Cisco Systems CCNA 2 Overview of TCP/IP Error Message, Icmp, Error reporting and error correction

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

8.1.1 ICMP

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.

8.1.2 Error reporting and error correction

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.

8.1.3 ICMP message delivery

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.

94 - 238 CCNA 2: Routers and Routing Basics v3.1 Instructor Guide – Module 8 Copyright ♥ 2004, Cisco Systems, Inc.