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Run idnits with the --verbose option for more detailed information about the items above. -------------------------------------------------------------------------------- 2 Network Working Group E. Ivov 3 Internet-Draft Jitsi 4 Intended status: Standards Track E. Rescorla 5 Expires: August 11, 2014 RTFM, Inc. 6 J. Uberti 7 Google 8 February 7, 2014 10 Trickle ICE: Incremental Provisioning of Candidates for the Interactive 11 Connectivity Establishment (ICE) Protocol 12 draft-ietf-mmusic-trickle-ice-01 14 Abstract 16 This document describes an extension to the Interactive Connectivity 17 Establishment (ICE) protocol that allows ICE agents to send and 18 receive candidates incrementally rather than exchanging complete 19 lists. With such incremental provisioning, ICE agents can begin 20 connectivity checks while they are still gathering candidates and 21 considerably shorten the time necessary for ICE processing to 22 complete. 24 The above mechanism is also referred to as "trickle ICE". 26 Status of This Memo 28 This Internet-Draft is submitted in full conformance with the 29 provisions of BCP 78 and BCP 79. 31 Internet-Drafts are working documents of the Internet Engineering 32 Task Force (IETF). Note that other groups may also distribute 33 working documents as Internet-Drafts. The list of current Internet- 34 Drafts is at http://datatracker.ietf.org/drafts/current/. 36 Internet-Drafts are draft documents valid for a maximum of six months 37 and may be updated, replaced, or obsoleted by other documents at any 38 time. It is inappropriate to use Internet-Drafts as reference 39 material or to cite them other than as "work in progress." 41 This Internet-Draft will expire on August 11, 2014. 43 Copyright Notice 45 Copyright (c) 2014 IETF Trust and the persons identified as the 46 document authors. All rights reserved. 48 This document is subject to BCP 78 and the IETF Trust's Legal 49 Provisions Relating to IETF Documents 50 (http://trustee.ietf.org/license-info) in effect on the date of 51 publication of this document. Please review these documents 52 carefully, as they describe your rights and restrictions with respect 53 to this document. Code Components extracted from this document must 54 include Simplified BSD License text as described in Section 4.e of 55 the Trust Legal Provisions and are provided without warranty as 56 described in the Simplified BSD License. 58 Table of Contents 60 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3 61 2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 4 62 3. Incompatibility with Standard ICE . . . . . . . . . . . . . . 5 63 4. Determining Support for Trickle ICE . . . . . . . . . . . . . 6 64 4.1. Unilateral Use of Trickle ICE (Half Trickle) . . . . . . 7 65 5. Sending the Initial Offer . . . . . . . . . . . . . . . . . . 8 66 5.1. Encoding the SDP . . . . . . . . . . . . . . . . . . . . 9 67 6. Receiving the Initial Offer . . . . . . . . . . . . . . . . . 9 68 6.1. Sending the Initial Answer . . . . . . . . . . . . . . . 10 69 6.2. Forming check lists and beginning connectivity 70 checks . . . . . . . . . . . . . . . . . . . . . . . . . 10 71 6.3. Encoding the SDP . . . . . . . . . . . . . . . . . . . . 11 72 7. Receiving the Initial Answer . . . . . . . . . . . . . . . . 11 73 8. Performing Connectivity Checks . . . . . . . . . . . . . . . 11 74 8.1. Check List and Timer State Updates . . . . . . . . . . . 11 75 9. Discovering and Sending Additional Local Candidates . . . . . 12 76 9.1. Pairing newly learned candidates and updating 77 check lists . . . . . . . . . . . . . . . . . . . . . . . 14 78 9.2. Encoding the SDP for Additional Candidates . . . . . . . 15 79 9.3. Announcing End of Candidates . . . . . . . . . . . . . . 15 80 10. Receiving Additional Remote Candidates . . . . . . . . . . . 17 81 11. Receiving an End Of Candidates Notification . . . . . . . . . 17 82 12. Trickle ICE and Peer Reflexive Candidates . . . . . . . . . . 17 83 13. Concluding ICE Processing . . . . . . . . . . . . . . . . . . 18 84 14. Subsequent Offer/Answer Exchanges . . . . . . . . . . . . . . 18 85 15. Interaction with ICE Lite . . . . . . . . . . . . . . . . . . 18 86 16. Example Flow . . . . . . . . . . . . . . . . . . . . . . . . 19 87 17. Security Considerations . . . . . . . . . . . . . . . . . . . 20 88 18. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 20 89 19. References . . . . . . . . . . . . . . . . . . . . . . . . . 20 90 19.1. Normative References . . . . . . . . . . . . . . . . . . 20 91 19.2. Informative References . . . . . . . . . . . . . . . . . 21 92 Appendix A. Open issues . . . . . . . . . . . . . . . . . . . . 22 93 A.1. MID/Stream Indices in SDP . . . . . . . . . . . . . . . . 22 94 A.2. Starting checks . . . . . . . . . . . . . . . . . . . . . 23 95 Appendix B. Changes From Earlier Versions . . . . . . . . . . . 23 96 B.1. Changes From draft-ivov-01 and draft-mmusic-00 . . . . . 23 97 B.2. Changes From draft-ivov-00 . . . . . . . . . . . . . . . 23 98 B.3. Changes From draft-rescorla-01 . . . . . . . . . . . . . 24 99 B.4. Changes From draft-rescorla-00 . . . . . . . . . . . . . 25 100 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 25 102 1. Introduction 104 The Interactive Connectivity Establishment (ICE) protocol [RFC5245] 105 describes mechanisms for gathering, candidates, prioritizing them, 106 choosing default ones, exchanging them with the remote party, pairing 107 them and ordering them into check lists. Once all of the above have 108 been completed, and only then, the participating agents can begin a 109 phase of connectivity checks and eventually select the pair of 110 candidates that will be used in the following session. 112 While the above sequence has the advantage of being relatively 113 straightforward to implement and debug once deployed, it may also 114 prove to be rather lengthy. Gathering candidates or candidate 115 harvesting would often involve things like querying STUN [RFC5389] 116 servers, discovering UPnP devices, and allocating relayed candidates 117 at TURN [RFC5766] servers. All of these can be delayed for a 118 noticeable amount of time and while they can be run in parallel, they 119 still need to respect the pacing requirements from [RFC5245], which 120 is likely to delay them even further. Some or all of the above would 121 also have to be completed by the remote agent. Both agents would 122 next perform connectivity checks and only then would they be ready to 123 begin streaming media. 125 All of the above could lead to relatively lengthy session 126 establishment times and degraded user experience. 128 The purpose of this document is to define an alternative mode of 129 operation for ICE implementations, also known as "trickle ICE", where 130 candidates can be exchanged incrementally. This would allow ICE 131 agents to exchange host candidates as soon as a session has been 132 initiated. Connectivity checks for a media stream would also start 133 as soon as the first candidates for that stream have become 134 available. 136 Trickle ICE allows reducing session establishment times in cases 137 where connectivity is confirmed for the first exchanged candidates 138 (e.g. where the host candidates for one of the agents are directly 139 reachable from the second agent). Even when this is not the case, 140 running candidate harvesting for both agents and connectivity checks 141 all in parallel allows to considerably reduce ICE processing times. 143 It is worth pointing out that before being introduced to the IETF, 144 trickle ICE had already been included in specifications such as XMPP 145 Jingle [XEP-0176] and it has been in use in various implementations 146 and deployments. 148 In addition to the basics of trickle ICE, this document also 149 describes how support for trickle ICE needs to be discovered, how 150 regular ICE processing needs to be modified when building and 151 updating check lists, and how trickle ICE implementations should 152 interoperate with agents that only implement [RFC5245] processing. 154 This specification does not define usage of trickle ICE with any 155 specific signalling protocol, contrary to [RFC5245] which contains a 156 usage for ICE with SIP. Such usages would have to be specified in 157 separate documents such as for example 158 [I-D.ivov-mmusic-trickle-ice-sip]. 160 Trickle ICE does however reuse and build upon the SDP syntax defined 161 by vanilla ICE. 163 2. Terminology 165 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 166 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this 167 document are to be interpreted as described in [RFC2119]. 169 This specification makes use of all terminology defined by the 170 protocol for Interactive Connectivity Establishment in [RFC5245]. 172 Vanilla ICE: The Interactive Connectivity Establishment protocol as 173 defined in [RFC5245]. 175 Candidate Harvester: A module used by an ICE agent to obtain local 176 candidates. Candidate harvesters use different mechanisms for 177 discovering local candidates. Some of them would typically make 178 use of protocols such as STUN or TURN. Others may also employ 179 techniques that are not referenced within [RFC5245]. UPnP based 180 port allocation and XMPP Jingle Relay Nodes [XEP-0278] are among 181 the possible examples. 183 Trickled Candidates: Candidates that a trickle ICE agent is sending 184 subsequently to but within the context defined by an offer or an 185 answer. Trickled candidates can be sent in parallel with 186 candidate harvesting and connectivity checks. 188 Trickling/Trickle (v.): The act of sending trickled candidates. 190 Half Trickle: A trickle ICE mode of operation where the offerer 191 gathers its first generation of candidates strictly before 192 creating and sending the offer. Once sent, that offer can be 193 processed by vanilla ICE agents and does not require support for 194 this specification. It also allows trickle ICE capable answerers 195 to still gather candidates and perform connectivity checks in a 196 non-blocking way, thus roughly offering "half" the advantages of 197 trickle ICE. The mechanism is mostly meant for use in cases where 198 support for trickle ICE cannot be confirmed prior to sending a 199 first offer. 201 Full Trickle: Regular mode of operation for trickle ICE agents, used 202 in opposition to the half trickle mode of operation. 204 3. Incompatibility with Standard ICE 206 The ICE protocol was designed to be fairly flexible so that it would 207 work in and adapt to as many network environments as possible. It is 208 hence important to point out at least some of the reasons why, 209 despite its flexibility, the specification in [RFC5245] would not 210 support trickle ICE. 212 [RFC5245] describes the conditions required to update check lists and 213 timer states while an ICE agent is in the Running state. These 214 conditions are verified upon transaction completion and one of them 215 stipulates that: 217 If there is not a pair in the valid list for each component of the 218 media stream, the state of the check list is set to Failed. 220 This could be a problem and cause ICE processing to fail prematurely 221 in a number of scenarios. Consider the following case: 223 o Alice and Bob are both located in different networks with Network 224 Address Translation (NAT). Alice and Bob themselves have 225 different address but both networks use the same [RFC1918] block. 227 o Alice sends Bob the candidate 10.0.0.10 which also happens to 228 correspond to an existing host on Bob's network. 230 o Bob creates a check list consisting solely of 10.0.0.10 and starts 231 checks. 233 o These checks reach the host at 10.0.0.10 in Bob's network, which 234 responds with an ICMP "port unreachable" error and per [RFC5245] 235 Bob marks the transaction as Failed. 237 At this point the check list only contains Failed candidates and the 238 valid list is empty. This causes the media stream and potentially 239 all ICE processing to Fail. 241 A similar race condition would occur if the initial offer from Alice 242 only contains candidates that can be determined as unreachable (per 243 [I-D.keranen-mmusic-ice-address-selection]) from any of the 244 candidates that Bob has gathered. This would be the case if Bob's 245 candidates only contain IPv4 addresses and the first candidate that 246 he receives from Alice is an IPv6 one. 248 Another potential problem could arise when a non-trickle ICE 249 implementation sends an offer to a trickle one. Consider the 250 following case: 252 o Alice's client has a non-trickle ICE implementation 254 o Bob's client has support for trickle ICE. 256 o Alice and Bob are behind NATs with address-dependent filtering 257 [RFC4787]. 259 o Bob has two STUN servers but one of them is currently unreachable 261 After Bob's agent receives Alice's offer it would immediately start 262 connectivity checks. It would also start gathering candidates, which 263 would take long because of the unreachable STUN server. By the time 264 Bob's answer is ready and sent to Alice, Bob's connectivity checks 265 may well have failed: until Alice gets Bob's answer, she won't be 266 able to start connectivity checks and punch holes in her NAT. The 267 NAT would hence be filtering Bob's checks as originating from an 268 unknown endpoint. 270 4. Determining Support for Trickle ICE 272 According to [RFC5245] every time an agent supporting trickle ICE 273 generates an offer or an answer, it MUST include the "trickle" token 274 in the ice-options attribute. Syntax for this token is defined in 275 Section 5.1. 277 Additionally, in order to avoid interoperability problems such as 278 those described in Section 3, it is important that trickle ICE 279 negotiation is only attempted in cases where the remote party 280 actually supports this specification. Agents that receive offers or 281 answers can verify support by examining them for the "trickle" ice- 282 options token. However, agents that are about to send a first offer, 283 have no immediate way of doing this. This means that usages of 284 trickle for specific protocols would need to either: 286 o Provide a way for agents to verify support of trickle ICE prior to 287 initiating a session. XMPP's Service discovery [XEP-0030] is an 288 example for one such mechanism; 290 o Make support for trickle ICE mandatory so that support could be 291 assumed the agents. 293 Alternately, for cases where a protocol provides neither of the 294 above, agents may either rely on provisioning/configuration, or use 295 the half trickle procedure described in Section 4.1. 297 Note that out-of-band discovery semantics and half trickle are only 298 necessary prior to session initiation, or in other words, when 299 sending the initial offer. Once a session is established and trickle 300 ICE support is confirmed for both parties, either agent can use full 301 trickle for subsequent offers. 303 4.1. Unilateral Use of Trickle ICE (Half Trickle) 305 The idea of using half trickle is about having the caller send a 306 regular, vanilla ICE offer, with a complete set of candidates. This 307 offer still indicates support for trickle ice, so the answerer is 308 able to respond with an incomplete set of candidates and continue 309 trickling the rest. Half trickle offers will typically contain an 310 end-of-candidates indication. 312 The mechanism can be used in cases where there is no way for an agent 313 to verify in advance whether a remote party supports trickle ice. 314 Because it contains a full set of candidates, its first offer can 315 thus be handled by a regular vanilla ICE agent, while still allowing 316 a trickle one to use the optimisation defined in this specification. 317 This prevents negotiation from failing in the former case while still 318 giving roughly half the trickle ICE benefits in the latter (hence the 319 name of the mechanism). 321 Use of half trickle is only necessary during an initial offer/answer 322 exchange. Once both parties have received a session description from 323 their peer, they can each reliably determine trickle ICE support and 324 use it for all subsequent offer/answer exchanges. 326 It is worth pointing out that using half trickle may actually bring 327 more than just half the improvement in terms of user experience. 328 This can happen in cases where an agent starts gathering candidates 329 upon user interface cues that a call is pending, such as activity on 330 a keypad or the phone going off hook. This would mean a part or all 331 candidate harvesting could have completed before the agent actually 332 needs to send the offer. Given that the answerer will be able to 333 trickle candidates, both agents will be able to start connectivity 334 checks and complete ICE processing earlier than with vanilla ICE and 335 potentially even as early as with full trickle. 337 However, such anticipation is not not always possible. For example, 338 a multipurpose user agent or a WebRTC web page where communication is 339 a non-central feature (e.g. calling a support line in case of a 340 problem with the main features) would not necessarily have a way of 341 distinguishing between call intentions and other user activity. 342 Still, even in these cases, using half trickle would be an 343 improvement over vanilla ICE as it would optimize performance for 344 answerers. 346 5. Sending the Initial Offer 348 An agent starts gathering candidates as soon as it has an indication 349 that communication is imminent (e.g. a user interface cue or an 350 explicit request to initiate a session). Contrary to vanilla ICE, 351 implementations of trickle ICE do not need to gather candidates in a 352 blocking manner. Therefore, unless half trickle is being used, 353 agents SHOULD generate and transmit their initial offer as early as 354 possible, in order to allow the remote party to start gathering and 355 trickling candidates. 357 Trickle ICE agents MAY include any set of candidates in an offer. 358 This includes the possibility of generating one with no candidates, 359 or one that contains all the candidates that the agent is planning on 360 using in the following session. 362 For optimal performance, it is RECOMMENDED that an initial offer 363 contains host candidates only. This would allow both agents to start 364 gathering server reflexive, relayed and other non-host candidates 365 simultaneously, and it would also enable them to begin connectivity 366 checks. 368 If the privacy implications of revealing host addresses are a 369 concern, agents MAY generate an offer that contains no candidates and 370 then only trickle candidates that do not reveal host addresses (e.g. 371 relayed candidates). 373 Prior to actually sending an initial offer, agents MAY verify if the 374 remote party supports trickle ICE, where such mechanisms actually 375 exist. If absence of such support is confirmed agents MUST fall back 376 to using vanilla ICE or abandon the entire session. 378 All trickle ICE offers and answers MUST indicate support of this 379 specification, as explained in Section 5.1. 381 Calculating priorities and foundations, as well as determining 382 redundancy of candidates work the same way they do with vanilla ICE. 384 5.1. Encoding the SDP 386 The process of encoding the SDP [RFC4566] is mostly the same as the 387 one used by vanilla ICE. Still, trickle ICE does require a few 388 differences described here. 390 Agents MUST indicate support for Trickle ICE by including the 391 "trickle" token for the "a=ice-options" attribute: 393 a=ice-options:trickle 395 As mentioned earlier in this section, Offers and Answers can contain 396 any set of candidates, which means that a trickle ICE session 397 description MAY contain no candidates at all. In such cases the 398 agent would still need to place an address in the "c=" line(s). If 399 the use of a host address there is undesirable (e.g. for privacy 400 reasons), the agent MAY set the connection address to IP6 ::. In this 401 case it MUST also set the port number to 9 (Discard). There is no 402 need to include a fictitious candidate for the IP6 :: address when 403 doing so. 405 It is worth noting that the use of IP6 :: has been selected over IP4 406 0.0.0.0, even though [RFC3264] already gives the latter semantics 407 appropriate for such use. The reason for this choice is the historic 408 use of 0.0.0.0 as a means of putting a stream on hold [RFC2543] and 409 the ambiguity that this may cause with legacy libraries and 410 applications. 412 It is also worth mentioning that use of IP6 :: here does not 413 constitute any kind of indication as to the actual use of IPv6 414 candidates in a session and it can very well appear in a negotiation 415 that only involves IPv4 candidates. 417 6. Receiving the Initial Offer 419 When an agent receives an initial offer, it will first check if it 420 indicates support for trickle ICE as explained in Section 4. If this 421 is not the case, the agent MUST process the offer according to the 422 [RFC5245] procedures or standard [RFC3264] processing in case no ICE 423 support is detected at all. 425 It is worth pointing out that in case support for trickle ICE is 426 confirmed, an agent will automatically assume support for vanilla ICE 427 as well even if the support verification procedure in [RFC5245] 428 indicates otherwise. Specifically, such verification would indicate 429 lack of support when the offer contains no candidates. The IP6 :: 430 address present in the c= line in that case would not "appear in a 431 candidate attribute". Obviously, a fallback to [RFC3264] is not 432 required when this happens. 434 If, the offer does indicate support for trickle ICE, the agent will 435 determine its role, start gathering and prioritizing candidates and, 436 while doing so it will also respond by sending its own answer, so 437 that both agents can start forming check lists and begin connectivity 438 checks. 440 6.1. Sending the Initial Answer 442 An agent can respond to an initial offer at any point while gathering 443 candidates. The answer can again contain any set of candidates 444 including none or all of them. Unless it is protecting host 445 addresses for privacy reasons, the agent would typically construct 446 this initial answer including only them, thus allowing the remote 447 party to also start forming checklists and performing connectivity 448 checks. 450 The answer MUST indicate support for trickle ICE as described by 451 Section 4. 453 6.2. Forming check lists and beginning connectivity checks 455 After exchanging offer and answer, and as soon as they have obtained 456 local and remote candidates, agents will begin forming candidate 457 pairs, computing their priorities and creating check lists according 458 to the vanilla ICE procedures described in [RFC5245]. Obviously in 459 order for candidate pairing to be possible, it would be necessary 460 that both the offer and the answer contained candidates. If this was 461 not the case agents will still create the check lists (so that their 462 Active/Frozen state could be monitored and updated) but they will 463 only populate them once they actually have the candidate pairs. 465 Initially, all check lists will have their Active/Frozen state set to 466 Frozen. 468 Trickle ICE agents will then inspect the first check list and attempt 469 to unfreeze all candidates belonging to the first component on the 470 first media stream (i.e. the first media stream that was reported to 471 the ICE implementation from the using application). If this 472 checklist is still empty however, agents will hold off further 473 processing until this is no longer the case. 475 Respecting the order in which lists have been reported to an ICE 476 implementation, or in other words, the order in which they appear in 477 SDP, is crucial to the frozen candidates algorithm and important when 478 making sure that connectivity checks are performed simultaneously by 479 both agents. 481 6.3. Encoding the SDP 483 The process for encoding the SDP at the answerer is identical to the 484 process followed by the offerer for both full and lite 485 implementations, as described in Section 5.1. 487 7. Receiving the Initial Answer 489 When receiving an answer, agents will follow vanilla ICE procedures 490 to determine their role and they would then form check lists (as 491 described in Section 6.2) and begin connectivity checks . 493 8. Performing Connectivity Checks 495 For the most part, trickle ICE agents perform connectivity checks 496 following vanilla ICE procedures. Of course, the asynchronous nature 497 of candidate harvesting in trickle ICE would impose a number of 498 changes described here. 500 8.1. Check List and Timer State Updates 502 The vanilla ICE specification requires that agents update check lists 503 and timer states upon completing a connectivity check transaction. 504 During such an update vanilla ICE agents would set the state of a 505 check list to Failed if the following two conditions are satisfied: 507 o all of the pairs in the check list are either in the Failed or 508 Succeeded state; 510 o if at least one of the components of the media stream has no pairs 511 in its valid list. 513 With trickle ICE, the above situation would often occur when 514 candidate harvesting and trickling are still in progress and it is 515 perfectly possible that future checks will succeed. For this reason 516 trickle ICE agents add the following conditions to the above list: 518 o all candidate harvesters have completed and the agent is not 519 expecting to discover any new local candidates; 521 o the remote agent has sent an end-of-candidates indication for that 522 check list as described in Section 9.3. 524 Vanilla ICE requires that agents then update all other check lists, 525 placing one pair in each of them into the Waiting state, effectively 526 unfreezing all remaining check lists. Given that with trickle ICE, 527 other check lists may still be empty at that point, a trickle ICE 528 agent SHOULD also maintain an explicit Active/Frozen state for every 529 check list, rather than deducing it from the state of the pairs it 530 contains. This state should be set to Active when unfreezing the 531 first pair in a list or when that couldn't happen because a list was 532 empty. 534 9. Discovering and Sending Additional Local Candidates 536 After an offer or an answer have been sent, agents will most likely 537 continue discovering new local candidates as STUN, TURN and other 538 non-host candidate harvesting mechanisms begin to yield results. 539 Whenever an agent discovers such a new candidate it will compute its 540 priority, type, foundation and component id according to normal 541 vanilla ICE procedures. 543 The new candidate is then checked for redundancy against the existing 544 list of local candidates. If its transport address and base match 545 those of an existing candidate, it will be considered redundant and 546 will be ignored. This would often happen for server reflexive 547 candidates that match the host addresses they were obtained from 548 (e.g. when the latter are public IPv4 addresses). Contrary to 549 vanilla ICE, trickle ICE agents will consider the new candidate 550 redundant regardless of its priority. 552 Next the client sends (i.e. trickles) the newly learnt candidate(s) 553 to the remote agent. The actual delivery of the new candidates will 554 be specified by using protocols such as SIP. Trickle ICE imposes no 555 restrictions on the way this is done or whether it is done at all. 556 For example, some applications may choose not to send trickle updates 557 for server reflexive candidates and rely on the discovery of peer 558 reflexive ones instead. 560 When trickle updates are sent however, each candidate MUST be 561 delivered to the receiving Trickle ICE implementation not more than 562 once and in the same order that they were sent. In other words, if 563 there are any candidate retransmissions, they must be hidden from the 564 ICE implementation. 566 Also, candidate trickling needs to be correlated to a specific ICE 567 negotiation session, so that if there is an ICE restart, any delayed 568 updates for a previous session can be recognized as such and ignored 569 by the receiving party. 571 One important aspect of Vanilla ICE is that connectivity checks for a 572 specific foundation and component be attempted simultaneously by both 573 agents, so that any firewalls or NATs fronting the agents would 574 whitelist both endpoints and allow all except for the first (suicide) 575 packets to go through. This is also crucial to unfreezing candidates 576 in the right time. 578 In order to preserve this feature here, when trickling candidates 579 agents MUST respect the order of the components as they appear 580 (implicitly or explicitly) in the Offer/Answer descriptions. 581 Therefore a candidate for a specific component MUST NOT be sent prior 582 to candidates for other components within the same foundation. 584 For example, the following session description contains two 585 components (RTP and RTCP), and two foundations (host and the server 586 reflexive): 588 v=0 589 o=jdoe 2890844526 2890842807 IN IP4 10.0.1.1 590 s= 591 c=IN IP4 10.0.1.1 592 t=0 0 593 a=ice-pwd:asd88fgpdd777uzjYhagZg 594 a=ice-ufrag:8hhY 595 m=audio 5000 RTP/AVP 0 596 a=rtpmap:0 PCMU/8000 597 a=candidate:1 1 UDP 2130706431 10.0.1.1 5000 typ host 598 a=candidate:1 2 UDP 2130706431 10.0.1.1 5001 typ host 599 a=candidate:2 1 UDP 1694498815 192.0.2.3 5000 typ srflx 600 raddr 10.0.1.1 rport 8998 601 a=candidate:2 2 UDP 1694498815 192.0.2.3 5001 typ srflx 602 raddr 10.0.1.1 rport 8998 604 For this description the RTCP host candidate MUST NOT be sent prior 605 to the RTP host candidate. Similarly the RTP server reflexive 606 candidate MUST be sent together with or prior to the RTCP server 607 reflexive candidate. 609 Note that the order restriction only applies among candidates that 610 belong to the same foundation. 612 It is also equally important to preserve this order across media 613 streams and this is covered by the requirement to always start 614 unfreezing candidates starting from the first media stream 615 Section 6.2. 617 Once the candidate has been sent to the remote party, the agent 618 checks if any remote candidates are currently known for this same 619 stream. If this is not the case the new candidate will simply be 620 added to the list of local candidates. 622 Otherwise, if the agent has already learned of one or more remote 623 candidates for this stream and component, it will begin pairing the 624 new local candidates with them and adding the pairs to the existing 625 check lists according to their priority. 627 9.1. Pairing newly learned candidates and updating check lists 629 Forming candidate pairs will work the way it is described by the 630 vanilla ICE specification. Actually adding the new pair to a check 631 list however, will happen according to the rules described below. 633 If the check list where the pair is to be added already contains the 634 maximum number of candidate pairs (100 by default as per [RFC5245]), 635 the new pair is discarded. 637 If the new pair's local candidate is server reflexive, the server 638 reflexive candidate MUST be replaced by its base before adding the 639 pair to the list. Once this is done, the agent examines the check 640 list looking for another pair that would be redundant with the new 641 one. If such a pair exists, the newly formed pair is ignored. 643 For all other pairs, including those with a server reflexive local 644 candidate that were not found to be redundant: 646 o if this check list is Frozen then the new pair will also be 647 assigned a Frozen state. 649 o else if the check list is Active and it is either empty or 650 contains only candidates in the Succeeded and Failed states, then 651 the new pair's state is set to Waiting. 653 o else if the check list is non-empty and Active, then the new pair 654 state will be set to 656 Frozen: if there is at least one pair in the list whose 657 foundation matches the one in the new pair and whose state is 658 neither Succeeded nor Failed (eventually the new pair will get 659 unfrozen after the the on-going check for the existing pair 660 concludes); 662 Waiting: if the list contains no pairs with the same foundation 663 as the new one, or, in case such pairs exist but they are all 664 in either the Succeeded or Failed states. 666 9.2. Encoding the SDP for Additional Candidates 668 To facilitate interoperability an ICE agent will encode additional 669 candidates using the vanilla ICE SDP syntax. For example: 671 a=candidate:2 1 UDP 1658497328 198.51.100.33 5000 typ host 673 Given that such lines do not provide a relationship between the 674 candidate and the m line that it relates to, signalling protocols 675 using trickle ICE MUST establish that relation themselves using an 676 MID [RFC3388]. Such MIDs use "media stream identification", as 677 defined in [RFC3388], to identify a corresponding m-line. When 678 creating candidate lines usages of trickle ICE MUST use the MID if 679 possible, or the m-line index if not. Obviously, agents MUST NOT 680 send individual candidates prior to generating the corresponding SDP 681 session description. 683 The exact means of transporting additional candidates to a remote 684 agent is left to the protocols using trickle ICE. It is important to 685 note, however, that these candidate exchanges are not part of the 686 offer/answer model. 688 9.3. Announcing End of Candidates 690 Once all candidate harvesters for a specific media stream complete, 691 or expire, the agents will generate an "end-of-candidates" indication 692 for that stream and send it to the remote agent via the signalling 693 channel. Such indications are sent in the form of a media-level 694 attribute that has the following form: end-of-candidates. 696 a=end-of-candidates 698 The end-of-candidates indications can be sent as part of an offer, 699 which would typically be the case with half trickle initial offers, 700 they can accompany the last candidate an agent can send for a stream, 701 and they can also be sent alone (e.g. after STUN Binding requests or 702 TURN Allocate requests to a server timeout and the agent has no other 703 active harvesters). 705 Controlled trickle ICE agents SHOULD always send end-of-candidates 706 indications once harvesting for a media stream has completed unless 707 ICE processing terminates before they've had a chance to do so. 708 Sending the indication is necessary in order to avoid ambiguities and 709 speed up ICE conclusion. This is necessary in order to avoid 710 ambiguities and speed up ICE conclusion. Controlling agents on the 711 other hand MAY sometimes conclude ICE processing prior to sending 712 end-of-candidates notifications for all streams. This would 713 typically be the case with aggressive nomination. Yet it is 714 RECOMMENDED that controlling agents do send such indications whenever 715 possible for the sake of consistency and keeping middle boxes and 716 controlled agents up-to-date on the state of ICE processing. 718 When sending end-of-candidates during trickling, rather than as a 719 part of an offer or an answer, it is the responsibility of the using 720 protocol to define means that can be used to relate the indication to 721 one or more specific m-lines. 723 Receiving an end-of-candidates notification allows an agent to update 724 check list states and, in case valid pairs do not exist for every 725 component in every media stream, determine that ICE processing has 726 failed. It also allows agents to speed ICE conclusion in cases where 727 a candidate pair has been validates but it involves the use of lower- 728 preference transports such as TURN. In such situations some 729 implementations may choose to wait in case higher-priority candidates 730 are received and end-of-candidates provides an indication that this 731 is not going to happen. 733 An agent MAY also choose to generate an end-of-candidates event 734 before candidate harvesting has actually completed, if the agent 735 determines that harvesting has continued for more than an acceptable 736 period of time. However, an agent MUST NOT send any more candidates 737 after it has send an end-of-candidates notification. 739 When performing half trickle agents SHOULD send end-of-candidates 740 together with their initial offer unless they are planning on 741 potentially sending additional candidates in case the remote party 742 turns out to actually support trickle ICE. 744 When end-of-candidates is sent as part of an offer or an answer it 745 can appear as a session-level attribute, which would be equivalent to 746 having it appear in all m-lines. 748 Once an agent sends the end-of-candidates event, it will update the 749 state of the corresponding check list as explained in section 750 Section 8.1. Past that point agents MUST NOT send any new 751 candidates. Once an agent has received an end-of-candidates 752 indication, it MUST also ignore any newly received candidates for 753 that media stream. Adding new candidates to the negotiation is hence 754 only possible through an ICE restart. 756 It is important to note that This specification does not override 757 vanilla ICE semantics for concluding ICE processing. This means that 758 even if end-of-candidates indications are sent agents will still have 759 to go through pair nomination. Also, if pairs have been nominated 760 for components and media streams, ICE processing will still conclude 761 even if end-of-candidate indications have not been received for all 762 streams. 764 10. Receiving Additional Remote Candidates 766 At any point of ICE processing, a trickle ICE agent may receive new 767 candidates from the remote agent. When this happens and no local 768 candidates are currently known for this same stream, the new remote 769 candidates are simply added to the list of remote candidates. 771 Otherwise, the new candidates are used for forming candidate pairs 772 with the pool of local candidates and they are added to the local 773 check lists as described in Section 9.1. 775 Once the remote agent has completed candidate harvesting, it will 776 send an end-of-candidates event. Upon receiving such an event, the 777 local agent MUST update check list states as per Section 8.1. This 778 may lead to some check lists being marked as Failed. 780 11. Receiving an End Of Candidates Notification 782 When an agent receives an end-of-candidates notification for a 783 specific check list, they will update its state as per Section 8.1. 784 In case the list is still in the Active state after the update, the 785 agent will persist the the fact that an end-of-candidates 786 notification has been received for and take it into account in future 787 list updates. 789 12. Trickle ICE and Peer Reflexive Candidates 791 Even though Trickle ICE does not explicitly modify the procedures for 792 handling peer reflexive candidates, their processing could be 793 impacted in implementations. With Trickle ICE, it is possible that 794 server reflexive candidates be discovered as peer reflexive in cases 795 where incoming connectivity checks are received from these candidates 796 before the trickle updates that carry them. 798 While this would certainly increase the number of cases where ICE 799 processing nominates and selects candidates discovered as peer- 800 reflexive it does not require any change in processing. 802 It is also likely that, some applications would prefer not to trickle 803 server reflexive candidates to entities that are known to be publicly 804 accessible and where sending a direct STUN binding request is likely 805 to reach the destination faster than the trickle update that travels 806 through the signalling path. 808 13. Concluding ICE Processing 810 This specification does not directly modify the procedures ending ICE 811 processing described in Section 8 of [RFC5245], and trickle ICE 812 implementations will follow the same rules. 814 14. Subsequent Offer/Answer Exchanges 816 Either agent MAY generate a subsequent offer at any time allowed by 817 [RFC3264]. When this happens agents will use [RFC5245] semantics to 818 determine whether or not the new offer requires an ICE restart. If 819 this is the case then agents would perform trickle ICE as they would 820 in an initial offer/answer exchange. 822 The only differences between an ICE restart and a brand new media 823 session are that: 825 o during the restart, media can continue to be sent to the 826 previously validated pair. 828 o both agents are already aware whether or not their peer supports 829 trickle ICE, and there is no longer need for performing half 830 trickle or confirming support with other mechanisms. 832 15. Interaction with ICE Lite 834 Behaviour of Trickle ICE capable ICE lite agents does not require any 835 particular rules other than those already defined in this 836 specification and [RFC5245]. This section is hence added with an 837 informational purpose only. 839 A Trickle ICE capable ICE Lite agent would generate offers or answers 840 as per [RFC5245]. Both will indicate support for trickle ICE 841 (Section 5.1) and given that they will contain a complete set of 842 candidates (the agent's host candidates) these offers and answers 843 would also be accompanied with an end-of-candidates notification. 845 When performing full trickle, a full ICE implementation could send an 846 offer or an answer with no candidates and an IP6 :: connection line 847 address. After receiving an answer that identifies the remote agent 848 as an ICE lite implementation, the offerer may very well choose to 849 not send any additional candidates. The same is also true in the 850 case when the ICE lite agent is making the offer and the full ICE one 851 is answering. In these cases the connectivity checks would be enough 852 for the ICE lite implementation to discover all potentially useful 853 candidates as peer reflexive. The following example illustrates one 854 such ICE session: 856 ICE Lite Bob 857 Agent 858 | Offer (a=ice-lite a=ice-options:trickle) | 859 |---------------------------------------------->| 860 | |no cand 861 | Answer (a=ice-options:trickle) |trickling 862 |<----------------------------------------------| 863 | Connectivity Checks | 864 |<--------------------------------------------->| 865 peer rflx| | 866 cand disco| | 867 | | 868 |<=============== MEDIA FLOWS =================>| 870 Figure 1: Example 872 In addition to reducing signaling traffic this approach also removes 873 the need to discover STUN bindings, or to make TURN or UPnP 874 allocations which may considerably lighten ICE processing. 876 16. Example Flow 878 A typical successful trickle ICE exchange with an Offer/Answer 879 protocol would look this way: 881 Alice Bob 882 | Offer | 883 |---------------------------------------------->| 884 | Additional Candidates | 885 |---------------------------------------------->| 886 | | 887 | Answer | 888 |<----------------------------------------------| 889 | Additional Candidates | 890 |<----------------------------------------------| 891 | | 892 | Additional Candidates and Connectivity Checks | 893 |<--------------------------------------------->| 894 | | 895 |<=============== MEDIA FLOWS =================>| 897 Figure 2: Example 899 17. Security Considerations 901 This specification inherits most of its semantics from [RFC5245] and 902 as a result all security considerations described there remain the 903 same. 905 18. Acknowledgements 907 The authors would like to thank Bernard Adoba, Christer Holmberg, 908 Dale R. Worley, Enrico Marocco, Flemming Andreasen, Jonathan Lennox 909 and Martin Thomson for their reviews and suggestions on improving 910 this document. 912 19. References 914 19.1. Normative References 916 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 917 Requirement Levels", BCP 14, RFC 2119, March 1997. 919 [RFC3264] Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model 920 with Session Description Protocol (SDP)", RFC 3264, June 921 2002. 923 [RFC4566] Handley, M., Jacobson, V., and C. Perkins, "SDP: Session 924 Description Protocol", RFC 4566, July 2006. 926 [RFC5245] Rosenberg, J., "Interactive Connectivity Establishment 927 (ICE): A Protocol for Network Address Translator (NAT) 928 Traversal for Offer/Answer Protocols", RFC 5245, April 929 2010. 931 19.2. Informative References 933 [I-D.ivov-mmusic-trickle-ice-sip] 934 Ivov, E., Marocco, E., and C. Holmberg, "A Session 935 Initiation Protocol (SIP) usage for Trickle ICE", draft- 936 ivov-mmusic-trickle-ice-sip-01 (work in progress), October 937 2013. 939 [I-D.keranen-mmusic-ice-address-selection] 940 Keraenen, A. and J. Arkko, "Update on Candidate Address 941 Selection for Interactive Connectivity Establishment 942 (ICE)", draft-keranen-mmusic-ice-address-selection-01 943 (work in progress), July 2012. 945 [RFC1918] Rekhter, Y., Moskowitz, R., Karrenberg, D., Groot, G., and 946 E. Lear, "Address Allocation for Private Internets", BCP 947 5, RFC 1918, February 1996. 949 [RFC2543] Handley, M., Schulzrinne, H., Schooler, E., and J. 950 Rosenberg, "SIP: Session Initiation Protocol", RFC 2543, 951 March 1999. 953 [RFC3261] Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston, 954 A., Peterson, J., Sparks, R., Handley, M., and E. 955 Schooler, "SIP: Session Initiation Protocol", RFC 3261, 956 June 2002. 958 [RFC3388] Camarillo, G., Eriksson, G., Holler, J., and H. 959 Schulzrinne, "Grouping of Media Lines in the Session 960 Description Protocol (SDP)", RFC 3388, December 2002. 962 [RFC3840] Rosenberg, J., Schulzrinne, H., and P. Kyzivat, 963 "Indicating User Agent Capabilities in the Session 964 Initiation Protocol (SIP)", RFC 3840, August 2004. 966 [RFC4787] Audet, F. and C. Jennings, "Network Address Translation 967 (NAT) Behavioral Requirements for Unicast UDP", BCP 127, 968 RFC 4787, January 2007. 970 [RFC5389] Rosenberg, J., Mahy, R., Matthews, P., and D. Wing, 971 "Session Traversal Utilities for NAT (STUN)", RFC 5389, 972 October 2008. 974 [RFC5766] Mahy, R., Matthews, P., and J. Rosenberg, "Traversal Using 975 Relays around NAT (TURN): Relay Extensions to Session 976 Traversal Utilities for NAT (STUN)", RFC 5766, April 2010. 978 [XEP-0030] 979 Hildebrand, J., Millard, P., Eatmon, R., and P. Saint- 980 Andre, "XEP-0030: Service Discovery", XEP XEP-0030, June 981 2008. 983 [XEP-0115] 984 Hildebrand, J., Saint-Andre, P., Troncon, R., and J. 985 Konieczny, "XEP-0115: Entity Capabilities", XEP XEP-0115, 986 February 2008. 988 [XEP-0176] 989 Beda, J., Ludwig, S., Saint-Andre, P., Hildebrand, J., 990 Egan, S., and R. McQueen, "XEP-0176: Jingle ICE-UDP 991 Transport Method", XEP XEP-0176, June 2009. 993 [XEP-0278] 994 Camargo, T., "XEP-0278: Jingle Relay Nodes", XEP XEP-0278, 995 June 2011. 997 Appendix A. Open issues 999 At the time of writing of this document the authors have no clear 1000 view on how and if the following list of issues should be addressed. 1002 A.1. MID/Stream Indices in SDP 1004 This specification does not currently define syntax for candidate-to- 1005 stream bindings although it says that they should be implemented with 1006 MID or a stream index. Yet, it is reasonable to assume that most 1007 usages would need to do this within the SDP and it may make sense to 1008 agree on the format. Here's one possible way to do this: 1010 a=mid:1 1011 a=candidate:1 1 UDP 1658497328 192.168.100.33 5000 typ host 1012 a=candidate:2 1 UDP 1658497328 96.1.2.3 5000 typ srflx 1013 a=mid:2 1014 a=candidate:2 1 UDP 1658497328 96.1.2.3 5002 typ srflx 1015 a=end-of-candidates 1017 A.2. Starting checks 1019 Normally Vanilla ICE implementations would first activate a check 1020 list, validate at least one pair in every component and only then 1021 unfreeze all other checklists. With trickle ICE this would be 1022 suboptimal since, candidates can arrive randomly and we would be 1023 wasting time waiting for a checklist to fill (almost as if we were 1024 doing vanilla ICE). We need to decide if unfreezing everything 1025 solely based on foundation is good enough. 1027 Appendix B. Changes From Earlier Versions 1029 Note to the RFC-Editor: please remove this section prior to 1030 publication as an RFC. 1032 B.1. Changes From draft-ivov-01 and draft-mmusic-00 1034 o Added a requirement to trickle candidates by order of components 1035 to avoid deadlocks in the unfreezing algorithm. 1037 o Added an informative note on peer-reflexive candidates explaining 1038 that nothing changes for them semantically but they do become a 1039 more likely occurrence for Trickle ICE. 1041 o Limit the number of pairs to 100 to comply with 5245. 1043 o Added clarifications on the non-importance of how newly discovered 1044 candidates are trickled/sent to the remote party or if this is 1045 done at all. 1047 o Added transport expectations for trickled candidates as per Dale 1048 Worley's recommendation. 1050 B.2. Changes From draft-ivov-00 1052 o Specified that end-of-candidates is a media level attribute which 1053 can of course appear as session level, which is equivalent to 1054 having it appear in all m-lines. Also made end-of-candidates 1055 optional for cases such as aggressive nomination for controlled 1056 agents. 1058 o Added an example for ICE lite and trickle ICE to illustrate how, 1059 when talking to an ICE lite agent doesn't need to send or even 1060 discover any candidates. 1062 o Added an example for ICE lite and trickle ICE to illustrate how, 1063 when talking to an ICE lite agent doesn't need to send or even 1064 discover any candidates. 1066 o Added wording that explicitly states ICE lite agents have to be 1067 prepared to receive no candidates over signalling and that they 1068 should not freak out if this happens. (Closed the corresponding 1069 open issue). 1071 o It is now mandatory to use MID when trickling candidates and using 1072 m-line indexes is no longer allowed. 1074 o Replaced use of 0.0.0.0 to IP6 :: in order to avoid potential 1075 issues with RFC2543 SDP libraries that interpret 0.0.0.0 as an on- 1076 hold operation. Also changed the port number here from 1 to 9 1077 since it already has a more appropriate meaning. (Port change 1078 suggested by Jonathan Lennox). 1080 o Closed the Open Issue about use about what to do with cands 1081 received after end-of-cands. Solution: ignore, do an ice restart 1082 if you want to add something. 1084 o Added more terminology, including trickling, trickled candidates, 1085 half trickle, full trickle, 1087 o Added a reference to the SIP usage for trickle ICE as requested at 1088 the Boston interim. 1090 B.3. Changes From draft-rescorla-01 1092 o Brought back explicit use of Offer/Answer. There are no more 1093 attempts to try to do this in an O/A independent way. Also 1094 removed the use of ICE Descriptions. 1096 o Added SDP specification for trickled candidates, the trickle 1097 option and 0.0.0.0 addresses in m-lines, and end-of-candidates. 1099 o Support and Discovery. Changed that section to be less abstract. 1100 As discussed in IETF85, the draft now says implementations and 1101 usages need to either determine support in advance and directly 1102 use trickle, or do half trickle. Removed suggestion about use of 1103 discovery in SIP or about letting implementing protocols do what 1104 they want. 1106 o Defined Half Trickle. Added a section that says how it works. 1107 Mentioned that it only needs to happen in the first o/a (not 1108 necessary in updates), and added Jonathan's comment about how it 1109 could, in some cases, offer more than half the improvement if you 1110 can pre-gather part or all of your candidates before the user 1111 actually presses the call button. 1113 o Added a short section about subsequent offer/answer exchanges. 1115 o Added a short section about interactions with ICE Lite 1116 implementations. 1118 o Added two new entries to the open issues section. 1120 B.4. Changes From draft-rescorla-00 1122 o Relaxed requirements about verifying support following a 1123 discussion on MMUSIC. 1125 o Introduced ICE descriptions in order to remove ambiguous use of 1126 3264 language and inappropriate references to offers and answers. 1128 o Removed inappropriate assumption of adoption by RTCWEB pointed out 1129 by Martin Thomson. 1131 Authors' Addresses 1133 Emil Ivov 1134 Jitsi 1135 Strasbourg 67000 1136 France 1138 Phone: +33 6 72 81 15 55 1139 Email: emcho@jitsi.org 1141 Eric Rescorla 1142 RTFM, Inc. 1143 2064 Edgewood Drive 1144 Palo Alto, CA 94303 1145 USA 1147 Phone: +1 650 678 2350 1148 Email: ekr@rtfm.com 1150 Justin Uberti 1151 Google 1152 747 6th St S 1153 Kirkland, WA 98033 1154 USA 1156 Phone: +1 857 288 8888 1157 Email: justin@uberti.name