AppleTalk and Macintosh

AppleTalk was developed by Apple Computers in the early 1980s to allow file and printer sharing and mail functionality between Macintosh computers. A Mac that shares resources is called a server, and the computer connecting to it a client. Like TCP/IP, AppleTalk is not just one protocol, but a suite of several protocols for different functions. It is built-in in every Macintosh computer and requires virtually no user interaction, therefore it is very easy to administer in small network environments. As with any other protocol, AppleTalk is best explained in correlation to the 7-layer OSI model.

At the Physical and Data Link layer several specifications are defined to allow AppleTalk to run over several network types with different media-access technologies. EtherTalk allows AppleTalk to run over Ethernet, TokenTalk allows AppleTalk to run over Token Ring, FDDITalk allows AppleTalk to run over FDDI, and LocalTalk is Apple's own media-access technology, and uses CSMA/CA as the access-method. LocalTalk uses UTP or STP cabling and has a maximum data transfer rate of 230 Kbps, you can still find this in today's networks, typically in very small environments for simple file and printer sharing. The image below shows a connector used in LocalTalk networks to connect network nodes. At one side it connects to a computer or printer using a min-din connector or DB-9 serial connector. The other side connects to a phone cable, which in turn, connects to another LocalTalk connector or a terminator. This type of media is known a PhoneNet, and is similar to building a 10Base2 bus network topology.



At the Network layer, two main protocols are defined:

Datagram Delivery Protocol (DDP)	A connectionless datagram protocol providing best-effort delivery and layer 3 addressing. Similar to the function of IP.
AppleTalk Address Resolution Protocol (AARP)	Maps (Network) layer 3 addresses to (Data Link) layer 2 MAC addresses. Analogous to the function of the ARP protocol in TCP/IP.

At the Transport layer a big difference with the TCP/IP suite becomes noticeable. In TCP/IP the routing protocols are defined at the Network layer, with AppleTalk this is not the case:

Routing Table Maintenance Protocol (RTMP)	Allows AppleTalk routers to exchange information and build their routing tables. The routing tables contain entries of possible routes in the network and their attributes. RTMP routers broadcast their routing table to neighboring routers every 10 seconds causing a lot of overhead. RTMP is the equivalent of the Routing Information Protocol (RIP) typically used in TCP/IP networks.
AppleTalk Update-based Routing Protocol (AURP)	Allows AppleTalk networks to be connected over a TCP/IP WAN link. AURP wraps AppleTalk datagrams into UDP datagrams allowing them to be tunneled over IP connections.
AppleTalk Echo Protocol (AEP)	Used to verify if remote hosts are reachable. This is similar to ICMPs Echo messages used by the PING utility in TCP/IP networks.
AppleTalk Transaction Protocol (ATP)	This is the transport protocol in AppleTalk. Provides reliable delivery service for transaction-oriented operations. ATP handles acknowledgements, flow control and sequencing.
Network Binding Protocol (NBP)	Maps AppleTalk names to AppleTalk network layer addresses. This protocol is largely responsible for the large overhead on AppleTalk networks because of the broadcast method it uses. NBP is somewhat similar to DNS and WINS in TCP/IP.

At the Session layer, the next 4 protocols are defined:

Printer Access Protocol (PAP)	A protocol created for client to printer communication, which manages the virtual connection to printers and print servers.
AppleTalk Data Stream Protocol (ADSP)	Provides a data channel between hosts. It is a full-duplex, connection-oriented protocol that provides its own transport layer services (therefore ADSP functions reside partly on OSI's Transport layer)
AppleTalk Session Protocol (ASP)	A Session protocol that manages sessions for higher layer protocols and uses the AppleTalk Transaction Protocol (ATP) for transport services.
Zone Information Protocol (ZIP)	Manages the relationship between network numbers and zone names and allows applications to use zones.

At the Presentation and Application layer the AppleTalk Filing Protocol (AFP) is defined. AFP provides an interface between an application and a file server. AFP allows a workstation on an AppleTalk network to access files on an AFP file server, such as an AppleShare file server. When the user opens a session with an AppleShare file server over the network, it appears as if the files were located on a local disk drive.


AppleTalk Addressing

The following network diagram shows an example of a simple AppleTalk network using EtherTalk:



An AppleTalk network consists of three main components:

Nodes	A uniquely identified host on the network, includes Macintosh computers, printers, Windows PCs and routers.
Networks	Multiple network numbers can be assigned to a single segment, known as an extended cable range.
Zones	Similar to the concept of VLANs, they are used to control broadcast traffic by dividing internetworks into logical groups. When a client request resources such as shares and printers, only those in the same zone of the client, will appear by default.

An AppleTalk address is 24 bits in length and as with all routable protocols needs a network and a host portion. The first 16 bits denote the network portion of the address, and is learned automatically from an AppleTalk router or computer. The other 8 bits denote the node portion. When a client is added to the network, it will make up the node portion itself and broadcasts requests to see if the number is already in use. If the number is in use, the client will generate a new number and start over again until an unused node number is found. The complete AppleTalk network address of node 37 is 58.37. The 16 bits network portion allows for 65000 networks and the 8 bits node portion allows for 254 hosts (0 can't be used, 255 is the broadcast address). The current version of AppleTalk is named AppleTalk phase 2, allowing multiple network numbers to be assigned to a single segment, known as an extended cable range, and eliminates the limit of 254 nodes per network. Sometimes the address includes the socket number, for example 58.37.254 or 58.37/254. An AppleTalk socket is similar to the concept of ports in TCP/IP.

Using the Network Binding Protocol's services, AppleTalk objects can be named. AppleTalk names consist of a object, type and zone field, where each of these three parts are limited to 32 characters in length. An example of a printer name could be: Finance1:LaserWriter@Executive, where Finance1 is the name configured for the object, LaserWriter the object type, and Executive the zone name.

MAC OS

The current operating system running on Macintosh computers is MAC OS X and supports TCP/IP. Macintosh computers are often used for graphical and other multimedia related purposes. MAC OS runs only on Macintosh computers, not on general x86 based machines. Mac OS X uses an access permission system based on a UNIX. Every file and folder on a hard disk has an associated set of permissions that determines who can do what. The three types of permissions are Read (r--), Write (-w-), and Execute (--x).