IP Fragmentation and PMTUD(Cisco)

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The graphic below shows an example of fragmentation. If you add up all the lengths of the IP fragments, the value exceeds the original IP datagram length by 60. The reason that the overall length is increased by 60 is because three additional IP headers were created, one for each fragment after the first fragment.　　The first fragment has an offset of 0, the length of this fragment is 1500; this includes 20 bytes for the slightly modified original IP header.
　　The second fragment has an offset of 185 (185 x 8 = 1480), which means that the data portion of this fragment starts 1480 bytes into the original IP datagram. The length of this fragment is 1500; this includes the additional IP header created for this fragment.
　　The third fragment has an offset of 370 (370 x 8 = 2960), which means that the data portion of this fragment starts 2960 bytes into the original IP datagram. The length of this fragment is 1500; this includes the additional IP header created for this fragment.
　　The fourth fragment has an offset of 555 (555 x 8 = 4440), which means that the data portion of this fragment starts 4440 bytes into the original IP datagram. The length of this fragment is 700 bytes; this includes the additional IP header created for this fragment.

　　It is only when the last fragment is received that the>
　　The fragment offset in the last fragment (555) gives a data offset of 4440 bytes into the original IP datagram. If you then add the data bytes from the last fragment (680 = 700 - 20), that gives you 5120 bytes, which is the data portion of the original IP datagram. Then, adding 20 bytes for an IP header equals the>

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