On Tue, Jun 26, 2007 at 11:44:28PM +0200,
 Andreas Mueller <mailinglists@andreas-mueller.com> wrote 
 a message of 37 lines which said:

> I fired up my Ultra1 (which has NetBSD 4.0_BETA2 on it), I used the
> same command as you and it doesn't show this problem.

Actually, Pascal Hambourg found the exact cause.

sysctl(3) says:

             tcp.mss_ifmtu
                     Returns 1 if TCP calculates the outgoing maximum segment
                     size based on the MTU of the appropriate interface.  Oth-
                     erwise, it is calculated based on the greater of the MTU
                     of the interface, and the largest (non-loopback) inter-
                     face MTU on the system.

And the offending machine had a:

% ifconfig pflog0
pflog0: flags=0 mtu 33136

which explains the problem, which is not 64-bits, nor NetBSD 3. (The
PC which does not show the problem did not have pflog0.)