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Destination Address (128 bits)
The Destination Address field 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 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
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 field.
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 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.
• 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 one 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.
01 Discard the packet.
10 Discard the packet and send an ICMP Parameter Problem Code 2 message to the
packet’s Source IP Address identifying the unknown option type.
IPv6 Routing
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