only allow cwnd to be reduced so often

include/libtorrent/settings_pack.hpp

@@ -1608,6 +1608,11 @@ namespace libtorrent
 			// systems.
 			close_file_interval,
 
+			// When uTP experiences packet loss, it will reduce the congestion
+			// window, and not reduce it again for this many milliseconds, even if
+			// experiencing another lost packet.
+			utp_cwnd_reduce_timer,
+
 			max_int_setting_internal
 		};
 

include/libtorrent/utp_socket_manager.hpp

@@ -87,6 +87,7 @@ namespace libtorrent
 		int connect_timeout() const { return m_sett.get_int(settings_pack::utp_connect_timeout); }
 		int min_timeout() const { return m_sett.get_int(settings_pack::utp_min_timeout); }
 		int loss_multiplier() const { return m_sett.get_int(settings_pack::utp_loss_multiplier); }
+		int cwnd_reduce_timer() const { return m_sett.get_int(settings_pack::utp_cwnd_reduce_timer); }
 
 		void mtu_for_dest(address const& addr, int& link_mtu, int& utp_mtu);
 		void set_sock_buf(int size);

src/settings_pack.cpp

@@ -366,6 +366,7 @@ namespace libtorrent
 		SET_NOPREV(urlseed_max_request_bytes, 16 * 1024 * 1024, 0),
 		SET_NOPREV(web_seed_name_lookup_retry, 1800, 0),
 		SET_NOPREV(close_file_interval, CLOSE_FILE_INTERVAL, &session_impl::update_close_file_interval),
+		SET_NOPREV(utp_cwnd_reduce_timer, 100, 0),
 	};
 
 #undef SET

src/utp_stream.cpp

@@ -359,7 +359,7 @@ struct utp_socket_impl
 	bool consume_incoming_data(
 		utp_header const* ph, boost::uint8_t const* ptr, int payload_size, time_point now);
 	void update_mtu_limits();
-	void experienced_loss(int seq_nr);
+	void experienced_loss(int seq_nr, time_point now);
 
 	void set_state(int s);
 
@@ -467,6 +467,10 @@ public:
 	// the last time we stepped the timestamp history
 	time_point m_last_history_step;
 
+	// the next time we allow a lost packet to halve cwnd. We only do this once every
+	// 100 ms
+	time_point m_next_loss;
+
 	// the max number of bytes in-flight. This is a fixed point
 	// value, to get the true number of bytes, shift right 16 bits
 	// the value is always >= 0, but the calculations performed on
@@ -1648,7 +1652,7 @@ void utp_socket_impl::parse_sack(boost::uint16_t const packet_ack, boost::uint8_
 		// the logic to handle a lost MTU probe is in resend_packet()
 		if (cut_cwnd && (pkt_seq != m_mtu_seq || m_mtu_seq == 0))
 		{
-			experienced_loss(pkt_seq);
+			experienced_loss(pkt_seq, now);
 			cut_cwnd = false;
 		}
 
@@ -2360,7 +2364,7 @@ bool utp_socket_impl::resend_packet(packet* p, bool fast_resend)
 	return !m_stalled;
 }
 
-void utp_socket_impl::experienced_loss(int const seq_nr)
+void utp_socket_impl::experienced_loss(int const seq_nr, time_point const now)
 {
 	INVARIANT_CHECK;
 
@@ -2380,11 +2384,17 @@ void utp_socket_impl::experienced_loss(int const seq_nr)
 	// same packet again, ignore it.
 	if (compare_less_wrap(seq_nr, m_loss_seq_nr + 1, ACK_MASK)) return;
 
+	// don't reduce cwnd more than once every 100ms
+	if (m_next_loss >= now) return;
+
+	m_next_loss = now + milliseconds(m_sm->cwnd_reduce_timer());
+
 	// cut window size in 2
 	m_cwnd = std::max(m_cwnd * m_sm->loss_multiplier() / 100
 		, boost::int64_t(m_mtu) * (1 << 16));
 	m_loss_seq_nr = m_seq_nr;
-	UTP_LOGV("%8p: Lost packet %d caused cwnd cut\n", static_cast<void*>(this), seq_nr);
+	UTP_LOGV("%8p: Lost packet %d caused cwnd cut. m_loss_seq_nr:%d\n"
+		, static_cast<void*>(this), seq_nr, m_seq_nr);
 
 	// if we happen to be in slow-start mode, we need to leave it
 	// note that we set ssthres to the window size _after_ reducing it. Next slow
@@ -3071,7 +3081,7 @@ bool utp_socket_impl::incoming_packet(boost::uint8_t const* buf, int size
 		{
 			// don't consider a lost probe as proper loss, it doesn't necessarily
 			// signal congestion
-			if (!p->mtu_probe) experienced_loss(m_fast_resend_seq_nr);
+			if (!p->mtu_probe) experienced_loss(m_fast_resend_seq_nr, receive_time);
 			resend_packet(p, true);
 			if (m_state == UTP_STATE_ERROR_WAIT || m_state == UTP_STATE_DELETE) return true;
 		}
@@ -3281,6 +3291,8 @@ bool utp_socket_impl::incoming_packet(boost::uint8_t const* buf, int size
 				if (m_state == UTP_STATE_ERROR_WAIT || m_state == UTP_STATE_DELETE) return true;
 			}
 
+			TORRENT_ASSERT(!compare_less_wrap(m_seq_nr, m_acked_seq_nr, ACK_MASK));
+
 #if TORRENT_UTP_LOG
 			if (sample && acked_bytes && prev_bytes_in_flight)
 			{