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

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RFC 4828                  TFRC: The SP Variant                April 2007


   TFRC-SP changes this behavior in the following ways.

   o  The nominal segment size: The nominal segment size s defaults to
      1460 bytes.  Following [RFC3714], this provides a goal of
      fairness, in terms of the sending rate in bytes per second, with a
      TCP flow with 1460 bytes of application data per packet but with
      the same packet drop rate.  If the endpoint knows the MTU (Maximum
      Transmission Unit) of the path and the derived MSS (Maximum
      Segment Size) is less than 1460 bytes, then the endpoint SHOULD
      set the nominal segment size s to MSS bytes.  In addition, if the
      endpoint knows the MTU of the path and the resulting MSS is less
      than 536 bytes, then the endpoint MUST set the nominal segment
      size s to MSS bytes.

      However, this document does not require that TFRC-SP endpoints
      determine the path MTU.  While most paths allow an MSS of 1460
      bytes, some paths have a slightly smaller MSS due to tunnels
      (e.g., IPv6 over IPv4).  In some specific cases, IPv4 paths may
      experience a much smaller path MTU.  Due to the complications of
      estimating the path MTU, and to the fact that most paths support
      an MSS of at least 536 bytes, TFRC-SP as a default uses a nominal
      segment size of 1460 bytes.  The nominal segment size is discussed
      in more detail in Section 4.5.3.

   o  Taking packet headers into account: The allowed transmit rate X in
      bytes per second is reduced by a factor that accounts for packet
      header size.  This gives the application some incentive, beyond
      the Min Interval, not to use unnecessarily small packets.  In
      particular, we introduce a new parameter H, which represents the
      expected size in bytes of network and transport headers to be used
      on the TFRC connection's packets.  This is used to reduce the
      allowed transmit rate X as follows:

      X := X * s_true / (s_true + H),

      where s_true is the true average data segment size for the
      connection in bytes, excluding the transport and network headers.
      Section 4.1 of RFC 3448 states that where the packet size varies
      naturally with the data, an estimate of the mean segment size can
      be used for s_true.  As suggested in Section 4.1 of [RFC3448bis],
      when an estimate of the mean segment size is used for s_true, the
      estimate SHOULD be calculated over at least the last four loss
      intervals.  However, this document does not specify a specific
      algorithm for estimating the mean segment size.

      The H parameter is set to the constant 40 bytes.  Thus, if the
      TFRC-SP application used 40-byte data segments, the allowed
      transmit rate X would be halved to account for the fact that half



Floyd & Kohler                Experimental                      [Page 6]