Add basic implementation.

api/src/main/java/com/velocitypowered/api/proxy/Player.java

@@ -1,6 +1,7 @@
 package com.velocitypowered.api.proxy;
 
 import com.velocitypowered.api.command.CommandSource;
+import com.velocitypowered.api.proxy.messages.ChannelMessageSource;
 import com.velocitypowered.api.server.ServerInfo;
 import com.velocitypowered.api.util.MessagePosition;
 import net.kyori.text.Component;
@@ -12,7 +13,7 @@ import java.util.UUID;
 /**
  * Represents a player who is connected to the proxy.
  */
-public interface Player extends CommandSource, InboundConnection {
+public interface Player extends CommandSource, InboundConnection, ChannelMessageSource {
     /**
      * Returns the player's current username.
      * @return the username

api/src/main/java/com/velocitypowered/api/proxy/ProxyServer.java

@@ -4,6 +4,7 @@ import com.velocitypowered.api.command.CommandSource;
 import com.velocitypowered.api.command.CommandManager;
 import com.velocitypowered.api.event.EventManager;
 import com.velocitypowered.api.plugin.PluginManager;
+import com.velocitypowered.api.proxy.messages.ChannelRegistrar;
 import com.velocitypowered.api.scheduler.Scheduler;
 import com.velocitypowered.api.server.ServerInfo;
 
@@ -101,4 +102,10 @@ public interface ProxyServer {
      * @return the scheduler instance
      */
     Scheduler getScheduler();
+
+    /**
+     * Gets the {@link ChannelRegistrar} instance.
+     * @return the channel registrar
+     */
+    ChannelRegistrar getChannelRegistrar();
 }

api/src/main/java/com/velocitypowered/api/proxy/ServerConnection.java

@@ -1,11 +1,12 @@
 package com.velocitypowered.api.proxy;
 
+import com.velocitypowered.api.proxy.messages.ChannelMessageSource;
 import com.velocitypowered.api.server.ServerInfo;
 
 /**
  * Represents a connection to a backend server from the proxy for a client.
  */
-public interface ServerConnection {
+public interface ServerConnection extends ChannelMessageSource {
     ServerInfo getServerInfo();
 
     Player getPlayer();

api/src/main/java/com/velocitypowered/api/proxy/messages/ChannelIdentifier.java

@@ -4,4 +4,5 @@ package com.velocitypowered.api.proxy.messages;
  * Represents a kind of channel identifier.
  */
 public interface ChannelIdentifier {
+    String getId();
 }

api/src/main/java/com/velocitypowered/api/proxy/messages/ChannelMessageSource.java

@@ -0,0 +1,4 @@
+package com.velocitypowered.api.proxy.messages;
+
+public interface ChannelMessageSource {
+}

api/src/main/java/com/velocitypowered/api/proxy/messages/ChannelRegistrar.java

@@ -1,4 +1,11 @@
 package com.velocitypowered.api.proxy.messages;
 
+/**
+ * Represents an interface to register and unregister {@link MessageHandler} instances for handling plugin messages from
+ * the client or the server.
+ */
 public interface ChannelRegistrar {
+    void register(MessageHandler handler, ChannelIdentifier... identifiers);
+
+    void unregister(ChannelIdentifier... identifiers);
 }

api/src/main/java/com/velocitypowered/api/proxy/messages/ChannelSide.java

@@ -0,0 +1,6 @@
+package com.velocitypowered.api.proxy.messages;
+
+public enum ChannelSide {
+    FROM_SERVER,
+    FROM_CLIENT
+}

api/src/main/java/com/velocitypowered/api/proxy/messages/LegacyChannelIdentifier.java

@@ -40,4 +40,9 @@ public final class LegacyChannelIdentifier implements ChannelIdentifier {
     public int hashCode() {
         return Objects.hash(name);
     }
+
+    @Override
+    public String getId() {
+        return name;
+    }
 }

api/src/main/java/com/velocitypowered/api/proxy/messages/MessageHandler.java

@@ -0,0 +1,10 @@
+package com.velocitypowered.api.proxy.messages;
+
+public interface MessageHandler {
+    ForwardStatus handle(ChannelMessageSource source, ChannelSide side, byte[] data);
+
+    enum ForwardStatus {
+        FORWARD,
+        HANDLED
+    }
+}

api/src/main/java/com/velocitypowered/api/proxy/messages/MinecraftChannelIdentifier.java

@@ -61,4 +61,9 @@ public final class MinecraftChannelIdentifier implements ChannelIdentifier {
     public int hashCode() {
         return Objects.hash(namespace, name);
     }
+
+    @Override
+    public String getId() {
+        return namespace + ":" + name;
+    }
 }