Cautious Growth of Window Size

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Networks Illustrated: Principles without Calculus

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Princeton University

Networks Illustrated: Principles without Calculus

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What makes WiFi faster at home than at a coffee shop? How does Google order its search results from the trillions of webpages on the Internet? Why does Verizon charge $15 for every GB of data we use? Is it really true that we are connected in six social steps or less? These are just a few of the many intriguing questions we can ask about the social and technical networks that form integral parts of our daily lives. This course is about exploring the answers, using a language that anyone can understand. We will focus on fundamental principles like “sharing is hard”, “crowds are wise”, and “network of networks” that have guided the design and sustainability of today’s networks, and summarize the theories behind everything from the social connections we make on platforms like Facebook to the technology upon which these websites run. Unlike other networking courses, the mathematics included here are no more complicated than adding and multiplying numbers. While mathematical details are necessary to fully specify the algorithms and systems we investigate, they are not required to understand the main ideas. We use illustrations, analogies, and anecdotes about networks as pedagogical tools in lieu of detailed equations. All the features of this course are available for free. It does not offer a certificate upon completion.

Из урока

Controlling Congestion in the Internet

The Internet has many important tasks to manage, like routing packets (discussed in the last module) and controlling congestion. This workload is modularized into different functional layers, each responsible for performing a different set of functions, as we will see in this lesson. We will also look at the principles of congestion control, managed at the transport layer.

Divide And Conquer2:10

Layered Protocol Stack4:13

Transport & Network Layers4:42

Processing Layers2:17

Controlling Congestion4:03

Traffic Jam & Bucket Analogy3:02

Sliding Window2:36

Cautious Growth of Window Size6:51

Inferring Congestion5:46

Congestion Control Versions2:16

Loss-Based Congestion Inference5:19

Delay-Based Congestion Inference: Part I6:43

Delay-Based Congestion Inference: Part II5:33

Delay-Based Congestion Inference: Part III4:51

Distributed Congestion Control5:14

Познакомьтесь с преподавателями

Christopher Brinton
Lecturer
Electrical Engineering
Mung Chiang
Professor
Electrical Engineering

But the window doesn't stay constant, right.

The, the window changes in size depending upon

the congestion conditions that the end host is infering.

so the CWND should grow for instance if there's no congestion we should be

able to increase it and that helps

us to more efficiently utilize the network, right.

So if there is Not, congestion right now, and everything's

working very well, then why not send more if we can.

And TCP advocates a linear increase

in the congestion window specifically increments by one each time.

So the idea is that if you send out three packets as

the sender, it goes through the receiver, the receiver gets all three, right.

time two over here, we're saying the receiver's getting all

three and sending the acknowledgements back, then at time three.

We receive all three of these acknowledgements in order, this would be

for this packet corresponds to this one, this packet would correspond to this

one, this packet would correspond to this one.

Within reason there's reordering and things that have

to go on but that's the main idea and

then we can increase the congestion window by

one right because we got all the packets back.

so let's increase it and send a little more.

Let's try to stretch the, how much we can send each time.

So, we increase it by one if all

the outstanding packets are received or equivalently understand

that we can increase it by one over

the current window size for each packet that comes

back and that would have the same effect. So, we do CWND

and then we add one over CWND and then there's CWND

packets, so it's really just adding one. This is just CWND, plus1 .

So for instance, if we have three outstanding packets, Right?

so we sent 3 right here.

then, For every one that comes back,

we would take 3, and we would add 1 3rd and then 1 3rd for the second one.

And then 1 3rd for the third one then this all becomes

one so then that just becomes 4 and then, if it was 5.

For instance, we take 5 plus 1 5th plus 1 5th plus 1 5th

plus 1 5th plus1 5th, then that becomes one, and that goes up to five.

So that's how you can implement it packet by packet and of course

you wouldn't see the effect of it until you got,

sorry this is 6 not 5, 5 plus 1 is 6.

[LAUGH]

So, but you wouldn't see the effect of this of course until you got

all the packets back, right, because you can't send just a fraction of a packet.

You can't send 1 5th of a packet because

they all have header and control information as well.

And note that this is happening in addition to sliding.

right, so.

We're sliding the window, right, and we are doing this addition, right.

So if we have three packets currently out right

now, then we would once we receive one acknowledgement

back we slide the window and we can already send another packet.

Then when we receive another acknowledgement

back we can send the next packet.

We receive another acknowledgement we can send the next packet.

Then the window goes up and that's at the point

that we send the fourth packet once the window goes up.

So then we have four outstanding packets but

it's, it's happening all at the same time.

So the window is sliding as we've said before.

The window slides each time we receive an acknowledgement.

And then you send more packets out, and then the

window size also will increase, if all the acknowledgements come back.

So, we can also visualize this in a

space-time diagram which is an interesting concept here.

This is that space is across between these two lines.

which denotes the network, space denotes the network.

And time is coming down, so this is what's happening over time.

Right, so initially supposed CWND is one.

So we send the data and we get an acknowledgement back.

That acknowledgement increases the window size to

two when we receive that acknowledgement we

send two packets out instead right away because now the windows has this too.

Now this packet comes back, right, and it gets increased

by 1 2, okay, then a, as soon as this

packet comes back we can already send the next packet out.

Then once this packet comes back we can send

the next one out because it is increased by a,

another, but it is also increased by another half which

means that we can now send one more packet out.

Right, and then all these three similarly, once this first packet

gets back, that's 1 3rd, so that would cause one more

packet to come out, when this packet gets back that's another

1 3rd which would cause one more to come out as well.

Because the

window slides, but then once the third

packet comes back The window slides, but then

also increments by one so we, then we have four packets out once cwnd is four.

so that's the idea.

Now if there's, there is congestion, right, so if we detect

that there is congestion, we have to reduce the window size right.

So we can't just Keep increasing it like this

or else we're going to come, go into something like

an arms race where we're all just trying to send as much as we can, at one time.

TCP advocates

a multiplicative decrease of the window size.

Okay, so we want to decrease it for instance by

half, right, so we want to half the window size.

kind of like this weighed binary exponential back

off worked, with when we talked about the CSMA.

So the cautious growth aspects of this window size is

that we want to additively increase it and multiplicatively decrease it, right.

And it's this is

obviously cautious if it was the other way around, it would be much more aggressive,

or we would have multiplicative increase an

additive decrease which wouldn't work well at all.

This is generally accepted as the better way to go, the more cautious size.

And we can draw another analogy to DVD rentals here, not Netflix specifically.

It come be some rental company does this, but so they increases your

allowance slowly as they return and they build more of a trust in you.

Or kind of like your credit card allowance,

how much you're allowed over time if you keep paying your bills

they'll increase the amount that you're allowed to spend on each credit card.

And then if you don't pay your bill on time they might reduce the amount

or charge you more money but, your DVD rentals as we talked about DVD before.

If you're currently, currently you're allowed five videos out and let's say you

return all of them and let's say now you can take six out, right.

So, then if you were to turn all of those six

eventually they would say okay well now you can turn seven out.

But along the way every time you return a movie

you can still take another one out but it's not

until when you return what the current window size is

of amount of movies that you can take only one out.

So, once you return the second, then you get a second one out.

If the window size is currently three then once you return the third one,

you can take a third one out and you can take a fourth one out.

Now, suppose, for instance, that you didn't return all of them, right?

So now,

suppose you currently had a window size of four, right?

So you had, four movies out and then you only

returned two of them and you never returned the other two.

Well, then they might say, okay, well, we're going to cut your allowance down

to two now as a penalty, 'cuz you never gave us those two back.

And then you can only have two out of negative at a time So you

return these two the window slides, right,and

you can have now two packets out only.

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