full-disclosure - Re: (ICMP attacks against TCP) (was Re: HPSBUX01137 SSRT5954 rev.4

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Date: Wed Jul 20 23:40:51 2005
From: avalon at caligula.anu.edu.au (Darren Reed)
Subject: Re: (ICMP attacks against TCP) (was Re:
	HPSBUX01137 SSRT5954 rev.4

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