The RTP payload type (PT) constants are defined in profiles rather
than this document. However, the octet of the RTP header which
contains the marker bit(s) and payload type must avoid the reserved
values 200 and 201 (decimal) to distinguish RTP packets from the RTCP
SR and RR packet types for the header validation procedure described
in Appendix A.1. For the standard definition of one marker bit and a
7-bit payload type field as shown in this specification, this
restriction means that payload types 72 and 73 are reserved.
Schulzrinne, et al Standards Track [Page 51]
RFC 1889 RTP January 1996
11.1 RTCP packet types
abbrev. name value
SR sender report 200
RR receiver report 201
SDES source description 202
BYE goodbye 203
APP application-defined 204
These type values were chosen in the range 200-204 for improved
header validity checking of RTCP packets compared to RTP packets or
other unrelated packets. When the RTCP packet type field is compared
to the corresponding octet of the RTP header, this range corresponds
to the marker bit being 1 (which it usually is not in data packets)
and to the high bit of the standard payload type field being 1 (since
the static payload types are typically defined in the low half). This
range was also chosen to be some distance numerically from 0 and 255
since all-zeros and all-ones are common data patterns.
Since all compound RTCP packets must begin with SR or RR, these codes
were chosen as an even/odd pair to allow the RTCP validity check to
test the maximum number of bits with mask and value.
Other constants are assigned by IANA. Experimenters are encouraged to
register the numbers they need for experiments, and then unregister
those which prove to be unneeded.
11.2 SDES types
