A Time Sliding Window Three Colour Marker (TSWTCM)
RFC 2859

Document Type RFC - Experimental (June 2000; No errata)
Last updated 2013-03-02
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Network Working Group                                             W. Fang
Request for Comments: 2859                           Princeton University
Category: Experimental                                         N. Seddigh
                                                                 B. Nandy
                                                          Nortel Networks
                                                                June 2000

           A Time Sliding Window Three Colour Marker (TSWTCM)

Status of this Memo

   This memo defines an Experimental Protocol for the Internet
   community.  It does not specify an Internet standard of any kind.
   Discussion and suggestions for improvement are requested.
   Distribution of this memo is unlimited.

Copyright Notice

   Copyright (C) The Internet Society (2000).  All Rights Reserved.


   This memo defines a Time Sliding Window Three Colour Marker (TSWTCM),
   which can be used as a component in a Diff-Serv traffic conditioner
   [RFC2475, RFC2474].  The marker is intended to mark packets that will
   be treated by the Assured Forwarding (AF) Per Hop Behaviour (PHB)
   [AFPHB] in downstream routers. The TSWTCM meters a traffic stream and
   marks packets to be either green, yellow or red based on the measured
   throughput relative to two specified rates: Committed Target Rate
   (CTR) and Peak Target Rate (PTR).

1.0 Introduction

   The Time Sliding Window Three Colour Marker (TSWTCM) is designed to
   mark packets of an IP traffic stream with colour of red, yellow or
   green. The marking is performed based on the measured throughput of
   the traffic stream as compared against the Committed Target Rate
   (CTR) and the Peak Target Rate (PTR). The TSWTCM is designed to mark
   packets contributing to sending rate below or equal to the CTR with
   green colour.  Packets contributing to the portion of the rate
   between the CTR and PTR are marked yellow. Packets causing the rate
   to exceed PTR are marked with red colour.

   The TSWTCM has been primarily designed for traffic streams that will
   be forwarded based on the AF PHB in core routers.

Fang, et al.                  Experimental                      [Page 1]
RFC 2859                         TSWTCM                        June 2000

   The TSWTCM operates based on simple control theory principles of
   proportionally regulated feedback control.

2.0 Overview of TSWTCM

   The TSWTCM consists of two independent components: a rate estimator,
   and a marker to associate a colour (drop precedence) with each
   packet.  The marker uses the algorithm specified in section 4. If the
   marker is used with the AF PHB, each colour would correspond to a
   level of drop precedence.

   The rate estimator provides an estimate of the running average
   bandwidth.  It takes into account burstiness and smoothes out its
   estimate to approximate the longer-term measured sending rate of the
   traffic stream.

   The marker uses the estimated rate to probabilistically associate
   packets with one of the three colours. Using a probabilistic function
   in the marker is beneficial to TCP flows as it reduces the likelihood
   of dropping multiple packets within a TCP window.  The marker also
   works correctly with UDP traffic, i.e., it associates the appropriate
   portion of the UDP packets with yellow or red colour marking if such
   flows transmit at a sustained level above the contracted rate.

                | Rate    | Rate
                |estimator| ==========
                |         |          |
                +---------+          |
                   ^                 V
                   |             +---------+
                   |             |         |
     Packet ====================>| Marker  |====> Marked packet stream
     Stream                      |         |    (Green, Yellow and Red)

                   Figure 1.  Block diagram for the TSWTCM

   The colour of the packet is translated into a DS field packet
   marking.  The colours red, yellow and green translate into DS
   codepoints representing drop precedence 2, 1 and 0 of a single AF
   class respectively.

   Based on feedback from four different implementations, the TSWTCM is
   simple and straightforward to implement.  The TSWTCM can be
   implemented in either software or hardware depending on the nature of
   the forwarding engine.

Fang, et al.                  Experimental                      [Page 2]
RFC 2859                         TSWTCM                        June 2000

3.0 Rate Estimator

   The Rate Estimator provides an estimate of the traffic stream's
   arrival rate.  This rate should approximate the running average
   bandwidth of the traffic stream over a specific period of time

   This memo does not specify a particular algorithm for the Rate
   Estimator.  However, different Rate Estimators should yield similar
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