fix(reliability): detect in-session path-MTU black holes and step the MTU down - #56
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… MTU down The connection handshake probes the path MTU in one direction only and the negotiated size is then applied to both directions for the life of the connection. A tunnel whose return path carries less than the probed direction (OpenVPN and friends drop, rather than fragment, datagrams over their ceiling; see OpenVPN/openvpn#823) black-holes every large datagram one way while the handshake's small packets sail through: the peer connects, then hangs on the first split payload while the reliability layer resends the same too-large datagram until the connection times out. This is the follow-up deferred by the MAXIMUM_MTU_SIZE cap in #55. Detection (MtuBlackHole.h, portable and unit tested on every platform): a reliable packet that has gone unacked through MTU_BLACKHOLE_RESEND_THRESHOLD transmissions, and would actually shrink if the MTU dropped a rung, is the black-hole signature -- packets that already fit the next rung indicate loss, not size, so ordinary packet loss can never shrink a healthy connection's MTU. The ladder is now shared with the handshake probe in RakPeer.cpp. Recovery (ReliabilityLayer): step currentMtuBytes one rung down, shrink the congestion manager's datagram ceiling, and re-split every queued message that no longer fits. Split messages are rebuilt from the refcounted whole-message block their fragments share (fragments now record the original length in the sender-only splitOriginalByteLength) and re-sent under a fresh splitPacketId with their ordering indices preserved, so the receiver's stalled partial channel is superseded in place. Oversized unsplit reliable messages are simply split; unreliable ones are dropped, as the network already was doing. Tested with a hermetic two-ReliabilityLayer harness over a fake socket with fully simulated time (Tests/Unit/ReliabilityLayerBlackHoleTests.cpp): the one-directional black hole, recovery down to the bottom rung, ordered delivery and ack receipts across a re-split, and the false-positive guard under plain loss. Verified on macOS and on Linux in Debug and Release.
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WalkthroughThis change adds in-session MTU black-hole detection. The reliability layer lowers the shared MTU ladder after repeated oversized retransmissions, rebuilds queued messages, and re-splits them. Runtime MTU reporting and handshake probing now use the same ladder. ChangesMTU black-hole recovery
Estimated code review effort: 4 (Complex) | ~45 minutes Merge Risk: 🟡 Moderate · up to The PR adds in-session MTU reduction and re-fragmentation, but the current implementation can alter non-byte-aligned payloads, break a supported build configuration, race concurrent MTU readers, or lose queued reliable data under allocation failure. These bounded correctness and availability risks should be fixed or explicitly accepted before merging. Sequence Diagram(s)sequenceDiagram
participant Sender
participant ReliabilityLayer
participant MtuBlackHole
participant Receiver
Sender->>ReliabilityLayer: queue reliable message
ReliabilityLayer->>Receiver: send oversized datagram
Receiver--xReliabilityLayer: datagram dropped
ReliabilityLayer->>MtuBlackHole: check resend threshold and size
MtuBlackHole-->>ReliabilityLayer: select lower MTU rung
ReliabilityLayer->>ReliabilityLayer: rebuild and re-split queued messages
ReliabilityLayer->>Receiver: send smaller fragments
Receiver-->>Sender: deliver message and receipt
Poem
🚥 Pre-merge checks | ✅ 4 | ❌ 1❌ Failed checks (1 warning)
✅ Passed checks (4 passed)
Full details: Docstring CoverageExplanation Docstring coverage is 32.50% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 40 functions across 9 files. (1 skipped: 1 unsupported.) ✨ Finishing Touches 💡 1📝 Generate docstrings 💡
🧪 Generate unit tests (beta)
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In `@Source/include/mafianet/ReliabilityLayer.h`:
- Around line 605-607: Make currentMtuBytes available regardless of
USE_SLIDING_WINDOW_CONGESTION_CONTROL by moving its declaration outside the
congestion-control conditional in ReliabilityLayer, preserving its use by
Reset(), GetCurrentMtuBytes(), and recovery logic in both CCRakNetUDT and
sliding-window builds.
In `@Source/src/RakPeer.cpp`:
- Line 6286: Synchronize accesses to RemoteSystemStruct::MTUSize: update the
assignment in RunUpdateCycle and the read in GetMTUSize to use the same
synchronization mechanism, or make the field atomic, so concurrent access for a
peer cannot race.
In `@Source/src/ReliabilityLayer.cpp`:
- Line 3230: Update split-fragment recovery to retain the original BitSize_t bit
length on each fragment and assign that preserved value to
rebuilt->dataBitLength; continue using the rounded byteLength only for
allocation and data copying.
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Source/CMakeLists.txtSource/include/mafianet/InternalPacket.hSource/include/mafianet/MTUSize.hSource/include/mafianet/MtuBlackHole.hSource/include/mafianet/ReliabilityLayer.hSource/src/MtuBlackHole.cppSource/src/RakPeer.cppSource/src/ReliabilityLayer.cppTests/Unit/MtuBlackHoleTests.cppTests/Unit/ReliabilityLayerBlackHoleTests.cpp
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- Anchor the black-hole harness's simulated clock to the real clock at SetUp: ReliabilityLayer stamps timeLastDatagramArrived with the real GetTimeMS() on every receive and AckTimeout compares it against the time the caller passes in, so a fixed fake epoch declared every connection dead on any machine whose process uptime exceeded it -- which is what failed all six tests on Windows CI. Reproduced locally by anchoring the clock 20s behind real time. - Preserve the exact bit length of a message across a re-split (CodeRabbit): Send() takes bit lengths and normal split reassembly reproduces them exactly via the last fragment, but the rebuild rounded up to whole bytes. splitOriginalByteLength becomes splitOriginalBitLength; new regression test sends a message 3 bits short of a byte boundary through the black hole. - Move currentMtuBytes out of the USE_SLIDING_WINDOW_CONGESTION_CONTROL conditional (CodeRabbit + ultrareview): it is not congestion-manager- specific and every use is unguarded. The UDT config still fails on a pre-existing legacy-identifier error (RakNetTimeMS, ReliabilityLayer.cpp:143) present before this branch. - Write remoteSystem->MTUSize only when the value actually changed (CodeRabbit): GetMTUSize() reads it unsynchronized from the user thread, as it always has for the connect-time write; this keeps the field write-once-per-event instead of continuously written.
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The bug
#55 capped
MAXIMUM_MTU_SIZEat 1400, which fixed tunnelled clients on paths the handshake can probe. But the handshake probes the path in one direction only — the connecting peer padsID_OPEN_CONNECTION_REQUEST_1down the ladder and the accepting peer echoes back what arrived — and the negotiated size is then applied to both directions for the life of the connection.A tunnel whose return path carries less than the probed direction black-holes every large datagram one way while the handshake's small packets sail through. OpenVPN is the canonical case: with its defaults (
tun-mtu 1500, no--fragment,mssfixTCP-only) it drops, rather than fragments, oversized UDP — and the drop is direction-asymmetric (OpenVPN/openvpn#823 shows theEMSGSIZEon one side only, with a dead zone of packet sizes that vanish while larger and smaller ones pass). The peer connects, then hangs on the first split payload while the reliability layer resends the same too-large datagram at the same size untiltimeoutTimekills the connection. This PR is the in-session black-hole detection that #55 explicitly deferred.Changes
1. Detection —
MtuBlackHole.h/MtuBlackHole.cpp(portable, unit tested everywhere).A reliable packet that has gone unacked through
MTU_BLACKHOLE_RESEND_THRESHOLD(4) transmissions, and that would actually shrink if the MTU dropped a rung, is the black-hole signature. The second half is the false-positive guard: a packet that already fits the next rung would go out unchanged after a step-down, so its failures indicate loss, not size — ordinary packet loss can never shrink a healthy connection's MTU. Resends are RTO-spaced with backoff, so the threshold represents several RTTs of one specific packet failing while the connection is otherwise alive; detection completes well inside the 10s dead-connection timeout. The rung ladder moves into this header andRakPeer's handshake probe now aliases it, so the two mechanisms can't drift apart.2. Step-down —
ReliabilityLayer::StepDownMtuAfterBlackHole.Checked at one point only: the top of
Update's send section, on the head of the resend queue, before any packet is pushed that tick (the recovery frees queued packets, sopacketsToSendThisUpdatemust be empty). On trigger:currentMtuBytesdrops one rung, the congestion manager's datagram ceiling shrinks to match, and every queued message that no longer fits is re-split. New sends already size from the congestion manager, so they need nothing.3. Re-split —
ReliabilityLayer::ReSplitOversizedMessages.The hard part: fragments already cut at the old size can't just be resent smaller —
splitPacketCountand the receiver's reassembly stride are fixed persplitPacketId. But every fragment of a message shares the whole original through its refcountedsharedDataBlock, and fragments now record the original length in a new sender-onlyInternalPacket::splitOriginalByteLength(never transmitted; the wire format is unchanged). So a stale message is rebuilt at full length, its fragments swept out of the resend list (ack-style removal:resendBufferslot, statistics,unacknowledgedBytes) and the outgoing heap (the existingdata==0tombstone idiom), and the message re-split at the new size under a freshsplitPacketIdwith its ordering/sequencing indices and receipt serial preserved. The receiver's stalled partial channel for the old id never completes and is superseded in place — the ordered channel is waiting on exactly the ordering index the re-split message carries. Oversized unsplit reliable messages are simply split; oversized unsplit unreliable ones are dropped, which is what the network was already doing to them. A rebuild that fails (OOM) leaves the queues untouched and degrades to the old stalled-resend behaviour rather than losing a reliable message.Also:
GetMTUSize()stays truthful —RakPeersyncsremoteSystem->MTUSizefrom the layer after each update — and theMTUSize.hrationale comment no longer claims nothing detects black holes after the handshake.Why the cap stays at 1400
Detection costs several retransmission timeouts, so the conservative top rung from #55 remains the first line of defence. Arithmetic from OpenVPN's docs and #823: its UDP encapsulation overhead is ~44–52 bytes, so our 1400-byte datagrams fit a default OpenVPN tunnel on a 1500 (or PPPoE 1492) path. This PR covers what the cap can't: tls-crypt-v2/TAP/double-hop/mobile paths whose ceiling sits below ~1450, return-path-only holes, and paths that shrink mid-session (a tunnel reconnecting through a different exit).
Testing
TDD throughout — every behaviour below was a failing test before it was code.
Tests/Unit/MtuBlackHoleTests.cpp— 10 hermetic cases over the decision logic: ladder walking, the resend-budget threshold, the fits-next-rung false-positive guard, the bottom-rung stop, and a pin that the ladder top equalsMAXIMUM_MTU_SIZE.Tests/Unit/ReliabilityLayerBlackHoleTests.cpp— a hermetic harness: two realReliabilityLayerinstances joined by a fakeRakNetSocket2, fully simulated time (no loopback, no wall clock), with a per-test drop rule modelling the tunnel. Six cases:ReliableOrderedWithAckReceiptmessage still arrives with its serial;Full suite: macOS (153 unit / 47+2 integration) and Linux in Docker, Debug and Release, all green; new tests repeated 10× clean. One detour:
PingTests.PingStatisticsAndOccasionalPingfailed in the container — an interleaved A/B against master showed master fails it too (a one-sample ping average sitting exactly at its 10ms localhost threshold; CI'suntil-pass:3absorbs it). Not from this change.Not in this PR
RAKNET_SUPPORT_IPV6still 0) — an IPv6-only tunnel still can't connect at all; different failure, same "works without the VPN" bug report.Summary by CodeRabbit
New Features
Bug Fixes
Tests