The client split into two main parts: a thread that gets the user's messages and another thread that pings the server occasionally for message updates. The first thread is just a wrapper around a BufferedReader that passes it over to a packet encoder. The second thread pings the server every second or so to ask if any new messages have arrived.
The server has a list of recent messages, which is updated anytime a message is recieved from a client. If a message update is requested, it bundles up the messages and sends them to the client.
The encoder uses netty.io ByteBufs to write create packets. Generally, the packets follow the following pattern:
[packet id] [length of item] [item] [length of item] [item] ...
The decoder verifies the packet id and then decodes the packets by reading lengths and items until the ByteBuf is consumed.