IPv6 Headers - Internet Protocol version 6 (IPv6)

What are the different headers of IPv6?

The header of IPv6 address is two times larger than that of IPv4. There is one Fixed Header and a zero or more Optional Headers. The fixed header contains all the important information required for a router. The optional information which enables the router in understanding how to handle a packet/flow is provided by the Extension Header.

Fixed Header

IPv6 Header

IPv6 fixed header is 40 bytes long and contains the following information.

Field & Description
Version(4-bits): The version of the Internet Protocol is represented, i.e. 0110.
Traffic Class(8-bits): The traffic class is divided into two parts – 6 bits and 2 bits. The 6 bits are used for the service types to let the router known about what services are provided to the packet. The 2 bits are used for Explicit Congestion Notification (ECN).
Flow Label(20-bits): The sequential flow of packets that belong to a communication are maintained by this label. The source labels the sequence to help the router identify that a particular packet belongs to a specific flow of information. Re-ordering of the data packets is avoided by this field.
Payload Length(16-bits):
The routers are informed about the information contained in a particular packet in the payload. The Payload includes Extension Headers and Upper Layer data. With 16 bits, up to 65535 bytes can be indicated; but if the Extension Headers contain Hop-by-Hop Extension Header, then the payload may exceed 65535 bytes and this field is set to 0.
Next Header(8-bits): Indicates the type of Extension Header and in the absence of Extension Header, the upper layer PDU is indicated. The values for the type of Upper Layer PDU are same as IPv4’s.
Hop Limit(8-bits): The packet is stopped from looping in the network, which is the same as TTL of IPv4. A link is passed as the value of the Hop Limit field decrements by 1. The packet is discarded when the field reaches to 0.
Source Address(128-bits): This field indicates the address of originator of the packet.
Destination Address(128-bits): This field provides the address of intended recipient of the packet.

Extension Headers

The extension headers contain all the information which is either not required or used rarely. A distinct value is used for identifying each of the Extension Header.

The fixed Header’s Next Header field points to the first Extension Header. In case of one more Extension Header, then the first Extension Header’s ‘Next-Header’ field points to the second one, and so on. The last Extension Header’s ‘Next-Header’ field points to the Upper Layer Header.

If the Next Header field contains the value 59, it indicates that there are no headers after this header, not even Upper Layer Header.

The following Extension Headers must be supported as per RFC 2460:

Extension Headers

The Extension Headers sequence is as follows:

Extension Headers Sequence

These headers:

  • Are processed by the First and subsequent destinations.
  • Are processed by Final Destination.

In the form of linked list, the Extension Headers are arranged one after the other as depicted below:

Extension Header Sequence

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Internet Protocol version 6 (IPv6) Topics