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Talk Daemon
Port 517 is assigned for the Talk service, an early Unix-based text communication protocol. It enabled real-time messaging between users on different systems using a split-screen interface, facilitating interactive conversations before the advent of modern instant messaging platforms..
The Talk protocol, traditionally operating over UDP port 517, was developed to allow two users on Unix systems to engage in real-time, interactive text conversations. The service employs a split-screen interface where each participant can simultaneously compose and view messages. Communication occurs by broadcasting requests and responses across the network, coordinating session initiation between client and server.
Behind the scenes, a client sends invitation packets over UDP port 517 to locate the intended recipient's talk daemon. The server daemon then mediates the connection establishment, negotiating which host acts as the listener. Once the connection is accepted, the actual chat data is transmitted over a separate negotiated channel, usually TCP port 518 or a dynamically assigned port, to allow continuous, bidirectional communication.
Due to its simplicity, the Talk protocol lacks advanced features or encryption. Its reliance on broadcast messages and unprotected traffic limits its scalability and security, which contributed to its gradual obsolescence with the emergence of more sophisticated chat systems.