draft-ietf-rmcat-sbd-04.txt		draft-ietf-rmcat-sbd-05.txt
	RTP Media Congestion Avoidance Techniques D. Hayes, Ed.		RTP Media Congestion Avoidance Techniques D. Hayes, Ed.
	Internet-Draft University of Oslo		Internet-Draft S. Ferlin
	Intended status: Experimental S. Ferlin		Intended status: Experimental Simula Research Laboratory
	Expires: September 22, 2016 Simula Research Laboratory		Expires: March 21, 2017 M. Welzl
	M. Welzl
	K. Hiorth		K. Hiorth
	University of Oslo		University of Oslo
	March 21, 2016		September 17, 2016
	Shared Bottleneck Detection for Coupled Congestion Control for RTP		Shared Bottleneck Detection for Coupled Congestion Control for RTP
	Media.		Media.
	draft-ietf-rmcat-sbd-04		draft-ietf-rmcat-sbd-05
	Abstract		Abstract
	This document describes a mechanism to detect whether end-to-end data		This document describes a mechanism to detect whether end-to-end data
	flows share a common bottleneck. It relies on summary statistics		flows share a common bottleneck. It relies on summary statistics
	that are calculated by a data receiver based on continuous		that are calculated based on continuous measurements and used as
	measurements and regularly fed to a grouping algorithm that runs		input to a grouping algorithm that runs wherever the knowledge is
	wherever the knowledge is needed. This mechanism complements the		needed. This mechanism complements the coupled congestion control
	coupled congestion control mechanism in draft-ietf-rmcat-coupled-cc.		mechanism in draft-ietf-rmcat-coupled-cc.
	Status of This Memo		Status of This Memo
	This Internet-Draft is submitted in full conformance with the		This Internet-Draft is submitted in full conformance with the
	provisions of BCP 78 and BCP 79.		provisions of BCP 78 and BCP 79.
	Internet-Drafts are working documents of the Internet Engineering		Internet-Drafts are working documents of the Internet Engineering
	Task Force (IETF). Note that other groups may also distribute		Task Force (IETF). Note that other groups may also distribute
	working documents as Internet-Drafts. The list of current Internet-		working documents as Internet-Drafts. The list of current Internet-
	Drafts is at http://datatracker.ietf.org/drafts/current/.		Drafts is at http://datatracker.ietf.org/drafts/current/.
	Internet-Drafts are draft documents valid for a maximum of six months		Internet-Drafts are draft documents valid for a maximum of six months
	and may be updated, replaced, or obsoleted by other documents at any		and may be updated, replaced, or obsoleted by other documents at any
	time. It is inappropriate to use Internet-Drafts as reference		time. It is inappropriate to use Internet-Drafts as reference
	material or to cite them other than as "work in progress."		material or to cite them other than as "work in progress."
	This Internet-Draft will expire on September 22, 2016.		This Internet-Draft will expire on March 21, 2017.
	Copyright Notice		Copyright Notice
	Copyright (c) 2016 IETF Trust and the persons identified as the		Copyright (c) 2016 IETF Trust and the persons identified as the
	document authors. All rights reserved.		document authors. All rights reserved.
	This document is subject to BCP 78 and the IETF Trust's Legal		This document is subject to BCP 78 and the IETF Trust's Legal
	Provisions Relating to IETF Documents		Provisions Relating to IETF Documents
	(http://trustee.ietf.org/license-info) in effect on the date of		(http://trustee.ietf.org/license-info) in effect on the date of
	publication of this document. Please review these documents		publication of this document. Please review these documents
	skipping to change at page 2, line 23 ¶		skipping to change at page 2, line 22 ¶
	1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3		1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
	1.1. The signals . . . . . . . . . . . . . . . . . . . . . . . 3		1.1. The signals . . . . . . . . . . . . . . . . . . . . . . . 3
	1.1.1. Packet Loss . . . . . . . . . . . . . . . . . . . . . 3		1.1.1. Packet Loss . . . . . . . . . . . . . . . . . . . . . 3
	1.1.2. Packet Delay . . . . . . . . . . . . . . . . . . . . 3		1.1.2. Packet Delay . . . . . . . . . . . . . . . . . . . . 3
	1.1.3. Path Lag . . . . . . . . . . . . . . . . . . . . . . 4		1.1.3. Path Lag . . . . . . . . . . . . . . . . . . . . . . 4
	2. Definitions . . . . . . . . . . . . . . . . . . . . . . . . . 4		2. Definitions . . . . . . . . . . . . . . . . . . . . . . . . . 4
	2.1. Parameters and their Effect . . . . . . . . . . . . . . . 7		2.1. Parameters and their Effect . . . . . . . . . . . . . . . 7
	2.2. Recommended Parameter Values . . . . . . . . . . . . . . 8		2.2. Recommended Parameter Values . . . . . . . . . . . . . . 8
	3. Mechanism . . . . . . . . . . . . . . . . . . . . . . . . . . 8		3. Mechanism . . . . . . . . . . . . . . . . . . . . . . . . . . 8
	3.1. SBD feedback requirements . . . . . . . . . . . . . . . . 9		3.1. SBD feedback requirements . . . . . . . . . . . . . . . . 9
	3.1.1. Feedback when all the logic is placed at		3.1.1. Feedback when all the logic is placed at the sender . 9
	the sender . . . . . . . . . . . . . . . . . . . . . 10		3.1.2. Feedback when the statistics are calculated at the
	3.1.2. Feedback when the statistics are		receiver and SBD performed at the sender . . . . . . 10
	calculated at the receiver and SBD at		3.1.3. Feedback when bottlenecks can be determined at both
	the sender . . . . . . . . . . . . . . . . . . . . . 10		senders and receivers . . . . . . . . . . . . . . . . 10
	3.1.3. Feedback when bottlenecks can be
	determined at both senders and
	receivers . . . . . . . . . . . . . . . . . . . . . . 11
	3.2. Key metrics and their calculation . . . . . . . . . . . . 11		3.2. Key metrics and their calculation . . . . . . . . . . . . 11
	3.2.1. Mean delay . . . . . . . . . . . . . . . . . . . . . 11		3.2.1. Mean delay . . . . . . . . . . . . . . . . . . . . . 11
	3.2.2. Skewness Estimate . . . . . . . . . . . . . . . . . . 11		3.2.2. Skewness Estimate . . . . . . . . . . . . . . . . . . 11
	3.2.3. Variability Estimate . . . . . . . . . . . . . . . . 12		3.2.3. Variability Estimate . . . . . . . . . . . . . . . . 12
	3.2.4. Oscillation Estimate . . . . . . . . . . . . . . . . 12		3.2.4. Oscillation Estimate . . . . . . . . . . . . . . . . 12
	3.2.5. Packet loss . . . . . . . . . . . . . . . . . . . . . 13		3.2.5. Packet loss . . . . . . . . . . . . . . . . . . . . . 13
	3.3. Flow Grouping . . . . . . . . . . . . . . . . . . . . . . 13		3.3. Flow Grouping . . . . . . . . . . . . . . . . . . . . . . 13
	3.3.1. Flow Grouping Algorithm . . . . . . . . . . . . . . . 13		3.3.1. Flow Grouping Algorithm . . . . . . . . . . . . . . . 13
	3.3.2. Using the flow group signal . . . . . . . . . . . . . 15		3.3.2. Using the flow group signal . . . . . . . . . . . . . 15
	3.4. Removing Noise from the Estimates . . . . . . . . . . . . 15		3.4. Removing Noise from the Estimates . . . . . . . . . . . . 15
	3.4.1. Oscillation noise . . . . . . . . . . . . . . . . . . 15		3.4.1. Oscillation noise . . . . . . . . . . . . . . . . . . 15
	3.4.2. Clock skew . . . . . . . . . . . . . . . . . . . . . 16		3.4.2. Clock skew . . . . . . . . . . . . . . . . . . . . . 15
	3.5. Reducing lag and Improving Responsiveness . . . . 16		3.5. Reducing lag and Improving Responsiveness . . . . . . . . 16
	3.5.1. Improving the response of the skewness estimate . 17		3.5.1. Improving the response of the skewness estimate . . . 16
	3.5.2. Improving the response of the variability estimate 19		3.5.2. Improving the response of the variability estimate . 19
	4. Measuring OWD . . . . . . . . . . . . . . . . . . . . . . . . 19		4. Measuring OWD . . . . . . . . . . . . . . . . . . . . . . . . 19
	4.1. Time stamp resolution . . . . . . . . . . . . . . . . . . 19		4.1. Time stamp resolution . . . . . . . . . . . . . . . . . . 19
	5. Implementation status . . . . . . . . . . . . . . . . . . . . 20		5. Expected feedback from experiments . . . . . . . . . . . . . 20
	6. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 20		6. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 20
	7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 20		7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 20
	8. Security Considerations . . . . . . . . . . . . . . . . . . . 20		8. Security Considerations . . . . . . . . . . . . . . . . . . . 20
	9. Change history . . . . . . . . . . . . . . . . . . . . . . . 20		9. Change history . . . . . . . . . . . . . . . . . . . . . . . 20
	10. References . . . . . . . . . . . . . . . . . . . . . . . . . 21		10. References . . . . . . . . . . . . . . . . . . . . . . . . . 21
	10.1. Normative References . . . . . . . . . . . . . . . . . . 21		10.1. Normative References . . . . . . . . . . . . . . . . . . 21
	10.2. Informative References . . . . . . . . . . . . . . . . . 21		10.2. Informative References . . . . . . . . . . . . . . . . . 21
	Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 22		Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 22
	1. Introduction		1. Introduction
	In the Internet, it is not normally known if flows (e.g., TCP		In the Internet, it is not normally known if flows (e.g., TCP
	connections or UDP data streams) traverse the same bottlenecks. Even		connections or UDP data streams) traverse the same bottlenecks. Even
	flows that have the same sender and receiver may take different paths		flows that have the same sender and receiver may take different paths
	and share a bottleneck or not. Flows that share a bottleneck link		and may or may not share a bottleneck. Flows that share a bottleneck
	usually compete with one another for their share of the capacity.		link usually compete with one another for their share of the
	This competition has the potential to increase packet loss and		capacity. This competition has the potential to increase packet loss
	delays. This is especially relevant for interactive applications		and delays. This is especially relevant for interactive applications
	that communicate simultaneously with multiple peers (such as multi-		that communicate simultaneously with multiple peers (such as multi-
	party video). For RTP media applications such as RTCWEB,		party video). For RTP media applications such as RTCWEB,
	[I-D.ietf-rmcat-coupled-cc] describes a scheme that combines the		[I-D.ietf-rmcat-coupled-cc] describes a scheme that combines the
	congestion controllers of flows in order to honor their priorities		congestion controllers of flows in order to honor their priorities
	and avoid unnecessary packet loss as well as delay. This mechanism		and avoid unnecessary packet loss as well as delay. This mechanism
	relies on some form of Shared Bottleneck Detection (SBD); here, a		relies on some form of Shared Bottleneck Detection (SBD); here, a
	measurement-based SBD approach is described.		measurement-based SBD approach is described.
	1.1. The signals		1.1. The signals
	The current Internet is unable to explicitly inform endpoints as to		The current Internet is unable to explicitly inform endpoints as to
	which flows share bottlenecks, so endpoints need to infer this from		which flows share bottlenecks, so endpoints need to infer this from
	whatever information is available to them. The mechanism described		whatever information is available to them. The mechanism described
	here currently utilises packet loss and packet delay, but is not		here currently utilizes packet loss and packet delay, but is not
	restricted to these.		restricted to these. As ECN becomes more prevalent it too will
			become a valuable base signal.
	1.1.1. Packet Loss		1.1.1. Packet Loss
	Packet loss is often a relatively rare signal. Therefore, on its own		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		it is of limited use for SBD, however, it is a valuable supplementary
	measure when it is more prevalent.		measure when it is more prevalent.
	1.1.2. Packet Delay		1.1.2. Packet Delay
	End-to-end delay measurements include noise from every device along		End-to-end delay measurements include noise from every device along
	the path in addition to the delay perturbation at the bottleneck		the path in addition to the delay perturbation at the bottleneck
	device. The noise is often significantly increased if the round-trip		device. The noise is often significantly increased if the round-trip
	time is used. The cleanest signal is obtained by using One-Way-Delay		time is used. The cleanest signal is obtained by using One-Way-Delay
	(OWD).		(OWD).
	Measuring absolute OWD is difficult since it requires both the sender		Measuring absolute OWD is difficult since it requires both the sender
	and receiver clocks to be synchronised. However, since the		and receiver clocks to be synchronized. However, since the
	statistics being collected are relative to the mean OWD, a relative		statistics being collected are relative to the mean OWD, a relative
	OWD measurement is sufficient. Clock skew is not usually significant		OWD measurement is sufficient. Clock skew is not usually significant
	over the time intervals used by this SBD mechanism (see [RFC6817] A.2		over the time intervals used by this SBD mechanism (see [RFC6817] A.2
	for a discussion on clock skew and OWD measurements). However, in		for a discussion on clock skew and OWD measurements). However, in
	circumstances where it is significant, Section 3.4.2 outlines a way		circumstances where it is significant, Section 3.4.2 outlines a way
	of adjusting the calculations to cater for it.		of adjusting the calculations to cater for it.
	Each packet arriving at the bottleneck buffer may experience very		Each packet arriving at the bottleneck buffer may experience very
	different queue lengths, and therefore different waiting times. A		different queue lengths, and therefore different waiting times. A
	single OWD sample does not, therefore, characterize the path well.		single OWD sample does not, therefore, characterize the path well.
	skipping to change at page 7, line 32 ¶		skipping to change at page 7, line 32 ¶
	increase the responsiveness of the SBD mechanism. If F is		increase the responsiveness of the SBD mechanism. If F is
	too small, statistics will be too noisy.		too small, statistics will be too noisy.
	c_s c_s is the threshold in skew_est used for determining whether		c_s c_s is the threshold in skew_est used for determining whether
	a flow is transiting a bottleneck or not. It should be		a flow is transiting a bottleneck or not. It should be
	slightly negative so that a very lightly loaded path does not		slightly negative so that a very lightly loaded path does not
	give a false indication. Setting c_s more negative makes the		give a false indication. Setting c_s more negative makes the
	SBD mechanism less sensitive to transient and slight		SBD mechanism less sensitive to transient and slight
	bottlenecks.		bottlenecks.
	c_h c_h adds hysteresis to the botteneck determination. It		c_h c_h adds hysteresis to the bottleneck determination. It
	should be large enough to avoid constant switching in the		should be large enough to avoid constant switching in the
	determination, but low enough to ensure that grouping is not		determination, but low enough to ensure that grouping is not
	attempted when there is no bottleneck and the delay and loss		attempted when there is no bottleneck and the delay and loss
	signals cannot be relied upon.		signals cannot be relied upon.
	p_v p_v determines the sensitivity of freq_est to noise. Making		p_v p_v determines the sensitivity of freq_est to noise. Making
	it smaller will yield higher but noisier values for freq_est.		it smaller will yield higher but noisier values for freq_est.
	Making it too large will render it ineffective for		Making it too large will render it ineffective for
	determining groups.		determining groups.
	skipping to change at page 8, line 34 ¶		skipping to change at page 8, line 34 ¶
	oscillation (estimate freq_est, see Section 3.2.4)		oscillation (estimate freq_est, see Section 3.2.4)
	with packet loss (estimate pkt_loss, see Section 3.2.5) used as a		with packet loss (estimate pkt_loss, see Section 3.2.5) used as a
	supplementary statistic.		supplementary statistic.
	Summary statistics help to address both the noise and the path lag		Summary statistics help to address both the noise and the path lag
	problems by describing the general shape over a relatively long		problems by describing the general shape over a relatively long
	period of time. Each summary statistic portrays a "view" of the		period of time. Each summary statistic portrays a "view" of the
	bottleneck link characteristics, and when used together, they provide		bottleneck link characteristics, and when used together, they provide
	a robust discrimination for grouping flows. They can be signalled		a robust discrimination for grouping flows. An RTP Media device may
	from a receiver, which measures the OWD and calculates the summary		be both a sender and a receiver and SBD can be performed at either a
	statistics, to a sender, which is the entity that is transmitting the		sender or a receiver or both.
	media stream. An RTP Media device may be both a sender and a
	receiver. SBD can be performed at either a sender or a receiver or
	both.
	+----+		+----+
	| H2 |		| H2 |
	+----+		+----+
	|		|
	| L2		| L2
	|		|
	+----+ L1 | L3 +----+		+----+ L1 | L3 +----+
	| H1 |------|------| H3 |		| H1 |------|------| H3 |
	+----+ +----+		+----+ +----+
	skipping to change at page 10, line 12 ¶		skipping to change at page 9, line 43 ¶
	both senders and receivers (beyond the current scope, but allows		both senders and receivers (beyond the current scope, but allows
	cooperative detection of bottlenecks).		cooperative detection of bottlenecks).
	3.1.1. Feedback when all the logic is placed at the sender		3.1.1. Feedback when all the logic is placed at the sender
	Having the sender calculate the summary statistics and determine the		Having the sender calculate the summary statistics and determine the
	shared bottlenecks based on them has the advantage of placing most of		shared bottlenecks based on them has the advantage of placing most of
	the functionality in one place -- the sender.		the functionality in one place -- the sender.
	The sender requires precise accurate OWD measurements for every		The sender requires precise accurate OWD measurements for every
	packet, along with the proportion of packets lost over the interval		packet, along with an indication of lost packets (or the proportion
	T, to be sent from the receivers to the senders every T.		of packets lost over the interval T). The mechanism performs its
			calculations every T and requires measurements to be available for
			this.
	An initialisation message may be required to agree on the feedback		An initialization message may be required to agree on the feedback
	interval.		interval.
	3.1.2. Feedback when the statistics are calculated at the receiver and		3.1.2. Feedback when the statistics are calculated at the receiver and
	SBD at the sender		SBD performed at the sender
	This scenario minimises feedback, but requires receivers to send		This scenario minimizes feedback, but requires receivers to send
	selected summary statistics at an agreed regular interval. We		selected summary statistics at an agreed regular interval. We
	envisage the following exchange of information to initialise the		envisage the following exchange of information to initialize the
	system:		system:
	o An initialization message from the sender to the receiver will		o An initialization message from the sender to the receiver will
	contain the following information:		contain the following information:
	* A protocol identifier (SBD=01). This is to future proof the		* A protocol identifier (SBD=01). This is to future proof the
	message exchange so that potential advances in SBD technology		message exchange so that potential advances in SBD technology
	can be easily deployed. All following initialisation elements		can be easily deployed. All following initialization elements
	relate to the mechanism outlined in this document which will		relate to the mechanism outlined in this document which will
	have the identifier SBD=01.		have the identifier SBD=01.
	* A list of which key metrics should be collected and relayed		* A list of which key metrics should be collected and relayed
	back to the sender out of a possibly extensible set (pkt_loss,		back to the sender out of a possibly extensible set (pkt_loss,
	var_est, skew_est, freq_est). The grouping algorithm described		var_est, skew_est, freq_est). The grouping algorithm described
	in this document requires all four of these metrics, and		in this document requires all four of these metrics, and
	receivers MUST be able to provide them, but future algorithms		receivers MUST be able to provide them, but future algorithms
	may be able to exploit other metrics (e.g. metrics based on		may be able to exploit other metrics (e.g. metrics based on
	explicit network signals).		explicit network signals).
	* The values of T, N, M, and the necessary resolution and		* The values of T, N, M, and the necessary resolution and
	precision of the relayed statistics.		precision of the relayed statistics.
	o A response message from the receiver acknowledges this message		o A response message from the receiver acknowledges this message
	with a list of key metrics it supports (subset of the senders		with a list of key metrics it supports (subset of the senders
	list) and is able to relay back to the sender.		list) and is able to relay back to the sender.
	This initialisation exchange may be repeated to finalize the agreed		This initialization exchange may be repeated to finalize the agreed
	metrics should not all be supported by all receivers.		metrics should not all be supported by all receivers.
	After initialisation the agreed summary statistics will be fed back		After initialization the agreed summary statistics will be fed back
	to the sender every T.		to the sender every T.
	3.1.3. Feedback when bottlenecks can be determined at both senders and		3.1.3. Feedback when bottlenecks can be determined at both senders and
	receivers		receivers
	This type of mechanism is currently beyond the scope of SBD in RMCAT.		This type of mechanism is currently beyond the scope of SBD in RMCAT.
	It is mentioned here to ensure more advanced sender/receiver		It is mentioned here to ensure more advanced sender/receiver
	cooperative shared bottleneck determination mechanisms remain		cooperative shared bottleneck determination mechanisms remain
	possible in the future.		possible in the future.
	It is envisaged that such a mechanism would be initialised in a		It is envisaged that such a mechanism would be initialized in a
	similar manner to that described in Section 3.1.2.		similar manner to that described in Section 3.1.2.
	After initialisation both summary statistics and shared bottleneck		After initialization both summary statistics and shared bottleneck
	determinations will need to be exchanged every T.		determinations should be exchanged every T.
	3.2. Key metrics and their calculation		3.2. Key metrics and their calculation
	Measurements are calculated over a base interval, T and summarized		Measurements are calculated over a base interval, T and summarized
	over N or M such intervals. All summary statistics can be calculated		over N or M such intervals. All summary statistics can be calculated
	incrementally.		incrementally.
	3.2.1. Mean delay		3.2.1. Mean delay
	The mean delay is not a useful signal for comparisons between flows		The mean delay is not a useful signal for comparisons between flows
	skipping to change at page 12, line 8 ¶		skipping to change at page 11, line 43 ¶
	3.2.2. Skewness Estimate		3.2.2. Skewness Estimate
	Skewness is difficult to calculate efficiently and accurately.		Skewness is difficult to calculate efficiently and accurately.
	Ideally it should be calculated over the entire period (M * T) from		Ideally it should be calculated over the entire period (M * T) from
	the mean OWD over that period. However this would require storing		the mean OWD over that period. However this would require storing
	every delay measurement over the period. Instead, an estimate is		every delay measurement over the period. Instead, an estimate is
	made over M * T based on a calculation every T using the previous T's		made over M * T based on a calculation every T using the previous T's
	calculation of mean_delay.		calculation of mean_delay.
	The base for the skewness calculation is estimated using a counter		The base for the skewness calculation is estimated using a counter
	initialised every T. It increments for one way delay samples (OWD)		initialized every T. It increments for one way delay samples (OWD)
	below the mean and decrements for OWD above the mean. So for each		below the mean and decrements for OWD above the mean. So for each
	OWD sample:		OWD sample:
	if (OWD < mean_delay) skew_base_T++		if (OWD < mean_delay) skew_base_T++
	if (OWD > mean_delay) skew_base_T--		if (OWD > mean_delay) skew_base_T--
	The mean_delay does not include the mean of the current T interval to		The mean_delay does not include the mean of the current T interval to
	enable it to be calculated iteratively.		enable it to be calculated iteratively.
	skipping to change at page 12, line 50 ¶		skipping to change at page 12, line 36 ¶
	var_est = MAD_MT = sum_MT(var_base_T)/num_MT(OWD)		var_est = MAD_MT = sum_MT(var_base_T)/num_MT(OWD)
	For calculation of freq_est p_v=0.7		For calculation of freq_est p_v=0.7
	For the grouping threshold p_mad=0.1		For the grouping threshold p_mad=0.1
	3.2.4. Oscillation Estimate		3.2.4. Oscillation Estimate
	An estimate of the low frequency oscillation of the delay signal is		An estimate of the low frequency oscillation of the delay signal is
	calculated by counting and normalising the significant mean,		calculated by counting and normalizing the significant mean,
	E_T(OWD), crossings of mean_delay:		E_T(OWD), crossings of mean_delay:
	freq_est = number_of_crossings / N		freq_est = number_of_crossings / N
	where we define a significant mean crossing as a crossing that		where we define a significant mean crossing as a crossing that
	extends p_v * var_est from mean_delay. In our experiments we		extends p_v * var_est from mean_delay. In our experiments we
	have found that p_v = 0.7 is a good value.		have found that p_v = 0.7 is a good value.
	Freq_est is a number between 0 and 1. Freq_est can be approximated		Freq_est is a number between 0 and 1. Freq_est can be approximated
	incrementally as follows:		incrementally as follows:
	skipping to change at page 15, line 33 ¶		skipping to change at page 15, line 16 ¶
	Grouping decisions can be made every T from the second T, however		Grouping decisions can be made every T from the second T, however
	they will not attain their full design accuracy until after the		they will not attain their full design accuracy until after the
	2*N'th T interval. We recommend that grouping decisions are not made		2*N'th T interval. We recommend that grouping decisions are not made
	until 2*M T intervals.		until 2*M T intervals.
	Network conditions, and even the congestion controllers, can cause		Network conditions, and even the congestion controllers, can cause
	bottlenecks to fluctuate. A coupled congestion controller MAY decide		bottlenecks to fluctuate. A coupled congestion controller MAY decide
	only to couple groups that remain stable, say grouped together 90% of		only to couple groups that remain stable, say grouped together 90% of
	the time, depending on its objectives. Recommendations concerning		the time, depending on its objectives. Recommendations concerning
	this are beyond the scope of this draft and will be specific to the		this are beyond the scope of this document and will be specific to
	coupled congestion controllers objectives.		the coupled congestion controllers objectives.
	3.4. Removing Noise from the Estimates		3.4. Removing Noise from the Estimates
	The following describe small changes to the calculation of the key		The following describe small changes to the calculation of the key
	metrics that help remove noise from them. Currently these "tweaks"		metrics that help remove noise from them. These "tweaks" are
	are described separately to keep the main description succinct. In		described separately to keep the main description succinct.
	future revisions of the draft these enhancements may replace the
	original key metric calculations.
	3.4.1. Oscillation noise		3.4.1. Oscillation noise
	When a path has no bottleneck, var_est will be very small and the		When a path has no bottleneck, var_est will be very small and the
	recorded significant mean crossings will be the result of path noise.		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		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		the point a link becomes a bottleneck and flows traversing it begin
	to be grouped.		to be grouped.
	To remove this source of noise from freq_est:		To remove this source of noise from freq_est:
	skipping to change at page 18, line 14 ¶		skipping to change at page 18, line 14 ¶
	To calculate this weighted skew_est incrementally:		To calculate this weighted skew_est incrementally:
	Notation: F_ - flat portion, D_ - declining portion, W_ - weighted		Notation: F_ - flat portion, D_ - declining portion, W_ - weighted
	component		component
	Initialise: sum_skewbase = 0, F_skewbase=0, W_D_skewbase=0		Initialise: sum_skewbase = 0, F_skewbase=0, W_D_skewbase=0
	skewbase_hist = buffer length M initialize to 0		skewbase_hist = buffer length M initialize to 0
	numsampT = buffer length M initialzed to 0		numsampT = buffer length M initialized to 0
	Steps per iteration:		Steps per iteration:
	1. old_skewbase = skewbase_hist(M)		1. old_skewbase = skewbase_hist(M)
	2. old_numsampT = numsampT(M)		2. old_numsampT = numsampT(M)
	3. cycle(skewbase_hist)		3. cycle(skewbase_hist)
	4. cycle(numsampT)		4. cycle(numsampT)
	skipping to change at page 19, line 36 ¶		skipping to change at page 19, line 36 ¶
	Var_est can be calculated incrementally in the same way as skew_est		Var_est can be calculated incrementally in the same way as skew_est
	in Section 3.5.1. However, note that the buffer numsampT is used for		in Section 3.5.1. However, note that the buffer numsampT is used for
	both calculations so the operations on it should not be repeated.		both calculations so the operations on it should not be repeated.
	4. Measuring OWD		4. Measuring OWD
	This section discusses the OWD measurements required for this		This section discusses the OWD measurements required for this
	algorithm to detect shared bottlenecks.		algorithm to detect shared bottlenecks.
	The SBD mechanism described in this draft relies on differences		The SBD mechanism described in this document relies on differences
	between OWD measurements to avoid the practical problems with		between OWD measurements to avoid the practical problems with
	measuring absolute OWD (see [Hayes-LCN14] section IIIC). Since all		measuring absolute OWD (see [Hayes-LCN14] section IIIC). Since all
	summary statistics are relative to the mean OWD and sender/receiver		summary statistics are relative to the mean OWD and sender/receiver
	clock offsets should be approximately constant over the measurement		clock offsets should be approximately constant over the measurement
	periods, the offset is subtracted out in the calculation.		periods, the offset is subtracted out in the calculation.
	4.1. Time stamp resolution		4.1. Time stamp resolution
	The SBD mechanism requires timing information precise enough to be		The SBD mechanism requires timing information precise enough to be
	able to make comparisons. As a rule of thumb, the time resolution		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		should be less than one hundredth of a typical path's range of
	delays. In general, the lower the time resolution, the more care		delays. In general, the lower the time resolution, the more care
	that needs to be taken to ensure rounding errors do not bias the		that needs to be taken to ensure rounding errors do not bias the
	skewness calculation.		skewness calculation.
	Typical RTP media flows use sub-millisecond timers, which should be		Typical RTP media flows use sub-millisecond timers, which should be
	adequate in most situations.		adequate in most situations.
	5. Implementation status		5. Expected feedback from experiments
	The University of Oslo is currently working on an implementation of		The algorithm described in this memo has so far been evaluated using
	this in the Chromium browser.		simulations. Real network tests using the proposed congestion
			control algorithms will help confirm the default parameter choice.
			For example, the time interval T may need to be made longer if the
			packet rate is very low. Implementers and testers are invited to
			document their findings in an Internet draft.
	6. Acknowledgements		6. Acknowledgments
	This work was part-funded by the European Community under its Seventh		This work was part-funded by the European Community under its Seventh
	Framework Programme through the Reducing Internet Transport Latency		Framework Programme through the Reducing Internet Transport Latency
	(RITE) project (ICT-317700). The views expressed are solely those of		(RITE) project (ICT-317700). The views expressed are solely those of
	the authors.		the authors.
	7. IANA Considerations		7. IANA Considerations
	This memo includes no request to IANA.		This memo includes no request to IANA.
	skipping to change at page 20, line 38 ¶		skipping to change at page 20, line 42 ¶
	Non-authenticated RTCP packets carrying shared bottleneck indications		Non-authenticated RTCP packets carrying shared bottleneck indications
	and summary statistics could allow attackers to alter the bottleneck		and summary statistics could allow attackers to alter the bottleneck
	sharing characteristics for private gain or disruption of other		sharing characteristics for private gain or disruption of other
	parties communication.		parties communication.
	9. Change history		9. Change history
	Changes made to this document:		Changes made to this document:
			WG-04->WG-05 : Fix ToC formatting. Add section on expected
			feedback from experiments replacing short section
			on implementation status. Added comment on ECN as
			a signal. Clarification of lost packet signaling.
			Change term "draft" to "document" where
			appropriate. American spelling. Some tightening
			of the text.
	WG-03->WG-04 : Add M to terminology table, suggest skew_est based		WG-03->WG-04 : Add M to terminology table, suggest skew_est based
	on previous T and no freq_est in clock skew		on previous T and no freq_est in clock skew
	section, feedback requirements as a separate sub		section, feedback requirements as a separate sub
	section.		section.
	WG-02->WG-03 : Correct misspelled author		WG-02->WG-03 : Correct misspelled author
	WG-01->WG-02 : Removed ambiguity associated with the term		WG-01->WG-02 : Removed ambiguity associated with the term
	"congestion". Expanded the description of		"congestion". Expanded the description of
	initialisation messages. Removed PDV metric.		initialisation messages. Removed PDV metric.
	skipping to change at page 22, line 29 ¶		skipping to change at page 23, line 4 ¶
	<http://www.rfc-editor.org/info/rfc6817>.		<http://www.rfc-editor.org/info/rfc6817>.
	[Zhang-Infocom02]		[Zhang-Infocom02]
	Zhang, L., Liu, Z., and H. Xia, "Clock synchronization		Zhang, L., Liu, Z., and H. Xia, "Clock synchronization
	algorithms for network measurements", Proc. the IEEE		algorithms for network measurements", Proc. the IEEE
	International Conference on Computer Communications		International Conference on Computer Communications
	(INFOCOM) pp160-169, September 2002,		(INFOCOM) pp160-169, September 2002,
	<http://dx.doi.org/10.1109/INFCOM.2002.1019257>.		<http://dx.doi.org/10.1109/INFCOM.2002.1019257>.
	Authors' Addresses		Authors' Addresses
	David Hayes (editor)		David Hayes (editor)
	University of Oslo		Simula Research Laboratory
	PO Box 1080 Blindern		P.O. Box 134
	Oslo N-0316		Lysaker 1325
	Norway		Norway
	Phone: +47 2284 5566		Phone: +47 2284 5566
	Email: davihay@ifi.uio.no		Email: davidh@simula.no
	Simone Ferlin		Simone Ferlin
	Simula Research Laboratory		Simula Research Laboratory
	P.O.Box 134		P.O.Box 134
	Lysaker 1325		Lysaker 1325
	Norway		Norway
	Phone: +47 4072 0702		Phone: +47 4072 0702
	Email: ferlin@simula.no		Email: ferlin@simula.no
	Michael Welzl		Michael Welzl
	University of Oslo		University of Oslo
	PO Box 1080 Blindern		PO Box 1080 Blindern
	Oslo N-0316		Oslo N-0316
	Norway		Norway
	Phone: +47 2285 2420		Phone: +47 2285 2420
	Email: michawe@ifi.uio.no		Email: michawe@ifi.uio.no
	Kristian Hiorth		Kristian Hiorth
End of changes. 38 change blocks.
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