TFTP C++未收到确认

#include <iostream>
#include <sys/socket.h>
#include <netinet/in.h>
#include <arpa/inet.h>
#include <unistd.h>
#include <cstring>

#define SERVER_IP "127.0.0.1"
#define SERVER_PORT 69
#define FILENAME "test.txt"

void send_rrq(int sockfd, const char* filename) {
    uint8_t buffer[4];
    buffer[0] = 0x00;
    buffer[1] = 0x01;
    strcpy(reinterpret_cast<char*>(buffer + 2), filename);
    buffer[2 + strlen(filename)] = '\0';
    buffer[3] = 0;

    sendto(sockfd, buffer, 4 + strlen(filename), 0, (struct sockaddr*)&server_addr, sizeof(server_addr));
}

void receive_data(int sockfd, const char* filename) {
    uint8_t buffer[516];
    struct sockaddr_in from_addr;
    socklen_t from_len = sizeof(from_addr);

    int recv_len = recvfrom(sockfd, buffer, sizeof(buffer), 0, (struct sockaddr*)&from_addr, &from_len);
    if (recv_len < 0) {
        perror("recvfrom");
        return;
    }

    FILE* fp = fopen(filename, "wb");
    if (!fp) {
        perror("fopen");
        return;
    }

    fwrite(buffer + 4, 1, recv_len - 4, fp);
    fclose(fp);
}

int main() {
    int sockfd = socket(AF_INET, SOCK_DGRAM, 0);
    if (sockfd < 0) {
        perror("socket");
        return -1;
    }

    memset(&server_addr, 0, sizeof(server_addr));
    server_addr.sin_family = AF_INET;
    server_addr.sin_port = htons(SERVER_PORT);
    inet_pton(AF_INET, SERVER_IP, &server_addr.sin_addr);

    send_rrq(sockfd, FILENAME);

    receive_data(sockfd, FILENAME);

    close(sockfd);
    return 0;
}