MSC pooling: add 'no allow-attach' for MSC off-loading

As in 3GPP TS 23.236, to offload an MSC, the BSC must be able to avoid
attaching new subscribers to it:

4.5a.1: "UEs being moved from one CN node are stopped from registering to the
same CN node again by an O&M command in BSCs and RNCs connected to the pool."

Change-Id: I6249201c15d0f6565aca643c21d2375c9ca58584

3 files changed, 40 insertions, 1 deletions

diff --git a/src/osmo-bsc/gsm_08_08.c b/src/osmo-bsc/gsm_08_08.c
index 46ef9afea..c002ebb96 100644
--- a/src/osmo-bsc/gsm_08_08.c
+++ b/src/osmo-bsc/gsm_08_08.c
@@ -280,7 +280,18 @@ static struct bsc_msc_data *bsc_find_msc(struct gsm_subscriber_connection *conn,
 
 		/* Figure out the next round-robin MSC. The MSCs may appear unsorted in net->mscs. Make sure to linearly
 		 * round robin the MSCs by number: pick the lowest msc->nr >= round_robin_next_nr, and also remember the
-		 * lowest available msc->nr to wrap back to that in case no next MSC is left. */
+		 * lowest available msc->nr to wrap back to that in case no next MSC is left.
+		 *
+		 * MSCs configured with `no allow-attach` do not accept new subscribers and hence must not be picked by
+		 * round-robin. Such an MSC still provides service for already attached subscribers: those that
+		 * successfully performed IMSI-Attach and have a TMSI with an NRI pointing at that MSC. We only avoid
+		 * adding IMSI-Attach of new subscribers. The idea is that the MSC is in a mode of off-loading
+		 * subscribers, and the MSC decides when each subscriber is off-loaded, by assigning the NULL-NRI in a
+		 * new TMSI (at the next periodical LU). So until the MSC decides to offload, an attached subscriber
+		 * remains attached to that MSC and is free to use its services.
+		 */
+		if (!msc->allow_attach)
+			continue;
 		if (!msc_round_robin_first || msc->nr < msc_round_robin_first->nr)
 			msc_round_robin_first = msc;
 		if (msc->nr >= round_robin_next_nr
diff --git a/src/osmo-bsc/osmo_bsc_msc.c b/src/osmo-bsc/osmo_bsc_msc.c
index ce0751806..157808e95 100644
--- a/src/osmo-bsc/osmo_bsc_msc.c
+++ b/src/osmo-bsc/osmo_bsc_msc.c
@@ -230,6 +230,7 @@ struct bsc_msc_data *osmo_msc_data_alloc(struct gsm_network *net, int nr)
 	msc_data->mgcp_ipa.local_port = 0; /* dynamic */
 
 	msc_data->nri_ranges = osmo_nri_ranges_alloc(msc_data);
+	msc_data->allow_attach = true;
 
 	return msc_data;
 }
diff --git a/src/osmo-bsc/osmo_bsc_vty.c b/src/osmo-bsc/osmo_bsc_vty.c
index c860cfef8..5c624dd1c 100644
--- a/src/osmo-bsc/osmo_bsc_vty.c
+++ b/src/osmo-bsc/osmo_bsc_vty.c
@@ -177,6 +177,9 @@ static void write_msc(struct vty *vty, struct bsc_msc_data *msc)
 	}
 
 	msc_write_nri(vty, msc, false);
+
+	if (!msc->allow_attach)
+		vty_out(vty, " no allow-attach%s", VTY_NEWLINE);
 }
 
 static int config_write_msc(struct vty *vty)
@@ -860,6 +863,28 @@ DEFUN(cfg_msc_nri_del, cfg_msc_nri_del_cmd,
 	return CMD_SUCCESS;
 }
 
+DEFUN(cfg_msc_allow_attach, cfg_msc_allow_attach_cmd,
+      "allow-attach",
+      "Allow this MSC to attach new subscribers (default).\n")
+{
+	struct bsc_msc_data *msc = bsc_msc_data(vty);
+	msc->allow_attach = true;
+	return CMD_SUCCESS;
+}
+
+DEFUN(cfg_msc_no_allow_attach, cfg_msc_no_allow_attach_cmd,
+      "no allow-attach",
+      NO_STR
+      "Do not assign new subscribers to this MSC."
+      " Useful if an MSC in an MSC pool is configured to off-load subscribers."
+      " The MSC will still be operational for already IMSI-Attached subscribers,"
+      " but the NAS node selection function will skip this MSC for new subscribers\n")
+{
+	struct bsc_msc_data *msc = bsc_msc_data(vty);
+	msc->allow_attach = false;
+	return CMD_SUCCESS;
+}
+
 static void msc_write_nri(struct vty *vty, struct bsc_msc_data *msc, bool verbose)
 {
 	struct osmo_nri_range *r;
@@ -963,6 +988,8 @@ int bsc_vty_init_extra(void)
 	install_element(MSC_NODE, &cfg_msc_nri_add_cmd);
 	install_element(MSC_NODE, &cfg_msc_nri_del_cmd);
 	install_element(MSC_NODE, &cfg_msc_show_nri_cmd);
+	install_element(MSC_NODE, &cfg_msc_allow_attach_cmd);
+	install_element(MSC_NODE, &cfg_msc_no_allow_attach_cmd);
 
 	/* Deprecated: ping time config, kept to support legacy config files. */
 	install_element(MSC_NODE, &cfg_net_msc_no_ping_time_cmd);