Each sender host transmits path messages downstream along the routes provided by the unicast and multicast routing protocols. Path messages follow the exact paths of application data, creating path states in the routers along the way, thus enabling routers to learn the previous-hop and next-hop node for the session. Path messages are sent periodically to refresh path states.
The refresh interval is controlled by a variable called the refresh-time, which is the periodical refresh timer expressed in seconds. A path state times out if a router does not receive a specified number of consecutive path messages. This number is specified by a variable called keep-multiplier. Path states are kept for ( (keep-multiplier + 0.5) x 1.5 x refresh-time ) seconds.