rfc3984

I have a confusion about the timestamp of h264 RTP packet. I know the wall clock rate of video is 90KHz which I defined in the SIP SDP. The frame rate of my encoder is not exactly 30 FPS, it is variable. It varies from 15 FPS to 30 FPS on the fly. So, I cannot use any fixed timestamp. Could any one tell me the timestamp of the following encoded packet. After 0 milisecond encoded RTP timestamp = 0 (Let the starting timestamp 0) After 50 milisecond encoded RTP timestamp = ? After 40 milisecond encoded RTP timestamp = ? After 33 milisecond encoded RTP timestamp = ? What is the formula when the