IPv6 Header Format

Introduction IPv6 Header Format

IPV6 header format is of 40 bytes in length, contains information essential to routing and delivery, consist of 8 fields, Version, Traffic Class, Flow Label, Payload length, next header, HOP limit, Source address and destination address, where each has its own features and provides essential data required to transmit the data. Version is 4 bit field; traffic class is of 8 bit, the flow label is of 20 bits, payload length is a 2-byte field, next header is of 8 bit, hop limit is an 8-bit field, the source address is of 16 bytes, and the destination address is of 16 bytes.

List of IPv6 Header Format Component

IPv6 Header Format Component, the data packet of IPv6 encompasses two main parts, i.e. header and the payload. The IPv6 consists of 40 bytes long fixed header which contains the following fields.

Let’s see the significance of the individual components of IPv6 Header in details-

1)Version:

It signifies the version of the Internet protocol in a 4-bit sequence, i.e. 0110. This field does not hold much importance as the IPv6, and IPv4 packets are not determined based on the version field but by the type of the protocol present inside the layer 2 envelopes.

2)Traffic Class:

This field is similar to the Service Field of the IPv4 packet. It signifies the priority of the IPv6 packet. It is responsible for handling the traffic based on the priority of the packet. In case of congestion on the router, it discards the packets with low priority.

It uses 8 bits of memory to control traffic congestion.

Priority	Meaning
0	No specific traffic
1	Background data
2	Unattended data traffic
3	reserved
4	Attended bulk data traffic
5	reserved
6	Interactive traffic
7	Control traffic

The source node can set the priorities, but the destination can’t expect the same set of priorities as the router can change the priorities on the way.

3)Flow label:

• This label ensures that the packets maintain the sequential flow belonging to the same communication. With the source’s help, the label router identifies which packet belongs to which flow of information. It also helps to avoid the reordering of the data packets.
• Flow label must be set to 0 if the router and host don’t support the flow label functionality. It uses 20 bits of memory for its functioning.

4)Payload Length (16-bits):

Payload length indicates the router about the size of the information contained by a particular packet. Payload length also consists of the upper layer packet and extension header (if any). If the payload length becomes greater than 65,535 bytes, then the payload length field becomes 0.

5)Next Header (8-bits):

The next Header signifies the Extension Header type; in some cases, when the Extension Header is not present, it signifies the protocols present inside the upper layer packet like UDP, TCP, etc. UDP (17) and TCP (6) are the most common Next Headers, but other types of headers are also possible. If compare with the IPv4 protocol, the Next Header is similar to the IPv4 protocol field.

6)Hop Limit (8-bits):

This field makes sure that the packet does not go into an infinite loop; every time the packet passes the link (router), this field is decremented by 1 and when it finally reaches where the package is discarded. This field also set an upper threshold on the maximum numbers of links between two nodes of the IPv6 protocol. It allows a maximum of 255 hops between the nodes, and anything after that will get discarded.

7)Source Address (128 bits):

This 128-bit source address field signifies the origin address of the package.

8)Destination Address (128 bits):

This 128-bit destination address field signifies the intended recipient address of the packet.

9)Extension Headers IPv6 Header Format:

• Extension Headers are introduced in IPv6 to overcome the limitation of the Options Field of IPv4. Unlike IPv4, In IPv6, the only information that is really necessary is defined in the Fixed Header, and all those information that are not so important or don’t get frequently used are defined in the Extension Header. Extension Header is present in between the Upper-Layer Header and the Fixed Header. Each Extension Header has a unique value that differentiates it from the other Extension Headers.
• In case extension headers are being used, then Fixed Header’s Next Header’s field will point to the first Extension Header. Similarly, if there are multiple Extension Header, then it works similarly.

IPv6 packet can have one or more than one extension headers; these headers should present in a specific sequence as mentioned below:

Order	Header Type	Next Header Code
1	Basic IPv6 Header	–
2	Hop by Hop option	0
3	Destination options (with routing options)	60
4	Routing Header	43
5	Fragment Header	44
6	Authentication Header	51
7	Encapsulation Security Payload Header	50
8	Destinations options	60
9	Mobility Header	135
	No next Header	59
Upper Layer	TCP	6
Upper Layer	UDP	17
Upper Layer	ICMPv6	58

Rules of Headers

Some predefined rules define the headers’ order; let’s have a look at these rule sets.

1. If Hop by Hop option is present, then it should be present after the IPv6 base Header.
2. Except for Destination Header, all other Headers can appear only once in the list.
3. In case the Destination Header is placed before the Routing Header, then the Destination Header will be examined by all the intermediate nodes present in the Routing Header.
4. In case the Destination Header is placed before Upper Layer, then the Destination Header will be examined only by the Destination Node.

Sequence of Headers

Let’s have a look at the sequence in which all the Extension Header should be arranged in an IPv6 packet.

Extension Header	Description
Hop by Hop Options	Examined by all devices on the path
Destination Options (with routing options)	Examined by the destination of the packet
Routing Header	Methods to take a routing decision.
Fragment Header	Contains parameters of fragmented datagram done by the source
Authentication Header	Verify authenticity
Encapsulation Security Payload	Carries Encrypted data

Conclusion

We have learned the IPv6 Header format and the different components present in the Header. We have seen each component’s significance and how these components are different from those of the IPv4 protocol. We have also learned the different rule sets that should be considered while sequencing the header type.

Recommended Articles

This has been a guide to IPv6 Header Format. Here we have discuss the basic concept, Components and the sequence where ipv6 packets are arranged. You can also go through our other suggested articles to learn more –

1. IPv4 vs IPv6
2. CCNA Commands
3. Introduction To Computer Network
4. Computer Network Interview Questions