Network Applications and Principles

Network Applications

• Email, web browsing, remote login, P2P file sharing, multi-user network games, streaming stored video (like YouTube), voice over IP (like Skype), real-time video conferencing, and social networking are common examples of network applications.

Principles of Network Applications

• Network application architectures define the overall design and structure of an application built to function on a computer network.
• Processes communicating refers to the interaction between multiple processes within a network.
• The interface between a process and the network describes the link between a process on a device and the underlying computer network.
• Transport services are services provided by the network stack that enable applications to communicate with each other.
• The internet provides two main transport services: TCP (Transmission Control Protocol) and UDP (User Datagram Protocol).
• Application-layer protocols are data exchange protocols that allow communication between applications running on different devices.

Client-Server Architecture

• A client-server network is designed for end users (clients) to access resources like songs, videos, etc., from a central computer called a server.
• The server manages major operations like security and network management.
• The server is responsible for managing resources such as files, directories, printers, etc.
• Clients communicate with each other through the central server.

HTTP Protocol

• A client initiates a TCP connection (creates a socket) to the server on port 80.
• The server accepts the TCP connection from the client.
• HTTP messages (application-layer protocol messages) are exchanged between the browser (HTTP client) and the Web server (HTTP server).
• HTTP is a stateless protocol, meaning the server does not maintain any information about past client requests.
• Non-persistent HTTP closes the connection after the server sends the requested object to the client.
• Persistent HTTP keeps the connection open for multiple requests and responses.

Non-Persistent & Persistent HTTP Connection

• Non-Persistent HTTP creates a new TCP connection for each request/response pair.
• Persistent HTTP uses the same TCP connection for all requests and responses.

HTTP Response Status Codes

• A status code appears on the first line of the server-to-client response message.
• Some protocols, like HTTP and SMTP, send control information in-band (within the same TCP connection) for both requests and response headers and data.

How to Use FTP

• Connect to the FTP server using an FTP client or the command line interface, providing the server's address, username, and password. Anonymous access might not require a username and password.
• Navigate directories using commands like:
  • ls: List directories
  • cd: Change directories
• Transfer files with commands:
  • get: Download files
  • put: Upload files
• Manage files using operations like:
  • Delete: Remove files
  • Rename: Change file names
  • Copy: Duplicate files
• Close the connection with the bye or quit command.

Simple Mail Transfer Protocol (SMTP)

• SMTP is a set of guidelines for software to transmit email over the internet.
• It operates at the application layer of the TCP/IP protocol stack.
• A client opens a TCP connection to the SMTP server on port 25 and sends the email over this connection.

Components of SMTP

• Mail User Agent (MUA): Creates email messages for transfer to the MTA.
• Mail Submission Agent (MSA): Receives mail from the MUA and interacts with the MTA for transfer.
• Mail Transfer Agent (MTA): Transfers mail from one system to another using SMTP.
• Mail Delivery Agent (MDA): Delivers mail to the local system.

How SMTP Works

• A user composes an email message using a Mail User Agent (MUA).
• The message has two parts: the body and the header.
• The email client submits the message to the SMTP server using port 25.
• The MTA checks the domain name of the recipient address and finds the target domain using MX records in the Domain Name System (DNS).
• The MTA connects to the exchange server for the recipient domain to relay the message.
• The incoming server (Mail Delivery Agent, MDA) stores the email for the recipient to retrieve it.
• The email can be accessed and retrieved using the MUA.

POP (Post Office Protocol)

• POP is a protocol used for retrieving email from a mail server.

Explore common network applications such as email, web browsing, and remote login, as well as the principles that guide network application architectures. Understand the interaction between processes, transport services, and the role of application-layer protocols. Test your knowledge of how these concepts apply to real-world scenarios.