Optimize PAGING-CS PDCH set selection when target MS is known

Before this patch, when a PAGING-GS was received in PCU from SGSN, it
would always forward the paging request to all PDCHs in all TRXs of all
BTS (well, it did some heuristics to avoid sending it in some PDCHs
where onyl repeated TBFs would be listening).

The previous behavior, didn't make much sense in the case where the PCU
is asked to page an MS which it knows (ie in which PDCHs is listening
to). Hence, in that case it makes sense to simply send the paging
request on 1 PDCH where the MS is listening, instead of sending it in a
big set of different PDCHs.

This commit also splits the old get_paging_mi() helper which was
erroneously created to parseboth CS/PS-PAGING requesst, since they
actually use a different set of target subscriber information (for
instance, CS-PAGING provides optionally a TLLI, and one provides P-TMSI
while the other provides TMSI).

In this patch, the handling of CS paging request is split into 2 parts:
1- A new helper "struct paging_req_cs" is introduced, where incoming
CS-PAGING requests (from both SGSN over BSSGP and BTS/BSC over PCUIF)
are parsed and information stored. Then, from available information, it
tries to find a target MS if avaialable
2- bts_add_paging() is called from both BSSGP and PCUIF paths with the
helper struct and the target MS (NULL if not found). If MS exists,
paging is forwarding only on 1 PDCH that MS is attached to. If no MS
exists, then the old heursitics are used to forward the request to all
MS.

Change-Id: Iea46d5321a29d800813b1aa2bf4ce175ce45e2cf

src/bts.cpp

@@ -337,11 +337,44 @@ void bts_set_current_block_frame_number(struct gprs_rlcmac_bts *bts, int fn)
 		bts_set_current_frame_number(bts, fn);
 }
 
-int bts_add_paging(struct gprs_rlcmac_bts *bts, uint8_t chan_needed, const struct osmo_mobile_identity *mi)
+/* Helper used by bts_add_paging() whenever the target MS is known */
+static void bts_add_paging_known_ms(struct GprsMs *ms, const struct osmo_mobile_identity *mi, uint8_t chan_needed)
+{
+	uint8_t ts;
+
+	if (ms->ul_tbf) {
+		for (ts = 0; ts < ARRAY_SIZE(ms->ul_tbf->pdch); ts++) {
+			if (ms->ul_tbf->pdch[ts]) {
+				LOGPDCH(ms->ul_tbf->pdch[ts], DRLCMAC, LOGL_INFO,
+					"Paging on PACCH for %s\n", tbf_name(ms->ul_tbf));
+				if (!ms->ul_tbf->pdch[ts]->add_paging(chan_needed, mi))
+					continue;
+				return;
+			}
+		}
+	}
+	if (ms->dl_tbf) {
+		for (ts = 0; ts < ARRAY_SIZE(ms->dl_tbf->pdch); ts++) {
+			if (ms->dl_tbf->pdch[ts]) {
+				LOGPDCH(ms->dl_tbf->pdch[ts], DRLCMAC, LOGL_INFO,
+					"Paging on PACCH for %s\n", tbf_name(ms->ul_tbf));
+				if (!ms->dl_tbf->pdch[ts]->add_paging(chan_needed, mi))
+					continue;
+				return;
+			}
+		}
+	}
+	LOGPMS(ms, DRLCMAC, LOGL_INFO, "Unable to page on PACCH, no available TBFs\n");
+	return;
+}
+
+/* ms is NULL if no specific taget was found */
+int bts_add_paging(struct gprs_rlcmac_bts *bts, const struct paging_req_cs *req, struct GprsMs *ms)
 {
 	uint8_t l, trx, ts, any_tbf = 0;
 	struct gprs_rlcmac_tbf *tbf;
 	struct llist_item *pos;
+	const struct osmo_mobile_identity *mi;
 	uint8_t slot_mask[8];
 	int8_t first_ts; /* must be signed */
 
@@ -351,13 +384,31 @@ int bts_add_paging(struct gprs_rlcmac_bts *bts, uint8_t chan_needed, const struc
 		NULL
 	};
 
+	/* First, build the MI used to page on PDCH from available subscriber info: */
+	if (req->mi_tmsi_present) {
+		mi = &req->mi_tmsi;
+	} else if (req->mi_imsi_present) {
+		mi = &req->mi_imsi;
+	} else {
+		LOGPMS(ms, DRLCMAC, LOGL_ERROR, "Unable to page on PACCH, no TMSI nor IMSI in request\n");
+		return -EINVAL;
+	}
+
 	if (log_check_level(DRLCMAC, LOGL_INFO)) {
 		char str[64];
 		osmo_mobile_identity_to_str_buf(str, sizeof(str), mi);
-		LOGP(DRLCMAC, LOGL_INFO, "Add RR paging: chan-needed=%d MI=%s\n", chan_needed, str);
+		LOGP(DRLCMAC, LOGL_INFO, "Add RR paging: chan-needed=%d MI=%s\n", req->chan_needed, str);
+	}
+
+	/* We known the target MS for the paging req, send the req only on PDCH
+	 * were that target MS is listening (first slot is enough), and we are done. */
+	if (ms) {
+		bts_add_paging_known_ms(ms, mi, req->chan_needed);
+		return 0;
 	}
 
-	/* collect slots to page
+	/* We don't know the target MS.
+	 * collect slots to page
 	 * Mark slots for every TBF, but only mark one of it.
 	 * Mark only the first slot found.
 	 * Don't mark, if TBF uses a different slot that is already marked. */
@@ -399,7 +450,7 @@ int bts_add_paging(struct gprs_rlcmac_bts *bts, uint8_t chan_needed, const struc
 		for (ts = 0; ts < 8; ts++) {
 			if ((slot_mask[trx] & (1 << ts))) {
 				/* schedule */
-				if (!bts->trx[trx].pdch[ts].add_paging(chan_needed, mi))
+				if (!bts->trx[trx].pdch[ts].add_paging(req->chan_needed, mi))
 					return -ENOMEM;
 
 				LOGPDCH(&bts->trx[trx].pdch[ts], DRLCMAC, LOGL_INFO, "Paging on PACCH\n");

src/bts.h

@@ -265,8 +265,17 @@ struct gprs_rlcmac_bts {
 extern "C" {
 #endif
 
+struct paging_req_cs {
+	uint8_t chan_needed;
+	uint32_t tlli; /* GSM_RESERVED_TMSI if not present */
+	bool mi_tmsi_present;
+	struct osmo_mobile_identity mi_tmsi;
+	bool mi_imsi_present;
+	struct osmo_mobile_identity mi_imsi;
+};
+
 struct GprsMs *bts_alloc_ms(struct gprs_rlcmac_bts *bts, uint8_t ms_class, uint8_t egprs_ms_class);
-int bts_add_paging(struct gprs_rlcmac_bts *bts, uint8_t chan_needed, const struct osmo_mobile_identity *mi);
+int bts_add_paging(struct gprs_rlcmac_bts *bts, const struct paging_req_cs *req, struct GprsMs *ms);
 
 uint32_t bts_rfn_to_fn(const struct gprs_rlcmac_bts *bts, int32_t rfn);
 

src/gprs_bssgp_pcu.c

@@ -171,8 +171,78 @@ static int gprs_bssgp_pcu_rx_dl_ud(struct msgb *msg, struct tlv_parsed *tp)
 			ms_class, egprs_ms_class, delay_csec, data, len);
 }
 
+/* 3GPP TS 48.018 Table 10.3.2. Returns 0 on success, suggested BSSGP cause otherwise */
+static unsigned int get_paging_cs_mi(struct paging_req_cs *req, const struct tlv_parsed *tp)
+{
+	int rc;
+
+	req->chan_needed = tlvp_val8(tp, BSSGP_IE_CHAN_NEEDED, 0);
+
+	if (!TLVP_PRESENT(tp, BSSGP_IE_IMSI)) {
+		LOGP(DBSSGP, LOGL_ERROR, "IMSI Mobile Identity mandatory IE not found\n");
+		return BSSGP_CAUSE_MISSING_MAND_IE;
+	}
+
+	rc = osmo_mobile_identity_decode(&req->mi_imsi, TLVP_VAL(tp, BSSGP_IE_IMSI),
+					 TLVP_LEN(tp, BSSGP_IE_IMSI), true);
+	if (rc < 0 || req->mi_imsi.type != GSM_MI_TYPE_IMSI) {
+		LOGP(DBSSGP, LOGL_ERROR, "Invalid IMSI Mobile Identity\n");
+		return BSSGP_CAUSE_INV_MAND_INF;
+	}
+	req->mi_imsi_present = true;
+
+	/* TIMSI is optional */
+	req->mi_tmsi_present = false;
+	if (TLVP_PRESENT(tp, BSSGP_IE_TMSI)) {
+		/* Be safe against an evil SGSN - check the length */
+		if (TLVP_LEN(tp, BSSGP_IE_TMSI) != GSM23003_TMSI_NUM_BYTES) {
+			LOGP(DBSSGP, LOGL_NOTICE, "TMSI IE has odd length (!= 4)\n");
+			return BSSGP_CAUSE_COND_IE_ERR;
+		}
+
+		/* NOTE: TMSI (unlike IMSI) IE comes without MI type header */
+		req->mi_tmsi = (struct osmo_mobile_identity){
+			.type = GSM_MI_TYPE_TMSI,
+		};
+		req->mi_tmsi.tmsi = osmo_load32be(TLVP_VAL(tp, BSSGP_IE_TMSI));
+		req->mi_tmsi_present = true;
+	}
+
+	if (TLVP_PRESENT(tp, BSSGP_IE_TLLI))
+		req->tlli = osmo_load32be(TLVP_VAL(tp, BSSGP_IE_TLLI));
+	else
+		req->tlli = GSM_RESERVED_TMSI;
+
+	return 0;
+}
+
+static int gprs_bssgp_pcu_rx_paging_cs(struct msgb *msg, const struct tlv_parsed *tp)
+{
+	struct paging_req_cs req;
+	struct gprs_rlcmac_bts *bts;
+	struct GprsMs *ms;
+	int rc;
+
+	if ((rc = get_paging_cs_mi(&req, tp)) > 0)
+		return bssgp_tx_status((enum gprs_bssgp_cause) rc, NULL, msg);
+
+	/* We need to page all BTSs since even if a BTS has a matching MS, it
+	 * may have already moved to a newer BTS. On Each BTS, if the MS is
+	 * known, then bts_add_paging() can optimize and page only on PDCHs the
+	 * target MS is using. */
+	llist_for_each_entry(bts, &the_pcu->bts_list, list) {
+		/* TODO: Match by TMSI before IMSI if present?! */
+		ms = bts_ms_by_tlli(bts, req.tlli, req.tlli);
+		if (!ms && req.mi_imsi_present)
+			ms = bts_ms_by_imsi(bts, req.mi_imsi.imsi);
+		bts_add_paging(bts, &req, ms);
+	}
+
+	return 0;
+}
+
 /* Returns 0 on success, suggested BSSGP cause otherwise */
-static unsigned int get_paging_mi(struct osmo_mobile_identity *mi, const struct tlv_parsed *tp)
+static unsigned int get_paging_ps_mi(struct osmo_mobile_identity *mi, const struct tlv_parsed *tp)
 {
 	/* Use TMSI (if present) or IMSI */
 	if (TLVP_PRESENT(tp, BSSGP_IE_TMSI)) {
@@ -202,22 +272,6 @@ static unsigned int get_paging_mi(struct osmo_mobile_identity *mi, const struct
 	return 0;
 }
 
-static int gprs_bssgp_pcu_rx_paging_cs(struct msgb *msg, const struct tlv_parsed *tp)
-{
-	struct osmo_mobile_identity mi;
-	struct gprs_rlcmac_bts *bts;
-	int rc;
-
-	if ((rc = get_paging_mi(&mi, tp)) > 0)
-		return bssgp_tx_status((enum gprs_bssgp_cause) rc, NULL, msg);
-
-	/* FIXME: look if MS is attached a specific BTS and then only page on that one? */
-	llist_for_each_entry(bts, &the_pcu->bts_list, list) {
-		bts_add_paging(bts, tlvp_val8(tp, BSSGP_IE_CHAN_NEEDED, 0), &mi);
-	}
-	return 0;
-}
-
 static int gprs_bssgp_pcu_rx_paging_ps(struct msgb *msg, const struct tlv_parsed *tp)
 {
 	struct osmo_mobile_identity mi_imsi;
@@ -242,7 +296,7 @@ static int gprs_bssgp_pcu_rx_paging_ps(struct msgb *msg, const struct tlv_parsed
 		return bssgp_tx_status(BSSGP_CAUSE_INV_MAND_INF, NULL, msg);
 	}
 
-	if ((rc = get_paging_mi(&paging_mi, tp)) > 0)
+	if ((rc = get_paging_ps_mi(&paging_mi, tp)) > 0)
 		return bssgp_tx_status((enum gprs_bssgp_cause) rc, NULL, msg);
 
 	/* FIXME: look if MS is attached a specific BTS and then only page on that one? */

src/pcu_l1_if.cpp

@@ -817,6 +817,9 @@ static int pcu_rx_time_ind(struct gprs_rlcmac_bts *bts, struct gsm_pcu_if_time_i
 static int pcu_rx_pag_req(struct gprs_rlcmac_bts *bts, struct gsm_pcu_if_pag_req *pag_req)
 {
 	struct osmo_mobile_identity mi;
+	struct GprsMs *ms = NULL;
+	struct paging_req_cs req = { .chan_needed = pag_req->chan_needed,
+				     .tlli = GSM_RESERVED_TMSI };
 	int rc;
 
 	LOGP(DL1IF, LOGL_DEBUG, "Paging request received: chan_needed=%d "
@@ -835,7 +838,23 @@ static int pcu_rx_pag_req(struct gprs_rlcmac_bts *bts, struct gsm_pcu_if_pag_req
 		return -EINVAL;
 	}
 
-	return bts_add_paging(bts, pag_req->chan_needed, &mi);
+	switch (mi.type) {
+	case GSM_MI_TYPE_TMSI:
+		req.mi_tmsi = mi;
+		req.mi_tmsi_present = true;
+		/* TODO: look up MS by TMSI? Derive TLLI? */
+		break;
+	case GSM_MI_TYPE_IMSI:
+		req.mi_imsi = mi;
+		req.mi_imsi_present = true;
+		ms = bts_ms_by_imsi(bts, req.mi_imsi.imsi);
+		break;
+	default:
+		LOGP(DL1IF, LOGL_ERROR, "Unexpected MI type %u\n", mi.type);
+		return -EINVAL;
+	}
+
+	return bts_add_paging(bts, &req, ms);
 }
 
 static int pcu_rx_susp_req(struct gprs_rlcmac_bts *bts, struct gsm_pcu_if_susp_req *susp_req)