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  • pg 1

 collection of networks interconnected to provide
  some sort of host to-host packet delivery service
 global internetwork to which a large percentage
  of networks are now connected
 An internetwork is an interconnected collection
  of such networks
 An internetwork is often referred to as a “network
  of networks” because it is made up of lots of
  smaller networks
• Ethernets, FDDI ring and a point-to-point link.
• Each of these is a single-technology network.
• The nodes that interconnect the networks are
  called routers or gateways,
• The Internet Protocol is the key tool used today
  to build scalable, heterogeneous internetworks.
  It was originally known as the Kahn-Cerf
• IP is that it runs on all the nodes (both hosts and
  routers) in a collection of networks and defines
  the infrastructure that allows these nodes and
   networks to function as a single logical
• Hosts H1 and H8 are logically connected
  by the internet ,including the protocol
  graph running on each node.
• Higher-level protocols, such as TCP and
  UDP, typically run on top of IP on the
                 Service Model
• A good place to start when you build an internetwork is
    to define its service model, ie the host-to-host services
    you want to provide.
• can provide a host-to-host service only if this service can
    somehow be provided over each of the underlying
    physical networks
 IP service model :
  two parts:
1. Addressing scheme
       - provides a way to identify all hosts in the
2. Datagram (connectionless) model (best effort )
     - IP makes every effort to deliver datagram's, it makes
    no guarantees.
                 Datagram Delivery
• The IP datagram is fundamental to the Internet Protocol.

• A datagram is a type of packet that happens to be sent in a
  connectionless manner over a network.

• Every datagram carries enough information to let the network
  forward the packet to its correct destination.

• there is no need for any advance setup mechanism to tell the
  network what to do when the packet arrives.

• If something goes wrong and the packet gets lost, corrupted,
  misdelivered, or in any way fails to reach its intended destination,
  the network does nothing—it made its best effort, and that is all it
  has to do.
• It does not make any attempt to recover from the failure. This is
  sometimes called an unreliable service.
Packet Format
                     Packet Format
• Version: (version of IP.)
        -The current version of IP is 4, (IPv4)

• Hlen: (Header Length)
      -length of the header in 32-bit words.

• TOS: (type of service)
        -field has had a number of different definitions over the years,
  but its basic function is to allow packets to be treated differently
  based on application needs.

• Length:(Length of the datagram)
      -including the header. The Length field counts bytes rather than
  words. The maximum size of an IP datagram is 65,535 bytes.
                     Packet Format
• TTL (time to live):
      -TTL was set to a specific number of seconds that the packet
  would be allowed to live.

• Protocol
       -is simply a demultiplexing key that identifies the higher-level
  protocol to which this IP packet should be passed.

• Checksum
       -is calculated by considering the entire IP header as a
  sequence of 16-bit words, adding them up using ones complement
  arithmetic, and taking the ones complement of the result.

• SourceAddr &DestinationAddr
      - Source address and destination address for the packet.
        Fragmentation and Reassembly
• Packets can be fragmented and reassembled when they
  are too big to go over a given network technology.
• Every network type has a maximum transmission unit
  (MTU), which is the largest IP datagram that it can carry
  in a frame
• When a host sends an IP datagram, therefore, it can
  choose any size that it wants. A reasonable choice is the
  MTU of the network to which the host is directly
• Then fragmentation will only be necessary if the path to
  the destination includes a network with a smaller MTU.
  Should the transport protocol that sits on top of IP give
  IP a packet larger than the local MTU, however, then the
  source host must fragment it.
• Assuming that the MTU is 1500 bytes for the two
  Ethernets, 4500 bytes for the FDDI network, and
  532 bytes for the point-to-point network, then a
  1420-byte datagram (20-byte IP header plus
  1400 bytes of data) sent from H1 makes it
  across the first Ethernet and the FDDI network
  without fragmentation but must be fragmented
  into three datagrams at router R2.
  These three fragments are then forwarded by
  router R3 across the second Ethernet to the
  destination host
•    The figure also serves to reinforce two important points:

1.   Each fragment is itself a self-contained IP datagram
     that is transmitted over a sequence of physical
     networks, independent of the other fragments.
2.   Each IP datagram is re encapsulated for each physical
     network over which it travels.
               Global Addresses
• If you want to be able to send data to any host
  on any network, there needs to be a way of
  identifying all the hosts.

• Thus, we need a global addressing scheme—
  one in which no two hosts have the same

• Global uniqueness is the first property that
  should be provided in an addressing scheme
• IP addresses are divided into three
  different classes
                IP Addressing
                              32 bits
Decimal             Network                    Host

Maximum       255          255           255           255
          1          8 9         16 17         24 25         32

Binary    11111111 11111111         11111111 11111111

Example     1
Decimal    172       16     122      204
Example 10101100 00010000 01111010 11001100
        IP Address Classes
             8 bits   8 bits   8 bits   8 bits

•Class A:   Network   Host     Host     Host

•Class B:   Network Network    Host     Host

•Class C:   Network Network Network     Host

•Class D:         Multicast
•Class E:   Research
             IP Address Classes
    Bits:    1          8 9              16 17           24 25            32
             0NNNNNNN            Host             Host             Host
Class A:
             Range (1-126)

    Bits:    1          8 9              16 17           24 25            32
             10NNNNNN          Network            Host             Host
Class B:
            Range (128-191)
             1           8 9             16 17             24 25          32
                 110NNNNN      Network           Network           Host
Class C:
            Range (192-223)
             1           8 9             16 17             24 25          32
              1110MMMM       Multicast Group Multicast Group Multicast Group
Class D:
            Range (224-239)
                          IP Address Classes

•    class D addresses that specify a multicast group, and
    class E addresses that are currently unused. In all
    cases, the address is 32 bits long.
•   The class of an IP address is identified in the most
    significant few bits.
•   If the first bit is 0, it is a class A address.
•    If the first bit is 1 and the second is 0, it is a class B
•    If the first two bits are 1 and the third is 0, it is a class C
    address .
•   Approximately 4 billion possible IP addresses, half are
    class A, one quarter are class B, and one-eighth are
    class C.
•    Each class allocates a certain number of bits for the
    network part of the address and the rest for the host part.
•   Class A networks have 7 bits for the network part and 24 bits for the host part,
    meaning that there can be only 126 class A networks (the values 0 and 127 are
                 IP Address Classes

1. Internet would consist of a small number of
   wide area networks (these would be class A
2. Modest number of site- (campus-) sized
   networks (these would be class B networks),
3. A large number of LANs (these would be class
   C networks).
                IP Address (Example)
• IP addresses are written as four decimal
  integers separated by dots.

• Each integer represents the decimal value
  contained in 1 byte of the address, starting at the
  most significant.

• For example, the address of the computer on
  which this sentence was typed is

• Domain names tend to be ASCII strings
  separated by dots, such as
   Datagram Forwarding in IP
• Forwarding is the process of taking a
  packet from an input and sending it out on
  the appropriate output.

• Routing is the process of building up the
  tables that allow the correct output for a
  packet to be determined.
           Forwarding of IP datagrams
• Every IP datagram contains the IP address of the
  destination host.

• The “network part” of an IP address uniquely identifies a
  single physical network that is part of the larger Internet.

• All hosts and routers that share the same network part of
  their address are connected to the same physical
  network and can thus communicate with each other by
  sending frames over that network.

• Every physical network that is part of the Internet has at
  least one router that, by definition, is also connected to
  at least one other physical network; this router can
  exchange packets with hosts or routers on either
Forwarding of IP datagrams can be handled in the following Way

  • A datagram is sent from a source host to a destination
    host, possibly passing through several routers along the
    way .
  • Any node, whether it is a host or a router, first tries to
    establish whether it is connected to the same physical
    network as the destination.
  • It compares the network part of the destination address
    with the network part of the address of each of its
    network interfaces.
  • If a match occurs, then that means that the destination
    lies on the same physical network as the interface, and
    the packet can be directly delivered over that network.
Forwarding of IP datagrams can be handled in the following Way

  • If the node is not connected to the same
    physical network as the destination node, then it
    needs to send the datagram to a router.
  • Each node will have a choice of several routers,
    and so it needs to pick the best one, or at least
    one that has a reasonable chance of getting the
    datagram closer to its destination.
  • The router that it chooses is known as the next
    hop router. The router finds the correct next hop
    by consulting its forwarding table .
Datagram forwarding algorithm
                      Working Principle
• H1 wants to send a datagram to H2.
        Since they are on the same physical network, H1 and H2 have
  the same network number in their IP address. Thus, H1deduce that
  it can deliver the datagram directly to H2 over the Ethernet.
        The one issue that needs to be resolved is how H1 finds out
  the correct Ethernet address for H2—this is the address resolution

• H1 wants to send a datagram to H8.
        Since these hosts are on different physical networks, they
  have different network numbers, so H1 deduces that it needs to
  send the datagram to a router.
       R1 is the only choice—the default router—so H1 sends the
  datagram over the Ethernet to R1.
                            Working Principle

•   R1 knows that it cannot deliver a datagram directly to H8 because
    neither of R1’s interfaces is on the same network as H8 .

•    R1’s default router is R2; R1 then sends the datagram to R2 over
    the token ring network. Assuming R2 has the forwarding table
• H8’s network number (network 1)and forwards the datagram to R3.
  Finally,R3, since it is on the same network as H8,forwards the
  datagram directly to H8.
• Example: the network interfaces of router R2 as interface 0 for the
  point-to point link (network 4) and interface 1 for the token ring
  (network 3). Then R2 would have the forwarding table
• Any network number that R2 encounters in a packet, it
  knows what to do. Either that network is directly
  connected to R2, in which case the packet can be
  delivered to its destination over that network, or the
  network is reachable via some next hop router that R2
  can reach over a network to which it is connected. In
  either case, R2 will use ARP, to find the MAC address of
  the node to which the packet is to be sent next.
• In example, R2 could store the information needed to
  reach all the hosts in the network in a four-entry table.
  Even if there were 100 hosts on each physical
  network,R2 would still only need those same four
  entries. This is a good first step in achieving scalability.
       The most important principles of building scalable networks:

• To achieve scalability, you need to reduce the amount of
  information that is stored in each node and that is
  exchanged between nodes. The most common way to
  do that is hierarchical aggregation.

• IP introduces a two-level hierarchy, with networks at the
  top level and nodes at the bottom level. We have
  aggregated information by letting routers deal only with
  reaching the right network; the information that a router
  needs to deliver a datagram to any node on a given
  network is represented by a single aggregated piece of

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