temporary storage until the switch is sure that it has been received properly by the next switch or by the end user. Unlike message switching, which uses the principle of store-and-forward switching, the copies of the packets are destroyed (actually, written over in memory) when the switch is confident that the packet has been successfully relayed. The flow of the message, in Fig. 6-15, is initiated by the trans- mission of packet 1 between user A and switch 1. When it fully re- ceives the first packet, switch 1, following a set of routing rules, transmits packet 1 toward its destination by sending it via switch 2. In the meantime, packet 2 is moving from user A into switch 1. Dur- ing this time the conditions in the network change (for instance, a large amount of traffic from switch 5 arrives at switch 2), so the second packet of the message from A to B is routed via switch 4. The third packet of the message, arriving at switch 1 soon after the second packet, is similarly routed via switch 4. After being received correctly by switch 4, the second packet is transmitted to the destination switch, switch 3. But during that trans- mission an error occurs. When switch 3 receives packet 2, the error- detection mechanism is able to detect the error and request a retrans- mission of packet 2. However, while this is occurring, packet 3 has been transmitted immediately behind the first (and errored) copy of packet 2. As a result, the second (correct) copy of packet 2 is received at switch 3 after packet 3. If we look at the network from the per- spective of switch 3, first packet 1 is received, then packet 3, and finally packet 2. If switch 3 delivered the packets to the destination (that is, to user B) in the same order they arrived at switch 3, user B would receive the packets in a different order than they entered the network. Differential delays along the many paths through the network in- NETWORK STRUCTURES 103 Message Reassembly Acknowledgments Error Retransmission Fig. 6-15. Packetized movement of messages in a net