Struct RoundtripTimeEstimator

pub(crate) struct RoundtripTimeEstimator {
    sendme_expected_from: VecDeque<Instant>,
    last_rtt: Duration,
    ewma_rtt: Duration,
    min_rtt: Duration,
    max_rtt: Duration,
    params: RoundTripEstimatorParams,
    clock_stalled: AtomicBool,
}

Provides an estimate of the round-trip time (RTT) of a Tor circuit.

Fields

sendme_expected_from: VecDeque<Instant>

A queue of times we sent a cell that we’d expect a SENDME for.

When a data cell is sent and for which we expect a SENDME next, the timestamp at the send is kept in this queue so we can use it to measure the RTT when the SENDME is received.

A queue is used here because the protocol allows to send all pending SENDMEs at once as long as it is within one congestion window.

last_rtt: Duration

The last measured round-trip time.

ewma_rtt: Duration

The current smoothed estimate of what the round-trip time is.

This is zero iff we have not managed to get any estimate yet.

min_rtt: Duration

The minimum observed value of last_rtt.

max_rtt: Duration

The maximum observed value of last_rtt.

params: RoundTripEstimatorParams

The network parameters we’re using.

clock_stalled: AtomicBool

A reference to a shared boolean for storing if the clock is stalled or not. Spec: CLOCK_HEURISTICS from prop324. See is_clock_stalled() for the implementation.

Implementations

impl RoundtripTimeEstimator

pub(crate) fn new(params: &RoundTripEstimatorParams) -> Self

Create a new RoundtripTimeEstimator, using a set of NetParameters and a shared boolean to cache clock stalled state in.

pub(crate) fn is_ready(&self) -> bool

Return true iff the estimator is ready to be used or read.

pub(crate) fn clock_stalled(&self) -> bool

Return the state of the clock stalled indicator.

pub(crate) fn ewma_rtt_usec(&self) -> u32

Return the EWMA RTT in usec or u32 MAX if we don’t have an estimate yet.

pub(crate) fn min_rtt_usec(&self) -> u32

Return the Minimum RTT in usec or u32 MAX value if we don’t have an estimate yet.

pub(crate) fn expect_sendme(&mut self, now: Instant)

Inform the estimator that we did (at time now) something that we’ll expect a SENDME to be received for.

fn can_crosscheck_with_current_estimate(&self, in_slow_start: bool) -> bool

Return whether we can use heuristics to sanity-check RTT values against our EWMA value. Spec: 2.1.1. Clock Jump Heuristics CLOCK_HEURISTICS

Used in is_clock_stalled, to check the sanity of a newly measured RTT value.

fn is_clock_stalled(&self, raw_rtt: Duration, in_slow_start: bool) -> bool

Given a raw RTT value we just observed, compute whether or not we think the clock has stalled or jumped, and we should throw it out as a result.

pub(crate) fn update( &mut self, now: Instant, state: &State, cwnd: &CongestionWindow, ) -> Result<(), Error>

Update the estimator on time now and at the congestion window cwnd.

Errors

Each call to this function removes an entry from sendme_expected_from (the entries are added using sendme_expected_from).

Returns an error if are not expecting any SENDMEs at this time (if expect_sendme was never called, or if we have exhausted all sendme_expected_from added by previous expect_sendme calls).

Spec: prop324 section 2.1 C-tor: congestion_control_update_circuit_rtt() in congestion_control_common.c

Trait Implementations

impl Debug for RoundtripTimeEstimator

fn fmt(&self, f: &mut Formatter<'_>) -> Result

Formats the value using the given formatter.