Dealing with fragmentation in Netty

Question

I am designing a binary protocol for client/server communication using Netty and the number of bytes sent is not fixed amount and can be arbitrary size.

The client is sending something like this:

first 4 bytes to represent an id number the remaining bytes is a String

In the example I have seen, the message has been a fixed size but the String content could be any size and need to ensure it is not fragmented when received by the server. How would I achieve this in Netty?

Thanks in advance.