Which mechanism removes the outermost MPLS label before forwarding a packet to an adjacent LER?

Penultimate Hop Popping (PHP) is a mechanism where the Label Edge Router (LER) immediately upstream from the egress LER removes the outermost MPLS label. This is done to prevent the egress LER from having to perform both an MPLS lookup and an IP lookup, thereby optimizing its processing. The egress LER receives a pure IP packet and forwards it based on its IP routing table. Explicit-null is a specific type of PHP where the penultimate LER replaces the service label with a special "explicit-null" label (label value 0 for IPv4, 2 for IPv6) before popping it, indicating to the egress LER that the packet is an IP packet. Label swap is the standard operation within the MPLS core where an incoming label is replaced with an outgoing label. Label imposition is the process of adding an initial MPLS label to an IP packet at the ingress LER.