* Verify tunnel MTU

   RFC 7676 section "3.2.  MTU Considerations"

   Before activating a GRE tunnel and periodically thereafter, the GRE
   ingress node MUST verify the tunnel's ability to carry a 1280-octet
   IPv6 payload packet from ingress to egress, without fragmenting the
   payload.  Having executed those procedures, the GRE ingress node MUST
   activate or deactivate the tunnel accordingly.

   Implementation details regarding the above-mentioned verification
   procedures are beyond the scope of this document.  However, a GRE
   ingress node can verify tunnel capabilities by sending a 1280-octet
   IPv6 packet addressed to itself through the tunnel under test.