Add SETTINGS_NO_RFC7540_PRIORITIES

Add SETTINGS_NO_RFC7540_PRIORITIES to disable RFC7540 priorities.  If
disabled, streams are served in FIFO.

lib/includes/nghttp2/nghttp2.h

@@ -703,7 +703,12 @@ typedef enum {
    * SETTINGS_ENABLE_CONNECT_PROTOCOL
    * (`RFC 8441 <https://tools.ietf.org/html/rfc8441>`_)
    */
-  NGHTTP2_SETTINGS_ENABLE_CONNECT_PROTOCOL = 0x08
+  NGHTTP2_SETTINGS_ENABLE_CONNECT_PROTOCOL = 0x08,
+  /**
+   * SETTINGS_NO_RFC7540_PRIORITIES (`RFC 9218
+   * <https://datatracker.ietf.org/doc/html/rfc9218>`_)
+   */
+  NGHTTP2_SETTINGS_NO_RFC7540_PRIORITIES = 0x09
 } nghttp2_settings_id;
 /* Note: If we add SETTINGS, update the capacity of
    NGHTTP2_INBOUND_NUM_IV as well */
@@ -2693,6 +2698,11 @@ nghttp2_option_set_max_deflate_dynamic_table_size(nghttp2_option *option,
  * This option prevents the library from retaining closed streams to
  * maintain the priority tree.  If this option is set to nonzero,
  * applications can discard closed stream completely to save memory.
+ *
+ * If
+ * :enum:`nghttp2_settings_id.NGHTTP2_SETTINGS_NO_RFC7540_PRIORITIES`
+ * of value of 1 is submitted via `nghttp2_submit_settings()`, any
+ * closed streams are not retained regardless of this option.
  */
 NGHTTP2_EXTERN void nghttp2_option_set_no_closed_streams(nghttp2_option *option,
                                                          int val);
@@ -3589,6 +3599,11 @@ NGHTTP2_EXTERN int nghttp2_session_consume_stream(nghttp2_session *session,
  * found, we use default priority instead of given |pri_spec|.  That
  * is make stream depend on root stream with weight 16.
  *
+ * If
+ * :enum:`nghttp2_settings_id.NGHTTP2_SETTINGS_NO_RFC7540_PRIORITIES`
+ * of value of 1 is submitted via `nghttp2_submit_settings()`, this
+ * function does nothing and returns 0.
+ *
  * This function returns 0 if it succeeds, or one of the following
  * negative error codes:
  *
@@ -3632,6 +3647,11 @@ nghttp2_session_change_stream_priority(nghttp2_session *session,
  * found, we use default priority instead of given |pri_spec|.  That
  * is make stream depend on root stream with weight 16.
  *
+ * If
+ * :enum:`nghttp2_settings_id.NGHTTP2_SETTINGS_NO_RFC7540_PRIORITIES`
+ * of value of 1 is submitted via `nghttp2_submit_settings()`, this
+ * function does nothing and returns 0.
+ *
  * This function returns 0 if it succeeds, or one of the following
  * negative error codes:
  *
@@ -3837,6 +3857,11 @@ nghttp2_priority_spec_check_default(const nghttp2_priority_spec *pri_spec);
  * :macro:`NGHTTP2_MAX_WEIGHT`, it becomes
  * :macro:`NGHTTP2_MAX_WEIGHT`.
  *
+ * If
+ * :enum:`nghttp2_settings_id.NGHTTP2_SETTINGS_NO_RFC7540_PRIORITIES`
+ * of value of 1 is received by a remote endpoint, |pri_spec| is
+ * ignored, and treated as if ``NULL`` is specified.
+ *
  * The |nva| is an array of name/value pair :type:`nghttp2_nv` with
  * |nvlen| elements.  The application is responsible to include
  * required pseudo-header fields (header field whose name starts with
@@ -4057,6 +4082,11 @@ NGHTTP2_EXTERN int nghttp2_submit_trailer(nghttp2_session *session,
  * :macro:`NGHTTP2_MIN_WEIGHT`.  If it is strictly greater than
  * :macro:`NGHTTP2_MAX_WEIGHT`, it becomes :macro:`NGHTTP2_MAX_WEIGHT`.
  *
+ * If
+ * :enum:`nghttp2_settings_id.NGHTTP2_SETTINGS_NO_RFC7540_PRIORITIES`
+ * of value of 1 is received by a remote endpoint, |pri_spec| is
+ * ignored, and treated as if ``NULL`` is specified.
+ *
  * The |nva| is an array of name/value pair :type:`nghttp2_nv` with
  * |nvlen| elements.  The application is responsible to include
  * required pseudo-header fields (header field whose name starts with
@@ -4184,6 +4214,11 @@ NGHTTP2_EXTERN int nghttp2_submit_data(nghttp2_session *session, uint8_t flags,
  * :macro:`NGHTTP2_MAX_WEIGHT`, it becomes
  * :macro:`NGHTTP2_MAX_WEIGHT`.
  *
+ * If
+ * :enum:`nghttp2_settings_id.NGHTTP2_SETTINGS_NO_RFC7540_PRIORITIES`
+ * of value of 1 is received by a remote endpoint, this function does
+ * nothing and returns 0.
+ *
  * This function returns 0 if it succeeds, or one of the following
  * negative error codes:
  *

lib/nghttp2_frame.c

@@ -1071,6 +1071,11 @@ int nghttp2_iv_check(const nghttp2_settings_entry *iv, size_t niv) {
         return 0;
       }
       break;
+    case NGHTTP2_SETTINGS_NO_RFC7540_PRIORITIES:
+      if (iv[i].value != 0 && iv[i].value != 1) {
+        return 0;
+      }
+      break;
     }
   }
   return 1;

lib/nghttp2_session.c

@@ -385,6 +385,7 @@ static void init_settings(nghttp2_settings_storage *settings) {
   settings->initial_window_size = NGHTTP2_INITIAL_WINDOW_SIZE;
   settings->max_frame_size = NGHTTP2_MAX_FRAME_SIZE_MIN;
   settings->max_header_list_size = UINT32_MAX;
+  settings->no_rfc7540_priorities = UINT32_MAX;
 }
 
 static void active_outbound_item_reset(nghttp2_active_outbound_item *aob,
@@ -398,6 +399,15 @@ static void active_outbound_item_reset(nghttp2_active_outbound_item *aob,
   aob->state = NGHTTP2_OB_POP_ITEM;
 }
 
+static int stream_less(const void *lhsx, const void *rhsx) {
+  const nghttp2_stream *lhs, *rhs;
+
+  lhs = nghttp2_struct_of(lhsx, nghttp2_stream, pq_entry);
+  rhs = nghttp2_struct_of(rhsx, nghttp2_stream, pq_entry);
+
+  return lhs->seq < rhs->seq;
+}
+
 int nghttp2_enable_strict_preface = 1;
 
 static int session_new(nghttp2_session **session_ptr,
@@ -442,6 +452,7 @@ static int session_new(nghttp2_session **session_ptr,
   (*session_ptr)->pending_local_max_concurrent_stream =
       NGHTTP2_DEFAULT_MAX_CONCURRENT_STREAMS;
   (*session_ptr)->pending_enable_push = 1;
+  (*session_ptr)->pending_no_rfc7540_priorities = UINT8_MAX;
 
   if (server) {
     (*session_ptr)->server = 1;
@@ -584,6 +595,12 @@ static int session_new(nghttp2_session **session_ptr,
     }
   }
 
+  rv = nghttp2_pq_init(&(*session_ptr)->ob_data, stream_less, mem);
+  if (rv != 0) {
+    assert(0);
+    abort();
+  }
+
   return 0;
 
 fail_aob_framebuf:
@@ -748,6 +765,7 @@ void nghttp2_session_del(nghttp2_session *session) {
     settings = next;
   }
 
+  nghttp2_pq_free(&session->ob_data);
   nghttp2_stream_free(&session->root);
 
   /* Have to free streams first, so that we can check
@@ -775,6 +793,7 @@ int nghttp2_session_reprioritize_stream(
   nghttp2_priority_spec pri_spec_default;
   const nghttp2_priority_spec *pri_spec = pri_spec_in;
 
+  assert(session->pending_no_rfc7540_priorities != 1);
   assert(pri_spec->stream_id != stream->stream_id);
 
   if (!nghttp2_stream_in_dep_tree(stream)) {
@@ -842,6 +861,104 @@ int nghttp2_session_reprioritize_stream(
   return 0;
 }
 
+static int session_ob_data_push(nghttp2_session *session,
+                                nghttp2_stream *stream) {
+  int rv;
+
+  assert(stream->flags & NGHTTP2_STREAM_FLAG_NO_RFC7540_PRIORITIES);
+  assert(stream->queued == 0);
+
+  rv = nghttp2_pq_push(&session->ob_data, &stream->pq_entry);
+  if (rv != 0) {
+    return rv;
+  }
+
+  stream->queued = 1;
+
+  return 0;
+}
+
+static int session_ob_data_remove(nghttp2_session *session,
+                                  nghttp2_stream *stream) {
+  assert(stream->flags & NGHTTP2_STREAM_FLAG_NO_RFC7540_PRIORITIES);
+  assert(stream->queued == 1);
+
+  nghttp2_pq_remove(&session->ob_data, &stream->pq_entry);
+
+  stream->queued = 0;
+
+  return 0;
+}
+
+static int session_attach_stream_item(nghttp2_session *session,
+                                      nghttp2_stream *stream,
+                                      nghttp2_outbound_item *item) {
+  int rv;
+
+  rv = nghttp2_stream_attach_item(stream, item);
+  if (rv != 0) {
+    return rv;
+  }
+
+  if (!(stream->flags & NGHTTP2_STREAM_FLAG_NO_RFC7540_PRIORITIES)) {
+    return 0;
+  }
+
+  return session_ob_data_push(session, stream);
+}
+
+static int session_detach_stream_item(nghttp2_session *session,
+                                      nghttp2_stream *stream) {
+  int rv;
+
+  rv = nghttp2_stream_detach_item(stream);
+  if (rv != 0) {
+    return rv;
+  }
+
+  if (!(stream->flags & NGHTTP2_STREAM_FLAG_NO_RFC7540_PRIORITIES) ||
+      !stream->queued) {
+    return 0;
+  }
+
+  return session_ob_data_remove(session, stream);
+}
+
+static int session_defer_stream_item(nghttp2_session *session,
+                                     nghttp2_stream *stream, uint8_t flags) {
+  int rv;
+
+  rv = nghttp2_stream_defer_item(stream, flags);
+  if (rv != 0) {
+    return rv;
+  }
+
+  if (!(stream->flags & NGHTTP2_STREAM_FLAG_NO_RFC7540_PRIORITIES) ||
+      !stream->queued) {
+    return 0;
+  }
+
+  return session_ob_data_remove(session, stream);
+}
+
+static int session_resume_deferred_stream_item(nghttp2_session *session,
+                                               nghttp2_stream *stream,
+                                               uint8_t flags) {
+  int rv;
+
+  rv = nghttp2_stream_resume_deferred_item(stream, flags);
+  if (rv != 0) {
+    return rv;
+  }
+
+  if (!(stream->flags & NGHTTP2_STREAM_FLAG_NO_RFC7540_PRIORITIES) ||
+      (stream->flags & NGHTTP2_STREAM_FLAG_DEFERRED_ALL)) {
+    return 0;
+  }
+
+  return session_ob_data_push(session, stream);
+}
+
 int nghttp2_session_add_item(nghttp2_session *session,
                              nghttp2_outbound_item *item) {
   /* TODO Return error if stream is not found for the frame requiring
@@ -863,7 +980,7 @@ int nghttp2_session_add_item(nghttp2_session *session,
       return NGHTTP2_ERR_DATA_EXIST;
     }
 
-    rv = nghttp2_stream_attach_item(stream, item);
+    rv = session_attach_stream_item(session, stream, item);
 
     if (rv != 0) {
       return rv;
@@ -1041,12 +1158,21 @@ nghttp2_stream *nghttp2_session_open_stream(nghttp2_session *session,
 
   if (stream) {
     assert(stream->state == NGHTTP2_STREAM_IDLE);
-    assert(nghttp2_stream_in_dep_tree(stream));
+    assert((stream->flags & NGHTTP2_STREAM_FLAG_NO_RFC7540_PRIORITIES) ||
+           nghttp2_stream_in_dep_tree(stream));
+
+    if (nghttp2_stream_in_dep_tree(stream)) {
+      assert(!(stream->flags & NGHTTP2_STREAM_FLAG_NO_RFC7540_PRIORITIES));
       nghttp2_session_detach_idle_stream(session, stream);
       rv = nghttp2_stream_dep_remove(stream);
       if (rv != 0) {
         return NULL;
       }
+
+      if (session->pending_no_rfc7540_priorities == 1) {
+        stream->flags |= NGHTTP2_STREAM_FLAG_NO_RFC7540_PRIORITIES;
+      }
+    }
   } else {
     stream = nghttp2_mem_malloc(mem, sizeof(nghttp2_stream));
     if (stream == NULL) {
@@ -1056,7 +1182,21 @@ nghttp2_stream *nghttp2_session_open_stream(nghttp2_session *session,
     stream_alloc = 1;
   }
 
-  if (pri_spec->stream_id != 0) {
+  if (session->pending_no_rfc7540_priorities == 1 ||
+      session->remote_settings.no_rfc7540_priorities == 1) {
+    /* For client which has not received server
+       SETTINGS_NO_RFC7540_PRIORITIES = 1, send a priority signal
+       opportunistically. */
+    if (session->server ||
+        session->remote_settings.no_rfc7540_priorities == 1) {
+      nghttp2_priority_spec_default_init(&pri_spec_default);
+      pri_spec = &pri_spec_default;
+    }
+
+    if (session->pending_no_rfc7540_priorities == 1) {
+      flags |= NGHTTP2_STREAM_FLAG_NO_RFC7540_PRIORITIES;
+    }
+  } else if (pri_spec->stream_id != 0) {
     dep_stream = nghttp2_session_get_stream_raw(session, pri_spec->stream_id);
 
     if (!dep_stream &&
@@ -1102,6 +1242,10 @@ nghttp2_stream *nghttp2_session_open_stream(nghttp2_session *session,
                         (int32_t)session->local_settings.initial_window_size,
                         stream_user_data, mem);
 
+    if (session->pending_no_rfc7540_priorities == 1) {
+      stream->seq = session->stream_seq++;
+    }
+
     rv = nghttp2_map_insert(&session->streams, stream_id, stream);
     if (rv != 0) {
       nghttp2_stream_free(stream);
@@ -1141,6 +1285,10 @@ nghttp2_stream *nghttp2_session_open_stream(nghttp2_session *session,
     }
   }
 
+  if (stream->flags & NGHTTP2_STREAM_FLAG_NO_RFC7540_PRIORITIES) {
+    return stream;
+  }
+
   if (pri_spec->stream_id == 0) {
     dep_stream = &session->root;
   }
@@ -1180,7 +1328,7 @@ int nghttp2_session_close_stream(nghttp2_session *session, int32_t stream_id,
 
     item = stream->item;
 
-    rv = nghttp2_stream_detach_item(stream);
+    rv = session_detach_stream_item(session, stream);
 
     if (rv != 0) {
       return rv;
@@ -1230,6 +1378,10 @@ int nghttp2_session_close_stream(nghttp2_session *session, int32_t stream_id,
   /* Closes both directions just in case they are not closed yet */
   stream->flags |= NGHTTP2_STREAM_FLAG_CLOSED;
 
+  if (session->pending_no_rfc7540_priorities == 1) {
+    return nghttp2_session_destroy_stream(session, stream);
+  }
+
   if ((session->opt_flags & NGHTTP2_OPTMASK_NO_CLOSED_STREAMS) == 0 &&
       session->server && !is_my_stream_id &&
       nghttp2_stream_in_dep_tree(stream)) {
@@ -2013,7 +2165,7 @@ static int session_prep_frame(nghttp2_session *session,
       if (stream) {
         int rv2;
 
-        rv2 = nghttp2_stream_detach_item(stream);
+        rv2 = session_detach_stream_item(session, stream);
 
         if (nghttp2_is_fatal(rv2)) {
           return rv2;
@@ -2032,7 +2184,7 @@ static int session_prep_frame(nghttp2_session *session,
          queue when session->remote_window_size > 0 */
       assert(session->remote_window_size > 0);
 
-      rv = nghttp2_stream_defer_item(stream,
+      rv = session_defer_stream_item(session, stream,
                                      NGHTTP2_STREAM_FLAG_DEFERRED_FLOW_CONTROL);
 
       if (nghttp2_is_fatal(rv)) {
@@ -2051,7 +2203,8 @@ static int session_prep_frame(nghttp2_session *session,
       return rv;
     }
     if (rv == NGHTTP2_ERR_DEFERRED) {
-      rv = nghttp2_stream_defer_item(stream, NGHTTP2_STREAM_FLAG_DEFERRED_USER);
+      rv = session_defer_stream_item(session, stream,
+                                     NGHTTP2_STREAM_FLAG_DEFERRED_USER);
 
       if (nghttp2_is_fatal(rv)) {
         return rv;
@@ -2062,7 +2215,7 @@ static int session_prep_frame(nghttp2_session *session,
       return NGHTTP2_ERR_DEFERRED;
     }
     if (rv == NGHTTP2_ERR_TEMPORAL_CALLBACK_FAILURE) {
-      rv = nghttp2_stream_detach_item(stream);
+      rv = session_detach_stream_item(session, stream);
 
       if (nghttp2_is_fatal(rv)) {
         return rv;
@@ -2078,7 +2231,7 @@ static int session_prep_frame(nghttp2_session *session,
     if (rv != 0) {
       int rv2;
 
-      rv2 = nghttp2_stream_detach_item(stream);
+      rv2 = session_detach_stream_item(session, stream);
 
       if (nghttp2_is_fatal(rv2)) {
         return rv2;
@@ -2339,6 +2492,10 @@ static int session_prep_frame(nghttp2_session *session,
 
 nghttp2_outbound_item *
 nghttp2_session_get_next_ob_item(nghttp2_session *session) {
+  nghttp2_outbound_item *item;
+  nghttp2_pq_entry *ent;
+  nghttp2_stream *stream;
+
   if (nghttp2_outbound_queue_top(&session->ob_urgent)) {
     return nghttp2_outbound_queue_top(&session->ob_urgent);
   }
@@ -2354,7 +2511,18 @@ nghttp2_session_get_next_ob_item(nghttp2_session *session) {
   }
 
   if (session->remote_window_size > 0) {
-    return nghttp2_stream_next_outbound_item(&session->root);
+    item = nghttp2_stream_next_outbound_item(&session->root);
+    if (item) {
+      return item;
+    }
+
+    ent = nghttp2_pq_top(&session->ob_data);
+    if (!ent) {
+      return NULL;
+    }
+
+    stream = nghttp2_struct_of(ent, nghttp2_stream, pq_entry);
+    return stream->item;
   }
 
   return NULL;
@@ -2363,6 +2531,8 @@ nghttp2_session_get_next_ob_item(nghttp2_session *session) {
 nghttp2_outbound_item *
 nghttp2_session_pop_next_ob_item(nghttp2_session *session) {
   nghttp2_outbound_item *item;
+  nghttp2_pq_entry *ent;
+  nghttp2_stream *stream;
 
   item = nghttp2_outbound_queue_top(&session->ob_urgent);
   if (item) {
@@ -2388,7 +2558,18 @@ nghttp2_session_pop_next_ob_item(nghttp2_session *session) {
   }
 
   if (session->remote_window_size > 0) {
-    return nghttp2_stream_next_outbound_item(&session->root);
+    item = nghttp2_stream_next_outbound_item(&session->root);
+    if (item) {
+      return item;
+    }
+
+    ent = nghttp2_pq_top(&session->ob_data);
+    if (!ent) {
+      return NULL;
+    }
+
+    stream = nghttp2_struct_of(ent, nghttp2_stream, pq_entry);
+    return stream->item;
   }
 
   return NULL;
@@ -2550,7 +2731,7 @@ static int session_after_frame_sent1(nghttp2_session *session) {
     }
 
     if (stream && aux_data->eof) {
-      rv = nghttp2_stream_detach_item(stream);
+      rv = session_detach_stream_item(session, stream);
       if (nghttp2_is_fatal(rv)) {
         return rv;
       }
@@ -2675,9 +2856,8 @@ static int session_after_frame_sent1(nghttp2_session *session) {
     }
   }
   case NGHTTP2_PRIORITY:
-    if (session->server) {
+    if (session->server || session->pending_no_rfc7540_priorities == 1) {
       return 0;
-      ;
     }
 
     stream = nghttp2_session_get_stream_raw(session, frame->hd.stream_id);
@@ -2852,7 +3032,7 @@ static int session_after_frame_sent2(nghttp2_session *session) {
      further data. */
   if (nghttp2_session_predicate_data_send(session, stream) != 0) {
     if (stream) {
-      rv = nghttp2_stream_detach_item(stream);
+      rv = session_detach_stream_item(session, stream);
 
       if (nghttp2_is_fatal(rv)) {
         return rv;
@@ -3150,7 +3330,7 @@ static ssize_t nghttp2_session_mem_send_internal(nghttp2_session *session,
       }
 
       if (rv == NGHTTP2_ERR_TEMPORAL_CALLBACK_FAILURE) {
-        rv = nghttp2_stream_detach_item(stream);
+        rv = session_detach_stream_item(session, stream);
 
         if (nghttp2_is_fatal(rv)) {
           return rv;
@@ -4091,6 +4271,8 @@ int nghttp2_session_on_priority_received(nghttp2_session *session,
   int rv;
   nghttp2_stream *stream;
 
+  assert(session->pending_no_rfc7540_priorities != 1);
+
   if (frame->hd.stream_id == 0) {
     return session_handle_invalid_connection(session, frame, NGHTTP2_ERR_PROTO,
                                              "PRIORITY: stream_id == 0");
@@ -4148,6 +4330,8 @@ static int session_process_priority_frame(nghttp2_session *session) {
   nghttp2_inbound_frame *iframe = &session->iframe;
   nghttp2_frame *frame = &iframe->frame;
 
+  assert(session->pending_no_rfc7540_priorities != 1);
+
   nghttp2_frame_unpack_priority_payload(&frame->priority, iframe->sbuf.pos);
 
   return nghttp2_session_on_priority_received(session, frame);
@@ -4214,8 +4398,8 @@ static int update_remote_initial_window_size_func(void *entry, void *ptr) {
   if (stream->remote_window_size > 0 &&
       nghttp2_stream_check_deferred_by_flow_control(stream)) {
 
-    rv = nghttp2_stream_resume_deferred_item(
+    rv = session_resume_deferred_stream_item(
-        stream, NGHTTP2_STREAM_FLAG_DEFERRED_FLOW_CONTROL);
+        arg->session, stream, NGHTTP2_STREAM_FLAG_DEFERRED_FLOW_CONTROL);
 
     if (nghttp2_is_fatal(rv)) {
       return rv;
@@ -4389,6 +4573,9 @@ int nghttp2_session_update_local_settings(nghttp2_session *session,
     case NGHTTP2_SETTINGS_ENABLE_CONNECT_PROTOCOL:
       session->local_settings.enable_connect_protocol = iv[i].value;
       break;
+    case NGHTTP2_SETTINGS_NO_RFC7540_PRIORITIES:
+      session->local_settings.no_rfc7540_priorities = iv[i].value;
+      break;
     }
   }
 
@@ -4548,7 +4735,29 @@ int nghttp2_session_on_settings_received(nghttp2_session *session,
       session->remote_settings.enable_connect_protocol = entry->value;
 
       break;
+    case NGHTTP2_SETTINGS_NO_RFC7540_PRIORITIES:
+
+      if (entry->value != 0 && entry->value != 1) {
+        return session_handle_invalid_connection(
+            session, frame, NGHTTP2_ERR_PROTO,
+            "SETTINGS: invalid SETTINGS_NO_RFC7540_PRIORITIES");
       }
+
+      if (session->remote_settings.no_rfc7540_priorities != UINT32_MAX &&
+          session->remote_settings.no_rfc7540_priorities != entry->value) {
+        return session_handle_invalid_connection(
+            session, frame, NGHTTP2_ERR_PROTO,
+            "SETTINGS: SETTINGS_NO_RFC7540_PRIORITIES cannot be changed");
+      }
+
+      session->remote_settings.no_rfc7540_priorities = entry->value;
+
+      break;
+    }
+  }
+
+  if (session->remote_settings.no_rfc7540_priorities == UINT32_MAX) {
+    session->remote_settings.no_rfc7540_priorities = 0;
   }
 
   if (!noack && !session_is_closing(session)) {
@@ -4833,8 +5042,8 @@ static int session_on_stream_window_update_received(nghttp2_session *session,
   if (stream->remote_window_size > 0 &&
       nghttp2_stream_check_deferred_by_flow_control(stream)) {
 
-    rv = nghttp2_stream_resume_deferred_item(
+    rv = session_resume_deferred_stream_item(
-        stream, NGHTTP2_STREAM_FLAG_DEFERRED_FLOW_CONTROL);
+        session, stream, NGHTTP2_STREAM_FLAG_DEFERRED_FLOW_CONTROL);
 
     if (nghttp2_is_fatal(rv)) {
       return rv;
@@ -5276,6 +5485,7 @@ static void inbound_frame_set_settings_entry(nghttp2_inbound_frame *iframe) {
   case NGHTTP2_SETTINGS_MAX_FRAME_SIZE:
   case NGHTTP2_SETTINGS_MAX_HEADER_LIST_SIZE:
   case NGHTTP2_SETTINGS_ENABLE_CONNECT_PROTOCOL:
+  case NGHTTP2_SETTINGS_NO_RFC7540_PRIORITIES:
     break;
   default:
     DEBUGF("recv: unknown settings id=0x%02x\n", iv.settings_id);
@@ -5995,6 +6205,8 @@ ssize_t nghttp2_session_mem_recv(nghttp2_session *session, const uint8_t *in,
 
         break;
       case NGHTTP2_PRIORITY:
+        if (session->pending_no_rfc7540_priorities != 1 &&
+            session->remote_settings.no_rfc7540_priorities != 1) {
           rv = session_process_priority_frame(session);
           if (nghttp2_is_fatal(rv)) {
             return rv;
@@ -6003,6 +6215,7 @@ ssize_t nghttp2_session_mem_recv(nghttp2_session *session, const uint8_t *in,
           if (iframe->state == NGHTTP2_IB_IGN_ALL) {
             return (ssize_t)inlen;
           }
+        }
 
         session_inbound_frame_reset(session);
 
@@ -6870,7 +7083,8 @@ int nghttp2_session_want_write(nghttp2_session *session) {
    */
   return session->aob.item || nghttp2_outbound_queue_top(&session->ob_urgent) ||
          nghttp2_outbound_queue_top(&session->ob_reg) ||
-         (!nghttp2_pq_empty(&session->root.obq) &&
+         ((!nghttp2_pq_empty(&session->root.obq) ||
+           !nghttp2_pq_empty(&session->ob_data)) &&
           session->remote_window_size > 0) ||
          (nghttp2_outbound_queue_top(&session->ob_syn) &&
           !session_is_outgoing_concurrent_streams_max(session));
@@ -7023,6 +7237,7 @@ int nghttp2_session_add_settings(nghttp2_session *session, uint8_t flags,
   int rv;
   nghttp2_mem *mem;
   nghttp2_inflight_settings *inflight_settings = NULL;
+  uint8_t no_rfc7540_pri = session->pending_no_rfc7540_priorities;
 
   mem = &session->mem;
 
@@ -7040,6 +7255,21 @@ int nghttp2_session_add_settings(nghttp2_session *session, uint8_t flags,
     return NGHTTP2_ERR_INVALID_ARGUMENT;
   }
 
+  for (i = 0; i < niv; ++i) {
+    if (iv[i].settings_id != NGHTTP2_SETTINGS_NO_RFC7540_PRIORITIES) {
+      continue;
+    }
+
+    if (no_rfc7540_pri == UINT8_MAX) {
+      no_rfc7540_pri = (uint8_t)iv[i].value;
+      continue;
+    }
+
+    if (iv[i].value != (uint32_t)no_rfc7540_pri) {
+      return NGHTTP2_ERR_INVALID_ARGUMENT;
+    }
+  }
+
   item = nghttp2_mem_malloc(mem, sizeof(nghttp2_outbound_item));
   if (item == NULL) {
     return NGHTTP2_ERR_NOMEM;
@@ -7114,6 +7344,12 @@ int nghttp2_session_add_settings(nghttp2_session *session, uint8_t flags,
     }
   }
 
+  if (no_rfc7540_pri == UINT8_MAX) {
+    session->pending_no_rfc7540_priorities = 0;
+  } else {
+    session->pending_no_rfc7540_priorities = no_rfc7540_pri;
+  }
+
   return 0;
 }
 
@@ -7242,7 +7478,9 @@ int nghttp2_session_pack_data(nghttp2_session *session, nghttp2_bufs *bufs,
     return rv;
   }
 
+  if (!(stream->flags & NGHTTP2_STREAM_FLAG_NO_RFC7540_PRIORITIES)) {
     reschedule_stream(stream);
+  }
 
   if (frame->hd.length == 0 && (data_flags & NGHTTP2_DATA_FLAG_EOF) &&
       (data_flags & NGHTTP2_DATA_FLAG_NO_END_STREAM)) {
@@ -7316,7 +7554,7 @@ int nghttp2_session_resume_data(nghttp2_session *session, int32_t stream_id) {
     return NGHTTP2_ERR_INVALID_ARGUMENT;
   }
 
-  rv = nghttp2_stream_resume_deferred_item(stream,
+  rv = session_resume_deferred_stream_item(session, stream,
                                            NGHTTP2_STREAM_FLAG_DEFERRED_USER);
 
   if (nghttp2_is_fatal(rv)) {
@@ -7424,6 +7662,8 @@ uint32_t nghttp2_session_get_remote_settings(nghttp2_session *session,
     return session->remote_settings.max_header_list_size;
   case NGHTTP2_SETTINGS_ENABLE_CONNECT_PROTOCOL:
     return session->remote_settings.enable_connect_protocol;
+  case NGHTTP2_SETTINGS_NO_RFC7540_PRIORITIES:
+    return session->remote_settings.no_rfc7540_priorities;
   }
 
   assert(0);
@@ -7447,6 +7687,8 @@ uint32_t nghttp2_session_get_local_settings(nghttp2_session *session,
     return session->local_settings.max_header_list_size;
   case NGHTTP2_SETTINGS_ENABLE_CONNECT_PROTOCOL:
     return session->local_settings.enable_connect_protocol;
+  case NGHTTP2_SETTINGS_NO_RFC7540_PRIORITIES:
+    return session->local_settings.no_rfc7540_priorities;
   }
 
   assert(0);
@@ -7730,6 +7972,10 @@ int nghttp2_session_change_stream_priority(
   nghttp2_stream *stream;
   nghttp2_priority_spec pri_spec_copy;
 
+  if (session->pending_no_rfc7540_priorities == 1) {
+    return 0;
+  }
+
   if (stream_id == 0 || stream_id == pri_spec->stream_id) {
     return NGHTTP2_ERR_INVALID_ARGUMENT;
   }
@@ -7762,6 +8008,10 @@ int nghttp2_session_create_idle_stream(nghttp2_session *session,
   nghttp2_stream *stream;
   nghttp2_priority_spec pri_spec_copy;
 
+  if (session->pending_no_rfc7540_priorities == 1) {
+    return 0;
+  }
+
   if (stream_id == 0 || stream_id == pri_spec->stream_id ||
       !session_detect_idle_stream(session, stream_id)) {
     return NGHTTP2_ERR_INVALID_ARGUMENT;

lib/nghttp2_session.h

@@ -165,6 +165,7 @@ typedef struct {
   uint32_t max_frame_size;
   uint32_t max_header_list_size;
   uint32_t enable_connect_protocol;
+  uint32_t no_rfc7540_priorities;
 } nghttp2_settings_storage;
 
 typedef enum {
@@ -202,6 +203,9 @@ struct nghttp2_session {
      response) frame, which are subject to
      SETTINGS_MAX_CONCURRENT_STREAMS limit. */
   nghttp2_outbound_queue ob_syn;
+  /* Queue for DATA frames which is used when
+     SETTINGS_NO_RFC7540_PRIORITIES is enabled. */
+  nghttp2_pq ob_data;
   nghttp2_active_outbound_item aob;
   nghttp2_inbound_frame iframe;
   nghttp2_hd_deflater hd_deflater;
@@ -227,6 +231,9 @@ struct nghttp2_session {
   /* Queue of In-flight SETTINGS values.  SETTINGS bearing ACK is not
      considered as in-flight. */
   nghttp2_inflight_settings *inflight_settings_head;
+  /* Sequential number across all streams to process streams in
+     FIFO. */
+  uint64_t stream_seq;
   /* The number of outgoing streams. This will be capped by
      remote_settings.max_concurrent_streams. */
   size_t num_outgoing_streams;
@@ -328,6 +335,9 @@ struct nghttp2_session {
   /* Unacked local ENABLE_CONNECT_PROTOCOL value.  We use this to
      accept :protocol header field before SETTINGS_ACK is received. */
   uint8_t pending_enable_connect_protocol;
+  /* Unacked local SETTINGS_NO_RFC7540_PRIORITIES value, which is
+     effective before it is acknowledged. */
+  uint8_t pending_no_rfc7540_priorities;
   /* Nonzero if the session is server side. */
   uint8_t server;
   /* Flags indicating GOAWAY is sent and/or received. The flags are

lib/nghttp2_stream.c

@@ -484,6 +484,10 @@ int nghttp2_stream_attach_item(nghttp2_stream *stream,
 
   stream->item = item;
 
+  if (stream->flags & NGHTTP2_STREAM_FLAG_NO_RFC7540_PRIORITIES) {
+    return 0;
+  }
+
   rv = stream_update_dep_on_attach_item(stream);
   if (rv != 0) {
     /* This may relave stream->queued == 1, but stream->item == NULL.
@@ -503,6 +507,10 @@ int nghttp2_stream_detach_item(nghttp2_stream *stream) {
   stream->item = NULL;
   stream->flags = (uint8_t)(stream->flags & ~NGHTTP2_STREAM_FLAG_DEFERRED_ALL);
 
+  if (stream->flags & NGHTTP2_STREAM_FLAG_NO_RFC7540_PRIORITIES) {
+    return 0;
+  }
+
   return stream_update_dep_on_detach_item(stream);
 }
 
@@ -514,6 +522,10 @@ int nghttp2_stream_defer_item(nghttp2_stream *stream, uint8_t flags) {
 
   stream->flags |= flags;
 
+  if (stream->flags & NGHTTP2_STREAM_FLAG_NO_RFC7540_PRIORITIES) {
+    return 0;
+  }
+
   return stream_update_dep_on_detach_item(stream);
 }
 
@@ -529,6 +541,10 @@ int nghttp2_stream_resume_deferred_item(nghttp2_stream *stream, uint8_t flags) {
     return 0;
   }
 
+  if (stream->flags & NGHTTP2_STREAM_FLAG_NO_RFC7540_PRIORITIES) {
+    return 0;
+  }
+
   return stream_update_dep_on_attach_item(stream);
 }
 

lib/nghttp2_stream.h

@@ -90,8 +90,10 @@ typedef enum {
   NGHTTP2_STREAM_FLAG_DEFERRED_USER = 0x08,
   /* bitwise OR of NGHTTP2_STREAM_FLAG_DEFERRED_FLOW_CONTROL and
      NGHTTP2_STREAM_FLAG_DEFERRED_USER. */
-  NGHTTP2_STREAM_FLAG_DEFERRED_ALL = 0x0c
+  NGHTTP2_STREAM_FLAG_DEFERRED_ALL = 0x0c,
-
+  /* Indicates that this stream is not subject to RFC7540
+     priorities scheme. */
+  NGHTTP2_STREAM_FLAG_NO_RFC7540_PRIORITIES = 0x10,
 } nghttp2_stream_flag;
 
 /* HTTP related flags to enforce HTTP semantics */

lib/nghttp2_submit.c

@@ -196,7 +196,8 @@ int32_t nghttp2_submit_headers(nghttp2_session *session, uint8_t flags,
 
   flags &= NGHTTP2_FLAG_END_STREAM;
 
-  if (pri_spec && !nghttp2_priority_spec_check_default(pri_spec)) {
+  if (pri_spec && !nghttp2_priority_spec_check_default(pri_spec) &&
+      session->remote_settings.no_rfc7540_priorities != 1) {
     rv = detect_self_dependency(session, stream_id, pri_spec);
     if (rv != 0) {
       return rv;
@@ -229,6 +230,10 @@ int nghttp2_submit_priority(nghttp2_session *session, uint8_t flags,
 
   mem = &session->mem;
 
+  if (session->remote_settings.no_rfc7540_priorities == 1) {
+    return 0;
+  }
+
   if (stream_id == 0 || pri_spec == NULL) {
     return NGHTTP2_ERR_INVALID_ARGUMENT;
   }
@@ -688,7 +693,8 @@ int32_t nghttp2_submit_request(nghttp2_session *session,
     return NGHTTP2_ERR_PROTO;
   }
 
-  if (pri_spec && !nghttp2_priority_spec_check_default(pri_spec)) {
+  if (pri_spec && !nghttp2_priority_spec_check_default(pri_spec) &&
+      session->remote_settings.no_rfc7540_priorities != 1) {
     rv = detect_self_dependency(session, -1, pri_spec);
     if (rv != 0) {
       return rv;

src/HttpServer.cc

@@ -114,7 +114,8 @@ Config::Config()
       hexdump(false),
       echo_upload(false),
       no_content_length(false),
-      ktls(false) {}
+      ktls(false),
+      no_rfc7540_pri(false) {}
 
 Config::~Config() {}
 
@@ -864,6 +865,12 @@ int Http2Handler::connection_made() {
     ++niv;
   }
 
+  if (config->no_rfc7540_pri) {
+    entry[niv].settings_id = NGHTTP2_SETTINGS_NO_RFC7540_PRIORITIES;
+    entry[niv].value = 1;
+    ++niv;
+  }
+
   r = nghttp2_submit_settings(session_, NGHTTP2_FLAG_NONE, entry.data(), niv);
   if (r != 0) {
     return r;

src/HttpServer.h

@@ -83,6 +83,7 @@ struct Config {
   bool echo_upload;
   bool no_content_length;
   bool ktls;
+  bool no_rfc7540_pri;
   Config();
   ~Config();
 };

src/app_helper.cc

@@ -79,6 +79,8 @@ const char *strsettingsid(int32_t id) {
     return "SETTINGS_MAX_HEADER_LIST_SIZE";
   case NGHTTP2_SETTINGS_ENABLE_CONNECT_PROTOCOL:
     return "SETTINGS_ENABLE_CONNECT_PROTOCOL";
+  case NGHTTP2_SETTINGS_NO_RFC7540_PRIORITIES:
+    return "SETTINGS_NO_RFC7540_PRIORITIES";
   default:
     return "UNKNOWN";
   }

src/nghttp.cc

@@ -123,7 +123,8 @@ Config::Config()
       no_push(false),
       expect_continue(false),
       verify_peer(true),
-      ktls(false) {
+      ktls(false),
+      no_rfc7540_pri(false) {
   nghttp2_option_new(&http2_option);
   nghttp2_option_set_peer_max_concurrent_streams(http2_option,
                                                  peer_max_concurrent_streams);
@@ -935,6 +936,12 @@ size_t populate_settings(nghttp2_settings_entry *iv) {
     ++niv;
   }
 
+  if (config.no_rfc7540_pri) {
+    iv[niv].settings_id = NGHTTP2_SETTINGS_NO_RFC7540_PRIORITIES;
+    iv[niv].value = 1;
+    ++niv;
+  }
+
   return niv;
 }
 } // namespace
@@ -2757,6 +2764,8 @@ Options:
               Suppress  warning  on  server  certificate  verification
               failure.
   --ktls      Enable ktls.
+  --no-rfc7540-pri
+              Disable RFC7540 priorities.
   --version   Display version information and exit.
   -h, --help  Display this help and exit.
 
@@ -2813,6 +2822,7 @@ int main(int argc, char **argv) {
         {"expect-continue", no_argument, &flag, 13},
         {"encoder-header-table-size", required_argument, &flag, 14},
         {"ktls", no_argument, &flag, 15},
+        {"no-rfc7540-pri", no_argument, &flag, 16},
         {nullptr, 0, nullptr, 0}};
     int option_index = 0;
     int c =
@@ -3044,6 +3054,10 @@ int main(int argc, char **argv) {
         // ktls option
         config.ktls = true;
         break;
+      case 16:
+        // no-rfc7540-pri option
+        config.no_rfc7540_pri = true;
+        break;
       }
       break;
     default:

src/nghttp.h

@@ -98,6 +98,7 @@ struct Config {
   bool expect_continue;
   bool verify_peer;
   bool ktls;
+  bool no_rfc7540_pri;
 };
 
 enum class RequestState { INITIAL, ON_REQUEST, ON_RESPONSE, ON_COMPLETE };

src/nghttpd.cc

@@ -179,6 +179,8 @@ Options:
   --no-content-length
               Don't send content-length header field.
   --ktls      Enable ktls.
+  --no-rfc7540-pri
+              Disable RFC7540 priorities.
   --version   Display version information and exit.
   -h, --help  Display this help and exit.
 
@@ -230,6 +232,7 @@ int main(int argc, char **argv) {
         {"no-content-length", no_argument, &flag, 10},
         {"encoder-header-table-size", required_argument, &flag, 11},
         {"ktls", no_argument, &flag, 12},
+        {"no-rfc7540-pri", no_argument, &flag, 13},
         {nullptr, 0, nullptr, 0}};
     int option_index = 0;
     int c = getopt_long(argc, argv, "DVb:c:d:ehm:n:p:va:w:W:", long_options,
@@ -413,6 +416,10 @@ int main(int argc, char **argv) {
         // tls option
         config.ktls = true;
         break;
+      case 13:
+        // no-rfc7540-pri option
+        config.no_rfc7540_pri = true;
+        break;
       }
       break;
     default:

tests/main.c

@@ -329,6 +329,8 @@ int main(void) {
                    test_nghttp2_session_no_closed_streams) ||
       !CU_add_test(pSuite, "session_set_stream_user_data",
                    test_nghttp2_session_set_stream_user_data) ||
+      !CU_add_test(pSuite, "session_no_rfc7540_priorities",
+                   test_nghttp2_session_no_rfc7540_priorities) ||
       !CU_add_test(pSuite, "http_mandatory_headers",
                    test_nghttp2_http_mandatory_headers) ||
       !CU_add_test(pSuite, "http_content_length",

tests/nghttp2_session_test.c

@@ -3651,6 +3651,29 @@ void test_nghttp2_session_on_settings_received(void) {
 
   nghttp2_session_del(session);
   nghttp2_option_del(option);
+
+  /* It is invalid to change SETTINGS_NO_RFC7540_PRIORITIES in the
+     following SETTINGS. */
+  nghttp2_session_client_new(&session, &callbacks, NULL);
+
+  session->remote_settings.no_rfc7540_priorities = 1;
+
+  iv[0].settings_id = NGHTTP2_SETTINGS_NO_RFC7540_PRIORITIES;
+  iv[0].value = 0;
+
+  nghttp2_frame_settings_init(&frame.settings, NGHTTP2_FLAG_NONE, dup_iv(iv, 1),
+                              1);
+
+  CU_ASSERT(0 == nghttp2_session_on_settings_received(session, &frame, 0));
+
+  nghttp2_frame_settings_free(&frame.settings, mem);
+
+  item = nghttp2_session_get_next_ob_item(session);
+
+  CU_ASSERT(NULL != item);
+  CU_ASSERT(NGHTTP2_GOAWAY == item->frame.hd.type);
+
+  nghttp2_session_del(session);
 }
 
 void test_nghttp2_session_on_push_promise_received(void) {
@@ -5796,6 +5819,37 @@ void test_nghttp2_submit_settings(void) {
   CU_ASSERT(50 == session->pending_local_max_concurrent_stream);
 
   nghttp2_session_del(session);
+
+  /* Bail out if there are contradicting
+     SETTINGS_NO_RFC7540_PRIORITIES in one SETTINGS. */
+  nghttp2_session_server_new(&session, &callbacks, &ud);
+
+  iv[0].settings_id = NGHTTP2_SETTINGS_NO_RFC7540_PRIORITIES;
+  iv[0].value = 1;
+  iv[1].settings_id = NGHTTP2_SETTINGS_NO_RFC7540_PRIORITIES;
+  iv[1].value = 0;
+
+  CU_ASSERT(NGHTTP2_ERR_INVALID_ARGUMENT ==
+            nghttp2_submit_settings(session, NGHTTP2_FLAG_NONE, iv, 2));
+
+  nghttp2_session_del(session);
+
+  /* Attempt to change SETTINGS_NO_RFC7540_PRIORITIES in the 2nd
+     SETTINGS. */
+  nghttp2_session_server_new(&session, &callbacks, &ud);
+
+  iv[0].settings_id = NGHTTP2_SETTINGS_NO_RFC7540_PRIORITIES;
+  iv[0].value = 1;
+
+  CU_ASSERT(0 == nghttp2_submit_settings(session, NGHTTP2_FLAG_NONE, iv, 1));
+
+  iv[0].settings_id = NGHTTP2_SETTINGS_NO_RFC7540_PRIORITIES;
+  iv[0].value = 0;
+
+  CU_ASSERT(NGHTTP2_ERR_INVALID_ARGUMENT ==
+            nghttp2_submit_settings(session, NGHTTP2_FLAG_NONE, iv, 1));
+
+  nghttp2_session_del(session);
 }
 
 void test_nghttp2_submit_settings_update_local_window_size(void) {
@@ -11118,6 +11172,101 @@ void test_nghttp2_session_set_stream_user_data(void) {
   nghttp2_session_del(session);
 }
 
+void test_nghttp2_session_no_rfc7540_priorities(void) {
+  nghttp2_session *session;
+  nghttp2_session_callbacks callbacks;
+  nghttp2_data_provider data_prd;
+  my_user_data ud;
+  nghttp2_outbound_item *item;
+  nghttp2_mem *mem;
+  nghttp2_settings_entry iv;
+  nghttp2_priority_spec pri_spec;
+
+  mem = nghttp2_mem_default();
+
+  memset(&callbacks, 0, sizeof(nghttp2_session_callbacks));
+  callbacks.send_callback = null_send_callback;
+
+  /* Do not use a dependency tree if SETTINGS_NO_RFC7540_PRIORITIES =
+     1. */
+  data_prd.read_callback = fixed_length_data_source_read_callback;
+
+  ud.data_source_length = 128 * 1024;
+  CU_ASSERT(0 == nghttp2_session_server_new(&session, &callbacks, &ud));
+
+  iv.settings_id = NGHTTP2_SETTINGS_NO_RFC7540_PRIORITIES;
+  iv.value = 1;
+
+  CU_ASSERT(0 == nghttp2_submit_settings(session, NGHTTP2_FLAG_NONE, &iv, 1));
+  CU_ASSERT(0 == nghttp2_session_send(session));
+
+  open_recv_stream2(session, 1, NGHTTP2_STREAM_OPENING);
+  CU_ASSERT(0 == nghttp2_submit_response(session, 1, resnv, ARRLEN(resnv),
+                                         &data_prd));
+  item = nghttp2_session_get_next_ob_item(session);
+  CU_ASSERT(ARRLEN(resnv) == item->frame.headers.nvlen);
+  assert_nv_equal(resnv, item->frame.headers.nva, item->frame.headers.nvlen,
+                  mem);
+
+  CU_ASSERT(0 == nghttp2_session_send(session));
+  CU_ASSERT(1 == nghttp2_pq_size(&session->ob_data));
+  CU_ASSERT(nghttp2_pq_empty(&session->root.obq));
+
+  nghttp2_session_del(session);
+
+  /* Priorities are sent as is before client receives
+     SETTINGS_NO_RFC7540_PRIORITIES = 1 from server. */
+  CU_ASSERT(0 == nghttp2_session_client_new(&session, &callbacks, NULL));
+
+  iv.settings_id = NGHTTP2_SETTINGS_NO_RFC7540_PRIORITIES;
+  iv.value = 1;
+
+  CU_ASSERT(0 == nghttp2_submit_settings(session, NGHTTP2_FLAG_NONE, &iv, 1));
+
+  pri_spec.stream_id = 5;
+  pri_spec.weight = 111;
+  pri_spec.exclusive = 1;
+
+  CU_ASSERT(1 == nghttp2_submit_request(session, &pri_spec, reqnv,
+                                        ARRLEN(reqnv), NULL, NULL));
+
+  item = nghttp2_outbound_queue_top(&session->ob_syn);
+
+  CU_ASSERT(NULL != item);
+  CU_ASSERT(NGHTTP2_HEADERS == item->frame.hd.type);
+  CU_ASSERT(pri_spec.stream_id == item->frame.headers.pri_spec.stream_id);
+  CU_ASSERT(pri_spec.weight == item->frame.headers.pri_spec.weight);
+  CU_ASSERT(pri_spec.exclusive == item->frame.headers.pri_spec.exclusive);
+
+  nghttp2_session_del(session);
+
+  /* Priorities are defaulted if client received
+     SETTINGS_NO_RFC7540_PRIORITIES = 1 from server. */
+  CU_ASSERT(0 == nghttp2_session_client_new(&session, &callbacks, NULL));
+
+  iv.settings_id = NGHTTP2_SETTINGS_NO_RFC7540_PRIORITIES;
+  iv.value = 1;
+
+  CU_ASSERT(0 == nghttp2_submit_settings(session, NGHTTP2_FLAG_NONE, &iv, 1));
+
+  session->remote_settings.no_rfc7540_priorities = 1;
+
+  pri_spec.stream_id = 5;
+  pri_spec.weight = 111;
+  pri_spec.exclusive = 1;
+
+  CU_ASSERT(1 == nghttp2_submit_request(session, &pri_spec, reqnv,
+                                        ARRLEN(reqnv), NULL, NULL));
+
+  item = nghttp2_outbound_queue_top(&session->ob_syn);
+
+  CU_ASSERT(NULL != item);
+  CU_ASSERT(NGHTTP2_HEADERS == item->frame.hd.type);
+  CU_ASSERT(nghttp2_priority_spec_check_default(&item->frame.headers.pri_spec));
+
+  nghttp2_session_del(session);
+}
+
 static void check_nghttp2_http_recv_headers_fail(
     nghttp2_session *session, nghttp2_hd_deflater *deflater, int32_t stream_id,
     int stream_state, const nghttp2_nv *nva, size_t nvlen) {

tests/nghttp2_session_test.h

@@ -162,6 +162,7 @@ void test_nghttp2_session_removed_closed_stream(void);
 void test_nghttp2_session_pause_data(void);
 void test_nghttp2_session_no_closed_streams(void);
 void test_nghttp2_session_set_stream_user_data(void);
+void test_nghttp2_session_no_rfc7540_priorities(void);
 void test_nghttp2_http_mandatory_headers(void);
 void test_nghttp2_http_content_length(void);
 void test_nghttp2_http_content_length_mismatch(void);