noq_proto/connection/

paths.rs

use std::{cmp, net::SocketAddr};

use identity_hash::IntMap;
use thiserror::Error;
use tracing::trace;

use super::{
    PathStats, PathStatus, SpaceKind,
    mtud::MtuDiscovery,
    pacing::Pacer,
    spaces::{PacketNumberSpace, SentPacket},
};
use crate::{
    ConnectionId, Duration, FourTuple, Instant, TIMER_GRANULARITY, TransportConfig,
    TransportErrorCode, VarInt,
    coding::{self, Decodable, Encodable},
    congestion,
    connection::{MAX_BACKOFF_EXPONENT, MAX_PTO_INTERVAL},
    frame::ObservedAddr,
};

#[cfg(feature = "qlog")]
use qlog::events::quic::RecoveryMetricsUpdated;

/// Id representing different paths when using multipath extension
#[cfg_attr(test, derive(test_strategy::Arbitrary))]
#[derive(Debug, PartialEq, Eq, PartialOrd, Ord, Clone, Copy, Default)]
pub struct PathId(pub(crate) u32);

impl std::hash::Hash for PathId {
    fn hash<H: std::hash::Hasher>(&self, state: &mut H) {
        state.write_u32(self.0);
    }
}

impl Decodable for PathId {
    fn decode<B: bytes::Buf>(r: &mut B) -> coding::Result<Self> {
        let v = VarInt::decode(r)?;
        let v = u32::try_from(v.0).map_err(|_| coding::UnexpectedEnd)?;
        Ok(Self(v))
    }
}

impl Encodable for PathId {
    fn encode<B: bytes::BufMut>(&self, w: &mut B) {
        VarInt(self.0.into()).encode(w)
    }
}

impl PathId {
    /// The maximum path ID allowed.
    pub const MAX: Self = Self(u32::MAX);

    /// The 0 path id.
    pub const ZERO: Self = Self(0);

    /// The number of bytes this [`PathId`] uses when encoded as a [`VarInt`]
    pub(crate) const fn size(&self) -> usize {
        VarInt(self.0 as u64).size()
    }

    /// Saturating integer addition. Computes self + rhs, saturating at the numeric bounds instead
    /// of overflowing.
    pub fn saturating_add(self, rhs: impl Into<Self>) -> Self {
        let rhs = rhs.into();
        let inner = self.0.saturating_add(rhs.0);
        Self(inner)
    }

    /// Saturating integer subtraction. Computes self - rhs, saturating at the numeric bounds
    /// instead of overflowing.
    pub fn saturating_sub(self, rhs: impl Into<Self>) -> Self {
        let rhs = rhs.into();
        let inner = self.0.saturating_sub(rhs.0);
        Self(inner)
    }

    /// Get the next [`PathId`]
    pub(crate) fn next(&self) -> Self {
        self.saturating_add(Self(1))
    }

    /// Get the underlying u32
    pub(crate) fn as_u32(&self) -> u32 {
        self.0
    }
}

impl std::fmt::Display for PathId {
    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
        self.0.fmt(f)
    }
}

impl<T: Into<u32>> From<T> for PathId {
    fn from(source: T) -> Self {
        Self(source.into())
    }
}

/// State needed for a single path ID.
///
/// A single path ID can migrate according to the rules in RFC9000 §9, either voluntary or
/// involuntary. We need to keep the [`PathData`] of the previously used such path available
/// in order to defend against migration attacks (see RFC9000 §9.3.1, §9.3.2 and §9.3.3) as
/// well as to support path probing (RFC9000 §9.1).
#[derive(Debug)]
pub(super) struct PathState {
    pub(super) data: PathData,
    pub(super) prev: Option<(ConnectionId, PathData)>,
}

impl PathState {
    /// Update counters to account for a packet becoming acknowledged, lost, or abandoned
    pub(super) fn remove_in_flight(&mut self, packet: &SentPacket) {
        // Visit known paths from newest to oldest to find the one `pn` was sent on
        for path_data in [&mut self.data]
            .into_iter()
            .chain(self.prev.as_mut().map(|(_, data)| data))
        {
            if path_data.remove_in_flight(packet) {
                return;
            }
        }
    }
}

#[derive(Debug)]
pub(super) struct SentChallengeInfo {
    /// When was the challenge sent on the wire.
    pub(super) sent_instant: Instant,
    /// The 4-tuple on which this path challenge was sent.
    pub(super) network_path: FourTuple,
}

/// State of particular network path 4-tuple within a [`PacketNumberSpace`].
///
/// With QUIC-Multipath a path is identified by a [`PathId`] and it is possible to have
/// multiple paths on the same 4-tuple. Furthermore a single QUIC-Multipath path can migrate
/// to a different 4-tuple, in a similar manner as an RFC9000 connection can use "path
/// migration" to move to a different 4-tuple. There are thus two states we keep for paths:
///
/// - [`PacketNumberSpace`]: The state for a single packet number space, i.e. [`PathId`], which
///   remains in place across path migrations to different 4-tuples.
///
///   This is stored in [`PacketSpace::number_spaces`] indexed on [`PathId`].
///
/// - [`PathData`]: The state we keep for each unique 4-tuple within a space. Of note is that a
///   single [`PathData`] can never belong to a different [`PacketNumberSpace`].
///
///   This is stored in [`Connection::paths`] indexed by the current [`PathId`] for which
///   space it exists. Either as the primary 4-tuple or as the previous 4-tuple just after a
///   migration.
///
/// It follows that there might be several [`PathData`] structs for the same 4-tuple if
/// several spaces are sharing the same 4-tuple. Note that during the handshake, the
/// Initial, Handshake and Data spaces for [`PathId::ZERO`] all share the same [`PathData`].
///
/// [`PacketSpace::number_spaces`]: super::spaces::PacketSpace::number_spaces
/// [`Connection::paths`]: super::Connection::paths
#[derive(Debug)]
pub(super) struct PathData {
    pub(super) network_path: FourTuple,
    pub(super) rtt: RttEstimator,
    /// Whether we're enabling ECN on outgoing packets
    pub(super) sending_ecn: bool,
    /// Congestion controller state
    pub(super) congestion: Box<dyn congestion::Controller>,
    /// Pacing state
    pub(super) pacing: Pacer,
    /// Whether the last `poll_transmit_on_path` call yielded no data because there was
    /// no outgoing application data.
    ///
    /// The RFC writes:
    /// > When bytes in flight is smaller than the congestion window and sending is not pacing
    /// > limited, the congestion window is underutilized. This can happen due to insufficient
    /// > application data or flow control limits. When this occurs, the congestion window SHOULD
    /// > NOT be increased in either slow start or congestion avoidance.
    ///
    /// (RFC9002, section 7.8)
    ///
    /// I.e. when app_limited is true, the congestion controller doesn't increase the congestion
    /// window.
    pub(super) app_limited: bool,

    /// Whether to trigger sending another PATH_CHALLENGE in the next poll_transmit.
    ///
    /// This is picked up by [`super::Connection::space_can_send`]. These are **not**
    /// retransmittable, which is why they are not part of the `PathRetransmits`.
    ///
    /// Only used for **on-path** challenges, like RFC9000-style path migration and
    /// multipath path validation (for opening).
    ///
    /// This is **not used** for n0 nat traversal challenge sending, which is off-path.
    pub(super) pending_challenge: bool,
    /// On-path path challenges sent that we didn't receive a path response for yet.
    unconfirmed_challenges: IntMap<u64, SentChallengeInfo>,
    /// How often we've deemed a path challenge to be lost.
    ///
    /// Similar to [`Self::pto_count`], but for on-path path challenges.
    /// Used to calculate exponential backoff for retrying path challenges.
    pub(super) lost_challenge_count: u32,
    /// Whether we're certain the peer can both send and receive on this address
    ///
    /// Initially equal to `use_stateless_retry` for servers, and becomes false again on every
    /// migration. Always true for clients.
    pub(super) validated: bool,
    /// Total size of all UDP datagrams sent on this path
    pub(super) total_sent: u64,
    /// Total size of all UDP datagrams received on this path
    pub(super) total_recvd: u64,
    /// The state of the MTU discovery process
    pub(super) mtud: MtuDiscovery,
    /// Packet number of the first packet sent after an RTT sample was collected on this path
    ///
    /// Used in persistent congestion determination.
    pub(super) first_packet_after_rtt_sample: Option<(SpaceKind, u64)>,
    /// The in-flight packets and bytes
    ///
    /// Note that this is across all spaces on this path
    pub(super) in_flight: InFlight,
    /// Queue of data that must be sent over this specific [`PathData::generation`] path.
    pub(super) pending: PathRetransmits,
    /// Observed address frame with the largest sequence number received from the peer on this
    /// path.
    pub(super) last_observed_addr_report: Option<ObservedAddr>,
    /// Number of the first packet sent on this path
    ///
    /// With RFC9000 §9 style migration (i.e. not multipath) the PathId does not change and
    /// hence packet numbers continue. This is used to determine whether a packet was sent
    /// on such an earlier path. Insufficient to determine if a packet was sent on a later
    /// path.
    first_packet: Option<u64>,
    /// The number of times a tail-loss probe has been sent without receiving an ack.
    ///
    /// This is incremented by one every time the [`LossDetection`] timer fires because a
    /// tail-loss probe needs to be sent. Once an acknowledgement for a packet is received
    /// again it is reset to 0. Used to compute the PTO duration.
    ///
    /// [`LossDetection`]: super::timer::PathTimer::LossDetection
    pub(super) pto_count: u32,

    //
    // Per-path idle & keep alive
    /// Idle timeout for the path
    ///
    /// If expired, the path will be abandoned.  This is different from the connection-wide
    /// idle timeout which closes the connection if expired.
    pub(super) idle_timeout: Option<Duration>,
    /// Keep alives to send on this path
    ///
    /// There is also a connection-level keep alive configured in the
    /// [`TransportParameters`].  This triggers activity on any path which can keep the
    /// connection alive.
    ///
    /// [`TransportParameters`]: crate::transport_parameters::TransportParameters
    pub(super) keep_alive: Option<Duration>,
    /// Whether to reset the idle timer when the next ack-eliciting packet is sent.
    ///
    /// Whenever we receive an authenticated packet the connection and path idle timers are
    /// reset if a maximum idle timeout was negotiated. However on the first ack-eliciting
    /// packet *sent* after this the idle timer also needs to be reset to avoid the idle
    /// timer firing while the sent packet is in-fight. See
    /// <https://www.rfc-editor.org/rfc/rfc9000.html#section-10.1>.
    pub(super) permit_idle_reset: bool,

    /// Whether we're currently draining the path after having abandoned it.
    ///
    /// This should only be true when a path discard timer is armed, and after the path was
    /// abandoned (and added to the abandoned_paths set).
    ///
    /// This will only ever be set from false to true.
    pub(super) draining: bool,

    /// Snapshot of the qlog recovery metrics
    #[cfg(feature = "qlog")]
    recovery_metrics: RecoveryMetrics,

    /// Tag uniquely identifying a path in a connection.
    ///
    /// When a migration happens on the same [`PathId`] we still detect a change in the
    /// 4-tuple and generate a new [`PathData`] for it. Each such generation has a unique
    /// value to keep track of which 4-tuple a packet belonged to.
    generation: u64,
}

impl PathData {
    pub(super) fn new(
        network_path: FourTuple,
        allow_mtud: bool,
        peer_max_udp_payload_size: Option<u16>,
        generation: u64,
        now: Instant,
        config: &TransportConfig,
    ) -> Self {
        let congestion = config
            .congestion_controller_factory
            .clone()
            .build(now, config.get_initial_mtu());
        Self {
            network_path,
            rtt: RttEstimator::new(config.initial_rtt),
            sending_ecn: true,
            pacing: Pacer::new(
                config.initial_rtt,
                congestion.initial_window(),
                config.get_initial_mtu(),
                config.max_outgoing_bytes_per_second,
                now,
            ),
            congestion,
            app_limited: false,
            unconfirmed_challenges: Default::default(),
            lost_challenge_count: 0,
            pending_challenge: false,
            validated: false,
            total_sent: 0,
            total_recvd: 0,
            mtud: config
                .mtu_discovery_config
                .as_ref()
                .filter(|_| allow_mtud)
                .map_or_else(
                    || MtuDiscovery::disabled(config.get_initial_mtu(), config.min_mtu),
                    |mtud_config| {
                        MtuDiscovery::new(
                            config.get_initial_mtu(),
                            config.min_mtu,
                            peer_max_udp_payload_size,
                            mtud_config.clone(),
                        )
                    },
                ),
            first_packet_after_rtt_sample: None,
            in_flight: InFlight::new(),
            pending: PathRetransmits::default(),
            last_observed_addr_report: None,
            first_packet: None,
            pto_count: 0,
            idle_timeout: config.default_path_max_idle_timeout,
            keep_alive: config.default_path_keep_alive_interval,
            permit_idle_reset: true,
            draining: false,
            #[cfg(feature = "qlog")]
            recovery_metrics: RecoveryMetrics::default(),
            generation,
        }
    }

    /// Create a new path from a previous one.
    ///
    /// This should only be called when migrating paths.
    pub(super) fn from_previous(
        network_path: FourTuple,
        prev: &Self,
        generation: u64,
        now: Instant,
    ) -> Self {
        let congestion = prev.congestion.clone_box();
        let smoothed_rtt = prev.rtt.get();
        Self {
            network_path,
            rtt: prev.rtt,
            pacing: Pacer::new(
                smoothed_rtt,
                congestion.window(),
                prev.current_mtu(),
                prev.pacing.max_bytes_per_second(),
                now,
            ),
            sending_ecn: true,
            congestion,
            app_limited: false,
            unconfirmed_challenges: Default::default(),
            lost_challenge_count: 0,
            pending_challenge: false,
            validated: false,
            total_sent: 0,
            total_recvd: 0,
            mtud: prev.mtud.clone(),
            first_packet_after_rtt_sample: prev.first_packet_after_rtt_sample,
            in_flight: InFlight::new(),
            pending: PathRetransmits::default(),
            last_observed_addr_report: None,
            first_packet: None,
            pto_count: 0,
            idle_timeout: prev.idle_timeout,
            keep_alive: prev.keep_alive,
            permit_idle_reset: true,
            draining: false,
            #[cfg(feature = "qlog")]
            recovery_metrics: prev.recovery_metrics.clone(),
            generation,
        }
    }

    /// Whether we're in the process of validating this path with PATH_CHALLENGEs
    pub(super) fn is_validating_path(&self) -> bool {
        !self.unconfirmed_challenges.is_empty() || self.pending_challenge
    }

    /// Indicates whether we're a server that hasn't validated the peer's address and hasn't
    /// received enough data from the peer to permit sending `bytes_to_send` additional bytes
    pub(super) fn anti_amplification_blocked(&self, bytes_to_send: u64) -> bool {
        !self.validated && self.total_recvd * 3 < self.total_sent + bytes_to_send
    }

    /// Returns the path's current MTU
    pub(super) fn current_mtu(&self) -> u16 {
        self.mtud.current_mtu()
    }

    /// Account for transmission of `packet` with number `pn` in `space`
    pub(super) fn sent(&mut self, pn: u64, packet: SentPacket, space: &mut PacketNumberSpace) {
        self.in_flight.insert(&packet);
        if self.first_packet.is_none() {
            self.first_packet = Some(pn);
        }
        if let Some(forgotten) = space.sent(pn, packet) {
            self.remove_in_flight(&forgotten);
        }
    }

    pub(super) fn record_path_challenge_sent(
        &mut self,
        now: Instant,
        token: u64,
        network_path: FourTuple,
    ) {
        let info = SentChallengeInfo {
            sent_instant: now,
            network_path,
        };
        debug_assert_eq!(network_path, self.network_path);
        self.unconfirmed_challenges.insert(token, info);
    }

    /// Remove `packet` with number `pn` from this path's congestion control counters, or return
    /// `false` if `pn` was sent before this path was established.
    pub(super) fn remove_in_flight(&mut self, packet: &SentPacket) -> bool {
        if packet.path_generation != self.generation {
            return false;
        }
        self.in_flight.remove(packet);
        true
    }

    /// Increment the total size of sent UDP datagrams
    pub(super) fn inc_total_sent(&mut self, inc: u64) {
        self.total_sent = self.total_sent.saturating_add(inc);
        if !self.validated {
            trace!(
                network_path = %self.network_path,
                anti_amplification_budget = %(self.total_recvd * 3).saturating_sub(self.total_sent),
                "anti amplification budget decreased"
            );
        }
    }

    /// Increment the total size of received UDP datagrams
    pub(super) fn inc_total_recvd(&mut self, inc: u64) {
        self.total_recvd = self.total_recvd.saturating_add(inc);
        if !self.validated {
            trace!(
                network_path = %self.network_path,
                anti_amplification_budget = %(self.total_recvd * 3).saturating_sub(self.total_sent),
                "anti amplification budget increased"
            );
        }
    }

    /// The earliest time at which an on-path challenge we sent is considered lost.
    pub(super) fn earliest_on_path_expiring_challenge(&self) -> Option<Instant> {
        if self.unconfirmed_challenges.is_empty() {
            return None;
        }
        let duration = self.on_path_challenge_pto();
        self.unconfirmed_challenges
            .values()
            .map(|info| info.sent_instant + duration)
            .min()
    }

    /// The duration after which a PTO expires for an on-path challenge, if sent now.
    ///
    /// Since challenges need an on-path response rather than just an ACK that can be sent
    /// on any path they need a different timer from the
    /// [`PathTimer::LossDetection`]. Functionally this behaves as the probe timeout
    /// however.
    ///
    /// [`PathTimer::LossDetection`]: super::timer::PathTimer::LossDetection
    pub(super) fn on_path_challenge_pto(&self) -> Duration {
        let backoff = 2u32.pow(self.lost_challenge_count.min(MAX_BACKOFF_EXPONENT));
        let duration = self.rtt.pto_base() * backoff;
        duration.min(MAX_PTO_INTERVAL)
    }

    /// Handle receiving a PATH_RESPONSE.
    pub(super) fn on_path_response_received(
        &mut self,
        now: Instant,
        token: u64,
    ) -> OnPathResponseReceived {
        // > § 8.2.3
        // > Path validation succeeds when a PATH_RESPONSE frame is received that contains the
        // > data that was sent in a previous PATH_CHALLENGE frame. A PATH_RESPONSE frame
        // > received on any network path validates the path on which the PATH_CHALLENGE was
        // > sent.
        //
        // At this point we have three potentially different network paths:
        // - current network path (`Self::network_path`)
        // - network path used to send the path challenge (`SentChallengeInfo::network_path`)
        // - network path over which the response arrived (not needed)
        //
        // As per above spec quote, this only validates the network path on which this was
        // *sent*, regardless of the path on which it was received in order to protect
        // against off-path packet forwarding attacks.
        match self.unconfirmed_challenges.remove(&token) {
            // Response to an on-path PathChallenge that validates this path.
            // The sent path should match the current path. However, it's possible that the
            // challenge was sent when no local_ip was known. This case is allowed as well.
            Some(info) if info.network_path.is_probably_same_path(&self.network_path) => {
                // Do not update or set the self.network_path.local_ip:
                // Connection::process_payload handles this later when required. We can mark
                // the path as validated though, because for a change in local_ip only we do
                // not need to re-validate the path.
                let sent_instant = info.sent_instant;
                if !std::mem::replace(&mut self.validated, true) {
                    trace!("new path validated");
                }
                // Clear any other on-path sent challenges and stop sending new ones.
                self.reset_on_path_challenges();

                // This RTT can only be used for the initial RTT, not as a normal
                // sample: https://www.rfc-editor.org/rfc/rfc9002#section-6.2.2-2.
                let rtt = now.saturating_duration_since(sent_instant);
                self.rtt.reset_initial_rtt(rtt);

                OnPathResponseReceived::OnPath
            }
            // Response to an on-path PathChallenge that does not validate this path.
            Some(info) => {
                // This is a valid path response, but this validates a 4-tuple we no longer
                // have in use. Keep only sent challenges for the current path.
                self.unconfirmed_challenges
                    .retain(|_token, i| i.network_path == self.network_path);

                // If there are no challenges for the current path, schedule one
                if !self.unconfirmed_challenges.is_empty() {
                    self.pending_challenge = true;
                }
                OnPathResponseReceived::Ignored {
                    sent_on: info.network_path,
                    current_path: self.network_path,
                }
            }
            None => {
                // Response to an unknown PathChallenge. Does not indicate failure.
                OnPathResponseReceived::Unknown
            }
        }
    }

    /// Removes all on-path challenges we remember and cancels sending new on-path challenges.
    pub(super) fn reset_on_path_challenges(&mut self) {
        self.unconfirmed_challenges.clear();
        self.pending_challenge = false;
        self.lost_challenge_count = 0;
    }

    #[cfg(feature = "qlog")]
    pub(super) fn qlog_recovery_metrics(
        &mut self,
        path_id: PathId,
    ) -> Option<RecoveryMetricsUpdated> {
        let controller_metrics = self.congestion.metrics();

        let metrics = RecoveryMetrics {
            min_rtt: Some(self.rtt.min),
            smoothed_rtt: Some(self.rtt.get()),
            latest_rtt: Some(self.rtt.latest),
            rtt_variance: Some(self.rtt.var),
            pto_count: Some(self.pto_count),
            bytes_in_flight: Some(self.in_flight.bytes),
            packets_in_flight: Some(self.in_flight.ack_eliciting),

            congestion_window: Some(controller_metrics.congestion_window),
            ssthresh: controller_metrics.ssthresh,
            pacing_rate: controller_metrics.pacing_rate,
        };

        let event = metrics.to_qlog_event(path_id, &self.recovery_metrics);
        self.recovery_metrics = metrics;
        event
    }

    /// Return how long we need to wait before sending `bytes_to_send`
    ///
    /// See [`Pacer::delay`].
    pub(super) fn pacing_delay(&mut self, bytes_to_send: u64, now: Instant) -> Option<Duration> {
        let smoothed_rtt = self.rtt.get();
        let metrics = self.congestion.metrics();
        self.pacing.delay(
            smoothed_rtt,
            bytes_to_send,
            self.current_mtu(),
            now,
            &metrics,
        )
    }

    /// Updates the last observed address report received on this path.
    ///
    /// If the address was updated, it's returned to be informed to the application.
    #[must_use = "updated observed address must be reported to the application"]
    pub(super) fn update_observed_addr_report(
        &mut self,
        observed: ObservedAddr,
    ) -> Option<SocketAddr> {
        match self.last_observed_addr_report.as_mut() {
            Some(prev) => {
                if prev.seq_no >= observed.seq_no {
                    // frames that do not increase the sequence number on this path are ignored
                    None
                } else if prev.ip == observed.ip && prev.port == observed.port {
                    // keep track of the last seq_no but do not report the address as updated
                    prev.seq_no = observed.seq_no;
                    None
                } else {
                    let addr = observed.socket_addr();
                    self.last_observed_addr_report = Some(observed);
                    Some(addr)
                }
            }
            None => {
                let addr = observed.socket_addr();
                self.last_observed_addr_report = Some(observed);
                Some(addr)
            }
        }
    }

    /// Tag uniquely identifying a path in a connection.
    ///
    /// When a migration happens on the same [`PathId`] we still detect a change in the
    /// 4-tuple and generate a new [`PathData`] for it. Each such generation has a unique
    /// value to keep track of which 4-tuple a packet belonged to.
    pub(super) fn generation(&self) -> u64 {
        self.generation
    }
}

pub(super) enum OnPathResponseReceived {
    /// This response validates the path on its current remote address.
    OnPath,
    /// The received token is unknown.
    Unknown,
    /// The response is valid but it's not usable for path validation.
    Ignored {
        sent_on: FourTuple,
        current_path: FourTuple,
    },
}

/// Congestion metrics as described in [`recovery_metrics_updated`].
///
/// [`recovery_metrics_updated`]: https://datatracker.ietf.org/doc/html/draft-ietf-quic-qlog-quic-events.html#name-recovery_metrics_updated
#[cfg(feature = "qlog")]
#[derive(Default, Clone, PartialEq, Debug)]
#[non_exhaustive]
struct RecoveryMetrics {
    pub min_rtt: Option<Duration>,
    pub smoothed_rtt: Option<Duration>,
    pub latest_rtt: Option<Duration>,
    pub rtt_variance: Option<Duration>,
    pub pto_count: Option<u32>,
    pub bytes_in_flight: Option<u64>,
    pub packets_in_flight: Option<u64>,
    pub congestion_window: Option<u64>,
    pub ssthresh: Option<u64>,
    pub pacing_rate: Option<u64>,
}

#[cfg(feature = "qlog")]
impl RecoveryMetrics {
    /// Retain only values that have been updated since the last snapshot.
    fn retain_updated(&self, previous: &Self) -> Self {
        macro_rules! keep_if_changed {
            ($name:ident) => {
                if previous.$name == self.$name {
                    None
                } else {
                    self.$name
                }
            };
        }

        Self {
            min_rtt: keep_if_changed!(min_rtt),
            smoothed_rtt: keep_if_changed!(smoothed_rtt),
            latest_rtt: keep_if_changed!(latest_rtt),
            rtt_variance: keep_if_changed!(rtt_variance),
            pto_count: keep_if_changed!(pto_count),
            bytes_in_flight: keep_if_changed!(bytes_in_flight),
            packets_in_flight: keep_if_changed!(packets_in_flight),
            congestion_window: keep_if_changed!(congestion_window),
            ssthresh: keep_if_changed!(ssthresh),
            pacing_rate: keep_if_changed!(pacing_rate),
        }
    }

    /// Emit a `MetricsUpdated` event containing only updated values
    fn to_qlog_event(&self, path_id: PathId, previous: &Self) -> Option<RecoveryMetricsUpdated> {
        let updated = self.retain_updated(previous);

        if updated == Self::default() {
            return None;
        }

        Some(RecoveryMetricsUpdated {
            min_rtt: updated.min_rtt.map(|rtt| rtt.as_micros() as f32 / 1000.0),
            smoothed_rtt: updated
                .smoothed_rtt
                .map(|rtt| rtt.as_micros() as f32 / 1000.0),
            latest_rtt: updated
                .latest_rtt
                .map(|rtt| rtt.as_micros() as f32 / 1000.0),
            rtt_variance: updated
                .rtt_variance
                .map(|rtt| rtt.as_micros() as f32 / 1000.0),
            pto_count: updated
                .pto_count
                .map(|count| count.try_into().unwrap_or(u16::MAX)),
            bytes_in_flight: updated.bytes_in_flight,
            packets_in_flight: updated.packets_in_flight,
            congestion_window: updated.congestion_window,
            ssthresh: updated.ssthresh,
            pacing_rate: updated.pacing_rate,
            path_id: Some(path_id.as_u32() as u64),
            ex_data: Default::default(),
        })
    }
}

/// RTT estimation for a particular network path
#[derive(Copy, Clone, Debug)]
pub struct RttEstimator {
    /// The most recent RTT measurement made when receiving an ack for a previously unacked packet
    latest: Duration,
    /// The smoothed RTT of the connection, computed as described in RFC6298
    smoothed: Option<Duration>,
    /// The RTT variance, computed as described in RFC6298
    var: Duration,
    /// The minimum RTT seen in the connection, ignoring ack delay.
    min: Duration,
}

impl RttEstimator {
    pub(crate) fn new(initial_rtt: Duration) -> Self {
        Self {
            latest: initial_rtt,
            smoothed: None,
            var: initial_rtt / 2,
            min: initial_rtt,
        }
    }

    /// Resets the estimator using a new initial_rtt value.
    ///
    /// This only resets the initial_rtt **if** no samples have been recorded yet. If there
    /// are any recorded samples the initial estimate can not be adjusted after the fact.
    ///
    /// This is useful when you receive a PATH_RESPONSE in the first packet received on a
    /// new path. In this case you can use the delay of the PATH_CHALLENGE-PATH_RESPONSE as
    /// the initial RTT to get a better expected estimation.
    ///
    /// A PATH_CHALLENGE-PATH_RESPONSE pair later in the connection should not be used
    /// explicitly as an estimation since PATH_CHALLENGE is an ACK-eliciting packet itself
    /// already.
    pub(crate) fn reset_initial_rtt(&mut self, initial_rtt: Duration) {
        if self.smoothed.is_none() {
            self.latest = initial_rtt;
            self.var = initial_rtt / 2;
            self.min = initial_rtt;
        }
    }

    /// The current best RTT estimation.
    pub fn get(&self) -> Duration {
        self.smoothed.unwrap_or(self.latest)
    }

    /// Conservative estimate of RTT
    ///
    /// Takes the maximum of smoothed and latest RTT, as recommended
    /// in 6.1.2 of the recovery spec (draft 29).
    pub fn conservative(&self) -> Duration {
        self.get().max(self.latest)
    }

    /// Minimum RTT registered so far for this estimator.
    pub fn min(&self) -> Duration {
        self.min
    }

    /// PTO computed as described in RFC9002#6.2.1.
    pub(crate) fn pto_base(&self) -> Duration {
        self.get() + cmp::max(4 * self.var, TIMER_GRANULARITY)
    }

    /// Records an RTT sample.
    pub(crate) fn update(&mut self, ack_delay: Duration, rtt: Duration) {
        self.latest = rtt;
        // https://www.rfc-editor.org/rfc/rfc9002.html#section-5.2-3:
        // min_rtt does not adjust for ack_delay to avoid underestimating.
        self.min = cmp::min(self.min, self.latest);
        // Based on RFC6298.
        if let Some(smoothed) = self.smoothed {
            let adjusted_rtt = if self.min + ack_delay <= self.latest {
                self.latest - ack_delay
            } else {
                self.latest
            };
            let var_sample = smoothed.abs_diff(adjusted_rtt);
            self.var = (3 * self.var + var_sample) / 4;
            self.smoothed = Some((7 * smoothed + adjusted_rtt) / 8);
        } else {
            self.smoothed = Some(self.latest);
            self.var = self.latest / 2;
            self.min = self.latest;
        }
    }
}

#[derive(Default, Debug)]
pub(crate) struct PathResponses {
    pending: Vec<PathResponse>,
}

impl PathResponses {
    pub(crate) fn push(&mut self, packet: u64, token: u64, network_path: FourTuple) {
        /// An arbitrary permissive limit to prevent abuse.
        ///
        /// If we've negotiated the n0 NAT Traversal extension, and one user might have a lot
        /// of addresses, e.g. because of having lots of interfaces (we've seen >25 interfaces
        /// on Macs with docker and other things), then we need to be able to process at least
        /// as many PATH_CHALLENGE frames as there are interfaces.
        /// On top of that, there are retries, which make it possible that we need to process
        /// even more.
        ///
        /// Considering that there can be up to 2 `PathData`s per active `PathId`, and
        /// reasonable default values for maximum concurrent multipath paths are ~8 and each
        /// `PathResponse` struct takes up 72 bytes at the moment this, means an attacker can
        /// cause us to keep `32 * 2 * 8 * 72 = ~37KB` of data around.
        const MAX_PATH_RESPONSES: usize = 32;
        let response = PathResponse {
            packet,
            token,
            network_path,
        };
        let existing = self
            .pending
            .iter_mut()
            .find(|x| x.network_path.remote == network_path.remote);
        if let Some(existing) = existing {
            // Update a queued response
            if existing.packet <= packet {
                *existing = response;
            }
            return;
        }
        if self.pending.len() < MAX_PATH_RESPONSES {
            self.pending.push(response);
        } else {
            // We don't expect to ever hit this with well-behaved peers, so we don't bother dropping
            // older challenges.
            trace!("ignoring excessive PATH_CHALLENGE");
        }
    }

    pub(crate) fn pop_off_path(&mut self, network_path: FourTuple) -> Option<(u64, FourTuple)> {
        let response = *self.pending.last()?;
        // We use an exact comparison here, because once we've received for the first time,
        // we really should either already have a local_ip, or we will never get one
        // (because our OS doesn't support it). And even if we get it wrong we are only
        // slightly less efficient and would not include other on-path data in the packet.
        if response.network_path == network_path {
            // We don't bother searching further because we expect that the on-path response will
            // get drained in the immediate future by a call to `pop_on_path`
            return None;
        }
        self.pending.pop();
        Some((response.token, response.network_path))
    }

    pub(crate) fn pop_on_path(&mut self, network_path: FourTuple) -> Option<u64> {
        let response = *self.pending.last()?;
        // Using an exact comparison. See explanation in `pop_off_path`.
        if response.network_path != network_path {
            // We don't bother searching further because we expect that the off-path response will
            // get drained in the immediate future by a call to `pop_off_path`
            return None;
        }
        self.pending.pop();
        Some(response.token)
    }

    /// Whether the next [`Self::pop_on_path`] will return something to send.
    pub(crate) fn has_pending_on_path(&self, network_path: FourTuple) -> bool {
        self.pending
            .last()
            .is_some_and(|response| response.network_path == network_path)
    }

    pub(crate) fn is_empty(&self) -> bool {
        self.pending.is_empty()
    }
}

#[derive(Copy, Clone, Debug)]
struct PathResponse {
    /// The packet number the corresponding PATH_CHALLENGE was received in
    packet: u64,
    /// The token of the PATH_CHALLENGE
    token: u64,
    /// The path the corresponding PATH_CHALLENGE was received from
    network_path: FourTuple,
}

/// Summary statistics of packets that have been sent on a particular path, but which have not yet
/// been acked or deemed lost
#[derive(Debug)]
pub(super) struct InFlight {
    /// Sum of the sizes of all sent packets considered "in flight" by congestion control
    ///
    /// The size does not include IP or UDP overhead. Packets only containing ACK frames do not
    /// count towards this to ensure congestion control does not impede congestion feedback.
    pub(super) bytes: u64,
    /// Number of packets in flight containing frames other than ACK and PADDING
    ///
    /// This can be 0 even when bytes is not 0 because PADDING frames cause a packet to be
    /// considered "in flight" by congestion control. However, if this is nonzero, bytes will
    /// always also be nonzero.
    pub(super) ack_eliciting: u64,
}

impl InFlight {
    fn new() -> Self {
        Self {
            bytes: 0,
            ack_eliciting: 0,
        }
    }

    fn insert(&mut self, packet: &SentPacket) {
        self.bytes += u64::from(packet.size);
        self.ack_eliciting += u64::from(packet.ack_eliciting);
    }

    /// Update counters to account for a packet becoming acknowledged, lost, or abandoned
    fn remove(&mut self, packet: &SentPacket) {
        self.bytes -= u64::from(packet.size);
        self.ack_eliciting -= u64::from(packet.ack_eliciting);
    }
}

/// Application events about paths
#[derive(Debug, Clone, PartialEq, Eq)]
#[non_exhaustive]
pub enum PathEvent {
    /// A new path has established connection with the peer.
    #[non_exhaustive]
    Established {
        /// The path which can now be used for application data.
        id: PathId,
    },
    /// A path was abandoned and is no longer usable.
    ///
    /// Note that this may be the first event for a path: If a path is abandoned
    /// before having been established, no [`Self::Established`] event is emitted.
    ///
    /// This event will always be followed by [`Self::Discarded`] after some time.
    #[non_exhaustive]
    Abandoned {
        /// The path that was abandoned.
        id: PathId,
        /// Reason why this path was abandoned.
        reason: PathAbandonReason,
    },
    /// A path was discarded and all remaining state for it has been removed.
    ///
    /// This event is the last event for a path, and is always emitted after [`Self::Abandoned`].
    #[non_exhaustive]
    Discarded {
        /// Which path had its state dropped
        id: PathId,
        /// The final path stats, they are no longer available via [`Connection::stats`]
        ///
        /// [`Connection::stats`]: super::Connection::stats
        path_stats: Box<PathStats>,
    },
    /// The remote changed the status of the path
    ///
    /// The local status is not changed because of this event. It is up to the application
    /// to update the local status, which is used for packet scheduling, when the remote
    /// changes the status.
    #[non_exhaustive]
    RemoteStatus {
        /// Path which has changed status
        id: PathId,
        /// The new status set by the remote
        status: PathStatus,
    },
    /// Received an observation of our external address from the peer.
    #[non_exhaustive]
    ObservedAddr {
        /// Path over which the observed address was reported, [`PathId::ZERO`] when multipath is
        /// not negotiated
        id: PathId,
        /// The address observed by the remote over this path
        addr: SocketAddr,
    },
}

/// Reason for why a path was abandoned.
#[derive(Debug, Clone, Eq, PartialEq)]
pub enum PathAbandonReason {
    /// The path was closed locally by the application.
    ApplicationClosed {
        /// The error code to be sent with the abandon frame.
        error_code: VarInt,
    },
    /// We didn't receive a path response in time after opening this path.
    ///
    /// This event is no longer emitted, when validation fails a path is only abandoned once
    /// there's a path timeout and the [`Self::TimedOut`] event will be emitted instead.
    #[deprecated(
        since = "1.1.0",
        note = "This event is no longer emitted, TimedOut will be emitted instead"
    )]
    ValidationFailed,
    /// We didn't receive any data from the remote within the path's idle timeout.
    TimedOut,
    /// The path became unusable after a local network change.
    UnusableAfterNetworkChange,
    /// The remote closed the path.
    RemoteAbandoned {
        /// The error that was sent with the abandon frame.
        error_code: VarInt,
    },
}

impl PathAbandonReason {
    /// Whether this abandon was initiated by the remote peer.
    pub(crate) fn is_remote(&self) -> bool {
        matches!(self, Self::RemoteAbandoned { .. })
    }

    /// Returns the error code to send with a PATH_ABANDON frame.
    pub(crate) fn error_code(&self) -> TransportErrorCode {
        match self {
            Self::ApplicationClosed { error_code } => (*error_code).into(),
            #[allow(deprecated)]
            Self::ValidationFailed | Self::TimedOut | Self::UnusableAfterNetworkChange => {
                TransportErrorCode::PATH_UNSTABLE_OR_POOR
            }
            Self::RemoteAbandoned { error_code } => (*error_code).into(),
        }
    }
}

/// Error from setting path status
#[derive(Debug, Error, Clone, PartialEq, Eq)]
pub enum SetPathStatusError {
    /// Error indicating that a path has not been opened or has already been abandoned
    #[error("closed path")]
    ClosedPath,
    /// Error indicating that this operation requires multipath to be negotiated whereas it hasn't
    /// been
    #[error("multipath not negotiated")]
    MultipathNotNegotiated,
}

/// Error indicating that a path has not been opened or has already been abandoned
#[derive(Debug, Default, Error, Clone, PartialEq, Eq)]
#[error("closed path")]
pub struct ClosedPath {
    pub(super) _private: (),
}

/// Retransmittable data specific to a [`PathData::generation`].
#[derive(Debug, Default, Clone)]
pub(super) struct PathRetransmits {
    /// Whether this path needs to report its remote address back to the peer.
    ///
    /// This only happens if both peers agree to do so based on their transport parameters.
    pub(super) observed_address: bool,
}

impl PathRetransmits {
    pub(super) fn is_empty(&self) -> bool {
        let Self { observed_address } = self;
        !observed_address
    }
}

impl std::ops::BitOrAssign for PathRetransmits {
    fn bitor_assign(&mut self, rhs: Self) {
        let Self { observed_address } = rhs;
        self.observed_address |= observed_address;
    }
}

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

    #[test]
    fn test_path_id_saturating_add() {
        // add within range behaves normally
        let large: PathId = u16::MAX.into();
        let next = u32::from(u16::MAX) + 1;
        assert_eq!(large.saturating_add(1u8), PathId::from(next));

        // outside range saturates
        assert_eq!(PathId::MAX.saturating_add(1u8), PathId::MAX)
    }
}