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I am not sure how many of you actually read Internet standards, but it is actually more boring than it sounds. Well, not to me, I enjoy it, but I can see how many people would rather watch paint dry or grass grow. If you truly want to understand how the internet really works, however, it is a great place to start. In this article we are going to take a quick look at the differences behind the headers in IPv4 and IPv6.

The Internet Engineering Task Force (IETF) is the standards body for the Internet. Actually, it is more of a collection of interested parties who have come together to develop suggested standards for the Internet as there is no real overall governing force that requires you to use a particular standard. The IETF utilizes a standards development process where Request for Comments (RFCs) are developed in draft form and discussed (debated, argued, etc.) in working groups and are finally voted on and sent to the IETF leadership for final approval. It is a fascinating process, and, if there is ever an IETF meeting close to you, I would suggest going, but read The Tao of IETF: A Novice’s Guide to the Internet Engineering Task Force before you get there as it will help you understand the procedures.

Now that we have the necessary background in place, on to the discussion at hand. For reference, the best place to find the protocol description for IPv4 is starting with RFC 791. There have been several updates to IPv4 over the years, but this is still the best place to read about it. IPv6 is described by RFC 2460. Each RFC will provide more details on the protocol header and what each of the fields is used for, but here is a quick comparison for you.

The two figures below show the headers for IPv4 and IPv6.

IPv4 Header

You can easily see that the IP header has been simplified in IPv6, with only 8 fields, as opposed to 13 in IPv4. One of the most significant changes is in the overall size of the header. The IPv4 header was approximately 20 bytes (I say 20 because there was an options and padding field and the header was not always 20 bytes. The header in IPv6 is 40 bytes, no variation. While this adds additional overhead in the length of the header, the IP header in IPv6 is now completely deterministic, and header compression and encryption can now be performed much faster in hardware based implementations.

The biggest increase in field size from IPv4 to IPv6 is in the size of the source and destination address fields. These have moved from 32 bits to 128 bits. You also see that IPv6 has removed and/or modified a number of fields from IPv4. One of the new additions in IPv6 is the extension header, where additional headers (some already define and other will be defined in the future) can be utilized.

The IPv6 header does not include the IPv4 feature of fragmentation, due to the desire to eliminate this feature in core networks and reduce overhead. Undesirable features, such as fragmentation, were moved to optional extension headers.

The IPv6 header includes the following:

Version: 4-bit version number (IPv6 = 0110 in binary),
Traffic Class: 8-bit field. This field is used to distinguish packet class or priority.
Flow Label: 20-bit field. This filed is planned to be used to label sequences of packets for special handling by IPv6 routers.
Payload Length: 16-bit unsigned integer. This is the length of the IPv6 payload following the header.
Next Header: 8-bit field identifying the type of header that immediately follows the IPv6 header. This provides the ability to specify an extension header, which can be chained together to provide multiple options.
Hop Limit: 8-bit unsigned integer. It is decreased (by one) as the packet moves through each hop in the network. The packet is discarded when the hop limit reaches zero.
Source Address: 128 bits field for the source address, and
Destination Address: 128 bits for the destination address.

Don’t forget to read our other Tech Tips on IPv6 addressing and IPv6 transition mechanisms.


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About this author

Dale Geesey (CISSP/PMP) is a security, networking and IT professional with over 20 years of experience working in the federal and carrier community. His primary focus is on IPv6, Cyber Security, Health IT and next generation technologies. He has been supporting Government and commercial IPv6 Transition activities since 2004. Specializing in the introduction of advanced and next generation technologies, he has supported numerous technology efforts for government organizations including VA, DoD, Army, SBA, DISA, NSA, SBA, Navy, NIST, OMB, FBI, and NATO. Dale is currently the Chief Operating Officer at Auspex Technologies.

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