[Make-wifi-fast] [Cake] [Bloat] [Cerowrt-devel] Little's Law mea culpa, but not invalidating my main point
David P. Reed
dpreed at deepplum.com
Mon Sep 20 17:44:07 EDT 2021
The top posting may be confusing, but "the example" here is the example of the > 100 TCP destinations and dozens of DNS queries that are needed (unless cached) to display the front page of CNN today.
On Monday, September 20, 2021 5:30pm, "David P. Reed" <dpreed at deepplum.com> said:
I use the example all the time, but not for interviewing. What's sad is that the answers seem to be quoting from some set of textbooks or popular explanations of the Internet that really have got it all wrong, but which many professionals seem to believe is true.
The same phenomenon appears in the various subfields of the design of radio communications at the physical and front end electronics level. The examples of mental models that are truly broken that are repeated by "experts" are truly incredible, and cover all fields. Two or three:
1. why do the AM commercial broadcast band (540-1600 kHz) signals you receive in your home travel farther than VHF band TV signals and UHF band TV signals? How does this explanation relate to the fact that we can see stars a million light-years away using receivers that respond to 500 Terahertz radio (visible light antennas)?
2. What is the "aperture" of an antenna system? Does it depend on frequency of the radiation? How does this relate to the idea of the size of an RF photon, and the mass of an RF photon? How big must a cellphone be to contain the antenna needed to receive and transmit signals in the 3G phone frequencies?
3. We can digitize the entire FM broadcast frequency band into a sequence of 14-bit digital samples at the Nyquist sampling rate of about 40 Mega-samples per second, which covers the 20 Mhz bandwidth of the FM band. Does this allow a receiver to use a digital receiver to tune into any FM station that can be received with an "analog FM radio" using the same antenna? Why or why not?
I'm sure Dick Roy understands all three of these questions, and what is going on. But I'm equally sure that the designers of WiFi radios or broadcast radios or even the base stations of cellular data systems include few who understand.
And literally no one at the FCC or CTIA understand how to answer these questions. But the problem is that they are *confident* that they know the answers, and that they are right.
The same is true about the packet layers and routing layers of the Internet. Very few engineers, much less lay people realize that what they have been told by "experts" is like how Einstein explained how radio works to a teenaged kid:
"Imagine a cat whose tail is in New York and his head is in Los Angeles. If you pinch his tail in NY, he howls in Los Angeles. Except there is no cat."
Though others have missed it, Einstein was not making a joke. The non-cat is the laws of quantum electrodynamics (or classically, the laws of Maxwell's Equations). The "cat" would be all the stories people talk about how radio works - beams of energy (or puffs of energy), modulated by some analog waveform, bouncing off of hard materials, going through less dense materials, "hugging the ground", "far field" and "near field" effects, etc.
Einstein's point was that there is no cat - that is, all the metaphors and models aren't accurate or equivalent to how radio actually works. But the underlying physical phenomenon supporting radio is real, and scientists do understand it pretty deeply.
Same with how packet networks work. There are no "streams" that behave like water in pipes, the connection you have to a shared network has no "speed" in megabits per second built in to it, A "website" isn't coming from one place in the world, and bits don't have inherent meaning.
There is NO CAT (not even a metaphorical one that behaves like the Internet actually works).
But in the case of the Internet, unlike radio communications, there is no deep mystery that requires new discoveries to understand it, because it's been built by humans. We don't need metaphors like "streams of water" or "sites in a place". We do it a disservice by making up these metaphors, which are only apt in a narrow context.
For example, congestion in a shared network is just unnecessary queuing delay caused by multiplexing the capacity of a particular link among different users. It can be cured by slowing down all the different packet sources in some more or less fair way. The simplest approach is just to discard from the queue excess packets that make that queue longer than can fit through the link Then there can't be any congestion. However, telling the sources to slow down somehow would be an improvement, hopefully before any discards are needed.
There is no "back pressure", because there is no "pressure" at all in a packet network. There are just queues and links that empty queues of packets at a certain rate. Thinking about back pressure comes from thinking about sessions and pipes. But 90% of the Internet has no sessions and no pipes. Just as there is "no cat" in real radio systems.
On Monday, September 20, 2021 12:09am, "David Lang" <david at lang.hm> said:
> On Mon, 20 Sep 2021, Valdis Klētnieks wrote:
> > On Sun, 19 Sep 2021 18:21:56 -0700, Dave Taht said:
> >> what actually happens during a web page load,
> > I'm pretty sure that nobody actually understands that anymore, in any
> > more than handwaving levels.
> This is my favorite interview question, it's amazing and saddning at the answers
> that I get, even from supposedly senior security and networking people.
> David Lang_______________________________________________
> Bloat mailing list
> Bloat at lists.bufferbloat.net
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