Fragmentation

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• IP provides best-effort delivery of an unreliable and connectionless nature
• delivery is not guaranteed
  • packet might be lost, delivered out of sequence, duplicated, or delayed
• possible that IP may fragment the packet into more manageable pieces to fit within the maximum transmission unit (MTU) of the Data Link protocol frame
  • due to limitations in the underlying network
• In IPv4, the ID, Flags, and Fragment Offset IP header fields are used to:
  • record the sequence in which the packets were sent
  • indicate whether the IP datagram has been split between multiple frames for transport over the underlying Data Link protocol
    • E.g., MTU of Ethernet frame is usually 1500 bytes
      • IP datagram larger than 1,500 would have to be fragmented across more than one Ethernet frame
      • datagram passing over an internetwork might have to be encapsulated in different Data Link frame types, each with different MTUs
• Systems try to avoid IP fragmentation
  • IPv6 does not allow routers to perform fragmentation
    • instead the host performs path MTU discovery to work out the MTU supported by each hop and crafts IP datagrams that will fit the smallest MTU