Linux network stack : adding protocols with an LKM and dev_add_pack

Question

Hello,

I have recently been trying to familiarize myself with the Linux Networking stack and device drivers (have both similarly named O'Reilly books) with the eventual goal of offloading UDP. I have already implemented UDP on the NIC but now the hard part...

Rather than ask for assistance on this larger goal I was hoping someone could clarify for me a particular snippet I found that is part of a LKM which registeres a new protocol (OTP) that acts as a filter between the device driver and network stack.

http://www.phrack.org/archives/55/p55_0x0c_Building%20Into%20The%20Linux%20Network%20Layer_by_lifeline%20&%20kossak.txt

(Note: this Phrack article contains three different modules, code for the OTP is at the bottom of the page)

In the init function of his example he has:

    otp_proto.type = htons(ETH_P_ALL); 
    otp_proto.func = otp_func;
    dev_add_pack(&otp_proto);

which (if I understand correctly) should register otp_proto as a packet sniffer and put it into the ptype_all data structure. My question is about the dev_add_pack.

Is it the case that the protocol being registered as a filter will always be placed at this layer between L2 and the device driver? Or, for instance could I make such a filtering occur between the application and transport layers (analyze socket parameters) using the same process?

I apologize if this is confusing - I am having some trouble wrapping my head around the bigger picture when it comes to modules altering kernel stack functionality.

Thanks