right. my point is that if you are selling the customer a service, and you say 'I will get you network connectivity' and then, for reasons outside your control, you don't get them network connectivity, it doesn't make much difference to the customer if the network is broken because you did something dumb or if the network is broken you are getting DDos'd from china. the point is that the network is broken.
last month I paid out almost fourteen grand in SLA credits because I didn't stop a DDos within my allowed 0.5% downtime. Was it my fault I got DDos'd? no. However, i was the only one in a position to do something about it. (and really, if I wasn't tired and generally an idiot, we would have been down for an hour rather than 8.)
You do need clear lines, though. if you need connectivity from point A to point B, that's easy, I can guarantee that. But defining connectivity to 'the internet' is harder. there are cases where I've got good connectivity to most places, but you can't get to some ISP in dallas, because they've hoarked up the routing table.
Right now, I play that sort of thing by ear. If only one customer is having the problem, I try to figure out where it is and if I can't figure it out, it's not that big of a deal to give them a credit. If many customers are having the problem, well, then I have a problem, and really, it's my job to figure out where that problem is and to work around it... even if that problem is a misconfigured router at some other ISP. I mean, really, what is the customer going to do about that sort of thing?
this is the point of having a SLA; it aligns the interests of the service provider with the interests of the customer.