IAB Concerns Regarding Congestion Control for Voice Traffic in the Internet
RFC 3714

Network Working Group                                      S. Floyd, Ed.
Request for Comments: 3714                                 J. Kempf, Ed.
Category: Informational                                      March 2004

             IAB Concerns Regarding Congestion Control for
                     Voice Traffic in the Internet

Status of this Memo

   This memo provides information for the Internet community.  It does
   not specify an Internet standard of any kind.  Distribution of this
   memo is unlimited.

Copyright Notice

   Copyright (C) The Internet Society (2004).  All Rights Reserved.

Abstract

   This document discusses IAB concerns about effective end-to-end
   congestion control for best-effort voice traffic in the Internet.
   These concerns have to do with fairness, user quality, and with the
   dangers of congestion collapse.  The concerns are particularly
   relevant in light of the absence of a widespread Quality of Service
   (QoS) deployment in the Internet, and the likelihood that this
   situation will not change much in the near term.  This document is
   not making any recommendations about deployment paths for Voice over
   Internet Protocol (VoIP) in terms of QoS support, and is not claiming
   that best-effort service can be relied upon to give acceptable
   performance for VoIP.  We are merely observing that voice traffic is
   occasionally deployed as best-effort traffic over some links in the
   Internet, that we expect this occasional deployment to continue, and
   that we have concerns about the lack of effective end-to-end
   congestion control for this best-effort voice traffic.

Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  2
   2.  An Example of the Potential for Trouble. . . . . . . . . . . .  4
   3.  Why are Persistent, High Drop Rates a Problem? . . . . . . . .  6
       3.1.  Congestion Collapse. . . . . . . . . . . . . . . . . . .  6
       3.2.  User Quality . . . . . . . . . . . . . . . . . . . . . .  7
       3.3.  The Amorphous Problem of Fairness. . . . . . . . . . . .  8
   4.  Current efforts in the IETF. . . . . . . . . . . . . . . . . . 10
       4.1.  RTP. . . . . . . . . . . . . . . . . . . . . . . . . . . 10
       4.2.  TFRC . . . . . . . . . . . . . . . . . . . . . . . . . . 11
       4.3.  DCCP . . . . . . . . . . . . . . . . . . . . . . . . . . 12

Floyd & Kempf                Informational                      [Page 1]
RFC 3714       IAB Concerns Regarding Congestion Control      March 2004

       4.4.  Adaptive Rate Audio Codecs . . . . . . . . . . . . . . . 12
       4.5.  Differentiated Services and Related Topics . . . . . . . 13
   5.  Assessing Minimum Acceptable Sending Rates . . . . . . . . . . 13
       5.1.  Drop Rates at 4.75 kbps Minimum Sending Rate . . . . . . 17
       5.2.  Drop Rates at 64 kbps Minimum Sending Rate . . . . . . . 18
       5.3.  Open Issues. . . . . . . . . . . . . . . . . . . . . . . 18
       5.4.  A Simple Heuristic . . . . . . . . . . . . . . . . . . . 19
   6. Constraints on VoIP Systems . . . . . . . . . . . . . . . . . . 20
   7.  Conclusions and Recommendations. . . . . . . . . . . . . . . . 20
   8.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 21
   9.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 21
       9.1.  Normative References . . . . . . . . . . . . . . . . . . 21
       9.2.  Informative References . . . . . . . . . . . . . . . . . 22
   10. Appendix - Sending Rates with Packet Drops . . . . . . . . . . 26
   11. Security Considerations. . . . . . . . . . . . . . . . . . . . 29
   12. IANA Considerations. . . . . . . . . . . . . . . . . . . . . . 29
   13. Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . 30
   14. Full Copyright Statement . . . . . . . . . . . . . . . . . . . 31

1.  Introduction

   While many in the telephony community assume that commercial VoIP
   service in the Internet awaits effective end-to-end QoS, in reality
   voice service over best-effort broadband Internet connections is an
   available service now with growing demand.  While some ISPs deploy
   QoS on their backbones, and some corporate intranets offer end-to-end
   QoS internally, end-to-end QoS is not generally available to
   customers in the current Internet.  Given the current commercial
   interest in VoIP on best-effort media connections, it seems prudent
   to examine the potential effect of real time flows on congestion.  In
   this document, we perform such an analysis.  Note, however, that this
   document is not making any recommendations about deployment paths for
   VoIP in terms of QoS support, and is not claiming that best-effort
   service can be relied upon to give acceptable performance for VoIP.
   This document is also not discussing signalling connections for VoIP.