fixes related to nat port mapping and having dht and bittorrent listen on the same port number

docs/manual.html

@@ -566,6 +566,21 @@ if it managed to open the socket, and false if it failed. If it fails, it will a
 generate an appropriate alert (<a class="reference" href="#listen-failed-alert">listen_failed_alert</a>).</p>
 <p>The interface parameter can also be a hostname that will resolve to the device you
 want to listen on.</p>
+<p>If you're also starting the DHT, it is a good idea to do that after you've called
+<tt class="docutils literal"><span class="pre">listen_on()</span></tt>, since the default listen port for the DHT is the same as the tcp
+listen socket. If you start the DHT first, it will assume the tcp port is free and
+open the udp socket on that port, then later, when <tt class="docutils literal"><span class="pre">listen_on()</span></tt> is called, it
+may turn out that the tcp port is in use. That results in the DHT and the bittorrent
+socket listening on different ports. If the DHT is active when <tt class="docutils literal"><span class="pre">listen_on</span></tt> is
+called, the udp port will be rebound to the new port, if it was configured to use
+the same port as the tcp socket, and if the listen_on call failed to bind to the
+same port that the udp uses.</p>
+<p>The reason why it's a good idea to run the DHT and the bittorrent socket on the same
+port is because that is an assumption that may be used to increase performance. One
+way to accelerate the connecting of peers on windows may be to first ping all peers
+with a DHT ping packet, and connect to those that responds first. On windows one
+can only connect to a few peers at a time because of a built in limitation (in XP
+Service pack 2).</p>
 </div>
 <div class="section">
 <h2><a id="pop-alert-set-severity-level" name="pop-alert-set-severity-level">pop_alert() set_severity_level()</a></h2>

docs/manual.rst

@@ -436,6 +436,22 @@ generate an appropriate alert (listen_failed_alert_).
 The interface parameter can also be a hostname that will resolve to the device you
 want to listen on.
 
+If you're also starting the DHT, it is a good idea to do that after you've called
+``listen_on()``, since the default listen port for the DHT is the same as the tcp
+listen socket. If you start the DHT first, it will assume the tcp port is free and
+open the udp socket on that port, then later, when ``listen_on()`` is called, it
+may turn out that the tcp port is in use. That results in the DHT and the bittorrent
+socket listening on different ports. If the DHT is active when ``listen_on`` is
+called, the udp port will be rebound to the new port, if it was configured to use
+the same port as the tcp socket, and if the listen_on call failed to bind to the
+same port that the udp uses.
+
+The reason why it's a good idea to run the DHT and the bittorrent socket on the same
+port is because that is an assumption that may be used to increase performance. One
+way to accelerate the connecting of peers on windows may be to first ping all peers
+with a DHT ping packet, and connect to those that responds first. On windows one
+can only connect to a few peers at a time because of a built in limitation (in XP
+Service pack 2).
 
 pop_alert() set_severity_level()
 --------------------------------

examples/client_test.cpp

@@ -590,6 +590,22 @@ int main(int ac, char* av[])
 		ses.add_extension(&create_metadata_plugin);
 		ses.add_extension(&create_ut_pex_plugin);
 
+		ses.set_max_uploads(upload_slots_limit);
+		ses.set_max_half_open_connections(half_open_limit);
+		ses.set_download_rate_limit(download_limit);
+		ses.set_upload_rate_limit(upload_limit);
+		ses.listen_on(std::make_pair(listen_port, listen_port + 10)
+			, bind_to_interface.c_str());
+		ses.set_settings(settings);
+		if (log_level == "debug")
+			ses.set_severity_level(alert::debug);
+		else if (log_level == "warning")
+			ses.set_severity_level(alert::warning);
+		else if (log_level == "fatal")
+			ses.set_severity_level(alert::fatal);
+		else
+			ses.set_severity_level(alert::info);
+
 #ifndef TORRENT_DISABLE_DHT
 		settings.use_dht_as_fallback = false;
 
@@ -613,22 +629,6 @@ int main(int ac, char* av[])
 			, 6881));
 #endif
 
-		ses.set_max_uploads(upload_slots_limit);
-		ses.set_max_half_open_connections(half_open_limit);
-		ses.set_download_rate_limit(download_limit);
-		ses.set_upload_rate_limit(upload_limit);
-		ses.listen_on(std::make_pair(listen_port, listen_port + 10)
-			, bind_to_interface.c_str());
-		ses.set_settings(settings);
-		if (log_level == "debug")
-			ses.set_severity_level(alert::debug);
-		else if (log_level == "warning")
-			ses.set_severity_level(alert::warning);
-		else if (log_level == "fatal")
-			ses.set_severity_level(alert::fatal);
-		else
-			ses.set_severity_level(alert::info);
-
 		// look for ipfilter.dat
 		// poor man's parser
 		// reads emule ipfilter files.

src/session_impl.cpp

@@ -1450,27 +1450,27 @@ namespace libtorrent { namespace detail
 		if (m_listen_socket)
 			m_listen_socket.reset();
 			
+		bool new_listen_address = m_listen_interface.address() != new_interface.address();
+
+		m_incoming_connection = false;
+		m_listen_interface = new_interface;
+
+		open_listen_port();
+
 #ifndef TORRENT_DISABLE_DHT
-		if ((m_listen_interface.address() != new_interface.address()
-			|| m_dht_same_port)
-			&& m_dht)
+		if ((new_listen_address || m_dht_same_port) && m_dht)
 		{
 			if (m_dht_same_port)
 				m_dht_settings.service_port = new_interface.port();
 			// the listen interface changed, rebind the dht listen socket as well
 			m_dht->rebind(new_interface.address()
 				, m_dht_settings.service_port);
-			if (m_listen_interface.address() != new_interface.address())
+			if (new_listen_address)
 				m_natpmp.rebind(new_interface.address());
 			m_natpmp.set_mappings(0, m_dht_settings.service_port);
 		}
 #endif
 
-		m_incoming_connection = false;
-		m_listen_interface = new_interface;
-
-		open_listen_port();
-
 #if defined(TORRENT_VERBOSE_LOGGING) || defined(TORRENT_LOGGING)
 		m_logger = create_log("main_session", listen_port(), false);
 		using boost::posix_time::second_clock;
@@ -1578,6 +1578,14 @@ namespace libtorrent { namespace detail
 			|| m_dht_same_port)
 		{
 			m_dht_same_port = true;
+			// if you hit this assert you are trying to start the
+			// DHT with the same port as the tcp listen port
+			// (which is default) _before_ you have opened the
+			// tcp listen port (so there is no configured port to use)
+			// basically, make sure you call listen_on() before
+			// start_dht(). See documentation for listen_on() for
+			// more information.
+			assert(m_listen_interface.port() > 0);
 			m_dht_settings.service_port = m_listen_interface.port();
 		}
 		m_external_udp_port = m_dht_settings.service_port;