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 Book is Here... and Now On Sale!

The whole site in one document for easy reference!
The TCP/IP Guide

Custom Search







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 Concepts and General Operation

Previous Topic/Section
ICMP Message Classes, Types and Codes
Previous Page
Pages in Current Topic/Section
12
3
Next Page
ICMP Common Message Format and Data Encapsulation
Next Topic/Section

ICMP Message Creation and Processing Conventions and Rules
(Page 3 of 3)

ICMP Message Processing Conventions

Message processing generally takes place as described in the topic on general operation, with the ICMP message delivered either to the IP software or other protocol software implementation as required. What is done with the message usually depends on its type. Some are destined only for the IP software itself, but many are intended for the higher-layer protocol that generated the datagram that led to the error. In the next topic we will see that ICMP error messages include information that allows the upper-layer protocol to be extracted for the purpose of passing the message to the appropriate software layer.

In IPv6, the class of message (error or informational) can be determined from the Type value. This knowledge can then be used to guide processing of ICMP messages with unknown Type values. The rule is that ICMP error messages with unknown Type values must be passed to the appropriate upper layer protocol. Informational messages with unknown Type values are discarded without taking action.

In addition to these general rules, there are specific rules put into place to guide the processing of some of the message types. I describe some of these conventions in the topics that discuss individual ICMP messages.

An important final point is that ICMP messages, especially error messages, are not considered "binding" on the device that processes them. To stick with the office analogy, they have the equivalent status in an office of only of an "FYI memo", not an "assignment". It is often the case that a device should take action upon processing an ICMP message, but the device is not required to. The exception, again, is when informational messages are used for specific purposes. For example, most of the messages that come in pairs are designed so that a Request results in the matching Reply and a Solicitation yields an Advertisement.

Key Concept: A device receiving an ICMP message is not required to take action unless a protocol using a message type dictates a specific response to a particular message type. In particular, devices are not mandated to perform any specific task when receiving an ICMP error message.



Previous Topic/Section
ICMP Message Classes, Types and Codes
Previous Page
Pages in Current Topic/Section
12
3
Next Page
ICMP Common Message Format and Data Encapsulation
Next Topic/Section

If you find The TCP/IP Guide useful, please consider making a small Paypal donation to help the site, using one of the buttons below. You can also donate a custom amount using the far right button (not less than $1 please, or PayPal gets most/all of your money!) In lieu of a larger donation, you may wish to consider purchasing a download license of The TCP/IP Guide. Thanks for your support!
Donate $2
Donate $5
Donate $10
Donate $20
Donate $30
Donate: $



Home - Table Of Contents - Contact Us

The TCP/IP Guide (http://www.TCPIPGuide.com)
Version 3.0 - Version Date: September 20, 2005

Copyright 2001-2005 Charles M. Kozierok. All Rights Reserved.
Not responsible for any loss resulting from the use of this site.