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b. TCP socket interface (See figures 6.3 and 6.4.)
•In TCP communication, a pipe-like virtual communications line (connection) is established
between sockets on two ports that are communicating (sending and receiving), and since the
protocol handles control of reception verification and retransmission processing, it provides a
highly reliable communication.
•There are two methods for establishing a connection: passive open and active open. In passive
open, the local socket is put into a state in which it is waiting for a request to establish a
connection from another port. In active open, a request is issued for the establishment of a
connection with a socket in the passive open state.
•When establishing a connection between two ports, one port must first open a socket in passive
open mode, and the other port must perform an active open on a local socket with respect to a
remote socket that is in the passive open state.
•Since the passive open socket port "provides" data transmission and other services, it is called
the "server."
•Since the active open socket port "requests" services, it is called the "client."
•A socket for which a connection has been established is unable to transfer data with any other
sockets unless that connection is first released.
•Since sockets are connected by connections, there is no need to specify the transmission source
or transmission destination with every data item transmitted.
Figure 6.3 TCP Socket (Connection)
Connection
Transmission data
Reception verification
Transmission destination port
TCP socket
Transmission source port
TCP socket
Server side
Passive open
Client side
Active open