Computer Networks: Chapter 2 - Application Layer

Computer Networks: Chapter 2 - Application Layer

• The Internet is designed for application-layer services, supported by the TCP/IP protocol suite's other layers.
• Communication happens via a logical connection, where application layers simulate a direct connection.

Chapter 2 Outline

• Introduction
• Application-Layer Paradigms
  • Client-Server Paradigm
  • Peer-to-Peer Paradigm
• Socket Interface
• Standard Applications
• Socket Interface Programming
• Standard Client-Server Applications

Introduction

• The Internet's design prioritizes application-layer services.
• The other layers support these application-layer services.
• Applications use a logical connection to communicate, assuming a direct connection for sending/receiving messages.

Standard and Nonstandard Protocols

• Protocols in the first four TCP/IP layers are standardized and documented, often integrated into operating systems like Windows or UNIX.
• Application-layer protocols can be either standard or nonstandard.

Nonstandard Application-Layer Protocols

• Programmers can create nonstandard application-layer programs to interact with the transport layer.
• Internet authorities don't require approval for nonstandard, privately-used protocols.

Application-Layer Paradigms

• Application programs need to communicate through the Internet architecture.
• Should they both request and provide services or do just one?
• The client-server and peer-to-peer paradigms exist for this communication issue.

Traditional Paradigm: Client-Server

• The client-server paradigm, once highly popular, involves a continuously running server process waiting for client requests.
• When clients need service, they connect to the server through the Internet.

Traditional Paradigm: Client-Server (Continued)

• The server manages all communication load and waits for client processes to initiate requests.
• The server needs to be a robust computer in this paradigm.
• Some services like HTTP, FTP, SSH, and email utilize this paradigm.

New Paradigm: Peer-to-Peer (P2P)

• The P2P paradigm eliminates the need for a constantly running server.
• Responsibilities for providing and requesting services are shared among peers.
• A single computer can both provide and receive services concurrently.
• Internet telephony is an example that fits this paradigm.

New Paradigm: Peer-to-Peer (P2P) (Continued)

• Security is a challenge in distributed P2P communication.
• Applicability is another consideration, as not all applications suit the P2P model.
• Examples of applications using this paradigm include BitTorrent, Skype, IPTV, and Internet telephony.

Client-Server Paradigm

• Communication occurs between two application programs (a client and a server).
• Clients initiate communication by sending requests to servers.
• Servers handle requests, process them, and return results to clients.

Application Programming Interface (API)

• To interact with other processes, programs need instructions that tell the lowest TCP/IP layers about the connection.
• Application Programming Interfaces (APIs) handle these instructions, providing a communication tool.
• Sockets are a commonly used API for network communication.

Socket Interface

• Sockets provide communication between the application layer and operating systems.
• It's a set of instructions for process-to-process communication.
• Sockets are critical for applications requiring internet or network connections (e.g., web browsers, email clients, file-sharing apps).

Socket Interface (Continued)

• Socket connections are two-way and require a local and a remote address pair.

Socket Addresses

• Socket addresses identify the computer and the specific process involved.
• An IP address (32-bit integer) defines the computer.
• A port number (16-bit integer) uniquely identifies a specific process on the computer.

Common Socket Interface Functions

• socket(): Creates a new socket.
• bind(): Assigns an IP address and port to a socket.
• listen(): Prepares a socket to accept incoming connections (server-side).
• connect(): Establishes a connection to a server (client-side).
• send(): Sends data.
• receive(): Receives data.
• close(): Closes the socket.

Uses of Sockets

• Web servers and browsers use sockets for HTTP requests and responses.
• Messaging services use sockets for managing continuous connections.
• File transfer protocols (like FTP) use sockets to establish data exchange.