Pre-release protocol

This page documents the changes from the last stable Minecraft release (currently 1.13.2, protocol 404) to the current pre-release (currently 18w43c, protocol 442). Note that this page contains bleeding-edge information that may not be completely or correctly documented.

One who wishes to commandeer the merging of this into Protocol when an update is made must be sure to respect any changes that may have occurred to the respective packets there.

Contents

Data types

Position

64-bit value split in to three parts

• x: 26 MSBs
• z: 26 LSBs middle bits
• y: 12 bits between them LSBs

Encoded as followed:

((x & 0x3FFFFFF) << 38) | ((y & 0xFFF) << 26) | (z & 0x3FFFFFF) ((z & 0x3FFFFFF) << 12) | (y & 0xFFF)

And decoded as:

val = read_unsigned_long();
x = val >> 38;
y = (val >> 26) & 0xFFF val & 0xFFF;
z = val << 38 >> 38 (val >> 12) & 0x3FFFFFF;

Note: The details of bit shifting are rather language dependent; the above may work in Java but probably won't in other languages without some tweaking. In particular, you will usually receive positive numbers even if the actual coordinates are negative. This can be fixed by adding something like the following:

if x >= 2^25 { x -= 2^26 }
if y >= 2^11 { y -= 2^12 }
if z >= 2^25 { z -= 2^26 }

Packets

ID	Packet name	Documentation
Handshaking serverbound
0x00	Handshake	Current	Pre
Play clientbound
0x22	Chunk Data	Current	Pre
0x4E	Entity Sound Effect		Pre
0x4E 0x4F	Player List Header And Footer	Current	(unchanged)
0x4F 0x50	Collect Item	Current	(unchanged)
0x50 0x51	Entity Teleport	Current	(unchanged)
0x51 0x52	Advancements	Current	(unchanged)
0x52 0x53	Entity Properties	Current	(unchanged)
0x53 0x54	Entity Effect	Current	(unchanged)
0x54 0x55	Declare Recipes	Current	(unchanged)
0x55 0x56	Tags	Current	Pre
0x57	Update Light		Pre

New/modified data types

No changes so far.

Entity Metadata

Mobs

Entity IDs have shifted with the addition of some new mobs.

Block Actions

No changes so far.

Play

Clientbound

Chunk Data

Main article: Chunk Format

See also: #Unload Chunk

The server only sends skylight information for chunk pillars in the Overworld, it's up to the client to know in which dimenison the player is currently located. You can also infer this information from the primary bitmask and the amount of uncompressed bytes sent. This packet also sends all block entities in the chunk (though sending them is not required; it is still legal to send them with Update Block Entity later).

Packet ID	State	Bound To	Field Name	Field Type	Notes
0x22	Play	Client	Chunk X	Int	Chunk coordinate (block coordinate divided by 16, rounded down)
			Chunk Z	Int	Chunk coordinate (block coordinate divided by 16, rounded down)
			Ground-Up Continuous	Boolean	See Chunk Format
			Primary Bit Mask	VarInt	Bitmask with bits set to 1 for every 16×16×16 chunk section whose data is included in Data. The least significant bit represents the chunk section at the bottom of the chunk column (from y=0 to y=15).
			Size	VarInt	Size of Data in bytes
			Data	Byte array	See data structure in Chunk Format
			Number of block entities	VarInt	Number of elements in the following array
			Block entities	Array of NBT Tag	All block entities in the chunk. Use the x, y, and z tags in the NBT to determine their positions.

Data structure

The data section of the packet contains most of the useful data for the chunk.

Field Name	Field Type	Notes
Data	Array of Chunk Section	The number of elements in the array is equal to the number of bits set in Primary Bit Mask. Sections are sent bottom-to-top, i.e. the first section, if sent, extends from Y=0 to Y=15.
Biomes	Optional Int Array	Only sent if Ground-Up Continuous is true; 256 entries if present

Chunk Section structure

A Chunk Section is defined in terms of other data types. A Chunk Section consists of the following fields:

Field Name	Field Type	Notes
Block count	Short	Number of non-air blocks present in the chunk section, for lighting purposes. "Non-air" is defined as any block other than air, cave air, and void air (in particular, note that fluids such as water are still counted).
Bits Per Block	Unsigned Byte	Determines how many bits are used to encode a block. Note that not all numbers are valid here.
Palette	Varies	See below for the format.
Data Array Length	VarInt	Number of longs in the following array
Data Array	Array of Long	Compacted list of 4096 indices pointing to state IDs in the Palette
Block Light	Byte Array	Half byte per block
Sky Light	Optional Byte Array	Only if in the Overworld; half byte per block

Data Array, Block Light, and Sky Light are is given for each block with increasing x coordinates, within rows of increasing z coordinates, within layers of increasing y coordinates. For the half-byte light arrays, even-indexed items (those with an even x coordinate, starting at 0) are packed into the low bits, odd-indexed into the high bits.

Entity Sound Effect

Plays a sound effect from an entity.

Packet ID	State	Bound To	Field Name	Field Type	Notes
0x4E	Play	Client	Sound ID	VarInt	ID of hardcoded sound event (events as of 1.13.2)
			Sound Category	VarInt Enum	The category that this sound will be played from (current categories)
			Entity ID	VarInt
			Volume	Float	1.0 is 100%, capped between 0.0 and 1.0 by Notchian clients
			Pitch	Float	Float between 0.5 and 2.0 by Notchian clients

Tags

Packet ID	State	Bound To	Field Name	Field Type	Notes
0x55 0x56	Play	Client	Block Tags	(See below)	IDs are block IDs
			Item Tags	(See below)	IDs are item IDs
			Fluid Tags	(See below)	IDs are fluid IDs
			Entity Tags	(See below)	IDs are entity type IDs

Update Light

Updates light levels for a chunk.

Packet ID	State	Bound To	Field Name	Field Type	Notes
0x57	Play	Client	Chunk X	VarInt	Chunk coordinate (block coordinate divided by 16, rounded down)
			Chunk Z	VarInt	Chunk coordinate (block coordinate divided by 16, rounded down)
			Sky Light Mask	VarInt	Mask containing 18 bits, with the lowest bit corresponding to chunk section -1 (in the void, y=-16 to y=-1) and the highest bit for chunk section 16 (above the world, y=256 to y=271)
			Block Light Mask	VarInt	Mask containing 18 bits, with the same order as sky light
			Sky Light arrays	Array of arrays of 2048 bytes	There is 1 array for each bit set to true in the sky light mask, starting with the lowest value. Half a byte per light value.
			Block Light arrays	Array of arrays of 2048 bytes	There is 1 array for each bit set to true in the block light mask, starting with the lowest value. Half a byte per light value.

Individual block or sky light arrays are is given for each block with increasing x coordinates, within rows of increasing z coordinates, within layers of increasing y coordinates. Even-indexed items (those with an even x coordinate, starting at 0) are packed into the low bits, odd-indexed into the high bits.

Serverbound

No changes so far.

Handshaking

Clientbound

There are no clientbound packets in the Handshaking state, since the protocol immediately switches to a different state after the client sends the first packet.

Serverbound

Handshake

This causes the server to switch into the target state.

Packet ID	State	Bound To	Field Name	Field Type	Notes
0x00	Handshaking	Server	Protocol Version	VarInt	See protocol version numbers (currently 404 442)
			Server Address	String	Hostname or IP, e.g. localhost or 127.0.0.1, that was used to connect. The Notchian server does not use this information.
			Server Port	Unsigned Short	Default is 25565. The Notchian server does not use this information.
			Next State	VarInt Enum	1 for status, 2 for login

Status

Clientbound

No changes so far.

Serverbound

No changes so far.

Login

Clientbound

No changes so far.

Serverbound

No changes so far.