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Re: MTU of the Internet?

  • From: Stephen Sprunk
  • Date: Fri Feb 06 11:40:35 1998

Sez Phil Howard:
> Turn the MTU down, way down, and see how it affects things.  At what point
> in number of connections does the problem happen with MTU=1500 and at what
> point does it happen with MTU=600 and even MTU=200.
> Of course changing MTU isn't the correct solution, nor is changing the
> number of connections.  But these are workarounds that do work, for now.

Changing the number of connections IS the correct answer in this case.
If you are a dialup user at 33.6k (or 56k even), you should be very
acutely aware of how puny your pipe is.  There is absolutely no reason
to expect 64 simultaneously connections to an OC-12 connected server
to behave normally.  HTTP 1.0 is known to be severely flawed in this
respect; it opens large numbers of connections and closes them before
slow-start and congestion avoidance can kick in.

If your primary complaint is interactive response while performing
large downloads, changing the MTU is the correct answer.  This is
simply a matter of the transmission time on large packets.

Does anyone have data to show if any terminal servers or client stacks
will honor TOS bits and/or known interactive port numbers when
ordering packets for transmission across slow links?

> point to do that at some point.  But having read the details on slow start
> as a result of this thread, I find it's not what I proposed, although I
> can now see that it, too, would have been explained as I explained my idea,
> so the explanation was flawed.  I might try to implement my ideas via UDP
> and see what kinds of effects I can really get over real networks of the
> 1990's, understanding that the original design was done in the 1980's and
> before with slow links and slow routers.

I'm sure we'd all like to see a reference implementation of your
ideas.  Keep in mind that one of the primary features of PMTUD is that
it requires no modifications to any hosts or routers, and that it uses
only one protocol (ICMP) which is required to be implemented on all


Stephen Sprunk          "Oops."                 Email: [email protected]
Network Consultant        -Albert Einstein      ICBM:  33.00151N 96.82326W