diff --git a/ryu/lib/track_instances.py b/ryu/lib/track_instances.py
new file mode 100644
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+++ b/ryu/lib/track_instances.py
@@ -0,0 +1,48 @@
+# Copyright (C) 2012 Isaku Yamahata <yamahata at valinux co jp>
+#
+# This program is free software: you can redistribute it and/or modify
+# it under the terms of the GNU General Public License as published by
+# the Free Software Foundation, version 3 of the License
+#
+# This program is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+# GNU General Public License for more details.
+#
+# You should have received a copy of the GNU General Public License
+# along with this program.  If not, see <http://www.gnu.org/licenses/>.
+
+# This is mixin class to track instances of a given class
+#
+# http://docs.python.org/faq/programming.html#how-do-i-get-a-list-of-all-instances-of-a-given-class
+# How do I get a list of all instances of a given class?
+# Python does not keep track of all instances of a class (or of a
+# built-in type). You can program the class's constructor to keep
+# track of all instances by keeping a list of weak references to each
+# instance.
+
+
+from collections import defaultdict
+
+# Although WeakSet can be used directory, WeakValueDictionary is used instead
+# for python 2.6 which is used by REHL6 because WeakSet is supported by
+# python 2.7+.
+# Correspondence:
+# wvd = WeakValueDictionary()           ws = WeakSet()
+# wvd[id(value)] = value                ws = value
+# wvd.values()                          ws: iterator
+
+from weakref import WeakValueDictionary
+
+
+class TrackInstances(object):
+    # weak reference is needed in order to make instances freeable by avoiding
+    # refrence count. Otherwise, instances will never be freed.
+    __refs__ = defaultdict(WeakValueDictionary)
+
+    def __init__(self):
+        self.__refs__[self.__class__][id(self)] = self
+
+    @classmethod
+    def all_instances(cls):
+        return cls.__refs__[cls].values()