five, three of five, and so on. The packets enter the network and proceed to make their way across the wilderness of the network fabric. Packets one, two, three, and five arrive without incident, although they do arrive out of order. Packet four, unfortunately, gets caught in a routing loop in New Mexico. The receive Transport Layer, tasked with delivering a complete, correct mes- sage to the layers above, puts everything on hold while it awaits the arrival of the errant packet. The layer, however, will wait only SO long. It has no idea where the packet is; it does, however, know where it is not. After some predetermined period of time, the receive Transport Layer assumes that the packet isn't going to make it and initiates recovery pro- cedures that result in the retransmission of the missing packet. Meanwhile, the lost packet has finally stopped and asked for directions, extricated itself from the traffic jams of Albuquerque, and made its way to the destination. It arrives, covered with dust, an "I've Seen Crystal Cav- erns" bumper sticker on its trailer, expecting to be incorporated into the original message. By this time, however, the packet has been replaced with the resent packet. Clearly, some kind of process must be in place to handle duplicate packet situations, which happen rather frequently. The Transport Layer then becomes the center point of message integrity. Transport Layer standards are diverse and numerous. ISO, the ITU-T, and the IETF publish recommendations for layer four. The ITU-T publishes X.224 and X.234, which detail the functions of both connection- oriented and connectionless networks. ISO publishes ISO 8073, which defines a transport protocol with five layers of functionality ranging from TPO through TP4, as shown. Class 0 (TPO): Simple class Class 1 (TP1): Basic error recovery class Class 2 (TP2): Multiplexing class Class 3 (TP3): Error recovery and multiplexing class Class 4 (TP4): Error detection and recovery class TPO has the least capability; it is roughly equivalent to the IETF's 