Real-time Transport Protocol (RTP) – a streaming media protocol, describing how audio and video packets should be formatted and transmitted over an Internet Protocol (IP) network. Typically it’s used in conjunction with the RTP Control Protocol (RTCP) protocol in a VoIP network.
RFC 1889 and RFC 1890 cover the Real-time Transport Protocol (RTP), which provides end-to-end delivery services for data with real-time characteristics, such as interactive audio and video. Services include payload type identification, sequence numbering, time stamping and delivery monitoring. The media gateways that digitize voice use the RTP to deliver the voice (bearer) traffic.
The RTP provides features for real-time applications, with the ability to reconstruct timing, loss detection, security, content delivery and identification of encoding schemes. For each participant, a particular pair of destination IP addresses defines the session between the two endpoints, which translates into a single RTP session for each phone call in progress. RTP is an application service built on user datagram protocol (UDP), so it is connectionless, with best-effort delivery. Although RTP is connectionless, it does have a sequencing system that allows for the detection of missing packets.
As part of its specification, the RTP Payload Type field includes the encoding scheme that the media gateway uses to digitize the voice content. This field identifies the RTP payload format and determines its interpretation by the CODEC in the media gateway. A profile specifies a default static mapping of payload type codes to payload formats. These mappings represent the International Telecommunication Union (ITU) G series of encoding schemes.
With the different types of encoding schemes and packet creation rates, RTP packets can vary in size and interval. Administrators must take RTP parameters into account when planning voice services. All the combined parameters of the RTP sessions dictate how much bandwidth is consumed by the voice bearer traffic. RTP traffic that carries voice traffic is the single greatest contributor to the VoIP network load.