Overview and Innovations

In 1998 a new standard named IPv6 was defined23 and is now supported by the Linux kernel in production quality. A full implementation of the protocol is located in the net/ipv6 directory. The modular and open structure of the network layer means that IPv6 can make use of the existing, mature infrastructure. As many aspects of IPv6 are similar to IPv4, a brief overview will suffice at this point.

A key change in IPv6 is a completely new packet format that uses 128-byte IP addresses, and is therefore easier and faster to process. The structure of an IPv6 packet is shown in Figure 12-21.


Traffic Class

Flow Label

Payload length

Next Header

Hop Limit

Source address

Destination address


Figure 12-21: Structure of an IPv6 packet.

Figure 12-21: Structure of an IPv6 packet.

The structure is much simpler than that in IPv4. There are only eight header fields instead of 14. Of particular note is the absence of the fragmentation field. Although IPv6 also supports the splitting of packet data into smaller units, the corresponding information is held in an extension header pointed to by the next header field. Support for a variable number of extension headers makes it easier to introduce new features.

The changes between IPv4 and IPv6 have also necessitated modification of the interface via which connections are programmed. Although sockets are still used, many old and familiar functions appear under a new name to support the new options. However, this is a problem faced by userspace and C libraries and will be ignored here.

The notation of IP addresses has also changed because of the increase in address length from 32 to 128 bits. Retaining the former notation (tuples of bytes) would have resulted in extremely long addresses. Preference was therefore given to hexadecimal notation for IPv6 addresses, for example, FEDC:BA98:7 654:3210:FEDC:BA98:7 654:3210 and 1080:0:0:0:8:800:200C:417A. A mixture of IPv4 and IPv6 formats resulting in addresses such as 0:0:0:0:0:FFFF: is also permitted.

Continue reading here: Implementation

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