Tuesday, September 20, 2016

checking your MTU (Maximum Transmission Unit) size

in computer networking, the maximum transmission unit (MTU) of a communications protocol of a layer is the size (in bytes or octets) of the largest protocol data unit that the layer can pass onwards.

an ethernet frame (layer 2) has a size of 1518 byte. 18 bytes are used for the header and checksum (18+4). so the MTU (maximum trasmission unit), space left for ip + data in one ethernet frame is 1500. normally.

in dsl networks there may be the pppoe protocol which needs 8 bytes. for that reason the MTU is capped to 1492 byte. 20 bytes are need for the IP headers and 8 bytes for ICMP.

to check your MTU you can use the ping command
ping -f -l <frame-size> <IP Address>

-f indicates do not fragment.
-l frame size

below you can see that my current MTU size is 1500 (1472+28 ip,icmp)

or you can just type the following command in windows cmd shell