feat(interceptor): add GCC sender-side bandwidth estimator

[nightness]

Summary

Add GccInterceptor — a Google Congestion Control (GCC, RFC 8698) bandwidth estimator that uses TWCC feedback to dynamically estimate available send bandwidth.

Key components:

• TrendlineFilter — sliding window (N=20) OLS linear regression on inter-packet delay gradients; dynamic overuse threshold [6ms, 600ms]
• AimdRateController — AIMD state machine: 1.08×/sec increase on Normal/Underusing, ×0.85 on Overuse; loss-based adjustment (> 10% loss → decrease)
• GccHandle — Arc<Mutex<GccShared>> output channel (no changes to framework's Eout = () contract)
• GccInterceptorBuilder::new() returns (builder, GccHandle) pair

Chain position (GCC must be inner to TwccSender):

TwccSender → GccInterceptor → SenderReport → NackResponder → NoopInterceptor

Usage:

let (gcc_builder, gcc_handle) = GccInterceptorBuilder::new();
let chain = Registry::new()
    .with(gcc_builder.build())
    .with(TwccSenderBuilder::new().build())
    .build();

// In app polling loop:
if let Some(bps) = gcc_handle.target_bitrate_bps() {
    encoder.set_bitrate(bps);
}

Also includes the JitterBuffer interceptor (receiver-side, RFC 3550 §A.8 adaptive playout delay) as a dependency.

Review feedback addressed

• Fix all clippy collapsible_if warnings (8 instances)
• Remove unused packet_size from send_times HashMap
• Avoid cloning TransportLayerCc packets; iterate by reference
• Eliminate per-chunk Vec allocation by inlining iteration
• Clamp initial_delay to [min_delay, max_delay] in JitterBufferStream::new()
• Fix permanently tightened bounds: use per-packet effective min/max
• Remove unnecessary Bytes::copy_from_slice; clone Bytes (cheap refcount bump)
• Eliminate temporary Vec<&TransportLayerCc> allocation; iterate inline
• Recover from mutex poison in target_bitrate_bps() and process_feedback() via unwrap_or_else with log::warn
• Fix playout-delay ordering: compute jitter-based release first, then clamp to per-packet sender bounds

Test plan

• cargo test -p rtc-interceptor — all 154 tests pass (7 new tests added)
• cargo clippy -p rtc-interceptor — zero warnings
• cargo fmt --check — clean

🤖 Generated with Claude Code

[Copilot]

Pull request overview

Note

Copilot was unable to run its full agentic suite in this review.

Adds sender-side GCC bandwidth estimation (TWCC-driven) and a receiver-side jitter buffer interceptor, exporting both via the rtc-interceptor crate API.

Changes:

• Introduces GccInterceptor (trendline overuse detection + AIMD rate controller) with a polling GccHandle.
• Adds JitterBufferInterceptor with adaptive playout delay (RFC 3550 jitter-based) and optional playout-delay RTP extension support.
• Re-exports new interceptors/builders and adds the bytes dependency for TWCC header extension parsing.

Reviewed changes

Copilot reviewed 7 out of 7 changed files in this pull request and generated 7 comments.

Show a summary per file

File	Description
rtc-interceptor/src/lib.rs	Registers new gcc and jitter_buffer modules and re-exports public builders/handles.
rtc-interceptor/src/jitter_buffer/mod.rs	Adds jitter buffer interceptor + builder, integrates with stream binding and scheduling.
rtc-interceptor/src/jitter_buffer/stream.rs	Implements per-SSRC jitter buffer logic, delay adaptation, and unit tests.
rtc-interceptor/src/gcc/mod.rs	Adds GCC interceptor, handle API, TWCC feedback parsing, and end-to-end tests.
rtc-interceptor/src/gcc/rate_controller.rs	Implements AIMD controller and tests.
rtc-interceptor/src/gcc/trendline.rs	Implements trendline filter + threshold adaptation and tests.
rtc-interceptor/Cargo.toml	Adds bytes workspace dependency.

Comments suppressed due to low confidence (1)

rtc-interceptor/src/gcc/mod.rs:1

• with_min_bitrate / with_max_bitrate allow creating an invalid configuration where min_bitrate_bps > max_bitrate_bps. With the current AimdRateController, this can produce estimates that exceed the intended max (e.g., decreases clamp with .max(min_bps) even if min_bps > max_bps). Please enforce min <= max at builder time (e.g., clamp, swap, or return a Result from build()/new() variants) so the public API cannot create a contradictory controller configuration.

//! GCC (Google Congestion Control) sender-side bandwidth estimator.

---

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[Copilot]

This clones every TransportLayerCc in the RTCP compound packet just to process them. TWCC feedback packets can be relatively large, and cloning them per read adds avoidable allocation/copy overhead. You should be able to iterate the RTCP packets by reference and call process_feedback on &TransportLayerCc directly without collecting/cloning first (process before forwarding to inner.handle_read(msg)).

Suggested change

	if let Packet::Rtcp(ref rtcp_pkts) = msg.message {
	let now = msg.now;
	// Collect feedback packets first to avoid borrow issues.
	let feedbacks: Vec<TransportLayerCc> = rtcp_pkts
	.iter()
	.filter_map(|pkt| {
	pkt.as_any()
	.downcast_ref::<TransportLayerCc>()
	.cloned()
	})
	.collect();
	for fb in &feedbacks {
	self.process_feedback(fb, now);
	}
	}
	let now = msg.now;
	let feedbacks: Vec<&TransportLayerCc> = {
	if let Packet::Rtcp(ref rtcp_pkts) = msg.message {
	rtcp_pkts
	.iter()
	.filter_map(|pkt| pkt.as_any().downcast_ref::<TransportLayerCc>())
	.collect()
	} else {
	Vec::new()
	}
	};
	for fb in feedbacks {
	self.process_feedback(fb, now);
	}

[nightness]

Addressed: eliminated the intermediate Vec<&TransportLayerCc> collection. Now iterates rtcp_pkts inline with filter_map + downcast_ref directly. Should be marked outdated.

[codecov]

Codecov Report

❌ Patch coverage is 87.71552% with 57 lines in your changes missing coverage. Please review.
✅ Project coverage is 71.33%. Comparing base (9feb4a3) to head (9371e86).

Files with missing lines	Patch %	Lines
rtc-interceptor/src/gcc/mod.rs	82.89%	26 Missing ⚠️
rtc-interceptor/src/jitter_buffer/stream.rs	78.50%	23 Missing ⚠️
rtc-interceptor/src/gcc/rate_controller.rs	94.00%	3 Missing ⚠️
rtc-interceptor/src/jitter_buffer/mod.rs	96.55%	3 Missing ⚠️
rtc-interceptor/src/gcc/trendline.rs	97.05%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master      #85      +/-   ##
==========================================
+ Coverage   71.17%   71.33%   +0.16%     
==========================================
  Files         442      447       +5     
  Lines       67330    67794     +464     
==========================================
+ Hits        47922    48361     +439     
- Misses      19408    19433      +25     

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[Copilot]

Pull request overview

Copilot reviewed 7 out of 7 changed files in this pull request and generated 5 comments.

---

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[Copilot]

target_bitrate_bps() silently returns None if the mutex is poisoned, which can hide bugs and cause the app to stop adapting bitrate without any signal. Consider recovering via unwrap_or_else(|e| e.into_inner()) (and/or logging) so callers still get the last known estimate.

[nightness]

Addressed: target_bitrate_bps() now recovers from poison via unwrap_or_else(|e| e.into_inner()) with a log::warn, so callers always get the last known estimate. Should be marked outdated.

[nightness]

Rebased onto PR #84's branch. Merge #84 first, then this PR applies cleanly. Previous branch structure has been cleaned up to avoid merge conflicts.