User buffer size to receive multicast packets?

Question

The below code is from Git. It joins a multicast group and receives packets.

Here we loop and receive the data in a buffer called msgbuf:

while (1) 
{
    char msgbuf[MSGBUFSIZE];
    const int addrlen = sizeof(addr);
    const int nbytes = recvfrom(fd, msgbuf, MSGBUFSIZE, 0, (struct sockaddr *) &addr, &addrlen);

How do I choose the size for the buffer msgBuf? Does it just have to be the max packet size? Or do I need to store multiple packets whilst I process the first?

Full code:

int main(int argc, char *argv[])
{
    if (argc != 3) {
       printf("Command line args should be multicast group and port\n");
       printf("(e.g. for SSDP, `listener 239.255.255.250 1900`)\n");
       return 1;
    }

    char* group = argv[1]; // e.g. 239.255.255.250 for SSDP
    int port = atoi(argv[2]); // 0 if error, which is an invalid port

    if(port <= 0)
    {
        perror("Invalid port");
        return 1;
    }


    // create what looks like an ordinary UDP socket
    //
    int fd = socket(AF_INET, SOCK_DGRAM, 0);
    if (fd < 0) 
    {
        perror("socket");
        return 1;
    }

    // allow multiple sockets to use the same PORT number
    //
    u_int yes = 1;
    if (setsockopt(fd, SOL_SOCKET, SO_REUSEADDR, (char*) &yes, sizeof(yes)) < 0)
    {
       perror("Reusing ADDR failed");
       return 1;
    }

        // set up destination address
    //
    struct sockaddr_in addr;
    memset(&addr, 0, sizeof(addr));
    addr.sin_family = AF_INET;
    addr.sin_addr.s_addr = htonl(INADDR_ANY); // differs from sender
    addr.sin_port = htons(port);

    // bind to receive address
    //
    if (bind(fd, (struct sockaddr*) &addr, sizeof(addr)) < 0) 
    {
        perror("bind");
        return 1;
    }

    // use setsockopt() to request that the kernel join a multicast group
    //
    struct ip_mreq mreq;
    mreq.imr_multiaddr.s_addr = inet_addr(group);
    mreq.imr_interface.s_addr = htonl(INADDR_ANY);

    if (setsockopt(fd, IPPROTO_IP, IP_ADD_MEMBERSHIP, (char*) &mreq, sizeof(mreq)) < 0)
    {
        perror("setsockopt");
        return 1;
    }

    // now just enter a read-print loop
    //
    while (1) 
    {
        char msgbuf[MSGBUFSIZE];
        const int addrlen = sizeof(addr);
        const int nbytes = recvfrom(fd, msgbuf, MSGBUFSIZE, 0, (struct sockaddr *) &addr, &addrlen);

        if (nbytes < 0) 
        {
            perror("recvfrom");
            return 1;
        }
        msgbuf[nbytes] = '\0';
        puts(msgbuf);
     }

    return 0;
}

Answer 1

Unlike TCP which combines packets into a stream, UDP respects packet boundaries so recvfrom only gets one packet at a time.

So MSGBUFSIZE only needs to be as big as a single packet. If you're not using jumbo packets that would be 1500, otherwise it would be 9000.

Answer 2

as noted by @Ingo, in this code you should be using:

    char msgbuf[MSGBUFSIZE + 1];

the + 1 is because recvfrom can write upto MSGBUFSIZE bytes into the array, and you then write another NUL byte at the end.

as far as choosing a value for MSGBUFSIZE, that would depend on the protocol specification. given that most physical networks would struggle to send more than 1500 bytes without fragmentation something like 2048 might be a reasonable value. you could also check for nbytes == MSGBUFSIZE (maybe also using MSG_TRUNC) and report a "packet truncated" warning, but this basically wouldn't happen for packets routed over the public internet

in response to:

do I need to store multiple packets whilst I process the first?

you'd normally let the network stack take care of that. recv maintains packet/datagram boundaries and hence will always start writing the next packet at the supplied buffer address. again it depends on the protocol how you detect and handle errors, e.g. missing or out-of-order packets, and timeouts