Re: How to use an IP-header bit for Path MTU discovery.
mogul (Jeffrey Mogul) Mon, 19 February 1990 21:24 UTC
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From: mogul (Jeffrey Mogul)
Message-Id: <9002192124.AA16738@acetes.pa.dec.com>
Date: 19 Feb 1990 1324-PST (Monday)
To: "John M. Wobus" <JMWOBUS%SUVM.BITNET@CORNELLC.cit.cornell.edu>
Cc: mtudwg
Subject: Re: How to use an IP-header bit for Path MTU discovery.
In-Reply-To: "John M. Wobus" <JMWOBUS%SUVM.BITNET@CORNELLC.cit.cornell.edu> /
Mon, 19 Feb 90 15:24:34 EST. <9002192029.AA13003@decwrl.dec.com>
Each host-route entry would include a flag meaning "the last oversized packet I received from this recipient was NOT fragmented". It should be turned on when the host route is created. A host would simply copy this bit into all the IP packets that it sends to the other host and would start out with its favorite large MTU. Recipients of this bit use it by simply switching to a smaller MTU (576) if the bit is clear. If they are talking to a host without the feature, they simply always switch down. This is clever, and (as far as I know) novel. It has some potential problems, including the ones you mentioned, but especially the problem common to all the RF-bit schemes: how many routers and end hosts will drop packets with the bit set? It would also break against those PCs that cannot reassemble fragments; at least the "handshaking RF" scheme solves this issue. I tend towards liking this proposal for the simplicity of the "two candidate MTU" approach; we already have some experience with this kind of behaviour (cf. the "subnetsarelocal" flag in some versions of BSD and BSD-derived systems). Basically, if one is forced to think of how many rational possibilities there are for the sender's segment size, one can easily come up with 2: "576" and the first-hop-net MTU. Coming up with a third choice is only necessary if we believe that it is too far from optimal to be stuck with these two choices. -Jeff
- How to use an IP-header bit for Path MTU discover… John M. Wobus
- Re: How to use an IP-header bit for Path MTU disc… Jeffrey Mogul
- Re: How to use an IP-header bit for Path MTU disc… jrd
- Re: How to use an IP-header bit for Path MTU disc… Philippe Prindeville
- Re: How to use an IP-header bit for Path MTU disc… Philippe Prindeville
- Re: How to use an IP-header bit for Path MTU disc… Philippe Prindeville
- Re: How to use an IP-header bit for Path MTU disc… Fred Bohle acc_gnsc
- Re: How to use an IP-header bit for Path MTU disc… Steve Deering
- Re: How to use an IP-header bit for Path MTU disc… Philippe Prindeville
- Re: How to use an IP-header bit for Path MTU disc… Philippe Prindeville