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[MUSIC PLAYING]

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SPEAKER: You type an address into
a browser, you send an email,

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you perhaps have a video
conference or a chat online.

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Have you ever stopped
to consider what exactly

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is going on underneath the hood, so
to speak, of those pieces of software?

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And really the entire
infrastructure that somehow

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connects you to the person or persons
with whom you're communicating.

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Well it turns out that there's
a whole stack, so to speak,

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of internet technologies that underline
the software that you and I use

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these days, every day.

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And indeed, the software
that we use, browsers

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and email clients and the
like, are really abstractions,

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very user friendly abstractions, on
top of some lower level implementation

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details.

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And these days, too, have we built
abstractions even above those so known

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as the cloud, an abstraction on top
of this underlying infrastructure

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that enables us to do most anything
we want computationally without even

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having that hardware locally.

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So let's see if we can't distill what
goes on when you do type an address

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or a URL into the address bar
of a browser and then hit Enter.

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Or you type out an email--

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specify someone's email
address and then hit Enter.

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What exactly is going
on underneath the hood?

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Well, at the end of the day, I dare say
that what your laptop, and my laptop,

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and our desktops, and even our
servers are capable of really

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is just sending messages in envelopes
back and forth across the internet.

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Virtual envelopes, if you will.

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Now in our human world, an envelope
needs a few things on the outside.

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If you want to send a letter or a card
or something old school to someone,

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you need to address it, of course.

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And you need to put, perhaps in
the middle, the recipient's name,

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and address, and other details.

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You might put in the top left hand
corner, by convention, your own name

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and or address.

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You might even put a
little memo in the bottom

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that specifies what's inside or
fragile or some other annotation.

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So this metaphor of the
physical world is actually

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pretty apt for what's going on
underneath the hood in computers.

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When you have a computer
plugged into a network

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or connected wirelessly
to a network, it really

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is just sending and receiving
envelopes, virtual envelopes,

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that at the end of the day are
just patterns of zeros and ones,

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but collectively, those zeros and ones
represent your email or the request

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that you've made of a web server,
the response you're getting back

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from that web server.

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So let's see if we can't formalize
exactly what these lower level

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primitives are, consider exactly how
they're layered on top of one another,

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because thereafter we can
build almost anything we want

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on top of this infrastructure once
we understand what those underlying

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building blocks actually are.

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So let's consider how
we actually address

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this envelope in the first place.

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After all, when I turn on my
laptop or turn on my phone

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or open up my desktop in the morning,
how does that computer or that phone

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even know what its own
address is on the internet?

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Because just as in our
human world, wherein

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you need to be uniquely
addressable in the physical world

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in order to even receive an
envelope or a card or a package,

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so do computers need to be uniquely
identifiable on the internet.

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Now for our purposes, now we
can consider the internet just

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to be an internetworked
collection of computers connected

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via wires, connected wirelessly.

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There's some kind of interconnectivity
among all of these devices

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and these days our phones and internet
of things devices and other things

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still.

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So let's just stipulate that
somehow or other there's

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a physical connection, or
even a wireless connection,

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between all of these various devices.

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So those devices all
need unique addresses,

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just like a building in the
human world needs an address.

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For instance, the computer science
building here on campus is at 33 Oxford

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Street, Cambridge,
Massachusetts, 02138, USA.

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With that precise information, can
you send us a real mail or a package

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or anything else through the
physical world in order for it

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to arrive on our doorstep?

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But what if you, instead,
wanted to send us an email

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and get it to that
building, or really me,

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wherever I am physically in the
world on my internet works device?

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You need to know my
computer's address, you

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need to know my phone's address,
or at least the mail server

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that's responsible for
receiving that message from you.

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Well, it turns out that most any
network on a campus, in a corporation,

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even at home these
days has a DHCP server.

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Stands for a Dynamic Host
Configuration Protocol,

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and that's just a fancy way of
describing a server that is constantly

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listening for new laptops, new desktops,
new phones, new other devices, to wake

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up or be turned on and to shout out
the digital equivalent of hello, world,

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what is my address?

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Because the purpose
in life of these DHCP

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servers is to answer that question.

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To say, David you're going to go
ahead and be address 1.2.3.4 today.

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Or David, you're going
to be 4.5.6.7 or 5.6.7.8.

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Any number of possibilities
can be used to represent

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uniquely my particular device.

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So DHCP servers are run by the
system administrators on a campus,

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in a company, in an
internet service provider.

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More generally, they're
run by whoever provides us

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with our internet connectivity.

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They just exist on our network.

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But these DHCP servers also
give us other information.

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After all, it's not really sufficient
just to know what my own address is.

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How do I know where anyone
else in the world is?

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Well, it turns out that
the internet is filled

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with devices called routers
whose purpose in life,

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as their name suggests, is to
route information from point A

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to point B to point C and so on.

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And those routers, similarly,
need to know these addresses

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so that they know upon receiving
some packet of information,

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some virtual envelope, in
which direction to send it off.

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So these DHCP servers also tell me not
just my address, but also the address

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of the next hop, so to speak.

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I, as a little old laptop
or phone or a desktop,

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I have no idea where 99.999 percent
of the computers in the world

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are, even higher than that perhaps.

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But I do need to know where the
next computer is on the internet,

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so that if I want to send
information that leaves this room,

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it needs to go to a router
whose purpose in life

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is to, again, route it further along.

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And generally there might be
one, two, maybe even 30 steps

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or hops in between me and my destination
for that email or virtual envelope,

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and those routers are all
configured by people who aren't me,

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system administrators beyond
this, beyond these walls

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to know how to route that data.

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So we can actually see evidence
of this that you yourself

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have had underneath your
fingertips all this time

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and you might not have
ever poked around.

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For instance, if you want
to see your own address,

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keep an eye out for a
number of this form.

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It's a number dot number dot number dot
number, and each of those place holders

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represents a specific value, either
starting at zero or ending at 255.

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In other words, each of these hashes
can be any value between 0 and 255,

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and that range 0 to 255 well
that's 256 total possible values.

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That's eight bits.

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Ergo, each of these place holders
represents 8 bits, 8 more bits, 8 more,

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8 more.

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So an IP address, by
definition, is 32 bits.

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And there it is.

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IP, an acronym you've
probably seen somewhere,

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even if you've not thought
hard about what it is,

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stands for Internet Protocol.

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Internet Protocol mandates that
every computer on the internet,

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at the risk of oversimplification, has
a unique address called an IP address.

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And those IP addresses look like this.

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If these IP addresses are composed of
32 bits, how many possible IPs are there

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and therefore how many possible
machines can we have on our internet?

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Well, 2 times 2 times 2, 2 to the 32,
so that's four billion, give or take.

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By design of IP addresses,
you can have four billion,

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give or take, possible permutations of
zeros and ones if you have 32 in total,

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and that gives you four
billion, maximally,

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computers and phones and internet
of things devices, and the like.

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Now that sounds big,
but not when each of us

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personally probably carries one IP
address in our pocket in our phone,

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maybe another on our wrist these
days, one or more computers

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in our life, not to mention all of the
other devices and servers in the world

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that need these addresses, too.

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So long story short,
this is version 4 of IP.

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It's decades old, but there's
also a newcomer on the field

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called IPv6, version 6.

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There isn't really to be a version 5.

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And IPv6 is only
finally gaining traction

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because we're running so
short on IPs that it's

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becoming a problem for campuses,
for companies, and beyond.

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But IPv6 will use 128
bits instead of 32,

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which gives us many, many, many,
many, more possibilities, bigger

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numbers than I can even pronounce.

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So that should cut it
for quite some time.

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But not every computer on the internet
needs a public IP address, only

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those envelopes, so to speak, that
need to leave my pocket, or my home,

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or my campus, or my company.

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It turns out, as a short term mechanism
to squeeze a bit more utility out

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of our 32-bit addresses,
which are still omnipresent

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and the most popular
among the versions, well

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we can actually distinguish
between public IP

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addresses that do actually go out on
the internet and private addresses.

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And indeed, if your
own IP address happens

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to start with the number 10
and then a dot or the number

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172.16 and then a dot, or the
number 162.168 and then a dot,

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and then something else, well, odds are,
your computer has a private IP address.

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And this is just a feature of the little
router that's probably in your home,

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or the bigger router on your
campus or corporate network,

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that enables you to have an IP address
that's only used within the company,

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only used within your home,
and cannot, by definition,

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be routed publicly beyond your
company, beyond your home,

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because the router will stop it.

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And so here we actually have the
beginnings of a firewalling mechanism,

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if you will.

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In the real world, a
firewall is a device

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that prevents fire from going from
one store to another, for instance.

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In the virtual world, a
firewall is a piece of software

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that prevents zeros and ones from
going from one place to another.

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And in this case do we
already have a mechanism

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via public and private addresses
of keeping some data securely,

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or with high probability
securely, within our company

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versus allowing it to
go out on the internet.

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So we'll see now some screenshots
of some actual computers from Mac OS

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and Windows alike that
reveal their IP addresses,

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and you yourself can see
this on your own machines.

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For instance, here on
Windows 10 is a screenshot

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of what your Network Preferences,
so to speak, might look like.

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And if you focus down here, it's
a bit arcane at first glance,

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but IPv4 address is 192168.1.139
when we took that screenshot.

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And indeed, it starts with
192168 which means it's private,

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and indeed, I took this screenshot
while we were within a home network,

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and so that suggests it can be used
to route among computers in that home

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but not beyond.

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You'll see, too, if we
move on to the next screen

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where you see more advanced
network properties,

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you can also see the dimension
of this default gateway, which is

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synonymous with router, default router.

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192168.1.1.

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So a default router or default
gateway is that first hop,

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so that if I want to send
an email outside of my home,

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I want to visit a web page
outside of my company,

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all I need do is hand that virtual
envelope containing that email

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or that web request off to the
machine on the local network that

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has that IP address.

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I have no idea where it's going to
go thereafter, to hops two and three

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and beyond, but that's why
we have this whole internet

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and even more routers out there.

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They, the routers, intercommunicate
and relay that data,

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hop to hop to hop, until it
finally reaches its destination.

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Now where did I get
my IPv4 address from,

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where did I get my default gateway from?

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From the DHCP server in my home,
in my company, or whatever network

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I happen to be on.

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And Mac OS is the same.

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If these screens are unfamiliar,
you might recognize this,

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under System Preferences in Mac OS.

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Here, while connected to
Harvard University's network,

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you can actually see that my
IP address was 10.254.16.242.

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00:11:53,560 --> 00:11:57,010
That number, too, starting with one
of those internal or private prefixes,

233
00:11:57,010 --> 00:11:59,350
indicative of the fact
that even within Harvard,

234
00:11:59,350 --> 00:12:03,190
where we keeping all of our Harvard
traffic internal to Harvard,

235
00:12:03,190 --> 00:12:05,328
and then not exposing that externally.

236
00:12:05,328 --> 00:12:07,870
And indeed, if we look in the
more advanced preferences here,

237
00:12:07,870 --> 00:12:13,060
we can see that the router
for my Mac was 10.254.16.1.

238
00:12:13,060 --> 00:12:16,390
Which is to say this Mac, when it's
ready to send something off campus,

239
00:12:16,390 --> 00:12:20,890
simply hands that envelope off
to this particular router here.

240
00:12:20,890 --> 00:12:25,480
And the router's job, ultimately, that
first hop, a border gateway or border

241
00:12:25,480 --> 00:12:27,400
router, literally
referring to a computer

242
00:12:27,400 --> 00:12:30,730
that physically or metaphorically
is on the edge of a campus

243
00:12:30,730 --> 00:12:35,230
or company, its purpose in
life is to simply change

244
00:12:35,230 --> 00:12:38,110
what's on that envelope
initially from the private IP

245
00:12:38,110 --> 00:12:41,800
address to one or more
public IP addresses, thereby

246
00:12:41,800 --> 00:12:43,180
maintaining this mapping.

247
00:12:43,180 --> 00:12:46,720
So this might result in everyone
else in the world thinking

248
00:12:46,720 --> 00:12:49,930
that I and you and everyone
else in my company or campus

249
00:12:49,930 --> 00:12:53,980
are all actually at the same IP
address, but that's not true.

250
00:12:53,980 --> 00:12:56,950
Each of our personal devices
has a private IP address

251
00:12:56,950 --> 00:12:59,830
and that router can actually
translate via something

252
00:12:59,830 --> 00:13:03,190
called network address translation,
or NAT, from a private address

253
00:13:03,190 --> 00:13:05,320
to public and back.

254
00:13:05,320 --> 00:13:09,400
And so in this way, too, can
a company help mask the origin

255
00:13:09,400 --> 00:13:13,000
or the identity of whoever it is that's
accessing some internet-based service.

256
00:13:13,000 --> 00:13:16,840
Of course, that same company could log
what it is that's leaving the company

257
00:13:16,840 --> 00:13:19,720
and coming back in, so via
subpoena or another mechanism,

258
00:13:19,720 --> 00:13:22,630
could someone certainly
figure out who was accessing

259
00:13:22,630 --> 00:13:25,532
that service at a particular
time, but the outside world

260
00:13:25,532 --> 00:13:26,740
would need help knowing that.

261
00:13:26,740 --> 00:13:29,600
And so here, even within Harvard,
it's done perhaps for that reason.

262
00:13:29,600 --> 00:13:32,710
But also perhaps in order
to use one public IP

263
00:13:32,710 --> 00:13:36,340
address among hundreds or
thousands of university affiliates,

264
00:13:36,340 --> 00:13:39,680
so that frankly we just don't
need as many IP addresses.

265
00:13:39,680 --> 00:13:42,100
So what might be both a
technological motivation

266
00:13:42,100 --> 00:13:46,550
can also have these policy
side effects as well.

267
00:13:46,550 --> 00:13:48,400
So IP itself.

268
00:13:48,400 --> 00:13:49,030
Protocol.

269
00:13:49,030 --> 00:13:50,488
Well, what does that actually mean?

270
00:13:50,488 --> 00:13:53,820
A protocol-- it's not a language, per
se, it's not a programming language.

271
00:13:53,820 --> 00:13:55,570
It's really just a set
of conventions that

272
00:13:55,570 --> 00:13:58,210
govern how computers intercommunicate.

273
00:13:58,210 --> 00:14:01,840
IP, specifically, says that if you
want to send a message on the internet,

274
00:14:01,840 --> 00:14:07,480
you shall write a sender address on
the envelope and a recipient address

275
00:14:07,480 --> 00:14:10,420
on the envelope, and that will
ensure that the routers know

276
00:14:10,420 --> 00:14:12,670
what to do with it, and
they'll send it back and forth

277
00:14:12,670 --> 00:14:14,140
in the appropriate directions.

278
00:14:14,140 --> 00:14:17,600
IP gives us some other features,
as well, fragmentation among them.

279
00:14:17,600 --> 00:14:20,680
It turns out for efficiency if you've
got a really big email or a really

280
00:14:20,680 --> 00:14:23,830
big file, whether a
PowerPoint file or video file,

281
00:14:23,830 --> 00:14:27,070
it's not really fair to everyone else
to kind of jam that onto the network

282
00:14:27,070 --> 00:14:30,820
and to the exclusion of other people's
data at any given point in time.

283
00:14:30,820 --> 00:14:34,960
And so IP tends to fragment
big files into smaller pieces

284
00:14:34,960 --> 00:14:37,540
and send them in multiple
envelopes that eventually

285
00:14:37,540 --> 00:14:41,210
get reassembled at the other end, so
that there is a compelling feature as

286
00:14:41,210 --> 00:14:41,710
well.

287
00:14:41,710 --> 00:14:44,710
But this leads, of course, to a
slippery slope of implications

288
00:14:44,710 --> 00:14:47,560
for net neutrality and for
companies or governments

289
00:14:47,560 --> 00:14:50,710
to actually then start to
distinguish between quality

290
00:14:50,710 --> 00:14:53,890
of service of this type of data
and this other type of data.

291
00:14:53,890 --> 00:14:54,850
Why can they do that?

292
00:14:54,850 --> 00:14:57,423
Well, it's all quantized at
a very small unit of measure,

293
00:14:57,423 --> 00:14:59,590
and within these packets
are additional information.

294
00:14:59,590 --> 00:15:04,510
Not just those addresses, but hints as
to what type of data is in the packet.

295
00:15:04,510 --> 00:15:08,320
Is it an email, is it a web page, is
it a video conference, is it Netflix,

296
00:15:08,320 --> 00:15:10,030
is it some competitor service?

297
00:15:10,030 --> 00:15:12,880
And so ISPs or companies or
governments can certainly

298
00:15:12,880 --> 00:15:18,130
distinguish among these types of packets
and treat them theoretically, and all

299
00:15:18,130 --> 00:15:20,350
to really these days, differently.

300
00:15:20,350 --> 00:15:23,230
So that they're derived simply
from these basic primitives.

301
00:15:23,230 --> 00:15:25,900
Now we can very quickly
go pretty low level.

302
00:15:25,900 --> 00:15:28,990
If you actually look back
at the formal definition

303
00:15:28,990 --> 00:15:33,580
that humans crafted decades ago for
what IP is, this is how they drew it.

304
00:15:33,580 --> 00:15:36,700
You might call this ASCII art,
to borrow a phrase from our look

305
00:15:36,700 --> 00:15:38,140
at computational thinking.

306
00:15:38,140 --> 00:15:41,140
It's sort of an artist's
rendition of some structure

307
00:15:41,140 --> 00:15:43,960
just by using the keys
on his or her keyboard.

308
00:15:43,960 --> 00:15:45,970
And so these dashes and
pluses, really, just

309
00:15:45,970 --> 00:15:48,430
are meant to draw a rectangular
picture, nothing more.

310
00:15:48,430 --> 00:15:52,700
The numbers on top represent
units of 10 bits at a time.

311
00:15:52,700 --> 00:15:57,610
Here's bit 0, here's 10, here's 20,
and over here is the 32nd such bit.

312
00:15:57,610 --> 00:16:02,140
So start at zero, and you count as
high as 31, so that's our 32nd bit.

313
00:16:02,140 --> 00:16:04,810
And we can see a few
details within here.

314
00:16:04,810 --> 00:16:08,320
We can see details like
the source address,

315
00:16:08,320 --> 00:16:12,190
and it's the whole width of this
picture indicating that indeed this

316
00:16:12,190 --> 00:16:17,210
is a 32-bit value that composes the
source address or the sender address.

317
00:16:17,210 --> 00:16:20,290
Destination address is just as
wide, so there's another 32 bits.

318
00:16:20,290 --> 00:16:22,240
There's options and other time to live.

319
00:16:22,240 --> 00:16:25,420
You can specify just how many
routers this can be handed off

320
00:16:25,420 --> 00:16:28,300
to before the router should
say, we just don't know where

321
00:16:28,300 --> 00:16:30,370
this destination is, we shall give up.

322
00:16:30,370 --> 00:16:32,710
And there's other
fields as well in here.

323
00:16:32,710 --> 00:16:34,330
Now what are we really looking at?

324
00:16:34,330 --> 00:16:36,430
This is just an artist's
rendition of what

325
00:16:36,430 --> 00:16:38,140
it means to send a pattern of bits.

326
00:16:38,140 --> 00:16:41,020
The first few bits
somehow relate to version.

327
00:16:41,020 --> 00:16:44,580
The next few bits relate to IHL and
type of service and total length.

328
00:16:44,580 --> 00:16:46,330
Eventually, the pattern
of bits represents

329
00:16:46,330 --> 00:16:48,500
source address and destination address.

330
00:16:48,500 --> 00:16:52,600
So any computer that's receiving
just a series of bits wirelessly

331
00:16:52,600 --> 00:16:57,160
or over the wire in the form of
wavelengths of light or of electricity

332
00:16:57,160 --> 00:17:01,990
on a wire, simply needs to realize,
oh, once I've received this many bits,

333
00:17:01,990 --> 00:17:04,270
I can infer that those bits
were my source address,

334
00:17:04,270 --> 00:17:06,069
those were my destination address.

335
00:17:06,069 --> 00:17:09,250
But again, this is so low level, it's
a lot more pleasant to sort of think

336
00:17:09,250 --> 00:17:11,230
about things at the virtual level.

337
00:17:11,230 --> 00:17:14,589
An envelope that just has this
information written on it, and let's

338
00:17:14,589 --> 00:17:18,640
not worry about an abstraction
level below this one, wherein

339
00:17:18,640 --> 00:17:21,250
we get into the weeds of this data.

340
00:17:21,250 --> 00:17:25,776
But it turns out that IP is not the
only protocol that drives the internet.

341
00:17:25,776 --> 00:17:28,359
In fact there's several, but
perhaps the other most common one

342
00:17:28,359 --> 00:17:30,410
that you've heard of is that one here.

343
00:17:30,410 --> 00:17:31,630
TCP.

344
00:17:31,630 --> 00:17:33,460
Transmission Control Protocol.

345
00:17:33,460 --> 00:17:36,700
Now this is just a protocol
that solves a different problem.

346
00:17:36,700 --> 00:17:40,660
Rather than simply focus on
addressing computers on the internet

347
00:17:40,660 --> 00:17:43,090
and ensuring data gets
from one point to another,

348
00:17:43,090 --> 00:17:46,630
TCP is about, among other
things, guaranteeing delivery.

349
00:17:46,630 --> 00:17:52,330
TCP adds some additional zeros and ones
to that envelope on the outside of it

350
00:17:52,330 --> 00:17:55,450
that helps us get that antelope
to its destination with much

351
00:17:55,450 --> 00:17:56,737
higher probability.

352
00:17:56,737 --> 00:17:58,570
In other words, the
internet's a busy place.

353
00:17:58,570 --> 00:18:01,870
Servers are constantly
getting new users,

354
00:18:01,870 --> 00:18:05,260
routers are receiving any number
of packets at any given time,

355
00:18:05,260 --> 00:18:07,360
and sometimes there are
spikes in connectivity.

356
00:18:07,360 --> 00:18:10,630
People might all be tuning into
some news broadcast online streaming

357
00:18:10,630 --> 00:18:13,630
lots of video, or downloading
the latest news all at once,

358
00:18:13,630 --> 00:18:15,745
or everyone's playing
the latest game online,

359
00:18:15,745 --> 00:18:17,620
and so there can be
these bursts of activity.

360
00:18:17,620 --> 00:18:22,690
And honestly humans don't necessarily
engineer with those bursts of activity

361
00:18:22,690 --> 00:18:26,680
in mind, and so routers get
busy, computers get busy.

362
00:18:26,680 --> 00:18:29,770
And when they get busy, they might
receive an envelope of information

363
00:18:29,770 --> 00:18:32,395
and realize, wait a minute, I
don't have enough hands for this,

364
00:18:32,395 --> 00:18:35,200
and packets get dropped, so to speak.

365
00:18:35,200 --> 00:18:39,070
In fact that's a term of ours, to
drop a packet just means to ignore it.

366
00:18:39,070 --> 00:18:42,490
You don't have enough memory,
enough RAM inside of your system

367
00:18:42,490 --> 00:18:46,360
to hang onto it for any length
of time, so you just ignore it.

368
00:18:46,360 --> 00:18:49,570
Now this would be pretty darn
frustrating if you send an email

369
00:18:49,570 --> 00:18:52,210
and only with some probability
does it go through.

370
00:18:52,210 --> 00:18:54,370
Now in practice that might
feel like it happens,

371
00:18:54,370 --> 00:18:56,787
especially when things get
caught up in spam and the like,

372
00:18:56,787 --> 00:19:00,520
but in practice you really do want
emails that are sent to be received.

373
00:19:00,520 --> 00:19:03,190
When you request a web page,
you want the entire web page.

374
00:19:03,190 --> 00:19:06,880
And even if those are big emails
or big web pages that are therefore

375
00:19:06,880 --> 00:19:10,780
chopped into fragments, you really
want to receive all of the fragments

376
00:19:10,780 --> 00:19:13,720
and not just only some of
the paragraphs in the email,

377
00:19:13,720 --> 00:19:16,540
or only some sections of the web page.

378
00:19:16,540 --> 00:19:21,670
So TCP ensures that you get all of
that data at the end of the day.

379
00:19:21,670 --> 00:19:24,620
Well hopefully not at the end
of the day, but ultimately.

380
00:19:24,620 --> 00:19:29,140
And so what TCP adds to the envelope
is essentially a little mental note

381
00:19:29,140 --> 00:19:32,740
that this is packet number one of
two, or one of three, or one of four,

382
00:19:32,740 --> 00:19:34,480
in the case of an even larger file.

383
00:19:34,480 --> 00:19:39,467
And so when the recipient of this email
or this web request gets the envelope

384
00:19:39,467 --> 00:19:42,550
and realizes, wait a minute, I've got
numbers two and three and four, wait

385
00:19:42,550 --> 00:19:44,620
a minute, I'm missing
the first envelope.

386
00:19:44,620 --> 00:19:47,800
TCP tells that Mac or
PC or other computer,

387
00:19:47,800 --> 00:19:50,410
go ahead and send a message
back to the sender saying, hey,

388
00:19:50,410 --> 00:19:53,850
I got everything except
packet one, please resend.

389
00:19:53,850 --> 00:19:55,850
That's going to take a
little bit of extra time,

390
00:19:55,850 --> 00:19:59,800
but that packet can be
resent and TCP knows

391
00:19:59,800 --> 00:20:01,600
how to reassemble them
in the proper order

392
00:20:01,600 --> 00:20:06,370
so that the human ultimately sees their
entire email or that entire web page

393
00:20:06,370 --> 00:20:09,530
and not just some portion thereof.

394
00:20:09,530 --> 00:20:11,072
So what does TCP really look like?

395
00:20:11,072 --> 00:20:13,780
Well, let's just take a quick peek
underneath the hood here, too.

396
00:20:13,780 --> 00:20:18,150
And here we see a similar pattern of
bits but not addresses, that, again,

397
00:20:18,150 --> 00:20:22,530
is handled by IP itself, but
you see mention of source port,

398
00:20:22,530 --> 00:20:23,820
and destination port.

399
00:20:23,820 --> 00:20:25,980
Sequence number, which
helps with the delivery,

400
00:20:25,980 --> 00:20:28,320
and then other options
as well, all of which

401
00:20:28,320 --> 00:20:31,660
we relate to the delivery
of that information.

402
00:20:31,660 --> 00:20:35,820
But these two up here, looks
like 16 bits each, source port

403
00:20:35,820 --> 00:20:37,080
and destination port.

404
00:20:37,080 --> 00:20:39,750
Those two have value, because
TCP does something else.

405
00:20:39,750 --> 00:20:42,820
It doesn't just guarantee that data
gets from one point to another,

406
00:20:42,820 --> 00:20:47,670
it also helps servers distinguish one
type of data from another, and in turn

407
00:20:47,670 --> 00:20:52,170
allows companies and universities
and internet service providers

408
00:20:52,170 --> 00:20:54,720
or governments to distinguish
different types of data

409
00:20:54,720 --> 00:20:57,510
because it's right there on
the outside of the envelope.

410
00:20:57,510 --> 00:21:01,890
In particular, TCP
specifies what protocol

411
00:21:01,890 --> 00:21:06,210
is being used to convey this packet
of information from one computer

412
00:21:06,210 --> 00:21:07,090
to another.

413
00:21:07,090 --> 00:21:09,632
In other words, there's lots of
internet services these days.

414
00:21:09,632 --> 00:21:12,190
There's email, there's chat,
there's video conferencing,

415
00:21:12,190 --> 00:21:13,650
there's web browsers, and more.

416
00:21:13,650 --> 00:21:16,930
So that's a lot of possibilities,
a lot of patterns of zeros and ones

417
00:21:16,930 --> 00:21:18,490
that can be in these envelopes.

418
00:21:18,490 --> 00:21:23,470
So how, upon receiving an envelope, does
a server know what type of information

419
00:21:23,470 --> 00:21:24,030
is in it?

420
00:21:24,030 --> 00:21:25,200
Especially big companies.

421
00:21:25,200 --> 00:21:27,480
Google, for instance, supports
all of those services.

422
00:21:27,480 --> 00:21:29,550
Video conferencing,
email, chat, and more.

423
00:21:29,550 --> 00:21:32,760
So when Google's servers
receives a packet of information,

424
00:21:32,760 --> 00:21:35,610
how does Google know that
this is an email from you,

425
00:21:35,610 --> 00:21:40,350
as opposed to a chat message from
you, as opposed to a video from you

426
00:21:40,350 --> 00:21:41,872
that you're uploading to YouTube?

427
00:21:41,872 --> 00:21:44,580
You need to be able to distinguish
these various services because

428
00:21:44,580 --> 00:21:47,010
at the end of the day,
they're just patterns of bits.

429
00:21:47,010 --> 00:21:49,560
Well, if we reserve some
of those bits, or really

430
00:21:49,560 --> 00:21:53,820
some of the markings on this virtual
envelope, for just one more number

431
00:21:53,820 --> 00:21:55,700
we can distinguish
services pretty easily.

432
00:21:55,700 --> 00:21:59,130
In fact, HTTP, an acronym
that you might not

433
00:21:59,130 --> 00:22:01,530
know what it means but
you've surely seen it a lot,

434
00:22:01,530 --> 00:22:03,720
since our hypertext
transfer protocol and it's

435
00:22:03,720 --> 00:22:07,890
the conventions via which browsers and
servers send web pages back and forth.

436
00:22:07,890 --> 00:22:09,870
Well, by convention,
humans decided years

437
00:22:09,870 --> 00:22:12,600
ago to call that service number 80.

438
00:22:12,600 --> 00:22:15,180
TCP port 80, so to speak.

439
00:22:15,180 --> 00:22:19,920
And the secure version of that, HTTPS,
they decided to number that 443,

440
00:22:19,920 --> 00:22:23,070
just because they'd already used quite
a few numbers in between those two

441
00:22:23,070 --> 00:22:23,700
values.

442
00:22:23,700 --> 00:22:27,630
INAP is the protocol via which
you can receive emails or check

443
00:22:27,630 --> 00:22:30,930
your email, that's used different ports
depending on whether you're using it

444
00:22:30,930 --> 00:22:33,480
security or insecurity like 143 or 993.

445
00:22:33,480 --> 00:22:38,820
SMTP, which is outbound email,
can use similarly 25, 465, or 587.

446
00:22:38,820 --> 00:22:41,970
And then, if familiar, there's
something called SSH, secure shell.

447
00:22:41,970 --> 00:22:44,070
This is what developers
might use at a lower level

448
00:22:44,070 --> 00:22:47,730
to connect from one computer,
say a laptop, to a remote server.

449
00:22:47,730 --> 00:22:49,750
That tends to use port 22.

450
00:22:49,750 --> 00:22:51,750
And there's hundreds,
there's actually thousands

451
00:22:51,750 --> 00:22:55,920
of others, as many as 65,000
possibilities, but only some of those

452
00:22:55,920 --> 00:22:57,720
are actually standardized.

453
00:22:57,720 --> 00:22:59,910
So this is to say what
ultimately is going

454
00:22:59,910 --> 00:23:03,600
on the outside of an envelope
is not just a user's address

455
00:23:03,600 --> 00:23:07,170
but when I as a computer send
a message to some other server

456
00:23:07,170 --> 00:23:11,700
and for instance my address
is 5.6.7.8 I'll write

457
00:23:11,700 --> 00:23:13,570
that in the top corner of the envelope.

458
00:23:13,570 --> 00:23:17,430
If the recipients of this envelope
are supposed to be 1.2.3.4

459
00:23:17,430 --> 00:23:19,810
I do write that in the
middle of the envelope,

460
00:23:19,810 --> 00:23:25,860
but I need to further specify IP
address 1.2.3.4 but port number,

461
00:23:25,860 --> 00:23:28,530
let's say, 80, if it's a
request for a web page.

462
00:23:28,530 --> 00:23:31,433
So conventionally you would do
:80 to distinguish that service.

463
00:23:31,433 --> 00:23:34,100
And then of course because of TCP
I need to number these things,

464
00:23:34,100 --> 00:23:37,380
so if it's a big request or a big
response I better write one of two,

465
00:23:37,380 --> 00:23:39,760
one of three, or one
of four, or the like.

466
00:23:39,760 --> 00:23:41,760
And so the envelope I'm
ultimately left with

467
00:23:41,760 --> 00:23:44,250
is something a little more like this.

468
00:23:44,250 --> 00:23:47,010
On the outside is this recipient's
address, on the outside

469
00:23:47,010 --> 00:23:49,290
is the sender's address,
and on the outside

470
00:23:49,290 --> 00:23:52,770
is the sequence number of
some sort that specifies

471
00:23:52,770 --> 00:23:56,830
how many packets I've actually sent
and hopefully will be received.

472
00:23:56,830 --> 00:24:00,840
So TCP then allows the recipient to
see this envelope, realize, oh this

473
00:24:00,840 --> 00:24:01,800
is for my web server.

474
00:24:01,800 --> 00:24:04,470
Google can hand it off to the
appropriate piece of software

475
00:24:04,470 --> 00:24:06,420
that governs its web
servers and so it's not

476
00:24:06,420 --> 00:24:09,330
confused for something else like
an email, a chat message, a voice

477
00:24:09,330 --> 00:24:11,910
conference, or the like.

478
00:24:11,910 --> 00:24:14,400
And again, all of these
features derive quite

479
00:24:14,400 --> 00:24:17,400
simply from these patterns of
bits that esoterically happen

480
00:24:17,400 --> 00:24:20,340
to be laid out in this way, but
if we abstract away from that

481
00:24:20,340 --> 00:24:24,340
and stipulate that just think about it
like the real world with an envelope,

482
00:24:24,340 --> 00:24:27,150
it's really just these numeric
values that somehow help

483
00:24:27,150 --> 00:24:31,770
us get data from one point to another.

484
00:24:31,770 --> 00:24:36,690
Collectively now, these two protocols,
which are so often used hand in hand,

485
00:24:36,690 --> 00:24:39,570
are generally very abbreviated TCP/IP.

486
00:24:39,570 --> 00:24:41,970
It's two separate protocols,
two separate conventions

487
00:24:41,970 --> 00:24:43,398
used in conjunction.

488
00:24:43,398 --> 00:24:45,940
Some of this information is just
written in different places,

489
00:24:45,940 --> 00:24:49,650
if you will, on the virtual
envelope, but TCP/IP settings are

490
00:24:49,650 --> 00:24:51,990
what you might look for
on a Mac or PC or server

491
00:24:51,990 --> 00:24:55,020
to actually configure
this level of detail.

492
00:24:55,020 --> 00:24:57,870
But of course, I've taken
some liberties here.

493
00:24:57,870 --> 00:25:01,170
If my goal is to send a
message from one computer

494
00:25:01,170 --> 00:25:04,230
to another, a chat message,
an email, anything else,

495
00:25:04,230 --> 00:25:07,320
you know what, I'm pretty
sure I have no idea what

496
00:25:07,320 --> 00:25:09,570
the IP address is of any colleague.

497
00:25:09,570 --> 00:25:12,960
And I have no idea what the IP
address is of Google or Facebook

498
00:25:12,960 --> 00:25:16,740
or any number of popular websites
that I might even visit daily.

499
00:25:16,740 --> 00:25:20,310
I don't even know people's phone numbers
anymore but that's another matter.

500
00:25:20,310 --> 00:25:23,280
In the context of words, though,
on the internet all of us,

501
00:25:23,280 --> 00:25:26,700
of course, type words, not numbers,
when we want to reach some destination.

502
00:25:26,700 --> 00:25:30,900
We go to facebook.com or gmail.com
or google.com or bing.com

503
00:25:30,900 --> 00:25:34,520
or any number of other
domain names, so to speak.

504
00:25:34,520 --> 00:25:36,270
And of course, that's
what you would write

505
00:25:36,270 --> 00:25:38,640
on the outside of an
envelope in the human world,

506
00:25:38,640 --> 00:25:43,530
ideally as many words as possible,
not just numbers let alone bits alone.

507
00:25:43,530 --> 00:25:46,170
And ideally our
computers would similarly

508
00:25:46,170 --> 00:25:49,770
express exactly what we humans
know, which is these domain

509
00:25:49,770 --> 00:25:52,380
names that are part of URLs.

510
00:25:52,380 --> 00:25:56,460
So it turns out we need the help of
at least one more service among all

511
00:25:56,460 --> 00:25:57,930
of these internet technologies.

512
00:25:57,930 --> 00:26:01,580
We need the help of a service
called DNS, domain name system.

513
00:26:01,580 --> 00:26:07,020
A DNS server is a server that quite
simply translates domain names

514
00:26:07,020 --> 00:26:11,670
like gmail.com and bing.com and
google.com into their corresponding IP

515
00:26:11,670 --> 00:26:12,390
addresses.

516
00:26:12,390 --> 00:26:15,023
We, the humans, might have
no idea what they are,

517
00:26:15,023 --> 00:26:17,940
but odds are there's at least one
human or more in the world, probably

518
00:26:17,940 --> 00:26:20,220
who works for those
companies, that does know.

519
00:26:20,220 --> 00:26:22,620
And provided he or she
configures their DNS

520
00:26:22,620 --> 00:26:26,370
servers to know that
association of domain name to IP

521
00:26:26,370 --> 00:26:29,940
address, the equivalent of just an
Excel file with one column with names

522
00:26:29,940 --> 00:26:33,990
and the other column with numbers, IP
addresses well their server can then

523
00:26:33,990 --> 00:26:36,330
answer questions from little old me.

524
00:26:36,330 --> 00:26:39,810
And indeed what my phone knows how to
do these days, what my Mac, my PC knows

525
00:26:39,810 --> 00:26:44,430
how to do is when my human types
in gmail.com and hits enter,

526
00:26:44,430 --> 00:26:47,730
the very first thing that my
browser, and in turn my operating

527
00:26:47,730 --> 00:26:51,510
system like Mac OS or Windows
does, is it asks the local DNS

528
00:26:51,510 --> 00:26:55,680
server for the conversion of
whatever I typed in, gmail.com,

529
00:26:55,680 --> 00:26:58,200
to the corresponding IP address.

530
00:26:58,200 --> 00:27:04,050
And hopefully, my own network be
it at home or on campus or in work,

531
00:27:04,050 --> 00:27:05,870
has the answer to that question.

532
00:27:05,870 --> 00:27:07,590
But the world's a big
place, and odds are

533
00:27:07,590 --> 00:27:11,100
my home does not know the IP address
of every server in the world.

534
00:27:11,100 --> 00:27:14,670
Odds are my campus or company doesn't
know the IP address of every server

535
00:27:14,670 --> 00:27:17,730
in the world, especially since
they're surely changing continually

536
00:27:17,730 --> 00:27:21,300
as new sites are coming online
and others are going offline.

537
00:27:21,300 --> 00:27:22,590
So how do we know?

538
00:27:22,590 --> 00:27:25,530
Well DNS is a whole
hierarchical system whereby

539
00:27:25,530 --> 00:27:30,360
you might have a small DNS server, so
to speak conceptually here on site.

540
00:27:30,360 --> 00:27:34,170
But then your internet service
provider or ISP, Comcast, Verizon,

541
00:27:34,170 --> 00:27:38,730
or some other entity, they probably have
a bigger DNS server with more memory,

542
00:27:38,730 --> 00:27:42,273
with a longer list of domain
names and IP addresses.

543
00:27:42,273 --> 00:27:44,440
And you know what, even if
they don't know everyone,

544
00:27:44,440 --> 00:27:47,280
there are probably what are
called root servers in the world,

545
00:27:47,280 --> 00:27:50,430
that much like the root of a
tree, is where everything starts.

546
00:27:50,430 --> 00:27:53,190
And indeed, you can find
out from these actual root

547
00:27:53,190 --> 00:27:56,100
servers on the internet,
the mapping, effectively,

548
00:27:56,100 --> 00:27:58,890
between all of the dot coms
and their IP addresses.

549
00:27:58,890 --> 00:28:01,680
All of the dot govs or the dot
nets and their IP addresses.

550
00:28:01,680 --> 00:28:05,700
And frankly, even if they don't know
the answer by definition of root server

551
00:28:05,700 --> 00:28:08,790
they will be configured
to know who knows.

552
00:28:08,790 --> 00:28:13,050
And so DNS is very hierarchical,
and it's also recursive.

553
00:28:13,050 --> 00:28:17,040
You might ask a local server, which
might ask a more remote server, which

554
00:28:17,040 --> 00:28:19,110
might ask and even further away server.

555
00:28:19,110 --> 00:28:22,440
That server might say, wait a minute,
I know, this server knows, and then

556
00:28:22,440 --> 00:28:24,870
the answer eventually
bubbles its way back to you.

557
00:28:24,870 --> 00:28:26,673
And long story short,
we can be efficient.

558
00:28:26,673 --> 00:28:28,590
We don't have to constantly
ask this question.

559
00:28:28,590 --> 00:28:30,660
We can cache those results locally.

560
00:28:30,660 --> 00:28:34,590
Remember them in my
browser, in my Mac or my PC.

561
00:28:34,590 --> 00:28:36,480
There's downsides there, though, too.

562
00:28:36,480 --> 00:28:39,630
By remembering that mapping
of domain name to IP address,

563
00:28:39,630 --> 00:28:43,680
I can save myself the trouble of asking
that same question multiple times a day

564
00:28:43,680 --> 00:28:45,810
or even per week or even per minute.

565
00:28:45,810 --> 00:28:48,690
The catch, though, is that if Google
changes something, or Facebook

566
00:28:48,690 --> 00:28:51,300
reconfigure something
and that IP changes,

567
00:28:51,300 --> 00:28:53,110
caching might actually be a bad thing.

568
00:28:53,110 --> 00:28:55,530
And so here, too, even at
the level of the internet

569
00:28:55,530 --> 00:28:57,300
do we see these series of trade-offs.

570
00:28:57,300 --> 00:29:01,860
You might save time by caching, but
you might sacrifice correctness,

571
00:29:01,860 --> 00:29:06,990
because now the servers recollection of
that IP address might become outdated.

572
00:29:06,990 --> 00:29:09,000
And so this is a whole
can of worms, ultimately,

573
00:29:09,000 --> 00:29:11,130
and speaks to what it really
means to be an engineer

574
00:29:11,130 --> 00:29:13,880
in the world of internet technologies
to anticipate to think about

575
00:29:13,880 --> 00:29:15,930
and ultimately to solve these problems.

576
00:29:15,930 --> 00:29:21,000
There is no sure fire solution other
than to expect that you'll need

577
00:29:21,000 --> 00:29:23,650
to accommodate these changes over time.

578
00:29:23,650 --> 00:29:25,440
So in Windows, can
you see this yourself?

579
00:29:25,440 --> 00:29:28,500
Well, if you open up those
same Wi-Fi properties or wired

580
00:29:28,500 --> 00:29:31,470
properties that you have, you'll see
again, not only your IPv4 address,

581
00:29:31,470 --> 00:29:35,550
but it was there all this
time, your IPv4 DNS servers one

582
00:29:35,550 --> 00:29:39,750
or more IP addresses turns out it's
exactly the same by coincidence

583
00:29:39,750 --> 00:29:44,640
but also by design on this computer
of my router or my default gateway

584
00:29:44,640 --> 00:29:47,070
192168.1.1.

585
00:29:47,070 --> 00:29:50,950
Which is to say that if this PC needs
to know an answer to the question,

586
00:29:50,950 --> 00:29:55,830
what is gmail.com's IP address it is
simply going to ask the local server

587
00:29:55,830 --> 00:29:59,310
that has that address and that
DNS server, and this is important,

588
00:29:59,310 --> 00:30:00,900
cannot have itself a name.

589
00:30:00,900 --> 00:30:04,320
We need to know what its IP
address is, otherwise, of course,

590
00:30:04,320 --> 00:30:05,580
we get into this endless loop.

591
00:30:05,580 --> 00:30:08,640
If we know only the name of our
DNS server but only the DNS server

592
00:30:08,640 --> 00:30:12,370
can convert that to an IP address, we'll
never actually answer that question.

593
00:30:12,370 --> 00:30:14,430
It's more of a catch-22.

594
00:30:14,430 --> 00:30:16,380
And even if it does
have a name, you need

595
00:30:16,380 --> 00:30:21,370
to know manually, via your DHCP server
somehow, what its IP address actually

596
00:30:21,370 --> 00:30:21,870
is.

597
00:30:21,870 --> 00:30:22,800
Mac OS, the same.

598
00:30:22,800 --> 00:30:26,370
And here on campus, Harvard happens to
have redundancy like most any company.

599
00:30:26,370 --> 00:30:29,470
They don't have just one DNS server
they have at least three here,

600
00:30:29,470 --> 00:30:33,250
128.103.1.1, and a couple
of others, as well.

601
00:30:33,250 --> 00:30:36,300
And again, I got these automatically
when I turned on my Mac or my phone

602
00:30:36,300 --> 00:30:41,220
or my PC via that local DHCP server.

603
00:30:41,220 --> 00:30:44,550
So let's see if we can't
mimic what it is my Mac,

604
00:30:44,550 --> 00:30:48,330
your PC, your phone is doing
everyday all day long, but rather

605
00:30:48,330 --> 00:30:49,710
unbeknownst to us.

606
00:30:49,710 --> 00:30:51,750
Here I have what's
called a terminal window.

607
00:30:51,750 --> 00:30:54,300
This is just a textual
interface to my computer here.

608
00:30:54,300 --> 00:30:57,660
Can exist on Macs, or PCs, or
other operating systems, as well.

609
00:30:57,660 --> 00:31:01,260
And it allows me to execute
by typing commands textually,

610
00:31:01,260 --> 00:31:04,620
only at my keyboard, no mouse,
exactly the types of commands

611
00:31:04,620 --> 00:31:08,850
that your browser and other software
are effectively executing or running

612
00:31:08,850 --> 00:31:09,570
for you.

613
00:31:09,570 --> 00:31:11,430
For instance, suppose
I genuinely do want

614
00:31:11,430 --> 00:31:12,990
to know the IP address of gmail.com.

615
00:31:12,990 --> 00:31:16,020
I can ask this program as follows.

616
00:31:16,020 --> 00:31:19,170
nslookup, for name server look
up, and then I can go ahead

617
00:31:19,170 --> 00:31:22,140
and type literally
gmail.com and hit Enter.

618
00:31:22,140 --> 00:31:28,620
Here, visually, we see on the screen
one answer that it's 172.217.3.37.

619
00:31:28,620 --> 00:31:31,740
And this comes from a server
whose IP address in this room

620
00:31:31,740 --> 00:31:36,270
is 10.0.0.2, which we know now
to be a private IP address,

621
00:31:36,270 --> 00:31:38,640
and indeed, here on
campus we have servers

622
00:31:38,640 --> 00:31:42,480
that are local only to this room, this
building, or this set of buildings

623
00:31:42,480 --> 00:31:42,987
here.

624
00:31:42,987 --> 00:31:44,820
Now this is a little
interesting because I'm

625
00:31:44,820 --> 00:31:48,000
pretty sure business is good
for Google, and surely they

626
00:31:48,000 --> 00:31:51,000
don't have just one server
and therefore one IP address.

627
00:31:51,000 --> 00:31:54,720
Well, it turns out that there's a whole
hierarchy of servers out there, most

628
00:31:54,720 --> 00:31:59,520
likely, that my data goes to and
thereafter through on Google's end.

629
00:31:59,520 --> 00:32:05,160
The one IP address that they're
telling me is theirs is 172.217.3.37,

630
00:32:05,160 --> 00:32:08,850
but once my packet of information gets
there to Mountain View, California,

631
00:32:08,850 --> 00:32:11,580
or wherever their servers
happen to be closest to me,

632
00:32:11,580 --> 00:32:16,290
then they might have any number of
servers, dozens, hundreds, thousands,

633
00:32:16,290 --> 00:32:18,800
that can actually
receive that packet next.

634
00:32:18,800 --> 00:32:22,380
This just happens to be the outward
facing IP that my own Mac or PC

635
00:32:22,380 --> 00:32:24,480
or phone actually sees.

636
00:32:24,480 --> 00:32:28,260
Well, let's see if we can't trace the
route to gmail.com via another command,

637
00:32:28,260 --> 00:32:32,550
literally traceroute, can I see
the packets of information line

638
00:32:32,550 --> 00:32:36,840
by line leaving my computer and making
their way, ultimately, to Google.

639
00:32:36,840 --> 00:32:39,270
I'm going to go ahead and
do this once, so dash q1

640
00:32:39,270 --> 00:32:41,502
means do one query, please, at a time.

641
00:32:41,502 --> 00:32:43,710
And then I'm going to go
ahead and say, quite simply,

642
00:32:43,710 --> 00:32:45,990
gmail.com, and then Enter.

643
00:32:45,990 --> 00:32:50,400
And we will see, line by line,
the sequence of IP addresses

644
00:32:50,400 --> 00:32:54,930
of every router that is to
say hop between me and Gmail.

645
00:32:54,930 --> 00:32:57,253
On occasion we'll see
these asterisks instead,

646
00:32:57,253 --> 00:32:59,670
which indicates that that
router isn't having any of this,

647
00:32:59,670 --> 00:33:03,660
it's not responding to my requests,
so we can't see its IP or anything

648
00:33:03,660 --> 00:33:04,650
else about it.

649
00:33:04,650 --> 00:33:09,420
But we can see that in 17
steps does data leave my laptop

650
00:33:09,420 --> 00:33:12,150
and end up at gmail.com,
and along the way

651
00:33:12,150 --> 00:33:16,830
it encounters all of these routers that
have these unique IP addresses but not

652
00:33:16,830 --> 00:33:18,870
names, it seems, and
the amount of time it

653
00:33:18,870 --> 00:33:21,420
takes for my data to get
from my laptop to gmail.com

654
00:33:21,420 --> 00:33:26,220
is, oh my, 0.967 milliseconds.

655
00:33:26,220 --> 00:33:29,940
Less than one millisecond is
required to get data or an email

656
00:33:29,940 --> 00:33:32,760
from my computer to gmail.com itself.

657
00:33:32,760 --> 00:33:36,450
Now what about all of these other
measurements of time up above?

658
00:33:36,450 --> 00:33:38,580
Each of these represents
the number of milliseconds

659
00:33:38,580 --> 00:33:43,410
it took during this process for data
to go from my laptop to this router,

660
00:33:43,410 --> 00:33:45,940
then to this router,
then to this router.

661
00:33:45,940 --> 00:33:48,390
Now, of course, it seems
strange that it takes more time

662
00:33:48,390 --> 00:33:52,390
to get these to these close routers
than it does to these further away.

663
00:33:52,390 --> 00:33:54,410
But there, too, if I
ran this all day long

664
00:33:54,410 --> 00:33:56,160
I would get different
numbers continually,

665
00:33:56,160 --> 00:33:58,737
it depends how busy those routers
are at that moment in time.

666
00:33:58,737 --> 00:34:00,570
It depends what else
everyone here on campus

667
00:34:00,570 --> 00:34:03,840
is doing, or other people in the
world at that moment in time.

668
00:34:03,840 --> 00:34:06,660
Routers might be a little slow
to respond because they're

669
00:34:06,660 --> 00:34:07,920
busy doing something else.

670
00:34:07,920 --> 00:34:10,050
My data might get
dropped in other contexts

671
00:34:10,050 --> 00:34:12,400
and need to be resent, which
is just going to take time,

672
00:34:12,400 --> 00:34:15,030
and I don't even see that
happening on the screen.

673
00:34:15,030 --> 00:34:18,870
But it's fair to say that these give
us a sense of the range of times

674
00:34:18,870 --> 00:34:21,270
it might take to go from
a point A to point B,

675
00:34:21,270 --> 00:34:26,040
and let's say 1 to 20 milliseconds
or even 32 milliseconds, somewhere

676
00:34:26,040 --> 00:34:29,370
in there is our average,
and that can vary over time.

677
00:34:29,370 --> 00:34:32,489
But that's pretty fast, and indeed,
even though it took a moment

678
00:34:32,489 --> 00:34:36,120
to run this whole test, this is why an
email can be sent from your computer

679
00:34:36,120 --> 00:34:38,460
and be received nearly
instantly by someone

680
00:34:38,460 --> 00:34:41,760
around the world, because at the end
of the day, we're limited, really,

681
00:34:41,760 --> 00:34:44,880
ultimately, by the speed
of light and little more.

682
00:34:44,880 --> 00:34:48,449
Well, to be fair, hardware and
cost and everything in between,

683
00:34:48,449 --> 00:34:52,469
but you can certainly transmit your
data faster than you can yourself.

684
00:34:52,469 --> 00:34:55,139
But what if we want to go
farther away than gmail.com?

685
00:34:55,139 --> 00:34:57,690
Odds are they probably do
have servers in California,

686
00:34:57,690 --> 00:35:01,380
but probably here on the east coast
of the US as well, let alone abroad.

687
00:35:01,380 --> 00:35:04,050
What if I deliberately try
to access a domain that is,

688
00:35:04,050 --> 00:35:06,150
in fact, abroad and go there?

689
00:35:06,150 --> 00:35:09,160
Well, let me go ahead
and visit via traceroute,

690
00:35:09,160 --> 00:35:16,230
say, www.cnn.co.jp, the domain
name for CNN's Japanese website.

691
00:35:16,230 --> 00:35:18,390
And then we'll add
just dash q1 this time

692
00:35:18,390 --> 00:35:22,020
at the end, which is fine, too,
to query the server just once.

693
00:35:22,020 --> 00:35:24,360
And here we see the
sequence of steps, one

694
00:35:24,360 --> 00:35:27,900
after another, whereby the data's
leaving my laptop and in turn campus,

695
00:35:27,900 --> 00:35:30,300
and then we see some
anonymous routers in between.

696
00:35:30,300 --> 00:35:36,540
But the 30th there seems to be
just in time, because within it

697
00:35:36,540 --> 00:35:40,710
seems 178 milliseconds do
we make our way to Japan.

698
00:35:40,710 --> 00:35:44,220
Now that's quite a few milliseconds
more, but that rather makes sense.

699
00:35:44,220 --> 00:35:47,400
Whereas it might take one
to 20 to 32 milliseconds

700
00:35:47,400 --> 00:35:50,940
to get from here to Gmail either
on the east coast or west coast,

701
00:35:50,940 --> 00:35:54,120
I'm kind of not surprised that
it takes an order of magnitude

702
00:35:54,120 --> 00:35:59,070
more, almost to factor of 10, to get
to Japan, because there's not only

703
00:35:59,070 --> 00:36:01,920
a whole continent between us
here in Cambridge and Japan,

704
00:36:01,920 --> 00:36:04,770
there's also an entire
Pacific Ocean between us.

705
00:36:04,770 --> 00:36:09,030
And indeed, there are Transatlantic,
Transpacific, and transoceanic cables

706
00:36:09,030 --> 00:36:12,000
all around the world these days
that actually transmit our data,

707
00:36:12,000 --> 00:36:15,450
not to mention all of the wireless
technologies we have, satellites

708
00:36:15,450 --> 00:36:16,385
and below.

709
00:36:16,385 --> 00:36:19,260
And so it does stand to reason that
even though none of these routers

710
00:36:19,260 --> 00:36:22,230
were paying attention to me at
that moment for privacy sake,

711
00:36:22,230 --> 00:36:26,460
this last one indicates that 200
milliseconds later we can get halfway

712
00:36:26,460 --> 00:36:28,950
across the world digitally.

713
00:36:28,950 --> 00:36:32,310
And so that does rather speak to
just how quickly these low level

714
00:36:32,310 --> 00:36:35,340
primitives operate, and
we can talk far longer

715
00:36:35,340 --> 00:36:38,400
about how these things work
than it actually takes time

716
00:36:38,400 --> 00:36:40,890
to actually get the data there.

717
00:36:40,890 --> 00:36:44,940
So then together we have
TCP/IP via DHCP can we

718
00:36:44,940 --> 00:36:49,410
get the addresses that we need to use
to address my envelopes and others,

719
00:36:49,410 --> 00:36:49,980
as well.

720
00:36:49,980 --> 00:36:54,180
Via DNS can we convert those domain
names into IP addresses and even back.

721
00:36:54,180 --> 00:36:56,910
And those internet
technologies are ultimately

722
00:36:56,910 --> 00:37:02,890
what govern how our data gets from point
A to point B. But what is the data?

723
00:37:02,890 --> 00:37:05,700
Indeed, everything thus far
is really just metadata.

724
00:37:05,700 --> 00:37:09,150
Information that helps our actual
data that we care about get from one

725
00:37:09,150 --> 00:37:10,192
point to another.

726
00:37:10,192 --> 00:37:12,900
But it's the data at the end of
the day that I really care about.

727
00:37:12,900 --> 00:37:16,140
The contents of my email, the
contents of my chat message,

728
00:37:16,140 --> 00:37:19,860
the voice that I'm sending over
a video conference, or even just

729
00:37:19,860 --> 00:37:21,275
the contents of a web page.

730
00:37:21,275 --> 00:37:24,150
Indeed, perhaps the most popular
service that you and I use every day

731
00:37:24,150 --> 00:37:27,270
is just that, pulling
up pages on the web.

732
00:37:27,270 --> 00:37:32,250
So just how is a web page
specifically requested and received?

733
00:37:32,250 --> 00:37:36,300
Well, it turns out that http://
that you've surely seen,

734
00:37:36,300 --> 00:37:39,450
but probably not typed for some
time, because your browser, odds are,

735
00:37:39,450 --> 00:37:42,840
just inserts it automatically
or even invisibly for you.

736
00:37:42,840 --> 00:37:47,970
That HTTP is yet another protocol in
this stack of internet technologies.

737
00:37:47,970 --> 00:37:50,550
Hypertext transfer protocol.

738
00:37:50,550 --> 00:37:55,830
A set of conventions that browsers and
web servers have agreed upon long ago

739
00:37:55,830 --> 00:37:57,900
to use when intercommunicating.

740
00:37:57,900 --> 00:38:00,540
And to be clear, then,
what exactly is a protocol?

741
00:38:00,540 --> 00:38:01,760
Well, it's just a convention.

742
00:38:01,760 --> 00:38:04,620
We humans have protocols even
though we might not call them such.

743
00:38:04,620 --> 00:38:07,453
When I meet someone new on the
street I might reach up to him or her

744
00:38:07,453 --> 00:38:09,420
and say, hello, my name is David.

745
00:38:09,420 --> 00:38:13,020
And that protocol results
in that other person,

746
00:38:13,020 --> 00:38:16,770
if polite, in extending their
hand too, reaching into mine

747
00:38:16,770 --> 00:38:20,430
and probably saying as well, hello,
nice to meet you or how are you.

748
00:38:20,430 --> 00:38:23,190
That's a human protocol that
we were taught some time ago,

749
00:38:23,190 --> 00:38:26,820
and culturally we have all agreed
here in the US to, generally speaking,

750
00:38:26,820 --> 00:38:28,740
greet each other in that manner.

751
00:38:28,740 --> 00:38:31,020
Computers, similarly,
have standardized what

752
00:38:31,020 --> 00:38:35,850
goes not only on the outside of these
envelopes but what goes in the inside,

753
00:38:35,850 --> 00:38:37,020
as well.

754
00:38:37,020 --> 00:38:39,270
And so if, for instance,
the goal at hand

755
00:38:39,270 --> 00:38:44,490
is to request a web page of a
canonical website like www.example.com,

756
00:38:44,490 --> 00:38:47,610
let's consider exactly what
is inside of this envelope.

757
00:38:47,610 --> 00:38:51,810
Well, first of all here we have a
proper URL, uniform resource locator.

758
00:38:51,810 --> 00:38:56,010
These days, your browser, whether it's
Chrome or Safari or Edge or Firefox,

759
00:38:56,010 --> 00:38:58,950
probably doesn't even show
you all of this information.

760
00:38:58,950 --> 00:39:02,280
In the interests of simpler
user interfaces or UIs,

761
00:39:02,280 --> 00:39:05,370
browsers have started to hide
these so-called protocol here

762
00:39:05,370 --> 00:39:09,210
at the left, even the ww here,
the hostname in the middle,

763
00:39:09,210 --> 00:39:12,780
leaving you oftentimes with just
example.com or the equivalent

764
00:39:12,780 --> 00:39:14,657
somewhere at the top of your screen.

765
00:39:14,657 --> 00:39:16,740
But if you click on that
address, typically you'll

766
00:39:16,740 --> 00:39:18,930
see more information such as that here.

767
00:39:18,930 --> 00:39:22,020
And sometimes there's more
information that's just implicit.

768
00:39:22,020 --> 00:39:26,670
It turns out if you try to
visit http://www.example.com

769
00:39:26,670 --> 00:39:31,310
or any similar domain name, what
you're likely reaching for is

770
00:39:31,310 --> 00:39:33,290
a very specific file on that server.

771
00:39:33,290 --> 00:39:34,550
But how do we reach it?

772
00:39:34,550 --> 00:39:37,850
Well, highlighted in yellow here is
what's called the domain name itself,

773
00:39:37,850 --> 00:39:39,260
example.com.

774
00:39:39,260 --> 00:39:41,690
This is something that
you buy, or really rent,

775
00:39:41,690 --> 00:39:45,620
on an annual basis via an internet
registrar, a company, that

776
00:39:45,620 --> 00:39:48,860
via the associations on the
internet that govern IP addresses

777
00:39:48,860 --> 00:39:51,560
domain names has been
authorized to sell, or really

778
00:39:51,560 --> 00:39:55,490
rent, you and anyone else a domain
name for some amount of time,

779
00:39:55,490 --> 00:39:58,280
usually one year or two
years or 10 or anywhere

780
00:39:58,280 --> 00:40:00,860
in between, for some dollar amount.

781
00:40:00,860 --> 00:40:03,680
And what you get, then, is the
ability, for that amount of time

782
00:40:03,680 --> 00:40:07,250
renewable thereafter, to use
that specific domain name.

783
00:40:07,250 --> 00:40:10,070
It might be dot com,
or dot net or dot org,

784
00:40:10,070 --> 00:40:15,050
or any number of hundreds of others
of TLDs, or top level domains.

785
00:40:15,050 --> 00:40:18,943
Indeed, that suffix there is what
represents the type of website,

786
00:40:18,943 --> 00:40:20,360
at least historically, that it is.

787
00:40:20,360 --> 00:40:24,680
Dot com for commercial, dot net
for network, dot edu for education,

788
00:40:24,680 --> 00:40:26,460
or dot gov for government.

789
00:40:26,460 --> 00:40:28,820
Of course, all of those
TLDs, or top level domains,

790
00:40:28,820 --> 00:40:32,120
were very US centric by
design, and so far it

791
00:40:32,120 --> 00:40:36,350
was generally a cohort of Americans
that designed a lot of this system

792
00:40:36,350 --> 00:40:37,070
initially.

793
00:40:37,070 --> 00:40:39,890
Of course, other countries
have shorter TLDs.

794
00:40:39,890 --> 00:40:44,150
Country codes, dot US, dot
JP and others that signify

795
00:40:44,150 --> 00:40:46,100
a specific country in which they're in.

796
00:40:46,100 --> 00:40:49,430
And these days anyone can
buy a dot com or dot net,

797
00:40:49,430 --> 00:40:53,930
but not everyone can buy a dot gov or
dot edu, or several other top level

798
00:40:53,930 --> 00:40:54,650
domains, as well.

799
00:40:54,650 --> 00:40:58,700
It depends on whoever controls
that particular suffix.

800
00:40:58,700 --> 00:41:03,380
This here we might call the hostname,
the name of the specific server

801
00:41:03,380 --> 00:41:06,380
that you were trying to visit that
lives within that domain name.

802
00:41:06,380 --> 00:41:09,050
In other contexts, you
might call this a subdomain,

803
00:41:09,050 --> 00:41:12,770
indicating what subdivision of a
company or university you're actually

804
00:41:12,770 --> 00:41:14,450
trying to access.

805
00:41:14,450 --> 00:41:19,070
And then down here on the right,
implicitly so to speak, is a file name.

806
00:41:19,070 --> 00:41:21,830
It is human convention, but
not required, that the name

807
00:41:21,830 --> 00:41:27,440
of the file that contains the web page
that a server serves up by default,

808
00:41:27,440 --> 00:41:30,350
happens to be traditionally index.html.

809
00:41:30,350 --> 00:41:35,060
It could also be index.htm or any
number of other names or extensions,

810
00:41:35,060 --> 00:41:37,140
but this is among the most common.

811
00:41:37,140 --> 00:41:40,450
So if you don't mention that
via just a slash, it's implied,

812
00:41:40,450 --> 00:41:43,070
and it's that file or
any other file, that's

813
00:41:43,070 --> 00:41:48,230
implied or even specified explicitly
that is inside of this envelope.

814
00:41:48,230 --> 00:41:51,740
That's the whole point of this
virtual packet of information,

815
00:41:51,740 --> 00:41:56,450
to encapsulate the request for a
page and the actual page itself.

816
00:41:56,450 --> 00:41:59,600
At the end of the day, it's
HTML, Hypertext Markup Language,

817
00:41:59,600 --> 00:42:03,620
an actual language in which pages are
written, that's inside that envelope,

818
00:42:03,620 --> 00:42:06,740
but it's transmitted there via HTTP.

819
00:42:06,740 --> 00:42:10,820
The protocol, the set of conventions
via which browser and server agree

820
00:42:10,820 --> 00:42:14,850
to send and receive that information.

821
00:42:14,850 --> 00:42:16,770
So what does that information look like?

822
00:42:16,770 --> 00:42:19,670
And just what have these
computers agreed on?

823
00:42:19,670 --> 00:42:22,400
It turns out that
inside of this envelope,

824
00:42:22,400 --> 00:42:27,080
when it represents a request for
a web page like my URL there,

825
00:42:27,080 --> 00:42:29,030
are these lines here.

826
00:42:29,030 --> 00:42:33,230
GET/HTTP/1.1, where
get is clearly a verb,

827
00:42:33,230 --> 00:42:35,900
by definition in all
caps in this protocol,

828
00:42:35,900 --> 00:42:40,500
slash means the default page of the
website index.html or something else.

829
00:42:40,500 --> 00:42:44,060
And then often a mention of host
colon and then the name of the host

830
00:42:44,060 --> 00:42:45,650
that you're actually looking for.

831
00:42:45,650 --> 00:42:47,990
Because it turns out servers
can do so many things.

832
00:42:47,990 --> 00:42:51,110
Not just Google servers with
voice and chat and other services,

833
00:42:51,110 --> 00:42:54,410
one web server can actually
serve up multiple websites.

834
00:42:54,410 --> 00:42:59,660
Example.com, acme.com,
Harvard.edu, google.com, all of us

835
00:42:59,660 --> 00:43:04,070
can actually have shared tendencies, so
to speak, on the same server in theory.

836
00:43:04,070 --> 00:43:08,510
And so by mentioning what actual
website you want inside of the envelope,

837
00:43:08,510 --> 00:43:12,560
the recipient of this envelope can make
sure that it serves you my home page

838
00:43:12,560 --> 00:43:14,030
and not someone else's.

839
00:43:14,030 --> 00:43:17,210
But beyond that, there needs to be
additional information, as well.

840
00:43:17,210 --> 00:43:20,030
You might explicitly specify
the name of the file.

841
00:43:20,030 --> 00:43:23,750
And again, we humans have nothing
to do with any of this, ultimately,

842
00:43:23,750 --> 00:43:25,760
we have just typed that URL.

843
00:43:25,760 --> 00:43:29,000
But it's our browser, on
Mac OS or Windows or phones,

844
00:43:29,000 --> 00:43:32,870
that's packaging up this information
inside of a virtual envelope

845
00:43:32,870 --> 00:43:36,470
and sending it out,
ultimately, on our behalf.

846
00:43:36,470 --> 00:43:40,430
And indeed, if all goes well and
that envelope reaches point B

847
00:43:40,430 --> 00:43:44,780
and it's opened up and it represents
the name of a web page that does,

848
00:43:44,780 --> 00:43:47,540
in fact, exist, the response
that I hope to get back

849
00:43:47,540 --> 00:43:51,230
in another envelope
from point B to point A

850
00:43:51,230 --> 00:43:54,590
is going to contain an
HTTP message like this.

851
00:43:54,590 --> 00:43:59,480
Literally the name of the protocol
again, HTTP/1.1, and then a number,

852
00:43:59,480 --> 00:44:01,140
and optionally a phrase.

853
00:44:01,140 --> 00:44:03,800
200 is perhaps a number
you've never actually seen,

854
00:44:03,800 --> 00:44:06,490
even though it is the best
possible response to get.

855
00:44:06,490 --> 00:44:09,560
200 means, quite literally, OK.

856
00:44:09,560 --> 00:44:11,480
The web page you
requested has been found

857
00:44:11,480 --> 00:44:15,170
and has been delivered in
this response envelope, OK.

858
00:44:15,170 --> 00:44:19,220
The type of content you've
received is in this case text/html.

859
00:44:19,220 --> 00:44:23,660
Which is to say inside of that
envelope is a clue to your browser

860
00:44:23,660 --> 00:44:26,240
what kind of content is inside deeper.

861
00:44:26,240 --> 00:44:29,120
Is it text.html, like the
contents of a web page?

862
00:44:29,120 --> 00:44:33,570
Is it an image/png like a graphic,
or image/gif, something animated,

863
00:44:33,570 --> 00:44:39,720
or video/mp4, an actual video file,
this so-called MIME type or content

864
00:44:39,720 --> 00:44:43,080
type is inside of the envelope for
your browser so as to provide a hint,

865
00:44:43,080 --> 00:44:45,660
so as to know how to
display it on the screen.

866
00:44:45,660 --> 00:44:48,750
There's so many other headers,
as well, but these two alone

867
00:44:48,750 --> 00:44:51,330
really specify almost
as much information

868
00:44:51,330 --> 00:44:54,950
as you need in order to render
that response for the user.

869
00:44:54,950 --> 00:44:56,700
Now as an aside, there
are other versions.

870
00:44:56,700 --> 00:44:59,370
And increasingly in vogue,
though not yet omnipresent,

871
00:44:59,370 --> 00:45:03,450
is HTTP2 which has additional
features, particularly for performance

872
00:45:03,450 --> 00:45:05,820
and getting data to
you even more quickly.

873
00:45:05,820 --> 00:45:08,580
It simply replaces that 1.1
with a two and the response,

874
00:45:08,580 --> 00:45:11,880
though, comes back almost the same.

875
00:45:11,880 --> 00:45:15,840
So let's consider an example
then, such as harvard.edu.

876
00:45:15,840 --> 00:45:23,310
It turns out that http://harvard.edu
is not where Harvard wants you to be.

877
00:45:23,310 --> 00:45:25,890
In fact, let me go ahead
and pull up my browser here

878
00:45:25,890 --> 00:45:28,630
and visit precisely that URL.

879
00:45:28,630 --> 00:45:33,480
http://harvard.edu, Enter.

880
00:45:33,480 --> 00:45:37,200
And within seconds do I find myself
not at harvard.edu, but rather

881
00:45:37,200 --> 00:45:45,480
at ww.harvard.edu and moreover
at https://www.harvard.edu.

882
00:45:45,480 --> 00:45:48,990
In other words, even though I
specified a protocol of HTTP,

883
00:45:48,990 --> 00:45:53,320
a domain name of harvard.edu,
and no hostname, so to speak,

884
00:45:53,320 --> 00:45:55,890
I have actually been whisked
away, seemingly magically,

885
00:45:55,890 --> 00:46:01,260
to this URL instead, for reasons both
technical and perhaps marketing alike.

886
00:46:01,260 --> 00:46:05,250
For today, though, let's focus
on exactly how this came to pass.

887
00:46:05,250 --> 00:46:08,100
Well, it turns out that inside of
the envelope with which Harvard,

888
00:46:08,100 --> 00:46:11,850
or any server, replies to me can
be additional metadata, as well.

889
00:46:11,850 --> 00:46:15,180
Not just 200 OK, but really
the equivalent of uh-uh,

890
00:46:15,180 --> 00:46:18,360
there's nothing to see
here, go here instead.

891
00:46:18,360 --> 00:46:22,410
So let me go ahead and run a program,
again in that black and white window

892
00:46:22,410 --> 00:46:24,480
known as my terminal
window, whereby I can

893
00:46:24,480 --> 00:46:27,240
pretend to be a browser
without all of the graphics

894
00:46:27,240 --> 00:46:29,970
and without all of the
distraction and focus only

895
00:46:29,970 --> 00:46:32,760
on the contents of
those digital envelopes.

896
00:46:32,760 --> 00:46:36,840
Here the program I'm going to run
is called curl for connect to a URL,

897
00:46:36,840 --> 00:46:41,820
and I'm going to specify dash I which
is to say I only want the HTTP headers.

898
00:46:41,820 --> 00:46:48,240
I'm going to go ahead now and say
http://harvard.edu, nothing more.

899
00:46:48,240 --> 00:46:51,030
When I hit Enter now, here
are the complete headers

900
00:46:51,030 --> 00:46:53,100
that come back from the server.

901
00:46:53,100 --> 00:46:56,130
No dot dot dot this time, we
see everything, in fact, here,

902
00:46:56,130 --> 00:46:57,510
but notice the first line.

903
00:46:57,510 --> 00:47:02,100
It's not 200 OK, but rather
301 moved permanently.

904
00:47:02,100 --> 00:47:03,510
Like, where did Harvard go?

905
00:47:03,510 --> 00:47:07,410
Well, it turns out that Harvard
has specified its new location

906
00:47:07,410 --> 00:47:13,950
down here as https://www.harvard.edu.

907
00:47:13,950 --> 00:47:16,080
Now there's other
lines of headers there,

908
00:47:16,080 --> 00:47:19,410
HTTP headers as they're called,
each of which starts with a word,

909
00:47:19,410 --> 00:47:23,010
perhaps with some punctuation, and
a colon, followed by the value.

910
00:47:23,010 --> 00:47:27,930
Location, value, go to this location
is the general paradigm there.

911
00:47:27,930 --> 00:47:31,650
But why might Harvard not want to
show me their web page at the address

912
00:47:31,650 --> 00:47:32,670
that I typed?

913
00:47:32,670 --> 00:47:36,420
Well, it turns out that HTTP
is by definition insecure.

914
00:47:36,420 --> 00:47:39,210
The extents to which
the message is encoded

915
00:47:39,210 --> 00:47:41,740
is quite literally in English
or English-like syntax,

916
00:47:41,740 --> 00:47:43,980
such as that we've been looking at here.

917
00:47:43,980 --> 00:47:47,050
It's just that text that's
inside the envelope.

918
00:47:47,050 --> 00:47:50,850
If instead, though, you want to encrypt
those contents so that no one knows

919
00:47:50,850 --> 00:47:53,370
what web page you're
requesting or receiving,

920
00:47:53,370 --> 00:47:57,690
and your employer and your university
administrator or your internet service

921
00:47:57,690 --> 00:48:00,900
provider or country does
not know what you're doing,

922
00:48:00,900 --> 00:48:04,380
be it for personal reasons,
financial, or otherwise, well then

923
00:48:04,380 --> 00:48:06,300
you want to use HTTPS.

924
00:48:06,300 --> 00:48:09,030
And Harvard University, like
so many companies today,

925
00:48:09,030 --> 00:48:12,030
is insistent that you
actually visit them securely,

926
00:48:12,030 --> 00:48:14,850
if only because it's best
practice, but it also

927
00:48:14,850 --> 00:48:18,390
prevents potentially private
information from leaking.

928
00:48:18,390 --> 00:48:20,640
And so here with this location
line is Harvard saying,

929
00:48:20,640 --> 00:48:24,930
no, we will not respond
to you with OK via HTTP,

930
00:48:24,930 --> 00:48:28,980
we have moved permanently to
a secure address at HTTPS,

931
00:48:28,980 --> 00:48:31,020
where the S denotes secure.

932
00:48:31,020 --> 00:48:32,320
But why the www?

933
00:48:32,320 --> 00:48:34,710
Back in the day, you
probably did have to type

934
00:48:34,710 --> 00:48:40,230
for many companies, www.example.com
instead of just going to example.com

935
00:48:40,230 --> 00:48:43,200
and hoping that you end
up in the right place.

936
00:48:43,200 --> 00:48:45,780
Well, humans have gotten more
comfortable with the internet

937
00:48:45,780 --> 00:48:48,240
over the past years, over
the past decades, and indeed,

938
00:48:48,240 --> 00:48:52,080
whereas years ago, in order to advertise
yourself effectively on the web,

939
00:48:52,080 --> 00:48:54,810
you might have indeed needed to
go to press on your business card

940
00:48:54,810 --> 00:49:00,500
or advertisement with
http://www.something.com.

941
00:49:00,500 --> 00:49:05,505
But all of us have kind of seen
HTTP enough, if not HTTPS as well,

942
00:49:05,505 --> 00:49:07,130
you don't need to tell me to type that.

943
00:49:07,130 --> 00:49:09,660
And indeed, my browser no
longer requires me to type

944
00:49:09,660 --> 00:49:13,080
that, so now you see business
cards and advertisements

945
00:49:13,080 --> 00:49:15,805
with just www.something.com.

946
00:49:15,805 --> 00:49:18,480
But you know what, I'm
not new to the internet.

947
00:49:18,480 --> 00:49:22,140
I know what ww is, and I
know what dot com is as well,

948
00:49:22,140 --> 00:49:27,780
don't even bother showing me or telling
me on your card or your website or ad

949
00:49:27,780 --> 00:49:32,740
that it's www.something.com,
just tell me something.com.

950
00:49:32,740 --> 00:49:35,328
And so browsers have been
getting more user friendly

951
00:49:35,328 --> 00:49:37,120
and humans have been
getting more familiar,

952
00:49:37,120 --> 00:49:40,540
and so we tend not to see
those prefixes anymore.

953
00:49:40,540 --> 00:49:44,500
But it turns out that for technical
reasons, for security reasons,

954
00:49:44,500 --> 00:49:47,370
it tends to be useful
to have a subdomain.

955
00:49:47,370 --> 00:49:49,120
As an aside, for things
like cookies, it's

956
00:49:49,120 --> 00:49:51,520
useful to keep cookies
in a subdomain as opposed

957
00:49:51,520 --> 00:49:56,330
to the domain itself just to narrow the
scope via which they can be accessed.

958
00:49:56,330 --> 00:49:58,960
But also for marketing
sake, it would be nice

959
00:49:58,960 --> 00:50:04,120
if everyone in the world, whether they
type harvard.edu or www.harvard.edu,

960
00:50:04,120 --> 00:50:08,020
ultimately end up in the same
location just because that's how we

961
00:50:08,020 --> 00:50:09,940
want to present ourselves to the world.

962
00:50:09,940 --> 00:50:13,150
And so for both technical and marketing
and security reasons alike might

963
00:50:13,150 --> 00:50:18,100
Harvard or a company want to
redirect to a URL like this one here.

964
00:50:18,100 --> 00:50:19,810
Now what does your browser know to do?

965
00:50:19,810 --> 00:50:23,440
Well, when your browser receives
not 200 OK, in which case

966
00:50:23,440 --> 00:50:27,460
it just shows you the page, but
instead receives 301 moved permanently,

967
00:50:27,460 --> 00:50:31,390
it instead looks for that location
line and takes you there instead,

968
00:50:31,390 --> 00:50:35,920
at which point then
you'll get that 200 OK.

969
00:50:35,920 --> 00:50:38,350
And so this, again, is with browsers do.

970
00:50:38,350 --> 00:50:42,820
HTTP is what they understand, and
know by definition of that protocol

971
00:50:42,820 --> 00:50:45,160
how to handle these cases.

972
00:50:45,160 --> 00:50:50,140
But not everything is always OK and not
always has something moved permanently.

973
00:50:50,140 --> 00:50:52,670
Sometimes something's just not found.

974
00:50:52,670 --> 00:50:55,210
And in fact, of all of these
numbers we've seen thus far,

975
00:50:55,210 --> 00:51:00,160
odds are you've not seen or cared
about 200 or even 301, but most of us

976
00:51:00,160 --> 00:51:03,070
have probably at least once seen 404.

977
00:51:03,070 --> 00:51:04,060
Why?

978
00:51:04,060 --> 00:51:06,640
Why in the world is that
the number we somehow

979
00:51:06,640 --> 00:51:11,140
see anytime you visit a web page that's
gone, or anytime you mistype an address

980
00:51:11,140 --> 00:51:12,610
and you reach a dead end?

981
00:51:12,610 --> 00:51:16,300
Well, for better or for worse the
designers of websites for years

982
00:51:16,300 --> 00:51:20,360
have exposed this value to end users
even though it's not all that useful.

983
00:51:20,360 --> 00:51:22,720
But it's indeed the
unique value that humans

984
00:51:22,720 --> 00:51:27,550
decided some years ago would uniquely
represent the notion of a page

985
00:51:27,550 --> 00:51:28,720
not being found.

986
00:51:28,720 --> 00:51:33,070
So if inside of that virtual envelope
comes back a message 404 not found,

987
00:51:33,070 --> 00:51:36,030
the browser can say that
literally or perhaps display

988
00:51:36,030 --> 00:51:41,140
a cute message to that effect, but
the reason that you're seeing that 404

989
00:51:41,140 --> 00:51:44,200
is because quite literally
and mind numbingly that

990
00:51:44,200 --> 00:51:49,480
is just the low level status code that
has come back from an HTTP server.

991
00:51:49,480 --> 00:51:50,890
And there's more of these, too.

992
00:51:50,890 --> 00:51:53,260
In fact, 200 OK is the
best you might get.

993
00:51:53,260 --> 00:51:55,300
301 moved permanently we've seen.

994
00:51:55,300 --> 00:51:57,910
302 found is another
form of redirection,

995
00:51:57,910 --> 00:51:59,640
but a temporary one instead.

996
00:51:59,640 --> 00:52:04,480
304 not modified is a response that
a server can send for efficiency.

997
00:52:04,480 --> 00:52:07,240
If you visited a web
page just a moment ago

998
00:52:07,240 --> 00:52:09,520
and you happen to hit
reload or click on a link

999
00:52:09,520 --> 00:52:13,060
and get back the same content again,
it's not terribly efficient or good

1000
00:52:13,060 --> 00:52:16,750
business for a company to incur
the time and perhaps financial cost

1001
00:52:16,750 --> 00:52:20,740
to retransmit all of those bits
to you, and so it might instead

1002
00:52:20,740 --> 00:52:25,540
respond with an envelope more
succinctly with 304 not modified

1003
00:52:25,540 --> 00:52:30,310
without anything else deeper in that
envelope, no additional content.

1004
00:52:30,310 --> 00:52:34,030
And so this way your browser will just
reline its own cache, its own copy,

1005
00:52:34,030 --> 00:52:36,550
so to speak, of the original request.

1006
00:52:36,550 --> 00:52:39,990
Meanwhile, if you're not allowed to
visit some web page because you've not

1007
00:52:39,990 --> 00:52:41,990
logged in or you don't
have authorization there,

1008
00:52:41,990 --> 00:52:45,400
too, well 401 unauthorized
might instead come back.

1009
00:52:45,400 --> 00:52:46,960
As might 403 forbidden.

1010
00:52:46,960 --> 00:52:49,420
404 not found means
there's just nothing there.

1011
00:52:49,420 --> 00:52:52,750
418 I'm a teapot was an
April Fool's joke some years

1012
00:52:52,750 --> 00:52:55,270
ago where someone went to the
lengths of actually writing

1013
00:52:55,270 --> 00:53:00,340
a formal specification for what a server
should say when it is in fact a teapot.

1014
00:53:00,340 --> 00:53:03,350
But the worst error you might see,
and most users would never see this,

1015
00:53:03,350 --> 00:53:08,620
but developers of software would
is five zero zero, 500, which

1016
00:53:08,620 --> 00:53:12,520
represents an internal server
error, and almost always represents

1017
00:53:12,520 --> 00:53:17,290
a logical or a syntactic error in
the code that someone has written,

1018
00:53:17,290 --> 00:53:21,410
be it in Python or any
number of other languages.

1019
00:53:21,410 --> 00:53:24,340
And now a fun example, perhaps,
to bring all this home.

1020
00:53:24,340 --> 00:53:28,510
It turns out that safetyschool.org
is an actual address on the web.

1021
00:53:28,510 --> 00:53:30,940
And indeed, it happens to
have been bought or rented

1022
00:53:30,940 --> 00:53:33,790
for years now by some Harvard alum.

1023
00:53:33,790 --> 00:53:36,580
And indeed, if you
visit safetyschool.org,

1024
00:53:36,580 --> 00:53:40,330
you shall find yourself
at this website here.

1025
00:53:40,330 --> 00:53:44,170
http://safetyschool.org.

1026
00:53:44,170 --> 00:53:49,060
We find ourselves whisked
away to www.yale.edu.

1027
00:53:49,060 --> 00:53:50,320
But how is that implemented?

1028
00:53:50,320 --> 00:53:52,420
Well, let's again turn
to our terminal window,

1029
00:53:52,420 --> 00:53:55,870
where we can see really the
contents of that virtual envelope.

1030
00:53:55,870 --> 00:53:59,710
And if in here in my terminal
window I again type curl dash I,

1031
00:53:59,710 --> 00:54:05,470
http://safetyschool.org,
well I see all of the headers

1032
00:54:05,470 --> 00:54:07,300
that are exactly coming back.

1033
00:54:07,300 --> 00:54:11,800
And indeed, here, safetyschool.org
has permanently moved for years now

1034
00:54:11,800 --> 00:54:17,740
to this location, http://www.yale.org.

1035
00:54:17,740 --> 00:54:21,220
A fun jab at our rivals that
some alum has been paying now

1036
00:54:21,220 --> 00:54:25,760
for years on an annual basis.

1037
00:54:25,760 --> 00:54:29,140
So we now have a pair of
protocols, TCP and IP,

1038
00:54:29,140 --> 00:54:32,020
via which we can get data,
any data, from point A

1039
00:54:32,020 --> 00:54:33,890
to point B on the internet.

1040
00:54:33,890 --> 00:54:38,800
Sometimes that data is itself HTTP
data that is a request for a web page

1041
00:54:38,800 --> 00:54:41,080
or a response with a web page.

1042
00:54:41,080 --> 00:54:45,880
But what if there are so many others
trying to access data at point B--

1043
00:54:45,880 --> 00:54:49,360
that is to say, business is good, and
a web server out there is receiving

1044
00:54:49,360 --> 00:54:54,400
so many packets per second that the
server cannot quite yet keep up?

1045
00:54:54,400 --> 00:54:57,700
The routers in between might very well
be able to handle that load perfectly

1046
00:54:57,700 --> 00:55:02,320
because those are much bigger servers,
conceptually and physically, with far

1047
00:55:02,320 --> 00:55:05,650
more CPUs and RAM and
therefore can handle that load,

1048
00:55:05,650 --> 00:55:10,930
but some business' server out
there is only finite in capacity.

1049
00:55:10,930 --> 00:55:15,250
And so what happens when you need
to scale to handle more users?

1050
00:55:15,250 --> 00:55:17,680
Well, you might have
initially just one server

1051
00:55:17,680 --> 00:55:20,150
such as that Dell server pictured here.

1052
00:55:20,150 --> 00:55:22,450
This is what's called
a rack server, insofar

1053
00:55:22,450 --> 00:55:26,170
as it's designed to exist on a rack
that you slide this thing into,

1054
00:55:26,170 --> 00:55:29,630
and it happens to be one
rack unit or 1.5 inches,

1055
00:55:29,630 --> 00:55:32,230
which is simply a
standardization thereof.

1056
00:55:32,230 --> 00:55:35,500
Inside of this rack server
is its hard drive, and RAM,

1057
00:55:35,500 --> 00:55:39,340
and CPU, and more pieces, but
it's exactly the same technology

1058
00:55:39,340 --> 00:55:41,770
that you might have in
a box under your desk

1059
00:55:41,770 --> 00:55:45,910
or even in the form factor of a
laptop, just bigger and faster.

1060
00:55:45,910 --> 00:55:48,400
And, to be fair, more expensive.

1061
00:55:48,400 --> 00:55:52,150
But it's only so big, indeed,
it's only 1.5 inches tall

1062
00:55:52,150 --> 00:55:54,790
and some number of inches deep,
which is to say there's only

1063
00:55:54,790 --> 00:55:57,160
a finite amount of RAM in there.

1064
00:55:57,160 --> 00:55:59,770
There's only a fixed
number of CPUs in there,

1065
00:55:59,770 --> 00:56:04,540
and there's only so many gigabytes,
presumably, of disk storage space.

1066
00:56:04,540 --> 00:56:07,480
At some point or other, we're
going to run out of one or more

1067
00:56:07,480 --> 00:56:08,830
of those resources.

1068
00:56:08,830 --> 00:56:13,600
And even though we've not really gotten
into the weeds of how a server handles

1069
00:56:13,600 --> 00:56:16,360
and reads these envelopes,
it certainly stands

1070
00:56:16,360 --> 00:56:19,780
to reason that it can only
read with finite resources

1071
00:56:19,780 --> 00:56:23,620
some finite number of
packets per unit of time,

1072
00:56:23,620 --> 00:56:26,290
be it second or minutes or days.

1073
00:56:26,290 --> 00:56:28,750
And so at some point
if business is booming,

1074
00:56:28,750 --> 00:56:32,080
we might receive at any
given point more packets

1075
00:56:32,080 --> 00:56:35,380
of information that we can handle and
indeed, like some routers, if they're

1076
00:56:35,380 --> 00:56:38,890
overwhelmed, we might just
drop these incoming packets,

1077
00:56:38,890 --> 00:56:43,210
or worse yet, not expect them and
just crash or freeze or somehow

1078
00:56:43,210 --> 00:56:45,580
behave unpredictably.

1079
00:56:45,580 --> 00:56:47,630
And that's probably not
good for our business.

1080
00:56:47,630 --> 00:56:49,960
So how can we go about
solving this problem?

1081
00:56:49,960 --> 00:56:54,430
Well, the easiest way, quite simply,
is to scale vertically, so to speak.

1082
00:56:54,430 --> 00:56:56,020
That is don't use that server.

1083
00:56:56,020 --> 00:57:01,000
Instead, buy one that's bigger with more
RAM and more CPUs and more disk space

1084
00:57:01,000 --> 00:57:03,970
and faster internet
connectivity, and really just

1085
00:57:03,970 --> 00:57:06,550
avoid that problem altogether.

1086
00:57:06,550 --> 00:57:08,030
Why is this compelling?

1087
00:57:08,030 --> 00:57:11,560
Well, cost of it aside, you
don't have to change your code,

1088
00:57:11,560 --> 00:57:14,260
you needn't change your
configuration in software,

1089
00:57:14,260 --> 00:57:20,050
you need only throw hardware and in
turn, to be fair, money at the problem.

1090
00:57:20,050 --> 00:57:23,470
Now that in and of itself might alone
be a deal breaker, the money alone,

1091
00:57:23,470 --> 00:57:27,640
but at some point if we want to handle
that business, we've got to scale up,

1092
00:57:27,640 --> 00:57:29,830
but even this is shortsighted.

1093
00:57:29,830 --> 00:57:35,620
Because at the end of the day, Dell only
sells servers that operate so quickly

1094
00:57:35,620 --> 00:57:37,660
and have so much disk space.

1095
00:57:37,660 --> 00:57:40,210
Those resources, too,
are ultimately finite.

1096
00:57:40,210 --> 00:57:42,790
And while next year there
might be an even bigger version

1097
00:57:42,790 --> 00:57:44,980
of this same machine
out there, this year

1098
00:57:44,980 --> 00:57:47,020
you might have the top of the line.

1099
00:57:47,020 --> 00:57:50,800
So at some point, one server,
even with so many resources,

1100
00:57:50,800 --> 00:57:54,910
might not be able to handle all of the
packets and business you're getting.

1101
00:57:54,910 --> 00:57:56,980
So what do you then do?

1102
00:57:56,980 --> 00:58:01,660
Well, there is an opportunity to
scale not vertically, so to speak,

1103
00:58:01,660 --> 00:58:04,120
but horizontally instead.

1104
00:58:04,120 --> 00:58:07,930
Focusing not on the top
tier machines, but instead,

1105
00:58:07,930 --> 00:58:12,820
two of the smaller ones, or as needed
three or four or more of the same.

1106
00:58:12,820 --> 00:58:15,940
In other words, spending
lower on that cost curve,

1107
00:58:15,940 --> 00:58:18,520
getting more hardware,
hopefully, for your money,

1108
00:58:18,520 --> 00:58:21,700
but such that the net effect
is even more CPU power

1109
00:58:21,700 --> 00:58:25,630
and more disk space and more RAM than
you might have gotten with that one

1110
00:58:25,630 --> 00:58:27,240
souped up machine itself.

1111
00:58:27,240 --> 00:58:28,990
And heck, if you really
need the capacity,

1112
00:58:28,990 --> 00:58:32,200
you can buy any number
of these big servers,

1113
00:58:32,200 --> 00:58:35,980
but you do somehow ultimately
have to interconnect them.

1114
00:58:35,980 --> 00:58:38,260
And here now is where
there's a trade-off.

1115
00:58:38,260 --> 00:58:40,660
Whereas money was
really the only barrier

1116
00:58:40,660 --> 00:58:44,140
to solving this problem initially,
though easier said than done,

1117
00:58:44,140 --> 00:58:48,370
now we have to re-engineer
our system, because no longer

1118
00:58:48,370 --> 00:58:51,910
are packets of internet data
coming in from our customers

1119
00:58:51,910 --> 00:58:54,670
and ending up in one place,
they now have to somehow

1120
00:58:54,670 --> 00:58:58,750
be spread across multiple servers.

1121
00:58:58,750 --> 00:59:00,910
So how might we do this back in the day?

1122
00:59:00,910 --> 00:59:04,780
Well, back in the late 90s, when
Larry and Sergey of Google fame

1123
00:59:04,780 --> 00:59:07,270
built out their first
cluster of servers,

1124
00:59:07,270 --> 00:59:09,970
they didn't have those
pretty Dell boxes, rather,

1125
00:59:09,970 --> 00:59:12,940
this was, now in Google's
museum, reportedly

1126
00:59:12,940 --> 00:59:15,550
one of their first racks of servers.

1127
00:59:15,550 --> 00:59:18,100
Notice there's no shiny
cases, let alone logos,

1128
00:59:18,100 --> 00:59:22,180
but instead, lots of circuit boards on
which are hard drive after hard drive

1129
00:59:22,180 --> 00:59:26,650
after hard drive and suffice to say
so many wires connecting everything.

1130
00:59:26,650 --> 00:59:30,240
And even though, ironically, this
picture seems to be vertical,

1131
00:59:30,240 --> 00:59:36,150
this is, perhaps, one of the earliest
examples in our internet era of scaling

1132
00:59:36,150 --> 00:59:37,230
horizontally.

1133
00:59:37,230 --> 00:59:40,110
Each of these servers, which is
represented by each of these boards,

1134
00:59:40,110 --> 00:59:43,260
is somehow interconnected in
such a way that those servers

1135
00:59:43,260 --> 00:59:45,100
can intercommunicate.

1136
00:59:45,100 --> 00:59:46,140
But how?

1137
00:59:46,140 --> 00:59:48,540
Well, let's consider, with
the proverbial engineering hat

1138
00:59:48,540 --> 00:59:51,750
on, how servers might
somehow intercommunicate.

1139
00:59:51,750 --> 00:59:54,690
If up here, for instance, is
just some artist's rendition

1140
00:59:54,690 --> 00:59:58,590
of someone's laptop, a potential
customer who's sending us packets,

1141
00:59:58,590 --> 01:00:03,780
that customer might previously have
been accessing our server, which we'll

1142
01:00:03,780 --> 01:00:06,270
represent here with a
box and just call it

1143
01:00:06,270 --> 01:00:09,250
A. Server A. There's no
other servers involved,

1144
01:00:09,250 --> 01:00:12,600
but there is some internet in between
us here, so we'll assume that this

1145
01:00:12,600 --> 01:00:14,880
is the so-called cloud, so to speak.

1146
01:00:14,880 --> 01:00:17,730
And I, as this laptop or the
customer, has a connection

1147
01:00:17,730 --> 01:00:20,940
and so does that one and
only server have the same.

1148
01:00:20,940 --> 01:00:22,570
This picture is fairly straightforward.

1149
01:00:22,570 --> 01:00:24,720
Now you request a web
page via this browser,

1150
01:00:24,720 --> 01:00:27,360
it somehow traverses the
internet via routers,

1151
01:00:27,360 --> 01:00:31,150
and then ultimately ends
up at that server A.

1152
01:00:31,150 --> 01:00:33,180
But what if, instead,
there's not just A,

1153
01:00:33,180 --> 01:00:35,580
even if it's top of the line,
because that's not enough,

1154
01:00:35,580 --> 01:00:40,710
but instead their servers A and
B together here for this website.

1155
01:00:40,710 --> 01:00:44,790
Well, here we might now have two boxes,
the same size or bigger or smaller,

1156
01:00:44,790 --> 01:00:46,860
but ultimately finite, as well.

1157
01:00:46,860 --> 01:00:51,330
And somehow we need to now
decide how to route information

1158
01:00:51,330 --> 01:00:54,460
from customer to server
A or B. In other words,

1159
01:00:54,460 --> 01:00:57,420
there is now virtually
a fork in the road,

1160
01:00:57,420 --> 01:01:00,700
left or right, that
packets need to traverse.

1161
01:01:00,700 --> 01:01:02,790
So how can we implement
this building block?

1162
01:01:02,790 --> 01:01:05,580
Well, again, as always, go
back to first principles.

1163
01:01:05,580 --> 01:01:07,710
We know from our stack
of internet technologies

1164
01:01:07,710 --> 01:01:10,710
that we already have a
mechanism via which to translate

1165
01:01:10,710 --> 01:01:13,080
domain names into IP addresses.

1166
01:01:13,080 --> 01:01:15,300
And if each of these
servers, by definition of IP,

1167
01:01:15,300 --> 01:01:21,180
has its own IP address, why not
just use DNS to solve this problem?

1168
01:01:21,180 --> 01:01:25,680
When a customer requests example.com,
perhaps answer that request

1169
01:01:25,680 --> 01:01:29,790
with the IP address of A. And then when
a second customer somewhere else out

1170
01:01:29,790 --> 01:01:34,110
there on his or her laptop
asks for example.com next,

1171
01:01:34,110 --> 01:01:39,060
return to them the IP address of
B and vise versa, again and again.

1172
01:01:39,060 --> 01:01:41,580
Literally adopting a
round robin technique

1173
01:01:41,580 --> 01:01:45,330
of sorts, whereby one time you
answer A, the next time you answer B,

1174
01:01:45,330 --> 01:01:47,580
and back and forth you go.

1175
01:01:47,580 --> 01:01:51,690
On average, you would like to think that
this uniform distribution of answers

1176
01:01:51,690 --> 01:01:55,110
will give you 50% load,
that is to say traffic,

1177
01:01:55,110 --> 01:01:58,020
on one server and 50% on the other.

1178
01:01:58,020 --> 01:02:00,840
But perhaps this customer is
more of a shopper than this one,

1179
01:02:00,840 --> 01:02:03,960
and they end up imposing even
more load on A than on B,

1180
01:02:03,960 --> 01:02:08,067
so there with that simple
heuristic you can get skew.

1181
01:02:08,067 --> 01:02:10,650
You might not even use round
robin, you could just use random,

1182
01:02:10,650 --> 01:02:15,390
but on there on average yes, you'll
send 50% traffic left and 50% right,

1183
01:02:15,390 --> 01:02:19,170
but some of those users might
be heavier users than other.

1184
01:02:19,170 --> 01:02:22,020
So perhaps we should have
some form of feedback loop,

1185
01:02:22,020 --> 01:02:24,910
and DNS alone might not be sufficient.

1186
01:02:24,910 --> 01:02:28,710
We really need there to be a
middle man, such as this dot here,

1187
01:02:28,710 --> 01:02:33,840
that decides more intelligently
whether to send data to A or to B.

1188
01:02:33,840 --> 01:02:37,560
And we'll call this thing here,
this dot now, a load balancer.

1189
01:02:37,560 --> 01:02:40,030
Aptly name insofar as
it balances load that's

1190
01:02:40,030 --> 01:02:42,270
incoming across multiple servers.

1191
01:02:42,270 --> 01:02:43,410
But how?

1192
01:02:43,410 --> 01:02:47,970
Well, if these connections between
A and B in this load balancer

1193
01:02:47,970 --> 01:02:50,460
are not uni-directional,
but bidirectional somehow,

1194
01:02:50,460 --> 01:02:54,360
literally a cable that allows
bits to flow left and right.

1195
01:02:54,360 --> 01:02:58,920
Could we perhaps have A just continually
report back to that load balancer

1196
01:02:58,920 --> 01:03:02,040
saying, I have capacity,
I have capacity?

1197
01:03:02,040 --> 01:03:04,800
Whereas B might say, I've
got too many customers.

1198
01:03:04,800 --> 01:03:08,850
And logically, then, this load balancer
can just start sending no traffic to B

1199
01:03:08,850 --> 01:03:11,280
and send all of it to A or vise versa.

1200
01:03:11,280 --> 01:03:13,350
Of course, logically,
we could find ourselves

1201
01:03:13,350 --> 01:03:18,270
in a situation where both A and B
are too busy, what then do we do?

1202
01:03:18,270 --> 01:03:21,390
Well, at some point we have
to throw money at the problem

1203
01:03:21,390 --> 01:03:23,760
and solve it by just adding hardware.

1204
01:03:23,760 --> 01:03:27,000
And so C might be added to
the mix with that same logic,

1205
01:03:27,000 --> 01:03:30,990
but the load balancer
just has to know about it.

1206
01:03:30,990 --> 01:03:32,910
So all fine and good,
we seem to have solved

1207
01:03:32,910 --> 01:03:37,680
the problem in a very straightforward
way, but as with computer science

1208
01:03:37,680 --> 01:03:40,740
more generally, there's probably
a price paid and a trade-off,

1209
01:03:40,740 --> 01:03:42,570
and not just financial.

1210
01:03:42,570 --> 01:03:47,760
Unfortunately, even though I have two,
maybe even three servers now, therefore

1211
01:03:47,760 --> 01:03:50,490
seemingly having high
availability of service,

1212
01:03:50,490 --> 01:03:53,340
that is any one of these servers
theoretically could go down

1213
01:03:53,340 --> 01:03:57,090
and I've still got 2/3 of my capacity.

1214
01:03:57,090 --> 01:04:02,520
But there's a single point of
failure here, an SPOF so to speak,

1215
01:04:02,520 --> 01:04:05,070
that could really derail
the whole process.

1216
01:04:05,070 --> 01:04:09,150
What happens if this load balancer,
which while pictorial is just a dot,

1217
01:04:09,150 --> 01:04:13,470
is actually a server
underneath the hood itself?

1218
01:04:13,470 --> 01:04:15,990
What if that load balancer
goes down, or what if that load

1219
01:04:15,990 --> 01:04:17,670
balancer itself gets overwhelmed?

1220
01:04:17,670 --> 01:04:22,290
It does not matter how many servers you
have here, A through Z, if none of them

1221
01:04:22,290 --> 01:04:23,710
can be reached.

1222
01:04:23,710 --> 01:04:28,050
So this simple architecture
alone is not a solution.

1223
01:04:28,050 --> 01:04:32,470
And indeed, this is what is meant
by architecting network itself.

1224
01:04:32,470 --> 01:04:36,320
This design is probably not the
best, especially for business.

1225
01:04:36,320 --> 01:04:40,300
And so let's start anew, at least
down here inside this company,

1226
01:04:40,300 --> 01:04:44,650
and consider if one load balancer is
not great, what's better than one?

1227
01:04:44,650 --> 01:04:46,510
Well, honestly, two.

1228
01:04:46,510 --> 01:04:48,550
And so let's now draw
them a bit bigger, where

1229
01:04:48,550 --> 01:04:51,820
here we have a load balancer on
the left, and here on the right,

1230
01:04:51,820 --> 01:04:53,770
and we'll number them 1 and 2.

1231
01:04:53,770 --> 01:04:58,330
Whereas our servers we'll continue to
name A and B and C and perhaps even

1232
01:04:58,330 --> 01:05:02,170
through Z. And now we just have
to ensure that we have connections

1233
01:05:02,170 --> 01:05:04,300
to both load balancers,
and that each load

1234
01:05:04,300 --> 01:05:09,460
balancer can connect to each server
in this sort of mesh network here.

1235
01:05:09,460 --> 01:05:12,730
It's wonderfully redundant
now, albeit a bit complex.

1236
01:05:12,730 --> 01:05:15,340
But because we have all of
these interconnections now,

1237
01:05:15,340 --> 01:05:21,280
we can ensure that even if one or two go
down, data can still reach A, B, or C.

1238
01:05:21,280 --> 01:05:25,660
But how to know whether load balancer
1 should be doing all of this

1239
01:05:25,660 --> 01:05:27,040
or load balancer 2?

1240
01:05:27,040 --> 01:05:30,850
You know what, why don't we draw
another connection between 1 and 2?

1241
01:05:30,850 --> 01:05:35,410
And a very common paradigm in
systems is heartbeats, quite simply.

1242
01:05:35,410 --> 01:05:40,120
Much like you and I have every second
or so a heartbeat saying we are alive,

1243
01:05:40,120 --> 01:05:42,730
we are alive, hello world,
hello world, if you will,

1244
01:05:42,730 --> 01:05:48,340
so here might load balancers 1
and 2, themselves just servers,

1245
01:05:48,340 --> 01:05:53,050
say I am alive, I am alive,
hello number 2, hello number 2.

1246
01:05:53,050 --> 01:05:57,010
And if 2 does not hear 1
eventually, or if 1 does not hear 2,

1247
01:05:57,010 --> 01:05:59,800
the other can just commandeer that role.

1248
01:05:59,800 --> 01:06:03,100
By default, only 1 will be load
balancing, but if it goes offline,

1249
01:06:03,100 --> 01:06:05,470
2 will presume to take over.

1250
01:06:05,470 --> 01:06:08,680
And now we have this property
generally known as high availability.

1251
01:06:08,680 --> 01:06:12,070
Even if we lose one or more
servers can we still stay up,

1252
01:06:12,070 --> 01:06:16,000
and there is no single point of
failure, at least here in this picture,

1253
01:06:16,000 --> 01:06:18,770
because we now have that
second load balancer.

1254
01:06:18,770 --> 01:06:21,250
But if we look a little
higher, it would seem

1255
01:06:21,250 --> 01:06:24,440
that we do actually have another
single point of failure in here,

1256
01:06:24,440 --> 01:06:26,380
and now we go down the rabbit hole.

1257
01:06:26,380 --> 01:06:29,500
If this line here to
the cloud, the internet,

1258
01:06:29,500 --> 01:06:33,520
represents my internet
connection, my ISP,

1259
01:06:33,520 --> 01:06:38,110
what if that ISP, Comcast, Verizon
or any other, itself goes down,

1260
01:06:38,110 --> 01:06:42,790
a big storm and a loss of power
might take my whole business offline.

1261
01:06:42,790 --> 01:06:44,830
Well, the best way to
solve that would be

1262
01:06:44,830 --> 01:06:48,040
to access someone else's
internet connectivity

1263
01:06:48,040 --> 01:06:49,820
and make sure you're connected to that.

1264
01:06:49,820 --> 01:06:54,460
And in fact, if we keep going
with load balancer 3 or even 4

1265
01:06:54,460 --> 01:06:59,770
or server D, E, and F, this picture very
quickly starts to get so intertwined.

1266
01:06:59,770 --> 01:07:01,240
But this is how you do it.

1267
01:07:01,240 --> 01:07:06,580
And not too long ago was this done
entirely with wires and hardware.

1268
01:07:06,580 --> 01:07:10,450
But these days this topology,
if you will, this architecture,

1269
01:07:10,450 --> 01:07:12,640
is increasingly done in software.

1270
01:07:12,640 --> 01:07:15,730
And indeed, the whole
thing is done in the cloud.

1271
01:07:15,730 --> 01:07:19,420
Less frequently do staff of
companies find themselves crawling

1272
01:07:19,420 --> 01:07:22,630
along the floor and in wiring
closets and in data centers,

1273
01:07:22,630 --> 01:07:25,510
so to speak, making these
connections possible,

1274
01:07:25,510 --> 01:07:28,390
but rather they do it
virtually in software.

1275
01:07:28,390 --> 01:07:30,920
And indeed, thus was born the cloud.

1276
01:07:30,920 --> 01:07:33,910
Well, it turns out that as
Moore's law, so to speak,

1277
01:07:33,910 --> 01:07:37,930
helps us in each passing year, we
seem to have computers that are

1278
01:07:37,930 --> 01:07:40,330
half as expensive and twice as fast.

1279
01:07:40,330 --> 01:07:44,230
Can we ride that sort of curve
of innovation in such a way

1280
01:07:44,230 --> 01:07:47,830
that we can solve even more
problems each year more quickly?

1281
01:07:47,830 --> 01:07:50,610
And yet, with each
passing year, I the human

1282
01:07:50,610 --> 01:07:53,560
am not getting any better or
faster at checking my email

1283
01:07:53,560 --> 01:07:57,490
or using the web, so we increasingly
have on our laptops and desks

1284
01:07:57,490 --> 01:08:00,370
and our server rooms
more computational power

1285
01:08:00,370 --> 01:08:03,440
frankly than we really know what to do.

1286
01:08:03,440 --> 01:08:08,500
And so increasingly in vogue these
days is to virtualize that hardware,

1287
01:08:08,500 --> 01:08:13,330
and to take physical hardware with so
many CPUs and so much RAM and so much

1288
01:08:13,330 --> 01:08:16,149
disk space and to write
software that runs

1289
01:08:16,149 --> 01:08:22,359
on it that creates the illusion that one
computer is two, or one computer is 10.

1290
01:08:22,359 --> 01:08:25,359
That is to say, through
software can you write

1291
01:08:25,359 --> 01:08:31,210
code that virtualizes that hardware,
thereby creating the illusion that you

1292
01:08:31,210 --> 01:08:36,340
can have one server per customer
but all 10 of those customers

1293
01:08:36,340 --> 01:08:38,529
are on the same machine.

1294
01:08:38,529 --> 01:08:42,120
Virtualization includes products
like VMware and Parallels

1295
01:08:42,120 --> 01:08:43,870
and other companies
as well, and it's just

1296
01:08:43,870 --> 01:08:45,580
software that runs on
top of the hardware

1297
01:08:45,580 --> 01:08:50,439
and creates this illusion, which
then is all the better for business.

1298
01:08:50,439 --> 01:08:53,200
If you can sell one piece
of hardware multiple times

1299
01:08:53,200 --> 01:08:55,960
but not necessarily in a way
that you're over-provisioning it

1300
01:08:55,960 --> 01:09:00,220
to multiple customers, but rather
you're isolating each of those customers

1301
01:09:00,220 --> 01:09:03,880
from one another, giving them not
only the illusion of their own machine

1302
01:09:03,880 --> 01:09:07,180
but indeed, the constraints
whereby my data can't

1303
01:09:07,180 --> 01:09:12,850
be accessed by another customer who
only has cloud access there, too.

1304
01:09:12,850 --> 01:09:17,470
And indeed, this is really in
part, why we have now this cloud.

1305
01:09:17,470 --> 01:09:20,000
The cloud is more of a buzz
word than anything technical.

1306
01:09:20,000 --> 01:09:23,770
Indeed, using the cloud just
means using servers somewhere else

1307
01:09:23,770 --> 01:09:25,750
that someone else is managing.

1308
01:09:25,750 --> 01:09:28,930
No longer do companies with as much
frequency have their own server

1309
01:09:28,930 --> 01:09:32,580
room in their office, or their own data
center in some warehouse somewhere.

1310
01:09:32,580 --> 01:09:35,790
Rather, they virtualized even
that piece of their product

1311
01:09:35,790 --> 01:09:38,729
using Amazon or Microsoft
or Google or others

1312
01:09:38,729 --> 01:09:41,207
out there that provide
you with access to servers

1313
01:09:41,207 --> 01:09:43,290
that they themselves
control, but they provide you

1314
01:09:43,290 --> 01:09:49,319
with access to the illusion of your very
own servers known as virtual machines.

1315
01:09:49,319 --> 01:09:52,590
And via this process can
we take ever more advantage

1316
01:09:52,590 --> 01:09:56,130
of so many of those new
CPUs and disk space and RAM

1317
01:09:56,130 --> 01:09:58,020
that otherwise might
frankly go to waste,

1318
01:09:58,020 --> 01:10:02,250
because there's only so much we can
typically do with one such machine.

1319
01:10:02,250 --> 01:10:06,540
Hence you might now think of this
design, this stack, so to speak,

1320
01:10:06,540 --> 01:10:07,200
as follows.

1321
01:10:07,200 --> 01:10:10,170
In green here pictured is
infrastructure, the physical hardware

1322
01:10:10,170 --> 01:10:11,130
that you have bought.

1323
01:10:11,130 --> 01:10:14,340
Here in blue is the
hypervisor, the software

1324
01:10:14,340 --> 01:10:18,660
called VMware or Parallels or something
else, that virtualizes this hardware

1325
01:10:18,660 --> 01:10:21,240
and creates the illusion
that you actually have

1326
01:10:21,240 --> 01:10:23,605
three machines, for instance, on one.

1327
01:10:23,605 --> 01:10:25,980
And within each of those
machines, which you can think of

1328
01:10:25,980 --> 01:10:29,220
is just a separate window on
that computer, double click

1329
01:10:29,220 --> 01:10:32,610
to open computer A so to speak
and computer B and computer

1330
01:10:32,610 --> 01:10:37,170
C, each of those virtual machines, you
can install your own operating system

1331
01:10:37,170 --> 01:10:39,750
differently in each of
those virtual machines.

1332
01:10:39,750 --> 01:10:42,360
Some version of Windows
in A, another in B,

1333
01:10:42,360 --> 01:10:46,590
and maybe Linux or Unix or something
else in C. And then within A, B, and C

1334
01:10:46,590 --> 01:10:51,240
can you install your own apps, your
own software, or so can your customers,

1335
01:10:51,240 --> 01:10:56,190
thereby being isolated, not so
much physically but virtually,

1336
01:10:56,190 --> 01:10:57,750
from everyone else.

1337
01:10:57,750 --> 01:10:59,970
But of course, there's always a price.

1338
01:10:59,970 --> 01:11:02,820
While this might take better
advantage of the increasing

1339
01:11:02,820 --> 01:11:06,180
computational resources
that we have in these boxes,

1340
01:11:06,180 --> 01:11:08,820
there seems to be some duplication here.

1341
01:11:08,820 --> 01:11:10,950
And indeed, in computer
science, anytime you

1342
01:11:10,950 --> 01:11:14,130
start duplicating resources
or efforts there's

1343
01:11:14,130 --> 01:11:16,710
probably an opportunity
for better design.

1344
01:11:16,710 --> 01:11:19,650
And while this technology
itself is still nascent,

1345
01:11:19,650 --> 01:11:22,680
there's a newcomer to the
field called containerization,

1346
01:11:22,680 --> 01:11:24,180
and it exists in multiple forms.

1347
01:11:24,180 --> 01:11:28,140
But containerization shares
more software, in some sense,

1348
01:11:28,140 --> 01:11:31,680
underneath the hood, so that you might
install an operating system not three

1349
01:11:31,680 --> 01:11:36,870
times but once, and share it across
those machines but in such a way that

1350
01:11:36,870 --> 01:11:39,330
one cannot access the other.

1351
01:11:39,330 --> 01:11:42,630
And on top of that layer,
here called Docker,

1352
01:11:42,630 --> 01:11:44,970
one of the most popular
incarnations thereof,

1353
01:11:44,970 --> 01:11:47,730
you have as before your
infrastructure, the actual hardware,

1354
01:11:47,730 --> 01:11:51,510
on top of which is your own operating
system, be it Windows or Linux.

1355
01:11:51,510 --> 01:11:55,230
On top of that is this program
called Docker that provides you

1356
01:11:55,230 --> 01:11:58,350
then with the ability
to run A through F apps

1357
01:11:58,350 --> 01:12:02,640
instead of say, just three
because the overhead, so to speak,

1358
01:12:02,640 --> 01:12:06,780
computationally is not quite as
much as with virtual machines.

1359
01:12:06,780 --> 01:12:10,620
Here we have three operating
systems, each installed independently

1360
01:12:10,620 --> 01:12:14,010
on the same hardware, we're
just surely consume time,

1361
01:12:14,010 --> 01:12:17,820
whereas here you have just one
operating system theoretically and then

1362
01:12:17,820 --> 01:12:21,000
more room for more apps there upon.

1363
01:12:21,000 --> 01:12:24,660
So whereas containerization
allows you ultimately

1364
01:12:24,660 --> 01:12:27,300
to isolate one app from
another, in virtual machines

1365
01:12:27,300 --> 01:12:30,390
allow you to isolate one
machine from another,

1366
01:12:30,390 --> 01:12:34,410
they do this through different
techniques and with disparate overhead.

1367
01:12:34,410 --> 01:12:37,200
And surely in the years to
come will this overhead only

1368
01:12:37,200 --> 01:12:39,660
get chipped away at as
we humans get better

1369
01:12:39,660 --> 01:12:44,940
about running more and more software
and less and less but more capable

1370
01:12:44,940 --> 01:12:46,290
hardware.

1371
01:12:46,290 --> 01:12:49,260
There than we have these internet
technologies all the way up

1372
01:12:49,260 --> 01:12:52,830
to cloud computing itself, whereas
the technologies we've looked at

1373
01:12:52,830 --> 01:12:57,360
are fairly low level protocols that
simply get zeros and ones from point A

1374
01:12:57,360 --> 01:13:02,340
to point B. Once we have that ability
and we can stipulate that we can do it,

1375
01:13:02,340 --> 01:13:05,510
we can build any number of
abstractions on top of it.

1376
01:13:05,510 --> 01:13:09,300
In HTTP, for instance, do we
have effectively an application,

1377
01:13:09,300 --> 01:13:14,220
known as web browsing, via which we
can transmit text and images and sounds

1378
01:13:14,220 --> 01:13:15,420
and so much more.

1379
01:13:15,420 --> 01:13:18,270
And via the cloud itself
do we have the ability now

1380
01:13:18,270 --> 01:13:21,400
to slice up individual machines
as though they are multiple

1381
01:13:21,400 --> 01:13:25,200
and that picture before can be
implemented not with two load

1382
01:13:25,200 --> 01:13:30,990
balancers and three servers physically,
but maybe, just maybe, with just one.

1383
01:13:30,990 --> 01:13:33,780
One server that's been
so virtualized or in turn

1384
01:13:33,780 --> 01:13:38,310
containerized so that you can have
different parts of its hardware each

1385
01:13:38,310 --> 01:13:41,490
implementing different pieces of
functionality that collectively

1386
01:13:41,490 --> 01:13:43,070
implement that architecture.

1387
01:13:43,070 --> 01:13:45,570
And so whereas back in the day
might you actually physically

1388
01:13:45,570 --> 01:13:49,080
wire all of those disparate
types of machines together, now

1389
01:13:49,080 --> 01:13:52,980
can you do it virtually in software
literally with keystrokes and mouse

1390
01:13:52,980 --> 01:13:56,430
clicks because someone has written
software that abstracts away

1391
01:13:56,430 --> 01:14:00,690
that underlying hardware in such a way
that you can think about it virtually.

1392
01:14:00,690 --> 01:14:03,810
Now at the end of the day, the
servers in Google's and Microsoft

1393
01:14:03,810 --> 01:14:06,750
and Amazon's closets are still
completely physical themselves

1394
01:14:06,750 --> 01:14:11,610
with so many cables, but you can reroute
information, those zeros and ones,

1395
01:14:11,610 --> 01:14:15,000
different ways virtually
thanks to these layers

1396
01:14:15,000 --> 01:14:19,550
that we've built on top of
these internet technologies.

1397
01:14:19,550 --> 01:14:20,738