Added VERB_ACK and VERB_QOS_MEASUREMENT, refined notion of path quality

18 files changed, 899 insertions, 665 deletions

diff --git a/include/ZeroTierDebug.h b/include/ZeroTierDebug.h
index 8e5366f0..a60179b7 100644
--- a/include/ZeroTierDebug.h
+++ b/include/ZeroTierDebug.h
@@ -86,7 +86,7 @@
 		#include <android/log.h>
 		#define ZT_LOG_TAG "ZTSDK"
 #endif
-#if defined(ZT_TRACE)
+#if defined(ZT_DEBUG_TRACE)
 	#if ZT_MSG_INFO == true
 		#if defined(__ANDROID__)
 			#define DEBUG_INFO(fmt, args...) ((void)__android_log_print(ANDROID_LOG_VERBOSE, ZT_LOG_TAG, \
diff --git a/make-linux.mk b/make-linux.mk
index 0f5ef384..749c24b8 100644
--- a/make-linux.mk
+++ b/make-linux.mk
@@ -45,6 +45,9 @@ ONE_OBJS+=ext/http-parser/http_parser.o
 ifeq ($(ZT_TRACE),1)
 	override DEFS+=-DZT_TRACE
 endif
+ifeq ($(ZT_DEBUG_TRACE),1)
+	DEFS+=-DZT_DEBUG_TRACE
+endif
 
 ifeq ($(ZT_RULES_ENGINE_DEBUGGING),1)
 	override DEFS+=-DZT_RULES_ENGINE_DEBUGGING
diff --git a/make-mac.mk b/make-mac.mk
index 1178437a..12357f68 100644
--- a/make-mac.mk
+++ b/make-mac.mk
@@ -43,7 +43,10 @@ ONE_OBJS+=ext/libnatpmp/natpmp.o ext/libnatpmp/getgateway.o ext/miniupnpc/connec
 
 # Build with address sanitization library for advanced debugging (clang)
 ifeq ($(ZT_SANITIZE),1)
-	SANFLAGS+=-fsanitize=address -DASAN_OPTIONS=symbolize=1
+	DEFS+=-fsanitize=address -DASAN_OPTIONS=symbolize=1
+endif
+ifeq ($(ZT_DEBUG_TRACE),1)
+	DEFS+=-DZT_DEBUG_TRACE
 endif
 # Debug mode -- dump trace output, build binary with -g
 ifeq ($(ZT_DEBUG),1)
diff --git a/node/Constants.hpp b/node/Constants.hpp
index ee2ff0a6..227497de 100644
--- a/node/Constants.hpp
+++ b/node/Constants.hpp
@@ -268,11 +268,6 @@
 #define ZT_PING_CHECK_INVERVAL 5000
 
 /**
- * Length of interface name
- */
-#define ZT_PATH_INTERFACE_NAME_SZ 16
-
-/**
  * How frequently to check for changes to the system's network interfaces. When
  * the service decides to use this constant it's because we want to react more
  * quickly to new interfaces that pop up or go down.
@@ -286,78 +281,95 @@
 #define ZT_MULTIPATH_PROPORTION_WIN_SZ 128
 
 /**
- * Threshold for flow to be considered balanced.
+ * How often we will sample packet latency. Should be at least greater than ZT_PING_CHECK_INVERVAL
+ * since we will record a 0 bit/s measurement if no valid latency measurement was made within this
+ * window of time.
  */
-#define ZT_MULTIPATH_FLOW_BALANCE_THESHOLD 0.80
+#define ZT_PATH_LATENCY_SAMPLE_INTERVAL ZT_MULTIPATH_PEER_PING_PERIOD * 2
 
 /**
- * Number of samples to consider when computing path statistics
+ * Interval used for rate-limiting the computation of path quality estimates. Set at 0
+ * to compute as new packets arrive with no delay.
  */
-#define ZT_PATH_QUALITY_METRIC_WIN_SZ 64
+#define ZT_PATH_QUALITY_COMPUTE_INTERVAL 1000
 
 /**
- * How often important path metrics are sampled (in ms). These metrics are later used
- * for path quality estimates
+ * Number of samples to consider when computing real-time path statistics
  */
-#define ZT_PATH_QUALITY_SAMPLE_INTERVAL 100
+#define ZT_PATH_QUALITY_METRIC_REALTIME_CONSIDERATION_WIN_SZ 128
 
 /**
- * How often new path quality estimates are computed
+ * Number of samples to consider when computing performing long-term path quality analysis.
+ * By default this value is set to ZT_PATH_QUALITY_METRIC_REALTIME_CONSIDERATION_WIN_SZ but can
+ * be set to any value greater than that to observe longer-term path quality behavior.
  */
-#define ZT_PATH_QUALITY_ESTIMATE_INTERVAL 100
+#define ZT_PATH_QUALITY_METRIC_WIN_SZ ZT_PATH_QUALITY_METRIC_REALTIME_CONSIDERATION_WIN_SZ
 
 /**
- * How often we will sample packet latency. Should be at least greater than ZT_PING_CHECK_INVERVAL
- * since we will record a 0 bit/s measurement if no valid latency measurement was made within this
- * window of time.
+ * Maximum acceptable Packet Delay Variance (PDV) over a path
  */
-#define ZT_PATH_LATENCY_SAMPLE_INTERVAL ZT_MULTIPATH_PEER_PING_PERIOD * 2
+#define ZT_PATH_MAX_PDV 1000
 
 /**
- * Interval used for rate-limiting the computation of path quality estimates. Set at 0
- * to compute as new packets arrive with no delay.
+ * Maximum acceptable time interval between expectation and receipt of at least one ACK over a path
+ */
+#define ZT_PATH_MAX_AGE 30000
+
+/**
+ * Maximum acceptable mean latency over a path
+ */
+#define ZT_PATH_MAX_MEAN_LATENCY 1000
+
+/**
+ * How much each factor contributes to the "stability" score of a path
+ */
+#define ZT_PATH_CONTRIB_PDV                    1.0 / 3.0
+#define ZT_PATH_CONTRIB_LATENCY                1.0 / 3.0
+#define ZT_PATH_CONTRIB_THROUGHPUT_DISTURBANCE 1.0 / 3.0
+
+/**
+ * How much each factor contributes to the "quality" score of a path
  */
-#define ZT_PATH_QUALITY_COMPUTE_INTERVAL 0
+#define ZT_PATH_CONTRIB_STABILITY  0.75 / 3.0
+#define ZT_PATH_CONTRIB_THROUGHPUT 1.50 / 3.0
+#define ZT_PATH_CONTRIB_SCOPE      0.75 / 3.0
 
 /**
- * Path error rate history window size. This is used to keep track of packet error
- * measurements over a path's medium-term history.
+ * Min and max acceptable sizes for a VERB_QOS_MEASUREMENT packet
  */
-#define ZT_PATH_ERROR_HIST_WIN_SZ 10
+#define ZT_PATH_MIN_QOS_PACKET_SZ 8 + 1
+#define ZT_PATH_MAX_QOS_PACKET_SZ 1400
 
 /**
- * The number of packet error measurements in each sample
+ * How many ID:sojourn time pairs in a single QoS packet
  */
-#define ZT_PATH_ERROR_SAMPLE_WIN_SZ 1024
+#define ZT_PATH_QOS_TABLE_SIZE (ZT_PATH_MAX_QOS_PACKET_SZ * 8) / (64 + 8)
 
 /**
- * How often a peer will prune its own paths. Pruning is important when multipath is
- * enabled because we want to prevent the allocation algorithms from sending anything
- * out on known dead paths. Additionally, quickly marking paths as dead helps when
- * a new path is learned and needs to replace an older path.
+ * How often the service tests the path throughput
  */
-#define ZT_CLOSED_PATH_PRUNING_INTERVAL 1000
+#define ZT_PATH_THROUGHPUT_MEASUREMENT_INTERVAL ZT_PATH_ACK_INTERVAL * 8
 
 /**
- * Datagram used to test link throughput. Contents are random.
+ * Minimum amount of time between each ACK packet
  */
-#define ZT_LINK_TEST_DATAGRAM_SZ 1024
+#define ZT_PATH_ACK_INTERVAL 250
 
 /**
- * Size of datagram expected as a reply to a link speed test
+ * How often a QoS packet is sent
  */
-#define ZT_LINK_TEST_DATAGRAM_RESPONSE_SZ 8
+#define ZT_PATH_QOS_INTERVAL 1000
 
 /**
- * Time before a link test datagram is considered lost. Any corresponding
- * timing records that would have been used to compute a RTT are purged.
+ * How often an aggregate link statistics report is emitted into this tracing system
  */
-#define ZT_LINK_TEST_TIMEOUT 10000
+#define ZT_PATH_AGGREGATE_STATS_REPORT_INTERVAL 60000
 
 /**
- * How often the service tests the link throughput.
+ * How much an aggregate link's component paths can vary from their target allocation
+ * before the link is considered to be in a state of imbalance.
  */
-#define ZT_LINK_SPEED_TEST_INTERVAL 1000
+#define ZT_PATH_IMBALANCE_THRESHOLD 0.20
 
 /**
  * How frequently to send heartbeats over in-use paths
diff --git a/node/IncomingPacket.cpp b/node/IncomingPacket.cpp
index 8f6dda63..e7227412 100644
--- a/node/IncomingPacket.cpp
+++ b/node/IncomingPacket.cpp
@@ -80,7 +80,7 @@ bool IncomingPacket::tryDecode(const RuntimeEnvironment *RR,void *tPtr)
 			if (!trusted) {
 				if (!dearmor(peer->key())) {
 					RR->t->incomingPacketMessageAuthenticationFailure(tPtr,_path,packetId(),sourceAddress,hops(),"invalid MAC");
-					_path->recordPacket(false);
+					_path->recordInvalidPacket();
 					return true;
 				}
 			}
@@ -90,15 +90,15 @@ bool IncomingPacket::tryDecode(const RuntimeEnvironment *RR,void *tPtr)
 				return true;
 			}
 
-			_path->recordPacket(true);
-
 			const Packet::Verb v = verb();
 			switch(v) {
 				//case Packet::VERB_NOP:
 				default: // ignore unknown verbs, but if they pass auth check they are "received"
-					peer->received(tPtr,_path,hops(),packetId(),v,0,Packet::VERB_NOP,false,0);
+					peer->received(tPtr,_path,hops(),packetId(),payloadLength(),v,0,Packet::VERB_NOP,false,0);
 					return true;
 				case Packet::VERB_HELLO:                      return _doHELLO(RR,tPtr,true);
+				case Packet::VERB_ACK:                        return _doACK(RR,tPtr,peer);
+				case Packet::VERB_QOS_MEASUREMENT:            return _doQOS_MEASUREMENT(RR,tPtr,peer);
 				case Packet::VERB_ERROR:                      return _doERROR(RR,tPtr,peer);
 				case Packet::VERB_OK:                         return _doOK(RR,tPtr,peer);
 				case Packet::VERB_WHOIS:                      return _doWHOIS(RR,tPtr,peer);
@@ -197,11 +197,55 @@ bool IncomingPacket::_doERROR(const RuntimeEnvironment *RR,void *tPtr,const Shar
 		default: break;
 	}
 
-	peer->received(tPtr,_path,hops(),packetId(),Packet::VERB_ERROR,inRePacketId,inReVerb,false,networkId);
+	peer->received(tPtr,_path,hops(),packetId(),payloadLength(),Packet::VERB_ERROR,inRePacketId,inReVerb,false,networkId);
+
+	return true;
+}
+
+bool IncomingPacket::_doACK(const RuntimeEnvironment *RR,void *tPtr,const SharedPtr<Peer> &peer)
+{
+	/* Dissect incoming ACK packet. From this we can estimate current throughput of the path, establish known
+	 * maximums and detect packet loss. */
+
+	if (RR->node->getMultipathMode() != ZT_MULTIPATH_NONE) {
+		int32_t ackedBytes;
+		memcpy(&ackedBytes, payload(), sizeof(int32_t));
+		_path->receivedAck(RR->node->now(), Utils::ntoh(ackedBytes));
+	}
 
 	return true;
 }
 
+bool IncomingPacket::_doQOS_MEASUREMENT(const RuntimeEnvironment *RR,void *tPtr,const SharedPtr<Peer> &peer)
+{
+
+	/* Dissect incoming QoS packet. From this we can compute latency values and their variance.
+	 * The latency variance is used as a measure of "jitter". */
+
+	if (RR->node->getMultipathMode() != ZT_MULTIPATH_NONE) {
+		if (payloadLength() < ZT_PATH_MAX_QOS_PACKET_SZ && payloadLength() > ZT_PATH_MIN_QOS_PACKET_SZ) {
+			const int64_t now = RR->node->now();
+			uint64_t rx_id[ZT_PATH_QOS_TABLE_SIZE];
+			uint8_t rx_ts[ZT_PATH_QOS_TABLE_SIZE];
+			char *begin = (char *)payload();
+			char *ptr = begin;
+			int count = 0;
+			int len = payloadLength();
+			// Read packet IDs and latency compensation intervals for each packet tracked by thie QoS packet
+			while (ptr < (begin + len)) {
+				memcpy((void*)&rx_id[count], ptr, sizeof(uint64_t));
+				rx_id[count] = Utils::ntoh(rx_id[count]);
+				ptr+=sizeof(uint64_t);
+				memcpy((void*)&rx_ts[count], ptr, sizeof(uint8_t));
+				ptr+=sizeof(uint8_t);
+				count++;
+			}
+			_path->receivedQoS(now, count, rx_id, rx_ts);
+		}
+	}
+	return true;
+}
+
 bool IncomingPacket::_doHELLO(const RuntimeEnvironment *RR,void *tPtr,const bool alreadyAuthenticated)
 {
 	const int64_t now = RR->node->now();
@@ -398,7 +442,7 @@ bool IncomingPacket::_doHELLO(const RuntimeEnvironment *RR,void *tPtr,const bool
 	_path->send(RR,tPtr,outp.data(),outp.size(),now);
 
 	peer->setRemoteVersion(protoVersion,vMajor,vMinor,vRevision); // important for this to go first so received() knows the version
-	peer->received(tPtr,_path,hops(),pid,Packet::VERB_HELLO,0,Packet::VERB_NOP,false,0);
+	peer->received(tPtr,_path,hops(),pid,payloadLength(),Packet::VERB_HELLO,0,Packet::VERB_NOP,false,0);
 
 	return true;
 }
@@ -448,8 +492,9 @@ bool IncomingPacket::_doOK(const RuntimeEnvironment *RR,void *tPtr,const SharedP
 				}
 			}
 
-			if (!hops())
+			if (!hops() && (RR->node->getMultipathMode() != ZT_MULTIPATH_NONE)) {
 				_path->updateLatency((unsigned int)latency, RR->node->now());
+			}
 
 			peer->setRemoteVersion(vProto,vMajor,vMinor,vRevision);
 
@@ -510,7 +555,7 @@ bool IncomingPacket::_doOK(const RuntimeEnvironment *RR,void *tPtr,const SharedP
 		default: break;
 	}
 
-	peer->received(tPtr,_path,hops(),packetId(),Packet::VERB_OK,inRePacketId,inReVerb,false,networkId);
+	peer->received(tPtr,_path,hops(),packetId(),payloadLength(),Packet::VERB_OK,inRePacketId,inReVerb,false,networkId);
 
 	return true;
 }
@@ -545,7 +590,7 @@ bool IncomingPacket::_doWHOIS(const RuntimeEnvironment *RR,void *tPtr,const Shar
 		_path->send(RR,tPtr,outp.data(),outp.size(),RR->node->now());
 	}
 
-	peer->received(tPtr,_path,hops(),packetId(),Packet::VERB_WHOIS,0,Packet::VERB_NOP,false,0);
+	peer->received(tPtr,_path,hops(),packetId(),payloadLength(),Packet::VERB_WHOIS,0,Packet::VERB_NOP,false,0);
 
 	return true;
 }
@@ -569,7 +614,7 @@ bool IncomingPacket::_doRENDEZVOUS(const RuntimeEnvironment *RR,void *tPtr,const
 		}
 	}
 
-	peer->received(tPtr,_path,hops(),packetId(),Packet::VERB_RENDEZVOUS,0,Packet::VERB_NOP,false,0);
+	peer->received(tPtr,_path,hops(),packetId(),payloadLength(),Packet::VERB_RENDEZVOUS,0,Packet::VERB_NOP,false,0);
 
 	return true;
 }
@@ -598,7 +643,7 @@ bool IncomingPacket::_doFRAME(const RuntimeEnvironment *RR,void *tPtr,const Shar
 		_sendErrorNeedCredentials(RR,tPtr,peer,nwid);
 	}
 
-	peer->received(tPtr,_path,hops(),packetId(),Packet::VERB_FRAME,0,Packet::VERB_NOP,trustEstablished,nwid);
+	peer->received(tPtr,_path,hops(),packetId(),payloadLength(),Packet::VERB_FRAME,0,Packet::VERB_NOP,trustEstablished,nwid);
 
 	return true;
 }
@@ -621,7 +666,7 @@ bool IncomingPacket::_doEXT_FRAME(const RuntimeEnvironment *RR,void *tPtr,const
 		if (!network->gate(tPtr,peer)) {
 			RR->t->incomingNetworkAccessDenied(tPtr,network,_path,packetId(),size(),peer->address(),Packet::VERB_EXT_FRAME,true);
 			_sendErrorNeedCredentials(RR,tPtr,peer,nwid);
-			peer->received(tPtr,_path,hops(),packetId(),Packet::VERB_EXT_FRAME,0,Packet::VERB_NOP,false,nwid);
+			peer->received(tPtr,_path,hops(),packetId(),payloadLength(),Packet::VERB_EXT_FRAME,0,Packet::VERB_NOP,false,nwid);
 			return true;
 		}
 
@@ -633,7 +678,7 @@ bool IncomingPacket::_doEXT_FRAME(const RuntimeEnvironment *RR,void *tPtr,const
 			const uint8_t *const frameData = (const uint8_t *)field(comLen + ZT_PROTO_VERB_EXT_FRAME_IDX_PAYLOAD,frameLen);
 
 			if ((!from)||(from == network->mac())) {
-				peer->received(tPtr,_path,hops(),packetId(),Packet::VERB_EXT_FRAME,0,Packet::VERB_NOP,true,nwid); // trustEstablished because COM is okay
+				peer->received(tPtr,_path,hops(),packetId(),payloadLength(),Packet::VERB_EXT_FRAME,0,Packet::VERB_NOP,true,nwid); // trustEstablished because COM is okay
 				return true;
 			}
 
@@ -644,19 +689,19 @@ bool IncomingPacket::_doEXT_FRAME(const RuntimeEnvironment *RR,void *tPtr,const
 							network->learnBridgeRoute(from,peer->address());
 						} else {
 							RR->t->incomingNetworkFrameDropped(tPtr,network,_path,packetId(),size(),peer->address(),Packet::VERB_EXT_FRAME,from,to,"bridging not allowed (remote)");
-							peer->received(tPtr,_path,hops(),packetId(),Packet::VERB_EXT_FRAME,0,Packet::VERB_NOP,true,nwid); // trustEstablished because COM is okay
+							peer->received(tPtr,_path,hops(),packetId(),payloadLength(),Packet::VERB_EXT_FRAME,0,Packet::VERB_NOP,true,nwid); // trustEstablished because COM is okay
 							return true;
 						}
 					} else if (to != network->mac()) {
 						if (to.isMulticast()) {
 							if (network->config().multicastLimit == 0) {
 								RR->t->incomingNetworkFrameDropped(tPtr,network,_path,packetId(),size(),peer->address(),Packet::VERB_EXT_FRAME,from,to,"multicast disabled");
-								peer->received(tPtr,_path,hops(),packetId(),Packet::VERB_EXT_FRAME,0,Packet::VERB_NOP,true,nwid); // trustEstablished because COM is okay
+								peer->received(tPtr,_path,hops(),packetId(),payloadLength(),Packet::VERB_EXT_FRAME,0,Packet::VERB_NOP,true,nwid); // trustEstablished because COM is okay
 								return true;
 							}
 						} else if (!network->config().permitsBridging(RR->identity.address())) {
 							RR->t->incomingNetworkFrameDropped(tPtr,network,_path,packetId(),size(),peer->address(),Packet::VERB_EXT_FRAME,from,to,"bridging not allowed (local)");
-							peer->received(tPtr,_path,hops(),packetId(),Packet::VERB_EXT_FRAME,0,Packet::VERB_NOP,true,nwid); // trustEstablished because COM is okay
+							peer->received(tPtr,_path,hops(),packetId(),payloadLength(),Packet::VERB_EXT_FRAME,0,Packet::VERB_NOP,true,nwid); // trustEstablished because COM is okay
 							return true;
 						}
 					}
@@ -676,9 +721,9 @@ bool IncomingPacket::_doEXT_FRAME(const RuntimeEnvironment *RR,void *tPtr,const
 			_path->send(RR,tPtr,outp.data(),outp.size(),RR->node->now());
 		}
 
-		peer->received(tPtr,_path,hops(),packetId(),Packet::VERB_EXT_FRAME,0,Packet::VERB_NOP,true,nwid);
+		peer->received(tPtr,_path,hops(),packetId(),payloadLength(),Packet::VERB_EXT_FRAME,0,Packet::VERB_NOP,true,nwid);
 	} else {
-		peer->received(tPtr,_path,hops(),packetId(),Packet::VERB_EXT_FRAME,0,Packet::VERB_NOP,false,nwid);
+		peer->received(tPtr,_path,hops(),packetId(),payloadLength(),Packet::VERB_EXT_FRAME,0,Packet::VERB_NOP,false,nwid);
 	}
 
 	return true;
@@ -698,7 +743,7 @@ bool IncomingPacket::_doECHO(const RuntimeEnvironment *RR,void *tPtr,const Share
 	outp.armor(peer->key(),true);
 	_path->send(RR,tPtr,outp.data(),outp.size(),RR->node->now());
 
-	peer->received(tPtr,_path,hops(),pid,Packet::VERB_ECHO,0,Packet::VERB_NOP,false,0);
+	peer->received(tPtr,_path,hops(),pid,payloadLength(),Packet::VERB_ECHO,0,Packet::VERB_NOP,false,0);
 
 	return true;
 }
@@ -743,7 +788,7 @@ bool IncomingPacket::_doMULTICAST_LIKE(const RuntimeEnvironment *RR,void *tPtr,c
 		}
 	}
 
-	peer->received(tPtr,_path,hops(),packetId(),Packet::VERB_MULTICAST_LIKE,0,Packet::VERB_NOP,trustEstablished,(network) ? network->id() : 0);
+	peer->received(tPtr,_path,hops(),packetId(),payloadLength(),Packet::VERB_MULTICAST_LIKE,0,Packet::VERB_NOP,trustEstablished,(network) ? network->id() : 0);
 
 	return true;
 }
@@ -866,7 +911,7 @@ bool IncomingPacket::_doNETWORK_CREDENTIALS(const RuntimeEnvironment *RR,void *t
 		}
 	}
 
-	peer->received(tPtr,_path,hops(),packetId(),Packet::VERB_NETWORK_CREDENTIALS,0,Packet::VERB_NOP,trustEstablished,(network) ? network->id() : 0);
+	peer->received(tPtr,_path,hops(),packetId(),payloadLength(),Packet::VERB_NETWORK_CREDENTIALS,0,Packet::VERB_NOP,trustEstablished,(network) ? network->id() : 0);
 
 	return true;
 }
@@ -892,7 +937,7 @@ bool IncomingPacket::_doNETWORK_CONFIG_REQUEST(const RuntimeEnvironment *RR,void
 		_path->send(RR,tPtr,outp.data(),outp.size(),RR->node->now());
 	}
 
-	peer->received(tPtr,_path,hopCount,requestPacketId,Packet::VERB_NETWORK_CONFIG_REQUEST,0,Packet::VERB_NOP,false,nwid);
+	peer->received(tPtr,_path,hopCount,requestPacketId,payloadLength(),Packet::VERB_NETWORK_CONFIG_REQUEST,0,Packet::VERB_NOP,false,nwid);
 
 	return true;
 }
@@ -913,7 +958,7 @@ bool IncomingPacket::_doNETWORK_CONFIG(const RuntimeEnvironment *RR,void *tPtr,c
 		}
 	}
 
-	peer->received(tPtr,_path,hops(),packetId(),Packet::VERB_NETWORK_CONFIG,0,Packet::VERB_NOP,false,(network) ? network->id() : 0);
+	peer->received(tPtr,_path,hops(),packetId(),payloadLength(),Packet::VERB_NETWORK_CONFIG,0,Packet::VERB_NOP,false,(network) ? network->id() : 0);
 
 	return true;
 }
@@ -956,7 +1001,7 @@ bool IncomingPacket::_doMULTICAST_GATHER(const RuntimeEnvironment *RR,void *tPtr
 		}
 	}
 
-	peer->received(tPtr,_path,hops(),packetId(),Packet::VERB_MULTICAST_GATHER,0,Packet::VERB_NOP,trustEstablished,nwid);
+	peer->received(tPtr,_path,hops(),packetId(),payloadLength(),Packet::VERB_MULTICAST_GATHER,0,Packet::VERB_NOP,trustEstablished,nwid);
 
 	return true;
 }
@@ -982,7 +1027,7 @@ bool IncomingPacket::_doMULTICAST_FRAME(const RuntimeEnvironment *RR,void *tPtr,
 		if (!network->gate(tPtr,peer)) {
 			RR->t->incomingNetworkAccessDenied(tPtr,network,_path,packetId(),size(),peer->address(),Packet::VERB_MULTICAST_FRAME,true);
 			_sendErrorNeedCredentials(RR,tPtr,peer,nwid);
-			peer->received(tPtr,_path,hops(),packetId(),Packet::VERB_MULTICAST_FRAME,0,Packet::VERB_NOP,false,nwid);
+			peer->received(tPtr,_path,hops(),packetId(),payloadLength(),Packet::VERB_MULTICAST_FRAME,0,Packet::VERB_NOP,false,nwid);
 			return true;
 		}
 
@@ -1006,19 +1051,19 @@ bool IncomingPacket::_doMULTICAST_FRAME(const RuntimeEnvironment *RR,void *tPtr,
 
 		if (network->config().multicastLimit == 0) {
 			RR->t->incomingNetworkFrameDropped(tPtr,network,_path,packetId(),size(),peer->address(),Packet::VERB_MULTICAST_FRAME,from,to.mac(),"multicast disabled");
-			peer->received(tPtr,_path,hops(),packetId(),Packet::VERB_MULTICAST_FRAME,0,Packet::VERB_NOP,false,nwid);
+			peer->received(tPtr,_path,hops(),packetId(),payloadLength(),Packet::VERB_MULTICAST_FRAME,0,Packet::VERB_NOP,false,nwid);
 			return true;
 		}
 
 		if ((frameLen > 0)&&(frameLen <= ZT_MAX_MTU)) {
 			if (!to.mac().isMulticast()) {
 				RR->t->incomingPacketInvalid(tPtr,_path,packetId(),source(),hops(),Packet::VERB_MULTICAST_FRAME,"destination not multicast");
-				peer->received(tPtr,_path,hops(),packetId(),Packet::VERB_MULTICAST_FRAME,0,Packet::VERB_NOP,true,nwid); // trustEstablished because COM is okay
+				peer->received(tPtr,_path,hops(),packetId(),payloadLength(),Packet::VERB_MULTICAST_FRAME,0,Packet::VERB_NOP,true,nwid); // trustEstablished because COM is okay
 				return true;
 			}
 			if ((!from)||(from.isMulticast())||(from == network->mac())) {
 				RR->t->incomingPacketInvalid(tPtr,_path,packetId(),source(),hops(),Packet::VERB_MULTICAST_FRAME,"invalid source MAC");
-				peer->received(tPtr,_path,hops(),packetId(),Packet::VERB_MULTICAST_FRAME,0,Packet::VERB_NOP,true,nwid); // trustEstablished because COM is okay
+				peer->received(tPtr,_path,hops(),packetId(),payloadLength(),Packet::VERB_MULTICAST_FRAME,0,Packet::VERB_NOP,true,nwid); // trustEstablished because COM is okay
 				return true;
 			}
 
@@ -1032,7 +1077,7 @@ bool IncomingPacket::_doMULTICAST_FRAME(const RuntimeEnvironment *RR,void *tPtr,
 					network->learnBridgeRoute(from,peer->address());
 				} else {
 					RR->t->incomingNetworkFrameDropped(tPtr,network,_path,packetId(),size(),peer->address(),Packet::VERB_MULTICAST_FRAME,from,to.mac(),"bridging not allowed (remote)");
-					peer->received(tPtr,_path,hops(),packetId(),Packet::VERB_MULTICAST_FRAME,0,Packet::VERB_NOP,true,nwid); // trustEstablished because COM is okay
+					peer->received(tPtr,_path,hops(),packetId(),payloadLength(),Packet::VERB_MULTICAST_FRAME,0,Packet::VERB_NOP,true,nwid); // trustEstablished because COM is okay
 					return true;
 				}
 			}
@@ -1055,10 +1100,10 @@ bool IncomingPacket::_doMULTICAST_FRAME(const RuntimeEnvironment *RR,void *tPtr,
 			}
 		}
 
-		peer->received(tPtr,_path,hops(),packetId(),Packet::VERB_MULTICAST_FRAME,0,Packet::VERB_NOP,true,nwid);
+		peer->received(tPtr,_path,hops(),packetId(),payloadLength(),Packet::VERB_MULTICAST_FRAME,0,Packet::VERB_NOP,true,nwid);
 	} else {
 		_sendErrorNeedCredentials(RR,tPtr,peer,nwid);
-		peer->received(tPtr,_path,hops(),packetId(),Packet::VERB_MULTICAST_FRAME,0,Packet::VERB_NOP,false,nwid);
+		peer->received(tPtr,_path,hops(),packetId(),payloadLength(),Packet::VERB_MULTICAST_FRAME,0,Packet::VERB_NOP,false,nwid);
 	}
 
 	return true;
@@ -1070,7 +1115,7 @@ bool IncomingPacket::_doPUSH_DIRECT_PATHS(const RuntimeEnvironment *RR,void *tPt
 
 	// First, subject this to a rate limit
 	if (!peer->rateGatePushDirectPaths(now)) {
-		peer->received(tPtr,_path,hops(),packetId(),Packet::VERB_PUSH_DIRECT_PATHS,0,Packet::VERB_NOP,false,0);
+		peer->received(tPtr,_path,hops(),packetId(),payloadLength(),Packet::VERB_PUSH_DIRECT_PATHS,0,Packet::VERB_NOP,false,0);
 		return true;
 	}
 
@@ -1094,7 +1139,7 @@ bool IncomingPacket::_doPUSH_DIRECT_PATHS(const RuntimeEnvironment *RR,void *tPt
 			case 4: {
 				const InetAddress a(field(ptr,4),4,at<uint16_t>(ptr + 4));
 				if (
-				    ((flags & ZT_PUSH_DIRECT_PATHS_FLAG_FORGET_PATH) == 0) && // not being told to forget
+					((flags & ZT_PUSH_DIRECT_PATHS_FLAG_FORGET_PATH) == 0) && // not being told to forget
 						(!( ((flags & ZT_PUSH_DIRECT_PATHS_FLAG_CLUSTER_REDIRECT) == 0) && (peer->hasActivePathTo(now,a)) )) && // not already known
 						(RR->node->shouldUsePathForZeroTierTraffic(tPtr,peer->address(),_path->localSocket(),a)) ) // should use path
 				{
@@ -1108,7 +1153,7 @@ bool IncomingPacket::_doPUSH_DIRECT_PATHS(const RuntimeEnvironment *RR,void *tPt
 			case 6: {
 				const InetAddress a(field(ptr,16),16,at<uint16_t>(ptr + 16));
 				if (
-				    ((flags & ZT_PUSH_DIRECT_PATHS_FLAG_FORGET_PATH) == 0) && // not being told to forget
+					((flags & ZT_PUSH_DIRECT_PATHS_FLAG_FORGET_PATH) == 0) && // not being told to forget
 						(!( ((flags & ZT_PUSH_DIRECT_PATHS_FLAG_CLUSTER_REDIRECT) == 0) && (peer->hasActivePathTo(now,a)) )) && // not already known
 						(RR->node->shouldUsePathForZeroTierTraffic(tPtr,peer->address(),_path->localSocket(),a)) ) // should use path
 				{
@@ -1123,7 +1168,7 @@ bool IncomingPacket::_doPUSH_DIRECT_PATHS(const RuntimeEnvironment *RR,void *tPt
 		ptr += addrLen;
 	}
 
-	peer->received(tPtr,_path,hops(),packetId(),Packet::VERB_PUSH_DIRECT_PATHS,0,Packet::VERB_NOP,false,0);
+	peer->received(tPtr,_path,hops(),packetId(),payloadLength(),Packet::VERB_PUSH_DIRECT_PATHS,0,Packet::VERB_NOP,false,0);
 
 	return true;
 }
@@ -1139,7 +1184,7 @@ bool IncomingPacket::_doUSER_MESSAGE(const RuntimeEnvironment *RR,void *tPtr,con
 		RR->node->postEvent(tPtr,ZT_EVENT_USER_MESSAGE,reinterpret_cast<const void *>(&um));
 	}
 
-	peer->received(tPtr,_path,hops(),packetId(),Packet::VERB_USER_MESSAGE,0,Packet::VERB_NOP,false,0);
+	peer->received(tPtr,_path,hops(),packetId(),payloadLength(),Packet::VERB_USER_MESSAGE,0,Packet::VERB_NOP,false,0);
 
 	return true;
 }
@@ -1163,7 +1208,7 @@ bool IncomingPacket::_doREMOTE_TRACE(const RuntimeEnvironment *RR,void *tPtr,con
 		}
 	}
 
-	peer->received(tPtr,_path,hops(),packetId(),Packet::VERB_REMOTE_TRACE,0,Packet::VERB_NOP,false,0);
+	peer->received(tPtr,_path,hops(),packetId(),payloadLength(),Packet::VERB_REMOTE_TRACE,0,Packet::VERB_NOP,false,0);
 
 	return true;
 }
diff --git a/node/IncomingPacket.hpp b/node/IncomingPacket.hpp
index 88f4f066..9144277c 100644
--- a/node/IncomingPacket.hpp
+++ b/node/IncomingPacket.hpp
@@ -125,6 +125,8 @@ private:
 	// been authenticated, decrypted, decompressed, and classified.
 	bool _doERROR(const RuntimeEnvironment *RR,void *tPtr,const SharedPtr<Peer> &peer);
 	bool _doHELLO(const RuntimeEnvironment *RR,void *tPtr,const bool alreadyAuthenticated);
+	bool _doACK(const RuntimeEnvironment *RR,void *tPtr,const SharedPtr<Peer> &peer);
+	bool _doQOS_MEASUREMENT(const RuntimeEnvironment *RR,void *tPtr,const SharedPtr<Peer> &peer);
 	bool _doOK(const RuntimeEnvironment *RR,void *tPtr,const SharedPtr<Peer> &peer);
 	bool _doWHOIS(const RuntimeEnvironment *RR,void *tPtr,const SharedPtr<Peer> &peer);
 	bool _doRENDEZVOUS(const RuntimeEnvironment *RR,void *tPtr,const SharedPtr<Peer> &peer);
diff --git a/node/Packet.hpp b/node/Packet.hpp
index 27da6fb5..6869691e 100644
--- a/node/Packet.hpp
+++ b/node/Packet.hpp
@@ -419,7 +419,7 @@ public:
 
 		template<unsigned int C2>
 		Fragment(const Buffer<C2> &b) :
-	 		Buffer<ZT_PROTO_MAX_PACKET_LENGTH>(b)
+			Buffer<ZT_PROTO_MAX_PACKET_LENGTH>(b)
 		{
 		}
 
@@ -930,7 +930,53 @@ public:
 		 */
 		VERB_PUSH_DIRECT_PATHS = 0x10,
 
-		// 0x11, 0x12 -- deprecated
+		// 0x11 -- deprecated
+
+		/**
+		 * An acknowledgement of receipt of a series of recent packets from another
+		 * peer. This is used to calculate relative throughput values and to detect
+		 * packet loss. Only VERB_FRAME and VERB_EXT_FRAME packets are counted.
+		 *
+		 * ACK response format:
+		 *  <[4] 32-bit number of bytes received since last ACK>
+		 *
+		 * Upon receipt of this packet, the local peer will verify that the correct
+		 * number of bytes were received by the remote peer. If these values do
+		 * not agree that could be an indicator of packet loss.
+		 *
+		 * Additionally, the local peer knows the interval of time that has
+		 * elapsed since the last received ACK. With this information it can compute
+		 * a rough estimate of the current throughput.
+		 *
+		 * This is sent at a maximum rate of once per every ZT_PATH_ACK_INTERVAL
+		 */
+		VERB_ACK = 0x12,
+
+		/**
+		 * A packet containing timing measurements useful for estimating path quality.
+		 * Composed of a list of <packet ID:internal sojourn time> pairs for an
+		 * arbitrary set of recent packets. This is used to sample for latency and
+		 * packet delay variance (PDV, "jitter").
+		 *
+		 * QoS record format:
+		 *
+		 *  <[8] 64-bit packet ID of previously-received packet>
+		 *  <[1] 8-bit packet sojourn time>
+		 *  <...repeat until end of max 1400 byte packet...>
+		 *
+		 * The number of possible records per QoS packet is: (1400 * 8) / 72 = 155
+		 * This packet should be sent very rarely (every few seconds) as it can be
+		 * somewhat large if the connection is saturated. Future versions might use
+		 * a bloom table to probablistically determine these values in a vastly
+		 * more space-efficient manner.
+		 *
+		 * Note: The 'internal packet sojourn time' is a slight misnomer as it is a
+		 * measure of the amount of time between when a packet was received and the
+		 * egress time of its tracking QoS packet.
+		 *
+		 * This is sent at a maximum rate of once per every ZT_PATH_QOS_INTERVAL
+		 */
+		VERB_QOS_MEASUREMENT = 0x13,
 
 		/**
 		 * A message with arbitrary user-definable content:
@@ -999,7 +1045,7 @@ public:
 
 	template<unsigned int C2>
 	Packet(const Buffer<C2> &b) :
- 		Buffer<ZT_PROTO_MAX_PACKET_LENGTH>(b)
+		Buffer<ZT_PROTO_MAX_PACKET_LENGTH>(b)
 	{
 	}
 
diff --git a/node/Path.hpp b/node/Path.hpp
index e6bcecf0..0278d919 100644
--- a/node/Path.hpp
+++ b/node/Path.hpp
@@ -102,24 +102,23 @@ public:
 		_latency(0xffff),
 		_addr(),
 		_ipScope(InetAddress::IP_SCOPE_NONE),
-		_currentPacketSampleCounter(0),
-		_meanPacketErrorRatio(0.0),
-		_meanLatency(0.0),
-		_lastLatencyUpdate(0),
-		_jitter(0.0),
-		_lastPathQualitySampleTime(0),
-		_lastComputedQuality(0.0),
-		_lastPathQualityEstimate(0),
-		_meanAge(0.0),
+		_lastAck(0),
+		_lastThroughputEstimation(0),
+		_lastQoSMeasurement(0),
+		_unackedBytes(0),
+		_expectingAckAsOf(0),
+		_packetsReceivedSinceLastAck(0),
 		_meanThroughput(0.0),
-		_packetLossRatio(0)
+		_maxLifetimeThroughput(0),
+		_bytesAckedSinceLastThroughputEstimation(0),
+		_meanLatency(0.0),
+		_packetDelayVariance(0.0),
+		_packetErrorRatio(0.0),
+		_packetLossRatio(0),
+		_lastComputedStability(0.0),
+		_lastComputedRelativeQuality(0)
 	{
-		memset(_ifname, 0, sizeof(_ifname));
-		memset(_addrString, 0, sizeof(_addrString));
-		_throughputSamples = new RingBuffer<uint64_t>(ZT_PATH_QUALITY_METRIC_WIN_SZ);
-		_ageSamples = new RingBuffer<uint64_t>(ZT_PATH_QUALITY_METRIC_WIN_SZ);
-		_latencySamples = new RingBuffer<uint32_t>(ZT_PATH_QUALITY_METRIC_WIN_SZ);
-		_errSamples = new RingBuffer<float>(ZT_PATH_QUALITY_METRIC_WIN_SZ);
+		prepareBuffers();
 	}
 
 	Path(const int64_t localSocket,const InetAddress &addr) :
@@ -131,37 +130,35 @@ public:
 		_latency(0xffff),
 		_addr(addr),
 		_ipScope(addr.ipScope()),
-		_currentPacketSampleCounter(0),
-		_meanPacketErrorRatio(0.0),
-		_meanLatency(0.0),
-		_lastLatencyUpdate(0),
-		_jitter(0.0),
-		_lastPathQualitySampleTime(0),
-		_lastComputedQuality(0.0),
-		_lastPathQualityEstimate(0),
-		_meanAge(0.0),
+		_lastAck(0),
+		_lastThroughputEstimation(0),
+		_lastQoSMeasurement(0),
+		_unackedBytes(0),
+		_expectingAckAsOf(0),
+		_packetsReceivedSinceLastAck(0),
 		_meanThroughput(0.0),
-		_packetLossRatio(0)
+		_maxLifetimeThroughput(0),
+		_bytesAckedSinceLastThroughputEstimation(0),
+		_meanLatency(0.0),
+		_packetDelayVariance(0.0),
+		_packetErrorRatio(0.0),
+		_packetLossRatio(0),
+		_lastComputedStability(0.0),
+		_lastComputedRelativeQuality(0)
 	{
-		memset(_ifname, 0, sizeof(_ifname));
-		memset(_addrString, 0, sizeof(_addrString));
-		_throughputSamples = new RingBuffer<uint64_t>(ZT_PATH_QUALITY_METRIC_WIN_SZ);
-		_ageSamples = new RingBuffer<uint64_t>(ZT_PATH_QUALITY_METRIC_WIN_SZ);
-		_latencySamples = new RingBuffer<uint32_t>(ZT_PATH_QUALITY_METRIC_WIN_SZ);
-		_errSamples = new RingBuffer<float>(ZT_PATH_QUALITY_METRIC_WIN_SZ);
+		prepareBuffers();
 	}
 
 	~Path()
 	{
 		delete _throughputSamples;
-		delete _ageSamples;
 		delete _latencySamples;
-		delete _errSamples;
-
+		delete _qualitySamples;
+		delete _packetValiditySamples;
 		_throughputSamples = NULL;
-		_ageSamples = NULL;
 		_latencySamples = NULL;
-		_errSamples = NULL;
+		_qualitySamples = NULL;
+		_packetValiditySamples = NULL;
 	}
 
 	/**
@@ -209,7 +206,6 @@ public:
 		else {
 			_latency = l;
 		}
-		_lastLatencyUpdate = now;
 		_latencySamples->push(l);
 	}
 
@@ -299,194 +295,273 @@ public:
 	}
 
 	/**
-	 * @return An estimate of path quality -- higher is better.
+	 * Take note that we're expecting a VERB_ACK on this path as of a specific time
+	 *
+	 * @param now Current time
+	 * @param packetId ID of the packet
+	 * @param payloadLength Number of bytes we're is expecting a reply to
 	 */
-	inline float computeQuality(const int64_t now)
+	inline void expectingAck(int64_t now, int64_t packetId, uint16_t payloadLength)
 	{
-		float latency_contrib    = _meanLatency ? (float)1.0 / _meanLatency : 0;
-		float jitter_contrib     = _jitter ? (float)1.0 / _jitter : 0;
-		float throughput_contrib = _meanThroughput ? _meanThroughput / 1000000 : 0; // in Mbps
-		float age_contrib        = _meanAge > 0 ? (float)sqrt(_meanAge) : 1;
-		float error_contrib      = (float)1.0 - _meanPacketErrorRatio;
-		float sum = (latency_contrib + jitter_contrib + throughput_contrib + error_contrib) / age_contrib;
-		_lastComputedQuality = sum * (long)((_ipScope) + 1);
-		return _lastComputedQuality;
+		_expectingAckAsOf = ackAge(now) > ZT_PATH_ACK_INTERVAL ? _expectingAckAsOf : now;
+		_unackedBytes += payloadLength;
+		_outgoingPacketRecords[packetId] = now;
 	}
 
 	/**
-	 * Since quality estimates can become expensive we should cache the most recent result for traffic allocation
-	 * algorithms which may need to reference this value multiple times through the course of their execution.
+	 * Record that we've received a VERB_ACK on this path, also compute throughput if required.
+	 *
+	 * @param now Current time
+	 * @param ackedBytes Number of bytes awknowledged by other peer
 	 */
-	inline float lastComputedQuality() {
-		return _lastComputedQuality;
+	inline void receivedAck(int64_t now, int32_t ackedBytes)
+	{
+		_expectingAckAsOf = 0;
+		_unackedBytes = (ackedBytes > _unackedBytes) ? 0 : _unackedBytes - ackedBytes;
+		int64_t timeSinceThroughputEstimate = (now - _lastThroughputEstimation);
+		if (timeSinceThroughputEstimate >= ZT_PATH_THROUGHPUT_MEASUREMENT_INTERVAL) {
+			uint64_t throughput = (float)(_bytesAckedSinceLastThroughputEstimation) / ((float)timeSinceThroughputEstimate / (float)1000);
+			_throughputSamples->push(throughput);
+			_maxLifetimeThroughput = throughput > _maxLifetimeThroughput ? throughput : _maxLifetimeThroughput;
+			_lastThroughputEstimation = now;
+			_bytesAckedSinceLastThroughputEstimation = 0;
+		} else {
+			_bytesAckedSinceLastThroughputEstimation += ackedBytes;
+		}
 	}
 
 	/**
-	 * @return A pointer to a cached copy of the human-readable name of the interface this Path's localSocket is bound to
+	 * @return Number of bytes this peer is responsible for ACKing since last ACK
 	 */
-	inline char *getName() { return _ifname; }
+	inline int32_t bytesToAck()
+	{
+		int32_t bytesToAck = 0;
+		for (int i=0; i<_packetsReceivedSinceLastAck; i++) {
+			bytesToAck += _recorded_len[i];
+		}
+		return bytesToAck;
+	}
 
 	/**
-	 * @return Estimated throughput in bps of this link
+	 * @return Number of bytes thusfar sent that have not been awknowledged by the remote peer
 	 */
-	inline uint64_t getThroughput() { return _phy->getThroughput((PhySocket *)((uintptr_t)_localSocket)); }
+	inline int64_t unackedSentBytes()
+	{
+		return _unackedBytes;
+	}
 
 	/**
-	 * @return Packet delay varience
+	 * Account for the fact that an ACK was just sent. Reset counters, timers, and clear statistics buffers
+	 *
+	 * @param Current time
 	 */
-	inline float jitter() { return _jitter; }
+	inline void sentAck(int64_t now)
+	{
+		memset(_recorded_id, 0, sizeof(_recorded_id));
+		memset(_recorded_ts, 0, sizeof(_recorded_ts));
+		memset(_recorded_len, 0, sizeof(_recorded_len));
+		_packetsReceivedSinceLastAck = 0;
+		_lastAck = now;
+	}
 
 	/**
-	 * @return Previously-computed mean latency
+	 * Receive QoS data, match with recorded egress times from this peer, compute latency
+	 * estimates.
+	 *
+	 * @param now Current time
+	 * @param count Number of records
+	 * @param rx_id table of packet IDs
+	 * @param rx_ts table of holding times
 	 */
-	inline float meanLatency() { return _meanLatency; }
+	inline void receivedQoS(int64_t now, int count, uint64_t *rx_id, uint8_t *rx_ts)
+	{
+		// Look up egress times and compute latency values for each record
+		for (int j=0; j<count; j++) {
+			std::map<uint64_t,uint64_t>::iterator it = _outgoingPacketRecords.find(rx_id[j]);
+			if (it != _outgoingPacketRecords.end()) {
+				uint16_t rtt = (uint16_t)(now - it->second);
+				uint16_t rtt_compensated = rtt - rx_ts[j];
+				float latency = rtt_compensated / 2.0;
+				updateLatency(latency, now);
+				_outgoingPacketRecords.erase(it);
+			}
+		}
+	}
 
 	/**
-	 * @return Packet loss rate
+	 * Generate the contents of a VERB_QOS_MEASUREMENT packet.
+	 *
+	 * @param now Current time
+	 * @param qosBuffer destination buffer
+	 * @return Size of payload
 	 */
-	inline float packetLossRatio() { return _packetLossRatio; }
+	inline int32_t generateQoSPacket(int64_t now, char *qosBuffer)
+	{
+		int32_t len = 0;
+		for (int i=0; i<_packetsReceivedSinceLastAck; i++) {
+			uint64_t id = _recorded_id[i];
+			memcpy(qosBuffer, &id, sizeof(uint64_t));
+			qosBuffer+=sizeof(uint64_t);
+			uint8_t holdingTime = (uint8_t)(now - _recorded_ts[i]);
+			memcpy(qosBuffer, &holdingTime, sizeof(uint8_t));
+			qosBuffer+=sizeof(uint8_t);
+			len+=sizeof(uint64_t)+sizeof(uint8_t);
+		}
+		return len;
+	}
 
 	/**
-	 * @return Mean packet error ratio
+	 * Account for the fact that a VERB_QOS_MEASUREMENT was just sent. Reset timers.
+	 *
+	 * @param Current time
 	 */
-	inline float meanPacketErrorRatio() { return _meanPacketErrorRatio; }
+	inline void sentQoS(int64_t now) { _lastQoSMeasurement = now; }
 
 	/**
-	 * @return Current packet error ratio (possibly incomplete sample set)
+	 * Record statistics on incoming packets. Used later to estimate QoS.
+	 *
+	 * @param now Current time
+	 * @param packetId
+	 * @param payloadLength
 	 */
-	inline float currentPacketErrorRatio() {
-		int errorsPerSample = 0;
-		for (int i=0; i<_currentPacketSampleCounter; i++) {
-			if (_packetValidity[i] == false) {
-				errorsPerSample++;
-			}
-		}
-		return (float)errorsPerSample / (float)ZT_PATH_ERROR_SAMPLE_WIN_SZ;
+	inline void recordIncomingPacket(int64_t now, int64_t packetId, int32_t payloadLength)
+	{
+		_recorded_ts[_packetsReceivedSinceLastAck] = now;
+		_recorded_id[_packetsReceivedSinceLastAck] = packetId;
+		_recorded_len[_packetsReceivedSinceLastAck] = payloadLength;
+		_packetsReceivedSinceLastAck++;
+		_packetValiditySamples->push(true);
 	}
 
 	/**
-	 * @return Whether the Path's local socket is in a CLOSED state
+	 * @param now Current time
+	 * @return Whether an ACK (VERB_ACK) packet needs to be emitted at this time
 	 */
-	inline bool isClosed() { return _phy->isClosed((PhySocket *)((uintptr_t)_localSocket)); }
+	inline bool needsToSendAck(int64_t now) {
+		return ((now - _lastAck) >= ZT_PATH_ACK_INTERVAL ||
+			(_packetsReceivedSinceLastAck == ZT_PATH_QOS_TABLE_SIZE)) && _packetsReceivedSinceLastAck;
+	}
 
 	/**
-	 * @return The state of a Path's local socket
+	 * @param now Current time
+	 * @return Whether a QoS (VERB_QOS_MEASUREMENT) packet needs to be emitted at this time
 	 */
-	inline int getState() { return _phy->getState((PhySocket *)((uintptr_t)_localSocket)); }
+	inline bool needsToSendQoS(int64_t now) {
+		return ((_packetsReceivedSinceLastAck >= ZT_PATH_QOS_TABLE_SIZE) ||
+			((now - _lastQoSMeasurement) > ZT_PATH_QOS_INTERVAL)) && _packetsReceivedSinceLastAck;
+	}
 
 	/**
-	 * @return Whether this socket may have been erased by the virtual physical link layer
+	 * How much time has elapsed since we've been expecting a VERB_ACK on this path. This value
+	 * is used to determine a more relevant path "age". This lets us penalize paths which are no
+	 * longer ACKing, but not those that simple aren't being used to carry traffic at the
+	 * current time.
 	 */
-	inline bool isValidState() { return _phy->isValidState((PhySocket *)((uintptr_t)_localSocket)); }
+	inline int64_t ackAge(int64_t now) { return _expectingAckAsOf ? now - _expectingAckAsOf : 0; }
 
 	/**
-	 * @return Whether the path quality monitors have collected enough data to provide a quality value
-	 * TODO: expand this
+	 * The maximum observed throughput for this path
 	 */
-	inline bool monitorsReady() {
-		return _latencySamples->count() && _ageSamples->count() && _throughputSamples->count();
-	}
+	inline uint64_t maxLifetimeThroughput() { return _maxLifetimeThroughput; }
 
 	/**
-	 * @return A pointer to a cached copy of the address string for this Path (For debugging only)
+	 * @return The mean throughput (in bits/s) of this link
 	 */
-	inline char *getAddressString() { return _addrString; }
+	inline float meanThroughput() { return _meanThroughput; }
 
 	/**
-	 * Handle path sampling, computation of quality estimates, and other periodic tasks
-	 * @param now Current time
+	 * Assign a new relative quality value for this path in the aggregate link
+	 *
+	 * @param rq Quality of this path in comparison to other paths available to this peer
 	 */
-	inline void measureLink(int64_t now) {
-		// Sample path properties and store them in a continuously-revolving buffer
-		if (now - _lastPathQualitySampleTime > ZT_PATH_QUALITY_SAMPLE_INTERVAL) {
-			_lastPathQualitySampleTime = now;
-			_throughputSamples->push(getThroughput()); // Thoughtput in bits/s
-			_ageSamples->push(now - _lastIn); // Age (time since last received packet)
-			if (now - _lastLatencyUpdate > ZT_PATH_LATENCY_SAMPLE_INTERVAL) {
-				_lastLatencyUpdate = now;
-				// Record 0 bp/s. Since we're using this to detect possible packet loss
-				updateLatency(0, now);
-			}
-		}
-		// Compute statistical values for use in link quality estimates
-		if (now - _lastPathQualityComputeTime > ZT_PATH_QUALITY_COMPUTE_INTERVAL) {
-			_lastPathQualityComputeTime = now;
-			// Cache Path address string
-			address().toString(_addrString);
-			_phy->getIfName((PhySocket *)((uintptr_t)_localSocket), _ifname, ZT_PATH_INTERFACE_NAME_SZ); // Cache Interface name
-			// Derived values
-			if (_throughputSamples->count()) {
-				_packetLossRatio = (float)_throughputSamples->zeroCount() / (float)_throughputSamples->count();
-			}
-			_meanThroughput = _throughputSamples->mean();
-			_meanAge = _ageSamples->mean();
-			_meanLatency = _latencySamples->mean();
-			// Jitter
-			// SEE: RFC 3393, RFC 4689
-			_jitter = _latencySamples->stddev();
-			_meanPacketErrorRatio = _errSamples->mean(); // Packet Error Ratio (PER)
-		}
-		// Periodically compute a path quality estimate
-		if (now - _lastPathQualityEstimate > ZT_PATH_QUALITY_ESTIMATE_INTERVAL) {
-			computeQuality(now);
-		}
-	}
+	inline void updateRelativeQuality(float rq) { _lastComputedRelativeQuality = rq; }
 
 	/**
-	 * @param buf Buffer to store resultant string
-	 * @return Description of path, in ASCII string format
+	 * @return Quality of this path compared to others in the aggregate link
 	 */
-	inline char *toString(char *buf) {
-		sprintf(buf,"%6s, q=%8.3f, %5.3f Mb/s, j=%8.2f, ml=%8.2f, meanAge=%8.2f, addr=%45s",
-			getName(),
-			lastComputedQuality(),
-			(float)meanThroughput() / (float)1000000,
-			jitter(),
-			meanLatency(),
-			meanAge(),
-			getAddressString());
-		return buf;
-	}
+	inline float relativeQuality() { return _lastComputedRelativeQuality; }
 
 	/**
-	 * Record whether a packet is considered invalid by MAC/compression/cipher checks. This
-	 * could be an indication of a bit error. This function will keep a running counter of
-	 * up to a given window size and with each counter overflow it will compute a mean error rate
-	 * and store that in a continuously shifting sample window.
-	 *
-	 * @param isValid Whether the packet in question is considered invalid
+	 * @return Stability estimates can become expensive to compute, we cache the most recent result.
 	 */
-	inline void recordPacket(bool isValid) {
-		if (_currentPacketSampleCounter < ZT_PATH_ERROR_SAMPLE_WIN_SZ) {
-			_packetValidity[_currentPacketSampleCounter] = isValid;
-			_currentPacketSampleCounter++;
-		}
-		else {
-			// Sample array is full, compute an mean and stick it in the ring buffer for trend analysis
-			_errSamples->push(currentPacketErrorRatio());
-			_currentPacketSampleCounter=0;
-		}
-	}
+	inline float lastComputedStability() { return _lastComputedStability; }
 
 	/**
-	 * @return The mean age (in ms) of this link
+	 * @return A pointer to a cached copy of the human-readable name of the interface this Path's localSocket is bound to
 	 */
-	inline float meanAge() { return _meanAge; }
+	inline char *getName() { return _ifname; }
 
 	/**
-	 * @return The mean throughput (in bits/s) of this link
+	 * @return Packet delay varience
 	 */
-	inline float meanThroughput() { return _meanThroughput; }
+	inline float packetDelayVariance() { return _packetDelayVariance; }
 
 	/**
-	 * @return True if this path is alive (receiving heartbeats)
+	 * @return Previously-computed mean latency
 	 */
-	inline bool alive(const int64_t now) const { return ((now - _lastIn) < (ZT_PATH_HEARTBEAT_PERIOD + 5000)); }
+	inline float meanLatency() { return _meanLatency; }
+
+	/**
+	 * @return Packet loss rate (PLR)
+	 */
+	inline float packetLossRatio() { return _packetLossRatio; }
+
+	/**
+	 * @return Packet error ratio (PER)
+	 */
+	inline float packetErrorRatio() { return _packetErrorRatio; }
+
+	/**
+	 * Record an invalid incoming packet. This packet failed MAC/compression/cipher checks and will now
+	 * contribute to a Packet Error Ratio (PER).
+	 */
+	inline void recordInvalidPacket() { _packetValiditySamples->push(false); }
 
 	/**
-	 * @return True if this path hasn't received a packet in a "significant" amount of time
+	 * @return A pointer to a cached copy of the address string for this Path (For debugging only)
 	 */
-	inline bool stale(const int64_t now) const { return ((now - _lastIn) > ZT_LINK_SPEED_TEST_INTERVAL * 10); }
+	inline char *getAddressString() { return _addrString; }
+
+	/**
+	 * Compute and cache stability and performance metrics. The resultant stability coefficint is a measure of how "well behaved"
+	 * this path is. This figure is substantially different from (but required for the estimation of the path's overall "quality".
+	 *
+	 * @param now Current time
+	 */
+	inline void processBackgroundPathMeasurements(int64_t now, const int64_t peerId) {
+		if (now - _lastPathQualityComputeTime > ZT_PATH_QUALITY_COMPUTE_INTERVAL) {
+			_lastPathQualityComputeTime = now;
+			_phy->getIfName((PhySocket *)((uintptr_t)_localSocket), _ifname, 16);
+			address().toString(_addrString);
+			_meanThroughput = _throughputSamples->mean();
+			_meanLatency = _latencySamples->mean();
+			_packetDelayVariance = _latencySamples->stddev(); // Similar to "jitter" (SEE: RFC 3393, RFC 4689)
+			// If no packet validity samples, assume PER==0
+			_packetErrorRatio = 1 - (_packetValiditySamples->count() ? _packetValiditySamples->mean() : 1);
+			// Compute path stability
+			// Normalize measurements with wildly different ranges into a reasonable range
+			float normalized_pdv = Utils::normalize(_packetDelayVariance, 0, ZT_PATH_MAX_PDV, 0, 10);
+			float normalized_la = Utils::normalize(_meanLatency, 0, ZT_PATH_MAX_MEAN_LATENCY, 0, 10);
+			float throughput_cv = _throughputSamples->mean() > 0 ? _throughputSamples->stddev() / _throughputSamples->mean() : 1;
+			// Form an exponential cutoff and apply contribution weights
+			float pdv_contrib = exp((-1)*normalized_pdv) * ZT_PATH_CONTRIB_PDV;
+			float latency_contrib = exp((-1)*normalized_la) * ZT_PATH_CONTRIB_LATENCY;
+			float throughput_disturbance_contrib = exp((-1)*throughput_cv) * ZT_PATH_CONTRIB_THROUGHPUT_DISTURBANCE;
+			// Obey user-defined ignored contributions
+			pdv_contrib = ZT_PATH_CONTRIB_PDV > 0.0 ? pdv_contrib : 1;
+			latency_contrib = ZT_PATH_CONTRIB_LATENCY > 0.0 ? latency_contrib : 1;
+			throughput_disturbance_contrib = ZT_PATH_CONTRIB_THROUGHPUT_DISTURBANCE > 0.0 ? throughput_disturbance_contrib : 1;
+			// Compute the quality product
+			_lastComputedStability = pdv_contrib + latency_contrib + throughput_disturbance_contrib;
+			_lastComputedStability *= 1 - _packetErrorRatio;
+			_qualitySamples->push(_lastComputedStability);
+		}
+	}
+
+	/**
+	 * @return True if this path is alive (receiving heartbeats)
+	 */
+	inline bool alive(const int64_t now) const { return ((now - _lastIn) < (ZT_PATH_HEARTBEAT_PERIOD + 5000)); }
 
 	/**
 	 * @return True if this path needs a heartbeat
@@ -508,6 +583,21 @@ public:
 	 */
 	inline int64_t lastTrustEstablishedPacketReceived() const { return _lastTrustEstablishedPacketReceived; }
 
+	/**
+	 * Initialize statistical buffers
+	 */
+	inline void prepareBuffers() {
+		_throughputSamples = new RingBuffer<uint64_t>(ZT_PATH_QUALITY_METRIC_WIN_SZ);
+		_latencySamples = new RingBuffer<uint32_t>(ZT_PATH_QUALITY_METRIC_WIN_SZ);
+		_qualitySamples = new RingBuffer<float>(ZT_PATH_QUALITY_METRIC_WIN_SZ);
+		_packetValiditySamples = new RingBuffer<bool>(ZT_PATH_QUALITY_METRIC_WIN_SZ);
+		memset(_ifname, 0, 16);
+		memset(_recorded_id, 0, sizeof(_recorded_id));
+		memset(_recorded_ts, 0, sizeof(_recorded_ts));
+		memset(_recorded_len, 0, sizeof(_recorded_len));
+		memset(_addrString, 0, sizeof(_addrString));
+	}
+
 private:
 	volatile int64_t _lastOut;
 	volatile int64_t _lastIn;
@@ -519,32 +609,42 @@ private:
 	InetAddress::IpScope _ipScope; // memoize this since it's a computed value checked often
 	AtomicCounter __refCount;
 
-	// Packet Error Ratio (PER)
-	int _packetValidity[ZT_PATH_ERROR_SAMPLE_WIN_SZ];
-	int _currentPacketSampleCounter;
-	volatile float _meanPacketErrorRatio;
+	uint64_t _recorded_id[ZT_PATH_QOS_TABLE_SIZE];
+	uint64_t _recorded_ts[ZT_PATH_QOS_TABLE_SIZE];
+	uint16_t _recorded_len[ZT_PATH_QOS_TABLE_SIZE];
 
-	// Latency and Jitter
-	volatile float _meanLatency;
-	int64_t _lastLatencyUpdate;
-	volatile float _jitter;
+	std::map<uint64_t, uint64_t> _outgoingPacketRecords;
+
+	int64_t _lastAck;
+	int64_t _lastThroughputEstimation;
+	int64_t _lastQoSMeasurement;
+
+	int64_t _unackedBytes;
+	int64_t _expectingAckAsOf;
+	int16_t _packetsReceivedSinceLastAck;
 
-	int64_t _lastPathQualitySampleTime;
-	float _lastComputedQuality;
-	int64_t _lastPathQualityEstimate;
-	float _meanAge;
 	float _meanThroughput;
+	uint64_t _maxLifetimeThroughput;
+	uint64_t _bytesAckedSinceLastThroughputEstimation;
 
-	// Circular buffers used to efficiently store large time series
-	RingBuffer<uint64_t> *_throughputSamples;
-	RingBuffer<uint32_t> *_latencySamples;
-	RingBuffer<uint64_t> *_ageSamples;
-	RingBuffer<float> *_errSamples;
+	volatile float _meanLatency;
+	float _packetDelayVariance;
 
+	float _packetErrorRatio;
 	float _packetLossRatio;
 
-	char _ifname[ZT_PATH_INTERFACE_NAME_SZ];
+	// cached estimates
+	float _lastComputedStability;
+	float _lastComputedRelativeQuality;
+
+	// cached human-readable strings for tracing purposes
+	char _ifname[16];
 	char _addrString[256];
+
+	RingBuffer<uint64_t> *_throughputSamples;
+	RingBuffer<uint32_t> *_latencySamples;
+	RingBuffer<float> *_qualitySamples;
+	RingBuffer<bool> *_packetValiditySamples;
 };
 
 } // namespace ZeroTier
diff --git a/node/Peer.cpp b/node/Peer.cpp
index c46ed751..8deaa362 100644
--- a/node/Peer.cpp
+++ b/node/Peer.cpp
@@ -24,8 +24,8 @@
  * of your own application.
  */
 
-#include "../version.h"
 
+#include "../version.h"
 #include "Constants.hpp"
 #include "Peer.hpp"
 #include "Node.hpp"
@@ -36,6 +36,7 @@
 #include "Trace.hpp"
 #include "InetAddress.hpp"
 #include "RingBuffer.hpp"
+#include "Utils.hpp"
 
 namespace ZeroTier {
 
@@ -61,13 +62,13 @@ Peer::Peer(const RuntimeEnvironment *renv,const Identity &myIdentity,const Ident
 	_id(peerIdentity),
 	_directPathPushCutoffCount(0),
 	_credentialsCutoffCount(0),
-	_linkBalanceStatus(false),
-	_linkRedundancyStatus(false)
+	_linkIsBalanced(false),
+	_linkIsRedundant(false),
+	_lastAggregateStatsReport(0)
 {
 	if (!myIdentity.agree(peerIdentity,_key,ZT_PEER_SECRET_KEY_LENGTH))
 		throw ZT_EXCEPTION_INVALID_ARGUMENT;
 	_pathChoiceHist = new RingBuffer<int>(ZT_MULTIPATH_PROPORTION_WIN_SZ);
-	_flowBalanceHist = new RingBuffer<float>(ZT_MULTIPATH_PROPORTION_WIN_SZ);
 }
 
 void Peer::received(
@@ -75,6 +76,7 @@ void Peer::received(
 	const SharedPtr<Path> &path,
 	const unsigned int hops,
 	const uint64_t packetId,
+	const unsigned int payloadLength,
 	const Packet::Verb verb,
 	const uint64_t inRePacketId,
 	const Packet::Verb inReVerb,
@@ -103,13 +105,13 @@ void Peer::received(
 	{
 		Mutex::Lock _l(_paths_m);
 		if (RR->node->getMultipathMode() != ZT_MULTIPATH_NONE) {
-			if ((now - _lastPathPrune) > ZT_CLOSED_PATH_PRUNING_INTERVAL) {
-				_lastPathPrune = now;
-				prunePaths();
+			recordIncomingPacket(tPtr, path, packetId, payloadLength, verb, now);
+			if (path->needsToSendQoS(now)) {
+				sendQOS_MEASUREMENT(tPtr, path, path->localSocket(), path->address(), now);
 			}
 			for(unsigned int i=0;i<ZT_MAX_PEER_NETWORK_PATHS;++i) {
 				if (_paths[i].p) {
-					_paths[i].p->measureLink(now);
+					_paths[i].p->processBackgroundPathMeasurements(now, _id.address().toInt());
 				}
 			}
 		}
@@ -117,7 +119,6 @@ void Peer::received(
 
 	if (hops == 0) {
 		// If this is a direct packet (no hops), update existing paths or learn new ones
-
 		bool havePath = false;
 		{
 			Mutex::Lock _l(_paths_m);
@@ -164,6 +165,19 @@ void Peer::received(
 					}
 				}
 
+				// If we find a pre-existing path with the same address, just replace it.
+				// If we don't find anything we can replace, just use the replacePath that we previously decided on.
+				if (RR->node->getMultipathMode() != ZT_MULTIPATH_NONE) {
+					for(unsigned int i=0;i<ZT_MAX_PEER_NETWORK_PATHS;++i) {
+						if (_paths[i].p) {
+							if ( _paths[i].p->address().ss_family == path->address().ss_family && _paths[i].p->address().ipsEqual2(path->address())) {
+								replacePath = i;
+								break;
+							}
+						}
+					}
+				}
+
 				if (replacePath != ZT_MAX_PEER_NETWORK_PATHS) {
 					if (verb == Packet::VERB_OK) {
 						RR->t->peerLearnedNewPath(tPtr,networkId,*this,path,packetId);
@@ -252,6 +266,117 @@ void Peer::received(
 	}
 }
 
+void Peer::recordOutgoingPacket(const SharedPtr<Path> &path, const uint64_t packetId,
+	uint16_t payloadLength, const Packet::Verb verb, int64_t now)
+{
+	if (RR->node->getMultipathMode() != ZT_MULTIPATH_NONE) {
+		if (verb == Packet::VERB_FRAME || verb == Packet::VERB_EXT_FRAME) {
+			path->expectingAck(now, packetId, payloadLength);
+		}
+	}
+}
+
+void Peer::recordIncomingPacket(void *tPtr, const SharedPtr<Path> &path, const uint64_t packetId,
+	uint16_t payloadLength, const Packet::Verb verb, int64_t now)
+{
+	if (verb == Packet::VERB_FRAME || verb == Packet::VERB_EXT_FRAME) {
+		if (path->needsToSendAck(now)) {
+			sendACK(tPtr, path, path->localSocket(), path->address(), now);
+		}
+		path->recordIncomingPacket(now, packetId, payloadLength);
+	}
+}
+
+float Peer::computeAggregateLinkRelativeQuality(int64_t now)
+{
+	float maxStability = 0;
+	float totalRelativeQuality = 0;
+	float maxThroughput = 1;
+	float maxScope = 0;
+	float relStability[ZT_MAX_PEER_NETWORK_PATHS];
+	float relThroughput[ZT_MAX_PEER_NETWORK_PATHS];
+	memset(&relStability, 0, sizeof(relStability));
+	memset(&relThroughput, 0, sizeof(relThroughput));
+	// Survey all paths
+	for(unsigned int i=0;i<ZT_MAX_PEER_NETWORK_PATHS;++i) {
+		if (_paths[i].p) {
+			relStability[i] = _paths[i].p->lastComputedStability();
+			relThroughput[i] = _paths[i].p->maxLifetimeThroughput();
+			maxStability = relStability[i] > maxStability ? relStability[i] : maxStability;
+			maxThroughput = relThroughput[i] > maxThroughput ? relThroughput[i] : maxThroughput;
+			maxScope = _paths[i].p->ipScope() > maxScope ? _paths[i].p->ipScope() : maxScope;
+		}
+	}
+	// Convert to relative values
+	for(unsigned int i=0;i<ZT_MAX_PEER_NETWORK_PATHS;++i) {
+		if (_paths[i].p) {
+			relStability[i] /= maxStability ? maxStability : 1;
+			relThroughput[i] /= maxThroughput ? maxThroughput : 1;
+			float normalized_ma = Utils::normalize(_paths[i].p->ackAge(now), 0, ZT_PATH_MAX_AGE, 0, 10);
+			float age_contrib = exp((-1)*normalized_ma);
+			float relScope = ((float)(_paths[i].p->ipScope()+1) / (maxScope + 1));
+			float relQuality =
+				(relStability[i] * ZT_PATH_CONTRIB_STABILITY)
+				+ (fmax(1, relThroughput[i]) * ZT_PATH_CONTRIB_THROUGHPUT)
+				+ relScope * ZT_PATH_CONTRIB_SCOPE;
+			relQuality *= age_contrib;
+			totalRelativeQuality += relQuality;
+			_paths[i].p->updateRelativeQuality(relQuality);
+		}
+	}
+	return (float)1.0 / totalRelativeQuality; // Used later to convert relative quantities into flow allocations
+}
+
+float Peer::computeAggregateLinkPacketDelayVariance()
+{
+	float pdv = 0.0;
+	for(unsigned int i=0;i<ZT_MAX_PEER_NETWORK_PATHS;++i) {
+		if (_paths[i].p) {
+			pdv += _paths[i].p->relativeQuality() * _paths[i].p->packetDelayVariance();
+		}
+	}
+	return pdv;
+}
+
+float Peer::computeAggregateLinkMeanLatency()
+{
+	float ml = 0.0;
+	for(unsigned int i=0;i<ZT_MAX_PEER_NETWORK_PATHS;++i) {
+		if (_paths[i].p) {
+			ml += _paths[i].p->relativeQuality() * _paths[i].p->meanLatency();
+		}
+	}
+	return ml;
+}
+
+int Peer::aggregateLinkPhysicalPathCount()
+{
+	std::map<std::string, bool> ifnamemap;
+	int pathCount = 0;
+	int64_t now = RR->node->now();
+	for(unsigned int i=0;i<ZT_MAX_PEER_NETWORK_PATHS;++i) {
+		if (_paths[i].p && _paths[i].p->alive(now)) {
+			if (!ifnamemap[_paths[i].p->getName()]) {
+				ifnamemap[_paths[i].p->getName()] = true;
+				pathCount++;
+			}
+		}
+	}
+	return pathCount;
+}
+
+int Peer::aggregateLinkLogicalPathCount()
+{
+	int pathCount = 0;
+	int64_t now = RR->node->now();
+	for(unsigned int i=0;i<ZT_MAX_PEER_NETWORK_PATHS;++i) {
+		if (_paths[i].p && _paths[i].p->alive(now)) {
+			pathCount++;
+		}
+	}
+	return pathCount;
+}
+
 SharedPtr<Path> Peer::getAppropriatePath(int64_t now, bool includeExpired)
 {
 	Mutex::Lock _l(_paths_m);
@@ -264,7 +389,7 @@ SharedPtr<Path> Peer::getAppropriatePath(int64_t now, bool includeExpired)
 	if (RR->node->getMultipathMode() == ZT_MULTIPATH_NONE) {
 		long bestPathQuality = 2147483647;
 		for(unsigned int i=0;i<ZT_MAX_PEER_NETWORK_PATHS;++i) {
-			if (_paths[i].p && _paths[i].p->isValidState()) {
+			if (_paths[i].p) {
 				if ((includeExpired)||((now - _paths[i].lr) < ZT_PEER_PATH_EXPIRATION)) {
 					const long q = _paths[i].p->quality(now) / _paths[i].priority;
 					if (q <= bestPathQuality) {
@@ -280,23 +405,14 @@ SharedPtr<Path> Peer::getAppropriatePath(int64_t now, bool includeExpired)
 		return SharedPtr<Path>();
 	}
 
-	if ((now - _lastPathPrune) > ZT_CLOSED_PATH_PRUNING_INTERVAL) {
-		_lastPathPrune = now;
-		prunePaths();
-	}
 	for(unsigned int i=0;i<ZT_MAX_PEER_NETWORK_PATHS;++i) {
 		if (_paths[i].p) {
-			_paths[i].p->measureLink(now);
+			_paths[i].p->processBackgroundPathMeasurements(now, _id.address().toInt());
 		}
 	}
 
 	/**
 	 * Randomly distribute traffic across all paths
-	 *
-	 * Behavior:
-	 *  - If path DOWN: Stop randomly choosing that path
-	 *  - If path UP: Start randomly choosing that path
-	 *  - If all paths are unresponsive: randomly choose from all paths
 	 */
 	int numAlivePaths = 0;
 	int numStalePaths = 0;
@@ -307,15 +423,13 @@ SharedPtr<Path> Peer::getAppropriatePath(int64_t now, bool includeExpired)
 		memset(&stalePaths, -1, sizeof(stalePaths));
 		for(unsigned int i=0;i<ZT_MAX_PEER_NETWORK_PATHS;++i) {
 			if (_paths[i].p) {
-				if (_paths[i].p->isValidState()) {
-					if (_paths[i].p->alive(now)) {
-						alivePaths[numAlivePaths] = i;
-						numAlivePaths++;
-					}
-					else {
-						stalePaths[numStalePaths] = i;
-						numStalePaths++;
-					}
+				if (_paths[i].p->alive(now)) {
+					alivePaths[numAlivePaths] = i;
+					numAlivePaths++;
+				}
+				else {
+					stalePaths[numStalePaths] = i;
+					numStalePaths++;
 				}
 			}
 		}
@@ -337,160 +451,104 @@ SharedPtr<Path> Peer::getAppropriatePath(int64_t now, bool includeExpired)
 	 * Proportionally allocate traffic according to dynamic path quality measurements
 	 */
 	if (RR->node->getMultipathMode() == ZT_MULTIPATH_PROPORTIONALLY_BALANCED) {
-		float relq[ZT_MAX_PEER_NETWORK_PATHS];
-		memset(&relq, 0, sizeof(relq));
 		float alloc[ZT_MAX_PEER_NETWORK_PATHS];
 		memset(&alloc, 0, sizeof(alloc));
-
-		// Survey
-		//
-		// Take a survey of all available link qualities. We use this to determine if we
-		// can skip this algorithm altogether and if not, to establish baseline for physical
-		// link quality used in later calculations.
-		//
-		// We find the min/max quality of our currently-active links so
-		// that we can form a relative scale to rank each link proportionally
-		// to each other link.
-		uint16_t alivePaths[ZT_MAX_PEER_NETWORK_PATHS];
-		uint16_t stalePaths[ZT_MAX_PEER_NETWORK_PATHS];
+		int numAlivePaths = 0;
+		int numStalePaths = 0;
+		int alivePaths[ZT_MAX_PEER_NETWORK_PATHS];
+		int stalePaths[ZT_MAX_PEER_NETWORK_PATHS];
 		memset(&alivePaths, -1, sizeof(alivePaths));
 		memset(&stalePaths, -1, sizeof(stalePaths));
-		uint16_t numAlivePaths = 0;
-		uint16_t numStalePaths = 0;
-		float minQuality = 10000;
-		float maxQuality = -1;
-		float currQuality;
-		for(uint16_t i=0;i<ZT_MAX_PEER_NETWORK_PATHS;++i) {
-			if (_paths[i].p && _paths[i].p->isValidState()) {
-				if (!_paths[i].p->monitorsReady()) {
-					// TODO: This should fix itself anyway but we should test whether forcing the use of a new path will
-					// aid in establishing flow balance more quickly.
-				}
-				// Compute quality here, going forward we will use lastComputedQuality()
-				currQuality = _paths[i].p->computeQuality(now);
-				if (!_paths[i].p->stale(now)) {
+		// Attempt to find an excuse not to use the rest of this algorithm
+		// Alive or Stale?
+		for(unsigned int i=0;i<ZT_MAX_PEER_NETWORK_PATHS;++i) {
+			if (_paths[i].p) {
+				if (_paths[i].p->alive(now)) {
+					alivePaths[numAlivePaths] = i;
 					numAlivePaths++;
-				}
-				else {
+				} else {
+					stalePaths[numStalePaths] = i;
 					numStalePaths++;
 				}
-				if (currQuality > maxQuality) {
-					maxQuality = currQuality;
-					bestPath = i;
-				}
-				if (currQuality < minQuality) {
-					minQuality = currQuality;
-				}
-				relq[i] = currQuality;
+				// Record a default path to use as a short-circuit for the rest of the algorithm
+				bestPath = i;
 			}
 		}
-
-		// Attempt to find an excuse not to use the rest of this algorithm
-		if (bestPath == ZT_MAX_PEER_NETWORK_PATHS || (numAlivePaths == 0 && numStalePaths == 0)) {
+		if (numAlivePaths == 0 && numStalePaths == 0) {
 			return SharedPtr<Path>();
-		} if (numAlivePaths == 1) {
-			//return _paths[bestPath].p;
-		} if (numStalePaths == 1) {
-			//return _paths[bestPath].p;
-		}
-
-		// Relative quality
-		//
-		// The strongest link will have a value of 1.0 whereas every other
-		// link will have a value which represents some fraction of the strongest link.
-		float totalRelativeQuality = 0;
-		for(unsigned int i=0;i<ZT_MAX_PEER_NETWORK_PATHS;++i) {
-			if (_paths[i].p && _paths[i].p->isValidState()) {
-				relq[i] /= maxQuality ? maxQuality : 1;
-				totalRelativeQuality += relq[i];
-			}
+		} if (numAlivePaths == 1 || numStalePaths == 1) {
+			return _paths[bestPath].p;
 		}
-
-		// Convert the relative quality values into flow allocations.
-		// Additionally, determine whether each path in the flow is
-		// contributing more or less than its target allocation. If
-		// it is contributing more than required, don't allow it to be
-		// randomly selected for the next packet. If however the path
-		// needs to contribute more to the flow, we should record
-		float imbalance = 0;
-		float qualityScalingFactor = (float)1.0 / totalRelativeQuality;
+		// Compare paths to each-other
+		float qualityScalingFactor = computeAggregateLinkRelativeQuality(now);
+		// Convert set of relative performances into an allocation set
 		for(uint16_t i=0;i<ZT_MAX_PEER_NETWORK_PATHS;++i) {
-			// Out of the last N packets to this peer, how many were sent by this path?
-			int numPktSentWithinWin = (int)_pathChoiceHist->countValue(i);
-			// Compute traffic allocation for each path in the flow
-			if (_paths[i].p && _paths[i].p->isValidState()) {
-				// Allocation
-				// This is the percentage of traffic we want to send over a given path
-				alloc[i] = relq[i] * qualityScalingFactor;
-				float currProportion = numPktSentWithinWin / (float)ZT_MULTIPATH_PROPORTION_WIN_SZ;
-				float targetProportion = alloc[i];
-				float diffProportion = currProportion - targetProportion;
-				// Imbalance
-				//
-				// This is the sum of the distances of each path's currently observed flow contributions
-				// from its most recent target allocation. In other words, this is a measure of how closely we
-				// are adhering to our desired allocations. It is worth noting that this value can be greater
-				// than 1.0 if a significant change to allocations is made by the algorithm, this will
-				// eventually correct itself.
-				imbalance += fabs(diffProportion);
-				if (diffProportion < 0) {
-					alloc[i] = targetProportion;
-				}
-				else {
-					alloc[i] = targetProportion;
-				}
-			}
-		}
-
-		// Compute and record current flow balance
-		float balance = (float)1.0 - imbalance;
-		if (balance >= ZT_MULTIPATH_FLOW_BALANCE_THESHOLD) {
-			if (!_linkBalanceStatus) {
-				_linkBalanceStatus = true;
-				RR->t->peerLinkBalanced(NULL,0,*this);
-			}
-		}
-		else {
-			if (_linkBalanceStatus) {
-				_linkBalanceStatus = false;
-				RR->t->peerLinkImbalanced(NULL,0,*this);
+			if (_paths[i].p) {
+				alloc[i] = _paths[i].p->relativeQuality() * qualityScalingFactor;
 			}
 		}
-
-		// Record the current flow balance. Later used for computing a mean flow balance value.
-		_flowBalanceHist->push(balance);
-
-		// Randomly choose path from allocated candidates
+		// Randomly choose path according to their allocations
 		unsigned int r;
 		Utils::getSecureRandom(&r, 1);
 		float rf = (float)(r %= 100) / 100;
 		for(int i=0;i<ZT_MAX_PEER_NETWORK_PATHS;++i) {
-			if (_paths[i].p && _paths[i].p->isValidState() && _paths[i].p->address().isV4()) {
-				if (alloc[i] > 0 && rf < alloc[i]) {
+			if (_paths[i].p) {
+				if (rf < alloc[i]) {
 					bestPath = i;
 					_pathChoiceHist->push(bestPath); // Record which path we chose
 					break;
 				}
-				if (alloc[i] > 0) {
-					rf -= alloc[i];
-				}
-				else {
-					rf -= alloc[i]*-1;
-				}
+				rf -= alloc[i];
 			}
 		}
 		if (bestPath < ZT_MAX_PEER_NETWORK_PATHS) {
 			return _paths[bestPath].p;
 		}
-		return SharedPtr<Path>();
 	}
+	return SharedPtr<Path>();
+}
 
-	// Adhere to a user-defined interface/allocation scheme
-	if (RR->node->getMultipathMode() == ZT_MULTIPATH_MANUALLY_BALANCED) {
-		// TODO
+char *Peer::interfaceListStr()
+{
+	std::map<std::string, int> ifnamemap;
+	char tmp[32];
+	const int64_t now = RR->node->now();
+	char *ptr = _interfaceListStr;
+	bool imbalanced = false;
+	memset(_interfaceListStr, 0, sizeof(_interfaceListStr));
+	int alivePathCount = aggregateLinkLogicalPathCount();
+	for(unsigned int i=0;i<ZT_MAX_PEER_NETWORK_PATHS;++i) {
+		if (_paths[i].p && _paths[i].p->alive(now)) {
+			int ipv = _paths[i].p->address().isV4();
+			// If this is acting as an aggregate link, check allocations
+			float targetAllocation = 1.0 / alivePathCount;
+			float currentAllocation = 1.0;
+			if (alivePathCount > 1) {
+				currentAllocation = (float)_pathChoiceHist->countValue(i) / (float)_pathChoiceHist->count();
+				if (fabs(targetAllocation - currentAllocation) > ZT_PATH_IMBALANCE_THRESHOLD) {
+					imbalanced = true;
+				}
+			}
+			char *ipvStr = ipv ? (char*)"ipv4" : (char*)"ipv6";
+			sprintf(tmp, "(%s, %s, %5.4f)", _paths[i].p->getName(), ipvStr, currentAllocation);
+			// Prevent duplicates
+			if(ifnamemap[_paths[i].p->getName()] != ipv) {
+				memcpy(ptr, tmp, strlen(tmp));
+				ptr += strlen(tmp);
+				*ptr = ' ';
+				ptr++;
+				ifnamemap[_paths[i].p->getName()] = ipv;
+			}
+		}
 	}
-
-	return SharedPtr<Path>();
+	ptr--; // Overwrite trailing space
+	if (imbalanced) {
+		sprintf(tmp, ", is IMBALANCED");
+		memcpy(ptr, tmp, sizeof(tmp));
+	} else {
+		*ptr = '\0';
+	}
+	return _interfaceListStr;
 }
 
 void Peer::introduce(void *const tPtr,const int64_t now,const SharedPtr<Peer> &other) const
@@ -614,6 +672,35 @@ void Peer::introduce(void *const tPtr,const int64_t now,const SharedPtr<Peer> &o
 	}
 }
 
+void Peer::sendACK(void *tPtr,const SharedPtr<Path> &path,const int64_t localSocket,const InetAddress &atAddress,int64_t now)
+{
+	Packet outp(_id.address(),RR->identity.address(),Packet::VERB_ACK);
+	uint32_t bytesToAck = path->bytesToAck();
+	outp.append<uint32_t>(bytesToAck);
+	if (atAddress) {
+		outp.armor(_key,false);
+		RR->node->putPacket(tPtr,localSocket,atAddress,outp.data(),outp.size());
+	} else {
+		RR->sw->send(tPtr,outp,false);
+	}
+	path->sentAck(now);
+}
+
+void Peer::sendQOS_MEASUREMENT(void *tPtr,const SharedPtr<Path> &path,const int64_t localSocket,const InetAddress &atAddress,int64_t now)
+{
+	Packet outp(_id.address(),RR->identity.address(),Packet::VERB_QOS_MEASUREMENT);
+	char qosData[ZT_PATH_MAX_QOS_PACKET_SZ];
+	path->generateQoSPacket(now,qosData);
+	outp.append(qosData,sizeof(qosData));
+	if (atAddress) {
+		outp.armor(_key,false);
+		RR->node->putPacket(tPtr,localSocket,atAddress,outp.data(),outp.size());
+	} else {
+		RR->sw->send(tPtr,outp,false);
+	}
+	path->sentQoS(now);
+}
+
 void Peer::sendHELLO(void *tPtr,const int64_t localSocket,const InetAddress &atAddress,int64_t now)
 {
 	Packet outp(_id.address(),RR->identity.address(),Packet::VERB_HELLO);
@@ -688,6 +775,25 @@ unsigned int Peer::doPingAndKeepalive(void *tPtr,int64_t now)
 	const bool sendFullHello = ((now - _lastSentFullHello) >= ZT_PEER_PING_PERIOD);
 	_lastSentFullHello = now;
 
+	// Emit traces regarding the status of aggregate links
+	if (RR->node->getMultipathMode() != ZT_MULTIPATH_NONE) {
+		int alivePathCount = aggregateLinkPhysicalPathCount();
+		if ((now - _lastAggregateStatsReport) > ZT_PATH_AGGREGATE_STATS_REPORT_INTERVAL) {
+			_lastAggregateStatsReport = now;
+			if (alivePathCount) {
+				RR->t->peerLinkAggregateStatistics(NULL,*this);
+			}
+		}
+		// Report link redundancy
+		if (alivePathCount < 2 && _linkIsRedundant) {
+			_linkIsRedundant = !_linkIsRedundant;
+			RR->t->peerLinkNoLongerRedundant(NULL,*this);
+		} if (alivePathCount > 1 && !_linkIsRedundant) {
+			_linkIsRedundant = !_linkIsRedundant;
+			RR->t->peerLinkNowRedundant(NULL,*this);
+		}
+	}
+
 	// Right now we only keep pinging links that have the maximum priority. The
 	// priority is used to track cluster redirections, meaning that when a cluster
 	// redirects us its redirect target links override all other links and we
@@ -726,22 +832,6 @@ unsigned int Peer::doPingAndKeepalive(void *tPtr,int64_t now)
 	return sent;
 }
 
-unsigned int Peer::prunePaths()
-{
-	unsigned int pruned = 0;
-	for(unsigned int i=0;i<ZT_MAX_PEER_NETWORK_PATHS;++i) {
-		if (_paths[i].p) {
-			if(_paths[i].p->isClosed() || !_paths[i].p->isValidState()) {
-				_paths[i].lr = 0;
-				_paths[i].p.zero();
-				_paths[i].priority = 1;
-				pruned++;
-			}
-		}
-	}
-	return pruned;
-}
-
 void Peer::clusterRedirect(void *tPtr,const SharedPtr<Path> &originatingPath,const InetAddress &remoteAddress,const int64_t now)
 {
 	SharedPtr<Path> np(RR->topology->getPath(originatingPath->localSocket(),remoteAddress));
diff --git a/node/Peer.hpp b/node/Peer.hpp
index 9873729b..2f723b07 100644
--- a/node/Peer.hpp
+++ b/node/Peer.hpp
@@ -68,9 +68,7 @@ public:
 	~Peer() {
 		Utils::burn(_key,sizeof(_key));
 		delete _pathChoiceHist;
-		delete _flowBalanceHist;
 		_pathChoiceHist = NULL;
-		_flowBalanceHist = NULL;
 	}
 
 	/**
@@ -114,6 +112,7 @@ public:
 		const SharedPtr<Path> &path,
 		const unsigned int hops,
 		const uint64_t packetId,
+		const unsigned int payloadLength,
 		const Packet::Verb verb,
 		const uint64_t inRePacketId,
 		const Packet::Verb inReVerb,
@@ -158,7 +157,74 @@ public:
 	}
 
 	/**
-	 * Get the most appropriate direct path based on current multipath configuration
+	 * Record statistics on outgoing packets
+	 *
+	 * @param path Path over which packet was sent
+	 * @param id Packet ID
+	 * @param len Length of packet payload
+	 * @param verb Packet verb
+	 * @param now Current time
+	 */
+	void recordOutgoingPacket(const SharedPtr<Path> &path, const uint64_t packetId, uint16_t payloadLength, const Packet::Verb verb, int64_t now);
+
+	/**
+	 * Record statistics on incoming packets
+	 *
+	 * @param path Path over which packet was sent
+	 * @param id Packet ID
+	 * @param len Length of packet payload
+	 * @param verb Packet verb
+	 * @param now Current time
+	 */
+	void recordIncomingPacket(void *tPtr, const SharedPtr<Path> &path, const uint64_t packetId, uint16_t payloadLength, const Packet::Verb verb, int64_t now);
+
+	/**
+	 * Send an ACK to peer for the most recent packets received
+	 *
+	 * @param tPtr Thread pointer to be handed through to any callbacks called as a result of this call
+	 * @param localSocket Raw socket the ACK packet will be sent over
+	 * @param atAddress Destination for the ACK packet
+	 * @param now Current time
+	 */
+	void sendACK(void *tPtr, const SharedPtr<Path> &path, const int64_t localSocket,const InetAddress &atAddress,int64_t now);
+
+	/**
+	 * Send a QoS packet to peer so that it can evaluate the quality of this link
+	 *
+	 * @param tPtr Thread pointer to be handed through to any callbacks called as a result of this call
+	 * @param localSocket Raw socket the QoS packet will be sent over
+	 * @param atAddress Destination for the QoS packet
+	 * @param now Current time
+	 */
+	void sendQOS_MEASUREMENT(void *tPtr, const SharedPtr<Path> &path, const int64_t localSocket,const InetAddress &atAddress,int64_t now);
+
+	/**
+	 * @return The relative quality values for each path
+	 */
+	float computeAggregateLinkRelativeQuality(int64_t now);
+
+	/**
+	 * @return The aggregate link Packet Delay Variance (PDV)
+	 */
+	float computeAggregateLinkPacketDelayVariance();
+
+	/**
+	 * @return The aggregate link mean latenct
+	 */
+	float computeAggregateLinkMeanLatency();
+
+	/**
+	 * @return The number of currently alive "physical" paths in the aggregate link
+	 */
+	int aggregateLinkPhysicalPathCount();
+
+	/**
+	 * @return The number of currently alive "logical" paths in the aggregate link
+	 */
+	int aggregateLinkLogicalPathCount();
+
+	/**
+	 * Get the most appropriate direct path based on current multipath and QoS configuration
 	 *
 	 * @param now Current time
 	 * @param includeExpired If true, include even expired paths
@@ -167,6 +233,12 @@ public:
 	SharedPtr<Path> getAppropriatePath(int64_t now, bool includeExpired);
 
 	/**
+	 * Generate a human-readable string of interface names making up the aggregate link, also include
+	 * moving allocation and IP version number for each (for tracing)
+	 */
+	char *interfaceListStr();
+
+	/**
 	 * Send VERB_RENDEZVOUS to this and another peer via the best common IP scope and path
 	 */
 	void introduce(void *const tPtr,const int64_t now,const SharedPtr<Peer> &other) const;
@@ -549,11 +621,12 @@ private:
 	AtomicCounter __refCount;
 
 	RingBuffer<int> *_pathChoiceHist;
-	RingBuffer<float> *_flowBalanceHist;
 
-	bool _linkBalanceStatus;
-	bool _linkRedundancyStatus;
+	bool _linkIsBalanced;
+	bool _linkIsRedundant;
+	uint64_t _lastAggregateStatsReport;
 
+	char _interfaceListStr[256]; // 16 characters * 16 paths in a link
 };
 
 } // namespace ZeroTier
diff --git a/node/RingBuffer.hpp b/node/RingBuffer.hpp
index cd384749..32ae037c 100644
--- a/node/RingBuffer.hpp
+++ b/node/RingBuffer.hpp
@@ -173,6 +173,11 @@ public:
 	}
 
 	/**
+	 * @return The most recently pushed element on the buffer
+	 */
+	T get_most_recent() { return *(buf + end); }
+
+	/**
 	 * @param dest Destination buffer
 	 * @param n Size (in terms of number of elements) of the destination buffer
 	 * @return Number of elements read from the buffer
@@ -218,10 +223,7 @@ public:
 	/**
 	 * @return The number of slots that are unused in the buffer
 	 */
-	size_t getFree()
-	{
-		return size - count();
-	}
+	size_t getFree() { return size - count(); }
 
 	/**
 	 * @return The arithmetic mean of the contents of the buffer
@@ -229,45 +231,67 @@ public:
 	float mean()
 	{
 		size_t iterator = begin;
-		float mean = 0;
-		for (size_t i=0; i<size; i++) {
-			iterator = (iterator + size - 1) % size;
-			mean += *(buf + iterator);
+		float subtotal = 0;
+		size_t curr_cnt = count();
+		for (size_t i=0; i<curr_cnt; i++) {
+			iterator = (iterator + size - 1) % curr_cnt;
+			subtotal += (float)*(buf + iterator);
 		}
-		return count() ? mean / (float)count() : 0;
+		return curr_cnt ? subtotal / (float)curr_cnt : 0;
 	}
 
 	/**
-	 * @return The sample standard deviation of the contents of the ring buffer
+	 * @return The arithmetic mean of the most recent 'n' elements of the buffer
 	 */
-	float stddev()
+	float mean(size_t n)
+	{
+		n = n < size ? n : size;
+		size_t iterator = begin;
+		float subtotal = 0;
+		size_t curr_cnt = count();
+		for (size_t i=0; i<n; i++) {
+			iterator = (iterator + size - 1) % curr_cnt;
+			subtotal += (float)*(buf + iterator);
+		}
+		return curr_cnt ? subtotal / (float)curr_cnt : 0;
+	}
+
+	/**
+	 * @return The sample standard deviation of element values
+	 */
+	float stddev() { return sqrt(variance()); }
+
+	/**
+	 * @return The variance of element values
+	 */
+	float variance()
 	{
 		size_t iterator = begin;
 		float cached_mean = mean();
+		size_t curr_cnt = count();
 		if (size) {
 			T sum_of_squared_deviations = 0;
-			for (size_t i=0; i<size; i++) {
-				iterator = (iterator + size - 1) % size;
+			for (size_t i=0; i<curr_cnt; i++) {
+				iterator = (iterator + size - 1) % curr_cnt;
 				float deviation = (buf[i] - cached_mean);
-				float sdev = deviation*deviation;
-				sum_of_squared_deviations += sdev;
+				sum_of_squared_deviations += (deviation*deviation);
 			}
-			float variance = sum_of_squared_deviations / (size - 1);
-			float sd = sqrt(variance);
-			return sd;
+			float variance = (float)sum_of_squared_deviations / (float)(size - 1);
+			return variance;
 		}
 		return 0;
 	}
 
 	/**
-	 * @return The number of elements of zero value, O(n)
+	 * @return The number of elements of zero value
 	 */
 	size_t zeroCount()
 	{
 		size_t iterator = begin;
 		size_t zeros = 0;
-		for (size_t i=0; i<size; i++) {
-			iterator = (iterator + size - 1) % size;
+		size_t curr_cnt = count();
+		for (size_t i=0; i<curr_cnt; i++) {
+			iterator = (iterator + size - 1) % curr_cnt;
 			if (*(buf + iterator) == 0) {
 				zeros++;
 			}
@@ -282,14 +306,15 @@ public:
 	size_t countValue(T value)
 	{
 		size_t iterator = begin;
-		size_t count = 0;
-		for (size_t i=0; i<size; i++) {
-			iterator = (iterator + size - 1) % size;
+		size_t cnt = 0;
+		size_t curr_cnt = count();
+		for (size_t i=0; i<curr_cnt; i++) {
+			iterator = (iterator + size - 1) % curr_cnt;
 			if (*(buf + iterator) == value) {
-				count++;
+				cnt++;
 			}
 		}
-		return count;
+		return cnt;
 	}
 
 	/**
@@ -301,10 +326,10 @@ public:
 		for (size_t i=0; i<size; i++) {
 			iterator = (iterator + size - 1) % size;
 			if (typeid(T) == typeid(int)) {
-				// DEBUG_INFO("buf[%2zu]=%2d", iterator, (int)*(buf + iterator));
+				 //DEBUG_INFO("buf[%2zu]=%2d", iterator, (int)*(buf + iterator));
 			}
 			else {
-				// DEBUG_INFO("buf[%2zu]=%2f", iterator, (float)*(buf + iterator));
+				 //DEBUG_INFO("buf[%2zu]=%2f", iterator, (float)*(buf + iterator));
 			}
 		}
 	}
diff --git a/node/Switch.cpp b/node/Switch.cpp
index 3f6c4e41..d53bf53e 100644
--- a/node/Switch.cpp
+++ b/node/Switch.cpp
@@ -666,6 +666,8 @@ bool Switch::_trySend(void *tPtr,Packet &packet,bool encrypt)
 	unsigned int chunkSize = std::min(packet.size(),mtu);
 	packet.setFragmented(chunkSize < packet.size());
 
+	peer->recordOutgoingPacket(viaPath, packet.packetId(), packet.payloadLength(), packet.verb(), now);
+
 	if (trustedPathId) {
 		packet.setTrusted(trustedPathId);
 	} else {
diff --git a/node/Trace.cpp b/node/Trace.cpp
index 01a8da55..f47a029b 100644
--- a/node/Trace.cpp
+++ b/node/Trace.cpp
@@ -106,24 +106,24 @@ void Trace::peerConfirmingUnknownPath(void *const tPtr,const uint64_t networkId,
 	}
 }
 
-void Trace::peerLinkNowRedundant(void *const tPtr,const uint64_t networkId,Peer &peer,const SharedPtr<Path> &newPath)
+void Trace::peerLinkNowRedundant(void *const tPtr,Peer &peer)
 {
-	ZT_LOCAL_TRACE(tPtr,RR,"link to peer %.10llx on network %.16llx is fully redundant",peer.address().toInt(),networkId);
+	ZT_LOCAL_TRACE(tPtr,RR,"link to peer %.10llx is fully redundant",peer.address().toInt());
 }
 
-void Trace::peerLinkNoLongerRedundant(void *const tPtr,const uint64_t networkId,Peer &peer,const SharedPtr<Path> &newPath)
+void Trace::peerLinkNoLongerRedundant(void *const tPtr,Peer &peer)
 {
-	ZT_LOCAL_TRACE(tPtr,RR,"link to peer %.10llx on network %.16llx is no longer redundant",peer.address().toInt(),networkId);
+	ZT_LOCAL_TRACE(tPtr,RR,"link to peer %.10llx is no longer redundant",peer.address().toInt());
 }
 
-void Trace::peerLinkBalanced(void *const tPtr,const uint64_t networkId,Peer &peer)
+void Trace::peerLinkAggregateStatistics(void *const tPtr,Peer &peer)
 {
-	ZT_LOCAL_TRACE(tPtr,RR,"link to peer %.10llx on network %.16llx is balanced",peer.address().toInt(),networkId);
-}
-
-void Trace::peerLinkImbalanced(void *const tPtr,const uint64_t networkId,Peer &peer)
-{
-	ZT_LOCAL_TRACE(tPtr,RR,"link to peer %.10llx on network %.16llx is unbalanced",peer.address().toInt(),networkId);
+	ZT_LOCAL_TRACE(tPtr,RR,"link to peer %.10llx is composed of (%d) physical paths %s, has packet delay variance (%.0f ms), mean latency (%.0f ms)",
+		peer.address().toInt(),
+		peer.aggregateLinkPhysicalPathCount(),
+		peer.interfaceListStr(),
+		peer.computeAggregateLinkPacketDelayVariance(),
+		peer.computeAggregateLinkMeanLatency());
 }
 
 void Trace::peerLearnedNewPath(void *const tPtr,const uint64_t networkId,Peer &peer,const SharedPtr<Path> &newPath,const uint64_t packetId)
diff --git a/node/Trace.hpp b/node/Trace.hpp
index b01163d6..734e84a5 100644
--- a/node/Trace.hpp
+++ b/node/Trace.hpp
@@ -122,10 +122,10 @@ public:
 
 	void peerConfirmingUnknownPath(void *const tPtr,const uint64_t networkId,Peer &peer,const SharedPtr<Path> &path,const uint64_t packetId,const Packet::Verb verb);
 
-	void peerLinkNowRedundant(void *const tPtr,const uint64_t networkId,Peer &peer,const SharedPtr<Path> &newPath);
-	void peerLinkNoLongerRedundant(void *const tPtr,const uint64_t networkId,Peer &peer,const SharedPtr<Path> &newPath);
-	void peerLinkBalanced(void *const tPtr,const uint64_t networkId,Peer &peer);
-	void peerLinkImbalanced(void *const tPtr,const uint64_t networkId,Peer &peer);
+	void peerLinkNowRedundant(void *const tPtr,Peer &peer);
+	void peerLinkNoLongerRedundant(void *const tPtr,Peer &peer);
+
+	void peerLinkAggregateStatistics(void *const tPtr,Peer &peer);
 
 	void peerLearnedNewPath(void *const tPtr,const uint64_t networkId,Peer &peer,const SharedPtr<Path> &newPath,const uint64_t packetId);
 	void peerRedirected(void *const tPtr,const uint64_t networkId,Peer &peer,const SharedPtr<Path> &newPath);
diff --git a/node/Utils.hpp b/node/Utils.hpp
index a24f2c9a..6ce67328 100644
--- a/node/Utils.hpp
+++ b/node/Utils.hpp
@@ -261,6 +261,14 @@ public:
 		return l;
 	}
 
+	static inline float normalize(float value, int64_t bigMin, int64_t bigMax, int32_t targetMin, int32_t targetMax)
+	{
+	    int64_t bigSpan = bigMax - bigMin;
+	    int64_t smallSpan = targetMax - targetMin;
+	    float valueScaled = (value - (float)bigMin) / (float)bigSpan;
+	    return (float)targetMin + valueScaled * (float)smallSpan;
+	}
+
 	/**
 	 * Generate secure random bytes
 	 *
diff --git a/osdep/Binder.hpp b/osdep/Binder.hpp
index 6e13836c..1f06021b 100644
--- a/osdep/Binder.hpp
+++ b/osdep/Binder.hpp
@@ -456,20 +456,6 @@ public:
 		return false;
 	}
 
-	/**
-	 * Get a list of socket pointers for all bindings.
-	 * 
-	 * @return A list of socket pointers for current bindings
-	 */
-	inline std::vector<PhySocket*> getBoundSockets()
-	{
-		std::vector<PhySocket*> sockets;
-		for (int i=0; i<ZT_BINDER_MAX_BINDINGS; i++) {
-			sockets.push_back(_bindings[i].udpSock);
-		}
-		return sockets;
-	}
-
 private:
 	_Binding _bindings[ZT_BINDER_MAX_BINDINGS];
 	std::atomic<unsigned int> _bindingCount;
diff --git a/osdep/Phy.hpp b/osdep/Phy.hpp
index 2e276a2a..5e659767 100644
--- a/osdep/Phy.hpp
+++ b/osdep/Phy.hpp
@@ -27,8 +27,6 @@
 #ifndef ZT_PHY_HPP
 #define ZT_PHY_HPP
 
-#include "../osdep/OSUtils.hpp"
-
 #include <stdio.h>
 #include <stdlib.h>
 #include <string.h>
@@ -88,22 +86,6 @@ namespace ZeroTier {
  */
 typedef void PhySocket;
 
-struct link_test_record
-{
-	link_test_record(PhySocket *_s, uint64_t _id, uint64_t _egress_time, uint32_t _length) :
-		s(_s),
-		id(_id),
-		egress_time(_egress_time),
-		length(_length)
-	{
-		//
-	}
-	PhySocket *s;
-	uint64_t id;
-	uint64_t egress_time;
-	uint32_t length;
-};
-
 /**
  * Simple templated non-blocking sockets implementation
  *
@@ -170,19 +152,13 @@ private:
 		ZT_PHY_SOCKET_UNIX_LISTEN = 0x08
 	};
 
-	struct PhySocketImpl
-	{
-		PhySocketImpl() :
-			throughput(0)
-		{
-			memset(ifname, 0, sizeof(ifname));
-		}
+	struct PhySocketImpl {
+		PhySocketImpl() { memset(ifname, 0, sizeof(ifname)); }
 		PhySocketType type;
 		ZT_PHY_SOCKFD_TYPE sock;
 		void *uptr; // user-settable pointer
 		ZT_PHY_SOCKADDR_STORAGE_TYPE saddr; // remote for TCP_OUT and TCP_IN, local for TCP_LISTEN, RAW, and UDP
 		char ifname[16];
-		uint64_t throughput;
 	};
 
 	std::list<PhySocketImpl> _socks;
@@ -198,7 +174,6 @@ private:
 
 	bool _noDelay;
 	bool _noCheck;
-	std::vector<struct link_test_record*> link_test_records;
 
 public:
 	/**
@@ -282,7 +257,9 @@ public:
 	 */
 	static inline void getIfName(PhySocket *s, char *nameBuf, int buflen)
 	{
-		memcpy(nameBuf, reinterpret_cast<PhySocketImpl *>(s)->ifname, buflen);
+		if (s) {
+			memcpy(nameBuf, reinterpret_cast<PhySocketImpl *>(s)->ifname, buflen);
+		}
 	}
 
 	/**
@@ -292,18 +269,9 @@ public:
 	 */
 	static inline void setIfName(PhySocket *s, char *ifname, int len)
 	{
-		memcpy(&(reinterpret_cast<PhySocketImpl *>(s)->ifname), ifname, len);
-	}
-
-	/**
-	 * Get result of most recent throughput test
-	 *
-	 * @param s Socket object
-	 */
-	inline uint64_t getThroughput(PhySocket *s)
-	{
-		PhySocketImpl *sws = (reinterpret_cast<PhySocketImpl *>(s));
-		return sws ? sws->throughput : 0;
+		if (s) {
+			memcpy(&(reinterpret_cast<PhySocketImpl *>(s)->ifname), ifname, len);
+		}
 	}
 
 	/**
@@ -339,105 +307,9 @@ public:
 	 */
 	inline bool isValidState(PhySocket *s)
 	{
-		PhySocketImpl *sws = (reinterpret_cast<PhySocketImpl *>(s));
-		return sws->type >= ZT_PHY_SOCKET_CLOSED && sws->type <= ZT_PHY_SOCKET_UNIX_LISTEN;
-	}
-
-	/**
-	 * Send a datagram of a known size to a selected peer and record egress time. The peer
-	 * shall eventually respond by echoing back a smaller datagram.
-	 *
-	 * @param s Socket object
-	 * @param remoteAddress Address of remote peer to receive link test packet
-	 * @param data Buffer containing random packet data
-	 * @param len Length of packet data buffer
-	 * @return Number of bytes successfully written to socket
-	 */
-	inline int test_link_speed(PhySocket *s, const struct sockaddr *to, void *data, uint32_t len) {
-		if (!reinterpret_cast<PhySocketImpl *>(s)) {
-			return 0;
-		}
-		uint64_t *buf = (uint64_t*)data;
-		uint64_t id = buf[0];
-		if (to->sa_family != AF_INET && to->sa_family != AF_INET6) {
-			return 0;
-		}
-		uint64_t egress_time = OSUtils::now();
-		PhySocketImpl *sws = (reinterpret_cast<PhySocketImpl *>(s));
-#if defined(_WIN32) || defined(_WIN64)
-		int w = ::sendto(sws->sock,reinterpret_cast<const char *>(data),len,0,to,(to->sa_family == AF_INET6) ? sizeof(struct sockaddr_in6) : sizeof(struct sockaddr_in))
-#else
-		int w = ::sendto(sws->sock,data,len,0,to,(to->sa_family == AF_INET6) ? sizeof(struct sockaddr_in6) : sizeof(struct sockaddr_in));
-#endif
-		if (w > 0) {
-			link_test_records.push_back(new link_test_record(s, id, egress_time, len));
-		}
-		return w;
-	}
-
-	/**
-	 * Remove link speed test records which have timed-out and record a 0 bits/s measurement 
-	 */
-	inline void refresh_link_speed_records()
-	{
-		for(size_t i=0;i<link_test_records.size();i++) {
-			if(OSUtils::now() - link_test_records[i]->egress_time > ZT_LINK_TEST_TIMEOUT) {
-				PhySocketImpl *sws = (reinterpret_cast<PhySocketImpl *>(link_test_records[i]->s));
-				if (sws) {
-					sws->throughput = 0;
-				}
-				link_test_records.erase(link_test_records.begin() + i);
-			}
-		}
-	}
-
-	/**
-	 * Upon receipt of a link speed test datagram we echo back only the identification portion
-	 *
-	 * @param s Socket object
-	 * @param from Address of remote peer that sent this datagram 
-	 * @param data Buffer containing datagram's contents
-	 * @param len Length of datagram
-	 * @return Number of bytes successfully written to socket in response
-	 */
-	inline int respond_to_link_test(PhySocket *s,const struct sockaddr *from,void *data,unsigned long len) {
-		PhySocketImpl *sws = (reinterpret_cast<PhySocketImpl *>(s));
-		uint64_t *id = (uint64_t*)data;
-#if defined(_WIN32) || defined(_WIN64)
-		int w = ::sendto(sws->sock,reinterpret_cast<const char *>(id),sizeof(id[0]),0,from,(from->sa_family == AF_INET6) ? sizeof(struct sockaddr_in6) : sizeof(struct sockaddr_in));
-#else
-		int w = ::sendto(sws->sock,id,sizeof(id[0]),0,from,(from->sa_family == AF_INET6) ? sizeof(struct sockaddr_in6) : sizeof(struct sockaddr_in));
-#endif
-		return w;
-	}
-
-	/**
-	 * Upon receipt of a response to our original link test datagram, correlate this new datagram with the record
-	 * of the one we sent. Compute the transit time and update the throughput field of the relevant socket. This
-	 * value will later be read by the path quality estimation logic located in Path.hpp.
-	 *
-	 * @param s Socket object
-	 * @param from Address of remote peer that sent this datagram
-	 * @param data Buffer containing datagram contents (ID of original link test datagram)
-	 * @param len Length of datagram
-	 * @return true if datagram correponded to previous record, false if otherwise
-	 */
-	inline bool handle_link_test_response(PhySocket *s,const struct sockaddr *from,void *data,unsigned long len) {
-		uint64_t *id = (uint64_t*)data;
-		for(size_t i=0;i<link_test_records.size();i++) {
-			if(link_test_records[i]->id == id[0]) {
-				float rtt  = (OSUtils::now()-link_test_records[i]->egress_time) / (float)1000; // s
-				uint32_t sz   = (link_test_records[i]->length) * 8; // bits
-				float transit_time = rtt / (float)2.0;
-				uint64_t raw = (uint64_t)(sz / transit_time);
-				PhySocketImpl *sws = (reinterpret_cast<PhySocketImpl *>(s));
-				if (sws) {
-					sws->throughput = raw;
-				}
-				delete link_test_records[i];
-				link_test_records.erase(link_test_records.begin() + i);
-				return true;
-			}
+		if (s) {
+			PhySocketImpl *sws = (reinterpret_cast<PhySocketImpl *>(s));
+			return sws->type >= ZT_PHY_SOCKET_CLOSED && sws->type <= ZT_PHY_SOCKET_UNIX_LISTEN;
 		}
 		return false;
 	}
diff --git a/service/OneService.cpp b/service/OneService.cpp
index 051629bc..cf2a6eda 100644
--- a/service/OneService.cpp
+++ b/service/OneService.cpp
@@ -37,7 +37,6 @@
 
 #include "../version.h"
 #include "../include/ZeroTierOne.h"
-#include "../include/ZeroTierDebug.h"
 
 #include "../node/Constants.hpp"
 #include "../node/Mutex.hpp"
@@ -458,9 +457,6 @@ public:
 	// Last potential sleep/wake event
 	uint64_t _lastRestart;
 
-	// Last time link throughput was tested
-	uint64_t _lastLinkSpeedTest;
-
 	// Deadline for the next background task service function
 	volatile int64_t _nextBackgroundTaskDeadline;
 
@@ -881,26 +877,6 @@ public:
 					lastMultipathModeUpdate = now;
 					_node->setMultipathMode(_multipathMode);
 				}
-				// Test link speeds
-				// TODO: This logic should eventually find its way into the core or as part of a passive
-				// measure within the protocol.
-				if (_multipathMode && ((now - _lastLinkSpeedTest) >= ZT_LINK_SPEED_TEST_INTERVAL)) {
-					_phy.refresh_link_speed_records();
-					_lastLinkSpeedTest = now;
-					// Generate random data to fill UDP packet
-					uint64_t pktBuf[ZT_LINK_TEST_DATAGRAM_SZ / sizeof(uint64_t)];
-					Utils::getSecureRandom(pktBuf, ZT_LINK_TEST_DATAGRAM_SZ);
-					ZT_PeerList *pl = _node->peers();
-					std::vector<PhySocket*> sockets = _binder.getBoundSockets();
-					for (int i=0; i<ZT_BINDER_MAX_BINDINGS; i++) {
-						for(size_t j=0;j<pl->peerCount;++j) {
-							for (int k=0; k<(ZT_MAX_PEER_NETWORK_PATHS/4); k++) {
-								Utils::getSecureRandom(pktBuf, 8); // generate one random integer for unique id
-								_phy.test_link_speed(sockets[i], (struct sockaddr*)&(pl->peers[j].paths[k].address), pktBuf, ZT_LINK_TEST_DATAGRAM_SZ);
-							}
-						}
-					}
-				}
 
 				// Run background task processor in core if it's time to do so
 				int64_t dl = _nextBackgroundTaskDeadline;
@@ -1799,15 +1775,6 @@ public:
 
 	inline void phyOnDatagram(PhySocket *sock,void **uptr,const struct sockaddr *localAddr,const struct sockaddr *from,void *data,unsigned long len)
 	{
-		if (_multipathMode) {
-			// Handle link test packets (should eventually be moved into the protocol itself)
-			if (len == ZT_LINK_TEST_DATAGRAM_SZ) {
-				_phy.respond_to_link_test(sock, from, data, len);
-			}
-			if (len == ZT_LINK_TEST_DATAGRAM_RESPONSE_SZ) {
-				_phy.handle_link_test_response(sock, from, data, len);
-			}
-		}
 		if ((len >= 16)&&(reinterpret_cast<const InetAddress *>(from)->ipScope() == InetAddress::IP_SCOPE_GLOBAL))
 			_lastDirectReceiveFromGlobal = OSUtils::now();
 		const ZT_ResultCode rc = _node->processWirePacket(