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Information on 'Tweaking' your TCP stack


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Tweak Myth Debunking

Some other tweak sites are promoting extremely long lists of registry updates, including the DefaultRcvWindow tweak as well. In an effort to dispell the myths, this handy little chart shows you which Tweaks you should be concerned about for Windows operating systems.

W95 = Windows 95 old version, without the Winsock2 update or the DUN1.3 update.
W95n = Windows 95, with the Winsock2 update / DUN1.3 update. Treat as per Windows 98 unless noted.
The other OS versions listed should be self explanatory!

GREEN A real tweak. A real (sometimes huge) performance improvement.
YELLOW A possible tweak. This could be because modem performance boosters have changed the setting from the correct default, or that a special problem can be cured with this tweak.
RED A tweak that should not be made, since it has either no impact on performance, or, the default setting is correct anyway.
Grey indicates this tweak is not applicable to this operating system version.
W98W95nW2kW95WNT
Receive Window
Dramatic speedups in download performance on long latency high speed connections
Default is 8192
make this large enough for maximum download performance
n/aDefault is 17520
make this large enough for maximum download performance
Default is 8192
Maximum is 65535 (DWORD)
Default is 8192
make this large enough for maximum download performance
MTU
MTU should be set to the maximum before fragmentation
Default 1500
Can be set lower
n/aDefault 1500
576 with PTMU-Discovery off. Can be set lower
Default 1500
Can be set lower
Default 1500
576 with PTMU-Discovery off. Can be set lower
RFC1323
Window Scaling, and Time stamp option (two flags in one key, Tcp1323Opts) are improvements to TCP spec
Off by default
With >64k RWIN sizes and ONLY if vtcp.386 has been patched see kb Q239705,Q236926
Off by default
Turning on causes a BUG with some websites (refer kb article 239699)
Off by default (kb article 224829), for windows size >64k
Create Tcp1323Opts as per kb article to turn on
n/aNot supported
NT Does not support large window sizes (as of SP6 anyway) refer kb 199947
RFC2018
Selective Acknowledgements improves throughput on packet lossy lines. If you find telnet becomes slow, the other server may not support Sack, and it should be turned off at your end..
On by default (source, microsoft kb 188307)
Can be turned Off by Creating SackOpts and making it 0 (resource kit help file shows incorrect path)
n/aOn by default (source, microsoft kb 224829)
Can be turned Off by Creating SackOpts and making it 0
n/an/a
MaxDupAcks
Faster retransmission when packet loss is encountered
Defaults to 3 (source, microsoft)
Can be changed with MaxDupAcks
n/aDefaults to 2 (source, microsoft kb 224829)
Can be changed with MaxDupAcks
n/aDefaults to 2 (source, microsoft kb 162179)
Can be changed with MaxDupAcks
Path MTU Discovery
Auto discover MTU size by monitoring fragmentation messages
Defaults to on (source, microsoft kb 158474)
Can be changed to off by using EnablePMTUDiscovery to 0
Defaults to on (source, microsoft kb 120642)
Can be changed to off by using EnablePMTUDiscovery to 0
Defaults to on (source, microsoft kb 120642)
Can be changed to off by set EnablePMTUDiscovery to 0
n/aDefaults to on (source, microsoft kb 120642)
Can be changed to off by using EnablePMTUDiscovery to 0
Black Hole Detection
Discover (rare) 'black hole routers'... a work around, where such routers can cause MTU Discovery and setting to be sub-optimal.
n/an/an/aDefaults to off
Can be changed to on using EnablePMTUBHDetect
Defaults to off
Can be changed to on using EnablePMTUBHDetect
TCPRetries
Windows 3.11 and 95 has a bug that gives up too soon if the remote end is busy. This bug is commonly tickled by 3270 mainframe connectivity.
n/an/an/aDefault is bad
Can be changed by setting MaxDataRetries to 64
n/a