Chapter 21 IP Encapsulation, Fragmentation, and Reassembly

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Chapter 21 IP Encapsulation, Fragmentation, and
Reassembly
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Encapsulation

• Refers to embedding of data
• When an IP datagram is encapsulated in a frame,
  the entire datagram is placed in the data area of
  a frame (fig 21.1)
• network hardware does not care what is inside the
  frame data area
• destination address in the frame is the physical
  address of the next hop to which the datagram
  should be sent whenever the destination computer
  is on a remote network.
• datagram is encapsulated in a frame appropriate
  to the network being traversed
• When the datagram crosses a router, the old frame
  header is discarded and a new frame header a
  prepended. (fig 21.2)

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Maximum Transmission Unit (MTU)

• a limitation placed by the network hardware
  technology on the size of a datagram (fig 21.3)
• eg. Ethernets MTU is 1500 bytes

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Fragmentation

• used by an IP router to solve the problem of
  different MTUs of networks
• When a router sees that a datagram is larger
  than the MTU of the network over which it must be
  sent, the router divides the datagram in smaller
  pieces called fragments, and sends each fragment
  independently (fig 21.4)
• A bit in the FLAGS field in the IP header
  indicates whether the datagram is a fragment or
  a complete datagram.
• FRAGMENT OFFSET field in the IP header of a
  fragment specifies where in the original datagram
  the fragment belongs.

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Reassembly

• process of recreating the original datagram from
  fragments
• Fragments are forwarded to the ultimate
  destination host, which reassembles them.
• MORE FRAMENTS bit in the FLAGS field tells the
  final host to know whether all fragments have
  arrived
• Intermediate routers need not reassemble
  fragments
• fragments may traverse different paths, making
  reassembly in the intermediate routers
  impossible

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Identifying the Datagram a Fragment Belongs

• each datagram is assigned a unique number by the
  source computer in the IDENTIFICATION field of
  IP header
• A copy of this number is copied into each
  fragment
• destination computer can reassemble the fragments
  to the proper datagrams by examining the source
  IP address, IDENTIFCATION field, and FRAGMENT
  OFFSET field.

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Fragment Loss

• if a fragment is lost, the destination computer
  discards the remaining fragments corresponding to
  the same datagram
• Sender will retransmit the entire datagram since
  it does not know how the datagram was fragmented
• when the datagram is retransmitted, it may
  traverse a different routing path and be
  fragmented differently.

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Fragmenting a Fragment

• an intermediate router with smaller MTUs may
  fragment an existing fragment by modifying the
  FRAGMENT OFFSET field
• The ultimate destination computer does not know
  whether an incoming fragment had be fragmented
  into subfragments.