The fixed fields in the IPv6 packet header are following:
- Version – This field contains a 4-bit binary value set to 0110 that represents this as an IPv6 packet.
- Traffic Class – This 8-bit field is equivalent to the IPv4 Differentiated Services (DS) field. These 8 bits are further divided into two parts. The first 6 bits are used for Type of Service to let the Router Known what services should be provided to this packet. The last 2 bits are used for Explicit Congestion Notification (ECN).
- Flow Label – This 20-bit field suggests that all packets with the same flow label receive the same type of handling by routers.The flow label is used to keep the sequential flow of the packets belonging to a communication. The source of the packet labels the sequence to help the router identify that a particular packet belongs to a specific flow of information. This field helps avoid re-ordering of data packets. It is designed for real-time media and streaming.
- Payload Length – This 16-bit field indicates the length of the data portion of the IPv6 packet. Payload length tells the routers how much information a particular packet contains in its payload.
- Next Header – This 8-bit field is equivalent to the IPv4 Protocol field.So that field indicates either the type of Extension Header or if the Extension Header is not present then it indicates the Upper Layer PDU. The values for the type of Upper Layer PDU are same as IPv4’s
- Hop Limit– This 8-bit field replaces the IPv4 TTL field. This field is used to stop packet to loop in the network infinitely This value is decremented by a value of 1 by each router that forwards the packet. When the value reaches 0; the packet is discarded, and an ICMPv6 Time Exceeded message is forwarded to the sending host, indicating that the packet did not reach its destination because the hop limit was exceeded.
- Source Address – This 128-bit field identifies the IPv6 address of the originator host.
- Destination Address – This 128-bit field identifies the IPv6 address of the destination host.
The above mention fields are fixed for IPv6 packet header. An IPv6 packet may also contain extension headers (EH), which give optional network layer information. Extension headers are optional and are placed between the IPv6 header and the payload. Extension Header are also used for security, fragmentation, routing header, hop by hop option header, to support mobility and more.