Re: [Cerowrt-devel] [Cake] active sensing queue management

[Sebastian Moeller <moeller0@gmx.de>]

Hi Dave,


On Jun 10, 2015, at 21:53 , Dave Taht <dave.taht@gmail.com> wrote:

> http://dl.ifip.org/db/conf/networking/networking2015/1570064417.pdf
> 
> gargoyle's qos system follows a similar approach, using htb + sfq, and
> a short ttl udp flow.
> 
> Doing this sort of measured, then floating the rate control with
> "cake" would be fairly easy (although it tends to be a bit more
> compute intensive not being on a fast path)
> 
> What is sort of missing here is trying to figure out which side of the
> bottleneck is the bottleneck (up or down).

	Yeah, they relay on having a reliable packet reflector upstream of the “bottleneck” so they get their timestamped probe packets returned. In the paper they used either uplink or downlink traffic so figuring where the bottleneck was easy at least this is how I interpret “Experiments were performed in the upload (data flowing from the users to the CDNs) as well as in the download direction.". At least this is what I get from their short description in glossing over the paper.
	Nice paper, but really not a full solution either. Unless the ISPs cooperate in supplying stable reflectors powerful enough to support all downstream customers. But if the ISPs cooperate, I would guess, they could eradicate downstream buffer bloat to begin with. Or the ISPs could have the reflector also add its own UTC time stamp which would allow to dissect the RTT into its constituting one-way delays to detect the currently bloated direction. (Think ICMP type 13/14 message pairs "on steroids", with higher resolution than milliseconds, but for buffer bloat detection ms resolution would probably be sufficient anyways). Currently, I hear that ISP equipment will not treat ICMP requests with priority though.
	Also I am confused what they actually simulated: “The modems and CMTS were equipped with ASQM, CoDel and PIE,” and “However, the problem pop- ularly called bufferbloat can move about among many queues some of which are resistant to traditional AQM such as Layer 2 MAC protocols used in cable/DSL links. We call this problem bufferbloat displacement.” seem to be slightly at odds. If modems and CTMS have decent AQMs all they need to do is not stuff their sub-IP layer queuesand be done with it. The way I understood the cable labs PIE story, they intended to do exactly that, so at least the “buffer displacement” remedy by ASQM reads a bit like a straw man argument. But as I am a) not of the cs field, and b) only glossed over the paper, most likely I am missing something important that is clearly in the paper...

Best Regards
	Sebastian

> 
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> Dave Täht
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