diff --git a/libsponge/tcp_connection.cc b/libsponge/tcp_connection.cc
index 3a54672..3939333 100644
--- a/libsponge/tcp_connection.cc
+++ b/libsponge/tcp_connection.cc
@@ -21,35 +21,30 @@ size_t TCPConnection::unassembled_bytes() const { return _receiver.unassembled_b
 size_t TCPConnection::time_since_last_segment_received() const { return _time_last_segment_received; }
 
 void TCPConnection::segment_received(const TCPSegment &seg) {
-    if (!_active)
-        return;
+    // god knows whether it should be active or not initially
     _time_last_segment_received = 0;
     if (seg.header().rst) {
-        _sender.stream_in().set_error();
-        _receiver.stream_out().set_error();
-        _active = false;
+        _set_rst();
         return;
     }
-    bool send_ack = seg.length_in_sequence_space();
     _receiver.segment_received(seg);
     if (seg.header().ack && _receiver.ackno().has_value()) {
         // filter out acks before syn
         _sender.ack_received(seg.header().ackno, seg.header().win);
-        // _sender.fill_window();
-        if (!_sender.segments_out().empty()) send_ack = false;
+        // _sender.fill_window(); !! This should be done in ack_received
     }
-    // std::cerr << seg.header().to_string();
-    // std::cerr << _receiver.stream_out().bytes_written() << " " << _receiver.unassembled_bytes() << std::endl;
-    // std::cerr << seg.payload().size() << std::endl;
-
-    if (send_ack) {
-        _sender.fill_window();
+    // if the inbound seg has len it should be responded
+    //  except that ack_received() has send a seg, so no more is needed
+    if (seg.length_in_sequence_space() && _sender.segments_out().empty()) {
+        _sender.fill_window();  // it is necessary to repond to SYN with a SYNACK
         if (_sender.segments_out().empty()) {
             _sender.send_empty_segment();
         }
     }
     _send_all();
     // TODO: Keep-alive but it seems to be use
+    // recver in FIN_RECV
+    // sender in SYN_ACKED
     if (_receiver.stream_out().input_ended() && !_sender.stream_in().eof()) {
         _linger_after_streams_finish = false;
     }
@@ -70,30 +65,19 @@ void TCPConnection::tick(const size_t ms_since_last_tick) {
     _time_last_segment_received += ms_since_last_tick;
     _sender.tick(ms_since_last_tick);
     if (_sender.consecutive_retransmissions() > TCPConfig::MAX_RETX_ATTEMPTS) {
-        _sender.send_empty_segment();
-        auto &seg_queue = _sender.segments_out();
-        auto seg = seg_queue.front();
-        seg_queue.pop();
-        if (_receiver.ackno().has_value()) {
-            seg.header().ack = true;
-            seg.header().ackno = _receiver.ackno().value();
-        }
-        seg.header().win = _receiver.window_size();
-        seg.header().rst = true;
-        _segments_out.push(seg);
-        _sender.stream_in().set_error();
-        _receiver.stream_out().set_error();
-        _active = false;
+        _send_rst();
+        _set_rst();
         return;
     }
     _send_all();
-    if (!_sender.bytes_in_flight() && _sender.stream_in().eof() && _receiver.stream_out().input_ended() &&
-        !_receiver.unassembled_bytes()) {
-        if (!_linger_after_streams_finish) {
-            _active = false;
-        } else if (_time_last_segment_received >= 10 * _cfg.rt_timeout) {
-            _active = false;
-        }
+    // actually, this long if could be represented by TCP states
+    // for sender, it is the FIN_ACKED state
+    // for recver, it is the FIN_RECV state
+    // and plus linger
+    if ((!_sender.bytes_in_flight() && _sender.stream_in().eof()) &&
+        (_receiver.stream_out().input_ended() && !_receiver.unassembled_bytes()) &&
+        (!_linger_after_streams_finish || _time_last_segment_received >= 10 * _cfg.rt_timeout)) {
+        _active = false;
     }
 }
 
@@ -112,20 +96,8 @@ TCPConnection::~TCPConnection() {
     try {
         if (active()) {
             cerr << "Warning: Unclean shutdown of TCPConnection\n";
-            _sender.send_empty_segment();
-            auto &seg_queue = _sender.segments_out();
-            auto seg = seg_queue.front();
-            seg_queue.pop();
-            if (_receiver.ackno().has_value()) {
-                seg.header().ack = true;
-                seg.header().ackno = _receiver.ackno().value();
-            }
-            seg.header().win = _receiver.window_size();
-            seg.header().rst = true;
-            _segments_out.push(seg);
-            _sender.stream_in().set_error();
-            _receiver.stream_out().set_error();
-            _active = false;
+            _send_rst();
+            _set_rst();
             // Your code here: need to send a RST segment to the peer
         }
     } catch (const exception &e) {
diff --git a/libsponge/tcp_connection.hh b/libsponge/tcp_connection.hh
index bee8aba..1131971 100644
--- a/libsponge/tcp_connection.hh
+++ b/libsponge/tcp_connection.hh
@@ -5,6 +5,7 @@
 #include "tcp_receiver.hh"
 #include "tcp_sender.hh"
 #include "tcp_state.hh"
+
 #include <algorithm>
 #include <limits>
 
@@ -22,21 +23,58 @@ class TCPConnection {
     //! for 10 * _cfg.rt_timeout milliseconds after both streams have ended,
     //! in case the remote TCPConnection doesn't know we've received its whole stream?
     bool _linger_after_streams_finish{true};
+
+    //! Private state for time_since_last_segment_received()
     size_t _time_last_segment_received{0};
+
+    //! Private state for active()
     bool _active{true};
+
+    //!\name Internal helper procedures
+    //!@{
+
+    //!\brief Send all available outbound segments
     void _send_all() {
-      auto &seg_queue = _sender.segments_out();
-      while (!seg_queue.empty()) {
-          auto seg = seg_queue.front();
-          seg_queue.pop();
-          if (_receiver.ackno().has_value()) {
-              seg.header().ack = true;
-              seg.header().ackno = _receiver.ackno().value();
-          }
-          seg.header().win = std::min(_receiver.window_size(), static_cast<size_t>(std::numeric_limits<uint16_t>::max()));
-          _segments_out.push(seg);
-      }
+        auto &seg_queue = _sender.segments_out();
+        while (!seg_queue.empty()) {
+            auto seg = seg_queue.front();
+            seg_queue.pop();
+            if (_receiver.ackno().has_value()) {
+                seg.header().ack = true;
+                seg.header().ackno = _receiver.ackno().value();
+            }
+            seg.header().win =
+                std::min(_receiver.window_size(), static_cast<size_t>(std::numeric_limits<uint16_t>::max()));
+            _segments_out.push(seg);
+        }
     }
+
+    //!\brief Deprecate all other packets in the queue, and send a RST segment
+    void _send_rst() {
+        auto &seg_queue = _sender.segments_out();
+        while (!seg_queue.empty()) {
+            seg_queue.pop();
+        }
+        _sender.send_empty_segment();
+        auto seg = seg_queue.front();
+        seg_queue.pop();
+        if (_receiver.ackno().has_value()) {
+            seg.header().ack = true;
+            seg.header().ackno = _receiver.ackno().value();
+        }
+        seg.header().win = _receiver.window_size();
+        seg.header().rst = true;
+        _segments_out.push(seg);
+    }
+
+    //!\brief Set connection to Dead State
+    void _set_rst() {
+        _sender.stream_in().set_error();
+        _receiver.stream_out().set_error();
+        _active = false;
+    }
+    //!@}
+
   public:
     //! \name "Input" interface for the writer
     //!@{