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diff --git a/src/lib/dhcpsrv/allocation_state.h b/src/lib/dhcpsrv/allocation_state.h
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+// Copyright (C) 2022-2023 Internet Systems Consortium, Inc. ("ISC")
+//
+// This Source Code Form is subject to the terms of the Mozilla Public
+// License, v. 2.0. If a copy of the MPL was not distributed with this
+// file, You can obtain one at http://mozilla.org/MPL/2.0/.
+
+#ifndef ALLOCATION_STATE_H
+#define ALLOCATION_STATE_H
+
+#include <dhcpsrv/lease.h>
+#include <boost/scoped_ptr.hpp>
+#include <boost/shared_ptr.hpp>
+#include <boost/date_time/posix_time/posix_time.hpp>
+#include <mutex>
+
+namespace isc {
+namespace dhcp {
+
+/// @brief Base class for representing allocation state in pools and subnets.
+///
+/// Allocators are used in Kea to implement different lease selection
+/// algorithms. They are stateful (i.e., they remember various information
+/// about the previous allocations) to work efficiently. For example, an
+/// iterative allocator must remember the last allocated address to pick
+/// the consecutive address when the new allocation request is issued.
+/// Allocation states differ between the allocators. State classes used
+/// by different allocators derive from this class.
+///
+/// The allocation states can be associated with pools and/or subnets.
+/// Both pool-specific and subnet-specific states derive from this class.
+class AllocationState {
+public:
+
+    /// @brief Virtual destructor.
+    virtual ~AllocationState() = default;
+};
+
+/// @brief Type of the pointer to the @c AllocationState.
+typedef boost::shared_ptr<AllocationState> AllocationStatePtr;
+
+/// @brief Common base class for subnet-specific allocation states.
+///
+/// All subnet-specific allocation states should derive from this class.
+/// It provides a mutex for thread-safe access to the class members.
+/// It maintains last allocation times for various lease types. These
+/// times are used by the shared networks to find the "preferred" subnet
+/// (i.e., a subnet from which the latest lease was assigned).
+class SubnetAllocationState : public AllocationState {
+public:
+
+    /// @brief Constructor.
+    ///
+    /// Initializes the mutex.
+    SubnetAllocationState();
+
+    /// @brief Returns last allocation time for the specified lease type.
+    ///
+    /// @return Last allocation time for the lease type or
+    /// @c boost::posix_time::neg_infin when no leases have been allocated
+    /// from this subnet yet.
+    boost::posix_time::ptime
+    getLastAllocatedTime() const;
+
+protected:
+
+    /// @brief Sets the last allocation time to current.
+    ///
+    /// This function should be called by derived classes. It should be
+    /// called in the thread-safe context.
+    void setCurrentAllocatedTimeInternal();
+
+    /// @brief Mutex used for thread-safe access to the state members.
+    boost::scoped_ptr<std::mutex> mutex_;
+
+    /// @brief Timestamp indicating when a lease has been last allocated
+    /// from the subnet.
+    boost::posix_time::ptime last_allocated_time_;
+};
+
+typedef boost::shared_ptr<SubnetAllocationState> SubnetAllocationStatePtr;
+
+} // end of namespace isc::dhcp
+} // end of namespace isc
+
+#endif // ALLOCATION_STATE_H