From Transmission to the Internet: A Guide to the TCP/IP Model

During 1970, the Defense Advanced Research Projects Agency (DARPA) was underway to create an open standard network model. This network model came to be known as the TCP/IP Model. Subsequently, by 1985, the TCP/IP model started gaining more importance and support from vendors, ultimately replacing the OSI model. Presently, the Internet Engineering Task Force (IETF) maintains the TCP/IP model and its related protocols.

The TCP/IP model was on the path of development when the OSI standard was published. The TCP/IP model is not the same as the OSI model. OSI is a seven-layered standard, but TCP/IP is a four-layered standard. The OSI model has been very important in the growth and development of the TCP/IP standard, and that is why much OSI terminology is applied to TCP/IP.

Both models are open to standard networking models.  However, the TCP/IP model has found more acceptance today and the TCP/IP protocol suite is more commonly used. Just like the OSI reference model, the TCP/IP model takes a layered approach. In this section, we will look at all the layers of the TCP/IP model and the various protocols used in those layers.

The TCP/IP model is a reduced version of the OSI reference model consisting of the following 4 layers:

• Application Layer
• Transport Layer
• Internet Layer
• Network Access Layer

The functions of these four layers are comparable to the functions of the seven layers of the OSI model. The figure below illustrates the comparison between the layers of the two models.

The following sections discuss the four layers and protocols in those layers in detail.

As we can see from the above figure, the presentation and session layers are not there in the TCP/IP model. Also, note that the Network Access Layer in the TCP/IP model combines the functions of the Datalink Layer and Physical Layer.

Application Layer of TCP/IP model

The application layer is the topmost layer of the TCP/IP model. Located atop the Transport layer, it defines TCP/IP application protocols and outlines how host programs interface with transport layer services to utilize the network.

The Application Layer of the TCP/IP Model encompasses various protocols performing functions equivalent to the OSI model’s Application, Presentation, and Session layers. This encompasses interaction with the application, data translation and encoding, dialogue control, and communication coordination between systems.

Encompassing higher-level protocols such as DNS (Domain Naming System), HTTP (Hypertext Transfer Protocol), Telnet, SSH, FTP (File Transfer Protocol), TFTP (Trivial File Transfer Protocol), SNMP (Simple Network Management Protocol), SMTP (Simple Mail Transfer Protocol), DHCP (Dynamic Host Configuration Protocol), X Windows, RDP (Remote Desktop Protocol), among others, the application layer plays a vital role in network communication. Notably, these protocols facilitate diverse functions crucial for effective and seamless data exchange across networks.

Transport Layer of TCP/IP model

The Transport Layer is the third layer of the TCP/IP model. Situated between the Application layer and the Internet layer, its purpose is to permit devices on the source and destination hosts to carry on a conversation. The transport layer defines the level of service and status of the connection used when transporting data. The main protocols included at the Transport layer are TCP (Transmission Control Protocol) and UDP (User Datagram Protocol).

The TCP/IP transport layer’s function is the same as the OSI layer’s transport layer. It is concerned with end-to-end transportation of data and sets up a logical connection between the hosts.

TCP is a connection-oriented and reliable protocol that uses windowing to control the flow and provides ordered delivery of the data in segments. On the other hand, UDP simply transfers the data without the bells and whistles. Though these two protocols are different in many ways, they do the same function of transferring data and they use a concept called port numbers to do this.

Internet Layer of TCP/IP model

Once TCP and UDP have segmented the data and have added their headers, they send the segment down to the Network layer. The destination host may reside in a different network far from the host, divided by multiple routers. It is the responsibility of the Internet Layer to ensure that the segment moves across the networks to the destination network.

The Internet layer packs data into data packets known as IP datagram, which contains source and destination address (logical address or IP address) information that forwards the datagram between hosts and across networks. The Internet layer is also responsible for the routing of  IP datagrams.

Packet switching network depends on a connectionless internetwork layer. This layer is known as the Internet layer. Its job is to allow hosts to insert packets into any network and have them deliver independently to the destination. At the destination side data; packets may seem in a different order than they were sent. It is the job of the higher layers to rearrange them to deliver them to proper network applications operating at the Application layer.

The main protocols included at the Internet layer are IP (Internet Protocol), ICMP (Internet Control Message Protocol), ARP (Address Resolution Protocol), RARP (Reverse Address Resolution Protocol), and IGMP (Internet Group Management Protocol).

The Internet layer of the TCP/IP model corresponds to the Network layer of the OSI reference model in function. It provides logical addressing, path determination, and forwarding.

Network Access Layer of  TCP/IP model

The Network Access Layer is the first layer of the TCP/IP model. Network Access Layer defines details of how data is physically sent through the network. It also includes how bits are electrically or optically signaled by hardware devices that interface directly with a network medium, such as coaxial cable, optical fiber, or twisted pair copper wire. The important protocols of the Network Access Layer are Ethernet, Token Ring, FDDI, X.25 also Frame Relay.

LAN architecture is the most popular among those listed above Ethernet. Ethernet uses an Access Method called CSMA/CD (Carrier Sense Multiple Access/Collision Detection) to access the media when Ethernet operates in a shared media.

IN the CSMA/CD Access Method, every host has equal access to the medium and can place data on the wire when the wire is free from traffic or in an idle position. When a host wants to place data on the wire. It will check the wire to find out whether another host is already using the medium.

If there is traffic already in the medium, the host will wait and if there is no traffic, it will place the data in the medium. But, if two systems place data on the medium at the same instance they will collide with each other, destroying the data. If the data is destroyed during transmission. This data will need to be re-transmitted. After the collision, each host will wait for a small interval of time and again the data will re-transmit.

FAQs

1. How did the TCP/IP model originate, and why did it gain more importance over the OSI model?

• The TCP/IP model was developed by DARPA in the 1970s and gained prominence by 1985, ultimately replacing the OSI model. It is currently maintained by the Internet Engineering Task Force (IETF).

2. What are the main differences between the OSI model and the TCP/IP model?

• The OSI model has seven layers, while the TCP/IP model has four. The presentation and session layers are absent in the TCP/IP model, and the Network Access Layer combines the functions of the Datalink Layer and Physical Layer.

3. What is the significance of the layered approach in the TCP/IP model?

• Just like the OSI model, the TCP/IP model takes a layered approach, which simplifies networking and ensures compatibility between different systems and devices.

4. Can you provide an overview of the four layers in the TCP/IP model and their functions?

• The TCP/IP model consists of the Application Layer, Transport Layer, Internet Layer, and Network Access Layer. These layers perform functions comparable to those of the seven layers in the OSI model.

5. What is the role of the Application Layer in the TCP/IP model, and which protocols are associated with it?

• The Application Layer defines TCP/IP application protocols and facilitates communication between host programs and transport layer services. Protocols like HTTP, FTP, and SMTP operate at this layer.

6. What does the Transport Layer in the TCP/IP model handle, and which protocols are commonly used in this layer?

• The Transport Layer enables communication between source and destination hosts and defines the level of service for data transportation. TCP and UDP are the primary protocols in this layer.

7. How does the Internet Layer function in the TCP/IP model, and what are the key protocols associated with it?

• The Internet Layer ensures data packets are correctly forwarded to the destination network, handling logical addressing, path determination, and forwarding. It involves protocols like IP, ICMP, ARP, and IGMP.

8. What is the purpose of the Network Access Layer in the TCP/IP model, and which hardware-specific protocols does it include?

• The Network Access Layer is responsible for physically sending data through the network and includes protocols like Ethernet, Token Ring, FDDI, and others. It deals with the details of hardware interface and signaling.

9. How does Ethernet’s CSMA/CD Access Method work, and what happens when data collisions occur?

• Ethernet uses the CSMA/CD Access Method, allowing hosts to access the medium when it’s free. Data collisions occur when two systems transmit data simultaneously, resulting in data loss. Hosts wait for a brief interval and re-transmit data in case of a collision.