The AAL5 trailer does not include a "type" field. Thus, an AAL5 frame does not identify its content. This means that either the two hosts at the ends of a virtual circuit must agree "a priori" that the circuit will be used for one specific protocol (e.g., the circuit will only be used to send IP datagrams), or the two hosts at the ends of a virtual circuit must agree "a priori" that some octets of the data area will be reserved for use as a type field to distinguish packets containing one protocol's data from packets containing another protocol's data.