RSVP messages

There are seven different RSVP messages shown in table .

Table: List of RSVP messages

Message type	Description
Path	RSVP process initiation: set up path state
Resv	Reservation request
Path Teardown	Deletion of a path state
Resv Teardown	Deletion of a reservation state
Path Error	Sent to the sender of a Path message which created an error
Resv Error	Sent to the sender of a Resv message which created an error
Confirmation	Confirm that a reservation has been done correctly

The two most important messages are the Path message and the Resv message. Two hosts willing to request a service from the network proceed as follows:

1. The sender (of the data flow) sends a Path message to the receiver. All along the path, every node supporting RSVP records this Path message and forwards it.

2. When the receiver receives the Path message, if it accepts the communication, it sends back a Resv message following the reverse path the Path message used, requesting the service for the session. Each node records this request, makes the corresponding reservation and forwards the message upstream.

As RSVP has a ``soft'' state, the sender and the receiver periodically send Path and Resv messages respectively to refresh the state. If those refreshes do not occur, the reservation is automatically deleted. The refresh period is calculated in such a way that a reservation will not be deleted because a UDP packet transporting an RSVP message got lost.

States can also be deleted explicitly downstream using a Path Teardown message, or upstream with a Resv Teardown message.

If an error occurs during the reservation process, the originator of the message is warned by an error message. As with other messages, every node supporting RSVP will do the necessary modifications in its tables if it gets an error message. If a Path message created the error, then a Path Error message is sent. A Resv Error will be generated in the case of an error due to a Resv message.

Finally, a host can ask to receive an acknowledgement once the reservation is made. A Confirmation message is used to inform it that the reservation was successful.