diff --git a/quiche/quic/core/quic_flags_list.h b/quiche/quic/core/quic_flags_list.h
index dc53509..d2a4308 100644
--- a/quiche/quic/core/quic_flags_list.h
+++ b/quiche/quic/core/quic_flags_list.h
@@ -47,6 +47,8 @@
 QUIC_FLAG(quic_reloadable_flag_quic_allow_client_enabled_bbr_v2, true)
 // If true, allow quic to use new ALPS codepoint to negotiate during handshake for H3 if client sends new ALPS codepoint.
 QUIC_FLAG(quic_reloadable_flag_quic_gfe_allow_alps_new_codepoint, false)
+// If true, allows QUIC control frames to be written while a packet is being processed.
+QUIC_FLAG(quic_restart_flag_quic_allow_control_frames_while_procesing, false)
 // If true, always bundle qpack decoder data with other frames opportunistically.
 QUIC_FLAG(quic_restart_flag_quic_opport_bundle_qpack_decoder_data2, false)
 // If true, an endpoint does not detect path degrading or blackholing until handshake gets confirmed.
diff --git a/quiche/quic/core/quic_packet_creator.cc b/quiche/quic/core/quic_packet_creator.cc
index 793f417..ed6e94b 100644
--- a/quiche/quic/core/quic_packet_creator.cc
+++ b/quiche/quic/core/quic_packet_creator.cc
@@ -117,6 +117,7 @@
       random_(random),
       have_diversification_nonce_(false),
       max_packet_length_(0),
+      next_max_packet_length_(0),
       server_connection_id_included_(CONNECTION_ID_PRESENT),
       packet_size_(0),
       server_connection_id_(server_connection_id),
@@ -155,8 +156,18 @@
 }
 
 void QuicPacketCreator::SetMaxPacketLength(QuicByteCount length) {
-  QUICHE_DCHECK(CanSetMaxPacketLength()) << ENDPOINT;
-
+  if (!GetQuicRestartFlag(quic_allow_control_frames_while_procesing)) {
+    QUICHE_DCHECK(CanSetMaxPacketLength()) << ENDPOINT;
+  } else {
+    QUIC_RESTART_FLAG_COUNT_N(quic_allow_control_frames_while_procesing, 2, 3);
+    if (!CanSetMaxPacketLength()) {
+      QUIC_RESTART_FLAG_COUNT_N(quic_allow_control_frames_while_procesing, 3,
+                                3);
+      // The new max packet length will be applied to the next packet.
+      next_max_packet_length_ = length;
+      return;
+    }
+  }
   // Avoid recomputing |max_plaintext_size_| if the length does not actually
   // change.
   if (length == max_packet_length_) {
@@ -473,6 +484,11 @@
   ClearPacket();
   RemoveSoftMaxPacketLength();
   delegate_->OnSerializedPacket(std::move(packet));
+  if (next_max_packet_length_ != 0) {
+    QUICHE_DCHECK(CanSetMaxPacketLength()) << ENDPOINT;
+    SetMaxPacketLength(next_max_packet_length_);
+    next_max_packet_length_ = 0;
+  }
 }
 
 void QuicPacketCreator::ClearPacket() {
diff --git a/quiche/quic/core/quic_packet_creator.h b/quiche/quic/core/quic_packet_creator.h
index bb2303e..1a4e725 100644
--- a/quiche/quic/core/quic_packet_creator.h
+++ b/quiche/quic/core/quic_packet_creator.h
@@ -623,6 +623,9 @@
   DiversificationNonce diversification_nonce_;
   // Maximum length including headers and encryption (UDP payload length.)
   QuicByteCount max_packet_length_;
+  // Value of max_packet_length_ to be applied for the next packet, if not 0.
+  QuicByteCount next_max_packet_length_;
+
   size_t max_plaintext_size_;
   // Whether the server_connection_id is sent over the wire.
   QuicConnectionIdIncluded server_connection_id_included_;
diff --git a/quiche/quic/core/quic_packet_creator_test.cc b/quiche/quic/core/quic_packet_creator_test.cc
index e1510f5..573e4ce 100644
--- a/quiche/quic/core/quic_packet_creator_test.cc
+++ b/quiche/quic/core/quic_packet_creator_test.cc
@@ -356,6 +356,76 @@
   ProcessPacket(serialized);
 }
 
+TEST_P(QuicPacketCreatorTest, SerializePacketWithPadding) {
+  creator_.set_encryption_level(ENCRYPTION_FORWARD_SECURE);
+
+  creator_.AddFrame(QuicFrame(QuicWindowUpdateFrame()), NOT_RETRANSMISSION);
+  creator_.AddFrame(QuicFrame(QuicPaddingFrame()), NOT_RETRANSMISSION);
+  EXPECT_CALL(delegate_, OnSerializedPacket(_))
+      .WillOnce(Invoke(this, &QuicPacketCreatorTest::SaveSerializedPacket));
+  creator_.FlushCurrentPacket();
+  ASSERT_TRUE(serialized_packet_->encrypted_buffer);
+
+  EXPECT_EQ(kDefaultMaxPacketSize, serialized_packet_->encrypted_length);
+
+  DeleteSerializedPacket();
+}
+
+TEST_P(QuicPacketCreatorTest, SerializeLargerPacketWithPadding) {
+  creator_.set_encryption_level(ENCRYPTION_FORWARD_SECURE);
+  const QuicByteCount packet_size = 100 + kDefaultMaxPacketSize;
+  creator_.SetMaxPacketLength(packet_size);
+
+  creator_.AddFrame(QuicFrame(QuicWindowUpdateFrame()), NOT_RETRANSMISSION);
+  creator_.AddFrame(QuicFrame(QuicPaddingFrame()), NOT_RETRANSMISSION);
+  EXPECT_CALL(delegate_, OnSerializedPacket(_))
+      .WillOnce(Invoke(this, &QuicPacketCreatorTest::SaveSerializedPacket));
+  creator_.FlushCurrentPacket();
+  ASSERT_TRUE(serialized_packet_->encrypted_buffer);
+
+  EXPECT_EQ(packet_size, serialized_packet_->encrypted_length);
+
+  DeleteSerializedPacket();
+}
+
+TEST_P(QuicPacketCreatorTest, IncreaseMaxPacketLengthWithFramesPending) {
+  if (!GetQuicRestartFlag(quic_allow_control_frames_while_procesing)) {
+    // When this flag is not set, the call to SetMaxPacketLength()
+    // is an error which triggers a QUICHE_DCHECK.
+    return;
+  }
+
+  creator_.set_encryption_level(ENCRYPTION_FORWARD_SECURE);
+  const QuicByteCount packet_size = 100 + kDefaultMaxPacketSize;
+
+  // Since the creator has a frame queued, the packet size will not change.
+  creator_.AddFrame(QuicFrame(QuicWindowUpdateFrame()), NOT_RETRANSMISSION);
+  creator_.SetMaxPacketLength(packet_size);
+  creator_.AddFrame(QuicFrame(QuicPaddingFrame()), NOT_RETRANSMISSION);
+  EXPECT_CALL(delegate_, OnSerializedPacket(_))
+      .WillOnce(Invoke(this, &QuicPacketCreatorTest::SaveSerializedPacket));
+  creator_.FlushCurrentPacket();
+  ASSERT_TRUE(serialized_packet_->encrypted_buffer);
+
+  EXPECT_EQ(kDefaultMaxPacketSize, serialized_packet_->encrypted_length);
+
+  DeleteSerializedPacket();
+
+  // Now that the previous packet was generated, the next on will use
+  // the new larger size.
+  creator_.AddFrame(QuicFrame(QuicWindowUpdateFrame()), NOT_RETRANSMISSION);
+  creator_.AddFrame(QuicFrame(QuicPaddingFrame()), NOT_RETRANSMISSION);
+  EXPECT_CALL(delegate_, OnSerializedPacket(_))
+      .WillOnce(Invoke(this, &QuicPacketCreatorTest::SaveSerializedPacket));
+  creator_.FlushCurrentPacket();
+  ASSERT_TRUE(serialized_packet_->encrypted_buffer);
+  EXPECT_EQ(packet_size, serialized_packet_->encrypted_length);
+
+  EXPECT_EQ(packet_size, serialized_packet_->encrypted_length);
+
+  DeleteSerializedPacket();
+}
+
 TEST_P(QuicPacketCreatorTest, ConsumeCryptoDataToFillCurrentPacket) {
   std::string data = "crypto data";
   QuicFrame frame;
diff --git a/quiche/quic/core/quic_session.cc b/quiche/quic/core/quic_session.cc
index a6e8ed5..4a805aa 100644
--- a/quiche/quic/core/quic_session.cc
+++ b/quiche/quic/core/quic_session.cc
@@ -880,11 +880,15 @@
     // Suppress the write before encryption gets established.
     return false;
   }
-  if (limit_sending_max_streams_ &&
-      connection_->framer().is_processing_packet()) {
-    QUIC_RELOADABLE_FLAG_COUNT_N(quic_limit_sending_max_streams2, 3, 3);
-    // The frame will be sent when OnCanWrite() is called.
-    return false;
+  if (GetQuicRestartFlag(quic_allow_control_frames_while_procesing)) {
+    QUIC_RESTART_FLAG_COUNT_N(quic_allow_control_frames_while_procesing, 1, 3);
+  } else {
+    if (limit_sending_max_streams_ &&
+        connection_->framer().is_processing_packet()) {
+      QUIC_RELOADABLE_FLAG_COUNT_N(quic_limit_sending_max_streams2, 3, 3);
+      // The frame will be sent when OnCanWrite() is called.
+      return false;
+    }
   }
   SetTransmissionType(type);
   QuicConnection::ScopedEncryptionLevelContext context(