[Live-devel] Reliable UDP -- does it make sense?

[Pannaman, Jonathan]

I am sure Ross will comment from the live555 perspective but from a TV perspective, and TV is a very common use for this, UDP is used because this is a streaming application, if the media does not get there as expected i.e. in time to display it in sequence for video playback, it is no longer needed and therefore why bother retransmitting. Worse still is if the system re-transmits and that delays or interferes with the next packet getting to the display system in time now there is further interruption to the stream.

Since the re-transmitting is not needed, TCP is not needed (actually is sometimes considered detrimental). The bigger issue with UDP is actually that there is unpredictable as to when it sends packets hence the use of RTP which gives the server controls that helps regulate the flow of packets and the receiving device to know when the frame is/was meant to be presented/delivered.

This is actually the area I was looking to live555 to help with. I have found very little help on how to do this. In my application I get transport stream data in large blocks that I can easily break up into 7 x TS packets for sending on Ethernet but I have not found the method of regulating the flow particularly clear either from the MPEG side or the live555 side.

If you ever dig into this I would be appreciate any information you may come up with.





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From: live-devel-bounces at ns.live555.com [mailto:live-devel-bounces at ns.live555.com] On Behalf Of Sharda Murthi
Sent: Friday, October 23, 2009 10:22 AM
To: live-devel at ns.live555.com
Subject: [Live-devel] Reliable UDP -- does it make sense?

Hi All,

We are trying to implement a retransmission mechanism for the live555 server over UDP, as part of our research work for a professor.

Well, we hit our first road block thinking why would someone want to do it at all? We do understand that UDP doesnt provide for reliable transmission. And that's the purpose! If retransmission is provided, then the overhead increases, which UDP cant afford.
So, why not just use the existing TCP mechanism, given that the developers of live555 have already done it?

Can someone point out as to why one would want to be doing this at all? Probably we're not seeing the subtler side of the idea. We're just too confused about this whole thing.
And if someone has done this before, could you please guide us as to where we can start from?

Thank you!



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