pack smaller packets as long as the encryption is not forward secure

handshake/crypto_setup_server.go

@@ -334,6 +334,8 @@ func (h *cryptoSetupServer) handleCHLO(sni string, data []byte, cryptoData map[T
 		return nil, err
 	}
 
+	h.aeadChanged <- protocol.EncryptionSecure
+
 	// Generate a new curve instance to derive the forward secure key
 	var fsNonce bytes.Buffer
 	fsNonce.Write(clientNonce)

handshake/crypto_setup_server_test.go

@@ -336,7 +336,11 @@ var _ = Describe("Crypto setup", func() {
 			Expect(err).NotTo(HaveOccurred())
 			Expect(stream.dataWritten.Bytes()).To(HavePrefix("SHLO"))
 			Expect(stream.dataWritten.Bytes()).ToNot(ContainSubstring("REJ"))
-			Expect(aeadChanged).To(Receive())
+			var encLevel protocol.EncryptionLevel
+			Expect(aeadChanged).To(Receive(&encLevel))
+			Expect(encLevel).To(Equal(protocol.EncryptionSecure))
+			Expect(aeadChanged).To(Receive(&encLevel))
+			Expect(encLevel).To(Equal(protocol.EncryptionForwardSecure))
 		})
 
 		It("recognizes inchoate CHLOs missing SCID", func() {

packet_packer.go

@@ -23,6 +23,8 @@ type packetPacker struct {
 	perspective  protocol.Perspective
 	version      protocol.VersionNumber
 	cryptoSetup  handshake.CryptoSetup
+	// as long as packets are not sent with forward-secure encryption, we limit the MaxPacketSize such that they can be retransmitted as a whole
+	isForwardSecure bool
 
 	packetNumberGenerator *packetNumberGenerator
 
@@ -105,6 +107,9 @@ func (p *packetPacker) packPacket(stopWaitingFrame *frames.StopWaitingFrame, lea
 		payloadFrames = []frames.Frame{p.controlFrames[0]}
 	} else {
 		maxSize := protocol.MaxFrameAndPublicHeaderSize - publicHeaderLength
+		if !p.isForwardSecure {
+			maxSize -= protocol.NonForwardSecurePacketSizeReduction
+		}
 		payloadFrames, err = p.composeNextPacket(stopWaitingFrame, maxSize)
 		if err != nil {
 			return nil, err
@@ -218,3 +223,7 @@ func (p *packetPacker) composeNextPacket(stopWaitingFrame *frames.StopWaitingFra
 func (p *packetPacker) QueueControlFrameForNextPacket(f frames.Frame) {
 	p.controlFrames = append(p.controlFrames, f)
 }
+
+func (p *packetPacker) SetForwardSecure() {
+	p.isForwardSecure = true
+}

packet_packer_test.go

@@ -60,6 +60,7 @@ var _ = Describe("Packet packer", func() {
 		publicHeaderLen = 1 + 8 + 2 // 1 flag byte, 8 connection ID, 2 packet number
 		maxFrameSize = protocol.MaxFrameAndPublicHeaderSize - publicHeaderLen
 		packer.version = protocol.Version34
+		packer.isForwardSecure = true
 	})
 
 	It("returns nil when no packet is queued", func() {
@@ -314,6 +315,18 @@ var _ = Describe("Packet packer", func() {
 			Expect(p.frames[0].(*frames.StreamFrame).DataLenPresent).To(BeFalse())
 		})
 
+		It("packs smaller packets when it is not yet forward-secure", func() {
+			packer.isForwardSecure = false
+			f := &frames.StreamFrame{
+				StreamID: 3,
+				Data:     bytes.Repeat([]byte{'f'}, int(protocol.MaxPacketSize)),
+			}
+			streamFramer.AddFrameForRetransmission(f)
+			p, err := packer.PackPacket(nil, nil, 0)
+			Expect(err).ToNot(HaveOccurred())
+			Expect(p.raw).To(HaveLen(int(protocol.MaxPacketSize - protocol.NonForwardSecurePacketSizeReduction)))
+		})
+
 		It("packs multiple small stream frames into single packet", func() {
 			f1 := &frames.StreamFrame{
 				StreamID: 5,

protocol/server_parameters.go

@@ -2,6 +2,10 @@ package protocol
 
 import "time"
 
+// NonForwardSecurePacketSizeReduction is the number of bytes a non forward-secure packet has to be smaller than a forward-secure packet
+// This makes sure that those packets can always be retransmitted without splitting the contained StreamFrames
+const NonForwardSecurePacketSizeReduction = 50
+
 // DefaultMaxCongestionWindow is the default for the max congestion window
 const DefaultMaxCongestionWindow = 1000
 

session.go

@@ -236,6 +236,9 @@ runLoop:
 			// begins with the public header and we never copy it.
 			putPacketBuffer(p.publicHeader.Raw)
 		case l := <-s.aeadChanged:
+			if l == protocol.EncryptionForwardSecure {
+				s.packer.SetForwardSecure()
+			}
 			s.tryDecryptingQueuedPackets()
 			s.cryptoChangeCallback(s, l == protocol.EncryptionForwardSecure)
 		}

session_test.go

@@ -1083,6 +1083,14 @@ var _ = Describe("Session", func() {
 		})
 	})
 
+	It("tells the packetPacker when forward-secure encryption is used", func() {
+		go sess.run()
+		sess.aeadChanged <- protocol.EncryptionSecure
+		Consistently(func() bool { return sess.packer.isForwardSecure }).Should(BeFalse())
+		sess.aeadChanged <- protocol.EncryptionForwardSecure
+		Eventually(func() bool { return sess.packer.isForwardSecure }).Should(BeTrue())
+	})
+
 	It("closes when crypto stream errors", func() {
 		go sess.run()
 		s, err := sess.GetOrOpenStream(3)