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What is ICMP?

By: Robert D. Thomson

The Internet Control Message Protocol, or ICMP, is a protocol complementary to Internet Protocol (IP).Both of these operate at the OSI Model’s Network Layer .

The specific purpose of ICMP is to send both control and test signals over IP networks.

ICMP, unlike either Transmission Control Protocol (TCP) or User Datagram Protocol (UDP), which operate at the Transport Layer (which is the layer above that of IP), coexists on the same layer as IP.

To successfully operate any network device that is compatible for use with IP, you must understand ICMP. Despite this, however, a significant number of security applications, including firewalls, will block or disable some part or all of ICMP functions for system security reasons.

Types of Messages Used With ICMP

This protocol functions by transmitting and receiving several types of messages. To learn which types are defined, please see the following: IANA ICMP Type Numbers.

The most frequently used types for ICMP messages include:

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Echo Request and Reply Messaging with ICMP

These two messages (eight and zero) are the most important used by ICMP. Both are used to test for network connections through the use of the ‘ping’ command. In this situation, the operator uses this command to transmit three Echo Request messages by 64-bye ICMP. The message’s target is www.freebsd.org. In response, three Echo Reply messages are received:

Bash-2.05a$ping -c 3 www.freebsd.org
PING www.freebsd.org (216.136.204.117): 56 data bytes
64 bytes from 216.136.204.117:icmp..seq=0 ttl=55 time=63.708 ms
64 bytes from 216.136.204.117:icmp..seq=1 ttl=55 time=62.725 ms
64 bytes from 216.136.204.117:icmp..seq=2 ttl=55 time=62.618 ms
---www.freebsd.org statistics---
3 packets transmitted, 3 packets received, 0% packet loss
Round-trip min/avg/max/stddey=62.618/63.017/63.708/0.491 ms

The data obtained allows us to determine that the network is able to successfully connect to www.freebsd.org. In addition, it relays the amount of time required for the return of each packet. ’Ping’ can be extremely useful when troubleshooting network problems.

To read more about ICMP, go to RFC 792: Internet Control Message Protocol.

Article Source: http://www.article-voip.com

Jane Hanson writes on topics such as ICMP , IP and Collision Domain for The Tech FAQ.

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