Bug in -s -S params?

hsalfeld@estec.esa.nl
Fri, 30 Jul 1999 14:41:00 +0100

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Hi Netperf community...,

I am currently experimenting with a satellite loopback scenario and using a
combination of NETPERF and TCPDUMP to evaluate the performance like the
throughput.

As being one of the key parameters in "Long Fat Networks" like satellite
links, I play with the Netperf parameters -s and -S simultanously to change
the RECV/SEND Socket sizes from 2048 to 65536 Bytes in steps of 2kB. I was
wondering why the throughput was always decreasing when going higher than
63488 Bytes for the -s and -S params.... (thus when reaching -s 65536 -S
65536).

I looked onto the traces of TCPDUMP to observe the Window size ("number of
bytes of receive buffer space available"), and I found some inconsistency:
the TCPDUMP Window size is not the same like I set with the -s -S params.! In
the following table (sorry, a bit long, but at least a detailed one!) one can
see the relationship between the NETPERF's -s -S parameters and the Window
size as detected by TCPDUMP:

(See attached file: table.gif)

So what does that mean? Just a misunderstanding on my side of the NETPERF's
Recv/Send _socket_ size parameters -s and -S, or of TCPDUMP's available
_buffer space_ output? Is the _Socket_ size (-s -S params) the same like the
_Window_ size?

Or is NETPERF simply not doing what it should do, namely to set the Window
size?

Sorry for any confusion, but maybe somebody knows an answer to this
phenomenon (if it's one). Any help is appreciated :)

Regards,
Heico
---------------------------------------------------------------------
Heico Salfeld
phone: +31-71-565-3136 mailto:hsalfeld@estec.esa.nl
fax: +31-71-565-4596 http://www.estec.esa.nl
---------------------------------------------------------------------
ESA - ESTEC
european space agency - european space research and technology centre
payload systems division - communication systems section (tos-etc)
Keplerlaan 1 - Postbox 299 - 2200 AG Noordwijk zh - The Netherlands
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