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1、RTP/RTCP Background; Overview; Basic concepts; RTP RTCP Applications of RTCP; Applications of RTP;Background December 1992, Henning Schulzrinne, GMD Berlin, published RTP version 1 Proposed (version 2) as standard November,1995 Widely used in multimedia communications; Background (contd)ALF Eliminat
2、ion of the improper packetization scheme and corresponding transmission controls at lower layers; Generalization of ALF: look for elegant ways to allow application visibility/participation in lower-level activities, e.g. QoS carry application intelligence to the point of QoS enforcement;Background (
3、contd)ALF ALF ALF ILP ILPThe typical multimedia transport protocol that adheres to the above requirements RTPReal-time Transport Protocol (RTP) RFC1889 RTP provides end-to-end delivery services for real-time data interactive audio, video source and payload type identification sequence numbering (res
4、eq, loss detection) time-stamping (for intra and inter-media sync) delivery monitoring and feedback Runs over UDP (multiplexing, checksum) Designed primarily for multicast of real-time data (also unicast, simplex, duplex)RTP Does not provide: quality of service guarantees synchronization among media
5、 streams Lower layers or application can provide such guarantees sequence numbers and timestamps can be used by application to provide synchronizationSession control RTP does NOT provide session control access control floor control etc. need different protocols for thoseComponents RTP; RTCP; Profile
6、 - application-oriented configuration; Format - media-oriented configuration;RTP components RTP: The real-time transport protocol that carries data with real-time properties Packet format; Functional entity; RTCP: RTP control protocol: monitors QoS, conveys info about participants “loosely controlle
7、d” sessions, e.g., no membership control more tightly controlled sessions require separate session control protocol uses one port higher than RTPRTP Architecture Very thin transport layer to avoid restricting the application designer RTP may often be integrated into the application rather than a sep
8、arate layer Definitions of several header fields deferred to A/V profile - e.g., marker bitProfile specification “Defines payload type codes and their mapping to payload formats (e.g., media encodings). A profile may also define extensions or modifications to RTP that are specific to to a particular
9、 class of applications. Typically, an application will operate under only one profile.”Payload specification which defines how a particular kind of payload data, such as H.261 encoded video, should be carried in RTP. These documents are typically titled “RTP payload Format for XYZ Audio/Video Encodi
10、ng“. Payload formats may be useful under multiple profiles and may therefore be defined independently of any particular profile. The profile documents are then responsible for assigning a default mapping of that format to a payload type value if needed.RTP protocol architectureRTPA/V profileH.261 vi
11、deoMPEG videoLayered videoPCM audioLPC audioLDCT videoPVH videoHVQ video.Interdependent specifications arranged in a class hierarchy to support the Application-Level framing architectureCore RTP specificationPayload formatFormat decompositionBasic conceptsRTP sessionThe association among a set of pa
12、rticipants communicating with RTP. For each participant, the session is defined by a particular pair of destination transport addresses (one network address plus a port pair for RTP and RTCP). The destination transport address pair may be common for all participants, as in the case of IP multicast,
13、or may be different for each, as in the case of individual unicast network addresses plus a common port pair. In a multimedia session, each medium is carried in a separate RTP session with its own RTCP packets. The multiple RTP sessions are distinguished by different port number pairs and/or differe
14、nt multicast addresses.Basic concepts RTP packetA data packet consisting of the fixed RTP header, a possibly empty list of contributing sources (who contribute the payload), and thepayload data. RTCP packetA control packet consisting of a fixed header part similar to that of RTP data packets, follow
15、ed by structured elements that vary depending upon the RTCP packet type. Typically, multiple RTCP packets are sent together as a compound RTCP packet in a single packet of the underlying protocol.Basic conceptsSynchronization source (SSRC): The source of a stream of RTP packets, identified by a 32-b
16、it numeric SSRC identifier carried in the RTP header so as not to be dependent upon the network address. All packets from a synchronization source form part of the same timing and sequence number space, so a receiver groups packets by synchronization source for playback. A synchronization source may change its data format, e.g., audio enco
