From mboxrd@z Thu Jan  1 00:00:00 1970
From: Florian Westphal <fw@strlen.de>
Subject: Re: NFTables Internals
Date: Thu, 27 Oct 2016 18:29:41 +0200
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To: mark diener <rpzrpzrpz@gmail.com>
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mark diener <rpzrpzrpz@gmail.com> wrote:
> https://workshop.netfilter.org/2016/wiki/images/7/78/Nft-tutorial.pdf
> 
> Now I just need to figure out how to interpet the "payload load 4b @
> network_header + 12" nomenclature to really get a handle on things.

It tells the payload expression to load 4 bytes, with offset 12,
realtive to network header (base).

> Any more documents or videos to understand the internals?
> (Instead of hours spent looking at source code!)

Whats your goal?

If you really want to learn the internal workings source code is indeed
best option.