IuUP: always allow Initialization

Do not refuse IuUP Initialization messages coming in on an RTP port.
If an IUFP conn is not yet configured (pre-Initialization), allow rx
from any remote address.

If we refuse the IuUP Initialization, a 3G RNC may fail to set up a RAB.
We will know the remote address only *after* assigning a RAB succeeded.
So the IuUP Initialization must be allowed before knowing all addresses.

At the time of writing, CRCX for IUFP are sent to osmo-mgw in either
LOOPBACK or in RECVONLY mode:
- current osmo-msc: recvonly
- osmo-msc <= v1.10.0: loopback
- osmo-hnbgw: loopback
IuUP Initialization should work regardless of that.
See also next patch I158dd046fdfcb10392cde3de8cc88dd095a05b40

IuUP is one layer below the loopback/send/recv decision for RTP; IuUP is
always terminated at the MGW, while the AMR payload carries through.

Decided for now that it's not worth the extra effort to make this more
restrictive; ideas would be:
- actually verify the incoming packet to have a valid IuUP Init header
  before permitting it to be received.
- as soon as the remote address is known, also validate the src for IuUP
  Initialization messages.
This patch is nice and simple and does the job.

Related: alternative patch Idd833997abce46886e9664505b2776fa5dadc8db
Related: SYS#6657
Change-Id: I6c365559a7bd197349f0ea99f7a13b56a4bb580b

src/libosmo-mgcp/mgcp_network.c

@@ -834,6 +834,15 @@ static int check_rtp_origin(struct mgcp_conn_rtp *conn, struct osmo_sockaddr *ad
 {
 	char ipbuf[INET6_ADDRSTRLEN];
 
+	/* Allow IuUP Initialization to get through even if we don't have a remote address set yet. */
+	if (mgcp_conn_rtp_is_iuup(conn) && !conn->iuup.configured) {
+		/* maybe todo: also verify that it is actually a valid IuUP Initialization header in the incoming msgb?
+		 * (though, why do we even care where the RTP is coming from) */
+		LOGPCONN(conn->conn, DRTP, LOGL_INFO, "Rx RTP from %s: allowing unknown src for IuUP Initialization\n",
+			 osmo_sockaddr_to_str(addr));
+		return 0;
+	}
+
 	if (osmo_sockaddr_is_any(&conn->end.addr) != 0) {
 		switch (conn->conn->mode) {
 		case MGCP_CONN_LOOPBACK: