Tuning TCP parameters to maximise throughput - Windows 2003 and 2008

Got 3 servers, A (Win 2008), B (Win 2003), and C (Win 2003) with network timeout issues between them.

Been testing with iperf and using various tcp window sizes.

A to B - works fine -high speed.
A to C  - worksfine - high speed.
B to C - very slow. Increase in TCP windows size in iperf makes difference (But still not as fast as previous 2).

B to A - very very slow. Increase in TCP window size in iperf makes big difference.
C to A - same.
C to B - same.

I can sort of understand why B to C is slow (both are Windows 2003). These have not had TCP parameters tuned at all which I understand is possible on Windows 2003. My plan was to try increasing the TCP window size on these servers.

But B to A is from 2003 to destination of Windows 2008. I thought TCP was all sorted in 2008 and there was nothing to tune?
paul williamsAsked:
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Dirk KotteSECommented:
you have a 1GBit WAN connection, between 3 locations, right?
at every location you have one server?
Which type of WAN do you use (dark fibre, provider MPLS, ...)
what are the latencies (or ping times) from
A to B
A to C
B to C
and the opposite direction?

if the lowness occurs only if WAN connections affected (within the locations the copy from server to client and backwards are fast?) the MTU is a possible problem too.
try reduce the MTU radical (i use MTU of 800 or 1000 for testing)

another possible problem is duplex mismatch. it is typical if one direction is fast the other very slow.
you should select the automatic duplex settings at all ports (servers,switches,routers, ... and VMWare also if used)
with wireshark you should see packet retransmissions...
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paul williamsAuthor Commented:
Ah. Would this by why? 2003 to 2008......

http://support.microsoft.com/kb/983528
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kevinhsiehCommented:
Is it too much to ask to just ditch Windows 2003? It's been around a long time and the end of support is very near. Not exactly a solution, but it's getting time to say goodbye. Networking in the later versions of Windows is greatly improved. I can actually push 1 GBs with the newer versions of Windows.
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pgm554Commented:
Is this over a WAN ,or LAN?
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Dirk KotteSECommented:
hi paul,
we need some more details ...

first - same network segment or routed
second - the question from pgm554: WAN or LAN connections
what do you mean with "works fine -high speed" and "very very slow"

i would suggest to not use iperf at the first steps.
copy a big file from one server to the other and look to the taskmanagers network-load. copy the file from the other server back to your server and look to the taskmanagers throughput.
post the results please.
if the difference from one direction to the other is really big (>50%) you have other problems than windows size. If one direction is OK, i think WS is not your problem. Changing WS is an option to optimize the throughput at WAN links but not the local LAN. Good description and calculation at http://bradhedlund.com/2008/12/19/how-to-calculate-tcp-throughput-for-long-distance-links/

We can search and find these errors if you provide us with many details as possible.
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pgm554Commented:
Here's my goto on tuning older versions of Windows:

http://www.psc.edu/index.php/networking/641-tcp-tune#WindowsXP
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paul williamsAuthor Commented:
Routed.
WAN
Fast is 600mbps, slow is under 10.
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