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!

Enjoy The TCP/IP Guide? Get the complete PDF!
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 Message Types and Formats
                     9  ICMP Version 4 (ICMPv4) Informational Message Types and Formats

Previous Topic/Section
ICMP Version 4 (ICMPv4) Informational Message Types and Formats
Previous Page
Pages in Current Topic/Section
1
2
3
Next Page
ICMPv4 Timestamp (Request) and Timestamp Reply Messages
Next Topic/Section

ICMPv4 Echo (Request) and Echo Reply Messages
(Page 2 of 3)

ICMPv4 Echo and Echo Reply Message Format

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


Table 96: ICMPv4 Echo and Echo Reply Message Format

Field Name

Size (bytes)

Description

Type

1

Type: Identifies the ICMP message type. For Echo messages the value is 8; for Echo Reply messages the value is 0.

Code

1

Code: Not used for Echo and Echo Reply messages; set to 0.

Checksum

2

Checksum: 16-bit checksum field for the ICMP header, as described in the topic on the ICMP common message format.

Identifier

2

Identifier: An identification field that can be used to help in matching Echo and Echo Reply messages.

Sequence Number

2

Sequence Number: A sequence number to help in matching Echo and Echo Reply messages.

Optional Data

Variable

Optional Data: Additional data to be sent along with the message (not specified.)



Figure 146: ICMPv4 Echo and Echo Reply Message Format

 


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.


Previous Topic/Section
ICMP Version 4 (ICMPv4) Informational Message Types and Formats
Previous Page
Pages in Current Topic/Section
1
2
3
Next Page
ICMPv4 Timestamp (Request) and Timestamp Reply Messages
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.