About AppleTalk
Every Macintosh computer includes AppleTalk hardware and software, so if your application needs to communicate with other Macintosh computers, you may want to use an AppleTalk protocol. AppleTalk includes protocols that handle Macintosh workstation-server interaction, LaserWriter and ImageWriter printing, data exchange through data streams or packets, and AppleTalk name lookups across a network.Although AppleTalk includes protocols that provide connection-oriented services, it is considered a connectionless network because all AppleTalk data is ultimately delivered by the Datagram Delivery Protocol (DDP), which implements connectionless packet delivery. Connection-oriented AppleTalk protocols that establish sessions and provide reliable delivery of data, such as the AppleTalk Data Stream Protocol (ADSP) and the AppleTalk Transaction Protocol (ATP), are built on top of the connectionless packet services that DDP provides. In the AppleTalk protocol stack, each protocol in a specific layer provides a set of functions and services to one or more protocols in a higher-level layer.
The AppleTalk architecture is closely aligned with the industry-standard Open Systems Interconnection (OSI) networking model. Figure 9-1 shows the AppleTalk protocols supported by Open Transport and shows how they relate to one another in the layers defined by the OSI model.
Figure 9-1 AppleTalk protocol stack and the OSI model
Here are some points worth noting about how AppleTalk under Open Transport maps to the OSI model:
- At the session layer, the AppleTalk Data Stream Protocol (ADSP) provides its own stream-based transport layer services that allow for full-duplex dialogs, while the Printer Access Protocol (PAP) uses the transaction-based services of the AppleTalk Transaction Protocol (ATP) to transport workstation commands to servers. The Zone Information Protocol (ZIP) is also at the session layer; a subset of its functions are available through AppleTalk service providers.
- At the transport layer, there are the AppleTalk Transaction Protocol (ATP) and Name-Binding Protocol (NBP), but NBP is accessible only through mapper providers. In addition to these two protocols, ADSP includes functions that span both the session and the transport layers.
- At the network layer, the Datagram Delivery Protocol (DDP) is AppleTalk's network delivery protocol.
- At the data-link layer, various link-access protocols support the underlying networking hardware. Open Transport provides standard Streams modules for the LocalTalk, Ethernet, token ring, and FDDI drivers.
AppleTalk Networks and Addresses
An AppleTalk network can be either a nonextended network or an extended network. Applications can use AppleTalk protocols across a single AppleTalk network or an AppleTalk internet, which is a number of interconnected AppleTalk networks. An AppleTalk internet can include a mix of LocalTalk, TokenTalk, EtherTalk, and FDDITalk networks, or it can consist of multiple networks of a single type, such as several LocalTalk networks. An AppleTalk internet can include both nonextended and extended networks.An AppleTalk nonextended network is one in which
LocalTalk is an example of a nonextended network. Each node on a nonextended network, such as LocalTalk, has a unique 8-bit node ID. Since there are 256 possible combinations of 8 bits, and three IDs are not available (ID 255 is reserved for broadcast messages and ID 0 and 254 are not allowed), a nonextended network can support up to 253 active nodes at a time.
- the network has one network number assigned to it
- the network supports only one zone
- all nodes on the network share the same network number and zone name
- each node on the network has a unique node ID
An AppleTalk extended network is one in which
Table 9-1 summarizes the identifiers that you use for AppleTalk addressing.
- the network has a range of network numbers assigned to it
- the network supports multiple zones
- each node on the network has a unique network number-node ID combination to identify it
Each network is assigned a network number so that an AppleTalk internet router can determine the packet's destination network number and forward the packet through an internet from one router to another until the packet arrives at its correct destination network. An extended network uses a range of network numbers. Nodes on an extended network can have different zone names and different network numbers within the network number range.
A node is a data-link addressable entity on an AppleTalk network; all physical devices on an AppleTalk network are nodes. When a node first connects to an AppleTalk network or is rebooted, AppleTalk dynamically assigns it a unique 8-bit node ID. For a node on an extended network, AppleTalk also assigns it a 16-bit network number within the range of numbers assigned to the extended network that the device is connected to. Once a packet arrives at its destination network, the packet is delivered to its destination node within that network, based on the node ID.
Note that because AppleTalk assigns node IDs dynamically whenever a node joins the network or is rebooted, a node's address on an AppleTalk network can change from time to time, although a computer attempts to reuse the node ID it last used. NBP provides a mapping of logical names (like those in the Chooser) to physical addresses in such a way that if the node ID changes, you can still find your application. This mapping is discussed further in the chapters "AppleTalk Addressing" and "AppleTalk Service Providers" in this book.
A zone is a logical grouping of nodes within an AppleTalk internet. The use of zones allows a network administrator to set up departmental or other logical sets of nodes in an internet. A single extended network can contain nodes belonging to any number of zones; an individual node on an extended network can belong to only one zone. Each zone is identified by a unique zone name.
A socket is an addressable data-link entity on a network. Endpoints exchange data with each other across an internet through sockets. Because each endpoint has its own socket address, a node can have multiple concurrent open connections, for example, one to a file server and one to a printer. A node can have several sockets open at the same time, so each endpoint on an AppleTalk network is associated with a unique 8-bit socket number.
AppleTalk sockets are divided into two groups: statically assigned sockets and dynamically assigned sockets. Statically assigned sockets are those sockets that are permanently reserved for a designated protocol or process. For example, socket 4 is always reserved as the echo socket, used for echoing packets across a network. Dynamically assigned sockets are those sockets arbitrarily assigned by DDP if you do not specify a socket number when binding an endpoint; DDP returns the socket number to you in the endpoint's address when the binding has completed.
In certain situations, you can bind multiple endpoints to a single socket. For connectionless endpoints, each must uses a different protocol. For connection-oriented endpoints, they can all use the same protocol, but each must establish a connection with a different remote endpoint.
Multinodes
AppleTalk's multinode architecture allows an application to acquire virtual node IDs, called multinode IDs. These multinode IDs allow the computer running your application to appear as multiple nodes on the network even though it is only one physical entity. Each acquired multinode is in addition to the standard node ID already assigned to the computer when it joined the network as a node.The prime example of a multinode application is Apple Remote Access (ARA).You can use a multinode to receive broadcast packets and any AppleTalk packets addressed to it through its multinode ID. You must then process the packets in a custom manner. A multinode ID is not connected to the AppleTalk protocol stack above the data-link layer, which means that an application that uses a multinode cannot use the services of higher-level protocols such as NBP, ATP, and ADSP, but instead must implement its own higher-level protocols if it expects packets for such protocols.
Handling Miscellaneous Events
In classic AppleTalk, you could use the AppleTalk Transition Queue (ATQ) to inform your application of miscellaneous events that occurred unexpectedly within AppleTalk. In Open Transport AppleTalk, this facility has been modified to allow your endpoint to receive only a few predefined events. Any applications that rely on the AppleTalk Transition Queue must use AppleTalk backward compatibility to handle them in the classic AppleTalk manner.In Open Transport AppleTalk, there are five miscellaneous events that you
can receive on your endpoint, which does not need to be bound. They are
as follows:To receive these events, your application must use the
OTIoctlfunction with a provider reference value, the constantkOTGetMiscellaneousEventsas its command, and the value of 1 as its data. For more information on theOTIoctlfunction, refer to the chapter "Providers" in this book.Configuring AppleTalk Protocol Providers
When you want to use a particular AppleTalk protocol, you open an endpoint configured for that protocol. To do this, you use specific constants as part of a configuration string that you pass to the Open Transport function for opening endpoints. This string specifies to Open Transport how to create the correct endpoint for you. For more information on the functions that you use to open endpoints, mappers, and AppleTalk service providers, refer to the chapters in this book on the specific type of provider; for more information about configuring providers, see the chapter "Configuration Management" in this book.Table 9-2 lists which constants to use to configure the AppleTalk providers. Note that these values are subject to change--they are included here only to provide an overview of how OpenTransport configures providers. Be sure to consult the AppleTalk header file for the current values.
There is one exception to the typical method of configuring providers. AppleTalk service providers do not have a string equivalent value. You configure an AppleTalk service provider with the constant
kDefaultAppleTalkServicesPath, which has a value of((OTConfiguration*)-3). The code for creating an AppleTalk service provider is as follows:
OTOpenEndpoint(kDefaultAppleTalkServicesPath, 0, &err)If you want to set an option as part of the configuration string, you need to know which protocols use which options and how to translate the option's constant name, given in the header files, into a string that the configuration functions can parse. For the AppleTalk options, Table 9-3 provides the constant name, the value used in the configuration string, and the protocols that use that option.To configure a provider with an option string, you put the string and its assigned value in parentheses after the protocol that uses it, as in the following lines of code:
OTOpenEndpoint(OTCreateConfiguration ("adsp,ddp(Checksum=1),ltlkB"), 0, NULL, &err) OTOpenEndpoint(OTCreateConfiguration (kADSPName"(EnableEOM=1)"), 0, NULL, &err);
