IPv6 was developed to solve some of the problems of IPv4, such as QoS. We know in IPV4 based on TOS values, a packet would be placed in a prioritized outgoing queue,or take a route with appropriate latency, throughput, or reliability. The IPV6 header has a structure that identifies the flow of packets (Flow Label Field) and thereby directs it to the router. So IPV6 and IPV4 both have fields for QoS. We know that QoS was almost ignored by routers in IPV4.How exactly IPV6 observe packets priority?How we are sure about it?
closed as not constructive by Michael Hampton, Brad Patton, Tog, Renan, 8088 May 2 '13 at 17:18
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It's worth noting that QoS in IPv4 is usually done using DSCP. DSCP is sort of an overhaul of the old TOS field: it occupies the same bits of the IPv4 header, but the meanings are different and generally better aligned with the kinds of QoS policies people really want to have.
Using DSCP basically makes IPv4 and IPv6 QoS the same, because the DSCP field is defined the same way for IPv4 and IPv6 (in different positions in the header).
QoS is not almost ignored by routers in IPv4. QoS is almost ignored by routers on the Internet. That goes for both IPv4 and IPv6. It's not a technical limitation, it's a trust & policy limitation. The Internet is a public network. Basically, none of the ISPs who forward your packets for you are inclined to trust you to tell them what kind of special treatment you want for your packets. As the originator of the packets, your self-interest would be to simply mark almost every one of your packets with the DSCP value that will result in the packet being placed in the best possible priority queue while hoping that at least some other Internet users won't do the same.
ISPs can and do classify traffic according to criteria which they control. For example, they might route all Internet traffic as best effort, route all traffic coming from leased private network customers using something better, and treat VoIP traffic related to the ISP's telco arm's internal network best of all. They might do it by encapsulating these different kinds of traffic with different MPLS labels, or they might rewrite DSCP bits in packets. Again, this is all IP-version-agnostic.
You can likewise mark traffic on your own internal network and have your own routers respect these markings to classify traffic into different priority queues. But you can't expect anyone else to respect your markings unless you are paying them do to so.