丢包超时（UDP）

Question

I'm trying to create a timeout using select() for UDP socket transfer. I want to send an int from client to server , wait 300ms, and if I don't get an ACK, resend the packet. I'm not sure how to set this up properly with the timeout. From what I've gathered online and on the notes I have from class, select should be used on the receiving end.

the client at the server send back and forth the numbers 1-100. I have a separate router simulated code that randomly drops packets

Here is the code i have for the client side

int sent = 1;
int received = 1;

    for (int i = 0; i < 100; i++)
    {
        string sent1 = to_string(sent);
        char const *pchar = sent1.c_str();
        if(!sendto(s, pchar, sizeof(pchar), 0, (struct  sockaddr*) &sa_in, sizeof(sa_in)))
            cout << "send NOT successful\n";
        else
        {
            cout << "Client sent " << sent << endl;
            sent++;
        }
        // receive
        fd_set readfds; //fd_set is a type
        FD_ZERO(&readfds); //initialize 
        FD_SET(s, &readfds); //put the socket in the set

        if(!(outfds = select (1 , &readfds, NULL, NULL, & timeouts))) 
            break;
        if (outfds == 1) //receive frame
        {
            if (!recvfrom(s, buffer2, sizeof(buffer2), 0, (struct sockaddr*) &client, &client_length))
                cout << "receive NOT successful\n";
            else
            {
                received = atoi(buffer2);
                cout << "Client received " << received << endl;
                received++;
            }
        }
    }

The code is identical for the receiving side except it is in reverse: receive first, then send

My code doesn't utilize the timeout at all. This is basically what I want to do:

send packet(N)
    if (timeout)
        resend packet(N)
    else
        send packet(N+1)

Answer 1

If the receiver gets a timeout it needs to tell the sender, or else not tell the sender. In other words you have to implement either a NACK-based protocol or an ACK-based protocol.