ONOS-6464 interfaces for protocol-independence (PI)

And also ONOS-6553, implementation of table entry classes

Change-Id: Ia7146b1bb0aac794e19b3fd8150c042e7ec70535
diff --git a/core/api/src/main/java/org/onosproject/net/pi/model/PiHeaderFieldModel.java b/core/api/src/main/java/org/onosproject/net/pi/model/PiHeaderFieldModel.java
new file mode 100644
index 0000000..c52a71d
--- /dev/null
+++ b/core/api/src/main/java/org/onosproject/net/pi/model/PiHeaderFieldModel.java
@@ -0,0 +1,39 @@
+/*
+ * Copyright 2017-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.onosproject.net.pi.model;
+
+import com.google.common.annotations.Beta;
+
+/**
+ * Model of a header's field instance in a protocol-independent pipeline.
+ */
+@Beta
+public interface PiHeaderFieldModel {
+    /**
+     * Returns the header instance of this field instance.
+     *
+     * @return a header instance
+     */
+    PiHeaderModel header();
+
+    /**
+     * Returns the type of this header's field instance.
+     *
+     * @return a field type value
+     */
+    PiHeaderFieldTypeModel type();
+}