[comp.sys.3b1] 3b/2 telnet checksum problem

terry@hq.af.mil (Terry Bernstein) (03/23/91)

I have a problem with a 3b/2 system that communicates fine with every
host over our network, except those connected via 3com cs200 terminal
servers.

The 3b/2 is connected to its own baseband ethernet. An IB/1, 3com
ethernet to broadband bridge, connects the ethernet to the broadband
backbone.  The 3b/2 works fine when connected to from any host on the
network, except cs200s.  When connecting from a cs200, it will
mysteriously hang -- sometimes recovering and sometimes timing out.
It usually hangs during the motd, but in other places as well.  In
looking at the sniffer protocol analyzer, I found that the packet it
hangs on has a bad TCP checksum.  This would make the recipient
discard the packet.  The 3b/2 continues to resend the packet (again
with bad TCP checksums) until the connection times out.  The IP
checksum is always correct and the packet looks ok.  It seems that
somehow the packet is being stamped with the wrong tcp checksum.

We have 3 other 3b/2 systems connected in exactly the same manner.
None of these exhibit this behavior.


The only difference I can think of when connecting with a cs200, is that the tcp/
options it negotiates would be different than those of other hosts.

The symptoms seem to rule out everything except intermittent software
(telnetd), or hardware failure -- but what exactly, and how do we find it?



Any ideas??????




-- terry --



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>>>>>>>>>>>>>>>>>>>>>   Terry Bernstein        <<terry@hq.af.mil<<<
>"Who is John Galt?">	          	       << The Pentagon  <<<
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