There is a new KB article with a correct procedure for finding the MTU
you require. Do not change your MTU unless this procedure indicates you
need this hack.
"Diagnoses and Treatment of Black Hole Routers (Q159211)" at
http://support.microsoft.com/support/kb/articles/Q159/2/11.ASP. Do not
use other KB articles as they are incorrect.
To change the client MTU for Win2K/XP machines:
1. Run REGEDIT or REGEDT32.
2. Go to
3. Find the branch with the interface ID that corresponds to the NIC
your ICS client uses on the home network. It should contain a
192.168.0.x IP address and a string value named DhcpDomain set to
4. Within this branch, create a new Dword value named MTU and give
it the value as determined by your ping tests.
5. Disable and enable the clients network card and attempt to access
the web resources again.
Windows Registry Editor Version 5.00
; increase the global TCP receive window size for broadband connections
; for Win2K and WinXP to 64240 bytes
; Ethernet TCP MSS is 1500-40=1460 bytes (IPMTU - TCP header)
; set TCP receive window to 64240 bytes (0xFAF0)
; to allow 44 segments to fit in 0xFFFF
AFAIK (I'm still checking), the GlobalMaxTcpWindowSize key is no longer
Kent W. England, MS MVP for WinXP Networking
> I have recently upgraded from win98 to XP. In 98 there were two keys
> that could be tweaked to make make it operate faster, see above. Now,
> my machine, loaded with the latest goodies is limping along at about
> 800kb/s download when it should be at least 2500(?). I cannot find the
> above keys in the XP registry. Do they still exist and if so, what are
> their registry paths? If not, what are their replacement keys and what
> are their paths?