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.