Side question: UDP seems to be the preferred protocol for VoIP-type of services. While the size of a UDP message is generally much larger than that of TCP, and some missed UDP messages are acceptable (analogous to cellular communication some signal weakness/loss is acceptable), any other strong reasons people prefer UDP over TCP in VoIP?
What happens when packets are dropped in UDP? A bit of audio corruption.
What happens when packets are dropped in TCP? A hard-to-predict amount of delay. And retransmission of audio data that you might just throw out anyway. (There are alternatives to discarding the data that should have already been heard, like temporal compression, but that's hard to do right, too.)
With TCP, when you drop a packet, you wait for it to requeue, and the application gets no data. You can see the effects in some poorly implemented video conferencing systems. You'll see someone speaking, then it'll pause for a few seconds, then the system will play the recovered video really fast to catch up. It's quite annoying.
Instead, for realtime applications, you make sure each packet is usable by itself, and make your system gracefully degrade in face of packet loss.
(Or, if you're from the insane world that designed Fax-over-IP, you transmit 2 or 3 packets for each packet, and hope that staves off packet loss.)
UDP is used in a lot of environments where a few dropped packets aren't an issue, because the package guarantees in TCP slow things down, and TCP is generally a heavier protocol so it's slower... because... ?
I think that I actually don't know any more than you about it.
* All packets arrive. (Reliable transport).
* All packets that arrive are in order. (Sequential transport).
Unfortunately, both of these aren't so necessary for real time audio (and somewhat less so, video). In fact, they get in the way.
Missing packets can often be replaced in VoIP settings with various kinds of interpolation. And out of order data can be buffered and used to build the interpolated data before playback. Interestingly enough, even packets that have been damaged still often are usable to VoIP applications, since they still contain some payload that can be used to improve the signal.
