[Cake] [lede-project/source] Add support for cake qdisc (#72)

[Jonathan Morton]

> On 1 Jun, 2016, at 15:25, Benjamin Cronce <bcronce at gmail.com> wrote:
> 
> 1) Ideally, regardless of platform, should an AQM or scheduler have the responsibility of changing anything other than ECN?

This was in part my original objection to having the squash/wash feature in Cake.

The other part is that if we are going to rewrite the rest of the TOS byte (not just the ECN bits), then we should do it properly, which requires a rather substantial extension to the configuration API, even if we only try to cover the most obvious use-cases.

This would then be a “semi proprietary” DSCP configurator, acting independently of Cake’s core AQM and shaping functions, which would have to be duplicated in other AQMs which had similar aims to Cake.  That’s not a good thing, and feeds back into the first point.

Hence the correct solution is to put DSCP rewriting elsewhere, making it reusable.

In Linux, doing ingress DSCP rewriting before it hits the ingress qdisc presently requires the rewriter itself to be a qdisc, but this can have Cake as a child qdisc.  For the simple “clearing” case iptables can be used instead, as long as Cake is configured to ignore the inbound DSCP using the “besteffort” flag.

 - Jonathan Morton