Keeping SSH sessions alive with Mac OS X Terminal

You can define that you want to keep the connection alive in the ssh configuration. You have to edit (create if it does not exist):

~/.ssh/config

Add these two lines to it:

ServerAliveInterval 300
ServerAliveCountMax 36

The value ServerAliveInterval defines the interval in seconds between two noop operations that keeps to connection alive. It is 5 minutes in this case. 
The value ServerAliveCountMax defines the number of times the noop is sent. In this case the connection will be kept open for 3 hours.