OSI 7계층과 TCP/IPThe OSI 7-Layer Model and TCP/IP

Early Network Environments

ARPANET (Advanced Research Projects Agency Network), run by the U.S. Department of Defense, was the first packet-switching network. It started in 1969 and became the origin of today’s internet.

In the 1960s and early ’70s, computer networks were implemented in each manufacturer’s own proprietary way, and these systems were not compatible with one another. As networks grew, communication between systems became a real problem.

Against this backdrop, discussion started around two main questions.

1. A systematic structure for managing complex network communication → the OSI 7 layers
2. Standardized rules for communication between different systems → protocols

Why the OSI 7-Layer Model Was Created

In the mid-1970s, network technology developed rapidly, and compatibility between different network systems became a serious problem. In response, the International Organization for Standardization (ISO) began developing a standard model for interconnecting open systems in 1977.

In 1984, ISO officially published the OSI (Open Systems Interconnection) reference model. Its goals were:

1. Manage complexity by splitting network communication into logical layers
2. Achieve compatibility across different systems through standardization of each layer
3. Provide a modular structure in which each layer can evolve independently

Structure of the OSI 7 Layers

The OSI model divides network communication into 7 layers. This layering made it possible to understand and manage the complex process of network communication systematically.

1. Physical Layer: defines the electrical and physical characteristics for transmitting bits
2. Data Link Layer: data transfer between nodes, error detection/correction
3. Network Layer: packet routing, logical addressing
4. Transport Layer: end-to-end communication control, reliable data delivery
5. Session Layer: setting up, maintaining and ending communication sessions
6. Presentation Layer: data format conversion, encryption
7. Application Layer: user interfaces, email, file transfer, etc.

> The OSI 7-layer structure

The Birth of the TCP/IP Protocol

A protocol is a set of rules that computers and network devices follow when communicating with each other. Early on, each company or institution developed its own protocols, which led to interoperability problems.

In 1974, Vint Cerf and Bob Kahn developed TCP (Transmission Control Protocol), which later evolved into TCP/IP. This protocol became the foundation of the “internet” concept: connecting many different networks together.

The OSI 7 Layers and the TCP/IP Model

The OSI model was solid in theory, but in practice the TCP/IP model became the standard of the internet. Given how few real implementations some layers have (the session and presentation layers in particular), it’s easy to see why the TCP/IP model was adopted more widely.

TCP/IP Protocol Structure

Bundling several layers that correspond to the OSI 7 layers is called a stack. The TCP/IP protocol stack consists of these 4 layers:

1. Network Access Layer: handles the connection to the physical network, such as MAC addresses and ARP.
2. Internet Layer: sets the path for data and delivers packets using IP.
3. Transport Layer: tunes the reliability or speed of data delivery with TCP and UDP.
4. Application Layer: protocols users interact with directly, such as HTTP, FTP and SMTP.

> The TCP/IP protocol stack

How the OSI 7 Layers Map to TCP/IP

The theoretical OSI model and the practical TCP/IP model each have their own role and meaning. OSI serves as an important reference model for network education and design, while TCP/IP is used as the standard for real internet communication.

> TCP/IP made the OSI 7-layer concept practical

Physical Layer + Data Link Layer = Network Access Layer

• Main functions
  • Physical connection and conversion to electrical signals
  • MAC address-based communication and error control
• Main protocols: MAC, ARP, Ethernet
• Main devices: hubs, switches, NICs (Network Interface Card)

Network Layer = Internet Layer

• Main functions
  • Routing (path selection)
  • Communication based on logical addresses (IP)
  • Packet fragmentation and reassembly
• Main protocols: IP, ICMP
• Main devices: routers

Transport Layer = Transport Layer

• Main functions
  • End-to-end communication control
  • Reliable data delivery (TCP)
  • Identifying processes by port number
• Main protocols: TCP, UDP
• Main devices: load balancers, firewalls

Session + Presentation + Application Layers = Application Layer

• Main functions
  • Setting up/maintaining/ending communication sessions, dialog control, managing synchronization points
  • Data format conversion, encryption/decryption, compression/decompression
  • Providing user services (web, email, file transfer), communication between applications, user interfaces
• Main protocols: HTTP, SMTP, FTP, DNS
• Main devices: application servers, L7 load balancers, WAFs (Web Application Firewall)

Wrapping Up

The OSI 7-layer model and TCP/IP each have their own role and meaning.

• The OSI 7-layer model is an important reference model for network education and design
• TCP/IP is the foundation of real internet communication

The OSI 7 layers and the TCP/IP model are the basics of networking, but they’re hard to grasp from theory alone. In upcoming posts, we’ll look more closely at each layer’s main protocols and how networks actually work, through hands-on practice.

Thanks for reading!😊

References

• Developer Kang Min-cheol - [Network] Binge-Watch - Part 1 (Korean)
• Introduction to Networking for IT Engineers - Go Jae-seong, Lee Sang-hoon | Gilbut (Korean)