Send - may block, buffer available on return, asynchronous

BSend - may block, uses supplied buffers, buffer available on return, asynchronous

SSend - may block, synchronous

RSend - may block, target must be receiving

ISend, IRecv - do not block, return request handle

Wait - block on request handle

Test - check request status

Cancel - cancel blocking wait

- plus B, S and R versions

- plus WaitAny, WaitAll, WaitSome

- plus TestAny, TestAll, TestSome

Probe, IProbe - block but do not receive