[nycbug-talk] TCP tuning between FreeBSD and Vista

Jonathan jonathan at kc8onw.net
Sun Nov 2 14:43:54 EST 2008

Alex Pilosov wrote:
> On Sun, 2 Nov 2008, Jonathan wrote:
>> I've been playing with it for two days and I can't for the life of me
>> get more than 7% utilization of a gigabit link between my laptop and my
>> file server.  I've gone through man tuning as well as a great deal of
>> Google searching and here are the settings I've tried tweaking with
>> their current values.  My current testing is transmitting from FreeBSD
>> to Vista so I've not messed with receive specific settings on BSD or the
>> transmit specific on Vista.
> 99% chance that either
> a) you have a duplex issue (check netstat -i)
> b) it's not gigabit.

Back to back gigabit with both interfaces autodetecting at 1Gbit full
duplex and no errors showing in netstat, no lost packets in the
wireshark trace either.

I did just find this though which is interesting:
"""There is no way to adjust the default TCP buffer in Vista, which is
64 KB. Also, the Windows Vista autotuning algorithm is not used unless
the RTT is greater than 1 ms, so single streamTCP will be throttled on a
LAN by this small default TCP buffer. """

With the systems back to back (no switch even) the RTT is much less than
1ms which makes me think Vista is using a default unscaled TCP window if
this is correct and given that when I force autotuning to disabled I get
the exact same throughput seems rather likely to be correct :(

[a bit later]
I tried adding a switch to the link but as I expected it didn't add
enough latency or make any other difference in the transmit rate.

Jonathan Stewart

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