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rfc:rfc5245

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.

Rosenberg Standards Track [Page 1] RFC 5245 ICE April 2010

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

Rosenberg Standards Track [Page 2] RFC 5245 ICE April 2010

   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

Rosenberg Standards Track [Page 4] RFC 5245 ICE April 2010

 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

Rosenberg Standards Track [Page 6] RFC 5245 ICE April 2010

 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.

Rosenberg Standards Track [Page 7] RFC 5245 ICE April 2010

 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

Rosenberg Standards Track [Page 8] RFC 5245 ICE April 2010

 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.

Rosenberg Standards Track [Page 9] RFC 5245 ICE April 2010

               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

Rosenberg Standards Track [Page 10] RFC 5245 ICE April 2010

 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.

Rosenberg Standards Track [Page 12] RFC 5245 ICE April 2010

 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.

Rosenberg Standards Track [Page 14] RFC 5245 ICE April 2010

 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.

Rosenberg Standards Track [Page 15] RFC 5245 ICE April 2010

 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.

Rosenberg Standards Track [Page 16] RFC 5245 ICE April 2010

 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.

Rosenberg Standards Track [Page 17] RFC 5245 ICE April 2010

 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.

Rosenberg Standards Track [Page 18] RFC 5245 ICE April 2010

 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

Rosenberg Standards Track [Page 19] RFC 5245 ICE April 2010

 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.

Rosenberg Standards Track [Page 20] RFC 5245 ICE April 2010

 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

Rosenberg Standards Track [Page 21] RFC 5245 ICE April 2010

 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.

Rosenberg Standards Track [Page 22] RFC 5245 ICE April 2010

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

Rosenberg Standards Track [Page 23] RFC 5245 ICE April 2010

 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.

Rosenberg Standards Track [Page 24] RFC 5245 ICE April 2010

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

Rosenberg Standards Track [Page 25] RFC 5245 ICE April 2010

 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,

Rosenberg Standards Track [Page 26] RFC 5245 ICE April 2010

 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):

Rosenberg Standards Track [Page 27] RFC 5245 ICE April 2010

     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.

Rosenberg Standards Track [Page 28] RFC 5245 ICE April 2010

 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.

Rosenberg Standards Track [Page 29] RFC 5245 ICE April 2010

 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.

Rosenberg Standards Track [Page 30] RFC 5245 ICE April 2010

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

Rosenberg Standards Track [Page 31] RFC 5245 ICE April 2010

 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.

Rosenberg Standards Track [Page 32] RFC 5245 ICE April 2010

     +------------------------------------------+
     |                                          |
     | +---------------------+                  |
     | |+----+ +----+ +----+ |   +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

Rosenberg Standards Track [Page 33] RFC 5245 ICE April 2010

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:

Rosenberg Standards Track [Page 34] RFC 5245 ICE April 2010

 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.

Rosenberg Standards Track [Page 35] RFC 5245 ICE April 2010

    +-----------+
    |           |
    |           |
    |  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.

Rosenberg Standards Track [Page 36] RFC 5245 ICE April 2010

 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

Rosenberg Standards Track [Page 37] RFC 5245 ICE April 2010

 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.

Rosenberg Standards Track [Page 38] RFC 5245 ICE April 2010

  • 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.

Rosenberg Standards Track [Page 39] RFC 5245 ICE April 2010

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

Rosenberg Standards Track [Page 40] RFC 5245 ICE April 2010

 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

Rosenberg Standards Track [Page 41] RFC 5245 ICE April 2010

 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.

Rosenberg Standards Track [Page 47] RFC 5245 ICE April 2010

 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

Rosenberg Standards Track [Page 48] RFC 5245 ICE April 2010

 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.

Rosenberg Standards Track [Page 49] RFC 5245 ICE April 2010

  • 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.

Rosenberg Standards Track [Page 51] RFC 5245 ICE April 2010

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

Rosenberg Standards Track [Page 53] RFC 5245 ICE April 2010

       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:

Rosenberg Standards Track [Page 62] RFC 5245 ICE April 2010

 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.

Rosenberg Standards Track [Page 66] RFC 5245 ICE April 2010

 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

Rosenberg Standards Track [Page 67] RFC 5245 ICE April 2010

 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

Rosenberg Standards Track [Page 68] RFC 5245 ICE April 2010

 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.

Rosenberg Standards Track [Page 69] RFC 5245 ICE April 2010

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.

Rosenberg Standards Track [Page 70] RFC 5245 ICE April 2010

 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.

Rosenberg Standards Track [Page 71] RFC 5245 ICE April 2010

 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.

Rosenberg Standards Track [Page 72] RFC 5245 ICE April 2010

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

Rosenberg Standards Track [Page 73] RFC 5245 ICE April 2010

    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.

Rosenberg Standards Track [Page 75] RFC 5245 ICE April 2010

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.

Rosenberg Standards Track [Page 77] RFC 5245 ICE April 2010

 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

Rosenberg Standards Track [Page 79] RFC 5245 ICE April 2010

 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):

Rosenberg Standards Track [Page 82] RFC 5245 ICE April 2010

     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:

Rosenberg Standards Track [Page 83] RFC 5245 ICE April 2010

     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

Rosenberg Standards Track [Page 84] RFC 5245 ICE April 2010

 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.

Rosenberg Standards Track [Page 85] RFC 5245 ICE April 2010

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

Rosenberg Standards Track [Page 86] RFC 5245 ICE April 2010

 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.

Rosenberg Standards Track [Page 87] RFC 5245 ICE April 2010

 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.

Rosenberg Standards Track [Page 88] RFC 5245 ICE April 2010

 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.

Rosenberg Standards Track [Page 89] RFC 5245 ICE April 2010

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

Rosenberg Standards Track [Page 90] RFC 5245 ICE April 2010

 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.

Rosenberg Standards Track [Page 91] RFC 5245 ICE April 2010

 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.

Rosenberg Standards Track [Page 92] RFC 5245 ICE April 2010

 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

Rosenberg Standards Track [Page 93] RFC 5245 ICE April 2010

 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

Rosenberg Standards Track [Page 94] RFC 5245 ICE April 2010

 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

Rosenberg Standards Track [Page 95] RFC 5245 ICE April 2010

 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.

Rosenberg Standards Track [Page 96] RFC 5245 ICE April 2010

 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.

Rosenberg Standards Track [Page 97] RFC 5245 ICE April 2010

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

Rosenberg Standards Track [Page 98] RFC 5245 ICE April 2010

 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.

Rosenberg Standards Track [Page 99] RFC 5245 ICE April 2010

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.

Rosenberg Standards Track [Page 100] RFC 5245 ICE April 2010

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.

Rosenberg Standards Track [Page 101] RFC 5245 ICE April 2010

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.

Rosenberg Standards Track [Page 102] RFC 5245 ICE April 2010

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.

Rosenberg Standards Track [Page 103] RFC 5245 ICE April 2010

 [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.

Rosenberg Standards Track [Page 104] RFC 5245 ICE April 2010

 [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.

Rosenberg Standards Track [Page 105] RFC 5245 ICE April 2010

 [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.

Rosenberg Standards Track [Page 106] RFC 5245 ICE April 2010

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,

Rosenberg Standards Track [Page 107] RFC 5245 ICE April 2010

 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:

Rosenberg Standards Track [Page 108] RFC 5245 ICE April 2010

            First Packets              Retransmits
                  |                        |
                  |                        |
           -------+------           -------+------
          /               \        /               \
         /                 \      /                 \
         +--+    +--+    +--+    +--+    +--+    +--+
         |A1|    |B1|    |C1|    |A2|    |B2|    |C2|
         +--+    +--+    +--+    +--+    +--+    +--+
  1. –+——-+——-+——-+——-+——-+———— 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:

Rosenberg Standards Track [Page 109] RFC 5245 ICE April 2010

        +----------+
        | 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

Rosenberg Standards Track [Page 110] RFC 5245 ICE April 2010

 (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.

Rosenberg Standards Track [Page 111] RFC 5245 ICE April 2010

 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.

Rosenberg Standards Track [Page 112] RFC 5245 ICE April 2010

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.

Rosenberg Standards Track [Page 113] RFC 5245 ICE April 2010

        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.

Rosenberg Standards Track [Page 114] RFC 5245 ICE April 2010

 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?

Rosenberg Standards Track [Page 115] RFC 5245 ICE April 2010

 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

Rosenberg Standards Track [Page 116] RFC 5245 ICE April 2010

 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

Rosenberg Standards Track [Page 117]

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