This is usually done using an MPEG transport stream (MTS). When MTS is used, there are a few encapsulation layers between the MPEG2 bitstream and the RTP packets that are transporting it. The bitstream generated by an MPEG2 encoder is called the elementary stream (ES). Its format is specified by the ITU H.262 standard. The ES is first encapsulated in a packetized elementary stream (PES). Next, the PES is encapsulated in an MTS. The format for both the PES and the MTS are specified in the ITU H.222.0 standard.
The MTS is divided into 188-byte packets. PES packets may be larger than this, so one PES packet may be divided between multiple MTS packets. MTS packets can be encapsulated into RTP packets. The format for this is specified in RFC 2250. RTP packet size can vary depending on the application, but the RTP payload size is usually larger than the 188-byte MTS packet size. RFC 2250 specifies that an integer number of MTS packets should be encapsulated into one RTP packet.
When SDP is used, the name “MP2T/90000” specifies an MPEG2 transport stream. It is usually given the RTP payload type number 33.