DTAP: Ensure proper DLCI is used in MSC-originated DTAP

The DLCI field of the DTAP header indicates the SAPI as well as the
data link (main DCCH or SACCH).  We must make sure to use the correct
DLCI when sending DTAP to the BSC.

We achieve this by
* storing the DLCI in the msgb->cb while parsing the DTAP header
* storing the received DLCI (from msgb->cb) in the transaction for
  mobile-originated transactions
* using the trans->dlci to sent msgb->cb when transmitting L3
* filling the DTAP DLCI value from msgb->cb when transmitting DTAP

For MSC-originated transactions, we choose a DLCI value corresponding
to the service (SAPI=0 for CC, SAPI=3 for SMS) and store that in
trans->dlci.

Closes: OS#3150
Change-Id: If511b20f52575054cab1346d99a8cb68d827fdbf

6 files changed, 16 insertions, 4 deletions

diff --git a/include/osmocom/msc/gsm_data.h b/include/osmocom/msc/gsm_data.h
index f3e1b94b1..6f0aca4b1 100644
--- a/include/osmocom/msc/gsm_data.h
+++ b/include/osmocom/msc/gsm_data.h
@@ -20,7 +20,8 @@
 
 #include "gsm_data_shared.h"
 
-
+/* TS 48.008 DLCI containing DCCH/ACCH + SAPI */
+#define OMSC_LINKID_CB(__msgb)   (__msgb)->cb[3]
 
 /** annotations for msgb ownership */
 #define __uses
diff --git a/include/osmocom/msc/transaction.h b/include/osmocom/msc/transaction.h
index 272fb3b2e..63b1cc519 100644
--- a/include/osmocom/msc/transaction.h
+++ b/include/osmocom/msc/transaction.h
@@ -31,6 +31,9 @@ struct gsm_trans {
 	/* The current transaction ID */
 	uint8_t transaction_id;
 
+	/* The DLCI (DCCH/ACCH + SAPI) of this transaction */
+	uint8_t dlci;
+
 	/* To whom we belong, unique identifier of remote MM entity */
 	struct vlr_subscr *vsub;
 
diff --git a/src/libmsc/a_iface.c b/src/libmsc/a_iface.c
index 2b1baf0fb..6f2000e00 100644
--- a/src/libmsc/a_iface.c
+++ b/src/libmsc/a_iface.c
@@ -145,14 +145,13 @@ int a_iface_tx_dtap(struct msgb *msg)
 	struct gsm_subscriber_connection *conn;
 	struct msgb *msg_resp;
 
-	/* FIXME: Set this to some meaninful value! */
-	uint8_t link_id = 0x00;
+	uint8_t link_id = OMSC_LINKID_CB(msg);
 	OSMO_ASSERT(msg);
 	conn = (struct gsm_subscriber_connection *)msg->dst;
 	OSMO_ASSERT(conn);
 	OSMO_ASSERT(conn->a.scu);
 
-	LOGPCONN(conn, LOGL_DEBUG, "Passing DTAP message from MSC to BSC\n");
+	LOGPCONN(conn, LOGL_DEBUG, "Passing DTAP message (DLCI=0x%02x) from MSC to BSC\n", link_id);
 
 	msg->l3h = msg->data;
 	msg_resp = gsm0808_create_dtap(msg, link_id);
diff --git a/src/libmsc/a_iface_bssap.c b/src/libmsc/a_iface_bssap.c
index 1665e2307..d0d6bc03e 100644
--- a/src/libmsc/a_iface_bssap.c
+++ b/src/libmsc/a_iface_bssap.c
@@ -610,6 +610,7 @@ static int rx_dtap(const struct osmo_sccp_user *scu, const struct a_conn_info *a
 {
 	struct gsm_network *network = a_conn_info->network;
 	struct gsm_subscriber_connection *conn;
+	struct dtap_header *dtap = (struct dtap_header *) msg->l2h;
 
 	conn = subscr_conn_lookup_a(network, a_conn_info->conn_id);
 	if (!conn) {
@@ -620,6 +621,7 @@ static int rx_dtap(const struct osmo_sccp_user *scu, const struct a_conn_info *a
 
 	/* msc_dtap expects the dtap payload in l3h */
 	msg->l3h = msg->l2h + 3;
+	OMSC_LINKID_CB(msg) = dtap->link_id;
 
 	/* Forward dtap payload into the msc */
 	msc_dtap(conn, conn->a.conn_id, msg);
diff --git a/src/libmsc/gsm_04_08.c b/src/libmsc/gsm_04_08.c
index a90ff172f..e5ddb44d4 100644
--- a/src/libmsc/gsm_04_08.c
+++ b/src/libmsc/gsm_04_08.c
@@ -164,6 +164,7 @@ static int gsm48_conn_sendmsg(struct msgb *msg, struct gsm_subscriber_connection
 	 * work that the caller no longer has to do */
 	if (trans) {
 		gh->proto_discr = trans->protocol | (trans->transaction_id << 4);
+		OMSC_LINKID_CB(msg) = trans->dlci;
 	}
 
 	return msc_tx_dtap(conn, msg);
@@ -3110,6 +3111,7 @@ int mncc_tx_to_cc(struct gsm_network *net, int msg_type, void *arg)
 
 		/* Assign conn */
 		trans->conn = msc_subscr_conn_get(conn, MSC_CONN_USE_TRANS_CC);
+		trans->dlci = 0x00; /* SAPI=0, not SACCH */
 		vlr_subscr_put(vsub);
 	} else {
 		/* update the subscriber we deal with */
@@ -3262,6 +3264,7 @@ static int gsm0408_rcv_cc(struct gsm_subscriber_connection *conn, struct msgb *m
 		}
 		/* Assign transaction */
 		trans->conn = msc_subscr_conn_get(conn, MSC_CONN_USE_TRANS_CC);
+		trans->dlci = OMSC_LINKID_CB(msg); /* DLCI as received from BSC */
 		cm_service_request_concludes(conn, msg);
 	}
 
diff --git a/src/libmsc/gsm_04_11.c b/src/libmsc/gsm_04_11.c
index f82aa6d55..aa6049002 100644
--- a/src/libmsc/gsm_04_11.c
+++ b/src/libmsc/gsm_04_11.c
@@ -137,6 +137,7 @@ static int gsm411_cp_sendmsg(struct msgb *msg, struct gsm_trans *trans,
 	/* Outgoing needs the highest bit set */
 	gh->proto_discr = trans->protocol | (trans->transaction_id<<4);
 	gh->msg_type = msg_type;
+	OMSC_LINKID_CB(msg) = trans->dlci;
 
 	DEBUGP(DLSMS, "sending CP message (trans=%x)\n", trans->transaction_id);
 
@@ -947,6 +948,7 @@ int gsm0411_rcv_sms(struct gsm_subscriber_connection *conn,
 			gsm411_rl_recv, gsm411_mn_send);
 
 		trans->conn = msc_subscr_conn_get(conn, MSC_CONN_USE_TRANS_SMS);
+		trans->dlci = OMSC_LINKID_CB(msg); /* DLCI as received from BSC */
 
 		new_trans = 1;
 		cm_service_request_concludes(conn, msg);
@@ -1029,6 +1031,8 @@ int gsm411_send_sms(struct gsm_subscriber_connection *conn, struct gsm_sms *sms)
 	trans->sms.sms = sms;
 
 	trans->conn = msc_subscr_conn_get(conn, MSC_CONN_USE_TRANS_SMS);
+	trans->dlci = 0x03;
+	/* FIXME: specify SACCH in case we already have active TCH */
 
 	/* Hardcode SMSC Originating Address for now */
 	data = (uint8_t *)msgb_put(msg, 8);