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TCP and mobile IP

Steinar H. Gunderson, sesse, has written an interesting article about TCP performance. I didn't find your blog's comment function, so I am commenting with a trackback. (note: which didn't work either, "The auto-discovered trackback URI does not match our target URI")

I frequently use mobile internet, using various of the German GSM/UMTS network operators, out of a moving train. As you have written, this frequently causes packet loss which is not only not caused by congestion, but sends the congestion avoidance algorithms on a false path.

For example, when the train passes through the 3575 m long Distelrasentunnel between Frankfurt and Fulda, my network link is broken for like two minutes. Passing through other parts of Germany sometimes gives me a ping response of hundreds of thousands of microseconds by virtue of the rather huge send buffer the UMTS equipment has.

In these circumstances, ssh sessions frequently take tens of minutes to notice that the network is back before the session is useable again. Frequently, it doesn't come up again before an hour has passed. And I have not found a way to work myself around this. Can you explain what's happening here, and do you have any ideas to solve the issue?

help needed for ATM support in ifupdown-scripts-zg2

I haven't been using ATM on Linux for some six years now. I neither have access to an ATM network any more nor do I have ATM hardware any more. Therefore, I plan to remove ATM support from ifupdown-scripts-zg2 in the next release which will be done in the next few weeks.

If anybody does still use ATM on Linux in conjunction with my scripts, you might want to offer help with the package if you want to have continued ATM support in ifdown-scripts-zg2. I cannot test the code any more and therefore cannot maintain it in the future.

Weird networking MTU issue

Given a simple, switched LAN with Debian Linux (sid, iputils-ping) and Microsoft Windows XP (Service Pack 2, with the Windows Firewall disabled). A user complains about the network being slow (meaning his Windows notebook). I quickly find out that I can ping the box from my Linux notebook if my -s parameter is < 1393 or > 1472. If it's between 1393 and 1472, no replies are received.

I spend the next hour with debugging this not so interesting phenomenon.

Continue reading "Weird networking MTU issue"