A protocol is a set of rules enabling two nodes of a network to communicate with each other, in order to automatically collaborate in accomplishing a common computational task. A protocol stack is a stack of layers of protocols which work together. This section introduces the Web over Internet protocol stack, to elaborate the basic concepts of communication in a computer network.
Protocol stack of the OSI model
The OSI (Open Systems Interconnection) model is a conceptual framework used to understand network interactions in seven layers.
| Layer | Name | Function | Examples |
|---|---|---|---|
| 7 | Application | Provides network services directly to end-users or applications | HTTP, FTP, SMTP, DNS |
| 6 | Presentation | Formats and encrypts data for the application layer | JPEG, MPEG, SSL/TLS |
| 5 | Session | Manages sessions between applications | NetBIOS, RPC |
| 4 | Transport | Provides end-to-end communication and reliability | TCP, UDP |
| 3 | Network | Routes data packets between different networks | IP, ICMP, OSPF |
| 2 | Data Link | Provides node-to-node communication and error detection | Ethernet, Wi-Fi, PPP |
| 1 | Physical | Transmits raw bit stream over physical medium | Cables, Switches, Hubs |
The following example shows the process of fetching a picture from a URL (e.g., https://www.example.com/pic.png) |
Application Layer (Layer 7)
- Request: Your web browser sends an HTTP GET request to
https://www.example.com/pic.png. - Response: The server responds with the image data.
Presentation Layer (Layer 6)
- Encryption: If using HTTPS, SSL/TLS encrypts the data before transmission.
Session Layer (Layer 5)
- Session Management: SSL/TLS session is established and maintained for the duration of the request.
Transport Layer (Layer 4)
- Segmentation: The image data is segmented into smaller chunks (TCP segments).
- Error Handling: TCP ensures all segments arrive correctly and reassembles them.
Network Layer (Layer 3)
- IP Packetization: Each TCP segment is encapsulated into an IP packet.
- Routing: The IP packet is routed from the source to the destination (web server to client).
Data Link Layer (Layer 2)
- Frame Encapsulation: IP packets are encapsulated into Ethernet frames (or Wi-Fi frames for wireless).
- MAC Addressing: Frames are addressed using MAC addresses for local network delivery.
Physical Layer (Layer 1)
- Bit Transmission: Frames are converted into electrical, optical, or radio signals and transmitted over the medium (e.g., copper wire, fiber optic cable, radio waves).
In the physical layer, data is transmitted as a sequence of bits. These bits are grouped and interpreted based on the physical layer protocol. For Ethernet, the frame structure includes:
- Preamble: A sequence of bits used for synchronization.
- Start Frame Delimiter (SFD): Indicates the start of the frame.
- Destination MAC Address: The MAC address of the receiving device.
- Source MAC Address: The MAC address of the sending device.
- Type/Length: Indicates the type of protocol (e.g., IPv4) or the length of the payload.
- Payload: The actual data being transmitted (e.g., a segment of the image file).
- Frame Check Sequence (FCS): Used for error checking to ensure data integrity.