header, followed by a Subnetwork Access Protocol (SNAP) header if necessary. This scheme has the advantage of allowing all traffic over the same circuit, but the disadvantage of requiring each packet to contain octets that identify the protocol type, which adds overhead. The scheme also has the disadvantage that packets from all protocols travel with the same delay and priority. RFC 2684 specifies that hosts can choose between the two methods of using AAL5. Both the sender and receiver must agree on how the circuit will be used. The agreement may involve manual configuration. Datagram encapsulation and IP MTU size Internet Protocol (IP) can use AAL5, combined with one of the encapsulation schemes described in RFC 2684, to transfer datagrams across an ATM network, as specified in . Before data can be sent, a virtual circuit (PVC or SVC) must be in place to the destination host and both ends must agree to use AAL5 on the circuit. To transfer a datagram, the sender passes it to AAL5 along with the VPI/VCI identifying the circuit. AAL5 generates a trailer, divides the datagram into cells, and transfers the cells across the network. At the receiving end, AAL5 reassembles the cells, checks the CRC to verify that no bits were lost or corrupted, extracts the datagram, and passes it to the IP layer. AAL5 uses a 16-bit length field, making it possible to send 65,535 (216−1) octets in a single packet. However, RFC 2225 ("Classical IP and ARP over ATM") specifies a default MTU of 9180 octets per datagram, so, unless the hosts on both ends of the virtual circuit negotiate a larger MTU, IP datagrams larger than 9180 octets will be fragmented. References Network protocols