Add keepalive support for clients in quic.Config

Fixes #525.

session.go

@@ -111,6 +111,9 @@ type session struct {
 	lastNetworkActivityTime time.Time
 
 	timer *utils.Timer
+	// keepAlivePingSent stores whether a Ping frame was sent to the peer or not
+	// it is reset as soon as we receive a packet from the peer
+	keepAlivePingSent bool
 }
 
 var _ Session = &session{}
@@ -302,6 +305,12 @@ runLoop:
 			s.sentPacketHandler.OnAlarm()
 		}
 
+		if s.config.KeepAlive && time.Since(s.lastNetworkActivityTime) >= s.idleTimeout()/2 {
+			// send the PING frame since there is no activity in the session
+			s.packer.QueueControlFrame(&frames.PingFrame{})
+			s.keepAlivePingSent = true
+		}
+
 		if err := s.sendPacket(); err != nil {
 			s.closeLocal(err)
 		}
@@ -333,7 +342,12 @@ func (s *session) WaitUntilClosed() {
 }
 
 func (s *session) maybeResetTimer() {
-	deadline := s.lastNetworkActivityTime.Add(s.idleTimeout())
+	var deadline time.Time
+	if s.config.KeepAlive && !s.keepAlivePingSent {
+		deadline = s.lastNetworkActivityTime.Add(s.idleTimeout() / 2)
+	} else {
+		deadline = s.lastNetworkActivityTime.Add(s.idleTimeout())
+	}
 
 	if ackAlarm := s.receivedPacketHandler.GetAlarmTimeout(); !ackAlarm.IsZero() {
 		deadline = utils.MinTime(deadline, ackAlarm)
@@ -373,6 +387,7 @@ func (s *session) handlePacketImpl(p *receivedPacket) error {
 	}
 
 	s.lastNetworkActivityTime = p.rcvTime
+	s.keepAlivePingSent = false
 	hdr := p.publicHeader
 	data := p.data