Network Speeds – MTU

I’ve been having an issue with copying data to my NAS for a while and I’ve finally had a chance to take a quick look at it. It’s an old NEtgear NAS Duo v1. I was only getting about 1/2 mb/s transfer to it, so I figured something was wrong there. I read a website from someone who had the same issue, their solution was to update the MTU for both the desktop and the NAS so I thought I would give it a try. OK so how do determine the optimal MTU setting for my desktop/nas?

Well that’s simple enough, you keep pinging a site with lower and lower packets until they stop being defragmented, add another number to it and bam, there you go.

Step 1
Open Command Prompt
Step 2
In the Command Prompt type in ping http://www..com -f -l 1472 and hit Enter. If you get ‘Packet needs to be fragmented but DF set.’ Message it means that the packet needs to be fragmented.
Step 3
Drop the test packet size down (10 or 12 bytes) and test again until your reach a packet size that does not fragment.
Step 4
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:
1440 Max packet size from Ping Test
+ 28 IP and ICMP headers
1468 Your optimum MTU Setting

In your case if the maximum packet size is 1452 the optimal MTU (Maximum Transmission Unit) is 1452+28 i.e. 1480. Here 1452 is the packet size of the data sent but the MTU is 1480.

To update your MTU on a windows machine, you can do this from the command line [I did it from a command line with administrative rights]

netsh interface ipv4 set subinterface “Local Area Connection” mtu=1458 store=persistent

There you go, sorted, updated the MTU on my NAS and now I’m getting 11/12mb/s which isn’t that much, but it’s an old NAS so I’ll forgive it.

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