Please Whitelist This Site? I know everyone hates ads. But please understand that I am providing premium content for free that takes hundreds of hours of time to research and write. I don't want to go to a pay-only model like some sites, but when more and more people block ads, I end up working for free. And I have a family to support, just like you. :) If you like The TCP/IP Guide, please consider the download version. It's priced very economically and you can read all of it in a convenient format without ads. If you want to use this site for free, I'd be grateful if you could add the site to the whitelist for Adblock. To do so, just open the Adblock menu and select "Disable on tcpipguide.com". Or go to the Tools menu and select "Adblock Plus Preferences...". Then click "Add Filter..." at the bottom, and add this string: "@@||tcpipguide.com^$document". Then just click OK. Thanks for your understanding! Sincerely, Charles Kozierok Author and Publisher, The TCP/IP Guide

NOTE: Using software to mass-download the site degrades the server and is prohibited. If you want to read The TCP/IP Guide offline, please consider licensing it. Thank you.

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

ICMP Version 4 (ICMPv4) Informational Message Types and Formats 1 2 3 ICMPv4 Timestamp (Request) and Timestamp Reply Messages

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.

ICMP Version 4 (ICMPv4) Informational Message Types and Formats 1 2 3 ICMPv4 Timestamp (Request) and Timestamp Reply Messages