1.3 EXTENSION HEADERS

When working with IPv6, optional Internet layer information is encoded within individual headers that can be inserted between the IPv6 header and the Upper-Layer header in the packet. In addition, there are only a small number of such extension headers. Each of these headers are distinguished by a single Next Header value. The IPv6 packet may have zero, one, or more extension headers, each is distinguished by the Next Header field of the header preceding header.

Any node does not usually process extension headers on a packet's delivery path until that packet has reached the node designated in the Destination Address field of the IPv6 header. Under these circumstances, demultiplexing the Next Header field of the IPv6 header asks the module to operate on the first extension header. However, there is no extension header there, so it must work on the Upper-Layer header. Both contents and semantics of each specific extension header determine when and if the next header is operated upon.

Extension headers need to be processed in the order they appear in the packet. However, the node that receives the information may not scan within a packet to look for a specific type of extension header and operate on that header before processing all previous headers.

There is one exception to this route, called the Hop-by-Hop Options header. It has information that must be looked at and operated on by each node throughout a packet's delivery path. This includes both the source and destination nodes.