MTU of the Internet?

Wed Feb 4 17:51:44 UTC 1998

On Wed, 4 Feb 1998, Perry E. Metzger wrote:

> > Several people have noted to the Microsoft Support and Product groups that
> > they want the Windows 95 PPP MTU to be set to 576 (down from 1500).  this
> > change is in Windows 98.

<snip>

> accept a large MTU, no matter what Windows would like. The entire
> story sounds, to say the least, fishy.

I think what is really going on is that people tweaking on the MTU setting
have discovered that for some unknown reason 576 just plain works better
over a dialup PPP connection than ~1500 or any other value for that
matter.

My guess would be that it is in some way related to the packet latency
generated by clocking in 1500 bytes over a ppp link (~500 ms PLUS the
V.whatever overhead)  A ~500 byte packet would be more like ~166 ms.  I
just did a real-world check of these numbers, and they seem pretty close
to reality for a 28.8 connection.   They're a little high for a 33.6.
(real 33.6 numbers were 250 (total clock+v.?) and 412 for 500 and 1500
respectively)

Now it's been a while since I looked at latency vs transfer rates, so
maybe someone who works on this on an everyday basis would like to comment
on what ~200 more ms of latency on a 28.8 link would do to throughput
end-to-end across the net (totals of something like 350 and 512 ms
end-to-end).

I'm pretty certain though, that the common "myth" that 576 works better
because of end-to-end MTU's is just that - a myth.   My network is all
1500 or better - not sure if I've EVER seen anything less than 1500 or
not in the last few years, but I doubt it.

- Forrest W. Christian (forrestc at imach.com) 
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