AVTCORE Working Group C. S. Perkins Internet-Draft University of Glasgow Updates: 3550 (if approved) V. Singh Intended status: Standards Track Aalto University Expires: June 07, 2015 December 04, 2014 Multimedia Congestion Control: Circuit Breakers for Unicast RTP Sessions draft-ietf-avtcore-rtp-circuit-breakers-08 Abstract The Real-time Transport Protocol (RTP) is widely used in telephony, video conferencing, and telepresence applications. Such applications are often run on best-effort UDP/IP networks. If congestion control is not implemented in the applications, then network congestion will deteriorate the user's multimedia experience. This document does not propose a congestion control algorithm; instead, it defines a minimal set of RTP "circuit-breakers". Circuit-breakers are conditions under which an RTP sender needs to stop transmitting media data in order to protect the network from excessive congestion. It is expected that, in the absence of severe congestion, all RTP applications running on best-effort IP networks will be able to run without triggering these circuit breakers. Any future RTP congestion control specification will be expected to operate within the constraints defined by these circuit breakers. 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 http://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 June 07, 2015. Copyright Notice Copyright (c) 2014 IETF Trust and the persons identified as the document authors. All rights reserved. Perkins & Singh Expires June 07, 2015 [Page 1] Internet-Draft RTP Circuit Breakers December 2014 This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents (http://trustee.ietf.org/license-info) in effect on the date of publication of this document. Please review these documents 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. Background . . . . . . . . . . . . . . . . . . . . . . . . . 3 4. RTP Circuit Breakers for Systems Using the RTP/AVP Profile . 6 4.1. RTP/AVP Circuit Breaker #1: Media Timeout . . . . . . . . 8 4.2. RTP/AVP Circuit Breaker #2: RTCP Timeout . . . . . . . . 8 4.3. RTP/AVP Circuit Breaker #3: Congestion . . . . . . . . . 9 4.4. RTP/AVP Circuit Breaker #4: Media Usability . . . . . . . 13 4.5. Ceasing Transmission . . . . . . . . . . . . . . . . . . 14 5. RTP Circuit Breakers for Systems Using the RTP/AVPF Profile . 14 6. Impact of RTCP Extended Reports (XR) . . . . . . . . . . . . 15 7. Impact of RTCP Reporting Groups . . . . . . . . . . . . . . . 15 8. Impact of Explicit Congestion Notification (ECN) . . . . . . 16 9. Impact of Bundled Media and Layered Coding . . . . . . . . . 16 10. Security Considerations . . . . . . . . . . . . . . . . . . . 16 11. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 17 12. Open Issues . . . . . . . . . . . . . . . . . . . . . . . . . 17 13. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 17 14. References . . . . . . . . . . . . . . . . . . . . . . . . . 17 14.1. Normative References . . . . . . . . . . . . . . . . . . 18 14.2. Informative References . . . . . . . . . . . . . . . . . 18 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 20 1. Introduction The Real-time Transport Protocol (RTP) [RFC3550] is widely used in voice-over-IP, video teleconferencing, and telepresence systems. Many of these systems run over best-effort UDP/IP networks, and can suffer from packet loss and increased latency if network congestion occurs. Designing effective RTP congestion control algorithms, to adapt the transmission of RTP-based media to match the available network capacity, while also maintaining the user experience, is a difficult but important problem. Many such congestion control and media adaptation algorithms have been proposed, but to date there is no consensus on the correct approach, or even that a single standard algorithm is desirable. Perkins & Singh Expires June 07, 2015 [Page 2] Internet-Draft RTP Circuit Breakers December 2014 This memo does not attempt to propose a new RTP congestion control algorithm. Rather, it proposes a minimal set of "circuit breakers"; conditions under which there is general agreement that an RTP flow is causing serious congestion, and ought to cease transmission. It is expected that future standards-track congestion control algorithms for RTP will operate within the envelope defined by this memo. 2. Terminology The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in RFC 2119 [RFC2119]. This interpretation of these key words applies only when written in ALL CAPS. Mixed- or lower-case uses of these key words are not to be interpreted as carrying special significance in this memo. 3. Background We consider congestion control for unicast RTP traffic flows. This is the problem of adapting the transmission of an audio/visual data flow, encapsulated within an RTP transport session, from one sender to one receiver, so that it matches the available network bandwidth. Such adaptation needs to be done in a way that limits the disruption to the user experience caused by both packet loss and excessive rate changes. Congestion control for multicast flows is outside the scope of this memo. Multicast traffic needs different solutions, since the available bandwidth estimator for a group of receivers will differ from that for a single receiver, and because multicast congestion control has to consider issues of fairness across groups of receivers that do not apply to unicast flows. Congestion control for unicast RTP traffic can be implemented in one of two places in the protocol stack. One approach is to run the RTP traffic over a congestion controlled transport protocol, for example over TCP, and to adapt the media encoding to match the dictates of the transport-layer congestion control algorithm. This is safe for the network, but can be suboptimal for the media quality unless the transport protocol is designed to support real-time media flows. We do not consider this class of applications further in this memo, as their network safety is guaranteed by the underlying transport. Perkins & Singh Expires June 07, 2015 [Page 3] Internet-Draft RTP Circuit Breakers December 2014 Alternatively, RTP flows can be run over a non-congestion controlled transport protocol, for example UDP, performing rate adaptation at the application layer based on RTP Control Protocol (RTCP) feedback. With a well-designed, network-aware, application, this allows highly effective media quality adaptation, but there is potential to disrupt the network's operation if the application does not adapt its sending rate in a timely and effective manner. We consider this class of applications in this memo. Congestion control relies on monitoring the delivery of a media flow, and responding to adapt the transmission of that flow when there are signs that the network path is congested. Network congestion can be detected in one of three ways: 1) a receiver can infer the onset of congestion by observing an increase in one-way delay caused by queue build-up within the network; 2) if Explicit Congestion Notification (ECN) [RFC3168] is supported, the network can signal the presence of congestion by marking packets using ECN Congestion Experienced (CE) marks; or 3) in the extreme case, congestion will cause packet loss that can be detected by observing a gap in the received RTP sequence numbers. Once the onset of congestion is observed, the receiver has to send feedback to the sender to indicate that the transmission rate needs to be reduced. How the sender reduces the transmission rate is highly dependent on the media codec being used, and is outside the scope of this memo. There are several ways in which a receiver can send feedback to a media sender within the RTP framework: o The base RTP specification [RFC3550] defines RTCP Reception Report (RR) packets to convey reception quality feedback information, and Sender Report (SR) packets to convey information about the media transmission. RTCP SR packets contain data that can be used to reconstruct media timing at a receiver, along with a count of the total number of octets and packets sent. RTCP RR packets report on the fraction of packets lost in the last reporting interval, the cumulative number of packets lost, the highest sequence number received, and the inter-arrival jitter. The RTCP RR packets also contain timing information that allows the sender to estimate the network round trip time (RTT) to the receivers. RTCP reports are sent periodically, with the reporting interval being determined by the number of SSRCs used in the session and a configured session bandwidth estimate (the number of SSRCs used is usually two in a unicast session, one for each participant, but can be greater if the participants send multiple media streams). The interval between reports sent from each receiver tends to be on the order of a few seconds on average, although it varies with the session bandwidth, and sub-second reporting intervals are possible in high bandwidth sessions, and it is randomised to avoid synchronisation Perkins & Singh Expires June 07, 2015 [Page 4] Internet-Draft RTP Circuit Breakers December 2014 of reports from multiple receivers. RTCP RR packets allow a receiver to report ongoing network congestion to the sender. However, if a receiver detects the onset of congestion part way through a reporting interval, the base RTP specification contains no provision for sending the RTCP RR packet early, and the receiver has to wait until the next scheduled reporting interval. o The RTCP Extended Reports (XR) [RFC3611] allow reporting of more complex and sophisticated reception quality metrics, but do not change the RTCP timing rules. RTCP extended reports of potential interest for congestion control purposes are the extended packet loss, discard, and burst metrics [RFC3611], [RFC7002], [RFC7097], [RFC7003], [RFC6958]; and the extended delay metrics [RFC6843], [RFC6798]. Other RTCP Extended Reports that could be helpful for congestion control purposes might be developed in future. o Rapid feedback about the occurrence of congestion events can be achieved using the Extended RTP Profile for RTCP-Based Feedback (RTP/AVPF) [RFC4585] (or its secure variant, RTP/SAVPF [RFC5124]) in place of the RTP/AVP profile [RFC3551]. This modifies the RTCP timing rules to allow RTCP reports to be sent early, in some cases immediately, provided the RTCP transmission rate keeps within its bandwidth allocation. It also defines transport-layer feedback messages, including negative acknowledgements (NACKs), that can be used to report on specific congestion events. RTP Codec Control Messages [RFC5104] extend the RTP/AVPF profile with additional feedback messages that can be used to influence that way in which rate adaptation occurs, but do not further change the dynamics of how rapidly feedback can be sent. Use of the RTP/AVPF profile is dependent on signalling. o Finally, Explicit Congestion Notification (ECN) for RTP over UDP [RFC6679] can be used to provide feedback on the number of packets that received an ECN Congestion Experienced (CE) mark. This RTCP extension builds on the RTP/AVPF profile to allow rapid congestion feedback when ECN is supported. In addition to these mechanisms for providing feedback, the sender can include an RTP header extension in each packet to record packet transmission times. There are two methods: [RFC5450] represents the transmission time in terms of a time-offset from the RTP timestamp of the packet, while [RFC6051] includes an explicit NTP-format sending timestamp (potentially more accurate, but a higher header overhead). Accurate sending timestamps can be helpful for estimating queuing delays, to get an early indication of the onset of congestion. Taken together, these various mechanisms allow receivers to provide feedback on the senders when congestion events occur, with varying Perkins & Singh Expires June 07, 2015 [Page 5] Internet-Draft RTP Circuit Breakers December 2014 degrees of timeliness and accuracy. The key distinction is between systems that use only the basic RTCP mechanisms, without RTP/AVPF rapid feedback, and those that use the RTP/AVPF extensions to respond to congestion more rapidly. 4. RTP Circuit Breakers for Systems Using the RTP/AVP Profile The feedback mechanisms defined in [RFC3550] and available under the RTP/AVP profile [RFC3551] are the minimum that can be assumed for a baseline circuit breaker mechanism that is suitable for all unicast applications of RTP. Accordingly, for an RTP circuit breaker to be useful, it needs to be able to detect that an RTP flow is causing excessive congestion using only basic RTCP features, without needing RTCP XR feedback or the RTP/AVPF profile for rapid RTCP reports. RTCP is a fundamental part of the RTP protocol, and the mechanisms described here rely on the implementation of RTCP. Implementations that claim to support RTP, but that do not implement RTCP, cannot use the circuit breaker mechanisms described in this memo. Such implementations SHOULD NOT be used on networks that might be subject to congestion unless equivalent mechanisms are defined using some non-RTCP feedback channel to report congestion and signal circuit breaker conditions. Three potential congestion signals are available from the basic RTCP SR/RR packets and are reported for each synchronisation source (SSRC) in the RTP session: 1. The sender can estimate the network round-trip time once per RTCP reporting interval, based on the contents and timing of RTCP SR and RR packets. 2. Receivers report a jitter estimate (the statistical variance of the RTP data packet inter-arrival time) calculated over the RTCP reporting interval. Due to the nature of the jitter calculation ([RFC3550], section 6.4.4), the jitter is only meaningful for RTP flows that send a single data packet for each RTP timestamp value (i.e., audio flows, or video flows where each packet comprises one video frame). 3. Receivers report the fraction of RTP data packets lost during the RTCP reporting interval, and the cumulative number of RTP packets lost over the entire RTP session. These congestion signals limit the possible circuit breakers, since they give only limited visibility into the behaviour of the network. Perkins & Singh Expires June 07, 2015 [Page 6] Internet-Draft RTP Circuit Breakers December 2014 RTT estimates are widely used in congestion control algorithms, as a proxy for queuing delay measures in delay-based congestion control or to determine connection timeouts. RTT estimates derived from RTCP SR and RR packets sent according to the RTP/AVP timing rules are too infrequent to be useful though, and don't give enough information to distinguish a delay change due to routing updates from queuing delay caused by congestion. Accordingly, we cannot use the RTT estimate alone as an RTP circuit breaker. Increased jitter can be a signal of transient network congestion, but in the highly aggregated form reported in RTCP RR packets, it offers insufficient information to estimate the extent or persistence of congestion. Jitter reports are a useful early warning of potential network congestion, but provide an insufficiently strong signal to be used as a circuit breaker. The remaining congestion signals are the packet loss fraction and the cumulative number of packets lost. If considered carefully, these can be effective indicators that congestion is occurring in networks where packet loss is primarily due to queue overflows, although loss caused by non-congestive packet corruption can distort the result in some networks. TCP congestion control [RFC5681] intentionally tries to fill the router queues, and uses the resulting packet loss as congestion feedback. An RTP flow competing with TCP traffic will therefore expect to see a non-zero packet loss fraction that has to be related to TCP dynamics to estimate available capacity. This behaviour of TCP is reflected in the congestion circuit breaker below, and will affect the design of any RTP congestion control protocol. Two packet loss regimes can be observed: 1) RTCP RR packets show a non-zero packet loss fraction, while the extended highest sequence number received continues to increment; and 2) RR packets show a loss fraction of zero, but the extended highest sequence number received does not increment even though the sender has been transmitting RTP data packets. The former corresponds to the TCP congestion avoidance state, and indicates a congested path that is still delivering data; the latter corresponds to a TCP timeout, and is most likely due to a path failure. A third condition is that data is being sent but no RTCP feedback is received at all, corresponding to a failure of the reverse path. We derive circuit breaker conditions for these loss regimes in the following. Perkins & Singh Expires June 07, 2015 [Page 7] Internet-Draft RTP Circuit Breakers December 2014 4.1. RTP/AVP Circuit Breaker #1: Media Timeout If RTP data packets are being sent, but the RTCP SR or RR packets reporting on that SSRC indicate a non-increasing extended highest sequence number received, this is an indication that those RTP data packets are not reaching the receiver. This could be a short-term issue affecting only a few packets, perhaps caused by a slow-to-open firewall or a transient connectivity problem, but if the issue persists, it is a sign of a more ongoing and significant problem. Accordingly, if a sender of RTP data packets receives three or more consecutive RTCP SR or RR packets from the same receiver, and those packets correspond to its transmission and have a non-increasing extended highest sequence number received field, then that sender SHOULD cease transmission (see Section 4.5). The extended highest sequence number received field is non-increasing if the sender receives at least three consecutive RTCP SR or RR packets that report the same value for this field, but it has sent RTP data packets that would have caused an increase in the reported value if they had reached the receiver. The reason for waiting for three or more consecutive RTCP packets with a non-increasing extended highest sequence number is to give enough time for transient reception problems to resolve themselves, but to stop problem flows quickly enough to avoid causing serious ongoing network congestion. A single RTCP report showing no reception could be caused by a transient fault, and so will not cease transmission. Waiting for more than three consecutive RTCP reports before stopping a flow might avoid some false positives, but could lead to problematic flows running for a long time period (potentially tens of seconds, depending on the RTCP reporting interval) before being cut off. Equally, an application that sends few packets when the packet loss rate is high runs the risk that the media timeout circuit breaker triggers inadvertently. The chosen timeout interval is a trade-off between these extremes. 4.2. RTP/AVP Circuit Breaker #2: RTCP Timeout In addition to media timeouts, as were discussed in Section 4.1, an RTP session has the possibility of an RTCP timeout. This can occur when RTP data packets are being sent, but there are no RTCP reports returned from the receiver. This is either due to a failure of the receiver to send RTCP reports, or a failure of the return path that is preventing those RTCP reporting from being delivered. In either case, it is not safe to continue transmission, since the sender has no way of knowing if it is causing congestion. Accordingly, an RTP sender that has not received any RTCP SR or RTCP RR packets reporting on the SSRC it is using for three or more of its RTCP reporting intervals SHOULD cease transmission (see Section 4.5). When Perkins & Singh Expires June 07, 2015 [Page 8] Internet-Draft RTP Circuit Breakers December 2014 calculating the timeout, the deterministic RTCP reporting interval, Td, without the randomization factor, and with a fixed minimum interval Tmin=5 seconds) SHOULD be used. The rationale for this choice of timeout is as described in Section 6.2 of RFC 3550 [RFC3550]. The choice of three RTCP reporting intervals as the timeout is made following Section 6.3.5 of RFC 3550 [RFC3550]. This specifies that participants in an RTP session will timeout and remove an RTP sender from the list of active RTP senders if no RTP data packets have been received from that RTP sender within the last two RTCP reporting intervals. Using a timeout of three RTCP reporting intervals is therefore large enough that the other participants will have timed out the sender if a network problem stops the data packets it is sending from reaching the receivers, even allowing for loss of some RTCP packets. If a sender is transmitting a large number of RTP media streams, such that the corresponding RTCP SR or RR packets are too large to fit into the network MTU, the receiver will generate RTCP SR or RR packets in a round-robin manner. In this case, the sender SHOULD treat receipt of an RTCP SR or RR packet corresponding to any SSRC it sent on the same 5-tuple of source and destination IP address, port, and protocol, as an indication that the receiver and return path are working, preventing the RTCP timeout circuit breaker from triggering. 4.3. RTP/AVP Circuit Breaker #3: Congestion If RTP data packets are being sent, and the corresponding RTCP SR or RR packets show non-zero packet loss fraction and increasing extended highest sequence number received, then those RTP data packets are arriving at the receiver, but some degree of congestion is occurring. The RTP/AVP profile [RFC3551] states that: If best-effort service is being used, RTP receivers SHOULD monitor packet loss to ensure that the packet loss rate is within acceptable parameters. Packet loss is considered acceptable if a TCP flow across the same network path and experiencing the same network conditions would achieve an average throughput, measured on a reasonable time scale, that is not less than the RTP flow is achieving. This condition can be satisfied by implementing congestion control mechanisms to adapt the transmission rate (or the number of layers subscribed for a layered multicast session), or by arranging for a receiver to leave the session if the loss rate is unacceptably high. The comparison to TCP cannot be specified exactly, but is intended as an "order-of-magnitude" comparison in time scale and Perkins & Singh Expires June 07, 2015 [Page 9] Internet-Draft RTP Circuit Breakers December 2014 throughput. The time scale on which TCP throughput is measured is the round-trip time of the connection. In essence, this requirement states that it is not acceptable to deploy an application (using RTP or any other transport protocol) on the best-effort Internet which consumes bandwidth arbitrarily and does not compete fairly with TCP within an order of magnitude. The phase "order of magnitude" in the above means within a factor of ten, approximately. In order to implement this, it is necessary to estimate the throughput a TCP connection would achieve over the path. For a long-lived TCP Reno connection, it has been shown that the TCP throughput can be estimated using the following equation [Padhye]: s X = -------------------------------------------------------------- R*sqrt(2*b*p/3) + (t_RTO * (3*sqrt(3*b*p/8) * p * (1+32*p^2))) where: X is the transmit rate in bytes/second. s is the packet size in bytes. If data packets vary in size, then the average size is to be used. R is the round trip time in seconds. p is the loss event rate, between 0 and 1.0, of the number of loss events as a fraction of the number of packets transmitted. t_RTO is the TCP retransmission timeout value in seconds, generally approximated by setting t_RTO = 4*R. b is the number of packets that are acknowledged by a single TCP acknowledgement; [RFC3448] recommends the use of b=1 since many TCP implementations do not use delayed acknowledgements. This is the same approach to estimated TCP throughput that is used in [RFC3448]. Under conditions of low packet loss the second term on the denominator is small, so this formula can be approximated with reasonable accuracy as follows [Mathis]: s X = ----------------- R * sqrt(2*b*p/3) Perkins & Singh Expires June 07, 2015 [Page 10] Internet-Draft RTP Circuit Breakers December 2014 It is RECOMMENDED that this simplified throughout equation be used, since the reduction in accuracy is small, and it is much simpler to calculate than the full equation. Measurements have shown that the simplified TCP throughput equation is effective as an RTP circuit breaker for multimedia flows sent to hosts on residential networks using ADSL and cable modem links [Singh]. The data shows that the full TCP throughput equation tends to be more sensitive to packet loss and triggers the RTP circuit breaker earlier than the simplified equation. Implementations that desire this extra sensitivity MAY use the full TCP throughput equation in the RTP circuit breaker. Initial measurements in LTE networks have shown that the extra sensitivity is helpful in that environment, with the full TCP throughput equation giving a more balanced circuit breaker response than the simplified TCP equation [Sarker]; other networks might see similar behaviour. No matter what TCP throughput equation is chosen, two parameters need to be estimated and reported to the sender in order to calculate the throughput: the round trip time, R, and the loss event rate, p (the packet size, s, is known to the sender). The round trip time can be estimated from RTCP SR and RR packets. This is done too infrequently for accurate statistics, but is the best that can be done with the standard RTCP mechanisms. Report blocks in RTCP SR or RR packets contain the packet loss fraction, rather than the loss event rate, so p cannot be reported (TCP typically treats the loss of multiple packets within a single RTT as one loss event, but RTCP RR packets report the overall fraction of packets lost, and does not report when the packet losses occurred). Using the loss fraction in place of the loss event rate can overestimate the loss. We believe that this overestimate will not be significant, given that we are only interested in order of magnitude comparison ([Floyd] section 3.2.1 shows that the difference is small for steady-state conditions and random loss, but using the loss fraction is more conservative in the case of bursty loss). The congestion circuit breaker is therefore: when a sender receives an RTCP SR or RR packet that contains a report block for an SSRC it is using, the sender MUST check the fraction lost field in the report block to determine if there is a non-zero packet loss rate. If the fraction lost field is zero, then continue sending as normal. If the fraction lost is greater than zero, then estimate the TCP throughput that would be achieved over the path using the chosen TCP throughput equation and the measured values of the round-trip time, R, the loss event rate, p (as approximated by the fraction lost), and the packet size, s. Compare this with the actual sending rate. If the actual sending rate has been more than ten times the TCP throughput estimate for three (or more) consecutive RTCP reporting intervals, then the congestion circuit breaker is triggered. Perkins & Singh Expires June 07, 2015 [Page 11] Internet-Draft RTP Circuit Breakers December 2014 When the congestion circuit breaker is triggered, the sender SHOULD cease transmission (see Section 4.5). However, if the sender is able to reduce its sending rate by a factor of (approximately) ten, then it MAY first reduce its sending rate by this factor (or some larger amount) to see if that resolves the congestion. If the sending rate is reduced in this way and the congestion circuit breaker triggers again after the next three RTCP reporting intervals, the sender MUST then cease transmission. An example of such a rate reduction might be a video conferencing system that backs off to sending audio only, before completely dropping the call. If such a reduction in sending rate resolves the congestion problem, the sender MAY gradually increase the rate at which it sends data after a reasonable amount of time has passed, provided it takes care not to cause the problem to recur ("reasonable" is intentionally not defined here). The congestion circuit breaker depends on the fraction of RTP data packets lost in a reporting interval. If the number of packets sent in the reporting interval is too low, this statistic loses meaning, and it is possible that a sampling error can give the appearance of high packet loss rates. Following the guidelines in [RFC5405], an RTP sender that sends not more than one RTP packet per RTT MAY ignore a single trigger of the congestion circuit breaker, on the basis that the packet loss rate estimate is unreliable with so few samples. However, if the congestion circuit breaker triggers again after the following three RTCP reporting intervals (i.e., if there have been six or more consecutive RTCP reporting intervals where the actual sending rate is more than ten times the estimated sending rate derived from the TCP throughput equation), then the sender SHOULD cease transmission (see Section 4.5). The RTCP reporting interval of the media sender does not affect how quickly congestion circuit breaker can trigger. The timing is based on the RTCP reporting interval of the receiver that generates the SR/ RR packets from which the loss rate and RTT estimate are derived (note that RTCP requires all participants in a session to have similar reporting intervals, else the participant timeout rules in [RFC3550] will not work, so this interval is likely similar to that of the sender). If the incoming RTCP SR or RR packets are using a reduced minimum RTCP reporting interval (as specified in Section 6.2 of RFC 3550 [RFC3550] or the RTP/AVPF profile [RFC4585]), then that reduced RTCP reporting interval is used when determining if the circuit breaker is triggered. Perkins & Singh Expires June 07, 2015 [Page 12] Internet-Draft RTP Circuit Breakers December 2014 As in Section 4.1 and Section 4.2, we use three reporting intervals to avoid triggering the circuit breaker on transient failures. This circuit breaker is a worst-case condition, and congestion control needs to be performed to keep well within this bound. It is expected that the circuit breaker will only be triggered if the usual congestion control fails for some reason. If there are more media streams that can be reported in a single RTCP SR or RR packet, or if the size of a complete RTCP SR or RR packet exceeds the network MTU, then the receiver will report on a subset of sources in each reporting interval, with the subsets selected round- robin across multiple intervals so that all sources are eventually reported [RFC3550]. When generating such round-robin RTCP reports, priority SHOULD be given to reports on sources that have high packet loss rates, to ensure that senders are aware of network congestion they are causing (this is an update to [RFC3550]). 4.4. RTP/AVP Circuit Breaker #4: Media Usability Applications that use RTP are generally tolerant to some amount of packet loss. How much packet loss can be tolerated will depend on the application, media codec, and the amount of error correction and packet loss concealment that is applied. There is an upper bound on the amount of loss can be corrected, however, beyond which the media becomes unusable. Similarly, many applications have some upper bound on the media capture to play-out latency that can be tolerated before the application becomes unusable. The latency bound will depend on the application, but typical values can range from the order of a few hundred milliseconds for voice telephony and interactive conferencing applications, up to several seconds for some video-on-demand systems. As a final circuit breaker, RTP senders SHOULD monitor the reported packet loss and delay to estimate whether the media is likely to be suitable for the intended purpose. If the packet loss rate and/or latency is such that the media has become unusable, and has remained unusable for a significant time period, then the application SHOULD cease transmission. Similarly, receivers SHOULD monitor the quality of the media they receive, and if the quality is unusable for a significant time period, they SHOULD terminate the session. This memo intentionally does not define a bound on the packet loss rate or latency that will result in unusable media, nor does it specify what time period is deemed significant, as these are highly application dependent. Sending media that suffers from such high packet loss or latency that it is unusable at the receiver is both wasteful of resources, and of no benefit to the user of the application. It also is highly likely to be congesting the network, and disrupting other applications. As Perkins & Singh Expires June 07, 2015 [Page 13] Internet-Draft RTP Circuit Breakers December 2014 such, the congestion circuit breaker will almost certainly trigger to stop flows where the media would be unusable due to high packet loss or latency. However, in pathological scenarios where the congestion circuit breaker does not stop the flow, it is desirable that the RTP application cease sending useless traffic. The role of the media usability circuit breaker is to protect the network in such cases. 4.5. Ceasing Transmission What it means to cease transmission depends on the application, but the intention is that the application will stop sending RTP data packets to a particular destination 3-tuple (transport protocol, destination port, IP address), until the user makes an explicit attempt to restart the call. It is important that a human user is involved in the decision to try to restart the call, since that user will eventually give up if the calls repeatedly trigger the circuit breaker. This will help avoid problems with automatic redial systems from congesting the network. Accordingly, RTP flows halted by the circuit breaker SHOULD NOT be restarted automatically unless the sender has received information that the congestion has dissipated. It is recognised that the RTP implementation in some systems might not be able to determine if a call set-up request was initiated by a human user, or automatically by some scripted higher-level component of the system. These implementations SHOULD rate limit attempts to restart a call to the same destination 3-tuple as used by a previous call that was recently halted by the circuit breaker. The chosen rate limit ought to not exceed the rate at which an annoyed human caller might redial a misbehaving phone. 5. RTP Circuit Breakers for Systems Using the RTP/AVPF Profile Use of the Extended RTP Profile for RTCP-based Feedback (RTP/AVPF) [RFC4585] allows receivers to send early RTCP reports in some cases, to inform the sender about particular events in the media stream. There are several use cases for such early RTCP reports, including providing rapid feedback to a sender about the onset of congestion. Receiving rapid feedback about congestion events potentially allows congestion control algorithms to be more responsive, and to better adapt the media transmission to the limitations of the network. It is expected that many RTP congestion control algorithms will adopt the RTP/AVPF profile for this reason, defining new transport layer feedback reports that suit their requirements. Since these reports are not yet defined, and likely very specific to the details of the congestion control algorithm chosen, they cannot be used as part of the generic RTP circuit breaker. Perkins & Singh Expires June 07, 2015 [Page 14] Internet-Draft RTP Circuit Breakers December 2014 Reduced-size RTCP reports sent under the RTP/AVPF early feedback rules that do not contain an RTCP SR or RR packet MUST be ignored by the congestion circuit breaker (they do not contain the information needed by the congestion circuit breaker algorithm), but MUST be counted as received packets for the RTCP timeout circuit breaker. Reduced-size RTCP reports sent under the RTP/AVPF early feedback rules that contain RTCP SR or RR packets MUST be processed by the congestion circuit breaker as if they were sent as regular RTCP reports, and counted towards the circuit breaker conditions specified in Section 4 of this memo. This will potentially make the RTP circuit breaker fire earlier than it would if the RTP/AVPF profile was not used. When using ECN with RTP (see Section 8), early RTCP feedback packets can contain ECN feedback reports. The count of ECN-CE marked packets contained in those ECN feedback reports is counted towards the number of lost packets reported if the ECN Feedback Report report is sent in an compound RTCP packet along with an RTCP SR/RR report packet. Reports of ECN-CE packets sent as reduced-size RTCP ECN feedback packets without an RTCP SR/RR packet MUST be ignored. These rules are intended to allow the use of low-overhead RTP/AVPF feedback for generic NACK messages without triggering the RTP circuit breaker. This is expected to make such feedback suitable for RTP congestion control algorithms that need to quickly report loss events in between regular RTCP reports. The reaction to reduced-size RTCP SR/RR packets is to allow such algorithms to send feedback that can trigger the circuit breaker, when desired. 6. Impact of RTCP Extended Reports (XR) RTCP Extended Report (XR) blocks provide additional reception quality metrics, but do not change the RTCP timing rules. Some of the RTCP XR blocks provide information that might be useful for congestion control purposes, others provided non-congestion-related metrics. With the exception of RTCP XR ECN Summary Reports (see Section 8), the presence of RTCP XR blocks in a compound RTCP packet does not affect the RTP circuit breaker algorithm. For consistency and ease of implementation, only the reception report blocks contained in RTCP SR packets, RTCP RR packets, or RTCP XR ECN Summary Report packets, are used by the RTP circuit breaker algorithm. 7. Impact of RTCP Reporting Groups An optimisation for grouping RTCP reception statistics and other feedback in RTP sessions with large numbers of participants is given in [I-D.ietf-avtcore-rtp-multi-stream-optimisation]. This allows one SSRC to act as a representative that sends reports on behalf of other Perkins & Singh Expires June 07, 2015 [Page 15] Internet-Draft RTP Circuit Breakers December 2014 SSRCs that are co-located in the same endpoint and see identical reception quality. When running the circuit breaker algorithms, an endpoint MUST treat a reception report from the representative of the reporting group as if a reception report was received from all members of that group. 8. Impact of Explicit Congestion Notification (ECN) The use of ECN for RTP flows does not affect the media timeout RTP circuit breaker (Section 4.1) or the RTCP timeout circuit breaker (Section 4.2), since these are both connectivity checks that simply determinate if any packets are being received. ECN-CE marked packets SHOULD be treated as if it were lost for the purposes of congestion control, when determining the optimal media sending rate for an RTP flow. If an RTP sender has negotiated ECN support for an RTP session, and has successfully initiated ECN use on the path to the receiver [RFC6679], then ECN-CE marked packets SHOULD be treated as if they were lost when calculating if the congestion- based RTP circuit breaker (Section 4.3) has been met. The count of ECN-CE marked RTP packets is returned in RTCP XR ECN summary report packets if support for ECN has been initiated for an RTP session. 9. Impact of Bundled Media and Layered Coding The RTP circuit breaker operates on a per-RTP session basis. An RTP sender that participates in several RTP sessions MUST treat each RTP session independently with regards to the RTP circuit breaker. An RTP sender can generate several media streams within a single RTP session, with each stream using a different SSRC. This can happen if bundled media are in use, when using simulcast, or when using layered media coding. By default, each SSRC will be treated independently by the RTP circuit breaker. However, the sender MAY choose to treat the flows (or a subset thereof) as a group, such that a circuit breaker trigger for one flow applies to the group of flows as a whole, and either causes the entire group to cease transmission, or the sending rate of the group to reduce by a factor of ten, depending on the RTP circuit breaker triggered. Grouping flows in this way is expected to be especially useful for layered flows sent using multiple SSRCs, as it allows the layered flow to react as a whole, ceasing transmission on the enhancement layers first to reduce sending rate if necessary, rather than treating each layer independently. 10. Security Considerations The security considerations of [RFC3550] apply. Perkins & Singh Expires June 07, 2015 [Page 16] Internet-Draft RTP Circuit Breakers December 2014 If the RTP/AVPF profile is used to provide rapid RTCP feedback, the security considerations of [RFC4585] apply. If ECN feedback for RTP over UDP/IP is used, the security considerations of [RFC6679] apply. If non-authenticated RTCP reports are used, an on-path attacker can trivially generate fake RTCP packets that indicate high packet loss rates, causing the circuit breaker to trigger and disrupting an RTP session. This is somewhat more difficult for an off-path attacker, due to the need to guess the randomly chosen RTP SSRC value and the RTP sequence number. This attack can be avoided if RTCP packets are authenticated; authentication options are discussed in [RFC7201]. Timely operation of the RTP circuit breaker depends on the choice of RTCP reporting interval. If the receiver has a reporting interval that is overly long, then the responsiveness of the circuit breaker decreases. In the limit, the RTP circuit breaker can be disabled for all practical purposes by configuring an RTCP reporting interval that is many minutes duration. This issue is not specific to the circuit breaker: long RTCP reporting intervals also prevent reception quality reports, feedback messages, codec control messages, etc., from being used. Implementations SHOULD impose an upper limit on the RTCP reporting interval they are willing to negotiate (based on the session bandwidth and RTCP bandwidth fraction) when using the RTP circuit breaker. An upper limit on the reporting interval on the order of 10 seconds is a reasonable bound. 11. IANA Considerations There are no actions for IANA. 12. Open Issues o Should the number of RTCP reporting intervals needed to trigger the media timeout and congestion circuit breakers scale with the duration of the RTCP reporting interval, so the circuit breaker triggers after a fixed duration, rather than after a fixed number of reporting intervals? 13. Acknowledgements The authors would like to thank Bernard Aboba, Harald Alvestrand, Gorry Fairhurst, Kevin Gross, Cullen Jennings, Randell Jesup, Jonathan Lennox, Matt Mathis, Stephen McQuistin, Eric Rescorla, Abheek Saha, and Fabio Verdicchio, for their valuable feedback. 14. References Perkins & Singh Expires June 07, 2015 [Page 17] Internet-Draft RTP Circuit Breakers December 2014 14.1. Normative References [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997. [RFC3448] Handley, M., Floyd, S., Padhye, J., and J. Widmer, "TCP Friendly Rate Control (TFRC): Protocol Specification", RFC 3448, January 2003. [RFC3550] Schulzrinne, H., Casner, S., Frederick, R., and V. Jacobson, "RTP: A Transport Protocol for Real-Time Applications", STD 64, RFC 3550, July 2003. [RFC3551] Schulzrinne, H. and S. Casner, "RTP Profile for Audio and Video Conferences with Minimal Control", STD 65, RFC 3551, July 2003. [RFC3611] Friedman, T., Caceres, R., and A. Clark, "RTP Control Protocol Extended Reports (RTCP XR)", RFC 3611, November 2003. [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, July 2006. 14.2. Informative References [Floyd] Floyd, S., Handley, M., Padhye, J., and J. Widmer, "Equation-Based Congestion Control for Unicast Applications", Proceedings of the ACM SIGCOMM conference, 2000, DOI 10.1145/347059.347397, August 2000. [I-D.ietf-avtcore-rtp-multi-stream-optimisation] Lennox, J., Westerlund, M., Wu, W., and C. Perkins, "Sending Multiple Media Streams in a Single RTP Session: Grouping RTCP Reception Statistics and Other Feedback", draft-ietf-avtcore-rtp-multi-stream-optimisation-04 (work in progress), August 2014. [Mathis] Mathis, M., Semke, J., Mahdavi, J., and T. Ott, "The macroscopic behavior of the TCP congestion avoidance algorithm", ACM SIGCOMM Computer Communication Review 27(3), DOI 10.1145/263932.264023, July 1997. Perkins & Singh Expires June 07, 2015 [Page 18] Internet-Draft RTP Circuit Breakers December 2014 [Padhye] Padhye, J., Firoiu, V., Towsley, D., and J. Kurose, "Modeling TCP Throughput: A Simple Model and its Empirical Validation", Proceedings of the ACM SIGCOMM conference, 1998, DOI 10.1145/285237.285291, August 1998. [RFC3168] Ramakrishnan, K., Floyd, S., and D. Black, "The Addition of Explicit Congestion Notification (ECN) to IP", RFC 3168, September 2001. [RFC5104] Wenger, S., Chandra, U., Westerlund, M., and B. Burman, "Codec Control Messages in the RTP Audio-Visual Profile with Feedback (AVPF)", RFC 5104, February 2008. [RFC5124] Ott, J. and E. Carrara, "Extended Secure RTP Profile for Real-time Transport Control Protocol (RTCP)-Based Feedback (RTP/SAVPF)", RFC 5124, February 2008. [RFC5405] Eggert, L. and G. Fairhurst, "Unicast UDP Usage Guidelines for Application Designers", BCP 145, RFC 5405, November 2008. [RFC5450] Singer, D. and H. Desineni, "Transmission Time Offsets in RTP Streams", RFC 5450, March 2009. [RFC5506] Johansson, I. and M. Westerlund, "Support for Reduced-Size Real-Time Transport Control Protocol (RTCP): Opportunities and Consequences", RFC 5506, April 2009. [RFC5681] Allman, M., Paxson, V., and E. Blanton, "TCP Congestion Control", RFC 5681, September 2009. [RFC6051] Perkins, C. and T. Schierl, "Rapid Synchronisation of RTP Flows", RFC 6051, November 2010. [RFC6679] Westerlund, M., Johansson, I., Perkins, C., O'Hanlon, P., and K. Carlberg, "Explicit Congestion Notification (ECN) for RTP over UDP", RFC 6679, August 2012. [RFC6798] Clark, A. and Q. Wu, "RTP Control Protocol (RTCP) Extended Report (XR) Block for Packet Delay Variation Metric Reporting", RFC 6798, November 2012. [RFC6843] Clark, A., Gross, K., and Q. Wu, "RTP Control Protocol (RTCP) Extended Report (XR) Block for Delay Metric Reporting", RFC 6843, January 2013. Perkins & Singh Expires June 07, 2015 [Page 19] Internet-Draft RTP Circuit Breakers December 2014 [RFC6958] Clark, A., Zhang, S., Zhao, J., and Q. Wu, "RTP Control Protocol (RTCP) Extended Report (XR) Block for Burst/Gap Loss Metric Reporting", RFC 6958, May 2013. [RFC7002] Clark, A., Zorn, G., and Q. Wu, "RTP Control Protocol (RTCP) Extended Report (XR) Block for Discard Count Metric Reporting", RFC 7002, September 2013. [RFC7003] Clark, A., Huang, R., and Q. Wu, "RTP Control Protocol (RTCP) Extended Report (XR) Block for Burst/Gap Discard Metric Reporting", RFC 7003, September 2013. [RFC7097] Ott, J., Singh, V., and I. Curcio, "RTP Control Protocol (RTCP) Extended Report (XR) for RLE of Discarded Packets", RFC 7097, January 2014. [RFC7201] Westerlund, M. and C. Perkins, "Options for Securing RTP Sessions", RFC 7201, April 2014. [Sarker] Sarker, Z., Singh, V., and C.S. Perkins, "An Evaluation of RTP Circuit Breaker Performance on LTE Networks", Proceedings of the IEEE Infocom workshop on Communication and Networking Techniques for Contemporary Video, 2014, April 2014. [Singh] Singh, V., McQuistin, S., Ellis, M., and C.S. Perkins, "Circuit Breakers for Multimedia Congestion Control", Proceedings of the International Packet Video Workshop, 2013, DOI 10.1109/PV.2013.6691439, December 2013. Authors' Addresses Colin Perkins University of Glasgow School of Computing Science Glasgow G12 8QQ United Kingdom Email: csp@csperkins.org Perkins & Singh Expires June 07, 2015 [Page 20] Internet-Draft RTP Circuit Breakers December 2014 Varun Singh Aalto University School of Electrical Engineering Otakaari 5 A Espoo, FIN 02150 Finland Email: varun@comnet.tkk.fi URI: http://www.netlab.tkk.fi/~varun/ Perkins & Singh Expires June 07, 2015 [Page 21]