connection.py

import binascii
import logging
import os
from collections import deque
from dataclasses import dataclass
from enum import Enum
from functools import partial
from typing import (
    Any,
    Callable,
    Deque,
    Dict,
    FrozenSet,
    List,
    Optional,
    Sequence,
    Set,
    Tuple,
)

from .. import tls
from ..buffer import (
    UINT_VAR_MAX,
    UINT_VAR_MAX_SIZE,
    Buffer,
    BufferReadError,
    size_uint_var,
)
from . import events
from .configuration import SMALLEST_MAX_DATAGRAM_SIZE, QuicConfiguration
from .congestion.base import K_GRANULARITY
from .crypto import CryptoError, CryptoPair, KeyUnavailableError
from .logger import QuicLoggerTrace
from .packet import (
    CONNECTION_ID_MAX_SIZE,
    NON_ACK_ELICITING_FRAME_TYPES,
    PACKET_TYPE_HANDSHAKE,
    PACKET_TYPE_INITIAL,
    PACKET_TYPE_ONE_RTT,
    PACKET_TYPE_RETRY,
    PACKET_TYPE_ZERO_RTT,
    PROBING_FRAME_TYPES,
    RETRY_INTEGRITY_TAG_SIZE,
    STATELESS_RESET_TOKEN_SIZE,
    QuicErrorCode,
    QuicFrameType,
    QuicProtocolVersion,
    QuicStreamFrame,
    QuicTransportParameters,
    get_retry_integrity_tag,
    get_spin_bit,
    is_draft_version,
    is_long_header,
    pull_ack_frame,
    pull_quic_header,
    pull_quic_transport_parameters,
    push_ack_frame,
    push_quic_transport_parameters,
)
from .packet_builder import (
    QuicDeliveryState,
    QuicPacketBuilder,
    QuicPacketBuilderStop,
)
from .recovery import QuicPacketRecovery, QuicPacketSpace
from .stream import FinalSizeError, QuicStream, StreamFinishedError

logger = logging.getLogger("quic")

CRYPTO_BUFFER_SIZE = 16384
EPOCH_SHORTCUTS = {
    "I": tls.Epoch.INITIAL,
    "H": tls.Epoch.HANDSHAKE,
    "0": tls.Epoch.ZERO_RTT,
    "1": tls.Epoch.ONE_RTT,
}
MAX_EARLY_DATA = 0xFFFFFFFF
SECRETS_LABELS = [
    [
        None,
        "CLIENT_EARLY_TRAFFIC_SECRET",
        "CLIENT_HANDSHAKE_TRAFFIC_SECRET",
        "CLIENT_TRAFFIC_SECRET_0",
    ],
    [
        None,
        None,
        "SERVER_HANDSHAKE_TRAFFIC_SECRET",
        "SERVER_TRAFFIC_SECRET_0",
    ],
]
STREAM_FLAGS = 0x07
STREAM_COUNT_MAX = 0x1000000000000000
UDP_HEADER_SIZE = 8
MAX_PENDING_RETIRES = 100

NetworkAddress = Any

# frame sizes
ACK_FRAME_CAPACITY = 64  # FIXME: this is arbitrary!
APPLICATION_CLOSE_FRAME_CAPACITY = 1 + 2 * UINT_VAR_MAX_SIZE  # + reason length
CONNECTION_LIMIT_FRAME_CAPACITY = 1 + UINT_VAR_MAX_SIZE
HANDSHAKE_DONE_FRAME_CAPACITY = 1
MAX_STREAM_DATA_FRAME_CAPACITY = 1 + 2 * UINT_VAR_MAX_SIZE
NEW_CONNECTION_ID_FRAME_CAPACITY = (
    1 + 2 * UINT_VAR_MAX_SIZE + 1 + CONNECTION_ID_MAX_SIZE + STATELESS_RESET_TOKEN_SIZE
)
PATH_CHALLENGE_FRAME_CAPACITY = 1 + 8
PATH_RESPONSE_FRAME_CAPACITY = 1 + 8
PING_FRAME_CAPACITY = 1
RESET_STREAM_FRAME_CAPACITY = 1 + 3 * UINT_VAR_MAX_SIZE
RETIRE_CONNECTION_ID_CAPACITY = 1 + UINT_VAR_MAX_SIZE
STOP_SENDING_FRAME_CAPACITY = 1 + 2 * UINT_VAR_MAX_SIZE
STREAMS_BLOCKED_CAPACITY = 1 + UINT_VAR_MAX_SIZE
TRANSPORT_CLOSE_FRAME_CAPACITY = 1 + 3 * UINT_VAR_MAX_SIZE  # + reason length


def EPOCHS(shortcut: str) -> FrozenSet[tls.Epoch]:
    return frozenset(EPOCH_SHORTCUTS[i] for i in shortcut)


def dump_cid(cid: bytes) -> str:
    return binascii.hexlify(cid).decode("ascii")


def get_epoch(packet_type: int) -> tls.Epoch:
    if packet_type == PACKET_TYPE_INITIAL:
        return tls.Epoch.INITIAL
    elif packet_type == PACKET_TYPE_ZERO_RTT:
        return tls.Epoch.ZERO_RTT
    elif packet_type == PACKET_TYPE_HANDSHAKE:
        return tls.Epoch.HANDSHAKE
    else:
        return tls.Epoch.ONE_RTT


def get_transport_parameters_extension(version: int) -> tls.ExtensionType:
    if is_draft_version(version):
        return tls.ExtensionType.QUIC_TRANSPORT_PARAMETERS_DRAFT
    else:
        return tls.ExtensionType.QUIC_TRANSPORT_PARAMETERS


def stream_is_client_initiated(stream_id: int) -> bool:
    """
    Returns True if the stream is client initiated.
    """
    return not (stream_id & 1)


def stream_is_unidirectional(stream_id: int) -> bool:
    """
    Returns True if the stream is unidirectional.
    """
    return bool(stream_id & 2)


class Limit:
    def __init__(self, frame_type: int, name: str, value: int):
        self.frame_type = frame_type
        self.name = name
        self.sent = value
        self.used = 0
        self.value = value


class QuicConnectionError(Exception):
    def __init__(self, error_code: int, frame_type: int, reason_phrase: str):
        self.error_code = error_code
        self.frame_type = frame_type
        self.reason_phrase = reason_phrase

    def __str__(self) -> str:
        s = "Error: %d, reason: %s" % (self.error_code, self.reason_phrase)
        if self.frame_type is not None:
            s += ", frame_type: %s" % self.frame_type
        return s


class QuicConnectionAdapter(logging.LoggerAdapter):
    def process(self, msg: str, kwargs: Any) -> Tuple[str, Any]:
        return "[%s] %s" % (self.extra["id"], msg), kwargs


@dataclass
class QuicConnectionId:
    cid: bytes
    sequence_number: int
    stateless_reset_token: bytes = b""
    was_sent: bool = False


class QuicConnectionState(Enum):
    FIRSTFLIGHT = 0
    CONNECTED = 1
    CLOSING = 2
    DRAINING = 3
    TERMINATED = 4


@dataclass
class QuicNetworkPath:
    addr: NetworkAddress
    bytes_received: int = 0
    bytes_sent: int = 0
    is_validated: bool = False
    local_challenge: Optional[bytes] = None
    remote_challenge: Optional[bytes] = None

    def can_send(self, size: int) -> bool:
        return self.is_validated or (self.bytes_sent + size) <= 3 * self.bytes_received


@dataclass
class QuicReceiveContext:
    epoch: tls.Epoch
    host_cid: bytes
    network_path: QuicNetworkPath
    quic_logger_frames: Optional[List[Any]]
    time: float


QuicTokenHandler = Callable[[bytes], None]

END_STATES = frozenset(
    [
        QuicConnectionState.CLOSING,
        QuicConnectionState.DRAINING,
        QuicConnectionState.TERMINATED,
    ]
)


class QuicConnection:
    """
    A QUIC connection.

    The state machine is driven by three kinds of sources:

    - the API user requesting data to be send out (see :meth:`connect`,
      :meth:`reset_stream`, :meth:`send_ping`, :meth:`send_datagram_frame`
      and :meth:`send_stream_data`)
    - data being received from the network (see :meth:`receive_datagram`)
    - a timer firing (see :meth:`handle_timer`)

    :param configuration: The QUIC configuration to use.
    """

    def __init__(
        self,
        *,
        configuration: QuicConfiguration,
        original_destination_connection_id: Optional[bytes] = None,
        retry_source_connection_id: Optional[bytes] = None,
        session_ticket_fetcher: Optional[tls.SessionTicketFetcher] = None,
        session_ticket_handler: Optional[tls.SessionTicketHandler] = None,
        token_handler: Optional[QuicTokenHandler] = None,
    ) -> None:
        assert configuration.max_datagram_size >= SMALLEST_MAX_DATAGRAM_SIZE, (
            "The smallest allowed maximum datagram size is "
            f"{SMALLEST_MAX_DATAGRAM_SIZE} bytes"
        )
        if configuration.is_client:
            assert (
                original_destination_connection_id is None
            ), "Cannot set original_destination_connection_id for a client"
            assert (
                retry_source_connection_id is None
            ), "Cannot set retry_source_connection_id for a client"
        else:
            assert token_handler is None, "Cannot set `token_handler` for a server"
            assert (
                configuration.token == b""
            ), "Cannot set `configuration.token` for a server"
            assert (
                configuration.certificate is not None
            ), "SSL certificate is required for a server"
            assert (
                configuration.private_key is not None
            ), "SSL private key is required for a server"
            assert (
                original_destination_connection_id is not None
            ), "original_destination_connection_id is required for a server"

        # configuration
        self._configuration = configuration
        self._is_client = configuration.is_client

        self._ack_delay = K_GRANULARITY
        self._close_at: Optional[float] = None
        self._close_event: Optional[events.ConnectionTerminated] = None
        self._connect_called = False
        self._cryptos: Dict[tls.Epoch, CryptoPair] = {}
        self._crypto_buffers: Dict[tls.Epoch, Buffer] = {}
        self._crypto_retransmitted = False
        self._crypto_streams: Dict[tls.Epoch, QuicStream] = {}
        self._events: Deque[events.QuicEvent] = deque()
        self._handshake_complete = False
        self._handshake_confirmed = False
        self._host_cids = [
            QuicConnectionId(
                cid=os.urandom(configuration.connection_id_length),
                sequence_number=0,
                stateless_reset_token=os.urandom(16) if not self._is_client else None,
                was_sent=True,
            )
        ]
        self.host_cid = self._host_cids[0].cid
        self._host_cid_seq = 1
        self._local_ack_delay_exponent = 3
        self._local_active_connection_id_limit = 8
        self._local_initial_source_connection_id = self._host_cids[0].cid
        self._local_max_data = Limit(
            frame_type=QuicFrameType.MAX_DATA,
            name="max_data",
            value=configuration.max_data,
        )
        self._local_max_stream_data_bidi_local = configuration.max_stream_data
        self._local_max_stream_data_bidi_remote = configuration.max_stream_data
        self._local_max_stream_data_uni = configuration.max_stream_data
        self._local_max_streams_bidi = Limit(
            frame_type=QuicFrameType.MAX_STREAMS_BIDI,
            name="max_streams_bidi",
            value=128,
        )
        self._local_max_streams_uni = Limit(
            frame_type=QuicFrameType.MAX_STREAMS_UNI, name="max_streams_uni", value=128
        )
        self._local_next_stream_id_bidi = 0 if self._is_client else 1
        self._local_next_stream_id_uni = 2 if self._is_client else 3
        self._loss_at: Optional[float] = None
        self._max_datagram_size = configuration.max_datagram_size
        self._network_paths: List[QuicNetworkPath] = []
        self._pacing_at: Optional[float] = None
        self._packet_number = 0
        self._parameters_received = False
        self._peer_cid = QuicConnectionId(
            cid=os.urandom(configuration.connection_id_length), sequence_number=None
        )
        self._peer_cid_available: List[QuicConnectionId] = []
        self._peer_cid_sequence_numbers: Set[int] = set([0])
        self._peer_retire_prior_to = 0
        self._peer_token = configuration.token
        self._quic_logger: Optional[QuicLoggerTrace] = None
        self._remote_ack_delay_exponent = 3
        self._remote_active_connection_id_limit = 2
        self._remote_initial_source_connection_id: Optional[bytes] = None
        self._remote_max_idle_timeout: Optional[float] = None  # seconds
        self._remote_max_data = 0
        self._remote_max_data_used = 0
        self._remote_max_datagram_frame_size: Optional[int] = None
        self._remote_max_stream_data_bidi_local = 0
        self._remote_max_stream_data_bidi_remote = 0
        self._remote_max_stream_data_uni = 0
        self._remote_max_streams_bidi = 0
        self._remote_max_streams_uni = 0
        self._retry_count = 0
        self._retry_source_connection_id = retry_source_connection_id
        self._spaces: Dict[tls.Epoch, QuicPacketSpace] = {}
        self._spin_bit = False
        self._spin_highest_pn = 0
        self._state = QuicConnectionState.FIRSTFLIGHT
        self._streams: Dict[int, QuicStream] = {}
        self._streams_queue: List[QuicStream] = []
        self._streams_blocked_bidi: List[QuicStream] = []
        self._streams_blocked_uni: List[QuicStream] = []
        self._streams_finished: Set[int] = set()
        self._version: Optional[int] = None
        self._version_negotiation_count = 0

        if self._is_client:
            self._original_destination_connection_id = self._peer_cid.cid
        else:
            self._original_destination_connection_id = (
                original_destination_connection_id
            )

        # logging
        self._logger = QuicConnectionAdapter(
            logger, {"id": dump_cid(self._original_destination_connection_id)}
        )
        if configuration.quic_logger:
            self._quic_logger = configuration.quic_logger.start_trace(
                is_client=configuration.is_client,
                odcid=self._original_destination_connection_id,
            )

        # loss recovery
        self._loss = QuicPacketRecovery(
            congestion_control_algorithm=configuration.congestion_control_algorithm,
            initial_rtt=configuration.initial_rtt,
            max_datagram_size=self._max_datagram_size,
            peer_completed_address_validation=not self._is_client,
            quic_logger=self._quic_logger,
            send_probe=self._send_probe,
            logger=self._logger,
        )

        # things to send
        self._close_pending = False
        self._datagrams_pending: Deque[bytes] = deque()
        self._handshake_done_pending = False
        self._ping_pending: List[int] = []
        self._probe_pending = False
        self._retire_connection_ids: List[int] = []
        self._streams_blocked_pending = False

        # callbacks
        self._session_ticket_fetcher = session_ticket_fetcher
        self._session_ticket_handler = session_ticket_handler
        self._token_handler = token_handler

        # frame handlers
        self.__frame_handlers = {
            0x00: (self._handle_padding_frame, EPOCHS("IH01")),
            0x01: (self._handle_ping_frame, EPOCHS("IH01")),
            0x02: (self._handle_ack_frame, EPOCHS("IH1")),
            0x03: (self._handle_ack_frame, EPOCHS("IH1")),
            0x04: (self._handle_reset_stream_frame, EPOCHS("01")),
            0x05: (self._handle_stop_sending_frame, EPOCHS("01")),
            0x06: (self._handle_crypto_frame, EPOCHS("IH1")),
            0x07: (self._handle_new_token_frame, EPOCHS("1")),
            0x08: (self._handle_stream_frame, EPOCHS("01")),
            0x09: (self._handle_stream_frame, EPOCHS("01")),
            0x0A: (self._handle_stream_frame, EPOCHS("01")),
            0x0B: (self._handle_stream_frame, EPOCHS("01")),
            0x0C: (self._handle_stream_frame, EPOCHS("01")),
            0x0D: (self._handle_stream_frame, EPOCHS("01")),
            0x0E: (self._handle_stream_frame, EPOCHS("01")),
            0x0F: (self._handle_stream_frame, EPOCHS("01")),
            0x10: (self._handle_max_data_frame, EPOCHS("01")),
            0x11: (self._handle_max_stream_data_frame, EPOCHS("01")),
            0x12: (self._handle_max_streams_bidi_frame, EPOCHS("01")),
            0x13: (self._handle_max_streams_uni_frame, EPOCHS("01")),
            0x14: (self._handle_data_blocked_frame, EPOCHS("01")),
            0x15: (self._handle_stream_data_blocked_frame, EPOCHS("01")),
            0x16: (self._handle_streams_blocked_frame, EPOCHS("01")),
            0x17: (self._handle_streams_blocked_frame, EPOCHS("01")),
            0x18: (self._handle_new_connection_id_frame, EPOCHS("01")),
            0x19: (self._handle_retire_connection_id_frame, EPOCHS("01")),
            0x1A: (self._handle_path_challenge_frame, EPOCHS("01")),
            0x1B: (self._handle_path_response_frame, EPOCHS("01")),
            0x1C: (self._handle_connection_close_frame, EPOCHS("IH01")),
            0x1D: (self._handle_connection_close_frame, EPOCHS("01")),
            0x1E: (self._handle_handshake_done_frame, EPOCHS("1")),
            0x30: (self._handle_datagram_frame, EPOCHS("01")),
            0x31: (self._handle_datagram_frame, EPOCHS("01")),
        }

    @property
    def configuration(self) -> QuicConfiguration:
        return self._configuration

    @property
    def original_destination_connection_id(self) -> bytes:
        return self._original_destination_connection_id

    def change_connection_id(self) -> None:
        """
        Switch to the next available connection ID and retire
        the previous one.

        .. aioquic_transmit::
        """
        if self._peer_cid_available:
            # retire previous CID
            self._retire_peer_cid(self._peer_cid)

            # assign new CID
            self._consume_peer_cid()

    def close(
        self,
        error_code: int = QuicErrorCode.NO_ERROR,
        frame_type: Optional[int] = None,
        reason_phrase: str = "",
    ) -> None:
        """
        Close the connection.

        .. aioquic_transmit::

        :param error_code: An error code indicating why the connection is
                           being closed.
        :param reason_phrase: A human-readable explanation of why the
                              connection is being closed.
        """
        if self._close_event is None and self._state not in END_STATES:
            self._close_event = events.ConnectionTerminated(
                error_code=error_code,
                frame_type=frame_type,
                reason_phrase=reason_phrase,
            )
            self._close_pending = True

    def connect(self, addr: NetworkAddress, now: float) -> None:
        """
        Initiate the TLS handshake.

        This method can only be called for clients and a single time.

        .. aioquic_transmit::

        :param addr: The network address of the remote peer.
        :param now: The current time.
        """
        assert (
            self._is_client and not self._connect_called
        ), "connect() can only be called for clients and a single time"
        self._connect_called = True

        self._network_paths = [QuicNetworkPath(addr, is_validated=True)]
        self._version = self._configuration.supported_versions[0]
        self._connect(now=now)

    def datagrams_to_send(self, now: float) -> List[Tuple[bytes, NetworkAddress]]:
        """
        Return a list of `(data, addr)` tuples of datagrams which need to be
        sent, and the network address to which they need to be sent.

        After calling this method call :meth:`get_timer` to know when the next
        timer needs to be set.

        :param now: The current time.
        """
        network_path = self._network_paths[0]

        if self._state in END_STATES:
            return []

        # build datagrams
        builder = QuicPacketBuilder(
            host_cid=self.host_cid,
            is_client=self._is_client,
            max_datagram_size=self._max_datagram_size,
            packet_number=self._packet_number,
            peer_cid=self._peer_cid.cid,
            peer_token=self._peer_token,
            quic_logger=self._quic_logger,
            spin_bit=self._spin_bit,
            version=self._version,
        )
        if self._close_pending:
            epoch_packet_types = []
            if not self._handshake_confirmed:
                epoch_packet_types += [
                    (tls.Epoch.INITIAL, PACKET_TYPE_INITIAL),
                    (tls.Epoch.HANDSHAKE, PACKET_TYPE_HANDSHAKE),
                ]
            epoch_packet_types.append((tls.Epoch.ONE_RTT, PACKET_TYPE_ONE_RTT))
            for epoch, packet_type in epoch_packet_types:
                crypto = self._cryptos[epoch]
                if crypto.send.is_valid():
                    builder.start_packet(packet_type, crypto)
                    self._write_connection_close_frame(
                        builder=builder,
                        epoch=epoch,
                        error_code=self._close_event.error_code,
                        frame_type=self._close_event.frame_type,
                        reason_phrase=self._close_event.reason_phrase,
                    )
            self._logger.info(
                "Connection close sent (code 0x%X, reason %s)",
                self._close_event.error_code,
                self._close_event.reason_phrase,
            )
            self._close_pending = False
            self._close_begin(is_initiator=True, now=now)
        else:
            # congestion control
            builder.max_flight_bytes = (
                self._loss.congestion_window - self._loss.bytes_in_flight
            )
            if (
                self._probe_pending
                and builder.max_flight_bytes < self._max_datagram_size
            ):
                builder.max_flight_bytes = self._max_datagram_size

            # limit data on un-validated network paths
            if not network_path.is_validated:
                builder.max_total_bytes = (
                    network_path.bytes_received * 3 - network_path.bytes_sent
                )

            try:
                if not self._handshake_confirmed:
                    for epoch in [tls.Epoch.INITIAL, tls.Epoch.HANDSHAKE]:
                        self._write_handshake(builder, epoch, now)
                self._write_application(builder, network_path, now)
            except QuicPacketBuilderStop:
                pass

        datagrams, packets = builder.flush()

        if datagrams:
            self._packet_number = builder.packet_number

            # register packets
            sent_handshake = False
            for packet in packets:
                packet.sent_time = now
                self._loss.on_packet_sent(
                    packet=packet, space=self._spaces[packet.epoch]
                )
                if packet.epoch == tls.Epoch.HANDSHAKE:
                    sent_handshake = True

                # log packet
                if self._quic_logger is not None:
                    self._quic_logger.log_event(
                        category="transport",
                        event="packet_sent",
                        data={
                            "frames": packet.quic_logger_frames,
                            "header": {
                                "packet_number": packet.packet_number,
                                "packet_type": self._quic_logger.packet_type(
                                    packet.packet_type
                                ),
                                "scid": (
                                    dump_cid(self.host_cid)
                                    if is_long_header(packet.packet_type)
                                    else ""
                                ),
                                "dcid": dump_cid(self._peer_cid.cid),
                            },
                            "raw": {"length": packet.sent_bytes},
                        },
                    )

            # check if we can discard initial keys
            if sent_handshake and self._is_client:
                self._discard_epoch(tls.Epoch.INITIAL)

        # return datagrams to send and the destination network address
        ret = []
        for datagram in datagrams:
            payload_length = len(datagram)
            network_path.bytes_sent += payload_length
            ret.append((datagram, network_path.addr))

            if self._quic_logger is not None:
                self._quic_logger.log_event(
                    category="transport",
                    event="datagrams_sent",
                    data={
                        "count": 1,
                        "raw": [
                            {
                                "length": UDP_HEADER_SIZE + payload_length,
                                "payload_length": payload_length,
                            }
                        ],
                    },
                )
        return ret

    def get_next_available_stream_id(self, is_unidirectional=False) -> int:
        """
        Return the stream ID for the next stream created by this endpoint.
        """
        if is_unidirectional:
            return self._local_next_stream_id_uni
        else:
            return self._local_next_stream_id_bidi

    def get_timer(self) -> Optional[float]:
        """
        Return the time at which the timer should fire or None if no timer is needed.
        """
        timer_at = self._close_at
        if self._state not in END_STATES:
            # ack timer
            for space in self._loss.spaces:
                if space.ack_at is not None and space.ack_at < timer_at:
                    timer_at = space.ack_at

            # loss detection timer
            self._loss_at = self._loss.get_loss_detection_time()
            if self._loss_at is not None and self._loss_at < timer_at:
                timer_at = self._loss_at

            # pacing timer
            if self._pacing_at is not None and self._pacing_at < timer_at:
                timer_at = self._pacing_at

        return timer_at

    def handle_timer(self, now: float) -> None:
        """
        Handle the timer.

        .. aioquic_transmit::

        :param now: The current time.
        """
        # end of closing period or idle timeout
        if now >= self._close_at:
            if self._close_event is None:
                self._close_event = events.ConnectionTerminated(
                    error_code=QuicErrorCode.INTERNAL_ERROR,
                    frame_type=QuicFrameType.PADDING,
                    reason_phrase="Idle timeout",
                )
            self._close_end()
            return

        # loss detection timeout
        if self._loss_at is not None and now >= self._loss_at:
            self._logger.debug("Loss detection triggered")
            self._loss.on_loss_detection_timeout(now=now)

    def next_event(self) -> Optional[events.QuicEvent]:
        """
        Retrieve the next event from the event buffer.

        Returns `None` if there are no buffered events.
        """
        try:
            return self._events.popleft()
        except IndexError:
            return None

    def _idle_timeout(self) -> float:
        # RFC 9000 section 10.1

        # Start with our local timeout.
        idle_timeout = self._configuration.idle_timeout
        if self._remote_max_idle_timeout is not None:
            # Our peer has a preference too, so pick the smaller timeout.
            idle_timeout = min(idle_timeout, self._remote_max_idle_timeout)
        # But not too small!
        return max(idle_timeout, 3 * self._loss.get_probe_timeout())

    def receive_datagram(self, data: bytes, addr: NetworkAddress, now: float) -> None:
        """
        Handle an incoming datagram.

        .. aioquic_transmit::

        :param data: The datagram which was received.
        :param addr: The network address from which the datagram was received.
        :param now: The current time.
        """
        # stop handling packets when closing
        if self._state in END_STATES:
            return

        # log datagram
        if self._quic_logger is not None:
            payload_length = len(data)
            self._quic_logger.log_event(
                category="transport",
                event="datagrams_received",
                data={
                    "count": 1,
                    "raw": [
                        {
                            "length": UDP_HEADER_SIZE + payload_length,
                            "payload_length": payload_length,
                        }
                    ],
                },
            )

        # for servers, arm the idle timeout on the first datagram
        if self._close_at is None:
            self._close_at = now + self._idle_timeout()

        buf = Buffer(data=data)
        while not buf.eof():
            start_off = buf.tell()
            try:
                header = pull_quic_header(
                    buf, host_cid_length=self._configuration.connection_id_length
                )
            except ValueError:
                if self._quic_logger is not None:
                    self._quic_logger.log_event(
                        category="transport",
                        event="packet_dropped",
                        data={
                            "trigger": "header_parse_error",
                            "raw": {"length": buf.capacity - start_off},
                        },
                    )
                return

            # RFC 9000 section 14.1 requires servers to drop all initial packets
            # contained in a datagram smaller than 1200 bytes.
            if (
                not self._is_client
                and header.packet_type == PACKET_TYPE_INITIAL
                and len(data) < SMALLEST_MAX_DATAGRAM_SIZE
            ):
                if self._quic_logger is not None:
                    self._quic_logger.log_event(
                        category="transport",
                        event="packet_dropped",
                        data={
                            "trigger": "initial_packet_datagram_too_small",
                            "raw": {"length": buf.capacity - start_off},
                        },
                    )
                return

            # check destination CID matches
            destination_cid_seq: Optional[int] = None
            for connection_id in self._host_cids:
                if header.destination_cid == connection_id.cid:
                    destination_cid_seq = connection_id.sequence_number
                    break
            if (
                self._is_client or header.packet_type == PACKET_TYPE_HANDSHAKE
            ) and destination_cid_seq is None:
                if self._quic_logger is not None:
                    self._quic_logger.log_event(
                        category="transport",
                        event="packet_dropped",
                        data={"trigger": "unknown_connection_id"},
                    )
                return

            # check protocol version
            if (
                self._is_client
                and self._state == QuicConnectionState.FIRSTFLIGHT
                and header.version == QuicProtocolVersion.NEGOTIATION
                and not self._version_negotiation_count
            ):
                # version negotiation
                versions = []
                while not buf.eof():
                    versions.append(buf.pull_uint32())
                if self._quic_logger is not None:
                    self._quic_logger.log_event(
                        category="transport",
                        event="packet_received",
                        data={
                            "frames": [],
                            "header": {
                                "packet_type": "version_negotiation",
                                "scid": dump_cid(header.source_cid),
                                "dcid": dump_cid(header.destination_cid),
                            },
                            "raw": {"length": buf.tell() - start_off},
                        },
                    )
                if self._version in versions:
                    self._logger.warning(
                        "Version negotiation packet contains %s" % self._version
                    )
                    return
                common = [
                    x for x in self._configuration.supported_versions if x in versions
                ]
                chosen_version = common[0] if common else None
                if self._quic_logger is not None:
                    self._quic_logger.log_event(
                        category="transport",
                        event="version_information",
                        data={
                            "server_versions": versions,
                            "client_versions": self._configuration.supported_versions,
                            "chosen_version": chosen_version,
                        },
                    )
                if chosen_version is None:
                    self._logger.error("Could not find a common protocol version")
                    self._close_event = events.ConnectionTerminated(
                        error_code=QuicErrorCode.INTERNAL_ERROR,
                        frame_type=QuicFrameType.PADDING,
                        reason_phrase="Could not find a common protocol version",
                    )
                    self._close_end()
                    return
                self._packet_number = 0
                self._version = QuicProtocolVersion(chosen_version)
                self._version_negotiation_count += 1
                self._logger.info("Retrying with %s", self._version)
                self._connect(now=now)
                return
            elif (
                header.version is not None
                and header.version not in self._configuration.supported_versions
            ):
                # unsupported version
                if self._quic_logger is not None:
                    self._quic_logger.log_event(
                        category="transport",
                        event="packet_dropped",
                        data={"trigger": "unsupported_version"},
                    )
                return

            # handle retry packet
            if header.packet_type == PACKET_TYPE_RETRY:
                if (
                    self._is_client
                    and not self._retry_count
                    and header.destination_cid == self.host_cid
                    and header.integrity_tag
                    == get_retry_integrity_tag(
                        buf.data_slice(
                            start_off, buf.tell() - RETRY_INTEGRITY_TAG_SIZE
                        ),
                        self._peer_cid.cid,
                        version=header.version,
                    )
                ):
                    if self._quic_logger is not None:
                        self._quic_logger.log_event(
                            category="transport",
                            event="packet_received",
                            data={
                                "frames": [],
                                "header": {
                                    "packet_type": "retry",
                                    "scid": dump_cid(header.source_cid),
                                    "dcid": dump_cid(header.destination_cid),
                                },
                                "raw": {"length": buf.tell() - start_off},
                            },
                        )

                    self._peer_cid.cid = header.source_cid
                    self._peer_token = header.token
                    self._retry_count += 1
                    self._retry_source_connection_id = header.source_cid
                    self._logger.info(
                        "Retrying with token (%d bytes)" % len(header.token)
                    )
                    self._connect(now=now)
                else:
                    # unexpected or invalid retry packet
                    if self._quic_logger is not None:
                        self._quic_logger.log_event(
                            category="transport",
                            event="packet_dropped",
                            data={"trigger": "unexpected_packet"},
                        )
                return

            crypto_frame_required = False
            network_path = self._find_network_path(addr)

            # server initialization
            if not self._is_client and self._state == QuicConnectionState.FIRSTFLIGHT:
                assert (
                    header.packet_type == PACKET_TYPE_INITIAL
                ), "first packet must be INITIAL"
                crypto_frame_required = True
                self._network_paths = [network_path]
                self._version = QuicProtocolVersion(header.version)
                self._initialize(header.destination_cid)

            # determine crypto and packet space
            epoch = get_epoch(header.packet_type)
            crypto = self._cryptos[epoch]
            if epoch == tls.Epoch.ZERO_RTT:
                space = self._spaces[tls.Epoch.ONE_RTT]
            else:
                space = self._spaces[epoch]

            # decrypt packet
            encrypted_off = buf.tell() - start_off
            end_off = buf.tell() + header.rest_length
            buf.seek(end_off)

            try:
                plain_header, plain_payload, packet_number = crypto.decrypt_packet(
                    data[start_off:end_off], encrypted_off, space.expected_packet_number
                )
            except KeyUnavailableError as exc:
                self._logger.debug(exc)
                if self._quic_logger is not None:
                    self._quic_logger.log_event(
                        category="transport",
                        event="packet_dropped",
                        data={"trigger": "key_unavailable"},
                    )

                # If a client receives HANDSHAKE or 1-RTT packets before it has
                # handshake keys, it can assume that the server's INITIAL was lost.
                if (
                    self._is_client
                    and epoch in (tls.Epoch.HANDSHAKE, tls.Epoch.ONE_RTT)
                    and not self._crypto_retransmitted
                ):
                    self._loss.reschedule_data(now=now)
                    self._crypto_retransmitted = True
                continue
            except CryptoError as exc:
                self._logger.debug(exc)
                if self._quic_logger is not None:
                    self._quic_logger.log_event(
                        category="transport",
                        event="packet_dropped",
                        data={"trigger": "payload_decrypt_error"},
                    )
                continue

            # check reserved bits
            if header.is_long_header:
                reserved_mask = 0x0C
            else:
                reserved_mask = 0x18
            if plain_header[0] & reserved_mask:
                self.close(
                    error_code=QuicErrorCode.PROTOCOL_VIOLATION,
                    frame_type=QuicFrameType.PADDING,
                    reason_phrase="Reserved bits must be zero",
                )
                return

            # log packet
            quic_logger_frames: Optional[List[Dict]] = None
            if self._quic_logger is not None:
                quic_logger_frames = []
                self._quic_logger.log_event(
                    category="transport",
                    event="packet_received",
                    data={
                        "frames": quic_logger_frames,
                        "header": {
                            "packet_number": packet_number,
                            "packet_type": self._quic_logger.packet_type(
                                header.packet_type
                            ),
                            "dcid": dump_cid(header.destination_cid),
                            "scid": dump_cid(header.source_cid),
                        },
                        "raw": {"length": end_off - start_off},
                    },
                )

            # raise expected packet number
            if packet_number > space.expected_packet_number:
                space.expected_packet_number = packet_number + 1

            # discard initial keys and packet space
            if not self._is_client and epoch == tls.Epoch.HANDSHAKE:
                self._discard_epoch(tls.Epoch.INITIAL)

            # update state
            if self._peer_cid.sequence_number is None:
                self._peer_cid.cid = header.source_cid
                self._peer_cid.sequence_number = 0

            if self._state == QuicConnectionState.FIRSTFLIGHT:
                self._remote_initial_source_connection_id = header.source_cid
                self._set_state(QuicConnectionState.CONNECTED)

            # update spin bit
            if not header.is_long_header and packet_number > self._spin_highest_pn:
                spin_bit = get_spin_bit(plain_header[0])
                if self._is_client:
                    self._spin_bit = not spin_bit
                else:
                    self._spin_bit = spin_bit
                self._spin_highest_pn = packet_number

                if self._quic_logger is not None:
                    self._quic_logger.log_event(
                        category="connectivity",
                        event="spin_bit_updated",
                        data={"state": self._spin_bit},
                    )

            # handle payload
            context = QuicReceiveContext(
                epoch=epoch,
                host_cid=header.destination_cid,
                network_path=network_path,
                quic_logger_frames=quic_logger_frames,
                time=now,
            )
            try:
                is_ack_eliciting, is_probing = self._payload_received(
                    context, plain_payload, crypto_frame_required=crypto_frame_required
                )
            except QuicConnectionError as exc:
                self._logger.warning(exc)
                self.close(
                    error_code=exc.error_code,
                    frame_type=exc.frame_type,
                    reason_phrase=exc.reason_phrase,
                )
            if self._state in END_STATES or self._close_pending:
                return

            # update idle timeout
            self._close_at = now + self._idle_timeout()

            # handle migration
            if (
                not self._is_client
                and context.host_cid != self.host_cid
                and epoch == tls.Epoch.ONE_RTT
            ):
                self._logger.debug(
                    "Peer switching to CID %s (%d)",
                    dump_cid(context.host_cid),
                    destination_cid_seq,
                )
                self.host_cid = context.host_cid
                self.change_connection_id()

            # update network path
            if not network_path.is_validated and epoch == tls.Epoch.HANDSHAKE:
                self._logger.debug(
                    "Network path %s validated by handshake", network_path.addr
                )
                network_path.is_validated = True
            network_path.bytes_received += end_off - start_off
            if network_path not in self._network_paths:
                self._network_paths.append(network_path)
            idx = self._network_paths.index(network_path)
            if idx and not is_probing and packet_number > space.largest_received_packet:
                self._logger.debug("Network path %s promoted", network_path.addr)
                self._network_paths.pop(idx)
                self._network_paths.insert(0, network_path)

            # record packet as received
            if not space.discarded:
                if packet_number > space.largest_received_packet:
                    space.largest_received_packet = packet_number
                    space.largest_received_time = now
                space.ack_queue.add(packet_number)
                if is_ack_eliciting and space.ack_at is None:
                    space.ack_at = now + self._ack_delay

    def request_key_update(self) -> None:
        """
        Request an update of the encryption keys.

        .. aioquic_transmit::
        """
        assert self._handshake_complete, "cannot change key before handshake completes"
        self._cryptos[tls.Epoch.ONE_RTT].update_key()

    def reset_stream(self, stream_id: int, error_code: int) -> None:
        """
        Abruptly terminate the sending part of a stream.

        .. aioquic_transmit::

        :param stream_id: The stream's ID.
        :param error_code: An error code indicating why the stream is being reset.
        """
        stream = self._get_or_create_stream_for_send(stream_id)
        stream.sender.reset(error_code)

    def send_ping(self, uid: int) -> None:
        """
        Send a PING frame to the peer.

        .. aioquic_transmit::

        :param uid: A unique ID for this PING.
        """
        self._ping_pending.append(uid)

    def send_datagram_frame(self, data: bytes) -> None:
        """
        Send a DATAGRAM frame.

        .. aioquic_transmit::

        :param data: The data to be sent.
        """
        self._datagrams_pending.append(data)

    def send_stream_data(
        self, stream_id: int, data: bytes, end_stream: bool = False
    ) -> None:
        """
        Send data on the specific stream.

        .. aioquic_transmit::

        :param stream_id: The stream's ID.
        :param data: The data to be sent.
        :param end_stream: If set to `True`, the FIN bit will be set.
        """
        stream = self._get_or_create_stream_for_send(stream_id)
        stream.sender.write(data, end_stream=end_stream)

    def stop_stream(self, stream_id: int, error_code: int) -> None:
        """
        Request termination of the receiving part of a stream.

        .. aioquic_transmit::

        :param stream_id: The stream's ID.
        :param error_code: An error code indicating why the stream is being stopped.
        """
        if not self._stream_can_receive(stream_id):
            raise ValueError(
                "Cannot stop receiving on a local-initiated unidirectional stream"
            )

        stream = self._streams.get(stream_id, None)
        if stream is None:
            raise ValueError("Cannot stop receiving on an unknown stream")

        stream.receiver.stop(error_code)

    # Private

    def _alpn_handler(self, alpn_protocol: str) -> None:
        """
        Callback which is invoked by the TLS engine when ALPN negotiation completes.
        """
        self._events.append(events.ProtocolNegotiated(alpn_protocol=alpn_protocol))

    def _assert_stream_can_receive(self, frame_type: int, stream_id: int) -> None:
        """
        Check the specified stream can receive data or raises a QuicConnectionError.
        """
        if not self._stream_can_receive(stream_id):
            raise QuicConnectionError(
                error_code=QuicErrorCode.STREAM_STATE_ERROR,
                frame_type=frame_type,
                reason_phrase="Stream is send-only",
            )

    def _assert_stream_can_send(self, frame_type: int, stream_id: int) -> None:
        """
        Check the specified stream can send data or raises a QuicConnectionError.
        """
        if not self._stream_can_send(stream_id):
            raise QuicConnectionError(
                error_code=QuicErrorCode.STREAM_STATE_ERROR,
                frame_type=frame_type,
                reason_phrase="Stream is receive-only",
            )

    def _consume_peer_cid(self) -> None:
        """
        Update the destination connection ID by taking the next
        available connection ID provided by the peer.
        """

        self._peer_cid = self._peer_cid_available.pop(0)
        self._logger.debug(
            "Switching to CID %s (%d)",
            dump_cid(self._peer_cid.cid),
            self._peer_cid.sequence_number,
        )

    def _close_begin(self, is_initiator: bool, now: float) -> None:
        """
        Begin the close procedure.
        """
        self._close_at = now + 3 * self._loss.get_probe_timeout()
        if is_initiator:
            self._set_state(QuicConnectionState.CLOSING)
        else:
            self._set_state(QuicConnectionState.DRAINING)

    def _close_end(self) -> None:
        """
        End the close procedure.
        """
        self._close_at = None
        for epoch in self._spaces.keys():
            self._discard_epoch(epoch)
        self._events.append(self._close_event)
        self._set_state(QuicConnectionState.TERMINATED)

        # signal log end
        if self._quic_logger is not None:
            self._configuration.quic_logger.end_trace(self._quic_logger)
            self._quic_logger = None

    def _connect(self, now: float) -> None:
        """
        Start the client handshake.
        """
        assert self._is_client

        if self._quic_logger is not None:
            self._quic_logger.log_event(
                category="transport",
                event="version_information",
                data={
                    "client_versions": self._configuration.supported_versions,
                    "chosen_version": self._version,
                },
            )
            self._quic_logger.log_event(
                category="transport",
                event="alpn_information",
                data={"client_alpns": self._configuration.alpn_protocols},
            )

        self._close_at = now + self._idle_timeout()
        self._initialize(self._peer_cid.cid)

        self.tls.handle_message(b"", self._crypto_buffers)
        self._push_crypto_data()

    def _discard_epoch(self, epoch: tls.Epoch) -> None:
        if not self._spaces[epoch].discarded:
            self._logger.debug("Discarding epoch %s", epoch)
            self._cryptos[epoch].teardown()
            self._loss.discard_space(self._spaces[epoch])
            self._spaces[epoch].discarded = True

    def _find_network_path(self, addr: NetworkAddress) -> QuicNetworkPath:
        # check existing network paths
        for idx, network_path in enumerate(self._network_paths):
            if network_path.addr == addr:
                return network_path

        # new network path
        network_path = QuicNetworkPath(addr)
        self._logger.debug("Network path %s discovered", network_path.addr)
        return network_path

    def _get_or_create_stream(self, frame_type: int, stream_id: int) -> QuicStream:
        """
        Get or create a stream in response to a received frame.
        """
        if stream_id in self._streams_finished:
            # the stream was created, but its state was since discarded
            raise StreamFinishedError

        stream = self._streams.get(stream_id, None)
        if stream is None:
            # check initiator
            if stream_is_client_initiated(stream_id) == self._is_client:
                raise QuicConnectionError(
                    error_code=QuicErrorCode.STREAM_STATE_ERROR,
                    frame_type=frame_type,
                    reason_phrase="Wrong stream initiator",
                )

            # determine limits
            if stream_is_unidirectional(stream_id):
                max_stream_data_local = self._local_max_stream_data_uni
                max_stream_data_remote = 0
                max_streams = self._local_max_streams_uni
            else:
                max_stream_data_local = self._local_max_stream_data_bidi_remote
                max_stream_data_remote = self._remote_max_stream_data_bidi_local
                max_streams = self._local_max_streams_bidi

            # check max streams
            stream_count = (stream_id // 4) + 1
            if stream_count > max_streams.value:
                raise QuicConnectionError(
                    error_code=QuicErrorCode.STREAM_LIMIT_ERROR,
                    frame_type=frame_type,
                    reason_phrase="Too many streams open",
                )
            elif stream_count > max_streams.used:
                max_streams.used = stream_count

            # create stream
            self._logger.debug("Stream %d created by peer" % stream_id)
            stream = self._streams[stream_id] = QuicStream(
                stream_id=stream_id,
                max_stream_data_local=max_stream_data_local,
                max_stream_data_remote=max_stream_data_remote,
                writable=not stream_is_unidirectional(stream_id),
            )
            self._streams_queue.append(stream)
        return stream

    def _get_or_create_stream_for_send(self, stream_id: int) -> QuicStream:
        """
        Get or create a QUIC stream in order to send data to the peer.

        This always occurs as a result of an API call.
        """
        if not self._stream_can_send(stream_id):
            raise ValueError("Cannot send data on peer-initiated unidirectional stream")

        stream = self._streams.get(stream_id, None)
        if stream is None:
            # check initiator
            if stream_is_client_initiated(stream_id) != self._is_client:
                raise ValueError("Cannot send data on unknown peer-initiated stream")

            # determine limits
            if stream_is_unidirectional(stream_id):
                max_stream_data_local = 0
                max_stream_data_remote = self._remote_max_stream_data_uni
                max_streams = self._remote_max_streams_uni
                streams_blocked = self._streams_blocked_uni
            else:
                max_stream_data_local = self._local_max_stream_data_bidi_local
                max_stream_data_remote = self._remote_max_stream_data_bidi_remote
                max_streams = self._remote_max_streams_bidi
                streams_blocked = self._streams_blocked_bidi

            # create stream
            is_unidirectional = stream_is_unidirectional(stream_id)
            stream = self._streams[stream_id] = QuicStream(
                stream_id=stream_id,
                max_stream_data_local=max_stream_data_local,
                max_stream_data_remote=max_stream_data_remote,
                readable=not is_unidirectional,
            )
            self._streams_queue.append(stream)
            if is_unidirectional:
                self._local_next_stream_id_uni = stream_id + 4
            else:
                self._local_next_stream_id_bidi = stream_id + 4

            # mark stream as blocked if needed
            if stream_id // 4 >= max_streams:
                stream.is_blocked = True
                streams_blocked.append(stream)
                self._streams_blocked_pending = True
        return stream

    def _handle_session_ticket(self, session_ticket: tls.SessionTicket) -> None:
        if (
            session_ticket.max_early_data_size is not None
            and session_ticket.max_early_data_size != MAX_EARLY_DATA
        ):
            raise QuicConnectionError(
                error_code=QuicErrorCode.PROTOCOL_VIOLATION,
                frame_type=QuicFrameType.CRYPTO,
                reason_phrase="Invalid max_early_data value %s"
                % session_ticket.max_early_data_size,
            )
        self._session_ticket_handler(session_ticket)

    def _initialize(self, peer_cid: bytes) -> None:
        # TLS
        self.tls = tls.Context(
            alpn_protocols=self._configuration.alpn_protocols,
            cadata=self._configuration.cadata,
            cafile=self._configuration.cafile,
            capath=self._configuration.capath,
            cipher_suites=self.configuration.cipher_suites,
            is_client=self._is_client,
            logger=self._logger,
            max_early_data=None if self._is_client else MAX_EARLY_DATA,
            server_name=self._configuration.server_name,
            verify_mode=self._configuration.verify_mode,
        )
        self.tls.certificate = self._configuration.certificate
        self.tls.certificate_chain = self._configuration.certificate_chain
        self.tls.certificate_private_key = self._configuration.private_key
        self.tls.handshake_extensions = [
            (
                get_transport_parameters_extension(self._version),
                self._serialize_transport_parameters(),
            )
        ]

        # TLS session resumption
        session_ticket = self._configuration.session_ticket
        if (
            self._is_client
            and session_ticket is not None
            and session_ticket.is_valid
            and session_ticket.server_name == self._configuration.server_name
        ):
            self.tls.session_ticket = self._configuration.session_ticket

            # parse saved QUIC transport parameters - for 0-RTT
            if session_ticket.max_early_data_size == MAX_EARLY_DATA:
                for ext_type, ext_data in session_ticket.other_extensions:
                    if ext_type == get_transport_parameters_extension(self._version):
                        self._parse_transport_parameters(
                            ext_data, from_session_ticket=True
                        )
                        break

        # TLS callbacks
        self.tls.alpn_cb = self._alpn_handler
        if self._session_ticket_fetcher is not None:
            self.tls.get_session_ticket_cb = self._session_ticket_fetcher
        if self._session_ticket_handler is not None:
            self.tls.new_session_ticket_cb = self._handle_session_ticket
        self.tls.update_traffic_key_cb = self._update_traffic_key

        # packet spaces
        def create_crypto_pair(epoch: tls.Epoch) -> CryptoPair:
            epoch_name = ["initial", "0rtt", "handshake", "1rtt"][epoch.value]
            secret_names = [
                "server_%s_secret" % epoch_name,
                "client_%s_secret" % epoch_name,
            ]
            recv_secret_name = secret_names[not self._is_client]
            send_secret_name = secret_names[self._is_client]
            return CryptoPair(
                recv_setup_cb=partial(self._log_key_updated, recv_secret_name),
                recv_teardown_cb=partial(self._log_key_retired, recv_secret_name),
                send_setup_cb=partial(self._log_key_updated, send_secret_name),
                send_teardown_cb=partial(self._log_key_retired, send_secret_name),
            )

        self._cryptos = dict(
            (epoch, create_crypto_pair(epoch))
            for epoch in (
                tls.Epoch.INITIAL,
                tls.Epoch.ZERO_RTT,
                tls.Epoch.HANDSHAKE,
                tls.Epoch.ONE_RTT,
            )
        )
        self._crypto_buffers = {
            tls.Epoch.INITIAL: Buffer(capacity=CRYPTO_BUFFER_SIZE),
            tls.Epoch.HANDSHAKE: Buffer(capacity=CRYPTO_BUFFER_SIZE),
            tls.Epoch.ONE_RTT: Buffer(capacity=CRYPTO_BUFFER_SIZE),
        }
        self._crypto_streams = {
            tls.Epoch.INITIAL: QuicStream(),
            tls.Epoch.HANDSHAKE: QuicStream(),
            tls.Epoch.ONE_RTT: QuicStream(),
        }
        self._spaces = {
            tls.Epoch.INITIAL: QuicPacketSpace(),
            tls.Epoch.HANDSHAKE: QuicPacketSpace(),
            tls.Epoch.ONE_RTT: QuicPacketSpace(),
        }

        self._cryptos[tls.Epoch.INITIAL].setup_initial(
            cid=peer_cid, is_client=self._is_client, version=self._version
        )

        self._loss.spaces = list(self._spaces.values())

    def _handle_ack_frame(
        self, context: QuicReceiveContext, frame_type: int, buf: Buffer
    ) -> None:
        """
        Handle an ACK frame.
        """
        ack_rangeset, ack_delay_encoded = pull_ack_frame(buf)
        if frame_type == QuicFrameType.ACK_ECN:
            buf.pull_uint_var()
            buf.pull_uint_var()
            buf.pull_uint_var()
        ack_delay = (ack_delay_encoded << self._remote_ack_delay_exponent) / 1000000

        # log frame
        if self._quic_logger is not None:
            context.quic_logger_frames.append(
                self._quic_logger.encode_ack_frame(ack_rangeset, ack_delay)
            )

        # check whether peer completed address validation
        if not self._loss.peer_completed_address_validation and context.epoch in (
            tls.Epoch.HANDSHAKE,
            tls.Epoch.ONE_RTT,
        ):
            self._loss.peer_completed_address_validation = True

        self._loss.on_ack_received(
            ack_rangeset=ack_rangeset,
            ack_delay=ack_delay,
            now=context.time,
            space=self._spaces[context.epoch],
        )

    def _handle_connection_close_frame(
        self, context: QuicReceiveContext, frame_type: int, buf: Buffer
    ) -> None:
        """
        Handle a CONNECTION_CLOSE frame.
        """
        error_code = buf.pull_uint_var()
        if frame_type == QuicFrameType.TRANSPORT_CLOSE:
            frame_type = buf.pull_uint_var()
        else:
            frame_type = None
        reason_length = buf.pull_uint_var()
        try:
            reason_phrase = buf.pull_bytes(reason_length).decode("utf8")
        except UnicodeDecodeError:
            reason_phrase = ""

        # log frame
        if self._quic_logger is not None:
            context.quic_logger_frames.append(
                self._quic_logger.encode_connection_close_frame(
                    error_code=error_code,
                    frame_type=frame_type,
                    reason_phrase=reason_phrase,
                )
            )

        self._logger.info(
            "Connection close received (code 0x%X, reason %s)",
            error_code,
            reason_phrase,
        )
        if self._close_event is None:
            self._close_event = events.ConnectionTerminated(
                error_code=error_code,
                frame_type=frame_type,
                reason_phrase=reason_phrase,
            )
            self._close_begin(is_initiator=False, now=context.time)

    def _handle_crypto_frame(
        self, context: QuicReceiveContext, frame_type: int, buf: Buffer
    ) -> None:
        """
        Handle a CRYPTO frame.
        """
        offset = buf.pull_uint_var()
        length = buf.pull_uint_var()
        if offset + length > UINT_VAR_MAX:
            raise QuicConnectionError(
                error_code=QuicErrorCode.FRAME_ENCODING_ERROR,
                frame_type=frame_type,
                reason_phrase="offset + length cannot exceed 2^62 - 1",
            )
        frame = QuicStreamFrame(offset=offset, data=buf.pull_bytes(length))

        # log frame
        if self._quic_logger is not None:
            context.quic_logger_frames.append(
                self._quic_logger.encode_crypto_frame(frame)
            )

        stream = self._crypto_streams[context.epoch]
        event = stream.receiver.handle_frame(frame)
        if event is not None:
            # pass data to TLS layer
            try:
                self.tls.handle_message(event.data, self._crypto_buffers)
                self._push_crypto_data()
            except tls.Alert as exc:
                raise QuicConnectionError(
                    error_code=QuicErrorCode.CRYPTO_ERROR + int(exc.description),
                    frame_type=frame_type,
                    reason_phrase=str(exc),
                )

            # parse transport parameters
            if (
                not self._parameters_received
                and self.tls.received_extensions is not None
            ):
                for ext_type, ext_data in self.tls.received_extensions:
                    if ext_type == get_transport_parameters_extension(self._version):
                        self._parse_transport_parameters(ext_data)
                        self._parameters_received = True
                        break
                if not self._parameters_received:
                    raise QuicConnectionError(
                        error_code=QuicErrorCode.CRYPTO_ERROR
                        + tls.AlertDescription.missing_extension,
                        frame_type=frame_type,
                        reason_phrase="No QUIC transport parameters received",
                    )

            # update current epoch
            if not self._handshake_complete and self.tls.state in [
                tls.State.CLIENT_POST_HANDSHAKE,
                tls.State.SERVER_POST_HANDSHAKE,
            ]:
                self._handshake_complete = True

                # for servers, the handshake is now confirmed
                if not self._is_client:
                    self._discard_epoch(tls.Epoch.HANDSHAKE)
                    self._handshake_confirmed = True
                    self._handshake_done_pending = True

                self._replenish_connection_ids()
                self._events.append(
                    events.HandshakeCompleted(
                        alpn_protocol=self.tls.alpn_negotiated,
                        early_data_accepted=self.tls.early_data_accepted,
                        session_resumed=self.tls.session_resumed,
                    )
                )
                self._unblock_streams(is_unidirectional=False)
                self._unblock_streams(is_unidirectional=True)
                self._logger.info(
                    "ALPN negotiated protocol %s", self.tls.alpn_negotiated
                )
        else:
            self._logger.info(
                "Duplicate CRYPTO data received for epoch %s", context.epoch
            )

            # if a server receives duplicate CRYPTO in an INITIAL packet,
            # it can assume the client did not receive the server's CRYPTO
            if (
                not self._is_client
                and context.epoch == tls.Epoch.INITIAL
                and not self._crypto_retransmitted
            ):
                self._loss.reschedule_data(now=context.time)
                self._crypto_retransmitted = True

    def _handle_data_blocked_frame(
        self, context: QuicReceiveContext, frame_type: int, buf: Buffer
    ) -> None:
        """
        Handle a DATA_BLOCKED frame.
        """
        limit = buf.pull_uint_var()

        # log frame
        if self._quic_logger is not None:
            context.quic_logger_frames.append(
                self._quic_logger.encode_data_blocked_frame(limit=limit)
            )

    def _handle_datagram_frame(
        self, context: QuicReceiveContext, frame_type: int, buf: Buffer
    ) -> None:
        """
        Handle a DATAGRAM frame.
        """
        start = buf.tell()
        if frame_type == QuicFrameType.DATAGRAM_WITH_LENGTH:
            length = buf.pull_uint_var()
        else:
            length = buf.capacity - start
        data = buf.pull_bytes(length)

        # log frame
        if self._quic_logger is not None:
            context.quic_logger_frames.append(
                self._quic_logger.encode_datagram_frame(length=length)
            )

        # check frame is allowed
        if (
            self._configuration.max_datagram_frame_size is None
            or buf.tell() - start >= self._configuration.max_datagram_frame_size
        ):
            raise QuicConnectionError(
                error_code=QuicErrorCode.PROTOCOL_VIOLATION,
                frame_type=frame_type,
                reason_phrase="Unexpected DATAGRAM frame",
            )

        self._events.append(events.DatagramFrameReceived(data=data))

    def _handle_handshake_done_frame(
        self, context: QuicReceiveContext, frame_type: int, buf: Buffer
    ) -> None:
        """
        Handle a HANDSHAKE_DONE frame.
        """
        # log frame
        if self._quic_logger is not None:
            context.quic_logger_frames.append(
                self._quic_logger.encode_handshake_done_frame()
            )

        if not self._is_client:
            raise QuicConnectionError(
                error_code=QuicErrorCode.PROTOCOL_VIOLATION,
                frame_type=frame_type,
                reason_phrase="Clients must not send HANDSHAKE_DONE frames",
            )

        # for clients, the handshake is now confirmed
        if not self._handshake_confirmed:
            self._discard_epoch(tls.Epoch.HANDSHAKE)
            self._handshake_confirmed = True
            self._loss.peer_completed_address_validation = True

    def _handle_max_data_frame(
        self, context: QuicReceiveContext, frame_type: int, buf: Buffer
    ) -> None:
        """
        Handle a MAX_DATA frame.

        This adjusts the total amount of we can send to the peer.
        """
        max_data = buf.pull_uint_var()

        # log frame
        if self._quic_logger is not None:
            context.quic_logger_frames.append(
                self._quic_logger.encode_connection_limit_frame(
                    frame_type=frame_type, maximum=max_data
                )
            )

        if max_data > self._remote_max_data:
            self._logger.debug("Remote max_data raised to %d", max_data)
            self._remote_max_data = max_data

    def _handle_max_stream_data_frame(
        self, context: QuicReceiveContext, frame_type: int, buf: Buffer
    ) -> None:
        """
        Handle a MAX_STREAM_DATA frame.

        This adjusts the amount of data we can send on a specific stream.
        """
        stream_id = buf.pull_uint_var()
        max_stream_data = buf.pull_uint_var()

        # log frame
        if self._quic_logger is not None:
            context.quic_logger_frames.append(
                self._quic_logger.encode_max_stream_data_frame(
                    maximum=max_stream_data, stream_id=stream_id
                )
            )

        # check stream direction
        self._assert_stream_can_send(frame_type, stream_id)

        stream = self._get_or_create_stream(frame_type, stream_id)
        if max_stream_data > stream.max_stream_data_remote:
            self._logger.debug(
                "Stream %d remote max_stream_data raised to %d",
                stream_id,
                max_stream_data,
            )
            stream.max_stream_data_remote = max_stream_data

    def _handle_max_streams_bidi_frame(
        self, context: QuicReceiveContext, frame_type: int, buf: Buffer
    ) -> None:
        """
        Handle a MAX_STREAMS_BIDI frame.

        This raises number of bidirectional streams we can initiate to the peer.
        """
        max_streams = buf.pull_uint_var()
        if max_streams > STREAM_COUNT_MAX:
            raise QuicConnectionError(
                error_code=QuicErrorCode.FRAME_ENCODING_ERROR,
                frame_type=frame_type,
                reason_phrase="Maximum Streams cannot exceed 2^60",
            )

        # log frame
        if self._quic_logger is not None:
            context.quic_logger_frames.append(
                self._quic_logger.encode_connection_limit_frame(
                    frame_type=frame_type, maximum=max_streams
                )
            )

        if max_streams > self._remote_max_streams_bidi:
            self._logger.debug("Remote max_streams_bidi raised to %d", max_streams)
            self._remote_max_streams_bidi = max_streams
            self._unblock_streams(is_unidirectional=False)

    def _handle_max_streams_uni_frame(
        self, context: QuicReceiveContext, frame_type: int, buf: Buffer
    ) -> None:
        """
        Handle a MAX_STREAMS_UNI frame.

        This raises number of unidirectional streams we can initiate to the peer.
        """
        max_streams = buf.pull_uint_var()
        if max_streams > STREAM_COUNT_MAX:
            raise QuicConnectionError(
                error_code=QuicErrorCode.FRAME_ENCODING_ERROR,
                frame_type=frame_type,
                reason_phrase="Maximum Streams cannot exceed 2^60",
            )

        # log frame
        if self._quic_logger is not None:
            context.quic_logger_frames.append(
                self._quic_logger.encode_connection_limit_frame(
                    frame_type=frame_type, maximum=max_streams
                )
            )

        if max_streams > self._remote_max_streams_uni:
            self._logger.debug("Remote max_streams_uni raised to %d", max_streams)
            self._remote_max_streams_uni = max_streams
            self._unblock_streams(is_unidirectional=True)

    def _handle_new_connection_id_frame(
        self, context: QuicReceiveContext, frame_type: int, buf: Buffer
    ) -> None:
        """
        Handle a NEW_CONNECTION_ID frame.
        """
        sequence_number = buf.pull_uint_var()
        retire_prior_to = buf.pull_uint_var()
        length = buf.pull_uint8()
        connection_id = buf.pull_bytes(length)
        stateless_reset_token = buf.pull_bytes(STATELESS_RESET_TOKEN_SIZE)
        if not connection_id or len(connection_id) > CONNECTION_ID_MAX_SIZE:
            raise QuicConnectionError(
                error_code=QuicErrorCode.FRAME_ENCODING_ERROR,
                frame_type=frame_type,
                reason_phrase="Length must be greater than 0 and less than 20",
            )

        # log frame
        if self._quic_logger is not None:
            context.quic_logger_frames.append(
                self._quic_logger.encode_new_connection_id_frame(
                    connection_id=connection_id,
                    retire_prior_to=retire_prior_to,
                    sequence_number=sequence_number,
                    stateless_reset_token=stateless_reset_token,
                )
            )

        # sanity check
        if retire_prior_to > sequence_number:
            raise QuicConnectionError(
                error_code=QuicErrorCode.PROTOCOL_VIOLATION,
                frame_type=frame_type,
                reason_phrase="Retire Prior To is greater than Sequence Number",
            )

        # only accept retire_prior_to if it is bigger than the one we know
        self._peer_retire_prior_to = max(retire_prior_to, self._peer_retire_prior_to)

        # determine which CIDs to retire
        change_cid = False
        retire = [
            cid
            for cid in self._peer_cid_available
            if cid.sequence_number < self._peer_retire_prior_to
        ]
        if self._peer_cid.sequence_number < self._peer_retire_prior_to:
            change_cid = True
            retire.insert(0, self._peer_cid)

        # update available CIDs
        self._peer_cid_available = [
            cid
            for cid in self._peer_cid_available
            if cid.sequence_number >= self._peer_retire_prior_to
        ]
        if (
            sequence_number >= self._peer_retire_prior_to
            and sequence_number not in self._peer_cid_sequence_numbers
        ):
            self._peer_cid_available.append(
                QuicConnectionId(
                    cid=connection_id,
                    sequence_number=sequence_number,
                    stateless_reset_token=stateless_reset_token,
                )
            )
            self._peer_cid_sequence_numbers.add(sequence_number)

        # retire previous CIDs
        for quic_connection_id in retire:
            self._retire_peer_cid(quic_connection_id)

        # assign new CID if we retired the active one
        if change_cid:
            self._consume_peer_cid()

        # check number of active connection IDs, including the selected one
        if 1 + len(self._peer_cid_available) > self._local_active_connection_id_limit:
            raise QuicConnectionError(
                error_code=QuicErrorCode.CONNECTION_ID_LIMIT_ERROR,
                frame_type=frame_type,
                reason_phrase="Too many active connection IDs",
            )

        # Check the number of retired connection IDs pending, though with a safer limit
        # than the 2x recommended in section 5.1.2 of the RFC.  Note that we are doing
        # the check here and not in _retire_peer_cid() because we know the frame type to
        # use here, and because it is the new connection id path that is potentially
        # dangerous.  We may transiently go a bit over the limit due to unacked frames
        # getting added back to the list, but that's ok as it is bounded.
        if len(self._retire_connection_ids) > min(
            self._local_active_connection_id_limit * 4, MAX_PENDING_RETIRES
        ):
            raise QuicConnectionError(
                error_code=QuicErrorCode.CONNECTION_ID_LIMIT_ERROR,
                frame_type=frame_type,
                reason_phrase="Too many pending retired connection IDs",
            )

    def _handle_new_token_frame(
        self, context: QuicReceiveContext, frame_type: int, buf: Buffer
    ) -> None:
        """
        Handle a NEW_TOKEN frame.
        """
        length = buf.pull_uint_var()
        token = buf.pull_bytes(length)

        # log frame
        if self._quic_logger is not None:
            context.quic_logger_frames.append(
                self._quic_logger.encode_new_token_frame(token=token)
            )

        if not self._is_client:
            raise QuicConnectionError(
                error_code=QuicErrorCode.PROTOCOL_VIOLATION,
                frame_type=frame_type,
                reason_phrase="Clients must not send NEW_TOKEN frames",
            )

        if self._token_handler is not None:
            self._token_handler(token)

    def _handle_padding_frame(
        self, context: QuicReceiveContext, frame_type: int, buf: Buffer
    ) -> None:
        """
        Handle a PADDING frame.
        """
        # consume padding
        pos = buf.tell()
        for byte in buf.data_slice(pos, buf.capacity):
            if byte:
                break
            pos += 1
        buf.seek(pos)

        # log frame
        if self._quic_logger is not None:
            context.quic_logger_frames.append(self._quic_logger.encode_padding_frame())

    def _handle_path_challenge_frame(
        self, context: QuicReceiveContext, frame_type: int, buf: Buffer
    ) -> None:
        """
        Handle a PATH_CHALLENGE frame.
        """
        data = buf.pull_bytes(8)

        # log frame
        if self._quic_logger is not None:
            context.quic_logger_frames.append(
                self._quic_logger.encode_path_challenge_frame(data=data)
            )

        context.network_path.remote_challenge = data

    def _handle_path_response_frame(
        self, context: QuicReceiveContext, frame_type: int, buf: Buffer
    ) -> None:
        """
        Handle a PATH_RESPONSE frame.
        """
        data = buf.pull_bytes(8)

        # log frame
        if self._quic_logger is not None:
            context.quic_logger_frames.append(
                self._quic_logger.encode_path_response_frame(data=data)
            )

        if data != context.network_path.local_challenge:
            raise QuicConnectionError(
                error_code=QuicErrorCode.PROTOCOL_VIOLATION,
                frame_type=frame_type,
                reason_phrase="Response does not match challenge",
            )
        self._logger.debug(
            "Network path %s validated by challenge", context.network_path.addr
        )
        context.network_path.is_validated = True

    def _handle_ping_frame(
        self, context: QuicReceiveContext, frame_type: int, buf: Buffer
    ) -> None:
        """
        Handle a PING frame.
        """
        # log frame
        if self._quic_logger is not None:
            context.quic_logger_frames.append(self._quic_logger.encode_ping_frame())

    def _handle_reset_stream_frame(
        self, context: QuicReceiveContext, frame_type: int, buf: Buffer
    ) -> None:
        """
        Handle a RESET_STREAM frame.
        """
        stream_id = buf.pull_uint_var()
        error_code = buf.pull_uint_var()
        final_size = buf.pull_uint_var()

        # log frame
        if self._quic_logger is not None:
            context.quic_logger_frames.append(
                self._quic_logger.encode_reset_stream_frame(
                    error_code=error_code, final_size=final_size, stream_id=stream_id
                )
            )

        # check stream direction
        self._assert_stream_can_receive(frame_type, stream_id)

        # check flow-control limits
        stream = self._get_or_create_stream(frame_type, stream_id)
        if final_size > stream.max_stream_data_local:
            raise QuicConnectionError(
                error_code=QuicErrorCode.FLOW_CONTROL_ERROR,
                frame_type=frame_type,
                reason_phrase="Over stream data limit",
            )
        newly_received = max(0, final_size - stream.receiver.highest_offset)
        if self._local_max_data.used + newly_received > self._local_max_data.value:
            raise QuicConnectionError(
                error_code=QuicErrorCode.FLOW_CONTROL_ERROR,
                frame_type=frame_type,
                reason_phrase="Over connection data limit",
            )

        # process reset
        self._logger.info(
            "Stream %d reset by peer (error code %d, final size %d)",
            stream_id,
            error_code,
            final_size,
        )
        try:
            event = stream.receiver.handle_reset(
                error_code=error_code, final_size=final_size
            )
        except FinalSizeError as exc:
            raise QuicConnectionError(
                error_code=QuicErrorCode.FINAL_SIZE_ERROR,
                frame_type=frame_type,
                reason_phrase=str(exc),
            )
        if event is not None:
            self._events.append(event)
        self._local_max_data.used += newly_received

    def _handle_retire_connection_id_frame(
        self, context: QuicReceiveContext, frame_type: int, buf: Buffer
    ) -> None:
        """
        Handle a RETIRE_CONNECTION_ID frame.
        """
        sequence_number = buf.pull_uint_var()

        # log frame
        if self._quic_logger is not None:
            context.quic_logger_frames.append(
                self._quic_logger.encode_retire_connection_id_frame(sequence_number)
            )

        if sequence_number >= self._host_cid_seq:
            raise QuicConnectionError(
                error_code=QuicErrorCode.PROTOCOL_VIOLATION,
                frame_type=frame_type,
                reason_phrase="Cannot retire unknown connection ID",
            )

        # find the connection ID by sequence number
        for index, connection_id in enumerate(self._host_cids):
            if connection_id.sequence_number == sequence_number:
                if connection_id.cid == context.host_cid:
                    raise QuicConnectionError(
                        error_code=QuicErrorCode.PROTOCOL_VIOLATION,
                        frame_type=frame_type,
                        reason_phrase="Cannot retire current connection ID",
                    )
                self._logger.debug(
                    "Peer retiring CID %s (%d)",
                    dump_cid(connection_id.cid),
                    connection_id.sequence_number,
                )
                del self._host_cids[index]
                self._events.append(
                    events.ConnectionIdRetired(connection_id=connection_id.cid)
                )
                break

        # issue a new connection ID
        self._replenish_connection_ids()

    def _handle_stop_sending_frame(
        self, context: QuicReceiveContext, frame_type: int, buf: Buffer
    ) -> None:
        """
        Handle a STOP_SENDING frame.
        """
        stream_id = buf.pull_uint_var()
        error_code = buf.pull_uint_var()  # application error code

        # log frame
        if self._quic_logger is not None:
            context.quic_logger_frames.append(
                self._quic_logger.encode_stop_sending_frame(
                    error_code=error_code, stream_id=stream_id
                )
            )

        # check stream direction
        self._assert_stream_can_send(frame_type, stream_id)

        # reset the stream
        stream = self._get_or_create_stream(frame_type, stream_id)
        stream.sender.reset(error_code=QuicErrorCode.NO_ERROR)

        self._events.append(
            events.StopSendingReceived(error_code=error_code, stream_id=stream_id)
        )

    def _handle_stream_frame(
        self, context: QuicReceiveContext, frame_type: int, buf: Buffer
    ) -> None:
        """
        Handle a STREAM frame.
        """
        stream_id = buf.pull_uint_var()
        if frame_type & 4:
            offset = buf.pull_uint_var()
        else:
            offset = 0
        if frame_type & 2:
            length = buf.pull_uint_var()
        else:
            length = buf.capacity - buf.tell()
        if offset + length > UINT_VAR_MAX:
            raise QuicConnectionError(
                error_code=QuicErrorCode.FRAME_ENCODING_ERROR,
                frame_type=frame_type,
                reason_phrase="offset + length cannot exceed 2^62 - 1",
            )
        frame = QuicStreamFrame(
            offset=offset, data=buf.pull_bytes(length), fin=bool(frame_type & 1)
        )

        # log frame
        if self._quic_logger is not None:
            context.quic_logger_frames.append(
                self._quic_logger.encode_stream_frame(frame, stream_id=stream_id)
            )

        # check stream direction
        self._assert_stream_can_receive(frame_type, stream_id)

        # check flow-control limits
        stream = self._get_or_create_stream(frame_type, stream_id)
        if offset + length > stream.max_stream_data_local:
            raise QuicConnectionError(
                error_code=QuicErrorCode.FLOW_CONTROL_ERROR,
                frame_type=frame_type,
                reason_phrase="Over stream data limit",
            )
        newly_received = max(0, offset + length - stream.receiver.highest_offset)
        if self._local_max_data.used + newly_received > self._local_max_data.value:
            raise QuicConnectionError(
                error_code=QuicErrorCode.FLOW_CONTROL_ERROR,
                frame_type=frame_type,
                reason_phrase="Over connection data limit",
            )

        # process data
        try:
            event = stream.receiver.handle_frame(frame)
        except FinalSizeError as exc:
            raise QuicConnectionError(
                error_code=QuicErrorCode.FINAL_SIZE_ERROR,
                frame_type=frame_type,
                reason_phrase=str(exc),
            )
        if event is not None:
            self._events.append(event)
        self._local_max_data.used += newly_received

    def _handle_stream_data_blocked_frame(
        self, context: QuicReceiveContext, frame_type: int, buf: Buffer
    ) -> None:
        """
        Handle a STREAM_DATA_BLOCKED frame.
        """
        stream_id = buf.pull_uint_var()
        limit = buf.pull_uint_var()

        # log frame
        if self._quic_logger is not None:
            context.quic_logger_frames.append(
                self._quic_logger.encode_stream_data_blocked_frame(
                    limit=limit, stream_id=stream_id
                )
            )

        # check stream direction
        self._assert_stream_can_receive(frame_type, stream_id)

        self._get_or_create_stream(frame_type, stream_id)

    def _handle_streams_blocked_frame(
        self, context: QuicReceiveContext, frame_type: int, buf: Buffer
    ) -> None:
        """
        Handle a STREAMS_BLOCKED frame.
        """
        limit = buf.pull_uint_var()
        if limit > STREAM_COUNT_MAX:
            raise QuicConnectionError(
                error_code=QuicErrorCode.FRAME_ENCODING_ERROR,
                frame_type=frame_type,
                reason_phrase="Maximum Streams cannot exceed 2^60",
            )

        # log frame
        if self._quic_logger is not None:
            context.quic_logger_frames.append(
                self._quic_logger.encode_streams_blocked_frame(
                    is_unidirectional=frame_type == QuicFrameType.STREAMS_BLOCKED_UNI,
                    limit=limit,
                )
            )

    def _log_key_retired(self, key_type: str, trigger: str) -> None:
        """
        Log a key retirement.
        """
        if self._quic_logger is not None:
            self._quic_logger.log_event(
                category="security",
                event="key_retired",
                data={"key_type": key_type, "trigger": trigger},
            )

    def _log_key_updated(self, key_type: str, trigger: str) -> None:
        """
        Log a key update.
        """
        if self._quic_logger is not None:
            self._quic_logger.log_event(
                category="security",
                event="key_updated",
                data={"key_type": key_type, "trigger": trigger},
            )

    def _on_ack_delivery(
        self, delivery: QuicDeliveryState, space: QuicPacketSpace, highest_acked: int
    ) -> None:
        """
        Callback when an ACK frame is acknowledged or lost.
        """
        if delivery == QuicDeliveryState.ACKED:
            space.ack_queue.subtract(0, highest_acked + 1)

    def _on_connection_limit_delivery(
        self, delivery: QuicDeliveryState, limit: Limit
    ) -> None:
        """
        Callback when a MAX_DATA or MAX_STREAMS frame is acknowledged or lost.
        """
        if delivery != QuicDeliveryState.ACKED:
            limit.sent = 0

    def _on_handshake_done_delivery(self, delivery: QuicDeliveryState) -> None:
        """
        Callback when a HANDSHAKE_DONE frame is acknowledged or lost.
        """
        if delivery != QuicDeliveryState.ACKED:
            self._handshake_done_pending = True

    def _on_max_stream_data_delivery(
        self, delivery: QuicDeliveryState, stream: QuicStream
    ) -> None:
        """
        Callback when a MAX_STREAM_DATA frame is acknowledged or lost.
        """
        if delivery != QuicDeliveryState.ACKED:
            stream.max_stream_data_local_sent = 0

    def _on_new_connection_id_delivery(
        self, delivery: QuicDeliveryState, connection_id: QuicConnectionId
    ) -> None:
        """
        Callback when a NEW_CONNECTION_ID frame is acknowledged or lost.
        """
        if delivery != QuicDeliveryState.ACKED:
            connection_id.was_sent = False

    def _on_ping_delivery(
        self, delivery: QuicDeliveryState, uids: Sequence[int]
    ) -> None:
        """
        Callback when a PING frame is acknowledged or lost.
        """
        if delivery == QuicDeliveryState.ACKED:
            self._logger.debug("Received PING%s response", "" if uids else " (probe)")
            for uid in uids:
                self._events.append(events.PingAcknowledged(uid=uid))
        else:
            self._ping_pending.extend(uids)

    def _on_retire_connection_id_delivery(
        self, delivery: QuicDeliveryState, sequence_number: int
    ) -> None:
        """
        Callback when a RETIRE_CONNECTION_ID frame is acknowledged or lost.
        """
        if delivery != QuicDeliveryState.ACKED:
            self._retire_connection_ids.append(sequence_number)

    def _payload_received(
        self,
        context: QuicReceiveContext,
        plain: bytes,
        crypto_frame_required: bool = False,
    ) -> Tuple[bool, bool]:
        """
        Handle a QUIC packet payload.
        """
        buf = Buffer(data=plain)

        crypto_frame_found = False
        frame_found = False
        is_ack_eliciting = False
        is_probing = None
        while not buf.eof():
            # get frame type
            try:
                frame_type = buf.pull_uint_var()
            except BufferReadError:
                raise QuicConnectionError(
                    error_code=QuicErrorCode.FRAME_ENCODING_ERROR,
                    frame_type=None,
                    reason_phrase="Malformed frame type",
                )

            # check frame type is known
            try:
                frame_handler, frame_epochs = self.__frame_handlers[frame_type]
            except KeyError:
                raise QuicConnectionError(
                    error_code=QuicErrorCode.FRAME_ENCODING_ERROR,
                    frame_type=frame_type,
                    reason_phrase="Unknown frame type",
                )

            # check frame type is allowed for the epoch
            if context.epoch not in frame_epochs:
                raise QuicConnectionError(
                    error_code=QuicErrorCode.PROTOCOL_VIOLATION,
                    frame_type=frame_type,
                    reason_phrase="Unexpected frame type",
                )

            # handle the frame
            try:
                frame_handler(context, frame_type, buf)
            except BufferReadError:
                raise QuicConnectionError(
                    error_code=QuicErrorCode.FRAME_ENCODING_ERROR,
                    frame_type=frame_type,
                    reason_phrase="Failed to parse frame",
                )
            except StreamFinishedError:
                # we lack the state for the stream, ignore the frame
                pass

            # update ACK only / probing flags
            frame_found = True

            if frame_type == QuicFrameType.CRYPTO:
                crypto_frame_found = True

            if frame_type not in NON_ACK_ELICITING_FRAME_TYPES:
                is_ack_eliciting = True

            if frame_type not in PROBING_FRAME_TYPES:
                is_probing = False
            elif is_probing is None:
                is_probing = True

        if not frame_found:
            raise QuicConnectionError(
                error_code=QuicErrorCode.PROTOCOL_VIOLATION,
                frame_type=QuicFrameType.PADDING,
                reason_phrase="Packet contains no frames",
            )

        # RFC 9000 - 17.2.2. Initial Packet
        # The first packet sent by a client always includes a CRYPTO frame.
        if crypto_frame_required and not crypto_frame_found:
            raise QuicConnectionError(
                error_code=QuicErrorCode.PROTOCOL_VIOLATION,
                frame_type=QuicFrameType.PADDING,
                reason_phrase="Packet contains no CRYPTO frame",
            )

        return is_ack_eliciting, bool(is_probing)

    def _replenish_connection_ids(self) -> None:
        """
        Generate new connection IDs.
        """
        while len(self._host_cids) < min(8, self._remote_active_connection_id_limit):
            self._host_cids.append(
                QuicConnectionId(
                    cid=os.urandom(self._configuration.connection_id_length),
                    sequence_number=self._host_cid_seq,
                    stateless_reset_token=os.urandom(16),
                )
            )
            self._host_cid_seq += 1

    def _retire_peer_cid(self, connection_id: QuicConnectionId) -> None:
        """
        Retire a destination connection ID.
        """
        self._logger.debug(
            "Retiring CID %s (%d) [%d]",
            dump_cid(connection_id.cid),
            connection_id.sequence_number,
            len(self._retire_connection_ids) + 1,
        )
        self._retire_connection_ids.append(connection_id.sequence_number)

    def _push_crypto_data(self) -> None:
        for epoch, buf in self._crypto_buffers.items():
            self._crypto_streams[epoch].sender.write(buf.data)
            buf.seek(0)

    def _send_probe(self) -> None:
        self._probe_pending = True

    def _parse_transport_parameters(
        self, data: bytes, from_session_ticket: bool = False
    ) -> None:
        """
        Parse and apply remote transport parameters.

        `from_session_ticket` is `True` when restoring saved transport parameters,
        and `False` when handling received transport parameters.
        """

        try:
            quic_transport_parameters = pull_quic_transport_parameters(
                Buffer(data=data)
            )
        except ValueError:
            raise QuicConnectionError(
                error_code=QuicErrorCode.TRANSPORT_PARAMETER_ERROR,
                frame_type=QuicFrameType.CRYPTO,
                reason_phrase="Could not parse QUIC transport parameters",
            )

        # log event
        if self._quic_logger is not None and not from_session_ticket:
            self._quic_logger.log_event(
                category="transport",
                event="parameters_set",
                data=self._quic_logger.encode_transport_parameters(
                    owner="remote", parameters=quic_transport_parameters
                ),
            )

        # validate remote parameters
        if not self._is_client:
            for attr in [
                "original_destination_connection_id",
                "preferred_address",
                "retry_source_connection_id",
                "stateless_reset_token",
            ]:
                if getattr(quic_transport_parameters, attr) is not None:
                    raise QuicConnectionError(
                        error_code=QuicErrorCode.TRANSPORT_PARAMETER_ERROR,
                        frame_type=QuicFrameType.CRYPTO,
                        reason_phrase="%s is not allowed for clients" % attr,
                    )

        if not from_session_ticket:
            if (
                quic_transport_parameters.initial_source_connection_id
                != self._remote_initial_source_connection_id
            ):
                raise QuicConnectionError(
                    error_code=QuicErrorCode.TRANSPORT_PARAMETER_ERROR,
                    frame_type=QuicFrameType.CRYPTO,
                    reason_phrase="initial_source_connection_id does not match",
                )
            if self._is_client and (
                quic_transport_parameters.original_destination_connection_id
                != self._original_destination_connection_id
            ):
                raise QuicConnectionError(
                    error_code=QuicErrorCode.TRANSPORT_PARAMETER_ERROR,
                    frame_type=QuicFrameType.CRYPTO,
                    reason_phrase="original_destination_connection_id does not match",
                )
            if self._is_client and (
                quic_transport_parameters.retry_source_connection_id
                != self._retry_source_connection_id
            ):
                raise QuicConnectionError(
                    error_code=QuicErrorCode.TRANSPORT_PARAMETER_ERROR,
                    frame_type=QuicFrameType.CRYPTO,
                    reason_phrase="retry_source_connection_id does not match",
                )
            if (
                quic_transport_parameters.active_connection_id_limit is not None
                and quic_transport_parameters.active_connection_id_limit < 2
            ):
                raise QuicConnectionError(
                    error_code=QuicErrorCode.TRANSPORT_PARAMETER_ERROR,
                    frame_type=QuicFrameType.CRYPTO,
                    reason_phrase="active_connection_id_limit must be no less than 2",
                )
            if (
                quic_transport_parameters.ack_delay_exponent is not None
                and quic_transport_parameters.ack_delay_exponent > 20
            ):
                raise QuicConnectionError(
                    error_code=QuicErrorCode.TRANSPORT_PARAMETER_ERROR,
                    frame_type=QuicFrameType.CRYPTO,
                    reason_phrase="ack_delay_exponent must be <= 20",
                )
            if (
                quic_transport_parameters.max_ack_delay is not None
                and quic_transport_parameters.max_ack_delay >= 2**14
            ):
                raise QuicConnectionError(
                    error_code=QuicErrorCode.TRANSPORT_PARAMETER_ERROR,
                    frame_type=QuicFrameType.CRYPTO,
                    reason_phrase="max_ack_delay must be < 2^14",
                )
            if quic_transport_parameters.max_udp_payload_size is not None and (
                quic_transport_parameters.max_udp_payload_size
                < SMALLEST_MAX_DATAGRAM_SIZE
            ):
                raise QuicConnectionError(
                    error_code=QuicErrorCode.TRANSPORT_PARAMETER_ERROR,
                    frame_type=QuicFrameType.CRYPTO,
                    reason_phrase=(
                        f"max_udp_payload_size must be >= {SMALLEST_MAX_DATAGRAM_SIZE}"
                    ),
                )

        # store remote parameters
        if not from_session_ticket:
            if quic_transport_parameters.ack_delay_exponent is not None:
                self._remote_ack_delay_exponent = self._remote_ack_delay_exponent
            if quic_transport_parameters.max_ack_delay is not None:
                self._loss.max_ack_delay = (
                    quic_transport_parameters.max_ack_delay / 1000.0
                )
            if (
                self._is_client
                and self._peer_cid.sequence_number == 0
                and quic_transport_parameters.stateless_reset_token is not None
            ):
                self._peer_cid.stateless_reset_token = (
                    quic_transport_parameters.stateless_reset_token
                )

        if quic_transport_parameters.active_connection_id_limit is not None:
            self._remote_active_connection_id_limit = (
                quic_transport_parameters.active_connection_id_limit
            )
        if quic_transport_parameters.max_idle_timeout is not None:
            self._remote_max_idle_timeout = (
                quic_transport_parameters.max_idle_timeout / 1000.0
            )
        self._remote_max_datagram_frame_size = (
            quic_transport_parameters.max_datagram_frame_size
        )
        for param in [
            "max_data",
            "max_stream_data_bidi_local",
            "max_stream_data_bidi_remote",
            "max_stream_data_uni",
            "max_streams_bidi",
            "max_streams_uni",
        ]:
            value = getattr(quic_transport_parameters, "initial_" + param)
            if value is not None:
                setattr(self, "_remote_" + param, value)

    def _serialize_transport_parameters(self) -> bytes:
        quic_transport_parameters = QuicTransportParameters(
            ack_delay_exponent=self._local_ack_delay_exponent,
            active_connection_id_limit=self._local_active_connection_id_limit,
            max_idle_timeout=int(self._configuration.idle_timeout * 1000),
            initial_max_data=self._local_max_data.value,
            initial_max_stream_data_bidi_local=self._local_max_stream_data_bidi_local,
            initial_max_stream_data_bidi_remote=self._local_max_stream_data_bidi_remote,
            initial_max_stream_data_uni=self._local_max_stream_data_uni,
            initial_max_streams_bidi=self._local_max_streams_bidi.value,
            initial_max_streams_uni=self._local_max_streams_uni.value,
            initial_source_connection_id=self._local_initial_source_connection_id,
            max_ack_delay=25,
            max_datagram_frame_size=self._configuration.max_datagram_frame_size,
            quantum_readiness=(
                b"Q" * SMALLEST_MAX_DATAGRAM_SIZE
                if self._configuration.quantum_readiness_test
                else None
            ),
            stateless_reset_token=self._host_cids[0].stateless_reset_token,
        )
        if not self._is_client:
            quic_transport_parameters.original_destination_connection_id = (
                self._original_destination_connection_id
            )
            quic_transport_parameters.retry_source_connection_id = (
                self._retry_source_connection_id
            )

        # log event
        if self._quic_logger is not None:
            self._quic_logger.log_event(
                category="transport",
                event="parameters_set",
                data=self._quic_logger.encode_transport_parameters(
                    owner="local", parameters=quic_transport_parameters
                ),
            )

        buf = Buffer(capacity=3 * self._max_datagram_size)
        push_quic_transport_parameters(buf, quic_transport_parameters)
        return buf.data

    def _set_state(self, state: QuicConnectionState) -> None:
        self._logger.debug("%s -> %s", self._state, state)
        self._state = state

    def _stream_can_receive(self, stream_id: int) -> bool:
        return stream_is_client_initiated(
            stream_id
        ) != self._is_client or not stream_is_unidirectional(stream_id)

    def _stream_can_send(self, stream_id: int) -> bool:
        return stream_is_client_initiated(
            stream_id
        ) == self._is_client or not stream_is_unidirectional(stream_id)

    def _unblock_streams(self, is_unidirectional: bool) -> None:
        if is_unidirectional:
            max_stream_data_remote = self._remote_max_stream_data_uni
            max_streams = self._remote_max_streams_uni
            streams_blocked = self._streams_blocked_uni
        else:
            max_stream_data_remote = self._remote_max_stream_data_bidi_remote
            max_streams = self._remote_max_streams_bidi
            streams_blocked = self._streams_blocked_bidi

        while streams_blocked and streams_blocked[0].stream_id // 4 < max_streams:
            stream = streams_blocked.pop(0)
            stream.is_blocked = False
            stream.max_stream_data_remote = max_stream_data_remote

        if not self._streams_blocked_bidi and not self._streams_blocked_uni:
            self._streams_blocked_pending = False

    def _update_traffic_key(
        self,
        direction: tls.Direction,
        epoch: tls.Epoch,
        cipher_suite: tls.CipherSuite,
        secret: bytes,
    ) -> None:
        """
        Callback which is invoked by the TLS engine when new traffic keys are
        available.
        """
        secrets_log_file = self._configuration.secrets_log_file
        if secrets_log_file is not None:
            label_row = self._is_client == (direction == tls.Direction.DECRYPT)
            label = SECRETS_LABELS[label_row][epoch.value]
            secrets_log_file.write(
                "%s %s %s\n" % (label, self.tls.client_random.hex(), secret.hex())
            )
            secrets_log_file.flush()

        crypto = self._cryptos[epoch]
        if direction == tls.Direction.ENCRYPT:
            crypto.send.setup(
                cipher_suite=cipher_suite, secret=secret, version=self._version
            )
        else:
            crypto.recv.setup(
                cipher_suite=cipher_suite, secret=secret, version=self._version
            )

    def _write_application(
        self, builder: QuicPacketBuilder, network_path: QuicNetworkPath, now: float
    ) -> None:
        crypto_stream: Optional[QuicStream] = None
        if self._cryptos[tls.Epoch.ONE_RTT].send.is_valid():
            crypto = self._cryptos[tls.Epoch.ONE_RTT]
            crypto_stream = self._crypto_streams[tls.Epoch.ONE_RTT]
            packet_type = PACKET_TYPE_ONE_RTT
        elif self._cryptos[tls.Epoch.ZERO_RTT].send.is_valid():
            crypto = self._cryptos[tls.Epoch.ZERO_RTT]
            packet_type = PACKET_TYPE_ZERO_RTT
        else:
            return
        space = self._spaces[tls.Epoch.ONE_RTT]

        while True:
            # apply pacing, except if we have ACKs to send
            if space.ack_at is None or space.ack_at >= now:
                self._pacing_at = self._loss._pacer.next_send_time(now=now)
                if self._pacing_at is not None:
                    break
            builder.start_packet(packet_type, crypto)

            if self._handshake_complete:
                # ACK
                if space.ack_at is not None and space.ack_at <= now:
                    self._write_ack_frame(builder=builder, space=space, now=now)

                # HANDSHAKE_DONE
                if self._handshake_done_pending:
                    self._write_handshake_done_frame(builder=builder)
                    self._handshake_done_pending = False

                # PATH CHALLENGE
                if (
                    not network_path.is_validated
                    and network_path.local_challenge is None
                ):
                    challenge = os.urandom(8)
                    self._write_path_challenge_frame(
                        builder=builder, challenge=challenge
                    )
                    network_path.local_challenge = challenge

                # PATH RESPONSE
                if network_path.remote_challenge is not None:
                    self._write_path_response_frame(
                        builder=builder, challenge=network_path.remote_challenge
                    )
                    network_path.remote_challenge = None

                # NEW_CONNECTION_ID
                for connection_id in self._host_cids:
                    if not connection_id.was_sent:
                        self._write_new_connection_id_frame(
                            builder=builder, connection_id=connection_id
                        )

                # RETIRE_CONNECTION_ID
                for sequence_number in self._retire_connection_ids[:]:
                    self._write_retire_connection_id_frame(
                        builder=builder, sequence_number=sequence_number
                    )
                    self._retire_connection_ids.pop(0)

                # STREAMS_BLOCKED
                if self._streams_blocked_pending:
                    if self._streams_blocked_bidi:
                        self._write_streams_blocked_frame(
                            builder=builder,
                            frame_type=QuicFrameType.STREAMS_BLOCKED_BIDI,
                            limit=self._remote_max_streams_bidi,
                        )
                    if self._streams_blocked_uni:
                        self._write_streams_blocked_frame(
                            builder=builder,
                            frame_type=QuicFrameType.STREAMS_BLOCKED_UNI,
                            limit=self._remote_max_streams_uni,
                        )
                    self._streams_blocked_pending = False

                # MAX_DATA and MAX_STREAMS
                self._write_connection_limits(builder=builder, space=space)

            # stream-level limits
            for stream in self._streams.values():
                self._write_stream_limits(builder=builder, space=space, stream=stream)

            # PING (user-request)
            if self._ping_pending:
                self._write_ping_frame(builder, self._ping_pending)
                self._ping_pending.clear()

            # PING (probe)
            if self._probe_pending:
                self._write_ping_frame(builder, comment="probe")
                self._probe_pending = False

            # CRYPTO
            if crypto_stream is not None and not crypto_stream.sender.buffer_is_empty:
                self._write_crypto_frame(
                    builder=builder, space=space, stream=crypto_stream
                )

            # DATAGRAM
            while self._datagrams_pending:
                try:
                    self._write_datagram_frame(
                        builder=builder,
                        data=self._datagrams_pending[0],
                        frame_type=QuicFrameType.DATAGRAM_WITH_LENGTH,
                    )
                    self._datagrams_pending.popleft()
                except QuicPacketBuilderStop:
                    break

            sent: Set[QuicStream] = set()
            discarded: Set[QuicStream] = set()
            try:
                for stream in self._streams_queue:
                    # if the stream is finished, discard it
                    if stream.is_finished:
                        self._logger.debug("Stream %d discarded", stream.stream_id)
                        self._streams.pop(stream.stream_id)
                        self._streams_finished.add(stream.stream_id)
                        discarded.add(stream)
                        continue

                    if stream.receiver.stop_pending:
                        # STOP_SENDING
                        self._write_stop_sending_frame(builder=builder, stream=stream)

                    if stream.sender.reset_pending:
                        # RESET_STREAM
                        self._write_reset_stream_frame(builder=builder, stream=stream)
                    elif not stream.is_blocked and not stream.sender.buffer_is_empty:
                        # STREAM
                        used = self._write_stream_frame(
                            builder=builder,
                            space=space,
                            stream=stream,
                            max_offset=min(
                                stream.sender.highest_offset
                                + self._remote_max_data
                                - self._remote_max_data_used,
                                stream.max_stream_data_remote,
                            ),
                        )
                        self._remote_max_data_used += used
                        if used > 0:
                            sent.add(stream)

            finally:
                # Make a new stream service order, putting served ones at the end.
                #
                # This method of updating the streams queue ensures that discarded
                # streams are removed and ones which sent are moved to the end even
                # if an exception occurs in the loop.
                self._streams_queue = [
                    stream
                    for stream in self._streams_queue
                    if not (stream in discarded or stream in sent)
                ]
                self._streams_queue.extend(sent)

            if builder.packet_is_empty:
                break
            else:
                self._loss._pacer.update_after_send(now=now)

    def _write_handshake(
        self, builder: QuicPacketBuilder, epoch: tls.Epoch, now: float
    ) -> None:
        crypto = self._cryptos[epoch]
        if not crypto.send.is_valid():
            return

        crypto_stream = self._crypto_streams[epoch]
        space = self._spaces[epoch]

        while True:
            if epoch == tls.Epoch.INITIAL:
                packet_type = PACKET_TYPE_INITIAL
            else:
                packet_type = PACKET_TYPE_HANDSHAKE
            builder.start_packet(packet_type, crypto)

            # ACK
            if space.ack_at is not None:
                self._write_ack_frame(builder=builder, space=space, now=now)

            # CRYPTO
            if not crypto_stream.sender.buffer_is_empty:
                if self._write_crypto_frame(
                    builder=builder, space=space, stream=crypto_stream
                ):
                    self._probe_pending = False

            # PING (probe)
            if (
                self._probe_pending
                and not self._handshake_complete
                and (
                    epoch == tls.Epoch.HANDSHAKE
                    or not self._cryptos[tls.Epoch.HANDSHAKE].send.is_valid()
                )
            ):
                self._write_ping_frame(builder, comment="probe")
                self._probe_pending = False

            if builder.packet_is_empty:
                break

    def _write_ack_frame(
        self, builder: QuicPacketBuilder, space: QuicPacketSpace, now: float
    ) -> None:
        # calculate ACK delay
        ack_delay = now - space.largest_received_time
        ack_delay_encoded = int(ack_delay * 1000000) >> self._local_ack_delay_exponent

        buf = builder.start_frame(
            QuicFrameType.ACK,
            capacity=ACK_FRAME_CAPACITY,
            handler=self._on_ack_delivery,
            handler_args=(space, space.largest_received_packet),
        )
        ranges = push_ack_frame(buf, space.ack_queue, ack_delay_encoded)
        space.ack_at = None

        # log frame
        if self._quic_logger is not None:
            builder.quic_logger_frames.append(
                self._quic_logger.encode_ack_frame(
                    ranges=space.ack_queue, delay=ack_delay
                )
            )

        # check if we need to trigger an ACK-of-ACK
        if ranges > 1 and builder.packet_number % 8 == 0:
            self._write_ping_frame(builder, comment="ACK-of-ACK trigger")

    def _write_connection_close_frame(
        self,
        builder: QuicPacketBuilder,
        epoch: tls.Epoch,
        error_code: int,
        frame_type: Optional[int],
        reason_phrase: str,
    ) -> None:
        # convert application-level close to transport-level close in early stages
        if frame_type is None and epoch in (tls.Epoch.INITIAL, tls.Epoch.HANDSHAKE):
            error_code = QuicErrorCode.APPLICATION_ERROR
            frame_type = QuicFrameType.PADDING
            reason_phrase = ""

        reason_bytes = reason_phrase.encode("utf8")
        reason_length = len(reason_bytes)

        if frame_type is None:
            buf = builder.start_frame(
                QuicFrameType.APPLICATION_CLOSE,
                capacity=APPLICATION_CLOSE_FRAME_CAPACITY + reason_length,
            )
            buf.push_uint_var(error_code)
            buf.push_uint_var(reason_length)
            buf.push_bytes(reason_bytes)
        else:
            buf = builder.start_frame(
                QuicFrameType.TRANSPORT_CLOSE,
                capacity=TRANSPORT_CLOSE_FRAME_CAPACITY + reason_length,
            )
            buf.push_uint_var(error_code)
            buf.push_uint_var(frame_type)
            buf.push_uint_var(reason_length)
            buf.push_bytes(reason_bytes)

        # log frame
        if self._quic_logger is not None:
            builder.quic_logger_frames.append(
                self._quic_logger.encode_connection_close_frame(
                    error_code=error_code,
                    frame_type=frame_type,
                    reason_phrase=reason_phrase,
                )
            )

    def _write_connection_limits(
        self, builder: QuicPacketBuilder, space: QuicPacketSpace
    ) -> None:
        """
        Raise MAX_DATA or MAX_STREAMS if needed.
        """
        for limit in (
            self._local_max_data,
            self._local_max_streams_bidi,
            self._local_max_streams_uni,
        ):
            if limit.used * 2 > limit.value:
                limit.value *= 2
                self._logger.debug("Local %s raised to %d", limit.name, limit.value)
            if limit.value != limit.sent:
                buf = builder.start_frame(
                    limit.frame_type,
                    capacity=CONNECTION_LIMIT_FRAME_CAPACITY,
                    handler=self._on_connection_limit_delivery,
                    handler_args=(limit,),
                )
                buf.push_uint_var(limit.value)
                limit.sent = limit.value

                # log frame
                if self._quic_logger is not None:
                    builder.quic_logger_frames.append(
                        self._quic_logger.encode_connection_limit_frame(
                            frame_type=limit.frame_type,
                            maximum=limit.value,
                        )
                    )

    def _write_crypto_frame(
        self, builder: QuicPacketBuilder, space: QuicPacketSpace, stream: QuicStream
    ) -> bool:
        frame_overhead = 3 + size_uint_var(stream.sender.next_offset)
        frame = stream.sender.get_frame(builder.remaining_flight_space - frame_overhead)
        if frame is not None:
            buf = builder.start_frame(
                QuicFrameType.CRYPTO,
                capacity=frame_overhead,
                handler=stream.sender.on_data_delivery,
                handler_args=(frame.offset, frame.offset + len(frame.data), False),
            )
            buf.push_uint_var(frame.offset)
            buf.push_uint16(len(frame.data) | 0x4000)
            buf.push_bytes(frame.data)

            # log frame
            if self._quic_logger is not None:
                builder.quic_logger_frames.append(
                    self._quic_logger.encode_crypto_frame(frame)
                )
            return True

        return False

    def _write_datagram_frame(
        self, builder: QuicPacketBuilder, data: bytes, frame_type: QuicFrameType
    ) -> bool:
        """
        Write a DATAGRAM frame.

        Returns True if the frame was processed, False otherwise.
        """
        assert frame_type == QuicFrameType.DATAGRAM_WITH_LENGTH
        length = len(data)
        frame_size = 1 + size_uint_var(length) + length

        buf = builder.start_frame(frame_type, capacity=frame_size)
        buf.push_uint_var(length)
        buf.push_bytes(data)

        # log frame
        if self._quic_logger is not None:
            builder.quic_logger_frames.append(
                self._quic_logger.encode_datagram_frame(length=length)
            )

        return True

    def _write_handshake_done_frame(self, builder: QuicPacketBuilder) -> None:
        builder.start_frame(
            QuicFrameType.HANDSHAKE_DONE,
            capacity=HANDSHAKE_DONE_FRAME_CAPACITY,
            handler=self._on_handshake_done_delivery,
        )

        # log frame
        if self._quic_logger is not None:
            builder.quic_logger_frames.append(
                self._quic_logger.encode_handshake_done_frame()
            )

    def _write_new_connection_id_frame(
        self, builder: QuicPacketBuilder, connection_id: QuicConnectionId
    ) -> None:
        retire_prior_to = 0  # FIXME

        buf = builder.start_frame(
            QuicFrameType.NEW_CONNECTION_ID,
            capacity=NEW_CONNECTION_ID_FRAME_CAPACITY,
            handler=self._on_new_connection_id_delivery,
            handler_args=(connection_id,),
        )
        buf.push_uint_var(connection_id.sequence_number)
        buf.push_uint_var(retire_prior_to)
        buf.push_uint8(len(connection_id.cid))
        buf.push_bytes(connection_id.cid)
        buf.push_bytes(connection_id.stateless_reset_token)

        connection_id.was_sent = True
        self._events.append(events.ConnectionIdIssued(connection_id=connection_id.cid))

        # log frame
        if self._quic_logger is not None:
            builder.quic_logger_frames.append(
                self._quic_logger.encode_new_connection_id_frame(
                    connection_id=connection_id.cid,
                    retire_prior_to=retire_prior_to,
                    sequence_number=connection_id.sequence_number,
                    stateless_reset_token=connection_id.stateless_reset_token,
                )
            )

    def _write_path_challenge_frame(
        self, builder: QuicPacketBuilder, challenge: bytes
    ) -> None:
        buf = builder.start_frame(
            QuicFrameType.PATH_CHALLENGE, capacity=PATH_CHALLENGE_FRAME_CAPACITY
        )
        buf.push_bytes(challenge)

        # log frame
        if self._quic_logger is not None:
            builder.quic_logger_frames.append(
                self._quic_logger.encode_path_challenge_frame(data=challenge)
            )

    def _write_path_response_frame(
        self, builder: QuicPacketBuilder, challenge: bytes
    ) -> None:
        buf = builder.start_frame(
            QuicFrameType.PATH_RESPONSE, capacity=PATH_RESPONSE_FRAME_CAPACITY
        )
        buf.push_bytes(challenge)

        # log frame
        if self._quic_logger is not None:
            builder.quic_logger_frames.append(
                self._quic_logger.encode_path_response_frame(data=challenge)
            )

    def _write_ping_frame(
        self, builder: QuicPacketBuilder, uids: List[int] = [], comment=""
    ):
        builder.start_frame(
            QuicFrameType.PING,
            capacity=PING_FRAME_CAPACITY,
            handler=self._on_ping_delivery,
            handler_args=(tuple(uids),),
        )
        self._logger.debug(
            "Sending PING%s in packet %d",
            " (%s)" % comment if comment else "",
            builder.packet_number,
        )

        # log frame
        if self._quic_logger is not None:
            builder.quic_logger_frames.append(self._quic_logger.encode_ping_frame())

    def _write_reset_stream_frame(
        self,
        builder: QuicPacketBuilder,
        stream: QuicStream,
    ) -> None:
        buf = builder.start_frame(
            frame_type=QuicFrameType.RESET_STREAM,
            capacity=RESET_STREAM_FRAME_CAPACITY,
            handler=stream.sender.on_reset_delivery,
        )
        frame = stream.sender.get_reset_frame()
        buf.push_uint_var(frame.stream_id)
        buf.push_uint_var(frame.error_code)
        buf.push_uint_var(frame.final_size)

        # log frame
        if self._quic_logger is not None:
            builder.quic_logger_frames.append(
                self._quic_logger.encode_reset_stream_frame(
                    error_code=frame.error_code,
                    final_size=frame.final_size,
                    stream_id=frame.stream_id,
                )
            )

    def _write_retire_connection_id_frame(
        self, builder: QuicPacketBuilder, sequence_number: int
    ) -> None:
        buf = builder.start_frame(
            QuicFrameType.RETIRE_CONNECTION_ID,
            capacity=RETIRE_CONNECTION_ID_CAPACITY,
            handler=self._on_retire_connection_id_delivery,
            handler_args=(sequence_number,),
        )
        buf.push_uint_var(sequence_number)

        # log frame
        if self._quic_logger is not None:
            builder.quic_logger_frames.append(
                self._quic_logger.encode_retire_connection_id_frame(sequence_number)
            )

    def _write_stop_sending_frame(
        self,
        builder: QuicPacketBuilder,
        stream: QuicStream,
    ) -> None:
        buf = builder.start_frame(
            frame_type=QuicFrameType.STOP_SENDING,
            capacity=STOP_SENDING_FRAME_CAPACITY,
            handler=stream.receiver.on_stop_sending_delivery,
        )
        frame = stream.receiver.get_stop_frame()
        buf.push_uint_var(frame.stream_id)
        buf.push_uint_var(frame.error_code)

        # log frame
        if self._quic_logger is not None:
            builder.quic_logger_frames.append(
                self._quic_logger.encode_stop_sending_frame(
                    error_code=frame.error_code, stream_id=frame.stream_id
                )
            )

    def _write_stream_frame(
        self,
        builder: QuicPacketBuilder,
        space: QuicPacketSpace,
        stream: QuicStream,
        max_offset: int,
    ) -> int:
        # the frame data size is constrained by our peer's MAX_DATA and
        # the space available in the current packet
        frame_overhead = (
            3
            + size_uint_var(stream.stream_id)
            + (
                size_uint_var(stream.sender.next_offset)
                if stream.sender.next_offset
                else 0
            )
        )
        previous_send_highest = stream.sender.highest_offset
        frame = stream.sender.get_frame(
            builder.remaining_flight_space - frame_overhead, max_offset
        )

        if frame is not None:
            frame_type = QuicFrameType.STREAM_BASE | 2  # length
            if frame.offset:
                frame_type |= 4
            if frame.fin:
                frame_type |= 1
            buf = builder.start_frame(
                frame_type,
                capacity=frame_overhead,
                handler=stream.sender.on_data_delivery,
                handler_args=(frame.offset, frame.offset + len(frame.data), frame.fin),
            )
            buf.push_uint_var(stream.stream_id)
            if frame.offset:
                buf.push_uint_var(frame.offset)
            buf.push_uint16(len(frame.data) | 0x4000)
            buf.push_bytes(frame.data)

            # log frame
            if self._quic_logger is not None:
                builder.quic_logger_frames.append(
                    self._quic_logger.encode_stream_frame(
                        frame, stream_id=stream.stream_id
                    )
                )

            return stream.sender.highest_offset - previous_send_highest
        else:
            return 0

    def _write_stream_limits(
        self, builder: QuicPacketBuilder, space: QuicPacketSpace, stream: QuicStream
    ) -> None:
        """
        Raise MAX_STREAM_DATA if needed.

        The only case where `stream.max_stream_data_local` is zero is for
        locally created unidirectional streams. We skip such streams to avoid
        spurious logging.
        """
        if (
            stream.max_stream_data_local
            and stream.receiver.highest_offset * 2 > stream.max_stream_data_local
        ):
            stream.max_stream_data_local *= 2
            self._logger.debug(
                "Stream %d local max_stream_data raised to %d",
                stream.stream_id,
                stream.max_stream_data_local,
            )
        if stream.max_stream_data_local_sent != stream.max_stream_data_local:
            buf = builder.start_frame(
                QuicFrameType.MAX_STREAM_DATA,
                capacity=MAX_STREAM_DATA_FRAME_CAPACITY,
                handler=self._on_max_stream_data_delivery,
                handler_args=(stream,),
            )
            buf.push_uint_var(stream.stream_id)
            buf.push_uint_var(stream.max_stream_data_local)
            stream.max_stream_data_local_sent = stream.max_stream_data_local

            # log frame
            if self._quic_logger is not None:
                builder.quic_logger_frames.append(
                    self._quic_logger.encode_max_stream_data_frame(
                        maximum=stream.max_stream_data_local, stream_id=stream.stream_id
                    )
                )

    def _write_streams_blocked_frame(
        self, builder: QuicPacketBuilder, frame_type: QuicFrameType, limit: int
    ) -> None:
        buf = builder.start_frame(frame_type, capacity=STREAMS_BLOCKED_CAPACITY)
        buf.push_uint_var(limit)

        # log frame
        if self._quic_logger is not None:
            builder.quic_logger_frames.append(
                self._quic_logger.encode_streams_blocked_frame(
                    is_unidirectional=frame_type == QuicFrameType.STREAMS_BLOCKED_UNI,
                    limit=limit,
                )
            )