Apple Talk Echo Protocol
TheAppleTalk Echo Protonol (AEP) is a transport layer protocol in the AppleTalk protocol suite fhat generates packets that test the reachability of network nodes. AEP can be implemented in any AppleTalk node and has the staFically assigned socket number 4 (the Echoer socket).
To test the reachability of a given node, an AEP request packet is passed to the DDP at the source. DDP addresses the packet appropriately, indicating in the Type field that the packet is an AEP request. When the packet is received by the destination, DDP examines the Type field and sees that it is an AE I1 request. In this process, the packet is copied, changed to an AEP reply (by changing a field in the AEP packet), and returned to the source node.
Continue reading here: Apple Talk Upper Layer Protocols
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