resource reservation protocol originally used for call admission control, this is why it deals with bandwidth, but later got te extensions which is just the capability to request a label towards a given pe (called tail end) with the path message, and signal the allocated label backards (to the head end) with the resv message. the protocol initially had the capabilities to describe the p routers the path need to traverse with the explicit route, probably this is why it was selected for te operations. later the protocol got the point to multipoint extension which is useful for effective multicast distribution. the rsvp peers does not form neighborship but request a path when needed, and this path need to be refreshed periodically, otherwise lsp expires resulting in an implicit path error. here are the captures for p2mp and mp2mp.
interestingly, rsvp te extension was proposed by juniper and they seems to prefer it over ldp with the automesh functionality which is a control plane construct. every given pe builds a te tunnel to all the service endpoints discovered from bgp advertisements. this results in an end to end full mesh of lsps between the pe routers.