TCP Friendly Rate Control (TFRC): The Small-Packet (SP) Variant

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RFC 4828                  TFRC: The SP Variant                April 2007


   The general lessons from the simulations are as follows.

   o  In scenarios where large and small packets receive similar packet
      drop rates, TFRC-SP gives roughly the desired sending rate
      (Appendix B.1, B.2).

   o  In scenarios where each *byte* is equally likely to be dropped,
      standard TFRC gives reasonable fairness between small-packet TFRC
      flows and large-packet TCP flows (Appendix B.2).

   o  In scenarios where small packets are less likely to be dropped
      than large packets, TFRC-SP does not give reasonable fairness
      between small-packet TFRC-SP flows and large-packet TCP flows;
      small-packet TFRC-SP flows can receive considerably more bandwidth
      than competing large-packet TCP flows, and in some cases large-
      packet TCP flows can be essentially starved by competing small-
      packet TFRC-SP flows (Appendix B.2, B.3, B.4).

   o  Scenarios where small packets are less likely to be dropped than
      large packets include those with Drop-Tail queues in bytes, and
      with AQM mechanisms in byte mode (Appendix B.3, B.4).  It has also
      been reported that wireless links are sometimes good enough to let
      small packets through, while larger packets have a much higher
      error rate, and hence a higher drop probability [S05].

8.  General Discussion

   Dropping rates for small packets: The goal of TFRC-SP is for small-
   packet TFRC-SP flows to have rough fairness with large-packet TCP
   flows in the sending rate in bps, in a scenario where each packet
   receives roughly the same probability of being dropped.  In a
   scenario where large packets are more likely to be dropped than small
   packets, or where flows with a bursty sending rate are more likely to
   have packets dropped than are flows with a smooth sending rate,
   small-packet TFRC-SP flows can receive significantly more bandwidth
   than competing large-packet TCP flows.

   The accuracy of the TCP response function used in TFRC: For
   applications with a maximum sending rate of 96 Kbps or less (i.e.,
   packets of at most 120 bytes), TFRC-SP only restricts the sending
   rate when the packet drop rate is fairly high, e.g., greater than
   10%.  [Derivation: A TFRC-SP flow with a 200 ms round-trip time and a
   maximum sending rate with packet headers of 128 Kbps would have a
   sending rate in bytes per second equivalent to a TCP flow with 1460-
   byte segments sending 2.2 packets per round-trip time.  From Table 1
   of RFC 3714, this sending rate can be sustained with a packet drop
   rate slightly greater than 10%.]  In this high-packet-drop regime,
   the performance of TFRC-SP is determined in part by the accuracy of



Floyd & Kohler                Experimental                     [Page 21]