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View Full Version : Path MTU Discovery off? Help!



Humboldt
10-05-00, 01:40 AM
OK, I ran the speedguide tweak test for my DSL connection, and got the message: "Problem, Path MTU Discovery is OFF
* Conclusion.. possible problem"
It recommended looking at a forum discussion on that problem and I read through all those but am still confused. What exactly is the problem with MTU Discovery being off, and how do I change it? Thanks folks

Humboldt
10-05-00, 01:50 AM
Btw, I downed DRTCP.exe, just not sure what the Path MTU Discovery on/off setting does, and am hesitant to go messing with settings when most things seem to work well. My DSL connection hangs sometimes after periods of non-use, but someone recommended some links a couple posts ago. The connection speed is good. Why is the tweak test red-flagging that setting? Thanks

spaceman
10-05-00, 04:47 AM
I keep mine off!! Don't worry about it. http://www.speedguide.net/ubb/cool.gif

Humboldt
10-05-00, 01:54 PM
Yah, trying not to worry, and everything does seem to run well. BUT, I figure the speedguide folks know a lot more about this then I do. Can someone please explain what this setting (Path MTU discovery) does and what benefits might be gained changing it from off to on? Thank you, I appreciate the help.

Bouncer
10-06-00, 12:51 AM
Okay, here's the deal...

PMTU Path Discovery is designed to adjust your packet size to the smallest link between you and your destination. Understand that not all links LIKE packets of X size. Some prefer them to be Y or Z sized. PMTU does a quick check and adjusts to the lowest common denominator for as long as you connect to the other site.

The reason this wasn't implemented before was that there was a bug in the MS TCP/IP stack that would adjust DOWN, but never up. So if you connect to a small pip[e to do something, and then connect to a different site which has bigger pipes you'd still be using packets optimized for the smaller pipe. Lots of problems can occur with something like that.

This has been fixed. It can improve eficiency to use it, and it won't hurt anything. It's also a part of the IPv6 standard, though you probably don't care. http://www.speedguide.net/ubb/smile.gif

Regards,
-Bouncer-

------------------
"Yeah Baby, YEAH!!!"

cablenut
10-06-00, 12:54 AM
PMTU Discovery rocks http://www.speedguide.net/ubb/smile.gif