2.1 Networks: Client-Server and Peer-to-Peer Models

This section explores the fundamental architectures of networked computers: the client-server model and the peer-to-peer (P2P) model. Understanding these models is crucial for comprehending how resources are shared and services are delivered in computer networks, including the internet.

Client-Server Model

Overview

The client-server model is a distributed application framework that divides tasks and responsibilities between two types of computers: clients and servers. Clients request services and resources, while servers provide those services and resources.

Components

• Client: A computer or device that requests services from a server. Examples include web browsers, email clients, and file-sharing applications.
• Server: A powerful computer or system that provides services to clients. These services can include file storage, web pages, email, and database access.
• Network: The communication infrastructure (e.g., Ethernet, Wi-Fi) that connects clients and servers.
• Protocol: A set of rules governing communication between clients and servers (e.g., HTTP, SMTP, FTP).

How it Works

1. A client initiates a request for a service or resource.
2. The request is transmitted over the network to the server.
3. The server processes the request and sends a response back to the client.
4. The client receives and uses the response.

Advantages

• Centralized Management: Resources are managed in a central location, simplifying administration and maintenance.
• Security: Security measures can be implemented on the server, protecting all clients.
• Scalability: Servers can be upgraded to handle increasing client demands.
• Reliability: Servers are typically designed for high availability and reliability.

Disadvantages

• Single Point of Failure: If the server fails, all clients are affected.
• Cost: Servers can be expensive to purchase and maintain.
• Bottlenecks: The server can become a bottleneck if it is overloaded with requests.

Examples

Role	Description	Example
Web Server	Stores and delivers web pages to clients.	Apache, Nginx
File Server	Stores and manages files accessible to clients.	Windows File Server, Samba
Email Server	Handles sending and receiving email messages.	Postfix, Sendmail

Peer-to-Peer (P2P) Model

Overview

In a P2P network, computers (peers) communicate directly with each other without relying on a central server. Each peer can act as both a client and a server, sharing resources directly with other peers.

Components

• Peers: Computers that share resources and communicate directly with each other.
• Network: The communication infrastructure connecting the peers.
• Protocols: Protocols facilitate direct communication between peers (e.g., BitTorrent, Gnutella).

How it Works

1. A peer requests a resource from another peer.
2. The requesting peer connects directly to the peer that has the resource.
3. The resource is transferred directly between the peers.

Advantages

• Cost-Effective: No need for expensive dedicated servers.
• Resilience: The network is more resilient to failures, as resources are distributed across multiple peers.
• Scalability: Easily scalable by adding more peers.
• Direct Access: Peers can directly access resources without going through a central server.

Disadvantages

• Security Risks: Increased security risks due to the lack of central control.
• Management Complexity: Difficult to manage and maintain the network.
• Legal Issues: Often associated with illegal file sharing.
• Performance Issues: Performance can be inconsistent depending on the availability and bandwidth of peers.

Examples

Application	Description	Example
File Sharing	Sharing files directly between users.	BitTorrent, eDonkey2000
Instant Messaging	Direct communication between users.	Skype, WhatsApp
Distributed Computing	Distributing computational tasks across multiple peers.	SETI@home

Comparison Table

Feature	Client-Server	Peer-to-Peer
Architecture	Centralized	Decentralized
Resource Location	Central Server	Distributed across peers
Management	Centralized	Decentralized
Security	Centralized security measures	Security relies on individual peers
Cost	Higher	Lower
Reliability	Dependent on server reliability	More resilient to failures