[c-nsp] STM-1 - TCP throughput

[sthaug at nethelp.no]

> Can anyone shed any light into the relationship between RTT and TCP
> throughput.

This is available from lots of textbooks. To a first approximation, max
TCP throughput = W / RTT where W is your window size.

> I have a STM-1 circuit where the Telco RTT SLA is ~ 30ms and I am getting
> a maximum TCP throughput of 20 M/s.
> When I do a test using UDP I am getting over 80 M/s.

Unless your TCP stack uses RFC 1323 extensions, the largest TCP window
is going to be 64 kByte, and maximum throughput at 30 ms RTT will be
64*1024*8 / 0.03 = 17.5 Mbps. Your measured throughput of 20 Mbps is just
close enough to this value that you could be hitting this exact problem.

Steinar Haug, Nethelp consulting, sthaug at nethelp.no