Sender Report RTCP Packets (SR)

  A Sender Report message consists of the header, the sender information block, a variable number of receiver report blocks, and potentially a profile-specific extensions.

The header:

 0               1               2               3
 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|V=2|P|    RC   |   PT=SR=200   |             length L          |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                         SSRC of sender                        |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

V, P:

As described for RTP data packets, see above.

RC:

5 bits
The number of reception report blocks contained in this packet.

PT:

8 bits
The packet type constant 200 designates an RTCP SR packet.

L:

16 bits
The length of this RTCP packet in 32-bit words minus one, including the header and any padding.

SSRC:

32 bits
The synchronisation source identifier for the sender of this SR packet.

The sender information block:

 0               1               2               3
 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|            NTP timestamp, most significant word NTS           |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|             NTP timestamp, least significant word             |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                       RTP timestamp RTS                       |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                   sender's packet count SPC                   |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                    sender's octet count SOC                   |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

NTS:

64 bits
The NTP timestamp indicates the point of time measured in wall clock time when this report was sent. In combination with timestamps returned in reception reports from the respective receivers, it can be used to estimate the round-trip propagation time to and from the receivers.

RTS:

32 bits
The RTP timestamp resembles the same time as the NTP timestamp (above), but is measured in the same units and with the same random offset as the RTP timestamps in data packets. This correspondence may be used for intra- and inter-media synchronisation for sources whose NTP timestamps are synchronised, and may be used by media-independent receivers to estimate the nominal RTP clock frequency.

SPC:

32 bits
The sender's packet count totals up the number of RTP data packets transmitted by the sender since joining the RTP session. This field can be used to estimate the average data packet rate.

SOC:

32 bits
The total number of payload octets (i.e., not including the header or any padding) transmitted in RTP data packets by the sender since starting up transmission. This field can be used to estimate the average payload data rate.

The receiver report blocks:

 0               1               2               3
 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                 SSRC_1 (SSRC of first source)                 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|fraction lost F|      cumulative number of packets lost C      |
-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|         extended highest sequence number received  EHSN       |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                    inter-arrival jitter J                     |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                          last SR LSR                          |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                    delay since last SR DLSR                   |
+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
|                 SSRC_2 (SSRC of second source)                |
:                               ...                             :
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

SSRC_n:

32 bits
The SSRC identifier of the sender whose reception is reported in this block.

F:

8 bits
The sender of the receiver report estimates the fraction of the RTP data packets from source SSRC_n that it assumes to be lost since it sent the previous SR or RR packet.

C:

24 bits
The sender of a receiver report blocks also tries to estimate the total number of RTP data packets from source SSRC_n that have been lost since the beginning of reception. Packets that arrive late are not counted as lost, and the loss may be negative if there are duplicates.

EHSN:

32 bits
The low 16 bits of the extended highest sequence number contain the highest sequence number received in an RTP data packet from source SSRC_n, and the most significant 16 bits extend that sequence number with the corresponding count of sequence number cycles.

J:

32 bits
An estimate of the statistical variance of the RTP data packet inter-arrival time, measured in timestamp units and expressed as an unsigned integer.

The extensions:

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                  profile-specific extensions                  |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+