Extension Header Fields - Dell C9000 Series Networking Configuration Manual

Hop Limit (8 bits)
The Hop Limit field shows the number of hops remaining for packet processing. In IPv4, this is known as the
Time to Live (TTL) field and uses seconds rather than hops.
Each time the packet moves through a forwarding router, this field 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 field contains the IPv6 address for the packet originator.
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)
IPv6 Routing 565