"Critical infrastructure is a term used by governments to describe assets that are essential for the functioning of a society and economy"  [http://en.wikipedia.org/wiki/Critical_infrastructure] 

In Australia both Thursday and Friday morning there was a widespread outage (more here and here) of about an hour with Telstra, which is one of the big telecommunications providers in Australia.    On Thursday it seemed that there was no international connection with Telstra at all and on Friday it seemed that root DNS servers were not available from the Telstra network.  Sites whose addresses were cached were accessible, those that required resolution were not (at least on the networks I was connected to).   

Many of you will probably read this and think, so what, it just one ISP.  But the situation is a little bit different here (and I don't mean upside down).  In Australia up until 1997 there was only one telecommunications provider, Telstra (under different names).  The company built all the infrastructure and to this day still owns and maintains a very large portion of the networks.  Pretty much every ISP and other Telecommunications company has varying degrees of dependency on Telstra.  

The end result is that the outage last Thursday and Friday morning was not limited to one ISP, but affected many ISPs who depend on the Telstra infrastructure or who resell Telstra services.   So when looking at critical infrastructure, whether just your organisation's or country, it may not always be obvious how services are provided and there may be dependencies that can affect you of which you were not aware.  When looking at critical infrastructure you have to start thinking outside the box and look at all the elements that make up a specific service and not limit yourself to only your organisation, but also examine the external influences on the infrastructure your company or country depends on. 

Mark H - Shearwater

SANS Critical Infrastructure Protection Course with Marc Sachs September 10-11 Canberra.