Learning Objectives
By the time you finish this room, you will have learned about the following:
- ISO OSI network model
- IP addresses, subnets, and routing
- TCP, UDP, and port numbers
- How to connect to an open TCP port from the command line
OSI Model
There are Seven Layers on OSI model, u can remember them easily by using mnemonic, “Please Do Not Throw Spinach Pizza Away”.
ERROR
I noticed above image is wrong, mnemonic shud start from bottom
Layer 1 : Physical Layer
The physical layer, also referred to as layer 1, deals with the physical connection between devices; this includes the medium, such as a wire, and the definition of the binary digits 0 and 1. Data transmission can be via an electrical, optical, or wireless signal. Consequently, we need data cables or antennas, depending on our physical medium.
In addition to Ethernet cable, shown in the illustration below, and optical fibre cable, examples of the physical layer medium include the WiFi radio bands, the 2.4 GHz band, the 5 GHz band, and the 6 GHz band.
Layer 2 : Data Link Layer
The Data Link Layer is the second layer (Layer 2) of the OSI model. It is responsible for node-to-node communication and ensures reliable data transfer over a physical link in the network. Here’s a simplified explanation of its key functions:
Key Responsibilities :
-
Framing:
- Converts raw bits into frames (structured chunks of data).
- Adds headers and trailers to help identify where frames start and end.
-
Error Detection and Correction:
- Detects and, in some cases, corrects errors that occur during transmission using mechanisms like checksums or CRC (Cyclic Redundancy Check).
-
Flow Control:
- Prevents a fast sender from overwhelming a slow receiver by managing the rate of data transfer.
-
Media Access Control (MAC):
- Determines how devices share access to the network medium, especially in shared mediums like Ethernet.
- Handles addressing through MAC (Media Access Control) addresses, unique hardware identifiers for devices on the same network.
-
Logical Link Control (LLC):
- Provides logical addressing and helps in identifying the protocol used by the upper layer.
Real-Life Examples :
- Ethernet: A common example of the Data Link Layer in action. It ensures proper data transfer between computers in a local network.
- Wi-Fi: Implements data link layer protocols to enable wireless communication.
Analogy :
Think of the Data Link Layer as the postal worker who ensures that the mail (data) is correctly packaged (framed), addressed (MAC address), and delivered reliably to the right house (device) in a neighborhood (local network).
We expect to see two MAC addresses in each frame in real network communication over Ethernet or WiFi. The packet in the screenshot below shows:
- The destination data-link address (MAC address) highlighted in yellow
- The source data link address (MAC address) is highlighted in blue
- The remaining bits show the data being sent
Layer 3 : Network Layer
The data link layer focuses on sending data between two nodes on the same network segment. The network layer, i.e., layer 3, is concerned with sending data between different networks. In more technical terms, the network layer handles logical addressing and routing, i.e., finding a path to transfer the network packets between the diverse networks.
- Routes data between different networks, potentially across cities or continents.
- Uses logical addressing (e.g., IP addresses) to identify networks and devices.
- Determines the best path for packets to take when traveling across multiple routers.
- Common protocols: IP, ICMP, and VPN protocols like IPSec or SSL/TLS.
Layer 4 : Transpot Layer
Layer 4, the transport layer, enables end-to-end communication between running applications on different hosts. Your web browser is connected to the TryHackMe web server over the transport layer, which can support various functions like flow control, segmentation, and error correction.
Examples of layer 4 are Transmission Control Protocol (TCP) and User Datagram Protocol (UDP).
TCP vs UDP
Layer 5 : Session Layer
The session layer is responsible for establishing, maintaining, and synchronising communication between applications running on different hosts. Establishing a session means initiating communication between applications and negotiating the necessary parameters for the session. Data synchronisation ensures that data is transmitted in the correct order and provides mechanisms for recovery in case of transmission failures.
Examples of the session layer are Network File System (NFS) and Remote Procedure Call (RPC).
Layer 6: Presentation Layer
The presentation layer ensures the data is delivered in a form the application layer can understand. Layer 6 handles data encoding, compression, and encryption. An example of encoding is character encoding, such as ASCII or Unicode.
Various standards are used at the presentation layer. Consider the scenario where we want to send an image via email. First, we use JPEG, GIF, and PNG to save our images; furthermore, although hidden from the user by the email client, we use MIME (Multipurpose Internet Mail Extensions) to attach the file to our email. MIME encodes a binary file using 7-bit ASCII characters.
Layer 7: Application Layer
The application layer provides network services directly to end-user applications. Your web browser would use the HTTP protocol to request a file, submit a form, or upload a file.
The application layer is the top layer, and you might have encountered many of its protocols as you use different applications.
Examples of Layer 7 protocols are HTTP, FTP, DNS, POP3, SMTP, and IMAP. Don’t worry if you are not familiar with all of them.
Summary
Reading about the ISO OSI model for the first time can be intimidating; however, it becomes easier as you progress in your study of networking protocols. To help with your studies, we have summarised the ISO OSI layers in the table below.
| Layer Number | Layer Name | Main Function | Example Protocols and Standards |
|---|---|---|---|
| Layer 7 | Application layer | Providing services and interfaces to applications | HTTP, FTP, DNS, POP3, SMTP, IMAP |
| Layer 6 | Presentation layer | Data encoding, encryption, and compression | Unicode, MIME, JPEG, PNG, MPEG |
| Layer 5 | Session layer | Establishing, maintaining, and synchronising sessions | NFS, RPC |
| Layer 4 | Transport layer | End-to-end communication and data segmentation | UDP, TCP |
| Layer 3 | Network layer | Logical addressing and routing between networks | IP, ICMP, IPSec |
| Layer 2 | Data link layer | Reliable data transfer between adjacent nodes | Ethernet (802.3), WiFi (802.11) |
| Layer 1 | Physical layer | Physical data transmission media | Electrical, optical, and wireless signals |
TCP/IP Model Overview
Now that we have covered the conceptual ISO OSI model, it is time to study an implemented model: the TCP/IP model. TCP/IP stands for Transmission Control Protocol/Internet Protocol and was developed in the 1970s by the Department of Defense (DoD). One of the strengths of this model is its ability to allow a network to continue functioning even if parts of it are out of service, such as during a military attack. This resilience is largely due to the design of the routing protocols, which can adapt as the network topology changes.
TCP/IP Model vs. ISO OSI Model
While the ISO OSI model consists of seven layers, the TCP/IP model simplifies this structure. Below is a comparison of the two models, illustrating how the TCP/IP layers correspond to the OSI layers.
Layer Mapping Table
| Layer Number | ISO OSI Model | TCP/IP Model (RFC 1122) | Protocols |
|---|---|---|---|
| 7 | Application Layer | Application Layer | HTTP, HTTPS, FTP, POP3, SMTP, IMAP, Telnet, SSH |
| 6 | Presentation Layer | Grouped into Application | |
| 5 | Session Layer | Grouped into Application | |
| 4 | Transport Layer | Transport Layer | TCP, UDP |
| 3 | Network Layer | Internet Layer | IP, ICMP, IPSec |
| 2 | Data Link Layer | Link Layer | Ethernet 802.3, WiFi 802.11 |
| 1 | Physical Layer | Physical Layer | Ethernet Cables, Fiber Optics, Wireless Signals |
Note: Many modern networking textbooks, such as Computer Networking: A Top-Down Approach (8th Edition) by Kurose and Ross, present the TCP/IP model with five layers by including the Physical layer separately:
- Application
- Transport
- Network
- Link
- Physical
TCP/IP Model Layers
Below is a visualization of the TCP/IP model layers compared to the OSI model using Mermaid diagrams.
Layer Comparison Diagram
TCP/IP Model Layer Diagram
Focus Areas
In the following sections, we will delve deeper into specific protocols within the TCP/IP model:
- Internet Layer: Understanding the IP protocol.
- Transport Layer: Exploring UDP and TCP protocols.
Key Protocols by Layer
Application Layer
- HTTP/HTTPS: HyperText Transfer Protocol (Secure)
- FTP: File Transfer Protocol
- POP3/IMAP: Email retrieval protocols
- SMTP: Simple Mail Transfer Protocol
- Telnet/SSH: Remote login protocols
Transport Layer
- TCP (Transmission Control Protocol): Connection-oriented protocol ensuring reliable data transmission.
- UDP (User Datagram Protocol): Connectionless protocol used for applications requiring speed over reliability.
Internet Layer
- IP (Internet Protocol): Core protocol for addressing and routing packets.
- ICMP (Internet Control Message Protocol): Used for diagnostic and error messages.
- IPSec: Suite for securing IP communications.
Link Layer
- Ethernet 802.3: Wired networking standard.
- WiFi 802.11: Wireless networking standard.
Physical Layer
- Ethernet Cables, Fiber Optics, Wireless Signals: Physical mediums for data transmission.
Questions
- To which layer does HTTP belong in the TCP/IP model?
- Application Layer
- How many layers of the OSI model does the application layer in the TCP/IP model cover?
- 3 (Application, Transport and Data Link)