Add max burst size limit to pacer

[algesten]

Summary

The pacer should add a max burst size limit to prevent overfilling socket buffers at high bitrates, similar to WebRTC's implementation.

Context

From PR #823 review discussion by @k0nserv on src/pacer/leaky.rs:290:

Maybe we should add in the max burst thing here too

WebRTC Reference

WebRTC's PacingController has a kMaxBurstSize constant of 63KB:

// modules/pacing/pacing_controller.h:89
static constexpr DataSize kMaxBurstSize = DataSize::Bytes(63 * 1000);

And uses it to cap burst intervals:

// modules/pacing/pacing_controller.cc:353-356
// Ensure that a burst of sent packet is not larger than kMaxBurstSize in
// order to not risk overfilling socket buffers at high bitrate.
TimeDelta send_burst_interval =
    std::min(send_burst_interval_, kMaxBurstSize / adjusted_media_rate_);

Current str0m code

In src/pacer/leaky.rs:290, the normal pacing path calculates drain time but does not cap the burst size:

let drain_debt_time = self.media_debt / self.adjusted_bitrate;
let next_send_offset = if drain_debt_time > PACING {
    drain_debt_time
} else {
    Duration::ZERO
};

Suggested fix

Add a max burst size constant and use it to limit the burst interval:

const MAX_BURST_SIZE: DataSize = DataSize::bytes(63_000);

// In poll_action:
let max_burst_interval = MAX_BURST_SIZE / self.adjusted_bitrate;
// Apply min with send_burst_interval

Related

• PR BWE V2 #823 (BWE V2)