Subject: How to change the Win2k MTU value of a PPP over a dial-up (serial) interface

From: "Mohammad, Alimuddin" <[email-masked]>
Date: Thu, 31 Aug 2000 14:13:46 -0700

Q: How to change the Win2k MTU value of a PPP over a dial-up (serial) interface?

I need to change the TCP frame size to 536 or the MTU size to 576 to validate a simulation model.  
With Windows2000, changing the registry KEY_LOCAL_MACHINE\System\ccs\Services\tcpip\Parameters\Interface  MTU value = 576 on the Ethernet interface, worked. However, this did not work for interfaces for an internal modem (Viking's v.90 ISA Controllerless modem) nor for an external modem (USRobatics' Courier v.Everything).   The TCP MTU size over the network captured by the netmonitor  were still 1500.   I also tried using the key  HKEY_LOCAL_MACHINE\System\ccs\Services\NdisWan\Parameters  IPMTU value=576, it still did not work.  I have the PMTU discovery  turned off even though there was no router/packet-cell switch in between the two modems.  I know MTU = 576 for PPP over a dial-up link is possible because we do have a benchmark log file that indicated the TCP frame size was 536 (i.e., MTU=576) using an unknown internal modem. 

Thanks.

---Alim