IETF RMCAT Working Group Z. Sarker Internet-Draft Ericsson AB Intended status: Standards Track C. Perkins Expires: March 6, 2021 University of Glasgow V. Singh callstats.io M. Ramalho September 2, 2020 RTP Control Protocol (RTCP) Feedback for Congestion Control draft-ietf-avtcore-cc-feedback-message-08 Abstract This document describes an RTCP feedback message intended to enable congestion control for interactive real-time traffic using RTP. The feedback message is designed for use with a sender-based congestion control algorithm, in which the receiver of an RTP flow sends RTCP feedback packets to the sender containing the information the sender needs to perform congestion control. Status of This Memo This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF). Note that other groups may also distribute working documents as Internet-Drafts. The list of current Internet- Drafts is at https://datatracker.ietf.org/drafts/current/. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress." This Internet-Draft will expire on March 6, 2021. Copyright Notice Copyright (c) 2020 IETF Trust and the persons identified as the document authors. All rights reserved. This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents (https://trustee.ietf.org/license-info) in effect on the date of publication of this document. Please review these documents Sarker, et al. Expires March 6, 2021 [Page 1] Internet-Draft Congestion Control Feedback in RTCP September 2020 carefully, as they describe your rights and restrictions with respect to this document. Code Components extracted from this document must include Simplified BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Simplified BSD License. Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2 2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3 3. RTCP Feedback for Congestion Control . . . . . . . . . . . . 3 3.1. RTCP Congestion Control Feedback Report . . . . . . . . . 4 4. Feedback Frequency and Overhead . . . . . . . . . . . . . . . 7 5. Response to Loss of Feedback Packets . . . . . . . . . . . . 8 6. SDP Signalling . . . . . . . . . . . . . . . . . . . . . . . 8 7. Relation to RFC 6679 . . . . . . . . . . . . . . . . . . . . 9 8. Design Rationale . . . . . . . . . . . . . . . . . . . . . . 9 9. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 11 10. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 11 11. Security Considerations . . . . . . . . . . . . . . . . . . . 12 12. References . . . . . . . . . . . . . . . . . . . . . . . . . 12 12.1. Normative References . . . . . . . . . . . . . . . . . . 12 12.2. Informative References . . . . . . . . . . . . . . . . . 14 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 15 1. Introduction For interactive real-time traffic, such as video conferencing flows, the typical protocol choice is the Real-time Transport Protocol (RTP) [RFC3550] running over the User Datagram Protocol (UDP). RTP does not provide any guarantee of Quality of Service (QoS), reliability, or timely delivery, and expects the underlying transport protocol to do so. UDP alone certainly does not meet that expectation. However, the RTP Control Protocol (RTCP) [RFC3550] provides a mechanism by which the receiver of an RTP flow can periodically send transport and media quality metrics to the sender of that RTP flow. This information can be used by the sender to perform congestion control. In the absence of standardized messages for this purpose, designers of congestion control algorithms have developed proprietary RTCP messages that convey only those parameters needed for their respective designs. As a direct result, the different congestion control designs are not interoperable. To enable algorithm evolution as well as interoperability across designs (e.g., different rate adaptation algorithms), it is highly desirable to have generic congestion control feedback format. To help achieve interoperability for unicast RTP congestion control, this memo proposes a common RTCP feedback packet format that can be Sarker, et al. Expires March 6, 2021 [Page 2] Internet-Draft Congestion Control Feedback in RTCP September 2020 used by NADA [RFC8698], SCReAM [RFC8298], Google Congestion Control [I-D.ietf-rmcat-gcc] and Shared Bottleneck Detection [RFC8382], and hopefully also by future RTP congestion control algorithms. 2. Terminology The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all capitals, as shown here. In addition the terminology defined in [RFC3550], [RFC3551], [RFC3611], [RFC4585], and [RFC5506] applies. 3. RTCP Feedback for Congestion Control Based on an analysis of NADA [RFC8698], SCReAM [RFC8298], Google Congestion Control [I-D.ietf-rmcat-gcc] and Shared Bottleneck Detection [RFC8382], the following per-RTP packet congestion control feedback information has been determined to be necessary: o RTP sequence number: The receiver of an RTP flow needs to feed the sequence numbers of the received RTP packets back to the sender, so the sender can determine which packets were received and which were lost. Packet loss is used as an indication of congestion by many congestion control algorithms. o Packet Arrival Time: The receiver of an RTP flow needs to feed the arrival time of each RTP packet back to the sender. Packet delay and/or delay variation (jitter) is used as a congestion signal by some congestion control algorithms. o Packet Explicit Congestion Notification (ECN) Marking: If ECN [RFC3168], [RFC6679] is used, it is necessary to feed back the 2-bit ECN mark in received RTP packets, indicating for each RTP packet whether it is marked not-ECT, ECT(0), ECT(1), or ECN-CE. If the path used by the RTP traffic is ECN capable the sender can use Congestion Experienced (ECN-CE) marking information as a congestion control signal. Every RTP flow is identified by its Synchronization Source (SSRC) identifier. Accordingly, the RTCP feedback format needs to group its reports by SSRC, sending one report block per received SSRC. As a practical matter, we note that host operating system (OS) process interruptions can occur at inopportune times. Accordingly, recording RTP packet send times at the sender, and the corresponding Sarker, et al. Expires March 6, 2021 [Page 3] Internet-Draft Congestion Control Feedback in RTCP September 2020 RTP packet arrival times at the receiver, needs to be done with deliberate care. This is because the time duration of host OS interruptions can be significant relative to the precision desired in the one-way delay estimates. Specifically, the send time needs to be recorded at the last opportunity prior to transmitting the RTP packet at the sender, and the arrival time at the receiver needs to be recorded at the earliest available opportunity. 3.1. RTCP Congestion Control Feedback Report Congestion control feedback can be sent as part of a regular scheduled RTCP report, or in an RTP/AVPF early feedback packet. If sent as early feedback, congestion control feedback MAY be sent in a non-compound RTCP packet [RFC5506] if the RTP/AVPF profile [RFC4585] or the RTP/SAVPF profile [RFC5124] is used. Irrespective of how it is transported, the congestion control feedback is sent as a Transport Layer Feedback Message (RTCP packet type 205). The format of this RTCP packet is shown in Figure 1: Sarker, et al. Expires March 6, 2021 [Page 4] Internet-Draft Congestion Control Feedback in RTCP September 2020 0 1 2 3 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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |V=2|P| FMT=CCFB | PT = 205 | length | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | SSRC of RTCP packet sender | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | SSRC of 1st RTP Stream | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | begin_seq | num_reports | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |L|ECN| Arrival time offset | ... . +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ . . . . . . +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | SSRC of nth RTP Stream | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | begin_seq | num_reports | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |L|ECN| Arrival time offset | ... | . . . . +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Report Timestamp (32bits) | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Figure 1: RTCP Congestion Control Feedback Packet Format The first eight octets comprise a standard RTCP header, with PT=205 and FMT=CCFB indicating that this is a congestion control feedback packet, and with the SSRC set to that of the sender of the RTCP packet. (NOTE TO RFC EDITOR: please replace CCFB here and in the above diagram with the IANA assigned RTCP feedback packet type, and remove this note) Section 6.1 of [RFC4585] requires the RTCP header to be followed by the SSRC of the RTP flow being reported upon. Accordingly, the RTCP header is followed by a report block for each SSRC from which RTP packets have been received, followed by a Report Timestamp. Each report block begins with the SSRC of the received RTP Stream on which it is reporting. Following this, the report block contains a 16-bit packet metric block for each RTP packet with sequence number in the range begin_seq to begin_seq+num_reports inclusive (calculated using arithmetic modulo 65536 to account for possible sequence number wrap-around). If the number of 16-bit packet metric blocks included Sarker, et al. Expires March 6, 2021 [Page 5] Internet-Draft Congestion Control Feedback in RTCP September 2020 in the report block is not a multiple of two, then 16 bits of zero padding MUST be added after the last packet metric block, to align the end of the packet metric blocks with the next 32 bit boundary. The value of num_reports MAY be zero, indicating that there are no packet metric blocks included for that SSRC. Each report block MUST NOT include more than 16384 packet metric blocks (i.e., it MUST NOT report on more than one quarter of the sequence number space in a single report). The contents of each 16-bit packet metric block comprises the L, ECN, and ATO fields as follows: o L (1 bit): is a boolean to indicate if the packet was received. 0 represents that the packet was not yet received and all the subsequent bits (ECN and ATO) are also set to 0. 1 represents that the packet was received and the subsequent bits in the block need to be parsed. o ECN (2 bits): is the echoed ECN mark of the packet. These are set to 00 if not received, or if ECN is not used. o Arrival time offset (ATO, 13 bits): is the arrival time of the RTP packet at the receiver, as an offset before the time represented by the Report Timestamp (RTS) field of this RTCP congestion control feedback report. The ATO field is in units of 1/1024 seconds (this unit is chosen to give exact offsets from the RTS field) so, for example, an ATO value of 512 indicates that the corresponding RTP packet arrived exactly half a second before the time instant represented by the RTS field. If the measured value is greater than 8189/1024 seconds (the value that would be coded as 0x1FFD), the value 0x1FFE MUST be reported to indicate an over- range measurement. If the measurement is unavailable, or if the arrival time of the RTP packet is after the time represented by the RTS field, then an ATO value of 0x1FFF MUST be reported for the packet. The RTCP congestion control feedback report packet concludes with the Report Timestamp field (RTS, 32 bits). This denotes the time instant on which this packet is reporting, and is the instant from which the arrival time offset values are calculated. The value of RTS field is derived from the same clock used to generate the NTP timestamp field in RTCP Sender Report (SR) packets. It is formatted as the middle 32 bits of an NTP format timestamp, as described in Section 4 of [RFC3550]. RTCP congestion control feedback packets SHOULD include a report block for every active SSRC. The sequence number ranges reported on in consecutive reports for a given SSRC will generally be contiguous, Sarker, et al. Expires March 6, 2021 [Page 6] Internet-Draft Congestion Control Feedback in RTCP September 2020 but overlapping reports MAY be sent (and need to be sent in cases where RTP packet reordering occurs across the boundary between consecutive reports). If reports covering overlapping sequence number ranges are sent, information in later reports updates that in sent previous reports for RTP packets included in both reports. If an RTP packet was reported as received in one report, that packet MUST also be reported as received in any overlapping reports sent later that cover its sequence number range. RTCP congestion control feedback packets can be large if they are sent infrequently relative to the number of RTP data packets. If an RTCP congestion control feedback packet is too large to fit within the path MTU, its sender SHOULD split it into multiple feedback packets. The RTCP reporting interval SHOULD be chosen such that feedback packets are sent often enough that they are small enough to fit within the path MTU ([I-D.ietf-rmcat-rtp-cc-feedback] discusses how to choose the reporting interval; specifications for RTP congestion control algorithms can also provide guidance). If duplicate copies of a particular RTP packet are received, then the arrival time of the first copy to arrive MUST be reported. If any of the copies of the duplicated packet are ECN-CE marked, then an ECN-CE mark MUST be reported that for packet; otherwise the ECN mark of the first copy to arrive is reported. If no packets are received from an SSRC in a reporting interval, a report block MAY be sent with begin_seq set to the highest sequence number previously received from that SSRC and num_reports set to zero (or, the report can simply to omitted). The corresponding SR/RR packet will have a non-increased extended highest sequence number received field that will inform the sender that no packets have been received, but it can ease processing to have that information available in the congestion control feedback reports too. A report block indicating that certain RTP packets were lost is not to be interpreted as a request to retransmit the lost packets. The receiver of such a report might choose to retransmit such packets, provided a retransmission payload format has been negotiated, but there is no requirement that it do so. 4. Feedback Frequency and Overhead There is a trade-off between speed and accuracy of reporting, and the overhead of the reports. [I-D.ietf-rmcat-rtp-cc-feedback] discusses this trade-off, suggests desirable RTCP feedback rates, and provides guidance on how to configure the RTCP bandwidth fraction, etc., to make appropriate use of the reporting block described in this memo. Sarker, et al. Expires March 6, 2021 [Page 7] Internet-Draft Congestion Control Feedback in RTCP September 2020 Specifications for RTP congestion control algorithms can also provide guidance. It is generally understood that congestion control algorithms work better with more frequent feedback. However, RTCP bandwidth and transmission rules put some upper limits on how frequently the RTCP feedback messages can be sent from an RTP receiver to the RTP sender. In many cases, sending feedback once per frame is an upper bound before the reporting overhead becomes excessive, although this will depend on the media rate and more frequent feedback might be needed with high-rate media flows [I-D.ietf-rmcat-rtp-cc-feedback]. Analysis [feedback-requirements] has also shown that some candidate congestion control algorithms can operate with less frequent feedback, using a feedback interval range of 50-200ms. Applications need to negotiate an appropriate congestion control feedback interval at session setup time, based on the choice of congestion control algorithm, the expected media bit rate, and the acceptable feedback overhead. 5. Response to Loss of Feedback Packets Like all RTCP packets, RTCP congestion control feedback packets might be lost. All RTP congestion control algorithms MUST specify how they respond to the loss of feedback packets. If only a single congestion control feedback packet is lost, an appropriate response is to assume that the level of congestion has remained roughly the same as the previous report. However, if multiple consecutive congestion control feedback packets are lost, then the sender SHOULD rapidly reduce its sending rate as this likely indicates a path failure. The RTP circuit breaker [RFC8083] provides further guidance. 6. SDP Signalling A new "ack" feedback parameter, "ccfb", is defined for use with the "a=rtcp-fb:" SDP extension to indicate the use of the RTP Congestion Control feedback packet format defined in Section 3. The ABNF definition of this SDP parameter extension is: rtcp-fb-ack-param = rtcp-fb-ack-param =/ ccfb-par ccfb-par = SP "ccfb" The payload type used with "ccfb" feedback MUST be the wildcard type ("*"). This implies that the congestion control feedback is sent for all payload types in use in the session, including any FEC and Sarker, et al. Expires March 6, 2021 [Page 8] Internet-Draft Congestion Control Feedback in RTCP September 2020 retransmission payload types. An example of the resulting SDP attribute is: a=rtcp-fb:* ack ccfb The offer/answer rules for these SDP feedback parameters are specified in Section 4.2 of the RTP/AVPF profile [RFC4585]. An SDP offer might indicate support for both the congestion control feedback mechanism specified in this memo and one or more alternative congestion control feedback mechanisms that offer substantially the same semantics. In this case, the answering party SHOULD include only one of the offered congestion control feedback mechanisms in its answer. If a re-invite offering the same set of congestion control feedback mechanisms is received, the generated answer SHOULD choose the same congestion control feedback mechanism as in the original answer where possible. When the SDP BUNDLE extension [I-D.ietf-mmusic-sdp-bundle-negotiation] is used for multiplexing, the "a=rtcp-fb:" attribute has multiplexing category IDENTICAL-PER-PT [I-D.ietf-mmusic-sdp-mux-attributes]. 7. Relation to RFC 6679 Use of Explicit Congestion Notification (ECN) with RTP is described in [RFC6679]. That specifies how to negotiate the use of ECN with RTP, and defines an RTCP ECN Feedback Packet to carry ECN feedback reports. It uses an SDP "a=ecn-capaable-rtp:" attribute to negotiate use of ECN, and the "a=rtcp-fb:" attributes with the "nack" parameter "ecn" to negotiate the use of RTCP ECN Feedback Packets. The RTCP ECN Feedback Packet is not useful when ECN is used with the RTP Congestion Control Feedback Packet defined in this memo since it provides duplicate information. Accordingly, when congestion control feedback is to be used with RTP and ECN, the SDP offer generated MUST include an "a=ecn-capable-rtp:" attribute to negotiate ECN support, along with an "a=rtcp-fb:" attribute with the "ack" parameter "ccfb" to indicate that the RTP Congestion Control Feedback Packet is to be used for feedback. The "a=rtcp-fb:" attribute MUST NOT include the "nack" parameter "ecn", so the RTCP ECN Feedback Packet will not be used. 8. Design Rationale The primary function of RTCP SR/RR packets is to report statistics on the reception of RTP packets. The reception report blocks sent in these packets contain information about observed jitter, fractional Sarker, et al. Expires March 6, 2021 [Page 9] Internet-Draft Congestion Control Feedback in RTCP September 2020 packet loss, and cumulative packet loss. It was intended that this information could be used to support congestion control algorithms, but experience has shown that it is not sufficient for that purpose. An efficient congestion control algorithm requires more fine-grained information on per-packet reception quality than is provided by SR/RR packets to react effectively. The feedback format defined in this memo provides such fine-grained feedback. Several other RTCP extensions also provide more detailed feedback than SR/RR packets: TMMBR: The Codec Control Messages for the RTP/AVPF profile [RFC5104] include a Temporary Maximum Media Bit Rate (TMMBR) message. This is used to convey a temporary maximum bit rate limitation from a receiver of RTP packets to their sender. Even though it was not designed to replace congestion control, TMMBR has been used as a means to do receiver based congestion control where the session bandwidth is high enough to send frequent TMMBR messages, especially when used with non-compound RTCP packets [RFC5506]. This approach requires the receiver of the RTP packets to monitor their reception, determine the level of congestion, and recommend a maximum bit rate suitable for current available bandwidth on the path; it also assumes that the RTP sender can/will respect that bit rate. This is the opposite of the sender-based congestion control approach suggested in this memo, so TMMBR cannot be used to convey the information needed for a sender-based congestion control. TMMBR could, however, be viewed a complementary mechanism that can inform the sender of the receiver's current view of acceptable maximum bit rate. The Received Estimated Maximum Bit-rate (REMB) mechanism [I-D.alvestrand-rmcat-remb] provides similar feedback. RTCP Extended Reports (XR): Numerous RTCP extended report (XR) blocks have been defined to report details of packet loss, arrival times [RFC3611], delay [RFC6843], and ECN marking [RFC6679]. It is possible to combine several such XR blocks into a compound RTCP packet, to report the detailed loss, arrival time, and ECN marking marking information needed for effective sender-based congestion control. However, the result has high overhead both in terms of bandwidth and complexity, due to the need to stack multiple reports. Transport-wide Congestion Control: The format defined in this memo provides individual feedback on each SSRC. An alternative is to add a header extension to each RTP packet, containing a single, transport-wide, packet sequence number, then have the receiver send RTCP reports giving feedback on these additional sequence numbers [I-D.holmer-rmcat-transport-wide-cc-extensions]. Such an Sarker, et al. Expires March 6, 2021 [Page 10] Internet-Draft Congestion Control Feedback in RTCP September 2020 approach adds the per-packet overhead of the header extension (8 octets per packet in the referenced format), but reduces the size of the feedback packets, and can simplify the rate calculation at the sender if it maintains a single rate limit that applies to all RTP packets sent irrespective of their SSRC. Equally, the use of transport-wide feedback makes it more difficult to adapt the sending rate, or respond to lost packets, based on the reception and/or loss patterns observed on a per-SSRC basis (for example, to perform differential rate control and repair for audio and video flows, based on knowledge of what packets from each flow were lost). Transport-wide feedback is also a less natural fit with the wider RTP framework, which makes extensive use of per-SSRC sequence numbers and feedback. Considering these issues, we believe it appropriate to design a new RTCP feedback mechanism to convey information for sender-based congestion control algorithms. The new congestion control feedback RTCP packet described in Section 3 provides such a mechanism. 9. Acknowledgements This document is based on the outcome of a design team discussion in the RTP Media Congestion Avoidance Techniques (RMCAT) working group. The authors would like to thank David Hayes, Stefan Holmer, Randell Jesup, Ingemar Johansson, Jonathan Lennox, Sergio Mena, Nils Ohlmeier, Magnus Westerlund, and Xiaoqing Zhu for their valuable feedback. 10. IANA Considerations The IANA is requested to register one new RTP/AVPF Transport-Layer Feedback Message in the table for FMT values for RTPFB Payload Types [RFC4585] as defined in Section 3.1: Name: CCFB Long name: RTP Congestion Control Feedback Value: (to be assigned by IANA) Reference: (RFC number of this document, when published) The IANA is also requested to register one new SDP "rtcp-fb" attribute "ack" parameter, "ccfb", in the SDP ("ack" and "nack" Attribute Values) registry: Value name: ccfb Long name: Congestion Control Feedback Usable with: ack Reference: (RFC number of this document, when published) Sarker, et al. Expires March 6, 2021 [Page 11] Internet-Draft Congestion Control Feedback in RTCP September 2020 11. Security Considerations The security considerations of the RTP specification [RFC3550], the applicable RTP profile (e.g., [RFC3551], [RFC3711], or [RFC4585]), and the RTP congestion control algorithm that is in use (e.g., [RFC8698], [RFC8298], [I-D.ietf-rmcat-gcc], or [RFC8382]) apply. A receiver that intentionally generates inaccurate RTCP congestion control feedback reports might be able trick the sender into sending at a greater rate than the path can support, thereby causing congestion on the path. This will negatively impact the quality of experience of that receiver. Since RTP is an unreliable transport, a sender can intentionally leave a gap in the RTP sequence number space without causing harm, to check that the receiver is correctly reporting losses. An on-path attacker that can modify RTCP congestion control feedback packets can change the reports to trick the sender into sending at either an excessively high or excessively low rate, leading to denial of service. The secure RTCP profile [RFC3711] can be used to authenticate RTCP packets to protect against this attack. 12. References 12.1. Normative References [I-D.ietf-mmusic-sdp-bundle-negotiation] Holmberg, C., Alvestrand, H., and C. Jennings, "Negotiating Media Multiplexing Using the Session Description Protocol (SDP)", draft-ietf-mmusic-sdp-bundle- negotiation-54 (work in progress), December 2018. [I-D.ietf-mmusic-sdp-mux-attributes] Nandakumar, S., "A Framework for SDP Attributes when Multiplexing", draft-ietf-mmusic-sdp-mux-attributes-19 (work in progress), August 2020. [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, March 1997, . [RFC3168] Ramakrishnan, K., Floyd, S., and D. Black, "The Addition of Explicit Congestion Notification (ECN) to IP", RFC 3168, DOI 10.17487/RFC3168, September 2001, . Sarker, et al. Expires March 6, 2021 [Page 12] Internet-Draft Congestion Control Feedback in RTCP September 2020 [RFC3550] Schulzrinne, H., Casner, S., Frederick, R., and V. Jacobson, "RTP: A Transport Protocol for Real-Time Applications", STD 64, RFC 3550, DOI 10.17487/RFC3550, July 2003, . [RFC3551] Schulzrinne, H. and S. Casner, "RTP Profile for Audio and Video Conferences with Minimal Control", STD 65, RFC 3551, DOI 10.17487/RFC3551, July 2003, . [RFC3611] Friedman, T., Ed., Caceres, R., Ed., and A. Clark, Ed., "RTP Control Protocol Extended Reports (RTCP XR)", RFC 3611, DOI 10.17487/RFC3611, November 2003, . [RFC3711] Baugher, M., McGrew, D., Naslund, M., Carrara, E., and K. Norrman, "The Secure Real-time Transport Protocol (SRTP)", RFC 3711, DOI 10.17487/RFC3711, March 2004, . [RFC4585] Ott, J., Wenger, S., Sato, N., Burmeister, C., and J. Rey, "Extended RTP Profile for Real-time Transport Control Protocol (RTCP)-Based Feedback (RTP/AVPF)", RFC 4585, DOI 10.17487/RFC4585, July 2006, . [RFC5124] Ott, J. and E. Carrara, "Extended Secure RTP Profile for Real-time Transport Control Protocol (RTCP)-Based Feedback (RTP/SAVPF)", RFC 5124, DOI 10.17487/RFC5124, February 2008, . [RFC5506] Johansson, I. and M. Westerlund, "Support for Reduced-Size Real-Time Transport Control Protocol (RTCP): Opportunities and Consequences", RFC 5506, DOI 10.17487/RFC5506, April 2009, . [RFC6679] Westerlund, M., Johansson, I., Perkins, C., O'Hanlon, P., and K. Carlberg, "Explicit Congestion Notification (ECN) for RTP over UDP", RFC 6679, DOI 10.17487/RFC6679, August 2012, . [RFC8083] Perkins, C. and V. Singh, "Multimedia Congestion Control: Circuit Breakers for Unicast RTP Sessions", RFC 8083, DOI 10.17487/RFC8083, March 2017, . Sarker, et al. Expires March 6, 2021 [Page 13] Internet-Draft Congestion Control Feedback in RTCP September 2020 [RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, May 2017, . 12.2. Informative References [feedback-requirements] "RMCAT Feedback Requirements", <://www.ietf.org/proceedings/95/slides/slides-95-rmcat- 1.pdf>. [I-D.alvestrand-rmcat-remb] Alvestrand, H., "RTCP message for Receiver Estimated Maximum Bitrate", draft-alvestrand-rmcat-remb-03 (work in progress), October 2013. [I-D.holmer-rmcat-transport-wide-cc-extensions] Holmer, S., Flodman, M., and E. Sprang, "RTP Extensions for Transport-wide Congestion Control", draft-holmer- rmcat-transport-wide-cc-extensions-01 (work in progress), October 2015. [I-D.ietf-rmcat-gcc] Holmer, S., Lundin, H., Carlucci, G., Cicco, L., and S. Mascolo, "A Google Congestion Control Algorithm for Real- Time Communication", draft-ietf-rmcat-gcc-02 (work in progress), July 2016. [I-D.ietf-rmcat-rtp-cc-feedback] Perkins, C., "RTP Control Protocol (RTCP) Feedback for Congestion Control in Interactive Multimedia Conferences", draft-ietf-rmcat-rtp-cc-feedback-05 (work in progress), November 2019. [RFC5104] Wenger, S., Chandra, U., Westerlund, M., and B. Burman, "Codec Control Messages in the RTP Audio-Visual Profile with Feedback (AVPF)", RFC 5104, DOI 10.17487/RFC5104, February 2008, . [RFC6843] Clark, A., Gross, K., and Q. Wu, "RTP Control Protocol (RTCP) Extended Report (XR) Block for Delay Metric Reporting", RFC 6843, DOI 10.17487/RFC6843, January 2013, . [RFC8298] Johansson, I. and Z. Sarker, "Self-Clocked Rate Adaptation for Multimedia", RFC 8298, DOI 10.17487/RFC8298, December 2017, . Sarker, et al. Expires March 6, 2021 [Page 14] Internet-Draft Congestion Control Feedback in RTCP September 2020 [RFC8382] Hayes, D., Ed., Ferlin, S., Welzl, M., and K. Hiorth, "Shared Bottleneck Detection for Coupled Congestion Control for RTP Media", RFC 8382, DOI 10.17487/RFC8382, June 2018, . [RFC8698] Zhu, X., Pan, R., Ramalho, M., and S. Mena, "Network- Assisted Dynamic Adaptation (NADA): A Unified Congestion Control Scheme for Real-Time Media", RFC 8698, DOI 10.17487/RFC8698, February 2020, . Authors' Addresses Zaheduzzaman Sarker Ericsson AB Torshamnsgatan 21 Stockholm 164 40 Sweden Phone: +46107173743 Email: zaheduzzaman.sarker@ericsson.com Colin Perkins University of Glasgow School of Computing Science Glasgow G12 8QQ United Kingdom Email: csp@csperkins.org Varun Singh CALLSTATS I/O Oy Annankatu 31-33 C 42 Helsinki 00100 Finland Email: varun.singh@iki.fi URI: http://www.callstats.io/ Sarker, et al. Expires March 6, 2021 [Page 15] Internet-Draft Congestion Control Feedback in RTCP September 2020 Michael A. Ramalho 6310 Watercrest Way Unit 203 Lakewood Ranch, FL 34202-5122 USA Phone: +1 732 832 9723 Email: mar42@cornell.edu URI: http://ramalho.webhop.info/ Sarker, et al. Expires March 6, 2021 [Page 16]