RTP refers to "Real-Time Transport Protocol". It is the protocol that is responsible for the encapsulation and transportation of real-time media data, such as audio and video, in the IMS network.

It provides features like sequence numbering, timestamping, and payload type identification to enable the receiver to properly reconstruct the real-time media stream.

RTP operates on top of the IP protocl in the network layer, which provides the necessary addressing and routing mechanisms for the real-time media data, to be transported across the IMS network.

And it works on the UDP protocol in the transport layer, which provides a lightweight communication, that is suitable for the delivery of time-sensitive multimedia data.