wire: apply the default value for the active_connection_id_limit (#3806)

connection.go

@@ -312,9 +312,14 @@ var newConnection = func(
 		DisableActiveMigration:          true,
 		StatelessResetToken:             &statelessResetToken,
 		OriginalDestinationConnectionID: origDestConnID,
-		ActiveConnectionIDLimit:         protocol.MaxActiveConnectionIDs,
+		// For interoperability with quic-go versions before May 2023, this value must be set to a value
-		InitialSourceConnectionID:       srcConnID,
+		// different from protocol.DefaultActiveConnectionIDLimit.
-		RetrySourceConnectionID:         retrySrcConnID,
+		// If set to the default value, it will be omitted from the transport parameters, which will make
+		// old quic-go versions interpret it as 0, instead of the default value of 2.
+		// See https://github.com/quic-go/quic-go/pull/3806.
+		ActiveConnectionIDLimit:   protocol.MaxActiveConnectionIDs,
+		InitialSourceConnectionID: srcConnID,
+		RetrySourceConnectionID:   retrySrcConnID,
 	}
 	if s.config.EnableDatagrams {
 		params.MaxDatagramFrameSize = protocol.MaxDatagramFrameSize
@@ -426,8 +431,13 @@ var newClientConnection = func(
 		MaxAckDelay:                    protocol.MaxAckDelayInclGranularity,
 		AckDelayExponent:               protocol.AckDelayExponent,
 		DisableActiveMigration:         true,
-		ActiveConnectionIDLimit:        protocol.MaxActiveConnectionIDs,
+		// For interoperability with quic-go versions before May 2023, this value must be set to a value
-		InitialSourceConnectionID:      srcConnID,
+		// different from protocol.DefaultActiveConnectionIDLimit.
+		// If set to the default value, it will be omitted from the transport parameters, which will make
+		// old quic-go versions interpret it as 0, instead of the default value of 2.
+		// See https://github.com/quic-go/quic-go/pull/3806.
+		ActiveConnectionIDLimit:   protocol.MaxActiveConnectionIDs,
+		InitialSourceConnectionID: srcConnID,
 	}
 	if s.config.EnableDatagrams {
 		params.MaxDatagramFrameSize = protocol.MaxDatagramFrameSize

internal/protocol/protocol.go

@@ -77,6 +77,9 @@ const MinConnectionIDLenInitial = 8
 // DefaultAckDelayExponent is the default ack delay exponent
 const DefaultAckDelayExponent = 3
 
+// DefaultActiveConnectionIDLimit is the default active connection ID limit
+const DefaultActiveConnectionIDLimit = 2
+
 // MaxAckDelayExponent is the maximum ack delay exponent
 const MaxAckDelayExponent = 20
 

internal/wire/transport_parameter_test.go

@@ -289,15 +289,16 @@ var _ = Describe("Transport Parameters", func() {
 		Expect(float32(dataLen) / num).To(BeNumerically("~", float32(defaultLen)/num+float32(entryLen), 1))
 	})
 
-	It("sets the default value for the ack_delay_exponent, when no value was sent", func() {
+	It("sets the default value for the ack_delay_exponent and max_active_connection_id_limit, when no values were sent", func() {
 		data := (&TransportParameters{
 			AckDelayExponent:        protocol.DefaultAckDelayExponent,
 			StatelessResetToken:     &protocol.StatelessResetToken{},
-			ActiveConnectionIDLimit: 2,
+			ActiveConnectionIDLimit: protocol.DefaultActiveConnectionIDLimit,
 		}).Marshal(protocol.PerspectiveServer)
 		p := &TransportParameters{}
 		Expect(p.Unmarshal(data, protocol.PerspectiveServer)).To(Succeed())
 		Expect(p.AckDelayExponent).To(BeEquivalentTo(protocol.DefaultAckDelayExponent))
+		Expect(p.ActiveConnectionIDLimit).To(BeEquivalentTo(protocol.DefaultActiveConnectionIDLimit))
 	})
 
 	It("errors when the varint value has the wrong length", func() {

internal/wire/transport_parameters.go

@@ -118,6 +118,7 @@ func (p *TransportParameters) unmarshal(r *bytes.Reader, sentBy protocol.Perspec
 	var (
 		readOriginalDestinationConnectionID bool
 		readInitialSourceConnectionID       bool
+		readActiveConnectionIDLimit         bool
 	)
 
 	p.AckDelayExponent = protocol.DefaultAckDelayExponent
@@ -139,6 +140,9 @@ func (p *TransportParameters) unmarshal(r *bytes.Reader, sentBy protocol.Perspec
 		}
 		parameterIDs = append(parameterIDs, paramID)
 		switch paramID {
+		case activeConnectionIDLimitParameterID:
+			readActiveConnectionIDLimit = true
+			fallthrough
 		case maxIdleTimeoutParameterID,
 			maxUDPPayloadSizeParameterID,
 			initialMaxDataParameterID,
@@ -148,7 +152,6 @@ func (p *TransportParameters) unmarshal(r *bytes.Reader, sentBy protocol.Perspec
 			initialMaxStreamsBidiParameterID,
 			initialMaxStreamsUniParameterID,
 			maxAckDelayParameterID,
-			activeConnectionIDLimitParameterID,
 			maxDatagramFrameSizeParameterID,
 			ackDelayExponentParameterID:
 			if err := p.readNumericTransportParameter(r, paramID, int(paramLen)); err != nil {
@@ -196,6 +199,9 @@ func (p *TransportParameters) unmarshal(r *bytes.Reader, sentBy protocol.Perspec
 		}
 	}
 
+	if !readActiveConnectionIDLimit {
+		p.ActiveConnectionIDLimit = protocol.DefaultActiveConnectionIDLimit
+	}
 	if !fromSessionTicket {
 		if sentBy == protocol.PerspectiveServer && !readOriginalDestinationConnectionID {
 			return errors.New("missing original_destination_connection_id")
@@ -402,7 +408,9 @@ func (p *TransportParameters) Marshal(pers protocol.Perspective) []byte {
 		}
 	}
 	// active_connection_id_limit
-	b = p.marshalVarintParam(b, activeConnectionIDLimitParameterID, p.ActiveConnectionIDLimit)
+	if p.ActiveConnectionIDLimit != protocol.DefaultActiveConnectionIDLimit {
+		b = p.marshalVarintParam(b, activeConnectionIDLimitParameterID, p.ActiveConnectionIDLimit)
+	}
 	// initial_source_connection_id
 	b = quicvarint.Append(b, uint64(initialSourceConnectionIDParameterID))
 	b = quicvarint.Append(b, uint64(p.InitialSourceConnectionID.Len()))