MMUSIC J. Rosenberg Internet-Draft Cisco Obsoletes: 4091 (if approved) March 26, 2007 Intended status: Standards Track
March 5, 2007Expires: September 6,27, 2007 Interactive Connectivity Establishment (ICE): A Methodology for Network Address Translator (NAT) Traversal for Offer/Answer Protocols draft-ietf-mmusic-ice-14draft-ietf-mmusic-ice-15 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 other groups may also distribute working documents as Internet- Drafts. Internet-Drafts are draft documents valid for a maximum of six months 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 September 6,27, 2007. Copyright Notice Copyright (C) The IETF Trust (2007). Abstract This document describes a protocol for Network Address Translator (NAT) traversal for multimedia sessions established with the offer/ answer model. This protocol is called Interactive Connectivity Establishment (ICE). ICE makes use of the Session Traversal Utilities for NAT (STUN) protocol, applying its binding discovery and relay usages, in addition to defining a new usage for checking connectivity between peers. ICE can be used by any protocol utilizing the offer/answer model, such as the Session Initiation Protocol (SIP). Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . .6 2. Overview of ICE . . . . . . . . . . . . . . . . . . . . . . . 7 2.1. Gathering Candidate Addresses . . . . . . . . . . . . . . 9 2.2. Connectivity Checks . . . . . . . . . . . . . . . . . . . 11 2.3. Sorting Candidates . . . . . . . . . . . . . . . . . . . .12 2.4. Frozen Candidates . . . . . . . . . . . . . . . . . . . . 13 2.5. Security for Checks . . . . . . . . . . . . . . . . . . . 14 2.6. Concluding ICE . . . . . . . . . . . . . . . . . . . . . .14 2.7. Lite Implementations . . . . . . . . . . . . . . . . . . .16 3. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 16 4. Sending the Initial Offer . . . . . . . . . . . . . . . . . . 19 4.1. Full Implementation Requirements . . . . . . . . . . . . .19 4.1.1. Gathering Candidates . . . . . . . . . . . . . . . . .19 184.108.40.206. Host Candidates . . . . . . . . . . . . . . . . . 2019 220.127.116.11. Server Reflexive and Relayed Candidates . . . . . 20 18.104.22.168. Eliminating Redundant Candidates . . . . . . . . .21 22.214.171.124. Computing Foundations . . . . . . . . . . . . . . 21 126.96.36.199. Keeping Candidates Alive . . . . . . . . . . . . .22 4.1.2. Prioritizing Candidates . . . . . . . . . . . . . . . 22 188.8.131.52. Recommended Formula . . . . . . . . . . . . . . . 22 184.108.40.206. Guidelines for Choosing Type and Local Preferences . . . . . . . . . . . . . . . . . . . 23 4.1.3. Choosing Default Candidates . . . . . . . . . . . . . 24 4.2. Lite Implementation . . . . . . . . . . . . . . . . . . . 25 4.3. Encoding the SDP . . . . . . . . . . . . . . . . . . . . .25 5. Receiving the Initial Offer . . . . . . . . . . . . . . . . . 27 5.1. Verifying ICE Support . . . . . . . . . . . . . . . . . . 27 5.2. Determining Role . . . . . . . . . . . . . . . . . . . . . 2728 5.3. Gathering Candidates . . . . . . . . . . . . . . . . . . .28 5.4. Prioritizing Candidates . . . . . . . . . . . . . . . . . 2829 5.5. Choosing Default Candidates . . . . . . . . . . . . . . . 2829 5.6. Encoding the SDP . . . . . . . . . . . . . . . . . . . . . 2829 5.7. Forming the Check Lists . . . . . . . . . . . . . . . . . 2829 5.7.1. Forming Candidate Pairs . . . . . . . . . . . . . . . 29 5.7.2. Computing Pair Priority and Ordering Pairs . . . . . . 3132 5.7.3. Pruning the Pairs . . . . . . . . . . . . . . . . . . 3132 5.7.4. Computing States . . . . . . . . . . . . . . . . . . . 3132 5.8. Performing Periodic Checks . . . . . . . . . . . . . . . . 3435 6. Receipt of the Initial Answer . . . . . . . . . . . . . . . . 3537 6.1. Verifying ICE Support . . . . . . . . . . . . . . . . . . 3637 6.2. Determining Role . . . . . . . . . . . . . . . . . . . . . 3637 6.3. Forming the Check List . . . . . . . . . . . . . . . . . . 3637 6.4. Performing Periodic Checks . . . . . . . . . . . . . . . . 3637 7. Performing Connectivity Checks . . . . . . . . . . . . . . . . 3637 7.1. Client Procedures . . . . . . . . . . . . . . . . . . . . 3738 7.1.1. Sending the Request . . . . . . . . . . . . . . . . . 3738 220.127.116.11. PRIORITY and USE-CANDIDATE . . . . . . . . . . . . 3738 18.104.22.168. ICE-CONTROLLED and ICE-CONTROLLING . . . . . . . 38 22.214.171.124. Forming Credentials . . . . . . . . . . . . . . . 37 126.96.36.199.39 188.8.131.52. DiffServ Treatment . . . . . . . . . . . . . . . . 3839 7.1.2. Processing the Response . . . . . . . . . . . . . . . 3839 184.108.40.206. Failure Cases . . . . . . . . . . . . . . . . . . 3839 220.127.116.11. Success Cases . . . . . . . . . . . . . . . . . . 3840 18.104.22.168.1. Discovering Peer Reflexive Candidates . . . . 3840 22.214.171.124.2. Updating Pair States . . . . . . . . . . . . . 3941 126.96.36.199.3. Constructing a Valid Pair . . . . . . . . . . 4042 188.8.131.52.4. Updating the Nominated Flag . . . . . . . . . 4042 184.108.40.206. Check List and Timer State Updates . . . . . . . 43 7.2. Server Procedures . . . . . . . . . . . . . . . . . . . . 4143 7.2.1. Additional Procedures for Full Implementations . . . . 4144 220.127.116.11. Detecting and Repairing Role Conflicts . . . . . 44 18.104.22.168. Computing Mapped Address . . . . . . . . . . . . . 41 22.214.171.124.45 126.96.36.199. Learning Peer Reflexive Candidates . . . . . . . . 42 188.8.131.52.45 184.108.40.206. Triggered Checks . . . . . . . . . . . . . . . . . 42 220.127.116.11.46 18.104.22.168. Updating the Nominated Flag . . . . . . . . . . . 4347 7.2.2. Additional Procedures for Lite Implementations . . . . 4347 8. Concluding ICE Processing . . . . . . . . . . . . . . . . . . 4347 8.1. Nominating Pairs . . . . . . . . . . . . . . . . . . . . . 4448 8.1.1. Regular Nomination . . . . . . . . . . . . . . . . . . 4448 8.1.2. Aggressive Nomination . . . . . . . . . . . . . . . . 4549 8.2. Updating States . . . . . . . . . . . . . . . . . . . . . 4549 9. Subsequent Offer/Answer Exchanges . . . . . . . . . . . . . . 4650 9.1. Generating the Offer . . . . . . . . . . . . . . . . . . . 4651 9.1.1. Procedures for All Implementations . . . . . . . . . . 4651 22.214.171.124. ICE Restarts . . . . . . . . . . . . . . . . . . . 4651 126.96.36.199. Removing a Media Stream . . . . . . . . . . . . . 4752 188.8.131.52. Adding a Media Stream . . . . . . . . . . . . . . 4752 9.1.2. Procedures for Full Implementations . . . . . . . . . 4752 184.108.40.206. Existing Media Streams with ICE Running . . . . . 4852 220.127.116.11. Existing Media Streams with ICE Completed . . . . 4853 9.1.3. Procedures for Lite Implementations . . . . . . . . . 4953 9.2. Receiving the Offer and Generating an Answer . . . . . . . 4953 9.2.1. Procedures for All Implementations . . . . . . . . . . 4953 18.104.22.168. Detecting ICE Restart . . . . . . . . . . . . . . 4954 22.214.171.124. New Media Stream . . . . . . . . . . . . . . . . . 5054 126.96.36.199. Removed Media Stream . . . . . . . . . . . . . . . 5054 9.2.2. Procedures for Full Implementations . . . . . . . . . 5054 188.8.131.52. Existing Media Streams with ICE Running and no remote-candidates . . . . . . . . . . . . . . . . 5055 184.108.40.206. Existing Media Streams with ICE Completed and no remote-candidates . . . . . . . . . . . . . . . 5055 220.127.116.11. Existing Media Streams and remote-candidates . . . 5055 9.2.3. Procedures for Lite Implementations . . . . . . . . . 5156 9.3. Updating the Check and Valid Lists . . . . . . . . . . . . 5256 9.3.1. Procedures for Full Implementations . . . . . . . . . 5256 18.104.22.168. ICE Restarts . . . . . . . . . . . . . . . . . . . 5256 22.214.171.124. New Media Stream . . . . . . . . . . . . . . . . . 5256 126.96.36.199. Removed Media Stream . . . . . . . . . . . . . . . 5256 188.8.131.52. ICE Continuing for Existing Media Stream . . . . . 5257 9.3.2. Procedures for Lite Implementations . . . . . . . . . 5357 10. Keepalives . . . . . . . . . . . . . . . . . . . . . . . . . . 5357 11. Media Handling . . . . . . . . . . . . . . . . . . . . . . . . 5458 11.1. Sending Media . . . . . . . . . . . . . . . . . . . . . . 5458 11.1.1. Procedures for Full Implementations . . . . . . . . . 5459 11.1.2. Procedures for Lite Implementations . . . . . . . . . 5559 11.1.3. Procedures for All Implementations . . . . . . . . . . 5560 11.2. Receiving Media . . . . . . . . . . . . . . . . . . . . . 5660 12. Usage with SIP . . . . . . . . . . . . . . . . . . . . . . . . 5660 12.1. Latency Guidelines . . . . . . . . . . . . . . . . . . . . 5660 12.1.1. Offer in INVITE . . . . . . . . . . . . . . . . . . . 5661 12.1.2. Offer in Response . . . . . . . . . . . . . . . . . . 5862 12.2. SIP Option Tags and Media Feature Tags . . . . . . . . . . 5863 12.3. Interactions with Forking . . . . . . . . . . . . . . . . 5863 12.4. Interactions with Preconditions . . . . . . . . . . . . . 5963 12.5. Interactions with Third Party Call Control . . . . . . . . 5963 13. UsageRelationship with ANAT . . . . . . . . . . . . . . . . . . . . . . . 5964 14. Extensibility Considerations . . . . . . . . . . . . . . . . . 6064 15. Grammar . . . . . . . . . . . . . . . . . . . . . . . . . . . 6165 15.1. "candidate" Attribute . . . . . . . . . . . . . . . . . . 6165 15.2. "remote-candidates" Attribute . . . . . . . . . . . . . . 6467 15.3. "ice-lite" and "ice-mismatch" Attributes . . . . . . . . . 6468 15.4. "ice-ufrag" and "ice-pwd" Attributes . . . . . . . . . . . 6468 15.5. "ice-options> Attribute . . . . . . . . . . . . . . . . . 6569 16. Example . . . . . . . . . . . . . . . . . . . . . . . . . . . 6569 17. Security Considerations . . . . . . . . . . . . . . . . . . . 7276 17.1. Attacks on Connectivity Checks . . . . . . . . . . . . . . 7276 17.2. Attacks on Address Gathering . . . . . . . . . . . . . . . 7478 17.3. Attacks on the Offer/Answer Exchanges . . . . . . . . . . 7579 17.4. Insider Attacks . . . . . . . . . . . . . . . . . . . . . 7579 17.4.1. The Voice Hammer Attack . . . . . . . . . . . . . . . 7580 17.4.2. STUN Amplification Attack . . . . . . . . . . . . . . 7680 17.5. Interactions with Application Layer Gateways and SIP . . . 7681 18. Definition of Connectivity Check Usage . . . . . . . . . . . . 7781 18.1. Applicability . . . . . . . . . . . . . . . . . . . . . . 7782 18.2. Client Discovery of Server . . . . . . . . . . . . . . . . 7882 18.3. Server Determination of Usage . . . . . . . . . . . . . . 7882 18.4. New Requests or Indications . . . . . . . . . . . . . . . 7882 18.5. New Attributes . . . . . . . . . . . . . . . . . . . . . . 7882 18.6. New Error Response Codes . . . . . . . . . . . . . . . . . 7883 18.7. Client Procedures . . . . . . . . . . . . . . . . . . . . 7883 18.8. Server Procedures . . . . . . . . . . . . . . . . . . . . 7883 18.9. Security Considerations for Connectivity Check . . . . . . 7983 19. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 7983 19.1. SDP Attributes . . . . . . . . . . . . . . . . . . . . . . 7984 19.1.1. candidate Attribute . . . . . . . . . . . . . . . . . 7984 19.1.2. remote-candidates Attribute . . . . . . . . . . . . . 7984 19.1.3. ice-lite Attribute . . . . . . . . . . . . . . . . . . 8085 19.1.4. ice-mismatch Attribute . . . . . . . . . . . . . . . . 8085 19.1.5. ice-pwd Attribute . . . . . . . . . . . . . . . . . . 8186 19.1.6. ice-ufrag Attribute . . . . . . . . . . . . . . . . . 8186 19.1.7. ice-options Attribute . . . . . . . . . . . . . . . . 8286 19.2. STUN Attributes . . . . . . . . . . . . . . . . . . . . . 8287 19.3. STUN Error Responses . . . . . . . . . . . . . . . . . . 87 20. IAB Considerations . . . . . . . . . . . . . . . . . . . . . . 8287 20.1. Problem Definition . . . . . . . . . . . . . . . . . . . . 8388 20.2. Exit Strategy . . . . . . . . . . . . . . . . . . . . . . 8388 20.3. Brittleness Introduced by ICE . . . . . . . . . . . . . . 8489 20.4. Requirements for a Long Term Solution . . . . . . . . . . 8489 20.5. Issues with Existing NAPT Boxes . . . . . . . . . . . . . 8590 21. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 8590 22. References . . . . . . . . . . . . . . . . . . . . . . . . . . 8691 22.1. Normative References . . . . . . . . . . . . . . . . . . . 8691 22.2. Informative References . . . . . . . . . . . . . . . . . . 8792 Appendix A. Lite and Full Implementations . . . . . . . . . . . . 8893 Appendix B. Design Motivations . . . . . . . . . . . . . . . . . 8994 B.1. Pacing of STUN Transactions . . . . . . . . . . . . . . . 9094 B.2. Candidates with Multiple Bases . . . . . . . . . . . . . . 9095 B.3. Purpose of the <rel-addr> and <rel-port> Attributes . . . 9297 B.4. Importance of the STUN Username . . . . . . . . . . . . . 9297 B.5. The Candidate Pair Sequence Number Formula . . . . . . . . 9398 B.6. The remote-candidates attribute . . . . . . . . . . . . . 9499 B.7. Why are Keepalives Needed? . . . . . . . . . . . . . . . . 95100 B.8. Why Prefer Peer Reflexive Candidates? . . . . . . . . . . 96101 B.9. Why Send an Updated Offer? . . . . . . . . . . . . . . . . 96101 B.10. Why are Binding Indications Used for Keepalives? . . . . . 96101 Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 97102 Intellectual Property and Copyright Statements . . . . . . . . . . 98103 1. Introduction RFC 3264  defines a two-phase exchange of Session Description Protocol (SDP) messages  for the purposes of establishment of multimedia sessions. This offer/answer mechanism is used by protocols such as the Session Initiation Protocol (SIP) . 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 the IP of media sources and sinks within their messages, which is known to be problematic through NAT . 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 , Simple Traversal Underneath NAT (STUN)  and its revision, retitled Session Traversal Utilities for NAT , the STUN Relay Usage , and Realm Specific IP   along with session description extensions needed to make them work, such as the Session Description Protocol (SDP)  attribute for the Real Time Control Protocol (RTCP) . 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 defines Interactive Connectivity Establishment (ICE) as a technique for NAT traversal for media streams established by the offer/answer model. ICE is an extension to the offer/answer model, and works by including a multiplicity of IP addresses and ports in SDP offers and answers, which are then tested for connectivity by peer-to-peer STUN exchanges. The IP addresses and ports included in the SDP are gathered using the STUN binding acquisition techniques in  and relay allocation procedures in . 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 protocol (such as SIP), by which they can perform an offer/answer exchange of SDP  messages. Note that ICE is not intended for NAT traversal for SIP, which is assumed to be provided via another mechanism .. At the beginning of the ICE process, the agents are ignorant of their own topologies. In particular, they might or might not be behind a NAT (or multiple tiers of NATs). ICE allows the agents to discover enough information about their topologies to potentially find one or more paths by which they can communicate. Figure 1 shows a typical environment for ICE deployment. The two endpoints are labelled L and R (for left and right, which helps visualize call flows). Both L and R are behind their own respective NATs though they may not be aware of it. The type of NAT and its properties are also unknown. Agents L and R are capable of engaging in an offer/answer exchange by which they can exchange SDP messages, whose purpose is to set up a media session between L and R. Typically, this exchange will occur through a SIP server. In addition to the agents, a SIP server and NATs, ICE is typically used in concert with STUN servers in the network. Each agent can have its own STUN server, or they can be the same. +-------+ | SIP | +-------+ | Srvr | +-------+ | STUN | | | | STUN | | Srvr | +-------+ | Srvr | | | / \ | | +-------+ / \ +-------+ / \ / \ / \ / \ / <- Signalling -> \ / \ / \ +--------+ +--------+ | NAT | | NAT | +--------+ +--------+ / \ / \ / \ +-------+ +-------+ | Agent | | Agent | | L | | R | | | | | +-------+ +-------+ Figure 1: ICE Deployment Scenario The basic idea behind ICE is as follows: each agent has a variety of candidate TRANSPORT ADDRESSES (combination of IP address and port) it could use to communicate with the other agent. These might include: o A transport address on a directly attached network interface or interfaces o A translated transport address on the public side of a NAT (a "server reflexive" address) o The transport 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 candidate transport addresses. In practice, however, many combinations will not work. For instance, if L and R are both behind NATs, their directly attached interface addresses are unlikely to be able to communicate directly (this is why ICE is needed, after all!). The purpose of ICE is to discover which pairs of addresses will work. The way that ICE does this is to systematically try all possible pairs (in a carefully sorted order) until it finds one or more that works. 2.1. Gathering Candidate Addresses In order to execute ICE, an agent has to identify all of its address candidates. A CANDIDATE is a transport address - a combination of IP address and port for a particular transport protocol. This document defines three types of candidates, some derived from physical or logical network interfaces, others discoverable via STUN. Naturally, one viable candidate is a transport address obtained directly from a local interface. Such a candidate is called a HOST CANDIDATE. The local interface could be ethernet or WiFi, or it could be one that is obtained through a tunnel mechanism, such as a Virtual Private Network (VPN) or Mobile IP (MIP). In all cases, such a network interface appears to the agent as a local interface from which ports (and thus a candidate) can be allocated. If an agent is multihomed, it obtains a candidate from each interface. Depending on the location of the PEER (the other agent in the session) on the IP network relative to the agent, the agent may be reachable by the peer through one or more of those interfaces. Consider, for example, an agent which has a local interface to a private net 10 network (I1), and a second connected to the public Internet (I2). A candidate from I1 will be directly reachable when communicating with a peer on the same private net 10 network, while a candidate from I2 will be directly reachable when communicating with a peer on the public Internet. Rather than trying to guess which interface will work prior to sending an offer, the offering agent includes both candidates in its offer. Next, the agent uses STUN to obtain additional candidates. These come in two flavors: translated addresses on the public side of a NAT (SERVER REFLEXIVE CANDIDATES) and addresses of media relays (RELAYED CANDIDATES). The relationship of these candidates to the host candidate is shown in Figure 2. Both types of candidates are discovered using STUN. To Internet | | | /------------ Relayed Y:y | / Address +--------+ | | | STUN | | Server | | | +--------+ | | | /------------ Server X1':x1'|/ Reflexive +------------+ Address | NAT | +------------+ | | /------------ Local X:x |/ Address +--------+ | | | Agent | | | +--------+ Figure 2: Candidate Relationships To find a server reflexive candidate, the agent sends a STUN Binding Request, using the Binding Discovery Usage  from each host candidate, to its STUN server. It is assumed that the address of the STUN server is manually configured or learned in some unspecified way. It is RECOMMENDED that when an agent has a choice of STUN servers (when, for example, they are learned through DNS records and multiple results are returned), an agent uses a single STUN server (based on its IP address) for all candidates for a particular session. This improves the performance of ICE. When the agent sends the Binding Request from IP address and port X:x, the NAT (assuming there is one) will allocate a binding X1':x1', mapping this server reflexive candidate to the host candidate X:x. Outgoing packets sent from the host candidate will be translated by the NAT to the server reflexive candidate. Incoming packets sent to the server relexive candidate will be translated by the NAT to the host candidate and forwarded to the agent. We call the host candidate associated with a given server reflexive candidate the BASE. NOTE: "Base" refers to the address an agent sends from for a particular candidate. Thus, as a degenerate case host candidates also have a base, but it's the same as the host candidate. When there are multiple NATs between the agent and the STUN server, the STUN request will create a binding on each NAT, but only the outermost server reflexive candidate will be discovered by the agent. If the agent is not behind a NAT, then the base candidate will be the same as the server reflexive candidate and the server reflexive candidate is redundant and will be eliminated. The final type of candidate is a RELAYED CANDIDATE. The STUN Relay Usage  allows a STUN server to act as a media relay, forwarding traffic between L and R. In order to send traffic to L, R sends traffic to the media relay at Y:y, and the relay forwards that to X1':x1', which passes through the NAT where it is mapped to X:x and delivered to L. 2.2. Connectivity Checks Once L has gathered all of its candidates, it orders them in highest to lowest priority and sends them to R over the signalling channel. The candidates are carried in attributes in the SDP offer. When R receives the offer, it performs the same gathering process and responds with its own list of candidates. At the end of this process, each agent has a complete list of both its candidates and its peer's candidates. It pairs them up, resulting in CANDIDATE PAIRS. To see which pairs work, the agent schedules a series of CHECKS. Each check is a STUN transaction that the client will perform on a particular candidate pair by sending a STUN request from the local candidate to the remote candidate. The basic principle of the connectivity checks is simple: 1. Sort the candidate pairs in priority order. 2. Send checks on each candidate pair in priority order. 3. Acknowledge checks received from the other agent. With both agents performing a check on a candidate pair, the result is a 4-way handshake: L R - - STUN request -> \ L's <- STUN response / check <- STUN request \ R's STUN response -> / check Figure 3: Basic Connectivity Check It is important to note that the STUN requests are sent to and from the exact same IP addresses and ports that will be used for media (e.g., RTP and RTCP). Consequently, agents demultiplex STUN and RTP/ RTCP using contents of the packets, rather than the port on which they are received. Fortunately, this demultiplexing is easy to do, especially for RTP and RTCP. Because STUN is used for the connectivity check, the STUN response will contain the agent's translated transport address on the public side any NATs between the agent and its peer. If this transport address is different from other candidates the agent already learned, it represents a new candidate, called a PEER REFLEXIVE CANDIDATE, which then gets tested by ICE just the same as any other candidate. As an optimization, as soon as R gets L's check message R immediately sends a check message to L on the same candidate pair. This accelerates the process of finding a valid candidate, and is called a TRIGGERED CHECK. At the end of this handshake, both L and R know that they can send (and receive) messages end-to-end in both directions. 2.3. Sorting Candidates Because the algorithm above searches all candidate pairs, if a working pair exists it will eventually find it no matter what order the candidates are tried in. In order to produce faster (and better) results, the candidates are sorted in a specified order. The resulting list of sorted candidate pairs is called the CHECK LIST. The algorithm is described in Section 4.1.2 but follows two general principles: o Each agent gives its candidates a numeric priority which is sent along with the candidate to the peer o The local and remote priorities are combined so that each agent has the same ordering for the candidate pairs. The second property is important for getting ICE to work when there are NATs in front of L and R. Frequently, NATs will not allow packets in from a host until the agent behind the NAT has sent a packet towards that host. Consequently, ICE checks in each direction will not succeed until both sides have sent a check through their respective NATs. The agent works through this check list by sending a STUN request for the next candidate pair on the list every 20ms. These are called PERIODIC CHECKS. In general the priority algorithm is designed so that candidates of similar type get similar priorities and so that more direct routes (that is, through fewer media relays and through fewer NATs) are preferred over indirect ones (ones with more media relays and more NATs). Within those guidelines, however, agents have a fair amount of discretion about how to tune their algorithms. 2.4. Frozen Candidates The previous description only addresses the case where the agents wish to establish a media session with one COMPONENT (a piece of a media stream requiring a single transport address; a media stream may require multiple components, each of which has to work for the media stream as a whole to be work). Typically, (e.g., with RTP and RTCP) the agents actually need to establish connectivity for more than one flow. The network properties are likely to be very similar for each component (especially because RTP and RTCP are sent and received from the same IP address). It is usually possible to leverage information from one media component in order to determine the best candidates for another. ICE does this with a mechanism called "frozen candidates." Each candidate is associated with a property called its FOUNDATION. Two candidates have the same foundation when they are "similar" - of the same type and obtained from the same interfaces and STUN servers. Otherwise, their foundation is different. A candidate pair has a foundation too, which is just the concatenation of the foundations of its two candidates. Initially, only the candidate pairs with unique foundations are tested. The other candidate pairs are marked "frozen". When the connectivity checks for a candidate pair succeed, the candidate pairs with the same foundation are unfrozen. This avoids repeated checking of components which are superficially more attractive but in fact are likely to fail. While we've described "frozen" here as a separate mechanism for expository purposes, in fact it is an integral part of ICE and the the ICE prioritization algorithm automatically ensures that the right candidates are unfrozen and checked in the right order. 2.5. Security for Checks Because ICE is used to discover which addresses can be used to send media between two agents, it is important to ensure that the process cannot be hijacked to send media to the wrong location. Each STUN connectivity check is covered by a message authentication code (MAC) computed using a key exchanged in the signalling channel. This MAC provides message integrity and data origin authentication, thus stopping an attacker from forging or modifying connectivity check messages. The MAC also aids in disambiguating ICE exchanges from forked calls when ICE is used with SIP . 2.6. Concluding ICE ICE checks are performed in a specific sequence, so that high priority candidate pairs are checked first, followed by lower priority ones. One way to conclude ICE is to declare victory as soon as a check for each component of each media stream completes successfully. Indeed, this is a reasonable algorithm, and details for it are provided below. However, it is possible that packet losses will cause a higher priority check to take longer to complete. In that case, allowing ICE to run a little longer might produce better results. More fundamentally, however, the prioritization defined by this specification may not yield "optimal" results. As an example, if the aim is to select low latency media paths, usage of a relay is a hint that latencies may be higher, but it is nothing more than a hint. An actual RTT measurement could be made, and it might demonstrate that a pair with lower priority is actually better than one with higher priority. Consequently, ICE assigns one of the agents in the role of the CONTROLLING AGENT, and the other of the CONTROLLED AGENT. The controlledcontrolling agent gets to nominate which candidate pairs will get used for media amongst the ones that are valid. It can do this in one of two ways - using REGULAR NOMINATION or AGGRESSIVE NOMINATION. With regular nomination, the controlling agent lets the checks continue until at least one valid candidate pair for each media stream is found. Then, it picks amongst those that are valid, and sends a second STUN request on its NOMINATED candidate pair, but this time with a flag set to tell the peer that this pair has been nominated for use. A This is shown in Figure 4. L R - - STUN request \ L's <- STUN response / check <- STUN request \ R's STUN response -> / check STUN request + flag \ L's <- STUN response / check Figure 4: Regular Nomination Once the STUN transaction with the flag completes, both sides cancel any future checks for that media stream. ICE will now send media using this pair. The pair an ICE agent is using for media is called the SELECTED PAIR. In aggressive nomination, the controlling agent puts the flag in every STUN request it sends. This way, once the first check succeeds, ICE processing is complete for that media stream and the controlling agent doesn't have to send a second STUN request. The selected pair will be the highest priority valid pair. Aggressive nomination is faster than regular nomination, but gives less flexibility. Aggressive nomination is shown in Figure 5. L R - - STUN request + flag \ L's <- STUN response / check <- STUN request \ R's STUN response -> / check Figure 5: Aggressive Nomination Once all of the media streams are completed, the controlling endpoint sends an updated offer if the candidates in the m and c lines for the media stream (called the DEFAULT CANDIDATES) don't match ICE's SELECTED CANDIDATES. Once ICE is concluded, it can be restarted at any time for one or all of the media streams by either agent. This is done by sending an updated offer indicating a restart. 2.7. Lite Implementations In order for ICE to be used in a call, both agents need to support it. However, certain agents will always be connected to the public Internet and have a public IP address at which it can receive packets from any correspondent. To make it easier for these devices to support ICE, ICE defines a special type of implementation called LITE (in contrast to the normal FULL implementation). A lite implementation doesn't gather candidates; it includes only host candidates for any media stream. 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. Additional guidance on when a lite implementation is appropriate, see the discussion in Appendix A. For an informational summary of ICE processing as seen by a lite agent, see .It is important to note that the lite implementation was added to this specification to provide a stepping stone to full implementation. Even for devices that are always connected to the public Internet, a full implementation is preferable if achievable. 3. Terminology The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in RFC 2119 . Readers should be familiar with the terminology defined in the offer/ answer model , STUN  and NAT Behavioral requirements for UDP  This specification makes use of the following additional terminology: Agent: As defined in RFC 3264, an agent is the protocol implementation involved in the offer/answer exchange. There are two agents involved in an offer/answer exchange. Peer: From the perspective of one of the agents in a session, its peer is the other agent. Specifically, from the perspective of the offerer, the peer is the answerer. From the perspective of the answerer, the peer is the offerer. Transport Address: The combination of an IP address and transport protocol (such as UDP or TCP) port. Candidate: A transport address that is to be tested by ICE procedures in order to determine its suitability for usage for receipt of media. Candidates also have properties - their type (server reflexive, relayed or host), priority, foundation, and base. Component: A component is a piece of a media stream requiring a single transport address; a media stream may require multiple components, each of which has to work for the media stream as a whole to work. For media streams based on RTP, there are two components per media stream - one for RTP, and one for RTCP. Host Candidate: A candidate obtained by binding to a specific port from an 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)  (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. The STUN server's 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. Relayed Candidate: A candidate obtained by sending a STUN Allocate request from a host candidate to a STUN server. The relayed candidate is resident on the STUN server, and the STUN server relays packets back towards the agent. Base: The base of a server reflexive candidate is the host candidate from which it was derived. A host candidate is also said to have a base, equal to that candidate itself. Similarly, the base of a relayed candidate is that candidate itself. Foundation: An arbitrary string that is the same for two candidates that have the same type, base IP address, and STUN server. If any of these are different then the foundation will be different. Two candidate pairs with the same foundation pairs are likely to have similar network characteristics. Foundations are used in the frozen algorithm. Local Candidate: A candidate that an agent has obtained and included in an offer or answer it sent. Remote Candidate: A candidate that an agent received in an offer or answer from its peer. Default Destination/Candidate: The default destination for a component of a media stream is the transport address that would be used by an agent that is not ICE aware. For the RTP component, the default IP address is in the c line of the SDP, and the port in the m line. For the RTCP component it is in the rtcp attribute when present, and when not present, the IP address in the c line and 1 plus the port in the m line. A default candidate for a component is one whose transport address matches the default destination for that component. Candidate Pair: A pairing containing a local candidate and a remote candidate. Check, Connectivity Check, STUN Check: A STUN Binding Request transaction for the purposes of verifying connectivity. A check is sent from the local candidate to the remote candidate of a candidate pair. Check List: An ordered set of candidate pairs that an agent will use to generate checks. 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. In any session, one agent is always controlling. The other is the controlled agent. Controlled Agent: A STUN agent which waits for the controlling agent to select the final choice of candidate pairs. Regular Nomination: The process of picking a valid candidate pair for media traffic by validating the pair with one STUN request, and then picking it by sending a second STUN request with a flag indicating its nomination. Aggressive Nomination: The process of picking a valid candidate pair for media traffic by including a flag in every STUN request, such that the first one to produce a valid candidate pair is used for media. Nominated: If a valid candidate pair has its nominated flag set, it means that it may be selected by ICE for sending and receiving media. Selected Pair, Selected Candidate: The candidate pair selected by ICE for sending and receiving media is called the selected pair, and each of its candidates is called the selected candidate. 4. Sending the Initial Offer In order to send the initial offer in an offer/answer exchange, an agent must (1) gather candidates, (2) prioritize them, (3) choose default candidates, and then (4) formulate and send the SDP. The first of these four steps differ for full and lite implementations. 4.1. Full Implementation Requirements 4.1.1. Gathering Candidates An agent gathers candidates when it believes that 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 server reflexive and relayed candidates are gathered using STUN's Binding Discovery and Relay Usages. The base of a candidate is the candidate that an agent must send from when using that candidate. 184.108.40.206. Host Candidates The first step is to gather host candidates. Host candidates are obtained by binding to ports (typically ephemeral) on an interface (physical or virtual, including VPN interfaces) on the host. The process for gathering host candidates depends on the transport protocol. Procedures are specified here for UDP. For each UDP media stream the agent wishes to use, the agent SHOULD obtain a candidate for each component of the media stream on each interface that the host has. It obtains each candidate by binding to a UDP port on the specific interface. A host candidate (and indeed every candidate) is always associated with a specific component for which it is a candidate. Each component has an ID assigned to it, called the component ID. For RTP-based media streams, the RTP itself has a component ID of 1, and RTCP a component ID of 2. If an agent is using RTCP it MUST obtain a candidate for it. If an agent is using both RTP and RTCP, it would end up with 2*K host candidates if an agent has K interfaces. The base for each host candidate is set to the candidate itself. 220.127.116.11. Server Reflexive and Relayed Candidates Agents SHOULD obtain relayed candidates and MUSTSHOULD obtain server reflexive candidates. The requirement to obtain relayed candidates isThese requirements are at SHOULD strength to allow for provider variation. Use of STUN servers may be unnecessary in closed networks where agents are never connected to the public Internet or to endpoints outside of the closed network. In such cases, a full implementation would be used for agents that are dual- stack or multi-homed, to select a host candidate. Use of relays is expensive, and when ICE is being used, relays will only be requiredutilized when both endpoints are behind NATs that perform address and port dependent mapping. Consequently, some deployments might consider this use case to be marginal, and elect not to use relays. If they arean agent does not used,gather server reflexive or relayed candidates, it is RECOMMENDED that itthe functionality be implemented and just disabled through configuration, so that it can re-enabled through configuration if conditions change in the future. The agent next pairs each host candidate with the STUN server with which it is configured or has discovered by some means. This specification only considers usage of a single STUN server. At that very instance, and then every Ta milliseconds thereafter, 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 using the relay usage . If the agent is using only server reflexive candidates, the request MUST be a STUN Binding request using the binding discovery usage . The value of Ta SHOULD be configurable, and SHOULD have a default of 20ms (see Appendix B.1 for a discussion on the selection of this value). Note that this pacing applies only to starting STUN transactions with source and destination transport addresses (i.e., 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 . 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 number of candidates which are gathered. The agent will receive a STUN Binding or Allocate response. A successful Allocate Response will provide the agent with a server reflexive candidate (obtained from the mapped address) and a relayed candidate in the RELAY-ADDRESS attribute. If the Allocate request is rejected because the server lacks resources to fulfill it, the agent SHOULD instead send a Binding Request to obtain a server reflexive candidate. A Binding Response will provide the agent with only a server reflexive candidate (also obtained from the mapped address). The base of the server reflexive candidate is the host candidate from which the Allocate or Binding request was sent. The base of a relayed candidate is that candidate itself. If a relayed candidate is identical to a host candidate (which can happen in rare cases), the relayed candidate MUST be discarded. Proper operation of ICE depends on eachcandidate having a unique base being unique.when their transport addresses are identical. 18.104.22.168. Eliminating Redundant Candidates Next, the agent eliminates redundant candidates. A candidate is redundant if its transport address equals another candidate, and its base equals the base of that other candidate. Note that two candidates can have the same transport address yet have different bases, and these would not be considered redundant. 22.214.171.124. Computing Foundations Finally, the agent assigns each candidate a foundation. The foundation is an identifier, scoped within a session. Two candidates MUST have the same foundation ID when all of the following are true: o they are of the same type (host, relayed, server reflexive, peer reflexive or relayed) o their bases have the same IP address (the ports can be different) o 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. 126.96.36.199. Keeping Candidates Alive Once server reflexive and relayed candidates are allocated, they MUST be kept alive until ICE processing has completed. For server reflexive candidates learned through the Binding Discovery usage, this MUST be another Binding Request from the Binding Discovery usage. For relayed candidates learned through the Relay Usage, this MUST be a new Allocate request. The Allocate request will also refresh the server reflexive candidate. 4.1.2. Prioritizing Candidates The prioritization process results in the assignment of a priority to each candidate. Each candidate for a media stream MUST have a unique priority that MUST be a positive integer between 1 and (2**32 - 1). This priority will be used by ICE to determine the order of the connectivity checks and the relative preference for candidates. An agent SHOULD compute this priority using the formula in Section 188.8.131.52 and choose its parameters using the guidelines in Section 184.108.40.206. If an agent elects to use a different formula, ICE will take longer to converge since both agents will not be coordinated in their checks. 220.127.116.11. Recommended Formula When using the formula, an agent computes the priority by determining a preference for each type of candidate (server reflexive, peer reflexive, relayed and host), and, when the agent is multihomed, choosing a preference for its interfaces. These two preferences are then combined to compute the priority for a candidate. That priority is computed using the following formula: priority = (2^24)*(type preference) + (2^8)*(local preference) + (2^0)*(256 - component ID) The type preference MUST be an integer from 0 to 126 inclusive, and represents the preference for the type of the candidate (where the types are local, server reflexive, peer reflexive and relayed). A 126 is the highest preference, and a 0 is the lowest. Setting the value to a 0 means that candidates of this type will only be used as a last resort. The type preference MUST be identical for all candidates of the same type and MUST be different for candidates of different types. The type preference for peer reflexive candidates MUST be higher than that of server reflexive candidates. Note that candidates gathered based on the procedures of Section 4.1.1 will never be peer reflexive candidates; candidates of these type are learned from the STUN connectivity checks performed by ICE. 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. More generally, if there are multiple candidates for a particular component for a particular media stream which have the same type, the local preference MUST be unique for each one. In this specification, this only happens for multi-homed hosts. The component ID is the component ID for the candidate, and MUST be between 1 and 256 inclusive. 18.104.22.168. Guidelines for Choosing Type and Local Preferences One criteria for selection of the type and local preference values is the use of an intermediary, such as a media relay. With an intermediary, if media is sent to that candidate, it will first transit the intermediary before being received. Relayed candidates are one type of candidate that involves 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 a media relay run by the provider. If these concerns are important, the type preference for relayed candidates SHOULD be lower than host candidates. The RECOMMENDED values are 126 for host candidates, 100 for server reflexive candidates, 110 for peer reflexive candidates, and 0 for relayed candidates. Furthermore, if an agent is 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) . It can also help with hosts that have both a native IPv6 address and a 6to4 address. In such a case, higher local preferences could be assigned to the v6 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 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. 4.1.3. Choosing Default Candidates A candidate is said to be default if it would be the target of media from a non-ICE peer; that target being called the DEFAULT DESTINATION. If the default candidates are not selected by the ICE algorithm when communicating with an ICE-aware peer, an updated offer/answer will be required after ICE processing completes in order to "correct" the SDP so that the default destination for media matches the candidates selected by ICE. If ICE happens to select the default candidates, no updated offer/answer is required. An agent MUST choose a set of candidates, one for each component of each in-use media stream, to be default. A media stream is in-use if it does not have a port of zero (which is used in RFC 3264 to reject a media stream). Consequently, a media stream is in-use even if it is marked as a=inactive  or has a bandwidth value of zero. It is RECOMMENDED that default candidates be chosen based on the likelihood of those candidates to work with the peer that is being contacted. It is RECOMMENDED that the default candidates are the relayed candidates (if relayed candidates are available), server reflexive candidates (if server reflexive candidates are available), and finally host candidates. 4.2. Lite Implementation 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 has multiple interfaces, it MUST choose one for each component of a particular media stream. With the lite implementation, ICE cannot be used to dynamically choose amongst candidates. 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, 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. If an agent has included two candidates for a component, the v4 candidate SHOULD be selected as the default. Since a lite implementation has a single candidate for a component, each of these candidates is considered to be default. 4.3. Encoding the SDP The process of encoding the SDP is identical between full and lite implementations. The agent will include an m-line for each media stream it wishes to use. The ordering of media streams in the SDP is relevant for ICE. ICE will perform its connectivity checks for the first m-line first, and consequently media will be able to flow for that stream first. Agents SHOULD place their most important media stream, if there is one, first in the SDP. There will be a candidate attribute for each candidate for a particular media stream. Section 15 provides detailed rules for constructing this attribute. The attribute carries the IP address, port and transport protocol for the candidate, in addition to its properties that need to be signaled to the peer for ICE to work: the priority, foundation, and component ID. The candidate attribute also carries information about the candidate that is useful for diagnostics and other functions: its type and related transport addresses. STUN connectivity checks between agents 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. The username fragment and password are exchanged in the ice-ufrag and ice-pwd attributes, respectively. In addition to providing security, the username provides disambiguation and correlation of checks to media streams. See Appendix B.4 for motivation. If an agent is a lite implementation, it MUST include an "a=ice-lite" session level attribute in its SDP. If an agent is a full implementation, it MUST NOT include this attribute. The default candidates are added to the SDP as the default destination for media. For streams based on RTP, this is done by placing the IP address and port of the RTP candidate into the c and m lines, respectively. If the agent is utilizing RTCP, it MUST encode the RTCP candidate using the a=rtcp attribute as defined in RFC 3605 . If RTCP is not in use, the agent MUST signal that using b=RS:0 and b=RR:0 as defined in RFC 3556 . The transport addresses that will be the default destination for media when communicating with non-ICE peers MUST also be present as candidates in one or more a=candidate lines. ICE provides for extensibility by allowing an offer or answer to contain a series of tokens which identify the ICE extensions used by that agent. If an agent supports an ICE extension, it MUST include the token defined for that extension in the ice-options attribute. The following is an example SDP message that includes ICE attributes (lines folded for readability): v=0 o=jdoe 2890844526 2890842807 IN IP4 10.0.1.1 s= c=IN IP4 192.0.2.3 t=0 0 a=ice-pwd:asd88fgpdd777uzjYhagZg a=ice-ufrag:8hhY m=audio 45664 RTP/AVP 0 b=RS:0 b=RR:0 a=rtpmap:0 PCMU/8000 a=candidate:1 1 UDP 2130706178 10.0.1.1 8998 typ localhost a=candidate:2 1 UDP 1694498562 192.0.2.3 45664 typ srflx raddr 10.0.1.1 rport 8998 Once an agent has sent its offer or sent its answer, that agent MUST be prepared to receive both STUN and media packets on each candidate. As discussed in Section 11.1, media packets can be sent to a candidate prior to its appearance as the default destination for media in an offer or answer. 5. Receiving the Initial Offer When an agent receives an initial offer, it will check if the offeror supports ICE,sufficient ICE functionality to proceed (i.e., if both offeror and answerer are lite implementations, ICE cannot proceed), determine its own role, gather candidates, prioritize them, choose default candidates, encode and send an answer, and for full implementations, form the check lists and begin connectivity checks. 5.1. Verifying ICE Support The answerer will proceed with the ICE procedures defined in this specification if the following are all true: o For each media stream, the default destination for at least one component of the media stream appears in a candidate attribute. For example, in the case of RTP, the IP address and port in the c and m line, respectively, appears in a candidate attribute, or the value in the rtcp attribute appears in a candidate attribute. o The offer omitted an a=ice-lite attribute or the answerer is a full implementation. In other words, if both agents are lite implementations, the agent does not proceed with ICE. If any of these conditions are not met, the agent MUST process the SDP based on normal RFC 3264 procedures, without using any of the ICE mechanisms described in the remainder of this specification with the following exceptions: 1. The agent MUST follow the rules of Section 10, which describe keepalive procedures for all agents. 2. If the agent is not proceeding with ICE because there were a=candidate attributes, but none that matched the default destination of the media stream, the agent MUST include an a=ice- mismatch attribute in its answer. 5.2. Determining Role For each session, each agent takes on a role. There are two roles - controlling, and controlled. The controlling agent is responsible for nominating the candidate pairs that can be used by ICE for each media stream, and for generating the updated offer based on ICE's selection, when needed. 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. If one of the agents is a lite implementation, it MUST assume the controlled role, and its peer (which will be full; if it was lite, ICE would have aborted) 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 onIn unusual cases it is possible for both agents to mistakenly believe they are controlled or controlling. To deal with such cases, at the time an agent determines its role, it MUST select a random number, called the tie-breaker, uniformly distributed between 0 and (2**64) - 1 (that is, a 64 bit positive integer). This number is used in STUN checks to detect and repair this definition, oncecase, as described in Section 22.214.171.124. Once roles are determined for a session, they persist unless ICE is restarted. A ICE restart (Section 9.1 causes a new selection of roles.roles and tie-breakers. 5.3. Gathering Candidates The process for gathering candidates at the answerer is identical to the process for the offerer as described in Section 4.1.1 for full implementations and Section 4.2 for lite implementations. It is RECOMMENDED that this process begin immediately on receipt of the offer, prior to alerting the user. Such gathering MAY begin when an agent starts. 5.4. Prioritizing Candidates The process for prioritizing candidates at the answerer is identical to the process followed by the offerer, as described in Section 4.1.2 for full implementations and Section 4.2 for lite implementations. 5.5. Choosing Default Candidates The process for selecting default candidates at the answerer is identical to the process followed by the offerer, as described in Section 4.1.3 for full implementations and Section 4.2 for lite implementations. 5.6. Encoding the SDP The process for encoding the SDP at the answerer is identical to the process followed by the offerer,offerer for both full and lite implementations, as described in Section 4.3. 5.7. Forming the Check Lists Forming check lists is done only by full implementations. Lite implementations MUST skip the steps defined in this section. There is one check list per in-use media stream resulting from the offer/answer exchange. To form the check list for a media stream, the agent forms candidate pairs, computes a candidate pair priority, orders the pairs by priority, prunes them, and sets their states. These steps are described in this section. 5.7.1. Forming Candidate Pairs First, the agent takes each of its candidates for a media stream (called LOCAL CANDIDATES) and pairs them with the candidates it received from its peer (called REMOTE CANDIDATES) for that media stream. In order to prevent the attacks described in Section 17.4.2, agents MAY limit the number of candidates they'll accept in an offer or answer. A local candidate is paired with a remote candidate if and only if the two candidates have the same component ID and have the same IP address version. It is possible that some of the local candidates don't get paired with a remote candidate, and some of the remote candidates don't get paired with local candidates. This can happen if one agent didn't include candidates for the all of the components for a media stream. If this happens, the number of components for that media stream is effectively reduced, and considered to be equal to the minimum across both agents of the maximum component ID provided by each agent across all components for the media stream. In the case of RTP, this would happen when one agent provided candidates for RTCP, and the other did not. As another example, the offerer can multiplex RTP and RTCP on the same port and signals it can do that in the SDP through some new attribute. However, since the offerer doesn't know if the answerer can perform such multiplexing, the offerer includes candidates for RTP and RTCP on separate ports, so that the offer has two components per media stream. If the answerer can perform such multiplexing, it would include just a single component for each candidate - for the combined RTP/RTCP mux. ICE would end up acting as if there was just a single component for this candidate. The candidate pairs whose local and remote candidates were both the default candidates for a particular component is called, unsurprisingly, the default candidate pair for that component. This is the pair that would be used to transmit media if both agents had not been ICE aware. In order to aid understanding, Figure 8 shows the relationships between several key concepts - transport addresses, candidates, candidate pairs, and check lists, in addition to indicating the main properties of candidates and candidate pairs. +------------------------------------------+ | | | +---------------------+ | | |+----+ +----+ +----+ | +Type | | || IP | |Port| |Tran| | +Priority | | ||Addr| | | | | | +Foundation | | |+----+ +----+ +----+ | +ComponentiD | | | Transport | +RelatedAddr | | | Addr | | | +---------------------+ +Base | | Candidate | +------------------------------------------+ * * * ************************************* * * +-------------------------------+ .| | | Local Remote | | +----+ +----+ +default? | | |Cand| |Cand| +valid? | | +----+ +----+ +nominated?| | +State | | | | | | Candidate Pair | +-------------------------------+ * * * ************ * * +------------------+ | Candidate Pair | +------------------+ +------------------+ | Candidate Pair | +------------------+ +------------------+ | Candidate Pair | +------------------+ Check List Figure 8: Conceptual Diagram of a Check List 5.7.2. Computing Pair Priority and Ordering Pairs Once the pairs are formed, a candidate pair priority is computed. Let O be the priority for the candidate provided by the offerer. Let A be the priority for the candidate provided by the answerer. The priority for a pair is computed as: pair priority = 2^32*MIN(O,A) + 2*MAX(O,A) + (O>A?1:0) Where O-P>A-P?1:0 is an expression whose value is 1 if O-P is greater than A-P, and 0 otherwise. This formula ensures a unique priority for each pair in most cases. Once the priority is assigned, the agent sorts the candidate pairs in decreasing order of priority. If two pairs have identical priority, the ordering amongst them is arbitrary. 5.7.3. Pruning the Pairs This sorted list of candidate pairs is used to determine a sequence of connectivity checks that will be performed. Each check involves sending a request from a local candidate to a remote candidate. Since an agent cannot send requests directly from a reflexive candidate, but only from its base, the agent next goes through the sorted list of candidate pairs. For each pair where the local candidate is server reflexive, the server reflexive candidate MUST be replaced by its base. Once this has been done, the agent MUST prune the list. This is done by removing a pair if its local and remote candidates are identical to the local and remote candidates of a pair higher up on the priority list. The result is a sequence of ordered candidate pairs, called the check list for that media stream. In addition, in order to limit the attacks described in Section 17.4.2, an agent SHOULD limit the total number of connectivity checks they perform across all check lists to 100, by discarding the lower priority candidate pairs until there are less than 100. 5.7.4. Computing States Each candidate pair in the check list has a foundation and a state. The foundation is the combination of the foundations of the local and remote candidates in the pair. The state is assigned once the check list for each media stream has been computed. There are five potential values that the state can have: Waiting: A check has not been performed for this pair, and can be performed as soon as it is the highest priority Waiting pair on the check list. In-Progress: A check has been sent for this pair, but the transaction is in progress. Succeeded: A check for this pair was already done and produced a successful result. Failed: A check for this pair was already done and failed, either never producing any response or producing an unrecoverable failure response. Frozen: A check for this pair hasn't been performed, and it can't yet be performed until some other check succeeds, allowing this pair to unfreeze and move into the Waiting state. As ICE runs, the pairs will move between states as shown in Figure 9. +-----------+ | | | | | Frozen | | | | | +-----------+ | |unfreeze | V +-----------+ +-----------+ | | | | | | perform | | | Waiting |-------->|In-Progress| | | | | | | | | +-----------+ +-----------+ / | // | // | // | / | // | failure // |success // | / | // | // | // | V V +-----------+ +-----------+ | | | | | | | | | Failed | | Succeeded | | | | | | | | | +-----------+ +-----------+ Figure 9: Pair State FSM The initial states for each pair in the check list are computed by performing the following sequence of steps: 1. The agent sets all of the pairs in each check list to the Frozen state. 2. It takes the first pair inThe agent examines the check list for the first media stream (a media stream is the first media stream when it is described by the first m-line in the SDP offer and answer), and sets its state to Waiting. 3. It findsanswer). For that media stream, it: * Groups together all of the otherpairs in that check listwith the same component ID, but different foundations, andfoundation, * For each group, sets allthe state of their statesthe pair with the lowest component ID to Waiting as well.Waiting. If there is more than one such pair, the one with the highest priority is used. One of the check lists will have some number of pairs in the Waiting state, and the other check lists will have all of their pairs in the Frozen state. A check list with at least one pair that is not FrozenWaiting is called an active check list, and a check list with all pairs frozen is called a frozen check list. The check list itself is associated with a state, which captures the state of ICE checks for that media stream. There are two states: Running: In this state, ICE checks are still in progress for this media stream. Completed: In this state, ICE checks have completed successfully for this media stream. Failed: In this state, the ICE checks have not completed successfully for this media stream. When a check list is first constructed as the consequence of an offer/answer exchange, it is placed in the Running state. ICE processing across all media streams also has a state associated with it. This state is equal to Running while checks are in progress. The state is Completed when all checks have been completed. Rules for transitioning between states are described below. 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 periodic checks and triggered checks. Periodic checks occur periodically for each media stream, and involve choosing the highest priority pair in the Waiting state from each check list, and sending a check on it. Triggered checks are performed on receipt of a connectivity check from the peer (see Section 126.96.36.199).188.8.131.52). This section describes how periodic checks are performed. 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/NTa*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 4.1.1. DividingMultiplying by N allows this aggregate check throughput to be split between all active check lists. 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 agent MUST: o Find the highest priority pair in that check list that is in the Waiting state. o If there is such a pair: * Send a STUN check from the local candidate of that pair to the remote candidate of that pair. The procedures for forming the STUN request for this purpose are described in Section 7.1.1. o If there is no such pair: * Find the highest priority pair in that check list that is in the Frozen state. * If there is such a pair: + Unfreeze the pair. + Perform a check for that pair, causing its state to transition to In-Progress. * If there is no such pair: + Set the state of the check list to Completed. +Terminate the timer for that check list. To compute the message integrity for the check, the agent uses the remote username fragment and password learned from the SDP from its peer. The local username fragment is known directly by the agent for its own candidate. 6. Receipt of the Initial Answer This section describes the procedures that an agent follows when it receives the answer from the peer. It verifies that its peer supports ICE, determines its role, and for full implementations, forms the check list and begins performing periodic checks. 6.1. Verifying ICE Support The offerer 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 10, which describes keepalive procedures. 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 default destination for media components without modifying the corresponding candidate attributes. See Section 17 for a discussion of cases where this can happen. This specification provides no guidance on how an agent should proceed in such a failure case. 6.2. Determining Role The offerer follows the same procedures described for the answerer in Section 5.2. 6.3. Forming the Check List Formation of check lists is performed only by full implementations. The offerer follows the same procedures described for the answerer in Section 5.7. 6.4. Performing Periodic Checks Periodic checks are performed only by full implementations. The offerer follows the same procedures described for the answerer in Section 5.8. 7. Performing Connectivity Checks This section describes how connectivity checks are performed. All ICE implementations are required to be compliant to , as opposed to the older . 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. 7.1. Client Procedures 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 check is generated by sending a Binding Request from a local candidate, to a remote candidate.  describes how Binding Requests are constructed and generated. This section defines additional procedures involving the PRIORITY and USE-CANDIDATE attributes, defined for the connectivity check usage, and details how credentials for message integrity and diffserv markings are computed. 184.108.40.206. PRIORITY and USE-CANDIDATE An agent MUST include the PRIORITY attribute in its Binding Request. The attribute MUST be set equal to the priority that would be assigned, based on the algorithm in Section 4.1.2, to a peer reflexive candidate, should one be learned as a consequence of this check (see Section 220.127.116.11.1 for how peer reflexive candidates are learned). This priority value will be computed identically to how the priority for the local candidate of the pair was computed, except that the type preference is set to the value for peer derived candidate types. The controlling agent MAY include the USE-CANDIDATE attribute in the Binding Request. The controlled agent MUST NOT include it in its Binding Request. This attribute signals that the controlling agent wishes to cease checks for this component, and use the candidate pair resulting from the check for this component. Section 8.1 provides guidance on determining when to include it. 18.104.22.168. ICE-CONTROLLED and ICE-CONTROLLING The agent MUST include the ICE-CONTROLLED attribute in the request if it is in the controlled role, and MUST include the ICE-CONTROLLING attribute in the request if it is in the controlling role. The content of either attribute MUST be the tie breaker that was determined in Section 5.2. 22.214.171.124. Forming Credentials A Binding Request serving as a connectivity check MUST utilize a STUN short term credential. The agent MUST include the USERNAME and MESSAGE-INTEGRITY attributes. An agent MUST NOT wait to be challenged for short term credentials. Rather, it MUST provide them in each Binding Request. Rather than being learned from a Shared Secret request, the short term credential is exchanged in the offer/answer procedures. In particular, the username is formed by concatenating the username fragment provided by the peer with the username fragment of the agent sending the request, separated by a colon (":"). The password is equal to the password provided by the peer. For example, consider the case where agent L is the offerer, and agent R is the answerer. Agent L included a username fragment of LFRAG for its candidates, and a password of LPASS. Agent R provided a username fragment of RFRAG and a password of RPASS. A connectivity check from L to R (and its response of course) utilize the username RFRAG:LFRAG and a password of RPASS. A connectivity check from R to L (and its response) utilize the username LFRAG:RFRAG and a password of LPASS. 126.96.36.199.188.8.131.52. DiffServ Treatment If the agent is using Diffserv Codepoint markings  in its media packets, it SHOULD apply those same markings to its connectivity checks. 7.1.2. Processing the Response When a Binding Response is received, it is correlated to its Binding Request using the transaction ID, as defined in , which then ties it to the candidate pair for which the Binding Request was sent. 184.108.40.206. Failure Cases If the STUN transaction generates a 487 (Role Conflict) error response, the agent checks whether it had included the ICE-CONTROLLED or ICE-CONTROLLING attribute in the Binding Request. If the request had contained the ICE-CONTROLLED attribute, the agent MUST switch to the controlling role if it has not already done so. If the request had contained the ICE-CONTROLLING attribute, the agent MUST switch to the controlled role if it has not already done so. Once it has switched, the agent MUST immediately retry the request with the ICE- CONTROLLING or ICE-CONTROLLED attribute reflecting its new role. Note, however, that the tie-breaker value MUST NOT be reselected. If the STUN transaction generates an ICMP error, or generates a STUN error response that is unrecoverable (as defined in , or times out, the agent sets the state of the pair to Failed. The agent MUST check that the source IP address and port of the response 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. In other words, the source and destination transport addresses in the request and responses are the symmetric. If they are not symmetric, the agent sets the state of the pair to Failed. 220.127.116.11. Success Cases IfA check is considered to be a success if all of the following are true: o the STUN transaction generated a success response between 200 and 299, ando the source IP address and port of the response equals the destination IP address and port that the Binding Request was sent to, andto o the destination IP address and port of the response match the source IP address and port that the Binding Request was sent from, the check was a success.from 18.104.22.168.1. Discovering Peer Reflexive Candidates The agent checks the mapped address from the STUN response. If the transport address does not match any of the local candidates that the agent knows about, the mapped address represents a new candidate - a peer reflexive candidate. Like other candidates, it has a type, base, priority and foundation. They are computed as follows: o Its type is equal to peer reflexive. o Its base is set equal to the local candidate of the candidate pair from which the STUN check was sent. o Its priority is set equal to the value of the PRIORITY attribute in the Binding Request. o Its foundation is selected as described in Section 4.1.1. This peer reflexive candidate is then added to the list of local candidates for the media stream. Its username fragment and password are the same as all other local candidates for that media stream. However, the peer reflexive candidate is not paired with other remote candidates. This is not necessary; a valid pair will be generated from it momentarily based on the procedures in Section 22.214.171.124.3. If an agent wishes to pair the peer reflexive candidate with other remote candidates besides the one in the valid pair that will be generated, the agent MAY generate an updated offer which includes the peer reflexive candidate. This will cause it to be paired with all other remote candidates. 126.96.36.199.2. Updating Pair States The agent sets the state of the pair that generated the check to succeeded.Succeeded. The agent sees if thesuccess forof this pair cancheck might also cause other pairs to be unfrozen. There are three cases: o Ifthe pair had a component IDstate of 1, theother checks to change as well. The agent MUST changeperform the following two steps: 1. The agent changes the states for all other Frozen pairs for the same media stream and same foundation, but different component IDs,foundation to Waiting. oTypically these other pairs will have different component IDs but not always. 2. If the pair hadthere is a component ID equal topair in the number of componentsvalid list for theevery component of this media stream (where this is the actual number of components being used, in cases where the number of components signaled in the SDP differs from offerer to answerer), the agent MUST change the statesuccess of this check may unfreeze checks for allother Frozen pairs for the first component of differentmedia streams (and thus in different check lists) butstreams. Note that this step is followed not just the same foundation, to Waiting. o Iffirst time the pair has anyvalid list under consideration has a pair for every component, but every subsequent time a check succeeds and adds yet another pair to that valid list. The agent examines the check list for each other component ID,media stream in turn: * If the check list is active, the agent changes the state of all Frozen pairs in that check list whose foundation matches a pair in the valid list under consideration, to Waiting. * If the check list is frozen, and there is at least one pair in the check list whose foundation matches a pair in the valid list under consideration, the state of all pairs in the check list whose foundation matches a pair in the valid list under consideration are set to Waiting. * If the check list is frozen, and there are no otherpairs can be unfrozen.in the check list whose foundation matches a pair in the valid list under consideration, the agent + Groups together all of the pairs with the same foundation, + For each group, sets the state of the pair with the lowest component ID to Waiting. If there is more than one such pair, the one with the highest priority is used. 188.8.131.52.3. Constructing a Valid Pair Next, the agent constructs a candidate pair whose local candidate equals the mapped address of the response, and whose remote candidate equals the destination address to which the request was sent. This is called a valid pair, since it has been validated by a STUN connectivity check. The valid pair may equal the pair that generated the check, may equal a different pair in the check list, or may be a pair not currently on any check list. If the pair equals the pair that generated the check or is on a check list currently, it is also added to the VALID LIST, which is maintained by the agent for each media stream. This list is empty at the start of ICE processing, and fills as checks are performed, resulting in valid candidate pairs. It will be very common that the pair will not be on any check list. Recall that the check list has pairs whose local candidates are never server reflexive; those pairs had their local candidates converted to the base of the server reflexive candidates, and then pruned if they were redundant. When the response to the STUN check arrives, the mapped address will be reflexive if there is a NAT between the two. In that case, the valid pair will have a local candidate that doesn't match any of the pairs in the check list. If the pair is not on any check list, the agent computes the priority for the pair based on the priority of each candidate, using the algorithm in Section 5.7. The priority of the local candidate depends on its type. If it is not peer reflexive, it is equal to the priority signaled for that candidate in the SDP. If it is peer reflexive, it is equal to the PRIORITY attribute the agent placed in the Binding Request which just completed. The priority of the remote candidate is taken from the SDP of the peer. If the candidate does not appear there, then the check must have been a triggered check to a new remote candidate. In that case, the priority is taken as the value of the PRIORITY attribute in the Binding Request which triggered the check that just completed. The pair is then added to the VALID LIST. 184.108.40.206.4. Updating the Nominated Flag If the agent was a controlling agent, and it had included a USE- CANDIDATE attribute in the Binding Request, the valid pair generated from that check has its nominated flag set to true. This flag indicates that this candidatevalid pair should be used for media if it is the highest priority one amongst those whose nominated flag is set. This may conclude ICE processing for this media stream or all media streams; see Section 8. If the agent is the controlled agent, the response may result in the valid pair having its nominated flag set. See Section 220.127.116.11.2.1.5 for the procedure. 18.104.22.168. Check List and Timer State Updates Regardless of whether the check was successful or failed, the completion of the transaction may require updating of check list and timer states. If all of the pairs in the check list are now either in the Failed or Succeeded state, and there is not a pair in the valid list for each component of the media stream, the state of the check list is set to Failed. For each frozen check list, the agent: o Groups together all of the pairs with the same foundation, o For each group, sets the state of the pair with the lowest component ID to Waiting. If there is more than one such pair, the one with the highest priority is used. If none of the pairs in the check list are in the Waiting or Frozen state, the check list is no longer considered active, and will not count towards the value of N in the computation of timers for periodic checks as described in Section 5.8. 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 base 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 MESSAGE-INTEGRITY is the output of a hash of the password and the STUN packet's contents. 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 receivingIf this answer, andhappens, the agent MUST generate a response generated. By doing this, ICE processing completes faster.(including computation of the mapped address as described in Section 22.214.171.124. Once the answer is received, it MUST proceed with the remaining steps required, namely Section 126.96.36.199, Section 188.8.131.52, and Section 184.108.40.206 for full implementations. In cases where multiple STUN requests are received before the answer, this may cause several triggered notifications to all be sent at the same time, If the agent is using Diffserv Codepoint markings  in its media packets, it SHOULD apply those same markings to its responses to Binding Requests. The same would apply to any layer 2 markings the endpoint might be applying to media packets. 7.2.1. Additional Procedures for Full Implementations This subsection defines the additional server procedures applicable to full implementations when generatingimplementations. 220.127.116.11. Detecting and Repairing Role Conflicts Normally, the rules for selection of a role in Section 5.2 will result in each agent selecting a different role - one controlling, and one controlled. However, in unusual call flows, typically utilizing third party call control, it is possible for both agents to select the same role. This section describes procedures for checking for this case and repairing it. An agent MUST examine the Binding Request for either the ICE- CONTROLLING or ICE-CONTROLLED attribute. It MUST follow these procedures: o If neither ICE-CONTROLLING or ICE-CONTROLLED are present in the request, there is no conflict. o If the agent is in the controlling role, and the ICE-CONTROLLING attribute is present in the request: * If the agent's tie-breaker is larger than or equal to the contents of the ICE-CONTROLLING attribute, the agent generates a Binding Error Response and includes an ERROR-CODE attribute with a value of 487 (Role Conflict) but retains its role. * If the agent's tie-breaker is less than the contents of the ICE-CONTROLLING attribute, the agent switches to the controlled role. o If the agent is in the controlled role, and the ICE-CONTROLLED attribute is present in the request: * If the agent's tie-breaker is larger than or equal to the contents of the ICE-CONTROLLED attribute, the agent switches to the controlling role. * If the agent's tie-breaker is less than the contents of the ICE-CONTROLLED attribute, the agent generates a Binding Error Response and includes an ERROR-CODE attribute with a value of 487 (Role Conflict) but retains its role. o If the agent is in the controlled role and the ICE-CONTROLLING attribute was present in the request, or the agent was in the controlling role and the ICE-CONTROLLED attribute was present in the request, there is no conflict. The remaining sections in Section 7.2.1 are followed if the server generated a successful response to athe Binding Request. 18.104.22.168.Request, even if the agent changed roles. 22.214.171.124. Computing Mapped Address For requests being received on a relayed candidate, the source transport address used for STUN processing (namely, generation of the XOR-MAPPED-ADDRESS attribute) is the transport address as seen by the 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 (a STUN relay delivers packets encapsulated in a Data Indication when no active destination is set). 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. 126.96.36.199.188.8.131.52. Learning Peer Reflexive Candidates If the source transport address of the request does not match any existing remote candidates, it represents a new peer reflexive remote candidate. This candidate is constructed as follows: o The full-mode agent givespriority of the candidate a priority equalis set to the PRIORITY attribute from the request. o The type of the candidate is equalset to peer reflexive. Itso The foundation of the candidate is set to an arbitrary value, different from the foundation for all other remote candidates. If any subsequent offer/answer exchanges contain this peer reflexive candidate in the SDP, it will signal the actual foundation for the candidate. o The component ID of this candidate is set to the component ID for the local candidate to which the request was sent. This candidate is thenadded to the list of remote candidates. However, the agent does not pair this candidate with any local candidates. 184.108.40.206.220.127.116.11. Triggered Checks Next, the agent constructs a pair whose local candidate is equal to the transport address on which the STUN request was received, and a remote candidate equal to the source transport address where the request came from (which may be peer-reflexive remote candidate that was just learned). Since both candidates are known to the agent, it can obtain their priorities and compute the candidate pair priority. This pair is then looked up in the check list. There can be one of several outcomes: o If the pair is already on the check list: * If the state of that pair is Waiting or Frozen, its state is changed to In-Progress and a check for that pair is performed immediately. This is called a triggered check. * If the state of that pair is In-Progress, the agent SHOULD 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. It is RECOMMENDED that, after the immediate retransmit, the next retransmission occur T milliseconds later, where T is the current STUN retransmit interval. If the immediate retransmit causes the total number of requests transmitted to equal the maximum value defined in , the agent SHOULD NOT send any further retransmits. * If the state of that pair is Failed or Succeeded, no triggered check is sent. o If the pair is not already on the check list: * The pair is inserted into the check list based on its priority * Its state is set to In-Progress * A triggered check for that pair is performed immediately. If a triggered check is to be generated, it is constructed and processed as described in Section 7.1.1. These procedures require the agent to know the transport address, username fragment and password for the peer. The username fragment for the remote candidate is equal to the part after the colon of the USERNAME in the Binding Request that was just received. Using that username fragment, the agent can check the SDP messages received from its peer (there may be more than one in cases of forking), and find this username fragment. The corresponding password is then selected. If agent has not yet received the username in an SDP (a likely case for the offerer in the initial offer/answer exchange), it MUST wait for the SDP to be received (since it won't have its peer's ICE password without it), and then proceed with the triggered check. 18.104.22.168.22.214.171.124. Updating the Nominated Flag If the Binding Request received by the agent had the USE-CANDIDATE attribute set, and the agent is in the controlled role, the agent looks at the state of the pair computed in Section 126.96.36.199:188.8.131.52: o If the state of this pair is succeeded, it means that the check generated by this pair produced a successful response. This would have caused the agent to construct a valid pair when that success response was received (see Section 184.108.40.206.3). The agent now sets the nominated flag in the valid pair to true. This may end ICE processing for this media stream; see Section 8. o If the state of this pair is In-Progress, if its check produces a successful result, the resulting valid pair has its nominated flag set when the response arrives. This may end ICE processing for this media stream when it arrives; see Section 8. 7.2.2. Additional Procedures for Lite Implementations 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 pair for that component of that media stream, called the valid list. The agent sets the nominated flag for that pair to true. ICE processing is considered complete for a media stream if the valid list contains a candidate pair for each component. 8. Concluding ICE Processing The processing rules in this section apply only to full implementations. Concluding ICE involves nominating pairs by the controlling agent and updating of state machinery 8.1. Nominating Pairs The controlling agent nominates pairs to be selected by ICE by using one of two techniques: regular nomination or aggressive nomination. If its peer has a lite implementation, an agent MUST use a regular nomination algorithm. If its peer is using ICE options (present in an ice-options attribute from the peer) that the agent does not understand, the agent MUST use a regular nomination algorithm. If its peer is a full implementation and isn't using any ICE options or is using ICE options understood by the agent, the agent MAY use either the aggressive or the regular nomination algorithm. However, the regular algorithm is RECOMMENDED since it provides greater stability. 8.1.1. Regular Nomination With regular nomination, the agent lets some number of checks complete, each of which omit the USE-CANDIDATE attribute. Once one or more checks complete successfully for a component of a media stream, valid pairs are generated and added to the valid list. The agent lets the checks continue until some stopping criteria is met, and then picks amongst the valid pairs based on an evaluation criteria. The criteria for stopping the checks and for evaluating the valid pairs is entirely a matter of local optimization. When the controlling agent selects the valid pair, it repeats the check that produced this valid pair, this time with the USE-CANDIDATE attribute. This check will succeed (since the previous did), causing the nominated flag of that and only that pair to be set. Consequently, there will be only a single nominated pair in the valid list, and when the state of the check list moves to completed, that exact pair is selected by ICE for sending and receiving media. Regular nomination provides the most flexibility, since the agent has control over the stopping and selection criteria for checks. The only requirement is that the agent MUST eventually pick one and only one candidate pair and generate a check for that pair with the USE- CANDIDATE attribute present. Regular nomination also improves ICE's resilience to variations in implementation (see Section 14.14). Regular nomination is also more stable, allowing both agents to converge on a single pair for media without any transient selections, which can happen with the aggressive algorithm. The drawback of regular nomination is that it is guaranteed to increase latencies because it requires an additional check to be done. 8.1.2. Aggressive Nomination With aggressive nomination, the controlling agent includes the USE- CANDIDATE attribute in every check it sends. Once the first check for a component succeeds, it will be added to the valid list, have its nominated flag set, and then cause ICE processing to cease for this check list. However, because the agent included the USE- CANDIDATE attribute in all of its checks, another check may yet complete, causing another valid pair to have its nominated flag set. ICE always selects the highest priority nominated candidate pair from the valid list as the one used for media. Consequently, the selected pair may actually change briefly as ICE checks complete, resulting in a set of transient selections until it stabilizes. 8.2. Updating States For both controlling and controlled agents, the state of ICE processing depends on the presence of nominated candidate pairs in the valid list and on the state of the check list: o If there are no nominated pairs in the valid list for a media stream,stream and the state of the check list is Running, ICE processing continues. o If there is at least one nominated pair in the valid list:list for a media stream and the state of the check list is Running: * The agent MUST remove all Waiting and Frozen pairs in the check list for the same component as the nominated pairs for that media stream * If an In-Progress pair in the check list is for the same component as a nominated pair, the agent SHOULD cease retransmissions for its check if its pair priority is lower than the lowest priority nominated pair for that component o Once there is at least one nominated pair in the valid list for every component of at least one media stream:stream and the state of the check list is Running: * The agent MUST change the state of processing for its check list for that media stream to Completed. * The agent MUST continue to respond to any checks it may still receive for that media stream, and MUST perform triggered checks if required by the processing of Section 7.2. * The agent MAY begin transmitting media for this media stream as described in Section 11.1 o Once there is at least one nominated pair inthe valid list for each componentstate of each media stream:check list is Completed: * The agent sets the state of ICE processing overall to Completed. * If an agent is controlling, it examines the highest priority nominated candidate pair for each component of each media stream. If any of those candidate pairs differ from the default candidate pairs in the most recent offer/answer exchange, the controlling agent MUST generate an updated offer as described in Section 9. If the controlling agent is using an aggressive nomination algorithm, this may result in several updated offers as the pairs selected for media change. An agent MAY delay sending the offer for a brief interval (one second is RECOMMENDED) in order to allow the selected pairs to stabilize. o If the state of the check list is Failed, ICE has not been able to complete for this media stream. The correct behavior depends on the state of the check lists for other media streams: * If all check lists are Failed, the agent SHOULD consider the session a failure, SHOULD NOT restart ICE, and the controlling agent SHOULD terminate the entire session. * If at least one of the check lists for other media streams is Completed, the controlling agent SHOULD remove the failed media stream from the session in its updated offer. * If none of the check lists for other media streams are Completed, but at least one is Running, the agent SHOULD let ICE continue. 9. Subsequent Offer/Answer Exchanges Either agent MAY generate a subsequent offer at any time allowed by RFC 3264 . The rules in Section 8 will cause the controlling agent to send an updated offer at the conclusion of ICE processing when ICE has selected different candidate pairs from the default pairs. This section defines rules for construction of subsequent offers and answers. 9.1. Generating the Offer 9.1.1. Procedures for All Implementations 220.127.116.11. ICE Restarts An agent MAY restart ICE processing for an existing media stream. An ICE restart, as the name implies, will cause all previous state of ICE processing to be flushed and checks to start anew. The only difference between an ICE restart and a brand new media session is that, during the restart, media can continue to be sent to the previously validated pair. An agent MUST restart ICE for a media stream if: o The offer is being generated for the purposes of changing the target of the media stream. In other words, if an agent wants to generated an updated offer which, had ICE not been in use, would result in a new value for the destination of a media component. o An agent is changing its implementation level. This typically only happens in third party call control use cases, where the entity performing the signaling is not the entity receiving the media, and it has changed the target of media mid-session to another entity that has a different ICE implementation. These rules imply that setting the IP address in the c line to 0.0.0.0 will cause an ICE restart. Consequently, ICE implementations MUST NOT utilize this mechanism for call hold, and instead MUST use a=inactive and a=sendonly as described in  To restart ICE, an agent MUST change both the ice-pwd and the ice- ufrag for the media stream in an offer. Note that it is permissible to use a session-level attribute in one offer, but to provide the same ice-pwd or ice-ufrag as a media-level attribute in a subsequent offer. This is not a change in password, just a change in its representation, and does not cause an ICE restart. An agent sets the rest of the fields in the SDP for this media stream as it would in an initial offer of this media stream (see Section 4.3). Consequently, the set of candidates MAY include some, none, or all of the previous candidates for that stream and MAY include a totally new set of candidates gathered as described in Section 4.1.1. 18.104.22.168. Removing a Media Stream If an agent removes a media stream by setting its port to zero, it MUST NOT include any candidate attributes for that media stream and SHOULD NOT include any other ICE-related attributes defined in Section 15 for that media stream. 22.214.171.124. Adding a Media Stream If an agent wishes to add a new media stream, it sets the fields in the SDP for this media stream as if this was an initial offer for that media stream (see Section 4.3). This will cause ICE processing to begin for this media stream. 9.1.2. Procedures for Full Implementations This section describes additional procedures for full implementations, covering existing media streams. The username fragments, password, and implementation level MUST remain the same as used previously. If an agent needs to change one of these it MUST restart ICE for that media stream. Additional behavior depends on the state ICE processing for that media stream. 126.96.36.199. Existing Media Streams with ICE Running If an agent generates an updated offer including media stream that was previously established, and for which ICE checks are in the Running state, the agent follows the procedures defined here. An agent MUST include candidate attributes for all local candidates it had signaled previously for that media stream. The properties of that candidate as signaled in SDP - the priority, foundation, type and related transport address SHOULD remain the same. The IP address, port and transport protocol, which fundamentally identify that candidate, MUST remain the same (if they change, it would be a new candidate). The component ID MUST remain the same. The agent MAY include additional candidates it did not offer previously, but which it has gathered since the last offer/answer exchange, including peer reflexive candidates. The agent MAY change the default destination for media. As with initial offers, there MUST be a set of candidate attributes in the offer matching this default destination. 188.8.131.52. Existing Media Streams with ICE Completed If an agent generates an updated offer including media stream that was previously established, and for which ICE checks are in the Completed state, the agent follows the procedures defined here. The default destination for media (i.e., the values of the IP addresses and ports in the m and c line used for that media stream) MUST be the local candidate from the highest priority nominated pair in the valid list for each component. This "fixes" the default destination for media to equal the destination ICE has selected for media. The agent MUST include a candidate attributes for candidates matching the default destination for each component of the media stream, and MUST NOT include any other candidates. In addition, if the agent is controlling, it MUST include the a=remote-candidates attribute for each media stream whose check list is in the Completed state. The attribute contains the remote candidates from the highest priority nominated pair in the valid list for each component of that media stream. It is needed to avoid a race condition whereby the controlling agent chooses its pairs, but the updated offer beats the connectivity checks to the controlled agent, which doesn't even know these pairs are valid, let alone selected. See Appendix B.6 for elaboration on this race condition. 9.1.3. Procedures for Lite Implementations This section describes procedures for lite implementations for existing streams for which ICE is running. A lite implementation MUST include its one and only candidate for each component of each media stream in an a=candidate attribute in any subsequent offer. This candidate is formed identically to the procedures for initial offers, as described in Section 4.2. The username fragments, password, and implementation level MUST remain the same as used previously. If an agent needs to change one of these it MUST restart ICE for that media stream. 9.2. Receiving the Offer and Generating an Answer 9.2.1. Procedures for All Implementations When receiving a subsequent offer within an existing session, an 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. 184.108.40.206. Detecting ICE Restart If the offer contained a change in the a=ice-ufrag or a=ice-pwd attributes compared to the previous SDP from the peer, it indicates that ICE is restarting for this media stream. If all media streams are restarting, than ICE is restarting overall. If ICE is restarting for a media stream: o The agent MUST change the a=ice-ufrag and a=ice-pwd attributes in the answer. o The agent MAY change its implementation level in the answer. An agent sets the rest of the fields in the SDP for this media stream as it would in an initial answer to this media stream (see Section 4.3). Consequently, the set of candidates MAY include some, none, or all of the previous candidates for that stream and MAY include a totally new set of candidates gathered as described in Section 4.1.1. 220.127.116.11. New Media Stream If the offer contains a new media stream, the agent sets the fields in the answer as if it had received an initial offer containing that media stream (see Section 4.3). This will cause ICE processing to begin for this media stream. 18.104.22.168. Removed Media Stream If an offer contains a media stream whose port is zero, the agent MUST NOT include any candidate attributes for that media stream in its answer and SHOULD NOT include any other ICE-related attributes defined in Section 15 for that media stream. 9.2.2. Procedures for Full Implementations The username fragments, password, and implementation level MUST remain the same as used previously. If an agent needs to change one of these it MUST restart ICE for that media stream by generating an offer; ICE cannot be restarted in an answer. Additional behaviors depend on the state of ICE processing for that media stream. 22.214.171.124. Existing Media Streams with ICE Running and no remote- candidates If ICE is running for a media stream, and the offer for that media stream lacked the remote-candidates attribute, the rules for construction of the answer are identical to those for the offerer as described in Section 126.96.36.199. 188.8.131.52. Existing Media Streams with ICE Completed and no remote- candidates If ICE is Completed for a media stream, and the offer for that media stream lacked the remote-candidates attribute, the rules for construction of the answer are identical to those for the offerer as described in Section 184.108.40.206, except that the answerer MUST NOT include the a=remote-candidates attribute in the answer. 220.127.116.11. Existing Media Streams and remote-candidates A controlled agent will receive an offer with the a=remote-candidates attribute for a media stream when its peer has concluded ICE processing for that media stream. This attribute is present in the offer to deal with a race condition between the receipt of the offer, and the receipt of the Binding Response which tells the answerer the candidate which will be selected by ICE. See Appendix B.6 for an explanation of this race condition. Consequently, processing of an offer with this attribute depends on the winner of the race. The agent forms a candidate pair for each component of the media stream by: o Setting the remote candidate equal to the offerers default destination for that component (e.g., the contents of the m and c-lines for RTP, and the a=rtcp attribute for RTCP) o Setting the local candidate equal to the transport address for that same component in the a=remote-candidates attribute in the offer. The agent then sees if each of these candidate pairs are present in the valid list. If a particular pair is not in the valid list, the check has "lost" the race. Call such a pair a "losing pair". The agent finds all the pairs in the check list whose remote candidates equal the remote candidate in the losing pair: o If none of the pairs are In-Progress, and at least one is Failed, it is most likely that a network failure, such as a network partition or serious packet loss, has occurred. The agent SHOULD generate an answer for this media stream as if the remote- candidates attribute had not been present, and then restart ICE for this stream. o If at least one of the pairs are In-Progress, the agent SHOULD wait for those checks to complete, and as each completes, redo the processing in this section until there are no losing pairs. Once there are no losing pairs, the agent can generate the answer. It MUST set the default destination for media to the candidates in the remote-candidates attribute from the offer (each of which will now be the local candidate of a candidate pair in the valid list). It MUST include a candidate attribute in the answer for each candidate in the remote-candidates attribute in the offer. 9.2.3. Procedures for Lite Implementations A lite implementation constructs its answer in the same way it does a subsequent offer as described in Section 9.1.3 9.3. Updating the Check and Valid Lists 9.3.1. Procedures for Full Implementations 18.104.22.168. ICE Restarts The agent MUST remember the highest priority nominated pairs in the Valid list for each component of the media stream, called the previous selected pairs, prior to the restart. The agent will continue to send media using these pairs, as described in Section 11.1. Once these destinations are noted, the agent MUST flush the valid and check lists, and then recompute the check list and its states as described in Section 5.7. 22.214.171.124. New Media Stream If the offer/answer exchange added a new media stream, the agent MUST create a new check list for it (and an empty Valid list to start of course), as described in Section 5.7. 126.96.36.199. Removed Media Stream If the offer/answer exchange removed a media stream, or an answer rejected an offered media stream, an agent MUST flush the Valid list for that media stream. It MUST terminate any STUN transactions in progress for that media stream. An agent MUST remove the check list for that media stream and cancel any pending periodic checks for it. 188.8.131.52. ICE Continuing for Existing Media Stream The valid list is not affected by an updated offer/answer exchange unless ICE is restarting. If an agent is in the Running state for that media stream, the check list is updated (the check list is irrelevant if the state is completed). To do that, the agent recomputes the check list using the procedures described in Section 5.7. If a pair on the new check list was also on the previous check list, and its state was Waiting, In-Progress, Succeeded or Failed, its state is copied over. Otherwise, its state is set to Frozen. If none of the check lists are active (meaning that the pairs in each check list are Frozen), the full-mode agent sets the first pair in the check list for the first media stream to Waiting, and then sets the state of all other pairs in that check list for the same component ID and with the same foundation to Waiting as well. Next, the agent goes through each check list, starting with the highest priority pair. If a pair has a state of Succeeded, and it has a component ID of 1, then all Frozen pairs in the same check list with the same foundation whose component IDs are not 1, have their state set to Waiting. If, for a particular check list, there are pairs for each component of that media stream in the Succeeded state, the agent moves the state of all Frozen pairs for the first component of all other media streams (and thus in different check lists) with the same foundation to Waiting. 9.3.2. Procedures for Lite Implementations If ICE is restarting for a media stream, the agent MUST start a new Valid list for that media stream. It MUST remember the pairs in the previous Valid list for each component of the media stream, called the previous selected pairs, and continue to send media there as described in Section 11.1. 10. Keepalives All endpoints MUST send keepalives for each media session. These keepalives serve the purpose of keeping NAT bindings alive for the media session. These keepalives MUST be sent regardless of whether the media stream is currently inactive, sendonly, recvonly or sendrecv, and regardless of the presence or value of the bandwidth attribute. These keepalives MUST be sent even if ICE is not being utilized for the session at all. The keepalive SHOULD be sent using a format 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  and RTP comfort noise . 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. If there has been no packet sent on the candidate pair ICE is using for a media component for Tr seconds (where packets include those defined for the component (RTP or RTCP) and previous keepalives), an agent MUST generate a keepalive on that pair. Tr SHOULD be configurable and SHOULD have a default of 15 seconds. Alternatively, if an agent has a dynamic way to discover the binding lifetimes of the intervening NATs, it can use that value to determine Tr. If STUN is being used for keepalives, a STUN Binding Indication is used . The Binding Indication SHOULD NOT contain integrity checks as the messages are simply discarded on receipt regardless of contents. The Indication SHOULD NOT contain the PRIORITY or USE- CANDIDATE attributes defined in this document. The Binding Indication is sent using the same local and remote candidates that are being used for media. An agent receiving 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 , but there is no impact on ICE processing otherwise. An agent MUST begin the keepalive processing once ICE has selected candidates for usage with media, or media begins to flow, whichever happens first. Keepalives end once the session terminates or the media stream is removed. 11. Media Handling 11.1. Sending Media Procedures for sending media differ for full and lite implementations. 11.1.1. Procedures for Full Implementations Agents always send media using a candidate pair, called the selected candidate pair. An agent will send media to the remote candidate in the selected pair (setting the destination address and port of the packet equal to that remote candidate), and will send it from the local candidate of the selected pair. When the local candidate is server or peer reflexive, media is originated from the base. Media sent from a relayed candidate is sent from the base through that relay, using procedures defined in . The selected pair for a component of a media stream is: o empty if the state of the check list for that media stream is Running, and there is no previous selected pair for that component due to an ICE restart o equal to the previous selected pair for a component of a media stream if the state of the check list for that media stream is Running, and there was a previous selected pair for that component due to an ICE restart o equal to the highest priority nominated pair for that component in the valid list if the state of the check list is Completed If the selected pair for at least one component of a media stream is empty, an agent MUST NOT send media for any component of that media stream. If the selected pair for each component of a media stream has a value, an agent MAY send media for all components of that media stream. Note that the selected pair for a component of a media stream may not equal the default pair for that same component from the most recent offer/answer exchange. When this happens, the selected pair is used for media, not the default pair. When ICE first completes, if the selected pairs aren't a match for the default pairs, the controlling agent sends an updated offer/answer exchange to remedy this disparity. However, until that updated offer arrives, there will not be a match. Furthermore, in very unusual cases, the default candidates in the updated offer/answer will not be a match. 11.1.2. Procedures for Lite Implementations A lite implementation MUST NOT send media until it has a Valid list that contains a candidate pair for each component of that media stream. Once that happens, the agent MAY begin sending media packets. To do that, it sends media to the remote candidate in the pair (setting the destination address and port of the packet equal to that remote candidate), and will send it from the local candidate. 11.1.3. Procedures for All Implementations ICE has interactions with jitter buffer adaptation mechanisms. An RTP stream can begin using one candidate, and switch to another one, though this happens rarely with ICE. The newer candidate may result in RTP packets taking a different path through the network - one with different delay characteristics. As discussed below, agents are encouraged to re-adjust jitter buffers when there are changes in source or destination address of media packets. Furthermore, many audio codecs use the marker bit to signal the beginning of a talkspurt, for the purposes of jitter buffer adaptation. For such codecs, it is RECOMMENDED that the sender set the marker bit  when an agent switches transmission of media from one candidate pair to another. 11.2. Receiving Media ICE implementations MUST be prepared to receive media on each component on any candidates provided for that component in the most recent offer/answer exchange (in the case of RTP, this would include both RTP and RTCP if candidates were provided for both). It is RECOMMENDED that, when an agent receives an RTP packet with a new source or destination IP address for a particular media stream, that the agent re-adjust its jitter buffers. RFC 3550  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. 12. Usage with SIP 12.1. Latency Guidelines ICE requires a series of STUN-based connectivity checks to take place between endpoints. These checks start from the answerer on generation of its answer, and start from the offerer when it receives the answer. These checks can take time to complete, and as such, the selection of messages to use with offers and answers can 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 successfully started ringing the phone of the called party. Two cases can be considered - one where the offer is present in the initial INVITE, and one where it is in a response. 12.1.1. Offer in INVITE To reduce post-dial delays, it is RECOMMENDED that the caller begin gathering candidates prior to actually sending its initial INVITE. This can be started upon user interface cues that a call is pending, such as activity on a keypad or the phone going offhook. If an offer is received in an INVITE request, the answerer SHOULD begin to gather its candidates on receipt of the offer and then generate an answer in a provisional response once it has completed that process. ICE requires that a provisional response with an SDP be transmitted reliably. This can be done through the existing PRACK mechanism , 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 (because receipt of a binding request indicates the offerer has received the answer) 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. Specifically, if the INVITE contained an offer, the same answer appears in all of the 1xx and in the 2xx response to the INVITE. Only after that 2xx has been sent can an updated offer/ answer exchange occur. This optimization SHOULD NOT be used if both agents support PRACK. Note that the optimization is very specific to provisional response carrying answers that start ICE processing; it is not a general technique for 1xx reliability. Alternatively, an agent MAY delay sending an answer until the 200 OK, however this results in a poor user experience and is NOT RECOMMENDED. Once the answer has been sent, the agent SHOULD begin its connectivity checks. Once candidate pairs for each component of a media stream enter the valid list, the answerer can begin sending media on that media stream. However, prior to this point, any media that needs to be sent towards the caller (such as SIP early media  MUST NOT be transmitted. For this reason, implementations SHOULD delay alerting the called party until candidates for each component of each media stream have entered the valid list. In the case of a PSTN gateway, this would mean that the setup message into the PSTN is delayed until this point. Doing this increases the post-dial delay, but has the effect of eliminating 'ghost rings'. Ghost rings are cases where the called party hears the phone ring, picks up, but hears nothing and cannot be heard. This technique works without requiring support for, or usage of, preconditions , 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. 12.1.2. Offer in Response In addition to uses where the offer is in an INVITE, and the answer is in the provisional and/or 200 OK response, ICE works with cases where the offer appears in the response. In such cases, which are common in third party call control , ICE agents SHOULD generate their offers in a reliable provisional response (which MUST utilize RFC 3262), and not alert the user on receipt of the INVITE. The answer will arrive in a PRACK. This allows for ICE processing to take place prior to alerting, so that there is no post-pickup delay, at the expense of increased call setup delays. Once ICE completes, the callee can alert the user and then generate a 200 OK when they answer. The 200 OK would contain no SDP, since the offer/answer exchange has completed. Alternatively, agents MAY place the offer in a 2xx instead (in which case the answer comes in the ACK). When this happens, the callee will alert the user on receipt of the INVITE, and the ICE exchanges will take place only after the user answers. This has the effect of reducing call setup delay, but can cause substantial post-pickup delays and media clipping. 12.2. SIP Option Tags and Media Feature Tags  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 intermediaries. 12.3. Interactions with Forking ICE interacts very well with forking. Indeed, ICE fixes some of the problems associated with forking. Without ICE, when a call forks and the caller receives multiple incoming media streams, it cannot determine which media stream corresponds to which callee. With ICE, this problem is resolved. The connectivity checks which occur prior to transmission of media carry username fragments, which in turn are correlated to a specific callee. Subsequent media packets which arrive on the same candidate pair as the connectivity check will be associated with that same callee. Thus, the caller can perform this correlation as long as it has received an answer. 12.4. Interactions with Preconditions Quality of Service (QoS) preconditions, which are defined in RFC 3312  and RFC 4032 , apply only to the transport addresses listed as the default targets for media in an offer/answer. If ICE changes the transport address where media is received, this change is reflected in an updated offer which changes the default destination for media to match ICE's selection. As such, it appears like any other re- INVITE would, and is fully treated in RFC 3312 and 4032, which apply without regard to the fact that the destination for media is changing due to ICE negotiations occurring "in the background". Indeed, an agent SHOULD NOT indicate that Qos preconditions have been met until the checks have completed and selected the candidate pairs to be used for media. ICE also has (purposeful) interactions with connectivity preconditions . 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 . 12.5. Interactions with Third Party Call Control ICE works with Flows I, III and IV as described in . Flow I works without the controller supporting or being aware of ICE. Flow IV will work as long as the controller passes along the ICE attributes without alteration. Flow II is fundamentally incompatible with ICE; each agent will believe itself to be the answerer and thus never generate a re-INVITE. The flows for continued operation, as described in Section 7 of RFC 3725, require additional behavior of ICE implementations to support. In particular, if an agent receives a mid-dialog re-INVITE that contains no offer, it MUST restart ICE for each media stream and go through the process of gathering new candidates. Furthermore, that list of candidates SHOULD include the ones currently being used for media. 13. Usage with ANAT RFC 4091  defines a mechanism for indicating that an agent can support both IPv4 and IPv6 for a media stream, and it does so by including two m-lines, one for v4, and one for v6. This is similar to ICE, which allows for an agent to indicate multiple transport addresses using the candidate attribute. However, ICE is not a replacement for ANAT. When an agent has a v4 and v6 interface and requires just a static choice of address - use v6 if both support v6, else v4 - ANAT alone is used. If an agent wishes the choice of v4 or v6 to be dynamic and depend on actual verification of connectivity, an agent would use ANAT in concert with ICE. To do that, The agent MUST include two media stream alternates, one for v4 and one for v6, as defined in RFC 4091. In addition, the agent MUST include a v4 candidate as a session attribute for the v4 stream alternate, and a v6 candidate as a session attribute of the v6 stream alternate. ICE will then perform its checks for each stream alternate. The agent MUST order the ICE selected pairs for each stream alternate based on their mid preference, and choose the highest one. This means that if ICE doesn't select any pair for a stream alternate (because, for example, no checks succeeded), the agent will choose the next highest preference pair which was selected. This allows v6 to be used if a v6 path can be verified, but to fallback to v4 ifcontains no offer, it cannot be verified. This extends naturally to multiple candidatesMUST restart ICE for each alternate. An agent MAY include multiple v4 candidates for the v4media stream alternateand multiple v6 candidates forgo through the v6 stream alternate. Allprocess of the candidates for a v4 stream alternate MUST be v4, and allgathering new candidates. Furthermore, that list of thecandidates SHOULD include the ones currently being used for media. 13. Relationship with ANAT RFC 4091  defines a v6 stream alternate MUST be v6. This will cause ICE to choose a v6 pair as long as one of the pairs works, else it will fall back to v4. Of course,mechanism for indicating that an agent can use ICE with v4support both IPv4 and v6 candidates without ANAT. In that mode, it would have justIPv6 for a singlemedia stream, with a default destination that is either v4 or v6. The candidates can include both v4and v6 candidates.it does so by including two m-lines, one for v4, and one for v6. This bringsis similar to ICE, which allows for an agent to indicate multiple transport addresses using the flexibility of choosing a v4 candidate even if a v6candidate validates, perhaps dueattribute. However, ANAT relies on static selection to differing path characteristics measured dynamicallypick between choices, rather than a dynamic connectivity check used by the agent. That kind of flexibility is not possible when ANATICE. This specification deprecates RFC 4091. Instead, agents wishing to support dual-stack will utilize ICE. Because a dual-stack agent will require at least two candidates, one for IPv4 and one for IPv6, dual- stack agents MUST be full implementations. However, agents that are implementing dual-stack and are running on closed networks where it is used.known that there are no NAT, MAY include only host candidates in their offers, skipping server reflexive and relayed candidates. 14. Extensibility Considerations This specification makes very specific choices about how both agents in a session coordinate to arrive at the set of candidate pairs that are selected for media. It is anticipated that future specifications will want to alter these algorithms, whether they are simple changes like timer tweaks, or larger changes like a revamp of the priority algorithm. When such a change is made, providing interoperability between the two agents in a session is critical. First, ICE provides the a=ice-options SDP attribute. Each extension or change to ICE is associated with a token. When an agent supporting such an extension or change generates an offer or an answer, it MUST include the token for that extension in this attribute. This allows each side to know what the other side is doing. This attribute MUST NOT be present if the agent doesn't support any ICE extensions or changes. At this time, no IANA registry or registration procedures are defined for these option tags. At time of writing, it is unclear whether ICE changes and extensions will be sufficiently common to warrrant a registry. One of the complications in achieving interoperability is that ICE relies on a distributed algorithm running on both agents to converge on an agreed set of candidate pairs. If the two agents run different algorithms, it can be difficult to guarantee convergence on the same candidate pairs. The regular nomination procedure described in Section 8 eliminates some of the tight coordination by delegating the selection algorithm completely to the controlling agent. Consequently, when a controlling agent is communicating with a peer that supports options it doesn't know about, the agent MUST run a regular nomination algorithm. When regular nomination is used, ICE will converge perfectly even when both agents use different pair prioritization algorithms. One of the keys to such convergence are triggered checks, which ensure that the nominated pair is validated by both agents. Consequently, any future ICE enhancements MUST preserve triggered checks. ICE is also extensible to other media streams beyond RTP, and for transport protocols beyond UDP. Extensions to ICE for non-RTP media streams need to specify how many components they utilize, and assign component IDSIDs to them, starting at 1 for the most important component ID. Specifications for new transport protocols must define how, if at all, various steps in the ICE processing differ from UDP. 15. Grammar This specification defines seven new SDP attributes - the "candidate", "remote-candidates", "ice-lite", "ice-mismatch", "ice- ufrag", "ice-pwd" and "ice-options" attributes. 15.1. "candidate" Attribute The candidate attribute is a media-level attribute only. It contains a transport address for a candidate that can be used for connectivity checks. The syntax of this attribute is defined using Augmented BNF as defined in RFC 4234 : candidate-attribute = "candidate" ":" foundation SP component-id SP transport SP priority SP connection-address SP ;from RFC 4566 port ;port from RFC 4566 SP cand-type [SP rel-addr] [SP rel-port] *(SP extension-att-name SP extension-att-value) foundation = 1*ice-char component-id = 1*DIGIT transport = "UDP" / transport-extension transport-extension = token ; from RFC 3261 priority = 1*DIGIT cand-type = "typ" SP candidate-types candidate-types = "host" / "srflx" / "prflx" / "relay" / token rel-addr = "raddr" SP connection-address rel-port = "rport" SP port extension-att-name = byte-string ;from RFC 4566 extension-att-value = byte-string ice-char = ALPHA / DIGIT / "+" / "/" This grammar encodes the primary information about a candidate: its IP address, port and transport protocol, and its properties: the foundation, component ID, priority, type, and related transport address: <connect-address>:<connection-address>: is taken from RFC 4566 . It is the IP address of the candidate, allowing for IPv4 addresses, IPv6 addresses and FQDNs. An IP address SHOULD be used, but an FQDN MAY be used in place of an IP address. In that case, when receiving an offer or answer containing an FQDN in an a=candidate attribute, the FQDN is looked up in the DNS using an A or AAAA record, and the resulting IP address is used for the remainder of ICE processing. <port>: is also taken from RFC 4566 . It is the port of the candidate. <transport>: indicates the transport protocol for the candidate. This specification only defines UDP. However, extensibility is provided to allow for future transport protocols to be used with ICE, such as TCP or the Datagram Congestion Control Protocol (DCCP) . <foundation>: is composed of one or more <ice-char>. It is an identifier that is equivalent for two candidates that are of the same type, share the same base, and come from the same STUN server. The foundation is used to optimize ICE performance in the Frozen algorithm. <component-id>: is a positive integer between 1 and 256 which identifies the specific component of the media stream for which this is a candidate. It MUST start at 1 and MUST increment by 1 for each component of a particular candidate. For media streams based on RTP, candidates for the actual RTP media MUST have a component ID of 1, and candidates for RTCP MUST have a component ID of 2. Other types of media streams which require multiple components MUST develop specifications which define the mapping of components to component IDs. See Section 14 for additional discussion on extending ICE to new media streams. <priority>: is a positive integer between 1 and (2**32 - 1). <cand-type>: encodes the type of candidate. This specification defines the values "host", "srflx", "prflx" and "relay" for host, server reflexive, peer reflexive and relayed candidates, respectively. The set of candidate types is extensible for the future. <rel-addr> and <rel-port>: convey transport addresses related to the candidate, useful for diagnostics and other purposes. <rel-addr> and <rel-port> MUST be present for server reflexive, peer reflexive and relayed candidates. If a candidate is server or peer reflexive, <rel-addr> and <rel-port> is equal to the base for that server or peer reflexive candidate. If the candidate is relayed, <rel-addr> and <rel-port> is equal to the mapped address in the Allocate Response that provided the client with that relayed candidate (see Appendix B.3 for a discussion of its purpose). If the candidate is a host candidate <rel-addr> and <rel-port> MUST be omitted. The candidate attribute can itself be extended. The grammar allows for new name/value pairs to be added at the end of the attribute. An implementation MUST ignore any name/value pairs it doesn't understand. 15.2. "remote-candidates" Attribute The syntax of the "remote-candidates" attribute is defined using Augmented BNF as defined in RFC 4234 . The remote-candidates attribute is a media level attribute only. remote-candidate-att = "remote-candidates" ":" remote-candidate 0*(SP remote-candidate) remote-candidate = component-ID SP connection-address SP port The attribute contains a connection-address and port for each component. The ordering of components is irrelevant. However, a value MUST be present for each component of a media stream. This attribute MUST be included in an offer by a controlling agent for a media stream that is Completed, and MUST NOT be included in any other case. 15.3. "ice-lite" and "ice-mismatch" Attributes The syntax of the "ice-lite" and "ice-mismatch" attributes, both of which are flags, is: ice-lite = "ice-lite" ice-mismatch = "ice-mismatch" "ice-lite" is a session level attribute only, and indicates that an agent is a lite implementation. "ice-mismatch" is a media level attribute only, and when present in an answer, indicates that the offer arrived with a default destination for a media component that didn't have a corresponding candidate attribute. 15.4. "ice-ufrag" and "ice-pwd" Attributes The "ice-ufrag" and "ice-pwd" attributes convey the username fragment and password used by ICE for message integrity. Their syntax is: ice-pwd-att = "ice-pwd" ":" password ice-ufrag-att = "ice-ufrag" ":" ufrag password = 22*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. Whether present at the session or media level, there MUST be an ice-pwd and ice-ufrag attribute for each media stream. If two media streams have identical ice-ufrag's, they MUST have identical ice-pwd's. The ice-ufrag and ice-pwd attributes MUST be chosen randomly at the beginning of a session. The ice-ufrag attribute MUST contain at least 24 bits of randomness, and the ice-pwd attribute MUST contain at least 128 bits of randomness. This means that the ice-ufrag attribute will be at least 4 characters long, and the ice-pwd at least 22 characters long, since the grammar for these attributes allows for 6 bits of randomness per character. The attributes MAY be longer than 4 and 22 characters respectively, of course. 15.5. "ice-options> Attribute The "ice-options" attribute is a session level attribute. It 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 16. Example The example is based on the simplified topology of Figure 15. +-----+ | | |STUN | | Srvr| +-----+ | +---------------------+ | | | Internet | | | | | +---------------------+ | | | | +---------+ | | NAT | | +---------+ | | | | | | | +-----+ +-----+ | | | | | L | | R | | | | | +-----+ +-----+ Figure 15: Example Topology 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 in private address space , and for agent R, 192.0.2.1 on the public Internet. Both are configured with the same STUN server (shown in this example for simplicity, although in practice the agents do not need to use the same STUN server), 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 endpoint independent mapping property and an address dependent filtering property. The public side of the NAT has an IP address of 192.0.2.3. To facilitate understanding, transport addresses are listed using variables that have mnemonic names. The format of the name is entity-type-seqno, where entity refers to the entity whose interface the transport address is on, and is one of "L", "R", "STUN", or "NAT". The type is either "PUB" for transport addresses that are public, and "PRIV" for transport addresses that are private. Finally, seq-no is a sequence number that is different for each transport address of the same type on a particular entity. Each variable has an IP address and port, denoted by varname.IP and varname.PORT, respectively, where varname is the name of the variable. The STUN server has advertised transport address STUN-PUB-1 (which is 192.0.2.2:3478) 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 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 | | | |------------->| | | |(3) STUN Res | | | |S=$STUN-PUB-1 | | | |D=$NAT-PUB-1 | | | |MA=$NAT-PUB-1 | | | |<-------------| | |(4) STUN Res | | | |S=$STUN-PUB-1 | | | |D=$L-PRIV-1 | | | |MA=$NAT-PUB-1 | | | |<-------------| | | |(5) Offer | | | |------------------------------------------->| | | | |RTP STUN alloc. | | |(6) STUN Req | | | |S=$R-PUB-1 | | | |D=$STUN-PUB-1 | | | |<-------------| | | |(7) STUN Res | | | |S=$STUN-PUB-1 | | | |D=$R-PUB-1 | | | |MA=$R-PUB-1 | | | |------------->| |(8) answer | | | |<-------------------------------------------| | |(9) Bind Req | | | |S=$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 | | | |<-------------| | | |RTP flows | | | | |(14) Bind Req | | | |S=$R-PUB-1 | | | |D=$NAT-PUB-1 | | | |<----------------------------| |(15) Bind Req | | | |S=$R-PUB-1 | | | |D=$L-PRIV-1 | | | |<-------------| | | |(16) Bind Res | | | |S=$L-PRIV-1 | | | |D=$R-PUB-1 | | | |MA=$R-PUB-1 | | | |------------->| | | | |(17) Bind Res | | | |S=$NAT-PUB-1 | | | |D=$R-PUB-1 | | | |MA=$R-PUB-1 | | | |---------------------------->| | | | |RTP flows Figure 16: Example Flow 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 this, the priority of the host candidate is 2130706178 and for the server reflexive candidate is 1694498562. The host candidate is assigned a foundation of 1, and the server reflexive, a foundation of 2. It chooses its server reflexive candidate as the default candidate, and encodes it into the m and c lines. The resulting offer (message 5) looks like (lines folded for clarity): v=0 o=jdoe 2890844526 2890842807 IN IP4 $L-PRIV-1.IP s= c=IN IP4 $NAT-PUB-1.IP t=0 0 a=ice-pwd:asd88fgpdd777uzjYhagZg a=ice-ufrag:8hhY m=audio $NAT-PUB-1.PORT RTP/AVP 0 b=RS:0 b=RR:0 a=rtpmap:0 PCMU/8000 a=candidate:1 1 UDP 2130706178 $L-PRIV-1.IP $L-PRIV-1.PORT typ localhost a=candidate:2 1 UDP 1694498562 $NAT-PUB-1.IP $NAT-PUB-1.PORT typ srflx raddr $L-PRIV-1.IP rport $L-PRIV-1.PORT The offer, with the variables replaced with their values, will look like (lines folded for clarity): v=0 o=jdoe 2890844526 2890842807 IN IP4 10.0.1.1 s= c=IN IP4 192.0.2.3 t=0 0 a=ice-pwd:asd88fgpdd777uzjYhagZg a=ice-ufrag:8hhY m=audio 45664 RTP/AVP 0 b=RS:0 b=RR:0 a=rtpmap:0 PCMU/8000 a=candidate:1 1 UDP 2130706178 10.0.1.1 8998 typ localhost a=candidate:2 1 UDP 1694498562 192.0.2.3 45664 typ srflx raddr 10.0.1.1 rport 8998 This offer is received at agent R. Agent R will obtain a host candidate, and from it, obtain a server reflexive candidate (messages 6-7). Since R is not behind a NAT, this candidate is identical to its host candidate, and they share the same base. It therefore discards this redundant candidate and ends up with a single host candidate. With identical type and local preferences as L, the priority for this candidate is 2130706178. It chooses a foundation of 1 for its single candidate. Its resulting answer looks like: v=0 o=bob 2808844564 2808844564 IN IP4 $R-PUB-1.IP s= c=IN IP4 $R-PUB-1.IP t=0 0 a=ice-pwd:YH75Fviy6338Vbrhrlp8Yh a=ice-ufrag:9uB6 m=audio $R-PUB-1.PORT RTP/AVP 0 b=RS:0 b=RR:0 a=rtpmap:0 PCMU/8000 a=candidate:1 1 UDP 2130706178 $R-PUB-1.IP $R-PUB-1.PORT typ localhost With the variables filled in: v=0 o=bob 2808844564 2808844564 IN IP4 192.0.2.1 s= c=IN IP4 192.0.2.1 t=0 0 a=ice-pwd:YH75Fviy6338Vbrhrlp8Yh a=ice-ufrag:9uB6 m=audio 3478 RTP/AVP 0 b=RS:0 b=RR:0 a=rtpmap:0 PCMU/8000 a=candidate:1 1 UDP 2130706178 192.0.2.1 3478 typ localhost Since neither side indicated that they are lite, the agent which sent the offer that began ICE processing (agent L) becomes the controlling agent. Agents L and R both pair up the candidates. They both initially have two pairs. However, agent L will prune the pair containing its server reflexive candidate, resulting in just one. At agent L, this pair has a local candidate of $L_PRIV_1 and remote candidate of $R_PUB_1, and has a candidate pair priority of 4.57566E+18 (note that an implementation would represent this as a 64 bit integer so as not to lose precision). At agent R, there are two pairs. The highest priority has a local candidate of $R_PUB_1 and remote candidate of $L_PRIV_1 and has a priority of 4.57566E+18, and the second has a local candidate of $R_PUB_1 and remote candidate of $NAT_PUB_1 and priority 3.63891E+18. Agent R begins its connectivity check (message 9) for the first pair (between the two host candidates). Since R is the controlled agent for this session, the check omits the USE-CANDIDATE attribute. The host candidate from agent L is private and behind a NAT, and thus this check won't be successful, because the packet cannot be routed from R to L. When agent L gets the answer, it performs its one and only connectivity check (messages 10-13). It implements the aggressive nomination algorithm, and thus includes a USE-CANDIDATE attribute in this check. Since the check succeeds, agent L creates a new pair, whose local candidate is from the mapped address in the binding response (NAT-PUB-1 from message 13) and whose remote candidate is the destination of the request (R-PUB-1 from message 10). This is added to the valid list. In addition, it is marked as selected since the Binding Request contained the USE-CANDIDATE attribute. Since there is a selected candidate in the Valid list for the one component of this media stream, ICE processing for this stream moves into the Completed state. Agent L can now send media if it so chooses. Upon receipt of the STUN Binding Request from agent L (message 11), agent R will generate its triggered check. This check happens to match the next one on its check list - from its host candidate to agent L's server reflexive candidate. This check (messages 14-17) will succeed. Consequently, agent R constructs a new candidate pair using the mapped address from the response as the local candidate (R-PUB-1) and the destination of the request (NAT-PUB-1) as the remote candidate. This pair is added to the Valid list for that media stream. Since the check was generated in the reverse direction of a check that contained the USE-CANDIDATE attribute, the candidate pair is marked as selected. Consequently, processing for this stream moves into the Completed state, and agent R can also send media. 17. Security Considerations There are several types of attacks possible in an ICE system. This section considers these attacks and their countermeasures. 17.1. Attacks on Connectivity Checks An attacker might attempt to disrupt the STUN connectivity checks. Ultimately, all of these attacks fool an agent into thinking something incorrect about the results of the connectivity checks. The possible false conclusions an attacker can try and cause are: False Invalid: An attacker can fool a pair of agents into thinking a candidate pair is invalid, when it isn't. This can be used to cause an agent to prefer a different candidate (such as one injected by the attacker), or to disrupt a call by forcing all candidates to fail. False Valid: An attacker can fool a pair of agents into thinking a candidate pair is valid, when it isn't. This can cause an agent to proceed with a session, but then not be able to receive any media. False Peer-Reflexive Candidate: An attacker can cause an agent to discover a new peer reflexive candidate, when it shouldn't have. This can be used to redirect media streams to a DoS target or to the attacker, for eavesdropping or other purposes. False Valid on False Candidate: An attacker has already convinced an agent that there is a candidate with an address that doesn't actually route to that agent (for example, by injecting a false peer reflexive candidate or false server reflexive candidate). It must then launch an attack that forces the agents to believe that this candidate is valid. Of the various techniques for creating faked STUN messages described in , many are not applicable for the connectivity checks. Compromises of STUN servers are not much of a concern, since the STUN servers are embedded in endpoints and distributed throughout the network. Thus, compromising the peer's embedded STUN server is equivalent to comprimising the endpoint, and if that happens, far more problematic attacks are possible than those against ICE. Similarly, DNS attacks are usually irrelevant since STUN servers are not typically discovered via DNS, they are normally signaled via IP addresses embedded in SDP. If the SDP does contain an FQDN for a host, then connectivity checks would be susceptible to the DNS vulnerabilities described in ; however it is far more common to use IP addresses. Injection of fake responses and relaying modified requests all can be handled in ICE with the countermeasures discussed below. To force the false invalid result, the attacker has to wait for the connectivity check from one of the agents to be sent. When it is, the attacker needs to inject a fake response with an unrecoverable error response, such as a 600. However, since the candidate is, in fact, valid, the original request may reach the peer agent, and result in a success response. The attacker needs to force this packet or its response to be dropped, through a DoS attack, layer 2 network disruption, or other technique. If it doesn't do this, the success response will also reach the originator, alerting it to a possible attack. Fortunately, this attack is mitigated completely through the STUN message integrity mechanism. The attacker needs to inject a fake response, and in order for this response to be processed, the attacker needs the password. If the offer/answer signaling is secured, the attacker will not have the password. Forcing the fake valid result works in a similar way. The agent needs to wait for the Binding Request from each agent, and inject a fake success response. The attacker won't need to worry about disrupting the actual response since, if the candidate is not valid, it presumably wouldn't be received anyway. However, like the fake invalid attack, this attack is mitigated completely through the STUN message integrity and offer/answer security techniques. Forcing the false peer reflexive candidate result can be done either with fake requests or responses, or with replays. We consider the fake requests and responses case first. It requires the attacker to send a Binding Request to one agent with a source IP address and port for the false candidate. In addition, the attacker must wait for a Binding Request from the other agent, and generate a fake response with a XOR-MAPPED-ADDRESS attribute containing the false candidate. Like the other attacks described here, this attack is mitigated by the STUN message integrity mechanisms and secure offer/answer exchanges. Forcing the false peer reflexive candidate result with packet replays is different. The attacker waits until one of the agents sends a check. It intercepts this request, and replays it towards the other agent with a faked source IP address. It must also prevent the original request from reaching the remote agent, either by launching a DoS attack to cause the packet to be dropped, or forcing it to be dropped using layer 2 mechanisms. The replayed packet is received at the other agent, and accepted, since the integrity check passes (the integrity check cannot and does not cover the source IP address and port). It is then responded to. This response will contain a XOR- MAPPED-ADDRESS with the false candidate, and will be sent to that false candidate. The attacker must then receive it and relay it towards the originator. The other agent will then initiate a connectivity check towards that false candidate. This validation needs to succeed. This requires the attacker to force a false valid on a false candidate. Injecting of fake requests or responses to achieve this goal is prevented using the integrity mechanisms of STUN and the offer/answer exchange. Thus, this attack can only be launched through replays. To do that, the attacker must intercept the check towards this false candidate, and replay it towards the other agent. Then, it must intercept the response and replay that back as well. This attack is very hard to launch unless the attacker is identified by the fake candidate. This is because it requires the attacker to intercept and replay packets sent by two different hosts. If both agents are on different networks (for example, across the public Internet), this attack can be hard to coordinate, since it needs to occur against two different endpoints on different parts of the network at the same time. If the attacker themself is identified by the fake candidate the attack is easier to coordinate. However, if SRTP is used , 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. 17.2. Attacks on Address Gathering ICE endpoints make use of STUN for gathering candidates from a STUN server in the network. This is corresponds to the Binding Discovery usage of STUN described in . As a consequence, the attacks against STUN itself that are described in that specification can still be used against the binding discovery usage when utilized with ICE. However, the additional mechanisms provided by ICE actually counteract such attacks, making binding discovery with STUN more secure when combined with ICE than without ICE. Consider an attacker which is able to provide an agent with a faked mapped address in a STUN Binding Request that is used for address gathering. This is the primary attack primitive described in . This address will be used as a server reflexive candidate in the ICE exchange. For this candidate to actually be used for media, the attacker must also attack the connectivity checks, and in particular, force a false valid on a false candidate. This attack is very hard to launch if the false address identifies a fourth party (neither the offerer, answerer, or attacker), since it requires attacking the checks generated by each agent in the session, and is prevented by SRTP if it identifies the attacker themself. If the attacker elects not to attack the connectivity checks, the worst it can do is prevent the server reflexive candidate from being used. However, if the peer agent has at least one candidate that is reachable by the agent under attack, the STUN connectivity checks themselves will provide a peer reflexive candidate that can be used for the exchange of media. Peer reflexive candidates are generally preferred over server reflexive candidates. As such, an attack solely on the STUN address gathering will normally have no impact on a session at all. 17.3. Attacks on the Offer/Answer Exchanges An attacker that can modify or disrupt the offer/answer exchanges themselves can readily launch a variety of attacks with ICE. They could direct media to a target of a DoS attack, they could insert themselves into the media stream, and so on. These are similar to the general security considerations for offer/answer exchanges, and the security considerations in RFC 3264  apply. These require techniques for message integrity and encryption for offers and answers, which are satisfied by the SIPS mechanism  when SIP is used. As such, the usage of SIPS with ICE is RECOMMENDED. 17.4. Insider Attacks In addition to attacks where the attacker is a third party trying to insert fake offers, answers or stun messages, there are several attacks possible with ICE when the attacker is an authenticated and valid participant in the ICE exchange. 17.4.1. The Voice Hammer Attack The voice hammer attack is an amplification attack. In this attack, the attacker initiates sessions to other agents, and maliciously includes the IP address and port of a DoS target as the destination for media traffic signaled in the SDP. This causes substantial amplification; a single offer/answer exchange can create a continuing flood of media packets, possibly at high rates (consider video sources). This attack is not specific to ICE, but ICE can help provide remediation. Specifically, if ICE is used, the agent receiving the malicious SDP will first perform connectivity checks to the target of media before sending media there. If this target is a third party host, the checks will not succeed, and media is never sent. Unfortunately, ICE doesn't help if its not used, in which case an attacker could simply send the offer without the ICE parameters. However, in environments where the set of clients are known, and limited to ones that support ICE, the server can reject any offers or answers that don't indicate ICE support. 17.4.2. 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. The attacker sends an offer with a large number of candidates, say 50. The answerer receives the offer, and starts its checks, which are directed at the target, and consequently, never generate a response. The answerer will start a new connectivity check every 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. 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. 17.5. 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 SDP. In this case, correctly means that the ALG does not modify the m and c lines or the rtcp attribute if they contain external addresses. If they contain internal addresses, the modification depends on the state of the ALG. If the ALG already has a binding established that maps an external port to an internal IP address and port in m and c lines or rtcp attribute , the ALG uses that binding instead of creating a new one. 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 and c lines and rtcp attributes. The ice- mismatch attribute is used for this purpose. ICE works best through ALGs when the signaling is run over TLS. This prevents the ALG from manipulating the SDP messages and interfering with ICE operation. Implementations 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 the SBC has requested it. If, however, the SBC passes the ICE attributes without modification, yet modifies the default destination for media (contained in the m and c lines and rtcp attribute), this will be detected as an ICE mismatch, and ICE processing is aborted for the call. It is outside of the scope of ICE for it to act as a tool for "working around" SBCs. If one is present, ICE will not be used and the SBC techniques take precedence. 18. Definition of Connectivity Check Usage STUN  requires that new usages provide a specific set of information as part of their formal definition. This section meets the requirements spelled out there. 18.1. Applicability This STUN usage provides a connectivity check between two peers participating in an offer/answer exchange. This check serves to validate a pair of candidates for usage of exchange of media. Connectivity checks also allow agents to discover reflexive candidates towards their peers, called peer reflexive candidates. Finally, this usage allows a Binding Indication to be used to keep NAT bindings alive. It is fundamental to this STUN usage that the addresses and ports used for media are the same ones used for the Binding Requests and responses. Consequently, it will be necessary to demultiplex STUN traffic from applications on that same port (e.g., RTP or RTCP). This demultiplexing is done using the techniques described in . 18.2. Client Discovery of Server The client does not follow the DNS-based procedures defined in . Rather, the remote candidate of the check to be performed is used as the transport address of the STUN server. Note that the STUN server is a logical entity, and is not a physically distinct server in this usage. 18.3. Server Determination of Usage The server is aware of this usage because it signaled transport addresses in its candidates on which it expects to receive STUN packets. Consequently, any STUN packets received on the base of a candidate offered in SDP will be for the connectivity check usage. 18.4. New Requests or Indications This usage does not define any new message types. 18.5. New Attributes This usage defines twofour new attributes, PRIORITYPRIORITY, USE-CANDIDATE, ICE- CONTROLLED and USE-CANDIDATE.ICE-CONTROLLING. The PRIORITY attribute indicates the priority that is to be associated with a peer reflexive candidate, should one be discovered by this check. It is a 32 bit unsigned integer, and has an attribute value of 0x0024. The USE-CANDIDATE attribute indicates that the candidate pair resulting from this check should be used for transmission of media. The attribute has no content (the Length field of the attribute is zero); it serves as a flag. It has an attribute value of 0x0025. The ICE-CONTROLLED attribute is present in a Binding Request, and indicates that the client believes it is currently in the controlled role. The content of the attribute is a 64 bit unsigned integer in network byte ordering, which contains a random number used for tie- breaking of role conflicts. The ICE-CONTROLLING attribute is present in a Binding Request, and indicates that the client believes it is currently in the controlling role. The content of the attribute is a 64 bit unsigned integer in network byte ordering, which contains a random number used for tie- breaking of role conflicts. 18.6. New Error Response Codes This usage does not define any newdefines a single error response codes.code: 487 (Role Conflict): The Binding Request contained either the ICE- CONTROLLING or ICE-CONTROLLED attribute, indicating a role that conflicted with the server. The server ran a tie-breaker based on the tie-breaker value in the request, and determined that the client needs to switch roles. 18.7. Client Procedures Client procedures are defined in Section 7.1. 18.8. Server Procedures Server procedures are defined in Section 7.2. 18.9. Security Considerations for Connectivity Check Security considerations for the connectivity check are discussed in Section 17. 19. IANA Considerations This specification registers new SDP attributes and new STUN attributes. 19.1. SDP Attributes This specification defines seven new SDP attributes per the procedures of Section 8.2.4 of . The required information for the registrations are included here. 19.1.1. candidate Attribute Contact Name: Jonathan Rosenberg, email@example.com. 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). Appropriate Values: See Section 15 of RFC XXXX [Note to RFC-ed: please replace XXXX with the RFC number of this specification]. 19.1.2. remote-candidates Attribute Contact Name: Jonathan Rosenberg, firstname.lastname@example.org. 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 15 of RFC XXXX [Note to RFC-ed: please replace XXXX with the RFC number of this specification]. 19.1.3. ice-lite Attribute Contact Name: Jonathan Rosenberg, email@example.com. Attribute Name: ice-lite Long Form: ice-lite Type of Attribute: session level Charset Considerations: The attribute is not subject to the charset attribute. Purpose: This attribute is used with Interactive Connectivity Establishment (ICE), and indicates that an agent has the minimum functionality required to support ICE inter-operation with a peer that has a full implementation. Appropriate Values: See Section 15 of RFC XXXX [Note to RFC-ed: please replace XXXX with the RFC number of this specification]. 19.1.4. ice-mismatch Attribute Contact Name: Jonathan Rosenberg, firstname.lastname@example.org. Attribute Name: ice-mismatch Long Form: ice-mismatch Type of Attribute: session level Charset Considerations: The attribute is not subject to the charset attribute. Purpose: This attribute is used with Interactive Connectivity Establishment (ICE), and indicates that an agent is ICE capable, but did not proceed with ICE due to a mismatch of candidates with the default destination for media signaled in the SDP. Appropriate Values: See Section 15 of RFC XXXX [Note to RFC-ed: please replace XXXX with the RFC number of this specification]. 19.1.5. ice-pwd Attribute Contact Name: Jonathan Rosenberg, email@example.com. Attribute Name: ice-pwd Long Form: ice-pwd Type of Attribute: session or media level Charset Considerations: The attribute is not subject to the charset attribute. Purpose: This attribute is used with Interactive Connectivity Establishment (ICE), and provides the password used to protect STUN connectivity checks. Appropriate Values: See Section 15 of RFC XXXX [Note to RFC-ed: please replace XXXX with the RFC number of this specification]. 19.1.6. ice-ufrag Attribute Contact Name: Jonathan Rosenberg, firstname.lastname@example.org. Attribute Name: ice-ufrag Long Form: ice-ufrag Type of Attribute: session or media level Charset Considerations: The attribute is not subject to the charset attribute. Purpose: This attribute is used with Interactive Connectivity Establishment (ICE), and provides the fragments used to construct the username in STUN connectivity checks. Appropriate Values: See Section 15 of RFC XXXX [Note to RFC-ed: please replace XXXX with the RFC number of this specification]. 19.1.7. ice-options Attribute Contact Name: Jonathan Rosenberg, email@example.com. 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 15 of RFC XXXX [Note to RFC-ed: please replace XXXX with the RFC number of this specification]. 19.2. STUN Attributes This section registers twofour new STUN attributes per the procedures in . 0x0024 PRIORITY 0x0025 USE-CANDIDATE 0x8029 ICE-CONTROLLED 0x802a ICE-CONTROLLING 19.3. STUN Error Responses This section registers one new STUN error response code per the procedures in . 487 Role Conflict 20. 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 . 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. 20.1. Problem Definition From RFC 3424 any UNSAF proposal must provide: Precise definition of a specific, limited-scope problem that is to be solved with the UNSAF proposal. A short term fix should not be generalized to solve other problems; this is why "short term fixes usually aren't". The specific problems being solved by ICE are: Provide a means for two peers to determine the set of transport addresses which can be used for communication. Provide a means for resolving many of the limitations of other UNSAF mechanisms by wrapping them in an additional layer of processing (the ICE methodology). Provide a means for a agent to determine an address that is reachable by another peer with which it wishes to communicate. 20.2. Exit Strategy From RFC 3424, any UNSAF proposal must provide: Description of an exit strategy/transition plan. The better short term fixes are the ones that will naturally see less and less use as the appropriate technology is deployed. ICE itself doesn't easily get phased out. However, it is useful even in a globally connected Internet, to serve as a means for detecting whether a router failure has temporarily disrupted connectivity, for example. ICE also helps prevent certain security attacks which have nothing to do with NAT. However, what ICE does is help phase out other UNSAF mechanisms. ICE effectively selects amongst those mechanisms, prioritizing ones that are better, and deprioritizing ones that are worse. Local IPv6 addresses can be preferred. As NATs begin to dissipate as IPv6 is introduced, server reflexive and relayed candidates (both forms of UNSAF mechanisms) simply never get used, because higher priority connectivity exists to the native host candidates. Therefore, the servers get used less and less, and can eventually be remove when their usage goes to zero. Indeed, ICE can assist in the transition from IPv4 to IPv6. It can be used to determine whether to use IPv6 or IPv4 when two dual-stack hosts communicate with SIP (IPv6 gets used). It can also allow a network with both 6to4 and native v6 connectivity to determine which address to use when communicating with a peer. 20.3. 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 ) 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 server. ICE makes use of a server for allocating unilateral addresses, but allows agents to directly connect if possible. Therefore, in some cases, the failure of a STUN server would still allow for a call to progress when ICE is used. Another point of brittleness in traditional STUN is that it assumes that the STUN server is on the public Internet. Interestingly, with ICE, that is not necessary. There can be a multitude of STUN servers in a variety of address realms. ICE will discover the one that has provided a usable address. The most troubling point of brittleness in traditional STUN is that it doesn't work in all network topologies. In cases where there is a shared NAT between each agent and the STUN server, traditional STUN may not work. With ICE, that restriction is removed. Traditional STUN also introduces some security considerations. Fortunately, those security considerations are also mitigated by ICE. Consequently, ICE serves to repair the brittleness introduced in other UNSAF mechanisms, and does not introduce any additional brittleness into the system. 20.4. Requirements for a Long Term Solution From RFC 3424, any UNSAF proposal must provide: Identify requirements for longer term, sound technical solutions -- contribute to the process of finding the right longer term solution. Our conclusions from STUN remain unchanged. However, we feel ICE actually helps because we believe it can be part of the long term solution. 20.5. Issues with Existing NAPT Boxes From RFC 3424, any UNSAF proposal must provide: Discussion of the impact of the noted practical issues with existing, deployed NA[P]Ts and experience reports. A number of NAT boxes are now being deployed into the market which try and provide "generic" ALG functionality. These generic ALGs hunt for IP addresses, either in text or binary form within a packet, and rewrite them if they match a binding. This interferes with traditional STUN. However, the update to STUN  uses an encoding which hides these binary addresses from generic ALGs. Existing NAPT boxes have non-deterministic and typically short expiration times for UDP-based bindings. This requires implementations to send periodic keepalives to maintain those bindings. ICE uses a default of 15s, which is a very conservative estimate. Eventually, over time, as NAT boxes become compliant to behave ,, 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. 21. Acknowledgements The authors would like to thank Dan Wing, Eric Rescorla, Flemming Andreasen, Rohan Mahy, Dean Willis, Eric Cooper, Jason Fischl, Douglas Otis, Tim Moore, Jean-Francois Mule, Jonathan Lennox and Francois Audet for their comments and input. A special thanks goes to Bill May, who suggested several of the concepts in this specification, Philip Matthews, who suggested many of the key performance optimizations in this specification, Eric Rescorla, who drafted the text in the introduction, and Magnus Westerlund, for doing several detailed reviews on the various revisions of this specification. 22. References 22.1. Normative References  Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997.  Huitema, C., "Real Time Control Protocol (RTCP) attribute in Session Description Protocol (SDP)", RFC 3605, October 2003.  Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston, A., Peterson, J., Sparks, R., Handley, M., and E. Schooler, "SIP: Session Initiation Protocol", RFC 3261, June 2002.  Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model with Session Description Protocol (SDP)", RFC 3264, June 2002.  Casner, S., "Session Description Protocol (SDP) Bandwidth Modifiers for RTP Control Protocol (RTCP) Bandwidth", RFC 3556, July 2003.  Camarillo, G., Marshall, W., and J. Rosenberg, "Integration of Resource Management and Session Initiation Protocol (SIP)", RFC 3312, October 2002.  Camarillo, G. and P. Kyzivat, "Update to the Session Initiation Protocol (SIP) Preconditions Framework", RFC 4032, March 2005.  Crocker, D. and P. Overell, "Augmented BNF for Syntax Specifications: ABNF", RFC 4234, October 2005.  Rosenberg, J. and H. Schulzrinne, "Reliability of Provisional Responses in Session Initiation Protocol (SIP)", RFC 3262, June 2002.  Handley, M., Jacobson, V., and C. Perkins, "SDP: Session Description Protocol", RFC 4566, July 2006.  Camarillo, G. and J. Rosenberg, "The Alternative Network Address Types (ANAT) Semantics for the Session Description Protocol (SDP) Grouping Framework", RFC 4091, June 2005.  Rosenberg, J., "Simple Traversal Underneath Network Address Translators (NAT) (STUN)", draft-ietf-behave-rfc3489bis-05 (work in progress), October 2006.  Rosenberg, J., "Obtaining Relay Addresses from Simple Traversal Underneath NAT (STUN)", draft-ietf-behave-turn-02 (work in progress), October 2006.  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. 22.2. Informative References  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.  Senie, D., "Network Address Translator (NAT)-Friendly Application Design Guidelines", RFC 3235, January 2002.  Srisuresh, P., Kuthan, J., Rosenberg, J., Molitor, A., and A. Rayhan, "Middlebox communication architecture and framework", RFC 3303, August 2002.  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.  Borella, M., Lo, J., Grabelsky, D., and G. Montenegro, "Realm Specific IP: Framework", RFC 3102, October 2001.  Borella, M., Grabelsky, D., Lo, J., and K. Taniguchi, "Realm Specific IP: Protocol Specification", RFC 3103, October 2001.  Daigle, L. and IAB, "IAB Considerations for UNilateral Self- Address Fixing (UNSAF) Across Network Address Translation", RFC 3424, November 2002.  Schulzrinne, H., Casner, S., Frederick, R., and V. Jacobson, "RTP: A Transport Protocol for Real-Time Applications", RFC 3550, July 2003.  Baugher, M., McGrew, D., Naslund, M., Carrara, E., and K. Norrman, "The Secure Real-time Transport Protocol (SRTP)", RFC 3711, March 2004.  Carpenter, B. and K. Moore, "Connection of IPv6 Domains via IPv4 Clouds", RFC 3056, February 2001.  Zopf, R., "Real-time Transport Protocol (RTP) Payload for Comfort Noise (CN)", RFC 3389, September 2002.  Camarillo, G. and H. Schulzrinne, "Early Media and Ringing Tone Generation in the Session Initiation Protocol (SIP)", RFC 3960, December 2004.  Blake, S., Black, D., Carlson, M., Davies, E., Wang, Z., and W. Weiss, "An Architecture for Differentiated Services", RFC 2475, December 1998.  Rekhter, Y., Moskowitz, R., Karrenberg, D., Groot, G., and E. Lear, "Address Allocation for Private Internets", BCP 5, RFC 1918, February 1996.  Audet, F. and C. Jennings, "Network Address Translation (NAT) Behavioral Requirements for Unicast UDP", BCP 127, RFC 4787, January 2007.  Andreasen, F., "Connectivity Preconditions for Session Description Protocol Media Streams", draft-ietf-mmusic-connectivity-precon-02 (work in progress), June 2006.  Andreasen, F., "A No-Op Payload Format for RTP", draft-ietf-avt-rtp-no-op-00 (work in progress), May 2005.  Kohler, E., Handley, M., and S. Floyd, "Datagram Congestion Control Protocol (DCCP)", RFC 4340, March 2006.  Hellstrom, G. and P. Jones, "RTP Payload for Text Conversation", RFC 4103, June 2005.  Audet, F. and C. Jennings, "NAT Behavioral Requirements for Unicast UDP", draft-ietf-behave-nat-udp-08 (work in progress), October 2006. Jennings, C. and R. Mahy, "Managing Client Initiated Connections in the Session Initiation Protocol (SIP)", draft-ietf-sip-outbound-07 (work in progress), January 2007.  Rescorla, E., "Overview of the Lite Implementation of Interactive Connectivity Establishment (ICE)", draft-ietf-mmusic-ice-lite-00.txt (work in progress), January 2007.Appendix A. Lite and Full Implementations ICE allows for two types of implementations. A full implementation supports the controlling and controlled roles in a session, and can also perform address gathering. In contrast, a lite implementation is a minimalist implementation that does little but respond to STUN checks. Because ICE requires both endpoints to support it in order to bring benefits to either endpoint, incremental deployment of ICE in a network is more complicated. Many sessions involve an endpoint which is, by itself, not behind a NAT and not one that would worry about NAT traversal. A very common case is to have one endpoint that requires NAT traversal (such as a VoIP hard phone or soft phone) make a call to one of these devices. Even if the phone supports a full ICE implementation, ICE won't be used at all if the other device doesn't support it. The lite implementation allows for a low-cost entry point for these devices. Once they support the lite implementation, full implementations can connect to them and get the full benefits of ICE. Consequently, a lite implementation is only appropriate for devices that will *always* be connected to the public Internet and have a public IP address at which it can receive packets from any correspondent. ICE will not function when a lite implementation is placed behind a NAT. 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 17 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 milliseconds, 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. B.2. Candidates with Multiple Bases 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 22:23: +----------+ | STUN Srvr| +----------+ | | ----- // \\ | | | B:net10 | | | \\ // ----- | | +----------+ | NAT | +----------+ | | ----- // \\ | A | |192.168/16 | | | \\ // ----- | | |192.168.1.1 ----- +----------+ // \\ +----------+ | | | | | | | Offerer |---------| C:net10 |---------| Answerer | | |10.0.1.1 | | 10.0.1.2 | | +----------+ \\ // +----------+ ----- Figure 22:23: Identical Candidates with Different Bases 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 net 10 private network, but not connected to network C. There is a STUN server on network B. The offerer obtains a host candidate on its interface on network C (10.0.1.1:2498) and a host candidate on its interface on network A (192.168.1.1:3344). It performs a STUN query to its configured STUN server from 192.168.1.1:3344. This query passes through the NAT, which happens to assign the binding 10.0.1.1:2498. The STUN server reflects this in the STUN Binding Response. Now, the offerer has obtained a server reflexive candidate with a transport address that is identical to a host candidate (10.0.1.1:2498). However, the server reflexive candidate has a base of 192.168.1.1:3344, and the host candidate has a base of 10.0.1.1:2498. B.3. Purpose of the <rel-addr> and <rel-port> Attributes The candidate attribute contains two values that are not used at all by ICE itself - <rel-addr> and <rel-port>. Why is it present? There are two motivations for its inclusion. The first is diagnostic. It is very useful to know the relationship between the different types of candidates. By including it, an agent can know which relayed candidate is associated with which reflexive candidate, which in turn is associated with a specific host candidate. When checks for one candidate succeed and not 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, 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 server reflexive candidate towards that relay 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 L, R, and Z. L and R are within private enterprise 1, which is using 10.0.0.0/8. Z is within private enterprise 2, which is also using 10.0.0.0/8. As it turns out, R and Z both have IP address 10.0.1.1. L sends an offer to Z. Z, in its answer, provides L with its host candidates. In this case, those candidates are 10.0.1.1:8866 and 10.0.1.1:8877. As it turns out, R is in a session at that same time, and is also using 10.0.1.1:8866 and 10.0.1.1:8877 as host candidates. This means that R is prepared to accept STUN messages on those ports, just as Z is. L will send a STUN request to 10.0.1.1:8866 and and another to 10.0.1.1:8877. However, these do not go to Z as expected. Instead, they go to R! If R just replied to them, L would believe it has connectivity to Z, when in fact it has connectivity to a completely different user, R. To fix this, the STUN short term credential mechanisms are used. The username fragments are sufficiently random that it is highly unlikely that R would be using the same values as Z. Consequently, R would reject the STUN request since the credentials were invalid. In essence, the STUN username fragments provide a form of transient host identifiers, bound to a particular offer/answer session. An unfortunate consequence of the non-uniqueness of IP addresses is that, in the above example, R might not even be an ICE agent. It could be any host, and the port to which the STUN packet is directed could be any ephemeral port on that host. If there is an application listening on this socket for packets, and it is not prepared to handle malformed packets for whatever protocol is in use, the operation of that application could be affected. Fortunately, since the ports exchanged in SDP are ephemeral and usually drawn from the dynamic or registered range, the odds are good that the port is not used to run a server on host R, but rather is the agent side of some protocol. This decreases the probability of hitting an allocated port, due to the transient nature of port usage in this range. However, the possibility of a problem does exist, and network deployers should be prepared for it. Note that this is not a problem specific to ICE; stray packets can arrive at a port at any time for any type of protocol, especially ones on the public Internet. As such, this requirement is just restating a general design guideline for Internet applications - be prepared for unknown packets on any port. 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) 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 the pair priority being a "concatenation" of the two component priorities. This creates the desired sorting property. B.6. The remote-candidates attribute The a=remote-candidates attribute exists to eliminate a race condition between the updated offer and the response to the STUN Binding Request that moved a candidate into the Valid list. This race condition is shown in Figure 23.24. On receipt of message 4, agent L adds a candidate pair to the valid list. If there was only a single media stream with a single component, agent L could now send an updated offer. However, the check from agent R has not yet generated a response, and agent R receives the updated offer (message 7) before getting the response (message 10).9). Thus, it does not yet know that this particular pair is valid. To eliminate this condition, the actual candidates at R that were selected by the offerer (the remote candidates) are included in the offer itself. Note, however, that agent R will not send mediaitself, and the answerer delays its answer until it has received this STUN response.those pairs validate. Agent LA Network Agent RB |(1) Offer | | |------------------------------------------>| |(2) Answer | | |<------------------------------------------| |(3) STUN Req. | | |------------------------------------------>| |(4) STUN Res. | | |<------------------------------------------| |(5) STUN Req. | | |<------------------------------------------| |(6) STUN Res. | | |-------------------->| | | |Lost | |(7) Offer | | |------------------------------------------>| |(8) Answer | | |<------------------------------------------| |(9)STUN Req. | | |<------------------------------------------| |(10)|(9) STUN Res. | | |------------------------------------------>| |(10) Answer | | |<------------------------------------------| Figure 23:24: Race Condition Flow B.7. Why are Keepalives Needed? Once media begins flowing on a candidate pair, it is still necessary to keep the bindings alive at intermediate NATs for the duration of the session. Normally, the media stream packets themselves (e.g., RTP) meet this objective. However, several cases merit further discussion. Firstly, in some RTP usages, such as SIP, the media streams can be "put on hold". This is accomplished by using the SDP "sendonly" or "inactive" attributes, as defined in RFC 3264 . 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 , 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 independently of media transmission. This makes its bandwidth requirements highly predictable, and thus amenable to QoS reservations. B.8. Why Prefer Peer Reflexive Candidates? Section 4.1.2 describes procedures for computing the priority of candidate based on its type and local preferences. That section requires that the type preference for peer reflexive candidates always be lower than server reflexive. Why is that? The reason has to do with the security considerations in Section 17. 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.9. Why Send an Updated Offer? Section 11.1 describes rules for sending media. Both agents can send media once ICE checks complete, without waiting for an updated offer. Indeed, the only purpose of the updated offer is to "correct" the SDP so that the default destination for media matches where media is being sent based on ICE procedures (which will be the highest priority nominated candidate pair). This begs the question - why is the updated offer/answer exchange needed at all? Indeed, in a pure offer/answer environment, it would not be. The offerer and answerer will agree on the candidates to use through ICE, and then can begin using them. As far as the agents themselves are concerned, the updated offer/answer provides no new information. However, in practice, numerous components along the signaling path look at the SDP information. These include entities performing off-path QoS reservations, NAT traversal components such as ALGs and Session Border Controllers (SBCs) and diagnostic tools that passively monitor the network. For these tools to continue to function without change, the core property of SDP - that the existing, pre-ICE definitions of the addresses used for media - the m and c lines and the rtcp attribute - must be retained. For this reason, an updated offer must be sent. B.10. Why are Binding Indications Used for Keepalives? Media keepalives are described in Section 10. These keepalives make use of STUN when both endpoints are ICE capable. However, rather than using a Binding Request transaction (which generates a response), the keepalives use an Indication. Why is that? The primary reason has to do with network QoS mechanisms. Once media begins flowing, network elements will assume that the media stream has a fairly regular structure, making use of periodic packets at fixed intervals, with the possibility of jitter. If an agent is sending media packets, and then receives a Binding Request, it would need to generate a response packet along with its media packets. This will increase the actual bandwidth requirements for the 5-tuple carrying the media packets, and introduce jitter in the delivery of those packets. Analysis has shown that this is a concern in certain layer 2 access networks that use fairly tight packet schedulers for media. Additionally, using a Binding Indication allows integrity to be disabled, allowing for better performance. This is useful for large scale endpoints, such as PSTN gateways and SBCs. Author's Address Jonathan Rosenberg Cisco Edison, NJ US Phone: +1 973 952-5000 Email: firstname.lastname@example.org URI: http://www.jdrosen.net Full Copyright Statement Copyright (C) The IETF Trust (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. 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