RMCAT WG V. Singh
Internet-Draft J. Ott
Intended status: Informational Aalto University
Expires: September 11, 2014 March 10, 2014
Evaluating Congestion Control for Interactive Real-time Media
draft-ietf-rmcat-eval-criteria-01
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.
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 September 11, 2014.
Copyright Notice
Copyright (c) 2014 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.
Singh & Ott Expires September 11, 2014 [Page 1]
Internet-Draft Evaluating Congestion Control for RMCAT March 2014
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Metrics . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
3.1. RTP Log Format . . . . . . . . . . . . . . . . . . . . . 4
4. Guidelines . . . . . . . . . . . . . . . . . . . . . . . . . 5
4.1. Avoiding Congestion Collapse . . . . . . . . . . . . . . 5
4.2. Stability . . . . . . . . . . . . . . . . . . . . . . . . 5
4.3. Media Traffic . . . . . . . . . . . . . . . . . . . . . . 5
4.4. Start-up Behaviour . . . . . . . . . . . . . . . . . . . 5
4.5. Diverse Environments . . . . . . . . . . . . . . . . . . 6
4.6. Varying Path Characteristics . . . . . . . . . . . . . . 6
4.7. Reacting to Transient Events or Interruptions . . . . . . 6
4.8. Fairness With Similar Cross-Traffic . . . . . . . . . . . 7
4.9. Impact on Cross-Traffic . . . . . . . . . . . . . . . . . 7
4.10. Extensions to RTP/RTCP . . . . . . . . . . . . . . . . . 7
5. Minimum Requirements for Evaluation . . . . . . . . . . . . . 7
6. Status of Proposals . . . . . . . . . . . . . . . . . . . . . 7
7. Security Considerations . . . . . . . . . . . . . . . . . . . 8
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 8
9. Contributors . . . . . . . . . . . . . . . . . . . . . . . . 8
10. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 8
11. References . . . . . . . . . . . . . . . . . . . . . . . . . 8
11.1. Normative References . . . . . . . . . . . . . . . . . . 8
11.2. Informative References . . . . . . . . . . . . . . . . . 9
Appendix A. Application Trade-off . . . . . . . . . . . . . . . 10
A.1. Measuring Quality . . . . . . . . . . . . . . . . . . . . 10
Appendix B. Change Log . . . . . . . . . . . . . . . . . . . . . 10
B.1. Changes in draft-ietf-rmcat-eval-criteria-01 . . . . . . 10
B.2. Changes in draft-ietf-rmcat-eval-criteria-00 . . . . . . 10
B.3. Changes in draft-singh-rmcat-cc-eval-04 . . . . . . . . . 10
B.4. Changes in draft-singh-rmcat-cc-eval-03 . . . . . . . . . 11
B.5. Changes in draft-singh-rmcat-cc-eval-02 . . . . . . . . . 11
B.6. Changes in draft-singh-rmcat-cc-eval-01 . . . . . . . . . 11
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 11
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.ietf-rmcat-cc-requirements]). 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].
Singh & Ott Expires September 11, 2014 [Page 2]
Internet-Draft Evaluating Congestion Control for RMCAT March 2014
The guidelines proposed in the document are intended to help prevent
a congestion collapse, promote fair capacity usage and optimize the
media flow's throughput. 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 of interest for interactive multimedia are:
o Throughput.
o Minimizing oscillations in the transmission rate (stability) when
the end-to-end capacity varies slowly.
o Delay.
o Reactivity to transient events.
o Packet losses and discards.
o Section 2.1 of [RFC5166] discusses the tradeoff between
throughput, delay and loss.
Each experiment is expected to log every incoming and outgoing packet
(the RTP logging format is described in Section 3.1). The logging
can be done inside the application or at the endpoints using pcap
(packet capture, e.g., tcpdump, wireshark). The following are
calculated based on the information in the packet logs:
1. Sending rate, Receiver rate, Goodput
2. Packet delay
Singh & Ott Expires September 11, 2014 [Page 3]
Internet-Draft Evaluating Congestion Control for RMCAT March 2014
3. Packet loss
4. If using, retransmission or FEC: residual loss
5. Packets discarded from the playout or de-jitter buffer
[Open issue (1): The "unfairness" test is (measured at 1s intervals):
1. Does not trigger the circuit breaker.
2. Over 3 times or less than 1/3 times the throughput for an RMCAT
media stream compared to identical RMCAT streams competing on a
bottleneck, for a case when the competing streams have similar RTTs.
3. Over 3 times delay compared to RTT measurements performed before
starting the RMCAT flow or for the case when competing with identical
RMCAT streams having similar RTTs.
]
[Open issue (2): Possibly using Jain-fairness index.]
Convergence time: the time taken to reach a stable rate at startup,
after the available link capacity changes, or when new flows get
added to the bottleneck link.
Bandwidth Utilization, defined as ratio of the instantaneous sending
rate to the instantaneous bottleneck capacity. This metric is useful
when an RMCAT flow is by itself or competing with similar cross-
traffic.
From the logs the statistical measures (min, max, mean, standard
deviation and variance) for the whole duration or any specific part
of the session can be calculated. Also the metrics (sending rate,
receiver rate, goodput, latency) can be visualized in graphs as
variation over time, the measurements in the plot are at 1 second
intervals. Additionally, from the logs it is possible to plot the
histogram or CDF of packet delay.
3.1. RTP Log Format
The log file is tab or comma separated containing the following
details:
Send or receive timestamp (unix)
RTP payload type
SSRC
RTP sequence no
RTP timestamp
marker bit
payload size
Singh & Ott Expires September 11, 2014 [Page 4]
Internet-Draft Evaluating Congestion Control for RMCAT March 2014
If the congestion control implements, retransmissions or FEC, the
evaluation should report both packet loss (before applying error-
resilience) and residual packet loss (after applying error-
resilience).
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
The congestion control algorithm is expected to take an action, such
as reducing the sending rate, when it detects congestion. Typically,
it should intervene before the circuit breaker
[I-D.ietf-avtcore-rtp-circuit-breakers] is engaged.
Does the congestion control propose any changes to (or diverge from)
the circuit breaker conditions defined in
[I-D.ietf-avtcore-rtp-circuit-breakers].
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 estimate too often or by too much may adversely affect
the application layer performance.
4.3. Media Traffic
The congestion control algorithm should be assessed with different
types of media behavior, i.e., the media should contain idle and
data-limited periods. For example, periods of silence for audio,
varying amount of motion for video, or bursty nature of I-frames.
The evaluation may be done in two stages. In the first stage, the
endpoint generates traffic at the rate calculated by the congestion
controller. In the second stage, real codecs or models of video
codecs are used to mimic application-limited data periods and varying
video frame sizes.
4.4. Start-up Behaviour
The congestion control algorithm should be assessed with different
start-rates. The main reason is to observe the behavior of the
congestion control in different test scenarios, such as when
Singh & Ott Expires September 11, 2014 [Page 5]
Internet-Draft Evaluating Congestion Control for RMCAT March 2014
competing with varying amount of cross-traffic or how quickly does
the congestion control algorithm achieve a stable sending rate.
4.5. Diverse Environments
The congestion control algorithm should be assessed in heterogeneous
environments, containing both wired and wireless paths. Examples of
wireless access technologies are: 802.11, GPRS, HSPA, or LTE. One of
the main challenges of the wireless environments for the congestion
control algorithm is to distinguish between congestion induced loss
and transmission (bit-error) loss. Congestion control algorithms may
incorrectly identify transmission loss as congestion loss and reduce
the media encoding rate by 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.
4.6. 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 bottleneck link and a
router's queue length. For the moment, only DropTail queues are
used. However, if new Active Queue Management (AQM) schemes become
available, the performance of the congestion control algorithm should
be again evaluated.
In an experiment, if the media only flows in a single direction, the
feedback path should also be tested with varying amounts of
impairments.
The main motivation for the previous and current criteria is to
identify situations in which the proposed congestion control is less
performant.
4.7. 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.
Singh & Ott Expires September 11, 2014 [Page 6]
Internet-Draft Evaluating Congestion Control for RMCAT March 2014
4.8. Fairness With Similar Cross-Traffic
The congestion control algorithm should be evaluated when competing
with other RTP flows using the same or another candidate congestion
control algorithm. The proposal should highlight the bottleneck
capacity share of each RTP flow.
4.9. 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.
The proposal should also measure the impact on varied number of
cross-traffic sources, i.e., few and many competing flows, or mixing
various amounts of TCP and similar cross-traffic.
4.10. 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 or defined tests/scenarios.]
6. 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).]
Singh & Ott Expires September 11, 2014 [Page 7]
Internet-Draft Evaluating Congestion Control for RMCAT March 2014
7. Security Considerations
Security issues have not been discussed in this memo.
8. IANA Considerations
There are no IANA impacts in this memo.
9. Contributors
The content and concepts within this document are a product of the
discussion carried out in the Design Team.
Michael Ramalho provided the text for a specific scenario, which is
now covered in [I-D.sarker-rmcat-eval-test].
10. Acknowledgements
Much of this document is derived from previous work on congestion
control at the IETF.
The authors would like to thank Harald Alvestrand, Luca De Cicco,
Wesley Eddy, Lars Eggert, Kevin Gross, Vinayak Hegde, Stefan Holmer,
Randell Jesup, Piers O'Hanlon, Colin Perkins, Michael Ramalho,
Zaheduzzaman Sarker, Timothy B. Terriberry, Michael Welzl, and Mo
Zanaty for providing valuable feedback on earlier versions of this
draft. Additionally, also thank the participants of the design team
for their comments and discussion related to the evaluation criteria.
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.
Singh & Ott Expires September 11, 2014 [Page 8]
Internet-Draft Evaluating Congestion Control for RMCAT March 2014
[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.ietf-rmcat-cc-requirements]
Jesup, R., "Congestion Control Requirements For RMCAT",
draft-ietf-rmcat-cc-requirements-02 (work in progress),
February 2014.
[I-D.ietf-avtcore-rtp-circuit-breakers]
Perkins, C. and V. Singh, "Multimedia Congestion Control:
Circuit Breakers for Unicast RTP Sessions", draft-ietf-
avtcore-rtp-circuit-breakers-05 (work in progress),
February 2014.
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.
[I-D.sarker-rmcat-eval-test]
Sarker, Z., Singh, V., Zhu, X., and M. Ramalho, "Test
Cases for Evaluating RMCAT Proposals", draft-sarker-rmcat-
eval-test-00 (work in progress), February 2014.
[SA4-EVAL]
R1-081955, 3GPP., "LTE Link Level Throughput Data for SA4
Evaluation Framework", 3GPP R1-081955, 5 2008.
[SA4-LR] S4-050560, 3GPP., "Error Patterns for MBMS Streaming over
UTRAN and GERAN", 3GPP S4-050560, 5 2008.
[TCP-eval-suite]
Lachlan, A., Marcondes, C., Floyd, S., Dunn, L., Guillier,
R., Gang, W., Eggert, L., Ha, S., and I. Rhee, "Towards a
Common TCP Evaluation Suite", Proc. PFLDnet. 2008, August
2008.
Singh & Ott Expires September 11, 2014 [Page 9]
Internet-Draft Evaluating Congestion Control for RMCAT March 2014
Appendix A. Application Trade-off
Application trade-off is yet to be defined. see RMCAT requirements
[I-D.ietf-rmcat-cc-requirements] document. Perhaps each experiment
should define the application's expectation or trade-off.
A.1. Measuring Quality
No quality metric is defined for performance evaluation, it is
currently an open issue. However, there is consensus that congestion
control algorithm should be able to show that it is useful for
interactive video by performing analysis using a real codec and video
sequences.
Appendix B. Change Log
Note to the RFC-Editor: please remove this section prior to
publication as an RFC.
B.1. Changes in draft-ietf-rmcat-eval-criteria-01
o Removed Appendix B.
o Removed Section on Evaluation Parameters.
B.2. Changes in draft-ietf-rmcat-eval-criteria-00
o Updated references.
o Resubmitted as WG draft.
B.3. Changes in draft-singh-rmcat-cc-eval-04
o Incorporate feedback from IETF 87, Berlin.
o Clarified metrics: convergence time, bandwidth utilization.
o Changed fairness criteria to fairness test.
o Added measuring pre- and post-repair loss.
o Added open issue of measuring video quality to appendix.
o clarified use of DropTail and AQM.
o Updated text in "Minimum Requirements for Evaluation"
Singh & Ott Expires September 11, 2014 [Page 10]
Internet-Draft Evaluating Congestion Control for RMCAT March 2014
B.4. Changes in draft-singh-rmcat-cc-eval-03
o Incorporate the discussion within the design team.
o Added a section on evaluation parameters, it describes the flow
and network characteristics.
o Added Appendix with self-fairness experiment.
o Changed bottleneck parameters from a proposal to an example set.
o
B.5. Changes in draft-singh-rmcat-cc-eval-02
o Added scenario descriptions.
B.6. Changes in draft-singh-rmcat-cc-eval-01
o Removed QoE metrics.
o Changed stability to steady-state.
o Added measuring impact against few and many flows.
o Added guideline for idle and data-limited periods.
o Added reference to TCP evaluation suite in example evaluation
scenarios.
Authors' Addresses
Varun Singh
Aalto University
School of Electrical Engineering
Otakaari 5 A
Espoo, FIN 02150
Finland
Email: varun@comnet.tkk.fi
URI: http://www.netlab.tkk.fi/~varun/
Singh & Ott Expires September 11, 2014 [Page 11]
Internet-Draft Evaluating Congestion Control for RMCAT March 2014
Joerg Ott
Aalto University
School of Electrical Engineering
Otakaari 5 A
Espoo, FIN 02150
Finland
Email: jo@comnet.tkk.fi
Singh & Ott Expires September 11, 2014 [Page 12]