Protocol Overview | Section |
6-2 Protocol Overview
6-2-1 Differences between TCP and UDP
There are differences in the socket services between TCP and UDP.
■TCP Communications
The following procedure is followed each time data is transmitted to ensure that the data arrives normally at the remote node:
1,2,3... 1. The remote node returns ACK when data is received normally.
2.The local node sends the next data after it receives ACK, or it resends the same data if ACK is not returned within the specified time.
Send request made.
Local node
Transmitted data
ACK (acknowledge)
Resent data
when ACK is not returned
Receive request made.
Remote node
With the TCP protocol, the remote IP address and remote TCP port number are specified when an open request is made for a socket. When a send request is made, the number of bytes to send and the send data are specified. When a receive request is made, the number of bytes to receive is specified.
With the TCP protocol, communications with another remote device are not possible until the socket that was opened has been closed.
■UDP Communications
Data is simply sent to the remote node. Unlike TCP, the reception of data is not checked and data is not resent. To increase communication reliability, data resends must be programmed by the user in user application.
Send request made.
Transmitted data
ACK (acknowledge: only when processed by application)
Receive request made.
Local node | Remote node |
With the UDP protocol, the remote IP address and remote UDP port number are not specified when an open request is made for a socket. When a send request is made, the remote IP address, the remote UDP port number, the number of bytes to send, and the send data are specified. When a receive request is made, the number of bytes to receive is specified. (The response data shows from which IP address and UDP port number the received data was sent.)
With the UDP protocol, communications with another remote device are possible even if the socket that was opened is not closed.
6-2-2 Opening TCP Sockets
To achieve highly reliable data communications, TCP establishes a virtual communications circuit between the two nodes before starting data transmissions. The virtual communications circuit is known as a “connection.”
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