What's your favorite MTU?
fab%saturn.ACC.COM@salt.acc.com (Fred Bohle acc_gnsc) Fri, 13 April 1990 22:16 UTC
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Date: Fri, 13 Apr 1990 18:15:54 -0500
From: fab%saturn.ACC.COM@salt.acc.com
Message-Id: <9004132315.AA02188@saturn.acc.com>
To: mtudwg
Subject: What's your favorite MTU?
After reviewing your notes I have the following comments: 1. How about a simple binary search to determine the MTU size? Looking at the list of real MTU sizes, they are very close to a fit to a simple halving to come up with an MTU size which will not fragment, e.g.: 64000 -> 32000 -> 16000 -> 8000 -> 4000 -> 2000 ->1000 -> 500 -> 250 -> 125 -> 62 This converges in the above sequence in 10 steps for the worst case, Hyperchannel to an undefined minimum MTU network. The starting point would not be 64000 every time, but the number less than the LOCAL network MTU value. The values along the way are not too bad except for FDDI and Ethernet. This is solved in the following points. 2. One objection to a simple convergence on a value for minimum MTU was the lack of notification when a larger MTU became available (due to a gateway coming back on line somewhere). The discussion on this list has lost track of this line of thinking. I suggest "probing" for a larger MTU after some amount of time/data has passed. In TCP a measure based on round trip times or some number of windows seems reasonable. Probably multiples of those, like maybe 100 RTT's or 10 windows. (Suggestions anyone?) 3. Probing would continue the binary search, only in an upward direction, remembering the last value which failed, and the last value which worked without fragmentation. Average them to get the size for the probe packet. We could do something tricky here to avoid holding up the data transfer for too long. Send the probe packet with "Don't Fragment" set, and if we get an ICMP Can't Fragment message, retransmit with DF turned off. 4. Continuing the binary search would converge on Ethernet numbers: 1000 -> 1500 ( which works for IP-E) 1500 -> 1250 ( if IP-IEEE 802.3, until the next probe) Converging on FDDI takes longer: 4000 -> 6000 ( which fragments) 6000 -> 5000 ( which also fragments) 5000 -> 4500 ( which also fragments) 4500 -> 4250 ( which works until the next probe) 5. Doing all this injects one extra packet every 10 (or whatever) windows. With the suggested numbers, each pleteau is visited in short order. Some bandwidth is unused until a probe sequence finds it. With binary searching, a value which does not fragment is found in typically 3 RTT's, maximum 10 RTT's. 6. When a gateway comes back up, the probe sequence will discover some of the unused bandwidth. To recover it all, we might try the last number which fragmented again, and resume our binary search if it still fragments. If it does not fragment, increase it some more (suggest percentages here), maybe MTU * 1.25?, MTU * 1.50, or even MTU * 2? Well, it has been a long day, and I can't think any more. Let me know where we take this idea from here. I still think we are on the right track with the DF bit, since it does not need a new bit in the IP header. Just having the new format of the ICMP message seems to do it. Fred ------------------------------------------------------------------------ Fred Bohle EMAIL: fab@saturn.acc.com Interlink Computer Sciences AT&T : 301-290-8100 10220 Old Columbia Road Columbia, MD 21046 ------------------------------------------------------------------------
- What's your favorite MTU? Jeffrey Mogul
- Re: What's your favorite MTU? Art Berggreen
- Re: What's your favorite MTU? Jeffrey Mogul
- Re: What's your favorite MTU? Drew Daniel Perkins
- Re: What's your favorite MTU? Drew Daniel Perkins
- What's your favorite MTU? Fred Bohle acc_gnsc
- Re: What's your favorite MTU? Jeffrey Mogul
- Re: What's your favorite MTU? Drew Daniel Perkins