improve robustness and performance of uTP PMTU discovery. fix duplicate ACK issue in uTP. demote an invariant check to 'expensive'

ChangeLog

@@ -1,6 +1,8 @@
 
 1.1.0 release
 
+	* improve robustness and performance of uTP PMTU discovery
+	* fix duplicate ACK issue in uTP
 	* support filtering which parts of session state are loaded by load_state()
 	* deprecate support for adding torrents by HTTP URL
 	* allow specifying which tracker to scrape in scrape_tracker

include/libtorrent/utp_socket_manager.hpp

@@ -95,6 +95,17 @@ namespace libtorrent
 		void defer_ack(utp_socket_impl* s);
 		void subscribe_drained(utp_socket_impl* s);
 
+		void restrict_mtu(int mtu)
+		{
+			m_restrict_mtu[m_mtu_idx] = mtu;
+			m_mtu_idx = (m_mtu_idx + 1) % m_restrict_mtu.size();
+		}
+
+		int restrict_mtu() const
+		{
+			return *std::max_element(m_restrict_mtu.begin(), m_restrict_mtu.end());
+		}
+
 		// used to keep stats of uTP events
 		// the counter is the enum from ``counters``.
 		void inc_stats_counter(int counter, int delta = 1);
@@ -154,6 +165,9 @@ namespace libtorrent
 		// stats counters
 		counters& m_counters;
 
+		boost::array<int, 3> m_restrict_mtu;
+		int m_mtu_idx;
+
 		// this is  passed on to the instantiate connection
 		// if this is non-null it will create SSL connections over uTP
 		void* m_ssl_context;

src/block_cache.cpp

@@ -1216,7 +1216,9 @@ int block_cache::pad_job(disk_io_job const* j, int blocks_in_piece
 void block_cache::insert_blocks(cached_piece_entry* pe, int block, file::iovec_t *iov
 	, int iov_len, disk_io_job* j, int flags)
 {
+#ifdef TORRENT_EXPENSIVE_INVARIANT_CHECKS
 	INVARIANT_CHECK;
+#endif
 
 	TORRENT_ASSERT(pe);
 	TORRENT_ASSERT(pe->in_use);

src/bt_peer_connection.cpp

@@ -3578,7 +3578,7 @@ namespace libtorrent
 		int next_barrier = m_enc_handler.encrypt(iovec);
 #ifndef TORRENT_DISABLE_LOGGING
 		if (next_barrier != 0)
-			peer_log(peer_log_alert::outgoing_message, "SEND_BARRIER"
+			peer_log(peer_log_alert::outgoing, "SEND_BARRIER"
 				, "encrypted block s = %d", next_barrier);
 #endif
 		return next_barrier;

src/peer_connection.cpp

@@ -1709,15 +1709,7 @@ namespace libtorrent
 		else if (m_ses.preemptive_unchoke())
 		{
 			// if the peer is choked and we have upload slots left,
-			// then unchoke it. Another condition that has to be met
-			// is that the torrent doesn't keep track of the individual
-			// up/down ratio for each peer (ratio == 0) or (if it does
-			// keep track) this particular connection isn't a leecher.
-			// If the peer was choked because it was leeching, don't
-			// unchoke it again.
-			// The exception to this last condition is if we're a seed.
-			// In that case we don't care if people are leeching, they
-			// can't pay for their downloads anyway.
+			// then unchoke it.
 
 			boost::shared_ptr<torrent> t = m_torrent.lock();
 			TORRENT_ASSERT(t);

src/utp_socket_manager.cpp

@@ -35,6 +35,7 @@ POSSIBILITY OF SUCH DAMAGE.
 #include "libtorrent/utp_socket_manager.hpp"
 #include "libtorrent/instantiate_connection.hpp"
 #include "libtorrent/socket_io.hpp"
+#include "libtorrent/socket.hpp" // for TORRENT_HAS_DONT_FRAGMENT
 #include "libtorrent/broadcast_socket.hpp" // for is_teredo
 #include "libtorrent/random.hpp"
 #include "libtorrent/performance_counters.hpp"
@@ -59,8 +60,11 @@ namespace libtorrent
 		, m_last_if_update(min_time())
 		, m_sock_buf_size(0)
 		, m_counters(cnt)
+		, m_mtu_idx(0)
 		, m_ssl_context(ssl_context)
-	{}
+	{
+		m_restrict_mtu.fill(65536);
+	}
 
 	utp_socket_manager::~utp_socket_manager()
 	{
@@ -128,7 +132,6 @@ namespace libtorrent
 			// the address field in the SOCKS header
 			if (addr.is_v4()) mtu -= 4;
 			else mtu -= 16;
-
 		}
 		else
 		{
@@ -136,7 +139,7 @@ namespace libtorrent
 			else mtu -= TORRENT_IPV6_HEADER;
 		}
 
-		utp_mtu = mtu;
+		utp_mtu = (std::min)(mtu, restrict_mtu());
 	}
 
 	void utp_socket_manager::send_packet(udp::endpoint const& ep, char const* p
@@ -159,12 +162,18 @@ namespace libtorrent
 #ifdef TORRENT_HAS_DONT_FRAGMENT
 		error_code tmp;
 		if (flags & utp_socket_manager::dont_fragment)
+		{
 			m_sock.set_option(libtorrent::dont_fragment(true), tmp);
+			TORRENT_ASSERT_VAL(!tmp, tmp.message());
+		}
 #endif
 		m_sock.send(ep, p, len, ec);
 #ifdef TORRENT_HAS_DONT_FRAGMENT
 		if (flags & utp_socket_manager::dont_fragment)
+		{
 			m_sock.set_option(libtorrent::dont_fragment(false), tmp);
+			TORRENT_ASSERT_VAL(!tmp, tmp.message());
+		}
 #endif
 	}
 

src/utp_stream.cpp

@@ -65,10 +65,7 @@ static struct utp_logger
 	FILE* utp_log_file;
 	mutex utp_log_mutex;
 
-	utp_logger() : utp_log_file(0)
-	{
-		utp_log_file = NULL;
-	}
+	utp_logger() : utp_log_file(NULL) {}
 	~utp_logger()
 	{
 		if (utp_log_file) fclose(utp_log_file);
@@ -806,7 +803,8 @@ void utp_socket_impl::update_mtu_limits()
 {
 	INVARIANT_CHECK;
 
-	TORRENT_ASSERT(m_mtu_floor <= m_mtu_ceiling);
+	if (m_mtu_floor > m_mtu_ceiling) m_mtu_floor = m_mtu_ceiling;
+
 	m_mtu = (m_mtu_floor + m_mtu_ceiling) / 2;
 
 	if ((m_cwnd >> 16) < m_mtu) m_cwnd = boost::int64_t(m_mtu) << 16;
@@ -1370,6 +1368,7 @@ void utp_socket_impl::send_syn()
 	p->size = sizeof(utp_header);
 	p->header_size = sizeof(utp_header);
 	p->num_transmissions = 0;
+	p->mtu_probe = false;
 #ifdef TORRENT_DEBUG
 	p->num_fast_resend = 0;
 #endif
@@ -1492,6 +1491,12 @@ void utp_socket_impl::send_reset(utp_header const* ph)
 	error_code ec;
 	m_sm->send_packet(udp::endpoint(m_remote_address, m_port)
 		, reinterpret_cast<char const*>(&h), sizeof(h), ec);
+	if (ec)
+	{
+		UTP_LOGV("%8p: socket error: %s\n"
+			, static_cast<void*>(this)
+			, ec.message().c_str());
+	}
 }
 
 std::size_t utp_socket_impl::available() const
@@ -1755,7 +1760,7 @@ bool utp_socket_impl::send_pkt(int flags)
 	INVARIANT_CHECK;
 #endif
 
-	bool force = (flags & pkt_ack) || (flags & pkt_fin);
+	bool const force = (flags & pkt_ack) || (flags & pkt_fin);
 
 //	TORRENT_ASSERT(m_state != UTP_STATE_FIN_SENT || (flags & pkt_ack));
 
@@ -1794,14 +1799,29 @@ bool utp_socket_impl::send_pkt(int flags)
 		if (sack > 32) sack = 32;
 	}
 
-	boost::uint32_t close_reason = m_close_reason;
+	boost::uint32_t const close_reason = m_close_reason;
 
-	int header_size = sizeof(utp_header)
+	// MTU DISCOVERY
+
+	// under these conditions, the next packet we send should be an MTU probe.
+	// MTU probes get to use the mid-point packet size, whereas other packets
+	// use a conservative packet size of the largest known to work. The reason
+	// for the cwnd condition is to make sure the probe is surrounded by non-
+	// probes, to be able to distinguish a loss of the probe vs. just loss in
+	// general.
+	bool const mtu_probe = (m_mtu_seq == 0
+		&& m_write_buffer_size >= m_mtu_floor * 3
+		&& m_seq_nr != 0
+		&& (m_cwnd >> 16) > m_mtu_floor * 3);
+
+	int const header_size = sizeof(utp_header)
 		+ (sack ? sack + 2 : 0)
 		+ (close_reason ? 6 : 0);
-	int payload_size = m_write_buffer_size;
-	if (m_mtu - header_size < payload_size)
-		payload_size = m_mtu - header_size;
+
+	// for non MTU-probes, use the conservative packet size
+	int const effective_mtu = mtu_probe ? m_mtu : m_mtu_floor;
+	int payload_size = (std::min)(m_write_buffer_size
+		, effective_mtu - header_size);
 
 	// if we have one MSS worth of data, make sure it fits in our
 	// congestion window and the advertized receive window from
@@ -1828,11 +1848,11 @@ bool utp_socket_impl::send_pkt(int flags)
 #if TORRENT_UTP_LOG
 			UTP_LOGV("%8p: skipping send seq_nr:%d ack_nr:%d "
 				"id:%d target:%s header_size:%d error:%s send_buffer_size:%d cwnd:%d "
-				"adv_wnd:%d in-flight:%d mtu:%d\n"
+				"adv_wnd:%d in-flight:%d mtu:%d effective-mtu:%d\n"
 				, static_cast<void*>(this), int(m_seq_nr), int(m_ack_nr)
 				, m_send_id, print_endpoint(udp::endpoint(m_remote_address, m_port)).c_str()
 				, header_size, m_error.message().c_str(), m_write_buffer_size, int(m_cwnd >> 16)
-				, m_adv_wnd, m_bytes_in_flight, m_mtu);
+				, m_adv_wnd, m_bytes_in_flight, m_mtu, effective_mtu);
 #endif
 			return false;
 		}
@@ -1876,8 +1896,8 @@ bool utp_socket_impl::send_pkt(int flags)
 		// need to keep the packet around (in the outbuf)
 		if (payload_size)
 		{
-			p = static_cast<packet*>(malloc(sizeof(packet) + m_mtu));
-			p->allocated = m_mtu;
+			p = static_cast<packet*>(malloc(sizeof(packet) + effective_mtu));
+			p->allocated = effective_mtu;
 			buf_holder.reset(reinterpret_cast<char*>(p));
 
 			m_sm->inc_stats_counter(counters::utp_payload_pkts_out);
@@ -1946,9 +1966,9 @@ bool utp_socket_impl::send_pkt(int flags)
 		else
 			sack = 0;
 
-		boost::int32_t size_left = p->allocated - p->size;
-		TORRENT_ASSERT(size_left > 0);
-		size_left = (std::min)(size_left, m_write_buffer_size);
+		boost::int32_t const size_left = (std::min)(p->allocated - p->size
+			, m_write_buffer_size);
+
 		write_payload(p->buf + p->size, size_left);
 		p->size += size_left;
 
@@ -2001,9 +2021,9 @@ bool utp_socket_impl::send_pkt(int flags)
 		// outstanding packet is acked, we'll send this
 		// payload
 		UTP_LOGV("%8p: NAGLE not enough payload send_buffer_size:%d cwnd:%d "
-			"adv_wnd:%d in-flight:%d mtu:%d\n"
+			"adv_wnd:%d in-flight:%d mtu:%d effective_mtu:%d\n"
 			, static_cast<void*>(this), m_write_buffer_size, int(m_cwnd >> 16)
-			, m_adv_wnd, m_bytes_in_flight, m_mtu);
+			, m_adv_wnd, m_bytes_in_flight, m_mtu, effective_mtu);
 		TORRENT_ASSERT(m_nagle_packet == NULL);
 		TORRENT_ASSERT(h->seq_nr == m_seq_nr);
 		m_nagle_packet = p;
@@ -2011,10 +2031,9 @@ bool utp_socket_impl::send_pkt(int flags)
 		return false;
 	}
 
-	// MTU DISCOVERY
-	if (m_mtu_seq == 0
-		&& p->size > m_mtu_floor
-		&& m_seq_nr != 0)
+	// for ST_DATA packets, payload size is 0. Such packets do not have unique
+	// sequence numbers and should never be used as mtu probes
+	if ((mtu_probe || p->mtu_probe) && payload_size > 0)
 	{
 		p->mtu_probe = true;
 		m_mtu_seq = m_seq_nr;
@@ -2063,7 +2082,9 @@ bool utp_socket_impl::send_pkt(int flags)
 	if (ec == error::message_size)
 	{
 #if TORRENT_UTP_LOG
-		UTP_LOGV("%8p: error sending packet: %s\n", static_cast<void*>(this), ec.message().c_str());
+		UTP_LOGV("%8p: error sending packet: %s\n"
+			, static_cast<void*>(this)
+			, ec.message().c_str());
 #endif
 		// if we fail even though this is not a probe, we're screwed
 		// since we'd have to repacketize
@@ -2074,8 +2095,7 @@ bool utp_socket_impl::send_pkt(int flags)
 		// resend the packet immediately without
 		// it being an MTU probe
 		p->mtu_probe = false;
-		if (m_mtu_seq == m_ack_nr)
-			m_mtu_seq = 0;
+		m_mtu_seq = 0;
 		ec.clear();
 
 #if TORRENT_UTP_LOG
@@ -2124,6 +2144,11 @@ bool utp_socket_impl::send_pkt(int flags)
 #endif
 		TORRENT_ASSERT(h->seq_nr == m_seq_nr);
 
+		// 0 is a special sequence number, since it's also used as "uninitialized".
+		// we never send an mtu probe for sequence number 0
+		TORRENT_ASSERT(p->mtu_probe == (m_seq_nr == m_mtu_seq)
+			|| m_seq_nr == 0);
+
 		// release the buffer, we're saving it in the circular
 		// buffer of outgoing packets
 		buf_holder.release();
@@ -2290,10 +2315,15 @@ bool utp_socket_impl::resend_packet(packet* p, bool fast_resend)
 	return !m_stalled;
 }
 
-void utp_socket_impl::experienced_loss(int seq_nr)
+void utp_socket_impl::experienced_loss(int const seq_nr)
 {
 	INVARIANT_CHECK;
 
+	// the window size could go below one MMS here, if it does,
+	// we'll get a timeout in about one second
+
+	m_sm->inc_stats_counter(counters::utp_packet_loss);
+
 	// since loss often comes in bursts, we only cut the
 	// window in half once per RTT. This is implemented
 	// by limiting which packets can cause us to cut the
@@ -2319,11 +2349,6 @@ void utp_socket_impl::experienced_loss(int seq_nr)
 		m_slow_start = false;
 		UTP_LOGV("%8p: experienced loss, slow_start -> 0\n", static_cast<void*>(this));
 	}
-
-	// the window size could go below one MMS here, if it does,
-	// we'll get a timeout in about one second
-
-	m_sm->inc_stats_counter(counters::utp_packet_loss);
 }
 
 void utp_socket_impl::set_state(int s)
@@ -2484,7 +2509,7 @@ bool utp_socket_impl::cancel_handlers(error_code const& ec, bool kill)
 
 	TORRENT_ASSERT(ec);
 	bool ret = m_read_handler || m_write_handler || m_connect_handler;
-	
+
 	// calling the callbacks with m_userdata being 0 will just crash
 	TORRENT_ASSERT((ret && bool(m_userdata)) || !ret);
 
@@ -2665,11 +2690,9 @@ void utp_socket_impl::init_mtu(int link_mtu, int 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;
+	// start in the middle of the PMTU search space
+	m_mtu = (m_mtu_ceiling + m_mtu_floor) / 2;
 	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
@@ -2886,8 +2909,13 @@ bool utp_socket_impl::incoming_packet(boost::uint8_t const* buf, int size
 
 	// if we get an ack for the same sequence number as
 	// was last ACKed, and we have outstanding packets,
-	// it counts as a duplicate ack
-	if (ph->ack_nr == m_acked_seq_nr && m_outbuf.size())
+	// it counts as a duplicate ack. The reason to not count ST_DATA packets as
+	// duplicate ACKs is because we may be receiving a stream of those
+	// regardless of our outgoing traffic, which makes their ACK number not
+	// indicative of a dropped packet
+	if (ph->ack_nr == m_acked_seq_nr
+		&& m_outbuf.size()
+		&& ph->get_type() == ST_STATE)
 	{
 		++m_duplicate_acks;
 	}
@@ -3548,6 +3576,15 @@ void utp_socket_impl::tick(time_point now)
 
 		if (m_outbuf.size()) ++m_num_timeouts;
 
+		UTP_LOGV("%8p: timeout num-timeouts: %d max-resends: %d confirmed: %d "
+			" acked-seq-num: %d mtu-seq: %d\n"
+			, static_cast<void*>(this)
+			, m_num_timeouts
+			, m_sm->num_resends()
+			, m_confirmed
+			, m_acked_seq_nr
+			, m_mtu_seq);
+
 		// a socket that has not been confirmed to actually have a live remote end
 		// (the IP may have been spoofed) fail on the first timeout. If we had
 		// heard anything from this peer, it would have been confirmed.
@@ -3590,7 +3627,7 @@ void utp_socket_impl::tick(time_point now)
 
 		m_timeout = now + milliseconds(packet_timeout());
 
-		UTP_LOGV("%8p: timeout resetting cwnd:%d\n"
+		UTP_LOGV("%8p: resetting cwnd:%d\n"
 			, static_cast<void*>(this), int(m_cwnd >> 16));
 
 		// we dropped all packets, that includes the mtu probe
@@ -3606,7 +3643,7 @@ void utp_socket_impl::tick(time_point now)
 		// we're very likely to have an ssthres set, which will make us leave
 		// slow start before inducing more delay or loss.
 		m_slow_start = true;
-		UTP_LOGV("%8p: timeout slow_start -> 1\n", static_cast<void*>(this));
+		UTP_LOGV("%8p: slow_start -> 1\n", static_cast<void*>(this));
 
 		// we need to go one past m_seq_nr to cover the case
 		// where we just sent a SYN packet and then adjusted for
@@ -3639,6 +3676,16 @@ void utp_socket_impl::tick(time_point now)
 					, static_cast<void*>(this), p->num_transmissions, socket_state_names[m_state]);
 #endif
 
+				if (p->size > m_mtu_floor)
+				{
+					// the packet that caused the connection to fail was an mtu probe
+					// (note that the mtu_probe field won't be set at this point because
+					// it's cleared when the packet is re-sent). This suggests that
+					// perhaps our network throws away oversized packets without
+					// fragmenting them. Tell the socket manager to be more conservative
+					// about mtu ceiling in the future
+					m_sm->restrict_mtu(m_mtu);
+				}
 				// the connection is dead
 				m_error = boost::asio::error::timed_out;
 				set_state(UTP_STATE_ERROR_WAIT);