idnits 2.17.1 draft-ietf-avtcore-cc-feedback-message-07.txt: Checking boilerplate required by RFC 5378 and the IETF Trust (see https://trustee.ietf.org/license-info): ---------------------------------------------------------------------------- No issues found here. Checking nits according to https://www.ietf.org/id-info/1id-guidelines.txt: ---------------------------------------------------------------------------- No issues found here. Checking nits according to https://www.ietf.org/id-info/checklist : ---------------------------------------------------------------------------- No issues found here. Miscellaneous warnings: ---------------------------------------------------------------------------- == The copyright year in the IETF Trust and authors Copyright Line does not match the current year -- The document date (June 10, 2020) is 1413 days in the past. Is this intentional? Checking references for intended status: Proposed Standard ---------------------------------------------------------------------------- (See RFCs 3967 and 4897 for information about using normative references to lower-maturity documents in RFCs) == Outdated reference: A later version (-19) exists of draft-ietf-mmusic-sdp-mux-attributes-17 == Outdated reference: A later version (-12) exists of draft-ietf-rmcat-rtp-cc-feedback-05 Summary: 0 errors (**), 0 flaws (~~), 3 warnings (==), 1 comment (--). Run idnits with the --verbose option for more detailed information about the items above. -------------------------------------------------------------------------------- 2 IETF RMCAT Working Group Z. Sarker 3 Internet-Draft Ericsson AB 4 Intended status: Standards Track C. Perkins 5 Expires: December 12, 2020 University of Glasgow 6 V. Singh 7 callstats.io 8 M. Ramalho 9 June 10, 2020 11 RTP Control Protocol (RTCP) Feedback for Congestion Control 12 draft-ietf-avtcore-cc-feedback-message-07 14 Abstract 16 This document describes an RTCP feedback message intended to enable 17 congestion control for interactive real-time traffic using RTP. The 18 feedback message is designed for use with a sender-based congestion 19 control algorithm, in which the receiver of an RTP flow sends RTCP 20 feedback packets to the sender containing the information the sender 21 needs to perform congestion control. 23 Status of This Memo 25 This Internet-Draft is submitted in full conformance with the 26 provisions of BCP 78 and BCP 79. 28 Internet-Drafts are working documents of the Internet Engineering 29 Task Force (IETF). Note that other groups may also distribute 30 working documents as Internet-Drafts. The list of current Internet- 31 Drafts is at https://datatracker.ietf.org/drafts/current/. 33 Internet-Drafts are draft documents valid for a maximum of six months 34 and may be updated, replaced, or obsoleted by other documents at any 35 time. It is inappropriate to use Internet-Drafts as reference 36 material or to cite them other than as "work in progress." 38 This Internet-Draft will expire on December 12, 2020. 40 Copyright Notice 42 Copyright (c) 2020 IETF Trust and the persons identified as the 43 document authors. All rights reserved. 45 This document is subject to BCP 78 and the IETF Trust's Legal 46 Provisions Relating to IETF Documents 47 (https://trustee.ietf.org/license-info) in effect on the date of 48 publication of this document. Please review these documents 49 carefully, as they describe your rights and restrictions with respect 50 to this document. Code Components extracted from this document must 51 include Simplified BSD License text as described in Section 4.e of 52 the Trust Legal Provisions and are provided without warranty as 53 described in the Simplified BSD License. 55 Table of Contents 57 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2 58 2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3 59 3. RTCP Feedback for Congestion Control . . . . . . . . . . . . 3 60 3.1. RTCP Congestion Control Feedback Report . . . . . . . . . 4 61 4. Feedback Frequency and Overhead . . . . . . . . . . . . . . . 7 62 5. Response to Loss of Feedback Packets . . . . . . . . . . . . 7 63 6. SDP Signalling . . . . . . . . . . . . . . . . . . . . . . . 8 64 7. Relation to RFC 6679 . . . . . . . . . . . . . . . . . . . . 8 65 8. Design Rationale . . . . . . . . . . . . . . . . . . . . . . 9 66 9. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 10 67 10. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 11 68 11. Security Considerations . . . . . . . . . . . . . . . . . . . 11 69 12. References . . . . . . . . . . . . . . . . . . . . . . . . . 11 70 12.1. Normative References . . . . . . . . . . . . . . . . . . 12 71 12.2. Informative References . . . . . . . . . . . . . . . . . 13 72 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 14 74 1. Introduction 76 For interactive real-time traffic, such as video conferencing flows, 77 the typical protocol choice is the Real-time Transport Protocol (RTP) 78 running over the User Datagram Protocol (UDP). RTP does not provide 79 any guarantee of Quality of Service (QoS), reliability, or timely 80 delivery, and expects the underlying transport protocol to do so. 81 UDP alone certainly does not meet that expectation. However, the RTP 82 Control Protocol (RTCP) provides a mechanism by which the receiver of 83 an RTP flow can periodically send transport and media quality metrics 84 to the sender of that RTP flow. This information can be used by the 85 sender to perform congestion control. In the absence of standardized 86 messages for this purpose, designers of congestion control algorithms 87 have developed proprietary RTCP messages that convey only those 88 parameters needed for their respective designs. As a direct result, 89 the different congestion control (i.e., rate adaptation) designs are 90 not interoperable. To enable algorithm evolution as well as 91 interoperability across designs (e.g., different rate adaptation 92 algorithms), it is highly desirable to have generic congestion 93 control feedback format. 95 To help achieve interoperability for unicast RTP congestion control, 96 this memo proposes a common RTCP feedback packet format that can be 97 used by NADA [RFC8698], SCReAM [RFC8298], Google Congestion Control 98 [I-D.ietf-rmcat-gcc] and Shared Bottleneck Detection [RFC8382], and 99 hopefully also by future RTP congestion control algorithms. 101 2. Terminology 103 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 104 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this 105 document are to be interpreted as described in [RFC2119]. 107 In addition the terminology defined in [RFC3550], [RFC3551], 108 [RFC3611], [RFC4585], and [RFC5506] applies. 110 3. RTCP Feedback for Congestion Control 112 Based on an analysis of NADA [RFC8698], SCReAM [RFC8298], Google 113 Congestion Control [I-D.ietf-rmcat-gcc] and Shared Bottleneck 114 Detection [RFC8382], the following per-RTP packet congestion control 115 feedback information has been determined to be necessary: 117 o RTP sequence number: The receiver of an RTP flow needs to feedback 118 the sequence numbers of the received RTP packets to the sender, so 119 the sender can determine which packets were received and which 120 were lost. Packet loss is used as an indication of congestion by 121 many congestion control algorithms. 123 o Packet Arrival Time: The receiver of an RTP flow needs to feedback 124 the arrival time of each RTP packet to the sender. Packet delay 125 and/or delay variation (jitter) is used as a congestion signal by 126 some congestion control algorithms. 128 o Packet Explicit Congestion Notification (ECN) Marking: If ECN 129 [RFC3168], [RFC6679] is used, it is necessary to feedback the 130 2-bit ECN mark in received RTP packets, indicating for each RTP 131 packet whether it is marked not-ECT, ECT(0), ECT(1), or ECN-CE. 132 If the path used by the RTP traffic is ECN capable the sender can 133 use Congestion Experienced (ECN-CE) marking information as a 134 congestion control signal. 136 Every RTP flow is identified by its Synchronization Source (SSRC) 137 identifier. Accordingly, the RTCP feedback format needs to group its 138 reports by SSRC, sending one report block per received SSRC. 140 As a practical matter, we note that host operating system (OS) 141 process interruptions can occur at inopportune times. Accordingly, 142 recording RTP packet send times at the sender, and the corresponding 143 RTP packet arrival times at the receiver, needs to be done with 144 deliberate care. This is because the time duration of host OS 145 interruptions can be significant relative to the precision desired in 146 the one-way delay estimates. Specifically, the send time needs to be 147 recorded at the last opportunity prior to transmitting the RTP packet 148 at the sender, and the arrival time at the receiver needs to be 149 recorded at the earliest available opportunity. 151 3.1. RTCP Congestion Control Feedback Report 153 Congestion control feedback can be sent as part of a regular 154 scheduled RTCP report, or in an RTP/AVPF early feedback packet. If 155 sent as early feedback, congestion control feedback MAY be sent in a 156 non-compound RTCP packet [RFC5506] if the RTP/AVPF profile [RFC4585] 157 or the RTP/SAVPF profile [RFC5124] is used. 159 Irrespective of how it is transported, the congestion control 160 feedback is sent as a Transport Layer Feedback Message (RTCP packet 161 type 205). The format of this RTCP packet is shown in Figure 1: 163 0 1 2 3 164 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 1 165 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 166 |V=2|P| FMT=CCFB | PT = 205 | length | 167 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 168 | SSRC of RTCP packet sender | 169 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 170 | SSRC of 1st RTP Stream | 171 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 172 | begin_seq | num_reports | 173 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 174 |L|ECN| Arrival time offset | ... . 175 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 176 . . 177 . . 178 . . 179 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 180 | SSRC of nth RTP Stream | 181 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 182 | begin_seq | num_reports | 183 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 184 |L|ECN| Arrival time offset | ... | 185 . . 186 . . 187 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 188 | Report Timestamp (32bits) | 189 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 191 Figure 1: RTCP Congestion Control Feedback Packet Format 193 The first eight octets comprise a standard RTCP header, with PT=205 194 and FMT=CCFB indicating that this is a congestion control feedback 195 packet, and with the SSRC set to that of the sender of the RTCP 196 packet. (NOTE TO RFC EDITOR: please replace CCFB here and in the 197 above diagram with the IANA assigned RTCP feedback packet type, and 198 remove this note) 200 Section 6.1 of [RFC4585] requires the RTCP header to be followed by 201 the SSRC of the RTP flow being reported upon. Accordingly, the RTCP 202 header is followed by a report block for each SSRC from which RTP 203 packets have been received, followed by a Report Timestamp. 205 Each report block begins with the SSRC of the received RTP Stream on 206 which it is reporting. Following this, the report block contains a 207 16-bit packet metric block for each RTP packet with sequence number 208 in the range begin_seq to begin_seq+num_reports inclusive (calculated 209 using arithmetic modulo 65536 to account for possible sequence number 210 wrap-around). If the number of 16-bit packet metric blocks included 211 in the report block is not a multiple of two, then 16 bits of zero 212 padding MUST be added after the last packet metric block, to align 213 the end of the packet metric blocks with the next 32 bit boundary. 214 The value of num_reports MAY be zero, indicating that there are no 215 packet metric blocks included for that SSRC. Each report block MUST 216 NOT include more than 16384 packet metric blocks (i.e., it MUST NOT 217 report on more than one quarter of the sequence number space in a 218 single report). 220 The contents of each 16-bit packet metric block comprises the L, ECN, 221 and ATO fields are as follows: 223 o L (1 bit): is a boolean to indicate if the packet was received. 0 224 represents that the packet was not yet received and all the 225 subsequent bits (ECN and ATO) are also set to 0. 1 represent the 226 packet was received and the subsequent bits in the block need to 227 be parsed. 229 o ECN (2 bits): is the echoed ECN mark of the packet. These are set 230 to 00 if not received, or if ECN is not used. 232 o Arrival time offset (ATO, 13 bits): is the arrival time of the RTP 233 packet at the receiver, as an offset before the time represented 234 by the Report Timestamp (RTS) field of this RTCP congestion 235 control feedback report. The ATO field is in units of 1/1024 236 seconds (this unit is chosen to give exact offsets from the RTS 237 field) so, for example, an ATO value of 512 indicates that the 238 corresponding RTP packet arrived exactly half a second before the 239 time instant represented by the RTS field. If the measured value 240 is greater than 8189/1024 seconds (the value that would be coded 241 as 0x1FFD), the value 0x1FFE MUST be reported to indicate an over- 242 range measurement. If the measurement is unavailable, or if the 243 arrival time of the RTP packet is after the time represented by 244 the RTS field, then an ATO value of 0x1FFF MUST be reported for 245 the packet. 247 The RTCP congestion control feedback report packet concludes with the 248 Report Timestamp field (RTS, 32 bits). This denotes the time instant 249 on which this packet is reporting, and is the instant from which the 250 arrival time offset values are calculated. The value of RTS field is 251 derived from the same clock used to generate the NTP timestamp field 252 in RTCP Sender Report (SR) packets. It is formatted as the middle 32 253 bits of an NTP format timestamp, as described in Section 4 of 254 [RFC3550]. 256 RTCP congestion control feedback packets SHOULD include a report 257 block for every active SSRC. The sequence number ranges reported on 258 in consecutive reports for a given SSRC will generally be contiguous, 259 but overlapping reports MAY be sent (and need to be sent in cases 260 where RTP packet reordering occurs across the boundary between 261 consecutive reports). If reports covering overlapping sequence 262 number ranges are sent, information in later reports updates that in 263 sent previous reports for RTP packets included in both reports. If 264 an RTP packet was reported as received in one report, that packet 265 MUST also be reported as received in any overlapping reports sent 266 later that cover its sequence number range. 268 RTCP congestion control feedback packets can be large if they are 269 sent infrequently relative to the number of RTP data packets. If an 270 RTCP congestion control feedback packet is too large to fit within 271 the path MTU, its sender SHOULD split it into multiple feedback 272 packets. The RTCP reporting interval SHOULD be chosen such that 273 feedback packets are sent often enough that they are small enough to 274 fit within the path MTU ([I-D.ietf-rmcat-rtp-cc-feedback] discusses 275 how to choose the reporting interval; specifications for RTP 276 congestion control algorithms can also provide guidance). 278 If duplicate copies of a particular RTP packet are received, then the 279 arrival time of the first copy to arrive MUST be reported. If any of 280 the copies of the duplicated packet are ECN-CE marked, then an ECN-CE 281 mark MUST be reported that for packet; otherwise the ECN mark of the 282 first copy to arrive is reported. 284 If no packets are received from an SSRC in a reporting interval, a 285 report block MAY be sent with begin_seq set to the highest sequence 286 number previously received from that SSRC and num_reports set to zero 287 (or, the report can simply to omitted). The corresponding SR/RR 288 packet will have a non-increased extended highest sequence number 289 received field that will inform the sender that no packets have been 290 received, but it can ease processing to have that information 291 available in the congestion control feedback reports too. 293 A report block indicating that certain RTP packets were lost is not 294 to be interpreted as a request to retransmit the lost packets. The 295 receiver of such a report might choose to retransmit such packets, 296 provided a retransmission payload format has been negotiated, but 297 there is no requirement that it do so. 299 4. Feedback Frequency and Overhead 301 There is a trade-off between speed and accuracy of reporting, and the 302 overhead of the reports. [I-D.ietf-rmcat-rtp-cc-feedback] discusses 303 this trade-off, suggests desirable RTCP feedback rates, and provides 304 guidance on how to configure the RTCP bandwidth fraction, etc., to 305 make appropriate use of the reporting block described in this memo. 306 Specifications for RTP congestion control algorithms can also provide 307 guidance. 309 It is generally understood that congestion control algorithms work 310 better with more frequent feedback. However, RTCP bandwidth and 311 transmission rules put some upper limits on how frequently the RTCP 312 feedback messages can be sent from an RTP receiver to the RTP sender. 313 In many cases, sending feedback once per frame is an upper bound 314 before the reporting overhead becomes excessive, although this will 315 depend on the media rate and more frequent feedback might be needed 316 with high-rate media flows [I-D.ietf-rmcat-rtp-cc-feedback]. 317 Analysis [feedback-requirements] has also shown that some candidate 318 congestion control algorithms can operate with less frequent 319 feedback, using a feedback interval range of 50-200ms. Applications 320 need to negotiate an appropriate congestion control feedback interval 321 at session setup time, based on the choice of congestion control 322 algorithm, the expected media bit rate, and the acceptable feedback 323 overhead. 325 5. Response to Loss of Feedback Packets 327 Like all RTCP packets, RTCP congestion control feedback packets might 328 be lost. All RTP congestion control algorithms MUST specify how they 329 respond to the loss of feedback packets. 331 If only a single congestion control feedback packet is lost, an 332 appropriate response is to assume that the level of congestion has 333 remained roughly the same as the previous report. However, if 334 multiple consecutive congestion control feedback packets are lost, 335 the sender SHOULD rapidly reduce its sending rate towards zero, as 336 this likely indicates a path failure. The RTP circuit breaker 337 [RFC8083] provides further guidance. 339 6. SDP Signalling 341 A new "ack" feedback parameter, "ccfb", is defined for use with the 342 "a=rtcp-fb:" SDP extension to indicate the use of the RTP Congestion 343 Control feedback packet format defined in Section 3. The ABNF 344 definition of this SDP parameter extension is: 346 rtcp-fb-ack-param = 347 rtcp-fb-ack-param =/ ccfb-par 348 ccfb-par = SP "ccfb" 350 When used with "ccfb" feedback, the wildcard payload type ("*") MUST 351 be used. This implies that the congestion control feedback is sent 352 for all payload types in use in the session, including any FEC and 353 retransmission payload types. An example of the resulting SDP 354 attribute is: 356 a=rtcp-fb:* ack ccfb 358 The offer/answer rules for these SDP feedback parameters are 359 specified in Section 4.2 of the RTP/AVPF profile [RFC4585]. 361 An SDP offer might indicate support for both the congestion control 362 feedback mechanism specified in this memo and one or more alternative 363 congestion control feedback mechanisms that offer substantially the 364 same semantics. In this case, the answering party SHOULD include 365 only one of the offered congestion control feedback mechanisms in its 366 answer. If a re-invite offering the same set of congestion control 367 feedback mechanisms is received, the generated answer SHOULD choose 368 the same congestion control feedback mechanism as in the original 369 answer where possible. 371 When the SDP BUNDLE extension 372 [I-D.ietf-mmusic-sdp-bundle-negotiation] is used for multiplexing, 373 the "a=rtcp-fb:" attribute has multiplexing category IDENTICAL-PER-PT 374 [I-D.ietf-mmusic-sdp-mux-attributes]. 376 7. Relation to RFC 6679 378 Use of Explicit Congestion Notification (ECN) with RTP is described 379 in [RFC6679]. That specifies how to negotiate the use of ECN with 380 RTP, and defines an RTCP ECN Feedback Packet to carry ECN feedback 381 reports. It uses an SDP "a=ecn-capaable-rtp:" attribute to negotiate 382 use of ECN, and the "a=rtcp-fb:" attributes with the "nack" parameter 383 "ecn" to negotiate the use of RTCP ECN Feedback Packets. 385 The RTCP ECN Feedback Packet is not useful when ECN is used with the 386 RTP Congestion Control Feedback Packet defined in this memo since it 387 provides duplicate information. Accordingly, when congestion control 388 feedback is to be used with RTP and ECN, the SDP offer generated MUST 389 include an "a=ecn-capable-rtp:" attribute to negotiate ECN support, 390 along with an "a=rtcp-fb:" attribute with the "ack" parameter "ccfb" 391 to indicate that the RTP Congestion Control Feedback Packet is to be 392 used for feedback. The "a=rtcp-fb:" attribute MUST NOT include the 393 "nack" parameter "ecn", so the RTCP ECN Feedback Packet will not be 394 used. 396 8. Design Rationale 398 The primary function of RTCP SR/RR packets is to report statistics on 399 the reception of RTP packets. The reception report blocks sent in 400 these packets contain information about observed jitter, fractional 401 packet loss, and cumulative packet loss. It was intended that this 402 information could be used to support congestion control algorithms, 403 but experience has shown that it is not sufficient for that purpose. 404 An efficient congestion control algorithm requires more fine grained 405 information on per packet reception quality than is provided by SR/RR 406 packets to react effectively. The feedback format defined in this 407 memo provides such fine grained feedback. 409 Several other RTCP extensions also provide more detailed feedback 410 than SR/RR packets: 412 TMMBR: The Codec Control Messages for the RTP/AVPF profile [RFC5104] 413 include a Temporary Maximum Media Bit Rate (TMMBR) message. This 414 is used to convey a temporary maximum bit rate limitation from a 415 receiver of RTP packets to their sender. Even though it was not 416 designed to replace congestion control, TMMBR has been used as a 417 means to do receiver based congestion control where the session 418 bandwidth is high enough to send frequent TMMBR messages, 419 especially when used with non-compound RTCP packets [RFC5506]. 420 This approach requires the receiver of the RTP packets to monitor 421 their reception, determine the level of congestion, and recommend 422 a maximum bit rate suitable for current available bandwidth on the 423 path; it also assumes that the RTP sender can/will respect that 424 bit rate. This is the opposite of the sender based congestion 425 control approach suggested in this memo, so TMMBR cannot be used 426 to convey the information needed for a sender based congestion 427 control. TMMBR could, however, be viewed a complementary 428 mechanism that can inform the sender of the receiver's current 429 view of acceptable maximum bit rate. The Received Estimated 430 Maximum Bit-rate (REMB) mechanism [I-D.alvestrand-rmcat-remb] 431 provides similar feedback. 433 RTCP Extended Reports (XR): Numerous RTCP extended report (XR) 434 blocks have been defined to report details of packet loss, arrival 435 times [RFC3611], delay [RFC6843], and ECN marking [RFC6679]. It 436 is possible to combine several such XR blocks into a compound RTCP 437 packet, to report the detailed loss, arrival time, and ECN marking 438 marking information needed for effective sender-based congestion 439 control. However, the result has high overhead both in terms of 440 bandwidth and complexity, due to the need to stack multiple 441 reports. 443 Transport-wide Congestion Control: The format defined in this memo 444 provides individual feedback on each SSRC. An alternative is to 445 add a header extension to each RTP packet, containing a single, 446 transport-wide, packet sequence number, then have the receiver 447 send RTCP reports giving feedback on these additional sequence 448 numbers [I-D.holmer-rmcat-transport-wide-cc-extensions]. Such an 449 approach adds the per-packet overhead of the header extension (8 450 octets per packet in the referenced format), but reduces the size 451 of the feedback packets, and can simplify the rate calculation at 452 the sender if it maintains a single rate limit that applies to all 453 RTP packets sent irrespective of their SSRC. Equally, the use of 454 transport-wide feedback makes it more difficult to adapt the 455 sending rate, or respond to lost packets, based on the reception 456 and/or loss patterns observed on a per-SSRC basis (for example, to 457 perform differential rate control and repair for audio and video 458 flows, based on knowledge of what packets from each flow were 459 lost). Transport-wide feedback is also a less natural fit with 460 the wider RTP framework, which makes extensive use of per-SSRC 461 sequence numbers and feedback. 463 Considering these issues, we believe it appropriate to design a new 464 RTCP feedback mechanism to convey information for sender based 465 congestion control algorithms. The new congestion control feedback 466 RTCP packet described in Section 3 provides such a mechanism. 468 9. Acknowledgements 470 This document is based on the outcome of a design team discussion in 471 the RTP Media Congestion Avoidance Techniques (RMCAT) working group. 472 The authors would like to thank David Hayes, Stefan Holmer, Randell 473 Jesup, Ingemar Johansson, Jonathan Lennox, Sergio Mena, Nils 474 Ohlmeier, Magnus Westerlund, and Xiaoqing Zhu for their valuable 475 feedback. 477 10. IANA Considerations 479 The IANA is requested to register one new RTP/AVPF Transport-Layer 480 Feedback Message in the table for FMT values for RTPFB Payload Types 481 [RFC4585] as defined in Section 3.1: 483 Name: CCFB 484 Long name: RTP Congestion Control Feedback 485 Value: (to be assigned by IANA) 486 Reference: (RFC number of this document, when published) 488 The IANA is also requested to register one new SDP "rtcp-fb" 489 attribute "ack" parameter, "ccfb", in the SDP ("ack" and "nack" 490 Attribute Values) registry: 492 Value name: ccfb 493 Long name: Congestion Control Feedback 494 Usable with: ack 495 Reference: (RFC number of this document, when published) 497 11. Security Considerations 499 The security considerations of the RTP specification [RFC3550], the 500 applicable RTP profile (e.g., [RFC3551], [RFC3711], or [RFC4585]), 501 and the RTP congestion control algorithm that is in use (e.g., 502 [RFC8698], [RFC8298], [I-D.ietf-rmcat-gcc], or [RFC8382]) apply. 504 A receiver that intentionally generates inaccurate RTCP congestion 505 control feedback reports might be able trick the sender into sending 506 at a greater rate than the path can support, thereby congesting the 507 path. This will negatively impact the quality of experience of that 508 receiver. Since RTP is an unreliable transport, a sender can 509 intentionally leave a gap in the RTP sequence number space without 510 causing harm, to check that the receiver is correctly reporting 511 losses. 513 An on-path attacker that can modify RTCP congestion control feedback 514 packets can change the reports to trick the sender into sending at 515 either an excessively high or excessively low rate, leading to denial 516 of service. The secure RTCP profile [RFC3711] can be used to 517 authenticate RTCP packets to protect against this attack. 519 12. References 520 12.1. Normative References 522 [I-D.ietf-mmusic-sdp-bundle-negotiation] 523 Holmberg, C., Alvestrand, H., and C. Jennings, 524 "Negotiating Media Multiplexing Using the Session 525 Description Protocol (SDP)", draft-ietf-mmusic-sdp-bundle- 526 negotiation-54 (work in progress), December 2018. 528 [I-D.ietf-mmusic-sdp-mux-attributes] 529 Nandakumar, S., "A Framework for SDP Attributes when 530 Multiplexing", draft-ietf-mmusic-sdp-mux-attributes-17 531 (work in progress), February 2018. 533 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 534 Requirement Levels", BCP 14, RFC 2119, 535 DOI 10.17487/RFC2119, March 1997, 536 . 538 [RFC3168] Ramakrishnan, K., Floyd, S., and D. Black, "The Addition 539 of Explicit Congestion Notification (ECN) to IP", 540 RFC 3168, DOI 10.17487/RFC3168, September 2001, 541 . 543 [RFC3550] Schulzrinne, H., Casner, S., Frederick, R., and V. 544 Jacobson, "RTP: A Transport Protocol for Real-Time 545 Applications", STD 64, RFC 3550, DOI 10.17487/RFC3550, 546 July 2003, . 548 [RFC3551] Schulzrinne, H. and S. Casner, "RTP Profile for Audio and 549 Video Conferences with Minimal Control", STD 65, RFC 3551, 550 DOI 10.17487/RFC3551, July 2003, 551 . 553 [RFC3611] Friedman, T., Ed., Caceres, R., Ed., and A. Clark, Ed., 554 "RTP Control Protocol Extended Reports (RTCP XR)", 555 RFC 3611, DOI 10.17487/RFC3611, November 2003, 556 . 558 [RFC3711] Baugher, M., McGrew, D., Naslund, M., Carrara, E., and K. 559 Norrman, "The Secure Real-time Transport Protocol (SRTP)", 560 RFC 3711, DOI 10.17487/RFC3711, March 2004, 561 . 563 [RFC4585] Ott, J., Wenger, S., Sato, N., Burmeister, C., and J. Rey, 564 "Extended RTP Profile for Real-time Transport Control 565 Protocol (RTCP)-Based Feedback (RTP/AVPF)", RFC 4585, 566 DOI 10.17487/RFC4585, July 2006, 567 . 569 [RFC5124] Ott, J. and E. Carrara, "Extended Secure RTP Profile for 570 Real-time Transport Control Protocol (RTCP)-Based Feedback 571 (RTP/SAVPF)", RFC 5124, DOI 10.17487/RFC5124, February 572 2008, . 574 [RFC5506] Johansson, I. and M. Westerlund, "Support for Reduced-Size 575 Real-Time Transport Control Protocol (RTCP): Opportunities 576 and Consequences", RFC 5506, DOI 10.17487/RFC5506, April 577 2009, . 579 [RFC6679] Westerlund, M., Johansson, I., Perkins, C., O'Hanlon, P., 580 and K. Carlberg, "Explicit Congestion Notification (ECN) 581 for RTP over UDP", RFC 6679, DOI 10.17487/RFC6679, August 582 2012, . 584 [RFC8083] Perkins, C. and V. Singh, "Multimedia Congestion Control: 585 Circuit Breakers for Unicast RTP Sessions", RFC 8083, 586 DOI 10.17487/RFC8083, March 2017, 587 . 589 12.2. Informative References 591 [feedback-requirements] 592 "RMCAT Feedback Requirements", 593 <://www.ietf.org/proceedings/95/slides/slides-95-rmcat- 594 1.pdf>. 596 [I-D.alvestrand-rmcat-remb] 597 Alvestrand, H., "RTCP message for Receiver Estimated 598 Maximum Bitrate", draft-alvestrand-rmcat-remb-03 (work in 599 progress), October 2013. 601 [I-D.holmer-rmcat-transport-wide-cc-extensions] 602 Holmer, S., Flodman, M., and E. Sprang, "RTP Extensions 603 for Transport-wide Congestion Control", draft-holmer- 604 rmcat-transport-wide-cc-extensions-01 (work in progress), 605 October 2015. 607 [I-D.ietf-rmcat-gcc] 608 Holmer, S., Lundin, H., Carlucci, G., Cicco, L., and S. 609 Mascolo, "A Google Congestion Control Algorithm for Real- 610 Time Communication", draft-ietf-rmcat-gcc-02 (work in 611 progress), July 2016. 613 [I-D.ietf-rmcat-rtp-cc-feedback] 614 Perkins, C., "RTP Control Protocol (RTCP) Feedback for 615 Congestion Control in Interactive Multimedia Conferences", 616 draft-ietf-rmcat-rtp-cc-feedback-05 (work in progress), 617 November 2019. 619 [RFC5104] Wenger, S., Chandra, U., Westerlund, M., and B. Burman, 620 "Codec Control Messages in the RTP Audio-Visual Profile 621 with Feedback (AVPF)", RFC 5104, DOI 10.17487/RFC5104, 622 February 2008, . 624 [RFC6843] Clark, A., Gross, K., and Q. Wu, "RTP Control Protocol 625 (RTCP) Extended Report (XR) Block for Delay Metric 626 Reporting", RFC 6843, DOI 10.17487/RFC6843, January 2013, 627 . 629 [RFC8298] Johansson, I. and Z. Sarker, "Self-Clocked Rate Adaptation 630 for Multimedia", RFC 8298, DOI 10.17487/RFC8298, December 631 2017, . 633 [RFC8382] Hayes, D., Ed., Ferlin, S., Welzl, M., and K. Hiorth, 634 "Shared Bottleneck Detection for Coupled Congestion 635 Control for RTP Media", RFC 8382, DOI 10.17487/RFC8382, 636 June 2018, . 638 [RFC8698] Zhu, X., Pan, R., Ramalho, M., and S. Mena, "Network- 639 Assisted Dynamic Adaptation (NADA): A Unified Congestion 640 Control Scheme for Real-Time Media", RFC 8698, 641 DOI 10.17487/RFC8698, February 2020, 642 . 644 Authors' Addresses 646 Zaheduzzaman Sarker 647 Ericsson AB 648 Torshamnsgatan 21 649 Stockholm 164 40 650 Sweden 652 Phone: +46107173743 653 Email: zaheduzzaman.sarker@ericsson.com 654 Colin Perkins 655 University of Glasgow 656 School of Computing Science 657 Glasgow G12 8QQ 658 United Kingdom 660 Email: csp@csperkins.org 662 Varun Singh 663 CALLSTATS I/O Oy 664 Annankatu 31-33 C 42 665 Helsinki 00100 666 Finland 668 Email: varun.singh@iki.fi 669 URI: http://www.callstats.io/ 671 Michael A. Ramalho 672 6310 Watercrest Way Unit 203 673 Lakewood Ranch, FL 34202-5122 674 USA 676 Phone: +1 732 832 9723 677 Email: mar42@cornell.edu 678 URI: http://ramalho.webhop.info/