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If these are generic example addresses, they should be changed to use any of the ranges defined in RFC 6890 (or successor): 192.0.2.x, 198.51.100.x or 203.0.113.x. == There are 1 instance of lines with non-RFC3849-compliant IPv6 addresses in the document. If these are example addresses, they should be changed. -- The document has examples using IPv4 documentation addresses according to RFC6890, but does not use any IPv6 documentation addresses. Maybe there should be IPv6 examples, too? -- The draft header indicates that this document obsoletes RFC5245, but the abstract doesn't seem to mention this, which it should. Miscellaneous warnings: ---------------------------------------------------------------------------- == The copyright year in the IETF Trust and authors Copyright Line does not match the current year == Using lowercase 'not' together with uppercase 'MUST', 'SHALL', 'SHOULD', or 'RECOMMENDED' is not an accepted usage according to RFC 2119. Please use uppercase 'NOT' together with RFC 2119 keywords (if that is what you mean). Found 'MUST not' in this paragraph: 2. The transport address from the peer for the default destination correspond to IP address values "0.0.0.0"/"::" and port value of "9". This MUST not be considered as a ICE failure by the peer agent and the ICE processing MUST continue as usual. == The document seems to contain a disclaimer for pre-RFC5378 work, but was first submitted on or after 10 November 2008. The disclaimer is usually necessary only for documents that revise or obsolete older RFCs, and that take significant amounts of text from those RFCs. If you can contact all authors of the source material and they are willing to grant the BCP78 rights to the IETF Trust, you can and should remove the disclaimer. Otherwise, the disclaimer is needed and you can ignore this comment. (See the Legal Provisions document at https://trustee.ietf.org/license-info for more information.) -- The document date (May 18, 2019) is 1799 days in the past. Is this intentional? 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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: November 19, 2019 A. Keranen 7 Ericsson 8 May 18, 2019 10 Session Description Protocol (SDP) Offer/Answer procedures for 11 Interactive Connectivity Establishment (ICE) 12 draft-ietf-mmusic-ice-sip-sdp-27 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 Status of This Memo 22 This Internet-Draft is submitted in full conformance with the 23 provisions of BCP 78 and BCP 79. 25 Internet-Drafts are working documents of the Internet Engineering 26 Task Force (IETF). Note that other groups may also distribute 27 working documents as Internet-Drafts. The list of current Internet- 28 Drafts is at https://datatracker.ietf.org/drafts/current/. 30 Internet-Drafts are draft documents valid for a maximum of six months 31 and may be updated, replaced, or obsoleted by other documents at any 32 time. It is inappropriate to use Internet-Drafts as reference 33 material or to cite them other than as "work in progress." 35 This Internet-Draft will expire on November 19, 2019. 37 Copyright Notice 39 Copyright (c) 2019 IETF Trust and the persons identified as the 40 document authors. All rights reserved. 42 This document is subject to BCP 78 and the IETF Trust's Legal 43 Provisions Relating to IETF Documents 44 (https://trustee.ietf.org/license-info) in effect on the date of 45 publication of this document. Please review these documents 46 carefully, as they describe your rights and restrictions with respect 47 to this document. Code Components extracted from this document must 48 include Simplified BSD License text as described in Section 4.e of 49 the Trust Legal Provisions and are provided without warranty as 50 described in the Simplified BSD License. 52 This document may contain material from IETF Documents or IETF 53 Contributions published or made publicly available before November 54 10, 2008. The person(s) controlling the copyright in some of this 55 material may not have granted the IETF Trust the right to allow 56 modifications of such material outside the IETF Standards Process. 57 Without obtaining an adequate license from the person(s) controlling 58 the copyright in such materials, this document may not be modified 59 outside the IETF Standards Process, and derivative works of it may 60 not be created outside the IETF Standards Process, except to format 61 it for publication as an RFC or to translate it into languages other 62 than English. 64 Table of Contents 66 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3 67 2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3 68 3. SDP Offer/Answer Procedures . . . . . . . . . . . . . . . . . 4 69 3.1. Introduction . . . . . . . . . . . . . . . . . . . . . . 4 70 3.2. Generic Procedures . . . . . . . . . . . . . . . . . . . 4 71 3.2.1. Encoding . . . . . . . . . . . . . . . . . . . . . . 4 72 3.2.2. RTP/RTCP Considerations . . . . . . . . . . . . . . . 6 73 3.2.3. Determining Role . . . . . . . . . . . . . . . . . . 6 74 3.2.4. STUN Considerations . . . . . . . . . . . . . . . . . 6 75 3.2.5. Verifying ICE Support Procedures . . . . . . . . . . 6 76 3.2.6. SDP Example . . . . . . . . . . . . . . . . . . . . . 7 77 3.3. Initial Offer/Answer Exchange . . . . . . . . . . . . . . 7 78 3.3.1. Sending the Initial Offer . . . . . . . . . . . . . . 7 79 3.3.2. Sending the Initial Answer . . . . . . . . . . . . . 8 80 3.3.3. Receiving the Initial Answer . . . . . . . . . . . . 9 81 3.3.4. Concluding ICE . . . . . . . . . . . . . . . . . . . 9 82 3.4. Subsequent Offer/Answer Exchanges . . . . . . . . . . . . 10 83 3.4.1. Sending Subsequent Offer . . . . . . . . . . . . . . 10 84 3.4.2. Sending Subsequent Answer . . . . . . . . . . . . . . 12 85 3.4.3. Receiving Answer for a Subsequent Offer . . . . . . . 14 86 4. Grammar . . . . . . . . . . . . . . . . . . . . . . . . . . . 16 87 4.1. "candidate" Attribute . . . . . . . . . . . . . . . . . . 16 88 4.2. "remote-candidates" Attribute . . . . . . . . . . . . . . 19 89 4.3. "ice-lite" and "ice-mismatch" Attributes . . . . . . . . 19 90 4.4. "ice-ufrag" and "ice-pwd" Attributes . . . . . . . . . . 20 91 4.5. "ice-pacing" Attribute . . . . . . . . . . . . . . . . . 21 92 4.6. "ice-options" Attribute . . . . . . . . . . . . . . . . . 21 93 5. Keepalives . . . . . . . . . . . . . . . . . . . . . . . . . 22 94 6. SIP Considerations . . . . . . . . . . . . . . . . . . . . . 22 95 6.1. Latency Guidelines . . . . . . . . . . . . . . . . . . . 22 96 6.1.1. Offer in INVITE . . . . . . . . . . . . . . . . . . . 23 97 6.1.2. Offer in Response . . . . . . . . . . . . . . . . . . 24 98 6.2. SIP Option Tags and Media Feature Tags . . . . . . . . . 24 99 6.3. Interactions with Forking . . . . . . . . . . . . . . . . 24 100 6.4. Interactions with Preconditions . . . . . . . . . . . . . 25 101 6.5. Interactions with Third Party Call Control . . . . . . . 25 102 7. Relationship with ANAT . . . . . . . . . . . . . . . . . . . 25 103 8. Security Considerations . . . . . . . . . . . . . . . . . . . 26 104 8.1. Attacks on the Offer/Answer Exchanges . . . . . . . . . . 26 105 8.2. Insider Attacks . . . . . . . . . . . . . . . . . . . . . 26 106 8.2.1. The Voice Hammer Attack . . . . . . . . . . . . . . . 26 107 8.2.2. Interactions with Application Layer Gateways and SIP 27 108 9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 28 109 9.1. SDP Attributes . . . . . . . . . . . . . . . . . . . . . 28 110 9.1.1. candidate Attribute . . . . . . . . . . . . . . . . . 28 111 9.1.2. remote-candidates Attribute . . . . . . . . . . . . . 29 112 9.1.3. ice-lite Attribute . . . . . . . . . . . . . . . . . 29 113 9.1.4. ice-mismatch Attribute . . . . . . . . . . . . . . . 30 114 9.1.5. ice-pwd Attribute . . . . . . . . . . . . . . . . . . 30 115 9.1.6. ice-ufrag Attribute . . . . . . . . . . . . . . . . . 31 116 9.1.7. ice-options Attribute . . . . . . . . . . . . . . . . 31 117 9.1.8. ice-pacing Attribute . . . . . . . . . . . . . . . . 32 118 9.2. Interactive Connectivity Establishment (ICE) Options 119 Registry . . . . . . . . . . . . . . . . . . . . . . . . 32 120 10. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 33 121 11. References . . . . . . . . . . . . . . . . . . . . . . . . . 33 122 11.1. Normative References . . . . . . . . . . . . . . . . . . 33 123 11.2. Informative References . . . . . . . . . . . . . . . . . 35 124 11.3. URIs . . . . . . . . . . . . . . . . . . . . . . . . . . 36 125 Appendix A. Examples . . . . . . . . . . . . . . . . . . . . . . 36 126 Appendix B. The remote-candidates Attribute . . . . . . . . . . 38 127 Appendix C. Why Is the Conflict Resolution Mechanism Needed? . . 39 128 Appendix D. Why Send an Updated Offer? . . . . . . . . . . . . . 40 129 Appendix E. Contributors . . . . . . . . . . . . . . . . . . . . 41 130 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 41 132 1. Introduction 134 This document describes how Interactive Connectivity Establishment 135 (ICE) is used with Session Description Protocol (SDP) offer/answer 136 [RFC3264]. The ICE specification [RFC8445] describes procedures that 137 are common to all usages of ICE and this document gives the 138 additional details needed to use ICE with SDP offer/answer. 140 2. Terminology 142 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 143 "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and 144 "OPTIONAL" in this document are to be interpreted as described in RFC 145 2119 [RFC2119]. 147 Readers should be familiar with the terminology defined in [RFC3264], 148 in [RFC8445] and the following: 150 Default Destination/Candidate: The default destination for a 151 component of a data stream is the transport address that would be 152 used by an agent that is not ICE aware. A default candidate for a 153 component is one whose transport address matches the default 154 destination for that component. For the RTP component, the 155 default connection address is in the "c=" line of the SDP, and the 156 port and transport protocol are in the "m=" line. For the RTCP 157 component, the address and port are indicated using the "a=rtcp" 158 attribute defined in [RFC3605], if present; otherwise, the RTCP 159 component address is same as the address of the RTP component, and 160 its port is one greater than the port of the RTP component. 162 3. SDP Offer/Answer Procedures 164 3.1. Introduction 166 [RFC8445] defines ICE candidate exchange as the process for ICE 167 agents (Initiator and Responder) to exchange their candidate 168 information required for ICE processing at the agents. For the 169 purposes of this specification, the candidate exchange process 170 corresponds to the [RFC3264] Offer/Answer protocol and the 171 terminologies offerer and answerer correspond to the initiator and 172 responder terminologies from [RFC8445] respectively. 174 Once the initiating agent has gathered, pruned and prioritized its 175 set of candidates [RFC8445], the candidate exchange with the peer 176 agent begins. 178 3.2. Generic Procedures 180 3.2.1. Encoding 182 Section 4 provides detailed rules for constructing various SDP 183 attributes defined in this specification. 185 3.2.1.1. Data Streams 187 Each data stream [RFC8445] is represented by an SDP media description 188 ("m=" section). 190 3.2.1.2. Candidates 192 With in a "m=" section, each candidate (including the default 193 candidate) associated with the data stream is represented by an SDP 194 candidate attribute. 196 Prior to nomination, the "c=" line associated with an "m=" section 197 contains the connection address of the default candidate, while the 198 "m=" line contains the port and transport protocol of the default 199 candidate for that "m=" section. 201 After nomination, the "c=" line for a given "m=" section contains the 202 connection address of the nominated candidate (the local candidate of 203 the nominated candidate pair) and the "m=" line contains the port and 204 transport protocol corresponding to the nominated candidate for that 205 "m=" section. 207 3.2.1.3. Username and Password 209 The ICE username is represented by an SDP ice-ufrag attribute and the 210 ICE password is represented by an SDP ice-pwd attribute. 212 3.2.1.4. Lite Implementations 214 An ICE lite implementation [RFC8445] MUST include an SDP ice-lite 215 attribute. A full implementation MUST NOT include that attribute. 217 3.2.1.5. ICE Extensions 219 An agent uses the SDP ice-options attribute to indicate support of 220 ICE extensions. 222 An agent compliant to this specification MUST include an SDP ice- 223 options attribute with an "ice2" attribute value. If an agent 224 receives an SDP offer or answer with ICE attributes but without the 225 "ice2" ice-options attribute value, the agent assumes that the peer 226 is compliant to [RFC5245]. 228 3.2.1.6. Inactive and Disabled Data Streams 230 If an "m=" section is marked as inactive [RFC4566], or has a 231 bandwidth value of zero [RFC4566], the agent MUST still include ICE 232 related SDP attributes. 234 If the port value associated with an "m=" section is set to zero 235 (implying a disabled stream) as defined in section 8.2 of [RFC3264], 236 the agent SHOULD NOT include ICE related SDP candidate attributes in 237 that "m=" section, unless an SDP extension specifying otherwise is 238 used. 240 3.2.2. RTP/RTCP Considerations 242 If an agent utilizes both RTP and RTCP, and separate ports are used 243 for RTP and RTCP, the agent MUST include SDP candidate attributes for 244 both the RTP and RTCP components and SDP rtcp attribute SHOULD be 245 included in the "m=" section, as described in [RFC3605] 247 In the cases where the port number for the RTCP is one higher than 248 the RTP port and RTCP component address is same as the address of the 249 RTP component, the SDP rtcp attribute MAY be omitted. 251 If the agent does not utilize RTCP, it indicates that by including 252 b=RS:0 and b=RR:0 SDP attributes, as described in [RFC3556]. 254 3.2.3. Determining Role 256 The offerer acts as the Initiating agent. The answerer acts as the 257 Responding agent. The ICE roles (controlling and controlled) are 258 determined using the procedures in [RFC8445]. 260 3.2.4. STUN Considerations 262 Once an agent has provided its local candidates to its peer in an SDP 263 offer or answer, the agent MUST be prepared to receive STUN 264 connectivity check Binding requests on those candidates. 266 3.2.5. Verifying ICE Support Procedures 268 The agents will proceed with the ICE procedures defined in [RFC8445] 269 and this specification if, for each data stream in the SDP it 270 received, the default destination for each component of that data 271 stream appears in a candidate attribute. For example, in the case of 272 RTP, the connection address, port and transport protocol are in the 273 "c=" and "m=" lines, respectively, appear in a candidate attribute 274 and the value in the rtcp attribute appears in a candidate attribute. 276 If this condition is not met, the agents MUST process the SDP based 277 on normal [RFC3264] procedures, without using any of the ICE 278 mechanisms described in the remainder of this specification with the 279 few exceptions noted below: 281 1. The presence of certain application layer gateways MAY modify the 282 transport address information as described in Section 8.2.2. The 283 behavior of the responding agent in such a situation is 284 implementation defined. Informally, the responding agent MAY 285 consider the mismatched transport address information as a 286 plausible new candidate learnt from the peer and continue its ICE 287 processing with that transport address included. Alternatively, 288 the responding agent MAY include an "a=ice-mismatch" attribute in 289 its answer and MAY also omit a=candidate attributes for such data 290 streams. 292 2. The transport address from the peer for the default destination 293 correspond to IP address values "0.0.0.0"/"::" and port value of 294 "9". This MUST not be considered as a ICE failure by the peer 295 agent and the ICE processing MUST continue as usual. 297 In some cases, controlling/initiator agent may receive the SDP answer 298 that may omit "a=candidate" attributes for the media streams, and 299 instead include a session level "a=ice-mismatch" attribute. This 300 signals to the offerer that the answerer supports ICE, but that ICE 301 processing was not used for this session. This specification 302 provides no guidance on how an agent should proceed in such a failure 303 case. 305 3.2.6. SDP Example 307 The following is an example SDP message that includes ICE attributes 308 (lines folded for readability): 310 v=0 311 o=jdoe 2890844526 2890842807 IN IP4 10.0.1.1 312 s= 313 c=IN IP4 192.0.2.3 314 t=0 0 315 a=ice-options:ice2 316 a=ice-pwd:asd88fgpdd777uzjYhagZg 317 a=ice-ufrag:8hhY 318 m=audio 45664 RTP/AVP 0 319 b=RS:0 320 b=RR:0 321 a=rtpmap:0 PCMU/8000 322 a=candidate:1 1 UDP 2130706431 10.0.1.1 8998 typ host 323 a=candidate:2 1 UDP 1694498815 192.0.2.3 45664 typ srflx raddr 324 10.0.1.1 rport 8998 326 3.3. Initial Offer/Answer Exchange 328 3.3.1. Sending the Initial Offer 330 When an offerer generates the initial offer, in each "m=" section it 331 MUST include SDP candidate attributes for each available candidate 332 associated with the "m=" section. In addition, the offerer MUST 333 include an SDP ice-ufrag and an SDP ice-pwd attribute in the offer. 335 It is valid for an offer "m=" line to include no SDP candidate 336 attributes and with default destination corresponding to the IP 337 address values "0.0.0.0"/"::" and port value of "9". This implies 338 that offering agent is only going to use peer reflexive candidates or 339 that additional candidates would be provided in subsequent signaling 340 messages. 342 Note: Within the scope of this document, "Initial Offer" refers to 343 the first SDP offer that is sent in order to negotiate usage of 344 ICE. It might, or might not, be the initial SDP offer of the SDP 345 session. 347 Note: The procedures in this document only consider "m=" sections 348 associated with data streams where ICE is used. 350 3.3.2. Sending the Initial Answer 352 When an answerer receives an initial offer that indicates that the 353 offerer supports ICE, and if the answerer accepts the offer and the 354 usage of ICE, in each "m=" section within the answer, it MUST include 355 SDP candidate attributes for each available candidate associated with 356 the "m=" section. In addition, the answerer MUST include an SDP ice- 357 ufrag and an SDP ice-pwd attribute in the answer. 359 In each "m=" line, the answerer MUST use the same transport protocol 360 as was used in the offer "m=" line. If none of the candidates in the 361 "m=" line in the answer use the same transport protocol as indicated 362 in the offer "m=" line, then, in order to avoid ICE mismatch, the 363 default destination MUST be set to IP address values "0.0.0.0"/"::" 364 and port value of "9". 366 It is also valid for an answer "m=" line to include no SDP candidate 367 attributes and with default destination corresponding to the IP 368 address values "0.0.0.0"/"::" and port value of "9". This implies 369 that answering agent is only going to use peer reflexive candidates 370 or that additional candidates would be provided in subsequent 371 signaling messages. 373 Once the answerer has sent the answer, it can start performing 374 connectivity checks towards the peer candidates that were provided in 375 the offer. 377 In certain scenarios when either no candidates were provided in the 378 offer or all the provided candidates were discarded (say, due to 379 unsupported address type or FQDN name resolution failure), the 380 answerer MUST NOT consider this as immediate ICE processing failure 381 but instead MUST wait for the peer reflexive candidates to arrive to 382 carryout the connectivity checks or eventually time out on the 383 connectivity checks (see [draft-holmberg-ice-pac]). 385 If the offer does not indicate support of ICE, the answerer MUST NOT 386 accept the usage of ICE. If the answerer still accepts the offer, 387 the answerer MUST NOT include any ICE related SDP attributes in the 388 answer. Instead the answerer will generate the answer according to 389 normal offer/answer procedures [RFC3264]. 391 If the answerer detects a possibility of the ICE mismatch, procedures 392 described in (Section 3.2.5) are followed. 394 3.3.3. Receiving the Initial Answer 396 When an offerer receives an initial answer that indicates that the 397 answerer supports ICE, it can start performing connectivity checks 398 towards the peer candidates that were provided in the answer. 400 In certain scenarios when either no candidates were provided in the 401 answer or all the provided candidates were discarded (say, due to 402 unsupported address type or FQDN name resolution failure), the 403 offerer MUST NOT consider this as immediate ICE processing failure 404 but instead MUST wait for the peer reflexive candidates to arrive to 405 carryout the connectivity checks or eventually time out on the 406 connectivity checks (see [draft-holmberg-ice-pac]). 408 If the answer does not indicate that the answerer supports ICE, or if 409 the offerer detects an ICE mismatch in the answer, the offerer MUST 410 terminate the usage of ICE. The subsequent actions taken by the 411 offerer are implementation dependent and are out of the scope of this 412 specification. 414 3.3.4. Concluding ICE 416 Once the state of each check list is Completed, and if the agent is 417 the controlling agent, it nominates a candidate pair [RFC8445] and 418 checks for each data stream whether the nominated pair matches the 419 default candidate pair. If there are one or more data streams with a 420 match, and the peer did not indicate support for the 'ice2' ice- 421 option, the controlling agent MUST generate a subsequent offer 422 (Section 3.4.1), in which the connection address, port and transport 423 protocol in the "c=" and "m=" lines associated with each data stream 424 match the corresponding local information of the nominated pair for 425 that data stream. 427 However, If the support for 'ice2' ice-option is in use, the 428 nominated candidate is noted and sent in the subsequent offer/answer 429 exchange as the default candidate and no updated offer is needed to 430 fix the default candidate. 432 Also as described in [RFC8445], once the controlling agent has 433 nominated a candidate pair for a data stream, the agent MUST NOT 434 nominate another pair for that data stream during the lifetime of the 435 ICE session (i.e. until ICE is restarted). 437 3.4. Subsequent Offer/Answer Exchanges 439 Either agent MAY generate a subsequent offer at any time allowed by 440 [RFC3264]. This section defines rules for construction of subsequent 441 offers and answers. 443 Should a subsequent offer fail, ICE processing continues as if the 444 subsequent offer had never been made. 446 3.4.1. Sending Subsequent Offer 448 3.4.1.1. Procedures for All Implementations 450 3.4.1.1.1. ICE Restarts 452 An agent MAY restart ICE processing for an existing data stream 453 [RFC8445]. 455 The rules governing the ICE restart imply that setting the connection 456 address in the "c=" line to 0.0.0.0 (for IPv4)/ :: (for IPv6) will 457 cause an ICE restart. Consequently, ICE implementations MUST NOT 458 utilize this mechanism for call hold, and instead MUST use a=inactive 459 and a=sendonly as described in [RFC3264]. 461 To restart ICE, an agent MUST change both the ice-pwd and the ice- 462 ufrag for the data stream in an offer. However, it is permissible to 463 use a session-level attribute in one offer, but to provide the same 464 ice-pwd or ice-ufrag as a media-level attribute in a subsequent 465 offer. This MUST NOT be considered as ICE restart. 467 An agent sets the rest of the ice related fields in the SDP for this 468 data stream as it would in an initial offer of this data stream (see 469 Section 3.2.1). Consequently, the set of candidates MAY include 470 some, none, or all of the previous candidates for that data stream 471 and MAY include a totally new set of candidates. 473 3.4.1.1.2. Removing a Data Stream 475 If an agent removes a data stream by setting its port to zero, it 476 MUST NOT include any candidate attributes for that data stream and 477 SHOULD NOT include any other ICE-related attributes defined in 478 Section 4 for that data stream. 480 3.4.1.1.3. Adding a Data Stream 482 If an agent wishes to add a new data stream, it sets the fields in 483 the SDP for this data stream as if this was an initial offer for that 484 data stream (see Section 3.2.1). This will cause ICE processing to 485 begin for this data stream. 487 3.4.1.2. Procedures for Full Implementations 489 This section describes additional procedures for full 490 implementations, covering existing data streams. 492 3.4.1.2.1. Before Nomination 494 When an offerer sends a subsequent offer; in each "m=" section for 495 which a candidate pair has not yet been nominated, the offer MUST 496 include the same set of ICE-related information that the offerer 497 included in the previous offer or answer. The agent MAY include 498 additional candidates it did not offer previously, but which it has 499 gathered since the last offer/ answer exchange, including peer 500 reflexive candidates. 502 The agent MAY change the default destination for media. As with 503 initial offers, there MUST be a set of candidate attributes in the 504 offer matching this default destination. 506 3.4.1.2.2. After Nomination 508 Once a candidate pair has been nominated for a data stream, the 509 connection address, port and transport protocol in each "c=" and "m=" 510 line associated with that data stream MUST match the data associated 511 with the nominated pair for that data stream. In addition, the 512 offerer only includes SDP candidates representing the local 513 candidates of the nominated candidate pair. The offerer MUST NOT 514 include any other SDP candidate attributes in the subsequent offer. 516 In addition, if the agent is controlling, it MUST include the 517 a=remote-candidates attribute for each data stream whose check list 518 is in the completed state. The attribute contains the remote 519 candidates corresponding to the nominated pair in the valid list for 520 each component of that data stream. It is needed to avoid a race 521 condition whereby the controlling agent chooses its pairs, but the 522 updated offer beats the connectivity checks to the controlled agent, 523 which doesn't even know these pairs are valid, let alone selected. 524 See Appendix B for elaboration on this race condition. 526 3.4.1.3. Procedures for Lite Implementations 528 If the ICE state is running, a lite implementation MUST include all 529 of its candidates for each component of each data stream in 530 a=candidate attribute in any subsequent offer. The candidates are 531 formed identical to the procedures for initial offers. 533 A lite implementation MUST NOT add additional host candidates in a 534 subsequent offer. If an agent needs to offer additional candidates, 535 it MUST restart ICE. Similarly, the username fragments or passwords 536 MUST remain the same as used previously. If an agent needs to change 537 one of these, it MUST restart ICE for that media stream. 539 If ICE has completed for a data stream and if the agent is 540 controlled, the default destination for that data stream MUST be set 541 to the remote candidate of the candidate pair for that component in 542 the valid list. For a lite implementation, there is always just a 543 single candidate pair in the valid list for each component of a data 544 stream. Additionally, the agent MUST include a candidate attribute 545 for each default destination. 547 If ICE state is completed and if the agent is controlling (which only 548 happens when both agents are lite), the agent MUST include the 549 a=remote-candidates attribute for each data stream. The attribute 550 contains the remote candidates from the candidate pairs in the valid 551 list (one pair for each component of each data stream). 553 3.4.2. Sending Subsequent Answer 555 If ICE is Completed for a data stream, and the offer for that data 556 stream lacked the a=remote-candidates attribute, the rules for 557 construction of the answer are identical to those for the offerer, 558 except that the answerer MUST NOT include the a=remote-candidates 559 attribute in the answer. 561 A controlled agent will receive an offer with the a=remote-candidates 562 attribute for a data stream when its peer has concluded ICE 563 processing for that data stream. This attribute is present in the 564 offer to deal with a race condition between the receipt of the offer, 565 and the receipt of the Binding Response that tells the answerer the 566 candidate that will be selected by ICE. See Appendix B for an 567 explanation of this race condition. Consequently, processing of an 568 offer with this attribute depends on the winner of the race. 570 The agent forms a candidate pair for each component of the data 571 stream by: 573 o Setting the remote candidate equal to the offerer's default 574 destination for that component (i.e. the contents of the "m=" and 575 "c=" lines for RTP, and the a=rtcp attribute for RTCP) 577 o Setting the local candidate equal to the transport address for 578 that same component in the a=remote-candidates attribute in the 579 offer. 581 The agent then sees if each of these candidate pairs is present in 582 the valid list. If a particular pair is not in the valid list, the 583 check has "lost" the race. Call such a pair a "losing pair". 585 The agent finds all the pairs in the check list whose remote 586 candidates equal the remote candidate in the losing pair: 588 o If none of the pairs are In-Progress, and at least one is Failed, 589 it is most likely that a network failure, such as a network 590 partition or serious packet loss, has occurred. The agent SHOULD 591 generate an answer for this data stream as if the remote- 592 candidates attribute had not been present, and then restart ICE 593 for this stream. 595 o If at least one of the pairs is In-Progress, the agent SHOULD wait 596 for those checks to complete, and as each completes, redo the 597 processing in this section until there are no losing pairs. 599 Once there are no losing pairs, the agent can generate the answer. 600 It MUST set the default destination for media to the candidates in 601 the remote-candidates attribute from the offer (each of which will 602 now be the local candidate of a candidate pair in the valid list). 603 It MUST include a candidate attribute in the answer for each 604 candidate in the remote-candidates attribute in the offer. 606 3.4.2.1. ICE Restart 608 If the offerer in a subsequent offer requested an ICE restart for a 609 data stream, and if the answerer accepts the offer, the answerer 610 follows the procedures for generating an initial answer. 612 For a given data stream, the answerer MAY include the same candidates 613 that were used in the previous ICE session, but it MUST change the 614 SDP ice-pwd and ice-ufrag attribute values. 616 3.4.2.2. Lite Implementation specific procedures 618 If the received offer contains the remote-candidates attribute for a 619 data stream, the agent forms a candidate pair for each component of 620 the data stream by: 622 o Setting the remote candidate equal to the offerer's default 623 destination for that component (i.e., the contents of the "m=" and 624 "c=" lines for RTP, and the a=rtcp attribute for RTCP). 626 o Setting the local candidate equal to the transport address for 627 that same component in the a=remote-candidates attribute in the 628 offer. 630 The state of ICE processing for that data stream is set to Completed. 632 Furthermore, if the agent believed it was controlling, but the offer 633 contained the a=remote-candidates attribute, both agents believe they 634 are controlling. In this case, both would have sent updated offers 635 around the same time. 637 However, the signaling protocol carrying the offer/answer exchanges 638 will have resolved this glare condition, so that one agent is always 639 the 'winner' by having its offer received before its peer has sent an 640 offer. The winner takes the role of controlling, so that the loser 641 (the answerer under consideration in this section) MUST change its 642 role to controlled. 644 Consequently, if the agent was going to send an updated offer since, 645 based on the rules in section 8.2 of [RFC8445], it was controlling, 646 it no longer needs to. 648 Besides the potential role change, change in the Valid list, and 649 state changes, the construction of the answer is performed 650 identically to the construction of an offer. 652 3.4.3. Receiving Answer for a Subsequent Offer 654 3.4.3.1. Procedures for Full Implementations 656 There may be certain situations where the offerer receives an SDP 657 answer that lacks ICE candidates although the initial answer did. 658 One example of such an "unexpected" answer might be happen when an 659 ICE-unaware B2BUA introduces a media server during call hold using 660 3rd party call-control procedures. Omitting further details how this 661 is done, this could result in an answer being received at the holding 662 UA that was constructed by the B2BUA. With the B2BUA being ICE- 663 unaware, that answer would not include ICE candidates. 665 Receiving an answer without ICE attributes in this situation might be 666 unexpected, but would not necessarily impair the user experience. 668 When the offerer receives an answer indicating support for ICE, the 669 offer performs on of the following actions: 671 o If the offer was a restart, the agent MUST perform ICE restart 672 procedures as specified in Section 3.4.3.1.1 674 o If the offer/answer exchange removed a data stream, or an answer 675 rejected an offered data stream, an agent MUST flush the Valid 676 list for that data stream. It MUST also terminate any STUN 677 transactions in progress for that data stream. 679 o If the offer/answer exchange added a new data stream, the agent 680 MUST create a new check list for it (and an empty Valid list to 681 start of course) which in turn triggers the candidate processing 682 procedures [RFC8445]. 684 o If ICE state is running for a given data stream, the agent 685 recomputes the check list. If a pair on the new check list was 686 also on the previous check list, and its state is not Frozen, its 687 state is copied over. Otherwise, its state is set to Frozen. If 688 none of the check lists are active (meaning that the pairs in each 689 check list are Frozen), appropriate procedures in [RFC8445] are 690 performed to move candidate(s) to the Waiting state to further 691 continue ICE processing. 693 o If ICE state is completed and the SDP answer conforms to 694 Section 3.4.2, the agent MUST reman in the ICE completed state. 696 However, if the ICE support is no longer indicated in the SDP answer, 697 the agent MUST fall-back to [RFC3264] procedures and SHOULD NOT drop 698 the dialog because of the missing ICE support or unexpected answer. 699 Once the agent sends a new offer later on, it MUST perform an ICE 700 restart. 702 3.4.3.1.1. ICE Restarts 704 The agent MUST remember the nominated pair in the Valid list for each 705 component of the data stream, called the previous selected pair prior 706 to the restart. The agent will continue to send media using this 707 pair, as described in section 12 of [RFC8445]. Once these 708 destinations are noted, the agent MUST flush the valid and check 709 lists, and then recompute the check list and its states, thus 710 triggering the candidate processing procedures [RFC8445] 712 3.4.3.2. Procedures for Lite Implementations 714 If ICE is restarting for a data stream, the agent MUST start a new 715 Valid list for that data stream. It MUST remember the nominated pair 716 in the previous Valid list for each component of the data stream, 717 called the previous selected pairs, and continue to send media there 718 as described in section 12 of [RFC8445]. The state of ICE processing 719 for each data stream MUST change to Running, and the state of ICE 720 processing MUST change to Running 722 4. Grammar 724 This specification defines eight new SDP attributes -- the 725 "candidate", "remote-candidates", "ice-lite", "ice-mismatch", "ice- 726 ufrag", "ice-pwd", "ice-pacing", and "ice-options" attributes. 728 This section also provides non-normative examples of the attributes 729 defined. 731 The syntax for the attributes follow Augmented BNF as defined in 732 [RFC5234]. 734 4.1. "candidate" Attribute 736 The candidate attribute is a media-level attribute only. It contains 737 a transport address for a candidate that can be used for connectivity 738 checks. 740 candidate-attribute = "candidate" ":" foundation SP component-id SP 741 transport SP 742 priority SP 743 connection-address SP ;from RFC 4566 744 port ;port from RFC 4566 745 SP cand-type 746 [SP rel-addr] 747 [SP rel-port] 748 *(SP extension-att-name SP 749 extension-att-value) 751 foundation = 1*32ice-char 752 component-id = 1*5DIGIT 753 transport = "UDP" / transport-extension 754 transport-extension = token ; from RFC 3261 755 priority = 1*10DIGIT 756 cand-type = "typ" SP candidate-types 757 candidate-types = "host" / "srflx" / "prflx" / "relay" / token 758 rel-addr = "raddr" SP connection-address 759 rel-port = "rport" SP port 760 extension-att-name = token 761 extension-att-value = *VCHAR 762 ice-char = ALPHA / DIGIT / "+" / "/" 764 This grammar encodes the primary information about a candidate: its 765 IP address, port and transport protocol, and its properties: the 766 foundation, component ID, priority, type, and related transport 767 address: 769 : is taken from RFC 4566 [RFC4566]. It is the 770 IP address of the candidate. When parsing this field, an agent 771 can differentiate an IPv4 address and an IPv6 address by presence 772 of a colon in its value -- the presence of a colon indicates IPv6. 773 An agent MUST ignore candidate lines that include candidates with 774 IP address versions that are not supported or recognized. An IP 775 address SHOULD be used, but an FQDN MAY be used in place of an IP 776 address. In that case, when receiving an offer or answer 777 containing an FQDN in an a=candidate attribute, the FQDN is looked 778 up in the DNS first using an AAAA record (assuming the agent 779 supports IPv6), and if no result is found or the agent only 780 supports IPv4, using an A record. If a FQDN returns multiple IP 781 addresses an agent MUST only use one of them throughout the 782 duration of the ICE session. Since an agent does not know whether 783 the peer listens to the chosen IP address and port, it is 784 RECOMMENDED to not use FQDNs that will resolve into multiple IP 785 addresses. 787 : is also taken from RFC 4566 [RFC4566]. It is the port of 788 the candidate. 790 : indicates the transport protocol for the candidate. 791 This specification only defines UDP. However, extensibility is 792 provided to allow for future transport protocols to be used with 793 ICE by extending the sub-registry "ICE Transport Protocols" under 794 "Interactive Connectivity Establishment (ICE)" registry. 796 : is composed of 1 to 32 s. It is an 797 identifier that is equivalent for two candidates that are of the 798 same type, share the same base, and come from the same STUN 799 server. The foundation is used to optimize ICE performance in the 800 Frozen algorithm as described in [RFC8445] 802 : is a positive integer between 1 and 256 (inclusive) 803 that identifies the specific component of the dta stream for which 804 this is a candidate. It MUST start at 1 and MUST increment by 1 805 for each component of a particular candidate. For data streams 806 based on RTP, candidates for the actual RTP media MUST have a 807 component ID of 1, and candidates for RTCP MUST have a component 808 ID of 2. See section 13 in [RFC8445] for additional discussion on 809 extending ICE to new data streams. 811 : is a positive integer between 1 and (2**31 - 1) 812 inclusive. The procedures for computing candidate's priority is 813 described in section 5.1.2 of [RFC8445]. 815 : encodes the type of candidate. This specification 816 defines the values "host", "srflx", "prflx", and "relay" for host, 817 server reflexive, peer reflexive, and relayed candidates, 818 respectively. Specifications for new candidate types MUST define 819 how, if at all, various steps in the ICE processing differ from 820 the ones defined by this specification. 822 and : convey transport addresses related to the 823 candidate, useful for diagnostics and other purposes. 824 and MUST be present for server reflexive, peer 825 reflexive, and relayed candidates. If a candidate is server or 826 peer reflexive, and are equal to the base 827 for that server or peer reflexive candidate. If the candidate is 828 relayed, and are equal to the mapped address 829 in the Allocate response that provided the client with that 830 relayed candidate (see Appendix B.3 of [RFC8445] for a discussion 831 of its purpose). If the candidate is a host candidate, 832 and MUST be omitted. 834 In some cases, e.g., for privacy reasons, an agent may not want to 835 reveal the related address and port. In this case the address 836 MUST be set to "0.0.0.0" (for IPv4 candidates) or "::" (for IPv6 837 candidates) and the port to zero. 839 The candidate attribute can itself be extended. The grammar allows 840 for new name/value pairs to be added at the end of the attribute. 841 Such extensions MUST be made through IETF Review or IESG Approval 842 [RFC5226] and the assignments MUST contain the specific extension and 843 a reference to the document defining the usage of the extension 845 An implementation MUST ignore any name/value pairs it doesn't 846 understand. 848 Example: SDP line for UDP server reflexive candidate attribute for the RTP component 850 a=candidate:2 1 UDP 1694498815 192.0.2.3 45664 typ srflx raddr 10.0.1.1 rport 8998 852 4.2. "remote-candidates" Attribute 854 The syntax of the "remote-candidates" attribute is defined using 855 Augmented BNF as defined in [RFC5234]. The remote-candidates 856 attribute is a media-level attribute only. 858 remote-candidate-att = "remote-candidates:" remote-candidate 859 0*(SP remote-candidate) 860 remote-candidate = component-ID SP connection-address SP port 862 The attribute contains a connection-address and port for each 863 component. The ordering of components is irrelevant. However, a 864 value MUST be present for each component of a data stream. This 865 attribute MUST be included in an offer by a controlling agent for a 866 data stream that is Completed, and MUST NOT be included in any other 867 case. 869 Example: Remote candidates SDP lines for the RTP and RTCP components: 871 a=remote-candidates:1 192.0.2.3 45664 872 a=remote-candidates:2 192.0.2.3 45665 874 4.3. "ice-lite" and "ice-mismatch" Attributes 876 The syntax of the "ice-lite" and "ice-mismatch" attributes, both of 877 which are flags, is: 879 ice-lite = "ice-lite" 880 ice-mismatch = "ice-mismatch" 881 "ice-lite" is a session-level attribute only, and indicates that an 882 agent is a lite implementation. "ice-mismatch" is a media-level 883 attribute only, and when present in an answer, indicates that the 884 offer arrived with a default destination for a media component that 885 didn't have a corresponding candidate attribute. 887 4.4. "ice-ufrag" and "ice-pwd" Attributes 889 The "ice-ufrag" and "ice-pwd" attributes convey the username fragment 890 and password used by ICE for message integrity. Their syntax is: 892 ice-pwd-att = "ice-pwd:" password 893 ice-ufrag-att = "ice-ufrag:" ufrag 894 password = 22*256ice-char 895 ufrag = 4*256ice-char 897 The "ice-pwd" and "ice-ufrag" attributes can appear at either the 898 session-level or media-level. When present in both, the value in the 899 media-level takes precedence. Thus, the value at the session-level 900 is effectively a default that applies to all data streams, unless 901 overridden by a media-level value. Whether present at the session or 902 media-level, there MUST be an ice-pwd and ice-ufrag attribute for 903 each data stream. If two data streams have identical ice-ufrag's, 904 they MUST have identical ice-pwd's. 906 The ice-ufrag and ice-pwd attributes MUST be chosen randomly at the 907 beginning of a session (the same applies when ICE is restarting for 908 an agent). 910 The ice-ufrag attribute MUST contain at least 24 bits of randomness, 911 and the ice-pwd attribute MUST contain at least 128 bits of 912 randomness. This means that the ice-ufrag attribute will be at least 913 4 characters long, and the ice-pwd at least 22 characters long, since 914 the grammar for these attributes allows for 6 bits of information per 915 character. The attributes MAY be longer than 4 and 22 characters, 916 respectively, of course, up to 256 characters. The upper limit 917 allows for buffer sizing in implementations. Its large upper limit 918 allows for increased amounts of randomness to be added over time. 919 For compatibility with the 512 character limitation for the STUN 920 username attribute value and for bandwidth conservation 921 considerations, the ice-ufrag attribute MUST NOT be longer than 32 922 characters when sending, but an implementation MUST accept up to 256 923 characters when receiving. 925 Example shows sample ice-ufrag and ice-pwd SDP lines: 927 a=ice-pwd:asd88fgpdd777uzjYhagZg 928 a=ice-ufrag:8hhY 930 4.5. "ice-pacing" Attribute 932 The "ice-pacing" is a session level attribute that indicates the 933 desired connectivity check pacing, in milliseconds, for this agent 934 (see section 14 of [RFC8445]). The syntax is: 936 ice-pacing-att = "ice-pacing:" pacing-value 937 pacing-value = 1*10DIGIT 939 Following the procedures defined in [RFC8445], a default value of 940 50ms is used for an agent when ice-pacing attribute is omitted in the 941 offer or the answer. 943 The same rule applies for ice-pacing attribute values lower than 944 50ms. This mandates that, if an agent includes the ice-pacing 945 attribute, its value MUST be greater than 50ms or else a value of 946 50ms is considered by default for that agent. 948 Also the larger of the ice-pacing attribute values between the offer 949 and the answer (determined either by the one provided in the ice- 950 pacing attribute or by picking the default value) MUST be considered 951 for a given ICE session. 953 Example shows ice-pacing value of 5 ms: 955 a=ice-pacing:5 957 4.6. "ice-options" Attribute 959 The "ice-options" attribute is a session- and media-level attribute. 960 It contains a series of tokens that identify the options supported by 961 the agent. Its grammar is: 963 ice-options = "ice-options:" ice-option-tag 964 0*(SP ice-option-tag) 965 ice-option-tag = 1*ice-char 967 The existence of an ice-option in an offer indicates that a certain 968 extension is supported by the agent and is willing to use it, if the 969 peer agent also includes the same extension in the answer. There 970 might be further extension specific negotiation needed between the 971 agents that determine how the extensions gets used in a given 972 session. The details of the negotiation procedures, if present, MUST 973 be defined by the specification defining the extension (see 974 Section 9.2). 976 Example shows 'rtp+ecn' ice-option SDP line from <>: 978 a=ice-options:rtp+ecn 980 5. Keepalives 982 All the ICE agents MUST follow the procedures defined in section 11 983 of [RFC8445] for sending keepalives. The keepalives MUST be sent 984 regardless of whether the data stream is currently inactive, 985 sendonly, recvonly, or sendrecv, and regardless of the presence or 986 value of the bandwidth attribute. An agent can determine that its 987 peer supports ICE by the presence of a=candidate attributes for each 988 media session. 990 6. SIP Considerations 992 Note that ICE is not intended for NAT traversal for SIP, which is 993 assumed to be provided via another mechanism [RFC5626]. 995 When ICE is used with SIP, forking may result in a single offer 996 generating a multiplicity of answers. In that case, ICE proceeds 997 completely in parallel and independently for each answer, treating 998 the combination of its offer and each answer as an independent offer/ 999 answer exchange, with its own set of local candidates, pairs, check 1000 lists, states, and so on. 1002 Once ICE processing has reached the Completed state for all peers for 1003 media streams using those candidates, the agent SHOULD wait an 1004 additional three seconds, and then it MAY cease responding to checks 1005 or generating triggered checks on that candidate. It MAY free the 1006 candidate at that time. Freeing of server reflexive candidates is 1007 never explicit; it happens by lack of a keepalive. The three-second 1008 delay handles cases when aggressive nomination is used, and the 1009 selected pairs can quickly change after ICE has completed. 1011 6.1. Latency Guidelines 1013 ICE requires a series of STUN-based connectivity checks to take place 1014 between endpoints. These checks start from the answerer on 1015 generation of its answer, and start from the offerer when it receives 1016 the answer. These checks can take time to complete, and as such, the 1017 selection of messages to use with offers and answers can affect 1018 perceived user latency. Two latency figures are of particular 1019 interest. These are the post-pickup delay and the post-dial delay. 1020 The post-pickup delay refers to the time between when a user "answers 1021 the phone" and when any speech they utter can be delivered to the 1022 caller. The post-dial delay refers to the time between when a user 1023 enters the destination address for the user and ringback begins as a 1024 consequence of having successfully started alerting the called user 1025 agent. 1027 Two cases can be considered -- one where the offer is present in the 1028 initial INVITE and one where it is in a response. 1030 6.1.1. Offer in INVITE 1032 To reduce post-dial delays, it is RECOMMENDED that the caller begin 1033 gathering candidates prior to actually sending its initial INVITE, so 1034 that the candidates can be provided in the INVITE. This can be 1035 started upon user interface cues that a call is pending, such as 1036 activity on a keypad or the phone going off-hook. 1038 On the receipt of the offer, the answerer SHOULD generate an answer 1039 in a provisional response as soon as it has completed gathering the 1040 candidates. ICE requires that a provisional response with an SDP be 1041 transmitted reliably. This can be done through the existing 1042 Provisional Response Acknowledgment (PRACK) mechanism [RFC3262] or 1043 through an ICE specific optimization, wherein, the agent retransmits 1044 the provisional response with the exponential backoff timers 1045 described in [RFC3262]. Such retransmissions MUST cease on receipt 1046 of a STUN Binding request with transport address matching candidate 1047 address for one of the data streams signaled in that SDP or on 1048 transmission of the answer in a 2xx response. If no Binding request 1049 is received prior to the last retransmit, the agent does not consider 1050 the session terminated. For the ICE lite peers , the agent MUST 1051 cease retransmitting the 18x after sending it four times since there 1052 will be no Binding request sent and the number four is arbitrarily 1053 chosen to limit the number of 18x retransmits ('poor man's version of 1054 [RFC3262]' basically). (ICE will actually work even if the peer 1055 never receives the 18x; however, experience has shown that sending it 1056 is important for middleboxes and firewall traversal). 1058 Once the answer has been sent, the agent SHOULD begin its 1059 connectivity checks. Once candidate pairs for each component of a 1060 data stream enter the valid list, the answerer can begin sending 1061 media on that data stream. 1063 However, prior to this point, any media that needs to be sent towards 1064 the caller (such as SIP early media [RFC3960]) MUST NOT be 1065 transmitted. For this reason, implementations SHOULD delay alerting 1066 the called party until candidates for each component of each data 1067 stream have entered the valid list. In the case of a PSTN gateway, 1068 this would mean that the setup message into the PSTN is delayed until 1069 this point. Doing this increases the post-dial delay, but has the 1070 effect of eliminating 'ghost rings'. Ghost rings are cases where the 1071 called party hears the phone ring, picks up, but hears nothing and 1072 cannot be heard. This technique works without requiring support for, 1073 or usage of, preconditions [RFC3312]. It also has the benefit of 1074 guaranteeing that not a single packet of media will get clipped, so 1075 that post-pickup delay is zero. If an agent chooses to delay local 1076 alerting in this way, it SHOULD generate a 180 response once alerting 1077 begins. 1079 6.1.2. Offer in Response 1081 In addition to uses where the offer is in an INVITE, and the answer 1082 is in the provisional and/or 200 OK response, ICE works with cases 1083 where the offer appears in the response. In such cases, which are 1084 common in third party call control [RFC3725], ICE agents SHOULD 1085 generate their offers in a reliable provisional response (which MUST 1086 utilize [RFC3262]), and not alert the user on receipt of the INVITE. 1087 The answer will arrive in a PRACK. This allows for ICE processing to 1088 take place prior to alerting, so that there is no post-pickup delay, 1089 at the expense of increased call setup delays. Once ICE completes, 1090 the callee can alert the user and then generate a 200 OK when they 1091 answer. The 200 OK would contain no SDP, since the offer/answer 1092 exchange has completed. 1094 Alternatively, agents MAY place the offer in a 2xx instead (in which 1095 case the answer comes in the ACK). When this happens, the callee 1096 will alert the user on receipt of the INVITE, and the ICE exchanges 1097 will take place only after the user answers. This has the effect of 1098 reducing call setup delay, but can cause substantial post-pickup 1099 delays and media clipping. 1101 6.2. SIP Option Tags and Media Feature Tags 1103 [RFC5768] specifies a SIP option tag and media feature tag for usage 1104 with ICE. ICE implementations using SIP SHOULD support this 1105 specification, which uses a feature tag in registrations to 1106 facilitate interoperability through signaling intermediaries. 1108 6.3. Interactions with Forking 1110 ICE interacts very well with forking. Indeed, ICE fixes some of the 1111 problems associated with forking. Without ICE, when a call forks and 1112 the caller receives multiple incoming data streams, it cannot 1113 determine which data stream corresponds to which callee. 1115 With ICE, this problem is resolved. The connectivity checks which 1116 occur prior to transmission of media carry username fragments, which 1117 in turn are correlated to a specific callee. Subsequent media 1118 packets that arrive on the same candidate pair as the connectivity 1119 check will be associated with that same callee. Thus, the caller can 1120 perform this correlation as long as it has received an answer. 1122 6.4. Interactions with Preconditions 1124 Quality of Service (QoS) preconditions, which are defined in 1125 [RFC3312] and [RFC4032], apply only to the transport addresses listed 1126 as the default targets for media in an offer/answer. If ICE changes 1127 the transport address where media is received, this change is 1128 reflected in an updated offer that changes the default destination 1129 for media to match ICE's selection. As such, it appears like any 1130 other re-INVITE would, and is fully treated in RFCs 3312 and 4032, 1131 which apply without regard to the fact that the destination for media 1132 is changing due to ICE negotiations occurring "in the background". 1134 Indeed, an agent SHOULD NOT indicate that QoS preconditions have been 1135 met until the checks have completed and selected the candidate pairs 1136 to be used for media. 1138 ICE also has (purposeful) interactions with connectivity 1139 preconditions [RFC5898]. Those interactions are described there. 1140 Note that the procedures described in Section 6.1 describe their own 1141 type of "preconditions", albeit with less functionality than those 1142 provided by the explicit preconditions in [RFC5898]. 1144 6.5. Interactions with Third Party Call Control 1146 ICE works with Flows I, III, and IV as described in [RFC3725]. Flow 1147 I works without the controller supporting or being aware of ICE. 1148 Flow IV will work as long as the controller passes along the ICE 1149 attributes without alteration. Flow II is fundamentally incompatible 1150 with ICE; each agent will believe itself to be the answerer and thus 1151 never generate a re-INVITE. 1153 The flows for continued operation, as described in Section 7 of 1154 [RFC3725], require additional behavior of ICE implementations to 1155 support. In particular, if an agent receives a mid-dialog re-INVITE 1156 that contains no offer, it MUST restart ICE for each data stream and 1157 go through the process of gathering new candidates. Furthermore, 1158 that list of candidates SHOULD include the ones currently being used 1159 for media. 1161 7. Relationship with ANAT 1163 [RFC4091], the Alternative Network Address Types (ANAT) Semantics for 1164 the SDP grouping framework, and [RFC4092], its usage with SIP, define 1165 a mechanism for indicating that an agent can support both IPv4 and 1166 IPv6 for a data stream, and it does so by including two "m=" lines, 1167 one for v4 and one for v6. This is similar to ICE, which allows for 1168 an agent to indicate multiple transport addresses using the candidate 1169 attribute. However, ANAT relies on static selection to pick between 1170 choices, rather than a dynamic connectivity check used by ICE. 1172 It is RECOMMENDED that ICE be used in realizing the dual-stack use- 1173 cases in agents that support ICE. 1175 8. Security Considerations 1177 8.1. Attacks on the Offer/Answer Exchanges 1179 An attacker that can modify or disrupt the offer/answer exchanges 1180 themselves can readily launch a variety of attacks with ICE. They 1181 could direct media to a target of a DoS attack, they could insert 1182 themselves into the data stream, and so on. These are similar to the 1183 general security considerations for offer/answer exchanges, and the 1184 security considerations in [RFC3264] apply. These require techniques 1185 for message integrity and encryption for offers and answers, which 1186 are satisfied by the TLS mechanism [RFC3261] when SIP is used. As 1187 such, the usage of TLS with ICE is RECOMMENDED. 1189 8.2. Insider Attacks 1191 In addition to attacks where the attacker is a third party trying to 1192 insert fake offers, answers, or STUN messages, there are several 1193 attacks possible with ICE when the attacker is an authenticated and 1194 valid participant in the ICE exchange. 1196 8.2.1. The Voice Hammer Attack 1198 The voice hammer attack is an amplification attack. In this attack, 1199 the attacker initiates sessions to other agents, and maliciously 1200 includes the connection address and port of a DoS target as the 1201 destination for media traffic signaled in the SDP. This causes 1202 substantial amplification; a single offer/answer exchange can create 1203 a continuing flood of media packets, possibly at high rates (consider 1204 video sources). This attack is not specific to ICE, but ICE can help 1205 provide remediation. 1207 Specifically, if ICE is used, the agent receiving the malicious SDP 1208 will first perform connectivity checks to the target of media before 1209 sending media there. If this target is a third-party host, the 1210 checks will not succeed, and media is never sent. 1212 Unfortunately, ICE doesn't help if it's not used, in which case an 1213 attacker could simply send the offer without the ICE parameters. 1214 However, in environments where the set of clients is known, and is 1215 limited to ones that support ICE, the server can reject any offers or 1216 answers that don't indicate ICE support. 1218 SIP User Agents (UA) [RFC3261] that are not willing to receive non- 1219 ICE answers MUST include an "ice" Option Tag in the SIP Require 1220 Header Field in their offer. UAs that rejects non-ICE offers SHOULD 1221 use a 421 response code, together with an Option Tag "ice" in the 1222 Require Header Field in the response. 1224 8.2.2. Interactions with Application Layer Gateways and SIP 1226 Application Layer Gateways (ALGs) are functions present in a Network 1227 Address Translation (NAT) device that inspect the contents of packets 1228 and modify them, in order to facilitate NAT traversal for application 1229 protocols. Session Border Controllers (SBCs) are close cousins of 1230 ALGs, but are less transparent since they actually exist as 1231 application-layer SIP intermediaries. ICE has interactions with SBCs 1232 and ALGs. 1234 If an ALG is SIP aware but not ICE aware, ICE will work through it as 1235 long as the ALG correctly modifies the SDP. A correct ALG 1236 implementation behaves as follows: 1238 o The ALG does not modify the "m=" and "c=" lines or the rtcp 1239 attribute if they contain external addresses. 1241 o If the "m=" and "c=" lines contain internal addresses, the 1242 modification depends on the state of the ALG: 1244 * If the ALG already has a binding established that maps an 1245 external port to an internal connection address and port 1246 matching the values in the "m=" and "c=" lines or rtcp 1247 attribute, the ALG uses that binding instead of creating a new 1248 one. 1250 * If the ALG does not already have a binding, it creates a new 1251 one and modifies the SDP, rewriting the "m=" and "c=" lines and 1252 rtcp attribute. 1254 Unfortunately, many ALGs are known to work poorly in these corner 1255 cases. ICE does not try to work around broken ALGs, as this is 1256 outside the scope of its functionality. ICE can help diagnose these 1257 conditions, which often show up as a mismatch between the set of 1258 candidates and the "m=" and "c=" lines and rtcp attributes. The ice- 1259 mismatch attribute is used for this purpose. 1261 ICE works best through ALGs when the signaling is run over TLS. This 1262 prevents the ALG from manipulating the SDP messages and interfering 1263 with ICE operation. Implementations that are expected to be deployed 1264 behind ALGs SHOULD provide for TLS transport of the SDP. 1266 If an SBC is SIP aware but not ICE aware, the result depends on the 1267 behavior of the SBC. If it is acting as a proper Back-to-Back User 1268 Agent (B2BUA), the SBC will remove any SDP attributes it doesn't 1269 understand, including the ICE attributes. Consequently, the call 1270 will appear to both endpoints as if the other side doesn't support 1271 ICE. This will result in ICE being disabled, and media flowing 1272 through the SBC, if the SBC has requested it. If, however, the SBC 1273 passes the ICE attributes without modification, yet modifies the 1274 default destination for media (contained in the "m=" and "c=" lines 1275 and rtcp attribute), this will be detected as an ICE mismatch, and 1276 ICE processing is aborted for the call. It is outside of the scope 1277 of ICE for it to act as a tool for "working around" SBCs. If one is 1278 present, ICE will not be used and the SBC techniques take precedence. 1280 9. IANA Considerations 1282 9.1. SDP Attributes 1284 The original ICE specification defined seven new SDP attributes per 1285 the procedures of Section 8.2.4 of [RFC4566]. The registration 1286 information from the original specification is included here with 1287 modifications to include Mux Category and also defines a new SDP 1288 attribute 'ice-pacing'. 1290 9.1.1. candidate Attribute 1292 Attribute Name: candidate 1294 Type of Attribute: media-level 1296 Subject to charset: No 1298 Purpose: This attribute is used with Interactive Connectivity 1299 Establishment (ICE), and provides one of many possible candidate 1300 addresses for communication. These addresses are validated with 1301 an end-to-end connectivity check using Session Traversal Utilities 1302 for NAT (STUN). 1304 Appropriate Values: See Section 4 of RFC XXXX. 1306 Contact Name: IESG 1308 Contact e-mail: iesg@ietf.org [1] 1310 Reference: RFCXXXX 1311 Mux Category: TRANSPORT 1313 9.1.2. remote-candidates Attribute 1315 Attribute Name: remote-candidates 1317 Type of Attribute: media-level 1319 Subject to charset: No 1321 Purpose: This attribute is used with Interactive Connectivity 1322 Establishment (ICE), and provides the identity of the remote 1323 candidates that the offerer wishes the answerer to use in its 1324 answer. 1326 Appropriate Values: See Section 4 of RFC XXXX. 1328 Contact Name: IESG 1330 Contact e-mail: iesg@ietf.org [2] 1332 Reference: RFCXXXX 1334 Mux Category: TRANSPORT 1336 9.1.3. ice-lite Attribute 1338 Attribute Name: ice-lite 1340 Type of Attribute: session-level 1342 Subject to charset: No 1344 Purpose: This attribute is used with Interactive Connectivity 1345 Establishment (ICE), and indicates that an agent has the minimum 1346 functionality required to support ICE inter-operation with a peer 1347 that has a full implementation. 1349 Appropriate Values: See Section 4 of RFC XXXX. 1351 Contact Name: IESG 1353 Contact e-mail: iesg@ietf.org [3] 1355 Reference: RFCXXXX 1357 Mux Category: NORMAL 1359 9.1.4. ice-mismatch Attribute 1361 Attribute Name: ice-mismatch 1363 Type of Attribute: media-level 1365 Subject to charset: No 1367 Purpose: This attribute is used with Interactive Connectivity 1368 Establishment (ICE), and indicates that an agent is ICE capable, 1369 but did not proceed with ICE due to a mismatch of candidates with 1370 the default destination for media signaled in the SDP. 1372 Appropriate Values: See Section 4 of RFC XXXX. 1374 Contact Name: IESG 1376 Contact e-mail: iesg@ietf.org [4] 1378 Reference: RFCXXXX 1380 Mux Category: NORMAL 1382 9.1.5. ice-pwd Attribute 1384 Attribute Name: ice-pwd 1386 Type of Attribute: session- or media-level 1388 Subject to charset: No 1390 Purpose: This attribute is used with Interactive Connectivity 1391 Establishment (ICE), and provides the password used to protect 1392 STUN connectivity checks. 1394 Appropriate Values: See Section 4 of RFC XXXX. 1396 Contact Name: IESG 1398 Contact e-mail: iesg@ietf.org [5] 1400 Reference: RFCXXXX 1402 Mux Category: TRANSPORT 1404 9.1.6. ice-ufrag Attribute 1406 Attribute Name: ice-ufrag 1408 Type of Attribute: session- or media-level 1410 Subject to charset: No 1412 Purpose: This attribute is used with Interactive Connectivity 1413 Establishment (ICE), and provides the fragments used to construct 1414 the username in STUN connectivity checks. 1416 Appropriate Values: See Section 4 of RFC XXXX. 1418 Contact Name: IESG 1420 Contact e-mail: iesg@ietf.org [6] 1422 Reference: RFCXXXX 1424 Mux Category: TRANSPORT 1426 9.1.7. ice-options Attribute 1428 Attribute Name: ice-options 1430 Long Form: ice-options 1432 Type of Attribute: session-level 1434 Subject to charset: No 1436 Purpose: This attribute is used with Interactive Connectivity 1437 Establishment (ICE), and indicates the ICE options or extensions 1438 used by the agent. 1440 Appropriate Values: See Section 4 of RFC XXXX. 1442 Contact Name: IESG 1444 Contact e-mail: iesg@ietf.org [7] 1446 Reference: RFCXXXX 1448 Mux Category: NORMAL 1450 9.1.8. ice-pacing Attribute 1452 This specification also defines a new SDP attribute, "ice-pacing" 1453 according to the following data: 1455 Attribute Name: ice-pacing 1457 Type of Attribute: session-level 1459 Subject to charset: No 1461 Purpose: This attribute is used with Interactive Connectivity 1462 Establishment (ICE) to indicate desired connectivity check pacing 1463 values. 1465 Appropriate Values: See Section 4 of RFC XXXX. 1467 Contact Name: IESG 1469 Contact e-mail: iesg@ietf.org [8] 1471 Reference: RFCXXXX 1473 Mux Category: NORMAL 1475 9.2. Interactive Connectivity Establishment (ICE) Options Registry 1477 IANA maintains a registry for ice-options identifiers under the 1478 Specification Required policy as defined in "Guidelines for Writing 1479 an IANA Considerations Section in RFCs" [RFC5226]. 1481 ICE options are of unlimited length according to the syntax in 1482 Section 4.6; however, they are RECOMMENDED to be no longer than 20 1483 characters. This is to reduce message sizes and allow for efficient 1484 parsing. ICE options are defined at the session leve.. 1486 A registration request MUST include the following information: 1488 o The ICE option identifier to be registered 1490 o Name, Email, and Address of a contact person for the registration 1492 o Organization or individuals having the change control 1494 o Short description of the ICE extension to which the option relates 1496 o Reference(s) to the specification defining the ICE option and the 1497 related extensions 1499 10. Acknowledgments 1501 A large part of the text in this document was taken from [RFC5245], 1502 authored by Jonathan Rosenberg. 1504 Some of the text in this document was taken from [RFC6336], authored 1505 by Magnus Westerlund and Colin Perkins. 1507 Many thanks to Christer Holmberg for providing text suggestions in 1508 Section 3 that aligns with [RFC8445] 1510 Thanks to Thomas Stach for text help, Roman Shpount for suggesting 1511 RTCP candidate handling and Simon Perreault for advising on IPV6 1512 address selection when candidate-address includes FQDN. 1514 Many thanks to Flemming Andreasen for shepherd review feedback. 1516 Thanks to following experts for their reviews and constructive 1517 feedback: Christer Holmberg, Adam Roach, Peter Saint-Andre and the 1518 MMUSIC WG. 1520 11. References 1522 11.1. Normative References 1524 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 1525 Requirement Levels", BCP 14, RFC 2119, 1526 DOI 10.17487/RFC2119, March 1997, 1527 . 1529 [RFC3261] Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston, 1530 A., Peterson, J., Sparks, R., Handley, M., and E. 1531 Schooler, "SIP: Session Initiation Protocol", RFC 3261, 1532 DOI 10.17487/RFC3261, June 2002, 1533 . 1535 [RFC3262] Rosenberg, J. and H. Schulzrinne, "Reliability of 1536 Provisional Responses in Session Initiation Protocol 1537 (SIP)", RFC 3262, DOI 10.17487/RFC3262, June 2002, 1538 . 1540 [RFC3264] Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model 1541 with Session Description Protocol (SDP)", RFC 3264, 1542 DOI 10.17487/RFC3264, June 2002, 1543 . 1545 [RFC3312] Camarillo, G., Ed., Marshall, W., Ed., and J. Rosenberg, 1546 "Integration of Resource Management and Session Initiation 1547 Protocol (SIP)", RFC 3312, DOI 10.17487/RFC3312, October 1548 2002, . 1550 [RFC3556] Casner, S., "Session Description Protocol (SDP) Bandwidth 1551 Modifiers for RTP Control Protocol (RTCP) Bandwidth", 1552 RFC 3556, DOI 10.17487/RFC3556, July 2003, 1553 . 1555 [RFC3605] Huitema, C., "Real Time Control Protocol (RTCP) attribute 1556 in Session Description Protocol (SDP)", RFC 3605, 1557 DOI 10.17487/RFC3605, October 2003, 1558 . 1560 [RFC4032] Camarillo, G. and P. Kyzivat, "Update to the Session 1561 Initiation Protocol (SIP) Preconditions Framework", 1562 RFC 4032, DOI 10.17487/RFC4032, March 2005, 1563 . 1565 [RFC4091] Camarillo, G. and J. Rosenberg, "The Alternative Network 1566 Address Types (ANAT) Semantics for the Session Description 1567 Protocol (SDP) Grouping Framework", RFC 4091, June 2005, 1568 . 1570 [RFC4092] Camarillo, G. and J. Rosenberg, "Usage of the Session 1571 Description Protocol (SDP) Alternative Network Address 1572 Types (ANAT) Semantics in the Session Initiation Protocol 1573 (SIP)", RFC 4092, June 2005, 1574 . 1576 [RFC4566] Handley, M., Jacobson, V., and C. Perkins, "SDP: Session 1577 Description Protocol", RFC 4566, DOI 10.17487/RFC4566, 1578 July 2006, . 1580 [RFC5226] Narten, T. and H. Alvestrand, "Guidelines for Writing an 1581 IANA Considerations Section in RFCs", BCP 26, RFC 5226, 1582 DOI 10.17487/RFC5226, May 2008, 1583 . 1585 [RFC5234] Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax 1586 Specifications: ABNF", STD 68, RFC 5234, 1587 DOI 10.17487/RFC5234, January 2008, 1588 . 1590 [RFC5768] Rosenberg, J., "Indicating Support for Interactive 1591 Connectivity Establishment (ICE) in the Session Initiation 1592 Protocol (SIP)", RFC 5768, DOI 10.17487/RFC5768, April 1593 2010, . 1595 [RFC6336] Westerlund, M. and C. Perkins, "IANA Registry for 1596 Interactive Connectivity Establishment (ICE) Options", 1597 RFC 6336, April 2010, 1598 . 1600 [RFC8445] Keranen, A., Holmberg, C., and J. Rosenberg, "Interactive 1601 Connectivity Establishment (ICE): A Protocol for Network 1602 Address Translator (NAT) Traversal", RFC 8445, 1603 DOI 10.17487/RFC8445, July 2018, 1604 . 1606 11.2. Informative References 1608 [draft-holmberg-ice-pac] 1609 Holmberg, C. and J. Uberti, "Interactive Connectivity 1610 Establishment Patiently Awaiting Connectivity (ICE PAC)", 1611 draft-holmberg-ice-pac-01 (work in progress), March 2019, 1612 . 1615 [RFC3725] Rosenberg, J., Peterson, J., Schulzrinne, H., and G. 1616 Camarillo, "Best Current Practices for Third Party Call 1617 Control (3pcc) in the Session Initiation Protocol (SIP)", 1618 BCP 85, RFC 3725, DOI 10.17487/RFC3725, April 2004, 1619 . 1621 [RFC3960] Camarillo, G. and H. Schulzrinne, "Early Media and Ringing 1622 Tone Generation in the Session Initiation Protocol (SIP)", 1623 RFC 3960, DOI 10.17487/RFC3960, December 2004, 1624 . 1626 [RFC5245] Rosenberg, J., "Interactive Connectivity Establishment 1627 (ICE): A Protocol for Network Address Translator (NAT) 1628 Traversal for Offer/Answer Protocols", RFC 5245, 1629 DOI 10.17487/RFC5245, April 2010, 1630 . 1632 [RFC5626] Jennings, C., Ed., Mahy, R., Ed., and F. Audet, Ed., 1633 "Managing Client-Initiated Connections in the Session 1634 Initiation Protocol (SIP)", RFC 5626, 1635 DOI 10.17487/RFC5626, October 2009, 1636 . 1638 [RFC5898] Andreasen, F., Camarillo, G., Oran, D., and D. Wing, 1639 "Connectivity Preconditions for Session Description 1640 Protocol (SDP) Media Streams", RFC 5898, 1641 DOI 10.17487/RFC5898, July 2010, 1642 . 1644 11.3. URIs 1646 [1] mailto:iesg@ietf.org 1648 [2] mailto:iesg@ietf.org 1650 [3] mailto:iesg@ietf.org 1652 [4] mailto:iesg@ietf.org 1654 [5] mailto:iesg@ietf.org 1656 [6] mailto:iesg@ietf.org 1658 [7] mailto:iesg@ietf.org 1660 [8] mailto:iesg@ietf.org 1662 [9] mailto:christer.holmberg@ericsson.com 1664 [10] mailto:rshpount@turbobridge.com 1666 [11] mailto:thomass.stach@gmail.com 1668 Appendix A. Examples 1670 For the example shown in section 15 of [RFC8445] the resulting offer 1671 (message 5) encoded in SDP looks like: 1673 v=0 1674 o=jdoe 2890844526 2890842807 IN IP6 $L-PRIV-1.IP 1675 s= 1676 c=IN IP6 $NAT-PUB-1.IP 1677 t=0 0 1678 a=ice-pwd:asd88fgpdd777uzjYhagZg 1679 a=ice-ufrag:8hhY 1680 m=audio $NAT-PUB-1.PORT RTP/AVP 0 1681 b=RS:0 1682 b=RR:0 1683 a=rtpmap:0 PCMU/8000 1684 a=candidate:1 1 UDP 2130706431 $L-PRIV-1.IP $L-PRIV-1.PORT typ host 1685 a=candidate:2 1 UDP 1694498815 $NAT-PUB-1.IP $NAT-PUB-1.PORT typ 1686 srflx raddr $L-PRIV-1.IP rport $L-PRIV-1.PORT 1688 The offer, with the variables replaced with their values, will look 1689 like (lines folded for clarity): 1691 v=0 1692 o=jdoe 2890844526 2890842807 IN IP6 fe80::6676:baff:fe9c:ee4a 1693 s= 1694 c=IN IP6 2001:420:c0e0:1005::61 1695 t=0 0 1696 a=ice-pwd:asd88fgpdd777uzjYhagZg 1697 a=ice-ufrag:8hhY 1698 m=audio 45664 RTP/AVP 0 1699 b=RS:0 1700 b=RR:0 1701 a=rtpmap:0 PCMU/8000 1702 a=candidate:1 1 UDP 2130706431 fe80::6676:baff:fe9c:ee4a 8998 typ host 1703 a=candidate:2 1 UDP 1694498815 2001:420:c0e0:1005::61 45664 typ srflx raddr 1704 fe80::6676:baff:fe9c:ee4a rport 8998 1706 The resulting answer looks like: 1708 v=0 1709 o=bob 2808844564 2808844564 IN IP4 $R-PUB-1.IP 1710 s= 1711 c=IN IP4 $R-PUB-1.IP 1712 t=0 0 1713 a=ice-pwd:YH75Fviy6338Vbrhrlp8Yh 1714 a=ice-ufrag:9uB6 1715 m=audio $R-PUB-1.PORT RTP/AVP 0 1716 b=RS:0 1717 b=RR:0 1718 a=rtpmap:0 PCMU/8000 1719 a=candidate:1 1 UDP 2130706431 $R-PUB-1.IP $R-PUB-1.PORT typ host 1720 With the variables filled in: 1722 v=0 1723 o=bob 2808844564 2808844564 IN IP4 192.0.2.1 1724 s= 1725 c=IN IP4 192.0.2.1 1726 t=0 0 1727 a=ice-pwd:YH75Fviy6338Vbrhrlp8Yh 1728 a=ice-ufrag:9uB6 1729 m=audio 3478 RTP/AVP 0 1730 b=RS:0 1731 b=RR:0 1732 a=rtpmap:0 PCMU/8000 1733 a=candidate:1 1 UDP 2130706431 192.0.2.1 3478 typ host 1735 Appendix B. The remote-candidates Attribute 1737 The a=remote-candidates attribute exists to eliminate a race 1738 condition between the updated offer and the response to the STUN 1739 Binding request that moved a candidate into the Valid list. This 1740 race condition is shown in Figure 1. On receipt of message 4, agent 1741 L adds a candidate pair to the valid list. If there was only a 1742 single data stream with a single component, agent L could now send an 1743 updated offer. However, the check from agent R has not yet generated 1744 a response, and agent R receives the updated offer (message 7) before 1745 getting the response (message 9). Thus, it does not yet know that 1746 this particular pair is valid. To eliminate this condition, the 1747 actual candidates at R that were selected by the offerer (the remote 1748 candidates) are included in the offer itself, and the answerer delays 1749 its answer until those pairs validate. 1751 Agent L Network Agent R 1752 |(1) Offer | | 1753 |------------------------------------------>| 1754 |(2) Answer | | 1755 |<------------------------------------------| 1756 |(3) STUN Req. | | 1757 |------------------------------------------>| 1758 |(4) STUN Res. | | 1759 |<------------------------------------------| 1760 |(5) STUN Req. | | 1761 |<------------------------------------------| 1762 |(6) STUN Res. | | 1763 |-------------------->| | 1764 | |Lost | 1765 |(7) Offer | | 1766 |------------------------------------------>| 1767 |(8) STUN Req. | | 1768 |<------------------------------------------| 1769 |(9) STUN Res. | | 1770 |------------------------------------------>| 1771 |(10) Answer | | 1772 |<------------------------------------------| 1774 Figure 1: Race Condition Flow 1776 Appendix C. Why Is the Conflict Resolution Mechanism Needed? 1778 When ICE runs between two peers, one agent acts as controlled, and 1779 the other as controlling. Rules are defined as a function of 1780 implementation type and offerer/answerer to determine who is 1781 controlling and who is controlled. However, the specification 1782 mentions that, in some cases, both sides might believe they are 1783 controlling, or both sides might believe they are controlled. How 1784 can this happen? 1786 The condition when both agents believe they are controlled shows up 1787 in third party call control cases. Consider the following flow: 1789 A Controller B 1790 |(1) INV() | | 1791 |<-------------| | 1792 |(2) 200(SDP1) | | 1793 |------------->| | 1794 | |(3) INV() | 1795 | |------------->| 1796 | |(4) 200(SDP2) | 1797 | |<-------------| 1798 |(5) ACK(SDP2) | | 1799 |<-------------| | 1800 | |(6) ACK(SDP1) | 1801 | |------------->| 1803 Figure 2: Role Conflict Flow 1805 This flow is a variation on flow III of RFC 3725 [RFC3725]. In fact, 1806 it works better than flow III since it produces fewer messages. In 1807 this flow, the controller sends an offerless INVITE to agent A, which 1808 responds with its offer, SDP1. The agent then sends an offerless 1809 INVITE to agent B, which it responds to with its offer, SDP2. The 1810 controller then uses the offer from each agent to generate the 1811 answers. When this flow is used, ICE will run between agents A and 1812 B, but both will believe they are in the controlling role. With the 1813 role conflict resolution procedures, this flow will function properly 1814 when ICE is used. 1816 At this time, there are no documented flows that can result in the 1817 case where both agents believe they are controlled. However, the 1818 conflict resolution procedures allow for this case, should a flow 1819 arise that would fit into this category. 1821 Appendix D. Why Send an Updated Offer? 1823 Section 11.1 describes rules for sending media. Both agents can send 1824 media once ICE checks complete, without waiting for an updated offer. 1825 Indeed, the only purpose of the updated offer is to "correct" the SDP 1826 so that the default destination for media matches where media is 1827 being sent based on ICE procedures (which will be the highest- 1828 priority nominated candidate pair). 1830 This begs the question -- why is the updated offer/answer exchange 1831 needed at all? Indeed, in a pure offer/answer environment, it would 1832 not be. The offerer and answerer will agree on the candidates to use 1833 through ICE, and then can begin using them. As far as the agents 1834 themselves are concerned, the updated offer/answer provides no new 1835 information. However, in practice, numerous components along the 1836 signaling path look at the SDP information. These include entities 1837 performing off-path QoS reservations, NAT traversal components such 1838 as ALGs and Session Border Controllers (SBCs), and diagnostic tools 1839 that passively monitor the network. For these tools to continue to 1840 function without change, the core property of SDP -- that the 1841 existing, pre-ICE definitions of the addresses used for media -- the 1842 "m=" and "c=" lines and the rtcp attribute -- must be retained. For 1843 this reason, an updated offer must be sent. 1845 Appendix E. Contributors 1847 Following experts have contributed textual and structural 1848 improvements for this work 1850 1. Christer Holmberg 1852 * Ericsson 1854 * Email: christer.holmberg@ericsson.com [9] 1856 2. Roman Shpount 1858 * TurboBridge 1860 * rshpount@turbobridge.com [10] 1862 3. Thomas Stach 1864 * thomass.stach@gmail.com [11] 1866 Authors' Addresses 1868 Marc Petit-Huguenin 1869 Impedance Mismatch 1871 Email: marc@petit-huguenin.org 1873 Suhas Nandakumar 1874 Cisco Systems 1875 707 Tasman Dr 1876 Milpitas, CA 95035 1877 USA 1879 Email: snandaku@cisco.com 1880 Ari Keranen 1881 Ericsson 1882 Jorvas 02420 1883 Finland 1885 Email: ari.keranen@ericsson.com