RFC 1144 (rfc1144) - Page 2 of 46

Compressing TCP/IP headers for low-speed serial links

   RFC 1144               Compressing TCP/IP Headers          February 1990


   than 100 to 200 ms.  Protocol headers interact with this threshold three
   ways:

   (1) If the line is too slow, it may be impossible to fit both the
       headers and data into a 200 ms window:  One typed character results
       in a 41 byte TCP/IP packet being sent and a 41 byte echo being
       received.  The line speed must be at least 4000 bps to handle these
       82 bytes in 200 ms.

   (2) Even with a line fast enough to handle packetized typing echo (4800
       bps or above), there may be an undesirable interaction between bulk
       data and interactive traffic:  For reasonable line efficiency the
       bulk data packet size needs to be 10 to 20 times the header size.
       I.e., the line maximum transmission unit or MTU should be 500 to
       1000 bytes for 40 byte TCP/IP headers.  Even with type-of-service
       queuing to give priority to interactive traffic, a telnet packet has
       to wait for any in-progress bulk data packet to finish.  Assuming
       data transfer in only one direction, that wait averages half the MTU
       or 500 ms for a 1024 byte MTU at 9600 bps.

   (3) Any communication medium has a maximum signalling rate, the Shannon
       limit.  Based on an AT&T study[2], the Shannon limit for a typical
       dialup phone line is around 22,000 bps.  Since a full duplex, 9600
       bps modem already runs at 80% of the limit, modem manufacturers are
       starting to offer asymmetric allocation schemes to increase
       effective bandwidth:  Since a line rarely has equivalent amounts of
       data flowing both directions simultaneously, it is possible to give
       one end of the line more than 11,000 bps by either time-division
       multiplexing a half-duplex line (e.g., the Telebit Trailblazer) or
       offering a low-speed `reverse channel' (e.g., the USR Courier
       HST)./3/ In either case, the modem dynamically tries to guess which
       end of the conversation needs high bandwidth by assuming one end of
       the conversation is a human (i.e., demand is limited to