Controlled Delay Approximate Fairness AQM
draft-morton-tsvwg-codel-approx-fair-01

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Transport Working Group                                        J. Morton
Internet-Draft                                                          
Intended status: Informational                                  P. Heist
Expires: 10 September 2020                                  9 March 2020

               Controlled Delay Approximate Fairness AQM
                draft-morton-tsvwg-codel-approx-fair-01

Abstract

   This note presents CodelAF, or Controlled Delay Approximate Fairness
   in full, as an alternative to single-queue AQM or Fair Queue
   implementations in the low-cost or high-speed network hardware
   spaces.  It builds on the seminal work in Codel [RFC8289], and guides
   multiple competing flows towards similar throughputs by differential
   congestion signalling, whilst requiring only a single FIFO queue.  It
   may also be combined with CNQ [I-D.morton-tsvwg-cheap-nasty-queueing]
   to provide a latency optimisation for sparse flows.

Status of This Memo

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   This Internet-Draft will expire on 10 September 2020.

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Morton & Heist          Expires 10 September 2020               [Page 1]
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   as described in Section 4.e of the Trust Legal Provisions and are
   provided without warranty as described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Background  . . . . . . . . . . . . . . . . . . . . . . . . .   3
   3.  The Codel Approximate Fairness Algorithm  . . . . . . . . . .   4
   4.  Extending CodelAF to Provide a Low Latency PHB  . . . . . . .   4
   5.  Security Considerations . . . . . . . . . . . . . . . . . . .   5
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   5
   7.  Informative References  . . . . . . . . . . . . . . . . . . .   5
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .   7

1.  Introduction

   For some years, the solution of choice for improving network
   performance as been the combination of Fair Queuing (FQ) with Active
   Queue Management (AQM) as demonstrated in FQ-Codel [RFC8290].
   However, concerns are legitimately raised over the difficulty of
   implementing FQ in hardware, making it a weak proposition for very
   low-cost and very high-speed network devices alike.  There is some
   evidence to suggest that implementing multiple AQM instances is not
   very difficult in hardware, but implementing multiple FIFOs can be
   prohibitive.

   CodelAF addresses this design space with a straightforward extension
   to the Codel AQM, allowing its target to be biased according to
   relative queue occupancy of a particular flow, and its signals
   applied only to that flow.  An arbitrary number of independent flows
   can then be signalled to more independently than a single AQM can,
   allowing convergence towards a fair-throughput state.

   This approach also successfully addresses the problem of allowing
   flows responding to dissimilar congestion signals to share the same
   FIFO queue without excessive bias.  In particular, it applies to Some
   Congestion Experienced [I-D.morton-tsvwg-sce] flows sharing a queue
   with conventional ECN [RFC3168] and Not-ECT flows.

   It is likely that a similar AF technique can also be applied to other
   AQMs that employ a target queue sojourn time, such as PIE and BLUE.

   Building on the basic CodelAF algorithm, this memo also shows how to
   provide a low-latency PHB through a twinned CodelAF configuration,
   requiring configuration of only a second set of AQM parameters and
   retaining approximate flow-fairness between the low-latency and best-
   effort traffic classes.

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