Added a WallClockTimestamp and associated interface/implementation for generating it. This will be used by the initial implementation of distributed host store
diff --git a/core/api/src/main/java/org/onlab/onos/net/host/HostClockService.java b/core/api/src/main/java/org/onlab/onos/net/host/HostClockService.java
new file mode 100644
index 0000000..49ef8f7
--- /dev/null
+++ b/core/api/src/main/java/org/onlab/onos/net/host/HostClockService.java
@@ -0,0 +1,17 @@
+package org.onlab.onos.net.host;
+
+import org.onlab.onos.store.Timestamp;
+import org.onlab.packet.MacAddress;
+
+/**
+ * Interface for a logical clock service that issues per host timestamps.
+ */
+public interface HostClockService {
+
+ /**
+ * Returns a new timestamp for the specified host mac address.
+ * @param hostMac host MAC address.
+ * @return timestamp.
+ */
+ public Timestamp getTimestamp(MacAddress hostMac);
+}