Is there a way to block on a socket send() until we get the ack for that packet?

Question

Or do I have to implement it at the application level?

Answer 1

I also faced the same problem few weeks ago when implementing a VoIP server. After spending several days I could come up with a solution. As many others mentioned, there is no any direct system call to do the job. Instead,

1. You can check if we have received the ACK after sending a packet with TCP_INFO option.
2. If we haven't received the ACK, wait for few milliseconds and check again.

This may continue until a time out reaches. You have to implement it as a wrapper function to send() call. You will need tcp_info struct from <netinet/tcp.h>. It is the data structure for holding information about your tcp connection.

Here is the pseudo code

int blockingSend(const char *msg, int msglen, int timeout) {

    std::lock_guard<std::mutex> lock(write_mutx);

    int sent = send(sock_fd, msg, msglen,  0); 

    tcp_info info;
    auto expireAt = chrono::system_clock::now() + chrono::milliseconds(timeout);
    do {
        this_thread::sleep_for(milliseconds(50));
        getsockopt(sock_fd,SOL_TCP, TCP_INFO, (void *) &info, sizeof(info));

      //wait till all packets acknowledged or time expires
    } while (info.tcpi_unacked > 0 && expireAt > system_clock::now());

    if(info.tcpi_unacked>0) {
        cerr << "no of unacked packets :" << info.tcpi_unacked << endl;
        return -1;
    }
    return sent;
}

Here tcpi_unacked member holds the number of packets unacknowledged of your connection. If you read it soon after the send() call, it will contain number of unacked packets which is equal to number of packets sent. With time, number of unacked packets will be decremented to zero. Therefore you need to periodically check the value of tcpi_unacked till it reaches zero. If the connection is half opened, you will never receive ACKs while causing a endless loop. For such scenarios you may need to add a timeout mechanism as implemented above.

Even though this question has been asked long ago, this answer may help some one who has faced the same problem. I must mention that there could be more accurate solutions to this problem than this solution. Since I am a newbie to system programming and C/C++ this is what I could come up with.

Answer 2

If you are talking about TCP, then no - no socket API I've seen allows you to do this.

You need to implement the ack in your application protocol if you need to be sure that the other end had received(and possibly processed) your data.