network performance measuring

Mark Blackman mark at
Mon Jan 26 23:58:09 GMT 2004

On 26 Jan 2004, at 21:04, Duncan Barclay wrote:

> Hi
> We've (a company I'm involved with) had a satellite network link 
> installed
> in the past few weeks (office in the middle of nowhere) but it doesn't 
> seem
> very "broadband". In fact, the throughput seems really bad.
> As such I'd like to gather some statistics about the data going over 
> the
> link so that we can "question" our supplier.
> Any recommendations as to good point testing tools and tools to log 
> traffic
> on the link over a period of time and to extract useful measures from 
> it.

To make sure you're actually using the available bandwidth,
you want to ensure the use of very large TCP window sizes for
both reception and transmission on every client. This is an
essential tuning step for a high bandwidth, high latency connection.

You'll have RTT around 500ms or worse I'd guess and so
you want at least 64k bytes in both directions (if not more).

In summary, the TCP window size is the number of bytes
before an ACK+window update packet of some variety must be seen. For

sysctl -w net.inet.tcp.recvspace=65535
sysctl -w net.inet.tcp.sendspace=65535

you can use a tool called 'cablenut' under windows to achieve
the same thing but you have to set several registry entries
to be sure.

To answer your direct question,


b) I quite like the active tool (requires server and client instance)
called 'netest' at
'netest' will try a variety of TCP window sizes as well to
find an optimum size for a given path. 'netest' is free,
but requires registration for the binaries (FreeBSD included).

Best Regards,
Mark Blackman
Exonetric       "The affordable, flexible Internet hosting provider"      Telehouse London colo. 40UKP+VAT/month

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