Module 8

TCP/IP Suite Error and

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 Internet Protocol (IP)
• IP is an unreliable method for delivery of network data.
• It is known as a best effort delivery mechanism.
• It has no built-in processes to ensure that data is delivered in
the event that problems exist with network communication.
• Nothing in its basic design allows IP to notify the sender that a
data transmission has failed.

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Internet Control Message Protocol (ICMP)
• ICMP is the component of the TCP/IP protocol stack that
addresses the basic limitation of IP.
– ICMP does not overcome the unreliability issues in IP.
– Reliability must be provided by upper layer protocols if it is
needed.
• It is an error reporting protocol for IP.
• When datagram delivery errors occur, ICMP is used to report
these errors back to the source of the datagram.

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Internet Control Message Protocol (ICMP)
• ICMP does not correct the encountered network problem; it
merely reports the problem.
• It does not propagate information about network changes to
routers.
• 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.
• For this reason, it is possible to have a datagram delivery error
that is never reported back to the sender

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 ICMP Messages
• There are two types of ICMP messages:
– Error messages - are the results of lost packets or error
conditions which occur during packet transmission
– Control messages- are used to inform hosts of conditions
such as network congestion or the existence of a better
gateway to a remote network.

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 Network Communication
Network communication depends upon certain basic conditions
being met.
• First, the sending and receiving devices must have the TCP/IP
protocol stack properly configured.
– installation of the TCP/IP protocol
– proper configuration of IP address and subnet mask
– A default gateway must also be configured if datagrams are
to travel outside of the local network.

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 Network Communication
Network communication depends upon certain basic conditions
being met.
• Second, intermediary devices (a router) must be in place to
route the datagram from the source device and its network to
the destination network. A router also must have the TCP/IP
protocol properly configured on its interfaces, and it must use
an appropriate routing protocol.

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 Destination Unreachable
• If a destination network is not accessible, it is said to be an
unreachable network.
• Possible causes of failure:
– the sending device may address the datagram to a non-
existent IP address
– Default gateway may not be configured correctly
– the destination device may be disconnected from its network
– routers interface is down
– the router does not have the information necessary to find
the destination network.

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 Destination Unreachable
• If a host or network is unreachable:
– A reply will be sent by the router to the sending host  Destination
Unreachable
– The packet will be dropped
– No further ICMP messages will be sent as a result of this
transaction

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 ICMP Echo Request (PING)
• The ICMP protocol can be used to test the availability of a
particular destination.
• An ICMP echo request message (initiated using the ping
command) is sent to the destination device.
• If the destination device receives the ICMP echo request, it
formulates an echo reply message to send back to the source
of the echo request.
• If the sender receives the echo reply, this confirms that the
destination device can be reached via the IP protocol.

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 ICMP Echo Request (PING)

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 ICMP Echo Request (PING)

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 ICMP Echo Request (PING)

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 ICMP Echo Request (PING)

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 ICMP Echo Request (PING)

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 ICMP Request / Reply Message
• As with any type of packet, ICMP messages have special
formats.
Sequence
Type Code Checksum Identifier Data
Number

• All ICMP message formats start with these same three fields:
– Type - indicates the type of ICMP message being sent
– Code - indicates the reason the packet could not be delivered
– Checksum - used to verify the integrity of the data
0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31
Type (8 bits) Code (8 bits) IMCP Header Checksum (16 bits)
Identifier (16 bits) Sequence Number (16 bits)

Optional Data (variable length)

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 ICMP Message Types

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 ICMP Message Types
• Common ICMP messages:
– Echo - used by the ping utility to test the connection between two
devices
– Echo reply - reply to a ping
– Destination unreachable - a variety of reasons cause a destination
to be unreachable (unavailable port, unknown network, etc.)
– Source quench - tells the data source to reduce its transmission
rate
– Redirect - informs the source of a better route to the destination
– Time exceeded - sent when a packet's time-to-live (TTL) reaches
zero

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 ICMP Codes

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 Destination Unreachable Message
• Datagrams cannot always be forwarded to their destinations.
• If ICMP delivers back to the sender a destination unreachable
message the problem could be:
– hardware failures
– improper protocol configuration
– down interfaces
– incorrect routing information
– packet fragmentation is required in order to forward a packet
– IP related services such as FTP or Web services are unavailable
(application layer processes)

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 ICMP redirect/change request messages
• Default gateways only send ICMP redirect/change request
messages if the following conditions are met:
– The interface on which the packet comes into the router is the
same interface on which the packet gets routed out.
– The subnet/network of the source IP address is the same
subnet/network of the next-hop IP address of the routed packet.
– The datagram is not source-routed.
– The route for the redirect is not another ICMP redirect or a default
route.
• The router is configured to send redirects. (By default, Cisco
routers send ICMP redirects. The interface subcommand no
ip redirects will disable ICMP redirects.)

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 ICMP redirect/change request messages
• Host B sends a packet to Host C
Host C on network
10.0.0.0/8.
• Since Host B is not
directly connected to the
Router Router
same network, it A B
forwards the packet to its
default gateway, Router
A.
Host B
• Router A finds the
correct route to network
10.0.0.0/8 by looking into
its route table.

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 ICMP redirect/change request messages
• It determines that the path Host C
to the network is back out
the same interface the
request to forward the
packet came from.
• It forwards the packet and Router Router
sends an ICMP A B
redirect/change request to
Host B telling it to use
Router B as the gateway to
forward all future requests Host B
to network 10.0.0.0/8.

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 ICMP Timestamp Message
• Hosts on different networks who are trying to communicate
using software that requires time synchronization can
sometimes encounter problems.
• The ICMP timestamp request message allows a host to ask for
the current time according to the remote host.
• The remote host uses an ICMP timestamp reply message to
respond to the request.
• All ICMP timestamp reply messages contain the originate,
receive and transmit timestamps.
• Using these three timestamps, the host can estimate transit
time across the network by subtracting the originate time from
the transit time.
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 ICMP Information Requests
and Reply Messages
• The ICMP information requests and reply messages were
originally intended to allow a host to determine its network
number.
• It is now considered obsolete.
• Other protocols such as BOOTP and Dynamic Host
Configuration Protocol (DHCP) are now used to allow hosts to
obtain their network numbers.

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 ICMP Address Request
• If a host does not know the subnet mask, it may send an
address mask request to the local router.
• If the address of the router is known, this request may be sent
directly to the router.
• Otherwise, the request will be broadcast.
• When the router receives the request, it will respond with an
address mask reply.
• This address mask reply will identify the correct subnet mask
from which the request was received.

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 ICMP Router Discovery
• When a host on the network boots, and the host has not been
manually configured with a default gateway, it can learn of
available routers through the process of router discovery.
• This process begins with the host sending a router solicitation
message to all routers, using the multicast address 224.0.0.2
as the destination address.
• When a router that supports the discovery process receives the
router discovery message, a router advertisement is sent in
return.

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 ICMP Source-Quench Messages
• ICMP source-quench messages are used to reduce the amount
of data lost due to congestion on the network.
• The source-quench message asks senders to reduce the rate
at which they are transmitting packets.
• ICMP source-quench messages can be sent by the host or
router (gateway).

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