add TODO

* Verify tunnel MTU

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+* 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.
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