Almost all of the complexity in MPI client/server routines addresses the question ``how does the client find out how to contact the server?'' The difficulty, of course, is that there is no existing communication channel between them, yet they must somehow agree on a rendezvous point where they will establish communication.
Agreeing on a rendezvous point always involves a third party. The third party may itself provide the rendezvous point or may communicate rendezvous information from server to client. Complicating matters might be the fact that a client does not really care what server it contacts, only that it be able to get in touch with one that can handle its request.
Ideally, MPI can accommodate a wide variety of run-time systems while retaining the ability to write simple, portable code. The following should be compatible with MPI:
A port_name is a system-supplied string that encodes a low-level network address at which a server can be contacted. Typically this is an IP address and a port number, but an implementation is free to use any protocol. The server establishes a port_name with the MPI_OPEN_PORT routine. It accepts a connection to a given port with MPI_COMM_ACCEPT. A client uses port_name to connect to the server.
By itself, the port_name mechanism is completely portable, but it may be clumsy to use because of the necessity to communicate port_name to the client. It would be more convenient if a server could specify that it be known by an application-supplied service_name so that the client could connect to that service_name without knowing the port_name.
An MPI implementation may allow the server to publish a ( port_name, service_name) pair with MPI_PUBLISH_NAME and the client to retrieve the port name from the service name with MPI_LOOKUP_NAME. This allows three levels of portability, with increasing levels of functionality.