The Message Send Protocol (MSP), more precisely referred to as Message Send Protocol 2, is an application layer protocol used to send a short message between nodes on a network.
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TCP-based service
One message send service is defined as a connection-based application on TCP. A service listens for TCP connections on port 18. Once a connection is established, a short message is transmitted from the sender to the receiver over the connection. The sender closes the connection after sending the message.
UDP-based service
Another message send service is defined as a datagram-based application on UDP. A service listens for UDP datagrams on port 18. When a datagram is received by the receiver, an answering datagram is sent back to the sender containing exactly the same data.
References
Message Send Protocol Wikipedia(Text) CC BY-SA