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# NGO 1.x over LiteNetLib — wire notes for the voice relay
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Derived entirely from public MIT sources: `Unity-Technologies/com.unity.netcode.gameobjects`
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(tag `ngo/1.14.1`, cross-checked against `ngo/1.6.0`) and
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`Unity-Technologies/multiplayer-community-contributions`
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(`Transports/com.community.netcode.transport.litenetlib`). No compatibility logic here is
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taken from a decompiled client; the client-side observations are listed separately as
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open questions to be settled by black-box capture.
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## 1. Transport layer
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LiteNetLib, **ProtocolId 13** — the `1.x` line. This is confirmed directly from a live
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capture: the client's `ConnectRequest` carries `protocolId = 13` at bytes `[1..4]`.
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Note this is **not** the LiteNetLib copy vendored inside the community transport package,
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which is still on the `0.9.5.x` / ProtocolId 11 line. A server built against the package's
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bundled sources will have every connect dropped in `NetManager` before any callback fires.
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Build against LiteNetLib 1.x.
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`ConnectRequest` layout (LNL 1.x, `NetConnectRequestPacket`, header 18 bytes):
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```
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[0] property (ConnectRequest = 5) | connectionNumber << 5
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[1..4] protocolId = 13
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[5..12] connectionTime (long)
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[13..16] peerId (int)
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[17] addrSize (16 for IPv4, 28 for IPv6)
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[18..] target SocketAddress, then connect data
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```
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Community transport behaviour the server must mirror:
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| aspect | value |
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|---|---|
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| connect payload | **3 bytes, observed `01 00 11`** — see below |
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| connection requests | accepted unconditionally (`request.Accept()`) |
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| clientId | `peer.Id`, `+1` on the server |
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| server clientId | `0` |
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| delivery mapping | Unreliable→`Unreliable`, …Sequenced→`Sequenced`, Reliable*→`ReliableOrdered` |
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| channel | always 0; LNL encodes `channel*4 + deliveryMethod` in its own header |
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The observed connect payload does **not** parse as LiteNetLib's `Put(string)` — an empty key
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writes `00 00`, and reading `01 00` as a length prefix (`size + 1 == 1`) leaves the trailing
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`11` unaccounted for. So the client is not using `Connect(address, port, key)` with a plain
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string; it passes a custom 3-byte blob. Because the transport accepts connection requests
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unconditionally, our server does not need to interpret it — but it must not reject on it.
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## 2. Batch framing
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Every datagram payload is one batch:
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```
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offset 0 ushort Magic = 0x1160
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offset 2 ushort BatchCount
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offset 4 int BatchSize (== padded total length)
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offset 8 ulong BatchHash (XXHash64, seed 0)
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offset 16 ... messages
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pad to a multiple of 8 bytes
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```
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`BatchHash` covers bytes `[16 .. BatchSize)` — i.e. **including the tail padding**.
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The receiver hashes `receivedLength - 16`, so sender and receiver agree only if the
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datagram arrives byte-exact.
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## 3. Message framing
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```
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bitpacked uint MessageType (index into the negotiated id space)
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bitpacked uint MessageSize (payload bytes, excluding this header)
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byte[] payload
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```
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### Bit-packed integers
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32-bit: low 3 bits of the first byte hold the byte count `n`.
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If the value needs ≥30 bits, the first byte is literal `5` followed by a raw LE `uint32`.
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Otherwise `v <<= 3`, `n = usedByteCount(v)`, write `v | n` in `n` LE bytes.
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64-bit: identical with 4 bits and a `9` escape; `v <<= 4`.
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Signed values are zig-zag encoded first (`(v << 1) ^ (v >> 31)`).
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Arrays written with `WriteValueSafe` are a **raw LE `int` length** followed by raw elements —
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not bit-packed.
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## 4. The id space (the important part)
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Message ids are **not** a fixed table. Each side builds a local list of its message types,
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sorted ordinal by `Type.FullName`, then moves `ConnectionApprovedMessage` and
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`ConnectionRequestMessage` to the front (`PrioritizeMessageOrder`). Because
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`…ConnectionApprovedMessage` sorts before `…ConnectionRequestMessage`:
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> **Index 0 is always ConnectionApproved and index 1 always ConnectionRequest**, for any
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> build, no matter which other messages were trimmed out.
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That is what lets the server parse the client's very first message without knowing anything
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about the client's message set.
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Then `ConnectionApprovedMessage` carries the server's hash list, and the client calls
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`SetServerMessageOrder` with it — **the server's ordering becomes the id space for the rest
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of the session**. The server therefore *chooses* the numbering and never has to discover
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Rec Room's internal ordering.
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Message identity is `XXHash32(Type.FullName)` over UTF-8, seed 0:
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| index | hash | type |
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|---|---|---|
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| 0 | `0xa5e29a7d` | `Unity.Netcode.ConnectionApprovedMessage` |
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| 1 | `0xb31736b2` | `Unity.Netcode.ConnectionRequestMessage` |
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| 2 | `0xb81cf6d3` | `Unity.Netcode.DisconnectReasonMessage` |
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| 3 | `0x978922a6` | `Unity.Netcode.NamedMessage` |
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| 4 | `0x179751e9` | `Unity.Netcode.ServerLogMessage` |
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| 5 | `0xc6887f42` | `Unity.Netcode.TimeSyncMessage` |
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| 6 | `0x46f5a397` | `Unity.Netcode.UnnamedMessage` |
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## 5. Handshake
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**Client → server, `ConnectionRequestMessage`** (id 1, version 0 in every 1.x release):
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```
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bitpacked int count
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count × { raw uint Hash ; bitpacked int Version }
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raw ulong ConfigHash
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[ byte[] ConnectionData ] only when the server has ConnectionApproval enabled
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```
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`ConfigHash` is `XXHash64` over, in order: `ProtocolVersion` (ushort, game-set),
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the literal `NetworkConstants.PROTOCOL_VERSION`, sorted prefab hashes (only when
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`ForceSamePrefabs`), `TickRate` (uint), then bools `ConnectionApproval`,
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`ForceSamePrefabs`, `EnableSceneManagement`, `EnsureNetworkVariableLengthSafety`, and
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`RpcHashSize`. A mismatch disconnects the client.
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`PROTOCOL_VERSION` is `"15.0.0"` across **all** of NGO 1.6 → 2.0, so the config hash does
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*not* identify the NGO version. It only constrains the game's own `ProtocolVersion`,
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tick rate and flags — a small enough tuple to brute-force from one captured hash.
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**Server → client, `ConnectionApprovedMessage`** (id 0):
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```
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bitpacked int count ← "forbidden segment", must come first
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count × { raw uint Hash ; bitpacked int Version }
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bitpacked ulong OwnerClientId
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bitpacked int NetworkTick
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raw int len + len × raw ulong ← ConnectedClientIds, only if targetVersion >= 1
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raw uint sceneObjectCount ← always 0 for us
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```
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Version discipline: serialize using the version *the client advertised* for that message.
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`ConnectionApprovedMessage.Version` is `0` in NGO 1.6–1.7 and `1`
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(`k_VersionAddClientIds`) from 1.8.0 onward — implement both branches and switch on the
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negotiated value rather than pinning a release. Every other message in our set is
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version `0` across the whole 1.x line.
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## 6. Session messages
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`TimeSyncMessage` — `bitpacked int Tick`, sent unreliable at tick rate.
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`NamedMessage` — `raw ulong Hash` then **opaque bytes to the end of the message**.
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Dissonance's own protocol rides inside those bytes; a proximity relay forwards them
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without parsing. `UnnamedMessage` is the same minus the leading hash.
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## 7c. Forked NGO batch format (from a decoded ConnectionRequest)
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The client's NGO batching is **not stock**. Decoded from a real 2041-byte ConnectionRequest
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(`docs/client-agent-request-3.md` has the capture). Layout:
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```
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[0:2] magic u16 = 0x1160
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[2:4] count u16 = 0 (stock BatchCount field, unused/zero)
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[4:8] size u32 = total datagram length
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[8:16] hash u64 = XXHash64 over [24:end] <-- NOTE: from 24, not 16
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[16:24] u64 = 1 (fork field; == message count in this sample)
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[24:] message stream
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```
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Message: `bitpacked type` (1 = ConnectionRequest in the client's local index),
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`bitpacked size`, then body. The hash covers the message stream `[24:end]` exactly.
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ConnectionRequest body (`[27:end]`, size 2014) vs stock:
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```
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0x81 <- 1 byte where stock writes bitpacked version-count (7 -> 0x39)
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7 × { u32 hash ; 1 byte version=0 } version table (our 7 message hashes, all v0)
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00 00 00 00 01 <- 5 bytes not present in stock, before ConfigHash
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u64 ConfigHash
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u32 ConnData length (1961)
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ConnData = Tachyon JSON (AI/AT/VB/CKA/CIA/CPK)
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```
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The `0x81` preamble and the 5-byte field are in the shared "version header" (forbidden
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segment), so ConnectionApproved must reproduce them. Their meaning, and the exact
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ConnectionApproved serialization, are the last unknowns — see `docs/client-agent-request-3.md`.
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## 7b. Live progress (real client)
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- **LiteNetLib handshake: SOLVED against the real client.** With the trailing-marker layer,
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the client reaches `Connected` and holds it — Ping/Pong and MTU discovery both flow, no
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timeout. The client only disconnects (`RemoteConnectionClose`) after ~10s of no NGO approval.
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- **NGO ConnectionRequest: reached.** The client sends a fragmented Channeled batch (3 LNL
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fragments, ~2041 bytes reassembled) carrying the Tachyon handshake JSON (base64 `CKA`/`CIA`/
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`CPK`, ends `=="}`).
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- **Open anomaly:** that batch parses as `magic=0x1160, count=0, size=2041`, and the batch hash
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does not match a stock XXHash64 over `[16..len]`. A count of 0 with a full message payload
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means the batch header layout and/or hash range differs from stock NGO 1.6–1.14, OR the fork
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frames the connection payload non-standardly. This must be resolved before ConnectionApproved,
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because a wrong batch header means the client rejects our approval too. Use `--dump-batches`
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to capture the full bytes for offline decode.
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## 7. State of the evidence
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Confirmed from the capture in `traffic.txt` (decode with `tools/decode_lnl.py`):
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- ProtocolId 13 → LiteNetLib 1.x.
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- A 3-byte connect payload `01 00 11`, not the stock empty key.
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- Target `66.228.47.217:4499`.
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- MTU probing at 1024, consistent with the LNL 1.x `PossibleMtu` table.
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The capture's failure mode is unambiguous and is **entirely below NGO**: 11 `ConnectRequest`s
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all carrying the *same* `connectionTime` (one connect attempt, resent), each answered by a
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`ConnectAccept`, pings going out with **no Pong and no MtuOk in reply**, and the ping
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sequence resetting 1‑4 → 1‑4 → 1‑4 as the peer times out at `DisconnectTimeout` (~5s) and
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retries. **Not one Channeled or Unreliable data packet appears**, so nothing in sections 2–6
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has yet been exercised against the real client.
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Whatever answered that capture accepts the connection and then stops speaking LiteNetLib.
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Using a real LiteNetLib library (LiteNetLibPP) in the server — rather than hand-rolling the handshake —
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makes Ping/Pong, MTU discovery and keepalive correct for free, which is exactly the failure
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that capture shows.
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Still open:
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1. **`ConfigHash` inputs.** Capture one `ConnectionRequestMessage` and brute-force the tuple
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in §5 to recover `ProtocolVersion`, `TickRate` and the flags exactly. Nothing in the
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current capture reaches this point.
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2. **Client message set and versions.** The advertised hash/version table in that request is
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a direct readout of the client's message set; the server logs it.
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3. **The truncated byte.** Upstream `LiteNetLibTransport.OnNetworkReceive` hands NGO
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`UserDataSize - 1`, with the comment "the last byte sent is used to indicate the channel"
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— but the matching `Send` stopped appending that byte in commit `b32ffdd`. LNL carries the
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channel in its own header, so upstream HEAD drops a real payload byte and the batch hash
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cannot verify. Rec Room's build must diverge somewhere. The server tests both hypotheses
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per packet and reports which one matches.
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4. **What the 3 connect-data bytes mean.** Not blocking, since the server accepts
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unconditionally, but more captures would settle it.
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## 8. Transport-level encryption (Rec Room fork) — CORRECTED
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An earlier revision of this doc argued from wire bytes that there was **no datagram-level
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packet layer**. That was wrong. Client static analysis (`LiteNetLibTransport.Initialize`)
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shows a `RecRoomClientEncryptLayer` is **always** installed, with
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`ExtraPacketSizeForLayer = 9`. It adds 9 bytes to every outbound datagram and strips 9 from
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every inbound one, **below** LiteNetLib — so it wraps the handshake, not just NGO payloads.
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The clean-looking captures were app-level packet logs taken below the layer, not raw wire.
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### Why this is the handshake failure
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- Client strips 9 bytes off every inbound datagram before LiteNetLib parses it.
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- A stock 15-byte ConnectAccept becomes 6 bytes → `NetConnectAcceptPacket.FromData` fails
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`Size != 15` → returns null → **silent ignore**. The client stays in `Outgoing`, resends
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ConnectRequest with the same `connectionTime`, and times out at ~5s. Exactly the observed loop.
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- A stock server, lacking the layer, still *accepts* the client's over-long ConnectRequest
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because the 9 extra bytes fold into variable-length connect data — which is why the server
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saw "peer connected" while the client never did.
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- Fix magnitude: a correct ConnectAccept is `15 + 9 = 24` bytes on the wire.
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Verified in the server: a `PacketLayerBase` with `ExtraPacketSizeForLayer = 9` on both ends
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completes the handshake; on the server only (stock client) it fails identically to the game.
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See `src/main.cpp` (marker lambdas + `patches/`) and `--encrypt-layer`.
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### The format is a trailing marker byte — SOLVED
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Read from the image (see `docs/tachyon-findings.md`). The layer is variable-length; the marker
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is the **last** byte of every datagram. Key-null (the whole handshake): outbound appends one
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byte `0x11`; inbound strips it. Key-set: append 8-byte nonce + marker `0x2A` and encrypt.
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Any other trailing marker is dropped. So a ConnectAccept must be `15 + 1 = 16` bytes on the
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wire with a trailing `0x11`. Implemented in `src/main.cpp` (marker lambdas + `patches/`)
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(`--encrypt-layer`) and verified end-to-end.
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### Payload encryption (post-handshake)
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Once `RecRoom_SetEncryptionInfo(clientId, key)` runs, the same layer encrypts NGO payloads
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with a split-key AES scheme: client contributes `CKA`/`CIA`, server contributes `SessionKeyB`
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(RSA-encrypted to the client's `CPK`), combined in a function at `0x8277880` (not yet read).
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The handshake to establish that key is JSON in the NGO connection payload — the Tachyon
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handshake, see `docs/tachyon-findings.md`.
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**Key material is deliberately not committed.** It is supplied at runtime.
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