Unlike TCP/IP and IPX/SPX, the AppleTalk suite has clearly defined Session Layer protocols. These include:
AppleTalk Data Stream Protocol (ADSP). ADSP establishes and maintains reliable full-duplex sessions between two AppleTalk sockets. It also handles flow control functions such as windowing. ADSP is symmetrical, meaning that both client sockets have equal access to the session.
AppleTalk Session Protocol (ASP). ASP is another Session Layer protocol that establishes, maintains, and closes connections between clients. ASP sessions are asymmetrical, with the client controlling session communication.
Printer Access Protocol (PAP). PAP is responsible for managing connection-oriented sessions to AppleTalk printers.
Zone Information Protocol (ZIP). The primary responsibility of ZIP is to maintain a network-wide mapping of networks (or cable ranges) to zone names. When a client starts and attempts to dynamically configure its address, it queries the local router using ZIP to find the valid network numbers on that segment
Presentation and Application Layers
At the Presentation and Application Layers of the OSI model, one primary AppleTalk protocol exists, the AppleTalk Filing Protocol (AFP). The responsibility of AFP is to manage file system access between a client and AppleShare server on an AppleTalk network. AFP creates an abstraction whereby a client accesses network files as if they were stored locally. AFP uses ASP as its Session layer protocol.