RTP Media Congestion Avoidance D. Hayes, Ed. Techniques University of Oslo Internet-Draft S. Ferlin Intended status: Experimental Simula Research Laboratory Expires:~~November 9, 2015~~January 2, 2016M. Welzl University of Oslo~~May 8,~~July 1,2015 Shared Bottleneck Detection for Coupled Congestion Control for RTP Media.~~draft-ietf-rmcat-sbd-00~~draft-ietf-rmcat-sbd-01Abstract This document describes a mechanism to detect whether end-to-end data flows share a common bottleneck. It relies on summary statistics that are calculated by a data receiver based on continuous measurements and regularly fed to a grouping algorithm that runs wherever the knowledge is needed. This mechanism complements the coupled congestion control mechanism in draft-welzl-rmcat-coupled-cc. 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~~November 9, 2015.~~January 2, 2016.Copyright Notice Copyright (c) 2015 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 (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 . . . . . . . . . . . . . . . . . . . . . . . . . 3 1.1. The signals . . . . . . . . . . . . . . . . . . . . . . . 3 1.1.1. Packet Loss . . . . . . . . . . . . . . . . . . . . . 3 1.1.2. Packet Delay . . . . . . . . . . . . . . . . . . . . . 3 1.1.3. Path Lag . . . . . . . . . . . . . . . . . . . . . . . 4 2. Definitions . . . . . . . . . . . . . . . . . . . . . . . . . 4 2.1.~~Parameter Values~~Parameters and their Effect . .. . . . . . . . . . . . .5 2.2. Recommended Parameter Values. . . . . . . .~~5~~. . . . . . . 73. Mechanism . . . . . . . . . . . . . . . . . . . . . . . . . .~~6~~73.1. Key metrics and their calculation . . . . . . . . . . . .~~7~~93.1.1. Mean delay . . . . . . . . . . . . . . . . . . . . . .~~7~~93.1.2. Skewness Estimate . . . . . . . . . . . . . . . . . .~~8~~93.1.3.~~Variance~~VariabilityEstimate . . . . . . . . . . . . . . . . .~~. 9~~103.1.4. Oscillation Estimate . . . . . . . . . . . . . . . . .~~9~~113.1.5. Packet loss . . . . . . . . . . . . . . . . . . . . .~~10~~113.2. Flow Grouping . . . . . . . . . . . . . . . . . . . . . .~~10~~123.2.1. Flow Grouping Algorithm . . . . . . . . . . . . . . .~~10~~123.2.2. Using the flow group signal . . . . . . . . . . . . .~~12~~133.3. Removing Noise from the Estimates . . . . . . . . . . . .~~12~~133.3.1.~~Oscillation~~PDVnoise . . . . . . . . . . . . . . . . . .~~12 3.3.2. Clock drift~~. . . .14 3.3.2. Oscillation noise .. . . . . . . . . . . . . . . . .~~13~~143.3.3.~~Bias in the skewness measure~~Clock skew. . . . . . . . . . . . .~~14~~. . . . . . . . . 153.4. Reducing lag and Improving Responsiveness . . . . . . . .~~14~~153.4.1. Improving the response of the skewness estimate . . .~~15~~163.4.2. Improving the response of the~~variance~~variabilityestimate . .~~. 15~~164. Measuring OWD . . . . . . . . . . . . . . . . . . . . . . . .~~16~~174.1. Time stamp resolution . . . . . . . . . . . . . . . . . .~~16~~175. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . .~~16~~176. IANA Considerations . . . . . . . . . . . . . . . . . . . . .~~16~~177. Security Considerations . . . . . . . . . . . . . . . . . . .~~16~~178. Change history . . . . . . . . . . . . . . . . . . . . . . . .~~17~~189. References . . . . . . . . . . . . . . . . . . . . . . . . . .~~17~~189.1. Normative References . . . . . . . . . . . . . . . . . . .~~17~~189.2. Informative References . . . . . . . . . . . . . . . . . .~~17~~18Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . .~~18~~191. Introduction In the Internet, it is not normally known if flows (e.g., TCP connections or UDP data streams) traverse the same bottlenecks. Even flows that have the same sender and receiver may take different paths and share a bottleneck or not. Flows that share a bottleneck link usually compete with one another for their share of the capacity. This competition has the potential to increase packet loss and delays. This is especially relevant for interactive applications that communicate simultaneously with multiple peers (such as multi- party video). For RTP media applications such as RTCWEB, [I-D.welzl-rmcat-coupled-cc] describes a scheme that combines the congestion controllers of flows in order to honor their priorities and avoid unnecessary packet loss as well as delay. This mechanism relies on some form of Shared Bottleneck Detection (SBD); here, a measurement-based SBD approach is described. 1.1. The signals The current Internet is unable to explicitly inform endpoints as to which flows share bottlenecks, so endpoints need to infer this from whatever information is available to them. The mechanism described here currently utilises packet loss and packet delay, but is not restricted to these. 1.1.1. Packet Loss Packet loss is often a relatively rare signal. Therefore, on its own it is of limited use for SBD, however, it is a valuable supplementary measure when it is more prevalent. 1.1.2. Packet Delay End-to-end delay measurements include noise from every device along the path in addition to the delay perturbation at the bottleneck device. The noise is often significantly increased if the round-trip time is used. The cleanest signal is obtained by using One-Way-Delay (OWD). Measuring absolute OWD is difficult since it requires both the sender and receiver clocks to be synchronised. However, since the statistics being collected are relative to the mean OWD, a relative OWD measurement is sufficient. Clock~~drift~~skewis not usually significant over the time intervals used by this SBD mechanism (see [RFC6817] A.2 for a discussion on clock~~drift~~skewand OWD measurements). However, in circumstances where it is significant, Section~~3.3.2~~3.3.3outlines a way of adjusting the calculations to cater for it. Each packet arriving at the bottleneck buffer may experience very different queue lengths, and therefore different waiting times. A single OWD sample does not, therefore, characterize the path well. However, multiple OWD measurements do reflect the distribution of delays experienced at the bottleneck. 1.1.3. Path Lag Flows that share a common bottleneck may traverse different paths, and these paths will often have different base delays. This makes it difficult to correlate changes in delay or loss. This technique uses the long term shape of the delay distribution as a base for comparison to counter this. 2. Definitions 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]. Acronyms used in this document: OWD -- One Way Delay PDV -- Packet Delay VariationMAD -- Mean Absolute DeviationRTT -- Round Trip Time SBD -- Shared Bottleneck Detection Conventions used in this document: T -- the base time interval over which measurements are made. N -- the number of base time, T, intervals used in some calculations. sum_T(...) -- summation of all the measurements of the variable in parentheses taken over the interval T sum(...) -- summation of terms of the variable in parentheses sum_N(...) -- summation of N terms of the variable in parentheses sum_NT(...) -- summation of all measurements taken over the interval N*T E_T(...) -- the expectation or mean of the measurements of the variable in parentheses over T E_N(...) --~~The~~theexpectation or mean of the last N values of the variable in parentheses E_M(...) --~~The~~theexpectation or mean of the last M values of the variable in parentheses, where M <= N. max_T(...) -- the maximum recorded measurement of the variable in parentheses taken over the interval T min_T(...) -- the minimum recorded measurement of the variable in parentheses taken over the interval T num_T(...) -- the count of measurements of the variable in parentheses taken in the interval T num_VM(...) -- the count of valid values of the variable in parentheses given M records PC -- a boolean variable indicating the particular flow was identified as experiencing congestion in the previous interval T (i.e. Previously Congested)~~CD_T~~skew_est--~~an estimate~~a measureof~~the effect~~skewness in a OWD distribution. var_est -- a measureof~~Clock Drift on the mean~~variability inOWD~~per T CD_Adj(...)~~measurements. freq_est--~~Mean~~a measure of low frequency oscillation in theOWD~~adjusted for clock drift~~measurements.p_l, p_f, p_pdv,p_mad,c_s, c_h, p_s, p_d, p_v -- various thresholds used in the~~mechanism. N, M,~~mechanism Mand F -- number of values~~(calculated over T). 2.1. Parameter Values Reference [Hayes-LCN14] uses T=350ms, N=50, p_l = 0.1. The other parameters have been tightened to reflect minor enhancements~~relatedto~~the algorithm outlined in Section 3.3: c_s = -0.01, p_f = p_s = p_d = 0.1, p_pdv = 0.2, p_v = 0.2. M=50, F=10,~~N 2.1. Parametersand~~c_h = 0.3 are additional parameters defined in the document. These are values~~their Effect T T should be long enough sothat~~seem to work well over~~there are enough packets received during T fora~~wide range~~useful estimateof~~practical Internet conditions, but are~~short term mean OWD and variation statistics. Making T too large can limitthe~~subject~~efficacyof~~ongoing tests. 3. Mechanism The mechanism described in this document is~~PDV and freq_est. It will also increase the response time of the mechanism. Making T too small will make the metrics noisier. N & M N should be large enough provide a stable estimate of oscillations in OWD and average PDV. Usually M=N, though having M<N may be beneficial in certain circumstances. M*T needs to be long enough provide stable estimates of skewness and MAD (if used). F F determines the number of intervals over which statistics are considered to be equally weighted. When F=M recent and older measurements are considered equal. Making F<M can increase the responsiveness of the SBD mechanism. If F is too small, statistics will be too noisy. c_s c_s is the threshold in skew_est used for determining whether a flow is experiencing congestion or not. It should be slightly negative so that a very lightly loaded path does not give a false indication. Setting c_s more negative makes the SBD mechanism less sensitive to transient and light congestion episodes. c_s c_h adds hysteresis to the congestion determination. It should be large enough to avoid constant switching in the determination, but low enough to ensure that grouping is not attempted when there is no congestion and the delay and loss signals cannot be relied upon. p_v p_v determines the sensitivity of freq_est to noise. Making it smaller will yield higher but noisier values for freq_est. Making it too large will render it ineffective for determining groups. p_* Flows are separated when the skew_est|var_est|freq_est measure is greater than p_s|p_f|p_d|(p_pdv|p_mad). Adjusting these is a compromise between false grouping of flows that do not share a bottleneck and false splitting of flows that do. Making them larger can help if the measures are very noisy, but reducing the noise in the statistical measures by adjusting T and N|M may be a better solution. 2.2. Recommended Parameter Values Reference [Hayes-LCN14] uses T=350ms, N=50, p_l = 0.1. The other parameters have been tightened to reflect minor enhancements to the algorithm outlined in Section 3.3: c_s = -0.01, p_f = p_s = p_d = 0.1, p_pdv = 0.2, p_v = 0.2 (or p_mad=0.1, p_v=0.7). M=50, F=25, and c_h = 0.3 are additional parameters defined in the document. These are values that seem to work well over a wide range of practical Internet conditions. 3. Mechanism The mechanism described in this document isbased on the observation that the distribution of delay measurements of packets~~from flows~~that~~share~~traversea common bottleneck have similar shape characteristics. These shape characteristics are described using 3 key summary statistics:~~variance~~variability(estimate var_est, see Section 3.1.3) skewness (estimate skew_est, see Section 3.1.2) oscillation (estimate freq_est, see Section 3.1.4) with packet loss (estimate pkt_loss, see Section 3.1.5) used as a supplementary statistic. Summary statistics help to address both the noise and the path lag problems by describing the general shape over a relatively long period of time. This is sufficient for their application in coupled congestion control for RTP Media. They can be signalled from a receiver, which measures the OWD and calculates the summary statistics, to a sender, which is the entity that is transmitting the media stream. An RTP Media device may be both a sender and a receiver. SBD can be performed at either~~Sender~~a senderorareceiver or both. +----+ | H2 | +----+ | | L2 | +----+ L1 | L3 +----+ | H1 |------|------| H3 | +----+ +----+ A network with 3 hosts (H1, H2, H3) and 3 links (L1, L2, L3). Figure 1 In Figure 1, there are two possible cases for shared bottleneck detection: a sender-based and a receiver-based case. 1. Sender-based: consider a situation where host H1 sends media streams to hosts H2 and H3, and L1 is a shared bottleneck. H2 and H3 measure the OWD and calculate summary statistics, which they send to H1 every T. H1, having this knowledge, can determine the shared bottleneck and accordingly control the send rates. 2. Receiver-based: consider that H2 is also sending media to H3, and L3 is a shared bottleneck. If H3 sends summary statistics to H1 and H2, neither H1 nor H2 alone obtain enough knowledge to detect this shared bottleneck; H3 can however determine it by combining the summary statistics related to H1 and H2, respectively. This case is applicable when send rates are controlled by the receiver; then, the signal from H3 to the senders contains the sending rate. A discussion of the required signalling for the receiver-based case is beyond the scope of this document. For the sender-based case, the messages and their data format will be defined here in future versions of this document. We envision that an initialization message from the sender to the receiver could specify which key metrics are requested out of a possibly extensible set (pkt_loss, var_est, skew_est, freq_est). The grouping algorithm described in this document requires all four of these metrics, and receivers MUST be able to provide them, but future algorithms may be able to exploit other metrics (e.g. metrics based on explicit network signals). Moreover, the initialization message could specify T, N, and the necessary resolution and precision (number of bits per field). 3.1. Key metrics and their calculation Measurements are calculated over a base interval, T. T should be long enough to provide enough samples for a good estimate of skewness, but short enough so that a measure of the oscillation can be made from N of these estimates. Reference [Hayes-LCN14] uses T = 350ms and N=M=50, which are values that seem to work well over a wide range of practical Internet conditions. 3.1.1. Mean delay The mean delay is not a useful signal for comparisons between flows since flows may traverse quite different paths and clocks will not necessarily be synchronized. However, it is a base measure for the 3 summary statistics. The mean delay, E_T(OWD), is the average one way delay measured over T. To facilitate the other calculations, the last N E_T(OWD) values will need to be stored in a cyclic buffer along with the moving average of E_T(OWD): mean_delay = E_M(E_T(OWD)) = sum_M(E_T(OWD)) / M where M <= N. Generally~~M=N,~~M=N:setting M to be less than N allows the mechanism to be more responsive to changes, but potentially at the expense of a higher error rate (see Section 3.4 for a discussion on improving the responsiveness of the mechanism.) 3.1.2. Skewness Estimate Skewness is difficult to calculate efficiently and accurately. Ideally it should be calculated over the entire period (M * T) from the mean OWD over that period. However this would require storing every delay measurement over the period. Instead, an estimate is made overM * T based on a calculation everyT using the previousT'scalculation of mean_delay.~~Comparisons are made using the mean of M skew estimates (an alternative that removes bias in the mean is given in Section 3.3.3).~~The skewness is estimated using two counters, counting the number of one way delay samples (OWD) above and below the mean:~~skew_est_T~~skew_base_T=~~(sum_T(OWD~~sum_T(OWD< mean_delay) - sum_T(OWD >~~mean_delay)) / num_T(OWD)~~mean_delay)where if (OWD < mean_delay) 1 else 0 if (OWD > mean_delay) 1 else 0~~skew_est_T is a number between -1~~and~~1 skew_est = E_M(skew_est_T) = sum_M(skew_est_T) / M For implementation ease,~~mean_delay does not include the mean of the current T interval.skew_est = sum_MT(skew_base_T)/num_MT(OWD) where skew_est is a number between -1 and 1Note: Care must be taken when implementing the comparisons to ensure that rounding does not bias skew_est. It is important that the mean is calculated with a higher precision than the samples. 3.1.3.~~Variance~~VariabilityEstimate Packet Delay Variation (PDV) ([RFC5481] and [ITU-Y1540]) is used as an estimator of the~~variance~~variabilityof the delay signal. We define PDV as follows: PDV = PDV_max = max_T(OWD) - E_T(OWD) var_est = E_M(PDV) = sum_M(PDV) / M This modifies PDV as outlined in [RFC5481] to provide a summary statistic version that best aids the grouping decisions of the algorithm (see [Hayes-LCN14] section IVB).Generally the maximum is sampled well during congestion, though it is more sensitive to path and operating system noise.The use of PDV = PDV_min = E_T(OWD) - min_T(OWD)~~is currently being investigated as an alternative that is~~would beless sensitive to~~noise. The drawback of using PDV_min~~this noise, butis~~that it does~~not~~distinguish between groups of flows with similar values of skew_est as~~well~~as PDV_max (see [Hayes-LCN14] section IVB).~~sampled during congestion at the bottleneck and therefore not recommended.3.1.4. Oscillation Estimate An estimate of the low frequency oscillation of the delay signal is calculated by counting and normalising the significant mean, E_T(OWD), crossings of mean_delay: freq_est = number_of_crossings / N~~Where~~wherewe define a significant mean crossing as a crossing that extends p_v * var_est from mean_delay. In our experiments we have found that p_v = 0.2 is a good value. Freq_est is a number between 0 and 1. Freq_est can be approximated incrementally as follows: With each new calculation of E_T(OWD) a decision is made as to whether this value of E_T(OWD) significantly crosses the current long term mean, mean_delay, with respect to the previous significant mean crossing. A cyclic buffer, last_N_crossings, records a 1 if there is a significant mean crossing, otherwise a 0. The counter, number_of_crossings, is incremented when there is a significant mean crossing and~~subtracted from~~decrementedwhen a non-zero value is removed from the last_N_crossings. This approximation of freq_est was not used in [Hayes-LCN14], which calculated freq_est every T using the current E_N(E_T(OWD)). Our tests show that this approximation of freq_est yields results that are almost identical to when the full calculation is performed every T. 3.1.5. Packet loss The proportion of packets lost is used as a supplementary measure: pkt_loss = sum_NT(lost packets) / sum_NT(total packets) Note: When pkt_loss is small it is very variable, however, when pkt_loss is high it becomes a stable measure for making grouping~~decisions.~~decisions..3.2. Flow Grouping 3.2.1. Flow Grouping Algorithm The following grouping algorithm is RECOMMENDED for SBD in the RMCAT context and is sufficient and efficient for small to moderate numbers of flows. For very large numbers of flows (e.g. hundreds), a more complex clustering algorithm may be substituted. Since no single metric is precise enough to group flows (due to noise), the algorithm uses multiple metrics. Each metric offers a different "view" of the bottleneck link characteristics, and used together they enable a more precise grouping of flows than would otherwise be possible. Flows determined to be experiencing congestion are successively divided into groups based on freq_est, var_est, and skew_est. The first step is to determine which flows are experiencing congestion. This is important, since if a flow is not experiencing congestion its delay based metrics will not describe the bottleneck, but the "noise" from the rest of the path. Skewness, with proportion of packets loss as a supplementary measure, is used to do this: 1. Grouping will be performed on flows where: skew_est < c_s || ( skew_est < c_h && PC ) || pkt_loss > p_l The parameter c_s controls how sensitive the mechanism is in detecting congestion. C_s = 0.0 was used in [Hayes-LCN14]. A value of c_s = 0.05 is a little more sensitive, and c_s = -0.05 is a little less sensitive. C_h controls the hysteresis on flows that were grouped as experiencing congestion last time. These flows, flows experiencing congestion, are then progressively divided into groups based on the freq_est, PDV, and skew_est summary statistics. The process proceeds according to the following steps: 2. Group flows whose difference in sorted freq_est is less than a threshold: diff(freq_est) < p_f 3. Group flows whose difference in sorted E_N(PDV) (highest to lowest) is less than a threshold: diff(var_est) < (p_pdv * var_est) The threshold, (p_pdv * var_est), is with respect to the highest value in the difference. 4. Group flows whose difference in sorted skew_est or pkt_loss is less than a threshold: if pkt_loss < p_l diff(skew_est) < p_s otherwise diff(pkt_loss) < (p_d * pkt_loss) The threshold, (p_d * pkt_loss), is with respect to the highest value in the difference. This procedure involves sorting estimates from highest to lowest. It is simple to implement, and efficient for small numbers of~~flows, such as are expected in RTCWEB.~~flows (up to 10-20).3.2.2. Using the flow group signal A grouping decisions is made every T from the second T, though they will not attain their full design accuracy until after the N'th T interval. Network conditions, and even the congestion controllers, can cause bottlenecks to fluctuate. A coupled congestion controller MAY decide only to couple groups that remain stable, say grouped together 90% of the time, depending on its objectives. Recommendations concerning this are beyond the scope of this draft and will be specific to the coupled congestion controllers objectives. 3.3. Removing Noise from the Estimates The following describe small changes to the calculation of the key metrics that help remove noise from them. Currently these "tweaks" are described separately to keep the~~main description succinct. In future revisions~~main description succinct. In future revisions of the draft these enhancements may replace the original key metric calculations. 3.3.1. PDV noise Usually during congestion the max_T(OWD) is quite well sampled as the delay distribution is skewed toward the maximum. However max_T(OWD) is subject to delay noise from other queues along the path as well as the host operating system. Min_T(OWD) is less prone to noise along the path and from the host operating system, but is not well sampled during congestion (i.e. when there is a bottleneck). Flows with very different packet send rates exacerbate the problem. An alternative delay variation measure that is less sensitive to extreme values and different send rates is Mean Absolute Deviation (MAD). It can be implemented in an online manner as follows: var_base_T = sum_T(|OWD - E_T(OWD)|) where |x| is the absolute value of x E_T(OWD) is the mean OWD calculated in the previous T var_est = MAD_MT = sum_MT(var_base_T)/num_MT(OWD) For calculation of freq_est p_v=0.7 (MAD is a smaller number than PDV) For the grouping threshold p_mad=0.1 insteadofp_pdv (MAD is less noisy sothe~~draft these enhancements may replace~~test can be tighter) Note thatthe~~original key metric calculations. 3.3.1.~~method for improving responsiveness of MAD_MT is the same as that described in Section 3.4.1 for skew_est. 3.3.2.Oscillation noise When a path has no congestion,~~the PDV~~var_estwill be very small and the recorded significant mean crossings will be the result of path noise. Thus up to N-1 meaningless mean crossings can be a source of error at the point a link becomes a bottleneck and flows traversing it begin to be grouped. To remove this source of noise from freq_est: 1. Set the current PDV to PDV = NaN (a value representing an invalid record,~~ie~~i.e.Not a Number) for flows that are deemed to not be experiencing congestion by the first skew_est based grouping test (see Section 3.2.1). 2. Then var_est = sum_M(PDV != NaN) / num_VM(PDV) 3. For freq_est, only record a significant mean crossing if flow is experiencing congestion. These three changes will remove the non-congestion noise from freq_est.~~3.3.2.~~A similar adjustment can be made for MAD based var_est. 3.3.3.Clock~~drift~~skewGenerally sender and receiver clock~~drift~~skewwill be too small to cause significant errors in the estimators. Skew_est is most sensitive to this type of noise. In circumstances where clock~~drift~~skewis high, making M < N can reduce this error. A better method is to estimate the effect the clock~~drift~~skewis having on the~~E_N(E_T(OWD)),~~summary statistics,and then adjust~~mean_delay~~statisticsaccordingly. A simpleonlinemethod of doing this~~follows: First divide the N E_T(OWD) values into two halves (N/2 in each) -- old and new. Calculate a mean of the old half: Older_mean = E_old(E_T(OWD)) / N/2 Calculate a mean of the new (most recent) half: Newer_mean = E_new(E_T(OWD)) / N/2 A linear estimate of the Clock Drift per T estimates is: CD_T = (Newer_mean - Older_mean)/N/2 An adjusted mean estimate then is: mean_delay = CD_Adj(E_M(E_T(OWD))) = E_M(E_T(OWD)) + CD_T * (M/2 + 0.5) CD_Adj can be thought of as a prediction of what the long term mean~~based on min_T(OWD)will bedescribed herein~~the current measurement period T. It is used as the basis for skew_est and freq_est. 3.3.3. Bias in the skewness measure If successive calculations of skew_est are made with very different numbers of samples (num_T(OWD)), the simple calculation~~a subsequent versionof~~E_M(skew_est) used for grouping decisions will be biased by the intervals that have few samples samples. This bias can be corrected if necessary as follows. skew_base_T = sum_T(OWD < mean_delay) - sum_T(OWD > mean_delay) skew_est = sum_MT(skew_base_T)/num_MT(OWD) This calculation requires slightly more state, since an implementation will need to maintain two cyclic buffers storing skew_base_T and num_T(OWD) respectively to manage the rolling summations (note only one cyclic buffer is needed for~~the~~calculation of skew_est outlined previously).~~draft.3.4. Reducing lag and Improving Responsiveness Measurement based shared bottleneck detection makes decisions in the present based on what has been measured in the past. This means that there is always a lag in responding to changing conditions. This mechanism is based on summary statistics taken over (N*T) seconds. This mechanism can be made more responsive to changing conditions by: 1. Reducing N and/or M -- but at the expense ofhavingless accurate metrics, and/or 2. Exploiting the fact that more recent measurements are more valuable than older measurements and weighting them accordingly. Although more recent measurements are more valuable, older measurements are still needed to gain an accurate estimate of the distribution descriptor we are measuring. Unfortunately, the simple exponentially weighted moving average weights drop off too quickly for our requirements and have an infinite tail. A simple linearly declining weighted moving average also does not provide enough weight to the most recent measurements. We propose a piecewise linear distribution of weights, such that the first section (samples 1:F) is flat as in a simple moving average, and the second section (samples F+1:M) is linearly declining weights to the end of the averaging window. We choose integer weights, which allows incremental calculation without introducing rounding errors. 3.4.1. Improving the response of the skewness estimate The weighted moving average for skew_est, based on skew_est in Section~~3.3.3,~~3.1.2,can be calculated as follows: skew_est = ((M-F+1)*sum(skew_base_T(1:F)) + sum([(M-F):1].*skew_base_T(F+1:M))) / ((M-F+1)*sum(numsampT(1:F)) + sum([(M-F):1].*numsampT(F+1:M))) where numsampT is an array of the number of OWD samples in each T~~(ie~~(i.e.num_T(OWD)), and numsampT(1) is the most recent; skew_base_T(1) is the most recent calculation of skew_base_T; 1:F refers to the integer values 1 through to F, and [(M-F):1] refers to an array of the integer values (M-F) declining through to 1; and ".*" is the array scalar dot product operator. 3.4.2. Improving the response of the~~variance~~variabilityestimate The weighted moving average for var_est can be calculated as follows: var_est = ((M-F+1)*sum(PDV(1:F)) + sum([(M-F):1].*PDV(F+1:M))) / (F*(M-F+1) + sum([(M-F):1]) where 1:F refers to the integer values 1 through to F, and [(M-F):1] refers to an array of the integer values (M-F) declining through to 1; and ".*" is the array scalar dot product operator. When removing oscillation noise (see Section~~3.3.1)~~3.3.2)this calculation must be adjusted to allow for invalid PDV records. 4. Measuring OWD This section discusses the OWD measurements required for this algorithm to detect shared bottlenecks. The SBD mechanism described in this draft relies on differences between OWD measurements to avoid the practical problems with measuring absolute OWD (see [Hayes-LCN14] section IIIC). Since all summary statistics are relative to the mean OWD and sender/receiver clock offsets should be approximately constant over the measurement periods, the offset is subtracted out in the calculation. 4.1. Time stamp resolution The SBD mechanism requires timing information precise enough to be able to make comparisons. As a rule of thumb, the time resolution should be less than one hundredth of a typical path's range of delays. In general, the lower the time resolution, the more care that needs to be taken to ensure rounding errors do not bias the skewness calculation. Typical RTP media flows use sub-millisecond timers, which should be adequate in most situations. 5. Acknowledgements This work was part-funded by the European Community under its Seventh Framework Programme through the Reducing Internet Transport Latency (RITE) project (ICT-317700). The views expressed are solely those of the authors. 6. IANA Considerations This memo includes no request to IANA. 7. Security Considerations The security considerations of RFC 3550 [RFC3550], RFC 4585 [RFC4585], and RFC 5124 [RFC5124] are expected to apply. Non-authenticated RTCP packets carrying shared bottleneck indications and summary statistics could allow attackers to alter the bottleneck sharing characteristics for private gain or disruption of other parties communication. 8. Change history Changes made to this document:WG-00->WG-01 : Moved unbiased skew section to replace skew estimate, more robust variability estimator, the term variance replaced with variability, clock drift term corrected to clock skew, revision to clock skew section with a place holder, description of parameters.02->WG-00 : Fixed missing 0.5 in 3.3.2 and missing brace in 3.3.3 01->02 : New section describing improvements to the key metric calculations that help to remove noise, bias, and reduce lag. Some revisions to the notation to make it clearer. Some tightening of the thresholds. 00->01 : Revisions to terminology for clarity 9. References 9.1. Normative References [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997. 9.2. Informative References [Hayes-LCN14] Hayes, D., Ferlin, S., and M. Welzl, "Practical Passive Shared Bottleneck Detection using Shape Summary Statistics", Proc. the IEEE Local Computer Networks (LCN) p150-158, September 2014, <http://heim.ifi.uio.no/ davihay/ hayes14__pract_passiv_shared_bottl_detec-abstract.html>. [I-D.welzl-rmcat-coupled-cc] Welzl, M., Islam, S., and S. Gjessing, "Coupled congestion control for RTP media", draft-welzl-rmcat-coupled-cc-04 (work in progress), October 2014. [ITU-Y1540] ITU-T, "Internet Protocol Data Communication Service - IP Packet Transfer and Availability Performance Parameters", Series Y: Global Information Infrastructure, Internet Protocol Aspects and Next-Generation Networks , March 2011, <http://www.itu.int/rec/T-REC-Y.1540-201103-I/en>. [RFC3550] Schulzrinne, H., Casner, S., Frederick, R., and V. Jacobson, "RTP: A Transport Protocol for Real-Time Applications", STD 64, RFC 3550, July 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. [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. [RFC5481] Morton, A. and B. Claise, "Packet Delay Variation Applicability Statement", RFC 5481, March 2009. [RFC6817] Shalunov, S., Hazel, G., Iyengar, J., and M. Kuehlewind, "Low Extra Delay Background Transport (LEDBAT)", RFC 6817, December 2012. Authors' Addresses David Hayes (editor) University of Oslo PO Box 1080 Blindern Oslo, N-0316 Norway Phone: +47 2284 5566 Email: davihay@ifi.uio.no Simone Ferlin Simula Research Laboratory P.O.Box 134 Lysaker, 1325 Norway Phone: +47 4072 0702 Email: ferlin@simula.no Michael Welzl University of Oslo PO Box 1080 Blindern Oslo, N-0316 Norway Phone: +47 2285 2420 Email: michawe@ifi.uio.no