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6-1
Section
Overview
89
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.
Client
TCP
socket
Active
open
Connection
Passive
open
Server
TCP
socket
Note
1. TCP sockets must be closed once a connection has been made before com-
munications are possible with other TCP sockets. This is true for other serv-
er and client sockets. Up to eight TCP sockets can be open simultaneously.
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 request-
ing 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 connec-
tion be opened and then transmits data. When the TCP protocol is used,
however, the client–server relationship does not need to be programmed in
the application because it is automatically handled by the protocol.
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.
Host computer
(server)
Passive open
Ethernet Unit
(client)
Connection requested
Active open
Send data
Data send request
Data receive request
ACK (acknowledge)
Send next data
Connection established
Close
Close
Send data
ACK (acknowledge)
Connection established
Data send request
Data receive request
Next data receive request
Passive OPEN and Active
OPEN
TCP Communications
Procedure