1. CIS 81 Fundamentals of Networking Chapter 8: IP Addressing Part 2 – ICMP (ICMPv4)
CCNA Introduction to Networking 5.0
Rick Graziani
Cabrillo College
Fall 2013

2. Chapter 8: Objectives Part 1
Describe the structure of an IPv4 address.
Describe the purpose of the subnet mask.
Compare the characteristics and uses of the unicast, broadcast and multicast IPv4 addresses.
Part 2
Describe ICMP and ICMPv4
Part 3
Explain the need for IPv6 addressing.
Describe the representation of an IPv6 address.
Describe types of IPv6 network addresses.
Configure global unicast addresses.
Describe ICMPv6

3. Testing the Network

4. IP IP is a best effort delivery system.
No mechanism to ensure that the data is delivered
So how do we know if a packet encountered a problem along the way?
Internet Control Message Protocol (ICMP)

5. Internet Control Message Protocol (ICMP)
ICMP is available for both IPv4 and IPv6.
ICMPv4 is the messaging protocol for IPv4.
ICMPv6 provides these same services for IPv6 but includes additional functionality.
ICMP messages common to ICMPv4 and ICMPv6 include:
Host confirmation
Destination or Service Unreachable
Route redirection
Time exceeded
ICMPv6 includes additional functionality.

6. ICMP ICMP is used for: Informational messages (ping, traceroute)
Error messages (network unreachable)
ICMP is a layer 3 protocol directly encapsulated in another layer 3 protocol IP.
No transport header
Knowledge of ICMP control messages is an essential part of network troubleshooting.

7. The ICMP packets are identified by type and code fields.

8. TCP/IP Suite Control Messages
The ICMPv4 packet identifies the type of control message in the “Type” field.

9. Host Confirmation (PING)
Ping is a utility used to verify connectivity to an IP host.
It measures the round-trip time for messages sent from the originating host to a destination computer.
Ping uses an ICMP Echo Message to determine if a host is reachable.
A host initiates a ping (ICMP Echo Request) and the destination replies (ICMP Echo Reply).
ICMP only reports on the status of the delivered packet to the source device.

10. Host Confirmation (PING)
The Echo Request (Type 8) is an ICMP message which sends a packet of data to the host and expects that data to be sent in return in an Echo Reply (Type 0).
The host must respond to all Echo Requests with an Echo Reply containing the exact data received in the request message.

11. Host Confirmation (PING)
The Echo Reply (Type 0) is an ICMP message generated in response to an ICMP Echo Request (Type 8) message, and is mandatory for all hosts and routers.

12. ICMP Packet

13. ICMP Packet

14. ICMP Packet

15. Use the ping command to help you troubleshoot / verify connectivity.
Apples-MacBook-Pro:~ rigrazia$ ping PING ( ): 56 data bytes 64 bytes from : icmp_seq=0 ttl=55 time= ms 64 bytes from : icmp_seq=1 ttl=55 time= ms ping statistics packets transmitted, 3 packets received, 0.0% packet loss round-trip min/avg/max/stddev = /31.963/42.574/7.529 ms Apples-MacBook-Pro:~ rigrazia$ ping PING ( ): 56 data bytes 64 bytes from : icmp_seq=0 ttl=55 time= ms 64 bytes from : icmp_seq=1 ttl=55 time= ms ping statistics --- round-trip min/avg/max/stddev = /22.291/23.232/0.704 ms Apples-MacBook-Pro:~ rigrazia$ ping PING ( ): 56 data bytes 64 bytes from : icmp_seq=0 ttl=64 time=7.207 ms 64 bytes from : icmp_seq=1 ttl=64 time=9.859 ms ping statistics --- round-trip min/avg/max/stddev = 7.207/8.507/9.859/1.083 ms Apples-MacBook-Pro:~ rigrazia$ ping PING ( ): 56 data bytes 64 bytes from : icmp_seq=0 ttl=64 time=0.073 ms 64 bytes from : icmp_seq=1 ttl=64 time=0.067 ms ping statistics --- round-trip min/avg/max/stddev = 0.067/0.071/0.073/0.003 ms Apples-MacBook-Pro:~ rigrazia$
Use the ping command to help you troubleshoot / verify connectivity.
Steps to verify:
Ping the remote host using the domain name
Ping the remote host using its IP address
Ping your default gateway
Ping your system

16. Ping a Remote Host

17. Ping the Default Gateway

18. Ping Yourself

19. Destination or Service Unreachable
When a router cannot deliver a packet, it sends an ICMP Destination Unreachable message to the source.
Message includes a code indicating why it could not be delivered.
Some of the Destination Unreachable codes for ICMPv4 are:
0 - net unreachable.
1 - host unreachable.
2 - protocol unreachable.
3 - port unreachable.
Note:
ICMPv6 has similar but slightly different codes for Destination Unreachable messages.

20. Destination or Service Unreachable
ICMP only reports on the status of the delivered packet to the source device.
For example:
PC1 pings PC2.
The ping crosses R1, then R2, but R3 encounters a link error.
Since the packet only contains the source and destination IP addresses, R3 may notify PC1 of the failure (optional).
It’s unaware of the exact path the packet took
No ICMP messages are sent to R1 or R2. R1 R2 R3
PC2
PC1

21. Destination or Service Unreachable
The Destination Unreachable message is generated by the router to inform the client that the destination host is unreachable.
Reasons:
The physical connection to the host does not exist (distance is infinite)
The indicated protocol or port is not active

22. Destination or Service Unreachable
The Destination Unreachable (Type 3) packet codes are:
0 = net unreachable
1 = host unreachable
2 = protocol unreachable
3 = port unreachable

23. Destination or Service Unreachable
Codes 0 (net unreachable) is sent when a router does not have the requested network.
Code 1 (host unreachable) is sent when a router receives a packet for which it has an attached route but is unable to deliver the packet to the host on the attached network.
Code 2 (protocol unreachable) and Code 3 (port unreachable) are used by an end host to indicate that the TCP segment or UDP datagram contained in a packet could not be delivered to the upper layer service.

24. Destination or Service Unreachable
C:\Users\Bob> ping
Pinging with 32 bytes of data:
Reply from : Destination net unreachable.
Ping statistics for :
Packets: Sent = 4, Received = 4, Lost = 0 (0% loss),
C:\Users\Bob>
Codes 0 (net unreachable) is sent when a router does not have the requested network.
C:\Users\Bob> ping
Pinging with 32 bytes of data:
Reply from : Destination host unreachable.
Ping statistics for :
Packets: Sent = 4, Received = 4, Lost = 0 (0% loss),
C:\Users\Bob>
Code 1 (host unreachable) is sent when a router receives a packet for which it has an attached route but is unable to deliver the packet to the host on the attached network.

25. Route Redirection
A router may use the ICMP Redirect Message to notify the hosts on a network that a better route is available for a particular destination.
This message may only be used when the source host is on the same physical network as both gateways.
Both ICMPv4 and ICMPv6 use route redirection messages.

26. Route Redirection
The Redirect Message (Type 5) informs a host to redirect its routing information (to send packets on an alternate route).
If the host tries to send data through a router (R1) and then another router (R2) to reach the host, and a direct path from the host to R2 is available, the redirect will inform the host of such a route.

27. Route Redirection
Host A sends a packet to Host B on network /8, but since they are not on the same network, it forwards it to the default gateway, R1.
R1 finds the correct route to network /8 is through the same interface and forwards out of its E0 interface to R2.
R1 also forwards an ICMP Redirect message to Host A telling it to use R2 as the gateway to forward all future requests to network /8. B A

28. Time Exceeded
An ICMPv4 Time Exceeded message is used by a router to indicate that a packet cannot be forwarded because the Time to Live (TTL) field of the packet was decremented to 0.
If a router receives a packet and decrements the TTL field in the IPv4 packet to zero, it discards the packet and sends a Time Exceeded message to the source host.
ICMPv6 also sends a Time Exceeded message if the router cannot forward an IPv6 packet because the packet has expired.
IPv6 does not have a TTL field; it uses the hop limit field to determine if the packet has expired.

29. Detecting Excessively Long Routes
Whether the actual path includes too many hops or a circular routing path exists, the packet will eventually reach the end of its life, known as its time-to-live (TTL).

30. Time Exceeded Message
The Time Exceeded (Type 11) message is an ICMP message which is generated by a gateway to inform the source of a datagram that the datagram has been discarded due to the time to live field reaching zero.
Time exceeded messages are used by the traceroute utility to identify gateways on the path between two hosts.

31. Traceroute
Ping is used to indicate the connectivity between two hosts.
Traceroute (tracert) is used to observe the path between these hosts.
The trace lists hops successfully reached along the way providing us with important verification and troubleshooting information.
If the data fails at some hop along the way, we have the address of the last router that responded to the trace indicating where the problem or security restrictions are. 31

32. Traceroute TTL 1 TTL 1 – 1 =0 ICMP Time Exceeded TTL 2 TTL 1
Traceroute
TTL 1
TTL 1 – 1 =0
ICMP Time Exceeded
TTL 2
TTL 1
TTL 2 – 1 =1
TTL 1 – 1 =0
ICMP Time Exceeded
TTL 3
TTL 2
TTL 1
TTL 3 – 1 =2
TTL 2 – 1 =1
TTL 2 – 1 =0
ICMP Time Exceeded

33. Three delay measurements.
Traceroute Example
C:\Users\Bob> tracert
Tracing route to eurocom.fr [ ]
over a maximum of 30 hops:
ms ms ms Home-Net [ ]
ms ms ms eastlink.ca [ ]
ms ms ms
ms * ms on-sdbr-dr001.on.eastlink.ca [ ]
ms ms ms eastlink.ca [ ]
ms ms ms eastlink.ca [ ]
ms ms ms xe mpr2.tor1.ca.above.net [ ]
ms ms ms xe cr1.lga5.us.above.net [ ]
ms ms ms xe cr1.dca2.us.above.net [ ]
ms ms ms xe mpr1.bwi9.us.above.net [ ]
ms ms ms t above.net [ ]
ms ms ms xe2-6-core1-d.paris.gandi.net [ ]
ms ms ms xe1-5-5-dist1-d.paris.gandi.net [ ]
ms ms ms
Trace complete.
C:\Users\Bob>
Three delay measurements.

34. Traceroute Example C:\Users\Bob> tracert cern.ch
Tracing route to cern.ch [ ]
over a maximum of 30 hops:
ms ms ms Home-Net [ ]
2 * * * Request timed out.
ms ms ms eastlink.ca [ ]
ms ms ms
5 * ms ms on-sdbr-dr001.on.eastlink.ca [ ]
ms ms ms eastlink.ca [ ]
ms ms ms te-4-3.car1.Montreal2.Level3.net [ ]
ms ms ms ae-5-5.ebr2.NewYork1.Level3.net [ ]
ms ms ms ae ebr2.NewYork2.Level3.net [ ]
ms ms ms ae ebr1.NewYork2.Level3.net [ ]
ms ms ms
ms ms ms ae ebr2.Paris1.Level3.net [ ]
ms ms ms ae-9-9.car1.Lyon1.Level3.net [ ]
ms ms ms ae-5-5.car1.Geneva1.Level3.net [ ]
ms ms ms h4allr1.safe-order.ch [ ]
ms ms ms e513-e-rbrxl-2-ne0.cern.ch [ ]
* * * Request timed out.
* * * Request timed out.
* * * Request timed out.
ms ms ms webr8.cern.ch [ ]
* * * Request timed out.
ms ms ms webr8.cern.ch [ ]
Trace complete.
C:\Users\Bob>

35. Traceroute Example C:\Users\Bob> tracert ehawaii.gov
Tracing route to ehawaii.gov [ ]
over a maximum of 30 hops:
ms ms ms Home-Net [ ]
2 * * * Request timed out.
ms ms ms eastlink.ca [ ]
ms ms ms
5 * * * Request timed out.
ms ms ms eastlink.ca [ ]
ms ms ms te-4-3.car1.Montreal2.Level3.net [ ]
ms ms ms ae-5-5.ebr2.NewYork1.Level3.net [ ]
ms ms ms
ms ms ms ae-2-52.edge3.NewYork2.Level3.net [ ]
ms ms ms jfk-brdr-04.inet.qwest.net [ ]
ms ms ms hnl-edge-04.inet.qwest.net [ ]
ms ms ms
ms ms ms
* * * Request timed out.
* * * Request timed out.
* * * Request timed out.
* * * Request timed out.
* * * Request timed out.

36. http://www.yougetsignal.com/tools/visual-tracert /

37. tracert -h 160 216.81.59.173 traceroute -m 100 -q 1 216.81.59.173

38. CIS 81 Fundamentals of Networking Chapter 8: IP Addressing Part 2 – ICMP (ICMPv4)
CCNA Introduction to Networking 5.0
Rick Graziani
Cabrillo College
Fall 2013