There is a limit on the size of the frame for both Ethernet encapsulation and 802.3 encapsulation. This limits the number of bytes of data to 1500 and 1492, respectively. This characteristic of the link layer is called the MTU, its maximum transmission unit. Most types of networks have an upper limit.
Path MTU
When two hosts are communicating across multiple networks, each link can have a different MTU. The important number is the smallest MTU of any data link that packets traverse between the two hosts. This is called the path MTU.
The path MTU between any two hosts need not be constant. It depends on the route being used at any time. Also, routing need not be symmetric (the route from A to B may not be the reverse of the route from B to A), hence the path MTU need not be the same in the two directions.
