A lot of tools can do this, it just depends on HOW you want to do it, WHERE and of course more importantly what information you want to see. NFSen/nTop come to mind at the minimum, but that is more because I have a Netflow-ish background, there are many many other tools though, just depends on what you like. If you have a tunnel interface, or a separate interface for IPv6 then you can just check the traffic counters of that interface. Otherwise, enable ip6tables on the host with one or more (accept/log/count/skip) rules and you will at least get counters that way. If you want to see more detail, configure more detailed/specific rules and/or install one of the above tools and start using that.