How do I measure data transfer across a network interface related to a specifc user?

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http://serverfault.com – I have an Ubuntu server and I am interested in measuring how much data is being transferred (both in and out) across a network interface, by a specific user. How do I do it? N.B. I know how to measure the total data transfer of the whole machine. I want to restrict my monitoring to a specific user. (HowTos)