CCNA R&S Article #3 – TCP/IP Networking Model
CCNA RnS Article #3 – TCP/IP Networking Model
Today let’s talk about TCP/IP Networking Model. If you want
to read more on the networking model, please click here.
TCP/IP networking model states the set of rules that allows
hardware or to be the more specific computer for example, when wired to the
correct cable network allows users to access applications communicating to
other devices on the network. For example, a web browser running on your laptop
or mobile allows you to access websites that are hosted on servers anywhere in
the globe. All this is possible because the hardware manufacturers and software
developers implement parts of TCP/IP. For instance, a network interface card
(NIC) or wireless adapter in your laptop, implements LAN standards of TCP/IP.
A big task can be completed when it is broken down into
pieces. In the same way, to understand a networking model, it is divided into
layers for users to get an understanding of what functions each layer performs
and the related protocols. TCP/IP networking model includes 5 layers and each
layer uses a common naming convention – for a user to easily understand its function.
Fig 1 : TCP/IP Networking Model
The top 2 layers focus on the applications that need to send
and receive data. And bottom 3 layers focus on how to transfer data over links.
Let’s see some of the protocols examples of TCP/IP models –
Figure 2: TCP/IP Model Protocols Examples
TCP/IP Application Layer
TCP/IP application layer defines the services to the
applications software running on your laptop. This layer does not specify how
to build an application but specifies the required services that the application
needs to effectively work and talk to other network systems. For example, it can
specify how the content is to be accessed and displayed to the user. The Application
layer is an interface through which software on your system interacts with the
network. The Web Browser is the most popular application example of the TCP/IP
model. More and more applications are being developed so that they can be accessed
through a web browser on a computer or mobile device.
TCP/IP Transport Layer
The Transport Layer provides the services to the Application
layer. TCP/IP Model Transport layer uses two most used protocols – the Transmission
Control Protocol (TCP) and the User Datagram Protocol (UDP).
Let’s look at a simple example to understand how it can
provide service to the upper layer (Application Layer). An application running on
a Web browser uses TCP protocols to ensure data is transferred reliably. This
reliable communication is ensured by TCP with one of the features called error recovery.
TCP/IP Network Layer
The network layer uses a major protocol – the Internet
Protocol (IP). TCP/IP Networking Model denotes the use of two major protocols separated
by a slash. IP protocol performs two most important functions – addressing and
routing.
As already stated, the Lower layers of the TCP/IP model provide
the delivery service to the upper layers. IP protocols identify the systems in the
network using logical addressing (IP Addressing) and performing the routing
function – which is sending traffic from source to destination across the network.
This addressing is required to identify the right network communication participants
and their locations whether they are on the same site or different sites. In a very
broad sense, routing is required when hosts are on a different network.
TCP/IP Datalink & Physical Layer
The Data-Link and physical layers define the protocols and
hardware and standards to deliver data across the physical network. They are referred
together sometimes as they work closely. Data-link layer helps the network
layer to identify the gateway or next-hop router to send the traffic so that it
can reach the destination. In short, Data-Link includes the protocols and
standards that control the use of the physical network.
The Physical layer defines the cabling and the energy specifications
that flow over the cables. Your laptop Ethernet Port is able to connect to a standard
RJ-45 cable because it implements the physical specification of the Ethernet Protocol.
TCP/IP Model Messages
In network communication, an application generates information
or Data. This data uses the layered approach comes to Transport, Network,
Datalink, and finally to Physical media from Sender. It makes the path from the
Physical layer to Data-Link, Network, Transport, and eventually to Application Layer
at the Receiver. This data that is generated by application referred by
different names at each layer and understanding of the same is required –
 |
| Figure 3: TCP/IP Model Messages Name |
Adjacent-layer Interaction & Same-layer Interaction
Adjacent-layer Interaction refers to how each layer in a
network model (like TCP/IP model) on the same computer work. For example, the HTTP
application interacts with TCP for reliable service. The TCP layer interacts
with the Network layer to identify hosts and deliver data to the right host. The
Data-Link layer helps IP protocols to know the next-hop router and send traffic
– it is the starting point for routing.
Same-Layer Interaction refers to how the same layer on one
computer communicates with the same layer on another computer. For example, two
computers are participating in network communication, and same-layer interaction
ensures traffic sent by an HTTP application on one is only received by an HTTP instance
on another computer, not to other applications that may be running on the
system – identification of the ports and protocol and the agreement between two
systems is an example of same-layer interaction.
TCP/IP vs OSI Model
TCP/IP won the battle and is the only networking model that exists today. A brief on OSI is still important as the protocols referred to by OSI still exist and are used. The OSI model has many similarities to the TCP/IP networking model. It also has layers and each layer defines related functions, protocols, and specifications.
Figure 4: TCP/IP vs OSI
OSI is generally is used as a reference model due to its terminology
when comparing other networking models today. It has 7 layers in total starting
from the physical layer. So, when someone refers to layer 3, it means the
Network Layer and NOT the Session Layer. The bottom 4 layers of the OSI and TCP/IP
model are the same in terms of name and functionality.
Remember the Application Layer protocols of the TCP/IP Model
are referred to as layer 7 protocol (as already said, the OSI layer is a reference
model of comparison to other networking model and inherit the layer numbering
conventions).
OSI layer uses the simple naming convention for data it uses
the format Layer n PDU (LnPDU) where n is the layer number. So, the Frame in the
OSI layer is called the L2PDU, Packet in OSI is called the L3PDU.
Hope you find this informative.
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