idnits 2.17.1 draft-ietf-mmusic-ice-sip-sdp-20.txt: Checking boilerplate required by RFC 5378 and the IETF Trust (see https://trustee.ietf.org/license-info): ---------------------------------------------------------------------------- No issues found here. Checking nits according to https://www.ietf.org/id-info/1id-guidelines.txt: ---------------------------------------------------------------------------- No issues found here. Checking nits according to https://www.ietf.org/id-info/checklist : ---------------------------------------------------------------------------- ** There are 3 instances of too long lines in the document, the longest one being 12 characters in excess of 72. == There are 3 instances of lines with private range IPv4 addresses in the document. 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 == 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 2215 days in the past. Is this intentional? Checking references for intended status: Proposed Standard ---------------------------------------------------------------------------- (See RFCs 3967 and 4897 for information about using normative references to lower-maturity documents in RFCs) -- Looks like a reference, but probably isn't: '1' on line 1287 -- Looks like a reference, but probably isn't: '2' on line 1310 -- Looks like a reference, but probably isn't: '3' on line 1333 -- Looks like a reference, but probably isn't: '4' on line 1356 -- Looks like a reference, but probably isn't: '5' on line 1378 -- Looks like a reference, but probably isn't: '6' on line 1400 -- Looks like a reference, but probably isn't: '7' on line 1424 -- Looks like a reference, but probably isn't: '8' on line 1449 -- Looks like a reference, but probably isn't: '9' on line 1832 -- Looks like a reference, but probably isn't: '10' on line 1838 -- Looks like a reference, but probably isn't: '11' on line 1842 == Outdated reference: A later version (-20) exists of draft-ietf-ice-rfc5245bis-00 ** Obsolete normative reference: RFC 4091 (Obsoleted by RFC 5245) ** Obsolete normative reference: RFC 4092 (Obsoleted by RFC 5245) ** Obsolete normative reference: RFC 4566 (Obsoleted by RFC 8866) ** Obsolete normative reference: RFC 5226 (Obsoleted by RFC 8126) ** Obsolete normative reference: RFC 5245 (Obsoleted by RFC 8445, RFC 8839) ** Obsolete normative reference: RFC 6336 (Obsoleted by RFC 8839) Summary: 7 errors (**), 0 flaws (~~), 5 warnings (==), 14 comments (--). 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-20 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. 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 . . . . . . . . . . . . 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 MAY be omitted. 252 If the agent does not utilize RTCP, it indicates that by including 253 b=RS:0 and b=RR:0 SDP attributes, as described in [RFC3556]. 255 3.2.3. Determining Role 257 The offerer acts as the Initiating agent. The answerer acts as the 258 Responding agent. The ICE roles (controlling and controlled) are 259 determined using the procedures in [ICE-BIS]. 261 3.2.4. STUN Considerations 263 Once an agent has provided its local candidates to its peer in an SDP 264 offer or answer, the agent MUST be prepared to receive STUN 265 connectivity check Binding requests on those candidates. 267 3.2.5. Verifying ICE Support Procedures 269 The agents will proceed with the ICE procedures defined in [ICE-BIS] 270 and this specification if, for each data stream in the SDP it 271 received, the default destination for each component of that data 272 stream appears in a candidate attribute. For example, in the case of 273 RTP, the IP address and port in the "c=" and "m=" lines, 274 respectively, appear in a candidate attribute and the value in the 275 rtcp attribute appears in a candidate attribute. 277 If this condition is not met, the agents MUST process the SDP based 278 on normal [RFC3264] procedures, without using any of the ICE 279 mechanisms described in the remainder of this specification with the 280 following exceptions: 282 1. A controlled/responding agent MUST follow the rules of section 11 283 of [ICE-BIS], which describe keepalive procedures for all agents. 284 Also, If the default candidates were relayed candidates learned 285 through a TURN server, the controlled agent MUST create 286 permissions in the TURN server for the IP addresses learned from 287 its peer in the offer SDP it just received. If this is not done, 288 initial packets in the data stream from the peer may be lost. 290 2. On the other hand, If the controlled agent is not proceeding with 291 ICE because there were a=candidate attributes, but none that 292 matched the default destination of the data stream, the agent 293 MUST include an a=ice-mismatch attribute in its answer and MAY 294 omit a=candidate attributes for such data streams. See 295 Section 9.2.2 for a discussion of cases where this can happen. 296 This specification provides no guidance on how an controlling/ 297 initiator agent should proceed in such a failure case. 299 3.2.6. SDP Example 301 The following is an example SDP message that includes ICE attributes 302 (lines folded for readability): 304 v=0 305 o=jdoe 2890844526 2890842807 IN IP4 10.0.1.1 306 s= 307 c=IN IP4 192.0.2.3 308 t=0 0 309 a=ice-options:ice2 310 a=ice-pwd:asd88fgpdd777uzjYhagZg 311 a=ice-ufrag:8hhY 312 m=audio 45664 RTP/AVP 0 313 b=RS:0 314 b=RR:0 315 a=rtpmap:0 PCMU/8000 316 a=candidate:1 1 UDP 2130706431 10.0.1.1 8998 typ host 317 a=candidate:2 1 UDP 1694498815 192.0.2.3 45664 typ srflx raddr 318 10.0.1.1 rport 8998 320 3.3. Initial Offer/Answer Exchange 322 3.3.1. Sending the Initial Offer 324 When an offerer generates the initial offer, in each "m=" section it 325 MUST include SDP candidate attributes for each available candidate 326 associated with the "m=" section. In addition, the offerer MUST 327 include an SDP ice-ufrag and an SDP ice-pwd attribute in the offer. 329 Note: Within the scope of this document, "Initial Offer" refers to 330 the first SDP offer that is sent in order to negotiate usage of 331 ICE. It might, or might not, be the initial SDP offer of the SDP 332 session. 334 Note: The procedures in this document only consider "m=" sections 335 associated with data streams where ICE is used. 337 3.3.2. Sending the Initial Answer 339 When an answerer receives an initial offer that indicates that the 340 offerer supports ICE, and if the answerer accepts the offer and the 341 usage of ICE, in each "m=" section within the answer, it MUST include 342 SDP candidate attributes for each available candidate associated with 343 the "m=" section. In addition, the answerer MUST include an SDP ice- 344 ufrag and an SDP ice-pwd attribute in the answer. 346 Once the answerer has sent the answer, it can start performing 347 connectivity checks towards the peer candidates that were provided in 348 the offer. 350 If the offer does not indicate support of ICE, the answerer MUST NOT 351 accept the usage of ICE. If the answerer still accepts the offer, 352 the answerer MUST NOT include any ICE related SDP attributes in the 353 answer. Instead the answerer will generate the answer according to 354 normal offer/answer procedures [RFC3264]. 356 If the answerer detects a possibility of the ICE mismatch, procedures 357 described in (Section 3.2.5) are followed. 359 3.3.3. Receiving the Initial Answer 361 When an offerer receives an initial answer that indicates that the 362 answerer supports ICE, it can start performing connectivity checks 363 towards the peer candidates that were provided in the answer. 365 If the answer does not indicate that the answerer supports ICE, or if 366 the offerer detects an ICE mismatch in the answer, the offerer MUST 367 terminate the usage of ICE. The subsequent actions taken by the 368 offerer are implementation dependent and are out of the scope of this 369 specification. 371 3.3.4. Concluding ICE 373 Once the state of each check list is Completed, and if the agent is 374 the controlling agent, it nominates a candidate pair [ICE-BIS] and 375 checks for each data stream whether the nominated pair matches the 376 default candidate pair. If there are one or more data streams with a 377 match, and the peer did not indicate support for the 'ice2' ice- 378 option, the controlling agent MUST generate a subsequent offer 379 (Section 3.4.1), in which the IP address, port and transport in the 380 "c=" and "m=" lines associated with each data stream match the 381 corresponding local information of the nominated pair for that data 382 stream. 384 However, If the support for 'ice2' ice-option is in use, the 385 nominated candidate is noted and sent in the subsequent offer/answer 386 exchange as the default candidate and no updated offer is needed to 387 fix the default candidate. 389 Also as described in [ICE-BIS], once the controlling agent has 390 nominated a candidate pair for a data stream, the agent MUST NOT 391 nominate another pair for that data stream during the lifetime of the 392 ICE session (i.e. until ICE is restarted). 394 3.4. Subsequent Offer/Answer Exchanges 396 Either agent MAY generate a subsequent offer at any time allowed by 397 [RFC3264]. This section defines rules for construction of subsequent 398 offers and answers. 400 Should a subsequent offer fail, ICE processing continues as if the 401 subsequent offer had never been made. 403 3.4.1. Sending Subsequent Offer 405 3.4.1.1. Procedures for All Implementations 407 3.4.1.1.1. ICE Restarts 409 An agent MAY restart ICE processing for an existing data stream 410 [ICE-BIS]. 412 The rules governing the ICE restart imply that setting the IP address 413 in the "c=" line to 0.0.0.0 will cause an ICE restart. Consequently, 414 ICE implementations MUST NOT utilize this mechanism for call hold, 415 and instead MUST use a=inactive and a=sendonly as described in 416 [RFC3264]. 418 To restart ICE, an agent MUST change both the ice-pwd and the ice- 419 ufrag for the data stream in an offer. Note that it is permissible 420 to use a session-level attribute in one offer, but to provide the 421 same ice-pwd or ice-ufrag as a media-level attribute in a subsequent 422 offer. This is not a change in password, just a change in its 423 representation, and does not cause an ICE restart. 425 An agent sets the rest of the ice related fields in the SDP for this 426 data stream as it would in an initial offer of this data stream (see 427 Section 3.2.1). Consequently, the set of candidates MAY include 428 some, none, or all of the previous candidates for that data stream 429 and MAY include a totally new set of candidates. 431 3.4.1.1.2. Removing a Data Stream 433 If an agent removes a data stream by setting its port to zero, it 434 MUST NOT include any candidate attributes for that data stream and 435 SHOULD NOT include any other ICE-related attributes defined in 436 Section 4 for that data stream. 438 3.4.1.1.3. Adding a Data Stream 440 If an agent wishes to add a new data stream, it sets the fields in 441 the SDP for this data stream as if this was an initial offer for that 442 data stream (see Section 3.2.1). This will cause ICE processing to 443 begin for this data stream. 445 3.4.1.2. Procedures for Full Implementations 447 This section describes additional procedures for full 448 implementations, covering existing data streams. 450 3.4.1.2.1. Before Nomination 452 When an offerer sends a subsequent offer; in each "m=" section for 453 which a candidate pair has not yet been nominated, the offer MUST 454 include the same set of ICE-related information that the offerer 455 included in the previous offer or answer. The agent MAY include 456 additional candidates it did not offer previously, but which it has 457 gathered since the last offer/ answer exchange, including peer 458 reflexive candidates. 460 The agent MAY change the default destination for media. As with 461 initial offers, there MUST be a set of candidate attributes in the 462 offer matching this default destination. 464 3.4.1.2.2. After Nomination 466 Once a candidate pair has been nominated for a data stream, the IP 467 address, port and transport in each "c=" and "m=" line associated 468 with that data stream MUST match the data associated with the 469 nominated pair for that data stream. In addition, the offerer only 470 includes SDP candidates representing the local candidates of the 471 nominated candidate pair. The offerer MUST NOT include any other SDP 472 candidate attributes in the subsequent offer. 474 In addition, if the agent is controlling, it MUST include the 475 a=remote-candidates attribute for each data stream whose check list 476 is in the completed state. The attribute contains the remote 477 candidates corresponding to the nominated pair in the valid list for 478 each component of that data stream. It is needed to avoid a race 479 condition whereby the controlling agent chooses its pairs, but the 480 updated offer beats the connectivity checks to the controlled agent, 481 which doesn't even know these pairs are valid, let alone selected. 482 See Appendix B for elaboration on this race condition. 484 3.4.1.3. Procedures for Lite Implementations 486 If the ICE state is running, a lite implementation MUST include all 487 of its candidates for each component of each data stream in 488 a=candidate attribute in any subsequent offer. The candidates are 489 formed identical to the procedures for initial offers. 491 A lite implementation MUST NOT add additional host candidates or 492 change username fragments or passwords in a subsequent offer. 493 Otherwise, it MUST restart ICE. 495 If ICE has completed for a data stream and if the agent is 496 controlled, the default destination for that data stream MUST be set 497 to the remote candidate of the candidate pair for that component in 498 the valid list. For a lite implementation, there is always just a 499 single candidate pair in the valid list for each component of a data 500 stream. Additionally, the agent MUST include a candidate attribute 501 for each default destination. 503 If ICE state is completed and if the agent is controlling (which only 504 happens when both agents are lite), the agent MUST include the 505 a=remote-candidates attribute for each data stream. The attribute 506 contains the remote candidates from the candidate pairs in the valid 507 list (one pair for each component of each data stream). 509 3.4.2. Sending Subsequent Answer 511 If ICE is Completed for a data stream, and the offer for that data 512 stream lacked the a=remote-candidates attribute, the rules for 513 construction of the answer are identical to those for the offerer, 514 except that the answerer MUST NOT include the a=remote-candidates 515 attribute in the answer. 517 A controlled agent will receive an offer with the a=remote-candidates 518 attribute for a data stream when its peer has concluded ICE 519 processing for that data stream. This attribute is present in the 520 offer to deal with a race condition between the receipt of the offer, 521 and the receipt of the Binding Response that tells the answerer the 522 candidate that will be selected by ICE. See Appendix B for an 523 explanation of this race condition. Consequently, processing of an 524 offer with this attribute depends on the winner of the race. 526 The agent forms a candidate pair for each component of the data 527 stream by: 529 o Setting the remote candidate equal to the offerer's default 530 destination for that component (i.e. the contents of the "m=" and 531 "c=" lines for RTP, and the a=rtcp attribute for RTCP) 533 o Setting the local candidate equal to the transport address for 534 that same component in the a=remote-candidates attribute in the 535 offer. 537 The agent then sees if each of these candidate pairs is present in 538 the valid list. If a particular pair is not in the valid list, the 539 check has "lost" the race. Call such a pair a "losing pair". 541 The agent finds all the pairs in the check list whose remote 542 candidates equal the remote candidate in the losing pair: 544 o If none of the pairs are In-Progress, and at least one is Failed, 545 it is most likely that a network failure, such as a network 546 partition or serious packet loss, has occurred. The agent SHOULD 547 generate an answer for this data stream as if the remote- 548 candidates attribute had not been present, and then restart ICE 549 for this stream. 551 o If at least one of the pairs is In-Progress, the agent SHOULD wait 552 for those checks to complete, and as each completes, redo the 553 processing in this section until there are no losing pairs. 555 Once there are no losing pairs, the agent can generate the answer. 556 It MUST set the default destination for media to the candidates in 557 the remote-candidates attribute from the offer (each of which will 558 now be the local candidate of a candidate pair in the valid list). 559 It MUST include a candidate attribute in the answer for each 560 candidate in the remote-candidates attribute in the offer. 562 3.4.2.1. Detecting ICE Restart 564 If the offerer in a subsequent offer requested an ICE restart for a 565 data stream, and if the answerer accepts the offer, the answerer 566 follows the procedures for generating an initial answer. 568 For a given data stream, the answerer MAY include the same candidates 569 that were used in the previous ICE session, but it MUST change the 570 SDP ice-pwd and ice-ufrag attribute values. 572 3.4.2.2. Lite Implementation specific procedures 574 If the received offer contains the remote-candidates attribute for a 575 data stream, the agent forms a candidate pair for each component of 576 the data stream by: 578 o Setting the remote candidate equal to the offerer's default 579 destination for that component (i.e., the contents of the "m=" and 580 "c=" lines for RTP, and the a=rtcp attribute for RTCP). 582 o Setting the local candidate equal to the transport address for 583 that same component in the a=remote-candidates attribute in the 584 offer. 586 The state of ICE processing for that data stream is set to Completed. 588 Furthermore, if the agent believed it was controlling, but the offer 589 contained the a=remote-candidates attribute, both agents believe they 590 are controlling. In this case, both would have sent updated offers 591 around the same time. 593 However, the signaling protocol carrying the offer/answer exchanges 594 will have resolved this glare condition, so that one agent is always 595 the 'winner' by having its offer received before its peer has sent an 596 offer. The winner takes the role of controlling, so that the loser 597 (the answerer under consideration in this section) MUST change its 598 role to controlled. 600 Consequently, if the agent was going to send an updated offer since, 601 based on the rules in section 8.2 of [ICE-BIS], it was controlling, 602 it no longer needs to. 604 Besides the potential role change, change in the Valid list, and 605 state changes, the construction of the answer is performed 606 identically to the construction of an offer. 608 3.4.3. Receiving Answer for a Subsequent Offer 610 3.4.3.1. Procedures for Full Implementations 612 There may be certain situations where the offerer receives an SDP 613 answer that lacks ICE candidates although the initial answer did. 614 One example of such an "unexpected" answer might be happen when an 615 ICE-unaware B2BUA introduces a media server during call hold using 616 3rd party call-control procedures. Omitting further details how this 617 is done, this could result in an answer being received at the holding 618 UA that was constructed by the B2BUA. With the B2BUA being ICE- 619 unaware, that answer would not include ICE candidates. 621 Receiving an answer without ICE attributes in this situation might be 622 unexpected, but would not necessarily impair the user experience. 624 When the offerer receives an answer indicating support for ICE, the 625 offer performs on of the following actions: 627 o If the offer was a restart, the agent MUST perform ICE restart 628 procedures as specified in Section 3.4.3.1.1 630 o If the offer/answer exchange removed a data stream, or an answer 631 rejected an offered data stream, an agent MUST flush the Valid 632 list for that data stream. It MUST also terminate any STUN 633 transactions in progress for that data stream. 635 o If the offer/answer exchange added a new data stream, the agent 636 MUST create a new check list for it (and an empty Valid list to 637 start of course) which in turn triggers the candidate processing 638 procedures [ICE-BIS]. 640 o If ICE state is running for a given data stream, the agent 641 recomputes the check list. If a pair on the new check list was 642 also on the previous check list, and its state was Waiting, In- 643 Progress, Succeeded, or Failed, its state is copied over. 644 Otherwise, its state is set to Frozen. If none of the check lists 645 are active (meaning that the pairs in each check list are Frozen), 646 appropriate procedures in [ICE-BIS] are performed to move 647 candidate(s) to the Waiting state to further continue ICE 648 processing. 650 o If ICE state is completed and the SDP answer conforms to 651 Section 3.4.2, the agent MUST reman in the ICE completed state. 653 However, if the ICE support is no longer indicated in the SDP answer, 654 the agent MUST fall-back to [RFC3264] procedures and SHOULD NOT drop 655 the dialog because of the missing ICE support or unexpected answer. 656 Once the agent sends a new offer later on, it MUST perform an ICE 657 restart. 659 3.4.3.1.1. ICE Restarts 661 The agent MUST remember the nominated pair in the Valid list for each 662 component of the data stream, called the previous selected pair prior 663 to the restart. The agent will continue to send media using this 664 pair, as described in section 12 of [ICE-BIS]. Once these 665 destinations are noted, the agent MUST flush the valid and check 666 lists, and then recompute the check list and its states, thus 667 triggering the candidate processing procedures [ICE-BIS] 669 3.4.3.2. Procedures for Lite Implementations 671 If ICE is restarting for a data stream, the agent MUST start a new 672 Valid list for that data stream. It MUST remember the nominated pair 673 in the previous Valid list for each component of the data stream, 674 called the previous selected pairs, and continue to send media there 675 as described in section 12 of [ICE-BIS]. The state of ICE processing 676 for each data stream MUST change to Running, and the state of ICE 677 processing MUST change to Running 679 4. Grammar 681 This specification defines eight new SDP attributes -- the 682 "candidate", "remote-candidates", "ice-lite", "ice-mismatch", "ice- 683 ufrag", "ice-pwd", "ice-pacing", and "ice-options" attributes. 685 This section also provides non-normative examples of the attributes 686 defined. 688 The syntax for the attributes follow Augmented BNF as defined in 689 [RFC5234]. 691 4.1. "candidate" Attribute 693 The candidate attribute is a media-level attribute only. It contains 694 a transport address for a candidate that can be used for connectivity 695 checks. 697 candidate-attribute = "candidate" ":" foundation SP component-id SP 698 transport SP 699 priority SP 700 connection-address SP ;from RFC 4566 701 port ;port from RFC 4566 702 SP cand-type 703 [SP rel-addr] 704 [SP rel-port] 705 *(SP extension-att-name SP 706 extension-att-value) 708 foundation = 1*32ice-char 709 component-id = 1*5DIGIT 710 transport = "UDP" / transport-extension 711 transport-extension = token ; from RFC 3261 712 priority = 1*10DIGIT 713 cand-type = "typ" SP candidate-types 714 candidate-types = "host" / "srflx" / "prflx" / "relay" / token 715 rel-addr = "raddr" SP connection-address 716 rel-port = "rport" SP port 717 extension-att-name = token 718 extension-att-value = *VCHAR 719 ice-char = ALPHA / DIGIT / "+" / "/" 721 This grammar encodes the primary information about a candidate: its 722 IP address, port and transport protocol, and its properties: the 723 foundation, component ID, priority, type, and related transport 724 address: 726 : is taken from RFC 4566 [RFC4566]. It is the 727 IP address of the candidate. When parsing this field, an agent 728 can differentiate an IPv4 address and an IPv6 address by presence 729 of a colon in its value -- the presence of a colon indicates IPv6. 730 An agent MUST ignore candidate lines that include candidates with 731 IP address versions that are not supported or recognized. An IP 732 address SHOULD be used, but an FQDN MAY be used in place of an IP 733 address. In that case, when receiving an offer or answer 734 containing an FQDN in an a=candidate attribute, the FQDN is looked 735 up in the DNS first using an AAAA record (assuming the agent 736 supports IPv6), and if no result is found or the agent only 737 supports IPv4, using an A record. The rules from section 6 of 738 [RFC6724] is followed by fixing the source address to be one from 739 the candidate pair to be matched against destination addresses 740 reported by FQDN, in cases where the DNS query returns more than 741 one IP address. 743 : is also taken from RFC 4566 [RFC4566]. It is the port of 744 the candidate. 746 : indicates the transport protocol for the candidate. 747 This specification only defines UDP. However, extensibility is 748 provided to allow for future transport protocols to be used with 749 ICE by extending the sub-registry "ICE Transport Protocols" under 750 "Interactive Connectivity Establishment (ICE)" registry. 752 : is composed of 1 to 32 s. It is an 753 identifier that is equivalent for two candidates that are of the 754 same type, share the same base, and come from the same STUN 755 server. The foundation is used to optimize ICE performance in the 756 Frozen algorithm as described in [ICE-BIS] 758 : is a positive integer between 1 and 256 (inclusive) 759 that identifies the specific component of the dta stream for which 760 this is a candidate. It MUST start at 1 and MUST increment by 1 761 for each component of a particular candidate. For data streams 762 based on RTP, candidates for the actual RTP media MUST have a 763 component ID of 1, and candidates for RTCP MUST have a component 764 ID of 2. See section 13 in [ICE-BIS] for additional discussion on 765 extending ICE to new data streams. 767 : is a positive integer between 1 and (2**31 - 1) 768 inclusive. The procedures for computing candidate's priority is 769 described in section 5.1.2 of [ICE-BIS]. 771 : encodes the type of candidate. This specification 772 defines the values "host", "srflx", "prflx", and "relay" for host, 773 server reflexive, peer reflexive, and relayed candidates, 774 respectively. Specifications for new candidate types MUST define 775 how, if at all, various steps in the ICE processing differ from 776 the ones defined by this specification. 778 and : convey transport addresses related to the 779 candidate, useful for diagnostics and other purposes. 780 and MUST be present for server reflexive, peer 781 reflexive, and relayed candidates. If a candidate is server or 782 peer reflexive, and are equal to the base 783 for that server or peer reflexive candidate. If the candidate is 784 relayed, and are equal to the mapped address 785 in the Allocate response that provided the client with that 786 relayed candidate (see Appendix B.3 of [ICE-BIS] for a discussion 787 of its purpose). If the candidate is a host candidate, 788 and MUST be omitted. 790 In some cases, e.g., for privacy reasons, an agent may not want to 791 reveal the related address and port. In this case the address 792 MUST be set to "0.0.0.0" (for IPv4 candidates) or "::" (for IPv6 793 candidates) and the port to zero. 795 The candidate attribute can itself be extended. The grammar allows 796 for new name/value pairs to be added at the end of the attribute. 797 Such extensions MUST be made through IETF Review or IESG Approval 798 [RFC5226] and the assignments MUST contain the specific extension and 799 a reference to the document defining the usage of the extension 801 An implementation MUST ignore any name/value pairs it doesn't 802 understand. 804 Example: SDP line for UDP server reflexive candidate attribute for the RTP component 806 a=candidate:2 1 UDP 1694498815 192.0.2.3 45664 typ srflx raddr 10.0.1.1 rport 8998 808 4.2. "remote-candidates" Attribute 810 The syntax of the "remote-candidates" attribute is defined using 811 Augmented BNF as defined in [RFC5234]. The remote-candidates 812 attribute is a media-level attribute only. 814 remote-candidate-att = "remote-candidates:" remote-candidate 815 0*(SP remote-candidate) 816 remote-candidate = component-ID SP connection-address SP port 818 The attribute contains a connection-address and port for each 819 component. The ordering of components is irrelevant. However, a 820 value MUST be present for each component of a data stream. This 821 attribute MUST be included in an offer by a controlling agent for a 822 data stream that is Completed, and MUST NOT be included in any other 823 case. 825 Example: Remote candidates SDP lines for the RTP and RTCP components: 827 a=remote-candidates:1 192.0.2.3 45664 828 a=remote-candidates:2 192.0.2.3 45665 830 4.3. "ice-lite" and "ice-mismatch" Attributes 832 The syntax of the "ice-lite" and "ice-mismatch" attributes, both of 833 which are flags, is: 835 ice-lite = "ice-lite" 836 ice-mismatch = "ice-mismatch" 838 "ice-lite" is a session-level attribute only, and indicates that an 839 agent is a lite implementation. "ice-mismatch" is a media-level 840 attribute only, and when present in an answer, indicates that the 841 offer arrived with a default destination for a media component that 842 didn't have a corresponding candidate attribute. 844 4.4. "ice-ufrag" and "ice-pwd" Attributes 846 The "ice-ufrag" and "ice-pwd" attributes convey the username fragment 847 and password used by ICE for message integrity. Their syntax is: 849 ice-pwd-att = "ice-pwd:" password 850 ice-ufrag-att = "ice-ufrag:" ufrag 851 password = 22*256ice-char 852 ufrag = 4*256ice-char 854 The "ice-pwd" and "ice-ufrag" attributes can appear at either the 855 session-level or media-level. When present in both, the value in the 856 media-level takes precedence. Thus, the value at the session-level 857 is effectively a default that applies to all data streams, unless 858 overridden by a media-level value. Whether present at the session or 859 media-level, there MUST be an ice-pwd and ice-ufrag attribute for 860 each data stream. If two data streams have identical ice-ufrag's, 861 they MUST have identical ice-pwd's. 863 The ice-ufrag and ice-pwd attributes MUST be chosen randomly at the 864 beginning of a session (the same applies when ICE is restarting for 865 an agent). 867 The ice-ufrag attribute MUST contain at least 24 bits of randomness, 868 and the ice-pwd attribute MUST contain at least 128 bits of 869 randomness. This means that the ice-ufrag attribute will be at least 870 4 characters long, and the ice-pwd at least 22 characters long, since 871 the grammar for these attributes allows for 6 bits of information per 872 character. The attributes MAY be longer than 4 and 22 characters, 873 respectively, of course, up to 256 characters. The upper limit 874 allows for buffer sizing in implementations. Its large upper limit 875 allows for increased amounts of randomness to be added over time. 876 For compatibility with the 512 character limitation for the STUN 877 username attribute value and for bandwidth conservation 878 considerations, the ice-ufrag attribute MUST NOT be longer than 32 879 characters when sending, but an implementation MUST accept up to 256 880 characters when receiving. 882 Example shows sample ice-ufrag and ice-pwd SDP lines: 884 a=ice-pwd:asd88fgpdd777uzjYhagZg 885 a=ice-ufrag:8hhY 887 4.5. "ice-pacing" Attribute 889 The "ice-pacing" is a session level attribute that indicates the 890 desired connectivity check pacing, in milliseconds, for this agent 891 (see section 14 of [ICE-BIS]). The syntax is: 893 ice-pacing-att = "ice-pacing:" pacing-value 894 pacing-value = 1*10DIGIT 896 Following the procedures defined in [ICE-BIS], a default value of 897 50ms is used for an agent when ice-pacing attribute is omitted in the 898 offer or the answer. 900 The same rule applies for ice-pacing attribute values lower than 901 50ms. This mandates that, if an agent includes the ice-pacing 902 attribute, its value MUST be greater than 50ms or else a value of 903 50ms is considered by default for that agent. 905 Also the larger of the ice-pacing attribute values between the offer 906 and the answer (determined either by the one provided in the ice- 907 pacing attribute or by picking the default value) MUST be considered 908 for a given ICE session. 910 Example shows ice-pacing value of 5 ms: 912 a=ice-pacing:5 914 4.6. "ice-options" Attribute 916 The "ice-options" attribute is a session- and media-level attribute. 917 It contains a series of tokens that identify the options supported by 918 the agent. Its grammar is: 920 ice-options = "ice-options:" ice-option-tag 921 0*(SP ice-option-tag) 922 ice-option-tag = 1*ice-char 924 The existence of an ice-option in an offer indicates that a certain 925 extension is supported by the agent and is willing to use it, if the 926 peer agent also includes the same extension in the answer. There 927 might be further extension specific negotiation needed between the 928 agents that determine how the extensions gets used in a given 929 session. The details of the negotiation procedures, if present, MUST 930 be defined by the specification defining the extension (see 931 Section 10.2). 933 Example shows 'rtp+ecn' ice-option SDP line from <>: 935 a=ice-options:rtp+ecn 937 5. Keepalives 939 All the ICE agents MUST follow the procedures defined in section 11 940 of [ICE-BIS] for sending keepalives. The keepalives MUST be sent 941 regardless of whether the data stream is currently inactive, 942 sendonly, recvonly, or sendrecv, and regardless of the presence or 943 value of the bandwidth attribute. An agent can determine that its 944 peer supports ICE by the presence of a=candidate attributes for each 945 media session. 947 6. SIP Considerations 949 Note that ICE is not intended for NAT traversal for SIP, which is 950 assumed to be provided via another mechanism [RFC5626]. 952 When ICE is used with SIP, forking may result in a single offer 953 generating a multiplicity of answers. In that case, ICE proceeds 954 completely in parallel and independently for each answer, treating 955 the combination of its offer and each answer as an independent offer/ 956 answer exchange, with its own set of local candidates, pairs, check 957 lists, states, and so on. 959 Once ICE processing has reached the Completed state for all peers for 960 media streams using those candidates, the agent SHOULD wait an 961 additional three seconds, and then it MAY cease responding to checks 962 or generating triggered checks on that candidate. It MAY free the 963 candidate at that time. Freeing of server reflexive candidates is 964 never explicit; it happens by lack of a keepalive. The three-second 965 delay handles cases when aggressive nomination is used, and the 966 selected pairs can quickly change after ICE has completed. 968 6.1. Latency Guidelines 970 ICE requires a series of STUN-based connectivity checks to take place 971 between endpoints. These checks start from the answerer on 972 generation of its answer, and start from the offerer when it receives 973 the answer. These checks can take time to complete, and as such, the 974 selection of messages to use with offers and answers can affect 975 perceived user latency. Two latency figures are of particular 976 interest. These are the post-pickup delay and the post-dial delay. 977 The post-pickup delay refers to the time between when a user "answers 978 the phone" and when any speech they utter can be delivered to the 979 caller. The post-dial delay refers to the time between when a user 980 enters the destination address for the user and ringback begins as a 981 consequence of having successfully started alerting the called user 982 agent. 984 Two cases can be considered -- one where the offer is present in the 985 initial INVITE and one where it is in a response. 987 6.1.1. Offer in INVITE 989 To reduce post-dial delays, it is RECOMMENDED that the caller begin 990 gathering candidates prior to actually sending its initial INVITE. 991 This can be started upon user interface cues that a call is pending, 992 such as activity on a keypad or the phone going off-hook. 994 On the receipt of the offer, the answerer SHOULD generate an answer 995 in a provisional response once it has completed candidate gathering. 996 ICE requires that a provisional response with an SDP be transmitted 997 reliably. This can be done through the existing Provisional Response 998 Acknowledgment (PRACK) mechanism [RFC3262] or through an ICE specific 999 optimization, wherein, the agent retransmits the provisional response 1000 with the exponential backoff timers described in [RFC3262]. Such 1001 retransmissions MUST cease on receipt of a STUN Binding request for 1002 one of the data streams signaled in that SDP or on transmission of 1003 the answer in a 2xx response. If no Binding request is received 1004 prior to the last retransmit, the agent does not consider the session 1005 terminated. For the ICE lite peers, the agent MUST cease 1006 retransmitting the 18x after sending it four times (ICE will actually 1007 work even if the peer never receives the 18x; however, experience has 1008 shown that sending it is important for middleboxes and firewall 1009 traversal). 1011 It should be noted that the ICE specific optimization is very 1012 specific to provisional response carrying answers that start ICE 1013 processing and it is not a general technique for 1xx reliability. 1014 Also such an optimization SHOULD NOT be used if both agents support 1015 PRACK. 1017 Despite the fact that the provisional response will be delivered 1018 reliably, the rules for when an agent can send an updated offer or 1019 answer do not change from those specified in [RFC3262]. 1020 Specifically, if the INVITE contained an offer, the same answer 1021 appears in all of the 1xx and in the 2xx response to the INVITE. 1022 Only after that 2xx has been sent can an updated offer/answer 1023 exchange occur. 1025 Alternatively, an agent MAY delay sending an answer until the 200 OK; 1026 however, this results in a poor user experience and is NOT 1027 RECOMMENDED. 1029 Once the answer has been sent, the agent SHOULD begin its 1030 connectivity checks. Once candidate pairs for each component of a 1031 data stream enter the valid list, the answerer can begin sending 1032 media on that data stream. 1034 However, prior to this point, any media that needs to be sent towards 1035 the caller (such as SIP early media [RFC3960]) MUST NOT be 1036 transmitted. For this reason, implementations SHOULD delay alerting 1037 the called party until candidates for each component of each data 1038 stream have entered the valid list. In the case of a PSTN gateway, 1039 this would mean that the setup message into the PSTN is delayed until 1040 this point. Doing this increases the post-dial delay, but has the 1041 effect of eliminating 'ghost rings'. Ghost rings are cases where the 1042 called party hears the phone ring, picks up, but hears nothing and 1043 cannot be heard. This technique works without requiring support for, 1044 or usage of, preconditions [RFC3312]. It also has the benefit of 1045 guaranteeing that not a single packet of media will get clipped, so 1046 that post-pickup delay is zero. If an agent chooses to delay local 1047 alerting in this way, it SHOULD generate a 180 response once alerting 1048 begins. 1050 6.1.2. Offer in Response 1052 In addition to uses where the offer is in an INVITE, and the answer 1053 is in the provisional and/or 200 OK response, ICE works with cases 1054 where the offer appears in the response. In such cases, which are 1055 common in third party call control [RFC3725], ICE agents SHOULD 1056 generate their offers in a reliable provisional response (which MUST 1057 utilize [RFC3262]), and not alert the user on receipt of the INVITE. 1058 The answer will arrive in a PRACK. This allows for ICE processing to 1059 take place prior to alerting, so that there is no post-pickup delay, 1060 at the expense of increased call setup delays. Once ICE completes, 1061 the callee can alert the user and then generate a 200 OK when they 1062 answer. The 200 OK would contain no SDP, since the offer/answer 1063 exchange has completed. 1065 Alternatively, agents MAY place the offer in a 2xx instead (in which 1066 case the answer comes in the ACK). When this happens, the callee 1067 will alert the user on receipt of the INVITE, and the ICE exchanges 1068 will take place only after the user answers. This has the effect of 1069 reducing call setup delay, but can cause substantial post-pickup 1070 delays and media clipping. 1072 6.2. SIP Option Tags and Media Feature Tags 1074 [RFC5768] specifies a SIP option tag and media feature tag for usage 1075 with ICE. ICE implementations using SIP SHOULD support this 1076 specification, which uses a feature tag in registrations to 1077 facilitate interoperability through signaling intermediaries. 1079 6.3. Interactions with Forking 1081 ICE interacts very well with forking. Indeed, ICE fixes some of the 1082 problems associated with forking. Without ICE, when a call forks and 1083 the caller receives multiple incoming data streams, it cannot 1084 determine which data stream corresponds to which callee. 1086 With ICE, this problem is resolved. The connectivity checks which 1087 occur prior to transmission of media carry username fragments, which 1088 in turn are correlated to a specific callee. Subsequent media 1089 packets that arrive on the same candidate pair as the connectivity 1090 check will be associated with that same callee. Thus, the caller can 1091 perform this correlation as long as it has received an answer. 1093 6.4. Interactions with Preconditions 1095 Quality of Service (QoS) preconditions, which are defined in 1096 [RFC3312] and [RFC4032], apply only to the transport addresses listed 1097 as the default targets for media in an offer/answer. If ICE changes 1098 the transport address where media is received, this change is 1099 reflected in an updated offer that changes the default destination 1100 for media to match ICE's selection. As such, it appears like any 1101 other re-INVITE would, and is fully treated in RFCs 3312 and 4032, 1102 which apply without regard to the fact that the destination for media 1103 is changing due to ICE negotiations occurring "in the background". 1105 Indeed, an agent SHOULD NOT indicate that QoS preconditions have been 1106 met until the checks have completed and selected the candidate pairs 1107 to be used for media. 1109 ICE also has (purposeful) interactions with connectivity 1110 preconditions [RFC5898]. Those interactions are described there. 1111 Note that the procedures described in Section 6.1 describe their own 1112 type of "preconditions", albeit with less functionality than those 1113 provided by the explicit preconditions in [RFC5898]. 1115 6.5. Interactions with Third Party Call Control 1117 ICE works with Flows I, III, and IV as described in [RFC3725]. Flow 1118 I works without the controller supporting or being aware of ICE. 1119 Flow IV will work as long as the controller passes along the ICE 1120 attributes without alteration. Flow II is fundamentally incompatible 1121 with ICE; each agent will believe itself to be the answerer and thus 1122 never generate a re-INVITE. 1124 The flows for continued operation, as described in Section 7 of 1125 [RFC3725], require additional behavior of ICE implementations to 1126 support. In particular, if an agent receives a mid-dialog re-INVITE 1127 that contains no offer, it MUST restart ICE for each data stream and 1128 go through the process of gathering new candidates. Furthermore, 1129 that list of candidates SHOULD include the ones currently being used 1130 for media. 1132 7. Relationship with ANAT 1134 [RFC4091], the Alternative Network Address Types (ANAT) Semantics for 1135 the SDP grouping framework, and [RFC4092], its usage with SIP, define 1136 a mechanism for indicating that an agent can support both IPv4 and 1137 IPv6 for a data stream, and it does so by including two "m=" lines, 1138 one for v4 and one for v6. This is similar to ICE, which allows for 1139 an agent to indicate multiple transport addresses using the candidate 1140 attribute. However, ANAT relies on static selection to pick between 1141 choices, rather than a dynamic connectivity check used by ICE. 1143 It is RECOMMENDED that ICE be used in realizing the dual-stack use- 1144 cases in agents that support ICE. 1146 8. Setting Ta and RTO for RTP Media Streams 1148 During the gathering phase of ICE and while ICE is performing 1149 connectivity checks, an agent sends STUN and TURN transactions. 1150 These transactions are paced at a rate of one every Ta milliseconds, 1151 and utilize a specific RTO. See Section 14 of [ICE-BIS] for details 1152 on how the values of Ta and RTO are computed with a real-time media 1153 stream of known maximum bandwidth to rate-control the ICE exchanges. 1155 9. Security Considerations 1157 9.1. Attacks on the Offer/Answer Exchanges 1159 An attacker that can modify or disrupt the offer/answer exchanges 1160 themselves can readily launch a variety of attacks with ICE. They 1161 could direct media to a target of a DoS attack, they could insert 1162 themselves into the data stream, and so on. These are similar to the 1163 general security considerations for offer/answer exchanges, and the 1164 security considerations in [RFC3264] apply. These require techniques 1165 for message integrity and encryption for offers and answers, which 1166 are satisfied by the TLS mechanism [RFC3261] when SIP is used. As 1167 such, the usage of TLS with ICE is RECOMMENDED. 1169 9.2. Insider Attacks 1171 In addition to attacks where the attacker is a third party trying to 1172 insert fake offers, answers, or STUN messages, there are several 1173 attacks possible with ICE when the attacker is an authenticated and 1174 valid participant in the ICE exchange. 1176 9.2.1. The Voice Hammer Attack 1178 The voice hammer attack is an amplification attack. In this attack, 1179 the attacker initiates sessions to other agents, and maliciously 1180 includes the IP address and port of a DoS target as the destination 1181 for media traffic signaled in the SDP. This causes substantial 1182 amplification; a single offer/answer exchange can create a continuing 1183 flood of media packets, possibly at high rates (consider video 1184 sources). This attack is not specific to ICE, but ICE can help 1185 provide remediation. 1187 Specifically, if ICE is used, the agent receiving the malicious SDP 1188 will first perform connectivity checks to the target of media before 1189 sending media there. If this target is a third-party host, the 1190 checks will not succeed, and media is never sent. 1192 Unfortunately, ICE doesn't help if it's not used, in which case an 1193 attacker could simply send the offer without the ICE parameters. 1194 However, in environments where the set of clients is known, and is 1195 limited to ones that support ICE, the server can reject any offers or 1196 answers that don't indicate ICE support. 1198 SIP User Agents (UA) [RFC3261] that are not willing to receive non- 1199 ICE answers MUST include an "ice" Option Tag in the SIP Require 1200 Header Field in their offer. UAs that rejects non-ICE offers SHOULD 1201 use a 421 response code, together with an Option Tag "ice" in the 1202 Require Header Field in the response. 1204 9.2.2. Interactions with Application Layer Gateways and SIP 1206 Application Layer Gateways (ALGs) are functions present in a Network 1207 Address Translation (NAT) device that inspect the contents of packets 1208 and modify them, in order to facilitate NAT traversal for application 1209 protocols. Session Border Controllers (SBCs) are close cousins of 1210 ALGs, but are less transparent since they actually exist as 1211 application-layer SIP intermediaries. ICE has interactions with SBCs 1212 and ALGs. 1214 If an ALG is SIP aware but not ICE aware, ICE will work through it as 1215 long as the ALG correctly modifies the SDP. A correct ALG 1216 implementation behaves as follows: 1218 o The ALG does not modify the "m=" and "c=" lines or the rtcp 1219 attribute if they contain external addresses. 1221 o If the "m=" and "c=" lines contain internal addresses, the 1222 modification depends on the state of the ALG: 1224 * If the ALG already has a binding established that maps an 1225 external port to an internal IP address and port matching the 1226 values in the "m=" and "c=" lines or rtcp attribute, the ALG 1227 uses that binding instead of creating a new one. 1229 * If the ALG does not already have a binding, it creates a new 1230 one and modifies the SDP, rewriting the "m=" and "c=" lines and 1231 rtcp attribute. 1233 Unfortunately, many ALGs are known to work poorly in these corner 1234 cases. ICE does not try to work around broken ALGs, as this is 1235 outside the scope of its functionality. ICE can help diagnose these 1236 conditions, which often show up as a mismatch between the set of 1237 candidates and the "m=" and "c=" lines and rtcp attributes. The ice- 1238 mismatch attribute is used for this purpose. 1240 ICE works best through ALGs when the signaling is run over TLS. This 1241 prevents the ALG from manipulating the SDP messages and interfering 1242 with ICE operation. Implementations that are expected to be deployed 1243 behind ALGs SHOULD provide for TLS transport of the SDP. 1245 If an SBC is SIP aware but not ICE aware, the result depends on the 1246 behavior of the SBC. If it is acting as a proper Back-to-Back User 1247 Agent (B2BUA), the SBC will remove any SDP attributes it doesn't 1248 understand, including the ICE attributes. Consequently, the call 1249 will appear to both endpoints as if the other side doesn't support 1250 ICE. This will result in ICE being disabled, and media flowing 1251 through the SBC, if the SBC has requested it. If, however, the SBC 1252 passes the ICE attributes without modification, yet modifies the 1253 default destination for media (contained in the "m=" and "c=" lines 1254 and rtcp attribute), this will be detected as an ICE mismatch, and 1255 ICE processing is aborted for the call. It is outside of the scope 1256 of ICE for it to act as a tool for "working around" SBCs. If one is 1257 present, ICE will not be used and the SBC techniques take precedence. 1259 10. IANA Considerations 1261 10.1. SDP Attributes 1263 The original ICE specification defined seven new SDP attributes per 1264 the procedures of Section 8.2.4 of [RFC4566]. The registration 1265 information from the original specification is included here with 1266 modifications to include Mux Category and also defines a new SDP 1267 attribute 'ice-pacing'. 1269 10.1.1. candidate Attribute 1271 Attribute Name: candidate 1273 Type of Attribute: media-level 1275 Subject to charset: No 1277 Purpose: This attribute is used with Interactive Connectivity 1278 Establishment (ICE), and provides one of many possible candidate 1279 addresses for communication. These addresses are validated with 1280 an end-to-end connectivity check using Session Traversal Utilities 1281 for NAT (STUN). 1283 Appropriate Values: See Section 4 of RFC XXXX. 1285 Contact Name: IESG 1287 Contact e-mail: iesg@ietf.org [1] 1289 Reference: RFCXXXX 1291 Mux Category: TRANSPORT 1293 10.1.2. remote-candidates Attribute 1295 Attribute Name: remote-candidates 1297 Type of Attribute: media-level 1299 Subject to charset: No 1301 Purpose: This attribute is used with Interactive Connectivity 1302 Establishment (ICE), and provides the identity of the remote 1303 candidates that the offerer wishes the answerer to use in its 1304 answer. 1306 Appropriate Values: See Section 4 of RFC XXXX. 1308 Contact Name: IESG 1310 Contact e-mail: iesg@ietf.org [2] 1312 Reference: RFCXXXX 1314 Mux Category: TRANSPORT 1316 10.1.3. ice-lite Attribute 1318 Attribute Name: ice-lite 1320 Type of Attribute: session-level 1322 Subject to charset: No 1324 Purpose: This attribute is used with Interactive Connectivity 1325 Establishment (ICE), and indicates that an agent has the minimum 1326 functionality required to support ICE inter-operation with a peer 1327 that has a full implementation. 1329 Appropriate Values: See Section 4 of RFC XXXX. 1331 Contact Name: IESG 1333 Contact e-mail: iesg@ietf.org [3] 1335 Reference: RFCXXXX 1337 Mux Category: NORMAL 1339 10.1.4. ice-mismatch Attribute 1341 Attribute Name: ice-mismatch 1343 Type of Attribute: media-level 1345 Subject to charset: No 1347 Purpose: This attribute is used with Interactive Connectivity 1348 Establishment (ICE), and indicates that an agent is ICE capable, 1349 but did not proceed with ICE due to a mismatch of candidates with 1350 the default destination for media signaled in the SDP. 1352 Appropriate Values: See Section 4 of RFC XXXX. 1354 Contact Name: IESG 1356 Contact e-mail: iesg@ietf.org [4] 1358 Reference: RFCXXXX 1360 Mux Category: NORMAL 1362 10.1.5. ice-pwd Attribute 1364 Attribute Name: ice-pwd 1366 Type of Attribute: session- or media-level 1368 Subject to charset: No 1370 Purpose: This attribute is used with Interactive Connectivity 1371 Establishment (ICE), and provides the password used to protect 1372 STUN connectivity checks. 1374 Appropriate Values: See Section 4 of RFC XXXX. 1376 Contact Name: IESG 1378 Contact e-mail: iesg@ietf.org [5] 1380 Reference: RFCXXXX 1382 Mux Category: TRANSPORT 1384 10.1.6. ice-ufrag Attribute 1386 Attribute Name: ice-ufrag 1388 Type of Attribute: session- or media-level 1390 Subject to charset: No 1392 Purpose: This attribute is used with Interactive Connectivity 1393 Establishment (ICE), and provides the fragments used to construct 1394 the username in STUN connectivity checks. 1396 Appropriate Values: See Section 4 of RFC XXXX. 1398 Contact Name: IESG 1400 Contact e-mail: iesg@ietf.org [6] 1402 Reference: RFCXXXX 1404 Mux Category: TRANSPORT 1406 10.1.7. ice-options Attribute 1408 Attribute Name: ice-options 1410 Long Form: ice-options 1412 Type of Attribute: session- or media-level 1414 Subject to charset: No 1416 Purpose: This attribute is used with Interactive Connectivity 1417 Establishment (ICE), and indicates the ICE options or extensions 1418 used by the agent. 1420 Appropriate Values: See Section 4 of RFC XXXX. 1422 Contact Name: IESG 1424 Contact e-mail: iesg@ietf.org [7] 1426 Reference: RFCXXXX 1428 Mux Category: NORMAL 1430 10.1.8. ice-pacing Attribute 1432 This specification also defines a new SDP attribute, "ice-pacing" 1433 according to the following data: 1435 Attribute Name: ice-pacing 1437 Type of Attribute: session-level 1439 Subject to charset: No 1441 Purpose: This attribute is used with Interactive Connectivity 1442 Establishment (ICE) to indicate desired connectivity check pacing 1443 values. 1445 Appropriate Values: See Section 4 of RFC XXXX. 1447 Contact Name: IESG 1449 Contact e-mail: iesg@ietf.org [8] 1451 Reference: RFCXXXX 1453 Mux Category: NORMAL 1455 10.2. Interactive Connectivity Establishment (ICE) Options Registry 1457 IANA maintains a registry for ice-options identifiers under the 1458 Specification Required policy as defined in "Guidelines for Writing 1459 an IANA Considerations Section in RFCs" [RFC5226]. 1461 ICE options are of unlimited length according to the syntax in 1462 Section 4.6; however, they are RECOMMENDED to be no longer than 20 1463 characters. This is to reduce message sizes and allow for efficient 1464 parsing. 1466 In [RFC5245] ICE options could only be defined at the session level. 1467 ICE options can now also be defined at the media level. This can be 1468 used when aggregating between different ICE agents in the same 1469 endpoint, but future options may require to be defined at the media- 1470 level. To ensure compatibility with legacy implementation, the 1471 media-level ICE options MUST be aggregated into a session-level ICE 1472 option. Because aggregation rules depend on the specifics of each 1473 option, all new ICE options MUST also define in their specification 1474 how the media-level ICE option values are aggregated to generate the 1475 value of the session-level ICE option. 1477 [RFC6679] defines the "rtp+ecn" ICE option. The aggregation rule for 1478 this ICE option is that if all aggregated media using ICE contain a 1479 media-level "rtp+ecn" ICE option then an "rtp+ecn" ICE option MUST be 1480 inserted at the session-level. If one of the media does not contain 1481 the option, then it MUST NOT be inserted at the session-level. 1483 Section 10 of [ICE-BIS] defines "ice2" ICE option. Since "ice2" is a 1484 session level ICE option, no aggregation rules apply. 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 11. 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 4 that aligns with [ICE-BIS] 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 and the MMUSIC WG. 1519 12. References 1521 12.1. Normative References 1523 [ICE-BIS] Keranen, A. and J. Rosenberg, "Interactive Connectivity 1524 Establishment (ICE): A Protocol for Network Address 1525 Translator (NAT) Traversal for Offer/Answer Protocols", 1526 draft-ietf-ice-rfc5245bis-00 (work in progress), March 1527 2015. 1529 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 1530 Requirement Levels", BCP 14, RFC 2119, 1531 DOI 10.17487/RFC2119, March 1997, 1532 . 1534 [RFC3261] Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston, 1535 A., Peterson, J., Sparks, R., Handley, M., and E. 1536 Schooler, "SIP: Session Initiation Protocol", RFC 3261, 1537 DOI 10.17487/RFC3261, June 2002, 1538 . 1540 [RFC3262] Rosenberg, J. and H. Schulzrinne, "Reliability of 1541 Provisional Responses in Session Initiation Protocol 1542 (SIP)", RFC 3262, DOI 10.17487/RFC3262, June 2002, 1543 . 1545 [RFC3264] Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model 1546 with Session Description Protocol (SDP)", RFC 3264, 1547 DOI 10.17487/RFC3264, June 2002, 1548 . 1550 [RFC3312] Camarillo, G., Ed., Marshall, W., Ed., and J. Rosenberg, 1551 "Integration of Resource Management and Session Initiation 1552 Protocol (SIP)", RFC 3312, DOI 10.17487/RFC3312, October 1553 2002, . 1555 [RFC3556] Casner, S., "Session Description Protocol (SDP) Bandwidth 1556 Modifiers for RTP Control Protocol (RTCP) Bandwidth", 1557 RFC 3556, DOI 10.17487/RFC3556, July 2003, 1558 . 1560 [RFC3605] Huitema, C., "Real Time Control Protocol (RTCP) attribute 1561 in Session Description Protocol (SDP)", RFC 3605, 1562 DOI 10.17487/RFC3605, October 2003, 1563 . 1565 [RFC4032] Camarillo, G. and P. Kyzivat, "Update to the Session 1566 Initiation Protocol (SIP) Preconditions Framework", 1567 RFC 4032, DOI 10.17487/RFC4032, March 2005, 1568 . 1570 [RFC4091] Camarillo, G. and J. Rosenberg, "The Alternative Network 1571 Address Types (ANAT) Semantics for the Session Description 1572 Protocol (SDP) Grouping Framework", RFC 4091, June 2005, 1573 . 1575 [RFC4092] Camarillo, G. and J. Rosenberg, "Usage of the Session 1576 Description Protocol (SDP) Alternative Network Address 1577 Types (ANAT) Semantics in the Session Initiation Protocol 1578 (SIP)", RFC 4092, June 2005, 1579 . 1581 [RFC4566] Handley, M., Jacobson, V., and C. Perkins, "SDP: Session 1582 Description Protocol", RFC 4566, DOI 10.17487/RFC4566, 1583 July 2006, . 1585 [RFC5226] Narten, T. and H. Alvestrand, "Guidelines for Writing an 1586 IANA Considerations Section in RFCs", BCP 26, RFC 5226, 1587 DOI 10.17487/RFC5226, May 2008, 1588 . 1590 [RFC5234] Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax 1591 Specifications: ABNF", STD 68, RFC 5234, 1592 DOI 10.17487/RFC5234, January 2008, 1593 . 1595 [RFC5245] Rosenberg, J., "Interactive Connectivity Establishment 1596 (ICE): A Protocol for Network Address Translator (NAT) 1597 Traversal for Offer/Answer Protocols", RFC 5245, 1598 DOI 10.17487/RFC5245, April 2010, 1599 . 1601 [RFC5768] Rosenberg, J., "Indicating Support for Interactive 1602 Connectivity Establishment (ICE) in the Session Initiation 1603 Protocol (SIP)", RFC 5768, DOI 10.17487/RFC5768, April 1604 2010, . 1606 [RFC6336] Westerlund, M. and C. Perkins, "IANA Registry for 1607 Interactive Connectivity Establishment (ICE) Options", 1608 RFC 6336, April 2010, 1609 . 1611 [RFC6679] Westerlund, M., Johansson, I., Perkins, C., O'Hanlon, P., 1612 and K. Carlberg, "Explicit Congestion Notification (ECN) 1613 for RTP over UDP", RFC 6679, DOI 10.17487/RFC6679, August 1614 2012, . 1616 [RFC6724] Thaler, D., Draves, R., Matsumoto, A., and T. Chown, 1617 "Default Address Selection for Internet Protocol Version 6 1618 (IPv6)", RFC 6724, September 2012, 1619 . 1621 12.2. Informative References 1623 [RFC3725] Rosenberg, J., Peterson, J., Schulzrinne, H., and G. 1624 Camarillo, "Best Current Practices for Third Party Call 1625 Control (3pcc) in the Session Initiation Protocol (SIP)", 1626 BCP 85, RFC 3725, DOI 10.17487/RFC3725, April 2004, 1627 . 1629 [RFC3960] Camarillo, G. and H. Schulzrinne, "Early Media and Ringing 1630 Tone Generation in the Session Initiation Protocol (SIP)", 1631 RFC 3960, DOI 10.17487/RFC3960, December 2004, 1632 . 1634 [RFC5626] Jennings, C., Ed., Mahy, R., Ed., and F. Audet, Ed., 1635 "Managing Client-Initiated Connections in the Session 1636 Initiation Protocol (SIP)", RFC 5626, 1637 DOI 10.17487/RFC5626, October 2009, 1638 . 1640 [RFC5898] Andreasen, F., Camarillo, G., Oran, D., and D. Wing, 1641 "Connectivity Preconditions for Session Description 1642 Protocol (SDP) Media Streams", RFC 5898, 1643 DOI 10.17487/RFC5898, July 2010, 1644 . 1646 Appendix A. Examples 1648 For the example shown in section 16 of [ICE-BIS] the resulting offer 1649 (message 5) encoded in SDP looks like: 1651 v=0 1652 o=jdoe 2890844526 2890842807 IN IP6 $L-PRIV-1.IP 1653 s= 1654 c=IN IP6 $NAT-PUB-1.IP 1655 t=0 0 1656 a=ice-pwd:asd88fgpdd777uzjYhagZg 1657 a=ice-ufrag:8hhY 1658 m=audio $NAT-PUB-1.PORT RTP/AVP 0 1659 b=RS:0 1660 b=RR:0 1661 a=rtpmap:0 PCMU/8000 1662 a=candidate:1 1 UDP 2130706431 $L-PRIV-1.IP $L-PRIV-1.PORT typ host 1663 a=candidate:2 1 UDP 1694498815 $NAT-PUB-1.IP $NAT-PUB-1.PORT typ 1664 srflx raddr $L-PRIV-1.IP rport $L-PRIV-1.PORT 1666 The offer, with the variables replaced with their values, will look 1667 like (lines folded for clarity): 1669 v=0 1670 o=jdoe 2890844526 2890842807 IN IP6 fe80::6676:baff:fe9c:ee4a 1671 s= 1672 c=IN IP6 2001:420:c0e0:1005::61 1673 t=0 0 1674 a=ice-pwd:asd88fgpdd777uzjYhagZg 1675 a=ice-ufrag:8hhY 1676 m=audio 45664 RTP/AVP 0 1677 b=RS:0 1678 b=RR:0 1679 a=rtpmap:0 PCMU/8000 1680 a=candidate:1 1 UDP 2130706431 fe80::6676:baff:fe9c:ee4a 8998 typ host 1681 a=candidate:2 1 UDP 1694498815 2001:420:c0e0:1005::61 45664 typ srflx raddr 1682 fe80::6676:baff:fe9c:ee4a rport 8998 1684 The resulting answer looks like: 1686 v=0 1687 o=bob 2808844564 2808844564 IN IP4 $R-PUB-1.IP 1688 s= 1689 c=IN IP4 $R-PUB-1.IP 1690 t=0 0 1691 a=ice-pwd:YH75Fviy6338Vbrhrlp8Yh 1692 a=ice-ufrag:9uB6 1693 m=audio $R-PUB-1.PORT RTP/AVP 0 1694 b=RS:0 1695 b=RR:0 1696 a=rtpmap:0 PCMU/8000 1697 a=candidate:1 1 UDP 2130706431 $R-PUB-1.IP $R-PUB-1.PORT typ host 1698 With the variables filled in: 1700 v=0 1701 o=bob 2808844564 2808844564 IN IP4 192.0.2.1 1702 s= 1703 c=IN IP4 192.0.2.1 1704 t=0 0 1705 a=ice-pwd:YH75Fviy6338Vbrhrlp8Yh 1706 a=ice-ufrag:9uB6 1707 m=audio 3478 RTP/AVP 0 1708 b=RS:0 1709 b=RR:0 1710 a=rtpmap:0 PCMU/8000 1711 a=candidate:1 1 UDP 2130706431 192.0.2.1 3478 typ host 1713 Appendix B. The remote-candidates Attribute 1715 The a=remote-candidates attribute exists to eliminate a race 1716 condition between the updated offer and the response to the STUN 1717 Binding request that moved a candidate into the Valid list. This 1718 race condition is shown in Figure 1. On receipt of message 4, agent 1719 L adds a candidate pair to the valid list. If there was only a 1720 single data stream with a single component, agent L could now send an 1721 updated offer. However, the check from agent R has not yet generated 1722 a response, and agent R receives the updated offer (message 7) before 1723 getting the response (message 9). Thus, it does not yet know that 1724 this particular pair is valid. To eliminate this condition, the 1725 actual candidates at R that were selected by the offerer (the remote 1726 candidates) are included in the offer itself, and the answerer delays 1727 its answer until those pairs validate. 1729 Agent L Network Agent R 1730 |(1) Offer | | 1731 |------------------------------------------>| 1732 |(2) Answer | | 1733 |<------------------------------------------| 1734 |(3) STUN Req. | | 1735 |------------------------------------------>| 1736 |(4) STUN Res. | | 1737 |<------------------------------------------| 1738 |(5) STUN Req. | | 1739 |<------------------------------------------| 1740 |(6) STUN Res. | | 1741 |-------------------->| | 1742 | |Lost | 1743 |(7) Offer | | 1744 |------------------------------------------>| 1745 |(8) STUN Req. | | 1746 |<------------------------------------------| 1747 |(9) STUN Res. | | 1748 |------------------------------------------>| 1749 |(10) Answer | | 1750 |<------------------------------------------| 1752 Figure 1: Race Condition Flow 1754 Appendix C. Why Is the Conflict Resolution Mechanism Needed? 1756 When ICE runs between two peers, one agent acts as controlled, and 1757 the other as controlling. Rules are defined as a function of 1758 implementation type and offerer/answerer to determine who is 1759 controlling and who is controlled. However, the specification 1760 mentions that, in some cases, both sides might believe they are 1761 controlling, or both sides might believe they are controlled. How 1762 can this happen? 1764 The condition when both agents believe they are controlled shows up 1765 in third party call control cases. Consider the following flow: 1767 A Controller B 1768 |(1) INV() | | 1769 |<-------------| | 1770 |(2) 200(SDP1) | | 1771 |------------->| | 1772 | |(3) INV() | 1773 | |------------->| 1774 | |(4) 200(SDP2) | 1775 | |<-------------| 1776 |(5) ACK(SDP2) | | 1777 |<-------------| | 1778 | |(6) ACK(SDP1) | 1779 | |------------->| 1781 Figure 2: Role Conflict Flow 1783 This flow is a variation on flow III of RFC 3725 [RFC3725]. In fact, 1784 it works better than flow III since it produces fewer messages. In 1785 this flow, the controller sends an offerless INVITE to agent A, which 1786 responds with its offer, SDP1. The agent then sends an offerless 1787 INVITE to agent B, which it responds to with its offer, SDP2. The 1788 controller then uses the offer from each agent to generate the 1789 answers. When this flow is used, ICE will run between agents A and 1790 B, but both will believe they are in the controlling role. With the 1791 role conflict resolution procedures, this flow will function properly 1792 when ICE is used. 1794 At this time, there are no documented flows that can result in the 1795 case where both agents believe they are controlled. However, the 1796 conflict resolution procedures allow for this case, should a flow 1797 arise that would fit into this category. 1799 Appendix D. Why Send an Updated Offer? 1801 Section 11.1 describes rules for sending media. Both agents can send 1802 media once ICE checks complete, without waiting for an updated offer. 1803 Indeed, the only purpose of the updated offer is to "correct" the SDP 1804 so that the default destination for media matches where media is 1805 being sent based on ICE procedures (which will be the highest- 1806 priority nominated candidate pair). 1808 This begs the question -- why is the updated offer/answer exchange 1809 needed at all? Indeed, in a pure offer/answer environment, it would 1810 not be. The offerer and answerer will agree on the candidates to use 1811 through ICE, and then can begin using them. As far as the agents 1812 themselves are concerned, the updated offer/answer provides no new 1813 information. However, in practice, numerous components along the 1814 signaling path look at the SDP information. These include entities 1815 performing off-path QoS reservations, NAT traversal components such 1816 as ALGs and Session Border Controllers (SBCs), and diagnostic tools 1817 that passively monitor the network. For these tools to continue to 1818 function without change, the core property of SDP -- that the 1819 existing, pre-ICE definitions of the addresses used for media -- the 1820 "m=" and "c=" lines and the rtcp attribute -- must be retained. For 1821 this reason, an updated offer must be sent. 1823 Appendix E. Contributors 1825 Following experts have contributed textual and structural 1826 improvements for this work 1828 1. Christer Holmberg 1830 * Ericsson 1832 * Email: christer.holmberg@ericsson.com [9] 1834 2. Roman Shpount 1836 * TurboBridge 1838 * rshpount@turbobridge.com [10] 1840 3. Thomas Stach 1842 * thomass.stach@gmail.com [11] 1844 Authors' Addresses 1846 Marc Petit-Huguenin 1847 Impedance Mismatch 1849 Email: marc@petit-huguenin.org 1851 Suhas Nandakumar 1852 Cisco Systems 1853 707 Tasman Dr 1854 Milpitas, CA 95035 1855 USA 1857 Email: snandaku@cisco.com 1858 Ari Keranen 1859 Ericsson 1860 Jorvas 02420 1861 Finland 1863 Email: ari.keranen@ericsson.com