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priority: don't let the best-path override pick the lowest index when links tie on quality - #27
In classic mode, every SRT retransmit goes to the first link in the IPs file, and so does every packet inside a keyframe critical window. The link's window, in-flight count and weight make no difference. With link weights (#24), retransmits therefore ignore the operator's preference. Without weights, one link carries all the recovery traffic, even when it is the link that is losing the packets.
Problem
After the normal scheduler picks a link, handle_srt_packet (src/sender/packet_handler.rs) re-routes two kinds of packet to srtla_core::priority::select_best_quality_idx: retransmits (R bit) and critical-window packets. That function ranks links by quality_cache.multiplier, and only enhanced selection refreshes that value (selection/enhanced.rs). In classic mode every link stays at the default 1.0. The strict > therefore returns the lowest eligible index, and the override sends the packet to link 0.
Change
select_best_quality_idx returns None when two or more eligible links have the same multiplier. There is no best path to prefer, so the caller keeps the scheduler's choice.
In classic mode the override is therefore a no-op. A retransmit is scheduled like any other packet, window / (in_flight + 1) × weight. This is what the C srtla_send and Moblin do.
Enhanced mode is unchanged except when every eligible link ties exactly. That case had the same lowest-index bias.
These cases are unchanged: a single eligible link, and one link with a strictly better multiplier.
One comment in packet_handler.rs explains when the override declines.
Tests
crates/srtla-core/src/priority.rs:
All links at the default multiplier (classic) → None.
A two-link tie → None. With one link made better → that link, whatever its index.
The existing held-out, disconnected and empty cases pass unchanged.
Both new tests fail without the change. These pass on the pinned toolchain:
10 % random loss on outbound UDP from link 0 only, for 60 s, between 45 s clean before and 60 s clean after.
Two runs before the change and two runs after.
To count retransmits per link we used a trace line that is not part of this PR.
Retransmits during the loss, link 0 (lossy) / link 1
critical override lines
Share link 0 / link 1, clean before
before, run 1
2858 / 0
927, all 1 -> 0
91 / 9
before, run 2
1933 / 0
788, all 1 -> 0
89 / 11
after, run 1
2872 / 444
0
93 / 7
after, run 2
2682 / 429
0
93 / 7
Before: every retransmit went to the lossy link. Every override moved a packet from link 1, which the scheduler had chosen, back to link 0.
After: 13–14 % of retransmits move to link 1 as link 0's window shrinks. Link 0 still carries most of them, as its weight asks. In clean air the weight is honoured (93/7).
At this loss rate and a 5 s buffer, SRT repaired the stream fully in all four runs. The receiver saw no damage and no sender drops in any of them. The change is shown in where the repairs travel, not in the picture.
When eligible links have equal quality, critical-window and retransmit traffic now follows normal scheduling instead of favoring a tied link. Links with a uniquely higher quality remain preferred, and traffic still uses normal scheduling when no link is eligible.
This makes routing behavior more consistent when links have equal quality.
… on a tie
handle_srt_packet re-routes SRT retransmits (R bit) and keyframe
critical-window packets to select_best_quality_idx after the scheduler
has picked a link. That ranks links by quality_cache.multiplier, which
only enhanced selection refreshes. In classic mode every link sits at the
default 1.0, so the strict `>` returns the lowest eligible index: every
retransmit and every critical-window packet went to link 0, whatever its
window, in-flight count or operator weight, including when link 0 was the
one losing the packets.
select_best_quality_idx now returns None when two or more eligible links
share the same multiplier: there is no best path to prefer, so the caller
keeps the scheduler's choice and a retransmit is scheduled like any other
packet (window / (in_flight + 1) x weight, as the C srtla_send and Moblin
do). A single eligible link, or one link with a strictly better
multiplier, is unchanged; in enhanced mode only the all-tied case changes.
Tests (priority.rs): all default multipliers -> None; a two-link tie ->
None, then one better link -> that link. Both fail without the change.
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Reviewing files that changed from the base of the PR and between 4bd3f3f and 02dd232.
Files selected for processing: 2
crates/srtla-core/src/priority.rs
src/sender/packet_handler.rs
Walkthrough
The selector now returns no preferred link when multiple eligible links have equal quality multipliers. It still selects the highest-multiplier link when qualities differ. Packet-handler comments describe normal scheduling when no preferred link is returned, and tests cover equal and differing multipliers.
Changes: Quality-based link selection
Layer / File(s)
Summary
Selector tie handling and fallback crates/srtla-core/src/priority.rs, src/sender/packet_handler.rs
The selector returns no preferred link when multiple eligible links have equal quality multipliers. Tests cover default and differing multipliers. Comments describe normal scheduling when the selector returns no link.
The change stops the quality override from forcing the lowest-index link when link quality is tied, which is the intended fix. One older gap remains: after switching from Enhanced to Classic at runtime, retransmits may still be steered by stale quality values. That is a bounded follow-up and does not make this change riskier.
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The title clearly and concisely describes the main change: preventing the best-path override from selecting the lowest-index link when quality ties.
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Docstring coverage is 100.00% which is sufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff with cyclomatic complexity of at least 2. Documentat…
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Actionable comments posted: 1
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Inline comments:
Review comments at @crates/srtla-core/src/priority.rs:
- Line 156: Update the quality override in select_best_quality_idx to apply only
when the configured SchedulingMode is Enhanced, so Classic mode uses the Classic
scheduler even with cached unequal multipliers. Add a regression test that
switches from Enhanced to Classic with unequal cached multipliers and verifies
the override is not used.
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Reviewing files that changed from the base of the PR and between 59a7d8b and 4bd3f3f.
Files selected for processing: 2
crates/srtla-core/src/priority.rs
src/sender/packet_handler.rs
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The tie no-op covers classic mode only while every multiplier is still at
the default. A runtime switch from enhanced to classic leaves the quality
cache with the unequal values enhanced selection last wrote, and classic
never refreshes them, so the override would keep ranking links on stale
data.
The packet handler now goes through select_best_path_override_idx, which
returns None in classic mode (the scheduler's choice stands) and is
select_best_quality_idx otherwise, tie no-op included.
Test (priority.rs): unequal cached multipliers -> None under Classic,
the better link under Enhanced.
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
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Why this matters to you
In classic mode, every SRT retransmit goes to the first link in the IPs file, and so does every packet inside a keyframe critical window. The link's window, in-flight count and weight make no difference. With link weights (#24), retransmits therefore ignore the operator's preference. Without weights, one link carries all the recovery traffic, even when it is the link that is losing the packets.
Problem
After the normal scheduler picks a link,
handle_srt_packet(src/sender/packet_handler.rs) re-routes two kinds of packet tosrtla_core::priority::select_best_quality_idx: retransmits (R bit) and critical-window packets. That function ranks links byquality_cache.multiplier, and only enhanced selection refreshes that value (selection/enhanced.rs). In classic mode every link stays at the default 1.0. The strict>therefore returns the lowest eligible index, and the override sends the packet to link 0.Change
select_best_quality_idxreturnsNonewhen two or more eligible links have the same multiplier. There is no best path to prefer, so the caller keeps the scheduler's choice.window / (in_flight + 1)× weight. This is what the C srtla_send and Moblin do.packet_handler.rsexplains when the override declines.Tests
crates/srtla-core/src/priority.rs:None.None. With one link made better → that link, whatever its index.Both new tests fail without the change. These pass on the pinned toolchain:
cargo +nightly fmt --all -- --checkcargo clippy --workspace --all-targets --all-features -- -D warningscargo test --workspace --all-featuresBench evidence
Setup:
critical overridelines1 -> 01 -> 0Before: every retransmit went to the lossy link. Every override moved a packet from link 1, which the scheduler had chosen, back to link 0.
After: 13–14 % of retransmits move to link 1 as link 0's window shrinks. Link 0 still carries most of them, as its weight asks. In clean air the weight is honoured (93/7).
At this loss rate and a 5 s buffer, SRT repaired the stream fully in all four runs. The receiver saw no damage and no sender drops in any of them. The change is shown in where the repairs travel, not in the picture.
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