noq_proto/

endpoint.rs

use std::{
    collections::{HashMap, hash_map},
    convert::TryFrom,
    fmt, mem,
    net::{IpAddr, SocketAddr},
    ops::{Index, IndexMut},
    sync::Arc,
};

use bytes::{Buf, BufMut, Bytes, BytesMut};
use rand::{Rng, RngExt, SeedableRng, rngs::StdRng};
use rustc_hash::FxHashMap;
use slab::Slab;
use thiserror::Error;
use tracing::{debug, error, trace, warn};

use crate::{
    Duration, FourTuple, INITIAL_MTU, Instant, MAX_CID_SIZE, MIN_INITIAL_SIZE, PathId,
    RESET_TOKEN_SIZE, ResetToken, Side, Transmit, TransportConfig, TransportError,
    cid_generator::ConnectionIdGenerator,
    coding::{BufMutExt, Decodable, Encodable, UnexpectedEnd},
    config::{ClientConfig, EndpointConfig, ServerConfig},
    connection::{Connection, ConnectionError, SideArgs},
    crypto::{self, Keys, UnsupportedVersion},
    frame,
    packet::{
        FixedLengthConnectionIdParser, Header, InitialHeader, InitialPacket, PacketDecodeError,
        PacketNumber, PartialDecode, ProtectedInitialHeader,
    },
    shared::{
        ConnectionEvent, ConnectionEventInner, ConnectionId, DatagramConnectionEvent, EcnCodepoint,
        EndpointEvent, EndpointEventInner, IssuedCid,
    },
    token::{IncomingToken, InvalidRetryTokenError, Token, TokenPayload},
    transport_parameters::{PreferredAddress, TransportParameters},
};

/// The main entry point to the library
///
/// This object performs no I/O whatsoever. Instead, it consumes incoming packets and
/// connection-generated events via `handle` and `handle_event`.
pub struct Endpoint {
    rng: StdRng,
    index: ConnectionIndex,
    connections: Slab<ConnectionMeta>,
    local_cid_generator: Box<dyn ConnectionIdGenerator>,
    config: Arc<EndpointConfig>,
    server_config: Option<Arc<ServerConfig>>,
    /// Whether the underlying UDP socket promises not to fragment packets
    allow_mtud: bool,
    /// Time at which a stateless reset was most recently sent
    last_stateless_reset: Option<Instant>,
    /// Buffered Initial and 0-RTT messages for pending incoming connections
    incoming_buffers: Slab<IncomingBuffer>,
    all_incoming_buffers_total_bytes: u64,
}

impl Endpoint {
    /// Create a new endpoint
    ///
    /// `allow_mtud` enables path MTU detection when requested by `Connection` configuration for
    /// better performance. This requires that outgoing packets are never fragmented, which can be
    /// achieved via e.g. the `IPV6_DONTFRAG` socket option.
    pub fn new(
        config: Arc<EndpointConfig>,
        server_config: Option<Arc<ServerConfig>>,
        allow_mtud: bool,
    ) -> Self {
        Self {
            rng: config
                .rng_seed
                .map_or_else(|| StdRng::from_rng(&mut rand::rng()), StdRng::from_seed),
            index: ConnectionIndex::default(),
            connections: Slab::new(),
            local_cid_generator: (config.connection_id_generator_factory.as_ref())(),
            config,
            server_config,
            allow_mtud,
            last_stateless_reset: None,
            incoming_buffers: Slab::new(),
            all_incoming_buffers_total_bytes: 0,
        }
    }

    /// Replace the server configuration, affecting new incoming connections only
    pub fn set_server_config(&mut self, server_config: Option<Arc<ServerConfig>>) {
        self.server_config = server_config;
    }

    /// Process `EndpointEvent`s emitted from related `Connection`s
    ///
    /// In turn, processing this event may return a `ConnectionEvent` for the same `Connection`.
    pub fn handle_event(
        &mut self,
        ch: ConnectionHandle,
        event: EndpointEvent,
    ) -> Option<ConnectionEvent> {
        use EndpointEventInner::*;
        match event.0 {
            NeedIdentifiers(path_id, now, n) => {
                return Some(self.send_new_identifiers(path_id, now, ch, n));
            }
            ResetToken(path_id, remote, token) => {
                if let Some(old) = self.connections[ch]
                    .reset_token
                    .insert(path_id, (remote, token))
                {
                    self.index.connection_reset_tokens.remove(old.0, old.1);
                }
                if self.index.connection_reset_tokens.insert(remote, token, ch) {
                    warn!("duplicate reset token");
                }
            }
            RetireResetToken(path_id) => {
                if let Some(old) = self.connections[ch].reset_token.remove(&path_id) {
                    self.index.connection_reset_tokens.remove(old.0, old.1);
                }
            }
            RetireConnectionId(now, path_id, seq, allow_more_cids) => {
                if let Some(cid) = self.connections[ch]
                    .local_cids
                    .get_mut(&path_id)
                    .and_then(|pcid| pcid.cids.remove(&seq))
                {
                    trace!(%path_id, "local CID retired {}: {}", seq, cid);
                    self.index.retire(cid);
                    if allow_more_cids {
                        return Some(self.send_new_identifiers(path_id, now, ch, 1));
                    }
                }
            }
            Drained => {
                if let Some(conn) = self.connections.try_remove(ch.0) {
                    self.index.remove(&conn);
                } else {
                    // This indicates a bug in downstream code, which could cause spurious
                    // connection loss instead of this error if the CID was (re)allocated prior to
                    // the illegal call.
                    error!(id = ch.0, "unknown connection drained");
                }
            }
        }
        None
    }

    /// Process an incoming UDP datagram
    pub fn handle(
        &mut self,
        now: Instant,
        network_path: FourTuple,
        ecn: Option<EcnCodepoint>,
        data: BytesMut,
        buf: &mut Vec<u8>,
    ) -> Option<DatagramEvent> {
        // Partially decode packet or short-circuit if unable
        let datagram_len = data.len();
        let mut event = match PartialDecode::new(
            data,
            &FixedLengthConnectionIdParser::new(self.local_cid_generator.cid_len()),
            &self.config.supported_versions,
            self.config.grease_quic_bit,
        ) {
            Ok((first_decode, remaining)) => DatagramConnectionEvent {
                now,
                network_path,
                path_id: PathId::ZERO, // Corrected later for existing paths
                ecn,
                first_decode,
                remaining,
            },
            Err(PacketDecodeError::UnsupportedVersion {
                src_cid,
                dst_cid,
                version,
            }) => {
                if self.server_config.is_none() {
                    debug!("dropping packet with unsupported version");
                    return None;
                }
                trace!("sending version negotiation");
                // Negotiate versions
                Header::VersionNegotiate {
                    random: self.rng.random::<u8>() | 0x40,
                    src_cid: dst_cid,
                    dst_cid: src_cid,
                }
                .encode(buf);
                // Grease with a reserved version
                buf.write::<u32>(match version {
                    0x0a1a_2a3a => 0x0a1a_2a4a,
                    _ => 0x0a1a_2a3a,
                });
                for &version in &self.config.supported_versions {
                    buf.write(version);
                }
                return Some(DatagramEvent::Response(Transmit {
                    destination: network_path.remote,
                    ecn: None,
                    size: buf.len(),
                    segment_size: None,
                    src_ip: network_path.local_ip,
                }));
            }
            Err(e) => {
                trace!("malformed header: {}", e);
                return None;
            }
        };

        let dst_cid = event.first_decode.dst_cid();

        if let Some(route_to) = self.index.get(&network_path, &event.first_decode) {
            event.path_id = match route_to {
                RouteDatagramTo::Incoming(_) => PathId::ZERO,
                RouteDatagramTo::Connection(_, path_id) => path_id,
            };
            match route_to {
                RouteDatagramTo::Incoming(incoming_idx) => {
                    let incoming_buffer = &mut self.incoming_buffers[incoming_idx];
                    let config = &self.server_config.as_ref().unwrap();

                    if incoming_buffer
                        .total_bytes
                        .checked_add(datagram_len as u64)
                        .is_some_and(|n| n <= config.incoming_buffer_size)
                        && self
                            .all_incoming_buffers_total_bytes
                            .checked_add(datagram_len as u64)
                            .is_some_and(|n| n <= config.incoming_buffer_size_total)
                    {
                        incoming_buffer.datagrams.push(event);
                        incoming_buffer.total_bytes += datagram_len as u64;
                        self.all_incoming_buffers_total_bytes += datagram_len as u64;
                    }

                    None
                }
                RouteDatagramTo::Connection(ch, _path_id) => Some(DatagramEvent::ConnectionEvent(
                    ch,
                    ConnectionEvent(ConnectionEventInner::Datagram(event)),
                )),
            }
        } else if event.first_decode.initial_header().is_some() {
            // Potentially create a new connection

            self.handle_first_packet(datagram_len, event, network_path, buf)
        } else if event.first_decode.has_long_header() {
            debug!(
                "ignoring non-initial packet for unknown connection {}",
                dst_cid
            );
            None
        } else if !event.first_decode.is_initial()
            && self.local_cid_generator.validate(dst_cid).is_err()
        {
            debug!("dropping packet with invalid CID");
            None
        } else if dst_cid.is_empty() {
            trace!("dropping unrecognized short packet without ID");
            None
        } else {
            // If we got this far, we're receiving a seemingly valid packet for an unknown
            // connection. Send a stateless reset if possible.
            self.stateless_reset(now, datagram_len, network_path, dst_cid, buf)
                .map(DatagramEvent::Response)
        }
    }

    /// Builds a stateless reset packet to respond with
    fn stateless_reset(
        &mut self,
        now: Instant,
        inciting_dgram_len: usize,
        network_path: FourTuple,
        dst_cid: ConnectionId,
        buf: &mut Vec<u8>,
    ) -> Option<Transmit> {
        if self
            .last_stateless_reset
            .is_some_and(|last| last + self.config.min_reset_interval > now)
        {
            debug!("ignoring unexpected packet within minimum stateless reset interval");
            return None;
        }

        /// Minimum amount of padding for the stateless reset to look like a short-header packet
        const MIN_PADDING_LEN: usize = 5;

        // Prevent amplification attacks and reset loops by ensuring we pad to at most 1 byte
        // smaller than the inciting packet.
        let max_padding_len = match inciting_dgram_len.checked_sub(RESET_TOKEN_SIZE) {
            Some(headroom) if headroom > MIN_PADDING_LEN => headroom - 1,
            _ => {
                debug!(
                    "ignoring unexpected {} byte packet: not larger than minimum stateless reset size",
                    inciting_dgram_len
                );
                return None;
            }
        };

        debug!(%dst_cid, %network_path.remote, "sending stateless reset");
        self.last_stateless_reset = Some(now);
        // Resets with at least this much padding can't possibly be distinguished from real packets
        const IDEAL_MIN_PADDING_LEN: usize = MIN_PADDING_LEN + MAX_CID_SIZE;
        let padding_len = if max_padding_len <= IDEAL_MIN_PADDING_LEN {
            max_padding_len
        } else {
            self.rng
                .random_range(IDEAL_MIN_PADDING_LEN..max_padding_len)
        };
        buf.reserve(padding_len + RESET_TOKEN_SIZE);
        buf.resize(padding_len, 0);
        self.rng.fill_bytes(&mut buf[0..padding_len]);
        buf[0] = 0b0100_0000 | (buf[0] >> 2);
        buf.extend_from_slice(&ResetToken::new(&*self.config.reset_key, dst_cid));

        debug_assert!(buf.len() < inciting_dgram_len);

        Some(Transmit {
            destination: network_path.remote,
            ecn: None,
            size: buf.len(),
            segment_size: None,
            src_ip: network_path.local_ip,
        })
    }

    /// Initiate a connection
    pub fn connect(
        &mut self,
        now: Instant,
        config: ClientConfig,
        remote: SocketAddr,
        server_name: &str,
    ) -> Result<(ConnectionHandle, Connection), ConnectError> {
        if self.cids_exhausted() {
            return Err(ConnectError::CidsExhausted);
        }
        if remote.port() == 0 || remote.ip().is_unspecified() {
            return Err(ConnectError::InvalidRemoteAddress(remote));
        }
        if !self.config.supported_versions.contains(&config.version) {
            return Err(ConnectError::UnsupportedVersion);
        }

        let remote_id = (config.initial_dst_cid_provider)();
        trace!(initial_dcid = %remote_id);

        let ch = ConnectionHandle(self.connections.vacant_key());
        let local_cid = self.new_cid(ch, PathId::ZERO);
        let params = TransportParameters::new(
            &config.transport,
            &self.config,
            self.local_cid_generator.as_ref(),
            local_cid,
            None,
            &mut self.rng,
        );
        let tls = config
            .crypto
            .start_session(config.version, server_name, &params)?;

        let conn = self.add_connection(
            ch,
            config.version,
            remote_id,
            local_cid,
            remote_id,
            FourTuple {
                remote,
                local_ip: None,
            },
            now,
            tls,
            config.transport,
            SideArgs::Client {
                token_store: config.token_store,
                server_name: server_name.into(),
            },
            &params,
        );
        Ok((ch, conn))
    }

    /// Generates new CIDs and creates message to send to the connection state
    fn send_new_identifiers(
        &mut self,
        path_id: PathId,
        now: Instant,
        ch: ConnectionHandle,
        num: u64,
    ) -> ConnectionEvent {
        let mut ids = vec![];
        for _ in 0..num {
            let id = self.new_cid(ch, path_id);
            let cid_meta = self.connections[ch].local_cids.entry(path_id).or_default();
            let sequence = cid_meta.issued;
            cid_meta.issued += 1;
            cid_meta.cids.insert(sequence, id);
            ids.push(IssuedCid {
                path_id,
                sequence,
                id,
                reset_token: ResetToken::new(&*self.config.reset_key, id),
            });
        }
        ConnectionEvent(ConnectionEventInner::NewIdentifiers(
            ids,
            now,
            self.local_cid_generator.cid_len(),
            self.local_cid_generator.cid_lifetime(),
        ))
    }

    /// Generate a connection ID for `ch`
    fn new_cid(&mut self, ch: ConnectionHandle, path_id: PathId) -> ConnectionId {
        loop {
            let cid = self.local_cid_generator.generate_cid();
            if cid.is_empty() {
                // Zero-length CID; nothing to track
                debug_assert_eq!(self.local_cid_generator.cid_len(), 0);
                return cid;
            }
            if let hash_map::Entry::Vacant(e) = self.index.connection_ids.entry(cid) {
                e.insert((ch, path_id));
                break cid;
            }
        }
    }

    fn handle_first_packet(
        &mut self,
        datagram_len: usize,
        event: DatagramConnectionEvent,
        network_path: FourTuple,
        buf: &mut Vec<u8>,
    ) -> Option<DatagramEvent> {
        let dst_cid = event.first_decode.dst_cid();
        let header = event.first_decode.initial_header().unwrap();

        let Some(server_config) = &self.server_config else {
            debug!("packet for unrecognized connection {}", dst_cid);
            return self
                .stateless_reset(event.now, datagram_len, network_path, dst_cid, buf)
                .map(DatagramEvent::Response);
        };

        if datagram_len < MIN_INITIAL_SIZE as usize {
            debug!("ignoring short initial for connection {}", dst_cid);
            return None;
        }

        let crypto = match server_config.crypto.initial_keys(header.version, dst_cid) {
            Ok(keys) => keys,
            Err(UnsupportedVersion) => {
                // This probably indicates that the user set supported_versions incorrectly in
                // `EndpointConfig`.
                debug!(
                    "ignoring initial packet version {:#x} unsupported by cryptographic layer",
                    header.version
                );
                return None;
            }
        };

        if let Err(reason) = self.early_validate_first_packet(header) {
            return Some(DatagramEvent::Response(self.initial_close(
                header.version,
                network_path,
                &crypto,
                header.src_cid,
                reason,
                buf,
            )));
        }

        let packet = match event.first_decode.finish(Some(&*crypto.header.remote)) {
            Ok(packet) => packet,
            Err(e) => {
                trace!("unable to decode initial packet: {}", e);
                return None;
            }
        };

        if !packet.reserved_bits_valid() {
            debug!("dropping connection attempt with invalid reserved bits");
            return None;
        }

        let Header::Initial(header) = packet.header else {
            panic!("non-initial packet in handle_first_packet()");
        };

        let server_config = self.server_config.as_ref().unwrap().clone();

        let token = match IncomingToken::from_header(&header, &server_config, network_path.remote) {
            Ok(token) => token,
            Err(InvalidRetryTokenError) => {
                debug!("rejecting invalid retry token");
                return Some(DatagramEvent::Response(self.initial_close(
                    header.version,
                    network_path,
                    &crypto,
                    header.src_cid,
                    TransportError::INVALID_TOKEN(""),
                    buf,
                )));
            }
        };

        let incoming_idx = self.incoming_buffers.insert(IncomingBuffer::default());
        self.index
            .insert_initial_incoming(header.dst_cid, incoming_idx);

        Some(DatagramEvent::NewConnection(Incoming {
            received_at: event.now,
            network_path,
            ecn: event.ecn,
            packet: InitialPacket {
                header,
                header_data: packet.header_data,
                payload: packet.payload,
            },
            rest: event.remaining,
            crypto,
            token,
            incoming_idx,
            improper_drop_warner: IncomingImproperDropWarner,
        }))
    }

    /// Attempt to accept this incoming connection (an error may still occur)
    // box err to avoid clippy::result_large_err
    pub fn accept(
        &mut self,
        mut incoming: Incoming,
        now: Instant,
        buf: &mut Vec<u8>,
        server_config: Option<Arc<ServerConfig>>,
    ) -> Result<(ConnectionHandle, Connection), Box<AcceptError>> {
        let remote_address_validated = incoming.remote_address_validated();
        incoming.improper_drop_warner.dismiss();
        let incoming_buffer = self.incoming_buffers.remove(incoming.incoming_idx);
        self.all_incoming_buffers_total_bytes -= incoming_buffer.total_bytes;

        let packet_number = incoming.packet.header.number.expand(0);
        let InitialHeader {
            src_cid,
            dst_cid,
            version,
            ..
        } = incoming.packet.header;
        let server_config =
            server_config.unwrap_or_else(|| self.server_config.as_ref().unwrap().clone());

        if server_config
            .transport
            .max_idle_timeout
            .is_some_and(|timeout| {
                incoming.received_at + Duration::from_millis(timeout.into()) <= now
            })
        {
            debug!("abandoning accept of stale initial");
            self.index.remove_initial(dst_cid);
            return Err(Box::new(AcceptError {
                cause: ConnectionError::TimedOut,
                response: None,
            }));
        }

        if self.cids_exhausted() {
            debug!("refusing connection");
            self.index.remove_initial(dst_cid);
            return Err(Box::new(AcceptError {
                cause: ConnectionError::CidsExhausted,
                response: Some(self.initial_close(
                    version,
                    incoming.network_path,
                    &incoming.crypto,
                    src_cid,
                    TransportError::CONNECTION_REFUSED(""),
                    buf,
                )),
            }));
        }

        if incoming
            .crypto
            .packet
            .remote
            .decrypt(
                PathId::ZERO,
                packet_number,
                &incoming.packet.header_data,
                &mut incoming.packet.payload,
            )
            .is_err()
        {
            debug!(packet_number, "failed to authenticate initial packet");
            self.index.remove_initial(dst_cid);
            return Err(Box::new(AcceptError {
                cause: TransportError::PROTOCOL_VIOLATION("authentication failed").into(),
                response: None,
            }));
        };

        let ch = ConnectionHandle(self.connections.vacant_key());
        let local_cid = self.new_cid(ch, PathId::ZERO);
        let mut params = TransportParameters::new(
            &server_config.transport,
            &self.config,
            self.local_cid_generator.as_ref(),
            local_cid,
            Some(&server_config),
            &mut self.rng,
        );
        params.stateless_reset_token = Some(ResetToken::new(&*self.config.reset_key, local_cid));
        params.original_dst_cid = Some(incoming.token.orig_dst_cid);
        params.retry_src_cid = incoming.token.retry_src_cid;
        let mut pref_addr_cid = None;
        if server_config.has_preferred_address() {
            let cid = self.new_cid(ch, PathId::ZERO);
            pref_addr_cid = Some(cid);
            params.preferred_address = Some(PreferredAddress {
                address_v4: server_config.preferred_address_v4,
                address_v6: server_config.preferred_address_v6,
                connection_id: cid,
                stateless_reset_token: ResetToken::new(&*self.config.reset_key, cid),
            });
        }

        let tls = server_config.crypto.start_session(version, &params);
        let transport_config = server_config.transport.clone();
        let mut conn = self.add_connection(
            ch,
            version,
            dst_cid,
            local_cid,
            src_cid,
            incoming.network_path,
            incoming.received_at,
            tls,
            transport_config,
            SideArgs::Server {
                server_config,
                pref_addr_cid,
                path_validated: remote_address_validated,
            },
            &params,
        );
        self.index.insert_initial(dst_cid, ch);

        match conn.handle_first_packet(
            incoming.received_at,
            incoming.network_path,
            incoming.ecn,
            packet_number,
            incoming.packet,
            incoming.rest,
        ) {
            Ok(()) => {
                trace!(id = ch.0, icid = %dst_cid, "new connection");

                for event in incoming_buffer.datagrams {
                    conn.handle_event(ConnectionEvent(ConnectionEventInner::Datagram(event)))
                }

                Ok((ch, conn))
            }
            Err(e) => {
                debug!("handshake failed: {}", e);
                self.handle_event(ch, EndpointEvent(EndpointEventInner::Drained));
                let response = match e {
                    ConnectionError::TransportError(ref e) => Some(self.initial_close(
                        version,
                        incoming.network_path,
                        &incoming.crypto,
                        src_cid,
                        e.clone(),
                        buf,
                    )),
                    _ => None,
                };
                Err(Box::new(AcceptError { cause: e, response }))
            }
        }
    }

    /// Check if we should refuse a connection attempt regardless of the packet's contents
    fn early_validate_first_packet(
        &mut self,
        header: &ProtectedInitialHeader,
    ) -> Result<(), TransportError> {
        let config = &self.server_config.as_ref().unwrap();
        if self.cids_exhausted() || self.incoming_buffers.len() >= config.max_incoming {
            return Err(TransportError::CONNECTION_REFUSED(""));
        }

        // RFC9000 §7.2 dictates that initial (client-chosen) destination CIDs must be at least 8
        // bytes. If this is a Retry packet, then the length must instead match our usual CID
        // length. If we ever issue non-Retry address validation tokens via `NEW_TOKEN`, then we'll
        // also need to validate CID length for those after decoding the token.
        if header.dst_cid.len() < 8
            && (header.token_pos.is_empty()
                || header.dst_cid.len() != self.local_cid_generator.cid_len())
        {
            debug!(
                "rejecting connection due to invalid DCID length {}",
                header.dst_cid.len()
            );
            return Err(TransportError::PROTOCOL_VIOLATION(
                "invalid destination CID length",
            ));
        }

        Ok(())
    }

    /// Reject this incoming connection attempt
    pub fn refuse(&mut self, incoming: Incoming, buf: &mut Vec<u8>) -> Transmit {
        self.clean_up_incoming(&incoming);
        incoming.improper_drop_warner.dismiss();

        self.initial_close(
            incoming.packet.header.version,
            incoming.network_path,
            &incoming.crypto,
            incoming.packet.header.src_cid,
            TransportError::CONNECTION_REFUSED(""),
            buf,
        )
    }

    /// Respond with a retry packet, requiring the client to retry with address validation
    ///
    /// Errors if `incoming.may_retry()` is false.
    pub fn retry(&mut self, incoming: Incoming, buf: &mut Vec<u8>) -> Result<Transmit, RetryError> {
        if !incoming.may_retry() {
            return Err(RetryError(Box::new(incoming)));
        }

        self.clean_up_incoming(&incoming);
        incoming.improper_drop_warner.dismiss();

        let server_config = self.server_config.as_ref().unwrap();

        // First Initial
        // The peer will use this as the DCID of its following Initials. Initial DCIDs are
        // looked up separately from Handshake/Data DCIDs, so there is no risk of collision
        // with established connections. In the unlikely event that a collision occurs
        // between two connections in the initial phase, both will fail fast and may be
        // retried by the application layer.
        let local_cid = self.local_cid_generator.generate_cid();

        let payload = TokenPayload::Retry {
            address: incoming.network_path.remote,
            orig_dst_cid: incoming.packet.header.dst_cid,
            issued: server_config.time_source.now(),
        };
        let token = Token::new(payload, &mut self.rng).encode(&*server_config.token_key);

        let header = Header::Retry {
            src_cid: local_cid,
            dst_cid: incoming.packet.header.src_cid,
            version: incoming.packet.header.version,
        };

        let encode = header.encode(buf);
        buf.put_slice(&token);
        buf.extend_from_slice(&server_config.crypto.retry_tag(
            incoming.packet.header.version,
            incoming.packet.header.dst_cid,
            buf,
        ));
        encode.finish(buf, &*incoming.crypto.header.local, None);

        Ok(Transmit {
            destination: incoming.network_path.remote,
            ecn: None,
            size: buf.len(),
            segment_size: None,
            src_ip: incoming.network_path.local_ip,
        })
    }

    /// Ignore this incoming connection attempt, not sending any packet in response
    ///
    /// Doing this actively, rather than merely dropping the [`Incoming`], is necessary to prevent
    /// memory leaks due to state within [`Endpoint`] tracking the incoming connection.
    pub fn ignore(&mut self, incoming: Incoming) {
        self.clean_up_incoming(&incoming);
        incoming.improper_drop_warner.dismiss();
    }

    /// Clean up endpoint data structures associated with an `Incoming`.
    fn clean_up_incoming(&mut self, incoming: &Incoming) {
        self.index.remove_initial(incoming.packet.header.dst_cid);
        let incoming_buffer = self.incoming_buffers.remove(incoming.incoming_idx);
        self.all_incoming_buffers_total_bytes -= incoming_buffer.total_bytes;
    }

    fn add_connection(
        &mut self,
        ch: ConnectionHandle,
        version: u32,
        init_cid: ConnectionId,
        local_cid: ConnectionId,
        remote_cid: ConnectionId,
        network_path: FourTuple,
        now: Instant,
        tls: Box<dyn crypto::Session>,
        transport_config: Arc<TransportConfig>,
        side_args: SideArgs,
        // Only used for qlog.
        params: &TransportParameters,
    ) -> Connection {
        let mut rng_seed = [0; 32];
        self.rng.fill_bytes(&mut rng_seed);
        let side = side_args.side();
        let pref_addr_cid = side_args.pref_addr_cid();

        let qlog =
            transport_config.create_qlog_sink(side_args.side(), network_path.remote, init_cid, now);

        qlog.emit_connection_started(
            now,
            local_cid,
            remote_cid,
            network_path.remote,
            network_path.local_ip,
            params,
        );

        let conn = Connection::new(
            self.config.clone(),
            transport_config,
            init_cid,
            local_cid,
            remote_cid,
            network_path,
            tls,
            self.local_cid_generator.as_ref(),
            now,
            version,
            self.allow_mtud,
            rng_seed,
            side_args,
            qlog,
        );

        let mut path_cids = PathLocalCids::default();
        path_cids.cids.insert(path_cids.issued, local_cid);
        path_cids.issued += 1;

        if let Some(cid) = pref_addr_cid {
            debug_assert_eq!(path_cids.issued, 1, "preferred address cid seq must be 1");
            path_cids.cids.insert(path_cids.issued, cid);
            path_cids.issued += 1;
        }

        let id = self.connections.insert(ConnectionMeta {
            init_cid,
            local_cids: FxHashMap::from_iter([(PathId::ZERO, path_cids)]),
            network_path,
            side,
            reset_token: Default::default(),
        });
        debug_assert_eq!(id, ch.0, "connection handle allocation out of sync");

        self.index.insert_conn(network_path, local_cid, ch, side);

        conn
    }

    fn initial_close(
        &mut self,
        version: u32,
        network_path: FourTuple,
        crypto: &Keys,
        remote_id: ConnectionId,
        reason: TransportError,
        buf: &mut Vec<u8>,
    ) -> Transmit {
        // We don't need to worry about CID collisions in initial closes because the peer
        // shouldn't respond, and if it does, and the CID collides, we'll just drop the
        // unexpected response.
        let local_id = self.local_cid_generator.generate_cid();
        let number = PacketNumber::U8(0);
        let header = Header::Initial(InitialHeader {
            dst_cid: remote_id,
            src_cid: local_id,
            number,
            token: Bytes::new(),
            version,
        });

        let partial_encode = header.encode(buf);
        let max_len =
            INITIAL_MTU as usize - partial_encode.header_len - crypto.packet.local.tag_len();
        frame::Close::from(reason).encoder(max_len).encode(buf);
        buf.resize(buf.len() + crypto.packet.local.tag_len(), 0);
        partial_encode.finish(
            buf,
            &*crypto.header.local,
            Some((0, Default::default(), &*crypto.packet.local)),
        );
        Transmit {
            destination: network_path.remote,
            ecn: None,
            size: buf.len(),
            segment_size: None,
            src_ip: network_path.local_ip,
        }
    }

    /// Access the configuration used by this endpoint
    pub fn config(&self) -> &EndpointConfig {
        &self.config
    }

    /// Number of connections that are currently open
    pub fn open_connections(&self) -> usize {
        self.connections.len()
    }

    /// Counter for the number of bytes currently used
    /// in the buffers for Initial and 0-RTT messages for pending incoming connections
    pub fn incoming_buffer_bytes(&self) -> u64 {
        self.all_incoming_buffers_total_bytes
    }

    #[cfg(test)]
    pub(crate) fn known_connections(&self) -> usize {
        let x = self.connections.len();
        debug_assert_eq!(x, self.index.connection_ids_initial.len());
        // Not all connections have known reset tokens
        debug_assert!(x >= self.index.connection_reset_tokens.0.len());
        // Not all connections have unique remotes, and 0-length CIDs might not be in use.
        debug_assert!(x >= self.index.incoming_connection_remotes.len());
        debug_assert!(x >= self.index.outgoing_connection_remotes.len());
        x
    }

    #[cfg(test)]
    pub(crate) fn known_cids(&self) -> usize {
        self.index.connection_ids.len()
    }

    /// Whether we've used up 3/4 of the available CID space
    ///
    /// We leave some space unused so that `new_cid` can be relied upon to finish quickly. We don't
    /// bother to check when CID longer than 4 bytes are used because 2^40 connections is a lot.
    fn cids_exhausted(&self) -> bool {
        let cid_len = self.local_cid_generator.cid_len();
        if cid_len == 0 || cid_len > 4 {
            return false;
        }

        // Keep this architecture-independent: on 32-bit targets, 2usize.pow(32) overflows.
        let bits = (cid_len * 8) as u32;
        let space = 1u64 << bits;
        let reserve = 1u64 << (bits - 2);
        let len = self.index.connection_ids.len() as u64;

        len > (space - reserve)
    }
}

impl fmt::Debug for Endpoint {
    fn fmt(&self, fmt: &mut fmt::Formatter<'_>) -> fmt::Result {
        fmt.debug_struct("Endpoint")
            .field("rng", &self.rng)
            .field("index", &self.index)
            .field("connections", &self.connections)
            .field("config", &self.config)
            .field("server_config", &self.server_config)
            // incoming_buffers too large
            .field("incoming_buffers.len", &self.incoming_buffers.len())
            .field(
                "all_incoming_buffers_total_bytes",
                &self.all_incoming_buffers_total_bytes,
            )
            .finish()
    }
}

/// Buffered Initial and 0-RTT messages for a pending incoming connection
#[derive(Default)]
struct IncomingBuffer {
    datagrams: Vec<DatagramConnectionEvent>,
    total_bytes: u64,
}

/// Part of protocol state incoming datagrams can be routed to
#[derive(Copy, Clone, Debug)]
enum RouteDatagramTo {
    Incoming(usize),
    Connection(ConnectionHandle, PathId),
}

/// Maps packets to existing connections
#[derive(Default, Debug)]
struct ConnectionIndex {
    /// Identifies connections based on the initial DCID the peer utilized
    ///
    /// Uses a standard `HashMap` to protect against hash collision attacks.
    ///
    /// Used by the server, not the client.
    connection_ids_initial: HashMap<ConnectionId, RouteDatagramTo>,
    /// Identifies connections based on locally created CIDs
    ///
    /// Uses a cheaper hash function since keys are locally created
    connection_ids: FxHashMap<ConnectionId, (ConnectionHandle, PathId)>,
    /// Identifies incoming connections with zero-length CIDs
    ///
    /// Uses a standard `HashMap` to protect against hash collision attacks.
    incoming_connection_remotes: HashMap<FourTuple, ConnectionHandle>,
    /// Identifies outgoing connections with zero-length CIDs
    ///
    /// We don't yet support explicit source addresses for client connections, and zero-length CIDs
    /// require a unique 4-tuple, so at most one client connection with zero-length local CIDs
    /// may be established per remote. We must omit the local address from the key because we don't
    /// necessarily know what address we're sending from, and hence receiving at.
    ///
    /// Uses a standard `HashMap` to protect against hash collision attacks.
    // TODO(matheus23): It's possible this could be changed now that we track the full 4-tuple on the client side, too.
    outgoing_connection_remotes: HashMap<SocketAddr, ConnectionHandle>,
    /// Reset tokens provided by the peer for the CID each connection is currently sending to
    ///
    /// Incoming stateless resets do not have correct CIDs, so we need this to identify the correct
    /// recipient, if any.
    connection_reset_tokens: ResetTokenTable,
}

impl ConnectionIndex {
    /// Associate an incoming connection with its initial destination CID
    fn insert_initial_incoming(&mut self, dst_cid: ConnectionId, incoming_key: usize) {
        if dst_cid.is_empty() {
            return;
        }
        self.connection_ids_initial
            .insert(dst_cid, RouteDatagramTo::Incoming(incoming_key));
    }

    /// Remove an association with an initial destination CID
    fn remove_initial(&mut self, dst_cid: ConnectionId) {
        if dst_cid.is_empty() {
            return;
        }
        let removed = self.connection_ids_initial.remove(&dst_cid);
        debug_assert!(removed.is_some());
    }

    /// Associate a connection with its initial destination CID
    fn insert_initial(&mut self, dst_cid: ConnectionId, connection: ConnectionHandle) {
        if dst_cid.is_empty() {
            return;
        }
        self.connection_ids_initial.insert(
            dst_cid,
            RouteDatagramTo::Connection(connection, PathId::ZERO),
        );
    }

    /// Associate a connection with its first locally-chosen destination CID if used, or otherwise
    /// its current 4-tuple
    fn insert_conn(
        &mut self,
        network_path: FourTuple,
        dst_cid: ConnectionId,
        connection: ConnectionHandle,
        side: Side,
    ) {
        match dst_cid.len() {
            0 => match side {
                Side::Server => {
                    self.incoming_connection_remotes
                        .insert(network_path, connection);
                }
                Side::Client => {
                    self.outgoing_connection_remotes
                        .insert(network_path.remote, connection);
                }
            },
            _ => {
                self.connection_ids
                    .insert(dst_cid, (connection, PathId::ZERO));
            }
        }
    }

    /// Discard a connection ID
    fn retire(&mut self, dst_cid: ConnectionId) {
        self.connection_ids.remove(&dst_cid);
    }

    /// Remove all references to a connection
    fn remove(&mut self, conn: &ConnectionMeta) {
        if conn.side.is_server() {
            self.remove_initial(conn.init_cid);
        }
        for cid in conn
            .local_cids
            .values()
            .flat_map(|pcids| pcids.cids.values())
        {
            self.connection_ids.remove(cid);
        }
        self.incoming_connection_remotes.remove(&conn.network_path);
        self.outgoing_connection_remotes
            .remove(&conn.network_path.remote);
        for (remote, token) in conn.reset_token.values() {
            self.connection_reset_tokens.remove(*remote, *token);
        }
    }

    /// Find the existing connection that `datagram` should be routed to, if any
    fn get(&self, network_path: &FourTuple, datagram: &PartialDecode) -> Option<RouteDatagramTo> {
        if !datagram.dst_cid().is_empty()
            && let Some(&(ch, path_id)) = self.connection_ids.get(&datagram.dst_cid())
        {
            return Some(RouteDatagramTo::Connection(ch, path_id));
        }
        if (datagram.is_initial() || datagram.is_0rtt())
            && let Some(&ch) = self.connection_ids_initial.get(&datagram.dst_cid())
        {
            return Some(ch);
        }
        if datagram.dst_cid().is_empty() {
            if let Some(&ch) = self.incoming_connection_remotes.get(network_path) {
                // Never multipath because QUIC-MULTIPATH 1.1 mandates the use of non-zero
                // length CIDs.  So this is always PathId::ZERO.
                return Some(RouteDatagramTo::Connection(ch, PathId::ZERO));
            }
            if let Some(&ch) = self.outgoing_connection_remotes.get(&network_path.remote) {
                // Like above, QUIC-MULTIPATH 1.1 mandates the use of non-zero length CIDs.
                return Some(RouteDatagramTo::Connection(ch, PathId::ZERO));
            }
        }
        let data = datagram.data();
        if data.len() < RESET_TOKEN_SIZE {
            return None;
        }
        // For stateless resets the PathId is meaningless since it closes the entire
        // connection regardless of path.  So use PathId::ZERO.
        self.connection_reset_tokens
            .get(network_path.remote, &data[data.len() - RESET_TOKEN_SIZE..])
            .cloned()
            .map(|ch| RouteDatagramTo::Connection(ch, PathId::ZERO))
    }
}

#[derive(Debug)]
pub(crate) struct ConnectionMeta {
    init_cid: ConnectionId,
    /// Locally issues CIDs for each path
    local_cids: FxHashMap<PathId, PathLocalCids>,
    /// Remote/local addresses the connection began with
    ///
    /// Only needed to support connections with zero-length CIDs, which cannot migrate, so we don't
    /// bother keeping it up to date.
    network_path: FourTuple,
    side: Side,
    /// Reset tokens provided by the peer for CIDs we're currently sending to
    ///
    /// Since each reset token is for a CID, it is also for a fixed remote address which is
    /// also stored. This allows us to look up which reset tokens we might expect from a
    /// given remote address, see [`ResetTokenTable`].
    ///
    /// Each path has its own active CID. We use the [`PathId`] as a unique index, allowing
    /// us to retire the reset token when a path is abandoned.
    // TODO(matheus23): Should be migrated to make reset tokens per 4-tuple instead of per remote addr
    reset_token: FxHashMap<PathId, (SocketAddr, ResetToken)>,
}

/// Local connection IDs for a single path
#[derive(Debug, Default)]
struct PathLocalCids {
    /// Number of connection IDs that have been issued in (PATH_)NEW_CONNECTION_ID frames
    ///
    /// Another way of saying this is that this is the next sequence number to be issued.
    issued: u64,
    /// Issues CIDs indexed by their sequence number.
    cids: FxHashMap<u64, ConnectionId>,
}

/// Internal identifier for a `Connection` currently associated with an endpoint
#[derive(Debug, Copy, Clone, Eq, PartialEq, Hash, Ord, PartialOrd)]
pub struct ConnectionHandle(pub usize);

impl From<ConnectionHandle> for usize {
    fn from(x: ConnectionHandle) -> Self {
        x.0
    }
}

impl Index<ConnectionHandle> for Slab<ConnectionMeta> {
    type Output = ConnectionMeta;
    fn index(&self, ch: ConnectionHandle) -> &ConnectionMeta {
        &self[ch.0]
    }
}

impl IndexMut<ConnectionHandle> for Slab<ConnectionMeta> {
    fn index_mut(&mut self, ch: ConnectionHandle) -> &mut ConnectionMeta {
        &mut self[ch.0]
    }
}

/// Event resulting from processing a single datagram
pub enum DatagramEvent {
    /// The datagram is redirected to its `Connection`
    ConnectionEvent(ConnectionHandle, ConnectionEvent),
    /// The datagram may result in starting a new `Connection`
    NewConnection(Incoming),
    /// Response generated directly by the endpoint
    Response(Transmit),
}

/// An incoming connection for which the server has not yet begun its part of the handshake.
#[derive(derive_more::Debug)]
pub struct Incoming {
    #[debug(skip)]
    received_at: Instant,
    network_path: FourTuple,
    ecn: Option<EcnCodepoint>,
    #[debug(skip)]
    packet: InitialPacket,
    #[debug(skip)]
    rest: Option<BytesMut>,
    #[debug(skip)]
    crypto: Keys,
    token: IncomingToken,
    incoming_idx: usize,
    #[debug(skip)]
    improper_drop_warner: IncomingImproperDropWarner,
}

impl Incoming {
    /// The local IP address which was used when the peer established the connection
    pub fn local_ip(&self) -> Option<IpAddr> {
        self.network_path.local_ip
    }

    /// The peer's UDP address
    pub fn remote_address(&self) -> SocketAddr {
        self.network_path.remote
    }

    /// Whether the socket address that is initiating this connection has been validated
    ///
    /// This means that the sender of the initial packet has proved that they can receive traffic
    /// sent to `self.remote_address()`.
    ///
    /// If `self.remote_address_validated()` is false, `self.may_retry()` is guaranteed to be true.
    /// The inverse is not guaranteed.
    pub fn remote_address_validated(&self) -> bool {
        self.token.validated
    }

    /// Whether it is legal to respond with a retry packet
    ///
    /// If `self.remote_address_validated()` is false, `self.may_retry()` is guaranteed to be true.
    /// The inverse is not guaranteed.
    pub fn may_retry(&self) -> bool {
        self.token.retry_src_cid.is_none()
    }

    /// The original destination connection ID sent by the client
    pub fn orig_dst_cid(&self) -> ConnectionId {
        self.token.orig_dst_cid
    }

    /// Decrypt the Initial packet payload
    ///
    /// This clones and decrypts the packet payload (~1200 bytes).
    /// Can be used to extract information from the TLS ClientHello without completing the handshake.
    pub fn decrypt(&self) -> Option<DecryptedInitial> {
        let packet_number = self.packet.header.number.expand(0);
        let mut payload = self.packet.payload.clone();
        self.crypto
            .packet
            .remote
            .decrypt(
                PathId::ZERO,
                packet_number,
                &self.packet.header_data,
                &mut payload,
            )
            .ok()?;
        Some(DecryptedInitial(payload.freeze()))
    }
}

/// Decrypted payload of a QUIC Initial packet
///
/// Obtained via [`Incoming::decrypt`]. Can be used to extract information from
/// the TLS ClientHello without completing the handshake.
pub struct DecryptedInitial(Bytes);

impl DecryptedInitial {
    /// Best-effort extraction of the ALPN protocols from the TLS ClientHello
    ///
    /// Parses the CRYPTO frames to extract the ALPN extension. This is intended
    /// for routing and filtering; it is not guaranteed to succeed if the
    /// ClientHello spans multiple packets. Returns `None` if parsing fails.
    pub fn alpns(&self) -> Option<IncomingAlpns> {
        let frames = frame::Iter::new(self.0.clone()).ok()?;
        let mut first = None;
        let mut rest = Vec::new();
        for frame in frames {
            match frame {
                Ok(frame::Frame::Crypto(crypto)) => match first {
                    None => first = Some(crypto),
                    Some(_) => rest.push(crypto),
                },
                Err(_) => return None,
                _ => {}
            }
        }
        let first = first?;

        // Fast path: single CRYPTO frame at offset 0 (no extra allocation)
        if rest.is_empty() && first.offset == 0 {
            let data = find_alpn_data(&first.data).ok()?;
            return Some(IncomingAlpns { data, pos: 0 });
        }

        // Slow path: reassemble multiple CRYPTO frames
        rest.push(first);
        let source = assemble_crypto_frames(&mut rest)?;
        let data = find_alpn_data(&source).ok()?;
        Some(IncomingAlpns { data, pos: 0 })
    }
}

/// TLS handshake type for ClientHello messages
/// <https://www.rfc-editor.org/rfc/rfc8446#section-4.1.2>
const TLS_HANDSHAKE_TYPE_CLIENT_HELLO: u8 = 0x01;
/// TLS extension type for Application-Layer Protocol Negotiation
/// <https://www.rfc-editor.org/rfc/rfc7301#section-3.1>
const TLS_EXTENSION_TYPE_ALPN: u16 = 0x0010;
/// Size of the fixed-length fields in a ClientHello (client_version + random)
/// <https://www.rfc-editor.org/rfc/rfc8446#section-4.1.2>
const TLS_CLIENT_HELLO_FIXED_LEN: usize = 2 + 32;

/// Iterator over ALPN protocol names from a TLS ClientHello
///
/// Yields protocol names as [`Bytes`] slices. On the common fast path (single
/// CRYPTO frame), the only allocation is the payload clone for decryption.
pub struct IncomingAlpns {
    data: Bytes,
    pos: usize,
}

impl Iterator for IncomingAlpns {
    type Item = Result<Bytes, UnexpectedEnd>;

    fn next(&mut self) -> Option<Self::Item> {
        if self.pos >= self.data.len() {
            return None;
        }
        let len = self.data[self.pos] as usize;
        self.pos += 1;
        if self.pos + len > self.data.len() {
            return Some(Err(UnexpectedEnd));
        }
        let proto = self.data.slice(self.pos..self.pos + len);
        self.pos += len;
        Some(Ok(proto))
    }
}

/// Sort CRYPTO frames by offset and concatenate into a contiguous `Bytes`
///
/// Returns `None` if there are gaps in the stream.
fn assemble_crypto_frames(frames: &mut [frame::Crypto]) -> Option<Bytes> {
    frames.sort_by_key(|f| f.offset);
    let capacity = frames.iter().map(|f| f.data.len()).sum();
    let mut buf = Vec::with_capacity(capacity);
    for f in frames.iter() {
        let start = f.offset as usize;
        if start > buf.len() {
            return None;
        }
        let end = start + f.data.len();
        if end > buf.len() {
            buf.extend_from_slice(&f.data[buf.len() - start..]);
        }
    }
    Some(Bytes::from(buf))
}

/// Locate the raw ALPN protocol list data within a TLS ClientHello message
///
/// Parses the ClientHello in `source` and returns a [`Bytes`] containing the
/// u8-length-prefixed protocol names (after the outer ProtocolNameList u16
/// length prefix). The returned `Bytes` is a zero-copy slice of `source`.
fn find_alpn_data(source: &Bytes) -> Result<Bytes, UnexpectedEnd> {
    let mut r = &**source;

    if u8::decode(&mut r)? != TLS_HANDSHAKE_TYPE_CLIENT_HELLO {
        return Err(UnexpectedEnd);
    }

    // Handshake message length (u24), scopes the remainder
    let len = decode_u24(&mut r)?;
    let mut body = take(&mut r, len)?;

    // Client version + random
    skip(&mut body, TLS_CLIENT_HELLO_FIXED_LEN)?;

    // Session ID, cipher suites, compression methods
    skip_u8_prefixed(&mut body)?;
    skip_u16_prefixed(&mut body)?;
    skip_u8_prefixed(&mut body)?;

    // Extensions
    let mut exts = take_u16_prefixed(&mut body)?;
    while exts.has_remaining() {
        let ext_type = u16::decode(&mut exts)?;
        let ext_data = take_u16_prefixed(&mut exts)?;
        if ext_type == TLS_EXTENSION_TYPE_ALPN {
            let list = take_u16_prefixed(&mut &*ext_data)?;
            return Ok(source.slice_ref(list));
        }
    }
    Err(UnexpectedEnd)
}

/// Decode a big-endian u24 as usize
fn decode_u24(r: &mut &[u8]) -> Result<usize, UnexpectedEnd> {
    let a = u8::decode(r)?;
    let b = u8::decode(r)?;
    let c = u8::decode(r)?;
    Ok(u32::from_be_bytes([0, a, b, c]) as usize)
}

/// Take `len` bytes from the front and return them as a sub-slice
fn take<'a>(r: &mut &'a [u8], len: usize) -> Result<&'a [u8], UnexpectedEnd> {
    if r.remaining() < len {
        return Err(UnexpectedEnd);
    }
    let data = &r[..len];
    r.advance(len);
    Ok(data)
}

/// Read a u16 length prefix and return the sub-slice it covers
fn take_u16_prefixed<'a>(r: &mut &'a [u8]) -> Result<&'a [u8], UnexpectedEnd> {
    let len = u16::decode(r)? as usize;
    take(r, len)
}

/// Advance past `n` bytes
fn skip(r: &mut &[u8], len: usize) -> Result<(), UnexpectedEnd> {
    take(r, len)?;
    Ok(())
}

/// Skip a u8-length-prefixed field
fn skip_u8_prefixed(r: &mut &[u8]) -> Result<(), UnexpectedEnd> {
    let len = u8::decode(r)? as usize;
    skip(r, len)
}

/// Skip a u16-length-prefixed field
fn skip_u16_prefixed(r: &mut &[u8]) -> Result<(), UnexpectedEnd> {
    let len = u16::decode(r)? as usize;
    skip(r, len)
}

struct IncomingImproperDropWarner;

impl IncomingImproperDropWarner {
    fn dismiss(self) {
        mem::forget(self);
    }
}

impl Drop for IncomingImproperDropWarner {
    fn drop(&mut self) {
        warn!(
            "noq_proto::Incoming dropped without passing to Endpoint::accept/refuse/retry/ignore \
               (may cause memory leak and eventual inability to accept new connections)"
        );
    }
}

/// Errors in the parameters being used to create a new connection
///
/// These arise before any I/O has been performed.
#[derive(Debug, Error, Clone, PartialEq, Eq)]
pub enum ConnectError {
    /// The endpoint can no longer create new connections
    ///
    /// Indicates that a necessary component of the endpoint has been dropped or otherwise disabled.
    #[error("endpoint stopping")]
    EndpointStopping,
    /// The connection could not be created because not enough of the CID space is available
    ///
    /// Try using longer connection IDs
    #[error("CIDs exhausted")]
    CidsExhausted,
    /// The given server name was malformed
    #[error("invalid server name: {0}")]
    InvalidServerName(String),
    /// The remote [`SocketAddr`] supplied was malformed
    ///
    /// Examples include attempting to connect to port 0, or using an inappropriate address family.
    #[error("invalid remote address: {0}")]
    InvalidRemoteAddress(SocketAddr),
    /// No default client configuration was set up
    ///
    /// Use `Endpoint::connect_with` to specify a client configuration.
    #[error("no default client config")]
    NoDefaultClientConfig,
    /// The local endpoint does not support the QUIC version specified in the client configuration
    #[error("unsupported QUIC version")]
    UnsupportedVersion,
}

/// Error type for attempting to accept an [`Incoming`]
#[derive(Debug)]
pub struct AcceptError {
    /// Underlying error describing reason for failure
    pub cause: ConnectionError,
    /// Optional response to transmit back
    pub response: Option<Transmit>,
}

/// Error for attempting to retry an [`Incoming`] which already bears a token from a previous retry
#[derive(Debug, Error)]
#[error("retry() with validated Incoming")]
pub struct RetryError(Box<Incoming>);

impl RetryError {
    /// Get the [`Incoming`]
    pub fn into_incoming(self) -> Incoming {
        *self.0
    }
}

/// Reset Tokens which are associated with peer socket addresses
///
/// The standard `HashMap` is used since both `SocketAddr` and `ResetToken` are
/// peer generated and might be usable for hash collision attacks.
#[derive(Default, Debug)]
struct ResetTokenTable(HashMap<SocketAddr, HashMap<ResetToken, ConnectionHandle>>);

impl ResetTokenTable {
    fn insert(&mut self, remote: SocketAddr, token: ResetToken, ch: ConnectionHandle) -> bool {
        self.0
            .entry(remote)
            .or_default()
            .insert(token, ch)
            .is_some()
    }

    fn remove(&mut self, remote: SocketAddr, token: ResetToken) {
        use std::collections::hash_map::Entry;
        match self.0.entry(remote) {
            Entry::Vacant(_) => {}
            Entry::Occupied(mut e) => {
                e.get_mut().remove(&token);
                if e.get().is_empty() {
                    e.remove_entry();
                }
            }
        }
    }

    fn get(&self, remote: SocketAddr, token: &[u8]) -> Option<&ConnectionHandle> {
        let token = ResetToken::from(<[u8; RESET_TOKEN_SIZE]>::try_from(token).ok()?);
        self.0.get(&remote)?.get(&token)
    }
}

#[cfg(test)]
mod tests {
    use super::*;

    #[test]
    fn assemble_contiguous() {
        let data = b"hello world";
        let mut frames = vec![
            frame::Crypto {
                offset: 0,
                data: Bytes::from_static(&data[..5]),
            },
            frame::Crypto {
                offset: 5,
                data: Bytes::from_static(&data[5..]),
            },
        ];
        assert_eq!(&assemble_crypto_frames(&mut frames).unwrap()[..], &data[..]);
    }

    #[test]
    fn assemble_out_of_order() {
        let data = b"hello world";
        let mut frames = vec![
            frame::Crypto {
                offset: 5,
                data: Bytes::from_static(&data[5..]),
            },
            frame::Crypto {
                offset: 0,
                data: Bytes::from_static(&data[..5]),
            },
        ];
        assert_eq!(&assemble_crypto_frames(&mut frames).unwrap()[..], &data[..]);
    }

    #[test]
    fn assemble_with_overlap() {
        let data = b"hello world";
        let mut frames = vec![
            frame::Crypto {
                offset: 0,
                data: Bytes::from_static(&data[..7]),
            },
            frame::Crypto {
                offset: 5,
                data: Bytes::from_static(&data[5..]),
            },
        ];
        assert_eq!(&assemble_crypto_frames(&mut frames).unwrap()[..], &data[..]);
    }

    #[test]
    fn assemble_with_gap() {
        let mut frames = vec![frame::Crypto {
            offset: 10,
            data: Bytes::from_static(b"world"),
        }];
        assert!(assemble_crypto_frames(&mut frames).is_none());
    }
}