What is meant by real-time?
The class of methods whose correctness depends not only on whether the result is the correct one, but also on the time at which the result is delivered.
To make things simpler, lets take an example. Say you want to listen to a song. When you are downloading it from a site, you don't care whether it is downloaded at the same rate or not. You just need a reliable download (preferably fast -:)). But what if want to listen to the song without downloading it? Then you are not only interested to get the whole data but also the rate at which you receive, otherwise the song loses its charm. Here you need a real-time transmission.
Note that the example is given only to show how the time-factor is important in transmission of data. Real-time transmission is more important in multimedia conferences.
RTP gives No Guarantee for timely Delivery
Confused?? I bet you are!
Well, the point is that RTP itself does not provide any mechanism to ensure timely delivery or provide other quality-of-service guarantees. It relies on lower-layer services (e.g. UDP, TCP) to do so. The dependence will be clearer when we discuss the RTP packet structure.
So how come is it called a real-time protocol?
RTP provides suitable functionality for carrying real-time content, e.g., a timestamp and control mechanisms for synchronizing different streams with timing properties. We will discuss those in more detail soon.