From 359af66d0d1ff6bf39a2233003785ae7f8f5d422 Mon Sep 17 00:00:00 2001
From: Arceliar <Arceliar@users.noreply.github.com>
Date: Tue, 12 Jun 2018 03:16:10 -0500
Subject: [PATCH] exponential dht throttle backoff, and make it based on when
 packets were sent as part of bootstrapping/maintenance, not when arbitrary
 packets were received

---
 src/yggdrasil/dht.go | 64 ++++++++++++++++++++++++++++++--------------
 1 file changed, 44 insertions(+), 20 deletions(-)

diff --git a/src/yggdrasil/dht.go b/src/yggdrasil/dht.go
index 3d341e71..dcd9a93d 100644
--- a/src/yggdrasil/dht.go
+++ b/src/yggdrasil/dht.go
@@ -41,10 +41,11 @@ type dhtInfo struct {
 	nodeID_hidden *NodeID
 	key           boxPubKey
 	coords        []byte
-	send          time.Time // When we last sent a message
-	recv          time.Time // When we last received a message
-	pings         int       // Decide when to drop
-	throttle      uint8     // Number of seconds to wait before pinging a node to bootstrap buckets, gradually increases up to 1 minute
+	send          time.Time     // When we last sent a message
+	recv          time.Time     // When we last received a message
+	pings         int           // Decide when to drop
+	throttle      time.Duration // Time to wait before pinging a node to bootstrap buckets, increases exponentially from 1 second to 1 minute
+	bootstrapSend time.Time     // The time checked/updated as part of throttle checks
 }
 
 // Returns the *NodeID associated with dhtInfo.key, calculating it on the fly the first time or from a cache all subsequent times.
@@ -141,12 +142,14 @@ func (t *dht) handleRes(res *dhtRes) {
 	if !isIn {
 		return
 	}
+	now := time.Now()
 	rinfo := dhtInfo{
-		key:      res.Key,
-		coords:   res.Coords,
-		send:     time.Now(), // Technically wrong but should be OK...
-		recv:     time.Now(),
-		throttle: 1,
+		key:           res.Key,
+		coords:        res.Coords,
+		send:          now, // Technically wrong but should be OK...
+		recv:          now,
+		throttle:      time.Second,
+		bootstrapSend: now,
 	}
 	// If they're already in the table, then keep the correct send time
 	bidx, isOK := t.getBucketIndex(rinfo.getNodeID())
@@ -157,13 +160,15 @@ func (t *dht) handleRes(res *dhtRes) {
 	for _, oldinfo := range b.peers {
 		if oldinfo.key == rinfo.key {
 			rinfo.send = oldinfo.send
-			rinfo.throttle += oldinfo.throttle
+			rinfo.throttle = oldinfo.throttle
+			rinfo.bootstrapSend = oldinfo.bootstrapSend
 		}
 	}
 	for _, oldinfo := range b.other {
 		if oldinfo.key == rinfo.key {
 			rinfo.send = oldinfo.send
-			rinfo.throttle += oldinfo.throttle
+			rinfo.throttle = oldinfo.throttle
+			rinfo.bootstrapSend = oldinfo.bootstrapSend
 		}
 	}
 	// Insert into table
@@ -266,8 +271,8 @@ func (t *dht) insert(info *dhtInfo, isPeer bool) {
 		// This speeds up bootstrapping
 		info.recv = info.recv.Add(-time.Hour)
 	}
-	if isPeer || info.throttle > 60 {
-		info.throttle = 60
+	if isPeer || info.throttle > time.Minute {
+		info.throttle = time.Minute
 	}
 	// First drop any existing entry from the bucket
 	b.drop(&info.key)
@@ -512,20 +517,39 @@ func (t *dht) doMaintenance() {
 			}
 		}
 		if oldest != nil && time.Since(oldest.recv) > time.Minute {
+			// Ping the oldest node in the DHT, but don't ping nodes that have been checked within the last minute
 			t.addToMill(oldest, nil)
-		} // if the DHT isn't empty
+		}
 		// Refresh buckets
 		if t.offset > last {
 			t.offset = 0
 		}
 		target := t.getTarget(t.offset)
-		for _, info := range t.lookup(target, true) {
-			if time.Since(info.recv) > time.Duration(info.throttle)*time.Second {
-				t.addToMill(info, target)
-				t.offset++
-				break
+		func() {
+			closer := t.lookup(target, false)
+			for _, info := range closer {
+				// Throttled ping of a node that's closer to the destination
+				if time.Since(info.recv) > info.throttle {
+					t.addToMill(info, target)
+					t.offset++
+					info.bootstrapSend = time.Now()
+					info.throttle *= 2
+					if info.throttle > time.Minute {
+						info.throttle = time.Minute
+					}
+					return
+				}
 			}
-		}
+			if len(closer) == 0 {
+				// If we don't know of anyone closer at all, then there's a hole in our dht
+				// Ping the closest node we know and ignore the throttle, to try to fill it
+				for _, info := range t.lookup(target, true) {
+					t.addToMill(info, target)
+					t.offset++
+					return
+				}
+			}
+		}()
 		//t.offset++
 	}
 	for len(t.rumorMill) > 0 {