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Run idnits with the --verbose option for more detailed information about the items above. -------------------------------------------------------------------------------- 2 MMUSIC M. Petit-Huguenin 3 Internet-Draft Impedance Mismatch 4 Obsoletes: 5245 (if approved) S. Nandakumar 5 Intended status: Standards Track Cisco Systems 6 Expires: December 5, 2019 A. Keranen 7 Ericsson 8 June 3, 2019 10 Session Description Protocol (SDP) Offer/Answer procedures for 11 Interactive Connectivity Establishment (ICE) 12 draft-ietf-mmusic-ice-sip-sdp-32 14 Abstract 16 This document describes Session Description Protocol (SDP) Offer/ 17 Answer procedures for carrying out Interactive Connectivity 18 Establishment (ICE) between the agents. 20 This document obsoletes RFC 5245. 22 Status of This Memo 24 This Internet-Draft is submitted in full conformance with the 25 provisions of BCP 78 and BCP 79. 27 Internet-Drafts are working documents of the Internet Engineering 28 Task Force (IETF). Note that other groups may also distribute 29 working documents as Internet-Drafts. The list of current Internet- 30 Drafts is at https://datatracker.ietf.org/drafts/current/. 32 Internet-Drafts are draft documents valid for a maximum of six months 33 and may be updated, replaced, or obsoleted by other documents at any 34 time. It is inappropriate to use Internet-Drafts as reference 35 material or to cite them other than as "work in progress." 37 This Internet-Draft will expire on December 5, 2019. 39 Copyright Notice 41 Copyright (c) 2019 IETF Trust and the persons identified as the 42 document authors. All rights reserved. 44 This document is subject to BCP 78 and the IETF Trust's Legal 45 Provisions Relating to IETF Documents 46 (https://trustee.ietf.org/license-info) in effect on the date of 47 publication of this document. Please review these documents 48 carefully, as they describe your rights and restrictions with respect 49 to this document. Code Components extracted from this document must 50 include Simplified BSD License text as described in Section 4.e of 51 the Trust Legal Provisions and are provided without warranty as 52 described in the Simplified BSD License. 54 This document may contain material from IETF Documents or IETF 55 Contributions published or made publicly available before November 56 10, 2008. The person(s) controlling the copyright in some of this 57 material may not have granted the IETF Trust the right to allow 58 modifications of such material outside the IETF Standards Process. 59 Without obtaining an adequate license from the person(s) controlling 60 the copyright in such materials, this document may not be modified 61 outside the IETF Standards Process, and derivative works of it may 62 not be created outside the IETF Standards Process, except to format 63 it for publication as an RFC or to translate it into languages other 64 than English. 66 Table of Contents 68 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3 69 2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 4 70 3. SDP Offer/Answer Procedures . . . . . . . . . . . . . . . . . 4 71 3.1. Introduction . . . . . . . . . . . . . . . . . . . . . . 4 72 3.2. Generic Procedures . . . . . . . . . . . . . . . . . . . 4 73 3.2.1. Encoding . . . . . . . . . . . . . . . . . . . . . . 4 74 3.2.2. RTP/RTCP Considerations . . . . . . . . . . . . . . . 6 75 3.2.3. Determining Role . . . . . . . . . . . . . . . . . . 6 76 3.2.4. STUN Considerations . . . . . . . . . . . . . . . . . 6 77 3.2.5. Verifying ICE Support Procedures . . . . . . . . . . 6 78 3.2.6. SDP Example . . . . . . . . . . . . . . . . . . . . . 7 79 3.3. Initial Offer/Answer Exchange . . . . . . . . . . . . . . 8 80 3.3.1. Sending the Initial Offer . . . . . . . . . . . . . . 8 81 3.3.2. Sending the Initial Answer . . . . . . . . . . . . . 8 82 3.3.3. Receiving the Initial Answer . . . . . . . . . . . . 9 83 3.3.4. Concluding ICE . . . . . . . . . . . . . . . . . . . 10 84 3.4. Subsequent Offer/Answer Exchanges . . . . . . . . . . . . 10 85 3.4.1. Sending Subsequent Offer . . . . . . . . . . . . . . 11 86 3.4.2. Sending Subsequent Answer . . . . . . . . . . . . . . 13 87 3.4.3. Receiving Answer for a Subsequent Offer . . . . . . . 15 88 4. Grammar . . . . . . . . . . . . . . . . . . . . . . . . . . . 16 89 4.1. "candidate" Attribute . . . . . . . . . . . . . . . . . . 17 90 4.2. "remote-candidates" Attribute . . . . . . . . . . . . . . 19 91 4.3. "ice-lite" and "ice-mismatch" Attributes . . . . . . . . 19 92 4.4. "ice-ufrag" and "ice-pwd" Attributes . . . . . . . . . . 20 93 4.5. "ice-pacing" Attribute . . . . . . . . . . . . . . . . . 21 94 4.6. "ice-options" Attribute . . . . . . . . . . . . . . . . . 21 95 5. Keepalives . . . . . . . . . . . . . . . . . . . . . . . . . 22 96 6. SIP Considerations . . . . . . . . . . . . . . . . . . . . . 22 97 6.1. Latency Guidelines . . . . . . . . . . . . . . . . . . . 22 98 6.1.1. Offer in INVITE . . . . . . . . . . . . . . . . . . . 23 99 6.1.2. Offer in Response . . . . . . . . . . . . . . . . . . 24 100 6.2. SIP Option Tags and Media Feature Tags . . . . . . . . . 24 101 6.3. Interactions with Forking . . . . . . . . . . . . . . . . 24 102 6.4. Interactions with Preconditions . . . . . . . . . . . . . 25 103 6.5. Interactions with Third Party Call Control . . . . . . . 25 104 7. Relationship with ANAT . . . . . . . . . . . . . . . . . . . 26 105 8. Security Considerations . . . . . . . . . . . . . . . . . . . 26 106 8.1. Attacks on the Offer/Answer Exchanges . . . . . . . . . . 26 107 8.2. Insider Attacks . . . . . . . . . . . . . . . . . . . . . 26 108 8.2.1. The Voice Hammer Attack . . . . . . . . . . . . . . . 26 109 8.2.2. Interactions with Application Layer Gateways and SIP 27 110 9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 28 111 9.1. SDP Attributes . . . . . . . . . . . . . . . . . . . . . 28 112 9.1.1. candidate Attribute . . . . . . . . . . . . . . . . . 28 113 9.1.2. remote-candidates Attribute . . . . . . . . . . . . . 29 114 9.1.3. ice-lite Attribute . . . . . . . . . . . . . . . . . 29 115 9.1.4. ice-mismatch Attribute . . . . . . . . . . . . . . . 30 116 9.1.5. ice-pwd Attribute . . . . . . . . . . . . . . . . . . 30 117 9.1.6. ice-ufrag Attribute . . . . . . . . . . . . . . . . . 31 118 9.1.7. ice-options Attribute . . . . . . . . . . . . . . . . 31 119 9.1.8. ice-pacing Attribute . . . . . . . . . . . . . . . . 32 120 9.2. Interactive Connectivity Establishment (ICE) Options 121 Registry . . . . . . . . . . . . . . . . . . . . . . . . 32 122 10. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 33 123 11. References . . . . . . . . . . . . . . . . . . . . . . . . . 33 124 11.1. Normative References . . . . . . . . . . . . . . . . . . 33 125 11.2. Informative References . . . . . . . . . . . . . . . . . 35 126 11.3. URIs . . . . . . . . . . . . . . . . . . . . . . . . . . 36 127 Appendix A. Examples . . . . . . . . . . . . . . . . . . . . . . 36 128 Appendix B. The remote-candidates Attribute . . . . . . . . . . 38 129 Appendix C. Why Is the Conflict Resolution Mechanism Needed? . . 39 130 Appendix D. Why Send an Updated Offer? . . . . . . . . . . . . . 40 131 Appendix E. Contributors . . . . . . . . . . . . . . . . . . . . 41 132 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 41 134 1. Introduction 136 This document describes how Interactive Connectivity Establishment 137 (ICE) is used with Session Description Protocol (SDP) offer/answer 138 [RFC3264]. The ICE specification [RFC8445] describes procedures that 139 are common to all usages of ICE and this document gives the 140 additional details needed to use ICE with SDP offer/answer. 142 2. Terminology 144 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 145 "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and 146 "OPTIONAL" in this document are to be interpreted as described in RFC 147 2119 [RFC2119]. 149 Readers should be familiar with the terminology defined in [RFC3264], 150 in [RFC8445] and the following: 152 Default Destination/Candidate: The default destination for a 153 component of a data stream is the transport address that would be 154 used by an agent that is not ICE aware. A default candidate for a 155 component is one whose transport address matches the default 156 destination for that component. For the RTP component, the 157 default connection address is in the "c=" line of the SDP, and the 158 port and transport protocol are in the "m=" line. For the RTCP 159 component, the address and port are indicated using the "a=rtcp" 160 attribute defined in [RFC3605], if present; otherwise, the RTCP 161 component address is the same as the address of the RTP component, 162 and its port is one greater than the port of the RTP component. 164 3. SDP Offer/Answer Procedures 166 3.1. Introduction 168 [RFC8445] defines ICE candidate exchange as the process for ICE 169 agents (Initiator and Responder) to exchange their candidate 170 information required for ICE processing at the agents. For the 171 purposes of this specification, the candidate exchange process 172 corresponds to the [RFC3264] Offer/Answer protocol and the 173 terminologies offerer and answerer correspond to the initiator and 174 responder terminologies from [RFC8445] respectively. 176 Once the initiating agent has gathered, pruned and prioritized its 177 set of candidates [RFC8445], the candidate exchange with the peer 178 agent begins. 180 3.2. Generic Procedures 182 3.2.1. Encoding 184 Section 4 provides detailed rules for constructing various SDP 185 attributes defined in this specification. 187 3.2.1.1. Data Streams 189 Each data stream [RFC8445] is represented by an SDP media description 190 ("m=" section). 192 3.2.1.2. Candidates 194 Within a "m=" section, each candidate (including the default 195 candidate) associated with the data stream is represented by an SDP 196 candidate attribute. 198 Prior to nomination, the "c=" line associated with an "m=" section 199 contains the connection address of the default candidate, while the 200 "m=" line contains the port and transport protocol of the default 201 candidate for that "m=" section. 203 After nomination, the "c=" line for a given "m=" section contains the 204 connection address of the nominated candidate (the local candidate of 205 the nominated candidate pair) and the "m=" line contains the port and 206 transport protocol corresponding to the nominated candidate for that 207 "m=" section. 209 3.2.1.3. Username and Password 211 The ICE username is represented by an SDP ice-ufrag attribute and the 212 ICE password is represented by an SDP ice-pwd attribute. 214 3.2.1.4. Lite Implementations 216 An ICE lite implementation [RFC8445] MUST include an SDP ice-lite 217 attribute. A full implementation MUST NOT include that attribute. 219 3.2.1.5. ICE Extensions 221 An agent uses the SDP ice-options attribute to indicate support of 222 ICE extensions. 224 An agent compliant to this specification MUST include an SDP ice- 225 options attribute with an "ice2" attribute value. If an agent 226 receives an SDP offer or answer with ICE attributes but without the 227 "ice2" ice-options attribute value, the agent assumes that the peer 228 is compliant to [RFC5245]. 230 3.2.1.6. Inactive and Disabled Data Streams 232 If an "m=" section is marked as inactive [RFC4566], or has a 233 bandwidth value of zero [RFC4566], the agent MUST still include ICE 234 related SDP attributes. 236 If the port value associated with an "m=" section is set to zero 237 (implying a disabled stream) as defined in section 8.2 of [RFC3264], 238 the agent SHOULD NOT include ICE related SDP candidate attributes in 239 that "m=" section, unless an SDP extension specifying otherwise is 240 used. 242 3.2.2. RTP/RTCP Considerations 244 If an agent utilizes both RTP and RTCP, and separate ports are used 245 for RTP and RTCP, the agent MUST include SDP candidate attributes for 246 both the RTP and RTCP components and SDP rtcp attribute SHOULD be 247 included in the "m=" section, as described in [RFC3605] 249 In the cases where the port number for the RTCP is one higher than 250 the RTP port and the RTCP component address is the same as the 251 address of the RTP component, the SDP rtcp attribute MAY be omitted. 253 If the agent does not utilize RTCP, it indicates that by including 254 b=RS:0 and b=RR:0 SDP attributes, as described in [RFC3556]. 256 3.2.3. Determining Role 258 The offerer acts as the Initiating agent. The answerer acts as the 259 Responding agent. The ICE roles (controlling and controlled) are 260 determined using the procedures in [RFC8445]. 262 3.2.4. STUN Considerations 264 Once an agent has provided its local candidates to its peer in an SDP 265 offer or answer, the agent MUST be prepared to receive STUN 266 connectivity check Binding requests on those candidates. 268 3.2.5. Verifying ICE Support Procedures 270 The agents will proceed with the ICE procedures defined in [RFC8445] 271 and this specification if, for each data stream in the SDP it 272 received, the default destination for each component of that data 273 stream appears in a candidate attribute. For example, in the case of 274 RTP, the connection address, port and transport protocol in the "c=" 275 and "m=" lines, respectively, appear in a candidate attribute and the 276 value in the rtcp attribute appears in a candidate attribute. 278 This specification provides no guidance on how an agent should 279 proceed in the cases where the above condition is not met with the 280 few exceptions noted below: 282 1. The presence of certain application layer gateways MAY modify the 283 transport address information as described in Section 8.2.2. The 284 behavior of the responding agent in such a situation is 285 implementation dependent. Informally, the responding agent MAY 286 consider the mismatched transport address information as a 287 plausible new candidate learnt from the peer and continue its ICE 288 processing with that transport address included. Alternatively, 289 the responding agent MAY include an "a=ice-mismatch" attribute in 290 its answer for such data streams. If an agent chooses to include 291 an "a=ice-mismatch" attribute in its answer for a data stream, 292 then it MUST also omit "a=candidate" attributes, MUST terminate 293 the usage of ICE procedures and [RFC3264] procedures MUST be used 294 instead for this data stream. 296 2. The transport address from the peer for the default destination 297 correspond to IPv4/IPv6 address values "0.0.0.0"/"::" and port 298 value of "9". This MUST NOT be considered as a ICE failure by 299 the peer agent and the ICE processing MUST continue as usual. 301 3. In some cases, the scontrolling/initiator agent may receive the 302 SDP answer that may omit "a=candidate" attributes for the data 303 stream, and instead include a media level "a=ice-mismatch" 304 attribute. This signals to the offerer that the answerer 305 supports ICE, but that ICE processing was not used for this data 306 stream. In this case, ICE processing MUST be terminated for this 307 data stream and [RFC3264] procedures MUST be followed instead. 309 4. The transport address from the peer for the default destination 310 is an FQDN. Regardless of the procedures used to resolve FQDN or 311 the resolution result, this MUST NOT be considered as a ICE 312 failure by the peer agent and the ICE processing MUST continue as 313 usual. 315 3.2.6. SDP Example 317 The following is an example SDP message that includes ICE attributes 318 (lines folded for readability): 320 v=0 321 o=jdoe 2890844526 2890842807 IN IP4 203.0.113.141 322 s= 323 c=IN IP4 192.0.2.3 324 t=0 0 325 a=ice-options:ice2 326 a=ice-pwd:asd88fgpdd777uzjYhagZg 327 a=ice-ufrag:8hhY 328 m=audio 45664 RTP/AVP 0 329 b=RS:0 330 b=RR:0 331 a=rtpmap:0 PCMU/8000 332 a=candidate:1 1 UDP 2130706431 203.0.113.141 8998 typ host 333 a=candidate:2 1 UDP 1694498815 192.0.2.3 45664 typ srflx raddr 334 203.0.113.141 rport 8998 336 3.3. Initial Offer/Answer Exchange 338 3.3.1. Sending the Initial Offer 340 When an offerer generates the initial offer, in each "m=" section it 341 MUST include SDP candidate attributes for each available candidate 342 associated with the "m=" section. In addition, the offerer MUST 343 include an SDP ice-ufrag and an SDP ice-pwd attribute in the offer. 345 It is valid for an offer "m=" line to include no SDP candidate 346 attributes and with default destination corresponding to the IP 347 address values "0.0.0.0"/"::" and port value of "9". This implies 348 that the offering agent is only going to use peer reflexive 349 candidates or that additional candidates would be provided in 350 subsequent signaling messages. 352 Note: Within the scope of this document, "Initial Offer" refers to 353 the first SDP offer that is sent in order to negotiate usage of 354 ICE. It might, or might not, be the initial SDP offer of the SDP 355 session. 357 Note: The procedures in this document only consider "m=" sections 358 associated with data streams where ICE is used. 360 3.3.2. Sending the Initial Answer 362 When an answerer receives an initial offer that indicates that the 363 offerer supports ICE, and if the answerer accepts the offer and the 364 usage of ICE, in each "m=" section within the answer, it MUST include 365 SDP candidate attributes for each available candidate associated with 366 the "m=" section. In addition, the answerer MUST include an SDP ice- 367 ufrag and an SDP ice-pwd attribute in the answer. 369 In each "m=" line, the answerer MUST use the same transport protocol 370 as was used in the offer "m=" line. If none of the candidates in the 371 "m=" line in the answer use the same transport protocol as indicated 372 in the offer "m=" line, then, in order to avoid ICE mismatch, the 373 default destination MUST be set to IP address values "0.0.0.0"/"::" 374 and port value of "9". 376 It is also valid for an answer "m=" line to include no SDP candidate 377 attributes and with default destination corresponding to the IP 378 address values "0.0.0.0"/"::" and port value of "9". This implies 379 that the answering agent is only going to use peer reflexive 380 candidates or that additional candidates would be provided in 381 subsequent signaling messages. 383 Once the answerer has sent the answer, it can start performing 384 connectivity checks towards the peer candidates that were provided in 385 the offer. 387 If the offer does not indicate support of ICE, the answerer MUST NOT 388 accept the usage of ICE. If the answerer still accepts the offer, 389 the answerer MUST NOT include any ICE related SDP attributes in the 390 answer. Instead the answerer will generate the answer according to 391 normal offer/answer procedures [RFC3264]. 393 If the answerer detects a possibility of the ICE mismatch, procedures 394 described in (Section 3.2.5) are followed. 396 Note: > provides guidance on finding working 397 candidate pairs and thus preventing premature declaration of ICE 398 failure in certain scenarios such as, if the peer has not provided 399 any candidates, or if all provided candidates have failed or have 400 been discarded. 402 3.3.3. Receiving the Initial Answer 404 When an offerer receives an initial answer that indicates that the 405 answerer supports ICE, it can start performing connectivity checks 406 towards the peer candidates that were provided in the answer. 408 If the answer does not indicate that the answerer supports ICE, or if 409 the answerer included "a=ice-mismatch" attributes for all the active 410 data streams in the answer, the offerer MUST terminate the usage of 411 ICE for the entire session and [RFC3264] procedures MUST be followed 412 instead. 414 On the other hand, if the answer indicates the support for ICE but 415 includes "a=ice-mismatch" in certain active data streams, then the 416 offerer MUST terminate the usage of ICE procedures and [RFC3264] 417 procedures MUST be used instead for only these data streams. Also, 418 ICE procedures MUST be used for data streams where an "a=ice- 419 mismatch" attribute was not included. 421 If the offerer detects an ICE mismatch for one or more data streams 422 in the answer, as described in (Section 3.2.5), the offerer MUST 423 terminate the usage of ICE for the entire session. The subsequent 424 actions taken by the offerer are implementation dependent and are out 425 of the scope of this specification. 427 Note: > provides guidance on finding working 428 candidate pairs and thus preventing premature declaration of ICE 429 failure in certain scenarios such as, if the peer has not provided 430 any candidates, or if all provided candidates have failed or have 431 been discarded. 433 3.3.4. Concluding ICE 435 Once the state of each check list is Completed, and if the agent is 436 the controlling agent, it nominates a candidate pair [RFC8445] and 437 checks for each data stream whether the nominated pair matches the 438 default candidate pair. If there are one or more data streams don't 439 match, and the peer did not indicate support for the 'ice2' ice- 440 option, the controlling agent MUST generate a subsequent offer 441 (Section 3.4.1), in which the connection address, port and transport 442 protocol in the "c=" and "m=" lines associated with each data stream 443 match the corresponding local information of the nominated pair for 444 that data stream. 446 However, if the support for 'ice2' ice-option is in use, the 447 nominated candidate is noted and sent in the subsequent offer/answer 448 exchange as the default candidate and no updated offer is needed to 449 fix the default candidate. 451 Also as described in [RFC8445], once the controlling agent has 452 nominated a candidate pair for a data stream, the agent MUST NOT 453 nominate another pair for that data stream during the lifetime of the 454 ICE session (i.e. until ICE is restarted). 456 3.4. Subsequent Offer/Answer Exchanges 458 Either agent MAY generate a subsequent offer at any time allowed by 459 [RFC3264]. This section defines rules for construction of subsequent 460 offers and answers. 462 Should a subsequent offer fail, ICE processing continues as if the 463 subsequent offer had never been made. 465 3.4.1. Sending Subsequent Offer 467 3.4.1.1. Procedures for All Implementations 469 3.4.1.1.1. ICE Restarts 471 An agent MAY restart ICE processing for an existing data stream 472 [RFC8445]. 474 The rules governing the ICE restart imply that setting the connection 475 address in the "c=" line to 0.0.0.0 (for IPv4)/ :: (for IPv6) will 476 cause an ICE restart. Consequently, ICE implementations MUST NOT 477 utilize this mechanism for call hold, and instead MUST use 478 "a=inactive" and "a=sendonly" as described in [RFC3264]. 480 To restart ICE, an agent MUST change both the ice-pwd and the ice- 481 ufrag for the data stream in an offer. However, it is permissible to 482 use a session-level attribute in one offer, but to provide the same 483 ice-pwd or ice-ufrag as a media-level attribute in a subsequent 484 offer. This MUST NOT be considered as ICE restart. 486 An agent sets the rest of the ice related fields in the SDP for this 487 data stream as it would in an initial offer of this data stream (see 488 Section 3.2.1). Consequently, the set of candidates MAY include 489 some, none, or all of the previous candidates for that data stream 490 and MAY include a totally new set of candidates. 492 3.4.1.1.2. Removing a Data Stream 494 If an agent removes a data stream by setting its port to zero, it 495 MUST NOT include any candidate attributes for that data stream and 496 SHOULD NOT include any other ICE-related attributes defined in 497 Section 4 for that data stream. 499 3.4.1.1.3. Adding a Data Stream 501 If an agent wishes to add a new data stream, it sets the fields in 502 the SDP for this data stream as if this was an initial offer for that 503 data stream (see Section 3.2.1). This will cause ICE processing to 504 begin for this data stream. 506 3.4.1.2. Procedures for Full Implementations 508 This section describes additional procedures for full 509 implementations, covering existing data streams. 511 3.4.1.2.1. Before Nomination 513 When an offerer sends a subsequent offer; in each "m=" section for 514 which a candidate pair has not yet been nominated, the offer MUST 515 include the same set of ICE-related information that the offerer 516 included in the previous offer or answer. The agent MAY include 517 additional candidates it did not offer previously, but which it has 518 gathered since the last offer/ answer exchange, including peer 519 reflexive candidates. 521 The agent MAY change the default destination for media. As with 522 initial offers, there MUST be a set of candidate attributes in the 523 offer matching this default destination. 525 3.4.1.2.2. After Nomination 527 Once a candidate pair has been nominated for a data stream, the 528 connection address, port and transport protocol in each "c=" and "m=" 529 line associated with that data stream MUST match the data associated 530 with the nominated pair for that data stream. In addition, the 531 offerer only includes SDP candidates representing the local 532 candidates of the nominated candidate pair. The offerer MUST NOT 533 include any other SDP candidate attributes in the subsequent offer. 535 In addition, if the agent is controlling, it MUST include the 536 "a=remote-candidates" attribute for each data stream whose check list 537 is in the completed state. The attribute contains the remote 538 candidates corresponding to the nominated pair in the valid list for 539 each component of that data stream. It is needed to avoid a race 540 condition whereby the controlling agent chooses its pairs, but the 541 updated offer beats the connectivity checks to the controlled agent, 542 which doesn't even know these pairs are valid, let alone selected. 543 See Appendix B for elaboration on this race condition. 545 3.4.1.3. Procedures for Lite Implementations 547 If the ICE state is running, a lite implementation MUST include all 548 of its candidates for each component of each data stream in 549 "a=candidate" attributes in any subsequent offer. The candidates are 550 formed identical to the procedures for initial offers. 552 A lite implementation MUST NOT add additional host candidates in a 553 subsequent offer. If an agent needs to offer additional candidates, 554 it MUST restart ICE. Similarly, the username fragments or passwords 555 MUST remain the same as used previously. If an agent needs to change 556 one of these, it MUST restart ICE for that data stream. 558 If ICE has completed for a data stream and if the agent is 559 controlled, the default destination for that data stream MUST be set 560 to the remote candidate of the candidate pair for that component in 561 the valid list. For a lite implementation, there is always just a 562 single candidate pair in the valid list for each component of a data 563 stream. Additionally, the agent MUST include a candidate attribute 564 for each default destination. 566 If ICE state is completed and if the agent is controlling (which only 567 happens when both agents are lite), the agent MUST include the 568 "a=remote-candidates" attribute for each data stream. The attribute 569 contains the remote candidates from the candidate pairs in the valid 570 list (one pair for each component of each data stream). 572 3.4.2. Sending Subsequent Answer 574 If ICE is Completed for a data stream, and the offer for that data 575 stream lacked the "a=remote-candidates" attribute, the rules for 576 construction of the answer are identical to those for the offerer, 577 except that the answerer MUST NOT include the "a=remote-candidates" 578 attribute in the answer. 580 A controlled agent will receive an offer with the "a=remote- 581 candidates" attribute for a data stream when its peer has concluded 582 ICE processing for that data stream. This attribute is present in 583 the offer to deal with a race condition between the receipt of the 584 offer, and the receipt of the Binding Response that tells the 585 answerer the candidate that will be selected by ICE. See Appendix B 586 for an explanation of this race condition. Consequently, processing 587 of an offer with this attribute depends on the winner of the race. 589 The agent forms a candidate pair for each component of the data 590 stream by: 592 o Setting the remote candidate equal to the offerer's default 593 destination for that component (i.e. the contents of the "m=" and 594 "c=" lines for RTP, and the "a=rtcp" attribute for RTCP) 596 o Setting the local candidate equal to the transport address for 597 that same component in the "a=remote-candidates" attribute in the 598 offer. 600 The agent then sees if each of these candidate pairs is present in 601 the valid list. If a particular pair is not in the valid list, the 602 check has "lost" the race. Call such a pair a "losing pair". 604 The agent finds all the pairs in the check list whose remote 605 candidates equal the remote candidate in the losing pair: 607 o If none of the pairs are In-Progress, and at least one is Failed, 608 it is most likely that a network failure, such as a network 609 partition or serious packet loss, has occurred. The agent SHOULD 610 generate an answer for this data stream as if the remote- 611 candidates attribute had not been present, and then restart ICE 612 for this stream. 614 o If at least one of the pairs is In-Progress, the agent SHOULD wait 615 for those checks to complete, and as each completes, redo the 616 processing in this section until there are no losing pairs. 618 Once there are no losing pairs, the agent can generate the answer. 619 It MUST set the default destination for media to the candidates in 620 the remote-candidates attribute from the offer (each of which will 621 now be the local candidate of a candidate pair in the valid list). 622 It MUST include a candidate attribute in the answer for each 623 candidate in the remote-candidates attribute in the offer. 625 3.4.2.1. ICE Restart 627 If the offerer in a subsequent offer requested an ICE restart for a 628 data stream, and if the answerer accepts the offer, the answerer 629 follows the procedures for generating an initial answer. 631 For a given data stream, the answerer MAY include the same candidates 632 that were used in the previous ICE session, but it MUST change the 633 SDP ice-pwd and ice-ufrag attribute values. 635 3.4.2.2. Lite Implementation specific procedures 637 If the received offer contains the remote-candidates attribute for a 638 data stream, the agent forms a candidate pair for each component of 639 the data stream by: 641 o Setting the remote candidate equal to the offerer's default 642 destination for that component (i.e., the contents of the "m=" and 643 "c=" lines for RTP, and the "a=rtcp" attribute for RTCP). 645 o Setting the local candidate equal to the transport address for 646 that same component in the "a=remote-candidates" attribute in the 647 offer. 649 The state of ICE processing for that data stream is set to Completed. 651 Furthermore, if the agent believed it was controlling, but the offer 652 contained the "a=remote-candidates" attribute, both agents believe 653 they are controlling. In this case, both would have sent updated 654 offers around the same time. 656 However, the signaling protocol carrying the offer/answer exchanges 657 will have resolved this glare condition, so that one agent is always 658 the 'winner' by having its offer received before its peer has sent an 659 offer. The winner takes the role of controlling, so that the loser 660 (the answerer under consideration in this section) MUST change its 661 role to controlled. 663 Consequently, if the agent was going to send an updated offer since, 664 based on the rules in section 8.2 of [RFC8445], it was controlling, 665 it no longer needs to. 667 Besides the potential role change, change in the Valid list, and 668 state changes, the construction of the answer is performed 669 identically to the construction of an offer. 671 3.4.3. Receiving Answer for a Subsequent Offer 673 3.4.3.1. Procedures for Full Implementations 675 There may be certain situations where the offerer receives an SDP 676 answer that lacks ICE candidates although the initial answer included 677 them. One example of such an "unexpected" answer might be happen 678 when an ICE-unaware Back-to-Back User Agent (B2BUA) introduces a 679 media server during call hold using 3rd party call-control 680 procedures. Omitting further details how this is done, this could 681 result in an answer being received at the holding UA that was 682 constructed by the B2BUA. With the B2BUA being ICE-unaware, that 683 answer would not include ICE candidates. 685 Receiving an answer without ICE attributes in this situation might be 686 unexpected, but would not necessarily impair the user experience. 688 When the offerer receives an answer indicating support for ICE, the 689 offer performs one of the following actions: 691 o If the offer was a restart, the agent MUST perform ICE restart 692 procedures as specified in Section 3.4.3.1.1 694 o If the offer/answer exchange removed a data stream, or an answer 695 rejected an offered data stream, an agent MUST flush the Valid 696 list for that data stream. It MUST also terminate any STUN 697 transactions in progress for that data stream. 699 o If the offer/answer exchange added a new data stream, the agent 700 MUST create a new check list for it (and an empty Valid list to 701 start of course) which in turn triggers the candidate processing 702 procedures [RFC8445]. 704 o If ICE state is running for a given data stream, the agent 705 recomputes the check list. If a pair on the new check list was 706 also on the previous check list, and its state is not Frozen, its 707 state is copied over. Otherwise, its state is set to Frozen. If 708 none of the check lists are active (meaning that the pairs in each 709 check list are Frozen), appropriate procedures in [RFC8445] are 710 performed to move candidate(s) to the Waiting state to further 711 continue ICE processing. 713 o If ICE state is completed and the SDP answer conforms to 714 Section 3.4.2, the agent MUST remain in the ICE completed state. 716 However, if the ICE support is no longer indicated in the SDP answer, 717 the agent MUST fall-back to [RFC3264] procedures and SHOULD NOT drop 718 the dialog because of the missing ICE support or unexpected answer. 719 Once the agent sends a new offer later on, it MUST perform an ICE 720 restart. 722 3.4.3.1.1. ICE Restarts 724 The agent MUST remember the nominated pair in the Valid list for each 725 component of the data stream, called the previous selected pair prior 726 to the restart. The agent will continue to send media using this 727 pair, as described in section 12 of [RFC8445]. Once these 728 destinations are noted, the agent MUST flush the valid and check 729 lists, and then recompute the check list and its states, thus 730 triggering the candidate processing procedures [RFC8445] 732 3.4.3.2. Procedures for Lite Implementations 734 If ICE is restarting for a data stream, the agent MUST start a new 735 Valid list for that data stream. It MUST remember the nominated pair 736 in the previous Valid list for each component of the data stream, 737 called the previous selected pairs, and continue to send media there 738 as described in section 12 of [RFC8445]. The state of ICE processing 739 for each data stream MUST change to Running, and the state of ICE 740 processing MUST change to Running 742 4. Grammar 744 This specification defines eight new SDP attributes -- the 745 "candidate", "remote-candidates", "ice-lite", "ice-mismatch", "ice- 746 ufrag", "ice-pwd", "ice-pacing", and "ice-options" attributes. 748 This section also provides non-normative examples of the attributes 749 defined. 751 The syntax for the attributes follow Augmented BNF as defined in 752 [RFC5234]. 754 4.1. "candidate" Attribute 756 The candidate attribute is a media-level attribute only. It contains 757 a transport address for a candidate that can be used for connectivity 758 checks. 760 candidate-attribute = "candidate" ":" foundation SP component-id SP 761 transport SP 762 priority SP 763 connection-address SP ;from RFC 4566 764 port ;port from RFC 4566 765 SP cand-type 766 [SP rel-addr] 767 [SP rel-port] 768 *(SP extension-att-name SP 769 extension-att-value) 771 foundation = 1*32ice-char 772 component-id = 1*5DIGIT 773 transport = "UDP" / transport-extension 774 transport-extension = token ; from RFC 3261 775 priority = 1*10DIGIT 776 cand-type = "typ" SP candidate-types 777 candidate-types = "host" / "srflx" / "prflx" / "relay" / token 778 rel-addr = "raddr" SP connection-address 779 rel-port = "rport" SP port 780 extension-att-name = token 781 extension-att-value = *VCHAR 782 ice-char = ALPHA / DIGIT / "+" / "/" 784 This grammar encodes the primary information about a candidate: its 785 IP address, port and transport protocol, and its properties: the 786 foundation, component ID, priority, type, and related transport 787 address: 789 : is taken from RFC 4566 [RFC4566]. It is the 790 IP address of the candidate, allowing for IPv4 addresses, IPv6 791 addresses, and fully qualified domain names (FQDNs). When parsing 792 this field, an agent can differentiate an IPv4 address and an IPv6 793 address by presence of a colon in its value - the presence of a 794 colon indicates IPv6. An agent generating local candidates MUST 795 NOT use FQDN addresses. An agent processing remote candidates 796 MUST ignore candidate lines that include candidates with FQDN or 797 IP address versions that are not supported or recognized. The 798 procedures for generation and handling of FQDN candidates, as well 799 as, how agents indicate support for such procedures, need to be 800 specified in an extension specification. 802 : is also taken from RFC 4566 [RFC4566]. It is the port of 803 the candidate. 805 : indicates the transport protocol for the candidate. 806 This specification only defines UDP. However, extensibility is 807 provided to allow for future transport protocols to be used with 808 ICE by extending the sub-registry "ICE Transport Protocols" under 809 "Interactive Connectivity Establishment (ICE)" registry. 811 : is composed of 1 to 32 s. It is an 812 identifier that is equivalent for two candidates that are of the 813 same type, share the same base, and come from the same STUN 814 server. The foundation is used to optimize ICE performance in the 815 Frozen algorithm as described in [RFC8445] 817 : is a positive integer between 1 and 256 (inclusive) 818 that identifies the specific component of the dta stream for which 819 this is a candidate. It MUST start at 1 and MUST increment by 1 820 for each component of a particular candidate. For data streams 821 based on RTP, candidates for the actual RTP media MUST have a 822 component ID of 1, and candidates for RTCP MUST have a component 823 ID of 2. See section 13 in [RFC8445] for additional discussion on 824 extending ICE to new data streams. 826 : is a positive integer between 1 and (2**31 - 1) 827 inclusive. The procedures for computing candidate's priority is 828 described in section 5.1.2 of [RFC8445]. 830 : encodes the type of candidate. This specification 831 defines the values "host", "srflx", "prflx", and "relay" for host, 832 server reflexive, peer reflexive, and relayed candidates, 833 respectively. Specifications for new candidate types MUST define 834 how, if at all, various steps in the ICE processing differ from 835 the ones defined by this specification. 837 and : convey transport addresses related to the 838 candidate, useful for diagnostics and other purposes. 839 and MUST be present for server reflexive, peer 840 reflexive, and relayed candidates. If a candidate is server or 841 peer reflexive, and are equal to the base 842 for that server or peer reflexive candidate. If the candidate is 843 relayed, and are equal to the mapped address 844 in the Allocate response that provided the client with that 845 relayed candidate (see Appendix B.3 of [RFC8445] for a discussion 846 of its purpose). If the candidate is a host candidate, 847 and MUST be omitted. 849 In some cases, e.g., for privacy reasons, an agent may not want to 850 reveal the related address and port. In this case the address 851 MUST be set to "0.0.0.0" (for IPv4 candidates) or "::" (for IPv6 852 candidates) and the port to zero. 854 The candidate attribute can itself be extended. The grammar allows 855 for new name/value pairs to be added at the end of the attribute. 856 Such extensions MUST be made through IETF Review or IESG Approval 857 [RFC5226] and the assignments MUST contain the specific extension and 858 a reference to the document defining the usage of the extension 860 An implementation MUST ignore any name/value pairs it doesn't 861 understand. 863 Example: SDP line for UDP server reflexive candidate attribute for the RTP component 865 a=candidate:2 1 UDP 1694498815 192.0.2.3 45664 typ srflx raddr 203.0.113.141 rport 8998 867 4.2. "remote-candidates" Attribute 869 The syntax of the "remote-candidates" attribute is defined using 870 Augmented BNF as defined in [RFC5234]. The remote-candidates 871 attribute is a media-level attribute only. 873 remote-candidate-att = "remote-candidates:" remote-candidate 874 0*(SP remote-candidate) 875 remote-candidate = component-ID SP connection-address SP port 877 The attribute contains a connection-address and port for each 878 component. The ordering of components is irrelevant. However, a 879 value MUST be present for each component of a data stream. This 880 attribute MUST be included in an offer by a controlling agent for a 881 data stream that is Completed, and MUST NOT be included in any other 882 case. 884 Example: Remote candidates SDP lines for the RTP and RTCP components: 886 a=remote-candidates:1 192.0.2.3 45664 887 a=remote-candidates:2 192.0.2.3 45665 889 4.3. "ice-lite" and "ice-mismatch" Attributes 891 The syntax of the "ice-lite" and "ice-mismatch" attributes, both of 892 which are flags, is: 894 ice-lite = "ice-lite" 895 ice-mismatch = "ice-mismatch" 897 "ice-lite" is a session-level attribute only, and indicates that an 898 agent is a lite implementation. "ice-mismatch" is a media-level 899 attribute and only reported in the answer. It indicates that the 900 offer arrived with a default destination for a media component that 901 didn't have a corresponding candidate attribute. Inclusion of 902 "a=ice-mismatch" attribute for a given data stream implies that even 903 though both agents support ICE, ICE procedures MUST NOT be used for 904 this data stream and [RFC3264] procedures MUST be used instead. 906 4.4. "ice-ufrag" and "ice-pwd" Attributes 908 The "ice-ufrag" and "ice-pwd" attributes convey the username fragment 909 and password used by ICE for message integrity. Their syntax is: 911 ice-pwd-att = "ice-pwd:" password 912 ice-ufrag-att = "ice-ufrag:" ufrag 913 password = 22*256ice-char 914 ufrag = 4*256ice-char 916 The "ice-pwd" and "ice-ufrag" attributes can appear at either the 917 session-level or media-level. When present in both, the value in the 918 media-level takes precedence. Thus, the value at the session-level 919 is effectively a default that applies to all data streams, unless 920 overridden by a media-level value. Whether present at the session or 921 media-level, there MUST be an ice-pwd and ice-ufrag attribute for 922 each data stream. If two data streams have identical ice-ufrag's, 923 they MUST have identical ice-pwd's. 925 The ice-ufrag and ice-pwd attributes MUST be chosen randomly at the 926 beginning of a session (the same applies when ICE is restarting for 927 an agent). 929 The ice-ufrag attribute MUST contain at least 24 bits of randomness, 930 and the ice-pwd attribute MUST contain at least 128 bits of 931 randomness. This means that the ice-ufrag attribute will be at least 932 4 characters long, and the ice-pwd at least 22 characters long, since 933 the grammar for these attributes allows for 6 bits of information per 934 character. The attributes MAY be longer than 4 and 22 characters, 935 respectively, of course, up to 256 characters. The upper limit 936 allows for buffer sizing in implementations. Its large upper limit 937 allows for increased amounts of randomness to be added over time. 938 For compatibility with the 512 character limitation for the STUN 939 username attribute value and for bandwidth conservation 940 considerations, the ice-ufrag attribute MUST NOT be longer than 32 941 characters when sending, but an implementation MUST accept up to 256 942 characters when receiving. 944 Example shows sample ice-ufrag and ice-pwd SDP lines: 946 a=ice-pwd:asd88fgpdd777uzjYhagZg 947 a=ice-ufrag:8hhY 949 4.5. "ice-pacing" Attribute 951 The "ice-pacing" is a session level attribute that indicates the 952 desired connectivity check pacing, in milliseconds, for this agent 953 (see section 14 of [RFC8445]). The syntax is: 955 ice-pacing-att = "ice-pacing:" pacing-value 956 pacing-value = 1*10DIGIT 958 Following the procedures defined in [RFC8445], a default value of 959 50ms is used for an agent when the ice-pacing attribute is omitted in 960 the offer or the answer. 962 The same rule applies for ice-pacing attribute values lower than 963 50ms. This mandates that, if an agent includes the ice-pacing 964 attribute, its value MUST be greater than 50ms or else a value of 965 50ms is considered by default for that agent. 967 Also the larger of the ice-pacing attribute values between the offer 968 and the answer (determined either by the one provided in the ice- 969 pacing attribute or by picking the default value) MUST be considered 970 for a given ICE session. 972 4.6. "ice-options" Attribute 974 The "ice-options" attribute is a session- and media-level attribute. 975 It contains a series of tokens that identify the options supported by 976 the agent. Its grammar is: 978 ice-options = "ice-options:" ice-option-tag 979 0*(SP ice-option-tag) 980 ice-option-tag = 1*ice-char 982 The existence of an ice-option in an offer indicates that a certain 983 extension is supported by the agent and it is willing to use it, if 984 the peer agent also includes the same extension in the answer. There 985 might be further extension specific negotiation needed between the 986 agents that determine how the extension gets used in a given session. 987 The details of the negotiation procedures, if present, MUST be 988 defined by the specification defining the extension (see 989 Section 9.2). 991 Example shows 'rtp+ecn' ice-option SDP line from <>: 993 a=ice-options:rtp+ecn 995 5. Keepalives 997 All the ICE agents MUST follow the procedures defined in section 11 998 of [RFC8445] for sending keepalives. The keepalives MUST be sent 999 regardless of whether the data stream is currently inactive, 1000 sendonly, recvonly, or sendrecv, and regardless of the presence or 1001 value of the bandwidth attribute. An agent can determine that its 1002 peer supports ICE by the presence of "a=candidate" attributes for 1003 each media session. 1005 6. SIP Considerations 1007 Note that ICE is not intended for NAT traversal for SIP, which is 1008 assumed to be provided via another mechanism [RFC5626]. 1010 When ICE is used with SIP, forking may result in a single offer 1011 generating a multiplicity of answers. In that case, ICE proceeds 1012 completely in parallel and independently for each answer, treating 1013 the combination of its offer and each answer as an independent offer/ 1014 answer exchange, with its own set of local candidates, pairs, check 1015 lists, states, and so on. 1017 Once ICE processing has reached the Completed state for all peers for 1018 media streams using those candidates, the agent SHOULD wait an 1019 additional three seconds, and then it MAY cease responding to checks 1020 or generating triggered checks on that candidate. It MAY free the 1021 candidate at that time. Freeing of server reflexive candidates is 1022 never explicit; it happens by lack of a keepalive. The three-second 1023 delay handles cases when aggressive nomination is used, and the 1024 selected pairs can quickly change after ICE has completed. 1026 6.1. Latency Guidelines 1028 ICE requires a series of STUN-based connectivity checks to take place 1029 between endpoints. These checks start from the answerer on 1030 generation of its answer, and start from the offerer when it receives 1031 the answer. These checks can take time to complete, and as such, the 1032 selection of messages to use with offers and answers can affect 1033 perceived user latency. Two latency figures are of particular 1034 interest. These are the post-pickup delay and the post-dial delay. 1035 The post-pickup delay refers to the time between when a user "answers 1036 the phone" and when any speech they utter can be delivered to the 1037 caller. The post-dial delay refers to the time between when a user 1038 enters the destination address for the user and ringback begins as a 1039 consequence of having successfully started alerting the called user 1040 agent. 1042 Two cases can be considered -- one where the offer is present in the 1043 initial INVITE and one where it is in a response. 1045 6.1.1. Offer in INVITE 1047 To reduce post-dial delays, it is RECOMMENDED that the caller begin 1048 gathering candidates prior to actually sending its initial INVITE, so 1049 that the candidates can be provided in the INVITE. This can be 1050 started upon user interface cues that a call is pending, such as 1051 activity on a keypad or the phone going off-hook. 1053 On the receipt of the offer, the answerer SHOULD generate an answer 1054 in a provisional response as soon as it has completed gathering the 1055 candidates. ICE requires that a provisional response with an SDP be 1056 transmitted reliably. This can be done through the existing 1057 Provisional Response Acknowledgment (PRACK) mechanism [RFC3262] or 1058 through an ICE specific optimization, wherein, the agent retransmits 1059 the provisional response with the exponential backoff timers 1060 described in [RFC3262]. Such retransmissions MUST cease on receipt 1061 of a STUN Binding request with transport address matching candidate 1062 address for one of the data streams signaled in that SDP or on 1063 transmission of the answer in a 2xx response. If no Binding request 1064 is received prior to the last retransmit, the agent does not consider 1065 the session terminated. For the ICE lite peers , the agent MUST 1066 cease retransmitting the 18x after sending it four times since there 1067 will be no Binding request sent and the number four is arbitrarily 1068 chosen to limit the number of 18x retransmits (poor man's version of 1069 [RFC3262] basically). (ICE will actually work even if the peer never 1070 receives the 18x; however, experience has shown that sending it is 1071 important for middleboxes and firewall traversal). 1073 Once the answer has been sent, the agent SHOULD begin its 1074 connectivity checks. Once candidate pairs for each component of a 1075 data stream enter the valid list, the answerer can begin sending 1076 media on that data stream. 1078 However, prior to this point, any media that needs to be sent towards 1079 the caller (such as SIP early media [RFC3960]) MUST NOT be 1080 transmitted. For this reason, implementations SHOULD delay alerting 1081 the called party until candidates for each component of each data 1082 stream have entered the valid list. In the case of a PSTN gateway, 1083 this would mean that the setup message into the PSTN is delayed until 1084 this point. Doing this increases the post-dial delay, but has the 1085 effect of eliminating 'ghost rings'. Ghost rings are cases where the 1086 called party hears the phone ring, picks up, but hears nothing and 1087 cannot be heard. This technique works without requiring support for, 1088 or usage of, preconditions [RFC3312]. It also has the benefit of 1089 guaranteeing that not a single packet of media will get clipped, so 1090 that post-pickup delay is zero. If an agent chooses to delay local 1091 alerting in this way, it SHOULD generate a 180 response once alerting 1092 begins. 1094 6.1.2. Offer in Response 1096 In addition to uses where the offer is in an INVITE, and the answer 1097 is in the provisional and/or 200 OK response, ICE works with cases 1098 where the offer appears in the response. In such cases, which are 1099 common in third party call control [RFC3725], ICE agents SHOULD 1100 generate their offers in a reliable provisional response (which MUST 1101 utilize [RFC3262]), and not alert the user on receipt of the INVITE. 1102 The answer will arrive in a PRACK. This allows for ICE processing to 1103 take place prior to alerting, so that there is no post-pickup delay, 1104 at the expense of increased call setup delays. Once ICE completes, 1105 the callee can alert the user and then generate a 200 OK when they 1106 answer. The 200 OK would contain no SDP, since the offer/answer 1107 exchange has completed. 1109 Alternatively, agents MAY place the offer in a 2xx instead (in which 1110 case the answer comes in the ACK). When this happens, the callee 1111 will alert the user on receipt of the INVITE, and the ICE exchanges 1112 will take place only after the user answers. This has the effect of 1113 reducing call setup delay, but can cause substantial post-pickup 1114 delays and media clipping. 1116 6.2. SIP Option Tags and Media Feature Tags 1118 [RFC5768] specifies a SIP option tag and media feature tag for usage 1119 with ICE. ICE implementations using SIP SHOULD support this 1120 specification, which uses a feature tag in registrations to 1121 facilitate interoperability through signaling intermediaries. 1123 6.3. Interactions with Forking 1125 ICE interacts very well with forking. Indeed, ICE fixes some of the 1126 problems associated with forking. Without ICE, when a call forks and 1127 the caller receives multiple incoming data streams, it cannot 1128 determine which data stream corresponds to which callee. 1130 With ICE, this problem is resolved. The connectivity checks which 1131 occur prior to transmission of media carry username fragments, which 1132 in turn are correlated to a specific callee. Subsequent media 1133 packets that arrive on the same candidate pair as the connectivity 1134 check will be associated with that same callee. Thus, the caller can 1135 perform this correlation as long as it has received an answer. 1137 6.4. Interactions with Preconditions 1139 Quality of Service (QoS) preconditions, which are defined in 1140 [RFC3312] and [RFC4032], apply only to the transport addresses listed 1141 as the default targets for media in an offer/answer. If ICE changes 1142 the transport address where media is received, this change is 1143 reflected in an updated offer that changes the default destination 1144 for media to match ICE's selection. As such, it appears like any 1145 other re-INVITE would, and is fully treated in RFCs 3312 and 4032, 1146 which apply without regard to the fact that the destination for media 1147 is changing due to ICE negotiations occurring "in the background". 1149 Indeed, an agent SHOULD NOT indicate that QoS preconditions have been 1150 met until the checks have completed and selected the candidate pairs 1151 to be used for media. 1153 ICE also has (purposeful) interactions with connectivity 1154 preconditions [RFC5898]. Those interactions are described there. 1155 Note that the procedures described in Section 6.1 describe their own 1156 type of "preconditions", albeit with less functionality than those 1157 provided by the explicit preconditions in [RFC5898]. 1159 6.5. Interactions with Third Party Call Control 1161 ICE works with Flows I, III, and IV as described in [RFC3725]. Flow 1162 I works without the controller supporting or being aware of ICE. 1163 Flow IV will work as long as the controller passes along the ICE 1164 attributes without alteration. Flow II is fundamentally incompatible 1165 with ICE; each agent will believe itself to be the answerer and thus 1166 never generate a re-INVITE. 1168 The flows for continued operation, as described in Section 7 of 1169 [RFC3725], require additional behavior of ICE implementations to 1170 support. In particular, if an agent receives a mid-dialog re-INVITE 1171 that contains no offer, it MUST restart ICE for each data stream and 1172 go through the process of gathering new candidates. Furthermore, 1173 that list of candidates SHOULD include the ones currently being used 1174 for media. 1176 7. Relationship with ANAT 1178 [RFC4091], the Alternative Network Address Types (ANAT) Semantics for 1179 the SDP grouping framework, and [RFC4092], its usage with SIP, define 1180 a mechanism for indicating that an agent can support both IPv4 and 1181 IPv6 for a data stream, and it does so by including two "m=" lines, 1182 one for v4 and one for v6. This is similar to ICE, which allows for 1183 an agent to indicate multiple transport addresses using the candidate 1184 attribute. However, ANAT relies on static selection to pick between 1185 choices, rather than a dynamic connectivity check used by ICE. 1187 It is RECOMMENDED that ICE be used in realizing the dual-stack use- 1188 cases in agents that support ICE. 1190 8. Security Considerations 1192 8.1. Attacks on the Offer/Answer Exchanges 1194 An attacker that can modify or disrupt the offer/answer exchanges 1195 themselves can readily launch a variety of attacks with ICE. They 1196 could direct media to a target of a DoS attack, they could insert 1197 themselves into the data stream, and so on. These are similar to the 1198 general security considerations for offer/answer exchanges, and the 1199 security considerations in [RFC3264] apply. These require techniques 1200 for message integrity and encryption for offers and answers, which 1201 are satisfied by the TLS mechanism [RFC3261] when SIP is used. As 1202 such, the usage of TLS with ICE is RECOMMENDED. 1204 8.2. Insider Attacks 1206 In addition to attacks where the attacker is a third party trying to 1207 insert fake offers, answers, or STUN messages, there are several 1208 attacks possible with ICE when the attacker is an authenticated and 1209 valid participant in the ICE exchange. 1211 8.2.1. The Voice Hammer Attack 1213 The voice hammer attack is an amplification attack. In this attack, 1214 the attacker initiates sessions to other agents, and maliciously 1215 includes the connection address and port of a DoS target as the 1216 destination for media traffic signaled in the SDP. This causes 1217 substantial amplification; a single offer/answer exchange can create 1218 a continuing flood of media packets, possibly at high rates (consider 1219 video sources). This attack is not specific to ICE, but ICE can help 1220 provide remediation. 1222 Specifically, if ICE is used, the agent receiving the malicious SDP 1223 will first perform connectivity checks to the target of media before 1224 sending media there. If this target is a third-party host, the 1225 checks will not succeed, and media is never sent. 1227 Unfortunately, ICE doesn't help if it's not used, in which case an 1228 attacker could simply send the offer without the ICE parameters. 1229 However, in environments where the set of clients is known, and is 1230 limited to ones that support ICE, the server can reject any offers or 1231 answers that don't indicate ICE support. 1233 SIP User Agents (UA) [RFC3261] that are not willing to receive non- 1234 ICE answers MUST include an "ice" Option Tag [RFC5768] in the SIP 1235 Require Header Field in their offer. UAs that rejects non-ICE offers 1236 SHOULD use a 421 response code, together with an Option Tag "ice" in 1237 the Require Header Field in the response. 1239 8.2.2. Interactions with Application Layer Gateways and SIP 1241 Application Layer Gateways (ALGs) are functions present in a Network 1242 Address Translation (NAT) device that inspect the contents of packets 1243 and modify them, in order to facilitate NAT traversal for application 1244 protocols. Session Border Controllers (SBCs) are close cousins of 1245 ALGs, but are less transparent since they actually exist as 1246 application-layer SIP intermediaries. ICE has interactions with SBCs 1247 and ALGs. 1249 If an ALG is SIP aware but not ICE aware, ICE will work through it as 1250 long as the ALG correctly modifies the SDP. A correct ALG 1251 implementation behaves as follows: 1253 o The ALG does not modify the "m=" and "c=" lines or the rtcp 1254 attribute if they contain external addresses. 1256 o If the "m=" and "c=" lines contain internal addresses, the 1257 modification depends on the state of the ALG: 1259 * If the ALG already has a binding established that maps an 1260 external port to an internal connection address and port 1261 matching the values in the "m=" and "c=" lines or rtcp 1262 attribute, the ALG uses that binding instead of creating a new 1263 one. 1265 * If the ALG does not already have a binding, it creates a new 1266 one and modifies the SDP, rewriting the "m=" and "c=" lines and 1267 rtcp attribute. 1269 Unfortunately, many ALGs are known to work poorly in these corner 1270 cases. ICE does not try to work around broken ALGs, as this is 1271 outside the scope of its functionality. ICE can help diagnose these 1272 conditions, which often show up as a mismatch between the set of 1273 candidates and the "m=" and "c=" lines and rtcp attributes. The ice- 1274 mismatch attribute is used for this purpose. 1276 ICE works best through ALGs when the signaling is run over TLS. This 1277 prevents the ALG from manipulating the SDP messages and interfering 1278 with ICE operation. Implementations that are expected to be deployed 1279 behind ALGs SHOULD provide for TLS transport of the SDP. 1281 If an SBC is SIP aware but not ICE aware, the result depends on the 1282 behavior of the SBC. If it is acting as a proper Back-to-Back User 1283 Agent (B2BUA), the SBC will remove any SDP attributes it doesn't 1284 understand, including the ICE attributes. Consequently, the call 1285 will appear to both endpoints as if the other side doesn't support 1286 ICE. This will result in ICE being disabled, and media flowing 1287 through the SBC, if the SBC has requested it. If, however, the SBC 1288 passes the ICE attributes without modification, yet modifies the 1289 default destination for media (contained in the "m=" and "c=" lines 1290 and rtcp attribute), this will be detected as an ICE mismatch, and 1291 ICE processing is aborted for the call. It is outside of the scope 1292 of ICE for it to act as a tool for "working around" SBCs. If one is 1293 present, ICE will not be used and the SBC techniques take precedence. 1295 9. IANA Considerations 1297 9.1. SDP Attributes 1299 The original ICE specification defined seven new SDP attributes per 1300 the procedures of Section 8.2.4 of [RFC4566]. The registration 1301 information from the original specification is included here with 1302 modifications to include Mux Category and also defines a new SDP 1303 attribute 'ice-pacing'. 1305 9.1.1. candidate Attribute 1307 Attribute Name: candidate 1309 Type of Attribute: media-level 1311 Subject to charset: No 1313 Purpose: This attribute is used with Interactive Connectivity 1314 Establishment (ICE), and provides one of many possible candidate 1315 addresses for communication. These addresses are validated with 1316 an end-to-end connectivity check using Session Traversal Utilities 1317 for NAT (STUN). 1319 Appropriate Values: See Section 4 of RFC XXXX. 1321 Contact Name: IESG 1323 Contact e-mail: iesg@ietf.org [1] 1325 Reference: RFCXXXX 1327 Mux Category: TRANSPORT 1329 9.1.2. remote-candidates Attribute 1331 Attribute Name: remote-candidates 1333 Type of Attribute: media-level 1335 Subject to charset: No 1337 Purpose: This attribute is used with Interactive Connectivity 1338 Establishment (ICE), and provides the identity of the remote 1339 candidates that the offerer wishes the answerer to use in its 1340 answer. 1342 Appropriate Values: See Section 4 of RFC XXXX. 1344 Contact Name: IESG 1346 Contact e-mail: iesg@ietf.org [2] 1348 Reference: RFCXXXX 1350 Mux Category: TRANSPORT 1352 9.1.3. ice-lite Attribute 1354 Attribute Name: ice-lite 1356 Type of Attribute: session-level 1358 Subject to charset: No 1360 Purpose: This attribute is used with Interactive Connectivity 1361 Establishment (ICE), and indicates that an agent has the minimum 1362 functionality required to support ICE inter-operation with a peer 1363 that has a full implementation. 1365 Appropriate Values: See Section 4 of RFC XXXX. 1367 Contact Name: IESG 1368 Contact e-mail: iesg@ietf.org [3] 1370 Reference: RFCXXXX 1372 Mux Category: NORMAL 1374 9.1.4. ice-mismatch Attribute 1376 Attribute Name: ice-mismatch 1378 Type of Attribute: media-level 1380 Subject to charset: No 1382 Purpose: This attribute is used with Interactive Connectivity 1383 Establishment (ICE), and indicates that an agent is ICE capable, 1384 but did not proceed with ICE due to a mismatch of candidates with 1385 the default destination for media signaled in the SDP. 1387 Appropriate Values: See Section 4 of RFC XXXX. 1389 Contact Name: IESG 1391 Contact e-mail: iesg@ietf.org [4] 1393 Reference: RFCXXXX 1395 Mux Category: NORMAL 1397 9.1.5. ice-pwd Attribute 1399 Attribute Name: ice-pwd 1401 Type of Attribute: session- or media-level 1403 Subject to charset: No 1405 Purpose: This attribute is used with Interactive Connectivity 1406 Establishment (ICE), and provides the password used to protect 1407 STUN connectivity checks. 1409 Appropriate Values: See Section 4 of RFC XXXX. 1411 Contact Name: IESG 1413 Contact e-mail: iesg@ietf.org [5] 1415 Reference: RFCXXXX 1416 Mux Category: TRANSPORT 1418 9.1.6. ice-ufrag Attribute 1420 Attribute Name: ice-ufrag 1422 Type of Attribute: session- or media-level 1424 Subject to charset: No 1426 Purpose: This attribute is used with Interactive Connectivity 1427 Establishment (ICE), and provides the fragments used to construct 1428 the username in STUN connectivity checks. 1430 Appropriate Values: See Section 4 of RFC XXXX. 1432 Contact Name: IESG 1434 Contact e-mail: iesg@ietf.org [6] 1436 Reference: RFCXXXX 1438 Mux Category: TRANSPORT 1440 9.1.7. ice-options Attribute 1442 Attribute Name: ice-options 1444 Long Form: ice-options 1446 Type of Attribute: session-level 1448 Subject to charset: No 1450 Purpose: This attribute is used with Interactive Connectivity 1451 Establishment (ICE), and indicates the ICE options or extensions 1452 used by the agent. 1454 Appropriate Values: See Section 4 of RFC XXXX. 1456 Contact Name: IESG 1458 Contact e-mail: iesg@ietf.org [7] 1460 Reference: RFCXXXX 1462 Mux Category: NORMAL 1464 9.1.8. ice-pacing Attribute 1466 This specification also defines a new SDP attribute, "ice-pacing" 1467 according to the following data: 1469 Attribute Name: ice-pacing 1471 Type of Attribute: session-level 1473 Subject to charset: No 1475 Purpose: This attribute is used with Interactive Connectivity 1476 Establishment (ICE) to indicate desired connectivity check pacing 1477 values. 1479 Appropriate Values: See Section 4 of RFC XXXX. 1481 Contact Name: IESG 1483 Contact e-mail: iesg@ietf.org [8] 1485 Reference: RFCXXXX 1487 Mux Category: NORMAL 1489 9.2. Interactive Connectivity Establishment (ICE) Options Registry 1491 IANA maintains a registry for ice-options identifiers under the 1492 Specification Required policy as defined in "Guidelines for Writing 1493 an IANA Considerations Section in RFCs" [RFC5226]. 1495 ICE options are of unlimited length according to the syntax in 1496 Section 4.6; however, they are RECOMMENDED to be no longer than 20 1497 characters. This is to reduce message sizes and allow for efficient 1498 parsing. ICE options are defined at the session level. 1500 A registration request MUST include the following information: 1502 o The ICE option identifier to be registered 1504 o Name, Email, and Address of a contact person for the registration 1506 o Organization or individuals having the change control 1508 o Short description of the ICE extension to which the option relates 1510 o Reference(s) to the specification defining the ICE option and the 1511 related extensions 1513 10. Acknowledgments 1515 A large part of the text in this document was taken from [RFC5245], 1516 authored by Jonathan Rosenberg. 1518 Some of the text in this document was taken from [RFC6336], authored 1519 by Magnus Westerlund and Colin Perkins. 1521 Many thanks to Christer Holmberg for providing text suggestions in 1522 Section 3 that aligns with [RFC8445] 1524 Thanks to Thomas Stach for text help, Roman Shpount for suggesting 1525 RTCP candidate handling and Simon Perreault for advising on IPV6 1526 address selection when candidate-address includes FQDN. 1528 Many thanks to Flemming Andreasen for shepherd review feedback. 1530 Thanks to following experts for their reviews and constructive 1531 feedback: Christer Holmberg, Adam Roach, Peter Saint-Andre and the 1532 MMUSIC WG. 1534 11. References 1536 11.1. Normative References 1538 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 1539 Requirement Levels", BCP 14, RFC 2119, 1540 DOI 10.17487/RFC2119, March 1997, 1541 . 1543 [RFC3261] Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston, 1544 A., Peterson, J., Sparks, R., Handley, M., and E. 1545 Schooler, "SIP: Session Initiation Protocol", RFC 3261, 1546 DOI 10.17487/RFC3261, June 2002, 1547 . 1549 [RFC3262] Rosenberg, J. and H. Schulzrinne, "Reliability of 1550 Provisional Responses in Session Initiation Protocol 1551 (SIP)", RFC 3262, DOI 10.17487/RFC3262, June 2002, 1552 . 1554 [RFC3264] Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model 1555 with Session Description Protocol (SDP)", RFC 3264, 1556 DOI 10.17487/RFC3264, June 2002, 1557 . 1559 [RFC3312] Camarillo, G., Ed., Marshall, W., Ed., and J. Rosenberg, 1560 "Integration of Resource Management and Session Initiation 1561 Protocol (SIP)", RFC 3312, DOI 10.17487/RFC3312, October 1562 2002, . 1564 [RFC3556] Casner, S., "Session Description Protocol (SDP) Bandwidth 1565 Modifiers for RTP Control Protocol (RTCP) Bandwidth", 1566 RFC 3556, DOI 10.17487/RFC3556, July 2003, 1567 . 1569 [RFC3605] Huitema, C., "Real Time Control Protocol (RTCP) attribute 1570 in Session Description Protocol (SDP)", RFC 3605, 1571 DOI 10.17487/RFC3605, October 2003, 1572 . 1574 [RFC4032] Camarillo, G. and P. Kyzivat, "Update to the Session 1575 Initiation Protocol (SIP) Preconditions Framework", 1576 RFC 4032, DOI 10.17487/RFC4032, March 2005, 1577 . 1579 [RFC4091] Camarillo, G. and J. Rosenberg, "The Alternative Network 1580 Address Types (ANAT) Semantics for the Session Description 1581 Protocol (SDP) Grouping Framework", RFC 4091, June 2005, 1582 . 1584 [RFC4092] Camarillo, G. and J. Rosenberg, "Usage of the Session 1585 Description Protocol (SDP) Alternative Network Address 1586 Types (ANAT) Semantics in the Session Initiation Protocol 1587 (SIP)", RFC 4092, June 2005, 1588 . 1590 [RFC4566] Handley, M., Jacobson, V., and C. Perkins, "SDP: Session 1591 Description Protocol", RFC 4566, DOI 10.17487/RFC4566, 1592 July 2006, . 1594 [RFC5226] Narten, T. and H. Alvestrand, "Guidelines for Writing an 1595 IANA Considerations Section in RFCs", BCP 26, RFC 5226, 1596 DOI 10.17487/RFC5226, May 2008, 1597 . 1599 [RFC5234] Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax 1600 Specifications: ABNF", STD 68, RFC 5234, 1601 DOI 10.17487/RFC5234, January 2008, 1602 . 1604 [RFC5768] Rosenberg, J., "Indicating Support for Interactive 1605 Connectivity Establishment (ICE) in the Session Initiation 1606 Protocol (SIP)", RFC 5768, DOI 10.17487/RFC5768, April 1607 2010, . 1609 [RFC6336] Westerlund, M. and C. Perkins, "IANA Registry for 1610 Interactive Connectivity Establishment (ICE) Options", 1611 RFC 6336, April 2010, 1612 . 1614 [RFC8445] Keranen, A., Holmberg, C., and J. Rosenberg, "Interactive 1615 Connectivity Establishment (ICE): A Protocol for Network 1616 Address Translator (NAT) Traversal", RFC 8445, 1617 DOI 10.17487/RFC8445, July 2018, 1618 . 1620 11.2. Informative References 1622 [draft-holmberg-ice-pac] 1623 Holmberg, C. and J. Uberti, "Interactive Connectivity 1624 Establishment Patiently Awaiting Connectivity (ICE PAC)", 1625 draft-holmberg-ice-pac-01 (work in progress), March 2019, 1626 . 1629 [RFC3725] Rosenberg, J., Peterson, J., Schulzrinne, H., and G. 1630 Camarillo, "Best Current Practices for Third Party Call 1631 Control (3pcc) in the Session Initiation Protocol (SIP)", 1632 BCP 85, RFC 3725, DOI 10.17487/RFC3725, April 2004, 1633 . 1635 [RFC3960] Camarillo, G. and H. Schulzrinne, "Early Media and Ringing 1636 Tone Generation in the Session Initiation Protocol (SIP)", 1637 RFC 3960, DOI 10.17487/RFC3960, December 2004, 1638 . 1640 [RFC5245] Rosenberg, J., "Interactive Connectivity Establishment 1641 (ICE): A Protocol for Network Address Translator (NAT) 1642 Traversal for Offer/Answer Protocols", RFC 5245, 1643 DOI 10.17487/RFC5245, April 2010, 1644 . 1646 [RFC5626] Jennings, C., Ed., Mahy, R., Ed., and F. Audet, Ed., 1647 "Managing Client-Initiated Connections in the Session 1648 Initiation Protocol (SIP)", RFC 5626, 1649 DOI 10.17487/RFC5626, October 2009, 1650 . 1652 [RFC5898] Andreasen, F., Camarillo, G., Oran, D., and D. Wing, 1653 "Connectivity Preconditions for Session Description 1654 Protocol (SDP) Media Streams", RFC 5898, 1655 DOI 10.17487/RFC5898, July 2010, 1656 . 1658 11.3. URIs 1660 [1] mailto:iesg@ietf.org 1662 [2] mailto:iesg@ietf.org 1664 [3] mailto:iesg@ietf.org 1666 [4] mailto:iesg@ietf.org 1668 [5] mailto:iesg@ietf.org 1670 [6] mailto:iesg@ietf.org 1672 [7] mailto:iesg@ietf.org 1674 [8] mailto:iesg@ietf.org 1676 [9] mailto:christer.holmberg@ericsson.com 1678 [10] mailto:rshpount@turbobridge.com 1680 [11] mailto:thomass.stach@gmail.com 1682 Appendix A. Examples 1684 For the example shown in section 15 of [RFC8445] the resulting offer 1685 (message 5) encoded in SDP looks like: 1687 v=0 1688 o=jdoe 2890844526 2890842807 IN IP6 $L-PRIV-1.IP 1689 s= 1690 c=IN IP6 $NAT-PUB-1.IP 1691 t=0 0 1692 a=ice-pwd:asd88fgpdd777uzjYhagZg 1693 a=ice-ufrag:8hhY 1694 m=audio $NAT-PUB-1.PORT RTP/AVP 0 1695 b=RS:0 1696 b=RR:0 1697 a=rtpmap:0 PCMU/8000 1698 a=candidate:1 1 UDP 2130706431 $L-PRIV-1.IP $L-PRIV-1.PORT typ host 1699 a=candidate:2 1 UDP 1694498815 $NAT-PUB-1.IP $NAT-PUB-1.PORT typ 1700 srflx raddr $L-PRIV-1.IP rport $L-PRIV-1.PORT 1702 The offer, with the variables replaced with their values, will look 1703 like (lines folded for clarity): 1705 v=0 1706 o=jdoe 2890844526 2890842807 IN IP6 fe80::6676:baff:fe9c:ee4a 1707 s= 1708 c=IN IP6 2001:DB8:8101:3a55:4858:a2a9:22ff:99b9 1709 t=0 0 1710 a=ice-pwd:asd88fgpdd777uzjYhagZg 1711 a=ice-ufrag:8hhY 1712 m=audio 45664 RTP/AVP 0 1713 b=RS:0 1714 b=RR:0 1715 a=rtpmap:0 PCMU/8000 1716 a=candidate:1 1 UDP 2130706431 fe80::6676:baff:fe9c:ee4a 8998 typ host 1717 a=candidate:2 1 UDP 1694498815 2001:DB8:8101:3a55:4858:a2a9:22ff:99b9 45664 typ srflx raddr 1718 fe80::6676:baff:fe9c:ee4a rport 8998 1720 The resulting answer looks like: 1722 v=0 1723 o=bob 2808844564 2808844564 IN IP4 $R-PUB-1.IP 1724 s= 1725 c=IN IP4 $R-PUB-1.IP 1726 t=0 0 1727 a=ice-pwd:YH75Fviy6338Vbrhrlp8Yh 1728 a=ice-ufrag:9uB6 1729 m=audio $R-PUB-1.PORT RTP/AVP 0 1730 b=RS:0 1731 b=RR:0 1732 a=rtpmap:0 PCMU/8000 1733 a=candidate:1 1 UDP 2130706431 $R-PUB-1.IP $R-PUB-1.PORT typ host 1734 With the variables filled in: 1736 v=0 1737 o=bob 2808844564 2808844564 IN IP4 192.0.2.1 1738 s= 1739 c=IN IP4 192.0.2.1 1740 t=0 0 1741 a=ice-pwd:YH75Fviy6338Vbrhrlp8Yh 1742 a=ice-ufrag:9uB6 1743 m=audio 3478 RTP/AVP 0 1744 b=RS:0 1745 b=RR:0 1746 a=rtpmap:0 PCMU/8000 1747 a=candidate:1 1 UDP 2130706431 192.0.2.1 3478 typ host 1749 Appendix B. The remote-candidates Attribute 1751 The "a=remote-candidates" attribute exists to eliminate a race 1752 condition between the updated offer and the response to the STUN 1753 Binding request that moved a candidate into the Valid list. This 1754 race condition is shown in Figure 1. On receipt of message 4, agent 1755 L adds a candidate pair to the valid list. If there was only a 1756 single data stream with a single component, agent L could now send an 1757 updated offer. However, the check from agent R has not yet generated 1758 a response, and agent R receives the updated offer (message 7) before 1759 getting the response (message 9). Thus, it does not yet know that 1760 this particular pair is valid. To eliminate this condition, the 1761 actual candidates at R that were selected by the offerer (the remote 1762 candidates) are included in the offer itself, and the answerer delays 1763 its answer until those pairs validate. 1765 Agent L Network Agent R 1766 |(1) Offer | | 1767 |------------------------------------------>| 1768 |(2) Answer | | 1769 |<------------------------------------------| 1770 |(3) STUN Req. | | 1771 |------------------------------------------>| 1772 |(4) STUN Res. | | 1773 |<------------------------------------------| 1774 |(5) STUN Req. | | 1775 |<------------------------------------------| 1776 |(6) STUN Res. | | 1777 |-------------------->| | 1778 | |Lost | 1779 |(7) Offer | | 1780 |------------------------------------------>| 1781 |(8) STUN Req. | | 1782 |<------------------------------------------| 1783 |(9) STUN Res. | | 1784 |------------------------------------------>| 1785 |(10) Answer | | 1786 |<------------------------------------------| 1788 Figure 1: Race Condition Flow 1790 Appendix C. Why Is the Conflict Resolution Mechanism Needed? 1792 When ICE runs between two peers, one agent acts as controlled, and 1793 the other as controlling. Rules are defined as a function of 1794 implementation type and offerer/answerer to determine who is 1795 controlling and who is controlled. However, the specification 1796 mentions that, in some cases, both sides might believe they are 1797 controlling, or both sides might believe they are controlled. How 1798 can this happen? 1800 The condition when both agents believe they are controlled shows up 1801 in third party call control cases. Consider the following flow: 1803 A Controller B 1804 |(1) INV() | | 1805 |<-------------| | 1806 |(2) 200(SDP1) | | 1807 |------------->| | 1808 | |(3) INV() | 1809 | |------------->| 1810 | |(4) 200(SDP2) | 1811 | |<-------------| 1812 |(5) ACK(SDP2) | | 1813 |<-------------| | 1814 | |(6) ACK(SDP1) | 1815 | |------------->| 1817 Figure 2: Role Conflict Flow 1819 This flow is a variation on flow III of RFC 3725 [RFC3725]. In fact, 1820 it works better than flow III since it produces fewer messages. In 1821 this flow, the controller sends an offerless INVITE to agent A, which 1822 responds with its offer, SDP1. The agent then sends an offerless 1823 INVITE to agent B, which it responds to with its offer, SDP2. The 1824 controller then uses the offer from each agent to generate the 1825 answers. When this flow is used, ICE will run between agents A and 1826 B, but both will believe they are in the controlling role. With the 1827 role conflict resolution procedures, this flow will function properly 1828 when ICE is used. 1830 At this time, there are no documented flows that can result in the 1831 case where both agents believe they are controlled. However, the 1832 conflict resolution procedures allow for this case, should a flow 1833 arise that would fit into this category. 1835 Appendix D. Why Send an Updated Offer? 1837 Section 11.1 describes rules for sending media. Both agents can send 1838 media once ICE checks complete, without waiting for an updated offer. 1839 Indeed, the only purpose of the updated offer is to "correct" the SDP 1840 so that the default destination for media matches where media is 1841 being sent based on ICE procedures (which will be the highest- 1842 priority nominated candidate pair). 1844 This begs the question -- why is the updated offer/answer exchange 1845 needed at all? Indeed, in a pure offer/answer environment, it would 1846 not be. The offerer and answerer will agree on the candidates to use 1847 through ICE, and then can begin using them. As far as the agents 1848 themselves are concerned, the updated offer/answer provides no new 1849 information. However, in practice, numerous components along the 1850 signaling path look at the SDP information. These include entities 1851 performing off-path QoS reservations, NAT traversal components such 1852 as ALGs and Session Border Controllers (SBCs), and diagnostic tools 1853 that passively monitor the network. For these tools to continue to 1854 function without change, the core property of SDP -- that the 1855 existing, pre-ICE definitions of the addresses used for media -- the 1856 "m=" and "c=" lines and the rtcp attribute -- must be retained. For 1857 this reason, an updated offer must be sent. 1859 Appendix E. Contributors 1861 Following experts have contributed textual and structural 1862 improvements for this work 1864 1. Christer Holmberg 1866 * Ericsson 1868 * Email: christer.holmberg@ericsson.com [9] 1870 2. Roman Shpount 1872 * TurboBridge 1874 * rshpount@turbobridge.com [10] 1876 3. Thomas Stach 1878 * thomass.stach@gmail.com [11] 1880 Authors' Addresses 1882 Marc Petit-Huguenin 1883 Impedance Mismatch 1885 Email: marc@petit-huguenin.org 1887 Suhas Nandakumar 1888 Cisco Systems 1889 707 Tasman Dr 1890 Milpitas, CA 95035 1891 USA 1893 Email: snandaku@cisco.com 1894 Ari Keranen 1895 Ericsson 1896 Jorvas 02420 1897 Finland 1899 Email: ari.keranen@ericsson.com