IBM TCP IP tutorial

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.Multicast and Multimedia 489ApplicationSender RTPUDPIPRTPRTCPApplication ApplicationRTP RTPReceiversUDP UDPIP IP3376\3376F6S9Figure 274.RTCP and RTP Packet DeliveryBecause RTCP packets can have different functions, there are different formatsdefined:SR Sender reportRR Receiver reportSDES Source description itemsBYE Indicates end of participationAPP Application-specific functions9.5.2.1 RTCP Sender ReportAn RTCP sender report will be sent by the sources of an RTP data stream toinform the receivers what they should have received.It is a multicast packet thatreaches all receivers.The sender report packet can be grouped in three parts.First the header, whichspecifies the RTCP packet type, packet length and the sender identification.Thesecond part contains the sender information and the third part the receiver reportblocks.Most times, the RTCP sender packet contains more than one receiverreport block.They allow the sender to report feedback for received RTP packetsfrom other senders.The following figure shows the format of an RTCP senderreport packet.490 TCP/IP Tutorial and Technical Overview0 1234567891011121314151617181920212223 2425262728293031V=2 RC PT=SR=200 lengthPSSRC of senderheaderNTP time stamp, most significant wordsenderinfoNTP time stamp, least significant wordRTP time stampsender's packet countsender's octet countsender's octet countSSRC_1 (SSRC of first source)reportblock 1fraction lost cumulative number of packets lostextended highest sequence number receivedinterarrival jitterlast SR (LSR)delay since last SR (DLSR)SSRC_2 (SSRC of first source) reportblock 2.profile-specific extensions3376\3376F6SBFigure 275.RTCP Sender Report Packet FormatWhere:V2-bit version field which is set to 1.PPadding bit.If this is set, there are one or more additional paddingbytes at the end of the packet.RC5-bit reception report count.Contains the number of reception reportblocks in this packet.Chapter 9.Multicast and Multimedia 491PT8-bit packet type field.For sender reports the value is 200.length16-bit packet length in bytes (including any added padding).SSRC of Sender32-bit field that contains the SSRC identifier of the host sending thispacket.NTP time stampTwo 32-bit fields containing the absolute time in the Network TimeProtocol (NTP) format.This timer counts the number of seconds sinceJanuary 1, 1900.RTP timestamp32-bit field that contains the time stamp from the RTP packets accordingto the sender.sender's packet count32-bit field with the total number of RTP data packets transmitted by thesender since the start of the transmission.sender's octet count32-bit field with the total number of payload bytes transmitted by thesender since the start of the transmission.SSRC_n (source identifier):32-bit field that contains the SSRC identifier of another RTP sender fromwhich this sender has received packets.The number of report blockswith different sender SSRCs depends on the number of other sourcesthat were heard by this sender since the last report.fraction lost8-bit field that represents the fraction of RTP data packets that was lostsince the previous SR or RR packet was sent from the source SSRC_n.cumulative number of packets lost24-bit field that represents the total number of lost RTP data packetsfrom source SSRC_n.extended highest sequence number received32-bit field.It contains the highest sequence number that was receivedin an RTP data packet from the source SSRC_n.interarrival jitter32-bit field that contains the estimated variance of the interarrival timesfrom the appropriate source.If the packets arrive regularly, the jittervalue is zero.If the packets arrive irregularly, the jitter value is high.last SR timestamp (LSR)32-bit field that contains the middle 32 bits from the 64-bit NTPtimestamp received with the last RTCP sender report packet from thesource SSRC_n.delay since last SR (DLSR)32-bit field.It contains the delay between receiving the last SR packetfrom the source SSRC_n and the sending of the current exceptionreport block in units of 1/65536 seconds.492 TCP/IP Tutorial and Technical Overview9.5.2.2 RTCP Receiver ReportThe RTCP Receiver Report (RR) packet is similar to the sender report packet.Thepacket type field contains the constant 201 and the sender information section(NTP and RTP timestamps/sender's packet and octet counts) is not included in thepacket [ Pobierz całość w formacie PDF ]

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