Understanding Multicast Addresses and Implementation

Posted on Feb 2, 2025 in Legal Sciences of Public Administrations

Multicast Addressing Explained

For multicasting, a specific multicast address (Class D addresses, ranging from 224.0.0.0 to 239.255.255.255) is used to target a specific group of receivers.

Key Multicast Address Ranges

• 224.0.0.0 – 224.0.0.255: Local network control block.
• 224.0.1.0 – 238.255.255.255: Globally scoped multicast.
• 239.0.0.0 – 239.255.255.255: Administratively scoped, used for local/private multicast within organizations.

Multicast Sender Implementation

#define MULTICAST_GROUP "239.0.0.1"  // Multicast group address (Class D)
#define MULTICAST_PORT 12345         // Port to send the message
#define MESSAGE "Hello, Multicast!"

int main() {
    int sockfd;
    struct sockaddr_in multicast_addr;
    char *multicast_message = MESSAGE;
    int loopback = 1;  // Enable loopback for local system to receive the datagrams

    // 1. Create a UDP socket (AF_INET, SOCK_DGRAM)
    if ((sockfd = socket(AF_INET, SOCK_DGRAM, 0)) < 0) {
        perror("socket");
        return 1;
    }

    // 2. Set the multicast address
    memset(&multicast_addr, 0, sizeof(multicast_addr));
    multicast_addr.sin_family = AF_INET;
    multicast_addr.sin_addr.s_addr = inet_addr(MULTICAST_GROUP);
    multicast_addr.sin_port = htons(MULTICAST_PORT);

    // 3. Enable loopback (optional, for local testing)
    if (setsockopt(sockfd, IPPROTO_IP, IP_MULTICAST_LOOP, &loopback, sizeof(loopback)) < 0) {
        perror("setsockopt");
        close(sockfd);
        return 1;
    }

    // 4. Send the multicast message
    if (sendto(sockfd, multicast_message, strlen(multicast_message), 0, (struct sockaddr *)&multicast_addr, sizeof(multicast_addr)) < 0) {
        perror("sendto");
        close(sockfd);
        return 1;
    }

    printf("Message sent: %s\n", multicast_message);

    close(sockfd);
    return 0;
}

Multicast Receiver Implementation

#define MULTICAST_GROUP "239.0.0.1"  // Multicast group address (Class D)
#define MULTICAST_PORT 12345         // Port to listen on
#define BUFFER_SIZE 1024

int main() {
    int sockfd;
    struct sockaddr_in local_addr;
    struct ip_mreq multicast_request;
    char buffer[BUFFER_SIZE];
    int reuse = 1;

    // 1. Create a UDP socket (AF_INET, SOCK_DGRAM)
    if ((sockfd = socket(AF_INET, SOCK_DGRAM, 0)) < 0) {
        perror("socket");
        return 1;
    }

    // 2. Allow multiple sockets to bind to the same address
    if (setsockopt(sockfd, SOL_SOCKET, SO_REUSEADDR, &reuse, sizeof(reuse)) < 0) {
        perror("setsockopt");
        close(sockfd);
        return 1;
    }

    // 3. Bind to the multicast port
    memset(&local_addr, 0, sizeof(local_addr));
    local_addr.sin_family = AF_INET;
    local_addr.sin_addr.s_addr = INADDR_ANY; // Listen on all interfaces
    local_addr.sin_port = htons(MULTICAST_PORT);

    if (bind(sockfd, (struct sockaddr *)&local_addr, sizeof(local_addr)) < 0) {
        perror("bind");
        close(sockfd);
        return 1;
    }

    // 4. Join the multicast group
    multicast_request.imr_multiaddr.s_addr = inet_addr(MULTICAST_GROUP);
    multicast_request.imr_interface.s_addr = INADDR_ANY; // Use default interface

    if (setsockopt(sockfd, IPPROTO_IP, IP_ADD_MEMBERSHIP, &multicast_request, sizeof(multicast_request)) < 0) {
        perror("setsockopt");
        close(sockfd);
        return 1;
    }

    printf("Listening for multicast messages on %s:%d\n", MULTICAST_GROUP, MULTICAST_PORT);

    // 5. Receive and print messages
    while (1) {
        socklen_t addr_len = sizeof(local_addr);
        int bytes_received = recvfrom(sockfd, buffer, BUFFER_SIZE - 1, 0, (struct sockaddr *)&local_addr, &addr_len);
        if (bytes_received < 0) {
            perror("recvfrom");
            close(sockfd);
            return 1;
        }
        buffer[bytes_received] = '\0';
        printf("Received: %s\n", buffer);
    }

    close(sockfd);
    return 0;
}