WEBVTT

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 So email, probably one of the most well-known
 popular network-based applications.

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 And you probably most likely have
 some familiarity with email.

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 I can almost guarantee you've used
 it in some form or fashion.

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 But if you're new to the world of computing,
 new to the world of networking,

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 you might know how to open up your email
 and check for messages, but you

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 probably don't know what's
 happening under the hood.

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 How's email actually
 do what it does?

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 And so I want to give you sort of an
 overview of that in this particular

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 section. So let's start with like a
 little history lesson here of email,

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 which will help sort of give us ideas
 where it's come from and where it's

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 gone to today. So back in the day, early
 computing, we had dumb terminals

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 that were connected to
 a common mainframe.

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 And let me just scroll forward
 here a little bit.

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 So I've got a good graphic.

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 If you've never heard that term dumb
 terminal before, this is an example.

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 Now this is from the telnet video
 and I'll show this again.

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 But this is an example
 of a dumb terminal.

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 So back in the day, you had this idea
 of a mainframe, which was a big

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 massive computer.

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 And what you as a human being sat
 in front of was a dumb terminal.

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 It had a keyboard.

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 It had possibly a mouse, but I don't
 think even mouse mice were invented

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 back then. And it had a monitor.

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 But that thing that you were sitting
 in front of didn't have any memory.

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 It didn't have any processor.

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 If you disconnected it,
 it couldn't do anything.

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 It had to have a cable leading back
 to that big mainframe because the

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 mainframe was the actual computer.


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 The dumb terminal was just the peripheral
 device, the device that gave

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 you as a human access
 to that computer.

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 And you can see here, back in those
 days, you could have several dozens

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 or even hundreds of terminals if you're
 talking about a government institution

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 or a large college or university all connected
 into this one central computer,

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 this large mainframe back then.

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 So that was back in
 early computers.

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 And back in these days, actually the
 first form of email was really simply

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 that on this mainframe, I would have
 some directories in memory, in that

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 same section of memory, you would have
 directories for whatever project

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 you were working on, Sally would have
 some directories, some files in

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 there. And maybe in my directory, I might
 have a subfolder called mailbox.

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 And so if you want to send email to me
 from your dumb terminal, you would

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 just open up my mailbox file
 and put a text file in there.

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 And so you're basically just putting
 a file into my directory and that

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 was the first early
 forms of email.

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 We're all connected to the same computer
 and we're just opening up each

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 other's files and putting text
 files in there to read them.

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 So it's just basically placing a message
 into another user's directory.

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 So the mailbox was the first email system
 used at MIT, the Massachusetts

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 Institute of Technology
 back in 1965.

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 And then another early program that
 was used, the send message program,

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 to send messages from one
 terminal to another.

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 So this is a way of sending
 a live message to someone.

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 You know, back in the day you could
 sit a dumb terminal, actually type

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 in a message and boop, it would show
 up on the other person's terminal

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 live real time. So if a crude form
 of actually instant messaging more

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 than email, then internet
 works involved.

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 Evolved. Now we had this idea
 of disparate computer systems.

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 You know, my terminal and my computer
 is different, geographically separate

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 from your terminal
 and your computer.

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 And so somebody said, okay, well, we need
 to have a way of putting electronic

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 messages between people,
 between devices.

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 And so we have to somehow have some
 way of putting these messages into

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 envelopes and addressing them so it can
 get to the right computer, because

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 we're not all connected to
 the same computer anymore.

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 So in 1972, one of the big moments in
 email history, Ray Tomlinson decided

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 that he was going to use the at symbol
 on the keyboard to denote sending

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 messages from one computer
 to another.

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 So something you can impress your friends
 with at your cocktail party

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 on Friday night, you know when the
 at symbol was first used in emails,

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 1972. And of course, they will think that
 you're just amazing and wonderful

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 that you know that knowledge.

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 So that's when that
 was first came out.

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 So now we're familiar with the
 email, kbogart at i and e.com.

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 And if we break that down, we probably
 know in case you didn't know this.

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 So kbogart, that is the name of the
 users that we now know that the at

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 symbol says this user is on a remote
 computer and i need.com is the name

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 of the email server that
 stores my messages.

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 Now the technical term is the user at
 and then we have domain.extension.

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 So i and e would be the domain and
 com.com would be the extension.

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 Okay, so what are some common protocols
 that are used with email?

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 A lot of email these days actually
 uses HTTP or HTTPS.

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 If you go into some web based email
 like Gmail or Hotmail or Microsoft

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 Exchange, you know, online,
 that's all HTTP.

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 It's all web based.

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 But if you have a standalone email application,
 then you're going to be

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 using one of these protocols.

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 You might be using SMTP, which is
 the simple mail transfer protocol.

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 SMTP is used for outgoing email.

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 So when you're pushing, when you're
 sending an outgoing email, SMTP is

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 what would be used for that.

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 It's not used for
 retrieving emails.

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 We'll look at that
 in just a second.

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 SMTP is TCP based.

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 All these email applications
 are TCP based.

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 It uses port number 25.

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 And if you want to look at the RFC for
 it, you can see here it's RFC 5321.

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 What protocols are actually
 used for retrieving email?

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 This might be new to you.

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 So you might be the first time you've
 ever heard there's actually one

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 protocol for sending email and another
 protocol for receiving emails for

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 sending. We have SMTP
 for receiving email.

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 There's actually a couple
 of protocols.

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 Once again, we're talking about
 not web based emails.

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 So one protocol for receiving emails
 is the post office protocol or POP3.

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 Also TCP based and you can see some
 facts about that right here.

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 The RFC number and the
 port numbers for that.

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 Another form of receiving emails is
 IMAP, the Internet Message Exchange

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 Protocol. So and I'll talk a little
 bit more in a little bit about what

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 the differences are between
 POP and IMAP.

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 I'm not going to spend
 time on that right now.

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 We'll get to that though
 in just a minute.

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 So we will get back to that.

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 So let's look at, let
 me zoom in here.

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 And so this process from
 a graphical perspective.

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 Okay, so in step number one, you are,
 or I should say should say I am

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 on a laptop. Here I am
 at Keith at INE.com.

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 So that's me. And I send an outgoing
 email to Joe at 777.com.

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 So in my laptop then, my laptop, now if
 I'm using a standalone, a standalone

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 email program, not web based,
 like Eudora as an example.

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 And that means I'm going to invoke
 the simple mail transfer protocol,

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 SMTP. So that's step
 number one here.

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 Now I'm going to be sending this outgoing
 message to what's called a mail

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 transport agent.

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 Mail transport agent.

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 That's the MTA. And this is a response
 for pushing outgoing emails out

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 of the company. So in step number two,
 the mail transport agent says,

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 okay, well, I need to find what the
 mail transport agent is at 777.com.

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 I'm going to have
 to do a DNS query.

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 So here in step number two
 is doing a DNS query.

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 Notice it's doing a query not for an
 A record because we're not looking

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 up a website. We're doing
 a query for an MX record.

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 We want to find the mail exchange
 server, 4777.com.

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 And so that's step number two.

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 Once the DNS server responds back with
 the IP address of this right here.

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 So this is the mail exchange server,
 otherwise known as the mail transfer

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 agent. Now the mail transfer agent for
 INE sends once again the SMTP that

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 outgoing email. That's
 step number three here.

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 So now it's going over a TCP IP connection
 SMTP to the mail transfer agent.

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 The mail transfer agent then in step
 number four sends it to the mail

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 delivery agent. That's what MDA
 stands for mail delivery agent.

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 So when you actually go into your email
 and you query for your email,

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 like we see right here in this next
 slide, you're not actually sending

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 a query to your mail
 transfer agent.

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 You only contact your mail transfer
 agent when you're trying to send a

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 message. But when you're saying,
 hey, what do you have for me?

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 Tell me what's in my inbox.

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 You're going to a different server,
 which is your mail delivery agent.

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 And so that's step number
 five and six right here.

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 So MUA is mail user agent.

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 So that's your laptop, your PC.

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 So the mail user agent
 is using pop or IMAP.

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 So now we're using a different protocol
 here to retrieve our email from

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 the MDA server, which then
 responds back to us.

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 So let's just look real quickly here
 at the differences between IMAP and

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 POP, because we can now see that IMAP
 and POP is used as we saw when you're

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 retrieving emails from your
 mail delivery agent.

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 So what's the difference
 between them?

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 Because if you've ever gone into a standalone
 email program like Eudora

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 or something, you had to set it up from
 scratch, one of the things that

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 probably asked you was, do you want
 to use POP3 or do you want to use

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 IMAP? And so where the difference is
 if you actually do have a choice.

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 So both IMAP and POP3, as it says, are
 protocols which allow you to receive

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 email messages from the
 mail transfer agent.

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 Actually hold on a second.

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 I don't think that is correct.

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 I think it should be the
 mail delivery agent.

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 Let me change that.

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 There we go. From the MDA.

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 Both support using third
 party email clients.

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 So IMAP and POP, if you actually looked
 in the specifications, it would

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 not say here are the drop down
 windows you should have.

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 Here's the buttons you should
 press on the GUI.

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 Here's the colors the
 GUI should use.

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 That's all up to the designer of the
 third party email client, such as

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 Eudora or Outlook or whatever.

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 Big main difference is here.

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 POP, the post office protocol, is designed
 that when you contact your

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 mail delivery agent and
 say, give me my email.

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 You're actually removing it from that
 server so the server no longer has

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 it and it's downloaded
 onto your laptop.

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 Remember your laptop or your tablet
 is considered the mail user agent.

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 That's you, the mail user agent.

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 So the default behavior of POP is that
 once you've downloaded your email,

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 the server no longer has it.

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 It's just on your
 own local device.

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 IMAP on the other hand, retains a copy
 of that message on the server while

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 temporarily caching a
 copy on your machine.

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 In addition to that, IMAP has the ability
 to mark a message as being either

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 read or unread. So your machine via
 IMAP will say, okay, I've read this

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 message or I've not read it or I've marked
 it as urgent or I've categorized

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 it in some way. POP doesn't do that
 but IMAP can because it's stored,

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 the original message is stored on the
 server for a predefined amount of

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 time. So what's sort of the differences
 between the workflow here?

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 Well, with the post office protocol,
 you connect to the server.

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 So that's step number one.

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 Then you retrieve your email, which
 means now it has been deleted.

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 You store the locally as new email,
 it's deleted from the server and you

0:13:09.660000 --> 0:13:13.680000
 disconnect. So that point, the
 server doesn't have it anymore.

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 Now POP does have the ability.

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 There are optional ways within POP to
 get it to store a copy of the email

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 even after you've downloaded it.

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 But that's not the
 default behavior.

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 Whereas with IMAP, you connect to the
 server, that's step number one.

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 You fetch the content
 and cache it locally.

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 For example, a list of the new
 messages, message summary.

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 Maybe you've seen that when you open
 up your email, you've got a message

0:13:39.260000 --> 0:13:42.340000
 summary but you don't actually see the
 whole thing until you double click

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 on the email. And then IMAP
 processes the user edits.

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 For example, when you read the email,
 it marks it as red or marks an email

0:13:52.700000 --> 0:13:57.760000
 as deleted. For example, gives that
 message back to the mail delivery

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 agent server and then
 you disconnect.

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 So where are some pros and cons of
 the differences between these?

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 Well, with the post office protocol,
 the mail is stored locally.

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 In other words, once you download
 it, it's on your laptop.

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 And some people have said
 this makes it more secure.

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 From a security perspective, it's more
 secure that's not saved on the

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 server that's now just
 on your laptop.

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 Other people say, oh, that's
 not really a big concern.

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 That's a bunch of overblown hype.

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 So that just depends on you.

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 With pop, we only need an internet connection
 to send and receive mail.

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 Once I've downloaded the mail, I don't
 need my internet connection anymore.

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 Once again, this is not a big deal in
 these days where most people have

0:14:43.380000 --> 0:14:46.980000
 always up connections like
 DSL or cable modem.

0:14:46.980000 --> 0:14:50.220000
 But back in the days when people had
 dial up connections using modems

0:14:50.220000 --> 0:14:54.260000
 or ISDN or something, this could be a
 big deal where you are charged every

0:14:54.260000 --> 0:14:56.820000
 single minute for your
 internet connectivity.

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 You want to minimize
 your internet usage.

0:14:59.620000 --> 0:15:04.060000
 So pop was good because you could grab
 your email, bring down your internet

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 connection, and now you could read
 your email locally on your machine.

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 So it saved server space because it
 was deleting emails from the server

0:15:12.620000 --> 0:15:14.480000
 as they were being downloaded.

0:15:14.480000 --> 0:15:20.760000
 But it had an option to leave a
 copy of the mail on the server.

0:15:20.760000 --> 0:15:24.740000
 Whereas iMap, the mail is stored
 on the remote server.

0:15:24.740000 --> 0:15:27.160000
 You need an internet connection
 to read email.

0:15:27.160000 --> 0:15:32.560000
 So the big benefit of this is with
 iMap, you can read your emails from

0:15:32.560000 --> 0:15:34.000000
 multiple different devices.

0:15:34.000000 --> 0:15:38.140000
 For example, I could open up an
 email on my tablet and read it.

0:15:38.140000 --> 0:15:42.360000
 And then later on, I could open up
 that same email on my smartphone if

0:15:42.360000 --> 0:15:43.760000
 I left my tablet at home.

0:15:43.760000 --> 0:15:47.640000
 Because the email was stored or
 cached on the server itself.

0:15:47.640000 --> 0:15:52.420000
 So I can access server with my smartphone,
 my laptop, my tablet, so I

0:15:52.420000 --> 0:15:56.480000
 can read and access my emails from
 multiple different devices.

0:15:56.480000 --> 0:16:01.380000
 So how would we troubleshoot some
 basic problems with email?

0:16:01.380000 --> 0:16:04.560000
 Well as far as the mail delivery
 agent is concerned, right?

0:16:04.560000 --> 0:16:08.840000
 So we're talking now about a standalone
 email application on your laptop

0:16:08.840000 --> 0:16:14.760000
 like Eudora or something, not
 web-based, not like Gmail.

0:16:14.760000 --> 0:16:20.300000
 Well we know that your client, the
 mail user agent, that's you, that's

0:16:20.300000 --> 0:16:24.600000
 your laptop, it has to know where the
 mail delivery agent is and how to

0:16:24.600000 --> 0:16:28.720000
 reach it. So maybe on your client you
 haven't given it that information.

0:16:28.720000 --> 0:16:33.260000
 You haven't given your client the DNS
 resolvable name or the IP address

0:16:33.260000 --> 0:16:36.340000
 of the mail delivery agent.

0:16:36.340000 --> 0:16:40.180000
 Or perhaps you have given it a
 name, but the name is wrong.

0:16:40.180000 --> 0:16:42.840000
 You mistyped it somehow.

0:16:42.840000 --> 0:16:48.360000
 Or maybe on the mail user agent, your
 client, your laptop, actually it

0:16:48.360000 --> 0:16:52.520000
 says here on the mail delivery agent there
 is no user name account configured.

0:16:52.520000 --> 0:16:55.520000
 In other words, the mail delivery
 agent doesn't know about you.

0:16:55.520000 --> 0:16:58.000000
 It doesn't have a mailbox
 set up for you.

0:16:58.000000 --> 0:17:01.560000
 You know this would be an example of a
 new employee who comes into a company

0:17:01.560000 --> 0:17:04.800000
 and their mail isn't set up yet because
 it has not yet been configured

0:17:04.800000 --> 0:17:07.820000
 on the mail delivery agent.

0:17:07.820000 --> 0:17:13.520000
 Certainly if the client has no internet
 connectivity, can't get your email.

0:17:13.520000 --> 0:17:19.200000
 If you typed in the name of the mail
 delivery agent correctly, but if

0:17:19.200000 --> 0:17:22.920000
 it can't be resolved via DNS, well
 then your client's not going to be

0:17:22.920000 --> 0:17:24.220000
 able to reach it.

0:17:24.220000 --> 0:17:26.640000
 Possibly the email server
 is just unreachable.

0:17:26.640000 --> 0:17:29.960000
 Maybe it's down right now or there's something
 in the network that's preventing

0:17:29.960000 --> 0:17:33.220000
 reachability. Just try pinging it.


0:17:33.220000 --> 0:17:35.460000
 Or maybe you're just simply
 using the wrong credentials.

0:17:35.460000 --> 0:17:40.360000
 Your user name and password are not
 right to get to that mail delivery

0:17:40.360000 --> 0:17:45.940000
 agent. So that concludes this basic
 overview here on email as a common

0:17:45.940000 --> 0:17:46.860000
 network application.

0:17:46.860000 --> 0:17:49.940000
 In the next video we're going
 to talk about web browsing.