Maximum transmission unit (MTU)
# a datagram can travel through different network thus the format and size of the sent/received frame depend on the protocol used by the physical network through which the frame is going to travel/has just traveled
# each data link layer protocol has its own frame format in most protocols. One of the field defined in the format is the maximum size of the data field
# when a datagram is encapsulated in a frame, the total size of the datagram must be less than the maximum size or maximum transmission unit (MTU)