Tài liệu Internet Control Message Protocol (ICMP) ppt

Internet Control Message Protocol (ICMP)

TCP and UDP exist primarily to deliver upper-layer data across a network. Whether

connection oriented or connectionless, fundamentally the process of delivering the data is

the same: identify the source and destination application ports, format the data

accordingly, and deliver the data to IP. This process works well when the network is

functioning without error and when systems are operating correctly and know how to

deliver data through routers and subnets to any destination, anywhere. The problem is

that the network does not always work, routes fail, and data may not be able to be

delivered (or it may need to be directed elsewhere to be successfully delivered). To

facilitate this process, the ICMP protocol was developed.

In many ways, ICMP functions like the traffic cop and policeman of the network.

Because IP (and UDP) lack any mechanism for identifying that failures may occur on the

network, they need an external protocol that can provide information about routing

failures and to report about delivery errors, congestion delays, and other conditions on the

network. Indeed, like the traffic cop notifying motorists about congestion delays or

blocked intersections (routers) on the street, ICMP provides a means to control the flow

of traffic in an effort to ensure that the data can be reliably delivered.

An important distinction to understand about ICMP is that it is an error-reporting

mechanism, not an error-correcting mechanism. That means that although ICMP can

notify hosts of error conditions, ICMP natively has no means or method of actually doing

anything about the error condition. Instead, ICMP relies on other protocols such as

routing protocols or reliable protocols such as TCP to account for and address the

particular error condition.

The most well-known use of ICMP is through the use of the ping application. Ping is a

network troubleshooting application that makes use of ICMP echo request and echo reply

(detailed in the next section) messages to determine whether a host is responding to

network traffic. This allows the user to determine the reachability and status of the target

host in a pretty simple manner. If a target host responds, it is reachable and available. If it

does not, depending on the echo reply message, either the target host, target network, or

network somewhere between the source and destination is unreachable and unavailable.

We talk more about ping in Chapter 13, "Troubleshooting Firewalls," and Appendix A,

"Firewall and Security Tools".

Note

RFC 0792 and RFC 0950 define ICMP.