CS 39006: Computer Networks Laboratory

POSIX Network Programming: UDP Sockets

INDIAN INSTITUTE OF TECHNOLOGY

KHARAGPUR

Sandip Chakraborty

sandipc@cse.iitkgp.ac.in

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Department of Computer Science and Engineering

Abhijnan Chakraborty

abhijnan@cse.iitkgp.ac.in

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Overview

Why Sockets?

• Enable process-to-process communication
• Basis of:
• Web (HTTP)
• DNS
• Streaming
• Multiplayer games
• IoT & sensor systems

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Today’s Focus

• POSIX sockets fundamentals
• UDP (datagram) sockets
• Writing a UDP client and server in ANSI C

Hardware

OS

APP11

APP12

Hardware

OS

APP11

APP12

Machine 1

Machine 1

Protocol Stack

Protocol Stack

Indian Institute of Technology Kharagpur

What Are POSIX Sockets?

Definition

• POSIX sockets = standardized API for network communication
• Same interface across:
• Linux
• macOS
• UNIX-like systems

Indian Institute of Technology Kharagpur

What Are POSIX Sockets?

Definition

• POSIX sockets = standardized API for network communication
• Same interface across:
• Linux
• macOS
• UNIX-like systems

Key Idea:

A socket is an endpoint for communication

Indian Institute of Technology Kharagpur

What Are POSIX Sockets?

Definition

• POSIX sockets = standardized API for network communication
• Same interface across:
• Linux
• macOS
• UNIX-like systems

​

Abstraction

• File-descriptor–like interface
• Uses standard system calls

Key Idea:

A socket is an endpoint for communication

Indian Institute of Technology Kharagpur

Socket Communication Model

Components

• IP Address → identifies host
• Port Number → identifies application
• Protocol → TCP or UDP

Indian Institute of Technology Kharagpur

Socket Communication Model

Components

• IP Address → identifies host
• Port Number → identifies application
• Protocol → TCP or UDP

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Socket Address

(IP address, Port, Protocol)

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Indian Institute of Technology Kharagpur

Socket Communication Model

Components

• IP Address → identifies host
• Port Number → identifies application
• Protocol → TCP or UDP

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Socket Address

(IP address, Port, Protocol)

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Typical Flow

Process → Socket → Network → Socket → Process

Indian Institute of Technology Kharagpur

TCP vs UDP

Indian Institute of Technology Kharagpur

TCP vs UDP

When to Use UDP?

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• Real-time systems
• DNS
• Video/Audio streaming
• Sensor data, telemetry

Indian Institute of Technology Kharagpur

UDP (Datagram) Sockets

Key Characteristics

• No connection setup
• Each message is an independent datagram
• Application handles:
• Loss
• Duplication
• Ordering

Indian Institute of Technology Kharagpur

UDP (Datagram) Sockets

Key Characteristics

• No connection setup
• Each message is an independent datagram
• Application handles:
• Loss
• Duplication
• Ordering

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POSIX Socket Type�

SOCK_DGRAM

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Indian Institute of Technology Kharagpur

UDP Socket API (High-Level)

Core System Calls

• socket()
• bind()
• sendto()
• recvfrom()
• close()

Indian Institute of Technology Kharagpur

UDP Socket API (High-Level)

Core System Calls

• socket()
• bind()
• sendto()
• recvfrom()
• close()

Header Files

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#include <sys/socket.h>

#include <netinet/in.h>

#include <arpa/inet.h>

#include <unistd.h>

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Indian Institute of Technology Kharagpur

Creating a UDP Socket

int sockfd;

sockfd = socket(AF_INET, SOCK_DGRAM, 0);

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Parameters

• AF_INET → IPv4
• SOCK_DGRAM → UDP
• 0 → default protocol (UDP)

Indian Institute of Technology Kharagpur

Creating a UDP Socket

int sockfd;

sockfd = socket(AF_INET, SOCK_DGRAM, 0);

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Parameters

• AF_INET → IPv4
• SOCK_DGRAM → UDP
• 0 → default protocol (UDP)

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Error Handling

if (sockfd < 0) {

perror("socket failed"); }

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Indian Institute of Technology Kharagpur

Socket Address Structure

struct sockaddr_in server_addr;

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Fields:

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server_addr.sin_family = AF_INET;

server_addr.sin_port = htons(8080);

server_addr.sin_addr.s_addr = INADDR_ANY;

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Indian Institute of Technology Kharagpur

Socket Address Structure

struct sockaddr_in server_addr;

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Fields:

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server_addr.sin_family = AF_INET;

server_addr.sin_port = htons(8080);

server_addr.sin_addr.s_addr = INADDR_ANY;

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Why htons()?�

• Converts host byte order → network byte order

Indian Institute of Technology Kharagpur

UDP Server: Binding the Socket

Purpose

• Associate socket with a port

bind(sockfd,

(struct sockaddr *)&server_addr,

sizeof(server_addr));

• Error Check

if (bind(...) < 0) {

perror("bind failed");

}

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Indian Institute of Technology Kharagpur

Receiving Data (UDP Server)

char buffer[1024];

struct sockaddr_in client_addr;

socklen_t len = sizeof(client_addr);

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int n = recvfrom(sockfd,

buffer,

sizeof(buffer),

0,

(struct sockaddr *)&client_addr,

&len);

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Indian Institute of Technology Kharagpur

Sending Data (UDP Server)

sendto(sockfd,

"Hello Client",

12,

0,

(struct sockaddr *)&client_addr,

len);

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Note

• Server replies using client’s address from recvfrom()

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Indian Institute of Technology Kharagpur

Minimal UDP Server (ANSI C)

int sockfd;

char buffer[1024];

struct sockaddr_in server_addr, client_addr;

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sockfd = socket(AF_INET, SOCK_DGRAM, 0);

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server_addr.sin_family = AF_INET;

server_addr.sin_port = htons(8080);

server_addr.sin_addr.s_addr = INADDR_ANY;

​

Indian Institute of Technology Kharagpur

Minimal UDP Server (ANSI C)

bind(sockfd,

(struct sockaddr *)&server_addr,

sizeof(server_addr));

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socklen_t len = sizeof(client_addr);

recvfrom(sockfd, buffer, sizeof(buffer), 0,

(struct sockaddr *)&client_addr, &len);

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sendto(sockfd, "ACK", 3, 0,

(struct sockaddr *)&client_addr, len);

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close(sockfd);

Indian Institute of Technology Kharagpur

UDP Client Basics

Differences from Server

• No bind() required (OS assigns port)
• Directly uses sendto()

Indian Institute of Technology Kharagpur

UDP Client Code

int sockfd;

struct sockaddr_in server_addr;

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sockfd = socket(AF_INET, SOCK_DGRAM, 0);

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server_addr.sin_family = AF_INET;

server_addr.sin_port = htons(8080);

inet_pton(AF_INET, "127.0.0.1", &server_addr.sin_addr);

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sendto(sockfd, "Hello Server", 12, 0, (struct sockaddr *)&server_addr,

sizeof(server_addr));

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close(sockfd);

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Common Pitfalls

• Forgetting htons() / inet_pton()
• Assuming delivery is guaranteed
• Buffer overflows
• Not checking return values
• Firewall blocking UDP ports

Indian Institute of Technology Kharagpur