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.
IPv6 Datagram Extension Headers
(Page 6 of 6)
IPv6 Extension Header Order
Each extension header appears only
once in any datagram (with one exception; see below). Also, extension
headers are only examined by the final recipients of the datagram, not
intermediate devices (again with one exception, which we will get to
momentarily). RFC 2460 specifies that when multiple headers appear,
they should be in the following order after the main header and before
the higher-layer encapsulated header in the IPv6 datagram payload:
- Hop-By-Hop Options
- Destination Options (for options
to be processed by the destination as well as devices specified in a
- Authentication Header
- Encapsulating Security Payload
- Destination Options (for options
processed only by the final destination)
Now let's look at those exceptions.
The only header that can appear twice is Destination Options.
Normally, it appears as the last header. However, a Destination Options
header may exist that contain options that must be examined by a list
of devices specified in a source route, in addition to the destination.
In this case, the Destination Options header for these options
is placed before the Routing header. A second such header containing
options only for the final destination may also appear.
Key Concept: Each extension header may appear only once in an IPv6 datagram, and they must appear in a fixed order. The exception is the Destination Options header, which may appear twice; near the start of the datagram for options to be processed by devices en route to the destination, and at the end of the extension headers for options intended only for the final destination.
The only header normally examined
by all intermediate devices is the Hop-By-Hop Options extension
header. It is used specifically to convey management information to
all routers in a route. The Hop-By-Hop Options extension header
must appear as the first extension header if present. Since it is the
only one that must be read by every router (which represents a performance
drain on routers) it is given top billing to make it easier
and faster to find and process.
Finally, note that all extension
headers must be a multiple of 8 bytes in length for alignment purposes.
Also, remember that the Next Header value for a particular extension
header appears in the Next Header field of the preceding
header, not the header itself.
|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!|
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.