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Re: (ICMP attacks against TCP) (was Re: HPSBUX01137 SSRT5954 rev.4
From: Darren Reed <avalon () caligula anu edu au>
Date: Thu, 21 Jul 2005 08:25:49 +1000 (Australia/ACT)

In some mail from Fernando Gont, sie said:
The IPv4 minimum MTU is 68, and not 576. If you blindly send packets larger 
than 68 with the DF bit set, in the case there's an intermmediate with an 
MTU lower that 576, the connection will stall.

And I think you can safely say that if you see any packets trying to
indicate that the MTU of a link is "68" then you should ignore it.

This came up some years ago in discussion about ... hmm... I think it
was what made a good (or sensible) "fragmentation required" ICMP message.

Ignoring quenches as a problem, if you try to send 10K of data to a
box that has an MTU of 68, 1200+ packets are required vs less than 10
for an ethernet MTU.  The problem is 1200 packets require a lot more
system time to send than 6 or 7.  A different kind of DoS attack.

I think it is reasonable to say anyone trying to advertise an MTU less
than 576 has nefarious purposes in mind.

oh, IPv6 guarantees a min. MTU of 1280.

Lets just stop using IPv4 already.

Darren


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