CTRL interface encapsulation over IPA link is missing when using libosmo-sigtran
When OsmoBSC uses libosmo-sigtran for the A interface transport, it misses the capabiliy to transport/encapsulate the MGCP messages inside the IPA multiplex (like used towards osmo-bsc_nat). This must be ported over from old OsmoBSC.
During a recent discussion it became aparrent how important this is for deployments behind NAT. There's one IPA/SCCPlite connection between BSC and BSC-NAT, and that connection must handle BSSMAP, CTRL, MGCP and possibly more.
Architecture-wise, we'll have to add some mode in which CTRL (in libosmoctrl) can register/bind to a sub-flow of the libosmonetif IPA stream creted by libosmo-sigtran. Not yet sure how it will look in the end.