Sketching more topology related stuff.
Added overview.html for Java API docs.
diff --git a/net/api/src/main/java/org/onlab/onos/net/topology/ClusterId.java b/net/api/src/main/java/org/onlab/onos/net/topology/ClusterId.java
new file mode 100644
index 0000000..af8f122
--- /dev/null
+++ b/net/api/src/main/java/org/onlab/onos/net/topology/ClusterId.java
@@ -0,0 +1,49 @@
+package org.onlab.onos.net.topology;
+
+import java.util.Objects;
+
+import static com.google.common.base.MoreObjects.toStringHelper;
+
+/**
+ * Representation of the topology cluster identity.
+ */
+public final class ClusterId {
+
+    private final int id;
+
+    // Public construction is prohibit
+    private ClusterId(int id) {
+        this.id = id;
+    }
+
+    /**
+     * Returns the cluster identifier, represented by the specified integer
+     * serial number.
+     *
+     * @param id integer serial number
+     * @return cluster identifier
+     */
+    public static ClusterId clusterId(int id) {
+        return new ClusterId(id);
+    }
+
+    @Override
+    public int hashCode() {
+        return Objects.hash(id);
+    }
+
+    @Override
+    public boolean equals(Object obj) {
+        if (obj instanceof ClusterId) {
+            final ClusterId other = (ClusterId) obj;
+            return Objects.equals(this.id, other.id);
+        }
+        return false;
+    }
+
+    @Override
+    public String toString() {
+        return toStringHelper(this).add("id", id).toString();
+    }
+
+}