A message consists of the data and the message envelope. The envelope provides information on how to match sends to receives. The three parameters used to specify the message envelope are:

Destination or source

This argument is set to a rank in a communicator (see below). Rank ranges from 0 to (size-1) where size is the number of processes in the communicator. Destination is specified by the send and is used to route the message to the appropriate process. Source is specified by the receive. Only messages coming from that source can be accepted by the receive call, but the receive can set source to MPI_ANY_SOURCE to indicate that any source is acceptable.

Tag

An arbitrary number to help distinguish among messages. The tags specified by the sender and receiver must match, but the receiver can specify MPI_ANY_TAG to indicate that any tag is acceptable.

Communicator

The communicator specified by the send must equal that specified by the receive. Communicators will be described in more depth later in this module. For now, it is enough to know that a communicator defines a communication "universe", and that processes may belong to more than one communicator. In this module, we will only be working with the predefined communicator MPI_COMM_WORLD which includes all processes in the application.

An Analogy

To help understand the message envelope parameters, consider the analogy of a bill collection agency that collects for several utilities. When sending a bill the agency must specify:

1. The person receiving the bill (more specifically, their ID number). This is the destination.
2. What month the bill covers. Since the person will get twelve bills each year, they need to know which month this bill covers. This is the tag.
3. Which utility is being collected for. The person needs to know whether this is their electric or phone bill. This is the communicator.