How would that work in a typical DSL scenario with PPPoE router connected by Ethernet (at least 10meg) to a DSL modem with let's say an 800K upload speed? If the router just sends the packets as fast as they arrive and the modem drops them if the uplink is still busy, then that means no possibility of upstream QoS on the router. Or to be more precise no possibility of prioritisation by the router.
Wouldn't you have that anyway? Where your PC, attached with a "gigabit ethernet" connection to the switch, sends too much for the switch to forward to the router, because it is only connected with a 100Mbps "fast ethernet" connection? Now your worry has moved into your switch, not your modem.
The truth is that the TCP protocol starts slow, and ramps up ... until packets start to be dropped somewhere (anywhere) along the route. It then falls back to a speed where everything was happy.
If the bottleneck is at the link between router and modem, as your example, then the router would never have been receiving stuff to send on at 10Mbps; once the modem even started to dump packets (at 800k if it is dumb; earlier if smart), TCP will have backed off at source, and the router will only end up receiving packets at (or below) 800k. A sustainable level for passing onward into the modem.
If a second PC starts trying to send, the two independent TCP stacks will fall back to around 400k.
TCP doesn't work by dumping a huge file as fast as it can into a router, and then waiting for the queue to empty. It attempts to maintain a steady-ish speed that can be sustained end-to-end.
If you want QoS to work, then all your network nodes that have any congestion control within them need to support the same QoS system; this gets used by every place that chooses what packets to dump, and what to favour. It builds on top of standard TCP congestion mechanisms.
It has always been my understanding that the Internet does not have QoS, so from your modem outbound, no QoS applies
"The internet" doesn't. But QoS exists on many, more private, networks. BT's network - which your packets traverse before arriving at your ISP and "the internet" - does employ QoS. As we go further forward, and your voice calls become a service on top of IP, you can be sure that BT will have QoS mechanisms that keep voice prioritised.
I thought it was well known that if you turn off QOS on a Huawei HG612, your upstream sync increases by 1 Mbps.
Yup. Even if you don't send any QoS information with your upstream data (and almost noone does), the HG612 would spend time looking for those QoS markers, to decide how to queue your packets, and to decide what order to drop things in. This check was enough to slow down upstream throughput. Wasted work... but work, nonetheless.