Protocol FAQ

People very, very often have questions regarding the Minecraft Modern Protocol, 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 #mcdevs on irc.libera.chat.

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 also collect information on the pre-release protocol changes, allowing us to quickly document new releases.

What's the normal login sequence for a client?

(This is about joining a Minecraft server. See Microsoft Authentication Scheme for how to log in to a Minecraft account, and Protocol Encryption for details on player authentication during server login.)

The recommended login sequence as of 1.21 looks like this, where C is the client and S is the server:

1. Client connects to the server
2. C→S: Handshake State=2
3. C→S: Login Start
4. S→C: Encryption Request (optional)
5. Client auth (Only if server sent Encryption Request)
6. C→S: Encryption Response (Only if server sent Encryption Request)
7. Server auth, both enable encryption (Only if server sent Encryption Request)
8. S → C: Set Compression (Optional, enables compression)
9. S → C: Login Success
10. C → S: Login Acknowledged
11. C → S: Serverbound Plugin Message (Optional, minecraft:brand with the client's brand)
12. C → S: Client Information (Optional)
13. S → C: Clientbound Plugin Message (Optional, minecraft:brand with the server's brand)
14. S → C: Feature Flags (Optional)
15. S → C: Clientbound Known Packs
16. C → S: Serverbound Known Packs
17. S → C: Registry Data (Multiple)
18. S → C: Update Tags (Optional)
19. S → C: Finish Configuration
20. C → S: Acknowledge Finish Configuration
21. S → C: Login (play)
22. S → C: Change Difficulty (Optional)
23. S → C: Player Abilities (Optional)
24. S → C: Set Held Item (Optional)
25. S → C: Update Recipes (Optional)
26. S → C: Entity Event (Optional, for the OP permission level; see Entity statuses#Player)
27. S → C: Commands (Optional)
28. S → C: Update Recipe Book (Optional)
29. S → C: Synchronize Player Position
30. C → S: Confirm Teleportation
31. C → S: Set Player Position and Rotation (Optional, to confirm the spawn position)
32. S → C: Server Data (Optional)
33. S → C: Player Info Update (Add Player action, all players except the one joining (the Notchian server separates these, you don't need to))
34. S → C: Player Info Update (Add Player action, joining player)
35. S → C: Initialize World Border (Optional)
36. S → C: Update Time (Optional)
37. S → C: Set Default Spawn Position (Optional, “home” spawn, not where the client will spawn on login)
38. S → C: Game Event (Start waiting for level chunks event, required for the client to spawn)
39. S → C: Set Ticking State (Optional)
40. S → C: Step Tick (Optional, the Notchian server sends this regardless of ticking state)
41. S → C: Set Center Chunk
42. S → C: Chunk Data and Update Light (One sent for each chunk in a circular area centered on the player's position)
43. S → C: inventory, entities, etc.

Offline mode

If the server is in offline mode, it will not send the Encryption Request packet, and likewise, the client should not send Encryption Response. In this case, encryption is never enabled, and no authentication is performed.

Clients can tell that a server is in offline mode if the server sends a Login Success without sending Encryption Request.

What does the normal status ping sequence look like?

When a Notchian client and server exchange information in a status ping, the exchange of packets will be as follows:

1. C → S: Handshake with Next State set to 1 (Status)
2. Client and Server set protocol state to Status.
3. C → S: Status Request
4. S → C: Status Response
5. C → S: Ping Request
6. S → C: Ping Response
7. Both sides close the connection

(Note that C is the Notchian client and S is the Notchian server).

I think I've done everything right, but…

…my player isn't spawning!

The Minecraft client will wait at the "Loading Terrain..." screen until late in the login sequence. As of 1.20.4, in order for the client to spawn, it must have received a Game Event packet with event 13 ("Start waiting for level chunks"), and at least one of the following conditions must be met:

• The player is in a loaded chunk (sent via Chunk Data and Update Light).
• The player is below the minimum world height or above the maximum world height (teleported via Synchronize Player Position).
• The player in spectator mode.
• The player is dead (set via Set Health or Combat Death).

In past versions, you could either (1.19.3 through 1.20.1) send the default spawn position packet or (pre-1.19.3) send the player position packet. In general, try sending packets that inform the client about the player's position in the world in order to get past the loading terrain screen.

As of 1.20.4, the minimum packets that need to be received and sent by the server in order to get the client past the loading terrain screen and into the world appear to be:

1. Receive Handshake
2. Receive Login Start
3. Send Login Success
4. Receive Login Acknowledged
5. Send Registry Data
6. Send Finish Configuration
7. Receive Acknowledge Finish Configuration
8. Send Login (Play)
9. Send Game Event, Start waiting for level chunks
10. Send Chunk Data and Update Light and/or Synchronize Player Position (see above)

The most difficult part of this may be sending any necessary NBTs in the Registry Data packet. Note that if you tell the Notchian client you have 0 known packs, it will blatantly lie to you, so you will probably need to record one from the standard server and replay it. Or you can find someone who has done that already, for example in Norbiros' gist. You can also find JSON representation of this packet in PrismarineJS/minecraft-data repo on GitHub.

…chunks are randomly showing and disappearing!

The Notchian client only reliably renders chunks surrounded by other loaded chunks on all sides. See Chunk Format#Tips and notes.

…the client is trying to send an invalid packet that begins with 0xFE01

The client is attempting a legacy ping, this happens if your server did not respond to the Server List Ping properly, including if it sent malformed JSON.

…the client disconnects after some time with a "Timed out" error

The server is expected to send a Keep Alive packet every 1-15 seconds (if the server does not send a keep alive within 20 seconds of state change to Play, the client will disconnect from the server for Timed Out), and the client should respond with the serverbound version of that packet. If either party does not receive keep alives for some period of time, they will disconnect.

...some of the packets I expect to receive seem to be missing or too short

You may be misusing the socket API. In particular, it is invalid to assume that the amount of data returned by calls to recv (or equivalent, depending on the API you're using) relates to packet boundaries in any way. There are no "borders" in a TCP data stream, only bytes. Regardless of how any two consecutive packets are sent, the receiver may get one packet, then the other, both at once, half of one, then the rest of both, or any other permutation of buffer sizes adding up to the total size of the packets. The only correct way to know where one packet ends and another begins is the packet length field, and you need to be prepared to handle multiple packets in one buffer, packets split across multiple buffers, etc. (as well as length fields split across multiple buffers!)

Similarly, depending on the API being used, a send call may also not guarantee that its whole input buffer is sent (consult the relevant documentation for details). This may also cause the connection to appear to hang during the login process, since the server will be left waiting indefinitely for the rest of the packet to arrive.

One reason why packets sent separately may arrive at once is Nagle's algorithm, a feature of many TCP implementations intended to improve efficiency particularly for applications making lots of small send calls. It is not the only reason, though, and disabling it should not be seen as a solution to the problem discussed here. Nonetheless, the delays it introduces are detrimental to real-time applications like Minecraft, so the Notchian client and server disable it, and you should too. This is typically done by enabling a socket option called TCP_NODELAY. Especially when disabling Nagle's algorithm, you should group multiple packets sent during the same tick in one send buffer for the best performance.

How do I open/save a command block?

The process to actually open the command block window clientside is somewhat complex; the client actually uses the Update Block Entity (0x09) packet to open it.

First, the client must have at least an OP permission level of 2, or else the client will refuse to open the command block. (The op permission level is set with the Entity Status packet)

To actually open the command block:

1. C→S: Player Block Placement (0x1C), with the position being the command block that was right-clicked.
2. S→C: Update Block Entity (0x09), with the NBT of the command block.

And to save it, use the MC|AutoCmd plugin channel.