QoS RSVP Path message
There are two fundamental RSVP message types Resv and Path.
A Path message is used by the source of the intended traffic for which RSVP will be employed, to indicate to the intended receiver, as well as to all intervening RSVP routers the required resource reservation for the traffic to be sent.
When a host wants to make a reservation it will send a RSVP reservation request using a RSVP Path message. This message is passed along the route towards the destination. When a router can guarantee the required bandwidth/delay it will forward the message to the next hop router until it reaches the intended receiver.
For more information about Quality of Service in general, take a look at QoS.
Links:
https://networklessons.com/quality-of-service/introduction-to-rsvp