• Flow Label (20 bits)• Payload Length (16 bits)• Next Header (8 bits)• Hop Limit (8 bits)• Source Address (128 bits)• Destination Address (128 bits)IPv6 provides for extension headers. Extension headers are used only if necessary. There can be no extension headers, one extensionheader or more than one extension header in an IPv6 packet. Extension headers are defined in the Next Header field of the preceding IPv6header.IPv6 Header FieldsThe 40 bytes of the IPv6 header are ordered, as shown in the following illustration.Figure 55. IPv6 Header FieldsVersion (4 bits)The Version field always contains the number 6, referring to the packet’s IP version.Traffic Class (8 bits)The Traffic Class field deals with any data that needs special handling. These bits define the packet priority and are defined by the packetSource. Sending and forwarding routers use this field to identify different IPv6 classes and priorities. Routers understand the prioritysettings and handle them appropriately during conditions of congestion.Flow Label (20 bits)The Flow Label field identifies packets requiring special treatment in order to manage real-time data traffic.The sending router can label sequences of IPv6 packets so that forwarding routers can process packets within the same flow withoutneeding to reprocess each packet’s header separately.NOTE: All packets in the flow must have the same source and destination addresses.Payload Length (16 bits)The Payload Length field specifies the packet payload. This is the length of the data following the IPv6 header. IPv6 Payload Length onlyincludes the data following the header, not the header itself.The Payload Length limit of 2 bytes requires that the maximum packet payload be 64 KB. However, the Jumbogram option type Extensionheader supports larger packet sizes when required.452 IPv6 Routing
