set uTP target delay to 100 ms. expose setting for packet loss multiplier for cwnd

docs/manual.rst

@@ -4526,6 +4526,7 @@ session_settings
 		int utp_connect_timeout;
 		int utp_delayed_ack;
 		bool utp_dynamic_sock_buf;
+		int utp_loss_multiplier;
 
 		enum bandwidth_mixed_algo_t
 		{
@@ -5349,6 +5350,11 @@ For RAM constrained systems, disabling this typically saves around 30kB in user
 and probably around 400kB in kernel socket buffers (it adjusts the send and receive
 buffer size on the kernel socket, both for IPv4 and IPv6).
 
+``utp_loss_multiplier`` controls how the congestion window is changed when a packet
+loss is experienced. It's specified as a percentage multiplier for ``cwnd``. By default
+it's set to 50 (i.e. cut in half). Do not change this value unless you know what
+you're doing. Never set it higher than 100.
+
 The ``mixed_mode_algorithm`` determines how to treat TCP connections when there are
 uTP connections. Since uTP is designed to yield to TCP, there's an inherent problem
 when using swarms that have both TCP and uTP connections. If nothing is done, uTP

include/libtorrent/session_settings.hpp

@@ -245,7 +245,7 @@ namespace libtorrent
 			, unchoke_slots_limit(8)
 			, half_open_limit(0)
 			, connections_limit(200)
-			, utp_target_delay(75) // milliseconds
+			, utp_target_delay(100) // milliseconds
 			, utp_gain_factor(1500) // bytes per rtt
 			, utp_min_timeout(500) // milliseconds
 			, utp_syn_resends(2)
@@ -254,6 +254,7 @@ namespace libtorrent
 			, utp_connect_timeout(3000) // milliseconds
 			, utp_delayed_ack(0) // milliseconds
 			, utp_dynamic_sock_buf(true)
+			, utp_loss_multiplier(50) // specified in percent
 			, mixed_mode_algorithm(peer_proportional)
 			, rate_limit_utp(false)
 			, listen_queue_size(5)
@@ -1008,6 +1009,11 @@ namespace libtorrent
 		// including loopback
 		bool utp_dynamic_sock_buf;
 
+		// what to multiply the congestion window by on packet loss.
+		// it's specified as a percent. The default is 50, i.e. cut
+		// in half
+		int utp_loss_multiplier;
+
 		enum bandwidth_mixed_algo_t
 		{
 			// disables the mixed mode bandwidth balancing

include/libtorrent/utp_socket_manager.hpp

@@ -78,6 +78,7 @@ namespace libtorrent
 		int connect_timeout() const { return m_sett.utp_connect_timeout; }
 		int delayed_ack() const { return m_sett.utp_delayed_ack; }
 		int min_timeout() const { return m_sett.utp_min_timeout; }
+		int loss_multiplier() const { return m_sett.utp_loss_multiplier; }
 		bool allow_dynamic_sock_buf() const { return m_sett.utp_dynamic_sock_buf; }
 
 		void mtu_for_dest(address const& addr, int& link_mtu, int& utp_mtu);

src/utp_stream.cpp

@@ -1804,7 +1804,7 @@ void utp_socket_impl::experienced_loss(int seq_nr)
 	if (compare_less_wrap(seq_nr, m_loss_seq_nr, ACK_MASK)) return;
 	
 	// cut window size in 2
-	m_cwnd = (std::max)(m_cwnd / 2, boost::int64_t(m_mtu << 16));
+	m_cwnd = (std::max)(m_cwnd * m_sm->loss_multiplier() / 100, boost::int64_t(m_mtu << 16));
 	m_loss_seq_nr = m_seq_nr;
 	UTP_LOGV("%8p: Lost packet %d caused cwnd cut\n", this, seq_nr);