Re: [aqm] floating a draft charter

[Jim Gettys <[e-mail]>]

There are a number of things I want to see:

1) packet scheduling is as important as drop/mark algorithms: it is the two
together that is such a win with fq_codel.  So I, for one, would be *very*
unhappy to see a charter that meant it was out of scope to discuss and
document scheduling and flow queuing strategies.  And packet scheduling can
interact with drop mark algorithms.

Optimal scheduling depends on where you are in the network (e.g. a host,
versus home router, vs. ISP head end, etc.); documenting what makes the
most sense where would likely help both implementors of software and
hardware and operators of those networks.  I've given some thought to the
area, but until we have running code, it's hand waving, but probably won't
be hand waving in a year or two.

2) many mark/drop algorithms can co-exist (and generating informational
RFC's to help customers write RFP's is worth while): but so far, there is
no universal "one size fits all" mark/drop algorithm that is optimally
effective everywhere.  Kathy and Van say a primary goal of CoDel was
"first, do no harm", rather than "optimal under all circumstances".  The
goal was an algorithm that never hurt you and would always be safe to have
enabled. CoDel's constants were chosen so that CoDel will work at the edge
of the network with planetary length paths in the face of highly variable
bandwidth.

But CoDel inside a datacenter (unless you mess with its constants) is not
effective in a timely fashion.

People have feared tuneable algorithms such as (W)RED can hurt them, so
what we have had for AQM is sometimes not present at all, or not enabled
(even in nodes that it would help greatly).  Note that one of Dave Taht's
discoveries was that RED had been broken in the Linux source tree for 5
years and *no one had noticed*.

Maybe someday we'll reach nirvana with a drop/mark algorithm that "just
works" everywhere well all the time and can never hurt and always works
perfectly, but that's a lofty goal left for research; in the meanwhile,
helping people understand what to use where in the network and the
tradeoffs is important.

But if CoDel (as we believe) or some other algorithm really is "first, do
no harm" without tuning,  it has great value to be on by default just to
prevent the disaster an operator not enabling any AQM at all; the world
will never be worse than that, and usually be radically better. Anything
that *requires* tuning of knobs to work is a not a candidate....

3) Some algorithms may be hard to retrofit into existing platforms.  For
example, if you can't timestamp a packet, it's really hard to use CoDel,
which is an algorithm easy to implement from scratch, but could be really
hard retrofit in an existing hardware design. I don't want to discourage
people doing things a lot better than nothing as a stop-gap, but people
need to understand that it may be a second best solution.

Again, advice as to what/where algorithms may be applicable will help
designers and operators both.  We face a 5-10 year transition here, even if
we succeed this time.

4) There are "feagtures" or lack of features that today's hardware does
that can/does make life for software very hard. Changes could make life
much easier.

For example, today's hardware makes it hard to do true head drop (which
hardware assist could make trivial).  Some have noticed that at low
bandwidth we may adjust CoDel parameters to work around the lack of head
drop and unaccounted for time in the driver's ring buffers (which CoDel
needs), since Linux's qdisc's can't see the time in the rings. So there are
a set of recommendations to hardware and OS designers that would help the
state of the network: this simple head drop feature, packet pacing, making
GSO/TSO less evil, and so on, war stories on all the places we've been
working on debloating Linux to guide other OS developers, etc.

This is really the other side of queue management: try to encourage traffic
and systems that is less bursty and gentle to the network, so the
AQM/scheduling algorithms don't have to work as hard.

Such things should go someplace in the IETF, and if not here, where?  Doing
nothing anywhere is what I want to avoid.

Best regards,
    Jim



On Thu, May 30, 2013 at 12:13 PM, Wesley Eddy <[e-mail]> wrote:

> On 5/30/2013 8:20 AM, Mirja Kuehlewind wrote:
> >
> > But my point is, we should not only to standardize more and more AQM
> > mechanisms because that's a large research area which maybe even should
> be
> > homed in the IRTF, but we need to find actually deployment strategies.
> Having
> > a working group on AQM will already help to make people aware of the
> topic
> > and maybe think about using AQM. But only standardizing more and more AQM
> > queue, might end up in investing a lot of working that no-one will ever
> use.
>
>
> I suggest appending something like this to the charter:
>
>   Many AQM algorithms have been proposed in academic literature, but
>   very few are widely implemented and deployed.  A goal of the working
>   group is to produce recommendations that will actually be used, and
>   algorithms that will actually be implemented, deployed in equipment,
>   and enabled.  Towards these ends, the group actively encourages
>   participation from operators and implementers, and will coordinate
>   with the IETF OPS area and other relevant parts of the IETF and
>   Internet community.  Wider research and evaluation of AQM mechanisms
>   shall be coordinated with the IRTF/ICCRG, and significant
>   participation in this WG from the academic and research community is
>   highly desirable, when it is directly relevant to implementation and
>   deployment.
>
>
> We will definitely need to get engagement from some of the operators
> that participate in the IETF, and should solicit participation more
> widely outside as well (e.g. advertise to NANOG list, bufferbloat
> list, etc.) in order to attract operators and implementers that aren't
> already aware of this activity.
>
> Would this help to address your concern?
>
>
> --
> Wes Eddy
> MTI Systems
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