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?

See Authentication for communication with Mojang's servers.

The recommended login sequence as of 1.20.4 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
5. Client auth
6. C→S: Encryption Response
7. Server auth, both enable encryption
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: Registry Data
16. S → C: Update Tags (Optional)
17. S → C: Finish Configuration
18. C → S: Acknowledge Finish Configuration
19. S → C: Login (play)
20. S → C: Change Difficulty (Optional)
21. S → C: Player Abilities (Optional)
22. S → C: Set Held Item (Optional)
23. S → C: Update Recipes (Optional)
24. S → C: Entity Event (Optional, for the OP permission level; see Entity statuses#Player)
25. S → C: Commands (Optional)
26. S → C: Update Recipe Book (Optional)
27. S → C: Synchronize Player Position
28. C → S: Confirm Teleportation
29. C → S: Set Player Position and Rotation (Optional, to confirm the spawn position)
30. S → C: Server Data (Optional)
31. S → C: Player Info Update (Add Player action, all players except the one joining (the Notchian server separates these, you don't need to))
32. S → C: Player Info Update (Add Player action, joining player)
33. S → C: Initialize World Border (Optional)
34. S → C: Update Time (Optional)
35. S → C: Set Default Spawn Position (Optional, “home” spawn, not where the client will spawn on login)
36. S → C: Game Event (Start waiting for level chunks action, required for the client to spawn)
37. S → C: Set Ticking State (Optional)
38. S → C: Step Tick (Optional, the Notchian server sends this regardless of ticking state)
39. S → C: Set Center Chunk
40. S → C: Chunk Data and Update Light (One sent for each chunk in a circular area centered on the player's position)
41. S → C: inventory, entities, etc.

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: Pong Response
7. Both sides close the connection

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

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.

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, the server must send the Set Default Spawn Position before the client will spawn the player and the player must be either in the loaded chunk (sent via Chunk Data and Update Light) or below the minimum world height or above the maximum world height (teleported via Synchronize Player Position). After that server needs to send Game Event with event 13 ("Start waiting for level chunks").

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 1.20.2, 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 (Configuration)
6. Send Finish Configuration (Clientbound)
7. Receive Finish Configuration (Serverbound)
8. Send Login (Play)
9. Send Chunk Data and Update Light and/or Synchronize Player Position (see above)
10. Send Set Default Spawn Position

The most difficult part of this may be sending any necessary NBTs in the Registry Data packet. 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.

…my client isn't receiving complete map chunks!

Main article: How to Write a Client

The standard Minecraft server sends full chunks only when your client is sending player status update packets (any of Player (0x03) through Player Position And Look (0x06)).

…chunks are randomly showing and disappearing!

For newer clients, your server needs to send 49 chunks ahead of time, not just one. Send a 7×7 square of chunks, centred on the connecting client's position, before spawning them. Also, edge chunks sometimes bug a bit, try adding empty chunks with air around existing ones to prevent this issue.

…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.

...my client isn't sending a Login Start packet!

By default, the Notchian server and client may group packets depending on if Nagle's TCP algorithm is enabled - the primary objective of Nagle's algorithm is to reduce the total number of packets needed to be sent over the network, increasing the efficiency. Nagle's algorithm is achieved by delaying each TCP packet to check if there are any more that are about to be sent to group them. You may not see a Login Start packet as you may not have parsed anything past the packet's length, because of this, you'll need to separate packets based off of the packet length.

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.