fix(transport): optionally accept reordered best-effort frames

[samconsidine]

Description

What does this PR do?

Adds an optional receive-side sequence-number window for unfragmented BestEffort Frames arriving over non-streamed links.

When transport.link.rx.best_effort_reorder_window is non-zero, Zenoh keeps a bounded bitmap of recently observed sequence numbers. A reordered Frame inside the window is delivered once, while duplicates and Frames older than the window are dropped. Frames are delivered immediately; this does not buffer packets or restore their original delivery order.

The default is 0, which preserves the existing strict monotonic validation. Reliable Frames, streamed links, multicast, and fragments retain their existing behavior.

Example:

{
  transport: {
    link: {
      rx: {
        best_effort_reorder_window: 1024,
      },
    },
  },
}

Why is this change needed?

Unordered transports such as QUIC DATAGRAM can deliver valid Frames out of sequence. The current receive path advances its sequence-number high-water mark when a later Frame arrives, then treats subsequently arriving earlier Frames as invalid. Forward batching or other network-induced reordering can therefore be amplified into substantial application-visible loss even when the packets reach the receiver.

In a controlled A/B reproduction using the same sender workload and network profile:

Mode	Sent	Received	Application loss	Invalid SN	RX qdisc drops
Strict/default	21,871	9,509	56.5%	10,024	0
Window = 1024	21,871	21,871	0%	0	0

With the window enabled, 9,603 late reordered Frames were accepted. The maximum observed reorder distance was 29 sequence numbers (mean 16.05).

The implementation also handles sequence-number rollover and suppresses duplicate delivery. Reordered fragments are deliberately excluded because the current defragmenter tracks a single sequential message.

Validation

• cargo test -p zenoh-config test_best_effort_reorder_window_config --lib
• cargo test -p zenoh-transport common::seq_num::tests --lib
• cargo test -p zenoh-transport --lib (30 passed, 1 ignored)
• cargo clippy -p zenoh-config -p zenoh-transport --lib -- -D warnings

The full local zenoh-config test suite has one unrelated existing failure: the TOML-format test runs although that local feature set reports only JSON, JSON5, and YAML support. The new configuration test passes both independently and within that suite.

Related Issues

No matching existing issue was found.

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Labels: enhancement

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[diogomatsubara]

@samconsidine Please, sign the Eclipse Contributor Agreement so we're able to consider this change.