Construct GraphDescription with online/active Device/Link only

Change-Id: I9312c0c8ae190bf0200bd040823b6f7a3e7a15e1
diff --git a/core/api/src/main/java/org/onlab/onos/net/link/LinkService.java b/core/api/src/main/java/org/onlab/onos/net/link/LinkService.java
index 2039aa5..58b01bf 100644
--- a/core/api/src/main/java/org/onlab/onos/net/link/LinkService.java
+++ b/core/api/src/main/java/org/onlab/onos/net/link/LinkService.java
@@ -41,6 +41,13 @@
     Iterable<Link> getLinks();
 
     /**
+     * Returns a collection of all active infrastructure links.
+     *
+     * @return all infrastructure links
+     */
+    Iterable<Link> getActiveLinks();
+
+    /**
      * Returns set of all infrastructure links leading to and from the
      * specified device.
      *