IPv6 Header Format

The IPv6 datagram header has changed slightly from the IPv4 datagram header. The number of header fields has been reduced, making it simpler to process and also keeps the bandwidth use to a minimum. The most important change is the introduction of extension headers, which allow a variable number of additional options to be added, without any impact on the performance of the routing of IPv6 packets. Figure 7.1 shows the IPv6 datagram format.

Figure 7.1. IPv6 datagram header format.

The header fields shown in the figure are as follows :

• Version ” A 4-bit field identifying the IP version number. For IPv6, the value is 6; for IPv4, the value is 4.

• Traffic class ” An 8-bit field that provides the means of identifying different classes or priorities of IPv6 packets.

• Flow label ” A 20-bit field that enables the sender to identify sequences of packets for which special handling by IPv6 routers is required, such as "real-time" service. It allows important packets, for example, to be delivered immediately, possibly ahead of other lower-priority packets.

• Payload length ” A 16-bit field that identifies the length of the IPv6 payload (the rest of the packet following the IPv6 header). Any extension headers that are included count toward the payload length, as well as the data itself.

• Next header ” An 8-bit field that identifies the type of extension header that follows the primary IPv6 header. This is similar to the Protocol field in the IPv4 datagram header.

• Hop limit ” An 8-bit integer field that is decremented by one each time the packet passes through a host that forwards it. If the value reaches zero, the packet is discarded.

• Source address ” The sender's 128-bit address.

• Destination address ” The destination's 128-bit address.

IPv6 headers also provide a second level of headers that contain optional Internet layer information. The extension header follows immediately after the primary IPv6 datagram header and is referred to by the Next header field. The advantage of using these headers is that the routers along the path that the packet takes do not examine the extension headers (with one exception: the hop-by-hop option, which defines a process that must be carried out each time the packet passes through a router), improving the performance of delivering the packet to its destination. Extension headers carry additional information relating to routing, fragmentation, or security.