I posted this like 2 years ago in other forums, maybe it can help:

Packet Format:
   (DWORD) 0x00
   (WORD) Checksum
   (WORD) Length
   (WORD) Sent
   (WORD) Recved
   (BYTE) Command Class
   (BYTE) Command
   (BYTE) Sender ID
   (BYTE) Resend
   (VOID) Packet


The header you are working with is specific to the Storm UDP protocol
and is ot ever assembled or viewed by the game protocol module itself.  Thus
only ontrol messages for the Storm UDP protocol use the command field.
 
The way this is laid out can be represented something like this:
 
-----------------
-- Storm Header--
-----------------
-- Data Payload -
-----------------
 
Storm.dll receives the data off the wire, and interprets class 0
control messages directly, instead of passing them on to the game (except
perhaps in the form of high level callbacks, such as a "player joined the game"
callback or event).

For non-class0 messages, the data payload is passed uninterpreted on to
the game protocol parser itself, whether that be in Diablo.exe,
Starcraft.exe,
etc:
 
-----------------
-- Data Payload -
-----------------

Since the main game module never sees the Storm header, command ids for
other than the internal control class can't be stored there.
 
Note that I am logically equating Storm.dll and the Storm network
service provider (SNP) as the same module.  In reality, the SNP is responsible
for sending and receiving the game data off of the wire, whether that be a
UDP socket or an IPX socket, or just an internal loopback (e.g. standard.snp).
 
Class 1 is used for messages that do not need to be synchronized with
the game state, such as chat commands.  Thus, class 1 commands can be sent
and received at any time.
 
Class 2 is used for messages that do need to be synchronized with the
game state, such as unit orders.  Thus, class 2 commands can only be sent
once each game turn (as a result they are typically queued internally inside
the game protocol module until the next turn is transmitted).
 
At this time I do not have a list of Starcraft game protocol commands
in other than source code form, which I am not prepared to distribute.

And also the UDP Checksum in VB:

Private Function RShift(ByVal pnValue As Long, ByVal pnShift As Long) As Double
On Error Resume Next
    RShift = CDbl(pnValue \ (2 ^ pnShift))
End Function

Private Function LShift(ByVal pnValue As Long, ByVal pnShift As Long) As Double
On Error Resume Next
    LShift = CDbl(pnValue * (2 ^ pnShift))
End Function

Private Function SubCheckSum(ByVal buf As String, ByVal length As Integer) As Long
    Dim sum1, sum2
    Dim i As Integer, iY As Integer
        For iY = 0 To length - 1
            i = length - iY
            sum2 = sum2 + Asc(Mid(buf, i, 1))
            If sum2 > &HFF Then
                sum2 = sum2 - &HFF
            End If
            sum1 = sum1 + sum2
        Next iY
        SubCheckSum = (LShift((sum2 And &HFF), 8)) Or ((sum1 Mod &HFF) And &HFF)
End Function

Private Function UDPCheckSum(buf As String) As Integer
    Dim subsum As Long, length As Integer
    Dim a As Long, b As Long, Ret As Integer
        CopyMemory length, ByVal Mid$(buf, 3, 2), 2
        length = length - 2
        subsum = SubCheckSum(Mid$(buf, 3), length)
        a = &HFF - ((subsum And &HFF) + (RShift(subsum, 8))) Mod &HFF
        b = CLng((((&HFF - (a + RShift(subsum, 8)) Mod &HFF) And &HFF) Or LShift(a, 8)))
        CopyMemory Ret, b, 2
        UDPCheckSum = Ret
End Function

This was from a research i did with Pianka years ago.

emulating Starcrafts game protocol

note for change* Command 1 0x4C = chat packet not 0x00

See http://code.google.com/p/vgce/source/browse/trunk/docs/Blizzard/Starcraft/packets2.txt

I know this topic is old, but I recently discovered some "Return codes" that are used internally(0x50 to 0x53).

I believe that the Lobby commands, return codes, and game commands are NOT related, but were initially related in Alpha/Beta, so I have merged them.