ack of header length field. In IPv6 the header is always fixed at 40 bytes. The Payload Length field includes the length of any possible IPv6 extension headers, as well as any other payload packet being carried. The 16 bits reserved for the length therefore limits IPv6 packet payload size to 65535 octets. Deploying IPv6 in 3GPP Networks Version Traffic Class Flow Label Payload Length Next Header Hop Limit Source Address Destination Address Figure 3.8 IPv6 header. Hop-by-hop Fragment IPv6 header header header Next Header: Next Header: Next Header: Fragment of UDP header Hop-by-hop Fragment and data Figure 3.9 Chaining of IPv6 headers. The Next Header field indicates what header follows the IPv6 header. These headers include transport protocol headers such as TCP and UDP, but also IPv6 extension headers. The Next Header field is also present in all IPv6 extension headers, thus creating a system that allows chaining of headers is illustrated in Figure 3.9. The Hop Limit field is essentially the same as IPv4's Time To Live, and is decremented by one every time an IPv6 packet is forwarded. If the decrement causes the Hop Limit to reach zero, the packet is dropped and an Internet Control Message Protocol version 6 (ICMPv6) Time Exceeded message is sent to the originator of the packet. The IPv6 header does not contain any fields for checksum, as the protocol suite design is such that relevant parts of the IPv6 header are included in the transport layer checksum (see more on this in Section 3.10.1). 3.2.1 Traffic Class and Flow Label RFC 2460 defines Traffic Class and Flow Label fields [21] for distinguishing packets belonging to the same categories, as described below. Traffic Class IPv6 header includes a Traffic Class field to allow the distinguishing of different IPv6 packets as different classes [21]. The semantics of the Traffic Class field are defined in 'Definition of the Differentiated Services Field in the IPv4 and IPv6 Headers' RFC Introduction to IPv6 2474 [22] and 'The Addition of Explicit Congestion Notifica