Network Services

Network Services: Communicate with other people/devices.

• e.g., file transfer, websites, databases, applications.

Client-Server Model

Client: Connects to servers (requester)

• e.g., web browser connects to web servers, email clients connect to email servers, etc.
• Request services over the Internet from servers.

Server: Offers services (responder)

• e.g., web sites use HTTP, secure shell uses SSH, email uses SMTP
• Constantly listen for client requests.

Remember: The server doesn’t initiate connection.

Domain Name System (DNS)

Distributed Database: Provides mapping from domain name to IP address.

• Hierarchical organization.
  • Top-level domains: .com, .edu, .org, .net
• Uses UDP port 53.

Why UDP?:

• Because we’re just doing a query, and don’t need a connection.

The World Wide Web

HTTP: Protocol used for requesting and responding.

HTML: Markup language to construct web pages.

WWW Models

TODO Old and Modern WWW Model

The old model was very simple, the modern model supports creating web pages in real-time, HTML forms, and database queries.

HTTP

TODO Read HTTP Wikipedia page.

File Transfer Protofol (FTP)

File Transfer Protocol: Transfers files between systems.

• PUT: Upload file.
• GET: Download file.
• Uses TCP ports 20 and 21

On SFTP:

• FTP doesn’t support encryption, so we use SFTP or SCP in the modern-day.

Secure Shell (SSH)

Secure Shell: Provides encrypted tunnel to a remote system.

• Uses port 22.
• Provides secure copy (SCP) for encrypted file transfer.

History:

• Replaced the old Telnet command, which wasnt’t encrypted.