RFC2765 - Stateless IP/ICMP Translation Algorithm (SIIT)

         Flags:
                 The More Fragments flag is copied from the M flag in
                 the Fragment header.  The Don't Fragments flag is set
                 to zero allowing this packet to be fragmented by IPv4
                 routers.

         Fragment Offset:
                 Copied from the Fragment Offset field in the Fragment
                 Header.

         Protocol:
                 Next Header value copied from Fragment header.

4.2.  Translating ICMPv6 Headers into ICMPv4 Headers

   All ICMP messages that are to be translated require that the ICMP
   checksum field be updated as part of the translation since ICMPv6,
   unlike ICMPv4, has a pseudo-header checksum just like UDP and TCP.

   In addition all ICMP packets need to have the Type value translated
   and for ICMP error messages the included IP header also needs
   translation.

   The actions needed to translate various ICMPv6 messages are:

      ICMPv6 informational messages:

        Echo Request and Echo Reply (Type 128 and 129)
           Adjust the type to 0 and 8, respectively, and adjust the ICMP
           checksum both to take the type change into account and to
           exclude the ICMPv6 pseudo-header.

        MLD Multicast Listener Query/Report/Done (Type 130, 131, 132)
           Single hop message.  Silently drop.

        Neighbor Discover messages (Type 133 through 137)
           Single hop message.  Silently drop.

        Unknown informational messages

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