Section 4.5 The OSI Model and Protocol Layering
Now that we’ve explored key protocols used across the TCP/IP model, it’s helpful to examine the Open Systems Interconnection (OSI model), which provides a conceptual framework for understanding how data moves across a network. The OSI model organizes communication into seven distinct layers, each with a specific function. Although modern systems commonly use the four-layer TCP/IP model, the OSI model offers a more detailed structure that helps clarify where different protocols operate and where vulnerabilities may occur.
The OSI model breaks down network communication into the following layers:
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Physical Layer – Transfers raw bits over a physical medium (e.g., cables, radio waves).
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Data Link Layer – Provides node-to-node data transfer and handles error correction from the physical layer.
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Network Layer – Manages data routing, addressing, and packet forwarding across networks.
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Transport Layer – Ensures complete data transfer with error recovery and flow control.
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Session Layer – Manages sessions between applications, maintaining state and synchronization.
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Presentation Layer – Transforms data into a format suitable for the application layer, handling encryption and compression.
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Application Layer – Provides network services to end-user applications, such as web browsers and email clients.
| OSI Layer | TCP/IP Layer | Protocol Examples |
|---|---|---|
| 1. Physical | Network Access | Wi-Fi |
| 2. Data Link | Internet | IP, ICMP |
| 3. Network | Internet | IP, ICMP, NAT, IPsec |
| 4. Transport | Transport | TCP, UDP |
| 5. Session | Application | SSH, RDP, Telnet |
| 6. Presentation | Application | TLS, SSL |
| 7. Application | Application | HTTP, HTTPS, DNS, FTP, DHCP, |
| SMTP, SNMP, IMAP, POP3, LDAP, NTP |
This table shows how the OSI model maps to the TCP/IP model and provides examples of protocols at each layer. The OSI model’s seven layers correspond to the four layers of the TCP/IP model, with some layers combining functions. It’s also helpful when trying to figure out where something is going wrong or where a system might be vulnerable, since you can narrow things down to a specific layer.
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