UDP Server Experiment

1. UDP Server Overview

UDP server is a "connectionless" service - simply bind to a port, and any client can send datagrams to that port. After the server receives data, it obtains the sender's address via remoteIP() and remotePort() for targeted replies. Compared to TCP servers, UDP doesn't require accept(), doesn't need to maintain connection state, and inherently supports multiple clients.

Typical Application Scenarios:

2. Core Concepts

2.1 UDP Server Processing Flow

bind(port) → Wait for datagram arrival → recvfrom() get data+sender address → Targeted reply

Key differences from TCP server:

2.2 remoteIP() / remotePort()

After parsePacket() receives a datagram, it internally records the sender's IP and port. Subsequent calls to remoteIP() and remotePort() retrieve this information for targeted replies. Unlike TCP's client.write(), UDP requires explicitly specifying beginPacket(remoteIP, remotePort) for each reply.

3. Hardware Dependencies

Pure software experiment. Need to connect to router via WIFI-STA first. Client and server must be on the same local network.

4. Project Architecture and Flow

​x
setup()
  ├─ connectWiFi()
  ├─ udp.begin(UDP_PORT)            // Bind listening port
  └─ Print server IP and port

loop()
  ├─ udp.parsePacket() > 0 ?        // Datagram arrived?
  │    ├─ udp.read(buf)             // Read payload
  │    ├─ Construct "Server Reply: xxx"
  │    └─ beginPacket(remoteIP(), remotePort()) → write → endPacket
  └─ delay(10)

5. Source Code Details

Complete source code: Source code → Arduino → 3.Network Communication → UDP_Server → UDP_Server.ino

5.1 Configuration and Global Variables

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#include <WiFiUDP.h>
WiFiUDP udp;
uint16_t UDP_PORT = 8080;

5.2 setup() — Start Server

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void setup() {
  Serial.begin(115200);
  connectWiFi();
  udp.begin(UDP_PORT);   // socket() → bind(8080)

  Serial.printf("UDP Server started | IP: %s Port: %d\n",
                WiFi.localIP().toString().c_str(), UDP_PORT);
}

5.3 loop() — Echo Processing

Reply to any received datagram with "Server Reply: ", targeted to the sender.

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void loop() {
  int packetSize = udp.parsePacket();
  if (packetSize > 0) {
    char buf[128];
    int len = udp.read(buf, sizeof(buf) - 1);
    buf[len] = '\0';
    Serial.printf("Received: %s\n", buf);

    char reply[160];
    sprintf(reply, "Server Reply: %s", buf);

    udp.beginPacket(udp.remoteIP(), udp.remotePort());  // Targeted reply
    udp.write(reply, strlen(reply));
    udp.endPacket();
  }
  delay(10);
}

remoteIP() and remotePort() are only valid after parsePacket() returns >0 - they correspond to the sender address of the last received datagram.

6. Operation Flow

6.1 Build and Flash

Arduino flashing process: See Arduino → 1. Before Development → 2. Build and Flash.

1. Build and flash

2. Use network debugging assistant or UDP client to send data to ServerIP:8080

6.2 Operation Steps

Step 1: Flash program, record server IP

Build and flash to ESP32-S3 development board. Open serial monitor (baud rate 115200), wait for WiFi connection success, and note down the server IP address printed in serial:

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UDP Server started | IP: 192.168.11.229 Port: 8080

Step 2: Open network debugging assistant, configure UDP client

Open network debugging assistant on PC, configure with following parameters:

Click "Connect" button.

Step 3: Send data, observe echo

Enter Hello from Client in the send area of network debugging assistant and click send. ESP32 will automatically reply with Server Reply: Hello from Client. Serial monitor and network debugging assistant will print respectively:

Network debugging assistant prints:

Serial monitor prints:

x
18:42:38.670 -> Connecting to WiFi: Yahboom2 (Attempt 1)
18:42:39.733 -> ...
18:42:40.734 -> WiFi Connected successfully!
18:42:40.734 -> IP: 192.168.11.181
18:42:40.734 -> ==================================
18:42:40.734 ->  UDP Server Startup successful!
18:42:40.734 ->  Server IP: 192.168.11.181
18:42:40.734 ->  Port: 8080
18:42:40.734 -> ==================================
18:48:30.356 -> UDP Receive: Hello Yahboom
18:48:30.356 -> UDP Send: Server Reply: Hello Yahboom
18:48:31.037 -> UDP Receive: Hello Yahboom
18:48:31.037 -> UDP Send: Server Reply: Hello Yahboom
18:48:31.988 -> UDP Receive: Hello Yahboom
18:48:31.988 -> UDP Send: Server Reply: Hello Yahboom
18:48:32.569 -> UDP Receive: Hello Yahboom
18:48:32.569 -> UDP Send: Server Reply: Hello Yahboom

Step 4: Verify multi-client support

On another device, also use UDP tool to send data to ESP32's 8080 port. Each client's datagrams will be processed independently and replied to, without interference.

6.3 Verification Methods

7. Common Issues