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As WAN bandwidths increase default network settings in older operating systems are less and less able to efficiently use the network.

A description of the problem with high bandwidth-delay product networks (also known as the long fat pipe problem) can be found here: http://www.psc.edu/networking/projects/tcptune/

Recent Linux releases do a good job of automatically enabling large windows and selective ACKs allowing better performance out of the box; however I have not seen any data on how recent Windows (or OSX) releases perform out of the box.

Does anyone know what the defaults for enhancements such as receive window size, window scaling and selective ACKs are in Windows 7?

As a secondary point of interest I would also like to know how Vista and recent OSX releases behave.

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2 Answers 2

Vista at least uses TCP Auto-Tuning by default, which scales the receive window dynamically. This has to be turned off manually if the user desires to tweak it manually.

I don't know how W7 does it, but I would expect it to behave the same. Don't know how the support is for jumbo frames, etc.

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up vote 2 down vote accepted

It seems that both Vista and Windows 7 ship with TCP Autotuning enabled and use a Microsoft adjustment to TCP congestion detection / avoidance algorithms called "Compound TCP (CTCP)

However it also appears that a lot of people have had problems with firewalls that don't support window scaling correctly and the recommendation is to turn it off (e.g. here and here).

The conclusion I am taking away from this is that for my current project (an evaluation of WAN Optimisation hardware) I will need to make sure I compare the improvement given by the hardware doing TCP acceleration compared to any performance improvement given by simply upgrading to Win7.

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