OSI Model Layer 5, 6, and 7 -- Session, Presentation, & Application Layers -- what they actually do
Summary
TLDRThis 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
🎶 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.
🌍 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.
📡 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.
Mindmap
Keywords
💡OSI Model
💡Layer 5 - Session Layer
💡Layer 6 - Presentation Layer
💡Layer 7 - Application Layer
💡TCP/IP Model
💡HTTP Cookies
💡Mainframes and Dumb Terminals
💡Base64 Encoding
💡TCP Ports
💡Abstraction
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
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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
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