TCPIP Suite Error and Control Messages

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TCP/IP Suite Error and Control Messages

• Module 8
• CCNA 2 Version 3.0

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IP and ICMP

• IP is known as a best effort delivery mechanism
• Considered unreliable
• has no built-in processes to ensure that data is
  delivered
• ICMP is an error reporting protocol for IP
• Used to send and receive error and control
  messages to hosts on a network

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ICMP Messaging

• ICMP is part of the TCP/IP protocol suite
• Two functions performed by ICMP
• Error messaging
• Control messaging

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ICMP Message Delivery

• ICMP messages are encapsulated into datagrams in
  the same way any other data is delivered using IP
• Errors created by ICMP messages do not generate
  their own ICMP messages
• It is possible to have a datagram delivery error
  that is never reported back to the sender of the
  data

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ICMP Message Types

• All IP implementations must include ICMP support
• ICMP messages have special formats with their own
  unique characteristics
• Control messages are used to inform hosts of
  conditions such as
• network congestion
• the existence of a better gateway to a remote
  network

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Error Reporting

• ICMP reports on the status of the delivered
  packet only to the source device. It does not
  propagate information about network changes to
  routers.

Router C notifies ONLY Workstation 1 that the
datagram could not be delivered
Suppose, Workstation 1 sends a packet to
Workstation 6
Interface Fa0/0 goes down on Router C
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TTL (Time to Live)

• If a circular routing path exists or there are
  too many hops to a destination, a packet will
  eventually exceed the maximum hop count
• This is also known as reaching its time-to-live
  (TTL)
• the TTL value typically matches the maximum hop
  count defined by the routing protocol
• A TTL value is defined in each datagram
• As each router processes the datagram, it
  decreases the TTL value by one
• When the TTL of the datagram value reaches zero,
  the packet is discarded
• ICMP uses a time exceeded message to notify the
  source device that the TTL of the datagram has
  been exceeded.

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Review of Network Communications

• Network communication depends upon certain basic
  conditions being met
• sending and receiving devices must have the
  TCP/IP protocol stack properly configured
• includes the installation of the TCP/IP protocol,
  proper configuration of IP address and subnet
  mask, and configuration of a default gateway if
  datagrams are to travel outside of the local
  network
• intermediary devices must be in place to route
  the datagram from the source device and its
  network to the destination network
• Routers serve this function (routers also must
  have the TCP/IP protocol properly configured on
  their interfaces and use an appropriate routing
  protocol)

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PING (Packet Internet Groper)

• ICMP can be used to test the availability of a
  particular destination (tests network
  connectivity)
• ICMP issues an echo request message to the
  destination device
• The echo request message is typically initiated
  using the ping command

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PING Contd

• In the following example, PING issues four echo
  requests and receives four echo replies,
  confirming IP connectivity between the two
  devices.

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Destination Unreachable

• ICMP destination unreachable message header
• The value of 3 in the type field indicates it is
  a destination unreachable message.

• The code value indicates the reason the packet
  could not be delivered.
• Below shows the meaning for each possible code
  value in a destination unreachable message

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Congestion and Flow Control Messages

• Congestion can occur when
• multiple computers try to access the same
  destination at the same time
• traffic from a high speed LAN reaches a slower
  WAN connection
• Dropped packets occur when there is too much
  congestion on a network
• ICMP source-quench messages are used to reduce
  the amount of data lost
• The source-quench message asks senders to reduce
  the rate at which they are transmitting packets

• A small office home office (SOHO) where several
  computers are sharing a slow Internet connection
  is a scenario where ICMP source-quench messages
  might be used effectively

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TCP/IP Suite Error and Control Messages
THE END

• Module 8
• CCNA 2 Version 3.0