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Re: TCP and WAN issue

  • From: Marshall Eubanks
  • Date: Wed Mar 28 08:38:37 2007


On Mar 28, 2007, at 5:12 AM, Andre Oppermann wrote:


Marshall Eubanks wrote: 
You might want to look at this classic by Stanislav Shalunov
http://shlang.com/writing/tcp-perf.html

The description on this website is very good.

Disclaimer: I'm a FreeBSD TCP/IP network stack kernel hacker.

To quickly sum up the facts and to dispell some misinformation:

- TCP is limited the delay bandwidth product and the socket buffer sizes.
- for a T3 with 70ms your socket buffer on both endss should be 450-512KB.
- TCP is also limited by the round trip time (RTT).
- if your application is working in a request/reply model no amount of
bandwidth will make a difference. The performance is then entirely
dominated by the RTT. The only solution would be to run multiple
sessions in parallel to fill the available bandwidth.
- Jumbo Frames have definately zero impact on your case as they don't
change any of the limiting parameters and don't make TCP go faster.
There are certain very high-speed and LAN (<5ms) case where it may
make a difference but not here.
- Your problem is not machine or network speed, only tuning.

Change these settings on both ends and reboot once to get better throughput:

[HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\Tcpip \Parameters]
"SackOpts"=dword:0x1 (enable SACK)
"TcpWindowSize"=dword:0x7D000 (512000 Bytes)
"Tcp1323Opts"=dword:0x3 (enable window scaling and timestamps)
"GlobalMaxTcpWindowSize"=dword:0x7D000 (512000 Bytes)

http://www.microsoft.com/technet/network/deploy/depovg/tcpip2k.mspx


And, of course, if you have Ethernet duplex or other mismatch issues anywhere along the
path, performance will be bad.

Regards
Marshall


--
Andre

Marshall
On Mar 27, 2007, at 4:26 PM, Philip Lavine wrote:

To all,

I have an east coast and west coast data center connected with a DS3. I am running into issues with streaming data via TCP and was wondering besides hardware acceleration, is there any options at increasing throughput and maximizing the bandwidth? How can I overcome the TCP stack limitations inherent in Windows (registry tweaks seem to not functions too well)?

Philip






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