Five main protocols exist at the AppleTalk Transport layer. These include:
Routing Table Maintenance Protocol (RTMP). RTMP is used by AppleTalk routers to exchange, establish, and maintain routing table entries.
Name Binding Protocol (NBP). NBP is used to dynamically map AppleTalk resource names (such as shared folders or printers) to their network address. NBP allows resources to be accessed by name rather than network address.
AppleTalk Update-Based Routing Protocol (AURP). AURP allows AppleTalk networks to be connected over a WAN by tunneling AppleTalk through a TCP/IP network. This is accomplished by defining AURP tunnels between routers, which encapsulate AppleTalk traffic destined for a remote network in UDP headers. The UDP segments are then encapsulated for IP (and whatever network technology the WAN uses), and sent to the other end of the AURP tunnel where they are de-encapsulated and forwarded. Both point-to-point and multipoint AURP tunnels can be created. Note that the AppleTalk suite only defines Data Link interfaces for Ethernet, Token Ring, FDDI and TokenTalk. The Cisco implementation of AppleTalk also supports AppleTalk encapsulation over a variety of WAN technologies.
AppleTalk Transaction Protocol (ATP). ATP is the AppleTalk Transport protocol that handles transaction requests and responses between systems. An example of a transaction request would be a socket on a client system asking a socket on a server to perform an action, such as a time request. ATP on each system will not only be sure that for each request sent a response is received, but will also handle common Transport layer functions such as data segmentation, sequencing, and acknowledgements. ATP is mainly used for transferring small amounts of data, and can be used as the upper-layer protocol that brings reliability to DDP.
AppleTalk Echo Protocol (AEP). AEP performs a similar function to an ICMP echo request and reply. It is used to test for reachability and round-trip transmission times with another AppleTalk node. When used, the source node sends out an AEP request, and the recipient sends back an AEP reply.