draft-ietf-mmusic-ice-13.txt   draft-ietf-mmusic-ice-14.txt 
MMUSIC J. Rosenberg MMUSIC J. Rosenberg
Internet-Draft Cisco Systems Internet-Draft Cisco
Expires: July 20, 2007 January 16, 2007 Intended status: Standards Track March 5, 2007
Expires: September 6, 2007
Interactive Connectivity Establishment (ICE): A Methodology for Network Interactive Connectivity Establishment (ICE): A Methodology for Network
Address Translator (NAT) Traversal for Offer/Answer Protocols Address Translator (NAT) Traversal for Offer/Answer Protocols
draft-ietf-mmusic-ice-13 draft-ietf-mmusic-ice-14
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Copyright (C) The Internet Society (2007). Copyright (C) The IETF Trust (2007).
Abstract Abstract
This document describes a protocol for Network Address Translator This document describes a protocol for Network Address Translator
(NAT) traversal for multimedia session signaling protocols based on (NAT) traversal for multimedia sessions established with the offer/
the offer/answer model, such as the Session Initiation Protocol answer model. This protocol is called Interactive Connectivity
(SIP). This protocol is called Interactive Connectivity
Establishment (ICE). ICE makes use of the Session Traversal Establishment (ICE). ICE makes use of the Session Traversal
Utilities for NAT (STUN) protocol, applying its binding discovery and Utilities for NAT (STUN) protocol, applying its binding discovery and
relay usages, in addition to defining a new usage for checking relay usages, in addition to defining a new usage for checking
connectivity between peers. connectivity between peers. ICE can be used by any protocol
utilizing the offer/answer model, such as the Session Initiation
Protocol (SIP).
Table of Contents Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 5 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 6
2. Overview of ICE . . . . . . . . . . . . . . . . . . . . . . . 5 2. Overview of ICE . . . . . . . . . . . . . . . . . . . . . . . 7
2.1. Gathering Candidate Addresses . . . . . . . . . . . . . . 7 2.1. Gathering Candidate Addresses . . . . . . . . . . . . . . 9
2.2. Connectivity Checks . . . . . . . . . . . . . . . . . . . 9 2.2. Connectivity Checks . . . . . . . . . . . . . . . . . . . 11
2.3. Sorting Candidates . . . . . . . . . . . . . . . . . . . . 10 2.3. Sorting Candidates . . . . . . . . . . . . . . . . . . . . 12
2.4. Frozen Candidates . . . . . . . . . . . . . . . . . . . . 11 2.4. Frozen Candidates . . . . . . . . . . . . . . . . . . . . 13
2.5. Security for Checks . . . . . . . . . . . . . . . . . . . 11 2.5. Security for Checks . . . . . . . . . . . . . . . . . . . 14
2.6. Concluding ICE . . . . . . . . . . . . . . . . . . . . . . 12 2.6. Concluding ICE . . . . . . . . . . . . . . . . . . . . . . 14
2.7. Lite Implementations . . . . . . . . . . . . . . . . . . . 13 2.7. Lite Implementations . . . . . . . . . . . . . . . . . . . 16
3. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 13 3. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 16
4. Sending the Initial Offer . . . . . . . . . . . . . . . . . . 16 4. Sending the Initial Offer . . . . . . . . . . . . . . . . . . 19
4.1. Full Implementation Requirements . . . . . . . . . . . . . 16 4.1. Full Implementation Requirements . . . . . . . . . . . . . 19
4.1.1. Gathering Candidates . . . . . . . . . . . . . . . . . 16 4.1.1. Gathering Candidates . . . . . . . . . . . . . . . . . 19
4.1.2. Prioritizing Candidates . . . . . . . . . . . . . . . 18 4.1.1.1. Host Candidates . . . . . . . . . . . . . . . . . 20
4.1.3. Choosing In-Use Candidates . . . . . . . . . . . . . . 20 4.1.1.2. Server Reflexive and Relayed Candidates . . . . . 20
4.2. Lite Implementation . . . . . . . . . . . . . . . . . . . 20 4.1.1.3. Eliminating Redundant Candidates . . . . . . . . . 21
4.3. Encoding the SDP . . . . . . . . . . . . . . . . . . . . . 21 4.1.1.4. Computing Foundations . . . . . . . . . . . . . . 21
5. Receiving the Initial Offer . . . . . . . . . . . . . . . . . 22 4.1.1.5. Keeping Candidates Alive . . . . . . . . . . . . . 22
5.1. Verifying ICE Support . . . . . . . . . . . . . . . . . . 23 4.1.2. Prioritizing Candidates . . . . . . . . . . . . . . . 22
5.2. Determining Role . . . . . . . . . . . . . . . . . . . . . 23 4.1.2.1. Recommended Formula . . . . . . . . . . . . . . . 22
5.3. Gathering Candidates . . . . . . . . . . . . . . . . . . . 24 4.1.2.2. Guidelines for Choosing Type and Local
5.4. Prioritizing Candidates . . . . . . . . . . . . . . . . . 24 Preferences . . . . . . . . . . . . . . . . . . . 23
5.5. Choosing In Use Candidates . . . . . . . . . . . . . . . . 24 4.1.3. Choosing Default Candidates . . . . . . . . . . . . . 24
5.6. Encoding the SDP . . . . . . . . . . . . . . . . . . . . . 24 4.2. Lite Implementation . . . . . . . . . . . . . . . . . . . 25
5.7. Forming the Check Lists . . . . . . . . . . . . . . . . . 24 4.3. Encoding the SDP . . . . . . . . . . . . . . . . . . . . . 25
5.8. Performing Periodic Checks . . . . . . . . . . . . . . . . 27 5. Receiving the Initial Offer . . . . . . . . . . . . . . . . . 27
6. Receipt of the Initial Answer . . . . . . . . . . . . . . . . 28 5.1. Verifying ICE Support . . . . . . . . . . . . . . . . . . 27
6.1. Verifying ICE Support . . . . . . . . . . . . . . . . . . 28 5.2. Determining Role . . . . . . . . . . . . . . . . . . . . . 27
6.2. Determining Role . . . . . . . . . . . . . . . . . . . . . 28 5.3. Gathering Candidates . . . . . . . . . . . . . . . . . . . 28
6.3. Forming the Check List . . . . . . . . . . . . . . . . . . 28 5.4. Prioritizing Candidates . . . . . . . . . . . . . . . . . 28
6.4. Performing Periodic Checks . . . . . . . . . . . . . . . . 28 5.5. Choosing Default Candidates . . . . . . . . . . . . . . . 28
7. Connectivity Checks . . . . . . . . . . . . . . . . . . . . . 28 5.6. Encoding the SDP . . . . . . . . . . . . . . . . . . . . . 28
7.1. Client Procedures . . . . . . . . . . . . . . . . . . . . 29 5.7. Forming the Check Lists . . . . . . . . . . . . . . . . . 28
7.1.1. Sending the Request . . . . . . . . . . . . . . . . . 29 5.7.1. Forming Candidate Pairs . . . . . . . . . . . . . . . 29
7.1.2. Processing the Response . . . . . . . . . . . . . . . 30 5.7.2. Computing Pair Priority and Ordering Pairs . . . . . . 31
7.2. Server Procedures . . . . . . . . . . . . . . . . . . . . 31 5.7.3. Pruning the Pairs . . . . . . . . . . . . . . . . . . 31
7.2.1. Additional Procedures for Full Implementations . . . . 32 5.7.4. Computing States . . . . . . . . . . . . . . . . . . . 31
7.2.2. Additional Procedures for Lite Implementations . . . . 34 5.8. Performing Periodic Checks . . . . . . . . . . . . . . . . 34
8. Concluding ICE . . . . . . . . . . . . . . . . . . . . . . . . 34 6. Receipt of the Initial Answer . . . . . . . . . . . . . . . . 35
9. Subsequent Offer/Answer Exchanges . . . . . . . . . . . . . . 35 6.1. Verifying ICE Support . . . . . . . . . . . . . . . . . . 36
9.1. Generating the Offer . . . . . . . . . . . . . . . . . . . 35 6.2. Determining Role . . . . . . . . . . . . . . . . . . . . . 36
9.1.1. Additional Procedures for Full Implementations . . . . 36 6.3. Forming the Check List . . . . . . . . . . . . . . . . . . 36
9.1.2. Additional Procedures for Lite Implementations . . . . 37 6.4. Performing Periodic Checks . . . . . . . . . . . . . . . . 36
9.2. Receiving the Offer and Generating an Answer . . . . . . . 37 7. Performing Connectivity Checks . . . . . . . . . . . . . . . . 36
9.2.1. Additional Procedures for Full Implementations . . . . 38 7.1. Client Procedures . . . . . . . . . . . . . . . . . . . . 37
9.3. Updating the Check and Valid Lists . . . . . . . . . . . . 38 7.1.1. Sending the Request . . . . . . . . . . . . . . . . . 37
9.3.1. Additional Procedures for Full Implementations . . . . 38 7.1.1.1. PRIORITY and USE-CANDIDATE . . . . . . . . . . . . 37
10. Keepalives . . . . . . . . . . . . . . . . . . . . . . . . . . 40 7.1.1.2. Forming Credentials . . . . . . . . . . . . . . . 37
11. Media Handling . . . . . . . . . . . . . . . . . . . . . . . . 41 7.1.1.3. DiffServ Treatment . . . . . . . . . . . . . . . . 38
11.1. Sending Media . . . . . . . . . . . . . . . . . . . . . . 41 7.1.2. Processing the Response . . . . . . . . . . . . . . . 38
11.1.1. Procedures for Full Implementations . . . . . . . . . 41 7.1.2.1. Failure Cases . . . . . . . . . . . . . . . . . . 38
11.1.2. Procedures for Lite Implementations . . . . . . . . . 42 7.1.2.2. Success Cases . . . . . . . . . . . . . . . . . . 38
11.2. Receiving Media . . . . . . . . . . . . . . . . . . . . . 42 7.1.2.2.1. Discovering Peer Reflexive Candidates . . . . 38
12. Usage with SIP . . . . . . . . . . . . . . . . . . . . . . . . 42 7.1.2.2.2. Updating Pair States . . . . . . . . . . . . . 39
12.1. Latency Guidelines . . . . . . . . . . . . . . . . . . . . 42 7.1.2.2.3. Constructing a Valid Pair . . . . . . . . . . 40
12.2. SIP Option Tags and Media Feature Tags . . . . . . . . . . 44 7.1.2.2.4. Updating the Nominated Flag . . . . . . . . . 40
12.3. Interactions with Forking . . . . . . . . . . . . . . . . 44 7.2. Server Procedures . . . . . . . . . . . . . . . . . . . . 41
12.4. Interactions with Preconditions . . . . . . . . . . . . . 45 7.2.1. Additional Procedures for Full Implementations . . . . 41
12.5. Interactions with Third Party Call Control . . . . . . . . 45 7.2.1.1. Computing Mapped Address . . . . . . . . . . . . . 41
13. Grammar . . . . . . . . . . . . . . . . . . . . . . . . . . . 45 7.2.1.2. Learning Peer Reflexive Candidates . . . . . . . . 42
14. Extensibility Considerations . . . . . . . . . . . . . . . . . 48 7.2.1.3. Triggered Checks . . . . . . . . . . . . . . . . . 42
15. Example . . . . . . . . . . . . . . . . . . . . . . . . . . . 49 7.2.1.4. Updating the Nominated Flag . . . . . . . . . . . 43
16. Security Considerations . . . . . . . . . . . . . . . . . . . 54 7.2.2. Additional Procedures for Lite Implementations . . . . 43
16.1. Attacks on Connectivity Checks . . . . . . . . . . . . . . 54 8. Concluding ICE Processing . . . . . . . . . . . . . . . . . . 43
16.2. Attacks on Address Gathering . . . . . . . . . . . . . . . 57 8.1. Nominating Pairs . . . . . . . . . . . . . . . . . . . . . 44
16.3. Attacks on the Offer/Answer Exchanges . . . . . . . . . . 57 8.1.1. Regular Nomination . . . . . . . . . . . . . . . . . . 44
16.4. Insider Attacks . . . . . . . . . . . . . . . . . . . . . 57 8.1.2. Aggressive Nomination . . . . . . . . . . . . . . . . 45
16.4.1. The Voice Hammer Attack . . . . . . . . . . . . . . . 58 8.2. Updating States . . . . . . . . . . . . . . . . . . . . . 45
16.4.2. STUN Amplification Attack . . . . . . . . . . . . . . 58 9. Subsequent Offer/Answer Exchanges . . . . . . . . . . . . . . 46
16.5. Interactions with Application Layer Gateways and SIP . . . 59 9.1. Generating the Offer . . . . . . . . . . . . . . . . . . . 46
17. Definition of Connectivity Check Usage . . . . . . . . . . . . 59 9.1.1. Procedures for All Implementations . . . . . . . . . . 46
17.1. Applicability . . . . . . . . . . . . . . . . . . . . . . 60 9.1.1.1. ICE Restarts . . . . . . . . . . . . . . . . . . . 46
17.2. Client Discovery of Server . . . . . . . . . . . . . . . . 60 9.1.1.2. Removing a Media Stream . . . . . . . . . . . . . 47
17.3. Server Determination of Usage . . . . . . . . . . . . . . 60 9.1.1.3. Adding a Media Stream . . . . . . . . . . . . . . 47
17.4. New Requests or Indications . . . . . . . . . . . . . . . 60 9.1.2. Procedures for Full Implementations . . . . . . . . . 47
17.5. New Attributes . . . . . . . . . . . . . . . . . . . . . . 60 9.1.2.1. Existing Media Streams with ICE Running . . . . . 48
17.6. New Error Response Codes . . . . . . . . . . . . . . . . . 61 9.1.2.2. Existing Media Streams with ICE Completed . . . . 48
17.7. Client Procedures . . . . . . . . . . . . . . . . . . . . 61 9.1.3. Procedures for Lite Implementations . . . . . . . . . 49
17.8. Server Procedures . . . . . . . . . . . . . . . . . . . . 61 9.2. Receiving the Offer and Generating an Answer . . . . . . . 49
17.9. Security Considerations for Connectivity Check . . . . . . 61 9.2.1. Procedures for All Implementations . . . . . . . . . . 49
18. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 61 9.2.1.1. Detecting ICE Restart . . . . . . . . . . . . . . 49
18.1. SDP Attributes . . . . . . . . . . . . . . . . . . . . . . 61 9.2.1.2. New Media Stream . . . . . . . . . . . . . . . . . 50
18.1.1. candidate Attribute . . . . . . . . . . . . . . . . . 61 9.2.1.3. Removed Media Stream . . . . . . . . . . . . . . . 50
18.1.2. remote-candidates Attribute . . . . . . . . . . . . . 62 9.2.2. Procedures for Full Implementations . . . . . . . . . 50
18.1.3. ice-lite Attribute . . . . . . . . . . . . . . . . . . 62 9.2.2.1. Existing Media Streams with ICE Running and no
18.1.4. ice-mismatch Attribute . . . . . . . . . . . . . . . . 63 remote-candidates . . . . . . . . . . . . . . . . 50
18.1.5. ice-pwd Attribute . . . . . . . . . . . . . . . . . . 63 9.2.2.2. Existing Media Streams with ICE Completed and
18.1.6. ice-ufrag Attribute . . . . . . . . . . . . . . . . . 63 no remote-candidates . . . . . . . . . . . . . . . 50
18.1.7. ice-options Attribute . . . . . . . . . . . . . . . . 64 9.2.2.3. Existing Media Streams and remote-candidates . . . 50
18.2. STUN Attributes . . . . . . . . . . . . . . . . . . . . . 64 9.2.3. Procedures for Lite Implementations . . . . . . . . . 51
19. IAB Considerations . . . . . . . . . . . . . . . . . . . . . . 65 9.3. Updating the Check and Valid Lists . . . . . . . . . . . . 52
19.1. Problem Definition . . . . . . . . . . . . . . . . . . . . 65 9.3.1. Procedures for Full Implementations . . . . . . . . . 52
19.2. Exit Strategy . . . . . . . . . . . . . . . . . . . . . . 65 9.3.1.1. ICE Restarts . . . . . . . . . . . . . . . . . . . 52
19.3. Brittleness Introduced by ICE . . . . . . . . . . . . . . 66 9.3.1.2. New Media Stream . . . . . . . . . . . . . . . . . 52
19.4. Requirements for a Long Term Solution . . . . . . . . . . 67 9.3.1.3. Removed Media Stream . . . . . . . . . . . . . . . 52
19.5. Issues with Existing NAPT Boxes . . . . . . . . . . . . . 67 9.3.1.4. ICE Continuing for Existing Media Stream . . . . . 52
20. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 68 9.3.2. Procedures for Lite Implementations . . . . . . . . . 53
21. References . . . . . . . . . . . . . . . . . . . . . . . . . . 68 10. Keepalives . . . . . . . . . . . . . . . . . . . . . . . . . . 53
21.1. Normative References . . . . . . . . . . . . . . . . . . . 68 11. Media Handling . . . . . . . . . . . . . . . . . . . . . . . . 54
21.2. Informative References . . . . . . . . . . . . . . . . . . 69 11.1. Sending Media . . . . . . . . . . . . . . . . . . . . . . 54
Appendix A. Lite and Full Implementations . . . . . . . . . . . . 71 11.1.1. Procedures for Full Implementations . . . . . . . . . 54
Appendix B. Design Motivations . . . . . . . . . . . . . . . . . 71 11.1.2. Procedures for Lite Implementations . . . . . . . . . 55
B.1. Pacing of STUN Transactions . . . . . . . . . . . . . . . 72 11.1.3. Procedures for All Implementations . . . . . . . . . . 55
B.2. Candidates with Multiple Bases . . . . . . . . . . . . . . 72 11.2. Receiving Media . . . . . . . . . . . . . . . . . . . . . 56
B.3. Purpose of the Translation . . . . . . . . . . . . . . . . 74 12. Usage with SIP . . . . . . . . . . . . . . . . . . . . . . . . 56
B.4. Importance of the STUN Username . . . . . . . . . . . . . 74 12.1. Latency Guidelines . . . . . . . . . . . . . . . . . . . . 56
B.5. The Candidate Pair Sequence Number Formula . . . . . . . . 75 12.1.1. Offer in INVITE . . . . . . . . . . . . . . . . . . . 56
B.6. The Frozen State . . . . . . . . . . . . . . . . . . . . . 76 12.1.2. Offer in Response . . . . . . . . . . . . . . . . . . 58
B.7. The remote-candidates attribute . . . . . . . . . . . . . 76 12.2. SIP Option Tags and Media Feature Tags . . . . . . . . . . 58
B.8. Why are Keepalives Needed? . . . . . . . . . . . . . . . . 77 12.3. Interactions with Forking . . . . . . . . . . . . . . . . 58
B.9. Why Prefer Peer Reflexive Candidates? . . . . . . . . . . 78 12.4. Interactions with Preconditions . . . . . . . . . . . . . 59
B.10. Why Send an Updated Offer? . . . . . . . . . . . . . . . . 78 12.5. Interactions with Third Party Call Control . . . . . . . . 59
B.11. Why are Binding Indications Used for Keepalives? . . . . . 78 13. Usage with ANAT . . . . . . . . . . . . . . . . . . . . . . . 59
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 80 14. Extensibility Considerations . . . . . . . . . . . . . . . . . 60
Intellectual Property and Copyright Statements . . . . . . . . . . 81 15. Grammar . . . . . . . . . . . . . . . . . . . . . . . . . . . 61
15.1. "candidate" Attribute . . . . . . . . . . . . . . . . . . 61
15.2. "remote-candidates" Attribute . . . . . . . . . . . . . . 64
15.3. "ice-lite" and "ice-mismatch" Attributes . . . . . . . . . 64
15.4. "ice-ufrag" and "ice-pwd" Attributes . . . . . . . . . . . 64
15.5. "ice-options> Attribute . . . . . . . . . . . . . . . . . 65
16. Example . . . . . . . . . . . . . . . . . . . . . . . . . . . 65
17. Security Considerations . . . . . . . . . . . . . . . . . . . 72
17.1. Attacks on Connectivity Checks . . . . . . . . . . . . . . 72
17.2. Attacks on Address Gathering . . . . . . . . . . . . . . . 74
17.3. Attacks on the Offer/Answer Exchanges . . . . . . . . . . 75
17.4. Insider Attacks . . . . . . . . . . . . . . . . . . . . . 75
17.4.1. The Voice Hammer Attack . . . . . . . . . . . . . . . 75
17.4.2. STUN Amplification Attack . . . . . . . . . . . . . . 76
17.5. Interactions with Application Layer Gateways and SIP . . . 76
18. Definition of Connectivity Check Usage . . . . . . . . . . . . 77
18.1. Applicability . . . . . . . . . . . . . . . . . . . . . . 77
18.2. Client Discovery of Server . . . . . . . . . . . . . . . . 78
18.3. Server Determination of Usage . . . . . . . . . . . . . . 78
18.4. New Requests or Indications . . . . . . . . . . . . . . . 78
18.5. New Attributes . . . . . . . . . . . . . . . . . . . . . . 78
18.6. New Error Response Codes . . . . . . . . . . . . . . . . . 78
18.7. Client Procedures . . . . . . . . . . . . . . . . . . . . 78
18.8. Server Procedures . . . . . . . . . . . . . . . . . . . . 78
18.9. Security Considerations for Connectivity Check . . . . . . 79
19. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 79
19.1. SDP Attributes . . . . . . . . . . . . . . . . . . . . . . 79
19.1.1. candidate Attribute . . . . . . . . . . . . . . . . . 79
19.1.2. remote-candidates Attribute . . . . . . . . . . . . . 79
19.1.3. ice-lite Attribute . . . . . . . . . . . . . . . . . . 80
19.1.4. ice-mismatch Attribute . . . . . . . . . . . . . . . . 80
19.1.5. ice-pwd Attribute . . . . . . . . . . . . . . . . . . 81
19.1.6. ice-ufrag Attribute . . . . . . . . . . . . . . . . . 81
19.1.7. ice-options Attribute . . . . . . . . . . . . . . . . 82
19.2. STUN Attributes . . . . . . . . . . . . . . . . . . . . . 82
20. IAB Considerations . . . . . . . . . . . . . . . . . . . . . . 82
20.1. Problem Definition . . . . . . . . . . . . . . . . . . . . 83
20.2. Exit Strategy . . . . . . . . . . . . . . . . . . . . . . 83
20.3. Brittleness Introduced by ICE . . . . . . . . . . . . . . 84
20.4. Requirements for a Long Term Solution . . . . . . . . . . 84
20.5. Issues with Existing NAPT Boxes . . . . . . . . . . . . . 85
21. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 85
22. References . . . . . . . . . . . . . . . . . . . . . . . . . . 86
22.1. Normative References . . . . . . . . . . . . . . . . . . . 86
22.2. Informative References . . . . . . . . . . . . . . . . . . 87
Appendix A. Lite and Full Implementations . . . . . . . . . . . . 88
Appendix B. Design Motivations . . . . . . . . . . . . . . . . . 89
B.1. Pacing of STUN Transactions . . . . . . . . . . . . . . . 90
B.2. Candidates with Multiple Bases . . . . . . . . . . . . . . 90
B.3. Purpose of the <rel-addr> and <rel-port> Attributes . . . 92
B.4. Importance of the STUN Username . . . . . . . . . . . . . 92
B.5. The Candidate Pair Sequence Number Formula . . . . . . . . 93
B.6. The remote-candidates attribute . . . . . . . . . . . . . 94
B.7. Why are Keepalives Needed? . . . . . . . . . . . . . . . . 95
B.8. Why Prefer Peer Reflexive Candidates? . . . . . . . . . . 96
B.9. Why Send an Updated Offer? . . . . . . . . . . . . . . . . 96
B.10. Why are Binding Indications Used for Keepalives? . . . . . 96
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 97
Intellectual Property and Copyright Statements . . . . . . . . . . 98
1. Introduction 1. Introduction
RFC 3264 [4] defines a two-phase exchange of Session Description RFC 3264 [4] defines a two-phase exchange of Session Description
Protocol (SDP) messages [10] for the purposes of establishment of Protocol (SDP) messages [10] for the purposes of establishment of
multimedia sessions. This offer/answer mechanism is used by multimedia sessions. This offer/answer mechanism is used by
protocols such as the Session Initiation Protocol (SIP) [3]. protocols such as the Session Initiation Protocol (SIP) [3].
Protocols using offer/answer are difficult to operate through Network Protocols using offer/answer are difficult to operate through Network
Address Translators (NAT). Because their purpose is to establish a Address Translators (NAT). Because their purpose is to establish a
flow of media packets, they tend to carry IP addresses within their flow of media packets, they tend to carry the IP of media sources and
messages, which is known to be problematic through NAT [15]. The sinks within their messages, which is known to be problematic through
protocols also seek to create a media flow directly between NAT [16]. The protocols also seek to create a media flow directly
participants, so that there is no application layer intermediary between participants, so that there is no application layer
between them. This is done to reduce media latency, decrease packet intermediary between them. This is done to reduce media latency,
loss, and reduce the operational costs of deploying the application. decrease packet loss, and reduce the operational costs of deploying
However, this is difficult to accomplish through NAT. A full the application. However, this is difficult to accomplish through
treatment of the reasons for this is beyond the scope of this NAT. A full treatment of the reasons for this is beyond the scope of
specification. this specification.
Numerous solutions have been proposed for allowing these protocols to Numerous solutions have been proposed for allowing these protocols to
operate through NAT. These include Application Layer Gateways operate through NAT. These include Application Layer Gateways
(ALGs), the Middlebox Control Protocol [16], Simple Traversal (ALGs), the Middlebox Control Protocol [17], Simple Traversal
Underneath NAT (STUN) [14] and its revision, retitled Session Underneath NAT (STUN) [15] and its revision, retitled Session
Traversal Utilities for NAT [11], the STUN Relay Usage [12], and Traversal Utilities for NAT [12], the STUN Relay Usage [13], and
Realm Specific IP [18] [19] along with session description extensions Realm Specific IP [19] [20] along with session description extensions
needed to make them work, such as the Session Description Protocol needed to make them work, such as the Session Description Protocol
(SDP) [10] attribute for the Real Time Control Protocol (RTCP) [2]. (SDP) [10] attribute for the Real Time Control Protocol (RTCP) [2].
Unfortunately, these techniques all have pros and cons which make Unfortunately, these techniques all have pros and cons which make
each one optimal in some network topologies, but a poor choice in each one optimal in some network topologies, but a poor choice in
others. The result is that administrators and implementors are others. The result is that administrators and implementors are
making assumptions about the topologies of the networks in which making assumptions about the topologies of the networks in which
their solutions will be deployed. This introduces complexity and their solutions will be deployed. This introduces complexity and
brittleness into the system. What is needed is a single solution brittleness into the system. What is needed is a single solution
which is flexible enough to work well in all situations. which is flexible enough to work well in all situations.
This specification provides that solution for media streams This specification defines Interactive Connectivity Establishment
established by signaling protocols based on the offer-answer model. (ICE) as a technique for NAT traversal for media streams established
It is called Interactive Connectivity Establishment, or ICE. ICE by the offer/answer model. ICE is an extension to the offer/answer
makes use of STUN and its relay extension, commonly called TURN, but model, and works by including a multiplicity of IP addresses and
uses them in a specific methodology which avoids many of the pitfalls ports in SDP offers and answers, which are then tested for
of using any one alone. connectivity by peer-to-peer STUN exchanges. The IP addresses and
ports included in the SDP are gathered using the STUN binding
acquisition techniques in [12] and relay allocation procedures in
[13].
2. Overview of ICE 2. Overview of ICE
In a typical ICE deployment, we have two endpoints (known as agents In a typical ICE deployment, we have two endpoints (known as AGENTS
in RFC 3264 terminology) which want to communicate. They are able to in RFC 3264 terminology) which want to communicate. They are able to
communicate indirectly via some signaling system such as SIP, by communicate indirectly via some signaling protocol (such as SIP), by
which they can perform an offer/answer exchange of SDP [4] messages. which they can perform an offer/answer exchange of SDP [4] messages.
Note that ICE is not intended for NAT traversal for SIP, which is Note that ICE is not intended for NAT traversal for SIP, which is
assumed to be provided via some other mechanism [32]. At the assumed to be provided via another mechanism [35]. At the beginning
beginning of the ICE process, the agents are ignorant of their own of the ICE process, the agents are ignorant of their own topologies.
topologies. In particular, they might or might not be behind a NAT In particular, they might or might not be behind a NAT (or multiple
(or multiple tiers of NATs). ICE allows the agents to discover tiers of NATs). ICE allows the agents to discover enough information
enough information about their topologies to find a path or paths by about their topologies to potentially find one or more paths by which
which they can communicate. they can communicate.
Figure Figure 1 shows a typical environment for ICE deployment. The Figure 1 shows a typical environment for ICE deployment. The two
two endpoints are labelled L and R (for left and right, which helps endpoints are labelled L and R (for left and right, which helps
visualize call flows). Both L and R are behind NATs -- though as visualize call flows). Both L and R are behind their own respective
mentioned before, they don't know that. The type of NAT and its NATs though they may not be aware of it. The type of NAT and its
properties are also unknown. Agents L and R are capable of engaging properties are also unknown. Agents L and R are capable of engaging
in an offer/answer exchange by which they can exchange SDP messages, in an offer/answer exchange by which they can exchange SDP messages,
whose purpose is to set up a media session between L and R. whose purpose is to set up a media session between L and R.
Typically, this exchange will occur through a SIP server. Typically, this exchange will occur through a SIP server.
In addition to the agents, a SIP server and NATs, ICE is typically In addition to the agents, a SIP server and NATs, ICE is typically
used in concert with STUN servers in the network. Each agent can used in concert with STUN servers in the network. Each agent can
have its own STUN server, or they can be the same. have its own STUN server, or they can be the same.
+-------+ +-------+
skipping to change at page 7, line 6 skipping to change at page 8, line 31
+--------+ +--------+ +--------+ +--------+
/ \ / \
/ \ / \
/ \ / \
+-------+ +-------+ +-------+ +-------+
| Agent | | Agent | | Agent | | Agent |
| L | | R | | L | | R |
| | | | | | | |
+-------+ +-------+ +-------+ +-------+
Figure 1 Figure 1: ICE Deployment Scenario
The basic idea behind ICE is as follows: each agent has a variety of The basic idea behind ICE is as follows: each agent has a variety of
candidate transport addresses it could use to communicate with the candidate TRANSPORT ADDRESSES (combination of IP address and port) it
other agent. These might include: could use to communicate with the other agent. These might include:
o It's directly attached network interface (or interfaces in the o A transport address on a directly attached network interface or
case of a multihomed machine interfaces
o A translated address on the public side of a NAT (a "server o A translated transport address on the public side of a NAT (a
reflexive" address) "server reflexive" address)
o The address of a media relay the agent is using. o The transport address of a media relay the agent is using.
Potentially, any of L's candidate transport addresses can be used to Potentially, any of L's candidate transport addresses can be used to
communicate with any of R's candidate transport addresses. In communicate with any of R's candidate transport addresses. In
practice, however, many combinations will not work. For instance, if practice, however, many combinations will not work. For instance, if
L and R are both behind NATs then their directly interface addresses L and R are both behind NATs, their directly attached interface
are unlikely to be able to communicate directly (this is why ICE is addresses are unlikely to be able to communicate directly (this is
needed, after all!). The purpose of ICE is to discover which pairs why ICE is needed, after all!). The purpose of ICE is to discover
of addresses will work. The way that ICE does this is to which pairs of addresses will work. The way that ICE does this is to
systematically try all possible pairs (in a carefully sorted order) systematically try all possible pairs (in a carefully sorted order)
until it finds one or more that works. until it finds one or more that works.
2.1. Gathering Candidate Addresses 2.1. Gathering Candidate Addresses
In order to execute ICE, an agent has to identify all of its address In order to execute ICE, an agent has to identify all of its address
candidates. Naturally, one viable candidate is one obtained directly candidates. A CANDIDATE is a transport address - a combination of IP
from a local interface the client has towards the network. Such a address and port for a particular transport protocol. This document
candidate is called a HOST CANDIDATE. The local interface could be defines three types of candidates, some derived from physical or
one on a local layer 2 network technology, such as ethernet or WiFi, logical network interfaces, others discoverable via STUN. Naturally,
or it could be one that is obtained through a tunnel mechanism, such one viable candidate is a transport address obtained directly from a
as a Virtual Private Network (VPN) or Mobile IP (MIP). In all cases, local interface. Such a candidate is called a HOST CANDIDATE. The
these appear to the agent as a local interface from which ports (and local interface could be ethernet or WiFi, or it could be one that is
thus a candidate) can be allocated. obtained through a tunnel mechanism, such as a Virtual Private
Network (VPN) or Mobile IP (MIP). In all cases, such a network
interface appears to the agent as a local interface from which ports
(and thus a candidate) can be allocated.
If an agent is multihomed, it can obtain a candidate from each If an agent is multihomed, it obtains a candidate from each
interface. Depending on the location of the peer on the IP network interface. Depending on the location of the PEER (the other agent in
relative to the agent, the agent may be reachable by the peer through the session) on the IP network relative to the agent, the agent may
one of those interfaces, or through another. Consider, for example, be reachable by the peer through one or more of those interfaces.
an agent which has a local interface to a private net 10 network, and Consider, for example, an agent which has a local interface to a
also to the public Internet. A candidate from the net10 interface private net 10 network (I1), and a second connected to the public
will be directly reachable when communicating with a peer on the same Internet (I2). A candidate from I1 will be directly reachable when
private net 10 network, while a candidate from the public interface communicating with a peer on the same private net 10 network, while a
will be directly reachable when communicating with a peer on the candidate from I2 will be directly reachable when communicating with
public Internet. Rather than trying to guess which interface will a peer on the public Internet. Rather than trying to guess which
work prior to sending an offer, the offering agent includes both interface will work prior to sending an offer, the offering agent
candidates in its offer. includes both candidates in its offer.
Once the agent has obtained host candidates, it uses STUN to obtain Next, the agent uses STUN to obtain additional candidates. These
additional candidates. These come in two flavors: translated come in two flavors: translated addresses on the public side of a NAT
addresses on the public side of a NAT (SERVER REFLEXIVE CANDIDATES) (SERVER REFLEXIVE CANDIDATES) and addresses of media relays (RELAYED
and addresses of media relays (RELAYED CANDIDATES). The relationship CANDIDATES). The relationship of these candidates to the host
of these candidates to the host candidate is shown in Figure 2. Both candidate is shown in Figure 2. Both types of candidates are
types of candidates are discovered using STUN. discovered using STUN.
To Internet To Internet
| |
| |
| /------------ Relayed | /------------ Relayed
| / Candidate Y:y | / Address
+--------+ +--------+
| | | |
| STUN | | STUN |
| Server | | Server |
| | | |
+--------+ +--------+
| |
| |
| /------------ Server | /------------ Server
|/ Reflexive X1':x1'|/ Reflexive
+------------+ Candidate +------------+ Address
| NAT | | NAT |
+------------+ +------------+
| |
| /------------ Host | /------------ Local
|/ Candidate X:x |/ Address
+--------+ +--------+
| | | |
| Agent | | Agent |
| | | |
+--------+ +--------+
Figure 2 Figure 2: Candidate Relationships
To find a server reflexive candidate, the agent sends a STUN Binding To find a server reflexive candidate, the agent sends a STUN Binding
Request, using the Binding Discovery Usage [11] from each host Request, using the Binding Discovery Usage [12] from each host
candidate, to its STUN server. (It is assumed that the address of candidate, to its STUN server. It is assumed that the address of the
the STUN server is configured, or learned in some way.) When the STUN server is manually configured or learned in some unspecified
agent sends the Binding Request, the NAT (assuming there is one) will way. It is RECOMMENDED that when an agent has a choice of STUN
allocate a binding, mapping this server reflexive candidate to the servers (when, for example, they are learned through DNS records and
host candidate. Outgoing packets sent from the host candidate will multiple results are returned), an agent uses a single STUN server
be translated by the NAT to the server reflexive candidate. Incoming (based on its IP address) for all candidates for a particular
packets sent to the server relexive candidate will be translated by session. This improves the performance of ICE.
the NAT to the host candidate and forwarded to the agent. We call
the host candidate associated with a given server reflexive candidate
the BASE.
Note When the agent sends the Binding Request from IP address and port
X:x, the NAT (assuming there is one) will allocate a binding X1':x1',
mapping this server reflexive candidate to the host candidate X:x.
Outgoing packets sent from the host candidate will be translated by
the NAT to the server reflexive candidate. Incoming packets sent to
the server relexive candidate will be translated by the NAT to the
host candidate and forwarded to the agent. We call the host
candidate associated with a given server reflexive candidate the
BASE.
"Base" refers to the address you'd send from for a particular NOTE: "Base" refers to the address an agent sends from for a
candidate. Thus, as a degenerate case host candidates also have a particular candidate. Thus, as a degenerate case host candidates
base, but it's the same as the host candidate. also have a base, but it's the same as the host candidate.
When there are multiple NATs between the agent and the STUN server, When there are multiple NATs between the agent and the STUN server,
the STUN request will create a binding on each NAT, but only the the STUN request will create a binding on each NAT, but only the
outermost server reflexive candidate will be discovered by the agent. outermost server reflexive candidate will be discovered by the agent.
If the agent is not behind a NAT, then the base candidate will be the 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 same as the server reflexive candidate and the server reflexive
candidate can be ignored. candidate is redundant and will be eliminated.
The final type of candidate is a RELAYED candidate. The STUN Relay The final type of candidate is a RELAYED CANDIDATE. The STUN Relay
Usage [12] allows a STUN server to act as a media relay, forwarding Usage [13] allows a STUN server to act as a media relay, forwarding
traffic between L and R. In order to send traffic to L, R sends traffic between L and R. In order to send traffic to L, R sends
traffic to the media relay which forwards it to L and vice versa. traffic to the media relay at Y:y, and the relay forwards that to
The same thing happens in the other direction. X1':x1', which passes through the NAT where it is mapped to X:x and
delivered to L.
Traffic from L to R has its addresses rewritten twice: first by the
NAT and second by the STUN relay server. Thus, the address that R
knows about and the one that it wants to send to is the one on the
STUN relay server. This address is the final kind of candidate,
which we call a RELAYED CANDIDATE.
2.2. Connectivity Checks 2.2. Connectivity Checks
Once L has gathered all of its candidates, it orders them highest to Once L has gathered all of its candidates, it orders them in highest
lowest priority and sends them to R over the signalling channel. The to lowest priority and sends them to R over the signalling channel.
candidates are carried in attributes in the SDP offer. When R The candidates are carried in attributes in the SDP offer. When R
receives the offer, it performs the same gathering process and receives the offer, it performs the same gathering process and
responds with its own list of candidates. At the end of this responds with its own list of candidates. At the end of this
process, each agent has a complete list of both its candidates and process, each agent has a complete list of both its candidates and
its peer's candidates and is ready to perform connectivity checks by its peer's candidates. It pairs them up, resulting in CANDIDATE
pairing up the candidates to see which pair works. PAIRS. To see which pairs work, the agent schedules a series of
CHECKS. Each check is a STUN transaction that the client will
perform on a particular candidate pair by sending a STUN request from
the local candidate to the remote candidate.
The basic principle of the connectivity checks is simple: The basic principle of the connectivity checks is simple:
1. Sort the candidate pairs in priority order. 1. Sort the candidate pairs in priority order.
2. Send checks on each candidate pair in priority order. 2. Send checks on each candidate pair in priority order.
3. Acknowledge checks received from the other agent. 3. Acknowledge checks received from the other agent.
A complete connectivity check for a single candidate pair is a simple With both agents performing a check on a candidate pair, the result
4-message handshake: is a 4-way handshake:
L R L R
- - - -
STUN request -> \ L's STUN request -> \ L's
<- STUN response / check <- STUN response / check
<- STUN request \ R's <- STUN request \ R's
STUN response -> / check STUN response -> / check
Figure 3 Figure 3: Basic Connectivity Check
As an optimization, as soon as R gets L's check message he It is important to note that the STUN requests are sent to and from
immediately sends his own check message to L on the same candidate the exact same IP addresses and ports that will be used for media
pair. This accelerates the process of finding a valid candidate, and (e.g., RTP and RTCP). Consequently, agents demultiplex STUN and RTP/
is called a triggered check. RTCP using contents of the packets, rather than the port on which
they are received. Fortunately, this demultiplexing is easy to do,
especially for RTP and RTCP.
Because STUN is used for the connectivity check, the STUN response
will contain the agent's translated transport address on the public
side any NATs between the agent and its peer. If this transport
address is different from other candidates the agent already learned,
it represents a new candidate, called a PEER REFLEXIVE CANDIDATE,
which then gets tested by ICE just the same as any other candidate.
As an optimization, as soon as R gets L's check message R immediately
sends a check message to L on the same candidate pair. This
accelerates the process of finding a valid candidate, and is called a
TRIGGERED CHECK.
At the end of this handshake, both L and R know that they can send At the end of this handshake, both L and R know that they can send
(and receive) messages end-to-end in both directions. (and receive) messages end-to-end in both directions.
2.3. Sorting Candidates 2.3. Sorting Candidates
Because the algorithm above searches all candidate pairs, if a Because the algorithm above searches all candidate pairs, if a
working pair exists it will eventually find it no matter what order working pair exists it will eventually find it no matter what order
the candidates are tried in. In order to produce faster (and better) the candidates are tried in. In order to produce faster (and better)
results, the candidates are sorted in a specified order. The results, the candidates are sorted in a specified order. The
algorithm is described in Section 4.1.2 but follows two general resulting list of sorted candidate pairs is called the CHECK LIST.
The algorithm is described in Section 4.1.2 but follows two general
principles: principles:
o Each agent gives its candidates a numeric priority which is sent o Each agent gives its candidates a numeric priority which is sent
along with the candidate to the peer along with the candidate to the peer
o The local and remote priorities are combined so that each agent o The local and remote priorities are combined so that each agent
has the same ordering for the candidate pairs. has the same ordering for the candidate pairs.
The second property is important for getting ICE to work when there The second property is important for getting ICE to work when there
are NATs in front of A and B. Frequently, NATs will not allow packets are NATs in front of L and R. Frequently, NATs will not allow packets
in from a host until the agent behind the NAT has sent a packet in from a host until the agent behind the NAT has sent a packet
towards that host. Consequently, ICE checks in each direction will towards that host. Consequently, ICE checks in each direction will
not succeed until both sides have sent a check through their not succeed until both sides have sent a check through their
respective NATs. respective NATs.
The agent works through this check list by sending a STUN request for
the next candidate pair on the list every 20ms. These are called
PERIODIC CHECKS.
In general the priority algorithm is designed so that candidates of In general the priority algorithm is designed so that candidates of
similar type get similar priorities and so that more direct routes similar type get similar priorities and so that more direct routes
are preferred over indirect ones. Within those guidelines, however, (that is, through fewer media relays and through fewer NATs) are
agents have a fair amount of discretion about how to tune their preferred over indirect ones (ones with more media relays and more
algorithms. NATs). Within those guidelines, however, agents have a fair amount
of discretion about how to tune their algorithms.
2.4. Frozen Candidates 2.4. Frozen Candidates
The previous description only addresses the case where the agents The previous description only addresses the case where the agents
wish to establish a single media component--i.e., a single flow with wish to establish a media session with one COMPONENT (a piece of a
a single host-port quartet. However, in many cases (in particular media stream requiring a single transport address; a media stream may
RTP and RTCP) the agents actually need to establish connectivity for require multiple components, each of which has to work for the media
more than one flow. stream as a whole to be work). Typically, (e.g., with RTP and RTCP)
the agents actually need to establish connectivity for more than one
flow.
The naive way to attack this problem would be to simply do The network properties are likely to be very similar for each
independent ICE exchanges for each media component. This is component (especially because RTP and RTCP are sent and received from
obviously inefficient because the network properties are likely to be the same IP address). It is usually possible to leverage information
very similar for each component (especially because RTP and RTCP are from one media component in order to determine the best candidates
typically run on adjacent ports). Thus, it should be possible to for another. ICE does this with a mechanism called "frozen
leverage information from one media component in order to determine candidates."
the best candidates for another. ICE does this with a mechanism
called "frozen candidates."
The basic principle behind frozen candidates is that initially only Each candidate is associated with a property called its FOUNDATION.
the candidates for a single media component are tested. The other Two candidates have the same foundation when they are "similar" - of
media components are marked "frozen". When the connectivity checks the same type and obtained from the same interfaces and STUN servers.
for the first component succeed, the corresponding candidates for the Otherwise, their foundation is different. A candidate pair has a
other components are unfrozen and checked immediately. This avoids foundation too, which is just the concatenation of the foundations of
repeated checking of components which are superficially more its two candidates. Initially, only the candidate pairs with unique
foundations are tested. The other candidate pairs are marked
"frozen". When the connectivity checks for a candidate pair succeed,
the candidate pairs with the same foundation are unfrozen. This
avoids repeated checking of components which are superficially more
attractive but in fact are likely to fail. attractive but in fact are likely to fail.
While we've described "frozen" here as a separate mechanism for While we've described "frozen" here as a separate mechanism for
expository purposes, in fact it is an integral part of ICE and the expository purposes, in fact it is an integral part of ICE and the
the ICE prioritization algorithm automatically ensures that the right the ICE prioritization algorithm automatically ensures that the right
candidates are unfrozen and checked in the right order. candidates are unfrozen and checked in the right order.
2.5. Security for Checks 2.5. Security for Checks
Because ICE is used to discover which addresses can be used to send Because ICE is used to discover which addresses can be used to send
media between two agents, it is important to ensure that the process media between two agents, it is important to ensure that the process
cannot be hijacked to send media to the wrong location. Each STUN cannot be hijacked to send media to the wrong location. Each STUN
connectivity check is covered by a message authentication code (MAC) connectivity check is covered by a message authentication code (MAC)
computed using a key exchanged in the signalling channel. This MAC computed using a key exchanged in the signalling channel. This MAC
provides message integrity and data origin authentication, thus provides message integrity and data origin authentication, thus
stopping an attacker from forging or modifying connectivity check stopping an attacker from forging or modifying connectivity check
messages. The MAC also aids in disambiguating ICE exchanges from messages. The MAC also aids in disambiguating ICE exchanges from
forked calls. forked calls when ICE is used with SIP [3].
2.6. Concluding ICE 2.6. Concluding ICE
ICE checks are performed in a specific sequence, so that high ICE checks are performed in a specific sequence, so that high
priority pairs are checked first, followed by lower priority ones. priority candidate pairs are checked first, followed by lower
One way to conclude ICE is to declare victory as soon as a check for priority ones. One way to conclude ICE is to declare victory as soon
each component of each media stream completes successfully. Indeed, as a check for each component of each media stream completes
this is a reasonable algorithm, and details for it are provided successfully. Indeed, this is a reasonable algorithm, and details
below. However, it is possible that packet losses will cause a for it are provided below. However, it is possible that packet
higher priority check to take longer to complete, and allowing ICE to losses will cause a higher priority check to take longer to complete.
run a little longer might produce better results. More In that case, allowing ICE to run a little longer might produce
fundamentally, however, the prioritization defined by this better results. More fundamentally, however, the prioritization
specification may not yield "optimal" results. As an example, if the defined by this specification may not yield "optimal" results. As an
aim is to select low latency media paths, usage of a relay is a hint example, if the aim is to select low latency media paths, usage of a
that latencies may be higher, but it is nothing more than a hint. An relay is a hint that latencies may be higher, but it is nothing more
actual RTT measurement could be made, and it might demonstrate that a than a hint. An actual RTT measurement could be made, and it might
pair with lower priority is actually better than one with higher demonstrate that a pair with lower priority is actually better than
priority. one with higher priority.
Consequently, ICE assigns one of the agents in the role of the Consequently, ICE assigns one of the agents in the role of the
controlling agent, and the other of the controlled agent. The CONTROLLING AGENT, and the other of the CONTROLLED AGENT. The
controlling agent runs a selection algorithm, through which it can controlled agent gets to nominate which candidate pairs will get used
decide when to conclude ICE checks, and which pairs get selected. for media amongst the ones that are valid. It can do this in one of
The one that is selected is called the favored candidate pair. When two ways - using REGULAR NOMINATION or AGGRESSIVE NOMINATION.
a controlling agent selects a pair for a particular component of a
media stream, it generates a check for that pair and includes a flag With regular nomination, the controlling agent lets the checks
in the check indicating that the pair has been selected. If the continue until at least one valid candidate pair for each media
controlled agent has already performed in a check in the reverse stream is found. Then, it picks amongst those that are valid, and
direction that succeeded, the controlled agent considers ICE sends a second STUN request on its NOMINATED candidate pair, but this
processing to be concluded for that component. Once there is a time with a flag set to tell the peer that this pair has been
selected pair for each component of a media stream, the ICE checks nominated for use. A This is shown in Figure 4.
for that media stream are considered to be completed. At this point,
further checks stop for that media stream - ICE is considered to be
done. Consequently, media can flow in each direction for that
stream, as shown in Figure 4. Once all of the media streams are
completed, the controlling endpoint sends an updated offer if the
currently in-use candidates don't match the ones it selected.
L R L R
- - - -
STUN request + flag -> \ L's STUN request \ L's
<- STUN response / check <- STUN response / check
-> RTP Data <- STUN request \ R's
<- RTP Data STUN response -> / check
STUN request + flag \ L's
<- STUN response / check
Figure 4: Regular Nomination
Once the STUN transaction with the flag completes, both sides cancel
any future checks for that media stream. ICE will now send media
using this pair. The pair an ICE agent is using for media is called
the SELECTED PAIR.
In aggressive nomination, the controlling agent puts the flag in
every STUN request it sends. This way, once the first check
succeeds, ICE processing is complete for that media stream and the
controlling agent doesn't have to send a second STUN request. The
selected pair will be the highest priority valid pair. Aggressive
nomination is faster than regular nomination, but gives less
flexibility. Aggressive nomination is shown in Figure 5.
L R
- -
STUN request + flag \ L's
<- STUN response / check
<- STUN request \ R's
STUN response -> / check
Figure 5: Aggressive Nomination
Once all of the media streams are completed, the controlling endpoint
sends an updated offer if the candidates in the m and c lines for the
media stream (called the DEFAULT CANDIDATES) don't match ICE's
SELECTED CANDIDATES.
Figure 4
Once ICE is concluded, it can be restarted at any time for one or all Once ICE is concluded, it can be restarted at any time for one or all
of the media streams by each agent. This is done by sending an of the media streams by either agent. This is done by sending an
updated offer indicating a restart. updated offer indicating a restart.
2.7. Lite Implementations 2.7. Lite Implementations
In order for ICE to be used in a call, both agents need to support In order for ICE to be used in a call, both agents need to support
it. However, certain agents, such as those in gateways to the PSTN, it. However, certain agents will always be connected to the public
media servers, conferencing servers, and voicemail servers, are known Internet and have a public IP address at which it can receive packets
to not be behind a NAT or firewall. To make it easier for these from any correspondent. To make it easier for these devices to
devices to support ICE, ICE defines a special type of implementation support ICE, ICE defines a special type of implementation called LITE
called "lite" (in contrast to the normal "full" implementation). A (in contrast to the normal FULL implementation). A lite
lite implementation doesn't gather candidates; it includes only its implementation doesn't gather candidates; it includes only host
host candidate for any media stream. When a lite implementation candidates for any media stream. When a lite implementation connects
connects with a full implementation, the full agent takes the role of with a full implementation, the full agent takes the role of the
the controlling agent, and the lite agent takes on the controlled controlling agent, and the lite agent takes on the controlled role.
role. In addition, lite agents do not need to generate connectivity In addition, lite agents do not need to generate connectivity checks,
checks, run the state machines, or compute candidate pairs. For an run the state machines, or compute candidate pairs. Additional
informational summary of ICE processing as seen by a lite agent, see guidance on when a lite implementation is appropriate, see the
[33]. discussion in Appendix A. For an informational summary of ICE
processing as seen by a lite agent, see [36].
It is important to note that the lite implementation was added to
this specification to provide a stepping stone to full
implementation. Even for devices that are always connected to the
public Internet, a full implementation is preferable if achievable.
3. Terminology 3. Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [1]. document are to be interpreted as described in RFC 2119 [1].
This specification makes use of the following terminology: Readers should be familiar with the terminology defined in the offer/
answer model [4], STUN [12] and NAT Behavioral requirements for UDP
[29]
This specification makes use of the following additional terminology:
Agent: As defined in RFC 3264, an agent is the protocol Agent: As defined in RFC 3264, an agent is the protocol
implementation involved in the offer/answer exchange. There are implementation involved in the offer/answer exchange. There are
two agents involved in an offer/answer exchange. two agents involved in an offer/answer exchange.
Peer: From the perspective of one of the agents in a session, its Peer: From the perspective of one of the agents in a session, its
peer is the other agent. Specifically, from the perspective of peer is the other agent. Specifically, from the perspective of
the offerer, the peer is the answerer. From the perspective of the offerer, the peer is the answerer. From the perspective of
the answerer, the peer is the offerer. the answerer, the peer is the offerer.
Transport Address: The combination of an IP address and port. Transport Address: The combination of an IP address and transport
protocol (such as UDP or TCP) port.
Candidate: A transport address that is to be tested by ICE procedures Candidate: A transport address that is to be tested by ICE
in order to determine its suitability for usage for receipt of procedures in order to determine its suitability for usage for
media. receipt of media. Candidates also have properties - their type
(server reflexive, relayed or host), priority, foundation, and
base.
Component: A component is a single transport address that is used to Component: A component is a piece of a media stream requiring a
support a media stream. For media streams based on RTP, there are single transport address; a media stream may require multiple
two components per media stream - one for RTP, and one for RTCP. components, each of which has to work for the media stream as a
whole to work. For media streams based on RTP, there are two
components per media stream - one for RTP, and one for RTCP.
Host Candidate: A candidate obtained by binding to a specific port Host Candidate: A candidate obtained by binding to a specific port
from an interface on the host. This includes both physical from an interface on the host. This includes both physical
interfaces and logical ones, such as ones obtained through Virtual interfaces and logical ones, such as ones obtained through Virtual
Private Networks (VPNs) and Realm Specific IP (RSIP) [18] (which Private Networks (VPNs) and Realm Specific IP (RSIP) [19] (which
lives at the operating system level). lives at the operating system level).
Server Reflexive Candidate: A candidate obtained by sending a STUN Server Reflexive Candidate: A candidate obtained by sending a STUN
request from a host candidate to a STUN server, distinct from the request from a host candidate to a STUN server, distinct from the
peer, whose address is configured or learned by the client prior peer. The STUN server's address is configured or learned by the
to an offer/answer exchange. client prior to an offer/answer exchange.
Peer Reflexive Candidate: A candidate obtained by sending a STUN Peer Reflexive Candidate: A candidate obtained by sending a STUN
request from a host candidate to the STUN server running on a request from a host candidate to the STUN server running on a
peer's candidate. peer's candidate.
Relayed Candidate: A candidate obtained by sending a STUN Allocate Relayed Candidate: A candidate obtained by sending a STUN Allocate
request from a host candidate to a STUN server. The relayed request from a host candidate to a STUN server. The relayed
candidate is resident on the STUN server, and the STUN server candidate is resident on the STUN server, and the STUN server
relays packets back towards the agent. relays packets back towards the agent.
Translation: The translation of a relayed candidate is the transport
address that the relay will forward a packet to, when one is
received at the relayed candidate. For relayed candidates learned
through the STUN Allocate request, the translation of the relayed
candidate is the server reflexive candidate returned by the
Allocate response.
Base: The base of a server reflexive candidate is the host candidate Base: The base of a server reflexive candidate is the host candidate
from which it was derived. A host candidate is also said to have from which it was derived. A host candidate is also said to have
a base, equal to that candidate itself. Similarly, the base of a a base, equal to that candidate itself. Similarly, the base of a
relayed candidate is that candidate itself. relayed candidate is that candidate itself.
Foundation: Each candidate has a foundation, which is an identifier Foundation: An arbitrary string that is the same for two candidates
that is distinct for two candidates that have different types, that have the same type, base IP address, and STUN server. If any
different interface IP addresses for their base, and different IP of these are different then the foundation will be different. Two
addresses for their STUN servers. Two candidates have the same candidate pairs with the same foundation pairs are likely to have
foundation when they are of the same type, their bases have the similar network characteristics. Foundations are used in the
same IP address, and, for server reflexive or relayed candidates, frozen algorithm.
they come from the same STUN server. Foundations are used to
correlate candidates, so that when one candidate is found to be
valid, candidates sharing the same foundation can be tested next,
as they are likely to also be valid.
Local Candidate: A candidate that an agent has obtained and included Local Candidate: A candidate that an agent has obtained and included
in an offer or answer it sent. in an offer or answer it sent.
Remote Candidate: A candidate that an agent received in an offer or Remote Candidate: A candidate that an agent received in an offer or
answer from its peer. answer from its peer.
In-Use Candidate: A candidate is in-use when it appears in the m/c- Default Destination/Candidate: The default destination for a
line of an active media stream. component of a media stream is the transport address that would be
used by an agent that is not ICE aware. For the RTP component,
the default IP address is in the c line of the SDP, and the port
in the m line. For the RTCP component it is in the rtcp attribute
when present, and when not present, the IP address in the c line
and 1 plus the port in the m line. A default candidate for a
component is one whose transport address matches the default
destination for that component.
Candidate Pair: A pairing containing a local candidate and a remote Candidate Pair: A pairing containing a local candidate and a remote
candidate. candidate.
Check: A candidate pair where the local candidate is a transport Check, Connectivity Check, STUN Check: A STUN Binding Request
address from which an agent can send a STUN connectivity check. transaction for the purposes of verifying connectivity. A check
is sent from the local candidate to the remote candidate of a
candidate pair.
Check List: An ordered set of STUN checks that an agent is to Check List: An ordered set of candidate pairs that an agent will use
generate towards a peer. to generate checks.
Periodic Check: A connectivity check generated by an agent as a Periodic Check: A connectivity check generated by an agent as a
consequence of a timer that fires periodically, instructing it to consequence of a timer that fires periodically, instructing it to
send a check. send a check.
Triggered Check: A connectivity check generated as a consequence of Triggered Check: A connectivity check generated as a consequence of
the receipt of a connectivity check from the peer. the receipt of a connectivity check from the peer.
Valid List: An ordered set of candidate pairs for a media stream that Valid List: An ordered set of candidate pairs for a media stream
have been validated by a successful STUN transaction. that have been validated by a successful STUN transaction.
Full: An ICE implementation that performs the complete set of Full: An ICE implementation that performs the complete set of
functionality defined by this specification. functionality defined by this specification.
Lite: An ICE implementation that omits certain functions, Lite: An ICE implementation that omits certain functions,
implementing only as much as is necessary for a peer implementing only as much as is necessary for a peer
implementation that is full to gain the benefits of ICE. Lite implementation that is full to gain the benefits of ICE. Lite
implementations can only act as the controlled agent in a session, implementations can only act as the controlled agent in a session,
and do not gather candidates. and do not gather candidates.
Controlling Agent: The STUN agent which is responsible for selecting Controlling Agent: The STUN agent which is responsible for selecting
the final choice of candidate pairs and signaling them through the final choice of candidate pairs and signaling them through
STUN and an updated offer, if needed. In any session, one agent STUN and an updated offer, if needed. In any session, one agent
is always controlling. The other is the controlled agent. is always controlling. The other is the controlled agent.
Controlled Agent: A STUN agent which waits for the controlling agent Controlled Agent: A STUN agent which waits for the controlling agent
to select the final choice of candidate pairs. to select the final choice of candidate pairs.
Regular Nomination: The process of picking a valid candidate pair
for media traffic by validating the pair with one STUN request,
and then picking it by sending a second STUN request with a flag
indicating its nomination.
Aggressive Nomination: The process of picking a valid candidate pair
for media traffic by including a flag in every STUN request, such
that the first one to produce a valid candidate pair is used for
media.
Nominated: If a valid candidate pair has its nominated flag set, it
means that it may be selected by ICE for sending and receiving
media.
Selected Pair, Selected Candidate: The candidate pair selected by
ICE for sending and receiving media is called the selected pair,
and each of its candidates is called the selected candidate.
4. Sending the Initial Offer 4. Sending the Initial Offer
In order to send the initial offer in an offer/answer exchange, an In order to send the initial offer in an offer/answer exchange, an
agent must gather candidates, priorize them, choose ones for agent must (1) gather candidates, (2) prioritize them, (3) choose
inclusion in the m/c-line, and then formulate and send the SDP. The default candidates, and then (4) formulate and send the SDP. The
first of these three steps differ for full and lite implementations. first of these four steps differ for full and lite implementations.
4.1. Full Implementation Requirements 4.1. Full Implementation Requirements
4.1.1. Gathering Candidates 4.1.1. Gathering Candidates
An agent gathers candidates when it believes that communications is An agent gathers candidates when it believes that communications is
imminent. An offerer can do this based on a user interface cue, or imminent. An offerer can do this based on a user interface cue, or
based on an explicit request to initiate a session. Every candidate based on an explicit request to initiate a session. Every candidate
is a transport address. It also has a type and a base. Three types is a transport address. It also has a type and a base. Three types
are defined and gathered by this specification - host candidates, are defined and gathered by this specification - host candidates,
server reflexive candidates, and relayed candidates. The base of a server reflexive candidates, and relayed candidates. The server
candidate is the candidate that an agent must send from when using reflexive and relayed candidates are gathered using STUN's Binding
that candidate. Discovery and Relay Usages. The base of a candidate is the candidate
that an agent must send from when using that candidate.
4.1.1.1. Host Candidates
The first step is to gather host candidates. Host candidates are The first step is to gather host candidates. Host candidates are
obtained by binding to ports (typically ephemeral) on an interface obtained by binding to ports (typically ephemeral) on an interface
(physical or virtual, including VPN interfaces) on the host. The (physical or virtual, including VPN interfaces) on the host. The
process for gathering host candidates depends on the transport process for gathering host candidates depends on the transport
protocol. Procedures are specified here for UDP. protocol. Procedures are specified here for UDP.
For each UDP media stream the agent wishes to use, the agent SHOULD For each UDP media stream the agent wishes to use, the agent SHOULD
obtain a candidate for each component of the media stream on each obtain a candidate for each component of the media stream on each
interface that the host has. It obtains each candidate by binding to interface that the host has. It obtains each candidate by binding to
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every candidate) is always associated with a specific component for every candidate) is always associated with a specific component for
which it is a candidate. Each component has an ID assigned to it, which it is a candidate. Each component has an ID assigned to it,
called the component ID. For RTP-based media streams, the RTP itself called the component ID. For RTP-based media streams, the RTP itself
has a component ID of 1, and RTCP a component ID of 2. If an agent has a component ID of 1, and RTCP a component ID of 2. If an agent
is using RTCP it MUST obtain a candidate for it. If an agent is is using RTCP it MUST obtain a candidate for it. If an agent is
using both RTP and RTCP, it would end up with 2*K host candidates if using both RTP and RTCP, it would end up with 2*K host candidates if
an agent has K interfaces. an agent has K interfaces.
The base for each host candidate is set to the candidate itself. The base for each host candidate is set to the candidate itself.
4.1.1.2. Server Reflexive and Relayed Candidates
Agents SHOULD obtain relayed candidates and MUST obtain server Agents SHOULD obtain relayed candidates and MUST obtain server
reflexive candidates. The requirement to obtain relayed candidates reflexive candidates. The requirement to obtain relayed candidates
is at SHOULD strength to allow for provider variation. If they are is at SHOULD strength to allow for provider variation. Use of relays
not used, it is RECOMMENDED that it be implemented and just disabled is expensive, and when ICE is being used, relays will only be
through configuration, so that it can re-enabled through required when both endpoints are behind NATs that perform address and
configuration if conditions change in the future. port dependent mapping. Consequently, some deployments might
consider this use case to be marginal, and elect not to use relays.
If they are not used, it is RECOMMENDED that it be implemented and
just disabled through configuration, so that it can re-enabled
through configuration if conditions change in the future.
The agent next pairs each host candidate with the STUN server with The agent next pairs each host candidate with the STUN server with
which it is configured or has discovered by some means. This which it is configured or has discovered by some means. This
specification only considers usage of a single STUN server. Every Ta specification only considers usage of a single STUN server. At that
seconds, the agent chooses another such pair (the order is very instance, and then every Ta milliseconds thereafter, the agent
inconsequential), and sends a STUN request to the server from that chooses another such pair (the order is inconsequential), and sends a
host candidate. If the agent is using both relayed and server STUN request to the server from that host candidate. If the agent is
reflexive candidates, this request MUST be a STUN Allocate request using both relayed and server reflexive candidates, this request MUST
from the relay usage [12]. If the agent is using only server be a STUN Allocate request using the relay usage [13]. If the agent
reflexive candidates, the request MUST be a STUN Binding request is using only server reflexive candidates, the request MUST be a STUN
using the binding discovery usage [11]. Binding request using the binding discovery usage [12].
The value of Ta SHOULD be configurable, and SHOULD have a default of The value of Ta SHOULD be configurable, and SHOULD have a default of
20ms. Note that this pacing applies only to starting STUN 20ms (see Appendix B.1 for a discussion on the selection of this
value). Note that this pacing applies only to starting STUN
transactions with source and destination transport addresses (i.e., transactions with source and destination transport addresses (i.e.,
the host candidate and STUN server respectively) for which a STUN the host candidate and STUN server respectively) for which a STUN
transaction has not previously been sent. Consequently, transaction has not previously been sent. Consequently,
retransmissions of a STUN request are governed entirely by the retransmissions of a STUN request are governed entirely by the
retransmission rules defined in [11]. Similarly, retries of a retransmission rules defined in [12]. Similarly, retries of a
request due to recoverable errors (such as an authentication request due to recoverable errors (such as an authentication
challenge) happen immediately and are not paced by timer Ta. Because challenge) happen immediately and are not paced by timer Ta. Because
of this pacing, it will take a certain amount of time to obtain all of this pacing, it will take a certain amount of time to obtain all
of the server reflexive and relayed candidates. Implementations of the server reflexive and relayed candidates. Implementations
should be aware of the time required to do this, and if the should be aware of the time required to do this, and if the
application requires a time budget, limit the amount of candidates application requires a time budget, limit the number of candidates
which are gathered. which are gathered.
An Allocate Response will provide the agent with a server reflexive The agent will receive a STUN Binding or Allocate response. A
candidate (obtained from the mapped address) and a relayed candidate successful Allocate Response will provide the agent with a server
in the RELAY-ADDRESS attribute. A Binding Response will provide the reflexive candidate (obtained from the mapped address) and a relayed
agent with only a server reflexive candidate (also obtained from the candidate in the RELAY-ADDRESS attribute. If the Allocate request is
mapped address). The base of the server reflexive candidate is the rejected because the server lacks resources to fulfill it, the agent
host candidate from which the Allocate or Binding request was sent. SHOULD instead send a Binding Request to obtain a server reflexive
The base of a relayed candidate is that candidate itself. A server candidate. A Binding Response will provide the agent with only a
reflexive candidate obtained from an Allocate response is the called server reflexive candidate (also obtained from the mapped address).
the "translation" of the relayed candidate obtained from the same The base of the server reflexive candidate is the host candidate from
response. The agent will need to remember the translation for the which the Allocate or Binding request was sent. The base of a
relayed candidate, since it is placed into the SDP. If a relayed relayed candidate is that candidate itself. If a relayed candidate
candidate is identical to a host candidate (which can happen in rare is identical to a host candidate (which can happen in rare cases),
cases), the relayed candidate MUST be discarded. Proper operation of the relayed candidate MUST be discarded. Proper operation of ICE
ICE depends on each base being unique. depends on each base being unique.
4.1.1.3. Eliminating Redundant Candidates
Next, the agent eliminates redundant candidates. A candidate is Next, the agent eliminates redundant candidates. A candidate is
redundant if its transport address equals another candidate, and its redundant if its transport address equals another candidate, and its
base equals the base of that other candidate. Note that two base equals the base of that other candidate. Note that two
candidates can have the same transport address yet have different candidates can have the same transport address yet have different
bases, and these would not be considered redundant. bases, and these would not be considered redundant.
4.1.1.4. Computing Foundations
Finally, the agent assigns each candidate a foundation. The Finally, the agent assigns each candidate a foundation. The
foundation is an identifier, scoped within a session. Two candidates foundation is an identifier, scoped within a session. Two candidates
MUST have the same foundation ID when they are of the same type MUST have the same foundation ID when all of the following are true:
(host, relayed, server reflexive, peer reflexive or relayed), their
bases have the same IP address (the ports can be different), and, for o they are of the same type (host, relayed, server reflexive, peer
reflexive and relayed candidates, the STUN servers used to obtain reflexive or relayed)
them have the same IP address. Similarly, two candidates MUST have
different foundations if their types are different, their bases have o their bases have the same IP address (the ports can be different)
different IP addresses, or the STUN servers used to obtain them have o for reflexive and relayed candidates, the STUN servers used to
different IP addresses. obtain them have the same IP address.
Similarly, two candidates MUST have different foundations if their
types are different, their bases have different IP addresses, or the
STUN servers used to obtain them have different IP addresses.
4.1.1.5. Keeping Candidates Alive
Once server reflexive and relayed candidates are allocated, they MUST
be kept alive until ICE processing has completed. For server
reflexive candidates learned through the Binding Discovery usage,
this MUST be another Binding Request from the Binding Discovery
usage. For relayed candidates learned through the Relay Usage, this
MUST be a new Allocate request. The Allocate request will also
refresh the server reflexive candidate.
4.1.2. Prioritizing Candidates 4.1.2. Prioritizing Candidates
The prioritization process results in the assignment of a priority to The prioritization process results in the assignment of a priority to
each candidate. Each candidate for a media stream MUST have a unique each candidate. Each candidate for a media stream MUST have a unique
priority. An agent SHOULD compute the priority by determining a priority that MUST be a positive integer between 1 and (2**32 - 1).
preference for each type of candidate (server reflexive, peer This priority will be used by ICE to determine the order of the
connectivity checks and the relative preference for candidates.
An agent SHOULD compute this priority using the formula in
Section 4.1.2.1 and choose its parameters using the guidelines in
Section 4.1.2.2. If an agent elects to use a different formula, ICE
will take longer to converge since both agents will not be
coordinated in their checks.
4.1.2.1. Recommended Formula
When using the formula, an agent computes the priority by determining
a preference for each type of candidate (server reflexive, peer
reflexive, relayed and host), and, when the agent is multihomed, reflexive, relayed and host), and, when the agent is multihomed,
choosing a preference for its interfaces. These two preferences are choosing a preference for its interfaces. These two preferences are
then combined to compute the priority for a candidate. That priority then combined to compute the priority for a candidate. That priority
SHOULD be computed using the following formula: is computed using the following formula:
priority = (2^24)*(type preference) + priority = (2^24)*(type preference) +
(2^8)*(local preference) + (2^8)*(local preference) +
(2^0)*(256 - component ID) (2^0)*(256 - component ID)
The type preference MUST be an integer from 0 to 126 inclusive, and The type preference MUST be an integer from 0 to 126 inclusive, and
represents the preference for the type of the candidate (where the represents the preference for the type of the candidate (where the
types are local, server reflexive, peer reflexive and relayed). A types are local, server reflexive, peer reflexive and relayed). A
126 is the highest preference, and a 0 is the lowest. Setting the 126 is the highest preference, and a 0 is the lowest. Setting the
value to a 0 means that candidates of this type will only be used as value to a 0 means that candidates of this type will only be used as
a last resort. The type preference MUST be identical for all a last resort. The type preference MUST be identical for all
candidates of the same type and MUST be different for candidates of candidates of the same type and MUST be different for candidates of
different types. The type preference for peer reflexive candidates different types. The type preference for peer reflexive candidates
MUST be higher than that of server reflexive candidates. Note that MUST be higher than that of server reflexive candidates. Note that
candidates gathered based on the procedures of Section 4.1.1 will candidates gathered based on the procedures of Section 4.1.1 will
never be peer reflexive candidates; candidates of these type are never be peer reflexive candidates; candidates of these type are
learned from the STUN connectivity checks performed by ICE. The learned from the STUN connectivity checks performed by ICE.
component ID is the component ID for the candidate, and MUST be
between 1 and 256 inclusive. The local preference MUST be an integer
from 0 to 65535 inclusive. It represents a preference for the
particular interface from which the candidate was obtained, in cases
where an agent is multihomed. 65535 represents the highest
preference, and a zero, the lowest. When there is only a single
interface, this value SHOULD be set to 65535. Generally speaking, if
there are multiple candidates for a particular component for a
particular media stream which have the same type, the local
preference MUST be unique for each one. In this specification, this
only happens for multi-homed hosts.
These rules guarantee that there is a unique priority for each The local preference MUST be an integer from 0 to 65535 inclusive.
candidate. This priority will be used by ICE to determine the order It represents a preference for the particular interface from which
of the connectivity checks and the relative preference for the candidate was obtained, in cases where an agent is multihomed.
candidates. Consequently, what follows are some guidelines for 65535 represents the highest preference, and a zero, the lowest.
selection of these values. When there is only a single interface, this value SHOULD be set to
65535. More generally, if there are multiple candidates for a
particular component for a particular media stream which have the
same type, the local preference MUST be unique for each one. In this
specification, this only happens for multi-homed hosts.
The component ID is the component ID for the candidate, and MUST be
between 1 and 256 inclusive.
4.1.2.2. Guidelines for Choosing Type and Local Preferences
One criteria for selection of the type and local preference values is One criteria for selection of the type and local preference values is
the use of an intermediary. That is, if media is sent to that the use of an intermediary, such as a media relay. With an
candidate, will the media first transit an intermediate server before intermediary, if media is sent to that candidate, it will first
being received? Relayed candidates are clearly one type of transit the intermediary before being received. Relayed candidates
candidates that involve an intermediary. Another are host candidates are one type of candidate that involves an intermediary. Another are
obtained from a VPN interface. When media is transited through an host candidates obtained from a VPN interface. When media is
intermediary, it can increase the latency between transmission and transited through an intermediary, it can increase the latency
reception. It can increase the packet losses, because of the between transmission and reception. It can increase the packet
additional router hops that may be taken. It may increase the cost losses, because of the additional router hops that may be taken. It
of providing service, since media will be routed in and right back may increase the cost of providing service, since media will be
out of an intermediary run by the provider. If these concerns are routed in and right back out of a media relay run by the provider.
important, the type preference for relayed candidates can be set If these concerns are important, the type preference for relayed
lower than the type preference for reflexive and host candidates. candidates SHOULD be lower than host candidates. The RECOMMENDED
Indeed, it is RECOMMENDED that in this case, host candidates have a values are 126 for host candidates, 100 for server reflexive
type preference of 126, server reflexive candidates have a type candidates, 110 for peer reflexive candidates, and 0 for relayed
preference of 100, peer reflexive have a type prefence of 110, and candidates. Furthermore, if an agent is multi-homed and has multiple
relayed candidates have a type preference of zero. Furthermore, if interfaces, the local preference for host candidates from a VPN
an agent is multi-homed and has multiple interfaces, the local interface SHOULD have a priority of 0.
preference for host candidates from a VPN interface SHOULD have a
priority of 0.
Another criteria for selection of preferences is IP address family. Another criteria for selection of preferences is IP address family.
ICE works with both IPv4 and IPv6. It therefore provides a ICE works with both IPv4 and IPv6. It therefore provides a
transition mechanism that allows dual-stack hosts to prefer transition mechanism that allows dual-stack hosts to prefer
connectivity over IPv6, but to fall back to IPv4 in case the v6 connectivity over IPv6, but to fall back to IPv4 in case the v6
networks are disconnected (due, for example, to a failure in a 6to4 networks are disconnected (due, for example, to a failure in a 6to4
relay) [23]. It can also help with hosts that have both a native relay) [24]. It can also help with hosts that have both a native
IPv6 address and a 6to4 address. In such a case, lower local IPv6 address and a 6to4 address. In such a case, higher local
preferences could be assigned to the v6 interface, followed by the preferences could be assigned to the v6 interface, followed by the
6to4 interfaces, followed by the v4 interfaces. This allows a site 6to4 interfaces, followed by the v4 interfaces. This allows a site
to obtain and begin using native v6 addresses immediately, yet still to obtain and begin using native v6 addresses immediately, yet still
fallback to 6to4 addresses when communicating with agents in other fallback to 6to4 addresses when communicating with agents in other
sites that do not yet have native v6 connectivity. sites that do not yet have native v6 connectivity.
Another criteria for selecting preferences is security. If a user is Another criteria for selecting preferences is security. If a user is
a telecommuter, and therefore connected to their corporate network a telecommuter, and therefore connected to their corporate network
and a local home network, they may prefer their voice traffic to be and a local home network, they may prefer their voice traffic to be
routed over the VPN in order to keep it on the corporate network when routed over the VPN in order to keep it on the corporate network when
skipping to change at page 20, line 14 skipping to change at page 24, line 30
a VPN interface would have a higher local preference than any other a VPN interface would have a higher local preference than any other
interface. interface.
Another criteria for selecting preferences is topological awareness. Another criteria for selecting preferences is topological awareness.
This is most useful for candidates that make use of relays. In those This is most useful for candidates that make use of relays. In those
cases, if an agent has preconfigured or dynamically discovered cases, if an agent has preconfigured or dynamically discovered
knowledge of the topological proximity of the relays to itself, it knowledge of the topological proximity of the relays to itself, it
can use that to assign higher local preferences to candidates can use that to assign higher local preferences to candidates
obtained from closer relays. obtained from closer relays.
4.1.3. Choosing In-Use Candidates 4.1.3. Choosing Default Candidates
A candidate is said to be "in-use" if it appears in the m/c-line of A candidate is said to be default if it would be the target of media
an offer or answer. When communicating with an ICE peer, being in- from a non-ICE peer; that target being called the DEFAULT
use implies that, should these candidates be selected by the ICE DESTINATION. If the default candidates are not selected by the ICE
algorithm, a re-INVITE will not be required after ICE processing algorithm when communicating with an ICE-aware peer, an updated
completes. When communicating with a peer that is not ICE-aware, the offer/answer will be required after ICE processing completes in order
in-use candidates will be used exclusively for the exchange of media, to "correct" the SDP so that the default destination for media
as defined in normal offer/answer procedures. matches the candidates selected by ICE. If ICE happens to select the
default candidates, no updated offer/answer is required.
An agent MUST choose a set of candidates, one for each component of An agent MUST choose a set of candidates, one for each component of
each active media stream, to be in-use. A media stream is active if each in-use media stream, to be default. A media stream is in-use if
it does not contain the a=inactive SDP attribute. it does not have a port of zero (which is used in RFC 3264 to reject
a media stream). Consequently, a media stream is in-use even if it
is marked as a=inactive [10] or has a bandwidth value of zero.
It is RECOMMENDED that in-use candidates be chosen based on the It is RECOMMENDED that default candidates be chosen based on the
likelihood of those candidates to work with the peer that is being likelihood of those candidates to work with the peer that is being
contacted. Unfortunately, it is difficult to ascertain which contacted. It is RECOMMENDED that the default candidates are the
candidates that might be. As an example, consider a user within an relayed candidates (if relayed candidates are available), server
enterprise. To reach non-ICE capable agents within the enterprise, reflexive candidates (if server reflexive candidates are available),
host candidates have to be used, since the enterprise policies may and finally host candidates.
prevent communication between elements using a relay on the public
network. However, when communicating to peers outside of the
enterprise, relayed candidates from a publically accessible STUN
server are needed.
Indeed, the difficulty in picking just one transport address that
will work is the whole problem that motivated the development of this
specification in the first place. As such, it is RECOMMENDED that
agents select relayed candidates to be in-use.
4.2. Lite Implementation 4.2. Lite Implementation
For each media stream, the agent allocates a single candidate for For each media stream, the agent allocates a single candidate for
each component of the media stream from one of its interfaces. If an each component of the media stream from one of its interfaces. If an
agent is multi-homed, it MUST choose one of its interfaces for a agent has multiple interfaces, it MUST choose one for each component
particular media stream; ICE cannot be used to dynamically choose of a particular media stream. With the lite implementation, ICE
one. Each component has an ID assigned to it, called the component cannot be used to dynamically choose amongst candidates. Each
ID. For RTP-based media streams, the RTP itself has a component ID component has an ID assigned to it, called the component ID. For
of 1, and RTCP a component ID of 2. If an agent is using RTCP it RTP-based media streams the RTP itself has a component ID of 1, and
MUST obtain a candidate for it. RTCP a component ID of 2. If an agent is using RTCP it MUST obtain a
candidate for it.
Each candidate is assigned a foundation. The foundation MUST be Each candidate is assigned a foundation. The foundation MUST be
different for two candidates from different interfaces (which can different for two candidates from different interfaces, and MUST be
occur if media streams are on different interfaces), and MUST be the the same otherwise. A simple integer that increments for each
same otherwise. A simple integer that increments for each interface interface will suffice. In addition, each candidate MUST be assigned
will suffice. In addition, each candidate MUST be assigned a unique a unique priority amongst all candidates for the same media stream.
priority amongst all candidates for the same media stream. This This priority SHOULD be equal to 2^24*(126) + 2^8*(65535) + 256 minus
priority SHOULD be equal to 2^24*(126) + 2^8*(65535) + 256 minus the the component ID, which is 2130706432 minus the component ID.
component ID, which is 2130706432 minus the component ID. Each of
these candidates is also considered to be "in-use", since they will If an agent has included two candidates for a component, the v4
be included in the m/c-line of an offer or answer. candidate SHOULD be selected as the default. Since a lite
implementation has a single candidate for a component, each of these
candidates is considered to be default.
4.3. Encoding the SDP 4.3. Encoding the SDP
The process of encoding the SDP is identical between full and lite The process of encoding the SDP is identical between full and lite
implementations. implementations.
The agent includes a single a=candidate media level attribute in the The agent will include an m-line for each media stream it wishes to
SDP for each candidate for that media stream. The a=candidate use. The ordering of media streams in the SDP is relevant for ICE.
attribute contains the IP address, port and transport protocol for ICE will perform its connectivity checks for the first m-line first,
that candidate. A Fully Qualified Domain Name (FQDN) for a host MAY and consequently media will be able to flow for that stream first.
be used in place of a unicast address. In that case, when receiving Agents SHOULD place their most important media stream, if there is
an offer or answer containing an FQDN in an a=candidate attribute, one, first in the SDP.
the FQDN is looked up in the DNS using an A or AAAA record, and the
resulting IP address is used for the remainder of ICE processing.
The candidate attribute also includes the component ID for that
candidate. For media streams based on RTP, candidates for the actual
RTP media MUST have a component ID of 1, and candidates for RTCP MUST
have a component ID of 2. Other types of media streams which require
multiple components MUST develop specifications which define the
mapping of components to component IDs, and these component IDs MUST
be between 1 and 256.
The candidate attribute also includes the priority and the There will be a candidate attribute for each candidate for a
foundation. The agent SHOULD include a type for each candidate by particular media stream. Section 15 provides detailed rules for
populating the candidate-types production with the appropriate value constructing this attribute. The attribute carries the IP address,
- "host" for host candidates, "srflx" for server reflexive port and transport protocol for the candidate, in addition to its
candidates, "prflx" for peer reflexive candidates (though these never properties that need to be signaled to the peer for ICE to work: the
appear in an initial offer/answer exchange), and "relay" for relayed priority, foundation, and component ID. The candidate attribute also
candidates. The related address MUST NOT be included if a type was carries information about the candidate that is useful for
not included. If a type was included, the related address SHOULD be diagnostics and other functions: its type and related transport
present for server reflexive, peer reflexive and relayed candidates. addresses.
If a candidate is server or peer reflexive, the related address is
equal to the base for that server or peer reflexive candidate. If
the candidate is relayed, the related address is equal to the
translation of the relayed address. If the candidiate is a host
candidate, there is no related address and the rel-addr production
MUST be omitted.
STUN connectivity checks between agents make use of a short term STUN connectivity checks between agents make use of a short term
credential that is exchanged in the offer/answer process. The credential that is exchanged in the offer/answer process. The
username part of this credential is formed by concatenating a username part of this credential is formed by concatenating a
username fragment from each agent, separated by a colon. Each agent username fragment from each agent, separated by a colon. Each agent
also provides a password, used to compute the message integrity for also provides a password, used to compute the message integrity for
requests it receives. As such, an SDP MUST contain the ice-ufrag and requests it receives. The username fragment and password are
ice-pwd attributes, containing the username fragment and password exchanged in the ice-ufrag and ice-pwd attributes, respectively. In
respectively. These can be either session or media level attributes, addition to providing security, the username provides disambiguation
and thus common across all candidates for all media streams, or all and correlation of checks to media streams. See Appendix B.4 for
candidates for a particular media stream, respectively. However, if motivation.
two media streams have identical ice-ufrag's, they MUST have
identical ice-pwd's. The ice-ufrag and ice-pwd attributes MUST be
chosen randomly at the beginning of a session. The ice-ufrag
attribute MUST contain at least 24 bits of randomness, and the ice-
pwd attribute MUST contain at least 128 bits of randomness. This
means that the ice-ufrag attribute will be at least 4 characters
long, and the ice-pwd at least 22 characters long, since the grammar
for these attributes allows for 6 bits of randomness per character.
The attributes MAY be longer than 4 and 22 characters respectively,
of course.
If an agent is a lite implementation, it MUST include an "a=ice-lite" If an agent is a lite implementation, it MUST include an "a=ice-lite"
session level attribute in its SDP. If an agent is a full session level attribute in its SDP. If an agent is a full
implementation, it MUST NOT include this attribute. implementation, it MUST NOT include this attribute.
The m/c-line is populated with the candidates that are in-use. For The default candidates are added to the SDP as the default
streams based on RTP, this is done by placing the RTP candidate into destination for media. For streams based on RTP, this is done by
the m and c lines respectively. If the agent is utilizing RTCP, it placing the IP address and port of the RTP candidate into the c and m
MUST encode the RTCP candidate into the m/c-line using the a=rtcp lines, respectively. If the agent is utilizing RTCP, it MUST encode
attribute as defined in RFC 3605 [2]. If RTCP is not in use, the the RTCP candidate using the a=rtcp attribute as defined in RFC 3605
agent MUST signal that using b=RS:0 and b=RR:0 as defined in RFC 3556 [2]. If RTCP is not in use, the agent MUST signal that using b=RS:0
[5]. and b=RR:0 as defined in RFC 3556 [5].
There MUST be a candidate attribute for each component of the media The transport addresses that will be the default destination for
stream in the m/c-line. media when communicating with non-ICE peers MUST also be present as
candidates in one or more a=candidate lines.
Once an offer or answer are sent, an agent MUST be prepared to ICE provides for extensibility by allowing an offer or answer to
receive both STUN and media packets on each candidate. As discussed contain a series of tokens which identify the ICE extensions used by
in Section 11.1, media packets can be sent to a candidate prior to that agent. If an agent supports an ICE extension, it MUST include
its appearence in the m/c-line. the token defined for that extension in the ice-options attribute.
The following is an example SDP message that includes ICE attributes
(lines folded for readability):
v=0
o=jdoe 2890844526 2890842807 IN IP4 10.0.1.1
s=
c=IN IP4 192.0.2.3
t=0 0
a=ice-pwd:asd88fgpdd777uzjYhagZg
a=ice-ufrag:8hhY
m=audio 45664 RTP/AVP 0
a=rtpmap:0 PCMU/8000
a=candidate:1 1 UDP 2130706178 10.0.1.1 8998 typ local
a=candidate:2 1 UDP 1694498562 192.0.2.3 45664 typ srflx raddr
10.0.1.1 rport 8998
Once an agent has sent its offer or sent its answer, that agent MUST
be prepared to receive both STUN and media packets on each candidate.
As discussed in Section 11.1, media packets can be sent to a
candidate prior to its appearance as the default destination for
media in an offer or answer.
5. Receiving the Initial Offer 5. Receiving the Initial Offer
When an agent receives an initial offer, it will check if the offeror When an agent receives an initial offer, it will check if the offeror
supports ICE, determine its role, gather candidates, prioritize them, supports ICE, determine its own role, gather candidates, prioritize
choose one for in-use, encode and send an answer, and for full them, choose default candidates, encode and send an answer, and for
implementations, form the check lists and begin connectivity checks. full implementations, form the check lists and begin connectivity
checks.
5.1. Verifying ICE Support 5.1. Verifying ICE Support
The answerer will proceed with the ICE procedures defined in this The answerer will proceed with the ICE procedures defined in this
specification if the following are true: specification if the following are all true:
o There is at least one a=candidate attribute for each media stream o For each media stream, the default destination for at least one
in the offer it just received. component of the media stream appears in a candidate attribute.
For example, in the case of RTP, the IP address and port in the c
and m line, respectively, appears in a candidate attribute, or the
value in the rtcp attribute appears in a candidate attribute.
o For each media stream, at least one of the candidates is a match o The offer omitted an a=ice-lite attribute or the answerer is a
for its respective in-use component in the m/c-line. full implementation.
If both of these conditions are not met, the agent MUST process the If any of these conditions are not met, the agent MUST process the
SDP based on normal RFC 3264 procedures, without using any of the ICE SDP based on normal RFC 3264 procedures, without using any of the ICE
mechanisms described in the remainder of this specification with two mechanisms described in the remainder of this specification with the
exceptions. First, in all cases, the agent MUST follow the rules of following exceptions:
Section 10, which describe keepalive procedures for all agents.
Secondly, if the agent is not proceeding with ICE because there were
a=candidate attributes, but none that matched the m/c-line of the
media stream, the agent MUST include an a=ice-mismatch attribute in
its answer. This mismatch occurs in cases where intermediary
elements modify the m/c-line, but don't modify candidate attributes.
By including this attribute in the response, diagnostic information
on the ICE failure is provided to the offeror and any intermediate
signaling entities.
In addition, if the offer contains the "a=ice-lite" attribute, and 1. The agent MUST follow the rules of Section 10, which describe
the answerer is also lite, the agent MUST process the SDP based on keepalive procedures for all agents.
normal RFC 3264 procedures, as if it didn't support ICE, with the
exception of Section 10, which describes keepalive procedures. 2. If the agent is not proceeding with ICE because there were
a=candidate attributes, but none that matched the default
destination of the media stream, the agent MUST include an a=ice-
mismatch attribute in its answer.
5.2. Determining Role 5.2. Determining Role
For each session, each agent takes on a role. There are two roles - For each session, each agent takes on a role. There are two roles -
controlling, and controlled. The controlling agent is responsible controlling, and controlled. The controlling agent is responsible
for selecting the candidate pairs to be used for each media stream, for nominating the candidate pairs that can be used by ICE for each
and for generating the updated offer based on that selection, when media stream, and for generating the updated offer based on ICE's
needed. The controlled agent is told which candidate pairs to use selection, when needed. The controlled agent is told which candidate
for each media stream, and does not generate an updated offer to pairs to use for each media stream, and does not generate an updated
signal this information in SIP. offer to signal this information.
If one of the agents is a lite implementation, it MUST assume the If one of the agents is a lite implementation, it MUST assume the
controlled role, and its peer (which will be full) MUST assume the controlled role, and its peer (which will be full; if it was lite,
controlling role. If the agent and its peer are both full ICE would have aborted) MUST assume the controlling role. If the
implementations, the agent which generated the offer which started agent and its peer are both full implementations, the agent which
the ICE processing takes on the controlling role, and the other takes generated the offer which started the ICE processing takes on the
the controlled role. controlling role, and the other takes the controlled role.
Based on this definition, once roles are determined for a session, Based on this definition, once roles are determined for a session,
they persist unless ICE is restarted, as discussed below. A restart they persist unless ICE is restarted. A ICE restart (Section 9.1
causes a new selection of roles. causes a new selection of roles.
5.3. Gathering Candidates 5.3. Gathering Candidates
The process for gathering candidates at the answerer is identical to The process for gathering candidates at the answerer is identical to
the process for the offerer as described in Section 4.1.1 for full the process for the offerer as described in Section 4.1.1 for full
implementations and Section 4.2 for lite implementations. It is implementations and Section 4.2 for lite implementations. It is
RECOMMENDED that this process begin immediately on receipt of the RECOMMENDED that this process begin immediately on receipt of the
offer, prior to user acceptance of a session. Such gathering MAY offer, prior to alerting the user. Such gathering MAY begin when an
even be done pre-emptively when an agent starts. agent starts.
5.4. Prioritizing Candidates 5.4. Prioritizing Candidates
The process for prioritizing candidates at the answerer is identical The process for prioritizing candidates at the answerer is identical
to the process followed by the offerer, as described in Section 4.1.2 to the process followed by the offerer, as described in Section 4.1.2
for full implementations and Section 4.2 for lite implementations. for full implementations and Section 4.2 for lite implementations.
5.5. Choosing In Use Candidates 5.5. Choosing Default Candidates
The process for selecting in-use candidates at the answerer is The process for selecting default candidates at the answerer is
identical to the process followed by the offerer, as described in identical to the process followed by the offerer, as described in
Section 4.1.3 for full implementations and Section 4.2 for lite Section 4.1.3 for full implementations and Section 4.2 for lite
implementations. implementations.
5.6. Encoding the SDP 5.6. Encoding the SDP
The process for encoding the SDP at the answerer is identical to the The process for encoding the SDP at the answerer is identical to the
process followed by the offerer, as described in Section 4.3. process followed by the offerer, as described in Section 4.3.
5.7. Forming the Check Lists 5.7. Forming the Check Lists
Forming check lists is done only by full implementations. Lite Forming check lists is done only by full implementations. Lite
implementations MUST skip the steps defined in this section. implementations MUST skip the steps defined in this section.
There is one check list per in-use media stream resulting from the There is one check list per in-use media stream resulting from the
offer/answer exchange. A media stream is in-use as long as its port offer/answer exchange. To form the check list for a media stream,
is non-zero (which is used in RFC 3264 to reject a media stream). the agent forms candidate pairs, computes a candidate pair priority,
Consequently, a media stream is in-use even if it is marked as orders the pairs by priority, prunes them, and sets their states.
a=inactive or has a bandwidth value of zero. Each check list is a These steps are described in this section.
sequence of STUN connectivity checks that are performed by the agent.
To form the check list for a media stream, the agent forms candidate 5.7.1. Forming Candidate Pairs
pairs, computes a candidate pair priority, orders the pairs by
priority, prunes them, and sets their states. These steps are
described in this section.
First, the agent takes each of its candidates for a media stream First, the agent takes each of its candidates for a media stream
(called local candidates) and pairs them with the candidates it (called LOCAL CANDIDATES) and pairs them with the candidates it
received from its peer (called remote candidates) for that media received from its peer (called REMOTE CANDIDATES) for that media
stream. A local candidate is paired with a remote candidate if and stream. In order to prevent the attacks described in Section 17.4.2,
only if the two candidates have the same component ID and have the agents MAY limit the number of candidates they'll accept in an offer
same IP address version. It is possible that some of the local or answer. A local candidate is paired with a remote candidate if
and only if the two candidates have the same component ID and have
the same IP address version. It is possible that some of the local
candidates don't get paired with a remote candidate, and some of the candidates don't get paired with a remote candidate, and some of the
remote candidates don't get paired with local candidates. This can remote candidates don't get paired with local candidates. This can
happen if one agent didn't include candidates for the all of the happen if one agent didn't include candidates for the all of the
components for a media stream. In the case of RTP, for example, this components for a media stream. If this happens, the number of
would happen when one agent provided candidates for RTCP, and the components for that media stream is effectively reduced, and
other did not. If this happens, the number of components for that considered to be equal to the minimum across both agents of the
media stream is effectively reduced, and considered to be equal to maximum component ID provided by each agent across all components for
the minimum across both agents of the maximum component ID provided the media stream.
by each agent across all components for the media stream.
In the case of RTP, this would happen when one agent provided
candidates for RTCP, and the other did not. As another example, the
offerer can multiplex RTP and RTCP on the same port and signals it
can do that in the SDP through some new attribute. However, since
the offerer doesn't know if the answerer can perform such
multiplexing, the offerer includes candidates for RTP and RTCP on
separate ports, so that the offer has two components per media
stream. If the answerer can perform such multiplexing, it would
include just a single component for each candidate - for the combined
RTP/RTCP mux. ICE would end up acting as if there was just a single
component for this candidate.
The candidate pairs whose local and remote candidates were both the
default candidates for a particular component is called,
unsurprisingly, the default candidate pair for that component. This
is the pair that would be used to transmit media if both agents had
not been ICE aware.
In order to aid understanding, Figure 8 shows the relationships
between several key concepts - transport addresses, candidates,
candidate pairs, and check lists, in addition to indicating the main
properties of candidates and candidate pairs.
+------------------------------------------+
| |
| +---------------------+ |
| |+----+ +----+ +----+ | +Type |
| || IP | |Port| |Tran| | +Priority |
| ||Addr| | | | | | +Foundation |
| |+----+ +----+ +----+ | +ComponentiD |
| | Transport | +RelatedAddr |
| | Addr | |
| +---------------------+ +Base |
| Candidate |
+------------------------------------------+
* *
* *************************************
* *
+-------------------------------+
.| |
| Local Remote |
| +----+ +----+ +default? |
| |Cand| |Cand| +valid? |
| +----+ +----+ +nominated?|
| +State |
| |
| |
| Candidate Pair |
+-------------------------------+
* *
* ************
* *
+------------------+
| Candidate Pair |
+------------------+
+------------------+
| Candidate Pair |
+------------------+
+------------------+
| Candidate Pair |
+------------------+
Check
List
Figure 8: Conceptual Diagram of a Check List
5.7.2. Computing Pair Priority and Ordering Pairs
Once the pairs are formed, a candidate pair priority is computed. Once the pairs are formed, a candidate pair priority is computed.
Let O-P be the priority for the candidate provided by the offerer. Let O be the priority for the candidate provided by the offerer. Let
Let A-P be the priority for the candidate provided by the answerer. A be the priority for the candidate provided by the answerer. The
The priority for a pair is computed as: priority for a pair is computed as:
pair priority = 2^32*MIN(O-P,A-P) + 2*MAX(O-P,A-P) + (O-P>A-P?1:0) pair priority = 2^32*MIN(O,A) + 2*MAX(O,A) + (O>A?1:0)
Where O-P>A-P?1:0 is an expression whose value is 1 if O-P is greater Where O-P>A-P?1:0 is an expression whose value is 1 if O-P is greater
than A-P, and 0 otherwise. This formula ensures a unique priority than A-P, and 0 otherwise. This formula ensures a unique priority
for each pair in most cases. One the priority is assigned, the agent for each pair in most cases. Once the priority is assigned, the
sorts the candidate pairs in decreasing order of priority. If two agent sorts the candidate pairs in decreasing order of priority. If
pairs have identical priority, the ordering amongst them is two pairs have identical priority, the ordering amongst them is
arbitrary. arbitrary.
5.7.3. Pruning the Pairs
This sorted list of candidate pairs is used to determine a sequence This sorted list of candidate pairs is used to determine a sequence
of connectivity checks that will be performed. Each check involves of connectivity checks that will be performed. Each check involves
sending a request from a local candidate to a remote candidate. sending a request from a local candidate to a remote candidate.
Since an agent cannot send requests directly from a reflexive Since an agent cannot send requests directly from a reflexive
candidate, but only from its base, the agent next goes through the candidate, but only from its base, the agent next goes through the
sorted list of candidate pairs. For each pair where the local sorted list of candidate pairs. For each pair where the local
candidate is server reflexive, the server reflexive candidate MUST be candidate is server reflexive, the server reflexive candidate MUST be
replaced by its base. Once this has been done, the agent MUST remove replaced by its base. Once this has been done, the agent MUST prune
redundant pairs. A pair is redundant if its local and remote the list. This is done by removing a pair if its local and remote
candidates are identical to the local and remote candidates of a pair candidates are identical to the local and remote candidates of a pair
higher up on the priority list. The result is called the check list higher up on the priority list. The result is a sequence of ordered
for that media stream, and each candidate pair on it is called a candidate pairs, called the check list for that media stream.
check.
Each check is also said to have a foundation, which is merely the In addition, in order to limit the attacks described in
combination of the foundations of the local and remote candidates in Section 17.4.2, an agent SHOULD limit the total number of
the check. connectivity checks they perform across all check lists to 100, by
discarding the lower priority candidate pairs until there are less
than 100.
Each check in the check list is associated with a state. This state 5.7.4. Computing States
is assigned once the check list for each media stream has been
computed. There are five potential values that the state can have:
Waiting: This check has not been performed, and can be performed as Each candidate pair in the check list has a foundation and a state.
soon as it is the highest priority Waiting check on the check The foundation is the combination of the foundations of the local and
list. remote candidates in the pair. The state is assigned once the check
list for each media stream has been computed. There are five
potential values that the state can have:
In-Progress: A request has been sent for this check, but the Waiting: A check has not been performed for this pair, and can be
performed as soon as it is the highest priority Waiting pair on
the check list.
In-Progress: A check has been sent for this pair, but the
transaction is in progress. transaction is in progress.
Succeeded: This check was already done and produced a successful Succeeded: A check for this pair was already done and produced a
result. successful result.
Failed: This check was already done and failed, either never Failed: A check for this pair was already done and failed, either
producing any response or producing an unrecoverable failure never producing any response or producing an unrecoverable failure
response. response.
Frozen: This check hasn't been performed, and it can't yet be Frozen: A check for this pair hasn't been performed, and it can't
performed until some other check succeeds, allowing it to move yet be performed until some other check succeeds, allowing this
into the Waiting state. pair to unfreeze and move into the Waiting state.
First, the agent sets all of the checks in each check list to the As ICE runs, the pairs will move between states as shown in Figure 9.
Frozen state. Then, it takes the first check in the check list for
the first media stream (a media stream is the first media stream when +-----------+
it is described by the first m-line in the SDP offer and answer), and | |
sets its state to Waiting. It then finds all of the other checks in | |
that check list with the same component ID, but different | Frozen |
foundations, and sets all of their states to Waiting as well. Once | |
this is done, one of the check lists will have some number of checks | |
in the Waiting state, and the other check lists will have all of +-----------+
their checks in the Frozen state. A check list with at least one |
check that is not Frozen is called an active check list. |unfreeze
|
V
+-----------+ +-----------+
| | | |
| | perform | |
| Waiting |-------->|In-Progress|
| | | |
| | | |
+-----------+ +-----------+
/ |
// |
// |
// |
/ |
// |
failure // |success
// |
/ |
// |
// |
// |
V V
+-----------+ +-----------+
| | | |
| | | |
| Failed | | Succeeded |
| | | |
| | | |
+-----------+ +-----------+
Figure 9: Pair State FSM
The initial states for each pair in the check list are computed by
performing the following sequence of steps:
1. The agent sets all of the pairs in each check list to the Frozen
state.
2. It takes the first pair in the check list for the first media
stream (a media stream is the first media stream when it is
described by the first m-line in the SDP offer and answer), and
sets its state to Waiting.
3. It finds all of the other pairs in that check list with the same
component ID, but different foundations, and sets all of their
states to Waiting as well.
One of the check lists will have some number of pairs in the Waiting
state, and the other check lists will have all of their pairs in the
Frozen state. A check list with at least one pair that is not Frozen
is called an active check list.
The check list itself is associated with a state, which captures the The check list itself is associated with a state, which captures the
state of ICE checks for that media stream. There are two states: state of ICE checks for that media stream. There are two states:
Running: In this state, ICE checks are still in progress for this Running: In this state, ICE checks are still in progress for this
media stream. media stream.
Completed: In this state, the controlling agent has signaled that a Completed: In this state, ICE checks have completed for this media
candidate pair has been selected for each component. stream.
Consequently, no further ICE checks are performed.
When a check list is first constructed as the consequence of an When a check list is first constructed as the consequence of an
offer/answer exchange, it is placed in the Running state. offer/answer exchange, it is placed in the Running state.
ICE processing across all media streams also has a state associated ICE processing across all media streams also has a state associated
with it. This state is equal to Running while checks are in with it. This state is equal to Running while checks are in
progress. The state is Completed when all checks have been progress. The state is Completed when all checks have been
completed. Rules for transitioning between states are described completed. Rules for transitioning between states are described
below. below.
5.8. Performing Periodic Checks 5.8. Performing Periodic Checks
Checks are generated only by full implementations. Lite Checks are generated only by full implementations. Lite
implementations MUST skip the steps described in this section. implementations MUST skip the steps described in this section.
An agent performs two types of checks. The first type are periodic An agent performs periodic checks and triggered checks. Periodic
checks. These checks occur periodically for each media stream, and checks occur periodically for each media stream, and involve choosing
involve choosing the highest priority check in the Waiting state from the highest priority pair in the Waiting state from each check list,
each check list, and performing it. The other type of check is and sending a check on it. Triggered checks are performed on receipt
called a triggered check. This is a check that is performed on of a connectivity check from the peer (see Section 7.2.1.3). This
receipt of a connectivity check from the peer. This section section describes how periodic checks are performed.
describes how periodic checks are performed.
Once the agent has computed the check lists as described in Once the agent has computed the check lists as described in
Section 5.7, it sets a timer for each active check list. The timer Section 5.7, it sets a timer for each active check list. The timer
fires every Ta/N seconds, where N is the number of active check lists fires every Ta/N seconds, where N is the number of active check lists
(initially, there is only one active check list). Implementations (initially, there is only one active check list). Implementations
MAY set the timer to fire less frequently than this. Ta is the same MAY set the timer to fire less frequently than this. Ta is the same
value used to pace the gathering of candidates, as described in value used to pace the gathering of candidates, as described in
Section 4.1.1. The first timer for each active check list fires Section 4.1.1. Dividing by N allows this aggregate check throughput
immediately, so that the agent performs a connectivity check the to be split between all active check lists. The first timer for each
moment the offer/answer exchange has been done, followed by the next active check list fires immediately, so that the agent performs a
periodic check Ta seconds later. connectivity check the moment the offer/answer exchange has been
done, followed by the next periodic check Ta seconds later.
When the timer fires, the agent MUST find the highest priority check When the timer fires, the agent MUST:
in that check list that is in the Waiting state. The agent then
sends a STUN check from the local candidate of that check to the
remote candidate of that check. The procedures for forming the STUN
request for this purpose are described in Section 7.1.1. If none of
the checks in that check list are in the Waiting state, but there are
checks in the Frozen state, the highest priority check in the Frozen
state is moved into the Waiting state, and that check is performed.
When a check is performed, its state is set to In-Progress. If there
are no checks in either the Waiting or Frozen state, then the timer
for that check list is stopped.
Performing the connectivity check requires the agent to know the o Find the highest priority pair in that check list that is in the
username fragment for the local and remote candidates, and the Waiting state.
password for the remote candidate. For periodic checks, the remote
username fragment and password are learned directly from the SDP o If there is such a pair:
received from the peer, and the local username fragment is known by
the agent. * Send a STUN check from the local candidate of that pair to the
remote candidate of that pair. The procedures for forming the
STUN request for this purpose are described in Section 7.1.1.
o If there is no such pair:
* Find the highest priority pair in that check list that is in
the Frozen state.
* If there is such a pair:
+ Unfreeze the pair.
+ Perform a check for that pair, causing its state to
transition to In-Progress.
* If there is no such pair:
+ Set the state of the check list to Completed.
+ Terminate the timer for that check list.
To compute the message integrity for the check, the agent uses the
remote username fragment and password learned from the SDP from its
peer. The local username fragment is known directly by the agent for
its own candidate.
6. Receipt of the Initial Answer 6. Receipt of the Initial Answer
This section describes the procedures that an agent follows when it This section describes the procedures that an agent follows when it
receives the answer from the peer. It verifies that its peer receives the answer from the peer. It verifies that its peer
supports ICE, determines its role, and for full implementations, supports ICE, determines its role, and for full implementations,
forms the check list and begins performing periodic checks. forms the check list and begins performing periodic checks.
6.1. Verifying ICE Support 6.1. Verifying ICE Support
The answerer will proceed with the ICE procedures defined in this The offerer will proceed with the ICE procedures defined in this
specification if there is at least one a=candidate attribute for each specification if there is at least one a=candidate attribute for each
media stream in the answer it just received. If this condition is media stream in the answer it just received. If this condition is
not met, the agent MUST process the SDP based on normal RFC 3264 not met, the agent MUST process the SDP based on normal RFC 3264
procedures, without using any of the ICE mechanisms described in the procedures, without using any of the ICE mechanisms described in the
remainder of this specification, with the exception of Section 10, remainder of this specification, with the exception of Section 10,
which describes keepalive procedures. which describes keepalive procedures.
In some cases, the answer may omit a=candidate attributes for the In some cases, the answer may omit a=candidate attributes for the
media streams, and instead include an a=ice-mismatch attribute for media streams, and instead include an a=ice-mismatch attribute for
one or more of the media streams in the SDP. This signals to the one or more of the media streams in the SDP. This signals to the
offerer that the answerer supports ICE, but that ICE processing was offerer that the answerer supports ICE, but that ICE processing was
not used for the session because an intermediary modified the m/c- not used for the session because an intermediary modified the default
lines without modifying the candidate attributes. See Section 16 for destination for media components without modifying the corresponding
a discussion of cases where this can happen. This specification candidate attributes. See Section 17 for a discussion of cases where
provides no guidance on how an agent should proceed in such a failure this can happen. This specification provides no guidance on how an
case. agent should proceed in such a failure case.
6.2. Determining Role 6.2. Determining Role
The offerer follows the same procedures described for the answerer in The offerer follows the same procedures described for the answerer in
Section 5.2. Section 5.2.
6.3. Forming the Check List 6.3. Forming the Check List
Formation of check lists is performed only by full implementations. Formation of check lists is performed only by full implementations.
The offerer follows the same procedures described for the answerer in The offerer follows the same procedures described for the answerer in
Section 5.7. Section 5.7.
6.4. Performing Periodic Checks 6.4. Performing Periodic Checks
Periodic checks are performed only by full implementations. The Periodic checks are performed only by full implementations. The
offerer follows the same procedures described for the answerer in offerer follows the same procedures described for the answerer in
Section 5.8. Section 5.8.
7. Connectivity Checks 7. Performing Connectivity Checks
This section describes how connectivity checks are performed. All This section describes how connectivity checks are performed. All
ICE implementations are required to be compliant to [11], as opposed ICE implementations are required to be compliant to [12], as opposed
to the older [14]. However, whereas a full implementation will both to the older [15]. However, whereas a full implementation will both
generate checks (acting as a STUN client) and receive them (acting as generate checks (acting as a STUN client) and receive them (acting as
a STUN server), a lite implementation will only ever receive checks, a STUN server), a lite implementation will only ever receive checks,
and thus will only act as a STUN server. and thus will only act as a STUN server.
7.1. Client Procedures 7.1. Client Procedures
These procedures define how an agent sends a connectivity check, These procedures define how an agent sends a connectivity check,
whether it is a periodic or a triggered check. These procedures are whether it is a periodic or a triggered check. These procedures are
only applicable to full implementations. only applicable to full implementations.
7.1.1. Sending the Request 7.1.1. Sending the Request
The agent acting as the client generates a connectivity check either The check is generated by sending a Binding Request from a local
periodically, or triggered. In either case, the check is generated candidate, to a remote candidate. [12] describes how Binding Requests
by sending a Binding Request from a local candidate, to a remote are constructed and generated. This section defines additional
candidate. The agent must know the username fragment for both procedures involving the PRIORITY and USE-CANDIDATE attributes,
candidates and the password for the remote candidate. defined for the connectivity check usage, and details how credentials
for message integrity and diffserv markings are computed.
A Binding Request serving as a connectivity check MUST utilize a STUN 7.1.1.1. PRIORITY and USE-CANDIDATE
short term credential. Rather than being learned from a Shared
Secret request, the short term credential is exchanged in the offer/
answer procedures. In particular, the username is formed by
concatenating the username fragment provided by the peer with the
username fragment of the agent sending the request, separated by a
colon (":"). The password is equal to the password provided by the
peer. For example, consider the case where agent A is the offerer,
and agent B is the answerer. Agent A included a username fragment of
AFRAG for its candidates, and a password of APASS. Agent B provided
a username fragment of BFRAG and a password of BPASS. A connectivity
check from A to B (and its response of course) utilize the username
BFRAG:AFRAG and a password of BPASS. A connectivity check from B to
A (and its response) utilize the username AFRAG:BFRAG and a password
of APASS.
An agent MUST include the PRIORITY attribute in its Binding Request. An agent MUST include the PRIORITY attribute in its Binding Request.
The attribute MUST be set equal to the priority that would be The attribute MUST be set equal to the priority that would be
assigned, based on the algorithm in Section 4.1.2, to a peer assigned, based on the algorithm in Section 4.1.2, to a peer
reflexive candidate learned from this check. Such a peer reflexive reflexive candidate, should one be learned as a consequence of this
candidate has a stream ID, component ID and local preference that are check (see Section 7.1.2.2.1 for how peer reflexive candidates are
equal to the host candidate from which the check is being sent, but a learned). This priority value will be computed identically to how
type preference equal to the value associated with peer reflexive the priority for the local candidate of the pair was computed, except
candidates. that the type preference is set to the value for peer derived
candidate types.
The Binding Request sent by an agent MUST include the USERNAME and
MESSAGE-INTEGRITY attributes. That is, an agent MUST NOT wait to be
challenged for short term credentials. Rather, it MUST provide them
in the Binding Request right away.
The controlling agent MAY include the USE-CANDIDATE attribute in the The controlling agent MAY include the USE-CANDIDATE attribute in the
Binding Request. The controlled agent MUST NOT include it in its Binding Request. The controlled agent MUST NOT include it in its
Binding Request. This attribute signals that the controlling agent Binding Request. This attribute signals that the controlling agent
wishes to cease checks for this component, and use the candidate pair wishes to cease checks for this component, and use the candidate pair
resulting from the check for this component. Section 8 provides resulting from the check for this component. Section 8.1 provides
guidance on determining when to include it. guidance on determining when to include it.
If the agent is using Diffserv Codepoint markings [26] in its media 7.1.1.2. Forming Credentials
A Binding Request serving as a connectivity check MUST utilize a STUN
short term credential. The agent MUST include the USERNAME and
MESSAGE-INTEGRITY attributes. An agent MUST NOT wait to be
challenged for short term credentials. Rather, it MUST provide them
in each Binding Request.
Rather than being learned from a Shared Secret request, the short
term credential is exchanged in the offer/answer procedures. In
particular, the username is formed by concatenating the username
fragment provided by the peer with the username fragment of the agent
sending the request, separated by a colon (":"). The password is
equal to the password provided by the peer. For example, consider
the case where agent L is the offerer, and agent R is the answerer.
Agent L included a username fragment of LFRAG for its candidates, and
a password of LPASS. Agent R provided a username fragment of RFRAG
and a password of RPASS. A connectivity check from L to R (and its
response of course) utilize the username RFRAG:LFRAG and a password
of RPASS. A connectivity check from R to L (and its response)
utilize the username LFRAG:RFRAG and a password of LPASS.
7.1.1.3. DiffServ Treatment
If the agent is using Diffserv Codepoint markings [27] in its media
packets, it SHOULD apply those same markings to its connectivity packets, it SHOULD apply those same markings to its connectivity
checks. checks.
7.1.2. Processing the Response 7.1.2. Processing the Response
If the STUN transaction generates an unrecoverable failure response When a Binding Response is received, it is correlated to its Binding
or times out, the agent sets the state of the check to Failed. The Request using the transaction ID, as defined in [12], which then ties
remainder of this section applies to processing of successful it to the candidate pair for which the Binding Request was sent.
responses (any response from 200 to 299).
7.1.2.1. Failure Cases
If the STUN transaction generates an ICMP error, or generates a STUN
error response that is unrecoverable (as defined in [12], or times
out, the agent sets the state of the pair to Failed.
The agent MUST check that the source IP address and port of the The agent MUST check that the source IP address and port of the
response equals the destination IP address and port that the Binding response equals the destination IP address and port that the Binding
Request was sent to, and that the destination IP address and port of Request was sent to, and that the destination IP address and port of
the response match the source IP address and port that the Binding the response match the source IP address and port that the Binding
Request was sent from. If these do not match, the processing Request was sent from. In other words, the source and destination
described in the remainder of this section MUST NOT be performed. In transport addresses in the request and responses are the symmetric.
addition, an agent sets the state of the check to Failed. If they are not symmetric, the agent sets the state of the pair to
Failed.
If the check succeeds, processing continues. The agent creates a 7.1.2.2. Success Cases
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 validated pair,
since it has been validated by a STUN connectivity check. It is very
important to note that this validated pair will often not be
identical to the check itself; in many cases, the local candidate
(learned through the mapped address in the response) will be
different than the local candidate the request was sent from.
Next, the agent computes the priority for the pair based on the If the STUN transaction generated a response between 200 and 299, and
priority of each candidate, using the algorithm in Section 5.7. The the source IP address and port of the response equals the destination
priority of the local candidate depends on its type. If it is not IP address and port that the Binding Request was sent to, and the
peer reflexive, it is equal to the priority signaled for that destination IP address and port of the response match the source IP
candidate in the SDP. If it is peer reflexive, it is equal to the address and port that the Binding Request was sent from, the check
PRIORITY attribute the agent placed in the Binding Request which just was a success.
completed. The priority of the remote candidate is taken from the
SDP of the peer. If the candidate does not appear there, then the
check must have been a triggered check to a new remote candidate. In
that case, the priority is taken as the value of the PRIORITY
attribute in the Binding Request which triggered the check that just
completed.
Once the priority of the candidate pair has been computed, the pair 7.1.2.2.1. Discovering Peer Reflexive Candidates
is added to the valid list for that media stream. If the agent was a
controlling agent, and the check had included a USE-CANDIDATE
attribute, the candidate pair is marked as "favored". If the agent
was a controlled agent, and the check was a triggered check, and the
request which caused the triggered check included the USE-CANDIDATE
attribute, the candidate pair is marked as "favored".
Next, the agent updates its ICE states. The agent checks the mapped The agent checks the mapped address from the STUN response. If the
address from the STUN response. If the transport address does not transport address does not match any of the local candidates that the
match any of the local candidates that the agent knows about, the agent knows about, the mapped address represents a new candidate - a
mapped address represents a new peer reflexive candidate. Its type peer reflexive candidate. Like other candidates, it has a type,
is equal to peer reflexive. Its base is set equal to the candidate base, priority and foundation. They are computed as follows:
from which the STUN check was sent. Its username fragment and
password are identical to the candidate from which the check was
sent. It is assigned the priority value that was placed in the
PRIORITY attribute of the request. Its foundation is selected as
described in Section 4.1.1. The peer reflexive candidate is then
added to the list of local candidates known by the agent (though it
is not paired with other remote candidates at this time).
Next, the agent changes the state for this check to Succeeded. The o Its type is equal to peer reflexive.
agent sees if the success of this check can cause other checks to be
unfrozen. If the check had a component ID of one, the agent MUST o Its base is set equal to the local candidate of the candidate pair
change the states for all other Frozen checks for the same media from which the STUN check was sent.
stream and same foundation, but different component IDs, to Waiting.
If the component ID for the check was equal to the number of o Its priority is set equal to the value of the PRIORITY attribute
components for the media stream (where this is the actual number of in the Binding Request.
o Its foundation is selected as described in Section 4.1.1.
This peer reflexive candidate is then added to the list of local
candidates for the media stream. Its username fragment and password
are the same as all other local candidates for that media stream.
However, the peer reflexive candidate is not paired with other remote
candidates. This is not necessary; a valid pair will be generated
from it momentarily based on the procedures in Section 7.1.2.2.3. If
an agent wishes to pair the peer reflexive candidate with other
remote candidates besides the one in the valid pair that will be
generated, the agent MAY generate an updated offer which includes the
peer reflexive candidate. This will cause it to be paired with all
other remote candidates.
7.1.2.2.2. Updating Pair States
The agent sets the state of the pair that generated the check to
succeeded. The agent sees if the success for this pair can cause
other pairs to be unfrozen. There are three cases:
o If the pair had a component ID of 1, the agent MUST change the
states for all other Frozen pairs for the same media stream and
same foundation, but different component IDs, to Waiting.
o If the pair had a component ID equal to the number of components
for the media stream (where this is the actual number of
components being used, in cases where the number of components components being used, in cases where the number of components
signaled in the SDP differs from offerer to answerer), the agent MUST signaled in the SDP differs from offerer to answerer), the agent
change the state for all other Frozen checks for the first component MUST change the state for all other Frozen pairs for the first
of different media streams (and thus in different check lists) but component of different media streams (and thus in different check
the same foundation, to Waiting. lists) but the same foundation, to Waiting.
o If the pair has any other component ID, no other pairs can be
unfrozen.
7.1.2.2.3. Constructing a Valid Pair
Next, the agent constructs a candidate pair whose local candidate
equals the mapped address of the response, and whose remote candidate
equals the destination address to which the request was sent. This
is called a valid pair, since it has been validated by a STUN
connectivity check. The valid pair may equal the pair that generated
the check, may equal a different pair in the check list, or may be a
pair not currently on any check list. If the pair equals the pair
that generated the check or is on a check list currently, it is also
added to the VALID LIST, which is maintained by the agent for each
media stream. This list is empty at the start of ICE processing, and
fills as checks are performed, resulting in valid candidate pairs.
It will be very common that the pair will not be on any check list.
Recall that the check list has pairs whose local candidates are never
server reflexive; those pairs had their local candidates converted to
the base of the server reflexive candidates, and then pruned if they
were redundant. When the response to the STUN check arrives, the
mapped address will be reflexive if there is a NAT between the two.
In that case, the valid pair will have a local candidate that doesn't
match any of the pairs in the check list.
If the pair is not on any check list, the agent computes the priority
for the pair based on the priority of each candidate, using the
algorithm in Section 5.7. The priority of the local candidate
depends on its type. If it is not peer reflexive, it is equal to the
priority signaled for that candidate in the SDP. If it is peer
reflexive, it is equal to the PRIORITY attribute the agent placed in
the Binding Request which just completed. The priority of the remote
candidate is taken from the SDP of the peer. If the candidate does
not appear there, then the check must have been a triggered check to
a new remote candidate. In that case, the priority is taken as the
value of the PRIORITY attribute in the Binding Request which
triggered the check that just completed. The pair is then added to
the VALID LIST.
7.1.2.2.4. Updating the Nominated Flag
If the agent was a controlling agent, and it had included a USE-
CANDIDATE attribute in the Binding Request, the valid pair generated
from that check has its nominated flag set to true. This flag
indicates that this candidate should be used for media if it is the
highest priority one amongst those whose nominated flag is set. This
may conclude ICE processing for this media stream or all media
streams; see Section 8.
If the agent is the controlled agent, the response may result in the
valid pair having its nominated flag set. See Section 7.2.1.4 for
the procedure.
7.2. Server Procedures 7.2. Server Procedures
An agent MUST be prepared to receive a Binding Request on the base of An agent MUST be prepared to receive a Binding Request on the base of
each candidate it included in its most recent offer or answer. each candidate it included in its most recent offer or answer.
Receipt of a Binding Request on a transport address that the agent Receipt of a Binding Request on a base is an indication that the
had included in a candidate attribute is an indication that the
connectivity check usage applies to the request. connectivity check usage applies to the request.
The agent MUST use a short term credential to authenticate the The agent MUST use a short term credential to authenticate the
request and perform a message integrity check. The agent MUST accept request and perform a message integrity check. The agent MUST accept
a credential if the username consists of two values separated by a a credential if the username consists of two values separated by a
colon, where the first value is equal to the username fragment colon, where the first value is equal to the username fragment
generated by the agent in an offer or answer for a session in- generated by the agent in an offer or answer for a session in-
progress, and the password is equal to the password for that username progress, and the MESSAGE-INTEGRITY is the output of a hash of the
fragment. It is possible (and in fact very likely) that an offeror password and the STUN packet's contents. It is possible (and in fact
will receive a Binding Request prior to receiving the answer from its very likely) that an offeror will receive a Binding Request prior to
peer. However, the request can be processed without receiving this receiving the answer from its peer. However, the request can be
answer, and a response generated. processed without receiving this answer, and a response generated.
By doing this, ICE processing completes faster.
If the agent is using Diffserv Codepoint markings [26] in its media If the agent is using Diffserv Codepoint markings [27] in its media
packets, it SHOULD apply those same markings to its responses to packets, it SHOULD apply those same markings to its responses to
Binding Requests. Binding Requests.
7.2.1. Additional Procedures for Full Implementations 7.2.1. Additional Procedures for Full Implementations
This subsection defines the additional server procedures applicable This subsection defines the additional server procedures applicable
to full implementations. to full implementations when generating a successful response to a
Binding Request.
7.2.1.1. Computing Mapped Address
For requests being received on a relayed candidate, the source For requests being received on a relayed candidate, the source
transport address used for STUN processing (namely, generation of the transport address used for STUN processing (namely, generation of the
XOR-MAPPED-ADDRESS attribute) is the transport address as seen by the XOR-MAPPED-ADDRESS attribute) is the transport address as seen by the
relay. That source transport address will be present in the REMOTE- relay. That source transport address will be present in the REMOTE-
ADDRESS attribute of a STUN Data Indication message, if the Binding ADDRESS attribute of a STUN Data Indication message, if the Binding
Request was delivered through a Data Indication. If the Binding Request was delivered through a Data Indication (a STUN relay
Request was not encapsulated in a Data Indication, that source delivers packets encapsulated in a Data Indication when no active
address is equal to the current active destination for the STUN relay destination is set). If the Binding Request was not encapsulated in
session. a Data Indication, that source address is equal to the current active
destination for the STUN relay session.
If the STUN request resulted in an error response, no further 7.2.1.2. Learning Peer Reflexive Candidates
processing is performed.
Assuming a success response, if the source transport address of the If the source transport address of the request does not match any
request does not match any existing remote candidates, it represents existing remote candidates, it represents a new peer reflexive remote
a new peer reflexive remote candidate. The full-mode agent gives the candidate. The full-mode agent gives the candidate a priority equal
candidate a priority equal to the PRIORITY attribute from the to the PRIORITY attribute from the request. The type of the
request. The type of the candidate is equal to peer reflexive. Its candidate is equal to peer reflexive. Its foundation is set to an
foundation is set to an arbitrary value, different from the arbitrary value, different from the foundation for all other remote
foundation for all other remote candidates. Note that any subsequent candidates. If any subsequent offer/answer exchanges contain this
offer/answer exchanges will contain this new peer reflexive candidate peer reflexive candidate in the SDP, it will signal the actual
in the SDP, and will signal the actual foundation for the candidate. foundation for the candidate. This candidate is then added to the
This candidate is then added to the list of remote candidates. list of remote candidates. However, the agent does not pair this
However, the agent does not pair this candidate with any local candidate with any local candidates.
candidates.
Next, the agent constructs a tentative check in the reverse 7.2.1.3. Triggered Checks
direction, called a triggered check. The triggered check has a local
candidate equal to the candidate 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 a new peer-
reflexive remote candidate). Since both candidates are known to the
agent, it can obtain their priorities and compute the candidate pair
priority. This tentative check is then looked up in the check list.
There can be one of several outcomes:
o If there is already a check on the check list with this same local Next, the agent constructs a pair whose local candidate is equal to
and remote candidates, and the state of that check is Waiting or the transport address on which the STUN request was received, and a
Frozen, its state is changed to In-Progress and the tentative remote candidate equal to the source transport address where the
check is performed. request came from (which may be peer-reflexive remote candidate that
was just learned). Since both candidates are known to the agent, it
can obtain their priorities and compute the candidate pair priority.
This pair is then looked up in the check list. There can be one of
several outcomes:
o If there is already a check on the check list with this same local o If the pair is already on the check list:
and remote candidates, and its state was In-Progress, the agent
SHOULD abandon the new tentative check and instead generate an
immediate retransmit of the Binding Request for the check in
progress. This is to facilitate rapid completion of ICE when both
agents are behind NAT.
o If there is already a check on the check list with this same local * If the state of that pair is Waiting or Frozen, its state is
and remote candidates, and its state was Succeeded, the new changed to In-Progress and a check for that pair is performed
tentative check is abandoned. If the Binding Request just immediately. This is called a triggered check.
received contained the USE-CANDIDATE attribute, it means that the
pair resulting from that previous check is favored by the peer
controlling agent. The agent MUST take the candidate pair in the
valid list that was learned from that previous successful check,
and mark it as favored.
o If there is already a check on the check list with this same local * If the state of that pair is In-Progress, the agent SHOULD
and remote candidates, and its state was Failed, the new tentative generate an immediate retransmit of the Binding Request for the
check is abandoned. check in progress. This is to facilitate rapid completion of
ICE when both agents are behind NAT.
o If there is no matching check on the check list, the new tentative * If the state of that pair is Failed or Succeeded, no triggered
check is inserted into the check list based on its priority, and check is sent.
its state is set to In-Progress.
If the tentative check is to be performed, it is constructed and o If the pair is not already on the check list:
* The pair is inserted into the check list based on its priority
* Its state is set to In-Progress
* A triggered check for that pair is performed immediately.
If a triggered check is to be generated, it is constructed and
processed as described in Section 7.1.1. These procedures require processed as described in Section 7.1.1. These procedures require
the agent to know the username fragment and password for the peer. the agent to know the transport address, username fragment and
They are readily determined from the SDP and from the check that was password for the peer. The username fragment for the remote
just received. The username fragment for the remote candidate is candidate is equal to the part after the colon of the USERNAME in the
equal to the bottom half (the part after the colon) of the USERNAME Binding Request that was just received. Using that username
in the Binding Request that was just received. Using that username
fragment, the agent can check the SDP messages received from its peer fragment, the agent can check the SDP messages received from its peer
(there may be more than one in cases of forking), and find this (there may be more than one in cases of forking), and find this
username fragment. The corresponding password is then selected. If username fragment. The corresponding password is then selected. If
agent has not yet received this SDP (a likely case for the offerer in agent has not yet received the username in an SDP (a likely case for
the initial offer/answer exchange), it MUST wait for the SDP to be the offerer in the initial offer/answer exchange), it MUST wait for
received, and then proceed with the triggered check. the SDP to be received (since it won't have its peer's ICE password
without it), and then proceed with the triggered check.
7.2.1.4. Updating the Nominated Flag
If the Binding Request received by the agent had the USE-CANDIDATE
attribute set, and the agent is in the controlled role, the agent
looks at the state of the pair computed in Section 7.2.1.3:
o If the state of this pair is succeeded, it means that the check
generated by this pair produced a successful response. This would
have caused the agent to construct a valid pair when that success
response was received (see Section 7.1.2.2.3). The agent now sets
the nominated flag in the valid pair to true. This may end ICE
processing for this media stream; see Section 8.
o If the state of this pair is In-Progress, if its check produces a
successful result, the resulting valid pair has its nominated flag
set when the response arrives. This may end ICE processing for
this media stream when it arrives; see Section 8.
7.2.2. Additional Procedures for Lite Implementations 7.2.2. Additional Procedures for Lite Implementations
If the check that was just received contained a USE-CANDIDATE If the check that was just received contained a USE-CANDIDATE
attribute, the agent constructs a candidate pair whose local attribute, the agent constructs a candidate pair whose local
candidate is equal to the transport address on which the request was candidate is equal to the transport address on which the request was
received, and whose remote candidate is equal to the source transport received, and whose remote candidate is equal to the source transport
address of the request that was received. This candidate pair is address of the request that was received. This candidate pair is
assigned an arbitrary priority, and placed into a list of valid assigned an arbitrary priority, and placed into a list of valid
candidates for that component of that media stream called the valid candidates pair for that component of that media stream, called the
list. In addition, it is marked as favored, since the peer agent has valid list. The agent sets the nominated flag for that pair to true.
indicated that it is to be used. ICE processing is considered ICE processing is considered complete for a media stream if the valid
complete for a media stream if the valid list contains a candidate list contains a candidate pair for each component.
pair for each component.
8. Concluding ICE 8. Concluding ICE Processing
The processing rules in this section apply only to full The processing rules in this section apply only to full
implementations. implementations. Concluding ICE involves nominating pairs by the
controlling agent and updating of state machinery
Concluding ICE involves selection of pairs by the controlling agent, 8.1. Nominating Pairs
updating of state machinery, and possibly the generation of an
updated offer by the controlling agent.
The controlling agent can use any algorithm it likes for deciding The controlling agent nominates pairs to be selected by ICE by using
when to select a candidate pair, called the favored pair, as the one one of two techniques: regular nomination or aggressive nomination.
that will be used for media. However, it MUST eventually include a If its peer has a lite implementation, an agent MUST use a regular
USE-CANDIDATE attribute in at least one successful check for each nomination algorithm. If its peer is using ICE options (present in
component of each media stream. an ice-options attribute from the peer) that the agent does not
understand, the agent MUST use a regular nomination algorithm. If
its peer is a full implementation and isn't using any ICE options or
is using ICE options understood by the agent, the agent MAY use
either the aggressive or the regular nomination algorithm. However,
the regular algorithm is RECOMMENDED since it provides greater
stability.
The most apparent way to utilize the USE-CANDIDATE attribute is to 8.1.1. Regular Nomination
run through a series of checks, each of which omit the flag. Once
one or more checks complete successfully for a component of a media
stream, the agent evaluates the choices based on some criteria, and
picks a candidate pair. The criteria for evaluation is a matter of
implementation and it allows for localized optimizations. The check
that yielded this pair is then repeated, this time with the USE-
CANDIDATE flag. This approach provides the most flexibility in terms
of algorithms, and also improves ICE's resilience to variations in
implementation (see Section 14. This approach is called
"introspective selection". The drawback of introspective selection
is that it is guaranteed to increase latencies because it requires an
additional check to be done.
An alternative is called "proactive selection". In this approach, With regular nomination, the agent lets some number of checks
the controlling agent includes the USE-CANDIDATE attribute in every complete, each of which omit the USE-CANDIDATE attribute. Once one
check it sends. Once the first check for a component succeeds, it is or more checks complete successfully for a component of a media
used by ICE. In this mode, the agent will end up using the candidate stream, valid pairs are generated and added to the valid list. The
pair which is highest priority based on ICE's prioritization agent lets the checks continue until some stopping criteria is met,
algorithm, instead of some other local optimization. It is possible and then picks amongst the valid pairs based on an evaluation
with proactive selection that multiple checks might succeed with the criteria. The criteria for stopping the checks and for evaluating
flag set; this is why ICE still applies its prioritization algorithm the valid pairs is entirely a matter of local optimization.
to pick amongst those pairs that have been favored.
If an agent is controlling and its peer has a lite implementation, an When the controlling agent selects the valid pair, it repeats the
agent MUST use an introspective selection algorithm. Of course, it check that produced this valid pair, this time with the USE-CANDIDATE
MAY select a favored pair based on ICE's prioritization. The key attribute. This check will succeed (since the previous did), causing
requirement is that the agent must complete a successful check before the nominated flag of that and only that pair to be set.
redoing it with the USE-CANDIDATE attribute. Consequently, there will be only a single nominated pair in the valid
list, and when the state of the check list moves to completed, that
exact pair is selected by ICE for sending and receiving media.
For both controlling and controlled agents, once a candidate pair in Regular nomination provides the most flexibility, since the agent has
the Valid list is marked as favored, an agent MUST NOT generate any control over the stopping and selection criteria for checks. The
further periodic checks for that component of that media stream, and only requirement is that the agent MUST eventually pick one and only
SHOULD cease any retransmissions in progress for checks for that one candidate pair and generate a check for that pair with the USE-
component of that media stream. Once there is at least one candidate CANDIDATE attribute present. Regular nomination also improves ICE's
pair for each component of a media stream that is favored, a full- resilience to variations in implementation (see Section 14. Regular
mode agent MUST change the state of processing for its check list to nomination is also more stable, allowing both agents to converge on a
Completed. Once all of the check lists for the media streams enter single pair for media without any transient selections, which can
the Completed state, the controlling agent takes the highest priority happen with the aggressive algorithm. The drawback of regular
favored candidate pair for each component of each media stream. If nomination is that it is guaranteed to increase latencies because it
any of those candidate pairs differ from the in-use candidates in requires an additional check to be done.
m/c-lines of the most recent offer/answer exchange, the controlling
agent MUST generate an updated offer as described in Section 9. 8.1.2. Aggressive Nomination
With aggressive nomination, the controlling agent includes the USE-
CANDIDATE attribute in every check it sends. Once the first check
for a component succeeds, it will be added to the valid list, have
its nominated flag set, and then cause ICE processing to cease for
this check list. However, because the agent included the USE-
CANDIDATE attribute in all of its checks, another check may yet
complete, causing another valid pair to have its nominated flag set.
ICE always selects the highest priority nominated candidate pair from
the valid list as the one used for media. Consequently, the selected
pair may actually change briefly as ICE checks complete, resulting in
a set of transient selections until it stabilizes.
8.2. Updating States
For both controlling and controlled agents, the state of ICE
processing depends on the presence of nominated candidate pairs in
the valid list:
o If there are no nominated pairs in the valid list for a media
stream, ICE processing continues.
o If there is at least one nominated pair in the valid list:
* The agent MUST remove all Waiting and Frozen pairs in the check
list for the same component as the nominated pairs for that
media stream
* If an In-Progress pair in the check list is for the same
component as a nominated pair, the agent SHOULD cease
retransmissions for its check if its pair priority is lower
than the lowest priority nominated pair for that component
o Once there is at least one nominated pair in the valid list for
every component of at least one media stream:
* The agent MUST change the state of processing for its check
list for that media stream to Completed.
* The agent MUST continue to respond to any checks it may still
receive for that media stream, and MUST perform triggered
checks if required by the processing of Section 7.2.
* The agent MAY begin transmitting media for this media stream as
described in Section 11.1
o Once there is at least one nominated pair in the valid list for
each component of each media stream:
* The agent sets the state of ICE processing overall to
Completed.
* If an agent is controlling, it examines the highest priority
nominated candidate pair for each component of each media
stream. If any of those candidate pairs differ from the
default candidate pairs in the most recent offer/answer
exchange, the controlling agent MUST generate an updated offer
as described in Section 9. If the controlling agent is using
an aggressive nomination algorithm, this may result in several
updated offers as the pairs selected for media change. An
agent MAY delay sending the offer for a brief interval (one
second is RECOMMENDED) in order to allow the selected pairs to
stabilize.
9. Subsequent Offer/Answer Exchanges 9. Subsequent Offer/Answer Exchanges
An agent MAY generate a subsequent offer at any time. However, the Either agent MAY generate a subsequent offer at any time allowed by
rules in Section 8 will cause the controlling agent to send an RFC 3264 [4]. The rules in Section 8 will cause the controlling
updated offer at the conclusion of ICE processing when ICE has agent to send an updated offer at the conclusion of ICE processing
selected different candidate pairs from the in-use pairs. This when ICE has selected different candidate pairs from the default
section defines rules for construction of subsequent offers and pairs. This section defines rules for construction of subsequent
answers. offers and answers.
9.1. Generating the Offer 9.1. Generating the Offer
An agent MAY change the ice-pwd and/or ice-ufrag for a media stream 9.1.1. Procedures for All Implementations
in an offer. Doing so is a signal to restart ICE processing for that
media stream. When an agent restarts ICE for a media stream, it MUST 9.1.1.1. ICE Restarts
NOT include the a=remote-candidates attribute, since the state of the
media stream would not be Completed at this point. Note that it is An agent MAY restart ICE processing for an existing media stream. An
permissible to use a session-level attribute in one offer, but to ICE restart, as the name implies, will cause all previous state of
provide the same password as a media-level attribute in a subsequent ICE processing to be flushed and checks to start anew. The only
difference between an ICE restart and a brand new media session is
that, during the restart, media can continue to be sent to the
previously validated pair.
An agent MUST restart ICE for a media stream if:
o The offer is being generated for the purposes of changing the
target of the media stream. In other words, if an agent wants to
generated an updated offer which, had ICE not been in use, would
result in a new value for the destination of a media component.
o An agent is changing its implementation level. This typically
only happens in third party call control use cases, where the
entity performing the signaling is not the entity receiving the
media, and it has changed the target of media mid-session to
another entity that has a different ICE implementation.
These rules imply that setting the IP address in the c line to
0.0.0.0 will cause an ICE restart. Consequently, ICE implementations
MUST NOT utilize this mechanism for call hold, and instead MUST use
a=inactive and a=sendonly as described in [4]
To restart ICE, an agent MUST change both the ice-pwd and the ice-
ufrag for the media stream in an offer. Note that it is permissible
to use a session-level attribute in one offer, but to provide the
same ice-pwd or ice-ufrag as a media-level attribute in a subsequent
offer. This is not a change in password, just a change in its offer. This is not a change in password, just a change in its
representation. representation, and does not cause an ICE restart.
An agent MUST restart ICE processing if the offer is being generated An agent sets the rest of the fields in the SDP for this media stream
for the purposes of changing the target of the media stream. In as it would in an initial offer of this media stream (see
other words, if an agent wants to generated an updated offer which, Section 4.3). Consequently, the set of candidates MAY include some,
had ICE not been in use, would result in a new value for the none, or all of the previous candidates for that stream and MAY
transport address in the m/c-line, the agent MUST restart ICE for include a totally new set of candidates gathered as described in
that media stream. This implies that setting the IP address in the c Section 4.1.1.
line to 0.0.0.0 will cause an ICE restart. Consequently, ICE
implementations SHOULD NOT utilize this mechanism for call hold, and 9.1.1.2. Removing a Media Stream
instead use a=inactive as described in [4]
If an agent removes a media stream by setting its port to zero, it If an agent removes a media stream by setting its port to zero, it
MUST NOT include any candidate attributes for that media stream. MUST NOT include any candidate attributes for that media stream and
SHOULD NOT include any other ICE-related attributes defined in
Section 15 for that media stream.
An agent MUST NOT signal a change in its implementation level (full 9.1.1.3. Adding a Media Stream
or lite) by adding or removing the a=ice-lite attribute from an
updated offer, unless ICE processing is being restarted for all media
streams in the offer. Of course, in normal cases the implementation
level is not dynamic and there would be no need to signal a change.
However, in applications like third party call control, which involve
a mid-session change in remote correspondent, this can happen and it
is permitted by ICE with a restart.
Note that an agent can add a new media stream at any time, even if If an agent wishes to add a new media stream, it sets the fields in
ICE has long finished for the existing media streams. Based on the the SDP for this media stream as if this was an initial offer for
rules described here, checks will begin for this new stream as if it that media stream (see Section 4.3). This will cause ICE processing
was in an initial offer. to begin for this media stream.
9.1.1. Additional Procedures for Full Implementations 9.1.2. Procedures for Full Implementations
This section describes additional procedures for full This section describes additional procedures for full
implementations. implementations, covering existing media streams.
When an agent generates an updated offer, the set of candidate The username fragments, password, and implementation level MUST
attributes to include for each media stream depend on the state of remain the same as used previously. If an agent needs to change one
ICE processing for that media stream. If the processing for that of these it MUST restart ICE for that media stream.
media stream is in the Completed state, a full-mode agent MUST
include a candidate attribute for the local candidate of each pair
that has been chosen for use by ICE for that media stream. A pair is
chosen if it is the highest priority favored pair in the valid list
for a component of that media stream. An agent SHOULD NOT include
any other candidate attributes for that media stream. If ICE
processing for a media stream is in the Running state, the agent MUST
include all current candidates (including peer reflexive candidates
learned through ICE processing) for that media stream. It MAY
include candidates it did not offer previously, but which it has
gathered since the last offer/answer exchange. If a media stream is
new or ICE checks are restarting for that stream, an agent includes
the set of candidates it wishes to utilize. This MAY include some,
none, or all of the previous candidates for that stream in the case
of a restart, and MAY include a totally new set of candidates
gathered as described in Section 4.1.1.
If a candidate was sent in a previous offer/answer exchange, it Additional behavior depends on the state ICE processing for that
SHOULD have the same priority. For a peer reflexive candidate, the media stream.
priority SHOULD be the same as determined by the processing in
Section 7.1.2. The foundation SHOULD be the same. The username
fragments and passwords for a media stream SHOULD remain the same as
the previous offer or answer.
Population of the m/c-lines also depends on the state of ICE 9.1.2.1. Existing Media Streams with ICE Running
processing. If ICE processing for a media stream is in the Completed
state, the m/c-line MUST use the local candidate from the highest If an agent generates an updated offer including media stream that
priority favored pair in the valid list for each component of that was previously established, and for which ICE checks are in the
media stream. If ICE processing is in the Running state, a full-mode Running state, the agent follows the procedures defined here.
agent SHOULD populate the m/c-line for that media stream based on the
considerations in Section 4.1.3. An agent MUST include candidate attributes for all local candidates
it had signaled previously for that media stream. The properties of
that candidate as signaled in SDP - the priority, foundation, type
and related transport address SHOULD remain the same. The IP
address, port and transport protocol, which fundamentally identify
that candidate, MUST remain the same (if they change, it would be a
new candidate). The component ID MUST remain the same. The agent
MAY include additional candidates it did not offer previously, but
which it has gathered since the last offer/answer exchange, including
peer reflexive candidates.
The agent MAY change the default destination for media. As with
initial offers, there MUST be a set of candidate attributes in the
offer matching this default destination.
9.1.2.2. Existing Media Streams with ICE Completed
If an agent generates an updated offer including media stream that
was previously established, and for which ICE checks are in the
Completed state, the agent follows the procedures defined here.
The default destination for media (i.e., the values of the IP
addresses and ports in the m and c line used for that media stream)
MUST be the local candidate from the highest priority nominated pair
in the valid list for each component. This "fixes" the default
destination for media to equal the destination ICE has selected for
media.
The agent MUST include a candidate attributes for candidates matching
the default destination for each component of the media stream, and
MUST NOT include any other candidates.
In addition, if the agent is controlling, it MUST include the In addition, if the agent is controlling, it MUST include the
a=remote-candidates attribute for each media stream that is in the a=remote-candidates attribute for each media stream whose check list
Completed state. The attribute contains the remote candidates from is in the Completed state. The attribute contains the remote
the highest priority favored pair in the valid list for each candidates from the highest priority nominated pair in the valid list
component of that media stream. for each component of that media stream. It is needed to avoid a
race condition whereby the controlling agent chooses its pairs, but
the updated offer beats the connectivity checks to the controlled
agent, which doesn't even know these pairs are valid, let alone
selected. See Appendix B.6 for elaboration on this race condition.
9.1.2. Additional Procedures for Lite Implementations 9.1.3. Procedures for Lite Implementations
A passive-only agent includes its one and only candidate for each This section describes procedures for lite implementations for
component of each media stream in an a=candidate attribute in any existing streams for which ICE is running.
subsequent offer. This candidate is formed identically to the
A lite implementation MUST include its one and only candidate for
each component of each media stream in an a=candidate attribute in
any subsequent offer. This candidate is formed identically to the
procedures for initial offers, as described in Section 4.2. procedures for initial offers, as described in Section 4.2.
The username fragments, password, and implementation level MUST
remain the same as used previously. If an agent needs to change one
of these it MUST restart ICE for that media stream.
9.2. Receiving the Offer and Generating an Answer 9.2. Receiving the Offer and Generating an Answer
9.2.1. Procedures for All Implementations
When receiving a subsequent offer within an existing session, an When receiving a subsequent offer within an existing session, an
agent MUST re-apply the verification procedures in Section 5.1 agent MUST re-apply the verification procedures in Section 5.1
without regard to the results of verification from any previous without regard to the results of verification from any previous
offer/answer exchanges. Indeed, it is possible that a previous offer/answer exchanges. Indeed, it is possible that a previous
offer/answer exchange resulted in ICE not being used, but it is used offer/answer exchange resulted in ICE not being used, but it is used
as a consequence of a subsequent exchange. as a consequence of a subsequent exchange.
9.2.1.1. Detecting ICE Restart
If the offer contained a change in the a=ice-ufrag or a=ice-pwd If the offer contained a change in the a=ice-ufrag or a=ice-pwd
attributes compared to the previous SDP from the peer, it is a signal attributes compared to the previous SDP from the peer, it indicates
that ICE is restarting for this media stream. If all media streams that ICE is restarting for this media stream. If all media streams
are restarting, than ICE is restarting overall. Procedures for ICE are restarting, than ICE is restarting overall.
restarts are discussed below. Unless ICE is restarting for that
media stream, an agent MUST NOT change the a=ice-ufrag or a=ice-pwd
attributes in an answer relative to the last SDP it provided. Such a
change can only take place in an offer. If ICE is restarting, the
a=ice-ufrag and a=ice-pwd attributes MUST be changed.
An agent MUST NOT change its implementation level from its previous If ICE is restarting for a media stream:
SDP unless, based on the offer, ICE procedures are being restarted
for all media streams in the offer. In that case, it MAY change its
level.
An agent MUST NOT include the a=remote-candidates attribute in an o The agent MUST change the a=ice-ufrag and a=ice-pwd attributes in
answer. the answer.
When the answerer generates its answer, it must decide what o The agent MAY change its implementation level in the answer.
candidates to include in the answer, how to populate the m/c-line,
and how to adjust the states of ICE processing. The rules for
inclusion of candidate attributes in an answer are identical to the
rules followed by the offerer as described in Section 9.1 for both
full and lite implementations. For lite implementations, those rules
also apply for setting the m/c-line. However, additional
considerations apply to full implementations.
9.2.1. Additional Procedures for Full Implementations An agent sets the rest of the fields in the SDP for this media stream
as it would in an initial answer to this media stream (see
Section 4.3). Consequently, the set of candidates MAY include some,
none, or all of the previous candidates for that stream and MAY
include a totally new set of candidates gathered as described in
Section 4.1.1.
The computation of the m/c-line additionally depends on the presence 9.2.1.2. New Media Stream
or absence of the a=remote-candidates attribute in a media stream.
If present, it means that the offerer (acting as the controlling
agent) believed that ICE processing has completed for that media
stream. In this case, the remote-candidates attribute contains the
candidates that the answerer is supposed to use. It is possible that
the agent doesn't even know of these candidates yet; they will be
discovered shortly through a response to an in-progress check. The
full-mode agent MUST populate the m/c-line with the candidates from
the a=remote-candidates attribute.
If the offer did not contain the a=remote-candidates attribute, the If the offer contains a new media stream, the agent sets the fields
agent follows the same procedures for populating the m/c-line as in the answer as if it had received an initial offer containing that
described for the offerer in Section 9.1. media stream (see Section 4.3). This will cause ICE processing to
begin for this media stream.
9.3. Updating the Check and Valid Lists 9.2.1.3. Removed Media Stream
If ICE is restarting for a media stream, the agent MUST start a new If an offer contains a media stream whose port is zero, the agent
Valid list for that media stream. However, it retains the old Valid MUST NOT include any candidate attributes for that media stream in
list for the purposes of sending media until ICE processing its answer and SHOULD NOT include any other ICE-related attributes
completes, at which point the old Valid list is discarded and the new defined in Section 15 for that media stream.
one is utilized to determine media and keepalive targets.
9.3.1. Additional Procedures for Full Implementations 9.2.2. Procedures for Full Implementations
The procedures in this section are applicable only to full The username fragments, password, and implementation level MUST
implementations. remain the same as used previously. If an agent needs to change one
of these it MUST restart ICE for that media stream by generating an
offer; ICE cannot be restarted in an answer.
Once the subsequent offer/answer exchange has completed, each agent Additional behaviors depend on the state of ICE processing for that
needs to determine the impact, if any, on the Check and Valid lists. media stream.
Unless there is an ICE restart, an offer/answer exchange has no
impact on the state of ICE processing for each media stream; that is
determined entirely by the checks themselves.
When ICE restarts, an agent MUST flush the check list for the 9.2.2.1. Existing Media Streams with ICE Running and no remote-
affected media streams, and then recompute the check list and its candidates
states as described in Section 5.7.
The remainder of this section describes processing when ICE is not If ICE is running for a media stream, and the offer for that media
restarting. stream lacked the remote-candidates attribute, the rules for
construction of the answer are identical to those for the offerer as
described in Section 9.1.2.1.
9.2.2.2. Existing Media Streams with ICE Completed and no remote-
candidates
If ICE is Completed for a media stream, and the offer for that media
stream lacked the remote-candidates attribute, the rules for
construction of the answer are identical to those for the offerer as
described in Section 9.1.2.2, except that the answerer MUST NOT
include the a=remote-candidates attribute in the answer.
9.2.2.3. Existing Media Streams and remote-candidates
A controlled agent will receive an offer with the a=remote-candidates
attribute for a media stream when its peer has concluded ICE
processing for that media stream. This attribute is present in the
offer to deal with a race condition between the receipt of the offer,
and the receipt of the Binding Response which tells the answerer the
candidate which will be selected by ICE. See Appendix B.6 for an
explanation of this race condition. Consequently, processing of an
offer with this attribute depends on the winner of the race.
The agent forms a candidate pair for each component of the media
stream by:
o Setting the remote candidate equal to the offerers default
destination for that component (e.g., the contents of the m and
c-lines for RTP, and the a=rtcp attribute for RTCP)
o Setting the local candidate equal to the transport address for
that same component in the a=remote-candidates attribute in the
offer.
The agent then sees if each of these candidate pairs are present in
the valid list. If a particular pair is not in the valid list, the
check has "lost" the race. Call such a pair a "losing pair".
The agent finds all the pairs in the check list whose remote
candidates equal the remote candidate in the losing pair:
o If none of the pairs are In-Progress, and at least one is Failed,
it is most likely that a network failure, such as a network
partition or serious packet loss, has occurred. The agent SHOULD
generate an answer for this media stream as if the remote-
candidates attribute had not been present, and then restart ICE
for this stream.
o If at least one of the pairs are In-Progress, the agent SHOULD
wait for those checks to complete, and as each completes, redo the
processing in this section until there are no losing pairs.
Once there are no losing pairs, the agent can generate the answer.
It MUST set the default destination for media to the candidates in
the remote-candidates attribute from the offer (each of which will
now be the local candidate of a candidate pair in the valid list).
It MUST include a candidate attribute in the answer for each
candidate in the remote-candidates attribute in the offer.
9.2.3. Procedures for Lite Implementations
A lite implementation constructs its answer in the same way it does a
subsequent offer as described in Section 9.1.3
9.3. Updating the Check and Valid Lists
9.3.1. Procedures for Full Implementations
9.3.1.1. ICE Restarts
The agent MUST remember the highest priority nominated pairs in the
Valid list for each component of the media stream, called the
previous selected pairs, prior to the restart. The agent will
continue to send media using these pairs, as described in
Section 11.1. Once these destinations are noted, the agent MUST
flush the valid and check lists, and then recompute the check list
and its states as described in Section 5.7.
9.3.1.2. New Media Stream
If the offer/answer exchange added a new media stream, the agent MUST If the offer/answer exchange added a new media stream, the agent MUST
create a new check list for it (and an empty Valid list to start of create a new check list for it (and an empty Valid list to start of
course), as described in Section 5.7. course), as described in Section 5.7.
9.3.1.3. Removed Media Stream
If the offer/answer exchange removed a media stream, or an answer If the offer/answer exchange removed a media stream, or an answer
rejected an offered media stream, an agent MUST flush the Valid list rejected an offered media stream, an agent MUST flush the Valid list
for that media stream. It MUST terminate any STUN transactions in for that media stream. It MUST terminate any STUN transactions in
progress for that media stream. An agent MUST remove the check list progress for that media stream. An agent MUST remove the check list
for that media stream and cancel any pending periodic checks for it. for that media stream and cancel any pending periodic checks for it.
If a media stream existed previously, and remains after the offer/ 9.3.1.4. ICE Continuing for Existing Media Stream
answer exchange, the agent MUST NOT modify the Valid list for that
media stream. However, if an agent is in the Running state for that
media stream, the check list is updated. To do that, the agent
recomputes the check lists using the procedures described in
Section 5.7. If a check on the new check lists was also on the
previous check lists, and its state was Waiting, In-Progress,
Succeeded or Failed, its state is copied over. If a check on the new
check lists does not have a state (because it's a new check on an
existing check list, or a check on a new check list, or the check was
on an old check list but its state was not copied over) its state is
set to Frozen.
If none of the check lists are active (meaning that the checks in The valid list is not affected by an updated offer/answer exchange
each check list are Frozen), the full-mode agent sets the first check unless ICE is restarting.
in the check list for the first media stream to Waiting, and then
sets the state of all other checks in that check list for the same If an agent is in the Running state for that media stream, the check
list is updated (the check list is irrelevant if the state is
completed). To do that, the agent recomputes the check list using
the procedures described in Section 5.7. If a pair on the new check
list was also on the previous check list, and its state was Waiting,
In-Progress, Succeeded or Failed, its state is copied over.
Otherwise, its state is set to Frozen.
If none of the check lists are active (meaning that the pairs in each
check list are Frozen), the full-mode agent sets the first pair in
the check list for the first media stream to Waiting, and then sets
the state of all other pairs in that check list for the same
component ID and with the same foundation to Waiting as well. component ID and with the same foundation to Waiting as well.
Next, the agent goes through each check list, starting with the Next, the agent goes through each check list, starting with the
highest priority check. If a check has a state of Succeeded, and it highest priority pair. If a pair has a state of Succeeded, and it
has a component ID of 1, then all Frozen checks in the same check has a component ID of 1, then all Frozen pairs in the same check list
list with the same foundation whose component IDs are not one, have with the same foundation whose component IDs are not 1, have their
their state set to Waiting. If, for a particular check list, there state set to Waiting. If, for a particular check list, there are
are checks for each component of that media stream in the Succeeded pairs for each component of that media stream in the Succeeded state,
state, the agent moves the state of all Frozen checks for the first the agent moves the state of all Frozen pairs for the first component
component of all other media streams (and thus in different check of all other media streams (and thus in different check lists) with
lists) with the same foundation to Waiting. the same foundation to Waiting.
9.3.2. Procedures for Lite Implementations
If ICE is restarting for a media stream, the agent MUST start a new
Valid list for that media stream. It MUST remember the pairs in the
previous Valid list for each component of the media stream, called
the previous selected pairs, and continue to send media there as
described in Section 11.1.
10. Keepalives 10. Keepalives
All endpoints MUST send keepalives for each media session. These All endpoints MUST send keepalives for each media session. These
keepalives serve the purpose of keeping NAT bindings active for the keepalives serve the purpose of keeping NAT bindings alive for the
media session. These keepalives MUST be sent regardless of whether media session. These keepalives MUST be sent regardless of whether
the media stream is currently inactive, sendonly, recvonly or the media stream is currently inactive, sendonly, recvonly or
sendrecv, and regardless of the presence or value of the bandwidth sendrecv, and regardless of the presence or value of the bandwidth
attribute. These keepalives MUST be sent even if ICE is not being attribute. These keepalives MUST be sent even if ICE is not being
utilized for the session at all. The keepalive SHOULD be sent using utilized for the session at all. The keepalive SHOULD be sent using
a format which is supported by its peer. ICE endpoints allow for a format which is supported by its peer. ICE endpoints allow for
STUN-based keepalives for UDP streams, and as such, STUN keepalives STUN-based keepalives for UDP streams, and as such, STUN keepalives
MUST be used when an agent is communicating with a peer that supports MUST be used when an agent is communicating with a peer that supports
ICE. An agent can determine that its peer supports ICE by the ICE. An agent can determine that its peer supports ICE by the
presence of a=candidate attributes for each media session. If the presence of a=candidate attributes for each media session. If the
peer does not support ICE, the choice of a packet format for peer does not support ICE, the choice of a packet format for
keepalives is a matter of local implementation. A format which keepalives is a matter of local implementation. A format which
allows packets to easily be sent in the absence of actual media allows packets to easily be sent in the absence of actual media
content is RECOMMENDED. Examples of formats which readily meet this content is RECOMMENDED. Examples of formats which readily meet this
goal are RTP No-Op [28] and RTP comfort noise [24]. If the peer goal are RTP No-Op [31] and RTP comfort noise [25]. If the peer
doesn't support any formats that are particularly well suited for doesn't support any formats that are particularly well suited for
keepalives, an agent SHOULD send RTP packets with an incorrect keepalives, an agent SHOULD send RTP packets with an incorrect
version number, or some other form of error which would cause them to version number, or some other form of error which would cause them to
be discarded by the peer. be discarded by the peer.
If there has been no packet sent on a candidate pair being used for If there has been no packet sent on the candidate pair ICE is using
media for Tr seconds (where packets include media and previous for a media component for Tr seconds (where packets include those
keepalives), an agent MUST generate a keepalive on that pair. Tr defined for the component (RTP or RTCP) and previous keepalives), an
SHOULD be configurable and SHOULD have a default of 15 seconds. agent MUST generate a keepalive on that pair. Tr SHOULD be
configurable and SHOULD have a default of 15 seconds. Alternatively,
if an agent has a dynamic way to discover the binding lifetimes of
the intervening NATs, it can use that value to determine Tr.
If STUN is being used for keepalives, a STUN Binding Indication is If STUN is being used for keepalives, a STUN Binding Indication is
used [11]. The Binding Indication SHOULD NOT contain integrity used [12]. The Binding Indication SHOULD NOT contain integrity
checks; since the messages are simply discarded on receipt regardless checks as the messages are simply discarded on receipt regardless of
of contents. The Indication SHOULD NOT contain the PRIORITY or USE- contents. The Indication SHOULD NOT contain the PRIORITY or USE-
CANDIDATE attributes defined here. The Binding Indication is sent CANDIDATE attributes defined in this document. The Binding
using the same local and remote candidates that are being used for Indication is sent using the same local and remote candidates that
media. An agent receipt a Binding Indication MUST discard it are being used for media. An agent receiving a Binding Indication
silently. Though Binding Indications are used for keepalives, an MUST discard it silently. Though Binding Indications are used for
agent MUST be prepared to receive Binding Requests as well. If a keepalives, an agent MUST be prepared to receive Binding Requests as
Binding Request is received, a response is generated as discussed in well. If a Binding Request is received, a response is generated as
[11], but there is no impact on ICE processing otherwise. discussed in [12], but there is no impact on ICE processing
otherwise.
An agent MUST begin the keepalive processing once ICE has selected An agent MUST begin the keepalive processing once ICE has selected
candidates for usage with media, or media begins to flow, whichever candidates for usage with media, or media begins to flow, whichever
happens first. Keepalives end once the session terminates or the happens first. Keepalives end once the session terminates or the
media stream is removed. media stream is removed.
11. Media Handling 11. Media Handling
11.1. Sending Media 11.1. Sending Media
Procedures for sending media differ for full and lite Procedures for sending media differ for full and lite
implementations. implementations.
11.1.1. Procedures for Full Implementations 11.1.1. Procedures for Full Implementations
Agents always send media using a candidate pair. An agent will send Agents always send media using a candidate pair, called the selected
media to the remote candidate in the pair (setting the destination candidate pair. An agent will send media to the remote candidate in
address and port of the packet equal to that remote candidate), and the selected pair (setting the destination address and port of the
will send it from the local candidate. When the local candidate is packet equal to that remote candidate), and will send it from the
local candidate of the selected pair. When the local candidate is
server or peer reflexive, media is originated from the base. Media server or peer reflexive, media is originated from the base. Media
sent from a relayed candidate is sent through that relay, using sent from a relayed candidate is sent from the base through that
procedures defined in [12]. relay, using procedures defined in [13].
If the state of a media stream is Running, there is no old Valid list The selected pair for a component of a media stream is:
for that media stream (which would be due to an ICE restart), an
agent MUST NOT send media.
When an agent sends media, it MUST send it using the highest priority o empty if the state of the check list for that media stream is
selected pair for each component in either the old Valid list for a Running, and there is no previous selected pair for that component
media stream (if it exists), else the new Valid list for that media due to an ICE restart
stream. In several cases, this will not be the same candidate pairs
present in the m/c-line. When ICE first completes, if the selected
pairs aren't a match for the m/c-line, an updated offer/answer
exchange will take place to remedy this disparity. However, until
that update offer arrives, there will not be a match. Furthermore,
in very unusual cases, the m/c-lines in the updated offer/answer will
not be a match.
ICE has interactions with jitter buffer adaptation mechanisms. An o equal to the previous selected pair for a component of a media
RTP stream can begin using one candidate, and switch to another one, stream if the state of the check list for that media stream is
though this happens rarely with ICE. The newer candidate may result Running, and there was a previous selected pair for that component
in RTP packets taking a different path through the network - one with due to an ICE restart
different delay characteristics. As discussed below, agents are
encouraged to re-adjust jitter buffers when there are changes in o equal to the highest priority nominated pair for that component in
source or destination address. Furthermore, many audio codecs use the valid list if the state of the check list is Completed
the marker bit to signal the beginning of a talkspurt, for the
purposes of jitter buffer adaptation. For such codecs, it is If the selected pair for at least one component of a media stream is
RECOMMENDED that the sender change the marker bit when an agent empty, an agent MUST NOT send media for any component of that media
switches transmission of media from one candidate pair to another. stream. If the selected pair for each component of a media stream
has a value, an agent MAY send media for all components of that media
stream.
Note that the selected pair for a component of a media stream may not
equal the default pair for that same component from the most recent
offer/answer exchange. When this happens, the selected pair is used
for media, not the default pair. When ICE first completes, if the
selected pairs aren't a match for the default pairs, the controlling
agent sends an updated offer/answer exchange to remedy this
disparity. However, until that updated offer arrives, there will not
be a match. Furthermore, in very unusual cases, the default
candidates in the updated offer/answer will not be a match.
11.1.2. Procedures for Lite Implementations 11.1.2. Procedures for Lite Implementations
A lite implementation MUST NOT send media until it has a Valid list A lite implementation MUST NOT send media until it has a Valid list
that contains a candidate pair for each component of that media that contains a candidate pair for each component of that media
stream. Once that happens, the agent MAY begin sending media stream. Once that happens, the agent MAY begin sending media
packets. To do that, it sends media to the remote candidate in the packets. To do that, it sends media to the remote candidate in the
pair (setting the destination address and port of the packet equal to pair (setting the destination address and port of the packet equal to
that remote candidate), and will send it from the local candidate. that remote candidate), and will send it from the local candidate.
In cases where there has been an ICE restart, there will be an old 11.1.3. Procedures for All Implementations
and a new Valid list. The old Valid list MUST be used by the agent
for sending media until the new one is complete, at which point the ICE has interactions with jitter buffer adaptation mechanisms. An
new one MUST be used, and the old one discarded. RTP stream can begin using one candidate, and switch to another one,
though this happens rarely with ICE. The newer candidate may result
in RTP packets taking a different path through the network - one with
different delay characteristics. As discussed below, agents are
encouraged to re-adjust jitter buffers when there are changes in
source or destination address of media packets. Furthermore, many
audio codecs use the marker bit to signal the beginning of a
talkspurt, for the purposes of jitter buffer adaptation. For such
codecs, it is RECOMMENDED that the sender set the marker bit [22]
when an agent switches transmission of media from one candidate pair
to another.
11.2. Receiving Media 11.2. Receiving Media
ICE implementations MUST be prepared to receive media on any ICE implementations MUST be prepared to receive media on each
candidates provided in the most recent offer/answer exchange. component on any candidates provided for that component in the most
recent offer/answer exchange (in the case of RTP, this would include
both RTP and RTCP if candidates were provided for both).
It is RECOMMENDED that, when an agent receives an RTP packet with a It is RECOMMENDED that, when an agent receives an RTP packet with a
new source or destination IP address for a particular media stream, new source or destination IP address for a particular media stream,
that the agent re-adjust its jitter buffers. that the agent re-adjust its jitter buffers.
RFC 3550 [21] describes an algorithm in Section 8.2 for detecting RFC 3550 [22] describes an algorithm in Section 8.2 for detecting
SSRC collisions and loops. These algorithms are based, in part, on SSRC collisions and loops. These algorithms are based, in part, on
seeing different source transport addresses with the same SSRC. seeing different source transport addresses with the same SSRC.
However, when ICE is used, such changes will sometimes occur as the However, when ICE is used, such changes will sometimes occur as the
media streams switch between candidates. An agent will be able to media streams switch between candidates. An agent will be able to
determine that a media stream is from the same peer as a consequence determine that a media stream is from the same peer as a consequence
of the STUN exchange that proceeds media transmission. Thus, if of the STUN exchange that proceeds media transmission. Thus, if
there is a change in source transport address, but the media packets there is a change in source transport address, but the media packets
come from the same peer agent, this SHOULD NOT be treated as an SSRC come from the same peer agent, this SHOULD NOT be treated as an SSRC
collision. collision.
skipping to change at page 43, line 4 skipping to change at page 56, line 38
12.1. Latency Guidelines 12.1. Latency Guidelines
ICE requires a series of STUN-based connectivity checks to take place ICE requires a series of STUN-based connectivity checks to take place
between endpoints. These checks start from the answerer on between endpoints. These checks start from the answerer on
generation of its answer, and start from the offerer when it receives generation of its answer, and start from the offerer when it receives
the answer. These checks can take time to complete, and as such, the the answer. These checks can take time to complete, and as such, the
selection of messages to use with offers and answers can effect selection of messages to use with offers and answers can effect
perceived user latency. Two latency figures are of particular perceived user latency. Two latency figures are of particular
interest. These are the post-pickup delay and the post-dial delay. interest. These are the post-pickup delay and the post-dial delay.
The post-pickup delay refers to the time between when a user "answers The post-pickup delay refers to the time between when a user "answers
the phone" and when any speech they utter can be delivered to the the phone" and when any speech they utter can be delivered to the
caller. The post-dial delay refers to the time between when a user caller. The post-dial delay refers to the time between when a user
enters the destination address for the user, and ringback begins as a enters the destination address for the user, and ringback begins as a
consequence of having succesfully started ringing the phone of the consequence of having successfully started ringing the phone of the
called party. called party.
Two cases can be considered - one where the offer is present in the
initial INVITE, and one where it is in a response.
12.1.1. Offer in INVITE
To reduce post-dial delays, it is RECOMMENDED that the caller begin To reduce post-dial delays, it is RECOMMENDED that the caller begin
gathering candidates prior to actually sending its initial INVITE. gathering candidates prior to actually sending its initial INVITE.
This can be started upon user interface cues that a call is pending, This can be started upon user interface cues that a call is pending,
such as activity on a keypad or the phone going offhook. such as activity on a keypad or the phone going offhook.
If an offer is received in an INVITE request, the callee SHOULD If an offer is received in an INVITE request, the answerer SHOULD
immediately gather its candidates and then generate an answer in a begin to gather its candidates on receipt of the offer and then
provisional response. ICE requires that a provisional response with generate an answer in a provisional response once it has completed
an SDP be transmitted reliably. This can be done through the that process. ICE requires that a provisional response with an SDP
existing PRACK mechanism [9], or through an optimization that is be transmitted reliably. This can be done through the existing PRACK
specific to ICE. With this optimization, provisional responses mechanism [9], or through an optimization that is specific to ICE.
containing an SDP answer that begins ICE processing for one or more With this optimization, provisional responses containing an SDP
media streams can be sent reliably without RFC 3264. To do this, the answer that begins ICE processing for one or more media streams can
agent retransmits the provisional response with th exponential be sent reliably without RFC 3264. To do this, the agent retransmits
backoff timers described in RFC 3262. Retransmits MUST cease on the provisional response with th exponential backoff timers described
receipt of a STUN Binding Request for one of the media streams in RFC 3262. Retransmits MUST cease on receipt of a STUN Binding
signaled in that SDP or on transmission of a 2xx response. If no Request for one of the media streams signaled in that SDP (because
Binding Request is received prior to the last retransmit, the agent receipt of a binding request indicates the offerer has received the
does not consider the session terminated. Despite the fact that the answer) or on transmission of a 2xx response. If no Binding Request
provisional response will be delivered reliably, the rules for when is received prior to the last retransmit, the agent does not consider
an agent can send an updated offer or answer do not change from those the session terminated. Despite the fact that the provisional
specified in RFC 3262. Specifically, if the INVITE contained an response will be delivered reliably, the rules for when an agent can
offer, the same answer appears in all of the 1xx and in the 2xx send an updated offer or answer do not change from those specified in
response to the INVITE. Only after that 2xx has been sent can an RFC 3262. Specifically, if the INVITE contained an offer, the same
updated offer/answer exchange occur. This optimization SHOULD NOT be answer appears in all of the 1xx and in the 2xx response to the
used if both agents support PRACK. Note that the optimization is INVITE. Only after that 2xx has been sent can an updated offer/
very specific to provisional response carrying answers that start ICE answer exchange occur. This optimization SHOULD NOT be used if both
processing; it is not a general technique for 1xx reliability. agents support PRACK. Note that the optimization is very specific to
provisional response carrying answers that start ICE processing; it
is not a general technique for 1xx reliability.
Alternatively, an agent MAY delay sending an answer until the 200 OK, Alternatively, an agent MAY delay sending an answer until the 200 OK,
however this results in a poor user experience and is NOT however this results in a poor user experience and is NOT
RECOMMENDED. RECOMMENDED.
Once the answer has been sent, the agent SHOULD begin its Once the answer has been sent, the agent SHOULD begin its
connectivity checks. Once candidate pairs for each component of a connectivity checks. Once candidate pairs for each component of a
media stream enter the valid list, the callee can begin sending media media stream enter the valid list, the answerer can begin sending
on that media stream. media on that media stream.
However, prior to this point, any media that needs to be sent towards However, prior to this point, any media that needs to be sent towards
the caller (such as SIP early media [25] cannot be transmitted. For the caller (such as SIP early media [26] MUST NOT be transmitted.
this reason, implementations SHOULD delay alerting the called party For this reason, implementations SHOULD delay alerting the called
until candidates for each component of each media stream have entered party until candidates for each component of each media stream have
the valid list. In the case of a PSTN gateway, this would mean that entered the valid list. In the case of a PSTN gateway, this would
the setup message into the PSTN is delayed until this point. Doing mean that the setup message into the PSTN is delayed until this
this increases the post-dial delay, but has the effect of eliminating point. Doing this increases the post-dial delay, but has the effect
'ghost rings'. Ghost rings are cases where the called party hears of eliminating 'ghost rings'. Ghost rings are cases where the called
the phone ring, picks up, but hears nothing and cannot be heard. party hears the phone ring, picks up, but hears nothing and cannot be
This technique works without requiring support for, or usage of, heard. This technique works without requiring support for, or usage
preconditions [6], since its a localized decision. It also has the of, preconditions [6], since its a localized decision. It also has
benefit of guaranteeing that not a single packet of media will get the benefit of guaranteeing that not a single packet of media will
clipped, so that post-pickup delay is zero. If an agent chooses to get clipped, so that post-pickup delay is zero. If an agent chooses
delay local alerting in this way, it SHOULD generate a 180 response to delay local alerting in this way, it SHOULD generate a 180
once alerting begins. response once alerting begins.
12.1.2. Offer in Response
In addition to uses where the offer is in an INVITE, and the answer In addition to uses where the offer is in an INVITE, and the answer
is in the provisional and/or 200 OK, ICE works with cases where the is in the provisional and/or 200 OK response, ICE works with cases
offer appears in the response. In such cases, which are common in where the offer appears in the response. In such cases, which are
third party call control, ICE agents SHOULD generate their offers in common in third party call control [18], ICE agents SHOULD generate
a reliable provisional response (which MUST utilize RFC 3262). In their offers in a reliable provisional response (which MUST utilize
that case, the answer will arrive in a PRACK. This allows for ICE RFC 3262), and not alert the user on receipt of the INVITE. The
processing to take place prior to alerting. Once ICE completes, the answer will arrive in a PRACK. This allows for ICE processing to
agent can alert the user and then generate a 200 OK. The 200 OK take place prior to alerting, so that there is no post-pickup delay,
would contain no SDP, since the offer/answer exchange has completed. at the expense of increased call setup delays. Once ICE completes,
Agents MAY place the offer in a 2xx instead (in which case the answer the callee can alert the user and then generate a 200 OK when they
comes in the ACK). This flow is simpler but results in a poorer user answer. The 200 OK would contain no SDP, since the offer/answer
experience. exchange has completed.
As discussed in Section 16, offer/answer exchanges SHOULD be secured Alternatively, agents MAY place the offer in a 2xx instead (in which
against eavesdropping and man-in-the-middle attacks. To do that, the case the answer comes in the ACK). When this happens, the callee
usage of SIPS [3] is RECOMMENDED when used in concert with ICE. will alert the user on receipt of the INVITE, and the ICE exchanges
will take place only after the user answers. This has the effect of
reducing call setup delay, but can cause substantial post-pickup
delays and media clipping.
12.2. SIP Option Tags and Media Feature Tags 12.2. SIP Option Tags and Media Feature Tags
[13] specifies a SIP option tag and media feature tag for usage with [14] specifies a SIP option tag and media feature tag for usage with
ICE. ICE implementations using SIP SHOULD support this ICE. ICE implementations using SIP SHOULD support this
specification, which uses a feature tag in registrations to specification, which uses a feature tag in registrations to
facilitate interoperability through gateways. facilitate interoperability through intermediaries.
12.3. Interactions with Forking 12.3. Interactions with Forking
ICE interacts very well with forking. Indeed, ICE fixes some of the ICE interacts very well with forking. Indeed, ICE fixes some of the
problems associated with forking. Without ICE, when a call forks and problems associated with forking. Without ICE, when a call forks and
the caller receives multiple incoming media streams, it cannot the caller receives multiple incoming media streams, it cannot
determine which media stream corresponds to which callee. determine which media stream corresponds to which callee.
With ICE, this problem is resolved. The connectivity checks which With ICE, this problem is resolved. The connectivity checks which
occur prior to transmission of media carry username fragments, which occur prior to transmission of media carry username fragments, which
in turn are correlated to a specific callee. Subsequent media in turn are correlated to a specific callee. Subsequent media
packets which arrive on the same 5-tuple as the connectivity check packets which arrive on the same candidate pair as the connectivity
will be associated with that same callee. Thus, the caller can check will be associated with that same callee. Thus, the caller can
perform this correlation as long as it has received an answer. perform this correlation as long as it has received an answer.
12.4. Interactions with Preconditions 12.4. Interactions with Preconditions
Quality of Service (QoS) preconditions, which are defined in RFC 3312 Quality of Service (QoS) preconditions, which are defined in RFC 3312
[6] and RFC 4032 [7], apply only to the transport addresses listed in [6] and RFC 4032 [7], apply only to the transport addresses listed as
the m/c lines in an offer/answer. If ICE changes the transport the default targets for media in an offer/answer. If ICE changes the
address where media is received, this change is reflected in the m/c transport address where media is received, this change is reflected
lines of a new offer/answer. As such, it appears like any other re- in an updated offer which changes the default destination for media
to match ICE's selection. As such, it appears like any other re-
INVITE would, and is fully treated in RFC 3312 and 4032, which apply INVITE would, and is fully treated in RFC 3312 and 4032, which apply
without regard to the fact that the m/c lines are changing due to ICE without regard to the fact that the destination for media is changing
negotiations ocurring "in the background". due to ICE negotiations occurring "in the background".
Indeed, an agent SHOULD NOT indicate that Qos preconditions have been Indeed, an agent SHOULD NOT indicate that Qos preconditions have been
met until the ICE checks have completed and selected the candidate met until the checks have completed and selected the candidate pairs
pairs to be used for media. to be used for media.
ICE also has (purposeful) interactions with connectivity ICE also has (purposeful) interactions with connectivity
preconditions [27]. Those interactions are described there. Note preconditions [30]. Those interactions are described there. Note
that the procedures described in Section 12.1 describe their own type that the procedures described in Section 12.1 describe their own type
of "preconditions", albeit with less functionality than those of "preconditions", albeit with less functionality than those
provided by the explicit preconditions in [27]. provided by the explicit preconditions in [30].
12.5. Interactions with Third Party Call Control 12.5. Interactions with Third Party Call Control
ICE works with Flows I, III and IV as described in [17]. Flow I ICE works with Flows I, III and IV as described in [18]. Flow I
works without the controller supporting or being aware of ICE. Flow works without the controller supporting or being aware of ICE. Flow
IV will work as long as the controller passes along the ICE IV will work as long as the controller passes along the ICE
attributes without alteration. Flow II is fundamentally incompatible attributes without alteration. Flow II is fundamentally incompatible
with ICE; each agent will believe itself to be the answerer and thus with ICE; each agent will believe itself to be the answerer and thus
never generate a re-INVITE. never generate a re-INVITE.
The flows for continued operation, as described in Section 7 of RFC The flows for continued operation, as described in Section 7 of RFC
3725, require additional behavior of ICE implementations to support. 3725, require additional behavior of ICE implementations to support.
In particular, if an agent receives a mid-dialog re-INVITE that In particular, if an agent receives a mid-dialog re-INVITE that
contains no offer, it MUST restart ICE for each media stream and go contains no offer, it MUST restart ICE for each media stream and go
through the process of gathering new candidates. Furthermore, that through the process of gathering new candidates. Furthermore, that
list of candidates SHOULD include the ones currently in-use. list of candidates SHOULD include the ones currently being used for
media.
13. Grammar 13. Usage with ANAT
RFC 4091 [11] defines a mechanism for indicating that an agent can
support both IPv4 and IPv6 for a media stream, and it does so by
including two m-lines, one for v4, and one for v6. This is similar
to ICE, which allows for an agent to indicate multiple transport
addresses using the candidate attribute.
However, ICE is not a replacement for ANAT. When an agent has a v4
and v6 interface and requires just a static choice of address - use
v6 if both support v6, else v4 - ANAT alone is used. If an agent
wishes the choice of v4 or v6 to be dynamic and depend on actual
verification of connectivity, an agent would use ANAT in concert with
ICE. To do that, The agent MUST include two media stream alternates,
one for v4 and one for v6, as defined in RFC 4091. In addition, the
agent MUST include a v4 candidate as a session attribute for the v4
stream alternate, and a v6 candidate as a session attribute of the v6
stream alternate. ICE will then perform its checks for each stream
alternate. The agent MUST order the ICE selected pairs for each
stream alternate based on their mid preference, and choose the
highest one. This means that if ICE doesn't select any pair for a
stream alternate (because, for example, no checks succeeded), the
agent will choose the next highest preference pair which was
selected. This allows v6 to be used if a v6 path can be verified,
but to fallback to v4 if it cannot be verified.
This extends naturally to multiple candidates for each alternate. An
agent MAY include multiple v4 candidates for the v4 stream alternate
and multiple v6 candidates for the v6 stream alternate. All of the
candidates for a v4 stream alternate MUST be v4, and all of the
candidates for a v6 stream alternate MUST be v6. This will cause ICE
to choose a v6 pair as long as one of the pairs works, else it will
fall back to v4.
Of course, an agent can use ICE with v4 and v6 candidates without
ANAT. In that mode, it would have just a single media stream, with a
default destination that is either v4 or v6. The candidates can
include both v4 and v6 candidates. This brings an agent the
flexibility of choosing a v4 candidate even if a v6 candidate
validates, perhaps due to differing path characteristics measured
dynamically by the agent. That kind of flexibility is not possible
when ANAT is used.
14. Extensibility Considerations
This specification makes very specific choices about how both agents
in a session coordinate to arrive at the set of candidate pairs that
are selected for media. It is anticipated that future specifications
will want to alter these algorithms, whether they are simple changes
like timer tweaks, or larger changes like a revamp of the priority
algorithm. When such a change is made, providing interoperability
between the two agents in a session is critical.
First, ICE provides the a=ice-options SDP attribute. Each extension
or change to ICE is associated with a token. When an agent
supporting such an extension or change generates an offer or an
answer, it MUST include the token for that extension in this
attribute. This allows each side to know what the other side is
doing. This attribute MUST NOT be present if the agent doesn't
support any ICE extensions or changes.
At this time, no IANA registry or registration procedures are defined
for these option tags. At time of writing, it is unclear whether ICE
changes and extensions will be sufficiently common to warrrant a
registry.
One of the complications in achieving interoperability is that ICE
relies on a distributed algorithm running on both agents to converge
on an agreed set of candidate pairs. If the two agents run different
algorithms, it can be difficult to guarantee convergence on the same
candidate pairs. The regular nomination procedure described in
Section 8 eliminates some of the tight coordination by delegating the
selection algorithm completely to the controlling agent.
Consequently, when a controlling agent is communicating with a peer
that supports options it doesn't know about, the agent MUST run a
regular nomination algorithm. When regular nomination is used, ICE
will converge perfectly even when both agents use different pair
prioritization algorithms. One of the keys to such convergence are
triggered checks, which ensure that the nominated pair is validated
by both agents. Consequently, any future ICE enhancements MUST
preserve triggered checks.
ICE is also extensible to other media streams beyond RTP, and for
transport protocols beyond UDP. Extensions to ICE for non-RTP media
streams need to specify how many components they utilize, and assign
component IDS to them, starting at 1 for the most important component
ID. Specifications for new transport protocols must define how, if
at all, various steps in the ICE processing differ from UDP.
15. Grammar
This specification defines seven new SDP attributes - the This specification defines seven new SDP attributes - the
"candidate", "remote-candidates", "ice-lite", "ice-ufrag", "ice-pwd" "candidate", "remote-candidates", "ice-lite", "ice-mismatch", "ice-
"ice-options" and "ice-mismatch" attributes. ufrag", "ice-pwd" and "ice-options" attributes.
15.1. "candidate" Attribute
The candidate attribute is a media-level attribute only. It contains The candidate attribute is a media-level attribute only. It contains
a transport address for a candidate that can be used for connectivity a transport address for a candidate that can be used for connectivity
checks. checks.
The syntax of this attribute is defined using Augmented BNF as The syntax of this attribute is defined using Augmented BNF as
defined in RFC 4234 [8]: defined in RFC 4234 [8]:
candidate-attribute = "candidate" ":" foundation SP component-id SP candidate-attribute = "candidate" ":" foundation SP component-id SP
transport SP transport SP
priority SP priority SP
connection-address SP ;from RFC 4566 connection-address SP ;from RFC 4566
port ;port from RFC 4566 port ;port from RFC 4566
[SP cand-type] SP cand-type
[SP rel-addr] [SP rel-addr]
[SP rel-port] [SP rel-port]
*(SP extension-att-name SP *(SP extension-att-name SP
extension-att-value) extension-att-value)
foundation = 1*ice-char foundation = 1*ice-char
component-id = 1*DIGIT component-id = 1*DIGIT
transport = "UDP" / transport-extension transport = "UDP" / transport-extension
transport-extension = token ; from RFC 3261 transport-extension = token ; from RFC 3261
priority = 1*DIGIT priority = 1*DIGIT
cand-type = "typ" SP candidate-types cand-type = "typ" SP candidate-types
candidate-types = "host" / "srflx" / "prflx" / "relay" / token candidate-types = "host" / "srflx" / "prflx" / "relay" / token
rel-addr = "raddr" SP connection-address rel-addr = "raddr" SP connection-address
rel-port = "rport" SP port rel-port = "rport" SP port
extension-att-name = byte-string ;from RFC 4566 extension-att-name = byte-string ;from RFC 4566
extension-att-value = byte-string extension-att-value = byte-string
ice-char = ALPHA / DIGIT / "+" / "/" ice-char = ALPHA / DIGIT / "+" / "/"
The foundation is composed of one or more ice-char. The component-id This grammar encodes the primary information about a candidate: its
is a positive integer, which identifies the specific component for IP address, port and transport protocol, and its properties: the
which the transport address is a candidate. It MUST start at 1 and foundation, component ID, priority, type, and related transport
MUST increment by 1 for each component of a particular candidate. address:
The connect-address production is taken from RFC 4566 [10], allowing
for IPv4 addresses, IPv6 addresses and FQDNs. The port production is
also taken from RFC 4566 [10]. The token production is taken from
RFC 3261 [3]. The transport production indicates the transport
protocol for the candidate. This specification only defines UDP.
However, extensibility is provided to allow for future transport
protocols to be used with ICE, such as TCP or the Datagram Congestion
Control Protocol (DCCP) [29].
The cand-type production encodes the type of candidate. This <connect-address>: is taken from RFC 4566 [10]. It is the IP
specification defines the values "host", "srflx", "prflx" and "relay" address of the candidate, allowing for IPv4 addresses, IPv6
for host, server reflexive, peer reflexive and relayed candidates, addresses and FQDNs. An IP address SHOULD be used, but an FQDN
MAY be used in place of an IP address. In that case, when
receiving an offer or answer containing an FQDN in an a=candidate
attribute, the FQDN is looked up in the DNS using an A or AAAA
record, and the resulting IP address is used for the remainder of
ICE processing.
<port>: is also taken from RFC 4566 [10]. It is the port of the
candidate.
<transport>: indicates the transport protocol for the candidate.
This specification only defines UDP. However, extensibility is
provided to allow for future transport protocols to be used with
ICE, such as TCP or the Datagram Congestion Control Protocol
(DCCP) [32].
<foundation>: is composed of one or more <ice-char>. It is an
identifier that is equivalent for two candidates that are of the
same type, share the same base, and come from the same STUN
server. The foundation is used to optimize ICE performance in the
Frozen algorithm.
<component-id>: is a positive integer between 1 and 256 which
identifies the specific component of the media stream for which
this is a candidate. It MUST start at 1 and MUST increment by 1
for each component of a particular candidate. For media streams
based on RTP, candidates for the actual RTP media MUST have a
component ID of 1, and candidates for RTCP MUST have a component
ID of 2. Other types of media streams which require multiple
components MUST develop specifications which define the mapping of
components to component IDs. See Section 14 for additional
discussion on extending ICE to new media streams.
<priority>: is a positive integer between 1 and (2**32 - 1).
<cand-type>: encodes the type of candidate. This specification
defines the values "host", "srflx", "prflx" and "relay" for host,
server reflexive, peer reflexive and relayed candidates,
respectively. The set of candidate types is extensible for the respectively. The set of candidate types is extensible for the
future. Inclusion of the candidate type is optional. The rel-addr future.
and rel-port productions convey information the related transport
addresses. Rules for inclusion of these values is described in
Section 4.3.
The a=candidate attribute can itself be extended. The grammar allows <rel-addr> and <rel-port>: convey transport addresses related to the
candidate, useful for diagnostics and other purposes. <rel-addr>
and <rel-port> MUST be present for server reflexive, peer
reflexive and relayed candidates. If a candidate is server or
peer reflexive, <rel-addr> and <rel-port> is equal to the base for
that server or peer reflexive candidate. If the candidate is
relayed, <rel-addr> and <rel-port> is equal to the mapped address
in the Allocate Response that provided the client with that
relayed candidate (see Appendix B.3 for a discussion of its
purpose). If the candidate is a host candidate <rel-addr> and
<rel-port> MUST be omitted.
The candidate attribute can itself be extended. The grammar allows
for new name/value pairs to be added at the end of the attribute. An for new name/value pairs to be added at the end of the attribute. An
implementation MUST ignore any name/value pairs it doesn't implementation MUST ignore any name/value pairs it doesn't
understand. understand.
15.2. "remote-candidates" Attribute
The syntax of the "remote-candidates" attribute is defined using The syntax of the "remote-candidates" attribute is defined using
Augmented BNF as defined in RFC 4234 [8]. The remote-candidates Augmented BNF as defined in RFC 4234 [8]. The remote-candidates
attribute is a media level attribute only. attribute is a media level attribute only.
remote-candidate-att = "remote-candidates" ":" remote-candidate remote-candidate-att = "remote-candidates" ":" remote-candidate
0*(SP remote-candidate) 0*(SP remote-candidate)
remote-candidate = component-ID SP connection-address SP port remote-candidate = component-ID SP connection-address SP port
The attribute contains a connection-address and port for each The attribute contains a connection-address and port for each
component. The ordering of components is irrelevant. However, a component. The ordering of components is irrelevant. However, a
value MUST be present for each component of a media stream. value MUST be present for each component of a media stream. This
attribute MUST be included in an offer by a controlling agent for a
media stream that is Completed, and MUST NOT be included in any other
case.
The syntax of the "ice-lite" and "ice-mismatch", both of which are 15.3. "ice-lite" and "ice-mismatch" Attributes
flags, is:
The syntax of the "ice-lite" and "ice-mismatch" attributes, both of
which are flags, is:
ice-lite = "ice-lite" ice-lite = "ice-lite"
ice-mismatch = "ice-mismatch" ice-mismatch = "ice-mismatch"
"ice-lite" is a session level attribute only, and "ice-mismatch" is a "ice-lite" is a session level attribute only, and indicates that an
media level attribute only. The syntax of the "ice-pwd" and "ice- agent is a lite implementation. "ice-mismatch" is a media level
ufrag" attributes are defined as: attribute only, and when present in an answer, indicates that the
offer arrived with a default destination for a media component that
didn't have a corresponding candidate attribute.
15.4. "ice-ufrag" and "ice-pwd" Attributes
The "ice-ufrag" and "ice-pwd" attributes convey the username fragment
and password used by ICE for message integrity. Their syntax is:
ice-pwd-att = "ice-pwd" ":" password ice-pwd-att = "ice-pwd" ":" password
ice-ufrag-att = "ice-ufrag" ":" ufrag ice-ufrag-att = "ice-ufrag" ":" ufrag
password = 22*ice-char password = 22*ice-char
ufrag = 4*ice-char ufrag = 4*ice-char
The "ice-pwd" and "ice-ufrag" attributes can appear at either the The "ice-pwd" and "ice-ufrag" attributes can appear at either the
session-level or media-level. When present in both, the value in the session-level or media-level. When present in both, the value in the
media-level takes precedence. Thus, the value at the session level media-level takes precedence. Thus, the value at the session level
is effectively a default that applies to all media streams, unless is effectively a default that applies to all media streams, unless
overriden by a media-level value. overriden by a media-level value. Whether present at the session or
media level, there MUST be an ice-pwd and ice-ufrag attribute for
each media stream. If two media streams have identical ice-ufrag's,
they MUST have identical ice-pwd's.
The ice-ufrag and ice-pwd attributes MUST be chosen randomly at the
beginning of a session. The ice-ufrag attribute MUST contain at
least 24 bits of randomness, and the ice-pwd attribute MUST contain
at least 128 bits of randomness. This means that the ice-ufrag
attribute will be at least 4 characters long, and the ice-pwd at
least 22 characters long, since the grammar for these attributes
allows for 6 bits of randomness per character. The attributes MAY be
longer than 4 and 22 characters respectively, of course.
15.5. "ice-options> Attribute
The "ice-options" attribute is a session level attribute. It The "ice-options" attribute is a session level attribute. It
contains a series of tokens which identify the options supported by contains a series of tokens which identify the options supported by
the agent. Its grammar is: the agent. Its grammar is:
ice-options = "ice-options" ":" ice-option-tag ice-options = "ice-options" ":" ice-option-tag
0*(SP ice-option-tag) 0*(SP ice-option-tag)
ice-option-tag = 1*ice-char ice-option-tag = 1*ice-char
14. Extensibility Considerations 16. Example
This specification makes very specific choices about how both agents
in a session coordinate to arrive at the set of candidate pairs that
are selected for media. It is anticipated that future specifications
will want to alter these algorithms, whether they are simple changes
like timer tweaks, or larger changes like a revamp of the priority
algorithm. When such a change is made, providing interoperability
between the two agents in a session is critical.
Firstly, ICE provides the a=ice-options SDP attribute. Each
extension or change to ICE is associated with a token. When an agent
supporting such an extension or change generates an offer or an
answer, it MUST include the token for that extension in this
attribute. This allows each side to know what the other side is
doing. This attribute MUST NOT be present if the agent doesn't
support any ICE extensions or changes.
At this time, no IANA registry or registration procedures are defined The example is based on the simplified topology of Figure 15.
for these option tags. At time of writing, it is unclear whether ICE
changes and extensions will be sufficiently common to warrrant a
registry.
One of the complications in achieving interoperability is that ICE +-----+
relies on a distributed algorithm running on both agents to converge | |
on an agreed set of candidate pairs. If the two agents run different |STUN |
algorithms, it can be difficult to guarantee convergence on the same | Srvr|
candidate pairs. The introspective selection procedure described in +-----+
Section 8 eliminates some of the tight coordination by delegating the |
selection algorithm completely to the controlling agent. +---------------------+
Consequently, when a controlling agent is communicating with a peer | |
that supports options it doesn't know about, the agent MUST run an | Internet |
introspective selection algorithm. When introspective selection is | |
used, ICE will converge perfectly even when both agents use different | |
pair prioritization algorithms. One of the keys to such convergence +---------------------+
are triggered checks, which ensure that the favored pair is validated | |
by both agents. Consequently, any future ICE enhancements MUST | |
preserve triggered checks. +---------+ |
| NAT | |
+---------+ |
| |
| |
| |
+-----+ +-----+
| | | |
| L | | R |
| | | |
+-----+ +-----+
15. Example Figure 15: Example Topology
Two agents, L and R, are using ICE. Both are full-mode ICE Two agents, L and R, are using ICE. Both are full-mode ICE
implementations. Both agents have a single IPv4 interface. For implementations. Both agents have a single IPv4 interface. For
agent L, it is 10.0.1.1, and for agent R, 192.0.2.1. Both are agent L, it is 10.0.1.1 in private address space [28], and for agent
configured with a single STUN server each (indeed, the same one for R, 192.0.2.1 on the public Internet. Both are configured with the
each), which is listening for STUN requests at an IP address of same STUN server (shown in this example for simplicity, although in
192.0.2.2 and port 3478. This STUN server supports only the Binding practice the agents do not need to use the same STUN server), which
Discovery usage; relays are not used in this example. Agent L is is listening for STUN requests at an IP address of 192.0.2.2 and port
behind a NAT, and agent R is on the public Internet. The NAT has an 3478. This STUN server supports only the Binding Discovery usage;
endpoint independent mapping property and an address dependent relays are not used in this example. Agent L is behind a NAT, and
filtering property. The public side of the NAT has an IP address of agent R is on the public Internet. The NAT has an endpoint
192.0.2.3. independent mapping property and an address dependent filtering
property. The public side of the NAT has an IP address of 192.0.2.3.
To facilitate understanding, transport addresses are listed using To facilitate understanding, transport addresses are listed using
variables that have mnemonic names. The format of the name is variables that have mnemonic names. The format of the name is
entity-type-seqno, where entity refers to the entity whose interface entity-type-seqno, where entity refers to the entity whose interface
the transport address is on, and is one of "L", "R", "STUN", or the transport address is on, and is one of "L", "R", "STUN", or
"NAT". The type is either "PUB" for transport addresses that are "NAT". The type is either "PUB" for transport addresses that are
public, and "PRIV" for transport addresses that are private. public, and "PRIV" for transport addresses that are private.
Finally, seq-no is a sequence number that is different for each Finally, seq-no is a sequence number that is different for each
transport address of the same type on a particular entity. Each transport address of the same type on a particular entity. Each
variable has an IP address and port, denoted by varname.IP and variable has an IP address and port, denoted by varname.IP and
skipping to change at page 51, line 29 skipping to change at page 69, line 8
|D=$R-PUB-1 | | | |D=$R-PUB-1 | | |
|MA=$R-PUB-1 | | | |MA=$R-PUB-1 | | |
|------------->| | | |------------->| | |
| |(17) Bind Res | | | |(17) Bind Res | |
| |S=$NAT-PUB-1 | | | |S=$NAT-PUB-1 | |
| |D=$R-PUB-1 | | | |D=$R-PUB-1 | |
| |MA=$R-PUB-1 | | | |MA=$R-PUB-1 | |
| |---------------------------->| | |---------------------------->|
| | | |RTP flows | | | |RTP flows
Figure 11 Figure 16: Example Flow
First, agent L obtains a host candidate from its local interface (not First, agent L obtains a host candidate from its local interface (not
shown), and from that, sends a STUN Binding Request to the STUN shown), and from that, sends a STUN Binding Request to the STUN
server to get a server reflexive candidate (messages 1-4). Recall server to get a server reflexive candidate (messages 1-4). Recall
that the NAT has the address and port independent mapping property. that the NAT has the address and port independent mapping property.
Here, it creates a binding of NAT-PUB-1 for this UDP request, and Here, it creates a binding of NAT-PUB-1 for this UDP request, and
this becomes the server reflexive candidate for RTP. this becomes the server reflexive candidate for RTP.
Agent L sets a type preference of 126 for the host candidate and 100 Agent L sets a type preference of 126 for the host candidate and 100
for the server reflexive. The local preference is 65535. Based on for the server reflexive. The local preference is 65535. Based on
this, the priority of the host candidate is 2130706178 and for the this, the priority of the host candidate is 2130706178 and for the
server reflexive candidate is 1694498562. The host candidate is server reflexive candidate is 1694498562. The host candidate is
assigned a foundation of 1, and the server reflexive, a foundation of assigned a foundation of 1, and the server reflexive, a foundation of
2. It chooses its server reflexive candidate as the in-use 2. It chooses its server reflexive candidate as the default
candidate, and encodes it into the m/c-line. The resulting offer candidate, and encodes it into the m and c lines. The resulting
(message 5) looks like (lines folded for clarity): offer (message 5) looks like (lines folded for clarity):
v=0 v=0
o=jdoe 2890844526 2890842807 IN IP4 $L-PRIV-1.IP o=jdoe 2890844526 2890842807 IN IP4 $L-PRIV-1.IP
s= s=
c=IN IP4 $NAT-PUB-1.IP c=IN IP4 $NAT-PUB-1.IP
t=0 0 t=0 0
a=ice-pwd:asd88fgpdd777uzjYhagZg a=ice-pwd:asd88fgpdd777uzjYhagZg
a=ice-ufrag:8hhY a=ice-ufrag:8hhY
m=audio $NAT-PUB-1.PORT RTP/AVP 0 m=audio $NAT-PUB-1.PORT RTP/AVP 0
a=rtpmap:0 PCMU/8000 a=rtpmap:0 PCMU/8000
skipping to change at page 52, line 38 skipping to change at page 70, line 22
m=audio 45664 RTP/AVP 0 m=audio 45664 RTP/AVP 0
a=rtpmap:0 PCMU/8000 a=rtpmap:0 PCMU/8000
a=candidate:1 1 UDP 2130706178 10.0.1.1 8998 typ local a=candidate:1 1 UDP 2130706178 10.0.1.1 8998 typ local
a=candidate:2 1 UDP 1694498562 192.0.2.3 45664 typ srflx raddr a=candidate:2 1 UDP 1694498562 192.0.2.3 45664 typ srflx raddr
10.0.1.1 rport 8998 10.0.1.1 rport 8998
This offer is received at agent R. Agent R will obtain a host This offer is received at agent R. Agent R will obtain a host
candidate, and from it, obtain a server reflexive candidate (messages candidate, and from it, obtain a server reflexive candidate (messages
6-7). Since R is not behind a NAT, this candidate is identical to 6-7). Since R is not behind a NAT, this candidate is identical to
its host candidate, and they share the same base. It therefore its host candidate, and they share the same base. It therefore
discards this candidate and ends up with a single host candidate. discards this redundant candidate and ends up with a single host
With identical type and local preferences as L, the priority for this candidate. With identical type and local preferences as L, the
candidate is 2130706178. It chooses a foundation of 1 for its single priority for this candidate is 2130706178. It chooses a foundation
candidate. Its resulting answer looks like: of 1 for its single candidate. Its resulting answer looks like:
v=0 v=0
o=bob 2808844564 2808844564 IN IP4 $R-PUB-1.IP o=bob 2808844564 2808844564 IN IP4 $R-PUB-1.IP
s= s=
c=IN IP4 $R-PUB-1.IP c=IN IP4 $R-PUB-1.IP
t=0 0 t=0 0
a=ice-pwd:YH75Fviy6338Vbrhrlp8Yh a=ice-pwd:YH75Fviy6338Vbrhrlp8Yh
a=ice-ufrag:9uB6 a=ice-ufrag:9uB6
m=audio $R-PUB-1.PORT RTP/AVP 0 m=audio $R-PUB-1.PORT RTP/AVP 0
a=rtpmap:0 PCMU/8000 a=rtpmap:0 PCMU/8000
skipping to change at page 53, line 19 skipping to change at page 71, line 4
v=0 v=0
o=bob 2808844564 2808844564 IN IP4 192.0.2.1 o=bob 2808844564 2808844564 IN IP4 192.0.2.1
s= s=
c=IN IP4 192.0.2.1 c=IN IP4 192.0.2.1
t=0 0 t=0 0
a=ice-pwd:YH75Fviy6338Vbrhrlp8Yh a=ice-pwd:YH75Fviy6338Vbrhrlp8Yh
a=ice-ufrag:9uB6 a=ice-ufrag:9uB6
m=audio 3478 RTP/AVP 0 m=audio 3478 RTP/AVP 0
a=rtpmap:0 PCMU/8000 a=rtpmap:0 PCMU/8000
a=candidate:1 1 UDP 2130706178 192.0.2.1 3478 typ local a=candidate:1 1 UDP 2130706178 192.0.2.1 3478 typ local
Since neither side indicated that they are lite, the agent which sent
Since neither side indicated that they are passive-only, the agent the offer that began ICE processing (agent L) becomes the controlling
which sent the offer that began ICE processing (agent L) becomes the agent.
controlling agent.
Agents L and R both pair up the candidates. They both initially have Agents L and R both pair up the candidates. They both initially have
two. However, agent L will prune the pair containing its server two pairs. However, agent L will prune the pair containing its
reflexive candidate, resulting in just one. At agent L, this pair server reflexive candidate, resulting in just one. At agent L, this
(the check) has a local candidate of $L_PRIV_1 and remote candidate pair has a local candidate of $L_PRIV_1 and remote candidate of
of $R_PUB_1, and has a candidate pair priority of 4.57566E+18 (note $R_PUB_1, and has a candidate pair priority of 4.57566E+18 (note that
that an implementation would represent this as a 64 bit integer so as an implementation would represent this as a 64 bit integer so as not
not to lose precision). At agent R, there are two checks. The to lose precision). At agent R, there are two pairs. The highest
highest priority has a local candidate of $R_PUB_1 and remote priority has a local candidate of $R_PUB_1 and remote candidate of
candidate of $L_PRIV_1 and has a priority of 4.57566E+18, and the $L_PRIV_1 and has a priority of 4.57566E+18, and the second has a
second has a local candidate of $R_PUB_1 and remote candidate of local candidate of $R_PUB_1 and remote candidate of $NAT_PUB_1 and
$NAT_PUB_1 and priority 3.63891E+18. priority 3.63891E+18.
Agent R begins its connectivity check (message 9) for the first pair Agent R begins its connectivity check (message 9) for the first pair
(between the two host candidates). Since R is the passive agent for (between the two host candidates). Since R is the controlled agent
this session, the check omits the USE-CANDIDATE attribute. The host for this session, the check omits the USE-CANDIDATE attribute. The
candidate from agent L is private and behind a different NAT, and host candidate from agent L is private and behind a NAT, and thus
thus this check is discarded. this check won't be successful, because the packet cannot be routed
from R to L.
When agent L gets the answer, it performs its one and only When agent L gets the answer, it performs its one and only
connectivity check (messages 10-13). It implements the default connectivity check (messages 10-13). It implements the aggressive
algorithm for candidate selection, and thus includes a USE-CANDIDATE nomination algorithm, and thus includes a USE-CANDIDATE attribute in
attribute in this check. Since the check succeeds, agent L creates a this check. Since the check succeeds, agent L creates a new pair,
new pair, whose local candidate is from the mapped address in the whose local candidate is from the mapped address in the binding
binding response (NAT-PUB-1 from message 13) and whose remote response (NAT-PUB-1 from message 13) and whose remote candidate is
candidate is the destination of the request (R-PUB-1 from message the destination of the request (R-PUB-1 from message 10). This is
10). This is added to the valid list. In addition, it is marked as added to the valid list. In addition, it is marked as selected since
selected since the Binding Request contained the USE-CANDIDATE the Binding Request contained the USE-CANDIDATE attribute. Since
attribute. Since there is a selected candidate in the Valid list for there is a selected candidate in the Valid list for the one component
the one component of this media stream, ICE processing for this of this media stream, ICE processing for this stream moves into the
stream moves into the Completed state. Agent L can now send media if Completed state. Agent L can now send media if it so chooses.
it so chooses.
Upon receipt of the check from agent L (message 11), agent R will Upon receipt of the STUN Binding Request from agent L (message 11),
generate its triggered check. This check happens to match the next agent R will generate its triggered check. This check happens to
one on its check list - from its host candidate to agent L's server match the next one on its check list - from its host candidate to
reflexive candidate. This check (messages 14-17) will succeed. agent L's server reflexive candidate. This check (messages 14-17)
Consequently, agent R constructs a new candidate pair using the will succeed. Consequently, agent R constructs a new candidate pair
mapped address from the response as the local candidate (R-PUB-1) and using the mapped address from the response as the local candidate
the destination of the request (NAT-PUB-1) as the remote candidate. (R-PUB-1) and the destination of the request (NAT-PUB-1) as the
This pair is added to the Valid list for that media stream. Since remote candidate. This pair is added to the Valid list for that
the check was generated in the reverse direction of a check that media stream. Since the check was generated in the reverse direction
contained the USE-CANDIDATE attribute, the candidate pair is marked of a check that contained the USE-CANDIDATE attribute, the candidate
as selected. Consequently, processing for this stream moves into the pair is marked as selected. Consequently, processing for this stream
Completed state, and agent R can also send media. moves into the Completed state, and agent R can also send media.
16. Security Considerations 17. Security Considerations
There are several types of attacks possible in an ICE system. This There are several types of attacks possible in an ICE system. This
section considers these attacks and their countermeasures. section considers these attacks and their countermeasures.
16.1. Attacks on Connectivity Checks 17.1. Attacks on Connectivity Checks
An attacker might attempt to disrupt the STUN connectivity checks. An attacker might attempt to disrupt the STUN connectivity checks.
Ultimately, all of these attacks fool an agent into thinking Ultimately, all of these attacks fool an agent into thinking
something incorrect about the results of the connectivity checks. something incorrect about the results of the connectivity checks.
The possible false conclusions an attacker can try and cause are: The possible false conclusions an attacker can try and cause are:
False Invalid: An attacker can fool a pair of agents into thinking a 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 candidate pair is invalid, when it isn't. This can be used to
cause an agent to prefer a different candidate (such as one cause an agent to prefer a different candidate (such as one
injected by the attacker), or to disrupt a call by forcing all injected by the attacker), or to disrupt a call by forcing all
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the attacker, for eavesdropping or other purposes. the attacker, for eavesdropping or other purposes.
False Valid on False Candidate: An attacker has already convinced an False Valid on False Candidate: An attacker has already convinced an
agent that there is a candidate with an address that doesn't agent that there is a candidate with an address that doesn't
actually route to that agent (for example, by injecting a false actually route to that agent (for example, by injecting a false
peer reflexive candidate or false server reflexive candidate). It peer reflexive candidate or false server reflexive candidate). It
must then launch an attack that forces the agents to believe that must then launch an attack that forces the agents to believe that
this candidate is valid. this candidate is valid.
Of the various techniques for creating faked STUN messages described Of the various techniques for creating faked STUN messages described
in [11], many are not applicable for the connectivity checks. in [12], many are not applicable for the connectivity checks.
Compromises of STUN servers are not much of a concern, since the STUN Compromises of STUN servers are not much of a concern, since the STUN
servers are embedded in endpoints and distributed throughout the servers are embedded in endpoints and distributed throughout the
network. Thus, compromising the STUN server is equivalent to network. Thus, compromising the peer's embedded STUN server is
comprimising the endpoint, and if that happens, far more problematic equivalent to comprimising the endpoint, and if that happens, far
attacks are possible than those against ICE. Similarly, DNS attacks more problematic attacks are possible than those against ICE.
are usually irrelevant since STUN servers are not typically Similarly, DNS attacks are usually irrelevant since STUN servers are
discovered via DNS, they are signaled via IP addresses embedded in not typically discovered via DNS, they are normally signaled via IP
SDP. Injection of fake responses and relaying modified requests all addresses embedded in SDP. If the SDP does contain an FQDN for a
can be handled in ICE with the countermeasures discussed below. host, then connectivity checks would be susceptible to the DNS
vulnerabilities described in [12]; however it is far more common to
use IP addresses. Injection of fake responses and relaying modified
requests all can be handled in ICE with the countermeasures discussed
below.
To force the false invalid result, the attacker has to wait for the 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, connectivity check from one of the agents to be sent. When it is,
the attacker needs to inject a fake response with an unrecoverable the attacker needs to inject a fake response with an unrecoverable
error response, such as a 600. However, since the candidate is, in error response, such as a 600. However, since the candidate is, in
fact, valid, the original request may reach the peer agent, and fact, valid, the original request may reach the peer agent, and
result in a success response. The attacker needs to force this result in a success response. The attacker needs to force this
packet or its response to be dropped, through a DoS attack, layer 2 packet or its response to be dropped, through a DoS attack, layer 2
network disruption, or other technique. If it doesn't do this, the network disruption, or other technique. If it doesn't do this, the
success response will also reach the originator, alerting it to a success response will also reach the originator, alerting it to a
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is different. The attacker waits until one of the agents sends a is different. The attacker waits until one of the agents sends a
check. It intercepts this request, and replays it towards the other check. It intercepts this request, and replays it towards the other
agent with a faked source IP address. It must also prevent the agent with a faked source IP address. It must also prevent the
original request from reaching the remote agent, either by launching original request from reaching the remote agent, either by launching
a DoS attack to cause the packet to be dropped, or forcing it to be 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 dropped using layer 2 mechanisms. The replayed packet is received at
the other agent, and accepted, since the integrity check passes (the the other agent, and accepted, since the integrity check passes (the
integrity check cannot and does not cover the source IP address and integrity check cannot and does not cover the source IP address and
port). It is then responded to. This response will contain a XOR- port). It is then responded to. This response will contain a XOR-
MAPPED-ADDRESS with the false candidate, and will be sent to that MAPPED-ADDRESS with the false candidate, and will be sent to that
false candidate. The attacker must then intercept it and relay it false candidate. The attacker must then receive it and relay it
towards the originator. towards the originator.
The other agent will then initiate a connectivity check towards that The other agent will then initiate a connectivity check towards that
false candidate. This validation needs to succeed. This requires false candidate. This validation needs to succeed. This requires
the attacker to force a false valid on a false candidate. Injecting the attacker to force a false valid on a false candidate. Injecting
of fake requests or responses to achieve this goal is prevented using of fake requests or responses to achieve this goal is prevented using
the integrity mechanisms of STUN and the offer/answer exchange. the integrity mechanisms of STUN and the offer/answer exchange.
Thus, this attack can only be launched through replays. To do that, Thus, this attack can only be launched through replays. To do that,
the attacker must intercept the check towards this false candidate, the attacker must intercept the check towards this false candidate,
and replay it towards the other agent. Then, it must intercept the and replay it towards the other agent. Then, it must intercept the
response and replay that back as well. response and replay that back as well.
This attack is very hard to launch unless the attacker themself is This attack is very hard to launch unless the attacker is identified
identified by the fake candidate. This is because it requires the by the fake candidate. This is because it requires the attacker to
attacker to intercept and replay packets sent by two different hosts. intercept and replay packets sent by two different hosts. If both
If both agents are on different networks (for example, across the agents are on different networks (for example, across the public
public Internet), this attack can be hard to coordinate, since it Internet), this attack can be hard to coordinate, since it needs to
needs to occur against two different endpoints on different parts of occur against two different endpoints on different parts of the
the network at the same time. network at the same time.
If the attacker themself is identified by the fake candidate the If the attacker themself is identified by the fake candidate the
attack is easier to coordinate. However, if SRTP is used [22], the attack is easier to coordinate. However, if SRTP is used [23], the
attacker will not be able to play the media packets, they will only attacker will not be able to play the media packets, they will only
be able to discard them, effectively disabling the media stream for be able to discard them, effectively disabling the media stream for
the call. However, this attack requires the agent to disrupt packets the call. However, this attack requires the agent to disrupt packets
in order to block the connectivity check from reaching the target. in order to block the connectivity check from reaching the target.
In that case, if the goal is to disrupt the media stream, its much In that case, if the goal is to disrupt the media stream, its much
easier to just disrupt it with the same mechanism, rather than attack easier to just disrupt it with the same mechanism, rather than attack
ICE. ICE.
16.2. Attacks on Address Gathering 17.2. Attacks on Address Gathering
ICE endpoints make use of STUN for gathering candidates rom a STUN ICE endpoints make use of STUN for gathering candidates from a STUN
server in the network. This is corresponds to the Binding Discovery server in the network. This is corresponds to the Binding Discovery
usage of STUN described in [11]. As a consequence, the attacks usage of STUN described in [12]. As a consequence, the attacks
against STUN itself that are described in that specification can against STUN itself that are described in that specification can
still be used against the binding discovery usage when utilized with still be used against the binding discovery usage when utilized with
ICE. ICE.
However, the additional mechanisms provided by ICE actually However, the additional mechanisms provided by ICE actually
counteract such attacks, making binding discovery with STUN more counteract such attacks, making binding discovery with STUN more
secure when combined with ICE than without ICE. secure when combined with ICE than without ICE.
Consider an attacker which is able to provide an agent with a faked Consider an attacker which is able to provide an agent with a faked
mapped address in a STUN Binding Request that is used for address mapped address in a STUN Binding Request that is used for address
gathering. This is the primary attack primitive described in [11]. gathering. This is the primary attack primitive described in [12].
This address will be used as a server reflexive candidate in the ICE This address will be used as a server reflexive candidate in the ICE
exchange. For this candidate to actually be used for media, the exchange. For this candidate to actually be used for media, the
attacker must also attack the connectivity checks, and in particular, attacker must also attack the connectivity checks, and in particular,
force a false valid on a false candidate. This attack is very hard force a false valid on a false candidate. This attack is very hard
to launch if the false address identifies a third party, and is to launch if the false address identifies a fourth party (neither the
prevented by SRTP if it identifies the attacker themself. offerer, answerer, or attacker), since it requires attacking the
checks generated by each agent in the session, and is prevented by
SRTP if it identifies the attacker themself.
If the attacker elects not to attack the connectivity checks, the If the attacker elects not to attack the connectivity checks, the
worst it can do is prevent the server reflexive candidate from being 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 used. However, if the peer agent has at least one candidate that is
reachable by the agent under attack, the STUN connectivity checks reachable by the agent under attack, the STUN connectivity checks
themselves will provide a peer reflexive candidate that can be used themselves will provide a peer reflexive candidate that can be used
for the exchange of media. Peer reflexive candidates are generally for the exchange of media. Peer reflexive candidates are generally
preferred over server reflexive candidates. As such, an attack preferred over server reflexive candidates. As such, an attack
solely on the STUN address gathering will normally have no impact on solely on the STUN address gathering will normally have no impact on
a session at all. a session at all.
16.3. Attacks on the Offer/Answer Exchanges 17.3. Attacks on the Offer/Answer Exchanges
An attacker that can modify or disrupt the offer/answer exchanges An attacker that can modify or disrupt the offer/answer exchanges
themselves can readily launch a variety of attacks with ICE. They themselves can readily launch a variety of attacks with ICE. They
could direct media to a target of a DoS attack, they could insert could direct media to a target of a DoS attack, they could insert
themselves into the media stream, and so on. These are similar to themselves into the media stream, and so on. These are similar to
the general security considerations for offer/answer exchanges, and the general security considerations for offer/answer exchanges, and
the security considerations in RFC 3264 [4] apply. These require the security considerations in RFC 3264 [4] apply. These require
techniques for message integrity and encryption for offers and techniques for message integrity and encryption for offers and
answers, which are satisfied by the SIPS mechanism [3] when SIP is answers, which are satisfied by the SIPS mechanism [3] when SIP is
used. As such, the usage of SIPS with ICE is RECOMMENDED. used. As such, the usage of SIPS with ICE is RECOMMENDED.
16.4. Insider Attacks 17.4. Insider Attacks
In addition to attacks where the attacker is a third party trying to In addition to attacks where the attacker is a third party trying to
insert fake offers, answers or stun messages, there are several insert fake offers, answers or stun messages, there are several
attacks possible with ICE when the attacker is an authenticated and attacks possible with ICE when the attacker is an authenticated and
valid participant in the ICE exchange. valid participant in the ICE exchange.
16.4.1. The Voice Hammer Attack 17.4.1. The Voice Hammer Attack
The voice hammer attack is an amplification attack. In this attack, The voice hammer attack is an amplification attack. In this attack,
the attacker initiates sessions to other agents, and includes the IP the attacker initiates sessions to other agents, and maliciously
address and port of a DoS target in the m/c-line of their SDP. This includes the IP address and port of a DoS target as the destination
causes substantial amplification; a single offer/answer exchange can for media traffic signaled in the SDP. This causes substantial
create a continuing flood of media packets, possibly at high rates amplification; a single offer/answer exchange can create a continuing
(consider video sources). This attack is not specific to ICE, but flood of media packets, possibly at high rates (consider video
ICE can help provide remediation. sources). This attack is not specific to ICE, but ICE can help
provide remediation.
Specifically, if ICE is used, the agent receiving the malicious SDP Specifically, if ICE is used, the agent receiving the malicious SDP
will first peform connectivity checks to the target of media before will first perform connectivity checks to the target of media before
sending it there. If this target is a third party host, the checks sending media there. If this target is a third party host, the
will not succeed, and media is never sent. checks will not succeed, and media is never sent.
Unfortunately, ICE doesn't help if its not used, in which case an Unfortunately, ICE doesn't help if its not used, in which case an
attacker could simply send the offer without the ICE parameters. attacker could simply send the offer without the ICE parameters.
However, in environments where the set of clients are known, and However, in environments where the set of clients are known, and
limited to ones that support ICE, the server can reject any offers or limited to ones that support ICE, the server can reject any offers or
answers that don't indicate ICE support. answers that don't indicate ICE support.
16.4.2. STUN Amplification Attack 17.4.2. STUN Amplification Attack
The STUN amplification attack is similar to the voice hammer. The STUN amplification attack is similar to the voice hammer.
However, instead of voice packets being directed to the target, STUN However, instead of voice packets being directed to the target, STUN
connectivity checks are directed to the target. This attack is connectivity checks are directed to the target. The attacker sends
accomplished by having the offerer send an offer with a large number an offer with a large number of candidates, say 50. The answerer
of candidates, say 50. The answerer receives the offer, and starts receives the offer, and starts its checks, which are directed at the
its checks, which are directed at the target, and consequently, never target, and consequently, never generate a response. The answerer
generate a response. The answerer will start a new connectivity will start a new connectivity check every 20ms, and each check is a
check every 20ms, and each check is a STUN transaction consisting of STUN transaction consisting of 7 transmissions of a message 65 bytes
7 transmissions of a message 65 bytes in length (plus 28 bytes for in length (plus 28 bytes for the IP/UDP header) that runs for 7.9
the IP/UDP header) that runs for 7.9 seconds, for a total of 58 seconds, for a total of 58 bytes/second per transaction on average.
bytes/second per transaction on average. In the worst case, there In the worst case, there can be 395 transactions in progress at once
can be 395 transactions in progress at once (7.9 seconds divided by (7.9 seconds divided by 20ms), for a total of 182 kbps, just for STUN
20ms), for a total of 182 kbps, just for STUN requests. requests.
It is impossible to eliminate the amplification, but the volume can It is impossible to eliminate the amplification, but the volume can
be reduced through a variety of heuristics. Agents SHOULD limit the be reduced through a variety of heuristics. Agents SHOULD limit the
total number of connectivity checks they perform to 100. total number of connectivity checks they perform to 100.
Additionally, agents MAY limit the number of candidates they'll Additionally, agents MAY limit the number of candidates they'll
accept in an offer or answer. accept in an offer or answer.
16.5. Interactions with Application Layer Gateways and SIP 17.5. Interactions with Application Layer Gateways and SIP
Application Layer Gateways (ALGs) are functions present in a NAT Application Layer Gateways (ALGs) are functions present in a NAT
device which inspect the contents of packets and modify them, in device which inspect the contents of packets and modify them, in
order to facilitate NAT traversal for application protocols. Session order to facilitate NAT traversal for application protocols. Session
Border Controllers (SBC) are close cousins of ALGs, but are less Border Controllers (SBC) are close cousins of ALGs, but are less
transparent since they actually exist as application layer SIP transparent since they actually exist as application layer SIP
intermediaries. ICE has interactions with SBCs and ALGs. intermediaries. ICE has interactions with SBCs and ALGs.
If an ALG is SIP aware but not ICE aware, ICE will work through it as If an ALG is SIP aware but not ICE aware, ICE will work through it as
long as the ALG correctly modifies the m/c-lines of SDP. In this long as the ALG correctly modifies the SDP. In this case, correctly
case, correctly means that the ALG does not modify m/c-lines with means that the ALG does not modify the m and c lines or the rtcp
external addresses. If the m/c-line contains internal addresses, but attribute if they contain external addresses. If they contain
ones for which a public binding exists, the ALG replaces the internal internal addresses, the modification depends on the state of the ALG.
address in the m/c-line with the public binding. Unfortunately, many If the ALG already has a binding established that maps an external
ALG are known to work poorly in these corner cases. ICE does not try port to an internal IP address and port in m and c lines or rtcp
to work around broken ALGs, as this is outside the scope of its attribute , the ALG uses that binding instead of creating a new one.
functionality. ICE can help diagnose these conditions, which often Unfortunately, many ALG are known to work poorly in these corner
show up as a mismatch between the set of candidates and the m/c-line. cases. ICE does not try to work around broken ALGs, as this is
The a=ice-mismatch parameter is used for this purpose. outside the scope of its functionality. ICE can help diagnose these
conditions, which often show up as a mismatch between the set of
candidates and the m and c lines and rtcp attributes. The ice-
mismatch attribute is used for this purpose.
ICE works best through ALGs when the signaling is run over TLS. This ICE works best through ALGs when the signaling is run over TLS. This
prevents the ALG from manipulating the SDP messages and interfering prevents the ALG from manipulating the SDP messages and interfering
with ICE operation. Implementations which are expected to be with ICE operation. Implementations which are expected to be
deployed behind ALGs SHOULD provide for TLS transport of the SDP. deployed behind ALGs SHOULD provide for TLS transport of the SDP.
If an SBC is SIP aware but not ICE aware, the result depends on the If an SBC is SIP aware but not ICE aware, the result depends on the
behavior of the SBC. If it is acting as a proper Back-to-Back User behavior of the SBC. If it is acting as a proper Back-to-Back User
Agent (B2BUA), the SBC will remove any SDP attributes it doesn't Agent (B2BUA), the SBC will remove any SDP attributes it doesn't
understand, including the ICE attributes. Consequently, the call understand, including the ICE attributes. Consequently, the call
will appear to both endpoints as if the other side doesn't support will appear to both endpoints as if the other side doesn't support
ICE. This will result in ICE being disabled, and media flowing ICE. This will result in ICE being disabled, and media flowing
through the SBC, if they SBC has requested it. If, however, the SBC through the SBC, if the SBC has requested it. If, however, the SBC
passes the ICE attributes without modification, yet modifies the m/c- passes the ICE attributes without modification, yet modifies the
lines, this will be detected as an ICE mismatch, and ICE processing default destination for media (contained in the m and c lines and
is aborted for the call. It is outside of the scope of ICE for it to rtcp attribute), this will be detected as an ICE mismatch, and ICE
act as a tool for "working around" SBCs. If one is present, ICE will processing is aborted for the call. It is outside of the scope of
not be used and the SBC techniques take precedence. ICE for it to act as a tool for "working around" SBCs. If one is
present, ICE will not be used and the SBC techniques take precedence.
17. Definition of Connectivity Check Usage 18. Definition of Connectivity Check Usage
STUN [11] requires that new usages provide a specific set of STUN [12] requires that new usages provide a specific set of
information as part of their formal definition. This section meets information as part of their formal definition. This section meets
the requirements spelled out there. the requirements spelled out there.
17.1. Applicability 18.1. Applicability
This STUN usage provides a connectivity check between two peers This STUN usage provides a connectivity check between two peers
participating in an offer/answer exchange. This check serves to participating in an offer/answer exchange. This check serves to
validate a pair of candidates for usage of exchange of media. validate a pair of candidates for usage of exchange of media.
Connectivity checks also allow agents to discover reflexive Connectivity checks also allow agents to discover reflexive
candidates towards their peers, called peer reflexive candidates. candidates towards their peers, called peer reflexive candidates.
Finally, connectivity checks serve to keep NAT bindings alive. Finally, this usage allows a Binding Indication to be used to keep
NAT bindings alive.
It is fundamental to this STUN usage that the addresses and ports It is fundamental to this STUN usage that the addresses and ports
used for media are the same ones used for the Binding Requests and used for media are the same ones used for the Binding Requests and
responses. Consequently, it will be necessary to demultiplex STUN responses. Consequently, it will be necessary to demultiplex STUN
traffic from whatever the media traffic is. This demultiplexing is traffic from applications on that same port (e.g., RTP or RTCP).
done using the techniques described in [11]. This demultiplexing is done using the techniques described in [12].
17.2. Client Discovery of Server 18.2. Client Discovery of Server
The client does not follow the DNS-based procedures defined in [11]. The client does not follow the DNS-based procedures defined in [12].
Rather, the remote candidate of the check to be performed is used as Rather, the remote candidate of the check to be performed is used as
the transport address of the STUN server. Note that the STUN server the transport address of the STUN server. Note that the STUN server
is a logical entity, and is not a physically distinct server in this is a logical entity, and is not a physically distinct server in this
usage. usage.
17.3. Server Determination of Usage 18.3. Server Determination of Usage
The server is aware of this usage because it signaled this port The server is aware of this usage because it signaled transport
through the offer/answer exchange. Any STUN packets received on this addresses in its candidates on which it expects to receive STUN
port will be for the connectivity check usage. packets. Consequently, any STUN packets received on the base of a
candidate offered in SDP will be for the connectivity check usage.
17.4. New Requests or Indications 18.4. New Requests or Indications
This usage does not define any new message types. This usage does not define any new message types.
17.5. New Attributes 18.5. New Attributes
This usage defines two new attributes, PRIORITY and USE-CANDIDATE. This usage defines two new attributes, PRIORITY and USE-CANDIDATE.
The PRIORITY attribute indicates the priority that is to be The PRIORITY attribute indicates the priority that is to be
associated with a peer reflexive candidate, should one be discovered associated with a peer reflexive candidate, should one be discovered
by this check. It is a 32 bit unsigned integer, and has an attribute by this check. It is a 32 bit unsigned integer, and has an attribute
type of 0x0024. value of 0x0024.
The USE-CANDIDATE attribute indicates that the candidate pair The USE-CANDIDATE attribute indicates that the candidate pair
resulting from this check should be used for transmission of media. resulting from this check should be used for transmission of media.
The attribute has no content (the Length field of the attribute is The attribute has no content (the Length field of the attribute is
zero); it serves as a flag. It has an attribute type of 0x0025. zero); it serves as a flag. It has an attribute value of 0x0025.
17.6. New Error Response Codes 18.6. New Error Response Codes
This usage does not define any new error response codes. This usage does not define any new error response codes.
17.7. Client Procedures 18.7. Client Procedures
Client procedures are defined in Section 7.1. Client procedures are defined in Section 7.1.
17.8. Server Procedures 18.8. Server Procedures
Server procedures are defined in Section 7.2. Server procedures are defined in Section 7.2.
17.9. Security Considerations for Connectivity Check 18.9. Security Considerations for Connectivity Check
Security considerations for the connectivity check are discussed in Security considerations for the connectivity check are discussed in
Section 16. Section 17.
18. IANA Considerations 19. IANA Considerations
This specification registers new SDP attributes and new STUN This specification registers new SDP attributes and new STUN
attributes. attributes.
18.1. SDP Attributes 19.1. SDP Attributes
This specification defines seven new SDP attributes per the This specification defines seven new SDP attributes per the
procedures of Section 8.2.4 of [10]. The required information for procedures of Section 8.2.4 of [10]. The required information for
the registrations are included here. the registrations are included here.
18.1.1. candidate Attribute 19.1.1. candidate Attribute
Contact Name: Jonathan Rosenberg, jdrosen@jdrosen.net. Contact Name: Jonathan Rosenberg, jdrosen@jdrosen.net.
Attribute Name: candidate Attribute Name: candidate
Long Form: candidate Long Form: candidate
Type of Attribute: media level Type of Attribute: media level
Charset Considerations: The attribute is not subject to the charset Charset Considerations: The attribute is not subject to the charset
attribute. attribute.
Purpose: This attribute is used with Interactive Connectivity Purpose: This attribute is used with Interactive Connectivity
Establishment (ICE), and provides one of many possible candidate Establishment (ICE), and provides one of many possible candidate
addresses for communication. These addresses are validated with addresses for communication. These addresses are validated with
an end-to-end connectivity check using Simple Traversal Underneath an end-to-end connectivity check using Simple Traversal Underneath
NAT (STUN). NAT (STUN).
Appropriate Values: See Section 13 of RFC XXXX [Note to RFC-ed: Appropriate Values: See Section 15 of RFC XXXX [Note to RFC-ed:
please replace XXXX with the RFC number of this specification]. please replace XXXX with the RFC number of this specification].
18.1.2. remote-candidates Attribute 19.1.2. remote-candidates Attribute
Contact Name: Jonathan Rosenberg, jdrosen@jdrosen.net. Contact Name: Jonathan Rosenberg, jdrosen@jdrosen.net.
Attribute Name: remote-candidates Attribute Name: remote-candidates
Long Form: remote-candidates Long Form: remote-candidates
Type of Attribute: media level Type of Attribute: media level
Charset Considerations: The attribute is not subject to the charset Charset Considerations: The attribute is not subject to the charset
attribute. attribute.
Purpose: This attribute is used with Interactive Connectivity Purpose: This attribute is used with Interactive Connectivity
Establishment (ICE), and provides the identity of the remote Establishment (ICE), and provides the identity of the remote
candidates that the offerer wishes the answerer to use in its candidates that the offerer wishes the answerer to use in its
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Type of Attribute: media level Type of Attribute: media level
Charset Considerations: The attribute is not subject to the charset Charset Considerations: The attribute is not subject to the charset
attribute. attribute.
Purpose: This attribute is used with Interactive Connectivity Purpose: This attribute is used with Interactive Connectivity
Establishment (ICE), and provides the identity of the remote Establishment (ICE), and provides the identity of the remote
candidates that the offerer wishes the answerer to use in its candidates that the offerer wishes the answerer to use in its
answer. answer.
Appropriate Values: See Section 13 of RFC XXXX [Note to RFC-ed: Appropriate Values: See Section 15 of RFC XXXX [Note to RFC-ed:
please replace XXXX with the RFC number of this specification]. please replace XXXX with the RFC number of this specification].
18.1.3. ice-lite Attribute 19.1.3. ice-lite Attribute
Contact Name: Jonathan Rosenberg, jdrosen@jdrosen.net. Contact Name: Jonathan Rosenberg, jdrosen@jdrosen.net.
Attribute Name: ice-lite Attribute Name: ice-lite
Long Form: ice-lite Long Form: ice-lite
Type of Attribute: session level Type of Attribute: session level
Charset Considerations: The attribute is not subject to the charset Charset Considerations: The attribute is not subject to the charset
attribute. attribute.
Purpose: This attribute is used with Interactive Connectivity Purpose: This attribute is used with Interactive Connectivity
Establishment (ICE), and indicates that an agent has the minimum Establishment (ICE), and indicates that an agent has the minimum
functionality required to support ICE inter-operation with a peer functionality required to support ICE inter-operation with a peer
that has a full implementation. that has a full implementation.
Appropriate Values: See Section 13 of RFC XXXX [Note to RFC-ed: Appropriate Values: See Section 15 of RFC XXXX [Note to RFC-ed:
please replace XXXX with the RFC number of this specification]. please replace XXXX with the RFC number of this specification].
18.1.4. ice-mismatch Attribute 19.1.4. ice-mismatch Attribute
Contact Name: Jonathan Rosenberg, jdrosen@jdrosen.net. Contact Name: Jonathan Rosenberg, jdrosen@jdrosen.net.
Attribute Name: ice-mismatch Attribute Name: ice-mismatch
Long Form: ice-mismatch Long Form: ice-mismatch
Type of Attribute: session level Type of Attribute: session level
Charset Considerations: The attribute is not subject to the charset Charset Considerations: The attribute is not subject to the charset
attribute. attribute.
Purpose: This attribute is used with Interactive Connectivity Purpose: This attribute is used with Interactive Connectivity
Establishment (ICE), and indicates that an agent is ICE capable, Establishment (ICE), and indicates that an agent is ICE capable,
but did not proceed with ICE due to a mismatch of candidates with but did not proceed with ICE due to a mismatch of candidates with
the values in the m/c-line. the default destination for media signaled in the SDP.
Appropriate Values: See Section 13 of RFC XXXX [Note to RFC-ed: Appropriate Values: See Section 15 of RFC XXXX [Note to RFC-ed:
please replace XXXX with the RFC number of this specification]. please replace XXXX with the RFC number of this specification].
18.1.5. ice-pwd Attribute 19.1.5. ice-pwd Attribute
Contact Name: Jonathan Rosenberg, jdrosen@jdrosen.net. Contact Name: Jonathan Rosenberg, jdrosen@jdrosen.net.
Attribute Name: ice-pwd Attribute Name: ice-pwd
Long Form: ice-pwd Long Form: ice-pwd
Type of Attribute: session or media level Type of Attribute: session or media level
Charset Considerations: The attribute is not subject to the charset Charset Considerations: The attribute is not subject to the charset
attribute. attribute.
Purpose: This attribute is used with Interactive Connectivity Purpose: This attribute is used with Interactive Connectivity
Establishment (ICE), and provides the password used to protect Establishment (ICE), and provides the password used to protect
STUN connectivity checks. STUN connectivity checks.
Appropriate Values: See Section 13 of RFC XXXX [Note to RFC-ed: Appropriate Values: See Section 15 of RFC XXXX [Note to RFC-ed:
please replace XXXX with the RFC number of this specification]. please replace XXXX with the RFC number of this specification].
18.1.6. ice-ufrag Attribute 19.1.6. ice-ufrag Attribute
Contact Name: Jonathan Rosenberg, jdrosen@jdrosen.net. Contact Name: Jonathan Rosenberg, jdrosen@jdrosen.net.
Attribute Name: ice-ufrag Attribute Name: ice-ufrag
Long Form: ice-ufrag Long Form: ice-ufrag
Type of Attribute: session or media level Type of Attribute: session or media level
Charset Considerations: The attribute is not subject to the charset Charset Considerations: The attribute is not subject to the charset
attribute. attribute.
Purpose: This attribute is used with Interactive Connectivity Purpose: This attribute is used with Interactive Connectivity
Establishment (ICE), and provides the fragments used to construct Establishment (ICE), and provides the fragments used to construct
the username in STUN connectivity checks. the username in STUN connectivity checks.
Appropriate Values: See Section 13 of RFC XXXX [Note to RFC-ed: Appropriate Values: See Section 15 of RFC XXXX [Note to RFC-ed:
please replace XXXX with the RFC number of this specification]. please replace XXXX with the RFC number of this specification].
18.1.7. ice-options Attribute 19.1.7. ice-options Attribute
Contact Name: Jonathan Rosenberg, jdrosen@jdrosen.net. Contact Name: Jonathan Rosenberg, jdrosen@jdrosen.net.
Attribute Name: ice-options Attribute Name: ice-options
Long Form: ice-options Long Form: ice-options
Type of Attribute: session level Type of Attribute: session level
Charset Considerations: The attribute is not subject to the charset Charset Considerations: The attribute is not subject to the charset
attribute. attribute.
Purpose: This attribute is used with Interactive Connectivity Purpose: This attribute is used with Interactive Connectivity
Establishment (ICE), and indicates the ICE options or extensions Establishment (ICE), and indicates the ICE options or extensions
used by the agent. used by the agent.
Appropriate Values: See Section 13 of RFC XXXX [Note to RFC-ed: Appropriate Values: See Section 15 of RFC XXXX [Note to RFC-ed:
please replace XXXX with the RFC number of this specification]. please replace XXXX with the RFC number of this specification].
18.2. STUN Attributes 19.2. STUN Attributes
This section registers two new STUN attributes per the procedures in This section registers two new STUN attributes per the procedures in
[11]. [12].
0x0024 PRIORITY 0x0024 PRIORITY
0x0025 USE-CANDIDATE 0x0025 USE-CANDIDATE
19. IAB Considerations 20. IAB Considerations
The IAB has studied the problem of "Unilateral Self Address Fixing", The IAB has studied the problem of "Unilateral Self Address Fixing",
which is the general process by which a agent attempts to determine which is the general process by which a agent attempts to determine
its address in another realm on the other side of a NAT through a its address in another realm on the other side of a NAT through a
collaborative protocol reflection mechanism [20]. ICE is an example collaborative protocol reflection mechanism [21]. ICE is an example
of a protocol that performs this type of function. Interestingly, of a protocol that performs this type of function. Interestingly,
the process for ICE is not unilateral, but bilateral, and the the process for ICE is not unilateral, but bilateral, and the
difference has a signficant impact on the issues raised by IAB. difference has a signficant impact on the issues raised by IAB.
Indeed, ICE can be considered a B-SAF (Bilateral Self-Address Fixing) Indeed, ICE can be considered a B-SAF (Bilateral Self-Address Fixing)
protocol, rather than an UNSAF protocol. Regardless, the IAB has protocol, rather than an UNSAF protocol. Regardless, the IAB has
mandated that any protocols developed for this purpose document a mandated that any protocols developed for this purpose document a
specific set of considerations. This section meets those specific set of considerations. This section meets those
requirements. requirements.
19.1. Problem Definition 20.1. Problem Definition
From RFC 3424 any UNSAF proposal must provide: From RFC 3424 any UNSAF proposal must provide:
Precise definition of a specific, limited-scope problem that is to Precise definition of a specific, limited-scope problem that is to
be solved with the UNSAF proposal. A short term fix should not be be solved with the UNSAF proposal. A short term fix should not be
generalized to solve other problems; this is why "short term fixes generalized to solve other problems; this is why "short term fixes
usually aren't". usually aren't".
The specific problems being solved by ICE are: The specific problems being solved by ICE are:
Provide a means for two peers to determine the set of transport Provide a means for two peers to determine the set of transport
addresses which can be used for communication. addresses which can be used for communication.
Provide a means for resolving many of the limitations of other Provide a means for resolving many of the limitations of other
UNSAF mechanisms by wrapping them in an additional layer of UNSAF mechanisms by wrapping them in an additional layer of
processing (the ICE methodology). processing (the ICE methodology).
Provide a means for a agent to determine an address that is Provide a means for a agent to determine an address that is
reachable by another peer with which it wishes to communicate. reachable by another peer with which it wishes to communicate.
19.2. Exit Strategy 20.2. Exit Strategy
From RFC 3424, any UNSAF proposal must provide: From RFC 3424, any UNSAF proposal must provide:
Description of an exit strategy/transition plan. The better short Description of an exit strategy/transition plan. The better short
term fixes are the ones that will naturally see less and less use term fixes are the ones that will naturally see less and less use
as the appropriate technology is deployed. as the appropriate technology is deployed.
ICE itself doesn't easily get phased out. However, it is useful even 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 in a globally connected Internet, to serve as a means for detecting
whether a router failure has temporarily disrupted connectivity, for whether a router failure has temporarily disrupted connectivity, for
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used, because higher priority connectivity exists to the native host used, because higher priority connectivity exists to the native host
candidates. Therefore, the servers get used less and less, and can candidates. Therefore, the servers get used less and less, and can
eventually be remove when their usage goes to zero. eventually be remove when their usage goes to zero.
Indeed, ICE can assist in the transition from IPv4 to IPv6. It can 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 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 hosts communicate with SIP (IPv6 gets used). It can also allow a
network with both 6to4 and native v6 connectivity to determine which network with both 6to4 and native v6 connectivity to determine which
address to use when communicating with a peer. address to use when communicating with a peer.
19.3. Brittleness Introduced by ICE 20.3. Brittleness Introduced by ICE
From RFC3424, any UNSAF proposal must provide: From RFC3424, any UNSAF proposal must provide:
Discussion of specific issues that may render systems more Discussion of specific issues that may render systems more
"brittle". For example, approaches that involve using data at "brittle". For example, approaches that involve using data at
multiple network layers create more dependencies, increase multiple network layers create more dependencies, increase
debugging challenges, and make it harder to transition. debugging challenges, and make it harder to transition.
ICE actually removes brittleness from existing UNSAF mechanisms. In ICE actually removes brittleness from existing UNSAF mechanisms. In
particular, traditional STUN (as described in RFC 3489 [14]) has particular, traditional STUN (as described in RFC 3489 [15]) has
several points of brittleness. One of them is the discovery process several points of brittleness. One of them is the discovery process
which requires a agent to try and classify the type of NAT it is which requires a agent to try and classify the type of NAT it is
behind. This process is error-prone. With ICE, that discovery behind. This process is error-prone. With ICE, that discovery
process is simply not used. Rather than unilaterally assessing the process is simply not used. Rather than unilaterally assessing the
validity of the address, its validity is dynamically determined by validity of the address, its validity is dynamically determined by
measuring connectivity to a peer. The process of determining measuring connectivity to a peer. The process of determining
connectivity is very robust. connectivity is very robust.
Another point of brittleness in traditional STUN and any other Another point of brittleness in traditional STUN and any other
unilateral mechanism is its absolute reliance on an additional unilateral mechanism is its absolute reliance on an additional
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shared NAT between each agent and the STUN server, traditional STUN shared NAT between each agent and the STUN server, traditional STUN
may not work. With ICE, that restriction is removed. may not work. With ICE, that restriction is removed.
Traditional STUN also introduces some security considerations. Traditional STUN also introduces some security considerations.
Fortunately, those security considerations are also mitigated by ICE. Fortunately, those security considerations are also mitigated by ICE.
Consequently, ICE serves to repair the brittleness introduced in Consequently, ICE serves to repair the brittleness introduced in
other UNSAF mechanisms, and does not introduce any additional other UNSAF mechanisms, and does not introduce any additional
brittleness into the system. brittleness into the system.
19.4. Requirements for a Long Term Solution 20.4. Requirements for a Long Term Solution
From RFC 3424, any UNSAF proposal must provide: From RFC 3424, any UNSAF proposal must provide:
Identify requirements for longer term, sound technical solutions Identify requirements for longer term, sound technical solutions
-- contribute to the process of finding the right longer term -- contribute to the process of finding the right longer term
solution. solution.
Our conclusions from STUN remain unchanged. However, we feel ICE Our conclusions from STUN remain unchanged. However, we feel ICE
actually helps because we believe it can be part of the long term actually helps because we believe it can be part of the long term
solution. solution.
19.5. Issues with Existing NAPT Boxes 20.5. Issues with Existing NAPT Boxes
From RFC 3424, any UNSAF proposal must provide: From RFC 3424, any UNSAF proposal must provide:
Discussion of the impact of the noted practical issues with Discussion of the impact of the noted practical issues with
existing, deployed NA[P]Ts and experience reports. existing, deployed NA[P]Ts and experience reports.
A number of NAT boxes are now being deployed into the market which A number of NAT boxes are now being deployed into the market which
try and provide "generic" ALG functionality. These generic ALGs hunt try and provide "generic" ALG functionality. These generic ALGs hunt
for IP addresses, either in text or binary form within a packet, and for IP addresses, either in text or binary form within a packet, and
rewrite them if they match a binding. This interferes with rewrite them if they match a binding. This interferes with
traditional STUN. However, the update to STUN [11] uses an encoding tradit