Handle packet loss UDP socket programming in C

Question

I want to send a byte array X times until I've reached the buffer size. My client send the byte array until the buffer size is full. But the problem is packet loss and the server only recieves like 8/10 of the buffer size.

Client:

while(writeSize < bufferSize)
{
    bytes_sent += sendto(servSocket, package, sizeof(package), 0, (struct sockaddr *)&server, sizeof(server));
    writeSize+= sizeof(package);    
}

Server:

while(packetSize < bufferSize)
{
    bytes_recv += recvfrom(servSocket, package, sizeof(package), 0, (struct sockaddr*)&client, &size);
    packetSize += sizeof(package);
    printf("Bytes recieved: %d\n", bytes_recv);
}

I can't really come up with a solution with this problem. When the client has sent all the packages, and the server suffers from package loss the while loop won't end, any ideas how I can solve this?

Many thanks

Answer 1

Hmpf. You are using UDP. For this protocol its perfectly ok to throw away packets if there is need to do so. So it's not reliable in terms of "what is sent will arrive". What you have to do in your client is to check wether all packets you need arrived, and if not, talk politely to your server to resend those packets you did not receive. To implement this stuff is not that easy, and you might finally end up with a solution wich might be less performant than just using plain old tcp-sockets, so my recomendation would be in the first instance to switch to a tcp connection.

Answer 2

If you have to use UDP to transfer a largish chunk of data, then design a small application-level protocol that would handle possible packet loss and re-ordering (that's part of what TCP does for you). I would go with something like this:

• Datagrams less then MTU (plus IP and UDP headers) in size (say 1024 bytes) to avoid IP fragmentation.
• Fixed-length header for each datagram that includes data length and a sequence number, so you can stitch data back together, and detect missed, duplicate, and re-ordered parts.
• Acknowledgements from the receiving side of what has been successfully received and put together.
• Timeout and retransmission on the sending side when these acks don't come within appropriate time.

Here you go. We started building TCP again ...