Subject: Re: NetBSD TCP strangeness (was: problems using nbcvs)
To: John Klos <>
From: Dennis Ferguson <>
List: tech-net
Date: 01/28/2003 16:25:53
> From behind IP NAT using IP Filter (tried NetBSD 1.5.2, 1.5.3, 1.6, 1.6
> release from two weeks ago, FreeBSD 4.6.2), all Mac OS X machines get
> anywhere from .5 k to 10 k/sec from my server. Note that all of the NATs
> tested were not PPPoE or anything that requires a reduced MTU.
> I don't have any OS X machines that are not behind NAT, but if anyone else
> can test this, I'd like to know if the problem is specifically due to IP
> Filter's NAT and how it works with OS X. Also, the OS X machines don't
> have any problems with most other servers.

I don't get the part about "most other servers".  Are there some ftp servers
that go fast and some that don't?

I've noticed that some OS X boxes have difficulty keeping up to their network
interfaces and drop packets that are too close together.  You may be able to
get some idea of what is bad and what isn't using ping -f between machines
behind the firewall.

If this is the case then the problem might be that the NetBSD NAT box is
sometimes clumping the packets together and bursting them out faster than
the OS X box can keep up to them.  You might try fixing the speed of the
inside interface on the NAT box to 10 Mbps to see if that helps.  If not,
a really good tcpdump trace of the file transfer taken on the wire between
the NetBSD NAT box and the losing OS X machine using a third machine might
be necessary to figure out what is happening.

Dennis Ferguson