1COMP 249 Advanced Distributed SystemsMultimedia Networkinghttp://www.cs.unc.edu/~jeffay/courses/comp249f99The Multimedia Control Protocol RTCPKevin JeffayDepartment of Computer ScienceUniversity of North Carolina at Chapel [email protected] 5, 19992The Multimedia Transport Protocol RTPOutlineu RTP concepts» Entities and abstractionsu Protocol definition» Header format and packet structureu Developing interoperable applications with RTP» RTP profilesu Quality-of-service monitoring and reporting» Real-time control protocol RTCP3The Real-Time Control Protocol RTCPOverviewu Senders & receivers periodically generate reports ofvarious session statistics and multicast to the groupRTPdataRTPMonitorRTCPreception reportsInternetworkInternetworku RTCP enables...» Diagnosis of faults in the multicast distribution tree» Congestion control» Third party performance monitoring & logging» (Simple) conference control4The Real-Time Control Protocol RTCPMessage typesu Sender reports (SR)» cumulative frame & bytecounts» wall clock/timestampvaluesu Receiver reports (RR)» frame loss/Framedelivery rateSenderReportsReceiver ReportsRTPMonitoru Source description (SDES)items» useful ASCII text strings(user & host name of participant,e-mail address, notes, ...)u “Bye” message» used to update participant’sSSRC tablesInternetworkInternetwork5The Real-Time Control Protocol RTCPMechanicsu RTCP messages are “stackable”» To amortize header overhead, multiple RTCP messages can becombined and sent in a compound RTCP messageu RTCP messages are always sent in (at least) pairs» Messages must always contain a sender/receiver report and asource description message containing the canonical name(CNAME) of the participantu RTCP messages are sent periodically with a period set toensure that control messages consume no more than 5% ofthe session bandwidth» Much of the contents of sender & receiver reports are includedso that participants can compute the RTCP sending interval6IP headerIP headerUDP headerUDP headerRTCP headerRTCP headerThe Real-Time Control Protocol RTCPMessage encapsulationUDPMessageCompoundRTCPMessageRTCP MessageReception Report1...Reception ReportnRTCP headerRTCP headerOptionalReportsRTCPSDESReportRTCPSender/ReceiverReportReportBlockCNAMERTCP headerRTCP header...7The Real-Time Control Protocol RTCPCommon sender/receiver report message headeru All report messages have the same 8 byte header» version number (same as RTP)» padding indicator» reception report count (5 bits)» RTCP message type (8 bits)» RTCP message length (16 bits)» SSRC for the sender of this report (32 bits)012345678901234567890123456789010123v = 2SSRC of report senderSSRC of report senderp RR count packet type message length8v = 2SSRC of report senderSSRC of report senderp RR count packet type=200 message lengthNTP timestamp (two 32-bit words)NTP timestamp (two 32-bit words)RTP timestampRTP timestampSender’s cumulative packet countSender’s cumulative packet countSender’s cumulative byte countSender’s cumulative byte countReception Report Block 1Reception Report Block 1Reception Report Block 2Reception Report Block 2Reception Report Block 2Reception Report Block 2The Real-Time Control Protocol RTCPSender reports — packet format012345678901234567890123456789010123...9The Real-Time Control Protocol RTCPReception report blocksu Each sender and receiver report should contain areception report block for each synchronizationsource heard from since the last RTCP reportu Contents:» source identifier for the block (SSRC)» fraction of RTP packets from this source lost since the lastreport» cumulative number of lost packets» extended highest sequence number received» estimated average RTP packet interarrival time jitter» last SR timestamp received from this source» delay since receiving the last SR report from this source10RTCP Reception Report BlocksLoss calculationu The number of lost packets is expressed as a fraction» where:nbr of packets lost = nbr packets expected – nbr packets receivednumber of packets expected = EHSNR – initial sequence numberEHSNR = extended highest sequence number received = number of sequence number cycles x 216 + last sequence number receivednumber of packets lostnumber of packets expectedfraction lost =11RTCP Reception Report BlocksJitter calculationu Interarrival time jitter is an estimate of the statisticalvariance of RTP data packet interarrival timesSenderReceiverDelay-jitterPerfect Deliveryjitternew = jitterold +instantaneous jitter – jitterold16instantaneous jitter = (reci – reci–1) – (senti – senti–1)reci – reci–1senti – senti–1» the smoothed mean absolute value of the difference between thesending interval at a source and the interarrival time at a receiver12RTCP Reception Report BlocksRound trip time calculationu The last-SR-timestamp received and delay-since-receiving-last-SR-report fields in the reception report block are used tocompute an estimate of the round-trip time from the receiver toa synchronization source RR received = time a source received this reception report SR sent = last SR timestamp received field delay = delay since last SR report fieldSSRCclockReceiver’sclockdelayRTTRTCPmessageestimated round-trip- time = RR received – SR sent – delay13v = 2message lengthmessage lengthThe Real-Time Control Protocol RTCPReceiver reports012345678901234567890123456789010123SSRC of report senderSSRC of report senderSSRC of first source heard fromSSRC of first source heard fromp RR count packet type=201cumulative number of lost packetscumulative number of lost packetsextended highest sequence number receivedextended highest sequence number receivedestimate RTP packet interarrival time jitterestimate RTP packet interarrival time jitterfraction losttimestamp of last SR report receivedtimestamp of last SR report receivedelapsed time since last SR report receivedelapsed time since last SR report received...Reception Report 2Reception Report 214SDES typeSDES lengthpacket type=202The Real-Time Control Protocol RTCPSource description item (SDES) messagesu An SDES message consists of one or more “chunks”of source description items» CNAME (user@host)» NAME» EMAIL012345678901234567890123456789010123SSRC or first CSRCSDES itemp chunk cnt message lengthv = 2SSRC or second CSRC...SDES type» PHONE» LOC» TOOL» NOTE» ...SDES length SDES item...15v = 2message lengthmessage lengthSource Description