Strict source route
# when a datagram is traveling, each router that processes the datagram compares the value of the pointer with the value of the length
# if the value of the pointer is greater than the value of the length, the datagram has visited all of the predefined routers
# if the value of the pointer is not greater than the value of the length, the router compares the destination IP address with its incoming IP address: if they are equal, it processes the datagram, swaps the IP address pointed by the pointer with the destination address, increments the pointer value by 4,and forward the datagram