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 Size, Maximum Transmission Unit (MTU), Fragmentation and Reassembly

Previous Topic/Section
IP Datagram Size, the Maximum Transmission Unit (MTU), and Fragmentation Overview
Previous Page
Pages in Current Topic/Section
1
234
Next Page
IP Message Reassembly Process
Next Topic/Section

IP Message Fragmentation Process
(Page 1 of 4)

When an IP datagram is too large for the maximum transmission unit (MTU) of the underlying data link layer technology used for the next leg of its journey, it must be fragmented before it can be sent across the network. The higher-layer message to be transmitted is not sent in a single IP datagram but rather broken down into pieces called fragments that are sent separately. In some cases, the fragments themselves may need to be fragmented further.

Fragmentation Issues and Concerns

Fragmentation is necessary to implement a network-layer internet that is independent of lower layer details, but introduces significant complexity to IP. Remember that IP is an unreliable, connectionless protocol. IP datagrams can take any of several routes on their way from the source to the destination, and some may not even make it to the destination at all. When we fragment a message we make a single datagram into many, which introduces several new issues to be concerned with:

  • Sequencing and Placement: The fragments will typically be sent in sequential order from the beginning of the message to the end, but they won't necessarily show up in the order in which they were sent. The receiving device must be able to determine the sequence of the fragments to reassemble them in the correct order. In fact, some implementations send the last fragment first, so the receiving device will immediately know the full size of the original complete datagram. This makes keeping track of the order of segments even more essential.

  • Separation of Fragmented Messages: A source device may need to send more than one fragmented message at a time; or, it may send multiple datagrams that are fragmented en route. This means the destination may be receiving multiple sets of fragments that must be put back together. Imagine a box into which the pieces from two, three or more jigsaw puzzles have been mixed and you understand this issue.

  • Completion: The destination device has to be able to tell when it has received all of the fragments so it knows when to start reassembly (or when to give up if it didn't get all the pieces).

To address these concerns and allow the proper reassembly of the fragmented message, IP includes several fields in the IP format header that convey information from the source to the destination about the fragments. Some of these contain a common value for all the fragments of the message, while others are different for each fragment.


Previous Topic/Section
IP Datagram Size, the Maximum Transmission Unit (MTU), and Fragmentation Overview
Previous Page
Pages in Current Topic/Section
1
234
Next Page
IP Message Reassembly Process
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.