Subject: Re: ip reassembly time exceeded?
To: Paul Goyette <>
From: Wiker, Raymond <>
List: current-users
Date: 01/27/1997 15:01:00
Paul Goyette writes:
 > On 27 Jan 1997, Matthias Scheler wrote:
 > > > I heard that this is the minimum length specified for IP packets.
 > > 
 > > No, the minimum length is 40 bytes.
 > 40 bytes is the minimum length of an IP packet, but 576 is the minimum MTU
 > that an IP transport link needs to support.  

	Um. I thought so as well, but according to "Interworking with
TCP/IP, volume 1" by Douglas Comer, the requirement is

"The IP specification states that routers must accept datagrams up to
the maximum of the MTU of networks to which they attach. In addition,
a router must always handle datagrams of up to 576 octets. (Hosts are
also required to accept, and reassemble if necessary, datagrams of at
least 576 octets.)"

	Note that the "datagram size" above is (according to my
understanding) the IP datagram size, not the medium datagram size less
medium headers (which is the MTU.)

	One possible source of conflict is the use of path MTU
discovery; if this process yields an MTU less than what the standard
requires for datagram size, it is possible that some IP stacks may get

	(The path MTU process works by sending IP packets with the "Do
not Fragment" bit set, varying the sizes of the IP packets and
checking what size of packets are successfully delivered to the
destination. Packets bigger than the path MTU result in an ICMP error
indication "Fragmentation needed and DF set".)