AppleTalk Network Protocol


Jarret W. Buse

AppleTalk Network Protocol

AppleTalk is a suite of network protocols designed by Apple for use by Mac systems. The protocol was released in 1985 for use by the original Macintosh. AppleTalk is used to connect Macs together in a Local Area Network (LAN). The network setup is practically Plug-n-Play since it manages it own addressing, namespace and routing (if needed for a Wide Area Network (WAN)).

Because of the popularity of the Internet and its use of TCP/IP, AppleTalk is no longer the default protocol for Apple systems. Support for AppleTalk was dropped in 2009 with the release of Mac OS X v10.6.

Apple also produced other devices which can use AppleTalk for network communications, such as the LaserWriter. Servers, such as the Macintosh Office, could be connected to the LAN.

Initially, AppleTalk ran on a Token Ring network, which did not actually require a physical ring. In 1987, Apple released EtherTalk and the previous Token Ring network was called TokenTalk. Where TokenTalk could handle a total of 32 systems on a LAN, EtherTalk could handle 255 systems on a LAN.

With the release of the EtherTalk network, Apple released the AppleShare system. AppleShare was a dedicated file server that took the place of the Macintosh Office that had low sales.

In 1989, AppleTalk Phase 2 was released to allow more than the previous 255 systems per LAN.

AppleTalk uses four bytes for addressing. Two bytes are used as a network number, one byte for the node number and one byte for a socket number. Network routers supply the network number and each node dynamically selects a node number. AppleTalk reserves socket numbers for specific uses, but socket numbers are dynamically assigned.

AppleTalk maps to the OSI Model as shown:

AppleTalk Figure 1.jpg

Let’s look over these items one at a time.

Apple Filing Protocol (AFP) allows a system to perform the following:
  • User authentication
  • Communicate with AppleShare systems
  • Connects to HFS file systems

The Session Layer of the OSI Model is made up of the following:
  • Zone Information Protocol (ZIP)
  • AppleTalk Session Protocol (ASP)
  • AppleTalk Data Stream Protocol (ADSP)

The Zone Information Protocol (ZIP) on the Session layer allows nodes to be set up. Nodes can be used to create subdivisions on a network, such as “Marketing” or “Sales”. Each node is given a number, part of the addressing, and the number is assigned to a preset name. The ZIP translates between the zone name and zone number.

Being the Session Layer, the AppleTalk Session Protocol (ASP) is used to open, manage and close sessions between system sockets.

The final part of the Session Layer is the AppleTalk Data Stream Protocol (ADSP). ADSP is a full duplex transport. That is, both clients have an open session between them, and both systems can send and receive data simultaneously. Any connection between clients must be open on both ends. Half open connections are not allowed, so when one system closes the session, both ends must be closed.

The Transport Layer is made up of four transports
  • AppleTalk Transaction Protocol (ATP)
  • AppleTalk Echo Protocol (AEP)
  • Name Binding Protocol (NBP)
  • Routing Table Maintenance Protocol (RTMP)

The AppleTalk Transaction Protocol (ATP) is a reliable transport. After a session is created, packets can be sent and then acknowledged by the receiving client. The acknowledgement includes which packets were received. If all packets arrived, then nothing is done (unless more packets need sent). If packets are missing, the acknowledgement includes which packets need to be retransmitted.

The AppleTalk Echo Protocol (AEP) is used to verify that the destination system is reachable. An AEP packet is sent to the receiving system, which if present, changes the AEP packet to an AEP Reply and sent back. When the packet is received back at the original system, it knows that the system exists and a session can be opened.

The Name Binding Protocol (NBP) is used to give names to system services. In this way, a user can access network resources with a name instead of having to know a system address (network number, node number and socket number). When a request is made to a network service by name, the NBP translates the name to an address. When a system comes online, it checks the network for an identical name to the one it was given. If a duplicate exists, an error would be generated at the system. When a service is required, the system is queried by NBP to verify that the service exists and maps the name to the address of the system.

To maintain the layout of the network topology, or where systems are located, the Routing Table Maintenance Protocol (RTMP) is used. Every ten seconds, a broadcast is sent out on a LAN listing each system address. Each routing device would check the list and update its table. The protocol allows routing devices to be aware of all systems on the network and the distance it was from the routing device.

The Network Layer consists of one protocol:
  • Datagram Delivery Protocol (DDP)

The Datagram Delivery Service (DDP) is a connectionless transport. Connectionless means that no session is necessarily required and the data sent is not required to be received. No acknowledgements or retransmission of lost data is needed. The protocol can be thought of as being similar to streaming music. Portions of the music stream may be lost or arrive out of order. When data is lost or out of order, it is merely skipped. Streaming music may sometimes have places where the music is “skipped” due to the connectionless nature of the transmission.

The Data Link Layer has four protocols:
  • EtherTalk Link Access Protocol (ELAP)
  • LocalTalk Link Access Protocol (LLAP)
  • TokenTalk Link Access Protocol (TLAP)
  • Fiber Distributed Data Interface (FDDI)

Each Link Access Protocol allows for AppleTalk to operate on the specified Network Topology as shown:
  • EtherTalk – Ethernet network
  • LocalTalk – Serial network implemented by Apple
  • TokenTalk – Token Ring network
  • Fiber – Network Topology using fiber-optic cable

The Physical Layer is the physical medium needed by the chosen Network Link Access Protocol. The medium can be wired or wireless as needed.


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