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→Message types: Sender Report description included an eeror claiming that absolute timestamps were in units of seconds since midnight UTC January 1 1970. According to RFC3550 timestamps are expressed in NTP timestamp format which is relative to midnight UTC January 1 1900 |
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* '''Version''': (2 bits) Identifies the version of RTP, which is the same in RTCP packets as in RTP data packets. The version defined by this specification is two (2).<ref name=RFC3550
* '''P (Padding)''': (1 bits) 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).<ref name=RFC3550/>
* '''RC (Reception report count )''': (5 bits) The number of reception report blocks contained in this packet. A value of zero is valid.<ref name=RFC3550/>
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RTCP distinguishes several types of packets: ''sender report'', ''receiver report'', ''source description'', and ''goodbye''. In addition, the protocol is extensible and allows application-specific RTCP packets. A standards-based extension of RTCP is the ''extended report'' packet type introduced by RFC 3611.<ref>RFC 3611, ''RTP Control Protocol Extended Reports (RTCP XR)'', T. Friedman (Ed.), R. Caceres, A. Clark (Ed.), The Internet Society (November 2003)</ref>
;Sender report (SR):The sender report is sent periodically by the active senders in a conference to report transmission and reception statistics for all RTP packets sent during the interval. The sender report includes two distinct timestamps, an absolute timestamp, represented using the timestamp format of the Network Time Protocol (NTP) (which is in seconds relative to midnight UTC on 1 January 1900) and an RTP timestamp that corresponds to the same time as the NTP timestamp, but in the same units and with the same random offset as the RTP timestamps in data packets described by this Sender Report
;Receiver report (RR): The receiver report is for passive participants, those that do not send RTP packets. The report informs the sender and other receivers about the quality of service.
;Source description (SDES): The Source Description message is used to send the CNAME item to session participants. It may also be used to provide additional information such as the name, e-mail address, telephone number, and address of the owner or controller of the source.
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