diff --git a/include/libtorrent/alert_types.hpp b/include/libtorrent/alert_types.hpp
index 02f599a67..77144313f 100644
--- a/include/libtorrent/alert_types.hpp
+++ b/include/libtorrent/alert_types.hpp
@@ -1401,7 +1401,7 @@ namespace libtorrent {
 	struct TORRENT_EXPORT portmap_error_alert final : alert
 	{
 		// internal
-		portmap_error_alert(aux::stack_allocator& alloc, int i
+		portmap_error_alert(aux::stack_allocator& alloc, port_mapping_t i
 			, portmap_transport t
 			, error_code const& e);
 
@@ -1413,7 +1413,7 @@ namespace libtorrent {
 
 		// refers to the mapping index of the port map that failed, i.e.
 		// the index returned from add_mapping().
-		int const mapping;
+		port_mapping_t const mapping;
 
 		// UPnP or NAT-PMP
 		portmap_transport map_transport;
@@ -1435,7 +1435,7 @@ namespace libtorrent {
 	struct TORRENT_EXPORT portmap_alert final : alert
 	{
 		// internal
-		portmap_alert(aux::stack_allocator& alloc, int i, int port
+		portmap_alert(aux::stack_allocator& alloc, port_mapping_t i, int port
 			, portmap_transport t, portmap_protocol protocol);
 
 		TORRENT_DEFINE_ALERT(portmap_alert, 51)
@@ -1445,7 +1445,7 @@ namespace libtorrent {
 
 		// refers to the mapping index of the port map that failed, i.e.
 		// the index returned from add_mapping().
-		int const mapping;
+		port_mapping_t const mapping;
 
 		// the external port allocated for the mapping.
 		int const external_port;
diff --git a/include/libtorrent/aux_/portmap.hpp b/include/libtorrent/aux_/portmap.hpp
index d76c023d3..b9442ac55 100644
--- a/include/libtorrent/aux_/portmap.hpp
+++ b/include/libtorrent/aux_/portmap.hpp
@@ -52,7 +52,7 @@ namespace aux {
 		// int: protocol (UDP, TCP)
 		// error_code: error, an empty error means success
 		// int: transport is 0 for NAT-PMP and 1 for UPnP
-		virtual void on_port_mapping(int mapping, address const& ip, int port
+		virtual void on_port_mapping(port_mapping_t mapping, address const& ip, int port
 			, portmap_protocol proto, error_code const& ec, portmap_transport transport) = 0;
 #ifndef TORRENT_DISABLE_LOGGING
 		virtual bool should_log_portmap(portmap_transport transport) const = 0;
diff --git a/include/libtorrent/aux_/session_impl.hpp b/include/libtorrent/aux_/session_impl.hpp
index ac6212e48..978cb5195 100644
--- a/include/libtorrent/aux_/session_impl.hpp
+++ b/include/libtorrent/aux_/session_impl.hpp
@@ -137,10 +137,10 @@ namespace aux {
 	{
 		listen_socket_t()
 		{
-			tcp_port_mapping[0] = -1;
-			tcp_port_mapping[1] = -1;
-			udp_port_mapping[0] = -1;
-			udp_port_mapping[1] = -1;
+			tcp_port_mapping[0] = port_mapping_t{-1};
+			tcp_port_mapping[1] = port_mapping_t{-1};
+			udp_port_mapping[0] = port_mapping_t{-1};
+			udp_port_mapping[1] = port_mapping_t{-1};
 		}
 
 		// listen_socket_t should not be copied or moved because
@@ -180,8 +180,8 @@ namespace aux {
 		int udp_external_port = 0;
 
 		// 0 is natpmp 1 is upnp
-		int tcp_port_mapping[2];
-		int udp_port_mapping[2];
+		port_mapping_t tcp_port_mapping[2];
+		port_mapping_t udp_port_mapping[2];
 
 		// indicates whether this is an SSL listen socket or not
 		transport ssl = transport::plaintext;
@@ -433,7 +433,7 @@ namespace aux {
 
 			// called when a port mapping is successful, or a router returns
 			// a failure to map a port
-			void on_port_mapping(int mapping, address const& ip, int port
+			void on_port_mapping(port_mapping_t mapping, address const& ip, int port
 				, portmap_protocol proto, error_code const& ec
 				, portmap_transport transport) override;
 
@@ -633,9 +633,9 @@ namespace aux {
 			void stop_natpmp();
 			void stop_upnp();
 
-			int add_port_mapping(portmap_protocol t, int external_port
+			port_mapping_t add_port_mapping(portmap_protocol t, int external_port
 				, int local_port);
-			void delete_port_mapping(int handle);
+			void delete_port_mapping(port_mapping_t handle);
 
 			int next_port() const;
 
diff --git a/include/libtorrent/natpmp.hpp b/include/libtorrent/natpmp.hpp
index 1d44e972b..a0d9ccbb1 100644
--- a/include/libtorrent/natpmp.hpp
+++ b/include/libtorrent/natpmp.hpp
@@ -55,9 +55,9 @@ struct TORRENT_EXTRA_EXPORT natpmp
 
 	// maps the ports, if a port is set to 0
 	// it will not be mapped
-	int add_mapping(portmap_protocol p, int external_port, tcp::endpoint local_ep);
-	void delete_mapping(int mapping_index);
-	bool get_mapping(int mapping_index, int& local_port, int& external_port
+	port_mapping_t add_mapping(portmap_protocol p, int external_port, tcp::endpoint local_ep);
+	void delete_mapping(port_mapping_t mapping_index);
+	bool get_mapping(port_mapping_t mapping_index, int& local_port, int& external_port
 		, portmap_protocol& protocol) const;
 
 	void close();
@@ -66,15 +66,15 @@ private:
 
 	std::shared_ptr<natpmp> self() { return shared_from_this(); }
 
-	void update_mapping(int i);
-	void send_map_request(int i);
+	void update_mapping(port_mapping_t i);
+	void send_map_request(port_mapping_t i);
 	void send_get_ip_address_request();
-	void resend_request(int i, error_code const& e);
+	void resend_request(port_mapping_t i, error_code const& e);
 	void on_reply(error_code const& e
 		, std::size_t bytes_transferred);
-	void try_next_mapping(int i);
+	void try_next_mapping(port_mapping_t i);
 	void update_expiration_timer();
-	void mapping_expired(error_code const& e, int i);
+	void mapping_expired(error_code const& e, port_mapping_t i);
 	void close_impl();
 
 	void disable(error_code const& ec);
@@ -100,7 +100,7 @@ private:
 
 	aux::portmap_callback& m_callback;
 
-	aux::vector<mapping_t> m_mappings;
+	aux::vector<mapping_t, port_mapping_t> m_mappings;
 
 	// the endpoint to the nat router
 	udp::endpoint m_nat_endpoint;
@@ -108,7 +108,7 @@ private:
 	// this is the mapping that is currently
 	// being updated. It is -1 in case no
 	// mapping is being updated at the moment
-	int m_currently_mapping = -1;
+	port_mapping_t m_currently_mapping{-1};
 
 	// current retry count
 	int m_retry_count = 0;
@@ -134,7 +134,7 @@ private:
 	deadline_timer m_refresh_timer;
 
 	// the mapping index that will expire next
-	int m_next_refresh = -1;
+	port_mapping_t m_next_refresh{-1};
 
 	bool m_disabled = false;
 
diff --git a/include/libtorrent/portmap.hpp b/include/libtorrent/portmap.hpp
index 30fd5536b..3462fa177 100644
--- a/include/libtorrent/portmap.hpp
+++ b/include/libtorrent/portmap.hpp
@@ -34,8 +34,12 @@ POSSIBILITY OF SUCH DAMAGE.
 #define TORRENT_PORTMAP_HPP_INCLUDED
 
 #include "libtorrent/config.hpp"
+#include "libtorrent/units.hpp"
 
 namespace libtorrent {
+namespace aux {
+	struct port_mapping_tag;
+}
 
 	enum class portmap_transport : std::uint8_t
 	{
@@ -47,6 +51,9 @@ namespace libtorrent {
 		none, tcp, udp
 	};
 
+	// this type represents an index referring to a port mapping
+	using port_mapping_t = aux::strong_typedef<int, aux::port_mapping_tag>;
+
 }
 
 #endif  //TORRENT_PORTMAP_HPP_INCLUDED
diff --git a/include/libtorrent/session_handle.hpp b/include/libtorrent/session_handle.hpp
index 35cf54e85..f0db1c766 100644
--- a/include/libtorrent/session_handle.hpp
+++ b/include/libtorrent/session_handle.hpp
@@ -1004,8 +1004,8 @@ namespace libtorrent {
 		// whichever is enabled. The return value is a handle referring to the
 		// port mapping that was just created. Pass it to delete_port_mapping()
 		// to remove it.
-		int add_port_mapping(portmap_protocol t, int external_port, int local_port);
-		void delete_port_mapping(int handle);
+		port_mapping_t add_port_mapping(portmap_protocol t, int external_port, int local_port);
+		void delete_port_mapping(port_mapping_t handle);
 
 		// This function is intended only for use by plugins. This type does
 		// not have a stable API and should be relied on as little as possible.
diff --git a/include/libtorrent/units.hpp b/include/libtorrent/units.hpp
index 5e94fc866..594e44917 100644
--- a/include/libtorrent/units.hpp
+++ b/include/libtorrent/units.hpp
@@ -43,7 +43,7 @@ POSSIBILITY OF SUCH DAMAGE.
 
 namespace libtorrent { namespace aux {
 	template <typename Tag>
-	struct difference_tag {};
+	struct difference_tag;
 
 	template<typename UnderlyingType, typename Tag
 		, typename Cond = typename std::enable_if<std::is_integral<UnderlyingType>::value>::type>
diff --git a/include/libtorrent/upnp.hpp b/include/libtorrent/upnp.hpp
index 547679188..2dce71a88 100644
--- a/include/libtorrent/upnp.hpp
+++ b/include/libtorrent/upnp.hpp
@@ -172,13 +172,13 @@ struct TORRENT_EXTRA_EXPORT upnp final
 	// portmap_alert_ respectively. If The mapping fails immediately, the return value
 	// is -1, which means failure. There will not be any error alert notification for
 	// mappings that fail with a -1 return value.
-	int add_mapping(portmap_protocol p, int external_port, tcp::endpoint local_ep);
+	port_mapping_t add_mapping(portmap_protocol p, int external_port, tcp::endpoint local_ep);
 
 	// This function removes a port mapping. ``mapping_index`` is the index that refers
 	// to the mapping you want to remove, which was returned from add_mapping().
-	void delete_mapping(int mapping_index);
+	void delete_mapping(port_mapping_t mapping_index);
 
-	bool get_mapping(int mapping_index, tcp::endpoint& local_ep, int& external_port
+	bool get_mapping(port_mapping_t mapping_index, tcp::endpoint& local_ep, int& external_port
 		, portmap_protocol& protocol) const;
 
 	void discover_device();
@@ -207,8 +207,8 @@ private:
 		, std::size_t bytes_transferred);
 
 	struct rootdevice;
-	void next(rootdevice& d, int i);
-	void update_map(rootdevice& d, int i);
+	void next(rootdevice& d, port_mapping_t i);
+	void update_map(rootdevice& d, port_mapping_t i);
 
 	void on_upnp_xml(error_code const& e
 		, libtorrent::http_parser const& p, rootdevice& d
@@ -218,22 +218,22 @@ private:
 		, http_connection& c);
 	void on_upnp_map_response(error_code const& e
 		, libtorrent::http_parser const& p, rootdevice& d
-		, int mapping, http_connection& c);
+		, port_mapping_t mapping, http_connection& c);
 	void on_upnp_unmap_response(error_code const& e
 		, libtorrent::http_parser const& p, rootdevice& d
-		, int mapping, http_connection& c);
+		, port_mapping_t mapping, http_connection& c);
 	void on_expire(error_code const& e);
 
 	void disable(error_code const& ec);
-	void return_error(int mapping, int code);
+	void return_error(port_mapping_t mapping, int code);
 #ifndef TORRENT_DISABLE_LOGGING
 	bool should_log() const;
 	void log(char const* msg, ...) const TORRENT_FORMAT(2,3);
 #endif
 
 	void get_ip_address(rootdevice& d);
-	void delete_port_mapping(rootdevice& d, int i);
-	void create_port_mapping(http_connection& c, rootdevice& d, int i);
+	void delete_port_mapping(rootdevice& d, port_mapping_t i);
+	void create_port_mapping(http_connection& c, rootdevice& d, port_mapping_t i);
 	void post(upnp::rootdevice const& d, char const* soap
 		, char const* soap_action);
 
@@ -278,7 +278,7 @@ private:
 		// either the WANIP namespace or the WANPPP namespace
 		std::string service_namespace;
 
-		aux::vector<mapping_t> mapping;
+		aux::vector<mapping_t, port_mapping_t> mapping;
 
 		// this is the hostname, port and path
 		// component of the url or the control_url
@@ -326,11 +326,11 @@ private:
 
 	struct upnp_state_t
 	{
-		std::vector<global_mapping_t> mappings;
+		aux::vector<global_mapping_t, port_mapping_t> mappings;
 		std::set<rootdevice> devices;
 	};
 
-	aux::vector<global_mapping_t> m_mappings;
+	aux::vector<global_mapping_t, port_mapping_t> m_mappings;
 
 	std::string m_user_agent;
 
diff --git a/src/alert.cpp b/src/alert.cpp
index 109762249..85658cbf9 100644
--- a/src/alert.cpp
+++ b/src/alert.cpp
@@ -1072,7 +1072,7 @@ namespace {
 	}
 
 	portmap_error_alert::portmap_error_alert(aux::stack_allocator&
-		, int i, portmap_transport const t, error_code const& e)
+		, port_mapping_t const i, portmap_transport const t, error_code const& e)
 		: mapping(i)
 		, map_transport(t)
 		, error(e)
@@ -1089,7 +1089,8 @@ namespace {
 			+ ": " + convert_from_native(error.message());
 	}
 
-	portmap_alert::portmap_alert(aux::stack_allocator&, int i, int port
+	portmap_alert::portmap_alert(aux::stack_allocator&, port_mapping_t const i
+		, int port
 		, portmap_transport const t
 		, portmap_protocol const proto)
 		: mapping(i)
diff --git a/src/natpmp.cpp b/src/natpmp.cpp
index 0d0eef2fe..e5bd99394 100644
--- a/src/natpmp.cpp
+++ b/src/natpmp.cpp
@@ -134,7 +134,7 @@ void natpmp::start()
 			|| i->act != portmap_action::none)
 			continue;
 		i->act = portmap_action::add;
-		update_mapping(int(i - m_mappings.begin()));
+		update_mapping(port_mapping_t(int(i - m_mappings.begin())));
 	}
 }
 
@@ -155,13 +155,13 @@ void natpmp::send_get_ip_address_request()
 	m_socket.send_to(boost::asio::buffer(buf, sizeof(buf)), m_nat_endpoint, 0, ec);
 }
 
-bool natpmp::get_mapping(int const index, int& local_port
+bool natpmp::get_mapping(port_mapping_t const index, int& local_port
 	, int& external_port, portmap_protocol& protocol) const
 {
 	TORRENT_ASSERT(is_single_thread());
 
-	TORRENT_ASSERT(index < int(m_mappings.size()) && index >= 0);
-	if (index >= int(m_mappings.size()) || index < 0) return false;
+	TORRENT_ASSERT(index < m_mappings.end_index() && index >= port_mapping_t{});
+	if (index >= m_mappings.end_index() || index < port_mapping_t{}) return false;
 	mapping_t const& m = m_mappings[index];
 	if (m.protocol == portmap_protocol::none) return false;
 	local_port = m.local_port;
@@ -216,19 +216,19 @@ void natpmp::disable(error_code const& ec)
 		if (i->protocol == portmap_protocol::none) continue;
 		portmap_protocol const proto = i->protocol;
 		i->protocol = portmap_protocol::none;
-		int const index = int(i - m_mappings.begin());
+		port_mapping_t const index(static_cast<int>(i - m_mappings.begin()));
 		m_callback.on_port_mapping(index, address(), 0, proto, ec
 			, portmap_transport::natpmp);
 	}
 	close_impl();
 }
 
-void natpmp::delete_mapping(int const index)
+void natpmp::delete_mapping(port_mapping_t const index)
 {
 	TORRENT_ASSERT(is_single_thread());
 
-	TORRENT_ASSERT(index < int(m_mappings.size()) && index >= 0);
-	if (index >= int(m_mappings.size()) || index < 0) return;
+	TORRENT_ASSERT(index < m_mappings.end_index() && index >= port_mapping_t{});
+	if (index >= m_mappings.end_index() || index < port_mapping_t{}) return;
 	mapping_t& m = m_mappings[index];
 
 	if (m.protocol == portmap_protocol::none) return;
@@ -243,12 +243,12 @@ void natpmp::delete_mapping(int const index)
 	update_mapping(index);
 }
 
-int natpmp::add_mapping(portmap_protocol const p, int const external_port
+port_mapping_t natpmp::add_mapping(portmap_protocol const p, int const external_port
 	, tcp::endpoint const local_ep)
 {
 	TORRENT_ASSERT(is_single_thread());
 
-	if (m_disabled) return -1;
+	if (m_disabled) return port_mapping_t{-1};
 
 	auto i = std::find_if(m_mappings.begin()
 		, m_mappings.end(), [] (mapping_t const& m) { return m.protocol == portmap_protocol::none; });
@@ -262,21 +262,21 @@ int natpmp::add_mapping(portmap_protocol const p, int const external_port
 	i->local_port = local_ep.port();
 	i->act = portmap_action::add;
 
-	int const mapping_index = int(i - m_mappings.begin());
+	port_mapping_t const mapping_index(static_cast<int>(i - m_mappings.begin()));
 #ifndef TORRENT_DISABLE_LOGGING
 	mapping_log("add",*i);
 #endif
 
 	update_mapping(mapping_index);
-	return mapping_index;
+	return port_mapping_t{mapping_index};
 }
 
-void natpmp::try_next_mapping(int const i)
+void natpmp::try_next_mapping(port_mapping_t const i)
 {
 	TORRENT_ASSERT(is_single_thread());
-	if (i < int(m_mappings.size()) - 1)
+	if (i < prev(m_mappings.end_index()))
 	{
-		update_mapping(i + 1);
+		update_mapping(next(i));
 		return;
 	}
 
@@ -296,13 +296,13 @@ void natpmp::try_next_mapping(int const i)
 		return;
 	}
 
-	update_mapping(int(m - m_mappings.begin()));
+	update_mapping(port_mapping_t(static_cast<int>(m - m_mappings.begin())));
 }
 
-void natpmp::update_mapping(int const i)
+void natpmp::update_mapping(port_mapping_t const i)
 {
 	TORRENT_ASSERT(is_single_thread());
-	if (i == int(m_mappings.size()))
+	if (i == m_mappings.end_index())
 	{
 		if (m_abort)
 		{
@@ -326,7 +326,7 @@ void natpmp::update_mapping(int const i)
 		return;
 	}
 
-	if (m_currently_mapping == -1)
+	if (m_currently_mapping == port_mapping_t{-1})
 	{
 		// the socket is not currently in use
 		// send out a mapping request
@@ -335,12 +335,12 @@ void natpmp::update_mapping(int const i)
 	}
 }
 
-void natpmp::send_map_request(int const i)
+void natpmp::send_map_request(port_mapping_t const i)
 {
 	TORRENT_ASSERT(is_single_thread());
 	using namespace libtorrent::detail;
 
-	TORRENT_ASSERT(m_currently_mapping == -1
+	TORRENT_ASSERT(m_currently_mapping == port_mapping_t{-1}
 		|| m_currently_mapping == i);
 	m_currently_mapping = i;
 	mapping_t& m = m_mappings[i];
@@ -360,7 +360,7 @@ void natpmp::send_map_request(int const i)
 	{
 		log("==> port map [ mapping: %d action: %s"
 			" proto: %s local: %u external: %u ttl: %u ]"
-			, i, to_string(m.act)
+			, static_cast<int>(i), to_string(m.act)
 			, to_string(m.protocol)
 			, m.local_port, m.external_port, ttl);
 	}
@@ -375,7 +375,7 @@ void natpmp::send_map_request(int const i)
 		// when we're shutting down, ignore the
 		// responses and just remove all mappings
 		// immediately
-		m_currently_mapping = -1;
+		m_currently_mapping = port_mapping_t{-1};
 		m.act = portmap_action::none;
 		try_next_mapping(i);
 	}
@@ -389,7 +389,7 @@ void natpmp::send_map_request(int const i)
 	}
 }
 
-void natpmp::resend_request(int const i, error_code const& e)
+void natpmp::resend_request(port_mapping_t const i, error_code const& e)
 {
 	TORRENT_ASSERT(is_single_thread());
 	COMPLETE_ASYNC("natpmp::resend_request");
@@ -400,7 +400,7 @@ void natpmp::resend_request(int const i, error_code const& e)
 	// to the next mapping
 	if (m_retry_count >= 9 || m_abort)
 	{
-		m_currently_mapping = -1;
+		m_currently_mapping = port_mapping_t{-1};
 		m_mappings[i].act = portmap_action::none;
 		// try again in two hours
 		m_mappings[i].expires = aux::time_now() + hours(2);
@@ -518,7 +518,7 @@ void natpmp::on_reply(error_code const& e
 #endif
 
 	mapping_t* m = nullptr;
-	int index = -1;
+	port_mapping_t index{-1};
 	for (auto i = m_mappings.begin(), end(m_mappings.end()); i != end; ++i)
 	{
 		if (private_port != i->local_port) continue;
@@ -526,7 +526,7 @@ void natpmp::on_reply(error_code const& e
 		if (!i->map_sent) continue;
 		if (!i->outstanding_request) continue;
 		m = &*i;
-		index = int(i - m_mappings.begin());
+		index = port_mapping_t(static_cast<int>(i - m_mappings.begin()));
 		break;
 	}
 
@@ -572,19 +572,19 @@ void natpmp::on_reply(error_code const& e
 
 		m->expires = aux::time_now() + hours(2);
 		portmap_protocol const proto = m->protocol;
-		m_callback.on_port_mapping(index, address(), 0, proto
+		m_callback.on_port_mapping(port_mapping_t{index}, address(), 0, proto
 			, ev, portmap_transport::natpmp);
 	}
 	else if (m->act == portmap_action::add)
 	{
 		portmap_protocol const proto = m->protocol;
-		m_callback.on_port_mapping(index, m_external_ip, m->external_port, proto
+		m_callback.on_port_mapping(port_mapping_t{index}, m_external_ip, m->external_port, proto
 			, errors::no_error, portmap_transport::natpmp);
 	}
 
 	if (m_abort) return;
 
-	m_currently_mapping = -1;
+	m_currently_mapping = port_mapping_t{-1};
 	m->act = portmap_action::none;
 	m_send_timer.cancel(ec);
 	update_expiration_timer();
@@ -598,20 +598,20 @@ void natpmp::update_expiration_timer()
 
 	time_point const now = aux::time_now() + milliseconds(100);
 	time_point min_expire = now + seconds(3600);
-	int min_index = -1;
+	port_mapping_t min_index{-1};
 	for (std::vector<mapping_t>::iterator i = m_mappings.begin()
 		, end(m_mappings.end()); i != end; ++i)
 	{
 		if (i->protocol == portmap_protocol::none
 			|| i->act != portmap_action::none) continue;
-		int const index = int(i - m_mappings.begin());
+		port_mapping_t const index(static_cast<int>(i - m_mappings.begin()));
 		if (i->expires < now)
 		{
 #ifndef TORRENT_DISABLE_LOGGING
-			log("mapping %u expired", index);
+			log("mapping %u expired", static_cast<int>(index));
 #endif
 			i->act = portmap_action::add;
-			if (m_next_refresh == index) m_next_refresh = -1;
+			if (m_next_refresh == index) m_next_refresh = port_mapping_t{-1};
 			update_mapping(index);
 		}
 		else if (i->expires < min_expire)
@@ -624,14 +624,14 @@ void natpmp::update_expiration_timer()
 	// this is already the mapping we're waiting for
 	if (m_next_refresh == min_index) return;
 
-	if (min_index >= 0)
+	if (min_index >= port_mapping_t{})
 	{
 #ifndef TORRENT_DISABLE_LOGGING
 		log("next expiration [ idx: %d ttl: %" PRId64 " ]"
-			, min_index, total_seconds(min_expire - aux::time_now()));
+			, static_cast<int>(min_index), total_seconds(min_expire - aux::time_now()));
 #endif
 		error_code ec;
-		if (m_next_refresh >= 0) m_refresh_timer.cancel(ec);
+		if (m_next_refresh >= port_mapping_t{}) m_refresh_timer.cancel(ec);
 
 		ADD_OUTSTANDING_ASYNC("natpmp::mapping_expired");
 		m_refresh_timer.expires_from_now(min_expire - now, ec);
@@ -640,16 +640,16 @@ void natpmp::update_expiration_timer()
 	}
 }
 
-void natpmp::mapping_expired(error_code const& e, int const i)
+void natpmp::mapping_expired(error_code const& e, port_mapping_t const i)
 {
 	TORRENT_ASSERT(is_single_thread());
 	COMPLETE_ASYNC("natpmp::mapping_expired");
 	if (e) return;
 #ifndef TORRENT_DISABLE_LOGGING
-	log("mapping %u expired", i);
+	log("mapping %u expired", static_cast<int>(i));
 #endif
 	m_mappings[i].act = portmap_action::add;
-	if (m_next_refresh == i) m_next_refresh = -1;
+	if (m_next_refresh == i) m_next_refresh = port_mapping_t{-1};
 	update_mapping(i);
 }
 
@@ -674,8 +674,8 @@ void natpmp::close_impl()
 	}
 	error_code ec;
 	m_refresh_timer.cancel(ec);
-	m_currently_mapping = -1;
-	update_mapping(0);
+	m_currently_mapping = port_mapping_t{-1};
+	update_mapping(port_mapping_t{});
 }
 
 } // namespace libtorrent
diff --git a/src/session_handle.cpp b/src/session_handle.cpp
index 3ef8a8b2e..ed4278a27 100644
--- a/src/session_handle.cpp
+++ b/src/session_handle.cpp
@@ -1201,13 +1201,13 @@ namespace {
 	}
 #endif // TORRENT_NO_DEPRECATE
 
-	int session_handle::add_port_mapping(portmap_protocol const t
+	port_mapping_t session_handle::add_port_mapping(portmap_protocol const t
 		, int external_port, int local_port)
 	{
-		return sync_call_ret<int>(&session_impl::add_port_mapping, t, external_port, local_port);
+		return sync_call_ret<port_mapping_t>(&session_impl::add_port_mapping, t, external_port, local_port);
 	}
 
-	void session_handle::delete_port_mapping(int handle)
+	void session_handle::delete_port_mapping(port_mapping_t handle)
 	{
 		async_call(&session_impl::delete_port_mapping, handle);
 	}
diff --git a/src/session_impl.cpp b/src/session_impl.cpp
index 0c037bcdd..b4bda03ca 100644
--- a/src/session_impl.cpp
+++ b/src/session_impl.cpp
@@ -2167,10 +2167,10 @@ namespace {
 	namespace {
 		template <typename MapProtocol, typename ProtoType, typename EndpointType>
 		void map_port(MapProtocol& m, ProtoType protocol, EndpointType const& ep
-			, int& map_handle)
+			, port_mapping_t& map_handle)
 		{
-			if (map_handle != -1) m.delete_mapping(map_handle);
-			map_handle = -1;
+			if (map_handle != port_mapping_t{-1}) m.delete_mapping(map_handle);
+			map_handle = port_mapping_t{-1};
 
 #if TORRENT_USE_IPV6
 			address const addr = ep.address();
@@ -5423,19 +5423,20 @@ namespace {
 
 	namespace {
 		bool find_tcp_port_mapping(portmap_transport const transport
-			, int mapping, std::shared_ptr<listen_socket_t> const& ls)
+			, port_mapping_t mapping, std::shared_ptr<listen_socket_t> const& ls)
 		{
 			return ls->tcp_port_mapping[static_cast<int>(transport)] == mapping;
 		}
 
 		bool find_udp_port_mapping(portmap_transport const transport
-			, int mapping, std::shared_ptr<listen_socket_t> const& ls)
+			, port_mapping_t mapping, std::shared_ptr<listen_socket_t> const& ls)
 		{
 			return ls->udp_port_mapping[static_cast<int>(transport)] == mapping;
 		}
 	}
 
-	void session_impl::on_port_mapping(int mapping, address const& ip, int port
+	void session_impl::on_port_mapping(port_mapping_t const mapping
+		, address const& ip, int port
 		, portmap_protocol const proto, error_code const& ec
 		, portmap_transport const transport)
 	{
@@ -6601,11 +6602,11 @@ namespace {
 		return m_upnp.get();
 	}
 
-	int session_impl::add_port_mapping(portmap_protocol const t
+	port_mapping_t session_impl::add_port_mapping(portmap_protocol const t
 		, int const external_port
 		, int const local_port)
 	{
-		int ret = 0;
+		port_mapping_t ret{-1};
 		if (m_upnp) ret = m_upnp->add_mapping(t, external_port
 			, tcp::endpoint({}, static_cast<std::uint16_t>(local_port)));
 		if (m_natpmp) ret = m_natpmp->add_mapping(t, external_port
@@ -6613,7 +6614,7 @@ namespace {
 		return ret;
 	}
 
-	void session_impl::delete_port_mapping(int handle)
+	void session_impl::delete_port_mapping(port_mapping_t handle)
 	{
 		if (m_upnp) m_upnp->delete_mapping(handle);
 		if (m_natpmp) m_natpmp->delete_mapping(handle);
@@ -6633,8 +6634,8 @@ namespace {
 		m_natpmp->close();
 		for (auto& s : m_listen_sockets)
 		{
-			s->tcp_port_mapping[0] = -1;
-			s->udp_port_mapping[0] = -1;
+			s->tcp_port_mapping[0] = port_mapping_t{-1};
+			s->udp_port_mapping[0] = port_mapping_t{-1};
 		}
 
 		m_natpmp.reset();
@@ -6647,8 +6648,8 @@ namespace {
 		m_upnp->close();
 		for (auto& s : m_listen_sockets)
 		{
-			s->tcp_port_mapping[1] = -1;
-			s->udp_port_mapping[1] = -1;
+			s->tcp_port_mapping[1] = port_mapping_t{-1};
+			s->udp_port_mapping[1] = port_mapping_t{-1};
 		}
 		m_upnp.reset();
 	}
diff --git a/src/upnp.cpp b/src/upnp.cpp
index 11a7d5f6b..b5a4095c4 100644
--- a/src/upnp.cpp
+++ b/src/upnp.cpp
@@ -181,7 +181,7 @@ void upnp::discover_device_impl()
 }
 
 // returns a reference to a mapping or -1 on failure
-int upnp::add_mapping(portmap_protocol const p, int const external_port
+port_mapping_t upnp::add_mapping(portmap_protocol const p, int const external_port
 	, tcp::endpoint const local_ep)
 {
 	TORRENT_ASSERT(is_single_thread());
@@ -194,7 +194,7 @@ int upnp::add_mapping(portmap_protocol const p, int const external_port
 		, external_port
 		, print_endpoint(local_ep).c_str(), m_disabled ? "DISABLED": "");
 #endif
-	if (m_disabled) return -1;
+	if (m_disabled) return port_mapping_t{-1};
 
 	auto mapping_it = std::find_if(m_mappings.begin(), m_mappings.end()
 		, [](global_mapping_t const& m) { return m.protocol == portmap_protocol::none; });
@@ -209,7 +209,7 @@ int upnp::add_mapping(portmap_protocol const p, int const external_port
 	mapping_it->external_port = external_port;
 	mapping_it->local_ep = local_ep;
 
-	int const mapping_index = int(mapping_it - m_mappings.begin());
+	port_mapping_t const mapping_index{static_cast<int>(mapping_it - m_mappings.begin())};
 
 	for (auto const& dev : m_devices)
 	{
@@ -217,7 +217,7 @@ int upnp::add_mapping(portmap_protocol const p, int const external_port
 		TORRENT_ASSERT(d.magic == 1337);
 
 		if (d.mapping.end_index() <= mapping_index)
-			d.mapping.resize(mapping_index + 1);
+			d.mapping.resize(static_cast<int>(mapping_index) + 1);
 		mapping_t& m = d.mapping[mapping_index];
 
 		m.act = portmap_action::add;
@@ -228,14 +228,14 @@ int upnp::add_mapping(portmap_protocol const p, int const external_port
 		if (!d.service_namespace.empty()) update_map(d, mapping_index);
 	}
 
-	return mapping_index;
+	return port_mapping_t{mapping_index};
 }
 
-void upnp::delete_mapping(int const mapping)
+void upnp::delete_mapping(port_mapping_t const mapping)
 {
 	TORRENT_ASSERT(is_single_thread());
 
-	if (mapping >= int(m_mappings.size())) return;
+	if (mapping >= m_mappings.end_index()) return;
 
 	global_mapping_t const& m = m_mappings[mapping];
 
@@ -259,14 +259,14 @@ void upnp::delete_mapping(int const mapping)
 	}
 }
 
-bool upnp::get_mapping(int const index
+bool upnp::get_mapping(port_mapping_t const index
 	, tcp::endpoint& local_ep
 	, int& external_port
 	, portmap_protocol& protocol) const
 {
 	TORRENT_ASSERT(is_single_thread());
-	TORRENT_ASSERT(index < int(m_mappings.size()) && index >= 0);
-	if (index >= int(m_mappings.size()) || index < 0) return false;
+	TORRENT_ASSERT(index < m_mappings.end_index() && index >= port_mapping_t{0});
+	if (index >= m_mappings.end_index() || index < port_mapping_t{0}) return false;
 	global_mapping_t const& m = m_mappings[index];
 	if (m.protocol == portmap_protocol::none) return false;
 	local_ep = m.local_ep;
@@ -706,7 +706,8 @@ void upnp::post(upnp::rootdevice const& d, char const* soap
 #endif
 }
 
-void upnp::create_port_mapping(http_connection& c, rootdevice& d, int const i)
+void upnp::create_port_mapping(http_connection& c, rootdevice& d
+	, port_mapping_t const i)
 {
 	TORRENT_ASSERT(is_single_thread());
 
@@ -716,7 +717,7 @@ void upnp::create_port_mapping(http_connection& c, rootdevice& d, int const i)
 	{
 		TORRENT_ASSERT(d.disabled);
 #ifndef TORRENT_DISABLE_LOGGING
-		log("mapping %u aborted", i);
+		log("mapping %u aborted", static_cast<int>(i));
 #endif
 		return;
 	}
@@ -750,12 +751,12 @@ void upnp::create_port_mapping(http_connection& c, rootdevice& d, int const i)
 	post(d, soap, soap_action);
 }
 
-void upnp::next(rootdevice& d, int const i)
+void upnp::next(rootdevice& d, port_mapping_t const i)
 {
 	TORRENT_ASSERT(is_single_thread());
-	if (i < num_mappings() - 1)
+	if (i < prev(m_mappings.end_index()))
 	{
-		update_map(d, i + 1);
+		update_map(d, lt::next(i));
 	}
 	else
 	{
@@ -763,11 +764,11 @@ void upnp::next(rootdevice& d, int const i)
 			, [](mapping_t const& m) { return m.act != portmap_action::none; });
 		if (j == d.mapping.end()) return;
 
-		update_map(d, int(j - d.mapping.begin()));
+		update_map(d, port_mapping_t{static_cast<int>(j - d.mapping.begin())});
 	}
 }
 
-void upnp::update_map(rootdevice& d, int const i)
+void upnp::update_map(rootdevice& d, port_mapping_t const i)
 {
 	TORRENT_ASSERT(is_single_thread());
 	TORRENT_ASSERT(d.magic == 1337);
@@ -787,7 +788,7 @@ void upnp::update_map(rootdevice& d, int const i)
 		|| m.protocol == portmap_protocol::none)
 	{
 #ifndef TORRENT_DISABLE_LOGGING
-		log("mapping %u does not need updating, skipping", i);
+		log("mapping %u does not need updating, skipping", static_cast<int>(i));
 #endif
 		m.act = portmap_action::none;
 		next(d, i);
@@ -835,7 +836,7 @@ void upnp::update_map(rootdevice& d, int const i)
 	m.act = portmap_action::none;
 }
 
-void upnp::delete_port_mapping(rootdevice& d, int const i)
+void upnp::delete_port_mapping(rootdevice& d, port_mapping_t const i)
 {
 	TORRENT_ASSERT(is_single_thread());
 
@@ -845,7 +846,7 @@ void upnp::delete_port_mapping(rootdevice& d, int const i)
 	{
 		TORRENT_ASSERT(d.disabled);
 #ifndef TORRENT_DISABLE_LOGGING
-		log("unmapping %u aborted", i);
+		log("unmapping %u aborted", static_cast<int>(i));
 #endif
 		return;
 	}
@@ -1082,7 +1083,7 @@ void upnp::disable(error_code const& ec)
 		if (i->protocol == portmap_protocol::none) continue;
 		portmap_protocol const proto = i->protocol;
 		i->protocol = portmap_protocol::none;
-		m_callback.on_port_mapping(int(i - m_mappings.begin()), address(), 0, proto, ec
+		m_callback.on_port_mapping(port_mapping_t(static_cast<int>(i - m_mappings.begin())), address(), 0, proto, ec
 			, portmap_transport::upnp);
 	}
 
@@ -1217,7 +1218,7 @@ void upnp::on_upnp_get_ip_address_response(error_code const& e
 				, convert_from_native(e.message()).c_str());
 		}
 #endif
-		if (num_mappings() > 0) update_map(d, 0);
+		if (num_mappings() > 0) update_map(d, port_mapping_t{0});
 		return;
 	}
 
@@ -1226,7 +1227,7 @@ void upnp::on_upnp_get_ip_address_response(error_code const& e
 #ifndef TORRENT_DISABLE_LOGGING
 		log("error while getting external IP address: incomplete http message");
 #endif
-		if (num_mappings() > 0) update_map(d, 0);
+		if (num_mappings() > 0) update_map(d, port_mapping_t{0});
 		return;
 	}
 
@@ -1239,7 +1240,7 @@ void upnp::on_upnp_get_ip_address_response(error_code const& e
 				, convert_from_native(p.message()).c_str());
 		}
 #endif
-		if (num_mappings() > 0) update_map(d, 0);
+		if (num_mappings() > 0) update_map(d, port_mapping_t{0});
 		return;
 	}
 
@@ -1284,11 +1285,11 @@ void upnp::on_upnp_get_ip_address_response(error_code const& e
 #endif
 	}
 
-	if (num_mappings() > 0) update_map(d, 0);
+	if (num_mappings() > 0) update_map(d, port_mapping_t{0});
 }
 
 void upnp::on_upnp_map_response(error_code const& e
-	, libtorrent::http_parser const& p, rootdevice& d, int const mapping
+	, libtorrent::http_parser const& p, rootdevice& d, port_mapping_t const mapping
 	, http_connection& c)
 {
 	TORRENT_ASSERT(is_single_thread());
@@ -1440,7 +1441,7 @@ void upnp::on_upnp_map_response(error_code const& e
 	next(d, mapping);
 }
 
-void upnp::return_error(int const mapping, int const code)
+void upnp::return_error(port_mapping_t const mapping, int const code)
 {
 	TORRENT_ASSERT(is_single_thread());
 	int num_errors = sizeof(error_codes) / sizeof(error_codes[0]);
@@ -1463,7 +1464,8 @@ void upnp::return_error(int const mapping, int const code)
 }
 
 void upnp::on_upnp_unmap_response(error_code const& e
-	, libtorrent::http_parser const& p, rootdevice& d, int const mapping
+	, libtorrent::http_parser const& p, rootdevice& d
+	, port_mapping_t const mapping
 	, http_connection& c)
 {
 	TORRENT_ASSERT(is_single_thread());
@@ -1546,7 +1548,7 @@ void upnp::on_expire(error_code const& ec)
 	{
 		rootdevice& d = const_cast<rootdevice&>(*i);
 		TORRENT_ASSERT(d.magic == 1337);
-		for (int m = 0; m < num_mappings(); ++m)
+		for (port_mapping_t m{0}; m < m_mappings.end_index(); ++m)
 		{
 			if (d.mapping[m].expires != max_time())
 				continue;
@@ -1596,9 +1598,9 @@ void upnp::close()
 				continue;
 			}
 			j->act = portmap_action::del;
-			m_mappings[int(j - d.mapping.begin())].protocol = portmap_protocol::none;
+			m_mappings[port_mapping_t{static_cast<int>(j - d.mapping.begin())}].protocol = portmap_protocol::none;
 		}
-		if (num_mappings() > 0) update_map(d, 0);
+		if (num_mappings() > 0) update_map(d, port_mapping_t{0});
 	}
 }
 
diff --git a/test/test_natpmp.cpp b/test/test_natpmp.cpp
index 16a8e2c2d..41e3564dd 100644
--- a/test/test_natpmp.cpp
+++ b/test/test_natpmp.cpp
@@ -43,7 +43,8 @@ namespace
 {
 	struct natpmp_callback : aux::portmap_callback
 	{
-		void on_port_mapping(int mapping, address const& ip, int port
+		void on_port_mapping(port_mapping_t const mapping
+			, address const& ip, int port
 			, portmap_protocol const protocol, error_code const& err
 			, portmap_transport const transport) override
 		{
@@ -84,7 +85,7 @@ int main(int argc, char* argv[])
 
 	deadline_timer timer(ios);
 
-	int const tcp_map = natpmp_handler->add_mapping(portmap_protocol::tcp
+	auto const tcp_map = natpmp_handler->add_mapping(portmap_protocol::tcp
 		, atoi(argv[1]), tcp::endpoint({}, atoi(argv[1])));
 	natpmp_handler->add_mapping(portmap_protocol::udp, atoi(argv[2])
 		, tcp::endpoint({}, atoi(argv[2])));
diff --git a/test/test_upnp.cpp b/test/test_upnp.cpp
index e488a733f..cbfd7dabc 100644
--- a/test/test_upnp.cpp
+++ b/test/test_upnp.cpp
@@ -106,7 +106,7 @@ void incoming_msearch(udp::endpoint const& from, char* buffer
 
 struct callback_info
 {
-	int mapping;
+	port_mapping_t mapping;
 	int port;
 	error_code ec;
 	bool operator==(callback_info const& e)
@@ -119,13 +119,15 @@ namespace
 {
 	struct upnp_callback : aux::portmap_callback
 	{
-		void on_port_mapping(int mapping, address const& ip, int port
+		void on_port_mapping(port_mapping_t const mapping
+			, address const& ip, int port
 			, portmap_protocol const protocol, error_code const& err
 			, portmap_transport const transport) override
 		{
 			callback_info info = {mapping, port, err};
 			callbacks.push_back(info);
-			std::cout << "mapping: " << mapping << ", port: " << port << ", IP: " << ip
+			std::cout << "mapping: " << static_cast<int>(mapping)
+				<< ", port: " << port << ", IP: " << ip
 				<< ", proto: " << static_cast<int>(protocol)
 				<< ", error: \"" << err.message() << "\"\n";
 		}
@@ -198,8 +200,8 @@ void run_upnp_test(char const* root_filename, char const* router_model, char con
 	std::cout << "router: " << upnp_handler->router_model() << std::endl;
 	TEST_EQUAL(upnp_handler->router_model(), router_model);
 
-	int const mapping1 = upnp_handler->add_mapping(portmap_protocol::tcp, 500, ep("127.0.0.1", 500));
-	int const mapping2 = upnp_handler->add_mapping(portmap_protocol::udp, 501, ep("127.0.0.1", 501));
+	auto const mapping1 = upnp_handler->add_mapping(portmap_protocol::tcp, 500, ep("127.0.0.1", 500));
+	auto const mapping2 = upnp_handler->add_mapping(portmap_protocol::udp, 501, ep("127.0.0.1", 501));
 
 	for (int i = 0; i < 40; ++i)
 	{