A re-writing of the SMLan Protocol, because I wish to understand it in
my own words. -aj

Protocol version: 0x04 (v4)
This has nothing to do with the server versions (see below)

==Client Versions==
(NETPROTOCOLVERSION in NetworkSyncManager.h)
1: StepMania 3.9 (SMLan?)
2: StepMania 3.95
3: StepMania 4 alphas/sm-ssc v1.0 RC1-Final

==Server Versions==
  1: SMLan (old)
128: SMOnline
129: SMOnline with "better song identification using chart hashes"

==Notes==
I use the term "client" and "player" interchangeably.
The original document used "client" and "user" interchangeably.

These two notes are from the SMOCaml project:
https://sourceforge.net/projects/smocaml/

Command S2C 004 (132): The data from miss to marvelous are reversed

Commands C2S 012-001 and 012-001: when there is no password the 'password' field
    isn't sended at all, not even a NULL character

==Information==
The default port is 8765/tcp, though this is actually assigned to Ultraseek HTTP
by the IANA (see http://www.iana.org/assignments/port-numbers).

A sm-ssc version of a online server for StepMania will use the currently
unassigned port 48040/tcp (and/or udp if the need arises).

The protocol is packaged in "EzSockets Data Packets".

A basic representation

 0123 4 5......  [Octets]
+----+-+---//--+
|ssss|C|Payload| [data]
+----+-+---//--+
where:
ssss	= Size of packet (handled by EzSockets if using C++)
C		= Command
Payload	= (variable size based on command)

The payload may contain more data than expected, for instance, if it's a new
protocol version.

"Size" refers to the size of the data chunk.
Some commonly referred to size types:
 NT = Null-terminated string
MSN = Most significant 4 bits (byte/16)
LSN = Least significant 4 bits (byte%16)

The primary player is 0x0, secondary is 0x1. This does not take into account
Multiplayer notefields.

==[ Client -> Server Communication Protocol ]===================================
0x00 (000) No Operation
This command will case the server to respond with a nop.

Payload: None
Expected response: Server 0x01 (001)
________________________________________________________________________________
0x01 (001) No Operation Response
This command is used to respond to a no operation.

Payload: None
Expected response: None
________________________________________________________________________________
0x02 (002) Hello
This is the first packet a client sends to the server.

Payload:
[Size] [Description]
  0x01 Client Protocol Version (see Client Versions above)
	NT StepMania build name (PRODUCT_ID)

Expected response: Server 0x02 (002)
________________________________________________________________________________
0x03 (003) Game Start Request

Payload:
[Size] [Description]
   MSN Primary Player Meter [0 if no player]
   LSN Secondary Player Meter [0 if no player]
   MSN Primary Player Difficulty
	   0 = Beginner
	   1 = Easy
	   2 = Medium
	   3 = Hard
	   4 = Expert
   LSN Secondary Player Difficulty (values same as primary)
   MSN Start Position:
	   0 = pre-sync
	   1 = sync
   LSN Reserved
   NT  Song Title
   NT  Song Subtitle
   NT  Song Artist
   NT  Course Title (null if doesn't exist)
   NT  Song options string
   NT  Primary player's options (null if doesn't exist)
   NT  Secondary player's options (null if doesn't exist)

Expected response: Server 0x03 (003)
________________________________________________________________________________
0x04 (004) Game Over Notice
Sent when end of game is encountered. I think technically this is round over.

Payload: None
Expected response: None (though later in the document you'd assume Server 0x04)
________________________________________________________________________________
0x05 (005) Game Status Update

The payload changes between protocols, and it will likely have to change again
to account for some more StepIDs.

Payload:
[Size] [Description]
   MSN Player Number
   LSN StepID
	   (if Protocol >= 3)
	   1 = HitMine
	   2 = AvoidMine
	   3 = Miss
	   4 = W5
	   5 = W4
	   6 = W3
	   7 = W2
	   8 = W1
	   9 = LetGo
	   10 = Held

	   (if Protocol <= 2; Don't support these in any new server software)
	   1 = Miss
	   2 = W5
	   3 = W4
	   4 = W3
	   5 = W2
	   6 = W1
	   7 = LetGo
	   8 = Held

   MSN Projected Grade (StepMania enum int, which can cause problems with themes.)
   LSN Reserved (so discuss it with people if changing)
  0x04 Score (net-order long)
  0x02 Combo (net-order int; songs with large combos may cause issues?)
  0x02 Health (net-order int)
  0x02 Offset (net-order number. huh)
		32767 would be DEAD on the note
		If the user is hitting late, the # will be higher.
		It if the user is exactly 0.25 seconds off, the number will be
		different by 500. With this pattern 0.5 seconds gives a difference of 1000.

Expected response: None
________________________________________________________________________________
0x06 (006) Style Update
!IMPORTANT! This command is unrelated to Server Command 0x06 (006). !IMPORTANT!
This is sent when a style is chosen.

Payload:
[Size] [Description]
 0x01 Number of enabled players (1 or 2)
 0x01 Player Number (0 = P1, 1 = P2)
   NT Player name for specified player number

Expected response: None
________________________________________________________________________________
0x07 (007) Chat Message
Send a general chat message.

Payload:
[Size] [Description]
	NT Message

Expected response: none?
________________________________________________________________________________
0x08 (008) Request Start Game, Tell server if Song exists or not.
The player selected a song on a net-enabled screen.

Payload:
[Size] [Description]
 0x01 Message Use:
	   0 = User has specified song (response to Server 0x08)
	   1 = User does not have specified song (response to Server 0x08)
	   2 = User has requested start game for the song
	NT Song Title		(GetTranslitMainTitle)
	NT Song Artist		(GetTranslitArtist)
	NT Song Subtitle	(GetTranslitSubTitle)

Expected response: none?
________________________________________________________________________________
0x09 (009) [Reserved]
________________________________________________________________________________
0x0A (010) User entered/exited a network screen

Payload:
[Size] [Description]
 0x01 Status:
	   0 = exited ScreenNetSelectMusic
	   1 = entered ScreenNetSelectMusic
	   2 = not sent
	   3 = entered Options
	   4 = exited ScreenNetEvaluation
	   5 = entered ScreenNetEvaluation
	   6 = exited ScreenNetRoom
	   7 = entered ScreenNetRoom

Expected response: none?
________________________________________________________________________________
0x0B (011) Player options changed

Payload:
[Size] [Description]
    NT Player 0's options (PLAYER_1)
    NT Player 1's options (PLAYER_2)

Expected response: none?
________________________________________________________________________________
0x0C (012) SMOnline Packet (Special Case)
The SMOnline Packet is not defined here.
SMLan packet 12 is a wrapper for the SMOnline packet. It is used for
"protection" of sorts for the SMO coders.

Payload:
[Size] [Description]
  0x01 SMOnline command
 <...> SMOnline data (<...> = "<VARIABLE>" in original docs)

Expected response: none?
________________________________________________________________________________
0x0D (013) [Reserved]
________________________________________________________________________________
0x0E (014) [Reserved]
________________________________________________________________________________
0x0F (015) XML Packet
This packet contains data in XML format.

Payload:
[Size] [Description]
    NT XML

Expected response: none?
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
==[ Server -> Client Communication Protocol ]===================================
The Server->Client Communication Protocol begins at 0x80 (128). References to
protocol numbers based on 000 will appear here as well.
________________________________________________________________________________
0x80 (128) [0x00 (000)] No Operation
This command makes the server respond with a nop.

Expected Response: Server 0x01 (001; so really 0x81/129?)
________________________________________________________________________________
0x81 (129) [0x01 (001)] No Operation Response
The response the server gives to a client nop.
________________________________________________________________________________
0x82 (130) [0x02 (002)] Server Hello Response
Introduces the server to the client.

Payload:
[Size] [Description]
 0x01 Server Protocol version (if >= 128; server is SMOnline.)
	NT Server Name
 0x04 Random Key (or Salt, iirc. Used for alternate login method.)
________________________________________________________________________________
0x83 (131) [0x03 (003)] Allow Start
Causes the client to start the game.
________________________________________________________________________________
0x84 (132) [0x04 (004)] Game over stats
Sent in response to the game over packet.
(Technically, this is the evaluation stats.)

Payload:
[Size] [Description]
  0x01 Number of players in this packet (active)
	   Send every player's information for a given field.
 0x01 First player's PlayerID
	 <other PlayerIDs>
 0x04 Score
	 <other Scores>
 0x01 Grade
	 <other Grades>
 0x01 Difficulty (0 = Beginner, &c.)
	 <other Difficulties>

	(the steps type section is reversed. We may need to do some packet analysis.)
 0x02 Judgement_Miss
	 <other Miss counts>
 0x02 Judgement_W5 (Boo)
	 <other W5s>
 0x02 Judgement_W4 (Good)
	 <other W4s>
 0x02 Judgement_W3 (Great)
	 <other W3s>
 0x02 Judgement_W2 (Perfect)
	 <other W2s>
 0x02 Judgement_W1 (Marvelous)
	 <other W1s>
 0x02 Judgement_Held (OK)
	 <other Helds>
	 (LetGo doesn't seem to be handled.)
 0x02 Max Combo
	 <other MaxCombos>
	NT Player options
	 <other Player Options>
________________________________________________________________________________
0x85 (133) [0x05 (005)] Scoreboard update
Update the client's scoreboard.

Payload:
[Size] [Description]
  0x01 Section
		0 = names
		1 = combos
		2 = grades (Projected)
  0x01 Number of players to display

The rest of the payload depends on section.
switch(section)
{
	case Names:
  0x01 Index of Player in 1st place
  0x01 Index of Player in 2nd place
	 ...
  0x01 Last player's index
	 break;

	case Combos:
  0x01 Player 1's combo
  0x01 Player 2's combo
	 ...
  0x01 Last player's combo
	 break;

	case Grades: // uses same format as other Grade reports
  0x01 Player 1's grade
  0x01 Player 2's grade
	 ...
  0x01 Last player's grade
	 break;
}
________________________________________________________________________________
0x86 (134) [0x06 (006)] System Message
!IMPORTANT! This command is unrelated to Server Command 0x06 (006). !IMPORTANT!
Send a system message to the user.

Payload:
[Size] [Description]
    NT Message
________________________________________________________________________________
0x87 (135) [0x07 (007)] Chat Message
Add a message to the chat window. ("on some StepMania screens.")

Payload:
[Size] [Description]
	NT Message
________________________________________________________________________________
0x88 (136) [0x08 (008)] Ask if client has song/Tell client to start song
The player selected a song on a net-enabled screen.

Payload:
[Size] [Description]
  0x01 Message Type
		0 = See if client has song.
		1 = See if client has song. If so, scroll to it.
		2 = See if client has song. If so, scroll to it and play it.
		3 = Blindly start song. (Probably assumes everyone has the song.)
	NT Song Title		(GetTranslitMainTitle)
	NT Song Artist		(GetTranslitArtist)
	NT Song Subtitle	(GetTranslitSubTitle)

________________________________________________________________________________
0x89 (137) [0x09 (009)] Update user list
Sends all the players currently connected.

Payload:
[Size] [Description]
  0x01 Max number of players
  0x01 Number of players in this packet
  0x01 Player 0's status
	NT Player 0's name (if no player, it should be an empty string ("").)
	 ...
  0x01 Last player's status
	NT Last player's name

The status flag:
0x00 = Inactive (user unknown)
0x01 = Active (user known)
0x02 = On a selection screen
0x02 = In options
0x02 = On Eval.
________________________________________________________________________________
0x8A (138) [0x0A (010)] Force change to NetSelectMusic

Payload:
[Size] [Description]
	NT Gametype to set
	NT Style to set
________________________________________________________________________________
0x8B (139) [0x0B (011)] Reserved
________________________________________________________________________________
0x8C (140) [0x0C (012)] SMOnline Packet (Special Case)
The SMOnline Packet is not defined here.
SMLan packet 12 is a wrapper for the SMOnline packet. It is used for
"protection" of sorts for the SMO coders.

Payload:
[Size] [Description]
	 1 SMOnline command
 <...> SMOnline data (<...> = "<VARIABLE>" in original docs)
________________________________________________________________________________
0x8D (141) [0x0D (013)] Formatted information packet
Send formatted information about the server back to the player.
This function is alternatively a broadcast packet (via UDP).

Payload:
[Size] [Description]
	NT Server name
  0x02 Server port
  0x02 Number of players connected
________________________________________________________________________________
0x8E (142) [0x0E (014)] Attack client

Payload:
[Size] [Description]
  0x01 Player number
  0x04 Attack length (milliseconds)
	NT Modifier string
________________________________________________________________________________
0x8F (143) [0x0F (015)] XML reply

Payload:
[Size] [Description]
	NT XML data

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
==[ Client SMOnline Packets ]===================================================
0x00 (000) Send Login Information

Payload:
[Size] [Description]
  0x01 Player Number
  0x01 Encryption Text:
	   0 = MD5 hash
	   1 = MD5(MD5 hash + salt), where salt is a plaintext base 10 string.
	NT Username
	NT Password

If the server's salt is 0, Encryption Text can't be 1. Block login.
________________________________________________________________________________
0x01 (001) Ask to enter room

Payload:
[Size] [Description]
  0x01 Enter or Exit?:
	   0 = Exit
	   1 = Enter
	NT Room Name (only when entering)
	NT Password (empty if unused)

________________________________________________________________________________
0x02 (002) Create a new room

Payload:
[Size] [Description]
  0x01 Room Type:
	   0 = Normal (with subrooms) 
	   1 = Game (no subrooms)
	NT Room Title
	NT Room Description
	NR Room Password (blank if no password)

________________________________________________________________________________
0x03 (003) Request room info

Payload:
[Size] [Description]
	NT Room name

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
==[ Server SMOnline Packets ]===================================================
0x00 (000) Login Response
Payload:
[Size] [Description]
  0x01 Approval status:
	   0 = Login OK
	   1 = Login Failed
	NT Login Response (plaintext)
________________________________________________________________________________
0x01 (001)

Payload:
The payload changes depending on the type of update.
[Size] [Description]
  0x01 Update Type:
	   0 = Change Room Title
	   1 = Update List of rooms/games

if Update Type == 0 (Change Room Title)
[Size] [Description]
	NT Room Title
	NT Room Description
  0x01 Room Type:
	   0 = Chat
	   1 = Game
  0x01 Allow Subroom creation?
	   0 = No
	   1 = Yes

if Update Type == 1 (Update room/game list)
[Size] [Description]
  0x01 Number of rooms
for 1 to Number of Rooms where room number = # do
	NT Room# Title
	NT Room# Description

Reverse compatiblity Room Status
[Size] [Description]
  0x01 Room# Status
	   0 = Normal
	   1 = unused
	   2 = Playing Game
	   3 = Song Selection Stage1
	   4 = Song Selection Stage2

Reverse compatiblity Room Flags
[Size] [Description]
  0x01 Room# Flags
	   if bit 0, Passworded
________________________________________________________________________________
0x02 (002) Request general information from server.

Payload:
[Size] [Description]
  0x01 Stats format
	   0 = Normal unformatted stats
________________________________________________________________________________
0x03 (003) Request room information

Payload:
[Size] [Description]
	NT Last Song Title
	NT Last Song Subtitle
	NT Last Song Artist
  0x01 Number of players
  0x01 Max players
	NT Player1 name
	 ...
	NT Last player (PlayerN) name