TrafficTreatment: add support for IP DSCP modifications

Allow modifying IP DSCP field in the headers.

Change-Id: Idd765f40f1baec810273536adcd56bf0e480217f
diff --git a/core/api/src/main/java/org/onosproject/net/flow/instructions/Instructions.java b/core/api/src/main/java/org/onosproject/net/flow/instructions/Instructions.java
index 33b7482..e0c58d3 100644
--- a/core/api/src/main/java/org/onosproject/net/flow/instructions/Instructions.java
+++ b/core/api/src/main/java/org/onosproject/net/flow/instructions/Instructions.java
@@ -485,6 +485,16 @@
     }
 
     /**
+     * Creates an IP DSCP modification.
+     *
+     * @param dscpValue the DSCP value to modify to
+     * @return a L3 modification
+     */
+    public static Instruction modIpDscp(byte dscpValue) {
+        return new L3ModificationInstruction.ModDscpInstruction(L3SubType.IP_DSCP, dscpValue);
+    }
+
+    /**
      * Creates an extension instruction.
      *
      * @param extension extension instruction