example for simplicity, although in
   practice the agents do not need to use the same STUN server), 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



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   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.
   "USE-CAND" implies the presence of the USE-CANDIDATE attribute.

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


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



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           |              |              |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    |              |              |
           |USE-CAND      |              |              |
           |------------->|              |              |
           |              |(11) Bind Req |              |
           |              |S=$NAT-PUB-1  |              |
           |              |D=$R-PUB-1    |              |
           |              |USE-CAND      |              |
           |              |---------------------------->|
           |              |(12) Bind Res |              |
           |              |S=$R-PUB-1    |              |
           |              |D=$NAT-PUB-1  |              |
           |              |MA=$NAT-PUB-1 |              |
           |              |<----------------------------|
           |(13) Bind Res |              |              |
           |S=$R-PUB-1    |              |              |
           |D=$L-PRIV-1   |              |              |
           |MA=$NAT-PUB-1 |              |              |
           |<-------------|              |              |
           |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    |              |



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           |              |MA=$R-PUB-1   |              |
           |              |---------------------------->|
           |              |              |              |RTP flows


                          Figure 16: Example Flow

   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 default
   candidate, and encodes it into the m and c lines.  The resulting
   offer (message 5) looks like (lines folded for clarity):


    v=0
    o=jdoe 2890844526 2890842807 IN IP4 $L-PRIV-1.IP
    s=
    c=IN IP4 $NAT-PUB-1.IP
    t=0 0
    a=ice-pwd:asd88fgpdd777uzjYhagZg
    a=ice-ufrag:8hhY
    m=audio $NAT-PUB-1.PORT RTP/AVP 0
    b=RS:0
    b=RR:0
    a=rtpmap:0 PCMU/8000
    a=candidate:1 1 UDP 2130706178 $L-PRIV-1.IP $L-PRIV-1.PORT typ host
    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):










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       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
       b=RS:0
       b=RR:0
       a=rtpmap:0 PCMU/8000
       a=candidate:1 1 UDP 2130706178 10.0.1.1 8998 typ host
       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 redundant 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
       b=RS:0
       b=RR:0
       a=rtpmap:0 PCMU/8000
       a=candidate:1 1 UDP 2130706178 $R-PUB-1.IP $R-PUB-1.PORT typ host

   With the variables filled in:












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       v=0
       o=bob 2808844564 2808844564 IN IP4 192.0.2.1
       s=
       c=IN IP4 192.0.2.1
       t=0 0
       a=ice-pwd:YH75Fviy6338Vbrhrlp8Yh
       a=ice-ufrag:9uB6
       m=audio 3478 RTP/AVP 0
       b=RS:0
       b=RR:0
       a=rtpmap:0 PCMU/8000
       a=candidate:1 1 UDP 2130706178 192.0.2.1 3478 typ host

   Since neither side indicated that they are lite, 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 pairs.  However, agent L will prune the pair containing its
   server reflexive candidate, resulting in just one.  At agent L, this
   pair 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 pairs.  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 controlled agent
   for this session, the check omits the USE-CANDIDATE attribute.  The
   host candidate from agent L is private and behind a NAT, and thus
   this check won't be successful, because the packet cannot be routed
   from R to L.

   When agent L gets the answer, it performs its one and only
   connectivity check (messages 10-13).  It implements the aggressive
   nomination algorithm, 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 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.



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   Upon receipt of the STUN Binding Request 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.


17.  Security Considerations

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

17.1.  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.

   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.



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   Of the various techniques for creating faked STUN messages described
   in [12], many are not applicable for the connectivity checks.
   Compromises of STUN servers are not much of a concern, since the STUN
   servers are embedded in endpoints and distributed throughout the
   network.  Thus, compromising the peer's embedded 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 normally signaled via IP
   addresses embedded in SDP.  If the SDP does contain an FQDN for a
   host, then connectivity checks would be susceptible to the DNS
   vulnerabilities described in [12]; however it is far more common to
   use IP addresses.  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
   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



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   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 receive 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 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 [23], 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.

17.2.  Attacks on Address Gathering

   ICE endpoints make use of STUN for gathering candidates from a STUN
   server in the network.  This is corresponds to the Binding Discovery



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   usage of STUN described in [12].  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 [12].
   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 fourth party (neither the
   offerer, answerer, or attacker), since it requires attacking the
   checks generated by each agent in the session, 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.

17.3.  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.

17.4.  Insider Attacks

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



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   valid participant in the ICE exchange.

17.4.1.  The Voice Hammer Attack

   The voice hammer attack is an amplification attack.  In this attack,
   the attacker initiates sessions to other agents, and maliciously
   includes the IP address and port of a DoS target as the destination
   for media traffic signaled in the 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 perform connectivity checks to the target of media before
   sending media 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.

17.4.2.  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.  The attacker sends
   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 20ms, and each check is a
   STUN transaction consisting of 7 transmissions 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 58 bytes/second per transaction on average.
   In the worst case, there can be 395 transactions in progress at once
   (7.9 seconds divided by 20ms), for a total of 182 kbps, just for STUN
   requests.

   It is impossible to eliminate the amplification, but the volume can
   be reduced through a variety of heuristics.  Agents SHOULD limit the
   total number of connectivity checks they perform to 100.
   Additionally, agents MAY limit the number of candidates they'll
   accept in an offer or answer.






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17.5.  Interactions with Application Layer Gateways and SIP

   Application Layer Gateways (ALGs) are functions present in a NAT
   device which inspect the contents of packets and modify them, in
   order to facilitate NAT traversal for application protocols.  Session
   Border Controllers (SBC) are close cousins of ALGs, but are less
   transparent since they actually exist as application layer SIP
   intermediaries.  ICE has interactions with SBCs and ALGs.

   If an ALG is SIP aware but not ICE aware, ICE will work through it as
   long as the ALG correctly modifies the SDP.  In this case, correctly
   means that the ALG does not modify the m and c lines or the rtcp
   attribute if they contain external addresses.  If they contain
   internal addresses, the modification depends on the state of the ALG.
   If the ALG already has a binding established that maps an external
   port to an internal IP address and port in m and c lines or rtcp
   attribute , the ALG uses that binding instead of creating a new one.
   Unfortunately, many ALG are known to work poorly in these corner
   cases.  ICE does not try to work around broken ALGs, as this is
   outside the scope of its functionality.  ICE can help diagnose these
   conditions, which often show up as a mismatch between the set of
   candidates and the m and c lines and rtcp attributes.  The ice-
   mismatch attribute is used for this purpose.

   ICE works best through ALGs when the signaling is run over TLS.  This
   prevents the ALG from manipulating the SDP messages and interfering
   with ICE operation.  Implementations which are expected to be
   deployed behind ALGs SHOULD provide for TLS transport of the SDP.

   If an SBC is SIP aware but not ICE aware, the result depends on the
   behavior of the SBC.  If it is acting as a proper Back-to-Back User
   Agent (B2BUA), the SBC will remove any SDP attributes it doesn't
   understand, including the ICE attributes.  Consequently, the call
   will appear to both endpoints as if the other side doesn't support
   ICE.  This will result in ICE being disabled, and media flowing
   through the SBC, if the SBC has requested it.  If, however, the SBC
   passes the ICE attributes without modification, yet modifies the
   default destination for media (contained in the m and c lines and
   rtcp attribute), this will be detected as an ICE mismatch, and ICE
   processing is aborted for the call.  It is outside of the scope of
   ICE for it to act as a tool for "working around" SBCs.  If one is
   present, ICE will not be used and the SBC techniques take precedence.


18.  Definition of Connectivity Check Usage

   STUN [12] requires that new usages provide a specific set of
   information as part of their formal definition.  This section meets



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   the requirements spelled out there.

18.1.  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, this usage allows a Binding Indication to be used 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 applications on that same port (e.g., RTP or RTCP).
   This demultiplexing is done using the techniques described in [12].

18.2.  Client Discovery of Server

   The client does not follow the DNS-based procedures defined in [12].
   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.

18.3.  Server Determination of Usage

   The server is aware of this usage because it signaled transport
   addresses in its candidates on which it expects to receive STUN
   packets.  Consequently, any STUN packets received on the base of a
   candidate offered in SDP will be for the connectivity check usage.

18.4.  New Requests or Indications

   This usage does not define any new message types.

18.5.  New Attributes

   This usage defines four new attributes, PRIORITY, USE-CANDIDATE, ICE-
   CONTROLLED and ICE-CONTROLLING.

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

   The USE-CANDIDATE attribute indicates that the candidate pair



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   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 value of 0x0025.

   The ICE-CONTROLLED attribute is present in a Binding Request, and
   indicates that the client believes it is currently in the controlled
   role.  The content of the attribute is a 64 bit unsigned integer in
   network byte ordering, which contains a random number used for tie-
   breaking of role conflicts.

   The ICE-CONTROLLING attribute is present in a Binding Request, and
   indicates that the client believes it is currently in the controlling
   role.  The content of the attribute is a 64 bit unsigned integer in
   network byte ordering, which contains a random number used for tie-
   breaking of role conflicts.

18.6.  New Error Response Codes

   This usage defines a single error response code:

   487 (Role Conflict):  The Binding Request contained either the ICE-
      CONTROLLING or ICE-CONTROLLED attribute, indicating a role that
      conflicted with the server.  The server ran a tie-breaker based on
      the tie-breaker value in the request, and determined that the
      client needs to switch roles.

18.7.  Client Procedures

   Client procedures are defined in Section 7.1.

18.8.  Server Procedures

   Server procedures are defined in Section 7.2.

18.9.  Security Considerations for Connectivity Check

   Security considerations for the connectivity check are discussed in
   Section 17.


19.  IANA Considerations

   This specification registers new SDP attributes and new STUN
   attributes.







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19.1.  SDP Attributes

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

19.1.1.  candidate Attribute

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

   Attribute Name:  candidate

   Long Form:  candidate

   Type of Attribute:  media level

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

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

   Appropriate Values:  See Section 15 of RFC XXXX [Note to RFC-ed:
      please replace XXXX with the RFC number of this specification].

19.1.2.  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.






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   Appropriate Values:  See Section 15 of RFC XXXX [Note to RFC-ed:
      please replace XXXX with the RFC number of this specification].

19.1.3.  ice-lite Attribute

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

   Attribute Name:  ice-lite

   Long Form:  ice-lite

   Type of Attribute:  session level

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

   Purpose:  This attribute is used with Interactive Connectivity
      Establishment (ICE), and indicates that an agent has the minimum
      functionality required to support ICE inter-operation with a peer
      that has a full implementation.

   Appropriate Values:  See Section 15 of RFC XXXX [Note to RFC-ed:
      please replace XXXX with the RFC number of this specification].

19.1.4.  ice-mismatch Attribute

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

   Attribute Name:  ice-mismatch

   Long Form:  ice-mismatch

   Type of Attribute:  session level

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

   Purpose:  This attribute is used with Interactive Connectivity
      Establishment (ICE), and indicates that an agent is ICE capable,
      but did not proceed with ICE due to a mismatch of candidates with
      the default destination for media signaled in the SDP.

   Appropriate Values:  See Section 15 of RFC XXXX [Note to RFC-ed:
      please replace XXXX with the RFC number of this specification].







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19.1.5.  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 15 of RFC XXXX [Note to RFC-ed:
      please replace XXXX with the RFC number of this specification].

19.1.6.  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.

   Appropriate Values:  See Section 15 of RFC XXXX [Note to RFC-ed:
      please replace XXXX with the RFC number of this specification].

19.1.7.  ice-options Attribute

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

   Attribute Name:  ice-options






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   Long Form:  ice-options

   Type of Attribute:  session level

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

   Purpose:  This attribute is used with Interactive Connectivity
      Establishment (ICE), and indicates the ICE options or extensions
      used by the agent.

   Appropriate Values:  See Section 15 of RFC XXXX [Note to RFC-ed:
      please replace XXXX with the RFC number of this specification].

19.2.  STUN Attributes

   This section registers four new STUN attributes per the procedures in
   [12].


      0x0024 PRIORITY
      0x0025 USE-CANDIDATE
      0x8029 ICE-CONTROLLED
      0x802a ICE-CONTROLLING

19.3.  STUN Error Responses

   This section registers one new STUN error response code per the
   procedures in [12].


      487   Role Conflict


20.  IAB Considerations

   The IAB has studied the problem of "Unilateral Self Address Fixing",
   which is the general process by which a agent attempts to determine
   its address in another realm on the other side of a NAT through a
   collaborative protocol reflection mechanism [21].  ICE is an example
   of a protocol that performs this type of function.  Interestingly,
   the process for ICE is not unilateral, but bilateral, and the
   difference has a signficant impact on the issues raised by IAB.
   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.



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20.1.  Problem Definition

   From RFC 3424 any UNSAF proposal must provide:

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

   The specific problems being solved by ICE are:

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

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

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

20.2.  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.



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20.3.  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 [15]) has
   several points of brittleness.  One of them is the discovery process
   which requires a agent to try and classify the type of NAT it is
   behind.  This process is error-prone.  With ICE, that discovery
   process is simply not used.  Rather than unilaterally assessing the
   validity of the address, its validity is dynamically determined by
   measuring connectivity to a peer.  The process of determining
   connectivity is very robust.

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

20.4.  Requirements for a Long Term Solution

   From RFC 3424, any UNSAF proposal must provide:




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      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.

20.5.  Issues with Existing NAPT Boxes

   From RFC 3424, any UNSAF proposal must provide:

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

   A number of NAT boxes are now being deployed into the market which
   try and provide "generic" ALG functionality.  These generic ALGs hunt
   for IP addresses, either in text or binary form within a packet, and
   rewrite them if they match a binding.  This interferes with
   traditional STUN.  However, the update to STUN [12] uses an encoding
   which hides these binary addresses from generic ALGs.

   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 [29], 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.


21.  Acknowledgements

   The authors would like to thank Dan Wing, Eric Rescorla, Flemming
   Andreasen, Rohan Mahy, Dean Willis, Eric Cooper, Jason Fischl,
   Douglas Otis, Tim Moore, Jean-Francois Mule, Jonathan Lennox 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.


22.  References



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22.1.  Normative References

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

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

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

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

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

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

   [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]  Camarillo, G. and J. Rosenberg, "The Alternative Network
         Address Types (ANAT) Semantics for the Session Description
         Protocol (SDP) Grouping Framework", RFC 4091, June 2005.

   [12]  Rosenberg, J., "Simple Traversal Underneath Network Address
         Translators (NAT) (STUN)", draft-ietf-behave-rfc3489bis-05
         (work in progress), October 2006.

   [13]  Rosenberg, J., "Obtaining Relay Addresses from Simple Traversal
         Underneath NAT (STUN)", draft-ietf-behave-turn-02 (work in
         progress), October 2006.




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   [14]  Rosenberg, J., "Indicating Support for Interactive Connectivity
         Establishment (ICE) in the  Session Initiation Protocol (SIP)",
         draft-ietf-sip-ice-option-tag-00 (work in progress),
         January 2007.

22.2.  Informative References

   [15]  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.

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

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

   [18]  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.

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

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

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

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

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

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

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

   [26]  Camarillo, G. and H. Schulzrinne, "Early Media and Ringing Tone



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         Generation in the Session Initiation Protocol (SIP)", RFC 3960,
         December 2004.

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

   [28]  Rekhter, Y., Moskowitz, R., Karrenberg, D., Groot, G., and E.
         Lear, "Address Allocation for Private Internets", BCP 5,
         RFC 1918, February 1996.

   [29]  Audet, F. and C. Jennings, "Network Address Translation (NAT)
         Behavioral Requirements for Unicast UDP", BCP 127, RFC 4787,
         January 2007.

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

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

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

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

   [34]  Jennings, C. and R. Mahy, "Managing Client Initiated
         Connections in the Session Initiation Protocol  (SIP)",
         draft-ietf-sip-outbound-07 (work in progress), January 2007.


Appendix A.  Lite and Full Implementations

   ICE allows for two types of implementations.  A full implementation
   supports the controlling and controlled roles in a session, and can
   also perform address gathering.  In contrast, a lite implementation
   is a minimalist implementation that does little but respond to STUN
   checks.

   Because ICE requires both endpoints to support it in order to bring
   benefits to either endpoint, incremental deployment of ICE in a
   network is more complicated.  Many sessions involve an endpoint which
   is, by itself, not behind a NAT and not one that would worry about
   NAT traversal.  A very common case is to have one endpoint that
   requires NAT traversal (such as a VoIP hard phone or soft phone) make



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   a call to one of these devices.  Even if the phone supports a full
   ICE implementation, ICE won't be used at all if the other device
   doesn't support it.  The lite implementation allows for a low-cost
   entry point for these devices.  Once they support the lite
   implementation, full implementations can connect to them and get the
   full benefits of ICE.

   Consequently, a lite implementation is only appropriate for devices
   that will *always* be connected to the public Internet and have a
   public IP address at which it can receive packets from any
   correspondent.  ICE will not function when a lite implementation is
   placed behind a NAT.

   It is important to note that the lite implementation was added to
   this specification to provide a stepping stone to full
   implementation.  Even for devices that are always connected to the
   public Internet, a full implementation is preferable if achievable.
   A full implementation will reduce call setup times.  Full
   implementations also obtain the security benefits of ICE unrelated to
   NAT traversal; in particular, the voice hammer attack described in
   Section 17 is prevented only for full implementations, not lite.
   Finally, it is often the case that a device which finds itself with a
   public address today will be placed in a network tomorrow where it
   will be behind a NAT.  It is difficult to definitively know, over the
   lifetime of a device or product, that it will always be used on the
   public Internet.  Full implementation provides assurance that
   communications will always work.


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 milliseconds, where Ta has a default of 20ms.
   Why are these transactions paced, and why was 20ms 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



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   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.

B.2.  Candidates with Multiple Bases

   Section 4.1.1 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 23:







































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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 23: Identical Candidates with Different Bases

   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 net 10 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 <rel-addr> and <rel-port> Attributes

   The candidate attribute contains two values that are not used at all
   by ICE itself - <rel-addr> and <rel-port>.  Why is it present?

   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 it, 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, 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 server reflexive candidate towards that relay
   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
   is actual required for correct operation of ICE in the first place.




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   Consider agents L, R, and Z. L and R are within private enterprise 1,
   which is using 10.0.0.0/8.  Z is within private enterprise 2, which
   is also using 10.0.0.0/8.  As it turns out, R and Z both have IP
   address 10.0.1.1.  L sends an offer to Z. Z, in its answer, provides
   L 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, R 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 R is prepared to accept STUN
   messages on those ports, just as Z is.  L 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 Z as expected.  Instead, they go to R!  If R just replied
   to them, L would believe it has connectivity to Z, when in fact it
   has connectivity to a completely different user, R. To fix this, the
   STUN short term credential mechanisms are used.  The username
   fragments are sufficiently random that it is highly unlikely that R
   would be using the same values as Z. Consequently, R 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, R 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 R, but rather is the agent side of some
   protocol.  This decreases the probability of hitting an allocated
   port, 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
   value, the resulting sorting has the MAX/MIN property.  This means



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   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 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 24.  On receipt of message 4, agent
   L adds a candidate pair to the valid list.  If there was only a
   single media stream with a single component, agent L could now send
   an updated offer.  However, the check from agent R has not yet
   generated a response, and agent R receives the updated offer (message
   7) before getting the response (message 9).  Thus, it does not yet
   know that this particular pair is valid.  To eliminate this
   condition, the actual candidates at R that were selected by the
   offerer (the remote candidates) are included in the offer itself, and
   the answerer delays its answer until those pairs validate.


























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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) STUN Req.        |                     |
             |<------------------------------------------|
             |(9) STUN Res.        |                     |
             |------------------------------------------>|
             |(10) Answer          |                     |
             |<------------------------------------------|


                      Figure 24: Race Condition Flow

B.7.  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 [33], 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
   independently of media transmission.  This makes its bandwidth
   requirements highly predictable, and thus amenable to QoS
   reservations.

B.8.  Why Prefer Peer Reflexive Candidates?

   Section 4.1.2 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 17.  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.9.  Why Send an Updated Offer?

   Section 11.1 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 SDP
   so that the default destination for media matches where media is
   being sent based on ICE procedures (which will be the highest
   priority nominated candidate pair).

   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
   existing, pre-ICE definitions of the addresses used for media - the m
   and c lines and the rtcp attribute - must be retained.  For this
   reason, an updated offer must be sent.

B.10.  Why are Binding Indications Used for Keepalives?

   Media keepalives are described in Section 10.  These keepalives make
   use of STUN when both endpoints are ICE capable.  However, rather
   than using a Binding Request transaction (which generates a
   response), the keepalives use an Indication.  Why is that?



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   The primary reason has to do with network QoS mechanisms.  Once media
   begins flowing, network elements will assume that the media stream
   has a fairly regular structure, making use of periodic packets at
   fixed intervals, with the possibility of jitter.  If an agent is
   sending media packets, and then receives a Binding Request, it would
   need to generate a response packet along with its media packets.
   This will increase the actual bandwidth requirements for the 5-tuple
   carrying the media packets, and introduce jitter in the delivery of
   those packets.  Analysis has shown that this is a concern in certain
   layer 2 access networks that use fairly tight packet schedulers for
   media.

   Additionally, using a Binding Indication allows integrity to be
   disabled, allowing for better performance.  This is useful for large
   scale endpoints, such as PSTN gateways and SBCs.


Author's Address

   Jonathan Rosenberg
   Cisco
   Edison, NJ
   US

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
























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Full Copyright Statement

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