autossh is a program to start a copy of ssh and monitor it, restarting
it as necessary should it die or stop passing traffic.

The original idea and the mechanism were from rstunnel (Reliable SSH
Tunnel). With this version the method changes: autossh uses ssh to
construct a loop of ssh forwardings (one from local to remote, one
from remote to local), and then sends test data that it expects to
get back. (The idea is thanks to Terrence Martin.)