Re: [Bloat] ipspace.net: "QUEUING MECHANISMS IN MODERN SWITCHES", > (Jonathan Morton)

Re: [Bloat] ipspace.net: "QUEUING MECHANISMS IN MODERN SWITCHES", > (Jonathan Morton)

[David Collier-Brown]

On 05/28/2014 11:33 AM, Jonathan Morton <chromatix99@gmail.com> wrote
> It's a mathematical truth for any topology that you can reduce to a black box with one or more inputs and one output, which you call a "queue" and which *does
not discard* packets.  Non-discarding queues don't exist in the real
world, of course.
> 
> The intuitive proof is that every time you promote a packet to be transmitted earlier, you must demote one to be transmitted later.  A non-FIFO queue tends to increase the maximum delay and decrease the minimum delay, but the average delay will remain constant.

A niggle: people working in queuing theory* make the simplifying
assumption that queues don't drop. When describing the real world, they
talk of "defections", the scenario where a human arrives at the tail of
the queue and "defects", either to another queue or to the exit door of
the store!

As you might guess, what I find intuitive the IP world finds wrong, and
vice versa.

--dave
[* as opposed, perhaps, to queuing networks (:-)]
-- 
David Collier-Brown,         | Always do right. This will gratify
System Programmer and Author | some people and astonish the rest
davecb@spamcop.net           |                      -- Mark Twain



-- 
David Collier-Brown,         | Always do right. This will gratify
System Programmer and Author | some people and astonish the rest
davecb@spamcop.net           |                      -- Mark Twain

Re: [Bloat] ipspace.net: "QUEUING MECHANISMS IN MODERN SWITCHES", > (Jonathan Morton)

[Neil Davies]

On 28 May 2014, at 18:29, David Collier-Brown <davec-b@rogers.com> wrote:

> On 05/28/2014 11:33 AM, Jonathan Morton <chromatix99@gmail.com> wrote
>> It's a mathematical truth for any topology that you can reduce to a black box with one or more inputs and one output, which you call a "queue" and which *does
> not discard* packets.  Non-discarding queues don't exist in the real
> world, of course.
>> 
>> The intuitive proof is that every time you promote a packet to be transmitted earlier, you must demote one to be transmitted later.  A non-FIFO queue tends to increase the maximum delay and decrease the minimum delay, but the average delay will remain constant.
> 
> A niggle: people working in queuing theory* make the simplifying
> assumption that queues don't drop. When describing the real world, they
> talk of "defections", the scenario where a human arrives at the tail of
> the queue and "defects", either to another queue or to the exit door of
> the store!

There is another mathematical approach that we've found very useful, actually the original work goes back to the 1950's (M/M/1/K/K). 

As mentioned in a reply just now in a different thread, it does give some interesting insights into the underlying two-degrees of freedom that are present in every finite queue. 


> As you might guess, what I find intuitive the IP world finds wrong, and
> vice versa.
> 
> --dave
> [* as opposed, perhaps, to queuing networks (:-)]
> -- 
> David Collier-Brown,         | Always do right. This will gratify
> System Programmer and Author | some people and astonish the rest
> davecb@spamcop.net           |                      -- Mark Twain
> 
> 
> 
> -- 
> David Collier-Brown,         | Always do right. This will gratify
> System Programmer and Author | some people and astonish the rest
> davecb@spamcop.net           |                      -- Mark Twain
> _______________________________________________
> Bloat mailing list
> Bloat@lists.bufferbloat.net
> https://lists.bufferbloat.net/listinfo/bloat

Re: [Bloat] ipspace.net: "QUEUING MECHANISMS IN MODERN SWITCHES", > (Jonathan Morton)

[Jonathan Morton]

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On 28 May 2014 20:31, "David Collier-Brown" <davec-b@rogers.com> wrote:
>
> On 05/28/2014 11:33 AM, Jonathan Morton <chromatix99@gmail.com> wrote
> > It's a mathematical truth for any topology that you can reduce to a
black box with one or more inputs and one output, which you call a "queue"
and which *does
> not discard* packets.  Non-discarding queues don't exist in the real
> world, of course.
> >
> > The intuitive proof is that every time you promote a packet to be
transmitted earlier, you must demote one to be transmitted later.  A
non-FIFO queue tends to increase the maximum delay and decrease the minimum
delay, but the average delay will remain constant.
>
> A niggle: people working in queuing theory* make the simplifying
> assumption that queues don't drop. When describing the real world, they
> talk of "defections", the scenario where a human arrives at the tail of
> the queue and "defects", either to another queue or to the exit door of
> the store!

I think my description of the black box is still valid: a "defection" must
imply a second output from the box, otherwise it will appear as either a
reordering (preserving the property) or a discard.

- Jonathan Morton

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Re: [Bloat] ipspace.net: "QUEUING MECHANISMS IN MODERN SWITCHES", > (Jonathan Morton)

[David Collier-Brown]

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On 05/29/2014 10:09 AM, Jonathan Morton wrote:
>
> On 28 May 2014 20:31, "David Collier-Brown" <davec-b@rogers.com
> <mailto:davec-b@rogers.com>> wrote:
> > A niggle: people working in queuing theory* make the simplifying
> > assumption that queues don't drop. When describing the real world, they
> > talk of "defections", the scenario where a human arrives at the tail of
> > the queue and "defects", either to another queue or to the exit door of
> > the store!
>
> I think my description of the black box is still valid: a "defection"
> must imply a second output from the box, otherwise it will appear as
> either a reordering (preserving the property) or a discard.
>
> - Jonathan Morton
>
Yes, I entirely agree: your black box is slightly larger than the usual
queuing box... which make your box think  "outside the box" (;-))

--dave

-- 
David Collier-Brown,         | Always do right. This will gratify
System Programmer and Author | some people and astonish the rest
davecb@spamcop.net           |                      -- Mark Twain


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