You pick up your iPhone while waiting in line at a coffee shop. You google a not-so-famous actor, get linked to a Wikipedia entry listing his recent movies and popular YouTube clips of several of them. You check out user reviews on Amazon and pick one, download that movie on BitTorrent or stream that in Netflix. But suddenly the WiFi logo on your phone is gone and you're on 3G. Video quality starts to degrade, but you don't know if it's the server getting crowded or the Internet is congested somewhere. In any case, it costs you $10 per Gigabyte, and you decide to stop watching the movie, and instead multitask between sending tweets and calling your friend on Skype, while songs stream from iCloud to your phone. You're happy with the call quality, but get a little irritated when you see there're no new followers on Twitter. You may wonder how they all kind of work, and why sometimes they don't. Take a look at the list of 20 questions below. Each question is selected not just for its relevance to our daily lives, but also for the core concepts in the field of networking illustrated by its answers. This course is about formulating and answering these 20 questions.
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Networks: Friends, Money, and Bytes
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Is It Fair that My Neighbor’s iPad Downloads Faster?
In this final lecture of the course, we will study a subject that we touched upon many times previously and forms an essential part of both social choice theory and technology network design: quantifying fairness of resource allocation.
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Mung ChiangProfessor
Electrical Engineering
Christopher BrintonLecturer
Electrical Engineering
Welcome to Lecture 20 of Networks, Friends, Money, and Bytes.
And this is the last lecture of the course.
We'll get to talk about something that we touched upon a few times in the course
but did not have the time to go through more detail.
And that is on the subject of fairness. Now, fairness is an important metric in
both social, economic kind of network, as well as
technological networks. So now, that we're into the very last
lecture of the course, we're going to spend an hour or so to
look at quantification of this notion of a fairness.
I just want to highlight that while we talk about fairness today, we'll talk
about fairness of resource allocation or performance path allocation.
Were not talking here about the fairness of liberty or rights,
okay?. the discussion around those issues will
be quite different. Alright.
What is a fair? Now this question had been studied so
many times in some many different subjects.
Let's start out with some basic thinking around this question.
First of all, is equality a performance and resource allocation fairness?
Now, of course, equality will be an interesting and intuitive way to quantify
fairness. But, this view of equality means fairness
is very problematic. For example, suppose I have to choose
between an allocation of one megabit per second or one megabit per second between
two iPad users versus one between 100 and 101 megabit per second between two users.
And which one would you prefer, which one would you think is more fair?
Clearly, this one is equal allocation. This one is actually a slight deviation
from equal allocation. If I plot this on a chart between the two
user's rate, one would be here,
another would be 45 degree line. Another would be like just a little above
it. Even though this one does not lie on the
45 degree line, almost all people will still prefer this,
and think that this is a better point, okay?
So clearly, when we say that a more fair point is actually a worse point that
doesn't give the word fair a very useful interpretation.
So if we say this one is more fair than this, that means we somehow have taken
into account some notion of efficiency, okay? We'll come back to this later in
today's lecture. And this also says that magnitude matters
and it is the absolute allocation, not just the relative proportion that makes
the notion of fairness useful. And this underlines something called the
difference principle in Rawl's theory of justice.
In a vast material part of the lecture, we'll be briefly going through this.
One of the most influential thinking in political philosophy in the last 20th
century on the subject of distributive justice or fairness of liberty and also
resource distribution. Now, of course, a tougher choice would
have been choosing between one, one and one, two let's say,
okay? But then, maybe magnitude matters,
okay? In fact, there's a interesting experiment that highlights why the
efficiency would matter and this would have been a tougher choice between these
two. And that's called the ultimatum the
ultimatum gain. Basically, there are two players in this
game, okay? And one player will be able to
issue the following. Since that here's a dollar, okay?
It just drops from the sky on our laps. And I can decide how to allocate this
dollar between the two of us. And I'll keep x, and then u, then have 1
- x. Now, this may remind you of the framework
of negotiation based on bargaining theory by Rubinstein back in lecture five,
okay? But in this case the choice is actually much sharper.
If I propose x and you get 1 - x, then you can only have two choices.
A, you accept it as is, or B, you reject it in which case the
game is over, okay? There's no further iterative
rounds. You will get nothing and I will get
nothing. So this was an experiment that carried
out in many different cultures and the demographics to see what x would people
pick, okay? And what x would the other party accept.
And it turns out that out of the many interesting implication, one is that the
magnitude matters. Whether this is dividing $1 or $1 billion
makes significant difference, okay? Alright. So, this is the first thinking
aloud of this fairness function, okay? Is equality fairness and probably
not. And if not, how do we understand the
impact of magnitude? How do we understand the tradeoff between
everybody much wealthier, except that some are more wealthy then others, versus
everybody start to, starve to death. and that will be very equal as people all
starve to death. Now, another way to think about this is
to think about the different contributions as well as different needs
by different people in this network, okay?
Whether it's a social economic network or a technological network.
For example, if this ISP glues together the rest of the internet,
okay? Maybe this is a very important node, a
very important link. Its contribution is significant,
okay? We may or may not appreciate how much
more contribution it is, but by some centrality measure for example, this
might be ten times more important than the others.
So, maybe this IP, ISP deserve higher allocation of resources.
And some people have more needs than others too,
okay? So, how do we think about that issue?
Shall we give it to the more capable contributing users?
Shall we give it to more needy users? And a good way to think about this is in
a course, okay?
Suppose in this course, I'm going to assign the grade,
and that is a B grade to all students. It doesn't matter who you are, what you
did, what you did not do, B grade for everyone,
okay? So, rather than differentiating into say two students get A+, and then 98
students would then get somewhere between A and D,
okay? Including quite a bunch in D, and maybe even a few getting an F grade.
I'll say, well, let's just be kind and generous.
I will just give B grade to everyone. First of all, this is problematic because
it may not be providing the right incentives to the students to work hard.
And second, this is hardly a fair allocation of grades, as most people
agree, even those who are getting a bump from D to B, they will think that hey, I
just got an unfair B grade. It somehow does not reflect that grades
should be different, depending on the performance.
I shouldn't have asked A+ students to subsidize my grades.
And third, this problem assumes that I can assign as
many B grades as I want. But often in a system, in a network, you
have a total resource constraints. There's only that many B grades you can
assign with, while leaving those constraints.
Alright. So, while we appreciate that different
people should receive different amount of performance and resources,
we also think that suppose there are some lazier workers.
Less capable workers, less underperforming workers, okay?
The fact that they are lazier and underperforming doesn't mean that they
should just be left to starve to death. And that their families should starve to
death. At the same time, there are few people
will think that these people should deserve the exact same pay, or even
anything remotely to similar pay as the others,
okay? So, if I play basketball I probably
should be getting the same pay as Michael Jordan, even though Michael Jordan, some
people may view as getting a ridiculous, unreasonable, unjustified high pay. But,
who would say? Well, he's that much better than many of us.
So now the question is, most people would agree that, just because they don't
perform as much as the others, they shouldn't be left to death.
At the same time, they shouldn't be getting the same as everyone else.
Now the question is, how much then should they get?
Most people say, well, they should get at least the basics.
For example, a very basic amount of, you know, food and shelter.
Now, how much is basic amount of food and shelter?
Is an annual vacation to the Bahamas part of a basic package?
That means, we have to somehow quantify. Because arguing with English usually lead
to nothing as we do not even agree on the scale of measurement.
So, how can we quantify the notion of fairness?
Now, there are quite a few different dimensions.
For example, procedural fairness. Too often the word fairness has been used
as a disguise to take away rights and liberty.
And too often the output what is fair enough viewed by somebody comes from a
procedure that is not, okay?
So, in legal study in a place called science, people study procedural fairness
a lot. Now we, unfortunately, will not have time
to talk about that. Even though that, I guess, is way more
important than whatever we will be talking about,
okay? then there are studies in a lot of fields
looking at the different metrics, okay?
To measure a given vector. I'll give you a vector of allocation,
okay? For example, one, one, okay? Or one, two, three between three people or
one, 10, 100 between three people, okay? And I asked you to give me some
metrics to measure it. And this has been done in quite a few
different ways. And we will be taking this view point,
okay? And c is that, as when mentioned, there's
also the notion of bargaining, alright? And the fairness in bargaining,
okay? In d, there are viewpoints on a certain kind of gains like the ultimatum
gain I mentioned like the divide and dollar gain,
okay? Like extensions of a fair cake cutting.
We'll mention some of these in homework problem but we won't be going through
this, in detail during the lecture, okay? So today, we're just going to focus
on particular angle. And that is, if you give me a vector, I would like turn that
into a scalar. Now, there's no shortage of metrics to do
exactly that. In fact if you look at different existing
notions, some will say this vector will give me a scalar of 0.33.
Some say it will give me a vector of 0.86.
This will turn into 0.01 or 0.41. These are not made of numbers.
They are actually what different matrix will map these two vectors into,
okay? So, even efficiency aside, this one
clearly gives more absolute value to everyone.
even ignoring that fact, just look at the relative representation of the three
users here. People probably agree that this is more
fair than this if you ignore efficiency. But how much more fair, than this?
And therefore, is it worth the sacrifice in efficiency?
That is clearly not understood. If I just compare this versus this,
it's 33 times more fair. But if I compare this with this, then
it's only twice more fair. And indeed, that there are many such
so-called diversity indices in statistics, in sociology in computer
science. For example, if I look at a ratio between
the smallest and largest entry that is one case, okay?
The ratio between the smallest and largest to spread. Or I can look at a
little bit more sophisticated like entropy function.
This used to measure the amount of uncertainty but also can be used to
measure the amount of variance. Well, speaking of variance, how about the
standard deviation, or may be the second order moment of this vector,
okay? Normalized.
And that's called the Jain's Index in certain networking literature,
okay? A lot of these try to normalize this so
that you going to map whatever vector into a number in the range between zero
and one, where one is most fair, okay?
Now, we have seen another related but subtly different approach where we
provide an objective functions and optimization problem, and then try to
understand what is the resulting optimizer's property.
For example, alpha fair utility function, okay? Utility parameterized by the
parameter alpha between zero and infinity says that I'm going to look at [SOUND]
one minus alpha. This function, if alpha is not one and by
L'Hopital's Rule this function if alpha is one,
okay? If I maximize a summation of these utility functions, I got a utility
function for the entire vector. And the resulting maximizer, x star,
satisfy the so called alpha fair notion. Including proportional fair that we
talked about, one alpha is one. Including max/min fair
that says, you cannot make one user's rate higher or resource higher without
making someone whose already getting a smaller amount of resource get even
smaller resource, okay? And that's called the max/min fair
which happens as alpha becomes very large, close to infinity,
okay? These two are special cases. As you can see already, these two
different approaches, a normalized diversity index and an alpha fair
objective function are actually different,
right? For example, the treatment of efficiency
is different. For alpha fair utility maximization,
efficiency is implicit and bad in it. But still in there and normalize the
index is not effected by the magnitude often,
okay? So, one, one the vector is the same as
100, 100th vector. Can these two approaches be unified?
And can more approaches be discovered? In fact, how many more approaches are
there? Well, let's try something that we saw
back in Lecture 6 when we talked about the axiomatic construction of a voting
theory and of bargaining theory. Back then, we looked at the axiomatics a
treatment of errors impossibility theorem, as well as
the axiomatic treatment of bargaining by Nash, in the advance material part of the
lecture, okay? So, in one case, a set of axioms
that led to impossibility result and another case led to a unique possibility
result. So, what kind of axioms are we talking
about here? We will see that in the advance material
of this lecture, the axiom of continuity. Very briefly it just says that, the
function that maps, a given vector of allocation in to some scalar.
The fairness value, okay?
Should be a continuous function, okay?
Of so called homogeneity that says, if I scale this x by say a factor of five,
five pi vector x, it should give the same as if I was
looking at x. In fact, doesn't matter if it was a five
or any positive constant. So, scale does not matter.
Another way to call that kind of function is called homogeneous function,
okay? So, inefficiency is automatically taken
out of consideration for the moment. And then, of saturation.
This is a tenical axiom where we will skip this on to the advanced material of
partition that says, you can grow the population size and the notion of
fairness still remaining well defined. And finally, of starvation that says,
the allocation of half, half between two users equal allocation should be no less
fair than the allocation of zero, one, which stops one of the two users,
okay? Notice this is not saying that you put allocations should by axiom be viewed
as most fair. It simply says, it is not less fair than
starvation, okay? So, it'ss a very weak statement,
therefore a very strong axiom. It turns out that if you believe these
five axioms, then skipping to the relation we will be able to derive a
unique family of functions F. And that's a vector of allocation to a
scaler representing the fairness value. And this scalar will allow us to do two
things. A, allow us to do quantitative
comparison, okay?
Between two vectors, which is more fair. And the second is scale.
Not only gives you a order comparison, but also provides a numerical scale to
talk about how much fair is one allocation with respect to the other.
So, it is our job now to go through that unique family of fairness function
constructed based just on these axioms of the function.
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