Now we come to the rtpmap lines for the formats included in the “m” lines above to map the payload types and the codecs. We also have a number of the fmtp lines to describe characteristics of the offered codecs. The order in which the formats are written in the media line is important as it defines the priority in which the browsers would like to use them.
The ptime (packetization time), refers to the media duration in ms to include in each RTP packet. This has important implications on the effects of packet loss and the amount overhead for the stream.