RSVP messages are passed between RSVP-capable routers to establish, remove, and confirm resource reservations along specified paths.
RSVP messages are sent as raw IP datagrams with protocol number 46. Each LSR along the path must process RSVP control messages so that it can maintain RSVP session state information. Therefore, most RSVP messages are transmitted with the IP Router Alert Option. Including the IP Router Alert provides a convenient mechanism allowing the IP routing hardware to intercept IP packets destined to a different IP address and deliver them to the RSVP control plane for processing. This is needed to set up and refresh RSVP-TE LSPs that follow an explicitly specified network path and thus may not use the normal routed next hop IP address. RSVP has two basic message types, path message and reserve message, as shown in the following figure.
RSVP has the following message types:
Path Message: The RSVP path message is used to store state information about each node in the path. Each RSVP sender transmits path messages downstream along routed paths to set up and maintain RSVP sessions. Path messages follow the exact same path as the data flow, creating path states in each LSR along the path. The IP source address of the path message must be an address of the sender it describes and the IP destination address must be the endpoint address for the session. The path message is transmitted with the IP Router Alert option since each router along the path must process the path message. Each LSR is responsible for refreshing its path status by periodically transmitting a path message to the downstream LSR.
In addition to the previous hop address, the path message contains the sender Tspec and Adspec. The reservation message carries the flowspec.
Reserve Message: Each receiver host transmits an RSVP reservation request to its upstream neighbor. Reserve messages carry reservation requests hop-by-hop along the reverse path. The IP destination address of a reserve message is the unicast address of the previous-hop LSR, obtained from the session's path state. The IP source address is the address of the node that originated the message. The reserve message creates and maintains a reserve state in each node on the path. Each LSR is responsible for refreshing its reserve status by periodically transmitting a reserve message to the upstream LSR.
Reserve messages are eventually delivered to the sender, so that the sender can configure appropriate traffic control parameters for the first hop node.
Path Tear Message: Path tear messages delete path state information reserved along the path. The message is initiated by the path sender or by any LSR in which a path state time-out occurs or an LSP is preempted (due to bandwidth reservations), and is sent downstream to the session's path endpoint. Path tear messages are transmitted with the IP Router Alert option and are routed exactly the same as path messages. The IP destination address must be the path endpoint and the source IP address must be the sender address obtained from the session's path state for the path that is being torn down.
When a path state is deleted as the result of the path tear message, the related reservation state must also be adjusted to maintain consistency in the node. The adjustment depends on the reservation style.
Reserve Tear Message: Reserve tear messages delete reservation state information. The message is initiated by the path endpoint or any node along the path in which a reservation state has timed out or an LSP is preempted (due to bandwidth reservations), and is sent upstream to the session's path sender. Reserve tear messages are routed exactly the same as reserve messages. The IP destination address of a reserve message is the unicast address of the previous-hop node, obtained from the session's reservation state. The IP source address is the address of the node that originated the message.
If no reservation state matches the reserve tear message, the message is discarded. The reserve tear message can delete any subset of the filter specification in FF-style or SE-style reservation state. Reservation styles are described in the following table.
Path Error Message: Path error messages are used to report processing errors for path messages. These messages are sent upstream to the sender that issued the path message. The message is routed hop-by-hop using the path state information maintained in each node. Path error messages are informational and do not modify the path state within any node.
Reserve Error Message: Reserve error messages are used to report processing errors for reserve messages. In addition, reserve error messages are used to report the spontaneous disruption of a reservation. Reserve error messages travel downstream to the endpoint of the session. The message is forwarded hop-by-hop using the reservation state information maintained in each node. Reserve error messages are informational and do not modify the reservation state within any node.
Reserve Confirm Message: Reserve confirm messages are optionally transmitted to acknowledge a reservation request. These messages are transmitted from the sender to the endpoint. The destination IP address is the IP address of the endpoint and the source IP address is the address of the sender. Since none of the intermediate path nodes need to process a reserve confirm message, the message is transmitted without the IP Router Alert option.