Buffer handling example?

[pinus]

If I understand MINA right I need to implement my own protocol encoder  
and decoder to support my protocol. The encoder looks quite simple but  
the decoder looks a bit strange.
If I understand it right the decoder should be aware that a message  
might be split to multiple doDecode() calls. The examples I found save  
some state in the session. I'm ok so far.
But I do nut understand the buffer handling. If I'm right, I'm allowed  
to read from position() to limit(). But what happens if I read to  
limit() and have no message complete? Do I get the same buffer next  
time again and can store the position where I stopped? If I set a  
mark() is it stored over multiple doDecode() calls? How much buffer is  
available? How do I mark the read part of the buffer as read?
Is there an example that explains how to handle the multiple  
doDecode() messages and the buffer handling?