Inside Macintosh: Networking With Open Transport / Part 1 - Open Transport Essentials

Chapter 17 - AppleTalk Transaction Protocol (ATP)

This chapter describes the programming interface to Open Transport's implementation of the AppleTalk Transaction Protocol (ATP). It explains how you can use ATP to send requests and responses between ATP endpoints, with one endpoint initiating the request and the other responding to it. You can create an endpoint that can both initiate and respond, or you can create one endpoint that only makes requests and another that only makes responses. Because ATP provides a connectionless transaction-based service, you do not incur the overhead entailed in establishing, maintaining, and breaking a connection that is associated with connection-oriented protocols, such as ADSP, but you can transfer only a limited amount of data using ATP.

You should read this chapter if you want to write an application that requires reliable delivery of data but does not require the transfer of large amounts of data. This chapter explains how you

• open and bind an ATP endpoint

• get information about an ATP endpoint

• use Open Transport functions to initiate and respond to a transaction

• specify ATP options to control connectionless transaction-based services

This chapter begins with a description of ATP and the services that it provides under Open Transport. The section "Using General Open Transport Functions With ATP" then gives detailed information about how ATP client applications use the endpoint functions that Open Transport provides for connectionless transaction-based protocols. For a more detailed explanation of endpoints and their functions, read the chapter "Endpoints".

For reference information about ATP options, see "ATP Reference". For an overview of ATP and how it fits within the AppleTalk protocol stack, read the chapter "Introduction to AppleTalk", which also introduces and defines some of the terminology used in this chapter. For a complete explanation of the ATP specification, see Inside AppleTalk, second edition.

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Chapter Contents

About ATP

Using ATP

At-Least-Once and Exactly-Once Transactions

Sending and Receiving ATP Data

Specifying ATP Options

The Retry Count and Interval Options

The Release Timer Option

Other ATP-Specific Options

Using the ATP Packet Header User Bytes

Using General Open Transport Functions With ATP

OTSndURequest

OTRcvURequest

OTSndUReply

OTRcvUReply