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7.2 IPv6 Infrastructure > 7.2.1 Protocol Mechanisms - Pg. 297

Using IPv6 to Support 3G VoIP 7.2 IPv6 Infrastructure 7.2.1 Protocol Mechanisms As we saw in Chapter 6, an IPv6 Protocol Data Unit (PDU) consists of an IPv6 header and an IPv6 pay- load, as depicted in Figure 7.5. The IPv6 header consists of two parts: the IPv6 base header, and optional extension headers. The optional extension headers are considered part of the IPv6 payload, as are the TCP/UDP/RTP PDUs (including the voice bit stream). Obviously, IPv4 headers and IPv6 headers are not interoperable; hence, a router operating in a mixed environment must use an implementation of both IPv4 and IPv6 in order to deal both header formats; this is also the case in VoIP arrangements that support both environments as well as transition environments. Figure 7.6 shows the flows of IPv6 PDUs in a VoIPv6 environment. IPv6 Packet IPv6 Header IPv6 Payload IPv6 Function Length Header Field Version Identifies the version of the protocol. For IPv6, the field is 6 (1010). 4 Bits Class Flow Label Intended for originatinh nodes and forwarding routers to identify and 8 Bits distinguish between different classes or priorities of IPv6 packets. Defines how traffic is handled and identified. A flow is a sequence 20 Bits