TCP retransmission and TCP Dup ACKs

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Sep 29th, 2010
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I have a windows 2003 server (192.168.30.1) behind a Linksys RV042 (192.168.30.254) router connected to a Comcast cable box.

I see some TCP retransmission and TCP Dup ACKs in wireshark when I access websites form that server.

What information do I need from the packet that would help me determine if the problem is with the router configuration or network card settings or cable modem perhaps?

Thanks

Do i need to adjust TCP window size?

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Ashley Georgeson Wed, 09/29/2010 - 14:13
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Hi Eugene,


I would say that a certain level of TCP retransmissions and duplicate acks are expected, especially over the internet with multiple routing paths and non-reliable delivery.


Of course, we do not want to see too high a proportion of these!  What overall rate of retransmissions do you have in this tcp stream?


Adjusting the window size would need to be done on both end-points - this would effectively increase the data pipe between the hosts before they require acknowledgement so in effect you would be able to increase the tcp throughput.  I'm not sure if this is a good idea over the internet however, as reliability is never guaranteed so you could see more lost acknowledgements but for larger windows, hence higher payload retransmissions required and degraded performance.


But you'll never know till you try!


Kind regards, Ash.

net tech Thu, 09/30/2010 - 06:23
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Ash,


Out of 8453 captured packets 27 are retransmissions and 355 are duplicate acks. Which comes out do be less than 1%. Do you know what would be the acceptable percentage of errors on the WAN connection?

Thanks


p.s. Capture is attached to my 1st post

Ashley Georgeson Thu, 09/30/2010 - 07:04
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Hi Eugene,


I have often asked myself this question - and it seems logical that the answer may be different applications have different levels of tolerance for the level of TCP retransmissions.  So there may not be any hard and fast acceptable threshold for retransmits (that I can find), I would be concerned at anything above 5-10% for sure!


See this post over on ethereal.com, discussing the same thing:


http://www.ethereal.com/lists/ethereal-users/200601/msg00166.html


So, I would seek to minimise the retransmissions where possible, but only if I have a cast-iron reason to do so.  Else my energies may be better spent elsewhere!


Regards, Ash.

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