Chapter 20' IP Datagrams and Datagram Forwarding

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Chapter 20. IP Datagrams and Datagram Forwarding

• Jing Wang
• Towson University

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20.1. Introduction

• Fundamental communication service in an internet
• The format of packets
• How routers process and forward packets

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20.2 Connectionless Service

• TCP/IP include protocols for both connectionless
  and connection-oriented services
• The fundamental delivery service is
  connectionless
• Add a reliable connection-oriented service that
  uses the underlying connectionless service

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20.3. Virtual Packets

• Because it can connect heterogeneous networks, a
  router cannot transmit a copy of a frame that
  arrives on one network across another.
• To accommodate heterogeneity, an internet must
  define a hardware-independent packet format.

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20.4. The IP Datagram

• Figure 20.1. The general form of an IP datagram
  with a header followed by data. The header
  contains information that controls where and how
  the datagram is to be sent.

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20.4. The IP Datagram

• The size of a datagram is determined by the
  application that sends data.
• Allowing the size of datagrams to vary makes IP
  adaptable to a variety of applications
• In IPv4, a datagram can contain as little as a
  single octet of data or as most 64K octets,
  including the header
• A packet sent across a TCP/IP internet is called
  an IP datagram.
• Each datagram consists of a header followed by
  data.
• Source and destination addresses in the datagram
  header are IP addresses.

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20.5. Forwarding An IP Datagram

• Figure 20.2. (a) An example internet with three
  routers connecting four physical networks, and
  (b) the conceptual routing table found in router
  R2. Each entry in the table lists a destination
  network and the next hop along a route to that
  network.

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20.5. Forwarding An IP Datagram

• Because each destination in a routing table
  corresponds to a network, the number of entries
  in a routing table is proportional to the number
  of networks in an internet.

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20.6. IP Addresses And Routing Table Entries

• Figure 20.3. (a) An internet of four networks and
  three routers with an IP address assigned to each
  router interface, and (b) the routing table found
  in the center router. Each entry in the table
  lists a destination, a mask, and the next hop
  used to reach the destination.

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20.6. IP Addresses And Routing Table Entries

• Destination field
• Network prefix of the destination network
• Address mask
• Specifies which bits of the destination
  correspond to the network prefix
• Next Hop field
• An IP address used to denotes a router

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20.7. The Mask Field And Datagram Forwarding

• The computation to examine the ith entry in the
  table can be expressed as
• If ( (Maski D) Destinationi )
• forward to NextHopi
• D - Datagram destination address

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20.8. Destination And Next-Hop Addresses

• The destination address in a datagram header
  always refers to the ultimate destination.
• When a router forwards the datagram to another
  router, the address of the next hop does not
  appear in the datagram header.
• After computing the address of a next hop, N, IP
  software uses the address binding to translate N
  to an equivalent hardware address for
  transmission.

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20.9. Best-Effort Delivery

• IP does not guarantee that it will handle the
  problems of
• Datagram duplication
• Delayed or out-of-order delivery
• Corruption of data
• Datagram loss

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20.9. Best-Effort Delivery

• Because IP is designed to operate over all types
  of network hardware, the underlying hardware may
  misbehave.
• As a result, IP datagrams may be lost,
  duplicated, delayed, delivered out of order, or
  delivered with corrupted data.
• Higher layers of protocol software are required
  to handle each of these errors.

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20.10. The IP Datagram Header Format

• Figure 20.4. Fields in the IP datagram header.
  Both the source and destination addresses are
  Internet addresses.

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20.11. Summary

• IP datagram
• Routing table