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Run idnits with the --verbose option for more detailed information about the items above. -------------------------------------------------------------------------------- 2 Network Working Group M. Westerlund 3 Internet-Draft I. Johansson 4 Intended status: Standards Track Ericsson 5 Expires: August 20, 2012 C. Perkins 6 University of Glasgow 7 P. O'Hanlon 8 UCL 9 K. Carlberg 10 G11 11 February 17, 2012 13 Explicit Congestion Notification (ECN) for RTP over UDP 14 draft-ietf-avtcore-ecn-for-rtp-06 16 Abstract 18 This memo specifies how Explicit Congestion Notification (ECN) can be 19 used with the Real-time Transport Protocol (RTP) running over UDP, 20 using RTP Control Protocol (RTCP) as a feedback mechanism. It 21 defines a new RTCP Extended Report (XR) block for periodic ECN 22 feedback, a new RTCP transport feedback message for timely reporting 23 of congestion events, and a Session Traversal Utilities for NAT 24 (STUN) extension used in the optional initialization method using 25 Interactive Connectivity Establishment (ICE). Signalling and 26 procedures for negotiation of capabilities and initialization methods 27 are also defined. 29 Status of this Memo 31 This Internet-Draft is submitted in full conformance with the 32 provisions of BCP 78 and BCP 79. 34 Internet-Drafts are working documents of the Internet Engineering 35 Task Force (IETF). Note that other groups may also distribute 36 working documents as Internet-Drafts. The list of current Internet- 37 Drafts is at http://datatracker.ietf.org/drafts/current/. 39 Internet-Drafts are draft documents valid for a maximum of six months 40 and may be updated, replaced, or obsoleted by other documents at any 41 time. It is inappropriate to use Internet-Drafts as reference 42 material or to cite them other than as "work in progress." 44 This Internet-Draft will expire on August 20, 2012. 46 Copyright Notice 48 Copyright (c) 2012 IETF Trust and the persons identified as the 49 document authors. All rights reserved. 51 This document is subject to BCP 78 and the IETF Trust's Legal 52 Provisions Relating to IETF Documents 53 (http://trustee.ietf.org/license-info) in effect on the date of 54 publication of this document. Please review these documents 55 carefully, as they describe your rights and restrictions with respect 56 to this document. Code Components extracted from this document must 57 include Simplified BSD License text as described in Section 4.e of 58 the Trust Legal Provisions and are provided without warranty as 59 described in the Simplified BSD License. 61 Table of Contents 63 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4 64 2. Conventions, Definitions and Acronyms . . . . . . . . . . . . 5 65 3. Discussion, Requirements, and Design Rationale . . . . . . . . 6 66 3.1. Requirements . . . . . . . . . . . . . . . . . . . . . . . 7 67 3.2. Applicability . . . . . . . . . . . . . . . . . . . . . . 8 68 3.3. Interoperability . . . . . . . . . . . . . . . . . . . . . 12 69 4. Overview of Use of ECN with RTP/UDP/IP . . . . . . . . . . . . 13 70 5. RTCP Extensions for ECN feedback . . . . . . . . . . . . . . . 16 71 5.1. RTP/AVPF Transport Layer ECN Feedback packet . . . . . . . 16 72 5.2. RTCP XR Report block for ECN summary information . . . . . 19 73 6. SDP Signalling Extensions for ECN . . . . . . . . . . . . . . 21 74 6.1. Signalling ECN Capability using SDP . . . . . . . . . . . 21 75 6.2. RTCP ECN Feedback SDP Parameter . . . . . . . . . . . . . 25 76 6.3. XR Block ECN SDP Parameter . . . . . . . . . . . . . . . . 25 77 6.4. ICE Parameter to Signal ECN Capability . . . . . . . . . . 26 78 7. Use of ECN with RTP/UDP/IP . . . . . . . . . . . . . . . . . . 26 79 7.1. Negotiation of ECN Capability . . . . . . . . . . . . . . 26 80 7.2. Initiation of ECN Use in an RTP Session . . . . . . . . . 27 81 7.3. Ongoing Use of ECN Within an RTP Session . . . . . . . . . 34 82 7.4. Detecting Failures . . . . . . . . . . . . . . . . . . . . 37 83 8. Processing ECN in RTP Translators and Mixers . . . . . . . . . 41 84 8.1. Transport Translators . . . . . . . . . . . . . . . . . . 41 85 8.2. Fragmentation and Reassembly in Translators . . . . . . . 41 86 8.3. Generating RTCP ECN Feedback in Media Transcoders . . . . 44 87 8.4. Generating RTCP ECN Feedback in Mixers . . . . . . . . . . 45 88 9. Implementation considerations . . . . . . . . . . . . . . . . 45 89 10. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 45 90 10.1. SDP Attribute Registration . . . . . . . . . . . . . . . . 46 91 10.2. RTP/AVPF Transport Layer Feedback Message . . . . . . . . 46 92 10.3. RTCP Feedback SDP Parameter . . . . . . . . . . . . . . . 46 93 10.4. RTCP XR Report blocks . . . . . . . . . . . . . . . . . . 46 94 10.5. RTCP XR SDP Parameter . . . . . . . . . . . . . . . . . . 47 95 10.6. STUN attribute . . . . . . . . . . . . . . . . . . . . . . 47 96 10.7. ICE Option . . . . . . . . . . . . . . . . . . . . . . . . 47 97 11. Security Considerations . . . . . . . . . . . . . . . . . . . 47 98 12. Examples of SDP Signalling . . . . . . . . . . . . . . . . . . 50 99 12.1. Basic SDP Offer/Answer . . . . . . . . . . . . . . . . . . 50 100 12.2. Declarative Multicast SDP . . . . . . . . . . . . . . . . 51 101 13. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 52 102 14. References . . . . . . . . . . . . . . . . . . . . . . . . . . 52 103 14.1. Normative References . . . . . . . . . . . . . . . . . . . 52 104 14.2. Informative References . . . . . . . . . . . . . . . . . . 53 105 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 55 107 1. Introduction 109 This memo outlines how Explicit Congestion Notification (ECN) 110 [RFC3168] can be used for Real-time Transport Protocol (RTP) 111 [RFC3550] flows running over UDP/IP which use RTP Control Protocol 112 (RTCP) as a feedback mechanism. The solution consists of feedback of 113 ECN congestion experienced markings to the sender using RTCP, 114 verification of ECN functionality end-to-end, and procedures for how 115 to initiate ECN usage. Since the initiation process has some 116 dependencies on the signalling mechanism used to establish the RTP 117 session, a specification for signalling mechanisms using Session 118 Description Protocol (SDP) [RFC4566] is included. 120 ECN is getting attention as a method to minimise the impact of 121 congestion on real-time multimedia traffic. The use of ECN provides 122 a way for the network to send a congestion control signal to a media 123 transport without having to impair the media. Unlike packet loss, 124 ECN signals unambiguously indicate congestion to the transport as 125 quickly as feedback delays allow, and without confusing congestion 126 with losses that might have occurred for other reasons such as 127 transmission errors, packet-size errors, routing errors, badly 128 implemented middleboxes, policy violations and so forth. 130 The introduction of ECN into the Internet requires changes to both 131 the network and transport layers. At the network layer, IP 132 forwarding has to be updated to allow routers to mark packets, rather 133 than discarding them in times of congestion [RFC3168]. In addition, 134 transport protocols have to be modified to inform the sender that ECN 135 marked packets are being received, so it can respond to the 136 congestion. The Transmission Control Protocol (TCP) [RFC3168], 137 Stream Control Transmission Protocol (SCTP) [RFC4960] and Datagram 138 Congestion Control Protocol (DCCP) [RFC4340] have been updated to 139 support ECN, but to date there is no specification how UDP-based 140 transports, such as RTP [RFC3550], can use ECN. This is due to the 141 lack of feedback mechanisms directly in UDP. Instead the signaling 142 control protocol on top of UDP needs to provide that feedback. For 143 RTP that feedback is provided by RTCP. 145 The remainder of this memo is structured as follows. We start by 146 describing the conventions, definitions and acronyms used in this 147 memo in Section 2, and the design rationale and applicability in 148 Section 3. Section 4 gives an overview of how ECN is used with RTP 149 over UDP. RTCP extensions for ECN feedback are defined in Section 5, 150 and SDP signalling extensions in Section 6. The details of how ECN 151 is used with RTP over UDP are defined in Section 7. In Section 8 we 152 describe how ECN is handled in RTP translators and mixers. Section 9 153 discusses some implementation considerations, Section 10 lists IANA 154 considerations, and Section 11 discusses security considerations. 156 2. Conventions, Definitions and Acronyms 158 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 159 "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and 160 "OPTIONAL" in this document are to be interpreted as described in RFC 161 2119 [RFC2119]. 163 Abbreviations and Definitions: 165 Sender: A sender of RTP packets carrying an encoded media stream. 166 The sender can change how the media transmission is performed by 167 varying the media coding or packetisation. It is one end-point of 168 the ECN control loop. 170 Receiver: A receiver of RTP packets with the intention to consume 171 the media stream. It sends RTCP feedback on the received stream. 172 It is the other end-point of the ECN control loop. 174 ECN Capable Host: A sender or receiver of a media stream that is 175 capable of setting and/or processing ECN marks. 177 ECN Capable Transport (ECT): A transport flow where both sender and 178 receiver are ECN capable hosts. Packets sent by an ECN Capable 179 Transport will be marked as ECT(0) or ECT(1) on transmission. See 180 [RFC3168] for the definition of the ECT(0) and ECT(1) marks. 182 ECN-CE: ECN Congestion Experienced mark (see [RFC3168]). 184 ECN Capable Packets: Packets with ECN mark set to either ECT(0), 185 ECT(1) or ECN-CE. 187 Not-ECT packets: Packets that are not sent by an ECN capable 188 transport, and are not ECN-CE marked. 190 ECN Oblivious Relay: A router or middlebox that treats ECN Capable 191 Packets no differently from Not-ECT packets. 193 ECN Capable Queue: A queue that supports ECN-CE marking of ECN- 194 Capable Packets to indicate congestion. 196 ECN Blocking Middlebox: A middlebox that discards ECN-Capable 197 Packets. 199 ECN Reverting Middlebox: A middlebox that changes ECN-Capable 200 Packets to Not-ECT packets by removing the ECN mark. 202 Note that RTP mixers or translators that operate in such a manner 203 that they terminate or split the ECN control loop will take on the 204 role of receivers or senders. This is further discussed in 205 Section 3.2. 207 3. Discussion, Requirements, and Design Rationale 209 ECN has been specified for use with TCP [RFC3168], SCTP [RFC4960], 210 and DCCP [RFC4340] transports. These are all unicast protocols which 211 negotiate the use of ECN during the initial connection establishment 212 handshake (supporting incremental deployment, and checking if ECN 213 marked packets pass all middleboxes on the path). ECN-CE marks are 214 immediately echoed back to the sender by the receiving end-point 215 using an additional bit in feedback messages, and the sender then 216 interprets the mark as equivalent to a packet loss for congestion 217 control purposes. 219 If RTP is run over TCP, SCTP, or DCCP, it can use the native ECN 220 support provided by those protocols. This memo does not concern 221 itself further with these use cases. However, RTP is more commonly 222 run over UDP. This combination does not currently support ECN, and 223 we observe that it has significant differences from the other 224 transport protocols for which ECN has been specified. These include: 226 Signalling: RTP relies on separate signalling protocols to negotiate 227 parameters before a session can be created, and doesn't include an 228 in-band handshake or negotiation at session set-up time (i.e., 229 there is no equivalent to the TCP three-way handshake in RTP). 231 Feedback: RTP does not explicitly acknowledge receipt of datagrams. 232 Instead, the RTP Control Protocol (RTCP) provides reception 233 quality feedback, and other back channel communication, for RTP 234 sessions. The feedback interval is generally on the order of 235 seconds, rather than once per network RTT (although the RTP/AVPF 236 profile [RFC4585] allows more rapid feedback in most cases). RTCP 237 is also very much oriented around counting packets, which makes 238 byte counting congestion algorithms difficult to utilize. 240 Congestion Response: While it is possible to adapt the transmission 241 of many audio/visual streams in response to network congestion, 242 and such adaptation is required by [RFC3550], the dynamics of the 243 congestion response may be quite different to those of TCP or 244 other transport protocols. 246 Middleboxes: The RTP framework explicitly supports the concept of 247 mixers and translators, which are middleboxes that are involved in 248 media transport functions. 250 Multicast: RTP is explicitly a group communication protocol, and was 251 designed from the start to support IP multicast (primarily Any 252 Source Multicast (ASM) [RFC1112], although a recent extension 253 supports Source Specific Multicast (SSM) [RFC3569] with unicast 254 feedback [RFC5760]). 256 Application Awareness: When ECN support is provided within the 257 transport protocol, the ability of the application to react to 258 congestion is limited, since it has little visibility into the 259 transport layer. By adding support of ECN to RTP using RTCP 260 feedback, the application is made aware of congestion, allowing a 261 wider range of reactions in response to that loss. 263 Counting vs Detecting Congestion: TCP, and the protocols derived 264 from, it are mainly designed to respond in the same way whether 265 they experience a burst of congestion indications within one RTT, 266 or just a single congestion indication. Whereas real-time 267 applications may be concerned with the amount of congestion 268 experienced, whether it is distributed smoothly or in bursts. 269 When feedback of ECN was added to TCP [RFC3168], the receiver was 270 designed to flip the echo congestion experienced (ECE) flag to 1 271 for a whole RTT then flop it back to zero. Whereas ECN feedback 272 in RTCP will need to report a count of how much congestion has 273 been experienced within an RTCP reporting period, irrespective of 274 round trip times. 276 These differences will significantly alter the shape of ECN support 277 in RTP-over-UDP compared to ECN support in TCP, SCTP, and DCCP, but 278 do not invalidate the need for ECN support. 280 ECN support is more important for RTP sessions than, for instance, is 281 the case for TCP. This is because the impact of packet loss in real- 282 time audio-visual media flows is highly visible to users. Effective 283 ECN support for RTP flows running over UDP will allow real-time 284 audio-visual applications to respond to the onset of congestion 285 before routers are forced to drop packets, allowing those 286 applications to control how they reduce their transmission rate, and 287 hence media quality, rather than responding to, and trying to conceal 288 the effects of unpredictable packet loss. Furthermore, widespread 289 deployment for ECN and active queue management in routers, should it 290 occur, can potentially reduce unnecessary queueing delays in routers, 291 lowering the round-trip time and benefiting interactive applications 292 of RTP, such as voice telephony. 294 3.1. Requirements 296 Considering ECN, transport protocols supporting ECN, and RTP based 297 applications one can create a set of requirements that must be 298 satisfied to at least some degree if ECN is to used by RTP over UDP. 300 o REQ 1: A mechanism MUST exist to negotiate and initiate the use of 301 ECN for RTP/UDP/IP sessions so that an RTP sender will not send 302 packets with ECT in the IP header unless it knows that all 303 potential receivers will understand any ECN-CE indications they 304 might receive. 306 o REQ 2: A mechanism MUST exist to feed back the reception of any 307 packets that are ECN-CE marked to the packet sender. 309 o REQ 3: The provided mechanism SHOULD minimise the possibility of 310 cheating (either by the sender or receiver). 312 o REQ 4: Some detection and fallback mechanism SHOULD exist to avoid 313 loss of communication due to the attempted usage of ECN in case an 314 intermediate node clears ECT or drops packets that are ECT marked. 316 o REQ 5: Negotiation of ECN SHOULD NOT significantly increase the 317 time taken to negotiate and set-up the RTP session (an extra RTT 318 before the media can flow is unlikely to be acceptable for some 319 use cases). 321 o REQ 6: Negotiation of ECN SHOULD NOT cause media clipping at the 322 start of a session. 324 The following sections describes how these requirements can be met 325 for RTP over UDP. 327 3.2. Applicability 329 The use of ECN with RTP over UDP is dependent on negotiation of ECN 330 capability between the sender and receiver(s), and validation of ECN 331 support in all elements of the network path(s) traversed. RTP is 332 used in a heterogeneous range of network environments and topologies, 333 with various different signalling protocols. The mechanisms defined 334 here make it possible to verify support for ECN in each of these 335 environments, and irrespective of the topology. 337 Due to the need for each RTP sender that intends to use ECN with RTP 338 to track all participants in the RTP session, the sub-sampling of the 339 group membership as specified by "Sampling of the Group Membership in 340 RTP" [RFC2762] MUST NOT be used. 342 The use of ECN is further dependent on a capability of the RTP media 343 flow to react to congestion signalled by ECN marked packets. 344 Depending on the application, media codec, and network topology, this 345 adaptation can occur in various forms and at various nodes. As an 346 example, the sender can change the media encoding, or the receiver 347 can change the subscription to a layered encoding, or either reaction 348 can be accomplished by a transcoding middlebox. RFC 5117 identifies 349 seven topologies in which RTP sessions may be configured, and which 350 may affect the ability to use ECN: 352 Topo-Point-to-Point: This utilises standard unicast flows. ECN may 353 be used with RTP in this topology in an analogous manner to its 354 use with other unicast transport protocols, with RTCP conveying 355 ECN feedback messages. 357 Topo-Multicast: This is either an any source multicast (ASM) group 358 [RFC3569] with potentially several active senders and multicast 359 RTCP feedback, or a source specific multicast (SSM) group 360 [RFC4607] with a single distribution source and unicast RTCP 361 feedback from receivers. RTCP is designed to scale to large group 362 sizes while avoiding feedback implosion (see Section 6.2 of 363 [RFC3550], [RFC4585], and [RFC5760]), and can be used by a sender 364 to determine if all its receivers, and the network paths to those 365 receivers, support ECN (see Section 7.2). It is somewhat more 366 difficult to determine if all network paths from all senders to 367 all receivers support ECN. Accordingly, we allow ECN to be used 368 by an RTP sender using multicast UDP provided the sender has 369 verified that the paths to all its known receivers support ECN, 370 and irrespective of whether the paths from other senders to their 371 receivers support ECN ("all its known receivers" are all the SSRCs 372 that the RTP sender has received RTP or RTCP from the last five 373 reporting intervals, i.e., they have not timed out). Note that 374 group membership may change during the lifetime of a multicast RTP 375 session, potentially introducing new receivers that are not ECN 376 capable or have a path that doesn't support ECN. Senders must use 377 the mechanisms described in Section 7.4 to check that all 378 receivers, and the network paths traversed to reach those 379 receivers, continue to support ECN, and they need to fallback to 380 non-ECN use if any receivers join that do not. 382 SSM groups that uses unicast RTCP feedback [RFC5760] do need a few 383 extra considerations. This topology can have multiple media 384 senders that provides traffic to the distribution source (DS) and 385 are separated from the DS. There can also be multiple feedback 386 targets. The requirement for using ECN for RTP in this topology 387 is that the media sender must be provided the feedback from the 388 receivers, it may be in aggregated form from the feedback targets. 389 We will not mention this SSM use case in the below text 390 specifically, but when actions are required by the media source, 391 they do apply also to case of SSM where the RTCP feedback goes to 392 the Feedback Target. 394 The mechanisms defined in this memo support multicast groups, but 395 are known to be conservative, and don't scale to large groups. 396 This is primarily because we require all members of the group to 397 demonstrate that they can make use of ECN before the sender is 398 allowed to send ECN-marked packets, since allowing some non-ECN 399 capable receivers causes fairness issues when the bottleneck link 400 is shared by ECN and non-ECN flows that we have not (yet) been 401 able to satisfactorily address. The rules regarding Determination 402 of ECN Support in Section 7.2.1 may be relaxed in a future version 403 of this specification to improve scaling once these issues have 404 been resolved. 406 Topo-Translator: An RTP translator is an RTP-level middlebox that is 407 invisible to the other participants in the RTP session (although 408 it is usually visible in the associated signalling session). 409 There are two types of RTP translator: those that do not modify 410 the media stream, and are concerned with transport parameters, for 411 example a multicast to unicast gateway; and those that do modify 412 the media stream, for example transcoding between different media 413 codecs. A single RTP session traverses the translator, and the 414 translator must rewrite RTCP messages passing through it to match 415 the changes it makes to the RTP data packets. A legacy, ECN- 416 unaware, RTP translator is expected to ignore the ECN bits on 417 received packets, and to set the ECN bits to not-ECT when sending 418 packets, so causing ECN negotiation on the path containing the 419 translator to fail (any new RTP translator that does not wish to 420 support ECN may do so similarly). An ECN aware RTP translator may 421 act in one of three ways: 423 * If the translator does not modify the media stream, it should 424 copy the ECN bits unchanged from the incoming to the outgoing 425 datagrams, unless it is overloaded and experiencing congestion, 426 in which case it may mark the outgoing datagrams with an ECN-CE 427 mark. Such a translator passes RTCP feedback unchanged. See 428 Section 8.1. 430 * If the translator modifies the media stream to combine or split 431 RTP packets, but does not otherwise transcode the media, it 432 must manage the ECN bits in a way analogous to that described 433 in Section 5.3 of [RFC3168], see Section 8.2 for details. 435 * If the translator is a media transcoder, or otherwise modifies 436 the content of the media stream, the output RTP media stream 437 may have radically different characteristics than the input RTP 438 media stream. Each side of the translator must then be 439 considered as a separate transport connection, with its own ECN 440 processing. This requires the translator interpose itself into 441 the ECN negotiation process, effectively splitting the 442 connection into two parts with their own negotiation. Once 443 negotiation has been completed, the translator must generate 444 RTCP ECN feedback back to the source based on its own 445 reception, and must respond to RTCP ECN feedback received from 446 the receiver(s) (see Section 8.3). 448 It is recognised that ECN and RTCP processing in an RTP translator 449 that modifies the media stream is non-trivial. 451 Topo-Mixer: A mixer is an RTP-level middlebox that aggregates 452 multiple RTP streams, mixing them together to generate a new RTP 453 stream. The mixer is visible to the other participants in the RTP 454 session, and is also usually visible in the associated signalling 455 session. The RTP flows on each side of the mixer are treated 456 independently for ECN purposes, with the mixer generating its own 457 RTCP ECN feedback, and responding to ECN feedback for data it 458 sends. Since unicast transport between the mixer and any end- 459 point are treated independently, it would seem reasonable to allow 460 the transport on one side of the mixer to use ECN, while the 461 transport on the other side of the mixer is not ECN capable, if 462 this is desired. See Section 8.4 for details in how mixers should 463 process ECN. 465 Topo-Video-switch-MCU: A video switching MCU receives several RTP 466 flows, but forwards only one of those flows onwards to the other 467 participants at a time. The flow that is forwarded changes during 468 the session, often based on voice activity. Since only a subset 469 of the RTP packets generated by a sender are forwarded to the 470 receivers, a video switching MCU can break ECN negotiation (the 471 success of the ECN negotiation may depend on the voice activity of 472 the participant at the instant the negotiation takes place - shout 473 if you want ECN). It also breaks congestion feedback and 474 response, since RTP packets are dropped by the MCU depending on 475 voice activity rather than network congestion. This topology is 476 widely used in legacy products, but is NOT RECOMMENDED for new 477 implementations and SHALL NOT be used with ECN. 479 Topo-RTCP-terminating-MCU: In this scenario, each participant runs 480 an RTP point-to-point session between itself and the MCU. Each of 481 these sessions is treated independently for the purposes of ECN 482 and RTCP feedback, potentially with some using ECN and some not. 484 Topo-Asymmetric: It is theoretically possible to build a middlebox 485 that is a combination of an RTP mixer in one direction and an RTP 486 translator in the other. To quote RFC 5117 "This topology is so 487 problematic and it is so easy to get the RTCP processing wrong, 488 that it is NOT RECOMMENDED to implement this topology." 490 These topologies may be combined within a single RTP session. 492 The ECN mechanism defined in this memo is applicable to both sender 493 and receiver controlled congestion algorithms. The mechanism ensures 494 that both senders and receivers will know about ECN-CE markings and 495 any packet losses. Thus the actual decision point for the congestion 496 control is not relevant. This is a great benefit as the rate of an 497 RTP session can be varied in a number of ways, for example a unicast 498 media sender might use TFRC [RFC5348] or some other algorithm, while 499 a multicast session could use a sender based scheme adapting to the 500 lowest common supported rate, or a receiver driven mechanism using 501 layered coding to support more heterogeneous paths. 503 To ensure timely feedback of ECN-CE marked packets when needed, this 504 mechanism requires support for the RTP/AVPF profile [RFC4585] or any 505 of its derivatives, such as RTP/SAVPF [RFC5124]. The standard RTP/ 506 AVP profile [RFC3551] does not allow any early or immediate 507 transmission of RTCP feedback, and has a minimal RTCP interval whose 508 default value (5 seconds) is many times the normal RTT between sender 509 and receiver. 511 3.3. Interoperability 513 The interoperability requirements for this specification are that 514 there is at least one common interoperability point for all 515 implementations. Since initialization using RTP and RTCP 516 (Section 7.2.1) is the one method that works in all cases, although 517 is not optimal for all uses, it is selected as mandatory to implement 518 this initialisation method. This method requires both the RTCP XR 519 extension and the ECN feedback format, which require the RTP/AVPF 520 profile to ensure timely feedback. 522 When one considers all the uses of ECN for RTP it is clear that there 523 exist congestion control mechanisms that are receiver driven only 524 (Section 7.3.3). These congestion control mechanism do not require 525 timely feedback of congestion events to the sender. If such a 526 congestion control mechanism is combined with an initialization 527 method that also doesn't require timely feedback using RTCP, like the 528 leap of faith or the ICE based method then neither the ECN feedback 529 format nor the RTP/AVPF profile would appear to be needed. However, 530 fault detection can be greatly improved by using receiver side 531 detection (Section 7.4.1) and early reporting of such cases using the 532 ECN feedback mechanism. 534 For interoperability we mandate the implementation of the RTP/AVPF 535 profile, with both RTCP extensions and the necessary signalling to 536 support a common operations mode. This specification recommends the 537 use of RTP/AVPF in all cases as negotiation of the common 538 interoperability point requires RTP/AVPF, mixed negotiation of RTP/ 539 AVP and RTP/AVPF depending on other SDP attributes in the same media 540 block is difficult, and the fact that fault detection can be improved 541 when using RTP/AVPF. 543 The use of the ECN feedback format is also recommended, but cases 544 exist where its use is not required due to no need for timely 545 feedback. These will be explicitly noted using the term "no timely 546 feedback required", and generally occur in combination with receiver 547 driven congestion control, and with the leap-of-faith and ICE-based 548 initialization methods. We also note that any receiver driven 549 congestion control solution that still requires RTCP for signalling 550 of any adaptation information to the sender will still require RTP/ 551 AVPF for timeliness. 553 4. Overview of Use of ECN with RTP/UDP/IP 555 The solution for using ECN with RTP over UDP/IP consists of four 556 different pieces that together make the solution work: 558 1. Negotiation of the capability to use ECN with RTP/UDP/IP 560 2. Initiation and initial verification of ECN capable transport 562 3. Ongoing use of ECN within an RTP session 564 4. Handling of dynamic behavior through failure detection, 565 verification and fallback 567 Before an RTP session can be created, a signalling protocol is used 568 to negotiate or at least configure session parameters (see 569 Section 7.1). In some topologies the signalling protocol can also be 570 used to discover the other participants. One of the parameters that 571 must be agreed is the capability of a participant to support ECN. 572 Note that all participants having the capability of supporting ECN 573 does not necessarily imply that ECN is usable in an RTP session, 574 since there may be middleboxes on the path between the participants 575 which don't pass ECN-marked packets (for example, a firewall that 576 blocks traffic with the ECN bits set). This document defines the 577 information that needs to be negotiated, and provides a mapping to 578 SDP for use in both declarative and offer/answer contexts. 580 When a sender joins a session for which all participants claim to 581 support ECN, it must verify if that support is usable. There are 582 three ways in which this verification can be done: 584 o The sender may generate a (small) subset of its RTP data packets 585 with the ECN field of the IP header set to ECT(0) or ECT(1). Each 586 receiver will then send an RTCP feedback packet indicating the 587 reception of the ECT marked RTP packets. Upon reception of this 588 feedback from each receiver it knows of, the sender can consider 589 ECN functional for its traffic. Each sender does this 590 verification independently. When a new receiver joins an existing 591 RTP session, it will send RTCP reports in the usual manner. If 592 those RTCP reports include ECN information, verification will have 593 succeeded and sources can continue to send ECT packets. If not, 594 verification fails and each sender MUST stop using ECN (see 595 Section 7.2.1 for details). 597 o Alternatively, ECN support can be verified during an initial end- 598 to-end STUN exchange (for example, as part of ICE connection 599 establishment). After having verified connectivity without ECN 600 capability an extra STUN exchange, this time with the ECN field 601 set to ECT(0) or ECT(1), is performed on the candidate path that 602 is about to be used. If successful the path's capability to 603 convey ECN marked packets is verified. A new STUN attribute is 604 defined to convey feedback that the ECT marked STUN request was 605 received (see Section 7.2.2), along with an ICE signalling option 606 (Section 6.4) to indicate that the check is to be performed. 608 o Thirdly, the sender may make a leap of faith that ECN will work. 609 This is only recommended for applications that know they are 610 running in controlled environments where ECN functionality has 611 been verified through other means. In this mode it is assumed 612 that ECN works, and the system reacts to failure indicators if the 613 assumption proved wrong. The use of this method relies on a high 614 confidence that ECN operation will be successful, or an 615 application where failure is not serious. The impact on the 616 network and other users must be considered when making a leap of 617 faith, so there are limitations on when this method is allowed 618 (see Section 7.2.3). 620 The first mechanism, using RTP with RTCP feedback, has the advantage 621 of working for all RTP sessions, but the disadvantages of potential 622 clipping if ECN marked RTP packets are discarded by middleboxes, and 623 slow verification of ECN support. The STUN-based mechanism is faster 624 to verify ECN support, but only works in those scenarios supported by 625 end-to-end STUN, such as within an ICE exchange. The third one, 626 leap-of-faith, has the advantage of avoiding additional tests or 627 complexities and enabling ECN usage from the first media packet. The 628 downside is that if the end-to-end path contains middleboxes that do 629 not pass ECN, the impact on the application can be severe: in the 630 worst case, all media could be lost if a middlebox that discards ECN 631 marked packets is present. A less severe effect, but still requiring 632 reaction, is the presence of a middlebox that re-marks ECT marked 633 packets to non-ECT, possibly marking packets with an ECN-CE mark as 634 non-ECT. This could result in increased levels of congestion due to 635 non-responsiveness, and impact media quality as applications end up 636 relying on packet loss as an indication of congestion. 638 Once ECN support has been verified (or assumed) to work for all 639 receivers, a sender marks all its RTP packets as ECT packets, while 640 receivers rapidly feed back reports on any ECN-CE marks to the sender 641 using RTCP in RTP/AVPF immediate or early feedback mode, unless no 642 timely feedback is required. Each feedback report indicates the 643 receipt of new ECN-CE marks since the last ECN feedback packet, and 644 also counts the total number of ECN-CE marked packets as a cumulative 645 sum. This is the mechanism to provide the fastest possible feedback 646 to senders about ECN-CE marks. On receipt of an ECN-CE marked 647 packet, the system must react to congestion as-if packet loss has 648 been reported. Section 7.3 describes the ongoing use of ECN within 649 an RTP session. 651 This rapid feedback is not optimised for reliability, so another 652 mechanism, RTCP XR ECN summary reports, is used to ensure more 653 reliable, but less timely, reporting of the ECN information. The ECN 654 summary report contains the same information as the ECN feedback 655 format, only packed differently for better efficiency with reports 656 for many sources. It is sent in a compound RTCP packet, along with 657 regular RTCP reception reports. By using cumulative counters for 658 observed ECN-CE, ECT, not-ECT, packet duplication, and packet loss 659 the sender can determine what events have happened since the last 660 report, independently of any RTCP packets having been lost. 662 RTCP reports MUST NOT be ECT marked, since ECT marked traffic may be 663 dropped if the path is not ECN compliant. RTCP is used to provide 664 feedback about what has been transmitted and what ECN markings that 665 are received, so it is important that it is received in cases when 666 ECT marked traffic is not getting through. 668 There are numerous reasons why the path the RTP packets take from the 669 sender to the receiver may change, e.g., mobility, link failure 670 followed by re-routing around it. Such an event may result in the 671 packet being sent through a node that is ECN non-compliant, thus re- 672 marking or dropping packets with ECT set. To prevent this from 673 impacting the application for longer than necessary, the operation of 674 ECN is constantly monitored by all senders (Section 7.4). Both the 675 RTCP XR ECN summary reports and the ECN feedback packets allow the 676 sender to compare the number of ECT(0), ECT(1), and non-ECT marked 677 packets received with the number that were sent, while also reporting 678 ECN-CE marked and lost packets. If these numbers do not agree, it 679 can be inferred that the path does not reliably pass ECN-marked 680 packets. A sender detecting a possible ECN non-compliance issue 681 should then stop sending ECT marked packets to determine if that 682 allows the packets to be correctly delivered. If the issues can be 683 connected to ECN, then ECN usage is suspended. 685 5. RTCP Extensions for ECN feedback 687 This memo defines two new RTCP extensions: one RTP/AVPF [RFC4585] 688 transport layer feedback format for reporting urgent ECN information, 689 and one RTCP XR [RFC3611] ECN summary report block type for regular 690 reporting of the ECN marking information. 692 5.1. RTP/AVPF Transport Layer ECN Feedback packet 694 This RTP/AVPF transport layer feedback format is intended for use in 695 RTP/AVPF early or immediate feedback modes when information needs to 696 urgently reach the sender. Thus its main use is to report reception 697 of an ECN-CE marked RTP packet so that the sender may perform 698 congestion control, or to speed up the initiation procedures by 699 rapidly reporting that the path can support ECN-marked traffic. The 700 feedback format is also defined with reduced size RTCP [RFC5506] in 701 mind, where RTCP feedback packets may be sent without accompanying 702 Sender or Receiver Reports that would contain the Extended Highest 703 Sequence number and the accumulated number of packet losses. Both 704 are important for ECN to verify functionality and keep track of when 705 CE marking does occur. 707 The RTP/AVPF transport layer feedback packet starts with the common 708 header defined by the RTP/AVPF profile [RFC4585] which is reproduced 709 in Figure 1. The FMT field takes the value [TBA1] to indicate that 710 the Feedback Control Information (FCI) contains ECN Feedback report, 711 as defined in Figure 2. 713 0 1 2 3 714 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 715 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 716 |V=2|P| FMT=TBA1| PT=RTPFB=205 | length | 717 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 718 | SSRC of packet sender | 719 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 720 | SSRC of media source | 721 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 722 : Feedback Control Information (FCI) : 723 : : 725 Figure 1: RTP/AVPF Common Packet Format for Feedback Messages 727 0 1 2 3 728 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 729 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 730 | Extended Highest Sequence Number | 731 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 732 | ECT (0) Counter | 733 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 734 | ECT (1) Counter | 735 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 736 | ECN-CE Counter | not-ECT Counter | 737 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 738 | Loss Packet Counter | Duplication Counter | 739 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 741 Figure 2: ECN Feedback Report Format 743 The ECN Feedback Report contains the following fields: 745 Extended Highest Sequence Number: The 32-bit Extended highest 746 sequence number received, as defined by [RFC3550]. Indicates the 747 highest RTP sequence number to which this report relates. 749 ECT(0) Counter: The 32-bit cumulative number of RTP packets with 750 ECT(0) received from this SSRC. 752 ECT(1) Counter: The 32-bit cumulative number of RTP packets with 753 ECT(1) received from this SSRC. 755 ECN-CE Counter: The cumulative number of RTP packets received from 756 this SSRC since the receiver joined the RTP session that were 757 ECN-CE marked, including ECN-CE marks in any duplicate packets. 758 The receiver should keep track of this value using a local 759 representation that is at least 32-bits, and only include the 16- 760 bits with least significance. In other words, the field will wrap 761 if more than 65535 ECN-CE marked packets have been received. 763 not-ECT Counter: The cumulative number of RTP packets received from 764 this SSRC since the receiver joined the RTP session that had an 765 ECN field value of not-ECT. The receiver should keep track of 766 this value using a local representation that is at least 32-bits, 767 and only include the 16-bits with least significance. In other 768 words, the field will wrap if more than 65535 not-ECT packets have 769 been received. 771 Lost Packets Counter: The cumulative number of RTP packets that the 772 receiver expected to receive minus the number of packets it 773 actually received that are not a duplicate of an already received 774 packet, from this SSRC since the receiver joined the RTP session. 776 Note that packets that arrive late are not counted as lost. The 777 receiver should keep track of this value using a local 778 representation that is at least 32-bits, and only include the 16- 779 bits with least significance. In other words, the field will wrap 780 if more than 65535 packets are lost. 782 Duplication Counter: The cumulative number of RTP packets received 783 that are a duplicate of an already received packet from this SSRC 784 since the receiver joined the RTP session. The receiver should 785 keep track of this value using a local representation that is at 786 least 32-bits, and only include the 16-bits with least 787 significance. In other words, the field will wrap if more than 788 65535 duplicate packets have been received. 790 All fields in the ECN Feedback Report are unsigned integers in 791 network byte order. Each ECN Feedback Report corresponds to a single 792 RTP source (SSRC). Multiple sources can be reported by including 793 multiple ECN Feedback Reports packets in an compound RTCP packet. 795 The counters SHALL be initiated to 0 for each new SSRC received. 796 This to enable detection of ECN-CE marks or Packet loss on the 797 initial report from a specific participant. 799 The use of at least 32-bit counters allows even extremely high packet 800 volume applications to not have wrapping of counters within any 801 timescale close to the RTCP reporting intervals. However, 32-bits 802 are not sufficiently large to disregard the fact that wrappings may 803 happen during the life time of a long-lived RTP session. Thus 804 handling of wrapping of these counters MUST be supported. It is 805 recommended that implementations uses local representation of these 806 counters that are longer than 32-bits to enable easy handling of 807 wraps. 809 There is a difference in packet duplication reports between the 810 packet loss counter that is defined in the Receiver Report Block 811 [RFC3550] and that defined here. To avoid holding state for what RTP 812 sequence numbers have been received, [RFC3550] specifies that one can 813 count packet loss by counting the number of received packets and 814 comparing it to the number of packets expected. As a result a packet 815 duplication can hide a packet loss. However, when populating the ECN 816 Feedback report, a receiver needs to track the sequence numbers 817 actually received and count duplicates and packet loss separately to 818 provide a more reliable indication. Reordering may however still 819 result in that packet loss is reported in one report and then removed 820 in the next. 822 The ECN-CE counter is robust for packet duplication. Adding each 823 received ECN-CE marked packet to the counter is not an issue, in fact 824 it is required to ensure complete tracking of the ECN state. If one 825 of the clones was ECN-CE marked that is still an indication of 826 congestion. Packet duplication has potential impact on the ECN 827 verification and thus there is a need to count the duplicates. 829 5.2. RTCP XR Report block for ECN summary information 831 This unilateral XR report block combined with RTCP SR or RR report 832 blocks carries the same information as the ECN Feedback Report and is 833 be based on the same underlying information. However, the ECN 834 Feedback Report is intended to report on an ECN-CE mark as soon as 835 possible, while this extended report is for the regular RTCP 836 reporting and continuous verification of the ECN functionality end- 837 to-end. 839 The ECN Summary report block consists of one RTCP XR report block 840 header, shown in Figure 3 followed by one or more ECN summary report 841 data blocks, as defined in Figure 4. 843 0 1 2 3 844 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 845 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 846 | BT=[TBA2] | Reserved | Block Length | 847 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 849 Figure 3: RTCP XR Report Header 851 0 1 2 3 852 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 853 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 854 | SSRC of Media Sender | 855 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 856 | ECT (0) Counter | 857 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 858 | ECT (1) Counter | 859 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 860 | ECN-CE Counter | not-ECT Counter | 861 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 862 | Loss Packet Counter | Duplication Counter | 863 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 865 Figure 4: RTCP XR ECN Summary Report 867 The RTCP XR ECN Summary Report contains the following fields: 869 BT: Block Type identifying the ECN summary report block. Value is 870 [TBA2]. 872 Reserved: All bits SHALL be set to 0 on transmission and ignored on 873 reception. 875 Block Length: The length of the report block. Used to indicate the 876 number of report data blocks present in the ECN summary report. 877 This length will be 5*n, where n is the number of ECN summary 878 report blocks, since blocks are a fixed size. The block length 879 MAY be zero if there is nothing to report. Receivers MUST discard 880 reports where the block length is not a multiple of five octets, 881 since these cannot be valid. 883 SSRC of Media Sender: The SSRC identifying the media sender this 884 report is for. 886 ECT(0) Counter: as in Section 5.1. 888 ECT(1) Counter: as in Section 5.1. 890 ECN-CE Counter: as in Section 5.1. 892 not-ECT Counter: as in Section 5.1. 894 Loss Packet Counter: as in Section 5.1. 896 Duplication Counter: as in Section 5.1. 898 The Extended Highest Sequence number counter for each SSRC is not 899 present in RTCP XR report, in contrast to the feedback version. The 900 reason is that this summary report will rely on the information sent 901 in the Sender Report (SR) or Receiver Report (RR) blocks part of the 902 same RTCP compound packet. The Extended Highest Sequence number is 903 available from the SR or RR. 905 All the SSRCs that are present in the SR or RR SHOULD also be 906 included in the RTCP XR ECN summary report. In cases where the 907 number of senders are so large that the combination of SR/RR and the 908 ECN summary for all the senders exceed the MTU, then only a subset of 909 the senders SHOULD be included so that the reports for the subset 910 fits within the MTU. The subsets SHOULD be selected round-robin 911 across multiple intervals so that all sources are periodically 912 reported. In case there are no SSRCs that currently are counted as 913 senders in the session, the report block SHALL still be sent with no 914 report block entry and a zero report block length to continuously 915 indicate to the other participants the receiver capability to report 916 ECN information. 918 6. SDP Signalling Extensions for ECN 920 This section defines a number of SDP signalling extensions used in 921 the negotiation of the ECN for RTP support when using SDP. This 922 includes one SDP attribute "ecn-capable-rtp" that negotiates the 923 actual operation of ECN for RTP. Two SDP signalling parameters are 924 defined to indicate the use of the RTCP XR ECN summary block and the 925 RTP/AVPF feedback format for ECN. One ICE option SDP representation 926 is also defined. 928 6.1. Signalling ECN Capability using SDP 930 One new SDP attribute, "a=ecn-capable-rtp", is defined. This is a 931 media level attribute, and MUST NOT be used at the session level. It 932 is not subject to the character set chosen. The aim of this 933 signalling is to indicate the capability of the sender and receivers 934 to support ECN, and to negotiate the method of ECN initiation to be 935 used in the session. The attribute takes a list of initiation 936 methods, ordered in decreasing preference. The defined values for 937 the initiation method are: 939 rtp: Using RTP and RTCP as defined in Section 7.2.1. 941 ice: Using STUN within ICE as defined in Section 7.2.2. 943 leap: Using the leap of faith method as defined in Section 7.2.3. 945 Further methods may be specified in the future, so unknown methods 946 MUST be ignored upon reception. 948 In addition, a number of OPTIONAL parameters may be included in the 949 "a=ecn-capable-rtp" attribute as follows: 951 mode: This parameter signals the endpoint's capability to set and 952 read ECN marks in UDP packets. An examination of various 953 operating systems has shown that end-system support for ECN 954 marking of UDP packets may be symmetric or asymmetric. By this we 955 mean that some systems may allow end points to set the ECN bits in 956 an outgoing UDP packet but not read them, while others may allow 957 applications to read the ECN bits but not set them. This 958 either/or case may produce an asymmetric support for ECN and thus 959 should be conveyed in the SDP signalling. The "mode=setread" 960 state is the ideal condition where an endpoint can both set and 961 read ECN bits in UDP packets. The "mode=setonly" state indicates 962 that an endpoint can set the ECT bit, but cannot read the ECN bits 963 from received UDP packets to determine if upstream congestion 964 occurred. The "mode=readonly" state indicates that the endpoint 965 can read the ECN bits to determine if congestion has occurred for 966 incoming packets, but it cannot set the ECT bits in outgoing UDP 967 packets. When the "mode=" parameter is omitted it is assumed that 968 the node has "setread" capabilities. This option can provide for 969 an early indication that ECN cannot be used in a session. This 970 would be case when both the offerer and answerer set the "mode=" 971 parameter to "setonly" or both set it to "readonly". 973 ect: This parameter makes it possible to express the preferred ECT 974 marking. This is either "random", "0", or "1", with "0" being 975 implied if not specified. The "ect" parameter describes a 976 receiver preference, and is useful in the case where the receiver 977 knows it is behind a link using IP header compression, the 978 efficiency of which would be seriously disrupted if it were to 979 receive packets with randomly chosen ECT marks. It is RECOMMENDED 980 that ECT(0) marking be used. 982 The ABNF [RFC5234] grammar for the "a=ecn-capable-rtp" attribute is 983 shown in Figure 5. 984 ecn-attribute = "a=ecn-capable-rtp:" SP init-list [SP parm-list] 985 init-list = init-value *("," init-value) 986 init-value = "rtp" / "ice" / "leap" / init-ext 987 init-ext = token 988 parm-list = parm-value *(";" SP parm-value) 989 parm-value = mode / ect / parm-ext 990 mode = "mode=" ("setonly" / "setread" / "readonly") 991 ect = "ect=" ("0" / "1" / "random") 992 parm-ext = parm-name "=" parm-value-ext 993 parm-name = token 994 parm-value-ext = token / quoted-string 995 quoted-string = DQUOTE *qdtext DQUOTE 996 qdtext = %x20-21 / %x23-7E / %x80-FF 997 ; any 8-bit ASCII except <"> 999 ; external references: 1000 ; token: from RFC 4566 1001 ; SP and DQUOTE from RFC 5234 1003 Figure 5: ABNF Grammar for the "a=ecn-capable-rtp" attribute 1005 6.1.1. Use of "a=ecn-capable-rtp:" with the Offer/Answer Model 1007 When SDP is used with the offer/answer model [RFC3264], the party 1008 generating the SDP offer MUST insert an "a=ecn-capable-rtp" attribute 1009 into the media section of the SDP offer of each RTP session for which 1010 it wishes to use ECN. The attribute includes one or more ECN 1011 initiation methods in a comma separated list in decreasing order of 1012 preference, with any number of optional parameters following. The 1013 answering party compares the list of initiation methods in the offer 1014 with those it supports in order of preference. If there is a match, 1015 and if the receiver wishes to attempt to use ECN in the session, it 1016 includes an "a=ecn-capable-rtp" attribute containing its single 1017 preferred choice of initiation method, and any optional parameters, 1018 in the media sections of the answer. If there is no matching 1019 initiation method capability, or if the receiver does not wish to 1020 attempt to use ECN in the session, it does not include an "a=ecn- 1021 capable-rtp" attribute in its answer. If the attribute is removed in 1022 the answer then ECN MUST NOT be used in any direction for that media 1023 flow. If there are initialization methods that are unknown, they 1024 MUST be ignored on reception and MUST NOT be included in an answer. 1026 The endpoints' capability to set and read ECN marks, as expressed by 1027 the optional "mode=" parameter, determines whether ECN support can be 1028 negotiated for flows in one or both directions: 1030 o If the "mode=setonly" parameter is present in the "a=ecn-capable- 1031 rtp" attribute of the offer and the answering party is also 1032 "mode=setonly", then there is no common ECN capability, and the 1033 answer MUST NOT include the "a=ecn-capable-rtp" attribute. 1034 Otherwise, if the offer is "mode=setonly" then ECN may only be 1035 initiated in the direction from the offering party to the 1036 answering party. 1038 o If the "mode=readonly" parameter is present in the "a=ecn-capable- 1039 rtp" attribute of the offer and the answering party is 1040 "mode=readonly", then there is no common ECN capability, and the 1041 answer MUST NOT include the "a=ecn-capable-rtp" attribute. 1042 Otherwise, if the offer is "mode=readonly" then ECN may only be 1043 initiated in the direction from the answering party to the 1044 offering party. 1046 o If the "mode=setread" parameter is present in the "a=ecn-capable- 1047 rtp" attribute of the offer and the answering party is "setonly", 1048 then ECN may only be initiated in the direction from the answering 1049 party to the offering party. If the offering party is 1050 "mode=setread" but the answering party is "mode=readonly", then 1051 ECN may only be initiated in the direction from the offering party 1052 to the answering party. If both offer and answer are 1053 "mode=setread", then ECN may be initiated in both directions. 1054 Note that "mode=setread" is implied by the absence of a "mode=" 1055 parameter in the offer or the answer. 1057 o An offer that does not include a "mode=" parameter MUST be treated 1058 as-if a "mode=setread" parameter had been included. 1060 In an RTP session using multicast and ECN, participants that intend 1061 to send RTP packets SHOULD support setting ECT marks in RTP packets 1062 (i.e., should be "mode=setonly" or "mode=setread"). Participants 1063 receiving data need the capability to read ECN marks on incoming 1064 packets. It is important that receivers can read ECN marks (are 1065 "mode=readonly" or "mode=setread"), since otherwise no sender in the 1066 multicast session will be able to enable ECN. Accordingly, receivers 1067 that are "mode=setonly" SHOULD NOT join multicast RTP sessions that 1068 use ECN. If session participants that are not aware of the ECN for 1069 RTP signalling are invited to a multicast session, and simply ignore 1070 the signalling attribute, the other party in the offer/answer 1071 exchange SHOULD terminate the SDP dialogue so that the participant 1072 leaves the session. 1074 The "ect=" parameter in the "a=ecn-capable-rtp" attribute is set 1075 independently in the offer and the answer. Its value in the offer 1076 indicates a preference for the sending behaviour of the answering 1077 party, and its value in the answer indicates a sending preference for 1078 the behaviour of the offering party. It will be the senders choice 1079 to honour the receivers preference for what to receive or not. In 1080 multicast sessions, all senders SHOULD set the ECT marks using the 1081 value declared in the "ect=" parameter. 1083 Unknown optional parameters MUST be ignored on reception, and MUST 1084 NOT be included in the answer. That way new parameters may be 1085 introduced and verified to be supported by the other end-point by 1086 having them include it in any answer. 1088 6.1.2. Use of "a=ecn-capable-rtp:" with Declarative SDP 1090 When SDP is used in a declarative manner, for example in a multicast 1091 session using the Session Announcement Protocol (SAP, [RFC2974]), 1092 negotiation of session description parameters is not possible. The 1093 "a=ecn-capable-rtp" attribute MAY be added to the session description 1094 to indicate that the sender will use ECN in the RTP session. The 1095 attribute MUST include a single method of initiation. Participants 1096 MUST NOT join such a session unless they have the capability to 1097 receive ECN-marked UDP packets, implement the method of initiation, 1098 and can generate RTCP ECN feedback. The mode parameter MAY also be 1099 included in declarative usage, to indicate the minimal capability is 1100 required by the consumer of the SDP. So for example in a SSM session 1101 the participants configured with a particular SDP will all be in a 1102 media receive only mode, thus mode=readonly will work as the 1103 capability of reporting on the ECN markings in the received is what 1104 is required. However, using "mode=readonly" also in ASM sessions is 1105 reasonable, unless all senders are required to attempt to use ECN for 1106 their outgoing RTP data traffic, in which case the mode needs to be 1107 set to "setread". 1109 6.1.3. General Use of the "a=ecn-capable-rtp:" Attribute 1111 The "a=ecn-capable-rtp" attribute MAY be used with RTP media sessions 1112 using UDP/IP transport. It MUST NOT be used for RTP sessions using 1113 TCP, SCTP, or DCCP transport, or for non-RTP sessions. 1115 As described in Section 7.3.3, RTP sessions using ECN require rapid 1116 RTCP ECN feedback, unless timely feedback is not required due to a 1117 receiver driven congestion control. To ensure that the sender can 1118 react to ECN-CE marked packets timely feedback is usually required. 1119 Thus, the use of the Extended RTP Profile for RTCP-Based Feedback 1120 (RTP/AVPF) [RFC4585] or other profile that inherits RTP/AVPF's 1121 signalling rules, MUST be signalled unless timely feedback is not 1122 required. If timely feedback is not required it is still RECOMMENDED 1123 to use RTP/AVPF. The signalling of an RTP/AVPF based profile is 1124 likely to be required even if the preferred method of initialization 1125 and the congestion control does not require timely feedback, as the 1126 common interoperable method is likely to be signalled or the improved 1127 fault reaction is desired. 1129 6.2. RTCP ECN Feedback SDP Parameter 1131 A new "nack" feedback parameter "ecn" is defined to indicate the 1132 usage of the RTCP ECN feedback packet format (Section 5.1). The ABNF 1133 [RFC5234] definition of the SDP parameter extension is: 1134 rtcp-fb-nack-param = 1135 rtcp-fb-nack-param /= ecn-fb-par 1136 ecn-fb-par = SP "ecn" 1138 The offer/answer rules for this SDP feedback parameters are specified 1139 in the RTP/AVPF profile [RFC4585]. 1141 6.3. XR Block ECN SDP Parameter 1143 A new unilateral RTCP XR block for ECN summary information is 1144 specified, thus the XR block SDP signalling also needs to be extended 1145 with a parameter. This is done in the same way as for the other XR 1146 blocks. The XR block SDP attribute as defined in Section 5.1 of the 1147 RTCP XR specification [RFC3611] is defined to be extensible. As no 1148 parameter values are needed for this ECN summary block, this 1149 parameter extension consists of a simple parameter name used to 1150 indicate support and intent to use the XR block. 1151 xr-format = 1152 xr-format /= ecn-summary-par 1153 ecn-summary-par = "ecn-sum" 1155 For SDP declarative and offer/answer usage, see the RTCP XR 1156 specification [RFC3611] and its description of how to handle 1157 unilateral parameters. 1159 6.4. ICE Parameter to Signal ECN Capability 1161 One new ICE [RFC5245] option, "rtp+ecn", is defined. This is used 1162 with the SDP session level "a=ice-options" attribute in an SDP offer 1163 to indicate that the initiator of the ICE exchange has the capability 1164 to support ECN for RTP-over-UDP flows (via "a=ice-options: rtp+ecn"). 1165 The answering party includes this same attribute at the session level 1166 in the SDP answer if it also has the capability, and removes the 1167 attribute if it does not wish to use ECN, or doesn't have the 1168 capability to use ECN. If the ICE initiation method (Section 7.2.2) 1169 is actually going to be used, it is also needs to be explicitly 1170 negotiated using the "a=ecn-capable-rtp" attribute. This ICE option 1171 SHALL be included when the ICE initiation method is offered or 1172 declared in the SDP. 1174 Note: This signalling mechanism is not strictly needed as long as 1175 the STUN ECN testing capability is used within the context of this 1176 document. It may however be useful if the ECN verification 1177 capability is used in additional contexts. 1179 7. Use of ECN with RTP/UDP/IP 1181 In the detailed specification of the behaviour below, the different 1182 functions in the general case will first be discussed. In case 1183 special considerations are needed for middleboxes, multicast usage 1184 etc, those will be specially discussed in related subsections. 1186 7.1. Negotiation of ECN Capability 1188 The first stage of ECN negotiation for RTP-over-UDP is to signal the 1189 capability to use ECN. An RTP system that supports ECN and uses SDP 1190 for its signalling MUST implement the SDP extension to signal ECN 1191 capability as described in Section 6.1, the RTCP ECN feedback SDP 1192 parameter defined in Section 6.2, and the XR Block ECN SDP parameter 1193 defined in Section 6.3. It MAY also implement alternative ECN 1194 capability negotiation schemes, such as the ICE extension described 1195 in Section 6.4. Other signalling systems will need to define 1196 signalling parameters corresponding to those defined for SDP. 1198 The "ecn-capable-rtp" SDP attribute MUST be used when employing ECN 1199 for RTP according to this specification in systems using SDP. As the 1200 RTCP XR ECN summary report is required independently of the 1201 initialization method or congestion control scheme, the "rtcp-xr" 1202 attribute with the "ecn-sum" parameter MUST also be used. The 1203 "rtcp-fb" attribute with the "nack" parameter "ecn" MUST be used 1204 whenever the initialization method or a congestion control algorithm 1205 requires timely sender side knowledge of received CE markings. If 1206 the congestion control scheme requires additional signalling, this 1207 should be indicated as appropriate. 1209 7.2. Initiation of ECN Use in an RTP Session 1211 Once the sender and the receiver(s) have agreed that they have the 1212 capability to use ECN within a session, they may attempt to initiate 1213 ECN use. All session participants connected over the same transport 1214 MUST use the same initiation method. RTP mixers or translators can 1215 use different initiation methods to different participants that are 1216 connected over different underlying transports. The mixer or 1217 translator will need to do individual signalling with each 1218 participant to ensure it is consistent with the ECN support in those 1219 cases where it does not function as one end-point for the ECN control 1220 loop. 1222 At the start of the RTP session, when the first few packets with ECT 1223 are sent, it is important to verify that IP packets with ECN field 1224 values of ECT or ECN-CE will reach their destination(s). There is 1225 some risk that the use of ECN will result in either reset of the ECN 1226 field, or loss of all packets with ECT or ECN-CE markings. If the 1227 path between the sender and the receivers exhibits either of these 1228 behaviours, the sender needs to stop using ECN immediately to protect 1229 both the network and the application. 1231 The RTP senders and receivers SHALL NOT ECT mark their RTCP traffic 1232 at any time. This is to ensure that packet loss due to ECN marking 1233 will not effect the RTCP traffic and the necessary feedback 1234 information it carries. 1236 An RTP system that supports ECN MUST implement the initiation of ECN 1237 using in-band RTP and RTCP described in Section 7.2.1. It MAY also 1238 implement other mechanisms to initiate ECN support, for example the 1239 STUN-based mechanism described in Section 7.2.2, or use the leap of 1240 faith option if the session supports the limitations provided in 1241 Section 7.2.3. If support for both in-band and out-of-band 1242 mechanisms are signalled, the sender when negotiating SHOULD offer 1243 detection of ECT using STUN with ICE with higher priority than 1244 detection of ECT using RTP and RTCP. 1246 No matter how ECN usage is initiated, the sender MUST continually 1247 monitor the ability of the network, and all its receivers, to support 1248 ECN, following the mechanisms described in Section 7.4. This is 1249 necessary because path changes or changes in the receiver population 1250 may invalidate the ability of the system to use ECN. 1252 7.2.1. Detection of ECT using RTP and RTCP 1254 The ECN initiation phase using RTP and RTCP to detect if the network 1255 path supports ECN comprises three stages. Firstly, the RTP sender 1256 generates some small fraction of its traffic with ECT marks to act as 1257 probe for ECN support. Then, on receipt of these ECT-marked packets, 1258 the receivers send RTCP ECN feedback packets and RTCP ECN summary 1259 reports to inform the sender that their path supports ECN. Finally, 1260 the RTP sender makes the decision to use ECN or not, based on whether 1261 the paths to all RTP receivers have been verified to support ECN. 1263 Generating ECN Probe Packets: During the ECN initiation phase, an 1264 RTP sender SHALL mark a small fraction of its RTP traffic as ECT, 1265 while leaving the reminder of the packets unmarked. The main 1266 reason for only marking some packets is to maintain usable media 1267 delivery during the ECN initiation phase in those cases where ECN 1268 is not supported by the network path. A secondary reason to send 1269 some not-ECT packets are to ensure that the receivers will send 1270 RTCP reports on this sender, even if all ECT marked packets are 1271 lost in transit. The not-ECT packets also provide a base-line to 1272 compare performance parameters against. A fourth reason for only 1273 probing with a small number of packets is to reduce the risk that 1274 significant numbers of congestion markings might be lost if ECT is 1275 cleared to Not-ECT by an ECN-Reverting Middlebox. Then any 1276 resulting lack of congestion response is likely to have little 1277 damaging effect on others. An RTP sender is RECOMMENDED to send a 1278 minimum of two packets with ECT markings per RTCP reporting 1279 interval. In case a random ECT pattern is intended to be used, at 1280 least one packet with ECT(0) and one with ECT(1) should be sent 1281 per reporting interval; in case a single ECT marking is to be 1282 used, only that ECT value SHOULD be sent. The RTP sender SHALL 1283 continue to send some ECT marked traffic as long as the ECN 1284 initiation phase continues. The sender SHOULD NOT mark all RTP 1285 packets as ECT during the ECN initiation phase. 1287 This memo does not mandate which RTP packets are marked with ECT 1288 during the ECN initiation phase. An implementation should insert 1289 ECT marks in RTP packets in a way that minimises the impact on 1290 media quality if those packets are lost. The choice of packets to 1291 mark is clearly very media dependent, but the use of RTP NO-OP 1292 payloads [I-D.ietf-avt-rtp-no-op], if supported, would be an 1293 appropriate choice. For audio formats, if would make sense for 1294 the sender to mark comfort noise packets or similar. For video 1295 formats, packets containing P- or B-frames (rather than I-frames) 1296 would be an appropriate choice. No matter which RTP packets are 1297 marked, those packets MUST NOT be sent in duplicate, with and 1298 without ECT, since the RTP sequence number is used to identify 1299 packets that are received with ECN markings. 1301 Generating RTCP ECN Feedback: If ECN capability has been negotiated 1302 in an RTP session, the receivers in the session MUST listen for 1303 ECT or ECN-CE marked RTP packets, and generate RTCP ECN feedback 1304 packets (Section 5.1) to mark their receipt. An immediate or 1305 early (depending on the RTP/AVPF mode) ECN feedback packet SHOULD 1306 be generated on receipt of the first ECT or ECN-CE marked packet 1307 from a sender that has not previously sent any ECT traffic. Each 1308 regular RTCP report MUST also contain an ECN summary report 1309 (Section 5.2). Reception of subsequent ECN-CE marked packets MUST 1310 result in additional early or immediate ECN feedback packets being 1311 sent unless no timely feedback is required. 1313 Determination of ECN Support: RTP is a group communication protocol, 1314 where members can join and leave the group at any time. This 1315 complicates the ECN initiation phase, since the sender must wait 1316 until it believes the group membership has stabilised before it 1317 can determine if the paths to all receivers support ECN (group 1318 membership changes after the ECN initiation phase has completed 1319 are discussed in Section 7.3). 1321 An RTP sender shall consider the group membership to be stable 1322 after it has been in the session and sending ECT-marked probe 1323 packets for at least three RTCP reporting intervals (i.e., after 1324 sending its third regularly scheduled RTCP packet), and when a 1325 complete RTCP reporting interval has passed without changes to the 1326 group membership. ECN initiation is considered successful when 1327 the group membership is stable, and all known participants have 1328 sent one or more RTCP ECN feedback packets or RTCP XR ECN summary 1329 reports indicating correct receipt of the ECT-marked RTP packets 1330 generated by the sender. 1332 As an optimisation, if an RTP sender is initiating ECN usage 1333 towards a unicast address, then it MAY treat the ECN initiation as 1334 provisionally successful if it receives an RTCP ECN feedback 1335 report or an RTCP XR ECN summary report indicating successful 1336 receipt of the ECT-marked packets, with no negative indications, 1337 from a single RTP receiver (where a single RTP receiver is 1338 considered as all SSRCs used by a single RTCP CNAME). After 1339 declaring provisional success, the sender MAY generate ECT-marked 1340 packets as described in Section 7.3, provided it continues to 1341 monitor the RTCP reports for a period of three RTCP reporting 1342 intervals from the time the ECN initiation started, to check if 1343 there are any other participants in the session. Thus as long as 1344 any additional SSRC that report on the ECN usage are using the 1345 same RTCP CNAME as the previous reports and they are all 1346 indicating functional ECN the sender may continue. If other 1347 participants are detected, i.e., other RTCP CNAMEs, the sender 1348 MUST fallback to only ECT-marking a small fraction of its RTP 1349 packets, while it determines if ECN can be supported following the 1350 full procedure described above. Different RTCP CNAMEs received 1351 over an unicast transport may occur when using translators in a 1352 multi-party RTP session (e.g., when using a centralised conference 1353 bridge). 1355 Note: The above optimization supports peer to peer unicast 1356 transport with several SSRCs multiplexed onto the same flow 1357 (e.g., a single participant with two video cameras, or SSRC 1358 multiplexed RTP retransmission [RFC4588]). It is desirable to 1359 be able to rapidly negotiate ECN support for such a session, 1360 but the optimisation above can fail if there are 1361 implementations that use the same CNAME for different parts of 1362 a distributed implementation that have different transport 1363 characteristics (e.g., if a single logical endpoint is split 1364 across multiple hosts). 1366 ECN initiation is considered to have failed at the instant the 1367 initiating RTP sender received an RTCP packet that doesn't contain 1368 an RTCP ECN feedback report or ECN summary report from any RTP 1369 session participant that has an RTCP RR with an extended RTP 1370 sequence number field that indicates that it should have received 1371 multiple (>3) ECT marked RTP packets. This can be due to failure 1372 to support the ECN feedback format by the receiver or some 1373 middlebox, or the loss of all ECT marked packets. Both indicate a 1374 lack of ECN support. 1376 If the ECN negotiation succeeds, this indicates that the path can 1377 pass some ECN-marked traffic, and that the receivers support ECN 1378 feedback. This does not necessarily imply that the path can robustly 1379 convey ECN feedback; Section 7.3 describes the ongoing monitoring 1380 that must be performed to ensure the path continues to robustly 1381 support ECN. 1383 When a sender or receiver detects ECN failures on paths they should 1384 log these to enable follow up and statistics gathering regarding 1385 broken paths. The logging mechanism used is implementation 1386 dependent. 1388 7.2.2. Detection of ECT using STUN with ICE 1390 This section describes an OPTIONAL method that can be used to avoid 1391 media impact and also ensure an ECN capable path prior to media 1392 transmission. This method is considered in the context where the 1393 session participants are using ICE [RFC5245] to find working 1394 connectivity. We need to use ICE rather than STUN only, as the 1395 verification needs to happen from the media sender to the address and 1396 port on which the receiver is listening. 1398 Note that this method is only applicable to sessions when the remote 1399 destinations are unicast addresses. In addition, transport 1400 translators that do not terminate the ECN control loop and may 1401 distribute received packets to more than one other receiver must 1402 either disallow this method (and use the RTP/RTCP method instead), or 1403 implement additional handling as discussed below. This is because 1404 the ICE initialization method verifies the underlying transport to 1405 one particular address and port. If the receiver at that address and 1406 port intends to use the received packets in a multi-point session 1407 then the tested capabilities and the actual session behavior are not 1408 matched. 1410 To minimise the impact of set-up delay, and to prioritise the fact 1411 that one has working connectivity rather than necessarily finding the 1412 best ECN capable network path, this procedure is applied after having 1413 performed a successful connectivity check for a candidate, which is 1414 nominated for usage. At that point an additional connectivity check 1415 is performed, sending the "ECN Check" attribute in a STUN packet that 1416 is ECT marked. On reception of the packet, a STUN server supporting 1417 this extension will note the received ECN field value, and send a 1418 STUN/UDP/IP packet in reply with the ECN field set to not-ECT and 1419 including an ECN check attribute. A STUN server that doesn't 1420 understand the extension, or is incapable of reading the ECN values 1421 on incoming STUN packets, should follow the rule in the STUN 1422 specification for unknown comprehension-optional attributes, and 1423 ignore the attribute, resulting in the sender receiving a STUN 1424 response without the ECN Check STUN attribute. 1426 The ECN STUN checks can be lost on the path, for example due to the 1427 ECT marking, but also due to various other non ECN related reasons 1428 causing packet loss. The goal is to detect when the ECT markings are 1429 rewritten or if it is the ECT marking that causes packet loss so that 1430 the path can be determined as not ECT. Other reasons for packet loss 1431 should not result in a failure to verify the path as ECT. Therefore 1432 a number of retransmissions should be attempted. But, the sender of 1433 ECN STUN checks will also have to set a criteria for when it gives up 1434 testing for ECN capability on the path. Since the ICE agent has 1435 successfully verified the path an RTT measurement for this path can 1436 be performed. To have a high probability of successfully verifying 1437 the path it is RECOMMENDED that the client retransmit the ECN STUN 1438 check at least 4 times. The transmission for that flow is stopped 1439 when an ECN Check STUN response has been received, which doesn't 1440 indicate a retransmission of the request due to a temporary error, or 1441 the maximum number of retransmissions has been sent. The ICE agent 1442 is recommended to give up on the ECN verification MAX(1.5*RTT, 20 ms) 1443 after the last ECN STUN check was sent. 1445 The transmission of the ECT marked STUN connectivity checks 1446 containing the ECN Check attribute can be done prior as well in 1447 parallel to actual media transmission. Both cases are supported, 1448 where the main difference is how aggressively the transmission of the 1449 STUN checks are done. The reason for this is to avoid adding 1450 additional startup delay until media can flow. If media is required 1451 immeditely after nomination has occured the STUN checks SHALL be done 1452 in parallel. If the application does not require media transmission 1453 immediately the verification of ECT SHOULD start using the aggresive 1454 mode. At any point in the process until ECT has been verified or 1455 found to not work media transmission MAY be started and the ICE agent 1456 SHALL transition from the aggressive mode to the parallel mode. 1458 The aggressive mode uses an interval between the retransmissions be 1459 based on the Ta timer as defined in Section 16.1 for RTP Media 1460 Streams in ICE [RFC5245]. The number of ECN STUN checks needing to 1461 be sent will depend on the number of ECN capable flows (N) that is to 1462 be established. The interval between each transmission of an ECN 1463 check packet MUST be Ta. In other words for a given flow being 1464 verified for ECT the RTO is set to Ta*N. 1466 The parallel mode uses transmission intervals that targets that the 1467 bit-rate increase due to the ECT verification checks shall not 1468 increase the total bit-rate more than 10% in addition to the media. 1469 As ICE's regular transmission schedule is mimicking a common voice 1470 call in amount, to meet that goal for most media flows, setting the 1471 retransmission interval to Ta*N*k where k=10 fulfills that goal. 1472 Thus the default behavior SHALL be to use k=10 when in parallel mode. 1473 In cases where the bit-rate of the STUN connectivity checks can be 1474 determined they MAY be sent with smaller values of k, but k MUST NOT 1475 be smaller than 1, as long as the total bit-rate for the connectivity 1476 checks are less than 10% of the used media bit-rate. The RTP media 1477 packets being sent in parallel mode SHALL NOT be ECT marked prior to 1478 verification of the path as ECT. 1480 The STUN ECN check attribute contains one field and a flag, as shown 1481 in Figure 6. The flag indicates whether the echo field contains a 1482 valid value or not. The field is the ECN echo field, and when valid 1483 contains the two ECN bits from the packet it echoes back. The ECN 1484 check attribute is a comprehension optional attribute. 1485 0 1 2 3 1486 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 1487 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1488 | Type | Length | 1489 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1490 | Reserved |ECF|V| 1491 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1493 Figure 6: ECN Check STUN Attribute 1495 V: Valid (1 bit) ECN Echo value field is valid when set to 1, and 1496 invalid when set 0. 1498 ECF: ECN Echo value field (2 bits) contains the ECN field value of 1499 the STUN packet it echoes back when field is valid. If invalid 1500 the content is arbitrary. 1502 Reserved: Reserved bits (29 bits) SHALL be set to 0 on transmission, 1503 and SHALL be ignored on reception. 1505 This attribute MAY be included in any STUN request to request the ECN 1506 field to be echoed back. In STUN requests the V bit SHALL be set to 1507 0. A compliant STUN server receiving a request with the ECN Check 1508 attribute SHALL read the ECN field value of the IP/UDP packet the 1509 request was received in. Upon forming the response the server SHALL 1510 include the ECN Check attribute setting the V bit to valid and 1511 include the read value of the ECN field into the ECF field. If the 1512 STUN responder was unable to ascertain, due to temporary errors, the 1513 ECN value of the STUN request, it SHALL set the V bit in the response 1514 to 0. The STUN client may retry immediately. 1516 The ICE based initialization method does require some special 1517 consideration when used by a translator. This is especially for 1518 transport translators and translators that fragment or reassemble 1519 packets, since they do not separate the ECN control loops between the 1520 end-points and the translator. When using ICE-based initiation, such 1521 a translator must ensure that any participants joining an RTP session 1522 for which ECN has been negotiated are successfully verified in the 1523 direction from the translator to the joining participant. 1524 Alternatively, it must correctly handle remarking of ECT RTP packets 1525 towards that participant. When a new participant joins the session, 1526 the translator will perform a check towards the new participant. If 1527 that is successfully completed the ECT properties of the session are 1528 maintained for the other senders in the session. If the check fails 1529 then the existing senders will now see a participant that fails to 1530 receive ECT. Thus the failure detection in those senders will 1531 eventually detect this. However to avoid misusing the network on the 1532 path from the translator to the new participant, the translator SHALL 1533 remark the traffic intended to be forwarded from ECT to non-ECT. Any 1534 packet intended to be forward that are ECN-CE marked SHALL be discard 1535 and not sent. In cases where the path from a new participant to the 1536 translator fails the ECT check then only that sender will not 1537 contribute any ECT marked traffic towards the translator. 1539 7.2.3. Leap of Faith ECT initiation method 1541 This method for initiating ECN usage is a leap of faith that assumes 1542 that ECN will work on the used path(s). The method is to go directly 1543 to "ongoing use of ECN" as defined in Section 7.3. Thus all RTP 1544 packets MAY be marked as ECT and the failure detection MUST be used 1545 to detect any case when the assumption that the path was ECT capable 1546 is wrong. This method is only recommended for controlled 1547 environments where the whole path(s) between sender and receiver(s) 1548 has been built and verified to be ECT. 1550 If the sender marks all packets as ECT while transmitting on a path 1551 that contains an ECN-blocking middlebox, then receivers downstream of 1552 that middlebox will not receive any RTP data packets from the sender, 1553 and hence will not consider it to be an active RTP SSRC. The sender 1554 can detect this and revert to sending packets without ECT marks, 1555 since RTCP SR/RR packets from such receivers will either not include 1556 a report for sender's SSRC, or will report that no packets have been 1557 received, but this takes at least one RTCP reporting interval. It 1558 should be noted that a receiver might generate its first RTCP packet 1559 immediately on joining a unicast session, or very shortly after 1560 joining a RTP/AVPF session, before it has had chance to receive any 1561 data packets. A sender that receives RTCP SR/RR packet indicating 1562 lack of reception by a receiver SHOULD therefore wait for a second 1563 RTCP report from that receiver to be sure that the lack of reception 1564 is due to ECT-marking. Since this recovery process can take several 1565 tens of seconds, during which time the RTP session is unusable for 1566 media, it is NOT RECOMMENDED that the leap-of-faith ECT initiation 1567 method be used in environments where ECN-blocking middleboxes are 1568 likely to be present. 1570 7.3. Ongoing Use of ECN Within an RTP Session 1572 Once ECN has been successfully initiated for an RTP sender, that 1573 sender begins sending all RTP data packets as ECT-marked, and its 1574 receivers send ECN feedback information via RTCP packets. This 1575 section describes procedures for sending ECT-marked data, providing 1576 ECN feedback information via RTCP, and responding to ECN feedback 1577 information. 1579 7.3.1. Transmission of ECT-marked RTP Packets 1581 After a sender has successfully initiated ECN use, it SHOULD mark all 1582 the RTP data packets it sends as ECT. The sender SHOULD mark packets 1583 as ECT(0) unless the receiver expresses a preference for ECT(1) or 1584 random using the "ect" parameter in the "a=ecn-capable-rtp" 1585 attribute. 1587 The sender SHALL NOT include ECT marks on outgoing RTCP packets, and 1588 SHOULD NOT include ECT marks on any other outgoing control messages 1589 (e.g., STUN [RFC5389] packets, DTLS [RFC6347] handshake packets, or 1590 ZRTP [RFC6189] control packets) that are multiplexed on the same UDP 1591 port. For control packets there might be exceptions, like the STUN 1592 based ECN check defined in Section 7.2.2. 1594 7.3.2. Reporting ECN Feedback via RTCP 1596 An RTP receiver that receives a packet with an ECN-CE mark, or that 1597 detects a packet loss, MUST schedule the transmission of an RTCP ECN 1598 feedback packet as soon as possible (subject to the constraints of 1599 [RFC4585] and [RFC3550]) to report this back to the sender unless no 1600 timely feedback is required. The feedback RTCP packet SHALL consist 1601 of at least one ECN feedback packet (Section 5.1) reporting on the 1602 packets received since the last ECN feedback packet, and will contain 1603 (at least) an RTCP SR/RR packet and an SDES packet, unless reduced 1604 size RTCP [RFC5506] is used. The RTP/AVPF profile in early or 1605 immediate feedback mode SHOULD be used where possible, to reduce the 1606 interval before feedback can be sent. To reduce the size of the 1607 feedback message, reduced size RTCP [RFC5506] MAY be used if 1608 supported by the end-points. Both RTP/AVPF and reduced size RTCP 1609 MUST be negotiated in the session set-up signalling before they can 1610 be used. 1612 Every time a regular compound RTCP packet is to be transmitted, an 1613 ECN-capable RTP receiver MUST include an RTCP XR ECN summary report 1614 as described in Section 5.2 as part of the compound packet. 1616 The multicast feedback implosion problem, that occurs when many 1617 receivers simultaneously send feedback to a single sender, must be 1618 considered. The RTP/AVPF transmission rules will limit the amount of 1619 feedback that can be sent, avoiding the implosion problem but also 1620 delaying feedback by varying degrees from nothing up to a full RTCP 1621 reporting interval. As a result, the full extent of a congestion 1622 situation may take some time to reach the sender, although some 1623 feedback should arrive in a reasonably timely manner, allowing the 1624 sender to react on a single or a few reports. 1626 7.3.3. Response to Congestion Notifications 1628 The reception of RTP packets with ECN-CE marks in the IP header is a 1629 notification that congestion is being experienced. The default 1630 reaction on the reception of these ECN-CE marked packets MUST be to 1631 provide the congestion control algorithm with a congestion 1632 notification that triggers the algorithm to react as if packet loss 1633 had occurred. There should be no difference in congestion response 1634 if ECN-CE marks or packet drops are detected. 1636 We note that there MAY be other reactions to ECN-CE specified in the 1637 future. Such an alternative reaction MUST be specified and 1638 considered to be safe for deployment under any restrictions 1639 specified. A potential example for an alternative reaction could be 1640 emergency communications (such as that generated by first responders, 1641 as opposed to the general public) in networks where the user has been 1642 authorized. A more detailed description of these other reactions, as 1643 well as the types of congestion control algorithms used by end-nodes, 1644 is outside of the scope of this document. 1646 Depending on the media format, type of session, and RTP topology 1647 used, there are several different types of congestion control that 1648 can be used: 1650 Sender-Driven Congestion Control: The sender is responsible for 1651 adapting the transmitted bit-rate in response to RTCP ECN 1652 feedback. When the sender receives the ECN feedback data it feeds 1653 this information into its congestion control or bit-rate 1654 adaptation mechanism so that it can react as if packet loss was 1655 reported. The congestion control algorithm to be used is not 1656 specified here, although TFRC [RFC5348] is one example that might 1657 be used. 1659 Receiver-Driven Congestion Control: In a receiver driven congestion 1660 control mechanism, the receivers can react to the ECN-CE marks 1661 themselves without providing ECN-CE feedback to the sender. This 1662 may allow faster response than sender-driven congestion control in 1663 some circumstances and also scale to large number of receivers and 1664 multicast usage. One example of receiver-driven congestion 1665 control is implemented by providing the content in a layered way, 1666 with each layer providing improved media quality but also 1667 increased bandwidth usage. The receiver locally monitors the 1668 ECN-CE marks on received packets to check if it experiences 1669 congestion with the current number of layers. If congestion is 1670 experienced, the receiver drops one layer, so reducing the 1671 resource consumption on the path towards itself. For example, if 1672 a layered media encoding scheme such as H.264 SVC is used, the 1673 receiver may change its layer subscription, and so reduce the bit 1674 rate it receives. The receiver MUST still send RTCP XR ECN 1675 Summary to the sender, even if it can adapt without contact with 1676 the sender, so that the sender can determine if ECN is supported 1677 on the network path. The timeliness of RTCP feedback is less of a 1678 concern with receiver driven congestion control, and regular RTCP 1679 reporting of ECN summary information is sufficient (without using 1680 RTP/AVPF immediate or early feedback). 1682 Hybrid: There might be mechanisms that utilize both some receiver 1683 behaviors and some sender side monitoring, thus requiring both 1684 feedback of congestion events to the sender and taking receiver 1685 decisions and possible signalling to the sender. In this case the 1686 congestion control algorithm needs to use the signalling to 1687 indicate which features of ECN for RTP are required. 1689 Responding to congestion indication in the case of multicast traffic 1690 is a more complex problem than for unicast traffic. The fundamental 1691 problem is diverse paths, i.e., when different receivers don't see 1692 the same path, and thus have different bottlenecks, so the receivers 1693 may get ECN-CE marked packets due to congestion at different points 1694 in the network. This is problematic for sender driven congestion 1695 control, since when receivers are heterogeneous in regards to 1696 capacity, the sender is limited to transmitting at the rate the 1697 slowest receiver can support. This often becomes a significant 1698 limitation as group size grows. Also, as group size increases the 1699 frequency of reports from each receiver decreases, which further 1700 reduces the responsiveness of the mechanism. Receiver-driven 1701 congestion control has the advantage that each receiver can choose 1702 the appropriate rate for its network path, rather than all receivers 1703 having to settle for the lowest common rate. 1705 We note that ECN support is not a silver bullet to improving 1706 performance. The use of ECN gives the chance to respond to 1707 congestion before packets are dropped in the network, improving the 1708 user experience by allowing the RTP application to control how the 1709 quality is reduced. An application which ignores ECN Congestion 1710 Experienced feedback is not immune to congestion: the network will 1711 eventually begin to discard packets if traffic doesn't respond. It 1712 is in the best interest of an application to respond to ECN 1713 congestion feedback promptly, to avoid packet loss. 1715 7.4. Detecting Failures 1717 Senders and receivers can deliberately ignore ECN-CE and thus get a 1718 benefit over behaving flows (cheating). The ECN Nonce [RFC3540] is 1719 an addition to TCP that attempts to solve this issue as long as the 1720 sender acts on behalf of the network. The assumption about the 1721 senders acting on the behalf of the network may be reduced due to the 1722 nature of peer-to-peer use of RTP. Still a significant portion of 1723 RTP senders are infrastructure devices (for example, streaming media 1724 servers) that do have an interest in protecting both service quality 1725 and the network. Even though there may be cases where the nonce may 1726 be applicable for RTP, it is not included in this specification. 1727 This is because a receiver interested in cheating would simply claim 1728 to not support the nonce, or even ECN itself. It is, however, worth 1729 mentioning that, as real-time media is commonly sensitive to 1730 increased delay and packet loss, it will be in both the media sender 1731 and receivers interest to minimise the number and duration of any 1732 congestion events as they will adversely affect media quality. 1734 RTP sessions can also suffer from path changes resulting in a non-ECN 1735 compliant node becoming part of the path. That node may perform 1736 either of two actions that has an effect on the ECN and application 1737 functionality. The gravest is if the node drops packets with the ECN 1738 field set to ECT(0), ECT(1), or ECN-CE. This can be detected by the 1739 receiver when it receives an RTCP SR packet indicating that a sender 1740 has sent a number of packets that it has not received. The sender 1741 may also detect such a middlebox based on the receiver's RTCP RR 1742 packet, when the extended sequence number is not advanced due to the 1743 failure to receive packets. If the packet loss is less than 100%, 1744 then packet loss reporting in either the ECN feedback information or 1745 RTCP RR will indicate the situation. The other action is to re-mark 1746 a packet from ECT or ECN-CE to not-ECT. That has less dire results, 1747 however it should be detected so that ECN usage can be suspended to 1748 prevent misusing the network. 1750 The RTCP XR ECN summary packet and the ECN feedback packet allow the 1751 sender to compare the number of ECT marked packets of different types 1752 received with the number it actually sent. The number of ECT packets 1753 received, plus the number of ECN-CE marked and lost packets, should 1754 correspond to the number of sent ECT marked packets plus the number 1755 of received duplicates. If these numbers don't agree there are two 1756 likely reasons, a translator changing the stream or not carrying the 1757 ECN markings forward, or that some node re-marks the packets. In 1758 both cases the usage of ECN is broken on the path. By tracking all 1759 the different possible ECN field values a sender can quickly detect 1760 if some non-compliant behavior is happening on the path. 1762 Thus packet losses and non-matching ECN field value statistics are 1763 possible indications of issues with using ECN over the path. The 1764 next section defines both sender and receiver reactions to these 1765 cases. 1767 7.4.1. Fallback mechanisms 1769 Upon the detection of a potential failure, both the sender and the 1770 receiver can react to mitigate the situation. 1772 A receiver that detects a packet loss burst MAY schedule an early 1773 feedback packet that includes at least the RTCP RR and the ECN 1774 feedback message to report this to the sender. This will speed up 1775 the detection of the loss at the sender, thus triggering sender side 1776 mitigation. 1778 A sender that detects high packet loss rates for ECT-marked packets 1779 SHOULD immediately switch to sending packets as not-ECT to determine 1780 if the losses are potentially due to the ECT markings. If the losses 1781 disappear when the ECT-marking is discontinued, the RTP sender should 1782 go back to initiation procedures to attempt to verify the apparent 1783 loss of ECN capability of the used path. If a re-initiation fails 1784 then the two possible actions exist: 1786 1. Periodically retry the ECN initiation to detect if a path change 1787 occurs to a path that is ECN capable. 1789 2. Renegotiate the session to disable ECN support. This is a choice 1790 that is suitable if the impact of ECT probing on the media 1791 quality is noticeable. If multiple initiations have been 1792 successful, but the following full usage of ECN has resulted in 1793 the fallback procedures, then disabling of the ECN support is 1794 RECOMMENDED. 1796 We foresee the possibility of flapping ECN capability due to several 1797 reasons: video switching MCU or similar middleboxes that selects to 1798 deliver media from the sender only intermittently; load balancing 1799 devices may in worst case result in that some packets take a 1800 different network path than the others; mobility solutions that 1801 switch underlying network path in a transparent way for the sender or 1802 receiver; and membership changes in a multicast group. It is however 1803 appropriate to mention that there are also issues such as re-routing 1804 of traffic due to a flappy route table or excessive reordering and 1805 other issues that are not directly ECN related but nevertheless may 1806 cause problems for ECN. 1808 7.4.2. Interpretation of ECN Summary information 1810 This section contains discussion on how the ECN summary report 1811 information can be used to detect various types of ECN path issues. 1812 We first review the information the RTCP reports provide on a per 1813 source (SSRC) basis: 1815 ECN-CE Counter: The number of RTP packets received so far in the 1816 session with an ECN field set to CE. 1818 ECT (0/1) Counters: The number of RTP packets received so far in the 1819 session with an ECN field set to ECT (0) and ECT (1) respectively. 1821 not-ECT Counter: The number of RTP packets received so far in the 1822 session with an ECN field set to not-ECT. 1824 Lost Packets counter: The number of RTP packets that where expected 1825 based on sequence numbers but never received. 1827 Duplication Counter: The number of received RTP packets that are 1828 duplicates of already received ones. 1830 Extended Highest Sequence number: The highest sequence number seen 1831 when sending this report, but with additional bits, to handle 1832 disambiguation when wrapping the RTP sequence number field. 1834 The counters will be initialised to zero to provide values for the 1835 RTP stream sender from the first report. After the first report, the 1836 changes between the last received report and the previous report are 1837 determined by simply taking the values of the latest minus the 1838 previous, taking wrapping into account. This definition is also 1839 robust to packet losses, since if one report is missing, the 1840 reporting interval becomes longer, but is otherwise equally valid. 1842 In a perfect world, the number of not-ECT packets received should be 1843 equal to the number sent minus the lost packets counter, and the sum 1844 of the ECT(0), ECT(1), and ECN-CE counters should be equal to the 1845 number of ECT marked packet sent. Two issues may cause a mismatch in 1846 these statistics: severe network congestion or unresponsive 1847 congestion control might cause some ECT-marked packets to be lost, 1848 and packet duplication might result in some packets being received, 1849 and counted in the statistics, multiple times (potentially with a 1850 different ECN-mark on each copy of the duplicate). 1852 The rate of packet duplication is tracked, allowing one to take the 1853 duplication into account. The value of the ECN field for duplicates 1854 will also be counted and when comparing the figures one needs to take 1855 some fraction of packet duplicates that are non-ECT and some fraction 1856 of packet duplicates being ECT into account into the calculation. 1857 Thus when only sending non-ECT then the number of sent packets plus 1858 reported duplicates equals the number of received non-ECT. When 1859 sending only ECT then number of sent ECT packets plus duplicates will 1860 equal ECT(0), ECT(1), ECN-CE and packet loss. When sending a mix of 1861 non-ECT and ECT then there is an uncertainty if any duplicate or 1862 packet loss was an non-ECT or ECT. If the packet duplication is 1863 completely independent of the usage of ECN, then the fraction of 1864 packet duplicates should be in relation to the number of non-ECT vs 1865 ECT packet sent during the period of comparison. This relation does 1866 not hold for packet loss, where higher rates of packet loss for non- 1867 ECT is expected than for ECT traffic. 1869 Detecting clearing of ECN field: If the ratio between ECT and not-ECT 1870 transmitted in the reports has become all not-ECT, or has 1871 substantially changed towards not-ECT, then this is clearly an 1872 indication that the path results in clearing of the ECT field. 1874 Dropping of ECT packets: To determine if the packet drop ratio is 1875 different between not-ECT and ECT marked transmission requires a mix 1876 of transmitted traffic. The sender should compare if the delivery 1877 percentage (delivered / transmitted) between ECT and not-ECT is 1878 significantly different. Care must be taken if the number of packets 1879 are low in either of the categories. One must also take into account 1880 the level of CE marking. A CE marked packet would have been dropped 1881 unless it was ECT marked. Thus, the packet loss level for not-ECT 1882 should be approximately equal to the loss rate for ECT when counting 1883 the CE marked packets as lost ones. A sender performing this 1884 calculation needs to ensure that the difference is statistically 1885 significant. 1887 If erroneous behavior is detected, it should be logged to enable 1888 follow up and statistics gathering. 1890 8. Processing ECN in RTP Translators and Mixers 1892 RTP translators and mixers that support ECN for RTP are required to 1893 process, and potentially modify or generate ECN marking in RTP 1894 packets. They also need to process, and potentially modify or 1895 generate RTCP ECN feedback packets for the translated and/or mixed 1896 streams. This includes both downstream RTCP reports generated by the 1897 media sender, and also reports generated by the receivers, flowing 1898 upstream back towards the sender. 1900 8.1. Transport Translators 1902 Some translators only perform transport level translations, like 1903 copying packets from one address domain, like unicast to multicast. 1904 It may also perform relaying like copying an incoming packet to a 1905 number of unicast receivers. This section details the ECN related 1906 actions for RTP and RTCP. 1908 For the RTP data packets the translator, which does not modify the 1909 media stream, SHOULD copy the ECN bits unchanged from the incoming to 1910 the outgoing datagrams, unless the translator itself is overloaded 1911 and experiencing congestion, in which case it may mark the outgoing 1912 datagrams with an ECN-CE mark. 1914 A Transport translator does not modify RTCP packets. It however MUST 1915 perform the corresponding transport translation of the RTCP packets 1916 as it does with RTP packets being sent from the same source/ 1917 end-point. 1919 8.2. Fragmentation and Reassembly in Translators 1921 An RTP translator may fragment or reassemble RTP data packets without 1922 changing the media encoding, and without reference to the congestion 1923 state of the networks it bridges. An example of this might be to 1924 combine packets of a voice-over-IP stream coded with one 20ms frame 1925 per RTP packet into new RTP packets with two 20ms frames per packet, 1926 thereby reducing the header overheads and so stream bandwidth, at the 1927 expense of an increase in latency. If multiple data packets are re- 1928 encoded into one, or vice versa, the RTP translator MUST assign new 1929 sequence numbers to the outgoing packets. Losses in the incoming RTP 1930 packet stream may also induce corresponding gaps in the outgoing RTP 1931 sequence numbers. An RTP translator MUST rewrite RTCP packets to 1932 make the corresponding changes to their sequence numbers, and to 1933 reflect the impact of the fragmentation or reassembly. This section 1934 describes how that rewriting is to be done for RTCP ECN feedback 1935 packets. Section 7.2 of [RFC3550] describes general procedures for 1936 other RTCP packet types. 1938 The processing of arriving RTP packets for this case is as follows. 1939 If an ECN marked packet is split into two, then both the outgoing 1940 packets MUST be ECN marked identically to the original; if several 1941 ECN marked packets are combined into one, the outgoing packet MUST be 1942 either ECN-CE marked or dropped if any of the incoming packets are 1943 ECN-CE marked. If the outgoing combined packet is not ECN-CE marked, 1944 then it MUST be ECT marked if any of the incoming packets were ECT 1945 marked. 1947 RTCP ECN feedback packets (Section 5.1) contain seven fields that are 1948 rewritten in an RTP translator that fragments or reassembles packets: 1949 the extended highest sequence number, the duplication counter, the 1950 lost packets counter, the ECN-CE counter, and not-ECT counter, the 1951 ECT(0) counter, and the ECT(1) counter. The RTCP XR report block for 1952 ECN summary information (Section 5.2) includes all of these fields 1953 except the extended highest sequence number which is present in the 1954 report block in an SR or RR packet. The procedures for rewriting 1955 these fields are the same for both RTCP ECN feedback packet and the 1956 RTCP XR ECN summary packet. 1958 When receiving an RTCP ECN feedback packet for the translated stream, 1959 an RTP translator first determines the range of packets to which the 1960 report corresponds. The extended highest sequence number in the RTCP 1961 ECN feedback packet (or in the RTCP SR/RR packet contained within the 1962 compound packet, in the case of RTCP XR ECN summary reports) 1963 specifies the end sequence number of the range. For the first RTCP 1964 ECN feedback packet received, the initial extended sequence number of 1965 the range may be determined by subtracting the sum of the lost 1966 packets counter, the ECN-CE counter, the not-ECT counter, the ECT(0) 1967 counter and the ECT(1) counter minus the duplication counter, from 1968 the extended highest sequence number. For subsequent RTCP ECN 1969 feedback packets, the starting sequence number may be determined as 1970 being one after the extended highest sequence number of the previous 1971 RTCP ECN feedback packet received from the same SSRC. These values 1972 are in the sequence number space of the translated packets. 1974 Based on its knowledge of the translation process, the translator 1975 determines the sequence number range for the corresponding original, 1976 pre-translation, packets. The extended highest sequence number in 1977 the RTCP ECN feedback packet is rewritten to match the final sequence 1978 number in the pre-translation sequence number range. 1980 The translator then determines the ratio, R, of the number of packets 1981 in the translated sequence number space (numTrans) to the number of 1982 packets in the pre-translation sequence number space (numOrig) such 1983 that R = numTrans / numOrig. The counter values in the RTCP ECN 1984 feedback report are then scaled by dividing each of them by R. For 1985 example, if the translation process combines two RTP packets into 1986 one, then numOrig will be twice numTrans, giving R=0.5, and the 1987 counters in the translated RTCP ECN feedback packet will be twice 1988 those in the original. 1990 The ratio, R, may have a value that leads to non-integer multiples of 1991 the counters when translating the RTCP packet. For example, a VoIP 1992 translator that combines two adjacent RTP packets into one if they 1993 contain active speech data, but passes comfort noise packets 1994 unchanged, would have an R values of between 0.5 and 1.0 depending on 1995 the amount of active speech. Since the counter values in the 1996 translated RTCP report are integer values, rounding will be necessary 1997 in this case. 1999 When rounding counter values in the translated RTCP packet, the 2000 translator should try to ensure that they sum to the number of RTP 2001 packets in the pre-translation sequence number space (numOrig). The 2002 translator should also try to ensure that no non-zero counter is 2003 rounded to a zero value, unless the pre-translated values are zero, 2004 since that will lose information that a particular type of event has 2005 occurred. It is recognised that it may be impossible to satisfy both 2006 of these constraints; in such cases, it is better to ensure that no 2007 non-zero counter is mapped to a zero value, since this preserves 2008 congestion adaptation and helps the RTCP-based ECN initiation 2009 process. 2011 One should be aware of the impact this type of translators have on 2012 the measurement of packet duplication. A translator performing 2013 aggregation and most likely also an fragmenting translator will 2014 suppress any duplication happening prior to itself. Thus the reports 2015 and what is being scaled will only represent packet duplication 2016 happening from the translator to the receiver reporting on the flow. 2018 It should be noted that scaling the RTCP counter values in this way 2019 is meaningful only on the assumption that the level of congestion in 2020 the network is related to the number of packets being sent. This is 2021 likely to be a reasonable assumption in the type of environment where 2022 RTP translators that fragment or reassemble packets are deployed, as 2023 their entire purpose is to change the number of packets being sent to 2024 adapt to known limitations of the network, but is not necessarily 2025 valid in general. 2027 The rewritten RTCP ECN feedback report is sent from the other side of 2028 the translator to that which it arrived (as part of a compound RTCP 2029 packet containing other translated RTCP packets, where appropriate). 2031 8.3. Generating RTCP ECN Feedback in Media Transcoders 2033 An RTP translator that acts as a media transcoder cannot directly 2034 forward RTCP packets corresponding to the transcoded stream, since 2035 those packets will relate to the non-transcoded stream, and will not 2036 be useful in relation to the transcoded RTP flow. Such a transcoder 2037 will need to interpose itself into the RTCP flow, acting as a proxy 2038 for the receiver to generate RTCP feedback in the direction of the 2039 sender relating to the pre-transcoded stream, and acting in place of 2040 the sender to generate RTCP relating to the transcoded stream, to be 2041 sent towards the receiver. This section describes how this proxying 2042 is to be done for RTCP ECN feedback packets. Section 7.2 of 2043 [RFC3550] describes general procedures for other RTCP packet types. 2045 An RTP translator acting as a media transcoder in this manner does 2046 not have its own SSRC, and hence is not visible to other entities at 2047 the RTP layer. RTCP ECN feedback packets and RTCP XR report blocks 2048 for ECN summary information that are received from downstream relate 2049 to the translated stream, and so must be processed by the translator 2050 as if it were the original media source. These reports drive the 2051 congestion control loop and media adaptation between the translator 2052 and the downstream receiver. If there are multiple downstream 2053 receivers, a logically separate transcoder instance must be used for 2054 each receiver, and must process RTCP ECN feedback and summary reports 2055 independently to the other transcoder instances. An RTP translator 2056 acting as a media transcoder in this manner MUST NOT forward RTCP ECN 2057 feedback packets or RTCP XR ECN summary reports from downstream 2058 receivers in the upstream direction. 2060 An RTP translator acting as a media transcoder will generate RTCP 2061 reports upstream towards the original media sender, based on the 2062 reception quality of the original media stream at the translator. 2063 The translator will run a separate congestion control loop and media 2064 adaptation between itself and the media sender for each of its 2065 downstream receivers, and must generate RTCP ECN feedback packets and 2066 RTCP XR ECN summary reports for that congestion control loop using 2067 the SSRC of that downstream receiver. 2069 8.4. Generating RTCP ECN Feedback in Mixers 2071 An RTP mixer terminates one-or-more RTP flows, combines them into a 2072 single outgoing media stream, and transmits that new stream as a 2073 separate RTP flow. A mixer has its own SSRC, and is visible to other 2074 participants in the session at the RTP layer. 2076 An ECN-aware RTP mixer must generate RTCP ECN feedback packets and 2077 RTCP XR report blocks for ECN summary information relating to the RTP 2078 flows it terminates, in exactly the same way it would if it were an 2079 RTP receiver. These reports form part of the congestion control loop 2080 between the mixer and the media senders generating the streams it is 2081 mixing. A separate control loop runs between each sender and the 2082 mixer. 2084 An ECN-aware RTP mixer will negotiate and initiate the use of ECN on 2085 the mixed RTP flows it generates, and will accept and process RTCP 2086 ECN feedback reports and RTCP XR report blocks for ECN relating to 2087 those mixed flows as if it were a standard media sender. A 2088 congestion control loop runs between the mixer and its receivers, 2089 driven in part by the ECN reports received. 2091 An RTP mixer MUST NOT forward RTCP ECN feedback packets or RTCP XR 2092 ECN summary reports from downstream receivers in the upstream 2093 direction. 2095 9. Implementation considerations 2097 To allow the use of ECN with RTP over UDP, an RTP implementation 2098 desiring to support receiving ECN controlled media streams must 2099 support reading the value of the ECT bits on received UDP datagrams, 2100 and an RTP implementation desiring to support sending ECN controlled 2101 media streams must support setting the ECT bits in outgoing UDP 2102 datagrams. The standard Berkeley sockets API pre-dates the 2103 specification of ECN, and does not provide the functionality which is 2104 required for this mechanism to be used with UDP flows, making this 2105 specification difficult to implement portably. 2107 10. IANA Considerations 2109 Note to RFC Editor: please replace "RFC XXXX" below with the RFC 2110 number of this memo, and remove this note. 2112 10.1. SDP Attribute Registration 2114 Following the guidelines in [RFC4566], the IANA is requested to 2115 register one new SDP attribute: 2117 o Contact name, email address and telephone number: Authors of 2118 RFCXXXX 2120 o Attribute-name: ecn-capable-rtp 2122 o Type of attribute: media-level 2124 o Subject to charset: no 2126 This attribute defines the ability to negotiate the use of ECT (ECN 2127 capable transport) for RTP flows running over UDP/IP. This attribute 2128 should be put in the SDP offer if the offering party wishes to 2129 receive an ECT flow. The answering party should include the 2130 attribute in the answer if it wish to receive an ECT flow. If the 2131 answerer does not include the attribute then ECT MUST be disabled in 2132 both directions. 2134 10.2. RTP/AVPF Transport Layer Feedback Message 2136 The IANA is requested to register one new RTP/AVPF Transport Layer 2137 Feedback Message in the table of FMT values for RTPFB Payload Types 2138 [RFC4585] as defined in Section 5.1: 2140 Name: RTCP-ECN-FB 2141 Long name: RTCP ECN Feedback 2142 Value: TBA1 2143 Reference: RFC XXXX 2145 10.3. RTCP Feedback SDP Parameter 2147 The IANA is requested to register one new SDP "rtcp-fb" attribute 2148 "nack" parameter "ecn" in the SDP ("ack" and "nack" Attribute Values) 2149 registry. 2150 Value name: ecn 2151 Long name: Explicit Congestion Notification 2152 Usable with: nack 2153 Reference: RFC XXXX 2155 10.4. RTCP XR Report blocks 2157 The IANA is requested to register one new RTCP XR Block Type as 2158 defined in Section 5.2: 2160 Block Type: TBA2 2161 Name: ECN Summary Report 2162 Reference: RFC XXXX 2164 10.5. RTCP XR SDP Parameter 2166 The IANA is requested to register one new RTCP XR SDP Parameter "ecn- 2167 sum" in the "RTCP XR SDP Parameters" registry. 2168 Parameter name XR block (block type and name) 2169 -------------- ------------------------------------ 2170 ecn-sum TBA2 ECN Summary Report Block 2172 10.6. STUN attribute 2174 A new STUN [RFC5389] attribute in the Comprehension-optional range 2175 under IETF Review (0x8000-0xFFFF) is request to be assigned to the 2176 STUN attribute defined in Section 7.2.2. The STUN attribute registry 2177 can currently be found at: http://www.iana.org/assignments/ 2178 stun-parameters/stun-parameters.xhtml. 2180 10.7. ICE Option 2182 A new ICE option "rtp+ecn" is registered in the registry that "IANA 2183 Registry for Interactive Connectivity Establishment (ICE) Options" 2184 [RFC6336] creates. 2186 11. Security Considerations 2188 The use of ECN with RTP over UDP as specified in this document has 2189 the following known security issues that need to be considered. 2191 External threats to the RTP and RTCP traffic: 2193 Denial of Service affecting RTCP: An attacker that can modify the 2194 traffic between the media sender and a receiver can achieve either 2195 of two things: 1) Report a lot of packets as being Congestion 2196 Experience marked, thus forcing the sender into a congestion 2197 response; or 2) Ensure that the sender disable the usage of ECN by 2198 reporting failures to receive ECN by changing the counter fields. 2199 This can also be accomplished by injecting false RTCP packets to 2200 the media sender. Reporting a lot of ECN-CE marked traffic is 2201 likely the more efficient denial of service tool as that may 2202 likely force the application to use lowest possible bit-rates. 2203 The prevention against an external threat is to integrity protect 2204 the RTCP feedback information and authenticate the sender. 2206 Information leakage: The ECN feedback mechanism exposes the 2207 receivers perceived packet loss, what packets it considers to be 2208 ECN-CE marked and its calculation of the ECN-none. This is mostly 2209 not considered as sensitive information. If it is considered 2210 sensitive the RTCP feedback should be encrypted. 2212 Changing the ECN bits: An on-path attacker that sees the RTP packet 2213 flow from sender to receiver and who has the capability to change 2214 the packets can rewrite ECT into ECN-CE thus forcing the sender or 2215 receiver to take congestion control response. This denial of 2216 service against the media quality in the RTP session is impossible 2217 for an end-point to protect itself against. Only network 2218 infrastructure nodes can detect this illicit re-marking. It will 2219 be mitigated by turning off ECN, however, if the attacker can 2220 modify its response to drop packets the same vulnerability exist. 2222 Denial of Service affecting the session set-up signalling: If an 2223 attacker can modify the session signalling it can prevent the 2224 usage of ECN by removing the signalling attributes used to 2225 indicate that the initiator is capable and willing to use ECN with 2226 RTP/UDP. This attack can be prevented by authentication and 2227 integrity protection of the signalling. We do note that any 2228 attacker that can modify the signalling has more interesting 2229 attacks they can perform than prevent the usage of ECN, like 2230 inserting itself as a middleman in the media flows enabling wire- 2231 tapping also for an off-path attacker. 2233 The following are threats that exist from misbehaving senders or 2234 receivers: 2236 Receivers cheating: A receiver may attempt to cheat and fail to 2237 report reception of ECN-CE marked packets. The benefit for a 2238 receiver cheating in its reporting would be to get an unfair bit- 2239 rate share across the resource bottleneck. It is far from certain 2240 that a receiver would be able to get a significant larger share of 2241 the resources. That assumes a high enough level of aggregation 2242 that there are flows to acquire shares from. The risk of cheating 2243 is that failure to react to congestion results in packet loss and 2244 increased path delay. 2246 Receivers misbehaving: A receiver may prevent the usage of ECN in an 2247 RTP session by reporting itself as non ECN capable, forcing the 2248 sender to turn off usage of ECN. In a point-to-point scenario 2249 there is little incentive to do this as it will only affect the 2250 receiver. Thus failing to utilise an optimisation. For multi- 2251 party session there exist some motivation why a receiver would 2252 misbehave as it can prevent also the other receivers from using 2253 ECN. As an insider into the session it is difficult to determine 2254 if a receiver is misbehaving or simply incapable, making it 2255 basically impossible in the incremental deployment phase of ECN 2256 for RTP usage to determine this. If additional information about 2257 the receivers and the network is known it might be possible to 2258 deduce that a receiver is misbehaving. If it can be determined 2259 that a receiver is misbehaving, the only response is to exclude it 2260 from the RTP session and ensure that is does not any longer have 2261 any valid security context to affect the session. 2263 Misbehaving Senders: The enabling of ECN gives the media packets a 2264 higher degree of probability to reach the receiver compared to 2265 not-ECT marked ones on a ECN capable path. However, this is no 2266 magic bullet and failure to react to congestion will most likely 2267 only slightly delay a network buffer over-run, in which its 2268 session also will experience packet loss and increased delay. 2269 There is some possibility that the media senders traffic will push 2270 other traffic out of the way without being affected too 2271 negatively. However, we do note that a media sender still needs 2272 to implement congestion control functions to prevent the media 2273 from being badly affected by congestion events. Thus the 2274 misbehaving sender is getting a unfair share. This can only be 2275 detected and potentially prevented by network monitoring and 2276 administrative entities. See Section 7 of [RFC3168] for more 2277 discussion of this issue. 2279 We note that the end-point security functions needed to prevent an 2280 external attacker from inferring with the signalling are source 2281 authentication and integrity protection. To prevent information 2282 leakage from the feedback packets encryption of the RTCP is also 2283 needed. For RTP there exist multiple solutions possible depending on 2284 the application context. Secure RTP (SRTP) [RFC3711] does satisfy 2285 the requirement to protect this mechanism despite only providing 2286 authentication if a entity is within the security context or not. 2287 IPsec [RFC4301] and DTLS [RFC6347] can also provide the necessary 2288 security functions. 2290 The signalling protocols used to initiate an RTP session also need to 2291 be source authenticated and integrity protected to prevent an 2292 external attacker from modifying any signalling. Here an appropriate 2293 mechanism to protect the used signalling needs to be used. For SIP/ 2294 SDP ideally S/MIME [RFC5751] would be used. However, with the 2295 limited deployment a minimal mitigation strategy is to require use of 2296 SIPS (SIP over TLS) [RFC3261] [RFC5630] to at least accomplish hop- 2297 by-hop protection. 2299 We do note that certain mitigation methods will require network 2300 functions. 2302 12. Examples of SDP Signalling 2304 This section contain a few different examples of the signalling 2305 mechanism defined in this specification in an SDP context. If there 2306 are discrepancies between these examples and the specification text, 2307 the specification text is definitive. 2309 12.1. Basic SDP Offer/Answer 2311 This example is a basic offer/answer SDP exchange, assumed done by 2312 SIP (not shown). The intention is to establish a basic audio session 2313 point to point between two users. 2315 The Offer: 2316 v=0 2317 o=jdoe 3502844782 3502844782 IN IP4 10.0.1.4 2318 s=VoIP call 2319 i=SDP offer for VoIP call with ICE and ECN for RTP 2320 b=AS:128 2321 b=RR:2000 2322 b=RS:2500 2323 a=ice-pwd:YH75Fviy6338Vbrhrlp8Yh 2324 a=ice-ufrag:9uB6 2325 a=ice-options:rtp+ecn 2326 t=0 0 2327 m=audio 45664 RTP/AVPF 97 98 99 2328 c=IN IP4 192.0.2.3 2329 a=rtpmap:97 G719/48000/1 2330 a=fmtp:97 maxred=160 2331 a=rtpmap:98 AMR-WB/16000/1 2332 a=fmtp:98 octet-align=1; mode-change-capability=2 2333 a=rtpmap:99 PCMA/8000/1 2334 a=maxptime:160 2335 a=ptime:20 2336 a=ecn-capable-rtp: ice rtp ect=0 mode=setread 2337 a=rtcp-fb:* nack ecn 2338 a=rtcp-fb:* trr-int 1000 2339 a=rtcp-xr:ecn-sum 2340 a=rtcp-rsize 2341 a=candidate:1 1 UDP 2130706431 10.0.1.4 8998 typ host 2342 a=candidate:2 1 UDP 1694498815 192.0.2.3 45664 typ srflx raddr 2343 10.0.1.4 rport 8998 2345 This SDP offer offers a single media stream with 3 media payload 2346 types. It proposes to use ECN with RTP, with the ICE based 2347 initialization as being preferred over the RTP/RTCP one. Leap of 2348 faith is not suggested to be used. The offerer is capable of both 2349 setting and reading the ECN bits. In addition the use of both the 2350 RTCP ECN feedback packet and the RTCP XR ECN summary report are 2351 supported. ICE is also proposed with two candidates. It also 2352 supports reduced size RTCP and can to use it. 2354 The Answer: 2355 v=0 2356 o=jdoe 3502844783 3502844783 IN IP4 198.51.100.235 2357 s=VoIP call 2358 i=SDP offer for VoIP call with ICE and ECN for RTP 2359 b=AS:128 2360 b=RR:2000 2361 b=RS:2500 2362 a=ice-pwd:asd88fgpdd777uzjYhagZg 2363 a=ice-ufrag:8hhY 2364 a=ice-options:rtp+ecn 2365 t=0 0 2366 m=audio 53879 RTP/AVPF 97 99 2367 c=IN IP4 198.51.100.235 2368 a=rtpmap:97 G719/48000/1 2369 a=fmtp:97 maxred=160 2370 a=rtpmap:99 PCMA/8000/1 2371 a=maxptime:160 2372 a=ptime:20 2373 a=ecn-capable-rtp: ice ect=0 mode=readonly 2374 a=rtcp-fb:* nack ecn 2375 a=rtcp-fb:* trr-int 1000 2376 a=rtcp-xr:ecn-sum 2377 a=candidate:1 1 UDP 2130706431 198.51.100.235 53879 typ host 2379 The answer confirms that only one media stream will be used. One RTP 2380 Payload type was removed. ECN capability was confirmed, and the 2381 initialization method will be ICE. However, the answerer is only 2382 capable of reading the ECN bits, which means that ECN can only be 2383 used for RTP flowing from the offerer to the answerer. ECT always 2384 set to 0 will be used in both directions. Both the RTCP ECN feedback 2385 packet and the RTCP XR ECN summary report will be used. Reduced size 2386 RTCP will not be used as the answerer has not indicated support for 2387 it in the answer. 2389 12.2. Declarative Multicast SDP 2391 The below session describes an any source multicast using session 2392 with a single media stream. 2394 v=0 2395 o=jdoe 3502844782 3502844782 IN IP4 198.51.100.235 2396 s=Multicast SDP session using ECN for RTP 2397 i=Multicasted audio chat using ECN for RTP 2398 b=AS:128 2399 t=3502892703 3502910700 2400 m=audio 56144 RTP/AVPF 97 2401 c=IN IP4 233.252.0.212/127 2402 a=rtpmap:97 g719/48000/1 2403 a=fmtp:97 maxred=160 2404 a=maxptime:160 2405 a=ptime:20 2406 a=ecn-capable-rtp: rtp mode=readonly; ect=0 2407 a=rtcp-fb:* nack ecn 2408 a=rtcp-fb:* trr-int 1500 2409 a=rtcp-xr:ecn-sum 2411 In the above example, as this is declarative we need to require 2412 certain functionality. As it is ASM the initialization method that 2413 can work here is the RTP/RTCP based one. So that is indicated. The 2414 ECN setting and reading capability to take part of this session is at 2415 least read. If one is capable of setting that is good, but not 2416 required as one can skip using ECN for anything one sends oneself. 2417 The ECT value is recommended to be set to 0 always. The ECN usage in 2418 this session requires both ECN feedback and the XR ECN summary 2419 report, so their use is also indicated. 2421 13. Acknowledgments 2423 The authors wish to thank the following persons for their reviews and 2424 comments: Thomas Belling, Bob Briscoe, Roni Even, Kevin P. Flemming, 2425 Tomas Frankkila, Christian Groves, Christer Holmgren, Cullen Jennings 2426 Tom Van Caenegem, Simo Veikkolainen, Bill Ver Steeg, Dan Wing, Qin 2427 Wu, and Lei Zhu. 2429 14. References 2431 14.1. Normative References 2433 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 2434 Requirement Levels", BCP 14, RFC 2119, March 1997. 2436 [RFC3168] Ramakrishnan, K., Floyd, S., and D. Black, "The Addition 2437 of Explicit Congestion Notification (ECN) to IP", 2438 RFC 3168, September 2001. 2440 [RFC3550] Schulzrinne, H., Casner, S., Frederick, R., and V. 2441 Jacobson, "RTP: A Transport Protocol for Real-Time 2442 Applications", STD 64, RFC 3550, July 2003. 2444 [RFC3611] Friedman, T., Caceres, R., and A. Clark, "RTP Control 2445 Protocol Extended Reports (RTCP XR)", RFC 3611, 2446 November 2003. 2448 [RFC5234] Crocker, D. and P. Overell, "Augmented BNF for Syntax 2449 Specifications: ABNF", STD 68, RFC 5234, January 2008. 2451 [RFC5245] Rosenberg, J., "Interactive Connectivity Establishment 2452 (ICE): A Protocol for Network Address Translator (NAT) 2453 Traversal for Offer/Answer Protocols", RFC 5245, 2454 April 2010. 2456 [RFC5348] Floyd, S., Handley, M., Padhye, J., and J. Widmer, "TCP 2457 Friendly Rate Control (TFRC): Protocol Specification", 2458 RFC 5348, September 2008. 2460 [RFC5389] Rosenberg, J., Mahy, R., Matthews, P., and D. Wing, 2461 "Session Traversal Utilities for NAT (STUN)", RFC 5389, 2462 October 2008. 2464 [RFC6336] Westerlund, M. and C. Perkins, "IANA Registry for 2465 Interactive Connectivity Establishment (ICE) Options", 2466 RFC 6336, July 2011. 2468 14.2. Informative References 2470 [I-D.ietf-avt-rtp-no-op] 2471 Andreasen, F., "A No-Op Payload Format for RTP", 2472 draft-ietf-avt-rtp-no-op-04 (work in progress), May 2007. 2474 [RFC1112] Deering, S., "Host extensions for IP multicasting", STD 5, 2475 RFC 1112, August 1989. 2477 [RFC2762] Rosenberg, J. and H. Schulzrinne, "Sampling of the Group 2478 Membership in RTP", RFC 2762, February 2000. 2480 [RFC2974] Handley, M., Perkins, C., and E. Whelan, "Session 2481 Announcement Protocol", RFC 2974, October 2000. 2483 [RFC3261] Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston, 2484 A., Peterson, J., Sparks, R., Handley, M., and E. 2485 Schooler, "SIP: Session Initiation Protocol", RFC 3261, 2486 June 2002. 2488 [RFC3264] Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model 2489 with Session Description Protocol (SDP)", RFC 3264, 2490 June 2002. 2492 [RFC3540] Spring, N., Wetherall, D., and D. Ely, "Robust Explicit 2493 Congestion Notification (ECN) Signaling with Nonces", 2494 RFC 3540, June 2003. 2496 [RFC3551] Schulzrinne, H. and S. Casner, "RTP Profile for Audio and 2497 Video Conferences with Minimal Control", STD 65, RFC 3551, 2498 July 2003. 2500 [RFC3569] Bhattacharyya, S., "An Overview of Source-Specific 2501 Multicast (SSM)", RFC 3569, July 2003. 2503 [RFC3711] Baugher, M., McGrew, D., Naslund, M., Carrara, E., and K. 2504 Norrman, "The Secure Real-time Transport Protocol (SRTP)", 2505 RFC 3711, March 2004. 2507 [RFC4301] Kent, S. and K. Seo, "Security Architecture for the 2508 Internet Protocol", RFC 4301, December 2005. 2510 [RFC4340] Kohler, E., Handley, M., and S. Floyd, "Datagram 2511 Congestion Control Protocol (DCCP)", RFC 4340, March 2006. 2513 [RFC4566] Handley, M., Jacobson, V., and C. Perkins, "SDP: Session 2514 Description Protocol", RFC 4566, July 2006. 2516 [RFC4585] Ott, J., Wenger, S., Sato, N., Burmeister, C., and J. Rey, 2517 "Extended RTP Profile for Real-time Transport Control 2518 Protocol (RTCP)-Based Feedback (RTP/AVPF)", RFC 4585, 2519 July 2006. 2521 [RFC4588] Rey, J., Leon, D., Miyazaki, A., Varsa, V., and R. 2522 Hakenberg, "RTP Retransmission Payload Format", RFC 4588, 2523 July 2006. 2525 [RFC4607] Holbrook, H. and B. Cain, "Source-Specific Multicast for 2526 IP", RFC 4607, August 2006. 2528 [RFC4960] Stewart, R., "Stream Control Transmission Protocol", 2529 RFC 4960, September 2007. 2531 [RFC5124] Ott, J. and E. Carrara, "Extended Secure RTP Profile for 2532 Real-time Transport Control Protocol (RTCP)-Based Feedback 2533 (RTP/SAVPF)", RFC 5124, February 2008. 2535 [RFC5506] Johansson, I. and M. Westerlund, "Support for Reduced-Size 2536 Real-Time Transport Control Protocol (RTCP): Opportunities 2537 and Consequences", RFC 5506, April 2009. 2539 [RFC5630] Audet, F., "The Use of the SIPS URI Scheme in the Session 2540 Initiation Protocol (SIP)", RFC 5630, October 2009. 2542 [RFC5751] Ramsdell, B. and S. Turner, "Secure/Multipurpose Internet 2543 Mail Extensions (S/MIME) Version 3.2 Message 2544 Specification", RFC 5751, January 2010. 2546 [RFC5760] Ott, J., Chesterfield, J., and E. Schooler, "RTP Control 2547 Protocol (RTCP) Extensions for Single-Source Multicast 2548 Sessions with Unicast Feedback", RFC 5760, February 2010. 2550 [RFC6189] Zimmermann, P., Johnston, A., and J. Callas, "ZRTP: Media 2551 Path Key Agreement for Unicast Secure RTP", RFC 6189, 2552 April 2011. 2554 [RFC6347] Rescorla, E. and N. Modadugu, "Datagram Transport Layer 2555 Security Version 1.2", RFC 6347, January 2012. 2557 Authors' Addresses 2559 Magnus Westerlund 2560 Ericsson 2561 Farogatan 6 2562 SE-164 80 Kista 2563 Sweden 2565 Phone: +46 10 714 82 87 2566 Email: magnus.westerlund@ericsson.com 2568 Ingemar Johansson 2569 Ericsson 2570 Laboratoriegrand 11 2571 SE-971 28 Lulea 2572 SWEDEN 2574 Phone: +46 73 0783289 2575 Email: ingemar.s.johansson@ericsson.com 2576 Colin Perkins 2577 University of Glasgow 2578 School of Computing Science 2579 Glasgow G12 8QQ 2580 United Kingdom 2582 Email: csp@csperkins.org 2584 Piers O'Hanlon 2585 University College London 2586 Computer Science Department 2587 Gower Street 2588 London WC1E 6BT 2589 United Kingdom 2591 Email: p.ohanlon@cs.ucl.ac.uk 2593 Ken Carlberg 2594 G11 2595 1600 Clarendon Blvd 2596 Arlington VA 2597 USA 2599 Email: carlberg@g11.org.uk