PC Bozo's World bites again (CNN, too)

Vadim Antonov avg at pluris.com
Thu May 28 07:09:35 UTC 1998

On Wed, 27 May 1998, Michael Dillon wrote:
> Most people are changing the MTU to speed up web browsing which is data
> intensive, not interactive. I think Karl's explanation of broken Windows
> TCP/IP stacks is more likely the root cause of the problem.
> But has anyone ever done a proper test of this with sniffers at both the
> client end of the network and the webserver end of the network?

I don't know, but dropped or corrupted large packets is the reliable
of the flow control problems on async lines.  I saw that million times,
that explanation is a lot more plausible than misterious bugs.  The
why broken flow control affects TCP performance is very simple: most
modems have buffers from 2 to 4 kilobytes.  Now, given the large
in speed of dialup line and the serial port, that buffer limits window
to 2-3 large packets, with single packet lost due to modem buffer
every round-trip time.  I.e. the steady-mode packet loss is 15-30%. 
MTU allows window to grow to 6-10 packets, thus reducing steady-mode
loss to 3-5%.

The real answer is: fix the !#@! CTS/RTS, so the buffering occurs in
the host memory. 
(BTW, the recommendation to reduce MTU to increase interactive
performance is
only valid when connection is shared between interactive and
traffic.  This is clearly not the case.)

Who still remembers debugging 4800 bps backbone lines.

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