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Modem Setting Change Can Increase Internet Speed

Compensates for bug in Sun Solaris Internet servers.

By Brian McWilliams, PCWorld   

According to telecommunications analyst John Navas, the default maximum setting for MTU in Windows 95 is 1500 bytes. Through extensive testing, Navas has found that changing that setting to 576 bytes can boost dial-up modem performance significantly.

Navas says: %dquotIf I put the fix on, I%squotll be downloading at 3.7-kbps, and if I take the fix off, I%squotll instantly drop down to 1.8 kbps. This affects FTP, World Wide Web, news server, just about anything.%dquot

The technical basis of this bug is complex. Navas says its roots actually lie not in Windows but in improperly configured operating system software on many Internet servers. Older versions of Sun%squots Solaris operating system, in particular, mistakenly assume that Windows 95 users are connecting from local area networks rather than over the open Internet using TCP/IP.

This assumption leads to problems when packets of data are lost during transmission over the Net. Navas says the servers don%squott wait long enough for the packets to get to their destination--your computer--and instead begin retransmitting them when they don%squott get an acknowledgement of receipt from your PC within a certain time period.

%dquotWhen the MTU is set to 1500 bytes, which is typical for an ethernet, [the Internet server] assumes that you are on a low-latency ethernet network, so the time-out value is set very low,%dquot says Navas. %dquotIf in fact you%squotre on a modem link, then the latency is too high and the timer expires, so the server starts retransmitting the file.%dquot

The right way to fix the retransmission bug, according to Navas, is at the server level. And the latest version of Solaris evidently does this. But because server upgrades sometimes happen slowly, Navas says Microsoft ought to issue a fix on the client side, in its Dial-Up Networking software. This could significantly improve the experience of many Internet users but, Navas says, Microsoft is strangely silent on MTU, and didn%squott address the issue in the new version of Dial-Up Networking that was released late last month.

Microsoft Windows product manager Stacey Breyfogle today acknowledged that default value of 1500 for MTU in Windows 95 does affect Internet performance for some users. But she said the company hasn%squott hard-wired a lower setting into Windows because that might impact performance when users are connecting to servers without the retransmission bug. Breyfogle said Microsoft is also looking into providing a way for users to adjust the MTU themselves. However, Breyfogle says, %dquotThat creates its own issues, because users who don%squott need to change it might tinker with it, and cause themselves problems.%dquot

Until the MTU issue is addressed in a future version of Dial-Up Networking, you can take advantage of a free control-panel applet Navas has written for changing the Windows 95 Registry settings for the MTU maximum. Click on the Navas link to your right to access Navas%squots FAQ.