Users Guide

Value Description
41 IPv6
43 Routing header
44 Fragmentation header
50 Encrypted Security
51 Authentication header
59 No Next Header
60 Destinations option header
NOTE: This table is not a comprehensive list of Next Header field values. For a complete and current listing, refer to the
Internet Assigned Numbers Authority (IANA) web page at .
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)
376
IPv6 Routing