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[MUSIC PLAYING]

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DAVID MALAN: So it's
after warriors of the net

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we have to go back in and clean
it up the explanation of how

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the internet works.

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But we actually talk
in CS50 more about HTTP

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as a representative
protocol than about TCP/IP.

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Even though with the whole set
up is about home networking

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and how the internet works.

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HTTP is really where we bring the
conversation back to application layer

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programming.

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DOUG LLOYD: Right.

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DAVID MALAN: For the web, in
this case and indeed with HTTP.

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We have an opportunity to
talk about what a protocol is.

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The example I always
give, if you'll humor me.

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Is, hello, nice to meet you.

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DOUG LLOYD: Nice to meet you as well.

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DAVID MALAN: And you acknowledge
that initiative of a protocol

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that I just commenced in.

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And silly little human tradition,
but it's an example of a protocol.

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I extend my hand.

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You extend yours.

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We shake.

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Transaction complete.

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So not unlike the TCP, pun
intended, three way handshake.

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But with HTTP/2, we
have some opportunities

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to discuss what it really is.

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And even though it is a protocol,
which is fancy sounding.

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We try to help students realize that
it's just textual messages inside

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of a virtual envelope, if you will.

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And one of the
demonstrations we'll often do

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is take out an envelope
and some sheets of paper.

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Write the messages on them.

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Put them inside.

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And if you really want
to go down that road.

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You can start talking about how the
HTTP packet is inside of the TCP packet,

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is inside of the IP
packet, and so forth.

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All the way down to that.

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DOUG LLOYD: You can
actually take a picture

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and cut it up into different pieces.

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[INTERPOSING VOICES]

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DOUG LLOYD: TCP reassemble it later on.

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DAVID MALAN: Indeed.

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DOUG LLOYD: Is there a
reason that you wanted

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to focus on the Hypertext Transfer
Protocol as opposed to SMTP, so email?

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I guess HTTP is familiar to students.

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DAVID MALAN: I'm just
more familiar, indeed.

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Most people assume or equate
the web with the internet.

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Even though there are certainly
any number of other services.

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Like SMTP and email more generally.

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So HTTP is also the domain
in which we're going

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to be playing with web programming.

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And so it just seems
to make sense there.

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And students have seen
it in various contexts.

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Sometimes you see cryptic
looking things in the URL.

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Sometimes you don't.

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Sometimes the browser warns you that
you're about to resubmit a form.

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So there's a lot of
ways now to tie together

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things they've seen in the real world.

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And actually explain what's
going on technologically.

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DOUG LLOYD: That's fair.

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You don't get those sort of
error messages or visual cues

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from SMTP or something.

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DAVID MALAN: [INAUDIBLE]
And this is nice too.

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Even though we throw away some of
the more distracting HTTP headers.

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You can start to wrap your mind
around these kinds of messages.

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And understand what
it is that's going on.

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DOUG LLOYD: And it's not
all that arcane to get.

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That's pretty intuitive
what it's going to do.

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Yeah.

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DAVID MALAN: Indeed.

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And what's nice too is because once
we start to reveal a few more headers.

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Like the set cookie
or the cookie headers,

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can we discuss how you can build on
top of an otherwise stateless protocol

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state.

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And then we can also talk about
security, session hijacking,

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or theft of cookies.

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And why encryption in
HTTPS is a good thing.

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Which brings us back then
to week 2 and our discussion

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early on of cryptography, albeit
with a more sophisticated cipher

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at this point.

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So there's a lot of nice
intersections with topics

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past that this allows us to
revisit and then go beyond.

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DOUG LLOYD: And one other great
thing about the fact that we do

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take a look at HTTP headers.

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Is it gives us a chance
to take a look in Chrome,

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the Chrome console of the Inspector.

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Which students will then later
use in particular with JavaScript.

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So it gives them a sense of that tool
that's been baked into their browser.

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That they might not have
had a chance to use before.

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DAVID MALAN: Indeed.

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