3.5.3. Recieve order

Each rank communicates only with those ranks that own tasks that are neighbors to the rank. The MPI protocol used ensures that these packages are received in the same order as they were sent, allowing copies of tasks (aka “virtual task”) to show the exact same update behavior as the real task on the owner rank. This is accomplished by using tags on the packages that encode sequence numbers, with each rank keeping track of the sequence numbers from other ranks in an atomic fashion.