Assuming SEND functionality is kept as is, exactly what is the
right layer to implement its security mechanisms? The possibilities
are as follows:

    1. AH + in part ND i.e. as in draft-ietf-send-ipsec-01.txt.
    2. CGA extension header + AH i.e. as in draft-nikander-send-ipsec-00.txt.
    3. Solely in ND i.e. as in draft-arkko-send-ndopt-00.txt.

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In IETF-57 we discussed this and decided to go with ND options.