How can a client process communicate with a server process? A computer program is a
set of predefined instructions that tells the computer what to do. A computer program
has a set of instructions for mathematical operations, another set of instructions for
string manipulation, still another set of instructions for input/output access. If we need
a program to be able to communicate with another program running on another
machine, we need a new set of instructions to tell the transport layer to open the connection,
send data to and receive data from the other end, and close the connection. A
set of instructions of this kind is normally referred to as an interface.