Session Description Protocol

The Session Description Protocol (SDP) is a format for describing streaming media communications parameters. The IETF published the original specification as a Proposed Standard in April 1998, and subsequently published a revised specification as RFC 4566 in July 2006. The Session Description Protocol (SDP) is a format for describing streaming media communications parameters. The IETF published the original specification as a Proposed Standard in April 1998, and subsequently published a revised specification as RFC 4566 in July 2006. SDP is used for describing multimedia communication sessions for the purposes of session announcement, session invitation, and parameter negotiation. SDP does not deliver any media by itself but is used between endpoints for negotiation of media type, format, and all associated properties. The set of properties and parameters are often called a session profile. SDP is designed to be extensible to support new media types and formats. SDP started off as a component of the Session Announcement Protocol (SAP), but found other uses in conjunction with Real-time Transport Protocol (RTP), Real-time Streaming Protocol (RTSP), Session Initiation Protocol (SIP) and even as a standalone format for describing multicast sessions. A session is described by a series of fields, one per line. The form of each field is as follows. Where <character> is a single case-sensitive character and <value> is structured text whose format depends upon attribute type. Values are typically a UTF-8 encoding. Whitespace is not allowed immediately to either side of the =.:Section 5 Within an SDP message there are three main sections, detailing the session, timing, and media descriptions. Each message may contain multiple timing and media descriptions. Names are only unique within the associated syntactic construct, i.e. within the session, time, or media. Optional values are specified with =* and each field must appear in the order shown below. Below is a sample session description from RFC 4566. This session is originated by the user 'jdoe', at IPv4 address 10.47.16.5. Its name is 'SDP Seminar' and extended session information ('A Seminar on the session description protocol') is included along with a link for additional information and an email address to contact the responsible party, Jane Doe. This session is specified to last for two hours using NTP timestamps, with a connection address (which indicates the address clients must connect to or — when a multicast address is provided, as it is here — subscribe to) specified as IPv4 224.2.17.12 with a TTL of 127. Recipients of this session description are instructed to only receive media. Two media descriptions are provided, both using RTP Audio Video Profile. The first is an audio stream on port 49170 using RTP/AVP payload type 0 (defined by RFC 3551 as PCMU), and the second is a video stream on port 51372 using RTP/AVP payload type 99 (defined as 'dynamic'). Finally, an attribute is included which maps RTP/AVP payload type 99 to format h263-1998 with a 90kHz clock rate. RTCP ports for the audio and video streams of 49171 and 51373, respectively, are implied. The SDP specification does not incorporate any transport protocol; it is purely a format for session description. It is intended to use different transport protocols as necessary, including SAP, SIP, and RTSP. SDP could even be transmitted by email or as an HTTP payload.