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From: hwb@MCR.UMICH.EDU (Hans-Werner Braun)
Newsgroups: comp.protocols.tcp-ip
Subject: Re: IP Datagram sizes
Message-ID: <8705241321.AA05020@MCR.UMICH.EDU>
Date: Sun, 24-May-87 09:21:55 EDT
Article-I.D.: MCR.8705241321.AA05020
Posted: Sun May 24 09:21:55 1987
Date-Received: Sun, 24-May-87 19:42:04 EDT
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I don't understand what this whole fuzz about messages to help negotiating
the MSS is all about.

First of all all the assumptions that the paths are symmetric in both
directions are not valid any more, in particular with the NSFNET which is
now running since almost a year and the upcoming networks from other
agencies (like NASA) and may be even the frequently quoted Interagency
Research Internet. The previous tree structure of the Internet is
certainly overtaken by events by now, or at least not guaranteed any
more. All new schemes we come up with have to survive in a real meshed
net of networks. Most if not all I have heard here so far assumes that
there are symmetric paths.

Second, and as someone else has pointed out before, we only have influence
on the MSS in the first packet exchange, i.e., as seen from the host. Any
extension to negotiating the MSS otherwise is therefore non-trivial and
needs to be well architectured, including that all the host implementations
will need to be changed.

Third, the Berkeley folks have changed their MSS attitude considerably with
the version 4.3bsd. The assumption now is to use local network sizes if
you can be reasonably sure that the packets stay on the local physical
network, and to use the only at least somewhat guaranteed maximum size
of 576 bytes otherwise. This strikes me as an excellent idea.

What are we really talking about? Most of what we are discussing implies
the difference from 576 bytes to 1500 bytes, i.e., the maximum record
size on an Ethernet. But 1500 bytes is less then three times the 576 bytes.
In the longer run, i.e., a very few years, what we REALLY need are much
larger packets then 1500 bytes. This will become imperative with the
expected appearance of very high speed networks. I cannot help myself
thinking that a reasonable thing to do for today, supposing you want to
reach other then your local net, is to rather stick with the 576 byte
limit (a limit that is spelled out all over the place) and rather design
future networks which allow at least 20K or 40K packets on very high speed
networks which might run at multiple hundreds of megabits per second or
higher. Even if the local speeds are much lower then this, there could be
a higher speed piece in the middle. These short packets are a in fact real 
problem already at much lower speeds, and they are killing the gateways
because of the overhead they impose.

	-- Hans-Wermeon