path: root/doc/rfc/rfc8445.txt

Internet Engineering Task Force (IETF)                        A. Keranen
Request for Comments: 8445                                   C. Holmberg
Obsoletes: 5245                                                 Ericsson
Category: Standards Track                                   J. Rosenberg
ISSN: 2070-1721                                              jdrosen.net
                                                               July 2018


             Interactive Connectivity Establishment (ICE):
       A Protocol for Network Address Translator (NAT) Traversal

Abstract

   This document describes a protocol for Network Address Translator
   (NAT) traversal for UDP-based communication.  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).

   This document obsoletes RFC 5245.

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 7841.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   https://www.rfc-editor.org/info/rfc8445.

















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Copyright Notice

   Copyright (c) 2018 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
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   publication of this document.  Please review these documents
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   This document may contain material from IETF Documents or IETF
   Contributions published or made publicly available before November
   10, 2008.  The person(s) controlling the copyright in some of this
   material may not have granted the IETF Trust the right to allow
   modifications of such material outside the IETF Standards Process.
   Without obtaining an adequate license from the person(s) controlling
   the copyright in such materials, this document may not be modified
   outside the IETF Standards Process, and derivative works of it may
   not be created outside the IETF Standards Process, except to format
   it for publication as an RFC or to translate it into languages other
   than English.

























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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   5
   2.  Overview of ICE . . . . . . . . . . . . . . . . . . . . . . .   6
     2.1.  Gathering Candidates  . . . . . . . . . . . . . . . . . .   8
     2.2.  Connectivity Checks . . . . . . . . . . . . . . . . . . .  10
     2.3.  Nominating Candidate Pairs and Concluding ICE . . . . . .  12
     2.4.  ICE Restart . . . . . . . . . . . . . . . . . . . . . . .  13
     2.5.  Lite Implementations  . . . . . . . . . . . . . . . . . .  13
   3.  ICE Usage . . . . . . . . . . . . . . . . . . . . . . . . . .  13
   4.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .  13
   5.  ICE Candidate Gathering and Exchange  . . . . . . . . . . . .  17
     5.1.  Full Implementation . . . . . . . . . . . . . . . . . . .  17
       5.1.1.  Gathering Candidates  . . . . . . . . . . . . . . . .  18
         5.1.1.1.  Host Candidates . . . . . . . . . . . . . . . . .  18
         5.1.1.2.  Server-Reflexive and Relayed Candidates . . . . .  20
         5.1.1.3.  Computing Foundations . . . . . . . . . . . . . .  21
         5.1.1.4.  Keeping Candidates Alive  . . . . . . . . . . . .  21
       5.1.2.  Prioritizing Candidates . . . . . . . . . . . . . . .  22
         5.1.2.1.  Recommended Formula . . . . . . . . . . . . . . .  22
         5.1.2.2.  Guidelines for Choosing Type and Local
                   Preferences . . . . . . . . . . . . . . . . . . .  23
       5.1.3.  Eliminating Redundant Candidates  . . . . . . . . . .  23
     5.2.  Lite Implementation Procedures  . . . . . . . . . . . . .  23
     5.3.  Exchanging Candidate Information  . . . . . . . . . . . .  24
     5.4.  ICE Mismatch  . . . . . . . . . . . . . . . . . . . . . .  26
   6.  ICE Candidate Processing  . . . . . . . . . . . . . . . . . .  26
     6.1.  Procedures for Full Implementation  . . . . . . . . . . .  26
       6.1.1.  Determining Role  . . . . . . . . . . . . . . . . . .  26
       6.1.2.  Forming the Checklists  . . . . . . . . . . . . . . .  28
         6.1.2.1.  Checklist State . . . . . . . . . . . . . . . . .  28
         6.1.2.2.  Forming Candidate Pairs . . . . . . . . . . . . .  28
         6.1.2.3.  Computing Pair Priority and Ordering Pairs  . . .  31
         6.1.2.4.  Pruning the Pairs . . . . . . . . . . . . . . . .  31
         6.1.2.5.  Removing Lower-Priority Pairs . . . . . . . . . .  31
         6.1.2.6.  Computing Candidate Pair States . . . . . . . . .  32
       6.1.3.  ICE State . . . . . . . . . . . . . . . . . . . . . .  36
       6.1.4.  Scheduling Checks . . . . . . . . . . . . . . . . . .  36
         6.1.4.1.  Triggered-Check Queue . . . . . . . . . . . . . .  36
         6.1.4.2.  Performing Connectivity Checks  . . . . . . . . .  36
     6.2.  Lite Implementation Procedures  . . . . . . . . . . . . .  38
   7.  Performing Connectivity Checks  . . . . . . . . . . . . . . .  38
     7.1.  STUN Extensions . . . . . . . . . . . . . . . . . . . . .  38
       7.1.1.  PRIORITY  . . . . . . . . . . . . . . . . . . . . . .  38
       7.1.2.  USE-CANDIDATE . . . . . . . . . . . . . . . . . . . .  38
       7.1.3.  ICE-CONTROLLED and ICE-CONTROLLING  . . . . . . . . .  39
     7.2.  STUN Client Procedures  . . . . . . . . . . . . . . . . .  39
       7.2.1.  Creating Permissions for Relayed Candidates . . . . .  39



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       7.2.2.  Forming Credentials . . . . . . . . . . . . . . . . .  39
       7.2.3.  Diffserv Treatment  . . . . . . . . . . . . . . . . .  40
       7.2.4.  Sending the Request . . . . . . . . . . . . . . . . .  40
       7.2.5.  Processing the Response . . . . . . . . . . . . . . .  40
         7.2.5.1.  Role Conflict . . . . . . . . . . . . . . . . . .  40
         7.2.5.2.  Failure . . . . . . . . . . . . . . . . . . . . .  41
           7.2.5.2.1.  Non-Symmetric Transport Addresses . . . . . .  41
           7.2.5.2.2.  ICMP Error  . . . . . . . . . . . . . . . . .  41
           7.2.5.2.3.  Timeout . . . . . . . . . . . . . . . . . . .  41
           7.2.5.2.4.  Unrecoverable STUN Response . . . . . . . . .  41
         7.2.5.3.  Success . . . . . . . . . . . . . . . . . . . . .  42
           7.2.5.3.1.  Discovering Peer-Reflexive Candidates . . . .  42
           7.2.5.3.2.  Constructing a Valid Pair . . . . . . . . . .  43
           7.2.5.3.3.  Updating Candidate Pair States  . . . . . . .  44
           7.2.5.3.4.  Updating the Nominated Flag . . . . . . . . .  44
         7.2.5.4.  Checklist State Updates . . . . . . . . . . . . .  44
     7.3.  STUN Server Procedures  . . . . . . . . . . . . . . . . .  45
       7.3.1.  Additional Procedures for Full Implementations  . . .  45
         7.3.1.1.  Detecting and Repairing Role Conflicts  . . . . .  46
         7.3.1.2.  Computing Mapped Addresses  . . . . . . . . . . .  47
         7.3.1.3.  Learning Peer-Reflexive Candidates  . . . . . . .  47
         7.3.1.4.  Triggered Checks  . . . . . . . . . . . . . . . .  47
         7.3.1.5.  Updating the Nominated Flag . . . . . . . . . . .  49
       7.3.2.  Additional Procedures for Lite Implementations  . . .  49
   8.  Concluding ICE Processing . . . . . . . . . . . . . . . . . .  50
     8.1.  Procedures for Full Implementations . . . . . . . . . . .  50
       8.1.1.  Nominating Pairs  . . . . . . . . . . . . . . . . . .  50
       8.1.2.  Updating Checklist and ICE States . . . . . . . . . .  51
     8.2.  Procedures for Lite Implementations . . . . . . . . . . .  52
     8.3.  Freeing Candidates  . . . . . . . . . . . . . . . . . . .  53
       8.3.1.  Full Implementation Procedures  . . . . . . . . . . .  53
       8.3.2.  Lite Implementation Procedures  . . . . . . . . . . .  53
   9.  ICE Restarts  . . . . . . . . . . . . . . . . . . . . . . . .  53
   10. ICE Option  . . . . . . . . . . . . . . . . . . . . . . . . .  54
   11. Keepalives  . . . . . . . . . . . . . . . . . . . . . . . . .  54
   12. Data Handling . . . . . . . . . . . . . . . . . . . . . . . .  55
     12.1.  Sending Data . . . . . . . . . . . . . . . . . . . . . .  55
       12.1.1.  Procedures for Lite Implementations  . . . . . . . .  56
     12.2.  Receiving Data . . . . . . . . . . . . . . . . . . . . .  56
   13. Extensibility Considerations  . . . . . . . . . . . . . . . .  57
   14. Setting Ta and RTO  . . . . . . . . . . . . . . . . . . . . .  57
     14.1.  General  . . . . . . . . . . . . . . . . . . . . . . . .  57
     14.2.  Ta . . . . . . . . . . . . . . . . . . . . . . . . . . .  58
     14.3.  RTO  . . . . . . . . . . . . . . . . . . . . . . . . . .  58
   15. Examples  . . . . . . . . . . . . . . . . . . . . . . . . . .  59
     15.1.  Example with IPv4 Addresses  . . . . . . . . . . . . . .  60
     15.2.  Example with IPv6 Addresses  . . . . . . . . . . . . . .  65




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   16. STUN Extensions . . . . . . . . . . . . . . . . . . . . . . .  69
     16.1.  Attributes . . . . . . . . . . . . . . . . . . . . . . .  69
     16.2.  New Error-Response Codes . . . . . . . . . . . . . . . .  70
   17. Operational Considerations  . . . . . . . . . . . . . . . . .  70
     17.1.  NAT and Firewall Types . . . . . . . . . . . . . . . . .  70
     17.2.  Bandwidth Requirements . . . . . . . . . . . . . . . . .  70
       17.2.1.  STUN and TURN Server-Capacity Planning . . . . . . .  71
       17.2.2.  Gathering and Connectivity Checks  . . . . . . . . .  71
       17.2.3.  Keepalives . . . . . . . . . . . . . . . . . . . . .  72
     17.3.  ICE and ICE-Lite . . . . . . . . . . . . . . . . . . . .  72
     17.4.  Troubleshooting and Performance Management . . . . . . .  72
     17.5.  Endpoint Configuration . . . . . . . . . . . . . . . . .  73
   18. IAB Considerations  . . . . . . . . . . . . . . . . . . . . .  73
     18.1.  Problem Definition . . . . . . . . . . . . . . . . . . .  73
     18.2.  Exit Strategy  . . . . . . . . . . . . . . . . . . . . .  74
     18.3.  Brittleness Introduced by ICE  . . . . . . . . . . . . .  74
     18.4.  Requirements for a Long-Term Solution  . . . . . . . . .  75
     18.5.  Issues with Existing NAPT Boxes  . . . . . . . . . . . .  75
   19. Security Considerations . . . . . . . . . . . . . . . . . . .  76
     19.1.  IP Address Privacy . . . . . . . . . . . . . . . . . . .  76
     19.2.  Attacks on Connectivity Checks . . . . . . . . . . . . .  77
     19.3.  Attacks on Server-Reflexive Address Gathering  . . . . .  80
     19.4.  Attacks on Relayed Candidate Gathering . . . . . . . . .  80
     19.5.  Insider Attacks  . . . . . . . . . . . . . . . . . . . .  81
       19.5.1.  STUN Amplification Attack  . . . . . . . . . . . . .  81
   20. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  82
     20.1.  STUN Attributes  . . . . . . . . . . . . . . . . . . . .  82
     20.2.  STUN Error Responses . . . . . . . . . . . . . . . . . .  82
     20.3.  ICE Options  . . . . . . . . . . . . . . . . . . . . . .  82
   21. Changes from RFC 5245 . . . . . . . . . . . . . . . . . . . .  83
   22. References  . . . . . . . . . . . . . . . . . . . . . . . . .  84
     22.1.  Normative References . . . . . . . . . . . . . . . . . .  84
     22.2.  Informative References . . . . . . . . . . . . . . . . .  85
   Appendix A.  Lite and Full Implementations  . . . . . . . . . . .  89
   Appendix B.  Design Motivations . . . . . . . . . . . . . . . . .  90
     B.1.  Pacing of STUN Transactions . . . . . . . . . . . . . . .  90
     B.2.  Candidates with Multiple Bases  . . . . . . . . . . . . .  92
     B.3.  Purpose of the Related-Address and Related-Port
           Attributes  . . . . . . . . . . . . . . . . . . . . . . .  94
     B.4.  Importance of the STUN Username . . . . . . . . . . . . .  95
     B.5.  The Candidate Pair Priority Formula . . . . . . . . . . .  96
     B.6.  Why Are Keepalives Needed?  . . . . . . . . . . . . . . .  96
     B.7.  Why Prefer Peer-Reflexive Candidates? . . . . . . . . . .  97
     B.8.  Why Are Binding Indications Used for Keepalives?  . . . .  97
     B.9.  Selecting Candidate Type Preference . . . . . . . . . . .  97
   Appendix C.  Connectivity-Check Bandwidth . . . . . . . . . . . .  99
   Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . . . 100
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . . 100



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1.  Introduction

   Protocols establishing communication sessions between peers typically
   involve exchanging IP addresses and ports for the data sources and
   sinks.  However, this poses challenges when operated through Network
   Address Translators (NATs) [RFC3235].  These protocols also seek to
   create a data flow directly between participants, so that there is no
   application-layer intermediary between them.  This is done to reduce
   data latency, decrease packet loss, and reduce the operational costs
   of deploying the application.  However, this is difficult to
   accomplish through NATs.  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 NATs.  These include Application Layer Gateways
   (ALGs), the Middlebox Control Protocol [RFC3303], the original Simple
   Traversal of UDP Through NAT (STUN) specification [RFC3489] (note
   that RFC 3489 has been obsoleted by RFC 5389), and Realm Specific IP
   [RFC3102] [RFC3103] along with session description extensions needed
   to make them work, such as the Session Description Protocol (SDP)
   attribute [RFC4566] for the Real-Time Control Protocol (RTCP)
   [RFC3605].  Unfortunately, these techniques all have pros and cons
   that make each one optimal in some network topologies, but a poor
   choice in others.  The result is that administrators and implementers
   are making assumptions about the topologies of the networks in which
   their solutions will be deployed.  This introduces complexity and
   brittleness into the system.

   This specification defines Interactive Connectivity Establishment
   (ICE) as a technique for NAT traversal for UDP-based data streams
   (though ICE has been extended to handle other transport protocols,
   such as TCP [RFC6544]).  ICE works by exchanging a multiplicity of IP
   addresses and ports, which are then tested for connectivity by
   peer-to-peer connectivity checks.  The IP addresses and ports are
   exchanged using ICE-usage-specific mechanisms (e.g., in an Offer/
   Answer exchange), and the connectivity checks are performed using
   STUN [RFC5389].  ICE also makes use of Traversal Using Relay 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.
   For this reason, RFC 5245 [RFC5245] deprecated the solutions
   previously defined in RFC 4091 [RFC4091] and RFC 4092 [RFC4092].

   Appendix B provides background information and motivations regarding
   the design decisions that were made when designing ICE.






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2.  Overview of ICE

   In a typical ICE deployment, there are two endpoints (ICE agents)
   that want to communicate.  Note that ICE is not intended for NAT
   traversal for the signaling protocol, which is assumed to be provided
   via another mechanism.  ICE assumes that the agents are able to
   establish a signaling connection between each other.

   Initially, the agents are ignorant of their own topologies.  In
   particular, the agents may or may not be behind NATs (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 establish a data session.

   Figure 1 shows a typical ICE deployment.  The agents are labeled L
   and R.  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.  L and R are capable of engaging in a candidate
   exchange process, whose purpose is to set up a data session between L
   and R.  Typically, this exchange will occur through a signaling
   server (e.g., a SIP proxy).

   In addition to the agents, a signaling 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.

                               +---------+
             +--------+        |Signaling|         +--------+
             | STUN   |        |Server   |         | STUN   |
             | Server |        +---------+         | Server |
             +--------+       /           \        +--------+
                             /             \
                            /               \
                           / <- Signaling -> \
                          /                   \
                   +--------+               +--------+
                   |  NAT   |               |  NAT   |
                   +--------+               +--------+
                      /                             \
                     /                               \
                 +-------+                       +-------+
                 | Agent |                       | Agent |
                 |   L   |                       |   R   |
                 +-------+                       +-------+

                     Figure 1: ICE Deployment Scenario




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   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
   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 Candidates

   In order to execute ICE, an ICE agent identifies and gathers one or
   more address candidates.  A candidate has a transport address -- a
   combination of IP address and port for a particular transport
   protocol (with only UDP specified here).  There are different types
   of candidates; some are derived from physical or logical network
   interfaces, and others are discoverable via STUN and TURN.

   The first category of candidates are those with a transport address
   obtained directly from a local interface.  Such a candidate is called
   a "host candidate".  The local interface could be Ethernet or Wi-Fi,
   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.

   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



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   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.

                      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 a 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.  The host candidate
   associated with a given server-reflexive candidate is 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.




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   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
   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 by highest-
   to-lowest priority and sends them to R over the signaling channel.
   When R receives the candidates from L, it performs the same gathering
   process and responds with its own list of candidates.  At the end of
   this process, each ICE 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 connectivity 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 STUN requests are sent to and from the
   exact same IP addresses and ports that will be used for data (e.g.,
   RTP, RTCP, or other protocols).  Consequently, agents demultiplex
   STUN and data using the contents of the packets rather than the port
   on which they are received.

   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 that of
   other candidates the agent already learned, it represents a new
   candidate (peer-reflexive candidate), which then gets tested by ICE
   just the same as any other candidate.

   Because the algorithm above searches all candidate pairs, if a
   working pair exists, the algorithm 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
   "checklist".

   The agent works through the checklist by sending a STUN request for
   the next candidate pair on the list periodically.  These are called
   "ordinary checks".  When a STUN transaction succeeds, one or more
   candidate pairs will become so-called "valid pairs" and will be added
   to a candidate-pair list called the "valid list".

   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 is called a "triggered check", and it accelerates the
   process of finding valid pairs.

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




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   In general, the priority algorithm is designed so that candidates of
   a similar type get similar priorities so that more direct routes
   (that is, routes without data relays or NATs) are preferred over
   indirect routes (routes with data relays or NATs).  Within those
   guidelines, however, agents have a fair amount of discretion about
   how to tune their algorithms.

   A data stream might consist of multiple components (pieces of a data
   stream that require their own set of candidates, e.g., RTP and RTCP).

2.3.  Nominating Candidate Pairs and Concluding ICE

   ICE assigns one of the ICE agents in the role of the controlling
   agent, and the other in the role of the controlled agent.  For each
   component of a data stream, the controlling agent nominates a valid
   pair (from the valid list) to be used for data.  The exact timing of
   the nomination is based on local policy.

   When nominating, the controlling agent lets the checks continue until
   at least one valid pair for each component of a data stream is found,
   and then it picks a valid pair and sends a STUN request on that pair,
   using an attribute to indicate to the controlled peer that it has
   been nominated.  This is shown in Figure 4.

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

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

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

                           Figure 4: Nomination

   Once the controlled agent receives the STUN request with the
   attribute, it will check (unless the check has already been done) the
   same pair.  If the transactions above succeed, the agents will set
   the nominated flag for the pairs and will cancel any future checks
   for that component of the data stream.  Once an agent has set the
   nominated flag for each component of a data stream, the pairs become
   the selected pairs.  After that, only the selected pairs will be used
   for sending and receiving data associated with that data stream.






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2.4.  ICE Restart

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

2.5.  Lite Implementations

   Certain ICE 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).  Lite agents only use
   host candidates and do not generate connectivity checks or run state
   machines, though they need to be able to respond to connectivity
   checks.

3.  ICE Usage

   This document specifies generic use of ICE with protocols that
   provide means to exchange candidate information between ICE agents.
   The specific details (i.e., how to encode candidate information and
   the actual candidate exchange process) for different protocols using
   ICE (referred to as "using protocol") are described in separate usage
   documents.

   One mechanism that allows agents to exchange candidate information is
   the utilization of Offer/Answer semantics (which are based on
   [RFC3264]) as part of the SIP protocol [RFC3261] [ICE-SIP-SDP].

   [RFC7825] defines an ICE usage for the Real-Time Streaming Protocol
   (RTSP).  Note, however, that the ICE usage is based on RFC 5245.

4.  Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
   "OPTIONAL" in this document are to be interpreted as described in
   BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
   capitals, as shown here.

   Readers need to be familiar with the terminology defined in [RFC5389]
   and NAT Behavioral requirements for UDP [RFC4787].








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   This specification makes use of the following additional terminology:

   ICE Session:  An ICE session consists of all ICE-related actions
      starting with the candidate gathering, followed by the
      interactions (candidate exchange, connectivity checks,
      nominations, and keepalives) between the ICE agents until all the
      candidates are released or an ICE restart is triggered.

   ICE Agent, Agent:  An ICE agent (sometimes simply referred to as an
      "agent") is the protocol implementation involved in the ICE
      candidate exchange.  There are two agents involved in a typical
      candidate exchange.

   Initiating Peer, Initiating Agent, Initiator:  An initiating agent is
      an ICE agent that initiates the ICE candidate exchange process.

   Responding Peer, Responding Agent, Responder:  A responding agent is
      an ICE agent that receives and responds to the candidate exchange
      process initiated by the initiating agent.

   ICE Candidate Exchange, Candidate Exchange:  The process where ICE
      agents exchange information (e.g., candidates and passwords) that
      is needed to perform ICE.  Offer/Answer with SDP encoding
      [RFC3264] is one example of a protocol that can be used for
      exchanging the candidate information.

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

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

   Data, Data Stream, Data Session:  When ICE is used to set up data
      sessions, the data is transported using some protocol.  Media is
      usually transported over RTP, composed of a stream of RTP packets.
      Data session refers to data packets that are exchanged between the
      peer on the path created and tested with ICE.

   Candidate, Candidate Information:  A transport address that is a
      potential point of contact for receipt of data.  Candidates also
      have properties -- their type (server reflexive, relayed, or
      host), priority, foundation, and base.






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   Component:  A component is a piece of a data stream.  A data stream
      may require multiple components, each of which has to work in
      order for the data stream as a whole to work.  For RTP/RTCP data
      streams, unless RTP and RTCP are multiplexed in the same port,
      there are two components per data stream -- one for RTP, and one
      for RTCP.  A component has a candidate pair, which cannot be used
      by other components.

   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 VPNs.

   Server-Reflexive Candidate:  A candidate whose IP address and port
      are a binding allocated by a NAT for an ICE agent after it sends a
      packet through the NAT to a server, such as a STUN server.

   Peer-Reflexive Candidate:  A candidate whose IP address and port are
      a binding allocated by a NAT for an ICE agent after it sends a
      packet through the NAT to its peer.

   Relayed Candidate:  A candidate obtained from a relay server, such as
      a TURN server.

   Base:  The transport address that an ICE agent sends from for a
      particular candidate.  For host, server-reflexive, and peer-
      reflexive candidates, the base is the same as the host candidate.
      For relayed candidates, the base is the same as the relayed
      candidate (i.e., the transport address used by the TURN server to
      send from).

   Related Address and Port:  A transport address related to a
      candidate, which is useful for diagnostics and other purposes.  If
      a candidate is server or peer reflexive, the related address and
      port is equal to the base for that server or peer-reflexive
      candidate.  If the candidate is relayed, the related address and
      port are equal to the mapped address in the Allocate response that
      provided the client with that relayed candidate.  If the candidate
      is a host candidate, the related address and port is identical to
      the host candidate.

   Foundation:  An arbitrary string used in the freezing algorithm to
      group similar candidates.  It 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.





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   Local Candidate:  A candidate that an ICE agent has obtained and may
      send to its peer.

   Remote Candidate:  A candidate that an ICE agent received from its
      peer.

   Default Destination/Candidate:  The default destination for a
      component of a data stream is the transport address that would be
      used by an ICE agent that is not ICE aware.  A default candidate
      for a component is one whose transport address matches the default
      destination for that component.

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

   Check, Connectivity Check, STUN Check:  A STUN Binding request for
      the purpose of verifying connectivity.  A check is sent from the
      base of the local candidate to the remote candidate of a candidate
      pair.

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

   Ordinary Check:  A connectivity check generated by an ICE 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 Pair:  A candidate pair whose local candidate equals the mapped
      address of a successful connectivity-check response and whose
      remote candidate equals the destination address to which the
      connectivity-check request was sent.

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

   Checklist Set:  The ordered list of all checklists.  The order is
      determined by each ICE usage.

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








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   Lite Implementation:  An ICE implementation that omits certain
      functions, implementing only as much as is necessary for a peer
      that is not a lite implementation 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 nominates a candidate pair.
      In any session, there is always one controlling agent and one
      controlled agent.

   Controlled Agent:  The ICE agent that waits for the controlling agent
      to nominate a candidate pair.

   Nomination:  The process of the controlling agent indicating to the
      controlled agent which candidate pair the ICE agents will use for
      sending and receiving data.  The nomination process defined in
      this specification was referred to as "regular nomination" in RFC
      5245.  The nomination process that was referred to as "aggressive
      nomination" in RFC 5245 has been deprecated in this specification.

   Nominated, Nominated Flag:  Once the nomination of a candidate pair
      has succeeded, the candidate pair has become nominated, and the
      value of its nominated flag is set to true.

   Selected Pair, Selected Candidate Pair:  The candidate pair used for
      sending and receiving data for a component of a data stream is
      referred to as the "selected pair".  Before selected pairs have
      been produced for a data stream, any valid pair associated with a
      component of a data stream can be used for sending and receiving
      data for the component.  Once there are nominated pairs for each
      component of a data stream, the nominated pairs become the
      selected pairs for the data stream.  The candidates associated
      with the selected pairs are referred to as "selected candidates".

   Using Protocol, ICE Usage:  The protocol that uses ICE for NAT
      traversal.  A usage specification defines the protocol-specific
      details on how the procedures defined here are applied to that
      protocol.

   Timer Ta:  The timer for generating new STUN or TURN transactions.

   Timer RTO (Retransmission Timeout):  The retransmission timer for a
      given STUN or TURN transaction.








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5.  ICE Candidate Gathering and Exchange

   As part of ICE processing, both the initiating and responding agents
   gather candidates, prioritize and eliminate redundant candidates, and
   exchange candidate information with the peer as defined by the using
   protocol (ICE usage).  Specifics of the candidate-encoding mechanism
   and the semantics of candidate information exchange is out of scope
   of this specification.

5.1.  Full Implementation

5.1.1.  Gathering Candidates

   An ICE agent gathers candidates when it believes that communication
   is imminent.  An initiating agent can do this based on a user
   interface cue or on an explicit request to initiate a session.  Every
   candidate has 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 process for gathering candidates at the responding agent is
   identical to the process for the initiating agent.  It is RECOMMENDED
   that the responding agent begin this process immediately on receipt
   of the candidate information, prior to alerting the user of the
   application associated with the ICE session.

5.1.1.1.  Host Candidates

   Host candidates are obtained by binding to ports on an IP address
   attached to an interface (physical or virtual, including VPN
   interfaces) on the host.

   For each component of each data stream the ICE agent wishes to use,
   the agent SHOULD obtain a candidate on each IP address that the host
   has, with the exceptions listed below.  The agent 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/RTCP data streams, unless both RTP and RTCP are multiplexed
   in the same UDP port (RTP/RTCP multiplexing), the RTP itself has a
   component ID of 1, and RTCP has a component ID of 2.  In case of RTP/
   RTCP multiplexing, a component ID of 1 is used for both RTP and RTCP.



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   When candidates are obtained, unless the agent knows for sure that
   RTP/RTCP multiplexing will be used (i.e., the agent knows that the
   other agent also supports, and is willing to use, RTP/RTCP
   multiplexing), or unless the agent only supports RTP/RTCP
   multiplexing, the agent MUST obtain a separate candidate for RTCP.
   If an agent has obtained a candidate for RTCP, and ends up using RTP/
   RTCP multiplexing, the agent does not need to perform connectivity
   checks on the RTCP candidate.  Absence of a component ID 2 as such
   does not imply use of RTCP/RTP multiplexing, as it could also mean
   that RTCP is not used.

   If an agent is using separate candidates for RTP and RTCP, it will
   end up with 2*K host candidates if an agent has K IP addresses.

   Note that the responding agent, when obtaining its candidates, will
   typically know if the other agent supports RTP/RTCP multiplexing, in
   which case it will not need to obtain a separate candidate for RTCP.
   However, absence of a component ID 2 as such does not imply use of
   RTCP/RTP multiplexing, as it could also mean that RTCP is not used.

   The use of multiple components, other than for RTP/RTCP streams, is
   discouraged as it increases the complexity of ICE processing.  If
   multiple components are needed, the component IDs SHOULD start with 1
   and increase by 1 for each component.

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

   The host candidates are gathered from all IP addresses with the
   following exceptions:

   o  Addresses from a loopback interface MUST NOT be included in the
      candidate addresses.

   o  Deprecated IPv4-compatible IPv6 addresses [RFC4291] and IPv6 site-
      local unicast addresses [RFC3879] MUST NOT be included in the
      address candidates.

   o  IPv4-mapped IPv6 addresses SHOULD NOT be included in the address
      candidates unless the application using ICE does not support IPv4
      (i.e., it is an IPv6-only application [RFC4038]).

   o  If gathering one or more host candidates that correspond to an
      IPv6 address that was generated using a mechanism that prevents
      location tracking [RFC7721], host candidates that correspond to
      IPv6 addresses that do allow location tracking, are configured on
      the same interface, and are part of the same network prefix MUST
      NOT be gathered.  Similarly, when host candidates corresponding to




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      an IPv6 address generated using a mechanism that prevents location
      tracking are gathered, then host candidates corresponding to IPv6
      link-local addresses [RFC4291] MUST NOT be gathered.

   The IPv6 default address selection specification [RFC6724] specifies
   that temporary addresses [RFC4941] are to be preferred over permanent
   addresses.

5.1.1.2.  Server-Reflexive and Relayed Candidates

   An ICE agent SHOULD gather server-reflexive and relayed candidates.
   However, use of STUN and TURN servers may be unnecessary in certain
   networks and use of TURN servers may be expensive, so some
   deployments may elect not to use them.  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.

   The agent pairs each host candidate with the STUN or TURN servers
   with which it is configured or has discovered by some means.  It is
   RECOMMENDED that a domain name be configured, 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.

   When multiple STUN or TURN servers are available (or when they are
   learned through DNS records and multiple results are returned), the
   agent MAY gather candidates for all of them and SHOULD gather
   candidates for at least one of them (one STUN server and one TURN
   server).  It does so by pairing host candidates with STUN or TURN
   servers, and for each pair, the agent sends a Binding or Allocate
   request to the server from the 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.

   The gathering process is controlled using a timer, Ta.  Every time Ta
   expires, the agent can generate another new STUN or TURN transaction.
   This transaction can be either 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 once
   per each ta expiration.  See Section 14 for guidance on how to set Ta
   and the STUN retransmit timer, RTO.



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

   If an IPv6-only agent is in a network that utilizes NAT64 [RFC6146]
   and DNS64 [RFC6147] technologies, it may also gather IPv4 server-
   reflexive and/or relayed candidates from IPv4-only STUN or TURN
   servers.  IPv6-only agents SHOULD also utilize IPv6 prefix discovery
   [RFC7050] to discover the IPv6 prefix used by NAT64 (if any) and
   generate server-reflexive candidates for each IPv6-only interface,
   accordingly.  The NAT64 server-reflexive candidates are prioritized
   like IPv4 server-reflexive candidates.

5.1.1.3.  Computing Foundations

   The ICE agent assigns each candidate a foundation.  Two candidates
   have the same foundation when all of the following are true:

   o  They have 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 (the IP address used
      by the agent to contact the STUN or TURN server).

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

   Similarly, two candidates 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 (the IP
   addresses used by the agent to contact the STUN or TURN server), or
   their transport protocols are different.







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5.1.1.4.  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.

   Host candidates do not time out, but the candidate addresses may
   change or disappear for a number of reasons.  An ICE agent SHOULD
   monitor the interfaces it uses, invalidate candidates whose base has
   gone away, and acquire new candidates as appropriate when new IP
   addresses (on new or currently used interfaces) appear.

5.1.2.  Prioritizing Candidates

   The prioritization process results in the assignment of a priority to
   each candidate.  Each candidate for a data 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.
   Higher-priority values give more priority over lower values.

   An ICE agent SHOULD compute this priority using the formula in
   Section 5.1.2.1 and choose its parameters using the guidelines in
   Section 5.1.2.2.  If an agent elects to use a different formula, ICE
   may take longer to converge since the agents will not be coordinated
   in their checks.

   The process for prioritizing candidates is common across the
   initiating and the responding agent.

5.1.2.1.  Recommended Formula

   The recommended formula combines a preference for the candidate type
   (server reflexive, peer reflexive, relayed, and host), a preference
   for the IP address for which the candidate was obtained, and a
   component ID 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 (lowest preference) to
   126 (highest preference) inclusive, MUST be identical for all
   candidates of the same type, and MUST be different for candidates of



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   different types.  The type preference for peer-reflexive candidates
   MUST be higher than that of server-reflexive candidates.  Setting the
   value to 0 means that candidates of this type will only be used as a
   last resort.  Note that candidates gathered based on the procedures
   of Section 5.1.1 will never be peer-reflexive candidates; candidates
   of this type are learned from the connectivity checks performed by
   ICE.

   The local preference MUST be an integer from 0 (lowest preference) to
   65535 (highest preference) inclusive.  When there is only a single IP
   address, this value SHOULD be set to 65535.  If there are multiple
   candidates for a particular component for a particular data stream
   that have the same type, the local preference MUST be unique for each
   one.  If an ICE agent is dual stack, the local preference SHOULD be
   set according to the current best practice described in [RFC8421].

   The component ID MUST be an integer between 1 and 256 inclusive.

5.1.2.2.  Guidelines for Choosing Type and Local Preferences

   The RECOMMENDED values for type preferences are 126 for host
   candidates, 110 for peer-reflexive candidates, 100 for server-
   reflexive candidates, and 0 for relayed candidates.

   If an ICE agent is multihomed and has multiple IP addresses, the
   recommendations in [RFC8421] SHOULD be followed.  If multiple TURN
   servers are used, local priorities for the candidates obtained from
   the TURN servers are chosen in a similar fashion as for multihomed
   local candidates: the local preference value is used to indicate a
   preference among different servers, but the preference MUST be unique
   for each one.

   When choosing type preferences, agents may take into account factors
   such as latency, packet loss, cost, network topology, security,
   privacy, and others.

5.1.3.  Eliminating Redundant Candidates

   Next, the ICE agents (initiating and responding) eliminate redundant
   candidates.  Two candidates can have the same transport address yet
   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.  A candidate is
   redundant if and only if its transport address and base equal those
   of another candidate.  The agent SHOULD eliminate the redundant
   candidate with the lower priority.





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5.2.  Lite Implementation Procedures

   Lite implementations only utilize host candidates.  For each IP
   address, independent of an IP address family, there MUST be zero or
   one candidate.  With the lite implementation, ICE cannot be used to
   dynamically choose amongst candidates.  Therefore, including more
   than one candidate from a particular IP address family is NOT
   RECOMMENDED, since only a connectivity check can truly determine
   whether to use one address or the other.  Instead, it is RECOMMENDED
   that agents that have multiple public IP addresses run full ICE
   implementations to ensure the best usage of its addresses.

   Each component has an ID assigned to it, called the "component ID".
   For RTP/RTCP data streams, unless RTCP is multiplexed in the same
   port with RTP, the RTP itself has a component ID of 1 and RTCP a
   component ID of 2.  If an agent is using RTCP without multiplexing,
   it MUST obtain candidates for it.  However, absence of a component ID
   2 as such does not imply use of RTCP/RTP multiplexing, as it could
   also mean that RTCP is not used.

   Each candidate is assigned a foundation.  The foundation MUST be
   different for two candidates allocated from different IP addresses;
   otherwise, it MUST be the same.  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 data
   stream.  If the formula in Section 5.1.2.1 is used to calculate the
   priority, the type preference value SHOULD be set to 126.  If a host
   is IPv4 only, the local preference value SHOULD be set to 65535.  If
   a host is IPv6 or dual stack, the local preference value SHOULD be
   set to the precedence value for IP addresses described in RFC 6724
   [RFC6724].

   Next, an agent chooses a default candidate for each component of each
   data stream.  If a host is IPv4 only, there would only be one
   candidate for each component of each data stream; therefore, that
   candidate is the default.  If a host is IPv6 only, the default
   candidate would typically be a globally scoped IPv6 address.  Dual-
   stack hosts SHOULD allow configuration whether IPv4 or IPv6 is used
   for the default candidate, and the configuration needs to be based on
   which one its administrator believes has a higher chance of success
   in the current network environment.

   The procedures in this section are common across the initiating and
   responding agents.







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5.3.  Exchanging Candidate Information

   ICE agents (initiating and responding) need the following information
   about candidates to be exchanged.  Each ICE usage MUST define how the
   information is exchanged with the using protocol.  This section
   describes the information that needs to be exchanged.

   Candidates:   One or more candidates.  For each candidate:

      Address:  The IP address and transport protocol port of the
         candidate.

      Transport:  The transport protocol of the candidate.  This MAY be
         omitted if the using protocol only runs over a single transport
         protocol.

      Foundation:  A sequence of up to 32 characters.

      Component ID:  The component ID of the candidate.  This MAY be
         omitted if the using protocol does not use the concept of
         components.

      Priority:  The 32-bit priority of the candidate.

      Type:  The type of the candidate.

      Related Address and Port:  The related IP address and port of the
         candidate.  These MAY be omitted or set to invalid values if
         the agent does not want to reveal them, e.g., for privacy
         reasons.

      Extensibility Parameters:  The using protocol might define means
         for adding new per-candidate ICE parameters in the future.

   Lite or Full:   Whether the agent is a lite agent or full agent.

   Connectivity-Check Pacing Value:  The pacing value for connectivity
      checks that the agent wishes to use.  This MAY be omitted if the
      agent wishes to use a defined default value.

   Username Fragment and Password:  Values used to perform connectivity
      checks.  The values MUST be unguessable, with at least 128 bits of
      random number generator output used to generate the password, and
      at least 24 bits of output to generate the username fragment.

   Extensions:  New media-stream or session-level attributes (ICE
      options).




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   If the using protocol is vulnerable to, and able to detect, ICE
   mismatch (Section 5.4), a way is needed for the detecting agent to
   convey this information to its peer.  It is a boolean flag.

   The using protocol may (or may not) need to deal with backwards
   compatibility with older implementations that do not support ICE.  If
   a fallback mechanism to non-ICE is supported and is being used, then
   presumably the using protocol provides a way of conveying the default
   candidate (its IP address and port) in addition to the ICE
   parameters.

   Once an agent has sent its candidate information, it MUST be prepared
   to receive both STUN and data packets on each candidate.  As
   discussed in Section 12.1, data packets can be sent to a candidate
   prior to its appearance as the default destination for data.

5.4.  ICE Mismatch

   Certain middleboxes, such as ALGs, can alter signaling information in
   ways that break ICE (e.g., by rewriting IP addresses in SDP).  This
   is referred to as "ICE mismatch".  If the using protocol is
   vulnerable to ICE mismatch, the responding agent needs to be able to
   detect it and inform the peer ICE agent about the ICE mismatch.

   Each using protocol needs to define whether the using protocol is
   vulnerable to ICE mismatch, how ICE mismatch is detected, and whether
   specific actions need to be taken when ICE mismatch is detected.

6.  ICE Candidate Processing

   Once an ICE agent has gathered its candidates and exchanged
   candidates with its peer (Section 5), it will determine its own role.
   In addition, full implementations will form checklists and begin
   performing connectivity checks with the peer.

6.1.  Procedures for Full Implementation

6.1.1.  Determining Role

   For each session, each ICE agent (initiating and responding) 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.  The sections below describe
   in detail the actual procedures followed by controlling and
   controlled agents.






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

   Both agents are full:  The initiating agent that started the ICE
      processing MUST take the controlling role, and the other MUST take
      the controlled role.  Both agents will form checklists, 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 become (if the connectivity checks
      associated with the nominations succeed) the selected pairs, and
      then both agents end ICE as described in Section 8.1.2.

   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 checklists, run the ICE state machines, and
      generate connectivity checks.  That agent will execute the logic
      in Section 8.1 to nominate pairs that will become (if the
      connectivity checks associated with the nominations succeed) the
      selected pairs 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 data stream is considered to be
      Running, and the state of ICE overall is Running.

   Both lite:  The initiating agent that 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 candidates are exchanged, 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 enabling
      the candidate exchange.  The state of ICE processing for each data
      stream is considered to be Running, and the state of ICE overall
      is Running.

   Once the roles are determined for a session, they persist throughout
   the lifetime of the session.  The roles can be redetermined as part
   of an ICE restart (Section 9), but an ICE agent MUST NOT redetermine
   the role as part of an ICE restart unless one or more of the
   following criteria is fulfilled:

   Full becomes lite:  If the controlling agent is full, and switches to
      lite, the roles MUST be redetermined if the peer agent is also
      full.





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   Role conflict:  If the ICE restart causes a role conflict, the roles
      might be redetermined due to the role conflict procedures in
      Section 7.3.1.1.

   NOTE: There are certain Third Party Call Control (3PCC) [RFC3725]
   scenarios where an ICE restart might cause a role conflict.

   NOTE: The agents need to inform each other whether they are full or
   lite before the roles are determined.  The mechanism for that is
   specific to the signaling protocol and outside the scope of the
   document.

   An agent MUST accept if the peer initiates a redetermination of the
   roles even if the criteria for doing so are not fulfilled.  This can
   happen if the peer is compliant with RFC 5245.

6.1.2.  Forming the Checklists

   There is one checklist for each data stream.  To form a checklist,
   initiating and responding ICE agents form candidate pairs, compute
   pair priorities, order pairs by priority, prune pairs, remove lower-
   priority pairs, and set checklist states.  If candidates are added to
   a checklist (e.g., due to detection of peer-reflexive candidates),
   the agent will re-perform these steps for the updated checklist.

6.1.2.1.  Checklist State

   Each checklist has a state, which captures the state of ICE checks
   for the data stream associated with the checklist.  The states are:

   Running:  The checklist is neither Completed nor Failed yet.
      Checklists are initially set to the Running state.

   Completed:  The checklist contains a nominated pair for each
      component of the data stream.

   Failed:  The checklist does not have a valid pair for each component
      of the data stream, and all of the candidate pairs in the
      checklist are in either the Failed or the Succeeded state.  In
      other words, at least one component of the checklist has candidate
      pairs that are all in the Failed state, which means the component
      has failed, which means the checklist has failed.

6.1.2.2.  Forming Candidate Pairs

   The ICE agent pairs each local candidate with each remote candidate
   for the same component of the same data stream with the same IP
   address family.  It is possible that some of the local candidates



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   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 all of the components for
   a data stream.  If this happens, the number of components for that
   data stream is effectively reduced and is considered to be equal to
   the minimum across both agents of the maximum component ID provided
   by each agent across all components for the data 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
   initiating agent can multiplex RTP and RTCP on the same port
   [RFC5761].  However, since the initiating agent doesn't know if the
   peer agent can perform such multiplexing, it includes candidates for
   RTP and RTCP on separate ports.  If the peer agent 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.

   With IPv6, it is common for a host to have multiple host candidates
   for each interface.  To keep the amount of resulting candidate pairs
   reasonable and to avoid candidate pairs that are highly unlikely to
   work, IPv6 link-local addresses MUST NOT be paired with other than
   link-local addresses.

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























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   Figure 5 shows the properties of and relationships between transport
   addresses, candidates, candidate pairs, and checklists.

              +--------------------------------------------+
              |                                            |
              | +---------------------+                    |
              | |+----+ +----+ +----+ |   +Type            |
              | || IP | |Port| |Tran| |   +Priority        |
              | ||Addr| |    | |    | |   +Foundation      |
              | |+----+ +----+ +----+ |   +Component ID    |
              | |      Transport      |   +Related Address |
              | |        Addr         |                    |
              | +---------------------+   +Base            |
              |             Candidate                      |
              +--------------------------------------------+
              *                                         *
              *    *************************************
              *    *
            +-------------------------------+
            |                               |
            | Local     Remote              |
            | +----+    +----+   +default?  |
            | |Cand|    |Cand|   +valid?    |
            | +----+    +----+   +nominated?|
            |                    +State     |
            |                               |
            |                               |
            |          Candidate Pair       |
            +-------------------------------+
            *                              *
            *                  ************
            *                  *
            +------------------+
            |  Candidate Pair  |
            +------------------+
            +------------------+
            |  Candidate Pair  |
            +------------------+
            +------------------+
            |  Candidate Pair  |
            +------------------+

                 Checklist


                Figure 5: Conceptual Diagram of a Checklist





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6.1.2.3.  Computing Pair Priority and Ordering Pairs

   The ICE agent computes a priority for each candidate pair.  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 follows:

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

   The agent sorts each checklist in decreasing order of candidate pair
   priority.  If two pairs have identical priority, the ordering amongst
   them is arbitrary.

6.1.2.4.  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 ICE agent cannot send requests directly from a reflexive
   candidate (server reflexive or peer reflexive), but only from its
   base, the agent next goes through the sorted list of candidate pairs.
   For each pair where the local candidate is reflexive, the candidate
   MUST be replaced by its base.

   The agent prunes each checklist.  This is done by removing a
   candidate pair if it is redundant with a higher-priority candidate
   pair in the same checklist.  Two candidate pairs are redundant if
   their local candidates have the same base and their remote candidates
   are identical.  The result is a sequence of ordered candidate pairs,
   called the "checklist" for that data stream.

6.1.2.5.  Removing Lower-Priority Pairs

   In order to limit the attacks described in Section 19.5.1, an ICE
   agent MUST limit the total number of connectivity checks the agent
   performs across all checklists in the checklist set.  This is done by
   limiting the total number of candidate pairs in the checklist set.
   The default limit of candidate pairs for the checklist set is 100,
   but the value MUST be configurable.  The limit is enforced by, within
   in each checklist, discarding lower-priority candidate pairs until
   the total number of candidate pairs in the checklist set is smaller
   than the limit value.  The discarding SHOULD be done evenly so that
   the number of candidate pairs in each checklist is reduced the same
   amount.

   It is RECOMMENDED that a lower-limit value than the default is picked
   when possible, and that the value is set to the maximum number of
   plausible candidate pairs that might be created in an actual



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   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.

6.1.2.6.  Computing Candidate Pair States

   Each candidate pair in the checklist has a foundation (the
   combination of the foundations of the local and remote candidates in
   the pair) and one of the following states:

   Waiting:  A check has not been sent for this pair, but the pair is
      not Frozen.

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

   Succeeded:  A check has been sent for this pair, and it produced a
      successful result.

   Failed:  A check has been sent for this pair, and it failed (a
      response to the check was never received, or a failure response
      was received).

   Frozen:  A check for this pair has not been sent, and it cannot be
      sent until the pair is unfrozen and moved into the Waiting state.


























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   Pairs move between states as shown in Figure 6.

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

              Figure 6: Pair State Finite State Machine (FSM)









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

   1.  The checklists are placed in an ordered list (the order is
       determined by each ICE usage), called the "checklist set".

   2.  The ICE agent initially places all candidate pairs in the Frozen
       state.

   3.  The agent sets all of the checklists in the checklist set to the
       Running state.

   4.  For each foundation, the agent sets the state of exactly one
       candidate pair to the Waiting state (unfreezing it).  The
       candidate pair to unfreeze is chosen by finding the first
       candidate pair (ordered by the lowest component ID and then the
       highest priority if component IDs are equal) in the first
       checklist (according to the usage-defined checklist set order)
       that has that foundation.

   NOTE: The procedures above are different from RFC 5245, where only
   candidate pairs in the first checklist were initially placed in the
   Waiting state.  Now it applies to candidate pairs in the first
   checklist that have that foundation, even if the checklist is not the
   first one in the checklist set.

   The table below illustrates an example.
























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   Table legend:

   Each row (m1, m2,...) represents a checklist associated with a
   data stream. m1 represents the first checklist in the checklist
   set.

   Each column (f1, f2,...) represents a foundation.  Every candidate
   pair within a given column share the same foundation.

   f-cp represents a candidate pair in the Frozen state.

   w-cp represents a candidate pair in the Waiting state.

   1.  The agent sets all of the pairs in the checklist set to the
       Frozen state.

         f1    f2    f3    f4    f5
       -----------------------------
   m1 | f-cp  f-cp  f-cp
      |
   m2 | f-cp  f-cp  f-cp  f-cp
      |
   m3 | f-cp                    f-cp


   2.  For each foundation, the candidate pair with the lowest
       component ID is placed in the Waiting state, unless a
       candidate pair associated with the same foundation has
       already been put in the Waiting state in one of the
       other examined checklists in the checklist set.

         f1    f2    f3    f4    f5
       -----------------------------
   m1 | w-cp  w-cp  w-cp
      |
   m2 | f-cp  f-cp  f-cp  w-cp
      |
   m3 | f-cp                    w-cp

                        Table 1: Pair State Example

   In the first checklist (m1), the candidate pair for each foundation
   is placed in the Waiting state, as no pairs for the same foundations
   have yet been placed in the Waiting state.

   In the second checklist (m2), the candidate pair for foundation f4 is
   placed in the Waiting state.  The candidate pair for foundations f1,
   f2, and f3 are kept in the Frozen state, as candidate pairs for those



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   foundations have already been placed in the Waiting state (within
   checklist m1).

   In the third checklist (m3), the candidate pair for foundation f5 is
   placed in the Waiting state.  The candidate pair for foundation f1 is
   kept in the Frozen state, as a candidate pair for that foundation has
   already been placed in the Waiting state (within checklist m1).

   Once each checklist have been processed, one candidate pair for each
   foundation in the checklist set has been placed in the Waiting state.

6.1.3.  ICE State

   The ICE agent has a state determined by the state of the checklists.
   The state is Completed if all checklists are Completed, Failed if all
   checklists are Failed, or Running otherwise.

6.1.4.  Scheduling Checks

6.1.4.1.  Triggered-Check Queue

   Once the ICE agent has computed the checklists and created the
   checklist set, as described in Section 6.1.2, the agent will begin
   performing connectivity checks (ordinary and triggered).  For
   triggered connectivity checks, the agent maintains a FIFO queue for
   each checklist, referred to as the "triggered-check queue", which
   contains candidate pairs for which checks are to be sent at the next
   available opportunity.  The triggered-check queue is initially empty.

6.1.4.2.  Performing Connectivity Checks

   The generation of ordinary and triggered connectivity checks is
   governed by timer Ta.  As soon as the initial states for the
   candidate pairs in the checklist set have been set, a check is
   performed for a candidate pair within the first checklist in the
   Running state, following the procedures in Section 7.  After that,
   whenever Ta fires the next checklist in the Running state in the
   checklist set is picked, and a check is performed for a candidate
   within that checklist.  After the last checklist in the Running state
   in the checklist set has been processed, the first checklist is
   picked again, etc.










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   Whenever Ta fires, the ICE agent will perform a check for a candidate
   pair within the checklist that was picked by performing the following
   steps:

   1.  If the triggered-check queue associated with the checklist
       contains one or more candidate pairs, the agent removes the top
       pair from the queue, performs a connectivity check on that pair,
       puts the candidate pair state to In-Progress, and aborts the
       subsequent steps.

   2.  If there is no candidate pair in the Waiting state, and if there
       are one or more pairs in the Frozen state, the agent checks the
       foundation associated with each pair in the Frozen state.  For a
       given foundation, if there is no pair (in any checklist in the
       checklist set) in the Waiting or In-Progress state, the agent
       puts the candidate pair state to Waiting and continues with the
       next step.

   3.  If there are one or more candidate pairs in the Waiting state,
       the agent picks the highest-priority candidate pair (if there are
       multiple pairs with the same priority, the pair with the lowest
       component ID is picked) in the Waiting state, performs a
       connectivity check on that pair, puts the candidate pair state to
       In-Progress, and aborts the subsequent steps.

   4.  If this step is reached, no check could be performed for the
       checklist that was picked.  So, without waiting for timer Ta to
       expire again, select the next checklist in the Running state and
       return to step #1.  If this happens for every single checklist in
       the Running state, meaning there are no remaining candidate pairs
       to perform connectivity checks for, abort these steps.

   Once the agent has picked a candidate pair for which a connectivity
   check is to be performed, the agent starts a check and sends the
   Binding request from the base associated with the local candidate of
   the pair to the remote candidate of the pair, as described in
   Section 7.2.4.

   Based on local policy, an agent MAY choose to terminate performing
   the connectivity checks for one or more checklists in the checklist
   set at any time.  However, only the controlling agent is allowed to
   conclude ICE (Section 8).

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




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6.2.  Lite Implementation Procedures

   Lite implementations skip most of the steps in Section 6 except for
   verifying the peer's ICE support and determining its role in the ICE
   processing.

   If the lite implementation is the controlling agent (which will only
   happen if the peer ICE agent is also a lite implementation), it
   selects a candidate pair based on the ones in the candidate exchange
   (for IPv4, there is only ever one pair) and then updates the peer
   with the new candidate information reflecting that selection, if
   needed (it is never needed for an IPv4-only host).

7.  Performing Connectivity Checks

   This section describes how connectivity checks are performed.

   An ICE agent MUST be compliant to [RFC5389].  A full implementation
   acts both as a STUN client and a STUN server, while a lite
   implementation only acts as a STUN server (as it does not generate
   connectivity checks).

7.1.  STUN Extensions

   ICE extends STUN with the attributes: PRIORITY, USE-CANDIDATE, ICE-
   CONTROLLED, and ICE-CONTROLLING.  These attributes are formally
   defined in Section 16.1.  This section describes the usage of the
   attributes.

   The attributes are only applicable to ICE connectivity checks.

7.1.1.  PRIORITY

   The PRIORITY attribute MUST be included in a Binding request and be
   set to the value computed by the algorithm in Section 5.1.2 for the
   local candidate, but with the candidate type preference of peer-
   reflexive candidates.

7.1.2.  USE-CANDIDATE

   The controlling agent MUST include the USE-CANDIDATE attribute in
   order to nominate a candidate pair (Section 8.1.1).  The controlled
   agent MUST NOT include the USE-CANDIDATE attribute in a Binding
   request.







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7.1.3.  ICE-CONTROLLED and ICE-CONTROLLING

   The controlling agent MUST include the ICE-CONTROLLING attribute in a
   Binding request.  The controlled agent MUST include the ICE-
   CONTROLLED attribute in a Binding request.

   The content of either attribute is used as tiebreaker values when an
   ICE role conflict occurs (Section 7.3.1.1).

7.2.  STUN Client Procedures

7.2.1.  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 ICE 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.

7.2.2.  Forming Credentials

   A connectivity-check Binding request 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 ICE 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 ICE agent L is the initiating
   agent and ICE agent R is the responding agent.  Agent L included a
   username fragment of LFRAG for its candidates and a password of
   LPASS.  Agent R provided a username fragment of RFRAG and a password
   of RPASS.  A connectivity check from L to R 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).





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7.2.3.  Diffserv Treatment

   If the agent is using Differentiated Services Code Point (DSCP)
   markings [RFC2475] in data packets that it will send, the agent
   SHOULD apply the same markings to Binding requests and responses that
   it will send.

   If multiple DSCP markings are used on the data packets, the agent
   SHOULD choose one of them for use with the connectivity check.

7.2.4.  Sending the Request

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

   Support for backwards compatibility with RFC 3489 MUST NOT be assumed
   when performing connectivity checks.  The FINGERPRINT mechanism MUST
   be used for connectivity checks.

7.2.5.  Processing the Response

   This section defines additional procedures for processing Binding
   responses specific to ICE connectivity checks.

   When a Binding response is received, it is correlated to the
   corresponding Binding request using the transaction ID [RFC5389],
   which then associates the response with the candidate pair for which
   the Binding request was sent.  After that, the response is processed
   according to the procedures for a role conflict, a failure, or a
   success, according to the procedures below.

7.2.5.1.  Role Conflict

   If the Binding request generates a 487 (Role Conflict) error response
   (Section 7.3.1.1), and if the ICE agent included an ICE-CONTROLLED
   attribute in the request, the agent MUST switch to the controlling
   role.  If the agent included an ICE-CONTROLLING attribute in the
   request, the agent MUST switch to the controlled role.

   Once the agent has switched its role, the agent MUST add the
   candidate pair whose check generated the 487 error response to the
   triggered-check queue associated with the checklist to which the pair
   belongs, and set the candidate pair state to Waiting.  When the
   triggered connectivity check is later performed, the ICE-CONTROLLING/
   ICE-CONTROLLED attribute of the Binding request will indicate the
   agent's new role.  The agent MUST change the tiebreaker value.



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   NOTE: A role switch requires an agent to recompute pair priorities
   (Section 6.1.2.3), since the priority values depend on the role.

   NOTE: A role switch will also impact whether the agent is responsible
   for nominating candidate pairs, and whether the agent is responsible
   for initiating the exchange of the updated candidate information with
   the peer once ICE is concluded.

7.2.5.2.  Failure

   This section describes cases when the candidate pair state is set to
   Failed.

   NOTE: When the ICE agent sets the candidate pair state to Failed as a
   result of a connectivity-check error, the agent does not change the
   states of other candidate pairs with the same foundation.

7.2.5.2.1.  Non-Symmetric Transport Addresses

   The ICE agent MUST check that the source and destination transport
   addresses in the Binding request and response are symmetric.  That
   is, the source IP address and port of the response MUST be equal to
   the destination IP address and port to which the Binding request was
   sent, and the destination IP address and port of the response MUST be
   equal to the source IP address and port from which the Binding
   request was sent.  If the addresses are not symmetric, the agent MUST
   set the candidate pair state to Failed.

7.2.5.2.2.  ICMP Error

   An ICE agent MAY support processing of ICMP errors for connectivity
   checks.  If the agent supports processing of ICMP errors, and if a
   Binding request generates a hard ICMP error, the agent SHOULD set the
   state of the candidate pair to Failed.  Implementers need to be aware
   that ICMP errors can be used as a method for Denial-of-Service (DoS)
   attacks when making a decision on how and if to process ICMP errors.

7.2.5.2.3.  Timeout

   If the Binding request transaction times out, the ICE agent MUST set
   the candidate pair state to Failed.

7.2.5.2.4.  Unrecoverable STUN Response

   If the Binding request generates a STUN error response that is
   unrecoverable [RFC5389], the ICE agent SHOULD set the candidate pair
   state to Failed.




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7.2.5.3.  Success

   A connectivity check is considered a success if each of the following
   criteria is true:

   o  The Binding request generated a success response; and

   o  The source and destination transport addresses in the Binding
      request and response are symmetric.

   If a check is considered a success, the ICE agent performs (in order)
   the actions described in the following sections.

7.2.5.3.1.  Discovering Peer-Reflexive Candidates

   The ICE agent MUST check 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, a
   peer-reflexive candidate has a type, base, priority, and foundation.
   They are computed as follows:

   o  The type is peer reflexive.

   o  The base is the local candidate of the candidate pair from which
      the Binding request was sent.

   o  The priority is the value of the PRIORITY attribute in the Binding
      request.

   o  The foundation is described in Section 5.1.1.3.

   The peer-reflexive candidate is then added to the list of local
   candidates for the data stream.  The username fragment and password
   are the same as for all other local candidates for that data stream.

   The ICE agent does not need to pair the peer-reflexive candidate with
   remote candidates, as a valid pair will be created due to the
   procedures in Section 7.2.5.3.2.  If an agent wishes to pair the
   peer-reflexive candidate with remote candidates other than the one in
   the valid pair that will be generated, the agent MAY provide updated
   candidate information to the peer that includes the peer-reflexive
   candidate.  This will cause the peer-reflexive candidate to be paired
   with all other remote candidates.







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7.2.5.3.2.  Constructing a Valid Pair

   The ICE 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".

   The valid pair might equal the pair that generated the connectivity
   check, a different pair in the checklist, or a pair currently not in
   the checklist.

   The agent maintains a separate list, referred to as the "valid list".
   There is a valid list for each checklist in the checklist set.  The
   valid list will contain valid pairs.  Initially, each valid list is
   empty.

   Each valid pair within the valid list has a flag, called the
   "nominated flag".  When a valid pair is added to a valid list, the
   flag value is set to 'false'.

   The valid pair will be added to a valid list as follows:

   1.  If the valid pair equals the pair that generated the check, the
       pair is added to the valid list associated with the checklist to
       which the pair belongs; or

   2.  If the valid pair equals another pair in a checklist, that pair
       is added to the valid list associated with the checklist of that
       pair.  The pair that generated the check is not added to a valid
       list; or

   3.  If the valid pair is not in any checklist, the agent computes the
       priority for the pair based on the priority of each candidate,
       using the algorithm in Section 6.1.2.  The priority of the local
       candidate depends on its type.  Unless the type is peer
       reflexive, the priority is equal to the priority signaled for
       that candidate in the candidate exchange.  If the type 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 candidate information of the
       peer.  If the candidate does not appear there, then the check has
       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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   NOTE: It will be very common that the valid pair will not be in any
   checklist.  Recall that the checklist has pairs whose local
   candidates are never reflexive; those pairs had their local
   candidates converted to the base of the reflexive candidates and were
   then pruned if they were redundant.  When the response to the Binding
   request 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 checklist.

7.2.5.3.3.  Updating Candidate Pair States

   The ICE agent sets the states of both the candidate pair that
   generated the check and the constructed valid pair (which may be
   different) to Succeeded.

   The agent MUST set the states for all other Frozen candidate pairs in
   all checklists with the same foundation to Waiting.

   NOTE: Within a given checklist, candidate pairs with the same
   foundations will typically have different component ID values.

7.2.5.3.4.  Updating the Nominated Flag

   If the controlling agent sends a Binding request with the USE-
   CANDIDATE attribute set, and if the ICE agent receives a successful
   response to the request, the agent sets the nominated flag of the
   pair to true.  If the request fails (Section 7.2.5.2), the agent MUST
   remove the candidate pair from the valid list, set the candidate pair
   state to Failed, and set the checklist state to Failed.

   If the controlled agent receives a successful response to a Binding
   request sent by the agent, and that Binding request was triggered by
   a received Binding request with the USE-CANDIDATE attribute set
   (Section 7.3.1.4), the agent sets the nominated flag of the pair to
   true.  If the triggered request fails, the agent MUST remove the
   candidate pair from the valid list, set the candidate pair state to
   Failed, and set the checklist state to Failed.

   Once the nominated flag is set for a component of a data stream, it
   concludes the ICE processing for that component (Section 8).

7.2.5.4.  Checklist State Updates

   Regardless of whether a connectivity check was successful or failed,
   the completion of the check may require updating of checklist states.
   For each checklist in the checklist set, if all of the candidate
   pairs are in either Failed or Succeeded state, and if there is not a
   valid pair in the valid list for each component of the data stream



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   associated with the checklist, the state of the checklist is set to
   Failed.  If there is a valid pair for each component in the valid
   list, the state of the checklist is set to Succeeded.

7.3.  STUN Server Procedures

   An ICE agent (lite or full) MUST be prepared to receive Binding
   requests on the base of each candidate it included in its most recent
   candidate exchange.

   The agent MUST use the short-term credential mechanism (i.e., the
   MESSAGE-INTEGRITY attribute) to authenticate the request and perform
   a message integrity check.  Likewise, the short-term credential
   mechanism MUST be used for the response.  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 a candidate exchange for a session in
   progress.  It is possible (and in fact very likely) that the
   initiating agent will receive a Binding request prior to receiving
   the candidates from its peer.  If this happens, the agent MUST
   immediately generate a response (including computation of the mapped
   address as described in Section 7.3.1.2).  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, see Sections
   7.3.1.3, 7.3.1.4, and 7.3.1.5 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 defined in RFC 5389
   (for working with the protocol in RFC 3489).  It MUST utilize the
   FINGERPRINT mechanism.

   If the agent is using DSCP markings [RFC2475] in its data packets, it
   SHOULD apply the same markings to Binding responses.  The same would
   apply to any Layer 2 markings the endpoint might be applying to data
   packets.

7.3.1.  Additional Procedures for Full Implementations

   This subsection defines the additional server procedures applicable
   to full implementations, when the full implementation accepts the
   Binding request.







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7.3.1.1.  Detecting and Repairing Role Conflicts

   In certain usages of ICE (such as 3PCC), both ICE agents may end up
   choosing the same role, resulting in a role conflict.  The section
   describes a mechanism for detecting and repairing role conflicts.
   The usage document MUST specify whether this mechanism is needed.

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

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

      *  If the agent's tiebreaker value 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 tiebreaker value 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 tiebreaker value is larger than or equal to the
         contents of the ICE-CONTROLLED attribute, the agent switches to
         the controlling role.

      *  If the agent's tiebreaker value 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 if 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 6.1.2.3), since those priorities are a function of role.
   The change in role will also impact whether the agent is responsible
   for selecting nominated pairs and initiating exchange with updated
   candidate information upon conclusion of ICE.






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   The remaining subsections in Section 7.3.1 are followed if the agent
   generated a successful response to the Binding request, even if the
   agent changed roles.

7.3.1.2.  Computing Mapped Addresses

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

7.3.1.3.  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 type is peer reflexive.

   o  The priority is the value of the PRIORITY attribute in the Binding
      request.

   o  The foundation is an arbitrary value, different from the
      foundations of all other remote candidates.  If any subsequent
      candidate exchanges contain this peer-reflexive candidate, it will
      signal the actual foundation for the candidate.

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

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

7.3.1.4.  Triggered Checks

   Next, the agent constructs a pair whose local candidate has the
   transport address (as seen by the agent) 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 be
   either 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



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   agent, it can obtain their priorities and compute the candidate pair
   priority.  This pair is then looked up in the checklist.  There can
   be one of several outcomes:

   o  When the pair is already on the checklist:

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

      *  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 Binding requests associated with the
         connectivity-check transaction, 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 enqueue the pair in the triggered checklist associated
         with the checklist, and set the state of the pair to Waiting,
         in order to trigger a new connectivity check of the pair.
         Creating a new connectivity check enables validating
         In-Progress pairs as soon as possible, without having to wait
         for retransmissions of the Binding requests associated with the
         original connectivity-check transaction.

      *  If the state of that pair is Waiting, Frozen, or Failed, the
         agent MUST enqueue the pair in the triggered checklist
         associated with the checklist (if not already present), and set
         the state of the pair to Waiting, in order to trigger a new
         connectivity check of the pair.  Note that a state change of
         the pair from Failed to Waiting might also trigger a state
         change of the associated checklist.

   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 checklist:

      *  The pair is inserted into the checklist 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 7.2.4.  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



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   candidates received from its peer (there may be more than one in
   cases of forking) and find this username fragment.  The corresponding
   password is then picked.

7.3.1.5.  Updating the Nominated Flag

   If the controlled agent receives a Binding request with the USE-
   CANDIDATE attribute set, and if the ICE agent accepts the request,
   the following action is based on the state of the pair computed in
   Section 7.3.1.4:

   o  If the state of this pair is Succeeded, it means that the check
      previously sent by this pair produced a successful response and
      generated a valid pair (Section 7.2.5.3.2).  The agent sets the
      nominated flag value of the valid pair to true.

   o  If the received Binding request triggered a new check to be
      enqueued in the triggered-check queue (Section 7.3.1.4), once the
      check is sent and if it generates a successful response, and
      generates a valid pair, the agent sets the nominated flag of the
      pair to true.  If the request fails (Section 7.2.5.2), the agent
      MUST remove the candidate pair from the valid list, set the
      candidate pair state to Failed, and set the checklist state to
      Failed.

   If the controlled agent does not accept the request from the
   controlling agent, the controlled agent MUST reject the nomination
   request with an appropriate error code response (e.g., 400)
   [RFC5389].

   Once the nominated flag is set for a component of a data stream, it
   concludes the ICE processing for that component.  See Section 8.

7.3.2.  Additional Procedures for Lite Implementations

   If the controlled agent receives a Binding request with the USE-
   CANDIDATE attribute set, and if the ICE agent accepts the request,
   the agent constructs a candidate pair whose local candidate has 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 the valid list of the associated checklist.
   The agent sets the nominated flag for that pair to true.

   Once the nominated flag is set for a component of a data stream, it
   concludes the ICE processing for that component.  See Section 8.





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8.  Concluding ICE Processing

   This section describes how an ICE agent completes ICE.

8.1.  Procedures for Full Implementations

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

8.1.1.  Nominating Pairs

   Prior to nominating, the controlling agent lets connectivity checks
   continue until some stopping criterion is met.  After that, based on
   an evaluation criterion, the controlling agent picks a pair among the
   valid pairs in the valid list for nomination.

   Once the controlling agent has picked a valid pair for nomination, it
   repeats the connectivity check that produced this valid pair (by
   enqueueing the pair that generated the check into the triggered-check
   queue), this time with the USE-CANDIDATE attribute
   (Section 7.2.5.3.4).  The procedures for the controlled agent are
   described in Section 7.3.1.5.

   Eventually, if the nominations succeed, both the controlling and
   controlled agents will have a single nominated pair in the valid list
   for each component of the data stream.  Once an ICE agent sets the
   state of the checklist to Completed (when there is a nominated pair
   for each component of the data stream), that pair becomes the
   selected pair for that agent and is used for sending and receiving
   data for that component of the data stream.

   If an agent is not able to produce selected pairs for each component
   of a data stream, the agent MUST take proper actions for informing
   the other agent, e.g., by removing the stream.  The exact actions are
   outside the scope of this specification.

   The criteria for stopping the connectivity checks and for picking a
   pair for nomination are outside the scope of this specification.
   They are a matter of local optimization.  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
   set.

   Once the controlling agent has successfully nominated a candidate
   pair (Section 7.2.5.3.4), the agent MUST NOT nominate another pair
   for same component of the data stream within the ICE session.  Doing
   so requires an ICE restart.




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   A controlling agent that does not support this specification (i.e.,
   it is implemented according to RFC 5245) might nominate more than one
   candidate pair.  This was referred to as "aggressive nomination" in
   RFC 5245.  If more than one candidate pair is nominated by the
   controlling agent, and if the controlled agent accepts multiple
   nominations requests, the agents MUST produce the selected pairs and
   use the pairs with the highest priority.

   The usage of the 'ice2' ICE option (Section 10) by endpoints
   supporting this specification is supposed to prevent controlling
   agents that are implemented according to RFC 5245 from using
   aggressive nomination.

   NOTE: In RFC 5245, usage of "aggressive nomination" allowed agents to
   continuously nominate pairs, before a pair was eventually selected,
   in order to allow sending of data on those pairs.  In this
   specification, data can always be sent on any valid pair, without
   nomination.  Hence, there is no longer a need for aggressive
   nomination.

8.1.2.  Updating Checklist and ICE States

   For both a controlling and a controlled agent, when a candidate pair
   for a component of a data stream gets nominated, it might impact
   other pairs in the checklist associated with the data stream.  It
   might also impact the state of the checklist:

   o  Once a candidate pair for a component of a data stream has been
      nominated, and the state of the checklist associated with the data
      stream is Running, the ICE agent MUST remove all candidate pairs
      for the same component from the checklist and from the triggered-
      check queue.  If the state of a pair is In-Progress, the agent
      cancels the In-Progress transaction.  Cancellation means that the
      agent will not retransmit the Binding requests associated with the
      connectivity-check transaction, will not treat the lack of
      response to be a failure, but will wait the duration of the
      transaction timeout for a response.

   o  Once candidate pairs for each component of a data stream have been
      nominated, and the state of the checklist associated with the data
      stream is Running, the ICE agent sets the state of the checklist
      to Completed.

   o  Once a candidate pair for a component of a data stream has been
      nominated, an agent MUST continue to respond to any Binding
      request it might still receive for the nominated pair and for any
      remaining candidate pairs in the checklist associated with the




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      data stream.  As defined in Section 7.3.1.4, when the state of a
      pair is Succeeded, an agent will no longer generate triggered
      checks when receiving a Binding request for the pair.

   Once the state of each checklist in the checklist set is Completed,
   the agent sets the state of the ICE session to Completed.

   If the state of a checklist is Failed, ICE has not been able to
   successfully complete the process for the data stream associated with
   the checklist.  The correct behavior depends on the state of the
   checklists in the checklist set.  If the controlling agent wants to
   continue the session without the data stream associated with the
   Failed checklist, and if there are still one or more checklists in
   Running or Completed mode, the agent can let the ICE processing
   continue.  The agent MUST take proper actions for removing the failed
   data stream.  If the controlling agent does not want to continue the
   session and MUST terminate the session, the state of the ICE session
   is set to Failed.

   If the state of each checklist in the checklist set is Failed, the
   state of the ICE session is set to Failed.  Unless the controlling
   agent wants to continue the session without the data streams, it MUST
   terminate the session.

8.2.  Procedures for Lite Implementations

   When ICE concludes, a lite ICE agent can free host candidates that
   were not used by ICE, as described in Section 8.3.

   If the peer is a full agent, once the lite agent accepts a nomination
   request for a candidate pair, the lite agent considers the pair
   nominated.  Once there are nominated pairs for each component of a
   data stream, the pairs become the selected pairs for the components
   of the data stream.  Once the lite agent has produced selected pairs
   for all components of all data streams, the ICE session state is set
   to Completed.

   If the peer is a lite agent, the agent pairs local candidates with
   remote candidates that are of the same data stream and have the same
   component, transport protocol, and IP address family.  For each
   component of each data stream, if there is only one candidate pair,
   that pair is added to the valid list.  If there is more than one
   pair, it is RECOMMENDED that an agent follow the procedures of RFC
   6724 [RFC6724] to select a pair and add it to the valid list.







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   If all of the components for all data streams had one pair, the state
   of ICE processing is Completed.  Otherwise, the controlling agent
   MUST send an updated candidate list to reconcile different agents
   selecting different candidate pairs.  ICE processing is complete
   after and only after the updated candidate exchange is complete.

8.3.  Freeing Candidates

8.3.1.  Full Implementation Procedures

   The rules in this section describe when it is safe for an agent to
   cease sending or receiving checks on a candidate that did not become
   a selected candidate (i.e., is not associated with a selected pair)
   and when to free the candidate.

   Once a checklist has reached the Completed state, the agent SHOULD
   wait an additional three seconds, and then it can cease responding to
   checks or generating triggered checks on all local candidates other
   than the ones that became selected candidates.  Once all ICE sessions
   have ceased using a given local candidate (a candidate may be used by
   multiple ICE sessions, e.g., in forking scenarios), the agent can
   free that candidate.  The three-second delay handles cases when
   aggressive nomination is used, and the selected pairs can quickly
   change after ICE has completed.

   Freeing of server-reflexive candidates is never explicit; it happens
   by lack of a keepalive.

8.3.2.  Lite Implementation Procedures

   A lite implementation can free candidates that did not become
   selected candidates as soon as ICE processing has reached the
   Completed state for all ICE sessions using those candidates.

9.  ICE Restarts

   An ICE agent MAY restart ICE for existing data streams.  An ICE
   restart causes all previous states of the data streams, excluding the
   roles of the agents, to be flushed.  The only difference between an
   ICE restart and a brand new data session is that during the restart,
   data can continue to be sent using existing data sessions, and a new
   data session always requires the roles to be determined.









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   The following actions can be accomplished only by using an ICE
   restart (the agent MUST use ICE restarts to do so):

   o  Change the destinations of data streams.

   o  Change from a lite implementation to a full implementation.

   o  Change from a full implementation to a lite implementation.

   To restart ICE, an agent MUST change both the password and the
   username fragment for the data stream(s) being restarted.

   When the ICE is restarted, the candidate set for the new ICE session
   might include some, none, or all of the candidates used in the
   current ICE session.

   As described in Section 6.1.1, agents MUST NOT redetermine the roles
   as part as an ICE restart, unless certain criteria that require the
   roles to be redetermined are fulfilled.

10.  ICE Option

   This section defines a new ICE option, 'ice2'.  When an ICE agent
   includes 'ice2' in a candidate exchange, the ICE option indicates
   that it is compliant to this specification.  For example, the agent
   will not use the aggressive nomination procedure defined in RFC 5245.
   In addition, it will ensure that a peer compliant with RFC 5245 does
   not use aggressive nomination either, as required by Section 14 of
   RFC 5245 for peers that receive unknown ICE options.

   An agent compliant to this specification MUST inform the peer about
   the compliance using the 'ice2' option.

   NOTE: The encoding of the 'ice2' option, and the message(s) used to
   carry it to the peer, are protocol specific.  The encoding for SDP
   [RFC4566] is defined in [ICE-SIP-SDP].

11.  Keepalives

   All endpoints MUST send keepalives for each data session.  These
   keepalives serve the purpose of keeping NAT bindings alive for the
   data session.  The keepalives 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
   ICE agent is a full ICE implementation and is communicating with a
   peer that supports ICE (lite or full).





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   An agent MUST send a keepalive on each candidate pair that is used
   for sending data if no packet has been sent on that pair in the last
   Tr seconds.  Agents SHOULD use a Tr value of 15 seconds.  Agents MAY
   use a bigger value but MUST NOT use a value smaller than 15 seconds.

   Once selected pairs have been produced for a data stream, keepalives
   are only sent on those pairs.

   An agent MUST stop sending keepalives on a data stream if the data
   stream is removed.  If the ICE session is terminated, an agent MUST
   stop sending keepalives on all data streams.

   An agent MAY use another value for Tr, e.g., based on configuration
   or network/NAT characteristics.  For example, 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.

   When 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 it 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 data.  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.

   Agents MUST by default use STUN keepalives.  Individual ICE usages
   and ICE extensions MAY specify usage-/extension-specific keepalives.

12.  Data Handling

12.1.  Sending Data

   An ICE agent MAY send data on any valid pair before selected pairs
   have been produced for the data stream.

   Once selected pairs have been produced for a data stream, an agent
   MUST send data on those pairs only.

   An agent sends data from the base of the local candidate to the
   remote candidate.  In the case of a local relayed candidate, data is
   forwarded through the base (located in the TURN server), using the
   procedures defined in [RFC5766].



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   If the local candidate is a relayed candidate, it is RECOMMENDED that
   an agent creates 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 data stream is:

   o  empty if the state of the checklist for that data 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 data
      stream if the state of the checklist for that data stream is
      Running, and there was a previous selected pair for that component
      due to an ICE restart

   Unless an agent is able to produce a selected pair for each component
   associated with a data stream, the agent MUST NOT continue sending
   data for any component associated with that data stream.

12.1.1.  Procedures for Lite Implementations

   A lite implementation MUST NOT send data until it has a valid list
   that contains a candidate pair for each component of that data
   stream.  Once that happens, the ICE agent MAY begin sending data
   packets.  To do that, it sends data 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 base associated with
   the candidate pair used for sending data.  In case of a relayed
   candidate, data is sent from the agent and forwarded through the base
   (located in the TURN server), using the procedures defined in
   [RFC5766].

12.2.  Receiving Data

   Even though ICE agents are only allowed to send data using valid
   candidate pairs (and, once selected pairs have been produced, only on
   the selected pairs), ICE implementations SHOULD by default be
   prepared to receive data on any of the candidates provided in the
   most recent candidate exchange with the peer.  ICE usages MAY define
   rules that differ from this, e.g., by defining that data will not be
   sent until selected pairs have been produced for a data stream.

   When an agent receives an RTP packet with a new source or destination
   IP address for a particular RTP/RTCP data stream, it is RECOMMENDED
   that the agent readjust its jitter buffers.





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   Section 8.2 of RFC 3550 [RFC3550] describes an algorithm for
   detecting synchronization source (SSRC) collisions and loops.  These
   algorithms are based, in part, on seeing different source transport
   addresses with the same SSRC.  However, when ICE is used, such
   changes will sometimes occur as the data streams switch between
   candidates.  An agent will be able to determine that a data stream is
   from the same peer as a consequence of the STUN exchange that
   proceeds media data transmission.  Thus, if there is a change in the
   source transport address, but the media data packets come from the
   same peer agent, this MUST NOT be treated as an SSRC collision.

13.  Extensibility Considerations

   This specification makes very specific choices about how both ICE
   agents in a session coordinate to arrive at the set of candidate
   pairs that are selected for data.  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 ICE option concept.  Each extension or change
   to ICE is associated with an ICE option.  When an agent supports such
   an extension or change, it provides the ICE option to the peer agent
   as part of the candidate exchange.

   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 nomination procedure described in Section 8
   eliminates some of the need for tight coordination by delegating the
   selection algorithm completely to the controlling agent, and 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.

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







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14.  Setting Ta and RTO

14.1.  General

   During the ICE gathering phase (Section 5.1.1) and while ICE is
   performing connectivity checks (Section 7), an ICE agent triggers
   STUN and TURN transactions.  These transactions are paced at a rate
   indicated by Ta, and the retransmission interval for each transaction
   is calculated based on the retransmission timer for the STUN
   transactions (RTO) [RFC5389].

   This section describes how the Ta and RTO values are computed during
   the ICE gathering phase and while ICE is performing connectivity
   checks.

   NOTE: Previously, in RFC 5245, different formulas were defined for
   computing Ta and RTO, depending on whether or not ICE was used for a
   real-time data stream (e.g., RTP).

   The formulas below 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 it 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 suboptimally, choosing lower-
   priority pairs over higher-priority pairs.

14.2.  Ta

   ICE agents SHOULD use a default Ta value, 50 ms, but MAY use another
   value based on the characteristics of the associated data.

   If an agent wants to use a Ta value other than the default value, the
   agent MUST indicate the proposed value to its peer during the
   establishment of the ICE session.  Both agents MUST use the higher
   value of the proposed values.  If an agent does not propose a value,
   the default value is used for that agent when comparing which value
   is higher.

   Regardless of the Ta value chosen for each agent, the combination of
   all transactions from all agents (if a given implementation runs
   several concurrent agents) MUST NOT be sent more often than once




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   every 5 ms (as though there were one global Ta value for pacing all
   agents).  See Appendix B.1 for the background of using a value of
   5 ms with ICE.

   NOTE: Appendix C shows examples of required bandwidth, using
   different Ta values.

14.3.  RTO

   During the ICE gathering phase, ICE agents SHOULD calculate the RTO
   value using the following formula:

     RTO = MAX (500ms, Ta * (Num-Of-Cands))

     Num-Of-Cands: the number of server-reflexive and relay candidates

   For connectivity checks, agents SHOULD calculate the RTO value using
   the following formula:

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

     N: the total number of connectivity checks to be performed.

     Num-Waiting: the number of checks in the checklist set in the
     Waiting state.

     Num-In-Progress: the number of checks in the checklist set 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.


   Agents MAY calculate the RTO value using other mechanisms than those
   described above.  Agents MUST NOT use an RTO value smaller than
   500 ms.

15.  Examples

   This section shows two ICE examples: one using IPv4 addresses and one
   using IPv6 addresses.

   To facilitate understanding, transport addresses are listed using
   variables that have mnemonic names.  The format of the name is
   entity-type-seqno: "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 or
   "PRIV" for transport addresses that are private [RFC1918].  Finally,



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   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.

   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 focus
   on a single data stream between two full implementations.

15.1.  Example with IPv4 Addresses

   The example below is using the topology shown in Figure 7.


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

                        Figure 7: Example Topology









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   In the example, ICE agents L and R are full ICE implementations.
   Both agents have a single IPv4 address, and both are configured with
   the same STUN server.  The NAT has an endpoint-independent mapping
   property and an address-dependent filtering property.  The IP
   addresses of the ICE agents, the STUN server, and the NAT are shown
   below:

   ENTITY                   IP Address  Mnemonic name
   --------------------------------------------------
   ICE Agent L:             10.0.1.1    L-PRIV-1
   ICE Agent R:             192.0.2.1   R-PUB-1
   STUN Server:             192.0.2.2   STUN-PUB-1
   NAT (Public):            192.0.2.3   NAT-PUB-1


             L             NAT           STUN             R
             |STUN alloc.   |              |              |
             |(1) STUN Req  |              |              |
             |S=$L-PRIV-1   |              |              |
             |D=$STUN-PUB-1 |              |              |
             |------------->|              |              |
             |              |(2) STUN Req  |              |
             |              |S=$NAT-PUB-1  |              |
             |              |D=$STUN-PUB-1 |              |
             |              |------------->|              |
             |              |(3) STUN Res  |              |
             |              |S=$STUN-PUB-1 |              |
             |              |D=$NAT-PUB-1  |              |
             |              |MA=$NAT-PUB-1 |              |
             |              |<-------------|              |
             |(4) STUN Res  |              |              |
             |S=$STUN-PUB-1 |              |              |
             |D=$L-PRIV-1   |              |              |
             |MA=$NAT-PUB-1 |              |              |
             |<-------------|              |              |
             |(5) L's Candidate Information|              |
             |------------------------------------------->|
             |              |              |              | 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   |
             |              |              |------------->|



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             |(8) R's Candidate Information|              |
             |<-------------------------------------------|
             |              |         (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    |              |              |
             |------------->|              |              |
             |              |(11) Bind Req |              |
             |              |S=$NAT-PUB-1  |              |
             |              |D=$R-PUB-1    |              |
             |              |---------------------------->|
             |              |(12) Bind Res |              |
             |              |S=$R-PUB-1    |              |
             |              |D=$NAT-PUB-1  |              |
             |              |MA=$NAT-PUB-1 |              |
             |              |<----------------------------|
             |(13) Bind Res |              |              |
             |S=$R-PUB-1    |              |              |
             |D=$L-PRIV-1   |              |              |
             |MA=$NAT-PUB-1 |              |              |
             |<-------------|              |              |
             |Data          |              |              |
             |===========================================>|
             |              |              |              |
             |              |(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   |              |
             |              |---------------------------->|
             |Data          |              |              |
             |<===========================================|



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             |              |              |              |
                                .......
             |              |              |              |
             |(18) Bind Req |              |              |
             |S=$L-PRIV-1   |              |              |
             |D=$R-PUB-1    |              |              |
             |USE-CAND      |              |              |
             |------------->|              |              |
             |              |(19) Bind Req |              |
             |              |S=$NAT-PUB-1  |              |
             |              |D=$R-PUB-1    |              |
             |              |USE-CAND      |              |
             |              |---------------------------->|
             |              |(20) Bind Res |              |
             |              |S=$R-PUB-1    |              |
             |              |D=$NAT-PUB-1  |              |
             |              |MA=$NAT-PUB-1 |              |
             |              |<----------------------------|
             |(21) Bind Res |              |              |
             |S=$R-PUB-1    |              |              |
             |D=$L-PRIV-1   |              |              |
             |MA=$NAT-PUB-1 |              |              |
             |<-------------|              |              |
             |              |              |              |

                          Figure 8: Example Flow

   Messages 1-4: Agent L gathers a host candidate from its local IP
   address, and from that it sends a STUN Binding request to the STUN
   server.  The request creates a NAT binding.  The NAT public IP
   address of the binding becomes agent L's server-reflexive candidate.

   Message 5: Agent L sends its local candidate information to agent R,
   using the signaling protocol associated with the ICE usage.

   Messages 6-7: Agent R gathers a host candidate from its local IP
   address, and from that it sends a STUN Binding request to the STUN
   server.  Since agent R is not behind a NAT, R's server-reflexive
   candidate will be identical to the host candidate.

   Message 8: Agent R sends its local candidate information to agent L,
   using the signaling protocol associated with the ICE usage.

   Since both agents are full ICE implementations, the initiating agent
   (agent L) becomes the controlling agent.






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   Agents L and R both pair up the candidates.  Both agents initially
   have two pairs.  However, agent L will prune the pair containing its
   server-reflexive candidate, resulting in just one (L1).  At agent L,
   this pair has a local candidate of $L_PRIV_1 and a remote candidate
   of $R_PUB_1.  At agent R, there are two pairs.  The highest-priority
   pair (R1) has a local candidate of $R_PUB_1 and a remote candidate of
   $L_PRIV_1, and the second pair (R2) has a local candidate of $R_PUB_1
   and a remote candidate of $NAT_PUB_1.  The pairs are shown below (the
   pair numbers are for reference purposes only):

                            Pairs
   ENTITY                   Local         Remote     Pair #     Valid
   ------------------------------------------------------------------
   ICE Agent L:             L_PRIV_1      R_PUB_1       L1

   ICE Agent R:             R_PUB_1       L_PRIV_1      R1
                            R_PUB_1       NAT_PUB_1     R2

   Message 9: Agent R initiates a connectivity check for pair #2.  As
   the remote candidate of the pair is the private address of agent L,
   the check will not be successful, as the request cannot be routed
   from R to L, and will be dropped by the network.

   Messages 10-13: Agent L initiates a connectivity check for pair L1.
   The check succeeds, and L creates a new pair (L2).  The local
   candidate of the new pair is $NAT_PUB_1, and the remote candidate is
   $R_PUB_1.  The pair (L2) is added to the valid list of agent L.
   Agent L can now send and receive data on the pair (L2) if it wishes.

                            Pairs
   ENTITY                   Local         Remote     Pair #     Valid
   ------------------------------------------------------------------
   ICE Agent L:             L_PRIV_1      R_PUB_1       L1
                            NAT_PUB_1     R_PUB_1       L2        X

   ICE Agent R:             R_PUB_1       L_PRIV_1      R1
                            R_PUB_1       NAT_PUB_1     R2

   Messages 14-17: When agent R receives the Binding request from agent
   L (message 11), it will initiate a triggered connectivity check.  The
   pair matches one of agent R's existing pairs (R2).  The check
   succeeds, and the pair (R2) is added to the valid list of agent R.
   Agent R can now send and receive data on the pair (R2) if it wishes.








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                            Pairs
   ENTITY                   Local         Remote     Pair #     Valid
   ------------------------------------------------------------------
   ICE Agent L:             L_PRIV_1      R_PUB_1       L1
                            NAT_PUB_1     R_PUB_1       L2        X

   ICE Agent R:             R_PUB_1       L_PRIV_1      R1
                            R_PUB_1       NAT_PUB_1     R2        X

   Messages 18-21: At some point, the controlling agent (agent L)
   decides to nominate a pair (L2) in the valid list.  It performs a
   connectivity check on the pair (L2) and includes the USE-CANDIDATE
   attribute in the Binding request.  As the check succeeds, agent L
   sets the nominated flag value of the pair (L2) to 'true', and agent R
   sets the nominated flag value of the matching pair (R2) to 'true'.
   As there are no more components associated with the stream, the
   nominated pairs become the selected pairs.  Consequently, processing
   for this stream moves into the Completed state.  The ICE process also
   moves into the Completed state.

15.2.  Example with IPv6 Addresses

   The example below is using the topology shown in Figure 9.

                                +-------+
                                |STUN   |
                                |Server |
                                +-------+
                                    |
                         +---------------------+
                         |                     |
                         |      Internet       |
                         |                     |
                         +---------------------+
                            |                |
                            |                |
                            |                |
                            |                |
                            |                |
                            |                |
                            |                |
                         +-----+          +-----+
                         |  L  |          |  R  |
                         +-----+          +-----+

                        Figure 9: Example Topology





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   In the example, ICE agents L and R are full ICE implementations.
   Both agents have a single IPv6 address, and both are configured with
   the same STUN server.  The IP addresses of the ICE agents and the
   STUN server are shown below:

   ENTITY                   IP Address  mnemonic name
   --------------------------------------------------
   ICE Agent L:             2001:db8::3   L-PUB-1
   ICE Agent R:             2001:db8::5   R-PUB-1
   STUN Server:             2001:db8::9   STUN-PUB-1


             L                           STUN             R
             |STUN alloc.                  |              |
             |(1) STUN Req                 |              |
             |S=$L-PUB-1                   |              |
             |D=$STUN-PUB-1                |              |
             |---------------------------->|              |
             |(2) STUN Res                 |              |
             | S=$STUN-PUB-1               |              |
             | D=$L-PUB-1                  |              |
             | MA=$L-PUB-1                 |              |
             |<----------------------------|              |
             |(3) L's Candidate Information|              |
             |------------------------------------------->|
             |                             |              | STUN
             |                             |              | alloc.
             |                             |(4) STUN Req  |
             |                             |S=$R-PUB-1    |
             |                             |D=$STUN-PUB-1 |
             |                             |<-------------|
             |                             |(5) STUN Res  |
             |                             |S=$STUN-PUB-1 |
             |                             |D=$R-PUB-1    |
             |                             |MA=$R-PUB-1   |
             |                             |------------->|
             |(6) R's Candidate Information|              |
             |<-------------------------------------------|
             |(7) Bind Req                 |              |
             |S=$L-PUB-1                   |              |
             |D=$R-PUB-1                   |              |
             |------------------------------------------->|
             |(8) Bind Res                 |              |
             |S=$R-PUB-1                   |              |
             |D=$L-PUB-1                   |              |
             |MA=$L-PUB-1                  |              |
             |<-------------------------------------------|




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             |Data                         |              |
             |===========================================>|
             |                             |              |
             |(9) Bind Req                 |              |
             |S=$R-PUB-1                   |              |
             |D=$L-PUB-1                   |              |
             |<-------------------------------------------|
             |(10) Bind Res                |              |
             |S=$L-PUB-1                   |              |
             |D=$R-PUB-1                   |              |
             |MA=$R-PUB-1                  |              |
             |------------------------------------------->|
             |Data                         |              |
             |<===========================================|
             |                             |              |
                                .......
             |                             |              |
             |(11) Bind Req                |              |
             |S=$L-PUB-1                   |              |
             |D=$R-PUB-1                   |              |
             |USE-CAND                     |              |
             |------------------------------------------->|
             |(12) Bind Res                |              |
             |S=$R-PUB-1                   |              |
             |D=$L-PUB-1                   |              |
             |MA=$L-PUB-1                  |              |
             |<-------------------------------------------|
             |              |              |              |

                          Figure 10: Example Flow

   Messages 1-2: Agent L gathers a host candidate from its local IP
   address, and from that it sends a STUN Binding request to the STUN
   server.  Since agent L is not behind a NAT, L's server-reflexive
   candidate will be identical to the host candidate.

   Message 3: Agent L sends its local candidate information to agent R,
   using the signaling protocol associated with the ICE usage.

   Messages 4-5: Agent R gathers a host candidate from its local IP
   address, and from that it sends a STUN Binding request to the STUN
   server.  Since agent R is not behind a NAT, R's server-reflexive
   candidate will be identical to the host candidate.

   Message 6: Agent R sends its local candidate information to agent L,
   using the signaling protocol associated with the ICE usage.





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   Since both agents are full ICE implementations, the initiating agent
   (agent L) becomes the controlling agent.

   Agents L and R both pair up the candidates.  Both agents initially
   have one pair each.  At agent L, the pair (L1) has a local candidate
   of $L_PUB_1 and a remote candidate of $R_PUB_1.  At agent R, the pair
   (R1) has a local candidate of $R_PUB_1 and a remote candidate of
   $L_PUB_1.  The pairs are shown below (the pair numbers are for
   reference purpose only):

                            Pairs
   ENTITY                   Local         Remote     Pair #     Valid
   ------------------------------------------------------------------
   ICE Agent L:             L_PUB_1       R_PUB_1       L1

   ICE Agent R:             R_PUB_1       L_PUB_1       R1

   Messages 7-8: Agent L initiates a connectivity check for pair L1.
   The check succeeds, and the pair (L1) is added to the valid list of
   agent L.  Agent L can now send and receive data on the pair (L1) if
   it wishes.

                            Pairs
   ENTITY                   Local         Remote     Pair #     Valid
   ------------------------------------------------------------------
   ICE Agent L:             L_PUB_1       R_PUB_1       L1         X

   ICE Agent R:             R_PUB_1       L_PUB_1       R1

   Messages 9-10: When agent R receives the Binding request from agent L
   (message 7), it will initiate a triggered connectivity check.  The
   pair matches agent R's existing pair (R1).  The check succeeds, and
   the pair (R1) is added to the valid list of agent R.  Agent R can now
   send and receive data on the pair (R1) if it wishes.

                            Pairs
   ENTITY                   Local         Remote     Pair #     Valid
   ------------------------------------------------------------------
   ICE Agent L:             L_PUB_1       R_PUB_1       L1         X

   ICE Agent R:             R_PUB_1       L_PUB_1       R1         X

   Messages 11-12: At some point, the controlling agent (agent L)
   decides to nominate a pair (L1) in the valid list.  It performs a
   connectivity check on the pair (L1) and includes the USE-CANDIDATE
   attribute in the Binding request.  As the check succeeds, agent L
   sets the nominated flag value of the pair (L1) to 'true', and agent R
   sets the nominated flag value of the matching pair (R1) to 'true'.



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   As there are no more components associated with the stream, the
   nominated pairs become the selected pairs.  Consequently, processing
   for this stream moves into the Completed state.  The ICE process also
   moves into the Completed state.

16.  STUN Extensions

16.1.  Attributes

   This specification defines four STUN 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, if one will 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 will be used for transmission of data.  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.  The
   attribute 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.  The
   number is used for solving role conflicts, when it is referred to as
   the "tiebreaker value".  An ICE agent MUST use the same number for
   all Binding requests, for all streams, within an ICE session, unless
   it has received a 487 response, in which case it MUST change the
   number (Section 7.2.5.1).  The agent MAY change the number when an
   ICE restart occurs.

   The ICE-CONTROLLING attribute is present in a Binding request.  The
   attribute 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.  As
   for the ICE-CONTROLLED attribute, the number is used for solving role
   conflicts.  An agent MUST use the same number for all Binding
   requests, for all streams, within an ICE session, unless it has
   received a 487 response, in which case it MUST change the number
   (Section 7.2.5.1).  The agent MAY change the number when an ICE
   restart occurs.








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16.2.  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 an ICE role
      that conflicted with the server.  The remote server compared the
      tiebreaker values of the client and the server and determined that
      the client needs to switch roles.

17.  Operational Considerations

   This section discusses issues relevant to operators operating
   networks where ICE will be used by endpoints.

17.1.  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 NATs.

   That said, ICE works best in environments where the NAT devices are
   "behave" compliant, meeting the recommendations defined in [RFC4787]
   and [RFC5382].  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.

17.2.  Bandwidth Requirements

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

17.2.1.  STUN and TURN Server-Capacity Planning

   First and foremost, ICE makes use of TURN and STUN servers, which
   would typically be located in data centers.  The STUN servers require
   relatively little bandwidth.  For each component of each data stream,
   there will be one or more STUN 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
   the caller and callee).  Each transaction is a single request and a
   single response, the former being 20 bytes long, and the latter, 28.





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   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 data.  The amount of calls requiring
   TURN for data relay is highly dependent on network topologies, and
   can and will vary over time.  In a network with 100% behave-compliant
   NATs, it is exactly zero.

   The planning considerations above become more significant in
   multimedia scenarios (e.g., audio and video conferences) and when the
   numbers of participants in a session grow.

17.2.2.  Gathering and Connectivity Checks

   The process of gathering candidates and performing connectivity
   checks can be bandwidth intensive.  ICE has been designed to pace
   both of these processes.  The gathering and connectivity-check phases
   are meant to generate traffic at roughly the same bandwidth as the
   data traffic itself will consume once the ICE process concludes.
   This was done to ensure that if a network is designed to support
   communication traffic of a certain type (voice, video, or just text),
   it will have sufficient capacity to support the ICE checks for that
   data.  Once ICE has concluded, the subsequent ICE keepalives will
   later cause a marginal increase in the total bandwidth 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 could send them.  Consequently, network
   operators need to ensure 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.

17.2.3.  Keepalives

   STUN keepalives (in the form of STUN Binding Indications) are sent in
   the middle of a data session.  However, they are sent only in the
   absence of actual data traffic.  In deployments with continuous media
   and without utilizing Voice Activity Detection (VAD), or deployments
   where VAD is utilized together with short interval (max 1 second)
   comfort noise, the keepalives are never used and there is no increase
   in bandwidth usage.  When VAD is being used without comfort noise,
   keepalives will be sent during silence periods.  This involves a



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   single packet every 15-20 seconds, far less than the packet every
   20-30 ms that is sent when there is voice.  Therefore, keepalives do
   not have any real impact on capacity planning.

17.3.  ICE and ICE-Lite

   Deployments utilizing a mix of ICE and ICE-lite interoperate with
   each other.  They have been explicitly designed to do so.

   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.

17.4.  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.
   Signaling servers, typically deployed in data centers of the network
   operator, will see the contents of the candidate 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
   candidate exchange takes place, signaling the selected address (and
   its type).  This updated signaling is performed exactly for the
   purposes of educating network equipment (such as a diagnostic tool
   attached to a signaling) about the results of ICE processing.

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

17.5.  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
   used to configure all of the other parameters in the endpoint.  For
   SIP phones, standard solutions such as the configuration framework
   [RFC6080] have been defined.





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18.  IAB Considerations

   The IAB has studied the problem of "Unilateral Self-Address Fixing"
   (UNSAF), which is the general process by which an ICE 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
   the IAB.  Indeed, ICE can be considered a Bilateral Self-Address
   Fixing (B-SAF) 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.

18.1.  Problem Definition

   From RFC 3424, any UNSAF proposal needs to 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.  Such generalizations lead to
      the the prolonged dependence on and usage of the supposed short
      term fix -- meaning that it is no longer accurate to call it
      "short term".

   The specific problems being solved by ICE are:

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

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

18.2.  Exit Strategy

   From RFC 3424, any UNSAF proposal needs to 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 picks amongst those



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   mechanisms, prioritizing ones that are better and deprioritizing ones
   that are worse.  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 removed 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.

18.3.  Brittleness Introduced by ICE

   From RFC 3424, any UNSAF proposal needs to 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 ICE 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 it
   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 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.






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   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 it does not introduce any additional brittleness
   into the system.

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

18.4.  Requirements for a Long-Term Solution

   From RFC 3424, any UNSAF proposal needs to provide the following:

      Identify 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.

18.5.  Issues with Existing NAPT Boxes

   From RFC 3424, any UNSAF proposal needs to 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, in either 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




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   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.

19.  Security Considerations

19.1.  IP Address Privacy

   The process of probing for candidates reveals the source addresses of
   the client and its peer to any on-network listening attacker, and the
   process of exchanging candidates reveals the addresses to any
   attacker that is able to see the negotiation.  Some addresses, such
   as the server-reflexive addresses gathered through the local
   interface of VPN users, may be sensitive information.  If these
   potential attacks cannot be mitigated, ICE usages can define
   mechanisms for controlling which addresses are revealed to the
   negotiation and/or probing process.  Individual implementations may
   also have implementation-specific rules for controlling which
   addresses are revealed.  For example, [WebRTC-IP-HANDLING] provides
   additional information about the privacy aspects of revealing IP
   addresses via ICE for WebRTC applications.  ICE implementations where
   such issues can arise are RECOMMENDED to provide a programmatic or
   user interface that provides control over which network interfaces
   are used to generate candidates.

   Based on the types of candidates provided by the peer, and the
   results of the connectivity tests performed against those candidates,
   the peer might be able to determine characteristics of the local
   network, e.g., if different timings are apparent to the peer.  Within
   the limit, the peer might be able to probe the local network.

   There are several types of attacks possible in an ICE system.  The
   subsections consider these attacks and their countermeasures.

19.2.  Attacks on Connectivity Checks

   An attacker might attempt to disrupt the STUN connectivity checks.
   Ultimately, all of these attacks fool an ICE agent into thinking
   something incorrect about the results of the connectivity checks.
   Depending on the type of attack, the attacker needs to have different
   capabilities.  In some cases, the attacker needs to be on the path of
   the connectivity checks.  In other cases, the attacker does not need
   to be on the path, as long as it is able to generate STUN
   connectivity checks.  While attacks on connectivity checks are
   typically performed by network entities, if an attacker is able to
   control an endpoint, it might be able to trigger connectivity-check
   attacks.  The possible false conclusions an attacker can try and
   cause are:



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   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
      data.

   False Peer-Reflexive Candidate:  An attacker can cause an agent to
      discover a new peer-reflexive candidate when it is not expected
      to.  This can be used to redirect data streams to a DoS target or
      to the attacker, for eavesdropping or other purposes.

   False Valid on False Candidate:  An attacker has already convinced an
      agent that there is a candidate with an address that does not
      actually route to that agent (e.g., by injecting a false peer-
      reflexive candidate or false server-reflexive candidate).  The
      attacker then launches 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), or drop the response so that it never
   reaches the agent.  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, a Layer 2 network disruption, or
   another technique.  If it doesn't do this, the success response will
   also reach the originator, alerting it to a possible attack.  The
   ability for the attacker to generate a fake response is mitigated
   through the STUN short-term credential mechanism.  In order for this
   response to be processed, the attacker needs the password.  If the
   candidate exchange signaling is secured, the attacker will not have
   the password, and its response will be discarded.

   Spoofed ICMP Hard Errors (Type 3, codes 2-4) can also be used to
   create false invalid results.  If an ICE agent implements a response
   to these ICMP errors, the attacker is capable of generating an ICMP
   message that is delivered to the agent sending the connectivity
   check.  The validation of the ICMP error message by the agent is its



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   only defense.  For Type 3 code=4, the outer IP header provides no
   validation, unless the connectivity check was sent with DF=0.  For
   codes 2 or 3, which are originated by the host, the address is
   expected to be any of the remote agent's host, reflexive, or relay
   candidate IP addresses.  The ICMP message includes the IP header and
   UDP header of the message triggering the error.  These fields also
   need to be validated.  The IP destination and UDP destination port
   need to match either the targeted candidate address and port or the
   candidate's base address.  The source IP address and port can be any
   candidate for the same base address of the agent sending the
   connectivity check.  Thus, any attacker having access to the exchange
   of the candidates will have the necessary information.  Hence, the
   validation is a weak defense, and the sending of spoofed ICMP attacks
   is also possible for off-path attackers from a node in a network
   without source address validation.

   Forcing the fake valid result works in a similar way.  The attacker
   needs to wait for the Binding request from each agent and inject a
   fake success response.  Again, due to the STUN short-term credential
   mechanism, in order for the attacker to inject a valid success
   response, the attacker needs the password.  Alternatively, the
   attacker can route (e.g., using a tunneling mechanism) a valid
   success response, which normally would be dropped or rejected by the
   network, to the agent.

   Forcing the false peer-reflexive candidate result can be done with
   either 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 needs to 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 candidate 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 also needs to prevent the
   original request from reaching the remote agent, by either 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 it will be sent to that
   false candidate.  The attacker then needs to receive it and relay it
   towards the originator.



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   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.  The
   injecting of fake requests or responses to achieve this goal is
   prevented using the integrity mechanisms of STUN and the candidate
   exchange.  Thus, this attack can only be launched through replays.
   To do that, the attacker needs to intercept the check towards this
   false candidate and replay it towards the other agent.  Then, it
   needs to intercept the response and replay that back as well.

   This attack is very hard to launch unless the attacker is identified
   by the fake candidate.  This is because it requires the attacker to
   intercept and replay packets sent by two different hosts.  If both
   agents are on different networks (e.g., 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 the data path is secured
   (e.g., using the Secure Real-time Transport Protocol (SRTP)
   [RFC3711]), the attacker will not be able to process the data
   packets, but will only be able to discard them, effectively disabling
   the data stream.  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 data stream,
   it's much easier to just disrupt it with the same mechanism, rather
   than attack ICE.

19.3.  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 DNSSEC is not required to address it.

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

   o  An attacker can compromise a STUN server and cause it to send
      responses with incorrect mapped addresses.



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   A false mapped address learned by these attacks will be used as a
   server-reflexive candidate in the establishment of the ICE session.
   For this candidate to actually be used for data, the attacker also
   needs to 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
   initiator, responder, nor attacker), since it requires attacking the
   checks generated by each ICE agent in the session and is prevented by
   SRTP if it identifies the attacker itself.

   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 data.  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.

19.4.  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.

   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 needs to launch a
   false valid on a false candidate, per above, which is a very
   difficult attack to coordinate.

19.5.  Insider Attacks

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








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19.5.1.  STUN Amplification Attack

   The STUN amplification attack is similar to a "voice hammer" attack,
   where the attacker causes other agents to direct voice packets to the
   attack target.  However, instead of voice packets being directed to
   the target, STUN connectivity checks are directed to the target.  The
   attacker sends a large number of candidates, say, 50.  The responding
   agent receives the candidate information and starts its checks, which
   are directed at the target, and consequently, never generate a
   response.  In the case of WebRTC, the user might not even be aware
   that this attack is ongoing, since it might be triggered in the
   background by malicious JavaScript code that the user has fetched.
   The answerer will start a new connectivity check every Ta ms (say,
   Ta=50ms).  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 data 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.  ICE agents SHOULD limit
   the total number of connectivity checks they perform to 100.
   Additionally, agents MAY limit the number of candidates they will
   accept.

   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 will be sent at the next
   opportunity, while in the former case, no further checks will be
   sent.









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20.  IANA Considerations

   The original ICE specification registered four STUN attributes and
   one new STUN error response.  The STUN attributes and error response
   are reproduced here.  In addition, this specification registers a new
   ICE option.

20.1.  STUN Attributes

   IANA has registered four STUN attributes:

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

20.2.  STUN Error Responses

   IANA has registered the following STUN error-response code:

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

20.3.  ICE Options

   IANA has registered the following ICE option in the "ICE Options"
   subregistry of the "Interactive Connectivity Establishment (ICE)"
   registry, following the procedures defined in [RFC6336].

   ICE Option name:
      ice2

   Contact:
      Name:    IESG
      Email:   iesg@ietf.org

   Change Controller:
      IESG

   Description:
      The ICE option indicates that the ICE agent using the ICE option
      is implemented according to RFC 8445.

   Reference:
      RFC 8445






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21.  Changes from RFC 5245

   The purpose of this updated ICE specification is to:

   o  Clarify procedures in RFC 5245.

   o  Make technical changes, due to discovered flaws in RFC 5245 and
      feedback from the community that has implemented and deployed ICE
      applications based on RFC 5245.

   o  Make the procedures independent of the signaling protocol, by
      removing the SIP and SDP procedures.  Procedures specific to a
      signaling protocol will be defined in separate usage documents.
      [ICE-SIP-SDP] defines ICE usage with SIP and SDP.

   The following technical changes have been done:

   o  Aggressive nomination removed.

   o  The procedures for calculating candidate pair states and
      scheduling connectivity checks modified.

   o  Procedures for calculation of Ta and RTO modified.

   o  Active checklist and Frozen checklist definitions removed.

   o  'ice2' ICE option added.

   o  IPv6 considerations modified.

   o  Usage with no-op for keepalives, and keepalives with non-ICE
      peers, removed.

22.  References

22.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <https://www.rfc-editor.org/info/rfc2119>.

   [RFC4941]  Narten, T., Draves, R., and S. Krishnan, "Privacy
              Extensions for Stateless Address Autoconfiguration in
              IPv6", RFC 4941, DOI 10.17487/RFC4941, September 2007,
              <https://www.rfc-editor.org/info/rfc4941>.





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   [RFC5389]  Rosenberg, J., Mahy, R., Matthews, P., and D. Wing,
              "Session Traversal Utilities for NAT (STUN)", RFC 5389,
              DOI 10.17487/RFC5389, October 2008,
              <https://www.rfc-editor.org/info/rfc5389>.

   [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,
              DOI 10.17487/RFC5766, April 2010,
              <https://www.rfc-editor.org/info/rfc5766>.

   [RFC6336]  Westerlund, M. and C. Perkins, "IANA Registry for
              Interactive Connectivity Establishment (ICE) Options",
              RFC 6336, DOI 10.17487/RFC6336, July 2011,
              <https://www.rfc-editor.org/info/rfc6336>.

   [RFC6724]  Thaler, D., Ed., Draves, R., Matsumoto, A., and T. Chown,
              "Default Address Selection for Internet Protocol Version 6
              (IPv6)", RFC 6724, DOI 10.17487/RFC6724, September 2012,
              <https://www.rfc-editor.org/info/rfc6724>.

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <https://www.rfc-editor.org/info/rfc8174>.

22.2.  Informative References

   [ICE-SIP-SDP]
              Petit-Huguenin, M., Nandakumar, S., and A. Keranen,
              "Session Description Protocol (SDP) Offer/Answer
              procedures for Interactive Connectivity Establishment
              (ICE)", Work in Progress,
              draft-ietf-mmusic-ice-sip-sdp-21, June 2018.

   [RFC1918]  Rekhter, Y., Moskowitz, B., Karrenberg, D., de Groot, G.,
              and E. Lear, "Address Allocation for Private Internets",
              BCP 5, RFC 1918, DOI 10.17487/RFC1918, February 1996,
              <https://www.rfc-editor.org/info/rfc1918>.

   [RFC2475]  Blake, S., Black, D., Carlson, M., Davies, E., Wang, Z.,
              and W. Weiss, "An Architecture for Differentiated
              Services", RFC 2475, DOI 10.17487/RFC2475, December 1998,
              <https://www.rfc-editor.org/info/rfc2475>.

   [RFC3102]  Borella, M., Lo, J., Grabelsky, D., and G. Montenegro,
              "Realm Specific IP: Framework", RFC 3102,
              DOI 10.17487/RFC3102, October 2001,
              <https://www.rfc-editor.org/info/rfc3102>.



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   [RFC3103]  Borella, M., Grabelsky, D., Lo, J., and K. Taniguchi,
              "Realm Specific IP: Protocol Specification", RFC 3103,
              DOI 10.17487/RFC3103, October 2001,
              <https://www.rfc-editor.org/info/rfc3103>.

   [RFC3235]  Senie, D., "Network Address Translator (NAT)-Friendly
              Application Design Guidelines", RFC 3235,
              DOI 10.17487/RFC3235, January 2002,
              <https://www.rfc-editor.org/info/rfc3235>.

   [RFC3261]  Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston,
              A., Peterson, J., Sparks, R., Handley, M., and E.
              Schooler, "SIP: Session Initiation Protocol", RFC 3261,
              DOI 10.17487/RFC3261, June 2002,
              <https://www.rfc-editor.org/info/rfc3261>.

   [RFC3264]  Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model
              with Session Description Protocol (SDP)", RFC 3264,
              DOI 10.17487/RFC3264, June 2002,
              <https://www.rfc-editor.org/info/rfc3264>.

   [RFC3303]  Srisuresh, P., Kuthan, J., Rosenberg, J., Molitor, A., and
              A. Rayhan, "Middlebox communication architecture and
              framework", RFC 3303, DOI 10.17487/RFC3303, August 2002,
              <https://www.rfc-editor.org/info/rfc3303>.

   [RFC3424]  Daigle, L., Ed. and IAB, "IAB Considerations for
              UNilateral Self-Address Fixing (UNSAF) Across Network
              Address Translation", RFC 3424, DOI 10.17487/RFC3424,
              November 2002, <https://www.rfc-editor.org/info/rfc3424>.

   [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,
              DOI 10.17487/RFC3489, March 2003,
              <https://www.rfc-editor.org/info/rfc3489>.

   [RFC3550]  Schulzrinne, H., Casner, S., Frederick, R., and V.
              Jacobson, "RTP: A Transport Protocol for Real-Time
              Applications", STD 64, RFC 3550, DOI 10.17487/RFC3550,
              July 2003, <https://www.rfc-editor.org/info/rfc3550>.

   [RFC3605]  Huitema, C., "Real Time Control Protocol (RTCP) attribute
              in Session Description Protocol (SDP)", RFC 3605,
              DOI 10.17487/RFC3605, October 2003,
              <https://www.rfc-editor.org/info/rfc3605>.





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   [RFC3711]  Baugher, M., McGrew, D., Naslund, M., Carrara, E., and K.
              Norrman, "The Secure Real-time Transport Protocol (SRTP)",
              RFC 3711, DOI 10.17487/RFC3711, March 2004,
              <https://www.rfc-editor.org/info/rfc3711>.

   [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, DOI 10.17487/RFC3725, April 2004,
              <https://www.rfc-editor.org/info/rfc3725>.

   [RFC3879]  Huitema, C. and B. Carpenter, "Deprecating Site Local
              Addresses", RFC 3879, DOI 10.17487/RFC3879, September
              2004, <https://www.rfc-editor.org/info/rfc3879>.

   [RFC4038]  Shin, M-K., Ed., Hong, Y-G., Hagino, J., Savola, P., and
              E. Castro, "Application Aspects of IPv6 Transition",
              RFC 4038, DOI 10.17487/RFC4038, March 2005,
              <https://www.rfc-editor.org/info/rfc4038>.

   [RFC4091]  Camarillo, G. and J. Rosenberg, "The Alternative Network
              Address Types (ANAT) Semantics for the Session Description
              Protocol (SDP) Grouping Framework", RFC 4091,
              DOI 10.17487/RFC4091, June 2005,
              <https://www.rfc-editor.org/info/rfc4091>.

   [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, DOI 10.17487/RFC4092, June 2005,
              <https://www.rfc-editor.org/info/rfc4092>.

   [RFC4103]  Hellstrom, G. and P. Jones, "RTP Payload for Text
              Conversation", RFC 4103, DOI 10.17487/RFC4103, June 2005,
              <https://www.rfc-editor.org/info/rfc4103>.

   [RFC4291]  Hinden, R. and S. Deering, "IP Version 6 Addressing
              Architecture", RFC 4291, DOI 10.17487/RFC4291, February
              2006, <https://www.rfc-editor.org/info/rfc4291>.

   [RFC4566]  Handley, M., Jacobson, V., and C. Perkins, "SDP: Session
              Description Protocol", RFC 4566, DOI 10.17487/RFC4566,
              July 2006, <https://www.rfc-editor.org/info/rfc4566>.

   [RFC4787]  Audet, F., Ed. and C. Jennings, "Network Address
              Translation (NAT) Behavioral Requirements for Unicast
              UDP", BCP 127, RFC 4787, DOI 10.17487/RFC4787, January
              2007, <https://www.rfc-editor.org/info/rfc4787>.



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   [RFC5245]  Rosenberg, J., "Interactive Connectivity Establishment
              (ICE): A Protocol for Network Address Translator (NAT)
              Traversal for Offer/Answer Protocols", RFC 5245,
              DOI 10.17487/RFC5245, April 2010,
              <https://www.rfc-editor.org/info/rfc5245>.

   [RFC5382]  Guha, S., Ed., Biswas, K., Ford, B., Sivakumar, S., and P.
              Srisuresh, "NAT Behavioral Requirements for TCP", BCP 142,
              RFC 5382, DOI 10.17487/RFC5382, October 2008,
              <https://www.rfc-editor.org/info/rfc5382>.

   [RFC5761]  Perkins, C. and M. Westerlund, "Multiplexing RTP Data and
              Control Packets on a Single Port", RFC 5761,
              DOI 10.17487/RFC5761, April 2010,
              <https://www.rfc-editor.org/info/rfc5761>.

   [RFC6080]  Petrie, D. and S. Channabasappa, Ed., "A Framework for
              Session Initiation Protocol User Agent Profile Delivery",
              RFC 6080, DOI 10.17487/RFC6080, March 2011,
              <https://www.rfc-editor.org/info/rfc6080>.

   [RFC6146]  Bagnulo, M., Matthews, P., and I. van Beijnum, "Stateful
              NAT64: Network Address and Protocol Translation from IPv6
              Clients to IPv4 Servers", RFC 6146, DOI 10.17487/RFC6146,
              April 2011, <https://www.rfc-editor.org/info/rfc6146>.

   [RFC6147]  Bagnulo, M., Sullivan, A., Matthews, P., and I. van
              Beijnum, "DNS64: DNS Extensions for Network Address
              Translation from IPv6 Clients to IPv4 Servers", RFC 6147,
              DOI 10.17487/RFC6147, April 2011,
              <https://www.rfc-editor.org/info/rfc6147>.

   [RFC6298]  Paxson, V., Allman, M., Chu, J., and M. Sargent,
              "Computing TCP's Retransmission Timer", RFC 6298,
              DOI 10.17487/RFC6298, June 2011,
              <https://www.rfc-editor.org/info/rfc6298>.

   [RFC6544]  Rosenberg, J., Keranen, A., Lowekamp, B., and A. Roach,
              "TCP Candidates with Interactive Connectivity
              Establishment (ICE)", RFC 6544, DOI 10.17487/RFC6544,
              March 2012, <https://www.rfc-editor.org/info/rfc6544>.

   [RFC6928]  Chu, J., Dukkipati, N., Cheng, Y., and M. Mathis,
              "Increasing TCP's Initial Window", RFC 6928,
              DOI 10.17487/RFC6928, April 2013,
              <https://www.rfc-editor.org/info/rfc6928>.





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   [RFC7050]  Savolainen, T., Korhonen, J., and D. Wing, "Discovery of
              the IPv6 Prefix Used for IPv6 Address Synthesis",
              RFC 7050, DOI 10.17487/RFC7050, November 2013,
              <https://www.rfc-editor.org/info/rfc7050>.

   [RFC7721]  Cooper, A., Gont, F., and D. Thaler, "Security and Privacy
              Considerations for IPv6 Address Generation Mechanisms",
              RFC 7721, DOI 10.17487/RFC7721, March 2016,
              <https://www.rfc-editor.org/info/rfc7721>.

   [RFC7825]  Goldberg, J., Westerlund, M., and T. Zeng, "A Network
              Address Translator (NAT) Traversal Mechanism for Media
              Controlled by the Real-Time Streaming Protocol (RTSP)",
              RFC 7825, DOI 10.17487/RFC7825, December 2016,
              <https://www.rfc-editor.org/info/rfc7825>.

   [RFC8421]  Martinsen, P., Reddy, T., and P. Patil, "Interactive
              Connectivity Establishment (ICE) Multihomed and IPv4/IPv6
              Dual-Stack Guidelines", RFC 8421, DOI 10.17487/RFC8421,
              July 2018, <https://www.rfc-editor.org/info/rfc8421>.

   [WebRTC-IP-HANDLING]
              Uberti, J. and G. Shieh, "WebRTC IP Address Handling
              Requirements", Work in Progress, draft-ietf-rtcweb-ip-
              handling-09, June 2018.


























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Appendix A.  Lite and Full Implementations

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

   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 6724, which is recommended by this specification.
   However, static mechanisms for address selection are always prone to
   error, since they can never reflect the actual topology or provide
   actual guarantees on connectivity.  They are always heuristics.
   Consequently, if an ICE 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.  Full implementations
   also obtain the security benefits of ICE unrelated to NAT traversal.
   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



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   lifetime of a device or product, if 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 necessary to understand for purposes of implementation, they
   are discussed here.  This appendix 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.  Discussions in the IETF
   ICE WG during the work on this specification concluded that once
   every 5 ms is well supported.  This is why Ta has a lower bound of
   5 ms.  Furthermore, transmission of these packets on the network
   makes use of bandwidth and needs to be rate limited by the ICE agent.
   Deployments based on earlier draft versions of [RFC5245] 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 MUST NOT use more
   bandwidth than the RTP itself will use, once data 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 data streams.  Of course, STUN has
   retransmits, and the desire is to pace those as well.  For this
   reason, RTO is set such that the first retransmit on the first
   transaction happens just as the first STUN request on the last
   transaction occurs.  Pictorially:








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              First Packets              Retransmits



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

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

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

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

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

   This mechanism of a global minimum pacing interval of 5 ms is not
   generally applicable to transport protocols, but it is applicable to
   ICE based on the following reasoning.

   o  Start with the following rules that would be generally applicable
      to transport protocols:

      1.  Let MaxBytes be the maximum number of bytes allowed to be
          outstanding in the network at startup, which SHOULD be 14600,
          as defined in Section 2 of [RFC6928].

      2.  Let HTO be the transaction timeout, which SHOULD be 2*RTT if
          RTT is known or 500 ms otherwise.  This is based on the RTO
          for STUN messages from [RFC5389] and the TCP initial RTO,
          which is 1 sec in [RFC6298].

      3.  Let MinPacing be the minimum pacing interval between
          transactions, which is 5 ms (see above).






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   o  Observe that agents typically do not know the RTT for ICE
      transactions (connectivity checks in particular), meaning that HTO
      will almost always be 500 ms.

   o  Observe that a MinPacing of 5 ms and HTO of 500 ms gives at most
      100 packets/HTO, which for a typical ICE check of less than 120
      bytes means a maximum of 12000 outstanding bytes in the network,
      which is less than the maximum expressed by rule 1.

   o  Thus, for ICE, the rule set reduces to just the MinPacing rule,
      which is equivalent to having a global Ta value.








































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B.2.  Candidates with Multiple Bases

   Section 5.1.3 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 ICE agent have two candidates that have the same IP address and
   port but different bases?  Consider the topology of Figure 11:

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

           Figure 11: Identical Candidates with Different Bases






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   In this case, the initiating agent is multihomed.  It has one IP
   address, 10.0.1.100, on network C, which is a net 10 private network.
   The responding agent is on this same network.  The initiating agent
   is also connected to network A, which is 192.168/16, and has an IP
   address of 192.168.1.100.  There is a NAT on this network, natting
   into network B, which is another net 10 private network, but it is
   not connected to network C.  There is a STUN server on network B.

   The initiating agent 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 (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
   initiating agent 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 Related-Address and Related-Port Attributes

   The candidate attribute contains two values that are not used at all
   by ICE itself -- related address and related port.  Why are they
   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 ICE 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 but 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 data traffic.  They can then interact, through policy servers,
   with access routers in the network, to establish guaranteed QoS for
   the data flows.  This QoS is provided by classifying the RTP traffic
   based on 5-tuple and then providing it a guaranteed rate, or marking
   its DSCP appropriately.  When a residential NAT is present, and a
   relayed candidate gets selected for data, 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




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   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
   candidate exchange.  The need for this mechanism goes beyond just
   security; it is actually required for correct operation of ICE in the
   first place.

   Consider ICE 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 candidates to Z.  Z
   responds to 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, STUN short-term credential mechanisms are used.  The username
   fragments are sufficiently random; thus 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 session established as part of the candidate
   exchange.

   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 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 need to be prepared for it.  Note that this is not a



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   problem specific to ICE; stray packets can arrive at a port at any
   time for any type of protocol, especially ones on the public
   Internet.  As such, this requirement is just restating a general
   design guideline for Internet applications -- be prepared for unknown
   packets on any port.

B.5.  The Candidate Pair 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 tiebreaker 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 ICE agent, a lower-priority candidate is never used
   until all higher-priority candidates have been tried.

B.6.  Why Are Keepalives Needed?

   Once data 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 data stream packets themselves (e.g.,
   RTP) meet this objective.  However, several cases merit further
   discussion.  Firstly, in some RTP usages, such as SIP, the data
   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
   data in these cases.  However, doing so may cause NAT bindings to
   time out, and data 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 data transmission to cease sufficiently long for NAT
   bindings to time out.





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   For these reasons, the data 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.7.  Why Prefer Peer-Reflexive Candidates?

   Section 5.1.2 describes procedures for computing the priority of a
   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 19.  It is much
   easier for an attacker to cause an ICE agent to use a false server-
   reflexive candidate rather than a false peer-reflexive candidate.
   Consequently, attacks against address gathering with Binding requests
   are thwarted by ICE by preferring the peer-reflexive candidates.

B.8.  Why Are Binding Indications Used for Keepalives?

   Data keepalives are described in Section 11.  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?

   The primary reason has to do with network QoS mechanisms.  Once data
   begins flowing, network elements will assume that the data stream has
   a fairly regular structure, making use of periodic packets at fixed
   intervals, with the possibility of jitter.  If an ICE agent is
   sending data packets, and then receives a Binding request, it would
   need to generate a response packet along with its data packets.  This
   will increase the actual bandwidth requirements for the 5-tuple
   carrying the data 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
   data.

   Additionally, using a Binding Indication allows integrity to be
   disabled, which may result in better performance.  This is useful for
   large-scale endpoints, such as Public Switched Telephone Network
   (PSTN) gateways and Session Border Controllers (SBCs).

B.9.  Selecting Candidate Type Preference

   One criterion for selecting type and local preference values is the
   use of a data intermediary, such as a TURN server, a tunnel service
   such as a VPN server, or NAT.  With a data intermediary, if data is
   sent to that candidate, it will first transit the data intermediary
   before being received.  One type of candidate that involves a data



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   intermediary is the relayed candidate.  Another type is the host
   candidate, which is obtained from a VPN interface.  When data is
   transited through a data intermediary, it can have a positive or
   negative effect on the latency between transmission and reception.
   It may or may not increase the packet losses, because of the
   additional router hops that may be taken.  It may increase the cost
   of providing service, since data will be routed in and right back out
   of a data intermediary run by a provider.  If these concerns are
   important, the type preference for relayed candidates needs to be
   carefully chosen.

   Another criterion for selecting preferences is the IP address family.
   ICE works with both IPv4 and IPv6.  It 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.  Implementation SHOULD follow the guidelines from
   [RFC8421] to avoid excessive delays in the connectivity-check phase
   if broken paths exist.

   Another criterion for selecting preferences is topological awareness.
   This is beneficial for candidates that make use of intermediaries.
   In those cases, if an ICE 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.

   Another criterion for selecting preferences might be security or
   privacy.  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 or similar tunnel in order to
   keep it on the corporate network when communicating within the
   enterprise but may 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.

















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Appendix C.  Connectivity-Check Bandwidth

   The tables below show, for IPv4 and IPv6, the bandwidth required for
   performing connectivity checks, using different Ta values (given in
   ms) and different ufrag sizes (given in bytes).

   The results were provided by Jusin Uberti (Google) on 11 April 2016.

                     IP version: IPv4
                     Packet len (bytes): 108 + ufrag
                          |
                       ms |     4     8    12    16
                     -----|------------------------
                      500 | 1.86k 1.98k 2.11k 2.24k
                      200 | 4.64k 4.96k 5.28k  5.6k
                      100 | 9.28k 9.92k 10.6k 11.2k
                       50 | 18.6k 19.8k 21.1k 22.4k
                       20 | 46.4k 49.6k 52.8k 56.0k
                       10 | 92.8k 99.2k  105k  112k
                        5 |  185k  198k  211k  224k
                        2 |  464k  496k  528k  560k
                        1 |  928k  992k 1.06M 1.12M

                     IP version: IPv6
                     Packet len (bytes): 128 + ufrag
                          |
                       ms |     4     8    12    16
                     -----|------------------------
                      500 | 2.18k  2.3k 2.43k 2.56k
                      200 | 5.44k 5.76k 6.08k  6.4k
                      100 | 10.9k 11.5k 12.2k 12.8k
                       50 | 21.8k 23.0k 24.3k 25.6k
                       20 | 54.4k 57.6k 60.8k 64.0k
                       10 |  108k  115k  121k  128k
                        5 |  217k  230k  243k  256k
                        2 |  544k  576k  608k  640k
                        1 | 1.09M 1.15M 1.22M 1.28M


                  Figure 12: Connectivity-Check Bandwidth











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Acknowledgements

   Most of the text in this document comes from the original ICE
   specification, RFC 5245.  The authors would like to thank everyone
   who has contributed to that document.  For additional contributions
   to this revision of the specification, we would like to thank Emil
   Ivov, Paul Kyzivat, Pal-Erik Martinsen, Simon Perrault, Eric
   Rescorla, Thomas Stach, Peter Thatcher, Martin Thomson, Justin
   Uberti, Suhas Nandakumar, Taylor Brandstetter, Peter Saint-Andre,
   Harald Alvestrand, and Roman Shpount.  Ben Campbell did the AD
   review.  Stephen Farrell did the sec-dir review.  Stewart Bryant did
   the gen-art review.  Qin We did the ops-dir review.  Magnus
   Westerlund did the tsv-art review.

Authors' Addresses

   Ari Keranen
   Ericsson
   Hirsalantie 11
   02420 Jorvas
   Finland

   Email: ari.keranen@ericsson.com


   Christer Holmberg
   Ericsson
   Hirsalantie 11
   02420 Jorvas
   Finland

   Email: christer.holmberg@ericsson.com


   Jonathan Rosenberg
   jdrosen.net
   Monmouth, NJ
   United States of America

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










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