Introduction to Open Transport
Open Transport is the networking architecture used by Apple Computer, Inc. for Macintosh computers. Whereas AppleTalk provided a proprietary networking system for Macintosh computers, the current Macintosh Operating System with Open Transport provides not only AppleTalk but also the industry-standard TCP/IP protocols and serial connections. In addition, the Open Transport architecture allows developers to add other networking systems to the Macintosh Operating System without altering the user's experience or the application programming interface (API).The independence of the APIs from the underlying networking or transport technology is called transport independence and is one of the cardinal features of Open Transport. Whereas the APIs are independent of the networking system in use, the specific set of functions you call does depend on the nature of the protocols. For example, you use different functions for a connection-oriented protocol like AppleTalk Data Stream Protocol (ADSP) than for a connectionless protocol like Datanet Delivery Protocol (DDP) or Internet Protocol (IP). Transport independence is described in more detail in "Transport Independence" on page 1-16.
Other important features of Open Transport are its support of multihoming and multinodes.
Multihoming allows multiple Ethernet, token ring, FDDI, and other network interface controller (NIC) cards to be active on a single node at the same time. In addition to selecting the type of network connection, the user can select a particular device to be used for the network connection.
Multinode architecture is an AppleTalk feature that allows an application to acquire node IDs that are additional to the standard node ID that is assigned to the system when the node joins an AppleTalk network. Multinode architecture is provided to meet the needs of special-purpose applications that receive and process AppleTalk packets in a custom manner instead of passing them directly on to a higher-level AppleTalk protocol for processing. Multinode IDs allow the system that is running your application to appear as multiple nodes on the network. The prime example of a multinode application is Apple Remote Access (ARA). The chapters "AppleTalk Addressing" and "Datagram Delivery Protocol" in this book describe the use of multinodes.