Chapter 11 IPsec VPN for FortiOS 5.0 : IPv6 for FortiOS 5.0 : IPv6 packet structure
  
IPv6 packet structure
Each IPv6 packet consists of a mandatory fixed header and optional extension headers, and carries a payload, which is typically either a datagram and/or Transport Layer information. The payload could also contain data for the Internet Layer or Link Layer. Unlike IPv4, IPv6 packets aren't fragmented by routers, requiring hosts to implement Maximum Transmission Unit (MTU) Path Discovery for MTUs larger than the smallest MTU (which is 1280 octets).