save the RTT in the session ticket and use it when accepting 0-RTT

internal/handshake/crypto_setup.go

@@ -87,6 +87,8 @@ type cryptoSetup struct {
 	// for clients: to see if a ServerHello is a HelloRetryRequest
 	writeRecord chan struct{}
 
+	rttStats *congestion.RTTStats
+
 	logger utils.Logger
 
 	perspective protocol.Perspective
@@ -198,6 +200,7 @@ func newCryptoSetup(
 		runner:                 runner,
 		ourParams:              tp,
 		paramsChan:             extHandler.TransportParameters(),
+		rttStats:               rttStats,
 		logger:                 logger,
 		perspective:            perspective,
 		handshakeDone:          make(chan struct{}),
@@ -464,7 +467,10 @@ func (h *cryptoSetup) GetSessionTicket() ([]byte, error) {
 	var appData []byte
 	// Save transport parameters to the session ticket if we're allowing 0-RTT.
 	if h.tlsConf.MaxEarlyData > 0 {
-		appData = (&sessionTicket{Parameters: h.ourParams}).Marshal()
+		appData = (&sessionTicket{
+			Parameters: h.ourParams,
+			RTT:        h.rttStats.SmoothedRTT(),
+		}).Marshal()
 	}
 	return h.conn.GetSessionTicket(appData)
 }
@@ -479,7 +485,8 @@ func (h *cryptoSetup) accept0RTT(sessionTicketData []byte) bool {
 	}
 	valid := h.ourParams.ValidFor0RTT(t.Parameters)
 	if valid {
-		h.logger.Debugf("Accepting 0-RTT.")
+		h.logger.Debugf("Accepting 0-RTT. Restoring RTT from session ticket: %s", t.RTT)
+		h.rttStats.SetInitialRTT(t.RTT)
 	} else {
 		h.logger.Debugf("Transport parameters changed. Rejecting 0-RTT.")
 	}

internal/handshake/session_ticket.go

@@ -4,19 +4,22 @@ import (
 	"bytes"
 	"errors"
 	"fmt"
+	"time"
 
 	"github.com/lucas-clemente/quic-go/internal/utils"
 )
 
-const sessionTicketRevision = 0
+const sessionTicketRevision = 1
 
 type sessionTicket struct {
 	Parameters *TransportParameters
+	RTT        time.Duration // to be encoded in mus
 }
 
 func (t *sessionTicket) Marshal() []byte {
 	b := &bytes.Buffer{}
 	utils.WriteVarInt(b, sessionTicketRevision)
+	utils.WriteVarInt(b, uint64(t.RTT.Microseconds()))
 	t.Parameters.MarshalForSessionTicket(b)
 	return b.Bytes()
 }
@@ -30,10 +33,15 @@ func (t *sessionTicket) Unmarshal(b []byte) error {
 	if rev != sessionTicketRevision {
 		return fmt.Errorf("unknown session ticket revision: %d", rev)
 	}
+	rtt, err := utils.ReadVarInt(r)
+	if err != nil {
+		return errors.New("failed to read RTT")
+	}
 	var tp TransportParameters
 	if err := tp.UnmarshalFromSessionTicket(b[len(b)-r.Len():]); err != nil {
 		return fmt.Errorf("unmarshaling transport parameters from session ticket failed: %s", err.Error())
 	}
 	t.Parameters = &tp
+	t.RTT = time.Duration(rtt) * time.Microsecond
 	return nil
 }

internal/handshake/session_ticket_test.go

@@ -2,6 +2,7 @@ package handshake
 
 import (
 	"bytes"
+	"time"
 
 	"github.com/lucas-clemente/quic-go/internal/utils"
 
@@ -11,14 +12,18 @@ import (
 
 var _ = Describe("Session Ticket", func() {
 	It("marshals and unmarshals a session ticket", func() {
-		params := &TransportParameters{
-			InitialMaxStreamDataBidiLocal:  1,
-			InitialMaxStreamDataBidiRemote: 2,
+		ticket := &sessionTicket{
+			Parameters: &TransportParameters{
+				InitialMaxStreamDataBidiLocal:  1,
+				InitialMaxStreamDataBidiRemote: 2,
+			},
+			RTT: 1337 * time.Microsecond,
 		}
 		var t sessionTicket
-		Expect(t.Unmarshal((&sessionTicket{Parameters: params}).Marshal())).To(Succeed())
+		Expect(t.Unmarshal(ticket.Marshal())).To(Succeed())
 		Expect(t.Parameters.InitialMaxStreamDataBidiLocal).To(BeEquivalentTo(1))
 		Expect(t.Parameters.InitialMaxStreamDataBidiRemote).To(BeEquivalentTo(2))
+		Expect(t.RTT).To(Equal(1337 * time.Microsecond))
 	})
 
 	It("refuses to unmarshal if the ticket is too short for the revision", func() {
@@ -31,6 +36,12 @@ var _ = Describe("Session Ticket", func() {
 		Expect((&sessionTicket{}).Unmarshal(b.Bytes())).To(MatchError("unknown session ticket revision: 1337"))
 	})
 
+	It("refuses to unmarshal if the RTT cannot be read", func() {
+		b := &bytes.Buffer{}
+		utils.WriteVarInt(b, sessionTicketRevision)
+		Expect((&sessionTicket{}).Unmarshal(b.Bytes())).To(MatchError("failed to read RTT"))
+	})
+
 	It("refuses to unmarshal if unmarshaling the transport parameters fails", func() {
 		b := &bytes.Buffer{}
 		utils.WriteVarInt(b, sessionTicketRevision)