This option applies to protocol version 2 only in protocol version 1 there is no mechanism to request a response from the server to the server alive messages, so disconnection is the responsibility of the TCP stack. If, for example, ServerAliveInterval (see below) is set to 15 and ServerAliveCountMax is left at the default, if the server becomes unresponsive, ssh will disconnect after approximately 45 seconds.
The server alive mechanism is valuable when the client or server depend on knowing when a connection has become inactive. The TCP keepalive option enabled by TCPKeepAlive is spoofable. The server alive messages are sent through the encrypted channel and therefore will not be spoofable. It is important to note that the use of server alive messages is very different from TCPKeepAlive (below). If this threshold is reached while server alive messages are being sent, ssh will disconnect from the server, terminating the session. ServerAliveCountMax Sets the number of server alive messages (see below) which may be sent without ssh(1) receiving any messages back from the server.
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