Use Chromium BUILD to approximate Blimp protocol version, and check it.

blimp/common/proto/protocol_control.proto

 // Copyright 2015 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 syntax = "proto2";
 
 option optimize_for = LITE_RUNTIME;
 
 package blimp;
 
 // Indicates the message-Id that the peer has most-recently processed.
 message CheckpointAckMessage {
   optional int64 checkpoint_id = 1;
 }
 
 // Contains Client->Engine information to establish an authenticated connection.
 message StartConnectionMessage {
   optional string client_token = 1;
   optional int32 protocol_version = 2;
 }
 
+// Indicates the reason for the peer ending the connection.
+message EndConnectionMessage {
+  enum Reason {
+    UNKNOWN = 0;
+    AUTHENTICATION_FAILED = 1;
+    PROTOCOL_MISMATCH = 2;
+  };
+  optional Reason reason = 1;
+}
+
 message ProtocolControlMessage {
   oneof connection_message {
     // Client => Server types.
     StartConnectionMessage start_connection = 41;
 
-    // Server => Client types.
+    // Bi-directional types.
     CheckpointAckMessage checkpoint_ack = 42;
+    EndConnectionMessage end_connection = 43;
   }
 }