4.2.1 Operation of RSVP

A sender sends path messages to the unicast or multicast destination address of the data. A path message contains the unicast address of the next RSVP-capable host upstream (towards the sender). A node that receives a path message saves this information as path state. It then passes the path message on with its own unicast address as the next upstream RSVP hop. Path messages build a trail of path state on the data path. Using this trail, Resv messages are routed hop-by-hop (unicast) to the next upstream RSVP node. Path messages also contain information about the sender (as a filterspec) and about traffic characteristics of the data flow that the sender will generate (a Tspec). Receivers use this information to choose an appropriate size for a reservation request. Path messages may also carry advertising information (an Adspec) which contains QoS information that is updated on every RSVP node. Receivers may then decide whether the advertised QoS is sufficient for their purposes.

Then, to make a reservation, a receiver sends a reservation request (Resv message) towards the source. Reservation requests specify the requested service (Rspec) and the size of the expected data flow (Tspec). The filterspec specifies which packets can use the reservation. As mentioned before, there are three reservation styles: fixed filter (FF), shared explicit (SE) and wildcard filter (WF). In fixed filter style, a reservation is made for packets sent by exactly one sender that is specified in the filterspec. In shared-explicit style, packets from several sources listed explicitly in the filterspec can use the reservation. In wildcard style, all sources sending to the multicast group address share the reservation. Shared and wildcard filters are useful for applications that are self-limiting in their bandwidth needs. These include audio sessions, because usually not more than one or two participants talk at the same time.

Each receiver makes a reservation according to its own needs. At the branch points in the multicast tree, the reservations from different receivers are merged so that only the larger reservation is passed on towards the sender. Thus, RSVP scales well with the number of receivers in a multicast session [5].