计算机网络——GBN协议实现

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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 numberasio::read(socket, asio::buffer(&data, 1), error);  // read dataif (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;}