How Can a Server Reliably Detect TCP Client Disconnections?

How to Detect TCP Client Disconnections

TCP, a reliable transmission protocol, ensures data integrity and delivery. However, when a client disconnects prematurely or loses network connectivity, it becomes crucial for the server to detect and handle such scenarios.

Detecting Orderly Disconnects

The only reliable method to detect an orderly client disconnect is through read operations. When the client sends a close command, the read() function will return 0, indicating the orderly termination of the connection.

Detecting Broken Connections

Unlike orderly disconnects, broken connections require a more active approach. Attempting to write data to a disconnected client will eventually trigger a response from TCP. The write operation will fail with an error code (ECONNRESET or "connection timed out") after multiple retries and timeouts, indicating a broken connection.

Read Timeout and Error Handling

To prevent hangs, a reasonable read timeout should be established. If a read operation exceeds this timeout, the connection should be terminated. Establishing a heartbeat mechanism can also help detect inactive clients.

Pitfalls

The ioctl() function with the FIONREAD argument is not an effective method for detecting disconnects. FIONREAD merely indicates the number of bytes in the socket receive buffer, which by itself is not a reliable indicator of client disconnection.

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