Cisco Systems CCNA 2 8.1 Overview of TCP/IP Error Message

8.1 Overview of TCP/IP Error Message

Essential Labs: None

Optional Labs: None

Core TIs: 8.1.1, 8.1.2, 8.1.4, 8.1.5, 8.1.6, and 8.1.8

Optional TIs: 8.1.3, 8.1.7, and 8.1.9

Course-Level Claim: Students can describe the operation of ICMP and identify the reasons,

types, and format of associated error and control messages.

Hands-on skills: none

IP is considered a “best effort” or unreliable method for the delivery of network data. If the data

does not reach its destination, the sender is not notified that the transmission has failed. ICMP

is the component of the TCP/IP protocol stack that addresses the limitations of IP. ICMP does

not overcome the unreliability issues in IP but ICMP does allow for testing. Reliability must be

provided by upper layer protocols. Explain the difference between a guaranteed method and a

best effort. This is a good time to show students how a successful ICMP works. Introduce

problems into the lab setup to demonstrate how ICMP relays messages in a network. Explain

that ICMP is a Layer 3 protocol of the TCP/IP suite. It is not an IP packet. It uses the IP

addressing scheme but has a different packet format than IP.

ICMP is an error reporting protocol for IP. When datagram delivery errors occur, ICMP is used

to report these errors back to the sender of the datagram.

This is an excellent opportunity to show the students this procedure in the lab setup. They

should be shown that ICMP does not correct the encountered network problem. ICMP just

reports on the status of the delivered packet to the sender. Its function is not to propagate

information about network changes.

ICMP is a message protocol for TCP/IP protocol suite. ICMP messages are encapsulated as

data in ICMP packets in the same way that IP data is delivered. ICMP messages have their

own header information. They are subject to the same failures as any other data. The students

should realize that ICMP is a Layer 3 protocol that does not use IP packets. ICMP uses IP

addressing but has a different structure than an IP packet. Show the students that this

scenario could generate more error reports and cause increased congestion on an already

ailing network. For this reason, errors created by ICMP messages do not generate their own

ICMP messages. Stress to the students that it is possible to have a datagram delivery error

that is never reported back to the sender of the data.