RFC2765 - Stateless IP/ICMP Translation Algorithm (SIIT)

                    IPv6-to-IPv4 Translation

   There are some differences between IPv6 and IPv4 in the area of
   fragmentation and the minimum link MTU that effect the translation.
   An IPv6 link has to have an MTU of 1280 bytes or greater.  The
   corresponding limit for IPv4 is 68 bytes.  Thus, unless there were
   special measures, it would not be possible to do end-to-end path MTU
   discovery when the path includes an IPv6-to-IPv4 translator since the
   IPv6 node might receive ICMP "packet too big" messages originated by
   an IPv4 router that report an MTU less than 1280.  However, [IPv6]
   requires that IPv6 nodes handle such an ICMP "packet too big" message
   by reducing the path MTU to 1280 and including an IPv6 fragment
   header with each packet.  This allows end-to-end path MTU discovery
   across the translator as long as the path MTU is 1280 bytes or
   greater.  When the path MTU drops below the 1280 limit the IPv6
   sender will originate 1280 byte packets that will be fragmented by
   IPv4 routers along the path after being translated to IPv4.

   The only drawback with this scheme is that it is not possible to use

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