diff --git a/lib/includes/nghttp2/nghttp2.h b/lib/includes/nghttp2/nghttp2.h
index 038030aa..1a92a520 100644
--- a/lib/includes/nghttp2/nghttp2.h
+++ b/lib/includes/nghttp2/nghttp2.h
@@ -443,7 +443,11 @@ typedef enum {
   /**
    * The ALTSVC frame.
    */
-  NGHTTP2_ALTSVC = 0x0a
+  NGHTTP2_ALTSVC = 0x0a,
+  /**
+   * The BLOCKED frame.
+   */
+  NGHTTP2_BLOCKED = 0x0b
 } nghttp2_frame_type;
 
 /**
@@ -982,6 +986,18 @@ typedef struct {
   uint16_t port;
 } nghttp2_altsvc;
 
+/**
+ * @struct
+ *
+ * The BLOCKED frame.  It has following members:
+ */
+typedef struct {
+  /**
+   * The frame header.
+   */
+  nghttp2_frame_hd hd;
+} nghttp2_blocked;
+
 /**
  * @union
  *
@@ -1034,6 +1050,10 @@ typedef union {
    * The ALTSVC frame.
    */
   nghttp2_altsvc altsvc;
+  /**
+   * The BLOCKED frame.
+   */
+  nghttp2_blocked blocked;
 } nghttp2_frame;
 
 /**
diff --git a/lib/nghttp2_frame.c b/lib/nghttp2_frame.c
index 0b8f7fb8..209328ad 100644
--- a/lib/nghttp2_frame.c
+++ b/lib/nghttp2_frame.c
@@ -216,6 +216,15 @@ void nghttp2_frame_altsvc_free(nghttp2_altsvc *frame)
   free(frame->protocol_id);
 }
 
+void nghttp2_frame_blocked_init(nghttp2_blocked *frame, int32_t stream_id)
+{
+  nghttp2_frame_set_hd(&frame->hd, 0, NGHTTP2_BLOCKED, NGHTTP2_FLAG_NONE,
+                       stream_id);
+}
+
+void nghttp2_frame_blocked_free(nghttp2_blocked *frame)
+{}
+
 void nghttp2_frame_data_init(nghttp2_data *frame, nghttp2_private_data *pdata)
 {
   frame->hd = pdata->hd;
@@ -797,6 +806,20 @@ int nghttp2_frame_unpack_altsvc_payload(nghttp2_altsvc *frame,
   return 0;
 }
 
+int nghttp2_frame_pack_blocked(nghttp2_bufs *bufs, nghttp2_blocked *frame)
+{
+  nghttp2_buf *buf;
+
+  assert(bufs->head == bufs->cur);
+
+  buf = &bufs->head->buf;
+  buf->pos -= NGHTTP2_FRAME_HDLEN;
+
+  nghttp2_frame_pack_frame_hd(buf->pos, &frame->hd);
+
+  return 0;
+}
+
 nghttp2_settings_entry* nghttp2_frame_iv_copy(const nghttp2_settings_entry *iv,
                                               size_t niv)
 {
diff --git a/lib/nghttp2_frame.h b/lib/nghttp2_frame.h
index 97939d6b..6c93173d 100644
--- a/lib/nghttp2_frame.h
+++ b/lib/nghttp2_frame.h
@@ -444,6 +444,16 @@ int nghttp2_frame_unpack_altsvc_payload(nghttp2_altsvc *frame,
                                         uint8_t *var_gift_payload,
                                         size_t var_gift_payloadlen);
 
+/*
+ * Packs BLOCKED frame |frame| in wire format and store it in |bufs|.
+ *
+ * The caller must make sure that nghttp2_bufs_reset(bufs) is called
+ * before calling this function.
+ *
+ * This function always returns 0.
+ */
+int nghttp2_frame_pack_blocked(nghttp2_bufs *bufs, nghttp2_blocked *frame);
+
 /*
  * Initializes HEADERS frame |frame| with given values.  |frame| takes
  * ownership of |nva|, so caller must not free it. If |stream_id| is
@@ -534,6 +544,10 @@ void nghttp2_frame_altsvc_init(nghttp2_altsvc *frame, int32_t stream_id,
 
 void nghttp2_frame_altsvc_free(nghttp2_altsvc *frame);
 
+void nghttp2_frame_blocked_init(nghttp2_blocked *frame, int32_t stream_id);
+
+void nghttp2_frame_blocked_free(nghttp2_blocked *frame);
+
 void nghttp2_frame_data_init(nghttp2_data *frame, nghttp2_private_data *pdata);
 
 /*
diff --git a/lib/nghttp2_session.c b/lib/nghttp2_session.c
index 8e820a8b..13d1c7ec 100644
--- a/lib/nghttp2_session.c
+++ b/lib/nghttp2_session.c
@@ -196,6 +196,9 @@ static void nghttp2_inbound_frame_reset(nghttp2_session *session)
   case NGHTTP2_ALTSVC:
     nghttp2_frame_altsvc_free(&iframe->frame.altsvc);
     break;
+  case NGHTTP2_BLOCKED:
+    nghttp2_frame_blocked_free(&iframe->frame.blocked);
+    break;
   }
   memset(&iframe->frame, 0, sizeof(nghttp2_frame));
 
@@ -628,6 +631,8 @@ int nghttp2_session_add_frame(nghttp2_session *session,
       break;
     case NGHTTP2_ALTSVC:
       break;
+    case NGHTTP2_BLOCKED:
+      break;
     }
 
     if(frame->hd.type == NGHTTP2_HEADERS &&
@@ -1292,6 +1297,41 @@ static int nghttp2_session_predicate_altsvc_send
   return 0;
 }
 
+/*
+ * This function checks BLOCKED with the stream ID |stream_id| can be
+ * sent at this time.  If |stream_id| is 0, BLOCKED frame is always
+ * allowed to send.
+ *
+ * This function returns 0 if it succeeds, or one of the following
+ * negative error codes:
+ *
+ * NGHTTP2_ERR_STREAM_CLOSED
+ *     The stream is already closed or does not exist.
+ * NGHTTP2_ERR_STREAM_CLOSING
+ *     RST_STREAM was queued for this stream.
+ */
+static int nghttp2_session_predicate_blocked_send
+(nghttp2_session *session, int32_t stream_id)
+{
+  nghttp2_stream *stream;
+
+  if(stream_id == 0) {
+    return 0;
+  }
+
+  stream = nghttp2_session_get_stream(session, stream_id);
+
+  if(stream == NULL) {
+    return NGHTTP2_ERR_STREAM_CLOSED;
+  }
+
+  if(stream->state == NGHTTP2_STREAM_CLOSING) {
+    return NGHTTP2_ERR_STREAM_CLOSING;
+  }
+
+  return 0;
+}
+
 /*
  * This function checks SETTINGS can be sent at this time.
  *
@@ -1436,6 +1476,69 @@ static int session_headers_add_pad(nghttp2_session *session,
   return 0;
 }
 
+/*
+ * Adds BLOCKED frame to outbound queue.  The |stream_id| could be 0,
+ * which means DATA is blocked by connection level flow control.
+ *
+ * This function returns 0 if it succeeds, or one of the following
+ * negative error codes:
+ *
+ * NGHTTP2_ERR_NOMEM
+ *     Out of memory
+ */
+static int session_add_blocked(nghttp2_session *session, int32_t stream_id)
+{
+  int rv;
+  nghttp2_frame *frame;
+
+  frame = malloc(sizeof(nghttp2_frame));
+
+  if(frame == NULL) {
+    return NGHTTP2_ERR_NOMEM;
+  }
+
+  nghttp2_frame_blocked_init(&frame->blocked, stream_id);
+
+  rv = nghttp2_session_add_frame(session, NGHTTP2_CAT_CTRL, frame, NULL);
+
+  if(rv != 0) {
+    nghttp2_frame_blocked_free(&frame->blocked);
+    free(frame);
+
+    return rv;
+  }
+  return 0;
+}
+
+/*
+ * Adds BLOCKED frame(s) to outbound queue if they are allowed to
+ * send.  We check BLOCKED frame can be sent for connection and stream
+ * individually.
+ *
+ * This function returns 0 if it succeeds, or one of the following
+ * negative error codes:
+ *
+ * NGHTTP2_ERR_NOMEM
+ *     Out of memory
+ */
+static int session_consider_blocked(nghttp2_session *session,
+                                    nghttp2_stream *stream)
+{
+  if(session->blocked_sent == 0 && session->remote_window_size <= 0) {
+    session->blocked_sent = 1;
+
+    return session_add_blocked(session, 0);
+  }
+
+  if(stream->blocked_sent == 0 && stream->remote_window_size <= 0) {
+    stream->blocked_sent = 1;
+
+    return session_add_blocked(session, stream->stream_id);
+  }
+
+  return 0;
+}
+
 /*
  * This function serializes frame for transmission.
  *
@@ -1646,6 +1749,21 @@ static int nghttp2_session_prep_frame(nghttp2_session *session,
         return framerv;
       }
 
+      break;
+    case NGHTTP2_BLOCKED:
+      rv = nghttp2_session_predicate_blocked_send(session,
+                                                  frame->hd.stream_id);
+      if(rv != 0) {
+        return rv;
+      }
+
+      framerv = nghttp2_frame_pack_blocked(&session->aob.framebufs,
+                                           &frame->blocked);
+
+      if(framerv < 0) {
+        return framerv;
+      }
+
       break;
     default:
       return NGHTTP2_ERR_INVALID_ARGUMENT;
@@ -1682,6 +1800,12 @@ static int nghttp2_session_prep_frame(nghttp2_session *session,
     next_readmax = nghttp2_session_next_data_read(session, stream);
 
     if(next_readmax == 0) {
+      rv = session_consider_blocked(session, stream);
+
+      if(nghttp2_is_fatal(rv)) {
+        return rv;
+      }
+
       rv = nghttp2_stream_defer_data(stream,
                                      NGHTTP2_STREAM_FLAG_DEFERRED_FLOW_CONTROL,
                                      &session->ob_pq);
@@ -2020,6 +2144,9 @@ static int nghttp2_session_after_frame_sent(nghttp2_session *session)
     case NGHTTP2_ALTSVC:
       /* nothing to do */
       break;
+    case NGHTTP2_BLOCKED:
+      /* nothing to do */
+      break;
     }
     nghttp2_active_outbound_item_reset(&session->aob);
     return 0;
@@ -2092,6 +2219,12 @@ static int nghttp2_session_after_frame_sent(nghttp2_session *session)
       next_readmax = nghttp2_session_next_data_read(session, stream);
 
       if(next_readmax == 0) {
+        rv = session_consider_blocked(session, stream);
+
+        if(nghttp2_is_fatal(rv)) {
+          return rv;
+        }
+
         rv = nghttp2_stream_defer_data
           (stream, NGHTTP2_STREAM_FLAG_DEFERRED_FLOW_CONTROL, &session->ob_pq);
 
@@ -3072,6 +3205,11 @@ static int nghttp2_update_remote_initial_window_size_func
     return nghttp2_session_terminate_session(arg->session,
                                              NGHTTP2_FLOW_CONTROL_ERROR);
   }
+
+  if(stream->remote_window_size > 0) {
+    stream->blocked_sent = 0;
+  }
+
   /* If window size gets positive, push deferred DATA frame to
      outbound queue. */
   if(nghttp2_stream_check_deferred_by_flow_control(stream) &&
@@ -3561,6 +3699,20 @@ static int session_process_altsvc_frame(nghttp2_session *session)
   return nghttp2_session_on_altsvc_received(session, frame);
 }
 
+int nghttp2_session_on_blocked_received(nghttp2_session *session,
+                                        nghttp2_frame *frame)
+{
+  return nghttp2_session_call_on_frame_received(session, frame);
+}
+
+static int session_process_blocked_frame(nghttp2_session *session)
+{
+  nghttp2_inbound_frame *iframe = &session->iframe;
+  nghttp2_frame *frame = &iframe->frame;
+
+  return nghttp2_session_on_blocked_received(session, frame);
+}
+
 static int nghttp2_push_back_deferred_data_func(nghttp2_map_entry *entry,
                                                 void *ptr)
 {
@@ -3609,6 +3761,8 @@ static int session_on_connection_window_update_received
   /* To queue the DATA deferred by connection-level flow-control, we
      have to check all streams. Bad. */
   if(session->remote_window_size > 0) {
+    session->blocked_sent = 0;
+
     rv = nghttp2_session_push_back_deferred_data(session);
     if(rv != 0) {
       /* FATAL */
@@ -3642,6 +3796,11 @@ static int session_on_stream_window_update_received
                                                  NGHTTP2_FLOW_CONTROL_ERROR);
   }
   stream->remote_window_size += frame->window_update.window_size_increment;
+
+  if(stream->remote_window_size > 0) {
+    stream->blocked_sent = 0;
+  }
+
   if(stream->remote_window_size > 0 &&
      session->remote_window_size > 0 &&
      nghttp2_stream_check_deferred_by_flow_control(stream)) {
@@ -4364,6 +4523,27 @@ ssize_t nghttp2_session_mem_recv(nghttp2_session *session,
         iframe->state = NGHTTP2_IB_READ_NBYTE;
         inbound_frame_set_mark(iframe, 8);
 
+        break;
+      case NGHTTP2_BLOCKED:
+        DEBUGF(fprintf(stderr, "recv: BLOCKED\n"));
+
+        iframe->frame.hd.flags = NGHTTP2_FLAG_NONE;
+
+        if(iframe->payloadleft != 0) {
+          busy = 1;
+
+          iframe->state = NGHTTP2_IB_FRAME_SIZE_ERROR;
+
+          break;
+        }
+
+        rv = session_process_blocked_frame(session);
+        if(nghttp2_is_fatal(rv)) {
+          return rv;
+        }
+
+        nghttp2_inbound_frame_reset(session);
+
         break;
       default:
         DEBUGF(fprintf(stderr, "recv: unknown frame\n"));
diff --git a/lib/nghttp2_session.h b/lib/nghttp2_session.h
index bcdb0467..16eecb22 100644
--- a/lib/nghttp2_session.h
+++ b/lib/nghttp2_session.h
@@ -199,6 +199,9 @@ struct nghttp2_session {
   /* Flags indicating GOAWAY is sent and/or recieved. The flags are
      composed by bitwise OR-ing nghttp2_goaway_flag. */
   uint8_t goaway_flags;
+  /* nonzero if blocked was sent and remote_window_size is still 0 or
+     negative */
+  uint8_t blocked_sent;
 };
 
 /* Struct used when updating initial window size of each active
@@ -573,6 +576,19 @@ int nghttp2_session_on_window_update_received(nghttp2_session *session,
 int nghttp2_session_on_altsvc_received(nghttp2_session *session,
                                        nghttp2_frame *frame);
 
+/*
+ * Called when BLOCKED is received, assuming |frame| is properly
+ * initialized.
+ *
+ * This function returns 0 if it succeeds, or one of the following
+ * negative error codes:
+ *
+ * NGHTTP2_ERR_CALLBACK_FAILURE
+ *   The callback function failed.
+ */
+int nghttp2_session_on_blocked_received(nghttp2_session *session,
+                                        nghttp2_frame *frame);
+
 /*
  * Called when DATA is received, assuming |frame| is properly
  * initialized.
diff --git a/lib/nghttp2_stream.c b/lib/nghttp2_stream.c
index 0e771710..ac67bee1 100644
--- a/lib/nghttp2_stream.c
+++ b/lib/nghttp2_stream.c
@@ -49,6 +49,7 @@ void nghttp2_stream_init(nghttp2_stream *stream, int32_t stream_id,
   stream->local_window_size = local_initial_window_size;
   stream->recv_window_size = 0;
   stream->recv_reduction = 0;
+  stream->blocked_sent = 0;
 
   stream->dep_prev = NULL;
   stream->dep_next = NULL;
diff --git a/lib/nghttp2_stream.h b/lib/nghttp2_stream.h
index e47d7498..ceb207b7 100644
--- a/lib/nghttp2_stream.h
+++ b/lib/nghttp2_stream.h
@@ -173,6 +173,9 @@ struct nghttp2_stream {
   uint8_t flags;
   /* Bitwise OR of zero or more nghttp2_shut_flag values */
   uint8_t shut_flags;
+  /* nonzero if blocked was sent and remote_window_size is still 0 or
+     negative */
+  uint8_t blocked_sent;
 };
 
 void nghttp2_stream_init(nghttp2_stream *stream, int32_t stream_id,
diff --git a/src/app_helper.cc b/src/app_helper.cc
index dc78792e..7ab1bd89 100644
--- a/src/app_helper.cc
+++ b/src/app_helper.cc
@@ -127,6 +127,8 @@ const char* strframetype(uint8_t type)
     return "WINDOW_UPDATE";
   case NGHTTP2_ALTSVC:
     return "ALTSVC";
+  case NGHTTP2_BLOCKED:
+    return "BLOCKED";
   default:
     return "UNKNOWN";
   }