diff --git a/libsponge/tcp_receiver.cc b/libsponge/tcp_receiver.cc
index 640f925..3a11f9c 100644
--- a/libsponge/tcp_receiver.cc
+++ b/libsponge/tcp_receiver.cc
@@ -10,8 +10,37 @@ void DUMMY_CODE(Targs &&.../* unused */) {}
 
 using namespace std;
 
-void TCPReceiver::segment_received(const TCPSegment &seg) { DUMMY_CODE(seg); }
+void TCPReceiver::segment_received(const TCPSegment &seg) {
+    if (seg.header().syn) {
+        _syn = true;
+        // there's no way to know whether connection has been established from bytestream status,
+        // so a special member is needed
+        _isn = seg.header().seqno;
+    }
+    if (!_syn)
+        return;  // abandon packets before SYN
+    uint64_t checkpoint = stream_out().bytes_written() ? stream_out().bytes_written() - 1 : 0;
+    // special case for initial SYN
+    uint64_t abs_seqno = unwrap(seg.header().seqno, _isn, checkpoint);
+    // In your TCP implementation, you’ll use the index of the last reassembled byte as the checkpoint.
+    // So, the bytes_written-1 (Stream Indices) should be this value
+    // Watch out for the case when no bytes have been assembled
+    if (abs_seqno == 0 && !seg.header().syn)
+        return;
+    // assertion: when abs_seqno is 0, this packet must be syn
+    uint64_t index = abs_seqno - (abs_seqno ? 1 : 0);  // special case for SYN&FIN
+    _reassembler.push_substring(seg.payload().copy(), index, seg.header().fin);
+    _abs_ackno = stream_out().bytes_written() + 1 + (stream_out().input_ended() ? 1 : 0);
+    _ackno = wrap(_abs_ackno, _isn);
+}
 
-optional<WrappingInt32> TCPReceiver::ackno() const { return {}; }
+optional<WrappingInt32> TCPReceiver::ackno() const {
+    if (_syn)
+        return _ackno;
+    return {};
+}
 
-size_t TCPReceiver::window_size() const { return {}; }
+size_t TCPReceiver::window_size() const {
+    // capacity minus the number of bytes holding in the byte stream
+    return _capacity - stream_out().buffer_size();
+}
diff --git a/libsponge/tcp_receiver.hh b/libsponge/tcp_receiver.hh
index 5b92180..68d8d3c 100644
--- a/libsponge/tcp_receiver.hh
+++ b/libsponge/tcp_receiver.hh
@@ -19,9 +19,10 @@ class TCPReceiver {
 
     //! The maximum number of bytes we'll store.
     size_t _capacity;
-    WrappingInt32 _seqno{0};
+    WrappingInt32 _isn{0};
     WrappingInt32 _ackno{0};
-    bool _is_start{};
+    uint64_t _abs_ackno{0};
+    bool _syn{};
 
   public:
     //! \brief Construct a TCP receiver
diff --git a/libsponge/wrapping_integers.cc b/libsponge/wrapping_integers.cc
index f08e552..846e13b 100644
--- a/libsponge/wrapping_integers.cc
+++ b/libsponge/wrapping_integers.cc
@@ -47,4 +47,10 @@ uint64_t unwrap(WrappingInt32 n, WrappingInt32 isn, uint64_t checkpoint) {
             return tmp + MODULO;
         }
     }
+    // actually, here is a tricky edge case:
+    //  what if two possible unwraps have the same distance to checkpoint?
+    //  For example, when n=2^16,isn=0,checkpoint=2^32
+    // I thought about this, and came up with a possible solution:
+    //  Mathematically, it is a problem, but in TCP's case, it is okay in that
+    //  it is impossible that window size reaches 2^16.
 }