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Overview Section 6-1
6-1-6 Opening TCP Sockets
To achieve highly reliable data communications, TCP establishes a virtual
communications circuit between the two nodes before starting data transmis-
sions. The virtual communications circuit is known as a “connection.”
Passive OPEN and Active
OPEN
An open command is executed for a node to establish a connection. The open
method differs depending on whether the node is a client or server. A passive
open method is used to open the node as a server and the active open
method is used to open the node as a client.
Note 1. TCP sockets must be closed once a connection has been made before
communications are possible with other TCP sockets. This is true for other
server and client sockets. Up to eight TCP sockets can be open simulta-
neously.
2. With UDP sockets, communications are possible with more than one other
UDP socket even without closing the connection.
3. When a connection is made between two nodes, the process at the node
providing a service is called the server, and the process at the node re-
questing the service is called the client. The server is started first and waits
for a service request from a client. The client requests to the server that a
connection be opened and then transmits data. When the TCP protocol is
used, however, the client–server relationship does not need to be pro-
grammed in the application because it is automatically handled by the pro-
tocol.
TCP Communications
Procedure
The communications procedure is shown below for communications between
a host computer and Ethernet Unit using a TCP socket. In this example, the
host computer is the server and the Ethernet Unit is the client.
Client
TCP
socket
Active
open
Connection
Passive
open
Server
TCP
socket