// 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.

// The entity that handles framing writes for a Quic client or server.
// Each QuicSession will have a connection associated with it.
//
// On the server side, the Dispatcher handles the raw reads, and hands off
// packets via ProcessUdpPacket for framing and processing.
//
// On the client side, the Connection handles the raw reads, as well as the
// processing.
//
// Note: this class is not thread-safe.

#ifndef QUICHE_QUIC_CORE_QUIC_CONNECTION_H_
#define QUICHE_QUIC_CORE_QUIC_CONNECTION_H_

#include <cstddef>
#include <cstdint>
#include <list>
#include <map>
#include <memory>
#include <string>
#include <vector>

#include "absl/strings/string_view.h"
#include "absl/types/optional.h"
#include "quiche/quic/core/congestion_control/rtt_stats.h"
#include "quiche/quic/core/crypto/quic_decrypter.h"
#include "quiche/quic/core/crypto/quic_encrypter.h"
#include "quiche/quic/core/crypto/quic_random.h"
#include "quiche/quic/core/crypto/transport_parameters.h"
#include "quiche/quic/core/frames/quic_ack_frequency_frame.h"
#include "quiche/quic/core/frames/quic_max_streams_frame.h"
#include "quiche/quic/core/frames/quic_new_connection_id_frame.h"
#include "quiche/quic/core/quic_alarm.h"
#include "quiche/quic/core/quic_alarm_factory.h"
#include "quiche/quic/core/quic_blocked_writer_interface.h"
#include "quiche/quic/core/quic_connection_context.h"
#include "quiche/quic/core/quic_connection_id.h"
#include "quiche/quic/core/quic_connection_id_manager.h"
#include "quiche/quic/core/quic_connection_stats.h"
#include "quiche/quic/core/quic_constants.h"
#include "quiche/quic/core/quic_framer.h"
#include "quiche/quic/core/quic_idle_network_detector.h"
#include "quiche/quic/core/quic_lru_cache.h"
#include "quiche/quic/core/quic_mtu_discovery.h"
#include "quiche/quic/core/quic_network_blackhole_detector.h"
#include "quiche/quic/core/quic_one_block_arena.h"
#include "quiche/quic/core/quic_packet_creator.h"
#include "quiche/quic/core/quic_packet_writer.h"
#include "quiche/quic/core/quic_packets.h"
#include "quiche/quic/core/quic_path_validator.h"
#include "quiche/quic/core/quic_ping_manager.h"
#include "quiche/quic/core/quic_sent_packet_manager.h"
#include "quiche/quic/core/quic_time.h"
#include "quiche/quic/core/quic_types.h"
#include "quiche/quic/core/uber_received_packet_manager.h"
#include "quiche/quic/platform/api/quic_export.h"
#include "quiche/quic/platform/api/quic_flags.h"
#include "quiche/quic/platform/api/quic_socket_address.h"
#include "quiche/common/platform/api/quiche_mem_slice.h"
#include "quiche/common/quiche_circular_deque.h"

namespace quic {

class QuicClock;
class QuicConfig;
class QuicConnection;

namespace test {
class QuicConnectionPeer;
}  // namespace test

// Class that receives callbacks from the connection when the path context is
// available.
class QUIC_EXPORT_PRIVATE MultiPortPathContextObserver {
 public:
  virtual void OnMultiPortPathContextAvailable(
      std::unique_ptr<QuicPathValidationContext>) = 0;

  virtual ~MultiPortPathContextObserver() = default;
};

// Class that receives callbacks from the connection when frames are received
// and when other interesting events happen.
class QUIC_EXPORT_PRIVATE QuicConnectionVisitorInterface {
 public:
  virtual ~QuicConnectionVisitorInterface() {}

  // A simple visitor interface for dealing with a data frame.
  virtual void OnStreamFrame(const QuicStreamFrame& frame) = 0;

  // Called when a CRYPTO frame containing handshake data is received.
  virtual void OnCryptoFrame(const QuicCryptoFrame& frame) = 0;

  // The session should process the WINDOW_UPDATE frame, adjusting both stream
  // and connection level flow control windows.
  virtual void OnWindowUpdateFrame(const QuicWindowUpdateFrame& frame) = 0;

  // A BLOCKED frame indicates the peer is flow control blocked
  // on a specified stream.
  virtual void OnBlockedFrame(const QuicBlockedFrame& frame) = 0;

  // Called when the stream is reset by the peer.
  virtual void OnRstStream(const QuicRstStreamFrame& frame) = 0;

  // Called when the connection is going away according to the peer.
  virtual void OnGoAway(const QuicGoAwayFrame& frame) = 0;

  // Called when |message| has been received.
  virtual void OnMessageReceived(absl::string_view message) = 0;

  // Called when a HANDSHAKE_DONE frame has been received.
  virtual void OnHandshakeDoneReceived() = 0;

  // Called when a NEW_TOKEN frame has been received.
  virtual void OnNewTokenReceived(absl::string_view token) = 0;

  // Called when a MAX_STREAMS frame has been received from the peer.
  virtual bool OnMaxStreamsFrame(const QuicMaxStreamsFrame& frame) = 0;

  // Called when a STREAMS_BLOCKED frame has been received from the peer.
  virtual bool OnStreamsBlockedFrame(const QuicStreamsBlockedFrame& frame) = 0;

  // Called when the connection is closed either locally by the framer, or
  // remotely by the peer.
  virtual void OnConnectionClosed(const QuicConnectionCloseFrame& frame,
                                  ConnectionCloseSource source) = 0;

  // Called when the connection failed to write because the socket was blocked.
  virtual void OnWriteBlocked() = 0;

  // Called once a specific QUIC version is agreed by both endpoints.
  virtual void OnSuccessfulVersionNegotiation(
      const ParsedQuicVersion& version) = 0;

  // Called when a packet has been received by the connection, after being
  // validated and parsed. Only called when the client receives a valid packet
  // or the server receives a connectivity probing packet.
  // |is_connectivity_probe| is true if the received packet is a connectivity
  // probe.
  virtual void OnPacketReceived(const QuicSocketAddress& self_address,
                                const QuicSocketAddress& peer_address,
                                bool is_connectivity_probe) = 0;

  // Called when a blocked socket becomes writable.
  virtual void OnCanWrite() = 0;

  // Called when the connection experiences a change in congestion window.
  virtual void OnCongestionWindowChange(QuicTime now) = 0;

  // Called when the connection receives a packet from a migrated client.
  virtual void OnConnectionMigration(AddressChangeType type) = 0;

  // Called when the peer seems unreachable over the current path.
  virtual void OnPathDegrading() = 0;

  // Called when forward progress made after path degrading.
  virtual void OnForwardProgressMadeAfterPathDegrading() = 0;

  // Called when the connection sends ack after
  // max_consecutive_num_packets_with_no_retransmittable_frames_ consecutive not
  // retransmittable packets sent. To instigate an ack from peer, a
  // retransmittable frame needs to be added.
  virtual void OnAckNeedsRetransmittableFrame() = 0;

  // Called when an AckFrequency frame need to be sent.
  virtual void SendAckFrequency(const QuicAckFrequencyFrame& frame) = 0;

  // Called to send a NEW_CONNECTION_ID frame.
  virtual void SendNewConnectionId(const QuicNewConnectionIdFrame& frame) = 0;

  // Called to send a RETIRE_CONNECTION_ID frame.
  virtual void SendRetireConnectionId(uint64_t sequence_number) = 0;

  // Called when server starts to use a server issued connection ID. Returns
  // true if this connection ID hasn't been used by another connection.
  virtual bool MaybeReserveConnectionId(
      const QuicConnectionId& server_connection_id) = 0;

  // Called when server stops to use a server issued connection ID.
  virtual void OnServerConnectionIdRetired(
      const QuicConnectionId& server_connection_id) = 0;

  // Called to ask if the visitor wants to schedule write resumption as it both
  // has pending data to write, and is able to write (e.g. based on flow control
  // limits).
  // Writes may be pending because they were write-blocked, congestion-throttled
  // or yielded to other connections.
  virtual bool WillingAndAbleToWrite() const = 0;

  // Called to ask if the connection should be kept alive and prevented
  // from timing out, for example if there are outstanding application
  // transactions expecting a response.
  virtual bool ShouldKeepConnectionAlive() const = 0;

  // Called to retrieve streams information for logging purpose.
  virtual std::string GetStreamsInfoForLogging() const = 0;

  // Called when a self address change is observed. Returns true if self address
  // change is allowed.
  virtual bool AllowSelfAddressChange() const = 0;

  // Called to get current handshake state.
  virtual HandshakeState GetHandshakeState() const = 0;

  // Called when a STOP_SENDING frame has been received.
  virtual void OnStopSendingFrame(const QuicStopSendingFrame& frame) = 0;

  // Called when a packet of encryption |level| has been successfully decrypted.
  virtual void OnPacketDecrypted(EncryptionLevel level) = 0;

  // Called when a 1RTT packet has been acknowledged.
  virtual void OnOneRttPacketAcknowledged() = 0;

  // Called when a packet of ENCRYPTION_HANDSHAKE gets sent.
  virtual void OnHandshakePacketSent() = 0;

  // Called when a key update has occurred.
  virtual void OnKeyUpdate(KeyUpdateReason reason) = 0;

  // Called to generate a decrypter for the next key phase. Each call should
  // generate the key for phase n+1.
  virtual std::unique_ptr<QuicDecrypter>
  AdvanceKeysAndCreateCurrentOneRttDecrypter() = 0;

  // Called to generate an encrypter for the same key phase of the last
  // decrypter returned by AdvanceKeysAndCreateCurrentOneRttDecrypter().
  virtual std::unique_ptr<QuicEncrypter> CreateCurrentOneRttEncrypter() = 0;

  // Called when connection is being closed right before a CONNECTION_CLOSE
  // frame is serialized, but only on the server and only if forward secure
  // encryption has already been established.
  virtual void BeforeConnectionCloseSent() = 0;

  // Called by the server to validate |token| in received INITIAL packets.
  // Consider the client address gets validated (and therefore remove
  // amplification factor) once the |token| gets successfully validated.
  virtual bool ValidateToken(absl::string_view token) = 0;

  // Called by the server to send another token.
  // Return false if the crypto stream fail to generate one.
  virtual bool MaybeSendAddressToken() = 0;

  // When bandwidth update alarms.
  virtual void OnBandwidthUpdateTimeout() = 0;

  // Runs OnMultiPortPathContextAvailable() from |context_observer| with context
  // needed for the connection to probe on the alternative path. The callback
  // must be called exactly once. May run OnMultiPortPathContextAvailable()
  // synchronously or asynchronously. If OnMultiPortPathContextAvailable() is
  // run asynchronously, it must be called on the same thread as QuicConnection
  // is not thread safe.
  virtual void CreateContextForMultiPortPath(
      std::unique_ptr<MultiPortPathContextObserver> context_observer) = 0;

  // Migrate to the multi-port path which is identified by |context|.
  virtual void MigrateToMultiPortPath(
      std::unique_ptr<QuicPathValidationContext> context) = 0;

  // Called when the client receives a preferred address from its peer.
  virtual void OnServerPreferredAddressAvailable(
      const QuicSocketAddress& server_preferred_address) = 0;
};

// Interface which gets callbacks from the QuicConnection at interesting
// points.  Implementations must not mutate the state of the connection
// as a result of these callbacks.
class QUIC_EXPORT_PRIVATE QuicConnectionDebugVisitor
    : public QuicSentPacketManager::DebugDelegate {
 public:
  ~QuicConnectionDebugVisitor() override {}

  // Called when a packet has been sent.
  virtual void OnPacketSent(QuicPacketNumber /*packet_number*/,
                            QuicPacketLength /*packet_length*/,
                            bool /*has_crypto_handshake*/,
                            TransmissionType /*transmission_type*/,
                            EncryptionLevel /*encryption_level*/,
                            const QuicFrames& /*retransmittable_frames*/,
                            const QuicFrames& /*nonretransmittable_frames*/,
                            QuicTime /*sent_time*/) {}

  // Called when a coalesced packet is successfully serialized.
  virtual void OnCoalescedPacketSent(
      const QuicCoalescedPacket& /*coalesced_packet*/, size_t /*length*/) {}

  // Called when a PING frame has been sent.
  virtual void OnPingSent() {}

  // Called when a packet has been received, but before it is
  // validated or parsed.
  virtual void OnPacketReceived(const QuicSocketAddress& /*self_address*/,
                                const QuicSocketAddress& /*peer_address*/,
                                const QuicEncryptedPacket& /*packet*/) {}

  // Called when the unauthenticated portion of the header has been parsed.
  virtual void OnUnauthenticatedHeader(const QuicPacketHeader& /*header*/) {}

  // Called when a packet is received with a connection id that does not
  // match the ID of this connection.
  virtual void OnIncorrectConnectionId(QuicConnectionId /*connection_id*/) {}

  // Called when an undecryptable packet has been received. If |dropped| is
  // true, the packet has been dropped. Otherwise, the packet will be queued and
  // connection will attempt to process it later.
  virtual void OnUndecryptablePacket(EncryptionLevel /*decryption_level*/,
                                     bool /*dropped*/) {}

  // Called when attempting to process a previously undecryptable packet.
  virtual void OnAttemptingToProcessUndecryptablePacket(
      EncryptionLevel /*decryption_level*/) {}

  // Called when a duplicate packet has been received.
  virtual void OnDuplicatePacket(QuicPacketNumber /*packet_number*/) {}

  // Called when the protocol version on the received packet doensn't match
  // current protocol version of the connection.
  virtual void OnProtocolVersionMismatch(ParsedQuicVersion /*version*/) {}

  // Called when the complete header of a packet has been parsed.
  virtual void OnPacketHeader(const QuicPacketHeader& /*header*/,
                              QuicTime /*receive_time*/,
                              EncryptionLevel /*level*/) {}

  // Called when a StreamFrame has been parsed.
  virtual void OnStreamFrame(const QuicStreamFrame& /*frame*/) {}

  // Called when a CRYPTO frame containing handshake data is received.
  virtual void OnCryptoFrame(const QuicCryptoFrame& /*frame*/) {}

  // Called when a QuicPaddingFrame has been parsed.
  virtual void OnPaddingFrame(const QuicPaddingFrame& /*frame*/) {}

  // Called when a Ping has been parsed.
  virtual void OnPingFrame(const QuicPingFrame& /*frame*/,
                           QuicTime::Delta /*ping_received_delay*/) {}

  // Called when a GoAway has been parsed.
  virtual void OnGoAwayFrame(const QuicGoAwayFrame& /*frame*/) {}

  // Called when a RstStreamFrame has been parsed.
  virtual void OnRstStreamFrame(const QuicRstStreamFrame& /*frame*/) {}

  // Called when a ConnectionCloseFrame has been parsed. All forms
  // of CONNECTION CLOSE are handled, Google QUIC, IETF QUIC
  // CONNECTION CLOSE/Transport and IETF QUIC CONNECTION CLOSE/Application
  virtual void OnConnectionCloseFrame(
      const QuicConnectionCloseFrame& /*frame*/) {}

  // Called when a WindowUpdate has been parsed.
  virtual void OnWindowUpdateFrame(const QuicWindowUpdateFrame& /*frame*/,
                                   const QuicTime& /*receive_time*/) {}

  // Called when a BlockedFrame has been parsed.
  virtual void OnBlockedFrame(const QuicBlockedFrame& /*frame*/) {}

  // Called when a NewConnectionIdFrame has been parsed.
  virtual void OnNewConnectionIdFrame(
      const QuicNewConnectionIdFrame& /*frame*/) {}

  // Called when a RetireConnectionIdFrame has been parsed.
  virtual void OnRetireConnectionIdFrame(
      const QuicRetireConnectionIdFrame& /*frame*/) {}

  // Called when a NewTokenFrame has been parsed.
  virtual void OnNewTokenFrame(const QuicNewTokenFrame& /*frame*/) {}

  // Called when a MessageFrame has been parsed.
  virtual void OnMessageFrame(const QuicMessageFrame& /*frame*/) {}

  // Called when a HandshakeDoneFrame has been parsed.
  virtual void OnHandshakeDoneFrame(const QuicHandshakeDoneFrame& /*frame*/) {}

  // Called when a version negotiation packet has been received.
  virtual void OnVersionNegotiationPacket(
      const QuicVersionNegotiationPacket& /*packet*/) {}

  // Called when the connection is closed.
  virtual void OnConnectionClosed(const QuicConnectionCloseFrame& /*frame*/,
                                  ConnectionCloseSource /*source*/) {}

  // Called when the version negotiation is successful.
  virtual void OnSuccessfulVersionNegotiation(
      const ParsedQuicVersion& /*version*/) {}

  // Called when a CachedNetworkParameters is sent to the client.
  virtual void OnSendConnectionState(
      const CachedNetworkParameters& /*cached_network_params*/) {}

  // Called when a CachedNetworkParameters are received from the client.
  virtual void OnReceiveConnectionState(
      const CachedNetworkParameters& /*cached_network_params*/) {}

  // Called when the connection parameters are set from the supplied
  // |config|.
  virtual void OnSetFromConfig(const QuicConfig& /*config*/) {}

  // Called when RTT may have changed, including when an RTT is read from
  // the config.
  virtual void OnRttChanged(QuicTime::Delta /*rtt*/) const {}

  // Called when a StopSendingFrame has been parsed.
  virtual void OnStopSendingFrame(const QuicStopSendingFrame& /*frame*/) {}

  // Called when a PathChallengeFrame has been parsed.
  virtual void OnPathChallengeFrame(const QuicPathChallengeFrame& /*frame*/) {}

  // Called when a PathResponseFrame has been parsed.
  virtual void OnPathResponseFrame(const QuicPathResponseFrame& /*frame*/) {}

  // Called when a StreamsBlockedFrame has been parsed.
  virtual void OnStreamsBlockedFrame(const QuicStreamsBlockedFrame& /*frame*/) {
  }

  // Called when a MaxStreamsFrame has been parsed.
  virtual void OnMaxStreamsFrame(const QuicMaxStreamsFrame& /*frame*/) {}

  // Called when an AckFrequencyFrame has been parsed.
  virtual void OnAckFrequencyFrame(const QuicAckFrequencyFrame& /*frame*/) {}

  // Called when |count| packet numbers have been skipped.
  virtual void OnNPacketNumbersSkipped(QuicPacketCount /*count*/,
                                       QuicTime /*now*/) {}

  // Called when a packet is serialized but discarded (i.e. not sent).
  virtual void OnPacketDiscarded(const SerializedPacket& /*packet*/) {}

  // Called for QUIC+TLS versions when we send transport parameters.
  virtual void OnTransportParametersSent(
      const TransportParameters& /*transport_parameters*/) {}

  // Called for QUIC+TLS versions when we receive transport parameters.
  virtual void OnTransportParametersReceived(
      const TransportParameters& /*transport_parameters*/) {}

  // Called for QUIC+TLS versions when we resume cached transport parameters for
  // 0-RTT.
  virtual void OnTransportParametersResumed(
      const TransportParameters& /*transport_parameters*/) {}

  // Called for QUIC+TLS versions when 0-RTT is rejected.
  virtual void OnZeroRttRejected(int /*reject_reason*/) {}

  // Called for QUIC+TLS versions when 0-RTT packet gets acked.
  virtual void OnZeroRttPacketAcked() {}

  // Called on peer address change.
  virtual void OnPeerAddressChange(AddressChangeType /*type*/,
                                   QuicTime::Delta /*connection_time*/) {}

  // Called after peer migration is validated.
  virtual void OnPeerMigrationValidated(QuicTime::Delta /*connection_time*/) {}

  // Called after an ClientHelloInner is encrypted and sent as a client.
  virtual void OnEncryptedClientHelloSent(absl::string_view /*client_hello*/) {}

  // Called after an ClientHelloInner is received and decrypted as a server.
  virtual void OnEncryptedClientHelloReceived(
      absl::string_view /*client_hello*/) {}
};

class QUIC_EXPORT_PRIVATE QuicConnectionHelperInterface {
 public:
  virtual ~QuicConnectionHelperInterface() {}

  // Returns a QuicClock to be used for all time related functions.
  virtual const QuicClock* GetClock() const = 0;

  // Returns a QuicRandom to be used for all random number related functions.
  virtual QuicRandom* GetRandomGenerator() = 0;

  // Returns a QuicheBufferAllocator to be used for stream send buffers.
  virtual quiche::QuicheBufferAllocator* GetStreamSendBufferAllocator() = 0;
};

class QUIC_EXPORT_PRIVATE QuicConnection
    : public QuicFramerVisitorInterface,
      public QuicBlockedWriterInterface,
      public QuicPacketCreator::DelegateInterface,
      public QuicSentPacketManager::NetworkChangeVisitor,
      public QuicNetworkBlackholeDetector::Delegate,
      public QuicIdleNetworkDetector::Delegate,
      public QuicPathValidator::SendDelegate,
      public QuicConnectionIdManagerVisitorInterface,
      public QuicPingManager::Delegate {
 public:
  // Constructs a new QuicConnection for |connection_id| and
  // |initial_peer_address| using |writer| to write packets. |owns_writer|
  // specifies whether the connection takes ownership of |writer|. |helper| must
  // outlive this connection.
  QuicConnection(QuicConnectionId server_connection_id,
                 QuicSocketAddress initial_self_address,
                 QuicSocketAddress initial_peer_address,
                 QuicConnectionHelperInterface* helper,
                 QuicAlarmFactory* alarm_factory, QuicPacketWriter* writer,
                 bool owns_writer, Perspective perspective,
                 const ParsedQuicVersionVector& supported_versions,
                 ConnectionIdGeneratorInterface& generator);
  QuicConnection(const QuicConnection&) = delete;
  QuicConnection& operator=(const QuicConnection&) = delete;
  ~QuicConnection() override;

  struct MultiPortStats {
    // general rtt stats of the multi-port path.
    RttStats rtt_stats;
    // rtt stats for the multi-port path when the default path is degrading.
    RttStats rtt_stats_when_default_path_degrading;
    // number of multi-port probe failures when path is not degrading
    size_t num_multi_port_probe_failures_when_path_not_degrading = 0;
    // number of multi-port probe failure when path is degrading
    size_t num_multi_port_probe_failures_when_path_degrading = 0;
    // number of total multi-port path creations in a connection
    size_t num_multi_port_paths_created = 0;
  };

  // Sets connection parameters from the supplied |config|.
  void SetFromConfig(const QuicConfig& config);

  // Apply |connection_options| for this connection. Unlike SetFromConfig, this
  // can happen at anytime in the life of a connection.
  // Note there is no guarantee that all options can be applied. Components will
  // only apply cherrypicked options that make sense at the time of the call.
  void ApplyConnectionOptions(const QuicTagVector& connection_options);

  // Called by the session when sending connection state to the client.
  virtual void OnSendConnectionState(
      const CachedNetworkParameters& cached_network_params);

  // Called by the session when receiving connection state from the client.
  virtual void OnReceiveConnectionState(
      const CachedNetworkParameters& cached_network_params);

  // Called by the Session when the client has provided CachedNetworkParameters.
  virtual void ResumeConnectionState(
      const CachedNetworkParameters& cached_network_params,
      bool max_bandwidth_resumption);

  // Called by the Session when a max pacing rate for the connection is needed.
  virtual void SetMaxPacingRate(QuicBandwidth max_pacing_rate);

  // Allows the client to adjust network parameters based on external
  // information.
  void AdjustNetworkParameters(
      const SendAlgorithmInterface::NetworkParams& params);
  void AdjustNetworkParameters(QuicBandwidth bandwidth, QuicTime::Delta rtt,
                               bool allow_cwnd_to_decrease);

  // Install a loss detection tuner. Must be called before OnConfigNegotiated.
  void SetLossDetectionTuner(
      std::unique_ptr<LossDetectionTunerInterface> tuner);
  // Called by the session when session->is_configured() becomes true.
  void OnConfigNegotiated();

  // Returns the max pacing rate for the connection.
  virtual QuicBandwidth MaxPacingRate() const;

  // Sends crypto handshake messages of length |write_length| to the peer in as
  // few packets as possible. Returns the number of bytes consumed from the
  // data.
  virtual size_t SendCryptoData(EncryptionLevel level, size_t write_length,
                                QuicStreamOffset offset);

  // Send the data of length |write_length| to the peer in as few packets as
  // possible. Returns the number of bytes consumed from data, and a boolean
  // indicating if the fin bit was consumed.  This does not indicate the data
  // has been sent on the wire: it may have been turned into a packet and queued
  // if the socket was unexpectedly blocked.
  virtual QuicConsumedData SendStreamData(QuicStreamId id, size_t write_length,
                                          QuicStreamOffset offset,
                                          StreamSendingState state);

  // Send |frame| to the peer. Returns true if frame is consumed, false
  // otherwise.
  virtual bool SendControlFrame(const QuicFrame& frame);

  // Called when stream |id| is reset because of |error|.
  virtual void OnStreamReset(QuicStreamId id, QuicRstStreamErrorCode error);

  // Closes the connection.
  // |connection_close_behavior| determines whether or not a connection close
  // packet is sent to the peer.
  virtual void CloseConnection(
      QuicErrorCode error, const std::string& details,
      ConnectionCloseBehavior connection_close_behavior);
  // Closes the connection, specifying the wire error code |ietf_error|
  // explicitly.
  virtual void CloseConnection(
      QuicErrorCode error, QuicIetfTransportErrorCodes ietf_error,
      const std::string& details,
      ConnectionCloseBehavior connection_close_behavior);

  QuicConnectionStats& mutable_stats() { return stats_; }

  // Returns statistics tracked for this connection.
  const QuicConnectionStats& GetStats();

  // Processes an incoming UDP packet (consisting of a QuicEncryptedPacket) from
  // the peer.
  // In a client, the packet may be "stray" and have a different connection ID
  // than that of this connection.
  virtual void ProcessUdpPacket(const QuicSocketAddress& self_address,
                                const QuicSocketAddress& peer_address,
                                const QuicReceivedPacket& packet);

  // QuicBlockedWriterInterface
  // Called when the underlying connection becomes writable to allow queued
  // writes to happen.
  void OnBlockedWriterCanWrite() override;

  bool IsWriterBlocked() const override {
    return writer_ != nullptr && writer_->IsWriteBlocked();
  }

  // Called when the caller thinks it's worth a try to write.
  // TODO(fayang): consider unifying this with QuicSession::OnCanWrite.
  virtual void OnCanWrite();

  // Called when an error occurs while attempting to write a packet to the
  // network.
  void OnWriteError(int error_code);

  // Whether |result| represents a MSG TOO BIG write error.
  bool IsMsgTooBig(const QuicPacketWriter* writer, const WriteResult& result);

  // If the socket is not blocked, writes queued packets.
  void WriteIfNotBlocked();

  // Set the packet writer.
  void SetQuicPacketWriter(QuicPacketWriter* writer, bool owns_writer) {
    QUICHE_DCHECK(writer != nullptr);
    if (writer_ != nullptr && owns_writer_) {
      delete writer_;
    }
    writer_ = writer;
    owns_writer_ = owns_writer;
  }

  // Set self address.
  void SetSelfAddress(QuicSocketAddress address) {
    default_path_.self_address = address;
  }

  // The version of the protocol this connection is using.
  QuicTransportVersion transport_version() const {
    return framer_.transport_version();
  }

  ParsedQuicVersion version() const { return framer_.version(); }

  // The versions of the protocol that this connection supports.
  const ParsedQuicVersionVector& supported_versions() const {
    return framer_.supported_versions();
  }

  // Mark version negotiated for this connection. Once called, the connection
  // will ignore received version negotiation packets.
  void SetVersionNegotiated() { version_negotiated_ = true; }

  // From QuicFramerVisitorInterface
  void OnError(QuicFramer* framer) override;
  bool OnProtocolVersionMismatch(ParsedQuicVersion received_version) override;
  void OnPacket() override;
  void OnVersionNegotiationPacket(
      const QuicVersionNegotiationPacket& packet) override;
  void OnRetryPacket(QuicConnectionId original_connection_id,
                     QuicConnectionId new_connection_id,
                     absl::string_view retry_token,
                     absl::string_view retry_integrity_tag,
                     absl::string_view retry_without_tag) override;
  bool OnUnauthenticatedPublicHeader(const QuicPacketHeader& header) override;
  bool OnUnauthenticatedHeader(const QuicPacketHeader& header) override;
  void OnDecryptedPacket(size_t length, EncryptionLevel level) override;
  bool OnPacketHeader(const QuicPacketHeader& header) override;
  void OnCoalescedPacket(const QuicEncryptedPacket& packet) override;
  void OnUndecryptablePacket(const QuicEncryptedPacket& packet,
                             EncryptionLevel decryption_level,
                             bool has_decryption_key) override;
  bool OnStreamFrame(const QuicStreamFrame& frame) override;
  bool OnCryptoFrame(const QuicCryptoFrame& frame) override;
  bool OnAckFrameStart(QuicPacketNumber largest_acked,
                       QuicTime::Delta ack_delay_time) override;
  bool OnAckRange(QuicPacketNumber start, QuicPacketNumber end) override;
  bool OnAckTimestamp(QuicPacketNumber packet_number,
                      QuicTime timestamp) override;
  bool OnAckFrameEnd(QuicPacketNumber start,
                     const absl::optional<QuicEcnCounts>& ecn_counts) override;
  bool OnStopWaitingFrame(const QuicStopWaitingFrame& frame) override;
  bool OnPaddingFrame(const QuicPaddingFrame& frame) override;
  bool OnPingFrame(const QuicPingFrame& frame) override;
  bool OnRstStreamFrame(const QuicRstStreamFrame& frame) override;
  bool OnConnectionCloseFrame(const QuicConnectionCloseFrame& frame) override;
  bool OnStopSendingFrame(const QuicStopSendingFrame& frame) override;
  bool OnPathChallengeFrame(const QuicPathChallengeFrame& frame) override;
  bool OnPathResponseFrame(const QuicPathResponseFrame& frame) override;
  bool OnGoAwayFrame(const QuicGoAwayFrame& frame) override;
  bool OnMaxStreamsFrame(const QuicMaxStreamsFrame& frame) override;
  bool OnStreamsBlockedFrame(const QuicStreamsBlockedFrame& frame) override;
  bool OnWindowUpdateFrame(const QuicWindowUpdateFrame& frame) override;
  bool OnBlockedFrame(const QuicBlockedFrame& frame) override;
  bool OnNewConnectionIdFrame(const QuicNewConnectionIdFrame& frame) override;
  bool OnRetireConnectionIdFrame(
      const QuicRetireConnectionIdFrame& frame) override;
  bool OnNewTokenFrame(const QuicNewTokenFrame& frame) override;
  bool OnMessageFrame(const QuicMessageFrame& frame) override;
  bool OnHandshakeDoneFrame(const QuicHandshakeDoneFrame& frame) override;
  bool OnAckFrequencyFrame(const QuicAckFrequencyFrame& frame) override;
  void OnPacketComplete() override;
  bool IsValidStatelessResetToken(
      const StatelessResetToken& token) const override;
  void OnAuthenticatedIetfStatelessResetPacket(
      const QuicIetfStatelessResetPacket& packet) override;
  void OnKeyUpdate(KeyUpdateReason reason) override;
  void OnDecryptedFirstPacketInKeyPhase() override;
  std::unique_ptr<QuicDecrypter> AdvanceKeysAndCreateCurrentOneRttDecrypter()
      override;
  std::unique_ptr<QuicEncrypter> CreateCurrentOneRttEncrypter() override;

  // Whether destination connection ID is required but missing in the packet
  // creator.
  bool IsMissingDestinationConnectionID() const;
  // QuicPacketCreator::DelegateInterface
  bool ShouldGeneratePacket(HasRetransmittableData retransmittable,
                            IsHandshake handshake) override;
  const QuicFrames MaybeBundleAckOpportunistically() override;
  QuicPacketBuffer GetPacketBuffer() override;
  void OnSerializedPacket(SerializedPacket packet) override;
  void OnUnrecoverableError(QuicErrorCode error,
                            const std::string& error_details) override;
  SerializedPacketFate GetSerializedPacketFate(
      bool is_mtu_discovery, EncryptionLevel encryption_level) override;

  // QuicSentPacketManager::NetworkChangeVisitor
  void OnCongestionChange() override;
  void OnPathMtuIncreased(QuicPacketLength packet_size) override;
  void OnInFlightEcnPacketAcked() override;
  void OnInvalidEcnFeedback() override;

  // QuicNetworkBlackholeDetector::Delegate
  void OnPathDegradingDetected() override;
  void OnBlackholeDetected() override;
  void OnPathMtuReductionDetected() override;

  // QuicIdleNetworkDetector::Delegate
  void OnHandshakeTimeout() override;
  void OnIdleNetworkDetected() override;
  void OnBandwidthUpdateTimeout() override;

  // QuicPingManager::Delegate
  void OnKeepAliveTimeout() override;
  void OnRetransmittableOnWireTimeout() override;

  // QuicConnectionIdManagerVisitorInterface
  void OnPeerIssuedConnectionIdRetired() override;
  bool SendNewConnectionId(const QuicNewConnectionIdFrame& frame) override;
  bool MaybeReserveConnectionId(const QuicConnectionId& connection_id) override;
  void OnSelfIssuedConnectionIdRetired(
      const QuicConnectionId& connection_id) override;

  // Please note, this is not a const function. For logging purpose, please use
  // ack_frame().
  const QuicFrame GetUpdatedAckFrame();

  // Called to send a new connection ID to client if the # of connection ID has
  // not exceeded the active connection ID limits.
  void MaybeSendConnectionIdToClient();

  // Called when the handshake completes. On the client side, handshake
  // completes on receipt of SHLO. On the server side, handshake completes when
  // SHLO gets ACKed (or a forward secure packet gets decrypted successfully).
  // TODO(fayang): Add a guard that this only gets called once.
  void OnHandshakeComplete();

  // Creates and probes an multi-port path if none exists.
  void MaybeCreateMultiPortPath();

  // Called in multi-port QUIC when the alternative path validation succeeds.
  // Stores the path validation context and prepares for the next validation.
  void OnMultiPortPathProbingSuccess(
      std::unique_ptr<QuicPathValidationContext> context, QuicTime start_time);

  // Probe the existing alternative path. Does not create a new alternative
  // path. This method is the callback for |multi_port_probing_alarm_|.
  virtual void MaybeProbeMultiPortPath();

  // Accessors
  void set_visitor(QuicConnectionVisitorInterface* visitor) {
    visitor_ = visitor;
  }
  void set_debug_visitor(QuicConnectionDebugVisitor* debug_visitor) {
    debug_visitor_ = debug_visitor;
    sent_packet_manager_.SetDebugDelegate(debug_visitor);
  }
  // Used in Chromium, but not internally.
  // Must only be called before ping_alarm_ is set.
  void set_keep_alive_ping_timeout(QuicTime::Delta keep_alive_ping_timeout);
  // Sets an initial timeout for the ping alarm when there is no retransmittable
  // data in flight, allowing for a more aggressive ping alarm in that case.
  void set_initial_retransmittable_on_wire_timeout(
      QuicTime::Delta retransmittable_on_wire_timeout);
  // Used in Chromium, but not internally.
  void set_creator_debug_delegate(QuicPacketCreator::DebugDelegate* visitor) {
    packet_creator_.set_debug_delegate(visitor);
  }
  const QuicSocketAddress& self_address() const {
    return default_path_.self_address;
  }
  const QuicSocketAddress& peer_address() const { return direct_peer_address_; }
  const QuicSocketAddress& effective_peer_address() const {
    return default_path_.peer_address;
  }

  // Returns the server connection ID used on the default path.
  const QuicConnectionId& connection_id() const {
    return default_path_.server_connection_id;
  }

  const QuicConnectionId& client_connection_id() const {
    return default_path_.client_connection_id;
  }
  void set_client_connection_id(QuicConnectionId client_connection_id);
  const QuicClock* clock() const { return clock_; }
  QuicRandom* random_generator() const { return random_generator_; }
  QuicByteCount max_packet_length() const;
  void SetMaxPacketLength(QuicByteCount length);

  size_t mtu_probe_count() const { return mtu_probe_count_; }

  bool connected() const { return connected_; }

  // Must only be called on client connections.
  const ParsedQuicVersionVector& server_supported_versions() const {
    QUICHE_DCHECK_EQ(Perspective::IS_CLIENT, perspective_);
    return server_supported_versions_;
  }

  bool HasQueuedPackets() const { return !buffered_packets_.empty(); }
  // Testing only. TODO(ianswett): Use a peer instead.
  size_t NumQueuedPackets() const { return buffered_packets_.size(); }

  // Returns true if the connection has queued packets or frames.
  bool HasQueuedData() const;

  // Sets the handshake and idle state connection timeouts.
  void SetNetworkTimeouts(QuicTime::Delta handshake_timeout,
                          QuicTime::Delta idle_timeout);

  void SetMultiPortProbingInterval(QuicTime::Delta probing_interval) {
    multi_port_probing_interval_ = probing_interval;
  }

  const MultiPortStats* multi_port_stats() const {
    return multi_port_stats_.get();
  }

  // Sets up a packet with an QuicAckFrame and sends it out.
  void SendAck();

  // Called when an RTO fires.  Resets the retransmission alarm if there are
  // remaining unacked packets.
  void OnRetransmissionTimeout();

  // Mark all sent 0-RTT encrypted packets for retransmission. Called when new
  // 0-RTT or 1-RTT key is available in gQUIC, or when 0-RTT is rejected in IETF
  // QUIC. |reject_reason| is used in TLS-QUIC to log why 0-RTT was rejected.
  void MarkZeroRttPacketsForRetransmission(int reject_reason);

  // Calls |sent_packet_manager_|'s NeuterUnencryptedPackets. Used when the
  // connection becomes forward secure and hasn't received acks for all packets.
  void NeuterUnencryptedPackets();

  // Changes the encrypter used for level |level| to |encrypter|.
  void SetEncrypter(EncryptionLevel level,
                    std::unique_ptr<QuicEncrypter> encrypter);

  // Called to remove encrypter of encryption |level|.
  void RemoveEncrypter(EncryptionLevel level);

  // SetNonceForPublicHeader sets the nonce that will be transmitted in the
  // header of each packet encrypted at the initial encryption level decrypted.
  // This should only be called on the server side.
  void SetDiversificationNonce(const DiversificationNonce& nonce);

  // SetDefaultEncryptionLevel sets the encryption level that will be applied
  // to new packets.
  void SetDefaultEncryptionLevel(EncryptionLevel level);

  // SetDecrypter sets the primary decrypter, replacing any that already exists.
  // If an alternative decrypter is in place then the function QUICHE_DCHECKs.
  // This is intended for cases where one knows that future packets will be
  // using the new decrypter and the previous decrypter is now obsolete. |level|
  // indicates the encryption level of the new decrypter.
  void SetDecrypter(EncryptionLevel level,
                    std::unique_ptr<QuicDecrypter> decrypter);

  // SetAlternativeDecrypter sets a decrypter that may be used to decrypt
  // future packets. |level| indicates the encryption level of the decrypter. If
  // |latch_once_used| is true, then the first time that the decrypter is
  // successful it will replace the primary decrypter.  Otherwise both
  // decrypters will remain active and the primary decrypter will be the one
  // last used.
  void SetAlternativeDecrypter(EncryptionLevel level,
                               std::unique_ptr<QuicDecrypter> decrypter,
                               bool latch_once_used);

  void InstallDecrypter(EncryptionLevel level,
                        std::unique_ptr<QuicDecrypter> decrypter);
  void RemoveDecrypter(EncryptionLevel level);

  // Discard keys for the previous key phase.
  void DiscardPreviousOneRttKeys();

  // Returns true if it is currently allowed to initiate a key update.
  bool IsKeyUpdateAllowed() const;

  // Returns true if packets have been sent in the current 1-RTT key phase but
  // none of these packets have been acked.
  bool HaveSentPacketsInCurrentKeyPhaseButNoneAcked() const;

  // Returns the count of packets received that appeared to attempt a key
  // update but failed decryption that have been received since the last
  // successfully decrypted packet.
  QuicPacketCount PotentialPeerKeyUpdateAttemptCount() const;

  // Increment the key phase. It is a bug to call this when IsKeyUpdateAllowed()
  // is false. Returns false on error.
  bool InitiateKeyUpdate(KeyUpdateReason reason);

  const QuicDecrypter* decrypter() const;
  const QuicDecrypter* alternative_decrypter() const;

  Perspective perspective() const { return perspective_; }

  // Allow easy overriding of truncated connection IDs.
  void set_can_truncate_connection_ids(bool can) {
    can_truncate_connection_ids_ = can;
  }

  // Returns the underlying sent packet manager.
  const QuicSentPacketManager& sent_packet_manager() const {
    return sent_packet_manager_;
  }

  // Returns the underlying sent packet manager.
  QuicSentPacketManager& sent_packet_manager() { return sent_packet_manager_; }

  UberReceivedPacketManager& received_packet_manager() {
    return uber_received_packet_manager_;
  }

  bool CanWrite(HasRetransmittableData retransmittable);

  // When the flusher is out of scope, only the outermost flusher will cause a
  // flush of the connection and set the retransmission alarm if there is one
  // pending.  In addition, this flusher can be configured to ensure that an ACK
  // frame is included in the first packet created, if there's new ack
  // information to be sent.
  class QUIC_EXPORT_PRIVATE ScopedPacketFlusher {
   public:
    explicit ScopedPacketFlusher(QuicConnection* connection);
    ~ScopedPacketFlusher();

   private:
    QuicConnection* connection_;
    // If true, when this flusher goes out of scope, flush connection and set
    // retransmission alarm if there is one pending.
    bool flush_and_set_pending_retransmission_alarm_on_delete_;
    // Latched connection's handshake_packet_sent_ on creation of this flusher.
    const bool handshake_packet_sent_;
  };

  class QUIC_EXPORT_PRIVATE ScopedEncryptionLevelContext {
   public:
    ScopedEncryptionLevelContext(QuicConnection* connection,
                                 EncryptionLevel level);
    ~ScopedEncryptionLevelContext();

   private:
    QuicConnection* connection_;
    // Latched current write encryption level on creation of this context.
    EncryptionLevel latched_encryption_level_;
  };

  QuicPacketWriter* writer() { return writer_; }
  const QuicPacketWriter* writer() const { return writer_; }

  // Sends an MTU discovery packet of size |target_mtu|.  If the packet is
  // acknowledged by the peer, the maximum packet size will be increased to
  // |target_mtu|.
  void SendMtuDiscoveryPacket(QuicByteCount target_mtu);

  // Sends a connectivity probing packet to |peer_address| with
  // |probing_writer|. If |probing_writer| is nullptr, will use default
  // packet writer to write the packet. Returns true if subsequent packets can
  // be written to the probing writer. If connection is V99, a padded IETF QUIC
  // PATH_CHALLENGE packet is transmitted; if not V99, a Google QUIC padded PING
  // packet is transmitted.
  virtual bool SendConnectivityProbingPacket(
      QuicPacketWriter* probing_writer, const QuicSocketAddress& peer_address);

  // Disable MTU discovery on this connection.
  void DisableMtuDiscovery();

  // Sends an MTU discovery packet and updates the MTU discovery alarm.
  void DiscoverMtu();

  // Sets the session notifier on the SentPacketManager.
  void SetSessionNotifier(SessionNotifierInterface* session_notifier);

  // Set data producer in framer.
  void SetDataProducer(QuicStreamFrameDataProducer* data_producer);

  // Set transmission type of next sending packets.
  void SetTransmissionType(TransmissionType type);

  // Tries to send |message| and returns the message status.
  // If |flush| is false, this will return a MESSAGE_STATUS_BLOCKED
  // when the connection is deemed unwritable.
  virtual MessageStatus SendMessage(QuicMessageId message_id,
                                    absl::Span<quiche::QuicheMemSlice> message,
                                    bool flush);

  // Returns the largest payload that will fit into a single MESSAGE frame.
  // Because overhead can vary during a connection, this method should be
  // checked for every message.
  QuicPacketLength GetCurrentLargestMessagePayload() const;
  // Returns the largest payload that will fit into a single MESSAGE frame at
  // any point during the connection.  This assumes the version and
  // connection ID lengths do not change.
  QuicPacketLength GetGuaranteedLargestMessagePayload() const;

  void SetUnackedMapInitialCapacity();

  virtual int GetUnackedMapInitialCapacity() const {
    return kDefaultUnackedPacketsInitialCapacity;
  }

  // Returns the id of the cipher last used for decrypting packets.
  uint32_t cipher_id() const;

  std::vector<std::unique_ptr<QuicEncryptedPacket>>* termination_packets() {
    return termination_packets_.get();
  }

  bool ack_frame_updated() const;

  QuicConnectionHelperInterface* helper() { return helper_; }
  const QuicConnectionHelperInterface* helper() const { return helper_; }
  QuicAlarmFactory* alarm_factory() { return alarm_factory_; }

  absl::string_view GetCurrentPacket();

  const QuicFramer& framer() const { return framer_; }

  const QuicPacketCreator& packet_creator() const { return packet_creator_; }

  EncryptionLevel encryption_level() const { return encryption_level_; }
  EncryptionLevel last_decrypted_level() const {
    return last_received_packet_info_.decrypted_level;
  }

  const QuicSocketAddress& last_packet_source_address() const {
    return last_received_packet_info_.source_address;
  }

  // This setting may be changed during the crypto handshake in order to
  // enable/disable padding of different packets in the crypto handshake.
  //
  // This setting should never be set to false in public facing endpoints. It
  // can only be set to false if there is some other mechanism of preventing
  // amplification attacks, such as ICE (plus its a non-standard quic).
  void set_fully_pad_crypto_handshake_packets(bool new_value) {
    packet_creator_.set_fully_pad_crypto_handshake_packets(new_value);
  }

  bool fully_pad_during_crypto_handshake() const {
    return packet_creator_.fully_pad_crypto_handshake_packets();
  }

  size_t min_received_before_ack_decimation() const;
  void set_min_received_before_ack_decimation(size_t new_value);

  // If |defer| is true, configures the connection to defer sending packets in
  // response to an ACK to the SendAlarm. If |defer| is false, packets may be
  // sent immediately after receiving an ACK.
  void set_defer_send_in_response_to_packets(bool defer) {
    defer_send_in_response_to_packets_ = defer;
  }

  // Sets the current per-packet options for the connection. The QuicConnection
  // does not take ownership of |options|; |options| must live for as long as
  // the QuicConnection is in use.
  void set_per_packet_options(PerPacketOptions* options) {
    per_packet_options_ = options;
  }

  bool IsPathDegrading() const { return is_path_degrading_; }

  // Attempts to process any queued undecryptable packets.
  void MaybeProcessUndecryptablePackets();

  // Queue a coalesced packet.
  void QueueCoalescedPacket(const QuicEncryptedPacket& packet);

  // Process previously queued coalesced packets. Returns true if any coalesced
  // packets have been successfully processed.
  bool MaybeProcessCoalescedPackets();

  enum PacketContent : uint8_t {
    NO_FRAMES_RECEIVED,
    // TODO(fkastenholz): Change name when we get rid of padded ping/
    // pre-version-99.
    // Also PATH CHALLENGE and PATH RESPONSE.
    FIRST_FRAME_IS_PING,
    SECOND_FRAME_IS_PADDING,
    NOT_PADDED_PING,  // Set if the packet is not {PING, PADDING}.
  };

  // Whether the handshake completes from this connection's perspective.
  bool IsHandshakeComplete() const;

  // Whether peer completes handshake. Only used with TLS handshake.
  bool IsHandshakeConfirmed() const;

  // Returns the largest received packet number sent by peer.
  QuicPacketNumber GetLargestReceivedPacket() const;

  // Sets the original destination connection ID on the connection.
  // This is called by QuicDispatcher when it has replaced the connection ID.
  void SetOriginalDestinationConnectionId(
      const QuicConnectionId& original_destination_connection_id);

  // Returns the original destination connection ID used for this connection.
  QuicConnectionId GetOriginalDestinationConnectionId() const;

  // Tells the visitor the serverside connection is no longer expecting packets
  // with the client-generated destination connection ID.
  void RetireOriginalDestinationConnectionId();

  // Called when ACK alarm goes off. Sends ACKs of those packet number spaces
  // which have expired ACK timeout. Only used when this connection supports
  // multiple packet number spaces.
  void SendAllPendingAcks();

  // Returns true if this connection supports multiple packet number spaces.
  bool SupportsMultiplePacketNumberSpaces() const;

  // For logging purpose.
  const QuicAckFrame& ack_frame() const;

  // Install encrypter and decrypter for ENCRYPTION_INITIAL using
  // |connection_id| as the first client-sent destination connection ID,
  // or the one sent after an IETF Retry.
  void InstallInitialCrypters(QuicConnectionId connection_id);

  // Called when version is considered negotiated.
  void OnSuccessfulVersionNegotiation();

  // Called when self migration succeeds after probing.
  void OnSuccessfulMigration(bool is_port_change);

  // Called for QUIC+TLS versions when we send transport parameters.
  void OnTransportParametersSent(
      const TransportParameters& transport_parameters) const;

  // Called for QUIC+TLS versions when we receive transport parameters.
  void OnTransportParametersReceived(
      const TransportParameters& transport_parameters) const;

  // Called for QUIC+TLS versions when we resume cached transport parameters for
  // 0-RTT.
  void OnTransportParametersResumed(
      const TransportParameters& transport_parameters) const;

  // Called after an ClientHelloInner is encrypted and sent as a client.
  void OnEncryptedClientHelloSent(absl::string_view client_hello) const;

  // Called after an ClientHelloInner is received and decrypted as a server.
  void OnEncryptedClientHelloReceived(absl::string_view client_hello) const;

  // Returns true if ack_alarm_ is set.
  bool HasPendingAcks() const;

  virtual void OnUserAgentIdKnown(const std::string& user_agent_id);

  // If now is close to idle timeout, returns true and sends a connectivity
  // probing packet to test the connection for liveness. Otherwise, returns
  // false.
  bool MaybeTestLiveness();

  // QuicPathValidator::SendDelegate
  // Send PATH_CHALLENGE using the given path information. If |writer| is the
  // default writer, PATH_CHALLENGE can be bundled with other frames, and the
  // containing packet can be buffered if the writer is blocked. Otherwise,
  // PATH_CHALLENGE will be written in an individual packet and it will be
  // dropped if write fails. |data_buffer| will be populated with the payload
  // for future validation.
  // Return false if the connection is closed thus the caller will not continue
  // the validation, otherwise return true.
  bool SendPathChallenge(const QuicPathFrameBuffer& data_buffer,
                         const QuicSocketAddress& self_address,
                         const QuicSocketAddress& peer_address,
                         const QuicSocketAddress& effective_peer_address,
                         QuicPacketWriter* writer) override;
  // If |writer| is the default writer and |peer_address| is the same as
  // peer_address(), return the PTO of this connection. Otherwise, return 3 *
  // kInitialRtt.
  QuicTime GetRetryTimeout(const QuicSocketAddress& peer_address_to_use,
                           QuicPacketWriter* writer_to_use) const override;

  // Start vaildating the path defined by |context| asynchronously and call the
  // |result_delegate| after validation finishes. If the connection is
  // validating another path, cancel and fail that validation before starting
  // this one.
  void ValidatePath(
      std::unique_ptr<QuicPathValidationContext> context,
      std::unique_ptr<QuicPathValidator::ResultDelegate> result_delegate,
      PathValidationReason reason);

  bool can_receive_ack_frequency_frame() const {
    return can_receive_ack_frequency_frame_;
  }

  void set_can_receive_ack_frequency_frame() {
    can_receive_ack_frequency_frame_ = true;
  }

  bool is_processing_packet() const { return framer_.is_processing_packet(); }

  bool HasPendingPathValidation() const;

  QuicPathValidationContext* GetPathValidationContext() const;

  void CancelPathValidation();

  // Returns true if the migration succeeds, otherwise returns false (e.g., no
  // available CIDs, connection disconnected, etc).
  bool MigratePath(const QuicSocketAddress& self_address,
                   const QuicSocketAddress& peer_address,
                   QuicPacketWriter* writer, bool owns_writer);

  // Called to clear the alternative_path_ when path validation failed on the
  // client side.
  void OnPathValidationFailureAtClient(
      bool is_multi_port, const QuicPathValidationContext& context);

  void SetSourceAddressTokenToSend(absl::string_view token);

  void SendPing() {
    SendPingAtLevel(framer().GetEncryptionLevelToSendApplicationData());
  }

  // Returns one server connection ID that associates the current session in the
  // session map.
  virtual QuicConnectionId GetOneActiveServerConnectionId() const;

  // Returns all server connection IDs that have not been removed from the
  // session map.
  virtual std::vector<QuicConnectionId> GetActiveServerConnectionIds() const;

  // Instantiates connection ID manager.
  void CreateConnectionIdManager();

  // Log QUIC_BUG if there is pending frames for the stream with |id|.
  void QuicBugIfHasPendingFrames(QuicStreamId id) const;

  QuicConnectionContext* context() { return &context_; }
  const QuicConnectionContext* context() const { return &context_; }

  void set_tracer(std::unique_ptr<QuicConnectionTracer> tracer) {
    context_.tracer.swap(tracer);
  }

  void set_bug_listener(std::unique_ptr<QuicBugListener> bug_listener) {
    context_.bug_listener.swap(bug_listener);
  }

  bool in_probe_time_out() const { return in_probe_time_out_; }

  QuicNetworkBlackholeDetector& blackhole_detector() {
    return blackhole_detector_;
  }

  // Ensures the network blackhole delay is longer than path degrading delay.
  static QuicTime::Delta CalculateNetworkBlackholeDelay(
      QuicTime::Delta blackhole_delay, QuicTime::Delta path_degrading_delay,
      QuicTime::Delta pto_delay);

  void DisableLivenessTesting() { liveness_testing_disabled_ = true; }

  void AddKnownServerAddress(const QuicSocketAddress& address);

  absl::optional<QuicNewConnectionIdFrame>
  MaybeIssueNewConnectionIdForPreferredAddress();

  // Kicks off validation of received server preferred address.
  void ValidateServerPreferredAddress();

  // Returns true if the client is validating the server preferred address which
  // hasn't been used before.
  bool IsValidatingServerPreferredAddress() const;

  // Called by client to start sending packets to the preferred address.
  // If |owns_writer| is true, the ownership of the writer in the |context| is
  // also passed in.
  void OnServerPreferredAddressValidated(QuicPathValidationContext& context,
                                         bool owns_writer);

  void set_sent_server_preferred_address(
      const QuicSocketAddress& sent_server_preferred_address) {
    sent_server_preferred_address_ = sent_server_preferred_address;
  }

  const QuicSocketAddress& sent_server_preferred_address() const {
    return sent_server_preferred_address_;
  }

  // True if received long packet header contains source connection ID.
  bool PeerIssuesConnectionIds() const {
    return peer_issued_cid_manager_ != nullptr;
  }

  bool ignore_gquic_probing() const { return ignore_gquic_probing_; }

  // Sets the ECN marking for all outgoing packets, assuming that the congestion
  // control supports that codepoint. QuicConnection will revert to sending
  // ECN_NOT_ECT if there is evidence the path is dropping ECN-marked packets,
  // or if the peer provides invalid ECN feedback. Returns false if the current
  // configuration prevents setting the desired codepoint.
  bool set_ecn_codepoint(QuicEcnCodepoint ecn_codepoint);

  QuicEcnCodepoint ecn_codepoint() const {
    return packet_writer_params_.ecn_codepoint;
  }

 protected:
  // Calls cancel() on all the alarms owned by this connection.
  void CancelAllAlarms();

  // Send a packet to the peer, and takes ownership of the packet if the packet
  // cannot be written immediately.
  virtual void SendOrQueuePacket(SerializedPacket packet);

  // Called after a packet is received from a new effective peer address and is
  // decrypted. Starts validation of effective peer's address change. Calls
  // OnConnectionMigration as soon as the address changed.
  void StartEffectivePeerMigration(AddressChangeType type);

  // Called when a effective peer address migration is validated.
  virtual void OnEffectivePeerMigrationValidated(bool is_migration_linkable);

  // Get the effective peer address from the packet being processed. For proxied
  // connections, effective peer address is the address of the endpoint behind
  // the proxy. For non-proxied connections, effective peer address is the same
  // as peer address.
  //
  // Notes for implementations in subclasses:
  // - If the connection is not proxied, the overridden method should use the
  //   base implementation:
  //
  //       return QuicConnection::GetEffectivePeerAddressFromCurrentPacket();
  //
  // - If the connection is proxied, the overridden method may return either of
  //   the following:
  //   a) The address of the endpoint behind the proxy. The address is used to
  //      drive effective peer migration.
  //   b) An uninitialized address, meaning the effective peer address does not
  //      change.
  virtual QuicSocketAddress GetEffectivePeerAddressFromCurrentPacket() const;

  // Selects and updates the version of the protocol being used by selecting a
  // version from |available_versions| which is also supported. Returns true if
  // such a version exists, false otherwise.
  bool SelectMutualVersion(const ParsedQuicVersionVector& available_versions);

  // Returns the current per-packet options for the connection.
  PerPacketOptions* per_packet_options() { return per_packet_options_; }

  AddressChangeType active_effective_peer_migration_type() const {
    return active_effective_peer_migration_type_;
  }

  // Sends a connection close packet to the peer and includes an ACK if the ACK
  // is not empty, the |error| is not PACKET_WRITE_ERROR, and it fits.
  // |ietf_error| may optionally be be used to directly specify the wire
  // error code. Otherwise if |ietf_error| is NO_IETF_QUIC_ERROR, the
  // QuicErrorCodeToTransportErrorCode mapping of |error| will be used.
  // Caller may choose to call SendConnectionClosePacket() directly instead of
  // CloseConnection() to notify peer that the connection is going to be closed,
  // for example, when the server is tearing down. Given
  // SendConnectionClosePacket() does not close connection, multiple connection
  // close packets could be sent to the peer.
  virtual void SendConnectionClosePacket(QuicErrorCode error,
                                         QuicIetfTransportErrorCodes ietf_error,
                                         const std::string& details);

  // Returns true if the packet should be discarded and not sent.
  virtual bool ShouldDiscardPacket(EncryptionLevel encryption_level);

  // Notify various components(Session etc.) that this connection has been
  // migrated.
  virtual void OnConnectionMigration();

  // Return whether the packet being processed is a connectivity probing.
  // A packet is a connectivity probing if it is a padded ping packet with self
  // and/or peer address changes.
  bool IsCurrentPacketConnectivityProbing() const;

  // Return true iff the writer is blocked, if blocked, call
  // visitor_->OnWriteBlocked() to add the connection into the write blocked
  // list.
  bool HandleWriteBlocked();

  // Whether connection enforces anti-amplification limit.
  bool EnforceAntiAmplificationLimit() const;

  void AddBytesReceivedBeforeAddressValidation(size_t length) {
    default_path_.bytes_received_before_address_validation += length;
  }

  bool defer_send_in_response_to_packets() const {
    return defer_send_in_response_to_packets_;
  }

  ConnectionIdGeneratorInterface& connection_id_generator() const {
    return connection_id_generator_;
  }

 private:
  friend class test::QuicConnectionPeer;

  enum RetransmittableOnWireBehavior {
    DEFAULT,                           // Send packet containing a PING frame.
    SEND_FIRST_FORWARD_SECURE_PACKET,  // Send 1st 1-RTT packet.
    SEND_RANDOM_BYTES  // Send random bytes which is an unprocessable packet.
  };

  enum class MultiPortStatusOnMigration {
    kNotValidated,
    kPendingRefreshValidation,
    kWaitingForRefreshValidation,
    kMaxValue,
  };

  struct QUIC_EXPORT_PRIVATE PendingPathChallenge {
    QuicPathFrameBuffer received_path_challenge;
    QuicSocketAddress peer_address;
  };

  struct QUIC_EXPORT_PRIVATE PathState {
    PathState() = default;

    PathState(const QuicSocketAddress& alternative_self_address,
              const QuicSocketAddress& alternative_peer_address,
              const QuicConnectionId& client_connection_id,
              const QuicConnectionId& server_connection_id,
              absl::optional<StatelessResetToken> stateless_reset_token)
        : self_address(alternative_self_address),
          peer_address(alternative_peer_address),
          client_connection_id(client_connection_id),
          server_connection_id(server_connection_id),
          stateless_reset_token(stateless_reset_token) {}

    PathState(PathState&& other);

    PathState& operator=(PathState&& other);

    // Reset all the members.
    void Clear();

    QuicSocketAddress self_address;
    // The actual peer address behind the proxy if there is any.
    QuicSocketAddress peer_address;
    QuicConnectionId client_connection_id;
    QuicConnectionId server_connection_id;
    absl::optional<StatelessResetToken> stateless_reset_token;
    // True if the peer address has been validated. Address is considered
    // validated when 1) an address token of the peer address is received and
    // validated, or 2) a HANDSHAKE packet has been successfully processed on
    // this path, or 3) a path validation on this path has succeeded.
    bool validated = false;
    // Used by the sever to apply anti-amplification limit after this path
    // becomes the default path if |peer_address| hasn't been validated.
    QuicByteCount bytes_received_before_address_validation = 0;
    QuicByteCount bytes_sent_before_address_validation = 0;
    // Points to the send algorithm on the old default path while connection is
    // validating migrated peer address. Nullptr otherwise.
    std::unique_ptr<SendAlgorithmInterface> send_algorithm;
    absl::optional<RttStats> rtt_stats;
    // If true, an ECN packet was acked on this path, so the path probably isn't
    // dropping ECN-marked packets.
    bool ecn_marked_packet_acked = false;
    // How many total PTOs have fired since the connection started sending ECN
    // on this path, but before an ECN-marked packet has been acked.
    uint8_t ecn_pto_count = 0;
  };

  using QueuedPacketList = std::list<SerializedPacket>;

  // BufferedPacket stores necessary information (encrypted buffer and self/peer
  // addresses) of those packets which are serialized but failed to send because
  // socket is blocked. From unacked packet map and send algorithm's
  // perspective, buffered packets are treated as sent.
  struct QUIC_EXPORT_PRIVATE BufferedPacket {
    BufferedPacket(const SerializedPacket& packet,
                   const QuicSocketAddress& self_address,
                   const QuicSocketAddress& peer_address,
                   QuicEcnCodepoint ecn_codepoint);
    BufferedPacket(const char* encrypted_buffer,
                   QuicPacketLength encrypted_length,
                   const QuicSocketAddress& self_address,
                   const QuicSocketAddress& peer_address,
                   QuicEcnCodepoint ecn_codepoint);
    // Please note, this buffered packet contains random bytes (and is not
    // *actually* a QUIC packet).
    BufferedPacket(QuicRandom& random, QuicPacketLength encrypted_length,
                   const QuicSocketAddress& self_address,
                   const QuicSocketAddress& peer_address);
    BufferedPacket(const BufferedPacket& other) = delete;
    BufferedPacket(const BufferedPacket&& other) = delete;

    ~BufferedPacket() = default;

    std::unique_ptr<char[]> data;
    const QuicPacketLength length;
    // Self and peer addresses when the packet is serialized.
    const QuicSocketAddress self_address;
    const QuicSocketAddress peer_address;
    QuicEcnCodepoint ecn_codepoint = ECN_NOT_ECT;
  };

  // ReceivedPacketInfo comprises the received packet information.
  // TODO(fayang): move more fields to ReceivedPacketInfo.
  struct QUIC_EXPORT_PRIVATE ReceivedPacketInfo {
    explicit ReceivedPacketInfo(QuicTime receipt_time);
    ReceivedPacketInfo(const QuicSocketAddress& destination_address,
                       const QuicSocketAddress& source_address,
                       QuicTime receipt_time, QuicByteCount length,
                       QuicEcnCodepoint ecn_codepoint);

    QuicSocketAddress destination_address;
    QuicSocketAddress source_address;
    QuicTime receipt_time = QuicTime::Zero();
    bool received_bytes_counted = false;
    QuicByteCount length = 0;
    QuicConnectionId destination_connection_id;
    // Fields below are only populated if packet gets decrypted successfully.
    // TODO(fayang): consider using absl::optional for following fields.
    bool decrypted = false;
    EncryptionLevel decrypted_level = ENCRYPTION_INITIAL;
    QuicPacketHeader header;
    absl::InlinedVector<QuicFrameType, 1> frames;
    QuicEcnCodepoint ecn_codepoint = ECN_NOT_ECT;
    // Stores the actual address this packet is received on when it is received
    // on the preferred address. In this case, |destination_address| will
    // be overridden to the current default self address.
    QuicSocketAddress actual_destination_address;
  };

  QUIC_EXPORT_PRIVATE friend std::ostream& operator<<(
      std::ostream& os, const QuicConnection::ReceivedPacketInfo& info);

  // UndecrytablePacket comprises a undecryptable packet and related
  // information.
  struct QUIC_EXPORT_PRIVATE UndecryptablePacket {
    UndecryptablePacket(const QuicEncryptedPacket& packet,
                        EncryptionLevel encryption_level,
                        const ReceivedPacketInfo& packet_info)
        : packet(packet.Clone()),
          encryption_level(encryption_level),
          packet_info(packet_info) {}

    std::unique_ptr<QuicEncryptedPacket> packet;
    EncryptionLevel encryption_level;
    ReceivedPacketInfo packet_info;
  };

  // Handles the reverse path validation result depending on connection state:
  // whether the connection is validating a migrated peer address or is
  // validating an alternative path.
  class ReversePathValidationResultDelegate
      : public QuicPathValidator::ResultDelegate {
   public:
    ReversePathValidationResultDelegate(
        QuicConnection* connection,
        const QuicSocketAddress& direct_peer_address);

    void OnPathValidationSuccess(
        std::unique_ptr<QuicPathValidationContext> context,
        QuicTime start_time) override;

    void OnPathValidationFailure(
        std::unique_ptr<QuicPathValidationContext> context) override;

   private:
    QuicConnection* connection_;
    QuicSocketAddress original_direct_peer_address_;
    // TODO(b/205023946) Debug-only fields, to be deprecated after the bug is
    // fixed.
    QuicSocketAddress peer_address_default_path_;
    QuicSocketAddress peer_address_alternative_path_;
    AddressChangeType active_effective_peer_migration_type_;
  };

  class ContextObserver final : public MultiPortPathContextObserver {
   public:
    explicit ContextObserver(QuicConnection* connection)
        : connection_(connection) {}

    void OnMultiPortPathContextAvailable(
        std::unique_ptr<QuicPathValidationContext> path_context) override;

   private:
    QuicConnection* connection_;
  };

  // Keeps an ongoing alternative path. The connection will not migrate upon
  // validation success.
  class MultiPortPathValidationResultDelegate
      : public QuicPathValidator::ResultDelegate {
   public:
    MultiPortPathValidationResultDelegate(QuicConnection* connection);

    void OnPathValidationSuccess(
        std::unique_ptr<QuicPathValidationContext> context,
        QuicTime start_time) override;

    void OnPathValidationFailure(
        std::unique_ptr<QuicPathValidationContext> context) override;

   private:
    QuicConnection* connection_;
  };

  // A class which sets and clears in_probe_time_out_ when entering
  // and exiting OnRetransmissionTimeout, respectively.
  class QUIC_EXPORT_PRIVATE ScopedRetransmissionTimeoutIndicator {
   public:
    // |connection| must outlive this indicator.
    explicit ScopedRetransmissionTimeoutIndicator(QuicConnection* connection);

    ~ScopedRetransmissionTimeoutIndicator();

   private:
    QuicConnection* connection_;  // Not owned.
  };

  // If peer uses non-empty connection ID, discards any buffered packets on path
  // change in IETF QUIC.
  void MaybeClearQueuedPacketsOnPathChange();

  // Notifies the visitor of the close and marks the connection as disconnected.
  // Does not send a connection close frame to the peer. It should only be
  // called by CloseConnection or OnConnectionCloseFrame, and
  // OnAuthenticatedIetfStatelessResetPacket. |ietf_error| may optionally be be
  // used to directly specify the wire error code. Otherwise if |ietf_error| is
  // NO_IETF_QUIC_ERROR, the QuicErrorCodeToTransportErrorCode mapping of
  // |error| will be used.
  void TearDownLocalConnectionState(QuicErrorCode error,
                                    QuicIetfTransportErrorCodes ietf_error,
                                    const std::string& details,
                                    ConnectionCloseSource source);
  void TearDownLocalConnectionState(const QuicConnectionCloseFrame& frame,
                                    ConnectionCloseSource source);

  // Replace server connection ID on the client side from retry packet or
  // initial packets with a different source connection ID.
  void ReplaceInitialServerConnectionId(
      const QuicConnectionId& new_server_connection_id);

  // Given the server_connection_id find if there is already a corresponding
  // client connection ID used on default/alternative path. If not, find if
  // there is an unused connection ID.
  void FindMatchingOrNewClientConnectionIdOrToken(
      const PathState& default_path, const PathState& alternative_path,
      const QuicConnectionId& server_connection_id,
      QuicConnectionId* client_connection_id,
      absl::optional<StatelessResetToken>* stateless_reset_token);

  // Returns true and sets connection IDs if (self_address, peer_address)
  // corresponds to either the default path or alternative path. Returns false
  // otherwise.
  bool FindOnPathConnectionIds(const QuicSocketAddress& self_address,
                               const QuicSocketAddress& peer_address,
                               QuicConnectionId* client_connection_id,
                               QuicConnectionId* server_connection_id) const;

  // Set default_path_ to the new_path_state and update the connection IDs in
  // packet creator accordingly.
  void SetDefaultPathState(PathState new_path_state);

  // Returns true if header contains valid server connection ID.
  bool ValidateServerConnectionId(const QuicPacketHeader& header) const;

  // Update the connection IDs when client migrates its own address
  // (with/without validation) or switches to server preferred address.
  // Returns false if required connection ID is not available.
  bool UpdateConnectionIdsOnMigration(const QuicSocketAddress& self_address,
                                      const QuicSocketAddress& peer_address);

  // Retire active peer issued connection IDs after they are no longer used on
  // any path.
  void RetirePeerIssuedConnectionIdsNoLongerOnPath();

  // When path validation fails, proactively retire peer issued connection IDs
  // no longer used on any path.
  void RetirePeerIssuedConnectionIdsOnPathValidationFailure();

  // Writes the given packet to socket, encrypted with packet's
  // encryption_level. Returns true on successful write, and false if the writer
  // was blocked and the write needs to be tried again. Notifies the
  // SentPacketManager when the write is successful and sets
  // retransmittable frames to nullptr.
  // Saves the connection close packet for later transmission, even if the
  // writer is write blocked.
  bool WritePacket(SerializedPacket* packet);

  // Enforce AEAD Confidentiality limits by iniating key update or closing
  // connection if too many packets have been encrypted with the current key.
  // Returns true if the connection was closed. Should not be called for
  // termination packets.
  bool MaybeHandleAeadConfidentialityLimits(const SerializedPacket& packet);

  // Flush packets buffered in the writer, if any.
  void FlushPackets();

  // Clears any accumulated frames from the last received packet.
  void ClearLastFrames();

  // Deletes and clears any queued packets.
  void ClearQueuedPackets();

  // Closes the connection if the sent packet manager is tracking too many
  // outstanding packets.
  void CloseIfTooManyOutstandingSentPackets();

  // Writes as many queued packets as possible.  The connection must not be
  // blocked when this is called.
  void WriteQueuedPackets();

  // Queues |packet| in the hopes that it can be decrypted in the
  // future, when a new key is installed.
  void QueueUndecryptablePacket(const QuicEncryptedPacket& packet,
                                EncryptionLevel decryption_level);

  // Sends any packets which are a response to the last packet, including both
  // acks and pending writes if an ack opened the congestion window.
  void MaybeSendInResponseToPacket();

  // Gets the least unacked packet number, which is the next packet number to be
  // sent if there are no outstanding packets.
  QuicPacketNumber GetLeastUnacked() const;

  // Sets the ping alarm to the appropriate value, if any.
  void SetPingAlarm();

  // Sets the retransmission alarm based on SentPacketManager.
  void SetRetransmissionAlarm();

  // Sets the MTU discovery alarm if necessary.
  // |sent_packet_number| is the recently sent packet number.
  void MaybeSetMtuAlarm(QuicPacketNumber sent_packet_number);

  HasRetransmittableData IsRetransmittable(const SerializedPacket& packet);
  bool IsTerminationPacket(const SerializedPacket& packet,
                           QuicErrorCode* error_code);

  // Set the size of the packet we are targeting while doing path MTU discovery.
  void SetMtuDiscoveryTarget(QuicByteCount target);

  // Returns |suggested_max_packet_size| clamped to any limits set by the
  // underlying writer, connection, or protocol.
  QuicByteCount GetLimitedMaxPacketSize(
      QuicByteCount suggested_max_packet_size);

  // Do any work which logically would be done in OnPacket but can not be
  // safely done until the packet is validated. Returns true if packet can be
  // handled, false otherwise.
  bool ProcessValidatedPacket(const QuicPacketHeader& header);

  // Returns true if received |packet_number| can be processed. Please note,
  // this is called after packet got decrypted successfully.
  bool ValidateReceivedPacketNumber(QuicPacketNumber packet_number);

  // Consider receiving crypto frame on non crypto stream as memory corruption.
  bool MaybeConsiderAsMemoryCorruption(const QuicStreamFrame& frame);

  // Check if the connection has no outstanding data to send and notify
  // congestion controller if it is the case.
  void CheckIfApplicationLimited();

  // Sets |current_packet_content_| to |type| if applicable. And
  // starts effective peer migration if current packet is confirmed not a
  // connectivity probe and |current_effective_peer_migration_type_| indicates
  // effective peer address change.
  // Returns true if connection is still alive.
  ABSL_MUST_USE_RESULT bool UpdatePacketContent(QuicFrameType type);

  // Called when last received ack frame has been processed.
  // |acked_new_packet| is true if a previously-unacked packet was acked.
  void PostProcessAfterAckFrame(bool acked_new_packet);

  // Updates the release time into the future.
  void UpdateReleaseTimeIntoFuture();

  // Sends generic path probe packet to the peer. If we are not IETF QUIC, will
  // always send a padded ping, regardless of whether this is a request or not.
  bool SendGenericPathProbePacket(QuicPacketWriter* probing_writer,
                                  const QuicSocketAddress& peer_address);

  // Called when an ACK is about to send. Resets ACK related internal states,
  // e.g., cancels ack_alarm_, resets
  // num_retransmittable_packets_received_since_last_ack_sent_ etc.
  void ResetAckStates();

  // Returns true if the ACK frame should be bundled with ACK-eliciting frame.
  bool ShouldBundleRetransmittableFrameWithAck() const;

  // Enables multiple packet number spaces support based on handshake protocol
  // and flags.
  void MaybeEnableMultiplePacketNumberSpacesSupport();

  // Called to update ACK timeout when an retransmittable frame has been parsed.
  void MaybeUpdateAckTimeout();

  // Tries to fill coalesced packet with data of higher packet space.
  void MaybeCoalescePacketOfHigherSpace();

  // Serialize and send coalesced_packet. Returns false if serialization fails
  // or the write causes errors, otherwise, returns true.
  bool FlushCoalescedPacket();

  // Returns the encryption level the connection close packet should be sent at,
  // which is the highest encryption level that peer can guarantee to process.
  EncryptionLevel GetConnectionCloseEncryptionLevel() const;

  // Called after an ACK frame is successfully processed to update largest
  // received packet number which contains an ACK frame.
  void SetLargestReceivedPacketWithAck(QuicPacketNumber new_value);

  // Called when new packets have been acknowledged or old keys have been
  // discarded.
  void OnForwardProgressMade();

  // Returns largest received packet number which contains an ACK frame.
  QuicPacketNumber GetLargestReceivedPacketWithAck() const;

  // Returns the largest packet number that has been sent.
  QuicPacketNumber GetLargestSentPacket() const;

  // Returns the largest sent packet number that has been ACKed by peer.
  QuicPacketNumber GetLargestAckedPacket() const;

  // Whether connection is limited by amplification factor.
  // If enforce_strict_amplification_factor_ is true, this will return true if
  // connection is amplification limited after sending |bytes|.
  bool LimitedByAmplificationFactor(QuicByteCount bytes) const;

  // Called before sending a packet to get packet send time and to set the
  // release time delay in |per_packet_options_|. Return the time when the
  // packet is scheduled to be released(a.k.a send time), which is NOW + delay.
  // Returns Now() and does not update release time delay if
  // |supports_release_time_| is false.
  QuicTime CalculatePacketSentTime();

  // If we have a previously validate MTU value, e.g. due to a write error,
  // revert to it and disable MTU discovery.
  // Return true iff we reverted to a previously validate MTU.
  bool MaybeRevertToPreviousMtu();

  QuicTime GetPathMtuReductionDeadline() const;

  // Returns path degrading deadline. QuicTime::Zero() means no path degrading
  // detection is needed.
  QuicTime GetPathDegradingDeadline() const;

  // Returns true if path degrading should be detected.
  bool ShouldDetectPathDegrading() const;

  // Returns network blackhole deadline. QuicTime::Zero() means no blackhole
  // detection is needed.
  QuicTime GetNetworkBlackholeDeadline() const;

  // Returns true if network blackhole should be detected.
  bool ShouldDetectBlackhole() const;

  // Returns retransmission deadline.
  QuicTime GetRetransmissionDeadline() const;

  // Validate connection IDs used during the handshake. Closes the connection
  // on validation failure.
  bool ValidateConfigConnectionIds(const QuicConfig& config);

  // Called when ACK alarm goes off. Try to bundle crypto data with ACKs.
  void MaybeBundleCryptoDataWithAcks();

  // Returns true if an undecryptable packet of |decryption_level| should be
  // buffered (such that connection can try to decrypt it later).
  bool ShouldEnqueueUnDecryptablePacket(EncryptionLevel decryption_level,
                                        bool has_decryption_key) const;

  // Returns string which contains undecryptable packets information.
  std::string UndecryptablePacketsInfo() const;

  // For Google Quic, if the current packet is connectivity probing packet, call
  // session OnPacketReceived() which eventually sends connectivity probing
  // response on server side. And no-op on client side. And for both Google Quic
  // and IETF Quic, start migration if the current packet is a non-probing
  // packet.
  // TODO(danzh) remove it when deprecating ignore_gquic_probing_.
  void MaybeRespondToConnectivityProbingOrMigration();

  // Called in IETF QUIC. Start peer migration if a non-probing frame is
  // received and the current packet number is largest received so far.
  void MaybeStartIetfPeerMigration();

  // Send PATH_RESPONSE to the given peer address.
  bool SendPathResponse(const QuicPathFrameBuffer& data_buffer,
                        const QuicSocketAddress& peer_address_to_send,
                        const QuicSocketAddress& effective_peer_address);

  // Update both connection's and packet creator's peer address.
  void UpdatePeerAddress(QuicSocketAddress peer_address);

  // Send PING at encryption level.
  void SendPingAtLevel(EncryptionLevel level);

  // Write the given packet with |self_address| and |peer_address| using
  // |writer|.
  bool WritePacketUsingWriter(std::unique_ptr<SerializedPacket> packet,
                              QuicPacketWriter* writer,
                              const QuicSocketAddress& self_address,
                              const QuicSocketAddress& peer_address,
                              bool measure_rtt);

  // Increment bytes sent/received on the alternative path if the current packet
  // is sent/received on that path.
  void MaybeUpdateBytesSentToAlternativeAddress(
      const QuicSocketAddress& peer_address, QuicByteCount sent_packet_size);
  void MaybeUpdateBytesReceivedFromAlternativeAddress(
      QuicByteCount received_packet_size);

  // TODO(danzh) pass in PathState of the incoming packet or the packet sent
  // once PathState is used in packet creator. Return true if the given self
  // address and peer address is the same as the self address and peer address
  // of the default path.
  bool IsDefaultPath(const QuicSocketAddress& self_address,
                     const QuicSocketAddress& peer_address) const;

  // Return true if the |self_address| and |peer_address| is the same as the
  // self address and peer address of the alternative path.
  bool IsAlternativePath(const QuicSocketAddress& self_address,
                         const QuicSocketAddress& peer_address) const;

  // Restore connection default path and congestion control state to the last
  // validated path and its state. Called after fail to validate peer address
  // upon detecting a peer migration.
  void RestoreToLastValidatedPath(
      QuicSocketAddress original_direct_peer_address);

  // Return true if the current incoming packet is from a peer address that is
  // validated.
  bool IsReceivedPeerAddressValidated() const;

  // Check the state of the multi-port alternative path and initiate path
  // migration.
  void MaybeMigrateToMultiPortPath();

  std::unique_ptr<QuicSelfIssuedConnectionIdManager>
  MakeSelfIssuedConnectionIdManager();

  // Called on peer IP change or restoring to previous address to reset
  // congestion window, RTT stats, retransmission timer, etc. Only used in IETF
  // QUIC.
  std::unique_ptr<SendAlgorithmInterface> OnPeerIpAddressChanged();

  // Process NewConnectionIdFrame either sent from peer or synsthesized from
  // preferred_address transport parameter.
  bool OnNewConnectionIdFrameInner(const QuicNewConnectionIdFrame& frame);

  // Called to patch missing client connection ID on default/alternative paths
  // when a new client connection ID is received.
  void OnClientConnectionIdAvailable();

  // Determines encryption level to send ping in `packet_number_space`.
  EncryptionLevel GetEncryptionLevelToSendPingForSpace(
      PacketNumberSpace space) const;

  // Returns true if |address| is known server address.
  bool IsKnownServerAddress(const QuicSocketAddress& address) const;

  // Retrieves the ECN codepoint to be sent on the next packet.
  QuicEcnCodepoint GetEcnCodepointToSend(
      const QuicSocketAddress& destination_address) const;

  // Writes the packet to |writer| with the ECN mark specified in
  // |ecn_codepoint|. Will also set last_ecn_sent_ appropriately.
  WriteResult SendPacketToWriter(const char* buffer, size_t buf_len,
                                 const QuicIpAddress& self_address,
                                 const QuicSocketAddress& destination_address,
                                 QuicPacketWriter* writer,
                                 const QuicEcnCodepoint ecn_codepoint);

  QuicConnectionContext context_;

  QuicFramer framer_;

  // TODO(danzh) remove below fields once quic_ignore_gquic_probing_ gets
  // deprecated. Contents received in the current packet, especially used to
  // identify whether the current packet is a padded PING packet.
  PacketContent current_packet_content_;
  // Set to true as soon as the packet currently being processed has been
  // detected as a connectivity probing.
  // Always false outside the context of ProcessUdpPacket().
  bool is_current_packet_connectivity_probing_;

  bool has_path_challenge_in_current_packet_;

  // Caches the current effective peer migration type if a effective peer
  // migration might be initiated. As soon as the current packet is confirmed
  // not a connectivity probe, effective peer migration will start.
  AddressChangeType current_effective_peer_migration_type_;
  QuicConnectionHelperInterface* helper_;  // Not owned.
  QuicAlarmFactory* alarm_factory_;        // Not owned.
  PerPacketOptions* per_packet_options_;   // Not owned.
  QuicPacketWriterParams packet_writer_params_;
  QuicPacketWriter* writer_;  // Owned or not depending on |owns_writer_|.
  bool owns_writer_;
  // Encryption level for new packets. Should only be changed via
  // SetDefaultEncryptionLevel().
  EncryptionLevel encryption_level_;
  const QuicClock* clock_;
  QuicRandom* random_generator_;

  // On the server, the connection ID is set when receiving the first packet.
  // This variable ensures we only set it this way once.
  bool client_connection_id_is_set_;

  // Whether we've already replaced our server connection ID due to receiving an
  // INITIAL packet with a different source connection ID. Only used on client.
  bool server_connection_id_replaced_by_initial_ = false;
  // Address on the last successfully processed packet received from the
  // direct peer.

  // Other than initialization, do not modify it directly, use
  // UpdatePeerAddress() instead.
  QuicSocketAddress direct_peer_address_;
  // The default path on which the endpoint sends non-probing packets.
  // The send algorithm and RTT stats of this path are stored in
  // |sent_packet_manager_| instead of in this object.
  PathState default_path_;

  // Records change type when the effective peer initiates migration to a new
  // address. Reset to NO_CHANGE after effective peer migration is validated.
  AddressChangeType active_effective_peer_migration_type_;

  // Records highest sent packet number when effective peer migration is
  // started.
  QuicPacketNumber highest_packet_sent_before_effective_peer_migration_;

  // True if Key Update is supported on this connection.
  bool support_key_update_for_connection_;

  // Tracks the lowest packet sent in the current key phase. Will be
  // uninitialized before the first one-RTT packet has been sent or after a
  // key update but before the first packet has been sent.
  QuicPacketNumber lowest_packet_sent_in_current_key_phase_;

  // TODO(rch): remove this when b/27221014 is fixed.
  const char* current_packet_data_;  // UDP payload of packet currently being
                                     // parsed or nullptr.
  bool should_last_packet_instigate_acks_;

  // Track some peer state so we can do less bookkeeping
  // Largest sequence sent by the peer which had an ack frame (latest ack info).
  // Do not read or write directly, use GetLargestReceivedPacketWithAck() and
  // SetLargestReceivedPacketWithAck() instead.
  QuicPacketNumber largest_seen_packet_with_ack_;
  // Largest packet number sent by the peer which had an ACK frame per packet
  // number space. Only used when this connection supports multiple packet
  // number spaces.
  QuicPacketNumber largest_seen_packets_with_ack_[NUM_PACKET_NUMBER_SPACES];

  // Largest packet number sent by the peer which had a stop waiting frame.
  QuicPacketNumber largest_seen_packet_with_stop_waiting_;

  // Collection of packets which were received before encryption was
  // established, but which could not be decrypted.  We buffer these on
  // the assumption that they could not be processed because they were
  // sent with the INITIAL encryption and the CHLO message was lost.
  std::deque<UndecryptablePacket> undecryptable_packets_;

  // Collection of coalesced packets which were received while processing
  // the current packet.
  quiche::QuicheCircularDeque<std::unique_ptr<QuicEncryptedPacket>>
      received_coalesced_packets_;

  // Maximum number of undecryptable packets the connection will store.
  size_t max_undecryptable_packets_;

  // Maximum number of tracked packets.
  QuicPacketCount max_tracked_packets_;

  // Contains the connection close packets if the connection has been closed.
  std::unique_ptr<std::vector<std::unique_ptr<QuicEncryptedPacket>>>
      termination_packets_;

  // Determines whether or not a connection close packet is sent to the peer
  // after idle timeout due to lack of network activity. During the handshake,
  // a connection close packet is sent, but not after.
  ConnectionCloseBehavior idle_timeout_connection_close_behavior_;

  // When > 0, close the QUIC connection after this number of RTOs.
  size_t num_rtos_for_blackhole_detection_;

  // Statistics for this session.
  QuicConnectionStats stats_;

  UberReceivedPacketManager uber_received_packet_manager_;

  // Indicates the retransmission alarm needs to be set.
  bool pending_retransmission_alarm_;

  // If true, defer sending data in response to received packets to the
  // SendAlarm.
  bool defer_send_in_response_to_packets_;

  // Arena to store class implementations within the QuicConnection.
  QuicConnectionArena arena_;

  // An alarm that fires when an ACK should be sent to the peer.
  QuicArenaScopedPtr<QuicAlarm> ack_alarm_;
  // An alarm that fires when a packet needs to be retransmitted.
  QuicArenaScopedPtr<QuicAlarm> retransmission_alarm_;
  // An alarm that is scheduled when the SentPacketManager requires a delay
  // before sending packets and fires when the packet may be sent.
  QuicArenaScopedPtr<QuicAlarm> send_alarm_;
  // An alarm that fires when an MTU probe should be sent.
  QuicArenaScopedPtr<QuicAlarm> mtu_discovery_alarm_;
  // An alarm that fires to process undecryptable packets when new decyrption
  // keys are available.
  QuicArenaScopedPtr<QuicAlarm> process_undecryptable_packets_alarm_;
  // An alarm that fires to discard keys for the previous key phase some time
  // after a key update has completed.
  QuicArenaScopedPtr<QuicAlarm> discard_previous_one_rtt_keys_alarm_;
  // An alarm that fires to discard 0-RTT decryption keys some time after the
  // first 1-RTT packet has been decrypted. Only used on server connections with
  // TLS handshaker.
  QuicArenaScopedPtr<QuicAlarm> discard_zero_rtt_decryption_keys_alarm_;
  // An alarm that fires to keep probing the multi-port path.
  QuicArenaScopedPtr<QuicAlarm> multi_port_probing_alarm_;
  // Neither visitor is owned by this class.
  QuicConnectionVisitorInterface* visitor_;
  QuicConnectionDebugVisitor* debug_visitor_;

  QuicPacketCreator packet_creator_;

  // Information about the last received QUIC packet, which may not have been
  // successfully decrypted and processed.
  ReceivedPacketInfo last_received_packet_info_;

  // Sent packet manager which tracks the status of packets sent by this
  // connection and contains the send and receive algorithms to determine when
  // to send packets.
  QuicSentPacketManager sent_packet_manager_;

  // Indicates whether connection version has been negotiated.
  // Always true for server connections.
  bool version_negotiated_;

  // Tracks if the connection was created by the server or the client.
  Perspective perspective_;

  // True by default.  False if we've received or sent an explicit connection
  // close.
  bool connected_;

  // Set to false if the connection should not send truncated connection IDs to
  // the peer, even if the peer supports it.
  bool can_truncate_connection_ids_;

  // If non-empty this contains the set of versions received in a
  // version negotiation packet.
  ParsedQuicVersionVector server_supported_versions_;

  // The number of MTU probes already sent.
  size_t mtu_probe_count_;

  // The value of |long_term_mtu_| prior to the last successful MTU increase.
  // 0 means either
  // - MTU discovery has never been enabled, or
  // - MTU discovery has been enabled, but the connection got a packet write
  //   error with a new (successfully probed) MTU, so it reverted
  //   |long_term_mtu_| to the value before the last increase.
  QuicPacketLength previous_validated_mtu_;
  // The value of the MTU regularly used by the connection. This is different
  // from the value returned by max_packet_size(), as max_packet_size() returns
  // the value of the MTU as currently used by the serializer, so if
  // serialization of an MTU probe is in progress, those two values will be
  // different.
  QuicByteCount long_term_mtu_;

  // The maximum UDP payload size that our peer has advertised support for.
  // Defaults to kDefaultMaxPacketSizeTransportParam until received from peer.
  QuicByteCount peer_max_packet_size_;

  // The size of the largest packet received from peer.
  QuicByteCount largest_received_packet_size_;

  // Indicates whether a write error is encountered currently. This is used to
  // avoid infinite write errors.
  bool write_error_occurred_;

  // Consecutive number of sent packets which have no retransmittable frames.
  size_t consecutive_num_packets_with_no_retransmittable_frames_;

  // After this many packets sent without retransmittable frames, an artificial
  // retransmittable frame(a WINDOW_UPDATE) will be created to solicit an ack
  // from the peer. Default to kMaxConsecutiveNonRetransmittablePackets.
  size_t max_consecutive_num_packets_with_no_retransmittable_frames_;

  // If true, bundle an ack-eliciting frame with an ACK if the PTO alarm have
  // previously fired.
  bool bundle_retransmittable_with_pto_ack_;

  // Id of latest sent control frame. 0 if no control frame has been sent.
  QuicControlFrameId last_control_frame_id_;

  // True if the peer is unreachable on the current path.
  bool is_path_degrading_;

  // True if an ack frame is being processed.
  bool processing_ack_frame_;

  // True if the writer supports release timestamp.
  bool supports_release_time_;

  std::unique_ptr<QuicPeerIssuedConnectionIdManager> peer_issued_cid_manager_;
  std::unique_ptr<QuicSelfIssuedConnectionIdManager> self_issued_cid_manager_;

  // Time this connection can release packets into the future.
  QuicTime::Delta release_time_into_future_;

  // Payloads that were received in the most recent probe. This needs to be a
  // Deque because the peer might no be using this implementation, and others
  // might send a packet with more than one PATH_CHALLENGE, so all need to be
  // saved and responded to.
  // TODO(danzh) deprecate this field when deprecating
  // --quic_send_path_response.
  quiche::QuicheCircularDeque<QuicPathFrameBuffer>
      received_path_challenge_payloads_;

  // When we receive a RETRY packet or some INITIAL packets, we replace
  // |server_connection_id_| with the value from that packet and save off the
  // original value of |server_connection_id_| into
  // |original_destination_connection_id_| for validation.
  absl::optional<QuicConnectionId> original_destination_connection_id_;

  // The connection ID that replaces original_destination_connection_id_.
  QuicConnectionId original_destination_connection_id_replacement_;

  // After we receive a RETRY packet, |retry_source_connection_id_| contains
  // the source connection ID from that packet.
  absl::optional<QuicConnectionId> retry_source_connection_id_;

  // Used to store content of packets which cannot be sent because of write
  // blocked. Packets' encrypted buffers are copied and owned by
  // buffered_packets_. From unacked_packet_map (and congestion control)'s
  // perspective, those packets are considered sent.
  std::list<BufferedPacket> buffered_packets_;

  // Used to coalesce packets of different encryption level into the same UDP
  // datagram. Connection stops trying to coalesce packets if a forward secure
  // packet gets acknowledged.
  QuicCoalescedPacket coalesced_packet_;

  QuicConnectionMtuDiscoverer mtu_discoverer_;

  QuicNetworkBlackholeDetector blackhole_detector_;

  QuicIdleNetworkDetector idle_network_detector_;

  bool blackhole_detection_disabled_ = false;

  const bool default_enable_5rto_blackhole_detection_ =
      GetQuicReloadableFlag(quic_default_enable_5rto_blackhole_detection2);

  // True if next packet is intended to consume remaining space in the
  // coalescer.
  bool fill_coalesced_packet_ = false;

  size_t anti_amplification_factor_ =
      GetQuicFlag(quic_anti_amplification_factor);

  // True if AckFrequencyFrame is supported.
  bool can_receive_ack_frequency_frame_ = false;

  // Indicate whether coalescing is done.
  bool coalescing_done_ = false;

  // Indicate whether any ENCRYPTION_HANDSHAKE packet has been sent.
  bool handshake_packet_sent_ = false;

  // Indicate whether to send an AckFrequencyFrame upon handshake completion.
  // The AckFrequencyFrame sent will updates client's max_ack_delay, which if
  // chosen properly can reduce the CPU and bandwidth usage for ACK frames.
  bool send_ack_frequency_on_handshake_completion_ = false;

  // Indicate whether AckFrequency frame has been sent.
  bool ack_frequency_sent_ = false;

  // True if a 0-RTT decrypter was or is installed at some point in the
  // connection's lifetime.
  bool had_zero_rtt_decrypter_ = false;

  // True after the first 1-RTT packet has successfully decrypted.
  bool have_decrypted_first_one_rtt_packet_ = false;

  // True if we are currently processing OnRetransmissionTimeout.
  bool in_probe_time_out_ = false;

  QuicPathValidator path_validator_;

  // Stores information of a path which maybe used as default path in the
  // future. On the client side, it gets created when the client starts
  // validating a new path and gets cleared once it becomes the default path or
  // the path validation fails or replaced by a newer path of interest. On the
  // server side, alternative_path gets created when server: 1) receives
  // PATH_CHALLENGE on non-default path, or 2) switches to a not yet validated
  // default path such that it needs to store the previous validated default
  // path.
  // Note that if alternative_path_ stores a validated path information (case
  // 2), do not override it on receiving PATH_CHALLENGE (case 1).
  PathState alternative_path_;

  // If true, upon seeing a new client address, validate the client address.
  bool validate_client_addresses_ = false;

  // Indicates whether we should proactively validate peer address on a
  // PATH_CHALLENGE received.
  bool should_proactively_validate_peer_address_on_path_challenge_ = false;

  // If true, send connection close packet on INVALID_VERSION.
  bool send_connection_close_for_invalid_version_ = false;

  // If true, disable liveness testing.
  bool liveness_testing_disabled_ = false;

  QuicPingManager ping_manager_;

  // Records first serialized 1-RTT packet.
  std::unique_ptr<BufferedPacket> first_serialized_one_rtt_packet_;

  std::unique_ptr<QuicPathValidationContext> multi_port_path_context_;

  QuicTime::Delta multi_port_probing_interval_;

  std::unique_ptr<MultiPortStats> multi_port_stats_;

  // Client side only.
  bool active_migration_disabled_ = false;

  const bool ignore_gquic_probing_ =
      GetQuicReloadableFlag(quic_ignore_gquic_probing);

  RetransmittableOnWireBehavior retransmittable_on_wire_behavior_ = DEFAULT;

  // Server addresses that are known to the client.
  std::vector<QuicSocketAddress> known_server_addresses_;

  // Stores received server preferred address in transport param. Client side
  // only.
  QuicSocketAddress received_server_preferred_address_;

  // Stores sent server preferred address in transport param. Server side only.
  QuicSocketAddress sent_server_preferred_address_;

  // If true, kicks off validation of server_preferred_address_ once it is
  // received. Also, send all coalesced packets on both paths until handshake is
  // confirmed.
  bool accelerated_server_preferred_address_ = false;

  // If true, throttle sending if next created packet will exceed amplification
  // limit.
  const bool enforce_strict_amplification_factor_ =
      GetQuicFlag(quic_enforce_strict_amplification_factor);

  ConnectionIdGeneratorInterface& connection_id_generator_;

  // This LRU cache records source addresses of packets received on server's
  // original address.
  QuicLRUCache<QuicSocketAddress, bool, QuicSocketAddressHash>
      received_client_addresses_cache_;

  // Endpoints should never mark packets with Congestion Experienced (CE), as
  // this is only done by routers. Endpoints cannot send ECT(0) or ECT(1) if
  // their congestion control cannot respond to these signals in accordance with
  // the spec, or ECN feedback doesn't conform to the spec. When true, the
  // connection will not verify that the requested codepoint adheres to these
  // policies. This is only accessible through QuicConnectionPeer.
  bool disable_ecn_codepoint_validation_ = false;

  // The ECN codepoint of the last packet to be sent to the writer, which
  // might be different from the next codepoint in per_packet_options_.
  QuicEcnCodepoint last_ecn_codepoint_sent_ = ECN_NOT_ECT;
};

}  // namespace quic

#endif  // QUICHE_QUIC_CORE_QUIC_CONNECTION_H_