diff --git a/connections/implementation/proto/offline_wire_formats.proto b/connections/implementation/proto/offline_wire_formats.proto
index b93c47b7..a60d339c 100644
--- a/connections/implementation/proto/offline_wire_formats.proto
+++ b/connections/implementation/proto/offline_wire_formats.proto
@@ -119,6 +119,11 @@ message ConnectionResponseFrame {
   }
   optional ResponseStatus response = 3;
   optional OsInfo os_info = 4;
+  // A bitmask value to indicate which medium supports Multiplex transmission
+  // feature. Each supporting medium could utilize one bit starting from the
+  // least significant bit in this field. Refer to ClientProxy.java for the bit
+  // usages.
+  optional int32 multiplex_socket_bitmask = 5;
 }
 
 message PayloadTransferFrame {
diff --git a/proto/errorcode/error_code_enums.proto b/proto/errorcode/error_code_enums.proto
index 3b328b79..120566b5 100644
--- a/proto/errorcode/error_code_enums.proto
+++ b/proto/errorcode/error_code_enums.proto
@@ -563,4 +563,12 @@ enum Description {
   FEATURE_USB_HOST_NOT_SUPPORTED = 177;
   FEATURE_USB_ACCESSORY_NOT_SUPPORTED = 178;
   FEATURE_USB_PORTS_NOT_FOUND = 179;
+  MULTIPLEX_SOCKET_UNKNOWN_ERROR = 180;
+  MULTIPLEX_SOCKET_NOT_LISTENING = 181;
+  MULTIPLEX_SOCKET_DISABLED = 182;
+  MULTIPLEX_SOCKET_TIMEOUT = 183;
+  MULTIPLEX_SOCKET_IOEXCEPTION = 184;
+  MULTIPLEX_SOCKET_UNKNOWN_RESPONSE_CODE = 185;
+  MULTIPLEX_SOCKET_EXECUTION_EXCEPTION = 186;
+  MULTIPLEX_SOCKET_INTERRUPTED_EXCEPTION = 187;
 }
diff --git a/proto/mediums/BUILD b/proto/mediums/BUILD
index 7e8acaaf..9f917770 100644
--- a/proto/mediums/BUILD
+++ b/proto/mediums/BUILD
@@ -16,6 +16,13 @@ licenses(["notice"])
 
 package(default_visibility = ["//visibility:public"])
 
+proto_library(
+    name = "multiplex_frames_proto",
+    srcs = [
+        "multiplex_frames.proto",
+    ],
+)
+
 proto_library(
     name = "nfc_frames_proto",
     srcs = [
diff --git a/proto/mediums/multiplex_frames.proto b/proto/mediums/multiplex_frames.proto
new file mode 100644
index 00000000..5eb09b15
--- /dev/null
+++ b/proto/mediums/multiplex_frames.proto
@@ -0,0 +1,84 @@
+// Copyright 2023 Google LLC
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+syntax = "proto2";
+
+package location.nearby.mediums;
+
+option optimize_for = LITE_RUNTIME;
+option java_outer_classname = "MultiplexFramesProto";
+option java_package = "com.google.location.nearby.mediums.proto";
+option objc_class_prefix = "GNCM";
+
+// The frame used to transmit any type of frame on a virtual socket.
+message MultiplexFrame {
+  enum MultiplexFrameType {
+    UNKNOWN_FRAME_TYPE = 0;
+    CONTROL_FRAME = 1;
+    DATA_FRAME = 2;
+  }
+
+  optional MultiplexFrameHeader header = 1;
+  optional MultiplexFrameType frame_type = 2;
+  oneof Frame {
+    MultiplexControlFrame control_frame = 3;
+    MultiplexDataFrame data_frame = 4;
+  }
+}
+
+message MultiplexFrameHeader {
+  optional bytes service_id_hash = 1;
+}
+
+// The control frame to establish, accept, reject or disconnect the virtual
+// socket.
+message MultiplexControlFrame {
+  enum MultiplexControlFrameType {
+    UNKNOWN_CONTROL_FRAME_TYPE = 0;
+    CONNECTION_REQUEST = 1;
+    CONNECTION_RESPONSE = 2;
+    DISCONNECTION = 3;
+  }
+
+  optional MultiplexControlFrameType control_frame_type = 1;
+
+  // Exactly one of the following fields will be set.
+  oneof Frame {
+    ConnectionRequestFrame connection_request_frame = 2;
+    ConnectionResponseFrame connection_response_frame = 3;
+    DisconnectFrame disconnect_frame = 4;
+  }
+}
+
+// The frame to request a virtual socket for the service ID.
+message ConnectionRequestFrame {}
+
+// The frame to accept or reject the CONNECTION_REQUEST.
+message ConnectionResponseFrame {
+  enum ConnectionResponseCode {
+    UNKNOWN_RESPONSE_CODE = 0;
+    CONNECTION_ACCEPTED = 1;
+    NOT_LISTENING = 2;
+  }
+
+  optional ConnectionResponseCode connection_response_code = 1;
+}
+
+// The frame to disconnect the virtual socket.
+message DisconnectFrame {}
+
+// The data frame used to transmit the data type bytes on a virtual socket.
+message MultiplexDataFrame {
+  optional bytes data = 1;
+}