The AURP packets are encapsulated in the header information required by the foreign network and sent over the tunnel to the destination exterior router, as shown in Figure 25-7 . At the destination exterior router, the AURP module removes the headers required by the foreign system from the AURP packets and sends AppleTalk data packets to their final destination. The exterior router uses the AURP packets that contain routing information to update its routing information tables but does not propagate that information to any other exterior router. As defined by Apple Computer, AURP converts RTMP and ZIP packets into AURP packets and vice versa. As implemented by Cisco, AURP converts Enhanced IGRP packets as well as RTMP and ZIP packets.
Transactions occur between two socket clients.
XO transactions are used in situations where performing the transaction more than once would be unacceptable. Banking transactions are examples of transactions that, if performed more than once, would result in invalid data. ATP is capable of most important transport-layer functions, including data acknowledgment and retransmission, packet sequencing, and fragmentation and reassembly. ATP limits message segmentation to 8 packets, and ATP packets cannot contain more than 578 data bytes. ADSP is a reliable protocol in that it guarantees that data bytes are delivered in the same order as sent and that they are not duplicated.