osmux: add osmux circuit ID management and resolve NAT problems

This patch includes several osmux fixes that are interdependent:

1) This adds Osmux circuit ID, this is allocated from the bsc-nat. This
   announces the circuit ID in the CRCX MGCP message. This aims to resolve
   the lack of uniqueness due to the use of endp->ci, which is local to
   the bsc. This ID is notified via X-Osmux: NUM where NUM is the osmux
   circuit ID.

2) The dummy load routines are now used to setup osmux both in bsc and
   bsc-nat to resolve source port NAT issues as suggested by Holger. The
   source port that is used from the bsc is not known until the first
   voice message is sent to the bsc-nat, therefore enabling osmux from
   the MGCP plane breaks when a different source port is used.

3) Add refcnt to struct osmux_handle, several endpoints can be using the
   same input RTP osmux handle to perform the batching. Remove it from the
   osmux handle list once nobody is using it anymore to clean it up.

4) Add a simple Osmux state-machine with three states. The initial state
   is disabled, then if the bsc-nat requests Osmux, both sides enters
   activating. The final enabled state is reached once the bsc-nat sees
   the dummy load message that tells what source port is used by the bsc.

5) The osmux input handle (which transforms RTP messages to one Osmux batch)
   is now permanently attached to the endpoint when Osmux is set up from the
   dummy load path, so we skip a lookup for each message. This simplifies
   osmux_xfrm_to_osmux().

After this patch, the workflow to setup Osmux is the following:

                    bsc                   bsc-nat
                     |                       |
                     |<------ CRCX ----------|
                     |      X-Osmux: 3       | (where 3 is the Osmux circuit ID
                     |                       |  that the bsc-nat has allocated)
                     |------- resp --------->|
                     |      X-Osmux: 3       | (the bsc confirm that it can
                     |                       |  use Osmux).
                     .                       .
                     |                       |
         setup osmux |----- dummy load ----->| setup osmux
                     |      Osmux CID: 3     |

In two steps:

1st) Allocate the Osmux Circuit ID (CID): The bsc-nat allocates an unique
     Osmux CID that is notified to the bsc through the 'X-Osmux:' extension.
     The bsc-nat annotates this circuit ID in the endpoint object. The bsc
     replies back with the 'X-Osmux:' to confirm that it agrees to use Osmux.
     If the bsc doesn't want to use Osmux, it doesn't include the extension
     so the bsc-nat knows that it has to use to RTP.

2nd) The dummy load is used to convey the Osmux CID. This needs to happen
     at this stage since the bsc-nat needs to know what source port the bsc
     uses to get this working since the bsc may use a different source
     port due to NAT. Unfortunately, this can't be done from the MGCP signal
     plane since the real source port is not known that the bsc uses is not
     known.

This patch also reverts the MDCX handling until it is clear that we need
this special handling for this case.

2 files changed, 18 insertions, 2 deletions

diff --git a/openbsc/include/openbsc/mgcp_internal.h b/openbsc/include/openbsc/mgcp_internal.h
index 2236b2a9b..3d308835e 100644
--- a/openbsc/include/openbsc/mgcp_internal.h
+++ b/openbsc/include/openbsc/mgcp_internal.h
@@ -174,7 +174,12 @@ struct mgcp_endpoint {
 	struct mgcp_rtp_tap taps[MGCP_TAP_COUNT];
 
 	struct {
-		int enable;
+		/* Osmux state: disabled, activating, active */
+		enum osmux_state state;
+		/* Allocated Osmux circuit ID for this endpoint */
+		uint8_t cid;
+		/* handle to batch messages */
+		struct osmux_in_handle *in;
 		/* handle to unbatch messages */
 		struct osmux_out_handle out;
 	} osmux;
diff --git a/openbsc/include/openbsc/osmux.h b/openbsc/include/openbsc/osmux.h
index 33456b76d..f4cb17abd 100644
--- a/openbsc/include/openbsc/osmux.h
+++ b/openbsc/include/openbsc/osmux.h
@@ -9,11 +9,22 @@ enum {
 };
 
 int osmux_init(int role, struct mgcp_config *cfg);
-int osmux_enable_endpoint(struct mgcp_endpoint *endp, int role);
+int osmux_enable_endpoint(struct mgcp_endpoint *endp, int role,
+			  struct in_addr *addr, uint16_t port);
+void osmux_disable_endpoint(struct mgcp_endpoint *endp);
 
 int osmux_xfrm_to_rtp(struct mgcp_endpoint *endp, int type, char *buf, int rc);
 int osmux_xfrm_to_osmux(int type, char *buf, int rc, struct mgcp_endpoint *endp);
 
 int osmux_send_dummy(struct mgcp_endpoint *endp);
 
+int osmux_get_cid(void);
+void osmux_put_cid(uint8_t osmux_cid);
+
+enum osmux_state {
+	OSMUX_STATE_DISABLED = 0,
+	OSMUX_STATE_ACTIVATING,
+	OSMUX_STATE_ENABLED,
+};
+
 #endif