Is it technically possible to change the mining protocol, so that a certain IP (solo miner or pool operator) cant have more than 49% hashpower ?
No.
#1 you cant force miners to update their client, so you are suggesting a hardfork
#2 you cant check how much hashingpower a certain IP has. You might allow only 49% of all block over a given time from a certain IP, but that would need a central place to deliver those blocks which can check where they come from. Thats not p2p, thats even more power on a single point. besides that: miners could just change the IP.
also: there allready is an emergency solution:
http://gavintech.blogspot.de/2012/05/neutralizing-51-attack.htmlLooks like this emergency solution is a push to centralization as already pointed out by some in the comment section. Old coins wont be there with new miners so mining will become the monopoly of the oldies.
Yes its a bad solution thats why its filed under "only use in case of emergency"
Old coins wont be there with new miners? I dont understand what you are trying to say.
This is what Gavins is saying...
Something like "ignore a longer chain orphaning the current best chain if the sum(priorities of transactions included in new chain) is much less than sum(priorities of transactions in the part of the current best chain that would be orphaned)" would mean a 51% attacker would have to have both lots of hashing power AND lots of old, high-priority bitcoins to keep up a transaction-denial-of-service attack. And they'd pretty quickly run out of old, high-priority bitcoins and would be forced to either include other people's transactions or have their chain rejected.
and this what my point is...
closing the door to participation in mining a little by including some kind of "trusted" bit in participation will make more sense as participation shifts from fully chain-aware wallets to intermediaries. The best way to do this might be closing participation in the public chain lengthening process, perhaps by all the current mining pools defining a closed communications channel and thereby shutting out newcomers, or at least vetting them a little. That way, if some major player with the ability to purpose hardware to deliver a 51% attack wanted to, they'd also need to join the club. Something like American anti-trust rules could be introduced, such as, a pool may not submit more than ten of every fifty blocks. With SMPPS, a pool that got lucky and is getting close to its limit can trivially distribute work for other pools instead to avoid forced downtime.
