Pre-release protocol

This page documents the changes from the last stable Minecraft release (currently 1.1) to the current pre-release

New packets

Entity Head Look (0x23)

Changes the direction an entity's head is facing.

Packet ID	Field Name	Field Type	Example	Notes
0x23	Entity ID	int
	Head Yaw	byte		Head yaw in steps of 2π/256
Total Size:	6 bytes

Update Tile Entity (0x84)

Essentially a block update on a tile entity.

Packet ID	Field Name	Field Type	Example	Notes
0x84	X	int
	Y	short
	Z	int
	Action	byte		The type of update to perform
	Custom 1	int		Varies
	Custom 2	int		Varies
	Custom 3	int		Varies
Total Size:	24 bytes

Actions

• 1: Set mob displayed inside a mob spawner. Custom 1 contains the mob type

Changed packets

Login Request (0x01)

World height field is no longer used, always 0.

Map seed field removed

Packet ID	Field Name	Field Type	Example	Notes
0x01	Entity ID	int	1298	The Players Entity ID
	Not used	string	(empty string)	Not used
	Level Type	string	default	default or SUPERFLAT; level-type in server.properties
	Server mode	int	0	0 for survival, 1 for creative
	Dimension	int	0	-1: The Nether, 0: The Overworld, 1: The End
	Difficulty	byte	1	0 thru 3 for Peaceful, Easy, Normal, Hard
	World height	byte	0	Probably not used anymore since Map Chunk is hardcoded to 256 blocks.
	Max players	unsigned byte	8	Used by the client to draw the player list
Total Size:	19 bytes + length of strings

Handshake (0x02)

Client to Server

The hostname and port were added to this packet, most likely to allow virtual hosting .(sample implementation)

Packet ID	Field Name	Field Type	Example	Notes
0x02	UsernameAndHost	string	TkTech;localhost:25565	The username of the player attempting to connect, and the host he is connecting to, seperated by a semicolon.
Total Size:	3 bytes + length of strings

Respawn (0x09)

Dimension is now an int, slightly bizarrely. I wonder if they intended to make world_height an int, in line with 0x01 login?

Map seed field removed

Packet ID	Field Name	Field Type	Example	Notes
0x09	Dimension	int	1	-1: The Nether, 0: The Overworld, 1: The End
	Difficulty	byte	1	0 thru 3 for Peaceful, Easy, Normal, Hard. 1 is always sent c->s
	Creative mode	byte	1	0 for survival, 1 for creative.
	World height	short	128	Defaults to 128
	Level Type	string	DEFAULT	See 0x01 login
Total Size:	11 bytes + length of string

Mob Spawn (0x18)

New byte field: head yaw

This needs confirmation!

Packet ID	Field Name	Field Type	Example	Notes
0x18	EID	int	446	Entity ID
	Type	byte	91	The type of mob. See Entities#Mobs
	X	int	13366	The Absolute Integer X Position of the object
	Y	int	2176	The Absolute Integer Y Position of the object
	Z	int	1680	The Absolute Integer Z Position of the object
	Yaw	byte	-27	The yaw in steps of 2π/256
	Pitch	byte	0	The pitch in steps of 2π/256
	Head Yaw	byte		Head yaw in steps of 2π/256
	Metadata	Metadata	127	Varies by mob, see Entities
Total Size:	21 bytes + Metadata (at least 1)

Map Chunk (0x33)

This is currently a best guess

Here's a packet capture: https://gist.github.com/1835702

Packet ID	Field Name	Field Type	Example	Notes
0x33	X	int		Chunk X Coordinate (*16 to get true X)
	Z	int		Chunk Z Coordinate (*16 to get true Z)
	Ground-up contiguous	boolean		This is True if the chunks consists of a contiguous column of non-air (i.e., indicated with a '1' in the bitmask) chunks starting at Y=0, followed by any number of unsent chunks containing only air.
	Primary bit map	short	15	Bitmask with 1 for every 16x16x16 chunk which data follows in the compressed data.
	Add bit map	short	0	Same as above, but this is used exclusively for the 'add' portion of the payload
	Compressed size	int		Size of compressed region data.
	Unused int	int	0	Doesn't seem to be used by the client. Always 0. I expect this is Mod API stuff.
	Compressed data	unsigned byte array	…	The region data is compressed using ZLib Deflate function.
Total Size:	22 bytes + Compressed chunk size

The payload is a set of 16x16x16 chunks, sharing the same X and Z chunk coordinates. What is and isn't sent is provided by the two bitmask fields. The least significant bit is '1' if the chunk spanning from Y=0 to Y=15 is not completely air, and so forth. For block IDs, metadata, and lighting, the primary bitmask is used. A secondary bitmask is used for 'add' data, which is mojang's means of provided Block IDs past 256. In vanilla minecraft, you can expect this to always be zero.

The data is compressed using the deflate() function in zlib. After uncompressing, the data consists of five (or six) sequential sections, in order:

• Block type array (1 byte per block, 4096 bytes per chunk)
• Block metadata array (half byte per block, 2048 bytes per chunk)
• Block light array (half byte per block, 2048 bytes per chunk)
• Sky light array (half byte per block, 2048 bytes per chunk)
• Add array (half byte per block, 2048 bytes per chunk, uses second bitmask)
• Biome array (1 byte per XZ column, 256 bytes total, only sent if 'ground up contiguous' is true)

In each section except biome data, your loop should look something like this:

 for (i=0;i<16;i++) {
   if (bitmask & 1 << i) {
     data = io.read(4096); //2048 for the other arrays, where you'll need to split the data
     
     for(int j=0; j<len(data); j++) {
       
       //Block data
       x = (j & 0xF0) >> 4;
       y = j & 0x0F;
       z = j >> 8;
       
       //Nibble arrays
       d1 = data[j] & 0x0F;
       d2 = data[j] >> 4;
       
       k = 2*j;
       x1 = (k & 0xF0) >> 4;
       y1 = k & 0x0F;
       z1 = k >> 8;
       
       k++;
       x2 = (k & 0xF0) >> 4;
       y2 = k & 0x0F;
       z2 = k >> 8;
      }
   }
 }

Each biome byte can have the values from 0 to 22, higher will cause the client to crash.

The 'add' array uses the second bitmask. Block IDs are presumed to be block_id | add << 8

Multi Block Change (0x34)

Further investigation shows that this is a multiple-block-change command; if you take the three arrays, and put together elements with the same index, and then decompose the short into coordinates (top 4 bits is X, next 4 bits is Z, bottom 8 bits is Y), you get things like [((8, 7, 4), 11, 0), ((7, 13, 6), 11, 0), ((13, 1, 8), 11, 0), ((7, 6, 6), 11, 0)].

See the Block Change command for description of the general format of a block change.

Packet ID	Field Name	Field Type	Example	Notes
0x34	Chunk X	int	-9	Chunk X Coordinate
	Chunk Z	int	12	Chunk Z Coordinate
	Array size	short	2	The total number of elements per array
	Unknown	int	40	Unknown
	Coordinate array	short array	…	The coordinates of the blocks to change
	Type array	byte array	…	The type for each block change
Metadata array	byte array	…	The Metadata for each block changed
Total Size:	11 bytes + Arrays

Protocol History

2012-02-23

• 12w08a
• Protocol version is now 28
• Updated packets: 0x01, 0x09, 0x34

2012-02-15

• 12w07a
• Protocol version is now 27
• Updated packets: 0x01, 0x09, 0x33

2012-02-09

• 12w06a
• Protocol version is now 25
• New packet: 0x84

2012-01-26

• 12w04a
• Protocol version has not been changed
• Handshake Packet by client (0x02) now contains the server host and port
• Window Open Packet (0x64): WindowTitle is now longer than accepted by older clients. Contains a keyword such as "container.furnace"

2012-01-19

• 12w03a
• Protocol version is now 24
• New packet: 0x23
• MobSpawn: new field, 1 byte inserted before metadata.