Understand network servers (file, web, mail, proxy)

Communications Technology – Network Servers (Cambridge IGCSE/A‑Level IT 9626 – Topic 14.3)

1. What is a Server?

A server is a computer programme or a dedicated machine that continuously offers a specific service to client devices over a network. Clients request the service using a defined protocol; the server processes the request and returns a response.

2. Summary of Server Families (Syllabus Overview)

3. Detailed Server Profiles

3.1 File Server

• Role: stores, shares and backs up files; enforces permissions and version control.
• Typical protocols & ports:
  • SMB/CIFS – TCP 445 (Windows) – also 139 (NetBIOS) for legacy systems.
  • NFS – TCP/UDP 2049 (Unix/Linux).
  • AFP – TCP 548 (Mac).
• Typical software: Open‑source: Samba, NetApp ONTAP (Free tier); Commercial: Microsoft Windows Server, Synology DSM.
• Key functions:
  • Centralised data repository for project teams.
  • Automated backup schedules.
  • Access‑control lists (ACLs) and share permissions.
• Real‑world example: A design studio keeps all CAD drawings on a Windows file server so every designer can open the latest version instantly.

3.2 Web Server

• Role: hosts web pages and web‑based applications.
• Typical protocols & ports:
  • HTTP – TCP 80.
  • HTTPS – TCP 443 (HTTP over TLS).
  • HTTP/2 and HTTP/3 (QUIC) use the same ports.
• Typical software: Open‑source: Apache HTTP Server, Nginx; Commercial: Microsoft IIS.
• Key functions:
  • Handles HTTP methods (GET, POST, PUT, DELETE).
  • Executes server‑side scripts (PHP, ASP.NET, Python, Node.js) for dynamic content.
  • Logs access and errors; can enforce SSL/TLS encryption.
• Real‑world example: The school’s intranet is hosted on an Apache web server; students log in to view timetables and submit assignments.

3.3 Mail Server

• Role: sends, routes, stores and retrieves e‑mail.
• Typical protocols & ports:
  • SMTP – TCP 25 (server‑to‑server), TCP 587 (client submission, usually with STARTTLS).
  • POP3 – TCP 110 (plain), TCP 995 (POP3 + TLS).
  • IMAP – TCP 143 (plain), TCP 993 (IMAP + TLS).
• Typical software: Open‑source: Postfix (MTA) + Dovecot (MDA); Commercial: Microsoft Exchange Server.
• Key functions:
  • Mail Transfer Agent (MTA) routes mail between domains (SMTP).
  • Mail Delivery Agent (MDA) stores mail in user mailboxes.
  • Client access via POP3 (download‑and‑delete) or IMAP (server‑side storage).
• Real‑world example: A company uses Microsoft Exchange to provide Outlook Web Access, calendar sharing and mobile sync for all staff.

3.4 Proxy Server

• Role: intercepts client requests; can cache, filter or anonymise traffic.
• Common types: forward proxy (client‑side), reverse proxy (server‑side), caching proxy.
• Typical protocols & ports:
  • HTTP – TCP 8080 (default for many forward proxies).
  • HTTPS – TCP 443 (usually via the CONNECT method).
  • SOCKS – TCP 1080 (generic TCP/UDP proxy).
  • Squid default – TCP 3128.
• Typical software: Open‑source: Squid; Commercial: Blue Coat ProxySG, Microsoft Forefront TMG.
• Key functions:
  • Caches frequently accessed web pages to reduce bandwidth.
  • Filters URLs or content according to organisational policy.
  • Hides internal IP addresses from the Internet (privacy).
• Real‑world example: A university deploys a Squid forward proxy so that students’ web traffic is cached and inappropriate sites are blocked.

3.5 Application Server

• Role: runs business logic and provides APIs or services to front‑end clients.
• Typical protocols & ports:
  • HTTP/HTTPS – 80/443 (REST, SOAP).
  • Custom ports for Java EE, .NET, CORBA, etc. (e.g., 8080, 8443).
• Typical software: Open‑source: Apache Tomcat, WildFly; Commercial: Microsoft .NET Core, IBM WebSphere.
• Key functions:
  • Executes Java EE, .NET, Node.js or similar applications.
  • Manages transactions, connection pools and session state.
  • Often sits behind a reverse proxy or load balancer.
• Real‑world example: An online retail site uses a Tomcat application server to process orders, calculate taxes and interact with the database.

3.6 Print Server

• Role: manages network printers and queues print jobs.
• Typical protocols & ports:
  • IPP (Internet Printing Protocol) – TCP 631.
  • LPR/LPD – TCP 515.
  • SMB – TCP 445 (Windows printer sharing).
• Typical software: Open‑source: CUPS; Commercial: Windows Print Services, HP Jetdirect.
• Key functions:
  • Spools print jobs and applies access controls.
  • Distributes printer drivers to client PCs.
• Real‑world example: A corporate print server hosts all office printers; users select “Company Printer” from any workstation.

3.7 FTP Server

• Role: transfers files reliably between client and server.
• Typical protocols & ports:
  • FTP – TCP 21 (control), TCP 20 (data – active mode).
  • FTPS (FTP over TLS) – 21 + TLS or explicit port 990.
  • SFTP (SSH File Transfer) – TCP 22.
• Typical software: Open‑source: vsftpd, FileZilla Server (free); Commercial: Microsoft IIS FTP.
• Key functions:
  • Upload and download of large files.
  • Directory listing and permission handling.
  • Secure variants (FTPS, SFTP) protect credentials.
• Real‑world example: A media company uses an SFTP server for secure delivery of video assets to partner broadcasters.

3.8 Virtual / Cloud Server

• Role: provides any of the above services as a virtualised instance that can be created, scaled and destroyed on demand.
• Key technologies: VMware ESXi, Microsoft Hyper‑V, KVM, Docker, Kubernetes, OpenStack, AWS EC2, Azure Virtual Machines, Google Compute Engine.
• Typical software (examples per service):
  • File service – Windows Server VM or Samba container.
  • Web service – Nginx container on AWS ECS.
  • Mail service – Postfix/Dovecot VM on Azure.
• Key functions:
  • Rapid provisioning – a new server can be created in minutes.
  • Scalable resources – CPU, RAM and storage can be adjusted on the fly.
  • Isolation – each VM or container runs its own OS or runtime.
• Real‑world example: A start‑up launches its web and database servers as Docker containers on AWS ECS, allowing automatic scaling during traffic spikes.

4. Request‑Response Cycle

1. Client initiates a request – e.g., a browser sends an HTTP GET, a file‑share client sends an SMB “Read”, or an e‑mail client issues an SMTP “MAIL FROM”.
2. Transport layer – the request travels over TCP (or UDP for NFS) using the appropriate port.
3. Server processes the request – authenticates the user, checks permissions, reads/writes data or generates a response.
4. Server sends a response – e.g., HTTP 200 OK, SMB “Read Data”, SMTP 250 OK.
5. Client receives the response – renders a web page, opens a file, or stores an e‑mail.

5. Server Farms, Load‑Balancing & Concurrency

• Server farm: a group of identical servers that provide the same service (e.g., multiple web servers) behind a load‑balancer.
• Load‑balancing methods:
  • Round‑robin – each new request goes to the next server in the list.
  • Least‑connections – the server with the fewest active sessions receives the request.
  • Resource‑based – decisions are made using CPU, memory or response‑time metrics.
• Concurrency handling: servers use multi‑threading, process pools or asynchronous I/O to serve many clients simultaneously. Session identifiers (cookies, tokens) keep track of individual users.
• Authentication & security:
  • Credentials are verified (username/password, Kerberos, LDAP).
  • Secure channels – TLS/SSL (HTTPS, SMTPS, FTPS) encrypt the request and response.
  • After successful login, the server often issues a session token to avoid re‑authenticating each request.

6. Comparison of Server Characteristics

7. Advantages & Disadvantages in Typical Scenarios