The AppleTalkTransactionProtocol (ATP) used on the transport layer is used for the unsecured data transport in an AppleTalk network. Functionally, it is comparable to the User Datagram Protocol
(UDP). The AppleTalk Transaction Protocol
is based onthe Datagram Delivery Protocol (DDP) and supports lossless transaction services between sockets. The services supported by the ATP protocol allow data exchange between two clients, where one client requests the other to perform a specific task. The protocol supports the aggregation of requests and responses
, providinga reliable exchange of requests and responses.
The reliable exchange of requests and responses is governed by two transaction services. Here, the requesting client repeats its requests until it receives a response or until it reaches a certain number of requests. In At-Least-Once (ALO), the request mechanism ensures that at least one request has been submitted. The second ATP transaction mechanism is Exactly-Once (XO).