IPv6 Packet Headers
IPv6 headers are different from IPv4 headers.
This section discusses the following topics that provide background information about IPv6 headers:
Header Structure
IPv6 packet headers contain many of the fields found in IPv4 packet headers; some of these fields have been modified from IPv4. The 40-byte IPv6 header consists of the following 8 fields:
- Traffic class—Class-of-service (CoS) priority of the packet. Previously the type-of-service (ToS) field in IPv4. However, the semantics of this field (for example, diffserv code points) are identical to IPv4.
- Destination address—Final destination node address for the packet.
- Flow label—Packet flows requiring a specific CoS. The flow label identifies all packets belonging to a specific flow, and routers can identify these packets and handle them in a similar fashion.
- Hop limit—Maximum number of hops allowed. Previously the time-to-live (TTL) field in IPv4.
- Next header—Next extension header to examine. Previously the protocol field in IPv4.
- Payload length—Length of the IPv6 payload. Previously the total length field in IPv4.
- Source address—Address of the source node sending the packet.
- Version—Version of the Internet Protocol.
Extension Headers
In IPv6, extension headers are used to encode optional Internet-layer information.
Extension headers are placed between the IPv6 header and the upper layer header in a packet.
Extension headers are chained together using the next header field in the IPv6 header. The next header field indicates to the router which extension header to expect next. If there are no more extension headers, the next header field indicates the upper layer header (TCP header, User Datagram Protocol [UDP] header, ICMPv6 header, an encapsulated IP packet, or other items).