IP Encapsulation, Fragmentation, and Reassembly

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IP Encapsulation, Fragmentation, and Reassembly
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Frames

• Datagrams must travel inside of frames
• Each network may well have different frame format
• Especially when transiting WANs
• Entire datagram encapsulated inside frame native
  to local network it is transiting
• Network pays no attention to datagram

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Encapsulation
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Encapsulation Continued

• Router translates next hop IP address into
  physical address
• Each hop encapsulated
• Datagram stored in router sans frame
• Frames do not accumulate
• Dont confuse frame destination address with
  datagrams destination address

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MTU

• Each network hardware has maximum frame size
• Termed Maximum Transmission Unit (MTU)
• If datagram larger, cannot be encapsulated

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MTU Example
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Fragmentation

• In order for datagram to transit network with
  small MTU, must be fragmented
• Datagram broken into parts (fragments) and each
  moved independently
• Fragment has same form as other datagrams
• Total length, flags, fragment offset changed

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Fragmentation Continued

• Routers do fragmentation
• They know MTUs
• Ultimate destination host does reassembly
• Termed opaque fragmentation
• Allows routes to change
• Fragments can be fragmented
• May have smaller and smaller MTUs
• Reassembly unaffected

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Fragmenting Fragments
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Reassembly

• Each original packet has unique identifier ID
  field in datagram header
• Each fragment has copy
• Each fragment also has offset as to where it was
  in original
• Last fragment does not have More Fragments bit set

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Reassembly Continued

• Fragments can be lost
• Cant wait forever for arrival
• Timer started, when times out, fragments
  discarded
• Cant ask for fragment resend as sender unaware
  of fragmentation
• Routes (and therefore level of fragmentation)
  will change too

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Important Header Fields for Fragmentation see
Figure 20.4 (page 324)

• Flags
• Flag telling if data-gram is a fragment
• Flag telling if more fragments (more fragments
  bit)
• Identification (ID) Field unique across all
  fragments of one data-gram
• Fragment offset field

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Questions

• 21.2, 21.3, 21.5, 21.8, 21.9, 21.10