Calculating required TCP buffer sizes

Regardless of operating system, TCP buffers must be appropriately dimensioned if they are to support long distance, high data rate flows. Victor Reijs's TCP Throughput calculator is able to determine the maximum achievable TCP throughput based on RTT, expected packet loss, segment size, re-transmission timeout (RTO) and buffer size. It can be found at:

http://people.heanet.ie/~vreijs/tcpcalculations.htm

Tolerance to Packet Re-ordering

As explained in the section on packet re-ordering, packets can be re-ordered wherever there is parellelism in a network. If such parellelism exists in a network (e.g. as caused by the internal architectire of the Juniper M160s used in the GEANT network) then TCP should be set to tolerate a larger number of re-ordered packets than the default, which is 3 (see here for an example for Linux). For large flows across GEANT a value of 20 should be sufficient - an alternative work-round is to use multiple parallel TCP flows.

Modifying TCP Behaviour

TCP Congestion control algorithms are often configured conservatively, to avoid 'slow start' overwhelming a network path. In a capacity rich network this may not be necessary, and may be over-ridden.

-- TobyRodwell - 24 Jan 2005

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Topic revision: r4 - 2007-05-17 - TobyRodwell
 
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