TCP Client Experiment

1. TCP Client Overview

TCP (Transmission Control Protocol) is a connection-oriented, reliable, byte-stream transport layer protocol. It establishes connections through three-way handshake, ensuring data arrives in order and intact.

TCP is the foundation of IoT communication. Whether it's HTTP web requests, MQTT message publishing, OTA firmware upgrades, or database remote connections, they all rely on TCP's reliable transmission at the underlying layer. In industrial automation, smart home, and remote monitoring scenarios, devices report sensor data and receive control commands, requiring "no data loss, no order disruption" - this is TCP's core value. UDP is fast but unreliable; TCP's three-way handshake overhead is worth it for the end-to-end acknowledgment and retransmission mechanism, which is irreplaceable in most IoT scenarios.

In this experiment, ESP32-S3 acts as a TCP client to actively connect to the server, sending one data message per second and receiving server responses. The WiFiClient class encapsulates lwIP's TCP socket, with connection, sending, receiving, and disconnection detection all completed through simple method calls.

Typical Application Scenarios:

2. Core Concepts

2.1 TCP Connection Lifecycle

A TCP client goes through 5 phases from connection to closure, each corresponding to a WiFiClient method:

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connect(ip, port)     establish connection
   ↓   underlying: three-way handshake SYN → SYN-ACK → ACK, blocking wait, returns true on success
write / print         send data
   ↓   full-duplex: both parties can send/receive simultaneously, write sends raw bytes, print sends formatted text
available() / read()  receive data
   ↓   available() returns number of readable bytes, non-blocking, returns 0 if no data
connected()           liveness check
   ↓   returns false if remote endpoint has closed (received FIN/RST) or network timeout
stop()                close connection
      underlying: four-way handshake FIN → ACK ← FIN → ACK, release socket resources

2.2 The "Packet Sticking" Problem of Stream Protocols

TCP is a stream (Stream) protocol, unlike UDP which preserves message boundaries. If the sender does two write("Hello") and write("World"), the receiver may receive "HelloWorld" in one read() (packet sticking), or may receive three fragments "Hel", "loWor", "ld" (fragmentation). The application layer must define message boundaries itself (such as newline characters, length prefixes, etc.).

2.3 write() vs print()

3. Hardware Dependencies

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

4. Project Architecture and Flow

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setup()
  ├─ connectWiFi()
  └─ client.connect(SERVER_IP, SERVER_PORT)  // three-way handshake

loop()
  ├─ if disconnected → stop() → mark for reconnect
  ├─ client.write(send_buf)                   // send raw bytes
  ├─ if client.available() → client.read()    // non-blocking read
  └─ delay(1000)

5. Source Code Analysis

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

5.1 Configuration and Global Variables

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#include <WiFi.h>
WiFiClient client;
const char* SERVER_IP = "192.168.11.183";
uint16_t    SERVER_PORT = 8080;

5.2 setup() — Establish TCP Connection

client.connect() initiates three-way handshake, returns true on success, returns false on failure (server not listening/firewall rejection).

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void setup() {
  Serial.begin(115200);
  connectWiFi();

  if (!client.connect(SERVER_IP, SERVER_PORT)) {
    Serial.println("Failed to connect to the server!");
    return;
  }
  Serial.println("Server Connected Successfully!");
}

5.3 loop() — Send and Receive

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void loop() {
  if (!client.connected()) {        // detect disconnection
    Serial.println("TCP Server disconnected!");
    client.stop();
    delay(2000);
    ESP.restart();
    return;
  }

  client.write("ESP32 TCP Client Data");  // send raw bytes

  if (client.available()) {               // non-blocking read
    char buf[128];
    int len = client.read(buf, sizeof(buf) - 1);
    buf[len] = '\0';
    Serial.printf("Receive: %s\n", buf);
  }
  delay(1000);
}

client.connected() returns false when: server FIN received, RST received, or multiple retransmissions with no ACK timeout. ESP.restart() is used for overall reset and reconnect after disconnection—this is the cleanest approach.

6. Operation Flow

6.1 Build and Flash

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

1. Modify server IP to actual address

2. First use network debugging assistant or TCP server program to listen on port 8080

3. Build and flash

6.2 Operation Steps

Step 1: Open network debugging assistant, configure TCP server

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

Click "Connect" button, network debugging assistant starts listening on TCP port 8080.

Step 2: Modify server IP in code, flash program

Open Arduino IDE, modify SERVER_IP in code to PC's actual LAN IP address, then compile and flash to ESP32-S3 development board.

Step 3: Open serial monitor, observe connection and send/receive

After flashing, open serial monitor (baud rate 115200), ESP32 will automatically connect to TCP server, after successful connection sends one data message per second and receives replies:

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11:30:16.823 -> Connecting to WiFi: Yahboom2 (Attempt 1)
11:30:17.877 -> ...
11:30:18.885 -> WiFi Connected successfully!
11:30:18.885 -> Device IP: 192.168.11.229
11:30:18.885 -> Target Server: 192.168.11.183:8080
11:30:18.930 -> Server Connected Successfully!
11:30:18.930 -> Send data: ESP32 TCP Client Data
11:30:19.942 -> Send data: ESP32 TCP Client Data
11:30:20.950 -> Send data: ESP32 TCP Client Data
...

At this point, enter any content in the send area of network debugging assistant and send, serial monitor will print data received by ESP32.

Data interaction:

Step 4: Test disconnection detection

Close TCP server of network debugging assistant (click "Disconnect"). After ESP32 detects disconnection, it prints TCP Server disconnected!, automatically restarts after 2 seconds and attempts to reconnect.

6.3 Verification Methods

7. Common Issues