void ConnectionManagerImpl::ActiveStream::decodeHeaders(RequestHeaderMapPtr&& headers,
                                                        bool end_stream) {
  ScopeTrackerScopeState scope(this,
                               connection_manager_.read_callbacks_->connection().dispatcher());
  request_headers_ = std::move(headers);
  filter_manager_.requestHeadersInitialized();
  if (request_header_timer_ != nullptr) {
    request_header_timer_->disableTimer();
    request_header_timer_.reset();
  }

  // Both saw_connection_close_ and is_head_request_ affect local replies: set
  // them as early as possible.
  const Protocol protocol = connection_manager_.codec_->protocol();
  state_.saw_connection_close_ = HeaderUtility::shouldCloseConnection(protocol, *request_headers_);

  // We need to snap snapped_route_config_ here as it's used in mutateRequestHeaders later.
  if (connection_manager_.config_.isRoutable()) {
    if (connection_manager_.config_.routeConfigProvider() != nullptr) {
      snapped_route_config_ = connection_manager_.config_.routeConfigProvider()->configCast();
    } else if (connection_manager_.config_.scopedRouteConfigProvider() != nullptr) {
      snapped_scoped_routes_config_ =
          connection_manager_.config_.scopedRouteConfigProvider()->config<Router::ScopedConfig>();
      snapScopedRouteConfig();
    }
  } else {
    snapped_route_config_ = connection_manager_.config_.routeConfigProvider()->configCast();
  }

  ENVOY_STREAM_LOG(debug, "request headers complete (end_stream={}):\n{}", *this, end_stream,
                   *request_headers_);

  // We end the decode here only if the request is header only. If we convert the request to a
  // header only, the stream will be marked as done once a subsequent decodeData/decodeTrailers is
  // called with end_stream=true.
  filter_manager_.maybeEndDecode(end_stream);

  // Drop new requests when overloaded as soon as we have decoded the headers.
  if (connection_manager_.random_generator_.bernoulli(
          connection_manager_.overload_stop_accepting_requests_ref_.value())) {
    // In this one special case, do not create the filter chain. If there is a risk of memory
    // overload it is more important to avoid unnecessary allocation than to create the filters.
    filter_manager_.skipFilterChainCreation();
    connection_manager_.stats_.named_.downstream_rq_overload_close_.inc();
    sendLocalReply(Http::Code::ServiceUnavailable, "envoy overloaded", nullptr, absl::nullopt,
                   StreamInfo::ResponseCodeDetails::get().Overload);
    return;
  }

  // This lambda should be erased when
  // `envoy.reloadable_features.http_100_continue_case_insensitive` is removed.
  auto is100Continue = [](absl::string_view request_expect) {
    return request_expect == Headers::get().ExpectValues._100Continue ||
           (Runtime::runtimeFeatureEnabled(
                "envoy.reloadable_features.http_100_continue_case_insensitive") &&
            // The Expect field-value is case-insensitive.
            // https://tools.ietf.org/html/rfc7231#section-5.1.1
            absl::EqualsIgnoreCase(request_expect, Headers::get().ExpectValues._100Continue));
  };

  if (!connection_manager_.config_.proxy100Continue() && request_headers_->Expect() &&
      is100Continue(request_headers_->Expect()->value().getStringView())) {
    // Note in the case Envoy is handling 100-Continue complexity, it skips the filter chain
    // and sends the 100-Continue directly to the encoder.
    chargeStats(continueHeader());
    response_encoder_->encode1xxHeaders(continueHeader());
    // Remove the Expect header so it won't be handled again upstream.
    request_headers_->removeExpect();
  }

  connection_manager_.user_agent_.initializeFromHeaders(*request_headers_,
                                                        connection_manager_.stats_.prefixStatName(),
                                                        connection_manager_.stats_.scope_);

  // Make sure we are getting a codec version we support.
  if (protocol == Protocol::Http10) {
    // Assume this is HTTP/1.0. This is fine for HTTP/0.9 but this code will also affect any
    // requests with non-standard version numbers (0.9, 1.3), basically anything which is not
    // HTTP/1.1.
    //
    // The protocol may have shifted in the HTTP/1.0 case so reset it.
    filter_manager_.streamInfo().protocol(protocol);
    if (!connection_manager_.config_.http1Settings().accept_http_10_) {
      // Send "Upgrade Required" if HTTP/1.0 support is not explicitly configured on.
      sendLocalReply(Code::UpgradeRequired, "", nullptr, absl::nullopt,
                     StreamInfo::ResponseCodeDetails::get().LowVersion);
      return;
    }
    if (!request_headers_->Host() &&
        !connection_manager_.config_.http1Settings().default_host_for_http_10_.empty()) {
      // Add a default host if configured to do so.
      request_headers_->setHost(
          connection_manager_.config_.http1Settings().default_host_for_http_10_);
    }
  }

  if (!request_headers_->Host()) {
    // Require host header. For HTTP/1.1 Host has already been translated to :authority.
    sendLocalReply(Code::BadRequest, "", nullptr, absl::nullopt,
                   StreamInfo::ResponseCodeDetails::get().MissingHost);
    return;
  }

  // Verify header sanity checks which should have been performed by the codec.
  ASSERT(HeaderUtility::requestHeadersValid(*request_headers_).has_value() == false);

  // Check for the existence of the :path header for non-CONNECT requests, or present-but-empty
  // :path header for CONNECT requests. We expect the codec to have broken the path into pieces if
  // applicable. NOTE: Currently the HTTP/1.1 codec only does this when the allow_absolute_url flag
  // is enabled on the HCM.
  if ((!HeaderUtility::isConnect(*request_headers_) || request_headers_->Path()) &&
      request_headers_->getPathValue().empty()) {
    sendLocalReply(Code::NotFound, "", nullptr, absl::nullopt,
                   StreamInfo::ResponseCodeDetails::get().MissingPath);
    return;
  }

  // Currently we only support relative paths at the application layer.
  if (!request_headers_->getPathValue().empty() && request_headers_->getPathValue()[0] != '/') {
    connection_manager_.stats_.named_.downstream_rq_non_relative_path_.inc();
    sendLocalReply(Code::NotFound, "", nullptr, absl::nullopt,
                   StreamInfo::ResponseCodeDetails::get().AbsolutePath);
    return;
  }

  // Path sanitization should happen before any path access other than the above sanity check.
  const auto action =
      ConnectionManagerUtility::maybeNormalizePath(*request_headers_, connection_manager_.config_);
  // gRPC requests are rejected if Envoy is configured to redirect post-normalization. This is
  // because gRPC clients do not support redirect.
  if (action == ConnectionManagerUtility::NormalizePathAction::Reject ||
      (action == ConnectionManagerUtility::NormalizePathAction::Redirect &&
       Grpc::Common::hasGrpcContentType(*request_headers_))) {
    connection_manager_.stats_.named_.downstream_rq_failed_path_normalization_.inc();
    sendLocalReply(Code::BadRequest, "", nullptr, absl::nullopt,
                   StreamInfo::ResponseCodeDetails::get().PathNormalizationFailed);
    return;
  } else if (action == ConnectionManagerUtility::NormalizePathAction::Redirect) {
    connection_manager_.stats_.named_.downstream_rq_redirected_with_normalized_path_.inc();
    sendLocalReply(
        Code::TemporaryRedirect, "",
        [new_path = request_headers_->Path()->value().getStringView()](
            Http::ResponseHeaderMap& response_headers) -> void {
          response_headers.addReferenceKey(Http::Headers::get().Location, new_path);
        },
        absl::nullopt, StreamInfo::ResponseCodeDetails::get().PathNormalizationFailed);
    return;
  }

  ASSERT(action == ConnectionManagerUtility::NormalizePathAction::Continue);
  auto optional_port = ConnectionManagerUtility::maybeNormalizeHost(
      *request_headers_, connection_manager_.config_, localPort());
  if (optional_port.has_value() &&
      requestWasConnect(request_headers_, connection_manager_.codec_->protocol())) {
    filter_manager_.streamInfo().filterState()->setData(
        Router::OriginalConnectPort::key(),
        std::make_unique<Router::OriginalConnectPort>(optional_port.value()),
        StreamInfo::FilterState::StateType::ReadOnly, StreamInfo::FilterState::LifeSpan::Request);
  }

  if (!state_.is_internally_created_) { // Only sanitize headers on first pass.
    // Modify the downstream remote address depending on configuration and headers.
    const auto mutate_result = ConnectionManagerUtility::mutateRequestHeaders(
        *request_headers_, connection_manager_.read_callbacks_->connection(),
        connection_manager_.config_, *snapped_route_config_, connection_manager_.local_info_);

    // IP detection failed, reject the request.
    if (mutate_result.reject_request.has_value()) {
      const auto& reject_request_params = mutate_result.reject_request.value();
      connection_manager_.stats_.named_.downstream_rq_rejected_via_ip_detection_.inc();
      sendLocalReply(reject_request_params.response_code, reject_request_params.body, nullptr,
                     absl::nullopt,
                     StreamInfo::ResponseCodeDetails::get().OriginalIPDetectionFailed);
      return;
    }

    filter_manager_.setDownstreamRemoteAddress(mutate_result.final_remote_address);
  }
  ASSERT(filter_manager_.streamInfo().downstreamAddressProvider().remoteAddress() != nullptr);

  ASSERT(!cached_route_);
  refreshCachedRoute();

  if (!state_.is_internally_created_) { // Only mutate tracing headers on first pass.
    filter_manager_.streamInfo().setTraceReason(
        ConnectionManagerUtility::mutateTracingRequestHeader(
            *request_headers_, connection_manager_.runtime_, connection_manager_.config_,
            cached_route_.value().get()));
  }

  filter_manager_.streamInfo().setRequestHeaders(*request_headers_);

  const bool upgrade_rejected = filter_manager_.createFilterChain() == false;

  // TODO if there are no filters when starting a filter iteration, the connection manager
  // should return 404. The current returns no response if there is no router filter.
  if (hasCachedRoute()) {
    // Do not allow upgrades if the route does not support it.
    if (upgrade_rejected) {
      // While downstream servers should not send upgrade payload without the upgrade being
      // accepted, err on the side of caution and refuse to process any further requests on this
      // connection, to avoid a class of HTTP/1.1 smuggling bugs where Upgrade or CONNECT payload
      // contains a smuggled HTTP request.
      state_.saw_connection_close_ = true;
      connection_manager_.stats_.named_.downstream_rq_ws_on_non_ws_route_.inc();
      sendLocalReply(Code::Forbidden, "", nullptr, absl::nullopt,
                     StreamInfo::ResponseCodeDetails::get().UpgradeFailed);
      return;
    }
    // Allow non websocket requests to go through websocket enabled routes.
  }

  if (hasCachedRoute()) {
    const Router::RouteEntry* route_entry = cached_route_.value()->routeEntry();
    if (route_entry != nullptr && route_entry->idleTimeout()) {
      // TODO(mattklein123): Technically if the cached route changes, we should also see if the
      // route idle timeout has changed and update the value.
      idle_timeout_ms_ = route_entry->idleTimeout().value();
      response_encoder_->getStream().setFlushTimeout(idle_timeout_ms_);
      if (idle_timeout_ms_.count()) {
        // If we have a route-level idle timeout but no global stream idle timeout, create a timer.
        if (stream_idle_timer_ == nullptr) {
          stream_idle_timer_ =
              connection_manager_.read_callbacks_->connection().dispatcher().createScaledTimer(
                  Event::ScaledTimerType::HttpDownstreamIdleStreamTimeout,
                  [this]() -> void { onIdleTimeout(); });
        }
      } else if (stream_idle_timer_ != nullptr) {
        // If we had a global stream idle timeout but the route-level idle timeout is set to zero
        // (to override), we disable the idle timer.
        stream_idle_timer_->disableTimer();
        stream_idle_timer_ = nullptr;
      }
    }
  }

  // Check if tracing is enabled at all.
  if (connection_manager_.config_.tracingConfig()) {
    traceRequest();
  }

  filter_manager_.decodeHeaders(*request_headers_, end_stream);

  // Reset it here for both global and overridden cases.
  resetIdleTimer();
}