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Date:      Thu, 25 Mar 1999 10:36:01 -0800
From:      Charles Henrich <henrich@flnet.com>
To:        freebsd-hackers@freebsd.org
Subject:   A Question about MTU and ICMP Need to Frag
Message-ID:  <19990325103601.C11010@orbit.flnet.com>

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I have a question.. I have a system with two interfaces fxp0 and fxp1, fxp1 is
the external interface (to the world) fxp0 is the internal interface (net 10).
If I alter the mtu on the external interface to something rediculous like 128
bytes, then connect to sites around the net, as far as I can tell, no ICMP
need to frag packets are being generated (watching with tcpdump), but
everything is working peachy keen.

However, if I alter the mtu on the internal interface.. Connections out to the
internet cause ICMP need to frag packets to be generated.

The question I have is, shouldnt an ICMP need to frag packet be generated in
all cases?  If not, what magic is going on to get the outside world to talk in
the smaller packet size?  Thanks!

-Crh

       Charles Henrich       Manex Visual Effects       henrich@flnet.com

                       http://orbit.flnet.com/~henrich


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