OT: Advice needed on dialup problems - TCP corruption? (windows users)

Wed Jul 10 19:29:50 UTC 2002

At 02:43 PM 7/10/2002 -0400, Jeffrey Wheat wrote:
>         I am seeing a strange problem on my network lately after adding a 
> new terminal server, a Lucent MAX TNT with madd modems in it. The 
> symptoms are that users can connect, they can ping and traceroute without 
> any trouble. Anything that is TCP based however is failing. In some 
> cases, disabling software compression fixes the problem for a short 
> period of time. I am not experienced enough with the MAX TNT units to 
> know how to debug the TCP packets as they come from the user. I am 
> running 10.0.0 TAOS on the unit. Can anyone please offer some advice or 
> suggestions? I am too young to go bald :)

It sounds like Van-Jacobsen TCP header compression is broken in the TAOS 
software release you're running. That would explain why UDP traffic works 
just fine, but TCP does not, since VJ compression only compresses the TCP 
header. VJ header compression is a PPP negotiated option. Almost all 
dial-up clients try to negotiate it by default, but turning it off is 
problematic on the client (as you have discovered), and not feasible when 
you have thousands of clients.

Remedies:

1) Find a version of TAOS where VJ compression is not broken
2) Although I have never done it on a TNT, you can usually configure a 
terminal server to refuse to negotiate VJ header compression. This would be 
a reasonable work-around until you can do (1).
3) Replace the TNT with a Cisco RAS. At the very least you get excellent 
debugging facilities for situations like these and a better connect success 
rate.

Cheers,

Mathew

>