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) Error Message Types and Formats

ICMP Version 4 (ICMPv4) Error Message Types and Formats 1 2 3 4 ICMPv4 Source Quench Messages

Interpretation of Destination Unreachable Messages

Finally, it's important to remember that just as IP is “best effort”, the reporting of unreachable destinations using ICMP is also “best effort”. For one things, these ICMP messages are themselves carried in IP datagrams. More than that, however, one must remember that there may be problems that prevent a router from detecting failure of delivery of an ICMP message, such as a low-level hardware problem. A router could, theoretically, also be precluded from sending an ICMP message even when failure of delivery is detected, for whatever reason.

For this reason, the sending of Destination Unreachable messages should be considered supplemental. There is no guarantee that every problem sending a datagram will result in a corresponding ICMP message. No device should count on receiving an ICMP Destination Unreachable for a failed delivery any more than it counts on the delivery in the first place. This is why the higher-layer mechanisms mentioned at the start of this topic are still important.

ICMP Version 4 (ICMPv4) Error Message Types and Formats 1 2 3 4 ICMPv4 Source Quench Messages