MTU Size Limits

As was mentioned previously, most likely the MTU size of all the networks between your clients and server is 1500. You can easily test this with the ping command. Use the “-l” (that’s the letter “L,” not the number “1”) option to specify the length of data to send in the ping packet. (The default is 32 bytes.) For example, you could try to send a packet that is 1500 bytes to see if it makes it. You’ll also need to use the”-f” option to mark the ping packet as “not fragmentable”—otherwise it could get broken into pieces and ruin your test.

For example, try the following command: ping yourservername –l 1472 –f

You’ll see that over the Internet (and in many routed corporate sites) the maximum length you can specify with the ping command is 1472. That’s because the IP and ICMP headers add 28 bytes to the packet, bringing the size up to an even 1500. If you get a response that says “Packet needs to be fragmented but DF set” then you know that the length you specified is bigger than the MTU size somewhere along the way. In this case, play a little “guess and check” until you find a length that results in a successful ping. Then, add 28 to that number and you have the MTU size for that route.