Re-assembled the topology subsystem.
diff --git a/core/api/src/main/java/org/onlab/onos/net/topology/GraphDescription.java b/core/api/src/main/java/org/onlab/onos/net/topology/GraphDescription.java
new file mode 100644
index 0000000..2b846a0
--- /dev/null
+++ b/core/api/src/main/java/org/onlab/onos/net/topology/GraphDescription.java
@@ -0,0 +1,34 @@
+package org.onlab.onos.net.topology;
+
+import com.google.common.collect.ImmutableSet;
+import org.onlab.onos.net.Description;
+
+/**
+ * Describes attribute(s) of a network graph.
+ */
+public interface GraphDescription extends Description {
+
+ /**
+ * Returns the creation timestamp of the graph description. This is
+ * expressed in system nanos to allow proper sequencing.
+ *
+ * @return graph description creation timestamp
+ */
+ long timestamp();
+
+ /**
+ * Returns the set of topology graph vertexes.
+ *
+ * @return set of graph vertexes
+ */
+ ImmutableSet<TopologyVertex> vertexes();
+
+ /**
+ * Returns the set of topology graph edges.
+ *
+ * @return set of graph edges
+ */
+ ImmutableSet<TopologyEdge> edges();
+
+}
+