OSI Model (7 Layers)
Encapsulation & Troubleshooting
Section titled “Encapsulation & Troubleshooting”The OSI model provides a framework for understanding network interactions. In practice, we focus on the TCP/IP model, but OSI terminology is standard for troubleshooting. What is the OSI Model? (Cloudflare).
The Layers & Troubleshooting
Section titled “The Layers & Troubleshooting”| Layer | Name | PDU | Key Concepts | Troubleshooting Context |
|---|---|---|---|---|
| L7 | Application | Data | HTTP, DNS, TLS, SSH | Status codes (502, 404), Headers, Payload corruption. |
| L6 | Presentation | Data | Encryption (TLS), Encoding | SSL Handshake failures, Certificate errors. |
| L5 | Session | Data | Session Mgmt | Keep-alives, Timeouts. |
| L4 | Transport | Segment | TCP, UDP, Ports | Syn/Ack, Retransmissions, Buffer bloat, Port exhaustion. |
| L3 | Network | Packet | IP, ICMP, BGP, IPSec | Routing loops, MTU/Fragmentation, Packet loss. |
| L2 | Data Link | Frame | Ethernet, MAC, VLAN | ARP issues, MAC flapping, CRC errors. |
| L1 | Physical | Bit | Fiber, Copper | Signal degradation, Cable cuts, Interface errors. |
Encapsulation Process
Section titled “Encapsulation Process”Data flows down the stack, with each layer adding a header (Encapsulation).
- L7: HTTP Request.
- L4: Adds TCP Header (Source/Dest Port).
- L3: Adds IP Header (Source/Dest IP).
- L2: Adds Ethernet Header (Source/Dest MAC).
Upon reception, the process is reversed (Decapsulation).
Cross-Layer Concepts
Section titled “Cross-Layer Concepts”- MTU (Maximum Transmission Unit): Typically 1500 bytes (Ethernet). If L3 packet > MTU, fragmentation occurs (bad for performance).
- MSS (Maximum Segment Size): L4 parameter to prevent fragmentation.