For example, an application may be designed to send only CNAME, NAME
and EMAIL and not any others. NAME might be given much higher
priority than EMAIL because the NAME would be displayed continuously
in the application's user interface, whereas EMAIL would be displayed
only when requested. At every RTCP interval, an RR packet and an SDES
packet with the CNAME item would be sent. For a small session
operating at the minimum interval, that would be every 5 seconds on
the average. Every third interval (15 seconds), one extra item would
be included in the SDES packet. Seven out of eight times this would
be the NAME item, and every eighth time (2 minutes) it would be the
EMAIL item.
When multiple applications operate in concert using cross-application
binding through a common CNAME for each participant, for example in a
multimedia conference composed of an RTP session for each medium, the
additional SDES information might be sent in only one RTP session.
The other sessions would carry only the CNAME item.
6.3 Sender and Receiver Reports
RTP receivers provide reception quality feedback using RTCP report
packets which may take one of two forms depending upon whether or not
the receiver is also a sender. The only difference between the sender
report (SR) and receiver report (RR) forms, besides the packet type
code, is that the sender report includes a 20-byte sender information
section for use by active senders. The SR is issued if a site has
sent any data packets during the interval since issuing the last
report or the previous one, otherwise the RR is issued.
Both the SR and RR forms include zero or more reception report
blocks, one for each of the synchronization sources from which this
receiver has received RTP data packets since the last report. Reports
are not issued for contributing sources listed in the CSRC list. Each
reception report block provides statistics about the data received
from the particular source indicated in that block. Since a maximum
of 31 reception report blocks will fit in an SR or RR packet,
additional RR packets may be stacked after the initial SR or RR
packet as needed to contain the reception reports for all sources
heard during the interval since the last report.
Schulzrinne, et al Standards Track [Page 22]
RFC 1889 RTP January 1996
The next sections define the formats of the two reports, how they may
be extended in a profile-specific manner if an application requires
additional feedback information, and how the reports may be used.
Details of reception reporting by translators and mixers is given in
Section 7.
6.3.1 SR: Sender report RTCP packet
