PMTUD By UDP

通过UDP探测MTU，并实现udp echo server

// Description: UDP echo server.
// g++ udp_echo_server.cc -o udp_echo_server
#include <iostream>
#include <cstring>
#include <arpa/inet.h>
#include <unistd.h>#define PORT                12345
#define BUFFER_SIZE         4096
#define IPV4_HEADER_SIZE    20
#define UDP_HEADER_SIZE     8int main() {int sockfd;char buffer[BUFFER_SIZE];struct sockaddr_in servaddr, cliaddr;socklen_t len;// 创建UDP套接字sockfd = socket(AF_INET, SOCK_DGRAM, 0);if (sockfd < 0) {std::cerr << "Socket creation failed" << std::endl;return 1;}// 设置不分片选项int dontfrag = IP_PMTUDISC_DO;if (setsockopt(sockfd, IPPROTO_IP, IP_MTU_DISCOVER, &dontfrag, sizeof(dontfrag)) < 0) {std::cerr << "Failed to set IP_MTU_DISCOVER option" << std::endl;close(sockfd);return 1;}// 清空地址结构memset(&servaddr, 0, sizeof(servaddr));memset(&cliaddr, 0, sizeof(cliaddr));// 填充服务器信息servaddr.sin_family = AF_INET;servaddr.sin_addr.s_addr = htonl(INADDR_ANY);servaddr.sin_port = htons(PORT);// 绑定套接字到指定端口if (bind(sockfd, (const struct sockaddr *)&servaddr, sizeof(servaddr)) < 0) {std::cerr << "Bind failed" << std::endl;close(sockfd);return 1;}std::cout << "UDP echo server started, listening on port " << PORT << "..." << std::endl;while (true) {len = sizeof(cliaddr);// 接收数据int n = recvfrom(sockfd, buffer, BUFFER_SIZE, 0, (struct sockaddr *)&cliaddr, &len);if (n > 0) {buffer[n] = '\0';char host_buffer[INET_ADDRSTRLEN];inet_ntop(AF_INET, &cliaddr.sin_addr, host_buffer, INET_ADDRSTRLEN);printf("RX from [%s:%u] -> %d B, mtu = %d\n", host_buffer, ntohs(cliaddr.sin_port), n, n + IPV4_HEADER_SIZE + UDP_HEADER_SIZE);// 发送回显数据sendto(sockfd, buffer, n, 0, (const struct sockaddr *)&cliaddr, len);} else {perror("recvfrom");}}close(sockfd);return 0;
}

// Description: UDP echo client with path MTU discovery.
// g++ udp_echo_client.cc -o udp_echo_client
#include <iostream>
#include <cstring>
#include <arpa/inet.h>
#include <netdb.h>
#include <unistd.h>#define SERVER_PORT 12345
#define BUFFER_SIZE 65536#define IPV4_HEADER_SIZE    20
#define UDP_HEADER_SIZE     8int discover_path_mtu(const struct sockaddr_in &servaddr)
{int sockfd;char probe_packet[BUFFER_SIZE];int32_t mtu_lbound, mtu_current, mtu_ubound, mtu_best;mtu_best = 68;mtu_lbound = 68;mtu_ubound = 65535;struct sockaddr_in remote_host;// 创建UDP套接字sockfd = socket(AF_INET, SOCK_DGRAM, 0);if (sockfd < 0) {std::cerr << "Socket creation failed" << std::endl;return -1;}// 设置不分片选项int dontfrag = IP_PMTUDISC_DO;if (setsockopt(sockfd, IPPROTO_IP, IP_MTU_DISCOVER, &dontfrag, sizeof(dontfrag)) < 0) {std::cerr << "Failed to set IP_MTU_DISCOVER option" << std::endl;close(sockfd);return -1;}struct timeval timeout;timeout.tv_sec = 0;timeout.tv_usec = 500000; // 500msif (setsockopt(sockfd, SOL_SOCKET, SO_RCVTIMEO, (char *)&timeout, sizeof(timeout)) != 0) {perror("Error in setsockopt(timeout/udp)");return -1;}while (mtu_lbound <= mtu_ubound) {mtu_current = (mtu_lbound + mtu_ubound) / 2;memset(probe_packet, 'a', mtu_current - IPV4_HEADER_SIZE - UDP_HEADER_SIZE); // 减去IPv4头（20字节）和UDP头（8字节）probe_packet[mtu_current - IPV4_HEADER_SIZE - UDP_HEADER_SIZE] = '\0';// 发送探测数据包int bytes = sendto(sockfd, probe_packet, mtu_current - IPV4_HEADER_SIZE - UDP_HEADER_SIZE, 0, (const struct sockaddr *)&servaddr, sizeof(servaddr));if (bytes < 0) {if (errno == EMSGSIZE) {printf("packet(%d) [%d, %d] too big for local interface\n", mtu_current, mtu_lbound, mtu_ubound);mtu_ubound = mtu_current - 1; // update rangecontinue;}perror("Error in sendto()");return -1;}char buffer[BUFFER_SIZE] = {0};socklen_t len = sizeof(remote_host);bytes = recvfrom(sockfd, buffer, BUFFER_SIZE, 0, (struct sockaddr *)&remote_host, &len);if (bytes < 0) {// timeoutif (errno == EAGAIN || errno == EWOULDBLOCK) {printf("timeout\n");mtu_ubound = mtu_current - 1;continue;}perror("Error in recvfrom()");return -1;} else if (bytes > 0) {printf("mtu = %d [%d, %d]\n", bytes + IPV4_HEADER_SIZE + UDP_HEADER_SIZE, mtu_lbound, mtu_ubound);if (strncmp(probe_packet, buffer, bytes) == 0) {mtu_lbound = mtu_current + 1;} else {mtu_ubound = mtu_current - 1;}if (mtu_current > mtu_best) {mtu_best = mtu_current;}} else {printf("recvfrom() returned 0\n");return -1;}}close(sockfd);std::cout << "The MTU of the path is: " << mtu_best << " B" << std::endl;return mtu_best;
}int main(int argc, char* argv[])
{int opt;const char *server_address = "127.0.0.1";int32_t port = SERVER_PORT;while ((opt = getopt(argc, argv, "s:p:")) != -1) {switch (opt) {case 's':server_address = optarg;break;case 'p':port = atoi(optarg);break;default: /* '?' */fprintf(stderr, "Usage: %s -s server_address -p port\n", argv[0]);return 0;}}// 解析服务器地址struct hostent* host = gethostbyname(server_address);if (host == nullptr) {std::cerr << "Failed to resolve hostname: " << server_address << std::endl;return 1;}int sockfd;char buffer[BUFFER_SIZE];struct sockaddr_in servaddr;// 创建UDP套接字sockfd = socket(AF_INET, SOCK_DGRAM, 0);if (sockfd < 0) {std::cerr << "Socket creation failed" << std::endl;return 1;}// 设置不分片选项int dontfrag = IP_PMTUDISC_DO;if (setsockopt(sockfd, IPPROTO_IP, IP_MTU_DISCOVER, &dontfrag, sizeof(dontfrag)) < 0) {std::cerr << "Failed to set IP_MTU_DISCOVER option" << std::endl;close(sockfd);return -1;}// 清空服务器地址结构memset(&servaddr, 0, sizeof(servaddr));// 填充服务器信息servaddr.sin_family = AF_INET;servaddr.sin_port = htons(port);memcpy(&servaddr.sin_addr, host->h_addr, host->h_length);// 探测路径MTUint mtu = discover_path_mtu(servaddr);if (mtu <= 0) {std::cerr << "Path MTU detection failed" << std::endl;}while (true) {std::cout << "Please enter the data to be sent:";std::string message;std::getline(std::cin, message);if (message.empty()) {break;}// 发送数据sendto(sockfd, message.c_str(), message.size(), 0, (const struct sockaddr *)&servaddr, sizeof(servaddr));// 接收回显数据socklen_t len = sizeof(servaddr);int n = recvfrom(sockfd, buffer, BUFFER_SIZE, 0, (struct sockaddr *)&servaddr, &len);buffer[n] = '\0';std::cout << "RX:" << buffer << std::endl;}close(sockfd);return 0;
}