My concern is that this sort of filtering would require a lot of fine tuning. How to create a FW rule that would restrict all the possible permutations that heavy downloading would use?
Real world issue for context... A weekend trailer camp has no DSL or CABLE available. Only wireless over an HSPA+ (cell phone) carrier. So BW is good, but there is a rather low monthly cap of 10 Gb to share. I've employed Gargoyle, which is largely invisible to the user. It works good. An IP is limited to 300Mb per weekend and another 300Mb for Mon-Fri. This is a reasonable amount for anything but video or heavy downloading.
I see people go to megavideo, youtube, or television streaming news or sports sites and burn up their quota in 2 hours or less. These are the types that your suggestion might stop successfully. OK... they shouldn't be going to a weekend trailer camp to watch TV
I also see laptops that suddenly invoke an MS OS upgrade. Just bad timing I guess. By both your suggestion for blocking certain types of streams or other suggestions of throttling that user to dial up speeds, it brings up the need for a secondary quota. First quota triggers a FW restriction blocking video and other high BW connections. Second quota would be required to shut that IP down completely in case they found a way around the FW rules or their particular heavy downloading application was missed? Similarly, the scheme to throtle the IP once quota is exceeded, per your observation, would require a second quota to shut them down in case they are happy to download at slow speed and watch later after the file is delivered.
In the weekend camp scenario, we only need to encourage people to do their downloading at home not at the camp. The assumption is that everyone has another location with a normal internet connection.
With the present "drop dead" quota implementation at the camp it takes folks a few quota breaches to figure out why their internet stopped working. They first blame the WiFi system. I've had to put up a notice on a bulletin board so the word about a quota gets out. Whatever the solution, there is a need for a splash page at the beginning of a session so users know what to expect and can act accordingly.
Alternatively, redirect their browser to a captive warning page when they hit their quota? I'm hoping that once the users are educated about the ISP monthly cap reality for the camp, they will consciously restrict themselves from high bandwidth activity. Of course, they might also learn to meter themselves to that quota limitation. If every IP device exercised their full quota, the monthly cap will be quickly exceeded and I'll have to adjust the quota smaller.
If you drill down and analyze what I'm trying to do with Gargoyle... I'm watching the traffic and trying to come up with a quota implementation that fairly distributes the monthly cap among uses. I'm trying to prevent overage charges from my ISP.
My 2 cents.