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.
P2P and Network Scaling
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From the course by Princeton University
Networks: Friends, Money, and Bytes
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From the lesson
How can Skype and BitTorrent be free?
The amount of content on the Internet continues to grow at a rapid pace. One of the ways that content distribution at such massive scale is made possible is through peer to peer (P2P) protocols. In this lecture, we will study P2P applications like Skype and BitTorrent. In doing so, we will see how peers in a network can share the workload of distributing content throughout a network.
Meet the Instructors
Mung ChiangProfessor
Electrical Engineering
Christopher BrintonLecturer
Electrical Engineering
Welcome to lecture fifteen of Networks, Friends, Money and Bytes.
And today we would talk about a peer-to-peer networks, such as Skype for
voice communication and BitTorrent for file sharing and video streaming.
And this starts a sequence of several lectures on the challenges faced in
technological networks. One big challenge is the massive amount of
content distribution. The content could be file,
Could be video, And we'll look at peer-to-peer and cloud
or data center technologies as solutions for this problem.
And another problem is a prevalent adoption of mobile wireless technologies
of various kinds, And we would be talking about this in
lecture eighteen. So, today, we'll focus on peer-to-peer for
massive amount of content distribution and we will look at two themes. One is the
theme on overlay networks. We will see the power of building overlay
networks based on basic connectivity services offered by the internet, by IP
networks, providing simple, stable, ubiquitous, and economical connectivity.
The second theme today is how to scale up a system?
We'll look at a different kind of scaling up method in the next lecture for cloud
and content, for cloud and the data center.
And today we'll look at how a peer-to-peer provide an intrinsically scalable
solution. Now P2P is a technology that's been
developing for more than fourteen years. Back in the late 90s, around 98 to 99,
Kazaa and Gnutella were provided and they became very, very popular around 90, 1999
to 2001. But they also face a few problems.
One of which is copyright issues, cuz a lot of content on these P2P networks
violated copyrights. Another is the issue of free riders. A lot
of users do not contribute the content or bandwidth and yet they consume from
others. These are the, if you will, first
generation of P2P networks on top of Internet.
And then, in the same year, 2001, two major companies came to existence.
One is Skype. Okay? It started from Kazaa, was equired
by eBay in 2006, for 2.6 billion and then it was, again, acquired by Microsoft a few
years later in 2011 for, this time, over $eight billion And today, there are over
700 million users worldwide using Skype, And it's reported that on average, every
single day, there's over 700 million minutes of Skype calls.
Now, another major company that took place in the same year is BitTorrent.
Okay? It is also a P2P technology, but this time
it is for content sharing and, it turns out that half of the Internet traffic in
the midnight 2000, Around 2005 to 2007,
Was estimated that nearly half of the all the Internet traffic were using BitTorrent
protocol, so there was a huge success of this technology.
Now, there are some major differences between Skype and BitTorrent.
Both are free, okay? They are free services, unless you need
to, need to, need to use, Skype to call a normal phone, landline phones or you want
to use a certain kind of conference call facilities.
Otherwise, Skype is free, and BitTorrent is free to install, and furthermore, the
content on BitTorrent is also free. So how could they be free?
Well, Skype uses P2P for signaling. Now, including locating each other, including
establishing connections. Okay. So this is a so-called control plane
use of P2P technology. In contrast, BitTorrent uses P2P for
actual content sharing in the data plane. Later in lecture nineteen, we'll talk more
about control versus data plane. Roughly speaking, you can think of data
plane as where, the place where the actual transmission of data takes places,
Whereas, control plane is where the control signals take place.
For example, in Netflix, when you go online to put some movies in your queue,
that is the control plane, And it carries out itself over the
Internet. And the actual DVD in rental services
arrives by, say, US Mail service. That's a completely different network,
then the Internet, And that is the data point, that's where
the actual movie arrives. Okay.
So BitTorrent actually uses Internet, P2P, for content distribution, including file
sharing, including video sharing. Okay? To share chunks of files and we will come to
more details in a minute. In fact, Skype's actual operation remains
a commercial secret. There were a few papers trying to reverse
engineer Skype operations in the late 2000s.
But the BitTorrent was founded by graduate student, and there were quite a few papers
written by the founding team about the BitTorrent operation.
So we'd know quite a bit more about the BitTorrent and we will say quite a bit
more about it in this lecture. Before we go there, I want to highlight
that P2P really underlines the tremendous potential of positive network effect.
Going all the way back to lecture seven, for example, when we were examining the
popularity of a new product and we were looking at the impact of, knowing there
are many other users of the same technology and how they influence your
decision to acquire the technology or product or service.
Now there is the famous Metcalfe's Law. Okay? Named after the inventor of the
Ethernet that connects networks, computers in a local area network.
And, Metcalfe's law has, quite a few different versions and misinterpretation,
But, popular version says that the benefits of joining a network grows like
the square of, say n, Which is the number of nodes in the
network right now. Okay?
So the benefit grows like N^2 order. And the underlying assumption is that
every link, okay, are equally important, Which is a problematic assumption, because
we know some links are much more important than others.
And, furthermore, if I have, five nodes, Pretty much every node talk to every other
node. So it forms a full mesh network,
And therefore, if you look at the number of connections in the network with n
nodes, it is N^2, or, you know, N N - one, which is in the order of N^2.
And more generally speaking, we need the number of lengths coming from a node to
grow at least linear order of n in order to enjoy this n^2 benefits of Metcalfe's
law. And this has been invoked in so many
studies in social sciences and as well as engineering.
But, do you really need the number of links per node to grow as a linear
function of N, not quite. In fact, in P2P law, we basically will see
a refinement of this Metcalfe's law. We'll see that even maintaining a number
of neighbors that is a constant can work as long as the peering relationships are
carefully designed. In the vast material part, we'll see a
really careful design through optimization, but even without
optimization, you'll see that reasonably smart peering relationship can provide
this P2P law. Okay? So, to enjoy the possible network
effect, you do not have to have a very dense network of connectivity.
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