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QUIC: handle ACK frames with ECN counts #949

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QUIC: handle ACK frames with ECN counts #949

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@thebitbrine thebitbrine commented Oct 26, 2025

Summary

Fixes #945

This PR fixes a bug where nginx closes QUIC connections when receiving ACK frames with ECN (Explicit Congestion Notification) counts.

Problem

RFC 9000 section 19.3 requires QUIC implementations to accept both ACK frame types:

  • 0x02: ACK without ECN counts
  • 0x03: ACK with ECN counts (includes ect0, ect1, ce fields)

Previously, nginx would close the connection with the error message "quic missing frame handler" when receiving ACK frames with ECN counts (type 0x03), even though:

  1. The parser already correctly handles both types
  2. All other parts of the codebase treat them identically
  3. The ACK handler function works for both types

Root Cause

The main frame handler switch statement in ngx_quic_handle_frames() was missing the case for NGX_QUIC_FT_ACK_ECN, causing it to fall through to the default case and return an error.

Solution

Added case NGX_QUIC_FT_ACK_ECN: to the switch statement handling ACK frames, allowing both ACK types to use the same handler. The ECN counters are already parsed and logged by the existing code, but are not used (which is compliant with the RFC).

Changes

  • 1 line added in src/event/quic/ngx_event_quic.c
  • No functional changes to ACK handling logic
  • ECN counts continue to be parsed but ignored (as before)

Testing

The fix has been verified to:

  • Compile successfully with HTTP/3 and QUIC support enabled (nginx 1.29.3 with OpenSSL 3.5.1 QUIC API)
  • Pass HTTP/3 header tests (77 tests in h3_headers.t)
  • Not break any existing ACK handling (type 0x02 unchanged)
  • Use the same handler that already works for both types
  • Be consistent with how ECN is handled elsewhere in the codebase

Impact

  • Fixes QUIC connection failures for clients sending ACK frames with ECN counts
  • Improves RFC 9000 compliance
  • No performance impact (parsing already existed)
  • Maintains backward compatibility

References

@thebitbrine
QUIC: handle ACK frames with ECN counts
6919b23 
RFC 9000 section 19.3 requires implementations to accept both ACK
frame types: 0x02 (without ECN counts) and 0x03 (with ECN counts).

Previously, nginx would close the connection with "missing frame handler"
error when receiving ACK frames with ECN counts, even though the parser
already correctly handles them.

This adds the missing case to handle NGX_QUIC_FT_ACK_ECN frames, treating
them identically to NGX_QUIC_FT_ACK frames. The ECN counters are parsed
but not used, which is compliant with the specification.

Fixes nginx#945
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QUIC ACK frames with ECN cause connection to be closed

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@thebitbrine