Unexpected messages

BMI's default mode of operation requires that each send operation be matched with a certain receive operation at the remote host in order to complete. This send and receive operation must match in terms of expected message size (more on this in section ), host address, and identication tag. Otherwise the communication will not complete. There is no mechanism for receiving from a ``wildcard'' address.

However, in order to loosen this restriction, BMI provides a special class of messages called unexpected messages. This type of message is sent without the receiving host explicitly requesting the communication. In other words, the receiving host does not post a matching receive for this type of message. Instead, it must periodically check to see if any unexpected messages have arrived in order to receive them successfully. This is the equivalent of ``listening'' for new requests in a more traditional networking system. Unexpected messages may come from any host on the network. Communication between two hosts is typically initiated by one of the hosts sending an unexpected message to the other.

Unexpected messages may be of any size less than a limit defined by the interface. When an unexpected message arrives, BMI will provide a buffer for it. This buffer is passed to the receiving process when it checks to see if unexpected messages have arrived. It is the responsibility of the caller to eventually free this buffer using the normal system free() function.