path: root/doc/rfc/rfc5245.txt

Internet Engineering Task Force (IETF)                      J. Rosenberg
Request for Comments: 5245                                   jdrosen.net
Obsoletes: 4091, 4092                                         April 2010
Category: Standards Track
ISSN: 2070-1721


             Interactive Connectivity Establishment (ICE):
     A Protocol for Network Address Translator (NAT) Traversal for
                         Offer/Answer Protocols

Abstract

   This document describes a protocol for Network Address Translator
   (NAT) traversal for UDP-based 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 and its extension,
   Traversal Using Relay NAT (TURN).  ICE can be used by any protocol
   utilizing the offer/answer model, such as the Session Initiation
   Protocol (SIP).

Status of This Memo

   This is an Internet Standards Track document.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Further information on
   Internet Standards is available in Section 2 of RFC 5741.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   http://www.rfc-editor.org/info/rfc5245.
















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Copyright Notice

   Copyright (c) 2010 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

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 . . . . . . . . . . . . . . . . .  20
         4.1.1.2.  Server Reflexive and Relayed Candidates . . . . .  20
         4.1.1.3.  Computing Foundations . . . . . . . . . . . . . .  22
         4.1.1.4.  Keeping Candidates Alive  . . . . . . . . . . . .  22
       4.1.2.  Prioritizing Candidates . . . . . . . . . . . . . . .  22
         4.1.2.1.  Recommended Formula . . . . . . . . . . . . . . .  23
         4.1.2.2.  Guidelines for Choosing Type and Local
                   Preferences . . . . . . . . . . . . . . . . . . .  23
       4.1.3.  Eliminating Redundant Candidates  . . . . . . . . . .  25
       4.1.4.  Choosing Default Candidates . . . . . . . . . . . . .  25
     4.2.  Lite Implementation Requirements  . . . . . . . . . . . .  25
     4.3.  Encoding the SDP  . . . . . . . . . . . . . . . . . . . .  26
   5.  Receiving the Initial Offer . . . . . . . . . . . . . . . . .  28
     5.1.  Verifying ICE Support . . . . . . . . . . . . . . . . . .  28
     5.2.  Determining Role  . . . . . . . . . . . . . . . . . . . .  29
     5.3.  Gathering Candidates  . . . . . . . . . . . . . . . . . .  30
     5.4.  Prioritizing Candidates . . . . . . . . . . . . . . . . .  30
     5.5.  Choosing Default Candidates . . . . . . . . . . . . . . .  31



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     5.6.  Encoding the SDP  . . . . . . . . . . . . . . . . . . . .  31
     5.7.  Forming the Check Lists . . . . . . . . . . . . . . . . .  31
       5.7.1.  Forming Candidate Pairs . . . . . . . . . . . . . . .  31
       5.7.2.  Computing Pair Priority and Ordering Pairs  . . . . .  34
       5.7.3.  Pruning the Pairs . . . . . . . . . . . . . . . . . .  34
       5.7.4.  Computing States  . . . . . . . . . . . . . . . . . .  34
     5.8.  Scheduling Checks . . . . . . . . . . . . . . . . . . . .  37
   6.  Receipt of the Initial Answer . . . . . . . . . . . . . . . .  39
     6.1.  Verifying ICE Support . . . . . . . . . . . . . . . . . .  39
     6.2.  Determining Role  . . . . . . . . . . . . . . . . . . . .  39
     6.3.  Forming the Check List  . . . . . . . . . . . . . . . . .  40
     6.4.  Performing Ordinary Checks  . . . . . . . . . . . . . . .  40
   7.  Performing Connectivity Checks  . . . . . . . . . . . . . . .  40
     7.1.  STUN Client Procedures  . . . . . . . . . . . . . . . . .  40
       7.1.1.  Creating Permissions for Relayed Candidates . . . . .  40
       7.1.2.  Sending the Request . . . . . . . . . . . . . . . . .  40
         7.1.2.1.  PRIORITY and USE-CANDIDATE  . . . . . . . . . . .  41
         7.1.2.2.  ICE-CONTROLLED and ICE-CONTROLLING  . . . . . . .  41
         7.1.2.3.  Forming Credentials . . . . . . . . . . . . . . .  41
         7.1.2.4.  DiffServ Treatment  . . . . . . . . . . . . . . .  42
       7.1.3.  Processing the Response . . . . . . . . . . . . . . .  42
         7.1.3.1.  Failure Cases . . . . . . . . . . . . . . . . . .  42
         7.1.3.2.  Success Cases . . . . . . . . . . . . . . . . . .  43
           7.1.3.2.1.  Discovering Peer Reflexive Candidates . . . .  43
           7.1.3.2.2.  Constructing a Valid Pair . . . . . . . . . .  44
           7.1.3.2.3.  Updating Pair States  . . . . . . . . . . . .  45
           7.1.3.2.4.  Updating the Nominated Flag . . . . . . . . .  46
         7.1.3.3.  Check List and Timer State Updates  . . . . . . .  46
     7.2.  STUN Server Procedures  . . . . . . . . . . . . . . . . .  46
       7.2.1.  Additional Procedures for Full Implementations  . . .  47
         7.2.1.1.  Detecting and Repairing Role Conflicts  . . . . .  47
         7.2.1.2.  Computing Mapped Address  . . . . . . . . . . . .  48
         7.2.1.3.  Learning Peer Reflexive Candidates  . . . . . . .  49
         7.2.1.4.  Triggered Checks  . . . . . . . . . . . . . . . .  49
         7.2.1.5.  Updating the Nominated Flag . . . . . . . . . . .  50
       7.2.2.  Additional Procedures for Lite Implementations  . . .  51
   8.  Concluding ICE Processing . . . . . . . . . . . . . . . . . .  51
     8.1.  Procedures for Full Implementations . . . . . . . . . . .  51
       8.1.1.  Nominating Pairs  . . . . . . . . . . . . . . . . . .  51
         8.1.1.1.  Regular Nomination  . . . . . . . . . . . . . . .  52
         8.1.1.2.  Aggressive Nomination . . . . . . . . . . . . . .  52
       8.1.2.  Updating States . . . . . . . . . . . . . . . . . . .  53
     8.2.  Procedures for Lite Implementations . . . . . . . . . . .  54
       8.2.1.  Peer Is Full  . . . . . . . . . . . . . . . . . . . .  54
       8.2.2.  Peer Is Lite  . . . . . . . . . . . . . . . . . . . .  55
     8.3.  Freeing Candidates  . . . . . . . . . . . . . . . . . . .  56
       8.3.1.  Full Implementation Procedures  . . . . . . . . . . .  56
       8.3.2.  Lite Implementation Procedures  . . . . . . . . . . .  56



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   9.  Subsequent Offer/Answer Exchanges . . . . . . . . . . . . . .  56
     9.1.  Generating the Offer  . . . . . . . . . . . . . . . . . .  57
       9.1.1.  Procedures for All Implementations  . . . . . . . . .  57
         9.1.1.1.  ICE Restarts  . . . . . . . . . . . . . . . . . .  57
         9.1.1.2.  Removing a Media Stream . . . . . . . . . . . . .  58
         9.1.1.3.  Adding a Media Stream . . . . . . . . . . . . . .  58
       9.1.2.  Procedures for Full Implementations . . . . . . . . .  58
         9.1.2.1.  Existing Media Streams with ICE Running . . . . .  58
         9.1.2.2.  Existing Media Streams with ICE Completed . . . .  59
       9.1.3.  Procedures for Lite Implementations . . . . . . . . .  59
         9.1.3.1.  Existing Media Streams with ICE Running . . . . .  59
         9.1.3.2.  Existing Media Streams with ICE Completed . . . .  60
     9.2.  Receiving the Offer and Generating an Answer  . . . . . .  60
       9.2.1.  Procedures for All Implementations  . . . . . . . . .  60
         9.2.1.1.  Detecting ICE Restart . . . . . . . . . . . . . .  60
         9.2.1.2.  New Media Stream  . . . . . . . . . . . . . . . .  61
         9.2.1.3.  Removed Media Stream  . . . . . . . . . . . . . .  61
       9.2.2.  Procedures for Full Implementations . . . . . . . . .  61
         9.2.2.1.  Existing Media Streams with ICE Running and no
                   remote-candidates . . . . . . . . . . . . . . . .  61
         9.2.2.2.  Existing Media Streams with ICE Completed and
                   no remote-candidates  . . . . . . . . . . . . . .  61
         9.2.2.3.  Existing Media Streams and remote-candidates  . .  61
       9.2.3.  Procedures for Lite Implementations . . . . . . . . .  62
     9.3.  Updating the Check and Valid Lists  . . . . . . . . . . .  63
       9.3.1.  Procedures for Full Implementations . . . . . . . . .  63
         9.3.1.1.  ICE Restarts  . . . . . . . . . . . . . . . . . .  63
         9.3.1.2.  New Media Stream  . . . . . . . . . . . . . . . .  63
         9.3.1.3.  Removed Media Stream  . . . . . . . . . . . . . .  64
         9.3.1.4.  ICE Continuing for Existing Media Stream  . . . .  64
       9.3.2.  Procedures for Lite Implementations . . . . . . . . .  64
   10. Keepalives  . . . . . . . . . . . . . . . . . . . . . . . . .  65
   11. Media Handling  . . . . . . . . . . . . . . . . . . . . . . .  66
     11.1. Sending Media . . . . . . . . . . . . . . . . . . . . . .  66
       11.1.1. Procedures for Full Implementations . . . . . . . . .  66
       11.1.2. Procedures for Lite Implementations . . . . . . . . .  67
       11.1.3. Procedures for All Implementations  . . . . . . . . .  67
     11.2. Receiving Media . . . . . . . . . . . . . . . . . . . . .  67
   12. Usage with SIP  . . . . . . . . . . . . . . . . . . . . . . .  68
     12.1. Latency Guidelines  . . . . . . . . . . . . . . . . . . .  68
       12.1.1. Offer in INVITE . . . . . . . . . . . . . . . . . . .  68
       12.1.2. Offer in Response . . . . . . . . . . . . . . . . . .  70
     12.2. SIP Option Tags and Media Feature Tags  . . . . . . . . .  70
     12.3. Interactions with Forking . . . . . . . . . . . . . . . .  70
     12.4. Interactions with Preconditions . . . . . . . . . . . . .  70
     12.5. Interactions with Third Party Call Control  . . . . . . .  71
   13. Relationship with ANAT  . . . . . . . . . . . . . . . . . . .  71
   14. Extensibility Considerations  . . . . . . . . . . . . . . . .  72



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   15. Grammar . . . . . . . . . . . . . . . . . . . . . . . . . . .  73
     15.1. "candidate" Attribute . . . . . . . . . . . . . . . . . .  73
     15.2. "remote-candidates" Attribute . . . . . . . . . . . . . .  75
     15.3. "ice-lite" and "ice-mismatch" Attributes  . . . . . . . .  75
     15.4. "ice-ufrag" and "ice-pwd" Attributes  . . . . . . . . . .  76
     15.5. "ice-options" Attribute . . . . . . . . . . . . . . . . .  76
   16. Setting Ta and RTO  . . . . . . . . . . . . . . . . . . . . .  76
     16.1. RTP Media Streams . . . . . . . . . . . . . . . . . . . .  77
     16.2. Non-RTP Sessions  . . . . . . . . . . . . . . . . . . . .  78
   17. Example . . . . . . . . . . . . . . . . . . . . . . . . . . .  79
   18. Security Considerations . . . . . . . . . . . . . . . . . . .  85
     18.1. Attacks on Connectivity Checks  . . . . . . . . . . . . .  86
     18.2. Attacks on Server Reflexive Address Gathering . . . . . .  88
     18.3. Attacks on Relayed Candidate Gathering  . . . . . . . . .  89
     18.4. Attacks on the Offer/Answer Exchanges . . . . . . . . . .  89
     18.5. Insider Attacks . . . . . . . . . . . . . . . . . . . . .  90
       18.5.1. The Voice Hammer Attack . . . . . . . . . . . . . . .  90
       18.5.2. STUN Amplification Attack . . . . . . . . . . . . . .  90
     18.6. Interactions with Application Layer Gateways and SIP  . .  91
   19. STUN Extensions . . . . . . . . . . . . . . . . . . . . . . .  92
     19.1. New Attributes  . . . . . . . . . . . . . . . . . . . . .  92
     19.2. New Error Response Codes  . . . . . . . . . . . . . . . .  93
   20. Operational Considerations  . . . . . . . . . . . . . . . . .  93
     20.1. NAT and Firewall Types  . . . . . . . . . . . . . . . . .  93
     20.2. Bandwidth Requirements  . . . . . . . . . . . . . . . . .  93
       20.2.1. STUN and TURN Server Capacity Planning  . . . . . . .  93
       20.2.2. Gathering and Connectivity Checks . . . . . . . . . .  94
       20.2.3. Keepalives  . . . . . . . . . . . . . . . . . . . . .  94
     20.3. ICE and ICE-lite  . . . . . . . . . . . . . . . . . . . .  95
     20.4. Troubleshooting and Performance Management  . . . . . . .  95
     20.5. Endpoint Configuration  . . . . . . . . . . . . . . . . .  95
   21. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  96
     21.1. SDP Attributes  . . . . . . . . . . . . . . . . . . . . .  96
       21.1.1. candidate Attribute . . . . . . . . . . . . . . . . .  96
       21.1.2. remote-candidates Attribute . . . . . . . . . . . . .  96
       21.1.3. ice-lite Attribute  . . . . . . . . . . . . . . . . .  97
       21.1.4. ice-mismatch Attribute  . . . . . . . . . . . . . . .  97
       21.1.5. ice-pwd Attribute . . . . . . . . . . . . . . . . . .  98
       21.1.6. ice-ufrag Attribute . . . . . . . . . . . . . . . . .  98
       21.1.7. ice-options Attribute . . . . . . . . . . . . . . . .  98
     21.2. STUN Attributes . . . . . . . . . . . . . . . . . . . . .  99
     21.3. STUN Error Responses  . . . . . . . . . . . . . . . . . .  99
   22. IAB Considerations  . . . . . . . . . . . . . . . . . . . . .  99
     22.1. Problem Definition  . . . . . . . . . . . . . . . . . . . 100
     22.2. Exit Strategy . . . . . . . . . . . . . . . . . . . . . . 100
     22.3. Brittleness Introduced by ICE . . . . . . . . . . . . . . 101
     22.4. Requirements for a Long-Term Solution . . . . . . . . . . 102
     22.5. Issues with Existing NAPT Boxes . . . . . . . . . . . . . 102



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   23. Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . 102
   24. References  . . . . . . . . . . . . . . . . . . . . . . . . . 103
     24.1. Normative References  . . . . . . . . . . . . . . . . . . 103
     24.2. Informative References  . . . . . . . . . . . . . . . . . 104
   Appendix A.  Lite and Full Implementations  . . . . . . . . . . . 107
   Appendix B.  Design Motivations . . . . . . . . . . . . . . . . . 108
     B.1.  Pacing of STUN Transactions . . . . . . . . . . . . . . . 108
     B.2.  Candidates with Multiple Bases  . . . . . . . . . . . . . 109
     B.3.  Purpose of the <rel-addr> and <rel-port> Attributes . . . 111
     B.4.  Importance of the STUN Username . . . . . . . . . . . . . 111
     B.5.  The Candidate Pair Priority Formula . . . . . . . . . . . 113
     B.6.  The remote-candidates Attribute . . . . . . . . . . . . . 113
     B.7.  Why Are Keepalives Needed?  . . . . . . . . . . . . . . . 114
     B.8.  Why Prefer Peer Reflexive Candidates? . . . . . . . . . . 115
     B.9.  Why Send an Updated Offer?  . . . . . . . . . . . . . . . 115
     B.10. Why Are Binding Indications Used for Keepalives?  . . . . 115
     B.11. Why Is the Conflict Resolution Mechanism Needed?  . . . . 116

1.  Introduction

   RFC 3264 [RFC3264] defines a two-phase exchange of Session
   Description Protocol (SDP) messages [RFC4566] for the purposes of
   establishment of multimedia sessions.  This offer/answer mechanism is
   used by protocols such as the Session Initiation Protocol (SIP)
   [RFC3261].

   Protocols using offer/answer are difficult to operate through Network
   Address Translators (NATs).  Because their purpose is to establish a
   flow of media packets, they tend to carry the IP addresses and ports
   of media sources and sinks within their messages, which is known to
   be problematic through NAT [RFC3235].  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 defined for allowing these protocols to
   operate through NAT.  These include Application Layer Gateways
   (ALGs), the Middlebox Control Protocol [RFC3303], the original Simple
   Traversal of UDP Through NAT (STUN) [RFC3489] specification, and
   Realm Specific IP [RFC3102] [RFC3103] along with session description
   extensions needed to make them work, such as the Session Description
   Protocol (SDP) [RFC4566] attribute for the Real Time Control Protocol
   (RTCP) [RFC3605].  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



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   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 that is flexible enough to work well in all
   situations.

   This specification defines Interactive Connectivity Establishment
   (ICE) as a technique for NAT traversal for UDP-based media streams
   (though ICE can be extended to handle other transport protocols, such
   as TCP [ICE-TCP]) 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 connectivity
   checks.  The IP addresses and ports included in the SDP and the
   connectivity checks are performed using the revised STUN
   specification [RFC5389], now renamed to Session Traversal Utilities
   for NAT.  The new name and new specification reflect its new role as
   a tool that is used with other NAT traversal techniques (namely ICE)
   rather than a standalone NAT traversal solution, as the original STUN
   specification was.  ICE also makes use of Traversal Using Relays
   around NAT (TURN) [RFC5766], an extension to STUN.  Because ICE
   exchanges a multiplicity of IP addresses and ports for each media
   stream, it also allows for address selection for multihomed and dual-
   stack hosts, and for this reason it deprecates RFC 4091 [RFC4091] and
   [RFC4092].

2.  Overview of ICE

   In a typical ICE deployment, we have two endpoints (known as AGENTS
   in RFC 3264 terminology) that 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 [RFC3264]
   messages.  Note that ICE is not intended for NAT traversal for SIP,
   which is assumed to be provided via another mechanism [RFC5626].  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.



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

                              +-------+
                              | SIP   |
           +-------+          | Srvr  |          +-------+
           | STUN  |          |       |          | STUN  |
           | Srvr  |          +-------+          | Srvr  |
           |       |         /         \         |       |
           +-------+        /           \        +-------+
                           /             \
                          /               \
                         /                 \
                        /                   \
                       /  <-  Signaling  ->  \
                      /                       \
                     /                         \
               +--------+                   +--------+
               |  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 for
   a particular transport protocol, which is always UDP in this
   specification)) it could use to communicate with the other agent.
   These might include:

   o  A transport address on a directly attached network interface

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

   o  A transport address allocated from a TURN server (a "relayed
      address").

   Potentially, any of L's candidate transport addresses can be used to
   communicate with any of R's candidate transport addresses.  In



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   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)
   until it finds one or more that work.

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 (with only
   UDP specified here).  This document defines three types of
   candidates, some derived from physical or logical network interfaces,
   others discoverable via STUN and TURN.  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 candidates) can be allocated.

   If an agent is multihomed, it obtains a candidate from each IP
   address.  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 IP addresses.
   Consider, for example, an agent that has a local IP address on 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 IP
   address will work prior to sending an offer, the offering agent
   includes both candidates in its offer.

   Next, the agent uses STUN or TURN to obtain additional candidates.
   These come in two flavors: translated addresses on the public side of
   a NAT (SERVER REFLEXIVE CANDIDATES) and addresses on TURN servers
   (RELAYED CANDIDATES).  When TURN servers are utilized, both types of
   candidates are obtained from the TURN server.  If only STUN servers
   are utilized, only server reflexive candidates are obtained from
   them.  The relationship of these candidates to the host candidate is
   shown in Figure 2.  In this figure, both types of candidates are
   discovered using TURN.  In the figure, the notation X:x means IP
   address X and UDP port x.




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                 To Internet

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

                     Figure 2: Candidate Relationships

   When the agent sends the TURN Allocate request from IP address and
   port X:x, the NAT (assuming there is one) will create 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 reflexive candidate will be translated by
   the NAT to the host candidate and forwarded to the agent.  We call
   the host candidate associated with a given server reflexive candidate
   the BASE.

      Note: "Base" refers to the address 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 TURN server,
   the TURN request will create a binding on each NAT, but only the
   outermost server reflexive candidate (the one nearest the TURN




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   server) 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 Allocate request then arrives at the TURN server.  The TURN
   server allocates a port y from its local IP address Y, and generates
   an Allocate response, informing the agent of this relayed candidate.
   The TURN server also informs the agent of the server reflexive
   candidate, X1':x1' by copying the source transport address of the
   Allocate request into the Allocate response.  The TURN server acts as
   a packet relay, forwarding traffic between L and R. In order to send
   traffic to L, R sends traffic to the TURN server at Y:y, and the TURN
   server forwards that to X1':x1', which passes through the NAT where
   it is mapped to X:x and delivered to L.

   When only STUN servers are utilized, the agent sends a STUN Binding
   request [RFC5389] to its STUN server.  The STUN server will inform
   the agent of the server reflexive candidate X1':x1' by copying the
   source transport address of the Binding request into the Binding
   response.

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 signaling 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, each agent schedules a series of
   CHECKS.  Each check is a STUN request/response 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.








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

   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 a STUN Binding request is used for the connectivity check,
   the STUN Binding response will contain the agent's translated
   transport address on the public side of 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 schedules
   a connectivity check message to be sent 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.



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

   The second property is important for getting ICE to work when there
   are NATs in front of 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 periodically.  These are called
   ORDINARY 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 host candidate and STUN
   server using the same protocol.  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 other candidate pairs with




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   the same foundation are unfrozen.  This avoids repeated checking of
   components that are superficially more attractive but in fact are
   likely to fail.

   While we've described "frozen" here as a separate mechanism for
   expository purposes, in fact it is an integral part of ICE and the
   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 signaling channel.  This MAC
   provides message integrity and data origin authentication, thus
   stopping an attacker from forging or modifying connectivity check
   messages.  Furthermore, if the SIP [RFC3261] caller is using ICE, and
   their call forks, the ICE exchanges happen independently with each
   forked recipient.  In such a case, the keys exchanged in the
   signaling help associate each ICE exchange with each forked
   recipient.

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 a packet
   loss 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 round-trip time (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.




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   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.  This is shown in Figure 4.

   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 whose check
   succeeded.  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.



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   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.

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.  Lite agents do not generate
   connectivity checks or run the state machines, though they need to be
   able to respond to connectivity checks.  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.  When two lite implementations connect, no checks are sent.

   For 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 [RFC2119].

   Readers should be familiar with the terminology defined in the offer/
   answer model [RFC3264], STUN [RFC5389], and NAT Behavioral
   requirements for UDP [RFC4787].

   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.








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

   Transport Address:  The combination of an IP address and transport
      protocol (such as UDP or TCP) port.

   Candidate:  A transport address that is a potential point of contact
      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 IP address on the host.  This includes IP addresses on
      physical interfaces and logical ones, such as ones obtained
      through Virtual Private Networks (VPNs) and Realm Specific IP
      (RSIP) [RFC3102] (which lives at the operating system level).

   Server Reflexive Candidate:  A candidate whose IP address and port
      are a binding allocated by a NAT for an agent when it sent a
      packet through the NAT to a server.  Server reflexive candidates
      can be learned by STUN servers using the Binding request, or TURN
      servers, which provides both a relayed and server reflexive
      candidate.

   Peer Reflexive Candidate:  A candidate whose IP address and port are
      a binding allocated by a NAT for an agent when it sent a STUN
      Binding request through the NAT to its peer.

   Relayed Candidate:  A candidate obtained by sending a TURN Allocate
      request from a host candidate to a TURN server.  The relayed
      candidate is resident on the TURN server, and the TURN 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.






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   Foundation:  An arbitrary string that is the same for two candidates
      that have the same type, base IP address, protocol (UDP, TCP,
      etc.), and STUN or TURN 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.

   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
      is in the m line.  For the RTCP component, it is in the rtcp
      attribute when present, and when not present, the IP address is in
      the c line and 1 plus the port is 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.

   Ordinary 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.





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   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 do not maintain any of the state machines and do
      not generate connectivity checks.

   Controlling Agent:  The ICE agent that 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.

   Controlled Agent:  An ICE agent that 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) eliminate
   redundant candidates, (4) choose default candidates, and then (5)
   formulate and send the SDP offer.  All but the last of these five
   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 communication 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




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   is a transport address.  It also has a type and a base.  Four types
   are defined and gathered by this specification -- host candidates,
   server reflexive candidates, peer reflexive candidates, and relayed
   candidates.  The server reflexive candidates are gathered using STUN
   or TURN, and relayed candidates are obtained through TURN.  Peer
   reflexive candidates are obtained in later phases of ICE, as a
   consequence of connectivity checks.  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 a IP address
   attached to an interface (physical or virtual, including VPN
   interfaces) on the host.

   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 IP
   address that the host has.  It obtains each candidate by binding to a
   UDP port on the specific IP address.  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 IP addresses.

   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 and TURN 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 multihomed, to select a host candidate.  Use of
   TURN servers is expensive, and when ICE is being used, they 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 TURN
   servers.  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 be re-enabled
   through configuration if conditions change in the future.




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   If an agent is gathering both relayed and server reflexive
   candidates, it uses a TURN server.  If it is gathering just server
   reflexive candidates, it uses a STUN server.

   The agent next pairs each host candidate with the STUN or TURN server
   with which it is configured or has discovered by some means.  If a
   STUN or TURN server is configured, it is RECOMMENDED that a domain
   name be configured, and the DNS procedures in [RFC5389] (using SRV
   records with the "stun" service) be used to discover the STUN server,
   and the DNS procedures in [RFC5766] (using SRV records with the
   "turn" service) be used to discover the TURN server.

   This specification only considers usage of a single STUN or TURN
   server.  When there are multiple choices for that single STUN or TURN
   server (when, for example, they are learned through DNS records and
   multiple results are returned), an agent SHOULD use a single STUN or
   TURN server (based on its IP address) for all candidates for a
   particular session.  This improves the performance of ICE.  The
   result is a set of pairs of host candidates with STUN or TURN
   servers.  The agent then chooses one pair, and sends a Binding or
   Allocate request to the server from that host candidate.  Binding
   requests to a STUN server are not authenticated, and any ALTERNATE-
   SERVER attribute in a response is ignored.  Agents MUST support the
   backwards compatibility mode for the Binding request defined in
   [RFC5389].  Allocate requests SHOULD be authenticated using a long-
   term credential obtained by the client through some other means.

   Every Ta milliseconds thereafter, the agent can generate another new
   STUN or TURN transaction.  This transaction can either be a retry of
   a previous transaction that failed with a recoverable error (such as
   authentication failure), or a transaction for a new host candidate
   and STUN or TURN server pair.  The agent SHOULD NOT generate
   transactions more frequently than one every Ta milliseconds.  See
   Section 16 for guidance on how to set Ta and the STUN retransmit
   timer, RTO.

   The agent will receive a 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 XOR-RELAYED-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




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   is identical to a host candidate (which can happen in rare cases),
   the relayed candidate MUST be discarded.

4.1.1.3.  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, or
      peer reflexive).

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

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

   o  they were obtained using the same transport protocol (TCP, UDP,
      etc.).

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

4.1.1.4.  Keeping Candidates Alive

   Once server reflexive and relayed candidates are allocated, they MUST
   be kept alive until ICE processing has completed, as described in
   Section 8.3.  For server reflexive candidates learned through a
   Binding request, the bindings MUST be kept alive by additional
   Binding requests to the server.  Refreshes for allocations are done
   using the Refresh transaction, as described in [RFC5766].  The
   Refresh requests 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**31 - 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.



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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 IP addresses.  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)

   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 connectivity checks performed by ICE.

   The local preference MUST be an integer from 0 to 65535 inclusive.
   It represents a preference for the particular IP address 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 IP address, this value SHOULD be set to
   65535.  More generally, if there are multiple candidates for a
   particular component for a particular media stream that have the same
   type, the local preference MUST be unique for each one.  In this
   specification, this only happens for multihomed hosts.  If a host is
   multihomed because it is dual stack, the local preference SHOULD be
   set equal to the precedence value for IP addresses described in RFC
   3484 [RFC3484].

   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 criterion for selection of the type and local preference values
   is the use of a media intermediary, such as a TURN server, VPN
   server, or NAT.  With a media intermediary, if media is sent to that




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   candidate, it will first transit the media intermediary before being
   received.  Relayed candidates are one type of candidate that involves
   a media intermediary.  Another are host candidates obtained from a
   VPN interface.  When media is transited through a media 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
   intermediary run by a 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 multihomed
   and has multiple IP addresses, the local preference for host
   candidates from a VPN interface SHOULD have a priority of 0.

   Another criterion for selection of preferences is IP address family.
   ICE works with both IPv4 and IPv6.  It therefore provides a
   transition mechanism that allows dual-stack hosts to prefer
   connectivity over IPv6, but to fall back to IPv4 in case the v6
   networks are disconnected (due, for example, to a failure in a 6to4
   relay) [RFC3056].  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 addresses, followed by the
   6to4 addresses, followed by the v4 addresses.  This allows a site to
   obtain and begin using native v6 addresses immediately, yet still
   fall back to 6to4 addresses when communicating with agents in other
   sites that do not yet have native v6 connectivity.

   Another criterion for selecting preferences is security.  If a user
   is a telecommuter, and therefore connected to a corporate network and
   a local home network, the user 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 address would have a higher local preference than any other
   address.

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







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4.1.3.  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.  Frequently, a
   server reflexive candidate and a host candidate will be redundant
   when the agent is not behind a NAT.  The agent SHOULD eliminate the
   redundant candidate with the lower priority.

4.1.4.  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 is 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 "fix up" 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 [RFC4566] 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
   relayed candidates (if relayed candidates are available), server
   reflexive candidates (if server reflexive candidates are available),
   and finally host candidates.

4.2.  Lite Implementation Requirements

   Lite implementations only utilize host candidates.  A lite
   implementation MUST, for each component of each media stream,
   allocate zero or one IPv4 candidates.  It MAY allocate zero or more
   IPv6 candidates, but no more than one per each IPv6 address utilized
   by the host.  Since there can be no more than one IPv4 candidate per
   component of each media stream, if an agent has multiple IPv4
   addresses, it MUST choose one for allocating the candidate.  If a
   host is dual stack, it is RECOMMENDED that it allocate one IPv4
   candidate and one global IPv6 address.  With the lite implementation,
   ICE cannot be used to dynamically choose amongst candidates.
   Therefore, including more than one candidate from a particular scope



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   is NOT RECOMMENDED, since only a connectivity check can truly
   determine whether to use one address or the other.

   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 candidates for it.

   Each candidate is assigned a foundation.  The foundation MUST be
   different for two candidates allocated from different IP addresses,
   and MUST be the same otherwise.  A simple integer that increments for
   each IP address 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:

   priority = (2^24)*(126) +
              (2^8)*(IP precedence) +
              (2^0)*(256 - component ID)

   If a host is v4-only, it SHOULD set the IP precedence to 65535.  If a
   host is v6 or dual stack, the IP precedence SHOULD be the precedence
   value for IP addresses described in RFC 3484 [RFC3484].

   Next, an agent chooses a default candidate for each component of each
   media stream.  If a host is IPv4 only, there would only be one
   candidate for each component of each media stream, and therefore that
   candidate is the default.  If a host is IPv6 or dual stack, the
   selection of default is a matter of local policy.  This default
   SHOULD be chosen such that it is the candidate most likely to be used
   with a peer.  For IPv6-only hosts, this would typically be a globally
   scoped IPv6 address.  For dual-stack hosts, the IPv4 address is
   RECOMMENDED.

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,



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

   STUN connectivity checks between agents are authenticated using the
   short-term credential mechanism defined for STUN [RFC5389].  This
   mechanism relies on a username and password that are exchanged
   through protocol machinery between the client and server.  With ICE,
   the offer/answer exchange is used to exchange them.  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
   [RFC3605].  If RTCP is not in use, the agent MUST signal that using
   b=RS:0 and b=RR:0 as defined in RFC 3556 [RFC3556].

   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 that 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 2130706431 10.0.1.1 8998 typ host
       a=candidate:2 1 UDP 1694498815 192.0.2.3 45664 typ srflx raddr
   10.0.1.1 rport 8998

   Once an agent has sent its offer or 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 offerer
   supports ICE, 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 agent will proceed with the ICE procedures defined in this
   specification if, for each media stream in the SDP it received, the
   default destination for each component of that media stream appears
   in a candidate attribute.  For example, in the case of RTP, the IP
   address and port in the c and m lines, respectively, appear in a
   candidate attribute and the value in the rtcp attribute appears in a
   candidate attribute.

   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 following
   exceptions:

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





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   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.

   3.  If the default candidates were relayed candidates learned through
       a TURN server, the agent MUST create permissions in the TURN
       server for the IP addresses learned from its peer in the SDP it
       just received.  If this is not done, initial packets in the media
       stream from the peer may be lost.

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
   the choice of the final candidate pairs used for communications.  For
   a full agent, this means 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.  For a lite
   implementation, being the controlling agent means selecting a
   candidate pair based on the ones in the offer and answer (for IPv4,
   there is only ever one pair), and then generating an updated offer
   reflecting that selection, when needed (it is never needed for an
   IPv4-only host).  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.  The sections below describe in detail
   the actual procedures followed by controlling and controlled nodes.

   The rules for determining the role and the impact on behavior are as
   follows:

   Both agents are full:  The agent that generated the offer which
      started the ICE processing MUST take the controlling role, and the
      other MUST take the controlled role.  Both agents will form check
      lists, run the ICE state machines, and generate connectivity
      checks.  The controlling agent will execute the logic in
      Section 8.1 to nominate pairs that will be selected by ICE, and
      then both agents end ICE as described in Section 8.1.2.  In
      unusual cases, described in Appendix B.11, it is possible for both
      agents to mistakenly believe they are controlled or controlling.
      To resolve this, each agent 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
      connectivity checks to detect and repair this case, as described
      in Section 7.1.2.2.






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   One agent full, one lite:  The full agent MUST take the controlling
      role, and the lite agent MUST take the controlled role.  The full
      agent will form check lists, run the ICE state machines, and
      generate connectivity checks.  That agent will execute the logic
      in Section 8.1 to nominate pairs that will be selected by ICE, and
      use the logic in Section 8.1.2 to end ICE.  The lite
      implementation will just listen for connectivity checks, receive
      them and respond to them, and then conclude ICE as described in
      Section 8.2.  For the lite implementation, the state of ICE
      processing for each media stream is considered to be Running, and
      the state of ICE overall is Running.

   Both lite:  The agent that generated the offer which started the ICE
      processing MUST take the controlling role, and the other MUST take
      the controlled role.  In this case, no connectivity checks are
      ever sent.  Rather, once the offer/answer exchange completes, each
      agent performs the processing described in Section 8 without
      connectivity checks.  It is possible that both agents will believe
      they are controlled or controlling.  In the latter case, the
      conflict is resolved through glare detection capabilities in the
      signaling protocol carrying the offer/answer exchange.  The state
      of ICE processing for each media stream is considered to be
      Running, and the state of ICE overall is Running.

   Once roles are determined for a session, they persist unless ICE is
   restarted.  An 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.

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.









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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 4.1.4 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 18.5.2,
   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 won't get paired with remote candidates, and some of the
   remote candidates won't get paired with local candidates.  This can
   happen if one agent doesn'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.

   In the case of RTP, this would happen when one agent provides
   candidates for RTCP, and the other does not.  As another example, the
   offerer can multiplex RTP and RTCP on the same port and signals that
   it can do that in the SDP through an SDP attribute [RFC5761].
   However, since the offerer doesn't know if the answerer can perform



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   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 are 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 6 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 6: 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 G be the priority for the candidate provided by the controlling
   agent.  Let D be the priority for the candidate provided by the
   controlled agent.  The priority for a pair is computed as:

      pair priority = 2^32*MIN(G,D) + 2*MAX(G,D) + (G>D?1:0)

   Where G>D?1:0 is an expression whose value is 1 if G is greater than
   D, and 0 otherwise.  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 18.5.2, an agent MUST limit the total number of connectivity
   checks the agent performs across all check lists to a specific value,
   and this value MUST be configurable.  A default of 100 is
   RECOMMENDED.  This limit is enforced by discarding the lower-priority
   candidate pairs until there are less than 100.  It is RECOMMENDED
   that a lower value be utilized when possible, set to the maximum
   number of plausible checks that might be seen in an actual deployment
   configuration.  The requirement for configuration is meant to provide
   a tool for fixing this value in the field if, once deployed, it is
   found to be problematic.

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 7.
































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

                         Figure 7: Pair State FSM

   The initial states for each pair in a 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:

       *  For all pairs with the same foundation, it 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 three states:

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

   Completed:  In this state, ICE checks have produced nominated pairs
      for each component of the media stream.  Consequently, ICE has
      succeeded and media can be sent.

   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 ICE processing is
   under way.  The state is Completed when ICE processing is complete
   and Failed if it failed without success.  Rules for transitioning
   between states are described below.

5.8.  Scheduling Checks

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

   An agent performs ordinary checks and triggered checks.  The
   generation of both checks is governed by a timer that fires
   periodically for each media stream.  The agent maintains a FIFO
   queue, called the triggered check queue, which contains candidate
   pairs for which checks are to be sent at the next available



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   opportunity.  When the timer fires, the agent removes the top pair
   from the triggered check queue, performs a connectivity check on that
   pair, and sets the state of the candidate pair to In-Progress.  If
   there are no pairs in the triggered check queue, an ordinary check is
   sent.

   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.  Implementations
   SHOULD take care to spread out these timers so that they do not fire
   at the same time for each media stream.  Ta and the retransmit timer
   RTO are computed as described in Section 16.  Multiplying by N allows
   this aggregate check throughput to be split between all active check
   lists.  The first timer fires immediately, so that the agent performs
   a connectivity check the moment the offer/answer exchange has been
   done, followed by the next check Ta seconds later (since there is
   only one active check list).

   When the timer fires and there is no triggered check to be sent, the
   agent MUST choose an ordinary check as follows:

   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 7.1.2.

      *  Set the state of the candidate pair to In-Progress.

   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.






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      *  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.

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 ordinary checks.

   When ICE is used with SIP, forking may result in a single offer
   generating a multiplicity of answers.  In that case, ICE proceeds
   completely in parallel and independently for each answer, treating
   the combination of its offer and each answer as an independent offer/
   answer exchange, with its own set of pairs, check lists, states, and
   so on.  The only case in which processing of one pair impacts another
   is freeing of candidates, discussed below in Section 8.3.

6.1.  Verifying ICE Support

   The logic at the offerer is identical to that of the answerer as
   described in Section 5.1, with the exception that an offerer would
   not ever generate a=ice-mismatch attributes in an SDP.

   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 a signaling intermediary modified
   the default destination for media components without modifying the
   corresponding candidate attributes.  See Section 18 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.







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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 Ordinary Checks

   Ordinary 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
   ICE implementations are required to be compliant to [RFC5389], as
   opposed to the older [RFC3489].  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 receive checks, and thus will only act as a STUN server.

7.1.  STUN Client Procedures

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

7.1.1.  Creating Permissions for Relayed Candidates

   If the connectivity check is being sent using a relayed local
   candidate, the client MUST create a permission first if it has not
   already created one previously.  It would have created one previously
   if it had told the TURN server to create a permission for the given
   relayed candidate towards the IP address of the remote candidate.  To
   create the permission, the agent follows the procedures defined in
   [RFC5766].  The permission MUST be created towards the IP address of
   the remote candidate.  It is RECOMMENDED that the agent defer
   creation of a TURN channel until ICE completes, in which case
   permissions for connectivity checks are normally created using a
   CreatePermission request.  Once established, the agent MUST keep the
   permission active until ICE concludes.

7.1.2.  Sending the Request

   The check is generated by sending a Binding request from a local
   candidate to a remote candidate.  [RFC5389] describes how Binding
   requests are constructed and generated.  A connectivity check MUST




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   utilize the STUN short-term credential mechanism.  Support for
   backwards compatibility with RFC 3489 MUST NOT be used or assumed
   with connectivity checks.  The FINGERPRINT mechanism MUST be used for
   connectivity checks.

   ICE extends STUN by defining several new attributes, including
   PRIORITY, USE-CANDIDATE, ICE-CONTROLLED, and ICE-CONTROLLING.  These
   new attributes are formally defined in Section 19.1, and their usage
   is described in the subsections below.  These STUN extensions are
   applicable only to connectivity checks used for ICE.

7.1.2.1.  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 7.1.3.2.1 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 reflexive
   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 8.1.1 provides
   guidance on determining when to include it.

7.1.2.2.  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.  These attributes are defined fully in
   Section 19.1.

7.1.2.3.  Forming Credentials

   A Binding request serving as a connectivity check MUST utilize the
   STUN short-term credential mechanism.  The username for the
   credential 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



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   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 utilizes the
   username RFRAG:LFRAG and a password of RPASS.  A connectivity check
   from R to L utilizes the username LFRAG:RFRAG and a password of
   LPASS.  The responses utilize the same usernames and passwords as the
   requests (note that the USERNAME attribute is not present in the
   response).

7.1.2.4.  DiffServ Treatment

   If the agent is using Diffserv Codepoint markings [RFC2475] in its
   media packets, it SHOULD apply those same markings to its
   connectivity checks.

7.1.3.  Processing the Response

   When a Binding response is received, it is correlated to its Binding
   request using the transaction ID, as defined in [RFC5389], which then
   ties it to the candidate pair for which the Binding request was sent.
   This section defines additional procedures for processing Binding
   responses specific to this usage of STUN.

7.1.3.1.  Failure Cases

   If the STUN transaction generates a 487 (Role Conflict) error
   response, the agent checks whether it included the ICE-CONTROLLED or
   ICE-CONTROLLING attribute in the Binding request.  If the request
   contained the ICE-CONTROLLED attribute, the agent MUST switch to the
   controlling role if it has not already done so.  If the request
   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 enqueue the candidate pair whose check generated the
   487 into the triggered check queue.  The state of that pair is set to
   Waiting.  When the triggered check is sent, it will contain an ICE-
   CONTROLLING or ICE-CONTROLLED attribute reflecting its new role.
   Note, however, that the tie-breaker value MUST NOT be reselected.

   A change in roles will require an agent to recompute pair priorities
   (Section 5.7.2), since those priorities are a function of controlling
   and controlled roles.  The change in role will also impact whether
   the agent is responsible for selecting nominated pairs and generating
   updated offers upon conclusion of ICE.

   Agents MAY support receipt of ICMP errors for connectivity checks.
   If the STUN transaction generates an ICMP error, the agent sets the
   state of the pair to Failed.  If the STUN transaction generates a




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   STUN error response that is unrecoverable (as defined in [RFC5389])
   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 equal the destination IP address and port to which the
   Binding request was sent, and that the destination IP address and
   port of the response match the source IP address and port from which
   the Binding request was sent.  In other words, the source and
   destination transport addresses in the request and responses are
   symmetric.  If they are not symmetric, the agent sets the state of
   the pair to Failed.

7.1.3.2.  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 to which the Binding request was
      sent.

   o  The destination IP address and port of the response match the
      source IP address and port from which the Binding request was
      sent.

7.1.3.2.1.  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 4.1.1.3.

   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.



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   However, the peer reflexive candidate is not paired with other remote
   candidates.  This is not necessary; a valid pair will be generated
   from it momentarily based on the procedures in Section 7.1.3.2.2.  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.

7.1.3.2.2.  Constructing a Valid Pair

   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 that 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 that triggered
   the check that just completed.  The pair is then added to the VALID
   LIST.






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7.1.3.2.3.  Updating Pair States

   The agent sets the state of the pair that *generated* the check to
   Succeeded.  Note that, the pair which *generated* the check may be
   different than the valid pair constructed in Section 7.1.3.2.2 as a
   consequence of the response.  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, but
       not always, these other pairs will have different component IDs.

   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 is set to Waiting.  This will cause the check
          list to become active, and ordinary checks will begin for it,
          as described in Section 5.8.

       *  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,
             and

          +  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.





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7.1.3.2.4.  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
   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 be the result
   of a triggered check that was sent in response to a request that
   itself had the USE-CANDIDATE attribute.  This case is described in
   Section 7.2.1.5, and may now result in setting the nominated flag for
   the pair learned from the original request.

7.1.3.3.  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:

   o  If 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.

   o  For each frozen check list, the agent

      *  groups together all of the pairs with the same foundation, and

      *  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
   ordinary checks as described in Section 5.8.

7.2.  STUN 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.  This
   requirement holds even if the peer is a lite implementation.





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   The agent MUST use a short-term credential to authenticate the
   request and perform a message integrity check.  The agent MUST
   consider the username to be valid if it 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.  It is possible (and in fact very likely) that an
   offerer will receive a Binding request prior to receiving the answer
   from its peer.  If this happens, the agent MUST immediately generate
   a response (including computation of the mapped address as described
   in Section 7.2.1.2).  The agent has sufficient information at this
   point to generate the response; the password from the peer is not
   required.  Once the answer is received, it MUST proceed with the
   remaining steps required, namely, 7.2.1.3, 7.2.1.4, and 7.2.1.5 for
   full implementations.  In cases where multiple STUN requests are
   received before the answer, this may cause several pairs to be queued
   up in the triggered check queue.

   An agent MUST NOT utilize the ALTERNATE-SERVER mechanism, and MUST
   NOT support the backwards-compatibility mechanisms to RFC 3489.  It
   MUST utilize the FINGERPRINT mechanism.

   If the agent is using Diffserv Codepoint markings [RFC2475] 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 nor ICE-CONTROLLED is present in the
      request, the peer agent may have implemented a previous version of
      this specification.  There may be a conflict, but it cannot be
      detected.



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   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
         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.

   A change in roles will require an agent to recompute pair priorities
   (Section 5.7.2), since those priorities are a function of controlling
   and controlled roles.  The change in role will also impact whether
   the agent is responsible for selecting nominated pairs and generated
   updated offers upon conclusion of ICE.

   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
   TURN server.  That source transport address will be present in the
   XOR-PEER-ADDRESS attribute of a Data Indication message, if the
   Binding request was delivered through a Data Indication.  If the




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   Binding request was delivered through a ChannelData message, the
   source transport address is the one that was bound to the channel.

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.

   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 the peer reflexive remote candidate
   that was just learned).  The local candidate will either be a host
   candidate (for cases where the request was not received through a
   relay) or a relayed candidate (for cases where it is received through
   a relay).  The local candidate can never be a server reflexive
   candidate.  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, a check for
         that pair is enqueued into the triggered check queue if not
         already present.





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      *  If the state of that pair is In-Progress, the agent cancels the
         in-progress transaction.  Cancellation means that the agent
         will not retransmit the request, will not treat the lack of
         response to be a failure, but will wait the duration of the
         transaction timeout for a response.  In addition, the agent
         MUST create a new connectivity check for that pair
         (representing a new STUN Binding request transaction) by
         enqueueing the pair in the triggered check queue.  The state of
         the pair is then changed to Waiting.

      *  If the state of the pair is Failed, it is changed to Waiting
         and the agent MUST create a new connectivity check for that
         pair (representing a new STUN Binding request transaction), by
         enqueueing the pair in the triggered check queue.

      *  If the state of that pair is Succeeded, nothing further is
         done.

      These steps are done to facilitate rapid completion of ICE when
      both agents are behind NAT.

   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 Waiting.

      *  The pair is enqueued into the triggered check queue.

   When a triggered check is to be sent, it is constructed and processed
   as described in Section 7.1.2.  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 (there may be more than one in
   cases of forking), and find this username fragment.  The
   corresponding password is then selected.

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



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      response was received (see Section 7.1.3.2.2).  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 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

   This section describes how an agent completes ICE.

8.1.  Procedures for Full Implementations

   Concluding ICE involves nominating pairs by the controlling agent and
   updating of state machinery.

8.1.1.  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.







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8.1.1.1.  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 criterion is met,
   and then picks amongst the valid pairs based on an evaluation
   criterion.  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 (by enqueuing the pair that
   generated the check into the triggered check queue), this time with
   the USE-CANDIDATE attribute.  This check should 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 for each component, and when the state of the check
   list moves to completed, that exact pair is selected by ICE for
   sending and receiving media for that component.

   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.

8.1.1.2.  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 and have
   its nominated flag set.  When all components have a nominated pair in
   the valid list, media can begin to flow using the highest priority
   nominated pair.  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.




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8.1.2.  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.  Note that, at any
   time, more than one of the following cases can apply:

   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 and triggered check queue 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.

      *  The agent MUST continue retransmitting any In-Progress checks
         for that check list.

      *  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



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         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, ICE processing overall is
         considered to be in the Failed state, and 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.

8.2.  Procedures for Lite Implementations

   Concluding ICE for a lite implementation is relatively
   straightforward.  There are two cases to consider:

      The implementation is lite, and its peer is full.

      The implementation is lite, and its peer is lite.

   The effect of ICE concluding is that the agent can free any allocated
   host candidates that were not utilized by ICE, as described in
   Section 8.3.

8.2.1.  Peer Is Full

   In this case, the agent will receive connectivity checks from its
   peer.  When an agent has received a connectivity check that includes
   the USE-CANDIDATE attribute for each component of a media stream, the
   state of ICE processing for that media stream moves from Running to
   Completed.  When the state of ICE processing for all media streams is
   Completed, the state of ICE processing overall is Completed.




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   The lite implementation will never itself determine that ICE
   processing has failed for a media stream; rather, the full peer will
   make that determination and then remove or restart the failed media
   stream in a subsequent offer.

8.2.2.  Peer Is Lite

   Once the offer/answer exchange has completed, both agents examine
   their candidates and those of its peer.  For each media stream, each
   agent pairs up its own candidates with the candidates of its peer for
   that media stream.  Two candidates are paired up when they are for
   the same component, utilize the same transport protocol (UDP in this
   specification), and are from the same IP address family (IPv4 or
   IPv6).

   o  If there is a single pair per component, that pair is added to the
      Valid list.  If all of the components for a media stream had one
      pair, the state of ICE processing for that media stream is set to
      Completed.  If all media streams are Completed, the state of ICE
      processing is set to Completed overall.  This will always be the
      case for implementations that are IPv4 only.

   o  If there is more than one pair per component:

      *  The agent MUST select a pair based on local policy.  Since this
         case only arises for IPv6, it is RECOMMENDED that an agent
         follow the procedures of RFC 3484 [RFC3484] to select a single
         pair.

      *  The agent adds the selected pair for each component to the
         valid list.  As described in Section 11.1, this will permit
         media to begin flowing.  However, it is possible (and in fact
         likely) that both agents have chosen different pairs.

      *  To reconcile this, the controlling agent MUST send an updated
         offer as described in Section 9.1.3, which will include the
         remote-candidates attribute.

      *  The agent MUST NOT update the state of ICE processing when the
         offer is sent.  If this subsequent offer completes, the
         controlling agent MUST change the state of ICE processing to
         Completed for all media streams, and the state of ICE
         processing overall to Completed.  The states for the controlled
         agent are set based on the logic in Section 9.2.3.







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8.3.  Freeing Candidates

8.3.1.  Full Implementation Procedures

   The procedures in Section 8 require that an agent continue to listen
   for STUN requests and continue to generate triggered checks for a
   media stream, even once processing for that stream completes.  The
   rules in this section describe when it is safe for an agent to cease
   sending or receiving checks on a candidate that was not selected by
   ICE, and then free the candidate.

   When ICE is used with SIP, and an offer is forked to multiple
   recipients, ICE proceeds in parallel and independently with each
   answerer, all using the same local candidates.  Once ICE processing
   has reached the Completed state for all peers for media streams using
   those candidates, the agent SHOULD wait an additional three seconds,
   and then it MAY cease responding to checks or generating triggered
   checks on that candidate.  It MAY free the candidate at that time.
   Freeing of server reflexive candidates is never explicit; it happens
   by lack of a keepalive.  The three-second delay handles cases when
   aggressive nomination is used, and the selected pairs can quickly
   change after ICE has completed.

8.3.2.  Lite Implementation Procedures

   A lite implementation MAY free candidates not selected by ICE as soon
   as ICE processing has reached the Completed state for all peers for
   all media streams using those candidates.

9.  Subsequent Offer/Answer Exchanges

   Either agent MAY generate a subsequent offer at any time allowed by
   RFC 3264 [RFC3264].  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.

   Should a subsequent offer be rejected, ICE processing continues as if
   the subsequent offer had never been made.











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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 states 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
      generate an updated offer that, 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 [RFC3264].

   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 a 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 a 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 lines 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 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

9.1.3.1.  Existing Media Streams with ICE Running

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

   A lite implementation MUST include all of its candidates for each
   component of each media stream in an a=candidate attribute in any
   subsequent offer.  These candidates are formed identically to the
   procedures for initial offers, as described in Section 4.2.

   A lite implementation MUST NOT add additional host candidates in a
   subsequent offer.  If an agent needs to offer additional candidates,
   it MUST restart ICE.

   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.





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9.1.3.2.  Existing Media Streams with ICE Completed

   If ICE has completed for a media stream, the default destination for
   that media stream MUST be set to the remote candidate of the
   candidate pair for that component in the valid list.  For a lite
   implementation, there is always just a single candidate pair in the
   valid list for each component of a media stream.  Additionally, the
   agent MUST include a candidate attribute for each default
   destination.

   Additionally, if the agent is controlling (which only happens when
   both agents are lite), the agent MUST include the a=remote-candidates
   attribute for each media stream.  The attribute contains the remote
   candidates from the candidate pairs in the valid list (one pair for
   each component of each 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 reapply the verification procedures in Section 5.1 without
   regard to the results of verification from any previous offer/answer
   exchanges.  Indeed, it is possible that a previous offer/answer
   exchange resulted in ICE not being used, but it is used as a
   consequence of a subsequent exchange.

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, then 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.




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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

   Unless the agent has detected an ICE restart from the offer, 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.

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,



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   and the receipt of the Binding response that tells the answerer the
   candidate that 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 offerer's 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 is 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
      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 is 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

   If the received offer contains the remote-candidates attribute for a
   media stream, the agent forms a candidate pair for each component of
   the media stream by:





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   o  Setting the remote candidate equal to the offerer's 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.

   It then places those candidates into the Valid list for the media
   stream.  The state of ICE processing for that media stream is set to
   Completed.

   Furthermore, if the agent believed it was controlling, but the offer
   contained the remote-candidates attribute, both agents believe they
   are controlling.  In this case, both would have sent updated offers
   around the same time.  However, the signaling protocol carrying the
   offer/answer exchanges will have resolved this glare condition, so
   that one agent is always the 'winner' by having its offer received
   before its peer has sent an offer.  The winner takes the role of
   controlled, so that the loser (the answerer under consideration in
   this section) MUST change its role to controlled.  Consequently, if
   the agent was going to send an updated offer since, based on the
   rules in Section 8.2.2, it was controlling, it no longer needs to.

   Besides the potential role change, change in the Valid list, and
   state changes, the construction of the answer is performed
   identically to the construction of an 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.



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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 ordinary checks for it.

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.  The state of ICE processing for each
   media stream MUST change to Running, and the state of ICE processing
   MUST change to Running.





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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 that is supported by its peer.  ICE endpoints allow for
   STUN-based keepalives for UDP streams, and as such, STUN keepalives
   MUST be used when an agent is a full ICE implementation and is
   communicating with a peer that supports ICE (lite or full).  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 that allows packets to easily be
   sent in the absence of actual media content is RECOMMENDED.  Examples
   of formats that readily meet this goal are RTP No-Op [NO-OP-RTP], and
   in cases where both sides support it, RTP comfort noise [RFC3389].
   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 that 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.  Tr MUST NOT be
   configured to less than 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.  Administrators
   deploying ICE in more controlled networking environments SHOULD set
   Tr to the longest duration possible in their environment.

   If STUN is being used for keepalives, a STUN Binding Indication is
   used [RFC5389].  The Indication MUST NOT utilize any authentication
   mechanism.  It SHOULD contain the FINGERPRINT attribute to aid in
   demultiplexing, but SHOULD NOT contain any other attributes.  It is
   used solely to keep the NAT bindings alive.  The Binding Indication
   is sent using the same local and remote candidates that are being
   used for media.  Though Binding Indications are used for keepalives,
   an agent MUST be prepared to receive a connectivity check as well.
   If a connectivity check is received, a response is generated as
   discussed in [RFC5389], but there is no impact on ICE processing
   otherwise.




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   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.

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 TURN
   server, using procedures defined in [RFC5766].

   If the local candidate is a relayed candidate, it is RECOMMENDED that
   an agent create a channel on the TURN server towards the remote
   candidate.  This is done using the procedures for channel creation as
   defined in Section 11 of [RFC5766].

   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.




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   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
   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
   [RFC3550] 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 [RFC3550] describes an algorithm in Section 8.2 for
   detecting synchronization source (SSRC) collisions and loops.  These
   algorithms are based, in part, on seeing different source transport



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   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
   selection of messages to use with offers and answers can affect
   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
   Provisional Response Acknowledgment (PRACK) mechanism [RFC3262] 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 3262.  To do this, the agent retransmits the
   provisional response with the exponential backoff timers described in
   RFC 3262.  Retransmits MUST cease on receipt of a STUN Binding



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   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 the answer in a 2xx response.  If the
   peer agent is lite, there will never be a STUN Binding request.  In
   such a case, the agent MUST cease retransmitting the 18x after
   sending it four times (ICE will actually work even if the peer never
   receives the 18x; however, experience has shown that sending it is
   important for middleboxes and firewall traversal).  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
   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 [RFC3960]) 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 [RFC3312], since it's a localized
   decision.  It also has the benefit of guaranteeing that not a single
   packet of media will get clipped, so that post-pickup delay is zero.
   If an agent chooses to delay local alerting in this way, it SHOULD
   generate a 180 response once alerting begins.







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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 [RFC3725], 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.

12.2.  SIP Option Tags and Media Feature Tags

   [RFC5768] 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 signaling 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 that 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
   [RFC3312] and RFC 4032 [RFC4032], apply only to the transport
   addresses listed as the default targets for media in an offer/answer.



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   If ICE changes the transport address where media is received, this
   change is reflected in an updated offer that 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 RFCs 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 [SDP-PRECON].  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 [SDP-PRECON].

12.5.  Interactions with Third Party Call Control

   ICE works with Flows I, III, and IV as described in [RFC3725].  Flow
   I works without the controller supporting or being aware of ICE.
   Flow IV will work as long as the controller passes along the ICE
   attributes without alteration.  Flow II is fundamentally incompatible
   with ICE; each agent will believe itself to be the answerer and thus
   never generate a re-INVITE.

   The flows for continued operation, as described in Section 7 of RFC
   3725, require additional behavior of ICE implementations to support.
   In particular, if an agent receives a mid-dialog re-INVITE that
   contains no offer, it MUST restart ICE for each media stream and go
   through the process of gathering new candidates.  Furthermore, that
   list of candidates SHOULD include the ones currently being used for
   media.

13.  Relationship with ANAT

   RFC 4091 [RFC4091], the Alternative Network Address Types (ANAT)
   Semantics for the SDP grouping framework, and RFC 4092 [RFC4092], its
   usage with SIP, define 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.





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   This specification deprecates RFC 4091 and RFC 4092.  Instead, agents
   wishing to support dual stack will utilize ICE.

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
   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 warrant 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 is
   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.



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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 5234 [RFC5234]:

   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*32ice-char
   component-id          = 1*5DIGIT
   transport             = "UDP" / transport-extension
   transport-extension   = token              ; from RFC 3261
   priority              = 1*10DIGIT
   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 [RFC4566].  It is the
      IP address of the candidate, allowing for IPv4 addresses, IPv6
      addresses, and fully qualified domain names (FQDNs).  When parsing
      this field, an agent can differentiate an IPv4 address and an IPv6



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      address by presence of a colon in its value - the presence of a
      colon indicates IPv6.  An agent MUST ignore candidate lines that
      include candidates with IP address versions that are not supported
      or recognized.  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 first using an AAAA record
      (assuming the agent supports IPv6), and if no result is found or
      the agent only supports IPv4, using an A.  If the DNS query
      returns more than one IP address, one is chosen, and then used for
      the remainder of ICE processing.

   <port>:  is also taken from RFC 4566 [RFC4566].  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) [RFC4340].

   <foundation>:  is composed of 1 to 32 <ice-char>s.  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 that
      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 that require multiple
      components MUST develop specifications that 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**31 - 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.






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   <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> are 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 5234 [RFC5234].  The remote-
   candidates attribute is a media-level attribute only.

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

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




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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*256ice-char
   ufrag                 = 4*256ice-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
   overridden 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
   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, up to 256
   characters.  The upper limit allows for buffer sizing in
   implementations.  Its large upper limit allows for increased amounts
   of randomness to be added over time.

15.5.  "ice-options" Attribute

   The "ice-options" attribute is a session-level attribute.  It
   contains a series of tokens that 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.  Setting Ta and RTO

   During the gathering phase of ICE (Section 4.1.1) and while ICE is
   performing connectivity checks (Section 7), an agent sends STUN and
   TURN transactions.  These transactions are paced at a rate of one
   every Ta milliseconds, and utilize a specific RTO.  This section
   describes how the values of Ta and RTO are computed.  This



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   computation depends on whether ICE is being used with a real-time
   media stream (such as RTP) or something else.  When ICE is used for a
   stream with a known maximum bandwidth, the computation in
   Section 16.1 MAY be followed to rate-control the ICE exchanges.  For
   all other streams, the computation in Section 16.2 MUST be followed.

16.1.  RTP Media Streams

   The values of RTO and Ta change during the lifetime of ICE
   processing.  One set of values applies during the gathering phase,
   and the other, for connectivity checks.

   The value of Ta SHOULD be configurable, and SHOULD have a default of:

   For each media stream i:

    Ta_i = (stun_packet_size / rtp_packet_size) * rtp_ptime

                           1
     Ta = MAX (20ms, ------------------- )
                           k
                         ----
                         \        1
                          >    ------
                         /       Ta_i
                         ----
                          i=1

   where k is the number of media streams.  During the gathering phase,
   Ta is computed based on the number of media streams the agent has
   indicated in its offer or answer, and the RTP packet size and RTP
   ptime are those of the most preferred codec for each media stream.
   Once an offer and answer have been exchanged, the agent recomputes Ta
   to pace the connectivity checks.  In that case, the value of Ta is
   based on the number of media streams that will actually be used in
   the session, and the RTP packet size and RTP ptime are those of the
   most preferred codec with which the agent will send.

   In addition, the retransmission timer for the STUN transactions, RTO,
   defined in [RFC5389], SHOULD be configurable and during the gathering
   phase, SHOULD have a default of:

     RTO = MAX (100ms, Ta * (number of pairs))

   where the number of pairs refers to the number of pairs of candidates
   with STUN or TURN servers.





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   For connectivity checks, RTO SHOULD be configurable and SHOULD have a
   default of:

     RTO = MAX (100ms, Ta*N * (Num-Waiting + Num-In-Progress))

   where Num-Waiting is the number of checks in the check list in the
   Waiting state, and Num-In-Progress is the number of checks in the In-
   Progress state.  Note that the RTO will be different for each
   transaction as the number of checks in the Waiting and In-Progress
   states change.

   These formulas are aimed at causing STUN transactions to be paced at
   the same rate as media.  This ensures that ICE will work properly
   under the same network conditions needed to support the media as
   well.  See Appendix B.1 for additional discussion and motivations.
   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 that are gathered.

   The formulas result in a behavior whereby an agent will send its
   first packet for every single connectivity check before performing a
   retransmit.  This can be seen in the formulas for the RTO (which
   represents the retransmit interval).  Those formulas scale with N,
   the number of checks to be performed.  As a result of this, ICE
   maintains a nicely constant rate, but becomes more sensitive to
   packet loss.  The loss of the first single packet for any
   connectivity check is likely to cause that pair to take a long time
   to be validated, and instead, a lower-priority check (but one for
   which there was no packet loss) is much more likely to complete
   first.  This results in ICE performing sub-optimally, choosing lower-
   priority pairs over higher-priority pairs.  Implementors should be
   aware of this consequence, but still should utilize the timer values
   described here.

16.2.  Non-RTP Sessions

   In cases where ICE is used to establish some kind of session that is
   not real time, and has no fixed rate associated with it that is known
   to work on the network in which ICE is deployed, Ta and RTO revert to
   more conservative values.  Ta SHOULD be configurable, SHOULD have a
   default of 500 ms, and MUST NOT be configurable to be less than 500
   ms.

   In addition, the retransmission timer for the STUN transactions, RTO,
   SHOULD be configurable and during the gathering phase, SHOULD have a
   default of:



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     RTO = MAX (500ms, Ta * (number of pairs))

   where the number of pairs refers to the number of pairs of candidates
   with STUN or TURN servers.

   For connectivity checks, RTO SHOULD be configurable and SHOULD have a
   default of:

     RTO = MAX (500ms, Ta*N * (Num-Waiting + Num-In-Progress))

17.  Example

   The example is based on the simplified topology of Figure 8.

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

                        Figure 8: Example Topology

   Two agents, L and R, are using ICE.  Both are full-mode ICE
   implementations and use aggressive nomination when they are
   controlling.  Both agents have a single IPv4 address.  For agent L,
   it is 10.0.1.1 in private address space [RFC1918], 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



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   practice the agents do not need to use the same STUN server), which
   is listening for STUN Binding requests at an IP address of 192.0.2.2
   and port 3478.  TURN servers 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 IP address
   the transport address is on, and is one of "L", "R", "STUN", or
   "NAT".  The type is either "PUB" for transport addresses that are
   public, and "PRIV" for transport addresses that are private.
   Finally, seq-no is a sequence number that is different for each
   transport address of the same type on a particular entity.  Each
   variable has an IP address and port, denoted by varname.IP and
   varname.PORT, respectively, where varname is the name of the
   variable.

   The STUN server has advertised transport address STUN-PUB-1 (which is
   192.0.2.2:3478).

   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 |              |
             |              |------------->|              |







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






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

                          Figure 9: Example Flow

   First, agent L obtains a host candidate from its local IP address
   (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 2130706431 and for the
   server reflexive candidate is 1694498815.  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):








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       v=0
       o=jdoe 2890844526 2890842807 IN IP4 $L-PRIV-1.IP
       s=
       c=IN IP4 $NAT-PUB-1.IP
       t=0 0
       a=ice-pwd:asd88fgpdd777uzjYhagZg
       a=ice-ufrag:8hhY
       m=audio $NAT-PUB-1.PORT RTP/AVP 0
       b=RS:0
       b=RR:0
       a=rtpmap:0 PCMU/8000
       a=candidate:1 1 UDP 2130706431 $L-PRIV-1.IP $L-PRIV-1.PORT typ
       host
       a=candidate:2 1 UDP 1694498815 $NAT-PUB-1.IP $NAT-PUB-1.PORT typ
        srflx raddr $L-PRIV-1.IP rport $L-PRIV-1.PORT

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

       v=0
       o=jdoe 2890844526 2890842807 IN IP4 10.0.1.1
       s=
       c=IN IP4 192.0.2.3
       t=0 0
       a=ice-pwd:asd88fgpdd777uzjYhagZg
       a=ice-ufrag:8hhY
       m=audio 45664 RTP/AVP 0
       b=RS:0
       b=RR:0
       a=rtpmap:0 PCMU/8000
       a=candidate:1 1 UDP 2130706431 10.0.1.1 8998 typ host
       a=candidate:2 1 UDP 1694498815 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 2130706431.  It chooses a foundation
   of 1 for its single candidate.  Its resulting answer looks like:









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       v=0
       o=bob 2808844564 2808844564 IN IP4 $R-PUB-1.IP
       s=
       c=IN IP4 $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 2130706431 $R-PUB-1.IP $R-PUB-1.PORT typ host

   With the variables filled in:

       v=0
       o=bob 2808844564 2808844564 IN IP4 192.0.2.1
       s=
       c=IN IP4 192.0.2.1
       t=0 0
       a=ice-pwd:YH75Fviy6338Vbrhrlp8Yh
       a=ice-ufrag:9uB6
       m=audio 3478 RTP/AVP 0
       b=RS:0
       b=RR:0
       a=rtpmap:0 PCMU/8000
       a=candidate:1 1 UDP 2130706431 192.0.2.1 3478 typ host

   Since neither side indicated that it is lite, the agent that 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




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   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.

   Soon after 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.

18.  Security Considerations

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

   o  Using ICE in conjunction with secure signaling techniques, such as
      SIPS.

   o  Limiting the total number of connectivity checks to 100, and
      optionally limiting the number of candidates they'll accept in an
      offer or answer.









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18.1.  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 Denial-of-Service
      (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.

      If an attacker can cause a false peer reflexive candidate or false
      valid on a false candidate, it can launch any of the attacks
      described in [RFC5389].

   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 400.  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 short-term credential 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



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   signaling is secured, the attacker will not have the password and its
   response will be discarded.

   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 by the STUN short-term
   credential mechanism in conjunction with a secure offer/answer
   exchange.

   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
   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.





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   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 itself is identified by the fake candidate, the
   attack is easier to coordinate.  However, if SRTP is used [RFC3711],
   the attacker will not be able to play the media packets, but 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,
   it's much easier to just disrupt it with the same mechanism, rather
   than attack ICE.

18.2.  Attacks on Server Reflexive Address Gathering

   ICE endpoints make use of STUN Binding requests for gathering server
   reflexive candidates from a STUN server.  These requests are not
   authenticated in any way.  As a consequence, there are numerous
   techniques an attacker can employ to provide the client with a false
   server reflexive candidate:

   o  An attacker can compromise the DNS, causing DNS queries to return
      a rogue STUN server address.  That server can provide the client
      with fake server reflexive candidates.  This attack is mitigated
      by DNS security, though DNS-SEC is not required to address it.

   o  An attacker that can observe STUN messages (such as an attacker on
      a shared network segment, like WiFi) can inject a fake response
      that is valid and will be accepted by the client.

   o  An attacker can compromise a STUN server by means of a virus, and
      cause it to send responses with incorrect mapped addresses.

   A false mapped address learned by these attacks 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, nor
   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.




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   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.

18.3.  Attacks on Relayed Candidate Gathering

   An attacker might attempt to disrupt the gathering of relayed
   candidates, forcing the client to believe it has a false relayed
   candidate.  Exchanges with the TURN server are authenticated using a
   long-term credential.  Consequently, injection of fake responses or
   requests will not work.  In addition, unlike Binding requests,
   Allocate requests are not susceptible to replay attacks with modified
   source IP addresses and ports, since the source IP address and port
   are not utilized to provide the client with its relayed candidate.

   However, TURN servers are susceptible to DNS attacks, or to viruses
   aimed at the TURN server, for purposes of turning it into a zombie or
   rogue server.  These attacks can be mitigated by DNS-SEC and through
   good box and software security on TURN servers.

   Even if an attacker has caused the client to believe in a false
   relayed candidate, the connectivity checks cause such a candidate to
   be used only if they succeed.  Thus, an attacker must launch a false
   valid on a false candidate, per above, which is a very difficult
   attack to coordinate.

18.4.  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 [RFC3264] apply.  These
   require techniques for message integrity and encryption for offers
   and answers, which are satisfied by the SIPS mechanism [RFC3261] when
   SIP is used.  As such, the usage of SIPS with ICE is RECOMMENDED.








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18.5.  Insider Attacks

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

18.5.1.  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 is known, and is
   limited to ones that support ICE, the server can reject any offers or
   answers that don't indicate ICE support.

18.5.2.  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 Ta ms (say, Ta=20ms).
   However, the retransmission timers are set to a large number due to
   the large number of candidates.  As a consequence, packets will be
   sent at an interval of one every Ta milliseconds, and then with
   increasing intervals after that.  Thus, STUN will not send packets at
   a rate faster than media would be sent, and the STUN packets persist
   only briefly, until ICE fails for the session.  Nonetheless, this is
   an amplification mechanism.

   It is impossible to eliminate the amplification, but the volume can
   be reduced through a variety of heuristics.  Agents SHOULD limit the



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   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.

   Frequently, protocols that wish to avoid these kinds of attacks force
   the initiator to wait for a response prior to sending the next
   message.  However, in the case of ICE, this is not possible.  It is
   not possible to differentiate the following two cases:

   o  There was no response because the initiator is being used to
      launch a DoS attack against an unsuspecting target that will not
      respond.

   o  There was no response because the IP address and port are not
      reachable by the initiator.

   In the second case, another check should be sent at the next
   opportunity, while in the former case, no further checks should be
   sent.

18.6.  Interactions with Application Layer Gateways and SIP

   Application Layer Gateways (ALGs) are functions present in a NAT
   device that inspect the contents of packets and modify them, in order
   to facilitate NAT traversal for application protocols.  Session
   Border Controllers (SBCs) 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.  A correct ALG
   implementation behaves as follows:

   o  The ALG does not modify the m and c lines or the rtcp attribute if
      they contain external addresses.

   o  If the m and c lines 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 matching the
         values in the m and c lines or rtcp attribute, the ALG uses
         that binding instead of creating a new one.

         If the ALG does not already have a binding, it creates a new
         one and modifies the SDP, rewriting the m and c lines and rtcp
         attribute.




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   Unfortunately, many ALGs 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 that 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.

19.  STUN Extensions

19.1.  New Attributes

   This specification 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
   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 order, which contains a random number used for tie-
   breaking of role conflicts.



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   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 order, which contains a random number used for tie-
   breaking of role conflicts.

19.2.  New Error Response Codes

   This specification 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.

20.  Operational Considerations

   This section discusses issues relevant to network operators looking
   to deploy ICE.

20.1.  NAT and Firewall Types

   ICE was designed to work with existing NAT and firewall equipment.
   Consequently, it is not necessary to replace or reconfigure existing
   firewall and NAT equipment in order to facilitate deployment of ICE.
   Indeed, ICE was developed to be deployed in environments where the
   Voice over IP (VoIP) operator has no control over the IP network
   infrastructure, including firewalls and NAT.

   That said, ICE works best in environments where the NAT devices are
   "behave" compliant, meeting the recommendations defined in [RFC4787]
   and [RFC5766].  In networks with behave-compliant NAT, ICE will work
   without the need for a TURN server, thus improving voice quality,
   decreasing call setup times, and reducing the bandwidth demands on
   the network operator.

20.2.  Bandwidth Requirements

   Deployment of ICE can have several interactions with available
   network capacity that operators should take into consideration.

20.2.1.  STUN and TURN Server Capacity Planning

   First and foremost, ICE makes use of TURN and STUN servers, which
   would typically be located in the network operator's data centers.
   The STUN servers require relatively little bandwidth.  For each
   component of each media stream, there will be one or more STUN



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   transactions from each client to the STUN server.  In a basic voice-
   only IPv4 VoIP deployment, there will be four transactions per call
   (one for RTP and one for RTCP, for both caller and callee).  Each
   transaction is a single request and a single response, the former
   being 20 bytes long, and the latter, 28.  Consequently, if a system
   has N users, and each makes four calls in a busy hour, this would
   require N*1.7bps.  For one million users, this is 1.7 Mbps, a very
   small number (relatively speaking).

   TURN traffic is more substantial.  The TURN server will see traffic
   volume equal to the STUN volume (indeed, if TURN servers are
   deployed, there is no need for a separate STUN server), in addition
   to the traffic for the actual media traffic.  The amount of calls
   requiring TURN for media relay is highly dependent on network
   topologies, and can and will vary over time.  In a network with 100%
   behave-compliant NAT, it is exactly zero.  At time of writing, large-
   scale consumer deployments were seeing between 5 and 10 percent of
   calls requiring TURN servers.  Considering a voice-only deployment
   using G.711 (so 80 kbps in each direction), with .2 erlangs during
   the busy hour, this is N*3.2 kbps.  For a population of one million
   users, this is 3.2 Gbps, assuming a 10% usage of TURN servers.

20.2.2.  Gathering and Connectivity Checks

   The process of gathering of candidates and performing of connectivity
   checks can be bandwidth intensive.  ICE has been designed to pace
   both of these processes.  The gathering phase and the connectivity
   check phase are meant to generate traffic at roughly the same
   bandwidth as the media traffic itself.  This was done to ensure that,
   if a network is designed to support multimedia traffic of a certain
   type (voice, video, or just text), it will have sufficient capacity
   to support the ICE checks for that media.  Of course, the ICE checks
   will cause a marginal increase in the total utilization; however,
   this will typically be an extremely small increase.

   Congestion due to the gathering and check phases has proven to be a
   problem in deployments that did not utilize pacing.  Typically,
   access links became congested as the endpoints flooded the network
   with checks as fast as they can send them.  Consequently, network
   operators should make sure that their ICE implementations support the
   pacing feature.  Though this pacing does increase call setup times,
   it makes ICE network friendly and easier to deploy.

20.2.3.  Keepalives

   STUN keepalives (in the form of STUN Binding Indications) are sent in
   the middle of a media session.  However, they are sent only in the
   absence of actual media traffic.  In deployments that are not



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   utilizing Voice Activity Detection (VAD), the keepalives are never
   used and there is no increase in bandwidth usage.  When VAD is being
   used, keepalives will be sent during silence periods.  This involves
   a single packet every 15-20 seconds, far less than the packet every
   20-30 ms that is sent when there is voice.  Therefore, keepalives
   don't have any real impact on capacity planning.

20.3.  ICE and ICE-lite

   Deployments utilizing a mix of ICE and ICE-lite interoperate
   perfectly.  They have been explicitly designed to do so, without loss
   of function.

   However, ICE-lite can only be deployed in limited use cases.  Those
   cases, and the caveats involved in doing so, are documented in
   Appendix A.

20.4.  Troubleshooting and Performance Management

   ICE utilizes end-to-end connectivity checks, and places much of the
   processing in the endpoints.  This introduces a challenge to the
   network operator -- how can they troubleshoot ICE deployments?  How
   can they know how ICE is performing?

   ICE has built-in features to help deal with these problems.  SIP
   servers on the signaling path, typically deployed in the data centers
   of the network operator, will see the contents of the offer/answer
   exchanges that convey the ICE parameters.  These parameters include
   the type of each candidate (host, server reflexive, or relayed),
   along with their related addresses.  Once ICE processing has
   completed, an updated offer/answer exchange takes place, signaling
   the selected address (and its type).  This updated re-INVITE is
   performed exactly for the purposes of educating network equipment
   (such as a diagnostic tool attached to a SIP server) about the
   results of ICE processing.

   As a consequence, through the logs generated by the SIP server, a
   network operator can observe what types of candidates are being used
   for each call, and what address was selected by ICE.  This is the
   primary information that helps evaluate how ICE is performing.

20.5.  Endpoint Configuration

   ICE relies on several pieces of data being configured into the
   endpoints.  This configuration data includes timers, credentials for
   TURN servers, and hostnames for STUN and TURN servers.  ICE itself
   does not provide a mechanism for this configuration.  Instead, it is
   assumed that this information is attached to whatever mechanism is



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   used to configure all of the other parameters in the endpoint.  For
   SIP phones, standard solutions such as the configuration framework
   [SIP-UA-FRMWK] have been defined.

21.  IANA Considerations

   This specification registers new SDP attributes, four new STUN
   attributes, and one new STUN error response.

21.1.  SDP Attributes

   This specification defines seven new SDP attributes per the
   procedures of Section 8.2.4 of [RFC4566].  The required information
   for the registrations is included here.

21.1.1.  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 Session Traversal Utilities
      for NAT (STUN)).

   Appropriate Values:  See Section 15 of RFC 5245.

21.1.2.  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.



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

   Appropriate Values:  See Section 15 of RFC 5245.

21.1.3.  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 5245.

21.1.4.  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 5245.





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21.1.5.  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 5245.

21.1.6.  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 5245.

21.1.7.  ice-options Attribute

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

   Attribute Name:  ice-options

   Long Form:  ice-options

   Type of Attribute:  session-level




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

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

   Appropriate Values:  See Section 15 of RFC 5245.

21.2.  STUN Attributes

   This section registers four new STUN attributes per the procedures in
   [RFC5389].

      0x0024 PRIORITY
      0x0025 USE-CANDIDATE
      0x8029 ICE-CONTROLLED
      0x802A ICE-CONTROLLING

21.3.  STUN Error Responses

   This section registers one new STUN error response code per the
   procedures in [RFC5389].

      487   Role Conflict: The client asserted an ICE role (controlling
      or
            controlled) that is in conflict with the role of the server.

22.  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 [RFC3424].  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 significant 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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22.1.  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 that can be used for communication.

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

22.2.  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 that 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 addresses) 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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22.3.  Brittleness Introduced by ICE

   >From RFC 3424, 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, classic STUN (as described in RFC 3489 [RFC3489]) has
   several points of brittleness.  One of them is the discovery process
   that requires an agent to try to 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 classic 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 classic 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 classic 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.

   Classic STUN also introduces some security considerations.
   Fortunately, those security considerations are also mitigated by ICE.

   Consequently, ICE serves to repair the brittleness introduced in
   classic STUN, and does not introduce any additional brittleness into
   the system.

   The penalty of these improvements is that ICE increases session
   establishment times.





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22.4.  Requirements for a Long-Term Solution

   From RFC 3424, any UNSAF proposal must provide:

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

   Our conclusions from RFC 3489 remain unchanged.  However, we feel ICE
   actually helps because we believe it can be part of the long-term
   solution.

22.5.  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 that try
   to 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 classic
   STUN.  However, the update to STUN [RFC5389] uses an encoding that
   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 15 s, which is a very conservative
   estimate.  Eventually, over time, as NAT boxes become compliant to
   behave [RFC4787], 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.

23.  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, Kevin Johns, 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.



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24.  References

24.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC3605]  Huitema, C., "Real Time Control Protocol (RTCP) attribute
              in Session Description Protocol (SDP)", RFC 3605,
              October 2003.

   [RFC3261]  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.

   [RFC3264]  Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model
              with Session Description Protocol (SDP)", RFC 3264,
              June 2002.

   [RFC3556]  Casner, S., "Session Description Protocol (SDP) Bandwidth
              Modifiers for RTP Control Protocol (RTCP) Bandwidth",
              RFC 3556, July 2003.

   [RFC3312]  Camarillo, G., Marshall, W., and J. Rosenberg,
              "Integration of Resource Management and Session Initiation
              Protocol (SIP)", RFC 3312, October 2002.

   [RFC4032]  Camarillo, G. and P. Kyzivat, "Update to the Session
              Initiation Protocol (SIP) Preconditions Framework",
              RFC 4032, March 2005.

   [RFC3262]  Rosenberg, J. and H. Schulzrinne, "Reliability of
              Provisional Responses in Session Initiation Protocol
              (SIP)", RFC 3262, June 2002.

   [RFC4566]  Handley, M., Jacobson, V., and C. Perkins, "SDP: Session
              Description Protocol", RFC 4566, July 2006.

   [RFC4091]  Camarillo, G. and J. Rosenberg, "The Alternative Network
              Address Types (ANAT) Semantics for the Session Description
              Protocol (SDP) Grouping Framework", RFC 4091, June 2005.

   [RFC4092]  Camarillo, G. and J. Rosenberg, "Usage of the Session
              Description Protocol (SDP) Alternative Network Address
              Types (ANAT) Semantics in the Session Initiation Protocol
              (SIP)", RFC 4092, June 2005.




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   [RFC3484]  Draves, R., "Default Address Selection for Internet
              Protocol version 6 (IPv6)", RFC 3484, February 2003.

   [RFC5234]  Crocker, D., Ed., and P. Overell, "Augmented BNF for
              Syntax Specifications: ABNF", STD 68, RFC 5234, January
              2008.

   [RFC5389]  Rosenberg, J., Mahy, R., Matthews, P., and D. Wing,
              "Session Traversal Utilities for NAT (STUN)", RFC 5389,
              October 2008.

   [RFC5766]  Mahy, R., Matthews, P., and J. Rosenberg, "Traversal Using
              Relays around NAT (TURN): Relay Extensions to Session
              Traversal Utilities for NAT (STUN)", RFC 5766, April 2010.

   [RFC5768]  Rosenberg, J., "Indicating Support for Interactive
              Connectivity Establishment (ICE) in the Session Initiation
              Protocol (SIP)", RFC 5768, April 2010.

24.2.  Informative References

   [RFC3489]  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.

   [RFC3235]  Senie, D., "Network Address Translator (NAT)-Friendly
              Application Design Guidelines", RFC 3235, January 2002.

   [RFC3303]  Srisuresh, P., Kuthan, J., Rosenberg, J., Molitor, A., and
              A. Rayhan, "Middlebox communication architecture and
              framework", RFC 3303, August 2002.

   [RFC3725]  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.

   [RFC3102]  Borella, M., Lo, J., Grabelsky, D., and G. Montenegro,
              "Realm Specific IP: Framework", RFC 3102, October 2001.

   [RFC3103]  Borella, M., Grabelsky, D., Lo, J., and K. Taniguchi,
              "Realm Specific IP: Protocol Specification", RFC 3103,
              October 2001.

   [RFC3424]  Daigle, L. and IAB, "IAB Considerations for UNilateral
              Self-Address Fixing (UNSAF) Across Network Address
              Translation", RFC 3424, November 2002.



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   [RFC3550]  Schulzrinne, H., Casner, S., Frederick, R., and V.
              Jacobson, "RTP: A Transport Protocol for Real-Time
              Applications", STD 64, RFC 3550, July 2003.

   [RFC3711]  Baugher, M., McGrew, D., Naslund, M., Carrara, E., and K.
              Norrman, "The Secure Real-time Transport Protocol (SRTP)",
              RFC 3711, March 2004.

   [RFC3056]  Carpenter, B. and K. Moore, "Connection of IPv6 Domains
              via IPv4 Clouds", RFC 3056, February 2001.

   [RFC3389]  Zopf, R., "Real-time Transport Protocol (RTP) Payload for
              Comfort Noise (CN)", RFC 3389, September 2002.

   [RFC3960]  Camarillo, G. and H. Schulzrinne, "Early Media and Ringing
              Tone Generation in the Session Initiation Protocol (SIP)",
              RFC 3960, December 2004.

   [RFC2475]  Blake, S., Black, D., Carlson, M., Davies, E., Wang, Z.,
              and W. Weiss, "An Architecture for Differentiated
              Services", RFC 2475, December 1998.

   [RFC1918]  Rekhter, Y., Moskowitz, R., Karrenberg, D., Groot, G., and
              E. Lear, "Address Allocation for Private Internets",
              BCP 5, RFC 1918, February 1996.

   [RFC4787]  Audet, F. and C. Jennings, "Network Address Translation
              (NAT) Behavioral Requirements for Unicast UDP", BCP 127,
              RFC 4787, January 2007.

   [SDP-PRECON]
              Andreasen, F., Camarillo, G., Oran, D., and D. Wing,
              "Connectivity Preconditions for Session Description
              Protocol Media Streams", Work in Progress, March 2010.

   [NO-OP-RTP]
              Andreasen, F., Oran, D., and D. Wing, "A No-Op Payload
              Format for RTP", Work in Progress, May 2007.

   [RFC5761]  Perkins, C. and M. Westerlund, "Multiplexing RTP Data and
              Control Packets on a Single Port", RFC 5761, April 2010.

   [RFC4340]  Kohler, E., Handley, M., and S. Floyd, "Datagram
              Congestion Control Protocol (DCCP)", RFC 4340, March 2006.

   [RFC4103]  Hellstrom, G. and P. Jones, "RTP Payload for Text
              Conversation", RFC 4103, June 2005.




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   [RFC5626]  Jennings, C., Mahy, R., and F. Audet, "Managing Client-
              Initiated Connections in the Session Initiation Protocol
              (SIP)", RFC 5626, October 2009.

   [RFC5382]  Guha, S., Biswas, K., Ford, B., Sivakumar, S., and P.
              Srisuresh, "NAT Behavioral Requirements for TCP", BCP 142,
              RFC 5382, October 2008.

   [SIP-UA-FRMWK]
              Petrie, D. and S. Channabasappa, Ed., "A Framework for
              Session Initiation Protocol User Agent Profile Delivery",
              Work in Progress, February 2010.

   [ICE-TCP]  Perreault, S., Ed. and J. Rosenberg, "TCP Candidates with
              Interactive Connectivity Establishment (ICE)", Work
              in Progress, October 2009.



































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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 that
   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
   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.

   ICE allows a lite implementation to have a single IPv4 host candidate
   and several IPv6 addresses.  In that case, candidate pairs are
   selected by the controlling agent using a static algorithm, such as
   the one in RFC 3484, which is recommended by this specification.
   However, static mechanisms for address selection are always prone to
   error, since they cannot ever reflect the actual topology and can
   never provide actual guarantees on connectivity.  They are always
   heuristics.  Consequently, if an agent is implementing ICE just to
   select between its IPv4 and IPv6 addresses, and none of its IP
   addresses are behind NAT, usage of full ICE is still RECOMMENDED in
   order to provide the most robust form of address selection possible.

   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 with just a single IPv4 address, a full
   implementation is preferable if achievable.  A full implementation
   will reduce call setup times, since ICE's aggressive mode can be
   used.  Full implementations also obtain the security benefits of ICE
   unrelated to NAT traversal; in particular, the voice hammer attack
   described in Section 18 is prevented only for full implementations,



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   not lite.  Finally, it is often the case that a device that 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 that may themselves be
   simple, but derive from complicated or non-obvious thinking or use
   cases that 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.  Each transaction, in turn, has a
   retransmission timer RTO that is a function of Ta as well.  Why are
   these transactions paced, and why are these formulas used?

   Sending of these STUN requests will often have the effect of creating
   bindings on NAT devices between the client and the STUN servers.
   Experience has shown that many NAT devices have upper limits on the
   rate at which they will create new bindings.  Experiments have shown
   that once every 20 ms is well supported, but not much lower than
   that.  This is why Ta has a lower bound of 20 ms.  Furthermore,
   transmission of these packets on the network makes use of bandwidth
   and needs to be rate limited by the agent.  Deployments based on
   earlier draft versions of this document tended to overload rate-
   constrained access links and perform poorly overall, in addition to
   negatively impacting the network.  As a consequence, the pacing
   ensures that the NAT device does not get overloaded and that traffic
   is kept at a reasonable rate.

   The definition of a "reasonable" rate is that STUN should not use
   more bandwidth than the RTP itself will use, once media starts
   flowing.  The formula for Ta is designed so that, if a STUN packet
   were sent every Ta seconds, it would consume the same amount of
   bandwidth as RTP packets, summed across all media streams.  Of
   course, STUN has retransmits, and the desire is to pace those as
   well.  For this reason, RTO is set such that the first retransmit on
   the first transaction happens just as the first STUN request on the
   last transaction occurs.  Pictorially:




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              First Packets              Retransmits



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

           +--+    +--+    +--+    +--+    +--+    +--+
           |A1|    |B1|    |C1|    |A2|    |B2|    |C2|
           +--+    +--+    +--+    +--+    +--+    +--+

        ---+-------+-------+-------+-------+-------+------------ Time
           0       Ta      2Ta     3Ta     4Ta     5Ta

   In this picture, there are three transactions that will be sent (for
   example, in the case of candidate gathering, there are three host
   candidate/STUN server pairs).  These are transactions A, B, and C.
   The retransmit timer is set so that the first retransmission on the
   first transaction (packet A2) is sent at time 3Ta.

   Subsequent retransmits after the first will occur even less
   frequently than Ta milliseconds apart, since STUN uses an exponential
   back-off on its retransmissions.

B.2.  Candidates with Multiple Bases

   Section 4.1.3 talks about eliminating candidates that have the same
   transport address and base.  However, candidates with the same
   transport addresses but different bases are not redundant.  When can
   an agent have two candidates that have the same IP address and port,
   but different bases?  Consider the topology of Figure 10:

















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          +----------+
          | STUN Srvr|
          +----------+
               |
               |
             -----
           //     \\
          |         |
         |  B:net10  |
          |         |
           \\     //
             -----
               |
               |
          +----------+
          |   NAT    |
          +----------+
               |
               |
             -----
           //     \\
          |    A    |
         |192.168/16 |
          |         |
           \\     //
             -----
               |
               |
               |192.168.1.100      -----
          +----------+           //     \\             +----------+
          |          |          |         |            |          |
          | Offerer  |---------|  C:net10  |-----------| Answerer |
          |          |10.0.1.100|         | 10.0.1.101 |          |
          +----------+           \\     //             +----------+
                                   -----

           Figure 10: Identical Candidates with Different Bases

   In this case, the offerer is multihomed.  It has one IP address,
   10.0.1.100, 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 IP address of
   192.168.1.100 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 IP address on network C
   (10.0.1.100:2498) and a host candidate on its IP address on network A



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   (192.168.1.100:3344).  It performs a STUN query to its configured
   STUN server from 192.168.1.100:3344.  This query passes through the
   NAT, which happens to assign the binding 10.0.1.100:2498.  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 (10.0.1.100:2498).  However,
   the server reflexive candidate has a base of 192.168.1.100:3344, and
   the host candidate has a base of 10.0.1.100:2498.

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 for 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 TURN
   server.  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 the TURN
   server 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 actually 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 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.












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B.5.  The Candidate Pair Priority Formula

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

      pair priority = 2^32*MIN(G,D) + 2*MAX(G,D) + (G>D?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
   that the pairs are first sorted based on decreasing value of the
   minimum of the two priorities.  For pairs that have the same value of
   the minimum priority, the maximum priority is used to sort amongst
   them.  If the max and the min priorities are the same, the
   controlling agent's 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 MAX/MIN sorting.  MAX/MIN ensures that,
   for a particular agent, a lower-priority candidate is never used
   until all higher-priority candidates have been tried.

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 11.  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 11: 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
   [RFC3264].  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 [RFC4103], 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.






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   For these reasons, the media packets themselves cannot be relied
   upon.  ICE defines a simple periodic keepalive utilizing STUN Binding
   indications.  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 higher than server reflexive.  Why is that?  The reason has
   to do with the security considerations in Section 18.  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
   address gathering with Binding requests 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.

B.11.  Why Is the Conflict Resolution Mechanism Needed?

   When ICE runs between two peers, one agent acts as controlled, and
   the other as controlling.  Rules are defined as a function of
   implementation type and offerer/answerer to determine who is
   controlling and who is controlled.  However, the specification
   mentions that, in some cases, both sides might believe they are
   controlling, or both sides might believe they are controlled.  How
   can this happen?

   The condition when both agents believe they are controlled shows up
   in third party call control cases.  Consider the following flow:

             A         Controller          B
             |(1) INV()     |              |
             |<-------------|              |
             |(2) 200(SDP1) |              |
             |------------->|              |
             |              |(3) INV()     |
             |              |------------->|
             |              |(4) 200(SDP2) |
             |              |<-------------|
             |(5) ACK(SDP2) |              |
             |<-------------|              |
             |              |(6) ACK(SDP1) |
             |              |------------->|

                       Figure 12: Role Conflict Flow

   This flow is a variation on flow III of RFC 3725 [RFC3725].  In fact,
   it works better than flow III since it produces fewer messages.  In
   this flow, the controller sends an offerless INVITE to agent A, which



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   responds with its offer, SDP1.  The agent then sends an offerless
   INVITE to agent B, which it responds to with its offer, SDP2.  The
   controller then uses the offer from each agent to generate the
   answers.  When this flow is used, ICE will run between agents A and
   B, but both will believe they are in the controlling role.  With the
   role conflict resolution procedures, this flow will function properly
   when ICE is used.

   At this time, there are no documented flows that can result in the
   case where both agents believe they are controlled.  However, the
   conflict resolution procedures allow for this case, should a flow
   arise that would fit into this category.

Author's Address

   Jonathan Rosenberg
   jdrosen.net
   Monmouth, NJ
   US

   Email: jdrosen@jdrosen.net
   URI:   http://www.jdrosen.net





























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