SplitHTTP client: Add xmux (multiplex controller) for H3 & H2 (#3613)

https://github.com/XTLS/Xray-core/pull/3613#issuecomment-2351954957

Closes https://github.com/XTLS/Xray-core/issues/3560#issuecomment-2247495778

---------

Co-authored-by: mmmray <142015632+mmmray@users.noreply.github.com>

infra/conf/transport_internet.go

@@ -231,6 +231,14 @@ type SplitHTTPConfig struct {
 	ScMinPostsIntervalMs *Int32Range       `json:"scMinPostsIntervalMs"`
 	NoSSEHeader          bool              `json:"noSSEHeader"`
 	XPaddingBytes        *Int32Range       `json:"xPaddingBytes"`
+	Xmux                 Xmux              `json:"xmux"`
+}
+
+type Xmux struct {
+	maxConnections *Int32Range `json:"maxConnections"`
+	maxConcurrency *Int32Range `json:"maxConcurrency"`
+	cMaxReuseTimes *Int32Range `json:"cMaxReuseTimes"`
+	cMaxLifetimeMs *Int32Range `json:"cMaxLifetimeMs"`
 }
 
 func splithttpNewRandRangeConfig(input *Int32Range) *splithttp.RandRangeConfig {
@@ -254,6 +262,19 @@ func (c *SplitHTTPConfig) Build() (proto.Message, error) {
 	} else if c.Host == "" && c.Headers["Host"] != "" {
 		c.Host = c.Headers["Host"]
 	}
+
+	if c.Xmux.maxConnections != nil && c.Xmux.maxConcurrency != nil {
+		return nil, errors.New("maxConnections cannot be specified together with maxConcurrency")
+	}
+
+	// Multiplexing config
+	muxProtobuf := splithttp.Multiplexing{
+		MaxConnections: splithttpNewRandRangeConfig(c.Xmux.maxConnections),
+		MaxConcurrency: splithttpNewRandRangeConfig(c.Xmux.maxConcurrency),
+		CMaxReuseTimes: splithttpNewRandRangeConfig(c.Xmux.cMaxReuseTimes),
+		CMaxLifetimeMs: splithttpNewRandRangeConfig(c.Xmux.cMaxLifetimeMs),
+	}
+
 	config := &splithttp.Config{
 		Path:                 c.Path,
 		Host:                 c.Host,
@@ -263,6 +284,7 @@ func (c *SplitHTTPConfig) Build() (proto.Message, error) {
 		ScMinPostsIntervalMs: splithttpNewRandRangeConfig(c.ScMinPostsIntervalMs),
 		NoSSEHeader:          c.NoSSEHeader,
 		XPaddingBytes:        splithttpNewRandRangeConfig(c.XPaddingBytes),
+		Xmux:                 &muxProtobuf,
 	}
 	return config, nil
 }

transport/internet/splithttp/client.go

@@ -30,8 +30,7 @@ type DialerClient interface {
 // implements splithttp.DialerClient in terms of direct network connections
 type DefaultDialerClient struct {
 	transportConfig *Config
-	download        *http.Client
+	client          *http.Client
-	upload          *http.Client
 	isH2            bool
 	isH3            bool
 	// pool of net.Conn, created using dialUploadConn
@@ -80,7 +79,7 @@ func (c *DefaultDialerClient) OpenDownload(ctx context.Context, baseURL string)
 
 		req.Header = c.transportConfig.GetRequestHeader()
 
-		response, err := c.download.Do(req)
+		response, err := c.client.Do(req)
 		gotConn.Close()
 		if err != nil {
 			errors.LogInfoInner(ctx, err, "failed to send download http request")
@@ -138,7 +137,7 @@ func (c *DefaultDialerClient) SendUploadRequest(ctx context.Context, url string,
 	req.Header = c.transportConfig.GetRequestHeader()
 
 	if c.isH2 || c.isH3 {
-		resp, err := c.upload.Do(req)
+		resp, err := c.client.Do(req)
 		if err != nil {
 			return err
 		}

transport/internet/splithttp/config.go

@@ -105,6 +105,49 @@ func (c *Config) GetNormalizedXPaddingBytes() RandRangeConfig {
 	return *c.XPaddingBytes
 }
 
+func (m *Multiplexing) GetNormalizedCMaxReuseTimes() RandRangeConfig {
+	if m.CMaxReuseTimes == nil {
+		return RandRangeConfig{
+			From: 0,
+			To:   0,
+		}
+	}
+
+	return *m.CMaxReuseTimes
+}
+
+func (m *Multiplexing) GetNormalizedCMaxLifetimeMs() RandRangeConfig {
+	if m.CMaxLifetimeMs == nil || m.CMaxLifetimeMs.To == 0 {
+		return RandRangeConfig{
+			From: 0,
+			To:   0,
+		}
+	}
+	return *m.CMaxLifetimeMs
+}
+
+func (m *Multiplexing) GetNormalizedMaxConnections() RandRangeConfig {
+	if m.MaxConnections == nil {
+		return RandRangeConfig{
+			From: 0,
+			To:   0,
+		}
+	}
+
+	return *m.MaxConnections
+}
+
+func (m *Multiplexing) GetNormalizedMaxConcurrency() RandRangeConfig {
+	if m.MaxConcurrency == nil {
+		return RandRangeConfig{
+			From: 0,
+			To:   0,
+		}
+	}
+
+	return *m.MaxConcurrency
+}
+
 func init() {
 	common.Must(internet.RegisterProtocolConfigCreator(protocolName, func() interface{} {
 		return new(Config)

transport/internet/splithttp/config.pb.go

@@ -33,6 +33,7 @@ type Config struct {
 	ScMinPostsIntervalMs *RandRangeConfig  `protobuf:"bytes,6,opt,name=scMinPostsIntervalMs,proto3" json:"scMinPostsIntervalMs,omitempty"`
 	NoSSEHeader          bool              `protobuf:"varint,7,opt,name=noSSEHeader,proto3" json:"noSSEHeader,omitempty"`
 	XPaddingBytes        *RandRangeConfig  `protobuf:"bytes,8,opt,name=xPaddingBytes,proto3" json:"xPaddingBytes,omitempty"`
+	Xmux                 *Multiplexing     `protobuf:"bytes,9,opt,name=xmux,proto3" json:"xmux,omitempty"`
 }
 
 func (x *Config) Reset() {
@@ -123,6 +124,13 @@ func (x *Config) GetXPaddingBytes() *RandRangeConfig {
 	return nil
 }
 
+func (x *Config) GetXmux() *Multiplexing {
+	if x != nil {
+		return x.Xmux
+	}
+	return nil
+}
+
 type RandRangeConfig struct {
 	state         protoimpl.MessageState
 	sizeCache     protoimpl.SizeCache
@@ -178,6 +186,77 @@ func (x *RandRangeConfig) GetTo() int32 {
 	return 0
 }
 
+type Multiplexing struct {
+	state         protoimpl.MessageState
+	sizeCache     protoimpl.SizeCache
+	unknownFields protoimpl.UnknownFields
+
+	MaxConnections *RandRangeConfig `protobuf:"bytes,1,opt,name=maxConnections,proto3" json:"maxConnections,omitempty"`
+	MaxConcurrency *RandRangeConfig `protobuf:"bytes,2,opt,name=maxConcurrency,proto3" json:"maxConcurrency,omitempty"`
+	CMaxReuseTimes *RandRangeConfig `protobuf:"bytes,3,opt,name=cMaxReuseTimes,proto3" json:"cMaxReuseTimes,omitempty"`
+	CMaxLifetimeMs *RandRangeConfig `protobuf:"bytes,4,opt,name=cMaxLifetimeMs,proto3" json:"cMaxLifetimeMs,omitempty"`
+}
+
+func (x *Multiplexing) Reset() {
+	*x = Multiplexing{}
+	if protoimpl.UnsafeEnabled {
+		mi := &file_transport_internet_splithttp_config_proto_msgTypes[2]
+		ms := protoimpl.X.MessageStateOf(protoimpl.Pointer(x))
+		ms.StoreMessageInfo(mi)
+	}
+}
+
+func (x *Multiplexing) String() string {
+	return protoimpl.X.MessageStringOf(x)
+}
+
+func (*Multiplexing) ProtoMessage() {}
+
+func (x *Multiplexing) ProtoReflect() protoreflect.Message {
+	mi := &file_transport_internet_splithttp_config_proto_msgTypes[2]
+	if protoimpl.UnsafeEnabled && x != nil {
+		ms := protoimpl.X.MessageStateOf(protoimpl.Pointer(x))
+		if ms.LoadMessageInfo() == nil {
+			ms.StoreMessageInfo(mi)
+		}
+		return ms
+	}
+	return mi.MessageOf(x)
+}
+
+// Deprecated: Use Multiplexing.ProtoReflect.Descriptor instead.
+func (*Multiplexing) Descriptor() ([]byte, []int) {
+	return file_transport_internet_splithttp_config_proto_rawDescGZIP(), []int{2}
+}
+
+func (x *Multiplexing) GetMaxConnections() *RandRangeConfig {
+	if x != nil {
+		return x.MaxConnections
+	}
+	return nil
+}
+
+func (x *Multiplexing) GetMaxConcurrency() *RandRangeConfig {
+	if x != nil {
+		return x.MaxConcurrency
+	}
+	return nil
+}
+
+func (x *Multiplexing) GetCMaxReuseTimes() *RandRangeConfig {
+	if x != nil {
+		return x.CMaxReuseTimes
+	}
+	return nil
+}
+
+func (x *Multiplexing) GetCMaxLifetimeMs() *RandRangeConfig {
+	if x != nil {
+		return x.CMaxLifetimeMs
+	}
+	return nil
+}
+
 var File_transport_internet_splithttp_config_proto protoreflect.FileDescriptor
 
 var file_transport_internet_splithttp_config_proto_rawDesc = []byte{
@@ -185,8 +264,8 @@ var file_transport_internet_splithttp_config_proto_rawDesc = []byte{
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+	0x65, 0x74, 0x2f, 0x73, 0x70, 0x6c, 0x69, 0x74, 0x68, 0x74, 0x74, 0x70, 0xaa, 0x02, 0x21, 0x58,
+	0x72, 0x61, 0x79, 0x2e, 0x54, 0x72, 0x61, 0x6e, 0x73, 0x70, 0x6f, 0x72, 0x74, 0x2e, 0x49, 0x6e,
+	0x74, 0x65, 0x72, 0x6e, 0x65, 0x74, 0x2e, 0x53, 0x70, 0x6c, 0x69, 0x74, 0x48, 0x74, 0x74, 0x70,
+	0x62, 0x06, 0x70, 0x72, 0x6f, 0x74, 0x6f, 0x33,
 }
 
 var (
@@ -252,23 +359,29 @@ func file_transport_internet_splithttp_config_proto_rawDescGZIP() []byte {
 	return file_transport_internet_splithttp_config_proto_rawDescData
 }
 
-var file_transport_internet_splithttp_config_proto_msgTypes = make([]protoimpl.MessageInfo, 3)
+var file_transport_internet_splithttp_config_proto_msgTypes = make([]protoimpl.MessageInfo, 4)
 var file_transport_internet_splithttp_config_proto_goTypes = []any{
 	(*Config)(nil),          // 0: xray.transport.internet.splithttp.Config
 	(*RandRangeConfig)(nil), // 1: xray.transport.internet.splithttp.RandRangeConfig
-	nil,                     // 2: xray.transport.internet.splithttp.Config.HeaderEntry
+	(*Multiplexing)(nil),    // 2: xray.transport.internet.splithttp.Multiplexing
+	nil,                     // 3: xray.transport.internet.splithttp.Config.HeaderEntry
 }
 var file_transport_internet_splithttp_config_proto_depIdxs = []int32{
-	2, // 0: xray.transport.internet.splithttp.Config.header:type_name -> xray.transport.internet.splithttp.Config.HeaderEntry
+	3,  // 0: xray.transport.internet.splithttp.Config.header:type_name -> xray.transport.internet.splithttp.Config.HeaderEntry
-	1, // 1: xray.transport.internet.splithttp.Config.scMaxConcurrentPosts:type_name -> xray.transport.internet.splithttp.RandRangeConfig
+	1,  // 1: xray.transport.internet.splithttp.Config.scMaxConcurrentPosts:type_name -> xray.transport.internet.splithttp.RandRangeConfig
-	1, // 2: xray.transport.internet.splithttp.Config.scMaxEachPostBytes:type_name -> xray.transport.internet.splithttp.RandRangeConfig
+	1,  // 2: xray.transport.internet.splithttp.Config.scMaxEachPostBytes:type_name -> xray.transport.internet.splithttp.RandRangeConfig
-	1, // 3: xray.transport.internet.splithttp.Config.scMinPostsIntervalMs:type_name -> xray.transport.internet.splithttp.RandRangeConfig
+	1,  // 3: xray.transport.internet.splithttp.Config.scMinPostsIntervalMs:type_name -> xray.transport.internet.splithttp.RandRangeConfig
-	1, // 4: xray.transport.internet.splithttp.Config.xPaddingBytes:type_name -> xray.transport.internet.splithttp.RandRangeConfig
+	1,  // 4: xray.transport.internet.splithttp.Config.xPaddingBytes:type_name -> xray.transport.internet.splithttp.RandRangeConfig
-	5, // [5:5] is the sub-list for method output_type
+	2,  // 5: xray.transport.internet.splithttp.Config.xmux:type_name -> xray.transport.internet.splithttp.Multiplexing
-	5, // [5:5] is the sub-list for method input_type
+	1,  // 6: xray.transport.internet.splithttp.Multiplexing.maxConnections:type_name -> xray.transport.internet.splithttp.RandRangeConfig
-	5, // [5:5] is the sub-list for extension type_name
+	1,  // 7: xray.transport.internet.splithttp.Multiplexing.maxConcurrency:type_name -> xray.transport.internet.splithttp.RandRangeConfig
-	5, // [5:5] is the sub-list for extension extendee
+	1,  // 8: xray.transport.internet.splithttp.Multiplexing.cMaxReuseTimes:type_name -> xray.transport.internet.splithttp.RandRangeConfig
-	0, // [0:5] is the sub-list for field type_name
+	1,  // 9: xray.transport.internet.splithttp.Multiplexing.cMaxLifetimeMs:type_name -> xray.transport.internet.splithttp.RandRangeConfig
+	10, // [10:10] is the sub-list for method output_type
+	10, // [10:10] is the sub-list for method input_type
+	10, // [10:10] is the sub-list for extension type_name
+	10, // [10:10] is the sub-list for extension extendee
+	0,  // [0:10] is the sub-list for field type_name
 }
 
 func init() { file_transport_internet_splithttp_config_proto_init() }
@@ -301,6 +414,18 @@ func file_transport_internet_splithttp_config_proto_init() {
 				return nil
 			}
 		}
+		file_transport_internet_splithttp_config_proto_msgTypes[2].Exporter = func(v any, i int) any {
+			switch v := v.(*Multiplexing); i {
+			case 0:
+				return &v.state
+			case 1:
+				return &v.sizeCache
+			case 2:
+				return &v.unknownFields
+			default:
+				return nil
+			}
+		}
 	}
 	type x struct{}
 	out := protoimpl.TypeBuilder{
@@ -308,7 +433,7 @@ func file_transport_internet_splithttp_config_proto_init() {
 			GoPackagePath: reflect.TypeOf(x{}).PkgPath(),
 			RawDescriptor: file_transport_internet_splithttp_config_proto_rawDesc,
 			NumEnums:      0,
-			NumMessages:   3,
+			NumMessages:   4,
 			NumExtensions: 0,
 			NumServices:   0,
 		},

transport/internet/splithttp/config.proto

@@ -15,9 +15,17 @@ message Config {
   RandRangeConfig scMinPostsIntervalMs = 6;
   bool noSSEHeader = 7;
   RandRangeConfig xPaddingBytes = 8;
+  Multiplexing xmux = 9;
 }
 
 message RandRangeConfig {
     int32 from = 1;
     int32 to = 2;
 }
+
+message Multiplexing {
+  RandRangeConfig maxConnections = 1;
+  RandRangeConfig maxConcurrency = 2;
+  RandRangeConfig cMaxReuseTimes = 3;
+  RandRangeConfig cMaxLifetimeMs = 4;
+}

transport/internet/splithttp/connection.go

@@ -11,6 +11,7 @@ type splitConn struct {
 	reader     io.ReadCloser
 	remoteAddr net.Addr
 	localAddr  net.Addr
+	onClose    func()
 }
 
 func (c *splitConn) Write(b []byte) (int, error) {
@@ -22,6 +23,10 @@ func (c *splitConn) Read(b []byte) (int, error) {
 }
 
 func (c *splitConn) Close() error {
+	if c.onClose != nil {
+		c.onClose()
+	}
+
 	err := c.writer.Close()
 	err2 := c.reader.Close()
 	if err != nil {

transport/internet/splithttp/dialer.go

@@ -41,32 +41,51 @@ type dialerConf struct {
 }
 
 var (
-	globalDialerMap    map[dialerConf]DialerClient
+	globalDialerMap    map[dialerConf]*muxManager
 	globalDialerAccess sync.Mutex
 )
 
-func getHTTPClient(ctx context.Context, dest net.Destination, streamSettings *internet.MemoryStreamConfig) DialerClient {
+func getHTTPClient(ctx context.Context, dest net.Destination, streamSettings *internet.MemoryStreamConfig) (DialerClient, *muxResource) {
 	if browser_dialer.HasBrowserDialer() {
-		return &BrowserDialerClient{}
+		return &BrowserDialerClient{}, nil
 	}
 
-	tlsConfig := tls.ConfigFromStreamSettings(streamSettings)
-	isH2 := tlsConfig != nil && !(len(tlsConfig.NextProtocol) == 1 && tlsConfig.NextProtocol[0] == "http/1.1")
-	isH3 := tlsConfig != nil && (len(tlsConfig.NextProtocol) == 1 && tlsConfig.NextProtocol[0] == "h3")
-
 	globalDialerAccess.Lock()
 	defer globalDialerAccess.Unlock()
 
 	if globalDialerMap == nil {
-		globalDialerMap = make(map[dialerConf]DialerClient)
+		globalDialerMap = make(map[dialerConf]*muxManager)
 	}
 
+	key := dialerConf{dest, streamSettings}
+
+	muxManager, found := globalDialerMap[key]
+
+	if !found {
+		transportConfig := streamSettings.ProtocolSettings.(*Config)
+		var mux Multiplexing
+		if transportConfig.Xmux != nil {
+			mux = *transportConfig.Xmux
+		}
+
+		muxManager = NewMuxManager(mux, func() interface{} {
+			return createHTTPClient(dest, streamSettings)
+		})
+		globalDialerMap[key] = muxManager
+	}
+
+	res := muxManager.GetResource(ctx)
+	return res.Resource.(DialerClient), res
+}
+
+func createHTTPClient(dest net.Destination, streamSettings *internet.MemoryStreamConfig) DialerClient {
+	tlsConfig := tls.ConfigFromStreamSettings(streamSettings)
+	isH2 := tlsConfig != nil && !(len(tlsConfig.NextProtocol) == 1 && tlsConfig.NextProtocol[0] == "http/1.1")
+	isH3 := tlsConfig != nil && (len(tlsConfig.NextProtocol) == 1 && tlsConfig.NextProtocol[0] == "h3")
+
 	if isH3 {
 		dest.Network = net.Network_UDP
 	}
-	if client, found := globalDialerMap[dialerConf{dest, streamSettings}]; found {
-		return client
-	}
 
 	var gotlsConfig *gotls.Config
 
@@ -74,6 +93,8 @@ func getHTTPClient(ctx context.Context, dest net.Destination, streamSettings *in
 		gotlsConfig = tlsConfig.GetTLSConfig(tls.WithDestination(dest))
 	}
 
+	transportConfig := streamSettings.ProtocolSettings.(*Config)
+
 	dialContext := func(ctxInner context.Context) (net.Conn, error) {
 		conn, err := internet.DialSystem(ctxInner, dest, streamSettings.SocketSettings)
 		if err != nil {
@@ -94,8 +115,7 @@ func getHTTPClient(ctx context.Context, dest net.Destination, streamSettings *in
 		return conn, nil
 	}
 
-	var downloadTransport http.RoundTripper
+	var transport http.RoundTripper
-	var uploadTransport http.RoundTripper
 
 	if isH3 {
 		quicConfig := &quic.Config{
@@ -107,7 +127,7 @@ func getHTTPClient(ctx context.Context, dest net.Destination, streamSettings *in
 			MaxIncomingStreams: -1,
 			KeepAlivePeriod:    h3KeepalivePeriod,
 		}
-		roundTripper := &http3.RoundTripper{
+		transport = &http3.RoundTripper{
 			QUICConfig:      quicConfig,
 			TLSClientConfig: gotlsConfig,
 			Dial: func(ctx context.Context, addr string, tlsCfg *gotls.Config, cfg *quic.Config) (quic.EarlyConnection, error) {
@@ -147,23 +167,20 @@ func getHTTPClient(ctx context.Context, dest net.Destination, streamSettings *in
 				return quic.DialEarly(ctx, udpConn, udpAddr, tlsCfg, cfg)
 			},
 		}
-		downloadTransport = roundTripper
-		uploadTransport = roundTripper
 	} else if isH2 {
-		downloadTransport = &http2.Transport{
+		transport = &http2.Transport{
 			DialTLSContext: func(ctxInner context.Context, network string, addr string, cfg *gotls.Config) (net.Conn, error) {
 				return dialContext(ctxInner)
 			},
 			IdleConnTimeout: connIdleTimeout,
 			ReadIdleTimeout: h2KeepalivePeriod,
 		}
-		uploadTransport = downloadTransport
 	} else {
 		httpDialContext := func(ctxInner context.Context, network string, addr string) (net.Conn, error) {
 			return dialContext(ctxInner)
 		}
 
-		downloadTransport = &http.Transport{
+		transport = &http.Transport{
 			DialTLSContext:  httpDialContext,
 			DialContext:     httpDialContext,
 			IdleConnTimeout: connIdleTimeout,
@@ -171,17 +188,12 @@ func getHTTPClient(ctx context.Context, dest net.Destination, streamSettings *in
 			// http.Client and our custom dial context.
 			DisableKeepAlives: true,
 		}
-		// we use uploadRawPool for that
-		uploadTransport = nil
 	}
 
 	client := &DefaultDialerClient{
-		transportConfig: streamSettings.ProtocolSettings.(*Config),
+		transportConfig: transportConfig,
-		download: &http.Client{
+		client: &http.Client{
-			Transport: downloadTransport,
+			Transport: transport,
-		},
-		upload: &http.Client{
-			Transport: uploadTransport,
 		},
 		isH2:           isH2,
 		isH3:           isH3,
@@ -189,7 +201,6 @@ func getHTTPClient(ctx context.Context, dest net.Destination, streamSettings *in
 		dialUploadConn: dialContext,
 	}
 
-	globalDialerMap[dialerConf{dest, streamSettings}] = client
 	return client
 }
 
@@ -223,7 +234,7 @@ func Dial(ctx context.Context, dest net.Destination, streamSettings *internet.Me
 	requestURL.Path = transportConfiguration.GetNormalizedPath() + sessionIdUuid.String()
 	requestURL.RawQuery = transportConfiguration.GetNormalizedQuery()
 
-	httpClient := getHTTPClient(ctx, dest, streamSettings)
+	httpClient, muxResource := getHTTPClient(ctx, dest, streamSettings)
 
 	maxUploadSize := scMaxEachPostBytes.roll()
 	// WithSizeLimit(0) will still allow single bytes to pass, and a lot of
@@ -231,7 +242,15 @@ func Dial(ctx context.Context, dest net.Destination, streamSettings *internet.Me
 	// uploadWriter wrapper, exact size limits can be enforced
 	uploadPipeReader, uploadPipeWriter := pipe.New(pipe.WithSizeLimit(maxUploadSize - 1))
 
+	if muxResource != nil {
+		muxResource.OpenRequests.Add(1)
+	}
+
 	go func() {
+		if muxResource != nil {
+			defer muxResource.OpenRequests.Add(-1)
+		}
+
 		requestsLimiter := semaphore.New(int(scMaxConcurrentPosts.roll()))
 		var requestCounter int64
 

transport/internet/splithttp/mux.go

@@ -0,0 +1,102 @@
+package splithttp
+
+import (
+	"context"
+	"math/rand"
+	"sync/atomic"
+	"time"
+
+	"github.com/xtls/xray-core/common/errors"
+)
+
+type muxResource struct {
+	Resource       interface{}
+	OpenRequests   atomic.Int32
+	leftUsage      int32
+	expirationTime time.Time
+}
+
+type muxManager struct {
+	newResourceFn func() interface{}
+	config        Multiplexing
+	concurrency   int32
+	connections   int32
+	instances     []*muxResource
+}
+
+func NewMuxManager(config Multiplexing, newResource func() interface{}) *muxManager {
+	return &muxManager{
+		config:        config,
+		concurrency:   config.GetNormalizedMaxConcurrency().roll(),
+		connections:   config.GetNormalizedMaxConnections().roll(),
+		newResourceFn: newResource,
+		instances:     make([]*muxResource, 0),
+	}
+}
+
+func (m *muxManager) GetResource(ctx context.Context) *muxResource {
+	m.removeExpiredConnections(ctx)
+
+	if m.connections > 0 && len(m.instances) < int(m.connections) {
+		errors.LogDebug(ctx, "xmux: creating client, connections=", len(m.instances))
+		return m.newResource()
+	}
+
+	if len(m.instances) == 0 {
+		errors.LogDebug(ctx, "xmux: creating client because instances is empty, connections=", len(m.instances))
+		return m.newResource()
+	}
+
+	clients := make([]*muxResource, 0)
+	if m.concurrency > 0 {
+		for _, client := range m.instances {
+			openRequests := client.OpenRequests.Load()
+			if openRequests < m.concurrency {
+				clients = append(clients, client)
+			}
+		}
+	} else {
+		clients = m.instances
+	}
+
+	if len(clients) == 0 {
+		errors.LogDebug(ctx, "xmux: creating client because concurrency was hit, total clients=", len(m.instances))
+		return m.newResource()
+	}
+
+	client := clients[rand.Intn(len(clients))]
+	if client.leftUsage > 0 {
+		client.leftUsage -= 1
+	}
+	return client
+}
+
+func (m *muxManager) newResource() *muxResource {
+	leftUsage := int32(-1)
+	if x := m.config.GetNormalizedCMaxReuseTimes().roll(); x > 0 {
+		leftUsage = x - 1
+	}
+	expirationTime := time.UnixMilli(0)
+	if x := m.config.GetNormalizedCMaxLifetimeMs().roll(); x > 0 {
+		expirationTime = time.Now().Add(time.Duration(x) * time.Millisecond)
+	}
+
+	client := &muxResource{
+		Resource:       m.newResourceFn(),
+		leftUsage:      leftUsage,
+		expirationTime: expirationTime,
+	}
+	m.instances = append(m.instances, client)
+	return client
+}
+
+func (m *muxManager) removeExpiredConnections(ctx context.Context) {
+	for i := 0; i < len(m.instances); i++ {
+		client := m.instances[i]
+		if client.leftUsage == 0 || (client.expirationTime != time.UnixMilli(0) && time.Now().After(client.expirationTime)) {
+			errors.LogDebug(ctx, "xmux: removing client, leftUsage = ", client.leftUsage, ", expirationTime = ", client.expirationTime)
+			m.instances = append(m.instances[:i], m.instances[i+1:]...)
+			i--
+		}
+	}
+}

transport/internet/splithttp/mux_test.go

@@ -0,0 +1,88 @@
+package splithttp_test
+
+import (
+	"context"
+	"testing"
+
+	. "github.com/xtls/xray-core/transport/internet/splithttp"
+)
+
+type fakeRoundTripper struct{}
+
+func TestMaxConnections(t *testing.T) {
+	config := Multiplexing{
+		MaxConnections: &RandRangeConfig{From: 4, To: 4},
+	}
+
+	mux := NewMuxManager(config, func() interface{} {
+		return &fakeRoundTripper{}
+	})
+
+	clients := make(map[interface{}]struct{})
+	for i := 0; i < 8; i++ {
+		clients[mux.GetResource(context.Background())] = struct{}{}
+	}
+
+	if len(clients) != 4 {
+		t.Error("did not get 4 distinct clients, got ", len(clients))
+	}
+}
+
+func TestCMaxReuseTimes(t *testing.T) {
+	config := Multiplexing{
+		CMaxReuseTimes: &RandRangeConfig{From: 2, To: 2},
+	}
+
+	mux := NewMuxManager(config, func() interface{} {
+		return &fakeRoundTripper{}
+	})
+
+	clients := make(map[interface{}]struct{})
+	for i := 0; i < 64; i++ {
+		clients[mux.GetResource(context.Background())] = struct{}{}
+	}
+
+	if len(clients) != 32 {
+		t.Error("did not get 32 distinct clients, got ", len(clients))
+	}
+}
+
+func TestMaxConcurrency(t *testing.T) {
+	config := Multiplexing{
+		MaxConcurrency: &RandRangeConfig{From: 2, To: 2},
+	}
+
+	mux := NewMuxManager(config, func() interface{} {
+		return &fakeRoundTripper{}
+	})
+
+	clients := make(map[interface{}]struct{})
+	for i := 0; i < 64; i++ {
+		client := mux.GetResource(context.Background())
+		client.OpenRequests.Add(1)
+		clients[client] = struct{}{}
+	}
+
+	if len(clients) != 32 {
+		t.Error("did not get 32 distinct clients, got ", len(clients))
+	}
+}
+
+func TestDefault(t *testing.T) {
+	config := Multiplexing{}
+
+	mux := NewMuxManager(config, func() interface{} {
+		return &fakeRoundTripper{}
+	})
+
+	clients := make(map[interface{}]struct{})
+	for i := 0; i < 64; i++ {
+		client := mux.GetResource(context.Background())
+		client.OpenRequests.Add(1)
+		clients[client] = struct{}{}
+	}
+
+	if len(clients) != 1 {
+		t.Error("did not get 1 distinct clients, got ", len(clients))
+	}
+}