--- 1/draft-ietf-mmusic-ice-12.txt 2007-01-17 22:12:42.000000000 +0100 +++ 2/draft-ietf-mmusic-ice-13.txt 2007-01-17 22:12:43.000000000 +0100 @@ -1,18 +1,18 @@ MMUSIC J. Rosenberg Internet-Draft Cisco Systems -Expires: April 26, 2007 October 23, 2006 +Expires: July 20, 2007 January 16, 2007 Interactive Connectivity Establishment (ICE): A Methodology for Network Address Translator (NAT) Traversal for Offer/Answer Protocols - draft-ietf-mmusic-ice-12 + draft-ietf-mmusic-ice-13 Status of this Memo By submitting this Internet-Draft, each author represents that any applicable patent or other IPR claims of which he or she is aware have been or will be disclosed, and any of which he or she becomes aware will be disclosed, in accordance with Section 6 of BCP 79. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF), its areas, and its working groups. Note that @@ -23,190 +23,205 @@ and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress." The list of current Internet-Drafts can be accessed at http://www.ietf.org/ietf/1id-abstracts.txt. The list of Internet-Draft Shadow Directories can be accessed at http://www.ietf.org/shadow.html. - This Internet-Draft will expire on April 26, 2007. + This Internet-Draft will expire on July 20, 2007. Copyright Notice - Copyright (C) The Internet Society (2006). + Copyright (C) The Internet Society (2007). Abstract This document describes a protocol for Network Address Translator (NAT) traversal for multimedia session signaling protocols based on the offer/answer model, such as the Session Initiation Protocol (SIP). This protocol is called Interactive Connectivity - Establishment (ICE). ICE makes use of the Simple Traversal - Underneath NAT (STUN) protocol, applying its binding discovery and + Establishment (ICE). ICE makes use of the Session Traversal + Utilities for NAT (STUN) protocol, applying its binding discovery and relay usages, in addition to defining a new usage for checking connectivity between peers. Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 5 2. Overview of ICE . . . . . . . . . . . . . . . . . . . . . . . 5 2.1. Gathering Candidate Addresses . . . . . . . . . . . . . . 7 2.2. Connectivity Checks . . . . . . . . . . . . . . . . . . . 9 2.3. Sorting Candidates . . . . . . . . . . . . . . . . . . . . 10 2.4. Frozen Candidates . . . . . . . . . . . . . . . . . . . . 11 2.5. Security for Checks . . . . . . . . . . . . . . . . . . . 11 - 2.6. Concluding ICE . . . . . . . . . . . . . . . . . . . . . . 11 - 2.7. Passive-Only Agents . . . . . . . . . . . . . . . . . . . 12 + 2.6. Concluding ICE . . . . . . . . . . . . . . . . . . . . . . 12 + 2.7. Lite Implementations . . . . . . . . . . . . . . . . . . . 13 3. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 13 - 4. Choosing a Mode . . . . . . . . . . . . . . . . . . . . . . . 15 - 5. Sending the Initial Offer . . . . . . . . . . . . . . . . . . 15 - 5.1. Gathering Candidates . . . . . . . . . . . . . . . . . . . 16 - 5.2. Prioritizing Candidates . . . . . . . . . . . . . . . . . 18 - 5.3. Choosing In-Use Candidates . . . . . . . . . . . . . . . . 20 - 5.4. Encoding the SDP . . . . . . . . . . . . . . . . . . . . . 20 - 6. Receiving the Initial Offer . . . . . . . . . . . . . . . . . 22 - 6.1. Verifying ICE Support . . . . . . . . . . . . . . . . . . 22 - 6.2. Determining Role . . . . . . . . . . . . . . . . . . . . . 23 - 6.3. Gathering Candidates . . . . . . . . . . . . . . . . . . . 23 - 6.4. Prioritizing Candidates . . . . . . . . . . . . . . . . . 23 - 6.5. Choosing In Use Candidates . . . . . . . . . . . . . . . . 23 - 6.6. Encoding the SDP . . . . . . . . . . . . . . . . . . . . . 23 - 6.7. Forming the Check Lists . . . . . . . . . . . . . . . . . 23 - 6.8. Performing Periodic Checks . . . . . . . . . . . . . . . . 26 - 7. Receipt of the Initial Answer . . . . . . . . . . . . . . . . 27 - 7.1. Verifying ICE Support . . . . . . . . . . . . . . . . . . 27 - 7.2. Determining Role . . . . . . . . . . . . . . . . . . . . . 27 - 7.3. Forming the Check List . . . . . . . . . . . . . . . . . . 27 - 7.4. Performing Periodic Checks . . . . . . . . . . . . . . . . 27 - 8. Connectivity Checks . . . . . . . . . . . . . . . . . . . . . 27 - 8.1. Client Procedures . . . . . . . . . . . . . . . . . . . . 28 - 8.1.1. Sending the Request . . . . . . . . . . . . . . . . . 28 - 8.1.2. Processing the Response . . . . . . . . . . . . . . . 29 - 8.2. Server Procedures . . . . . . . . . . . . . . . . . . . . 30 - 9. Concluding ICE . . . . . . . . . . . . . . . . . . . . . . . . 32 - 10. Subsequent Offer/Answer Exchanges . . . . . . . . . . . . . . 33 - 10.1. Generating the Offer . . . . . . . . . . . . . . . . . . . 33 - 10.2. Receiving the Offer and Generating an Answer . . . . . . . 34 - 10.3. Updating the Check and Valid Lists . . . . . . . . . . . . 35 - 11. Keepalives . . . . . . . . . . . . . . . . . . . . . . . . . . 37 - 12. Media Handling . . . . . . . . . . . . . . . . . . . . . . . . 38 - 12.1. Sending Media . . . . . . . . . . . . . . . . . . . . . . 38 - 12.2. Receiving Media . . . . . . . . . . . . . . . . . . . . . 39 - 13. Usage with SIP . . . . . . . . . . . . . . . . . . . . . . . . 39 - 13.1. Latency Guidelines . . . . . . . . . . . . . . . . . . . . 39 - 13.2. Interactions with Forking . . . . . . . . . . . . . . . . 40 - 13.3. Interactions with Preconditions . . . . . . . . . . . . . 41 - 13.4. Interactions with Third Party Call Control . . . . . . . . 41 - 14. Grammar . . . . . . . . . . . . . . . . . . . . . . . . . . . 42 - 15. Example . . . . . . . . . . . . . . . . . . . . . . . . . . . 44 - 16. Security Considerations . . . . . . . . . . . . . . . . . . . 49 - 16.1. Attacks on Connectivity Checks . . . . . . . . . . . . . . 49 - 16.2. Attacks on Address Gathering . . . . . . . . . . . . . . . 52 - 16.3. Attacks on the Offer/Answer Exchanges . . . . . . . . . . 52 - 16.4. Insider Attacks . . . . . . . . . . . . . . . . . . . . . 52 - 16.4.1. The Voice Hammer Attack . . . . . . . . . . . . . . . 53 - 16.4.2. STUN Amplification Attack . . . . . . . . . . . . . . 53 - 17. Definition of Connectivity Check Usage . . . . . . . . . . . . 54 - 17.1. Applicability . . . . . . . . . . . . . . . . . . . . . . 54 - 17.2. Client Discovery of Server . . . . . . . . . . . . . . . . 54 - 17.3. Server Determination of Usage . . . . . . . . . . . . . . 54 - 17.4. New Requests or Indications . . . . . . . . . . . . . . . 54 - 17.5. New Attributes . . . . . . . . . . . . . . . . . . . . . . 54 - 17.6. New Error Response Codes . . . . . . . . . . . . . . . . . 55 - 17.7. Client Procedures . . . . . . . . . . . . . . . . . . . . 55 - 17.8. Server Procedures . . . . . . . . . . . . . . . . . . . . 55 - 17.9. Security Considerations for Connectivity Check . . . . . . 55 - 18. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 55 - 18.1. SDP Attributes . . . . . . . . . . . . . . . . . . . . . . 55 - 18.1.1. candidate Attribute . . . . . . . . . . . . . . . . . 55 - 18.1.2. remote-candidates Attribute . . . . . . . . . . . . . 56 - 18.1.3. ice-passive Attribute . . . . . . . . . . . . . . . . 56 - 18.1.4. ice-pwd Attribute . . . . . . . . . . . . . . . . . . 57 - 18.1.5. ice-ufrag Attribute . . . . . . . . . . . . . . . . . 57 - 18.2. STUN Attributes . . . . . . . . . . . . . . . . . . . . . 58 - 19. IAB Considerations . . . . . . . . . . . . . . . . . . . . . . 58 - 19.1. Problem Definition . . . . . . . . . . . . . . . . . . . . 58 - 19.2. Exit Strategy . . . . . . . . . . . . . . . . . . . . . . 59 - 19.3. Brittleness Introduced by ICE . . . . . . . . . . . . . . 59 - 19.4. Requirements for a Long Term Solution . . . . . . . . . . 60 - 19.5. Issues with Existing NAPT Boxes . . . . . . . . . . . . . 60 - 20. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 61 - 21. References . . . . . . . . . . . . . . . . . . . . . . . . . . 61 - 21.1. Normative References . . . . . . . . . . . . . . . . . . . 61 - 21.2. Informative References . . . . . . . . . . . . . . . . . . 62 - Appendix A. Passive-Only ICE . . . . . . . . . . . . . . . . . . 64 - Appendix B. Design Motivations . . . . . . . . . . . . . . . . . 66 - B.1. Pacing of STUN Transactions . . . . . . . . . . . . . . . 66 - B.2. Candidates with Multiple Bases . . . . . . . . . . . . . . 67 - B.3. Purpose of the Translation . . . . . . . . . . . . . . . . 69 - B.4. Importance of the STUN Username . . . . . . . . . . . . . 69 - B.5. The Candidate Pair Sequence Number Formula . . . . . . . . 70 - B.6. The Frozen State . . . . . . . . . . . . . . . . . . . . . 71 - B.7. The remote-candidates attribute . . . . . . . . . . . . . 71 - B.8. Why are Keepalives Needed? . . . . . . . . . . . . . . . . 72 - B.9. Why Prefer Peer Reflexive Candidates? . . . . . . . . . . 73 - B.10. Why Send an Updated Offer? . . . . . . . . . . . . . . . . 73 - Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 74 - Intellectual Property and Copyright Statements . . . . . . . . . . 75 + 4. Sending the Initial Offer . . . . . . . . . . . . . . . . . . 16 + 4.1. Full Implementation Requirements . . . . . . . . . . . . . 16 + 4.1.1. Gathering Candidates . . . . . . . . . . . . . . . . . 16 + 4.1.2. Prioritizing Candidates . . . . . . . . . . . . . . . 18 + 4.1.3. Choosing In-Use Candidates . . . . . . . . . . . . . . 20 + 4.2. Lite Implementation . . . . . . . . . . . . . . . . . . . 20 + 4.3. Encoding the SDP . . . . . . . . . . . . . . . . . . . . . 21 + 5. Receiving the Initial Offer . . . . . . . . . . . . . . . . . 22 + 5.1. Verifying ICE Support . . . . . . . . . . . . . . . . . . 23 + 5.2. Determining Role . . . . . . . . . . . . . . . . . . . . . 23 + 5.3. Gathering Candidates . . . . . . . . . . . . . . . . . . . 24 + 5.4. Prioritizing Candidates . . . . . . . . . . . . . . . . . 24 + 5.5. Choosing In Use Candidates . . . . . . . . . . . . . . . . 24 + 5.6. Encoding the SDP . . . . . . . . . . . . . . . . . . . . . 24 + 5.7. Forming the Check Lists . . . . . . . . . . . . . . . . . 24 + 5.8. Performing Periodic Checks . . . . . . . . . . . . . . . . 27 + 6. Receipt of the Initial Answer . . . . . . . . . . . . . . . . 28 + 6.1. Verifying ICE Support . . . . . . . . . . . . . . . . . . 28 + 6.2. Determining Role . . . . . . . . . . . . . . . . . . . . . 28 + 6.3. Forming the Check List . . . . . . . . . . . . . . . . . . 28 + 6.4. Performing Periodic Checks . . . . . . . . . . . . . . . . 28 + 7. Connectivity Checks . . . . . . . . . . . . . . . . . . . . . 28 + 7.1. Client Procedures . . . . . . . . . . . . . . . . . . . . 29 + 7.1.1. Sending the Request . . . . . . . . . . . . . . . . . 29 + 7.1.2. Processing the Response . . . . . . . . . . . . . . . 30 + 7.2. Server Procedures . . . . . . . . . . . . . . . . . . . . 31 + 7.2.1. Additional Procedures for Full Implementations . . . . 32 + 7.2.2. Additional Procedures for Lite Implementations . . . . 34 + 8. Concluding ICE . . . . . . . . . . . . . . . . . . . . . . . . 34 + 9. Subsequent Offer/Answer Exchanges . . . . . . . . . . . . . . 35 + 9.1. Generating the Offer . . . . . . . . . . . . . . . . . . . 35 + 9.1.1. Additional Procedures for Full Implementations . . . . 36 + 9.1.2. Additional Procedures for Lite Implementations . . . . 37 + 9.2. Receiving the Offer and Generating an Answer . . . . . . . 37 + 9.2.1. Additional Procedures for Full Implementations . . . . 38 + 9.3. Updating the Check and Valid Lists . . . . . . . . . . . . 38 + 9.3.1. Additional Procedures for Full Implementations . . . . 38 + 10. Keepalives . . . . . . . . . . . . . . . . . . . . . . . . . . 40 + 11. Media Handling . . . . . . . . . . . . . . . . . . . . . . . . 41 + 11.1. Sending Media . . . . . . . . . . . . . . . . . . . . . . 41 + 11.1.1. Procedures for Full Implementations . . . . . . . . . 41 + 11.1.2. Procedures for Lite Implementations . . . . . . . . . 42 + 11.2. Receiving Media . . . . . . . . . . . . . . . . . . . . . 42 + 12. Usage with SIP . . . . . . . . . . . . . . . . . . . . . . . . 42 + 12.1. Latency Guidelines . . . . . . . . . . . . . . . . . . . . 42 + 12.2. SIP Option Tags and Media Feature Tags . . . . . . . . . . 44 + 12.3. Interactions with Forking . . . . . . . . . . . . . . . . 44 + 12.4. Interactions with Preconditions . . . . . . . . . . . . . 45 + 12.5. Interactions with Third Party Call Control . . . . . . . . 45 + 13. Grammar . . . . . . . . . . . . . . . . . . . . . . . . . . . 45 + 14. Extensibility Considerations . . . . . . . . . . . . . . . . . 48 + 15. Example . . . . . . . . . . . . . . . . . . . . . . . . . . . 49 + 16. Security Considerations . . . . . . . . . . . . . . . . . . . 54 + 16.1. Attacks on Connectivity Checks . . . . . . . . . . . . . . 54 + 16.2. Attacks on Address Gathering . . . . . . . . . . . . . . . 57 + 16.3. Attacks on the Offer/Answer Exchanges . . . . . . . . . . 57 + 16.4. Insider Attacks . . . . . . . . . . . . . . . . . . . . . 57 + 16.4.1. The Voice Hammer Attack . . . . . . . . . . . . . . . 58 + 16.4.2. STUN Amplification Attack . . . . . . . . . . . . . . 58 + 16.5. Interactions with Application Layer Gateways and SIP . . . 59 + 17. Definition of Connectivity Check Usage . . . . . . . . . . . . 59 + 17.1. Applicability . . . . . . . . . . . . . . . . . . . . . . 60 + 17.2. Client Discovery of Server . . . . . . . . . . . . . . . . 60 + 17.3. Server Determination of Usage . . . . . . . . . . . . . . 60 + 17.4. New Requests or Indications . . . . . . . . . . . . . . . 60 + 17.5. New Attributes . . . . . . . . . . . . . . . . . . . . . . 60 + 17.6. New Error Response Codes . . . . . . . . . . . . . . . . . 61 + 17.7. Client Procedures . . . . . . . . . . . . . . . . . . . . 61 + 17.8. Server Procedures . . . . . . . . . . . . . . . . . . . . 61 + 17.9. Security Considerations for Connectivity Check . . . . . . 61 + 18. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 61 + 18.1. SDP Attributes . . . . . . . . . . . . . . . . . . . . . . 61 + 18.1.1. candidate Attribute . . . . . . . . . . . . . . . . . 61 + 18.1.2. remote-candidates Attribute . . . . . . . . . . . . . 62 + 18.1.3. ice-lite Attribute . . . . . . . . . . . . . . . . . . 62 + 18.1.4. ice-mismatch Attribute . . . . . . . . . . . . . . . . 63 + 18.1.5. ice-pwd Attribute . . . . . . . . . . . . . . . . . . 63 + 18.1.6. ice-ufrag Attribute . . . . . . . . . . . . . . . . . 63 + 18.1.7. ice-options Attribute . . . . . . . . . . . . . . . . 64 + 18.2. STUN Attributes . . . . . . . . . . . . . . . . . . . . . 64 + 19. IAB Considerations . . . . . . . . . . . . . . . . . . . . . . 65 + 19.1. Problem Definition . . . . . . . . . . . . . . . . . . . . 65 + 19.2. Exit Strategy . . . . . . . . . . . . . . . . . . . . . . 65 + 19.3. Brittleness Introduced by ICE . . . . . . . . . . . . . . 66 + 19.4. Requirements for a Long Term Solution . . . . . . . . . . 67 + 19.5. Issues with Existing NAPT Boxes . . . . . . . . . . . . . 67 + 20. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 68 + 21. References . . . . . . . . . . . . . . . . . . . . . . . . . . 68 + 21.1. Normative References . . . . . . . . . . . . . . . . . . . 68 + 21.2. Informative References . . . . . . . . . . . . . . . . . . 69 + Appendix A. Lite and Full Implementations . . . . . . . . . . . . 71 + Appendix B. Design Motivations . . . . . . . . . . . . . . . . . 71 + B.1. Pacing of STUN Transactions . . . . . . . . . . . . . . . 72 + B.2. Candidates with Multiple Bases . . . . . . . . . . . . . . 72 + B.3. Purpose of the Translation . . . . . . . . . . . . . . . . 74 + B.4. Importance of the STUN Username . . . . . . . . . . . . . 74 + B.5. The Candidate Pair Sequence Number Formula . . . . . . . . 75 + B.6. The Frozen State . . . . . . . . . . . . . . . . . . . . . 76 + B.7. The remote-candidates attribute . . . . . . . . . . . . . 76 + B.8. Why are Keepalives Needed? . . . . . . . . . . . . . . . . 77 + B.9. Why Prefer Peer Reflexive Candidates? . . . . . . . . . . 78 + B.10. Why Send an Updated Offer? . . . . . . . . . . . . . . . . 78 + B.11. Why are Binding Indications Used for Keepalives? . . . . . 78 + Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 80 + Intellectual Property and Copyright Statements . . . . . . . . . . 81 1. Introduction RFC 3264 [4] defines a two-phase exchange of Session Description Protocol (SDP) messages [10] for the purposes of establishment of multimedia sessions. This offer/answer mechanism is used by protocols such as the Session Initiation Protocol (SIP) [3]. Protocols using offer/answer are difficult to operate through Network Address Translators (NAT). Because their purpose is to establish a flow of media packets, they tend to carry IP addresses within their - messages, which is known to be problematic through NAT [14]. The + messages, which is known to be problematic through NAT [15]. The protocols also seek to create a media flow directly between participants, so that there is no application layer intermediary between them. This is done to reduce media latency, decrease packet loss, and reduce the operational costs of deploying the application. However, this is difficult to accomplish through NAT. A full treatment of the reasons for this is beyond the scope of this specification. Numerous solutions have been proposed for allowing these protocols to operate through NAT. These include Application Layer Gateways - (ALGs), the Middlebox Control Protocol [15], Simple Traversal - Underneath NAT (STUN) [13] and its revision [11], the STUN Relay - Usage [12], and Realm Specific IP [17] [18] along with session - description extensions needed to make them work, such as the Session - Description Protocol (SDP) [10] attribute for the Real Time Control - Protocol (RTCP) [2]. Unfortunately, these techniques all have pros - and cons which make each one optimal in some network topologies, but - a poor choice in others. The result is that administrators and - implementors are making assumptions about the topologies of the - networks in which their solutions will be deployed. This introduces - complexity and brittleness into the system. What is needed is a - single solution which is flexible enough to work well in all - situations. + (ALGs), the Middlebox Control Protocol [16], Simple Traversal + Underneath NAT (STUN) [14] and its revision, retitled Session + Traversal Utilities for NAT [11], the STUN Relay Usage [12], and + Realm Specific IP [18] [19] along with session description extensions + needed to make them work, such as the Session Description Protocol + (SDP) [10] attribute for the Real Time Control Protocol (RTCP) [2]. + Unfortunately, these techniques all have pros and cons which make + each one optimal in some network topologies, but a poor choice in + others. The result is that administrators and implementors are + making assumptions about the topologies of the networks in which + their solutions will be deployed. This introduces complexity and + brittleness into the system. What is needed is a single solution + which is flexible enough to work well in all situations. This specification provides that solution for media streams established by signaling protocols based on the offer-answer model. It is called Interactive Connectivity Establishment, or ICE. ICE makes use of STUN and its relay extension, commonly called TURN, but uses them in a specific methodology which avoids many of the pitfalls of using any one alone. 2. Overview of ICE In a typical ICE deployment, we have two endpoints (known as agents in RFC 3264 terminology) which want to communicate. They are able to communicate indirectly via some signaling system such as SIP, by 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 - assumed to be provided via some other mechanism [31]. At the + assumed to be provided via some other mechanism [32]. At the beginning of the ICE process, the agents are ignorant of their own topologies. In particular, they might or might not be behind a NAT (or multiple tiers of NATs). ICE allows the agents to discover enough information about their topologies to find a path or paths by which they can communicate. Figure Figure 1 shows a typical environment for ICE deployment. The two endpoints are labelled L and R (for left and right, which helps visualize call flows). Both L and R are behind NATs -- though as mentioned before, they don't know that. The type of NAT and its @@ -253,24 +268,24 @@ o It's directly attached network interface (or interfaces in the case of a multihomed machine o A translated address on the public side of a NAT (a "server reflexive" address) o The address of a media relay the agent is using. Potentially, any of L's candidate transport addresses can be used to - communicate with any of R's transport addresses. In practice, - however, many combinations will not work. For instance, if L and R - are both behind NATs then their directly interface addresses are - unlikely to be able to communicate directly (this is why ICE is + communicate with any of R's candidate transport addresses. In + practice, however, many combinations will not work. For instance, if + L and R are both behind NATs then their directly interface addresses + are unlikely to be able to communicate directly (this is why ICE is needed, after all!). The purpose of ICE is to discover which pairs of addresses will work. The way that ICE does this is to systematically try all possible pairs (in a carefully sorted order) until it finds one or more that works. 2.1. Gathering Candidate Addresses In order to execute ICE, an agent has to identify all of its address candidates. Naturally, one viable candidate is one obtained directly from a local interface the client has towards the network. Such a @@ -396,50 +411,51 @@ STUN request -> \ L's <- STUN response / check <- STUN request \ R's STUN response -> / check Figure 3 As an optimization, as soon as R gets L's check message he immediately sends his own check message to L on the same candidate - pair. This accelerates the process of finding a valid candidate. + pair. This accelerates the process of finding a valid candidate, and + is called a triggered check. At the end of this handshake, both L and R know that they can send (and receive) messages end-to-end in both directions. 2.3. Sorting Candidates Because the algorithm above searches all candidate pairs, if a working pair exists it will eventually find it no matter what order the candidates are tried in. In order to produce faster (and better) results, the candidates are sorted in a specified order. The - algorithm is described in Section 5.2 but follows two general + algorithm is described in Section 4.1.2 but follows two general principles: o Each agent gives its candidates a numeric priority which is sent along with the candidate to the peer o The local and remote priorities are combined so that each agent has the same ordering for the candidate pairs. The second property is important for getting ICE to work when there are NATs in front of A and B. Frequently, NATs will not allow packets in from a host until the agent behind the NAT has sent a packet towards that host. Consequently, ICE checks in each direction will not succeed until both sides have sent a check through their respective NATs. In general the priority algorithm is designed so that candidates of similar type get similar priorities and so that more direct routes - are favored over indirect ones. Within those guidelines, however, + are preferred over indirect ones. Within those guidelines, however, agents have a fair amount of discretion about how to tune their algorithms. 2.4. Frozen Candidates The previous description only addresses the case where the agents wish to establish a single media component--i.e., a single flow with a single host-port quartet. However, in many cases (in particular RTP and RTCP) the agents actually need to establish connectivity for more than one flow. @@ -490,56 +506,67 @@ run a little longer might produce better results. More fundamentally, however, the prioritization defined by this specification may not yield "optimal" results. As an example, if the aim is to select low latency media paths, usage of a relay is a hint that latencies may be higher, but it is nothing more than a hint. An actual RTT measurement could be made, and it might demonstrate that a pair with lower priority is actually better than one with higher priority. Consequently, ICE assigns one of the agents in the role of the - controlling agent, and the other as passive. The controlling agent - runs a selection algorithm, through which it can decide when to - conclude ICE checks, and which pairs get selected. When a - controlling agent selects a pair for a particular component of a + controlling agent, and the other of the controlled agent. The + controlling agent runs a selection algorithm, through which it can + decide when to conclude ICE checks, and which pairs get selected. + The one that is selected is called the favored candidate pair. When + 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 - in the check indicating that the pair has been selected. This will - cause the passive agent to cease any other checks it has lined up for - that component, and mark the pair validated by that check as - "selected". Once there is a selected pair for each component of a - media stream, the ICE checks for that media stream are considered to - be completed, and 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. + in the check indicating that the pair has been selected. If the + controlled agent has already performed in a check in the reverse + direction that succeeded, the controlled agent considers ICE + processing to be concluded for that component. Once there is a + selected pair for each component of a media stream, the ICE checks + 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 - - STUN request + flag -> \ L's <- STUN response / check -> RTP Data <- RTP Data Figure 4 + 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 + updated offer indicating a restart. -2.7. Passive-Only Agents +2.7. Lite Implementations - ICE requires both sides of a call to support it. However, certain - agents, such as those in gateways to the PSTN, media servers, - conferencing servers, and voicemail servers, are known to not be - behind a NAT or firewall. To make it easier for these devices to - support ICE, they can operate in a "passive-only" mode (in contrast - to a "full" mode). In passive-only mode, they don't need to gather - candidates and don't act as the controlling agent. They only need to - respond to checks, generate triggered checks, and follow the rules - for sending media and keepalives. + 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, + media servers, conferencing servers, and voicemail servers, are known + to not be behind a NAT or firewall. To make it easier for these + devices to support ICE, ICE defines a special type of implementation + called "lite" (in contrast to the normal "full" implementation). A + lite implementation doesn't gather candidates; it includes only its + host candidate for any media stream. When a lite implementation + connects with a full implementation, the full agent takes the role of + the controlling agent, and the lite agent takes on the controlled + role. In addition, lite agents do not need to generate connectivity + checks, run the state machines, or compute candidate pairs. For an + informational summary of ICE processing as seen by a lite agent, see + [33]. 3. Terminology The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in RFC 2119 [1]. This specification makes use of the following terminology: Agent: As defined in RFC 3264, an agent is the protocol @@ -557,21 +584,21 @@ in order to determine its suitability for usage for receipt of media. Component: A component is a single transport address that is used to support a media stream. For media streams based on RTP, there are two components per media stream - one for RTP, and one for RTCP. Host Candidate: A candidate obtained by binding to a specific port from an interface on the host. This includes both physical interfaces and logical ones, such as ones obtained through Virtual - Private Networks (VPNs) and Realm Specific IP (RSIP) [17] (which + Private Networks (VPNs) and Realm Specific IP (RSIP) [18] (which lives at the operating system level). Server Reflexive Candidate: A candidate obtained by sending a STUN request from a host candidate to a STUN server, distinct from the peer, whose address is configured or learned by the client prior to an offer/answer exchange. Peer Reflexive Candidate: A candidate obtained by sending a STUN request from a host candidate to the STUN server running on a peer's candidate. @@ -625,57 +652,47 @@ Periodic Check: A connectivity check generated by an agent as a consequence of a timer that fires periodically, instructing it to send a check. Triggered Check: A connectivity check generated as a consequence of the receipt of a connectivity check from the peer. Valid List: An ordered set of candidate pairs for a media stream that have been validated by a successful STUN transaction. + Full: An ICE implementation that performs the complete set of + functionality defined by this specification. + + Lite: An ICE implementation that omits certain functions, + implementing only as much as is necessary for a peer + implementation that is full to gain the benefits of ICE. Lite + implementations can only act as the controlled agent in a session, + and do not gather candidates. + Controlling Agent: The STUN agent which is responsible for selecting the final choice of candidate pairs and signaling them through - STUN and an updated offer, if needed. + STUN and an updated offer, if needed. In any session, one agent + is always controlling. The other is the controlled agent. - Passive Agent: The 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. -4. Choosing a Mode - - The first step in ICE processing is selection of a mode. An ICE - agent can operate in either full mode or passive-only mode. An agent - MUST NOT act in passive-only mode unless the following are all true: - - 1. The device definitively knows that it has a public IP address. - Usage of tests and heuristics like those defined in RFC 3489 [13] - are not sufficient to make this determination. Rather, knowledge - comes from explicit configuration due to known location in the - network. Typically, this limits passive-only mode to devices - like PSTN gateways, conferencing servers, voicemail servers and - so on. - - 2. The device will only provide one candidate for each component of - each media stream, matching the values in the m/c-line for each - media stream. - - Full mode is meant for general purpose endpoints, such as softphones, - hard-phones, and other devices that may or may not be placed in - networks with public addresses. - -5. Sending the Initial Offer +4. Sending the Initial Offer In order to send the initial offer in an offer/answer exchange, an agent must gather candidates, priorize them, choose ones for - inclusion in the m/c-line, and then formulate and send the SDP. Each - of these steps is described in the subsections below. + inclusion in the m/c-line, and then formulate and send the SDP. The + first of these three steps differ for full and lite implementations. -5.1. Gathering Candidates +4.1. Full Implementation Requirements + +4.1.1. Gathering Candidates An agent gathers candidates when it believes that communications is imminent. An offerer can do this based on a user interface cue, or based on an explicit request to initiate a session. Every candidate is a transport address. It also has a type and a base. Three types are defined and gathered by this specification - host candidates, server reflexive candidates, and relayed candidates. The base of a candidate is the candidate that an agent must send from when using that candidate. @@ -692,134 +709,134 @@ every candidate) is always associated with a specific component for which it is a candidate. Each component has an ID assigned to it, called the component ID. For RTP-based media streams, the RTP itself has a component ID of 1, and RTCP a component ID of 2. If an agent is using RTCP it MUST obtain a candidate for it. If an agent is using both RTP and RTCP, it would end up with 2*K host candidates if an agent has K interfaces. The base for each host candidate is set to the candidate itself. - Agents implementing passive-only mode MUST NOT gather server - reflexive or relayed candidates. Agents implementing full mode - SHOULD obtain relayed candidates and MUST obtain server reflexive - candidates. The requirement to obtain relayed candidates is at - SHOULD strength to allow for provider variation. If they are not - used, it is RECOMMENDED that it be implemented and just disabled + Agents SHOULD obtain relayed candidates and MUST obtain server + reflexive candidates. The requirement to obtain relayed candidates + is at SHOULD strength to allow for provider variation. 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 full-mode agent next pairs each host candidate with the STUN - server with which it is configured or has discovered by some means. - This specification only considers usage of a single STUN server. - Every Ta seconds, the full-mode agent chooses another such pair (the - order is inconsequential), and sends a STUN request to the server - from that host candidate. If the full-mode agent is using both - relayed and server reflexive candidates, this request MUST be a STUN - Allocate request from the relay usage [12]. If the full-mode agent - is using only server reflexive candidates, the request MUST be a STUN - Binding request using the binding discovery usage [11]. + The agent next pairs each host candidate with the STUN server with + which it is configured or has discovered by some means. This + specification only considers usage of a single STUN server. Every Ta + seconds, the agent chooses another such pair (the order is + inconsequential), and sends a STUN request to the server from that + host candidate. If the agent is using both relayed and server + reflexive candidates, this request MUST be a STUN Allocate request + from the relay usage [12]. If the agent is using only server + reflexive candidates, the request MUST be a STUN Binding request + using the binding discovery usage [11]. The value of Ta SHOULD be configurable, and SHOULD have a default of 20ms. Note that this pacing applies only to starting STUN transactions with source and destination transport addresses (i.e., the host candidate and STUN server respectively) for which a STUN transaction has not previously been sent. Consequently, retransmissions of a STUN request are governed entirely by the retransmission rules defined in [11]. Similarly, retries of a request due to recoverable errors (such as an authentication 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 the server reflexive and relayed candidates. Implementations should be aware of the time required to do this, and if the application requires a time budget, limit the amount of candidates which are gathered. - An Allocate Response will provide the client with a server reflexive + An Allocate Response will provide the agent with a server reflexive candidate (obtained from the mapped address) and a relayed candidate in the RELAY-ADDRESS attribute. A Binding Response will provide the - client with a only server reflexive candidate (also obtained from the + agent with only a server reflexive candidate (also obtained from the mapped address). The base of the server reflexive candidate is the host candidate from which the Allocate or Binding request was sent. The base of a relayed candidate is that candidate itself. A server reflexive candidate obtained from an Allocate response is the called the "translation" of the relayed candidate obtained from the same response. The agent will need to remember the translation for the relayed candidate, since it is placed into the SDP. If a relayed candidate is identical to a host candidate (which can happen in rare cases), the relayed candidate MUST be discarded. Proper operation of ICE depends on each base being unique. - Next, a full-mode agent eliminates redundant candidates. A candidate - is redundant if its transport address equals another candidate, and - its base equals the base of that other candidate. Note that two + Next, the agent eliminates redundant candidates. A candidate is + redundant if its transport address equals another candidate, and its + base equals the base of that other candidate. Note that two candidates can have the same transport address yet have different bases, and these would not be considered redundant. - Finally, all agents assign each candidate a foundation. The + Finally, the agent assigns each candidate a foundation. The foundation is an identifier, scoped within a session. Two candidates MUST have the same foundation ID when they are of the same type (host, relayed, server reflexive, peer reflexive or relayed), their bases have the same IP address (the ports can be different), and, for reflexive and relayed candidates, the STUN servers used to 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. -5.2. Prioritizing Candidates +4.1.2. Prioritizing Candidates The prioritization process results in the assignment of a priority to - each candidate. An agent does this by determining a preference for - each type of candidate (server reflexive, peer reflexive, relayed and - host), and, when the agent is multihomed, choosing a preference for - its interfaces. These two preferences are then combined to compute - the priority for a candidate. That priority MUST be computed using - the following formula: + each candidate. Each candidate for a media stream MUST have a unique + priority. An agent SHOULD compute the priority by determining a + preference for each type of candidate (server reflexive, peer + reflexive, relayed and host), and, when the agent is multihomed, + choosing a preference for its interfaces. These two preferences are + then combined to compute the priority for a candidate. That priority + SHOULD be computed using the following formula: priority = (2^24)*(type preference) + (2^8)*(local preference) + (2^0)*(256 - component ID) The type preference MUST be an integer from 0 to 126 inclusive, and represents the preference for the type of the candidate (where the types are local, server reflexive, peer reflexive and relayed). A 126 is the highest preference, and a 0 is the lowest. Setting the value to a 0 means that candidates of this type will only be used as a last resort. The type preference MUST be identical for all candidates of the same type and MUST be different for candidates of different types. The type preference for peer reflexive candidates MUST be higher than that of server reflexive candidates. Note that - candidates gathered based on the procedures of Section 5.1 will never - be peer reflexive candidates; candidates of these type are learned - from the STUN connectivity checks performed by ICE. The 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. + candidates gathered based on the procedures of Section 4.1.1 will + never be peer reflexive candidates; candidates of these type are + learned from the STUN connectivity checks performed by ICE. The + 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 candidate. This priority will be used by ICE to determine the order of the connectivity checks and the relative preference for candidates. Consequently, what follows are some guidelines for selection of these values. 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 candidate, will the media first transit an intermediate server before - being received. Relayed candidates are clearly one type of + being received? Relayed candidates are clearly one type of candidates that involve an intermediary. Another are host candidates obtained from a VPN interface. When media is transited through an intermediary, it can increase the latency between transmission and reception. It can increase the packet losses, because of the additional router hops that may be taken. It may increase the cost of providing service, since media will be routed in and right back out of an intermediary run by the provider. If these concerns are important, the type preference for relayed candidates can be set lower than the type preference for reflexive and host candidates. Indeed, it is RECOMMENDED that in this case, host candidates have a @@ -828,54 +845,51 @@ relayed candidates have a type preference of zero. Furthermore, if an agent is multi-homed and has multiple interfaces, the local preference for host candidates from a VPN interface SHOULD have a priority of 0. Another criteria for selection of preferences is IP address family. ICE works with both IPv4 and IPv6. It therefore provides a transition mechanism that allows dual-stack hosts to prefer connectivity over IPv6, but to fall back to IPv4 in case the v6 networks are disconnected (due, for example, to a failure in a 6to4 - relay) [22]. It can also help with hosts that have both a native + relay) [23]. It can also help with hosts that have both a native IPv6 address and a 6to4 address. In such a case, lower local preferences could be assigned to the v6 interface, followed by the 6to4 interfaces, followed by the v4 interfaces. This allows a site to obtain and begin using native v6 addresses immediately, yet still fallback to 6to4 addresses when communicating with agents in other sites that do not yet have native v6 connectivity. Another criteria for selecting preferences is security. If a user is a telecommuter, and therefore connected to their corporate network 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 communicating within the enterprise, but use the local network when communicating with users outside of the enterprise. In such a case, a VPN interface would have a higher local preference than any other - interfaces. + interface. Another criteria for selecting preferences is topological awareness. This is most useful for candidates that make use of relays. In those cases, if an agent has preconfigured or dynamically discovered knowledge of the topological proximity of the relays to itself, it can use that to assign higher local preferences to candidates obtained from closer relays. -5.3. Choosing In-Use Candidates +4.1.3. Choosing In-Use Candidates A candidate is said to be "in-use" if it appears in the m/c-line of an offer or answer. When communicating with an ICE peer, being in- use implies that, should these candidates be selected by the ICE - algorithm, bidirectional media can flow and the candidates can be - used. If a candidate is selected by ICE but is not in-use, only - unidirectional media can flow and only for a brief time; the - candidate must be made in-use through an updated offer/answer - exchange. When communicating with a peer that is not ICE-aware, the + algorithm, a re-INVITE will not be required after ICE processing + completes. When communicating with a peer that is not ICE-aware, the in-use candidates will be used exclusively for the exchange of media, as defined in normal offer/answer procedures. 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 it does not contain the a=inactive SDP attribute. It is RECOMMENDED that in-use candidates be chosen based on the likelihood of those candidates to work with the peer that is being contacted. Unfortunately, it is difficult to ascertain which @@ -883,177 +897,215 @@ enterprise. To reach non-ICE capable agents within the enterprise, host candidates have to be used, since the enterprise policies may 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 - full mode agents select relayed candidates to be in-use. Passive- - only agents will, naturally, select their only candidates - the host - candidates - to be in use. + agents select relayed candidates to be in-use. -5.4. Encoding the SDP +4.2. Lite Implementation + + For each media stream, the agent allocates a single candidate for + 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 + particular media stream; ICE cannot be used to dynamically choose + one. Each component has an ID assigned to it, called the component + ID. For RTP-based media streams, the RTP itself has a component ID + of 1, and RTCP a component ID of 2. If an agent is using RTCP it + MUST obtain a candidate for it. + + Each candidate is assigned a foundation. The foundation MUST be + different for two candidates from different interfaces (which can + occur if media streams are on different interfaces), and MUST be the + same otherwise. A simple integer that increments for each interface + will suffice. In addition, each candidate MUST be assigned a unique + priority amongst all candidates for the same media stream. This + priority SHOULD be equal to 2^24*(126) + 2^8*(65535) + 256 minus the + component ID, which is 2130706432 minus the component ID. Each of + these candidates is also considered to be "in-use", since they will + be included in the m/c-line of an offer or answer. + +4.3. Encoding the SDP + + The process of encoding the SDP is identical between full and lite + implementations. The agent includes a single a=candidate media level attribute in the SDP for each candidate for that media stream. The a=candidate attribute contains the IP address, port and transport protocol for that candidate. A Fully Qualified Domain Name (FQDN) for a host MAY be used in place of a unicast 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. 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, which is the - value determined for the candidate as described in Section 5.2, and - the foundation, which is the value determined for the candidate as - described in Section 5.1. The agent SHOULD include a type for each - candidate by populating the candidate-types production with the - appropriate value - "host" for host candidates, "srflx" for server - reflexive candidates, "prflx" for peer reflexive candidates (though - these never appear in an initial offer/answer exchange), and "relay" - for relayed candidates. The related address MUST NOT be included if - a type was not included. If a type was included, the related address - SHOULD be present for server reflexive, peer reflexive and relayed - candidates. 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. + The candidate attribute also includes the priority and the + foundation. The agent SHOULD include a type for each candidate by + populating the candidate-types production with the appropriate value + - "host" for host candidates, "srflx" for server reflexive + candidates, "prflx" for peer reflexive candidates (though these never + appear in an initial offer/answer exchange), and "relay" for relayed + candidates. The related address MUST NOT be included if a type was + not included. If a type was included, the related address SHOULD be + present for server reflexive, peer reflexive and relayed candidates. + 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 credential that is exchanged in the offer/answer process. The username part of this credential is formed by concatenating a username fragment from each agent, separated by a colon. Each agent also provides a password, used to compute the message integrity for requests it receives. As such, an SDP MUST contain the ice-ufrag and ice-pwd attributes, containing the username fragment and password respectively. These can be either session or media level attributes, and thus common across all candidates for all media streams, or all candidates for a particular media stream, respectively. However, 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 21 characters respectively, + The attributes MAY be longer than 4 and 22 characters respectively, of course. - If an agent is operating in passive-only mode, it MUST include the - "a=ice-passive" session level attribute in its offer. If an agent is - in full mode, it MUST NOT include this attribute. + If an agent is a lite implementation, it MUST include an "a=ice-lite" + session level attribute in its SDP. If an agent is a full + implementation, it MUST NOT include this attribute. The m/c-line is populated with the candidates that are in-use. For streams based on RTP, this is done by placing the RTP candidate into the m and c lines respectively. If the agent is utilizing RTCP, it MUST encode the RTCP candidate into the m/c-line using the a=rtcp attribute as defined in RFC 3605 [2]. If RTCP is not in use, the agent MUST signal that using b=RS:0 and b=RR:0 as defined in RFC 3556 [5]. There MUST be a candidate attribute for each component of the media stream in the m/c-line. Once an offer or answer are sent, an agent MUST be prepared to receive both STUN and media packets on each candidate. As discussed - in Section 12.1, media packets can be sent to a candidate prior to + in Section 11.1, media packets can be sent to a candidate prior to its appearence in the m/c-line. -6. Receiving the Initial Offer +5. Receiving the Initial Offer When an agent receives an initial offer, it will check if the offeror supports ICE, determine its role, gather candidates, prioritize them, - choose one for in-use, encode and send an answer, and then form the - check lists and begin connectivity checks. + choose one for in-use, encode and send an answer, and for full + implementations, form the check lists and begin connectivity checks. -6.1. Verifying ICE Support +5.1. Verifying ICE Support The answerer will proceed with the ICE procedures defined in this specification if the following are true: o There is at least one a=candidate attribute for each media stream in the offer it just received. o For each media stream, at least one of the candidates is a match for its respective in-use component in the m/c-line. If both of these conditions are not met, the agent MUST process the SDP based on normal RFC 3264 procedures, without using any of the ICE - mechanisms described in the remainder of this specification, with the - exception of Section 11, which describes keepalive procedures. + mechanisms described in the remainder of this specification with two + exceptions. First, in all cases, the agent MUST follow the rules of + 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-passive" attribute, and - the answerer is also passive-only, the agent MUST process the SDP - based on normal RFC 3264 procedures, as if it didn't support ICE, - with the exception of Section 11, which describes keepalive - procedures. + In addition, if the offer contains the "a=ice-lite" attribute, and + the answerer is also lite, the agent MUST process the SDP based on + normal RFC 3264 procedures, as if it didn't support ICE, with the + exception of Section 10, which describes keepalive procedures. -6.2. Determining Role +5.2. Determining Role - If the agent is in passive-only mode, it assumes the passive role for - this session. If the agent is in full-mode, but its peer is in - passive-only mode (as indicated by the a=ice-passive attribute in the - SDP), the agent assumes the controlling role for this session. If - the agent and its peer are both in full-mode, the agent which - generated the offer which started the ICE processing takes on the - controlling role, and the other takes the passive role. + For each session, each agent takes on a role. There are two roles - + controlling, and controlled. The controlling agent is responsible + for selecting the candidate pairs to be used for each media stream, + and for generating the updated offer based on that selection, when + needed. The controlled agent is told which candidate pairs to use + for each media stream, and does not generate an updated offer to + signal this information in SIP. + + 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 + controlling role. If the agent and its peer are both full + implementations, the agent which generated the offer which started + the ICE processing takes on the controlling role, and the other takes + the controlled role. Based on this definition, once roles are determined for a session, they persist unless ICE is restarted, as discussed below. A restart causes a new selection of roles. -6.3. Gathering Candidates +5.3. Gathering Candidates The process for gathering candidates at the answerer is identical to - the process for the offerer as described in Section 5.1. It is + the process for the offerer as described in Section 4.1.1 for full + implementations and Section 4.2 for lite implementations. It is RECOMMENDED that this process begin immediately on receipt of the offer, prior to user acceptance of a session. Such gathering MAY even be done pre-emptively when an agent starts. -6.4. Prioritizing Candidates +5.4. Prioritizing Candidates The process for prioritizing candidates at the answerer is identical - to the process followed by the offerer, as described in Section 5.2. + to the process followed by the offerer, as described in Section 4.1.2 + for full implementations and Section 4.2 for lite implementations. -6.5. Choosing In Use Candidates +5.5. Choosing In Use Candidates The process for selecting in-use candidates at the answerer is identical to the process followed by the offerer, as described in - Section 5.3. + Section 4.1.3 for full implementations and Section 4.2 for lite + implementations. -6.6. Encoding the SDP +5.6. Encoding the SDP The process for encoding the SDP at the answerer is identical to the - process followed by the offerer, as described in Section 5.4. + process followed by the offerer, as described in Section 4.3. -6.7. Forming the Check Lists +5.7. Forming the Check Lists - A full-mode agent MUST form the check lists as described in this - section. A passive-only agent MAY do so, but there is no need. + Forming check lists is done only by full implementations. Lite + implementations MUST skip the steps defined in this section. There is one check list per in-use media stream resulting from the offer/answer exchange. A media stream is in-use as long as its port is non-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 or has a bandwidth value of zero. Each check list is a sequence of STUN connectivity checks that are performed by the agent. To form the check list for a media stream, the agent forms candidate pairs, computes a candidate pair priority, orders the pairs by priority, prunes them, and sets their states. These steps are described in this section. First, the agent takes each of its candidates for a media stream (called local candidates) and pairs them with the candidates it @@ -1069,23 +1121,23 @@ other did not. If this happens, the number of components for that media stream is effectively reduced, and considered to be equal to the minimum across both agents of the maximum component ID provided by each agent across all components for the media stream. 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 A-P be the priority for the candidate provided by the answerer. The 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-P,A-P) + 2*MAX(O-P,A-P) + (O-P>A-P?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 for each pair in most cases. One the priority is assigned, the agent sorts the candidate pairs in decreasing order of priority. If two pairs have identical priority, the ordering amongst them is arbitrary. This sorted list of candidate pairs is used to determine a sequence of connectivity checks that will be performed. Each check involves sending a request from a local candidate to a remote candidate. Since an agent cannot send requests directly from a reflexive @@ -1146,106 +1198,126 @@ Completed: In this state, the controlling agent has signaled that a candidate pair has been selected for each component. Consequently, no further ICE checks are performed. When a check list is first constructed as the consequence of an offer/answer exchange, it is placed in the Running state. ICE processing across all media streams also has a state associated with it. This state is equal to Running while checks are in 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. -6.8. Performing Periodic Checks +5.8. Performing Periodic Checks + + Checks are generated only by full implementations. Lite + implementations MUST skip the steps described in this section. An agent performs two types of checks. The first type are periodic checks. These checks occur periodically for each media stream, and involve choosing the highest priority check in the Waiting state from each check list, and performing it. The other type of check is called a triggered check. This is a check that is performed on - receipt of a connectivity check from the peer. Full mode agents MUST - generate periodic checks, and all agents MUST generate triggered - checks. This section describes how periodic checks are performed, - and thus applies only to full mode agents. + receipt of a connectivity check from the peer. This section + describes how periodic checks are performed. - Once the full-mode agent has computed the check lists as described in - Section 6.7, it sets a timer for each active check list. The timer + Once the agent has computed the check lists as described in + Section 5.7, it sets a timer for each active check list. The timer fires every Ta/N seconds, where N is the number of active check lists (initially, there is only one active check list). Implementations MAY set the timer to fire less frequently than this. Ta is the same value used to pace the gathering of candidates, as described in - Section 5.1. The first timer for each active check list fires + Section 4.1.1. The first timer for each active check list fires immediately, so that the agent performs a 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 full-mode agent MUST find the highest - priority check 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 8.1.1. 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. + When the timer fires, the agent MUST find the highest priority check + 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 username fragment for the local and remote candidates, and the password for the remote candidate. For periodic checks, the remote username fragment and password are learned directly from the SDP received from the peer, and the local username fragment is known by the agent. -7. Receipt of the Initial Answer +6. Receipt of the Initial Answer This section describes the procedures that an agent follows when it receives the answer from the peer. It verifies that its peer - supports ICE, determines its role, forms the check list and begins - performing periodic checks. + supports ICE, determines its role, and for full implementations, + forms the check list and begins performing periodic checks. -7.1. Verifying ICE Support +6.1. Verifying ICE Support The answerer will proceed with the ICE procedures defined in this specification if there is at least one a=candidate attribute for each 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 procedures, without using any of the ICE mechanisms described in the - remainder of this specification, with the exception of Section 11, + remainder of this specification, with the exception of Section 10, which describes keepalive procedures. -7.2. Determining Role + In some cases, the answer may omit a=candidate attributes for the + media streams, and instead include an a=ice-mismatch attribute for + one or more of the media streams in the SDP. This signals to the + offerer that the answerer supports ICE, but that ICE processing was + not used for the session because an intermediary modified the m/c- + lines without modifying the candidate attributes. See Section 16 for + a discussion of cases where this can happen. This specification + provides no guidance on how an agent should proceed in such a failure + case. + +6.2. Determining Role The offerer follows the same procedures described for the answerer in - Section 6.2. + Section 5.2. -7.3. Forming the Check List +6.3. Forming the Check List + Formation of check lists is performed only by full implementations. The offerer follows the same procedures described for the answerer in - Section 6.7. + Section 5.7. -7.4. Performing Periodic Checks +6.4. Performing Periodic Checks - The offerer follows the same procedures described for the answerer in - Section 6.8. + Periodic checks are performed only by full implementations. The + offerer follows the same procedures described for the answerer in + Section 5.8. -8. Connectivity Checks +7. Connectivity Checks This section describes how connectivity checks are performed. All ICE implementations are required to be compliant to [11], as opposed - to the older [13]. + to the older [14]. However, whereas a full implementation will both + generate checks (acting as a STUN client) and receive them (acting as + a STUN server), a lite implementation will only ever receive checks, + and thus will only act as a STUN server. -8.1. Client Procedures +7.1. Client Procedures -8.1.1. Sending the Request + These procedures define how an agent sends a connectivity check, + whether it is a periodic or a triggered check. These procedures are + only applicable to full implementations. + +7.1.1. Sending the Request The agent acting as the client generates a connectivity check either periodically, or triggered. In either case, the check is generated by sending a Binding Request from a local candidate, to a remote candidate. The agent must know the username fragment for both candidates and the password for the remote candidate. A Binding Request serving as a connectivity check MUST utilize a STUN short term credential. Rather than being learned from a Shared Secret request, the short term credential is exchanged in the offer/ @@ -1255,242 +1327,307 @@ 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. - A full-mode agent MUST include the PRIORITY attribute in its Binding - Request. This attribute MAY be omitted for passive-only agents. The - attribute MUST be set equal to the priority that would be assigned, - based on the algorithm in Section 5.2, to a peer reflexive candidate - learned from this check. Such a peer reflexive candidate has a - stream ID, component ID and local preference that are equal to the - host candidate from which the check is being sent, but a type - preference equal to the value associated with peer reflexive + An agent MUST include the PRIORITY attribute in its Binding Request. + The attribute MUST be set equal to the priority that would be + assigned, based on the algorithm in Section 4.1.2, to a peer + reflexive candidate learned from this check. Such a peer reflexive + candidate has a stream ID, component ID and local preference that are + equal to the host candidate from which the check is being sent, but a + type preference equal to the value associated with peer reflexive candidates. - The Binding Request by an agent MUST include the USERNAME and + 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 - Binding Request. The passive 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 wishes to cease checks for this component, and use the candidate pair - resulting from the check for this component. Section 9 provides + resulting from the check for this component. Section 8 provides guidance on determining when to include it. - If the agent is using Diffserv Codepoint markings [25] in its media + If the agent is using Diffserv Codepoint markings [26] in its media packets, it SHOULD apply those same markings to its connectivity checks. -8.1.2. Processing the Response +7.1.2. Processing the Response If the STUN transaction generates an unrecoverable failure response - or times out, a full-mode agent sets the state of the check to Failed - (passive-only agents do not maintain the state machinery). The + or times out, the agent sets the state of the check to Failed. The remainder of this section applies to processing of successful responses (any response from 200 to 299). The agent MUST check that the source IP address and port of the response equals the destination IP address and port that the Binding 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 Request was sent from. If these do not match, the processing described in the remainder of this section MUST NOT be performed. In - addition, a full-mode agent sets the state of the check to Failed. + addition, an agent sets the state of the check to Failed. If the check succeeds, processing continues. The agent creates 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 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 - priority of each candidate, using the algorithm in Section 6.7. For - a passive-only agent, the priority of the local candidate is the one - it signaled for the candidate in its SDP, and the priority of the - remote candidate is known either from the SDP, or if not there, from - the value of the PRIORITY attribute in the Binding Request which - triggered the check that just completed. For a full-mode agent, if - the local candidate was not one it signaled in its SDP, the priority - of the local candidate might additionally be equal to the PRIORITY - attribute the agent placed in the Binding Request which just + 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. Once the priority of the candidate pair has been computed, the pair - is added to the valid list for that media stream. If the response is - a consequence of a triggered check, and the request which caused the - triggered check included the USE-CANDIDATE attribute, the candidate - pair is additionally marked as selected. If a full-mode agent had - included the USE-CANDIDATE attribute in the request that produced the - success response, the agent marks the candidate pair as selected. + 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, a full-mode agent updates its ICE states. The full-mode agent - checks the mapped address from the STUN response. If the transport - address does not match any of the local candidates that the agent - knows about, the mapped address representes a new peer reflexive - candidate. Its type is equal to peer reflexive. Its base is set - equal to the candidate 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 5.1. 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 updates its ICE states. The agent checks the mapped + address from the STUN response. If the transport address does not + match any of the local candidates that the agent knows about, the + mapped address represents a new peer reflexive candidate. Its type + is equal to peer reflexive. Its base is set equal to the candidate + 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 full-mode agent changes the state for this check to - Succeeded. The full-mode 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 full-mode agent MUST change the states for all other - Frozen checks for the same media stream and same foundation, but - different component IDs, to Waiting. If the component ID for the - check was equal to the number of components for the media stream, the - full-mode agent MUST change the state for all other Frozen checks for - the first component of different media streams (and thus in different - check lists) but the same foundation, to Waiting. + Next, the agent changes the state for this check to Succeeded. The + 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 + change the states for all other Frozen checks for the same media + stream and same foundation, but different component IDs, to Waiting. + If the component ID for the check was 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 + signaled in the SDP differs from offerer to answerer), the agent MUST + change the state for all other Frozen checks for the first component + of different media streams (and thus in different check lists) but + the same foundation, to Waiting. -8.2. Server Procedures +7.2. Server Procedures An agent MUST be prepared to receive a Binding Request on the base of each candidate it included in its most recent offer or answer. Receipt of a Binding Request on a transport address that the agent had included in a candidate attribute is an indication that the connectivity check usage applies to the request. The agent MUST use a short term credential to authenticate the request and perform a message integrity check. The agent MUST accept a credential if the username consists of two values separated by a colon, where the first value is equal to the username fragment generated by the agent in an offer or answer for a session in- progress, and the password is equal to the password for that username fragment. It is possible (and in fact very likely) that an offeror will receive a Binding Request prior to receiving the answer from its peer. However, the request can be processed without receiving this answer, and a response generated. + If the agent is using Diffserv Codepoint markings [26] in its media + packets, it SHOULD apply those same markings to its responses to + Binding Requests. + +7.2.1. Additional Procedures for Full Implementations + + This subsection defines the additional server procedures applicable + to full implementations. + For requests being received on a relayed candidate, the source transport address used for STUN processing (namely, generation of the XOR-MAPPED-ADDRESS attribute) is the transport address as seen by the relay. That source transport address will be present in the REMOTE- ADDRESS attribute of a STUN Data Indication message, if the Binding Request was delivered through a Data Indication. If the Binding Request was not encapsulated in a Data Indication, that source address is equal to the current active destination for the STUN relay session. - If the agent is using Diffserv Codepoint markings [25] in its media - packets, it SHOULD apply those same markings to its responses to - Binding Requests. - If the STUN request resulted in an error response, no further processing is performed. - Otherwise, the agent MUST generate a triggered check in the reverse - directon if it has not already sent such a 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 not be - amongst the candidates signaled previously from the peer in its SDP). - The username fragment and password of the peer are readily determined - from the SDP and from the check that was just received. The username - fragment for this candidate is equal to the bottom half (the part - after the colon) of the USERNAME in the Binding Request that was just - received. Using that username fragment, the agent can check the SDP - messages received from its peer (there may be more than one in cases - of forking), and find this username fragment. The corresponding - password is then selected. If agent has not yet received this SDP (a - likely case for the offerer in the initial offer/answer exchange), it - MUST wait for the SDP to be received, and then proceed with the - triggered check and the rest of the processing described in the - remainder of this section. + Assuming a success response, if the source transport address of the + request does not match any existing remote candidates, it represents + a new peer reflexive remote candidate. The full-mode agent gives the + candidate a priority equal to the PRIORITY attribute from the + request. The type of the candidate is equal to peer reflexive. Its + foundation is set to an arbitrary value, different from the + foundation for all other remote candidates. Note that any subsequent + offer/answer exchanges will contain this new peer reflexive candidate + in the SDP, and will signal the actual foundation for the candidate. + This candidate is then added to the list of remote candidates. + However, the agent does not pair this candidate with any local + candidates. - The remainder of the processing in this section applies to state - updates performed by full-mode agents. + Next, the agent constructs a tentative check in the reverse + 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: - If the source transport address of the request does not match any - existing remote candidates, it represents a new peer reflexive remote - candidate. The full-mode agent gives the candidate a priority equal - to the PRIORITY attribute from the request. The type of the - candidate is equal to peer reflexive. Its foundation is set to an - arbitrary value, different from the foundation for all other remote - candidates. This candidate is then added to the list of remote - candidates. However, the full-mode agent does not pair this - candidate with any local candidates. + o If there is already a check on the check list with this same local + and remote candidates, and the state of that check is Waiting or + Frozen, its state is changed to In-Progress and the tentative + check is performed. - A full-mode agent knows the priorities for the local and remote - candidates of the triggered check described above, and so can compute - the priority for the check itself. If there is already a check on - the check list with this same local and remote candidates, and the - state of that check is Waiting or Frozen, its state is changed to In- - Progress. If there was already a check on the check list with this - same local and remote candidates, and its state was In-Progress, the - agent SHOULD generate an immediate retransmit of the Binding Request. - This is to facilitate rapid completion of ICE when both agents are - behind NAT. If there was a check in the list already and its state - was Succeeded, and the Binding Request just received contained the - USE-CANDIDATE attribute, it means that the pair resulting from that - previous check has been selected. The agent MUST take the candidate - pair in the valid list that was learned from that previous successful - check, and mark it as selected. If there was a check on the check - list with this same local and remote candidates, and its state was - Failed, nothing further is done. If there was no matching check on - the check list, it is inserted into the check list based on its - priority, and its state is set to In-Progress. + o If there is already a check on the check list with this same local + 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. -9. Concluding ICE + o If there is already a check on the check list with this same local + and remote candidates, and its state was Succeeded, the new + tentative check is abandoned. If the Binding Request just + 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 + and remote candidates, and its state was Failed, the new tentative + check is abandoned. + + o If there is no matching check on the check list, the new tentative + check is inserted into the check list based on its priority, and + its state is set to In-Progress. + + If the tentative check is to be performed, it is constructed and + processed as described in Section 7.1.1. These procedures require + the agent to know the username fragment and password for the peer. + They are readily determined from the SDP and from the check that was + just received. The username fragment for the remote candidate is + equal to the bottom half (the part after the colon) of the USERNAME + in the Binding Request that was just received. Using that username + fragment, the agent can check the SDP messages received from its peer + (there may be more than one in cases of forking), and find this + username fragment. The corresponding password is then selected. If + agent has not yet received this SDP (a likely case for the offerer in + the initial offer/answer exchange), it MUST wait for the SDP to be + received, and then proceed with the triggered check. + +7.2.2. Additional Procedures for Lite Implementations + + If the check that was just received contained a USE-CANDIDATE + attribute, the agent constructs a candidate pair whose local + candidate is equal to the transport address on which the request was + received, and whose remote candidate is equal to the source transport + address of the request that was received. This candidate pair is + assigned an arbitrary priority, and placed into a list of valid + candidates for that component of that media stream called the valid + list. In addition, it is marked as favored, since the peer agent has + indicated that it is to be used. ICE processing is considered + complete for a media stream if the valid list contains a candidate + pair for each component. + +8. Concluding ICE + + The processing rules in this section apply only to full + implementations. Concluding ICE involves selection of pairs by the controlling agent, - updating of state machinery by full-mode agents, and possibly the - generation of an updated offer by the controlling agent. Since a - passive-only agent can never be in the controlling role, the - processing in this section only applies to full-mode agents. + 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 - when to select a candidate pair. However, it MUST eventually include - a USE-CANDIDATE attribute in a check for each component of each media - stream. The following trivial algorithm chooses the first candidate - pair that validates for each media stream: the controlling agent - includes the USE-CANDIDATE attribute in every check it sends. + when to select a candidate pair, called the favored pair, as the one + that will be used for media. However, it MUST eventually include a + USE-CANDIDATE attribute in at least one successful check for each + component of each media stream. - Once a candidate pair in the Valid list is marked as selected, a - full-mode agent MUST NOT generate any further periodic checks for - that component of that media stream, and SHOULD cease any - retransmissions in progress for checks for that component of that - media stream. Once there is at least one candidate pair for each - component of a media stream that is marked as selected, a full-mode - agent MUST change the state of processing for its check list to - Completed. Once all of the media streams enter the Completed state, - the controlling agent takes the highest priority candidate pair for - each component of each media stream which has been marked as - selected. If any of those candidate pairs differ from the in-use - candidates in m/c-lines of the most recent offer/answer exchange, the - controlling agent MUST generate an updated offer as described in - Section 10. + The most apparent way to utilize the USE-CANDIDATE attribute is to + 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. -10. Subsequent Offer/Answer Exchanges + An alternative is called "proactive selection". In this approach, + the controlling agent includes the USE-CANDIDATE attribute in every + check it sends. Once the first check for a component succeeds, it is + used by ICE. In this mode, the agent will end up using the candidate + pair which is highest priority based on ICE's prioritization + algorithm, instead of some other local optimization. It is possible + with proactive selection that multiple checks might succeed with the + flag set; this is why ICE still applies its prioritization algorithm + to pick amongst those pairs that have been favored. + + If an agent is controlling and its peer has a lite implementation, an + agent MUST use an introspective selection algorithm. Of course, it + MAY select a favored pair based on ICE's prioritization. The key + requirement is that the agent must complete a successful check before + redoing it with the USE-CANDIDATE attribute. + + For both controlling and controlled agents, once a candidate pair in + the Valid list is marked as favored, an agent MUST NOT generate any + further periodic checks for that component of that media stream, and + SHOULD cease any retransmissions in progress for checks for that + component of that media stream. Once there is at least one candidate + pair for each component of a media stream that is favored, a full- + mode agent MUST change the state of processing for its check list to + Completed. Once all of the check lists for the media streams enter + the Completed state, the controlling agent takes the highest priority + favored candidate pair for each component of each media stream. If + any of those candidate pairs differ from the in-use candidates in + m/c-lines of the most recent offer/answer exchange, the controlling + agent MUST generate an updated offer as described in Section 9. + +9. Subsequent Offer/Answer Exchanges An agent MAY generate a subsequent offer at any time. However, the - rules in Section 9 will cause the controlling agent to send an + rules in Section 8 will cause the controlling agent to send an updated offer at the conclusion of ICE processing when ICE has - selected different candidate pairs from the in-use pairs. + selected different candidate pairs from the in-use pairs. This + section defines rules for construction of subsequent offers and + answers. -10.1. Generating the Offer +9.1. Generating the Offer An agent MAY change the ice-pwd and/or ice-ufrag for a media stream 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 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 permissible to use a session-level attribute in one offer, but to provide the same password as a media-level attribute in a subsequent offer. This is not a change in password, just a change in its representation. @@ -1501,251 +1638,271 @@ had ICE not been in use, would result in a new value for the transport address in the m/c-line, the agent MUST restart ICE for that media stream. This implies that setting the IP address in the c line to 0.0.0.0 will cause an ICE restart. Consequently, ICE implementations SHOULD NOT utilize this mechanism for call hold, and instead use a=inactive as described in [4] If an agent removes a media stream by setting its port to zero, it MUST NOT include any candidate attributes for that media stream. - When a full-mode agent generates an updated offer, the set of - candidate attributes to include for each media stream depend on the - state of ICE processing for that media stream. If the processing for - that media stream is in the Completed state, a full-mode agent MUST + An agent MUST NOT signal a change in its implementation level (full + 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 + ICE has long finished for the existing media streams. Based on the + rules described here, checks will begin for this new stream as if it + was in an initial offer. + +9.1.1. Additional Procedures for Full Implementations + + This section describes additional procedures for full + implementations. + + When an agent generates an updated offer, the set of candidate + attributes to include for each media stream depend on the state of + ICE processing for that media stream. If the processing for that + 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 selected pair in the valid list - for a component of that media stream. A full-mode agent SHOULD NOT - include any other candidate attributes for that media stream. If ICE + 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, a full-mode 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 5.1. - - A passive-only agent includes its one and only candidate for each - component of each media stream in an a=candidate attribute in any - subsequent offer. + 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 SHOULD have the same priority. For a peer reflexive candidate, the priority SHOULD be the same as determined by the processing in - Section 8.1.2. The foundation SHOULD be the same. The username + 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 for full-mode agents also depends on the - state of ICE 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 priority selected pair in the valid list for each - component of that media stream. If ICE processing is in the Running - state, a full-mode agent SHOULD populate the m/c-line for that media - stream based on the considerations in Section 5.3. - - A passive agent populates the m/c-lines with its one and only one - candidate for each component of each media stream. + Population of the m/c-lines also depends on the state of ICE + 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 + priority favored pair in the valid list for each component of that + media stream. If ICE processing is in the Running state, a full-mode + agent SHOULD populate the m/c-line for that media stream based on the + considerations in Section 4.1.3. - In addition, the controlling agent MUST include the a=remote- - candidates attribute for each media stream that is in the Completed - state. The attribute contains the remote candidates from the highest - priority selected pair in the valid list for each component of that - media stream. + In addition, if the agent is controlling, it MUST include the + a=remote-candidates attribute for each media stream that is in the + Completed state. The attribute contains the remote candidates from + the highest priority favored pair in the valid list for each + component of that media stream. - An agent MUST NOT change its mode (passive-only or full) by adding or - removing the a=ice-passive attribute from an updated offer, unless - ICE processing is being restarted for all media streams in the offer. +9.1.2. Additional Procedures for Lite Implementations - Note that an agent can add a new media stream at any time, even if - ICE has long finished for the existing media streams. Based on the - rules described here, checks will begin for this new stream as if it - was in an initial offer. + A passive-only agent includes 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. -10.2. Receiving the Offer and Generating an Answer +9.2. Receiving the Offer and Generating an Answer When receiving a subsequent offer within an existing session, an - agent MUST re-apply the verification procedures in Section 6.1 + agent MUST re-apply the verification procedures in Section 5.1 without regard to the results of verification from any previous offer/answer exchanges. Indeed, it is possible that a previous offer/answer exchange resulted in ICE not being used, but it is used as a consequence of a subsequent exchange. + 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 + that ICE is restarting for this media stream. If all media streams + are restarting, than ICE is restarting overall. Procedures for ICE + 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 + 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 + answer. + When the answerer generates its answer, it must decide what candidates to include in the answer, how to populate the m/c-line, - and how to adjust the states of ICE processing. + 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. - The rules for inclusion of candidate attributes in an answer are - identical to the rules followed by the offerer as described in - Section 10.1. +9.2.1. Additional Procedures for Full Implementations - However, the rules for setting the contents of the m/c-line are - different. For a full-mode agent, processing of the offer depends on - the presence 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. + The computation of the m/c-line additionally depends on the presence + 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, or - the agent is a passive-only agent, the agent follows the same - procedures for populating the m/c-line as described for the offerer - in Section 10.1. + If the offer did not contain the a=remote-candidates attribute, the + agent follows the same procedures for populating the m/c-line as + described for the offerer in Section 9.1. - An agent MUST NOT include the a=remote-candidates attribute in an - answer. 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. However, 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 that ICE - is restarting for this media stream. +9.3. Updating the Check and Valid Lists - An agent MUST NOT change its mode from a previous answer unless, - based on the offer, ICE procedures are being restarted for all media - streams in the offer. In that case, it MAY change its mode. + If ICE is restarting for a media stream, the agent MUST start a new + Valid list for that media stream. However, it retains the old Valid + list for the purposes of sending media until ICE processing + completes, at which point the old Valid list is discarded and the new + one is utilized to determine media and keepalive targets. -10.3. Updating the Check and Valid Lists +9.3.1. Additional Procedures for Full Implementations + + The procedures in this section are applicable only to full + implementations. Once the subsequent offer/answer exchange has completed, each agent needs to determine the impact, if any, on the Check and Valid lists. 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. An updated offer/ - answer exchange can impact the transmission rules for media, as - described in Section 12.1. + determined entirely by the checks themselves. - If the offer had a change in the ice-ufrag and/or ice-pwd for a media - stream, the agent MUST start a new Valid list for that media stream. - However, it retains the old Valid list for the purposes of sending - media until ICE processing completes, at which point the old Valid - list is discarded and the new one is utilized to determine media and - keepalive targets. A full-mode agent MUST also flush the check list - for the affected media streams, and then recompute the check list and - its states as described in Section 6.7. + When ICE restarts, an agent MUST flush the check list for the + affected media streams, and then recompute the check list and its + states as described in Section 5.7. - If the subsequent offer added a new media stream, a full-mode agent - MUST create a new check list for it (and an empty Valid list to start - of course), as described in Section 6.7. + The remainder of this section describes processing when ICE is not + restarting. - If the subsequent offer removed a media stream, or an answer rejected - an offered media stream, an agent MUST flush the Valid list for that - media stream. It MUST terminate any STUN transactions in progress - for that media stream. A full-mode agent MUST remove the check list + If the offer/answer exchange added a new media stream, the agent MUST + create a new check list for it (and an empty Valid list to start of + course), as described in Section 5.7. + + If the offer/answer exchange removed a media stream, or an answer + rejected an offered media stream, an agent MUST flush the Valid list + for that media stream. It MUST terminate any STUN transactions in + progress for that media stream. An agent MUST remove the check list for that media stream and cancel any pending periodic checks for it. If a media stream existed previously, and remains after the offer/ answer exchange, the agent MUST NOT modify the Valid list for that - media stream. However, if a full-mode agent is in the Running state - for that media stream, the check list is updated. To do that, the - full-mode agent recomputes the check lists using the procedures - described in Section 6.7. If a check on the new check lists was also - on the previous check lists, and its state was Waiting, In-Progress, + 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 its a new check on an + 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 each check list are Frozen), the full-mode agent sets the first check 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 component ID and with the same foundation to Waiting as well. - Next, the full-mode agent goes through each check list, starting with - the highest priority check. If a check has a state of Succeeded, and - it has a component ID of 1, then all Frozen checks in the same check + Next, the agent goes through each check list, starting with the + highest priority check. If a check has a state of Succeeded, and it + has a component ID of 1, then all Frozen checks in the same check list with the same foundation whose component IDs are not one, have their state set to Waiting. If, for a particular check list, there are checks for each component of that media stream in the Succeeded state, the agent moves the state of all Frozen checks for the first component of all other media streams (and thus in different check lists) with the same foundation to Waiting. -11. Keepalives - - STUN connectivity checks are also used to keep NAT bindings open once - ICE processing has completed. This is accomplished by periodically - generating a check on the candidate pair currently being used for - media. - - Specifically, once ICE processing allows media to begin flowing, as - described in Section 12.1, the agent sets a timer to fire in Tr - seconds. Tr SHOULD be configurable and SHOULD have a default of 15 - seconds. When Tr fires, the agent creates a connectivity check for - each component of that media stream. This check is sent on the - candidate pair currently being used to send media, as described in - Section 12.1. - - This specification makes no recommendations on the behaviors should - the keepalive itself fail. However, an agent SHOULD NOT blindly - restart ICE processing for that stream; if the keepalive was lost due - to congestion, the ICE restart will only aggravate the problem. - - When an ICE agent is communicating with an agent that is not ICE- - aware, keepalives still need to be utilized. Indeed, these - keepalives are essential even if neither endpoint implements ICE. As - such, this specification defines keepalive behavior generally, for - endpoints that support ICE, and those that do not. +10. Keepalives All endpoints MUST send keepalives for each media session. These - keepalives MUST be sent regardless of whether the media stream is - currently inactive, sendonly, recvonly or sendrecv, and regardless of - the presence or value of the bandwidth attribute. The keepalive - SHOULD be sent using a format which is supported by its peer. ICE - endpoints allow for STUN-based keepalives for UDP streams, and as - such, STUN keepalives MUST be used when an agent is communicating - with a peer that supports ICE. An agent can determine that its peer - supports ICE by the presence of a=candidate attributes for each media - session. If the peer does not support ICE, the choice of a packet - format for keepalives is a matter of local implementation. A format - which allows packets to easily be sent in the absence of actual media + keepalives serve the purpose of keeping NAT bindings active for the + media session. These keepalives MUST be sent regardless of whether + the media stream is currently inactive, sendonly, recvonly or + sendrecv, and regardless of the presence or value of the bandwidth + attribute. These keepalives MUST be sent even if ICE is not being + utilized for the session at all. The keepalive SHOULD be sent using + a format which is supported by its peer. ICE endpoints allow for + STUN-based keepalives for UDP streams, and as such, STUN keepalives + MUST be used when an agent is communicating with a peer that supports + ICE. An agent can determine that its peer supports ICE by the + presence of a=candidate attributes for each media session. If the + peer does not support ICE, the choice of a packet format for + keepalives is a matter of local implementation. A format which + allows packets to easily be sent in the absence of actual media content is RECOMMENDED. Examples of formats which readily meet this - goal are RTP No-Op [27] and RTP comfort noise [23]. If the peer + goal are RTP No-Op [28] and RTP comfort noise [24]. If the peer doesn't support any formats that are particularly well suited for keepalives, an agent SHOULD send RTP packets with an incorrect version number, or some other form of error which would cause them to be discarded by the peer. - STUN-based keepalives will be sent periodically every Tr seconds as - described above. If STUN keepalives are not in use (because the peer - does not support ICE), an agent SHOULD ensure that a media packet is - sent every Tr seconds. If one is not sent as a consequence of normal - media communications, a keepalive packet using one of the formats - discussed above SHOULD be sent. + If there has been no packet sent on a candidate pair being used for + media for Tr seconds (where packets include media and previous + keepalives), an agent MUST generate a keepalive on that pair. Tr + SHOULD be configurable and SHOULD have a default of 15 seconds. -12. Media Handling + If STUN is being used for keepalives, a STUN Binding Indication is + used [11]. The Binding Indication SHOULD NOT contain integrity + checks; since the messages are simply discarded on receipt regardless + of contents. The Indication SHOULD NOT contain the PRIORITY or USE- + CANDIDATE attributes defined here. The Binding Indication is sent + using the same local and remote candidates that are being used for + media. An agent receipt a Binding Indication MUST discard it + silently. Though Binding Indications are used for keepalives, an + agent MUST be prepared to receive Binding Requests as well. If a + Binding Request is received, a response is generated as discussed in + [11], but there is no impact on ICE processing otherwise. -12.1. Sending Media + An agent MUST begin the keepalive processing once ICE has selected + candidates for usage with media, or media begins to flow, whichever + happens first. Keepalives end once the session terminates or the + media stream is removed. + +11. Media Handling + +11.1. Sending Media + + Procedures for sending media differ for full and lite + implementations. + +11.1.1. Procedures for Full Implementations Agents always send media using a candidate pair. An agent will send media to the remote candidate in the pair (setting the destination address and port of the packet equal to that remote candidate), and will send it from the local candidate. When the local candidate is server or peer reflexive, media is originated from the base. Media sent from a relayed candidate is sent through that relay, using procedures defined in [12]. If the state of a media stream is Running, there is no old Valid list - for that media stream (which would be due to an ICE restart), a full- - mode agent MUST NOT send media. For passive-only agents, which do - not retain states about ICE processing, it MUST NOT send media until - there is a selected candidate pair in either the old or new Valid - list for each component of the media stream. + 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 selected pair for each component in either the old Valid list for a media stream (if it exists), else the new Valid list for that media 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 @@ -1756,51 +1913,66 @@ 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. 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 change the marker bit when an agent switches transmission of media from one candidate pair to another. -12.2. Receiving Media +11.1.2. Procedures for Lite Implementations + + A lite implementation MUST NOT send media until it has a Valid list + that contains a candidate pair for each component of that media + stream. Once that happens, the agent MAY begin sending media + packets. To do that, it sends media to the remote candidate in the + pair (setting the destination address and port of the packet equal to + that remote candidate), and will send it from the local candidate. + + In cases where there has been an ICE restart, there will be an old + 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 + new one MUST be used, and the old one discarded. + +11.2. Receiving Media ICE implementations MUST be prepared to receive media on any candidates provided in the most recent offer/answer exchange. It is RECOMMENDED that, when an agent receives an RTP packet with a new source or destination IP address for a particular media stream, that the agent re-adjust its jitter buffers. - RFC 3550 [20] describes an algorithm in Section 8.2 for detecting + RFC 3550 [21] describes an algorithm in Section 8.2 for detecting SSRC collisions and loops. These algorithms are based, in part, on seeing different source transport addresses with the same SSRC. However, when ICE is used, such changes will sometimes occur as the media streams switch between candidates. An agent will be able to determine that a media stream is from the same peer as a consequence of the STUN exchange that proceeds media transmission. Thus, if there is a change in source transport address, but the media packets come from the same peer agent, this SHOULD NOT be treated as an SSRC collision. -13. Usage with SIP +12. Usage with SIP -13.1. Latency Guidelines +12.1. Latency Guidelines ICE requires a series of STUN-based connectivity checks to take place between endpoints. These checks start from the answerer on generation of its answer, and start from the offerer when it receives the answer. These checks can take time to complete, and as such, the selection of messages to use with offers and answers can effect perceived user latency. Two latency figures are of particular interest. These are the post-pickup delay and the post-dial delay. + The post-pickup delay refers to the time between when a user "answers the phone" and when any speech they utter can be delivered to the caller. The post-dial delay refers to the time between when a user enters the destination address for the user, and ringback begins as a consequence of having succesfully started ringing the phone of the called party. To reduce post-dial delays, it is RECOMMENDED that the caller begin gathering candidates prior to actually sending its initial INVITE. This can be started upon user interface cues that a call is pending, @@ -1801,117 +1973,147 @@ consequence of having succesfully started ringing the phone of the called party. To reduce post-dial delays, it is RECOMMENDED that the caller begin gathering candidates prior to actually sending its initial INVITE. This can be started upon user interface cues that a call is pending, such as activity on a keypad or the phone going offhook. If an offer is received in an INVITE request, the callee SHOULD immediately gather its candidates and then generate an answer in a - provisional response. When reliable provisional responses are not - used, the SDP in the provisional response is the answer, and that - exact same answer reappears in the 200 OK. To deal with possible - losses of the provisional response, it SHOULD be retransmitted until - some indication of receipt. This indication can either be through - PRACK [9], or through the receipt of a successful STUN Binding - Request. Even if PRACK is not used, the provisional response SHOULD - be retransmitted using the exponential backoff and timers described - in [9]. Note, however, that if PRACK is not used, the rules for when + provisional response. ICE requires that a provisional response with + an SDP be transmitted reliably. This can be done through the + existing PRACK mechanism [9], or through an optimization that is + specific to ICE. With this optimization, provisional responses + containing an SDP answer that begins ICE processing for one or more + media streams can be sent reliably without RFC 3264. To do this, the + agent retransmits the provisional response with th exponential + backoff timers described in RFC 3262. Retransmits MUST cease on + receipt of a STUN Binding Request for one of the media streams + signaled in that SDP or on transmission of a 2xx response. If no + Binding Request is received prior to the last retransmit, the agent + does not consider the session terminated. Despite the fact that the + provisional response will be delivered reliably, the rules for when an agent can send an updated offer or answer do not change from those - specified in RFC 3262, even though the provisional response has been - delivered "reliably". Specifically, if the offer contained an - INVITE, the same answer appears in all of the 1xx and in the 2xx + specified in RFC 3262. Specifically, if the INVITE contained an + offer, the same answer appears in all of the 1xx and in the 2xx response to the INVITE. Only after that 2xx has been sent can an - updated offer/answer exchange occur. + updated offer/answer exchange occur. This optimization SHOULD NOT be + used if both agents support PRACK. Note that the optimization is + very specific to provisional response carrying answers that start ICE + processing; it is not a general technique for 1xx reliability. + + Alternatively, an agent MAY delay sending an answer until the 200 OK, + however this results in a poor user experience and is NOT + RECOMMENDED. Once the answer has been sent, the agent SHOULD begin its connectivity checks. Once candidate pairs for each component of a media stream enter the valid list, the callee can begin sending media on that media stream. However, prior to this point, any media that needs to be sent towards - the caller (such as SIP early media [24] cannot be transmitted. For + the caller (such as SIP early media [25] cannot be transmitted. For this reason, implementations SHOULD delay alerting the called party until candidates for each component of each media stream have entered the valid list. In the case of a PSTN gateway, this would mean that the setup message into the PSTN is delayed until this point. Doing this increases the post-dial delay, but has the effect of eliminating 'ghost rings'. Ghost rings are cases where the called party hears the phone ring, picks up, but hears nothing and cannot be heard. This technique works without requiring support for, or usage of, preconditions [6], since its a localized decision. It also has the benefit of guaranteeing that not a single packet of media will get clipped, so that post-pickup delay is zero. If an agent chooses to delay local alerting in this way, it SHOULD generate a 180 response once alerting begins. + 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 + offer appears in the response. In such cases, which are common in + third party call control, ICE agents SHOULD generate their offers in + a reliable provisional response (which MUST utilize RFC 3262). In + that case, the answer will arrive in a PRACK. This allows for ICE + processing to take place prior to alerting. Once ICE completes, the + agent can alert the user and then generate a 200 OK. The 200 OK + would contain no SDP, since the offer/answer exchange has completed. + Agents MAY place the offer in a 2xx instead (in which case the answer + comes in the ACK). This flow is simpler but results in a poorer user + experience. + As discussed in Section 16, offer/answer exchanges SHOULD be secured against eavesdropping and man-in-the-middle attacks. To do that, the usage of SIPS [3] is RECOMMENDED when used in concert with ICE. -13.2. Interactions with Forking +12.2. SIP Option Tags and Media Feature Tags + + [13] specifies a SIP option tag and media feature tag for usage with + ICE. ICE implementations using SIP SHOULD support this + specification, which uses a feature tag in registrations to + facilitate interoperability through gateways. + +12.3. Interactions with Forking ICE interacts very well with forking. Indeed, ICE fixes some of the problems associated with forking. Without ICE, when a call forks and the caller receives multiple incoming media streams, it cannot determine which media stream corresponds to which callee. With ICE, this problem is resolved. The connectivity checks which occur prior to transmission of media carry username fragments, which in turn are correlated to a specific callee. Subsequent media packets which arrive on the same 5-tuple as the connectivity check will be associated with that same callee. Thus, the caller can perform this correlation as long as it has received an answer. -13.3. Interactions with Preconditions +12.4. Interactions with Preconditions Quality of Service (QoS) preconditions, which are defined in RFC 3312 [6] and RFC 4032 [7], apply only to the transport addresses listed in the m/c lines in an offer/answer. If ICE changes the transport address where media is received, this change is reflected in the m/c lines of a new offer/answer. As such, it appears like any other re- 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 negotiations ocurring "in the background". Indeed, an agent SHOULD NOT indicate that Qos preconditions have been met until the ICE checks have completed and selected the candidate pairs to be used for media. ICE also has (purposeful) interactions with connectivity - preconditions [26]. Those interactions are described there. Note - that the procedures described in Section 13.1 describe their own type + preconditions [27]. Those interactions are described there. Note + that the procedures described in Section 12.1 describe their own type of "preconditions", albeit with less functionality than those - provided by the explicit preconditions in [26]. + provided by the explicit preconditions in [27]. -13.4. 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 [16]. Flow I + ICE works with Flows I, III and IV as described in [17]. Flow I works without the controller supporting or being aware of ICE. Flow IV will work as long as the controller passes along the ICE attributes without alteration. Flow II is fundamentally incompatible with ICE; each agent will believe itself to be the answerer and thus never generate a re-INVITE. The flows for continued operation, as described in Section 7 of RFC 3725, require additional behavior of ICE implementations to support. In particular, if an agent receives a mid-dialog re-INVITE that contains no offer, it MUST restart ICE for each media stream and go through the process of gathering new candidates. Furthermore, that list of candidates SHOULD include the ones currently in-use. -14. Grammar +13. Grammar - This specification defines five new SDP attributes - the "candidate", - "remote-candidates", "ice-passive", "ice-ufrag" and "ice-pwd" - attributes. + This specification defines seven new SDP attributes - the + "candidate", "remote-candidates", "ice-lite", "ice-ufrag", "ice-pwd" + "ice-options" and "ice-mismatch" attributes. The candidate attribute is a media-level attribute only. It contains a transport address for a candidate that can be used for connectivity checks. The syntax of this attribute is defined using Augmented BNF as defined in RFC 4234 [8]: candidate-attribute = "candidate" ":" foundation SP component-id SP transport SP @@ -1939,69 +2141,118 @@ The foundation is composed of one or more ice-char. The component-id is a positive integer, which identifies the specific component for which the transport address is a candidate. It MUST start at 1 and MUST increment by 1 for each component of a particular candidate. 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) [28]. + Control Protocol (DCCP) [29]. The cand-type production encodes the type of candidate. This specification defines the values "host", "srflx", "prflx" and "relay" for host, server reflexive, peer reflexive and relayed candidates, respectively. The set of candidate types is extensible for the future. Inclusion of the candidate type is optional. The rel-addr and rel-port productions convey information the related transport addresses. Rules for inclusion of these values is described in - Section 5.4. + Section 4.3. The a=candidate attribute can itself be extended. The grammar allows for new name/value pairs to be added at the end of the attribute. An implementation MUST ignore any name/value pairs it doesn't understand. The syntax of the "remote-candidates" attribute is defined using Augmented BNF as defined in RFC 4234 [8]. The remote-candidates attribute is a media level attribute only. remote-candidate-att = "remote-candidates" ":" remote-candidate 0*(SP remote-candidate) remote-candidate = component-ID SP connection-address SP port The attribute contains a connection-address and port for each component. The ordering of components is irrelevant. However, a value MUST be present for each component of a media stream. - The syntax of the "ice-passive" candidate is: + The syntax of the "ice-lite" and "ice-mismatch", both of which are + flags, is: - ice-passive = "ice-passive" + ice-lite = "ice-lite" + ice-mismatch = "ice-mismatch" - The syntax of the "ice-pwd" and "ice-ufrag" attributes are defined - as: + "ice-lite" is a session level attribute only, and "ice-mismatch" is a + media level attribute only. The syntax of the "ice-pwd" and "ice- + ufrag" attributes are defined as: ice-pwd-att = "ice-pwd" ":" password ice-ufrag-att = "ice-ufrag" ":" ufrag password = 22*ice-char ufrag = 4*ice-char The "ice-pwd" and "ice-ufrag" attributes can appear at either the session-level or media-level. When present in both, the value in the media-level takes precedence. Thus, the value at the session level is effectively a default that applies to all media streams, unless overriden by a media-level value. + The "ice-options" attribute is a session level attribute. It + contains a series of tokens which identify the options supported by + the agent. Its grammar is: + + ice-options = "ice-options" ":" ice-option-tag + 0*(SP ice-option-tag) + ice-option-tag = 1*ice-char + +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. + + 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 + 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 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 + 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. + 15. Example Two agents, L and R, are using ICE. Both are full-mode ICE 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 configured with a single STUN server each (indeed, the same one for each), which is listening for STUN requests at an IP address of 192.0.2.2 and port 3478. This STUN server supports only the Binding Discovery usage; relays are not used in this example. Agent L is behind a NAT, and agent R is on the public Internet. The NAT has an @@ -2022,23 +2273,25 @@ variable. The STUN server has advertised transport address STUN-PUB-1 (which is 192.0.2.2:3478) for the binding discovery usage. In the call flow itself, STUN messages are annotated with several attributes. The "S=" attribute indicates the source transport address of the message. The "D=" attribute indicates the destination transport address of the message. The "MA=" attribute is used in STUN Binding Response messages and refers to the mapped address. + "USE-CAND" implies the presence of the USE-CANDIDATE attribute. The call flow examples omit STUN authentication operations and RTCP, - and focus on RTP for a single media stream. + and focus on RTP for a single media stream between two full + implementations. L NAT STUN R |RTP STUN alloc. | | |(1) STUN Req | | | |S=$L-PRIV-1 | | | |D=$STUN-PUB-1 | | | |------------->| | | | |(2) STUN Req | | | |S=$NAT-PUB-1 | | | |D=$STUN-PUB-1 | | @@ -2068,24 +2321,26 @@ |(8) answer | | | |<-------------------------------------------| | |(9) Bind Req | | | |S=$R-PUB-1 | | | |D=L-PRIV-1 | | | |<----------------------------| | |Dropped | | |(10) Bind Req | | | |S=$L-PRIV-1 | | | |D=$R-PUB-1 | | | + |USE-CAND | | | |------------->| | | | |(11) Bind Req | | | |S=$NAT-PUB-1 | | | |D=$R-PUB-1 | | + | |USE-CAND | | | |---------------------------->| | |(12) Bind Res | | | |S=$R-PUB-1 | | | |D=$NAT-PUB-1 | | | |MA=$NAT-PUB-1 | | | |<----------------------------| |(13) Bind Res | | | |S=$R-PUB-1 | | | |D=$L-PRIV-1 | | | |MA=$NAT-PUB-1 | | | @@ -2104,21 +2359,21 @@ |D=$R-PUB-1 | | | |MA=$R-PUB-1 | | | |------------->| | | | |(17) Bind Res | | | |S=$NAT-PUB-1 | | | |D=$R-PUB-1 | | | |MA=$R-PUB-1 | | | |---------------------------->| | | | |RTP flows - Figure 10 + Figure 11 First, agent L obtains a host candidate from its local interface (not shown), and from that, sends a STUN Binding Request to the STUN server to get a server reflexive candidate (messages 1-4). Recall that the NAT has the address and port independent mapping property. Here, it creates a binding of NAT-PUB-1 for this UDP request, and this becomes the server reflexive candidate for RTP. Agent L sets a type preference of 126 for the host candidate and 100 for the server reflexive. The local preference is 65535. Based on @@ -2347,21 +2602,21 @@ This attack is very hard to launch unless the attacker themself is identified by the fake candidate. This is because it requires the attacker to intercept and replay packets sent by two different hosts. If both agents are on different networks (for example, across the public Internet), this attack can be hard to coordinate, since it needs to occur against two different endpoints on different parts of the network at the same time. If the attacker themself is identified by the fake candidate the - attack is easier to coordinate. However, if SRTP is used [21], the + attack is easier to coordinate. However, if SRTP is used [22], the attacker will not be able to play the media packets, they will only be able to discard them, effectively disabling the media stream for the call. However, this attack requires the agent to disrupt packets in order to block the connectivity check from reaching the target. In that case, if the goal is to disrupt the media stream, its much easier to just disrupt it with the same mechanism, rather than attack ICE. 16.2. Attacks on Address Gathering @@ -2438,35 +2693,71 @@ 16.4.2. STUN Amplification Attack The STUN amplification attack is similar to the voice hammer. However, instead of voice packets being directed to the target, STUN connectivity checks are directed to the target. This attack is accomplished by having the offerer send an offer with a large number of candidates, say 50. The answerer receives the offer, and starts its checks, which are directed at the target, and consequently, never generate a response. The answerer will start a new connectivity - check every 50ms, and each check is a STUN transaction consisting of - 9 retransmits of a message 65 bytes in length (plus 28 bytes for the - IP/UDP header) that runs for 7.9 seconds, for a total of 105 bytes/ - second per transaction on average. In the worst case, there can be - 158 transactions in progress at once (7.9 seconds divided by 50ms), - for a total of 132 kbps, just for STUN requests. + check every 20ms, and each check is a STUN transaction consisting of + 7 transmissions of a message 65 bytes in length (plus 28 bytes for + the IP/UDP header) that runs for 7.9 seconds, for a total of 58 + bytes/second per transaction on average. In the worst case, there + can be 395 transactions in progress at once (7.9 seconds divided by + 20ms), for a total of 182 kbps, just for STUN requests. It is impossible to eliminate the amplification, but the volume can - be reduced through a variety of heuristics. For example, agents can - limit the number of candidates they'll accept in an offer or answer, - they can increase the value of Ta, or exponentially increase Ta as - time goes on. All of these ultimately trade off the time for the ICE - exchanges to complete, with the amount of traffic that gets sent. + be reduced through a variety of heuristics. Agents SHOULD limit the + total number of connectivity checks they perform to 100. + Additionally, agents MAY limit the number of candidates they'll + accept in an offer or answer. - OPEN ISSUE: Need better remediation for this. +16.5. Interactions with Application Layer Gateways and SIP + + Application Layer Gateways (ALGs) are functions present in a NAT + device which inspect the contents of packets and modify them, in + order to facilitate NAT traversal for application protocols. Session + Border Controllers (SBC) are close cousins of ALGs, but are less + transparent since they actually exist as application layer SIP + intermediaries. ICE has interactions with SBCs and ALGs. + + If an ALG is SIP aware but not ICE aware, ICE will work through it as + long as the ALG correctly modifies the m/c-lines of SDP. In this + case, correctly means that the ALG does not modify m/c-lines with + external addresses. If the m/c-line contains internal addresses, but + ones for which a public binding exists, the ALG replaces the internal + address in the m/c-line with the public binding. Unfortunately, many + ALG are known to work poorly in these corner cases. ICE does not try + to work around broken ALGs, as this is outside the scope of its + functionality. ICE can help diagnose these conditions, which often + show up as a mismatch between the set of candidates and the m/c-line. + The a=ice-mismatch parameter is used for this purpose. + + ICE works best through ALGs when the signaling is run over TLS. This + prevents the ALG from manipulating the SDP messages and interfering + with ICE operation. Implementations which are expected to be + deployed behind ALGs SHOULD provide for TLS transport of the SDP. + + If an SBC is SIP aware but not ICE aware, the result depends on the + behavior of the SBC. If it is acting as a proper Back-to-Back User + Agent (B2BUA), the SBC will remove any SDP attributes it doesn't + understand, including the ICE attributes. Consequently, the call + will appear to both endpoints as if the other side doesn't support + ICE. This will result in ICE being disabled, and media flowing + through the SBC, if they SBC has requested it. If, however, the SBC + passes the ICE attributes without modification, yet modifies the m/c- + lines, this will be detected as an ICE mismatch, and ICE processing + is aborted for the call. It is outside of the scope of ICE for it to + act as a tool for "working around" SBCs. If one is present, ICE will + not be used and the SBC techniques take precedence. 17. Definition of Connectivity Check Usage STUN [11] requires that new usages provide a specific set of information as part of their formal definition. This section meets the requirements spelled out there. 17.1. Applicability This STUN usage provides a connectivity check between two peers @@ -2513,49 +2804,50 @@ resulting from this check should be used for transmission of media. The attribute has no content (the Length field of the attribute is zero); it serves as a flag. It has an attribute type of 0x0025. 17.6. New Error Response Codes This usage does not define any new error response codes. 17.7. Client Procedures - Client procedures are defined in Section 8.1. + Client procedures are defined in Section 7.1. 17.8. Server Procedures - Server procedures are defined in Section 8.2. + Server procedures are defined in Section 7.2. 17.9. Security Considerations for Connectivity Check Security considerations for the connectivity check are discussed in Section 16. 18. IANA Considerations This specification registers new SDP attributes and new STUN attributes. 18.1. SDP Attributes - This specification defines five new SDP attributes per the procedures - of Section 8.2.4 of [10]. The required information for the - registrations are included here. + This specification defines seven new SDP attributes per the + procedures of Section 8.2.4 of [10]. The required information for + the registrations are included here. 18.1.1. candidate Attribute Contact Name: Jonathan Rosenberg, jdrosen@jdrosen.net. Attribute Name: candidate Long Form: candidate + Type of Attribute: media level Charset Considerations: The attribute is not subject to the charset attribute. Purpose: This attribute is used with Interactive Connectivity Establishment (ICE), and provides one of many possible candidate addresses for communication. These addresses are validated with an end-to-end connectivity check using Simple Traversal Underneath NAT (STUN). @@ -2553,117 +2845,160 @@ Charset Considerations: The attribute is not subject to the charset attribute. Purpose: This attribute is used with Interactive Connectivity Establishment (ICE), and provides one of many possible candidate addresses for communication. These addresses are validated with an end-to-end connectivity check using Simple Traversal Underneath NAT (STUN). - Appropriate Values: See Section 14 of RFC XXXX [Note to RFC-ed: + Appropriate Values: See Section 13 of RFC XXXX [Note to RFC-ed: please replace XXXX with the RFC number of this specification]. 18.1.2. remote-candidates Attribute Contact Name: Jonathan Rosenberg, jdrosen@jdrosen.net. Attribute Name: remote-candidates Long Form: remote-candidates Type of Attribute: media level Charset Considerations: The attribute is not subject to the charset attribute. Purpose: This attribute is used with Interactive Connectivity Establishment (ICE), and provides the identity of the remote candidates that the offerer wishes the answerer to use in its answer. - Appropriate Values: See Section 14 of RFC XXXX [Note to RFC-ed: + Appropriate Values: See Section 13 of RFC XXXX [Note to RFC-ed: please replace XXXX with the RFC number of this specification]. -18.1.3. ice-passive Attribute +18.1.3. ice-lite Attribute Contact Name: Jonathan Rosenberg, jdrosen@jdrosen.net. - Attribute Name: ice-passive + Attribute Name: ice-lite - Long Form: ice-passive + Long Form: ice-lite Type of Attribute: session level + Charset Considerations: The attribute is not subject to the charset attribute. Purpose: This attribute is used with Interactive Connectivity - Establishment (ICE), and indicates that an agent can only operate - in ICE's passive mode. + Establishment (ICE), and indicates that an agent has the minimum + functionality required to support ICE inter-operation with a peer + that has a full implementation. - Appropriate Values: See Section 14 of RFC XXXX [Note to RFC-ed: + Appropriate Values: See Section 13 of RFC XXXX [Note to RFC-ed: please replace XXXX with the RFC number of this specification]. -18.1.4. ice-pwd Attribute +18.1.4. ice-mismatch Attribute + + Contact Name: Jonathan Rosenberg, jdrosen@jdrosen.net. + + Attribute Name: ice-mismatch + + Long Form: ice-mismatch + + Type of Attribute: session level + + Charset Considerations: The attribute is not subject to the charset + attribute. + + Purpose: This attribute is used with Interactive Connectivity + Establishment (ICE), and indicates that an agent is ICE capable, + but did not proceed with ICE due to a mismatch of candidates with + the values in the m/c-line. + + Appropriate Values: See Section 13 of RFC XXXX [Note to RFC-ed: + please replace XXXX with the RFC number of this specification]. + +18.1.5. ice-pwd Attribute Contact Name: Jonathan Rosenberg, jdrosen@jdrosen.net. Attribute Name: ice-pwd Long Form: ice-pwd Type of Attribute: session or media level Charset Considerations: The attribute is not subject to the charset attribute. Purpose: This attribute is used with Interactive Connectivity Establishment (ICE), and provides the password used to protect STUN connectivity checks. - Appropriate Values: See Section 14 of RFC XXXX [Note to RFC-ed: + Appropriate Values: See Section 13 of RFC XXXX [Note to RFC-ed: please replace XXXX with the RFC number of this specification]. -18.1.5. ice-ufrag Attribute +18.1.6. ice-ufrag Attribute Contact Name: Jonathan Rosenberg, jdrosen@jdrosen.net. Attribute Name: ice-ufrag Long Form: ice-ufrag Type of Attribute: session or media level Charset Considerations: The attribute is not subject to the charset attribute. Purpose: This attribute is used with Interactive Connectivity Establishment (ICE), and provides the fragments used to construct the username in STUN connectivity checks. - Appropriate Values: See Section 14 of RFC XXXX [Note to RFC-ed: + Appropriate Values: See Section 13 of RFC XXXX [Note to RFC-ed: + please replace XXXX with the RFC number of this specification]. + +18.1.7. ice-options Attribute + + Contact Name: Jonathan Rosenberg, jdrosen@jdrosen.net. + + Attribute Name: ice-options + + Long Form: ice-options + + Type of Attribute: session level + + Charset Considerations: The attribute is not subject to the charset + attribute. + + Purpose: This attribute is used with Interactive Connectivity + Establishment (ICE), and indicates the ICE options or extensions + used by the agent. + + Appropriate Values: See Section 13 of RFC XXXX [Note to RFC-ed: please replace XXXX with the RFC number of this specification]. 18.2. STUN Attributes This section registers two new STUN attributes per the procedures in [11]. 0x0024 PRIORITY 0x0025 USE-CANDIDATE 19. IAB Considerations The IAB has studied the problem of "Unilateral Self Address Fixing", which is the general process by which a agent attempts to determine its address in another realm on the other side of a NAT through a - collaborative protocol reflection mechanism [19]. ICE is an example + collaborative protocol reflection mechanism [20]. ICE is an example of a protocol that performs this type of function. Interestingly, the process for ICE is not unilateral, but bilateral, and the difference has a signficant impact on the issues raised by IAB. Indeed, ICE can be considered a B-SAF (Bilateral Self-Address Fixing) protocol, rather than an UNSAF protocol. Regardless, the IAB has mandated that any protocols developed for this purpose document a specific set of considerations. This section meets those requirements. 19.1. Problem Definition @@ -2718,21 +3053,21 @@ 19.3. Brittleness Introduced by ICE From RFC3424, any UNSAF proposal must provide: Discussion of specific issues that may render systems more "brittle". For example, approaches that involve using data at multiple network layers create more dependencies, increase debugging challenges, and make it harder to transition. ICE actually removes brittleness from existing UNSAF mechanisms. In - particular, traditional STUN (as described in RFC 3489 [13]) has + particular, traditional STUN (as described in RFC 3489 [14]) has 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 behind. This process is error-prone. With ICE, that discovery process is simply not used. Rather than unilaterally assessing the validity of the address, its validity is dynamically determined by measuring connectivity to a peer. The process of determining connectivity is very robust. Another point of brittleness in traditional STUN and any other unilateral mechanism is its absolute reliance on an additional @@ -2785,21 +3120,21 @@ traditional STUN. However, the update to STUN [11] uses an encoding which hides these binary addresses from generic ALGs. Since [11] is required for all ICE implementations, this NAPT problem does not impact ICE. Existing NAPT boxes have non-deterministic and typically short expiration times for UDP-based bindings. This requires implementations to send periodic keepalives to maintain those bindings. ICE uses a default of 15s, which is a very conservative estimate. Eventually, over time, as NAT boxes become compliant to - behave [30], this minimum keepalive will become deterministic and + behave [31], this minimum keepalive will become deterministic and well-known, and the ICE timers can be adjusted. Having a way to discover and control the minimum keepalive interval would be far better still. 20. Acknowledgements The authors would like to thank Flemming Andreasen, Rohan Mahy, Dean Willis, Eric Cooper, Dan Wing, Douglas Otis, Tim Moore, and Francois Audet for their comments and input. A special thanks goes to Bill May, who suggested several of the concepts in this specification, @@ -2840,269 +3175,183 @@ Specifications: ABNF", RFC 4234, October 2005. [9] Rosenberg, J. and H. Schulzrinne, "Reliability of Provisional Responses in Session Initiation Protocol (SIP)", RFC 3262, June 2002. [10] Handley, M., Jacobson, V., and C. Perkins, "SDP: Session Description Protocol", RFC 4566, July 2006. [11] Rosenberg, J., "Simple Traversal Underneath Network Address - Translators (NAT) (STUN)", draft-ietf-behave-rfc3489bis-04 - (work in progress), July 2006. + Translators (NAT) (STUN)", draft-ietf-behave-rfc3489bis-05 + (work in progress), October 2006. [12] Rosenberg, J., "Obtaining Relay Addresses from Simple Traversal Underneath NAT (STUN)", draft-ietf-behave-turn-02 (work in progress), October 2006. + [13] Rosenberg, J., "Indicating Support for Interactive Connectivity + Establishment (ICE) in the Session Initiation Protocol (SIP)", + draft-ietf-sip-ice-option-tag-00 (work in progress), + January 2007. + 21.2. Informative References - [13] Rosenberg, J., Weinberger, J., Huitema, C., and R. Mahy, "STUN + [14] Rosenberg, J., Weinberger, J., Huitema, C., and R. Mahy, "STUN - Simple Traversal of User Datagram Protocol (UDP) Through Network Address Translators (NATs)", RFC 3489, March 2003. - [14] Senie, D., "Network Address Translator (NAT)-Friendly + [15] Senie, D., "Network Address Translator (NAT)-Friendly Application Design Guidelines", RFC 3235, January 2002. - [15] Srisuresh, P., Kuthan, J., Rosenberg, J., Molitor, A., and A. + [16] Srisuresh, P., Kuthan, J., Rosenberg, J., Molitor, A., and A. Rayhan, "Middlebox communication architecture and framework", RFC 3303, August 2002. - [16] Rosenberg, J., Peterson, J., Schulzrinne, H., and G. Camarillo, + [17] Rosenberg, J., Peterson, J., Schulzrinne, H., and G. Camarillo, "Best Current Practices for Third Party Call Control (3pcc) in the Session Initiation Protocol (SIP)", BCP 85, RFC 3725, April 2004. - [17] Borella, M., Lo, J., Grabelsky, D., and G. Montenegro, "Realm + [18] Borella, M., Lo, J., Grabelsky, D., and G. Montenegro, "Realm Specific IP: Framework", RFC 3102, October 2001. - [18] Borella, M., Grabelsky, D., Lo, J., and K. Taniguchi, "Realm + [19] Borella, M., Grabelsky, D., Lo, J., and K. Taniguchi, "Realm Specific IP: Protocol Specification", RFC 3103, October 2001. - [19] Daigle, L. and IAB, "IAB Considerations for UNilateral Self- + [20] Daigle, L. and IAB, "IAB Considerations for UNilateral Self- Address Fixing (UNSAF) Across Network Address Translation", RFC 3424, November 2002. - [20] Schulzrinne, H., Casner, S., Frederick, R., and V. Jacobson, + [21] Schulzrinne, H., Casner, S., Frederick, R., and V. Jacobson, "RTP: A Transport Protocol for Real-Time Applications", RFC 3550, July 2003. - [21] Baugher, M., McGrew, D., Naslund, M., Carrara, E., and K. + [22] Baugher, M., McGrew, D., Naslund, M., Carrara, E., and K. Norrman, "The Secure Real-time Transport Protocol (SRTP)", RFC 3711, March 2004. - [22] Carpenter, B. and K. Moore, "Connection of IPv6 Domains via + [23] Carpenter, B. and K. Moore, "Connection of IPv6 Domains via IPv4 Clouds", RFC 3056, February 2001. - [23] Zopf, R., "Real-time Transport Protocol (RTP) Payload for + [24] Zopf, R., "Real-time Transport Protocol (RTP) Payload for Comfort Noise (CN)", RFC 3389, September 2002. - [24] Camarillo, G. and H. Schulzrinne, "Early Media and Ringing Tone + [25] Camarillo, G. and H. Schulzrinne, "Early Media and Ringing Tone Generation in the Session Initiation Protocol (SIP)", RFC 3960, December 2004. - [25] Blake, S., Black, D., Carlson, M., Davies, E., Wang, Z., and W. + [26] Blake, S., Black, D., Carlson, M., Davies, E., Wang, Z., and W. Weiss, "An Architecture for Differentiated Services", RFC 2475, December 1998. - [26] Andreasen, F., "Connectivity Preconditions for Session + [27] Andreasen, F., "Connectivity Preconditions for Session Description Protocol Media Streams", draft-ietf-mmusic-connectivity-precon-02 (work in progress), June 2006. - [27] Andreasen, F., "A No-Op Payload Format for RTP", + [28] Andreasen, F., "A No-Op Payload Format for RTP", draft-ietf-avt-rtp-no-op-00 (work in progress), May 2005. - [28] Kohler, E., Handley, M., and S. Floyd, "Datagram Congestion + [29] Kohler, E., Handley, M., and S. Floyd, "Datagram Congestion Control Protocol (DCCP)", RFC 4340, March 2006. - [29] Hellstrom, G. and P. Jones, "RTP Payload for Text + [30] Hellstrom, G. and P. Jones, "RTP Payload for Text Conversation", RFC 4103, June 2005. - [30] Audet, F. and C. Jennings, "NAT Behavioral Requirements for + [31] Audet, F. and C. Jennings, "NAT Behavioral Requirements for Unicast UDP", draft-ietf-behave-nat-udp-08 (work in progress), October 2006. - [31] Jennings, C. and R. Mahy, "Managing Client Initiated + [32] Jennings, C. and R. Mahy, "Managing Client Initiated Connections in the Session Initiation Protocol (SIP)", - draft-ietf-sip-outbound-04 (work in progress), June 2006. - -Appendix A. Passive-Only ICE - - ICE allows for two modes of operation in an agent - passive-only and - full. Passive-only mode is applicable to entities like PSTN - gateways, media servers and conferencing servers that are always - publicly connected and are not behind a firewall or NAT. - - This leads to an important question - why would such an endpoint even - bother with ICE? If it has a public IP address, what additional - value do the ICE procedures bring? There are many, actually. - - First, doing so greatly facilitates NAT traversal for clients that - connect to it. Consider a PC softphone behind a NAT whose mapping - policy is address and port dependent. The softphone initiates a call - through a gateway that implements ICE. The gateway doesn't obtain - any server reflexive or relayed candidates, but it implements ICE, - and consequently, is prepared to receive STUN connectivity checks on - its host candidates. The softphone will send a STUN connectivity - check to the gateway, which passes through the intervending NAT. - This causes the NAT to allocate a new binding for the softphone. The - connectivity is received by the gateway, and will cause it gateway to - send a check back to the softphone, at this newly created candidate. - A successful response confirms that this candidate is usable, and the - gateway can send media immediately to the softphone. This allows - direct media transmission between the gateway and softphone, without - the need for relays, even though the softphone was behind a 'bad' - NAT. - - Second, implementation of the STUN connectivity checks allows for NAT - bindings along the way to be kept open. Keeping these bindings open - is essential for continued communications between the gateway and - softphone. - - Third, ICE prevents a fairly destructive attack in multimedia - systems, called the voice hammer. The STUN connectivity check used - by an ICE endpoint allows it to be certain that the target of media - packets is, in fact, the same entity that requested the packets - through the offer/answer exchange. See Section 16 for a more - complete discussion on this attack. - - Because of the benefits of implementing ICE in endpoints that don't - themselves require NAT traversal, ICE reduces the cost of - implementation by allowing them to run in passive-only mode. The - rules for passive-only endpoints are described throughout the - specification. What follows is an informative summary to give - implementors a good sense of what is required: - - o A passive-only agent obtains candidates just from its host - interfaces, just like it would do without ICE. It doesn't need to - implement the STUN Binding Discovery usage [11] or the relay usage - [12] to gather server reflexive or relayed candidates. It needs - to assign its candidates a foundation ID; however it can use the - IP address itself as the foundation ID. - - o The prioritization in Section 5.2 is trivially accomplished for - passive-only agents utilizing RTP. The type preference is set to - 126 and the local preference to 65535, resulting in a priority of - 2130706431 for RTP and 2130706430 for RTCP. - - o In use candidates Section 5.3 are trivially selected - they are - equal to the host candidates. - - o A passive-only agent will need to select a username and password - for each session. An SDP offer (and answer) constructed by an - RTP-based audio-only agent will contain two a=candidate lines, - which mirror the RTP and RTCP transport addresses in the m/c-line. - Each a=candidate line contains the priority and foundation - computed above, and indicates that it is a host candidate - Section 5.4. + draft-ietf-sip-outbound-07 (work in progress), January 2007. - o A passive-only agent doesn't need to construct check lists or - maintain the states of ICE processing Section 6.7. It only needs - to maintain the valid list, which are the list of checks it has - completed. Once it places its candidate lines into an offer or - answer, it waits for the receipt of checks. + [33] Rescorla, E., "Overview of the Lite Implementation of + Interactive Connectivity Establishment (ICE)", + draft-ietf-mmusic-ice-lite-00.txt (work in progress), + January 2007. - o A passive-only agent doesn't generate periodic checks. It only - generates triggered checks, which are checks that are created as a - consequence of receiving a check. A passive-only agent does need - to be able to respond to a STUN check it receives. +Appendix A. Lite and Full Implementations - o A passive-only agent does not add the PRIORITY or USE-CANDIDATE - attributes to its STUN requests. Its STUN requests only contain - the USERNAME and MESSAGE-INTEGRITY attributes, set based on the - username fragments and passwords exchanged in the offer and - answer. + ICE allows for two types of implementations. A full implementation + supports the controlling and controlled roles in a session, and can + also perform address gathering. In contrast, a lite implementation + is a minimalist implementation that does little but respond to STUN + checks. - o Handling of subsequent offer/answer exchanges is done trivially - - the passive-only agent includes its one and only candidate for - each component of each media stream in an a=candidate attribute - and in the m/c-line, just like an initial offer or answer. + Because ICE requires both endpoints to support it in order to bring + benefits to either endpoint, incremental deployment of ICE in a + network is more complicated. Many sessions involve an endpoint which + is, by itself, not behind a NAT and not one that would worry about + NAT traversal. Examples include gateways to the PSTN, media servers, + conference bridges, and application servers. A very common case is + to have one endpoint that requires NAT traversal (such as a VoIP hard + phone or soft phone) make a call through one of these devices. Even + if the phone supports a full ICE implementation, ICE won't be used at + all if the other device doesn't support it. The lite implementation + allows for a low-cost entry point for these devices. Once they + support the lite implementation, full implementations can connect to + them and get the full benefits of ICE. - o A passive-only agent never needs to compute or include the - a=remote-candidates attribute in any offer it sends. It never - needs to generate an updated offer as a consequence of ICE - processing. + Consequently, a lite implementation is only appropriate for devices + that will always be connected to the public Internet and have a + public IP address at which it can receive packets from any + correspondent. ICE will not function when a lite implementation is + placed behind a NAT. - o A passive-only agent sends media once a selected candidate pair - appears in its Valid list for that media stream. + 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. + A full implementation will reduce call setup times. Full + implementations also obtain the security benefits of ICE unrelated to + NAT traversal; in particular, the voice hammer attack described in + Section 16 is prevented only for full implementations, not lite. + Finally, it is often the case that a device which finds itself with a + public address today will be placed in a network tomorrow where it + will be behind a NAT. It is difficult to definitively know, over the + lifetime of a device or product, that it will always be used on the + public Internet. Full implementation provides assurance that + communications will always work. Appendix B. Design Motivations ICE contains a number of normative behaviors which may themselves be simple, but derive from complicated or non-obvious thinking or use cases which merit further discussion. Since these design motivations are not neccesary to understand for purposes of implementation, they are discussed here in an appendix to the specification. This section is non-normative. B.1. Pacing of STUN Transactions STUN transactions used to gather candidates and to verify connectivity are paced out at an approximate rate of one new - transaction every Ta seconds, where Ta has a default of 50ms. Why - are these transactions paced, and why was 50ms chosen as default? + transaction every Ta seconds, where Ta has a default of 20ms. Why + are these transactions paced, and why was 20ms chosen as default? Sending of these STUN requests will often have the effect of creating bindings on NAT devices between the client and the STUN servers. Experience has shown that many NAT devices have upper limits on the rate at which they will create new bindings. Furthermore, transmission of these packets on the network makes use of bandwidth and needs to be rate limited by the agent. As a consequence, the pacing ensures that the NAT devices does not get overloaded and that traffic is kept at a reasonable rate. - Another aspect of the STUN requests is their bandwidth usage. In - ICE, each STUN request contains the STUN 20 byte header, in addition - to the USERNAME, MESSAGE-INTEGRITY and PRIORITY attributes. The - USERNAME attribute contains a 4-byte attribute overhead, plus the - username value itself. This username is the concatenation of the two - fragments, plus a colon. Each fragment is supposed to be at least 4 - bytes long, making the total length of the USERNAME attribute (4*2 + - 1 + 4) = 13 bytes. The MESSAGE-INTEGRITY attribute is 4 bytes of - overhead plus 20 bytes value, for 24 bytes. The PRIORITY attribute - is 4 bytes of overhead plus 4 bytes of value, for 8 bytes. Thus, the - total length of the STUN Binding Request is (20 + 13 + 24 + 8) = 65 - bytes, with 28 bytes of overhead for IP and UDP for a total of 93 - bytes. The response contains the STUN 20 byte header, the XOR- - MAPPED-ADDRESS, and MESSAGE-INTEGRITY attributes. XOR-MAPPED-ADDRESS - has 4 bytes overhead plus an 8 byte value, for a total of 12 bytes. - Thus, each STUN response is (20 + 12 + 24) = 56 bytes plus 28 bytes - of UDP/IP overhead for a total of 84 bytes. Checks typically fall - into one of two cases. If a check works, each transaction has a - single request and a single response, for a total of 2 packets and - 177 bytes over one RTT interval. Assuming a fairly agressive RTT of - 70ms, this produces 20.23 kbps, but only briefly. If a check fails - because the pair is invalid, there will be nine requests and no - responses. This produces 837 bytes over 7.9s, for a total of 105.9 - bps, but over a long period of time. - - OPEN ISSUE: The bandwidth computations are pretty complex because - ICE is not a CBR stream, and its bandwidth utilization depends on - how many transactions it ends up generating before it finishes. - Need to work this model more. - - Given that these numbers are close to, if not greater than, the - bandwidths utilized by many voice codecs, this seems a reasonable - value to use. - - OPEN ISSUE: There is some debate about whether to reduce this - pacing interval smaller, say 20ms, to speed up ICE, or perhaps - make it equal to the bandwidth that would be utilized by the media - streams themselves. - B.2. Candidates with Multiple Bases - Section 5.1 talks about merging together candidates that are + Section 4.1.1 talks about merging together candidates that are identical but have different bases. When can an agent have two candidates that have the same IP address and port, but different - bases? Consider the topology of Figure 16: + bases? Consider the topology of Figure 17: +----------+ | STUN Srvr| +----------+ | | ----- // \\ | | | B:net10 | @@ -3126,21 +3375,21 @@ | | |192.168.1.1 ----- +----------+ // \\ +----------+ | | | | | | | Offerer |---------| C:net10 |---------| Answerer | | |10.0.1.1 | | 10.0.1.2 | | +----------+ \\ // +----------+ ----- - Figure 16 + Figure 17 In this case, the offerer is multi-homed. It has one interface, 10.0.1.1, on network C, which is a net 10 private network. The Answerer is on this same network. The offerer is also connected to network A, which is 192.168/16. The offerer has an interface of 192.168.1.1 on this network. There is a NAT on this network, natting into network B, which is another net10 private network, but not connected to network C. There is a STUN server on network B. The offerer obtains a host candidate on its interface on network C @@ -3162,36 +3411,35 @@ There are two motivations for its inclusion. The first is diagnostic. It is very useful to know the relationship between the different types of candidates. By including the translation, an agent can know which relayed candidate is associated with which reflexive candidate, which in turn is associated with a specific host candidate. When checks for one candidate succeed and not the others, this provides useful diagnostics on what is going on in the network. The second reason has to do with off-path Quality of Service (QoS) - mechanisms. When ICE is used in environments such as PacketCable 2.0 - [[TODO: need PC2.0 reference]], proxies will, in addition to - performing normal SIP operations, inspect the SDP in SIP messages, - and extract the IP address and port for media traffic. They can then - interact, through policy servers, with access routers in the network, - to establish guaranteed QoS for the media flows. This QoS is - provided by classifying the RTP traffic based on 5-tuple, and then - providing it a guaranteed rate, or marking its Diffserv codepoints - appropriately. When a residential NAT is present, and a relayed - candidate gets selected for media, this relayed candidate will be a - transport address on an actual STUN relay. That address says nothing - about the actual transport address in the access router that would be - used to classify packets for QoS treatment. Rather, the translation - of that relayed address is needed. By carrying the translation in - the SDP, the proxy can use that transport address to request QoS from - the access router. + mechanisms. When ICE is used in environments such as PacketCable + 2.0, proxies will, in addition to performing normal SIP operations, + inspect the SDP in SIP messages, and extract the IP address and port + for media traffic. They can then interact, through policy servers, + with access routers in the network, to establish guaranteed QoS for + the media flows. This QoS is provided by classifying the RTP traffic + based on 5-tuple, and then providing it a guaranteed rate, or marking + its Diffserv codepoints appropriately. When a residential NAT is + present, and a relayed candidate gets selected for media, this + relayed candidate will be a transport address on an actual STUN + relay. That address says nothing about the actual transport address + in the access router that would be used to classify packets for QoS + treatment. Rather, the translation of that relayed address is + needed. By carrying the translation in the SDP, the proxy can use + that transport address to request QoS from the access router. B.4. Importance of the STUN Username ICE requires the usage of message integrity with STUN using its short term credential functionality. The actual short term credential is formed by exchanging username fragments in the SDP offer/answer exchange. The need for this mechanism goes beyond just security; it is actual required for correct operation of ICE in the first place. Consider agents A, B, and C. A and B are within private enterprise 1, @@ -3230,21 +3478,21 @@ be prepared for it. Note that this is not a problem specific to ICE; stray packets can arrive at a port at any time for any type of protocol, especially ones on the public Internet. As such, this requirement is just restating a general design guideline for Internet applications - be prepared for unknown packets on any port. B.5. The Candidate Pair Sequence Number Formula The sequence number for a candidate pair has an odd form. It is: - 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-P,A-P) + 2*MAX(O-P,A-P) + (O-P>A-P?1:0) Why is this? When the candidate pairs are sorted based on this value, the resulting sorting has the MAX/MIN property. This means that the pairs are first sorted based on decreasing value of the maximum of the two sequence numbers. For pairs that have the same value of the maximum sequence number, the minimum sequence number is used to sort amongst them. If the max and the min sequence numbers are the same, the offerers priority is used as the tie breaker in the last part of the expression. The factor of 2*32 is used since the priority of a single candidate is always less than 2*32, resulting in @@ -3264,21 +3512,21 @@ results of a check for one media stream, and applies them to another. For example, if only the relayed candidates for audio (which were the last resort candidates) succeed, ICE will check the relayed candidates for video first. B.7. The remote-candidates attribute The a=remote-candidates attribute exists to eliminate a race condition between the updated offer and the response to the STUN Binding Request that moved a candidate into the Valid list. This - race condition is shown in Figure 17. On receipt of message 4, agent + race condition is shown in Figure 18. On receipt of message 4, agent A adds a candidate pair to the valid list. If there was only a single media stream with a single component, agent A could now send an updated offer. However, the check from agent B has not yet generated a response, and agent B receives the updated offer (message 7) before getting the response (message 10). Thus, it does not yet know that this particular pair is valid. To eliminate this condition, the actual candidates at B that were selected by the offerer (the remote candidates) are included in the offer itself. Note, however, that agent B will not send media until it has received this STUN response. @@ -3299,84 +3547,107 @@ | |Lost | |(7) Offer | | |------------------------------------------>| |(8) Answer | | |<------------------------------------------| |(9) STUN Req. | | |<------------------------------------------| |(10) STUN Res. | | |------------------------------------------>| - Figure 17 + Figure 18 B.8. Why are Keepalives Needed? Once media begins flowing on a candidate pair, it is still necessary to keep the bindings alive at intermediate NATs for the duration of the session. Normally, the media stream packets themselves (e.g., RTP) meet this objective. However, several cases merit further discussion. Firstly, in some RTP usages, such as SIP, the media streams can be "put on hold". This is accomplished by using the SDP "sendonly" or "inactive" attributes, as defined in RFC 3264 [4]. RFC 3264 directs implementations to cease transmission of media in these cases. However, doing so may cause NAT bindings to timeout, and media won't be able to come off hold. Secondly, some RTP payload formats, such as the payload format for - text conversation [29], may send packets so infrequently that the + text conversation [30], may send packets so infrequently that the interval exceeds the NAT binding timeouts. Thirdly, if silence suppression is in use, long periods of silence may cause media transmission to cease sufficiently long for NAT bindings to time out. For these reasons, the media packets themselves cannot be relied upon. ICE defines a simple periodic keepalive that operates - indpendently of media transmission. This makes its bandwidth + independently of media transmission. This makes its bandwidth requirements highly predictable, and thus amenable to QoS reservations. B.9. Why Prefer Peer Reflexive Candidates? - Section 5.2 describes procedures for computing the priority of + Section 4.1.2 describes procedures for computing the priority of candidate based on its type and local preferences. That section requires that the type preference for peer reflexive candidates always be lower than server reflexive. Why is that? The reason has to do with the security considerations in Section 16. It is much easier for an attacker to cause an agent to use a false server reflexive candidate than it is for an attacker to cause an agent to use a false peer reflexive candidate. Consequently, attacks against the STUN binding discovery usage are thwarted by ICE by preferring the peer reflexive candidates. B.10. Why Send an Updated Offer? - Section 12.1 describes rules for sending media. Both agents can send + Section 11.1 describes rules for sending media. Both agents can send media once ICE checks complete, without waiting for an updated offer. Indeed, the only purpose of the updated offer is to "correct" the m/c-line so that it matches where media is being sent, based on ICE procedures. This begs the question - why is the updated offer/answer exchange needed at all? Indeed, in a pure offer/answer environment, it would not be. The offerer and answerer will agree on the candidates to use through ICE, and then can begin using them. As far as the agents themselves are concerned, the updated offer/answer provides no new information. However, in practice, numerous components along the signaling path look at the SDP information. These include entities performing off-path QoS reservations, NAT traversal components such as ALGs and Session Border Controllers (SBCs) and diagnostic tools that passively monitor the network. For these tools to continue to function without change, the core property of SDP - that the m/c- lines represent the addresses used for media - must be retained. For this reason, an updated offer must be sent. +B.11. Why are Binding Indications Used for Keepalives? + + Media keepalives are described in Section 10. These keepalives make + use of STUN when both endpoints are ICE capable. However, rather + than using a Binding Request transaction (which generates a + response), the keepalives use an Indication. Why is that? + + The primary reason has to do with network QoS mechanisms. Once media + begins flowing, network elements will assume that the media stream + has a fairly regular structure, making use of periodic packets at + fixed intervals, with the possibility of jitter. If an agent is + sending media packets, and then receives a Binding Request, it would + need to generate a response packet along with its media packets. + This will increase the actual bandwidth requirements for the 5-tuple + carrying the media packets, and introduce jitter in the delivery of + those packets. Analysis has shown that this is a concern in certain + layer 2 access networks that use fairly tight packet schedulers for + media. + + Additionally, using a Binding Indication allows integrity to be + disabled, allowing for better performance. This is useful for large + scale endpoints, such as PSTN gateways. + Author's Address Jonathan Rosenberg Cisco Systems 600 Lanidex Plaza Parsippany, NJ 07054 US Phone: +1 973 952-5000 Email: jdrosen@cisco.com @@ -3411,18 +3682,18 @@ This document and the information contained herein are provided on an "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE INTERNET ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE. Copyright Statement - Copyright (C) The Internet Society (2006). This document is subject + Copyright (C) The Internet Society (2007). This document is subject to the rights, licenses and restrictions contained in BCP 78, and except as set forth therein, the authors retain all their rights. Acknowledgment Funding for the RFC Editor function is currently provided by the Internet Society.