计算机网络——GBN协议实现

实验目的

编程模拟实现GBN可靠传输软件

实验内容

C++ 程序模拟实现Go-Back-N可靠数据传输，需要编写一个发送端程序和一个测试端程序来模拟传输过程

具体流程

1. 编写发送端程序，调用库实现socket连接，然后主要实现滑动窗口，接收ACK确认帧和超时重传的功能
2. 编写接收端，调用监听一个端口，因为在本地实验，要模拟丢包的过程，使用随机数模拟出百分之十的丢包率。编写丢包前后不同的ACK确认帧逻辑

关键代码

发送端，先建立连接，然后接收用户在命令行的输入，获取要发送的消息和滑动窗口。然后开始按照GBN协议发送，发送完一帧后就滑动窗口往下。发送完一帧的同时还会开启一个计时器，计时器延时一秒钟，没有收到ACK就会从超时的位置开始重传

    asio::io_service io_service;
    tcp::socket socket(io_service);
    socket.connect(tcp::endpoint(asio::ip::address::from_string("127.0.0.1"), 12345));

    std::string message;
    std::cout << "Input message: ";
    std::getline(std::cin, message);

    int window_size;
    std::cout << "Window size: ";
    std::cin >> window_size;

    std::vector<char> window(window_size);
    int base = 0;
    int next_seq_num = 0;

    asio::steady_timer timer(io_service);
    timer.expires_from_now(std::chrono::seconds(1));

    while (base < message.size()) {
        while (next_seq_num < base + window_size && next_seq_num < message.size()) {
            window[next_seq_num % window_size] = message[next_seq_num];
            char seq_num_char = static_cast<char>(next_seq_num);
            asio::write(socket, asio::buffer(&seq_num_char, 1));
            asio::write(socket, asio::buffer(&window[next_seq_num % window_size], 1)); 
            std::cout << "Seq_num: " << next_seq_num << "  Send: " << window[next_seq_num % window_size] << std::endl;
            next_seq_num++;
        }

        char ack;
        asio::async_read(socket, asio::buffer(&ack, 1), [&](const asio::error_code& error, std::size_t length) {
            if (!error) {
                for (int i = 0; i < 25; i++) {
                    std::cout << " ";
                }
                std::cout << "Receive ACK: " << static_cast<int>(ack) << std::endl;
                base = ack + 1;

                if (base == next_seq_num) {
                    timer.cancel();
                } 
                else {
                    timer.expires_from_now(std::chrono::seconds(1));
                }
            } 
            else {
                std::cout << "Error reading ACK: " << error.message() << std::endl;
            }
        });

        timer.async_wait([&](const asio::error_code& error) {
            if (!error) {
                std::cout << "Timeout resend from: " << base << std::endl;
                for (int i = base; i < next_seq_num; i++) {
                    char seq_num_char = static_cast<char>(i);
                    asio::write(socket, asio::buffer(&seq_num_char, 1));
                    asio::write(socket, asio::buffer(&window[i % window_size], 1));
                }
            }
        });

        io_service.run();
        io_service.reset();
    }

接收端，先开启端口的监听，然后开始接收消息，中间模拟随机百分之十的丢包率，如果发生丢包只会发送重复最后收到的连续的帧。还有一段处理错误的代码，接收结束消息，安全退出

    asio::io_service io_service;
    tcp::acceptor acceptor(io_service, tcp::endpoint(tcp::v4(), 12345));

    tcp::socket socket(io_service);
    acceptor.accept(socket);

    srand(time(0));

    int expected_seq_num = -1;
    while (true) {
        char seq_num_char;
        char data;
        asio::error_code error;
        asio::read(socket, asio::buffer(&seq_num_char, 1), error);  // read sequence number
        asio::read(socket, asio::buffer(&data, 1), error);  // read data

        if (error) {
            if (error == asio::error::eof) {
                std::cout << "\nAll message have been received, connection closed" << std::endl;
            } 
            else {
                std::cout << "Error reading: " << error.message() << std::endl;
            }
            break;
        }

        int seq_num = static_cast<int>(seq_num_char);

        if (expected_seq_num == -1) {
            int random_num = rand() % 10;
            if (random_num == 0) {
                std::cout << "Loss Seq_num: " << seq_num<< std::endl;
                expected_seq_num = seq_num;
                continue;
            }
            std::cout << "Seq_num: " << seq_num << "  Receive: " << static_cast<char>(data) << std::endl;
        }
        else {
            std::cout << "Seq_num: " << seq_num << "  Receive: " << static_cast<char>(data) << std::endl;
            if (seq_num == expected_seq_num) {
                expected_seq_num = -1;
            }
            else {
                seq_num = expected_seq_num - 1;
            }
        }

        asio::write(socket, asio::buffer(&seq_num, 1));
        for (int i = 0; i < 25; i++) {
            std::cout << " ";
        }
        std::cout << "Send ACK: " << seq_num << std::endl;
    }