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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
                     9  ICMP Version 6 (ICMPv6) Informational Message Types and Formats

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ICMP Version 6 (ICMPv6) Informational Message Types and Formats
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ICMPv6 Router Advertisement and Router Solicitation Messages
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ICMPv6 Echo Request and Echo Reply Messages
(Page 1 of 2)

IP is a relatively simple protocol that does not itself include any method for performing tests between devices to help in diagnosing internetwork problems. This means that this job, like other support tasks, falls to ICMP. The simplest test performed when there is a problem using TCP/IP is usually a check that a pair of devices is able to send datagrams to each other. This is most often done by an initiating device sending a test message to a second device, which receives it and replies back to tell the first device it received the message.

Like ICMPv4, ICMPv6 includes a pair of messages specifically for connection testing. To use them, Device A begins the test process by sending an ICMPv4 Echo Request message to Device B. Device B responds back to Device A with an Echo Reply message. When Device A receives this message, it knows that it is able to communicate (both send and receive) successfully with Device B.

Note: As a side note, in ICMPv4, the first message type was named just Echo but was often called Echo Request instead. In ICMPv6 they finally just added the “Request” to the message name and were done with it. A modest but useful improvement from a clarity standpoint.



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Version 3.0 - Version Date: September 20, 2005

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