OSI Layer

The OSI (Open Systems Interconnection) Model is a conceptual framework developed by the ISO to standardize the functions of telecommunication and computing systems. It ensures communication between different systems and devices, promoting interoperability across a wide range of networking technologies. The OSI model divides network communication into seven abstraction layers, each representing a specific functionality.

The seven layers are:

• Physical Layer: Deals with the transmission of raw bit streams over a physical medium (e.g., cables, WiFi).

• Data Link Layer: Ensures error-free data transfer between adjacent network nodes by managing frames.

• Network Layer: Handles routing and forwarding of data packets across different networks (e.g., IP).

• Transport Layer: Ensures reliable data transfer between hosts, handling error correction and flow control (e.g., TCP/UDP).

• Session Layer: Manages sessions or connections between applications.

• Presentation Layer: Translates data formats between applications and the network (e.g., encryption, data compression).

• Application Layer: Provides network services directly to end-user applications (e.g., HTTP, SMTP).

Each layer performs a unique role and communicates with the layers directly above and below it, ensuring smooth data exchange across diverse systems.

Function & Usage

Layer	Function	Example
Application	Provides services and interfaces for end-user applications to access the network. It handles high-level protocols like file transfers and email.	HTTP, FTP, SMTP, DNS, Telnet
Presentation	Ensures data is in a usable format for the application layer by handling translation, encryption, and compression.	Encryption (TLS/SSL), JPEG, PNG, ASCII
Session	Manages and controls connections between computers, ensuring proper start, maintenance, and termination of sessions.	Session management, RPC, NetBIOS
Transport	Ensures reliable data transfer between hosts, managing error detection, retransmissions, and flow control.	TCP, UDP, SPX
Network	Responsible for routing and forwarding packets between different networks, ensuring they reach their destination.	IP, ICMP, IPX, Routers
Data Link	Handles node-to-node data transfer by framing data, detecting errors, and controlling how devices share the physical medium.	Ethernet, Wi-Fi (802.11), Switches, MAC, ARP
Physical	Transfers raw bits over a physical medium (cables, wireless), ensuring electrical or optical signals are transmitted correctly.	Cables (Ethernet, Fiber Optic), Wi-Fi

Security on Each Layer

Layer	Security Defenses	Common Attacks
Application	Web Application Firewalls (WAF) Secure Socket Layer (SSL)/Transport Layer Security (TLS) Regular software updates and patch management Input validation and sanitization	SQL Injection Cross-Site Scripting (XSS) Cross-Site Request Forgery (CSRF) Buffer Overflow
Presentation	Encryption (e.g., AES, RSA) Data Integrity Checks (e.g., hashes) Secure data format conversions	Data Interception (Man-in-the-Middle attacks) Data Tampering Format String Attacks
Session	Secure Session Management Use of Session Tokens Regular monitoring and session timeouts	Session Hijacking Session Fixation Man-in-the-Middle Attacks (session interception)
Transport	Transport Layer Security (TLS) Secure protocols (e.g., TCP with proper configurations) Port filtering and monitoring	TCP SYN Flood UDP Flood Session Hijacking Man-in-the-Middle Attacks (e.g., packet sniffing)
Network	IPsec Network Firewalls Network Segmentation Routing Protocol Security (e.g., BGP)	IP Spoofing Routing Attacks (e.g., BGP Hijacking) DDoS Attacks Packet Sniffing
Data Link	MAC Address Filtering VLANs (Virtual Local Area Networks) Network Access Control (NAC) Frame Relay Security	MAC Spoofing ARP Spoofing VLAN Hopping Frame Injection
Physical	Physical Security Measures (e.g., secure access to facilities) Network Segregation Monitoring and Surveillance	Cable Tapping Physical Theft of Equipment Signal Interception Electromagnetic Interference (EMI)

The OSI model serves as a guideline for developing and understanding network protocols by organizing the complex task of network communication into manageable and structured layers. It is a conceptual model that helps in breaking down network functions into seven distinct layers, each responsible for specific aspects of communication.

While the OSI model provides a framework for understanding and designing network architectures, it is not a strict blueprint for every networking system. Instead, it acts as a reference model that guides the development and implementation of network protocols and systems, promoting a standardized approach to network communication.