Skip to main content

RFC 3550 - 12. Summary of Protocol Constants

  1. Summary of Protocol Constants

This section contains a summary listing of the constants defined in this specification.

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.

12.1 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.

Additional RTCP packet types may be registered through IANA (see Section 15).

12.2 SDES Types

abbrev. name value END end of SDES list 0 CNAME canonical name 1 NAME user name 2 EMAIL user's electronic mail address 3 PHONE user's phone number 4 LOC geographic user location 5 TOOL name of application or tool 6 NOTE notice about the source 7 PRIV private extensions 8

Additional SDES types may be registered through IANA (see Section 15).