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IP Fragmentation

Learn about IP fragmentation, Maximum Transmission Unit (MTU), packet fragmentation, and defragmentation in computer networking. Discover how data fields are divided and reassembled along the network route.

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IP Fragmentation

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  1. IP Fragmentation

  2. MTU • Maximum Transmission Unit (MTU) • Largest IP packet a network will accept • Arriving IP packet may be larger MTU IP Packet

  3. IP Fragmentation • If IP packet is longer than the MTU, the router fragments the packet • Extracts data field from incoming packet • Divides data field into smaller fragments • Sends each fragment in its own IP packet MTU IP Packet 2 1 3 IP Packets Fragmentation

  4. IP Fragmentation • Each packet is smaller than the MTU • Suppose MTU is 500 octets • IP header is 20 octets if there are no options • Only 480 octets available for data in each fragmented IP packet MTU (500 Octets) IP Data Field (480) IP Header (20)

  5. Multiple Fragmentations • Original packet may be fragmented multiple times along its route Source Host Internet Process Destination Host Internet Process Fragmentation

  6. Defragmentation • Internet layer process on destination host defragments, restoring the original packet • Defragmentation only occurs once Source Host Internet Process Destination Host Internet Process Defragmentation

  7. Fragmentation and IP Fields • More Fragments field (1 bit) • 1 if more fragments, 0 if not • Source host internet process sets more fragments field to 0 • If router fragments, keeps More Fragments field in last fragment 0 • In all other fragments, sets to 1 0 0 1 1 Original IP Packet Fragments

  8. Fragmentation and IP Fields • More Fragments field (1 bit) • If another fragmentation • If MF field is 1, make all MF fields 1 in fragment IP packets • IF MF field is zero, make all MF fields 1 in fragment IP packets except the last one. Make that 0. 1 1 1 1 0 0 1 1 Original IP Packet Fragments

  9. Identification Field • IP packet has a 16-bit Identification field • Source host internet process places a random number in the Identification field • Different for each IP packet Identification Field 47 Original IP Packet

  10. Identification Field • IP packet has a 16-bit Identification field • If router fragments, places the original Identification field value in the Identification field of each fragment Identification Fields 47 47 47 47 Original IP Packet Fragments

  11. Identification Field • Purpose • Allows receiving host’s internet layer process know what fragments belong to each original packet • Works even if an IP packet is fragmented several times 47 47 47 47 Original IP Packet Fragments

  12. Fragment Offset Field • Used to reorder fragments with the same Identification field • Contains the data field’s starting point (in bytes) from the start of the data field in the original IP packet First byte is 731st from start of fragment 212 730 0 Fragment Offset Field

  13. Fragment Offset Field • Receiving host’s internet layer process assembles fragments in order of increasing fragment offset field value • This works even if fragments arrive out of order! • Works even if fragmentation occurs multiple times 2nd Fragment 3rd Fragment 1st Fragment 212 730 0

  14. Fragmentation: Recap • Fragmentation • Large IP is packet broken into a series of smaller IP packets fitting a network’s MTU • Fragmentation is done by routers • Fragmentation may be done multiple times along the route

  15. Fragmentation: Recap • Fragmentation • Defragmentation (reassembly) is done once, by destination host’s internet layer process

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