North American Network Operators Group Date Prev | Date Next | Date Index | Thread Index | Author Index | Historical Re: MTU path discovery and IPSec
On Thu, Dec 04, 2003 at 05:54:42PM -0500, [email protected] wrote: > On Thu, 04 Dec 2003 16:40:45 EST, Joe Maimon <[email protected]> said: > > I was wondering would it not be wiser for fraggers to frag in half > > instead of just the overflow? > > There's 2 cases here: > > 1) This is the final frag on the path - if PMTUD is in use, we want to frag > right at the overflow so the connection can use the max (so if we're fragging > from 1500 down to 1410, they end up with 1410 rather than 750). > > 2) There's an even more restrictive frag further downstream. We frag from 1500 > to 1460, and somebody else frags from 1460 down to 1410. If you frag at overflow, > you end up with a PMTU of 1410. If you fragged it in half, you avoid the second > frag but end up with a PMTU of 750. > > After several dozen packets, the difference between 750 and 1410 will start to become > noticable..... That's not how PMTUD works. If DF is set, you discard the packet and report back with ICMP. If DF is not set, you frag the packet - but that's not PMTUD, because no report ever goes back to the sender. -- Barney Wolff http://www.databus.com/bwresume.pdf I'm available by contract or FT, in the NYC metro area or via the 'Net.
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