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TCP Half-Open & Half-Closed Connections

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TCP close is not symmetric

Most developers think of a TCP connection as a single thing that opens and closes. In reality, TCP has separate send and receive paths — each can be closed independently. This creates half-closed connections, half-open connections, and the dreaded CLOSE_WAIT bug.

Understanding these states is essential for debugging connection leaks, keepalive timeouts, and RST responses.

Packets in motion — click to inspect:

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