diff --git a/src/main/java/org/opencord/cordvtn/api/Constants.java b/src/main/java/org/opencord/cordvtn/api/Constants.java
new file mode 100644
index 0000000..b4c7329
--- /dev/null
+++ b/src/main/java/org/opencord/cordvtn/api/Constants.java
@@ -0,0 +1,37 @@
+/*
+ * Copyright 2016-present Open Networking Laboratory
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package org.opencord.cordvtn.api;
+
+/**
+ * Provides constants used in CORD VTN services.
+ */
+public final class Constants {
+
+    private Constants() {
+    }
+
+    public static final String CORDVTN_APP_ID = "org.opencord.vtn";
+
+    public static final String ERROR_XOS_ACCESS = "XOS access is not configured";
+    public static final String ERROR_OPENSTACK_ACCESS = "OpenStack access is not configured";
+    public static final String MSG_OK = "OK";
+    public static final String MSG_NO = "NO";
+
+    public static final String PORT_NAME = "portName";
+    public static final String DEFAULT_TUNNEL = "vxlan";
+    public static final String DEFAULT_BRIDGE = "br-int";
+    public static final String VPORT_PREFIX = "tap";
+}
diff --git a/src/main/java/org/opencord/cordvtn/api/CordVtnService.java b/src/main/java/org/opencord/cordvtn/api/DependencyService.java
similarity index 76%
rename from src/main/java/org/opencord/cordvtn/api/CordVtnService.java
rename to src/main/java/org/opencord/cordvtn/api/DependencyService.java
index 16d26f7..48905da 100644
--- a/src/main/java/org/opencord/cordvtn/api/CordVtnService.java
+++ b/src/main/java/org/opencord/cordvtn/api/DependencyService.java
@@ -18,11 +18,9 @@
 import org.onosproject.xosclient.api.VtnServiceId;
 
 /**
- * Service for provisioning overlay virtual networks on compute nodes.
+ * Provides dependency services.
  */
-public interface CordVtnService {
-
-    String CORDVTN_APP_ID = "org.opencord.vtn";
+public interface DependencyService {
 
     /**
      * Creates dependencies for a given tenant service.
@@ -31,8 +29,7 @@
      * @param pServiceId id of the service which provide dependency
      * @param isBidirectional true to enable bidirectional connectivity between two services
      */
-    void createServiceDependency(VtnServiceId tServiceId, VtnServiceId pServiceId,
-                                 boolean isBidirectional);
+    void createDependency(VtnServiceId tServiceId, VtnServiceId pServiceId, boolean isBidirectional);
 
     /**
      * Removes all dependencies from a given tenant service.
@@ -40,5 +37,5 @@
      * @param tServiceId id of the service which has a dependency
      * @param pServiceId id of the service which provide dependency
      */
-    void removeServiceDependency(VtnServiceId tServiceId, VtnServiceId pServiceId);
+    void removeDependency(VtnServiceId tServiceId, VtnServiceId pServiceId);
 }
diff --git a/src/main/java/org/opencord/cordvtn/api/InstanceService.java b/src/main/java/org/opencord/cordvtn/api/InstanceService.java
new file mode 100644
index 0000000..a8e0f77
--- /dev/null
+++ b/src/main/java/org/opencord/cordvtn/api/InstanceService.java
@@ -0,0 +1,57 @@
+/*
+ * Copyright 2016-present Open Networking Laboratory
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package org.opencord.cordvtn.api;
+
+import org.onosproject.net.ConnectPoint;
+import org.onosproject.net.HostId;
+import org.onosproject.net.host.HostDescription;
+
+/**
+ * Provides service instance addition or removal.
+ */
+public interface InstanceService {
+
+    /**
+     * Adds a service instance on a given connect point.
+     *
+     * @param connectPoint connect point of the instance
+     */
+    void addInstance(ConnectPoint connectPoint);
+
+    /**
+     * Removes a service instance from a given connect point.
+     *
+     * @param connectPoint connect point
+     */
+    void removeInstance(ConnectPoint connectPoint);
+
+    /**
+     * Adds a nested instance with given host ID and host description.
+     * Nested instance can be a container inside a virtual machine, for example.
+     * DHCP is not supported for the nested instance.
+     *
+     * @param hostId host id
+     * @param description host description
+     */
+    void addNestedInstance(HostId hostId, HostDescription description);
+
+    /**
+     * Removes nested instance with a given host ID.
+     *
+     * @param hostId host id
+     */
+    void removeNestedInstance(HostId hostId);
+}