SDP - VoIP-Info

SDP is intended for describing multimedia sessions for the purposes of session announcement, session invitation, and other forms of multimedia session initiation. SDP is used from VoIP signalling protocols like SIP, H.323 and some minor VoIP protocols to transfer media setup information about a multi media client from A to B.

SDP is used by SAP – the Session Announcement Protocol.
SDP is used by SIP

IETF RFCs

RFC 4566: Session Description Protocol
obsolete RFC2327: Session Description Protocol
RFC 3388: Grouping of Media Lines in the Session Description Protocol (SDP)
IANA SDP parameters: https://www.iana.org/assignments/sdp-parameters/sdp-parameters.xhtml

Fields

Optional items are marked with a `*’.

Session description

v= (protocol version)
o= (owner/creator and session identifier).
s= (session name)
i=* (session information)
u=* (URI of description)
e=* (email address)
p=* (phone number)
c=* (connection information – not required if included in all media)
b=* (bandwidth information)
One or more time descriptions (see below)
z=* (time zone adjustments)
k=* (encryption key)
a=* (zero or more session attribute lines)
Zero or more media descriptions (see below)

Time description

t= (time the session is active)
r=* (zero or more repeat times)

Media description

m= (media name and transport address)
i=* (media title)
c=* (connection information – optional if included at session-level)
b=* (bandwidth information)
k=* (encryption key)
a=* (zero or more media attribute lines)

RTP Payload Type Numbers

RTP payload type numbers appear in the m= and rtpmap lines of the SDP, but these numbers are not defined in the SDP RFCs, there is more information here: RTP.