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//! The intervals the protocol runs on, derived from the keepalive.
use std::time::Duration;
use crate::backoff::Backoff;
/// First wait for a sync answer; generous, since a transport may take a while with large
/// messages (Mycelium sends them in many small packets).
const SYNC_REPLY_TIMEOUT: Duration = Duration::from_secs(10);
/// Upper bound, as a multiple of the first wait: 5 minutes, about as long as a transport
/// tries to deliver a message (`Transport::delivery_limit`).
const SYNC_REPLY_TIMEOUT_MAX_FACTOR: u32 = 30;
/// Upper bound on the wait between probes of an unreachable peer, as a multiple of
/// `dormant_probe`: an hour at the default keepalive (1m). A returning node HELLOs its own
/// neighbours, so this only bounds how long one we never hear from stays out.
const PROBE_MAX_FACTOR: u32 = 6;
#[derive(Clone, Copy, Debug)]
pub(crate) struct Timing {
pub(crate) keepalive: Duration,
pub(crate) dormant_after: Duration,
pub(crate) resync_after: Duration,
pub(crate) dormant_probe: Duration,
/// While catching up: send the source a STATUS after this much silence (`swarm::source`).
pub(crate) catch_up_probe: Duration,
/// First wait for an answer to a sync message before sending again; doubles while
/// answers stay out (`SyncSession::reply_backoff`).
pub(crate) sync_reply_timeout: Duration,
/// How long the transport tries to deliver a message before dropping it
/// (`Transport::delivery_limit`): a peer silent for longer has nothing on the way to
/// us any more.
pub(crate) delivery_limit: Duration,
}
impl Timing {
/// How long a round-trip sample stays meaningful (`swarm::rtt`): a few keepalive
/// intervals, since nodes move.
pub(crate) fn rtt_fresh_for(&self) -> Duration {
self.keepalive * 5
}
/// The timings for `--keepalive`.
///
/// Each node keeps about `k` neighbours (`--neighbours`) whatever the swarm's size,
/// and only one side of a pair sends them, so idle, a node sends about `k` messages and
/// receives about `k` (STATUSes and STATUS_REPLYs) per keepalive interval (more on a stable
/// member others prefer). The interval trades that against how soon a silent peer is
/// noticed (`dormant_after`, 4 intervals) and a gap closes (within an interval).
/// Recommended:
///
/// | Environment | Keepalive | Notes |
/// |---|---|---|
/// | Tests | 250 ms – 2 s | dormancy within seconds keeps tests short |
/// | LAN, servers | 15–30 s | a returning or silent peer is noticed within a minute or two |
/// | WAN, mobile, metered (the default, 1m) | 60–120 s | about 0.4 messages per second per node at `k` = 12 and 1m |
///
/// `delivery_limit`: the transport's (`Transport::delivery_limit`).
pub(crate) fn from_keepalive(keepalive: Duration, delivery_limit: Duration) -> Self {
Self::new(keepalive, SYNC_REPLY_TIMEOUT, delivery_limit)
}
/// [`from_keepalive`](Self::from_keepalive) with another first sync reply timeout:
/// for tests, whose messages don't travel for seconds.
pub(crate) fn new(
keepalive: Duration,
sync_reply_timeout: Duration,
delivery_limit: Duration,
) -> Self {
Self {
keepalive,
dormant_after: keepalive * 4, // dormant_after >= 4* keepalive to tolerate three lost STATUSes in a row
// >= 2 keepalives, so a second exchange confirms the difference; and >= twice
// the first sync reply timeout, so a lost message's sender resends it first.
resync_after: (keepalive * 2).max(sync_reply_timeout * 2),
dormant_probe: keepalive * 10, // >= 10*: dormant peers cost almost nothing, but still rejoin within minutes
// Seconds, not a keepalive interval: a dead source should cost little time.
// Only the source gets these, and only while catching up.
catch_up_probe: (keepalive / 30).clamp(Duration::from_secs(1), Duration::from_secs(5)),
sync_reply_timeout,
delivery_limit,
}
}
/// Waits between probes of an unreachable peer: `dormant_probe`, doubling up to
/// `PROBE_MAX_FACTOR` times as much.
pub(crate) fn probe_backoff(&self) -> Backoff {
Backoff::new(self.dormant_probe, self.dormant_probe * PROBE_MAX_FACTOR)
}
/// Waits for sync answers: `sync_reply_timeout`, doubling up to 30 times as much.
pub(crate) fn sync_reply_backoff(&self) -> Backoff {
Backoff::new(
self.sync_reply_timeout,
self.sync_reply_timeout * SYNC_REPLY_TIMEOUT_MAX_FACTOR,
)
}
}
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