[Cake] More on 'target' corner cases - rate/target/interval confusion?

Toke Høiland-Jørgensen toke at toke.dk
Tue Nov 3 07:46:39 EST 2015

Kevin Darbyshire-Bryant <kevin at darbyshire-bryant.me.uk> writes:

> Can someone please explain if the above is actually correct and why.
> i.e. I'm confused why voice traffic has a target latency of 72mS over
> a measurement interval of 1/2 second when I'd have thought keeping
> voice latency as low as possible would be the ideal. I'm sure I'm
> being exceptionally stupid so I await education please :-)

Well, I *think* this comes from the 1.5*MTU calculation. I.e. 1.5 * 1500
bytes at 250Kbps is 72 ms. And interval is adjusted to always be

Now whether or not this is *correct* I'm not so sure about. The minimum
value set from the MTU is the Linux counterpart to the CoDel NS2 model's
"always keep one packet in the queue". The reasoning is, basically, that
there will be at least one packet in the layers below the qdisc which
we will have to wait for before we can dequeue; so CoDel shouldn't react
until queueing time goes *above* this minimum of waiting for one packet.

Now, in this case we have the rate limiter built right in to the qdisc,
so the queued packet will in fact *not* be in a layer below (because
that is probably going to be a link with a physical rate that is quite a
bit higher than what we're setting here; or if not, we're not doing the
MTU adjustment anyway). So it's not quite clear to me what the right
thing to do here is; as you rightly point out this leads to a very high
target for the VoIP queue which is not necessarily a good idea. Capping
the target before doing the per-tin calculation might be a better idea
(i.e. using the total bandwidth limit as the input to the calculation).
Will think about it some more.

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