c＃UDP客户端服务器问题

Question

I have a udp client and a udp server. The udp client and server is really disconnected physically. So I can only send messages to the udp server but I can't get acknowledgements whether the data was receieved properly.

So what I am doing is to send a header of 4 bytes that will specify the amount of data I am about to send. And then I read the data according to the size I sent earlier.

// Sending Header(header is the size of the data)
byte[] header = BitConverter.GetBytes(data.Count());
socket.SendTo(header, 0, header.Count(), SocketFlags.None, outEP);
// Sending Data
socket.SendTo(data, 0, data.Count(), SocketFlags.None, outEP);

// Receiving input size
int receivedCount = socket.EndReceive(result);
// Header is filled on BeginReceive
Int32 count = BitConverter.ToInt32(header, 0);
if(count != receivedCount) throw;
// Then I receive the count relevent
socket.Receive(data, 0, count, SocketFlags.None);

A couple problems come up:

1. count != receivedCount

   This is why I did the check, but it sometimes happens and sometimes it doesn't..I do need a maximum guarantee. I found out that when the count is different its not because of fallen bytes or network issues. I just sometimes don't receive the header - only the data and when I read the 4 bytes I actually read the 4 bytes of the data thats why it fails.

   The data just sometimes Overrides the header. Why is that? and how can I solve this?

2. Sometimes I also get the following error on the socket.Receive

   A message sent on a datagram socket was larger than the internal message buffer or some other network limit, or the buffer used to receive a datagram into was smaller than the datagram itself

   The problem is that is sometimes happens and most of the times it doesn't... I can just receive it all in a max buffer..but then I would have memory problems.

   This should be in high performance and guaranteed delivery as much as possible.. This must be udp because the server side can't physically contact the client side It is one way - client to server.

Answer 1

UDP doesn't guarantee the packets arrive in the order you send them (or that they arrive at all). I doubt anything gets overridden / overwritten. You'll have to send the header and payload in one packet.

Have you considered using TCP? You may argue that UDP is faster, but if you are going to implement packet reordening yourself, you'll lose that benefit. (missed that it must be UDP).

If you look at TCP (network layer handles retransmissions) or SIP over UDP (software handles retransmissions), there are means to detect lost packets and retransmit them. You could for instance number your packets and have the receiver acknowledge receiving each one.

Answer 2

You will need a two way communication when using UDP and ensure that there are no lost, double or disordered packets.
Have you looked at protocols like PGM - Pragmatic Multicast?
Furthermore you might need a single thread just for cleaning the network stack quickly and post then the data into a queue or some other memory data structure. The network stack is a lot smaller than you can imagine and fuller faster than you ever thought.
Edit: The problem with UDP is that by definition, you might have lossed or duplicate packates or changes in the order, even if the receiver detects them, how should the receiver request them to be corrected when no communication is possible.