RFC 3465 (rfc3465) - Page 2 of 10

TCP Congestion Control with Appropriate Byte Counting (ABC)

RFC 3465            TCP Congestion Control with ABC        February 2003


   network path, therefore providing a more measured response to ACKs
   that cover only small amounts of data (less than a full segment size)
   than ACK counting.  This more appropriate cwnd growth can improve
   both performance and can prevent inappropriate cwnd growth in
   response to a misbehaving receiver.  On the other hand, in some cases
   the modified cwnd growth algorithm causes larger bursts of segments
   to be sent into the network.  In some cases this can lead to a non-
   negligible increase in the drop rate and reduced performance (see
   section 4 for a larger discussion of the issues).

   This document is organized as follows.  Section 2 outlines the
   modified algorithm for increasing TCP's congestion window.  Section 3
   discusses the advantages of using the modified algorithm.  Section 4
   discusses the disadvantages of the approach outlined in this
   document.  Section 5 outlines some of the fairness issues that must
   be considered for the modified algorithm.  Section 6 discusses
   security considerations.

   Statement of Intent

      This specification contains an algorithm improving the performance
      of TCP which is understood to be effective and safe, but which has
      not been widely deployed.  One goal of publication as an
      Experimental RFC is to be prudent, and encourage use and
      deployment prior to publication in the standards track.  It is the
      intent of the Transport Area to re-submit this specification as an
      IETF Proposed Standard in the future, after more experience has
      been gained.

2   A Modified Algorithm for Increasing the Congestion Window

   As originally outlined in [Jac88] and specified in [RFC 2581], TCP
   uses two algorithms for increasing the congestion window.  During
   steady-state, TCP uses the Congestion Avoidance algorithm to linearly
   increase the value of cwnd.  At the beginning of a transfer, after a
   retransmission timeout or after a long idle period (in some
   implementations), TCP uses the Slow Start algorithm to increase cwnd
   exponentially.  According to RFC 2581, slow start bases the cwnd
   increase on the number of incoming acknowledgments.  During
   congestion avoidance RFC 2581 allows more latitude in increasing
   cwnd, but traditionally implementations have based the increase on
   the number of arriving ACKs.  In the following two subsections, we
   detail modifications to these algorithms to increase cwnd based on
   the number of bytes being acknowledged by each arriving ACK, rather
   than by the number of ACKs that arrive.  We call these changes
   "Appropriate Byte Counting" (ABC) [All99].





Allman                        Experimental