Final answer:
NorthStar uses the Path Computation Element Protocol (PCEP) to establish paths between PE devices in an SDN environment.
Step-by-step explanation:
The protocol required for NorthStar to establish paths between Provider Edge (PE) devices is the Path Computation Element Protocol (PCEP).
NorthStar is a Software-Defined Networking (SDN) controller that utilizes PCEP to communicate with network devices for the purpose of path computation. PCEP enables the NorthStar Controller to instruct routers on setting up Label Switched Paths (LSPs) in MPLS networks, which are critical for managing traffic flow and ensuring efficient network operation.