Inserting a layer between TCP and IP

Hi,
I am not sure if this is the right platform for my question. I am not
very familiar with linux driver development. If I am wrong here, please
be so kind and tell me, where I should better go to. 
I want to insert an additional layer between the tcp and ip layer with
some special functionality and I am not sure how to do this. I fear it
cannot be done with system_calls, can it? That would be very
comfortable. I could write a loadable kernel module which registers
itself on insert and deregisters itself on remove. 
Is it possible to insert my special layer at runtime without kernel
changes? 
Or do I have to build a special kernel? 
Which ist the best way to insert a layer between the existing tcp and ip
layer?
Thanx, Helmut