When processing RFC 2833 DTMF, accomodate increasing timestamps in End events

While endpoints should not be changing the source timestamp between DTMF event
packets, the fact is there exists those endpoints that do exactly that.  To
work around this, we absorb timestamps within the expected re-transmit period.
Note that this period only affects End of Event packets, so it should not
prevent the detection of new DTMF digits that happen to arrive right on top
of each other.

(closes issue ASTERISK-20424)
Reported by: Vladimir Mikhelson
Tested by: mjordan, Vladimir Mikhelson

Review: https://reviewboard.asterisk.org/r/2124


git-svn-id: https://origsvn.digium.com/svn/asterisk/branches/1.8@373236 65c4cc65-6c06-0410-ace0-fbb531ad65f3

res/res_rtp_asterisk.c

@@ -1587,8 +1587,9 @@ static void process_dtmf_rfc2833(struct ast_rtp_instance *instance, unsigned cha
 		new_duration = (new_duration & ~0xFFFF) | samples;
 
 		if (event_end & 0x80) {
-			/* End event */
-			if ((rtp->last_seqno != seqno) && (timestamp > rtp->last_end_timestamp)) {
+			/* End event.  Absorb re-transmits, and account for some endpoints
+			 * that erroneously increment the timestamp during re-transmissions */
+			if ((seqno != rtp->last_seqno) && (timestamp > rtp->last_end_timestamp + 320)) {
 				rtp->last_end_timestamp = timestamp;
 				rtp->dtmf_duration = new_duration;
 				rtp->resp = resp;
@@ -1598,7 +1599,7 @@ static void process_dtmf_rfc2833(struct ast_rtp_instance *instance, unsigned cha
 				rtp->dtmf_duration = rtp->dtmf_timeout = 0;
 				AST_LIST_INSERT_TAIL(frames, f, frame_list);
 			} else if (rtpdebug) {
-				ast_debug(1, "Dropping duplicate or out of order DTMF END frame (seqno: %d, ts %d, digit %c)\n",
+				ast_debug(1, "Dropping re-transmitted, duplicate, or out of order DTMF END frame (seqno: %d, ts %d, digit %c)\n",
 					seqno, timestamp, resp);
 			}
 		} else {