Path: utzoo!utgpu!jarvis.csri.toronto.edu!mailrus!tut.cis.ohio-state.edu!ucbvax!PESCADERO.STANFORD.EDU!deering
From: deering@PESCADERO.STANFORD.EDU (Steve Deering)
Newsgroups: comp.protocols.tcp-ip
Subject: Re: Re-fragmenting IP Datagrams
Message-ID: <89.02.23.1432.658@pescadero.stanford.edu>
Date: 23 Feb 89 22:32:00 GMT
Sender: daemon@ucbvax.BERKELEY.EDU
Organization: The Internet
Lines: 21

> I believe the specifications also require all networks to be able to
> forward a 576 octet datagram without fragmentation.

That's incorrect, Guru.  From page 25 of the IP spec (RFC 791):

	Every internet module must be able to forward a datagram of
	68 octets without further fragmentation.  This is because an
	internet header may 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.

Therefore, every network must be able to deliver 68 bytes of IP in one
piece.  This is still too long for the silly ATM stuff, but as Craig
pointed out, transparent fragmentation and reassembly below the IP
level is allowed by the IP architecture (and recommended for networks
with ridiculously small MTUs).

Steve