Compare RPC call semantics with the semantics of WS-ReliableMessaging:

i) State the entities to which each refers.

ii) Compare the differing meanings of the available semantics (for example, at-least-once,
at-most-once, exactly-once).

In WS-ReliableMessaging, there is a message exchange between and application source and an application destination and the goal is to provide a given semantics over this message delivery. The semantics are as follows:

At-least-once: The message is delivered at least once (with possible duplicate messages), but an error is reported if it cannot be delivered.

At-most-once: The message is delivered at most once (with no duplicate messages, but without any error report if it cannot be delivered.

Exactly-once: The message is delivered exactly once, but an error is reported if it cannot be delivered.

In RPC call semantics, there is a two way interaction between the client and the server with the client sending a request, the server executing the (remote) procedure and the server sending a reply back to the client. The semantics are defined over this complete path and not just the delivery of the initial message (the request):

At-least-once: The client receives a result, in which case the requested procedure is executed at least once (with possible duplicate executions), or receives an error, with no result obtained.

At-most-once: The client receives a result, in which case the requested procedure is executed at most once (no duplicates, but possible failure in executing the procedure), or receives an error, with no result obtained.

Exactly-once: The client receives a result, in which case the requested procedure is executed exactly once (no duplicates, and no failure part way through), or receives an error, with no result obtained.