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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: If an agent uses separate ports for RTP and RTCP, the agent MUST include an SDP rtcp attribute in the "m=" section, as described in [RFC3605]. In the cases where the port number for the RTCP is one higher than the RTP port and RTCP component address is same as the address of the RTP component, the SDP rtcp attribute MUST not be included. == 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 (April 1, 2018) is 2188 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: October 3, 2018 A. Keranen 7 Ericsson 8 April 1, 2018 10 Session Description Protocol (SDP) Offer/Answer procedures for 11 Interactive Connectivity Establishment (ICE) 12 draft-ietf-mmusic-ice-sip-sdp-19 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 http://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 October 3, 2018. 37 Copyright Notice 39 Copyright (c) 2018 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 (http://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 . . . . . . . . . . . . . . . . . . . . . . . . . 4 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. ICE Mismatch . . . . . . . . . . . . . . . . . . . . 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 . . . . . . . . . . . . 8 81 3.3.4. Concluding ICE . . . . . . . . . . . . . . . . . . . 8 82 3.4. Subsequent Offer/Answer Exchanges . . . . . . . . . . . . 9 83 3.4.1. Sending Subsequent Offer . . . . . . . . . . . . . . 9 84 3.4.2. Sending Subsequent Answer . . . . . . . . . . . . . . 11 85 3.4.3. Receiving Answer for a Subsequent Offer . . . . . . . 13 86 4. Grammar . . . . . . . . . . . . . . . . . . . . . . . . . . . 15 87 4.1. "candidate" Attribute . . . . . . . . . . . . . . . . . . 15 88 4.2. "remote-candidates" Attribute . . . . . . . . . . . . . . 18 89 4.3. "ice-lite" and "ice-mismatch" Attributes . . . . . . . . 18 90 4.4. "ice-ufrag" and "ice-pwd" Attributes . . . . . . . . . . 19 91 4.5. "ice-pacing" Attribute . . . . . . . . . . . . . . . . . 19 92 4.6. "ice-options" Attribute . . . . . . . . . . . . . . . . . 20 93 5. Keepalives . . . . . . . . . . . . . . . . . . . . . . . . . 21 94 6. SIP Considerations . . . . . . . . . . . . . . . . . . . . . 21 95 6.1. Latency Guidelines . . . . . . . . . . . . . . . . . . . 21 96 6.1.1. Offer in INVITE . . . . . . . . . . . . . . . . . . . 22 97 6.1.2. Offer in Response . . . . . . . . . . . . . . . . . . 23 98 6.2. SIP Option Tags and Media Feature Tags . . . . . . . . . 23 99 6.3. Interactions with Forking . . . . . . . . . . . . . . . . 24 100 6.4. Interactions with Preconditions . . . . . . . . . . . . . 24 101 6.5. Interactions with Third Party Call Control . . . . . . . 24 102 7. Relationship with ANAT . . . . . . . . . . . . . . . . . . . 25 103 8. Setting Ta and RTO for RTP Media Streams . . . . . . . . . . 25 104 9. Security Considerations . . . . . . . . . . . . . . . . . . . 25 105 9.1. Attacks on the Offer/Answer Exchanges . . . . . . . . . . 25 106 9.2. Insider Attacks . . . . . . . . . . . . . . . . . . . . . 25 107 9.2.1. The Voice Hammer Attack . . . . . . . . . . . . . . . 26 108 9.2.2. Interactions with Application Layer Gateways and SIP 26 109 10. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 27 110 10.1. SDP Attributes . . . . . . . . . . . . . . . . . . . . . 27 111 10.1.1. candidate Attribute . . . . . . . . . . . . . . . . 28 112 10.1.2. remote-candidates Attribute . . . . . . . . . . . . 28 113 10.1.3. ice-lite Attribute . . . . . . . . . . . . . . . . . 29 114 10.1.4. ice-mismatch Attribute . . . . . . . . . . . . . . . 29 115 10.1.5. ice-pwd Attribute . . . . . . . . . . . . . . . . . 30 116 10.1.6. ice-ufrag Attribute . . . . . . . . . . . . . . . . 30 117 10.1.7. ice-options Attribute . . . . . . . . . . . . . . . 31 118 10.1.8. ice-pacing Attribute . . . . . . . . . . . . . . . . 31 119 10.2. Interactive Connectivity Establishment (ICE) Options 120 Registry . . . . . . . . . . . . . . . . . . . . . . . . 32 121 11. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 33 122 12. References . . . . . . . . . . . . . . . . . . . . . . . . . 33 123 12.1. Normative References . . . . . . . . . . . . . . . . . . 33 124 12.2. Informative References . . . . . . . . . . . . . . . . . 35 125 12.3. URIs . . . . . . . . . . . . . . . . . . . . . . . . . . 36 126 Appendix A. Examples . . . . . . . . . . . . . . . . . . . . . . 36 127 Appendix B. The remote-candidates Attribute . . . . . . . . . . 38 128 Appendix C. Why Is the Conflict Resolution Mechanism Needed? . . 39 129 Appendix D. Why Send an Updated Offer? . . . . . . . . . . . . . 40 130 Appendix E. Contributors . . . . . . . . . . . . . . . . . . . . 41 131 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 41 133 1. Introduction 135 This document describes how Interactive Connectivity Establishment 136 (ICE) is used with Session Description Protocol (SDP) offer/answer 137 [RFC3264]. The ICE specification [ICE-BIS] describes procedures that 138 are common to all usages of ICE and this document gives the 139 additional details needed to use ICE with SDP offer/answer. 141 2. Terminology 143 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 144 "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and 145 "OPTIONAL" in this document are to be interpreted as described in RFC 146 2119 [RFC2119]. 148 Readers should be familiar with the terminology defined in [RFC3264], 149 in [ICE-BIS] and the following: 151 Default Destination/Candidate: The default destination for a 152 component of a data stream is the transport address that would be 153 used by an agent that is not ICE aware. A default candidate for a 154 component is one whose transport address matches the default 155 destination for that component. For the RTP component, the 156 default IP address is in the "c=" line of the SDP, and the port is 157 in the "m=" line. For the RTCP component, the address and port 158 are indicated using the "a=rtcp" attribute defined in [RFC3605], 159 if present; otherwise, the RTCP component address is same as the 160 address of the RTP component, and its port is one greater than the 161 port of the RTP component. 163 3. SDP Offer/Answer Procedures 165 3.1. Introduction 167 [ICE-BIS] defines ICE candidate exchange as the process for ICE 168 agents (Initiator and Responder) to exchange their candidate 169 information required for ICE processing at the agents. For the 170 purposes of this specification, the candidate exchange process 171 corresponds to the [RFC3264] Offer/Answer protocol and the 172 terminologies offerer and answerer correspond to the initiator and 173 responder terminologies from [ICE-BIS] respectively. 175 Once the initiating agent has gathered, pruned and prioritized its 176 set of candidates [ICE-BIS], the candidate exchange with the peer 177 agent begins. 179 3.2. Generic Procedures 181 3.2.1. Encoding 183 Section 4 provides detailed rules for constructing various SDP 184 attributes defined in this specification. 186 3.2.1.1. Data Streams 188 Each data stream [ICE-BIS] is represented by an SDP media description 189 ("m=" section). 191 3.2.1.2. Candidates 193 With in a "m=" section, each candidate (including the default 194 candidate) associated with the data stream is represented by an SDP 195 candidate attribute. 197 Prior to nomination, the "c=" line associated with an "m=" section 198 contains the IP address of the default candidate, while the "m=" line 199 contains the port and transport of the default candidate for that 200 "m=" section. 202 After nomination, the "c=" line for a given "m=" section contains the 203 IP address of the nominated candidate (the local candidate of the 204 nominated candidate pair) and the "m=" line contains the port and 205 transport corresponding to the nominated candidate for that "m=" 206 section. 208 3.2.1.3. Username and Password 210 The ICE username is represented by an SDP ice-ufrag attribute and the 211 ICE password is represented by an SDP ice-pwd attribute. 213 3.2.1.4. Lite Implementations 215 An ICE lite implementation [ICE-BIS] MUST include an SDP ice-lite 216 attribute. A full implementation MUST NOT include that attribute. 218 3.2.1.5. ICE Extensions 220 An agent uses the SDP ice-options attribute to indicate support of 221 ICE extensions. 223 An agent compliant to this specification MUST include an SDP ice- 224 options attribute with an "ice2" attribute value. If an agent 225 receives an SDP offer or answer with ICE attributes but without the 226 "ice2" ice-options attribute value, the agent assumes that the peer 227 is compliant to [RFC5245]. 229 3.2.1.6. Inactive and Disabled Data Streams 231 If an "m=" section is marked as inactive [RFC4566], or has a 232 bandwidth value of zero [RFC4566], the agent MUST still include ICE 233 related SDP attributes. 235 If the port value associated with an "m=" section is set to zero 236 (implying a disabled stream), the agent SHOULD NOT include ICE 237 related SDP candidate attributes in that "m=" section, unless an SDP 238 extension specifying otherwise is used. 240 3.2.2. RTP/RTCP Considerations 242 If an agent utilizes both RTP and RTCP, the agent MUST include SDP 243 candidate attributes for both the RTP and RTCP components in the "m=" 244 section. 246 If an agent uses separate ports for RTP and RTCP, the agent MUST 247 include an SDP rtcp attribute in the "m=" section, as described in 248 [RFC3605]. In the cases where the port number for the RTCP is one 249 higher than the RTP port and RTCP component address is same as the 250 address of the RTP component, the SDP rtcp attribute MUST not be 251 included. 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 [ICE-BIS]. 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. ICE Mismatch 270 The agent will proceed with the ICE procedures defined in [ICE-BIS] 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 IP address and port in the "c=" and "m=" lines, 275 respectively, appear in a candidate attribute and the value in the 276 rtcp attribute appears in a candidate attribute. 278 If this condition is not met, the agent MUST process the SDP based on 279 normal [RFC3264] procedures, without using any of the ICE mechanisms 280 described in the remainder of this specification with the following 281 exceptions: 283 1. The agent MUST follow the rules of section 11 of [ICE-BIS], which 284 describe keepalive procedures for all agents. 286 2. If the agent is not proceeding with ICE because there were 287 a=candidate attributes, but none that matched the default 288 destination of the data stream, the agent MUST include an a=ice- 289 mismatch attribute in its answer and MAY omit a=candidate 290 attributes for such data streams. See Section 9.2.2 for a 291 discussion of cases where this can happen. This specification 292 provides no guidance on how an agent should proceed in such a 293 failure case. 295 3. If the default candidates were relayed candidates learned through 296 a TURN server, the agent MUST create permissions in the TURN 297 server for the IP addresses learned from its peer in the SDP it 298 just received. If this is not done, initial packets in the data 299 stream from the peer may be lost. 301 3.2.6. SDP Example 303 The following is an example SDP message that includes ICE attributes 304 (lines folded for readability): 306 v=0 307 o=jdoe 2890844526 2890842807 IN IP4 10.0.1.1 308 s= 309 c=IN IP4 192.0.2.3 310 t=0 0 311 a=ice-options:ice2 312 a=ice-pwd:asd88fgpdd777uzjYhagZg 313 a=ice-ufrag:8hhY 314 m=audio 45664 RTP/AVP 0 315 b=RS:0 316 b=RR:0 317 a=rtpmap:0 PCMU/8000 318 a=candidate:1 1 UDP 2130706431 10.0.1.1 8998 typ host 319 a=candidate:2 1 UDP 1694498815 192.0.2.3 45664 typ srflx raddr 320 10.0.1.1 rport 8998 322 3.3. Initial Offer/Answer Exchange 324 3.3.1. Sending the Initial Offer 326 When an offerer generates the initial offer, in each "m=" section it 327 MUST include SDP candidate attributes for each available candidate 328 associated with the "m=" section. In addition, the offerer MUST 329 include an SDP ice-ufrag and an SDP ice-pwd attribute in the offer. 331 Note: Within the scope of this document, "Initial Offer" refers to 332 the first SDP offer that is sent in order to negotiate usage of 333 ICE. It might, or might not, be the initial SDP offer of the SDP 334 session. 336 Note: The procedures in this document only consider "m=" sections 337 associated with data streams where ICE is used. 339 3.3.2. Sending the Initial Answer 341 When an answerer receives an initial offer that indicates that the 342 offerer supports ICE, and if the answerer accepts the offer and the 343 usage of ICE, in each "m=" section within the answer, it MUST include 344 SDP candidate attributes for each available candidate associated with 345 the "m=" section. In addition, the answerer MUST include an SDP ice- 346 ufrag and an SDP ice-pwd attribute in the answer. 348 Once the answerer has sent the answer, it can start performing 349 connectivity checks towards the peer candidates that were provided in 350 the offer. 352 If the offer does not indicate support of ICE, the answerer MUST NOT 353 accept the usage of ICE. If the answerer still accepts the offer, 354 the answerer MUST NOT include any ICE related SDP attributes in the 355 answer. Instead the answerer will generate the answer according to 356 normal offer/answer procedures [RFC3264]. 358 If the answerer detects a possibility of the ICE mismatch, procedures 359 described in (Section 3.2.5) are followed. 361 3.3.3. Receiving the Initial Answer 363 When an offerer receives an initial answer that indicates that the 364 answerer supports ICE, it can start performing connectivity checks 365 towards the peer candidates that were provided in the answer. 367 If the answer does not indicate that the answerer supports ICE, or if 368 the offerer detects an ICE mismatch in the answer, the offerer MUST 369 terminate the usage of ICE. The subsequent actions taken by the 370 offerer are implementation dependent and are out of the scope of this 371 specification. 373 3.3.4. Concluding ICE 375 Once the state of each check list is Completed, and if the agent is 376 the controlling agent, it nominates a candidate pair [ICE-BIS] and 377 checks for each data stream whether the nominated pair matches the 378 default candidate pair. If there are one or more data streams with a 379 match, and the peer did not indicate support for the 'ice2' ice- 380 option, the controlling agent MUST generate a subsequent offer 381 (Section 3.4.1), in which the IP address, port and transport in the 382 "c=" and "m=" lines associated with each data stream match the 383 corresponding local information of the nominated pair for that data 384 stream. 386 However, If the support for 'ice2' ice-option is in use, the 387 nominated candidate is noted and sent in the subsequent offer/answer 388 exchange as the default candidate and no updated offer is needed to 389 fix the default candidate. 391 Also as described in [ICE-BIS], once the controlling agent has 392 nominated a candidate pair for a data stream, the agent MUST NOT 393 nominate another pair for that data stream during the lifetime of the 394 ICE session (i.e. until ICE is restarted). 396 3.4. Subsequent Offer/Answer Exchanges 398 Either agent MAY generate a subsequent offer at any time allowed by 399 [RFC3264]. This section defines rules for construction of subsequent 400 offers and answers. 402 Should a subsequent offer fail, ICE processing continues as if the 403 subsequent offer had never been made. 405 3.4.1. Sending Subsequent Offer 407 3.4.1.1. Procedures for All Implementations 409 3.4.1.1.1. ICE Restarts 411 An agent MAY restart ICE processing for an existing data stream 412 [ICE-BIS]. 414 The rules governing the ICE restart imply that setting the IP address 415 in the "c=" line to 0.0.0.0 will cause an ICE restart. Consequently, 416 ICE implementations MUST NOT utilize this mechanism for call hold, 417 and instead MUST use a=inactive and a=sendonly as described in 418 [RFC3264]. 420 To restart ICE, an agent MUST change both the ice-pwd and the ice- 421 ufrag for the data stream in an offer. Note that it is permissible 422 to use a session-level attribute in one offer, but to provide the 423 same ice-pwd or ice-ufrag as a media-level attribute in a subsequent 424 offer. This is not a change in password, just a change in its 425 representation, and does not cause an ICE restart. 427 An agent sets the rest of the ice related fields in the SDP for this 428 data stream as it would in an initial offer of this data stream (see 429 Section 3.2.1). Consequently, the set of candidates MAY include 430 some, none, or all of the previous candidates for that data stream 431 and MAY include a totally new set of candidates. 433 3.4.1.1.2. Removing a Data Stream 435 If an agent removes a data stream by setting its port to zero, it 436 MUST NOT include any candidate attributes for that data stream and 437 SHOULD NOT include any other ICE-related attributes defined in 438 Section 4 for that data stream. 440 3.4.1.1.3. Adding a Data Stream 442 If an agent wishes to add a new data stream, it sets the fields in 443 the SDP for this data stream as if this was an initial offer for that 444 data stream (see Section 3.2.1). This will cause ICE processing to 445 begin for this data stream. 447 3.4.1.2. Procedures for Full Implementations 449 This section describes additional procedures for full 450 implementations, covering existing data streams. 452 3.4.1.2.1. Before Nomination 454 When an offerer sends a subsequent offer; in each "m=" section for 455 which a candidate pair has not yet been nominated, the offer MUST 456 include the same set of ICE-related information that the offerer 457 included in the previous offer or answer. The agent MAY include 458 additional candidates it did not offer previously, but which it has 459 gathered since the last offer/ answer exchange, including peer 460 reflexive candidates. 462 The agent MAY change the default destination for media. As with 463 initial offers, there MUST be a set of candidate attributes in the 464 offer matching this default destination. 466 3.4.1.2.2. After Nomination 468 Once a candidate pair has been nominated for a data stream, the IP 469 address, port and transport in each "c=" and "m=" line associated 470 with that data stream MUST match the data associated with the 471 nominated pair for that data stream. In addition, the offerer only 472 includes SDP candidates representing the local candidates of the 473 nominated candidate pair. The offerer MUST NOT include any other SDP 474 candidate attributes in the subsequent offer. 476 In addition, if the agent is controlling, it MUST include the 477 a=remote-candidates attribute for each data stream whose check list 478 is in the completed state. The attribute contains the remote 479 candidates corresponding to the nominated pair in the valid list for 480 each component of that data stream. It is needed to avoid a race 481 condition whereby the controlling agent chooses its pairs, but the 482 updated offer beats the connectivity checks to the controlled agent, 483 which doesn't even know these pairs are valid, let alone selected. 484 See Appendix B for elaboration on this race condition. 486 3.4.1.3. Procedures for Lite Implementations 488 If the ICE state is running, a lite implementation MUST include all 489 of its candidates for each component of each data stream in 490 a=candidate attribute in any subsequent offer. The candidates are 491 formed identical to the procedures for initial offers. 493 A lite implementation MUST NOT add additional host candidates or 494 change username fragments or passwords in a subsequent offer. 495 Otherwise, it MUST restart ICE. 497 If ICE has completed for a data stream and if the agent is 498 controlled, the default destination for that data stream MUST be set 499 to the remote candidate of the candidate pair for that component in 500 the valid list. For a lite implementation, there is always just a 501 single candidate pair in the valid list for each component of a data 502 stream. Additionally, the agent MUST include a candidate attribute 503 for each default destination. 505 If ICE state is completed and if the agent is controlling (which only 506 happens when both agents are lite), the agent MUST include the 507 a=remote-candidates attribute for each data stream. The attribute 508 contains the remote candidates from the candidate pairs in the valid 509 list (one pair for each component of each data stream). 511 3.4.2. Sending Subsequent Answer 513 If ICE is Completed for a data stream, and the offer for that data 514 stream lacked the a=remote-candidates attribute, the rules for 515 construction of the answer are identical to those for the offerer, 516 except that the answerer MUST NOT include the a=remote-candidates 517 attribute in the answer. 519 A controlled agent will receive an offer with the a=remote-candidates 520 attribute for a data stream when its peer has concluded ICE 521 processing for that data stream. This attribute is present in the 522 offer to deal with a race condition between the receipt of the offer, 523 and the receipt of the Binding Response that tells the answerer the 524 candidate that will be selected by ICE. See Appendix B for an 525 explanation of this race condition. Consequently, processing of an 526 offer with this attribute depends on the winner of the race. 528 The agent forms a candidate pair for each component of the data 529 stream by: 531 o Setting the remote candidate equal to the offerer's default 532 destination for that component (i.e. the contents of the "m=" and 533 "c=" lines for RTP, and the a=rtcp attribute for RTCP) 535 o Setting the local candidate equal to the transport address for 536 that same component in the a=remote-candidates attribute in the 537 offer. 539 The agent then sees if each of these candidate pairs is present in 540 the valid list. If a particular pair is not in the valid list, the 541 check has "lost" the race. Call such a pair a "losing pair". 543 The agent finds all the pairs in the check list whose remote 544 candidates equal the remote candidate in the losing pair: 546 o If none of the pairs are In-Progress, and at least one is Failed, 547 it is most likely that a network failure, such as a network 548 partition or serious packet loss, has occurred. The agent SHOULD 549 generate an answer for this data stream as if the remote- 550 candidates attribute had not been present, and then restart ICE 551 for this stream. 553 o If at least one of the pairs is In-Progress, the agent SHOULD wait 554 for those checks to complete, and as each completes, redo the 555 processing in this section until there are no losing pairs. 557 Once there are no losing pairs, the agent can generate the answer. 558 It MUST set the default destination for media to the candidates in 559 the remote-candidates attribute from the offer (each of which will 560 now be the local candidate of a candidate pair in the valid list). 561 It MUST include a candidate attribute in the answer for each 562 candidate in the remote-candidates attribute in the offer. 564 3.4.2.1. Detecting ICE Restart 566 If the offerer in a subsequent offer requested an ICE restart for a 567 data stream, and if the answerer accepts the offer, the answerer 568 follows the procedures for generating an initial answer. 570 For a given data stream, the answerer MAY include the same candidates 571 that were used in the previous ICE session, but it MUST change the 572 SDP ice-pwd and ice-ufrag attribute values. 574 3.4.2.2. Lite Implementation specific procedures 576 If the received offer contains the remote-candidates attribute for a 577 data stream, the agent forms a candidate pair for each component of 578 the data stream by: 580 o Setting the remote candidate equal to the offerer's default 581 destination for that component (i.e., the contents of the "m=" and 582 "c=" lines for RTP, and the a=rtcp attribute for RTCP). 584 o Setting the local candidate equal to the transport address for 585 that same component in the a=remote-candidates attribute in the 586 offer. 588 The state of ICE processing for that data stream is set to Completed. 590 Furthermore, if the agent believed it was controlling, but the offer 591 contained the a=remote-candidates attribute, both agents believe they 592 are controlling. In this case, both would have sent updated offers 593 around the same time. 595 However, the signaling protocol carrying the offer/answer exchanges 596 will have resolved this glare condition, so that one agent is always 597 the 'winner' by having its offer received before its peer has sent an 598 offer. The winner takes the role of controlling, so that the loser 599 (the answerer under consideration in this section) MUST change its 600 role to controlled. 602 Consequently, if the agent was going to send an updated offer since, 603 based on the rules in section 8.2 of [ICE-BIS], it was controlling, 604 it no longer needs to. 606 Besides the potential role change, change in the Valid list, and 607 state changes, the construction of the answer is performed 608 identically to the construction of an offer. 610 3.4.3. Receiving Answer for a Subsequent Offer 612 3.4.3.1. Procedures for Full Implementations 614 There may be certain situations where the offerer receives an SDP 615 answer that lacks ICE candidates although the initial answer did. 616 One example of such an "unexpected" answer might be happen when an 617 ICE-unaware B2BUA introduces a media server during call hold using 618 3rd party call-control procedures. Omitting further details how this 619 is done, this could result in an answer being received at the holding 620 UA that was constructed by the B2BUA. With the B2BUA being ICE- 621 unaware, that answer would not include ICE candidates. 623 Receiving an answer without ICE attributes in this situation might be 624 unexpected, but would not necessarily impair the user experience. 626 When the offerer receives an answer indicating support for ICE, the 627 offer performs on of the following actions: 629 o If the offer was a restart, the agent MUST perform ICE restart 630 procedures as specified in Section 3.4.3.1.1 632 o If the offer/answer exchange removed a data stream, or an answer 633 rejected an offered data stream, an agent MUST flush the Valid 634 list for that data stream. It MUST also terminate any STUN 635 transactions in progress for that data stream. 637 o If the offer/answer exchange added a new data stream, the agent 638 MUST create a new check list for it (and an empty Valid list to 639 start of course) which in turn triggers the candidate processing 640 procedures [ICE-BIS]. 642 o If ICE state is running for a given data stream, the agent 643 recomputes the check list. If a pair on the new check list was 644 also on the previous check list, and its state was Waiting, In- 645 Progress, Succeeded, or Failed, its state is copied over. 646 Otherwise, its state is set to Frozen. If none of the check lists 647 are active (meaning that the pairs in each check list are Frozen), 648 appropriate procedures in [ICE-BIS] are performed to move 649 candidate(s) to the Waiting state to further continue ICE 650 processing. 652 o If ICE state is completed and the SDP answer conforms to 653 Section 3.4.2, the agent MUST reman in the ICE completed state. 655 However, if the ICE support is no longer indicated in the SDP answer, 656 the agent MUST fall-back to [RFC3264] procedures and SHOULD NOT drop 657 the dialog because of the missing ICE support or unexpected answer. 658 Once the agent sends a new offer later on, it MUST perform an ICE 659 restart. 661 3.4.3.1.1. ICE Restarts 663 The agent MUST remember the nominated pair in the Valid list for each 664 component of the data stream, called the previous selected pair prior 665 to the restart. The agent will continue to send media using this 666 pair, as described in section 12 of [ICE-BIS]. Once these 667 destinations are noted, the agent MUST flush the valid and check 668 lists, and then recompute the check list and its states, thus 669 triggering the candidate processing procedures [ICE-BIS] 671 3.4.3.2. Procedures for Lite Implementations 673 If ICE is restarting for a data stream, the agent MUST start a new 674 Valid list for that data stream. It MUST remember the nominated pair 675 in the previous Valid list for each component of the data stream, 676 called the previous selected pairs, and continue to send media there 677 as described in section 12 of [ICE-BIS]. The state of ICE processing 678 for each data stream MUST change to Running, and the state of ICE 679 processing MUST change to Running 681 4. Grammar 683 This specification defines eight new SDP attributes -- the 684 "candidate", "remote-candidates", "ice-lite", "ice-mismatch", "ice- 685 ufrag", "ice-pwd", "ice-pacing", and "ice-options" attributes. 687 This section also provides non-normative examples of the attributes 688 defined. 690 The syntax for the attributes follow Augmented BNF as defined in 691 [RFC5234]. 693 4.1. "candidate" Attribute 695 The candidate attribute is a media-level attribute only. It contains 696 a transport address for a candidate that can be used for connectivity 697 checks. 699 candidate-attribute = "candidate" ":" foundation SP component-id SP 700 transport SP 701 priority SP 702 connection-address SP ;from RFC 4566 703 port ;port from RFC 4566 704 SP cand-type 705 [SP rel-addr] 706 [SP rel-port] 707 *(SP extension-att-name SP 708 extension-att-value) 710 foundation = 1*32ice-char 711 component-id = 1*5DIGIT 712 transport = "UDP" / transport-extension 713 transport-extension = token ; from RFC 3261 714 priority = 1*10DIGIT 715 cand-type = "typ" SP candidate-types 716 candidate-types = "host" / "srflx" / "prflx" / "relay" / token 717 rel-addr = "raddr" SP connection-address 718 rel-port = "rport" SP port 719 extension-att-name = token 720 extension-att-value = *VCHAR 721 ice-char = ALPHA / DIGIT / "+" / "/" 723 This grammar encodes the primary information about a candidate: its 724 IP address, port and transport protocol, and its properties: the 725 foundation, component ID, priority, type, and related transport 726 address: 728 : is taken from RFC 4566 [RFC4566]. It is the 729 IP address of the candidate. When parsing this field, an agent 730 can differentiate an IPv4 address and an IPv6 address by presence 731 of a colon in its value -- the presence of a colon indicates IPv6. 732 An agent MUST ignore candidate lines that include candidates with 733 IP address versions that are not supported or recognized. An IP 734 address SHOULD be used, but an FQDN MAY be used in place of an IP 735 address. In that case, when receiving an offer or answer 736 containing an FQDN in an a=candidate attribute, the FQDN is looked 737 up in the DNS first using an AAAA record (assuming the agent 738 supports IPv6), and if no result is found or the agent only 739 supports IPv4, using an A record. The rules from section 6 of 740 [RFC6724] is followed by fixing the source address to be one from 741 the candidate pair to be matched against destination addresses 742 reported by FQDN, in cases where the DNS query returns more than 743 one IP address. 745 : is also taken from RFC 4566 [RFC4566]. It is the port of 746 the candidate. 748 : indicates the transport protocol for the candidate. 749 This specification only defines UDP. However, extensibility is 750 provided to allow for future transport protocols to be used with 751 ICE by extending the sub-registry "ICE Transport Protocols" under 752 "Interactive Connectivity Establishment (ICE)" registry. 754 : is composed of 1 to 32 s. It is an 755 identifier that is equivalent for two candidates that are of the 756 same type, share the same base, and come from the same STUN 757 server. The foundation is used to optimize ICE performance in the 758 Frozen algorithm as described in [ICE-BIS] 760 : is a positive integer between 1 and 256 (inclusive) 761 that identifies the specific component of the dta stream for which 762 this is a candidate. It MUST start at 1 and MUST increment by 1 763 for each component of a particular candidate. For data streams 764 based on RTP, candidates for the actual RTP media MUST have a 765 component ID of 1, and candidates for RTCP MUST have a component 766 ID of 2. See section 13 in [ICE-BIS] for additional discussion on 767 extending ICE to new data streams. 769 : is a positive integer between 1 and (2**31 - 1) 770 inclusive. The procedures for computing candidate's priority is 771 described in section 5.1.2 of [ICE-BIS]. 773 : encodes the type of candidate. This specification 774 defines the values "host", "srflx", "prflx", and "relay" for host, 775 server reflexive, peer reflexive, and relayed candidates, 776 respectively. Specifications for new candidate types MUST define 777 how, if at all, various steps in the ICE processing differ from 778 the ones defined by this specification. 780 and : convey transport addresses related to the 781 candidate, useful for diagnostics and other purposes. 782 and MUST be present for server reflexive, peer 783 reflexive, and relayed candidates. If a candidate is server or 784 peer reflexive, and are equal to the base 785 for that server or peer reflexive candidate. If the candidate is 786 relayed, and are equal to the mapped address 787 in the Allocate response that provided the client with that 788 relayed candidate (see Appendix B.3 of [ICE-BIS] for a discussion 789 of its purpose). If the candidate is a host candidate, 790 and MUST be omitted. 792 In some cases, e.g., for privacy reasons, an agent may not want to 793 reveal the related address and port. In this case the address 794 MUST be set to "0.0.0.0" (for IPv4 candidates) or "::" (for IPv6 795 candidates) and the port to zero. 797 The candidate attribute can itself be extended. The grammar allows 798 for new name/value pairs to be added at the end of the attribute. 799 Such extensions MUST be made through IETF Review or IESG Approval 800 [RFC5226] and the assignments MUST contain the specific extension and 801 a reference to the document defining the usage of the extension 803 An implementation MUST ignore any name/value pairs it doesn't 804 understand. 806 Example: SDP line for UDP server reflexive candidate attribute for the RTP component 808 a=candidate:2 1 UDP 1694498815 192.0.2.3 45664 typ srflx raddr 10.0.1.1 rport 8998 810 4.2. "remote-candidates" Attribute 812 The syntax of the "remote-candidates" attribute is defined using 813 Augmented BNF as defined in [RFC5234]. The remote-candidates 814 attribute is a media-level attribute only. 816 remote-candidate-att = "remote-candidates:" remote-candidate 817 0*(SP remote-candidate) 818 remote-candidate = component-ID SP connection-address SP port 820 The attribute contains a connection-address and port for each 821 component. The ordering of components is irrelevant. However, a 822 value MUST be present for each component of a data stream. This 823 attribute MUST be included in an offer by a controlling agent for a 824 data stream that is Completed, and MUST NOT be included in any other 825 case. 827 Example: Remote candidates SDP lines for the RTP and RTCP components: 829 a=remote-candidates:1 192.0.2.3 45664 830 a=remote-candidates:2 192.0.2.3 45665 832 4.3. "ice-lite" and "ice-mismatch" Attributes 834 The syntax of the "ice-lite" and "ice-mismatch" attributes, both of 835 which are flags, is: 837 ice-lite = "ice-lite" 838 ice-mismatch = "ice-mismatch" 840 "ice-lite" is a session-level attribute only, and indicates that an 841 agent is a lite implementation. "ice-mismatch" is a media-level 842 attribute only, and when present in an answer, indicates that the 843 offer arrived with a default destination for a media component that 844 didn't have a corresponding candidate attribute. 846 4.4. "ice-ufrag" and "ice-pwd" Attributes 848 The "ice-ufrag" and "ice-pwd" attributes convey the username fragment 849 and password used by ICE for message integrity. Their syntax is: 851 ice-pwd-att = "ice-pwd:" password 852 ice-ufrag-att = "ice-ufrag:" ufrag 853 password = 22*256ice-char 854 ufrag = 4*256ice-char 856 The "ice-pwd" and "ice-ufrag" attributes can appear at either the 857 session-level or media-level. When present in both, the value in the 858 media-level takes precedence. Thus, the value at the session-level 859 is effectively a default that applies to all data streams, unless 860 overridden by a media-level value. Whether present at the session or 861 media-level, there MUST be an ice-pwd and ice-ufrag attribute for 862 each data stream. If two data streams have identical ice-ufrag's, 863 they MUST have identical ice-pwd's. 865 The ice-ufrag and ice-pwd attributes MUST be chosen randomly at the 866 beginning of a session (the same applies when ICE is restarting for 867 an agent). 869 The ice-ufrag attribute MUST contain at least 24 bits of randomness, 870 and the ice-pwd attribute MUST contain at least 128 bits of 871 randomness. This means that the ice-ufrag attribute will be at least 872 4 characters long, and the ice-pwd at least 22 characters long, since 873 the grammar for these attributes allows for 6 bits of information per 874 character. The attributes MAY be longer than 4 and 22 characters, 875 respectively, of course, up to 256 characters. The upper limit 876 allows for buffer sizing in implementations. Its large upper limit 877 allows for increased amounts of randomness to be added over time. 878 For compatibility with the 512 character limitation for the STUN 879 username attribute value and for bandwidth conservation 880 considerations, the ice-ufrag attribute MUST NOT be longer than 32 881 characters when sending, but an implementation MUST accept up to 256 882 characters when receiving. 884 Example shows sample ice-ufrag and ice-pwd SDP lines: 886 a=ice-pwd:asd88fgpdd777uzjYhagZg 887 a=ice-ufrag:8hhY 889 4.5. "ice-pacing" Attribute 891 The "ice-pacing" is a session level attribute that indicates the 892 desired connectivity check pacing, in milliseconds, for this agent 893 (see section 14 of [ICE-BIS]). The syntax is: 895 ice-pacing-att = "ice-pacing:" pacing-value 896 pacing-value = 1*10DIGIT 898 Following the procedures defined in [ICE-BIS], a default value of 899 50ms is used for an agent when ice-pacing attribute is omitted in the 900 offer or the answer. 902 The same rule applies for ice-pacing attribute values lower than 903 50ms. This mandates that, if an agent includes the ice-pacing 904 attribute, its value MUST be greater than 50ms or else a value of 905 50ms is considered by default for that agent. 907 Also the larger of the ice-pacing attribute values between the offer 908 and the answer (determined either by the one provided in the ice- 909 pacing attribute or by picking the default value) MUST be considered 910 for a given ICE session. 912 Example shows ice-pacing value of 5 ms: 914 a=ice-pacing:5 916 4.6. "ice-options" Attribute 918 The "ice-options" attribute is a session- and media-level attribute. 919 It contains a series of tokens that identify the options supported by 920 the agent. Its grammar is: 922 ice-options = "ice-options:" ice-option-tag 923 0*(SP ice-option-tag) 924 ice-option-tag = 1*ice-char 926 The existence of an ice-option in an offer indicates that a certain 927 extension is supported by the agent and is willing to use it, if the 928 peer agent also includes the same extension in the answer. There 929 might be further extension specific negotiation needed between the 930 agents that determine how the extensions gets used in a given 931 session. The details of the negotiation procedures, if present, MUST 932 be defined by the specification defining the extension (see 933 Section 10.2). 935 Example shows 'rtp+ecn' ice-option SDP line from <>: 937 a=ice-options:rtp+ecn 939 5. Keepalives 941 All the ICE agents MUST follow the procedures defined in section 11 942 of [ICE-BIS] for sending keepalives. The keepalives MUST be sent 943 regardless of whether the data stream is currently inactive, 944 sendonly, recvonly, or sendrecv, and regardless of the presence or 945 value of the bandwidth attribute. An agent can determine that its 946 peer supports ICE by the presence of a=candidate attributes for each 947 media session. 949 6. SIP Considerations 951 Note that ICE is not intended for NAT traversal for SIP, which is 952 assumed to be provided via another mechanism [RFC5626]. 954 When ICE is used with SIP, forking may result in a single offer 955 generating a multiplicity of answers. In that case, ICE proceeds 956 completely in parallel and independently for each answer, treating 957 the combination of its offer and each answer as an independent offer/ 958 answer exchange, with its own set of local candidates, pairs, check 959 lists, states, and so on. 961 Once ICE processing has reached the Completed state for all peers for 962 media streams using those candidates, the agent SHOULD wait an 963 additional three seconds, and then it MAY cease responding to checks 964 or generating triggered checks on that candidate. It MAY free the 965 candidate at that time. Freeing of server reflexive candidates is 966 never explicit; it happens by lack of a keepalive. The three-second 967 delay handles cases when aggressive nomination is used, and the 968 selected pairs can quickly change after ICE has completed. 970 6.1. Latency Guidelines 972 ICE requires a series of STUN-based connectivity checks to take place 973 between endpoints. These checks start from the answerer on 974 generation of its answer, and start from the offerer when it receives 975 the answer. These checks can take time to complete, and as such, the 976 selection of messages to use with offers and answers can affect 977 perceived user latency. Two latency figures are of particular 978 interest. These are the post-pickup delay and the post-dial delay. 979 The post-pickup delay refers to the time between when a user "answers 980 the phone" and when any speech they utter can be delivered to the 981 caller. The post-dial delay refers to the time between when a user 982 enters the destination address for the user and ringback begins as a 983 consequence of having successfully started alerting the called user 984 agent. 986 Two cases can be considered -- one where the offer is present in the 987 initial INVITE and one where it is in a response. 989 6.1.1. Offer in INVITE 991 To reduce post-dial delays, it is RECOMMENDED that the caller begin 992 gathering candidates prior to actually sending its initial INVITE. 993 This can be started upon user interface cues that a call is pending, 994 such as activity on a keypad or the phone going off-hook. 996 On the receipt of the offer, the answerer SHOULD generate an answer 997 in a provisional response once it has completed candidate gathering. 998 ICE requires that a provisional response with an SDP be transmitted 999 reliably. This can be done through the existing Provisional Response 1000 Acknowledgment (PRACK) mechanism [RFC3262] or through an ICE specific 1001 optimization, wherein, the agent retransmits the provisional response 1002 with the exponential backoff timers described in [RFC3262]. Such 1003 retransmissions MUST cease on receipt of a STUN Binding request for 1004 one of the data streams signaled in that SDP or on transmission of 1005 the answer in a 2xx response. If no Binding request is received 1006 prior to the last retransmit, the agent does not consider the session 1007 terminated. For the ICE lite peers, the agent MUST cease 1008 retransmitting the 18x after sending it four times (ICE will actually 1009 work even if the peer never receives the 18x; however, experience has 1010 shown that sending it is important for middleboxes and firewall 1011 traversal). 1013 It should be noted that the ICE specific optimization is very 1014 specific to provisional response carrying answers that start ICE 1015 processing and it is not a general technique for 1xx reliability. 1016 Also such an optimization SHOULD NOT be used if both agents support 1017 PRACK. 1019 Despite the fact that the provisional response will be delivered 1020 reliably, the rules for when an agent can send an updated offer or 1021 answer do not change from those specified in [RFC3262]. 1022 Specifically, if the INVITE contained an offer, the same answer 1023 appears in all of the 1xx and in the 2xx response to the INVITE. 1024 Only after that 2xx has been sent can an updated offer/answer 1025 exchange occur. 1027 Alternatively, an agent MAY delay sending an answer until the 200 OK; 1028 however, this results in a poor user experience and is NOT 1029 RECOMMENDED. 1031 Once the answer has been sent, the agent SHOULD begin its 1032 connectivity checks. Once candidate pairs for each component of a 1033 data stream enter the valid list, the answerer can begin sending 1034 media on that data stream. 1036 However, prior to this point, any media that needs to be sent towards 1037 the caller (such as SIP early media [RFC3960]) MUST NOT be 1038 transmitted. For this reason, implementations SHOULD delay alerting 1039 the called party until candidates for each component of each data 1040 stream have entered the valid list. In the case of a PSTN gateway, 1041 this would mean that the setup message into the PSTN is delayed until 1042 this point. Doing this increases the post-dial delay, but has the 1043 effect of eliminating 'ghost rings'. Ghost rings are cases where the 1044 called party hears the phone ring, picks up, but hears nothing and 1045 cannot be heard. This technique works without requiring support for, 1046 or usage of, preconditions [RFC3312]. It also has the benefit of 1047 guaranteeing that not a single packet of media will get clipped, so 1048 that post-pickup delay is zero. If an agent chooses to delay local 1049 alerting in this way, it SHOULD generate a 180 response once alerting 1050 begins. 1052 6.1.2. Offer in Response 1054 In addition to uses where the offer is in an INVITE, and the answer 1055 is in the provisional and/or 200 OK response, ICE works with cases 1056 where the offer appears in the response. In such cases, which are 1057 common in third party call control [RFC3725], ICE agents SHOULD 1058 generate their offers in a reliable provisional response (which MUST 1059 utilize [RFC3262]), and not alert the user on receipt of the INVITE. 1060 The answer will arrive in a PRACK. This allows for ICE processing to 1061 take place prior to alerting, so that there is no post-pickup delay, 1062 at the expense of increased call setup delays. Once ICE completes, 1063 the callee can alert the user and then generate a 200 OK when they 1064 answer. The 200 OK would contain no SDP, since the offer/answer 1065 exchange has completed. 1067 Alternatively, agents MAY place the offer in a 2xx instead (in which 1068 case the answer comes in the ACK). When this happens, the callee 1069 will alert the user on receipt of the INVITE, and the ICE exchanges 1070 will take place only after the user answers. This has the effect of 1071 reducing call setup delay, but can cause substantial post-pickup 1072 delays and media clipping. 1074 6.2. SIP Option Tags and Media Feature Tags 1076 [RFC5768] specifies a SIP option tag and media feature tag for usage 1077 with ICE. ICE implementations using SIP SHOULD support this 1078 specification, which uses a feature tag in registrations to 1079 facilitate interoperability through signaling intermediaries. 1081 6.3. Interactions with Forking 1083 ICE interacts very well with forking. Indeed, ICE fixes some of the 1084 problems associated with forking. Without ICE, when a call forks and 1085 the caller receives multiple incoming data streams, it cannot 1086 determine which data stream corresponds to which callee. 1088 With ICE, this problem is resolved. The connectivity checks which 1089 occur prior to transmission of media carry username fragments, which 1090 in turn are correlated to a specific callee. Subsequent media 1091 packets that arrive on the same candidate pair as the connectivity 1092 check will be associated with that same callee. Thus, the caller can 1093 perform this correlation as long as it has received an answer. 1095 6.4. Interactions with Preconditions 1097 Quality of Service (QoS) preconditions, which are defined in 1098 [RFC3312] and [RFC4032], apply only to the transport addresses listed 1099 as the default targets for media in an offer/answer. If ICE changes 1100 the transport address where media is received, this change is 1101 reflected in an updated offer that changes the default destination 1102 for media to match ICE's selection. As such, it appears like any 1103 other re-INVITE would, and is fully treated in RFCs 3312 and 4032, 1104 which apply without regard to the fact that the destination for media 1105 is changing due to ICE negotiations occurring "in the background". 1107 Indeed, an agent SHOULD NOT indicate that QoS preconditions have been 1108 met until the checks have completed and selected the candidate pairs 1109 to be used for media. 1111 ICE also has (purposeful) interactions with connectivity 1112 preconditions [RFC5898]. Those interactions are described there. 1113 Note that the procedures described in Section 6.1 describe their own 1114 type of "preconditions", albeit with less functionality than those 1115 provided by the explicit preconditions in [RFC5898]. 1117 6.5. Interactions with Third Party Call Control 1119 ICE works with Flows I, III, and IV as described in [RFC3725]. Flow 1120 I works without the controller supporting or being aware of ICE. 1121 Flow IV will work as long as the controller passes along the ICE 1122 attributes without alteration. Flow II is fundamentally incompatible 1123 with ICE; each agent will believe itself to be the answerer and thus 1124 never generate a re-INVITE. 1126 The flows for continued operation, as described in Section 7 of 1127 [RFC3725], require additional behavior of ICE implementations to 1128 support. In particular, if an agent receives a mid-dialog re-INVITE 1129 that contains no offer, it MUST restart ICE for each data stream and 1130 go through the process of gathering new candidates. Furthermore, 1131 that list of candidates SHOULD include the ones currently being used 1132 for media. 1134 7. Relationship with ANAT 1136 [RFC4091], the Alternative Network Address Types (ANAT) Semantics for 1137 the SDP grouping framework, and [RFC4092], its usage with SIP, define 1138 a mechanism for indicating that an agent can support both IPv4 and 1139 IPv6 for a data stream, and it does so by including two "m=" lines, 1140 one for v4 and one for v6. This is similar to ICE, which allows for 1141 an agent to indicate multiple transport addresses using the candidate 1142 attribute. However, ANAT relies on static selection to pick between 1143 choices, rather than a dynamic connectivity check used by ICE. 1145 It is RECOMMENDED that ICE be used in realizing the dual-stack use- 1146 cases in agents that support ICE. 1148 8. Setting Ta and RTO for RTP Media Streams 1150 During the gathering phase of ICE and while ICE is performing 1151 connectivity checks, an agent sends STUN and TURN transactions. 1152 These transactions are paced at a rate of one every Ta milliseconds, 1153 and utilize a specific RTO. See Section 14 of [ICE-BIS] for details 1154 on how the values of Ta and RTO are computed with a real-time media 1155 stream of known maximum bandwidth to rate-control the ICE exchanges. 1157 9. Security Considerations 1159 9.1. Attacks on the Offer/Answer Exchanges 1161 An attacker that can modify or disrupt the offer/answer exchanges 1162 themselves can readily launch a variety of attacks with ICE. They 1163 could direct media to a target of a DoS attack, they could insert 1164 themselves into the data stream, and so on. These are similar to the 1165 general security considerations for offer/answer exchanges, and the 1166 security considerations in [RFC3264] apply. These require techniques 1167 for message integrity and encryption for offers and answers, which 1168 are satisfied by the TLS mechanism [RFC3261] when SIP is used. As 1169 such, the usage of TLS with ICE is RECOMMENDED. 1171 9.2. Insider Attacks 1173 In addition to attacks where the attacker is a third party trying to 1174 insert fake offers, answers, or STUN messages, there are several 1175 attacks possible with ICE when the attacker is an authenticated and 1176 valid participant in the ICE exchange. 1178 9.2.1. The Voice Hammer Attack 1180 The voice hammer attack is an amplification attack. In this attack, 1181 the attacker initiates sessions to other agents, and maliciously 1182 includes the IP address and port of a DoS target as the destination 1183 for media traffic signaled in the SDP. This causes substantial 1184 amplification; a single offer/answer exchange can create a continuing 1185 flood of media packets, possibly at high rates (consider video 1186 sources). This attack is not specific to ICE, but ICE can help 1187 provide remediation. 1189 Specifically, if ICE is used, the agent receiving the malicious SDP 1190 will first perform connectivity checks to the target of media before 1191 sending media there. If this target is a third-party host, the 1192 checks will not succeed, and media is never sent. 1194 Unfortunately, ICE doesn't help if it's not used, in which case an 1195 attacker could simply send the offer without the ICE parameters. 1196 However, in environments where the set of clients is known, and is 1197 limited to ones that support ICE, the server can reject any offers or 1198 answers that don't indicate ICE support. 1200 SIP User Agents (UA) [RFC3261] that are not willing to receive non- 1201 ICE answers MUST include an "ice" Option Tag in the SIP Require 1202 Header Field in their offer. UAs that rejects non-ICE offers SHOULD 1203 use a 421 response code, together with an Option Tag "ice" in the 1204 Require Header Field in the response. 1206 9.2.2. Interactions with Application Layer Gateways and SIP 1208 Application Layer Gateways (ALGs) are functions present in a Network 1209 Address Translation (NAT) device that inspect the contents of packets 1210 and modify them, in order to facilitate NAT traversal for application 1211 protocols. Session Border Controllers (SBCs) are close cousins of 1212 ALGs, but are less transparent since they actually exist as 1213 application-layer SIP intermediaries. ICE has interactions with SBCs 1214 and ALGs. 1216 If an ALG is SIP aware but not ICE aware, ICE will work through it as 1217 long as the ALG correctly modifies the SDP. A correct ALG 1218 implementation behaves as follows: 1220 o The ALG does not modify the "m=" and "c=" lines or the rtcp 1221 attribute if they contain external addresses. 1223 o If the "m=" and "c=" lines contain internal addresses, the 1224 modification depends on the state of the ALG: 1226 * If the ALG already has a binding established that maps an 1227 external port to an internal IP address and port matching the 1228 values in the "m=" and "c=" lines or rtcp attribute, the ALG 1229 uses that binding instead of creating a new one. 1231 * If the ALG does not already have a binding, it creates a new 1232 one and modifies the SDP, rewriting the "m=" and "c=" lines and 1233 rtcp attribute. 1235 Unfortunately, many ALGs are known to work poorly in these corner 1236 cases. ICE does not try to work around broken ALGs, as this is 1237 outside the scope of its functionality. ICE can help diagnose these 1238 conditions, which often show up as a mismatch between the set of 1239 candidates and the "m=" and "c=" lines and rtcp attributes. The ice- 1240 mismatch attribute is used for this purpose. 1242 ICE works best through ALGs when the signaling is run over TLS. This 1243 prevents the ALG from manipulating the SDP messages and interfering 1244 with ICE operation. Implementations that are expected to be deployed 1245 behind ALGs SHOULD provide for TLS transport of the SDP. 1247 If an SBC is SIP aware but not ICE aware, the result depends on the 1248 behavior of the SBC. If it is acting as a proper Back-to-Back User 1249 Agent (B2BUA), the SBC will remove any SDP attributes it doesn't 1250 understand, including the ICE attributes. Consequently, the call 1251 will appear to both endpoints as if the other side doesn't support 1252 ICE. This will result in ICE being disabled, and media flowing 1253 through the SBC, if the SBC has requested it. If, however, the SBC 1254 passes the ICE attributes without modification, yet modifies the 1255 default destination for media (contained in the "m=" and "c=" lines 1256 and rtcp attribute), this will be detected as an ICE mismatch, and 1257 ICE processing is aborted for the call. It is outside of the scope 1258 of ICE for it to act as a tool for "working around" SBCs. If one is 1259 present, ICE will not be used and the SBC techniques take precedence. 1261 10. IANA Considerations 1263 10.1. SDP Attributes 1265 The original ICE specification defined seven new SDP attributes per 1266 the procedures of Section 8.2.4 of [RFC4566]. The registration 1267 information from the original specification is included here with 1268 modifications to include Mux Category and to align with the recent 1269 recommendations for populating Contact information. 1271 10.1.1. candidate Attribute 1273 Attribute Name: candidate 1275 Type of Attribute: media-level 1277 Subject to charset: No 1279 Purpose: This attribute is used with Interactive Connectivity 1280 Establishment (ICE), and provides one of many possible candidate 1281 addresses for communication. These addresses are validated with 1282 an end-to-end connectivity check using Session Traversal Utilities 1283 for NAT (STUN). 1285 Appropriate Values: See Section 4 of RFC XXXX. 1287 Contact Name: IESG 1289 Contact e-mail: iesg@ietf.org [1] 1291 Reference: RFCXXXX 1293 Mux Category: TRANSPORT 1295 10.1.2. remote-candidates Attribute 1297 Attribute Name: remote-candidates 1299 Type of Attribute: media-level 1301 Subject to charset: No 1303 Purpose: This attribute is used with Interactive Connectivity 1304 Establishment (ICE), and provides the identity of the remote 1305 candidates that the offerer wishes the answerer to use in its 1306 answer. 1308 Appropriate Values: See Section 4 of RFC XXXX. 1310 Contact Name: IESG 1312 Contact e-mail: iesg@ietf.org [2] 1314 Reference: RFCXXXX 1316 Mux Category: TRANSPORT 1318 10.1.3. ice-lite Attribute 1320 Attribute Name: ice-lite 1322 Type of Attribute: session-level 1324 Subject to charset: No 1326 Purpose: This attribute is used with Interactive Connectivity 1327 Establishment (ICE), and indicates that an agent has the minimum 1328 functionality required to support ICE inter-operation with a peer 1329 that has a full implementation. 1331 Appropriate Values: See Section 4 of RFC XXXX. 1333 Contact Name: IESG 1335 Contact e-mail: iesg@ietf.org [3] 1337 Reference: RFCXXXX 1339 Mux Category: NORMAL 1341 10.1.4. ice-mismatch Attribute 1343 Attribute Name: ice-mismatch 1345 Type of Attribute: media-level 1347 Subject to charset: No 1349 Purpose: This attribute is used with Interactive Connectivity 1350 Establishment (ICE), and indicates that an agent is ICE capable, 1351 but did not proceed with ICE due to a mismatch of candidates with 1352 the default destination for media signaled in the SDP. 1354 Appropriate Values: See Section 4 of RFC XXXX. 1356 Contact Name: IESG 1358 Contact e-mail: iesg@ietf.org [4] 1360 Reference: RFCXXXX 1362 Mux Category: NORMAL 1364 10.1.5. ice-pwd Attribute 1366 Attribute Name: ice-pwd 1368 Type of Attribute: session- or media-level 1370 Subject to charset: No 1372 Purpose: This attribute is used with Interactive Connectivity 1373 Establishment (ICE), and provides the password used to protect 1374 STUN connectivity checks. 1376 Appropriate Values: See Section 4 of RFC XXXX. 1378 Contact Name: IESG 1380 Contact e-mail: iesg@ietf.org [5] 1382 Reference: RFCXXXX 1384 Mux Category: TRANSPORT 1386 10.1.6. ice-ufrag Attribute 1388 Attribute Name: ice-ufrag 1390 Type of Attribute: session- or media-level 1392 Subject to charset: No 1394 Purpose: This attribute is used with Interactive Connectivity 1395 Establishment (ICE), and provides the fragments used to construct 1396 the username in STUN connectivity checks. 1398 Appropriate Values: See Section 4 of RFC XXXX. 1400 Contact Name: IESG 1402 Contact e-mail: iesg@ietf.org [6] 1404 Reference: RFCXXXX 1406 Mux Category: TRANSPORT 1408 10.1.7. ice-options Attribute 1410 Attribute Name: ice-options 1412 Long Form: ice-options 1414 Type of Attribute: session- or media-level 1416 Subject to charset: No 1418 Purpose: This attribute is used with Interactive Connectivity 1419 Establishment (ICE), and indicates the ICE options or extensions 1420 used by the agent. 1422 Appropriate Values: See Section 4 of RFC XXXX. 1424 Contact Name: IESG 1426 Contact e-mail: iesg@ietf.org [7] 1428 Reference: RFCXXXX 1430 Mux Category: NORMAL 1432 10.1.8. ice-pacing Attribute 1434 This specification also defines a new SDP attribute, "ice-pacing" 1435 according to the following data: 1437 Attribute Name: ice-pacing 1439 Type of Attribute: session-level 1441 Subject to charset: No 1443 Purpose: This attribute is used with Interactive Connectivity 1444 Establishment (ICE) to indicate desired connectivity check pacing 1445 values. 1447 Appropriate Values: See Section 4 of RFC XXXX. 1449 Contact Name: IESG 1451 Contact e-mail: iesg@ietf.org [8] 1453 Reference: RFCXXXX 1455 Mux Category: TRANSPORT 1457 10.2. Interactive Connectivity Establishment (ICE) Options Registry 1459 IANA maintains a registry for ice-options identifiers under the 1460 Specification Required policy as defined in "Guidelines for Writing 1461 an IANA Considerations Section in RFCs" [RFC5226]. 1463 ICE options are of unlimited length according to the syntax in 1464 Section 4.6; however, they are RECOMMENDED to be no longer than 20 1465 characters. This is to reduce message sizes and allow for efficient 1466 parsing. 1468 In [RFC5245] ICE options could only be defined at the session level. 1469 ICE options can now also be defined at the media level. This can be 1470 used when aggregating between different ICE agents in the same 1471 endpoint, but future options may require to be defined at the media- 1472 level. To ensure compatibility with legacy implementation, the 1473 media-level ICE options MUST be aggregated into a session-level ICE 1474 option. Because aggregation rules depend on the specifics of each 1475 option, all new ICE options MUST also define in their specification 1476 how the media-level ICE option values are aggregated to generate the 1477 value of the session-level ICE option. 1479 [RFC6679] defines the "rtp+ecn" ICE option. The aggregation rule for 1480 this ICE option is that if all aggregated media using ICE contain a 1481 media-level "rtp+ecn" ICE option then an "rtp+ecn" ICE option MUST be 1482 inserted at the session-level. If one of the media does not contain 1483 the option, then it MUST NOT be inserted at the session-level. 1485 Section 10 of [ICE-BIS] defines "ice2" ICE option. Since "ice2" is a 1486 session level ICE option, no aggregation rules apply. 1488 A registration request MUST include the following information: 1490 o The ICE option identifier to be registered 1492 o Name, Email, and Address of a contact person for the registration 1494 o Organization or individuals having the change control 1496 o Short description of the ICE extension to which the option relates 1498 o Reference(s) to the specification defining the ICE option and the 1499 related extensions 1501 11. Acknowledgments 1503 A large part of the text in this document was taken from [RFC5245], 1504 authored by Jonathan Rosenberg. 1506 Some of the text in this document was taken from [RFC6336], authored 1507 by Magnus Westerlund and Colin Perkins. 1509 Many thanks to Christer Holmberg for providing text suggestions in 1510 Section 4 that aligns with [ICE-BIS] 1512 Thanks to Thomas Stach for text help, Roman Shpount for suggesting 1513 RTCP candidate handling and Simon Perreault for advising on IPV6 1514 address selection when candidate-address includes FQDN. 1516 Many thanks to Flemming Andreasen for shepherd review feedback. 1518 Thanks to following experts for their reviews and constructive 1519 feedback: Christer Holmberg, Adam Roach and the MMUSIC WG. 1521 12. References 1523 12.1. Normative References 1525 [ICE-BIS] Keranen, A. and J. Rosenberg, "Interactive Connectivity 1526 Establishment (ICE): A Protocol for Network Address 1527 Translator (NAT) Traversal for Offer/Answer Protocols", 1528 draft-ietf-ice-rfc5245bis-00 (work in progress), March 1529 2015. 1531 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 1532 Requirement Levels", BCP 14, RFC 2119, 1533 DOI 10.17487/RFC2119, March 1997, 1534 . 1536 [RFC3261] Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston, 1537 A., Peterson, J., Sparks, R., Handley, M., and E. 1538 Schooler, "SIP: Session Initiation Protocol", RFC 3261, 1539 DOI 10.17487/RFC3261, June 2002, 1540 . 1542 [RFC3262] Rosenberg, J. and H. Schulzrinne, "Reliability of 1543 Provisional Responses in Session Initiation Protocol 1544 (SIP)", RFC 3262, DOI 10.17487/RFC3262, June 2002, 1545 . 1547 [RFC3264] Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model 1548 with Session Description Protocol (SDP)", RFC 3264, 1549 DOI 10.17487/RFC3264, June 2002, 1550 . 1552 [RFC3312] Camarillo, G., Ed., Marshall, W., Ed., and J. Rosenberg, 1553 "Integration of Resource Management and Session Initiation 1554 Protocol (SIP)", RFC 3312, DOI 10.17487/RFC3312, October 1555 2002, . 1557 [RFC3556] Casner, S., "Session Description Protocol (SDP) Bandwidth 1558 Modifiers for RTP Control Protocol (RTCP) Bandwidth", 1559 RFC 3556, DOI 10.17487/RFC3556, July 2003, 1560 . 1562 [RFC3605] Huitema, C., "Real Time Control Protocol (RTCP) attribute 1563 in Session Description Protocol (SDP)", RFC 3605, 1564 DOI 10.17487/RFC3605, October 2003, 1565 . 1567 [RFC4032] Camarillo, G. and P. Kyzivat, "Update to the Session 1568 Initiation Protocol (SIP) Preconditions Framework", 1569 RFC 4032, DOI 10.17487/RFC4032, March 2005, 1570 . 1572 [RFC4091] Camarillo, G. and J. Rosenberg, "The Alternative Network 1573 Address Types (ANAT) Semantics for the Session Description 1574 Protocol (SDP) Grouping Framework", RFC 4091, June 2005, 1575 . 1577 [RFC4092] Camarillo, G. and J. Rosenberg, "Usage of the Session 1578 Description Protocol (SDP) Alternative Network Address 1579 Types (ANAT) Semantics in the Session Initiation Protocol 1580 (SIP)", RFC 4092, June 2005, 1581 . 1583 [RFC4566] Handley, M., Jacobson, V., and C. Perkins, "SDP: Session 1584 Description Protocol", RFC 4566, DOI 10.17487/RFC4566, 1585 July 2006, . 1587 [RFC5226] Narten, T. and H. Alvestrand, "Guidelines for Writing an 1588 IANA Considerations Section in RFCs", BCP 26, RFC 5226, 1589 DOI 10.17487/RFC5226, May 2008, 1590 . 1592 [RFC5234] Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax 1593 Specifications: ABNF", STD 68, RFC 5234, 1594 DOI 10.17487/RFC5234, January 2008, 1595 . 1597 [RFC5245] Rosenberg, J., "Interactive Connectivity Establishment 1598 (ICE): A Protocol for Network Address Translator (NAT) 1599 Traversal for Offer/Answer Protocols", RFC 5245, 1600 DOI 10.17487/RFC5245, April 2010, 1601 . 1603 [RFC5768] Rosenberg, J., "Indicating Support for Interactive 1604 Connectivity Establishment (ICE) in the Session Initiation 1605 Protocol (SIP)", RFC 5768, DOI 10.17487/RFC5768, April 1606 2010, . 1608 [RFC6336] Westerlund, M. and C. Perkins, "IANA Registry for 1609 Interactive Connectivity Establishment (ICE) Options", 1610 RFC 6336, April 2010, 1611 . 1613 [RFC6679] Westerlund, M., Johansson, I., Perkins, C., O'Hanlon, P., 1614 and K. Carlberg, "Explicit Congestion Notification (ECN) 1615 for RTP over UDP", RFC 6679, DOI 10.17487/RFC6679, August 1616 2012, . 1618 [RFC6724] Thaler, D., Draves, R., Matsumoto, A., and T. Chown, 1619 "Default Address Selection for Internet Protocol Version 6 1620 (IPv6)", RFC 6724, September 2012, 1621 . 1623 12.2. Informative References 1625 [RFC3725] Rosenberg, J., Peterson, J., Schulzrinne, H., and G. 1626 Camarillo, "Best Current Practices for Third Party Call 1627 Control (3pcc) in the Session Initiation Protocol (SIP)", 1628 BCP 85, RFC 3725, DOI 10.17487/RFC3725, April 2004, 1629 . 1631 [RFC3960] Camarillo, G. and H. Schulzrinne, "Early Media and Ringing 1632 Tone Generation in the Session Initiation Protocol (SIP)", 1633 RFC 3960, DOI 10.17487/RFC3960, December 2004, 1634 . 1636 [RFC5626] Jennings, C., Ed., Mahy, R., Ed., and F. Audet, Ed., 1637 "Managing Client-Initiated Connections in the Session 1638 Initiation Protocol (SIP)", RFC 5626, 1639 DOI 10.17487/RFC5626, October 2009, 1640 . 1642 [RFC5898] Andreasen, F., Camarillo, G., Oran, D., and D. Wing, 1643 "Connectivity Preconditions for Session Description 1644 Protocol (SDP) Media Streams", RFC 5898, 1645 DOI 10.17487/RFC5898, July 2010, 1646 . 1648 12.3. URIs 1650 [1] mailto:iesg@ietf.org 1652 [2] mailto:iesg@ietf.org 1654 [3] mailto:iesg@ietf.org 1656 [4] mailto:iesg@ietf.org 1658 [5] mailto:iesg@ietf.org 1660 [6] mailto:iesg@ietf.org 1662 [7] mailto:iesg@ietf.org 1664 [8] mailto:iesg@ietf.org 1666 [9] mailto:christer.holmberg@ericsson.com 1668 [10] mailto:rshpount@turbobridge.com 1670 [11] mailto:thomass.stach@gmail.com 1672 Appendix A. Examples 1674 For the example shown in section 16 of [ICE-BIS] the resulting offer 1675 (message 5) encoded in SDP looks like: 1677 v=0 1678 o=jdoe 2890844526 2890842807 IN IP6 $L-PRIV-1.IP 1679 s= 1680 c=IN IP6 $NAT-PUB-1.IP 1681 t=0 0 1682 a=ice-pwd:asd88fgpdd777uzjYhagZg 1683 a=ice-ufrag:8hhY 1684 m=audio $NAT-PUB-1.PORT RTP/AVP 0 1685 b=RS:0 1686 b=RR:0 1687 a=rtpmap:0 PCMU/8000 1688 a=candidate:1 1 UDP 2130706431 $L-PRIV-1.IP $L-PRIV-1.PORT typ host 1689 a=candidate:2 1 UDP 1694498815 $NAT-PUB-1.IP $NAT-PUB-1.PORT typ 1690 srflx raddr $L-PRIV-1.IP rport $L-PRIV-1.PORT 1692 The offer, with the variables replaced with their values, will look 1693 like (lines folded for clarity): 1695 v=0 1696 o=jdoe 2890844526 2890842807 IN IP6 fe80::6676:baff:fe9c:ee4a 1697 s= 1698 c=IN IP6 2001:420:c0e0:1005::61 1699 t=0 0 1700 a=ice-pwd:asd88fgpdd777uzjYhagZg 1701 a=ice-ufrag:8hhY 1702 m=audio 45664 RTP/AVP 0 1703 b=RS:0 1704 b=RR:0 1705 a=rtpmap:0 PCMU/8000 1706 a=candidate:1 1 UDP 2130706431 fe80::6676:baff:fe9c:ee4a 8998 typ host 1707 a=candidate:2 1 UDP 1694498815 2001:420:c0e0:1005::61 45664 typ srflx raddr 1708 fe80::6676:baff:fe9c:ee4a rport 8998 1710 The resulting answer looks like: 1712 v=0 1713 o=bob 2808844564 2808844564 IN IP4 $R-PUB-1.IP 1714 s= 1715 c=IN IP4 $R-PUB-1.IP 1716 t=0 0 1717 a=ice-pwd:YH75Fviy6338Vbrhrlp8Yh 1718 a=ice-ufrag:9uB6 1719 m=audio $R-PUB-1.PORT RTP/AVP 0 1720 b=RS:0 1721 b=RR:0 1722 a=rtpmap:0 PCMU/8000 1723 a=candidate:1 1 UDP 2130706431 $R-PUB-1.IP $R-PUB-1.PORT typ host 1724 With the variables filled in: 1726 v=0 1727 o=bob 2808844564 2808844564 IN IP4 192.0.2.1 1728 s= 1729 c=IN IP4 192.0.2.1 1730 t=0 0 1731 a=ice-pwd:YH75Fviy6338Vbrhrlp8Yh 1732 a=ice-ufrag:9uB6 1733 m=audio 3478 RTP/AVP 0 1734 b=RS:0 1735 b=RR:0 1736 a=rtpmap:0 PCMU/8000 1737 a=candidate:1 1 UDP 2130706431 192.0.2.1 3478 typ host 1739 Appendix B. The remote-candidates Attribute 1741 The a=remote-candidates attribute exists to eliminate a race 1742 condition between the updated offer and the response to the STUN 1743 Binding request that moved a candidate into the Valid list. This 1744 race condition is shown in Figure 1. On receipt of message 4, agent 1745 L adds a candidate pair to the valid list. If there was only a 1746 single data stream with a single component, agent L could now send an 1747 updated offer. However, the check from agent R has not yet generated 1748 a response, and agent R receives the updated offer (message 7) before 1749 getting the response (message 9). Thus, it does not yet know that 1750 this particular pair is valid. To eliminate this condition, the 1751 actual candidates at R that were selected by the offerer (the remote 1752 candidates) are included in the offer itself, and the answerer delays 1753 its answer until those pairs validate. 1755 Agent L Network Agent R 1756 |(1) Offer | | 1757 |------------------------------------------>| 1758 |(2) Answer | | 1759 |<------------------------------------------| 1760 |(3) STUN Req. | | 1761 |------------------------------------------>| 1762 |(4) STUN Res. | | 1763 |<------------------------------------------| 1764 |(5) STUN Req. | | 1765 |<------------------------------------------| 1766 |(6) STUN Res. | | 1767 |-------------------->| | 1768 | |Lost | 1769 |(7) Offer | | 1770 |------------------------------------------>| 1771 |(8) STUN Req. | | 1772 |<------------------------------------------| 1773 |(9) STUN Res. | | 1774 |------------------------------------------>| 1775 |(10) Answer | | 1776 |<------------------------------------------| 1778 Figure 1: Race Condition Flow 1780 Appendix C. Why Is the Conflict Resolution Mechanism Needed? 1782 When ICE runs between two peers, one agent acts as controlled, and 1783 the other as controlling. Rules are defined as a function of 1784 implementation type and offerer/answerer to determine who is 1785 controlling and who is controlled. However, the specification 1786 mentions that, in some cases, both sides might believe they are 1787 controlling, or both sides might believe they are controlled. How 1788 can this happen? 1790 The condition when both agents believe they are controlled shows up 1791 in third party call control cases. Consider the following flow: 1793 A Controller B 1794 |(1) INV() | | 1795 |<-------------| | 1796 |(2) 200(SDP1) | | 1797 |------------->| | 1798 | |(3) INV() | 1799 | |------------->| 1800 | |(4) 200(SDP2) | 1801 | |<-------------| 1802 |(5) ACK(SDP2) | | 1803 |<-------------| | 1804 | |(6) ACK(SDP1) | 1805 | |------------->| 1807 Figure 2: Role Conflict Flow 1809 This flow is a variation on flow III of RFC 3725 [RFC3725]. In fact, 1810 it works better than flow III since it produces fewer messages. In 1811 this flow, the controller sends an offerless INVITE to agent A, which 1812 responds with its offer, SDP1. The agent then sends an offerless 1813 INVITE to agent B, which it responds to with its offer, SDP2. The 1814 controller then uses the offer from each agent to generate the 1815 answers. When this flow is used, ICE will run between agents A and 1816 B, but both will believe they are in the controlling role. With the 1817 role conflict resolution procedures, this flow will function properly 1818 when ICE is used. 1820 At this time, there are no documented flows that can result in the 1821 case where both agents believe they are controlled. However, the 1822 conflict resolution procedures allow for this case, should a flow 1823 arise that would fit into this category. 1825 Appendix D. Why Send an Updated Offer? 1827 Section 11.1 describes rules for sending media. Both agents can send 1828 media once ICE checks complete, without waiting for an updated offer. 1829 Indeed, the only purpose of the updated offer is to "correct" the SDP 1830 so that the default destination for media matches where media is 1831 being sent based on ICE procedures (which will be the highest- 1832 priority nominated candidate pair). 1834 This begs the question -- why is the updated offer/answer exchange 1835 needed at all? Indeed, in a pure offer/answer environment, it would 1836 not be. The offerer and answerer will agree on the candidates to use 1837 through ICE, and then can begin using them. As far as the agents 1838 themselves are concerned, the updated offer/answer provides no new 1839 information. However, in practice, numerous components along the 1840 signaling path look at the SDP information. These include entities 1841 performing off-path QoS reservations, NAT traversal components such 1842 as ALGs and Session Border Controllers (SBCs), and diagnostic tools 1843 that passively monitor the network. For these tools to continue to 1844 function without change, the core property of SDP -- that the 1845 existing, pre-ICE definitions of the addresses used for media -- the 1846 "m=" and "c=" lines and the rtcp attribute -- must be retained. For 1847 this reason, an updated offer must be sent. 1849 Appendix E. Contributors 1851 Following experts have contributed a textual and structural 1852 suggestions for this work 1854 1. Christer Holmberg 1856 * Ericsson 1858 * Email: christer.holmberg@ericsson.com [9] 1860 2. Roman Shpount 1862 * TurboBridge 1864 * rshpount@turbobridge.com [10] 1866 3. Thomas Stach 1868 * thomass.stach@gmail.com [11] 1870 Authors' Addresses 1872 Marc Petit-Huguenin 1873 Impedance Mismatch 1875 Email: marc@petit-huguenin.org 1877 Suhas Nandakumar 1878 Cisco Systems 1879 707 Tasman Dr 1880 Milpitas, CA 95035 1881 USA 1883 Email: snandaku@cisco.com 1884 Ari Keranen 1885 Ericsson 1886 Jorvas 02420 1887 Finland 1889 Email: ari.keranen@ericsson.com