1.14.4 What does the discriminator byte do in PacketBuffers?

[Keheck]

I am currently setting up some networking to synchronize data between Clients and Servers, and I've been running into a problem where I got an invalid discriminator byte on my channel.

I've fixed that by "shifting" the data written to the PacketBuffer in my encoding method by one byte.

To know what exactly what I'm talking about, you can take a look at this and that class, they contain the important stuff (Mod's main class and the Message class, respectively). The packet is sent off here . (An Item class' onItemRightClick method)

 

Or you can open the spoiler:

 

 

 

 

 

 

 

 

Spoiler

In the mod's main class (io.github.kehek.mobfighters.Mobfighters)

private static int MSG_ID = 0;
// CHANNEL is a SimpleChannel instance named "mobfighters:main" and a simple version cheking of "1".equals("1") and a Supplier<String> returning "1"
public void doCommonSetup(FMLCommonSetupEvent event)
{                               //Originally MSG_ID++, doesn't change the outcome in any notable ways though
    MobfightersNetworkHandler.CHANNEL.registerMessage(++MSG_ID, FightStartPacket.class, FightStartPacket::encode, FightStartPacket::decode, FightStartPacket::handle);
}

In the message class (io.github.keheck.mobfighters.util.network.FightStartPacket)

// Offset the data by one byte, to get a valid discriminator at index 0
// Was previously not present, and caused invalid discriminator problems
private static final int off = 1;
public static void encode(FightStartPacket packet, PacketBuffer buffer)
{
    for(int i = 0; i < 4; i++)
    {
        // Put the least significant bits in the buffer
        boolean putLeast = i%2 == 0;

        // Put the player in the buffer
        if(i < 2)
        {
            if(putLeast)
                buffer.setLong(i*Long.BYTES+off, packet.playerUUID.getLeastSignificantBits());
            else
                buffer.setLong(i*Long.BYTES+off, packet.playerUUID.getMostSignificantBits());
        }
        // Put the enemy in the buffer
        else
        {
            if(putLeast)
                buffer.setLong(i*Long.BYTES+off, packet.enemyUUID.getLeastSignificantBits());
            else
                buffer.setLong(i*Long.BYTES+off, packet.enemyUUID.getMostSignificantBits());
        }
    }
    buffer.setInt(Long.BYTES*4+off, packet.fightID);
    buffer.setBoolean(Long.BYTES*4 + Integer.BYTES+off, packet.enemyWild);
}

In the item's class (io.github.keheck.mobfighters.item.FightInitiatorItem)

@Override
public ActionResult<ItemStack> onItemRightClick(World worldIn, PlayerEntity playerIn, Hand handIn)
{
    // Disclaimer: Simplified for readability
    if(!worldIn.isRemote())
    {
        ServerPlayerEntity serverPlayer = (ServerPlayerEntity)playerIn;

        // I know, there are convenience-methods for sending packets to clients.
        MobfightersNetworkHandler.CHANNEL.sendTo
                (
                        new FightStartPacket(1, playerIn, playerIn),
                        serverPlayer.connection.getNetworkManager(),
                        NetworkDirection.PLAY_TO_CLIENT
                );
    }

    return super.onItemRightClick(worldIn, playerIn, handIn);
}

 

 

It seems like the discriminator is somehow tied to the id of the Message registered on the channel but I would like some confirmation/correction on that theory.

What is the discriminator actually used for?

[Keheck]

So if I understood correctly, the first byte of a packet payload is always(?) the id of the message in the specific channel. It is to correctly identify the message that the payload needs to be decoded into, since there is no (more efficient) way to store the message type itself into the payload, correct?

[Keheck]

3 minutes ago, diesieben07 said:

You could use newEventChannel and write your own implementation, if you need it.

When would it be better to use that instead of a SimpleChannel? Like, what are some example cases you could name?