Source Address (128 bits)
The Source Address eld contains the IPv6 address for the packet originator.
Destination Address (128 bits)
The Destination Address eld contains the intended recipient’s IPv6 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 identied by the Next Header eld in the IPv6 header that precedes it. Extension headers are viewed only by the
destination router identied in the Destination Address eld. 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 by 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 eld.
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).
When a Hop-by-Hop Options header is not included, the router knows that it does not have to process any router specic information and
immediately processes the packet to its nal 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 eld identies the type of header following the Hop-by-Hop Options header and uses the same values.
• Header Extension Length (1 byte)
This eld identies the length of the Hop-by-Hop Options header in 8-byte units, but does not include the rst 8 bytes. Consequently, if the
header is less than 8 bytes, the value is 0 (zero).
• Options (size varies)
This eld can contain one or more options. The rst byte if the eld identies the Option type, and directs the router how to handle the
option.
00
Skip and continue processing.
01 Discard the packet.
380 IPv6 Routing
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