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The TCP/IP Guide  9  TCP/IP Lower-Layer (Interface, Internet and Transport) Protocols (OSI Layers 2, 3 and 4)       9  TCP/IP Internet Layer (OSI Network Layer) Protocols            9  Internet Protocol (IP/IPv4, IPng/IPv6) and IP-Related Protocols (IP NAT, IPSec, Mobile IP)                 9  Internet Protocol Version 4 (IP, IPv4)                      9  IP Datagram Size, Maximum Transmission Unit (MTU), Fragmentation and Reassembly

IP Datagram Size, Maximum Transmission Unit (MTU), Fragmentation and Reassembly 1 2 3 4 IP Message Fragmentation Process

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.)

IP Datagram Size, Maximum Transmission Unit (MTU), Fragmentation and Reassembly 1 2 3 4 IP Message Fragmentation Process

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