Starting to experiment with flow tracking.
diff --git a/core/api/src/main/java/org/onlab/onos/net/LinkKey.java b/core/api/src/main/java/org/onlab/onos/net/LinkKey.java
index dee4e88..d3ff0f4 100644
--- a/core/api/src/main/java/org/onlab/onos/net/LinkKey.java
+++ b/core/api/src/main/java/org/onlab/onos/net/LinkKey.java
@@ -6,6 +6,7 @@
 
 // TODO Consider renaming.
 // it's an identifier for a Link, but it's not ElementId, so not using LinkId.
+
 /**
  * Immutable representation of a link identity.
  */
@@ -43,6 +44,15 @@
         this.dst = dst;
     }
 
+    /**
+     * Creates a link identifier for the specified link.
+     *
+     * @param link link descriptor
+     */
+    public LinkKey(Link link) {
+        this(link.src(), link.dst());
+    }
+
     @Override
     public int hashCode() {
         return Objects.hash(src(), dst);