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This FSM is similar to the already existing ran_peer_fsm in osmo-msc, which already had better logic around SCCP and RANAP state handling. Similarly, osmo-sgsn's struct ranap_iu_rnc maps to osmo-msc's struct ran_peer. With this FSM we can currently track the RANAP link state towards a given RNC peer: * Reject (RANAP Error Indication) all UE-related messages until a RANAP RESET from RNC is received first. * Tear down all subsriber connections whenever the RANAP peer sends us a RESET message. * Tear down all subscriber connections whenever the SCCP link towards RNC becomes unavailable. * Send a RESET towards RNC peer once the SCCP link towrdards it becomes available again. This commit only implements so far the Rx path of the FSM, ie. when receiving events/messages from a peer over SCCP and pushing them locally up the stack (RANAP). The Tx side will be implemented in a follow-up commit, which will allow discarding messages if the lower layers towards a given RNC are known to be down. Related: OS#3403 Change-Id: I18b7803500163e78ff6a684095194174b0fb6ee1