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

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RFC 4828                  TFRC: The SP Variant                April 2007


   packet drop rate of at most 15%, keeping the TCP connection out of
   the regime of exponential backoffs of the retransmit timer, then this
   requires B*S = 0.15, or S = 0.15/B.

   In the high-congestion regime of Table 10, with high congestion and
   with byte drop rates of 0.01 and higher, the TCP performance is
   dominated by the exponential backoff of the retransmit timer
   regardless of the segment size.  Even a 40-byte packet with a zero-
   byte data segment would have a packet drop rate of at least 33%.  In
   this regime, the optimal TCP *sending* rate comes with a large,
   1460-byte data segment, but the TCP sending rate is low with any
   segment size, considerably less than one packet per round-trip time.

   We note that in this regime, while a larger packet gives a higher TCP
   *sending* rate, a smaller packet gives a better *goodput* rate.

   In general, Tables 8 and 9 show good performance for TFRC-SP in
   environments with stable packet drop rates, where the decision to
   drop a packet is independent of the packet size.  However, in some
   environments the packet size might affect the likelihood that a
   packet is dropped.  For example, with heavy congestion and a Drop
   Tail queue with a fixed number of bytes rather than a fixed number of
   packet-sized buffers, small packets might be more likely than large
   packets to find room at the end of an almost-full queue.  As a
   further complication, in a scenario with Active Queue Management, the
   AQM mechanism could either be in packet mode, dropping each packet
   with equal probability, or in byte mode, dropping each byte with
   equal probability.  Sections B.3 and B.4 show simulations with
   packets dropped at Drop-Tail or AQM queues, rather that from a
   probabilistic process.

B.3.  Packet Dropping Behavior at Routers with Drop-Tail Queues

   One of the problems with comparing the throughput of two flows using
   different packet sizes is that the packet size itself can influence
   the packet drop rate [V00, WBL04].

   The default TFRC was designed for rough fairness with TCP, for TFRC
   and TCP flows with the same packet size and experiencing the same
   packet drop rate.  When the issue of fairness between flows with
   different packets sizes is addressed, it matters whether the packet
   drop rates experienced by the flows is related to the packet size.
   That is, are small packets just as likely to be dropped as large TCP
   packets, or are the smaller packets less likely to be dropped
   [WBL04]?  And what is the relationship between the packet-dropping
   behavior of the path, and the loss event measurements of TFRC?





Floyd & Kohler                Experimental                     [Page 32]