last-hop routers, raising the MTU
smb@research.att.com Sat, 09 December 1989 19:12 UTC
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From: smb@research.att.com
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To: mtudwg
Subject: last-hop routers, raising the MTU
Date: Sat, 09 Dec 1989 12:10:33 -0500
>From: hector!smb
When Jeff proposed that the last-hop router could send back the requisite message, I objected. I no longer consider my objections to be valid. First, I said that the router might not know the host's MTU. It has to, or it wouldn't know how to fragment packets addressed to that host. In fact, theoretically it needs to know that hosts can handle, say, 1500 bytes on an Ethernet-to-Ethernet relay, or it would be obligated to use 576 bytes. Second, I said that what appeared to be the last hop might not be the last hop, if ARP-fakery is going on. Again, this doesn't matter much. Both confused router and the smarter one doing the fakery can send back MTU messages; the originating host should simply believe the lowest MTU reported to it. On another matter, in response to my suggestion that perhaps it wasn't necessary to ever raise an MTU, several people suggested slowing down the rate drastically, say to every 30 minutes. I don't buy that. The programming complexity necessary to reprobe is the same whether the interval is every 30 seconds or every 30 minutes. Then we have to worry about what rate is right for what net, or for the metric as reported by the router, how to make it adaptive (slow-start MTU discovery?), etc. I'd be much happier with a passive mechanism that only reduced the MTU, and never mind trying to increase it later during the connection. --Steve Bellovin
- last-hop routers, raising the MTU smb
- Re: last-hop routers, raising the MTU Noel Chiappa
- Re: last-hop routers, raising the MTU Philippe Prindeville
- Re: last-hop routers, raising the MTU Noel Chiappa
- Re: last-hop routers, raising the MTU Jeffrey Mogul