Codec and Headers

I have a question regarding Custom Codecs and Headers. I have a message
format that is is a 28 byte header followed by XML. I am using the MINA2
component to receive the data and implemented a custom codec to read the
data and add the header and XML to the body as a byte[]. I would like my
codec to parse the header and set that data in the Message header and have
the Message body be only the XML. I have not been able to find out how to
do that. Will I need to send the byte[] to a processor after the codec to
separate it out or can I do it from the codec itself?

Thanks!

- Tim