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msc {
	hscale=3;
	ms [label="MS"], bts [label="BTS"], bsc[label="BSC"], bsc_gscon[label="BSC conn FSM"], bsc_mgcp[label="BSC mgcp FSM"], mgw[label="MGW"], msc_[label="MSC"];

	ms note msc_ [label="lchan allocation sequence for BSSMAP Assignment Request"];

	bsc <= msc_ [label="BSSMAP Assignment Request"];
	bsc box bsc [label="bssmap_handle_assignm_req()"];
	bsc -> bsc_gscon [label="GSCON_EV_A_ASSIGNMENT_CMD"];

	--- [label="is the chan_mode a speech mode?"];

	bsc_gscon abox bsc_gscon [label="ST_WAIT_CRCX_BTS (MGCP_MGW_TIMEOUT = 4s)"];
	bsc_gscon -> bsc_mgcp [label="mgcp_conn_create()"];
	bsc_mgcp => mgw [label="CRCX (for BTS)"];
	bsc_mgcp abox bsc_mgcp [label="ST_CRCX (MGCP_MGW_TIMEOUT = 4s)"];
	bsc_gscon note bsc_mgcp [label="two timeouts running in parallel"];
	bsc_gscon note bsc_mgcp [label="note: #define MGCP_MGW_TIMEOUT exists twice,
				       once in libosmo-mgcp-client,
				       once in bsc_subscr_conn_fsm.c"];
	bsc_mgcp -> bsc_gscon [label="mgcp_conn_create() exits"];
	bsc_gscon -> bsc [label="bssmap_handle_assignm_req() exits"];
	...;
	--- [label="On Timeout"];
	bsc_gscon note bsc_gscon [label="The conn FSM likely timeouts first"];
	bsc_gscon => msc_ [label="BSSMAP Assignment Failure"];
	bsc_gscon abox bsc_gscon [label="ST_ACTIVE"];
	bsc_mgcp note bsc_mgcp [label="The MGCP FSM will timeout right after that, and terminate itself,
				      emitting the parent_term event set upon mgcp_conn_create():"];
	bsc_mgcp -> bsc_gscon [label="GSCON_EV_MGW_FAIL_BTS"];
	bsc_gscon note bsc_gscon [label="GSCON_EV_MGW_FAIL_BTS is handled by the conn FSM allstate
					handler. It sets conn->user_plane.fi_bts = NULL. There is code
					that would emit a BSSMAP Assignment Failure, but not in
					ST_ACTIVE"];
	--- [label="end: 'On Timeout'"];
	...;

	bsc_mgcp <= mgw [label="CRCX OK (for BTS)"];
	bsc_mgcp box bsc_mgcp [label="libosmo-mgcp-client fsm_crcx_resp_cb()"];
	bsc_mgcp -> bsc_gscon [label="GSCON_EV_MGW_CRCX_RESP_BTS"];
	--- [label="end: 'is the chan_mode a speech mode?'"];

	bsc_gscon note bsc_gscon [label="for mode=sign, we're still handling GSCON_EV_A_ASSIGNMENT_CMD;
					 for speech mode, we're handling GSCON_EV_MGW_CRCX_RESP_BTS"];
	bsc <- bsc_gscon [label="gsm0808_assign_req()"];

	bsc box bsc [label="lchan_alloc(): pick available lchan"];
	bsc box bsc [label="rsl_chan_activate_lchan()"];

	--- [label="is the chosen lchan on dynamic timeslot that is currently used as PDCH?"];
	bts <= bsc [label="i) RSL RF Chan Release of PDCH (Osmocom dyn TS)"];
	bts <= bsc [label="OR ii) RSL PDCH Deact (ip.access dyn TS)"];
	bsc -> bsc_gscon [label="gsm0808_assign_req() returns early"];
	bsc_gscon abox bsc_gscon [label="ST_WAIT_ASS_COMPL (GSM0808_T10_VALUE=6s)"];
	...;
	bts note bsc_gscon [linecolor="red",
		label="Osmocom style dyn TS use lchan->act_timer to watch over RF Chan Release, but there
		seems to be no timer watching over PDCH Deact!"];
	...;
	bts => bsc [label="i) RSL RF Chan Release ACK (Osmocom dyn TS)"];
	bts => bsc [label="OR ii) RSL PDCH Deact ACK (ip.access dyn TS)"];
	bsc box bsc [label="rsl_chan_activate_lchan() re-invoked"];
	bsc box bsc [label="lchan->state = LCHAN_S_ACT_REQ"];
	bts <= bsc [label="RSL Chan Activ"];
	--- [label="else (no dyn TS switchover)"];

	bsc box bsc [label="lchan->state = LCHAN_S_ACT_REQ"];
	bts <= bsc [label="RSL Chan Activ"];
	bsc -> bsc_gscon [label="gsm0808_assign_req() returns"];
	bsc_gscon abox bsc_gscon [label="ST_WAIT_ASS_COMPL (GSM0808_T10_VALUE=6s)"];
	---;

	...;
	--- [label="On Timeout"];
	bsc_gscon => msc_ [label="BSSMAP Assignment Failure"];
	bsc_gscon abox bsc_gscon [label="ST_ACTIVE"];
	bsc_gscon note bsc_mgcp [linecolor="red",
	  label="The mgcp FSM from CRCX above apparently lacks a cleanup action for this case.
	         It should be cleaned up eventually when the conn is torn down, but we should
		 release RTP endpoints as soon as possible."];
	--- [label="end: 'On Timeout'"];
	...;

	bts => bsc [label="RSL Chan Activ ACK"];
	bsc box bsc [label="rsl_rx_chan_act_ack()"];
	bsc box bsc [label="Stop lchan->act_timer"];
	bsc box bsc [label="lchan->state = LCHAN_S_ACTIVE"];
	bsc -> bsc [label="S_LCHAN_ACTIVATE_ACK"];
	bsc box bsc [label="bsc_api.c handle_chan_ack()"];
	ms <= bsc [label="RR Assignment Command"];

	...;
	ms note bsc_gscon [label="We rely on the overall conn FSM ST_WAIT_ASS_COMPL timeout."];
	...;

	ms => bsc [label="RR Assignment Complete"];
	bsc box bsc [label="handle_ass_compl()"];
	--- [label="Release old lchan"];
	bsc box bsc [label="_lchan_handle_release(sacch_deact=0)"];
	bsc box bsc [label="rsl_release_sapis_from(start=1)"];
	bts <= bsc [label="RSL Release Request (Local End)..."];
	bts <= bsc [label="...for each SAPI except link_id=0"];
	bsc box bsc [label="rsl_release_request(link_id=0)"];
	bts <= bsc [label="RSL Release Request (Local End) for link_id=0"];
	bsc box bsc [label="_lchan_handle_release() returns here, the remaining release is asynchronous;
	                    see `End: 'Release old lchan'` below."];
	...;
	bts note bsc_gscon [linecolor="red",
	  label="There seems to be no timer watching over RSL Release Request!"];
	...;
	bts => bsc [label="RSL Release Confirm..."];
	bts => bsc [label="...for each SAPI and link_id=0"];
	bsc abox bsc [label="start T3111"];
	...;
	bsc box bsc [label="T3111 expires"];
	bsc abox bsc [label="Start lchan->act_timer with lchan_deact_tmr_cb"];
	bts <= bsc [label="RSL RF Channel Release"];
	...;
	--- [label="On timeout"];
	bsc box bsc [label="lchan_deact_tmr_cb()"];
	bsc box bsc [label="rsl_lchan_mark_broken(): state=LCHAN_S_BROKEN"];
	bsc box bsc [label="lchan_free()"];
	bsc -> bsc [label="S_LCHAN_UNEXPECTED_RELEASE"];
	bsc box bsc [label="bsc_api.c handle_release()"];
	bsc box bsc [label="bsc->assign_fail()"];
	bsc -> bsc_gscon [label="GSCON_EV_RR_ASS_FAIL"];
	bsc note bsc_gscon [linecolor="orange",
	  label="The name 'RR_ASS_FAIL' might suggest the event means an actual RR Assignment
		 Failure message being received. Maybe this should be called GSCON_EV_ASSIGNMENT_ERROR."];
	...;
	bsc box bsc [label="bsc->clear_request()"];
	bsc box bsc [label="bsc_clear_request encodes a BSSMAP Clear Request message and passes it on
	                    to the conn FSM as data argument via:"];
	bsc -> bsc_gscon [label="GSCON_EV_TX_SCCP"];
	bsc_gscon => msc_ [label="BSSMAP Clear Request"];
	bsc note bsc_gscon [linecolor="red",
	  label="Instead of sending an arbitrary message, the conn FSM should
		 be explicitly instructed to clear the connection, to be able
		 to notice if the MSC failed to respond to the Clear Request.
		 Currently, this relies on the MSC responding with a Clear
		 Command, hopefully, some time later."];
	--- [label="End: 'On timeout'"];
	...;
	bts => bsc [label="RSL RF Channel Release Ack"];
	bsc box bsc [label="Stop lchan->act_timer"];
	bsc box bsc [label="Stop lchan->T3111"];
	--- [label="End: 'Release old lchan'"];
	bsc box bsc [label="still in handle_ass_compl()"];
	bsc note bsc [label="officially take over new lchan: conn->lchan = conn->secondary_lchan"];
	--- [label="is BTS using IPA Abis? (osmo-bts, ip.access)"];
	bts <= bsc [label="IPACC CRCX"];
	---;
	bsc -> bsc [label="handle_ass_compl() calls bsc_api->assign_compl()"];
	--- [label="is BTS using IPA Abis? (osmo-bts, ip.access) && conn->user_plane.rtp_ip"];
	bsc box bsc [label="bsc_assign_compl()"];
	bsc note bsc [label="set ass_compl.valid = true,
			    postponing GSCON_EV_RR_ASS_COMPL until after the
			    IPACC MDCX ACK received in osmo_bsc_audio.c"];
	bsc box bsc [label="exit early: bsc_assign_compl()"];
	bsc box bsc [label="exit early: handle_ass_compl()"];
	bsc box bsc [label="osmo_bsc_audio.c"];
	bts => bsc [label="IPACC CRCX ACK"];
	bts <= bsc [label="IPACC MDCX"];
	bts => bsc [label="IPACC MDCX ACK"];
	bsc box bsc [label="handle_abisip_signal()"];
	bsc -> bsc_gscon [label="GSCON_EV_RR_ASS_COMPL"];
	--- [label="else"];
	bsc box bsc [label="bsc_assign_compl()"];
	bsc -> bsc_gscon [label="GSCON_EV_RR_ASS_COMPL"];
	--- ;

	--- [label="is chan_mode a speech mode?"];
	bsc_gscon abox bsc_gscon [label="ST_WAIT_MDCX_BTS"];
	bsc_gscon -> bsc_mgcp [label="mgcp_conn_modify()"];
	bsc_mgcp note bsc_mgcp [label="same mgcp FSM as above, for BTS side"];
	bsc_mgcp => mgw [label="MDCX (for BTS)"];
	bsc_mgcp <= mgw [label="MDCX OK"];
	bsc_mgcp -> bsc_gscon [label="GSCON_EV_MGW_MDCX_RESP_BTS"];
	bsc_gscon abox bsc_gscon [label="ST_WAIT_CRCX_MSC"];
	bsc_gscon -> bsc_mgcp [label="mgcp_conn_create()"];
	bsc_mgcp note bsc_mgcp [label="second mgcp FSM for MSC side"];
	bsc_mgcp => mgw [label="CRCX (for MSC)"];
	bsc_mgcp <= mgw [label="CRCX OK (for MSC)"];
	bsc_gscon <- bsc_mgcp [label="GSCON_EV_MGW_CRCX_RESP_MSC"];
	---;

	bsc_gscon => msc_ [label="BSSMAP Assignment Complete"];

	bsc_gscon abox bsc_gscon [label="ST_ACTIVE"];
}