RSVP uses unidirectional and simplex flows through the network to perform its function. The inbound router initiates an RSVP path message and sends it downstream to the outbound router. The path message contains information about the resources needed for the connection. Each router along the path begins to maintain information about a reservation.
When the path message reaches the outbound router, resource reservation begins. The outbound router sends a reservation message upstream to the inbound router. Each router along the path receives the reservation message and sends it upstream, following the path of the original path message. When the inbound router receives the reservation message, the unidirectional network path is established.
The established path remains open as long as the RSVP session is active. The session is maintained by the transmission of additional path and reservation messages that report the session state every 30 seconds. If a router does not receive the maintenance messages for 3 minutes, it terminates the RSVP session and reroutes the LSP through another active router.