src/network/connection.cpp

@@ -49,7 +49,7 @@ namespace con
 // TODO: Clean this up.
 #define LOG(a) a
 
-#define PING_TIMEOUT 5.0
+#define PING_TIMEOUT 5.0f
 
 u16 BufferedPacket::getSeqnum() const
 {
@@ -548,6 +548,15 @@ ConnectionCommandPtr ConnectionCommand::disconnect_peer(session_t peer_id)
 	return c;
 }
 
+ConnectionCommandPtr ConnectionCommand::resend_one(session_t peer_id)
+{
+	auto c = create(CONNCMD_RESEND_ONE);
+	c->peer_id = peer_id;
+	c->channelnum = 0; // must be same as createPeer
+	c->reliable = true;
+	return c;
+}
+
 ConnectionCommandPtr ConnectionCommand::send(session_t peer_id, u8 channelnum,
 	NetworkPacket *pkt, bool reliable)
 {
@@ -984,12 +993,12 @@ void UDPPeer::reportRTT(float rtt)
 bool UDPPeer::Ping(float dtime,SharedBuffer<u8>& data)
 {
 	m_ping_timer += dtime;
-	if (m_ping_timer >= PING_TIMEOUT)
+	if (!isHalfOpen() && m_ping_timer >= PING_TIMEOUT)
 	{
 		// Create and send PING packet
 		writeU8(&data[0], PACKET_TYPE_CONTROL);
 		writeU8(&data[1], CONTROLTYPE_PING);
-		m_ping_timer = 0.0;
+		m_ping_timer = 0.0f;
 		return true;
 	}
 	return false;
@@ -1608,6 +1617,12 @@ void Connection::DisconnectPeer(session_t peer_id)
 	putCommand(ConnectionCommand::disconnect_peer(peer_id));
 }
 
+void Connection::doResendOne(session_t peer_id)
+{
+	assert(peer_id != PEER_ID_INEXISTENT);
+	putCommand(ConnectionCommand::resend_one(peer_id));
+}
+
 void Connection::sendAck(session_t peer_id, u8 channelnum, u16 seqnum)
 {
 	assert(channelnum < CHANNEL_COUNT); // Pre-condition
@@ -1634,6 +1649,7 @@ UDPPeer* Connection::createServerPeer(Address& address)
 	}
 
 	UDPPeer *peer = new UDPPeer(PEER_ID_SERVER, address, this);
+	peer->SetFullyOpen();
 
 	{
 		MutexAutoLock lock(m_peers_mutex);

src/network/connection.h

@@ -313,7 +313,8 @@ enum ConnectionCommandType{
 	CONNCMD_SEND,
 	CONNCMD_SEND_TO_ALL,
 	CONCMD_ACK,
-	CONCMD_CREATE_PEER
+	CONCMD_CREATE_PEER,
+	CONNCMD_RESEND_ONE
 };
 
 struct ConnectionCommand;
@@ -336,6 +337,7 @@ struct ConnectionCommand
 	static ConnectionCommandPtr connect(Address address);
 	static ConnectionCommandPtr disconnect();
 	static ConnectionCommandPtr disconnect_peer(session_t peer_id);
+	static ConnectionCommandPtr resend_one(session_t peer_id);
 	static ConnectionCommandPtr send(session_t peer_id, u8 channelnum, NetworkPacket *pkt, bool reliable);
 	static ConnectionCommandPtr ack(session_t peer_id, u8 channelnum, const Buffer<u8> &data);
 	static ConnectionCommandPtr createPeer(session_t peer_id, const Buffer<u8> &data);
@@ -520,6 +522,9 @@ class Peer {
 		void ResetTimeout()
 			{MutexAutoLock lock(m_exclusive_access_mutex); m_timeout_counter = 0.0; };
 
+		bool isHalfOpen() const { return m_half_open; }
+		void SetFullyOpen() { m_half_open = false; }
+
 		bool isTimedOut(float timeout);
 
 		unsigned int m_increment_packets_remaining = 0;
@@ -590,6 +595,15 @@ class Peer {
 		rttstats m_rtt;
 		float m_last_rtt = -1.0f;
 
+		/*
+			Until the peer has communicated with us using their assigned peer id
+			the connection is considered half-open.
+			During this time we inhibit re-sending any reliables or pings. This
+			is to avoid spending too many resources on a potential DoS attack
+			and to make sure Minetest servers are not useful for UDP amplificiation.
+		*/
+		bool m_half_open = true;
+
 		// current usage count
 		unsigned int m_usage = 0;
 
@@ -736,6 +750,8 @@ protected:
 
 	void SetPeerID(session_t id) { m_peer_id = id; }
 
+	void doResendOne(session_t peer_id);
+
 	void sendAck(session_t peer_id, u8 channelnum, u16 seqnum);
 
 	std::vector<session_t> getPeerIDs()

src/network/connectionthreads.cpp

@@ -46,11 +46,11 @@ namespace con
 
 #define WINDOW_SIZE 5
 
-static session_t readPeerId(const u8 *packetdata)
+static inline session_t readPeerId(const u8 *packetdata)
 {
 	return readU16(&packetdata[4]);
 }
-static u8 readChannel(const u8 *packetdata)
+static inline u8 readChannel(const u8 *packetdata)
 {
 	return readU8(&packetdata[6]);
 }
@@ -220,28 +220,21 @@ void ConnectionSendThread::runTimeouts(float dtime)
 			channel.UpdatePacketLossCounter(timed_outs.size());
 			g_profiler->graphAdd("packets_lost", timed_outs.size());
 
-			m_iteration_packets_avaialble -= timed_outs.size();
-
-			for (const auto &k : timed_outs) {
-				u8 channelnum = readChannel(k->data);
-				u16 seqnum = k->getSeqnum();
-
-				channel.UpdateBytesLost(k->size());
-
-				LOG(derr_con << m_connection->getDesc()
-					<< "RE-SENDING timed-out RELIABLE to "
-					<< k->address.serializeString()
-					<< "(t/o=" << resend_timeout << "): "
-					<< "count=" << k->resend_count
-					<< ", channel=" << ((int) channelnum & 0xff)
-					<< ", seqnum=" << seqnum
-					<< std::endl);
-
-				rawSend(k.get());
-
-				// do not handle rtt here as we can't decide if this packet was
-				// lost or really takes more time to transmit
+			// Note that this only happens during connection setup, it would
+			// break badly otherwise.
+			if (peer->isHalfOpen()) {
+				if (!timed_outs.empty()) {
+					dout_con << m_connection->getDesc() <<
+						"Skipping re-send of " << timed_outs.size() <<
+						" timed-out reliables to peer_id " << udpPeer->id
+						<< " (half-open)." << std::endl;
 				}
+				continue;
+			}
+
+			m_iteration_packets_avaialble -= timed_outs.size();
+			for (const auto &k : timed_outs)
+				resendReliable(channel, k.get(), resend_timeout);
 
 			channel.UpdateTimers(dtime);
 		}
@@ -270,8 +263,36 @@ void ConnectionSendThread::runTimeouts(float dtime)
 	}
 }
 
+void ConnectionSendThread::resendReliable(Channel &channel, const BufferedPacket *k, float resend_timeout)
+{
+	assert(k);
+	u8 channelnum = readChannel(k->data);
+	u16 seqnum = k->getSeqnum();
+
+	channel.UpdateBytesLost(k->size());
+
+	derr_con << m_connection->getDesc()
+		<< "RE-SENDING timed-out RELIABLE to "
+		<< k->address.serializeString();
+	if (resend_timeout >= 0)
+		derr_con << "(t/o=" << resend_timeout << "): ";
+	else
+		derr_con << "(force): ";
+	derr_con
+		<< "count=" << k->resend_count
+		<< ", channel=" << ((int) channelnum & 0xff)
+		<< ", seqnum=" << seqnum
+		<< std::endl;
+
+	rawSend(k);
+
+	// do not handle rtt here as we can't decide if this packet was
+	// lost or really takes more time to transmit
+}
+
 void ConnectionSendThread::rawSend(const BufferedPacket *p)
 {
+	assert(p);
 	try {
 		m_connection->m_udpSocket.Send(p->address, p->data, p->size());
 		LOG(dout_con << m_connection->getDesc()
@@ -398,6 +419,23 @@ void ConnectionSendThread::processReliableCommand(ConnectionCommandPtr &c)
 			}
 			return;
 
+		case CONNCMD_RESEND_ONE: {
+			LOG(dout_con << m_connection->getDesc()
+				<< "UDP processing reliable CONNCMD_RESEND_ONE" << std::endl);
+
+			PeerHelper peer = m_connection->getPeerNoEx(c->peer_id);
+			if (!peer)
+				return;
+			Channel &channel = dynamic_cast<UDPPeer *>(&peer)->channels[c->channelnum];
+
+			auto timed_outs = channel.outgoing_reliables_sent.getTimedOuts(0, 1);
+
+			if (!timed_outs.empty())
+				resendReliable(channel, timed_outs.front().get(), -1);
+
+			return;
+		}
+
 		case CONNCMD_SERVE:
 		case CONNCMD_CONNECT:
 		case CONNCMD_DISCONNECT:
@@ -457,6 +495,7 @@ void ConnectionSendThread::processNonReliableCommand(ConnectionCommandPtr &c_ptr
 			sendAsPacket(c.peer_id, c.channelnum, c.data, true);
 			return;
 		case CONCMD_CREATE_PEER:
+		case CONNCMD_RESEND_ONE:
 			FATAL_ERROR("Got command that should be reliable as unreliable command");
 		default:
 			LOG(dout_con << m_connection->getDesc()
@@ -918,21 +957,25 @@ void ConnectionReceiveThread::receive(SharedBuffer<u8> &packetdata,
 			return;
 		}
 
-		/* Try to identify peer by sender address (may happen on join) */
+		const bool knew_peer_id = peer_id != PEER_ID_INEXISTENT;
+
+		if (!m_connection->ConnectedToServer()) {
+			// Try to identify peer by sender address
 			if (peer_id == PEER_ID_INEXISTENT) {
 				peer_id = m_connection->lookupPeer(sender);
-			// We do not have to remind the peer of its
-			// peer id as the CONTROLTYPE_SET_PEER_ID
-			// command was sent reliably.
+				if (peer_id != PEER_ID_INEXISTENT) {
+					/* During join it can happen that the CONTROLTYPE_SET_PEER_ID
+					 * packet is lost. Since resends are not active at this stage
+					 * we need to remind the peer manually. */
+					m_connection->doResendOne(peer_id);
+				}
 			}
 
+			// Someone new is trying to talk to us. Add them.
 			if (peer_id == PEER_ID_INEXISTENT) {
-			/* Ignore it if we are a client */
-			if (m_connection->ConnectedToServer())
-				return;
-			/* The peer was not found in our lists. Add it. */
 				peer_id = m_connection->createPeer(sender, MTP_MINETEST_RELIABLE_UDP, 0);
 			}
+		}
 
 		PeerHelper peer = m_connection->getPeerNoEx(peer_id);
 		if (!peer) {
@@ -959,6 +1002,9 @@ void ConnectionReceiveThread::receive(SharedBuffer<u8> &packetdata,
 			return;
 		}
 
+		if (knew_peer_id)
+			peer->SetFullyOpen();
+
 		peer->ResetTimeout();
 
 		Channel *channel = nullptr;

src/network/connectionthreads.h

@@ -70,6 +70,7 @@ public:
 
 private:
 	void runTimeouts(float dtime);
+	void resendReliable(Channel &channel, const BufferedPacket *k, float resend_timeout);
 	void rawSend(const BufferedPacket *p);
 	bool rawSendAsPacket(session_t peer_id, u8 channelnum,
 			const SharedBuffer<u8> &data, bool reliable);