Re: won't talk to 2.4.0-test7 system

From: Matti Aarnio (
Date: Sat Sep 02 2000 - 09:59:30 EST

On Sat, Sep 02, 2000 at 04:12:04PM +0200, Elmer Joandi wrote:
> There is another big problem like that...
> tunnels actually do not work on todays real internet...
> MTU 1500 is so much a standard that it starts killing tunnels.
> MTU < 1500 is not a working solution today thanks to (mostly
> linux based ? ) broken firewalls

        It is due to knee-jerk ICMP blocking settings at firewalls/routers.

        From personal experience I can say that even cisco router training
        includes example of: "block ALL of ICMP", which of course makes
        TCP PMTU discovery non-functional.
        (That was 3 years ago when PMTU discovery was a new thing, but still..)

        Another issue are still existing SLIP links with mismatching
        link MTUs at each side.

> elmer.

/Matti Aarnio
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