send an ack if we receive a packet and don't have any return traffic, keeping a legacy 4-second keep-alive in case there's no traffic at all to send (to be removed later, after nodes have upgraded), ideally we should either remove ReadTimeout or use it for the switch idle timeout instead

src/util/util.go

@@ -3,6 +3,7 @@ package util
 // These are misc. utility functions that didn't really fit anywhere else
 
 import "runtime"
+import "time"
 
 // A wrapper around runtime.Gosched() so it doesn't need to be imported elsewhere.
 func Yield() {
@@ -44,3 +45,14 @@ func PutBytes(bs []byte) {
 	default:
 	}
 }
+
+// This is a workaround to go's broken timer implementation
+func TimerStop(t *time.Timer) bool {
+	if !t.Stop() {
+		select {
+		case <-t.C:
+		default:
+		}
+	}
+	return true
+}

src/yggdrasil/link.go

@@ -20,6 +20,7 @@ type link struct {
 	mutex      sync.RWMutex // protects interfaces below
 	interfaces map[linkInfo]*linkInterface
 	awdl       awdl // AWDL interface support
+	// TODO timeout (to remove from switch), read from config.ReadTimeout
 }
 
 type linkInfo struct {
@@ -78,8 +79,6 @@ func (l *link) create(msgIO linkInterfaceMsgIO, name, linkType, local, remote st
 		incoming: incoming,
 		force:    force,
 	}
-	//l.interfaces[intf.name] = &intf
-	//go intf.start()
 	return &intf, nil
 }
 
@@ -172,41 +171,49 @@ func (intf *linkInterface) handler() error {
 	go intf.peer.linkLoop()
 	// Start the writer
 	signalReady := make(chan struct{}, 1)
+	signalSent := make(chan struct{}, 1)
+	sendAck := make(chan struct{}, 1)
 	go func() {
 		defer close(signalReady)
+		defer close(signalSent)
 		interval := 4 * time.Second
-		timer := time.NewTimer(interval)
+		tcpTimer := time.NewTimer(interval) // used for backwards compat with old tcp
-		clearTimer := func() {
+		defer util.TimerStop(tcpTimer)
-			if !timer.Stop() {
+		send := func(bs []byte) {
-				select {
+			intf.msgIO.writeMsg(bs)
-				case <-timer.C:
+			select {
-				default:
+			case signalSent <- struct{}{}:
-				}
+			default:
 			}
 		}
-		defer clearTimer()
 		for {
 			// First try to send any link protocol traffic
 			select {
 			case msg := <-intf.peer.linkOut:
-				intf.msgIO.writeMsg(msg)
+				send(msg)
 				continue
 			default:
 			}
 			// No protocol traffic to send, so reset the timer
-			clearTimer()
+			util.TimerStop(tcpTimer)
-			timer.Reset(interval)
+			tcpTimer.Reset(interval)
 			// Now block until something is ready or the timer triggers keepalive traffic
 			select {
-			case <-timer.C:
+			case <-tcpTimer.C:
-				intf.msgIO.writeMsg(nil)
+				intf.link.core.log.Debugf("Sending (legacy) keep-alive to %s: %s, source %s",
+					strings.ToUpper(intf.info.linkType), themString, intf.info.local)
+				send(nil)
+			case <-sendAck:
+				intf.link.core.log.Debugf("Sending ack to %s: %s, source %s",
+					strings.ToUpper(intf.info.linkType), themString, intf.info.local)
+				send(nil)
 			case msg := <-intf.peer.linkOut:
 				intf.msgIO.writeMsg(msg)
 			case msg, ok := <-out:
 				if !ok {
 					return
 				}
-				intf.msgIO.writeMsg(msg)
+				send(msg)
 				util.PutBytes(msg)
 				select {
 				case signalReady <- struct{}{}:
@@ -217,27 +224,23 @@ func (intf *linkInterface) handler() error {
 	}()
 	//intf.link.core.switchTable.idleIn <- intf.peer.port // notify switch that we're idle
 	// Used to enable/disable activity in the switch
-	signalAlive := make(chan struct{}, 1)
+	signalAlive := make(chan bool, 1) // True = real packet, false = keep-alive
 	defer close(signalAlive)
 	go func() {
 		var isAlive bool
 		var isReady bool
-		interval := 6 * time.Second // TODO set to ReadTimeout from the config, reset if it gets changed
+		var ackTimerRunning bool
-		timer := time.NewTimer(interval)
+		timeout := 6 * time.Second // TODO set to ReadTimeout from the config, reset if it gets changed
-		clearTimer := func() {
+		ackTime := time.Second
-			if !timer.Stop() {
+		timer := time.NewTimer(timeout)
-				select {
+		defer util.TimerStop(timer)
-				case <-timer.C:
+		ackTimer := time.NewTimer(ackTime)
-				default:
+		defer util.TimerStop(ackTimer)
-				}
-			}
-		}
-		defer clearTimer()
 		for {
-			clearTimer()
+			util.TimerStop(timer)
-			timer.Reset(interval)
+			timer.Reset(timeout)
 			select {
-			case _, ok := <-signalAlive:
+			case gotMsg, ok := <-signalAlive:
 				if !ok {
 					return
 				}
@@ -249,6 +252,24 @@ func (intf *linkInterface) handler() error {
 						intf.link.core.switchTable.idleIn <- intf.peer.port
 					}
 				}
+				if gotMsg && !ackTimerRunning {
+					util.TimerStop(ackTimer)
+					ackTimer.Reset(ackTime)
+					ackTimerRunning = true
+				}
+			case _, ok := <-signalSent:
+				// Stop any running ack timer
+				if !ok {
+					return
+				}
+				util.TimerStop(ackTimer)
+				ackTimerRunning = false
+			case <-ackTimer.C:
+				// We haven't sent anything in the past ackTimeout, so signal a send of a nil packet
+				select {
+				case sendAck <- struct{}{}:
+				default:
+				}
 			case _, ok := <-signalReady:
 				if !ok {
 					return
@@ -275,7 +296,7 @@ func (intf *linkInterface) handler() error {
 			return err
 		}
 		select {
-		case signalAlive <- struct{}{}:
+		case signalAlive <- len(msg) > 0:
 		default:
 		}
 	}