Reference Guide

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.
11 Discard the packet and send an ICMP Parameter Problem, Code 2 message to the packet’s
Source IP Address only if the Destination IP Address is not a multicast address.
The second byte contains the Option Data Length.
The third byte specifies whether the information can change en route to the destination. The value is 1 if it can change;
the value is 0 if it cannot change.
Addressing
IPv6 addresses are normally written as eight groups of four hexadecimal digits, where each group is separated by a
colon (:).
For example, 2001:0db8:0000:0000:0000:0000:1428:57ab is a valid IPv6 address. If one or more four-digit group(s) is 0000,
the zeros may be omitted and replaced with two colons(::). For example, 2001:0db8:0000:0000:0000:0000:1428:57ab can be
shortened to 2001:0db8::1428:57ab. Only one set of double colons is supported in a single address. Any number of
consecutive 0000 groups may be reduced to two colons, as long as there is only one double colon used in an address.
Leading and/or trailing zeros in a group can also be omitted (as in ::1 for localhost, 1:: for network addresses and :: for
unspecified addresses).
All the addresses in the following list are all valid and equivalent.
• 2001:0db8:0000:0000:0000:0000:1428:57ab
• 2001:0db8:0000:0000:0000::1428:57ab
• 2001:0db8:0:0:0:0:1428:57ab
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