I know routers today have the ability to prioritize traffic, but last I heard, these controls are not often used for user traffic (let's not discuss net neutrality here).

Are they used for control (e.g., routing) traffic?

They are used for BUSINESS traffic. Also, since these controls make routers work harder, there is no point in using them where there are no traffic problems. Most providers build their core networks with enough headroom so that there are no traffic problems. That leaves the last mile connection to tinker with, however, the cost problem remains. The only way to solve the cost problem is where a business customer will pay for traffic controls to be implemented on the provider's edge routers. Bottom line is that use of QOS is very common on CPE routers and edge routers of business customers and this is usually part of a total managed network package such as a VPN or Extranet. I have seen QOS implemented for control traffic but that was a scenario where the company allowed business customers access to a tool which would ping through to the CPE router. In order to prevent the customer from DOSing themself with pings, they capped the traffic with CAR or similar. It can be a bad idea to apply QOS to control traffic since it is generally easier and cheaper to add bandwidth. Just because the machine has lots of bright buttons and knobs on the control panel doesn't mean that it is a good idea to play with them all. And the fundamental problem of QOS means that you only use it where you have to. QOS works by delaying or discarding packets. It is hard to sell that as a valuable service to ordinary users. --Michael Dillon