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My opinions are US specific. I don't know enough about most foreign countries constitutions to make such harsh judgments about this subject. If you live in China sorry but you can't complain (mostly because you would get thrown into prison) if you want access to unmonitored web traffic move or overthrow your government.

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ISPs should not be allowed to censor traffic that goes across their networks. But they should be allowed to take administrative actions when directed to by an authorized authority. For example I don't think an ISP should automatically take down a website they "believe" is hosting illegal content. They should certainly report it to the proper authorities and take action if directed to do so. This concept would also extend to network traffic ISPs shouldn't be allowed to view traffic at such a low level that they can distinguish between legal and illegal content. If a proper authority authorizes them to capture that information for law enforcement reasons that is acceptable.
ISPs would still retain the right to manage their network traffic to keep the network running properly. If they are seeing a DOS traffic that is negatively effecting their network they have the right to drop that traffic. If a customer requests certain traffic to be blocked from their network that isn't a problem. I would even go so far as to say if an ISP wanted to block all P2P traffic through their network that is acceptable as long as they aren't required or being pressured to do so for reasons other then sound network management. If an ISP is willing to tell it's customers that they do not allow P2P traffic they should be able to block that traffic. They may lose customers but that is their choice.