RMCAT Working Group V. Singh
Internet-Draft J. Ott
Intended status: Informational Aalto University
Expires: April 18, 2013 October 15, 2012
Evaluating Congestion Control for Interactive Real-time Media.
draft-singh-rmcat-cc-eval-00.txt
Abstract
The Real-time Transport Protocol (RTP) is used to transmit media in
telephony and video conferencing applications. This document
describes the guidelines to evaluate new congestion control
algorithms for interactive point-to-point real-time media.
Status of this Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
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This Internet-Draft will expire on April 18, 2013.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Metrics . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
4. Guidelines . . . . . . . . . . . . . . . . . . . . . . . . . . 4
4.1. Avoiding Congestion Collapse . . . . . . . . . . . . . . . 4
4.2. Stability . . . . . . . . . . . . . . . . . . . . . . . . . 5
4.3. Diverse Environments . . . . . . . . . . . . . . . . . . . 5
4.4. Varying Path Characteristics . . . . . . . . . . . . . . . 5
4.5. Reacting to Transient Events or Interruptions . . . . . . . 5
4.6. Fairness With Similar Cross-Traffic . . . . . . . . . . . . 5
4.7. Impact on Cross-Traffic . . . . . . . . . . . . . . . . . . 6
4.8. Extensions to RTP/RTCP . . . . . . . . . . . . . . . . . . 6
5. Minimum Requirements for Evaluation . . . . . . . . . . . . . . 6
6. Example Evaluation Scenarios . . . . . . . . . . . . . . . . . 6
7. Status of Proposals . . . . . . . . . . . . . . . . . . . . . . 7
8. Security Considerations . . . . . . . . . . . . . . . . . . . . 7
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 7
10. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 7
11. References . . . . . . . . . . . . . . . . . . . . . . . . . . 8
11.1. Normative References . . . . . . . . . . . . . . . . . . . 8
11.2. Informative References . . . . . . . . . . . . . . . . . . 8
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 9
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1. Introduction
This memo describes the guidelines to help with evaluating new
congestion control algorithms for interactive point-to-point real
time media. The requirements for the congestion control algorithm
are outlined in [I-D.jesup-rtp-congestion-reqs]). This document
builds upon previous work at the IETF: Specifying New Congestion
Control Algorithms [RFC5033] and Metrics for the Evaluation of
Congestion Control Algorithms [RFC5166].
The guidelines proposed in the document are intended to prevent a
congestion collapse, promote fair capacity usage and optimize the
media flow's throughput, delay, loss and quality. Furthermore, the
proposed algorithms are expected to operate within the envelope of
the circuit breakers defined in
[I-D.ietf-avtcore-rtp-circuit-breakers].
This document only provides broad-level criteria for evaluating a new
congestion control algorithm and the working group should expect a
thorough scientific study to make its decision. The results of the
evaluation are not expected to be included within the internet-draft
but should be cited in the document.
2. Terminology
The terminology defined in RTP [RFC3550], RTP Profile for Audio and
Video Conferences with Minimal Control [RFC3551], RTCP Extended
Report (XR) [RFC3611], Extended RTP Profile for RTCP-based Feedback
(RTP/AVPF) [RFC4585] and Support for Reduced-Size RTCP [RFC5506]
apply.
3. Metrics
[RFC5166] describes the basic metrics for congestion control.
Metrics that are important to interactive multimedia are:
* Delay
* Throughput
* Minimizing oscillations in encoding rate (stability)
* Reactivity to transient events
* Packet loss and discard rate
* Users' quality of experience
[Editor's Note: measurement interval and statistical measures
(min, max, mean, median) are yet to be specified.]
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Section 2.1 of [RFC5166] discusses the tradeoff between throughput,
delay and loss.
(i) Bandwidth Utilization: is the ratio of the encoding rate to
the (available) end-to-end path capacity.
* Under-utilization: is the period of time when the endpoint's
encoding rate is lower than the end-to-end capacity, i.e., the
bandwidth utilization is less than 1.
* Overuse: is the period of time when the endpoint's encoding
rate is higher than the end-to-end capacity, i.e., the
bandwidth utilization is greater than 1.
* Stability: is the period of time when the endpoint's encoding
rate is relatively stable, i.e., the bandwidth utilization is
constant.
(ii) Packet Loss and Discard Rate.
(iii) Fair Share.
[Editor's Note: This metric should match the one defined in the
RMCAT requirements [I-D.jesup-rtp-congestion-reqs] document.]
(iv) Quality: There are many different types of quality metrics
for audio and video. Audio quality is often expressed by a MOS
("Mean Opinion Score") and can be calculated using an objective
algorithm [I-D.ietf-xrblock-rtcp-xr-qoe]. Section 4.7 of
[RFC3611] can also be used for VoIP metrics. Similarly, there
exist several metrics to measure video quality, for example Peak
Signal to Noise Ratio (PSNR).
[Editor's Note: Should the algorithm compare average PSNR of test
video sequences or what other video quality metric can be used?]
4. Guidelines
A congestion control algorithm should be tested in simulation or a
testbed environment, and the experiments should be repeated multiple
times to infer statistical significance. The following guidelines
are considered for evaluation:
4.1. Avoiding Congestion Collapse
Does the congestion control propose any changes to (or diverge from)
the circuit breaker conditions defined in
[I-D.ietf-avtcore-rtp-circuit-breakers].
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4.2. Stability
The congestion control should be assessed for its stability when the
path characteristics do not change over time. Changing the media
encoding rate too often or by too much may adversely affect the
users' quality of experience.
4.3. Diverse Environments
The congestion control algorithm should be assessed in heterogeneous
environments, containing both wired and wireless paths. Examples of
wireless access technologies are: 802.11x, HSPA, WCDMA, or GPRS. One
of the main challenges of the wireless environments is the inability
to distinguish congestion induced loss from transmission (bit-error)
loss. Congestion control algorithms may incorrectly identify
transmission loss as congestion loss and reduce the media encoding
rate too much, which may cause oscillatory behavior and deteriorate
the users' quality of experience. Furthermore, packet loss may
induce additional delay in networks with wireless paths due to link-
layer retransmissions.
4.4. Varying Path Characteristics
The congestion control algorithm should be evaluated for a range of
path characteristics such as, different end-to-end capacity and
latency, varying amount of cross traffic on a bottle-neck link and a
router's queue length. [Editor's Note: Different types of queueing
mechanisms? Random Early Detection or only DropTail?]. The main
motivation for the previous and current criteria is to determine
under which circumstances will the proposed congestion control
algorithm break down and also determine the operational range of the
algorithm.
4.5. Reacting to Transient Events or Interruptions
The congestion control algorithm should be able to handle changes in
end-to-end capacity and latency. Latency may change due to route
updates, link failures, handovers etc. In mobile environment the
end-to-end capacity may vary due to the interference, fading,
handovers, etc. In wired networks the end-to-end capacity may vary
due to changes in resource reservation.
4.6. Fairness With Similar Cross-Traffic
The congestion control algorithm should be evaluated when competing
with other RTP flows using the same congestion control algorithm.
The proposal should highlight the bottleneck capacity share of each
RTP flow.
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4.7. Impact on Cross-Traffic
The congestion control algorithm should be evaluated when competing
with standard TCP. Short TCP flows may be considered as transient
events and the RTP flow may give way to the short TCP flow to
complete quickly. However, long-lived TCP flows may starve out the
RTP flow depending on router queue length. In the latter case the
proposed congestion control for RTP should be as aggressive as
standard TCP [RFC5681].
4.8. Extensions to RTP/RTCP
The congestion control algorithm should indicate if any protocol
extensions are required to implement it and should carefully describe
the impact of the extension.
5. Minimum Requirements for Evaluation
[Editor's Note: If needed, a minimum evaluation criteria can be based
on the above guidelines]
6. Example Evaluation Scenarios
In the scenarios listed below, all RTP flows are bi-directional and
point-to-point.
[S1] RTP flow on a fixed link: This scenario evaluates the ramp-up
to the bottleneck capacity and the stability of the proposed
congestion control algorithm.
[S2] RTP flow on a variable capacity link: This scenario evaluates
the reactivity of the proposed congestion control algorithm to
transient network events due to interference and handovers in
mobile environments. Sample 3G bandwidth traces are available at
[3GPP.R1.081955].
[S3] Fairness to RTP flows running the same congestion control
algorithm: This scenario shows if the proposed algorithm can share
the bottleneck link equitably, irrespective of number of flows.
[S4] Competing with long-lived TCP flows: In this scenario the
proposed algorithm is expected to be TCP-friendly, i.e., it should
neither starve out the competing TCP flows (causing a congestion
collapse) nor should it be starved out by TCP.
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[S5] Competing with short TCP flows: Depending on the level of
statistical multiplexing on the bottleneck link, the proposed
algorithm may behave differently. If there are a few short TCP
flows then the proposed algorithm may observe these flows as
transient events and let them complete quickly. Alternatively, if
there are many short flows then the proposed algorithm may have to
compete with the flows as if they were long lived TCP flows.
[Editor's Note: many and few short TCP flows may depend on the
bottleneck link capacity.]
7. Status of Proposals
Congestion control algorithms are expected to be published as
"Experimental" documents until they are shown to be safe to deploy.
An algorithm published as a draft should be experimented in
simulation, or a controlled environment (testbed) to show its
applicability. Every congestion control algorithm should include a
note describing the environments in which the algorithm is tested and
safe to deploy. It is possible that an algorithm is not recommended
for certain environments or perform sub-optimally for the user.
[Editor's Note: Should there be a distinction between "Informational"
and "Experimental" drafts for congestion control algorithms in RMCAT.
[RFC5033] describes Informational proposals as algorithms that are
not safe for deployment but are proposals to experiment with in
simulation/testbeds. While Experimental algorithms are ones that are
deemed safe in some environments but require a more thorough
evaluation (from the community).]
8. Security Considerations
Security issues have not been discussed in this memo.
9. IANA Considerations
There are no IANA impacts in this memo.
10. Acknowledgements
Much of this document is derived from previous work on congestion
control at the IETF.
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11. References
11.1. Normative References
[RFC3550] Schulzrinne, H., Casner, S., Frederick, R., and V.
Jacobson, "RTP: A Transport Protocol for Real-Time
Applications", STD 64, RFC 3550, July 2003.
[RFC3551] Schulzrinne, H. and S. Casner, "RTP Profile for Audio and
Video Conferences with Minimal Control", STD 65, RFC 3551,
July 2003.
[RFC3611] Friedman, T., Caceres, R., and A. Clark, "RTP Control
Protocol Extended Reports (RTCP XR)", RFC 3611,
November 2003.
[RFC4585] Ott, J., Wenger, S., Sato, N., Burmeister, C., and J. Rey,
"Extended RTP Profile for Real-time Transport Control
Protocol (RTCP)-Based Feedback (RTP/AVPF)", RFC 4585,
July 2006.
[RFC5506] Johansson, I. and M. Westerlund, "Support for Reduced-Size
Real-Time Transport Control Protocol (RTCP): Opportunities
and Consequences", RFC 5506, April 2009.
[I-D.jesup-rtp-congestion-reqs]
Jesup, R. and H. Alvestrand, "Congestion Control
Requirements For Real Time Media",
draft-jesup-rtp-congestion-reqs-00 (work in progress),
March 2012.
[I-D.ietf-avtcore-rtp-circuit-breakers]
Perkins, C. and V. Singh, "RTP Congestion Control: Circuit
Breakers for Unicast Sessions",
draft-ietf-avtcore-rtp-circuit-breakers-00 (work in
progress), October 2012.
11.2. Informative References
[RFC5033] Floyd, S. and M. Allman, "Specifying New Congestion
Control Algorithms", BCP 133, RFC 5033, August 2007.
[RFC5166] Floyd, S., "Metrics for the Evaluation of Congestion
Control Mechanisms", RFC 5166, March 2008.
[RFC5681] Allman, M., Paxson, V., and E. Blanton, "TCP Congestion
Control", RFC 5681, September 2009.
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[I-D.ietf-xrblock-rtcp-xr-qoe]
Hunt, G., Clark, A., Wu, W., Schott, R., and G. Zorn,
"RTCP XR Blocks for QoE Metric Reporting",
draft-ietf-xrblock-rtcp-xr-qoe-02 (work in progress),
July 2012.
[3GPP.R1.081955]
R1-081955, 3GPP., "LTE Link Level Throughput Data for SA4
Evaluation Framework", 3GPP R1-081955, 5 2008.
Authors' Addresses
Varun Singh
Aalto University
School of Electrical Engineering
Otakaari 5 A
Espoo, FIN 02150
Finland
Email: varun@comnet.tkk.fi
Joerg Ott
Aalto University
School of Electrical Engineering
Otakaari 5 A
Espoo, FIN 02150
Finland
Email: jo@comnet.tkk.fi
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