quic: handle h3 max header size option

jasnell · aduh95 · commit 9701a82a78aa · 2026-05-27T11:35:47.000+02:00
Signed-off-by: James M Snell <jasnell@gmail.com> PR-URL: #63483 Reviewed-By: Stephen Belanger <admin@stephenbelanger.com> Reviewed-By: Matteo Collina <matteo.collina@gmail.com>
diff --git a/src/quic/application.cc b/src/quic/application.cc
@@ -115,6 +115,13 @@ Maybe<Session::Application_Options> Session::Application_Options::From(
 
 #undef SET
 
+  // Ensure the advertised max_field_section_size in SETTINGS is at least
+  // as large as max_header_length. Otherwise the peer would be told to
+  // restrict headers to a smaller size than what CanAddHeader accepts.
+  if (options.max_field_section_size < options.max_header_length) {
+    options.max_field_section_size = options.max_header_length;
+  }
+
   return Just<Application_Options>(options);
 }
 
diff --git a/src/quic/session.h b/src/quic/session.h
@@ -72,7 +72,11 @@ class Session final : public AsyncWrap, private SessionTicket::AppData::Source {
     uint64_t max_header_length = DEFAULT_MAX_HEADER_LENGTH;
 
     // HTTP/3 specific options.
-    uint64_t max_field_section_size = 0;
+    // The maximum header section size advertised to the peer in SETTINGS.
+    // Defaults to match max_header_length so the SETTINGS frame accurately
+    // reflects the enforcement limit. A value of 0 would incorrectly tell
+    // the peer not to send any headers at all.
+    uint64_t max_field_section_size = DEFAULT_MAX_HEADER_LENGTH;
     uint64_t qpack_max_dtable_capacity = 4096;
     uint64_t qpack_encoder_max_dtable_capacity = 4096;
     uint64_t qpack_blocked_streams = 100;
