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TCP Port Numbers
TCP provides an interprocess delivery system,
so it needs to identify processes in the two "end systems" which it
connects.
To do this, it defines an abstract address called
a port number. Two processes can communicate by
agreeing on the port numbers they will use for communications. Port
numbers are the addresses of the TCP protocol -- each segment
contains port numbers for each of the sending and receiving
processes.
In order to set up a TCP connection, a process notifies the TCP
software that it is waiting for connections "at" a certain port
number. By definition, such a process is called a
server. A client process which
needs to connect to the server asks its local TCP software to
allocate an unused port number and establish the connection. Once
the connection is established, the two processes can
communicate.
In order to manage TCP connections, a group of port numbers (0 to
1023) have been defined to be used by processes providing
well known services. Most Unix systems provide
server processes corresponding to all the well-known services.
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