relnote: Allow individual QUIC writers to limit the maximum packet size.

net/quic/quic_connection.cc

 // Copyright (c) 2012 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "net/quic/quic_connection.h"
 
 #include <string.h>
 #include <sys/types.h>
 
 #include <algorithm>
@@ skipping 189 matching lines @@
 class MtuDiscoveryAckListener : public QuicAckNotifier::DelegateInterface {
  public:
   MtuDiscoveryAckListener(QuicConnection* connection, QuicByteCount probe_size)
       : connection_(connection), probe_size_(probe_size) {}
 
   void OnAckNotification(int /*num_retransmittable_packets*/,
                          int /*num_retransmittable_bytes*/,
                          QuicTime::Delta /*delta_largest_observed*/) override {
     // Since the probe was successful, increase the maximum packet size to that.
     if (probe_size_ > connection_->max_packet_length()) {
-      connection_->set_max_packet_length(probe_size_);
+      connection_->SetMaxPacketLength(probe_size_);
     }
   }
 
  protected:
   // MtuDiscoveryAckListener is ref counted.
   ~MtuDiscoveryAckListener() override {}
 
  private:
   QuicConnection* connection_;
   QuicByteCount probe_size_;
@@ skipping 99 matching lines @@
       next_mtu_probe_at_(kPacketsBetweenMtuProbesBase),
       largest_received_packet_size_(0),
       goaway_sent_(false),
       goaway_received_(false) {
   DVLOG(1) << ENDPOINT << "Created connection with connection_id: "
            << connection_id;
   framer_.set_visitor(this);
   framer_.set_received_entropy_calculator(&received_packet_manager_);
   stats_.connection_creation_time = clock_->ApproximateNow();
   sent_packet_manager_.set_network_change_visitor(this);
-  if (perspective_ == Perspective::IS_SERVER) {
-    set_max_packet_length(kDefaultServerMaxPacketSize);
-  }
+  // Allow the packet writer to potentially reduce the packet size to a value
+  // even smaller than kDefaultMaxPacketSize.
+  SetMaxPacketLength(perspective_ == Perspective::IS_SERVER
+                         ? kDefaultServerMaxPacketSize
+                         : kDefaultMaxPacketSize);
 }
 
 QuicConnection::~QuicConnection() {
   if (owns_writer_) {
     delete writer_;
   }
   STLDeleteElements(&undecryptable_packets_);
   STLDeleteValues(&group_map_);
   ClearQueuedPackets();
 }
@@ skipping 31 matching lines @@
     packet_generator_.set_fec_send_policy(FecSendPolicy::FEC_ALARM_TRIGGER);
   }
   if (config.HasClientSentConnectionOption(kFRTT, perspective_)) {
     // TODO(rtenneti): Delete this code after the 0.25 RTT FEC experiment.
     const float kFecTimeoutRttMultiplier = 0.25;
     packet_generator_.set_rtt_multiplier_for_fec_timeout(
         kFecTimeoutRttMultiplier);
   }
 
   if (config.HasClientSentConnectionOption(kMTUH, perspective_)) {
-    mtu_discovery_target_ = kMtuDiscoveryTargetPacketSizeHigh;
+    SetMtuDiscoveryTarget(kMtuDiscoveryTargetPacketSizeHigh);
   }
   if (config.HasClientSentConnectionOption(kMTUL, perspective_)) {
-    mtu_discovery_target_ = kMtuDiscoveryTargetPacketSizeLow;
+    SetMtuDiscoveryTarget(kMtuDiscoveryTargetPacketSizeLow);
   }
 }
 
 void QuicConnection::OnSendConnectionState(
     const CachedNetworkParameters& cached_network_params) {
   if (debug_visitor_ != nullptr) {
     debug_visitor_->OnSendConnectionState(cached_network_params);
   }
 }
 
@@ skipping 1012 matching lines @@
 
     // Store in case we want to migrate connection in ProcessValidatedPacket.
     migrating_peer_ip_ = peer_address.address();
     migrating_peer_port_ = peer_address.port();
   }
 
   if (!self_address.address().empty() && !self_address_.address().empty()) {
     self_ip_changed_ = (self_address.address() != self_address_.address());
     self_port_changed_ = (self_address.port() != self_address_.port());
   }
+
+  // TODO(vasilvv): reset maximum packet size on connection migration. Whenever
+  // the connection is migrated, it usually ends up being on a different path,
+  // with possibly smaller MTU.  This means the max packet size has to be reset
+  // and MTU discovery mechanism re-initialized.  The main reason the code does
+  // not do it now is that the retransmission code currently cannot deal with
+  // the case when it needs to resend a packet created with larger MTU (see
+  // b/22172803).
 }
 
 void QuicConnection::OnCanWrite() {
   DCHECK(!writer_->IsWriteBlocked());
 
   WriteQueuedPackets();
   WritePendingRetransmissions();
 
   // Sending queued packets may have caused the socket to become write blocked,
   // or the congestion manager to prohibit sending.  If we've sent everything
@@ skipping 54 matching lines @@
   DVLOG(1) << ENDPOINT << "time of last received packet: "
            << time_of_last_received_packet_.ToDebuggingValue();
 
   if (last_size_ > largest_received_packet_size_) {
     largest_received_packet_size_ = last_size_;
   }
 
   if (perspective_ == Perspective::IS_SERVER &&
       encryption_level_ == ENCRYPTION_NONE &&
       last_size_ > packet_generator_.GetMaxPacketLength()) {
-    set_max_packet_length(last_size_);
+    SetMaxPacketLength(last_size_);
   }
   return true;
 }
 
 void QuicConnection::WriteQueuedPackets() {
   DCHECK(!writer_->IsWriteBlocked());
 
   if (pending_version_negotiation_packet_) {
     SendVersionNegotiationPacket();
   }
@@ skipping 631 matching lines @@
     group_map_.erase(it);
     delete fec_group;
     it = next;
   }
 }
 
 QuicByteCount QuicConnection::max_packet_length() const {
   return packet_generator_.GetMaxPacketLength();
 }
 
-void QuicConnection::set_max_packet_length(QuicByteCount length) {
-  return packet_generator_.SetMaxPacketLength(length, /*force=*/false);
+void QuicConnection::SetMaxPacketLength(QuicByteCount length) {
+  return packet_generator_.SetMaxPacketLength(LimitMaxPacketSize(length),
+                                              /*force=*/false);
 }
 
 bool QuicConnection::HasQueuedData() const {
   return pending_version_negotiation_packet_ ||
       !queued_packets_.empty() || packet_generator_.HasQueuedFrames();
 }
 
 bool QuicConnection::CanWriteStreamData() {
   // Don't write stream data if there are negotiation or queued data packets
   // to send. Otherwise, continue and bundle as many frames as possible.
@@ skipping 209 matching lines @@
     return false;
   }
   for (const QuicFrame& frame : retransmittable_frames->frames()) {
     if (frame.type == CONNECTION_CLOSE_FRAME) {
       return true;
     }
   }
   return false;
 }
 
+void QuicConnection::SetMtuDiscoveryTarget(QuicByteCount target) {
+  mtu_discovery_target_ = LimitMaxPacketSize(target);
+}
+
+QuicByteCount QuicConnection::LimitMaxPacketSize(
+    QuicByteCount suggested_max_packet_size) {
+  if (FLAGS_quic_allow_oversized_packets_for_test) {
+    return suggested_max_packet_size;
+  }
+
+  if (!FLAGS_quic_limit_mtu_by_writer) {
+    return suggested_max_packet_size;
+  }
+
+  if (peer_address_.address().empty()) {
+    LOG(DFATAL) << "Attempted to use a connection without a valid peer address";
+    return suggested_max_packet_size;
+  }
+
+  const QuicByteCount writer_limit = writer_->GetMaxPacketSize(peer_address());
+
+  QuicByteCount max_packet_size = suggested_max_packet_size;
+  if (max_packet_size > writer_limit) {
+    max_packet_size = writer_limit;
+  }
+  if (max_packet_size > kMaxPacketSize) {
+    max_packet_size = kMaxPacketSize;
+  }
+  return max_packet_size;
+}
+
 void QuicConnection::SendMtuDiscoveryPacket(QuicByteCount target_mtu) {
+  // Currently, this limit is ensured by the caller.
+  DCHECK_EQ(target_mtu, LimitMaxPacketSize(target_mtu));
+
   // Create a listener for the new probe.  The ownership of the listener is
   // transferred to the AckNotifierManager.  The notifier will get destroyed
   // before the connection (because it's stored in one of the connection's
   // subfields), hence |this| pointer is guaranteed to stay valid at all times.
   scoped_refptr<MtuDiscoveryAckListener> last_mtu_discovery_ack_listener(
       new MtuDiscoveryAckListener(this, target_mtu));
 
   // Send the probe.
   packet_generator_.GenerateMtuDiscoveryPacket(
       target_mtu, last_mtu_discovery_ack_listener.get());
@@ skipping 16 matching lines @@
       packet_number_of_last_sent_packet_ + packets_between_mtu_probes_ + 1;
   ++mtu_probe_count_;
 
   DVLOG(2) << "Sending a path MTU discovery packet #" << mtu_probe_count_;
   SendMtuDiscoveryPacket(mtu_discovery_target_);
 
   DCHECK(!mtu_discovery_alarm_->IsSet());
 }
 
 }  // namespace net