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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)
Internet Header + First 64 bits of datagram
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
Internet Header + First 64 bits of datagram
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
Identifier Sequencenumber
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
Type (15 or 16) Code (0) Checksum
Identifier Sequencenumber
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
Type (17 or 18) Code (0) Checksum
Identifier Sequencenumber
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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Q&AQ&A