when forcing a retransmittable packet, bundle the PING with other frames

We're sending a retransmittable packet every 20 packets (if there are no
other frames to send). To make a packet retransmittable, we add a PING
frame. We should bundle this PING with an ACK.

packet_packer.go

@@ -27,12 +27,13 @@ type packetPacker struct {
 	packetNumberGenerator *packetNumberGenerator
 	streamFramer          *streamFramer
 
-	controlFrames    []wire.Frame
-	stopWaiting      *wire.StopWaitingFrame
-	ackFrame         *wire.AckFrame
-	leastUnacked     protocol.PacketNumber
-	omitConnectionID bool
-	hasSentPacket    bool // has the packetPacker already sent a packet
+	controlFrames                 []wire.Frame
+	stopWaiting                   *wire.StopWaitingFrame
+	ackFrame                      *wire.AckFrame
+	leastUnacked                  protocol.PacketNumber
+	omitConnectionID              bool
+	hasSentPacket                 bool // has the packetPacker already sent a packet
+	makeNextPacketRetransmittable bool
 }
 
 func newPacketPacker(connectionID protocol.ConnectionID,
@@ -153,6 +154,15 @@ func (p *packetPacker) PackPacket() (*packedPacket, error) {
 	if len(payloadFrames) == 1 && p.stopWaiting != nil {
 		return nil, nil
 	}
+	// check if this packet only contains an ACK and / or STOP_WAITING
+	if !ackhandler.HasRetransmittableFrames(payloadFrames) {
+		if p.makeNextPacketRetransmittable {
+			payloadFrames = append(payloadFrames, &wire.PingFrame{})
+			p.makeNextPacketRetransmittable = false
+		}
+	} else { // this packet already contains a retransmittable frame. No need to send a PING
+		p.makeNextPacketRetransmittable = false
+	}
 	p.stopWaiting = nil
 	p.ackFrame = nil
 
@@ -369,3 +379,7 @@ func (p *packetPacker) SetLeastUnacked(leastUnacked protocol.PacketNumber) {
 func (p *packetPacker) SetOmitConnectionID() {
 	p.omitConnectionID = true
 }
+
+func (p *packetPacker) MakeNextPacketRetransmittable() {
+	p.makeNextPacketRetransmittable = true
+}