Plugin channels

Plugin channels are implemented using packet 0xFA, allowing client mods and server plugins to communicate without cluttering up chat. This post by Dinnerbone is a good introduction and basic documentation.

Reserved channels

REGISTER

Two-way

Allows the client to register for one or more custom channels, indicating that the server should send data on those channels if the appropriate plugin is installed. Payload is a null (\00) separated list of strings.

UNREGISTER

Two-way

Allows the client to unregister from one or more custom channels, indicating that the server should stop sending data on those channels. Payload is a null-separated list of strings.

Channels internal to Minecraft

As of 1.3, Minecraft itself started using plugin channels to implement new features. These internal channel names are prefixed by MC|. They are not formally registered using the REGISTER channel. The vanilla Minecraft server will send these packets regardless, and the vanilla client will accept them.

MC|BEdit

Client to server

When a player edits an unsigned book.

This payload is simply a set of bytes corresponding to an ItemStack. It is serialized / deserialized exactly the same as itemstacks in packets.

MC|BSign

Client to server

When a player signs a book. This payload is simply a set of bytes corresponding to an ItemStack. It is serialized / deserialized exactly the same as itemstacks in packets.

MC|TPack

Server to client

Remote texture packs. This contains a string representing the url to get the texture pack from, followed by null (\00) and then '16', all in ASCII.

MC|TrList

Server to client

The list of trades a villager NPC is offering.

MC|Beacon

Client to server

To integers corresponding to the 2 effects a user wishes to have active

MC|TrSel

Client to server

Notable community plugin channels

Channels listed in this section are not Mojang-sanctioned. This is just a likely-incomplete list of channels used by mods/plugins popular within the Minecraft community.

ML|OpenTE

Server to client

Used by ModLoader to support custom GUI windows. Does not use the REGISTER channel.

WECUI

Two-way

Used by the server-side WorldEdit and the client-side WorldEditCUI to coordinate selections.