Audio

m=audio 54278 RTP/SAVPF 111 103 104 0 8 106 105 13 126

m means it is a media line –  it condenses a lot of information on the media attributes of the stream. In this order, it tells us:

• audio - the media type is going to be used for the session (media types are registered at the IANA),
• 54278 – the port is going to be used for SRTP (and for RTCP if RTCP multiplex is supported by the other peer),
• RTP/SAVPF - the transport protocol is going to be used for the session, and last but not least
• 111 103 104 0 8 106 105 13 126 - the media format descriptions are supported by the browser to send and receive media.

RTP/SAVPF is defined in RFC5124. In short it requires the use of SRTP and SRTCP and RTCP Feedback packets.

The media format descriptions, with protocol RTP/SAVPF, gives the RTP payload numbers which are going to be used for the different formats.  Payload numbers lower than 96 are mapped to encoding formats by the IANA.  In our SDP 0 maps to G711U and 8 to G711A. Format numbers larger than 95 are dynamic and there are a=rtpmap: attribute to map from the RTP payload type numbers to media encoding names.  There are also a=fmtp:  attributes which specify format parameters