network not performing - where to start?

Erik Norgaard norgaard at
Mon Feb 18 12:44:45 UTC 2008

Boldra wrote:

> Can you point me to a man page or something for your suggestion "use 
> packet filtering statistics" ?  Where do I start?

If you have a firewall enabled this usually creates statistics on the 
packets sent and received. If not, you can create a simple filter that 
just passes everything. With packet filter something like

pass on ed1 all

should do, where ed1 is the name of you interface.

It can show you just how much data is actually sent when you transfer 
that 100Mbyte file.

I'm a bit outdated on the current situation, it's been a long time since 
I've had anything but FreeBSD to worry about - that is, I don't care 
about optimal performance for those who choose to use windows :)

You will always loose something due to overhead in protocols on 
different levels, for example tcp send back ACK packets, using scp your 
system will also do encryption and decryption, etc.

I recall once also experiencing performance issues when transfering data 
between linux and freebsd - that was more than 5 years ago, but I would 
always assume performance to be best between to equivalent systems.

I don't know your brand of interface, but while the interface may 
formally support 100Mbit/s, in reality other things also affect 
performance, particularly cheap NICs perform bad in peak load, although 
they do communicate at 100Mbit. Don't ask me about details, but 
buffering is one of the issues AFAIK.

Also, check MTU on the windows machines, that can be a killer if it 
forces fragmentation. I recall once that some set the MTU to 1492 or 
some other value. It can have significant impact on performance if MTUs 
differ and packets are fragmented.

There is the MTU ping test:

DOS> ping ip.n.u.m -f -l 1492


Cheers, Erik

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