dimensionless way to track how often nodes are faster than the current parent

src/yggdrasil/switch.go

@@ -21,7 +21,7 @@ import (
 const switch_timeout = time.Minute
 const switch_updateInterval = switch_timeout / 2
 const switch_throttle = switch_updateInterval / 2
-const switch_max_time = time.Hour
+const switch_faster_threshold = 2880 // 1 update per 30 seconds for 24 hours
 
 // The switch locator represents the topology and network state dependent info about a node, minus the signatures that go with it.
 // Nodes will pick the best root they see, provided that the root continues to push out updates with new timestamps.
@@ -119,13 +119,13 @@ func (x *switchLocator) isAncestorOf(y *switchLocator) bool {
 
 // Information about a peer, used by the switch to build the tree and eventually make routing decisions.
 type peerInfo struct {
-	key       sigPubKey     // ID of this peer
-	locator   switchLocator // Should be able to respond with signatures upon request
-	degree    uint64        // Self-reported degree
-	time      time.Time     // Time this node was last seen
-	firstSeen time.Time
-	port      switchPort // Interface number of this peer
-	msg       switchMsg  // The wire switchMsg used
+	key     sigPubKey     // ID of this peer
+	locator switchLocator // Should be able to respond with signatures upon request
+	degree  uint64        // Self-reported degree
+	time    time.Time     // Time this node was last seen
+	faster  uint16        // Counter of how often a node is faster than the current parent, penalized extra if slower
+	port    switchPort    // Interface number of this peer
+	msg     switchMsg     // The wire switchMsg used
 }
 
 // This is just a uint64 with a named type for clarity reasons.
@@ -252,7 +252,7 @@ func (t *switchTable) forgetPeer(port switchPort) {
 		return
 	}
 	for _, info := range t.data.peers {
-		t.unlockedHandleMsg(&info.msg, info.port)
+		t.unlockedHandleMsg(&info.msg, info.port, true)
 	}
 }
 
@@ -326,7 +326,7 @@ func (t *switchTable) checkRoot(msg *switchMsg) bool {
 func (t *switchTable) handleMsg(msg *switchMsg, fromPort switchPort) {
 	t.mutex.Lock()
 	defer t.mutex.Unlock()
-	t.unlockedHandleMsg(msg, fromPort)
+	t.unlockedHandleMsg(msg, fromPort, false)
 }
 
 // This updates the switch with information about a peer.
@@ -334,7 +334,8 @@ func (t *switchTable) handleMsg(msg *switchMsg, fromPort switchPort) {
 // That happens if this node is already our parent, or is advertising a better root, or is advertising a better path to the same root, etc...
 // There are a lot of very delicate order sensitive checks here, so its' best to just read the code if you need to understand what it's doing.
 // It's very important to not change the order of the statements in the case function unless you're absolutely sure that it's safe, including safe if used along side nodes that used the previous order.
-func (t *switchTable) unlockedHandleMsg(msg *switchMsg, fromPort switchPort) {
+// Set the third arg to true if you're reprocessing an old message, e.g. to find a new parent after one disconnects, to avoid updating some timing related things.
+func (t *switchTable) unlockedHandleMsg(msg *switchMsg, fromPort switchPort, reprocessing bool) {
 	// TODO directly use a switchMsg instead of switchMessage + sigs
 	now := time.Now()
 	// Set up the sender peerInfo
@@ -350,10 +351,7 @@ func (t *switchTable) unlockedHandleMsg(msg *switchMsg, fromPort switchPort) {
 	}
 	sender.msg = *msg
 	oldSender, isIn := t.data.peers[fromPort]
-	if !isIn {
-		oldSender.firstSeen = now
-	}
-	sender.firstSeen = oldSender.firstSeen
+	sender.faster = oldSender.faster
 	sender.port = fromPort
 	sender.time = now
 	// Decide what to do
@@ -374,9 +372,36 @@ func (t *switchTable) unlockedHandleMsg(msg *switchMsg, fromPort switchPort) {
 	doUpdate := false
 	if !equiv(&sender.locator, &oldSender.locator) {
 		doUpdate = true
-		sender.firstSeen = now
 	}
+	// Check if faster than the current parent, and update sender.faster accordingly
+	switch {
+	case reprocessing:
+		// Don't change anything if we're just reprocessing old messages.
+	case !isIn:
+		// Not known, sender.faster == 0, but set it explicitly just to make that obvious to the reader.
+		sender.faster = 0
+	case msg.Root != oldSender.locator.root:
+		// This is a new root.
+		// Honestly not sure if we should reset or do something else. For now, we'll just leave it alone.
+	case sender.port == t.parent:
+		// This is the current parent. If roots change, there's a good chance that they're still the best route to the root, so we probably don't want them to converge towards 0.
+		// If we leae them alone, then when a different node gets parented, this one will get penalized by a couple of points, so it hopefully shouldn't flap too hard to leave this alone for now.
+	case sender.locator.tstamp <= t.data.locator.tstamp:
+		// This timestamp came in slower than our parent's, so we should penalize them by more than we reward faster nodes.
+		if sender.faster > 1 {
+			sender.faster -= 2
+		} else {
+			// If exactly 1, don't let it roll under
+			sender.faster = 0
+		}
+	default:
+		// They sent us an update faster than our parent did, so reward them.
+		// FIXME make sure this can't ever roll over. It shouldn't be possible, we'd switch to them as a parent first, but still...
+		sender.faster++
+	}
+	// Update sender
 	t.data.peers[fromPort] = sender
+	// Decide if we should also update our root info to make the sender our parent
 	updateRoot := false
 	oldParent, isIn := t.data.peers[t.parent]
 	noParent := !isIn
@@ -391,20 +416,8 @@ func (t *switchTable) unlockedHandleMsg(msg *switchMsg, fromPort switchPort) {
 		}
 		return true
 	}()
-	// Get the time we've known about the sender (or old parent's) current coords, up to a maximum of `switch_max_time`.
-	sTime := now.Sub(sender.firstSeen)
-	if sTime > switch_max_time {
-		sTime = switch_max_time
-	}
-	pTime := now.Sub(oldParent.firstSeen)
-	if pTime > switch_max_time {
-		pTime = switch_max_time
-	}
-	// Really want to compare sLen/sTime and pLen/pTime
-	// Cross multiplied to avoid divide-by-zero
-	cost := float64(len(sender.locator.coords)) * pTime.Seconds()
-	pCost := float64(len(t.data.locator.coords)) * sTime.Seconds()
 	dropTstamp, isIn := t.drop[sender.locator.root]
+	// Decide if we need to update info about the root or change parents.
 	switch {
 	case !noLoop:
 		// This route loops, so we can't use the sender as our parent.
@@ -420,8 +433,16 @@ func (t *switchTable) unlockedHandleMsg(msg *switchMsg, fromPort switchPort) {
 	case noParent:
 		// We currently have no working parent, and at this point in the switch statement, anything is better than nothing.
 		updateRoot = true
-	case cost < pCost:
-		// The sender has a better combination of path length and reliability than the current parent.
+	case sender.faster > switch_faster_threshold:
+		// The is reliably faster than the current parent.
+		updateRoot = true
+	case reprocessing && len(sender.locator.coords) < len(oldParent.locator.coords):
+		// We're reprocessing old messages to find a new parent.
+		// That means we're in the middle of a route flap.
+		// We don't know how often each node is faster than the others, only relative to the old parent.
+		// If any of them was faster than the old parent, then we'd probably already be using them.
+		// So the best we can really do is pick the shortest route and hope it's OK as a starting point.
+		// TODO: Find some way to reliably store relative order between all peers. Basically a pxp "faster" matrix, more likely a faster port->uint map per peer, but preferably not literally that, since it'd be tedious to manage and probably slows down updates.
 		updateRoot = true
 	case sender.port != t.parent:
 		// Ignore further cases if the sender isn't our parent.
@@ -432,9 +453,9 @@ func (t *switchTable) unlockedHandleMsg(msg *switchMsg, fromPort switchPort) {
 		// Then reprocess *all* messages to look for a better parent.
 		// This is so we don't keep using this node as our parent if there's something better.
 		t.parent = 0
-		t.unlockedHandleMsg(msg, fromPort)
+		t.unlockedHandleMsg(msg, fromPort, true)
 		for _, info := range t.data.peers {
-			t.unlockedHandleMsg(&info.msg, info.port)
+			t.unlockedHandleMsg(&info.msg, info.port, true)
 		}
 	case now.Sub(t.time) < switch_throttle:
 		// We've already gotten an update from this root recently, so ignore this one to avoid flooding.