Payload Type (PT): The assignment of an RTP payload type for this new
packet format is outside the scope of this document, and will not be
specified here. It is expected that the RTP profile for a particular
class of applications will assign a payload type for this encoding,
or if that is not done then a payload type in the dynamic range SHALL
be chosen by means of an out of band signaling protocol (e.g., H.245,
SIP, etc).
RFC3016里的原文,看来你自己随便定义一个好了,能收到数据包就行呗