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// See RFC 5348: "TCP Friendly Rate Control (TFRC): Protocol Specification"
// This is a constant-time overhead implementation of the loss rate computation, where only the
// most recent loss interval is updated each time a new frame is acked or nacked. Currently, no
// attempt is made to fill holes in the loss history as acks are received for previous nacks. In
// the future, an approximation to hole-filling will be performed which does not require iteration
// over the loss history.
use std::collections::VecDeque;
#[derive(Debug)]
struct LossInterval {
end_time_ms: u64,
length: u32,
}
#[derive(Debug)]
pub struct LossIntervalQueue {
entries: VecDeque<LossInterval>,
}
impl LossIntervalQueue {
// See section 5.4
const WEIGHTS: [f64; 8] = [ 1.0, 1.0, 1.0, 1.0, 0.8, 0.6, 0.4, 0.2 ];
pub fn new() -> Self {
Self {
entries: VecDeque::new()
}
}
pub fn reset(&mut self, initial_p: f64) {
// Because the loss interval queue is truncated here, the particular loss pattern of the
// first feedback message is ignored. That is, if frames were acked/nacked as follows:
//
// first nack
// v
// 111111111100100111000110111
// a) +------------------
// b) +------------------+--+----+----+---
//
// Only the single loss interval (a) will be generated, with a length initialized according
// to the throughput equation, and ignoring any subsequent initial loss events. Standard
// TFRC would not ignore those loss events, as shown in (b).
//
// By ignoring subsequent loss intervals, the throughput equation phase will begin at half
// of the maximum send rate regardless of the initial loss pattern. This effects a slightly
// improved response to initial packet drops, but any difference in behavior relative to
// standard TFRC should be marginal.
self.entries.truncate(1);
self.entries[0].length = (Self::WEIGHTS[0] / initial_p).clamp(0.0, u32::MAX as f64).round() as u32;
}
pub fn push_ack(&mut self) {
if let Some(last_interval) = self.entries.front_mut() {
// Acks always contribute to previous loss interval
last_interval.length = last_interval.length.saturating_add(1);
}
}
pub fn push_nack(&mut self, send_time_ms: u64, rtt_ms: u64) {
if let Some(last_interval) = self.entries.front_mut() {
if send_time_ms >= last_interval.end_time_ms {
// This nack marks a new loss interval
self.entries.push_front(LossInterval {
end_time_ms: send_time_ms + rtt_ms,
length: 1,
});
self.entries.truncate(9);
} else {
// This nack falls under previous loss interval
last_interval.length = last_interval.length.saturating_add(1);
}
} else {
// This nack marks a new loss interval
self.entries.push_front(LossInterval {
end_time_ms: send_time_ms + rtt_ms,
length: 1,
});
}
}
pub fn compute_loss_rate(&self) -> f64 {
// See section 5.4
if self.entries.len() > 0 {
let mut i_total_0 = 0.0;
let mut i_total_1 = 0.0;
let mut w_total = 0.0;
if self.entries.len() > 1 {
for i in 0..self.entries.len()-1 {
i_total_0 += self.entries[i].length as f64 * Self::WEIGHTS[i];
w_total += Self::WEIGHTS[i];
}
for i in 1..self.entries.len() {
i_total_1 += self.entries[i].length as f64 * Self::WEIGHTS[i - 1];
}
return w_total / i_total_0.max(i_total_1);
} else {
return Self::WEIGHTS[0] / (self.entries[0].length as f64 * Self::WEIGHTS[0]);
}
} else {
return 0.0;
}
}
}
#[cfg(test)]
mod tests {
use super::*;
/* Tests from previous incarnation. Because frames are now consumed greedily from the reorder
* buffer, some modification will be necessary.
*
#[test]
fn basic() {
let mut fbc = FeedbackComp::new(0);
fbc.log_frame(0, false, 53, 0);
fbc.log_frame(1, false, 71, 0);
fbc.log_frame(2, false, 89, 0);
fbc.log_frame(3, false, 107, 10);
fbc.acknowledge_frames(frame::FrameAck { base_id: 0, bitfield: 0b1111, nonce: false }, 100);
let feedback = fbc.pending_feedback().unwrap();
assert_eq!(feedback.last_send_time_ms, 10);
assert_eq!(feedback.total_ack_size, 320);
assert_eq!(feedback.loss_rate, 0.0);
assert_eq!(feedback.rate_limited, false);
assert_eq!(fbc.pending_feedback(), None);
}
#[test]
fn bad_nonce() {
let mut fbc = FeedbackComp::new(0);
fbc.log_frame(0, false, 53, 0);
fbc.log_frame(1, true, 71, 0);
fbc.log_frame(2, false, 89, 0);
fbc.log_frame(3, true, 107, 10);
fbc.acknowledge_frames(frame::FrameAck { base_id: 0, bitfield: 0b1111, nonce: true }, 100);
assert_eq!(fbc.pending_feedback(), None);
}
#[test]
fn rate_limited() {
let mut fbc = FeedbackComp::new(0);
fbc.log_frame(0, false, 53, 0);
fbc.log_rate_limited();
fbc.log_frame(1, false, 71, 0);
fbc.log_frame(2, false, 89, 0);
fbc.log_frame(3, false, 107, 10);
fbc.acknowledge_frames(frame::FrameAck { base_id: 0, bitfield: 0b1001, nonce: false }, 100);
let feedback = fbc.pending_feedback().unwrap();
assert_eq!(feedback.last_send_time_ms, 10);
assert_eq!(feedback.total_ack_size, 160);
assert_eq!(feedback.rate_limited, true);
}
#[test]
fn loss_intervals_reorder() {
let mut fbc = FeedbackComp::new(0);
fbc.log_frame( 0, false, 64, 0);
fbc.log_frame( 1, false, 64, 0);
fbc.log_frame( 2, false, 64, 0);
fbc.log_frame( 3, false, 64, 0);
fbc.log_frame( 4, false, 64, 0);
fbc.log_frame( 5, false, 64, 0);
fbc.log_frame( 6, false, 64, 0);
fbc.log_frame( 7, false, 64, 0);
fbc.log_frame( 8, false, 64, 0);
fbc.log_frame( 9, false, 64, 0);
fbc.acknowledge_frames(frame::FrameAck { base_id: 0, bitfield: 0b0000001001, nonce: false }, 100);
assert_eq!(fbc.loss_intervals.entries.iter().map(|entry| entry.length).collect::<Vec<_>>(), vec![ ]);
fbc.acknowledge_frames(frame::FrameAck { base_id: 0, bitfield: 0b0001001001, nonce: false }, 100);
assert_eq!(fbc.loss_intervals.entries.iter().map(|entry| entry.length).collect::<Vec<_>>(), vec![ ]);
fbc.acknowledge_frames(frame::FrameAck { base_id: 0, bitfield: 0b1001001001, nonce: false }, 100);
assert_eq!(fbc.loss_intervals.entries.iter().map(|entry| entry.length).collect::<Vec<_>>(), vec![ 3 ]);
}
#[test]
fn loss_intervals_rtt() {
let mut fbc = FeedbackComp::new(0);
fbc.log_frame( 0, false, 64, 0); // ack
fbc.log_frame( 1, false, 64, 0);
fbc.log_frame( 2, false, 64, 1);
fbc.log_frame( 3, false, 64, 99);
fbc.log_frame( 4, false, 64, 100);
fbc.log_frame( 5, false, 64, 200);
fbc.log_frame( 6, false, 64, 200);
fbc.log_frame( 7, false, 64, 200); // ack
fbc.log_frame( 8, false, 64, 200); // ack
fbc.log_frame( 9, false, 64, 200); // ack
fbc.acknowledge_frames(frame::FrameAck { base_id: 0, bitfield: 0b1110000001, nonce: false }, 100);
assert_eq!(fbc.loss_intervals.entries.iter().map(|entry| entry.length).collect::<Vec<_>>(), vec![ 3, 1, 3 ]);
}
#[test]
fn loss_intervals_reorder_forget_partial() {
let mut fbc = FeedbackComp::new(0);
fbc.log_frame( 0, false, 64, 0);
fbc.log_frame( 1, false, 64, 0);
fbc.log_frame( 2, false, 64, 100); // ack
fbc.log_frame( 3, false, 64, 100);
fbc.log_frame( 4, false, 64, 150);
fbc.log_frame( 5, false, 64, 150); // ack
fbc.log_frame( 6, false, 64, 199);
fbc.log_frame( 7, false, 64, 200);
fbc.log_frame( 8, false, 64, 300);
fbc.acknowledge_frames(frame::FrameAck { base_id: 0, bitfield: 0b000000100, nonce: false }, 100);
assert_eq!(fbc.loss_intervals.entries.iter().map(|entry| entry.length).collect::<Vec<_>>(), vec![ ]);
fbc.acknowledge_frames(frame::FrameAck { base_id: 0, bitfield: 0b000100000, nonce: false }, 100);
assert_eq!(fbc.loss_intervals.entries.iter().map(|entry| entry.length).collect::<Vec<_>>(), vec![ ]);
assert_eq!(fbc.next_ack_id, 0);
assert_eq!(fbc.frame_log.base_id(), 0);
assert_eq!(fbc.frame_log.next_id(), 9);
fbc.forget_frames(150, 100);
assert_eq!(fbc.loss_intervals.entries.iter().map(|entry| entry.length).collect::<Vec<_>>(), vec![ 1, 3 ]);
assert_eq!(fbc.next_ack_id, 4);
assert_eq!(fbc.frame_log.base_id(), 4);
assert_eq!(fbc.frame_log.next_id(), 9);
}
#[test]
fn loss_intervals_reorder_forget_full() {
let mut fbc = FeedbackComp::new(0);
fbc.log_frame( 0, false, 64, 0);
fbc.log_frame( 1, false, 64, 0);
fbc.log_frame( 2, false, 64, 100); // ack
fbc.log_frame( 3, false, 64, 100);
fbc.log_frame( 4, false, 64, 100);
fbc.log_frame( 5, false, 64, 100); // ack
fbc.log_frame( 6, false, 64, 199);
fbc.log_frame( 7, false, 64, 200);
fbc.log_frame( 8, false, 64, 300);
fbc.acknowledge_frames(frame::FrameAck { base_id: 0, bitfield: 0b000000100, nonce: false }, 100);
assert_eq!(fbc.loss_intervals.entries.iter().map(|entry| entry.length).collect::<Vec<_>>(), vec![ ]);
fbc.acknowledge_frames(frame::FrameAck { base_id: 0, bitfield: 0b000100000, nonce: false }, 100);
assert_eq!(fbc.loss_intervals.entries.iter().map(|entry| entry.length).collect::<Vec<_>>(), vec![ ]);
assert_eq!(fbc.next_ack_id, 0);
assert_eq!(fbc.frame_log.base_id(), 0);
assert_eq!(fbc.frame_log.next_id(), 9);
fbc.forget_frames(300, 100);
assert_eq!(fbc.loss_intervals.entries.iter().map(|entry| entry.length).collect::<Vec<_>>(), vec![ 1, 4, 3 ]);
assert_eq!(fbc.next_ack_id, 8);
assert_eq!(fbc.frame_log.base_id(), 8);
assert_eq!(fbc.frame_log.next_id(), 9);
}
#[test]
fn max_loss_intervals() {
let mut fbc = FeedbackComp::new(0);
fbc.log_frame( 0, false, 64, 0); // ack
fbc.log_frame( 1, false, 64, 0);
fbc.log_frame( 2, false, 64, 100);
fbc.log_frame( 3, false, 64, 200);
fbc.log_frame( 4, false, 64, 200);
fbc.log_frame( 5, false, 64, 300);
fbc.log_frame( 6, false, 64, 400);
fbc.log_frame( 7, false, 64, 500);
fbc.log_frame( 8, false, 64, 500);
fbc.log_frame( 9, false, 64, 500);
fbc.log_frame(10, false, 64, 600);
fbc.log_frame(11, false, 64, 700);
fbc.log_frame(12, false, 64, 800);
fbc.log_frame(13, false, 64, 800);
fbc.log_frame(14, false, 64, 900);
fbc.log_frame(15, false, 64, 900);
fbc.log_frame(16, false, 64, 900); // ack
fbc.log_frame(17, false, 64, 1000); // ack
fbc.log_frame(18, false, 64, 1000); // ack
fbc.acknowledge_frames(frame::FrameAck { base_id: 0, bitfield: 0x70001, nonce: false }, 100);
assert_eq!(fbc.loss_intervals.entries.iter().map(|entry| entry.length).collect::<Vec<_>>(), vec![ 3, 2, 1, 1, 3, 1, 1, 2, 1 ]);
}
#[test]
fn loss_intervals_changing_rtt() {
let mut fbc = FeedbackComp::new(0);
fbc.log_frame( 0, false, 64, 0); // ack @ 100ms RTT
fbc.log_frame( 1, false, 64, 0);
fbc.log_frame( 2, false, 64, 0); // ack @ 100ms RTT
fbc.log_frame( 3, false, 64, 0); // ack @ 100ms RTT
fbc.log_frame( 4, false, 64, 0); // ack @ 100ms RTT
fbc.log_frame( 5, false, 64, 99);
fbc.log_frame( 6, false, 64, 100);
fbc.log_frame( 7, false, 64, 150);
fbc.log_frame( 8, false, 64, 200); // ack @ 50ms RTT
fbc.log_frame( 9, false, 64, 200); // ack @ 50ms RTT
fbc.log_frame(10, false, 64, 200); // ack @ 50ms RTT
fbc.acknowledge_frames(frame::FrameAck { base_id: 0, bitfield: 0b00000011101, nonce: false }, 100);
assert_eq!(fbc.loss_intervals.entries.iter().map(|entry| entry.length).collect::<Vec<_>>(), vec![ 2 ]);
fbc.acknowledge_frames(frame::FrameAck { base_id: 0, bitfield: 0b11100000000, nonce: false }, 50);
assert_eq!(fbc.loss_intervals.entries.iter().map(|entry| entry.length).collect::<Vec<_>>(), vec![ 2, 1, 5 ]);
}
*/
}