disable support for large MTUs in uTP until it is more reliable

ChangeLog

@@ -1,6 +1,7 @@
 	* fix uTP edge case where udp socket buffer fills up
 	* fix nagle implementation in uTP
 
+	* disable support for large MTUs in uTP until it is more reliable
 	* expose post_torrent_updates and state_update_alert to python bindings
 	* fix incorrect SSL error messages
 	* fix windows build of shared library with openssl

src/session.cpp

@@ -1263,7 +1263,7 @@ namespace libtorrent
 #ifndef TORRENT_NO_DEPRECATE
 		, utp_delayed_ack(0) // milliseconds
 #endif
-		, utp_dynamic_sock_buf(true)
+		, utp_dynamic_sock_buf(false) // this doesn't seem quite reliable yet
 		, utp_loss_multiplier(50) // specified in percent
 		, mixed_mode_algorithm(peer_proportional)
 		, rate_limit_utp(true)

src/utp_stream.cpp

@@ -2340,8 +2340,14 @@ void utp_socket_impl::init_mtu(int link_mtu, int utp_mtu)
 		link_mtu -= decrease;
 	}
 
-	m_mtu = utp_mtu;
+	// set the ceiling to what we found out from the interface
 	m_mtu_ceiling = utp_mtu;
+
+	// however, start the search from a more conservative MTU
+	int overhead = link_mtu - utp_mtu;
+	m_mtu = TORRENT_ETHERNET_MTU - overhead;
+	if (m_mtu > m_mtu_ceiling) m_mtu = m_mtu_ceiling;
+
 	if (m_mtu_floor > utp_mtu) m_mtu_floor = utp_mtu;
 
 	// if the window size is smaller than one packet size