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!

Get The TCP/IP Guide for your own computer.
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  TCP/IP IPv6 Neighbor Discovery Protocol (ND)

Previous Topic/Section
IPv6 ND Host-Router Discovery Functions: Router Discovery, Prefix Discovery, Parameter Discovery and Address Autoconfiguration
Previous Page
Pages in Current Topic/Section
12
3
Next Page
IPv6 ND Redirect Function
Next Topic/Section

IPv6 ND Host-Host Communication Functions: Address Resolution, Next-Hop Determination, Neighbor Unreachability Detection and Duplicate Address Detection
(Page 3 of 3)

Neighbor Unreachability Detection and the Neighbor Cache

Neighbor Solicitation and Neighbor Advertisement messages are most often associated with address resolution, but also have other purposes. One of these is Neighbor Unreachability Detection. Each device maintains information about each of its neighbors and updates it dynamically as network conditions change. The information is kept for both host and router devices that are neighbors on the local network.

Knowing that a device has become unreachable is important because a host can adapt its behavior accordingly. In the case of an unreachable host, a device may wait a certain period of time before trying to send datagrams to an unreachable host instead of flooding the network with repeated attempts to send to the host. An unreachable router, on the other hand, is a signal that the device needs to find a new router to use, if an alternate is available.

Each host maintains a neighbor cache that contains information about neighboring devices. Each time a host receives a datagram from a neighbor, it knows the neighbor is reachable at that particular moment, so the device makes an entry in the cache for the neighbor to indicate this. Of course, receiving a datagram from a neighbor now only means the neighbor is reachable now; the more time that elapses since the last datagram was received, the greater the chance that something has happened to make the neighbor no longer reachable.

For this reason, neighbor reachability information must be considered temporary. Each time a neighbor is entered into the cache as reachable, a timer is started. When the timer expires, the reachability information for that neighbor is considered stale, and reachability is no longer assumed for that neighbor. When a new datagram is received from the neighbor in question the timer is reset and the cache is again set to indicate that the device is reachable. The amount of time a host should consider a neighbor reachable before expiring it is communicated by a local router using a field in a Router Advertisement message.

A host can also dynamically seek out a neighbor if it needs to know its reachability status. It sends a Neighbor Solicitation to the device and waits for a Neighbor Advertisement in response. It then updates the cache accordingly.

Duplicate Address Detection

The last use of the two messages we have been discussing here is for Duplicate Address Detection. When a host uses the IPv6 autoconfiguration facility, one of the steps in the process is to ensure that the address it is trying to use doesn't already exist on the network. This is done by sending a Neighbor Solicitation message to the address the device wishes to use. If a Neighbor Advertisement is received in reply the address is already in use.

Key Concept: The second of the two main functional groups of the Neighbor Discovery protocol is the set of host-host communication functions. Two ICMPv6 messages are defined, Neighbor Advertisement and Neighbor Solicitation, which enable a variety of types of essential communication between adjacent hosts on a local network. These include address resolution, determining the next hop to which a datagram should be sent, and also the assessment of a neighboring device’s reachability.



Previous Topic/Section
IPv6 ND Host-Router Discovery Functions: Router Discovery, Prefix Discovery, Parameter Discovery and Address Autoconfiguration
Previous Page
Pages in Current Topic/Section
12
3
Next Page
IPv6 ND Redirect Function
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.