Make HTTP/2 session and stream windows configurable

Issue #8199 outlines the issue. This PR adds a new setting to allow for
setting the session window separately from the stream window.

Closes #8199

Author: bneradt

doc/admin-guide/files/records.config.en.rst

@@ -4137,7 +4137,17 @@ HTTP/2 Configuration
    :reloadable:
    :units: bytes
 
-   The initial window size for inbound connections.
+   The initial window size for inbound connection streams.
+
+.. ts:cv:: CONFIG proxy.config.http2.session_initial_window_size_in INT 0
+   :reloadable:
+
+   The initial window size for inbound connection session.  HTTP/2 provides both
+   a per stream window and a session wide window.  Each data byte exchanged decrements
+   the window of the associated stream and the session window.  To allow for multiple
+   active streams, the session window should be larger than the stream window.
+   |TS| verifies that the session initial window is always at least as large as the
+   stream initial window.
 
 .. ts:cv:: CONFIG proxy.config.http2.max_frame_size INT 16384
    :reloadable:

mgmt/RecordsConfig.cc

@@ -1299,6 +1299,8 @@ static const RecordElement RecordsConfig[] =
   ,
   {RECT_CONFIG, "proxy.config.http2.initial_window_size_in", RECD_INT, "65535", RECU_DYNAMIC, RR_NULL, RECC_STR, "^[0-9]+$", RECA_NULL}
   ,
+  {RECT_CONFIG, "proxy.config.http2.session_initial_window_size_in", RECD_INT, "0", RECU_DYNAMIC, RR_NULL, RECC_STR, "^[0-9]+$", RECA_NULL}
+  ,
   {RECT_CONFIG, "proxy.config.http2.max_frame_size", RECD_INT, "16384", RECU_DYNAMIC, RR_NULL, RECC_STR, "^[0-9]+$", RECA_NULL}
   ,
   {RECT_CONFIG, "proxy.config.http2.header_table_size", RECD_INT, "4096", RECU_DYNAMIC, RR_NULL, RECC_STR, "^[0-9]+$", RECA_NULL}

proxy/http2/HTTP2.cc

@@ -571,6 +571,7 @@ uint32_t Http2::max_active_streams_in           = 0;
 bool Http2::throttling                          = false;
 uint32_t Http2::stream_priority_enabled         = 0;
 uint32_t Http2::initial_window_size             = 65535;
+uint32_t Http2::session_initial_window_size     = 65535;
 uint32_t Http2::max_frame_size                  = 16384;
 uint32_t Http2::header_table_size               = 4096;
 uint32_t Http2::max_header_list_size            = 4294967295;
@@ -603,6 +604,13 @@ Http2::init()
   REC_EstablishStaticConfigInt32U(max_active_streams_in, "proxy.config.http2.max_active_streams_in");
   REC_EstablishStaticConfigInt32U(stream_priority_enabled, "proxy.config.http2.stream_priority_enabled");
   REC_EstablishStaticConfigInt32U(initial_window_size, "proxy.config.http2.initial_window_size_in");
+  REC_EstablishStaticConfigInt32U(session_initial_window_size, "proxy.config.http2.session_initial_window_size_in");
+
+  // The session window must be at least as big as the stream window
+  if (session_initial_window_size < initial_window_size) {
+    session_initial_window_size = initial_window_size;
+  }
+
   REC_EstablishStaticConfigInt32U(max_frame_size, "proxy.config.http2.max_frame_size");
   REC_EstablishStaticConfigInt32U(header_table_size, "proxy.config.http2.header_table_size");
   REC_EstablishStaticConfigInt32U(max_header_list_size, "proxy.config.http2.max_header_list_size");

proxy/http2/HTTP2.h

@@ -374,6 +374,7 @@ class Http2
   static bool throttling;
   static uint32_t stream_priority_enabled;
   static uint32_t initial_window_size;
+  static uint32_t session_initial_window_size;
   static uint32_t max_frame_size;
   static uint32_t header_table_size;
   static uint32_t max_header_list_size;

proxy/http2/Http2ConnectionState.cc

@@ -1040,7 +1040,7 @@ void
 Http2ConnectionState::init(Http2CommonSession *ssn)
 {
   session            = ssn;
-  this->_server_rwnd = Http2::initial_window_size;
+  this->_server_rwnd = Http2::session_initial_window_size;
 
   local_hpack_handle  = new HpackHandle(HTTP2_HEADER_TABLE_SIZE);
   remote_hpack_handle = new HpackHandle(HTTP2_HEADER_TABLE_SIZE);
@@ -1073,8 +1073,10 @@ Http2ConnectionState::send_connection_preface()
 
   send_settings_frame(configured_settings);
 
-  if (server_settings.get(HTTP2_SETTINGS_INITIAL_WINDOW_SIZE) > HTTP2_INITIAL_WINDOW_SIZE) {
-    send_window_update_frame(0, server_settings.get(HTTP2_SETTINGS_INITIAL_WINDOW_SIZE) - HTTP2_INITIAL_WINDOW_SIZE);
+  // If the session window size is non-default, send a window update right away
+
+  if (Http2::session_initial_window_size > HTTP2_INITIAL_WINDOW_SIZE) {
+    send_window_update_frame(0, Http2::session_initial_window_size - HTTP2_INITIAL_WINDOW_SIZE);
   }
 }
 
@@ -1417,7 +1419,7 @@ Http2ConnectionState::restart_streams()
 void
 Http2ConnectionState::restart_receiving(Http2Stream *stream)
 {
-  uint32_t initial_rwnd = this->server_settings.get(HTTP2_SETTINGS_INITIAL_WINDOW_SIZE);
+  uint32_t initial_rwnd = Http2::session_initial_window_size;
   uint32_t min_rwnd     = std::min(initial_rwnd, this->server_settings.get(HTTP2_SETTINGS_MAX_FRAME_SIZE));
 
   // Connection level WINDOW UPDATE
@@ -1438,6 +1440,8 @@ Http2ConnectionState::restart_receiving(Http2Stream *stream)
     return;
   }
 
+  // Update the window size for the stream
+  initial_rwnd              = Http2::initial_window_size;
   Http2WindowSize diff_size = initial_rwnd - std::max(static_cast<int64_t>(stream->server_rwnd()), data_size);
   stream->increment_server_rwnd(diff_size);
   this->send_window_update_frame(stream->get_id(), diff_size);