7.2.2: Transmission Control Protocol/ Internet Protocol Model

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The OSI model presents a great high-level overview of network communications but as mentioned earlier it is very theoretical and in many cases networking in the industry does not follow the OSI model.

However, the Transmission Control Protocol, Internet Protocol (TCP/IP) model or more commonly referred to as the TCP/IP Protocol Suite is used. Many experts describe the OSI model and theoretical whereas the TCP/IP model as practical.

The TCP/IP Model has 4 layers: Application; Transport; Internetwork; Data Link. — Figure \(\PageIndex{1}\): TCP/IP Architecture. ("Information Security" by Umar Khokhar Binh Tran, OpenALG is licensed under CC BY 4.0)

A “protocol” is not specific to networking but refers to a set of rules and procedures for communication or behavior. For example, an employee must follow a set of protocols to be employed at an organization. Two people communicating must agree on a certain language or “protocol” to successfully communicate. In the past there were several protocols that were used in networking such as Windows specific NetBEUI, or Novell NetWare’s IPX/SPX but both are now obsolete. The TCP/IP protocol suite is the protocol of the Internet and the one that all current operating systems and systems run.

The TCP/IP protocol suite uses a similar layered approach like the OSI model but it is condensed into four layers instead of seven namely Application, Transport, Internetwork and Network access.

Application-Layer Protocols
The Application layer provides network services to user applications as well as provide authentication and data-formatting, data encryption and translation. Common protocols which we will discuss in further detail later include HTTP/HTTPS: Protocols of the World Wide Web and Email Protocols: POP3, IMAP and SMTP. DHCP and DNS are also very important Application layer protocols to overall network and Internet operation.
Transport-Layer Protocols
The Transport layer protocols include TCP and UDP just like in the OSI Transport layer and its role is to provide reliability to flow control to transfer large amounts of data. Segmenting, sequencing, flow control, acknowledgement and hand-shaking occur here.
Internetwork-Layer Protocols
The Internetwork layer protocols is where network configuration and the Internet Protocol (IP) operate. The layer defines and verifies IP addresses, routes packets through the networks, resolves MAC addresses to IP addresses, delivers packets effectively and efficiently. IPv4, IPv6, ICMP, ARP and IPSec are the more common protocols that operate at this layer.
Network Access-Layer Protocols
The Network Access layer technically does not include any protocols but rather technologies such as Ethernet. The layer is responsible for MAC address confirmation, defining of media access rules, de- encapsulation of frame, checking for errors and converting the signals to bits whether it is electrical, light pulses or radio waves.

As you can already see the TCP/IP suite more accurately aligns its protocols and functionality with real network services thus it is the more practical model used today.