Tweak3D.net - Modem

Written By: Keith "Farrel" McClellan
Posted: February 19, 2000

Optimizing Other TCP/IP Settings Continued

Determining Your MTU

Depending on your particular modem/line/ISP, you may have more success with other settings. There is, however, an easy way to determine your optimal MTU, which should make your tinkering a lot less complicated. Start by opening a dos prompt, then enter the command "ping -f -l [trial MTU number] www.[your-isp].com". Start with 548 and go up or down depending on the message (if it tells you that the packets are being fragmented, you need to go down - you want the highest setting that doesn't cause packet fragmentation). To derive your MTU from the ping data, add 28 to the highest number that worked (packet size+28). Then, to derive your MSS from your MTU number, take MTU-40 and insert that value for MSS. The other settings will probably require a bit more tinkering though.

Windows 98 Modem:

Windows 98 made leaps and bounds when it comes to its TCP/IP stack (as the driver protocol is referred to), but there are still some problems with it - particularly when using it in conjunction with iSpeed. If you already have iSpeed installed, use the program to reset the windows defaults and uninstall the software. Then follow the following steps to tweak your TCP/IP settings.

- Enter the control panel and start up the Network applet
- Click on the icon representing your network card and click on the Properties button
- Go to the Advanced Tab
- Make sure that IP Packet Size is set to Large
- If you mostly use the internet for downloading files and viewing webpages, enable IP Header Compression - otherwise, leave it off.

Why is this so easy? Well, Windows 98 has a new feature called MTU Auto discover. This will automatically determine the optimal MTU (and will derive all of the other settings from that) on connect. The problem is, if you set the IP Packet size to something smaller than large (auto assumes 576, even though it shouldn't), you limit your maximum MTU because Windows assumes that that setting is as high as it needs to go. Also, if you set the settings manually (through the registry, as iSpeed does), you lose Windows' ability to dynamically choose the optimal settings on connect as well - and servers have been known to change these settings frequently to handle more traffic. My local ISP even changes the settings daily to allow more users, because of the heinous bottleneck between Malaysia and California. Special thanks goes out to Rob at BXBoards for the heads up on this issue.

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