MTU Optimization

[Bride]

Hi,
I have a doubt about the MTU optimization process, following the procedure below.
I have to verify the parameters on my network equiments, any advice will be appreciated, I'm here for learn if possible:

ADSL WAN ROUTER - MTU 1444
ASUS RT-N66U ROUTER - MTU 1416
WINDOWS CLIENTS - MTU 1388
VPN SOFTWARE - MTU 1360


To find the correct MTU for your configuration you must run a simple DOS Ping test. You will simply send out ping requests and progressively lower your packet size until the packet no longer needs to be fragmented.

Please reference the following steps: The command for this ping test is ping www.snbforums.com -f -l xxxx where "xxxx" is the MTU size that you are testing, you can use also any well known, pingable domain like ping www.google.com

Step 1 Open a DOS prompt screen by clicking on Start>Programs>MSDOS-PROMPT. You can also use the Run Command by clicking on Start>Run then type in "cmd"

Step 2 At the DOS Prompt type in ping www.snbforums.com -f -l 1472 and hit Enter. Notice that the packet needs to be fragmented (packet needs to be fragmented but df set).

Step 3 Drop the test packet size down (10 or 12 bytes) and test again. Notice that the packet still needs to be fragmented.

Step 4 Drop the test packet size down more and test again until your reach a packet size that does not fragment. (reply from...).

Step 5 Once you have a test packet that is not fragmented increase your packet size in small increments and retest until you find the largest possible packet that doesn´t fragment.

Step 6 Take the maximum packet size from the ping test and add 28. You add 28 bytes because 20 bytes are reserved for the IP header and 8 bytes must be allocated for the ICMP Echo Request header. Remember: You must add 28 to your results from the ping test!

An example: 1444 Max packet size from Ping Test + 28 IP and ICMP headers 1472 is your optimal MTU Setting

 

[ColinTaylor]

That process is correct. The MTU for my RT-N66U is 1500 (1472+28).

C:\Users\Colin>ping www.snbforums.com -f -l 1472

Pinging snbforums.com [104.25.235.15] with 1472 bytes of data:
Reply from 104.25.235.15: bytes=1472 time=18ms TTL=57
Reply from 104.25.235.15: bytes=1472 time=14ms TTL=57
Reply from 104.25.235.15: bytes=1472 time=15ms TTL=57
Reply from 104.25.235.15: bytes=1472 time=16ms TTL=57

Ping statistics for 104.25.235.15:
    Packets: Sent = 4, Received = 4, Lost = 0 (0% loss),
Approximate round trip times in milli-seconds:
    Minimum = 14ms, Maximum = 18ms, Average = 15ms

 

[sfx2000]

Hi,
I have a doubt about the MTU optimization process, following the procedure below.
I have to verify the parameters on my network equiments, any advice will be appreciated, I'm here for learn if possible:

ADSL WAN ROUTER - MTU 1444
NETGEAR RT-N66U ROUTER - MTU 1416
WINDOWS CLIENTS - MTU 1388
VPN SOFTWARE - MTU 1360

You're probably worrying too much about this - TCP will do what it does, and it does it pretty well...

There's a few exceptions perhaps, but this is based on broken end-points...

(good example is your VPN, you're slowing it down if you're doing TCP - if you do UDP, you can build huge packets around it)

 

[Bride]

Thank you guys, I'm just trying to understand if the configuration between devices need a different MTU size or not. Probably a configuration like this one can to be ok:

ADSL WAN ROUTER - MTU 1444
ASUS RT-N66U ROUTER - MTU 1444
WINDOWS CLIENTS - MTU 1416
VPN SOFTWARE - MTU 1388

by the way, for avoid collisions, the difference must to be of 28 bytes? or can to be just 1 bytes of difference? between the router ADSL and the router ASUS, keeping the same values, there is any problem? thanks in advance

 

[ColinTaylor]

Set everything at 1500 and then use an MTU discovery tool like mturoute (http://www.elifulkerson.com/projects/mturoute.php) or tracepath (Linux).

 

[Bride]

Set everything at 1500 and then use an MTU discovery tool like mturoute (http://www.elifulkerson.com/projects/mturoute.php) or tracepath (Linux).

thanks a lot, I'll follow your advice

 

[System Error Message]

The problem i see here is that you can only optimise MTU for either internet or LAN. LAN benefits from very high MTUs while internet, especially if you use vpns or PPP or vlans will need lower.

If your MTU is at 9K and router's LAN MTU is at 9K the router will repack into smaller packets so it could be less efficient due to more packets not filling up to the MTU of internet.

 

[Bride]

The problem i see here is that you can only optimise MTU for either internet or LAN. LAN benefits from very high MTUs while internet, especially if you use vpns or PPP or vlans will need lower.

If your MTU is at 9K and router's LAN MTU is at 9K the router will repack into smaller packets so it could be less efficient due to more packets not filling up to the MTU of internet.

Actually I'm working at

ADSL WAN ROUTER - MTU 1444
ASUS RT-N66U ROUTER - MTU 1444
WINDOWS CLIENTS - MTU 1416
VPN SOFTWARE - MTU 1388

and everything looks ok. On the windows clients a difference of 12 bytes was suggested (1432) but I think keep 28 bytes still good...

 

[ColinTaylor]

On the windows clients a difference of 12 bytes was suggested (1432) but I think keep 28 bytes still good...

I can't think of any logical reason why you would need to set the MTU of the Windows clients to something different than the router.

 

[sfx2000]

Shouldn't have to worry about MTU - the Stack will do path discovery, and should just generally work.

The only exception I've seen first hand is a couple of PPOE implementations that did need some manual intervention as the MTU was advertised at 1500, but didn't catch the overhead of encapsulation of the TCP frames once inside a PPP frame...

 

[Bride]

I can't think of any logical reason why you would need to set the MTU of the Windows clients to something different than the router.

Well, so you can explain to me why not, with an useful advice... by the way the Auslogic Internet Optimizer set it...

 

[ColinTaylor]

Well, so you can explain to me why not, with an useful advice... by the way the Auslogic Internet Optimizer set it...

The only reason you would need to have a lower MTU on your Windows client compared to the router would be if the IP packets are being encapsulated within another protocol.

This is the case with a software VPN client for example. Cisco's VPN Windows client sets the MTU at 1300 because it is a "safe" value. But for a "normal" connection between a PC and the router there is no encapsulation and therefore the MTU's of both devices should match.

That's not to say that there isn't a good reason for you setting it the way you have, it's just that you haven't said anything that would suggest you need to.

 

[Bride]

The only reason you would need to have a lower MTU on your Windows client compared to the router would be if the IP packets are being encapsulated within another protocol.

This is the case with a software VPN client for example. Cisco's VPN Windows client sets the MTU at 1300 because it is a "safe" value. But for a "normal" connection between a PC and the router there is no encapsulation and therefore the MTU's of both devices should match.

That's not to say that there isn't a good reason for you setting it the way you have, it's just that you haven't said anything that would suggest you need to.

Thank you for the detailed answer. I know that aren't always useful the softwares like Auslogic Internet Optimizer, but why this one setup my Windows MTU at 1432 when my router is on 1444?