[Cake] cake exploration

Jonathan Morton chromatix99 at gmail.com
Sun Apr 12 19:14:39 EDT 2015

> On 11 Apr, 2015, at 21:47, Dave Taht <dave.taht at gmail.com> wrote:
> 14) strict priority queues. Some CBR techniques, notably IPTV, want 0
> packet loss, but run at a rate determined by the provider to be below
> what the subscriber will use. Sharing that "fairly" will lead to loss
> of packets to those applications.
> I do not like strict priority queues. I would prefer, for example,
> that the CBR application be marked with ECN, and ignored, vs the the
> high probability someone will abuse a strict priority queue.

The new priority mechanism in cake3 actually still supports a hard rate-limit function, albeit with a small amount of slop in it.  You would simply need to force the “bandwidth share” quantum value to zero, which would mean that the class involved only gets quanta when it’s running within its limit.

A sufficiently large “priority share” quantum value would also behave an awful lot like strict priority.  This is aided by the fact that cake3 still charges the bandwidth of high priority classes to all lower priority classes - but note that if the normal strictly-decreasing structure of the classes is violated, it becomes possible to force some of the high-priority classes to operate permanently in bandwidth-sharing mode, robbing them of most of their original benefit.

I feel fairly strongly that this type of traffic should be handled in one of two ways:

- Mark it with an appropriate DSCP, such as CS3 or VA, and accept contention if it occurs.

- Permanently mark off the CBR bandwidth as unavailable to normal traffic, and configure cake to use the remainder; use a separate mechanism to have the CBR traffic bypass cake.  This would be particularly appropriate for a shared broadcast stream.

As a variation on the second option, it may be that the CBR stream is only present intermittently.  In that case, cake can be reconfigured on the fly by an external mechanism, to use either the full or reduced bandwidth; the bypass mechanism should remain in place meanwhile.

 - Jonathan Morton

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