The MTU setting controls the maximum ethernet packet size your PC will send (you did know the internet works in packets, didn't you?). Why a limit? because although larger packets can be constructed and sent, Your ISP and Internet backbone routers and equipment will chop up (fragment) any larger than their limit, then these parts are reassembled by the target equipment before reading. This fragmentation, and reassembly is not optimal. MTU and Windows and Defaults
Unless otherwise set, windows defaults MTU to 1500, or a lower value of 576 for external networks. 1500 is ok unless you are running PPPoE, or want to use IPSec (Secure VPNs), or both, (it is then too big). 576 is not efficient for the broadband/internet, (it is too small). Finding the largest MTU, by EXPERIMENT
If your MTU is too low already, (maybe 576), the following method will not be able to detect whether you can switch to an optimal size..... So first follow "CHANGING MTU for PPPoE" to reset MTU to 1500, reboot, then come back to this! The best value for MTU is that value just before your packets get fragmented. How do you find out that? By using Ping at an MSDOS command prompt.
ping -f -l 1472 www.jarm.net
Press Enter. Then reduce 1472 by 10 until you no longer get the "packet needs to be fragmented" error message. Then increase by 1 until you are 1 less from getting "packet need to be fragmented" message again.
Add 28 more to this (since you specified ping packet size, not including IP/ICMP header of 28 bytes) And this is your MaxMTU.
Note:If you can ping through with the number at 1472, you are done! Stop right there. Add 28 and your MaxMTU is 1500. For PPPoE, your MaxMTU should be no more than 1492, to allow space for the 8 byte PPPoE "wrapper", but again, experiment to find the optimal value.. For PPPoE, the stakes are high: if you get MTU wrong, you may not just be sub-optimal, things like UPLOADING, or web pages, may stall, or not work at all! |
