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

net/quic/quic_connection.cc

Index: net/quic/quic_connection.cc
diff --git a/net/quic/quic_connection.cc b/net/quic/quic_connection.cc
index 1bfc6ff5211c823369c8120d1d0449a75196f889..a8763794fe9c755f62ad43559ca73a52017edad3 100644
--- a/net/quic/quic_connection.cc
+++ b/net/quic/quic_connection.cc
@@ -207,7 +207,7 @@ class MtuDiscoveryAckListener : public QuicAckNotifier::DelegateInterface {
                          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_);
     }
   }
 
@@ -327,9 +327,11 @@ QuicConnection::QuicConnection(QuicConnectionId connection_id,
   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() {
@@ -381,10 +383,10 @@ void QuicConnection::SetFromConfig(const QuicConfig& config) {
   }
 
   if (config.HasClientSentConnectionOption(kMTUH, perspective_)) {
-    mtu_discovery_target_ = kMtuDiscoveryTargetPacketSizeHigh;
+    SetMtuDiscoveryTarget(kMtuDiscoveryTargetPacketSizeHigh);
   }
   if (config.HasClientSentConnectionOption(kMTUL, perspective_)) {
-    mtu_discovery_target_ = kMtuDiscoveryTargetPacketSizeLow;
+    SetMtuDiscoveryTarget(kMtuDiscoveryTargetPacketSizeLow);
   }
 }
 
@@ -1417,6 +1419,14 @@ void QuicConnection::CheckForAddressMigration(
     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() {
@@ -1491,7 +1501,7 @@ bool QuicConnection::ProcessValidatedPacket() {
   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;
 }
@@ -2143,8 +2153,9 @@ 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 {
@@ -2374,7 +2385,41 @@ bool QuicConnection::IsConnectionClose(const QueuedPacket& packet) {
   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