Classic Protocol Extension

Classic Protocol Extension (CPE) is a project to augment the Minecraft Classic network protocol with new and improved functionality.

Extensions are designed to keep extended clients and servers compatible with standard clients and servers. Standard clients and extended clients can play on the same server side-by-side. Extensions are designed to be modular: custom clients and servers can selectively implement any subset of extensions, and only mutually-supported extensions will be used.

Support

Custom servers that fully support CPE: MCGalaxy ProCraft

Custom servers that partially support CPE: D3PP, MCYeti, Obsidian, ProjectObsidian, D3, fCraft, FemtoCraft, LegendCraft, GemsCraft, Hypercube, MCSong, ClassiCH

Custom servers that plan to add support: 800Craft, cloudBox, GGS

Custom clients that fully support CPE: ClassiCube Client, ClassicalSharp

Custom clients that plan to add support: Charged Miners, GemsClient

Negotiation

When CPE-capable client connects to a CPE-capable server, a brief negotiation needs to happen before any extensions are used. Client and server declare their capabilities and determine which extensions are mutually supported. All CPE-capable software is required to support this.

Client behavior: Extended clients must use magic number of 0x42 (66 decimal) for the padding byte of the PlayerIdentification packet. It must then await a response. If server responds with any packet other than ExtInfo, client must assume that the server does NOT support CPE. If the server responds with an ExtInfo packet, client must parse it and any ExtEntry packets that follow. Client must then compare its locally-supported set of extensions with the list of extensions provided by the server, and find an intersection of these sets. These are the mutually-supported extensions. Client must now send ExtInfo packet of its own, followed by a list of zero or more client-supported extensions. After sending the last of ExtEntry packets, client should activate all mutually-supported extensions and resume normal login procedure.

Server behavior: When a client connects, server must read the PlayerIdentification packet and check its padding byte. If this byte is set to 0x42 (66 decimal), assume that the client supports CPE. If this byte is set to any other value, assume that the client does NOT support CPE.

Server must immediately reply to CPE clients with an ExtInfo packet, followed by zero or more ExtEntry packets, and await a response from the client. Client will respond with one ExtInfo and zero or more ExtEntry packets. Server must then compare its locally-supported set of extensions with the list of extensions provided by the client, and find an intersection of these sets. These are the mutually-supported extensions. After receiving the last of ExtEntry packets, server should activate all mutually-supported extensions and resume normal login procedure.

Note 1: All standard/non-extended clients use 0x00 for the padding byte. All standard servers ignore this padding byte. Therefore, this negotiation process does not affect compatibility with standard software.

Note 2: Do not make any assumptions about supported functionality based on the AppName field of ExtInfo packet. It's for logging purposes only.

Note 3: Do not declare support for an extension until it is FULLY implemented, except for debugging.

ExtInfo packet

ExtEntry packet

Extensions

Note: The section heading is the name of the extension. Packet names are not same as extension names. For example, the first extension listed here is named "ClickDistance" and not "SetClickDistance".

ClickDistance

Used to extend or restrict the distance at which client may click blocks, controlled by the server. Click range is given in player-space units (32 units per block). In Minecraft Classic, the default range is 160.

Motivation: This extension allows trusted players to have a wider or virtually-unlimited reach. It may simplify operation of certain bots. Restricting the reach may allow new games/mini-games.

Client Behavior: Upon receiving a SetClickDistance packet, client should immediately apply the change. Distance should persist between worlds/maps. If 0 is given for Distance, client should not be able to click blocks at all.

Server Behavior: Server should send SetClickDistance packet when the client connects, or whenever their permitted click distance changes. Server should allow for some margin-of-error (+/- 1 block) when enforcing click distance restrictions.

SetClickDistance packet

Server to Client

CustomBlocks

Used to add support for custom block types. Custom block IDs start at 50 (0x32). New block types will be added in batches, a few at a time. Both client and server declare which batch they support, and use the lower of the two versions. Claiming to support a batch implies fully implementing all the batches that came before it. If either server or client do not support this extension, only the standard 50 block types should be used.

Motivation: Adding new visually distinct blocks, to enhance Classic players' experience.

Client behavior: Client must expect a CustomBlockSupportLevel packet from a compatible server immediately after sending the last ExtEntry packet. It should then reply with its own CustomBlockSupportLevel packet, containing its actual support level. Client must then use the lower of the two levels in operation. Client must not send any block types that are not defined by the current support level. Client should expect ServerIdentification packet only AFTER sending its CustomBlockSupportLevel packet.

Server behavior: Server must send a CustomBlockSupportLevel packet to compatible clients immediately after receiving the last ExtEntry packet from the client. It should then wait to receive a CustomBlockSupportLevel packet from the client before sending the ServerIdentification packet. Server must then use the lower of the two levels in operation. If this level is lower than the server's, it has to filter data sent to the client, to make sure that the client never receives any block types that it does not support. Each level will define what substitutions to use.

CustomBlockSupportLevel packet

Bidirectional

Blocks in support level 1

Client must be able to receive/render all 16 custom blocks to claim support. Server must be able to receive/store all 16 custom blocks to claim support.

Block IDs for future support levels are guaranteed to be assigned monotonically, incrementally, and permanently.

HeldBlock

Provides a way for the client to notify the server about the blocktype that it is currently holding, and for the server to change the currently-held block type.

Motivation: This allows server to know which block player is holding, for example for drawing commands, without needing to wait for the player's click. It also allows for features like /Spectate to show what block a spectated player is holding.

Client behavior: When this extension is mutually supported, client should use the PlayerID field of the PositionAndOrientation packet (currently unused) to indicate which blocktype the client is currently holding. It should be ready to accept HoldThis packets to change the block that the player is holding. If an unrecognized blocktype is given, no action is needed.

Server behavior: The server can use HoldThis packet to force the client to hold the desired block type. It should be done sparingly.

HoldThis packet

Server to Client

EmoteFix

This extension indicates that the client can render emotes (ASCII control characters) in chat properly, without padding or suffixes that are required for vanilla client. This extension does not define any new packets.

Motivation: To improve appearance of emotes in chat.

Client behavior: Client should not emulate vanilla client's quirks.

Server behavior: Server should not pad or suffix emotes in chat.

TextHotKey

This extension allows the server to define "hotkeys" for certain commands.

Motivation: To speed up and simplify access to commonly-used commands and command macros by providing server-defined client-side hotkeys.

Client behavior: Client should not try to persist previously-defined hotkeys between sessions. When a defined hotkey is activated by the user, client should open up a text prompt and type in contents of the Action field. A newline character (\n) in the Action field indicates that whatever is currently typed-in should be sent to the server. If Action does not end with a newline, text prompt should be left open, for the user to complete. Client may provide a way for the user to see a list of currently-defined hotkeys, and a way to notify the user when a hotkey was activated.

Server behavior: The server should send a definition of each hotkey (SetTextHotKey packet) once per connection.

SetTextHotKey packet

Server to Client

ExtPlayerList

Motivation: Provides more flexibility in naming of players and loading of skins, autocompletion, and player tab-list display. Separates tracking of in-game entities (spawned player models) and names on the player list. ExtAddPlayerName/ExtRemovePlayerName packets take over managing the player names list (tab-list), and ExtAddEntity2/DespawnPlayer packets are used only to manage in-game entities.

Version 1 (Deprecated since 28 August 2014)

This version of the extension has been deprecated and replaced with version 2.

Version 2

Version 2 of this extension replaces ExtAddEntity packet with ExtAddEntity2, allows using full URLs for SkinName, and clarifies interaction with ChangeModel extension.

Client Behavior

When ExtAddPlayerName packet is received for an unrecognized NameID, a new name must be added to the player-name list. When receiving ExtAddPlayerName packet for an already-listed NameID, client must update its ListName, GroupName, and GroupRank. Player-name list must persist when client changes worlds/maps.

When an ExtAddEntity2 packet is received, it must be treated as the SpawnPlayer packet. A player model must be spawned in-game at the given location, with InGameName text drawn above it. Skin should be loaded using the given SkinName for a player name. When client receives ExtAddEntity2 packet for an already-spawned player, a duplicate entity must not be spawned and existing entity's position must not be changed. Instead their InGameName and SkinName must be updated. If a negative EntityID is given for ExtAddEntity2, client must update player's own spawn point, InGameName, and SkinName. The client must ignore regular SpawnPlayer packets, if any are received.

Player entity's skin should be loaded using the given SkinName. If SkinName appears to be a player name, skin should be downloaded from the default skin server. If SkinName appears to be a full URL to a PNG image (starts with http:// or https:// and ends with .png) then skin should be downloaded from that URL. If image is correctly sized/proportioned to use as a skin for the current model, it should be used. If a blank or unrecognized value is given for SkinName, or if given image could not be downloaded or used, then the default skin should be used.

Names on the player-name list should be grouped by GroupName in the player-name list. Names within a GroupName should be sorted by GroupRank (in ascending order). Names with the same GroupName and GroupRank should be sorted alphabetically by ListName. Color codes may be either drawn or stripped from ListName, GroupName, and InGameName.

When a standard DespawnPlayer packet is received for a recognized EntityID, player model must be removed from a world. When ExtRemovePlayerName packet is received for a recognized NameID, their name must be removed from player-name list. Packets with out-of-range or unrecognized NameIDs must be ignored.

In-game entities must never be affected by ExtAddPlayerName or ExtRemovePlayerName packets. Player name list must never be affected by ExtAddEntity2 or DespawnPlayer packets.

Server Behavior

Unique NameID between 0 and 255 should be assigned to every online player. When a new player connects to the server, ExtAddPlayerName must be sent. GroupName and GroupRank can be used in any way, for example to group players by map/world or rank/class/faction. Server must use ExtAddEntity2 in place of standard SpawnPlayer packet. Server should re-send ExtAddPlayerName packet, using the identical NameID, when player's ListName, GroupName, or GroupRank change. Server must reliably send an ExtRemovePlayerName packet when the player disconnects. Color codes are permitted in ListName, GroupName, and InGameName.

ExtAddPlayerName packet

Server to Client

ExtAddEntity2 packet

Server to Client

ExtRemovePlayerName packet

Server to Client

EnvColors

This extension allows server to alter some of the colors used by the client in environment rendering.

Motivation: To allow the server to give worlds/maps a unique feel: time-of-day, weather/climate, lighting effect, etc.

Client behavior: Client must check for EnvSetColor packets right before LevelFinalize packet, and apply these changes before the map is displayed. Client must be able to read this packet at other times, but it is not required to then apply the changes immediately. If an unrecognized or unsupported Variable field is given, no action is needed. If an out-of-range color is given by the server (i.e. if any of Red, Green, or Blue is less than 0 or greater than 255), then the specified Variable should be reset to its default value.

Server behavior: Server should normally only use EnvSetColor packets right before the LevelFinalize packet. To reset a variable to its default value, the server should send a packet with value -1 for Red, Green, and Blue.

EnvSetColor packet

Server to Client

SelectionCuboid

Motivation: Allows the server to highlight parts of a world. Applications include zoning, previewing draw commands, previewing undo commands.

Coordinates: {StartX,StartY,StartZ} are coordinates of the block inside the selection that is closest to the map origin. {EndX,EndY,EndZ} are coordinates of the block inside the selection that is furthest from the map origin. Therefore, the resulting selection has dimensions {EndX-StartX+1, EndY-StartY+1, EndZ-StartZ+1).

Client behavior: Client should be ready to receive MakeSelection packets any time after LevelFinalize packet. Upon receiving the packet, a translucent cuboid should appear in the world. The cuboid may feature a plain or "grid" texture. Selections that extend outside the map may be either ignored or clipped to fit. Selections with inconsistent coordinates (e.g. where StartX<EndX) may either be ignored or re-ordered. Out-of-range values for Red, Green, Blue, and Opacity should be clipped to fit the valid range. Supporting Opacity is optional: the client may opt to provide fixed opacity instead. When map changes (i.e. when LevelInitialize packet is received), all existing selections should be removed. RemoveSelection packets that refer to non-existent SelectionIDs should be ignored.

Server behavior: All given coordinates must be contained within the map. End coordinates should be higher or equal than start coordinates.

MakeSelection packet

Server to Client

RemoveSelection packet

Server to Client

BlockPermissions

This extension allows the server to instruct the player that certain block types are allowed/disallowed to be placed or deleted.

Motivation: To prevent players from inadvertently placing or removing prohibited block types (e.g. water, lava, grass, admincrete), before it even reaches the server.

Client behavior: Client should prevent placement of prohibited block types (by graying out or hiding blocks in block-selection screen, or any other effective means). Client should prevent player from deleting prohibited block types. Client must be ready to receive SetBlockPermission packet after map load (LevelFinalize packet). Permission changes should take effect as soon as packet is received. Admincrete (solid block) permissions set by SetBlockPermission must always override permission set by ServerIdentification and UpdateUserType packets. If BlockType is set to 0, the client behaviour is undefined. Permissions must persist between map changes. Client may optionally warn the player attempting to place/delete prohibited blocks via sound effect, visual effect, chat message, etc.

Server behavior: Server may send SetBlockPermission packets any time after map load (LevelFinalize packet). Any valid block ID may be specified for BlockType, including custom blocks (if CustomBlocks extension is mutually supported). Server must not assume that client is compliant/obedient, and server must still verify each SetBlock packet coming from the client. What to do with non-complying clients (kick or warn) is up to you.

SetBlockPermission packet

Server to Client

ChangeModel

Allows changing appearance of player models in supporting clients.

Client Behavior

The client will receive an EntityID and a string value containing the model name. The client will then change the model of the player whose ID is the same as the received AddEntity, ExtAddEntity, or ExtAddEntity2 packet. The model name will be parsed by the model manager and the model changed in game. If the model does not exists in the model manager or is 0-length, change the model back to humanoid. Alternatively, you can send the client an int converted to a string which represents a valid Block ID. An EntityID of -1 (255 unsigned) indicates the player's own model. If model is set to humanoid, player-specific skin should be used. For all other models, default model-specific textures should be used.

Server Behavior

The server may send an EntityID and then a ModelName to the client for a desired entity. The model name must be chosen from the list below. To reset the model to default, server should set ModelName to humanoid or to an empty string.

Interaction with ExtPlayerList

If an ExtAddEntity2 packet is received for a non-humanoid entity and SkinName specifies a full URL, that image should be downloaded and used as a texture for the currently model. Receiving another ExtAddEntity2 packet should either update this texture (if SkinName is a full URL) or reset it back to default (if SkinName is anything else). Receiving another ChangeModel packet with a non-humanoid ModelName should reset texture back to model-specific default.

ChangeModel packet

Available models

Client can render any or none of the below, but it is down to the client to handle what can and cannot be rendered if the packet is received

EnvMapAppearance

This extension allows the server to specify custom terrain textures, and tweak appearance of map edges.

Motivation: To provide more ways to customize map appearance, including functionality that's currently provided by World of Minecraft's scheme.

All versions of this extension have been deprecated and replaced with EnvMapAspect.

EnvWeatherType

This extension allows the server to trigger special weather conditions (like rain and snow) on demand.

Motivation: To allow the server to give worlds/maps a unique feel with weather effects, e.g. for adventure maps.

Client behavior: Client must be able to receive EnvWeather packets at any time after the first LevelDataChunk packet is received, and apply these changes immediately. If an unrecognized or unsupported WeatherType field is given, no action is needed.

Server behavior: Server may send EnvSetColor packets to connected clients at any time after the first LevelDataChunk packet is sent.

EnvSetWeatherType packet

Server to Client

HackControl

This extension allows the server to control cheats/hacks in the client.

Motivation: To allow fine-grained control over cheats/hacks on multi-world servers.

Client behavior: Client must be able to receive HackControl packets at any time after the first LevelDataChunk packet is received, and apply these changes immediately. Unrecognized or unsupported values for any field should be ignored. Clients may approximate JumpHeight by rounding down to the nearest half-block (i.e. nearest multiple of 16), if needed. If a negative value is given for JumpHeight, client should reset jump height to its default setting (around 40 in vanilla Minecraft).

Server behavior: Server may send HackControl packets to connected clients at any time after the first LevelDataChunk packet is sent.

HackControl packet

Server to Client

MessageTypes

This extension adds new ways of presenting messages in the client.

Motivation: To enhance the display of announcements and status information, and to reduce chat clutter.

Client behavior: When this extension is mutually supported, the PlayerID field of the standard server-to-client Message packet should be treated as a MessageType code. Unrecognized or unsupported codes may be ignored (in which case the message should be presented as a regular chat message). When connected to non-supporting servers, this field should be ignored.

Server behavior: Server may use the PlayerID field of the Message packet to set a MessageType. For non-supporting clients, this field should always be set to 0.

This extension does not define any new packets.

PlayerClick

This extension lets the server receive details of every mouse click a player makes, including targeting information.

Motivation: To allow for the creation of more interactive maps: interactive buttons, cannons, ability to "tag" or "shoot" other players, ability to drag-to-draw at a distance, etc.

Client Behavior: When this extension is mutually supported, a PlayerClicked packet should be sent to the server, as soon as reasonably possible, every time the user presses or releases a mouse button. Mouse interactions that happen while player is interacting with chat, with GUI menus, or with any other application must NOT be sent. A matching "released" event must be sent for every "pressed" event.

High resolution up-to-date Yaw and Pitch for the player should be transmitted with every packet. If player is currently targeting another entity/player TargetEntityID fields should be set. If player is currently targeting a block (within permitted ClickDistance) TargetBlockX, TargetBlockY, TargetBlockZ, and TargetBlockFace should be set. An out-of-range value should be used for these fields when they are not in use. These packets should not be sent during level transitions (after receiving LevelInitialize and until receiving LevelFinalize packets).

Server Behavior: Server must be ready to accept any number of PlayerClicked packets at any time after level sending has completed. What to do with this information is entirely up to the server.

PlayerClicked packet

Client to Server

LongerMessages

This extension allows clients to accept messages longer than 64 characters, and send them to the server in parts.

Motivation: To allow players to write longer messages!

Client Behavior: When a regular-length message is sent, a single Message packet is sent, with the unused PlayerID (aka MessageType) byte set to 0. When an extra-long message is sent, multiple Message packets are sent. All packets except the last one should have the unused byte set to 1, meaning "this message continues in the next Message packet".

Server Behavior: Server should check the unused PlayerID (aka MessageType) byte of every incoming Message packet. If the value is non-zero, it should treat it as a partial message. Partial messages should be stored and prepended to the next message received from this player, until a message with a 0 byte is received. Server should preserve leading and trailing whitespace in message packets with a 1 byte. Server should preserve leading whitespace, but trim trailing whitespace from the last packet of a partial message.

No new packets are defined, but treatment of Message packet is altered.

FullCP437

This extension allows players to send and receive chat with all characters in code page 437, rather than just the 0 to 127 characters.

Motivation: To allow players to use more characters for different purposes.

Client Behavior: When a message packet is received, clients must treat all characters in that message as unsigned bytes that represent indices into code page 437. Clients may optionally provide a way of directly entering code page 437 characters into chat.

Server Behavior: Servers must support being able to send and receive chat message packets which include unsigned byte values ranging from 0 to 255. Servers must also provide a fallback mechanism for converting unsigned bytes in the 128-255 range into the 0 to 127 range for clients that do not support this extension. (e.g converting all unsigned bytes in the 128 to 255 range to a '?' symbol)

Note: Ideally, servers should also provide some method of escaping that allows users to enter a token that is then converted into a code page 437 index, but that is not required to claim support for this extension.

As an example of an escaping mechanism, when "{infinity}" is entered by the player, it becomes a single unsigned byte with a value of '236' after being processed by the server. When the chat message packet is later received by the client, it renders the unsigned byte with the value of '236' as ∞.

BlockDefinitions

This extension allows servers to define new block types with custom IDs and appearance, as well as the ability to redefine the properties of standard block types.

Motivation: To allow defining block types without client modifications. To make full use of custom textures.

Client Behavior: Client must be able to receive any number of DefineBlock and RemoveBlockDefinition packets at any time. Servers should prefer to send these packets during level loading (after sending LevelInitialize and before sending LevelFinalize), as sending these after level loading may cause the client to perform expensive operations such as redrawing the map. Client must keep track of all defined blocks between level changes or until they are individually removed with RemoveBlockDefinition.

Client must be able to accept and render blocks as specified in DefineBlock packets. Clients must accept any valid definitions with BlockID between 1 and 255, inclusive. If a new definition is accepted for an already-defined BlockID, the old definition should be replaced. If RemoveBlockDefinition is received for a non-defined BlockID, it should be ignored. Otherwise, the properties of that block should be reset to their defaults.

Server Behavior: Server may send any number of DefineBlock packets at any time. Server should only use IDs of currently-defined blocks in block data sent in LevelDataChunk packets, SetBlock packets, or any other packets that expect block IDs.

Texturing: New textures are defined by altering terrain.png and specifying it with SetMapEnvUrl packet. DefineBlock fields TopTextureID, SideTextureID, and BottomTextureID refer to a tile number within terrain.png. Tiles are numbered 0 (top-left corner) through 255 (bottom-right corner), increasing left-to-right within each row. Refer to this image for a list of standard tiles, and a graphical list of tile indices.

Compatibility: For non-supporting clients, the server must convert custom block ids into fallback block ids (e.g. 'lamp' fallbacks to lava) that lie within the client's set of supported blocks. (TODO: Define how the client should store map files with block definitions. Maybe integrate this with ClassicWorld definition?) 0 is not allowed to be redefined.

DefineBlock packet

Server to Client

RemoveBlockDefinition packet

Server to Client

BlockDefinitionsExt

This extension allows servers to define new block types with custom IDs and appearance, as well as the ability to redefine the properties of standard block types.

Motivation: To allow defining custom block types that do not have minX/Y/Z values of 0, and maxX/Z values of 1.

Client Behavior: See the BlockDefinitions specification. This extension depends on BlockDefinitions extension, and must only be enabled if that extension is also mutually supported.

Version 1 (Deprecated since 9 May 2016)

This version of the extension has been deprecated and replaced with version 2.

Version 2

Texturing: See the BlockDefinitions specification. The MinX/Y/Z and MaxX/Y/Z fields also affect the location within the terrain tile for a given face that the block should sample texels from. (e.g. if minX and minZ was 8, and maxX and maxZ was 16, then for the terrain.png tile with pixels from (0, 0) to (16, 16), the top face would be drawn with the subset of pixels from (8, 8) to (16, 16))

Server Behavior: See the BlockDefinitions specification.

This packet is the same as DefineBlock packet, except the 'shape field' is replaced by 6 fields indicating the bounding box of the block in pixel coordinates, and the 'side texture' is replaced by 4 fields indicating the individual textures for each horizontal side face. Note that this packet does not support sprites, you must send a regular DefineBlock packet for that.

DefineBlockExt packet

Server to Client

BulkBlockUpdate

Allows servers to send a single optimised packet that contains 256 block updates.

Motivation: Reduce bandwidth usage and increase responsiveness to clients during times of heavy block updates, e.g. for large cuboids or lava survival servers.

Client Behavior: When this extension is mutually supported, treat the packet in the same manner as a list of up to 256 SetBlock packets.

Server Behavior: When this extension is mutually supported, server may send BlockBulkUpdate packets to the client after the LevelFinalize packet for the current level has been sent to the client.

BulkBlockUpdate packet

Server to Client

TextColors

This extension allows the server to define custom text colors.

Motivation: To allow parts of chat to be more distinctive, allow for more colorful chat, etc.

Client Behavior: Client must be able to receive SetTextColor packets at any time. Client must then draw text using the color specified in the packet, when the color code specified by the packet is encountered in any Message packet.

An alpha/opacity of 0 is treated as removing the custom colour code from the client's list of custom color codes.

Server Behavior: Server may send SetTextColor packets at any time.

Compatibility: Clients may choose to always draw text fully opaque, even if the custom text colour specifies a non fully-opaque color. These clients shall still be considered as compliant with the specification.

For maximum compatibility, it is recommended that the color code in the SetTextColor be greater than 0x20(space) and less than 0x7F(delete). The color code must not be '&', '%', ' ', or '\0'.

Clients may support overriding /replacing the standard hex color codes, when the code in the range 0-9, a-f, or A-F.

SetTextColor packet

Server to client

Example: Suppose the server sends a SetTextColor packet of color #123456 and color code 'G'. When the client receives a Message packet with the text &fFoo&GBar, Foo is drawn using the color #FFFFFF and Bar is drawn using the color #123456.

EnvMapAspect

This extension allows the server to specify custom texture packs, and tweak appearance of a map.

Client behavior: Client must be able to receive SetMapEnvUrl and SetMapEnvProperty packets at any time after the LevelInitalise packet is received for a level. If the TexturePackURL field in a SetMapEnvUrl packet is blank or if the given file could not be loaded for any reason, then the texture pack should be reset to the client's default. If an unsupported block ID is given for SideBlock or EdgeBlock, it should be ignored. Client should keep using the texture pack for future maps, unless specified otherwise by the server.

Server behavior: Server may send SetMapEnvUrl and SetMapEnvProperty packets after the LevelInitalise packet is sent for a level. Server should not use any custom block IDs unless the client declared the appropriate CustomBlocks support level. To reset the texture pack to the client's default one, server should send an SetMapEnvUrl packet with empty string for TexturePackURL. To reset other properties, server should simply use the default values (listed below).

Block type restrictions: Only solid blocks are allowed to be used for SideBlock and EdgeBlock properties. Sprites (Sapling, Dandelion, Rose, BrownMushroom, RedMushroom, Rope, Fire) partial-height blocks (Slab, CobblestoneSlab, Snow), and transparent blocks (Air, Leaves, Glass) cannot be used for those fields.

SetMapEnvUrl packet

Server to Client

Texture pack .zip format

Note: It is up to clients which files (except for terrain.png) they support in a texture pack. These clients are still considered to be compliant with the specification.

Note: Texture packs may also contain other files not listed below. If clients recognise these other files, then they may perform whatever is desired with them.

Note: If a texture pack does not contain a certain texture, clients should continue to use the existing texture from the previous texture pack.

animations.txt format

SetMapEnvProperty packet

Server to Client

EntityProperty

Allows servers to change the properties of entities, beyond changing their position and yaw/pitch.

Motivation: Allow for greater customisation, while also allowing for extension to more properties in the future.

Client Behavior: Client must be able to receive SetEntityProperty packets after the LevelFinalize packet for the current level has been received from the server.

Server Behavior: Server may send SetEntityProperty packets to the client after the LevelFinalize packet for the current level has been sent to the client.

SetEntityProperty packet

Server to Client

ExtEntityPositions

Expands entity positions from the range of [-1024, 1023] to [-67,108,864, 67,108,863].

Motivation: Allows for players to interact and see other players properly on very large/tall/long worlds.

Client Behavior: Client must read 32 instead of 16 bit integers for the AddEntity, EntityTeleport, ExtAddEntity2, ExtEntityTeleport, and SetSpawnpoint packets. It must write 32 instead of 16 bit integers for the Position and Orientation packet sent to the server.

Server Behavior: Server must write 32 instead of 16 bit integers for the AddEntity, EntityTeleport, ExtAddEntity2, ExtEntityTeleport, and SetSpawnpoint packets. It must read 32 instead of 16 bit integers for the Position and Orientation packet received from the client.

Note: No new packets are defined by this extension, instead, the reading and writing behaviour of existing packets is modified. The behaviour change only applies to the X, Y, Z fields of the aforementioned packets.

TwoWayPing

Allows servers and clients to send identifiable ping packets.

Motivation: Allows clients and servers to calculate ping statistics.

Client Behavior: Periodic TwoWayPing packets should be sent to the server, with a Direction of 0. Behavior upon receipt of a TwoWayPing packet depends on Direction:

• If 0, update ping statistics. (Typically using difference between current time and time a ping packet with a matching Data was sent)
• Else, immediately send the packet back to the server.

Server Behavior: Periodic TwoWayPing packets should be sent to the client, with a Direction of 1. Behavior upon receipt of a TwoWayPing packet depends on Direction:

• If 0, immediately send the packet back to the client.
• Else update ping statistics. (Typically using difference between current time and time a ping packet with a matching Data was sent)

TwoWayPing packet

Bidirectional

InventoryOrder

Allows servers to explicitly control the inventory's layout.

Motivation: Allows users greater over the inventory, such as grouping related blocks, hiding blocks, etc.

Client Behavior: If Order is 0, client must remove the given block from the inventory. Otherwise, the given inventory slot must be set to the given block (e.g. Stone by default has slot 1). Clients must not perform any reordering or eliding of slots. As such, there may be gaps between inventory slots, or the same block put in multiple inventory slots. (See the note below.) Clients may optionally choose to not render completely empty rows in the inventory.

Server Behavior: Servers decide how to handle multiple blocks being assigned to the same inventory slot, before sending SetInventoryOrder packets to the client. For example, servers may decide to do nothing, in which case the client shows the inventory slot as the block from the last packet sent. Alternatively, server may decide to move conflicting blocks into other free inventory slots, or some other method.

Note: The SetInventoryOrder packet was previously documented here with the fields reversed, but implementations in practice specify the BlockId field first. Additionally, ClassiCube as of version 1.3.8 does not allow for the same block in multiple slots.¹

SetInventoryOrder packet

Server to client

InstantMOTD

Indicates a client supports receiving Server Identification packets at any time, not just before a map is sent.

Motivation: Sending Server Identification packets is useful for changing MOTD variables (such as hacks statuses). However, some clients behave unpredictably when receiving a Server Identification packet without the whole map being resent afterwards.

Client Behavior: Clients must be prepared to receive Server Identification packets at any time. Do not assume that a new map will be sent after receiving this packet.

Server Behavior: Server must keep track of which clients it can safely send only Server Identification packets to, and which clients require the whole map to be resent afterwards.

Note: No new packets are defined by this extension.

ExtendedBlocks

Allows using more than 256 unique block types.

Motivation: Allows for more detailed maps by allowing up to 768 instead of 256 different block types in a map.

Client Behavior: Clients must adjust how they read the map as described in the Affect on map sending section. Clients must also adjust how they read and write several fields of the packets described in the Affect on other packets section. When a client detects that the secondary map blocks array is used, it must switch from accessing blocks via map.blocks[index] to map1.blocks[index] | (map2.blocks[index] << 8) instead.

Server Behavior: Servers must adjust how they write the map as described in the Affect on map sending section. Servers must also adjust how they read and write several fields of the packets described in the Affect on other packets section. Ideally, servers should avoid sending a secondary map blocks array when possible to minimise client memory usage.

Affect on map sending

Essentially, instead of LevelDataChunk packets just being used to send the compressed form of the map blocks array, it is repurposed to send both the compressed form of the 'main' map blocks array and optionally a secondary map blocks array. To accomplish this, the Percent Complete field in the LevelDataChunk packet is repurposed. A value of 0 indicates that this LevelDataChunk packet contains part of the compressed data for the 'main' map blocks array, while a value of 1 indicates this LevelDataChunk packet contains part of the compressed data for the optional secondary map blocks array.

The client and server should treat the LevelDataChunk packets used for the 'main' map blocks array as completely separate from the secondary map blocks array, i.e. the 'main' map blocks array should be compressed and decompressed completely independently of the optional secondary map blocks array.

Note: Even if the server only sends a 'main' map blocks array, the client must still be prepared to allocate a secondary map blocks array at any time after map load. For example:

• Client A goes to map XYZ
• Server sends client A just a main blocks array for map XYZ
• A minute later, client B places a block with ID 510 on map XYZ
• Server then broadcasts this block change to all clients on map XYZ

Since a secondary map blocks array was not sent during map load by the server, Client A must now allocate a secondary map blocks array.

Affect on other packets

Several fields in some packets are expanded in size. These are:

Affect on BulkBlockUpdate

Although the affect on most packets is straightforward, the changes to BulkBlockUpdate requires a more thorough explanation:

• The first 256 bytes store the lower 8 bits of the ID for each of the 256 blocks.
• The remaining 64 bytes store the higher 2 bits of the ID for each of the 256 blocks, with 4 of these packed into 1 byte.

Code example for how a client would read the block ids from BulkBlockUpdate when ExtendedBlocks is mutually supported:

var block_ids = new short[256];
for (var i = 0; i < count; i++)
    block_ids[i] = Blocks[i]
end for

for (var i = 0; i < count; i += 4) {
    var flags = Blocks[256 + (i >> 2)]
    block_ids[i + 0] |= ((flags & 0x03) >> 0) << 8
    block_ids[i + 1] |= ((flags & 0x0C) >> 2) << 8
    block_ids[i + 2] |= ((flags & 0x30) >> 4) << 8
    block_ids[i + 3] |= ((flags & 0xC0) >> 6) << 8
end for

FastMap

Reduces load on clients and servers by reducing the complexity of sending the map.

Motivation: Clients and servers no longer need to compute or validate a CRC32 for the map data. Clients can also pre-allocate output buffers for decompression.

Client Behavior: LevelInit packet data is changed to consist of a 32 bit integer (instead of no data), representing the size of the map. Data in LevelDataChunk packets must be decompressed using DEFLATE instead of GZIP algorithm.

Server Behavior: Server must write a 32 bit integer for the LevelInit packet data, representing the size of the map. Data in LevelDataChunk packets must be compressed using DEFLATE instead of GZIP algorithm.

Note: The uncompressed data is no longer prefixed by the size of the map, it purely consists of the data of the map.

Note: DEFLATE means raw deflate, no ZLib header or footer. Behaviour of clients when decompressed data size does not equal the size specified in LevelInit packet is unspecified.

ExtendedTextures

Allows using more than 256 unique textures.

Motivation: Allows for more detailed maps, since each block can use up to six different textures.

Client Behavior: Clients must read unsigned 16 bit integers (instead of 8 bit) for the TopTextureID,SideTextureID,BottomTextureID fields in a DefineBlock packet, and the TopTextureID,LeftTextureId,RightTextureId,FrontTextureId,BackTextureId,BottomTextureID fields in a DefineBlockExt packet. The texture ID should be read modulo client's maximum number of supported textures. (i.e. If client supports up to 512 textures, use ReadU16() & 0x1FF)

Server Behavior: Servers must write unsigned 16 bit integers (instead of 8 bit) for the aforementioned fields in DefineBlock and DefineBlockExt packets. Texture IDs should be written as 8 bits modulo 256 (i.e. WriteU8(id & 0xFF)) for non-supporting clients.

Note: Displaying more textures is achieved by increasing terrain.png to be 256x512 instead of 256x256 in size (or a multiple of that). Hence, EnvMapAppearance extension (deprecated) or EnvMapAspect extension must be mutually supported for this extension to be useful.

SetHotbar

Allows servers to set the contents of a player's hotbar.

Motivation: Gives some control to the server over what the player holds.

Client Behavior: Clients must set the block at the given hotbar position upon receiving the SetHotbar packet.

Server Behavior: Servers may send the SetHotbar packet to change the block at a specific position in a player's hotbar.

SetHotbar packet

Server to client

SetSpawnpoint

Allows servers to directly set the spawn position and orientation of players.

Motivation: Allows setting the spawn of a player without changing their current position. This is to overcome the problem where sending a AddEntity packet with an EntityID of 255 to update's a client spawn also causes them to be repositioned back several blocks if they are currently moving due to latency, which is noticeably jarring and annoying to users.

Client Behavior: Clients must update their spawn using the exact same logic that they use to update their spawn upon receiving a AddEntity or ExtAddEntity2 packet with an EntityID of 255.

Server Behavior: Servers may send the SetSpawnpoint packet to anytime to update the client's spawn. (Note that clients will still continue to update their spawn upon receiving a AddEntity or ExtAddEntity2 packet with an EntityID of 255)

SetSpawnpoint packet

Server to Client

VelocityControl

Allows servers to affect the velocity of players.

Motivation: Allows the server to affect the velocity of a client to make that client jump, knockback that client, etc.

Client Behavior: Clients must adjust their X, Y, and Z velocity based on the fields read from the VelocityControl packet.

Server Behavior: Servers may send the VelocityControl packet at anytime (except during map load) to update the client's velocity.

VelocityControl packet

Server to Client

As an example, to make the client jump up 1.233 blocks without affecting horizontal velocity, send a VelocityControl packet with: VelocityX = 0, VelocityY = 12330, VelocityZ = 0, ModeX = 0, ModeY = 1, ModeZ = 0

CustomParticles

Allows servers to define and display particle effects in the game.

Motivation: The ability to show custom particles can be used to visually enhance map environments, such as making leaves fall from trees or making water splash.

Client Behavior: Clients must be able to receive the packets defined by this extention at any time after level loading. Clients must render particles with the proper attributes defined by the DefineEffect packet upon reception of a SpawnEffect packet. Clients should ignore SpawnEffect packets with an EffectID that is not associated with a defined particle.

Server Behavior: Servers may send any of these packets at any time after level loading. Servers should only send SpawnEffect packets with an EffectID associated with an effect defined by the DefineEffect packet.

DefineEffect packet

Server to client

SpawnEffect packet

Server to client

CustomModels v2

Allows servers to define custom entity models.

Motivation: The ability to show custom models can be used to visually enhance entities.

Client Behavior: Clients must be able to receive the packets defined by this extension at any time. Clients should ignore DefineModelPart and UndefineModel packets with an ModelID that is not associated with a defined model.

Server Behavior: Servers may send any of these packets at any time. Servers should only send DefineModelPart packets with an ModelID associated with a custom model defined by the DefineModel packet.

DefineModel packet

Server to client

DefineModelPart packet

Server to client

UndefineModel packet

Server to client

Data structures

PluginMessage

Allows server plugins and client plugins to communicate with each other.

Motivation: To enable custom client-side and server-side plugins to exchange information.

Client Behavior: When the client receives this packet, it should trigger an event for its plugins, passing along the channel and the string message. Clients can also send this packet when a plugin needs to send a message to the server.

Server Behavior: When the server receives this packet, it should trigger an event for its plugins, passing along the channel and the string message. Servers can also send this packet when a plugin needs to send a message to the client.

PluginMessage packet

Bidirectional

ExtEntityTeleport

Allows server greater control over how the client teleports entities.

Motivation: Increased possibilities when teleporting as well as improved visual result.

Client Behavior: Client must be able to receive this packet any time it can receive a Player Teleport packet (0x08) and teleport as specified by the packet.

Server Behavior: Servers may send the ExtEntityTeleport packet to teleport entities.

ExtEntityTeleport packet

Server to client

Data structure

Unsigned byte holding four fields within its 8 bits.

1 = least significant bit (rightmost)

8 = most significant bit (leftmost)

LightingMode

Indicates support for various ways to light maps clientside and allows server greater control over which lighting mode should be used.

Motivation: Improved visual results, especially in dark levels. Allowing map creators to ensure their level is lit in whatever way looks best.

Client Behavior: Client must be able to receive this packet any time and switch to the lighting mode specified.

Server Behavior: Servers may send the LightingMode packet to change a client's lighting mode.

LightingMode packet

Server to client

Classic lighting mode

• The world is lit just like Minecraft Classic.
• There is a sun color and a shadow color and blocks can cast shadows directly downward.

Fancy lighting mode

• Sunlight and shadow act identically to Classic lighting mode.
• Bright blocks are able to cast a much wider (15 blocks in both directions at most) range of light.
• Bright blocks are able to cast 1 of 2 new environment colors: LavaLight and LampLight.
• With EnvColors extension, the server may send LavaLight and LampLight colors using the enumeration values of 6 and 7 respectively.
• LavaLight should use a default color of #FFEBC6 and LampLight should use a default color of #FFFFFF
• By default, the client should give bright blocks such as lava a LavaLight strength of 15.

Fancy lighting mode interacts with the "BlockDefinitions" extension and changes how the FullBright field from DefineBlock packet should be read

• If FullBright is zero, the block should be treated as usual.
• Else if the leftmost (most significant) bit of FullBright is 0, and the overall value is non-zero, the block should be treated as casting LavaLight strength of 15.
• Else if the leftmost (most significant) bit of FullBright is 1, then the FullBright value should be treated as a LightInfo Structure.
• If the "BlockDefinitionsExt" extension is enabled, this also applies to the FullBright field in the DefineBlockExt packet.

LightInfo structure

Unsigned byte holding three fields within its 8 bits.

1 = least significant bit (rightmost)

8 = most significant bit (leftmost)

CinematicGui

Changes properties of the player's gui to achieve cinematic effects.

Motivation: Allows servers to add black bars to signify to the player that they have initiated a cinematic. Can also be used for things like drowning/health indicators by setting aperture size to full and decreasing the opacity.

Client Behavior: Must hide hand/hotbar/crosshair if those are specified to be disabled. Must render cinematic bars over the game with the supplied aperture/RGB/opacity values.

Server Behavior: Servers may control whether hand/hotbar/crosshair is supposed to be hidden. Server may also configure the appearance of the cinematic bars.

CinematicGui packet

Server to client

NotifyAction

Allows servers to detect when useful events happen in the client.

Motivation: Primary motivations consist of detecting level saves or respawns/setspawns.

Client Behavior: Clients must send a packet when certain events happen, along with any data relevant to the event.

Server Behavior: Servers may listen for NotifyAction packets.

NotifyAction packet

Client to server

NotifyPositionAction packet

Client to server

ToggleBlockList

Allows servers to open or close the block list menu for the player.

Motivation: Enables custom block list GUI control, letting servers open or close the block list GUI when needed.

Client Behavior: Must open or close the block list menu depending on the value of the toggle field.

Server Behavior: Servers may send this packet to control the visibility of the block list GUI.

ToggleBlockList packet

Server to client

Category:Java Edition Classic protocol