[Help-gnunet] bandwidth on ADSL

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Hi fellow GNUnetters,

I have finally set up a dedicated GNUnet box, and it's cheerfully chatting
away with other nodes, but I'm not sure how best to control its bandwidth.

I have free upstream traffic, but I pay for downloads after a certain
threshold is reached.

Do MAXNETBPSDOWN and MAXNETBPSUP work averaged over the uptime? 
And how accurately (should I rely on it)?

Will MAXNETBPSDOWN really keep my downloads within quota, or will other
nodes (even non-malicious ones) tend to overshoot the mark?

I don't really care about upstream (since it's free), so should I set that
really high, or will it increase my downstream indirectly?

Finally, does anyone know of some way (in GNUnet or otherwise) to regulate
downloads so that any non-local traffic going out of this box will be
strictly limited to N x gig per month?

Many Thanks.