People very, very often have questions regarding the Minecraft Alpha protocol documentation, so we'll try to address some of the most common ones on this document. If you're still having trouble, join us on IRC, channel #mcc on irc.esper.net.
Is the protocol documentation complete?
Depending on your definition, yes! All packet types are known and their layout documented. Some finer details are missing, but everything you need to make functional programs is present. We have a script that monitors for new versions of the client and notifies us, allowing us to quickly document new packets.
What's the normal login sequence for a client?
The recommended login sequence looks like this, where C is the client and S is the server:
C ↔ S : Connects
C → S : Sends handshake
C ← S : Sends handshake response
C → S : After authenticating (if needed), sends the login packet
C ← S : Either kicks (invalid login) or sends a login response
C ← S : Sends pre-chunks and chunks
C ← S : Sends spawn position
C ← S : Sends inventory (once for each of the three types)
C ← S : Tell the client they're ready to spawn by sending a position + look packet. Note: The stance and Y should be swapped when the server sends it to the client (See Protocol Page)!
C → S : Sends a position + look packet to confirm the spawn position, with the stance and Y swapped back to the correct positions
I think I've done everything right, but my player isn't spawning