TCP vs UDP on video stream
I just came home from my exam in network-programming, and one of the question they asked us was "If you are going to stream video, would you use TCP or UDP? Give an explanation for both stored video and live video-streams". To this question they simply expected a short answer of TCP for stored video and UDP for live video, but I thought about this on my way home, and is it necessarily better to use UDP for streaming live video? I mean, if you have the bandwidth for it, and say you are streaming a soccer match, or concert for that matter, do you really need to use UDP?
Lets say that while you are streaming this concert or whatever using TCP you start losing packets (something bad happened in some network between you and the sender), and for a whole minute you don't get any packets. The video-stream will pause, and after the minute is gone packets start to get through again (IP found a new route for you). What would then happen is that TCP would retransmit the minute you lost and continue sending you the live stream. As an assumption the bandwidth is higher than the bit-rate on the stream, and the ping is not too high, so in a short amount of time, the one minute you lost will act as a buffer for the stream for you, that way, if packet-loss happens again, you won't notice.
Now, I can think of some appliances where this wouldn't be a good idea, like for instance video-conferences, where you need to always be at the end of the stream, because delay during a video-chat is just horrible, but during a soccer-match, or a concert what does it matter if you are a single minute behind the stream? Plus, you are guaranteed that you get all the data and it would be better to save for later viewing when it's coming in without any errors.
So this brings me to my question. Are there any drawbacks that I don't know of about using TCP for live-streaming? Or should it really be, that if you have the bandwidth for it you should go for TCP given that it is "nicer" to the network (flow-control)?
Answer
Drawbacks of using TCP for live video:
- Typically live video-streaming appliances are not designed with TCP streaming in mind. If you use TCP, the OS must buffer the unacknowledged segments for every client. This is undesirable, particularly in the case of live events; presumably your list of simultaneous clients is long due to the singularity of the event. Pre-recorded video-casts typically don't have as much of a problem with this because viewers stagger their replay activity; therefore TCP is more appropriate for replaying a video-on-demand.
- IP multicast significantly reduces video bandwidth requirements for large audiences; TCP prevents the use of IP multicast, but UDP is well-suited for IP multicast.
- Live video is normally a constant-bandwidth stream recorded off a camera; pre-recorded video streams come off a disk. The loss-backoff dynamics of TCP make it harder to serve live video when the source streams are at a constant bandwidth (as would happen for a live-event). If you buffer to disk off a camera, be sure you have enough buffer for unpredictable network events and variable TCP send/backoff rates. UDP gives you much more control for this application since UDP doesn't care about network transport layer drops.
FYI, please don't use the word "packages" when describing networks. Networks send "packets".