diff --git a/src/leaf.hpp b/src/leaf.hpp
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+/* -*- Mode: C++; c-file-style: "gnu"; indent-tabs-mode:nil -*- */
+/*
+ * Copyright (c) 2012-2014 University of California, Los Angeles
+ *
+ * This file is part of ChronoSync, synchronization library for distributed realtime
+ * applications for NDN.
+ *
+ * ChronoSync 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, either
+ * version 3 of the License, or (at your option) any later version.
+ *
+ * ChronoSync 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
+ * ChronoSync, e.g., in COPYING.md file.  If not, see <http://www.gnu.org/licenses/>.
+ *
+ * @author Zhenkai Zhu <http://irl.cs.ucla.edu/~zhenkai/>
+ * @author Chaoyi Bian <bcy@pku.edu.cn>
+ * @author Alexander Afanasyev <http://lasr.cs.ucla.edu/afanasyev/index.html>
+ * @author Yingdi Yu <yingdi@cs.ucla.edu>
+ */
+
+#ifndef CHRONOSYNC_LEAF_HPP
+#define CHRONOSYNC_LEAF_HPP
+
+#include "common.hpp"
+#include <ndn-cxx/util/digest.hpp>
+
+namespace chronosync {
+
+typedef uint64_t SeqNo;
+
+/**
+ * @brief Sync tree leaf
+ *
+ * The leaf node should be copyable when used to construct diff between two states.
+ */
+class Leaf
+{
+public:
+  Leaf(const Name& sessionName, const SeqNo& seq);
+
+  Leaf(const Name& userPrefix, uint64_t session, const SeqNo& seq);
+
+  virtual
+  ~Leaf();
+
+  const Name&
+  getSessionName() const
+  {
+    return m_sessionName;
+  }
+
+  const SeqNo&
+  getSeq() const
+  {
+    return m_seq;
+  }
+
+  ndn::ConstBufferPtr
+  getDigest() const;
+
+  /**
+   * @brief Update sequence number of the leaf
+   * @param seq Sequence number
+   *
+   * If seq is no greater than getSeq(), this operation has no effect.
+   */
+  virtual void
+  setSeq(const SeqNo& seq);
+
+private:
+  void
+  updateDigest();
+
+private:
+  Name     m_sessionName;
+  SeqNo    m_seq;
+
+  mutable ndn::util::Sha256 m_digest;
+};
+
+typedef boost::shared_ptr<Leaf> LeafPtr;
+typedef boost::shared_ptr<const Leaf> ConstLeafPtr;
+
+std::ostream&
+operator<<(std::ostream& os, const Leaf& leaf);
+
+} // namespace chronosync
+
+#endif // CHRONOSYNC_LEAF_HPP