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IP Datagram Size, the Maximum Transmission Unit (MTU), and Fragmentation Overview
(Page 3 of 4)
Multiple-Stage Fragmentation
While the fragments above are in
transit, they may need to pass over a hop between two routers where
the physical network's MTU is only 1,300 bytes. In this case, each of
the fragments will again need to be fragmented. The 3,300 byte fragments
will end up in three pieces each (two of about 1,300 bytes and one of
around 700 bytes) and the final 2,100-byte fragment will become a 1300-byte
and 800-byte fragment. So instead of having four fragments, we will
end up with eleven (3*3+1*2)! This is illustrated in Figure 89.
Figure 89: IPv4 Datagram Fragmentation This example shows illustrates a two-step fragmentation of a large IP datagram. The boxes represent datagrams or datagram fragments and are shown to scale. The original datagram is 12,000 bytes in size, represented by the large gray box. To transmit this data over the first local link, Device A splits it into four fragments, shown at left in four primary colors. The first router must fragment each of these into smaller fragments to send them over the 1,300-byte MTU link, as shown on the bottom. Note that the second router does not reassemble the 1,300-byte fragments, even though its link to Device B has an MTU of 3,300 bytes.
(Figure 90 shows the process by which the fragments in this example are created.)
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