What countermeasures does Bitcoin currently have in place to limit Sybil attacks (ie of full nodes [not miners]) and eclipse attacks?
In particular, it seems like an important countermeasure would be to attempt to give your connections only about as much resources (bandwidth/processing/etc) as they're giving back to you, so as to ensure that a sybil attacker must cost approximately the same amount as an honest node. But I get the feeling this doesn't exist. Does it?
Are there other countermeasures in place currently?