I remember back in the early 1980s, when I first
started learning networking.
And I'd studied the OSI reference model.
I was pleased as punch when I would read an article,
and it would talk about the application
layer services of XYZ, or something at layer three,
or a new layer two device.
Because I knew the functionality of those OSI
reference model layers.
And I thought to myself, "OK.
Great.
That kind of gives me an idea of what
that product or service does.
But then it also occurred to me, pretty much at the same time,
that we would be hard pressed to find a computer that's actually
running any type of an OSI reference protocol.
Because the OSI reference model is just that.
It's a reference model.
It's not a common, real protocol that we're using today.
So I thought to myself, "Self, what are we using?"
well, back in the day, we had lots of other protocols,
including IPX.
But we also had this new and upcoming protocol
called TCP/IP.
And TCP/IP was courtesy of the government.
The Department of Defense created it.
And this TCP/IP IP was actually a literal protocol stack,
a suite of protocols working together
to provide the functionalities that were described as part
of the OSI reference model.
And so, if we put them side by side here,
all the functionality in the OSI reference model,
regarding layer seven, six, and five,
the application layer with network services,
the presentation layer dealing with encryption and decryption
and compression, and the session layer,
dealing with the establishment, and the set up of sessions,
all that functionality can be found in the TCP/IP Protocol
Suite, in the one layer called the application layer.
And then, layer four, the transport layer of the OSI
reference model, we usually refer
to that as the transport layer here,
as well, on the TCP/IP protocol suite.
Also, it's sometimes referred to as the host to host layer.
In either case, it's providing the middle manager
functionality for reliable or unreliable delivery,
based on the application we're using,
as well as the potential for chunking up large messages
into smaller parcels or pieces.
But the functionality from the model and the TCP/IP Protocol
Suite are the same at that layer.
And as we go down to the network layer of the OSI reference
model, and the TCP/IP Protocol Suite is referred
to as the internet layer.
However, the responsibilities and functions
are the same as the OSI reference
models, which include adding address information,
like for example, a street name, and a house number.
Or, in the case of the TCP/IP Protocol Suite,
it would be a logical IP address,
which we can think of as a house number and street name that
allows a packet to be delivered over a network
to the device it was intended to go to.
And then as we continue to go down the TCP/IP protocol stack,
we have this one section called Network Access, which
includes the functionality of the datalink layer
and the physical layer of the OSI reference model, which
would be our envelope stuffer, and the correct framing
for the messages, and the actual physical transmission
and sending of the information over the network itself.
But there was one more challenge I discovered.
And that was, well, if we're using the TCP/IP protocol
suite, and I'm reading this article,
and it's talking about a layer two device,
how does that fit in?
And so, for the actual practical purposes for you
and I, as we work with TCP/IP networks, which
is the major protocol in the world today, when we refer
to the actual names and layers, for layer purposes,
we're going to borrow the names of physical and datalink
for layers one and two, even though we're actually
using TCP/IP.
And one of the benefits of that, it
allows us to be very specific when
we're talking about components.
For example, if we a new network adapter,
and we plug it into a computer, and it's
physically connecting us to the rest of the network,
we can say that that network adapter, for that purpose,
is a layer one device, a physical layer device.
Also, for TCP/IP, at layer three, we refer to layer three
as the network layer.
So we're leveraging the name from the OSI reference model,
actually renaming the TCP/IP Protocol
Suite, the original name, for that layer three,
which was Internet, and simply calling it,
once again, Network.
And also, at layer four, we're hanging
onto the name of Transport as the transport layer
in the TCP/IP protocol suite.
And then, for everything above that,
which would be layers five, six, and seven from the OSI
reference model, we just refer to that one big chunk
of functions and features as the application layer.
So as we discuss the actual use of a protocol
stack, and going back to our analogy
with our King A and King B, or a user like Bob, sitting
in front of a browser, to access the network services,
those are being provided by the application layer of our TCP/IP
Pericles stack.
And as that information is passed down this protocol
stack, and each of the responsible parties
are doing their work, and it's being sent over the network
we can use more specific terms for the data.
For example, if we have the middle manager, the transport
layer, that was currently working with or looking
at some data, instead of calling that data,
we could refer to that information
at that layer as a segment.
For example, a segment of data.
And as that information gets passed down to the mailroom,
down to the network layer or three,
we could refer to the data at that layer as packets.
And as the network layer passes that down
to the envelope stuffers, the datalink layer,
we can refer to the data at that layer, at layer two, as frames.
Frames of data.
And then, as that information is actually transmitted or sent,
the actual data could be referred
to as individual bits that are being sent
over the network at that point.
So the question might come up, "Well Keith,
does that mean that anybody who says the word packet always
is talking specifically about Layer three of the TCP/IP
protocol suite?"
And the answer is, no.
Because some people use terms loosely.
We might have to take things in context.
But the beautiful thing is this.
You and I, going forward, if we say segment, versus packet,
versus frame, because we know these terms,
we can assume that if one of us says the word segment of data,
we're referring to the data here at layer four of the TCP/IP
Protocol Suite.
And following that logic, if you and I were
to talk about frames, we would be
referring to data at layer two of the protocol stack.
I hope this has been informative for you,
and I'd like to thank you for viewing.