RFC 2859 (rfc2859) - Page 2 of 9

A Time Sliding Window Three Colour Marker (TSWTCM)

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