diff --git a/peerserver.js b/peerserver.js
index cd4016d..87f323f 100644
--- a/peerserver.js
+++ b/peerserver.js
@@ -3,8 +3,8 @@
 // SPDX-License-Identifier: Apache-2.0
 
 // Prepare for web server
-var fs = require("fs");
-var path = require("path");
+var fs = require('fs');
+var path = require('path');
 var url = require('url');
 var config = require('./config');
 var account = require('./vendormodule');
@@ -17,19 +17,80 @@ var httpsOptions = {
 
 var app = require('express')();
 var server = app.listen(config.port.plain);
-var servers = require("https").createServer(httpsOptions, app).listen(config.port.secured);
+var servers = require('https').createServer(httpsOptions, app).listen(config.port.secured);
 var io = require('socket.io').listen(server);
 var ios = require('socket.io').listen(servers);
 
 var sessionMap = {};  // Key is uid, and value is session object.
+var instanceList = new Array();
+var clientId = 1000;
+
+function addInstance(uid){
+  var i = 0;
+  while(i < instanceList.length && instanceList[i] != uid){
+    ++i;
+  }
+  if(i == instanceList.length){
+    instanceList.push({uid:uid, available:1});
+  }
+}
+
+function deleteInstance(uid){
+  for(var i = 0; i < instanceList.length; ++i){
+    if(instanceList && instanceList[i].uid == uid){
+      instanceList.splice(i, 1);
+      return;
+    }
+  }
+}
+
+function changeToIdle(uid){
+  for(var i = 0; i < instanceList.length; ++i){
+    if(instanceList && instanceList[i].uid == uid){
+      instanceList[i].available = 1;
+      return;
+    }
+  }
+}
+
+function changeToOccupy(uid, state){
+  for(var i = 0; i < instanceList.length; ++i){
+    if(instanceList && instanceList[i].uid == uid){
+      instanceList[i].available = 0;
+      return;
+    }
+  }
+}
+
+function isExist(uid){
+  if(!instanceList){
+    return -1;
+  }
+  for(var i = 0; i < instanceList.length; ++i){
+    if(instanceList[i].uid == uid){
+      if(instanceList[i].available == 1){
+        return 1;
+      }else{
+        return 2;
+      }
+    }
+  }
+  return 0;
+}
+
+function logNumber(){
+  var sessionMapkeys = Object.keys(sessionMap);
+  var clientNum = sessionMapkeys.length - instanceList.length;
+  console.log('Client number: ' + clientNum + ' ; Instance number: ' + instanceList.length);
+}
 
 // Check user's token from partner
 function validateUser(token, successCallback, failureCallback){
   // TODO: Should check token first, replace this block when engagement with different partners.
   if(token){
-    account.authentication(token,function(uid){
+    account.authentication(token, function(uid){
       successCallback(uid);
-    },function(){
+    }, function(){
       console.log('Account system return false.');
       failureCallback(0);
     });
@@ -39,11 +100,11 @@ function validateUser(token, successCallback, failureCallback){
 }
 
 function disconnectClient(uid){
-  if(sessionMap[uid]!==undefined){
-    var session=sessionMap[uid];
+  if(sessionMap[uid] !== undefined){
+    var session = sessionMap[uid];
     session.emit('server-disconnect');
     session.disconnect();
-    console.log('Force disconnected '+uid);
+    console.log('Force disconnected ' + uid);
   }
 }
 
@@ -56,7 +117,7 @@ function createUuid(){
 
 function emitChatEvent(targetUid, eventName, message, successCallback, failureCallback){
   if(sessionMap[targetUid]){
-    sessionMap[targetUid].emit(eventName,message);
+    sessionMap[targetUid].emit(eventName, message);
     if(successCallback)
       successCallback();
   }
@@ -66,10 +127,15 @@ function emitChatEvent(targetUid, eventName, message, successCallback, failureCa
 }
 
 function authorization(socket, next){
-  var query=url.parse(socket.request.url,true).query;
-  var token=query.token;
-  var clientVersion=query.clientVersion;
-  var clientType=query.clientType;
+  var query = url.parse(socket.request.url, true).query;
+  var clientVersion = query.clientVersion;
+  var clientType = query.clientType;
+  var isClient = query.isClient;
+  if(isClient){
+    token = (clientId++) + '';
+  }else{
+    token = query.token;
+  }
   switch(clientVersion){
     case '4.2':
     case '4.2.1':
@@ -77,55 +143,96 @@ function authorization(socket, next){
       // socket.user stores session related information.
       if(token){
         validateUser(token, function(uid){  // Validate user's token successfully.
-          socket.user={id:uid};
-          console.log(uid+' authentication passed.');
-        },function(error){
-            // Invalid login.
+          socket.user = {id:uid, isClient:isClient, instanceId:null};
+          if(!isClient){
+            console.log('Instance ' + uid + ' authentication passed.');
+            addInstance(uid);
+          }else{
+            console.log('Client ' + uid + ' authentication passed.');
+          }
+        }, function(error){
+          // Invalid login.
             console.log('Authentication failed.');
             next();
         });
       }else{
-        socket.user=new Object();
-        socket.user.id=createUuid()+'@anonymous';
-        console.log('Anonymous user: '+socket.user.id);
+        socket.user = new Object();
+        socket.user.id = createUuid() + '@anonymous';
+        console.log('Anonymous user: ' + socket.user.id);
       }
       next();
       break;
     default:
       next(new Error('2103'));
-      console.log('Unsupported client. Client version: '+query.clientVersion);
+      console.log('Unsupported client. Client version: ' + query.clientVersion);
       break;
   }
 }
 
 function onConnection(socket){
   // Disconnect previous session if this user already signed in.
-  var uid=socket.user.id;
+  var uid = socket.user.id;
+  var isClient = socket.user.isClient;
   disconnectClient(uid);
-  sessionMap[uid]=socket;
-  socket.emit('server-authenticated',{uid:uid});  // Send current user's id to client.
-  console.log('A new client has connected. Online user number: '+Object.keys(sessionMap).length);
+  sessionMap[uid] = socket;
+  socket.emit('server-authenticated', {uid:uid});  // Send current user's id to client.
+  logNumber();
+  if(isClient){
+    socket.on('disconnect-instance', function (data) {
+      console.log('Client ' + uid + ' disconnect with instance ' + data.to);
+      changeToIdle(data.to);
+    });
+  }
 
-  socket.on('disconnect',function(){
+  socket.on('disconnect', function(){
     if(socket.user){
-      var uid=socket.user.id;
+      var uid = socket.user.id;
+      var isClient = socket.user.isClient;
       // Delete session
-      if(socket===sessionMap[socket.user.id]){
+      if(socket === sessionMap[socket.user.id]){
         delete sessionMap[socket.user.id];
       }
-      console.log(uid+' has disconnected. Online user number: '+Object.keys(sessionMap).length);
+      if(isClient){
+        console.log('Client ' + uid + ' disconnect with server.');
+        if(socket.user.instanceId != null){
+          changeToIdle(socket.user.instanceId);
+        }
+      }else{
+        console.log('Instance ' + uid + ' disconnect with server.');
+        deleteInstance(uid);
+      }
+      logNumber();
+    }
+  });
+
+  socket.on('build-p2p-connect', function(data, callback){
+    var m = isExist(data.to);
+    callback(false, m);
+    if(m == -1){
+      console.log('Error: instanceList does not exist.');
+    }else if(m == 0){
+      console.log('Instance ' + data.to + ' does not exist.');
+    }else if(m == 1){
+      console.log('Instance ' + data.to + ' is available.');
+      changeToOccupy(data.to);
+    }else if(m == 2){
+      console.log('Instance ' + data.to + ' is occupied.');
     }
   });
 
   // Forward events
-  var forwardEvents=['owt-message'];
-  for (var i=0;i<forwardEvents.length;i++){
-    socket.on(forwardEvents[i],(function(i){
+  var forwardEvents = ['owt-message'];
+  for(var i = 0; i < forwardEvents.length; i++){
+    socket.on(forwardEvents[i], (function(i){
       return function(data, ackCallback){
-        data.from=socket.user.id;
-        var to=data.to;
+        console.log('Received ' + forwardEvents[i] + ' : ' + JSON.stringify(data));
+        data.from = socket.user.id;
+        var to = data.to;
+        if(socket.user.isClient){
+          socket.user.instanceId = to;
+        }
         delete data.to;
-        emitChatEvent(to,forwardEvents[i],data,function(){
+        emitChatEvent(to, forwardEvents[i], data, function(){
           if(ackCallback)
             ackCallback();
         },function(errorCode){
@@ -139,7 +246,7 @@ function onConnection(socket){
 
 function listen(io) {
   io.use(authorization);
-  io.on('connection',onConnection);
+  io.on('connection', onConnection);
 }
 
 listen(io);