Subject: Re: Does window scale solve long fat pipe issue?

From: Mark Allman <[email-masked]>
Date: Thu, 28 Oct 1999 12:01:13 -0400

> I did a few simulations of client-server ftp sessions over
> geostationary on-board ATM processed satellite using OPNET
> simulation tool with almost all default values provided by OPNET
> Modeler6.0, and find that window scale extension described in
> RFC1323 does not provide the throughput great than 1Mbps
> (64Kbytes/500ms) on a 2Mbps link.

Window scaling (and the associated algorithms) does provide for
throughputs much greater than 1 Mbit/sec.  For instance, experiments
conducted here at NASA GRC have seen throughputs in excess of 500
Mbits/sec with Solaris boxes over a GEO hop.  See RFC 2488 for a
discussion and pointers to experiments that show the benefits of the
RFC 1323 extensions.

allman

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From: Eric Travis <[email-masked]>
Date: Fri, 29 Oct 1999 00:51:18 -0400 (EDT)

> (64Kbytes/500ms) on a 2Mbps link.

Large windows *can* make it possible to achieve full link utilization,
but they alone are not sufficient. You need to be transferring enough
data to allow the connection to "come up to speed".

If the OPNET models don't allow you to observe the instantaneous cwnd
of transport connections during a run, then pass *large* amounts of 
data (per connection) and see if the throughput increases with the
transfer size...

You don't mention how large your simulated ftp transfers were, but 
because of slow-start, is it possible that:

   - Your transfers were too small for the TCP connections to
     fully open cwnd to allow for the transmission rate to 
     reach 2Mbps

   - Your transfers *were* large enough to allow the cwnd to
     fully open by the end of the transfers, yet they were
     not large enough to make the cost of slow-start a minimal
     in the throughput calculation.

I'm guessing/hoping that your simulations didn't incur any 
losses over the path (either due to congestion or corruption); 
If there are losses involved, your throughput will suffer
(as you are reducing your cwnd - and thus the transmission
rate in response to a loss).

Eric