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 Protocol (IP/IPv4, IPng/IPv6) and IP-Related Protocols (IP NAT, IPSec, Mobile IP)
                9  Internet Protocol Version 4 (IP, IPv4)
                     9  IP Datagram Encapsulation and Formatting

Previous Topic/Section
IP Datagram Encapsulation
Previous Page
Pages in Current Topic/Section
1
2
3
Next Page
IP Datagram Options and Option Format
Next Topic/Section

IP Datagram General Format
(Page 2 of 3)

Time To Live (TTL) Field

Since IP datagrams are sent from router to router as they travel across an internetwork, it is possible that a situation could result where a datagram gets passed from router A to router B to router C and then back to router A. Router loops are not supposed to happen, and rarely do, but are possible.

To ensure that datagrams don't circle around endlessly, the TTL field was intended to be filled in with a time value (in seconds) when a datagram was originally sent. Routers would decrease the time value periodically, and if it ever hit zero, the datagram would be destroyed. This was also intended to be used to ensure that time-critical datagrams wouldn’t linger past the point where they would be “stale”.

In practice, this field is not used in exactly this manner. Routers today are fast and usually take far less than a second to forward a datagram; measuring the time that a datagram “lives” would be impractical. Instead, this field is used as a “maximum hop count” for the datagram. Each time a router processes a datagram, it reduces the value of the TTL field by one. If doing this results in the field being zero, the datagram is said to have expired. It is dropped, and usually an ICMP Time Exceeded message is sent to inform the originator of the message that this happened.

The TTL field is one of the primary mechanisms by which networks are protected from router loops (see the description of ICMP Time Exceeded messages for more on how TTL helps IP handle router loops.)


Previous Topic/Section
IP Datagram Encapsulation
Previous Page
Pages in Current Topic/Section
1
2
3
Next Page
IP Datagram Options and Option Format
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.