Re: Simulating smaller MTU? ie sending small packets.

On Wed, 17 Aug 2005, Ed W wrote:

> > This is probably a moot venture. The smaller the packets are, the
> > lower the overall throughput is going to be. This is due to the
> > fact that TCP packet have to be acknowledged.
>
> Be careful with your generalisation. The point of my experiment is
> to test an unreliable (and very slow) satellite network to determine
> whether faster speed would be achieved using smaller MTU due to less
> retranmissions. 1500 bytes represents up to 7 seconds of
> transmission time...

Yes, but the acknowledgement doesn't have to be for every individual
packet. Check the "window" parameter. You may however need some very
large buffers if you hope to improve performance. One sees a similar
effect without the satellite, if trying to get good bulk data
throughtput on a transatlantic cable link: despite having at least
1Gbit/sec paths at all points between the hosts at each end, and the
hosts themselves being adequate to the purpose, the throughput looks
quite miserable unless some serious tuning of the TCP parameters is
done.

However, this would be better explored on a networking group, I think,
than right here on c.l.p.misc.

And google for * tcp tuning throughput * and similar combinations of
terms. Anything recent-ish which comes back with LBL.gov and/or
internet2 in the URL is likely to be worth a look.

> I'm not sure I can see how window size affects things, but it's
> interesting to see that I can influence it on a per connection
> basis?

The acknowledgements are serial numbered as to which packets they
relate to, so you can be acknowledging a packet which was quite some
time back while you have all the intervening packets "up the spout" or
in transit, at least for the major part of the transfer (at the ends
of course it sorts itself out).

hope this helps.
.

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