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

ICMPv4 Echo and Echo Reply Message Format

Table 96 and Figure 146 show the format for both ICMPv4 Echo and Echo Reply messages.

It is possible that a source device may want to send more than one Echo message to either a single destination or multiple destinations. Conversely, a single destination might receive Echo messages from more than one source. It is essential that a device receiving an Echo Reply knows which Echo prompted it to be sent.

Two special fields are used within the format of these messages to allow Echo and Echo Reply messages to be matched together, and to allow a sequence of messages to be exchanged. The Identifier field was envisioned as being used as a higher-level label, like a session identifier, while the Sequence Number was seen as something to identify individual test messages within a series. However, the use of these fields is up to the particular implementation. In some cases, the Identifier field is filled in with the process number of the application that is using the Echo or Echo Reply message, to allow several users to use utilities like ping without interference.

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