What is traffic painting in QoS?

Question

QoS

Good afternoon, I'm trying to understand the essence of the QoS mechanisms and stuck in a misunderstanding of such concepts as traffic painting, coloring, color marking. Everywhere in the articles just three green yellow red flowers are mentioned, I understand that this is some kind of action with packages after passing the classification, but I don’t understand what exactly it does and due to what.

Answer 1

When packets pass through the port of the network equipment, they fall into a certain queue. There can be several queues on a port (some hardware, some software). Queues are configured, for them it is determined what they should do with packets when certain thresholds are reached (read about the basket token algorithm). For each queue, a traffic light is set (your colors are red, yellow, green) - boundary values. For each boundary value, there are actions (shaping, discarding, shifting to another queue, etc.).
Coloring a packet means assigning some QoS field to it (IPP/DSCP at the IP level, 802.1p on ethernet, or EXP on MPLS). Based on this coloring, the packet is placed in one of the queues on the interface. It has nothing to do with the red-yellow-green colors.

Answer 2

Traffic painting is a manipulation with DSCP, the DS (ToS) field in the IP packet, in addition to it, the ECN field is also there. There may still be IP-precedence, it's all there but from ancient documents.
At the second layer of Ethernet, this is the manipulation of the CoS (802.1p) field that appears in the 802.1q Wealan specification. There are
corresponding headers in many other protocols, in MPLS, for example, this is the TC field.
The paint here is nothing more than a rough simplification. The number of colors for CoS is 8, and for DSCP as many as 64. And you won’t get off with 3 colors here. Of course, there are certain agreements on what traffic to label, in general, the higher the number of topics, the traffic is more priority.
But it doesn't really mean anything, it's just a number. As traffic moves forward, each individual node on the path decides for itself what to do with traffic marked in one way or another.