There are two protocols at the network layer:
• AppleTalk Address-Resolution Protocol (AARP)—Associates AppleTalk network addresses with hardware addresses
• Datagram Delivery Protocol (DDP)—Provides a best-effort connectionless datagram service between AppleTalk sockets
Five key implementations exist at the transport layer of the AppleTalk protocol suite:
• Routing Table Maintenance Protocol (RTMP)—Responsible for establishing and maintaining routing tables for AppleTalk routers. RTMP is a distance-vector protocol typically used in AppleTalk LANs; it uses hop count as its metric.
• Name-Binding Protocol (NBP)—Maps the addresses used at lower layers to AppleTalk names.
• AppleTalk Update-Based Routing Protocol (AURP)—Allows two or more AppleTalk internetworks to be interconnected through a TCP/IP network to form an AppleTalk WAN. AURP encapsulates packets in User Datagram Protocol (UDP) headers, allowing them to be transported transparently through a TCP/IP network; this creates a virtual data link between the AppleTalk networks. AURP is also the routing protocol used on this virtual data link; in this capacity, it is similar to distance vector routing protocols but is designed to handle routing update traffic over WAN links more efficiently than RTMP by only sending changed information.
• AppleTalk Transaction Protocol (ATP)—Handles transactions between two AppleTalk sockets.
• AppleTalk Echo Protocol (AEP)—Generates packets that test the reachability of network nodes.
The session layer protocol implementations supported by AppleTalk include:
• AppleTalk Data-Stream Protocol (ADSP)—Establishes and maintains full-duplex communication between two AppleTalk sockets
• Zone-Information Protocol (ZIP)—Maintains network number-to-zone name mappings in AppleTalk routers
• AppleTalk Session Protocol (ASP)—Establishes and maintains sessions between AppleTalk clients and servers
• Printer-Access Protocol (PAP)—Allows client workstations to establish connections with servers, particularly printers
The AppleTalk Filing Protocol (AFP) is implemented at the presentation and application layers of the AppleTalk protocol suite. AFP permits AppleTalk workstations to share files across a network.
AppleTalk utilizes addresses to identify and locate devices on a network in a manner similar to the process utilized by protocols such as TCP/IP and IPX. These addresses, which are assigned dynamically, are composed of three elements:
• Network number—A 16-bit value that identifies a specific AppleTalk network (either a nonextended network or from an extended cable range)
• Node number—An 8-bit value that identifies a particular AppleTalk node attached to the specified network
• Socket number—An 8-bit number that identifies a specific socket running on a network node
AppleTalk addresses usually are written as decimal values separated by a period. For example, 10.1.50 means network 10, node 1, socket 50. This also might be represented as 10.1, socket 50.
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