diff --git a/nsync/src/sync-seq-no.h b/nsync/src/sync-seq-no.h
new file mode 100644
index 0000000..bf21704
--- /dev/null
+++ b/nsync/src/sync-seq-no.h
@@ -0,0 +1,198 @@
+/* -*- Mode: C++; c-file-style: "gnu"; indent-tabs-mode:nil -*- */
+/*
+ * Copyright (c) 2012 University of California, Los Angeles
+ *
+ * This program is free software; you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License version 2 as
+ * published by the Free Software Foundation;
+ *
+ * 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, write to the Free Software
+ * Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA  02111-1307  USA
+ *
+ * Author: Zhenkai Zhu <zhenkai@cs.ucla.edu>
+ *         Chaoyi Bian <bcy@pku.edu.cn>
+ *	   Alexander Afanasyev <alexander.afanasyev@ucla.edu>
+ *         Yingdi Yu <yingdi@cs.ucla.edu>
+ */
+
+#ifndef SYNC_SEQ_NO_H
+#define SYNC_SEQ_NO_H
+
+#include <boost/cstdint.hpp>
+#include "sync-digest.h"
+
+namespace Sync {
+
+/**
+ * @ingroup sync
+ * @brief Sequence number abstraction
+ */
+class SeqNo
+{
+public:
+  /**
+   * @brief Default constructor.  Creates an zero sequence number with zero session ID (basically is an invalid object)
+   */
+  SeqNo ()
+    : m_valid (false)
+    , m_session (0)
+    , m_seq (0)
+  {
+  }
+  
+  /**
+   * @brief Copy constructor
+   * @param seq sequence number object to copy from
+   */
+  SeqNo (const SeqNo &seq)
+  {
+    *this = seq;
+  }
+
+  /**
+   * @brief Assignment operator
+   * @param seq sequence number object to copy from
+   */
+  SeqNo &
+  operator = (const SeqNo &seq)
+  {
+    m_valid = seq.m_valid;
+    m_session = seq.m_session;
+    m_seq = seq.m_seq;
+
+    return *this;
+  }
+
+  /**
+   * @brief Constructor with just sequence number. Session assumed to be zero
+   * @param seq Sequence number
+   */
+  SeqNo (uint64_t seq)
+    : m_valid (true)
+    , m_session (0)
+    , m_seq (seq)
+  { }
+
+  /**
+   * @brief Constructor with session and sequence id
+   * @param session Session ID
+   * @param seq Sequence number
+   */
+  SeqNo (uint64_t session, uint64_t seq)
+    : m_valid (true)
+    , m_session (session)
+    , m_seq (seq)
+  { }
+
+  /**
+   * @brief Get sequence number digest
+   *
+   * Digest will be calculated every time it is requested
+   */
+  DigestConstPtr
+  getDigest () const;
+
+  /**
+   * @brief Compare if one sequence number is lower
+   * @param seq Another sequence number to compare with
+   *
+   * tuple (session1, seq1) is less than (session2, seq2) in two cases:
+   * 1. session1 < session2
+   * 2. session1 == session2 and seq1 < seq2
+   */
+  bool
+  operator < (const SeqNo &seq) const
+  {
+    return m_session < seq.m_session || (m_session == seq.m_session && m_seq < seq.m_seq);
+  }
+
+  /**
+   * @brief Compare if two sequence numbers are equal
+   * @param seq Another sequence number to compare with
+   */
+  bool
+  operator == (const SeqNo &seq) const
+  {
+    return m_session == seq.m_session && m_seq == seq.m_seq;
+  }
+
+  bool
+  operator <= (const SeqNo &seq) const
+  {
+    return m_session == seq.m_session && m_seq <= seq.m_seq;
+  }
+
+  SeqNo &
+  operator ++ ()
+  {
+    if (m_valid) {
+      m_seq ++;
+    }
+    else {
+      m_valid = true;
+    }
+    return *this;
+  }
+
+  bool
+  isValid () const
+  {
+    return m_valid;
+  }
+  
+  /**
+   * @brief Get session id
+   */
+  uint64_t getSession () const
+  { return m_session; }
+
+  /**
+   * @brief Get sequence number
+   */
+  uint64_t getSeq () const
+  { return m_seq; }
+
+  /**
+   * @brief Set sequence number
+   */
+   void
+   setSeq(uint64_t seq)
+   { m_seq = seq; }
+  
+private:
+  bool m_valid;
+  
+  /**
+   * @brief Session ID (e.g., after crash, application will choose new session ID.
+   *
+   * Note that session IDs for the same name should always increase. So, the good choice
+   * for the session ID is client's timestamp
+   */
+  uint64_t m_session;
+
+  /**
+   * @brief Sequence number
+   *
+   * Sequence number for a session always starts with 0 and goes to max value.
+   *
+   * For now, wrapping sequence number after max to zero is not supported
+   */
+  uint64_t m_seq;
+};
+
+inline std::ostream &
+operator << (std::ostream &os, const SeqNo &seqno)
+{
+  os << "<session>" << seqno.getSession () << "</session><seqno>" << seqno.getSeq () << "</seqno>";
+  return os;
+}
+
+} // Sync
+
+#endif // SYNC_SEQ_NO_H