Question: What is the mandatory encapsulation method for establishing a bi-directional tunnel between the Mobile Anchor Gateway (MAG) and the Local Mobility Anchor (LMA) for routing UE's PDN connection user-plane traffic?
Option 1: IP-in-IP Encapsulation
Option 2: MPLS (Multiprotocol Label Switching)
Option 3: GRE (Generic Routing Encapsulation) with GRE Key Identifier Option always present
Option 4: L2TP (Layer 2 Tunneling Protocol)
Explanation: According to PMIPv6 specifications, GRE encapsulation is mandated for establishing tunnels between MAG and LMA, with the GRE Key Identifier Option always present in PBU messages, ensuring consistent tunneling for user-plane traffic across different transport networks.