Semester 2 v3.1.1: Routers and Routing Basics

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Semester 2 v3.1.1Routers and Routing Basics

• MODULE 8
• TCP/IP Suite Error and Control Messages

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IP is for Isnt Perfect

• TCP/IP is regarded as a reliable form of data
  delivery.
• Reliable, connection-oriented services provided
  by TCP at Layer 4.
• IP at Layer 3 is an unreliable, best-effort
  delivery protocol.
• No mechanisms built in to IP to tell it if
  something goes wrong.
• Where service reliability is taken care of by
  TCP, error reporting and service control is
  handled by ICMP.

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ICMP apologises for the delay

• Internet Control Message Protocol (ICMP) reports
  delivery errors to the source of the undelivered
  datagram.
• ICMP can only report errors, not correct errors.
• When preparing to report back to the source
  device about an undelivered datagram, ICMP adds
  its own header and data to the datagram before
  sending.
• No ICMP errors are generated from undeliverable
  ICMP error messages, hence no recursive ICMP
  message generation.

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Dude, wheres my network?

• Internetwork communication wont take place
  unless
• TCP/IP stack is installed and functioning on all
  hosts.
• IP addresses, subnet masks, and default gateway
  addresses properly configured.
• At least one functioning router is in place to
  route data between networks.
• What happens if
• A host doesnt have a properly configured TCP/IP
  stack? The host is unreachable.
• An interface on a router goes down? The attached
  network is unreachable.
• Too many routers are between source and
  destination? The hop count for the routing
  protocol in use may be exceeded.

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Youve been pinged!

• To test if a host is reachable, issue the ping
  command.
• Ping sends an ICMP echo request to the specified
  destination host.
• By default, ping sends five ICMP echo requests.
  The number of ICMP echo requests and timeout
  values can be changed in IOS.
• Upon receipt of an ICMP echo request, the
  destination device sends back an ICMP echo reply.

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ICMP Redirect Requests

• In addition to reporting on delivery errors, ICMP
  can inform hosts of changing network conditions.
• One of the more important control messages is
  ICMP redirect request.
• Issued where a better gateway than the default
  gateway exists for access to a certain specific
  network.

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ICMP has it all under control

• Other ICMP control messages include
• ICMP timestamp
• Synchronises system clocks of two hosts.
• Largely rendered obsolete by Network Time
  Protocol (NTP).
• ICMP information request/reply
• Allows a host to determine its network number.
• Largely rendered obsolete by DHCP, RARP, and
  BootP.
• ICMP address mask request/reply
• Allows a host to discover its subnet mask.
• ICMP router discovery/advertisement message
• Allows a host not configured with a default
  gateway to discover routers via multicasting on
  224.0.0.2.

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Relieving a Blocked Node

• Dropped packets are the result of the network
  becoming too congested.
• Too many hosts are trying to access the same
  network resource at the same time.
• Traffic from a high speed network is being routed
  onto a lower speed network.
• ICMP source quench messages request to senders of
  dropped packets that they reduce their rate of
  data transmission until congestion subsides.

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Welcome to my world )

• Daniel Comarmond
• CCNP, CCDP, CCSP, CCAI
• Cisco Networking Academy Instructor
• Systems Engineer Cisco Systems
• E-Mail dcom_at_it.uts.edu.au
• MSN dcom82_at_dcom82.com
• Phone [phone]
• Website http//www-staff.it.uts.edu.au/dcom
• Take care, and SMILE!!! )