TCP Connection Retransmition Problem

[Joe von Habsburg]

I am working on a project. in this project I am using tcp client.

When I send any message to the server, the server transmits messages to me in packets (total message length is always constant) . But most of the time there is a freeze. When I check via wireshark, I see that I get TCP Retransmission - TCP Dup ACK errors.

C++ code :

#include "client.h"

Client::Client(QObject *parent)
    : QObject{parent}
{
    _client = new QTcpSocket(this);

    connect(_client, &QTcpSocket::readyRead, this, &Client::readyRead);

}

Client::~Client()
{
    _client->abort();
    _client->close();
}

void Client::connectToHost(QString address, int port)
{
    if(_client->isOpen()){
        _client->disconnect();
    }

    REMOTE_ADDRESS = address;
    REMOTE_PORT = port;

    _client->connectToHost(REMOTE_ADDRESS, REMOTE_PORT);

    QTimer::singleShot(3000, this, &Client::sendHi);
}

void Client::disconnect()
{
    _client->close();
    _client->abort();
    _client->flush();
}

void Client::readyRead()
{
    if(_data.size() < _max_len){
        _data += _client->read(_max_len - _data.size());
    }

    if(_data.size() == _max_len){
        emit sendData(_data);
        _data.clear();
        QByteArray data = QString("hi").toUtf8();
        _client->write(data, data.size());
    }
    else if(_data.size() > _max_len){
        _data.clear();
        QByteArray data = QString("hi").toUtf8();
        _client->write(data);
    }
}

void Client::sendHi()
{
    if(!_client->isOpen()){
        qDebug() << "Could not connect";
        return;
    }

    QByteArray data = QString("hi").toUtf8();
    _client->write(data,data.size());
    qDebug() << "First 'hi' has been sent : " << QDateTime::currentDateTime();
}

also when I run the same process on a simple python code, I get the same error less than my qt c++ code.

Python code :

import socket

HOST = '192.168.1.10' 
PORT = 5001             

def receive_data(sock, length):
   
    data = b''
    while len(data) < length:
        more = sock.recv(length - len(data))
        if not more:
            raise EOFError('ERR')
        data += more
    return data

sock = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
sock.setsockopt(socket.IPPROTO_TCP, socket.TCP_NODELAY, 1)
sock.connect((HOST, PORT))
print(f'{HOST}:{PORT} connected')

num_samples = 4096

for _ in range(10000):
    sock.sendall(b'hi')
    data = receive_data(sock, num_samples*4*2)
    time.sleep(.001)

time.sleep(1)
sock.close()

why am i getting this error and how can i handle this error? what is your suggestion?