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Reassembled PDUs / Fragmentented TCP packets

Posted on 2006-11-27
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Hi,

I am doing database queries over a relatively slow line and became aware, that only some 1/6th of the available bandwidth is used.

The server is Windows 2000 Server, the database is SQLBase ( please see also this question: http://www.experts-exchange.com/Databases/Q_22073042.html ).

When doing a network trace with ethereal, I noticed that basically all packets sent from the server  are split into fragments, with Ethereal  noting: TCP Segment of a Reassembled PDU. Also, the payload of those packets is really small, usually less than 300 bytes and never more than 600, resulting in a huge number of packets for a small amount of data.

I played with the MTU settings (which didn't show any effect at all), along with the various window sizes and so on. The strange part is:a dump on the server itself shows this behaviour, too. If, on the other side, I just download a file from the server via SMB, everything is fine anf nearly the whole bandwidth is used, though.

I also tried connecting from another server in the same network, but basically it is the same. Lots of PDU segments, just less notible performance impact, because the line is faster.

Any ideas? Is this a problem of SQLBase? One of Windows 2000? Any wrong settings? Thanks for your suggestions.
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Question by:QTCCK
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FixingStuff earned 2000 total points
ID: 18032460
Is your "relatively slow" connection made via VPN or any other encryption methods? VPN and most encryption causes packet fragmentation because of the encapsulation of data packets. It's something to check.
FS-
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Author Comment

by:QTCCK
ID: 18035952
It indeed is a VPN connection, but the troubles basically only occurs with small network packages. I now am looking into TcpDelAckTicks & Message Queuing, to see if it has an effect.
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Author Comment

by:QTCCK
ID: 18043736
Adjusted the TcpDelAckTicks to a lower value, and indeed it had an effect - it slowed it down. Sadly, setting it higher than to the defaut value did not have a positive influence on the performance, but it was same as before.
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Expert Comment

by:FixingStuff
ID: 18058006
Have you performed the basic point to point packet loss and latency tests? I have to assume so, but thought I would ask. Never know if you may be fighting a bad switch port or NIC.
fs-
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Author Comment

by:QTCCK
ID: 18172077
I think I got to live with the fact that it primarily is a SQL base problem. I do not think that much can be done about it. Thanks for your help though.
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