How to buffer entire file while streaming

Hi,

I have a service running off a FMS and I was wondering how I would get a stream to keep downloading the rest of the file even while streaming. For example, like YouTube downloads the entire video while you're watching it, much like Progressive Download does but I want that from my streaming server. How would I go about doing this?

Or even better than that, if I could buffer at least 20 seconds ahead of the current stream position. If I use NetStream.setBufferTime(20) - the stream doesn't play until at least 20 seconds has been buffered, but I want it so that it will play even though it hasn't buffered completely, but will keep buffering while streaming.

Thanks