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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 4 (ICMPv4) Informational Message Types and Formats

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ICMP Version 4 (ICMPv4) Informational Message Types and Formats
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ICMPv4 Timestamp (Request) and Timestamp Reply Messages
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ICMPv4 Echo (Request) and Echo Reply Messages
(Page 1 of 3)

One of the main purposes of ICMP informational messages is to enable testing and diagnostics, to help identify and correct problems on an internetwork. The most basic test that can be conducted between two devices is simply checking if they are capable of sending datagrams to each other. The usual way that this is done is to have one device send a test message to a second device, which receives the message and replies back to tell the first device it received the message.

ICMPv4 includes a pair of messages specifically for connection testing. Suppose Device A wants to see if it can reach Device B. Device A begins the test process by sending an ICMPv4 Echo message to B. Device B, when it receives the Echo, 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: The name of the first message in this pair is often given as Echo Request. While this does convey the paired nature of the Echo and Echo Reply messages, the formal name used in the standards is simply an Echo message.



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ICMP Version 4 (ICMPv4) Informational Message Types and Formats
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ICMPv4 Timestamp (Request) and Timestamp Reply Messages
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