[e2e] Fwd: Re: Question the other way round:
touch at isi.edu
Thu Nov 21 09:56:33 PST 2013
Forwarded for David Reed.
Joe (list admin)
-------- Original Message --------
Subject: Re: [e2e] Question the other way round:
Date: Thu, 21 Nov 2013 11:21:40 -0500 (EST)
From: dpreed at reed.com
To: Jon Crowcroft <Jon.Crowcroft at cl.cam.ac.uk>
CC: Joe Touch <touch at isi.edu>, jon.crowcroft at cl.cam.ac.uk, Ted Faber
<faber at isi.edu>, end2end-interest at postel.org
(please forward, Joe, if this is OK)
We don't actually cause congestion to discover the rate, Jon.
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.
Only Ph.D. theses spend an enormous amount of effort on the totally
congested "corner cases". 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
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
> 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
> 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:
> >>>>> ?
> >>>>> In particular, I think it's opening sentence is quite
> >>>> I agree, but it jumps in assuming packets. Given packets, it's
> easy to
> >>>> assume that oversubscription is the natural consequence of
> >>>> 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
> >>>> attempting to avoid oversubscription of any of the processing or
> >>>> capabilities of the intermediate nodes and networks and taking
> >>>> 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
> >>(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.
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