[Bloat] Remarkably simple bloat managing use by a UK ISP

[Dave Taht]

On Fri, Jun 5, 2015 at 9:35 AM, Jonathan Morton <chromatix99 at gmail.com> wrote:
> Going back to fundamentals, there's a clear distinction between traffic
> which is latency sensitive and traffic which is bandwidth sensitive. Perhaps
> surprisingly, web traffic tends to fall into the former category, unless the
> link bandwidth is very low by current standards (analogue modem territory).
>
> It sounds like that router's default settings attempt to make that
> distinction based on port number, and then perform strict prioritization.
> The example of SSH demonstrates why that doesn't work; interactive shell
> over SSH is latency sensitive, but SCP and rsync-over-SSH are bandwidth
> sensitive, and they all use the same port. The need for explicit
> configuration (a database of port numbers) is also a black mark against it,
> especially since they managed to leave out such a common protocol as DNS.
>
> Packet size is a better heuristic than port number. Most latency sensitive
> protocols do tend to use small packets, and nearly all bandwidth sensitive
> traffic uses the largest packets it can. SSH also naturally switches between
> small and large packets depending on the type of traffic it's carrying. If
> you simply sort your traffic by packet size, you don't even need to
> configure it - but otherwise you just have a threshold to set and forget.
> Cunningly, this also naturally performs ack and ping promotion.

I do not regard ping promotion as a "feature". I think ping should be
a measurement of worser-case latency. Openwrt does both synflood and
ping flood protection in their firewall implementation badly with a
fixed limit too low for synflood (it used to be 25/sec), and too high
for ping flood.(1000/sec and extensive filtering)

> The downside of packet size as a heuristic is that it's possible to game it,
> especially with strict priority in force. All you have to do is send packets
> below the threshold if there is one, or slightly smaller than the competing
> traffic if there isn't one. The receiver can influence this by setting the
> MSS low during TCP handshakes.

There is a natural game theory influence pushing packet size larger
for bulkier traffic, and that is header overhead gets pretty
significant as you get below 300 bytes.

>
> That is why fq_codel uses the sparse/saturating flow distinction for
> priority. It's a much more robust heuristic than packet size, requires no
> configuration at all, and is not so easy to game. The downside? It's only
> available if you're already doing flow isolation, which basically solves the
> core problem on its own. Still, making that distinction does help with new
> flow startup and jitter reduction.

Yep. A godawful amount of thought and data went into all this...

... and I stress in
http://www.bufferbloat.net/projects/cerowrt/wiki/Wondershaper_Must_Die

that in 10 years someone might be cursing us for using any of these
techniques so universally because it interacts badly with the latest
brain to network direct connect with high speed voice response

https://www.youtube.com/watch?v=M1ONXea0mXg

(or something like that)

> - Jonathan Morton
>
>
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-- 
Dave Täht
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