The wonder of IPv6 lies in its header. IPv6 address is 4 times
larger than IPv4 but the IPv6 header is only 2 times larger than that of
IPv4. IPv6 headers have one Fixed Header and zero or more Optional
(Extension) Headers. All necessary information which is essential for a
router is kept in Fixed Header. Extension Header contains optional
information which helps routers to understand how to handle a
packet/flow.
[Image: IPv6 Fixed Header]IPv6 fixed header is 40 bytes long and contains the following information.
S.N.
Field & Description
1
Version (4-bits): This represents the version of Internet Protocol, i.e. 0110.
2
Traffic Class (8-bits): These 8 bits are
divided into two parts. Most significant 6 bits are used for Type of
Service, which tells the Router what services should be provided to this
packet. Least significant 2 bits are used for Explicit Congestion
Notification (ECN).
3
Flow Label (20-bits): This label is used to
maintain the sequential flow of the packets belonging to a
communication. The source labels the sequence which helps the router to
identify that this packet belongs to a specific flow of information.
This field helps to avoid re-ordering of data packets. It is designed
for streaming/real-time media.
4
Payload Length (16-bits): This field is used
to tell the routers how much information this packet contains in its
payload. Payload is composed of Extension Headers and Upper Layer data.
With 16 bits, up to 65535 bytes can be indicated but if Extension
Headers contain Hop-by-Hop Extension Header than payload may exceed
65535 bytes and this field is set to 0.
5
Next Header (8-bits): This field is used to
indicate either the type of Extension Header, or if Extension Header is
not present then it indicates the Upper Layer PDU. The values for the
type of Upper Layer PDU is same as IPv4’s.
6
Hop Limit (8-bits): This field is used to
stop packet to loop in the network infinitely. This is same as TTL in
IPv4. The value of Hop Limit field is decremented by 1 as it passes a
link (router/hop). When the field reaches 0 the packet is discarded.
7
Source Address (128-bits): This field indicates the address of originator of the packet.
8
Destination Address (128-bits): This field provides the address of intended recipient of the packet.
Extension Headers
In IPv6, the Fixed Header contains only information which is
necessary and avoiding information which is either not required or is
rarely used. All such information, is put between the Fixed Header and
Upper layer header in the form of Extension Headers. Each Extension
Header is identified by a distinct value.
When Extension Headers are used, IPv6 Fixed Header’s Next Header
field points to the first Extension Header. If there is one more
Extension Header, then first Extension Header’s ‘Next-Header’ field
point to the second one, and so on. The last Extension Header’s
‘Next-Header’ field point to Upper Layer Header. Thus all headers from
point to the next one in a linked list manner.
If the Next Header field contains value 59, it indicates that there’s no header after this header, not even Upper Layer Header.
The following Extension Headers must be supported as per RFC 2460:
The sequence of Extension Headers should be:
These headers:
1. Should be processed by First and subsequent destinations.
2. Should be processed by Final Destination.
Extension Headers are arranged one after another in a Linked list manner, as depicted in the diagram below:
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