RSVP operates over an IPv4 or IPv6 and provides receiver-initiated setup of resource reservations for multicast or unicast data flows.
RSVP can be used by hosts and routers to request or deliver specific levels of quality of service (QoS) for application data streams.
In Multiprotocol Label Switching (MPLS) a flow is defined as a label-switched path (LSP).
An RSVP host that needs to send a data flow with specific QoS will transmit an RSVP path message every 30 seconds that will travel along the unicast or multicast routes pre-established by the working routing protocol.
Those who want to listen to them send a corresponding resv (short for reserve) message which then traces the path back to the sender.
A simple filter spec could be just the sender’s IP address and optionally its UDP or TCP port.