hostapd: Add Power Constraint element

Add Power Constraint information element to Beacon and Probe Response
frames when hostapd is configured on 5 GHz band and Country information
element is also added. According to IEEE Std 802.11-2012 a STA shall
determine a local maximum transmit power for the current channel based
on information derived from Country and Power Constraint elements.

In order to add Power Constraint element ieee80211d option need to be
enabled and new local_pwr_constraint config option need to be set to
unsigned value in units of decibels. For now this value is statically
configured but the future goal is to implement dynamic TPC algorithm
to control local power constraint.

Signed-hostap: Srinivasan <srinivasanb@posedge.com>
Signed-hostap: Chaitanya T K <chaitanyatk@posedge.com>
Signed-hostap: Marek Puzyniak <marek.puzyniak@tieto.com>

hostapd/config_file.c

@@ -2907,6 +2907,14 @@ static int hostapd_config_fill(struct hostapd_config *conf,
 					   "sae_groups value '%s'", line, pos);
 				return 1;
 			}
+		} else if (os_strcmp(buf, "local_pwr_constraint") == 0) {
+			int val = atoi(pos);
+			if (val < 0 || val > 255) {
+				wpa_printf(MSG_ERROR, "Line %d: Invalid local_pwr_constraint %d (expected 0..255)",
+					   line, val);
+				return 1;
+			}
+			conf->local_pwr_constraint = val;
 		} else {
 			wpa_printf(MSG_ERROR, "Line %d: unknown configuration "
 				   "item '%s'", line, buf);

hostapd/hostapd.conf

@@ -108,6 +108,13 @@ ssid=test
 # (default: 0 = disabled)
 #ieee80211h=1
 
+# Add Power Constraint element to Beacon and Probe Response frames
+# This config option adds Power Constraint element when applicable and Country
+# element is added. Power Constraint element is required by Transmit Power
+# Control. This can be used only with ieee80211d=1.
+# Valid values are 0..255.
+#local_pwr_constraint=3
+
 # Operation mode (a = IEEE 802.11a, b = IEEE 802.11b, g = IEEE 802.11g,
 # ad = IEEE 802.11ad (60 GHz); a/g options are used with IEEE 802.11n, too, to
 # specify band)

src/ap/ap_config.c

@@ -154,6 +154,8 @@ struct hostapd_config * hostapd_config_defaults(void)
 	conf->rts_threshold = -1; /* use driver default: 2347 */
 	conf->fragm_threshold = -1; /* user driver default: 2346 */
 	conf->send_probe_response = 1;
+	/* Set to invalid value means do not add Power Constraint IE */
+	conf->local_pwr_constraint = -1;
 
 	conf->wmm_ac_params[0] = ac_be;
 	conf->wmm_ac_params[1] = ac_bk;
@@ -829,6 +831,12 @@ int hostapd_config_check(struct hostapd_config *conf, int full_config)
 		return -1;
 	}
 
+	if (full_config && conf->local_pwr_constraint != -1 &&
+	    !conf->ieee80211d) {
+		wpa_printf(MSG_ERROR, "Cannot add Power Constraint element without Country element");
+		return -1;
+	}
+
 	for (i = 0; i < conf->num_bss; i++) {
 		if (hostapd_config_check_bss(conf->bss[i], conf, full_config))
 			return -1;

src/ap/ap_config.h

@@ -519,6 +519,13 @@ struct hostapd_config {
 
 	int ieee80211h; /* DFS */
 
+	/*
+	 * Local power constraint is an octet encoded as an unsigned integer in
+	 * units of decibels. Invalid value -1 indicates that Power Constraint
+	 * element will not be added.
+	 */
+	int local_pwr_constraint;
+
 	struct hostapd_tx_queue_params tx_queue[NUM_TX_QUEUES];
 
 	/*

src/ap/beacon.c

@@ -102,6 +102,36 @@ static u8 * hostapd_eid_erp_info(struct hostapd_data *hapd, u8 *eid)
 }
 
 
+static u8 * hostapd_eid_pwr_constraint(struct hostapd_data *hapd, u8 *eid)
+{
+	u8 *pos = eid;
+
+	if (hapd->iconf->local_pwr_constraint == -1 ||
+	    hapd->iface->current_mode == NULL ||
+	    hapd->iface->current_mode->mode != HOSTAPD_MODE_IEEE80211A)
+		return eid;
+
+	/*
+	 * A STA that is not an AP shall use a transmit power less than or
+	 * equal to the local maximum transmit power level for the channel.
+	 * The local maximum transmit power can be calculated from the formula:
+	 * local max TX pwr = max TX pwr - local pwr constraint
+	 * Where max TX pwr is maximum transmit power level specified for
+	 * channel in Country element and local pwr constraint is specified
+	 * for channel in this Power Constraint element.
+	 */
+
+	/* Element ID */
+	*pos++ = WLAN_EID_PWR_CONSTRAINT;
+	/* Length */
+	*pos++ = 1;
+	/* Local Power Constraint */
+	*pos++ = hapd->iconf->local_pwr_constraint;
+
+	return pos;
+}
+
+
 static u8 * hostapd_eid_country_add(u8 *pos, u8 *end, int chan_spacing,
 				    struct hostapd_channel_data *start,
 				    struct hostapd_channel_data *prev)
@@ -315,6 +345,9 @@ static u8 * hostapd_gen_probe_resp(struct hostapd_data *hapd,
 
 	pos = hostapd_eid_country(hapd, pos, epos - pos);
 
+	/* Power Constraint element */
+	pos = hostapd_eid_pwr_constraint(hapd, pos);
+
 	/* ERP Information element */
 	pos = hostapd_eid_erp_info(hapd, pos);
 
@@ -721,6 +754,9 @@ int ieee802_11_build_ap_params(struct hostapd_data *hapd,
 	tailpos = hostapd_eid_country(hapd, tailpos,
 				      tail + BEACON_TAIL_BUF_SIZE - tailpos);
 
+	/* Power Constraint element */
+	tailpos = hostapd_eid_pwr_constraint(hapd, tailpos);
+
 	/* ERP Information element */
 	tailpos = hostapd_eid_erp_info(hapd, tailpos);