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
indicator
of the flow control problems on async lines. I saw that million times,
and
that explanation is a lot more plausible than misterious bugs. The
reason
why broken flow control affects TCP performance is very simple: most
dialup
modems have buffers from 2 to 4 kilobytes. Now, given the large
disparity
in speed of dialup line and the serial port, that buffer limits window
size
to 2-3 large packets, with single packet lost due to modem buffer
overflow
every round-trip time. I.e. the steady-mode packet loss is 15-30%.
Reducing
MTU allows window to grow to 6-10 packets, thus reducing steady-mode
packet
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
non-interactive
traffic. This is clearly not the case.)
--vadim
Who still remembers debugging 4800 bps backbone lines.
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