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|
//! Round-trip times to peers, measured from traffic we send anyway: PING → PONG and
//! HELLO → HELLO_REPLY. No probes of our own.
//!
//! A sample only counts when exactly one request was outstanding: with two, we can't tell
//! which one the answer belongs to (an answer to the older one would look too fast).
//! Samples are smoothed (exponential moving average, weight 1/4) to ride out jitter.
//!
//! What we use is the *effective* round-trip time: the smoothed value, raised to the age
//! of the oldest request still unanswered (the round trip takes at least that long), so
//! a peer that stops answering drifts away by itself. Without a recent sample (nodes
//! move; an old value says little) it is unknown. Silence alone doesn't count: we don't
//! ask non-neighbours anything, so their quiet is ours.
use std::collections::HashMap;
use std::time::Duration;
use tokio::time::Instant;
#[derive(Debug, Default)]
pub(crate) struct RttTable {
pings: HashMap<String, Outstanding>,
hellos: HashMap<String, Outstanding>,
smoothed: HashMap<String, Sample>,
}
#[derive(Debug, Clone, Copy)]
struct Sample {
rtt: Duration,
at: Instant,
}
#[derive(Debug, Clone, Copy)]
struct Outstanding {
sent: Instant,
count: u32,
}
impl RttTable {
pub(crate) fn ping_sent(&mut self, peer: &str, now: Instant) {
Self::sent(&mut self.pings, peer, now);
}
pub(crate) fn pong_received(&mut self, peer: &str, now: Instant) {
if let Some(sample) = Self::answered(&mut self.pings, peer, now) {
self.record(peer, sample, now);
}
}
pub(crate) fn hello_sent(&mut self, peer: &str, now: Instant) {
Self::sent(&mut self.hellos, peer, now);
}
pub(crate) fn hello_reply_received(&mut self, peer: &str, now: Instant) {
if let Some(sample) = Self::answered(&mut self.hellos, peer, now) {
self.record(peer, sample, now);
}
}
/// The effective round-trip time: the smoothed value if sampled within `fresh_for`,
/// raised to the age of the oldest unanswered request; None without a recent sample.
pub(crate) fn effective(
&self,
peer: &str,
now: Instant,
fresh_for: Duration,
) -> Option<Duration> {
let sample = self
.smoothed
.get(peer)
.filter(|sample| now.saturating_duration_since(sample.at) < fresh_for)?;
let waiting = [self.pings.get(peer), self.hellos.get(peer)]
.into_iter()
.flatten()
.map(|outstanding| now.saturating_duration_since(outstanding.sent))
.max()
.unwrap_or_default();
Some(sample.rtt.max(waiting))
}
/// The effective round-trip time of every peer that has one.
pub(crate) fn all_effective(
&self,
now: Instant,
fresh_for: Duration,
) -> impl Iterator<Item = (&str, Duration)> {
self.smoothed.keys().filter_map(move |peer| {
self.effective(peer, now, fresh_for)
.map(|rtt| (peer.as_str(), rtt))
})
}
fn sent(map: &mut HashMap<String, Outstanding>, peer: &str, now: Instant) {
map.entry(peer.to_owned())
.and_modify(|outstanding| outstanding.count += 1)
.or_insert(Outstanding {
sent: now,
count: 1,
});
}
fn answered(
map: &mut HashMap<String, Outstanding>,
peer: &str,
now: Instant,
) -> Option<Duration> {
let outstanding = map.remove(peer)?;
(outstanding.count == 1).then(|| now.saturating_duration_since(outstanding.sent))
}
fn record(&mut self, peer: &str, sample: Duration, now: Instant) {
let rtt = match self.smoothed.get(peer) {
Some(old) => (old.rtt * 3 + sample) / 4,
None => sample,
};
self.smoothed
.insert(peer.to_owned(), Sample { rtt, at: now });
}
}
#[cfg(test)]
mod tests {
use super::*;
const FRESH: Duration = Duration::from_secs(300);
#[test]
fn samples_only_unambiguous_answers_and_smooths() {
let mut table = RttTable::default();
let start = Instant::now();
let at = |ms| start + Duration::from_millis(ms);
table.ping_sent("a", start);
table.pong_received("a", at(100));
assert_eq!(
table.effective("a", at(100), FRESH),
Some(Duration::from_millis(100))
);
table.ping_sent("a", at(100));
table.pong_received("a", at(300));
assert_eq!(
table.effective("a", at(300), FRESH),
Some(Duration::from_millis(125))
);
// Two PINGs out: the PONG can't be attributed.
table.ping_sent("a", at(300));
table.ping_sent("a", at(400));
table.pong_received("a", at(450));
assert_eq!(
table.effective("a", at(450), FRESH),
Some(Duration::from_millis(125))
);
// An unsolicited PONG changes nothing.
table.pong_received("b", start);
assert_eq!(table.effective("b", start, FRESH), None);
}
#[test]
fn unanswered_requests_raise_it_and_old_samples_expire() {
let mut table = RttTable::default();
let start = Instant::now();
table.ping_sent("a", start);
table.pong_received("a", start + Duration::from_millis(50));
// A PING unanswered for 40 s: at least 40 s.
let sent = start + Duration::from_secs(60);
table.ping_sent("a", sent);
let later = sent + Duration::from_secs(40);
assert_eq!(
table.effective("a", later, FRESH),
Some(Duration::from_secs(40))
);
// No sample within the freshness window: unknown.
let stale = start + FRESH + Duration::from_secs(1);
assert_eq!(table.effective("a", stale, FRESH), None);
}
}
|
