IP Encapsulation, Fragmentation, And Reassembly

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IP Encapsulation, Fragmentation, And Reassembly

• Computer Science
• Prof. Martins

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Introduction

• Previous chapter
• IP datagram format
• Routing
• This chapter
• IP datagram transmission

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Datagram Transmission

• IP software selects the next hop to Transmit the
  datagram across the physical network
• Network hardware does not understand datagram
  format or internet addressing
• Hardware understands frames
• Frames may differ from network to network

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Encapsulation

• How can a datagram be transmitted across a
  network that does not understand the datagram
  format?
• The answer Encapsulation
• When an IP datagram is encapsulated in a frame,
  the entire datagram is placed in the data area of
  a frame.

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Encapsulation

• Figure 21.1 - An IP datagram encapsulated in
  a hardware frame. The entire datagram resides in
  the frame data area. In practice, the frame
  format used with some technologies includes a
  frame trailer as well as frame header.

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Encapsulation

• How does a receiver know whether the data area in
  an incoming frame contains an IP datagram or
  other data?
• Answer The sender and receiver agree on the
  value used in the frame type field.
• A special value is reserved for IP.

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Encapsulation

• In addition to placing a datagram in the data
  area of a frame, encapsulation requires the
  sender to supply the physical address of the next
  computer
• Address binding computes the appropriate
  hardware address (ARP - chapter 19)
• The binding translates the IP address of the next
  hop into an equivalent hardware address.

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Summary

• A datagram is encapsulated in a frame
• The destination address in the frame is the
  address of the next hop
• The destination of the next hop is obtained by
  translating the IP address of the next hop to an
  equivalent hardware address.

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Transmission Across An Internet
Figure 21.2 - An IP datagram as it appears at
each step during a trip across an internet.
Whenever it travels across a physical network,
the datagram is encapsulated in a frame
appropriate to the network.
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Transmission Across An Internet

• Encapsulation applies to one transmission at a
  time
• After the sender selects a next hop, the sender
  encapsulates the datagram in a frame and
  transmits the result across the physical network
  to the next hop.
• When the frame receives the next hop, the
  receiving software removes the IP datagram and
  discards the frame.

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Transmission Across An Internet

• Frame headers do not accumulate during a trip
  through the internet.
• Before a datagram is transmitted across a given
  network, the datagram is encapsulated
• When the frame arrives at the next hop, the
  datagram is removed from the incoming frame,
  before being routed and encapsulated in an
  outgoing frame.

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Transmission Across An Internet

• When the datagram reaches its final destination,
  the frame that carried the datagram is discarded
• The datagram appears exactly the same as when it
  was originally sent
• When a datagram arrives in a network frame, the
  receiver extracts the datagram from the frame
  data area and discards the frame header.

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MTU, Datagram Size, and Encapsulation

• MTU is the Maximum Transmission Unit the
  maximum amount of data that a frame can carry.
• In an Internet that connects heterogeneous
  networks, MTU restrictions can cause a problem

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MTU, Datagram Size, and Encapsulation
Figure 21.3 - An example of a router that
connects two networks with different MTU values.
A frame that travels across the network 1 can
contain 1500 octets of data, while a frame that
travels across network 2 can contain at most 1000
octets of data.
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MTU, Datagram Size, and Encapsulation

• An IP router uses a technique known as
  fragmentation to solve the problem of
  heterogeneous MTUs.
• When a datagram is larger than the MTU, the
  router divides the datagram into smaller pieces
  called fragments.
• Each fragment is sent separately.

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MTU, Datagram Size, and Encapsulation

• A bit in the FLAGS field indicate whether a
  datagram is a fragment or a complete datagram.
• A fragment has the same format as the other
  datagrams.
• The FRAGMENT OFFSET field in the header of a
  fragment specifies where in the original datagram
  the fragment belongs.

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MTU, Datagram Size, and Encapsulation
Figure 21.4 - An IP datagram divided into three
fragments. Each fragment carries some data from
the original datagram, and has an IP header
similar to the original datagram.
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Summary

• A datagram cannot be larger than the MTU of a
  network over which it is sent.
• A router divides the datagram into smaller pieces
  called fragments.
• Each fragment uses the IP datagram format, but
  carries only parts of the data.

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Reassembly

• The process of creating a copy of the original
  datagram from fragments is called reassembly.
• All fragments have the same destination address
  as the original datagram.
• The fragment that carries the final piece of data
  has an additional bit set in the header.
• A receiver performing reassembly can tell whether
  all fragments have arrived successfully.

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Reassembly
Figure 21.5 - An example internet in which hosts
can generate datagrams that require
fragmentation. Once a datagram has been
fragmented, the fragments are forwarded to the
final destination, which reassembles them.
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Identifying A Datagram

• Since IP does not guarantee delivery, some
  fragments can be lost or arrive out of order.
• How does IP reassemble fragments that arrive out
  of order?
• IDENTIFICATION field a unique ID number of each
  outgoing datagram.

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Identifying A Datagram

• When a router fragments the datagram, the router
  copies the ID number into each fragment.
• The FRAGMENT OFFSET field tells a receiver how to
  order fragments within a given datagram

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Fragment Loss

• Recall that IP does not guarantee datagram
  delivery
• Some fragments may be delayed or lost
• Datagrams with lost fragments cannot be
  reassembled
• Fragments may be saved temporarily.
• IP specifies a maximum time to hold fragments.
• After a timer expires, saved fragments are
  discarded.

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Fragmenting a Fragment

• If a fragment reaches another network that has a
  smaller MTU, it is possible to further fragment a
  fragment.
• IP does not distinguish between original
  fragments and sub fragments.
• The advantage of making all fragments the same is
  that a receiver can perform reassembly without
  first reassembling sub fragments.

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Summary

• An IP datagram is encapsulated in a network frame
  for transmission across a hardware network.
• To encapsulate a datagram, the sender places the
  entire datagram in the data area of a network
  frame.
• Each network technology defines the maximum
  amount of data (MTU) accepted.

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Summary

• When a router receives a datagram that is larger
  than the network MTU, the router divides the
  datagram into fragments.
• Each fragment travels to the ultimate
  destination, which is responsible for
  reassembling fragments into the original
  datagram.