Service Manual

Hop Limit (8 bits)
The Hop Limit eld shows the number of hops remaining for packet processing. In IPv4, this is known as the Time to Live (TTL) eld
and uses seconds rather than hops.
Each time the packet moves through a forwarding router, this eld decrements by 1. If a router receives a packet with a Hop Limit of
1, it decrements it to 0 (zero). The router discards the packet and sends an ICMPv6 message back to the sending router indicating
that the Hop Limit was exceeded in transit.
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.
IPv6 Routing
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