Re: Another proposal to think about
Steve Deering <deering@pescadero.stanford.edu> Fri, 24 November 1989 01:41 UTC
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Date: Thu, 23 Nov 1989 17:05:00 -0000
From: Steve Deering <deering@pescadero.stanford.edu>
Subject: Re: Another proposal to think about
To: Philippe Prindeville <philipp@gipsi.gipsi.fr>
Cc: mtudwg
Message-Id: <89/11/23
In-Reply-To: Philippe Prindeville's message of Thu, 23 Nov 89 152708 -0100
Philip writes: > ...if the packet was a jumbogram and more than 2 derived packets > resulted, I think that any fragment with the MF-bit set (ie. all > but the tail) should be usable [as an indicator of the path MTU], > since they might arrive out of order. No, you don't want to do that. Imagine this scenario: [source]------->[gateway]------->[gateway]------->[destination] MTU MTU MTU 600 400 300 The source sends a 600 byte packet. At the first gateway, it is fragmented into 400 + 200. At the second gateway, they become 300 + 100 + 200. If the 100-byte fragment should arrive first at the destination, it should *not* be used for MTU determination. Only fragment 0 should be assumed to be the size of the path MTU, regardless of the order of reception. (Think of the initial fragment being trimmed at various gateways, by arbitrary amounts, until it equals the path MTU. The other fragments are just the "trimmings".) Even when packets are reordered, you expect fragment 0 to show up eventually (very soon, actually), so it's OK to wait for fragment 0 for MTU determination. Steve
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