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//! Implementation of the Tor Vegas congestion control algorithm.
//!
//! This is used by the circuit reactor in order to decide when to send data and SENDMEs.
//! Spec: prop324 section 3.3 (TOR_VEGAS)
use super::{
params::VegasParams, rtt::RoundtripTimeEstimator, CongestionControlAlgorithm,
CongestionSignals, CongestionWindow, State,
};
use crate::Result;
use tor_error::{error_report, internal};
/// Bandwidth-Delay Product (BDP) estimator.
///
/// Spec: prop324 section 3.1 (BDP_ESTIMATION).
#[derive(Clone, Debug, Default)]
pub(crate) struct BdpEstimator {
/// The BDP value of this estimator.
bdp: u32,
}
impl BdpEstimator {
/// Return the current BDP value.
fn get(&self) -> u32 {
self.bdp
/// Update the estimator with the given congestion window, RTT estimator and any condition
/// signals that we are currently experiencing.
/// C-tor: congestion_control_update_circuit_bdp() in congestion_control_common.c
fn update(
&mut self,
cwnd: &CongestionWindow,
rtt: &RoundtripTimeEstimator,
signals: &CongestionSignals,
) {
// Stalled clock means our RTT value is invalid so set the BDP to the cwnd.
if rtt.clock_stalled() {
self.bdp = if signals.channel_blocked {
// Set the BDP to the cwnd minus the outbound queue size, capping it to the minimum
// cwnd.
cwnd.get()
.saturating_sub(signals.channel_outbound_size)
.max(cwnd.min())
} else {
// Congestion window based BDP will respond to changes in RTT only, and is relative to
// cwnd growth. It is useful for correcting for BDP overestimation, but if BDP is
// higher than the current cwnd, it will underestimate it.
//
// To clarify this is equivalent to: cwnd * min_rtt / ewma_rtt.
self.bdp = cwnd
.get()
.saturating_mul(rtt.min_rtt_usec())
.saturating_div(rtt.ewma_rtt_usec());
/// Congestion control Vegas algorithm.
/// TCP Vegas control algorithm estimates the queue lengths at relays by subtracting the current
/// BDP estimate from the current congestion window.
/// This object implements CongestionControlAlgorithm trait used by the ['CongestionControl'].
/// Spec: prop324 section 3.3 (TOR_VEGAS)
/// C-tor: Split between congestion_control_vegas.c and the congestion_control_t struct.
#[derive(Clone, Debug)]
pub(crate) struct Vegas {
/// Congestion control parameters.
params: VegasParams,
/// Bandwidth delay product.
/// C-tor: "bdp"
bdp: BdpEstimator,
/// Congestion window.
/// C-tor: "cwnd", "cwnd_inc_pct_ss", "cwnd_inc", "cwnd_min", "cwnd_inc_rate", "cwnd_full",
cwnd: CongestionWindow,
/// Number of cells expected before we send a SENDME resulting in more data.
num_cell_until_sendme: u32,
/// The number of SENDME until we will acknowledge a congestion event again.
/// C-tor: "next_cc_event"
num_sendme_until_cwnd_update: u32,
/// Counts down until we process a cwnd worth of SENDME acks. Used to track full cwnd status.
/// C-tor: "next_cwnd_event"
num_sendme_per_cwnd: u32,
/// Number of cells in-flight (sent but awaiting SENDME ack).
/// C-tor: "inflight"
num_inflight: u32,
/// Indicate if we noticed we were blocked on channel during an algorithm run. This is used to
/// notice a change from blocked to non-blocked in order to reset the num_sendme_per_cwnd.
/// C-tor: "blocked_chan"
is_blocked_on_chan: bool,
impl Vegas {
/// Create a new [`Vegas`] from the specified parameters, state, and cwnd.
pub(crate) fn new(params: &VegasParams, state: &State, cwnd: CongestionWindow) -> Self {
Self {
params: params.clone(),
bdp: BdpEstimator::default(),
num_cell_until_sendme: cwnd.sendme_inc(),
num_inflight: 0,
num_sendme_per_cwnd: 0,
num_sendme_until_cwnd_update: cwnd.update_rate(state),
cwnd,
is_blocked_on_chan: false,
impl CongestionControlAlgorithm for Vegas {
fn uses_stream_sendme(&self) -> bool {
// Not allowed as in Vegas doesn't need them.
false
fn is_next_cell_sendme(&self) -> bool {
// Matching inflight number to the SENDME increment, time to send a SENDME. Contrary to
// C-tor, this is called after num_inflight is incremented.
self.num_inflight % self.cwnd.sendme_inc() == 0
fn can_send(&self) -> bool {
self.num_inflight < self.cwnd.get()
fn cwnd(&self) -> Option<&CongestionWindow> {
Some(&self.cwnd)
/// Called when a SENDME cell is received.
/// This is where the Vegas algorithm magic happens entirely. For every SENDME we get, the
/// entire state is updated which usually result in the congestion window being changed.
/// An error is returned if there is a protocol violation with regards to flow or congestion
/// control.
/// C-tor: congestion_control_vegas_process_sendme() in congestion_control_vegas.c
fn sendme_received(
state: &mut State,
rtt: &mut RoundtripTimeEstimator,
signals: CongestionSignals,
) -> Result<()> {
// Update the countdown until we need to update the congestion window.
self.num_sendme_until_cwnd_update = self.num_sendme_until_cwnd_update.saturating_sub(1);
// We just got a SENDME so decrement the amount of expected SENDMEs for a cwnd.
self.num_sendme_per_cwnd = self.num_sendme_per_cwnd.saturating_sub(1);
// From here, C-tor proceeds to update the RTT and BDP (circuit estimates). The RTT is
// updated before this is called and so the "rtt" object is up to date with the latest. As
// for the BDP, we update it now. See C-tor congestion_control_update_circuit_estimates().
// Update the BDP estimator even if the RTT estimator is not ready. If that is the case,
// we'll estimate a BDP value to bootstrap.
self.bdp.update(&self.cwnd, rtt, &signals);
// Evaluate if we changed state on the blocked chan. This is done in the BDP update function
// in C-tor. Instead, we do it now after the update of the BDP value.
if rtt.is_ready() {
if signals.channel_blocked {
// Going from non blocked to block, it is an immediate congestion signal so reset the
// number of sendme per cwnd because we are about to reevaluate it.
if !self.is_blocked_on_chan {
self.num_sendme_until_cwnd_update = 0;
// Going from blocked to non block, need to reevaluate the cwnd and so reset num
// sendme.
if self.is_blocked_on_chan {
self.is_blocked_on_chan = signals.channel_blocked;
// Only run the algorithm if the RTT estimator is ready or we have a blocked channel.
if !rtt.is_ready() && !self.is_blocked_on_chan {
// The inflight value can never be below a sendme_inc because every time a cell is sent,
// inflight is incremented and we only end up decrementing if we receive a valid
// authenticated SENDME which is always after the sendme_inc value that we get that.
debug_assert!(self.num_inflight >= self.cwnd.sendme_inc());
self.num_inflight = self.num_inflight.saturating_sub(self.cwnd.sendme_inc());
return Ok(());
// The queue use is the amount in which our cwnd is above BDP;
// if it is below, then 0 queue use.
let queue_use = self.cwnd.get().saturating_sub(self.bdp.get());
// Evaluate if the congestion window has became full or not.
self.cwnd.eval_fullness(
self.num_inflight,
self.params.cwnd_full_gap(),
self.params.cwnd_full_min_pct().as_percent(),
);
// Spec: See the pseudocode of TOR_VEGAS with RFC3742
if state.in_slow_start() {
if queue_use < self.params.cell_in_queue_params().gamma() && !self.is_blocked_on_chan {
// If the congestion window is not fully in use, skip any increment in slow start.
if self.cwnd.is_full() {
// This is the "Limited Slow Start" increment.
let inc = self
.cwnd
.rfc3742_ss_inc(self.params.cell_in_queue_params().ss_cwnd_cap());
// Check if inc is less than what we would do in steady-state avoidance. Note
// that this is likely never to happen in practice. If so, exit slow start.
if (inc * self.cwnd.sendme_per_cwnd())
<= (self.cwnd.increment() * self.cwnd.increment_rate())
{
*state = State::Steady;
// Congestion signal: Set cwnd to gamma threshold
self.cwnd
.set(self.bdp.get() + self.params.cell_in_queue_params().gamma());
// Exit slow start due to congestion signal.
// Max the window and exit slow start.
if self.cwnd.get() >= self.params.ss_cwnd_max() {
self.cwnd.set(self.params.ss_cwnd_max());
} else if self.num_sendme_until_cwnd_update == 0 {
// Once in steady state, we only update once per window.
if queue_use > self.params.cell_in_queue_params().delta() {
// Above delta threshold, drop cwnd down to the delta.
self.cwnd.set(
self.bdp.get() + self.params.cell_in_queue_params().delta()
- self.cwnd.increment(),
} else if queue_use > self.params.cell_in_queue_params().beta()
|| self.is_blocked_on_chan
// Congestion signal: Above beta or if channel is blocked, decrement window.
self.cwnd.dec();
} else if self.cwnd.is_full() && queue_use < self.params.cell_in_queue_params().alpha()
// Congestion window is full and the queue usage is below alpha, increment.
self.cwnd.inc();
// Reset our counters if they reached their bottom.
if self.num_sendme_until_cwnd_update == 0 {
self.num_sendme_until_cwnd_update = self.cwnd.update_rate(state);
if self.num_sendme_per_cwnd == 0 {
self.num_sendme_per_cwnd = self.cwnd.sendme_per_cwnd();
// Decide if enough time has passed to reset the cwnd.
if self.params.cwnd_full_per_cwnd() != 0 {
if self.num_sendme_per_cwnd == self.cwnd.sendme_per_cwnd() {
self.cwnd.reset_full();
} else if self.num_sendme_until_cwnd_update == self.cwnd.update_rate(state) {
// Finally, update the inflight now that we have a SENDME.
Ok(())
fn sendme_sent(&mut self) -> Result<()> {
// SENDME is on the wire, set our counter until next one.
self.num_cell_until_sendme = self.cwnd.sendme_inc();
fn data_received(&mut self) -> Result<bool> {
if self.num_cell_until_sendme == 0 {
// This is not a protocol violation, it is a code flow error and so don't close the
// circuit by sending back an Error. Catching this prevents from sending two SENDMEs
// back to back. We recover from this but scream very loudly.
error_report!(internal!("Congestion control unexptected data cell"), "");
return Ok(false);
// Decrement the expected window.
self.num_cell_until_sendme = self.num_cell_until_sendme.saturating_sub(1);
// Reaching zero, lets inform the caller a SENDME needs to be sent. This counter is reset
// when the SENDME is actually sent.
Ok(self.num_cell_until_sendme == 0)
fn data_sent(&mut self) -> Result<()> {
// This can be above cwnd because that cwnd can shrink while we are still sending data.
self.num_inflight = self.num_inflight.saturating_add(1);
#[cfg(feature = "conflux")]
fn inflight(&self) -> Option<u32> {
Some(self.num_inflight)
#[cfg(test)]
fn send_window(&self) -> u32 {
self.cwnd.get()
#[allow(clippy::print_stderr)]
pub(crate) mod test {
// @@ begin test lint list maintained by maint/add_warning @@
#![allow(clippy::bool_assert_comparison)]
#![allow(clippy::clone_on_copy)]
#![allow(clippy::dbg_macro)]
#![allow(clippy::mixed_attributes_style)]
#![allow(clippy::print_stderr)]
#![allow(clippy::print_stdout)]
#![allow(clippy::single_char_pattern)]
#![allow(clippy::unwrap_used)]
#![allow(clippy::unchecked_duration_subtraction)]
#![allow(clippy::useless_vec)]
#![allow(clippy::needless_pass_by_value)]
//! <!-- @@ end test lint list maintained by maint/add_warning @@ -->
use std::{
collections::VecDeque,
time::{Duration, Instant},
use tor_units::Percentage;
use super::*;
use crate::congestion::{
params::VegasParamsBuilder,
test_utils::{new_cwnd, new_rtt_estimator},
/// Set the number of inflight cell.
pub(crate) fn set_inflight(&mut self, v: u32) {
self.num_inflight = v;
/// Return the state of the blocked on chan flag.
fn is_blocked_on_chan(&self) -> bool {
self.is_blocked_on_chan
/// Set the state of the blocked on chan flag.
fn set_is_blocked_on_chan(&mut self, v: bool) {
self.is_blocked_on_chan = v;
/// The test vector parameters. They have the exact same name as in C-tor in order to help
/// matching them and avoid confusion.
#[derive(Debug)]
struct TestVectorParams {
// Inbound parameters.
sent_usec_in: u64,
got_sendme_usec_in: u64,
or_conn_blocked_in: bool,
inflight_in: u32,
// Expected outbound parameters.
ewma_rtt_usec_out: u32,
min_rtt_usec_out: u32,
cwnd_out: u32,
in_slow_start_out: bool,
cwnd_full_out: bool,
blocked_chan_out: bool,
impl From<[u32; 10]> for TestVectorParams {
fn from(arr: [u32; 10]) -> Self {
sent_usec_in: u64::from(arr[0]),
got_sendme_usec_in: u64::from(arr[1]),
or_conn_blocked_in: arr[2] == 1,
inflight_in: arr[3],
ewma_rtt_usec_out: arr[4],
min_rtt_usec_out: arr[5],
cwnd_out: arr[6],
in_slow_start_out: arr[7] == 1,
cwnd_full_out: arr[8] == 1,
blocked_chan_out: arr[9] == 1,
struct VegasTest {
params: VecDeque<TestVectorParams>,
rtt: RoundtripTimeEstimator,
state: State,
vegas: Vegas,
impl VegasTest {
fn new(vec: Vec<[u32; 10]>) -> Self {
let mut params = VecDeque::new();
for values in vec {
params.push_back(values.into());
let state = State::default();
params,
rtt: new_rtt_estimator(),
vegas: Vegas::new(&build_vegas_params(), &state, new_cwnd()),
state,
fn run_once(&mut self, p: &TestVectorParams) {
eprintln!("Testing vector: {:?}", p);
// Set the inflight and channel blocked value from the test vector.
self.vegas.set_inflight(p.inflight_in);
self.vegas.set_is_blocked_on_chan(p.or_conn_blocked_in);
let now = Instant::now();
self.rtt
.expect_sendme(now + Duration::from_micros(p.sent_usec_in));
let ret = self.rtt.update(
now + Duration::from_micros(p.got_sendme_usec_in),
&self.state,
self.vegas.cwnd().expect("No CWND"),
assert!(ret.is_ok());
let signals = CongestionSignals::new(p.or_conn_blocked_in, 0);
let ret = self
.vegas
.sendme_received(&mut self.state, &mut self.rtt, signals);
assert_eq!(self.rtt.ewma_rtt_usec(), p.ewma_rtt_usec_out);
assert_eq!(self.rtt.min_rtt_usec(), p.min_rtt_usec_out);
assert_eq!(self.vegas.cwnd().expect("No CWND").get(), p.cwnd_out);
assert_eq!(
self.vegas.cwnd().expect("No CWND").is_full(),
p.cwnd_full_out
assert_eq!(self.state.in_slow_start(), p.in_slow_start_out);
assert_eq!(self.vegas.is_blocked_on_chan(), p.blocked_chan_out);
fn run(&mut self) {
while let Some(param) = self.params.pop_front() {
self.run_once(¶m);
pub(crate) fn build_vegas_params() -> VegasParams {
const OUTBUF_CELLS: u32 = 62;
VegasParamsBuilder::default()
.cell_in_queue_params(
(
3 * OUTBUF_CELLS, // alpha
4 * OUTBUF_CELLS, // beta
5 * OUTBUF_CELLS, // delta
3 * OUTBUF_CELLS, // gamma
600, // ss_cap
)
.into(),
.ss_cwnd_max(5_000)
.cwnd_full_gap(4)
.cwnd_full_min_pct(Percentage::new(25))
.cwnd_full_per_cwnd(1)
.build()
.expect("Unable to build Vegas parameters")
#[test]
fn test_vectors() {
let vec1 = vec![
[100000, 200000, 0, 124, 100000, 100000, 155, 1, 0, 0],
[200000, 300000, 0, 155, 100000, 100000, 186, 1, 1, 0],
[350000, 500000, 0, 186, 133333, 100000, 217, 1, 1, 0],
[500000, 550000, 0, 217, 77777, 77777, 248, 1, 1, 0],
[600000, 700000, 0, 248, 92592, 77777, 279, 1, 1, 0],
[700000, 750000, 0, 279, 64197, 64197, 310, 1, 0, 0], // Fullness expiry
[750000, 875000, 0, 310, 104732, 64197, 341, 1, 1, 0],
[875000, 900000, 0, 341, 51577, 51577, 372, 1, 1, 0],
[900000, 950000, 0, 279, 50525, 50525, 403, 1, 1, 0],
[950000, 1000000, 0, 279, 50175, 50175, 434, 1, 1, 0],
[1000000, 1050000, 0, 279, 50058, 50058, 465, 1, 1, 0],
[1050000, 1100000, 0, 279, 50019, 50019, 496, 1, 1, 0],
[1100000, 1150000, 0, 279, 50006, 50006, 527, 1, 1, 0],
[1150000, 1200000, 0, 279, 50002, 50002, 558, 1, 1, 0],
[1200000, 1250000, 0, 550, 50000, 50000, 589, 1, 1, 0],
[1250000, 1300000, 0, 550, 50000, 50000, 620, 1, 0, 0], // Fullness expiry
[1300000, 1350000, 0, 550, 50000, 50000, 635, 1, 1, 0],
[1350000, 1400000, 0, 550, 50000, 50000, 650, 1, 1, 0],
[1400000, 1450000, 0, 150, 50000, 50000, 650, 1, 0, 0], // cwnd not full
[1450000, 1500000, 0, 150, 50000, 50000, 650, 1, 0, 0], // cwnd not full
[1500000, 1550000, 0, 550, 50000, 50000, 664, 1, 1, 0], // cwnd full
[1500000, 1600000, 0, 550, 83333, 50000, 584, 0, 1, 0], // gamma exit
[1600000, 1650000, 0, 550, 61111, 50000, 585, 0, 1, 0], // alpha
[1650000, 1700000, 0, 550, 53703, 50000, 586, 0, 1, 0],
[1700000, 1750000, 0, 100, 51234, 50000, 586, 0, 0, 0], // alpha, not full
[1750000, 1900000, 0, 100, 117078, 50000, 559, 0, 0, 0], // delta, not full
[1900000, 2000000, 0, 100, 105692, 50000, 558, 0, 0, 0], // beta, not full
[2000000, 2075000, 0, 500, 85230, 50000, 558, 0, 1, 0], // no change
[2075000, 2125000, 1, 500, 61743, 50000, 557, 0, 1, 1], // beta, blocked
[2125000, 2150000, 0, 500, 37247, 37247, 558, 0, 1, 0], // alpha
[2150000, 2350000, 0, 500, 145749, 37247, 451, 0, 1, 0], // delta
];
VegasTest::new(vec1).run();
let vec2 = vec![
[500000, 550000, 1, 217, 77777, 77777, 403, 0, 1, 1], // ss exit, blocked
[600000, 700000, 0, 248, 92592, 77777, 404, 0, 1, 0], // alpha
[700000, 750000, 1, 404, 64197, 64197, 403, 0, 0, 1], // blocked beta
[750000, 875000, 0, 403, 104732, 64197, 404, 0, 1, 0],
VegasTest::new(vec2).run();
let vec3 = vec![
[18258527, 19002938, 0, 83, 744411, 744411, 155, 1, 0, 0],
[18258580, 19254257, 0, 52, 911921, 744411, 186, 1, 1, 0],
[20003224, 20645298, 0, 164, 732023, 732023, 217, 1, 1, 0],
[20003367, 21021444, 0, 133, 922725, 732023, 248, 1, 1, 0],
[20003845, 21265508, 0, 102, 1148683, 732023, 279, 1, 1, 0],
[20003975, 21429157, 0, 71, 1333015, 732023, 310, 1, 0, 0],
[20004309, 21707677, 0, 40, 1579917, 732023, 310, 1, 0, 0],
VegasTest::new(vec3).run();
let vec4 = vec![
[358297091, 358854163, 0, 83, 557072, 557072, 155, 1, 0, 0],
[358297649, 359123845, 0, 52, 736488, 557072, 186, 1, 1, 0],
[359492879, 359995330, 0, 186, 580463, 557072, 217, 1, 1, 0],
[359493043, 360489243, 0, 217, 857621, 557072, 248, 1, 1, 0],
[359493232, 360489673, 0, 248, 950167, 557072, 279, 1, 1, 0],
[359493795, 360489971, 0, 279, 980839, 557072, 310, 1, 0, 0],
[359493918, 360490248, 0, 310, 991166, 557072, 341, 1, 1, 0],
[359494029, 360716465, 0, 341, 1145346, 557072, 372, 1, 1, 0],
[359996888, 360948867, 0, 372, 1016434, 557072, 403, 1, 1, 0],
[359996979, 360949330, 0, 403, 973712, 557072, 434, 1, 1, 0],
[360489528, 361113615, 0, 434, 740628, 557072, 465, 1, 1, 0],
[360489656, 361281604, 0, 465, 774841, 557072, 496, 1, 1, 0],
[360489837, 361500461, 0, 496, 932029, 557072, 482, 0, 1, 0],
[360489963, 361500631, 0, 482, 984455, 557072, 482, 0, 1, 0],
[360490117, 361842481, 0, 482, 1229727, 557072, 481, 0, 1, 0],
VegasTest::new(vec4).run();