mailing list archives
Re: (ICMP attacks against TCP) (was Re: HPSBUX01137 SSRT5954 rev.4
From: Fernando Gont <fernando () frh utn edu ar>
Date: Wed, 20 Jul 2005 19:35:55 -0300
At 07:25 p.m. 20/07/2005, Darren Reed wrote:
In some mail from Fernando Gont, sie said:
> The IPv4 minimum MTU is 68, and not 576. If you blindly send packets
> 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.
Yes. But what about 296?
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.
That of "more system time" required was listed as one of the effects of the
PMTUD attack in one of the e-mails I sent today.
Not sure what you are saying about ICMP Source Quenches....
I think it is reasonable to say anyone trying to advertise an MTU less
than 576 has nefarious purposes in mind.
There are still some radio links with MTUs of 296 bytes.