Question

When I use ssh to log in a remote server and open vim, if I don’t type any words the session will timeout and I have to log in again.
But if I run command like top the session will never timeout?
What’s the reason?

Answer 1

Note that the behavior you’re seeing isn’t related to vim or to top. Chances are good some router along the way is culling “dead” TCP sessions. This is often done by a NAT firewall or a stateful firewall to reduce memory pressure and protect against simple denial of service attacks.

Probably the ServerAliveInterval configuration option can keep your idle-looking sessions from being reaped:

 ServerAliveInterval
         Sets a timeout interval in seconds after which if no
         data has been received from the server, ssh(1) will
         send a message through the encrypted channel to request
         a response from the server.  The default is 0,
         indicating that these messages will not be sent to the
         server, or 300 if the BatchMode option is set.  This
         option applies to protocol version 2 only.
         ProtocolKeepAlives and SetupTimeOut are Debian-specific
         compatibility aliases for this option.

Try adding ServerAliveInterval 180 to your ~/.ssh/config file. This will ask for the keepalive probes every three minutes, which should be faster than many firewall timeouts.