networking - TCP vs UDP on video stream

Question

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 1

Drawbacks of using TCP for live video:

1. As you mentioned, TCP buffers the unacknowledged segments for every client. In some cases this is undesirable, such as TCP streaming for very popular live events: your list of simultaneous clients (and buffering requirements) are large in this case. Pre-recorded video-casts typically don't have as much of a problem with this because viewers tend to stagger their replay activity.

2. TCP's delivery guarantees are a blocking function which isn't helpful in interactive conversations. Assume your network connection drops for 15 seconds. When we miss part of a conversation, we naturally ask the person to repeat (or the other party will proactively repeat if it seems like you missed something). UDP doesn't care if you missed part of a conversation for the last 15 seconds; it keeps working as if nothing happened. On the other hand, the app could be designed for TCP to replay the last 15 seconds (and the person on the other end may not want or know about that). Such a replay by TCP aggravates the problem, and makes it more difficult to stay in sync with other parties in the conversation. Comparing TCP and UDP’s behavior in the face of packet loss, one could say that it’s easier for UDP to stay in sync with the state of an interactive conversation.

3. IP multicast significantly reduces video bandwidth requirements for large audiences; multicast requires UDP (and is incompatible with TCP). Note - multicast is generally restricted to private networks. Please note that multicast over the internet is not common. I would also point out that operating multicast networks is more complicated than operating typical unicast networks.

FYI, please don't use the word "packages" when describing networks. Networks send "packets".