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Review of Network Programming Theory
To complete our picture of RPC processing, we'll need to review some network programming theory.
In order for two processes running on separate computers to exchange data, an association needs to be formed on each host. An association is defined as the following 5-tuple: {protocol, local-address, local-process, foreign-address, foreign-process}
The protocol is the transport mechanism (typically TCP or UDP) that is used to move the data between hosts. This, of course, is the part that needs to be common to both host computers. For either host computer, the local-address/process pair defines the endpoint on the host computer running that process. The foreign-address/process pair refers to the endpoint at the opposite end of the connection.
Breaking this down further, the term address refers to the network address assigned to the host. This would typically be an Internet Protocol (IP) address. The term process refers not to an actual process identifier (such as a Unix PID) but to some integer identifier required to transport the data to the correct process once it has arrived at the correct host computer. This is generally referred to as a port. Port numbers are used because it is not practical for a process running on a remote host to know the PID of a particular server. Standard port numbers are assigned to well known services such as Telnet (port 23) and FTP (port 21).