 It has the advantage of not requiring additional protocol-identifying information in a packet, which minimizes the overhead.For example, if the hosts agree to transfer IP, a sender can pass each datagram directly to AAL5 to transfer, nothing needs to be sent besides the datagram and the AAL5 trailer.This reduction in overhead tends to further improve efficiency (e.g., an IPv4 datagram containing a TCP ACK-only packet with neither IP nor TCP options exactly fits into a single ATM cell).