Pre-release protocol

This page documents the changes from the last stable Minecraft release (currently 1.17.1, protocol 756) to the current pre-release (currently 21w44a, protocol Snapshot 48). 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

No changes so far.

Packets

ID	Packet name	Documentation
Play clientbound
0x0A	Block Entity Data	Current	Pre
0x22	Chunk Data and Update Light	Current	Pre

New/modified data types

No changes so far.

Entity Metadata

No changes so far.

Entity

No changes so far.

Block Actions

No changes so far.

Inventories

No changes so far.

Plugin Channels

No changes so far.

Play

Clientbound

Block Entity Data

Sets the block entity associated with the block at the given location.

Packet ID	State	Bound To	Field Name	Field Type	Notes
0x0A	Play	Client	Location	Position
			Action	Unsigned Byte	The type of update to perform, see below.
			Type	VarInt	The type of block entity
			NBT Data	NBT Tag	Data to set. May be a TAG_END (0), in which case the block entity at the given location is removed (though this is not required since the client will remove the block entity automatically on chunk unload or block removal).

Chunk Data and Update Light

Main article: Chunk Format

See also: #Unload Chunk

	The following information needs to be added to this page:
	How do biomes work now? The biome change happened at the same time as the seed change, but it's not clear how/if biomes could be computed given that it's not the actual seed... (/r/mojira discussion which notes that it seems to be some kind of interpolation, and 3D biomes are only used in the nether)

The server only sends skylight information for chunk pillars in the Overworld, it's up to the client to know in which dimension 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
0x20 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)
			Bit Mask Length		VarInt		Length of the following array
			Primary Bit Mask		Array of Long		BitSet with bits (world height in blocks / 16) 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 the lowest y to 15 blocks above).
			Heightmaps		NBT		Compound containing one long array named MOTION_BLOCKING, which is a heightmap for the highest solid block at each position in the chunk (as a compacted long array with 256 entries at 9 bits per entry totaling 36 longs). The Notchian server also adds a WORLD_SURFACE long array, the purpose of which is unknown, but it's not required for the chunk to be accepted.
			Biomes length		VarInt		Size of the following array; should always be 1024. Not present if full chunk is false.
			Biomes		Array of VarInt		1024 biome IDs, ordered by x then z then y, in 4×4×4 blocks. Not present if full chunk is false. See Chunk Format § Biomes.
			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.
			Block Entity	Packed XZ	Array	Byte	(blockZ & 15)
				Y		Short	The height relative to the world
				Type		VarInt	The type of block entity
				Data		NBT	The block entity's data, without the X, Y, and Z values
			Trust Edges		Boolean		If edges should be trusted for light updates.
			Length 1		VarInt		Length of the following array
			Sky Light Mask		Array of Long		BitSet containing bits for each sections in the world + 2 (lowest bit 16-1 blocks below min world height, highest 1-16 above max world height)
			Length 2		VarInt		Length of the following array
			Block Light Mask		Array of Long		BitSet containing bits for each sections in the world + 2, same order as sky light
			Length 3		VarInt		Length of the following array
			Empty Sky Light Mask		Array of Long		BitSet containing a variable amount of bits (see sky/block light) which indicates sections that have 0 for all their sky light values
			Length 4		VarInt		Length of the following array
			Empty Block Light Mask		Array of Long		BitSet containing a variable amount of bits (see sky/block light) which indicates sections that have 0 for all their block light values
			Sky Light array count		VarInt		Number of entries in the following array
			Sky Light arrays	Length	Array	VarInt	Length of the following array in bytes (always 2048)
				Sky Light array		Array 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 array count		VarInt		Number of entries in the following array
			Block Light arrays	Length	Array	VarInt	Length of the following array in bytes (always 2048)
				Block Light array		Array 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.

Note that the Notchian client requires an Update View Position packet when it crosses a chunk border, otherwise it'll only display render distance + 2 chunks around the chunk it spawned in.

The compacted array format has been adjusted so that individual entries no longer span across multiple longs, affecting the main data array and heightmaps.

New format, 5 bits per block, containing the following references to blocks in a palette (not shown): 122344566480743131516914101202

0020863148418841 0000000000100000100001100011000101001000010000011000100001000001
01018A7260F68C87 0000000100000001100010100111001001100000111101101000110010000111

Serverbound

No changes so far.

Status

Clientbound

No changes so far.

Serverbound

No changes so far.

Login

Clientbound

No changes so far.

Serverbound

No changes so far.