Network documentation

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Header

All of SRB2's packets start out with a specific 8-byte header, consisting of four values and a padding byte:

Data name	Data type	Description
checksum	Unsigned 32-bit Integer	The checksum.
ack	Unsigned 8-bit Integer	The acknowledgment variable. If not null, the server must send whatever is contained here in the next part of the header structure.
ackreturn	Unsigned 8-bit Integer	The acknowledgment variable's return.
packettype	Unsigned 8-bit Integer	The packet type.
reserved	Null byte	Padding.

Checksum calculation

The checksum for all packets is calculated in the function named NetbufferChecksum. This checksum is analyzed when the server receives the packet, and if it does not match what the server is expecting, it discards the packet without a reply.

List of packet types

Decimal ID	Hex ID	Name	Sender → receiver	Purpose
0	0x00	PT_NOTHING	Server → Client Client → Server	Sends a no-op through the network.
1	0x01	PT_SERVERCFG	Server → Client	Sends server configuration to the client. Is considered a positive response to a request to join the server.
2	0x02	PT_CLIENTCMD	Client → Server	Contains a client's ticcmd.
3	0x03	PT_CLIENTMIS	Client → Server	Contains a client's ticcmd. Is used when the server missed a ticcmd and the client is sending it again.
4	0x04	PT_CLIENT2CMD	Client → Server	Contains two clients' ticcmds for splitscreen.
5	0x05	PT_CLIENT2MIS	Client → Server	Contains two clients' ticcmds for splitscreen. Is used when the server missed a ticcmd and the clients are sending it again.
6	0x06	PT_NODEKEEPALIVE	Client → Server	Sent to the server to tell that the client is still alive but has nothing to send for this tic.
7	0x07	PT_NODEKEEPALIVEMIS	Client → Server	Sent to the server to tell that the client is still alive but has nothing to send for this tic. Is used when the server missed a ticcmd and the client is sending it again.
8	0x08	PT_SERVERTICS	Server → Client	Server sending all the clients' ticcmds and net commands to each other.
9	0x09	PT_SERVERREFUSE	Server → Client	Sends a refusal notice to a client that wishes to connect, which contains a reason.
10	0x0A	PT_SERVERSHUTDOWN	Server → Client	Sent when the server shuts down, disconnecting all clients in the process.
11	0x0B	PT_CLIENTQUIT	Client → Server	Sent when a client disconnects from the server.
12	0x0C	PT_ASKINFO	Client → Server	Sent when a client requests info from a server.
13	0x0D	PT_SERVERINFO	Server → Client	Sent by the server as one of two replies to PT_ASKINFO. Contains server information.
14	0x0E	PT_PLAYERINFO	Server → Client	Sent by the server as the other one of two replies to PT_ASKINFO. Contains information on all players in the server.
15	0x0F	PT_REQUESTFILE	Client → Server	Sent by the client to request the transfer all important files from the server that they do not already have. If the client has a different version of any needed file (the MD5 sum is bad), the file will also be requested here.
16	0x10	PT_ASKINFOVIAMS	N/A	Unused – this packet appears to have been identical to PT_ASKINFO, except it tells the server to send PT_SERVERINFO/PT_PLAYERINFO packets to the IP address contained in the packet. However, nothing is designed to send this packet type anymore, if ever at all, so it should be considered obsolete.
17	0x11	PT_WILLRESENDGAMESTATE	Server → Client	"Hey Client, I am about to resend you the gamestate!"
18	0x12	PT_CANRECEIVEGAMESTATE	Client → Server	"Okay Server, I'm ready to receive it, you can go ahead."
19	0x13	PT_RECEIVEDGAMESTATE	Client → Server	"Thank you Server, I am ready to play again!"
20	0x14	PT_SENDINGLUAFILE	Server → Client	Server telling a client Lua needs to open a file
21	0x15	PT_ASKLUAFILE	Client → Server	Client telling the server they don't have the file
22	0x16	PT_HASLUAFILE	Client → Server	Client telling the server they have the file
23	0x17	PT_CANFAIL	The packet types below this line can fail to be sent. In addition, these packet types can't occupy all the available ack slots.
23	0x17	PT_FILEFRAGMENT	Server → Client	A fragment of a file sent by the server to the client. This is either a fragment of an important file sent in reply to PT_REQUESTFILE, or a fragment of $$$.sav (the netgame's savefile).
24	0x18	PT_FILEACK
25	0x19	PT_FILERECEIVED
26	0x1A	PT_TEXTCMD	Client → Server	Extra net commands from the client, e.g. SAY commands.
27	0x1B	PT_TEXTCMD2	Client → Server	Extra net commands from a splitscreen client.
28	0x1C	PT_CLIENTJOIN	Client → Server	Sent by a client to request to join a server.
29	0x1D	PT_NODETIMEOUT	Server/Client → self	Sent by a client or server, to themselves, to indicate to the game that a client has lost connection to the server.
30	0x1E	PT_LOGIN	Client → Server	Sent by a client to attempt a login. If successful, the player is awarded admin privileges. This is sent when a player uses the login command.
31	0x1F	PT_PING	Server → Client	Sent by the server to inform a client of all the other clients' pings.