Credit where credit is due
mogul (Jeffrey Mogul) Mon, 16 April 1990 18:59 UTC
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From: mogul (Jeffrey Mogul)
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Date: 16 Apr 1990 1059-PST (Monday)
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Subject: Credit where credit is due
In the process of editing Steve's draft proposal, I went through the material I used 2 years ago when writing "Fragmentation Considered Harmful", and found this message: Return-Path: <tcp-ip-RELAY@SRI-NIC.ARPA> Received: from SRI-NIC.ARPA by navajo.stanford.edu with TCP; Sun, 24 May 87 02:42:34 PDT Received: from decwrl.dec.com by SRI-NIC.ARPA with TCP; Sun 24 May 87 00:15:43-PDT Received: by decwrl.dec.com (5.54.3/4.7.34) id AA16654; Sun, 24 May 87 00:15:45 PDT Received: by imagen.uucp (5.51/Imagen-1.1) id AA04028; Sat, 23 May 87 22:20:55 PDT Return-Path: <geof> Received: by apolling.imagen.uucp (4.12/Imagen-1.1) id AA01407; Sat, 23 May 87 22:17:05 pdt Date: Sat, 23 May 87 22:17:05 pdt From: imagen!apolling!geof@decwrl.DEC.COM (Geof Cooper) Message-Id: <8705240517.AA01407@apolling.imagen.uucp> To: tcp-ip@sri-nic.ARPA Reply-To: imagen!geof@decwrl.DEC.COM Phone: (408) 986-9400 (work) Postal-Address: IMAGEN, 2650 San Thomas Expressway, Santa Clara, CA 95052 Subject: Re: IP Datagram sizes I like the idea of an IP-level solution to the fragmentation problem since it has application to UDP protocols (I know that none that exist today could use it, but that's no excuse for ignoring UDP). Isn't there a destination unreachable message with the reason being "can't fragment and had to" (sorry my ICMP spec is at the office)? If not, we could certainly add one. In that case, the idea is to always send TCP packets with the "don't fragment" bit set. Use the scheme suggested that keeps track of MTU's in the routing cache. Update the cache based on DU's received (decrease the MTU a bit and try again) -- time out the entry on a long timer to be able to detect new routes. The obvious improvement is to have the ICMP message also include the MTU restriction that is appropriate -- that requires changing ICMP, of course, but it would probably be a good idea.
- Credit where credit is due Jeffrey Mogul