I know, a bit unusual question.

Is there any TCP/IP parameter or program that you know of that can limit the transfer rate of a TCP/IP connection on purpose?

I have tuned the registry in XP to get the highest transfer rate like >20 mb/s on a direct link.

I's like to intentionally slow down a PPP link that uses a specific COM port.
A bandwidth limiter, so to speak.

First, a stupid question: Why can't you set port rate on COM port to whatever you need it to be?

See if NetLimiter (http://www.netlimiter.com/) will do what you need. Another possible option is using traffic shaping capabilities of a router. What's the actual device configuration of what you're doing?

The Traffic Shapers that I looked at that are free will only monitor. The ones that actually let you apply policies to your traffic cost a little money.

Here are a few more. Some of them out there will only "divy up" the bandwidth in equal portions. Keep an eye out for that.

Bandwidth management and traffic shaping software for Windows (http://www.softperfect.com/products/bandwidth/)
Traffic Shaper Software (http://bandwidthcontroller.com/traffic-shaping/traffic-shaper.html)

I just finished evaluating Traffic Shaper XP. Messes up the ability to access DNS. Errors in the log file. Had to reboot and it would work for a while. Never found if it would work for me. Had to remove it.

I just finished evaluating Traffic Shaper XP. Messes up the ability to access DNS. Errors in the log file. Had to reboot and it would work for a while. Never found if it would work for me. Had to remove it.

I tried that one too. I can definitely tell you that it DOES NOT work on Vista. BSOD after install.

Doesn't work on XP either.

Try BMF from Bandwidth Management and Firewall, Traffic Shaping Software for Windows (http://www.softinengines.com/) it provides firewall and also traffic shaping and works on WXP/W2K3/Vista/W2K8 all 32 and 64 bit.

Rooeus

Netlimiter works. It just doesn't appear smart enough to limit one adapter.