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
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.