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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 Message Types and Formats

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ICMPv4 Parameter Problem Messages
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ICMPv4 Echo (Request) and Echo Reply Messages
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ICMP Version 4 (ICMPv4) Informational Message Types and Formats

The five ICMP error message types communicate important information about error or problem conditions encountered during the operation of an IP internetwork. In contrast, the other class of ICMP messages contains those messages that are informational. They are not sent in response to some issue with a regular IP datagram, but are used on they own to implement various support functions for IP. Informational messages are used for testing and diagnostic purposes, as well as to let devices share critical information they need to function correctly.

In this section I describe nine different ICMP version 4 informational messages. Since many of these messages are used in functional sets, pairs of related messages are described in the same topic. I begin with a discussion of the Echo (Request) and Echo Reply messages used for network testing, and Timestamp and Timestamp Reply messages used for clock synchronization. I explain the use and format of Router Advertisement and Router Solicitation messages, which allow hosts to discover the identity of local routers and learn important information about them. I also describe the Address Mask Request and Address Mask Reply messages that communicate subnet mask information. I conclude with a look at the Traceroute message, which implements a more sophisticated version of the traceroute utility.

Note: The original ICMP standard also defined two more informational message types: Information Request and Information Reply. These were intended to allow devices to determine an IP address and possibly other configuration information. This function was later implemented using protocols such as RARP, BOOTP and DHCP, and these message types obsoleted. Therefore, they are not discussed in this section. (Well, I guess I did just discuss them there... you know what I mean!)


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