Working on model annotations; still in progress.

+package org.onlab.onos.net;
+
+import com.google.common.collect.ImmutableMap;
+
+import java.util.Map;
+import java.util.Objects;
+import java.util.Set;
+
+/**
+ * Represents a set of simple annotations that can be used to add arbitrary
+ * attributes to various parts of the data model.
+ */
+public final class Annotations {
+
+    private final Map<String, String> map;
+
+    /**
+     * Creates a new set of annotations using the specified immutable map.
+     *
+     * @param map immutable map of key/value pairs
+     */
+    private Annotations(ImmutableMap<String, String> map) {
+        this.map = map;
+    }
+
+    /**
+     * Creates a new annotations builder.
+     *
+     * @return new annotations builder
+     */
+    public static Builder builder() {
+        return new Builder();
+    }
+
+    /**
+     * Returns the set of keys for available annotations. Note that this set
+     * includes keys for any attributes tagged for removal.
+     *
+     * @return annotation keys
+     */
+    public Set<String> keys() {
+        return map.keySet();
+    }
+
+    /**
+     * Returns the value of the specified annotation.
+     *
+     * @param key annotation key
+     * @return annotation value
+     */
+    public String value(String key) {
+        String value = map.get(key);
+        return Objects.equals(Builder.REMOVED, value) ? null : value;
+    }
+
+    /**
+     * Indicates whether the specified key has been tagged as removed. This is
+     * used to for merging sparse annotation sets.
+     *
+     * @param key annotation key
+     * @return true if the previous annotation has been tagged for removal
+     */
+    public boolean isRemoved(String key) {
+        return Objects.equals(Builder.REMOVED, map.get(key));
+    }
+
+    /**
+     * Facility for gradually building model annotations.
+     */
+    public static final class Builder {
+
+        private static final String REMOVED = "~rEmOvEd~";
+        private final ImmutableMap.Builder<String, String> builder = ImmutableMap.builder();
+
+        // Private construction is forbidden.
+        private Builder() {
+        }
+
+        /**
+         * Adds the specified annotation. Any previous value associated with
+         * the given annotation key will be overwritten.
+         *
+         * @param key   annotation key
+         * @param value annotation value
+         * @return self
+         */
+        public Builder set(String key, String value) {
+            builder.put(key, value);
+            return this;
+        }
+
+        /**
+         * Adds the specified annotation. Any previous value associated with
+         * the given annotation key will be tagged for removal.
+         *
+         * @param key annotation key
+         * @return self
+         */
+        public Builder remove(String key) {
+            builder.put(key, REMOVED);
+            return this;
+        }
+
+        /**
+         * Returns immutable annotations built from the accrued key/values pairs.
+         *
+         * @return annotations
+         */
+        public Annotations build() {
+            return new Annotations(builder.build());
+        }
+    }
+}

@@ -2,12 +2,13 @@ package org.onlab.onos.net;
 
 import static com.google.common.base.MoreObjects.toStringHelper;
 
+import java.util.Map;
 import java.util.Objects;
 
 /**
  * Default port implementation.
  */
-public class DefaultPort implements Port {
+public class DefaultPort extends AbstractAnnotated implements Port {
 
     private final Element element;
     private final PortNumber number;
@@ -19,9 +20,13 @@ public class DefaultPort implements Port {
      * @param element     parent network element
      * @param number      port number
      * @param isEnabled   indicator whether the port is up and active
+     * @param annotations optional key/value annotations
      */
+    @SafeVarargs
     public DefaultPort(Element element, PortNumber number,
-                       boolean isEnabled) {
+                       boolean isEnabled,
+                       Map<String, String>... annotations) {
+        super(annotations);
         this.element = element;
         this.number = number;
         this.isEnabled = isEnabled;

@@ -3,7 +3,7 @@ package org.onlab.onos.net;
 /**
  * Base abstraction of a network element, i.e. an infrastructure device or an end-station host.
  */
-public interface Element extends Provided {
+public interface Element extends Annotated, Provided {
 
     /**
      * Returns the network element identifier.

@@ -3,7 +3,7 @@ package org.onlab.onos.net;
 /**
  * Abstraction of a network infrastructure link.
  */
-public interface Link extends Provided {
+public interface Link extends Annotated, Provided {
 
     /**
      * Coarse representation of the link type.

@@ -4,7 +4,7 @@ package org.onlab.onos.net;
 /**
  * Abstraction of a network port.
  */
-public interface Port {
+public interface Port extends Annotated {
 
     /**
      * Returns the port number.