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Inside Macintosh: Networking /

Chapter 6 - AppleTalk Transaction Protocol (ATP) / About ATP

The Buffer Data Structure

The responder application needs to provide space to store the data to be sent to the requester until the requester application has received all of the data. The requester application needs to provide space to receive the data that it expects to receive as a result of the transaction. Each response can include up to eight packets. To handle the storage of these packets, the ATP client application at each end of the transaction provides a buffer data structure. The buffer data structure is designed to allow ATP to easily manage reliable transfer of multiple packets belonging to a single response message. A buffer data structure consists of an array of eight elements, each of which contains a pointer to a record of type BDSElement.

Each record contains a field for the size of the buffer created to hold the data and a pointer to that buffer. It also contains fields for the size of the data in the response packet and the user bytes that were passed in the packet header, if these bytes were used to communicate additional information. You can create your own buffer data structures,
or you can use the ATP utility provided for this purpose. For a description of the BDS data type, see "The Buffer Data Structure" on page 6-20. For a description of the utility that you can use to build the buffer data structure, see "BuildBDS" on page 6-44.