SCTP客户端多端口连接同一服务端端口技术咨询

我用C语言写了一套SCTP客户端-服务端程序：客户端创建两个线程，分别将套接字绑定到本地端口8888和9999，再连接同一SCTP服务端端口。但服务端解析SCTP头获取客户端端口时，得到的是完全不同的端口，且两个连接显示的客户端端口相同。

想咨询三个问题：

1. 是否可以从SCTP客户端的不同端口连接同一服务端端口？
2. SCTP是否支持服务端多路复用？
3. SCTP是否支持客户端到服务端的多对一连接？

客户端代码 #

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>
#include <libgen.h>
#include <sys/socket.h>
#include <sys/types.h>
#include <netinet/in.h>
#include <netinet/sctp.h>
#include <arpa/inet.h>
#include <pthread.h>


static void die(const char *s) {
        perror(s);
        exit(1);
}

void* client_thread(void* arg) 
{
    int clientSocket;
    struct sockaddr_in clientAddress;
    struct sockaddr_in serverAddress;
    const char *msg = "Hello, Server!";
    int ret;
    int clientPort = *((int*)arg);


    // Create the client socket
    clientSocket = socket(AF_INET, SOCK_STREAM, IPPROTO_SCTP);
    if (clientSocket == -1) {
        perror("Failed to create socket");
        exit(EXIT_FAILURE);
    }

    // Set up the client address and port
    clientAddress.sin_family = AF_INET;
    clientAddress.sin_port = htons(clientPort);  // Specify the source port number here
    clientAddress.sin_addr.s_addr = INADDR_ANY;

    // Bind the client socket to the local address and port
    if (bind(clientSocket, (struct sockaddr *)&clientAddress, sizeof(clientAddress)) < 0) {
        perror("Failed to bind socket");
        exit(EXIT_FAILURE);
    }

    // Set up the server address and port
    serverAddress.sin_family = AF_INET;
    serverAddress.sin_port = htons(62325);  // Specify the destination port number here
    serverAddress.sin_addr.s_addr = inet_addr("127.0.0.1");  // Specify the server IP address here

    // Connect to the server
    if (connect(clientSocket, (struct sockaddr *)&serverAddress, sizeof(serverAddress)) < 0) {
        perror("Failed to connect to server");
        exit(EXIT_FAILURE);
    }
        
    char buffer[45];

    int length = snprintf(buffer, sizeof(buffer), "Hello, Server! The value of client port %d", clientPort);
    ret = sctp_sendmsg(clientSocket, (void *) buffer, length + 1, NULL, 0, 0, 0, 0, 0, 0 );
    if (ret < 0)
            die("sctp_sendmsg");

    close(clientSocket);
    return NULL;
}


int main(int argc, char **argv) 
{
    pthread_t thread1, thread2;
    int client_port1 = 8888;
    int client_port2 = 9999;

    // Create thread 1
    if (pthread_create(&thread1, NULL, client_thread, &client_port1) != 0) {
        perror("Failed to create thread 1");
        exit(EXIT_FAILURE);
    }

    // Create thread 2
    if (pthread_create(&thread2, NULL, client_thread, &client_port2) != 0) {
        perror("Failed to create thread 2");
        exit(EXIT_FAILURE);
    }

    // Wait for threads to finish
    if (pthread_join(thread1, NULL) != 0) {
        perror("Failed to join thread 1");
        exit(EXIT_FAILURE);
    }
    if (pthread_join(thread2, NULL) != 0) {
        perror("Failed to join thread 2");
        exit(EXIT_FAILURE);
    }

    return 0;
}

服务端代码 #

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>
#include <libgen.h>
#include <sys/socket.h>
#include <sys/types.h>
#include <netinet/in.h>
#include <netinet/sctp.h>
#include <arpa/inet.h>

#define MY_PORT_NUM 62325

static void die(const char *s) {
        perror(s);
        exit(1);
}

static void server(void) {
        int listen_fd, conn_fd, flags, ret, in;        
        struct sctp_rcvinfo rcvinfo;
        struct sockaddr_in servaddr = {
                .sin_family = AF_INET,
                .sin_addr.s_addr = htonl(INADDR_ANY),
                .sin_port = htons(MY_PORT_NUM),
        };
        struct sctp_initmsg initmsg = {
                .sinit_num_ostreams = 5,
                .sinit_max_instreams = 5,
                .sinit_max_attempts = 4,
        };

        listen_fd = socket(AF_INET, SOCK_STREAM, IPPROTO_SCTP);
        if (listen_fd < 0)
                die("socket");

        ret = bind(listen_fd, (struct sockaddr *) &servaddr, sizeof(servaddr));
        if (ret < 0)
                die("bind");

        ret = setsockopt(listen_fd, IPPROTO_SCTP, SCTP_INITMSG, &initmsg, sizeof(initmsg));
        if (ret < 0)
                die("setsockopt");

        ret = listen(listen_fd, initmsg.sinit_max_instreams);
        if (ret < 0)
                die("listen");

        for (;;) {
                char buffer[1024];

                printf("Waiting for connection\n");
                fflush(stdout);

                conn_fd = accept(listen_fd, (struct sockaddr *) NULL, NULL);
                if(conn_fd < 0)
                        die("accept()");

                printf("New client connected\n");
                fflush(stdout);
               
                in = sctp_recvmsg(conn_fd, buffer, sizeof(buffer), NULL, 0, &rcvinfo, &flags);
                if (in > 0) {
                        printf("Received data: %s\n", buffer);
                        // Extract the source port from the SCTP header
                        uint16_t sourcePort = ntohs(rcvinfo.rcv_sid);
                        printf("Source port: %u\n", sourcePort);                        
                        fflush(stdout);
                }

                close(conn_fd);
        }
}


int main(int argc, char **argv) 
{
   server();
   return 0;
}

运行输出 #

New client connected
Received data: Hello, Server! The value of client port 9999
Source port: 61608         ------------------------------------> using 9999 client port but shows 61608
Waiting for connection
New client connected
Received data: Hello, Server! The value of client port 8888
Source port: 61608         ------------------------------------> using 8888 client port but shows 61608
Waiting for connection

问题解答 #

1. 是否可以从SCTP客户端的不同端口连接同一服务端端口？ #

可以。和TCP逻辑一致，只要本地端口未被占用，客户端就能绑定到指定端口后发起连接，多个不同本地端口的套接字可以同时连接同一个服务端端口。你的客户端绑定逻辑是正确的，问题出在服务端获取端口的方式错误。

2. SCTP是否支持服务端多路复用？ #

支持。SCTP本身就是面向连接的多路复用协议，服务端通过单个监听套接字，就能同时处理多个客户端的关联（Association），你的代码中用accept逐个处理新连接的方式，就是多路复用的典型实现。

3. SCTP是否支持客户端到服务端的多对一连接？ #

支持，包含两种常见场景：

• 单个客户端进程通过多个本地端口（多个套接字）连接同一个服务端端口，就是你代码中的场景，完全可行；
• 单个SCTP关联内的多流传输，这是SCTP的核心特性之一，一个连接内可以同时传输多个独立的数据流，无需建立多个关联。

代码错误修复 #

你服务端获取客户端端口的逻辑完全错误：

uint16_t sourcePort = ntohs(rcvinfo.rcv_sid);

rcv_sid是SCTP流ID，和客户端源端口毫无关系！要正确获取客户端端口，有两种可靠方式：

方式一：在accept时保存客户端地址

修改服务端的accept调用，捕获客户端的地址信息：

struct sockaddr_in client_addr;
socklen_t client_len = sizeof(client_addr);
conn_fd = accept(listen_fd, (struct sockaddr *)&client_addr, &client_len);
// 解析客户端端口
uint16_t sourcePort = ntohs(client_addr.sin_port);

方式二：用sctp_getpaddrs获取远端地址

在sctp_recvmsg后，通过SCTP提供的接口获取远端地址：

struct sockaddr *addr;
int addrcnt = sctp_getpaddrs(conn_fd, &rcvinfo, &addr);
if (addrcnt > 0) {
    struct sockaddr_in *remote_addr = (struct sockaddr_in *)addr;
    uint16_t sourcePort = ntohs(remote_addr->sin_port);
    printf("Source port: %u\n", sourcePort);
    sctp_freepaddrs(addr); // 必须释放内存
}

采用任意一种方式修改后，服务端就能正确获取到客户端绑定的8888和9999端口。

内容的提问来源于stack exchange，提问作者myquest9 sh