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By optimizing the MaxMTU & MaxMSS settings along with the RWIN and TTL settings you can increase dialup modem and broadband connection reliability and performance by as much as 200%.

What is the MTU and MSS?

MTU stands for Maximum Transmission Unit and in basic terms, it defines the maximum size of a packet that can be transferred in one frame over a network.

MSS or Maximum Segment Size defines how much actual data can put in that packet, and it is therefore a marginally smaller value than MTU to allow for packet addressing and other information.

Why do the default values need to be changed?

By default Windows hard codes the value of MTU to equal 1500 bytes. This value is adequate for normal LAN networking, but when your connecting to the Internet with a modem this value is too high. This can show itself in different ways, the most noticeable affect is less than peak performance.

By adjusting this value to more closely match the requirements of a modem Internet connection huge performance gains can be achieved.

Below are the instructions on how to modify these values for yourself. Please remember though, modifying the Windows registry is not for the faint hearted. Always remember to back it up first!

1. To change the settings you first have to find the details of your Dial-Up Adapter. Open your registry and find the key [HKEY_LOCAL_MACHINE\Enum\Root\Net] and within there will be one or more sub-keys such as 0000, 0001 ... 000x
   
   
2. Open each sub-key and find the one that contains the value of 'DeviceDesc' equal to 'Dial-Up Adapter'.
   
   
3. Within that key there will also be another sub-key named 'Bindings'. Open the 'Bindings' sub-key and you will find a string name like 'MSTCP\0000'. Take note of the four digit number after 'MSTCP' (in this case 0000) and use it in the next step.
   
   
4. Now go to the key [HKEY_LOCAL_MACHINE\Enum\Network\MSTCP] and expand the sub-key with the same four digit number as from the previous step.
   
   
5. In this key you will find a value called 'Driver' and it will be equal to something similar to 'NetTrans\0002'. Take note of this four digit number (in this case 0002) and use it in the next step.
   
   
6. Now open the key [HKEY_LOCAL_MACHINE\System\CurrentControlSet\Services\Class\NetTrans] below there will be one or more sub-key's. Expand the key with the name equal to the four digit number from the previous step.
   
   
7. Within this key create two new string value named 'MaxMTU' and 'MaxMSS' and set them to the values from the list below. Note: MaxMSS is usually equal to MaxMTU - 40 (the standard IP header size).
   • Dialup Modem
     MaxMTU = "576", MaxMSS = "536"
   • Broadband Cable or DSL
     MaxMTU = "1500", MaxMSS = "1460"
   • Broadband (with PPPOE)
     MaxMTU = "1492", MaxMSS = "1452"

Note: In some versions of the Windows TCP/IP stack the MaxMSS value may be unnecessary but it is included here for completeness.

		(Default)	REG_SZ	(value not set)
		MaxMTU	REG_SZ	"576"
		MaxMSS	REG_SZ	"536"

	HKEY_LOCAL_MACHINE\System\CurrentControlSet\Services\Class\NetTrans

Related Tweaks

• Cable Modem and DSL Speed Tweak to Increase Performance (Windows 95/98/Me)
• Optimize the DefaultRcvWindow & DefaultTTL Settings (Windows 95/98/Me)
• Change the Maximum Transmission Unit (MTU) Size (Windows NT/2000/XP)

Disclaimer: Modifying the registry can cause serious problems that may require you to reinstall your operating system. We cannot guarantee that problems resulting from modifications to the registry can be solved. Use the information provided at your own risk.