diff --git a/src/engine_robust.cpp b/src/engine_robust.cpp
index 9ed2a31fb..3382d189d 100644
--- a/src/engine_robust.cpp
+++ b/src/engine_robust.cpp
@@ -1,6 +1,6 @@
 /*!
  * \file engine_robust.cpp
- * \brief Robust implementation of AllReduce 
+ * \brief Robust implementation of AllReduce
  *   using TCP non-block socket and tree-shape reduction.
  *
  *   This implementation considers the failure of nodes
@@ -35,8 +35,10 @@ class AllReduceManager : public IEngine {
   // constant one byte out of band message to indicate error happening
   // and mark for channel cleanup
   const static char kOOBReset = 95;
+  // and mark for channel cleanup, after OOB signal
+  const static char kResetMark = 97;
   // and mark for channel cleanup
-  const static char kOOBResetAck = 97;
+  const static char kResetAck = 97;
 
   AllReduceManager(void) {
     master_uri = "NULL";
@@ -173,7 +175,6 @@ class AllReduceManager : public IEngine {
                          size_t count,           
                          ReduceFunction reducer) {
     while (true) {
-      if (rank == rand() % 3) TryResetLinks();
       ReturnType ret = TryAllReduce(sendrecvbuf_, type_nbytes, count, reducer);
       if (ret == kSuccess) return;
       if (ret == kSockError) {
@@ -280,119 +281,95 @@ class AllReduceManager : public IEngine {
     for (int i = 0; i < nlink; ++i) {
       links[i].InitBuffer(sizeof(int), 1 << 10, reduce_buffer_size);
       links[i].ResetSize();
+      links[i].except = false;
     }
-    printf("[%d] start to reset link\n", rank);    
+    // read and discard data from all channels until pass mark
     while (true) {
-      printf("[%d] loop\n", rank);
-      bool finished = true;
       for (int i = 0; i < nlink; ++i) {
         if (links[i].sock.BadSocket()) continue;
         if (links[i].size_write == 0) {
           char sig = kOOBReset;
           ssize_t len = links[i].sock.Send(&sig, sizeof(sig), MSG_OOB);
           // error will be filtered in next loop
-          if (len != -1) {
-            links[i].size_write += len;
-            printf("[%d] send OOB success\n", rank);
+          if (len == sizeof(sig)) links[i].size_write = 1;
+        }
+        if (links[i].size_write == 1) {
+          char sig = kResetMark;
+          ssize_t len = links[i].sock.Send(&sig, sizeof(sig));
+          if (len == sizeof(sig)) links[i].size_write = 2;
+        }
+        if (links[i].size_read == 0) {
+          int atmark = links[i].sock.AtMark();
+          if (atmark < 0) {
+            utils::Assert(links[i].sock.BadSocket(), "must already gone bad");
+          } else if (atmark > 0) {           
+            links[i].size_read = 1;
+          } else {
+            // no at mark, read and discard data
+            ssize_t len = links[i].sock.Recv(links[i].buffer_head, links[i].buffer_size);
+            // zero length, remote closed the connection, close socket
+            if (len == 0) links[i].sock.Close();
           }
         }
-        // need to send OOB to every other link
-        if (links[i].size_write == 0) finished = false;
       }
-      if (finished) break;
-    }
-    printf("[%d] finish send all OOB\n", rank);
-    // wait for incoming except from all links
-    for (int i = 0; i < nlink; ++ i) {
-      if (links[i].sock.BadSocket()) continue;
-      printf("[%d] wait except\n", rank);
-      if (utils::SelectHelper::WaitExcept(links[i].sock) == -1) {
-        utils::Socket::Error("select");
-      }
-      printf("[%d] finish wait except\n", rank);
-    }
-    printf("[%d] start to discard link\n", rank);
-    // read and discard data from all channels until pass mark
-    while (true) {
       utils::SelectHelper rsel;
       bool finished = true;
       for (int i = 0; i < nlink; ++i) {
-        if (links[i].sock.BadSocket()) continue;
-        if (links[i].size_read == 0) {
-          int atmark = links[i].sock.AtMark();
-          if (atmark < 0)  return kSockError;
-          if (atmark == 1) {
-            char oob_msg;
-            ssize_t len = links[i].sock.Recv(&oob_msg, sizeof(oob_msg), MSG_OOB);
-            if (len == -1 && errno != EAGAIN && errno != EWOULDBLOCK) {
-              finished = false; continue;
-            }
-            utils::Assert(oob_msg == kOOBReset, "wrong oob msg");
-            links[i].size_read = 1;
-          } else  {
-            finished = false;
-            rsel.WatchRead(links[i].sock);
-          }
+        if (links[i].size_write != 2 && !links[i].sock.BadSocket()) {
+            rsel.WatchWrite(links[i].sock); finished = false;
+        }
+        if (links[i].size_read == 0 && !links[i].sock.BadSocket()) {
+          rsel.WatchRead(links[i].sock); finished = false;
         }
       }
       if (finished) break;
       // wait to read from the channels to discard data
       rsel.Select();
-      printf("[%d] select finish read from\n", rank);
-      for (int i = 0; i < nlink; ++i) {
-        if (links[i].sock.BadSocket()) continue;
-        if (rsel.CheckRead(links[i].sock)) {                        
-          ssize_t len = links[i].sock.Recv(links[i].buffer_head, links[i].buffer_size);
-          // zero length, remote closed the connection, close socket
-          if (len == 0) {
-            links[i].sock.Close();
-          } else if (len == -1) {
-            // when error happens here, oob_clear will remember
-            if (errno == EAGAIN && errno == EWOULDBLOCK) printf("would block\n");
-          } else {
-            printf("[%d] discard %ld bytes\n", rank, len);
-          }
-        }
-      }
     }
-    printf("[%d] discard all success\n", rank);
-    // start synchronization step
+    // start synchronization, use blocking I/O to avoid select
     for (int i = 0; i < nlink; ++i) {
-      links[i].ResetSize();
-    }
-    while (true) {
-      // selecter for TryResetLinks
-      utils::SelectHelper rsel;
-      for (int i = 0; i < nlink; ++i) {
-        if (links[i].sock.BadSocket()) continue;
-        if (links[i].size_read == 0) rsel.WatchRead(links[i].sock);
-        if (links[i].size_write == 0) rsel.WatchWrite(links[i].sock);
-      }
-      printf("[%d] before select\n", rank);
-      rsel.Select();
-      printf("[%d] after select\n", rank);
-      bool finished = true;
-      for (int i = 0; i < nlink; ++i) {
-        if (links[i].sock.BadSocket()) continue;
-        if (links[i].size_read == 0 && rsel.CheckRead(links[i].sock)) {
-          char ack;
-          links[i].ReadToArray(&ack, sizeof(ack));
-          if (links[i].size_read != 0) {
-            utils::Assert(ack == kOOBResetAck, "expect ack message");
+      if (!links[i].sock.BadSocket()) {
+        char oob_mark;
+        links[i].sock.SetNonBlock(false);
+        ssize_t len = links[i].sock.Recv(&oob_mark, sizeof(oob_mark), MSG_WAITALL);
+        if (len == 0) {
+          links[i].sock.Close(); continue;
+        } else if (len > 0) {
+          utils::Assert(oob_mark == kResetMark, "wrong oob msg");
+          utils::Assert(!links[i].sock.AtMark(), "should already read past mark");
+        } else {
+          utils::Assert(errno != EAGAIN|| errno != EWOULDBLOCK, "BUG");
+        }
+        // send out ack
+        char ack = kResetAck;
+        while (true) {
+          len = links[i].sock.Send(&ack, sizeof(ack));
+          if (len == sizeof(ack)) break;
+          if (len == -1) {
+            if (errno != EAGAIN && errno != EWOULDBLOCK) break;
           }
         }
-        if (links[i].size_write == 0 && rsel.CheckWrite(links[i].sock)) {
-          char ack = kOOBResetAck;
-          links[i].WriteFromArray(&ack, sizeof(ack));
-        }
-        if (links[i].size_read == 0 || links[i].size_write == 0) finished = false;
       }
-      if (finished) break;
     }
-    printf("[%d] after the read write data success\n", rank);
+    // wait all ack
+    for (int i = 0; i < nlink; ++i) {
+      if (!links[i].sock.BadSocket()) {
+        char ack;
+        ssize_t len = links[i].sock.Recv(&ack, sizeof(ack), MSG_WAITALL);
+        if (len == 0) {
+          links[i].sock.Close(); continue;
+        } else if (len > 0) {
+          utils::Assert(ack == kResetAck, "wrong Ack MSG");
+        } else {
+          utils::Assert(errno != EAGAIN|| errno != EWOULDBLOCK, "BUG");
+        }
+        // set back to nonblock mode
+        links[i].sock.SetNonBlock(true);
+      }
+    }
     for (int i = 0; i < nlink; ++i) {
       if (links[i].sock.BadSocket()) return kSockError;
-    }    
+    }
     return kSuccess;
   }
   // Run AllReduce, return if success
@@ -540,9 +517,12 @@ class AllReduceManager : public IEngine {
     // pointer to buffer head
     char *buffer_head;
     // buffer size, in bytes
-    size_t buffer_size;    
+    size_t buffer_size;
+    // exception
+    bool except;
     // constructor
     LinkRecord(void) {}
+
     // initialize buffer
     inline void InitBuffer(size_t type_nbytes, size_t count, size_t reduce_buffer_size) {
       size_t n = (type_nbytes * count + 7)/ 8;
@@ -587,7 +567,7 @@ class AllReduceManager : public IEngine {
      * \return true if it is an successful read, false if there is some error happens, check errno
      */
     inline bool ReadToArray(void *recvbuf_, size_t max_size) {
-      if (max_size == size_read ) return true;
+      if (max_size == size_read) return true;
       char *p = static_cast<char*>(recvbuf_);
       ssize_t len = sock.Recv(p + size_read, max_size - size_read);
       // length equals 0, remote disconnected
diff --git a/test/test_allreduce.cpp b/test/test_allreduce.cpp
index 9afdc6d03..40c85ea0b 100644
--- a/test/test_allreduce.cpp
+++ b/test/test_allreduce.cpp
@@ -76,9 +76,9 @@ int main(int argc, char *argv[]) {
 
   printf("[%d] start at %s\n", rank, name.c_str());
   TestMax(mock, n);
-  printf("[%d] TestMax pass\n", rank);
+  printf("[%d] !!!TestMax pass\n", rank);
   TestSum(mock, n);
-  printf("[%d] TestSum pass\n", rank);
+  printf("[%d] !!!TestSum pass\n", rank);
   sync::Finalize();
   printf("[%d] all check pass\n", rank);
   return 0;