7.8.1.0 - Force10 Networks

Destination Address (128 bits)
The Destination Address field contains the intended recipient’s IP address. This can be either the ultimate
destination or the address of the next hop router.
Extension Header fields
Extension headers are used only when necessary. Due to the streamlined nature of the IPv6 header, adding
extension headers do not severely impact performance. Each Extension headers’s lengths vary, but they are
always a multiple of 8 bytes.
Each extension header is identified by the Next Header field in the IPv6 header that precedes it. Extension
headers are viewed only by the destination router identified in the Destination Address field. If the
Destination Address is a multicast address, the Extension headers are examined by all the routers in that
multicast group.
However, if the Destination Address is a Hop-by-Hop options header, the Extension header is examined
bye every forwarding router along the packet’s route. The Hop-by-Hop options header must immediately
follow the IPv6 header, and is noted by the value 0 (zero) in the Next Header field (Table 33 ).
Extension headers are processed in the order in which they appear in the packet header.
Hop-by-Hop Options header
The Hop-by-Hop options header contains information that is examined by every router along the packet’s
path. It follows the IPv6 header and is designated by the Next Header value 0 (zero) (Table 33 ).
When a Hop-by-Hop Options header is not included, the router knows that it does not have to process any
router specific information and immediately processes the packet to its final destination.
When a Hop-by-Hop Options header is present, the router only needs this extension header and does not
need to take the time to view further into the packet.
The Hop-by-Hop Options header contains:
• Next Header (1 byte)
This field identifies the type of header following the Hop-by-Hop Options header and uses the same
values shown in Table 33 .
• Header Extension Length (1 byte)
This field identifies the length of the Hop-by-Hop Options header in 8-byte units, but does not include
the first 8 bytes. Consequently, if the header is less than 8 bytes, the value is 0 (zero).
• Options (size varies)
This field can contain 1 or more options. The first byte if the field identifies the Option type, and
directs the router how to handle the option.
00 Skip and continue processing
412 IPv6 Introduction