[e2e] Answer to Dave Reed Re: Fwd: Re: Question the other way round:

Fri Nov 22 07:08:14 PST 2013

> Forwarded for David Reed.
>
>
>
> (please forward, Joe, if this is OK)
>
> We don't actually cause congestion to discover the rate, Jon.

I'm sorry David - it is exactly what happens.

(IIRC a bit "hidden" in BSD Unix because IIRC a TCP socket which
attempts to enqueue a packet in case of seeing an occupied queue halves
its window and postpones the packet, I don't remember the details, it is
a bit more sophisticated than a silent discard, however the consequences
are the same.)

>   Typically, we try to build networks that have adequate capacity
> (factors of 10 or 100 are needed for things like the "Mother's Day"
> effect, or 9/11-scale community need to spread and filter news quickly.)
>
> We encounter congestion rarely - and we fix it by building in "factors
> of safety" in every portion of an underlying network.

Hm. To my understanding, the main mechanism for finding the appropriate
window size is inducing congestion.

>
> Only Ph.D. theses spend an enormous amount of effort on the totally
> congested "corner cases".

So, congestion is no real problem?
> It's like a little puzzle that is easy to
> state, easy to solve, and makes the solver work hard.  It's kind of like
> a "rite of passage", so that is good, I guess.
>
> But if you are building a datacenter (AWS) or an access network or a
> transport network, you build for the worst case, and expect it to happen

What is the "worst case"? What is .the largest number of parallel flows
possible?

Detlef
> rarely.  The systems that depend on the network to actually work for
> people's needs never want a congested network, and don't actually want
> the network to operate at its local minimum cost/bit/sec.  They want the
> network to never be in the way, and the cost they really care about is
> the cost of getting congested for the wrong reasons.
>
>
>
> On Thursday, November 21, 2013 2:29am, "Jon Crowcroft"
> <Jon.Crowcroft at cl.cam.ac.uk> said:
>
>  > i think we're mixing up two discussions here
>  >
>  > 1. congestion was the original cause of the cwnd mech in tcp, BUT the
>  > rate adaption using feedback as a way to distributed resource
>  > allocation is the solution of the optimisation problem of net + user
>  > addressed by several researchers (kelly/voice et al, also folks at
>  > caltech) - these aren't the same thing - they got conflated in
>  > protocols in practice because we couldn't get ECN out there completely
>  > (yet) - ECN (when implemented with some decent queue (see 3 below) can
>  > be part of an efficient decentralised rate allocation
>  >
>  > congestion is bad - avoiding it is good
>  >
>  > distributed rate allocation for flows
>  > that have increasing utility for higher rate transfer
>  > is also good (actually its betterer:)
>  >
>  > 2. for flows that have an a priori known rate, distributed rate
>  > allocation is a daft idea, a priori - so some sort of admission
>  > control for the flow seems better (but you can do probe/measurement
>  > based admission control if you like, and are allergic to complex
>  > signaling protocols)
>  >
>  > 3. orthogonal to both 1&2 is policing and fairness - flow state
> means you
>  > can do somewhat better in fairness for 1 (e.g. do fair queus, a la
>  > keshav), and a lot better for policing for 2...
>  >
>  > but then we've been round the best effort, integrated service,
>  > differentated service, core stateless fair queue, probe based
>  > admission control, ecn, pcn loop about 6 times since this list
>  > existed:)
>  >
>  > yes, to detlef's original point, causing congestion (and buffer
>  > overrun) to find out the rate is a bit of a sad story...
>  >
>  > In missive <528CFE15.7070808 at isi.edu>, Joe Touch typed:
>  >
>  > >>
>  > >>
>  > >>On 11/19/2013 7:14 PM, Ted Faber wrote:
>  > >>> On 11/19/2013 10:15, Joe Touch wrote:
>  > >>>> On 11/19/2013 10:09 AM, Dave Crocker wrote:
>  > >>>>> Given the complete generality of the question that was
>  > asked, is there
>  > >>>>> something fundamentally deficient in the answer in:
>  > >>>>>
>  > >>>>>
>  > http://en.wikipedia.org/wiki/Congestion_control#Congestion_control
>  > >>>>>
>  > >>>>> ?
>  > >>>>>
>  > >>>>> In particular, I think it's opening sentence is quite
>  > reasonable.
>  > >>>>
>  > >>>> I agree, but it jumps in assuming packets. Given packets, it's
>  > easy to
>  > >>>> assume that oversubscription is the natural consequence of
>  > avoiding
>  > >>>> congestion.
>  > >>>
>  > >>> Unless someone's edited it, you should read the first sentence
>  > again. I
>  > >>> see:
>  > >>>
>  > >>>> Congestion control concerns controlling traffic entry into a
>  > >>>> telecommunications network, so as to avoid congestive collapse
>  > by
>  > >>>> attempting to avoid oversubscription of any of the processing or
>  > link
>  > >>>> capabilities of the intermediate nodes and networks and taking
>  > resource
>  > >>>> reducing steps, such as reducing the rate of sending packets.
>  > >>>
>  > >>> I read the reference to packets as an example.
>  > >>
>  > >>Me too.
>  > >>
>  > >>But circuits don't have a collapse or oversubscription. They simply
>  > >>reject calls that aren't compatible with available capacity.
>  > >>
>  > >>I'm not disagreeing with the definition; I'm disagreeing with the
>  > >>assumption that having a network implies congestion and thus the
> need
>  > >>for congestion control.
>  > >>
>  > >>There are a variety of mechanisms that avoid congestion,
> typically by
>  > >>a-priori reservation (circuits), or by limiting resource use
> implicitly
>  > >>(e.g., ischemic control). These are a kind of proactive control that
>  > >>avoid congestion in the first place.
>  > >>
>  > >>That's not to say whether these mechanisms are scalable or efficient
>  > >>compared to the resource sharing afforded by packet multiplexing.
>  > >>
>  > >>Joe
>  >
>  > cheers
>  >
>  > jon
>  >
>  >
>
>
>

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