[Contents] [Back] [<< Prev] [Next >>]

AppleTalk
AppleTalk is the computing architecture developed by Apple Computer for the Macintosh family of personal computers. Although AppleTalk originally supported only Apple’s proprietary LocalTalk cabling system, the suite has been expanded to incorporate both Ethernet and Token Ring Physical layers.

AppleTalk originally supported networks of limited scope. The AppleTalk Phase 2 specification issued in 1989, however, extended the scope of AppleTalk to enterprise networks. The Phase 2 specification also enabled AppleTalk to coexist on networks with other protocol suites. Figure 5.5 presents a layered perspective of the AppleTalk protocols.

The LocalTalk, EtherTalk, and TokenTalk Link Access Protocols (LLAP, ELAP, and TLAP) integrate AppleTalk upper-layer protocols with the LocalTalk, Ethernet, and Token Ring environments.

Apple’s Datagram Deliver Protocol (DDP) is a Network layer protocol that provides connectionless service between two sockets. A socket is the AppleTalk term for a service address. A combination of a device address, network address, and socket uniquely identifies each process.

DDP performs network routing and consults routing tables maintained by Routing Table Maintenance Protocol (RTMP) to determine routing. Packet delivery is performed by the data link protocol operating on a given destination network.

The AppleTalk Transaction Protocol (ATP) is a connectionless Transport layer protocol. Reliable service is provided through a system of acknowledgments and retransmissions. Retransmissions are initiated automatically if an acknowledgment is not received within a specified time interval. ATP reliability is based on transactions. A transaction consists of a request followed by a reply. ATP is responsible for segment development and performs fragmentation and reassembly of packets that exceed the specifications for lower-layer protocols. Packets include sequence numbers that enable message reassembly and retransmission of lost packets. Only damaged or lost packets are retransmitted.

The AppleTalk File Protocol (AFP) provides file services and is responsible for translating local file service requests into formats required for network file services. AFP directly translates command syntax and enables applications to perform file format translations. AFP is responsible for file system security and verifies and encrypts logon names and passwords during connection setup.

AppleShare is a client/server system for Macintosh. AppleShare provides three primary application services: