From 8ea663d04ad0c8e020aa9aa6a49f5094d4f6c6bc Mon Sep 17 00:00:00 2001
From: Marten Seemann <martenseemann@gmail.com>
Date: Mon, 7 Sep 2020 17:22:26 +0700
Subject: [PATCH] skip a packet number when sending a 1-RTT PTO packet

---
 internal/ackhandler/sent_packet_handler.go      |  2 ++
 internal/ackhandler/sent_packet_handler_test.go | 17 +++++++++++++++++
 2 files changed, 19 insertions(+)

diff --git a/internal/ackhandler/sent_packet_handler.go b/internal/ackhandler/sent_packet_handler.go
index 6b441443..d970b61f 100644
--- a/internal/ackhandler/sent_packet_handler.go
+++ b/internal/ackhandler/sent_packet_handler.go
@@ -632,6 +632,8 @@ func (h *sentPacketHandler) onVerifiedLossDetectionTimeout() error {
 		case protocol.EncryptionHandshake:
 			h.ptoMode = SendPTOHandshake
 		case protocol.Encryption1RTT:
+			// skip a packet number in order to elicit an immediate ACK
+			_ = h.PopPacketNumber(protocol.Encryption1RTT)
 			h.ptoMode = SendPTOAppData
 		default:
 			return fmt.Errorf("PTO timer in unexpected encryption level: %s", encLevel)
diff --git a/internal/ackhandler/sent_packet_handler_test.go b/internal/ackhandler/sent_packet_handler_test.go
index 9b428809..489631d2 100644
--- a/internal/ackhandler/sent_packet_handler_test.go
+++ b/internal/ackhandler/sent_packet_handler_test.go
@@ -692,6 +692,23 @@ var _ = Describe("SentPacketHandler", func() {
 			Expect(handler.SendMode()).ToNot(Equal(SendPTOAppData))
 		})
 
+		It("skips a packet number for 1-RTT PTOs", func() {
+			handler.ReceivedPacket(protocol.EncryptionHandshake)
+			handler.SetHandshakeConfirmed()
+			var lostPackets []protocol.PacketNumber
+			pn := handler.PopPacketNumber(protocol.Encryption1RTT)
+			handler.SentPacket(ackElicitingPacket(&Packet{
+				PacketNumber: pn,
+				SendTime:     time.Now().Add(-time.Hour),
+				Frames: []Frame{
+					{Frame: &wire.PingFrame{}, OnLost: func(wire.Frame) { lostPackets = append(lostPackets, 1) }},
+				},
+			}))
+			Expect(handler.OnLossDetectionTimeout()).To(Succeed())
+			Expect(handler.SendMode()).To(Equal(SendPTOAppData))
+			Expect(handler.PopPacketNumber(protocol.Encryption1RTT)).To(Equal(pn + 2))
+		})
+
 		It("only counts ack-eliciting packets as probe packets", func() {
 			handler.ReceivedPacket(protocol.EncryptionHandshake)
 			handler.SetHandshakeConfirmed()