General discussion



By dirv9 ·
Hi! I recently received help here for a connection problem, and in searching for answers came across a program called DrTCP. It's a tweak tool to optimize TCP settings in the registry. I followed instructions and did a ping command---ping -f -l 1500, to figure out my MTU setting. The result stated: Packet needs to be fragmented but DF set. What that does that mean? Is something wrong in my settings somewhere? I was looking for the latency (average ping time in ms x 1.5) so I could set The RWin (the TCP receive window) in DrTCP. It says the default for XP ( which I'm using) is 17520, but it can be set higher to increase download speeds. The example picture has it set at 32767. But there were no reply lines in the result of the ping command, just the above packet message. So, is there something wrong I need to correct, or should I try the 32767 setting for the RWin? Any response would be greatly appreciated. Thanks

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-f is the 'do not fragment' option

by robo_dev In reply to DrTCP

The actual MTU is 'Packet size' + 28, you need to allow for the header when using ping command to determine MTU. So when you give a length of 1500, you're trying to send 1528 byte packets which is too big.

To test for 1500 byte packets, the magic number is 1472

But, ping -f -l 1472 is not likely to work either, in most cases about 1460 is the most that will typically fly over the internet. You typically should be able to get a 1472 byte ping response from your own router, however.

The MTU and the RWIN are not as directly linked as one would think because both pathMTU discovery, which is active in many internet routers, pretty much makes things very optimal most of the time.

RWIN is the TCP windowing size. This is the amount of data that a sender on a network will send before requiring an acknowledgment from the receiver. This is usually the parameter that gets tweaked on 'speed up' software for slower links.

A higher RWIN number allows a sender to send more data over a slow link, before requiring an acknowledge packet.

BUT, if the link drops a lot of packets, a high RWIN can slow things down, since it will have to resend all data after the dropped packet is resent.

The reality is that it really doesn't matter. With a sufficiently high RWIN, you shouldn't really have to worry about the occasional partial packet because of the MTU setting. And, as I said, most systems automatically set MTU, so you really can't control it anyway, except to make it smaller. Some systems set RWIN dynamically as well.

So the long answer to a short question is that there is nothing wrong with your configuration. A RWIN of 32767 is totally OK. Personally I've never noticed a dramatic difference after tweaking mtu/rwin settings.

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Use TCPOptimizer

by Avila In reply to DrTCP


I recommend you to use TCPOptimizer (just google it for download, it's free), if remember correctly it sets the best configuration for your computer.


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It doesn't really matter

by JackOfAllTech In reply to DrTCP

Unless you're on dial-up or really slow DSL, changing any of these settings is not going to make much of a difference. As another poster said, most of the 'net is self-optimizing now.

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