openttd-client/docs/PROTOCOL.md

Protocol Internals

This client supports the modern OpenTTD Game Port protocol (TCP 3979), specifically as implemented in JGRPP.

X25519 PAKE Authentication

OpenTTD 14+ and JGRPP use a Password-Authenticated Key Exchange to prevent plaintext password leakage.

Key Derivation (KDF)

We use Blake2b (64-byte digest) to derive two 32-byte session keys.

• Input: SharedSecret (32) + ServerPublicKey (32) + ClientPublicKey (32) + Password (string)
• Output:
  • 0..31: Client-to-Server Key
  • 32..63: Server-to-Client Key

Handshake Nonces

The server provides a 24-byte nonce in the ServerAuthenticationRequest. This nonce is used for the AEAD challenge during the auth response and for the initial stream encryption setup.

Admin Network (TCP 3977)

The Admin Network allows external applications to monitor and control the server. It supports both unsecured and secure (X25519 PAKE) authentication.

Secure Authentication

Similar to the Game Port, the Admin Network uses X25519 PAKE for secure authentication.

• Packet: AdminJoinSecure starts the handshake.
• Encryption: Once enabled via ServerEnableEncryption, all subsequent traffic is encrypted using XChaCha20-Poly1305.

Update Frequencies

Admins can subscribe to various updates (Date, Client Info, Company Info, etc.) at different frequencies (Poll, Daily, Weekly, Monthly, Quarterly, Annually, Automatic).

Vehicle Listing

The Admin Network has no native packet or AdminUpdateType for listing individual vehicles — ServerCompanyStats only reports aggregate per-company vehicle counts (trains/lorries/buses/planes/ships). To retrieve an actual vehicle list, this client sends a list_vehicles command over the GameScript JSON channel (AdminGamescript/ServerGamescript) via list_vehicles(). This requires a companion GameScript running server-side that understands the list_vehicles command and replies with vehicle data through ServerGamescript.

Important: the server only forwards ServerGamescript packets to admins that have subscribed with update_frequency(AdminUpdateType.Gamescript, AdminUpdateFrequency.Automatic) (enforced server-side in NetworkAdminGameScript, which checks update_frequency[ADMIN_UPDATE_GAMESCRIPT]). Call update_frequency() for Gamescript before list_vehicles(), or the response is silently dropped.

When a list_vehicles request carries a request_id field, the AdminBridge GameScript echoes it back in the reply (backward compatible: absent otherwise).

Timetable Query

The stock GameScript API has no timetable getters, so this project patches the server (see docker/patches/) to add read-only getters to GSOrder (GetTimetableWaitTime, GetTimetableTravelTime, IsWaitTimetabled, IsTravelTimetabled, IsWaitFixed, IsTravelFixed, GetLeaveType, GetTimetableMaxSpeed, GetTimetableLateness, GetTimetableStartTick, GetCurrentOrderTime, GetTimetableTotalDuration). On top of that, get_timetable() sends a request over the same GameScript JSON channel as vehicle listing and awaits the correlated reply — an authoritative snapshot of the live game state, unlike the passive observer on the game port (see below).

• Request: {"command": "get_timetable", "vehicle_id": N, "request_id": X} — request_id is a client-side monotonic counter used to match the reply to the awaiting caller.
• Reply (success): {"command": "get_timetable", "vehicle_id": N, "request_id": X, "lateness": ..., "start_tick": ..., "current_order_time": ..., "total_duration": ..., "orders": [{"position", "wait_time", "travel_time", "wait_timetabled", "travel_timetabled", "wait_fixed", "travel_fixed", "leave_type", "max_speed"}, ...]} (booleans encoded as 0/1).
• Reply (error): same envelope with an "error" field instead of the data: "invalid_vehicle" (no such vehicle) or "response_too_large" (the reply exceeded the admin packet size limit, possible with very many orders). get_timetable() raises ValueError for these.

Replies carrying a request_id that matches a pending request resolve that request and are not delivered to the on_gamescript callback; all other ServerGamescript traffic reaches the callback unchanged. The same update_frequency subscription requirement applies (get_timetable() subscribes automatically on first use). Since GameScripts do not tick while the game is paused, a query against a paused server times out (asyncio.TimeoutError).

Station Listing

Like vehicles, the Admin Network has no native packet for enumerating individual stations (ServerCompanyStats only reports an aggregate per-company station count). list_stations() sends a list_stations command over the same GameScript JSON channel and the companion AdminBridge GameScript replies with station data through ServerGamescript ({"command": "list_stations", "stations": [{"id", "name", ...}, ...]}). It is fire-and-forget, so the reply is delivered to the on_gamescript callback — subscribe to Gamescript updates first, exactly as for list_vehicles(). An optional company_id field scopes the list to one company.

Station Query

get_station() fetches an authoritative snapshot of one station's live cargo state over the same GameScript JSON channel, awaiting the correlated reply — the station analogue of get_timetable(). Unlike timetables, the getters it relies on (GSStation.GetCargoWaiting, GetCargoPlanned, GetCargoRating) are part of the stock GameScript API, so this needs no server patch. The per-cargo reply exposes both the real-time amount currently waiting and the planned amount routed through the station by the cargodist link graph.

• Request: {"command": "get_station", "station_id": N, "request_id": X} — request_id is the same client-side monotonic counter used by get_timetable(), matching the reply to the awaiting caller.
• Reply (success): {"command": "get_station", "station_id": N, "request_id": X, "name": ..., "location": <tile>, "owner": <company_id>, "cargo": [{"cargo_id", "waiting", "planned", "rating"}, ...]} — waiting is the real-time units at the station (GetCargoWaiting), planned is the link-graph planned flow (GetCargoPlanned, 0 when cargo distribution is off for that cargo), and rating is the acceptance rating as a percentage (0-100, GetCargoRating) or null when the station has no rating for that cargo yet. Only cargo the station has handled appears.
• Reply (error): same envelope with an "error" field instead of the data: "invalid_station" (no such station) or "response_too_large". get_station() raises ValueError for these.

Correlation, the update_frequency subscription requirement (auto-subscribed on first use), and the paused-game timeout behave exactly as described for the Timetable Query above.

Station Cargo Flow Breakdown

get_station_cargo() drills into a single cargo type at one station and returns how its waiting (real-time) and planned amounts split across the cargo distribution (cargodist) link graph. Cargodist tags every unit with a source station (from, where it was first loaded) and a next hop (via, the next station it travels to toward its final destination). There is no per-station store of the final destination — the routing destination is the next hop — so the breakdown is offered along those two axes. The GS reads them with the stock GSStation.GetCargoWaiting{From,Via,FromVia} / GetCargoPlanned{From,Via,FromVia} scalars and the GSStationList_Cargo{Waiting,Planned}By{From,Via} (and …ViaByFrom / …FromByVia) list classes — again no server patch.

• Request: {"command": "get_station_cargo", "station_id": N, "cargo_id": C, "request_id": X}, optionally with "from_station" and/or "via_station" filters.
• Reply (success): {"command": "get_station_cargo", "station_id": N, "cargo_id": C, "request_id": X, "waiting": ..., "planned": ..., "waiting_by_from": [{"station", "amount"}, ...], "planned_by_from": [...], "waiting_by_via": [...], "planned_by_via": [...]}. waiting/planned are the (filtered) totals; each *_by_from list groups by source station and each *_by_via list groups by next hop (zero-amount entries omitted). A station of 65535 (STATION_INVALID) means the source was deleted or — as a next hop — the cargo has no onward routing / is consumed here (also the only next hop for cargo using manual, non-cargodist distribution). Any supplied from_station/via_station filter is echoed back.
• Filters: via_station restricts the query (and the *_by_from breakdowns) to cargo whose next hop is that station; from_station restricts it (and the *_by_via breakdowns) to cargo from that source; supplying both makes waiting/planned the exact source-and-next-hop amount. Pass 65535 to target STATION_INVALID.
• Reply (error): same envelope with an "error" field: "invalid_station", "invalid_cargo", "invalid_from_station"/"invalid_via_station" (a filter that is neither a valid station nor STATION_INVALID), or "response_too_large". get_station_cargo() raises ValueError for these.

Vehicle Timetables (Game Port DoCommands)

Unlike vehicle listing, timetables have no stock GameScript API surface (this project adds read-only getters via a server patch — see "Timetable Query" above; writing still has none). Reading and modifying them requires real engine commands (DoCommands) sent over the game port (TCP 3979) via ClientCommand/ServerCommand packets, not the Admin Network. This section covers the wire format; for how to call the methods and what each parameter means, see the Vehicle Timetables Usage Guide.

Command envelope

Both ClientCommand and ServerCommand share this body: company (uint8), cmd (uint16 LE, index into the Commands enum), error_msg (uint16 LE, StringID, use 0), tile (uint32 LE, always 0 for these commands), payload_len (uint16 LE), payload (payload_len bytes), callback (uint8, use 0), callback_param (uint32 LE, only present if callback != 0). ServerCommand additionally appends frame (uint32 LE) and my_cmd (uint8 bool), and is a broadcast echo of the request (no success/failure code) sent to every joined client, not just the sender.

Timetable command IDs and payload tuples

Method	cmd	payload
change_timetable()	174 (ChangeTimetable)	VehicleID (varuint), VehicleOrderID (uint16), ModifyTimetableFlags (uint8), value (varuint), ModifyTimetableCtrlFlags (uint8)
set_vehicle_on_time()	176 (SetVehicleOnTime)	VehicleID (varuint), apply_to_group (uint8 bool)
autofill_timetable()	177 (AutofillTimetable)	VehicleID (varuint), bool (uint8), bool (uint8)
set_timetable_start()	180 (SetTimetableStart)	VehicleID (varuint), bool (uint8), StateTicks (signed varuint)

VehicleID and other 4/8-byte fields use OpenTTD's custom varuint scheme (write_varuint/read_varuint in protocol.py) — a UTF-8-like prefix encoding, not LEB128; signed fields (StateTicks) use zigzag on top of it (write_varuint_signed/read_varuint_signed).

Ownership requirement

A command is rejected (and the client kicked) unless it's issued by the company that owns the target vehicle — join that company via join_company() with a real company id (not 255/spectator) before calling any timetable method.

Reading timetables — no query command exists on the game port

There is no getter DoCommand for orders/timetables anywhere in the protocol. get_vehicle_timetable() works by passively decoding ServerCommand broadcasts (including the sender's own) as they arrive — it only reflects changes made after the client joined. A vehicle's pre-existing timetable (set before this client connected) is invisible until something changes it again; seeing it upfront would require parsing the ORDR/VEHS chunks of the initial savegame transfer (ServerMapData), which this client does not implement. For an authoritative read, use the Admin Network's get_timetable() instead (see "Timetable Query" above).

Stream Encryption (AEAD)

Once ServerEnableEncryption is received, all subsequent packets use XChaCha20-Poly1305 (Authenticated Encryption with Associated Data).

Encrypted Packet Format

On the wire, encrypted packets have the following structure:

1. Length (2 bytes): Big-endian uint16 of the entire remaining packet.
2. MAC (16 bytes): The Poly1305 authentication tag.
3. Ciphertext (variable): The encrypted payload.

Decryption Logic

The OpenTTDProtocol layer uses an IncrementalAuthenticatedEncryption state from the Monocypher library. It maintains the nonce state internally. If a MAC check fails (indicating corruption or a wrong key), the client immediately closes the connection (SocketClosed).

Keep-Alive (Simulation Synchronization)

OpenTTD is a lockstep simulation. The server sends ServerFrame packets periodically.

• Client Requirement: You must respond with a ClientAck containing the frame number and a one-time token provided in the frame packet.
• Timeout: If the server does not receive an ACK for several in-game days, it will disconnect the client with error code 17 (TimeoutComputer).