Originating nodes and forwarding routers can use the 8-bit Traffic Class field in the IPv6 header to identify and distinguish between different classes or priorities of IPv6 packets.
The service interface to the IPv6 service within a node must provide a means for an upper-layer protocol to supply the value of the Traffic Class bits in packets that were originated by that upper-layer protocol. The default value must be zero for all 8 bits.
Nodes that support a specific use of some or all of the Traffic Class bits can change the value of those bits in packets that they originate, forward, or receive, as required for that specific use. Nodes should ignore and leave unchanged any bits of the Traffic Class field for which they do not support a specific use.