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Table Of Contents  The TCP/IP Guide
 9  TCP/IP Lower-Layer (Interface, Internet and Transport) Protocols (OSI Layers 2, 3 and 4)
      9  TCP/IP Internet Layer (OSI Network Layer) Protocols
           9  Internet Control Message Protocol (ICMP/ICMPv4 and ICMPv6)
                9  ICMP Concepts and General Operation

Previous Topic/Section
ICMP Overview, History, Versions and Standards
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2
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ICMP Message Classes, Types and Codes
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ICMP General Operation
(Page 1 of 2)

ICMP is one of the simplest protocols in the TCP/IP protocol suite. Most protocols implement a particular type of functionality to either facilitate basic operation of a part of the network stack, or an application. To this end they include many specific algorithms and tasks that define the protocol, which is where most of the complexity lies. ICMP, in contrast, is exactly what its name suggests: a protocol that defines control messages. As such, pretty much all of what ICMP is about is providing a mechanism for any IP device to send control messages to another device.

The ICMP Message-Passing Service

Various message types are defined in ICMP that allow different types of information to be exchanged. These are usually either generated for the purpose of reporting errors, or for exchanging important information of different sorts that is needed to keep IP operating smoothly. ICMP itself doesn't define how all the different ICMP messages are used; this is done by the protocols that use the messages. In this manner, ICMP describes a simple message-passing service to other protocols.

Key Concept: ICMP is not like most other TCP/IP protocols in that it does not perform a specific task. It defines a mechanism by which various control messages can be transmitted and received to implement a variety of functions.


As mentioned in the preceding overview topic, ICMP is considered an integral part of IP, even though it uses IP to sends its messages. Typically, the operation of ICMP involves some portion of the TCP/IP protocol software on a machine detecting a condition that causes it to generate an ICMP message. This is often the IP layer itself, though it may be some other part of the software. The message is then encapsulated and transmitted like any other TCP/IP message, and is given no special treatment compared to other IP datagrams. The message is sent over the internetwork to the IP layer at the receiving device, as shown in Figure 137.


Figure 137: ICMP General Operation

A typical use of ICMP is to provide a feedback mechanism when an IP message is sent. In this example, device A is trying to send an IP datagram to device B. However, when it gets to router R3 a problem of some sort is detected that causes the datagram to be dropped. R3 sends an ICMP message back to A to tell it that something happened, hopefully with enough information to let A correct the problem, if possible. R3 can only send the ICMP message back to A, not to R2 or R1.

 


Again, since many of the ICMP messages are actually intended to convey information to a device's IP software, the IP layer itself may be the “ultimate destination” of an ICMP message once a recipient gets it. In other cases, the ultimate destination may be some other part of the TCP/IP protocol software, which is determined by the type of message received. ICMP does not use ports like UDP or TCP to direct its messages to different applications on a host; the software recognizes the message type and directs it accordingly within the software.

ICMP was originally designed with the idea that most messages would be sent by routers, but they can be sent by both routers and by regular hosts as well, depending on the message type. Some are obviously only sent by routers, such as Redirect messages, while others may be sent by either routers or hosts. Many of the ICMP messages are used in matched pairs, especially various kinds of Request and Reply messages, and Advertisement and Solicitation messages.


Previous Topic/Section
ICMP Overview, History, Versions and Standards
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Pages in Current Topic/Section
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2
Next Page
ICMP Message Classes, Types and Codes
Next Topic/Section

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Version 3.0 - Version Date: September 20, 2005

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