Max MTU: How do I find mine?

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 packets larger than their limit. These parts are then 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, want to use IPSec (Secure VPNs) or both, then it’s too big. 576 is not efficient for the broadband/Internet; it’s 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 and then come back to this!

The best value for MTU is that value just before your packets get fragmented. How do you find that out? By using Ping at an command prompt.

Windows 2000/XP users:

Go to Start/ Programs/ Accessories/ Command Prompt and type the following:

ping -f -l 1472
(That is a dash lower case “L,” not a dash “1.” Also note the spaces in between the sections.)

Linux users:

ping -s 1472

OS X users:

ping -D -s 1472

Linux and OS X commands are case sensitive.

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 away from getting the “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 your MTU wrong, you may not just be sub-optimal, things like UPLOADING or web pages may stall or not work at all!

(TCP, IP, MTU and MSS magic numbers)

1500 The biggest-sized IP packet that can normally traverse the Internet without getting fragmented. Typical MTU for non-PPPoE, non-VPN connections.
1492 The maximum MTU recommended for Internet PPPoE implementations.
1472 The maximum ping data payload before fragmentation errors are received on non-PPPoE, non-VPN connections.
1460 TCP Data size (MSS) when MTU is 1500 and not using PPPoE.
1464 The maximum ping data payload before fragmentation errors are received when using a PPPoE-connected machine.
1452 TCP Data size (MSS) when MTU is 1492 and using PPPoE.
576 Typically recommended as the MTU for dial-up type applications, leaving 536 bytes of TCP data.
48 The sum of IP, TCP and PPPoE headers.
40 The sum of IP and TCP headers.
28 The sum of IP and ICMP headers.

Windows 98/98SE (or 95 with DUN 1.3) – CHANGING MTU
Use drTCP to change MTU.

MTU and other OS
For Linux, it is a matter of using the ifconfig command (read the main page for ifconfig).

For Macintosh, your PPPoE software (MacPOET) will set the MTU correctly.

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