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
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Internet-Drafts are draft documents valid for a maximum of six months Internet-Drafts are draft documents valid for a maximum of six months
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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.
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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 |
+----+ +----+ +----+ +----+
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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
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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.
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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:
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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:
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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
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