bugtraq - Re: (ICMP attacks against TCP) (was Re: HPSBUX01137 SSRT5954 rev.4 - HP-UX TCP/IP Remote Denial of Service (DoS))

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Message-ID: <6.1.2.0.0.20050721193634.02ba2a20@pop.informatik.uni-bremen.de>
Date: Thu, 21 Jul 2005 19:36:41 +0200
From: Dennis Lubert <plasmahh@...ormatik.uni-bremen.de>
To: bugtraq@...urityfocus.com
Cc: Fernando Gont <fernando@....utn.edu.ar>
Subject: Re: (ICMP attacks against TCP) (was Re: HPSBUX01137 SSRT5954
  rev.4 - HP-UX TCP/IP Remote Denial of Service (DoS))


At 00:09 20.07.2005, Fernando Gont wrote:

>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.
>
>576 is the minimum reassembly buffer size. That is the minimum packet size 
>every *end-system* should be able to reassemble, and NOT the minimum 
>packet size that can get to destination without fragmentation.

To be completely correct
<quote RFC 791>
Every internet module must be able to forward a datagram of 68 octets 
without further fragmentation. This is because an internet header my be up 
to 60 octets, and the minimum fragment is 8 octets.
Every internet destination must be able to receive a datagram of 576 octets 
either in one piece or in fragments to be reassembled.
</quote>

So 576 is the minimum packet size you can get to a destination without 
fragmentation


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