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!

Read offline with no ads or diagram watermarks!
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 Overview, History, Versions and Standards
Previous Page
Pages in Current Topic/Section
1
2
Next Page
ICMP Message Classes, Types and Codes
Next Topic/Section

ICMP General Operation
(Page 2 of 2)

ICMP Error-Reporting Limitations

One interesting general characteristic of ICMP's operation is that when errors are detected, they can be reported using ICMP, but only back to the original source of a datagram. This is actually a big drawback in how ICMP works. Refer back to Figure 137 and consider again client host A sending a message to server host B, with a problem detected in the datagram by router R3. Even if R3 suspects that the problem was caused by one of the preceding routers that handled the message, such as R2, it cannot send a problem report to R2. It can only send an ICMP message back to host A.

This limitation is an artifact of how the Internet Protocol works. You may recall from looking at the IP datagram format that the only address fields are for the original source and ultimate destination of the datagram. (The only exception is if the IP Record Route option is used, but devices cannot count on this.) When R3 receives a datagram from R2 that R2 in turn received from R1 (and prior to that, from A), it is only A's address in the datagram. Thus, R3 must send a problem report back to A, and A must decide what to do with it. Device A may decide to change the route it uses, or to generate an error report that an administrator can use to troubleshoot the R2 router.

In addition to this basic limitation, several special rules and conventions have been put in place to govern the circumstances under which ICMP messages are generated, sent and processed.

Key Concept: ICMP error-reporting messages sent in response to a problem seen in an IP datagram can only be sent back to the originating device. Intermediate devices cannot be the recipient of an ICMP message because their addresses are normally not carried in the IP datagram’s header.



Previous Topic/Section
ICMP Overview, History, Versions and Standards
Previous Page
Pages in Current Topic/Section
1
2
Next Page
ICMP Message Classes, Types and Codes
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.