network protocols handbook.pdf

144Protocols GuideVoice Over IP(VOIP) - Media / CODECProtocol NameRTP: Real-Time TransportProtocolProtocol DescriptionThe Real-time Transport Protocol (RTP) provides end-to-enddelivery services for data with real-time characteristics, such asinteractive audio and video or simulation data, over multicast orunicast network services. Applications typically run RTP on topof UDP to make use of its multiplexing and checksum services;both protocols contribute parts of the transport protocol functionality.However, RTP may be used with other suitable underlyingnetwork or transport protocols. RTP supports data transfer tomultiple destinations using multicast distribution if provided bythe underlying network.RTP itself does not provide any mechanism to ensure timelydelivery or provide other quality-of-service guarantees but relieson lower-layer services to do so. It does not guarantee deliveryor prevent out-of-order delivery, nor does it assume that theunderlying network is reliable. It delivers packets in sequence.The sequence numbers included in RTP allow the receiver to reconstructthe sender’s packet sequence but sequence numbersmight also be used to determine the proper location of a packet,for example in video decoding, without necessarily decodingpackets in sequence.RTP consists of two closely-linked parts:The real-time transport protocol (RTP), to carry data that hasreal-time properties.The RTP control protocol (RTCP), to monitor the quality of serviceand to convey information about the participants in an on-goingsession. The latter aspect of RTCP may be sufficient for “looselycontrolled” sessions, i.e., where there is no explicit membershipcontrol and set-up, but it is not necessarily intended to supportall of an application’s control communication requirements.Protocol Structure• CSRC count -Contains the number of CSRC identifiersthat follow the fixed header.• M - Marker. The interpretation of the marker is definedby a profile. It is intended to allow significantevents such as frame boundaries to be marked in thepacket stream.• Payload type - Identifies the format of the RTP payloadand determines its interpretation by the application.A profile specifies a default static mapping ofpayload type codes to payload formats. Additionalpayload type codes may be defined dynamicallythrough non-RTP means.• Sequence number - Increments by one for each RTPdata packet sent, and may be used by the receiver todetect packet loss and to restore packet sequence.• Timestamp - Reflects the sampling instant of the firstoctet in the RTP data packet. The sampling instantmust be derived from a clock that increments monotonicallyand linearly in time to allow synchronizationand jitter calculations.• SSRC - Synchronization source. This identifier ischosen randomly, with the intent that no two synchronizationsources within the same RTP session willhave the same SSRC identifier.• CSRC - Contributing source identifiers list. Identifiesthe contributing sources for the payload contained inthis packet.Related protocolsRTCP, RTSP, UDP, TCP, IPSponsor SourceRTP is defined by IETF (www.ietf.org ) in RFC 3550 and 3551.Referencehttp://www.javvin.com/protocol/rfc3550.pdfRTP: A Transport Protocol for Real-Time Applicationshttp://www.javvin.com/protocol/rfc3551.pdfRTP Profile for Audio and Video Conferences with Minimal Control2 3 4 8 9 16bitV P X CSRC count M Payload typeSequence numberTimestampSSRCCSRC (variable 0 – 15 items32bits each)• V - Version. Identifies the RTP version.• P - Padding. When set, the packet contains one ormore additional padding octets at the end which arenot part of the payload.• X - Extension bit. When set, the fixed header is followedby exactly one header extension, with a definedformat.