Clients are applications that actively connect to remote
applications in order to obtain a service from the remote
application. The remote application is the Server.
Servers are applications that wait for and then reply to incoming requests
Clients need to locate servers, and the simplest way to achieve this is by
having the server bind to a pre-agreed address (other more complex methods
Servers do not need to know the client's address in advance, they just
send their replies to the address from which the client's request originated.
Therefore clients can (normally) bind to nearly any port number, although high port
numbers in the range 0xC000 to 0xFFFF are reserved for use by FreeRTOS+TCP
itself, and many low port numbers are (by convention only)
reserved for common network services.
The standard echo service
provides a convenient example. Echo servers simply echo back the data
sent to them by clients. Echo servers use port 7 by convention. The
sequence diagram below shows a socket being created and bound on both
an echo client and an echo server, and then a single echo transaction between
the client and the server (it should be noted that the sockets only need
to be created and bound once, not for each transaction).
A simple transaction between an echo client and an echo server