network performance measuring

Mark Blackman mark at
Tue Jan 27 07:22:18 GMT 2004

Mark Blackman wrote:

> 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.

An important caveat that I left out is that this will only
make best use of whatever TCP send/receive window
the machine on the other side is using. If the other end
is using 4k windows, your settings won't help.

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

FreeBSD out-of-the-box tends to come with 64k and 40k respectively,
so a FreeBSD box won't necessarily need tuning. You can set this
according to the old classic 2*[one-way delay]*[bandwidth.]

> 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,
> a)
> 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
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