Calling semantics

Traditionally, if you call a procedure and it returns, you know it executed exactly once
What about RPC?
- A request is made, and no response is received
  - Server is dead
  - Server is slow
  - Server never got your request (UDP)
  - Client never received response (UDP)
- Then a response is received
  - if the client retransmitted the request, how many times did it execute? (UDP)