Fix BBR MinRtt validity checks

[Wizmann]

Summary

Fix BBR's invalid MinRtt handling to consistently use the existing MinRttTimestampValid state instead of relying on an incorrect sentinel comparison.

Motivation

BBR's model depends on two independent path estimates:

• bottleneck bandwidth (BtlBw)
• round-trip propagation time (RTprop, represented here by MinRtt)

The BBR paper describes RTprop as a windowed minimum RTT estimate. BBR should only use that estimate after a valid RTT sample has established it. Until then, calculations based on BDP (BtlBw * RTprop) should use startup-safe fallback behavior instead of consuming an uninitialized sentinel value.

In this implementation, MinRtt is initialized to UINT64_MAX, while validity is separately tracked by MinRttTimestampValid. However, some code paths checked MinRtt == UINT32_MAX to decide whether the estimate was unavailable. Since UINT64_MAX != UINT32_MAX, those paths could incorrectly treat an invalid MinRtt as valid.

This matters because an invalid RTprop estimate can affect:

• target cwnd calculation
• paced send allowance fallback
• ProbeRTT entry decisions

ProbeRTT is meant to refresh an existing RTprop estimate after it expires. If no valid MinRtt sample has ever existed, entering ProbeRTT is not useful; BBR should remain in startup/model-building behavior until a valid RTT sample exists.

Change

This change replaces invalid MinRtt checks with !Bbr->MinRttTimestampValid.

That makes the code match the actual state model:

• MinRttTimestampValid == FALSE: no valid MinRtt/RTprop sample exists yet
• MinRttTimestampValid == TRUE: MinRtt can safely participate in BDP, pacing, and ProbeRTT decisions

Specifically, the change ensures:

• target cwnd falls back safely when no MinRtt sample exists
• send allowance does not use paced calculations before MinRtt is valid
• BBR does not enter ProbeRTT before it has a valid MinRtt estimate to refresh

Tests

Updated and added BBR unit coverage:

• verifies send allowance falls back to available congestion window before a valid MinRtt sample exists
• verifies target cwnd falls back to the initial congestion window when bandwidth exists but MinRtt is still invalid
• verifies BBR remains in STARTUP instead of entering ProbeRTT when no valid MinRtt sample exists

[Wizmann]

@microsoft-github-policy-service agree

[Copilot]

Pull request overview

This PR fixes BBR’s handling of an “invalid/uninitialized MinRtt” by removing an incorrect sentinel comparison (MinRtt == UINT32_MAX) and consistently using the existing validity flag (MinRttTimestampValid). This aligns BBR’s control logic with its internal state model so BDP- and pacing-based computations don’t consume MinRtt before any valid RTT sample exists.

Changes:

• Replace incorrect MinRtt == UINT32_MAX checks with !MinRttTimestampValid in target cwnd and send allowance calculations.
• Prevent transitions to BBR_STATE_PROBE_RTT unless a valid MinRtt sample has been established.
• Update/add BBR unit tests to validate safe fallback behavior when MinRtt is invalid (including ensuring BBR stays in STARTUP instead of entering PROBE_RTT).

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
src/core/bbr.c	Uses MinRttTimestampValid to gate target cwnd, pacing send allowance, and ProbeRTT entry to avoid consuming an invalid MinRtt sentinel.
src/core/unittest/BbrTest.cpp	Updates the send allowance test expectation and adds coverage verifying target cwnd fallback + no premature ProbeRTT when MinRtt is invalid.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 85.55%. Comparing base (51d449b) to head (1c4d205).
⚠️ Report is 6 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #6118      +/-   ##
==========================================
- Coverage   86.15%   85.55%   -0.61%     
==========================================
  Files          60       60              
  Lines       18857    18911      +54     
==========================================
- Hits        16247    16179      -68     
- Misses       2610     2732     +122     

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[guhetier]

Thanks for the PR! LGTM, minor nitpick to clarify a comment.

[Wizmann]

Suggestion patched. Ready to merge :)