draft-singh-rmcat-cc-eval-00.txt

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.

   Internet-Drafts are working documents of the Internet Engineering
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   This Internet-Draft will expire on April 18, 2013.

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   described in the Simplified BSD License.




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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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