A.2 RTCP Header Validity Checks

The following checks can be applied to RTCP packets.

• RTP version field must equal 2.

• The payload type field of the first RTCP packet in a compound packet must be equal to SR or RR.

• The padding bit (P) should be zero for the first packet of a compound RTCP packet because only the last should possibly need padding.

• The length fields of the individual RTCP packets must total to the overall length of the compound RTCP packet as received. This is a fairly strong check.

The code fragment below performs all of these checks. The packet type is not checked for subsequent packets since unknown packet types may be present and should be ignored.

       u_int32 len;        /* length of compound RTCP packet in words */
       rtcp_t *r;          /* RTCP header */
       rtcp_t *end;        /* end of compound RTCP packet */

       if ((*(u_int16 *)r & RTCP_VALID_MASK) != RTCP_VALID_VALUE) {
           /* something wrong with packet format */
       }
       end = (rtcp_t *)((u_int32 *)r + len);

       do r = (rtcp_t *)((u_int32 *)r + r->common.length + 1);
       while (r < end && r->common.version == 2);

       if (r != end) {
           /* something wrong with packet format */
       }