Win 95 Updates

Microsoft Dial Up Networking

What are MTU and RWIN? And why do I want to change them?

MTU and RWIN are settings used by the dial up networking. They control the size of your send and receive packet sizes. By default, Win95 uses the largest value of MTU possible for the chosen media type. These default values are not the best for 28,800 internet dial up. NOTE: Optimum MTU and RWIN settings differ based on how you access internet. The settings recommended here are for 28.8 dial up connection. If you access the internet through a network connection, using the values given below may actually degrade your performance.

WARNING! Be careful when making changes to your Registry file. If you are not clear on the material presented here, seek help from someone familiar with making changes to the Registry. A bad or blown Registry can bring Windows to it's knees.

MTU and RWIN are hidden in two different places in the Registry. MTU can be set for each protocol-adapter binding; RWIN is set globally.

For MTU, open the Registry to:


Each instance of each protocol you have loaded will have it's own folder. These folders Are labeled 000n where n is the number of the instance. To figure out which 000n is the TCP/IP protocol for your DUN connection, look for the entry called DriverDesc with a value of TCP/IP.

Inside that 000n, create a new string variable called "MaxMTU" and enter your value. 1500 is the default; some terminal servers work better with 1002; lowest you should ever need is 552. In general, use the highest MTU your machine can handle without overruns. The setting that worked the best for me was 576.

For RWIN, open the Registry to:


Create a new string variable called "DefaultRcvWindow" with a value 4 times (MTU + 24). For example, if you use 576 for the MTU your RWIN entry would be 2328.

It might also help to turn off your modem compression features; consult your modem manual, and enter an init string into DUN Modem Advanced Properties\Extra Settings.

Default TTL

While we have you making changes, let's make one more! Default TTL settings in Win95 are set to 32. With this setting in place, you may not reach servers that are more than 32 hops away. With the internet growing like it is, it is becoming common to have more than 32 hops. Setting the DefaultTTL to 128 should help a great deal.

Open regedit and go to:


Create a string variable named "DefaultTTL" with a value of 128.

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