OSI Model Layer 5, 6, and 7 -- Session, Presentation, & Application Layers -- what they actually do

Summary

This video explores the often-overlooked layers 5, 6, and 7 of the OSI model, emphasizing their practical importance in networking. The speaker demystifies how these layers contribute to the functionality of the internet, from managing user sessions (layer 5), interpreting data (layer 6), to defining application commands (layer 7). Using relatable examples like HTTP cookies, the video shows how these layers work in harmony. It also highlights why the TCP/IP model combines these layers into a single application layer, encouraging viewers to see networking models as tools for understanding abstraction in computer networks.

Takeaways

• 💡 The OSI model is often misunderstood and seen as something just to memorize, but it's crucial for understanding how the internet works.
• 📶 Layers 5, 6, and 7 are often called the 'Forgotten Layers' because they're not as well covered as Layers 1 through 4.
• 🌍 The goal of networking is to allow data sharing between two users via computers, with each OSI layer playing a role in achieving this.
• 🖥️ Layer 5, the session layer, distinguishes between different user sessions, even if Layer 2, 3, or 4 information is the same.
• 🍪 HTTP cookies are an example of Layer 5, allowing users to maintain their session across different networks without logging in repeatedly.
• 🔢 Layer 6, the presentation layer, interprets the ones and zeros in a packet, converting them into characters or data formats like ASCII or Base64.
• 🌐 HTTP uses extended ASCII encoding at Layer 6 to translate binary data into readable characters.
• 🔎 Layer 7, the application layer, defines how the interpreted data is used, such as executing commands in HTTP to request a webpage.
• ⚙️ Different protocols (like FTP and HTTP) implement Layers 5, 6, and 7 differently based on their application needs.
• 📊 While OSI has seven layers, the TCP/IP model often combines Layers 5, 6, and 7 into one 'application layer' for simplicity, as protocols may handle these layers uniquely.

Q & A

What is the main focus of the video script?

The video focuses on explaining layers 5, 6, and 7 of the OSI model, which are often overlooked or combined into a single application layer in other models like TCP/IP.

Why are layers 5, 6, and 7 of the OSI model referred to as the 'Forgotten layers'?

They are referred to as 'Forgotten layers' because most resources and discussions on the OSI model focus on layers 1 through 4, and the TCP/IP model combines layers 5, 6, and 7 into a single application layer.

What problem does layer 5 of the OSI model solve?

Layer 5, the session layer, solves the problem of distinguishing between different user sessions. It allows networking protocols to keep track of user-specific data and separate different sessions, even if the underlying layer 2, 3, or 4 addresses are the same.

Can you provide an example of how layer 5 functions in modern computing?

An example of layer 5's function is HTTP cookies. These cookies store user-specific information that allows websites to identify users independently from changes in layer 2, 3, or 4 information, such as IP addresses, when switching networks.

What role does layer 6, the presentation layer, play in the OSI model?

Layer 6 determines how to interpret the ones and zeros received by a network. It manages tasks like data encoding, compression, and encryption by deciding how data should be formatted or interpreted, such as using Base64, hexadecimal, or ASCII encoding.

How does layer 7, the application layer, function in the OSI model?

Layer 7, the application layer, defines the application commands that are carried out after the data is processed and interpreted. For example, in HTTP, the 'GET' command is used to request web pages, making layer 7 responsible for executing such high-level functions.

Why are layers 5, 6, and 7 combined into a single application layer in the TCP/IP model?

Layers 5, 6, and 7 are combined in the TCP/IP model because each protocol or application handles these layers' responsibilities (session, presentation, and application) in its own way. Since there's no strict need to separate them, they are grouped as a single 'application layer' for simplicity.

How do different protocols like HTTP and FTP implement layers 5, 6, and 7 differently?

Different protocols implement these layers based on their specific needs. For example, HTTP uses cookies for session management (layer 5), extended ASCII encoding for data (layer 6), and commands like 'GET' for application operations (layer 7). In contrast, FTP may not have session management (layer 5) and uses its own set of application commands in layer 7.

What is the significance of understanding the OSI model in layers of abstraction?

Understanding the OSI model in layers of abstraction allows you to see how different networking responsibilities are divided among layers. Each layer focuses on a specific function, making networking more manageable by ensuring that higher layers do not have to worry about the details handled by lower layers.

Why is it important not to try to fit all networking knowledge strictly into the OSI model?

The OSI model and TCP/IP models are abstractions meant to teach networking concepts. Networking in practice can be more complex, and it's not always necessary to fit everything into one of these layers. These models serve as guidelines rather than strict rules.

Outlines

00:00

🎶 Misconceptions About OSI Model Layers 5, 6, and 7

The video begins by addressing the misinformation about OSI model layers 5, 6, and 7, often dismissed as unimportant. The creator emphasizes a more practical understanding of these layers, highlighting their crucial role in networking. The video aims to unpack these layers in detail, continuing from previous videos on layers 1-4, which received viewer requests for further exploration of the remaining layers.

05:01

🌍 Understanding Networking Layers and Their Goals

The OSI model has seven layers, each playing a specific role to enable networking, which allows users to share data between computers. When each layer functions correctly, networking works seamlessly, which in turn powers the internet. While layers 1-4 are commonly covered in resources, layers 5-7 are often overlooked. The TCP/IP model even combines these layers into one, which simplifies the model but also obscures their unique contributions.

10:02

📡 The Role of Layer 5: Session Management

Layer 5, or the session layer, manages user sessions independently of the network addresses (layer 2, 3, or 4). In legacy systems, such as mainframes and dumb terminals, this layer distinguished between users sharing computing resources. Even today, with technologies like HTTP cookies, layer 5 plays a crucial role in identifying users across network changes, ensuring seamless browsing without requiring users to log in again after switching networks.

🔐 Layer 6: Interpreting Data Formats

Layer 6, known as the presentation layer, deals with how to interpret the data received, ensuring that the bits transmitted across the network are converted into meaningful information. This layer decides how to group the binary data—whether into base64 encoding, hexadecimal, or other formats like ASCII. In the case of HTTP, for example, it interprets bits into ASCII characters to make sense of the data, translating them into readable text like 'GET' requests.

🖥️ Layer 7: Application-Level Commands

Layer 7, the application layer, defines the actions performed once the data has been processed. In the case of HTTP, for instance, the presentation layer has already interpreted the bits, and now the application layer handles specific commands like fetching a webpage using 'GET' requests. This layer dictates the interaction between user applications and networking services, ensuring commands are executed correctly.

📂 Comparing Protocols in Layers 5-7: HTTP vs FTP

Not all protocols handle layers 5-7 in the same way. While HTTP uses cookies for session management, FTP does not have a built-in mechanism for managing user sessions across network changes, requiring users to log in again if their IP changes. Both HTTP and FTP use extended ASCII encoding for data interpretation, but each has its own set of application-layer commands specific to its protocol.

🔄 OSI vs TCP/IP: Aggregated Layers and Flexibility

The TCP/IP model combines OSI layers 5-7 into one 'application layer' because different protocols can handle these layers in varying ways. This flexibility allows different networking models to work efficiently without adhering strictly to the OSI layer structure. Both models are valuable as conceptual tools, not rigid frameworks, and should be used to understand the abstraction and responsibilities of each layer in networking.

💡 Key Takeaways on Layers 5, 6, and 7

In conclusion, layers 5-7 play important, albeit often aggregated, roles in networking models. Their functions allow for session management, data format interpretation, and application-level commands. Understanding these layers helps learners appreciate how abstraction works in networking, making concepts like tunneling and overlays easier to grasp. The creator invites viewers to explore more through their course on networking and encourages engagement with the video.

Keywords

💡OSI Model

The OSI (Open Systems Interconnection) model is a conceptual framework used to understand how different networking protocols interact across seven layers to enable communication between computers. In the video, it is presented as a critical model for understanding networking, with a focus on layers 5, 6, and 7, which are often overlooked.

💡Layer 5 - Session Layer

Layer 5 of the OSI model is responsible for establishing, managing, and terminating communication sessions between devices. In the video, it is described as handling user sessions, ensuring that data streams for different users are kept separate even if they share the same MAC, IP, or port addresses. An example is HTTP cookies used to track user sessions across different networks.

💡Layer 6 - Presentation Layer

The presentation layer, or Layer 6, is tasked with translating data between the application layer and the network format, ensuring that the data sent from one system is readable by another. The video explains how Layer 6 handles encoding methods like Base64 or ASCII, ensuring that raw data (ones and zeros) is interpreted correctly, such as transforming binary into readable text.

💡Layer 7 - Application Layer

Layer 7, the application layer, defines how software interacts with the network. This is where protocols like HTTP and FTP reside. In the video, Layer 7 is shown to manage specific commands like 'GET' in HTTP, which tells a server to retrieve a specific webpage. It plays a crucial role in defining how applications utilize the network.

💡TCP/IP Model

The TCP/IP model is a more streamlined networking framework that condenses layers 5, 6, and 7 of the OSI model into a single 'application layer.' The video explains why this model is more popular today, as many modern applications handle the responsibilities of session management, data representation, and application commands in a unified way.

💡HTTP Cookies

HTTP cookies are small pieces of data sent from a server and stored on a user's device to maintain session information across different networks. The video uses cookies as an example of how Layer 5 operates, allowing users to stay logged into a website even when their network changes, distinguishing them independently of their IP or port.

💡Mainframes and Dumb Terminals

Mainframes are large, powerful computers that were once common for centralized computing, with 'dumb terminals' serving as simple input/output devices. The video uses this historical context to explain how the session layer was first developed to separate user data when many users connected to the same mainframe.

💡Base64 Encoding

Base64 is a method for encoding binary data into text format, often used in email and web technologies. In the video, Base64 is used as an example of how Layer 6 (the presentation layer) interprets raw data. It demonstrates how the same ones and zeros can be represented in different formats depending on the encoding method.

💡TCP Ports

TCP ports are virtual endpoints for communication at the transport layer (Layer 4) that help identify specific processes or services on a networked computer. The video mentions Layer 4 addresses like ports in its explanation of why Layer 5 is essential for managing sessions, ensuring data from different users doesn’t get confused when sharing the same transport endpoints.

💡Abstraction

Abstraction refers to the concept of simplifying complex systems by focusing on specific layers of functionality while hiding the details of others. The OSI model and TCP/IP model both use layers to abstract different responsibilities in networking. The video stresses that understanding the models as layers of abstraction helps learners grasp how complex systems like the internet function without being overwhelmed by every detail.

Highlights

Layers 5, 6, and 7 of the OSI model are often overlooked, yet crucial to understanding networking.

The OSI model is not just something to memorize, but a way to understand how different components work together to make the internet function.

Layer 5, known as the session layer, distinguishes between user sessions, allowing multiple users to be identified separately despite sharing the same network addresses.

HTTP cookies are a modern example of how layer 5 identifies users independently from lower-layer data like IP addresses and ports.

Layer 6, the presentation layer, focuses on how to interpret the data received, like deciding whether to interpret bits as text, numbers, or other formats such as Base64 or Hexadecimal.

The presentation layer allows protocols like HTTP to group ones and zeros into meaningful characters using extended ASCII encoding.

Layer 7, the application layer, defines what should be done with the interpreted data, such as HTTP requests containing commands like GET to fetch web pages.

Each network protocol, such as HTTP or FTP, implements layers 5, 6, and 7 differently, which is why TCP/IP combines these into a single 'application layer.'

TCP/IP’s aggregation of layers 5, 6, and 7 into a single application layer simplifies protocol-specific implementations of these layers.

The OSI and TCP/IP models are abstractions designed to help understand how different layers of networking work together, not rigid structures.

Layered abstractions, such as the OSI model, allow each layer to focus on its specific role without worrying about the responsibilities of other layers.

Legacy systems like mainframes and dumb terminals illustrate the original importance of the session layer in keeping user data streams separate.

HTTP cookies help maintain user sessions even when changing networks (such as switching Wi-Fi networks) by identifying users independently of lower-layer addresses.

The presenter emphasizes not fitting everything into these layers but using them as a teaching tool for networking concepts.

The flexibility of networking protocols allows them to implement layers 5, 6, and 7 in ways that make the most sense for their specific applications, contributing to the overall efficiency of internet communication.

Transcripts

[Music]

there's a lot of bad information out

there about layers 5 six and seven of

the OSI model in fact there are many

resources out there that describe the

OSI model as merely something to

memorize I think that's a huge

disservice there's a way of

understanding The OSI model from a

practical perspective as a series of

components that all contribute towards

making the internet work that's the

strategy I took when I published my two

videos on the osm those videos focused

on layers 1 through 4 the most common

request I got in the coms ments of those

videos was to continue with lers 56 and

7 so here we

[Music]

are the OSI model consists of seven

different layers that each have a

specific responsibility which contribute

to the overall goal of networking which

is to allow two users to use computers

to share data between each other if each

layer is doing its responsibility

successfully then the goal of networking

is attained which means the internet

works there are many resources that

unpack layers 1 2 3 and four but very

few that really pick apart layers 5 six

and 7even I call these the Forgotten

layers of the OSI model part of that is

because the other popular networking

models the tcpip models simply combine

these layers as a single aggregated

application layer now there's good

reason for that and you'll understand

those reasons towards the end of this

video what I want to do in this video is

unpack the details of layer 5 6 and 7

and specifically how each of those

layers contribute towards the goal of

networking before we can understand

these layers however we have to

understand the layers beneath it

specifically you must understand the

terms hop to hop end to end and service

to service and how each of those apply

towards the goal of networking if you're

unfamiliar with those terms then please

check out my other two videos on the OSI

model I promise it'll be the best take

on not only understanding The OSI model

but also how to explain it to others

with that said let's get into the

Forgotten layers of the OSI model

starting with layer five to understand

layer five of The OSI model you have to

understand that this model was created

long before the days of personal

computers or

PCS back in those days Computing was

done on large massive machines called

mainframes and these were often so big

that they took up entire rooms the

Computing resources on these enormous

mainframes were then accessed by people

using what is known as dumb terminals

which were effectively just a monitor

and a keyboard plugged into these main

frames no actual processing of data was

done on these dump terminals all the

actual Computing was done on the

mainframes fun fact this was also the

model that Linux was created in this is

why Linux is often described as a

multi-user operating system in any case

all the Computing was done on these

large mainframes and users would connect

to these mainframes using dump Terminals

and then those main frames were then

connected to other main frames which

also had their own set of users using

their own set of dump

Terminals and here we can Define the

problem that layer five is meant to

solve notice this blue user is using

this Mainframe to speak to this blue

user using that main frame at the same

time these two green users are also

speaking to each other using these two

main frames well if both of those main

frames are doing all the Computing and

sending of the data between each other

then the packets that are sent from this

main frame to the other are going to

have the same layer 2 address like a MAC

address the same layer three address

like an IP address and even potentially

the same layer four addresses like ports

which then begs the question how is the

data between these two blue users going

to be kept separate from the data

between these two green users that's

where layer five comes into play layer

five is there to distinguish between

user sessions it allows networking

protocols to identify a user

independently from their layer 2 layer 3

or layer 4

addresses okay so that makes sense but

is layer five simply an artifact of

Legacy Computing on these mainframe

computers well no you actually use

something that I would categorize as a

layer five component every day when

you're browsing the web let me show you

let's say these two blue users are not

using main frames to speak to each other

and instead this user is actually a web

server for the site site.com and this

user is simply someone using a mobile

phone this mobile phone is currently

logged into a Wi-Fi network at this

user's house and this phone uses this

connection to log into the site

site.com but what happens if this user

then decides that they want to go to a

coffee shop and connect to the Wi-Fi

network there since this is a different

network they're going to have a new IP

address which means this information is

going to change and if the website is

identifying this us user with layer 1

through 4 information then this user is

going to have to log back into the

website every time they connect to a new

Wi-Fi network because in each case

they're going to be getting a new IP

address instead since we're using HTTP

to access this website HTTP has a

functionality known as HTTP cookies

these cookies are simply arbitrary text

strings that store users specific

information that is created by the

server what happens is when the user

logs into the website the the server

generates one of these cookies and sends

it to the user this cookie then stays

with the user and if this user logs into

a new Wi-Fi network with new layer 234

information that user still has the same

cookie and the website can still

identify this user independent from

whatever layer 2 layer three and layer

four addresses that that user might

maintain so you see HTTP cookies are a

perfect example of layer five's

responsibilities in the OSI model they

allow the network to identify user

independently from the layer 2 three and

four

addresses which then brings us to layer

six to understand the presentation layer

we're going to simulate a packet

arriving on the web server for site.com

recall that every packet is really

nothing more than a series of ones and

zeros and because of the rest of the

layers of the OSI model we know that

this packet successfully delivered to

the web server arrive to the right Mac

address arrive to the right IP address

arrive to the right port and arrive to

the right user session which now brings

us to layer six what layer six does is

it tells us how to interpret the ones

and zeros there are many ways to

interpret those ones and zeros for

example should we group those ones and

zeros in sets of six bits as base 64

encoding would require this would mean

each combination of six bits would turn

into a new base 64 character or should

we interpret those ones and zeros in

groups of four bits this is what heximal

would require where each group of four

ones and zeros represents a new heximal

character or should we interpret those

ones and zeros as long numbers or

integers if we were to interpret all

these ones and zeros as 32-bit numbers

this number would be 1 billion in change

and this number would be 796 million in

change alternatively maybe we're

interpreting this as a single large 64

pit number in which case this string of

64 ones and zeros could be translated to

this huge number in decimal so you see

that's what layer six tells us it tells

us what do we do with these ones and

zeros now earlier we use HTTP as an

example and we can continue to do so

HTTP as a protocol uses extended asky

encoding extended asy takes all the ones

and zeros and groups them into sets of

eight bits and interprets each of those

eight bits as a different ASE character

this combination of ones and zeros

translates to the letter G and this

combination of ones and zeros translates

to the letter e and this combination

translates to the letter T and so on and

that's what layer six provides it tells

us what to do with the received ones and

zeros that have been successfully

processed through the OSI model and

delivered to the right user session

which then brings us to layer seven if

layer six told us how to interpret the

ones and zeros that have been received

layer seven will tell us what do we do

with the interpreted characters or said

another way layer 7 will actually Define

the application commands these three

bytes of ones and zeros turn into the

letters g e and t and the command get in

HTTP is the command you would use to

fetch a particular web page what you're

seeing here is actually the first eight

characters of an HTTP request asking for

the page simple. HTML using http P 1.1

from the host packet.net

and if that looks familiar that's

because it's the exact HTTP request we

made in my last video where I showed you

how to make an HTTP request using tet

and if you haven't seen that video I

think that one plus this video work well

together to really explain how

networking protocols work either way

that is what layer seven of the OSI

model does and that actually concludes

my description of what the Forgotten

layers of the OSI model actually do but

before I let you go there's a few more

ideas I have to leave you with

throughout this lesson we've been using

HTTP as our example protocol we told you

that HTTP to accomplish the goals of

session management uses HTTP cookies and

for layer six HTTP uses extended asking

coded and for layer seven HTTP has its

own set of application commands but

there are other protocols that exist

Beyond just http for example FTP well it

doesn't really make sense for FTP to use

HTTP cookies to accomplish its layer

five goals FTP is in fact free to choose

how to implement layer five in any way

it deems necessary or to not implement

it at all FTP in fact doesn't have a way

to identify a user session independent

from the layers below it FTP uses layer

three and layer four information to

identify a user which means if you are

in the middle of an FTP session and your

IP address changes you're going to have

to log back in to that FTP server for

layer six FTP also uses extended asky

encoding and for layer 7 FTP has its own

set of application commands so that's

just HTTP and FTP two of the maybe

thousands of protocols that exist and

you can see that each of those protocols

can Implement layers 5 six and seven in

whatever way that makes the most sense

for that particular application that is

why the the tcpip models combine those

layers into a single aggregate

application layer since every protocol

an application is going to implement

their own way of doing these three goals

it makes sense to combine those as a

single application layer so in the end

whether you're using the seven layer OSI

model the five layer tcpip model or even

the somewhat Legacy four layer tcpip

model one way or another each of these

are merely models they are simply a way

of understanding computer networking

don't try too hard to fit everything you

learn in the networking world into one

of these layers or one of these models

that's not the goal these are simply

abstractions to teach you how computer

networking can work in the end each of

these models are there to teach you the

concept of layers of abstraction one

layer has responsibility and that

precludes the other layers from having

to worry about that

responsibility layer three can do its

job of endtoend delivery without having

to worry about how the ones and zeros

are going to be delivered across every

router in the path and HTTP cookies work

the same whether you're using ipv4 or

IPv6 each layer's goals are abstracted

from the other layers understanding The

OSI model or the tcpip model in that way

will set you up for Success when you're

learning deeper things in the networking

world like tunneling and underlays and

overlays

so that's my take on layers 5 six and

7even of the OSI model hopefully you now

have a more solid understanding of how

these layers work and why they are often

simply aggregated as a single

application layer if you enjoyed this

lesson then check out my new course

called networking the internet the cloud

and everything in between it's currently

under construction and you can join now

at a discounted rate to get permanent

access and help shape this into the best

networking course

possible otherwise if you think more

people should understand and explain

layers 5 6 and 7even this way and help

me out by liking commenting and

subscribing that would help me with the

YouTube algorithm or if so inclined

please share this video amongst your

peers that would really help me out and

I would appreciate it greatly thank you

for watching this video I hope you got a

lot out of it and I'll see you in the

next

[Music]

one

[Music]

oh