[ubuntu] How to measure volume of internet use

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http://ubuntuforums.org – I want to measure how much Internet I use on a day-to-day basis. I don't mean how much time I spend on the Internet. I mean volume of data; i.e. how many Mb of data I download and upload, including everything: surfing, on-line backups, emails, IM, VOIP, updates, streaming, and so forth. I don't need this broken down by type (though it would be nice); I just need totals per 24 hour periods. How can I get these statistics? Do I need to install a special program? TIA (HowTos)