Inner Payload Size = IP Packet Size - IP Overhead
  Outer Payload Size = MTU - IPsec Overhead

                Inner Payload Size
  NF0 = ----------------------------------
         Outer Payload Size - IP Overhead

  NF = CEILING(NF0)

  OH = NF * (IP Overhead + IPsec Overhead)
       - IP Overhead
       + NF * (Outer Payload Size - IP Overhead)
       - Inner Payload Size

  OH = NF * (IPsec Overhead + Outer Payload Size)
       - (IP Overhead + Inner Payload Size)

  OH = NF * (IPsec Overhead + Outer Payload Size)
       - Inner Packet Size