DirectPlay system messages

Summary

DirectPlay delivers system traffic from player id 0. Lords of the Realm 2 reads the first DWORD of the receive buffer as dwType and routes every DirectPlay 3 DPSYS_* message it recognizes to lotr2_net_handle_system_message. All claims here are inferred from client code unless otherwise noted.

Evidence

Claim	Level
System messages arrive with sender id 0	inferred from client code
dwType is read from receive buffer offset 0x00	inferred from client code
Eight DPSYS_* values are routed	inferred from client code
Fields after dwType use stock DirectPlay 3 SDK layouts	inferred from SDK layout; not verified on wire

Session Context

System messages are processed inside lotr2_net_poll_messages after IDirectPlay3::Receive fills the shared receive buffer at DAT_0052d490. Messages from lotr2_net_local_player_id are treated as self-echoes and ignored. Messages from any other nonzero sender are custom app frames; see dplay-app-messages.md.

Message Routing

dwType	DirectPlay constant	Internal index	Client action
0x00000003	DPSYS_CREATEPLAYERORGROUP	1	Logs create; reads created DPID from DAT_0052d498.
0x00000005	DPSYS_DESTROYPLAYERORGROUP	2	Removes slot; before gameplay, broadcasts app opcode 3 roster sync.
0x00000007	DPSYS_ADDPLAYERTOGROUP	5	Logs only.
0x00000021	DPSYS_DELETEPLAYERFROMGROUP	6	Logs only.
0x00000031	DPSYS_SESSIONLOST	3	Logs session lost.
0x00000101	DPSYS_HOST	4	Logs host migration and sets lotr2_net_is_host = 1.
0x00000102	DPSYS_SETPLAYERORGROUPDATA	7	Logs only.
0x00000103	DPSYS_SETPLAYERORGROUPNAME	8	Logs only.

Generic Field Layouts

DirectPlay system messages are standard DirectPlay payloads, not the custom lotr2_net_msg_header frame. The client only reads dwType in the receive loop and, for create/destroy, the DPID at offset 0x08.

DPSYS_CREATEPLAYERORGROUP (0x00000003)

SDK payload: DPMSG_CREATEPLAYERORGROUP.

Field	Offset	Size	Type	Description
dwType	0x00	4	uint32le	DPSYS_CREATEPLAYERORGROUP.
dpId	0x04	4	uint32le	Created player or group id.
dwPlayerType	0x08	4	uint32le	DirectPlay player/group type.
lpData	0x0c	4	pointer	Player data pointer in local process.
dwDataSize	0x10	4	uint32le	Player data byte count.
dpnName	0x14	16	DPNAME	DirectPlay name structure.

• Direction: DirectPlay system -> client.
• Trigger: another player or group is created in the session.
• Handler (parse): lotr2_net_handle_system_message // was FUN_004480ef @ 0x004480ef.
• Compatibility notes: the client's router passes DAT_0052d498, which is the DPID slot used by both create and destroy system messages in this export.

DPSYS_DESTROYPLAYERORGROUP (0x00000005)

SDK payload: DPMSG_DESTROYPLAYERORGROUP.

Field	Offset	Size	Type	Description
dwType	0x00	4	uint32le	DPSYS_DESTROYPLAYERORGROUP.
dpId	0x04	4	uint32le	Destroyed player or group id.
dwPlayerType	0x08	4	uint32le	DirectPlay player/group type.
lpLocalData	0x0c	4	pointer	Local player data pointer.
dwLocalDataSize	0x10	4	uint32le	Local data size.
lpRemoteData	0x14	4	pointer	Remote player data pointer.
dwRemoteDataSize	0x18	4	uint32le	Remote data size.
dpnName	0x1c	16	DPNAME	DirectPlay name structure.

• Direction: DirectPlay system -> client.
• Trigger: another player or group leaves or is destroyed.
• Handler (parse): lotr2_net_handle_system_message // was FUN_004480ef @ 0x004480ef.
• Compatibility notes: before the game reaches state DAT_0055c02c >= 3, the client rebroadcasts custom opcode 3 after removing the player slot.

DPSYS_ADDPLAYERTOGROUP (0x00000007)

SDK payload: DPMSG_ADDPLAYERTOGROUP.

Field	Offset	Size	Type	Description
dwType	0x00	4	uint32le	DPSYS_ADDPLAYERTOGROUP.
dpIdGroup	0x04	4	uint32le	Target group id.
dpIdPlayer	0x08	4	uint32le	Added player id.

• Direction: DirectPlay system -> client.
• Trigger: a DirectPlay group membership changes.
• Handler (parse): lotr2_net_handle_system_message.
• Compatibility notes: the observed client only logs this message.

DPSYS_DELETEPLAYERFROMGROUP (0x00000021)

SDK payload: DPMSG_DELETEPLAYERFROMGROUP.

Field	Offset	Size	Type	Description
dwType	0x00	4	uint32le	DPSYS_DELETEPLAYERFROMGROUP.
dpIdGroup	0x04	4	uint32le	Target group id.
dpIdPlayer	0x08	4	uint32le	Removed player id.

• Direction: DirectPlay system -> client.
• Trigger: a player leaves a DirectPlay group.
• Handler (parse): lotr2_net_handle_system_message.
• Compatibility notes: the observed client only logs this message.

DPSYS_SESSIONLOST (0x00000031)

SDK payload: DPMSG_GENERIC.

Field	Offset	Size	Type	Description
dwType	0x00	4	uint32le	DPSYS_SESSIONLOST.

• Direction: DirectPlay system -> client.
• Trigger: the DirectPlay session is lost.
• Handler (parse): lotr2_net_handle_system_message.
• Compatibility notes: the handler logs; teardown is handled elsewhere in session control paths.

DPSYS_HOST (0x00000101)

SDK payload: DPMSG_HOST.

Field	Offset	Size	Type	Description
dwType	0x00	4	uint32le	DPSYS_HOST.
dpIdNewHost	0x04	4	uint32le	New DirectPlay host id.

• Direction: DirectPlay system -> client.
• Trigger: DirectPlay host migration.
• Handler (parse): lotr2_net_handle_system_message.
• Compatibility notes: the client does not compare dpIdNewHost; receiving the system message is enough to set lotr2_net_is_host = 1.

DPSYS_SETPLAYERORGROUPDATA (0x00000102)

SDK payload: DPMSG_SETPLAYERORGROUPDATA.

Field	Offset	Size	Type	Description
dwType	0x00	4	uint32le	DPSYS_SETPLAYERORGROUPDATA.
dpId	0x04	4	uint32le	Player or group id.
dwPlayerType	0x08	4	uint32le	DirectPlay player/group type.
lpData	0x0c	4	pointer	New data pointer.
dwDataSize	0x10	4	uint32le	New data size.

• Direction: DirectPlay system -> client.
• Trigger: player or group data changes through DirectPlay.
• Handler (parse): lotr2_net_handle_system_message.
• Compatibility notes: the observed client only logs this message.

DPSYS_SETPLAYERORGROUPNAME (0x00000103)

SDK payload: DPMSG_SETPLAYERORGROUPNAME.

Field	Offset	Size	Type	Description
dwType	0x00	4	uint32le	DPSYS_SETPLAYERORGROUPNAME.
dpId	0x04	4	uint32le	Player or group id.
dwPlayerType	0x08	4	uint32le	DirectPlay player/group type.
dpnName	0x0c	16	DPNAME	New DirectPlay name structure.

• Direction: DirectPlay system -> client.
• Trigger: player or group name changes through DirectPlay.
• Handler (parse): lotr2_net_handle_system_message.
• Compatibility notes: the observed client only logs this message.

Receive HRESULTs

The receive loop treats DP_OK as success, returns quietly on the no-message HRESULT, and logs several DirectPlay error codes before returning. These are transport status values, not message types.

Open Questions

• Confirm exact DirectPlay SDK struct packing against a Win95 DirectX SDK.
• Capture system traffic on a stock-host join and disconnect.

Captures Needed

• LAN trace covering create-player, host migration if possible, player destroy, and session lost.