account for lost frames during ptime detection

This allows the "broken ptime" detection to work correctly when packet
loss is present on the wire. In addition to the timestamps this patch
adds frame sequence tracking and corrects the timestamp difference
only as needed and according to the number of lost packets.

FS-6898 #resolve

src/switch_core_media.c

@@ -95,6 +95,7 @@ typedef struct switch_rtp_engine_s {
 	switch_codec_implementation_t write_impl;
 
 	switch_size_t last_ts;
+	switch_size_t last_seq;
 	uint32_t check_frames;
 	uint32_t mismatch_count;
 	uint32_t last_codec_ms;
@@ -1743,6 +1744,7 @@ SWITCH_DECLARE(switch_status_t) switch_core_media_read_frame(switch_core_session
 			
 			engine->check_frames = 0;
 			engine->last_ts = 0;
+			engine->last_seq = 0;
 
 			do_cng = 1;
 		}
@@ -1869,8 +1871,14 @@ SWITCH_DECLARE(switch_status_t) switch_core_media_read_frame(switch_core_session
 							uint32_t codec_ms = (int) (engine->read_frame.timestamp -
 													   engine->last_ts) / (engine->read_impl.samples_per_second / 1000);
 
+							if (engine->last_seq && (int) (engine->read_frame.seq - engine->last_seq) > 1) {
+								switch_log_printf(SWITCH_CHANNEL_SESSION_LOG(session), SWITCH_LOG_DEBUG, "Correcting calculated ptime value from %d to %d to compensate for %d lost packet(s)\n", codec_ms, codec_ms / (int) (engine->read_frame.seq - engine->last_seq), (int) (engine->read_frame.seq - engine->last_seq - 1));
+								codec_ms = codec_ms / (int) (engine->read_frame.seq - engine->last_seq);
+							}
+
 							if ((codec_ms % 10) != 0 || codec_ms > engine->read_impl.samples_per_packet * 10) {
 								engine->last_ts = 0;
+								engine->last_seq = 0;
 								goto skip;
 							}
 
@@ -1918,11 +1926,13 @@ SWITCH_DECLARE(switch_status_t) switch_core_media_read_frame(switch_core_session
 						}
 
 						engine->last_ts = engine->read_frame.timestamp;
+						engine->last_seq = engine->read_frame.seq;
 
 
 					} else {
 						engine->mismatch_count = 0;
 						engine->last_ts = 0;
+						engine->last_seq = 0;
 					}
 				}
 
@@ -4560,6 +4570,7 @@ SWITCH_DECLARE(void) switch_core_media_reset_autofix(switch_core_session_t *sess
 
 	engine->check_frames = 0;
 	engine->last_ts = 0;
+	engine->last_seq = 0;
 }