The Real-time Transport Protocol (RTP) is a network protocol for delivering audio and video over IP networks. RTP is used extensively in communication and entertainment systems that involve streaming media, such as telephony, video teleconference applications, television services and web-based push-to-talk features.
RTP is used in conjunction with the RTP Control Protocol (RTCP). While RTP carries the media streams (e.g., audio and video), RTCP is used to monitor transmission statistics and quality of service (QoS) and aids synchronization of multiple streams. RTP is one of the technical foundations of Voice over IP and in this context is often used in conjunction with a signaling protocol such as the Session Initiation Protocol (SIP) which establishes connections across the network.
RTP is designed for end-to-end, real-time, transfer of streaming media. The protocol provides facilities for jitter compensation and detection of out of sequence arrival in data, which are common during transmissions on an IP network. RTP allows data transfer to multiple destinations through IP multicast. RTP is regarded as the primary standard for audio/video transport in IP networks and is used with an associated profile and payload format.
Real-time multimedia streaming applications require timely delivery of information and often can tolerate some packet loss to achieve this goal. For example, loss of a packet in audio application may result in loss of a fraction of a second of audio data, which can be made unnoticeable with suitable error concealment algorithms. The Transmission Control Protocol (TCP), although standardized for RTP use, is not normally used in RTP applications because TCP favors reliability over timeliness. Instead the majority of the RTP implementations are built on the User Datagram Protocol (UDP). Other transport protocols specifically designed for multimedia sessions are SCTP and DCCP, although, as of 2010, they are not in widespread use.
RTP was developed by the Audio/Video Transport working group of the IETF standards organization. RTP is used in conjunction with other protocols such as H.323 and RTSP. The RTP standard defines a pair of protocols: RTP and RTCP. RTP is used for transfer of multimedia data, and the RTCP is used to periodically send control information and QoS parameters.
The RTP specification describes two sub-protocols, RTP and RTCP.
The data transfer protocol, RTP, facilitates the transfer of real-time data. Information provided by this protocol include timestamps (for synchronization), sequence numbers (for packet loss and reordering detection) and the payload format which indicates the encoded format of the data.
The control protocol RTCP is used to specify quality of service (QoS) feedback and synchronization between the media streams. The bandwidth of RTCP traffic compared to RTP is small, typically around 5%.
RTP sessions are typically initiated between communicating peers using a signaling protocol, such as H.323, the Session Initiation Protocol (SIP), or Jingle (XMPP). These protocols may use the Session Description Protocol to negotiate the parameters for the sessions.
An RTP session is established for each multimedia stream. A session consists of an IP address with a pair of ports for RTP and RTCP. For example, audio and video streams use separate RTP sessions, enabling a receiver to deselect a particular stream. The ports which form a session are negotiated using other protocols such as RTSP (using SDP in the setup method) and SIP.
According to the specification, an RTP port should be even and the RTCP port is the next higher odd port number.:68[a] It is however also possible to use a single port for both RTP and RTCP. RTP and RTCP typically use unprivileged UDP ports (1024 to 65535), but may use other transport protocols (most notably, SCTP and DCCP) as well, as the protocol design is transport independent.
Profiles and payload formats
See also: RTP audio video profile
One of the design considerations of RTP was to carry a range of multimedia formats (such as H.264, MPEG-4, MJPEG, MPEG, etc.) and allow new formats to be added without revising the RTP standard. The design of RTP is based on the architectural principle known as application level framing (ALF). The information required by a specific application's needs is not included in the generic RTP header, but is instead provided through RTP profiles and payload formats. For each class of application (e.g., audio, video), RTP defines a profile and one or more associated payload formats. A complete specification of RTP for a particular application usage will require a profile and payload format specification(s).:71
The profile defines the codecs used to encode the payload data and their mapping to payload format codes in the Payload Type (PT) field of the RTP header (see below). Each profile is accompanied by several payload format specifications, each of which describes the transport of a particular encoded data. The audio payload formats include G.711, G.723, G.726, G.729, GSM, QCELP, MP3, and DTMF, and the video payload formats include H.261, H.263, H.264, and MPEG-4.
Examples of RTP Profiles include:
- The RTP profile for Audio and video conferences with minimal control (RFC 3551) defines a set of static payload type assignments, and a mechanism for mapping between a payload format, and a payload type identifier (in header) using Session Description Protocol (SDP).
- The Secure Real-time Transport Protocol (SRTP) (RFC 3711) defines a profile of RTP that provides cryptographic services for the transfer of payload data.
- The experimental Control Data Profile for RTP (RTP/CDP) for machine-to-machine communications.
RTP packet header
|96||CSRC identifiers ...|
|96+32×CC||Profile-specific extension header ID||Extension header length|
|128+32×CC||Extension header ...|
The RTP header has a minimum size of 12 bytes. After the header, optional header extensions may be present. This is followed by the RTP payload, the format of which is determined by the particular class of application. The fields in the header are as follows:
- Version: (2 bits) Indicates the version of the protocol. Current version is 2.
- P (Padding): (1 bit) Used to indicate if there are extra padding bytes at the end of the RTP packet. A padding might be used to fill up a block of certain size, for example as required by an encryption algorithm. The last byte of the padding contains the number of padding bytes that were added (including itself).:12
- X (Extension): (1 bit) Indicates presence of an Extension header between standard header and payload data. This is application or profile specific.
- CC (CSRC count): (4 bits) Contains the number of CSRC identifiers (defined below) that follow the fixed header.:12
- M (Marker): (1 bit) Used at the application level and defined by a profile. If it is set, it means that the current data has some special relevance for the application.:13
- PT (Payload type): (7 bits) Indicates the format of the payload and determines its interpretation by the application. This is specified by an RTP profile. For example, see RTP Profile for audio and video conferences with minimal control (RFC 3551).
- Sequence number: (16 bits) The sequence number is incremented by one for each RTP data packet sent and is to be used by the receiver to detect packet loss and to restore packet sequence. The RTP does not specify any action on packet loss; it is left to the application to take appropriate action. For example, video applications may play the last known frame in place of the missing frame. According to RFC 3550, the initial value of the sequence number should be random to make known-plaintext attacks on encryption more difficult.:13 RTP provides no guarantee of delivery, but the presence of sequence numbers makes it possible to detect missing packets.
- Timestamp: (32 bits) Used to enable the receiver to play back the received samples at appropriate intervals. When several media streams are present, the timestamps are independent in each stream, and may not be relied upon for media synchronization. The granularity of the timing is application specific. For example, an audio application that samples data once every 125 µs (8 kHz, a common sample rate in digital telephony) would use that value as its clock resolution. The clock granularity is one of the details that is specified in the RTP profile for an application.
- SSRC: (32 bits) Synchronization source identifier uniquely identifies the source of a stream. The synchronization sources within the same RTP session will be unique.:15
- CSRC: (32 bits each) Contributing source IDs enumerate contributing sources to a stream which has been generated from multiple sources.:15
- Header extension: (optional) The first 32-bit word contains a profile-specific identifier (16 bits) and a length specifier (16 bits) that indicates the length of the extension (EHL = extension header length) in 32-bit units, excluding the 32 bits of the extension header.:17
A functional network-based system includes other protocols and standards in conjunction with RTP. Protocols such as SIP, Jingle, RTSP, H.225 and H.245 are used for session initiation, control and termination. Other standards, such as H.264, MPEG and H.263, are used to encode the payload data as specified via RTP Profile.
An RTP sender captures the multimedia data, then encodes, frames and transmits it as RTP packets with appropriate timestamps and increasing sequence numbers. Depending on the RTP profile in use, the sender may set the Payload Type field. The RTP receiver captures the RTP packets, detects missing packets, and may reorder packets. It decodes the frames according to the payload format and presents the stream to its user.