mgcp: Deal with receiving another payload type

In case we get offered G729 and G711 we might have selected
G729 as the audio codec. The first packet we receive might be
G711 though. In that case we will need to change. But only if
we have a matching alternate codec payload_type. E.g. in the
case of comfort noise we will receive the PT=11 and we don't
want to change.

1 files changed, 53 insertions, 1 deletions

diff --git a/openbsc/src/libmgcp/mgcp_transcode.c b/openbsc/src/libmgcp/mgcp_transcode.c
index e961ba6cc..38daeb8ae 100644
--- a/openbsc/src/libmgcp/mgcp_transcode.c
+++ b/openbsc/src/libmgcp/mgcp_transcode.c
@@ -397,11 +397,62 @@ static int encode_audio(struct mgcp_process_rtp_state *state,
 	return nbytes;
 }
 
+static struct mgcp_rtp_end *source_for_dest(struct mgcp_endpoint *endp,
+					struct mgcp_rtp_end *dst_end)
+{
+	if (&endp->bts_end == dst_end)
+		return &endp->net_end;
+	else if (&endp->net_end == dst_end)
+		return &endp->bts_end;
+	OSMO_ASSERT(0);
+}
+
+/*
+ * With some modems we get offered multiple codecs
+ * and we have selected one of them. It might not
+ * be the right one and we need to detect this with
+ * the first audio packets. One difficulty is that
+ * we patch the rtp payload type in place, so we
+ * need to discuss this.
+ */
+struct mgcp_process_rtp_state *check_transcode_state(
+				struct mgcp_endpoint *endp,
+				struct mgcp_rtp_end *dst_end,
+				struct rtp_hdr *rtp_hdr)
+{
+	struct mgcp_rtp_end *src_end;
+
+	/* Only deal with messages from net to bts */
+	if (&endp->bts_end != dst_end)
+		goto done;
+
+	src_end = source_for_dest(endp, dst_end);
+
+	/* Already patched */
+	if (rtp_hdr->payload_type == dst_end->codec.payload_type)
+		goto done;
+	/* The payload we expect */
+	if (rtp_hdr->payload_type == src_end->codec.payload_type)
+		goto done;
+	/* The matching alternate payload type? Then switch */
+	if (rtp_hdr->payload_type == src_end->alt_codec.payload_type) {
+		struct mgcp_config *cfg = endp->cfg;
+		struct mgcp_rtp_codec tmp_codec = src_end->alt_codec;
+		src_end->alt_codec = src_end->codec;
+		src_end->codec = tmp_codec;
+		cfg->setup_rtp_processing_cb(endp, &endp->net_end, &endp->bts_end);
+		cfg->setup_rtp_processing_cb(endp, &endp->bts_end, &endp->net_end);
+	}
+
+done:
+	return dst_end->rtp_process_data;
+}
+
 int mgcp_transcoding_process_rtp(struct mgcp_endpoint *endp,
 				struct mgcp_rtp_end *dst_end,
 			     char *data, int *len, int buf_size)
 {
-	struct mgcp_process_rtp_state *state = dst_end->rtp_process_data;
+	struct mgcp_process_rtp_state *state;
 	const size_t rtp_hdr_size = sizeof(struct rtp_hdr);
 	struct rtp_hdr *rtp_hdr = (struct rtp_hdr *) data;
 	char *payload_data = (char *) &rtp_hdr->data[0];
@@ -414,6 +465,7 @@ int mgcp_transcoding_process_rtp(struct mgcp_endpoint *endp,
 	uint32_t ts_no;
 	int rc;
 
+	state = check_transcode_state(endp, dst_end, rtp_hdr);
 	if (!state)
 		return 0;