This is a part of a tcpdump from a PPP connection over ISDN. The problem is 
reproduceable, interactive traffics with smaller packets works (i.e. telnet),
but shows some "bad tcp chksum"s too.

The same software (ISDN4BSD, using it's own version of in-kernel
PPP-over-ISDN, the isp* driver) works for everyone else, so this must be a
very well hidden bug, pilot error or bad hardware. Bad hardware is unliekly,
the lower (ISDN) levels don't report any errors and using raw IP frames over
ISDN works with the same hardware.

When I request the same file transfered in this http transaction from my
home system, I receive it fast and well, but with most packets of size 
1460 (ifconfig isp0 shows a MTU of 1500). This happens with a point-to-point
connection between two NetBSD systems as well, no blocking routers involved.


Strange, isn't it?

Does this ring a bell for someone? Any hints?


Thanks in advance & merry christmas


Martin

00:02:27.219991 192.168.1.1.63591 > 192.168.1.2.www: S [tcp sum ok] 2518473171:2518473171(0) win 16384 <mss 1460,nop,wscale 0,nop,nop,timestamp 367153 0> (ttl 64, id 22812)
00:02:27.283137 192.168.1.2.www > 192.168.1.1.63591: S [tcp sum ok] 2509707400:2509707400(0) ack 2518473172 win 32120 <mss 1460,nop,nop,timestamp 19992564 367153,nop,wscale 0> (DF) (ttl 52, id 36125)
00:02:27.283323 192.168.1.1.63591 > 192.168.1.2.www: . [tcp sum ok] ack 1 win 17520 <nop,nop,timestamp 367153 19992564> (ttl 64, id 22813)
00:02:27.284627 192.168.1.1.63591 > 192.168.1.2.www: P [tcp sum ok] 1:144(143) ack 1 win 17520 <nop,nop,timestamp 367153 19992564> (ttl 64, id 22815)
00:02:27.365849 192.168.1.2.www > 192.168.1.1.63591: . [tcp sum ok] ack 144 win 32120 <nop,nop,timestamp 19992572 367153> (DF) (ttl 52, id 36126)
00:02:28.251681 192.168.1.2.www > 192.168.1.1.63591: P [bad tcp cksum 1fdf!] 1:1449(1448) ack 144 win 32120 <nop,nop,timestamp 19992641 367153> (DF) (ttl 52, id 36127)
00:02:28.444028 192.168.1.2.www > 192.168.1.1.63591: P [bad tcp cksum fe9e!] 1449:2897(1448) ack 144 win 32120 <nop,nop,timestamp 19992641 367153> (DF) (ttl 52, id 36128)
00:02:31.243922 192.168.1.2.www > 192.168.1.1.63591: P [bad tcp cksum 1fdf!] 1:1449(1448) ack 144 win 32120 <nop,nop,timestamp 19992941 367153> (DF) (ttl 52, id 36133)
00:02:37.243954 192.168.1.2.www > 192.168.1.1.63591: P [bad tcp cksum 1fdf!] 1:1449(1448) ack 144 win 32120 <nop,nop,timestamp 19993541 367153> (DF) (ttl 52, id 36187)
00:02:49.245222 192.168.1.2.www > 192.168.1.1.63591: P [bad tcp cksum 1fdf!] 1:1449(1448) ack 144 win 32120 <nop,nop,timestamp 19994741 367153> (DF) (ttl 52, id 36223)
00:03:13.248135 192.168.1.2.www > 192.168.1.1.63591: P [bad tcp cksum 1fdf!] 1:1449(1448) ack 144 win 32120 <nop,nop,timestamp 19997141 367153> (DF) (ttl 52, id 36406)