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IP provides a best-effort delivery service. I believe this implies, among other things, that it offers no QoS to the users.

However, IP has a byte in its headers (ToS in v4 and Trafic Class in v6) that I think does indeed offer some sort of QoS.

So does IP offer QoS or not?

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IPv4 has the DSCP (formerly ToS) field, and IPv6 has the Traffic Class field. Those are just fields in the IP headers. IP itself does not have any QoS. QoS is implemented by network devices, often based on those fields in the IP headers. The fields are to facilitate QoS, but QoS in not part of IP.

For example, a VoIP phone could mark its packets as EF (Expedited Forwarding). The network routers could then be configured with QoS and priority queues for traffic marked EF, but the routers may not have such a configuration, so even though the IP packets are marked, there is no QoS.

You define the QoS on your network to have the policies that you want. You could even place EF packets in a low-priority queue if you want. Everyone has a different view of how QoS should be implemented, and that is why QoS doesn't work over the public Internet.

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    Just a couple of adds. First, QoS doesn’t really do anything at all if your links (or devices) aren’t oversubscribed. It is only when links (or devices) are saturated that QoS actually makes a difference. Second, the biggest reason QoS doesn’t work on the Internet has to do with most networks rewriting DSCP on inbound traffic (this often includes inbound traffic from both up/down stream and end station ports). Since any traffic can be marked in any way, if the values were simply trusted it would be too easy to abuse QoS for personal advantage.
    – YLearn
    Feb 15, 2018 at 23:34

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