* Sequence number wrapped - count another 64K cycle.
*/
s->cycles += RTP_SEQ_MOD;
}
s->max_seq = seq;
} else if (udelta <= RTP_SEQ_MOD - MAX_MISORDER) {
/* the sequence number made a very large jump */
if (seq == s->bad_seq) {
/*
* Two sequential packets -- assume that the other side
* restarted without telling us so just re-sync
* (i.e., pretend this was the first packet).
*/
init_seq(s, seq);
}
else {
s->bad_seq = (seq + 1) & (RTP_SEQ_MOD-1);
return 0;
}
} else {
/* duplicate or reordered packet */
}
s->received++;
return 1;
}
The validity check can be made stronger requiring more than two
packets in sequence. The disadvantages are that a larger number of
initial packets will be discarded and that high packet loss rates
could prevent validation. However, because the RTCP header validation
is relatively strong, if an RTCP packet is received from a source
before the data packets, the count could be adjusted so that only two
packets are required in sequence. If initial data loss for a few
seconds can be tolerated, an application could choose to discard all
data packets from a source until a valid RTCP packet has been
received from that source.
Depending on the application and encoding, algorithms may exploit
additional knowledge about the payload format for further validation.
For payload types where the timestamp increment is the same for all
packets, the timestamp values can be predicted from the previous
packet received from the same source using the sequence number
difference (assuming no change in payload type).
A strong "fast-path" check is possible since with high probability
the first four octets in the header of a newly received RTP data
packet will be just the same as that of the previous packet from the
same SSRC except that the sequence number will have increased by one.
Schulzrinne, et al Standards Track [Page 62]
RFC 1889 RTP January 1996
Similarly, a single-entry cache may be used for faster SSRC lookups
in applications where data is typically received from one source at a
time.
A.2 RTCP Header Validity Checks
The following checks can be applied to RTCP packets.
o RTP version field must equal 2.
