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Fragmentation and Re-assembly services

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• Maximum payload size in physical networks

  • Fact:

    Each type of physical network has a maximum frame size.

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  • Example:

    The payload (= data portion) size of an Ethernet frame is at most 1500 bytes: Graphically: 7 bytes start 48 bit 48 bit 16 b n <= 1500 B 32b +-----------+----------+-------+------+--------+--------------+-----+ | Preamble | 10101011 | Src | Dest | Length | Payload | CRC | +-----------+----------+-------+------+--------+--------------+-----+ The payload (= data portion) size of the FDDI token ring frame is at most 4500 bytes.

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• The need to break up an IP packet
  • The need to fragment an IP Packet:

    When an IP packet is transmitted across different types of networks that has different maximum sizes payloads we may need to fragment (divide) the payload

    Example:

    A node on an FDDI Token ring network transmits an IP packet of size 2000 bytes to a destination on an Ethernet network: The IP packet (payload) is too big to be transmitted inside an Ethernet frame:

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• How to transmit over-sized IP packets
  • How to transmit an over-sized IP packet:

    The IP packet is received by a router: The IP packet is divided into multiple IP packets: The IP header (= 40 bytes) is duplicated in every IP packet (with some modification --- see below) Each IP packet is is transmitted in a Ethernet frame:

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• Where does fragmentation and re-assembly take place

  • Packet fragmentation happens at:

    In a router that forward an IP packet: The destination network uses a smaller maximum payload size

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  • IP packet re-assembly happens at:

    Packet re-assembly only takes place at the destination (computer)

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  • Note:

    Every IP protocol software module must include the fragmentation and re-assembly functions.