Reassembly on a host is not considered a problem because the host has the time and memory resources to devote to this task. This is done efficiently because the information needed to create the fragments is immediately available.įragmentation causes more overhead for the receiver when reassembling the fragments because the receiver must allocate memory for the arriving fragments and coalesce them back into one datagram after all of the fragments are received. The creation of fragments involves the creation of fragment headers and copies the original datagram into the fragments. This is true for the sender and for a router in the path between a sender and a receiver. IPv4 fragmentation results in a small increase in CPU and memory overhead to fragment an IPv4 datagram.
The addition of 20 bytes for an IPv4 header equals the size of the original IPv4 datagram (4440 + 680 + 20 = 5140) as shown in the images. The sum of the data bytes from the last fragment (680 = 700 - 20) yields 5120 bytes, which is the data portion of the original IPv4 datagram. The fragment offset in the last fragment (555) gives a data offset of 4440 bytes into the original IPv4 datagram. It is only when the last fragment is received that the size of the original IPv4 datagram can be determined. The length of this fragment is 700 bytes this includes the additional IPv4 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 IPv4 datagram. The third fragment has an offset of 370 (370 x 8 = 2960) the data portion of this fragment starts 2960 bytes into the original IPv4 datagram. The length of this fragment is 1500 this includes the additional IPv4 header created for this fragment.
The second fragment has an offset of 185 (185 x 8 = 1480) the data portion of this fragment starts 1480 bytes into the original IPv4 datagram. The first fragment has an offset of 0, the length of this fragment is 1500 this includes 20 bytes for the slightly modified original IPv4 header. The reason that the overall length is increased by 60 is because three additional IPv4 headers were created, one for each fragment after the first fragment. If the lengths of the IPv4 fragments are added, the value exceeds the original IPv4 datagram length by 60. The "do not fragment" (DF) bit determines whether or not a packet is allowed to be fragmented.īit 0 is reserved and is always set to 0.īit 1 is the DF bit (0 = "can fragment", 1 = "do not fragment").īit 2 is the MF bit (0 = "last fragment," 1 = "more fragments"). There are 3 bits for control flags in the flags field of the IPv4 header. The fragment offset is 13 bits and indicates where a fragment belongs in the original IPv4 datagram. This aids in the reassembly of the fragments of a datagram. The identification is 16 bits and is a value assigned by the sender of an IPv4 datagram. This image depicts the layout of an IPv4 header. The IPv4 source, destination, identification, total length, and fragment offset fields, along with "more fragments" (MF) and "do not fragment" (DF) flags in the IPv4 header, are used for IPv4 fragmentation and reassembly.įor more information about the mechanics of IPv4 fragmentation and reassembly, see RFC 791. IPv4 fragmentation breaks a datagram into pieces that are reassembled later.
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The receiving station is responsible for the reassembly of the fragments into the original, full size IPv4 datagram. The design of IPv4 accommodates MTU differences because it allows routers to fragment IPv4 datagrams as necessary.
The MTU value depends on the transmission link. IPv4 Fragmentation and ReassemblyĪlthough the maximum length of an IPv4 datagram is 65535, most transmission links enforce a smaller maximum packet length limit, called an MTU. Background InformationĪlso discussed are scenarios that involve the behavior of PMTUD when combined with different combinations of IPv4 tunnels. This document describes how IPv4 Fragmentation and Path Maximum Transmission Unit Discovery (PMTUD) work.
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