Illustrated TCP/IP by Matthew G. Naugle Wiley Computer Publishing, John Wiley & Sons, Inc. ISBN: 0471196568   Pub Date: 11/01/98

Chapter 146
IPv6 Extension Headers

Since the fields on the preceeding slide were deleted from use in the IPv6 header how do we continue their use? For example, how do we indicate the protocol of the next header?

The IPv6 header is straightforward. Some of the options for IPv4 were better served by other TCP/IP protocols and some were kept as a part of IPv6 and are now known as IPv6 extension headers. These extension headers allow for IPv6 to become extensible beyond a specified (and limited) options field. It can be modified at later dates to include other options. The current IPv6 specification calls for the following headers (in the order they should appear in the datagram):

IPv6 header (not directly part of the extensions but shown here to show header order).

Hop-by-Hop Options (RFC 1883). The Hop-by-Hop Options header is used to carry optional information that must be examined by every node along a packet’s delivery path. One of the options is the jumbo datagram option.

Destination Options (RFC 1883). The Destination Options header is used to carry optional information that needs be examined only by a packet’s destination node(s).

Routing (Type 0) (RFC 1883). The Routing header is used by an IPv6 source to list one or more intermediate nodes to be “visited” on the way to a packet’s destination. This function is very similar to IPv4’s Source Route options.

Fragment (RFC 1883). The Fragment header is used by an IPv6 source to send packets larger than would fit in the path MTU to their destinations.

Authentication (RFC 1826).

Encapsulating Security Payload (RFC 1827).

Upper-layer header. (not part of the extension header, but shown here to show order).

IPv6 Extension Headers

From end-to-end communication, these fields should be ignored by all stations that may receive it. These fields are generally built and consumed by the source and destination stations only. The exception is the hop-by-hop options field, which may be reviewed by routers in the path to the destination.