Matt Bennett [off-list ref] wrote:

I'm emailing this list for feedback on the feasibility of increasing
skb->mark or adding a new field for marking. Perhaps this extension
could be done under a new CONFIG option. Perhaps there are other ways we
could achieve the desired behaviour?

Well I pointed you towards connlabels which provide 128 bit of space
in the conntrack extension area but you did not tell me why you cannot
use it.