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

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RFC 4828                  TFRC: The SP Variant                April 2007


   available bandwidth, to the exclusion of the competing TCP or TFRC-SP
   flow.

   All IPv4 *end hosts* are required to accept and reassemble IP packets
   of size 576 bytes [RFC791], but IPv4 *links* do not necessarily have
   to support this packet size.  In slow networks, the largest possible
   packet may take a considerable amount of time to send [RFC3819], and
   a smaller MTU may be desirable, e.g., hundreds of bytes.  If the
   first-hop link had a small MTU, then TCP would choose an
   appropriately small MSS [RFC879].  [RFC1144] quotes cases of very low
   link speeds where the MSS may be tens of bytes (and notes this is an
   extreme case).  We note that if TFRC-SP is used over a path with an
   MTU considerably smaller than 576 bytes, and the TFRC-SP flow uses a
   nominal segment size s of 1460 bytes, then the TFRC-SP flow could
   receive considerably more than three times the bandwidth of competing
   TCP flows.

   If TFRC-SP is used with a nominal segment size s of less than 536
   bytes (because the path MTU is known to be less than 576 bytes), then
   TFRC-SP is likely to be of minimal benefit to applications.  If
   TFRC-SP was to be used on paths that have a path MTU of considerably
   less than 576 bytes, and the transport protocol was not required to
   keep track of the path MTU, this could result in the TFRC-SP flow
   using the default nominal segment size of 1460 bytes, and as a result
   receiving considerably more bandwidth than competing TCP flows.  As a
   result, TFRC-SP is not recommended for use with transport protocols
   that don't maintain some knowledge of the path MTU.

4.6.  The Loss Interval Length for Short Loss Intervals

   For a TFRC-SP receiver, the guidelines from Section 6 of RFC 3448
   govern when the receiver should send feedback messages.  In
   particular, from [RFC3448], "a feedback packet should ... be sent
   whenever a new loss event is detected without waiting for the end of
   an RTT".  In addition, feedback packets are sent at least once per
   RTT.

   For a TFRC-SP connection with a short current loss interval (less
   than two round-trip times), it is possible for the loss interval
   length calculated for that loss interval to change in odd ways as
   additional packet losses in that loss interval are detected.  To
   prevent unnecessary oscillations in the average loss interval,
   Section 3 specifies that the current loss interval can be included in
   the calculation of the average loss interval only if the current loss
   interval is longer than two round-trip times.






Floyd & Kohler                Experimental                     [Page 18]