[jggimi]

I guess my memory was playing tricks, and I have mistated the probable cause. From http://tools.ietf.org/html//rfc4303

   In tunnel mode, the "inner" IP header carries the ultimate (IP)
   source and destination addresses, while an "outer" IP header contains
   the addresses of the IPsec "peers", e.g., addresses of security
   gateways.  Mixed inner and outer IP versions are allowed, i.e., IPv6
   over IPv4 and IPv4 over IPv6.  In tunnel mode, ESP protects the
   entire inner IP packet, including the entire inner IP header.  The
   position of ESP in tunnel mode, relative to the outer IP header, is
   the same as for ESP in transport mode.  The following diagram
   illustrates ESP tunnel mode positioning for typical IPv4 and IPv6
   packets.

                 BEFORE APPLYING ESP
            ----------------------------
      IPv4  |orig IP hdr  |     |      |
            |(any options)| TCP | Data |
            ----------------------------

                 AFTER APPLYING ESP

            -----------------------------------------------------------
      IPv4  | new IP hdr* |     | orig IP hdr*  |   |    | ESP   | ESP|
            |(any options)| ESP | (any options) |TCP|Data|Trailer| ICV|
            -----------------------------------------------------------
                                |<--------- encryption --------->|
                          |<------------- integrity ------------>|