IPv6 header fields -- simply elegant

By Robert Currier, ITworld.com |  Networking

In my last column, I explained preparing your network for IPv6 is a good idea. In this
edition, we'll peek under the hood of the IPv6 header and see how it's put together.

The IPv6 header is, surprisingly, less complicated than the IPv4 header. This is a
credit to its designers, who have substantially improved functionality while reducing
complexity.

The length of an IPv4 packet's header varies, and thus requires the use of a header
length field. IPv6 uses 40 bytes in an eight-field header. A fixed-length header makes
it much easier for routers to process the packet.

Three of the fields are the same in both versions:

  • Version (4 bits) is used to tell routers what protocol is in
    use; the default is 6.

  • Source address and destination address (128
    bits each) are the IPv6 addresses of the sending and destination hosts.

The other five fields in the IPv6 header are new. I don't have room to go into great
detail; if you're interested in digging deeper, check out href="http://www.ietf.org/rfc/rfc2460.txt">the IPv6 RFC.

  • The traffic class field (8 bits) allows devices to differentiate
    between latency-sensitive traffic (like video and voice) and low-priority data (like
    email and Web traffic). There are several groups currently developing ways to best
    utilize this field, with the
    Differentiated Services project currently in the lead.

  • The flow label field (20 bits) can be used by a host to request
    special handling from IPv6-compliant routers. The ability to manage flows -- traffic
    between end stations -- is important in providing quality of service. This field allows
    IPv6 to operate in a manner similar to the IPv4 leader in flow management, href="http://www.ietf.org/html.charters/mpls-charter.html">Multiprotocol Label
    Switching.
  • The next header field (8 bits) alerts routers about additional
    headers that need to be examined. While the IPv6 header's length is fixed, the protocol
    can add other headers to the main header. These additional headers provide features
    such as source routing, encryption, and authentication.
  • The payload length field (16 bits) describes the length in
    octets of the payload (data) portion of the IPv6 packet. The 16-bit field length (2^16)
    lets version 6 support payloads in excess of 64,000 octets.
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