--- a/sip-manual-out/rtp.c	Fri Mar 17 01:14:57 2023 -0800
+++ b/sip-manual-out/rtp.c	Fri Mar 17 09:54:22 2023 -0800
@@ -27,6 +27,38 @@
 static uint16_t rtp_last_seq;
 static int got_some_rtcp;
 
+static const uint8_t hdr_pattern[20] =	{0, 1, 0, 1, 0, 1, 1, 0, 1, 0,
+					 0, 1, 1, 0, 1, 0, 1, 0, 0, 1};
+
+static uint8_t is_hunt_buf[320];
+
+static void
+reset_is_hunt()
+{
+	memset(is_hunt_buf, 0xFF, 320);
+}
+
+static void
+is_hunt_proc(input, input_pos)
+	uint8_t *input;
+	unsigned input_pos;
+{
+	unsigned offset, n;
+
+	memmove(is_hunt_buf, is_hunt_buf + 16, 304);
+	memcpy(is_hunt_buf + 304, input, 16);
+	for (offset = 0; offset < 16; offset++) {
+		for (n = 0; n < 20; n++)
+			if ((is_hunt_buf[offset + n*16] & 1) != hdr_pattern[n])
+				break;
+		if (n == 20)
+			break;
+	}
+	if (n == 20)
+		printf("Found IS_Header, last bit offset %u\n",
+			input_pos * 16 + offset);
+}
+
 void
 obtain_rtp_endp()
 {
@@ -39,6 +71,7 @@
 	bcopy(&res.pstn_addr, &rtp_local_addr, sizeof(struct sockaddr_in));
 	rtp_udp_fd = res.pstn_rtp_fd;
 	rtcp_udp_fd = res.pstn_rtcp_fd;
+	reset_is_hunt();
 }
 
 void
@@ -50,6 +83,7 @@
 	int16_t seq_delta;
 	int32_t ts_delta;
 	int rc;
+	unsigned is_chunk;
 
 	addrlen = sizeof(struct sockaddr_in);
 	rc = recvfrom(rtp_udp_fd, &pkt, sizeof pkt, 0,
@@ -77,6 +111,7 @@
 			printf("Rx RTP stream changed SSRC\n");
 			rtp_ssrc_chg_flag = 1;
 		}
+		reset_is_hunt();
 	} else if (rtp_start_flag) {
 		seq_delta = ntohs(pkt.seq) - rtp_last_seq;
 		ts_delta = ntohl(pkt.tstamp) - rtp_last_ts;
@@ -99,11 +134,13 @@
 				printf("Rx RTP stream seq break\n");
 				rtp_seq_brk_flag = 1;
 			}
+			reset_is_hunt();
 		} else if (ts_delta != 160) {
 			if (!rtp_ts_brk_flag) {
 				printf("Rx RTP stream tstamp break\n");
 				rtp_ts_brk_flag = 1;
 			}
+			reset_is_hunt();
 		}
 	}
 	rtp_ssrc = pkt.ssrc;
@@ -114,6 +151,8 @@
 			rtp_last_seq, rtp_last_ts);
 		rtp_start_flag = 1;
 	}
+	for (is_chunk = 0; is_chunk < 10; is_chunk++)
+		is_hunt_proc(pkt.payload + is_chunk * 16, is_chunk);
 }
 
 void