RSVP is a label signalling protocol that was originally designed to provided end to end quality of services guarantees within IP networks. The protocol has since been extended to provide label signalling and constraint based traffic engineering capabilities with MPLS networks. RSVP should not be confused with more advanced traffic engineering features that involve path determination using CSPF calculation.
RSVP uses path and resv message to signal bidirectional LSPs and carries specific constraint based parameters as objects within these messages.
The primary function of RSVP is to establish an LSP from a head end router to a tail end; to achieve this used primarily to signal LSPs usingbut can also be used to manually steer an LSP path through an MPLS network using a feature called ERO (Explicit Route Object) or to reserve end to end link bandwidth specific LSPs.
Traffic Engineering (ERO)
In addition to providing basic bandwidth reservation and traffic engineering features the extensions to RSVP allow the protocol to carry additional fields within the Resv packet to allocate MPLS labels forming bidirectional LSPs within an MPLS network.