Editing AppleTalk (section)

=== AppleTalk Transaction Protocol ===
The AppleTalk Transaction Protocol (ATP) was the original reliable transport-level protocol for AppleTalk, built on top of DDP. At the time it was being developed, a full, reliable connection-oriented protocol like TCP was considered to be too expensive to implement for most of the intended uses of AppleTalk. Thus, ATP was a simple request/response exchange, with no need to set up or tear down connections.

An ATP ''request'' packet could be answered by up to eight ''response'' packets. The requestor then sent an ''acknowledgement'' packet containing a bit mask indicating which of the response packets it received, so the responder could retransmit the remainder.

ATP could operate in either "at-least-once" mode or "exactly-once" mode. Exactly-once mode was essential for operations which were not [[idempotent]]; in this mode, the responder kept a copy of the response buffers in memory until successful receipt of a ''release'' packet from the requestor, or until a timeout elapsed. This way, it could respond to duplicate requests with the same transaction ID by resending the same response data, without performing the actual operation again.<ref>{{Cite web |title=AppleTalk Transaction Protocol |url=https://developer.apple.com/library/archive/documentation/mac/pdf/Networking/ATP.pdf}}</ref>