In 1998, the Internet Engineering Task Force (IETF) published the specification for a Session Description Protocol or SDP as a format that describes parameters for streaming media. The original IETF Proposed Standard was updated in 2006 as RFC 4566. Although the SDP was created as a feature of Session Announcement Protocol (SAP), it can be used with Real-time Transport Protocol, Real-time Streaming Protocol (RTSP), and Session Initiation Protocol, as well as a standalone protocol. Parameter negotiation, session announcement and session invitation are included in the descriptive sessions from the SDP protocol. Rather than transmit data like other types of protocols, an SDP negotiates between media type endpoints, format and properties involved. A session begins when a connection is established, and the session is terminated only after every endpoint is no longer participating.
The parameters of a session are described through the protocol by using a field on a line in the form of <character>=<value>. In this form, <character> is a single character and the <value> is a structured text that depends on the attribute type. An SDP message uses three sections including describing the session, timing and descriptions of media, and these messages can contain many descriptions of timing and media. The protocol uses attributes in the form of property or value. A property form (a=<flag>) portrays a session's property, whereas a value form (a=<attribute>:<value>) gives a parameter's name. Parameters can also be the time, date and name of a session, as well as a session's purpose, the ports of all participating endpoints, data formats that will be used, and any bandwidth requirements in order to transfer data. Generally, SDP is used in wide-area networks (WANs), but local area networks (LANs) and metropolitan area networks (MANs) can often support the protocol.
SDP works with Session Initiation Protocol-based VoIP calls in order for endpoint participants to know the details (parameters) of the session. These include the session description, time description and media description.