Adding model abstractions to help flesh-out the concepts.
diff --git a/net/api/src/main/java/org/onlab/onos/net/topology/TopologyListener.java b/net/api/src/main/java/org/onlab/onos/net/topology/TopologyListener.java
new file mode 100644
index 0000000..bcc2cf3
--- /dev/null
+++ b/net/api/src/main/java/org/onlab/onos/net/topology/TopologyListener.java
@@ -0,0 +1,9 @@
+package org.onlab.onos.net.topology;
+
+import org.onlab.onos.event.EventListener;
+
+/**
+ * Entity capable of receiving network topology related events.
+ */
+public interface TopologyListener extends EventListener<TopologyEvent> {
+}