A GRE tunnel initiator and a tunnel endpoint can exchange GRE Heartbeat messages
to provide status information about the tunnel endpoint. The tunnel endpoint can
send the tunnel initiator a GRE Heartbeat message through an established GRE tunnel.
The GRE Heartbeat messages can be sent at different times while the GRE tunnel
is active, and they can indicate to the GRE tunnel initiator that the tunnel endpoint
remains active.