IPV6 brings quality of service that is required for several new applications such as IP telephony, video/audio, interactive games or ecommerce. Whereas IPv4 is a best effort service, IPv6 ensures QoS, a set of service requirements to deliver performance guarantee while transporting traffic over the network.
For networking traffic, the quality refers to data loss, latency (jitter) or bandwidth. In order to implement QOS marking, IPv6 provides a traffic-class field (8 bits) in the IPv6 header. It also has a 20-bit flow label.