Comparison With TCP

At a first glance, there are many similarities between TCP and our protocol, e.g. sliding window, sequence numbers, sender-based retransmission, and acknowledgments. However, there are three differences worthwhile noting:

• Unlike in TCP/IP, there is no guarantee that the protocol at the receiver's side is serviced by the CPU when the sender sends. This makes the use of retransmission timers difficult. An expired timer is not a good indicator of packet loss -- the application on the other side could be off doing computations, for instance because of load imbalance.

• For small messages, a similar issue exists on the sender side. Performance reasons (and for the non-blocking case the MPI semantics) mandate a release of control by the ADI layer, which in turn makes it impossible to build a TCP-style data transfer with a round trip estimator.
• Whereas TCP provides a reliable byte stream, we operate on a per message basis. This requires a smaller sequence number space and results in a smaller header.