Try to avoid some unnecessary roaming

When multiple APs are present in scan results with similar signal
strength, wpa_supplicant may end up bounching between them frequently
whenever new scan results are available (e.g., due to periodic scans
requested by NetworkManager). This can result in unnecessary roaming
and in case of the current cfg80211 version, to frequent network
disconnections.

Do not request a roam if the current BSS is still present in the scan
results and the selected BSS is in the same ESS and has only a slighly
stronger signal strength.

wpa_supplicant/events.c

@@ -776,6 +776,72 @@ static void wpa_supplicant_rsn_preauth_scan_results(
 }
 
 
+static int wpa_supplicant_need_to_roam(struct wpa_supplicant *wpa_s,
+				       struct wpa_bss *selected,
+				       struct wpa_ssid *ssid,
+				       struct wpa_scan_results *scan_res)
+{
+	size_t i;
+	struct wpa_scan_res *current_bss = NULL;
+	int min_diff;
+
+	if (wpa_s->reassociate)
+		return 1; /* explicit request to reassociate */
+	if (wpa_s->wpa_state < WPA_ASSOCIATED)
+		return 1; /* we are not associated; continue */
+	if (wpa_s->current_ssid == NULL)
+		return 1; /* unknown current SSID */
+	if (wpa_s->current_ssid != ssid)
+		return 1; /* different network block */
+
+	for (i = 0; i < scan_res->num; i++) {
+		struct wpa_scan_res *res = scan_res->res[i];
+		const u8 *ie;
+		if (os_memcmp(res->bssid, wpa_s->bssid, ETH_ALEN) != 0)
+			continue;
+
+		ie = wpa_scan_get_ie(res, WLAN_EID_SSID);
+		if (ie == NULL)
+			continue;
+		if (ie[1] != wpa_s->current_ssid->ssid_len ||
+		    os_memcmp(ie + 2, wpa_s->current_ssid->ssid, ie[1]) != 0)
+			continue;
+		current_bss = res;
+		break;
+	}
+
+	if (!current_bss)
+		return 1; /* current BSS not seen in scan results */
+
+	wpa_printf(MSG_DEBUG, "Considering within-ESS reassociation");
+	wpa_printf(MSG_DEBUG, "Current BSS: " MACSTR " level=%d",
+		   MAC2STR(current_bss->bssid), current_bss->level);
+	wpa_printf(MSG_DEBUG, "Selected BSS: " MACSTR " level=%d",
+		   MAC2STR(selected->bssid), selected->level);
+
+	min_diff = 2;
+	if (current_bss->level < 0) {
+		if (current_bss->level < -85)
+			min_diff = 1;
+		else if (current_bss->level < -80)
+			min_diff = 2;
+		else if (current_bss->level < -75)
+			min_diff = 3;
+		else if (current_bss->level < -70)
+			min_diff = 4;
+		else
+			min_diff = 5;
+	}
+	if (abs(current_bss->level - selected->level) < min_diff) {
+		wpa_printf(MSG_DEBUG, "Skip roam - too small difference in "
+			   "signal level");
+		return 0;
+	}
+
+	return 1;
+}
+
+
 static void wpa_supplicant_event_scan_results(struct wpa_supplicant *wpa_s,
 					      union wpa_event_data *data)
 {
@@ -832,11 +898,17 @@ static void wpa_supplicant_event_scan_results(struct wpa_supplicant *wpa_s,
 	wpa_supplicant_rsn_preauth_scan_results(wpa_s, scan_res);
 
 	selected = wpa_supplicant_pick_network(wpa_s, scan_res, &ssid);
-	wpa_scan_results_free(scan_res);
 
 	if (selected) {
+		int skip;
+		skip = !wpa_supplicant_need_to_roam(wpa_s, selected, ssid,
+						    scan_res);
+		wpa_scan_results_free(scan_res);
+		if (skip)
+			return;
 		wpa_supplicant_connect(wpa_s, selected, ssid);
 	} else {
+		wpa_scan_results_free(scan_res);
 		wpa_printf(MSG_DEBUG, "No suitable network found");
 		ssid = wpa_supplicant_pick_new_network(wpa_s);
 		if (ssid) {