DarkflameServer/docs/PacketArchitecture.md

Packet and Game Message Architecture

This document describes how DLU reads and writes network packets and game messages, the struct-based architecture everything is being moved to, and the plan for getting there.

Hard rule: the wire format never changes as part of a conversion. The LEGO Universe 1.10.64 client depends on every message ID, enum value, field order, field width and bit of padding. A conversion may only change how the server produces the bytes, never the bytes. Every conversion proves this with byte-equality tests against the old code before the old code is deleted (see Verification). Where DLU's bytes are known to be wrong, the fix is its own clearly labelled change, verified against the client, never part of a conversion.

Sources for layouts, in order of trust: the 1.10.64 client in Ghidra, then lu_packets, then lcdr-utils packet definitions. When they disagree, the client wins.

1. Where we are

1.1 Final state

Every packet and every game message is a struct. The migration in section 4 is complete.

Family	Base	Where	Dispatch
Game messages (wire)	GameMessages::NetGameMsg	17 per-domain files dGame/dGameMessages/<Domain>Messages.{h,cpp}: Activity, Building, Combat, Effects, Inventory, Mission, Movement, Object, Pet, Player, Property, QuickBuild, Racing, Skill, Trade, Vendor, Zone (about 280 structs)	g_MessageHandlers in GameMessageHandler.cpp (118 inbound messages); no switch. Unknown IDs are logged at debug level, messages that fail Deserialize are logged and dropped.
Game messages (internal)	GameMessages::GameMsg, NetGameMsgEvent<Msg>	GameMessages.h (base types and the server-internal events only)	Entity::RegisterMsg / HandleMsg
Packets	LUBitStream	one file pair per ServiceType in dNet/ (CommonPackets, AuthPackets, ChatPackets, WorldPackets, ClientPackets, MasterPackets + master/*), Stamps, WorldRoutePacket, Mail in dGame/dUtilities/Mail.*	per-service maps and PacketDispatcher

GameMessages.cpp holds only the base code (GameMsg::Send, NetGameMsg::WritePacket/ReadPacketHeader/Send/ SendToClient/BroadcastExcept). There are no GameMessages::Send*/Handle* functions left. Inbound messages are handled by the struct's Handle, which delegates to a component method where the behaviour belongs to one component (for example PossessorComponent::OnDismountComplete, InventoryComponent::On*) or keeps logic that spans several entities (platform resyncs, rails, activities) in the handler.

Removed: the CBITSTREAM, CMSGHEADER, CINSTREAM, CINSTREAM_SKIP_HEADER, SEND_PACKET, SEND_PACKET_BROADCAST and HEADER_SIZE macros, the free BitStreamUtils::WriteHeader (use LUBitStream::WriteHeader) and PacketUtils. The frozen oracles in tests/**/Legacy/ still use the macros verbatim through the test-only tests/dGameTests/LegacyPacketMacros.h.

ChatPackets::SendSystemMessage stays as a thin helper: it builds the general chat struct and sends it.

1.2 What still touches raw bytes, and why

Where	What	Why it stays
AuthServer, ChatServer, MasterServer, WorldServer, dServer HandlePacket	packet->data[0] compared with RakNet IDs (ID_USER_PACKET_ENUM, ID_DISCONNECTION_NOTIFICATION, ID_CONNECTION_LOST, ID_NEW_INCOMING_CONNECTION, ...)	RakNet's own connection messages, not LU packets. Everything after the RakNet ID is read with LUBitStream::ReadHeader and a struct.
EntityManager	ID_REPLICA_MANAGER_CONSTRUCTION/SERIALIZE/DESTRUCTION headers written before the components	Replica serialization, out of scope (see below).
dGame/dBehaviors/*, the sBitStream of skill messages	Behavior bit streams	The skill payload is its own format, carried as bytes inside the skill structs.
MessageInspector	Copies the payload bytes of sent/received game messages	A capture tap; the header is read with NetGameMsg::ReadPacketHeader.
PacketCapture, RakPeer::Send hook	Copies whole packets for packet captures (CaptureReplay.md)	A capture tap; reads only the 8 byte LU header, and rewrites packets with secrets through their structs (PacketDecoder::Redact).

Out of scope: replica/component serialization (Component::Serialize) and LDF/AMF, which are separate formats with their own tests.

Before the migration (survey of origin/main @ 129199e4) there were 161 hand written GameMessages::Send* functions, 101 Handle* functions behind a 112-case switch, about 120 hand written packet functions across the servers, 90 raw header writes and 58 CINSTREAM / 29 packet->data[i] reads.

1.3 Inconsistencies inside the new style, and how they are settled

#	Inconsistency	Decision
1	Three packet base designs in flight: LUBitStream on main, the unmerged auth-packet-re-write redesign (per-service bases), the old local packet-refactor branch (dNet/packets/*).	LUBitStream as on main. The other branches are not used.
2	GameMessageHandler ignored the result of Deserialize.	Drop and log messages that fail to deserialize (PR 0).
3	The same GameMsg type was used for wire messages and server-internal events; msg.Send() and msg.Send(sysAddr) went to different places.	Split into GameMsg (internal) and NetGameMsg (wire); mixing them is a compile error (PR 0).
4	Registration is ad hoc: one GM map + switch, a Mail map, switches or if-chains in every server.	One map per message family (see 2.1, 2.2).
5	Struct locations: 34 structs in the 1000-line GameMessages.h, 6 one-off headers, Mail in Mail.h, chat structs partly outside any namespace.	Per-domain files (see 2.3).
6	Naming: some fields use the client's names, some don't; stream parameters are stream/bitStream/bitstream/inStream; some overrides lack override.	Client names, bitStream, always override (new code; old structs as they are touched).
7	Optional ("default flag") fields written by hand; this produced a wire bug in UnSmash (flag checks duration, value guarded by builderID != 3.0f).	BitStreamUtils::WriteOptional/ReadOptional (PR 0). UnSmash fixed in its own wire-change commit.
8	Length-prefixed strings written with per-character loops.	BitStreamUtils::WriteLengthPrefixed/ReadLengthPrefixed (PR 0).
9	73 Send* functions broadcast and then call Send(UNASSIGNED, broadcast = false), which RakNet rejects (harmless).	Goes away with NetGameMsg::Send.
10	dServerMock::Send kept a pointer to the caller's stack BitStream; LUBitStream had virtual functions but no virtual destructor while dispatchers delete through the base.	Fixed in PR 0.

2. Target architecture

2.1 Game messages

There are two kinds of game message and they are different types:

• GameMessages::NetGameMsg - goes on the wire. Send(sysAddr) (UNASSIGNED broadcasts), SendToClient(sysAddr) (one client only, never broadcasts; used where the old function only did SEND_PACKET), WritePacket, Serialize, Deserialize, Handle(Entity&, sysAddr), requiredGmLevel.
• GameMessages::GameMsg - a server-internal event. Send() / Send(target) deliver it to the handlers entities, components and scripts registered with RegisterMsg. It cannot reach a client.
• GameMessages::NetGameMsgEvent<Msg> (aliases such as RequestUseEvent, DropClientLootEvent) wraps a copy of a wire message so local handlers can react to it. GameMessages::DeliverLocally(msg) sends one to msg.target.

Every wire message is a NetGameMsg subclass in a per-domain file dGame/dGameMessages/<Domain>Messages.{h,cpp} (pilot: ActivityMessages). Fields use the client's names and are declared in wire order. Every wire message implements both Serialize and Deserialize, even if DLU only uses one direction, so it can be round-trip tested.

// dGame/dGameMessages/ActivityMessages.h
namespace GameMessages {
	// Server -> client.
	struct ShowActivityCountdown : public NetGameMsg {
		ShowActivityCountdown() : NetGameMsg(MessageType::Game::SHOW_ACTIVITY_COUNTDOWN) {}
		void Serialize(RakNet::BitStream& bitStream) const override;
		bool Deserialize(RakNet::BitStream& bitStream) override;

		bool bPlayAdditionalSound{};
		bool bPlayCountdownSound{};
		std::u16string sndName{};
		int32_t stateToPlaySoundOn{};
	};

	// Client -> server.
	struct RequestActivityExit : public NetGameMsg {
		RequestActivityExit() : NetGameMsg(MessageType::Game::REQUEST_ACTIVITY_EXIT) {}
		void Serialize(RakNet::BitStream& bitStream) const override;
		bool Deserialize(RakNet::BitStream& bitStream) override;
		void Handle(Entity& entity, const SystemAddress& sysAddr) override;

		bool bUserCancel{};
		LWOOBJID userID{};
	};
}

// dGame/dGameMessages/ActivityMessages.cpp
void ShowActivityCountdown::Serialize(RakNet::BitStream& bitStream) const {
	bitStream.Write(bPlayAdditionalSound);
	bitStream.Write(bPlayCountdownSound);
	BitStreamUtils::WriteLengthPrefixed<uint32_t>(bitStream, sndName);
	bitStream.Write(stateToPlaySoundOn);
}

bool ShowActivityCountdown::Deserialize(RakNet::BitStream& bitStream) {
	VALIDATE_READ(bitStream.Read(bPlayAdditionalSound));
	VALIDATE_READ(bitStream.Read(bPlayCountdownSound));
	VALIDATE_READ(BitStreamUtils::ReadLengthPrefixed<uint32_t>(bitStream, sndName));
	VALIDATE_READ(bitStream.Read(stateToPlaySoundOn));
	return true;
}

Sending replaces the Send* free function:

GameMessages::ShowActivityCountdown countdown;
countdown.target = self->GetObjectID();
countdown.Send(sender->GetSystemAddress()); // UNASSIGNED_SYSTEM_ADDRESS broadcasts

Inbound messages are registered with one line in GameMessageHandler.cpp; the GM-level gate comes from the struct, and a message whose Deserialize fails is logged and dropped before Handle:

std::map<MessageType::Game, MessageCreator> g_MessageHandlers = {
	...
	{ REQUEST_ACTIVITY_EXIT, []() { return std::make_unique<RequestActivityExit>(); } },
};

When every case is converted the switch is deleted and GameMessages.cpp goes away.

2.2 Packets (LUBitStream, as on main)

Every non-game-message packet is an LUBitStream subclass. The base owns the full header:

• LUBitStream(ServiceType, packetId) - WriteHeader writes 0x53, the ServiceType (u16), the packet id (u32) and one pad byte; ReadHeader reads and checks the same.
• WritePacket(bitStream) = WriteHeader + Serialize; Send(sysAddr) sends that (UNASSIGNED broadcasts via Broadcast()). Tests use WritePacket.
• Leaf structs only (de)serialize their own fields. A feature with its own sub-header (Mail's eMessageID) has an intermediate base whose Serialize/Deserialize handle the sub-header, and leaf structs call it first (MailLUBitStream is the model).
• Handle() takes no arguments; the dispatcher sets context members (sysAddr, and per-service extras such as Mail's player) before calling Deserialize and Handle.
• Each service has one dispatch function with a std::map<MessageType::X, factory>, exactly like Mail::HandleMail: read the id, create, set context, Deserialize (log and drop on failure), Handle. Each server's HandlePacket only routes by ServiceType.
• One file pair per ServiceType; a packet goes in the file of the ServiceType written in its header, in that namespace: CommonPackets (COMMON), AuthPackets (AUTH), ChatPackets (CHAT), WorldPackets (WORLD: what a client sends to a world server), ClientPackets (CLIENT: what any server sends to the client), MasterPackets (MASTER), all in dNet/. So the auth server's login response is in ClientPackets, and a world server's message to chat is in ChatPackets. Anything used by several services (for example Stamps, which rides in auth, master and client packets, or generic helpers) gets its own small file named for what it is (dNet/Stamps.h), not one service's file. A packet found in the wrong file is moved (a pure move, same bytes). Mail stays in dGame/dUtilities/Mail.*. Handlers that need game or server state stay in the server that owns that state (dChatServer, dMasterServer, dWorldServer, dAuthServer) and are attached to the struct by overriding Handle in that server's translation unit.
• Fixed-width strings use LUString/LUWString with the width spelled out (LUWString password(41)).
• PacketDispatcher<MessageType::X> (dNet/PacketDispatcher.h) is the dispatch map for servers where the same struct is handled differently depending on who receives it (chat, master, the dashboard): each entry names the struct and a handler function (const Msg&, const SystemAddress&). Dispatch(packet, service) reads the header, reads the struct and drops (and logs) packets that fail to Deserialize.
• Structs go in the file of the ServiceType in their header (ChatPackets for CHAT, ClientPackets for CLIENT, MasterPackets for MASTER, ...), whoever sends them. What chat sends a client in the CLIENT service (friends, ignore list and team responses, and the team game messages it writes as ClientPackets::TeamGameMsg, since chat doesn't link dGame) is in ClientPackets; chat-service packets the client receives are in ChatPackets::Client.
• Chat -> client packets are wrapped in ChatPackets::WorldRoutePacket (dNet/WorldRoutePacket.h: target object ID
  • the inner packet of any service); the world passes the inner bytes on unchanged.
• MASTER structs: MasterPackets.h for the core ones (session keys, zone transfer, private zones, player counts, world ready, shutdown, server list); topic groups (dashboard player actions, data changes, positions and announcements, message capture, instance migration) in dNet/master/<Topic>.h, all included by MasterPackets.h.

// dNet/ChatPackets.h
struct AchievementNotify : public LUBitStream {
	LUWString targetPlayerName{};
	uint32_t missionEmailID{};
	LWOOBJID earningPlayerID{};
	LUWString earnerName{};

	AchievementNotify() : LUBitStream(ServiceType::CHAT, MessageType::Chat::ACHIEVEMENT_NOTIFY) {}
	void Serialize(RakNet::BitStream& bitStream) const override;
	bool Deserialize(RakNet::BitStream& bitStream) override;
};

2.3 Conventions

• Wire order = declaration order. Comment the direction (// Server -> client.) and anything odd (padding, unknown fields: write them as named constants, never drop them).
• Explicit widths for everything written: bitStream.Write<uint32_t>(x.size()), never size_t, int, long or float_t. Enums keep their declared underlying type.
• Deserialize returns false on the first failed read (VALIDATE_READ) and bounds-checks lengths (ReadLengthPrefixed rejects negative sizes and anything above MAX_MESSAGE_LENGTH by default).
• Optional/default-flag fields: BitStreamUtils::WriteOptional(bs, value, default) / ReadOptional.
• Length-prefixed strings: BitStreamUtils::WriteLengthPrefixed<LenT> / ReadLengthPrefixed<LenT>.
• No new CBITSTREAM / CMSGHEADER / SEND_PACKET macros or Send* wrapper functions; build the struct at the call site.
• Message IDs are append-only: never renumber, reuse or delete a value. tests/dCommonTests/MessageIdPinTests.cpp and tests/dGameTests/MailIdPinTests.cpp pin every enumerator at compile time.
• Behaviour changes and wire fixes are separate, clearly labelled commits, never part of a conversion.

2.4 Testing helpers

• tests/dGameTests/GameDependencies.h: dServerMock copies every sent packet (GetSentPackets(), each with bytes, exact bit count, address and broadcast flag).
• tests/dGameTests/PacketTestUtils.h: Capture(fn), FromBitStream, FromHex, ToHex, EXPECT_PACKET_EQ (compares bit count and bytes and prints both as hex on failure).
• NetGameMsg::WritePacket / LUBitStream::WritePacket produce exactly what Send puts on the wire.

3. Verification

Every conversion carries these tests, in tests/dGameTests/dGameMessagesTests/<Domain>MessagesTests.cpp (or a dNet test directory for packets):

1. Frozen oracle. Copy the old functions verbatim into tests/.../Legacy/<Domain>Legacy.h (namespace Legacy*; only the namespace changes). A temporary test sends the same inputs through the production function and the oracle and requires identical bytes; it is deleted in the commit that deletes the production code. From then on the oracle pins the old bytes.
2. Byte equality. For a grid of inputs (every bool combination, empty/ASCII/non-ASCII/long strings, 0/negative/max numbers, default and non-default values of every optional field) send through the oracle and through the struct, to one client and as a broadcast. Require identical bits, identical bytes and the same effective destination.
3. Golden bytes. At least one hand-computed hex packet per message family, independent of both implementations.
4. Round trip. Serialize -> Deserialize -> Serialize gives the same bytes and fields and consumes every bit.
5. Inbound. For handlers, compare the struct's Deserialize with the oracle's read sequence, and check a truncated stream returns false.
6. Mutation check while writing the tests: break one field width and confirm the tests fail.
7. Live captures, locally only. Decoding real captured packets with the struct and re-serializing them is a useful extra check, but it is done with a local script outside the repository. No capture-derived bytes are ever committed, not even anonymized.
8. Smoke test the affected feature with the real 1.10.64 client before merging (tests cannot cover handler behaviour).

4. Migration plan

Small, self-contained PRs by subsystem: add structs + oracle + tests, switch callers, delete old code. Each PR is a clean range of commits on the working branch. Target: under ~1.5k changed lines per PR.

#	PR	Contents	Size
0	Foundations	Test infra (dServerMock, PacketTestUtils, WritePacket), string and optional helpers, compile-time pins of every message ID enum, drop GMs that fail to deserialize, LUBitStream virtual destructor and WritePacket, the GameMsg/NetGameMsg split, this document.	S
W1	Wire fix: UnSmash	The one intended wire change: stop writing duration when it has its default, matching the client.	XS
1	Activity GMs	ActivityMessages: 8 outbound + RequestActivityExit inbound.	S
2	GMs: racing and vehicles	SendVehicle*, SendRacing*, SendNotifyRacingClient, HandleRacing*, HandleVehicle*, module assembly	M
3	GMs: effects, audio, animation, UI text	FX, animations, ND audio, 2D ambient, UI messages, message boxes, chat bubbles, billboards, cinematics, tooltips	M
4	GMs: missions, flags, levels	Offer/notify mission(+task), respond, dialog OK, linked missions, flags, collectibles, level rewards	M
5	GMs: inventory and items	Add/remove/move/equip/unequip, inventory size and groups, use/consume item, UpdateInventoryUi	M
6	GMs: vendors, donation, trade	Vendor window/status/transactions, buyback, donation vendor, trade	M
7	GMs: combat and skills	Skill add/remove, stun, buffs, die/resurrect/smash, knockback; the one-off EchoStartSkill/EchoSyncSkill/StartSkill/SyncSkill/projectile classes become NetGameMsgs	M
8	GMs: pets	Taming minigame, pet naming, commands	M
9	GMs: property and building	Property management, models, BBB, modular build, PropertyDataMessage, PropertySelectQueryProperty, ControlBehaviors (may split in two)	L
10	GMs: remaining + switch removal	Movement, teleport, platforms, rails, camera, control scheme, misc, team GMs sent from TeamContainer; delete the switch and GameMessages.cpp (done: Movement, Zone, Player, Object, QuickBuild files; GameMessages.cpp keeps only the base code)	M
11	Common + Auth packets	CommonPackets (version confirm, disconnect notify, general notify; dServer::Disconnect), AuthPackets login request, ClientPackets login response + stamps; AuthServer dispatch map	S
12	World packets	Validation, character list/create/delete/rename, world login, level load complete, position update, string check, general chat, route packet, top-5, funness, and the ClientPackets responses they send; handlers move out of the WorldServer.cpp switch into a dispatch map	M
13	Chat packets	Friends, ignore list, teams, who/show-all, private/general chat, routing (WORLD_ROUTE_PACKET wraps an LUBitStream), achievement notify, GM announce/mute, plus the chat-side of WorldServer's chat cases (may split friends/teams)	L
14	Master packets	Session keys, zone transfer, private zones, player added/removed, world ready, prep zone, shutdown; MasterServer switch becomes a dispatch map; InstanceManager/ZoneInstanceManager senders	M
15	Cleanup	Remove CBITSTREAM/CMSGHEADER/SEND_PACKET*/CINSTREAM*, the free BitStreamUtils::WriteHeader, PacketUtils; update this document (done)	S

Game-message PRs (2-10) are independent of each other and of the packet PRs (11-14), so they can be reviewed in any order after PR 0.

Risks

• Silent wire fixes. A struct written from the client's definition can differ from what DLU has been sending (e.g. UnSmash). The oracle test catches it; the conversion must reproduce the old bytes, and the fix goes in its own commit.
• Implicit widths at old call sites (int, char, size_t, float_t, enum underlying types, 1-bit bools vs uint8_t). The byte-equality grid catches these; that is why the oracle is copied verbatim.
• Default arguments. Old Send* defaults must become the struct's member initializers exactly.
• Handler behaviour. Keep each handler's logic verbatim in the conversion; behaviour changes are separate.
• Merge conflicts with feature work in GameMessages.cpp: small PRs, landed domain by domain.
• Personal data. Captures contain account names, chat and IDs; they stay out of the repository entirely.

5. Known wire discrepancies

Found while converting; none of them is fixed by a conversion (the structs reproduce DLU's bytes, pinned by the oracle tests). Each fix, if wanted, is its own labelled wire change verified against the client. Addresses are in the 1.10.64 client.

Game messages

Message	DLU	Client / reference
PlaceModelResponse	Fixed (wire fix, see docs/BuildWorkflow.md): used to write a 4-byte response where the client expects the rotation.	The client reads a 16-byte w, x, y, z quaternion when the rotation isn't identity (0x00dc0170); a live server echoed the rotation the client placed the model with.
NotifyPetTamingPuzzleSelected	Written as the client's Serialize (0x00db6880) writes it.	The client's own Deserialize (0x00e3a7c0) reads an extra u32 its Serialize never writes.
SetBuildModeConfirmed	Always sends the default flags.	The client has non-default flag fields.
NotifyNotEnoughInvSpace	Fixed (wire fix): used to be sent with message ID VEHICLE_NOTIFY_FINISHED_RACE (1396).	Its ID is NOTIFY_NOT_ENOUGH_INV_SPACE (1516, 0x00545c90); payload read at 0x00d8b850.
MoveInventoryBatch	Now follows the client layout.
UnEquipInventory	The trailing optional replacementObjectID is never read.	The client can send it.
SetStatusImmunity	Fixed (wire fix): used to write the flags in DLU's order (BasicAttack, DOT, Knockback, Interrupt, Speed, ImaginationGain, ImaginationLoss, QuickbuildInterrupt, PullToPoint).	The client uses alphabetical order: BasicAttack, DOT, ImaginationGain, ImaginationLoss, Interrupt, Knockback, PullToPoint, QuickbuildInterrupt, Speed (0x00d8f140; field offsets named by 0x00d8f410).
RequestDie	Read with the Die layout.	Starts with one bDieAccepted bit and has a mandatory lootOwnerID (0x00e02d90).
SetCurrency	Fixed: sourceTradeID is an optional object ID (8 bytes), set for trades.	The client reads an object ID.
FireEventClientSide	Fixed: writes param1/param2. RocketEquipped carries the property clone in param1 and a LUP entrance's world index in param2, and goes to every client (live: 1,196 of 1,265 captured were other players' launches).	Optional i64 param1 (default 0) and i32 param2 (default -1).
PickupCurrency	Fixed: reads the position after the amount (echoed in SetCurrency for the coin animation).	The client always sends it.
MatchUpdate, MatchRequest	Name-value text is widened/narrowed one byte per UTF-16 unit, so non-ASCII names are garbled.	UTF-16 text.
ScriptNetworkVarUpdate	The text goes through ASCIIToUTF16.	UTF-16 text (non-ASCII values are garbled).
SetShootingGalleryParams	Removed: had no callers, and its field order was a guess ("No clue about the order here").	Not verified.

Behaviour changes that come with dropping malformed messages: ParseChatMessage longer than MAX_MESSAGE_LENGTH is dropped instead of truncated; PLAYER_LOADED (0x00dc36f0), READY_FOR_UPDATES and MISSION_DIALOGUE_CANCELLED (0x00d9cc10) now read the fields the client sends (DLU ignores them) and would be dropped if they were missing.

Packets

Packet	DLU	Client / reference
VERSION_CONFIRM (server -> client)	Fixed (labelled wire change): after serviceType it sends the build identifier (below) instead of the old fixed 8 bytes (ASCII "0.1.3"). unknown stays 861228100 ("DLU3").	Reads only netVersion (+0x7) and serviceType (+0xf) (0x00b30c90); unknown (+0xb) is never read and packetLength is unused, so trailing bytes are ignored. The only length check is payload > 6 bytes in LwoNetClient::Update (0x00b34730).
LoadStaticZone	Always sends clone 0.	The zone's clone ID.
ChatModerationString	The accepted byte is segments.empty().
StringCheck	Keeps 42 narrowed characters, including whatever garbage follows the text.
Route packets	Forwarded from byte 23, using the low byte of the routed packet ID.

VERSION_CONFIRM build identifier (CommonPackets::ServerVersionConfirm, values from dCommon/BuildInfo.h):

Offset (after header)	Size	Field
0	u32	netVersion (client_net_version, default 171022)
4	u32	unknown = 861228100 ("DLU3")
8	u32	serviceType
12	3 x u8	version major, minor, patch (CMakeVariables.txt)
15	u8	flags: bits 0-1 build kind (0 unknown, 1 local, 2 CI, 3 release), bit 2 dirty
16	4 bytes	first 32 bits of the commit hash, most significant byte first (0 without git)
20	u16 + chars	build string, e.g. 3.0.0-experimental+g1a2b3c4d-dirty; optional when reading

The build kind comes from DLU_BUILD_KIND=release|ci|local in the build's environment, else CI=true means CI, else local. cmake/BuildInfo.cmake regenerates the data on every build and only rewrites it when it changed.