Legacy Document

Inside Macintosh: Networking /

Chapter 1 - Introduction to AppleTalk / Deciding Which AppleTalk Protocol to Use

Using a Session Protocol to Send and Receive Data

AppleTalk includes two session protocols that you can use to send and receive data:

• ADSP provides a symmetrical session.
• ASP provides an asymmetrical session.

Most applications use ADSP, which was made available after ASP.

AppleTalk Data Stream Protocol

Your application can use ADSP to set up and maintain a connection with another application over an internet. Through this connection, both applications can send and receive streams of data at any time. Because ADSP allows for the continuous exchange of data, any application that needs to support the exchange of more than a small amount
of data should use ADSP. In addition to providing for a duplex data stream, ADSP also provides an application with a means of sending attention messages to pass control information between the two communicating applications without disrupting the main flow of data.

In most cases, ADSP is the protocol that Apple recommends applications use for sending and receiving data. In addition to ensuring reliable delivery of data, ADSP provides a peer-to-peer connection, that is, both ends of the connection can exert equal control over the exchange of data.

Note

Because ADSP is connection-oriented, it entails additional processing and memory usage in setting up and maintaining the connection between the two applications. Therefore, if your application needs to send a small amount of data, such as a request that the other end perform a task and report the result in response, and you don't want to incur the overhead involved in establishing, maintaining, and breaking a connection, you should consider using ATP rather than ADSP for
data transfer.

ADSP appears to its clients to maintain an open pipeline between the two entities at either end. Either entity can write a stream of bytes to the pipeline or read data bytes from the pipeline. However, because ADSP, like all other higher-level AppleTalk protocols, is a client of DDP, the data is actually sent as datagrams. This allows ADSP to correct transmission errors in a way that would not be possible for a true data stream connection. Thus, ADSP retains many of the advantages of a connectionless protocol while providing to its clients a connection-oriented full-duplex data stream.

An application that uses ADSP can treat the data to be transferred as continuous streams of data, or it can treat it as discrete messages to be interpreted individually. Applications that might use ADSP include server software applications such as mail servers, terminal emulation programs, or any application that requires two-way communication between computers. ADSP also includes features that let you authenticate the identity of the party at the other end of the connection and send encrypted data across the session, which is then decrypted at the other end. The authentication and encryption features of ADSP are referred to as AppleTalk Secure Data Stream Protocol (ASDSP).

AppleTalk Session Protocol

You can use the AppleTalk Session Protocol (ASP) to implement workstation applica-
tions that require an asymmetrical dialog with a server in which the workstation application initiates and controls the dialog. The workstation application tells the server application what to do and the server responds. ASP provides for the setting up, main-
taining, and closing down of a session between a workstation and a server.

A workstation application that requires a state-dependent service should use ASP instead of ATP. State dependence means that the response to a request is dependent on
a previous request. Consider the example of a workstation application connecting to a file server to read a file: before the application can read the file, it must have first issued
a request to open the file. (For example, the AppleTalk Filing Protocol [AFP] uses ASP. However, only the client side of ASP is implemented on the Macintosh.) When a dialog is state dependent, all requests must be delivered in order and duplicate packets must not be sent: ASP provides for this.

An ATP transaction-based request, such as a workstation application requesting a server to return the time of day, is independent of other requests and not state dependent.

ASP assigns each session a unique identifier called a session reference number that allows more than one workstation to establish a session with the same server at the same time. For example, a server might use session reference numbers to distinguish between commands received from various clients of sessions.

ASP ensures that commands from a workstation are delivered without duplication and in the same order in which they were sent. ASP conveys the results of these commands back to the workstation. As long as the session is open, the workstation can request directory information, change filenames, and so forth. The file server must respond to the workstation's commands and cannot initiate any actions on its own.