[Cake] active sensing queue management

Daniel Havey dhavey at gmail.com
Wed Jun 10 20:10:36 EDT 2015

Hmmm, maybe I can help clarify.  Bufferbloat occurs in the slowest
queue on the path.  This is because the other queues are faster and
will drain.  AQM algorithms work only if they are placed where the
packets pile up (e.g. the slowest queue in the path).  This is
documented in Kathy and Van's CoDel paper.

This is usually all well and good because we know where the bottleneck
(the slowest queue in the path) is.  It is the IP layer in the modem
where the ISP implements their rate limiter.  That is why algorithms
such as PIE and CoDel are implemented in the IP layer on the modem.

Suppose the full committed rate of the token bucket rate limiter is 8
Mbps.  This means that the queue at the IP layer in the modem is
capable of emitting packets at 8 Mbps sustained rate.  The problem
occurs during peak hours when the ISP is not providing the full
committed rate of 8 Mbps or that some queue in the system (probably in
the access link) is providing something less than 8 Mbps (say for sake
of discussion that the number is 7.5 Mbps).

We know that (see Kathy and Van's paper) that AQM algorithms only work
when they are placed at the slowest queue.  However, the AQM is placed
at the queue that is capable of providing 8 Mbps and this is not the
slowest queue.  The AQM algorithm will not work in these conditions.

This is what is shown in the paper where the CoDel and PIE performance
goes to hell in a handbasket.  The ASQM algorithm is designed to
address this problem.

On Wed, Jun 10, 2015 at 1:54 PM, Sebastian Moeller <moeller0 at gmx.de> wrote:
> Hi Dave,
> On Jun 10, 2015, at 21:53 , Dave Taht <dave.taht at 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, we never did figure out how to separate the up from the
downlink.  However, we just consider the access link as a whole (up +
down) and mark/drop according to ratios of queuing time.  Overall it
seems to work well, but, we never did a mathematical analysis.  Kind
of like saying it's not a "bug", it is a feature.  And it this case it
is true since both sides can experience bloat.

>         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.

Not exactly.  We thought this through for some time and considered
many angles.  Each method has its advantages and disadvantages.

We decided not to use ICMP at all because of the reasons you stated
above.  We also decided not to use a "reflector" although as you said
it would allow us to separate upload queue time from download.  We
decided not to use this because it would be difficult to get ISPs to
do this.

Are final choice for the paper was "magic" IP packets.  This consists
of an IP packet header and the timestamp.  The IP packet is "self
addressed" and we trick the iptables to emit the packet on the correct
interface.  This packet will be returned to us as soon as it reaches
another IP layer (typically at the CMTS).

Here's a quick summary:
ICMP -- Simple, but, needs the ISP's cooperation (good luck :)
Reflector  -- Separates upload queue time from download queue time,
but, requires the ISP to cooperate and to build something for us.
(good luck :)
Magic IP packets -- Requires nothing from the ISP (YaY!  We have a
winner!), but, is a little more complex.

>         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...

Good point!  However, once again it's not quite that simple.  Queues
are necessary to absorb short term variations in packet arrival rate
(or bursts).  The queue required for any flow is given by the
bandwidth delay product.  Since we don't know the delay we can't
predict the queue size in advance.  What I'm getting at is the
equipment manufacturers aren't putting in humongous queues because
they are stupid, they are putting them there because in some cases you
might really need that large of a queue.

Statically sizing the queues is not the answer.  Managing the size of
the queue with an algorithm is the answer.  :)

> Best Regards
>         Sebastian
>> --
>> Dave Täht
>> What will it take to vastly improve wifi for everyone?
>> https://plus.google.com/u/0/explore/makewififast
>> _______________________________________________
>> Cake mailing list
>> Cake at lists.bufferbloat.net
>> https://lists.bufferbloat.net/listinfo/cake

More information about the Cake mailing list