Illustrated TCP/IP by Matthew G. Naugle Wiley Computer Publishing, John Wiley & Sons, Inc. ISBN: 0471196568   Pub Date: 11/01/98

Chapter 212
Sender Reports

Reports for RTCP allow senders and receivers to communicate with each other for quality reasons. Each sender sends a report giving statistics for its transmissions. Receivers consume this data and also send out reports to indicate how well they are receiving data. The senders use this data to tune their transmissions.

There are two types of reports that RTP receivers may send: Sender and Receiver reports (SR and RR, respectively). The type of report sent depends on whether the receiver is also a sender. RTP receivers provide reception quality feedback using the RTCP reports. The sender report tells receivers what they should have received. The V is for the version number, which should be set to 2. The R Cnt is the receiver block count and contains the number of receiver blocks in the message. The Length field indicates the length of the packet in bytes.

Both types of reports are very similar. The only difference between the two reports, besides the packet type, is the 20-byte sender information section in the Sender report. The Sender report is issued to let the receiver know what should have been received. Both the SR and RR can include from 0 to 31 blocks (notice the blocks are simply replicated); 1 block for each of the synchronization sources from which the receiver has received RTP data packets since its last report.

The SSRC field ties the Sender report to any RTP data packets the source may have sent. The NTP timestamp is the actual time of day. It is used as a control to measure the delta for timestamps extracted from reception reports. This allows for an estimate to be made as to the round-trip propagation delay to those receivers. The RTP timestamp allows receivers to put this message in an order relative to the RTP packets. The last fields indicate the number of packets and bytes the sender has transmitted.

Sender Reports

The next sections are receiver blocks. In the Sender report, they allow the sender to report its transmitted data but also any RTP data that is has received. Each block represents one remote source. The block indicates the fraction of packets from that source that were lost since the last report and the total number of packets lost since inception. The Extended Highest Sequence Number Received is the highest sequence number from that source. The Interarrival Jitter field allows for the receiver to estimate the variance of the source’s interarrival times. A high value indicates that this receiver is receiving a stream of packets irregularly.

The last two fields indicate when the last report from this source arrived.