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limit the exponential PTO backoff to 60s #3595

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merged 4 commits into from
Nov 15, 2022
Merged

limit the exponential PTO backoff to 60s #3595

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bb179ed
limiting exponential pto delay to a constant
cliffc-spirent Oct 14, 2022
f5d43b0
updating maximum pto to 60s, referring to rfc
cliffc-spirent Oct 17, 2022
de80d4a
updating exponential backoff test for truncation
cliffc-spirent Oct 20, 2022
9ec71c1
Apply suggestions from code review
marten-seemann Nov 15, 2022
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18 changes: 14 additions & 4 deletions internal/ackhandler/sent_packet_handler.go
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Original file line number Diff line number Diff line change
Expand Up @@ -23,6 +23,8 @@ const (
amplificationFactor = 3
// We use Retry packets to derive an RTT estimate. Make sure we don't set the RTT to a super low value yet.
minRTTAfterRetry = 5 * time.Millisecond
// PTO duration is exponential but truncated to the following maximum as allowed by RFC 8961 Section 4.4.
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maxPTODuration = 60 * time.Second
)

type packetNumberSpace struct {
Expand Down Expand Up @@ -457,6 +459,14 @@ func (h *sentPacketHandler) getLossTimeAndSpace() (time.Time, protocol.Encryptio
return lossTime, encLevel
}

func (h *sentPacketHandler) getScaledPTO(includeMaxAckDelay bool) time.Duration {
pto := h.rttStats.PTO(includeMaxAckDelay) << h.ptoCount
if (pto > maxPTODuration) || (pto <= 0) {
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return maxPTODuration
}
return pto
}

// same logic as getLossTimeAndSpace, but for lastAckElicitingPacketTime instead of lossTime
func (h *sentPacketHandler) getPTOTimeAndSpace() (pto time.Time, encLevel protocol.EncryptionLevel, ok bool) {
// We only send application data probe packets once the handshake is confirmed,
Expand All @@ -465,7 +475,7 @@ func (h *sentPacketHandler) getPTOTimeAndSpace() (pto time.Time, encLevel protoc
if h.peerCompletedAddressValidation {
return
}
t := time.Now().Add(h.rttStats.PTO(false) << h.ptoCount)
t := time.Now().Add(h.getScaledPTO(false))
if h.initialPackets != nil {
return t, protocol.EncryptionInitial, true
}
Expand All @@ -475,18 +485,18 @@ func (h *sentPacketHandler) getPTOTimeAndSpace() (pto time.Time, encLevel protoc
if h.initialPackets != nil {
encLevel = protocol.EncryptionInitial
if t := h.initialPackets.lastAckElicitingPacketTime; !t.IsZero() {
pto = t.Add(h.rttStats.PTO(false) << h.ptoCount)
pto = t.Add(h.getScaledPTO(false))
}
}
if h.handshakePackets != nil && !h.handshakePackets.lastAckElicitingPacketTime.IsZero() {
t := h.handshakePackets.lastAckElicitingPacketTime.Add(h.rttStats.PTO(false) << h.ptoCount)
t := h.handshakePackets.lastAckElicitingPacketTime.Add(h.getScaledPTO(false))
if pto.IsZero() || (!t.IsZero() && t.Before(pto)) {
pto = t
encLevel = protocol.EncryptionHandshake
}
}
if h.handshakeConfirmed && !h.appDataPackets.lastAckElicitingPacketTime.IsZero() {
t := h.appDataPackets.lastAckElicitingPacketTime.Add(h.rttStats.PTO(true) << h.ptoCount)
t := h.appDataPackets.lastAckElicitingPacketTime.Add(h.getScaledPTO(true))
if pto.IsZero() || (!t.IsZero() && t.Before(pto)) {
pto = t
encLevel = protocol.Encryption1RTT
Expand Down
12 changes: 11 additions & 1 deletion internal/ackhandler/sent_packet_handler_test.go
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Original file line number Diff line number Diff line change
Expand Up @@ -687,6 +687,14 @@ var _ = Describe("SentPacketHandler", func() {
handler.ptoCount = 2
handler.setLossDetectionTimer()
Expect(handler.GetLossDetectionTimeout().Sub(sendTime)).To(Equal(4 * timeout))
// truncated when the exponential gets too large
handler.ptoCount = 20
handler.setLossDetectionTimer()
Expect(handler.GetLossDetectionTimeout().Sub(sendTime)).To(Equal(maxPTODuration))
// protected from rollover
handler.ptoCount = 100
handler.setLossDetectionTimer()
Expect(handler.GetLossDetectionTimeout().Sub(sendTime)).To(Equal(maxPTODuration))
})

It("reset the PTO count when receiving an ACK", func() {
Expand Down Expand Up @@ -1036,7 +1044,7 @@ var _ = Describe("SentPacketHandler", func() {
})

It("correctly sets the timer after the Initial packet number space has been dropped", func() {
handler.SentPacket(initialPacket(&Packet{PacketNumber: 1, SendTime: time.Now().Add(-42 * time.Second)}))
handler.SentPacket(initialPacket(&Packet{PacketNumber: 1, SendTime: time.Now().Add(-19 * time.Second)}))
_, err := handler.ReceivedAck(
&wire.AckFrame{AckRanges: []wire.AckRange{{Smallest: 1, Largest: 1}}},
protocol.EncryptionInitial,
Expand All @@ -1048,6 +1056,8 @@ var _ = Describe("SentPacketHandler", func() {

pto := handler.rttStats.PTO(false)
Expect(pto).ToNot(BeZero())
// pto is approximately 19 * 3. Using a number > 19 above will
// run into the maxPTODuration limit
Expect(handler.GetLossDetectionTimeout()).To(BeTemporally("~", time.Now().Add(pto), 10*time.Millisecond))
})

Expand Down