尽管未插入LAN电缆，但OutputStream不会引发IOException

Question

I established a TCP Connection with Java. The Server sends an "ALIVE"-Message to the Client every second to detect a broken connection.

but when I plug of the LAN-cable the IOException is thrown exactly after 23seconds, and not immediately after the send attempt.

My code to send looks like that

// out is an OutputStream
try {
    out.write(String.format("%s\r\n", encryptedCommand).getBytes(Charset.forName("UTF-8")));
} catch (IOException e) {
    fireConnectionClosed();
}

I'm pretty sure that this was working already, but now it doesn't any more

Answer 1

When a TCP packet fails to get through, the TCP stack resends the packet a few times. This allows the connection to stay up even if an occasional packet is lost. That prevents the problem of connections dropping whenever there's the least bit of congestion on the network. You can test this by unplugging the network for 5 or 10 seconds, then plugging it back in - the connection should be fine.

However, this feature also means that the TCP connection won't be recognized as having been lost until after a few retries.