The nodes that originate a packet must identify different classes or different priorities of IPv6 packets. The nodes use the Traffic Class field in the IPv6 header to make this identification. The routers that forward the packets also use the Traffic Class field for the same purpose.
The following general requirements apply to the Traffic Class field:
The service interface to the IPv6 service within a node must supply the value of the Traffic Class bits for an upper-layer protocol. The Traffic Class bits must be in packets that are originated by that upper-layer protocol. The default value must be zero for all 8 bits.
Nodes that support some or all of the Traffic Class bits can change the value of those bits. The nodes can change only the values in packets that the nodes 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 the nodes do not support a specific use.
The Traffic Class bits in a received packet might not be the same value that is sent by the packet's source. Therefore, the upper-layer protocol must not assume that the values are the same.