Chapter 11 Layer 3 Protocols

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Chapter 11Layer 3 - Protocols
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Lecture Objective (Week 12)

• After finishing this lecture, students should be
  able to
• Assign IP address by using RARP, DHCP, or BOOTP
• Explain how does ARP work
• Differentiate Routed protocol, non-routed
  protocol, and routing protocol
• Describe various routing methods
• Explain how router works

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Unique Network Numbers

• Send data from network A to network B. When data
  (frames), coming from network A, reaches the
  router, the router performs the following
  functions
• It strips off the data link header, carried by
  the frame.
• It examines the network layer address to
  determine the destination network.
• It consults its routing tables to determine which
  of its interfaces it will use to send the data,
  in order for it to reach its destination network.
  
• the router determines that it should send the
  data from its interface, with address B1.
• Before actually sending the data out interface
  B1, the router would encapsulate the data in the
  appropriate data link frame.

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Router Interface/port

• A routers attachment to a network is called an
  interface it may also be referred to as a port.
• In IP routing, each interface must have a
  separate, unique network (or subnetwork) address.

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Methods for Assigning an IP Address

• Static Addressing
• you must go to each individual device and
  configure it with an IP address.
• keep very meticulous records to avoid duplicated
  IP addresses.
• Dynamic Addressing
• Dynamic Host Configuration Protocol (DHCP)
• a defined range of IP addresses on a DHCP server.
• As hosts come online they contact the DHCP server
  and request an address.
• The DHCP server chooses an address and allocates
  it to that host.
• With DHCP, the entire computers configuration
  can be obtained in one message (e.g. along with
  the IP address, the server can also send a subnet
  mask).
• Reverse Address Resolution Protocol (RARP)
• BOOTstrap Protocol (BOOTP)

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Methods for Assigning an IP Address

• Reverse Address Resolution Protocol (RARP)
• binds MAC addresses to IP addresses.
• A network device such as a diskless workstation
  might know its MAC address, but not its IP
  address.
• Devices using RARP require that a RARP server be
  present on the network to answer RARP requests.
• Example
• the source knows its own MAC address, but is
  unable to locate its own IP address in its ARP
  table.
• the source initiates a process called a RARP
  request, which helps it detect its own IP
  address.
• To ensure that all devices see the RARP request
  on the network, it uses a broadcast IP address.
• The RARP packet format contains places for MAC
  addresses of both destination and source. The
  source IP address field is empty. The broadcast
  goes to all devices on the network therefore the
  destination IP address will be set to all binary
  1s. Workstations running RARP have codes in ROM
  that direct them to start the RARP process, and
  locate the RARP server.

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Methods for Assigning an IP Address

• BOOTstrap Protocol (BOOTP)
• A device uses BOOTstrap protocol (BOOTP) when it
  starts up, to obtain an IP address.
• A computer uses BOOTP to send a broadcast IP
  datagram (using a destination IP address of all
  1s - 255.255.255.255).
• A BOOTP server receives the broadcast and then
  sends a broadcast.
• The client receives a datagram and checks the MAC
  address. If it finds its own MAC address in the
  destination address field, then it takes the IP
  address in that datagram.
• Like RARP, BOOTP operates in a client-server
  environment, and only requires a single packet
  exchange.
• However, unlike RARP, which only sends back a 4
  octet IP address, BOOTP datagrams can include the
  IP address, the address of a router (default
  gateway), the address of a server, and a
  vendor-specific field.
• One of the problems with BOOTP is that it was not
  designed to provide dynamic address assignment.
  With BOOTP you create a configuration file that
  specifies the parameters for each device.

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DHCP Initialization Sequence

• initialize state
• sends DHCPDISCOVER broadcast messages, which are
  UDP packets with the port number set to the BOOTP
  port.
• select state
• collects DHCPOFFER responses from DHCP server.
• The client then selects the first response it
  receives and negotiates lease time (the length of
  time it can keep the address without renewing it)
  with the DHCP server by sending a DHCPREQUEST
  packet.
• request state
• The DHCP server acknowledges a client request
  with a DHCPACK packet.
• bound state
• begin using the address

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IP Key Components

• ARP
• can automatically obtain the MAC address of the
  computer that is associated with an IP address.
• Note
• The basic unit of data transfer in IP is the IP
  packet. Packet processing occurs in software,
  which means that content and format are not
  hardware dependent.
• A packet is divided into two major components
  the header, which includes source and destination
  addresses and the data.
• Internet Control Message Protocol (ICMP)
• used by a device to report a problem to the
  sender of a message.
• E.g. echo-request/echo-reply, which is a
  component that tests whether a packet can reach a
  destination by pinging the destination.

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Function of the ARP

• A data packet must contain both a destination MAC
  address and a destination IP address.
• After devices determine the IP addresses of the
  destination devices, they can add the destination
  MAC addresses to the data packets. 

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Determination of the destination MAC address

• keep tables that contain all the MAC addresses
  and IP addresses of other devices that are
  connected to the same LAN. They are called
  Address Resolution Protocol (ARP) tables, and
  they map IP addresses to the corresponding MAC
  addresses.
• ARP tables are sections of RAM memory, in which
  the cached memory is maintained automatically on
  each of the devices.
• Each computer on a network maintains its own ARP
  table.
• Whenever a network device wants to send data
  across a network, it uses information provided by
  its ARP table.

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ARP Operation Within a Subnet

• If it is unable to locate a MAC address for the
  destination in its own ARP table, the host
  initiates a process called an ARP request to
  discover the destination MAC address.
• A host builds an ARP request packet and sends it
  to all devices on the network. To ensure that all
  devices see the ARP request, the source uses a
  broadcast MAC address (FF-FF-FF-FF-FF-FF).
• all devices on the local network receive the
  packets and pass them up to the network layer for
  further examination. If the IP address of a
  device matches the destination IP address in the
  ARP request, that device responds by sending the
  source its MAC address. This is known as the ARP
  reply.

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Default Gateway

• In order for a device to communicate with another
  device on another network, you must supply it
  with a default gateway.
• A default gateway is the IP address of the
  interface on the router that connects to the
  network segment on which the source host is
  located.
• The default gateways IP address must be in the
  same network segment as the source host.
• The computer that sends the data does a
  comparison between the IP address of the
  destination and its own ARP table. If it finds no
  match, it must have a default IP address to use.
• Without a default gateway, the source computer
  has no destination MAC address, and the message
  is undeliverable communication is possible
  only on the devices own logical network segment.

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How ARP Sends Data to Remote Networks

• ARP uses broadcast packets to accomplish its
  function.
• Routers, however, do not forward broadcast
  packets.
• The source host compares the destination IP
  address and its own IP address to determine if
  the two IP addresses are located on the same
  segment.
• If the receiving host is not on the same segment,
  the source host sends the data to the default
  gateway.

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Proxy ARP

• intermediate device (e.g. router) sends an ARP
  response, on behalf of an end node, to the
  requesting host.
• Routers running proxy ARP capture ARP packets.
  They respond with their MAC addresses for those
  requests in which the IP address is not in the
  range of addresses of the local subnet.
• data is sent to a host on a different subnet and
  the source host does not have a default gateway
  configured
• it sends an ARP request.
• All hosts on the segment, including the router,
  receive the ARP request
• The router compares the IP destination address
  with the IP subnet address to determine if the
  destination IP address is on the same subnet as
  the source host.
• If the subnet address is the same, the router
  discards the packet.
• If the subnet address is different, the router
  will respond with its own MAC address for the
  interface that is directly connected to the
  segment on which the source host is located.
• Then the router can forward the data packets
  (based on the destination IP address) to the
  proper subnet for delivery.

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Routable and Non-routable Protocols

• Protocols that provide support for the network
  layer are called routed or routable protocols.
• IP
• IPX/SPX
• AppleTalk.
• Protocols that do not support Layer 3 are classed
  as non-routable protocols.
• NetBEUI is a small, fast, and efficient protocol
  that is limited to running on one segment.

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Characteristics of a Routable Protocol

• routable protocol it must provide the ability to
  assign a network number, as well as a host
  number, to each individual device.
• IPX
• only require that you assign a network number,
  because they use a host's MAC address for the
  physical number.
• IP
• require that you provide a complete address, as
  well as a subnet mask. The network address is
  obtained by ANDing the address with the subnet
  mask.

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Routing Protocols

• Routing protocols (Note Do not confuse with
  routed protocols.) determine the paths that
  routed protocols follow to their destinations.
• Examples
• Routing Information Protocol (RIP)
• Interior Gateway Routing Protocol (IGRP)
• Enhanced Interior Gateway Routing Protocol
  (EIGRP)
• Open Shortest Path First (OSPF)
• Routing protocols enable routers that are
  connected to create a map, internally, of other
  routers in the network or on the Internet.
• Routers use routing protocols to exchange routing
  tables and to share routing information.
• This allows routing (i.e. selecting the best
  path, and switching) to occur. Such maps become
  part of each router's routing table.

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Routing Information Protocol (RIP)

• RIP enables routers to update their routing
  tables at programmable intervals, usually every
  30 seconds.
• Distance-Vector
• calculates distances to a destination host in
  terms of how many hops (i.e. how many routers) a
  packet must pass through
• the path with the least number of hops would be
  the path chosen by the router.
• Because hop count is the only routing metric (a
  measurement for making decisions) used by RIP, it
  doesnt necessarily select the fastest path to a
  destination.
• RIP are very popular due primarily to the fact
  that it was one of the earliest routing protocols
  to be developed.
• problem posed by the use of RIP
• they are constantly connecting to neighboring
  routers to update their routing tables, thus
  creating large amounts of network traffic.
• When using RIP, the maximum number of hops that
  data can be forwarded through is fifteen. The
  destination network is considered unreachable if
  it is more than fifteen router hops away.

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Routing Encapsulation Sequence

• At the data link layer, an IP datagram is
  encapsulated into a frame.The datagram, including
  the IP header, is treated as data.
• A router receives the frame, strips off the frame
  header, then checks the destination IP address in
  the IP header.
• The router then looks for that destination IP
  address in its routing table, encapsulates the
  data in a data link layer frame, and sends it out
  to the appropriate interface.
• If it does not find the destination IP address,
  it may drop the packet.

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Multi-protocol Routing

• Routers are capable of concurrently supporting
  multiple independent routing protocols, and of
  maintaining routing tables for several routed
  protocols.
• This capability allows a router to deliver
  packets from several routed protocols over the
  same data links.

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Connectionless Connection-oriented Network
Services

• connectionless delivery system (packet switched)
• treat each packet separately, and send it on its
  way through the network.
• packets may take different paths to get through
  the network, but are reassembled when they arrive
  at the destination.
• the destination is not contacted before a packet
  is sent. A good analogy for a connectionless
  system is a postal system.
• connection-oriented systems (circuit switched)
• a connection is established between the sender
  and the recipient before any data is transferred.
  An example of a connection-oriented network is
  the telephone system.

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Connectionless and Connection-oriented Network
Processes

• Connectionless network processes
• packets pass from source to destination, they can
  switch to different paths, as well as (possibly)
  arrive out of order. Devices make the path
  determination for each packet based on a variety
  of criteria. Some of the criteria (e.g. available
  bandwidth) may differ from packet to packet.
• Connection-oriented network processes
• establish a connection with the recipient, first,
  and then begin the data transfer. All packets
  travel sequentially across the same physical
  circuit, or more commonly, across the same
  virtual circuit.
• The Internet is one huge connectionless network
  in which all packet deliveries are handled by IP.
  
• TCP (Layer 4) adds connection-oriented services
  on top of IP (Layer 3). TCP provides
  connection-oriented session services to reliably
  deliver data.

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Comparing Router ARP Tables With ARP Tables Kept
by Other Networking Devices

• a typical device contains mapping information
  pertaining only to devices on its own network. It
  knows very little about devices beyond its LAN.
• Routers build tables that describe all networks
  connected to them. ARP tables kept by routers can
  contain IP addresses and MAC addresses of devices
  located on more than one network.
• In addition to mapping IP addresses to MAC
  addresses, router tables also map ports IP
  addresses to network addresses.

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Other Router Issues

• In addition to IP addresses and MAC addresses of
  devices located on networks to which it connects,
  a router also possesses IP addresses and MAC
  addresses of other routers. It uses these
  addresses to direct data toward its final
  destination.
• If a router receives a packet whose destination
  address is not in its routing table, it forwards
  it to the address of another router that most
  likely does contain information about the
  destination host in its routing table.
• When a router does not know the MAC address of
  the next-hop router, the source router (router
  that has the data to be sent on) issues an ARP
  request. A router that is connected to the same
  segment as the source router receives the ARP
  request. This router issues an ARP reply to the
  router that originated the ARP request. The reply
  contains the MAC address of the non-local router.
• Can a device on one subnetwork find the MAC
  address of a device on another subnetwork? The
  answer is yes, provided the source directs its
  question to the router. Working through a third
  party is called proxy ARP, and it allows the
  router to act as a default gateway.

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Indirect Routing

• To obtain the services of a default gateway, a
  source encapsulates the data so that it contains
  the destination MAC address of the router.
• A source uses the destination IP address of the
  host device, and not that of a router, in the IP
  header, because it wants the data delivered to
  the host device and not to a router.
• When a router picks up data, it strips off the
  data link layer information that is used in the
  encapsulation.
• It then passes the data up to the network layer
  where the router examines the destination IP
  address. It compares the destination IP address
  with information contained in its routing tables.
  
• If the router locates the mapped destination IP
  address and the MAC address, and learns that the
  location of the destination network is attached
  to one of its ports, it encapsulates the data
  with the new MAC address information, and
  forwards it to the correct destination.
• If the router cannot locate the mapped
  destination address and MAC address of the device
  of the final target device, it locates the MAC
  address of another router that can perform this
  function, and forwards the data to that router.
  This type of routing is referred to as indirect
  routing.

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Interior Gateway Protocols (IGP) and Exterior
Gateway Protocol (EGP)

• Exterior Gateway Protocols route data between
  autonomous systems.
• BGP (Border Gateway Protocol) used in the
  Internet.
• Interior Gateway Protocols route data in an
  autonomous system.
• OSPF(open shortest path first)uses several
  criteria to determine the best route to a
  destination. These criteria include cost metrics,
  which factor in such things as route speed,
  traffic, reliability, and security. actually
• RIP
• IGRP (developed by Cisco proprietary routing
  protocols )
• used for routing in large multi-vendor networks
• is a distance-vector protocol however, when
  determining the best path, it also takes into
  consideration such things as bandwidth, load,
  delay, and reliability. Network administrators
  can determine the importance given to any one of
  these metrics, or, allow IGRP to automatically
  calculate the optimal path.
• EIGRP is an advanced version of IGRP
• provides superior operating efficiency and
  combines the advantages of link-state protocols
  with those of distance-vector protocols.

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Static Routing

• The network administrator can manually enter the
  route information in the router.
• useful whenever a network administrator wants to
  control which path a router will select.
• For example, routing tables that are based on
  static information could be used to test a
  particular link in the network, or to conserve
  wide area bandwidth.
• Static routing is also the preferred method for
  maintaining routing tables when there is only one
  path to a destination network.
• prevent routers from trying to find another way
  to this stub network if its connection fails.

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Dynamic or Adaptive Routing

• Routes learned automatically
• routers send periodic routing update messages to
  each other. Each time a router receives a message
  containing new information, it recalculates the
  new best route, and sends the new updated
  information to other routers.
• By using dynamic routing, routers can adjust to
  changing network conditions.
• Dynamic routing eliminates the need for network
  administrators or vendors to manually enter
  information into routing tables.
• It works best when bandwidth and large amounts of
  network traffic are not issues.
• RIP, IGRP, EIGRP, and OSPF are all examples of
  dynamic routing protocols because they allow this
  process to occur.
• Without dynamic routing protocols, the Internet
  would be impossible.

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How Routers Route Data Through a Network

• You have a Class B network that is divided into
  eight subnetworks that are connected by three
  routers.
• Host A has data it wants to send to host Z.

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How Routers Route Data Through a Network

• When the data reaches the network layer, source A
  uses its own IP address and the destination IP
  address of host Z
• At the data link layer, source A places the
  destination MAC address of the router1(default
  gateway), to which it is connected, and its own
  MAC address in the MAC header.
• The data packet continues along subnetwork 1
  until it reaches router 1.
• Router 1 picks the packet up, because it
  recognizes that its own MAC address is the same
  as the destination MAC address.
• Router 1 strips off the MAC header of the data
  and passes the data up to the network layer where
  it looks at the destination IP address in the IP
  header.
• The router then searches its routing tables in
  order to map a route for the network address of
  the destination, to the MAC address of the router
  that is connected to subnetwork 8.
• The router is using RIP as its routing protocol,
  therefore, it determines that the best path for
  the data is one that places the destination only
  three hops away.

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How Routers Route Data Through a Network

• Then, the router determines that it must send the
  data packet through the port attached to
  subnetwork 4, in order for the data packet to
  reach its destination via the selected path.
• The router passes the data down to the data link
  layer, where it places a new MAC header on the
  data packet. The new MAC header contains the
  destination MAC address of router 2, and the MAC
  address of the first router that became the new
  source. The IP header remains unchanged.
• The first router passes the data packet through
  the port that it selects, and on to subnetwork 4.
  
• The data packet continues along subnetwork 4
  until it reaches router 2.
• Router 2 picks the data packet up because it
  recognizes that its own MAC address is the same
  as the destination MAC address.
• At the data link layer, the router strips off the
  MAC header and passes the data up to the network
  layer. There, it examines the destination network
  IP address and looks in its routing table.
• The router, using RIP as its routing protocol,
  determines that the best path for the data is one
  that places the destination only two hops away.

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How Routers Route Data Through a Network

• the router determines that it must send the data
  packet through the port attached to subnetwork 5,
  in order for the data packet to reach its
  destination via the selected path.
• The router passes the data down to the data link
  layer where it places a new MAC header on the
  data packet. The new MAC header contains the
  destination MAC address of router 2, and the MAC
  address of the first router becomes the new
  source MAC. The IP header remains unchanged.
• The router 2 passes the data packet through the
  port that it selects and on to subnetwork 5.
• The data packet continues along subnetwork 5
  until it reaches router 3.
• Router 3 picks the data packet up because it
  recognizes that its own MAC address is the same
  as the destination MAC address.
• At the data link layer, the router strips off the
  MAC header and passes it up to the network layer.
  There, it sees that the destination IP address in
  the IP header matches that of a host that is
  located on one of the subnetworks to which it is
  attached.

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How Routers Route Data Through a Network

• the router determines that it must send the data
  packet through the port attached to subnetwork 8
• It places a new MAC which contains the
  destination MAC address of host Z, and the source
  MAC address of router 3 on the data.
• The IP header remains unchanged. Router 3 sends
  the data through the port that is attached to
  subnetwork 8.
• The data packet travels along subnetwork 8 and
  reaches host Z
• Host Z picks it up because it sees that its MAC
  address matches the destination MAC address
  carried in the MAC header of the data packet.
• Host Z strips off the MAC header and passes the
  data to the network layer.
• At the network layer, host Z sees that its IP
  address, and the destination IP address carried
  in the IP header, match. Host Z strips off the IP
  header and passes the data up to the transport
  layer of the OSI model.
• Host Z continues to strip off the layers that
  encapsulate the data packet and then passes the
  data to the next layer of the OSI model. This
  continues until the data finally arrives at the
  application layer of the OSI model.