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CCNA Semester 2

Chapter 08Chap

ter 08

TCP/IP SUITE ERRORTCP/IP SUITE ERRORANDAND

CONTROL MESSAGESCONTROL MESSAGES

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ObjectivesObjectives

Describe ICMP

Describe the ICMP message format

Identify ICMP error message types

Identify potential causes of specific ICMPerror messages

Describe ICMP control messages

Identify a variety of ICMP control messagesused in networks today

Determine the causes for ICMP control

messages

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Table of ContentTable of Content

1 Overview of TCP/IP Error Message2 TCP/IP Suite Control Messages

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OVERVIEWOVERVIEW

OFOF

TCP/IP ERROR MESSAGESTCP/IP ERROR MESSAGES

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Error reporting and error correctionError reporting and error correction

Workstation 6

CA

Workstation 1

Fa0/0

ICMP

Destinationunreachable

IP

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Internet Control Message Protocol (ICMP)Internet Control Message Protocol (ICMP)

Transp

or

t

Ne

twor

k

IP

TCP UDP

ICMPARP RARP

DestinationunreachableEcho (Ping)Others

ICMP simply sends error message to the sender of thedata, indicating that problems occurred with datadelivery

ICMP is the component of the TCP/IP protocol stackthat addresses this basic limitation of IP.

It does not propagate information about networkchanges to routers.

Does not correct the encountered network problem

ICMP do not overcome the unreliabilit limitation that

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Encapsulation of an ICMP in an IP packetEncapsulation of an ICMP in an IP packet

Frame Header Datagram Header ICMP Header ICMP Data

0 8 16 31

Type Code Checksum

Option Header

Option Data

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Destination unreachableDestination unreachable

CA

To Z

Send DataTo Z

I don notknow

How to get toZ!

Send ICMP

Data network

Destination

unreachable

An ICMP destination unreachable message issend if:

Host or port unreachable.

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Destination unreachable messageDestination unreachable message

0 8 16 31

Type(3) Code(0-12) Checksum

Unused (must be zero)

Internet Header + First 64 bits of datagram

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.

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Code values for destination unreachable messageCode values for destination unreachable message

Code Description

0Net unreachable.

1Host unreachable.

2Protocol unreachable.

3Port unreachable.

4Fragmentation needed and DF set.

5Source route failed.

6Destination network unknown

7Destination host unknown

8Source Host Isolated

9Communication with destination network administrativelyprohibited

10Communication with destination network administrativelyprohibited

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Using ping to test destination reachabilityUsing ping to test destination reachability

A CA

Is Breachable

Yes, I amhere.

B

ICMP echo reply

ICMP echo request

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ICMP echo messagesICMP echo messages

0 8 16 31

Type (0 or 8) Code (0) Checksum

Identifier Sequencenumber

Option Data

The value of 0 in the type field indicates it isthe echo request.

The value of 8 in the type field indicates it isthe echo reply.

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Miscellaneous error reportingMiscellaneous error reporting

0 8 16 31

Type (12) Code (0-2) Checksum

Pointer Unused ( Mustbe zero)


Parameter problem. When the code value is 0, the pointer field

indicates the octet of the datagram thatproduced the error.

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Detecting excessively long routesDetecting excessively long routes

When the TTL of the datagram value reacheszero, the packet is discarded.

ICMP uses a time exceeded message to notifythe source device that the TTL of the datagramhas been exceeded

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TCP/IP SUITE CONTROL MESSAGESTCP/IP SUITE CONTROL MESSAGES

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ICMP Type of Control messagesICMP Type of Control messages

Type Description

0Echo reply.

3Destination unreachable.

4Source quench.

5Redirect / Change request

8Echo Request

9Routers advertisment

10Router Selection

11Time exceeded.

12Parameter problem.

13Timestamp.

14Timestamp reply.

15Information Request

16Information Reply

17Address Mask Request

18Address Mask Reply

Unlike error messages, controlmessages are used to informhosts of conditions such asnetwork congestion or theexistence of a better gatewayto a remote network.

Like all ICMP messages, ICMPcontrol messages areencapsulated within an IP

datagram.

ICMP uses IP datagrams inorder to traverse multiplenetworks.

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ICMP redirect/change requestsICMP redirect/change requests

Router A

172.16.1.100 172.16.1.200E0

E0

172.16.1.1/24

Default GW:172.16.1.100

10.0.0.1/8C

B

Router B

Router A sends an ICMP redirect/changerequest to Host B telling it to use Router B as

the gateway to forward all future requests to

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Conditions to send ICMP redirect/change requestConditions to send ICMP redirect/change request

Default gateways only send ICMP redirect/change requestmessages if the following conditions are met:

The interface on which the packet comes into the routeris the same interface on which the packet gets routed out.

The subnet/network of the source IP address is the samesubnet/network of the next-hop IP address of the routedpacket.

The datagram is not source-routed.

The route for the redirect is not another ICMP redirect or adefault 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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The ICMP redirect/change request messageThe ICMP redirect/change request message

0 8 16 31

Type (5) Code (0-3) Checksum

Router Internet address


The Router Internet Address field in the ICMPredirect is the IP address that should be usedas the default gateway for a particularnetwork.

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Clock synchronization and transit timeClock synchronization and transit time

estimationestimation0 8 16 31

Type (13 or14)

Code (0) Checksum


Originate Timestamp

Receive Timestamp

Transit Timestamp

Allows a host to ask for the current time

according to the remote host.More robust protocols such as Network Time

Protocol (NTP) at the upper layers of theTCP/IP protocol stack perform clocksynchronization in a more reliable manner.

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Information requests and reply message formatsInformation requests and reply message formats

0 8 16 31



Originally intended to allow a host todetermine its network number, is consideredobsolete.

Other protocols such as BOOTP and DynamicHost Configuration Protocol (DHCP) are nowused to allow hosts to obtain their network

numbers.

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Address mask requirementsAddress mask requirements

0 8 16 31



Address Mask

Subnet mask is crucial in identifying network,subnet, and host bits in an IP address.

If a host does not know the subnet mask, it

may send an address mask request to thelocal router or broadcast.

When the router receives the request, it willrespond with an address mask reply. Thisaddress mask reply will identify the correct

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Router discovery messageRouter discovery message

0 8 16 31

Type (9) Code (0) Checksum

Number ofaddresses

Address entrysize

Lifetime

Router address 1

Preferences Level 1Router address 2

Preferences Level 2

Hosts use router discovery message to learn of

available routers (gateway).Using the multicast address 224.0.0.2 as thedestination address. May also be broadcast.

If router that does not support the discoveryrocess, the solicitation will o unanswered.

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Router solicitation messageRouter solicitation message

0 8 16 31

Type (10) Code (0) Checksum

Reversed

A host generates an ICMP router solicitation

message in response to a missing default gateway.

This message is sent via multicast and it is the firsstep in the router discovery process.

A local router will respond with a routeradvertisement identifying the default gateway forthe local host.

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Congestion and flow control messagesCongestion and flow control messages

High speedLAN

Slow

link

Network congestion isexperienced at the WAN link

ICMP source-quench messages are used toreduce the amount of data lost.

The source-quench message asks senders toreduce the rate at which they are transmittingpackets.

Most Cisco routers do not send source-quench

messages by default

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SummarySummary

IP uses Internet Control Message Protocol (ICMP) tonotify the sender of data that there was an error inthe delivery process.

ICMP messages are transmitted using the IP protocolso their delivery is unreliable.

ICMP echo request and echo reply messages allowthe network administrator to test IP connectivity toaid in the troubleshooting process.

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