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rfc:rfc8445

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.

Keranen, et al. Standards Track [Page 1] RFC 8445 ICE July 2018

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

Keranen, et al. Standards Track [Page 2] RFC 8445 ICE July 2018

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

Keranen, et al. Standards Track [Page 3] RFC 8445 ICE July 2018

     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

Keranen, et al. Standards Track [Page 4] RFC 8445 ICE July 2018

 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

Keranen, et al. Standards Track [Page 5] RFC 8445 ICE July 2018

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.

Keranen, et al. Standards Track [Page 6] RFC 8445 ICE July 2018

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

Keranen, et al. Standards Track [Page 7] RFC 8445 ICE July 2018

 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

Keranen, et al. Standards Track [Page 8] RFC 8445 ICE July 2018

 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.

Keranen, et al. Standards Track [Page 9] RFC 8445 ICE July 2018

 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.

Keranen, et al. Standards Track [Page 10] RFC 8445 ICE July 2018

 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.

Keranen, et al. Standards Track [Page 11] RFC 8445 ICE July 2018

 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.

Keranen, et al. Standards Track [Page 12] RFC 8445 ICE July 2018

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].

Keranen, et al. Standards Track [Page 13] RFC 8445 ICE July 2018

 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.

Keranen, et al. Standards Track [Page 14] RFC 8445 ICE July 2018

 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.

Keranen, et al. Standards Track [Page 15] RFC 8445 ICE July 2018

 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.

Keranen, et al. Standards Track [Page 16] RFC 8445 ICE July 2018

 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.

Keranen, et al. Standards Track [Page 17] RFC 8445 ICE July 2018

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.

Keranen, et al. Standards Track [Page 18] RFC 8445 ICE July 2018

 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

Keranen, et al. Standards Track [Page 19] RFC 8445 ICE July 2018

    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.

Keranen, et al. Standards Track [Page 20] RFC 8445 ICE July 2018

 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.

Keranen, et al. Standards Track [Page 21] RFC 8445 ICE July 2018

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

Keranen, et al. Standards Track [Page 22] RFC 8445 ICE July 2018

 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.

Keranen, et al. Standards Track [Page 23] RFC 8445 ICE July 2018

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.

Keranen, et al. Standards Track [Page 24] RFC 8445 ICE July 2018

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).

Keranen, et al. Standards Track [Page 25] RFC 8445 ICE July 2018

 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.

Keranen, et al. Standards Track [Page 26] RFC 8445 ICE July 2018

 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.

Keranen, et al. Standards Track [Page 27] RFC 8445 ICE July 2018

 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

Keranen, et al. Standards Track [Page 28] RFC 8445 ICE July 2018

 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.

Keranen, et al. Standards Track [Page 29] RFC 8445 ICE July 2018

 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

Keranen, et al. Standards Track [Page 30] RFC 8445 ICE July 2018

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

Keranen, et al. Standards Track [Page 31] RFC 8445 ICE July 2018

 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.

Keranen, et al. Standards Track [Page 32] RFC 8445 ICE July 2018

 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)

Keranen, et al. Standards Track [Page 33] RFC 8445 ICE July 2018

 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.

Keranen, et al. Standards Track [Page 34] RFC 8445 ICE July 2018

 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

Keranen, et al. Standards Track [Page 35] RFC 8445 ICE July 2018

 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.

Keranen, et al. Standards Track [Page 36] RFC 8445 ICE July 2018

 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.

Keranen, et al. Standards Track [Page 37] RFC 8445 ICE July 2018

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.

Keranen, et al. Standards Track [Page 38] RFC 8445 ICE July 2018

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).

Keranen, et al. Standards Track [Page 39] RFC 8445 ICE July 2018

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.

Keranen, et al. Standards Track [Page 40] RFC 8445 ICE July 2018

 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.

Keranen, et al. Standards Track [Page 41] RFC 8445 ICE July 2018

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.

Keranen, et al. Standards Track [Page 42] RFC 8445 ICE July 2018

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.

Keranen, et al. Standards Track [Page 43] RFC 8445 ICE July 2018

 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

Keranen, et al. Standards Track [Page 44] RFC 8445 ICE July 2018

 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.

Keranen, et al. Standards Track [Page 45] RFC 8445 ICE July 2018

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.

Keranen, et al. Standards Track [Page 46] RFC 8445 ICE July 2018

 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

Keranen, et al. Standards Track [Page 47] RFC 8445 ICE July 2018

 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

Keranen, et al. Standards Track [Page 48] RFC 8445 ICE July 2018

 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.

Keranen, et al. Standards Track [Page 49] RFC 8445 ICE July 2018

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.

Keranen, et al. Standards Track [Page 50] RFC 8445 ICE July 2018

 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

Keranen, et al. Standards Track [Page 51] RFC 8445 ICE July 2018

    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.

Keranen, et al. Standards Track [Page 52] RFC 8445 ICE July 2018

 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.

Keranen, et al. Standards Track [Page 53] RFC 8445 ICE July 2018

 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).

Keranen, et al. Standards Track [Page 54] RFC 8445 ICE July 2018

 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].

Keranen, et al. Standards Track [Page 55] RFC 8445 ICE July 2018

 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.

Keranen, et al. Standards Track [Page 56] RFC 8445 ICE July 2018

 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.

Keranen, et al. Standards Track [Page 57] RFC 8445 ICE July 2018

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

Keranen, et al. Standards Track [Page 58] RFC 8445 ICE July 2018

 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,

Keranen, et al. Standards Track [Page 59] RFC 8445 ICE July 2018

 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

Keranen, et al. Standards Track [Page 60] RFC 8445 ICE July 2018

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

Keranen, et al. Standards Track [Page 61] RFC 8445 ICE July 2018

           |(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          |              |              |
           |<===========================================|

Keranen, et al. Standards Track [Page 62] RFC 8445 ICE July 2018

           |              |              |              |
                              .......
           |              |              |              |
           |(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.

Keranen, et al. Standards Track [Page 63] RFC 8445 ICE July 2018

 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.

Keranen, et al. Standards Track [Page 64] RFC 8445 ICE July 2018

                          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

Keranen, et al. Standards Track [Page 65] RFC 8445 ICE July 2018

 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                  |              |
           |<-------------------------------------------|

Keranen, et al. Standards Track [Page 66] RFC 8445 ICE July 2018

           |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.

Keranen, et al. Standards Track [Page 67] RFC 8445 ICE July 2018

 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'.

Keranen, et al. Standards Track [Page 68] RFC 8445 ICE July 2018

 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.

Keranen, et al. Standards Track [Page 69] RFC 8445 ICE July 2018

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.

Keranen, et al. Standards Track [Page 70] RFC 8445 ICE July 2018

 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

Keranen, et al. Standards Track [Page 71] RFC 8445 ICE July 2018

 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.

Keranen, et al. Standards Track [Page 72] RFC 8445 ICE July 2018

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

Keranen, et al. Standards Track [Page 73] RFC 8445 ICE July 2018

 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.

Keranen, et al. Standards Track [Page 74] RFC 8445 ICE July 2018

 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

Keranen, et al. Standards Track [Page 75] RFC 8445 ICE July 2018

 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:

Keranen, et al. Standards Track [Page 76] RFC 8445 ICE July 2018

 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

Keranen, et al. Standards Track [Page 77] RFC 8445 ICE July 2018

 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.

Keranen, et al. Standards Track [Page 78] RFC 8445 ICE July 2018

 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.

Keranen, et al. Standards Track [Page 79] RFC 8445 ICE July 2018

 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.

Keranen, et al. Standards Track [Page 80] RFC 8445 ICE July 2018

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.

Keranen, et al. Standards Track [Page 81] RFC 8445 ICE July 2018

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

Keranen, et al. Standards Track [Page 82] RFC 8445 ICE July 2018

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>.

Keranen, et al. Standards Track [Page 83] RFC 8445 ICE July 2018

 [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>.

Keranen, et al. Standards Track [Page 84] RFC 8445 ICE July 2018

 [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>.

Keranen, et al. Standards Track [Page 85] RFC 8445 ICE July 2018

 [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>.

Keranen, et al. Standards Track [Page 86] RFC 8445 ICE July 2018

 [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>.

Keranen, et al. Standards Track [Page 87] RFC 8445 ICE July 2018

 [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.

Keranen, et al. Standards Track [Page 88] RFC 8445 ICE July 2018

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

Keranen, et al. Standards Track [Page 89] RFC 8445 ICE July 2018

 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:

Keranen, et al. Standards Track [Page 90] RFC 8445 ICE July 2018

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

0 Ta 2Ta 3Ta 4Ta 5Ta

 In this picture, there are three transactions that will be sent (for
 example, in the case of candidate gathering, there are three host
 candidate/STUN server pairs).  These are transactions A, B, and C.
 The retransmit timer is set so that the first retransmission on the
 first transaction (packet A2) is sent at time 3Ta.
 Subsequent retransmits after the first will occur even less
 frequently than Ta milliseconds apart, since STUN uses an exponential
 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).

Keranen, et al. Standards Track [Page 91] RFC 8445 ICE July 2018

 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.

Keranen, et al. Standards Track [Page 92] RFC 8445 ICE July 2018

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

Keranen, et al. Standards Track [Page 93] RFC 8445 ICE July 2018

 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

Keranen, et al. Standards Track [Page 94] RFC 8445 ICE July 2018

 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

Keranen, et al. Standards Track [Page 95] RFC 8445 ICE July 2018

 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.

Keranen, et al. Standards Track [Page 96] RFC 8445 ICE July 2018

 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

Keranen, et al. Standards Track [Page 97] RFC 8445 ICE July 2018

 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.

Keranen, et al. Standards Track [Page 98] RFC 8445 ICE July 2018

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

Keranen, et al. Standards Track [Page 99] RFC 8445 ICE July 2018

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

Keranen, et al. Standards Track [Page 100]

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