No. I'm talking per account. If an account does what I mentioned above, that account's fees scale or get delayed.
And so the attacker moves accounts and continues his attack?
Depends on the algorithm. Assume the network notices there are 10tx/s at any given moment. It delays transactions spreading them throughout random future blocks and readjusts that according to the amount of transaction each block is having atm. Assume it reaches 10tx/s then it starts readjusting them to the next few blocks, if all those blocks are getting filled with transactions surpassing a given limit it extends the next ones throughout the next blocks and gives priority to accounts with single transactions. It would have to compare the number of all transactions made by all accounts in the last X blocks though, that might cause some performance issues..
Of course I don't know about the feasibility of this. If it notices the same account having multiple transactions on an interval of 5 or 10 blocks then rises the fee for that account. Of course he can do this with more accounts. Then you prioritize accounts with less transactions. Assume there are more 100 accounts doing the same, those would get their transactions delayed in future blocks where normal non-attacker accounts would get their transactions prioritized.
The challenge here is identifying attacking accounts.
Imagine each account being flagged if the number of transactions in any given number of blocks surpasses a pre defined limit. Let's say 100 transactions in 10 blocks or 20 transactions per 3 blocks, whatever number we see fit. If an account hits that, it gets flagged and restricted for the following X blocks. From the moment an account gets flagged, all other accounts get a limited amount of transactions per blocks.
This would mean that if the attacker has 100s of accounts, now instead of for example them being able to do each 100tx per 10 blocks they can only do 10. Normal users don't really need to spam the network with tons of transactions, 1 per block is fine per user I guess?
As for big businesses in the future having the need to do more transactions per block, they get a different flag that allows them to have a bigger or no limit at all.
Just throwing some random stuff there. I think the last example I gave makes more sense.. Then any number of accounts would have their number of transactions restricted. I think you might still argue we're making his job easier, but we really only have to find the right limits from here I think. It can be more, or less. It's "just" a matter of figuring it out.
Once again I'm not a technical guy, just throwing some stuff that makes sense in my head, until you come with a counter argument. If everyone brainstormed we might get something.