IPv6 Packet Headers
An IPv6 packet is a block of data that contains a header and a payload. The header is the information necessary to deliver the packet to a destination address; the payload is the data that you want to deliver. IPv6 packets can use a standard or an extended format.
IPv4 and IPv6 Header Differences
The main difference between IPv4 and IPv6 resides in their headers. Figure 3-1 provides a comparison between the two protocol versions.
Standard IPv6 Headers
IPv6 packet headers contain many of the fields found in IPv4 packet headers; some of these fields differ from IPv4. (See Figure 3-1.)
The 40-byte IPv6 header consists of the following eight fields:
- Version - Indicates the version of the Internet Protocol.
- Traffic class - Previously the type-of-service (ToS) field in IPv4, the traffic class field defines the class-of-service (CoS) priority of the packet. However, the semantics for this field (for example, diffserv code points) are identical to IPv4.
- Flow label - The flow label identifies all packets belonging to a specific flow (that is, packet flows requiring a specific class of service [CoS]); routers can identify these packets and handle them in a similar fashion.
- Payload length - Previously the total length field in IPv4, the payload length field specifies the length of the IPv6 payload.
- Next header - Previously the protocol field in IPv4, the Next Header field indicates the next extension header to examine.
- Hop limit - Previously the time-to-live (TTL) field in IPv4, the hop limit indicates the maximum number of hops allowed.
- Source address - Identifies the address of the source node sending the packet.
- Destination address - Identifies the final destination node address for the packet.
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.
IPv6 allows you to chain extension headers together by using the next header field. The next header field, located in the IPv6 header, 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, UDP header, ICMPv6 header, an encapsulated IP packet, or other items).