Cyber Security Interview Prep. Q1: "The OSI Model"

Category: General IT

"The OSI model. What is it, how many layers does it have, and can you name them for me?"

My answer:

OSI is a model used to understand how computer systems communicate over a network. You can often better troubleshoot networking issues if you break it down and look through it systematically.

The layers are from top to bottom:

7. Application layer- End user facing part of the software. eg email client, web browser. Protocol examples: DNS, FTP, HTTP, POP, SNMP, SMTP.

6. Presentation layer- Layer that preps data for app layer. Defines encoding, encryption and compression of the data being sent, if any required. Also takes data from app layer and preps it to be sent over session layer.

5. Session layer- Here a communication channel is opened between the client and server. Layer is responsible for opening, maintaining and closing a session. Also makes checkpoints so if session dies data transfer can resume rather than start again. Protocol examples are ARP and ICMP.

4. Transport layer- Data is broken into segments, transferred and then reformed on the other side. Here flow control is carried out- data sent is matched to the connection speed of the receiving device. if error controls and data checks are done, its done here. This is layer of protocols TCP and UDP.

3. Network layer. Has two functions:

1) Break up data into packets and reassemble on other side.

2) Discovering best path from source to destination for the packets to be routed through. IP addresses will be used here.

2. Data link layer- opens and closes connection between 2 physically connected nodes. Packets further broken into frames. Two parts make up data link:

1) Logical link control (performs error checks and syncs frames)

2) Media access control (which uses MAC addresses to connect devices and define their transmit and receive permissions)

1. Physical layer- transfers raw data in binary. Defines the connector-what physical medium is being used to transfer data between nodes like cable or wireless.