mgcp: Attempt to count missing RTP packets with a basic calculation

This code compares the UDP sequence numbers of two RTP messages
and guesses if packets are missing. It is guessing in two ways:

	1.) by default the sequence number is 0, so on the first
	    value we ignore the jump... we might ignore a real issue
	    in case of a wrap around which is easily possible as the
	    sequence should be a random number.
	2.) the UDP stream might have been reordered on the network
	    and we would see the jump...

In any case these two shortcomings are acceptable for the feature
that is meant to provide some basic analysis..

openbsc/include/openbsc/mgcp_internal.h

@@ -61,6 +61,12 @@ struct mgcp_endpoint {
 	/* statistics */
 	unsigned int in_bts;
 	unsigned int in_remote;
+
+	/* sequence bits */
+	uint16_t net_seq_no;
+	uint16_t bts_seq_no;
+	int net_lost_no;
+	int bts_lost_no;
 };
 
 #define ENDPOINT_NUMBER(endp) abs(endp - endp->cfg->endpoints)

openbsc/src/mgcp/mgcp_network.c

@@ -94,17 +94,29 @@ int mgcp_send_dummy(struct mgcp_endpoint *endp)
 			endp->net_rtp, buf, 1);
 }
 
-static void patch_payload(int payload, char *data, int len)
+static void patch_and_count(uint16_t *last_seq, int *lost, int payload, char *data, int len)
 {
+	uint16_t seq;
 	struct rtp_hdr *rtp_hdr;
 
 	if (len < sizeof(*rtp_hdr))
 		return;
 
+	rtp_hdr = (struct rtp_hdr *) data;
+	seq = ntohs(rtp_hdr->sequence);
+
+	/* 0 is assumed to be not set */
+	if (*last_seq == 0)
+		*last_seq = seq;
+	else if (*last_seq + 1 != seq)
+		*lost += abs(*last_seq - seq);
+
+	*last_seq = seq;
+
+
 	if (payload < 0)
 		return;
 
-	rtp_hdr = (struct rtp_hdr *) data;
 	rtp_hdr->payload_type = payload;
 }
 
@@ -194,13 +206,15 @@ static int rtp_data_cb(struct bsc_fd *fd, unsigned int what)
 
 	if (dest == DEST_NETWORK) {
 		if (proto == PROTO_RTP)
-			patch_payload(endp->net_payload_type, buf, rc);
+			patch_and_count(&endp->bts_seq_no, &endp->bts_lost_no,
+					endp->net_payload_type, buf, rc);
 		return udp_send(fd->fd, &endp->remote,
 			     proto == PROTO_RTP ? endp->net_rtp : endp->net_rtcp,
 			     buf, rc);
 	} else {
 		if (proto == PROTO_RTP)
-			patch_payload(endp->bts_payload_type, buf, rc);
+			patch_and_count(&endp->net_seq_no, &endp->net_lost_no,
+					endp->bts_payload_type, buf, rc);
 		return udp_send(fd->fd, &endp->bts,
 			     proto == PROTO_RTP ? endp->bts_rtp : endp->bts_rtcp,
 			     buf, rc);

openbsc/src/mgcp/mgcp_protocol.c

@@ -764,4 +764,7 @@ void mgcp_free_endp(struct mgcp_endpoint *endp)
 	endp->in_bts = endp->in_remote = 0;
 	memset(&endp->remote, 0, sizeof(endp->remote));
 	memset(&endp->bts, 0, sizeof(endp->bts));
+
+	endp->net_seq_no = endp->bts_seq_no = 0;
+	endp->net_lost_no = endp->bts_lost_no = 0;
 }