IPv6 Essentials

Extension Headers

The IPv4 header can be extended from a minimum of 20 bytes to 60 bytes in order to specify options such as Security Options, Source Routing, or Timestamping. This capacity has rarely been used because it causes a performance hit. For example, IPv4 hardware forwarding implementations have to pass the packet containing options to the main processor (software handling).

The simpler a packet header, the faster the processing. IPv6 has a new way to deal with options that has substantially improved processing. It handles options in additional headers called Extension headers.

The current IPv6 specification (RFC 2460) defines six Extension headers:

• Hop-by-Hop Options header

• Routing header

• Fragment header

• Destination Options header

• Authentication header

• Encrypted Security Payload header

There can be zero, one, or more than one Extension header between the IPv6 header and the upper-layer protocol header. Each Extension header is identified by the Next Header field in the preceding header. The Extension headers are examined or processed only by the node identified in the Destination Address field of the IPv6 header. If the address in the Destination Address field is a multicast address, the Extension headers are examined and processed by all the nodes belonging to that multicast group. Extension headers must be strictly processed in the order they appear in the packet header.

There is an exception to the above rule: only the destination node will process an Extension ...