On Jul 12, 2007, at 1:38 PM, Stephen Clark wrote:
The MTU is actually defined in reference to a network segment such
as an "ethernet collision domain", and applies to all machines
sending traffic to that segment. If the MTU is really 1280,
nobody else should be sending larger packets, and the drivers
will drop any larger packets they receive and generate the
appropriate ICMP error....
First thanks for responding but thats the problem,
this did't generate an icmp when the packet was dropped.
kernel: rl0: discard oversize frame (ether type 800 flags 3 len
1514 > max
1294)
This message did not result in any icmp packet.
I was running tcpdump looking for them.
Taking a quick look at ether_input() in src/sys/net/if_ethersubr.c
suggests that you are right-- if the incoming packet exceeds the MTU
being set, the input errors count for that interface is incremented,
but no ICMP_UNREACH_NEEDFRAG is generated even if DF flag is set.
You might file a PR and see whether you can get Andre or one of the
other networking gurus interested in fixing this. Or maybe I'll give
it a try myself if I can get some free time.... :-)
--
-Chuck
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