multipart-form-stream | transform files and parameters into a multipart/form-data | Form library

I know, this topic's title is pretty general and I thought this problem was easy to solve (I made it on Android very easily). But on iOS... Well, this question exists so....

I have a device and I want to stream data (audio data to be precise) on it. The protocol I have to use is HTTP. Actually, I have to build ans send a single HTTP request containing some data in its body, then, as and when my "producer" (here, a process that resample/encode audio data) gives new data to transmit, send it through this request body. It sounds weird for HTTP, but it's like HTTP multipart without boundary (data are just sent one behind the others).

For example, in Java I initialize a HttpUrlConnection with URL and headers, I precise the Content-Length header to "9999999" (as it is wrote in the targeted device's documentation) with /*my connection*/.setFixedLengthStreamingMode(9999999);. I don't call connect()method but I get the output stream (getOutputStream()) and when my producer gives new data, I just call /*my output stream*/.write(/*my data*/); then /*my output stream*/.flush();

So, I tried with NSURLSession and with low-level APIs NSInputStreamand NSOutputStream. But in each case, I can't achieve that:

• With NSURLSession I called uploadTaskWithStreamedRequest: but the callback needNewBodyStream: is only called once (or twice in case of new auth). So I only send one data packet in the input stream.

• With NSInputStreamand NSOutputStream connection is close by targeted device without error before I can send any data with standard TCP ending on input stream (except HTTP headers).

I think this is something a lot of people did and they is a way I didn't see, no? Please help me by give me examples (this'll be perfect) or avises about what API I have to choose or a specific parameter to apply?

Thanks!

EDIT

My problem is similar to this but it's an other (shorter) way to explain it.

EDIT 2

Here is my code when I use NSURLSession (I prefer keep using that if possible):