packet lib: add packet_utils.py

Add a helper function to calculate the checksum.

Signed-off-by: FUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>

ryu/lib/packet/packet_utils.py

@@ -0,0 +1,27 @@
+# Copyright (C) 2012 Nippon Telegraph and Telephone Corporation.
+#
+# 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.
+
+
+def carry_around_add(a, b):
+    c = a + b
+    return (c & 0xffff) + (c >> 16)
+
+
+def checksum(data):
+    s = 0
+    for i in range(0, len(data), 2):
+        w = data[i] + (data[i + 1] << 8)
+        s = carry_around_add(s, w)
+    return ~s & 0xffff