enable Early Immediate Assignment

When an Immediate Assignment comes in targeting an lchan that is not yet
active, then hold back the RR Immediate Assignment until the channel
becomes active.

This allows the BSC to send the Immediate Assignment before first
waiting for the Channel Activation ACK, saving one Abis roundtrip, and
helping avoid double allocation on high latency Abis links.

Related: SYS#5559
Related: I56c25cde152040fb66bdba44399bd37671ae3df2 (osmo-bsc)
Related: Ifb2c62431a91dafa6116b5d6b9410930f00a6e18 (osmo-ttcn3-hacks)
Change-Id: Ie52765b238b01f22fb327fe12327fbf10abcad4c

1 files changed, 24 insertions, 0 deletions

diff --git a/src/common/rsl.c b/src/common/rsl.c
index 18c03491..f5a963ce 100644
--- a/src/common/rsl.c
+++ b/src/common/rsl.c
@@ -1085,6 +1085,30 @@ static int rsl_rx_imm_ass(struct gsm_bts_trx *trx, struct msgb *msg)
 	msg->l2h = NULL;
 	msg->len = TLVP_LEN(&tp, RSL_IE_FULL_IMM_ASS_INFO);
 
+	/* Early Immediate Assignment: when there is a lot of latency on Abis, the Abis roundtrip of Chan Activ -> Chan
+	 * Activ ACK -> Immediate Assignment may take so long that each MS sends a second RACH for Chan Rqd, reserving
+	 * two SDCCH for each request but using only one. To help with that, the Early IA feature in osmo-bsc sends the
+	 * Immediate Assignment without waiting for the Channel Activation ACK. This may then be too early, and the MS
+	 * may not be able to establish a channel. So to help with Early IA, look up whether the target lchan is already
+	 * active. If not, then hold back the RR Immediate Assignment message, and send it once L1 has confirmed that
+	 * the channel is active. Hence we still wait for the activation, but don't need the Abis roundtrip of Activ ACK
+	 * -> Immediate Assignment via the BSC.
+	 * If anything is wrong with the sizes or the lchan lookup, behave normally, i.e. do not do the RR IA caching,
+	 * but just send the RR message to the MS as-is. */
+	if (msg->len >= sizeof(struct gsm48_imm_ass)) {
+		struct gsm48_imm_ass *rr_ia = (void*)msg->data;
+		struct gsm_lchan *ia_target_lchan = lchan_lookup(trx, rr_ia->chan_desc.chan_nr, "Early IA check: ");
+		if (ia_target_lchan && ia_target_lchan->state != LCHAN_S_ACTIVE) {
+			/* Target lchan is not yet active. Cache the IA.
+			 * If a previous IA is still lingering, free it. */
+			msgb_free(ia_target_lchan->early_rr_ia);
+			ia_target_lchan->early_rr_ia = msg;
+
+			/* return 1 means: don't msgb_free() the msg */
+			return 1;
+		}
+	}
+
 	/* put into the AGCH queue of the BTS */
 	if (bts_agch_enqueue(trx->bts, msg) < 0) {
 		/* if there is no space in the queue: send DELETE IND */