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.

TCP/IP Networking Model
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

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 –

TCP/IP Model Messages
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
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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