The OSI Model: Part 4 - Transport Layer
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35 hours 25 minutes
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Video Transcription
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>> We now find ourselves at layer
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4 of the OSI reference model,
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and that is the transport layer.
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The transport layer is going to be
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responsible for end to end delivery,
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and it's going to determine whether or not we have
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reliable or unreliable delivery.
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We're going to look at each of those elements.
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But the phrase I really want you to have for layer 4
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is end-to-end data transport services.
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This ultimately is what establishes
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the connection between the computer systems,
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and all upper layer protocols
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are going to piggyback on a layer 4 protocol.
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Here, we get that connection of, hey,
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let's communicate, I'm ready to communicate as well.
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We have to have that means
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of determining that a communication is
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going to happen and that's
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what transport layer protocols do.
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I always think of this again as
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the pony express layer of the OSI model.
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Upper layer protocols and services piggyback
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on one of the layer 4 protocols.
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Now, SSL and TLS that
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we talked about in the cryptography domain,
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those work between layers 4-7.
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Now, I've seen them placed at layer 4,
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but I really think the better answer is to say
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they work from layers 4-7.
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Those could be, but the main protocols at
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layer 4 are TCP and UDP.
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Let's look at TCP first.
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Connection oriented, guaranteed delivery,
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reliable, those are
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phrases you're going to associate with TCP.
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Let me give you an example.
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Let's say I'm an administrative assistant
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and I get a phone call for Janita.
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I can pick up the phone and say,
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"Janita, you have a call on line 1."
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I can hang it up and I can go to lunch.
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I did my job right out threw the message out there.
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Hope you get it, but I'm going,
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I did that for speed.
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Now an alternative to that is I
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can dial Janita's extension.
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Say, "Hey, can you take a call?"
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Janita comes back and says,
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''Yes, I'd be happy to take the call.''
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I say, "Okay, here's your call."
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She says, "Thank you for sending me the call."
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I say, "Okay, I'm going to send you another call."
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Now. I make sure that she gets the message.
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But there's a lot of back and forth, so that's slower.
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Just like always, we trade speed for guaranteed delivery,
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better security, more reliability.
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TCP guaranteed delivery uses a process called
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handshaking.. Handshaking with TCP
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is known as a three-way handshake.
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The very first communication that says, ''Hey,
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let's talk'' is to synchronize.
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The client, sends the server a
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send message SYN for synchronize,
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that basically says, "Hey, I want to communicate."
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The server comes back and says SYN-ACK,
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I acknowledge you want to communicate,
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that's fine with me.
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Then the client sends back
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an ACK that said here we are communicating.
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That handshake process takes time,
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but it makes sure that we're configured to send and
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receive and it makes sure that what I
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sent you is what's received.
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TCP uses
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a three-way handshaking process to guarantee delivery.
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I'll just let you appreciate that cartoon,
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no narration needed.
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Now UDP, on the other hand,
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connectionless, unreliable, no handshaking.
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With UDP, just transmit the data, hope you get it.
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I don't know if you did.
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Don't care, it's time for lunch.
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It's designed to be fast, quick.
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Now you may be asking yourself,
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how do I determine whether I want to use TCP or UDP?
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The answer is you don't determine that.
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But if I were a developer writing a network application,
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I've got to decide do we want to use TCP or UDP.
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The majority of times our developer wants to use
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TCP because it incorporates
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the function that I wouldn't
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necessarily need for communication.
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If I choose instead to use UDP,
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I'm going to have to add additional functionality
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into my application.
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But I may do that when speed is essential.
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Anytime you're looking at real-time transfer,
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UDP is generally going to be preferable.
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When you think about media streaming especially,
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UDP is more desirable.
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If you've ever set somewhere and you're watching a movie,
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and then it goes to buffering, then you know,
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UDP where we get as best performance as
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possible as opposed to
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a slower transfer of data, that's what we want.
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Now, lifetime chat, gaming,
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anything that has to be
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real time or what we sometimes call
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near real-time that's probably going to use UDP.
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Just to mention, most people have heard
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of FTP, file transfer protocol.
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Its job is to transfer files
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from an FTP server to a client.
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TCP is reliable, has guaranteed delivery,
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though it's a little bit slow,
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that's because FTP piggybacks on TCP.
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Now FTP has a cousin that's still transfers files,
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but it's designed for larger files and has to be fast.
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That's called TFTP,
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trivial file transfer protocol and
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TFTP piggybacks on UDP.
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I'm going to give you the best joke ever.
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What's the best thing about a UDP joke?
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I don't care if you get it or not [LAUGHTER].
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Wiping the tears from my eyes.
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That's a great joke folks,
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and that is your value added reason to choose Cybrary.
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Where else are you going to get comedy gold like this?
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Nowhere. That wraps up the transport layer,
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layer 4 of the OSI model.
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We talked about TCP versus UDP.
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We talked about TCP being reliable,
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giving guaranteed delivery, whereas
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UDP is unreliable, but it's fast.
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