[c-nsp] STM-1 - TCP throughput

[Frank Bulk]

This is slight off-topic, but while we're talking about TCP latency and
bandwidth: if a cable modem provider starts offering 10 Mbps downstream but
the RTT is 80 msec it looks like they can't much beyond 7 Mbps in TCP
throughput.  I can see how FiOS, which likely has lower latencies, could
have a leg up.

Frank

-----Original Message-----
From: cisco-nsp-bounces at puck.nether.net
[mailto:cisco-nsp-bounces at puck.nether.net] On Behalf Of sthaug at nethelp.no
Sent: Saturday, December 02, 2006 6:10 AM
To: zhassan at gmx.net
Cc: cisco-nsp at puck.nether.net
Subject: Re: [c-nsp] STM-1 - TCP throughput

> 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
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