AppleTalk is a suite of protocols developed by Apple Computer for computer networking.
Design
The design fairly rigorously followed the OSI model of protocol layering. Unlike most other early LAN systems, AppleTalk was not built on the archetypal Xerox XNS system, as the intended target was not Ethernet and did not have 48-bit addresses to route. Nevertheless many portions of the AppleTalk system have direct analogs in XNS.
One key differentiator for AppleTalk was that the system contained two protocols aimed at making the system completely self-configuring. The AppleTalk address resolution protocol (AARP) allowed AppleTalk hosts to automatically generate their own network addresses, and the Name Binding Protocol (NBP) was essentially a dynamic DNS system which mapped network addresses to user-readable names. Although systems similar to AARP existed in other systems, Banyan VINES for instance, nothing like NBP has existed until recently.
Both AARP and NBP had defined ways to allow "controller" devices override the default mechanisms. The concept here was to allow routers to provide all of this information, or additionally "hardwire" the system to known addresses and names. On larger networks where AARP could cause problems as new nodes searched for free addresses, the addition of a router could dramatically reduce "chattiness".
Together AARP and NBP made AppleTalk perhaps the easiest to use networking system yet developed. New machines were added to the network simply by plugging them in, and optionally giving them a name. The NBP lists were examined and displayed by a program known as the Chooser (originally because it allowed you to choose your default printer) which would display a list of machines on the local network, divided into classes such as fileservers and printers. All of this was completely automated.
One problem for AppleTalk was that it was originally intended to be part of a project known as Macintosh Office, which would consist a host machine providing routing, printer sharing and file sharing. However this project was cancelled in 1986, and while the LaserWriter included AppleTalk built-in and could be easily dropped on a network, file sharing and routing was left to third parties. Support for a long time was spotty at best, and did not arrive in convincing form until the early 1990s when much of the market was writing off Apple as dead.
AppleTalk is now considered clunky and often called 'verbose', notably on larger networks and WANs where the naming services generated considerable unwanted traffic.
Today AppleTalk support is provided for backward compatibility in many products, but the default networking on the Mac is TCP/IP. Under Mac OS X versions after v10.2, Apple Bonjour provides discovery and naming services similar to NBP, while standard DHCP provides setup similar to AARP.
Addressing
An AppleTalk address was a four-byte quantity. This consisted of a two-byte network number, a one-byte node number, and a one-byte socket number. Of these, only the network number required any configuration, being obtained from a router. Each node dynamically chose its own node number, according to a protocol which handled contention between different nodes accidentally choosing the same number. For socket numbers, a few well-known numbers were reserved for special purposes specific to the AppleTalk protocol itself. Apart from these, all application-level protocols were expected to use dynamically-assigned socket numbers at both the client and server end.
Because of this dynamism, users could not be expected to access services by specifying their address. Instead, all services had names which, being chosen by humans, could be expected to be meaningful to users, and also could be sufficiently long enough to minimize the chance of conflicts.
Note that, because a name translated to an address which included a socket number as well as a node number, a name in AppleTalk mapped directly to a service being provided by a machine, which was entirely separate from the name of the machine itself. Thus, services could be moved to a different machine and, so long as they kept the same service name, there was no need for users to do anything different to continue accessing the service. And the same machine could host any number of instances of services of the same type, without any network connection conflicts.
Contrast this with A records in the DNS, where a name translates only to a machine address, not including the port number that might be providing a service. Thus, if people are accustomed to using a particular machine name to access a particular service, their access will break when the service is moved to a different machine. This can be mitigated somewhat by insistence on using CNAME records indicating service rather than actual machine names to refer to the service, but there is no way of guaranteeing that users will follow such a convention. (Some newer protocols, such as Kerberos and Active Directory use DNS SRV records to identify services by name, which is much closer to the AppleTalk model.)
Protocols
AppleTalk Address Resolution Protocol
AARP resolves AppleTalk addresses to physical layer, usually MAC, addresses. It is functionally equivalent to ARP.
AARP is a fairly simple system. When powered on, an AppleTalk machine broadcasts an AARP probe packet asking for a network address, intending to hear back from controllers such as routers. If no address is provided, one is picked at random from the "base subnet", 0. It then broadcasts another packet saying "I am selecting this address", and then waits to see if anyone else on the network complains. If another machine has that address, it will pick another address, and keep trying until it finds a free one. On a network with many machines it may take several tries before a free address is found, so for performance purposes the successful address is "written down" in NVRAM and used as the default address in the future. This means that in most real-world setups where machines are added a few at a time, only one or two tries are needed before the address effectively become constant.
AppleTalk Data Stream Protocol
This was a comparatively late addition to the AppleTalk protocol suite, done when it became clear that a TCP-style reliable connection-oriented transport was needed. Significant differences from TCP were:
- a connection attempt could be rejected
- there were no "half-open" connections; once one end initiated a tear-down of the connection, the whole connection would be closed (i.e., ADSP is full duplex, not dual simplex ).
Apple Filing Protocol
The Apple Filing Protocol (AFP) is the protocol for communicating with AppleShare file servers. Built on top of ASP, it provided services for authenticating users (extensible to different authentication methods including two-way random-number exchange) and for performing operations specific to the Macintosh HFS filesystem.
AppleTalk Session Protocol
ASP was an intermediate protocol, built on top of ATP, which in turn was the foundation of AFP. It provided basic services for requesting responses to arbitrary commands and performing out-of-band status queries. It also allowed the server to send asynchronous attention messages to the client.
AppleTalk Transaction Protocol
ATP was the original reliable session-level protocol for AppleTalk, built on top of DDP. At the time it was being developed, a full, reliable connection-oriented protocol like TCP was considered to be too expensive to implement for most of the intended uses of AppleTalk. Thus, ATP was a simple request/response exchange, with no need to set up or tear down connections.
An ATP request packet could be answered by up to eight response packets. The requestor then sent an acknowledgement packet containing a bit mask indicating which of the response packets it received, so the responder could retransmit the remainder.
ATP could operate in either "at-least-once" mode or "exactly-once" mode. Exactly-once mode was essential for operations which were not idempotent; in this mode, the responder kept a copy of the response buffers in memory until successful receipt of a release packet from the requestor, or until a timeout elapsed. This way, it could respond to duplicate requests with the same transaction ID by resending the same response data, without performing the actual operation again.
Datagram Delivery Protocol
DDP was the lowest-level data-link-independent transport protocol. It provided a datagram service with no guarantees of delivery. All application-level protocols, including the infrastructure protocols NBP, RTMP and ZIP, were built on top of DDP.
Name Binding Protocol
NBP was a dynamic, distributed system for managing AppleTalk names. When a service started up on a machine, it registered a name for itself on that machine, as chosen by a human administrator. At this point, NBP provided a system for checking that no other machine had already registered the same name. Then later, when a client wanted to access that service, it used NBP to query machines to find that service. NBP provided browseability ("what are the names of all the services available?") as well as the ability to find a service with a particular name.
As would be expected from Apple, names were truly human readable, containing spaces, upper and lower case letters, and including support for searching.
Printer Access Protocol
PAP was the standard way of communicating with PostScript printers. It was built on top of ATP. When a PAP connection was opened, each end sent the other an ATP request which basically meant "send me more data". The client's response to the server was to send a block of PostScript code, while the server could respond with any diagnostic messages that might be generated as a result, after which another "send-more-data" request was sent. This use of ATP provided automatic flow control; each end could only send data to the other end if there was an outstanding ATP request to respond to.
PAP also provided for out-of-band status queries, handled by separate ATP transactions. Even while it was busy servicing a print job from one client, a PAP server could continue to respond to status requests from any number of other clients. This allowed other Macintoshes on the LAN that were waiting to print to display status messages indicating that the printer was busy, and what the job was that it was busy with.
Routing Table Maintenance Protocol
RTMP was the protocol by which routers kept each other informed about the topology of the network. This was the only part of AppleTalk that required periodic unsolicited broadcasts: every 10 seconds, each router had to send out a list of all the network numbers it knew about and how far away it thought they were.
Zone Information Protocol
ZIP was the protocol by which AppleTalk network numbers were associated with zone names. A zone was a subdivision of the network that made sense to humans (for example, "Accounting Department"); but while a network number had to be assigned to a topologically-contiguous section of the network, a zone could include several different discontiguous portions of the network.
Implementations
The initial default hardware implementation for AppleTalk was a high-speed serial protocol known as LocalTalk that used the Macintosh's built-in RS-422 ports at 230.4 kbit/s. LocalTalk used a splitter box in the RS-422 port to provide an upstream and downstream cable from a single port. The system was slow by today's standards, but at the time the additional cost and complexity of networking on PC machines was such that it was common that Macs were the only networked machines in the office.
Other physical implementations were also available. One common replacement for LocalTalk was PhoneNet, a 3rd party solution (from a company called Farallon) that also used the RS-422 port and was indistinguishable from LocalTalk as far as Apple's LocalTalk port drivers were concerned, but ran over two unused wires in existing phone cabling. PhoneNet was considerably less expensive to install and maintain, and it is perhaps surprising that Apple did not move to this solution as well. Ethernet and TokenRing was also supported, known as EtherTalk and TokenTalk respectively. EtherTalk in particular gradually became the dominant implementation method for AppleTalk as Ethernet became generally popular in the PC industry throughout the 1990s.
The BSD and Linux operating systems support AppleTalk through an open source project called Netatalk, which implements the complete protocol suite and allows them to both act as native file or print servers for Macintoshes, and print to LocalTalk printers over the network.
Networking Model
| OSI Model |
Corresponding AppleTalk layers |
|
Application
|
Apple Filing Protocol (AFP) |
|
Presentation
|
Apple Filing Protocol (AFP) |
|
Session
|
Zone Information Protocol (ZIP)
AppleTalk Session Protocol (ASP)
AppleTalk Data Stream Protocol (ADSP) |
|
Transport
|
AppleTalk Transaction Protocol (ATP)
AppleTalk Echo Protocol (AEP)
Name Binding Protocol (NBP)
Routing Table Maintenance Protocol (RTMP) |
|
Network
|
Datagram Delivery Protocol (DDP) |
|
Data link
|
EtherTalk Link Access Protocol (ELAP)
LocalTalk Link Access Protocol (LLAP)
TokenTalk Link Access Protocol (TLAP)
Fiber Distributed Data Interface (FDDI) |
|
Physical
|
LocalTalk driver
Ethernet driver
Token Ring driver
FDDI driver |
References
Inside AppleTalk - original specification for the AppleTalk suite of protocols
Last updated: 09-01-2005 05:59:44
