IDG logo

Advertise with InfoWorld

SiteMap News Test Center Opinions Forums Careers Stock Quote Subject Indexes About Us Search Subscribe Home [Window Manager]

July 21, 1997

Double your Net download speed with MaxMTU

Windows 95 suffers from default settings that can severely hinder your Internet download speeds, according to Internet users who have experimented with their configurations.

But if you know the secret, it's easy to change those default settings. Scores of Windows 95 users have reported 50-percent cuts in the time spent waiting for downloads from some Internet sites.

Extensive research on this problem has been performed by NetPro NorthWest, a consulting service based in Portland, Ore. Its Web site at describes the behavior in dozens of pages of technical prose.

The following is my interpretation of the problem and its potential solution.

Data on the Internet is sent in chunks called packets. The Internet standard packet size is 576 bytes (including 40 bytes for address information). This packet size is known as the Maximum Transmission Unit, or MTU. The 536 bytes of data remaining in the packet after the address information is subtracted are known as the Maximum Segment Size (MSS).

Unfortunately, the default setting for Windows 95 Dial-Up Networking (using the Winsock.dll file provided by Microsoft) is an MTU of 1,500 bytes.

This setting is appropriate for Internet connections that are made through LANs, many of which have a default MTU value of 1,500 bytes. But if you are connecting to the Internet though a dial-up connection (a modem), your download times can be slowed as packets are re-sent to accommodate the mismatched packet sizes.

The following steps provide a fix for this situation. You should try this if you use Dial-Up Networking and your Internet service provider gives you a Point to Point Protocol, or PPP, connection (as opposed to a less efficient SLIP connection). The procedure is not recommended if you have more than one network installed.

  • Step 1. Back up your Windows 95 Registry files by running Cfgback.exe, located in your Windows folder. This saves duplicates of your Registry, where they can be restored by running Cfgback.exe again if need be.

  • Step 2. Download the Zip file and save it in a separate folder on your hard drive. Unzip this file. You should get two Registry merge files, Mtu_edit.reg and Rwn_edit.reg, plus a document in HTML format.

  • Step 3. In the Windows Explorer, right-click Mtu_edit.reg, then click Merge. This creates a MaxMTU value of 576 in a new Registry key, HKEY_LOCAL_MACHINE\System\CurrentControlSet\Services\Class\NetTrans\0002\MaxMTU.

  • Step 4. Right-click Rwn_edit.reg, then click Merge. This creates a value of 2,144 in a new Registry key, HKEY_LOCAL_MACHINE\System\CurrentControlSet\Services\VxD\MSTCP\DefaultRcvWindow. This value is four times larger than your MSS of 536. This number specifies your Internet Receive Window, the data packets your PC should receive.

Restart Windows and test your Internet throughput after Step 3 and Step 4. If you see no benefit, you can delete one or both keys using Regedit.exe.

I'll print in future columns a summary of findings by readers and the InfoWorld Test Center.

I'll write next week about tools you can use to test your Net throughput.

Brian Livingston is the co-author of Windows 95 Secrets Gold and four other Windows books (IDG Books). Send tips to or fax: (206) 282-1248.

Missed a column? Go back for more.

Copyright © 1997 by InfoWorld Publishing Company


Copyright © 2002. InfoWorld Media Group, Inc. is a member of complies with the ASME guidelines with IDG extensions For New media.