Chapter 11 IPsec VPN for FortiOS 5.0 : IPv6 for FortiOS 5.0 : IPv6 packet structure : Fragmentation and reassembly
  
Fragmentation and reassembly
As noted, packets that are too large for the MTU require hosts to perform MTU Path Discovery to determine the maximum size of packets to send. Packets that are too large require a 'Fragment' extension header, to divide the payload into segments that are 8 octets in length (except for the last fragment, which is smaller). Packets are reassembled according to the extension header and the fragment offset.