Consider the RPC mechanism.Des

If an RPC mechanism could not support either the“at most once”or“at least once”semantics,then the RPC server can not guarantee that a remote procedure will not be invoked multiple occurrences.Consider if a remote procedure were withdrawing money from a bank account on a system that did not support these semantics.It is possible that a single invocation of the remote procedure might lead to multiple withdrawals on the server.

For a system to support either of these semantics generally requires the server maintain some form of client state such as the timestamp described in the text.

If a system were unable to support either of these sematics,then such a system could only safely provide remote procedures that do not alter data or provide time-sensitive results.Using our bank accunt as an example,we certainly require“at most once”or“at least once”semantics for performing a withdrawal (or deposit!)However,an inquiry into an account balance or other accunt information such as name,address,etc.does not require these semantics.