path computation element protocol is used by pe routers to ask to calculate lsps for te or sr. originally it was invented to overcome the problem of inter-area or inter-level te that is, the headend have no visibility to distant parts of the network so cannot calculate the lsps alone. alternatively, it could be used for inter-as te. later extended for sr, in this case no rsvp nodes but explicit labels are returned as a result. with the sdn era, it could be used to centralize path computation on a controller. here is the capture for pcep.